Internet Engineering Task Force Nancy Greene INTERNET DRAFT Nortel Networks Michael Ramalho Category: Informational Telcordia Expires: October 18, 1999 Brian Rosen Fore Systems Media Gateway Control Protocol Architecture and Requirements Nancy Greene, Michael Ramalho, Brian Rosen Status of this memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working docu- ments of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working docu- ments 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 protocol requirements for the Media Gateway Con- trol protocol between a Media Gateway Controller and a Media Gateway. Table of Contents -to be added Greene, Rosen, Ramalho [Page 1] Internet draft MEGACO Requirements 18 April 1999 1. Introduction This document describes requirements to be placed on the Megaco proto- col. When the word protocol is used on its own in this document it implicitly means the Megaco protocol. 2. Terminology In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and indi- cate requirement levels for the Media Gateway control protocol. 3. Definitions * Context Under the control of a Media Gateway Controller (MGC), the Media Gateway (MG) realizes contexts. Contexts are associations of resources hosted by the MG. They typically involve two terminations, but may involve more. * Line or Loop An analogue or digital access connection from a user terminal which car- ries user media content and telephony access signalling (DP, DTMF, BRI, proprietary business set). * Media Gateway unit - physical entity (MG-unit): An MG-unit contains an MG function and may also contain other functions, e.g. an SG function. * Media Gateway (MG) A Media Gateway (MG) function provides the media mapping and/or tran- scoding functions between dissimilar networks, one of which is presumed to be a packet, frame or cell network. For example, an MG might ter- minate switched circuit network (SCN) facilities (trunks, loops), pack- etize the media stream, if it is not already packetized, and deliver packetized traffic to a packet network. It would perform these func- tions in the reverse order for media streams flowing from the packet network to the SCN. Media Gateways are not limited to SCN - packet/frame/cell functions. A canonical MG would have a physical interface to a non-packet/cell/frame Media stream, and a physical interface to a packet/frame/cell network, but is not restricted to such an arrangement. A conference bridge with Greene, Rosen, Ramalho [Page 2] Internet draft MEGACO Requirements 18 April 1999 all packet interfaces could be an MG, as well as an IVR unit or a voice recognition system with a cell interface. * Media Gateway Controller (MGC) A Media Gateway Controller (MGC) function controls a MG. * Media Resource Examples of media resources are codecs, announcements, tones, and modems, IVR units, bridges, etc. Media resources * Signaling Gateway (SG) An SG function receives/sends SCN native signalling at the edge of a data network. For example the SG function may relay, translate or ter- minate SS7 signaling in an SS7-Internet Gateway. The SG function may also be co-resident with the MG function to process SCN signalling asso- ciated with line or trunk terminations controlled by the MG, such as the "D" channel of an ISDN PRI trunk. * Termination A termination is a point of entry and/or exit of media flows relative to MG. When an MG is asked to connect two or more terminations, it under- stands how the flows entering and leaving each termination are related to each other. Terminations are, for instance, DS0's, ATM VCs and RTP ports. * Termination Class A Termination Class is defined as the general protocol requirements (or a subset of them), plus any new specific requirements needed for that Termination Class. A Termination Class provides the detailed require- ments for its particular application/bearer type. A particular class of Media Gateway, for example, would implement a particular set of Megaco Termination Classes. * Trunk An analog or digital connection from a circuit switch which carries user media content and may carry telephony signalling (MF, R2, etc.). Digi- tal trunks may be transported and may appear at the Media Gateway as channels within a framed bit stream, or as an ATM cell stream. Trunks are typically provisioned in groups, each member of which provides equivalent routing and service. Greene, Rosen, Ramalho [Page 3] Internet draft MEGACO Requirements 18 April 1999 4. Specific functions assumed within the MG This section provides an environment for the definition of the general Megaco protocol requirements. MGs can be architected in many different ways depending where the media conversions and transcoding (if required) are performed, the level of programmability of terminations, how conferences are supported, and how associated signalling is treated. The functions assumed to be within the MG must not be biased towards a particular architecture. For instance, announcements in a MG could be provided by media resources or by the termination itself. Further, this difference must not be visi- ble to MGC, the MGC must be able to issue the same request to two dif- ferent implementations and achieve the same result. Terminations are clearly the cornerstone of the MG. The operation of other media resources must be captured only as they relate to terminations in an implementation independent manner. Depending on the application of the MG (e.g., trunking, residential), some functions will be more prominent than others, and in some cases functions may even disappear. Although media adaptation is the essence of the MG, it is not necessary for every context to involve media adaptation. A context may join two terminations of the same type (i.e., the MG behaves as a switch). The required media conversion depends on the media type supported by the terminations within a context. In addition to media adaptation function, terminations have a number of unique properties, for instance: * certain types of terminations have associated signalling capabili- ties (e.g., PRI signalling, DTMF), * some terminations perform maintenance functions (e.g., continuity tests), * the MGC needs to know the state changes of terminations (e.g., a trunk group going out of service), * the MG retains some control over the allocation and control of some resources (e.g., termination name space: RTP port numbers). Therefore, an MG realizes contexts (point-to-point and conferences), and supports several termination functions. These functions include media conversion, resource allocation and management, and event notifi- cations. Handling termination associated signalling is either done using Greene, Rosen, Ramalho [Page 4] Internet draft MEGACO Requirements 18 April 1999 event notifications, or is handled by the signalling backhaul part of a MG-unit (i.e. NOT directly handled by the MG). MGs must also support some level system related functions, such as, establishing and maintaining some kind of MG-MGC association. This is essential for MGC redundancy, fail-over and resource sharing. Therefore, an MG is assumed to contain these functions: * Reservation and release, of terminations * Ability to provide state of terminations * Maintenance of terminations - It must be possible to make mainte- nance operations independent of other termination functions, for instance, some maintenance states should not affect the resources associated with the termination. Examples of maintenance functions are loopbacks and continuity tests. * Context management, including context state. * Media processing, using media resources: these provide services such as transcoding, conferencing, IVR units. Media resources may or may not be directly part of terminations. * Incoming digit analysis for terminations, interpretation of scripts for terminations * Event detection and signal insertion for per-channel signalling * Ability to configure signalling backhauls (Sigtran) * Management of the association between the MGC and MG, or between the MGC and MG terminations. 5. Per-Call Requirements 5.1. Resource Reservation The Protocol must: a. Support reservation of bearer terminations and media resources for use by a particular call and support their subsequent release (which may be implicit or explicit). b. Allow release of all resources associated with a given context in a single exchange of messages. Greene, Rosen, Ramalho [Page 5] Internet draft MEGACO Requirements 18 April 1999 5.2. Connection Requirements The Protocol must: a. Support connections involving packet and circuit bearer termina- tions in any combination. b. Support connections involving TDM, Analogue, ATM, IP or FR tran- sport in any combination. c. Allow the specification of bearer plane (e.g. Frame Relay, IP, etc.) on a call by call basis. d. Support unidirectional, symmetric bi-directional, and asymmetric bi-directional flows of media. Support multiple media types (e.g. audio, video, T.120). Support point-to-point and point-to- multipoint connections. g. Support creation and modification of more complex flow topologies e.g. conference bridge capabilities. h. Support inclusion of media resources into contexts as required. Depending on the protocol and resource type, media resources may be implicitly included, class-assigned, or individually assigned. i. Provide unambiguous specification of which media flows pass through a point and which are blocked at time, if the protocol permits mul- tiple flows to pass through the same point. j. Support "Hairpin" connections (connections between two circuit con- nections within the same MG). k. Allow modifications of an existing termination, such as use of higher compression to compensate for insufficient bandwidth, or changing transport network connections. Such a mechanism may, for instance, be used to maintain QoS of an established service without making the end-user aware of resource changes. l. Allow a priority marking to flag a given connection as having a higher priority. 5.3. Media Transformations The Protocol must: a. Support mediation/adaptation of flows between different types of transport Greene, Rosen, Ramalho [Page 6] Internet draft MEGACO Requirements 18 April 1999 b. Support invocation of additional processing such as echo cancella- tion. c. Support mediation of flows between different content encoding (codecs, encryption/decryption) d. Allow the MGC to specify whether FAX/data modem traffic is to be terminated at the MG, modulated/demodulated, and converted to pack- ets, or is to be passed on by the MG in the media flow as voice band traffic. e. Allow the MGC to specify that MF/DTMF be extracted and encoded in a separate RTP stream, extracted and converted into packets, or ter- minated at the MG. [Editor's Note: A fourth alternative was mentioned at the Megaco meeting - what was it?] f. Allow the MGC to specify signalled flow characteristics on circuit as well as on packet bearer connections, e.g. u-law/a-law. g. Allow for packet/cell trans-adaptation only (no media adaptation) e.g. mid-stream (packet-to-packet) codec, ATM/AAL5/IP/UDP/RTP to ATM/AAL2, etc. Transport audio normalization levels as a setup parameter, e.g confer- ence bridging. 5.4. Signal/Event Processing and Scripting The Protocol must: a. Allow the MGC to enable/disable monitoring for specific supervision events at specific circuit terminations b. Allow the MGC to enable/disable monitoring for specific events within specified media streams c. Allow reporting of detected events on the MG to the MGC. The proto- col should provide the means to minimize the messaging required to report commonly-occurring event sequences. d. Allow the MGC to specify other actions (besides reporting) that the MG should take upon detection of specified signalling events. e. Allow the MGC to enable and/or mask events. Greene, Rosen, Ramalho [Page 7] Internet draft MEGACO Requirements 18 April 1999 f. Provide a way for MGC to positively acknowledge event notification. g. Allow the MGC to specify signals (e.g., supervision, ringing) to be applied at circuit terminations. h. Allow the MGC to specify content of extended duration (announce- ments, continuous tones) to be inserted into specified media flows. i. Allow the MGC to specify alternative conditions (detection of specific events, timeouts) under which the insertion of extended- duration signals should cease. j. Allow the MGC to download, and specify a script to be invoked on the occurrence of an event. k. Specify common events and signals to maximize MG/MGC interworking. l. Provide an extension mechanism for implementation defined events and signals with, for example, IANA registration procedures. 5.5. QoS/CoS The Protocol must: a. Support the establishment of a bearer channel with a specified QoS/CoS. b. Support the ability to specify QoS for the connection as a whole, and by direction. c Support a means to change QoS as a whole and by direction. d. Provide a means to change some aspects of QoS by flow direction at different times throughout the life of the connection. e. Allow the MGC to set QOS thresholds and receive notification when such thresholds cannot be maintained. 5.6. Test Support The protocol must: a. Support of the different types of PSTN Continuity Testing (COT) for both the originating and terminating ends of the circuit connection (2-wire and 4- wire). b. Specifically support 103, 105, and 108 test line operation. Greene, Rosen, Ramalho [Page 8] Internet draft MEGACO Requirements 18 April 1999 c. Support general test capabilities, for example loopbacks through the MG to test DSP operation. 5.7. Accounting The protocol must: a. Support a common identifier to mark terminations related to one call. b. Support collection of specified accounting information from MGs. c. Provide the mechanism for the MGC to specify that the MG report Accounting information automatically at end of call, in mid-call upon request, at specific time intervals as specified by the MGC and at unit usage thresholds as specified by the MGC. d. Specifically support collection of: * start and stop time, by media flow, * volume of content carried, by media flow, * QOS statistics, by media flow. e. Allow the MGC to have some controls on which statistics are reported, to enable it to manage the amount of information transferred. 5.8. Signalling Control Establishment and provisioning of signalling backhaul channels (via SIGTRAN for example is out of scope. However, the MG must be capable of supporting detection of events, and application of signals associated with basic analogue line, and CAS type signalling. The MEGACO protocol must: a. Support the signalling requirements of analogue lines and Channel Associated Signaling (CAS). b. Support national variations of such signalling. c. Provide mechanisms to support signalling without requiring MG-MGC timing constraints beyond that specified in this document. Greene, Rosen, Ramalho [Page 9] Internet draft MEGACO Requirements 18 April 1999 6. Resource Control 6.1. Resource Status Management The Protocol must: a. Allow the MG to report changes in status of physical entities sup- porting bearer terminations, media resources, and facility- associated signalling channels, due to failures, recovery, or administrative action. It must be able to report whether a termina- tion is in service or out of service. b. Support administrative blocking and release of TDM circuit termina- tions. [Editor's Note: as the above point only relates to ISUP-controlled cir- cuits, it may be unnecessary to require this since the MGC controls their use. However, it may be meaningful for MF and R2-signalled trunks, where supervisory states are set to make the trunks unavailable at the far end.] c. Provide a method for the MGC to request that the MG release all resources currently in use, or reserved, for any or all contexts. d. Provide an MG Resource Discovery mechanism which must allow an MGC to discover what resources the MG has. Expressing resources can be an arbitrarily difficult problem and the initial release of the protocol may have a simplistic view of resource discovery. At a minimum, resource discovery must enumerate the names and quan- tities of available terminations and the allowed values for parame- ters supported by terminations. The protocol should be defined so that simple gateways could respond with a relatively short, pre-stored response to the discovery request mechanism. In general, if the protocol defines a mechanism that allows the MGC to specify a setting or parameter for a resource or connection in the MG, and MGs are not required to support all possible values for that setting or parameter, then the discovery mechanism should provide the MGC with a method to deter- mine what possible values such settings or parameters are supported in a particular MG. e. Provide a mechanism to discover the current available resources in the MG, where resources are dynamically consumed by connections and the MGC cannot reasonable track the consumption of such resources itself. It should also be possible to discover resources currently in use, in order to clean up inconsistencies between the MGC and Greene, Rosen, Ramalho [Page 10] Internet draft MEGACO Requirements 18 April 1999 the MG. f. Not require an MGC to implement an SNMP manager function in order to discover capabilities of an MG that may be specified during con- text establishment. 6.2. Resource Assignment The Protocol must: a. Provide a way for the MG to indicate that it was unable to perform a requested action because of resource exhaustion, or because of temporary resource unavailability. b. Provide an ability for the MGC to indicate to an MG the resource to use for a call (e.g. DS0) exactly, or indicate a set of resources (e.g. pick a DS0 on a T1 line or a list of codec types) via a "wild card" mechanism from which the MG can select a specific resource for a call (e.g. the 16th timeslot, or G.723). c. Allow the use of DNS names and IP addresses to identify network entities, MGs and MGCs. 7. Operational/Management Requirements 7.1. Assurance of Control/Connectivity The Protocol must: a. Provide the means to minimize duration of loss of control due to loss of contact or state mismatches. b. Support detection and recovery from loss of contact due to failure/congestion of communication links or due to MG or MGC failure. Note that failover arrangements are one of the mechanisms which could be used to meet this requirement. c. Support detection and recovery from loss of synchronized view of resource and connection states between MGCs and MGs. (e.g. through the use of audits). d. Provide a means for MGC and MG to provide each other with booting and reboot indications, and what the MG's configuration is. Greene, Rosen, Ramalho [Page 11] Internet draft MEGACO Requirements 18 April 1999 7.2. Resiliency The Protocol must: a. Allow an MG to recover from the failure of its current controlling MGC. b. Allow the MG and MGC to recover gracefully from failure of the net- work path between them. c. Permit more than one backup MGC and provide an orderly way for the MG to contact one of its backups. d. Provide for an orderly switchback to the primary MGC after it recovers. How MGCs coordinate resources between themselves is out- side the scope of the protocol. e. Provide a mechanism so that when an MGC fails, connections already established can be maintained. The protocol does not have to pro- vide a capability to maintain connections in the process of being connected, but not actually connected when the failure occurs. f. Provide a connection discovery mechanism for a backup MGC to deter- mine what connections are established, what parameters are applied to the terminations and/or connection, as well as what MG resources they are using. g. Allow the MGC to recover gracefully from the failure of an MG under its control. 7.3. Error Control The Protocol must: a. Allow for the MG to report cause codes for abnormal failure of lower layer connections e.g. TDM circuit failure, ATM VCC failure. b. Allow for the MG to report Usage Parameter Control. (UPC) events. c. Provide a mechanism to avoid failures on an MG from generating "showers" of supervision events. d. Allow the MG to notify the MGC that a termination was terminated and communicate a reason when a terminations is taken out-of- service unilaterally by the MG due to abnormal events. e. Allow the MGC to acknowledge that a termination has been taken Greene, Rosen, Ramalho [Page 12] Internet draft MEGACO Requirements 18 April 1999 out-of-service. f. Allow the MG to request the MGC to release a Termination and com- municate a reason. g. Allow the MGC to specify, as a result of such a request its deci- sion to take termination down, leave it as is or modify it. 7.4. MIB Requirements The Protocol must define a common MG MIB, which must be extensible, but must: a. Provide information on: * mapping between resources and supporting physical entities. * statistics on quality of service on the control and signalling backhaul interfaces. * statistics required for traffic engineering within the MG. b. MGC MIB should provide ... to be completed. [Editor's Note: Discussion: MIB requirements should focus solely on the management of the operation of the Media Gateway control protocol itself. Other MIBs cover the topics suggested here, except possibly for the traffic engineering statistics. The point was raised that the MGC should not have to implement a manager function, because of the compli- cations this would pose for security administration. This raises a requirement for the MGC to be able to discover the resources and other necessary information pertaining to a given MG by means of the Megaco protocol. A suggestion was also made that the MG needs to discover cer- tain information about the MGC. The mailing list is invited to comment, both on the proper Media Control MIB requirements and on the requirements for discovery.] 8. General Protocol Requirements The Protocol must: a. Support multiple operations to be invoked in one message and treated as a single transaction. b. Be both modular and extensible. Not all implementations may wish to Greene, Rosen, Ramalho [Page 13] Internet draft MEGACO Requirements 18 April 1999 support all of the possible extensions for the protocol. This will permit lightweight implementations for specialized tasks where pro- cessing resources are constrained. [Editor's Note: consolidate this with a definition of profile, and find a new name for profile]. c. Be flexible in allocation of intelligence between MG and MGC [Editor's Note: Discussion: this may have validity as a general require- ment, but it is preferable to give details specific instances where the flexibility should be offered.] d. Support scalability from very small to very large MGs: The protocol must support MGs with capacities ranging from one to a few tens of thousands of terminations. e. Support scalability from very small to very large MGC span of con- trol: The protocol should support MGCs that from one MG to several million MGs. Editor's note: millions seems impossibly large. Should it not be "a few tens of thousands"? f. Support the needs of a residential gateway that supports one to a few lines, and the needs of a large PSTN gateway supporting tens of thousands of lines. Protocol mechanisms favoring one extreme or the other should be minimized in favor of more general purpose mechan- ism applicable to a wide range of MGs. Where special purpose mechanisms are proposed to optimize a subset of implementations, such mechanisms should be defined as optional, and should have minimal impact on the rest of the protocol. g. Facilitate MG and MGC version upgrades independently of one another. The protocol must include a version identifier in the ini- tial message exchange. h. Facilitate the discovery of the protocol capabilities of the one entity to the other. i. Specify commands as optional (they can be ignored) or mandatory (the command must be rejected), and within a command, to specify parameters as optional (they can be ignored) or mandatory (the com- mand must be rejected). Greene, Rosen, Ramalho [Page 14] Internet draft MEGACO Requirements 18 April 1999 8.1. MG-MGC Association Requirements The Protocol must: a. Support the establishment of a control relationship between an MGC and an MG. b. Allow multiple MGCs to send control messages to an MG. Thus, the protocol must allow control messages from multiple IP addresses to a single MG. (See also .....) c. Provide a method for the MG to tell an MGC that the MG received a command for a resource that is under the control of a different MGC. d. Support a method for the MG to control the rate of requests it receives from the MGC (e.g. windowing techniques, exponential back- off). e. Support a method for the MG to tell an MGC that it cannot handle any more requests. 8.2. Performance Requirements ITU E.500 series recommendation that specifies performance requirements for telephony GW operations. The protocol must: a. Minimize message exchanges between MG and MGC, for example during boot/reboot, and during continuity tests. b. Support Continuity test constraints which are a maximum of 200ms cross-MGC IAM (IAM is the name given to an SS7 connection setup msg) propagation delay, and a maximum of 200ms from end of dialing to IAM emission). c. Make efficient use of the underlying transport mechanism. For exam- ple, protocol PDU sizes vs. transport MTU sizes needs to be con- sidered in designing the protocol. d. Support peak calling rates in the order of 140 calls/second at the MGC on a moderately loaded IP network. e. Allow for default/provisioned settings so that commands need only contain non-default parameters. Greene, Rosen, Ramalho [Page 15] Internet draft MEGACO Requirements 18 April 1999 9. Transport Requirements The Protocol must assume that the underlying network: a. May be over large shared networks: proximity assumptions are not allowed. b. Does not assure reliable delivery of messages. c. Does not guarantee ordering of messages: Sequenced delivery of mes- sages associated with the same source of events is not assumed. d. Does not prevent duplicate transmissions. The protocol must: a. Provide the ability to abort delivery of obsolete messages at the sending end if their transmission has not been successfully com- pleted. For example, aborting a command which has been overtaken by events. b. Support priority messages: The protocol shall allow a command pre- cedence to allow priority messages to supercede non-priority mes- sages. c. Support of large fan-out at the MGC d. Provide a way for one entity to correlate commands and responses with the other entity e. Provide a reason for any command failure. f. Provide that loss of a packet not stall messages not related to the message(s) contained in the packet lost. Note that there may be enough protocol reliability requirements here to warrant a separate reliable transport layer be written apart from the Megaco protocol. Also need to compare Megaco reliable transport require- ments with similar Sigtran requirements. 10. Security Requirements Security mechanisms may be specified as provided in underlying transport mechanisms, such as IPSEC. The Protocol, or such mechanisms, must: a. Allow for mutual authentication at the start of an MGC-MG associa- tion Greene, Rosen, Ramalho [Page 16] Internet draft MEGACO Requirements 18 April 1999 b. Allow for preservation of the of control messages once the associa- tion has been Established. c. Allow for optional confidentiality protection of control messages. The mechanism should allow a choice in the algorithm to be used. d. Operate across untrusted domains in a secure fashion. e. Support non-repudiation for a customer-located MG talking to a net- work operator's MGC. g. Define mechanisms to mitigate denial of service attacks Note: the protocol document will need to include an extended discussion of security requirements, offering more precision on each threat and giving a complete picture of the defense including non-protocol measures such as configuration. 11. Requirements specific to Termination Classes The terminations listed in Table 1 can be packaged into different types of MGs. Examples are listed in the following sections. How they are packaged is outside the scope of the general Megaco protocol, but would be defined by different termination classes. The Megaco protocol must support all types of terminations listed in Table 1. Table 1: Termination Classes Termination Class Applications Transit Network ==================================================================== Trunk+ISUP trunking/access IP, ATM, FR Voice, Fax, NAS Trunk+MF trunking/access IP, ATM, FR Voice, Fax, NAS ISDN trunking/access IP, ATM, FR Voice, Fax,NAS Analogue Voice,Fax IP, ATM, FR Termination in a Restricted Voice,Fax IP, ATM, FR Capability Gateway Application Termination IVR, Announcement Server, Voice Recognition Server, Wiretap,... Greene, Rosen, Ramalho [Page 17] Internet draft MEGACO Requirements 18 April 1999 11.1. Media-specific Termination Classes This section describes requirements for handling terminations attached to specific types of networks. 11.1.1. Requirements for TDM PSTN (Circuit) This termination class is applicable to a Trunking GW, Access GW, ... The Megaco protocol MUST allow: a. the MGC to specify the encoding to use on the attached circuit. In general, if something is set by a global signalling protocol (e.g. ISUP allows mu-Law or A-Law to be signalled using ISUP) then it must be settable by the protocol. c. TDM attributes: * Echo cancellation, * PCM encoding or other voice compression (e.g. mu-law or A-law), * encryption, * rate adaptation (e.g. V.110, or V.120). d. for incoming calls, identification of a specific TDM circuit (timeslot and facility). e. for calls outgoing to the circuit network, identification of a specific circuit or identification of a circuit group with the indication that the MG must select and return the identification of an available member of that group. f. specification of the default encoding of content passing to and from a given circuit, possibly on a logical or physical circuit group basis. g. specification at any point during the life of a connection of vari- able aspects of the content encoding, particularly including chan- nel information capacity. h. specification at any point during the life of a connection of loss padding to be applied to incoming and outgoing media streams at the circuit termination. Greene, Rosen, Ramalho [Page 18] Internet draft MEGACO Requirements 18 April 1999 i. specification at any point during the life of a connection of the applicability of echo cancellation to the outgoing media stream. j. Multi-rate calls to/from the SCN. k. H-channel (n x 64K) calls to/from the SCN. The Megaco protocol MAY also allow: l. specification of sub-channel media streams, m. specification of multi-channel media streams. 11.1.2. Packet Termination Class The Megaco protocol must be able to specify: a. ingress and egress coding (i.e. the way packets coming in and out are encoded) (including encryption). b. Near and far-end ports and other session parameters for RTP and RTCP. The Megaco protocol must support reporting of: c. re-negotiation of codec for cause - for further study d. on Trunking and Access Gateways, resources capable of more than one active connection at a time must also be capable of mixing and packet duplication. The Megaco protocol must allow: e. specification of parameters for outgoing and incoming packet flows at separate points in the life of the connection (because far-end port addresses are typically obtained through a separate signalling exchange before or after the near-end port addresses are assigned). f. the possibility for each Media Gateway to allocate the ports on which it will receive packet flows (including RTCP as well as media streams) and report its allocations to the Media Gateway Controller for signalling to the far end. Note that support of different IP backbone providers on a per call basis would require that the ports on which packets flow be selected by the MGC. (but only if the IP address of the MG is different for each backbone provider). g. the specification at any point during the life of a connection of Greene, Rosen, Ramalho [Page 19] Internet draft MEGACO Requirements 18 April 1999 RTP payload type and RTP session number for each RTP-encapsulated media flow. h. indication that the Media Gateway must detect DTMF on the circuit side of a connection, replace it by silence, and either report the detected DTMF to the Media Gateway Controller or transmit it as a separate RTP flow. i. the ability to specify whether outgoing flows are to be uni-cast or multi-cast. Note that on an IP network this information is implicit in the destination address, but in other networks this is a connec- tion parameter. j. invoking of encryption/decryption on media flows and specification of the associated algorithm and key. The Megaco protocol SHOULD also allow: k. the MGC to configure non-RTP (proprietary or other) encapsulated packet flows. 11.1.3. Termination class requirements for ATM This termination class is applicable to Trunking GW, Access GW, .... 11.1.3.1. Addressing a. The protocol must be able to specify the following Termination attributes: * VC identifier, * VC identifier plus AAL2 slot, and wildcarded variant of these, * remote termination network address, remote MG name. b. Allow specification of an ATM termination which is to be assigned to a Media Gateway connection as a VC identifier, a VC identifier plus AAL2 slot, a wild-carded variant of either of these. A remote termination network address, or a remote Media Gateway name could be used also when the MG can select the Virtual Circuit and change the VC during the life of the connection by using ATM signalling. c. Provide an indication by the Media Gateway of the VC identifier and possibly AAL2 slot of the termination actually assigned to a con- nection Greene, Rosen, Ramalho [Page 20] Internet draft MEGACO Requirements 18 April 1999 d. Provide a means to refer subsequently to that termination e. Refer to an existing VCC as the physical interface + Virtual Path Identifier (VPI) + Virtual Circuit Identifier (VCI). f. Where the VCC is locally established (SVCs signalled by the Gateway through UNI or PNNI signalling or similar), the VCC must be indirectly referred to in terms which are of significance to both ends of the VCC. For example, a global name or the ATM address of the ATM devices at each end of the VCC. However, it is possible/ probable that there may be several VCCs between a given pair of ATM devices. Therefore the ATM address pair must be further resolved by a VCC identifier unambiguous within the context of the ATM address pair. g. refer to a VCC as the Remote GW ATM End System Address + VCCI. h. allow the VCCI to be selected by the MG or imposed on the MG. i. support all ATM addressing variants (e.g. NSAP and E.164). 11.1.3.2. Connection related requirements The Protocol must: a. Allow for the de-coupling of creation/deletion of the narrow-band connection from the creation/deletion of the underlying VCC. b. Allow for efficient disconnection of all connections associated with a physical port or VCC. As an example, this could aggregate disconnections across a broadband circuit which experienced a phy- sical error. c. Allow the connection established using this protocol to be carried over a VCC, which may be a: * PVC or SPVC, * an SVC established on demand, either by the MGC itself or by a broker acting on its behalf or, * an SVC originated as required by the local MG, or by the remote end to the local MG through UNI or PNNI signalling. d. Allow ATM transport parameters and QoS parameters to be passed to the MG. Greene, Rosen, Ramalho [Page 21] Internet draft MEGACO Requirements 18 April 1999 Where a VCC is required to be established on a per narrow-band call basis, the Protocol SHOULD allow all necessary information to be passed in 1 message. 11.1.3.3. Media adaptation Note that no specific requirements for AAL1 or AAL5 have been defined here yet. The Protocol must a. Allow AAL parameters to be passed to the MG. b. Allow AAL2 multiple narrow-band calls may be mapped to a single VCC. These calls are differentiated within each VCC by a AAL2 chan- nel identifier. An AAL2 connection may span more than 1 VCC and transit AAL2 switching devices. ATMF standards are working on an end-to-end AAL2 connection identifier as part of the AAL2 signal- ling set. c. Allow unambiguous binding of a narrow band call to an AAL2 connec- tion identifier within the specified VCC. d. Allow the AAL2 connection identifier to be selected by the MG or imposed on the MG e. Allow AAL2 channel identifier (cid) instead of AAL2 connection identifier f. Allow the AAL2 voice profile to be imposed or negotiated before the start of the connection. AAL2 allows for variable length packets and varying packet rates, with multiple codecs possible within a given profile. Thus a given call may upgrade or downgrade the codec within the lifetime of the call. Idle channels may generate zero bandwidth. Thus an AAL2 VCC may vary in bandwidth and possibly exceed its contract. Congestion controls within a gateway may react to congestion by modifying codec rates/types. g. Allow the MGC to instruct the MG of how individual narrow-band calls behave under congestion. 11.1.3.4. Reporting requirements The protocol SHOULD: a. Allow any end-of-call statistics to show loss/restoration of under- lying VCC within the calls duration, together with duration of loss. Greene, Rosen, Ramalho [Page 22] Internet draft MEGACO Requirements 18 April 1999 b. Allow notification, as requested by MGC, of any congestion avoidance actions taken by the MG. The Protocol must: c. Allow ATM VCCs to be audited by the MGC. d. Allow changes in status of ATM VCCs to be notified as requested by the MGC. e. Allow the MGC to query the resource & endpoint availability. Resources may include VCCs, & DSPs. VCCs may be up or down. End- points may be connection-free, connected or unavailable. 11.1.3.5. Functional requirements The Protocol must: a. Allow an MGC to reserve a bearer, and specify a route for it through the network. 11.1.4. Termination class requirements for Frame Relay The Frame Relay Termination Class is applicable to Trunking GW, Access GW, ... 11.1.5. Termination Class Requirements for an Analogue Gateway 11.1.6. Termination Class Requirements for an ISDN Gateway 11.2. Application-Specific Requirements 11.2.1. Trunking Gateway A Trunking Gateway is an interface between SCN networks and Voice over IP or Voice over ATM networks. Such gateways typically interface to SS7 or other NNI signalling on the SCN and manage a large number of digital circuits. The protocol must: a. Provide Circuit and Packet-side loopback. Greene, Rosen, Ramalho [Page 23] Internet draft MEGACO Requirements 18 April 1999 b Provide Circuit-side n x 64kbs connections. c Provide subrate and multirate connections -for further study. d. Provide lawful Wiretap capability e. Provide the capability to support Reporting/generation of per-trunk CAS signalling (DP, DTMF, MF, R2, J2, and national variants). f. Provide the capability to support reporting of detected DTMF events either digit-by-digit, as a sequence of detected digits with a flexible mechanism For the MG to determine the likely end of dial string, or in a separate RTP stream. g. Provide the capability to support ANI and DNIS generation and reception. 11.2.2. Access Gateway An Access Gateway connects UNI interfaces like ISDN (PRI and BRI) or traditional analog voice terminal interfaces, to a Voice over IP or Voice over ATM network, or Voice over Frame Relay network. The Protocol must: a. Support detection and generation of analog line signaling (hook- state, ring generation). b. Provide the capability to support reporting of detected DTMF events either digit-by-digit, as a sequence of detected digits with a flexible mechanism For the MG to determine the likely end of dial string, or in a separate RTP stream. c. Not require scripting mechanisms, event buffering, digit map storage when implementing restricted function (1-2 line) gateways with very limited capabilities. d. Provide the capability to support CallerID generation and recep- tion. Proxying of the protocol is for further study. 11.2.3. Trunking/Access Gateway with fax ports Greene, Rosen, Ramalho [Page 24] Internet draft MEGACO Requirements 18 April 1999 11.2.4. Trunking/Access Gateway with conference ports (Multipoint Pro- cessing Unit) 11.2.5. Network Access Server Below is a reference architecture for a Network Access Server (NAS). +-------+ +-------+ Signaling | | | | -----------+ MGC + | AAA | | | | | +---+---+ +--+----+ | Megaco | | | +---+---+ ~~~~~~ Bearer | | ( ) -----------+ NAS +-------( IP ) | | ( ) +-------+ ~~~~~~ Signaling comes into the MGC and the MGC controls the NAS. A NAS is an access gateway, or Media Gateway (MG), which terminates modem signals or synchronous HDLC connections from an SCN network and provide data access to the packet network. A NAS is assumed to occupy a position in network similar to a trunking gateway. Hence it must handle the same set of trunk circuit bearers, and the same trunk signalling. Only those requirements specific to a NAS are described here. The Protocol must support: a. Callback capabilities: * PPP Callback * Callback based on Login b. Modem calls. The protocol must be able to specify the modem type(s) to be used for the call. c. Synchronous HDLC calls. The protocol must be able to specify the data rate of the TDM connection (e.g., 64 kbit/s, 56 kbit/s, 384 kbit/s), if this is available from the SCN. Greene, Rosen, Ramalho [Page 25] Internet draft MEGACO Requirements 18 April 1999 d. Rate Adaptation: The protocol must be able to specify the type of rate adaptation to be used for the call including indicating the subrate, if this is available from the SCN (e.g. 56K, or V.110 sig- nalled in Bearer capabilities with subsrate connection of 19.2kbit/s. e. Adaptable NASes: The protocol must be able to support multiple options for an incoming call to allow the NAS to dynamically select the proper type of call. For example, an incoming ISDN call coded for "Speech" Bearer Capability could actually be a voice, modem, fax or 56 kbit/s synchronous call. The protocol should allow the NAS to report back to the MGC the actual type of call once it is detected. f. Passing of information received from a AAA server by the MGC to the NAS. This would be necessary if it is the MGC that queries a AAA server. g. Passage of Called and Calling Party Number information to the NAS from the MGC. h. Passing of information required by the AAA Server from the NAS to the MGC, for example, username, calling number, called number, PIN number based on an IVR prompt (Note: This could be done via RADIUS with the MGC acting as proxy. This would require the least development on the NAS). i. Specification of per-call configuration parameters from the MGC to NAS (e.g., based on ANI and DNIS). These may include: * L2TP Tunnel Parameters * AAA Server to use for the call * Other attributes defined in RFC2138 (RADIUS). j. Specification of QoS parameters to the NAS to use for a session. It should be able to specify QoS parameters based on the authentica- tion parameters. k. Delivery of call information from the NAS to the MGC. NASes already provide accounting information via AAA. The information to be delivered is equivalent to that identified in RFC 2139 (RADIUS Accounting). l. Be able to report Capability information, for example, connection types (V.34/V90/Synch ISDN..), AAA mechanism (RADIUS/DIAMETER/..), access type (PPP/SLIP/..) after restart or upgrade. Greene, Rosen, Ramalho [Page 26] Internet draft MEGACO Requirements 18 April 1999 m. Support the following connection types: - Circuit-modem (onward routing implicit) - Circuit-specified L2TP tunnel via modem - Circuit-circuit (reroute to VoIP MG) n. NAT mapping, port filters or tunnel parameters for a particular user may need to be communicated to the MG. o. Sending disconnect information and reason from the NAS to the MGC. 11.2.6. Analog Gateway See requirements for restricted capability gateway. 11.2.7. Restricted Capability Gateway The requirements here may also be applied to small analog gateways, and to cable/xDSL modems. See also the section on access gateways. The Protocol must support: a. The ability to provide a scaled down version of the protocol. When features of the protocol are not supported, an appropriate error message must be sent. Appropriate default action must be defined. Where this is defined may be outside the scope of the Megaco proto- col. b. The ability to provide device capability information to the MGC with respect to the use of Megaco protocol. 11.2.8. ISDN Gateway add text 11.2.9. H.320 Gateway The protocol must have sufficient capability to support an H.320 gate- way. H.320 is characterized by a single data stream with multiple media streams multiplexed on it If the mapping is from H.320 on the circuit side, and H.323 on the packet side, it is assumed that the MG knows how to map respective sub- channels from H.320 side to streams on packet side. If extra information is required when connecting two terminations, then it must be supplied so that the connections are not ambiguous. Greene, Rosen, Ramalho [Page 27] Internet draft MEGACO Requirements 18 April 1999 11.2.10. IVR Unit An IVR Unit, or IVR MG, provides automatic voice response and switching services in response to DTMF signals from the SCN. A scripting require- ment for IVR would involve IVR-controlling extensions to basic scripting packages. To support Media Gateways functioning as IVR systems, the Protocol must: a. Be able to provide these basic operations: - request an IVR MG to play an announcement. - request an IVR MG to play an announcement and then collect DTMF digits. - request an IVR MG to play an announcement and then record voice. b. Be able to specify these play characteristics: - Play volume - Play speed - Play iterations - Interval between play iterations - Play duration c. Be able to specify these event collection characteristics: - The number of attempts to give the user to enter a valid DTMF pat- tern. - The number of attempts to give the user to make a recording. d. Provide a base set of voice variables including date, digits, dura- tion, money, month, number, silence, string, display text, text to voice, time, generated tone, and weekday. Given a variable type and value, the IVR MG must dynamically assemble the phrases required for its playback. For example, given a type of "US Currency" and a value of "1995," the IVR MG should play "Nineteen dollars and ninty-five cents." e. Using the terminology that a segment is a discrete unit of playable speech, support these types of segments: Greene, Rosen, Ramalho [Page 28] Internet draft MEGACO Requirements 18 April 1999 - A provisioned recording. - A block of text to be converted to speech. - A block of text to be displayed on a device. - A length of silence to be played. - An algorithmically generated tone. - A voice variable, specified by type and value. Given a variable type and value, the IVR would dynamically assemble the phrases required for its playback. - A provisioned sequence of segments, referenced by a single unique id. - A provisioned set of segments, referenced by unique id and a selec- tor specifying a particular segment in the set. f. Provide two different methods of audio specification: - Direct specification of the audio components to be played by speci- fying the sequence of segments in the command itself. - Indirect specification of the audio components to be played by reference to a single identifier that resolves to a provisioned sequence of audio segments. The terminology of sequence and set can be used with segments. A sequence is a provisioned sequence of one or more audio segments. Com- ponent segments are not necessarily all of the same type. Every sequence is assigned a unique segment id. On playback, a sequence id reference is deconstructed into its individual parts, each of which is played in order. A set is a provisioned collection of audio segments with an associated selector. On playback, the selector value is resolved to a particular set element. Selector types need not be defined in the protocol itself, but may be defined by the user. Some examples of selector types that a user might define are: language, codec, audio file format, gender, accent, customer, and voice talent. g. Using the above set and sequence terminology, provide for the fol- lowing uses of segments: - Sequences of heterogeneous segments Greene, Rosen, Ramalho [Page 29] Internet draft MEGACO Requirements 18 April 1999 - Nesting of sequences - Nesting of sets. - Nesting of sets with in sequences and sequences within sets. - Use of set selectors to implement properties such as language, codec, audio format, voice talent, customer, gender, accent, etc. h. With respect to digit timers, allow the specification of: - Time allowed to enter the first digit. - Time allowed for user to enter each digit subsequent to the first digit. - Time allowed for user to enter a digit once the maximum expected number of digits has been entered. i. With respect to recording timers, allow the specification of: - Time to wait for the user to initially speak. - The amount of silence necessary following the last speech segment for the recording to be considered complete. - The maximum allowable length of the recording (not including pre- and post- speech silence). j. To be able to allow multiple prompt operations for digit collection and voice recording, provide the following types of prompts: - Initial Prompt - Reprompt - Error prompt - Failure announcement - Success announcement. k. To allow digit pattern matching, allow the specification of: - maximum number of digits to collect. - minimum number of digits to collect. Greene, Rosen, Ramalho [Page 30] Internet draft MEGACO Requirements 18 April 1999 - a digit pattern using a regular expression. l. To allow digit buffer control, allow the specification of: - Ability to clear digit buffer prior to playing initial prompt (default is not to clear buffer). - Default clearing of buffer following playing of uninterruptible announcement segment. - Default clearing of buffer before playing a reprompt in response to previous invalid input. m. Provide a method to specify DTMF interruptibility on a per audio segment basis. n. Allow the specification of definable key sequences for digit collection/recording to: - Discard collected digits/recording in progress, replay the prompt, and resume digit collection/recording (digit collection only). - Discard collected digits/recording in progress and resume digit collection/recording (digit collection only). - Terminate the current operation and return the terminating key sequence to the MGC. o. Provide a way to ask the IVR MG to support the following definable keys for digit collection and recording. These keys would then be able to be acted upon by the IVR MG: - A key to terminate playing of an announcement in progress. - A set of one or more keys that can be accepted as the first digit to be collected (digit collection only). - A key that signals the end of user input (digit collection or recording). The key may or may not be returned to the MGC along with the input already collected. - Keys to stop playing the current announcement and resume playing at the beginning of the first segment of the announcement, last seg- ment of the announcement, previous segment of the announcement, next segment of the announcement, or the current announcement seg- ment. p. While audio prompts are usually provisioned in IVR MGs, support Greene, Rosen, Ramalho [Page 31] Internet draft MEGACO Requirements 18 April 1999 changing the provisioned prompts in a voice session rather than a data session. In particular, with respect to audio management: - A method to replace provisioned audio with audio recorded during a call. The newly recorded audio must be accessible using the iden- tifier of the audio it replaces. - A method to revert from replaced audio to the original provisioned audio. - A method to take audio recorded during a call and store it such that it is accessible to the current call only through its own newly created unique identifier. - A method to take audio recorded during a call and store it such that it is accessible to any subsequent call through its own newly created identifier. The Protocol need not support the following: q. While inflection (rising, flat, and falling) must be supported by the IVR MG, specification of inflection is outside the scope of this protocol. 11.2.11. Other Application-specific Gateways (Announcement Server, Voice Recognition Server...) add text 12. Requirements proposed to be removed Editor's note. We believe these requirements to now be superfluous, they will be deleted in the next revision unless justification is pro- vided. Allow for different control relationship profiles, then define a very few that cover the majority of industry needs. Explicit activation of terminations, and of termination programs by MGC on boot/reboot Appropriate handling of termination program activity status on failover. An MG may provide services for MG configuration, for assigning itself an IP address, for name lookup to find an MGC, and for providing a secure link between itself and an MGC. In some cases, existing protocols may be employed (SNMP, DHC, DNS, IPSEC, ...) and in some cases the MG may refer to be pre-provisioned with the information it needs. Greene, Rosen, Ramalho [Page 32] Internet draft MEGACO Requirements 18 April 1999 The Megaco protocol should support MGs of both greater and lesser intel- ligence. As an example, in some cases the MG is able to manage its own resources, as for instance if it is able to select an available outgoing trunk given the trunk group name. This sort of intelligence is essen- tial to meet the upper range of sizes for MGCs. In other cases the Media Gateway Controller will do the detailed manage- ment of Media Gateway resources; in the trunking example, the MGC will specify the exact circuit termination to use in an outgoing call. The MG may have the power to infer behaviours required of the devices it hosts (e.g., tone collection, IVR interworking). It is also possible that distributed call processing over two MGs may be required to address issues such as post-dial delay and open dialing plans. This needs to be developed further. [Editor's Note: this is an area in which not much debate has occurred, therefore the protocol requirements might not yet be completely identi- fied.] Editor's Note: The following requirement goes beyond what we have currently defined the minimum signal requirements. Specifically, it defines a Requirement that signal sources can come from outside the MG, and the MG is responsible for connecting to such a resource. Network/Media resources: - naming of announcements to be played: for example, announcementx@domain - need to resolve the address, and then who decides whether to set up an IP connection to a specialized server, or whether it can be handled by a media resource, or by an termination in the MG? - it may be an termination in a MG, or the MGC could set up the connection to an AS, or the MG could set up the connection to an AS. In any case, we need a location independent specification of network resources 13. References Note: Since references to internet-drafts cannot remain in an RFC, internet- draft references below will be removed at some point. They are included here in the meantime, to indicate the source of some of the requirements in this document. [1] Cuervo, Greene, Holdrege, Ong, Huitema, "SS7-Internet Interworking - Architectural Framework", work in progress. [2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Greene, Rosen, Ramalho [Page 33] Internet draft MEGACO Requirements 18 April 1999 Transport Protocol for Real-Time Applications", RFC 1889, January 1996. [3] Schulzrinne, H., "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 1890, January 1996. [4] Vandenameele, J., "Requirements for the Reference Point ("N") between Media Gateway Controller and Media Gateway", ETSI Tiphon WG2 working document and Internet-Draft, , January, 1999. [5] Holdrege, M., "Multiservice Switching Forum requirements input to MEGACO", , April, 1999. [6] Cromwell, D., Durling, M., "Requirements For Control Of An IVR Function", , April, 1999. 14. Acknowledgements The authors would like to acknowledge the many contributors who debated the media gateway control architecture and requirements on the IETF Megaco and Sigtran mailing lists. Greene, Rosen, Ramalho [Page 34] Internet draft MEGACO Requirements 18 April 1999 15. Authors' addresses Nancy Greene Nortel Networks P.O. Box 3511 Stn C Ottawa, ON, Canada K1Y 4H7 Tel: (514) 271-7221 Email: ngreene@nortelnetworks.com Michael Ramalho Telcordia Email: mramalho@telcordia.com Brian Rosen Fore Systems 1000 FORE Drive, Warrendale, PA 15086 Tel: (724) 742-6826 Email: brosen@eng.fore.com Greene, Rosen, Ramalho [Page 35]