Internet DRAFT - draft-andreasen-mgcp-rfc2705bis

draft-andreasen-mgcp-rfc2705bis





Internet Engineering Task Force                            F. Andreasen 
Internet Draft                                                B. Foster 
Document: draft-andreasen-mgcp-rfc2705bis-05.txt          Cisco Systems 
Category: Informational                                       
                                                           May 29, 2002 
 
 
                 Media Gateway Control Protocol (MGCP) 
                              Version 1.0 
    
Status of this Memo 
    
   This document is an Internet-Draft and is in full conformance with 
   all provisions of Section 10 of RFC2026 [1].  
    
   Internet-Drafts are working documents of the Internet Engineering 
   Task Force (IETF), its areas, and its working groups. Note that 
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of 
   six months and may be updated, replaced, or obsoleted by other 
   documents at any time. It is inappropriate to use Internet-Drafts as 
   reference material or to cite them other than as "work in progress."  
    
   The list of current Internet-Drafts can be accessed at 
   http://www.ietf.org/ietf/1id-abstracts.txt  
    
   The list of Internet-Draft Shadow Directories can be accessed at 
   http://www.ietf.org/shadow.html. 
    
Abstract 
    
   This document describes an application programming interface and a 
   corresponding protocol (MGCP) which is used between elements of a 
   decomposed multimedia gateway. The decomposed multimedia gateway 
   consists of a Call Agent, which contains the call control 
   "intelligence", and a media gateway which contains the media 
   functions, e.g. conversion from TDM voice to Voice over IP.  
    
   Media gateways contain endpoints on which the Call Agent can create, 
   modify and delete connections in order to establish and control 
   media sessions with other multimedia endpoints. Also, the Call Agent 
   can instruct the endpoints to detect certain events and generate 
   signals. The endpoints automatically communicate changes in service 
   state to the Call Agent. Furthermore, the Call Agent can audit 
   endpoints as well as the connections on endpoints.  
    
   The basic and general MGCP protocol is defined in this document, 
   however most media gateways will need to implement one or more MGCP 
   packages, which define extensions to the protocol suitable for use 
   with specific types of media gateways. Such packages are defined in 
   separate documents.  
    
 
                                                                        
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Table of Contents 
 
1. INTRODUCTION.......................................................5 
 1.1 Relation with the H.323 Standards...............................6 
 1.2 Relation with the IETF Standards................................7 
 1.3 Definitions.....................................................9 
 1.4 Conventions used in this Document...............................9 
2. MEDIA GATEWAY CONTROL INTERFACE...................................10 
 2.1 Model and Naming Conventions...................................10 
   2.1.1 Types of Endpoints.........................................10 
   2.1.2 Endpoint Identifiers.......................................14 
   2.1.3 Calls and Connections......................................16 
   2.1.4 Names of Call Agents and Other Entities....................22 
   2.1.5 Digit Maps.................................................23 
   2.1.6 Packages...................................................26 
   2.1.7 Events and Signals.........................................27 
 2.2 Usage of SDP...................................................31 
 2.3 Gateway Control Commands.......................................32 
   2.3.1 Overview of Commands.......................................32 
   2.3.2 EndpointConfiguration......................................34 
   2.3.3 NotificationRequest........................................35 
   2.3.4 Notify.....................................................42 
   2.3.5 CreateConnection...........................................43 
   2.3.6 ModifyConnection...........................................49 
   2.3.7 DeleteConnection (from the Call Agent).....................51 
   2.3.8 DeleteConnection (from the gateway)........................54 
   2.3.9 DeleteConnection (multiple connections from the Call Agent) 55 
   2.3.10 AuditEndpoint.............................................56 
   2.3.11 AuditConnection...........................................60 
   2.3.12 RestartInProgress.........................................62 
 2.4 Return Codes and Error Codes...................................64 
 2.5 Reason Codes...................................................69 
 2.6 Use of Local Connection Options and Connection Descriptors.....69 
 2.7 Resource Reservations..........................................71 
3. MEDIA GATEWAY CONTROL PROTOCOL....................................72 
 3.1 General Description............................................72 
 3.2 Command Header.................................................73 
   3.2.1 Command Line...............................................73 
   3.2.2 Parameter Lines............................................76 
 3.3 Format of response headers.....................................92 
   3.3.1 CreateConnection Response..................................95 
   3.3.2 ModifyConnection Response..................................96 
   3.3.3 DeleteConnection Response..................................96 
   3.3.4 NotificationRequest Response...............................96 
   3.3.5 Notify Response............................................96 
   3.3.6 AuditEndpoint Response.....................................97 
   3.3.7 AuditConnection Response...................................97 
   3.3.8 RestartInProgress Response.................................98 
 3.4 Encoding of the Session Description (SDP)......................98 
   3.4.1 Usage of SDP for an Audio Service.........................100 
                                                                        
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   3.4.2 Usage of SDP for LOCAL Connections........................100 
 3.5 Transmission over UDP.........................................101 
   3.5.1 Providing the At-Most-Once Functionality..................101 
   3.5.2 Transaction Identifiers and Three Ways Handshake..........102 
   3.5.3 Computing Retransmission Timers...........................103 
   3.5.4 Maximum Datagram Size, Fragmentation and Reassembly.......104 
   3.5.5 Piggybacking..............................................105 
   3.5.6 Provisional Responses.....................................105 
4. STATES, FAILOVER AND RACE CONDITIONS.............................108 
 4.1 Failover Assumptions and Highlights...........................108 
 4.2 Communicating with Gateways...................................110 
 4.3 Retransmission, and Detection of Lost Associations:...........110 
 4.4 Race Conditions...............................................114 
   4.4.1 Quarantine List...........................................114 
   4.4.2 Explicit Detection........................................119 
   4.4.3 Transactional Semantics...................................120 
   4.4.4 Ordering of Commands, and Treatment of Misorder...........122 
   4.4.5 Endpoint Service States...................................124 
   4.4.6 Fighting the Restart Avalanche............................126 
   4.4.7 Disconnected Endpoints....................................128 
   4.4.8 Load Control in General...................................131 
5. SECURITY REQUIREMENTS............................................133 
 5.1 Protection of Media Connections...............................133 
6. PACKAGES.........................................................135 
 6.1 Actions.......................................................136 
 6.2 BearerInformation.............................................136 
 6.3 ConnectionModes...............................................137 
 6.4 ConnectionParameters..........................................137 
 6.5 DigitMapLetters...............................................137 
 6.6 Events and Signals............................................138 
   6.6.1 Default and Reserved Events...............................140 
 6.7 ExtensionParameters...........................................141 
 6.8 LocalConnectionOptions........................................142 
 6.9 Reason Codes..................................................142 
 6.10 RestartMethods...............................................143 
 6.11 Return Codes.................................................143 
7. VERSIONS AND COMPATIBILITY.......................................144 
 7.1 Changes from RFC 2705.........................................144 
8. SECURITY CONSIDERATIONS..........................................149 

9. ACKNOWLEDGEMENTS.................................................150 

10. REFERENCES......................................................151 

11. AUTHORS' ADDRESSES..............................................153 

APPENDIX A: FORMAL SYNTAX DESCRIPTION OF THE PROTOCOL...............154 

                                                                        
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APPENDIX B: BASE PACKAGE............................................161 
 B.1 Events........................................................161 
 B.2 Extension Parameters..........................................161 
   B.2.1 PersistentEvents..........................................161 
   B.2.2 NotificationState.........................................162 
 B.3 Verbs.........................................................163 
APPENDIX C: IANA CONSIDERATIONS.....................................165 
 C.1 New MGCP Package Sub-Registry.................................165 
 C.2 New MGCP Package..............................................165 
 C.3 New MGCP LocalConnectionOptions Sub-Registry..................165 
APPENDIX D: MODE INTERACTIONS.......................................167 

APPENDIX E: ENDPOINT NAMING CONVENTIONS.............................169 
 E.1 Analog Access Line Endpoints..................................169 
 E.2 Digital Trunks................................................169 
 E.3 Virtual Endpoints.............................................169 
 E.4 Media Gateway.................................................170 
 E.5 Range Wildcards...............................................171 
APPENDIX F: EXAMPLE COMMAND ENCODINGS...............................172 
 F.1 NotificationRequest...........................................172 
 F.2 Notify........................................................172 
 F.3 CreateConnection..............................................173 
 F.4 ModifyConnection..............................................175 
 F.5 DeleteConnection (from the Call Agent)........................176 
 F.6 DeleteConnection (from the gateway)...........................176 
 F.7 DeleteConnection (multiple connections from the Call Agent)...176 
 F.8 AuditEndpoint.................................................177 
 F.9 AuditConnection...............................................178 
 F.10 RestartInProgress............................................179 
APPENDIX G: EXAMPLE CALL FLOWS......................................181 
 G.1 Restart.......................................................181 
   G.1.1 Residential Gateway Restart...............................181 
   G.1.2 Call Agent Restart........................................184 
 G.2 Connection Creation...........................................186 
   G.2.1 Residential Gateway to Residential Gateway................186 
 G.3 Connection Deletion...........................................191 
   G.3.1 Residential Gateway to Residential Gateway................191 
FULL COPYRIGHT STATEMENT............................................194 

ACKNOWLEDGEMENT.....................................................195 
 
 
 
 



                                                                        
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1. Introduction 
 
   This document describes an abstract application programming 
   interface (MGCI) and a corresponding protocol (MGCP) for controlling 
   media gateways from external call control elements called media 
   gateway controllers or Call Agents. A media gateway is typically a 
   network element that provides conversion between the audio signals 
   carried on telephone circuits and data packets carried over the 
   Internet or over other packet networks. Example of media gateways 
   are: 
 
   *  Trunking gateways, that interface between the telephone network 
      and a Voice over IP network. Such gateways typically manage a 
      large number of digital circuits. 
    
   *  Voice over ATM gateways, which operate much the same way as voice 
      over IP trunking gateways, except that they interface to an ATM 
      network. 
    
   *  Residential gateways, that provide a traditional analog (RJ11) 
      interface to a Voice over IP network. Examples of residential 
      gateways include cable modem/cable set-top boxes, xDSL devices, 
      broad-band wireless devices 
    
   *  Access gateways, that provide a traditional analog (RJ11) or 
      digital PBX interface to a Voice over IP network. Examples of 
      access gateways include small-scale voice over IP gateways. 
    
   *  Business gateways, that provide a traditional digital PBX 
      interface or an integrated "soft PBX" interface to a Voice over 
      IP network. 
    
   *  Network Access Servers, that can attach a "modem" to a telephone 
      circuit and provide data access to the Internet. We expect that, 
      in the future, the same gateways will combine Voice over IP 
      services and Network Access services. 
    
   *  Circuit switches, or packet switches, which can offer a control 
      interface to an external call control element. 
    
   MGCP assumes a call control architecture where the call control 
   "intelligence" is outside the gateways and handled by external call 
   control elements known as Call Agents. The MGCP assumes that these 
   call control elements, or Call Agents, will synchronize with each 
   other to send coherent commands and responses to the gateways under 
   their control. If this assumption is violated, inconsistent behavior 
   should be expected. MGCP does not define a mechanism for 
   synchronizing Call Agents. MGCP is, in essence, a master/slave 
   protocol, where the gateways are expected to execute commands sent 
   by the Call Agents. In consequence, this document specifies in great 
   detail the expected behavior of the gateways, but only specifies 
   those parts of a Call Agent implementation, such as timer 
   management, that are mandated for proper operation of the protocol. 
                                                                        
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   MGCP assumes a connection model where the basic constructs are 
   endpoints and connections. Endpoints are sources and/or sinks of 
   data and can be physical or virtual. Examples of physical endpoints 
   are: 
    
   *  An interface on a gateway that terminates a trunk connected to a 
      PSTN switch (e.g., Class 5, Class 4, etc.). A gateway that 
      terminates trunks is called a trunking gateway. 
    
   *  An interface on a gateway that terminates an analog POTS 
      connection to a phone, key system, PBX, etc. A gateway that 
      terminates residential POTS lines (to phones) is called a 
      residential gateway. 
    
   An example of a virtual endpoint is an audio source in an audio-
   content server. Creation of physical endpoints requires hardware 
   installation, while creation of virtual endpoints can be done by 
   software. 
    
   Connections may be either point to point or multipoint. A point to 
   point connection is an association between two endpoints with the 
   purpose of transmitting data between these endpoints. Once this 
   association is established for both endpoints, data transfer between 
   these endpoints can take place. A multipoint connection is 
   established by connecting the endpoint to a multipoint session. 
    
   Connections can be established over several types of bearer 
   networks, for example: 
    
   *  Transmission of audio packets using RTP and UDP over an IP 
      network. 
   
   *  Transmission of audio packets using AAL2, or another adaptation 
      layer, over an ATM network. 
   
   *  Transmission of packets over an internal connection, for example 
      the TDM backplane or the interconnection bus of a gateway. This 
      is used, in particular, for "hairpin" connections, connections 
      that terminate in a gateway but are immediately rerouted over the 
      telephone network. 
 
   For point-to-point connections the endpoints of a connection could 
   be in separate gateways or in the same gateway. 
    
1.1 Relation with the H.323 Standards 
    
   MGCP is designed as an internal protocol within a distributed system 
   that appears to the outside as a single VoIP gateway. This system is 
   composed of a Call Agent, that may or may not be distributed over 
   several computer platforms, and of a set of gateways, including at 
   least one "media gateway" that perform the conversion of media 
   signals between circuits and packets, and at least one "signaling 
                                                                        
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   gateway" when connecting to an SS7 controlled network. In a typical 
   configuration, this distributed gateway system will interface on one 
   side with one or more telephony (i.e. circuit) switches, and on the 
   other side with H.323 conformant systems, as indicated in the 
   following table: 
    
    ------------------------------------------------------------------ 
   | Functional|  Phone     |  Terminating    |  H.323 conformant     | 
   | Plane     |  switch    |  Entity         |  systems              | 
   |-----------|------------|-----------------|-----------------------| 
   | Signaling |  Signaling |  Call agent     |  Signaling exchanges  | 
   | Plane     |  exchanges |                 |  with the Call Agent  | 
   |           |  through   |                 |  through H.225/RAS and| 
   |           |  SS7/ISUP  |                 |  H.225/Q.931.         | 
   |-----------|------------|-----------------|-----------------------| 
   |           |            |                 |  Possible negotiation | 
   |           |            |                 |  of logical channels  | 
   |           |            |                 |  and transmission     | 
   |           |            |                 |  parameters through   | 
   |           |            |                 |  H.245 with the call  | 
   |           |            |                 |  agent.               | 
   |-----------|------------|-----------------|-----------------------| 
   |           |            |  Internal       |                       | 
   |           |            |  synchronization|                       | 
   |           |            |  through MGCP   |                       | 
   |-----------|------------|-----------------|-----------------------| 
   | Bearer    |  Connection|  Telephony      |  Transmission of VoIP | 
   | Data      |  through   |  gateways       |  data using RTP       | 
   | Transport |  high speed|                 |  directly between the | 
   | Plane     |  trunk     |                 |  H.323 station and the| 
   |           |  groups    |                 |  gateway.             | 
    ------------------------------------------------------------------ 
    
   In the MGCP model, the gateways focus on the audio signal 
   translation function, while the Call Agent handles the call 
   signaling and call processing functions. As a consequence, the Call 
   Agent implements the "signaling" layers of the H.323 standard, and 
   presents itself as an "H.323 Gatekeeper" or as one or more "H.323 
   Endpoints" to the H.323 systems. 
    
1.2  Relation with the IETF Standards 
    
   While H.323 is the recognized standard for VoIP terminals, the IETF 
   has also produced specifications for other types of multi-media 
   applications. These other specifications include: 
    
   *  the Session Description Protocol (SDP), RFC 2327, 
    
   *  the Session Announcement Protocol (SAP), RFC 2974 
    
   *  the Session Initiation Protocol (SIP), RFC 2543 
    
   *  the Real Time Streaming Protocol (RTSP), RFC 2326. 
                                                                        
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   The latter three specifications are in fact alternative signaling 
   standards that allow for the transmission of a session description 
   to an interested party. SAP is used by multicast session managers to 
   distribute a multicast session description to a large group of 
   recipients, SIP is used to invite an individual user to take part in 
   a point-to-point or unicast session, RTSP is used to interface a 
   server that provides real time data. In all three cases, the session 
   description is described according to SDP; when audio is 
   transmitted, it is transmitted through the Real-time Transport 
   Protocol, RTP. 
    
   The distributed gateway systems and MGCP will enable PSTN telephony 
   users to access sessions set up using SAP, SIP or RTSP. The Call 
   Agent provides for signaling conversion, according to the following 
   table: 
    
    ------------------------------------------------------------------ 
   | Functional|  Phone     |  Terminating  |  IETF conforming systems| 
   | Plane     |  switch    |  Entity       |                         | 
   |-----------|------------|---------------|-------------------------| 
   | Signaling |  Signaling |  Call agent   |  Signaling exchanges    | 
   | Plane     |  exchanges |               |  with the Call Agent    | 
   |           |  through   |               |  through SAP, SIP or    | 
   |           |  SS7/ISUP  |               |  RTSP.                  | 
   |-----------|------------|---------------|-------------------------| 
   |           |            |               |  Negotiation of session | 
   |           |            |               |  description parameters | 
   |           |            |               |  through SDP (telephony | 
   |           |            |               |  gateway terminated but | 
   |           |            |               |  passed via the call    | 
   |           |            |               |  agent to and from the  | 
   |           |            |               |  IETF conforming system)| 
   |-----------|------------|---------------|-------------------------| 
   |           |            | Internal syn- |                         | 
   |           |            | chronization  |                         | 
   |           |            | through MGCP  |                         | 
   |-----------|------------|---------------|-------------------------| 
   | Bearer    |  Connection|  Telephony    |  Transmission of VoIP   | 
   | Data      |  through   |  gateways     |  data using RTP,        | 
   | Transport |  high speed|               |  directly between the   | 
   | Plane     |  trunk     |               |  remote IP end system   | 
   |           |  groups    |               |  and the gateway.       | 
    ------------------------------------------------------------------ 
    
   The SDP standard has a pivotal status in this architecture. We will 
   see in the following description that we also use it to carry 
   session descriptions in MGCP. 
    




                                                                        
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1.3 Definitions 
    
   Trunk: A communication channel between two switching systems, e.g., 
   a DS0 on a T1 or E1 line. 
    
1.4 Conventions used in this Document 
    
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED, "MAY", and 
   "OPTIONAL" in this document are to be interpreted as described in 
   RFC-2119 [2]. 
 









































                                                                        
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2. Media Gateway Control Interface 
    
   The interface functions provide for connection control and endpoint 
   control. Both use the same system model and the same naming 
   conventions. 
    
2.1 Model and Naming Conventions 
    
   The MGCP assumes a connection model where the basic constructs are 
   endpoints and connections. Connections are grouped in calls. One or 
   more connections can belong to one call. Connections and calls are 
   set up at the initiative of one or more Call Agents. 
    
2.1.1 Types of Endpoints 
    
   In the introduction, we presented several classes of gateways. Such 
   classifications, however, can be misleading. Manufacturers can 
   arbitrarily decide to provide several types of services in a single 
   package. A single product could well, for example, provide some 
   trunk connections to telephony switches, some primary rate 
   connections and some analog line interfaces, thus sharing the 
   characteristics of what we described in the introduction as 
   "trunking", "access" and "residential" gateways. MGCP does not make 
   assumptions about such groupings. We simply assume that media 
   gateways support collections of endpoints. The type of the endpoint 
   determines its functionality. Our analysis, so far, has led us to 
   isolate the following basic endpoint types: 
    
   *  Digital channel (DS0), 
    
   *  Analog line, 
    
   *  Announcement server access point, 
    
   *  Interactive Voice Response access point, 
    
   *  Conference bridge access point, 
    
   *  Packet relay, 
    
   *  ATM "trunk side" interface. 
    
   In this section, we will describe the expected behavior of such 
   endpoints. 
    
   This list is not final. There may be other types of endpoints 
   defined in the future, for example test endpoints that could be used 
   to check network quality, or frame-relay endpoints that could be 
   used to manage audio channels multiplexed over a frame-relay virtual 
   circuit. 
    


                                                                        
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2.1.1.1 Digital Channel (DS0) 
    
   Digital channels provide a 64 Kbps service. Such channels are found 
   in trunk and ISDN interfaces. They are typically part of digital 
   multiplexes, such as T1, E1, T3 or E3 interfaces. Media gateways 
   that support such channels are capable of translating the digital 
   signals received on the channel, which may be encoded according to 
   A-law or mu-law, using either the complete set of 8 bits per sample 
   or only 7 of these bits, into audio packets. When the media gateway 
   also supports a Network Access Server (NAS) service, the gateway 
   shall be capable of receiving either audio-encoded data (modem 
   connection) or binary data (ISDN connection) and convert them into 
   data packets. 
    
                                         +------- 
                           +------------+| 
              (channel) ===|DS0 endpoint| -------- Connections 
                           +------------+| 
                                         +------- 
    
   Media gateways should be able to establish several connections 
   between the endpoint and the packet networks, or between the 
   endpoint and other endpoints in the same gateway. The signals 
   originating from these connections shall be mixed according to the 
   connection "mode", as specified later in this document. The precise 
   number of connections that an endpoint supports is a characteristic 
   of the gateway, and may in fact vary according to the allocation of 
   resources within the gateway. 
    
   In some cases, digital channels are used to carry signaling. This is 
   the case for example for SS7 "F" links, or ISDN "D" channels. Media 
   gateways that support these signaling functions shall be able to 
   send and receive the signaling packets to and from a Call Agent, 
   using the "backhaul" procedures defined by the SIGTRAN working group 
   of the IETF. Digital channels are sometimes used in conjunction with 
   channel associated signaling, such as "MF R2". Media gateways that 
   support these signaling functions shall be able to detect and 
   produce the corresponding signals, such as for example "wink" or 
   "A", according to the event signaling and reporting procedures 
   defined in MGCP. 
    
2.1.1.2 Analog Line 
    
   Analog lines can be used either as a "client" interface, providing 
   service to a classic telephone unit, or as a "service" interface, 
   allowing the gateway to send and receive analog calls. When the 
   media gateway also supports a NAS service, the gateway shall be 
   capable of receiving audio-encoded data (modem connection) and 
   convert them into data packets. 
    



                                                                        
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                                         +------- 
                        +---------------+| 
              (line) ===|analog endpoint| -------- Connections 
                        +---------------+| 
                                         +------- 
    
   Media gateways should be able to establish several connections 
   between the endpoint and the packet networks, or between the 
   endpoint and other endpoints in the same gateway. The audio signals 
   originating from these connections shall be mixed according to the 
   connection "mode", as specified later in this document. The precise 
   number of connections that an endpoint supports is a characteristic 
   of the gateway, and may in fact vary according to the allocation of 
   resources within the gateway. A typical gateway should however be 
   able to support two or three connections per endpoint, in order to 
   support services such as "call waiting" or "three way calling". 
    
2.1.1.3 Announcement Server Access Point 
    
   An announcement server endpoint provides access to an announcement 
   service. Under requests from the Call Agent, the announcement server 
   will "play" a specified announcement. The requests from the Call 
   Agent will follow the event signaling and reporting procedures 
   defined in MGCP. 
    
                  +----------------------+ 
                  | Announcement endpoint| -------- Connection 
                  +----------------------+ 
    
   A given announcement endpoint is not expected to support more than 
   one connection at a time. If several connections were established to 
   the same endpoint, then the same announcements would be played 
   simultaneously over all the connections. 
    
   Connections to an announcement server are typically one way, or 
   "half duplex" -- the announcement server is not expected to listen 
   to the audio signals from the connection. 
    
2.1.1.4 Interactive Voice Response Access Point 
    
   An Interactive Voice Response (IVR) endpoint provides access to an 
   IVR service. Under requests from the Call Agent, the IVR server will 
   "play" announcements and tones, and will "listen" to responses, such 
   as DTMF input or voice messages, from the user. The requests from 
   the Call Agent will follow the event signaling and reporting 
   procedures defined in MGCP. 
    
                      +-------------+ 
                      | IVR endpoint| -------- Connection 
                      +-------------+ 
    
   A given IVR endpoint is not expected to support more than one 
   connection at a time. If several connections were established to the 
                                                                        
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   same endpoint, then the same tones and announcements would be played 
   simultaneously over all the connections. 
    
2.1.1.5 Conference Bridge Access Point 
    
   A conference bridge endpoint is used to provide access to a specific 
   conference. 
    
                                           +------- 
               +--------------------------+| 
               |Conference bridge endpoint| -------- Connections 
               +--------------------------+| 
                                           +------- 
    
   Media gateways should be able to establish several connections 
   between the endpoint and the packet networks, or between the 
   endpoint and other endpoints in the same gateway. The signals 
   originating from these connections shall be mixed according to the 
   connection "mode", as specified later in this document. The precise 
   number of connections that an endpoint supports is a characteristic 
   of the gateway, and may in fact vary according to the allocation of 
   resources within the gateway. 
    
2.1.1.6 Packet Relay 
    
   A packet relay endpoint is a specific form of conference bridge, 
   that typically only supports two connections. Packets relays can be 
   found in firewalls between a protected and an open network, or in 
   transcoding servers used to provide interoperation between 
   incompatible gateways, for example gateways that do not support 
   compatible compression algorithms, or gateways that operate over 
   different transmission networks such as IP and ATM. 
    
                                           +------- 
                   +---------------------+ | 
                   |Packet relay endpoint|  2 connections 
                   +---------------------+ | 
                                           +------- 
    
    
2.1.1.7 ATM "trunk side" Interface. 
    
   ATM "trunk side" endpoints are typically found when one or several 
   ATM permanent virtual circuits are used as a replacement for the 
   classic "TDM" trunks linking switches. When ATM/AAL2 is used, 
   several trunks or channels are multiplexed on a single virtual 
   circuit; each of these trunks correspond to a single endpoint. 
    





                                                                        
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                                          +------- 
                      +------------------+| 
          (channel) = |ATM trunk endpoint| -------- Connections 
                      +------------------+| 
                                          +------- 
    
   Media gateways should be able to establish several connections 
   between the endpoint and the packet networks, or between the 
   endpoint and other endpoints in the same gateway. The signals 
   originating from these connections shall be mixed according to the 
   connection "mode", as specified later in this document. The precise 
   number of connections that an endpoint supports is a characteristic 
   of the gateway, and may in fact vary according to the allocation of 
   resources within the gateway. 
    
2.1.2 Endpoint Identifiers 
    
   Endpoint identifiers have two components that both are case-
   insensitive: 
    
   *  the domain name of the gateway that is managing the endpoint, 
    
   *  a local name within that gateway, 
    
   Endpoint names are of the form: 
    
      local-endpoint-name@domain-name 
    
   where domain-name is an absolute domain-name as defined in RFC 1034 
   and includes a host portion, thus an example domain-name could be: 
    
      mygateway.whatever.net 
    
   Also, domain-name may be an IP-address of the form defined for 
   domain name in RFC 821, thus another example could be (see RFC 821 
   for details):  
    
      [192.168.1.2] 
    
   Both IPv4 and IPv6 addresses can be specified, however use of IP 
   addresses as endpoint identifiers is generally discouraged. 
    
   Note that since the domain name portion is part of the endpoint 
   identifier, different forms or different values referring to the 
   same entity are not freely interchangeable. The most recently 
   supplied form and value MUST always be used.  
    
   The local endpoint name is case-insensitive. The syntax of the local 
   endpoint name is hierarchical, where the least specific component of 
   the name is the leftmost term, and the most specific component is 
   the rightmost term. The precise syntax depends on the type of 
   endpoint being named and MAY start with a term that identifies the 

                                                                        
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   endpoint type. In any case, the local endpoint name MUST adhere to 
   the following naming rules: 
    
   1) The individual terms of the naming path MUST be separated by a 
      single slash ("/", ASCII 2F hex). 
    
   2) The individual terms are character strings composed of letters, 
      digits or other printable characters, with the exception of 
      characters used as delimiters ("/", "@"), characters used for 
      wildcarding ("*", "$") and white spaces. 
    
   3) Wild-carding is represented either by an asterisk ("*") or a 
      dollar sign ("$") for the terms of the naming path which are to 
      be wild-carded. Thus, if the full local endpoint name is of the 
      form: 
       
          term1/term2/term3 
       
      then the entity name field looks like this depending on which 
      terms are wild-carded: 
       
          */term2/term3 if term1 is wild-carded 
          term1/*/term3 if term2 is wild-carded 
          term1/term2/* if term3 is wild-carded 
          term1/*/*     if term2 and term3 are wild-carded, etc. 
       
      In each of these examples a dollar sign could have appeared 
      instead of an asterisk. 
    
   4) A term represented by an asterisk ("*") is to be interpreted as: 
      "use ALL values of this term known within the scope of the Media 
      Gateway". Unless specified otherwise, this refers to all 
      endpoints configured for service, regardless of their actual 
      service state, i.e., in-service or out-of-service.  
    
   5) A term represented by a dollar sign ("$") is to be interpreted 
      as: "use ANY ONE value of this term known within the scope of the 
      Media Gateway". Unless specified otherwise, this only refers to 
      endpoints that are in-service.  
    
   Furthermore, it is RECOMMENDED that Call Agents adhere to the 
   following: 
    
   *  Wild-carding should only be done from the right, thus if a term 
      is wild-carded, then all terms to the right of that term should 
      be wild-carded as well. 
    
   *  In cases where mixed dollar sign and asterisk wild-cards are 
      used, dollar-signs should only be used from the right, thus if a 
      term had a dollar sign wild-card, all terms to the right of that 
      term should also contain dollar sign wild-cards. 
    

                                                                        
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   The description of a specific command may add further criteria for 
   selection within the general rules given above.  
    
   Note, that wild-cards may be applied to more than one term in which 
   case they shall be evaluated from left to right. For example, if we 
   have the endpoint names "a/1", "a/2", "b/1", and "b/2", then "$/*" 
   (which is not recommended) will evaluate to either "a/1, a/2", or 
   "b/1, b/2". However, "*/$" may evaluate to "a/1, b/1", "a/1, b/2", 
   "a/2, b/1", or "a/2, b/2". The use of mixed wild-cards in a command 
   is considered error prone and is consequently discouraged. 
    
   A local name that is composed of only a wildcard character refers to 
   either all (*) or any ($) endpoints within the media gateway. 
    
2.1.3 Calls and Connections 
    
   Connections are created on the Call Agent on each endpoint that will 
   be involved in the "call". In the classic example of a connection 
   between two "DS0" endpoints (EP1 and EP2), the Call Agents 
   controlling the endpoints will establish two connections (C1 and 
   C2): 
    
                  +---+                            +---+ 
    (channel1) ===|EP1|--(C1)--...        ...(C2)--|EP2|===(channel2) 
                  +---+                            +---+ 
    
   Each connection will be designated locally by an endpoint unique 
   connection identifier, and will be characterized by connection 
   attributes. 
    
   When the two endpoints are located on gateways that are managed by 
   the same Call Agent, the creation is done via the three following 
   steps: 
    
   1) The Call Agent asks the first gateway to "create a connection" on 
      the first endpoint. The gateway allocates resources to that 
      connection, and responds to the command by providing a "session 
      description". The session description contains the information 
      necessary for a third party to send packets towards the newly 
      created connection, such as for example IP address, UDP port, and 
      codec parameters. 
    
   2) The Call Agent then asks the second gateway to "create a 
      connection" on the second endpoint. The command carries the 
      "session description" provided by the first gateway. The gateway 
      allocates resources to that connection, and responds to the 
      command by providing its own "session description". 
    
   3) The Call Agent then uses a "modify connection" command to provide 
      this second "session description" to the first endpoint. Once 
      this is done, communication can proceed in both directions. 
    

                                                                        
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   When the two endpoints are located on gateways that are managed by 
   two different Call Agents, the Call Agents exchange information 
   through a Call-Agent to Call-Agent signaling protocol, e.g. SIP [7], 
   in order to synchronize the creation of the connection on the two 
   endpoints. 
    
   Once a connection has been established, the connection parameters 
   can be modified at any time by a "modify connection" command. The 
   Call Agent may for example instruct the gateway to change the codec 
   used on a connection, or to modify the IP address and UDP port to 
   which data should be sent, if a connection is "redirected". 
    
   The Call Agent removes a connection by sending a "delete connection" 
   command to the gateway. The gateway may also, under some 
   circumstances, inform a gateway that a connection could not be 
   sustained. 
    
   The following diagram provides a view of the states of a connection, 
   as seen from the gateway: 
    

































                                                                        
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           Create connection 
              received 
                  | 
                  V 
         +-------------------+ 
         |resource allocation|-(failed)-+ 
         +-------------------+          | 
                  |           (connection refused) 
            (successful) 
                  | 
                  v 
     +----------->+ 
     |            | 
     |   +-------------------+ 
     |   |  remote session   | 
     |   |   description     |----------(yes)--------+ 
     |   |    available ?    |                       | 
     |   +-------------------+                       | 
     |            |                                  | 
     |          (no)                                 | 
     |            |                                  | 
     |      +-----------+                         +------+ 
     | +--->| half open |------> Delete   <-------| open |<----------+ 
     | |    |  (wait)   |      Connection         |(wait)|           | 
     | |    +-----------+       received          +------+           | 
     | |          |                 |                |               | 
     | |   Modify Connection        |         Modify Connection      | 
     | |      received              |            received            | 
     | |          |                 |                |               | 
     | | +--------------------+     |       +--------------------+   | 
     | | |assess modification |     |       |assess modification |   | 
     | | +--------------------+     |       +--------------------+   | 
     | |    |             |         |          |             |       | 
     | |(failed)     (successful)   |      (failed)     (successful) | 
     | |    |             |         |          |             |       | 
     | +<---+             |         |          +-------------+-------+ 
     |                    |         | 
     +<-------------------+         | 
                                    | 
                           +-----------------+ 
                           | Free connection | 
                           | resources.      | 
                           | Report.         | 
                           +-----------------+ 
                                    | 
                                    V 
    
2.1.3.1 Names of Calls 
    
   One of the attributes of each connection is the "call identifier", 
   which as far as the MGCP protocol is concerned has little semantic 
   meaning, and is mainly retained for backwards compatibility. 
    
                                                                        
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   Calls are identified by unique identifiers, independent of the 
   underlying platforms or agents. Call identifiers are hexadecimal 
   strings, which are created by the Call Agent. The maximum length of 
   call identifiers is 32 characters. 
    
   Call identifiers are expected to be unique within the system, or at 
   a minimum, unique within the collection of Call Agents that control 
   the same gateways. From the gateway's perspective, the Call 
   identifier is thus unique. When a Call Agent builds several 
   connections that pertain to the same call, either on the same 
   gateway or in different gateways, these connections that belong to 
   the same call should share the same call-id. This identifier can 
   then be used by accounting or management procedures, which are 
   outside the scope of MGCP. 
    
2.1.3.2 Names of Connections 
    
   Connection identifiers are created by the gateway when it is 
   requested to create a connection. They identify the connection 
   within the context of an endpoint. Connection identifiers are 
   treated in MGCP as hexadecimal strings. The gateway MUST make sure 
   that a proper waiting period, at least 3 minutes, elapses between 
   the end of a connection that used this identifier and its use in a 
   new connection for the same endpoint (gateways MAY decide to use 
   identifiers that are unique within the context of the gateway). The 
   maximum length of a connection identifier is 32 characters. 
    
2.1.3.3 Management of Resources, Attributes of Connections 
    
   Many types of resources will be associated to a connection, such as 
   specific signal processing functions or packetization functions. 
   Generally, these resources fall in two categories: 
    
   1) Externally visible resources, that affect the format of "the bits 
      on the network" and must be communicated to the second endpoint 
      involved in the connection. 
    
   2) Internal resources, that determine which signal is being sent 
      over the connection and how the received signals are processed by 
      the endpoint. 
    
   The resources allocated to a connection, and more generally the 
   handling of the connection, are chosen by the gateway under 
   instructions from the Call Agent. The Call Agent will provide these 
   instructions by sending two sets of parameters to the gateway: 
    
   1) The local directives instruct the gateway on the choice of 
      resources that should be used for a connection, 
    
   2) When available, the "session description" provided by the other 
      end of the connection (referred to as the remote session 
      description). 
    
                                                                        
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   The local directives specify such parameters as the mode of the 
   connection (e.g. send-only, or send-receive), preferred coding or 
   packetization methods, usage of echo cancellation or silence 
   suppression. (A detailed list can be found in the specification of 
   the LocalConnectionOptions parameter of the CreateConnection 
   command.) Depending on the parameter, the Call Agent MAY either 
   specify a value, a range of values, or no value at all. This allows 
   various implementations to implement various levels of control, from 
   a very tight control where the Call Agent specifies minute details 
   of the connection handling to a very loose control where the Call 
   Agent only specifies broad guidelines, such as the maximum 
   bandwidth, and lets the gateway choose the detailed values subject 
   to the guidelines. 
    
   Based on the value of the local directives, the gateway will 
   determine the resources to allocate to the connection. When this is 
   possible, the gateway will choose values that are in line with the 
   remote session description - but there is no absolute requirement 
   that the parameters be exactly the same.  
    
   Once the resources have been allocated, the gateway will compose a 
   "session description" that describes the way it intends to send and 
   receive packets. Note that the session description may in some cases 
   present a range of values. For example, if the gateway is ready to 
   accept one of several compression algorithms, it can provide a list 
   of these accepted algorithms. 
    


























                                                                        
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                 Local Directives 
                (from Call Agent 1) 
                        | 
                        V 
                 +-------------+ 
                 | resource    | 
                 | allocation  | 
                 | (gateway 1) | 
                 +-------------+ 
                   |         | 
                   V         | 
                 Local       | 
              Parameters     V 
                   |      Session 
                   |    Description               Local Directives 
                   |         |                   (from Call Agent 2) 
                   |         +---> Transmission----+      | 
                   |                (CA to CA)     |      | 
                   |                               V      V 
                   |                           +-------------+ 
                   |                           | resource    | 
                   |                           | allocation  | 
                   |                           | (gateway 2) | 
                   |                           +-------------+ 
                   |                               |      | 
                   |                               |      V 
                   |                               |    Local 
                   |                               |  Parameters 
                   |                            Session 
                   |                          Description 
                   |         +---- Transmission<---+ 
                   |         |      (CA to CA) 
                   V         V 
                 +-------------+ 
                 | modification| 
                 | (gateway 1) | 
                 +-------------+ 
                   | 
                   V 
                 Local 
              Parameters 
    
      -- Information flow: local directives & session descriptions -- 
    
2.1.3.4 Special Case of Local Connections 
    
   Large gateways include a large number of endpoints which are often 
   of different types. In some networks, we may often have to set-up 
   connections between endpoints that are located within the same 
   gateway. Examples of such connections may be: 
    
   *  Connecting a call to an Interactive Voice-Response unit, 
    
                                                                        
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   *  Connecting a call to a Conferencing unit, 
    
   *  Routing a call from one endpoint to another, something often 
      described as a "hairpin" connection. 
    
   Local connections are much simpler to establish than network 
   connections. In most cases, the connection will be established 
   through some local interconnecting device, such as for example a TDM 
   bus. 
    
   When two endpoints are managed by the same gateway, it is possible 
   to specify the connection in a single command that conveys the names 
   of the two endpoints that will be connected. The command is 
   essentially a "Create Connection" command which includes the name of 
   the second endpoint in lieu of the "remote session description". 
    
2.1.4 Names of Call Agents and Other Entities 
    
   The media gateway control protocol has been designed to allow the 
   implementation of redundant Call Agents, for enhanced network 
   reliability. This means that there is no fixed binding between 
   entities and hardware platforms or network interfaces. 
    
   Call Agent names consist of two parts, similar to endpoint names. 
   Semantically, the local portion of the name does not exhibit any 
   internal structure. An example Call Agent name is: 
    
      ca1@ca.whatever.net 
    
   Reliability can be improved by using the following procedures: 
    
   *  Entities such as endpoints or Call Agents are identified by their 
      domain name, not their network addresses. Several addresses can 
      be associated with a domain name. If a command or a response 
      cannot be forwarded to one of the network addresses, 
      implementations MUST retry the transmission using another 
      address. 
    
   *  Entities MAY move to another platform. The association between a 
      logical name (domain name) and the actual platform is kept in the 
      domain name service. Call Agents and Gateways MUST keep track of 
      the time-to-live of the record they read from the DNS. They MUST 
      query the DNS to refresh the information if the time to live has 
      expired. 
    
   In addition to the indirection provided by the use of domain names 
   and the DNS, the concept of "notified entity" is central to 
   reliability and fail-over in MGCP. The "notified entity" for an 
   endpoint is the Call Agent currently controlling that endpoint. At 
   any point in time, an endpoint has one, and only one, "notified 
   entity" associated with it. The "notified entity" determines where 
   the endpoint will send commands to; when the endpoint needs to send 
   a command to the Call Agent, it MUST send the command to its current 
                                                                        
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   "notified entity". The "notified entity" however does not determine 
   where commands can be received from; any Call Agent can send 
   commands to the endpoint. Please refer to Section 5 for the relevant 
   security considerations.  
    
   Upon startup, the "notified entity" MUST be set to a provisioned 
   value. Most commands sent by the Call Agent include the ability to 
   explicitly name the "notified entity" through the use of a 
   "NotifiedEntity" parameter. The "notified entity" will stay the same 
   until either a new "NotifiedEntity" parameter is received or the 
   endpoint does a warm or cold restart.  
    
   If a "NotifiedEntity" parameter is sent with an "empty" value, the 
   "notified entity" for the endpoint will be set to empty. If the 
   "notified entity" for an endpoint is empty or has not been set 
   explicitly (neither by a command nor by provisioning), the "notified 
   entity" will then default to the source address (i.e., IP address 
   and UDP port number) of the last successful non-audit command 
   received for the endpoint. Auditing will thus not change the 
   "notified entity". Use of an empty "NotifiedEntity" parameter value 
   is strongly discouraged as it is error prone and eliminates the DNS-
   based fail-over and reliability mechanisms. 
    
2.1.5 Digit Maps 
    
   The Call Agent can ask the gateway to collect digits dialed by the 
   user. This facility is intended to be used with residential gateways 
   to collect the numbers that a user dials; it can also be used with 
   trunking gateways and access gateways alike, to collect access 
   codes, credit card numbers and other numbers requested by call 
   control services. 
    
   One procedure is for the gateway to notify the Call Agent of each 
   individual dialed digit, as soon as they are dialed. However, such a 
   procedure generates a large number of interactions. It is preferable 
   to accumulate the dialed numbers in a buffer, and to transmit them 
   in a single message. 
    
   The problem with this accumulation approach, however, is that it is 
   hard for the gateway to predict how many numbers it needs to 
   accumulate before transmission. For example, using the phone on our 
   desk, we can dial the following numbers: 
    










                                                                        
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        ------------------------------------------------------ 
       |  0                     |  Local operator             | 
       |  00                    |  Long distance operator     | 
       |  xxxx                  |  Local extension number     | 
       |  8xxxxxxx              |  Local number               | 
       |  #xxxxxxx              |  Shortcut to local number at| 
       |                        |  other corporate sites      | 
       |  *xx                   |  Star services              | 
       |  91xxxxxxxxxx          |  Long distance number       | 
       |  9011 + up to 15 digits|  International number       | 
        ------------------------------------------------------ 
    
   The solution to this problem is to have the Call Agent load the 
   gateway with a digit map that may correspond to the dial plan. This 
   digit map is expressed using a syntax derived from the Unix system 
   command, egrep. For example, the dial plan described above results 
   in the following digit map: 
    
      (0T|00T|[1-7]xxx|8xxxxxxx|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T) 
    
   The formal syntax of the digit map is described by the DigitMap rule 
   in the formal syntax description of the protocol (see Appendix A) - 
   support for basic digit map letters is REQUIRED while support for 
   extension digit map letters is OPTIONAL. A gateway receiving a digit 
   map with an extension digit map letter not supported SHOULD return 
   error code 537 (unknown digit map extension).  
    
   A digit map, according to this syntax, is defined either by a (case 
   insensitive) "string" or by a list of strings. Each string in the 
   list is an alternative numbering scheme, specified either as a set 
   of digits or timers, or as an expression over which the gateway will 
   attempt to find a shortest possible match. The following constructs 
   can be used in each numbering scheme: 
    
   * Digit:       A digit from "0" to "9".  
   * Timer:       The symbol "T" matching a timer expiry.  
   * DTMF:        A digit, a timer, or one of the symbols "A", "B",  
                  "C", "D", "#", or "*". Extensions may be defined. 
   * Wildcard:    The symbol "x" which matches any digit ("0" to "9"). 
   * Range:       One or more DTMF symbols enclosed between square  
                  brackets ("[" and "]").  
   * Subrange:    Two digits separated by hyphen ("-") which matches  
                  any digit between and including the two. The subrange  
                  construct can only be used inside a range construct,  
                  i.e. between "[" and "]". 
   * Position:    A period (".") which matches an arbitrary number,  
                  including zero, of occurrences of the preceding  
                  construct. 
    
   A gateway that detects events to be matched against a digit map MUST 
   do the following: 
    

                                                                        
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   1) Add the event code as a token to the end of an internal state 
      variable for the endpoint called the "current dial string". 
    
   2) Apply the current dial string to the digit map table, attempting 
      a match to each expression in the digit map.  
    
   3) If the result is under-qualified (partially matches at least one 
      entry in the digit map and doesn't completely match another 
      entry), do nothing further. 
    
   If the result matches an entry, or is over-qualified (i.e. no 
   further digits could possibly produce a match), send the list of 
   accumulated events to the Call Agent. A match, in this 
   specification, can be either a "perfect match," exactly matching one 
   of the specified alternatives, or an impossible match, which occurs 
   when the dial string does not match any of the alternatives. 
   Unexpected timers, for example, can cause "impossible matches". Both 
   perfect matches and impossible matches trigger notification of the 
   accumulated digits (which may include other events - see Section 
   2.3.3).  
    
   The following example illustrates the above. Assume we have the 
   digit map: 
    
      (xxxxxxx|x11) 
    
   and a current dial string of "41". Given the input "1" the current 
   dial string becomes "411". We have a partial match with "xxxxxxx", 
   but a complete match with "x11", and hence we send "411" to the Call 
   Agent.  
    
   The following digit map example is more subtle: 
    
     (0[12].|00|1[12].1|2x.#) 
    
   Given the input "0", a match will occur immediately since position 
   (".") allows for zero occurrences of the preceding construct. The 
   input "00" can thus never be produced in this digit map. 
    
   Given the input "1", only a partial match exists. The input "12" is 
   also only a partial match, however both "11" and "121" are a match.  
    
   Given the input "2", a partial match exists. A partial match also 
   exists for the input "23", "234", "2345", etc. A full match does not 
   occur here until a "#" is generated, e.g. "2345#". The input "2#" 
   would also have been a match. 
    
   Note that digit maps simply define a way of matching sequences of 
   event codes against a grammar. Although digit maps as defined here 
   are for DTMF input, extension packages can also be defined so that 
   digit maps can be used for other types of input represented by event 
   codes that adhere to the digit map syntax already defined for these 
   event codes (e.g. "1" or "T"). Where such usage is envisioned, the 
                                                                        
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   definition of the particular event(s) SHOULD explicitly state that 
   in the package definition.  
    
   Since digit maps are not bounded in size, it is RECOMMENDED that 
   gateways support digit maps up to at least 2048 bytes per endpoint.  
    
2.1.6 Packages 
    
   MGCP is a modular and extensible protocol, however with 
   extensibility comes the need to manage, identify, and name the 
   individual extensions. This is achieved by the concept of packages, 
   which are simply well-defined groupings of extensions. For example, 
   one package may support a certain group of events and signals, e.g. 
   off-hook and ringing, for analog access lines. Another package may 
   support another group of events and signals for analog access lines 
   or for another type of endpoint such as video. One or more packages 
   may be supported by a given endpoint. 
    
   MGCP allows the following types of extensions to be defined in a 
   package:  
    
   *  BearerInformation 
       
   *  LocalConnectionOptions 
       
   *  ExtensionParameters 
       
   *  ConnectionModes 
       
   *  Events 
       
   *  Signals 
       
   *  Actions 
       
   *  DigitMapLetters 
       
   *  ConnectionParameters 
       
   *  RestartMethods 
       
   *  ReasonCodes 
       
   *  Return codes 
    
   each of which will be explained in more detail below. The rules for 
   defining each of these extensions in a package are described in 
   Section 6, and the encoding and syntax are defined in Section 3 and 
   Appendix A. 
    
   With the exception of DigitMapLetters, a package defines a separate 
   name space for each type of extension by adding the package name as 
   a prefix to the extension, i.e.: 
                                                                        
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      package-name/extension 
    
   Thus the package-name is followed by a slash ("/") and the name of 
   the extension.  
    
   An endpoint supporting one or more packages may define one of those 
   packages as the default package for the endpoint. Use of the package 
   name for events and signals in the default package for an endpoint 
   is OPTIONAL, however it is RECOMMENDED to always include the package 
   name. All other extensions, except DigitMapLetter, defined in the 
   package MUST include the package-name when referring to the 
   extension. 
    
   Package names are case insensitive strings of letters, hyphens and 
   digits, with the restriction that hyphens shall never be the first 
   or last character in a name. Examples of package names are "D", "T", 
   and "XYZ". Package names are not case sensitive - names such as 
   "XYZ", "xyz", and "xYz" are equal. 
    
   Package definitions will be provided in other documents and with 
   package names and extensions names registered with IANA. For more 
   details, refer to section 6.  
    
   Implementers can gain experience by using experimental packages. The 
   name of an experimental package MUST start with the two characters 
   "x-"; the IANA SHALL NOT register package names that start with 
   these characters, or the characters "x+", which are reserved. A 
   gateway that receives a command referring to an unsupported package 
   MUST return an error (error code 518 - unsupported package, is 
   RECOMMENDED). 
    
2.1.7 Events and Signals 
    
   The concept of events and signals is central to MGCP. A Call Agent 
   may ask to be notified about certain events occurring in an endpoint 
   (e.g. off-hook events) by including the name of the event in a 
   RequestedEvents parameter (in a NotificationRequest command - see 
   Section 2.3.3). 
    
   A Call Agent may also request certain signals to be applied to an 
   endpoint (e.g. dial-tone) by supplying the name of the event in a 
   SignalRequests parameter. 
    
   Events and signals are grouped in packages, within which they share 
   the same name space which we will refer to as event names in the 
   following. Event names are case insensitive strings of letters, 
   hyphens and digits, with the restriction that hyphens SHALL NOT be 
   the first or last character in a name. Some event codes may need to 
   be parameterized with additional data, which is accomplished by 
   adding the parameters between a set of parentheses. Event names are 
   not case sensitive - values such as "hu", "Hu", "HU" or "hU" are 
   equal. 
                                                                        
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   Examples of event names can be "hu" (off hook or "hang-up" 
   transition), "hf" (hook-flash) or "0" (the digit zero). 
    
   The package name is OPTIONAL for events in the default package for 
   an endpoint, however it is RECOMMENDED to always include the package 
   name. If the package name is excluded from the event name, the 
   default package name for that endpoint MUST be assumed. For example, 
   for an analog access line which has the line package ("L") as a 
   default with dial-tone ("dl") as one of the events in that package, 
   the following two event names are equal: 
    
      L/dl 
    
   and 
    
      dl 
    
   For any other non-default packages that are associated with that 
   endpoint, (such as the generic package for an analog access 
   endpoint-type for example), the package name MUST be included with 
   the event name. Again, unconditional inclusion of the package name 
   is RECOMMENDED. 
    
   Digits, or letters, are supported in some packages, notably "DTMF". 
   Digits and letters are defined by the rules "Digit" and "Letter" in 
   the definition of digit maps. This definition refers to the digits 
   (0 to 9), to the asterisk or star ("*") and orthotrope, number or 
   pound sign ("#"), and to the letters "A", "B", "C" and "D", as well 
   as the timer indication "T". These letters can be combined in "digit 
   string" that represents the keys that a user punched on a dial. In 
   addition, the letter "X" can be used to represent all digits (0 to 
   9). Also, extensions MAY define use of other letters. The need to 
   easily express the digit strings in earlier versions of the protocol 
   has a consequence on the form of event names: 
    
   An event name that does not denote a digit MUST always contain at 
   least one character that is neither a digit, nor one of the letters 
   A, B, C, D, T or X (such names also MUST NOT just contain the 
   special signs "*", or "#"). 
    
   A Call Agent may often have to ask a gateway to detect a group of 
   events. Two conventions can be used to denote such groups: 
    
   *  The "*" and "all" wildcard conventions (see below) can be used to 
      detect any event belonging to a package, or a given event in many 
      packages, or any event in any package supported by the gateway. 
    
   *  The regular expression Range notation can be used to detect a 
      range of digits. 
    


                                                                        
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   The star sign (*) can be used as a wildcard instead of a package 
   name, and the keyword "all" can be used as a wildcard instead of an 
   event name: 
    
   *  A name such as "foo/all" denotes all events in package "foo". 
   *  A name such as "*/bar" denotes the event "bar" in any package 
      supported by the gateway. 
   *  The name "*/all" denotes all events supported by the endpoint. 
    
   This specification purposely does not define any additional detail 
   for the "all packages" and "all events" wildcards. They provide 
   limited benefits, but introduce significant complexity along with 
   the potential for errors. Their use is consequently strongly 
   discouraged. 
    
   The Call Agent can ask a gateway to detect a set of digits or 
   letters either by individually describing those letters, or by using 
   the "range" notation defined in the syntax of digit strings. For 
   example, the Call Agent can: 
    
   Use the letter "x" to denote" digits from 0 to 9. Use the notation 
   "[0-9#]" to denote the digits 0 to 9 and the pound sign. The 
   individual event codes are still defined in a package though (e.g., 
   the "DTMF" package).  
    
   Events can by default only be generated and detected on endpoints, 
   however events can be also be defined so they can be generated or 
   detected on connections rather than on the endpoint itself (see 
   Section 6.6). For example, gateways may be asked to provide a 
   ringback tone on a connection. When an event is to be applied on a 
   connection, the name of the connection MUST be added to the name of 
   the event, using an "at" sign (@) as a delimiter, as in: 
    
      G/rt@0A3F58 
    
   where "G" is the name of the package and "rt" is the name of the 
   event. Should the connection be deleted while an event or signal is 
   being detected or applied on it, that particular event detection or 
   signal generation simply stops. Depending on the signal, this may 
   generate a failure (see below).  
    
   The wildcard character "*" (star) can be used to denote "all 
   connections". When this convention is used, the gateway will 
   generate or detect the event on all the connections that are 
   connected to the endpoint. This applies to existing as well as 
   future connections created on the endpoint. An example of this 
   convention could be: 
    
      R/qa@* 
    
   where "R" is the name of the package and "qa" is the name of the 
   event.  
    
                                                                        
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   When processing a command using the "all connections" wildcard, the 
   "*" wildcard character applies to all current and future connections 
   on the endpoint, however it will not be expanded. If a subsequent 
   command either explicitly (e.g. by auditing) or implicitly (e.g. by 
   persistence) refers to such an event, the "*" value will be used. 
   However, when the event is actually observed, that particular 
   occurrence of the event will include the name of the specific 
   connection it occurred on. 
    
   The wildcard character "$" can be used to denote "the current 
   connection". It can only be used by the Call Agent, when the event 
   notification request is "encapsulated" within a connection creation 
   or modification command. When this convention is used, the gateway 
   will generate or detect the event on the connection that is 
   currently being created or modified. An example of this convention 
   is: 
    
      G/rt@$ 
    
   When processing a command using the "current connection" wildcard, 
   the "$" wildcard character will be expanded to the value of the 
   current connection. If a subsequent command either explicitly (e.g. 
   by auditing) or implicitly (e.g. by persistence) refers to such an 
   event, the expanded value will be used. In other words, the "current 
   connection" wildcard is expanded once, which is at the initial 
   processing of the command in which it was explicitly included.  
    
   The connection id, or a wildcard replacement, can be used in 
   conjunction with the "all packages" and "all events" conventions. 
   For example, the notation: 
    
      */all@* 
    
   can be used to designate all events on all current and future 
   connections on the endpoint. However, as mentioned before, the use 
   of the "all packages" and "all events" wildcards are strongly 
   discouraged. 
    
   Signals are divided into different types depending on their 
   behavior: 
    
   *  On/off (OO):       Once applied, these signals last until they 
      are turned off. This can only happen as the result of a reboot or 
      a new SignalRequests where the signal is explicitly turned off 
      (see later). Signals of type OO are defined to be idempotent, 
      thus multiple requests to turn a given OO signal on (or off) are 
      perfectly valid and MUST NOT result in any errors. An On/Off 
      signal could be a visual message-waiting indicator (VMWI). Once 
      turned on, it MUST NOT be turned off until explicitly instructed 
      to by the Call Agent, or as a result of an endpoint restart, i.e. 
      these signals will not turn off as a result the arrival of a 
      requested event 
    
                                                                        
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   *  Time-out (TO):     Once applied, these signals last until they 
      are either cancelled (by the occurrence of an event or by not 
      being included in a subsequent (possibly empty) list of signals), 
      or a signal-specific period of time has elapsed. A TO signal that 
      times out will generate an "operation complete" event. A TO 
      signal could be "ringback" timing out after 180 seconds. If an 
      event occurs prior to the 180 seconds, the signal will, by 
      default, be stopped (the "Keep signals active" action - see 
      Section 2.3.3 - will override this behavior). If the signal is 
      not stopped, the signal will time out, stop and generate an 
      "operation complete" event, about which the Call Agent may or may 
      not have requested to be notified. If the Call Agent has asked 
      for the "operation complete" event to be notified, the "operation 
      complete" event sent to the Call Agent SHALL include the name(s) 
      of the signal(s) that timed out (note that if parameters were 
      passed to the signal, the parameters will not be reported). If 
      the signal was generated on a connection, the name of the 
      connection SHALL be included as described above. Time-out signals 
      have a default time-out value defined for them, which MAY be 
      altered by the provisioning process. Also, the time-out period 
      may be provided as a parameter to the signal (see Section 
      3.2.2.4). A value of zero indicates that the time-out period is 
      infinite. A TO signal that fails after being started, but before 
      having generated an "operation complete" event will generate an 
      "operation failure" event which will include the name of the 
      signal that failed. Deletion of a connection with an active TO 
      signal will result in such a failure. 
    
   *  Brief (BR):        The duration of these signals is normally so 
      short that they stop on their own. If a signal stopping event 
      occurs, or a new SignalRequests is applied, a currently active BR 
      signal will not stop. However, any pending BR signals not yet 
      applied MUST be cancelled. As an example, a brief tone could be a 
      DTMF digit. If the DTMF digit "1" is currently being played, and 
      a signal stopping event occurs, the "1" would play to completion. 
                                                
   Signal(s) generated on a connection MUST include the name of that 
   connection. 
    
2.2 Usage of SDP 
    
   The Call Agent uses the MGCP to provide the endpoint with the 
   description of connection parameters such as IP addresses, UDP port 
   and RTP profiles. These descriptions will follow the conventions 
   delineated in the Session Description Protocol which is now an IETF 
   proposed standard, documented in RFC 2327. 
    






                                                                        
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2.3 Gateway Control Commands 
    
2.3.1 Overview of Commands 
    
   This section describes the commands of the MGCP. The service 
   consists of connection handling and endpoint handling commands. 
   There are currently nine commands in the protocol: 
    
   *  The Call Agent can issue an EndpointConfiguration command to a 
      gateway, instructing the gateway about the coding characteristics 
      expected by the "line-side" of the endpoint. 
    
   *  The Call Agent can issue a NotificationRequest command to a 
      gateway, instructing the gateway to watch for specific events 
      such as hook actions or DTMF tones on a specified endpoint. 
    
   *  The gateway will then use the Notify command to inform the Call 
      Agent when the requested events occur. 
    
   *  The Call Agent can use the CreateConnection command to create a 
      connection that terminates in an "endpoint" inside the gateway. 
    
   *  The Call Agent can use the ModifyConnection command to change the 
      parameters associated with a previously established connection. 
    
   *  The Call Agent can use the DeleteConnection command to delete an 
      existing connection. The DeleteConnection command may also be 
      used by a gateway to indicate that a connection can no longer be 
      sustained. 
    
   *  The Call Agent can use the AuditEndpoint and AuditConnection 
      commands to audit the status of an "endpoint" and any connections 
      associated with it. Network management beyond the capabilities 
      provided by these commands is generally desirable. Such 
      capabilities are expected to be supported by the use of the 
      Simple Network Management Protocol (SNMP) and definition of a MIB 
      which is outside the scope of this specification. 
    
   *  The Gateway can use the RestartInProgress command to notify the 
      Call Agent that a group of endpoints managed by the gateway is 
      being taken out-of-service or is being placed back in-service. 
    
   These services allow a controller (normally, the Call Agent) to 
   instruct a gateway on the creation of connections that terminate in 
   an "endpoint" attached to the gateway, and to be informed about 
   events occurring at the endpoint. An endpoint may be for example: 
    
   *  A specific trunk circuit, within a trunk group terminating in a 
      gateway, 
    
   *  A specific announcement handled by an announcement server. 
    

                                                                        
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   Connections are logically grouped into "calls" (the concept of a 
   "call" has however little semantic meaning in MGCP itself). Several 
   connections, that may or may not belong to the same call, can 
   terminate in the same endpoint. Each connection is qualified by a 
   "mode" parameter, which can be set to "send only" (sendonly), 
   "receive only" (recvonly), "send/receive" (sendrecv), "conference" 
   (confrnce), "inactive" (inactive), "loopback", "continuity test" 
   (conttest), "network loop back" (netwloop) or "network continuity 
   test" (netwtest).  
    
   Media generated by the endpoint is sent on connections whose mode is 
   either "send only", "send/receive", or "conference", unless the 
   endpoint has a connection in "loopback" or "continuity test" mode. 
   However, media generated by applying a signal to a connection is 
   always sent on the connection, regardless of the mode.  
    
   The handling of the media streams received on connections is 
   determined by the mode parameters: 
    
   *  Media streams received through connections in "receive", 
      "conference" or "send/receive" mode are mixed and sent to the 
      endpoint, unless the endpoint has another connection in 
      "loopback" or "continuity test" mode. 
    
   *  Media streams originating from the endpoint are transmitted over 
      all the connections whose mode is "send", "conference" or 
      "send/receive", unless the endpoint has another connection in 
      "loopback" or "continuity test" mode. 
    
   *  In addition to being sent to the endpoint, a media stream 
      received through a connection in "conference" mode is forwarded 
      to all the other connections whose mode is "conference". This 
      also applies when the endpoint has a connection in "loopback" or 
      "continuity test" mode. The details of this forwarding, e.g., RTP 
      translator or mixer, is outside the scope of this document. 
    
   Note that in order to detect events on a connection, the connection 
   must by default be in one of the modes "receive", "conference", 
   "send/receive", "network loopback" or "network continuity test". The 
   event detection only applies to the incoming media. Connections in 
   "sendonly", "inactive", "loopback", or "continuity test" mode will 
   thus normally not detect any events, although requesting to do so is 
   not considered an error. 
    
   The "loopback" and "continuity test" modes are used during 
   maintenance and continuity test operations. An endpoint may have 
   more than one connection in either "loopback" or "continuity test" 
   mode. As long as there is one connection in that particular mode, 
   and no other connection on the endpoint is placed in a different 
   maintenance or test mode, the maintenance or test operation shall 
   continue undisturbed. There are two flavors of continuity test, one 
   specified by ITU and one used in the US. In the first case, the test 
   is a loopback test. The originating switch will send a tone (the go 
                                                                        
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   tone) on the bearer circuit and expects the terminating switch to 
   loopback the tone. If the originating switch sees the same tone 
   returned (the return tone), the COT has passed. If not, the COT has 
   failed. In the second case, the go and return tones are different. 
   The originating switch sends a certain go tone. The terminating 
   switch detects the go tone, it asserts a different return tone in 
   the backwards direction. When the originating switch detects the 
   return tone, the COT is passed. If the originating switch never 
   detects the return tone, the COT has failed. 
    
   If the mode is set to "loopback", the gateway is expected to return 
   the incoming signal from the endpoint back into that same endpoint. 
   This procedure will be used, typically, for testing the continuity 
   of trunk circuits according to the ITU specifications. If the mode 
   is set to "continuity test", the gateway is informed that the other 
   end of the circuit has initiated a continuity test procedure 
   according to the GR specification (see [22]). The gateway will place 
   the circuit in the transponder mode required for dual-tone 
   continuity tests. 
    
   If the mode is set to "network loopback", the audio signals received 
   from the connection will be echoed back on the same connection. The 
   media is not forwarded to the endpoint. 
    
   If the mode is set to "network continuity test", the gateway will 
   process the packets received from the connection according to the 
   transponder mode required for dual-tone continuity test, and send 
   the processed signal back on the connection. The media is not 
   forwarded to the endpoint. The "network continuity test" mode is 
   included for backwards compatibility only and use of it is 
   discouraged. 
    
2.3.2 EndpointConfiguration  
    
   The EndpointConfiguration command can be used to specify the 
   encoding of the signals that will be received by the endpoint. For 
   example, in certain international telephony configurations, some 
   calls will carry mu-law encoded audio signals, while others will use 
   A-law. The Call Agent can use the EndpointConfiguration command to 
   pass this information to the gateway. The configuration may vary on 
   a call by call basis, but can also be used in the absence of any 
   connection. 
    
         ReturnCode 
         [, PackageList] 
         <-- EndpointConfiguration(EndpointId, 
                                   [BearerInformation]) 
    
   EndpointId is the name of the endpoint(s) in the gateway where 
   EndpointConfiguration executes. The "any of" wildcard convention 
   MUST NOT be used. If the "all of" wildcard convention is used, the 
   command applies to all the endpoints whose name matches the 
   wildcard. 
                                                                        
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   BearerInformation is a parameter defining the coding of the data 
   sent to and received from the line side. The information is encoded 
   as a list of sub-parameters. The only sub-parameter defined in this 
   version of the specification is the encoding method, whose value can 
   be set to "A-law" or "mu-law". The set of sub-parameters may be 
   extended. 
    
   In order to allow for extensibility, while remaining backwards 
   compatible, the BearerInformation parameter is conditionally 
   optional based on the following conditions: 
    
   *  if Extension Parameters (vendor, package or other) are not used, 
      the BearerInformation parameter is REQUIRED, 
   *  otherwise, the BearerInformation parameter is OPTIONAL.  
    
   When omitted, BearerInformation MUST retain its current value. 
    
   ReturnCode is a parameter returned by the gateway. It indicates the 
   outcome of the command and consists of an integer number optionally 
   followed by commentary. 
    
   PackageList is a list of supported packages that MAY be included 
   with error code 518 (unsupported package).  
    
2.3.3 NotificationRequest  
    
   The NotificationRequest command is used to request the gateway to 
   send notifications upon the occurrence of specified events in an 
   endpoint. For example, a notification may be requested for when a 
   gateway detects that an endpoint is receiving tones associated with 
   fax communication. The entity receiving this notification may then 
   decide to specify use of a different type of encoding method in the 
   connections bound to this endpoint and instruct the gateway 
   accordingly with a ModifyConnection Command. 
    
         ReturnCode, 
         [PackageList] 
         <-- NotificationRequest(EndpointId, 
                                 [NotifiedEntity,] 
                                 [RequestedEvents,] 
                                 RequestIdentifier, 
                                 [DigitMap,] 
                                 [SignalRequests,] 
                                 [QuarantineHandling,] 
                                 [DetectEvents,] 
                                 [encapsulated EndpointConfiguration]) 
    
   EndpointId is the identifier for the endpoint(s) in the gateway 
   where the NotificationRequest. The "any of" wildcard MUST NOT be 
   used. 
    

                                                                        
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   NotifiedEntity is an optional parameter that specifies a new 
   "notified entity" for the endpoint.   
    
   RequestIdentifier is used to correlate this request with the 
   notifications that it triggers. It will be repeated in the 
   corresponding Notify command. 
    
   RequestedEvents is a list of events, possibly qualified by event 
   parameters (see Section 3.2.2.4), that the gateway is requested to 
   detect and report. Such events may include, for example, fax tones, 
   continuity tones, or on-hook transition. Unless otherwise specified, 
   events are detected on the endpoint, however some events can be 
   detected on a connection. A given event MUST NOT appear more than 
   once in a RequestedEvents. If the parameter is omitted, it defaults 
   to empty.  
    
   To each event is associated one or more actions, which can be: 
    
   *  Notify the event immediately, together with the accumulated list 
      of observed events, 
    
   *  Swap audio, 
    
   *  Accumulate the event in an event buffer, but don't notify yet, 
    
   *  Accumulate according to Digit Map, 
    
   *  Keep Signal(s) active, 
    
   *  Process the Embedded Notification Request, 
    
   *  Ignore the event. 
    
   Support for Notify, Accumulate, Keep Signal(s) Active, Embedded 
   Notification Request, and Ignore is REQUIRED. Support for Accumulate 
   according to Digit Map is REQUIRED on any endpoint capable of 
   detecting DTMF. Support for any other action is OPTIONAL. The set of 
   actions can be extended. 
    
   A given action can by default be specified for any event, although 
   some actions will not make sense for all events. For example, an 
   off-hook event with the Accumulate according to Digit Map action is 
   valid, but will of course immediately trigger a digit map mismatch 
   when the off-hook event occurs. Needless to say, such practice is 
   discouraged. 
    
   Some actions can be combined as shown in the table below, where "Y" 
   means the two actions can be combined, and "N" means they cannot: 
    




                                                                        
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       -------------------------------------------------------------- 
      |       | Notif | Swap | Accum | AccDi | KeSiA | EmbNo | Ignor | 
      |--------------------------------------------------------------| 
      | Notif |   N   |   Y  |   N   |   N   |   Y   |   Y*  |   N   | 
      | Swap  |   -   |   N  |   Y   |   N   |   N   |   N   |   Y   | 
      | Accum |   -   |   -  |   N   |   N   |   Y   |   Y   |   N   | 
      | AccDi |   -   |   -  |   -   |   N   |   Y   |   N   |   N   | 
      | KeSiA |   -   |   -  |   -   |   -   |   N   |   Y   |   Y   | 
      | EmbNo |   -   |   -  |   -   |   -   |   -   |   N   |   N   | 
      | Ignor |   -   |   -  |   -   |   -   |   -   |   -   |   N   | 
       -------------------------------------------------------------- 
      Note (*): The "Embedded Notification Request" can only be 
      combined with "Notify", if the gateway is allowed to issue more 
      than one Notify command per Notification request (see below and 
      Section 4.4.1). 
    
   If no action is specified, the Notify action will be applied. If one 
   or more actions are specified, only those actions apply. When two or 
   more actions are specified, each action MUST be combinable with all 
   the other actions as defined by the table above - the individual 
   actions are assumed to occur simultaneously.  
    
   If a client receives a request with an invalid or unsupported action 
   or an illegal combination of actions, it MUST return an error to the 
   Call Agent (error code 523 - unknown or illegal combination of 
   actions, is RECOMMENDED).  
    
   In addition to the RequestedEvents parameter specified in the 
   command, some MGCP packages may contain "persistent events" (this is 
   generally discouraged though - see Appendix B for an alternative). 
   Persistent events in a given package are always detected on an 
   endpoint that implements that package. If a persistent event is not 
   included in the list of RequestedEvents, and the event occurs, the 
   event will be detected anyway and processed like all other events, 
   as if the persistent event had been requested with a Notify action. 
   A NotificationRequest MUST still be in place for a persistent event 
   to trigger a Notify though. Thus, informally, persistent events can 
   be viewed as always being implicitly included in the list of 
   RequestedEvents with an action to Notify, although no glare 
   detection, etc., will be performed.  
    
   Non-persistent events are those events that need to be explicitly 
   included in the RequestedEvents list. The (possibly empty) list of 
   requested events completely replaces the previous list of requested 
   events. In addition to the persistent events, only the events 
   specified in the requested events list will be detected by the 
   endpoint. If a persistent event is included in the RequestedEvents 
   list, the action specified will replace the default action 
   associated with the event for the life of the RequestedEvents list, 
   after which the default action is restored. For example, if "off-
   hook" was a persistent event, the "Ignore off-hook" action was 
   specified, and a new request without any off-hook instructions were 
   received, the default "Notify off-hook" operation would be restored.  
                                                                        
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   The gateway will detect the union of the persistent events and the 
   requested events. If an event is not included in either list, it 
   will be ignored. 
    
   The Call Agent can send a NotificationRequest with an empty (or 
   omitted) RequestedEvents list to the gateway. The Call Agent can do 
   so, for example, to a gateway when it does not want to collect any 
   more DTMF digits. However, persistent events will still be detected 
   and notified. 
    
   The Swap Audio action can be used when a gateway handles more than 
   one connection on an endpoint. This will be the case for call 
   waiting, and possibly other feature scenarios. In order to avoid the 
   round-trip to the Call Agent when just changing which connection is 
   attached to the audio functions of the endpoint, the 
   NotificationRequest can map an event (usually hook flash, but could 
   be some other event) to a local swap audio function, which selects 
   the "next" connection in a round robin fashion. If there is only one 
   connection, this action is effectively a no-op. If there are more 
   than two connections, the order is undefined. If the endpoint has 
   exactly two connections, one of which is "inactive", the other of 
   which is in "send/receive" mode, then swap audio will attempt to 
   make the "send/receive" connection "inactive", and vice versa. This 
   specification intentionally does not provide any additional detail 
   on the swap audio action.  
    
   If signal(s) are desired to start when an event being looked for 
   occurs, the "Embedded NotificationRequest" action can be used. The 
   embedded NotificationRequest may include a new list of 
   RequestedEvents, SignalRequests and a new digit map as well. The 
   semantics of the embedded NotificationRequest is as if a new 
   NotificationRequest was just received with the same NotifiedEntity, 
   RequestIdentifier, QuarantineHandling and DetectEvents. When the 
   "Embedded NotificationRequest" is activated, the "current dial 
   string" will be cleared; however the list of observed events and the 
   quarantine buffer will be unaffected (if combined with a Notify, the 
   Notify will clear the list of observed events though - see Section 
   4.4.1). Note, that the Embedded NotificationRequest action does not 
   accumulate the triggering event, however it can be combined with the 
   Accumulate action to achieve that. If the Embedded 
   NotificationRequest fails, an Embedded NotificationRequest failure 
   event SHOULD be generated (see Appendix B). 
    
   MGCP implementations SHALL be able to support at least one level of 
   embedding. An embedded NotificationRequest that respects this 
   limitation MUST NOT contain another Embedded NotificationRequest. 
    
   DigitMap is an optional parameter that allows the Call Agent to 
   provision the endpoint with a digit map according to which digits 
   will be accumulated. If this optional parameter is absent, the 
   previously defined value is retained. This parameter MUST be 
   defined, either explicitly or through a previous command, if the 
                                                                        
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   RequestedEvents parameter contains a request to "accumulate 
   according to the digit map". The collection of these digits will 
   result in a digit string. The digit string is initialized to a null 
   string upon reception of the NotificationRequest, so that a 
   subsequent notification only returns the digits that were collected 
   after this request. Digits that were accumulated according to the 
   digit map are reported as any other accumulated event, in the order 
   in which they occur. It is therefore possible that other events 
   accumulated are found in between the list of digits. If the gateway 
   is requested to "accumulate according to digit map" and the gateway 
   currently does not have a digit map for the endpoint in question, 
   the gateway MUST return an error (error code 519 - endpoint does not 
   have a digit map, is RECOMMENDED). 
    
   SignalRequests is an optional parameter that contains the set of 
   signals that the gateway is asked to apply. When omitted, it 
   defaults to empty. When multiple signals are specified, the signals 
   MUST be applied in parallel. Unless otherwise specified, signals are 
   applied to the endpoint. However some signals can be applied to a 
   connection. Signals are identified by their name, which is an event 
   name, and may be qualified by signal parameters (see Section 
   3.2.2.4). The following are examples of signals: 
    
   *  Ringing, 
    
   *  Busy tone, 
    
   *  Call waiting tone, 
    
   *  Off hook warning tone, 
    
   *  Ringback tones on a connection 
    
   Names and descriptions of signals are defined in the appropriate 
   package. 
    
   Signals are, by default, applied to endpoints. If a signal applied 
   to an endpoint results in the generation of a media stream (audio, 
   video, etc.), then by default the media stream MUST NOT be forwarded 
   on any connection associated with that endpoint, regardless of the 
   mode of the connection. For example, if a call-waiting tone is 
   applied to an endpoint involved in an active call, only the party 
   using the endpoint in question will hear the call-waiting tone. 
   However, individual signals may define a different behavior. 
    
   When a signal is applied to a connection that has received a 
   RemoteConnectionDescriptor, the media stream generated by that 
   signal will be forwarded on the connection regardless of the current 
   mode of the connection (including loopback and continuity test). If 
   a RemoteConnectionDescriptor has not been received, the gateway MUST 
   return an error (error code 527 - missing 
   RemoteConnectionDescriptor, is RECOMMENDED). Note that this 
   restriction does not apply to detecting events on a connection. 
                                                                        
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   When a (possibly empty) list of signal(s) is supplied, this list 
   completely replaces the current list of active time-out signals. 
   Currently active time-out signals that are not provided in the new 
   list MUST be stopped and the new signal(s) provided will now become 
   active. Currently active time-out signals that are provided in the 
   new list of signals MUST remain active without interruption, thus 
   the timer for such time-out signals will not be affected. 
   Consequently, there is currently no way to restart the timer for a 
   currently active time-out signal without turning the signal off 
   first. If the time-out signal is parameterized, the original set of 
   parameters MUST remain in effect, regardless of what values are 
   provided subsequently. A given signal MUST NOT appear more than once 
   in a SignalRequests. Note that applying a signal S to an endpoint, 
   connection C1 and connection C2, constitutes three different and 
   independent signals. 
    
   The action triggered by the SignalRequests is synchronized with the 
   collection of events specified in the RequestedEvents parameter. For 
   example, if the NotificationRequest mandates "ringing" and the 
   RequestedEvents asks to look for an "off-hook" event, the ringing 
   SHALL stop as soon as the gateway detects an off-hook event. The 
   formal definition is that the generation of all "Time Out" signals 
   SHALL stop as soon as one of the requested events is detected, 
   unless the "Keep signals active" action is associated to the 
   detected event. The RequestedEvents and SignalRequests may refer to 
   the same event definitions. In one case, the gateway is asked to 
   detect the occurrence of the event, and in the other case it is 
   asked to generate it. The specific events and signals that a given 
   endpoint can detect or perform are determined by the list of 
   packages that are supported by that endpoint. Each package specifies 
   a list of events and signals that can be detected or performed. A 
   gateway that is requested to detect or perform an event belonging to 
   a package that is not supported by the specified endpoint MUST 
   return an error (error code 518 - unsupported or unknown package, is 
   RECOMMENDED). When the event name is not qualified by a package 
   name, the default package name for the endpoint is assumed. If the 
   event name is not registered in this default package, the gateway 
   MUST return an error (error code 522 - no such event or signal, is 
   RECOMMENDED). 
    
   The Call Agent can send a NotificationRequest whose requested signal 
   list is empty. It will do so for example when a time-out signal(s) 
   should stop. 
    
   If signal(s) are desired to start as soon as a "looked-for" event 
   occurs, the "Embedded NotificationRequest" action can be used. The 
   embedded NotificationRequest may include a new list of 
   RequestedEvents, SignalRequests and a new Digit Map as well. The 
   embedded NotificationRequest action allows the Call Agent to set up 
   a "mini-script" to be processed by the gateway immediately following 
   the detection of the associated event. Any SignalRequests specified 
   in the embedded NotificationRequest will start immediately. 
                                                                        
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   Considerable care must be taken to prevent discrepancies between the 
   Call Agent and the gateway. However, long-term discrepancies should 
   not occur as a new SignalRequests completely replaces the old list 
   of active time-out signals, and BR-type signals always stop on their 
   own. Limiting the number of On/Off-type signals is encouraged. It is 
   considered good practice for a Call Agent to occasionally turn on 
   all On/Off signals that should be on, and turn off all On/Off 
   signals that should be off. 
    
   The Ignore action can be used to ignore an event, e.g., to prevent a 
   persistent event from being notified. However, the synchronization 
   between the event and an active time-out signal will still occur by 
   default (e.g. a time-out dial-tone signal will stop when an off-hook 
   occurs even if off-hook was a requested event with action "Ignore"). 
   To prevent this synchronization from happening, the "Keep Signal(s) 
   Active" action will have to be specified as well. 
    
   The optional QuarantineHandling parameter specifies the handling of 
   "quarantine" events, i.e., events that have been detected by the 
   gateway before the arrival of this NotificationRequest command, but 
   have not yet been notified to the Call Agent. The parameter provides 
   a set of handling options (see Section 4.4.1 for details): 
    
   *  whether the quarantined events should be processed or discarded 
      (the default is to process them.) 
    
   *  whether the gateway is expected to generate at most one 
      notification (step by step), or multiple notifications (loop), in 
      response to this request (the default is at most one.) 
    
   When the parameter is absent, the default value is assumed. 
    
   We should note that the quarantine-handling parameter also governs 
   the handling of events that were detected and processed but not yet 
   notified when the command is received. 
    
   DetectEvents is an optional parameter, possibly qualified by event 
   parameters, that specifies a list of events that the gateway is 
   requested to detect during the quarantine period. When this 
   parameter is absent, the events to be detected in the quarantine 
   period are those listed in the last received DetectEvents list. In 
   addition, the gateway will also detect persistent events and the 
   events specified in the RequestedEvents list, including those for 
   which the "ignore" action is specified. 
    
   Some events and signals, such as the in-line ringback or the quality 
   alert, are performed or detected on connections terminating in the 
   endpoint rather than on the endpoint itself. The structure of the 
   event names (see Section 2.1.7) allows the Call Agent to specify the 
   connection(s) on which the events should be performed or detected. 
    
   The NotificationRequest command may carry an encapsulated 
   EndpointConfiguration command, that will apply to the same 
                                                                        
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   endpoint(s). When this command is present, the parameters of the 
   EndpointConfiguration command are included with the normal 
   parameters of the NotificationRequest, with the exception of the 
   EndpointId, which is not replicated. 
    
   The encapsulated EndpointConfiguration command shares the fate of 
   the NotificationRequest command. If the NotificationRequest is 
   rejected, the EndpointConfiguration is not executed.  
    
   ReturnCode is a parameter returned by the gateway. It indicates the 
   outcome of the command and consists of an integer number optionally 
   followed by commentary. 
    
   PackageList is a list of supported packages that MAY be included 
   with error code 518 (unsupported package). 
    
2.3.4 Notify 
    
   Notifications with the observed events are sent by the gateway via 
   the Notify command when a triggering event occurs. 
    
         ReturnCode, 
         [PackageList] 
         <-- Notify(EndpointId, 
                    [NotifiedEntity,] 
                    RequestIdentifier, 
                    ObservedEvents) 
    
   EndpointId is the name for the endpoint in the gateway which is 
   issuing the Notify command. The identifier MUST be a fully qualified 
   endpoint identifier, including the domain name of the gateway. The 
   local part of the name MUST NOT use any of the wildcard conventions. 
    
   NotifiedEntity is an optional parameter that identifies the entity 
   which requested the notification. This parameter is equal to the 
   NotifiedEntity parameter of the NotificationRequest that triggered 
   this notification. The parameter is absent if there was no such 
   parameter in the triggering request. Regardless of the value of the 
   NotifiedEntity parameter, the notification MUST be sent to the 
   current "notified entity" for the endpoint. 
    
   RequestIdentifier is a parameter that repeats the RequestIdentifier 
   parameter of the NotificationRequest that triggered this 
   notification. It is used to correlate this notification with the 
   request that triggered it. Persistent events will be viewed here as 
   if they had been included in the last NotificationRequest. An 
   implicit NotificationRequest MAY be in place right after restart - 
   the RequestIdentifier used for it will be zero ("0") - see Section 
   4.4.1 for details. 
    
   ObservedEvents is a list of events that the gateway detected and 
   accumulated. A single notification may report a list of events that 
   will be reported in the order in which they were detected (FIFO). 
                                                                        
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   The list will only contain the identification of events that were 
   requested in the RequestedEvents parameter of the triggering 
   NotificationRequest. It will contain the events that were either 
   accumulated (but not notified) or treated according to digit map 
   (but no match yet), and the final event that triggered the 
   notification or provided a final match in the digit map. It should 
   be noted that digits MUST be added to the list of observed events as 
   they are accumulated, irrespective of whether they are accumulated 
   according to the digit map or not. For example, if a user enters the 
   digits "1234" and some event E is accumulated between the digits "3" 
   and "4" being entered, the list of observed events would be "1, 2, 
   3, E, 4". Events that were detected on a connection SHALL include 
   the name of that connection as in "R/qa@0A3F58" (see Section 2.1.7).  
    
   If the list of ObservedEvents reaches the capacity of the endpoint, 
   an ObservedEvents Full event (see Appendix B) SHOULD be generated 
   (the endpoint shall ensure it has capacity to include this event in 
   the list of ObservedEvents). If the ObservedEvents Full event is not 
   used to trigger a Notify, event processing continues as before 
   (including digit map matching); however, the subsequent events will 
   not be included in the list of ObservedEvents.  
    
   ReturnCode is a parameter returned by the Call Agent. It indicates 
   the outcome of the command and consists of an integer number 
   optionally followed by commentary. 
    
   PackageList is a list of supported packages that MAY be included 
   with error code 518 (unsupported package). 
    
2.3.5 CreateConnection 
    
   This command is used to create a connection between two endpoints. 
    
         ReturnCode, 
         [ConnectionId,] 
         [SpecificEndPointId,] 
         [LocalConnectionDescriptor,] 
         [SecondEndPointId,] 
         [SecondConnectionId,] 
         [PackageList] 
         <-- CreateConnection(CallId, 
                              EndpointId, 
                              [NotifiedEntity,] 
                              [LocalConnectionOptions,] 
                              Mode, 
                              [{RemoteConnectionDescriptor | 
                              SecondEndpointId}, ] 
                              [Encapsulated NotificationRequest,] 
                              [Encapsulated EndpointConfiguration]) 
    
   A connection is defined by its endpoints. The input parameters in 
   CreateConnection provide the data necessary to build a gateway's 
   "view" of a connection. 
                                                                        
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   CallId is a parameter that identifies the call (or session) to which 
   this connection belongs. This parameter SHOULD, at a minimum, be 
   unique within the collection of Call Agents that control the same 
   gateways. Connections that belong to the same call SHOULD share the 
   same call-id. The call-id has little semantic meaning in the 
   protocol; however it can be used to identify calls for reporting and 
   accounting purposes. It does not affect the handling of connections 
   by the gateway. 
    
   EndpointId is the identifier for the connection endpoint in the 
   gateway where CreateConnection executes. The EndpointId can be 
   fully-specified by assigning a value to the parameter EndpointId in 
   the function call or it may be under-specified by using the "any of" 
   wildcard convention. If the endpoint is underspecified, the endpoint 
   identifier SHALL be assigned by the gateway and its complete value 
   returned in the SpecificEndPointId parameter of the response. When 
   the "any of" wildcard is used, the endpoint assigned MUST be in-
   service and MUST NOT already have any connections on it. If no such 
   endpoint is available, error code 410 (no endpoint available) SHOULD 
   be returned. The "all of" wildcard MUST NOT be used.  
    
   The NotifiedEntity is an optional parameter that specifies a new 
   "notified entity" for the endpoint.  
    
   LocalConnectionOptions is an optional structure used by the Call 
   Agent to direct the handling of the connection by the gateway. The 
   fields contained in a LocalConnectionOptions structure may include 
   one or more of the following (each field MUST NOT be supplied more 
   than once):  
    
   *  Codec compression algorithm: One or more codecs, listed in order 
      of preference. For interoperability, it is RECOMMENDED to support 
      G.711 mu-law encoding ("PCMU"). See Section 2.6 for details on 
      the codec selection process. 
    
   *  Packetization period:        A single millisecond value or a 
      range may be specified. The packetization period SHOULD NOT 
      contradict the specification of the codec compression algorithm. 
      If a codec is specified that has a frame size which is 
      inconsistent with the packetization period, and that codec is 
      selected, the gateway is authorized to use a packetization period 
      that is consistent with the frame size even if it is different 
      from that specified. In so doing, the gateway SHOULD choose a 
      non-zero packetization period as close to that specified as 
      possible. If a packetization period is not specified, the 
      endpoint SHOULD use the default packetization period(s) for the 
      codec(s) selected. 
    
   *  Bandwidth:                   The allowable bandwidth, i.e. 
      payload plus any header overhead from the transport layer and up, 
      e.g. IP, UDP, and RTP. The bandwidth specification SHOULD NOT 
      contradict the specification of codec compression algorithm or 
                                                                        
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      packetization period. If a codec is specified, then the gateway 
      is authorized to use it, even if it results in the usage of a 
      larger bandwidth than specified. Any discrepancy between the 
      bandwidth and codec specification will not be reported as an 
      error. 
    
   *  Type of Service:             This indicates the class of service 
      to be used for this connection. When the Type of Service is not 
      specified, the gateway SHALL use a default value of zero unless 
      provisioned otherwise.  
    
   *  Usage of echo cancellation:  By default, the telephony gateways 
      always perform echo cancellation on the endpoint. However, it may 
      be necessary, for some calls, to turn off these operations. The 
      echo cancellation parameter can have two values, "on" (when the 
      echo cancellation is requested) and "off" (when it is turned 
      off). The parameter is optional. If the parameter is omitted when 
      creating a connection and there are no other connections on the 
      endpoint, the endpoint SHALL apply echo cancellation initially. 
      If the parameter is omitted when creating a connection and there 
      are existing connections on the endpoint, echo cancellation is 
      unchanged. The endpoint SHOULD subsequently enable or disable 
      echo cancellation when voiceband data is detected - see e.g. ITU-
      T recommendation V.8, V.25, and G.168. Following termination of 
      voiceband data, the handling of echo cancellation SHALL then 
      revert to the current value of the echo cancellation parameter. 
      It is RECOMMENDED that echo cancellation handling is left to the 
      gateway rather than having this parameter specified by the Call 
      Agent. 
    
   *  Silence Suppression:         The telephony gateways may perform 
      voice activity detection, and avoid sending packets during 
      periods of silence. However, it is necessary, for example for 
      modem calls, to turn off this detection. The silence suppression 
      parameter can have two values, "on" (when the detection is 
      requested) and "off" (when it is not requested). The default is 
      "off" (unless provisioned otherwise). Upon detecting voiceband 
      data, the endpoint SHOULD disable silence suppression. Following 
      termination of voiceband data, the handling of silence 
      suppression SHALL then revert to the current value of the silence 
      suppression parameter. 
    
   *  Gain Control:                The telephony gateways may perform 
      gain control on the endpoint, in order to adapt the level of the 
      signal. However, it is necessary, for example for some modem 
      calls, to turn off this function. The gain control parameter may 
      either be specified as "automatic", or as an explicit number of 
      decibels of gain. The gain specified will be added to media sent 
      out over the endpoint (as opposed to the connection) and 
      subtracted from media received on the endpoint. The parameter is 
      optional. When there are no other connections on the endpoint, 
      and the parameter is omitted, the default is to not perform gain 
      control (unless provisioned otherwise), which is equivalent to 
                                                                        
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      specifying a gain of 0 decibels. If there are other connections 
      on the endpoint, and the parameter is omitted, gain control is 
      unchanged. Upon detecting voiceband data, the endpoint SHOULD 
      disable gain control if needed. Following termination of 
      voiceband data, the handling of gain control SHALL then revert to 
      the current value of the gain control parameter. It should be 
      noted, that handling of gain control is normally best left to the 
      gateway and hence use of this parameter is NOT RECOMMENDED. 
    
   *  RTP security:                The Call agent can request the 
      gateway to enable encryption of the audio Packets. It does so by 
      providing a key specification, as specified in RFC 2327. By 
      default, encryption is not performed. 
    
   *  Network Type:                The Call Agent may instruct the 
      gateway to prepare the connection on a specified type of network. 
      If absent, the value is based on the network type of the gateway 
      being used. 
    
   *  Resource reservation:        The Call Agent may instruct the 
      gateway to use network resource reservation for the connection. 
      See Section 2.7 for details.  
    
   The Call Agent specifies the relevant fields it cares about in the 
   command and leaves the rest to the discretion of the gateway. For 
   those of the above parameters that were not explicitly included, the 
   gateway SHOULD use the default values if possible. For a detailed 
   list of local connection options included with this specification 
   refer to section 3.2.2.10. The set of local connection options can 
   be extended.  
    
   The Mode indicates the mode of operation for this side of the 
   connection. The basic modes are "send", "receive", "send/receive", 
   "conference", "inactive", "loopback", "continuity test", "network 
   loop back" and "network continuity test". The expected handling of 
   these modes is specified in the introduction of the "Gateway Control 
   Commands", Section 2.3. Note that signals applied to a connection do 
   not follow the connection mode. Some endpoints may not be capable of 
   supporting all modes. If the command specifies a mode that the 
   endpoint does not support, an error SHALL be returned (error 517 - 
   unsupported mode, is RECOMMENDED). Also, if a connection has not yet 
   received a RemoteConnectionDescriptor, an error MUST be returned if 
   the connection is attempted to be placed in any of the modes "send 
   only", "send/receive", "conference", "network loopback", "network 
   continuity test", or if a signal (as opposed to detecting an event) 
   is to be applied to the connection (error code 527 - missing 
   RemoteConnectionDescriptor, is RECOMMENDED). The set of modes can be 
   extended. 
    
   The gateway returns a ConnectionId, that uniquely identifies the 
   connection within the endpoint, and a LocalConnectionDescriptor, 
   which is a session description that contains information about the 

                                                                        
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   connection, e.g. IP address and port for the media, as defined in 
   SDP.  
    
   The SpecificEndPointId is an optional parameter that identifies the 
   responding endpoint. It is returned when the EndpointId argument 
   referred to an "any of" wildcard name and the command succeeded. 
   When a SpecificEndPointId is returned, the Call Agent SHALL use it 
   as the EndpointId value in successive commands referring to this 
   connection. 
    
   The SecondEndpointId can be used instead of the 
   RemoteConnectionDescriptor to establish a connection between two 
   endpoints located on the same gateway. The connection is by 
   definition a local connection. The SecondEndpointId can be fully- 
   specified by assigning a value to the parameter SecondEndpointId in 
   the function call or it may be under-specified by using the "any of" 
   wildcard convention. If the SecondEndpointId is underspecified, the 
   second endpoint identifier will be assigned by the gateway and its 
   complete value returned in the SecondEndPointId parameter of the 
   response. 
    
   When a SecondEndpointId is specified, the command really creates two 
   connections that can be manipulated separately through 
   ModifyConnection and DeleteConnection commands. In addition to the 
   ConnectionId and LocalConnectionDescriptor for the first connection, 
   the response to the creation provides a SecondConnectionId parameter 
   that identifies the second connection. The second connection is 
   established in "send/receive" mode.  
    
   After receiving a "CreateConnection" request that did not include a 
   RemoteConnectionDescriptor parameter, a gateway is in an ambiguous 
   situation. Because it has exported a LocalConnectionDescriptor 
   parameter, it can potentially receive packets. Because it has not 
   yet received the RemoteConnectionDescriptor parameter of the other 
   gateway, it does not know whether the packets that it receives have 
   been authorized by the Call Agent. It must thus navigate between two 
   risks, i.e., clipping some important announcements or listening to 
   insane data. The behavior of the gateway is determined by the value 
   of the Mode parameter: 
    
   *  If the mode was set to ReceiveOnly, the gateway MUST accept the 
      media and transmit them through the endpoint. 
    
   *  If the mode was set to Inactive, Loopback, or Continuity Test, 
      the gateway MUST NOT transmit the media through to the endpoint. 
    
   Note that the mode values SendReceive, Conference, SendOnly, Network 
   Loopback and Network Continuity Test do not make sense in this 
   situation. They MUST be treated as errors, and the command MUST be 
   rejected (error code 527 - missing RemoteConnectionDescriptor, is 
   RECOMMENDED). 
    

                                                                        
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   The command may optionally contain an encapsulated Notification 
   Request command, which applies to the EndpointId, in which case a 
   RequestIdentifier parameter must be present, as well as, optionally, 
   other parameters of the NotificationRequest with the exception of 
   the EndpointId, which is not replicated. The encapsulated 
   NotificationRequest is executed simultaneously with the creation of 
   the connection. For example, when the Call Agent wants to initiate a 
   call to a residential gateway, it could: 
    
   *  ask the residential gateway to prepare a connection, in order to 
      be sure that the user can start speaking as soon as the phone 
      goes off hook, 
    
   *  ask the residential gateway to start ringing, 
    
   *  ask the residential gateway to notify the Call Agent when the 
      phone goes off-hook. 
    
   This can be accomplished in a single CreateConnection command, by 
   also transmitting the RequestedEvents parameters for the off-hook 
   event, and the SignalRequests parameter for the ringing signal. 
    
   When these parameters are present, the creation and the 
   NotificationRequest MUST be synchronized, which means that both MUST 
   be accepted, or both MUST be refused. In our example, the 
   CreateConnection may be refused if the gateway does not have 
   sufficient resources, or cannot get adequate resources from the 
   local network access, and the off-hook NotificationRequest can be 
   refused in the glare condition, if the user is already off-hook. In 
   this example, the phone must not ring if the connection cannot be 
   established, and the connection must not be established if the user 
   is already off-hook. 
    
   The NotifiedEntity parameter, if present, defines the new "notified 
   entity" for the endpoint. 
    
   The command may carry an encapsulated EndpointConfiguration command, 
   which applies to the EndpointId. When this command is present, the 
   parameters of the EndpointConfiguration command are included with 
   the normal parameters of the CreateConnection with the exception of 
   the EndpointId, which is not replicated. The EndpointConfiguration 
   command may be encapsulated together with an encapsulated 
   NotificationRequest command. Note that both of these apply to the 
   EndpointId only. 
    
   The encapsulated EndpointConfiguration command shares the fate of 
   the CreateConnection command. If the CreateConnection is rejected, 
   the EndpointConfiguration is not executed. 
    
   ReturnCode is a parameter returned by the gateway. It indicates the 
   outcome of the command and consists of an integer number optionally 
   followed by commentary. 
    
                                                                        
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   PackageList is a list of supported packages that MAY be included 
   with error code 518 (unsupported package). 
    
2.3.6 ModifyConnection 
    
   This command is used to modify the characteristics of a gateway's 
   "view" of a connection. This "view" of the call includes both the 
   local connection descriptor as well as the remote connection 
   descriptor. 
    
         ReturnCode, 
         [LocalConnectionDescriptor,] 
         [PackageList] 
         <-- ModifyConnection(CallId, 
                              EndpointId, 
                              ConnectionId, 
                              [NotifiedEntity,] 
                              [LocalConnectionOptions,] 
                              [Mode,] 
                              [RemoteConnectionDescriptor,] 
                              [Encapsulated NotificationRequest,] 
                              [Encapsulated EndpointConfiguration]) 
    
   The parameters used are the same as in the CreateConnection command, 
   with the addition of a ConnectionId that identifies the connection 
   within the endpoint. This parameter was returned by the 
   CreateConnection command, in addition to the local connection 
   descriptor. It uniquely identifies the connection within the context 
   of the endpoint. The CallId used when the connection was created 
   MUST be included as well. 
    
   The EndpointId MUST be a fully qualified endpoint identifier. The 
   local name MUST NOT use the wildcard conventions. 
    
   The ModifyConnection command can be used to affect parameters of a 
   connection in the following ways: 
    
   *  Provide information about the other end of the connection, 
      through the RemoteConnectionDescriptor. If the parameter is 
      omitted, it retains its current value. 
    
   *  Activate or deactivate the connection, by changing the value of 
      the Mode parameter. This can occur at any time during the 
      connection, with arbitrary parameter values. If the parameter is 
      omitted, it retains its current value. 
    
   *  Change the parameters of the connection through the 
      LocalConnectionOptions, for example by switching to a different 
      coding scheme, changing the packetization period, or modifying 
      the handling of echo cancellation. If one or more 
      LocalConnectionOptions parameters are omitted, then the gateway 
      SHOULD refrain from changing that parameter from its current 
      value, unless another parameter necessitating such a change is 
                                                                        
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      explicitly provided. For example, a codec change might require a 
      change in silence suppression. Note that if a 
      RemoteConnectionDescriptor is supplied, then only the 
      LocalConnectionOptions actually supplied with the 
      ModifyConnection command will affect the codec negotiation (as 
      described in Section 2.6). 
    
   Connections can only be fully activated if the 
   RemoteConnectionDescriptor has been provided to the gateway. The 
   receive-only mode, however, can be activated without the provision 
   of this descriptor. 
    
   The command will only return a LocalConnectionDescriptor if the 
   local connection parameters, such as RTP ports, were modified. Thus, 
   if, for example, only the mode of the connection is changed, a 
   LocalConnectionDescriptor will not be returned. Note however, that 
   inclusion of LocalConnectionOptions in the command is not a 
   prerequisite for local connection parameter changes to occur. If a 
   connection parameter is omitted, e.g. silence suppression, the old 
   value of that parameter will be retained if possible. If a parameter 
   change necessitates a change in one or more unspecified parameters, 
   the gateway is free to choose suitable values for the unspecified 
   parameters that must change. This can for instance happen if the 
   packetization period was not specified. If the new codec supported 
   the old packetization period, the value of this parameter would not 
   change, as a change would not be necessary. However, if it did not 
   support the old packetization period, it would choose a suitable 
   value. 
    
   The command may optionally contain an encapsulated Notification 
   Request command, in which case a RequestIdentifier parameter must be 
   present, as well as, optionally, other parameters of the 
   NotificationRequest with the exception of the EndpointId, which is 
   not replicated. The encapsulated NotificationRequest is executed 
   simultaneously with the modification of the connection. For example, 
   when a connection is accepted, the calling gateway should be 
   instructed to place the circuit in send-receive mode and to stop 
   providing ringing tones. This can be accomplished in a single 
   ModifyConnection command, by also transmitting the RequestedEvents 
   parameters, for the on-hook event, and an empty SignalRequests 
   parameter, to stop the provision of ringing tones. 
    
   When these parameters are present, the modification and the 
   NotificationRequest MUST be synchronized, which means that both MUST 
   be accepted, or both MUST be refused.  
    
   The NotifiedEntity parameter, if present, defines the new "notified 
   entity" for the endpoint. 
    
   The command may carry an encapsulated EndpointConfiguration command, 
   that will apply to the same endpoint. When this command is present, 
   the parameters of the EndpointConfiguration command are included 
   with the normal parameters of the ModifyConnection with the 
                                                                        
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   exception of the EndpointId, which is not replicated. The 
   EndpointConfiguration command may be encapsulated together with an 
   encapsulated NotificationRequest command. 
    
   The encapsulated EndpointConfiguration command shares the fate of 
   the ModifyConnection command. If the ModifyConnection is rejected, 
   the EndpointConfiguration is not executed. 
    
   ReturnCode is a parameter returned by the gateway. It indicates the 
   outcome of the command and consists of an integer number optionally 
   followed by commentary. 
    
   PackageList is a list of supported packages that MAY be included 
   with error code 518 (unsupported package). 
    
2.3.7 DeleteConnection (from the Call Agent) 
    
   This command is used to terminate a connection. As a side effect, it 
   collects statistics on the execution of the connection. 
    
         ReturnCode, 
         ConnectionParameters, 
         [PackageList] 
         <-- DeleteConnection(CallId, 
                              EndpointId, 
                              ConnectionId, 
                              [NotifiedEntity,] 
                              [Encapsulated NotificationRequest,] 
                              [Encapsulated EndpointConfiguration]) 
    
   The endpoint identifier, in this form of the DeleteConnection 
   command, SHALL be fully qualified. Wildcard conventions SHALL NOT be 
   used. 
    
   The ConnectionId identifies the connection to be deleted. The CallId 
   used when the connection was created is included as well. 
    
   The NotifiedEntity parameter, if present, defines the new "notified 
   entity" for the endpoint. 
    
   In the case of IP multicast, connections can be deleted individually 
   and independently. However, in the unicast case where a connection 
   has two ends, a DeleteConnection command has to be sent to both 
   gateways involved in the connection. After the connection has been 
   deleted, media streams previously supported by the connection are no 
   longer available. Any media packets received for the old connection 
   are simply discarded and no new media packets for the stream are 
   sent.  
    
   After the connection has been deleted, any loopback that has been 
   requested for the connection must be cancelled (unless the endpoint 
   has another connection requesting loopback).  
    
                                                                        
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   In response to the DeleteConnection command, the gateway returns a 
   list of connection parameters that describe statistics for the 
   connection.  
    
   When the connection was for an Internet media stream, these 
   parameters are: 
    
   Number of packets sent: 
       
      The total number of media packets transmitted by the sender since 
      starting transmission on this connection. In the case of RTP, the 
      count is not reset if the sender changes its synchronization 
      source identifier (SSRC, as defined in RTP), for example as a 
      result of a ModifyConnection command. The value is zero if the 
      connection was always set in "receive only" mode and no signals 
      were applied to the connection. 
    
   Number of octets sent: 
       
      The total number of payload octets (i.e., not including header or 
      padding) transmitted in media packets by the sender since 
      starting transmission on this connection. In the case of RTP, the 
      count is not reset if the sender changes its SSRC identifier, for 
      example as a result of a ModifyConnection command. The value is 
      zero if the connection was always set in "receive only" mode and 
      no signals were applied to the connection. 
    
   Number of packets received: 
       
      The total number of media packets received by the sender since 
      starting reception on this connection. In the case of RTP, the 
      count includes packets received from different SSRC, if the 
      sender used several values. The value is zero if the connection 
      was always set in "send only" mode. 
    
   Number of octets received: 
       
      The total number of payload octets (i.e., not including header, 
      e.g. RTP, or padding) transmitted in media packets by the sender 
      since starting transmission on this connection. In the case of 
      RTP, the count includes packets received from different SSRC, if 
      the sender used several values. The value is zero if the 
      connection was always set in "send only" mode. 
    
   Number of packets lost: 
       
      The total number of media packets that have been lost since the 
      beginning of reception. This number is defined to be the number 
      of packets expected less the number of packets actually received, 
      where the number of packets received includes any which are late 
      or duplicates. For RTP, the count includes packets received from 
      different SSRC, if the sender used several values. Thus packets 
      that arrive late are not counted as lost, and the loss may be 
                                                                        
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      negative if there are duplicates. The count includes packets 
      received from different SSRC, if the sender used several values. 
      The number of packets expected is defined to be the extended last 
      sequence number received, as defined next, less the initial 
      sequence number received. The count includes packets received 
      from different SSRC, if the sender used several values. The value 
      is zero if the connection was always set in "send only" mode.  
    
   Interarrival jitter: 
       
      An estimate of the statistical variance of the media packet 
      interarrival time measured in milliseconds and expressed as an 
      unsigned integer. For RTP, the interarrival jitter J is defined 
      to be the mean deviation (smoothed absolute value) of the 
      difference D in packet spacing at the receiver compared to the 
      sender for a pair of packets. Detailed computation algorithms are 
      found in RFC 1889. The count includes packets received from 
      different SSRC, if the sender used several values. The value is 
      zero if the connection was always set in "send only" mode.  
    
   Average transmission delay: 
       
      An estimate of the network latency, expressed in milliseconds. 
      For RTP, this is the average value of the difference between the 
      NTP timestamp indicated by the senders of the RTCP messages and 
      the NTP timestamp of the receivers, measured when the messages 
      are received. The average is obtained by summing all the 
      estimates, then dividing by the number of RTCP messages that have 
      been received. When the gateway's clock is not synchronized by 
      NTP, the latency value can be computed as one half of the round 
      trip delay, as measured through RTCP. When the gateway cannot 
      compute the one way delay or the round trip delay, the parameter 
      conveys a null value. 
    
   For a detailed definition of these variables, refer to RFC 1889. 
    
   When the connection was set up over a LOCAL interconnect, the 
   meaning of these parameters is defined as follows: 
    
   Number of packets sent: 
      Not significant - MAY be omitted. 
    
   Number of octets sent: 
      The total number of payload octets transmitted over the local 
      connection. 
    
   Number of packets received: 
      Not significant - MAY be omitted. 
    
   Number of octets received: 
      The total number of payload octets received over the connection. 
    

                                                                        
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   Number of packets lost: 
      Not significant - MAY be omitted. A value of zero is assumed. 
    
   Interarrival jitter: 
      Not significant - MAY be omitted. A value of zero is assumed. 
    
   Average transmission delay: 
      Not significant - MAY be omitted. A value of zero is assumed. 
    
   The set of connection parameters can be extended. Also, the meaning 
   may be further defined by other types of networks which MAY 
   furthermore elect to not return all, or even any, of the above 
   specified parameters. 
    
   The command may optionally contain an encapsulated Notification 
   Request command, in which case a RequestIdentifier parameter MUST be 
   present, as well as, optionally, other parameters of the 
   NotificationRequest with the exception of the EndpointId, which is 
   not replicated. The encapsulated NotificationRequest is executed 
   simultaneously with the deletion of the connection. For example, 
   when a user hang-up is notified, the gateway should be instructed to 
   delete the connection and to start looking for an off-hook event. 
   This can be accomplished in a single DeleteConnection command, by 
   also transmitting the RequestedEvents parameters, for the off-hook 
   event, and an empty SignalRequests parameter. 
    
   When these parameters are present, the DeleteConnection and the 
   NotificationRequest must be synchronized, which means that both MUST 
   be accepted, or both MUST be refused. 
    
   The command may carry an encapsulated EndpointConfiguration command, 
   that will apply to the same endpoint. When this command is present, 
   the parameters of the EndpointConfiguration command are included 
   with the normal parameters of the DeleteConnection with the 
   exception of the EndpointId, which is not replicated. The 
   EndpointConfiguration command may be encapsulated together with an 
   encapsulated NotificationRequest command. 
    
   The encapsulated EndpointConfiguration command shares the fate of 
   the DeleteConnection command. If the DeleteConnection is rejected, 
   the EndpointConfiguration is not executed. 
    
   ReturnCode is a parameter returned by the gateway. It indicates the 
   outcome of the command and consists of an integer number optionally 
   followed by commentary. 
    
   PackageList is a list of supported packages that MAY be included 
   with error code 518 (unsupported package). 
    
2.3.8 DeleteConnection (from the gateway) 
    
   In some rare circumstances, a gateway may have to clear a 
   connection, for example because it has lost the resource associated 
                                                                        
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   with the connection, or because it has detected that the endpoint no 
   longer is capable or willing to send or receive media. The gateway 
   may then terminate the connection by using a variant of the 
   DeleteConnection command: 
    
         ReturnCode, 
         [PackageList] 
         <-- DeleteConnection(CallId, 
                              EndpointId, 
                              ConnectionId, 
                              ReasonCode, 
                              Connection-parameters) 
    
   The EndpointId, in this form of the DeleteConnection command, MUST 
   be fully qualified. Wildcard conventions MUST NOT be used. 
    
   The ReasonCode is a text string starting with a numeric reason code 
   and optionally followed by a descriptive text string. The reason 
   code indicates the cause of the DeleteConnection. A list of reason 
   codes can be found in Section 2.5. 
    
   In addition to the call, endpoint and connection identifiers, the 
   gateway will also send the connection parameters that would have 
   been returned to the Call Agent in response to a DeleteConnection 
   command. 
    
   ReturnCode is a parameter returned by the Call Agent. It indicates 
   the outcome of the command and consists of an integer number 
   optionally followed by commentary. 
    
   PackageList is a list of supported packages that MAY be included 
   with error code 518 (unsupported package). 
    
   Note that use of this command is generally discouraged and should 
   only be done as a last resort. If a connection can be sustained, 
   deletion of it should be left to the discretion of the Call Agent 
   which is in a far better position to make intelligent decisions in 
   this area. 
    
2.3.9 DeleteConnection (multiple connections from the Call Agent) 
    
   A variation of the DeleteConnection function can be used by the Call 
   Agent to delete multiple connections at the same time. Note that 
   encapsulating other commands with this variation of the 
   DeleteConnection command is not permitted. The command can be used 
   to delete all connections that relate to a Call for an endpoint: 
    
         ReturnCode, 
         [PackageList] 
         <-- DeleteConnection(CallId, 
                              EndpointId) 
    

                                                                        
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   The EndpointId, in this form of the DeleteConnection command, MUST 
   NOT use the "any of" wildcard. All connections for the endpoint(s) 
   with the CallId specified will be deleted. Note that the command 
   will still succeed if there were no connections with the CallId 
   specified, as long as the EndpointId was valid. However, if the 
   EndpointId is invalid, the command will fail. The command does not 
   return any individual statistics or call parameters. 
    
   It can also be used to delete all connections that terminate in a 
   given endpoint: 
    
         ReturnCode, 
         [PackageList] 
         <-- DeleteConnection(EndpointId) 
    
   The EndpointId, in this form of the DeleteConnection command, MUST 
   NOT use the "any of" wildcard. Again, the command succeeds even if 
   there were no connections on the endpoint(s). 
    
   Finally, Call Agents can take advantage of the hierarchical 
   structure of endpoint names to delete all the connections that 
   belong to a group of endpoints. In this case, the "local name" 
   component of the EndpointId will be specified using the "all of" 
   wildcarding convention. The "any of" convention SHALL NOT be used. 
   For example, if endpoint names are structured as the combination of 
   a physical interface name and a circuit number, as in "X35V3+A4/13", 
   the Call Agent may replace the circuit number by the "all of" wild 
   card character "*", as in "X35V3+A4/*". This "wildcard" command 
   instructs the gateway to delete all the connections that where 
   attached to circuits connected to the physical interface "X35V3+A4". 
    
   After all the connections have been deleted, any loopback that has 
   been requested for the connections MUST be cancelled by the gateway. 
    
   This command does not return any individual statistics or call 
   parameters. 
    
   ReturnCode is a parameter returned by the gateway. It indicates the 
   outcome of the command and consists of an integer number optionally 
   followed by commentary. 
    
   PackageList is a list of supported packages that MAY be included 
   with error code 518 (unsupported package). 
    
2.3.10 AuditEndpoint 
    
   The AuditEndPoint command can be used by the Call Agent to find out 
   the status of a given endpoint. 
    




                                                                        
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         ReturnCode, 
         EndPointIdList,|{ 
         [RequestedEvents,] 
         [QuarantineHandling,] 
         [DigitMap,] 
         [SignalRequests,] 
         [RequestIdentifier,] 
         [NotifiedEntity,] 
         [ConnectionIdentifiers,] 
         [DetectEvents,] 
         [ObservedEvents,] 
         [EventStates,] 
         [BearerInformation,] 
         [RestartMethod,] 
         [RestartDelay,] 
         [ReasonCode,] 
         [MaxMGCPDatagram,] 
         [Capabilities]} 
         [PackageList] 
         <-- AuditEndPoint(EndpointId, 
                           [RequestedInfo]) 
    
   The EndpointId identifies the endpoint(s) being audited. The "any 
   of" wildcard convention MUST NOT be used.  
    
   The EndpointId identifies the endpoint(s) being audited. The "all 
   of" wildcard convention can be used to start auditing of a group of 
   endpoints (regardless of their service-state). If this convention is 
   used, the gateway SHALL return the list of endpoint identifiers that 
   match the wildcard in the EndPointIdList parameter, which is simply 
   one or more SpecificEndpointIds (each supplied separately). In the 
   case where the "all of" wildcard is used, RequestedInfo SHOULD NOT 
   be included (if it is included, it MUST be ignored). Note that the 
   use of the "all of" wildcard can potentially generate a large 
   EndPointIdList. If the resulting EndPointIdList is considered too 
   large, the gateway returns an error (error code 533 - response too 
   large, is RECOMMENDED). 
    
   When a non-wildcard EndpointId is specified, the (possibly empty) 
   RequestedInfo parameter describes the information that is requested 
   for the EndpointId specified. The following endpoint info can be 
   audited with this command: 
    
     RequestedEvents, DigitMap, SignalRequests, RequestIdentifier, 
     QuarantineHandling, NotifiedEntity, ConnectionIdentifiers, 
     DetectEvents, ObservedEvents, EventStates, BearerInformation, 
     RestartMethod, RestartDelay, ReasonCode, PackageList, 
     MaxMGCPDatagram, and Capabilities. 
    
   The list may be extended by extension parameters. The response will 
   in turn include information about each of the items for which 
   auditing info was requested. Supported parameters with empty values 
   MUST always be returned. However, if an endpoint is queried about a 
                                                                        
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   parameter it does not understand, the endpoint MUST NOT generate an 
   error; instead the parameter MUST be omitted from the response: 
    
   *  RequestedEvents: The current value of RequestedEvents the 
      endpoint is using including the action(s) and event parameters 
      associated with each event. Persistent events are included in the 
      list. 
    
   *  DigitMap: the digit map the endpoint is currently using. The 
      parameter will be empty if the endpoint does not have a digit 
      map. 
    
   *  SignalRequests: A list of the; Time-Out signals that are 
      currently active, On/Off signals that are currently "on" for the 
      endpoint (with or without parameter), and any pending Brief 
      signals. Time-Out signals that have timed-out, and currently 
      playing Brief signals are not included. Any signal parameters 
      included in the original SignalRequests will be included. 
    
   *  RequestIdentifier: the RequestIdentifier for the last 
      NotificationRequest received by this endpoint (includes 
      NotificationRequests encapsulated in other commands). If no 
      NotificationRequest has been received since reboot, the value 
      zero will be returned. 
    
   *  QuarantineHandling: the QuarantineHandling for the last 
      NotificationRequest received by this endpoint. If 
      QuarantineHandling was not included, or no notification request 
      has been received, the default parameters will be returned. 
    
   *  DetectEvents: the value of the most recently received 
      DetectEvents parameter plus any persistent events implemented by 
      the endpoint. If no DetectEvents parameter has been received, the 
      (possibly empty) list only includes persistent events. 
    
   *  NotifiedEntity: the current "notified entity" for the endpoint. 
    
   *  ConnectionIdentifiers: the list of ConnectionIdentifiers for all 
      connections that currently exist for the specified endpoint. 
    
   *  ObservedEvents: the current list of observed events for the 
      endpoint. 
    
   *  EventStates: For events that have auditable states associated 
      with them, the event corresponding to the state the endpoint is 
      in, e.g., off-hook if the endpoint is off-hook. Note that the 
      definition of the individual events will state if the event in 
      question has an auditable state associated with it.  
    
   *  BearerInformation: the value of the last received 
      BearerInformation parameter for this endpoint (this includes the 
      case where BearerInformation was provisioned). The parameter will 

                                                                        
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      be empty if the endpoint has not received a BearerInformation 
      parameter and a value was also not provisioned. 
    
   *  RestartMethod: "restart" if the endpoint is in-service and 
      operation is normal, or if the endpoint is in the process of 
      becoming in-service (a non-zero RestartDelay will indicate the 
      latter). Otherwise, the value of the restart method parameter in 
      the last RestartInProgress command issued (or should have been 
      issued) by the endpoint. Note that a "disconnected" endpoint will 
      thus only report "disconnected" as long as it actually is 
      disconnected, and "restart" will be reported once it is no longer 
      disconnected. Similarly, "cancel-graceful" will not be reported, 
      but "graceful" might (see Section 4.4.5 for further details). 
    
   *  RestartDelay: the value of the restart delay parameter if a 
      RestartInProgress command was to be issued by the endpoint at the 
      time of this response, or zero if the command would not include 
      this parameter. 
    
   *  ReasonCode: the value of the ReasonCode parameter in the last 
      RestartInProgress or DeleteConnection command issued by the 
      gateway for the endpoint, or the special value 000 if the 
      endpoint's state is normal. 
    
   *  PackageList: the packages supported by the endpoint including 
      package version numbers. For backwards compatibility, support for 
      the parameter is OPTIONAL although implementations with package 
      versions higher than zero SHOULD support it.  
    
   *  MaxMGCPDatagram: The maximum size of an MGCP datagram in bytes 
      that can be received by the endpoint (see Section 3.5.4). The 
      value excludes any lower layer overhead. For backwards 
      compatibility, support for this parameter is OPTIONAL. The 
      default maximum MGCP datagram size SHOULD be assumed if a value 
      is not returned. 
    
   *  Capabilities: the capabilities for the endpoint similar to the 
      LocalConnectionOptions parameter and including packages and 
      connection modes. Extensions MAY be included as well. If any 
      unknown capabilities are reported, they MUST simply be ignored. 
      If there is a need to specify that some parameters, such as e.g., 
      silence suppression, are only compatible with some codecs, then 
      the gateway MUST return several capability sets, each of which 
      may include: 
       
      -  Compression Algorithm: A list of supported codecs. The rest of 
         the parameters in the capability set will apply to all codecs 
         specified in this list. 
       
      -  Packetization Period: A single value or a range may be 
         specified. 
       

                                                                        
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      -  Bandwidth: A single value or a range corresponding to the 
         range for packetization periods may be specified (assuming no 
         silence suppression). 
       
      -  Echo Cancellation: Whether echo cancellation is supported or 
         not for the endpoint. 
       
      -  Silence Suppression: Whether silence suppression is supported 
         or not. 
       
      -  Gain Control: Whether gain control is supported or not. 
       
      -  Type of Service: Whether type of service is supported or not. 
       
      -  Resource Reservation: Whether resource reservation is 
         supported or not. 
       
      -  Security: Whether media encryption is supported or not.  
       
      -  Type of network: The type(s) of network supported. 
       
      -  Packages: A list of packages supported. The first package in 
         the list will be the default package. 
       
      -  Modes: A list of supported connection modes. 
       
   The Call Agent may then decide to use the AuditConnection command to 
   obtain further information about the connections. 
    
   If no info was requested and the EndpointId refers to a valid 
   endpoint (in-service or not), the gateway simply returns a positive 
   acknowledgement.  
    
   ReturnCode is a parameter returned by the gateway. It indicates the 
   outcome of the command and consists of an integer number optionally 
   followed by commentary. 
    
   Note that PackageList MAY also be included with error code 518 
   (unsupported package). 
    
2.3.11 AuditConnection 
    
   The AuditConnection command can be used by the Call Agent to 
   retrieve the parameters attached to a connection. 
    








                                                                        
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         ReturnCode, 
         [CallId,] 
         [NotifiedEntity,] 
         [LocalConnectionOptions,] 
         [Mode,] 
         [RemoteConnectionDescriptor,] 
         [LocalConnectionDescriptor,] 
         [ConnectionParameters,] 
         [PackageList] 
         <-- AuditConnection(EndpointId, 
                             ConnectionId, 
                             RequestedInfo) 
    
   The EndpointId parameter specifies the endpoint that handles the 
   connection. The wildcard conventions SHALL NOT be used.  
    
   The ConnectionId parameter is the identifier of the audited 
   connection, within the context of the specified endpoint. 
    
   The (possibly empty) RequestedInfo describes the information that is 
   requested for the ConnectionId within the EndpointId specified. The 
   following connection info can be audited with this command: 
    
     CallId, NotifiedEntity, LocalConnectionOptions, Mode, 
     RemoteConnectionDescriptor, LocalConnectionDescriptor, 
     ConnectionParameters 
    
   The AuditConnection response will in turn include information about 
   each of the items auditing info was requested for: 
    
   *  CallId, the CallId for the call the connection belongs to. 
    
   *  NotifiedEntity, the current "notified entity" for the Connection. 
      Note this is the same as the "notified entity" for the endpoint 
      (included here for backwards compatibility). 
    
   *  LocalConnectionOptions, the most recent LocalConnectionOptions 
      parameters that was actually supplied for the connection 
      (omitting LocalConnectionOptions from a command thus does not 
      change this value). Note that default parameters omitted from the 
      LocalConnectionOptions will not be included.  
    
   *  Mode, the current mode of the connection. 
    
   *  RemoteConnectionDescriptor, the RemoteConnectionDescriptor that 
      was supplied to the gateway for the connection. 
    
   *  LocalConnectionDescriptor, the LocalConnectionDescriptor the 
      gateway supplied for the connection. 
    
   *  ConnectionParameters, the current values of the connection 
      parameters for the connection. 
    
                                                                        
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   If no info was requested and the EndpointId is valid, the gateway 
   simply checks that the connection exists, and if so returns a 
   positive acknowledgement. Note, that by definition, the endpoint 
   must be in-service for this to happen, as out-of-service endpoints 
   do not have any connections. 
    
   ReturnCode is a parameter returned by the gateway. It indicates the 
   outcome of the command and consists of an integer number optionally 
   followed by commentary. 
    
   PackageList is a list of supported packages that MAY be included 
   with error code 518 (unsupported package). 
    
2.3.12 RestartInProgress 
    
   The RestartInProgress command is used by the gateway to signal that 
   an endpoint, or a group of endpoints, is put in-service or out-of-
   service. 
    
         ReturnCode, 
         [NotifiedEntity,] 
         [PackageList] 
         <-- RestartInProgress(EndPointId, 
                               RestartMethod, 
                               [RestartDelay,] 
                               [ReasonCode]) 
    
   The EndPointId identifies the endpoint(s) that are put in-service or 
   out-of-service. The "all of" wildcard convention may be used to 
   apply the command to a group of endpoints managed by the same Call 
   Agent, such as for example all endpoints that are attached to a 
   specified interface, or even all endpoints that are attached to a 
   given gateway. The "any of" wildcard convention SHALL NOT be used. 
    
   The RestartMethod parameter specifies the type of restart. The 
   following values have been defined: 
    
   *  A "graceful" restart method indicates that the specified 
      endpoints will be taken out-of-service after the specified delay.  
      The established connections are not yet affected, but the Call 
      Agent SHOULD refrain from establishing new connections, and 
      SHOULD try to gracefully tear down the existing connections. 
    
   *  A "forced" restart method indicates that the specified endpoints 
      are taken abruptly out-of-service. The established connections, 
      if any, are lost. 
    
   *  A "restart" method indicates that service will be restored on the 
      endpoints after the specified "restart delay", i.e. the endpoints 
      will be in-service. The endpoints are in their clean default 
      state and there are no connections that are currently established 
      on the endpoints. 
    
                                                                        
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   *  A "disconnected" method indicates that the endpoint has become 
      disconnected and is now trying to establish connectivity (see 
      Section 4.4.7). The "restart delay" specifies the number of 
      seconds the endpoint has been disconnected. Established 
      connections are not affected. 
    
   *  A "cancel-graceful" method indicates that a gateway is canceling 
      a previously issued "graceful" restart command. The endpoints are 
      still in-service. 
    
   The list of restart methods may be extended.  
    
   The optional "restart delay" parameter is expressed as a number of 
   seconds. If the number is absent, the delay value MUST be considered 
   null (i.e. zero). In the case of the "graceful" method, a null delay 
   indicates that the Call Agent SHOULD simply wait for the natural 
   termination of the existing connections, without establishing new 
   connections. The restart delay is always considered null in the case 
   of the "forced" and "cancel-graceful" methods, and hence the 
   "restart delay" parameter MUST NOT be used with these restart 
   methods. When the gateway sends a "restart" or "graceful" 
   RestartInProgress message with a non-zero restart delay, the gateway 
   SHOULD send an updated RestartInProgress message after the "restart 
   delay" has passed.  
    
   A restart delay of null for the "restart" method indicates that 
   service has already been restored. This typically will occur after 
   gateway startup/reboot. To mitigate the effects of a gateway IP 
   address change as a result of a re-boot, the Call Agent MAY wish to 
   either flush its DNS cache for the gateway's domain name or resolve 
   the gateway's domain name by querying the DNS regardless of the TTL 
   of a current DNS resource record for the restarted gateway. 
    
   The optional reason code parameter indicates the cause of the 
   restart. 
    
   Gateways SHOULD send a "graceful" or "forced" RestartInProgress 
   message (for the relevant endpoints) as a courtesy to the Call Agent 
   when they are taken out-of-service, e.g., by being shutdown, or 
   taken out-of-service by a network management system, however the 
   Call Agent cannot rely on always receiving such a message. Gateways 
   MUST send a "restart" RestartInProgress message (for the relevant 
   endpoints) with a null delay to their Call Agent when they are back 
   in-service according to the restart procedure specified in Section 
   4.4.6 - Call Agents can rely on receiving this message. Also, 
   gateways MUST send a "disconnected" RestartInProgress message (for 
   the relevant endpoints) to their current "notified entity" according 
   to the "disconnected" procedure specified in Section 4.4.7.  
    
   The RestartInProgress message will be sent to the current "notified 
   entity" for the EndpointId in question. It is expected that a 
   default Call Agent, i.e., "notified entity", has been provisioned so 
   that after a reboot, the default Call Agent will always be the 
                                                                        
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   "notified entity" for the endpoint. Gateways SHOULD take full 
   advantage of wild-carding to minimize the number of 
   RestartInProgress messages generated when multiple endpoints in a 
   gateway restart and the endpoints are managed by the same Call 
   Agent. 
    
   ReturnCode is a parameter returned by the Call Agent. It indicates 
   the outcome of the command and consists of an integer number 
   optionally followed by commentary. 
    
   A NotifiedEntity may additionally be returned with the response to 
   the RestartInProgress from the Call Agent - this SHOULD normally 
   only be done in response to "restart" or "disconnected" (see also 
   Section 4.4.6 and 4.4.7): 
    
   *  If the response indicated success (return code 200 - transaction 
      executed), the restart in question completed successfully, and 
      the NotifiedEntity returned is the new "notified entity" for the 
      endpoint(s).  
    
   *  If the response from the Call Agent indicated an error, the 
      restart in question did not complete successfully. If a 
      NotifiedEntity parameter was included in the response returned, 
      it specifies a new "notified entity" for the endpoint(s), which 
      MUST be used when retrying the restart in question (as a new 
      transaction). This SHOULD only be done with error code 521 
      (endpoint redirected).  
       
   Note that the above behavior for returning a NotifiedEntity in the 
   response is only defined for RestartInProgress responses and SHOULD 
   NOT be done for responses to other commands. Any other behavior is 
   undefined. 
    
   PackageList is a list of supported packages that MAY be included 
   with error code 518 (unsupported package). 
 
2.4 Return Codes and Error Codes. 
    
   All MGCP commands are acknowledged. The acknowledgment carries a 
   return code, which indicates the status of the command. The return 
   code is an integer number, for which the following ranges of values 
   have been defined: 
    
   *  values between 000 and 099 indicate a response acknowledgement, 
    
   *  values between 100 and 199 indicate a provisional response, 
    
   *  values between 200 and 299 indicate a successful completion, 
    
   *  values between 400 and 499 indicate a transient error 
    
   *  values between 500 and 599 indicate a permanent error. 
    
                                                                        
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   *  values between 800 and 899 are package specific response codes. 
    
   A broad description of transient errors (4XX error codes) versus 
   permanent errors (5XX error codes) is as follows: 
    
   *  If a Call Agent receives a transient error, there is the 
      expectation of the possibility that a future similar request will 
      be honored by the endpoint. In some cases, this may require some 
      state change in the environment of the endpoint (e.g. hook state 
      as in the case of error codes 401 or 402; resource availability 
      as in the case of error code 403, or bandwidth availability as in 
      the case of error code 404). 
       
   *  Permanent errors (error codes 500 to 599) indicate one or more 
      permanent conditions either due to protocol error or 
      incompatibility between the endpoint and the Call Agent, or 
      because of some error condition over which the Call Agent has no 
      control. Examples are protocol errors, requests for endpoint 
      capabilities that do not exist, errors on interfaces associated 
      with the endpoint, missing or incorrect information in the 
      request or any number of other conditions which will simply not 
      disappear with time. 
    
   The values that have been already defined are the following: 
    
   000   Response Acknowledgement. 
    
   100   The transaction is currently being executed. An actual 
         completion message will follow later. 
    
   101   The transaction has been queued for execution. An actual 
         completion message will follow later. 
    
   200   The requested transaction was executed normally. This return 
         code can be used for a successful response to any command.  
    
   250   The connection was deleted. This return code can only be used 
         for a successful response to a DeleteConnection command. 
    
   400   The transaction could not be executed, due to some unspecified 
         transient error. 
    
   401   The phone is already off hook. 
    
   402   The phone is already on hook. 
    
   403   The transaction could not be executed, because the endpoint 
         does not have sufficient resources at this time. 
    
   404   Insufficient bandwidth at this time. 
    
   405   The transaction could not be executed, because the endpoint is 
         "restarting". 
                                                                        
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   406   Transaction time-out. The transaction did not complete in a 
         reasonable period of time and has been aborted. 
          
   407   Transaction aborted. The transaction was aborted by some 
         external action, e.g. a ModifyConnection command aborted by a 
         DeleteConnection command.  
    
   409   The transaction could not be executed because of internal 
         overload. 
    
   410   No endpoint available. A valid "any of" wildcard was used, 
         however there was no endpoint available to satisfy the 
         request.  
    
   500   The transaction could not be executed, because the endpoint is 
         unknown. 
    
   501   The transaction could not be executed, because the endpoint is 
         not ready. This includes the case where the endpoint is out-
         of-service.  
    
   502   The transaction could not be executed, because the endpoint 
         does not have sufficient resources (permanent condition). 
    
   503   "All of" wildcard too complicated. 
    
   504   Unknown or unsupported command. 
    
   505   Unsupported RemoteConnectionDescriptor. This SHOULD be used 
         when one or more mandatory parameters or values in the 
         RemoteConnectionDescriptor is not supported. 
    
   506   Unable to satisfy both LocalConnectionOptions and 
         RemoteConnectionDescriptor. This SHOULD be used when the 
         LocalConnectionOptions and RemoteConnectionDescriptor contain 
         one or more mandatory parameters or values that conflict with 
         each other and/or cannot be supported at the same time (except 
         for codec negotiation failure - see error code 534).  
    
   508   Unknown or unsupported quarantine handling. 
    
   509   Error in RemoteConnectionDescriptor. This SHOULD be used when 
         there is a syntax or semantic error in the 
         RemoteConnectionDescriptor. 
    
   510   The transaction could not be executed, because some 
         unspecified protocol error was detected. Automatic recovery 
         from such an error will be very difficult, and hence this code 
         SHOULD only be used as a last resort.  
    


                                                                        
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   511   The transaction could not be executed, because the command 
         contained an unrecognized extension. This code SHOULD be used 
         for unsupported critical parameter extensions ("X+").  
    
   512   The transaction could not be executed, because the gateway is 
         not equipped to detect one of the requested events. 
    
   513   The transaction could not be executed, because the gateway is 
         not equipped to generate one of the requested signals. 
    
   514   The transaction could not be executed, because the gateway 
         cannot send the specified announcement. 
    
   515   The transaction refers to an incorrect connection-id (may have 
         been already deleted). 
    
   516   The transaction refers to an unknown call-id, or the call-id 
         supplied is incorrect (e.g. connection-id not associated with 
         this call-id). 
    
   517   Unsupported or invalid mode. 
    
   518   Unsupported or unknown package. It is RECOMMENDED to include a 
         PackageList parameter with the list of supported packages in 
         the response, especially if the response is generated by the 
         Call Agent. 
    
   519   Endpoint does not have a digit map. 
    
   520   The transaction could not be executed, because the endpoint is 
         "restarting". In most cases this would be a transient error, 
         in which case, error code 405 SHOULD be used instead. The 
         error code is only included here for backwards compatibility.  
    
   521   Endpoint redirected to another Call Agent. The associated 
         redirection behavior is only well-defined when this response 
         is issued for a RestartInProgress command. 
    
   522   No such event or signal. The request referred to an event or 
         signal that is not defined in the relevant package (which 
         could be the default package).  
    
   523   Unknown action or illegal combination of actions. 
    
   524   Internal inconsistency in LocalConnectionOptions. 
    
   525   Unknown extension in LocalConnectionOptions. This code SHOULD 
         be used for unsupported mandatory vendor extensions ("x+"). 
    
   526   Insufficient bandwidth. In cases where this is a transient 
         error, error code 404 SHOULD be used instead. 
    
   527   Missing RemoteConnectionDescriptor. 
                                                                        
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   528   Incompatible protocol version. 
    
   529   Internal hardware failure. 
    
   530   CAS signaling protocol error. 
    
   531   Failure of a grouping of trunks (e.g. facility failure). 
    
   532   Unsupported value(s) in LocalConnectionOptions. 
    
   533   Response too large. 
    
   534   Codec negotiation failure. 
    
   535   Packetization period not supported. 
    
   536   Unknown or unsupported RestartMethod. 
    
   537   Unknown or unsupported digit map extension. 
    
   538   Event/signal parameter error (e.g. missing, erroneous, 
         unsupported, unknown, etc.). 
    
   539   Invalid or unsupported command parameter. This code SHOULD 
         only be used when the parameter is neither a package or vendor 
         extension parameter. 
    
   540   Per endpoint connection limit exceeded.  
    
   541   Invalid or unsupported LocalConnectionOptions. This code 
         SHOULD only be used when the LocalConnectionOptions is neither 
         a package nor a vendor extension LocalConnectionOptions. 
    
    
   The set of return codes may be extended in a future version of the 
   protocol. Implementations that receive an unknown or unsupported 
   return code SHOULD treat the return code as follows: 
    
   *  Unknown 0xx code treated as 000. 
    
   *  Unknown 1xx code treated as 100. 
    
   *  Unknown 2xx code treated as 200. 
    
   *  Unknown 3xx code treated as 521. 
    
   *  Unknown 4xx code treated as 400. 
    
   *  Unknown 5xx-9xx code treated as 510. 
           
    

                                                                        
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2.5 Reason Codes 
    
   Reason codes are used by the gateway when deleting a connection to 
   inform the Call Agent about the reason for deleting the connection. 
   They may also be used in a RestartInProgress command, to inform the 
   gateway of the reason for the RestartInProgress.  
    
   The reason code is an integer number, and the following values have 
   been defined: 
    
   000   Endpoint state is normal (this code is only used in response 
         to audit requests). 
    
   900   Endpoint malfunctioning. 
    
   901   Endpoint taken out-of-service. 
    
   902   Loss of lower layer connectivity (e.g., downstream sync). 
    
   903   QoS resource reservation was lost. 
    
   904   Manual intervention.  
    
   905   Facility failure (e.g. DS-0 failure). 
    
   The set of reason codes can be extended. 
    
2.6 Use of Local Connection Options and Connection Descriptors 
    
   As indicated previously, the normal sequence in setting up a bi-
   directional connection involves at least 3 steps: 
    
   1) The Call Agent asks the first gateway to "create a connection" on 
      an endpoint. The gateway allocates resources to that connection, 
      and responds to the command by providing a "session description" 
      (referred to as its LocalConnectionDescriptor). The session 
      description contains the information necessary for another party 
      to send packets towards the newly created connection. 
    
   2) The Call Agent then asks the second gateway to "create a 
      connection" on an endpoint. The command carries the "session 
      description" provided by the first gateway (now referred to as 
      the RemoteConnectionDescriptor). The gateway allocates resources 
      to that connection, and responds to the command by providing its 
      own "session description" (LocalConnectionDescriptor). 
    
   3) The Call Agent uses a "modify connection" command to provide this 
      second "session description" (now referred to as the 
      RemoteConnectionDescriptor ) to the first endpoint. Once this is 
      done, communication can proceed in both directions. 
    
   When the Call Agent issues a Create or Modify Connection command, 
   there are thus three parameters that determine the media supported 
                                                                        
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   by that connection: 
    
   *  LocalConnectionOptions:      Supplied by the Call Agent to 
      control the media parameters used by the gateway for the 
      connection. When supplied, the gateway MUST conform to these 
      media parameters until either the connection is deleted, or a 
      ModifyConnection command with new media parameters 
      (LocalConnectionOptions or RemoteConnectionDescriptor) is 
      received. 
       
   *  RemoteConnectionDescriptor:  Supplied by the Call Agent to convey 
      the media parameters supported by the other side of the 
      connection. When supplied, the gateway MUST conform to these 
      media parameters until either the connection is deleted, or a 
      ModifyConnection command with new media parameters 
      (LocalConnectionOptions or RemoteConnectionDescriptor) is 
      received. 
       
   *  LocalConnectionDescriptor:   Supplied by the gateway to the Call 
      Agent to convey the media parameters it supports for the 
      connection. When supplied, the gateway MUST honor the media 
      parameters until either the connection is deleted, or the gateway 
      issues a new LocalConnectionDescriptor for that connection. 
    
   In determining which codec(s) to provide in the 
   LocalConnectionDescriptor, there are three lists of codecs that a 
   gateway needs to consider: 
    
   *  A list of codecs allowed by the LocalConnectionOptions in the 
      current command (either explicitly by encoding method or 
      implicitly by bandwidth and/or packetization period). 
    
   *  A list of codecs in the RemoteConnectionDescriptor in the current 
      command. 
    
   *  An internal list of codecs that the gateway can support for the 
      connection. A gateway MAY support one or more codecs for a given 
      connection. 
    
   Codec selection (incl. all relevant media parameters) can then be 
   described by the following steps: 
    
   1. An approved list of codecs is formed by taking the intersection 
      of the internal list of codecs and codecs allowed by the 
      LocalConnectionOptions. If LocalConnectionOptions were not 
      provided in the current command, the approved list of codecs thus 
      contains the internal list of codecs. 
    
   2. If the approved list of codecs is empty, a codec negotiation 
      failure has occurred and an error response is generated (error 
      code 534 - codec negotiation failure, is RECOMMENDED) 
    

                                                                        
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   3. Otherwise, a negotiated list of codecs is formed by taking the 
      intersection of the approved list of codecs and codecs allowed by 
      the RemoteConnectionDescriptor. If a RemoteConnectionDescriptor 
      was not provided in the current command, the negotiated list of 
      codecs thus contains the approved list of codecs.  
    
   4. If the negotiated list of codecs is empty, a codec negotiation 
      failure has occurred and an error response is generated (error 
      code 534 - codec negotiation failure, is RECOMMENDED) 
    
   5. Otherwise, codec negotiation has succeeded, and the negotiated 
      list of codecs is returned in the LocalConnectionDescriptor.  
    
   Note that both LocalConnectionOptions and the 
   RemoteConnectionDescriptor can contain a list of codecs ordered by 
   preference. When both are supplied in the current command, the 
   gateway MUST adhere to the preferences provided in the 
   LocalConnectionOptions.  
    
2.7 Resource Reservations 
    
   The gateways can be instructed to perform a reservation, for example 
   using RSVP, on a given connection. When a reservation is needed, the 
   call agent will specify the reservation profile to be used, which is 
   either "controlled load" or "guaranteed service". The absence of 
   reservation can be indicated by asking for the "best effort" 
   service, which is the default value of this parameter in a 
   CreateConnection command. For a ModifyConnection command, the 
   default is simply to retain the current value. When reservation has 
   been asked on a connection, the gateway will: 
    
   *  start emitting RSVP "PATH" messages if the connection is in 
      "send-only", "send-receive", "conference", "network loop back" or 
      "network continuity test" mode (if a suitable remote connection 
      descriptor has been received,) 
    
   *  start emitting RSVP "RESV" messages as soon as it receives "PATH" 
      messages if the connection is in "receive-only", "send-receive", 
      "conference", "network loop back" or "network continuity test" 
      mode. 
    
   The RSVP filters will be deduced from the characteristics of the 
   connection. The RSVP resource profiles will be deduced from the 
   connection's codecs, bandwidth and packetization period. 
    








                                                                        
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3. Media Gateway Control Protocol 
    
   The Media Gateway Control Protocol (MGCP) implements the media 
   gateway control interface as a set of transactions. The transactions 
   are composed of a command and a mandatory response. There are nine 
   commands: 
    
   *  EndpointConfiguration 
    
   *  CreateConnection 
    
   *  ModifyConnection 
    
   *  DeleteConnection 
    
   *  NotificationRequest 
    
   *  Notify 
    
   *  AuditEndpoint 
    
   *  AuditConnection 
    
   *  RestartInProgress 
    
   The first five commands are sent by the Call Agent to a gateway. The 
   Notify command is sent by the gateway to the Call Agent. The gateway 
   may also send a DeleteConnection as defined in Section 2.3.8. The 
   Call Agent may send either of the Audit commands to the gateway, and 
   the gateway may send a RestartInProgress command to the Call Agent. 
    
3.1 General Description 
    
   All commands are composed of a Command header, optionally followed 
   by a session description. 
    
   All responses are composed of a Response header, optionally followed 
   by session description information. 
    
   Headers and session descriptions are encoded as a set of text lines, 
   separated by a carriage return and line feed character (or, 
   optionally, a single line-feed character). The session descriptions 
   are preceded by an empty line. 
    
   MGCP uses a transaction identifier to correlate commands and 
   responses. The transaction identifier is encoded as a component of 
   the command header and repeated as a component of the response 
   header (see sections 3.2.1.2 and 3.3). 
    
   Note that an ABNF grammar for MGCP is provided in Appendix A. 
   Commands and responses SHALL be encoded in accordance with the 
   grammar, which, per RFC 2234, is case-insensitive except for the SDP 
   part. Similarly, implementations SHALL be capable of decoding 
                                                                        
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   commands and responses that follow the grammar. Additionally, it is 
   RECOMMENDED that implementations tolerate additional linear white 
   space.  
    
   Some productions allow for use of quoted strings, which can be 
   necessary to avoid syntax problems. Where the quoted string form is 
   used, the contents will be UTF-8 encoded [20], and the actual value 
   provided is the unquoted string (UTF-8 encoded). Where both a quoted 
   and unquoted string form is allowed, either form can be used 
   provided it does not otherwise violate the grammar.  
    
   In the following, we provide additional detail on the format of MGCP 
   commands and responses.  
    
3.2 Command Header 
    
   The command header is composed of: 
    
   *  A command line, identifying the requested action or verb, the 
      transaction identifier, the endpoint towards which the action is 
      requested, and the MGCP protocol version, 
    
   *  A set of zero or more parameter lines, composed of a parameter 
      name followed by a parameter value. 
    
   Unless otherwise noted or dictated by other referenced standards 
   (e.g., SDP), each component in the command header is case 
   insensitive. This goes for verbs as well as parameters and values, 
   and hence all comparisons MUST treat upper and lower case as well as 
   combinations of these as being equal. 
    
3.2.1 Command Line 
    
   The command line is composed of: 
    
   *  The name of the requested verb, 
    
   *  The identification of the transaction, 
    
   *  The name of the endpoint(s) that are to execute the command (in 
      notifications or restarts, the name of the endpoint(s) that is 
      issuing the command), 
    
   *  The protocol version. 
    
   These four items are encoded as strings of printable ASCII 
   characters, separated by white spaces, i.e. the ASCII space (0x20) 
   or tabulation (0x09) characters. It is RECOMMENDED to use exactly 
   one ASCII space separator. However, MGCP entities MUST be able to 
   parse messages with additional white space characters. 
    


                                                                        
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3.2.1.1 Coding of the Requested Verb 
    
   The verbs that can be requested are encoded as four letter upper or 
   lower case ASCII codes (comparisons SHALL be case insensitive) as 
   defined in the following table: 
    
                  ----------------------------- 
                 |       Verb           | Code | 
                 |----------------------|------| 
                 | EndpointConfiguration| EPCF | 
                 | CreateConnection     | CRCX | 
                 | ModifyConnection     | MDCX | 
                 | DeleteConnection     | DLCX | 
                 | NotificationRequest  | RQNT | 
                 | Notify               | NTFY | 
                 | AuditEndpoint        | AUEP | 
                 | AuditConnection      | AUCX | 
                 | RestartInProgress    | RSIP | 
                  ----------------------------- 
    
   The transaction identifier is encoded as a string of up to 9 decimal 
   digits. In the command line, it immediately follows the coding of 
   the verb. 
    
   New verbs may be defined in further versions of the protocol. It may 
   be necessary, for experimentation purposes, to use new verbs before 
   they are sanctioned in a published version of this protocol. 
   Experimental verbs MUST be identified by a four letter code starting 
   with the letter X, such as for example XPER. 
    
3.2.1.2 Transaction Identifiers 
    
   MGCP uses a transaction identifier to correlate commands and 
   responses. A gateway supports two separate transaction identifier 
   name spaces: 
    
   * a transaction identifier name space for sending transactions, and 
    
   * a transaction identifier name space for receiving transactions. 
    
   At a minimum, transaction identifiers for commands sent to a given 
   gateway MUST be unique for the maximum lifetime of the transactions 
   within the collection of Call Agents that control that gateway. 
   Thus, regardless of the sending Call Agent, gateways can always 
   detect duplicate transactions by simply examining the transaction 
   identifier. The coordination of these transaction identifiers 
   between Call Agents is outside the scope of this specification 
   though. 
    
   Transaction identifiers for all commands sent from a given gateway 
   MUST be unique for the maximum lifetime of the transactions 
   regardless of which Call Agent the command is sent to. Thus, a Call 
   Agent can always detect a duplicate transaction from a gateway by 
                                                                        
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   the combination of the domain-name of the endpoint and the 
   transaction identifier. 
    
   The transaction identifier is encoded as a string of up to nine 
   decimal digits. In the command lines, it immediately follows the 
   coding of the verb.  
    
   Transaction identifiers have values between 1 and 999999999. 
   Transaction identifiers SHOULD NOT use any leading zeroes, although 
   equality is based on numerical value, i.e. leading zeroes are 
   ignored. An MGCP entity MUST NOT reuse a transaction identifier more 
   quickly than three minutes after completion of the previous command 
   in which the identifier was used. 
    
3.2.1.3 Coding of the Endpoint Identifiers and Entity Names 
    
   The endpoint identifiers and entity names are encoded as case 
   insensitive e-mail addresses, as defined in RFC 821, although with 
   some syntactic restrictions on the local part of the name. 
   Furthermore, both the local endpoint name part and the domain name 
   part can each be up to 255 characters. In these addresses, the 
   domain name identifies the system where the endpoint is attached, 
   while the left side identifies a specific endpoint or entity on that 
   system. 
    
   Examples of such addresses are: 
    
    ------------------------------------------------------------------ 
   | hrd4/56@gw23.example.net     |  Circuit number 56 in             | 
   |                              |  interface "hrd4" of the Gateway  | 
   |                              |  23 of the "Example" network      | 
   | Call-agent@ca.example.net    |  Call Agent for the               | 
   |                              |  "example" network                | 
   | Busy-signal@ann12.example.net|  The "busy signal" virtual        | 
   |                              |  endpoint in the announcement     | 
   |                              |  server number 12.                | 
    ------------------------------------------------------------------ 
    
   The name of a notified entity is expressed with the same syntax, 
   with the possible addition of a port number as in: 
    
      Call-agent@ca.example.net:5234 
    
   In case the port number is omitted from the notified entity, the 
   default MGCP Call Agent port (2727) MUST be used. 
    
3.2.1.4 Coding of the Protocol Version 
    
   The protocol version is coded as the keyword MGCP followed by a 
   white space and the version number, and optionally followed by a 
   profile name. The version number is composed of a major version, 
   coded by a decimal number, a dot, and a minor version number, coded 

                                                                        
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   as a decimal number. The version described in this document is 
   version 1.0. 
    
   The profile name, if present, is represented by white-space 
   separated strings of visible (printable) characters extending to the 
   end of the line. Profile names may be defined for user communities 
   who want to apply restrictions or other profiling to MGCP. 
    
   In the initial messages, the version will be coded as: 
    
      MGCP 1.0 
    
   An entity that receives a command with a protocol version it does 
   not support, MUST respond with an error (error code 528 - 
   incompatible protocol version, is RECOMMENDED). Note that this 
   applies to unsupported profiles as well. 
    
3.2.2 Parameter Lines 
    
   Parameter lines are composed of a parameter name, which in most 
   cases is composed of one or two characters, followed by a colon, 
   optional white space(s) and the parameter value. The parameters that 
   can be present in commands are defined in the following table: 
    
    ------------------------------------------------------------------ 
   |Parameter name        | Code |  Parameter value                   | 
   |----------------------|------|------------------------------------| 
   |BearerInformation     |   B  |  See description (3.2.2.1).        | 
   |CallId                |   C  |  See description (3.2.2.2).        | 
   |Capabilities          |   A  |  See description (3.2.2.3).        | 
   |ConnectionId          |   I  |  See description (3.2.2.5).        | 
   |ConnectionMode        |   M  |  See description (3.2.2.6).        | 
   |ConnectionParameters  |   P  |  See description (3.2.2.7).        | 
   |DetectEvents          |   T  |  See description (3.2.2.8).        | 
   |DigitMap              |   D  |  A text encoding of a digit map.   | 
   |EventStates           |   ES |  See description (3.2.2.9).        | 
   |LocalConnectionOptions|   L  |  See description (3.2.2.10).       | 
   |MaxMGCPDatagram       |   MD |  See description (3.2.2.11).       |        
   |NotifiedEntity        |   N  |  An identifier, in RFC 821 format, | 
   |                      |      |  composed of an arbitrary string   | 
   |                      |      |  and of the domain name of the     | 
   |                      |      |  requesting entity, possibly com-  | 
   |                      |      |  pleted by a port number, as in:   | 
   |                      |      |    Call-agent@ca.example.net:5234  | 
   |                      |      |  See also Section 3.2.1.3.         | 
   |ObservedEvents        |   O  |  See description (3.2.2.12).       | 
   |PackageList           |   PL |  See description (3.2.2.13).       | 
   |QuarantineHandling    |   Q  |  See description (3.2.2.14).       | 
   |ReasonCode            |   E  |  A string with a 3 digit integer   | 
   |                      |      |  optionally followed by a set of   | 
   |                      |      |  arbitrary characters (3.2.2.15).  | 
   |RequestedEvents       |   R  |  See description (3.2.2.16).       | 
   |RequestedInfo         |   F  |  See description (3.2.2.17).       | 
                                                                        
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   |RequestIdentifier     |   X  |  See description (3.2.2.18).       | 
   |ResponseAck           |   K  |  See description (3.2.2.19).       | 
   |RestartDelay          |   RD |  A number of seconds, encoded as   | 
   |                      |      |  a decimal number.                 | 
   |RestartMethod         |   RM |  See description (3.2.2.20).       | 
   |SecondConnectionId    |   I2 |  Connection Id.                    | 
   |SecondEndpointId      |   Z2 |  Endpoint Id.                      | 
   |SignalRequests        |   S  |  See description (3.2.2.21).       | 
   |SpecificEndPointId    |   Z  |  An identifier, in RFC 821 format, | 
   |                      |      |  composed of an arbitrary string,  | 
   |                      |      |  followed by an "@" followed by    | 
   |                      |      |  the domain name of the gateway to | 
   |                      |      |  which this endpoint is attached.  | 
   |                      |      |  See also Section 3.2.1.3.         | 
   |----------------------|------|------------------------------------| 
   |RemoteConnection-     |   RC |  Session Description.              | 
   |         Descriptor   |      |                                    | 
   |LocalConnection-      |   LC |  Session Description.              | 
   |         Descriptor   |      |                                    | 
    ------------------------------------------------------------------ 
    
   The parameters are not necessarily present in all commands. The 
   following table provides the association between parameters and 
   commands. The letter M stands for mandatory, O for optional and F 
   for forbidden. Unless otherwise specified, a parameter MUST NOT be 
   present more than once.  
    
    ------------------------------------------------------------------ 
   | Parameter name      | EP | CR | MD | DL | RQ | NT | AU | AU | RS | 
   |                     | CF | CX | CX | CX | NT | FY | EP | CX | IP | 
   |---------------------|----|----|----|----|----|----|----|----|----| 
   | BearerInformation   |  O*|  O |  O |  O |  O |  F |  F |  F |  F | 
   | CallId              |  F |  M |  M |  O |  F |  F |  F |  F |  F | 
   | Capabilities        |  F |  F |  F |  F |  F |  F |  F |  F |  F | 
   | ConnectionId        |  F |  F |  M |  O |  F |  F |  F |  M |  F | 
   | ConnectionMode      |  F |  M |  O |  F |  F |  F |  F |  F |  F | 
   | Connection-         |  F |  F |  F |  O*|  F |  F |  F |  F |  F | 
   |   Parameters        |    |    |    |    |    |    |    |    |    | 
   | DetectEvents        |  F |  O |  O |  O |  O |  F |  F |  F |  F | 
   | DigitMap            |  F |  O |  O |  O |  O |  F |  F |  F |  F | 
   | EventStates         |  F |  F |  F |  F |  F |  F |  F |  F |  F | 
   | LocalConnection-    |  F |  O |  O |  F |  F |  F |  F |  F |  F | 
   |            Options  |    |    |    |    |    |    |    |    |    | 
   | MaxMGCPDatagram     |  F |  F |  F |  F |  F |  F |  F |  F |  F | 
   | NotifiedEntity      |  F |  O |  O |  O |  O |  O |  F |  F |  F | 
   | ObservedEvents      |  F |  F |  F |  F |  F |  M |  F |  F |  F | 
   | PackageList         |  F |  F |  F |  F |  F |  F |  F |  F |  F | 
   | QuarantineHandling  |  F |  O |  O |  O |  O |  F |  F |  F |  F | 
   | ReasonCode          |  F |  F |  F |  O |  F |  F |  F |  F |  O | 
   | RequestedEvents     |  F |  O |  O |  O |  O*|  F |  F |  F |  F | 
   | RequestIdentifier   |  F |  O*|  O*|  O*|  M |  M |  F |  F |  F | 
   | RequestedInfo       |  F |  F |  F |  F |  F |  F |  O |  M |  F | 

                                                                        
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   | ResponseAck         |  O |  O |  O |  O |  O |  O |  O |  O |  O | 
   | RestartDelay        |  F |  F |  F |  F |  F |  F |  F |  F |  O | 
   | RestartMethod       |  F |  F |  F |  F |  F |  F |  F |  F |  M | 
   | SecondConnectionId  |  F |  F |  F |  F |  F |  F |  F |  F |  F | 
   | SecondEndpointId    |  F |  O |  F |  F |  F |  F |  F |  F |  F | 
   | SignalRequests      |  F |  O |  O |  O |  O*|  F |  F |  F |  F | 
   | SpecificEndpointId  |  F |  F |  F |  F |  F |  F |  F |  F |  F | 
   |---------------------|----|----|----|----|----|----|----|----|----| 
   | RemoteConnection-   |  F |  O |  O |  F |  F |  F |  F |  F |  F | 
   |          Descriptor |    |    |    |    |    |    |    |    |    | 
   | LocalConnection-    |  F |  F |  F |  F |  F |  F |  F |  F |  F | 
   |          Descriptor |    |    |    |    |    |    |    |    |    | 
    ------------------------------------------------------------------ 
    
   Notes (*): 
    
   * The BearerInformation parameter is only conditionally optional as 
     explained in Section 2.3.2. 
      
   * The RequestIdentifier parameter is optional in connection 
     creation, modification and deletion commands, however it becomes 
     REQUIRED if the command contains an encapsulated notification 
     request. 
    
   * The RequestedEvents and SignalRequests parameters are optional in 
     the NotificationRequest. If these parameters are omitted the 
     corresponding lists will be considered empty. 
      
   * The ConnectionParameters parameter is only valid in a 
     DeleteConnection request sent by the gateway.  
    
   The set of parameters can be extended in two different ways: 
    
   *  Package Extension Parameters (preferred) 
    
   *  Vendor Extension Parameters  
    
   Package Extension Parameters are defined in packages which provides 
   the following benefits: 
    
   *  a registration mechanism (IANA) for the package name. 
    
   *  a separate name space for the parameters.  
    
   *  a convenient grouping of the extensions. 
    
   *  a simple way to determine support for them through auditing. 
    
   The package extension mechanism is the preferred extension method.  
    
   Vendor extension parameters can be used if implementers need to 
   experiment with new parameters, for example when developing a new 
   application of MGCP. Vendor extension parameters MUST be identified 
                                                                        
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   by names that start with the string "X-" or "X+", such as for 
   example: 
    
      X-Flower: Daisy 
    
   Parameter names that start with "X+" are critical parameter 
   extensions. An MGCP entity that receives a critical parameter 
   extension that it cannot understand MUST refuse to execute the 
   command. It SHOULD respond with error code 511 (unrecognized 
   extension). 
    
   Parameter names that start with "X-" are non-critical parameter 
   extensions. An MGCP entity that receives a non-critical parameter 
   extension that it cannot understand MUST simply ignore that 
   parameter. 
    
   Note that vendor extension parameters use an unmanaged name space, 
   which implies a potential for name clashing. Vendors are 
   consequently encouraged to include some vendor specific string, e.g. 
   vendor name, in their vendor extensions.  
    
3.2.2.1 BearerInformation 
    
   The values of the bearer information are encoded as a comma 
   separated list of attributes, which are represented by an attribute 
   name, and possibly followed by a colon and an attribute value. 
    
   The only attribute that is defined is the "encoding" (code "e") 
   attribute, which MUST have one of the values "A" (A-law) or "mu" 
   (mu-law). 
    
   An example of bearer information encoding is: 
    
      B: e:mu 
    
   The set of bearer information attributes may be extended through 
   packages. 
    
3.2.2.2 CallId 
    
   The Call Identifier is encoded as a hexadecimal string, at most 32 
   characters in length. Call Identifiers are compared as strings 
   rather than numerical values. 
    
3.2.2.3 Capabilities 
    
   Capabilities inform the Call Agent about endpoints' capabilities 
   when audited. The encoding of capabilities is based on the Local 
   Connection Options encoding for the parameters that are common to 
   both, although a different parameter line code is used ("A"). In 
   addition, capabilities can also contain a list of supported 
   packages, and a list of supported modes.  
    
                                                                        
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   The parameters used are: 
    
   A list of supported codecs.  
      The following parameters will apply to all codecs specified in 
      this list. If there is a need to specify that some parameters, 
      such as e.g. silence suppression, are only compatible with some 
      codecs, then the gateway will return several Capability 
      parameters; one for each set of codecs. 
    
   Packetization Period: 
      A range may be specified. 
    
   Bandwidth: 
      A range corresponding to the range for packetization periods may 
      be specified (assuming no silence suppression). If absent, the 
      values will be deduced from the codec type. 
    
   Echo Cancellation: 
      "on" if echo cancellation is supported, "off" otherwise. The 
      default is support. 
    
   Silence Suppression: 
      "on" if silence suppression is supported for this codec, "off" 
      otherwise. The default is support. 
    
   Gain Control: 
      "0" if gain control is not supported, all other values indicate 
      support for gain control. The default is support. 
    
   Type of Service: 
      The value "0" indicates no support for type of service, all other 
      values indicate support for type of service. The default is 
      support. 
    
   Resource Reservation Service: 
      The parameter indicates the reservation services that are 
      supported, in addition to best effort. The value "g" is encoded 
      when the gateway supports both the guaranteed and the controlled 
      load service, "cl" when only the controlled load service is 
      supported. The default is "best effort". 
    
   Encryption Key: 
      Encoding any value indicates support for encryption. Default is 
      no support which is implied by omitting the parameter. 
    
   Type of network: 
      The keyword "nt", followed by a colon and a semicolon separated 
      list of supported network types. This parameter is optional. 
    
   Packages: 
      The packages supported by the endpoint encoded as the keyword 
      "v", followed by a colon and a character string. If a list of 
      values is specified, these values will be separated by a 
                                                                        
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      semicolon. The first value specified will be the default package 
      for the endpoint. 
    
   Modes: 
      The modes supported by this endpoint encoded as the keyword "m", 
      followed by a colon and a semicolon-separated list of supported 
      connection modes for this endpoint. 
    
   Lack of support for a capability can also be indicated by excluding 
   the parameter from the capability set.  
    
   An example capability is: 
      
     A: a:PCMU;G728, p:10-100, e:on, s:off, t:1, v:L,                  
                              m:sendonly;recvonly;sendrecv;inactive 
    
   The carriage return above is included for formatting reasons only 
   and is not permissible in a real implementation. 
    
   If multiple capabilities are to be returned, each will be returned 
   as a separate capability line. 
    
   Since Local Connection Options can be extended, the list of 
   capability parameters can also be extended. Individual extensions 
   may define how they are reported as capabilities. If no such 
   definition is provided, the following defaults apply: 
    
   *  Package Extension attributes: The individual attributes are not 
      reported. Instead, the name of the package is simply reported in 
      the list of supported packages. 
    
   *  Vendor Extension attributes: The name of the attribute is 
      reported without any value. 
    
   *  Other Extension attributes: The name of the attribute is reported 
      without any value. 
 
3.2.2.4 Coding of Event Names 
    
   Event names are composed of an optional package name, separated by a 
   slash (/) from the name of the actual event (see Section 2.1.7). The 
   wildcard character star ("*") can be use to refer to all packages. 
   The event name can optionally be followed by an at sign (@) and the 
   identifier of a connection (possibly using a wildcard) on which the 
   event should be observed. Event names are used in the 
   RequestedEvents, SignalRequests, ObservedEvents, DetectEvents, and 
   EventStates parameters.  
    
   Events and signals may be qualified by parameters defined for the 
   event/signal. Such parameters may be enclosed in double-quotes (in 
   fact, some parameters MUST be enclosed in double-quotes due to 
   syntactic restrictions) in which case they are UTF-8 encoded [20]. 

                                                                        
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   The parameter name "!" (exclamation point) is reserved for future 
   use for both events and signals.  
    
   Each signal has one of the following signal-types associated with 
   it: On/Off (OO), Time-out (TO), or Brief (BR). (These signal types 
   are specified in the package definitions, and are not present in the 
   messages.) On/Off signals can be parameterized with a "+" to turn 
   the signal on, or a "-" to turn the signal off. If an on/off signal 
   is not parameterized, the signal is turned on. Both of the following 
   will turn the vmwi signal (from the line package "L") on: 
    
      L/vmwi(+) 
      L/vmwi 
    
   In addition to "!", "+" and "-", the signal parameter "to" is 
   reserved as well. It can be used with Time-Out signals to override 
   the default time-out value for the current request. A decimal value 
   in milliseconds will be supplied. The individual signal and/or 
   package definition SHOULD indicate if this parameter is supported 
   for one or more TO signals in the package. If not indicated, TO 
   signals in package version zero are assumed to not support it, 
   whereas TO signals in package versions one or higher are assumed to 
   support it. By default, a supplied time-out value MAY be rounded to 
   the nearest non-zero value divisible by 1000, i.e. whole second. The 
   individual signal and/or package definition may define other 
   rounding rules. All new package and TO signal definitions are 
   strongly encouraged to support the "to" signal parameter.  
    
   The following example illustrates how the "to" parameter can be used 
   to apply a signal for 6 seconds:  
    
      L/rg(to=6000) 
      L/rg(to(6000)) 
    
   The following are examples of event names: 
    
      ----------------------------------------------------------- 
     | L/hu        |   on-hook transition, in the line package   | 
     | F/0         |   digit 0 in the MF package                 | 
     | hf          |   Hook-flash, assuming that the line package| 
     |             |   is the default package for the endpoint.  | 
     | G/rt@0A3F58 |   Ring back signal on connection "0A3F58"   | 
      ----------------------------------------------------------- 
    
   In addition, the range and wildcard notation of events can be used, 
   instead of individual names, in the RequestedEvents and DetectEvents 
   parameters. The event code "all" is reserved and refers to all 
   events or signals in a package. The star sign ("*") can be used to 
   denote "all connections", and the dollar sign ("$") can be used to 
   denote the "current" connection (see Section 2.1.7 for details). 
    
   The following are examples of such notations: 
    
                                                                        
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      --------------------------------------------------------- 
     | M/[0-9]   |   Digits 0 to 9 in the MF package.          | 
     | hf        |   Hook-flash, assuming that the line package| 
     |           |   is a default package for the endpoint.    | 
     | [0-9*#A-D]|   All digits and letters in the DTMF        | 
     |           |   packages (default for endpoint).          | 
     | T/all     |   All events in the trunk package.          | 
     | R/qa@*    |   The quality alert event on all            | 
     |           |   connections.                              | 
     | G/rt@$    |   Ringback on current connection.           | 
      --------------------------------------------------------- 
 
3.2.2.5 ConnectionId 
    
   The Connection Identifier is encoded as a hexadecimal string, at 
   most 32 characters in length. Connection Identifiers are compared as 
   strings rather than numerical values. 
 
3.2.2.6 ConnectionMode 
    
   The connection mode describes the mode of operation of the 
   connection. The possible values are: 
    
      -------------------------------------------------------- 
     |    Mode     |               Meaning                    | 
     |-------------|------------------------------------------| 
     | M: sendonly |  The gateway should only send packets    | 
     | M: recvonly |  The gateway should only receive packets | 
     | M: sendrecv |  The gateway should send                 | 
     |             |  and receive packets                     | 
     | M: confrnce |  The gateway should place                | 
     |             |  the connection in conference mode       | 
     | M: inactive |  The gateway should neither              | 
     |             |  send nor receive packets                | 
     | M: loopback |  The gateway should place                | 
     |             |  the circuit in loopback mode.           | 
     | M: conttest |  The gateway should place                | 
     |             |  the circuit in test mode.               | 
     | M: netwloop |  The gateway should place                | 
     |             |  the connection in network loopback mode.| 
     | M: netwtest |  The gateway should place the connection | 
     |             |  in network continuity test mode.        | 
      -------------------------------------------------------- 
    
   Note that irrespective of the connection mode, signals applied to 
   the connection will still result in packets being sent (see Section 
   2.3.1).  
    
   The set of connection modes can be extended through packages. 
 



                                                                        
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3.2.2.7 ConnectionParameters 
    
   Connection parameters are encoded as a string of type and value 
   pairs, where the type is either a two-letter identifier of the 
   parameter or an extension type, and the value a decimal integer. 
   Types are separated from value by an '=' sign. Parameters are 
   separated from each other by a comma. Connection parameter values 
   can contain up to nine digits. If the maximum value is reached, the 
   counter is no longer updated, i.e. it doesn't wrap or overflow. 
    
   The connection parameter types are specified in the following table: 
    
    ----------------------------------------------------------------- 
   | Connection parameter| Code |  Connection parameter              | 
   | name                |      |  value                             | 
   |---------------------|------|------------------------------------| 
   | Packets sent        |  PS  |  The number of packets that        | 
   |                     |      |  were sent on the connection.      | 
   | Octets sent         |  OS  |  The number of octets that         | 
   |                     |      |  were sent on the connection.      | 
   | Packets received    |  PR  |  The number of packets that        | 
   |                     |      |  were received on the connection.  | 
   | Octets received     |  OR  |  The number of octets that         | 
   |                     |      |  were received on the connection.  | 
   | Packets lost        |  PL  |  The number of packets that        | 
   |                     |      |  were lost on the connection       | 
   |                     |      |  as deduced from gaps in the       | 
   |                     |      |  RTP sequence number.              | 
   | Jitter              |  JI  |  The average inter-packet arrival  | 
   |                     |      |  jitter, in milliseconds,          | 
   |                     |      |  expressed as an integer number.   | 
   | Latency             |  LA  |  Average latency, in milliseconds, | 
   |                     |      |  expressed as an integer number.   | 
    ----------------------------------------------------------------- 
    
   The set of connection parameters can be extended in two different 
   ways: 
    
   *  Package Extension Parameters (preferred) 
    
   *  Vendor Extension Parameters  
    
   Package Extension Connection Parameters are defined in packages 
   which provides the following benefits: 
    
   *  A registration mechanism (IANA) for the package name. 
    
   *  A separate name space for the parameters.  
    
   *  A convenient grouping of the extensions. 
    
   *  A simple way to determine support for them through auditing. 
    
                                                                        
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   The package extension mechanism is the preferred extension method.  
    
   Vendor extension parameters names are composed of the string "X-" 
   followed by a two or more letters extension parameter name.  
    
   Call agents that receive unrecognized package or vendor connection 
   parameter extensions SHALL silently ignore these parameters. 
    
   An example of connection parameter encoding is: 
    
      P: PS=1245, OS=62345, PR=0, OR=0, PL=0, JI=0, LA=48 
    
3.2.2.8 DetectEvents 
    
   The DetectEvents parameter is encoded as a comma separated list of 
   events (see Section 3.2.2.4), such as for example: 
    
      T: L/hu,L/hd,L/hf,D/[0-9#*] 
    
   It should be noted, that no actions can be associated with the 
   events, however event parameters may be provided. 
    
3.2.2.9 EventStates 
    
   The EventStates parameter is encoded as a comma separated list of 
   events (see Section 3.2.2.4), such as for example: 
    
      ES: L/hu 
    
   It should be noted, that no actions can be associated with the 
   events, however event parameters may be provided. 
 
3.2.2.10 LocalConnectionOptions 
    
   The local connection options describe the operational parameters 
   that the Call Agent provides to the gateway in connection handling 
   commands. These include: 
    
   *  The allowed codec(s), encoded as the keyword "a", followed by a 
      colon and a character string. If the Call Agent specifies a list 
      of values, these values will be separated by a semicolon. For 
      RTP, audio codecs SHALL be specified by using encoding names 
      defined in the RTP AV Profile [4] or its replacement, or by 
      encoding names registered with the IANA. Non-audio media 
      registered as a MIME type MUST use the "<MIME type>/<MIME 
      subtype>" form, as in "image/t38".  
    
   *  The packetization period in milliseconds, encoded as the keyword 
      "p", followed by a colon and a decimal number. If the Call Agent 
      specifies a range of values, the range will be specified as two 
      decimal numbers separated by a hyphen (as specified for the 
      "ptime" parameter for SDP). 
    
                                                                        
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   *  The bandwidth in kilobits per second (1000 bits per second), 
      encoded as the keyword "b", followed by a colon and a decimal 
      number. If the Call Agent specifies a range of values, the range 
      will be specified as two decimal numbers separated by a hyphen. 
    
   *  The type of service parameter, encoded as the keyword "t", 
      followed by a colon and the value encoded as two hexadecimal 
      digits. When the connection is transmitted over an IP network, 
      the parameters encode the 8-bit type of service value parameter 
      of the IP header (a.k.a. DiffServ field). The left-most "bit" in 
      the parameter corresponds to the least significant bit in the IP 
      header.  
    
   *  The echo cancellation parameter, encoded as the keyword "e", 
      followed by a colon and the value "on" or "off". 
    
   *  The gain control parameter, encoded as the keyword "gc", followed 
      by a colon and a value which can be either the keyword "auto" or 
      a decimal number (positive or negative) representing the number 
      of decibels of gain. 
    
   *  The silence suppression parameter, encoded as the keyword "s", 
      followed by a colon and the value "on" or "off". 
    
   *  The resource reservation parameter, encoded as the keyword "r", 
      followed by a colon and the value "g" (guaranteed service), "cl" 
      (controlled load) or "be" (best effort). 
    
   *  The encryption key, encoded as the keyword "k" followed by a 
      colon and a key specification, as defined for the parameter "K" 
      in SDP (RFC 2327). 
    
   *  The type of network, encoded as the keyword "nt" followed by a 
      colon and the type of network encoded as the keyword "IN" 
      (internet), "ATM", "LOCAL" (for a local connection), or possibly 
      another type of network registered with the IANA as per SDP (RFC 
      2327). 
    
   *  The resource reservation parameter, encoded as the keyword "r", 
      followed by a colon and the value "g" (guaranteed service), "cl" 
      (controlled load) or "be" (best effort). 
    
   The encoding of the first three attributes, when they are present, 
   will be compatible with the SDP and RTP profiles. Note that each of 
   the attributes is optional. When several attributes are present, 
   they are separated by a comma. 
    
   Examples of local connection options are: 
    
      L: p:10, a:PCMU 
      L: p:10, a:G726-32 
      L: p:10-20, b:64 
      L: b:32-64, e:off 
                                                                        
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   The set of Local Connection Options attributes can be extended in 
   three different ways: 
    
   *  Package Extension attributes (preferred) 
       
   *  Vendor Extension attributes 
       
   *  Other Extension attributes 
    
   Package Extension Local Connection Options attributes are defined in 
   packages which provides the following benefits: 
    
   *  A registration mechanism (IANA) for the package name. 
    
   *  A separate name space for the attributes.  
    
   *  A convenient grouping of the extensions. 
    
   *  A simple way to determine support for them through auditing. 
    
   The package extension mechanism is the preferred extension method. 
    
   Vendor extension attributes are composed of an attribute name, and 
   possibly followed by a colon and an attribute value. The attribute 
   name MUST start with the two characters "x+", for a mandatory 
   extension, or "x-", for a non-mandatory extension. If a gateway 
   receives a mandatory extension attribute that it does not recognize, 
   it MUST reject the command (error code 525 - unknown extension in 
   LocalConnectionOptions, is RECOMMENDED). 
    
   Note that vendor extension attributes use an unmanaged name space, 
   which implies a potential for name clashing. Vendors are 
   consequently encouraged to include some vendor specific string, e.g. 
   vendor name, in their vendor extensions.  
    
   Finally, for backwards compatibility with some existing 
   implementations, MGCP allows for other extension attributes as well 
   (see grammar in Appendix A). Note however, that these attribute 
   extensions do not provide the package extension attribute benefits. 
   Use of this mechanism for new extensions is discouraged. 
    
3.2.2.11 MaxMGCPDatagram 
 
   The MaxMGCPDatagram can only be used for auditing, i.e. it is a 
   valid RequestedInfo code and can be provided as a response 
   parameter.  
    
   In responses, the MaxMGCPDatagram value is encoded as a string of up 
   to nine decimal digits -
                          - leading zeroes are not permitted. The 
   following example illustrates the use of this parameter: 
    
      MD: 8100 
                                                                        
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3.2.2.12 ObservedEvents 
    
   The observed events parameter provides the list of events that have 
   been observed. The event codes are the same as those used in the 
   NotificationRequest. Events that have been accumulated according to 
   the digit map may be grouped in a single string, however such 
   practice is discouraged; they SHOULD be reported as lists of 
   isolated events if other events where detected during the digit 
   accumulation. Examples of observed events are: 
    
      O: L/hu 
      O: D/8295555T 
      O: D/8,D/2,D/9,D/5,D/5,L/hf,D/5,D/5,D/T 
      O: L/hf, L/hf, L/hu 
    
3.2.2.13 PackageList 
    
   The Package List can only be used for auditing, i.e., it is a valid 
   RequestedInfo code and can be provided as a response parameter.  
    
   The response parameter will consist of a comma separated list of 
   packages supported. The first package returned in the list is the 
   default package. Each package in the list consists of the package 
   name followed by a colon, and the highest version number of the 
   package supported.  
    
   An example of a package list is: 
    
     PL: L:1,G:1,D:0,FOO:2,T:1 
    
   Note that for backwards compatibility, support for this parameter is 
   OPTIONAL. 
    
3.2.2.14 QuarantineHandling 
    
   The quarantine handling parameter contains a list of comma separated 
   keywords: 
    
   *  The keyword "process" or "discard" to indicate the treatment of 
      quarantined and observed events. If neither "process" or 
      "discard" is present, "process" is assumed. 
    
   *  The keyword "step" or "loop" to indicate whether at most one 
      notification per NotificationRequest is allowed, or whether 
      multiple notifications per NotificationRequest are allowed. If 
      neither "step" nor "loop" is present, "step" is assumed. 
    
   The following values are valid examples: 
    
      Q: loop 
      Q: process 
      Q: loop,discard 
                                                                        
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3.2.2.15 ReasonCode 
    
   Reason codes are three-digit numeric values. The reason code is 
   optionally followed by a white space and commentary, e.g.: 
    
      E: 900 Endpoint malfunctioning 
    
   A list of reason codes can be found in Section 2.5. 
    
   The set of reason codes can be extended through packages. 
    
3.2.2.16 RequestedEvents 
    
   The RequestedEvents parameter provides the list of events that are 
   requested. The event codes are described in Section 3.2.2.4. 
    
   Each event can be qualified by a requested action, or by a list of 
   actions. The actions, when specified, are encoded as a list of 
   keywords, enclosed in parenthesis and separated by commas. The codes 
   for the various actions are: 
    
                ------------------------------------- 
               |          Action              | Code | 
               |------------------------------|------| 
               | Notify immediately           |  N   | 
               | Accumulate                   |  A   | 
               | Treat according to digit map |  D   | 
               | Swap                         |  S   | 
               | Ignore                       |  I   | 
               | Keep Signal(s) active        |  K   | 
               | Embedded Notification Request|  E   | 
                ------------------------------------- 
    
   When no action is specified, the default action is to notify the 
   event. This means that, for example, ft and ft(N) are equivalent. 
   Events that are not listed are ignored (unless they are persistent). 
    
   The digit-map action SHOULD only be specified for the digits, 
   letters and interdigit timers in packages that define the encoding 
   of digits, letters, and timers (incl. extension digit map letters). 
    
   The requested events list is encoded on a single line, with 
   event/action groups separated by commas. Examples of RequestedEvents 
   encodings are: 
    
      R: L/hu(N), L/hf(S,N) 
      R: L/hu(N), D/[0-9#T](D) 
    
   In the case of the "Embedded Notification Request" action, the 
   embedded notification request parameters are encoded as a list of up 
   to three parameter groups separated by commas. Each group starts by 
   a one letter identifier, followed by a list of parameters enclosed 
                                                                        
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   between parentheses. The first optional parameter group, identified 
   by the letter "R", is the value of the embedded RequestedEvents 
   parameter. The second optional group, identified by the letter "S", 
   is the embedded value of the SignalRequests parameter. The third 
   optional group, identified by the letter "D", is the embedded value 
   of the DigitMap. (Note that some existing implementations and 
   profiles may encode these three components in a different order. 
   Implementers are encouraged to accept such encodings, but they 
   SHOULD NOT generate them.) 
    
   If the RequestedEvents parameter is not present, the parameter will 
   be set to a null value. If the SignalRequests parameter is not 
   present, the parameter will be set to a null value. If the DigitMap 
   is absent, the current value MUST be used. The following are valid 
   examples of embedded requests: 
    
      R: L/hd(E(R(D/[0-9#T](D),L/hu(N)),S(L/dl),D([0-9].[#T]))) 
      R: L/hd(E(R(D/[0-9#T](D),L/hu(N)),S(L/dl))) 
    
   Some events can be qualified by additional event parameters. Such 
   event parameters will be separated by commas and enclosed within 
   parenthesis. Event parameters may be enclosed in double-quotes (in 
   fact, some event parameters MUST be enclosed in double-quotes due to 
   syntactic restrictions), in which case the quoted string itself is 
   UTF-8 encoded. Please refer to Section 3.2.2.4 for additional detail 
   on event parameters.  
    
   The following example shows the foobar event with an event parameter 
   "epar" 
    
      R: X/foobar(N)(epar=2) 
    
   Notice that the Action was included even though it is the default 
   Notify action - this is required by the grammar. 
    
3.2.2.17 RequestedInfo 
    
   The RequestedInfo parameter contains a comma separated list of 
   parameter codes, as defined in Section 3.2.2. For example, if one 
   wants to audit the value of the NotifiedEntity, RequestIdentifier, 
   RequestedEvents, SignalRequests, DigitMap, QuarantineHandling and 
   DetectEvents parameters, the value of the RequestedInfo parameter 
   will be: 
    
      F: N,X,R,S,D,Q,T 
    
   Note that extension parameters in general can be audited as well. 
   The individual extension will define the auditing operation. 
    
   The capabilities request, in the AuditEndPoint command, is encoded 
   by the parameter code "A", as in: 
    
      F: A 
                                                                        
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3.2.2.18 RequestIdentifier 
    
   The request identifier correlates a Notify command with the 
   NotificationRequest that triggered it. A RequestIdentifier is a 
   hexadecimal string, at most 32 characters in length. 
   RequestIdentifiers are compared as strings rather than numerical 
   value. The string "0" is reserved for reporting of persistent events 
   in the case where a NotificationRequest has not yet been received 
   after restart. 
    
3.2.2.19 ResponseAck 
    
   The response acknowledgement parameter is used to manage the "at- 
   most-once" facility described in Section 3.5. It contains a comma 
   separated list of "confirmed transaction-id ranges". 
    
   Each "confirmed transaction-id range" is composed of either one 
   decimal number, when the range includes exactly one transaction, or 
   two decimal numbers separated by a single hyphen, describing the 
   lower and higher transaction identifiers included in the range. 
    
   An example of a response acknowledgement is: 
    
      K: 6234-6255, 6257, 19030-19044 
    
3.2.2.20 RestartMethod 
    
   The RestartMethod parameter is encoded as one of the keywords 
   "graceful", "forced", "restart", "disconnected" or "cancel-graceful" 
   as for example: 
    
      RM: restart 
    
   The set of restart methods can be extended through packages. 
    
3.2.2.21 SignalRequests 
    
   The SignalRequests parameter provides the name of the signal(s) that 
   have been requested. Each signal is identified by a name, as 
   described in Section 3.2.2.4. 
    
   Some signals, such as for example announcement or ADSI display, can 
   be qualified by additional parameters, e.g.: 
    
   *  the name and parameters of the announcement, 
    
   *  the string that should be displayed. 
    
   Such parameters will be separated by commas and enclosed within 
   parenthesis, as in: 
    
      S: L/adsi("123456 Francois Gerard") 
                                                                        
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      S: A/ann(http://ann.example.net/no-such-number.au, 1234567) 
    
   When a quoted-string is provided, the string itself is UTF-8 encoded 
   [20]. 
    
   When several signals are requested, their codes are separated by a 
   comma, as in: 
    
      S: L/adsi("123456 Your friend"), L/rg 
    
   Please refer to Section 3.2.2.4 for additional detail on signal 
   parameters. 
    
3.3 Format of response headers 
    
   The response header is composed of a response line, optionally 
   followed by headers that encode the response parameters. 
    
   An example of a response header could be: 
    
      200 1203 OK 
    
   The response line starts with the response code, which is a three 
   digit numeric value. The code is followed by a white space, and the 
   transaction identifier. Response codes defined in packages (8xx) are 
   followed by white space, a slash ("/") and the package name. All 
   response codes may furthermore be followed by optional commentary 
   preceded by a white space. 
    
   The following table describes the parameters whose presence is 
   mandatory or optional in a response header, as a function of the 
   command that triggered the response. The letter M stands for 
   mandatory, O for optional and F for forbidden. Unless otherwise 
   specified, a parameter MUST NOT be present more than once. Note that 
   the table only reflects the default for responses that have not 
   defined any other behavior. If a response is received with a 
   parameter that is either not understood or marked as forbidden, the 
   offending parameter(s) MUST simply be ignored.  
    














                                                                        
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    ------------------------------------------------------------------ 
   | Parameter name      | EP | CR | MD | DL | RQ | NT | AU | AU | RS | 
   |                     | CF | CX | CX | CX | NT | FY | EP | CX | IP | 
   |---------------------|----|----|----|----|----|----|----|----|----| 
   | BearerInformation   |  F |  F |  F |  F |  F |  F |  O |  F |  F | 
   | CallId              |  F |  F |  F |  F |  F |  F |  F |  O |  F | 
   | Capabilities        |  F |  F |  F |  F |  F |  F |  O*|  F |  F | 
   | ConnectionId        |  F |  O*|  F |  F |  F |  F |  O*|  F |  F | 
   | ConnectionMode      |  F |  F |  F |  F |  F |  F |  F |  O |  F | 
   | Connection-         |  F |  F |  F |  O*|  F |  F |  F |  O |  F | 
   |   Parameters        |    |    |    |    |    |    |    |    |    | 
   | DetectEvents        |  F |  F |  F |  F |  F |  F |  O |  F |  F | 
   | DigitMap            |  F |  F |  F |  F |  F |  F |  O |  F |  F | 
   | EventStates         |  F |  F |  F |  F |  F |  F |  O |  F |  F | 
   | LocalConnection-    |  F |  F |  F |  F |  F |  F |  F |  O |  F | 
   |            Options  |    |    |    |    |    |    |    |    |    | 
   | MaxMGCPDatagram     |  F |  F |  F |  F |  F |  F |  O |  F |  F | 
   | NotifiedEntity      |  F |  F |  F |  F |  F |  F |  O |  O |  O | 
   | ObservedEvents      |  F |  F |  F |  F |  F |  F |  O |  F |  F | 
   | QuarantineHandling  |  F |  F |  F |  F |  F |  F |  O |  F |  F | 
   | PackageList         |  O*|  O*|  O*|  O*|  O*|  O*|  O |  O*|  O*| 
   | ReasonCode          |  F |  F |  F |  F |  F |  F |  O |  F |  F | 
   | RequestIdentifier   |  F |  F |  F |  F |  F |  F |  O |  F |  F | 
   | ResponseAck         |  O*|  O*|  O*|  O*|  O*|  O*|  O*|  O*|  O*| 
   | RestartDelay        |  F |  F |  F |  F |  F |  F |  O |  F |  F | 
   | RestartMethod       |  F |  F |  F |  F |  F |  F |  O |  F |  F | 
   | RequestedEvents     |  F |  F |  F |  F |  F |  F |  O |  F |  F | 
   | RequestedInfo       |  F |  F |  F |  F |  F |  F |  F |  F |  F | 
   | SecondConnectionId  |  F |  O |  F |  F |  F |  F |  F |  F |  F | 
   | SecondEndpointId    |  F |  O |  F |  F |  F |  F |  F |  F |  F | 
   | SignalRequests      |  F |  F |  F |  F |  F |  F |  O |  F |  F | 
   | SpecificEndpointId  |  F |  O |  F |  F |  F |  F |  O*|  F |  F | 
   |---------------------|----|----|----|----|----|----|----|----|----| 
   | LocalConnection-    |  F |  O*|  O |  F |  F |  F |  F |  O*|  F | 
   |         Descriptor  |    |    |    |    |    |    |    |    |    | 
   | RemoteConnection-   |  F |  F |  F |  F |  F |  F |  F |  O*|  F | 
   |         Descriptor  |    |    |    |    |    |    |    |    |    | 
    ------------------------------------------------------------------ 
    
   Notes (*): 
    
   *  The PackageList parameter is only allowed with return code 518 
      (unsupported package), except for AuditEndpoint, where it may 
      also be returned if audited. 
    
   *  The ResponseAck parameter MUST NOT be used with any other 
      responses than a final response issued after a provisional 
      response for the transaction in question. In that case, the 
      presence of the ResponseAck parameter SHOULD trigger a Response 
      Acknowledgement - any ResponseAck values provided will be 
      ignored. 
    

                                                                        
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   *  In the case of a CreateConnection message, the response line is 
      followed by a Connection-Id parameter and a 
      LocalConnectionDescriptor. It may also be followed a Specific-
      Endpoint-Id parameter, if the creation request was sent to a 
      wildcarded Endpoint-Id. The connection-Id and 
      LocalConnectionDescriptor parameter are marked as optional in the 
      Table. In fact, they are mandatory with all positive responses, 
      when a connection was created, and forbidden when the response is 
      negative, and no connection was created. 
    
   *  A LocalConnectionDescriptor MUST be transmitted with a positive 
      response (code 200) to a CreateConnection. It MUST also be 
      transmitted in response to a ModifyConnection command, if the 
      modification resulted in a modification of the session 
      parameters. The LocalConnectionDescriptor is encoded as a 
      "session description", as defined in section 3.4. It is separated 
      from the response header by an empty line. 
    
   *  Connection-Parameters are only valid in a response to a non-
      wildcarded DeleteConnection command sent by the Call Agent.  
    
   *  Multiple ConnectionId, SpecificEndpointId, and Capabilities 
      parameters may be present in the response to an AuditEndpoint 
      command. 
       
   *  When several session descriptors are encoded in the same 
      response, they are encoded one after each other, separated by an 
      empty line. This is the case for example when the response to an 
      audit connection request carries both a local session description 
      and a remote session description, as in: 
       
          200 1203 OK 
          C: A3C47F21456789F0 
          N: [128.96.41.12] 
          L: p:10, a:PCMU;G726-32 
          M: sendrecv 
          P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27,LA=48 
       
          v=0 
          o=- 25678 753849 IN IP4 128.96.41.1 
          s=- 
          c=IN IP4 128.96.41.1 
          t=0 0 
          m=audio 1296 RTP/AVP 0 
       
          v=0 
          o=- 33343 346463 IN IP4 128.96.63.25 
          s=- 
          c=IN IP4 128.96.63.25 
          t=0 0 
          m=audio 1296 RTP/AVP 0 96 
          a=rtpmap:96 G726-32/8000 
       
                                                                        
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      In this example, according to the SDP syntax, each description 
      starts with a "version" line, (v=...). The local description is 
      always transmitted before the remote description. If a connection 
      descriptor is requested, but it does not exist for the connection 
      audited, that connection descriptor will appear with the SDP 
      protocol version field only. 
    
   The response parameters are described for each of the commands in 
   the following. 
    
3.3.1 CreateConnection Response 
    
   In the case of a CreateConnection message, the response line is 
   followed by a Connection-Id parameter with a successful response 
   (code 200). A LocalConnectionDescriptor is furthermore transmitted 
   with a positive response. The LocalConnectionDescriptor is encoded 
   as a "session description", as defined by SDP (RFC 2327). It is 
   separated from the response header by an empty line, e.g.: 
    
      200 1204 OK 
      I: FDE234C8 
       
      v=0 
      o=- 25678 753849 IN IP4 128.96.41.1 
      s=- 
      c=IN IP4 128.96.41.1 
      t=0 0 
      m=audio 3456 RTP/AVP 96 
      a=rtpmap:96 G726-32/8000 
    
   When a provisional response has been issued previously, the final 
   response SHOULD furthermore contain the Response Acknowledgement 
   parameter (final responses issued by entities adhering to this 
   specification will include the parameter, but older RFC 2705 
   implementations MAY not): 
    
      200 1204 OK 
      K: 
      I: FDE234C8 
       
      v=0 
      o=- 25678 753849 IN IP4 128.96.41.1 
      s=- 
      c=IN IP4 128.96.41.1 
      t=0 0 
      m=audio 3456 RTP/AVP 96 
      a=rtpmap:96 G726-32/8000 
    
   The final response SHOULD then be acknowledged by a Response 
   Acknowledgement: 
    
      000 1204 
    
                                                                        
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3.3.2 ModifyConnection Response 
    
   In the case of a successful ModifyConnection message, the response 
   line is followed by a LocalConnectionDescriptor, if the modification 
   resulted in a modification of the session parameters (e.g., changing 
   only the mode of a connection does not alter the session 
   parameters). The LocalConnectionDescriptor is encoded as a "session 
   description", as defined by SDP. It is separated from the response 
   header by an empty line.  
    
      200 1207 OK 
        
      v=0 
      o=- 25678 753849 IN IP4 128.96.41.1 
      s=- 
      c=IN IP4 128.96.41.1 
      t=0 0 
      m=audio 3456 RTP/AVP 0 
       
   When a provisional response has been issued previously, the final 
   response SHOULD furthermore contain the Response Acknowledgement 
   parameter as in: 
    
      200 1207 OK 
      K: 
    
   The final response SHOULD then be acknowledged by a Response 
   Acknowledgement: 
    
      000 1207 OK 
       
3.3.3 DeleteConnection Response 
    
   Depending on the variant of the DeleteConnection message, the 
   response line may be followed by a Connection Parameters parameter 
   line, as defined in Section 3.2.2.7. 
    
      250 1210 OK 
      P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48 
    
3.3.4 NotificationRequest Response 
    
   A successful NotificationRequest response does not include any 
   additional response parameters. 
    
3.3.5 Notify Response 
    
   A successful Notify response does not include any additional 
   response parameters.  
    



                                                                        
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3.3.6 AuditEndpoint Response 
    
   In the case of a successful AuditEndPoint the response line may be 
   followed by information for each of the parameters requested - each 
   parameter will appear on a separate line. Parameters for which no 
   value currently exists, e.g., digit map, will still be provided but 
   with an empty value. Each local endpoint name "expanded" by a 
   wildcard character will appear on a separate line using the 
   "SpecificEndPointId" parameter code, e.g.: 
    
      200 1200 OK 
      Z: aaln/1@rgw.whatever.net 
      Z: aaln/2@rgw.whatever.net 
       
   When connection identifiers are audited and multiple connections 
   exist on the endpoint, a comma-separated list of connection 
   identifiers SHOULD be returned as in: 
      
      200 1200 OK 
      I: FDE234C8, DFE233D1 
    
   Alternatively, multiple connection id parameter lines may be 
   returned - the two forms should not be mixed although doing so does 
   not constitute an error. 
    
   When capabilities are audited, the response may include multiple 
   capabilities parameter lines as in: 
    
      200 1200 OK 
      A: a:PCMU;G728, p:10-100, e:on, s:off, t:1, v:L, 
          m:sendonly;recvonly;sendrecv;inactive 
      A: a:G729, p:30-90, e:on, s:on, t:1, v:L,
          m:sendonly;recvonly;sendrecv;inactive;confrnce 
    
   Note: The carriage return for Capabilities shown above is present 
   for formatting reasons only. It is not permissible in a real command 
   encoding. 
    
3.3.7 AuditConnection Response 
    
   In the case of a successful AuditConnection, the response may be 
   followed by information for each of the parameters requested. 
   Parameters for which no value currently exists will still be 
   provided. Connection descriptors will always appear last and each 
   will be preceded by an empty line, as for example:  
    







                                                                        
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      200 1203 OK 
      C: A3C47F21456789F0 
      N: [128.96.41.12] 
      L: p:10, a:PCMU;G728 
      M: sendrecv 
      P: PS=622, OS=31172, PR=390, OR=22561, PL=5, JI=29, LA=50 
       
      v=0 
      o=- 4723891 7428910 IN IP4 128.96.63.25  
      s=- 
      c=IN IP4 128.96.63.25 
      t=0 0 
      m=audio 1296 RTP/AVP 96 
      a=rtpmap:96 G726-32/8000 
    
   If both a local and a remote connection descriptor are provided, the 
   local connection descriptor will be the first of the two. If a 
   connection descriptor is requested, but it does not exist for the 
   connection audited, that connection descriptor will appear with the 
   SDP protocol version field only ("v=0"), as for example: 
    
      200 1203 OK 
       
      v=0 
       
3.3.8 RestartInProgress Response 
    
   A successful RestartInProgress response may include a NotifiedEntity 
   parameter, but otherwise does not include any additional response 
   parameters.  
    
   Also, a 521 response to a RestartInProgress MUST include a 
   NotifiedEntity parameter with the name of another Call Agent to 
   contact when the first Call Agent redirects the endpoint to another 
   Call Agent as in: 
    
      521 1204 Redirect 
      N: CA-1@whatever.net 
    
3.4 Encoding of the Session Description (SDP) 
    
   The session description (SDP) is encoded in conformance with the 
   session description protocol, SDP. MGCP implementations are REQUIRED 
   to be fully capable of parsing any conformant SDP message, and MUST 
   send session descriptions that strictly conform to the SDP standard.  
    
   The general description and explanation of SDP parameters can be 
   found in RFC 2327 (or its successor). In particular, it should be 
   noted that the  
    
   *  Origin ("o="),  
    
   *  Session Name ("s="), and  
                                                                        
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   *  Time active ("t=") 
    
   are all mandatory in RFC 2327. While they are of little use to MGCP, 
   they MUST be provided in conformance with RFC 2327 nevertheless. The 
   following suggests values to be used for each of the fields, however 
   the reader is encouraged to consult RFC 2327 (or its successor) for 
   details: 
    
   Origin 
   o = <username> <session id> <version> <network type> <address type> 
       <address> 
   
   *  The username SHOULD be set to hyphen ("-"). 
   *  The session id is RECOMMENDED to be an NTP timestamp as suggested 
      in RFC 2327. 
   *  The version is a version number that MUST increment with each 
      change to the SDP. A counter initialized to zero or an NTP 
      timestamp as suggested in RFC 2327 is RECOMMENDED.  
   *  The network type defines the type of network. For RTP sessions 
      the network type SHOULD be "IN".  
   *  The address type defines the type of address. For RTP sessions 
      the address type SHOULD be "IP4" (or "IP6"). 
   *  The address SHOULD be the same address as provided in the 
      connection information ("c=") field. 
    
   Session Name      
   s = <session name> 
    
   The session name should be hyphen ("-").  
    
   Time active 
   t = <start time> <stop time> 
    
   *  The start time may be set to zero. 
   *  The stop time should be set to zero.  
    
   Each of the three fields can be ignored upon reception.  
    
   To further accommodate the extensibility principles of MGCP, 
   implementations are ENCOURAGED to support the PINT "a=require" 
   attribute - please refer to RFC 2848 for further details. 
    
   The usage of SDP actually depends on the type of session that is 
   being established. Below we describe usage of SDP for an audio 
   service using the RTP/AVP profile [4], or the LOCAL interconnect 
   defined in this document. In case of any conflicts between what is 
   described below and SDP (RFC 2327 or its successor), the SDP 
   specification takes precedence. 
    



                                                                        
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3.4.1 Usage of SDP for an Audio Service 
    
   In a telephony gateway, we only have to describe sessions that use 
   exactly one media, audio. The usage of SDP for this is 
   straightforward and described in detail in RFC 2327.  
    
   The following is an example of an RFC 2327 conformant session 
   description for an audio connection: 
    
      v=0 
      o=- A7453949499 0 IN IP4 128.96.41.1 
      s=- 
      c=IN IP4 128.96.41.1 
      t=0 0 
      m=audio 3456 RTP/AVP 0 96 
      a=rtpmap:96 G726-32/8000 
       
3.4.2 Usage of SDP for LOCAL Connections 
    
   When MGCP is used to set up internal connections within a single 
   gateway, the SDP format is used to encode the parameters of that 
   connection. The connection and media parameters will be used as 
   follows: 
    
   *  The connection parameter (c=) will specify that the connection is 
      local, using the keyword "LOCAL" as network type, the keyword 
      "EPN" (endpoint name) as address type, and the local name of the 
      endpoint as the connection-address.  
    
   *  The "m=audio" parameter will specify a port number, which will 
      always be set to 0, the type of protocol, always set to the 
      keyword LOCAL, and the type of encoding, using the same 
      conventions used for the RTP AVP profile(RTP payload numbers). 
      The type of encoding should normally be set to 0 (PCMU). 
    
   A session-level attribute identifying the connection MAY furthermore 
   be present. This enables endpoints to support multiple LOCAL 
   connections. Use of this attribute is OPTIONAL and indeed 
   unnecessary for endpoints that only support a single LOCAL 
   connection. The attribute is defined as follows: 
    
   a=MGCPlocalcx:<ConnectionID> 
      The MGCP Local Connection attribute is a session level only case-
      insensitive attribute that identifies the MGCP LOCAL connection, 
      on the endpoint identified in the connection information, to 
      which the SDP applies. The ConnectionId is a hexadecimal string 
      containing at most 32 characters. The ConnectionId itself is 
      case-insensitive. The MGCP Local Connection attribute is not 
      subject to the charset attribute.  
    
   An example of a LOCAL session description could be: 
    

                                                                        
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      v=0 
      o=- A7453949499 0 LOCAL EPN X35V3+A4/13 
      s=- 
      c=LOCAL EPN X35V3+A4/13 
      t=0 0 
      a=MGCPlocalcx:FDE234C8 
      m=audio 0 LOCAL 0 
    
   Note that the MGCP Local Connection attribute is specified at the 
   session level and that it could have been omitted in case only a 
   single LOCAL connection per endpoint is supported.  
    
3.5 Transmission over UDP 
    
   MGCP messages are transmitted over UDP. Commands are sent to one of 
   the IP addresses defined in the DNS for the specified endpoint. The 
   responses are sent back to the source address (i.e., IP address and 
   UDP port number) of the commands - the response may or may not 
   arrive from the same address as the command was sent to.  
    
   When no port is specified for the endpoint, the commands MUST by 
   default be sent: 
    
   *  by the Call Agents, to the default MGCP port for gateways, 2427. 
    
   *  by the Gateways, to the default MGCP port for Call Agents, 2727. 
    
3.5.1 Providing the At-Most-Once Functionality 
    
   MGCP messages, being carried over UDP, may be subject to losses. In 
   the absence of a timely response, commands are retransmitted. Most 
   MGCP commands are not idempotent. The state of the gateway would 
   become unpredictable if, for example, CreateConnection commands were 
   executed several times. The transmission procedures MUST thus 
   provide an "at-most-once" functionality. 
    
   MGCP entities are expected to keep in memory a list of the responses 
   that they sent to recent transactions, and a list of the 
   transactions that are currently being executed. The numerical value 
   of transaction identifiers of incoming commands are compared to the 
   transaction identifiers of the recent responses. If a match is 
   found, the MGCP entity does not execute the transaction again, but 
   simply resends the response. The remaining commands will be compared 
   to the list of current transactions, i.e. transactions received 
   previously which have not yet finished executing. If a match is 
   found, the MGCP entity does not execute the transaction again, but a 
   provisional response (Section 3.5.5) SHOULD be issued to acknowledge 
   receipt of the command.  
    
   The procedure uses a long timer value, noted T-HIST in the 
   following. The timer MUST be set larger than the maximum duration of 
   a transaction, which MUST take into account the maximum number of 
   repetitions, the maximum value of the repetition timer and the 
                                                                        
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   maximum propagation delay of a packet in the network. A suggested 
   value is 30 seconds. 
    
   The copy of the responses MAY be destroyed either T-HIST seconds 
   after the response is issued, or when the gateway (or the Call 
   Agent) receives a confirmation that the response has been received, 
   through the "Response Acknowledgement". For transactions that are 
   acknowledged through this attribute, the gateway SHALL keep a copy 
   of the transaction-id (as opposed to the entire transaction 
   response) for T-HIST seconds after the response is issued, in order 
   to detect and ignore duplicate copies of the transaction request 
   that could be produced by the network. 
    
3.5.2 Transaction Identifiers and Three Ways Handshake 
    
   Transaction identifiers are integer numbers in the range from 1 to 
   999,999,999. Call-agents may decide to use a specific number space 
   for each of the gateways that they manage, or to use the same number 
   space for all gateways that belong to some arbitrary group. Call 
   agents may decide to share the load of managing a large gateway 
   between several independent processes. These processes MUST then 
   share the transaction number space. There are multiple possible 
   implementations of this sharing, such as having a centralized 
   allocation of transaction identifiers, or pre-allocating non-
   overlapping ranges of identifiers to different processes. The 
   implementations MUST guarantee that unique transaction identifiers 
   are allocated to all transactions that originate from a logical call 
   agent, as defined in Section 4. Gateways can simply detect duplicate 
   transactions by looking at the transaction identifier only. 
    
   The Response Acknowledgement Attribute can be found in any command. 
   It carries a set of "confirmed transaction-id ranges" for final 
   responses received - provisional responses MUST NOT be confirmed. A 
   given response SHOULD NOT be confirmed in two separate messages. 
    
   MGCP entities MAY choose to delete the copies of the responses (but 
   not the transaction-id) to transactions whose id is included in 
   "confirmed transaction-id ranges" received in the Response 
   Confirmation messages (command or response). They SHOULD then 
   silently discard further commands from that entity when the 
   transaction-id falls within these ranges, and the response was 
   issued less than T-HIST seconds ago. 
    
   Entities MUST exercise due caution when acknowledging responses. In 
   particular, a response SHOULD only be acknowledged if the response 
   acknowledgement is sent to the same entity as the corresponding 
   command (i.e. the command whose response is being acknowledged) was 
   sent to. 
    
   Likewise, entities SHOULD NOT blindly accept a response 
   acknowledgement for a given response. However it is considered safe 
   to accept a response acknowledgement for a given response, when that 

                                                                        
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   response acknowledgement is sent by the same entity as the command 
   that generated that response.  
    
   It should be noted, that use of response acknowledgements in 
   commands (as opposed to the Response Acknowledgement response 
   following a provisional response) is OPTIONAL. The benefit of using 
   it is that it reduces overall memory consumption. However, in order 
   to avoid large messages, implementations SHOULD NOT generate large 
   response acknowledgement lists. One strategy is to manage responses 
   to commands on a per endpoint basis. A command for an endpoint can 
   confirm a response to an older command for that same endpoint. 
   Responses to commands with wildcarded endpoint names can be 
   confirmed selectively with due consideration to message sizes, or 
   alternatively simply not be acknowledged (unless the response 
   explicitly required a Response Acknowledgement). Care must be taken 
   to not confirm the same response twice or a response that is more 
   than T-HIST seconds old. 
    
   The "confirmed transaction-id ranges" values SHALL NOT be used if 
   more than T-HIST seconds have elapsed since the entity issued its 
   last response to the other entity, or when an entity resumes 
   operation. In this situation, commands MUST be accepted and 
   processed, without any test on the transaction-id. 
    
   Commands that carry the "Response Acknowledgement attribute" may be 
   transmitted in disorder. The union of the "confirmed transaction-id 
   ranges" received in recent messages SHALL be retained. 
    
3.5.3 Computing Retransmission Timers 
    
   It is the responsibility of the requesting entity to provide 
   suitable time outs for all outstanding commands, and to retry 
   commands when time outs have been exceeded. Furthermore, when 
   repeated commands fail to be acknowledged, it is the responsibility 
   of the requesting entity to seek redundant services and/or clear 
   existing or pending associations. 
    
   The specification purposely avoids specifying any value for the 
   retransmission timers. These values are typically network dependent. 
   The retransmission timers SHOULD normally estimate the timer by 
   measuring the time spent between the sending of a command and the 
   return of the first response to the command. At a minimum, a 
   retransmission strategy involving exponential backoff MUST be 
   implemented. One possibility is to use the algorithm implemented in 
   TCP-IP, which uses two variables: 
    
   *  the average acknowledgement delay, AAD, estimated through an 
      exponentially smoothed average of the observed delays, 
    
   *  the average deviation, ADEV, estimated through an exponentially 
      smoothed average of the absolute value of the difference between 
      the observed delay and the current average 
    
                                                                        
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   The retransmission timer, RTO, in TCP, is set to the sum of the 
   average delay plus N times the average deviation, where N is a 
   constant. In MGCP, the maximum value of the timer SHOULD however be 
   bounded, in order to guarantee that no repeated packet will be 
   received by the gateways after T-HIST seconds. A suggested maximum 
   value for RTO (RTO-MAX) is 4 seconds. Implementers SHOULD consider 
   bounding the minimum value of this timer as well [19]. 
    
   After any retransmission, the MGCP entity SHOULD do the following: 
    
   *  It should double the estimated value of the acknowledgement delay 
      for this transaction, T-DELAY. 
    
   *  It should compute a random value, uniformly distributed between 
      0.5 T-DELAY and T-DELAY. 
    
   *  It should set the retransmission timer (RTO) to the minimum of: 
      - the sum of that random value and N times the average deviation, 
      - RTO-MAX. 
    
   This procedure has two effects. Because it includes an exponentially 
   increasing component, it will automatically slow down the stream of 
   messages in case of congestion. Because it includes a random 
   component, it will break the potential synchronization between 
   notifications triggered by the same external event. 
    
   Note that the estimators AAD and ADEV SHOULD NOT be updated for 
   transactions that involve retransmissions. Also, the first new 
   transmission following a successful retransmission SHOULD use the 
   RTO for that last retransmission. If this transmission succeeds 
   without any retransmissions, the AAD and ADEV estimators are updated 
   and RTO is determined as usual again. See, e.g., [18] for further 
   details. 
    
3.5.4 Maximum Datagram Size, Fragmentation and Reassembly 
    
   MGCP messages being transmitted over UDP rely on IP for 
   fragmentation and reassembly of large datagrams. The maximum 
   theoretical size of an IP datagram is 65535 bytes. With a 20-byte IP 
   header and an 8-byte UDP header, this leaves us with a maximum 
   theoretical MGCP message size of 65507 bytes when using UDP. 
    
   However, IP does not require a host to receive IP datagrams larger 
   than 576 bytes [21], which would provide an unacceptably small MGCP 
   message size. Consequently, MGCP mandates that implementations MUST 
   support MGCP datagrams up to at least 4000 bytes, which requires the 
   corresponding IP fragmentation and reassembly to be supported. Note, 
   that the 4000 byte limit applies to the MGCP level. Lower layer 
   overhead will require support for IP datagrams that are larger than 
   this: UDP and IP overhead will be at least 28 bytes, and, e.g., use 
   of IPSec will add additional overhead. 
    

                                                                        
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   It should be noted, that the above applies to both Call Agents and 
   endpoints. Call Agents can audit endpoints to determine if they 
   support larger MGCP datagrams than specified above. Endpoints do 
   currently not have a similar capability to determine if a Call Agent 
   supports larger MGCP datagram sizes. 
    
3.5.5 Piggybacking 
    
   There are cases when a Call Agent will want to send several messages 
   at the same time to the same gateways, and vice versa. When several 
   MGCP messages have to be sent in the same datagram, they MUST be 
   separated by a line of text that contains a single dot, as in for 
   example: 
    
      200 2005 OK 
      . 
      DLCX 1244 card23/21@tgw-7.example.net MGCP 1.0 
      C: A3C47F21456789F0 
      I: FDE234C8 
    
   The piggybacked messages MUST be processed exactly as if they had 
   been received one at a time in several separate datagrams. Each 
   message in the datagram MUST be processed to completion and in order 
   starting with the first message, and each command MUST be responded 
   to. Errors encountered in a message that was piggybacked MUST NOT 
   affect any of the other messages received in that datagram - each 
   message is processed on its own. 
    
   Piggybacking can be used to achieve two things: 
    
   * Guaranteed in-order delivery and processing of messages. 
      
   * Fate sharing of message delivery. 
    
   When piggybacking is used to guarantee in-order delivery of 
   messages, entities MUST ensure that this in-order delivery property 
   is retained on retransmissions of the individual messages. An 
   example of this is when multiple Notify's are sent using 
   piggybacking (as described in Section 4.4.1).  
    
   Fate sharing of message delivery ensures that either all the 
   messages are delivered, or none of them are delivered. When 
   piggybacking is used to guarantee this fate-sharing, entities MUST 
   also ensure that this property is retained upon retransmission. For 
   example, upon receiving a Notify from an endpoint operating in 
   lockstep mode, the Call Agent may wish to send the response and a 
   new NotificationRequest command in a single datagram to ensure 
   message delivery fate-sharing of the two. 
    
3.5.6 Provisional Responses 
    
   Executing some transactions may require a long time. Long execution 
   times may interact with the timer based retransmission procedure. 
                                                                        
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   This may result either in an inordinate number of retransmissions, 
   or in timer values that become too long to be efficient. 
    
   Gateways (and Call Agents) that can predict that a transaction will 
   require a long execution time SHOULD send a provisional response 
   with response code 100. As a guideline, a transaction that requires 
   external communication to complete, e.g. network resource 
   reservation, SHOULD issue a provisional response. Furthermore 
   entities SHOULD send a provisional response if they receive a 
   repetition of a transaction that has not yet finished executing. 
    
   Gateways (or Call Agents) that start building up queues of 
   transactions to be executed may send a provisional response with 
   response code 101 to indicate this (see Section 4.4.8 for further 
   details).  
    
   Pure transactional semantics would imply, that provisional responses 
   SHOULD NOT return any other information than the fact that the 
   transaction is currently executing, however an optimistic approach 
   allowing some information to be returned enables a reduction in the 
   delay that would otherwise be incurred in the system.  
    
   In order to reduce the delay in the system, it is RECOMMENDED to 
   include a connection identifier and session description in a 100 
   provisional response to the CreateConnection command. If a session 
   description would be returned by the ModifyConnection command, the 
   session description SHOULD be included in the provisional response 
   here as well. If the transaction completes successfully, the 
   information returned in the provisional response MUST be repeated in 
   the final response. It is considered a protocol error not to repeat 
   this information or to change any of the previously supplied 
   information in a successful response. If the transaction fails, an 
   error code is returned - the information returned previously is no 
   longer valid. 
    
   A currently executing CreateConnection or ModifyConnection 
   transaction MUST be cancelled if a DeleteConnection command for the 
   endpoint is received. In that case, a final response for the 
   cancelled transaction SHOULD still be returned automatically (error 
   code 407 - transaction aborted, is RECOMMENDED), and a final 
   response for the cancelled transaction MUST be returned if a 
   retransmission of the cancelled transaction is detected (see also 
   Section 4.4.4). 
    
   MGCP entities that receive a provisional response SHALL switch to a 
   longer repetition timer (LONGTRAN-TIMER) for that transaction. The 
   purpose of this timer is primarily to detect processing failures. 
   The default value of LONGTRAN-TIMER is 5 seconds, however the 
   provisioning process may alter this. Note, that retransmissions MUST 
   still satisfy the timing requirements specified in Section 3.5.1 and 
   3.5.3. Consequently LONGTRAN-TIMER MUST be smaller than T-HIST (it 
   should in fact be considerably smaller). Also, entities MUST NOT let 
   a transaction run forever. A transaction that is timed out by the 
                                                                        
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   entity SHOULD return error code 406 (transaction time-out). Per the 
   definition of T-HIST (Section 3.5.1), the maximum transaction 
   execution time is smaller than T-HIST (in a network with low delay, 
   it can reasonably safely be approximated as T-HIST minus T-MAX), and 
   a final response should be received no more than T-HIST seconds 
   after the command was sent initially. Nevertheless, entities SHOULD 
   wait for 2*T-HIST seconds before giving up on receiving a final 
   response. Retransmission of the command MUST still cease after T-MAX 
   seconds though. If a response is not received, the outcome of the 
   transaction is not known. If the entity sending the command was a 
   gateway, it now becomes "disconnected" and SHALL initiate the 
   "disconnected" procedure (see Section 4.4.7). 
    
   When the transaction finishes execution, the final response is sent 
   and the by now obsolete provisional response is deleted. In order to 
   ensure rapid detection of a lost final response, final responses 
   issued after provisional responses for a transaction SHOULD be 
   acknowledged (unfortunately older RFC 2705 implementations may not 
   do this, which is the only reason it is not an absolute 
   requirement).  
    
   The endpoint SHOULD therefore include an empty "ResponseAck" 
   parameter in those, and only those, final responses. The presence of 
   the "ResponseAck" parameter in the final response SHOULD trigger a 
   "Response Acknowledgement" response to be sent back to the endpoint. 
   The Response Acknowledgement" response will then include the 
   transaction-id of the response it acknowledges in the response 
   header. Note that, for backwards compatibility, entities cannot 
   depend on receiving such a "response acknowledgement", however it is 
   strongly RECOMMENDED to support this behavior, as excessive delays 
   in case of packet loss as well as excessive retransmissions may 
   occur otherwise.  
    
   Receipt of a "Response Acknowledgement" response is subject to the 
   same time-out and retransmission strategies and procedures as 
   responses to commands, i.e., the sender of the final response will 
   retransmit it if a "Response Acknowledgement" is not received in 
   time. For backwards compatibility, failure to receive a "response 
   acknowledgement" SHOULD NOT affect the roundtrip time estimates for 
   subsequent commands, and furthermore MUST NOT lead to the endpoint 
   becoming "disconnected". The "Response Acknowledgment" response is 
   never acknowledged. 
    










                                                                        
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4. States, Failover and Race Conditions. 
    
   In order to implement proper call signaling, the Call Agent must 
   keep track of the state of the endpoint, and the gateway must make 
   sure that events are properly notified to the Call Agent. Special 
   conditions exist when the gateway or the Call Agent are restarted: 
   the gateway must be redirected to a new Call Agent during "failover" 
   procedures, the Call Agent must take special action when the gateway 
   is taken offline, or restarted. 
    
4.1 Failover Assumptions and Highlights 
    
   The following protocol highlights are important to understanding 
   Call Agent fail-over mechanisms: 
    
   *  Call Agents are identified by their domain name (and optional 
      port), not their network addresses, and several addresses can be 
      associated with a domain name.  
    
   *  An endpoint has one and only one Call Agent associated with it at 
      any given point in time. The Call Agent associated with an 
      endpoint is the current value of the "notified entity". The 
      "notified entity" determines where the gateway will send it's 
      commands. If the "notified entity" does not include a port 
      number, the default Call Agent port number (2727) is assumed. 
    
   *  NotifiedEntity is a parameter sent by the Call Agent to the 
      gateway to set the "notified entity" for the endpoint.  
    
   *  The "notified entity" for an endpoint is the last value of the 
      NotifiedEntity parameter received for this endpoint. If no 
      explicit NotifiedEntity parameter has ever been received, the 
      "notified entity" defaults to a provisioned value. If no value 
      was provisioned or an empty NotifiedEntity parameter was provided 
      (both strongly discouraged) thereby making the "notified entity" 
      empty, the "notified entity" is set to the source address of the 
      last non-audit command for the endpoint. Thus auditing will not 
      change the "notified entity". 
    
   *  Responses to commands are sent to the source address of the 
      command, regardless of the current "notified entity". When a 
      Notify message needs to be piggybacked with the response, the 
      datagram is still sent to the source address of the new command 
      received, regardless of the current "notified entity". 
    
   The ability for the "notified entity" to resolve to multiple network 
   addresses, allows a "notified entity" to represent a Call Agent with 
   multiple physical interfaces on it and/or a logical Call Agent made 
   up of multiple physical systems. The order of network addresses when 
   a DNS name resolves to multiple addresses is non-deterministic so 
   Call Agent fail-over schemes MUST NOT depend on any order (e.g. a 
   gateway MUST be able to send a "Notify" to any of the resolved 
   network addresses). On the other hand, the system is likely to be 
                                                                        
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   most efficient if the gateway sends commands to the interface with 
   which it already has a current association. It is RECOMMENDED that 
   gateways use the following algorithm to achieve that goal: 
    
   *  If the "notified entity" resolves to multiple network addresses, 
      and the source address of the request is one of those addresses, 
      that network address is the preferred destination address for 
      commands. 
    
   *  If on the other hand, the source address of the request is not 
      one of the resolved addresses, the gateway must choose one of the 
      resolved addresses for commands. 
    
   *  If the gateway fails to contact the network address chosen, it 
      MUST try the alternatives in the resolved list as described in 
      Section 4.3. 
    
   If an entire Call Agent becomes unavailable, the endpoints managed 
   by that Call Agent will eventually become "disconnected". The only 
   way for these endpoints to become connected again is either for the 
   failed Call Agent to become available, or for a backup call agent to 
   contact the affected endpoints with a new "notified entity". 
    
   When a backup Call Agent has taken over control of a group of 
   endpoints, it is assumed that the failed Call Agent will communicate 
   and synchronize with the backup Call Agent in order to transfer 
   control of the affected endpoints back to the original Call Agent. 
   Alternatively, the failed Call Agent could simply become the backup 
   Call Agent. 
    
   We should note that handover conflict resolution between separate 
   CA's is not in place - we are relying strictly on the CA's knowing 
   what they are doing and communicating with each other (although 
   AuditEndpoint can be used to learn about the current 
   NotifiedEntity). If this is not the case, unexpected behavior may 
   occur.  
    
   Note that as mentioned earlier, the default "notified entity" is 
   provisioned and may include both domain name and port. For small 
   gateways, provisioning may be done on a per endpoint basis. For much 
   larger gateways, a single provisioning element may be provided for 
   multiple endpoints or even for the entire gateway itself. In either 
   case, once the gateway powers up, each endpoint MUST have its own 
   "notified entity", so provisioned values for an aggregation of 
   endpoints MUST be copied to the "notified entity" for each endpoint 
   in the aggregation before operation proceeds. Where possible, the 
   RestartInProgress command on restart SHOULD be sent to the 
   provisioned "notified entity" based on an aggregation that allows 
   the "all of" wild-card to be used. This will reduce the number of 
   RestartInProgress messages. 
    
   Another way of viewing the use of "notified entity" is in terms of 
   associations between gateways and Call Agents. The "notified entity" 
                                                                        
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   is a means to set up that association, and governs where the gateway 
   will send commands to. Commands received by the gateway however may 
   come from any source. The association is initially provisioned with 
   a provisioned "notified entity", so that on power up 
   RestartInProgress and persistent events that occur prior to the 
   first NotificationRequest from Call Agents will be sent to the 
   provisioned Call Agent. Once a Call Agent makes a request, however 
   it may include the NotifiedEntity parameter and set up a new 
   association. Since the "notified entity" persists across calls, the 
   association remains intact until a new "notified entity" is 
   provided.  
    
4.2 Communicating with Gateways 
    
   Endpoint names in gateways include a local name indicating the 
   specific endpoint and a domain name indicating the host/gateway 
   where the endpoint resides. Gateways may have several interfaces for 
   redundancy.  
    
   In gateways that have routing capability, the domain name may 
   resolve to a single network address with internal routing to that 
   address from any of the gateway's interfaces. In others, the domain 
   name may resolve to multiple network addresses, one for each 
   interface. In the latter case, if a Call Agent fails to contact the 
   gateway on one of the addresses, it MUST try the alternates. 
    
4.3 Retransmission, and Detection of Lost Associations: 
    
   The media gateway control protocol is organized as a set of 
   transactions, each of which is composed of a command and a response, 
   commonly referred to as an acknowledgement. The MGCP messages, being 
   carried over UDP, may be subject to losses. In the absence of a 
   timely response, commands are retransmitted. MGCP entities MUST keep 
   in memory a list of the responses that they sent to recent 
   transactions, i.e., a list of all the responses they sent over the 
   last T-HIST seconds, and a list of the transactions that have not 
   yet finished executing.  
    
   The transaction identifiers of incoming commands are compared to the 
   transaction identifiers of the recent responses. If a match is 
   found, the MGCP entity does not execute the transaction, but simply 
   repeats the response. If a match to a previously responded to 
   transaction is not found, the transaction identifier of the incoming 
   command is compared to the list of transactions that have not yet 
   finished executing. If a match is found, the MGCP entity does not 
   execute the transaction again, but SHOULD simply sends a provisional 
   response - a final response will be provided when the execution of 
   the command is complete (see Section 3.5.6 for further detail). 
    
   The repetition mechanism is used to guard against four types of 
   possible errors: 
    

                                                                        
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   *  transmission errors, when for example a packet is lost due to 
      noise on a line or congestion in a queue, 
    
   *  component failure, when for example an interface to a Call Agent 
      becomes unavailable, 
    
   *  Call Agent failure, when for example an entire Call Agent becomes 
      unavailable, 
    
   *  failover, when a new Call Agent is "taking over" transparently. 
    
   The elements should be able to derive from the past history an 
   estimate of the packet loss rate due to transmission errors. In a 
   properly configured system, this loss rate should be very low, 
   typically less than 1%. If a Call Agent or a gateway has to repeat a 
   message more than a few times, it is very legitimate to assume that 
   something other than a transmission error is occurring. For example, 
   given a loss rate of 1%, the probability that 5 consecutive 
   transmission attempts fail is 1 in 100 billion, an event that should 
   occur less than once every 10 days for a Call Agent that processes 
   1,000 transactions per second. (Indeed, the number of 
   retransmissions that is considered excessive should be a function of 
   the prevailing packet loss rate.) We should note that the "suspicion 
   threshold", which we will call "Max1", is normally lower than the 
   "disconnection threshold", which we will call "Max2". Max2 MUST be 
   set to a larger value than Max1.  
    
   The MGCP retransmission algorithm is illustrated in the Figure below 
   and explained further in the following: 
























                                                                        
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      Command issued: N=0, T=0 
              | 
              |  +------------ retransmission: N++ <--------------+ 
              |  |                                                | 
              |  |     if T <= T-Max then                         | 
              |  |      transmission                              | 
              |  |  +-- to new address, <-+<----------------------|--+ 
              |  |  |       N=0           |                       |  | 
              V  V  V                     |                       |  | 
          +-----------+                   |                       |  | 
      +-->| awaiting  |- new Call Agent ->+   +------------+      |  | 
      |   |  response |--- timer elapsed  --->| T > T-Max ?|      |  | 
      |   +-----------+                       +------------+      ^  ^ 
      |          |                             |    |             |  | 
      |          v             +-----(yes)-----+   (no)           |  |  
      |      (response         |                    |             |  | 
      |       received)        |              +------------+      |  | 
      |          |             |              | N >= Max1 ?|-(no)>+  | 
      |          v             |              +------------+      ^  ^ 
      |      +--------+        |                    |             |  | 
      +<(no)-| final ?|        |                  (yes)           |  | 
      ^      +--------+        |                    |             |  | 
      |          |             |     (if first address & N=Max1,  |  | 
      |          v             |      or last address & N=Max2    |  | 
      |        (yes)           |               check DNS)         |  | 
      |          |             |                    |             |  | 
      |          v             V           +---------------+      |  | 
      |        (end)           |           |more addresses?|(yes)-|->+ 
      |                        |           +---------------+      | 
      |                        |                    |             ^ 
      |                        |                  (no)            | 
      |                        |                    |             | 
      |                        |              +------------+      | 
      |                        |              | N >= Max2 ?|(no)--+ 
      |                        |              +------------+ 
      |                        |                    | 
      |                        |                  (yes)   
      |                        |                    |              
      |                        |            +----------------+      
      |                        +----------->| T >= 2*T-HIST ?|  
      |                                     +----------------+ 
      |                                       |       |               
      |                                     (no)    (yes)  
      +---------------<-----------------------+       | 
                                                      v 
                                                (disconnected) 
    
    
   A classic retransmission algorithm would simply count the number of 
   successive repetitions, and conclude that the association is broken 
   after re-transmitting the packet an excessive number of times 
   (typically between 7 and 11 times.) In order to account for the 

                                                                        
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   possibility of an undetected or in-progress "failover", we modify 
   the classic algorithm as follows: 
    
   *  We require that the gateway always checks for the presence of a 
      new Call Agent. It can be noticed either by: 
    
      -  receiving a command where the NotifiedEntity points to the new 
         Call Agent, or 
    
      -  receiving a redirection response pointing to a new Call Agent. 
    
      If a new Call Agent is detected, the gateway MUST start 
      retransmitting outstanding commands for the endpoint(s) 
      redirected to that new Call Agent. Responses to new or old 
      commands are still transmitted to the source address of the 
      command.  
    
   *  Prior to any retransmission, it is checked that the time elapsed 
      since the sending of the initial datagram is no greater than T-
      MAX. If more than T-MAX time has elapsed, then retransmissions 
      MUST cease. If more than 2*T-HIST has elapsed, then the endpoint 
      becomes disconnected. 
    
   *  If the number of repetitions for this Call Agent is equal to 
      "Max1", and its domain name was not resolved recently (e.g. 
      within the last 5 seconds or otherwise provisioned), and it is 
      not in the process of being resolved, then the gateway MAY 
      actively query the domain name server in order to detect the 
      possible change of the Call Agent interfaces. Note that the first 
      repetition is the second transmission. 
    
   *  The gateway may have learned several IP addresses for the call 
      agent. If the number of repetitions for this IP address is 
      greater than or equal to "Max1" and lower than "Max2", and there 
      are more addresses that have not been tried, then the gateway 
      MUST direct the retransmissions to alternate addresses. Also, 
      receipt of explicit network notifications such as, e.g., ICMP 
      network, host, protocol, or port unreachable SHOULD lead the 
      gateway to try alternate addresses. 
    
   *  If there are no more interfaces to try, and the number of 
      repetitions for this address is Max2, then the gateway SHOULD 
      contact the DNS one more time to see if any other interfaces have 
      become available, unless the domain name was resolved recently 
      (e.g. within the last 5 seconds or otherwise provisioned), or it 
      is already in the process of being resolved. If there still are 
      no more interfaces to try, the gateway is then disconnected and 
      MUST initiate the "disconnected" procedure (see Section 4.4.7). 
    
   In order to automatically adapt to network load, MGCP specifies 
   exponentially increasing timers. If the initial timer is set to 200 
   milliseconds, the loss of a fifth retransmission will be detected 
   after about 6 seconds. This is probably an acceptable waiting delay 
                                                                        
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   to detect a failover. The repetitions should continue after that 
   delay not only in order to perhaps overcome a transient connectivity 
   problem, but also in order to allow some more time for the execution 
   of a failover - waiting a total delay of 30 seconds is probably 
   acceptable. 
    
   It is however important that the maximum delay of retransmissions be 
   bounded. Prior to any retransmission, it is checked that the time 
   (T) elapsed since the sending of the initial datagram is no greater 
   than T-MAX. If more than T-MAX time has elapsed, retransmissions 
   MUST cease. If more than 2*T-HIST time has elapsed, the endpoint 
   becomes disconnected. The value T-MAX is related to the T-HIST 
   value: the T-HIST value MUST be greater than or equal to T-MAX plus 
   the maximum propagation delay in the network. 
    
   The default value for T-MAX is 20 seconds. Thus, if the assumed 
   maximum propagation delay is 10 seconds, then responses to old 
   transactions would have to be kept for a period of at least 30 
   seconds. The importance of having the sender and receiver agree on 
   these values cannot be overstated.  
    
   The default value for Max1 is 5 retransmissions and the default 
   value for Max2 is 7 retransmissions. Both of these values may be 
   altered by the provisioning process. 
    
   The provisioning process MUST be able to disable one or both of the 
   Max1 and Max2 DNS queries.  
    
4.4 Race Conditions 
    
   MGCP deals with race conditions through the notion of a "quarantine 
   list" and through explicit detection of desynchronization, e.g., for 
   mismatched hook state due to glare for an endpoint. 
    
   MGCP does not assume that the transport mechanism will maintain the 
   order of commands and responses. This may cause race conditions, 
   that may be obviated through a proper behavior of the Call Agent. 
   (Note that some race conditions are inherent to distributed systems; 
   they would still occur, even if the commands were transmitted in 
   strict order.) 
    
   In some cases, many gateways may decide to restart operation at the 
   same time. This may occur, for example, if an area loses power or 
   transmission capability during an earthquake or an ice storm. When 
   power and transmission are reestablished, many gateways may decide 
   to send "RestartInProgress" commands simultaneously, leading to very 
   unstable operation. 
    
4.4.1 Quarantine List 
    
   MGCP controlled gateways will receive "notification requests" that 
   ask them to watch for a list of "events". The protocol elements that 
   determine the handling of these events are the "Requested Events" 
                                                                        
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   list, the "Digit Map", the "Quarantine Handling", and the "Detect 
   Events" list. 
    
   When the endpoint is initialized, the requested events list only 
   consists of persistent events for the endpoint, and the digit map is 
   assumed empty. At this point, the endpoint MAY use an implicit 
   NotificationRequest with the reserved RequestIdentifier zero ("0") 
   to detect and report a persistent event, e.g. off-hook. A pre-
   existing off-hook condition MUST here result in the off-hook event 
   being generated as well. 
    
   The endpoint awaits the reception of a NotificationRequest command, 
   after which the gateway starts observing the endpoint for 
   occurrences of the events mentioned in the list, including 
   persistent events. 
    
   The events are examined as they occur. The action that follows is 
   determined by the "action" parameter associated with the event in 
   the list of requested events, and also by the digit map. The events 
   that are defined as "accumulate" or "accumulate according to digit 
   map" are accumulated in a list of events, the events that are marked 
   as "accumulate according to the digit map" will additionally be 
   accumulated in the "current dial string". This will go on until one 
   event is encountered that triggers a notification which will be sent 
   to the current "notified entity". 
    
   The gateway, at this point, will transmit the Notify command and 
   will place the endpoint in a "notification" state. As long as the 
   endpoint is in this notification state, the events that are to be 
   detected on the endpoint are stored in a "quarantine" buffer (FIFO) 
   for later processing. The events are, in a sense, "quarantined". All 
   events that are specified by the union of the RequestedEvents 
   parameter and the most recently received DetectEvents parameter or, 
   in the absence of the latter, all events that are referred to in the 
   RequestedEvents, SHALL be detected and quarantined, regardless of 
   the action associated with the event. Persistent events are here 
   viewed as implicitly included in RequestedEvents. If the quarantine 
   buffer reaches the capacity of the endpoint, a Quarantine Buffer 
   Overflow event (see Appendix B) SHOULD be generated (when this event 
   is supported, the endpoint MUST ensure it has capacity to include 
   the event in the quarantine buffer). Excess events will now be 
   discarded. 
    
   The endpoint exits the "notification state" when the response 
   (whether success or failure) to the Notify command is received. The 
   Notify command may be retransmitted in the "notification state", as 
   specified in Section 3.5 and 4. If the endpoint is or becomes 
   disconnected (see Section 4.3) during this, a response to the Notify 
   command will never be received. The Notify command is then lost and 
   hence no longer considered pending, yet the endpoint is still in the 
   "notification state". Should that occur, completion of the 
   disconnected procedure specified in Section 4.4.7 SHALL then lead 
   the endpoint to exit the "notification state". 
                                                                        
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   When the endpoint exits the "notification state" it resets the list 
   of observed events and the "current dial string" of the endpoint to 
   a null value. 
    
   Following that point, the behavior of the gateway depends on the 
   value of the QuarantineHandling parameter in the triggering 
   NotificationRequest command: 
    
   If the Call Agent had specified, that it expected at most one 
   notification in response to the notification request command, then 
   the gateway SHALL simply keep on accumulating events in the 
   quarantine buffer until it receives the next notification request 
   command. 
    
   If, however, the gateway is authorized to send multiple successive 
   Notify commands, it will proceed as follows. When the gateway exits 
   the "notification state", it resets the list of observed events and 
   the "current dial string" of the endpoint to a null value and starts 
   processing the list of quarantined events, using the already 
   received list of requested events and digit map. When processing 
   these events, the gateway may encounter an event which triggers a 
   Notify command to be sent. If that is the case, the gateway can 
   adopt one of the two following behaviors: 
    
   *  it can immediately transmit a Notify command that will report all 
      events that were accumulated in the list of observed events until 
      the triggering event, included, leaving the unprocessed events in 
      the quarantine buffer, 
    
   *  or it can attempt to empty the quarantine buffer and transmit a 
      single Notify command reporting several sets of events (in a 
      single list of observed events) and possibly several dial 
      strings. The "current dial string" is reset to a null value after 
      each triggering event. The events that follow the last triggering 
      event are left in the quarantine buffer. 
    
   If the gateway transmits a Notify command, the endpoint will reenter 
   and remain in the "notification state" until the acknowledgement is 
   received (as described above). If the gateway does not find a 
   quarantined event that triggers a Notify command, it places the 
   endpoint in a normal state. Events are then processed as they come, 
   in exactly the same way as if a Notification Request command had 
   just been received. 
    
   A gateway may receive at any time a new Notification Request command 
   for the endpoint, including the case where the endpoint is 
   disconnected. Activating an embedded Notification Request is here 
   viewed as receiving a new Notification Request as well, except that 
   the current list of ObservedEvents remains unmodified rather than 
   being processed again. When a new notification request is received 
   in the notification state, the gateway SHALL ensure that the pending 
   Notify is received by the Call Agent prior to a new Notify (note 
                                                                        
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   that a Notify that was lost due to being disconnected, is no longer 
   considered pending). It does so by using the "piggybacking" 
   functionality of the protocol. The messages will then be sent in a 
   single packet to the current "notified entity". The steps involved 
   are the following: 
    
   a) the gateway sends a response to the new notification request.  
    
   b) the endpoint is then taken out of the "notification state" 
      without waiting for the acknowledgement of the pending Notify 
      command. 
    
   c) a copy of the unacknowledged Notify command is kept until an 
      acknowledgement is received. If a timer elapses, the Notify will 
      be retransmitted. 
    
   d) If the gateway has to transmit a new Notify before the previous 
      Notify(s) is acknowledged, it constructs a packet that piggybacks 
      a repetition of the old Notify(s) and the new Notify (ordered by 
      age with the oldest first). This datagram will be sent to the 
      current "notified entity". 
    
   f) Gateways that cannot piggyback several messages in the same 
      datagram and hence guarantee in-order delivery of two (or more) 
      Notify's SHALL leave the endpoint in the "notification" state as 
      long as the last Notify is not acknowledged. 
    
   The procedure is illustrated by the following diagram: 
    
























                                                                        
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    +-------------------+ 
    | Processing Events |<--------------------------------------+ 
    +-------------------+                                       | 
             |                                                  | 
     Need to send NTFY                                          | 
             |                                                  | 
             v                                                  | 
    +-------------------+                                       | 
    | Outstanding NTFY  |---- No -------+                       | 
    |                   |               |                       | 
    +-------------------+               v                       | 
             |                    +-----------+                 | 
            Yes                   | Send NTFY |                 | 
             |                    +-----------+                 | 
             v                          |                       | 
    +--------------------+              v                       | 
    | Piggyback new NTFY |     +--------------------+           | 
    | w. old outstanding |---->| Notification State |           | 
    | NTFY(s)            |     +--------------------+           | 
    +--------------------+       |               |              | 
                             new RQNT        NTFY response      | 
                             received        received           | 
                                 |               |              | 
                                 |               v              | 
                                 |        +-------------+       | 
                                 |        | Step mode ? |- No ->+ 
                                 |        +-------------+       ^ 
                                 |               |              | 
                                 |              Yes             | 
                                 |               |              | 
                                 |               v              | 
                                 |       +---------------+      | 
                                 |       | Wait for RQNT |      | 
                                 |       +---------------+      | 
                                 |               |              | 
                                 |         RQNT received        | 
                                 |               |              | 
                                 |               v              | 
                                 |       +---------------+      | 
                                 +------>| Apply RQNT and|----->+ 
                                         | send response | 
                                         +---------------+ 
    
    
    
   Gateways may also attempt to deliver the pending Notify prior to a 
   successful response to the new NotificationRequest by using the 
   "piggybacking" functionality of the protocol. This was in fact 
   required behavior in RFC 2705, however there are several 
   complications in doing this, and the benefits are questionable. In 
   particular, the RFC 2705 mechanism did not guarantee in-order 
   delivery of Notify's and responses to NotificationRequests in 
   general, and hence Call Agents had to handle out-of-order delivery 
                                                                        
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   of these messages anyway. The change to optional status is thus 
   backwards compatible while greatly reducing complexity.  
   
   After receiving the Notification Request command, the requested 
   events list and digit map (if a new one was provided) are replaced 
   by the newly received parameters, and the current dial string is 
   reset to a null value. Furthermore, when the Notification Request 
   was received in the "notification state", the list of observed 
   events is reset to a null value. The subsequent behavior is 
   conditioned by the value of the QuarantineHandling parameter. The 
   parameter may specify that quarantined events (and observed events 
   which in this case is now an empty list), should be discarded, in 
   which case they will be. If the parameter specifies that the 
   quarantined (and observed) events are to be processed, the gateway 
   will start processing the list of quarantined (and observed) events, 
   using the newly received list of requested events and digit map (if 
   provided). When processing these events, the gateway may encounter 
   an event which requires a Notify command to be sent. If that is the 
   case, the gateway will immediately transmit a Notify command that 
   will report all events that were accumulated in the list of observed 
   events until the triggering event, included leaving the unprocessed 
   events in the quarantine buffer, and will enter the "notification 
   state". 
    
   A new notification request may be received while the gateway has 
   accumulated events according to the previous notification request, 
   but has not yet detected a notification-triggering events, i.e. the 
   endpoint is not in the "notification state". The handling of not-
   yet-notified events is determined, as with the 
   quarantined events, by the quarantine handling parameter: 
    
   *  If the quarantine-handling parameter specifies that quarantined 
      events shall be ignored, the observed events list is simply 
      reset. 
    
   *  If the quarantine-handling parameter specifies that quarantined 
      events shall be processed, the observed event list is transferred 
      to the quarantined event list. The observed event list is then 
      reset, and the quarantined event list is processed. 
    
   Call Agents controlling endpoints in lockstep mode SHOULD provide 
   the response to a successful Notify message and the new 
   NotificationRequest in the same datagram using the piggybacking 
   mechanism. 
    
4.4.2 Explicit Detection 
    
   A key element of the state of several endpoints is the position of 
   the hook. A race condition may occur when the user decides to go 
   off-hook before the Call Agent has the time to ask the gateway to 
   notify an off-hook event (the "glare" condition well known in 
   telephony), or if the user goes on-hook before the Call Agent has 
   the time to request the event's notification. 
                                                                        
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   To avoid this race condition, the gateway MUST check the condition 
   of the endpoint before acknowledging a NotificationRequest. It MUST 
   return an error: 
    
   1. If the gateway is requested to notify an "off-hook" transition 
      while the phone is already off-hook, (error code 401 - phone off 
      hook) 
    
   2. If the gateway is requested to notify an "on-hook" or "flash 
      hook" condition while the phone is already on-hook (error code 
      402 - phone on hook). 
    
   Additionally, individual signal definitions can specify that a 
   signal will only operate under certain conditions, e.g., ringing may 
   only be possible if the phone is already off-hook. If such 
   prerequisites exist for a given signal, the gateway MUST return the 
   error specified in the signal definition if the prerequisite is not 
   met. 
    
   It should be noted, that the condition check is performed at the 
   time the notification request is received, whereas the actual event 
   that caused the current condition may have either been reported, or 
   ignored earlier, or it may currently be quarantined. 
    
   The other state variables of the gateway, such as the list of 
   RequestedEvents or list of requested signals, are entirely replaced 
   after each successful NotificationRequest, which prevents any long 
   term discrepancy between the Call Agent and the gateway. 
    
   When a NotificationRequest is unsuccessful, whether it is included 
   in a connection-handling command or not, the gateway MUST simply 
   continue as if the command had never been received. As all other 
   transactions, the NotificationRequest MUST operate as an atomic 
   transaction, thus any changes initiated as a result of the command 
   MUST be reverted. 
    
   Another race condition may occur when a Notify is issued shortly 
   before the reception by the gateway of a NotificationRequest. The 
   RequestIdentifier is used to correlate Notify commands with 
   NotificationRequest commands thereby enabling the Call Agent to  
   determine if the Notify command was generated before or after the  
   gateway received the new NotificationRequest. This is especially 
   important to avoid deadlocks in "step" mode. 
    
4.4.3 Transactional Semantics 
    
   As the potential transaction completion times increase, e.g. due to 
   external resource reservations, a careful definition of the 
   transactional semantics becomes increasingly important. In 
   particular the issue of race conditions, e.g. as it relates to hook-
   state, must be defined carefully. 
    
                                                                        
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   An important point to consider is, that the status of a pre-
   condition (e.g. hook-state) may in fact change between the time a 
   transaction starts and the time it either completes successfully 
   (transaction commit) or fails. In general, we can say that the 
   successful execution of a transaction depends on one or more pre-
   conditions where the status of one or more of the pre-conditions may 
   change dynamically between the transaction start and transaction 
   commit.  
    
   The simplest semantics for this is simply to require that all pre-
   conditions be met from the time the transaction is initiated until 
   the transaction commits. If any pre-condition is not met before the 
   completion of the transaction, the transaction will also fail.  
    
   As an example, consider a transaction that includes a request for 
   the "off-hook" event. When the transaction is initiated the phone is 
   "on-hook" and this pre-condition is therefore met. If the hook-state 
   changes to "off-hook" before the transaction completes, the pre-
   condition is no longer met, and the transaction therefore 
   immediately fails.  
    
   Finally, we need to consider the point in time when a new 
   transaction takes effect and endpoint processing according to an old 
   transaction stops. For example, assume that transaction T1 has been 
   executed successfully and event processing is currently being done 
   according to transaction T1. Now we receive a new transaction T2 
   specifying new event processing (for example a CreateConnection with 
   an encapsulated NotificationRequest). Since we don't know whether T2 
   will complete successfully or not, we cannot start processing events 
   according to T2 until the outcome of T2 is known. While we could 
   suspend all event processing until the outcome of T2 is known, this 
   would make for a less responsive system and hence SHOULD NOT be 
   done. Instead, when a new transaction Ty is received and Ty modifies 
   processing according to an old transaction Tx, processing according 
   to Tx SHOULD remain active for as long as possible, until a 
   successful outcome of Ty is known to occur. If Ty fails, then 
   processing according to Tx will of course continue as usual. Any 
   changes incurred by Ty logically takes effect when Ty commits. Thus, 
   if the endpoint was in the notification state when Ty commits, and 
   Ty contained a NotificationRequest, the endpoint will be taken out 
   of the notification state when Ty commits. Note that this is 
   independent of whether the endpoint was in the notification state 
   when Ty was initiated. For example, a Notify could be generated  due 
   to processing according to Tx between the start and commit of Ty. If 
   the commit of Ty leads to the endpoint entering the notification 
   state, a new NotificationRequest (Tz) is needed to exit the 
   notification state. This follows from the fact that transaction 
   execution respects causal order.  
    
   Another related issue is the use of wildcards, especially the "all 
   of" wildcard, which may match more than one endpoint. When a command 
   is requested, and the endpoint identifier matches more than one 
   endpoint, transactional semantics still apply. Thus, the command 
                                                                        
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   MUST either succeed for all the endpoints, or it MUST fail for all 
   of them. A single response is consequently always issued. 
    
4.4.4 Ordering of Commands, and Treatment of Misorder 
    
   MGCP does not mandate that the underlying transport protocol 
   guarantees in-order delivery of commands to a gateway or an 
   endpoint. This property tends to maximize the timeliness of actions, 
   but it has a few drawbacks. For example: 
    
   *  Notify commands may be delayed and arrive at the Call Agent after 
      the transmission of a new Notification Request command, 
    
   *  If a new NotificationRequest is transmitted before a previous one 
      is acknowledged, there is no guarantee that the previous one will 
      not be received and executed after the new one. 
    
   Call Agents that want to guarantee consistent operation of the 
   endpoints can use the following rules: 
    
   1) When a gateway handles several endpoints, commands pertaining to 
      the different endpoints can be sent in parallel, for example 
      following a model where each endpoint is controlled by its own 
      process or its own thread. 
    
   2) When several connections are created on the same endpoint, 
      commands pertaining to different connections can be sent in 
      parallel. 
    
   3) On a given connection, there should normally be only one 
      outstanding command (create or modify). However, a 
      DeleteConnection command can be issued at any time. In 
      consequence, a gateway may sometimes receive a ModifyConnection 
      command that applies to a previously deleted connection. Such 
      commands will fail, and an error code MUST be returned (error 
      code 515 - incorrect connection-id, is RECOMMENDED). 
    
   4) On a given endpoint, there should normally be only one 
      outstanding NotificationRequest command at any time. The 
      RequestId parameter MUST be used to correlate Notify commands 
      with the triggering notification request. 
    
   5) In some cases, an implicitly or explicitly wildcarded 
      DeleteConnection command that applies to a group of endpoints can 
      step in front of a pending CreateConnection command. The Call 
      Agent should individually delete all connections whose completion 
      was pending at the time of the global DeleteConnection command. 
      Also, new CreateConnection commands for endpoints named by the 
      wild-carding SHOULD NOT be sent until the wild-carded 
      DeleteConnection command is acknowledged. 
    
   6) When commands are embedded within each other, sequencing 
      requirements for all commands must be adhered to. For example a 
                                                                        
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      Create Connection command with a Notification Request in it must 
      adhere to the sequencing requirements associated with both 
      CreateConnection and NotificationRequest at the same time. 
    
   7) AuditEndpoint and AuditConnection are not subject to any 
      sequencing requirements. 
    
   8) RestartInProgress MUST always be the first command sent by an 
      endpoint as defined by the restart procedure. Any other command 
      or response, except for responses to auditing, MUST be delivered 
      after this RestartInProgress command (piggybacking allowed). 
    
   9) When multiple messages are piggybacked in a single packet, the 
      messages are always processed in order. 
    
   10) On a given endpoint, there should normally be only one 
      outstanding EndpointConfiguration command at any time. 
    
   Gateways MUST NOT make any assumptions as to whether Call Agents 
   follow these rules or not. Consequently gateways MUST always respond 
   to commands, regardless of whether they adhere to the above rules or 
   not. To ensure consistent operation, gateways SHOULD behave as 
   specified below when one or more of the above rules are not 
   followed: 
    
   * Where a single outstanding command is expected (ModifyConnection, 
     NotificationRequest, and EndpointConfiguration), but the same 
     command is received in a new transaction before the old finishes 
     executing, the gateway SHOULD fail the previous command. This 
     includes the case where one or more of the commands were 
     encapsulated. The use of error code 407 (transaction aborted) is 
     RECOMMENDED. 
      
   * If a ModifyConnection command is received for a pending 
     CreateConnection command, the ModifyConnection command SHOULD 
     simply be rejected. The use of error code 400 (transient error) is 
     RECOMMENDED. Note that this situation constitutes a Call Agent 
     programming error. 
      
   * If a DeleteConnection command is received for a pending 
     CreateConnection or ModifyConnection command, the pending command 
     MUST be aborted. The use of error code 407 (transaction aborted) 
     is RECOMMENDED.  
      
   Note, that where reception of a new command leads to aborting an old 
   command, the old command SHOULD be aborted regardless of whether the 
   new command succeeds or not. For example, if a ModifyConnection 
   command is aborted by a DeleteConnection command which itself fails 
   due to an encapsulated NotificationRequest, the ModifyConnection 
   command is still aborted. 
    


                                                                        
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4.4.5 Endpoint Service States 
    
   As described earlier, endpoints configured for operation may be 
   either in-service or out-of-service. The actual service-state of the 
   endpoint is reflected by the combination of the RestartMethod and 
   RestartDelay parameters, which are sent with RestartInProgress 
   commands (Section 2.3.12) and furthermore may be audited in 
   AuditEndpoint commands (Section 2.3.10). 
    
   The service-state of an endpoint affects how it processes a command. 
   An endpoint in-service MUST process any command received, whereas an 
   endpoint that is out-of-service MUST reject non-auditing commands, 
   but SHOULD process auditing commands if possible. For backwards 
   compatibility, auditing commands for an out-of-service endpoint may 
   alternatively be rejected as well. Any command rejected due to an 
   endpoint being out-of-service SHOULD generate error code 501 
   (endpoint not ready/out-of-service). 
    
   Note that (per Section 2.1.2), unless otherwise specified for a 
   command, endpoint names containing the "any of" wildcard only refer 
   to endpoints in-service, whereas endpoint names containing the "all 
   of" wildcard refer to all endpoints, regardless of service state.  
    
   The above relationships are illustrated in the table below which 
   shows the current service-states and gateway processing of commands 
   as a function of the RestartInProgress command sent and the response 
   (if any) received to it. The last column also lists (in parentheses) 
   the RestartMethod to be returned if audited: 
    
























                                                                        
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    ------------------------------------------------------------------ 
   | Restart-  | Restart- |    2xx    | Service- |   Response to      | 
   |    Method |    Delay | received ?|    State |   new command      |  
   |------------------------------------------------------------------| 
   | graceful  |   zero   |   Yes/No  |   In     | non-audit: 2xx     | 
   |           |          |           |          | audit:     2xx     | 
   |           |          |           |          |        (graceful)  | 
   |-----------+----------+-----------+----------+--------------------| 
   | graceful  | non-zero |   Yes/No  |   In*    | non-audit: 2xx     | 
   |           |          |           |          | audit:     2xx     | 
   |           |          |           |          |        (graceful)  | 
   |-----------+----------+-----------+----------+--------------------| 
   | forced    |   N/A    |   Yes/No  |   Out    | non-audit: 501     | 
   |           |          |           |          | audit:     2xx     | 
   |           |          |           |          |         (forced)   | 
   |-----------+----------+-----------+----------+--------------------| 
   | restart   |   zero   |    No     |   In     | non-audit: 2xx,405*| 
   |           |          |           |          | audit:     2xx     | 
   |           |          |           |          |         (restart)  | 
   |-----------+----------+-----------+----------+--------------------| 
   | restart   |   zero   |    Yes    |   In     | non-audit: 2xx     | 
   |           |          |           |          | audit:     2xx     | 
   |           |          |           |          |         (restart)  | 
   |-----------+----------+-----------+----------+--------------------| 
   | restart   | non-zero |    No     |   Out*   | non-audit: 501*    | 
   |           |          |           |          | audit:     2xx     | 
   |           |          |           |          |         (restart)  | 
   |-----------+----------+-----------+----------+--------------------| 
   | restart   | non-zero |    Yes    |   Out*   | non-audit: 501*    | 
   |           |          |           |          | audit:     2xx     | 
   |           |          |           |          |         (restart)  | 
   |-----------+----------+-----------+----------+--------------------| 
   | discon-   |   zero/  |    No     |   In     | non-audit: 2xx,    | 
   |    nected | non-zero |           |          | audit:     2xx     | 
   |           |          |           |          |      (disconnected)| 
   |-----------+----------+-----------+----------+--------------------| 
   | discon-   |   zero/  |    Yes    |   In     | non-audit: 2xx     | 
   |    nected | non-zero |           |          | audit:     2xx     | 
   |           |          |           |          |         (restart)  | 
   |-----------+----------+-----------+----------+--------------------| 
   | cancel-   |   N/A    |   Yes/No  |   In     | non-audit: 2xx     | 
   |  graceful |          |           |          | audit:     2xx     | 
   |           |          |           |          |         (restart)  | 
    ------------------------------------------------------------------ 
   Notes (*):   
    
   * The three service-states marked with "*" will change after the 
     expiration of the RestartDelay at which time an updated 
     RestartInProgress command SHOULD be sent. 
      
   * If the endpoint returns 2xx when the restart procedure has not yet 
     completed, then in-order delivery MUST still be satisfied, i.e. 
     piggy-backing is to be used. If instead, the command is not 
                                                                        
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     processed, 405 SHOULD be returned.  
      
   * Following a "restart" RestartInProgress with a non-zero 
     RestartDelay, error code 501 is only returned until the endpoint 
     goes in-service, i.e. until the expiration of the RestartDelay.  
    
4.4.6 Fighting the Restart Avalanche 
    
   Let's suppose that a large number of gateways are powered on 
   simultaneously. If they were to all initiate a RestartInProgress 
   transaction, the Call Agent would very likely be swamped, leading to 
   message losses and network congestion during the critical period of 
   service restoration. In order to prevent such avalanches, the 
   following behavior is REQUIRED: 
    
   1) When a gateway is powered on, it MUST initiate a restart timer to 
      a random value, uniformly distributed between 0 and a maximum 
      waiting delay (MWD). Care should be taken to avoid synchronicity 
      of the random number generation between multiple gateways that 
      would use the same algorithm. 
    
   2) The gateway MUST then wait for either the end of this timer, the 
      reception of a command from the Call Agent, or the detection of a 
      local user activity, such as for example an off-hook transition 
      on a residential gateway. 
    
   3) When the timer elapses, when a command is received, or when an 
      activity is detected, the gateway MUST initiate the restart 
      procedure. 
    
   The restart procedure simply requires the endpoint to guarantee that 
   the first  
    
   *  non-audit command, or  
    
   *  non-restart response (i.e. error codes other than 405, 501, and 
      520) to a non-audit command 
    
   that the Call Agent sees from this endpoint is a "restart" 
   RestartInProgress command. The endpoint is free to take full 
   advantage of piggybacking to achieve this. Endpoints that are 
   considered in-service will have a RestartMethod of "restart", 
   whereas endpoints considered out-of-service will have a 
   RestartMethod of "forced" (also see Section 4.4.5). Commands 
   rejected due to an endpoint not yet having completed the restart 
   procedure SHOULD use error code 405 (endpoint "restarting").  
    
   The restart procedure is complete once a success response has been 
   received. If an error response is received, the subsequent behavior 
   depends on the error code in question: 
    
   *  If the error code indicates a transient error (4xx), then the 
      restart procedure MUST be initiated again (as a new transaction). 
                                                                        
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   *  If the error code is 521, then the endpoint is redirected, and 
      the restart procedure MUST be initiated again (as a new 
      transaction). The 521 response MUST have included a 
      NotifiedEntity which then is the "notified entity" towards which 
      the restart is initiated. If it did not include a NotifiedEntity, 
      the response is treated as any other permanent error (see below). 
    
   *  If the error is any other permanent error (5xx), and the endpoint 
      is not able to rectify the error, then the endpoint no longer 
      initiates the restart procedure on its own (until rebooted) 
      unless otherwise specified. If a command is received for the 
      endpoint, the endpoint MUST initiate the restart procedure again.  
    
   Note that if the RestartInProgress is piggybacked with the response 
   (R) to a command received while restarting, then retransmission of 
   the RestartInProgress does not require piggybacking of the response 
   R. However, while the endpoint is restarting, a resend of the 
   response R does require the RestartInProgress to be piggybacked to 
   ensure in-order delivery of the two.  
    
   Should the gateway enter the "disconnected" state while carrying out 
   the restart procedure, the disconnected procedure specified in 
   Section 4.4.7 MUST be carried out, except that a "restart" rather 
   than "disconnected" message is sent during the procedure. 
    
   Each endpoint in a gateway will have a provisionable Call Agent, 
   i.e., "notified entity", to direct the initial restart message 
   towards. When the collection of endpoints in a gateway is managed by 
   more than one Call Agent, the above procedure MUST be performed for 
   each collection of endpoints managed by a given Call Agent. The 
   gateway MUST take full advantage of wild-carding to minimize the 
   number of RestartInProgress messages generated when multiple 
   endpoints in a gateway restart and the endpoints are managed by the 
   same Call Agent. Note that during startup, it is possible for 
   endpoints to start out as being out-of-service, and then become in-
   service as part of the gateway initialization procedure. A gateway 
   may thus choose to send first a "forced" RestartInProgress for all 
   its endpoints, and subsequently a "restart" RestartInProgress for 
   the endpoints that come in-service. Alternatively, the gateway may 
   simply send "restart" RestartInProgress for only those endpoints 
   that are in-service, and "forced" RestartInProgress for the specific 
   endpoints that are out-of-service. Wild-carding MUST still be used 
   to minimize the number of messages sent though. 
    
   The value of MWD is a configuration parameter that depends on the 
   type of the gateway. The following reasoning can be used to 
   determine the value of this delay on residential gateways. 
    
   Call agents are typically dimensioned to handle the peak hour 
   traffic load, during which, in average, 10% of the lines will be 
   busy, placing calls whose average duration is typically 3 minutes. 
   The processing of a call typically involves 5 to 6 MGCP transactions 
                                                                        
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   between each endpoint and the Call Agent. This simple calculation 
   shows that the Call Agent is expected to handle 5 to 6 transactions 
   for each endpoint, every 30 minutes on average, or, to put it 
   otherwise, about one transaction per endpoint every 5 to 6 minutes 
   on average. This suggest that a reasonable value of MWD for a 
   residential gateway would be 10 to 12 minutes. In the absence of 
   explicit configuration, residential gateways should adopt a value of 
   600 seconds for MWD. 
    
   The same reasoning suggests that the value of MWD should be much 
   shorter for trunking gateways or for business gateways, because they 
   handle a large number of endpoints, and also because the usage rate 
   of these endpoints is much higher than 10% during the peak busy 
   hour, a typical value being 60%. These endpoints, during the peak 
   hour, are thus expected to contribute about one transaction per 
   minute to the Call Agent load. A reasonable algorithm is to make the 
   value of MWD per "trunk" endpoint six times shorter than the MWD per 
   residential gateway, and also inversely proportional to the number 
   of endpoints that are being restarted. for example MWD should be set 
   to 2.5 seconds for a gateway that handles a T1 line, or to 60 
   milliseconds for a gateway that handles a T3 line. 
    
4.4.7 Disconnected Endpoints 
    
   In addition to the restart procedure, gateways also have a 
   "disconnected" procedure, which MUST be initiated when an endpoint 
   becomes "disconnected" as described in Section 4.3. It should here 
   be noted, that endpoints can only become disconnected when they 
   attempt to communicate with the Call Agent. The following steps MUST 
   be followed by an endpoint that becomes "disconnected": 
    
   1. A "disconnected" timer is initialized to a random value, 
      uniformly distributed between 1 and a provisionable 
      "disconnected" initial waiting delay (Tdinit), e.g., 15 seconds. 
      Care MUST be taken to avoid synchronicity of the random number 
      generation between multiple gateways and endpoints that would use 
      the same algorithm. 
    
   2. The gateway then waits for either the end of this timer, the 
      reception of a command for the endpoint from the Call Agent, or 
      the detection of a local user activity for the endpoint, such as 
      for example an off-hook transition. 
    
   3. When the "disconnected" timer elapses for the endpoint, when a 
      command is received for the endpoint, or when local user activity 
      is detected for the endpoint, the gateway initiates the 
      "disconnected" procedure for the endpoint - if a disconnected 
      procedure was already in progress for the endpoint, it is simply 
      replaced by the new one. Furthermore, in the case of local user 
      activity, a provisionable "disconnected" minimum waiting delay 
      (Tdmin) MUST have elapsed since the endpoint became disconnected 
      or the last time it ended the "disconnected" procedure in order 
      to limit the rate at which the procedure is performed. If Tdmin 
                                                                        
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      has not passed, the endpoint simply proceeds to step 2 again, 
      without affecting any disconnected procedure already in progress. 
    
   4. If the "disconnected" procedure still left the endpoint 
      disconnected, the "disconnected" timer is then doubled, subject 
      to a provisionable "disconnected" maximum waiting delay (Tdmax), 
      e.g., 600 seconds, and the gateway proceeds with step 2 again 
      (using a new transaction-id). 
    
   The "disconnected" procedure is similar to the restart procedure in 
   that it simply states that the endpoint MUST send a 
   RestartInProgress command to the Call Agent informing it that the 
   endpoint was disconnected. Furthermore, the endpoint MUST guarantee 
   that the first non-audit message (non-audit command or response to 
   non-audit command) that the Call Agent sees from this endpoint MUST 
   inform the Call Agent that the endpoint is disconnected (unless the 
   endpoint goes out-of-service). When a command (C) is received, this 
   is achieved by sending a piggy-backed datagram with a "disconnected" 
   RestartInProgress command and the response to command C to the 
   source address of command C as opposed to the current "notified 
   entity". This piggy-backed RestartInProgress is not automatically 
   retransmitted by the endpoint but simply relies on fate-sharing with 
   the piggy-backed response to guarantee the in-order delivery 
   requirement. The Call Agent still sends a response to the piggy-
   backed RestartInProgress, however, as usual, the response may be 
   lost. In addition to the piggy-backed RestartInProgress command, a 
   new "disconnected" procedure is triggered by the command received. 
   This will lead to a non piggy-backed copy (i.e. same transaction) of 
   the "disconnected" RestartInProgress command being sent reliably to 
   the current "notified entity".  
    
   When the Call Agent learns that the endpoint is disconnected, the 
   Call Agent may then for instance decide to audit the endpoint, or 
   simply clear all connections for the endpoint. Note that each such 
   "disconnected" procedure will result in a new RestartInProgress 
   command, which will be subject to the normal retransmission 
   procedures specified in Section 4.3. At the end of the procedure, 
   the endpoint may thus still be "disconnected". Should the endpoint 
   go out-of-service while being disconnected, it SHOULD send a 
   "forced" RestartInProgress message as described in Section 2.3.12. 
    
   The disconnected procedure is complete once a success response has 
   been received. Error responses are handled similarly to the restart 
   procedure (Section 4.4.6). If the "disconnected" procedure is to be 
   initiated again following an error response, the rate-limiting timer 
   considerations specified above still apply.  
    
   Note, that if the RestartInProgress is piggybacked with the response 
   (R) to a command received while being disconnected, then 
   retransmission of this particular RestartInProgress does not require 
   piggybacking of the response R. However, while the endpoint is 
   disconnected, resending the response R does require the 

                                                                        
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   RestartInProgress to be piggybacked with the response to ensure the 
   in-order delivery of the two.  
    
   If a set of disconnected endpoints have the same "notified entity", 
   and the set of endpoints can be named with a wildcard, the gateway 
   MAY replace the individual disconnected procedures with a suitably 
   wildcarded disconnected procedure instead. In that case, the Restart 
   Delay for the wildcarded "disconnected" RestartInProgress command 
   SHALL be the Restart Delay corresponding to the oldest disconnected 
   procedure replaced. Note that if only a subset of these endpoints 
   subsequently have their "notified entity" changed and/or are no 
   longer disconnected, then that wildcarded disconnected procedure can 
   no longer be used. The remaining individual disconnected procedures 
   MUST then be resumed again. 
    
   A disconnected endpoint may wish to send a command (besides 
   RestartInProgress) while it is disconnected. Doing so will only 
   succeed once the Call Agent is reachable again, which raises the 
   question of what to do with such a command meanwhile. At one 
   extreme, the endpoint could drop the command right away, however 
   that would not work very well when the Call Agent was in fact 
   available, but the endpoint had not yet completed the "disconnected" 
   procedure (consider for example the case where a NotificationRequest 
   was just received which immediately resulted in a Notify being 
   generated). To prevent such scenarios, disconnected endpoints SHALL 
   NOT blindly drop new commands to be sent for a period of T-MAX 
   seconds after they receive a non-audit command.  
    
   One way of satisfying this requirement is to employ a temporary 
   buffering of commands to be sent, however in doing so, the endpoint 
   MUST ensure, that it: 
    
   *  does not build up a long queue of commands to be sent, 
    
   *  does not swamp the Call Agent by rapidly sending too many 
      commands once it is connected again. 
       
   Buffering commands for T-MAX seconds and, once the endpoint is 
   connected again, limiting the rate at which buffered commands are 
   sent to one outstanding command per endpoint is considered 
   acceptable (see also Section 4.4.8, especially if using wildcards). 
   If the endpoint is not connected within T-MAX seconds, but a 
   "disconnected" procedure is initiated within T-MAX seconds, the 
   endpoint MAY piggyback the buffered command(s) with that 
   RestartInProgress. Note, that once a command has been sent, 
   regardless of whether it was buffered initially, or piggybacked 
   earlier, retransmission of that command MUST cease T-MAX seconds 
   after the initial send as described in Section 4.3.  
    
   This specification purposely does not specify any additional 
   behavior for a disconnected endpoint. Vendors MAY for instance 
   choose to provide silence, play reorder tone, or even enable a 
   downloaded wav file to be played. 
                                                                        
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   The default value for Tdinit is 15 seconds, the default value for 
   Tdmin, is 15 seconds, and the default value for Tdmax is 600 
   seconds. 
    
4.4.8 Load Control in General 
 
   The previous sections have described several MGCP mechanisms to deal 
   with congestion and overload, namely: 
    
   *  the UDP retransmission strategy which adapts to network and call 
      agent congestion on a per endpoint basis, 
    
   *  the guidelines on the ordering of commands which limit the number 
      of commands issued in parallel,  
    
   *  the restart procedure which prevents flooding in case of a 
      restart avalanche, and 
    
   *  the disconnected procedure which prevents flooding in case of a 
      large number of disconnected endpoints.  
    
   It is however still possible for a given set of endpoints, either on 
   the same or different gateways, to issue one or more commands at a 
   given point in time. Although it can be argued, that Call Agents 
   should be sized to handle one message per served endpoint at any 
   given point in time, this may not always be the case in practice. 
   Similarly, gateways may not be able to handle a message for all of 
   its endpoints at any given point in time. In general, such issues 
   can be dealt with through the use of a credit-based mechanism, or by 
   monitoring and automatically adapting to the observed behavior. We 
   opt for the latter approach as follows.  
    
   Conceptually, we assume that Call Agents and gateways maintain a 
   queue of incoming transactions to be executed. Associated with this 
   transaction queue is a high-water and a low-water mark. Once the 
   queue length reaches the high-water mark, the entity SHOULD start 
   issuing 101 provisional responses (transaction queued) until the 
   queue length drops to the low-water mark. This applies to new 
   transactions as well as to retransmissions. If the entity is unable 
   to process any new transactions at this time, it SHOULD return error 
   code 409 (processing overload). 
    
   Furthermore, gateways SHOULD adjust the sending rate of new commands 
   to a given Call Agent by monitoring the observed response times from 
   that Call Agent to a *set* of endpoints. If the observed smoothed 
   average response time suddenly rises significantly over some 
   threshold, or the gateway receives a 101 (transaction queued) or 409 
   (overload) response, the gateway SHOULD adjust the sending rate of 
   new commands to that Call Agent accordingly. The details of the 
   smoothing average algorithm, the rate adjustments, and the 
   thresholds involved are for further study, however they MUST be 
   configurable. 
                                                                        
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   Similarly, Call Agents SHOULD adjust the sending rate of new 
   transactions to a given gateway by monitoring the observed response 
   times from that gateway for a *set* of endpoints. If the observed 
   smoothed average response time suddenly rises significantly over 
   some threshold, or the Call Agent receives a 101 (transaction 
   queued) or 409 (overloaded), the Call Agent SHOULD adjust the 
   sending rate of new commands to that gateway accordingly. The 
   details of the smoothing average algorithm, the rate adjustments, 
   and the thresholds involved are for further study, however they MUST 
   be configurable. 










































                                                                        
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5. Security Requirements 
    
   Any entity can send a command to an MGCP endpoint. If unauthorized 
   entities could use the MGCP, they would be able to set-up 
   unauthorized calls, or to interfere with authorized calls. We expect 
   that MGCP messages will always be carried over secure Internet 
   connections, as defined in the IP security architecture as defined 
   in RFC 2401, using either the IP Authentication Header, defined in 
   RFC 2402, or the IP Encapsulating Security Payload, defined in RFC 
   2406. The complete MGCP protocol stack would thus include the 
   following layers: 
    
                ------------------------------- 
               |              MGCP             | 
               |-------------------------------| 
               |              UDP              | 
               |-------------------------------| 
               |          IP security          | 
               | (authentication or encryption)| 
               |-------------------------------| 
               |              IP               | 
               |-------------------------------| 
               |       transmission media      | 
                ------------------------------- 
    
   Adequate protection of the connections will be achieved if the 
   gateways and the Call Agents only accept messages for which IP 
   security provided an authentication service. An encryption service 
   will provide additional protection against eavesdropping, thus 
   forbidding third parties from monitoring the connections set up by a 
   given endpoint 
    
   The encryption service will also be requested if the session 
   descriptions are used to carry session keys, as defined in SDP. 
    
   These procedures do not necessarily protect against denial of 
   service attacks by misbehaving gateways or misbehaving Call Agents. 
   However, they will provide an identification of these misbehaving 
   entities, which should then be deprived of their authorization 
   through maintenance procedures. 
    
5.1 Protection of Media Connections 
    
   MGCP allows Call Agent to provide gateways with "session keys" that 
   can be used to encrypt the audio messages, protecting against 
   eavesdropping. 
    
   A specific problem of packet networks is "uncontrolled barge-in". 
   This attack can be performed by directing media packets to the IP 
   address and UDP port used by a connection. If no protection is 
   implemented, the packets will be decompressed and the signals will 
   be played on the "line side". 
    
                                                                        
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   A basic protection against this attack is to only accept packets 
   from known sources, however this tends to conflict with RTP 
   principles. This also has two inconveniences: it slows down 
   connection establishment and it can be fooled by source spoofing: 
    
   *  To enable the address-based protection, the Call Agent must 
      obtain the source address of the egress gateway and pass it to 
      the ingress gateway. This requires at least one network round 
      trip, and leaves us with a dilemma: either allow the call to 
      proceed without waiting for the round trip to complete, and risk 
      for example "clipping" a remote announcement, or wait for the 
      full round trip and settle for slower call-set-up procedures. 
    
   *  Source spoofing is only effective if the attacker can obtain 
      valid pairs of source and destination addresses and ports, for 
      example by listening to a fraction of the traffic. To fight 
      source spoofing, one could try to control all access points to 
      the network. But this is in practice very hard to achieve. 
    
   An alternative to checking the source address is to encrypt and 
   authenticate the packets, using a secret key that is conveyed during 
   the call set-up procedure. This will not slow down the call set-up, 
   and provides strong protection against address spoofing. 
    





























                                                                        
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6. Packages 
    
   As described in Section 2.1.6, packages are the preferred way of 
   extending MGCP. In this section we describe the requirements 
   associated with defining a package. 
    
   A package MUST have a unique package name defined. The package name 
   MUST be registered with the IANA, unless it starts with the 
   characters "x-" or "x+" which are reserved for experimental 
   packages. Please refer to Appendix C for IANA considerations. 
    
   A package MUST also have a version defined which is simply a non-
   negative integer. The default and initial version of a package is 
   zero, the next version is one, etc. New package versions MUST be 
   completely backwards compatible, i.e. a new version of a package 
   MUST NOT redefine or remove any of the extensions provided in an 
   earlier version of the package. If such a need arises, a new package 
   name MUST be used instead. 
    
   Packages containing signals of type time-out MAY indicate if the 
   "to" parameter is supported for all the time-out signals in the 
   package as well as the default rounding rules associated with these 
   (see Section 3.2.2.4). If no such definition is provided, each time-
   out signal SHOULD provide these definitions. 
    
   A package defines one or more of the following extensions: 
    
   *  Actions 
       
   *  BearerInformation 
       
   *  ConnectionModes 
       
   *  ConnectionParameters 
       
   *  DigitMapLetters 
       
   *  Events and Signals 
       
   *  ExtensionParameters 
       
   *  LocalConnectionOptions 
       
   *  ReasonCodes 
       
   *  RestartMethods 
       
   *  Return codes 
       
   For each of the above types of extensions supported by the package, 
   the package definition MUST contain a description of the extension 
   as defined in the following sections. Please note, that package 

                                                                        
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   extensions, just like any other extension, MUST adhere to the MGCP 
   grammar.  
    
6.1 Actions 
    
   Extension Actions SHALL include: 
    
   *  The name and encoding of the extension action. 
       
   *  If the extension action takes any action parameters, then the 
      name, encoding, and possible values of those parameters. 
       
   *  A description of the operation of the extension action.  
       
   *  A listing of the actions in this specification the extension can 
      be combined with. If such a listing is not provided, it is 
      assumed that the extension action cannot be combined with any 
      other action in this specification.  
       
   *  If more than one extension action is defined in the package, then 
      a listing of the actions in the package the extension can be 
      combined with. If such a listing is not provided, it is assumed 
      that the extension action cannot be combined with any other 
      action in the package. 
    
   Extension actions defined in two or more different packages SHOULD 
   NOT be used simultaneously, unless very careful consideration to 
   their potential interaction and side-effects has been given. 
    
6.2 BearerInformation 
    
   BearerInformation extensions SHALL include:  
    
   *  The name and encoding of the BearerInformation extension. 
       
   *  The possible values and encoding of those values that can be 
      assigned to the BearerInformation extension. 
       
   *  A description of the operation of the BearerInformation 
      extension. As part of this description the default value (if any) 
      if the extension is omitted in an EndpointConfiguration command 
      MUST be defined. It may be necessary to make a distinction 
      between the default value before and after the initial 
      application of the parameter, for example if the parameter 
      retains its previous value once specified, until explicitly 
      altered. If default values are not described, then the extension 
      parameter simply defaults to empty in all EndpointConfiguration 
      commands. 
    
   Note that the extension SHALL be included in the result for an 
   AuditEndpoint command auditing the BearerInformation. 
    

                                                                        
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6.3 ConnectionModes 
    
   Extension Connection Modes SHALL include: 
    
   *  The name and encoding of the extension connection mode. 
       
   *  A description of the operation of the extension connection mode.  
       
   *  A description of the interaction a connection in the extension 
      connection mode will have with other connections in each of the 
      modes defined in this specification. If such a description is not 
      provided, the extension connection mode MUST NOT have any 
      interaction with other connections on the endpoint. 
    
   Extension connection modes SHALL NOT be included in the list of 
   modes in a response to an AuditEndpoint for Capabilities, since the 
   package will be reported in the list of packages. 
    
6.4 ConnectionParameters 
    
   Extension Connection Parameters SHALL include: 
    
   *  The name and encoding of the connection parameter extension. 
       
   *  The possible values and encoding of those values that can be 
      assigned to the connection parameter extension. 
       
   *  A description of how those values are derived. 
    
   Note that the extension connection parameter MUST be included in the 
   result for an AuditConnection command auditing the connection 
   parameters. 
    
6.5 DigitMapLetters 
    
   Extension Digit Map Letters SHALL include: 
    
   *  The name and encoding of the extension digit map letter(s). 
       
   *  A description of the meaning of the extension digit map 
      letter(s).  
    
   Note that extension DigitMapLetters in a digit map do not follow the 
   normal naming conventions for extensions defined in packages. More 
   specifically the package name and slash ("/") will not be part of 
   the extension name, thereby forming a flat and limited name space 
   with potential name clashing.  
    
   Therefore, a package SHALL NOT define a digit map letter extension 
   whose encoding has already been used in another package. If two 
   packages have used the same encoding for a digit map letter 
   extension, and those two packages are supported by the same 

                                                                        
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   endpoint, the result of using that digit map letter extension is 
   undefined. 
    
   Note that although an extension DigitMapLetter does not include the 
   package name prefix and slash ("/") as part of the extension name 
   within a digit map, the package name prefix and slash are included 
   when the event code for the event that matched the DigitMapLetter is 
   reported as an observed event. In other words, the digit map just 
   define the matching rule(s), but the event is still reported like 
   any other event. 
    
6.6 Events and Signals 
    
   The event/signal definition SHALL include the precise name of the 
   event/signal (i.e., the code used in MGCP), a plain text definition 
   of the event/signal, and, when appropriate, the precise definition 
   of the corresponding events/signals, for example the exact 
   frequencies of audio signals such as dial tones or DTMF tones. 
    
   The package description MUST provide, for each event/signal, the 
   following information: 
    
   *  The description of the event/signal and its purpose, which SHOULD 
      include the actual signal that is generated by the client (e.g., 
      xx ms FSK tone) as well as the resulting user observed result 
      (e.g., Message Waiting light on/off). 
    
   *  The event code used for the event/signal.  
    
   *  The detailed characteristics of the event/signal, such as for 
      example frequencies and amplitude of audio signals, modulations 
      and repetitions. Such details may be country specific. 
    
   *  The typical and maximum duration of the event/signal if 
      applicable. 
       
   *  If the signal or event can be applied to a connection (across a 
      media stream), it MUST be indicated explicitly. If no such 
      indication is provided, it is assumed that the signal or event 
      cannot be applied to a connection. 
    
   For events, the following MUST be provided as well: 
    
   *  An indication if the event is persistent. By default, events are 
      not persistent - defining events as being persistent is 
      discouraged (see Appendix B for a preferred alternative). Note 
      that persistent events will automatically trigger a Notify when 
      they occur, unless the Call Agent explicitly instructed the 
      endpoint otherwise. This not only violates the normal MGCP model, 
      but also assumes the Call Agent supports the package in question. 
      Such an assumption is unlikely to hold in general. 
       

                                                                        
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   *  An indication if there is an auditable event-state associated 
      with the event. By default, events do not have auditable event-
      states. 
    
   *  If event parameters are supported, it MUST be stated explicitly. 
      The precise syntax and semantics of these MUST then be provided 
      (subject to the grammar provided in Appendix A). It SHOULD also 
      be specified whether these parameters apply to RequestedEvents, 
      ObservedEvents, DetectEvents and EventStates. If not specified 
      otherwise, it is assumed that: 
      *   they do not apply to RequestedEvents, 
      *   they do apply to ObservedEvents, 
      *   they apply in the same way to DetectEvents as they do to  
          RequestedEvents for a given event parameter, 
      *   they apply in the same way to EventStates as they do to  
          ObservedEvents for a given event parameter. 
       
   *  If the event is expected to be used in digit map matching, it 
      SHOULD explicitly state so. Note that only events with single 
      letter or digit parameter codes can do this. See Section 2.1.5 
      for further details. 
    
   For signals, the following MUST be provided as well: 
    
   *  The type of signal (OO, TO, BR).  
    
   *  Time-Out signals SHOULD have an indication of the default time-
      out value. In some cases, time-out values may be variable (if 
      dependent on some action to complete such as out-pulsing digits). 
       
   *  If signal parameters are supported, it MUST be stated explicitly. 
      The precise syntax and semantics of these MUST then be provided 
      (subject to the grammar provided in Appendix A). 
    
   *  Time-Out signals may also indicate whether the "to" parameter is 
      supported or not as well as what the rounding rules associated 
      with them are. If omitted from the signal definition, the 
      package-wide definition is assumed (see Section 6). If the 
      package definition did not specify this, rounding rules default 
      to the nearest non-zero second, whereas support for the "to" 
      parameter defaults to "no" for package version zero, and "yes" 
      for package versions one and higher.  
    
   The following format is RECOMMENDED for defining events and signals 
   in conformance with the above: 
    
    ------------------------------------------------------------------ 
   | Symbol  |   Definition               |  R  |   S     Duration    | 
   |---------|----------------------------|-----|---------------------| 
   |         |                            |     |                     | 
   |         |                            |     |                     | 
    ------------------------------------------------------------------ 
    
                                                                        
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   where: 
     
   *  Symbol indicates the event code used for the event/signal, e.g. 
      "hd". 
       
   *  Definition gives a brief definition of the event/signal 
       
   *  R contains an "x" if the event can be detected or one or more of 
      the following symbols: 
      -  "P" if the event is persistent. 
      -  "S" if the events is an event-state that may be audited. 
      -  "C" if the event can be detected on a connection. 
    
   *  S contains one of the following if it is a signal: 
      -  "OO" if the signal is On/Off signal. 
      -  "TO" if the signal is a Time-Out signal. 
      -  "BR" if the signal is a Brief signal. 
       
   *  S also contains:  
      -  "C" if the signal can be applied on a connection. 
    
   The table SHOULD then be followed by a more comprehensive 
   description of each event/signal defined.  
    
6.6.1 Default and Reserved Events  
    
   All packages that contain Time-Out type signals contain the 
   operation failure ("of") and operation complete ("oc") events, 
   irrespective of whether they are provided as part of the package 
   description or not. These events are needed to support Time-Out 
   signals and cannot be overridden in packages with Time-Out signals. 
   They MAY be extended if necessary, however such practice is 
   discouraged. 
    
   If a package without Time-Out signals does contain definitions for 
   the "oc" and "of" events, the event definitions provided in the 
   package MAY over-ride those indicated here. Such practice is however 
   discouraged and is purely allowed to avoid potential backwards 
   compatibility problems.  
    
   It is considered good practice to explicitly mention that the two 
   events are supported in accordance with their default definitions, 
   which are as follows: 
    
    ------------------------------------------------------------------ 
   | Symbol  |   Definition               |  R  |   S     Duration    | 
   |---------|----------------------------|-----|---------------------| 
   | oc      | Operation Complete         |  x  |                     | 
   | of      | Operation Failure          |  x  |                     | 
    ------------------------------------------------------------------ 
    
   Operation complete (oc):   The operation complete event is generated 
   when the gateway was asked to apply one or several signals of type 
                                                                        
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   TO on the endpoint or connection, and one or more of those signals 
   completed without being stopped by the detection of a requested 
   event such as off-hook transition or dialed digit. The completion 
   report should carry as a parameter the name of the signal that came 
   to the end of its live time, as in:  
    
      O: G/oc(G/rt) 
    
   In this case, the observed event occurred because the "rt" signal in 
   the "G" package timed out. 
    
   If the reported signal was applied on a connection, the parameter 
   supplied will include the name of the connection as well, as in: 
    
      O: G/oc(G/rt@0A3F58) 
    
   When the operation complete event is requested, it cannot be 
   parameterized with any event parameters. When the package name is 
   omitted (which is discouraged) as part of the signal name, the 
   default package is assumed. 
    
    
   Operation failure (of):    The operation failure is generated when 
   the endpoint was asked to apply one or several signals of type TO on 
   the endpoint or connection, and one or more of those signals failed 
   prior to timing out. The completion report should carry as a 
   parameter the name of the signal that failed, as in: 
    
      O: G/of(G/rt) 
    
   In this case a failure occurred in producing the "rt" signal in the 
   "G" package. 
    
   When the reported signal was applied on a connection, the parameter 
   supplied will include the name of the connection as well, as in: 
    
      O: G/of(G/rt@0A3F58) 
    
   When the operation failure event is requested, event parameters can 
   not be specified. When the package name is omitted (which is 
   discouraged), the default package name is assumed. 
    
6.7 ExtensionParameters 
    
   Extension parameter extensions SHALL include 
    
   *  The name and encoding of the extension parameter. 
       
   *  The possible values and encoding of those values that can be 
      assigned to the extension parameter. 
       
   *  For each of the commands defined in this specification, whether 
      the extension parameter is Mandatory, Optional, or Forbidden in 
                                                                        
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      requests as well as responses. Note that extension parameters 
      SHOULD NOT normally be mandatory. 
       
   *  A description of the operation of the extension parameter. As 
      part of this description the default value (if any) if the 
      extension is omitted in a command MUST be defined. It may be 
      necessary to make a distinction between the default value before 
      and after the initial application of the parameter, for example 
      if the parameter retains its previous value once specified, until 
      explicitly altered. If default values are not described, then the 
      extension parameter simply defaults to empty in all commands. 
       
   *  Whether the extension can be audited in AuditEndpoint and/or 
      AuditConnection as well as the values returned. If nothing is 
      specified, then auditing of the extension parameter can only be 
      done for AuditEndpoint, and the value returned SHALL be the 
      current value for the extension. Note that this may be empty. 
    
6.8 LocalConnectionOptions 
    
   LocalConnectionOptions extensions SHALL include: 
    
   *  The name and encoding of the LocalConnectionOptions extension. 
       
   *  The possible values and encoding of those values that can be 
      assigned to the LocalConnectionOptions extension. 
       
   *  A description of the operation of the LocalConnectionOptions 
      extension. As part of this description the following MUST be 
      specified: 
       
      - The default value (if any) if the extension is omitted in a 
        CreateConnection command.  
         
      - The default value if omitted in a ModifyConnection command. 
        This may be to simply retain the previous value (if any) or to 
        apply the default value. If nothing is specified, the current 
        value is retained if possible.  
         
      - If Auditing of capabilities will result in the extension being 
        returned, then a description to that effect as well as with 
        what possible values and their encoding (note that the package 
        itself will always be returned). If nothing is specified, the 
        extension SHALL NOT be returned when auditing capabilities. 
    
   Also note, that the extension MUST be included in the result for an 
   AuditConnection command auditing the LocalConnectionOptions. 
    
6.9 Reason Codes 
    
   Extension reason codes SHALL include: 
    

                                                                        
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   *  The number for the reason code. The number MUST be in the range 
      800 to 899. 
       
   *  A description of the extension reason code including the 
      circumstances that leads to the generation of the reason code. 
      Those circumstances SHOULD be limited to events caused by another 
      extension defined in the package to ensure the recipient will be 
      able to interpret the extension reason code correctly. 
    
   Note that the extension reason code may have to be provided in the 
   result for an AuditEndpoint command auditing the reason code. 
    
6.10 RestartMethods 
    
   Extension Restart Methods SHALL include: 
    
   *  The name and encoding for the restart method.  
       
   *  A description of the restart method including the circumstances 
      that leads to the generation of the restart method. Those 
      circumstances SHOULD be limited to events caused by another 
      extension defined in the package to ensure the recipient will be 
      able to interpret the extension restart method correctly. 
    
   *  An indication of whether the RestartDelay parameter is to be used 
      with the extension. If nothing is specified, it is assumed that 
      it is not to be used. In that case, RestartDelay MUST be ignored 
      if present.  
    
   *  If the restart method defines a service state, the description 
      MUST explicitly state and describe this. In that case, the 
      extension restart method can then be provided in the result for 
      an AuditEndpoint command auditing the restart method. 
    
6.11 Return Codes 
    
   Extension Return Codes SHALL include: 
    
   *  The number for the extension return code. The number MUST be in 
      the range 800 to 899. 
       
   *  A description of the extension return code including the 
      circumstances that leads to the generation of the extension 
      return code. Those circumstances SHOULD be limited to events 
      caused by another extension defined in the package to ensure the 
      recipient will be able to interpret the extension return code 
      correctly. 
    





                                                                        
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7. Versions and Compatibility 
 
7.1 Changes from RFC 2705 
    
   RFC 2705 was issued in October 1999, as the last update of draft 
   version 0.5. This updated document benefits from further 
   implementation experience. The main changes from RFC 2705 are: 
    
   *  Contains several clarifications, editorial changes and resolution 
      of known inconsistencies. 
    
   *  Firmed up specification language in accordance with RFC 2119 and 
      added RFC 2119 conventions section.  
       
   *  Clarified behavior of mixed wild-carding in endpoint names. 
       
   *  Deleted naming requirement about having first term identify the 
      physical gateway when the gateway consists of multiple physical 
      gateways. Also added recommendations on wild-carding naming usage 
      from the right only, as well as mixed wildcard usage. 
    
   *  Clarified that synonymous forms and values for endpoint names are 
      not freely interchangeable.  
    
   *  Allowed IPv6 addresses in endpoint names. 
    
   *  Clarified Digit Map matching rules. 
       
   *  Added missing semantics for symbols used in digit maps.  
    
   *  Added Timer T description in Digit Maps. 
    
   *  Added recommendation to support digit map sizes of at least 2048 
      bytes per endpoint. 
    
   *  Clarified use of wildcards in several commands. 
       
   *  Event and Signal Parameters formally defined for events and 
      signals. 
       
   *  Persistent events now allowed in base MGCP protocol. 
       
   *  Added additional detail on connection wildcards.  
       
   *  Clarified behavior of loopback, and continuity test connection 
      modes for mixing and multiple connections in those modes. 
       
   *  Modified BearerInformation to be conditional optional in the 
      EndpointConfiguration command. 
       
   *  Clarified "swap audio" action operation for one specific scenario 
      and noted that operation for other scenarios is undefined. 
    
                                                                        
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   *  Added recommendation that all implementations support PCMU 
      encoding for interoperability. 
    
   *  Changed Bandwidth LocalConnectionOptions value from excluding to 
      including overhead from the IP layer and up for consistency with 
      SDP. 
    
   *  Clarified that mode of second connection in a CreateConnection 
      command will be set to "send/receive". 
       
   *  Type of service default changed to zero. 
       
   *  Additional detail on echo cancellation, silence suppression, and 
      gain control. Also added recommendation for Call Agents not to 
      specify handling of echo cancellation and gain control. 
       
   *  Added requirement for a connection to have a 
      RemoteConnectionDescriptor in order to use the "network loopback" 
      and "network continuity test" modes.  
    
   *  Removed procedures and specification for NAS's (will be provided 
      as package instead).  
       
   *  Removed procedures and specification for ATM (will be provided as 
      package instead). 
       
   *  Added missing optional NotifiedEntity parameter to the 
      DeleteConnection (from the Call Agent) MGCI command. 
    
   *  Added optional new MaxMGCPDatagram RequestedInfo code for 
      AuditEndpoint to enable auditing of maximum size of MGCP 
      datagrams supported.  
    
   *  Added optional new PackageList RequestedInfo code for 
      AuditEndpoint to enable auditing of packages with a package 
      version number. PackageList parameter also allowed with return 
      code 518 (unsupported package).  
       
   *  Added missing attributes in Capabilities. 
       
   *  Clarified that at the expiration of a non-zero restart delay, an 
      updated RestartInProgress should be sent. Also clarified that a 
      new NotifiedEntity can only be returned in response to a 
      RestartInProgress command. 
    
   *  Added Response Acknowledgement response (return code 000) and 
      included in three-way handshake.  
       
   *  ResponseAck parameter changed to be allowed in all commands. 
       
   *  Added return codes 101, 405, 406, 407, 409, 410, 503, 504, 505, 
      506, 508, 509, 533, 534, 535, 536, 537, 538, 539, 540, 541, and 
      defined return codes in range 800-899 to be package specific 
                                                                        
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      return codes. Additional text provided for some return codes and 
      additional detail on how to handle unknown return codes added. 
       
   *  Added reason code 903, 904, 905 and defined reason codes 800-899 
      to be package specific reason codes. 
       
   *  Added section clarifying codec negotiation procedure. 
       
   *  Clarified that resource reservation parameters in a 
      ModifyConnection command defaults to the current value used. 
    
   *  Clarified that connection mode is optional in ModifyConnection 
      commands. 
       
   *  Corrected LocalConnectionDescriptor to be optional in response to 
      CreateConnection commands (in case of failure). 
       
   *  Clarified that quoted-strings are UTF-8 encoded and 
      interchangeability of quoted strings and unquoted strings. 
    
   *  Clarified that Transaction Identifiers are compared as numerical 
      values. 
       
   *  Clarified bit-ordering for Type Of Service 
      LocalConnectionOptions.  
    
   *  Clarified the use of RequestIdentifier zero. 
    
   *  Added example sections for commands, responses, and some call 
      flows. 
       
   *  Corrected usage of and requirements for SDP to be strictly RFC 
      2327 compliant. 
       
   *  Added requirement that all MGCP implementations must support MGCP 
      datagrams up to at least 4000 bytes. Also added new section on 
      Maximum Datagram Size, Fragmentation and reassembly.  
    
   *  Generalized piggybacking retransmission scheme to only state 
      underlying requirements to be satisfied. 
    
   *  Clarified the section on computing retransmission timers. 
       
   *  Clarified operation of long-running transactions, incl. 
      provisional responses, retransmissions and failures.  
    
   *  Enhanced description of provisional responses and interaction 
      with three-way handshake. 
    
   *  Enhanced description of fail-over and the role of "notified 
      entity". An empty "notified entity" has been allowed, although 
      strongly discouraged.  
       
                                                                        
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   *  Clarified retransmission procedure and removed "wrong key" 
      considerations from it. Also fixed inconsistencies between Max1 
      and Max2 retransmission boundaries and the associated flow 
      diagram. 
       
   *  Updated domain name resolution for retransmission procedure to 
      incur less overhead when multiple endpoints are retransmitting. 
    
   *  Removed requirement for in-order delivery of NotificationRequests 
      response and Notify commands. Notify commands are still delivered 
      in-order. 
    
   *  Clarified that activating an embedded Notification Request does 
      not clear the list of ObservedEvents.  
    
   *  Defined interactions between disconnected state and notification 
      state. 
       
   *  Added section on transactional semantics. 
       
   *  Defined gateway behavior when multiple interacting transactions 
      are received. 
       
   *  Additional details provided on service states. Clarified 
      relationship between endpoint service states, restart methods, 
      and associated processing of commands. 
       
   *  Clarified operation for transitioning from "restart procedure" to 
      "disconnected state". 
       
   *  Allowed auditing commands and responses to bypass the "restart" 
      and "disconnected" procedures. 
       
   *  Clarified operation of "disconnected procedure" and in particular 
      the operation of piggy-backed "disconnected" RestartInProgress 
      messages.  
    
   *  Added option to aggregate "disconnected" RestartInProgress 
      messages under certain conditions to reduce message volume. 
    
   *  Defined additional behavior for endpoints wishing to send 
      commands while in the "disconnected" state. 
    
   *  Added new section on Load Control in General which includes two 
      new error codes (101 and 409) to handle overload. 
    
   *  Deleted the "Proposed MoveConnection command". 
       
   *  Removed packages from protocol specification (will be provided in 
      separate documents instead).  
       
   *  Package concept formally extended to be primary extension 
      mechanism now allowing extensions for the following to be defined 
                                                                        
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      in packages as well: 
       
      -  BearerInformation  
          
      -  LocalConnectionOptions  
          
      -  ExtensionParameters 
          
      -  Connection Modes  
          
      -  Actions 
          
      -  Digit Map Letters 
          
      -  Connection Parameters 
          
      -  Restart Methods 
          
      -  Reason Codes 
          
      -  Return Codes 
          
   *  Requirements and suggested format for package definitions added. 
    
   *  Defined "operation complete" and "operation failure" events to be 
      automatically present in packages with Time-Out signals. 
       
   *  Deleted list of differences that were prior to RFC 2705.  
    
   *  Added Base Package to deal with quarantine buffer overflow, 
      ObservedEvents overflow, embedded NotificationRequest failure, 
      and to enable events to be requested persistently. A new 
      "Message" command is included as well. 
       
   *  IANA registration procedures for packages and other extensions 
      added.  
       
   *  Updated grammar to fix known errors and support new extensions in 
      a backwards compatible manner. Added new (optional) PackageList 
      and MaxMGCPDatagram for auditing. Changed explicit white space 
      rules in some productions to make grammar more consistent. 
    
   *  Connection Mode interaction table added. 
    
   *  Added additional detail on virtual endpoint naming conventions. 
      Also added suggested gateway endpoint convention and a "Range 
      Wildcard" option to the Endpoint Naming Conventions.  
    





                                                                        
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8. Security Considerations 
    
   Security issues are discussed in section 5. 
    

















































                                                                        
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9. Acknowledgements 
    
   Special thanks are due to the authors of the original MGCP 1.0 
   specification: Mauricio Arango, Andrew Dugan, Isaac Elliott, 
   Christian Huitema, and Scott Picket.  
    
   We also want to thank the many reviewers who provided advice on the 
   design of SGCP and then MGCP, notably Sankar Ardhanari, Francois 
   Berard, David Auerbach, Bob Biskner, David Bukovinsky, Charles 
   Eckel, Mario Edini, Ed Guy, Barry Hoffner, Jerry Kamitses, Oren 
   Kudevitzki, Rajesh Kumar, Troy Morley, Dave Oran, Jeff Orwick, John 
   Pickens, Lou Rubin, Chip Sharp, Paul Sijben, Kurt Steinbrenner, Joe 
   Stone, and Stuart Wray. 
    
   The version 0.1 of MGCP was heavily inspired by the "Internet 
   Protocol Device Control" (IPDC) designed by the Technical Advisory 
   Committee set up by Level 3 Communications. Whole sets of text were 
   retrieved from the IP Connection Control protocol, IP Media Control 
   protocol, and IP Device Management. The authors wish to acknowledge 
   the contribution to these protocols made by Ilya Akramovich, Bob 
   Bell, Dan Brendes, Peter Chung, John Clark, Russ Dehlinger, Andrew 
   Dugan, Isaac Elliott, Cary FitzGerald, Jan Gronski, Tom Hess, Geoff 
   Jordan, Tony Lam, Shawn Lewis, Dave Mazik, Alan Mikhak, Pete 
   O'Connell, Scott Pickett, Shyamal Prasad, Eric Presworsky, Paul 
   Richards, Dale Skran, Louise Spergel, David Sprague, Raj Srinivasan, 
   Tom Taylor and Michael Thomas. 
    


























                                                                        
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10. References 
 
   [1]  Bradner, S., "The Internet Standards Process -- Revision 3", 
        BCP 9, RFC 2026, October 1996. 
         
   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement 
        Levels", BCP 14, RFC 2119, March 1997 
         
   [3]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, 
        "RTP: A Transport Protocol for Real-Time Applications", RFC 
        1889, January 1996. 
         
   [4]  Schulzrinne, H., "RTP Profile for Audio and Video Conferences 
        with Minimal Control", RFC 1890, January 1996. 
         
   [5]  Handley, M and V. Jacobson, "SDP: Session Description 
        Protocol", RFC 2327, April 1998. 
         
   [6]  Handley, M., "SAP - Session Announcement Protocol", Work in 
        Progress. 
         
   [7]  Handley, M., Schulzrinne, H. and E. Schooler, "Session 
        Initiation Protocol (SIP)", RFC 2543, March 1999. 
         
   [8]  Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming 
        Protocol (RTSP)", RFC 2326, April 1998. 
         
   [9]  ITU-T, Recommendation Q.761, "FUNCTIONAL DESCRIPTION OF THE 
        ISDN USER PART OF SIGNALING SYSTEM No. 7", (Malaga-
        Torremolinos, 1984; modified at Helsinki, 1993). 
         
   [10] ITU-T, Recommendation Q.762, "GENERAL FUNCTION OF MESSAGES AND 
        SIGNALS OF THE ISDN USER PART OF SIGNALING SYSTEM No. 7", 
        (MalagaTorremolinos, 1984; modified at Helsinki, 1993). 
         
   [11] ITU-T, Recommendation H.323 (02/98), "PACKET-BASED MULTIMEDIA 
        COMMUNICATIONS SYSTEMS". 
         
   [12] ITU-T, Recommendation H.225, "Call Signaling Protocols and 
        Media Stream Packetization for Packet Based Multimedia 
        Communications Systems". 
         
   [13] ITU-T, Recommendation H.245 (02/98), "CONTROL PROTOCOL FOR 
        MULTIMEDIA COMMUNICATION". 
         
   [14] Kent, S. and R. Atkinson, "Security Architecture for the 
        Internet Protocol", RFC 2401, November 1998. 
         
   [15] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402, 
        November 1998. 
         


                                                                        
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   [16] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload 
        (ESP)", RFC 2406, November 1998. 
         
   [17] Crocker, D. and P. Overell, "Augmented BNF for Syntax 
        Specifications: ABNF", RFC 2234, November 1997. 
         
   [18] Stevens, W. Richard, "TCP/IP Illustrated, Volume 1, The 
        Protocols", Addison-Wesley, 1994. 
         
   [19] Allman, M., Paxson, V. "On Estimating End-to-End Network Path 
        Properties", Proc. SIGCOMM'99, 1999. 
         
   [20] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 
        2279, January 1998. 
         
   [21] Braden, R., "Requirements for Internet Hosts -- Communication 
        Layers", RFC 1122, October 1989. 
         
   [22] Bellcore, "LSSGR: Switching System Generic Requirements for 
        Call Control Using the Integrated Services Digital Network User 
        Part (ISDNUP)", GR-317-CORE, Issue 2, December 1997. 
    































                                                                        
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11. Authors' Addresses 
    
   Flemming Andreasen 
   Cisco Systems 
   499 Thornall Street, 8th Floor 
   Edison, NJ 08837 
   Phone: +1 732 452 1667 
   EMail: fandreas@cisco.com 
    
    
   Bill Foster 
   Cisco Systems 
   771 Alder Drive 
   Milpitas, CA 95035 
   Phone: +1 408 527 8791 
   EMail: bfoster@cisco.com 
    
    
    
    
    
































                                                                        
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Appendix A: Formal Syntax Description of the Protocol 
    
   In this section, we provide a formal description of the protocol 
   syntax, following the "Augmented BNF for Syntax Specifications" 
   defined in RFC 2234. The syntax makes use of the core rules defined 
   in RFC 2234, Section 6.1, which are not included here. Furthermore, 
   the syntax follows the case-sensitivity rules of RFC 2234, i.e. MGCP 
   is case-insensitive (but SDP is not). It should be noted, that ABNF 
   does not provide for implicit specification of linear white space 
   and MGCP messages MUST thus follow the explicit linear white space 
   rules provided in the grammar below. However, in line with general 
   robustness principles, implementers are strongly encouraged to 
   tolerate additional linear white space in messages received.  
    
MGCPMessage = MGCPCommand / MGCPResponse  
 
MGCPCommand = MGCPCommandLine 0*(MGCPParameter) [EOL *SDPinformation]  
 
MGCPCommandLine = MGCPVerb 1*(WSP) transaction-id 1*(WSP)  
                        endpointName 1*(WSP) MGCPversion EOL  
 
MGCPVerb = "EPCF" / "CRCX" / "MDCX" / "DLCX" / "RQNT"  
         / "NTFY" / "AUEP" / "AUCX" / "RSIP" / extensionVerb  
 
extensionVerb  = ALPHA 3(ALPHA / DIGIT) ; experimental starts with X 
 
transaction-id = 1*9(DIGIT)  
 
endpointName      = LocalEndpointName "@" DomainName  
LocalEndpointName = LocalNamePart 0*("/" LocalNamePart)  
LocalNamePart     = AnyName / AllName / NameString  
AnyName           = "$"  
AllName           = "*"  
NameString        = 1*(range-of-allowed-characters)  
; VCHAR except "$", "*", "/", "@" 
range-of-allowed-characters  = %x21-23 / %x25-29 / %x2B-2E  
                             / %x30-3F / %x41-7E 
 
DomainName = 1*255(ALPHA / DIGIT / "." / "-")    ; as defined  
           / "#" number                          ; in RFC 821 
           / "[" IPv4address / IPv6address "]"   ; see RFC 2373 
 
; Rewritten to ABNF from RFC 821 
number =  1*DIGIT  
 
;From RFC 2373 
IPv6address = hexpart [ ":" IPv4address ] 
IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT 
; this production, while occurring in RFC2373, is not referenced 
; IPv6prefix  = hexpart "/" 1*2DIGIT 
hexpart = hexseq / hexseq "::" [ hexseq ] / "::" [ hexseq ] 
hexseq  = hex4 *( ":" hex4) 
hex4    = 1*4HEXDIG 
                                                                        
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MGCPversion = "MGCP" 1*(WSP) 1*(DIGIT) "." 1*(DIGIT)  
                            [1*(WSP) ProfileName]  
ProfileName = VCHAR *( WSP / VCHAR)    
 
MGCPParameter = ParameterValue EOL  
 
; Check infoCode if more parameter values defined  
; Most optional values can only be omitted when auditing 
ParameterValue = ("K"  ":" 0*(WSP)  [ResponseAck])   
               / ("B"  ":" 0*(WSP)  [BearerInformation])   
               / ("C"  ":" 0*(WSP)  CallId)   
               / ("I"  ":" 0*(WSP)  [ConnectionId])  
               / ("N"  ":" 0*(WSP)  [NotifiedEntity])   
               / ("X"  ":" 0*(WSP)  [RequestIdentifier])   
               / ("L"  ":" 0*(WSP)  [LocalConnectionOptions])   
               / ("M"  ":" 0*(WSP)  ConnectionMode)   
               / ("R"  ":" 0*(WSP)  [RequestedEvents])   
               / ("S"  ":" 0*(WSP)  [SignalRequests])   
               / ("D"  ":" 0*(WSP)  [DigitMap])   
               / ("O"  ":" 0*(WSP)  [ObservedEvents])   
               / ("P"  ":" 0*(WSP)  [ConnectionParameters])   
               / ("E"  ":" 0*(WSP)  ReasonCode)   
               / ("Z"  ":" 0*(WSP)  [SpecificEndpointID])   
               / ("Z2" ":" 0*(WSP)  SecondEndpointID)   
               / ("I2" ":" 0*(WSP)  SecondConnectionID)   
               / ("F"  ":" 0*(WSP)  [RequestedInfo])   
               / ("Q"  ":" 0*(WSP)  QuarantineHandling)   
               / ("T"  ":" 0*(WSP)  [DetectEvents])   
               / ("RM" ":" 0*(WSP)  RestartMethod)   
               / ("RD" ":" 0*(WSP)  RestartDelay)   
               / ("A"  ":" 0*(WSP)  [Capabilities])   
               / ("ES" ":" 0*(WSP)  [EventStates]) 
               / ("PL" ":" 0*(WSP)  [PackageList])     ; Auditing only 
               / ("MD" ":" 0*(WSP)  MaxMGCPDatagram)   ; Auditing only 
               / (extensionParameter ":" 0*(WSP) [parameterString])  
 
; A final response may include an empty ResponseAck 
ResponseAck =  confirmedTransactionIdRange     
               *( ","  0*(WSP) confirmedTransactionIdRange ) 
 
confirmedTransactionIdRange = transaction-id ["-" transaction-id]  
 
BearerInformation = BearerAttribute 0*("," 0*(WSP) BearerAttribute)  
BearerAttribute   = ("e" ":" BearerEncoding)  
                  / (BearerExtensionName [":" BearerExtensionValue]) 
BearerExtensionName  = PackageLCOExtensionName 
BearerExtensionValue = LocalOptionExtensionValue 
BearerEncoding = "A" / "mu"          
 
CallId = 1*32(HEXDIG)  
 

                                                                        
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; The audit request response may include a list of identifiers  
ConnectionId = 1*32(HEXDIG) 0*("," 0*(WSP) 1*32(HEXDIG))  
SecondConnectionID = ConnectionId  
 
NotifiedEntity = [LocalName "@"] DomainName [":" portNumber]  
LocalName  = LocalEndpointName            ; No internal structure 
 
portNumber = 1*5(DIGIT)  
 
RequestIdentifier = 1*32(HEXDIG)  
 
LocalConnectionOptions = LocalOptionValue 0*(WSP)  
                           0*("," 0*(WSP) LocalOptionValue 0*(WSP))   
LocalOptionValue = ("p"  ":" packetizationPeriod)  
                 / ("a"  ":" compressionAlgorithm)  
                 / ("b"  ":" bandwidth)  
                 / ("e"  ":" echoCancellation)  
                 / ("gc" ":" gainControl)  
                 / ("s"  ":" silenceSuppression)  
                 / ("t"  ":" typeOfService)  
                 / ("r"  ":" resourceReservation)  
                 / ("k"  ":" encryptiondata)  
                 / ("nt" ":" ( typeOfNetwork /  
                                    supportedTypeOfNetwork)) 
                 / (LocalOptionExtensionName  
                         [":" LocalOptionExtensionValue])   
 
Capabilities    =  CapabilityValue 0*(WSP)  
                     0*("," 0*(WSP) CapabilityValue 0*(WSP))    
CapabilityValue = LocalOptionValue  
                / ("v" ":" supportedPackages)  
                / ("m" ":" supportedModes)  
 
PackageList     = pkgNameAndVers 0*("," pkgNameAndVers) 
pkgNameAndVers  = packageName ":" packageVersion                                  
packageVersion  = 1*(DIGIT) 
 
packetizationPeriod  = 1*4(DIGIT) ["-" 1*4(DIGIT)]  
compressionAlgorithm = algorithmName 0*(";" algorithmName)  
algorithmName        = 1*(SuitableLCOCharacter)  
bandwidth            = 1*4(DIGIT) ["-" 1*4(DIGIT)]  
echoCancellation     = "on" / "off"  
gainControl          = "auto" / ["-"] 1*4(DIGIT)  
silenceSuppression   = "on" / "off"  
typeOfService        = 1*2(HEXDIG)     ; 1 hex only for capabilities 
resourceReservation  = "g" / "cl" / "be"  
 
;encryption parameters are coded as in SDP (RFC 2327)  
;NOTE: encryption key may contain an algorithm as specified in RFC 1890 
encryptiondata = ( "clear" ":" encryptionKey )  
               / ( "base64" ":" encodedEncryptionKey )  
               / ( "uri" ":" URItoObtainKey )  
               / ( "prompt" ) ; defined in SDP, not usable in MGCP!  
                                                                        
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encryptionKey = 1*(SuitableLCOCharacter) / quotedString    
; See RFC 2045 
encodedEncryptionKey = 1*(ALPHA / DIGIT / "+" / "/" / "=")   
URItoObtainKey = 1*(SuitableLCOCharacter) / quotedString  
 
typeOfNetwork = "IN" / "ATM" / "LOCAL" / OtherTypeOfNetwork 
; Registered with IANA - see RFC 2327  
OtherTypeOfNetwork     = 1*(SuitableLCOCharacter) 
supportedTypeOfNetwork = typeOfNetwork *(";" typeOfNetwork) 
 
supportedModes    = ConnectionMode 0*(";" ConnectionMode)  
 
supportedPackages = packageName 0*(";" packageName)  
 
packageName = 1*(ALPHA / DIGIT / HYPHEN) ; Hyphen neither first or last 
 
LocalOptionExtensionName = VendorLCOExtensionName  
                         / PackageLCOExtensionName  
                         / OtherLCOExtensionName 
VendorLCOExtensionName   = "x" ("+"/"-") 1*32(SuitableExtLCOCharacter)  
PackageLCOExtensionName  = packageName "/"  
                            1*32(SuitablePkgExtLCOCharacter) 
; must not start with "x-" or "x+" 
OtherLCOExtensionName    = 1*32(SuitableExtLCOCharacter)   
     
LocalOptionExtensionValue = (1*(SuitableExtLCOValChar)  
                                                    / quotedString) 
                              *(";" (1*(SuitableExtLCOValChar)  
                                                      / quotedString))  
 
;Note: No "data" mode. 
ConnectionMode = "sendonly" / "recvonly" / "sendrecv"  
               / "confrnce" / "inactive" / "loopback"  
               / "conttest" / "netwloop" / "netwtest"  
               / ExtensionConnectionMode      
ExtensionConnectionMode = PkgExtConnectionMode 
PkgExtConnectionMode    = packageName "/" 1*(ALPHA / DIGIT) 
 
RequestedEvents = requestedEvent 0*("," 0*(WSP) requestedEvent)  
requestedEvent  = (eventName ["(" requestedActions ")"])  
                / (eventName "(" requestedActions ")" 
                                       "(" eventParameters ")" ) 
eventName = [(packageName / "*") "/"]  
                (eventId / "all" / eventRange  
                                        / "*" / "#") ; for DTMF 
                              ["@" (ConnectionId / "$" / "*")]  
eventId = 1*(ALPHA / DIGIT / HYPHEN)   ; Hyphen neither first nor last 
eventRange = "[" 1*(DigitMapLetter / (DIGIT "-" DIGIT) / 
                        (DTMFLetter "-" DTMFLetter)) "]"  
DTMFLetter = "A" / "B" / "C" / "D"         
 


                                                                        
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requestedActions = requestedAction 0*("," 0*(WSP) requestedAction)  
requestedAction  = "N" / "A" / "D" / "S" / "I" / "K"  
                 / "E" "(" EmbeddedRequest ")"  
                 / ExtensionAction 
ExtensionAction  = PackageExtAction         
PackageExtAction = packageName "/" Action ["(" ActionParameters ")"] 
Action           = 1*ALPHA 
ActionParameters = eventParameters        ; May contain actions     
 
;NOTE: Should tolerate different order when receiving, e.g. for NCS.  
EmbeddedRequest = (      "R" "(" EmbeddedRequestList ")"  
                    ["," 0*(WSP) "S" "(" EmbeddedSignalRequest ")"]  
                    ["," 0*(WSP) "D" "(" EmbeddedDigitMap ")"]      )  
                / (      "S" "(" EmbeddedSignalRequest ")"  
                    ["," 0*(WSP) "D" "(" EmbeddedDigitMap ")"] )  
                / (      "D" "(" EmbeddedDigitMap ")" )  
 
EmbeddedRequestList   = RequestedEvents  
EmbeddedSignalRequest = SignalRequests  
EmbeddedDigitMap = DigitMap  
 
SignalRequests   = SignalRequest 0*("," 0*(WSP) SignalRequest )  
SignalRequest    = eventName [ "(" eventParameters ")" ]  
 
eventParameters  = eventParameter 0*("," 0*(WSP) eventParameter)  
eventParameter   = eventParameterValue  
                 / eventParameterName "=" eventParameter 
                 / eventParameterName "(" eventParameters ")"  
eventParameterString = 1*(SuitableEventParamCharacter)  
eventParameterName   = eventParameterString 
eventParameterValue  = eventParameterString / quotedString 
 
DigitMap           = DigitString  / "(" DigitStringList ")"  
DigitStringList    = DigitString 0*( "|" DigitString )  
DigitString        = 1*(DigitStringElement)  
DigitStringElement = DigitPosition ["."]  
DigitPosition      = DigitMapLetter / DigitMapRange  
; NOTE "X" is now included 
DigitMapLetter     = DIGIT / "#" / "*" / "A" / "B" / "C" / "D" / "T"  
                   / "X" / ExtensionDigitMapLetter        
ExtensionDigitMapLetter = "E" / "F" / "G" / "H" / "I" / "J" / "K"   
                        / "L" / "M" / "N" / "O" / "P" / "Q" / "R"  
                        / "S" / "U" / "V" / "W" / "Y" / "Z" 
; NOTE "[x]" is now allowed 
DigitMapRange = "[" 1*DigitLetter "]"  
DigitLetter   = *((DIGIT "-" DIGIT) / DigitMapLetter)  
 
ObservedEvents = SignalRequests  
 
EventStates    = SignalRequests  
 
ConnectionParameters = ConnectionParameter  
                        0*( "," 0*(WSP) ConnectionParameter )  
                                                                        
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ConnectionParameter  = ( "PS" "=" packetsSent )  
                     / ( "OS" "=" octetsSent )  
                     / ( "PR" "=" packetsReceived )  
                     / ( "OR" "=" octetsReceived )  
                     / ( "PL" "=" packetsLost )  
                     / ( "JI" "=" jitter )  
                     / ( "LA" "=" averageLatency )  
                     / ( ConnectionParameterExtensionName  
                              "=" ConnectionParameterExtensionValue ) 
packetsSent     = 1*9(DIGIT) 
octetsSent      = 1*9(DIGIT)  
packetsReceived = 1*9(DIGIT)  
octetsReceived  = 1*9(DIGIT)  
packetsLost     = 1*9(DIGIT)  
jitter          = 1*9(DIGIT)  
averageLatency  = 1*9(DIGIT)  
 
ConnectionParameterExtensionName = VendorCPExtensionName  
                                 /    PackageCPExtensionName  
VendorCPExtensionName  = "X" "-" 2*ALPHA  
PackageCPExtensionName = packageName "/" CPName 
CPName = 1*(ALPHA / DIGIT / HYPHEN) 
ConnectionParameterExtensionValue = 1*9(DIGIT)       
 
MaxMGCPDatagram = 1*9(DIGIT) 
 
ReasonCode = 3DIGIT  
             [1*(WSP) "/" packageName]     ; Only for 8xx 
             [WSP 1*(%x20-7E)]  
 
SpecificEndpointID = endpointName  
SecondEndpointID   = endpointName  
 
RequestedInfo = infoCode 0*("," 0*(WSP) infoCode) 
 
infoCode = "B" / "C" / "I" / "N" / "X" / "L" / "M" / "R" / "S"  
         / "D" / "O" / "P" / "E" / "Z" / "Q" / "T" / "RC" / "LC"  
         / "A" / "ES" / "RM" / "RD" / "PL" / "MD" / extensionParameter 
 
QuarantineHandling = loopControl / processControl  
                   / (loopControl "," 0*(WSP) processControl )  
loopControl    = "step" / "loop"  
processControl = "process" / "discard"  
 
DetectEvents = SignalRequests 
 
RestartMethod = "graceful" / "forced" / "restart" / "disconnected"  
              / "cancel-graceful" / extensionRestartMethod 
extensionRestartMethod = PackageExtensionRM     
PackageExtensionRM     = packageName "/" 1*32(ALPHA / DIGIT / HYPHEN)  
RestartDelay = 1*6(DIGIT)  
 

                                                                        
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extensionParameter = VendorExtensionParameter  
                   / PackageExtensionParameter  
                   / OtherExtensionParameter 
VendorExtensionParameter  = "X" ("-"/"+") 1*6(ALPHA / DIGIT)  
PackageExtensionParameter = packageName "/"  
                            1*32(ALPHA / DIGIT / HYPHEN) 
; must not start with "x-" or x+" 
OtherExtensionParameter   = 1*32(ALPHA / DIGIT / HYPHEN)              
 
;If first character is a double-quote, then it is a quoted-string 
parameterString = (%x21 / %x23-7F) *(%x20-7F) ; first and last must not  
                                              ; be white space 
                    / quotedString  
 
MGCPResponse = MGCPResponseLine 0*(MGCPParameter)  
                                        *2(EOL *SDPinformation) 
 
MGCPResponseLine = responseCode 1*(WSP) transaction-id  
                        [1*(WSP) "/" packageName]    ; Only for 8xx 
                             [WSP responseString] EOL  
responseCode = 3DIGIT  
responseString = *(%x20-7E)  
 
SuitablePkgExtLCOCharacter =  SuitableLCOCharacter 
 
SuitableExtLCOCharacter = DIGIT / ALPHA / "+" / "-" / "_" / "&"  
              / "!" / "'" / "|" / "=" / "#" / "?"       
              / "." / "$" / "*" /       "@" / "[" / "]"  
              / "^" / "`" / "{" / "}" / "~" 
 
SuitableLCOCharacter   = SuitableExtLCOCharacter / "/" 
 
SuitableExtLCOValChar  = SuitableLCOCharacter / ":" 
 
; VCHAR except """, "(", ")", ",", and "=" 
SuitableEventParamCharacter = %x21 / %x23-27 / %x2A-2B  
                            / %x2D-3C / %x3E-7E 
 
; NOTE: UTF8 encoded 
quotedString  = DQUOTE 0*(quoteEscape / quoteChar) DQUOTE  
quoteEscape   = DQUOTE DQUOTE  
quoteChar = (%x00-21 / %x23-FF)  
 
EOL = CRLF / LF 
 
HYPHEN = "-" 
 
; See RFC 2327 for proper SDP grammar instead.  
SDPinformation = SDPLine CRLF *(SDPLine CRLF)        ; see RFC 2327 
SDPLine        = 1*(%x01-09 / %x0B / %x0C / %x0E-FF) ; for proper def. 
 
 
 
                                                                        
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Appendix B: Base Package 
    
   Package name: B 
   Version: 0 
    
   The MGCP specification defines a base package which contains a set 
   of events and extension parameters that are of general use to the 
   protocol. Although not required, it is highly RECOMMENDED to support 
   this package as it provides important functionality for the base 
   protocol. 
    
B.1 Events 
    
   The table below lists the events: 
    
    ------------------------------------------------------------------ 
   | Symbol  |   Definition               |  R  |   S     Duration    | 
   |---------|----------------------------|-----|---------------------| 
   | enf(##) | embedded RQNT failure      |  x  |                     | 
   | oef     | observed events full       |  x  |                     | 
   | qbo     | quarantine buffer overflow |  x  |                     | 
    ------------------------------------------------------------------ 
    
   The events are defined as follows: 
    
   Embedded NotificationRequest failure (enf): 
     The Embedded NotificationRequest Failure (enf) event is generated 
     when an embedded Notification Request failure occurs. When the 
     event is requested, it should be as part of the Embedded 
     NotificationRequest itself. When the event is reported, it may be 
     parameterized with an error code (see Section 2.4) detailing the 
     error that occurred. When requested, it cannot be parameterized. 
      
   Observed events full (oef): 
     The event is generated when the endpoint is unable to accumulate 
     any more events in the list of ObservedEvents. If this event 
     occurs, and it is not used to trigger a Notify, subsequent events 
     that should have been added to the list will be lost.  
    
   Quarantine buffer overflow (qbo):  
     The event is generated when the quarantine buffer overflows and 
     one or more events have been lost.  
      
B.2 Extension Parameters 
    
B.2.1 PersistentEvents 
    
   PersistentEvents:     A list of events that the gateway is requested 
   to detect and report persistently. The parameter is optional but can 
   be provided in any command where the DetectEvents parameter can be 
   provided. The initial default value of the parameter is empty. When 
   the parameter is omitted from a command, it retains its current 
   value. When the parameter is provided, it completely replaces the 
                                                                        
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   current value. Providing an event in this list, is similar (but 
   preferable) to defining that particular event as being persistent. 
   The current list of PersistentEvents will implicitly apply to the 
   current as well as subsequent NotificationRequests, however no glare 
   detection etc. will be performed (similarly to DetectEvents). If an 
   event provided in this list is included in a RequestedEvents list, 
   the action and event parameters used in the RequestedEvents will 
   replace the action and event parameters associated with the event in 
   the PersistentEvents list for the life of the RequestedEvents list, 
   after which the PersistentEvents action and event parameters are 
   restored. Events with event states requested through this parameter 
   will be included in the list of EventStates if audited. 
    
   PersistentEvents can also be used to detect events on connections. 
   Use of the "all connections" wildcard is straightforward, whereas 
   using PersistentEvents with one or more specific connections must be 
   considered carefully. Once the connection in question is deleted, a 
   subsequent NotificationRequest without a new PersistentEvents value 
   will fail (error code 515 - incorrect connection-id, is 
   RECOMMENDED), as it implicitly refers to the deleted connection.  
    
   The parameter generates the relevant error codes from the base 
   protocol, e.g. error code 512 if an unknown event is specified. 
    
   The PersistentEvents parameter can be audited, in which case it will 
   return its current value. Auditing of RequestedEvents is not 
   affected by this extension, i.e. events specified in this list are 
   not automatically reported when auditing RequestedEvents. 
    
   The parameter name for PersistentEvents is "PR" and it is defined by 
   the production: 
    
     PersistentEvents = "PR" ":" 0*WSP  [RequestedEvents] 
    
   The following example illustrates the use of the parameter: 
    
     B/PR: L/hd(N), L/hf(N), L/hu(N), B/enf, B/oef, B/qbo 
    
   which instructs the endpoint to persistently detect and report off-
   hook, hook-flash, and on-hook. It also instructs the endpoint to 
   persistently detect and report Embedded Notification Request 
   failure, Observed events full, and Quarantine buffer overflow. 
    
B.2.2 NotificationState 
    
   NotificationState is a RequestedInfo parameter that can be audited 
   with the AuditEndpoint command. It can be used to determine if the 
   endpoint is in the notification state or not.  
    
   The parameter is forbidden in any command. In responses, it is a 
   valid response parameter for AuditEndpoint only.  
    
   It is defined by the following grammar: 
                                                                        
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     NotificationState        = "NS" ":" 0*WSP NotificationStateValue 
     NotificationStateValue   = "yes" / "no" 
    
   It is requested as part of auditing by including the parameter code 
   in RequestedInfo, as in: 
    
     F: B/NS 
    
   The response parameter will contain the value "yes" if the endpoint 
   is in the "notification state" or the value "no" if it is not, as 
   for example:  
    
     B/NS: yes 
      
B.3 Verbs 
    
   MGCP packages are not intended to define new commands, however an 
   exception is made in this case in order to add an important general 
   capability currently missing, namely the ability for the gateway to 
   send a generic message to the Call Agent.  
    
   The definition of the new command is: 
    
          ReturnCode 
          <-- Message(EndpointId  
                         [, ...]) 
    
   EndpointId is the name for the endpoint(s) in the gateway which is 
   issuing the Message command. The identifier MUST be a fully 
   qualified endpoint identifier, including the domain name of the 
   gateway. The local part of the endpoint name MUST NOT use the ććany 
   of╦╦ wildcard.  
    
   The only parameter specified in the definition of the Message 
   command is the EndpointId, however, it is envisioned that extensions 
   will define additional parameters to be used with the Message 
   command. Such extensions MUST NOT alter or otherwise interfere with 
   the normal operation of the basic MGCP protocol. They may however 
   define additional capabilities above and beyond that provided by the 
   basic MGCP protocol. For example, an extension to enable the gateway 
   to audit the packages supported by the Call Agent could be defined, 
   whereas using the Message command as an alternative way of reporting 
   observed events would be illegal, as that would alter the normal 
   MGCP protocol behavior.  
    
   In order to not interfere with normal MGCP operation, lack of a 
   response to the Message command MUST NOT lead the endpoint to become 
   disconnected. The endpoint(s) MUST be prepared to handle this 
   transparently and continue normal processing unaffected. 
    
   If the endpoint(s) receive a response indicating that the Call Agent 
   does not support the Message command, the endpoint(s) MUST NOT send 
                                                                        
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   a Message command again until the current ććnotified entity╦╦ has 
   changed. Similarly, if the endpoint(s) receive a response indicating 
   that the Call Agent does not support one or more parameters in the 
   Message command, the endpoint(s) MUST NOT send a Message command 
   with those parameters again until the current ććnotified entity╦╦ has 
   changed.  
    
   The ććMessage╦╦ command is encoded as ććMESG╦╦, as shown in the 
   following example: 
    
      MESG 1200 aaln/1@rgw.whatever.net MGCP 1.0 
       
    
    
    
    





































                                                                        
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Appendix C: IANA Considerations 
    
C.1 New MGCP Package Sub-Registry 
    
   Upon publication of this document as an RFC, IANA is instructed to 
   establish a new sub-registry for MGCP packages under    
   http://www.iana.org/assignments/mgcp-packages.  
    
   Packages can be registered with the IANA according to the following 
   procedure: 
    
   The package MUST have a unique string name which MUST NOT start with 
   the two characters "x-" or "x+". 
    
   The package name and version (zero assumed by default) MUST be 
   registered with IANA as well as a reference to the document that 
   describes the package. The document MUST have a stable URL and MUST 
   be contained on a public web server. 
    
   Packages may define one or more Extension Digit Map Letters, however 
   these are taken from a limited and flat name space. To prevent name 
   clashing, IANA SHALL NOT register a package that defines an 
   Extension Digit Map Letter already defined in another package 
   registered by IANA. To ease this task, such packages SHALL contain 
   the line "Extension Digit Map Letters:  " followed by a list of the 
   Extension Digit Map Letters defined in the package at the beginning 
   of the package definition. 
    
   A contact name, e-mail and postal address for the package MUST be 
   provided. The contact information SHALL be updated by the defining 
   organization as necessary. 
    
C.2 New MGCP Package 
 
   This document defines a new MGCP Base Package in Appendix B, which 
   is to be registered with IANA.  
    
C.3 New MGCP LocalConnectionOptions Sub-Registry 
    
   Upon publication of this document as an RFC, IANA is instructed to 
   establish a new sub-registry for MGCP LocalConnectionOptions under    
   http://www.iana.org/assignments/mgcp-localconnectionoptions. 
    
   Packages are the preferred extension mechanism, however for 
   backwards compatibility, local connection options beyond those 
   provided in this specification can be registered with IANA. Each 
   such local connection option MUST have a unique string name which 
   MUST NOT start with "x-" or "x+". The name of the local connection 
   option MUST be registered with IANA as well as a reference to the 
   document that describes the local connection option. The document 
   MUST have a stable URL and MUST be contained on a public web server. 
    

                                                                        
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   A contact name, e-mail and postal address for the local connection 
   option MUST be provided. The contact information SHALL be updated by 
   the defining organization as necessary. 
    
    
    















































                                                                        
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Appendix D: Mode Interactions 
    
   An MGCP endpoint can establish one or more media streams. These 
   streams are either incoming (from a remote endpoint) or outgoing 
   (generated at the handset microphone). The "connection mode" 
   parameter establishes the direction and generation of these streams. 
   When there is only one connection to an endpoint, the mapping of 
   these streams is straightforward; the handset plays the incoming 
   stream over the handset speaker and generates the outgoing stream 
   from the handset microphone signal, depending on the mode parameter.  
    
   However, when several connections are established to an endpoint, 
   there can be many incoming and outgoing streams. Depending on the 
   connection mode used, these streams may interact differently with 
   each other and the streams going to/from the handset.  
    
   The table below describes how different connections SHALL be mixed 
   when one or more connections are concurrently "active". An active 
   connection is here defined as a connection that is in one of the 
   following modes:  
    
   *  "send/receive" 
   *  "send only" 
   *  "receive only" 
   *  "conference" 
    
   Connections in "network loopback", "network continuity test", or 
   "inactive" modes are not affected by connections in the "active" 
   modes. The Table uses the following conventions: 
    
   *  Ai is the incoming media stream from Connection A 
   *  Bi is the incoming media stream from Connection B 
   *  Hi is the incoming media stream from the Handset Microphone 
   *  Ao is the outgoing media stream to Connection A 
   *  Bo is the outgoing media stream to Connection B 
   *  Ho is the outgoing media stream to the Handset earpiece 
   *  NA indicates No Stream whatever 
    
   "netw" in the following table indicates either "netwloop" or 
   "netwtest" mode. 
    












                                                                        
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     ------------------------------------------------------------- 
    |       |               Connection A Mode                     | 
    |       |----------------------------------------------------- 
    |       |sendonly|recvonly|sendrecv|confrnce|inactive|  netw  | 
    |-------|-----------------------------------------------------| 
    | |Send | Ao=Hi  | Ao=NA  | Ao=Hi  | Ao=Hi  | Ao=NA  | Ao=Ai  | 
    |C|only | Bo=Hi  | Bo=Hi  | Bo=Hi  | Bo=Hi  | Bo=Hi  | Bo=Hi  | 
    |o|     | Ho=NA  | Ho=Ai  | Ho=Ai  | Ho=Ai  | Ho=NA  | Ho=NA  | 
    |n|----------------------------------------------------------- 
    |n|recv |        |Ao=NA   |Ao=Hi   |Ao=Hi   | Ao=NA  | Ao=Ai  | 
    |e|only |        |Bo=NA   |Bo=NA   |Bo=NA   | Bo=NA  | Bo=NA  | 
    |c|     |        |Ho=Ai+Bi|Ho=Ai+Bi|Ho=Ai+Bi| Ho=Bi  | Ho=Bi  | 
    |t|-----------------------------------------------------------| 
    |i|send |        |        |Ao=Hi   |Ao=Hi   | Ao=NA  | Ao=Ai  | 
    |o|recv |        |        |Bo=Hi   |Bo=Hi   | Bo=Hi  | Bo=Hi  | 
    |n|     |        |        |Ho=Ai+Bi|Ho=Ai+Bi| Ho=Bi  | Ho=Bi  | 
    | |-----------------------------------------------------------| 
    |B|conf |        |        |        |Ao=Hi+Bi| Ao=NA  | Ao=Ai  | 
    | |rnce |        |        |        |Bo=Hi+Ai| Bo=Hi  | Bo=Hi  | 
    |M|     |        |        |        |Ho=Ai+Bi| Ho=Bi  | Ho=Bi  | 
    |o|-----------------------------------------------------------| 
    |d|Inac |        |        |        |        | Ao=NA  | Ao=Ai  | 
    |e|tive |        |        |        |        | Bo=NA  | Bo=NA  | 
    | |     |        |        |        |        | Ho=NA  | Ho=NA  | 
    | |-----------------------------------------------------------| 
    | |netw |        |        |        |        |        | Ao=Ai  | 
    | |     |        |        |        |        |        | Bo=Bi  | 
    | |     |        |        |        |        |        | Ho=NA  | 
     ------------------------------------------------------------- 
    
   If there are three or more "active" channels they will still 
   interact as defined in the table above with the outgoing media 
   streams mixed for each interaction (union of all streams). If 
   internal resources are used up and the streams cannot be mixed, the 
   gateway MUST return an error (error code 403 or 502, not enough 
   resources, are RECOMMENDED). 
    
    















                                                                        
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Appendix E: Endpoint Naming Conventions 
    
   The following sections provide some RECOMMENDED endpoint naming 
   conventions. 
    
E.1 Analog Access Line Endpoints 
    
   The string "aaln", should be used as the first term in a local 
   endpoint name for analog access line endpoints. Terms following 
   "aaln" should follow the physical hierarchy of the gateway so that 
   if the gateway has a number of RJ11 ports, the local endpoint name 
   could look like the following: 
    
      aaln/#    
    
   where "#" is the number of the analog line (RJ11 port) on the 
   gateway.  
    
   On the other hand, the gateway may have a number of physical plug-in 
   units, each of which contain some number of RJ11 ports, in which 
   case, the local endpoint name might look like the following: 
    
      aaln/<unit #>/# 
    
   where <unit #> is the number of the plug in unit in the gateway and 
   "#" is the number of the analog line (RJ11 port) on that unit. 
   Leading zeroes MUST NOT be used in any of the numbers ("#") above. 
    
E.2 Digital Trunks 
    
   The string "ds" should be used for the first term of digital 
   endpoints with a naming convention that follows the physical and 
   digital hierarchy such as: 
    
      ds/<unit-type1>-<unit #>/<unit-type2>-<unit #>/.../<channel #> 
    
   where: <unit-type> identifies the particular hierarchy level. Some 
   example values of <unit-type> are: "s", "su", "oc3", "ds3", "e3", 
   "ds2", "e2", "ds1", "e1" where "s" indicates a slot number and "su" 
   indicates a sub-unit within a slot. Leading zeroes MUST NOT be used 
   in any of the numbers ("#") above. 
    
   The <unit #> is a decimal number which is used to reference a 
   particular instance of a <unit-type> at that level of the hierarchy. 
   The number of levels and naming of those levels is based on the 
   physical hierarchy within the media gateway. 
    
E.3 Virtual Endpoints 
    
   Another type of endpoint is one that is not associated with a 
   physical interface (such as an analog or digital endpoint). This 
   type of endpoint is called a virtual endpoint and is often used to 
   represent some DSP resources that gives the endpoint some 
                                                                        
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   capability. Examples are announcement, IVR or conference bridge 
   devices. These devices may have multiple instances of DSP functions 
   so that a possible naming convention is: 
    
      <virtual-endpoint-type>/<endpoint-#> 
    
   where <virtual-endpoint-type> may be some string representing the 
   type of endpoint (such as "ann" for announcement server or "cnf" for 
   conference server) and <endpoint-#> would identify a particular 
   virtual endpoint within the device. Leading zeroes MUST NOT be used 
   in the number ("#") above. If the physical hierarchy of the server 
   includes plug-in DSP cards, another level of hierarchy in the local 
   endpoint name may be used to describe the plug in unit. 
    
   A virtual endpoint may be created as the result of using the "any 
   of" wildcard. Similarly, a virtual endpoint may cease to exist once 
   the last connection on the virtual endpoint is deleted. The 
   definition of the virtual endpoint MUST detail both of these 
   aspects.  
    
   When a <virtual-endpoint-type> creates and deletes virtual endpoints 
   automatically, there will be cases where no virtual endpoints exist 
   at the time a RestartInProgress command is to be issued. In such 
   cases, the gateway SHOULD simply use the "all of" wildcard in lieu 
   of any specific <endpoint-#> as in, e.g.: 
    
     ann/*@mygateway.whatever.net 
    
   If the RestartInProgress command refers to all endpoints in the 
   gateway (virtual or not), the <virtual-endpoint-id> can be omitted 
   as in, e.g.: 
    
     *@mygateway.whatever.net 
    
   Commands received by the gateway will still have to refer to an 
   actual endpoint (possibly created by that command by use of the "any 
   of" wildcard) in order for the command to be processed though. 
    
E.4 Media Gateway 
    
   MGCP only defines operation on endpoints in a media gateway. It may 
   be beneficial to define an endpoint that represents the gateway 
   itself as opposed to the endpoints managed by the gateway. 
   Implementations that wish to do so should use the local endpoint 
   name "mg" (for media gateway) as in: 
    
     mg@mygateway.whatever.net 
    
   Note that defining such an endpoint does not change any of the 
   protocol semantics, i.e., the "mg" endpoint and other endpoints 
   (e.g. digital trunks) in the gateway are still independent endpoints 
   and MUST be treated as such. For example, RestartInProgress commands 
   MUST still be issued for all endpoints in the gateway as usual.  
                                                                        
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E.5 Range Wildcards  
 
   As described in Section 2.1.2, the MGCP endpoint naming scheme 
   defines the "all of" and "any of" wildcards for the individual terms 
   in a local endpoint name. While the "all of" wildcard is very useful 
   for reducing the number of messages, it can by definition only be 
   used when we wish to refer to all instances of a given term in the 
   local endpoint name. Furthermore, in the case where a command is to 
   be sent by the gateway to the Call Agent, the "all of" wildcard can 
   only be used if all of the endpoints named by it have the same 
   "notified entity". Implementations that prefer a finer-grained 
   wildcarding scheme can use the range wildcarding scheme described 
   here.  
    
   A range wildcard is defined as follows: 
    
   RangeWildcard    = "[" NumericalRange *( "," NumericalRange ) "]" 
   NumericalRange   = 1*(DIGIT) [ "-" 1*(DIGIT) ] 
    
   Note that white space is not permitted. Also, since range wildcards 
   use the character "[" to indicate the start of a range, the "[" 
   character MUST NOT be used in endpoint names that use range 
   wildcards. The length of a range wildcard SHOULD be bounded to a 
   reasonably small value, e.g. 128 characters.  
    
   Range wildcards can be used anywhere an "all of" wildcard can be 
   used. The semantics are identical for the endpoints named. However, 
   it MUST be noted, that use of the range wildcarding scheme requires 
   support on both the gateway and the Call Agent. Therefore, a gateway 
   MUST NOT assume that it's Call Agent supports range wildcarding and 
   vice versa. In practice, this typically means that both the gateway 
   and Call Agent will need to be provisioned consistently in order to 
   use range wildcards. Also, if a gateway or Call Agent using range 
   wildcards receives an error response that could indicate a possible 
   endpoint naming problem, they MUST be able to automatically revert 
   to not using range wildcards.  
    
   The following examples illustrates the use of range wildcards: 
    
      ds/ds1-1/[1-12] 
      ds/ds1-1/[1,3,20-24] 
      ds/ds1-[1-2]/* 
      ds/ds3-1/[1-96] 
      
   The following example illustrates how to use it in a command: 
    
      RSIP 1204 ds/ds3-1/[1-96]@tgw-18.whatever.net MGCP 1.0  
      RM: restart 
      RD: 0 



                                                                        
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Appendix F: Example Command Encodings 
    
   This appendix provides examples of commands and responses shown with 
   the actual encoding used. Examples are provided for each command. 
   All commentary shown in the commands and responses is optional. 
    
F.1 NotificationRequest 
    
   The first example illustrates a NotificationRequest that will ring a 
   phone and look for an off-hook event: 
    
      RQNT 1201 aaln/1@rgw-2567.whatever.net MGCP 1.0 
      N: ca@ca1.whatever.net:5678 
      X: 0123456789AC 
      R: l/hd(N) 
      S: l/rg 
 
   The response indicates that the transaction was successful: 
    
      200 1201 OK 
    
   The second example illustrates a NotificationRequest that will look 
   for and accumulate an off-hook event, and then provide dial-tone and 
   accumulate digits according to the digit map provided. The "notified 
   entity" is set to "ca@ca1.whatever.net:5678", and since the 
   SignalRequests parameter is empty (it could have been omitted as 
   well), all currently active TO signals will be stopped. All events 
   in the quarantine buffer will be processed, and the list of events 
   to detect in the "notification" state will include fax tones in 
   addition to the "requested events" and persistent events: 
    
      RQNT 1202 aaln/1@rgw-2567.whatever.net MGCP 1.0 
      N: ca@ca1.whatever.net:5678 
      X: 0123456789AC 
      R: L/hd(A, E(S(L/dl),R(L/oc, L/hu, D/[0-9#*T](D)))) 
      D: (0T|00T|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)  
      S: 
      Q: process 
      T: G/ft 
    
   The response indicates that the transaction was successful: 
    
      200 1202 OK 
    
F.2 Notify 
    
   The example below illustrates a Notify message that notifies an off-
   hook event followed by a 12-digit number beginning with "91". A 
   transaction identifier correlating the Notify with the 
   NotificationRequest it results from is included. The command is sent 
   to the current "notified entity", which typically will be the actual 
   value supplied in the NotifiedEntity parameter, i.e.,  

                                                                        
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   "ca@ca1.whatever.net:5678" - a failover situation could have changed 
   this: 
    
      NTFY 2002 aaln/1@rgw-2567.whatever.net MGCP 1.0 
      N: ca@ca1.whatever.net:5678 
      X: 0123456789AC 
      O: L/hd,D/9,D/1,D/2,D/0,D/1,D/8,D/2,D/9,D/4,D/2,D/6,D/6 
    
   The Notify response indicates that the transaction was successful: 
    
      200 2002 OK 
    
F.3 CreateConnection 
    
   The first example illustrates a CreateConnection command to create a 
   connection on the endpoint specified. The connection will be part of 
   the specified CallId. The LocalConnectionOptions specify that G.711 
   mu-law will be the codec used and the packetization period will be 
   10 ms. The connection mode will be "receive only":  
    
      CRCX 1204 aaln/1@rgw-2567.whatever.net MGCP 1.0 
      C: A3C47F21456789F0 
      L: p:10, a:PCMU 
      M: recvonly 
    
   The response indicates that the transaction was successful, and a 
   connection identifier for the newly created connection is therefore 
   included. A session description for the new connection is included 
   as well - note that it is preceded by an empty line. 
    
      200 1204 OK 
      I: FDE234C8 
       
      v=0 
      o=- 25678 753849 IN IP4 128.96.41.1 
      s=- 
      c=IN IP4 128.96.41.1 
      t=0 0 
      m=audio 3456 RTP/AVP 0  
    
   The second example illustrates a CreateConnection command containing 
   a notification request and a RemoteConnectionDescriptor: 
    










                                                                        
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      CRCX 1205 aaln/1@rgw-2569.whatever.net MGCP 1.0  
      C: A3C47F21456789F0 
      L: p:10, a:PCMU 
      M: sendrecv 
      X: 0123456789AD 
      R: L/hd 
      S: L/rg 
       
      v=0 
      o=- 25678 753849 IN IP4 128.96.41.1 
      s=- 
      c=IN IP4 128.96.41.1 
      t=0 0 
      m=audio 3456 RTP/AVP 0 
    
   The response indicates that the transaction failed, because the 
   phone was already off-hook. Consequently, neither a connection-id 
   nor a session description is returned: 
    
      401 1205 Phone off-hook  
    
   Our third example illustrates the use of the provisional response 
   and the three-way handshake. We create another connection and 
   acknowledge the previous response received by using the response 
   acknowledgement parameter: 
    
      CRCX 1206 aaln/1@rgw-2569.whatever.net MGCP 1.0 
      K: 1205 
      C: A3C47F21456789F0 
      L: p:10, a:PCMU  
      M: inactive 
       
      v=0 
      o=- 25678 753849 IN IP4 128.96.41.1 
      s=- 
      c=IN IP4 128.96.41.1 
      t=0 0 
      m=audio 3456 RTP/AVP 0  
    
   A provisional response is returned initially: 
    
      100 1206 Pending 
      I: DFE233D1 
       
      v=0 
      o=- 4723891 7428910 IN IP4 128.96.63.25 
      s=- 
      c=IN IP4 128.96.63.25 
      t=0 0 
      m=audio 3456 RTP/AVP 0  
    
   A little later, the final response is received: 
    
                                                                        
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      200 1206 OK 
      K: 
      I: DFE233D1 
       
      v=0 
      o=- 4723891 7428910 IN IP4 128.96.63.25 
      s=- 
      c=IN IP4 128.96.63.25 
      t=0 0 
      m=audio 3456 RTP/AVP 0  
    
   The Call Agent acknowledges the final response as requested: 
    
      000 1206 
    
   and the transaction is complete. 
    
F.4 ModifyConnection 
    
   The first example shows a ModifyConnection command that simply sets 
   the connection mode of a connection to "send/receive" - the 
   "notified entity" is set as well: 
    
      MDCX 1209 aaln/1@rgw-2567.whatever.net MGCP 1.0 
      C: A3C47F21456789F0 
      I: FDE234C8 
      N: ca@ca1.whatever.net  
      M: sendrecv 
    
   The response indicates that the transaction was successful: 
    
      200 1209 OK 
    
   In the second example, we pass a session description and include a 
   notification request with the ModifyConnection command. The endpoint 
   will start playing ring-back tones to the user: 
    
      MDCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0 
      C: A3C47F21456789F0 
      I: FDE234C8 
      M: recvonly 
      X: 0123456789AE 
      R: L/hu  
      S: G/rt 
       
      v=0 
      o=- 4723891 7428910 IN IP4 128.96.63.25 
      s=- 
      c=IN IP4 128.96.63.25 
      t=0 0 
      m=audio 3456 RTP/AVP 0  
    
   The response indicates that the transaction was successful:  
                                                                        
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      200 1206 OK 
    
F.5 DeleteConnection (from the Call Agent) 
    
   In this example, the Call Agent simply instructs the gateway to 
   delete the connection "FDE234C8" on the endpoint specified: 
    
      DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0 
      C: A3C47F21456789F0 
      I: FDE234C8 
    
   The response indicates success, and that the connection was deleted. 
   Connection parameters for the connection are therefore included as 
   well: 
    
      250 1210 OK 
      P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48 
    
F.6 DeleteConnection (from the gateway) 
    
   In this example, the gateway sends a DeleteConnection command to the 
   Call Agent to instruct it that a connection on the specified 
   endpoint has been deleted. The ReasonCode specifies the reason for 
   the deletion, and Connection Parameters for the connection are 
   provided as well: 
    
      DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0 
      C: A3C47F21456789F0 
      I: FDE234C8 
      E: 900 - Hardware error 
      P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48 
    
   The Call Agent sends a success response to the gateway: 
    
      200 1210 OK 
    
F.7 DeleteConnection (multiple connections from the Call Agent) 
    
   In the first example, the Call Agent instructs the gateway to delete 
   all connections related to call "A3C47F21456789F0" on the specified 
   endpoint: 
    
      DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0 
      C: A3C47F21456789F0 
    
   The response indicates success and that the connection(s) were 
   deleted: 
    
      250 1210 OK 
    
   In the second example, the Call Agent instructs the gateway to 
   delete all connections related to all of the endpoints specified: 
                                                                        
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      DLCX 1210 aaln/*@rgw-2567.whatever.net MGCP 1.0 
    
   The response indicates success: 
    
      250 1210 OK 
    
F.8 AuditEndpoint 
    
   In the first example, the Call Agent wants to learn what endpoints 
   are present on the gateway specified, hence the use of the "all of" 
   wild-card for the local portion of the endpoint-name: 
    
      AUEP 1200 *@rgw-2567.whatever.net MGCP 1.0 
    
   The gateway indicates success and includes a list of endpoint names: 
    
      200 1200 OK 
      Z: aaln/1@rgw-2567.whatever.net 
      Z: aaln/2@rgw-2567.whatever.net 
    
   In the second example, the capabilities of one of the endpoints is 
   requested: 
    
      AUEP 1201 aaln/1@rgw-2567.whatever.net MGCP 1.0  
      F: A 
    
   The response indicates success and the capabilities as well. Two 
   codecs are supported, however with different capabilities. 
   Consequently two separate capability sets are returned: 
    
      200 1201 OK 
      A: a:PCMU, p:10-100, e:on, s:off, v:L;S, m:sendonly; 
               recvonly;sendrecv;inactive;netwloop;netwtest 
      A: a:G729, p:30-90, e:on, s:on, v:L;S, m:sendonly; 
               recvonly;sendrecv;inactive;confrnce;netwloop 
    
   Note that the carriage return in the Capabilities lines are shown 
   for formatting reasons only - they are not permissible in a real 
   implementation. 
    
   In the third example, the Call Agent audits several types of 
   information for the endpoint: 
    
      AUEP 2002 aaln/1@rgw-2567.whatever.net MGCP 1.0  
      F: R,D,S,X,N,I,T,O,ES 
    
   The response indicates success: 
    




                                                                        
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      200 2002 OK 
      R: L/hu,L/oc(N),D/[0-9](N)  
      D: 
      S: L/vmwi(+) 
      X: 0123456789B1 
      N: [128.96.41.12] 
      I: 32F345E2 
      T: G/ft 
      O: L/hd,D/9,D/1,D/2 
      ES: L/hd 
    
   The list of requested events contains three events. Where no package 
   name is specified, the default package is assumed. The same goes for 
   actions, so the default action - Notify - must therefore be assumed 
   for the "L/hu" event. The omission of a value for the "digit map" 
   means the endpoint currently does not have a digit map. There are 
   currently no active time-out signals, however the OO signal "vmwi" 
   is currently on and is consequently included - in this case it was 
   parameterized, however the parameter could have been excluded. The 
   current "notified entity" refers to an IP-address and only a single 
   connection exists for the endpoint. The current value of 
   DetectEvents is "G/ft", and the list of ObservedEvents contains the 
   four events specified. Finally, the event-states audited reveals 
   that the phone was off-hook at the time the transaction was 
   processed. 
    
F.9 AuditConnection 
    
   The first example shows an AuditConnection command where we audit 
   the CallId, NotifiedEntity, LocalConnectionOptions, Connection Mode, 
   LocalConnectionDescriptor, and the Connection Parameters: 
    
      AUCX 2003 aaln/1@rgw-2567.whatever.net MGCP 1.0 
      I: 32F345E2 
      F: C,N,L,M,LC,P 
    
   The response indicates success and includes information for the 
   RequestedInfo: 
    
      200 2003 OK 
      C: A3C47F21456789F0 
      N: ca@ca1.whatever.net 
      L: p:10, a:PCMU 
      M: sendrecv 
      P: PS=395, OS=22850, PR=615, OR=30937, PL=7, JI=26, LA=47 
       
      v=0 
      o=- 4723891 7428910 IN IP4 128.96.63.25 
      s=- 
      c=IN IP4 128.96.63.25 
      t=0 0 
      m=audio 1296 RTP/AVP 0  
    
                                                                        
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   In the second example, we request to audit 
   RemoteConnectionDescriptor and LocalConnectionDescriptor: 
    
      AUCX 1203 aaln/2@rgw-2567.whatever.net MGCP 1.0  
      I: FDE234C8 
      F: RC,LC 
    
   The response indicates success, and includes information for the 
   RequestedInfo. In this case, no RemoteConnectionDescriptor exists, 
   hence only the protocol version field is included for the 
   RemoteConnectionDescriptor: 
    
      200 1203 OK 
        
      v=0 
      o=- 4723891 7428910 IN IP4 128.96.63.25 
      s=- 
      c=IN IP4 128.96.63.25 
      t=0 0 
      m=audio 1296 RTP/AVP 0 
       
      v=0 
    
F.10 RestartInProgress 
    
   The first example illustrates a RestartInProgress message sent by an 
   gateway to inform the Call Agent that the specified endpoint will be 
   taken out-of-service in 300 seconds: 
    
      RSIP 1200 aaln/1@rgw-2567.whatever.net MGCP 1.0 
      RM: graceful 
      RD: 300 
    
   The Call Agent╦s response indicates that the transaction was 
   successful: 
    
      200 1200 OK 
    
   In the second example, the RestartInProgress message sent by the 
   gateway informs the Call Agent, that all of the gateway╦s endpoints 
   are being placed in-service in 0 seconds, i.e., they are currently 
   in service. The restart delay could have been omitted as well:  
    
      RSIP 1204 *@rgw-2567.whatever.net MGCP 1.0  
      RM: restart 
      RD: 0 
    
   The Call Agent╦s response indicates success, and furthermore 
   provides the endpoints in question with a new "notified entity": 
    
      200 1204 OK 
      N: CA-1@whatever.net 
    
                                                                        
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   Alternatively, the command could have failed with a new "notified 
   entity" as in: 
    
      521 1204 OK 
      N: CA-1@whatever.net 
    
   In that case, the command would then have to be retried in order to 
   satisfy the "restart procedure", this time going to Call Agent "CA-
   1@whatever.net". 
    











































                                                                        
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Appendix G: Example Call Flows 
    
   The message flow tables in this section use the following 
   abbreviations: 
    
   *  rgw = Residential Gateway 
   *  ca  = Call Agent 
   *  n+  = step 'n' is repeated one or more times 
    
   Note that any use of upper and lower case within the text of the 
   messages is to aid readability and is not in any way a requirement. 
   The only requirement involving case is to be case insensitive at all 
   times. 
    
G.1 Restart 
    
G.1.1 Residential Gateway Restart 
    
   The following table shows a message sequence that might occur when a 
   call agent (ca) is contacted by two independent residential gateways 
   (rgw1 and rgw2) which have restarted. 
    
                 Table F.1: Residential Gateway Restart 
 --------------------------------------------------------------------- 
|step#|    usr1    |    rgw1    |     ca     |    rgw2    |    usr2   | 
|=====|============|============|============|============|===========| 
|  1  |            |    rsip -> |            |            |           | 
|     |            |            | <- ack     |            |           | 
|-----|------------|------------|------------|------------|-----------| 
|  2  |            |            | <- auep    |            |           | 
|     |            |     ack -> |            |            |           | 
|-----|------------|------------|------------|------------|-----------| 
|  3+ |            |            | <- rqnt    |            |           | 
|     |            |     ack -> |            |            |           | 
|-----|------------|------------|------------|------------|-----------| 
|  4  |            |            |            | <- rsip    |           | 
|     |            |            |     ack -> |            |           | 
|-----|------------|------------|------------|------------|-----------| 
|  5  |            |            |    auep -> |            |           | 
|     |            |            |            | <- ack     |           | 
|-----|------------|------------|------------|------------|-----------| 
|  6+ |            |            |    rqnt -> |            |           | 
|     |            |            |            | <- ack     |           | 
 --------------------------------------------------------------------- 
    
   Step 1 - RestartInProgress (rsip) from rgw1 to ca 
    
   rgw1 uses DNS to determine the domain name of ca and send to the 
   default port of 2727. The command consists of the following: 
    
      rsip 1 *@rgw1.whatever.net mgcp 1.0 
      rm: restart 
    
                                                                        
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   The "*" is used to inform ca that all endpoints of rgw1 are being 
   restarted, and "restart" is specified as the restart method. The 
   Call Agent "ca" acknowledges the command with an acknowledgement 
   message containing the transaction-id (in this case 1) for the 
   command. It sends the acknowledgement to rgw1 using the same port 
   specified as the source port for the rsip. If none was indicated, it 
   uses the default port of 2727. 
    
      200 1 ok 
    
   A response code is mandatory. In this case, "200", indicates "the 
   requested transaction was executed normally". The response string is 
   optional. In this case, "ok" is included as an additional 
   description. 
    
   Step 2 - AuditEndpoint (auep) from ca to rgw1 
    
   The command consists of the following: 
    
      auep 153 *@rgw1.whatever.net mgcp 1.0 
    
   The "*" is used to request audit information from rgw1 of all its 
   endpoints. rgw1 acknowledges the command with an acknowledgement 
   message containing the transaction-id (in this case 153) of the 
   command, and it includes a list of its endpoints. In this example, 
   rgw1 has two endpoints, aaln/1 and aaln/2. 
    
      200 153 ok 
      Z: aaln/1@rgw1.whatever.net  
      Z: aaln/2@rgw1.whatever.net 
    
   Once it has the list of endpoint ids, ca may send individual 
   AuditEndpoint commands in which the "*" is replaced by the id of the 
   given endpoint. As its response, rgw1 would replace the endpoint id 
   list returned in the example with the info requested for the 
   endpoint. This optional message exchange is not shown in this 
   example. 
    
   Step 3 - NotificationRequest (rqnt) from ca to each endpoint of rgw1 
    
   In this case, ca sends two rqnts, one for aaln/1: 
    
      rqnt 154 aaln/1@rgw1.whatever.net mgcp 1.0 
      r: l/hd(n) 
      x: 3456789a0 
    
   and a second for aaln/2:  
    
      rqnt 155 aaln/2@rgw1.whatever.net mgcp 1.0 
      r: l/hd(n) 
      x: 3456789a1 
    

                                                                        
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   Note that in the requested events parameter line, the event is fully 
   specified as "l/hd", i.e. with the package name, in order to avoid 
   any potential ambiguity. This is the recommended behavior. For the 
   sake of clarity, the action, which in this case is to Notify, is 
   explicitly specified by including the "(n)". If no action is 
   specified, Notify is assumed as the default regardless of the event. 
   If any other action is desired, it must be stated explicitly. 
    
   The expected response from rgw1 to these requests is an 
   acknowledgement from aaln/1 as follows: 
    
      200 154 ok 
    
   and from aaln/2: 
    
      200 155 ok 
    
   Step 4 RestartInProgress (rsip) from rgw2 to ca  
    
      rsip 0 *@rgw2.whatever.net mgcp 1.0 
      rm: restart 
    
   followed by the acknowledgement from ca: 
    
      200 0 ok 
    
   Step 5 - AuditEndpoint (auep) from ca to rgw2 
    
      auep 156 *@rgw2.whatever.net mgcp 1.0 
    
   followed by an acknowledgement from rgw2: 
    
      200 156 ok 
      z: aaln/1@rgw2.whatever.net 
      z: aaln/2@rgw2.whatever.net 
    
   Step 6 - NotificationRequest (rqnt) from ca to each endpoint of rgw2 
    
      rqnt 157 aaln/1@rgw2.whatever.net mgcp 1.0 
      r: l/hd(n) 
      x: 3456789a2 
    
   followed by: 
    
      rqnt 158 aaln/2@rgw2.whatever.net mgcp 1.0 
      r: l/hd(n) 
      x: 3456789a3 
    
   with rgw2 acknowledging for aaln/1: 
    
      200 157 ok 
    
   and for aaln/2: 
                                                                        
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      200 158 ok 
    
G.1.2 Call Agent Restart 
    
   The following table shows the message sequence which occurs when a 
   call agent (ca) restarts. How it determines the address information 
   of the gateways, in this case rgw1 and rgw2, is not covered in this 
   document. For interoperability, it is RECOMMENDED to provide the 
   ability to configure the call agent to send AUEP (*) to specific 
   addresses and ports. 
    
                 Table F.2: Residential Gateway Restart 
 --------------------------------------------------------------------- 
| # |     usr1    |    rgw1    |     ca     |    rgw2    |     usr2   | 
|===|=============|============|============|============|============| 
| 1 |             |            | <- auep    |            |            | 
|   |             |     ack -> |            |            |            | 
|---|-------------|------------|------------|------------|------------| 
| 2+|             |            | <- rqnt    |            |            | 
|   |             |     ack -> |            |            |            | 
|---|-------------|------------|------------|------------|------------| 
| 3 |             |            |    auep -> |            |            | 
|   |             |            |            | <- ack     |            | 
|---|-------------|------------|------------|------------|------------| 
| 4+|             |            |    rqnt -> |            |            | 
|   |             |            |            | <- ack     |            | 
 --------------------------------------------------------------------- 
    
   Step 1 - AuditEndpoint (auep) from ca to rgw1 
    
   The command consists of the following: 
    
      auep 0 *@rgw1.whatever.net mgcp 1.0 
    
   The "*" is used to request audit information from rgw1 of all its 
   endpoints. rgw1 acknowledges the command with an acknowledgement 
   message containing the transaction id (in this case 0) of the 
   command, and it includes a list of its endpoints. In this example, 
   rgw1 has two endpoints, aaln/1 and aaln/2. 
    
      200 0 ok 
      z: aaln/1@rgw1.whatever.net 
      z: aaln/2@rgw1.whatever.net 
    
   Once it has the list of endpoint ids, ca may send individual 
   AuditEndpoint commands in which the "*" is replaced by the id of the 
   given endpoint. As its response, rgw1 would replace the endpoint id 
   list returned in the example with the info requested for the 
   endpoint. This optional message exchange is not shown in this 
   example. 
    
   Step 2 - NotificationRequest (rqnt) off-hook from ca to rgw1 
                                                                        
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   In this case, ca sends two rqnts, one for aaln/1: 
    
      rqnt 1 aaln/1@rgw1.whatever.net mgcp 1.0 
      r: l/hd(n) 
      x: 234567890 
    
   and a second for aaln/2:  
    
      rqnt 2 aaln/2@rgw1.whatever.net mgcp 1.0 
      r: l/hd(n) 
      x: 234567891 
    
   The expected response from rgw1 to these requests is an 
   acknowledgement from aaln/1 as follows: 
    
      200 1 ok 
    
   and from aaln/2: 
    
      200 2 ok 
    
   Step 3 - AuditEndpoint (auep) from ca to rgw2 
    
      auep 3 *@rgw2.whatever.net mgcp 1.0 
    
   followed by an acknowledgement from rgw2: 
    
      200 3 ok 
      z: aaln/1@rgw2.whatever.net 
      z: aaln/2@rgw2.whatever.net 
    
   Step 4 - NotificationRequest (rqnt) from ca to each endpoint of rgw2 
    
      rqnt 4 aaln/1@rgw2.whatever.net mgcp 1.0 
      r: l/hd(n) 
      x: 234567892 
    
   followed by: 
    
      rqnt 5 aaln/2@rgw2.whatever.net mgcp 1.0 
      r: l/hd(n) 
      x: 234567893 
    
   with rgw2 acknowledging for aaln/1: 
    
      200 4 ok 
    
   and for aaln/2: 
    
      200 5 ok 
    

                                                                        
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G.2 Connection Creation 
    
G.2.1 Residential Gateway to Residential Gateway 
    
   The following table shows the message sequence which occurs when a 
   user (usr1) makes a call through a residential gateway (rgw1) to a 
   user served by another residential gateway (rgw2). This example 
   illustrates the communication between the residential gateways and 
   the call agent (ca) only. The local name of the endpoints in this 
   example is aaln/1 for both gateways, and references within the 
   description of the steps to rgw1 and rgw2 can be assumed to refer to 
   aaln/1 of rgw1 and aaln/1 of rgw2. Note that this is only an example 
   and is not the only legal call scenario. 
    







































                                                                        
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           Table F.3: Residential Gateway Connection Creation 
 --------------------------------------------------------------------- 
| # |     usr1    |    rgw1    |     ca     |    rgw2    |     usr2   | 
|===|=============|============|============|============|============| 
| 1 |  offhook -> |    ntfy -> |            |            |            | 
|   |             |            | <- ack     |            |            | 
|---|-------------|------------|------------|------------|------------| 
| 2 | <- dialtone |            | <- rqnt    |            |            | 
|   |             |     ack -> |            |            |            | 
|---|-------------|------------|------------|------------|------------| 
| 3 |   digits -> |    ntfy -> |            |            |            | 
|   |             |            | <- ack     |            |            | 
|---|-------------|------------|------------|------------|------------| 
| 4 |             |            | <- rqnt    |            |            | 
|   |             |     ack -> |            |            |            | 
|---|-------------|------------|------------|------------|------------| 
| 5 | <- recvonly |            | <- crcx    |            |            | 
|   |             |     ack -> |            |            |            | 
|---|-------------|------------|------------|------------|------------| 
| 6 |             |            |    crcx -> |            | sendrcv -> | 
|   |             |            |            | <- ack     |            | 
|---|-------------|------------|------------|------------|------------| 
| 7 | <- recvonly |            | <- mdcx    |            |            | 
|   |             |     ack -> |            |            |            | 
|---|-------------|------------|------------|------------|------------| 
| 8 | <- ringback |            | <- rqnt    |            |            | 
|   |             |     ack -> |            |            |            | 
|---|-------------|------------|------------|------------|------------| 
| 9 |             |            |    rqnt -> |            | ringing -> | 
|   |             |            |            | <- ack     |            | 
|---|-------------|------------|------------|------------|------------| 
|10 |             |            |            | <- ntfy    | <- offhook | 
|   |             |            |     ack -> |            |            | 
|---|-------------|------------|------------|------------|------------| 
|11 |             |            |    rqnt -> |            |            | 
|   |             |            |            | <- ack     |            | 
|---|-------------|------------|------------|------------|------------| 
|12 |             |            | <- rqnt    |            |            | 
|   |             |     ack -> |            |            |            | 
|---|-------------|------------|------------|------------|------------| 
|13 | <- sendrcv  |            | <- mdcx    |            |            | 
|   |             |     ack -> |            |            |            | 
 --------------------------------------------------------------------- 
    
   Step 1 - Notify (ntfy) offhook from rgw1 to ca 
    
   This ntfy is the result of usr1 going offhook and assumes ca had 
   previously sent an rqnt with RequestId "445678944" to rgw1 
   requesting notification in the event of an offhook: 
    
      ntfy 12 aaln/1@rgw1.whatever.net mgcp 1.0 
      o: l/hd 
      x: 445678944 
                                                                        
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   Acknowledgement from ca: 
    
      200 12 ok 
    
   Step 2 - Request Notification (rqnt) for digits from ca to rgw1 
    
   Request rgw1 to notify if on-hook and collect digits according to 
   the digit map, and to provide dialtone: 
    
      rqnt 1057 aaln/1@rgw1.whatever.net mgcp 1.0 
      r: l/hu(n), d/[0-9#*T](d) 
      s: l/dl 
      x: 445678945 
      d: 5xxx  
    
   Acknowledgement from rgw1: 
    
      200 1057 ok 
    
   Step 3 - Notify (ntfy) digits from rgw1 to ca  
    
      ntfy 13 aaln/1@rgw1.whatever.net mgcp 1.0 
      o: d/5, d/0, d/0, d/1 
      x: 445678945 
    
   Acknowledgement from ca: 
    
      200 13 ok 
    
   Step 4 - Request Notification (rqnt) from ca to rgw1 
    
   Request rgw1 to notify in the event of an on-hook transition: 
    
      rqnt 1058 aaln/1@rgw1.whatever.net mgcp 1.0 
      r: l/hu(n) 
      x: 445678946 
    
   Acknowledgement from rgw1: 
    
      200 1058 ok 
    
   Step 5 - Create Connection (crcx) from ca to rgw1  
    
   Request a new connection on rgw1 with the specified local connection 
   options, including 20 msec as the packetization period, G.711 mu-law 
   as the codec, and receive only as the mode: 
    
      crcx 1059 aaln/1@rgw1.whatever.net mgcp 1.0 
      c: 9876543210abcdef 
      l: p:20, a:PCMU 
      m: recvonly 
    
                                                                        
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   Acknowledgement from rgw1 that a new connection, "456789fedcba5", 
   has been created; followed by a blank line and then the SDP 
   parameters: 
    
      200 1059 ok 
      i: 456789fedcba5 
       
      v=0 
      o=- 23456789 98765432 IN IP4 192.168.5.7 
      s=- 
      c=IN IP4 192.168.5.7  
      t=0 0 
      m=audio 6058 RTP/AVP 0 
    
   Step 6 - Create Connection (crcx) from ca to rgw2  
    
   Request a new connection on rgw2. The request includes the session 
   description returned by rgw1 such that a two way connection can be 
   initiated: 
    
      crcx 2052 aaln/1@rgw2.whatever.net mgcp 1.0 
      c: 9876543210abcdef 
      l: p:20, a:PCMU 
      m: sendrecv 
       
      v=0 
      o=- 23456789 98765432 IN IP4 192.168.5.7 
      s=- 
      c=IN IP4 192.168.5.7 
      t=0 0 
      m=audio 6058 RTP/AVP 0 
       
   Acknowledgement from rgw2 that a new connection, "67890af54c9", has 
   been created; followed by a blank line and then the SDP parameters: 
    
      200 2052 ok  
      i: 67890af54c9 
       
      v=0 
      o=- 23456889 98865432 IN IP4 192.168.5.8 
      s=- 
      c=IN IP4 192.168.5.8 
      t=0 0 
      m=audio 6166 RTP/AVP 0 
    
   Step 7 - Modify Connection (mdcx) from ca to rgw1 
    
   Request rgw1 to modify the existing connection, "456789fedcba5", to 
   use the session description returned by rgw2 establishing a half 
   duplex connection which, though not used in this example, could be 
   used to provide usr1 with in band ringback tone, announcements, etc: 
    

                                                                        
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      mdcx 1060 aaln/1@rgw1.whatever.net mgcp 1.0 
      c: 9876543210abcdef 
      i: 456789fedcba5 
      l: p:20, a:PCMU 
      M: recvonly 
       
      v=0 
      o=- 23456889 98865432 IN IP4 192.168.5.8 
      s=- 
      c=IN IP4 192.168.5.8 
      t=0 0 
      m=audio 6166 RTP/AVP 0 
    
   Acknowledgement from rgw1: 
    
      200 1060 ok 
    
   Step 8 - Request Notification (rqnt) from ca for rgw1 to provide 
   ringback 
    
   Request rgw1 to notify in the event of an on-hook transition, and 
   also to provide ringback tone: 
    
      rqnt 1061 aaln/1@rgw1.whatever.net mgcp 1.0 
      r: l/hu(n) 
      s: g/rt 
      x: 445678947 
       
   Acknowledgement from rgw1: 
    
      200 1061 ok 
    
   Step 9 - Request Notification (rqnt) from ca to rgw2 to provide 
   ringing 
    
   Request rgw2 to continue to look for offhook and provide ringing: 
    
      rqnt 2053 aaln/1@rgw2.whatever.net mgcp 1.0 
      r: l/hd(n) 
      s: l/rg 
      x: 445678948 
    
   Acknowledgement from rgw2: 
    
      200 2053 ok 
    
   Step 10 - Notify (ntfy) offhook from rgw2 to ca  
    
      ntfy 27 aaln/1@rgw2.whatever.net mgcp 1.0 
      o: l/hd 
      x: 445678948 
    
   Acknowledgement from ca: 
                                                                        
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      200 27 ok 
    
   Step 11 - Request Notification (rqnt) of on-hook from ca to rgw2 
    
      rqnt 2054 aaln/1@rgw2.whatever.net mgcp 1.0 
      r: l/hu(n) 
      x: 445678949 
    
   Acknowledgement from rgw2: 
    
      200 2054 ok 
    
   Step 12 - Request Notification (rqnt) of on-hook from ca to rgw1 
    
      rqnt 1062 aaln/1@rgw1.whatever.net mgcp 1.0 
      r: l/hu(n) 
      x: 445678950 
    
   Acknowledgement from rgw1: 
    
      200 1062 ok 
    
   Step 13 - Modify Connection (mdcx) from ca to rgw1 
    
   Request rgw1 to modify the existing connection, "456789fedcba5", to 
   sendrecv such that a full duplex connection is initiated: 
    
      mdcx 1063 aaln/1@rgw1.whatever.net mgcp 1.0 
      c: 9876543210abcdef 
      i: 456789fedcba5 
      m: sendrecv 
    
   Acknowledgement from rgw1: 
    
      200 1063 ok 
    
G.3 Connection Deletion 
    
G.3.1 Residential Gateway to Residential Gateway 
    
   The following table shows the message sequence which occurs when a 
   user (usr2) initiates the deletion of an existing connection on a 
   residential gateway (rgw2) with a user served by another residential 
   gateway (rgw1). This example illustrates the communication between 
   the residential gateways and the call agent (ca) only. The local 
   name of the endpoints in this example is aaln/1 for both gateways, 
   and references within the description of the steps to rgw1 and rgw2 
   can be assumed to refer to aaln/1 of rgw1 and aaln/1 of rgw2. 
    



                                                                        
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            Table F.4: Residential Gateway Connection Deletion 
 --------------------------------------------------------------------- 
| # |     usr1    |    rgw1    |     ca     |    rgw2    |     usr2   | 
|===|=============|============|============|============|============| 
| 1 |             |            |            | <- ntfy    | <- on-hook | 
|   |             |            |     ack -> |            |            | 
|---|-------------|------------|------------|------------|------------| 
| 2 |             |            |    dlcx -> |            |            | 
|   |             |            |            | <- ack     |            | 
|---|-------------|------------|------------|------------|------------| 
| 3 |             |            | <- dlcx    |            |            | 
|   |             |     ack -> |            |            |            | 
|---|-------------|------------|------------|------------|------------| 
| 4 |             |            |    rqnt -> |            |            | 
|   |             |            |            | <- ack     |            | 
|---|-------------|------------|------------|------------|------------| 
| 5 |  on-hook -> |    ntfy -> |            |            |            | 
|   |             |            | <- ack     |            |            | 
|---|-------------|------------|------------|------------|------------| 
| 6 |             |            | <- rqnt    |            |            | 
|   |             |     ack -> |            |            |            | 
 --------------------------------------------------------------------- 
    
   Step 1 - Notify (ntfy) offhook from rgw1 to ca 
    
   This ntfy is the result of usr2 going on-hook and assumes that ca 
   had previously sent an rqnt to rgw2 requesting notification in the 
   event of an on-hook (see end of Connection Creation sequence): 
    
      ntfy 28 aaln/1@rgw2.whatever.net mgcp 1.0 
      o: l/hu 
      x: 445678949 
    
   Acknowledgement from ca: 
    
      200 28 ok 
    
   Step 2 - Delete Connection (dlcx) from ca to rgw2 
    
   Requests rgw2 to delete the connection "67890af54c9": 
    
      dlcx 2055 aaln/1@rgw1.whatever.net mgcp 1.0 
      c: 9876543210abcdef 
      i: 67890af54c9 
    
   Acknowledgement from rgw2. Note the response code of "250" meaning 
   "the connection was deleted": 
    
      250 2055 ok 
    
   Step 3 - Delete Connection (dlcx) from ca to rgw1 
    
   Requests rgw1 to delete the connection "456789fedcba5": 
                                                                        
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      dlcx 1064 aaln/1@rgw1.whatever.net mgcp 1.0 
      c: 9876543210abcdef 
      i: 456789fedcba5 
    
   Acknowledgement from rgw1: 
    
      250 1064 ok 
    
   Step 4 - NotificationRequest (rqnt) from ca to rgw2 
    
   Requests rgw2 to notify ca in the event of an offhook transition: 
    
      rqnt 2056 aaln/1@rgw2.whatever.net mgcp 1.0 
      r: l/hd(n) 
      x: 445678951 
    
   Acknowledgement from rgw2: 
    
      200 2056 ok 
    
   Step 5 - Notify (ntfy) on-hook from rgw1 to ca 
    
   Notify ca that usr1 at rgw1 went back on-hook: 
    
      ntfy 15 aaln/1@rgw1.whatever.net mgcp 1.0 
      o: l/hu 
      x: 445678950 
    
   Acknowledgement from ca: 
    
      200 15 ok 
    
   Step 6 - NotificationRequest (rqnt) offhook from ca to rgw1 
    
   Requests rgw1 to notify ca in the event of an offhook transition: 
    
      rqnt 1065 aaln/1@rgw1.whatever.net mgcp 1.0 
      r: l/hd(n) 
      x: 445678952 
    
   Acknowledgement from rgw1: 
    
      200 1065 ok 
    
    
    






                                                                        
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Acknowledgement 
    
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