Internet Engineering Task Force Mauricio Arango
INTERNET DRAFT RSL COM
October 27, 1998 Andrew Dugan
Expires April 27, 1999 Level3 Communications
<draft-huitema-MGCP-v0r1-00.txt> Isaac Elliott
Level3 Communications
Christian Huitema
Bellcore
Scott Pickett
Vertical Networks
Media Gateway Control Protocol (MGCP)
Mauricio Arango, Andrew Dugan, Isaac Elliott,
Christian Huitema, Scott Pickett
Version 0.1 draft
October 27, 1998
Status of this document
This document is an Internet-Draft. Internet-Drafts are working docu-
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Abstract
This document describes an application programming interface and a
corresponding protocol (MGCP) for controlling Voice over IP (VoIP) Gate-
ways from external call control elements. The MGCP assumes a call con-
trol architecture where the call control "intelligence" is outside the
gateways and handled by external call control elements.
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The document is structured in 6 main sections:
* The introduction presents the basic assumptions and the relation to
other protocols such as H.323, RTSP, SAP or SIP.
* The interface section presents a conceptual overview of the MGCP,
presenting the naming conventions, the usage of the session
description protocol SDP, and the five procedures that compose
MGCP: Notifications Request, Notification, Create Connection,
Modify Connection, Delete Connection, AuditEndpoint, AuditConnec-
tion and RestartInProgress.
* The protocol description section presents the MGCP encodings, which
are based on simple text formats, and the transmission procedure
over UDP.
* The security section presents the security requirement of MGCP, and
its usage of IP security services (IPSEC).
* The event packages section provides an initial definition of pack-
ages and event names.
* The description of the changes made in combining SGCP 1.1 and IPDC
to create MGCP 0.1.
1. Introduction
This document describes an abstract application programming interface
and a corresponding protocol (MGCP) for controlling Telephony Gateways
from external call control elements. A telephony gateway is 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 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 gate-
ways include cable modem/cable set-top boxes, xDSL devices, broad-
band wireless devices
* Access gateways, that provide a traditional analog (RJ11) or
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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 "intel-
ligence" is outside the gateways and handled by external call control
elements. The MGCP assumes that these call control elements, or Call
Agents, will synchronize with each other to send coherent commands to
the gateways under their control. MGCP does not define a mechanism for
synchronizing Call Agents.
MGCP assumes a connection model where the basic constructs are endpoints
and connections. Endpoints are sources or sinks of data and could 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 ter-
minates trunks is called a trunk 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:
* Transmission of audio packets using RTP and UDP over a TCP/IP net-
work.
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* 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 tele-
phone network.
For point-to-point connections the endpoints of a connection could be in
separate gateways or in the same gateway.
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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. In a typical con-
figuration, 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 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 Gate-
keeper" 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
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applications. These other specifications include:
* the Session Description Protocol (SDP), RFC 2327,
* the Session Announcement Protocol (SAP),
* the Session Initiation Protocol (SIP),
* the Real Time Streaming Protocol (RTSP), RFC 2326.
The latter three specifications are in fact alternative signaling stan-
dards that allow for the transmission of a session description to an
interested party. SAP is used by multicast session managers to distri-
bute 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.
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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 | |
| | | synchronization| |
| | | 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.
1.3. Definitions
Trunk: A communication channel between two switching systems. E.g., a
DS0 on a T1 or E1 line.
2. Media Gateway Control Interface
The interface functions provide for connection control and endpoint con-
trol. Both use the same system model and the same naming conventions.
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2.1. Model and naming conventions.
The MGCP assumes a connection model where the basic constructs are end-
points 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 several Call Agents.
2.1.1. Names of endpoints
Endpoints names have two components:
* the domain name of the gateway that is managing the endpoint ,
* a local name within that gateway,
The syntax of the local name depends on the type of endpoint being
named. However, the local name for each of these types is naturally
hierarchical, beginning with a term which identifies the physical gate-
way containing the given endpoint and ending in a term which specifies
the individual endpoint concerned. With this in mind, the following
rules for construction and interpretation of the Entity Name field for
these entity types MUST be supported:
1. The individual terms of the naming path MUST be separated by a sin-
gle slash ("/", ASCII 2F hex).
2. The individual terms are character strings composed of letters,
digits or other printable characters, with the exception of charac-
ters used as delimitors ("/", "@"), 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 naming path looks like
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
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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".
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". The description of a specific command or AVP may add
further criteria for selection within the general rules given here.
If the Media Gateway controls multiple physical gateways, the first term
of the naming MUST identify the physical gateway containing the desired
entity. If the Media Gateway controls only a single physical gateway,
the first term of the naming string MAY identify that physical gateway,
depending on local practice. A local name that is composed of only a
wildcard character refers to either all (*) or any ($) endpoints within
the media gateway.
In the case of trunking gateways, endpoints are trunk circuits linking a
gateway to a telephone switch. These circuits are typically grouped into
a digital multiplex, that is connected to the gateway by a physical
interface. Such circuits are named in three contexts:
* In the ISUP protocol, trunks are grouped into trunk groups, identi-
fied by the SS7 point codes of the switches that the group con-
nects. Circuits within a trunk group are identified by a circuit
number (CIC in ISUP).
* In the gateway configuration files, physical interfaces are typi-
cally identified by the name of the interface, an arbitrary text
string. When the interface multiplexes several circuits, individual
circuits are typically identified by a circuit number.
* In MGCP, the endpoints are identified by an endpoint name.
The Call Agents use configuration databases to map ranges of circuit
numbers within an ISUP trunk group to corresponding ranges of circuits
in a multiplex connected to a gateway through a physical interface. The
gateway will be identified, in MGCP, by a domain name. The local name
will be structured to encode both the name of the physical interface,
for example X35V3+A4, and the circuit number within the multiplex con-
nected to the interface, for example 13. The circuit number will be
separated from the name of the interface by a fraction bar, as in:
X35V3+A4/13
The circuit number can be omitted when the physical interface only
includes one circuit, or if the Call Agent requests the gateway to
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choose one available circuit within a multiplex.
Other types of endpoints will use different conventions. For example, an
endpoint that produces an "all lines busy" announcement could be named:
all-lines-busy/loop@announce-23.whatever.net
The exact syntax of such names should be specified in the corresponding
server specification.
2.1.2. Names of calls
Calls are identified by unique identifiers, independent of the underly-
ing platforms or agents. These identifiers are created by the Call
Agent. They are treated in MGCP as unstructured octet strings.
Call identifiers are expected to be unique within the system. When a
Call Agent builds several connections that pertain to the same call,
either on the same gateway or in different gateways, these connections
will all be linked to the same call through the globally unique identif-
ier. This identifier can then be used by accounting or management pro-
cedures, which are outside the scope of MGCP.
2.1.3. 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. They are treated in MGCP as unstructured octet strings.
The gateway should make sure that a proper waiting period, at least 3
minutes, elapses between the end of a connection that used this identif-
ier 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.)
2.1.4. Names of Call Agents and other entities
The media gateway control protocol has been designed to allow the imple-
mentation of redundant Call Agents, for enhanced network reliability.
This means that there is no fixed binding between entities and hardware
platforms or network interfaces.
Reliability can be improved by the following precautions:
* 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 should
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retry the transmission using another address.
* Entities may move to another platform. The association between a
logical name (domain name) and the actual platform are kept in the
domain name service. Call Agents and Gateways should keep track of
the time-to-live of the record they read from the DNS. They should
query the DNS to refresh the information if the time to live has
expired.
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 col-
lect the numbers that a user dials; it may also be used with trunking
gateways and access gateways alike, to collect the access codes, credit
card numbers and other numbers requested by call control services.
An alternative procedure is for the gateway to notify the Call Agent of
the dialed digits, 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 mes-
sage.
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:
_______________________________________________________
| 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 load the gateway with a digit map
that 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)
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The formal syntax of the digit map is described by the following BNF
notation:
Digit ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" |
"7" | "8" | "9"
Timer ::= "T" -- matches the detection of a timer
Letter::= Digit | Timer | "#" | "*" | "A" | "B" | "C"
| "D"
Range ::= "x" -- matches any digit
| "[" Letters "]" -- matches any of the specified letters
Letters ::= Subrange | Subrange Letters
Subrange ::= Letter -- matches the specified letter
| Digit "-" Digit -- matches any digit between first and
-- last
Position ::= Letter
| Range
StringElement::= Position -- matches an occurrence of
-- the position
| Position "." -- matches an arbitrary number of
-- occurrences
-- of the position, including 0
String::= StringElement | StringElement String
StringList::= String | String "|" StringList
DigitMap::= String | "(" StringList ")"
A DigitMap, according to this syntax, is defined either by a "string" or
by a list of strings. Each string in the list is an alternative number-
ing scheme. A gateway that detects digits, letters or timers will:
1. Add the event parameter code as a token to the end of an internal
state variable called the "current dial string"
2. Apply the current dial string to the digit map table, attempting a
match to each regular expression in the Digit Map in lexical order
3. If the result is under-qualified (partially matches at least one
entry in the digit map), do nothing further.
If the result matches, or is over-qualified (i.e. no further digits
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could possibly produce a match), send the current digit string to the
Call Agent.
Digit maps are provided to the gateway by the Call Agent, whenever the
Call Agent instructs the gateway to listen for digits.
2.1.6. Names of events
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, and a call agent may request certain signals to be
applied to an endpoint, e.g. dial-tone.
Events and signals are grouped in packages within which they share the
same namespace which we will refer to as event names in the following.
Packages are groupings of the events and signals supported by a particu-
lar type of endpoint. For instance, one package may support a certain
group of events and signals for analog access lines, and another package
may support another group of events and signals for video lines. One or
more packages may exist for a given endpoint-type.
Event names are case insensitive and are composed of two logical parts,
a package name and an event name. Both names are strings of letters,
hyphens and digits, with the restriction that hyphens shall never be the
first or last characters in a name. Package or event names are not case
sensitive - values such as "hu", "Hu", "HU" or "hU" should be considered
equal.
Examples of package names are "D" (DTMF), "M" (MF), "T" (Trunk) or "L"
(Line). Examples of event names can be "hu" (off hook or "hang-up" tran-
sition), "hf" (flash hook) or "0" (the digit zero).
In textual representations, the package name, when present, is separated
from the event name by a slash ("/"). The package name is in fact
optional. Each endpoint-type has a default package associated with it,
and if the package name is excluded from the event name, the default
package name for that endpoint-type is assumed. For example, for an ana-
log access line, the following two event names are equal:
l/dl dial-tone in the line package for an analog access line.
dl dial-tone in the line package (default) for an analog access line.
This document defines a basic set of package names and event names.
Additional package names and event names can be registered with the
IANA. A package definition shall define the name of the package, and the
definition of each event belonging to the package. The event definition
shall include the precise name of the event (i.e., the code used in
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MGCP), a plain text defnition of the event, and, went appropriate, the
precise definition of the corresponding signals, for example the exact
frequencies of audio signal such as dial tones or DTMF tones.
In addition, implementers can gain experience by using experimental
packages. The names of experimental packages must start with the two
characters "x-"; the IANA shall not register package names that start
with these characters.
Digits, or letters, are supported in many packages, notably "DTMF", "MF"
and "pulse". Digits and letters are defined by the rules "Digit" and
"Letter" in the definition of digit maps. These definition refers to the
digits (0 to 9), to the asterisk ("*") and orthothrop 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
represent the keys that a user punched on a dial. In addition, the
letter "X" can be used to represent all digits, and the sign "$" can be
used in wildcard notations. The need to easily express the digit strings
has a consequence on the form of event names:
An event name that does not denote a digit should 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 should not contain the special
signs "*", "#", "/" or "$".)
A Call Agent may often have to ask a gateway to detect a group of
events. Two conventions can be used to denote such groups:
* The wildcard convention can be used to detect any event belonging
to a package, or a given event in many packages, or event any event
in any package supported by the gateway.
* The regular expression Range notation can be used to detect a range
of digits.
The dollar sign ("$") can be used as a wildcard instead of a package
name or an event name:
A name such as "foo/$" denotes all events in package "foo"
A name such as "$/bar" denotes the event bar in any package sup-
ported by the gateway
The names "$" or "$/$" denote all events supported by the gateway.
The call agent can ask a gateway to detect a set of digits or letters
either by individually decribing those letters, or by using the "range"
notation defined in the syntax of digit strings. For example, the call
agent can:
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Use the letter "x" to denote "any letter or digit."
Use the notation "[0-9#]" to denote the digits 0 to 9 and the pound
sign.
Events and signals are described in packages. The package description
must provide, for each events, the following informations:
* The description of the event and its purpose,
* The detailed characteristics of the event, such as for example fre-
quencies and amplitude of audio signals, modulations and repeti-
tions,
* The typical and maximum duration of the event.
Signals are divided into different types depending on their behavior:
* On/off (OO)
Once applied, these signals last forever until they are turned off.
This may happen either as the result of an event or a new SignalRe-
quests (see later).
* Time-out (TO)
Once applied, these signals last until they are either turned off
(by an event or SignalRequests) or a signal specific period of time
has elapsed. Currently, a signal that times out does not generate
any events.
* Brief (BR)
The duration of these signals is so short, that they stop on their
own. If an event occurs the signal will not stop, however if a new
SignalRequests is applied, the signal will stop.
TO signals are normally used to alert the endpoints' users, to signal
them that they are expected to perform a specific action, such as hang
down the phone (ringing). Transmission of these signals should typi-
cally be interrupted as soon as the first of the requested events has
been produced.
Package descriptions should describe, for all signals, their type (OO,
TO, BR). They should also describe the maximum duration of the TO sig-
nals.
2.2. Usage of SDP
The Call Agent uses the MGCP to provision the gateways with the descrip-
tion of connection parameters such as IP addresses, UDP port and RTP
profiles. These descriptions will follow the conventions delineated in
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the Session Description Protocol which is now an IETF proposed standard,
documented in RFC 2327.
SDP allows for description of multimedia conferences. This version lim-
its SDP usage to the setting of audio circuits and data access circuits.
The initial session descriptions contain the description of exactly one
media, of type "audio" for audio connections, "nas" for data access.
2.3. Gateway Control Functions
This section describes the commands of the MGCP. The service consists of
connection handling and endpoint handling commands. There are eight com-
mands in the protocol:
* The Call Agent can issue a NotificationRequest command to a gate-
way, 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 to 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 sus-
tained.
* The Call Agent can use the AuditEndpoint and AuditConnection com-
mands to audit the status of an "endpoint" and any connections
associated with it. Network management beyond the capabilities pro-
vided by these commands are generally desirable, e.g. information
about the status of the embedded client. Such capabilities are
expected to be supported by the use of the Simple Network Manage-
ment 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 the gateway, or 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"
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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.
Connections are grouped into "calls". 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", "receive only", "send/receive", "conference", "data",
"inactive", "loopback", "continuity test", "network loop back" or "net-
work continuity test."
The handling of the audio signals received on these connections is
determined by the mode parameters:
* Audio signals received in data packets through connections in
"receive", "conference" or "send/receive" mode are mixed and sent
to the endpoint.
* Audio signals originating from the endpoint are transmitted over
all the connections whose mode is "send", "conference" or
"send/receive."
* In addition to being sent to the endpoint, audio signals received
in data packets through connections in "conference" mode are repli-
cated to all the other connections whose mode is "conference."
The "loopback" and "continuity test" modes are used during maintenance
and continuity test operations. 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 tone) on the bearer circuit and expect the terminating switch to
loopback the circuit. 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.
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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. 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.
If the mode is set to "network continuity test", the gateway will pro-
cess the packets received from the conenction according to the tran-
sponder mode required for dual-tone continuity test, and send the pro-
cessed signal back on the connection.
2.3.1. NotificationRequest
The NotificationRequest commands are 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 decide to use a different
type of encoding method in the connections bound to this endpoint.
NotificationRequest( EndpointId,
NotifiedEntity,
RequestedEvents,
RequestIdentifier,
DigitMap,
SignalRequests,
QuarantineHandling,
DetectEvents)
EndpointId is the name for the endpoint in the gateway where Notifica-
tionRequest executes, as defined in section 2.1.1.
NotifiedEntity is an optional parameter that specifies where the notifi-
cations should be sent. When this parameter is absent, the notifications
should be sent to the originator of the NotificationRequest.
RequestIdentifier is used to correlate this request with the notifica-
tions that it triggers.
RequestedEvents is a list of events that the gateway is requested to
detect and report. Such events include, for example, fax tones, con-
tinuity tones, or on-hook transition. To each event is associated an
action, which can be:
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* 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.
Some actions can be combined. In particular:
* The "swap audio" action can be combined with "Notify", "Accumulate"
and "Ignore."
* The "keep signal active" action can be combined with "Notify",
"Accumulate", "Accumulate acording to Digit Map" and "Embedded
Notification Request."
* The "Embedded Notification Request" can be combined with "Accumu-
late." It can also be combined with Notify, if the gateway is
allowed to issue several Notify commands in response to a single
Notification request.
Events that are not specified in the list will, by default, be ignored.
The Swap Audio action can be used when a gateway handles more than one
active connection on an endpoint. This will be the case for three-way
calling, call waiting, and possibly other feature scenarios. In order to
avoid the round-trip to the Call Agent when just changing which connec-
tion is attached to the audio functions of the endpoint, the Notifica-
tionRequest can map an event (usually hook flash, but could be some
other event) to a local function swap audio, which selects the "next"
connection in a round robin fashion. If there is only one connection,
this action is effectively a no-op.
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, SignalRe-
quests and a new digit map as well. The semantics of the embedded Noti-
ficationRequest is as if a new NotificationRequest was just received
with the same NotifiedEntity, and RequestIdentifier. The quarantine
buffer will not be cleared (see later).
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Internet draft Media Gateway Control Protocol (MGCP) October 27, 1998
MGCP implementations shall be able to support at least one level of
embedding. An embedded NotificationRequest that respects this limita-
tion shall not contain another Embedded NotificationRequest.
DigitMap is an optional parameter that allows the Call Agent to provi-
sion the gateways 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 expli-
citly or through a previous command, if the RequestedEvent parameters
contain an request to "accumulate according to the digit map." The col-
lection of these digits will result in a digit string. The digit string
is initialized to a null string upon reception of the NotificationRe-
quest, so that a subsequent notification only returns the digits that
were collected after this request.
SignalRequests is a parameter that contains the set of signals that the
gateway is asked to apply to the endpoint , such as, for example ring-
ing, or continuity tones. Signals are identified by their name, which is
an event name, and may be qualified by parameters.
The action triggered by the SignalRequests is synchronized with the col-
lection of events specified in the RequestedEvents parameter. For exam-
ple, if the NotificationRequest mandates "ringing" and the event request
ask to look for an "off-hook" event, the ringing shall stop as soon as
the gateway detect 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 specified event.
The specific definition of actions that are requested via these Signal-
Requests, such as the duration of and frequency of a DTMF digit, is out-
side the scope of MGCP. This definition may vary from location to loca-
tion and hence from gateway to gateway.
The RequestedEvents and SignalRequests refer to the same events. 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 sig-
nals that a given endpoint can detect or perform are determined by the
list of event packages that are supported by that end point. Each pack-
age specifies a list of events and actions that can be detected or per-
formed. A gateway that is requested to detect or perform an event
belonging to a package that is not supported by the specify endpoint
shall return an error. When the event name is not qualified by a package
name, the default package name for the end point is assumed. If the
event name is not registered in this default package, the gateway shall
return an error.
The Call Agent can send a NotificationRequest whose requested signal
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list is empty. It will do so for example when tone generation should
stop.
The optional QuarantineHandling parameter specifies a set of handling
options:
* whether the quarantined events should be processed or discarded
(the default is to discard them.)
* whether the gateway is expected to generate at most one notifica-
tion (step by step), or multiple notifications (loop), in response
to this request (the default is exactly one.)
When the parameter is absent, the default value is assumed.
DetectEvents is an optional parameter that specifies a list of events
that the gateway is requested to detect during the quarantine period.
When this parameter is absent, the events that should be detected in the
quarantine period are those listed in the request 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 end
point rather than on the endpoint itself. The following rules apply:
* when the notification request ask for a signal to be sent "from"
the endpoint, as is the case for example of inline ringback, then
the corresponding audio signals should be sent over all the connec-
tions that would otherwise receive audio signals from the endpoint.
* when the notification request ask for an event to be observed on
incoming connections, then the endpoint would try to detect the
event on all the active connections. The following notify command
will specify the identifier of the connection on which the signal
was observed.
2.3.2. Notifications
Notifications are sent via the Notify command and are sent by the gate-
way when the observed events occur.
Notify( EndpointId,
NotifiedEntity,
RequestIdentifier,
ObservedEvents)
EndpointId is the name for the endpoint in the gateway which is issuing
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the Notify command, as defined in section 2.1.1. The identifier should
be a fully qualified endpoint name, including the domain name of the
gateway. The local part of the name shall not use the wildcard conven-
tion.
NotifiedEntity is an optional parameter that identifies the entity to
which the notifications is sent. This parameter is equal to the Noti-
fiedEntity parameter of the NotificationRequest that triggered this
notification. The parameter is absent if there was no such parameter in
the triggering request. In this case, the notification is sent to the
entity from which the request was received.
RequestIdentifier is parameter that repeats the RequestIdentifier param-
eter of the NotificationRequest that triggered this notification. It is
used to correlate this notification with the request that triggered it.
ObservedEvents is a list of events that the gateway detected. A single
notification may report a list of events that will be reported in the
order in which they were detected. The list may only contain the iden-
tification of events that were requested in the RequestedEvents parame-
ter 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
detection or provided a final match in the digit map.
2.3.3. CreateConnection
This command is used to create a connection.
ConnectionId,
[SpecificEndPointId,]
[LocalConnectionDescriptor]
<--- CreateConnection(CallId,
EndpointId,
NotifiedEntity,
LocalConnectionOptions,
Mode,
RemoteConnectionDescriptor,
RequestedEvents,
RequestIdentifier,
DigitMap,
SignalRequests,
QuarantineHandling,
DetectEvents)
This function is used to create a connection between two endpoints. A
connection is defined by its endpoints. The input parameters in
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CreateConnection provide the data necessary to build a gateway's "view"
of a connection.
CallId is a globally unique parameter that identifies the call (or ses-
sion) to which this connection belongs. This parameter is unique within
the whole network of gateways; connections that belong to the same call
share the same call-id. The call-id can be used to identify calls for
reporting and accounting purposes.
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, either by using the "anyone" wildcard conven-
tion or by referring to a "generic" endpoint name. If the endpoint is
underspecified, the endpoint name will be assigned by the gateway and
its complete value returned in the SpecificEndPointId parameter of the
response.
The NotifiedEntity is an optional parameter that specifies where the
Notify or DeleteConnection commands should be sent. If the parameter is
absent, the Notify or DeleteConnection commands should be sent to the
originator of the CreateConnection command.
LocalConnectionOptions is a parameter used by the Call Agent to direct
the handling of the connection by the gateway. The fields contained in
LocalConnectionOptions are the following:
* Encoding Method,
* Packetization period,
* Bandwidth,
* Type of Service,
* Usage of echo cancellation,
* Usage of silence suppression or voice activity detection,
* Usage of signal level adaptation and noise level reduction, or
"gain control."
* Usage of reservation service,
* Usage of RTP security,
* Type of network used to carry the connection.
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The values of several of these fields are defined in the SDP standard.
For each of the first three fields, the Call Agent has three options:
* It may state exactly one value, which the gateway will then use for
the connection,
* It may provide a loose specification, such as a list of allowed
encoding methods or a range of packetization periods,
* It may simply provide a bandwidth indication, leaving the choice of
encoding method and packetization period to the gateway.
The bandwidth specification shall not contradict the specification of
encoding methods and packetization period. If an encoding method is
specified, then the gateway is authorized to use it, even if it results
in the usage of a larger bandwidth than specified.
The LocalConnectionOptions parameter may be absent in the case of a data
call.
The Type of Service specifies the class of service that will be used for
the connection. When the connection is transmitted over an IP network,
the parameters encodes the 8-bit type of service value parameter of the
IP header. When the Type of Service is not specified, the gateway shall
use a default or configured value.
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 that should 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. When reservation has been
asked on a connection, the gateway will:
* start emitting RSVP "PATH" messages if the connection is in "sen-
donly", "send-receive", "conference", "network loop back" or "net-
work continuity test" mode,
* start emitting RSVP "RSVP" 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, mode.
The RSVP filters will be deduced from the characteristics of the connec-
tion. The RSVP resource profiles will be deduced from the connection's
bandwidth and packetization period.
By default, the telephony gateways always perform echo cancellation.
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However, it is 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 telephony gateways may perform gain control, in order to adapt the
level of the signal. However, it is necessary, for example for 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 default is to not perform gain control, which is equivalent
to specifying a gain of 0 decibels.
The telephony gateways may perform , 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 turned off.) The default is "off."
The Call agent can request the gateway to enable encryption of the audio
Packets. It does so by providing an key specification, as specified in
RFC 2327. By default, encryption is not used.
The Call Agent may instruct the gateway to prepare the connection on a
specified type of network. The type of network is encoded as in the
"connection-field" parameter of the SDP standard. Possible values are
IN (Internet), ATM and LOCAL. The parameter is optional; if absent, the
network is determined by the type of gateway.
RemoteConnectionDescriptor is the connection descriptor for the remote
side of a connection, on the other side of the IP network. It includes
the same fields as in the LocalConnectionDescriptor, i.e. the fields
that describe a session according to the SDP standard. This parameter
may have a null value when the information for the remote end is not
known yet. This occurs because the entity that builds a connection
starts by sending a CreateConnection to one of the two gateways involved
in it. For the first CreateConnection issued, there is no information
available about the other side of the connection. This information may
be provided later via a ModifyConnection call. In the case of data con-
nections (mode=data), this parameter describes the characteristics of
the data connection.
Mode indicates the mode of operation for this side of the connection.
The mode are "send", "receive", "send/receive", "conference", "data",
"inactive", "loopback", "continuity test", "network loop back" or "net-
work continuity test." The expected handling of these modes is specified
in the introduction of the "Gateway Handling Function" section. Some end
points may not be capable of supporting all modes. If the command
specifies a mode that the endpoint cannot support, and error shall be
returned.
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The gateway returns a ConnectionId, that uniquely identifies the connec-
tion within one endpoint , and a LocalConnectionDescriptor, which is a
session description that contains information about addresses and RTP
ports, as defined in SDP. The LocalConnectionDescriptor is not returned
in the case of data connections. The SpecificEndPointId is an optional
parameter that identifies the responding endpoint. It can be used when
the EndpointId argument referred to a generic endpoint name or a "any
of" wildcard name. When a SpecificEndPointId is returned, the Call Agent
should use it as the EndpointId value is successive commands referring
to this call.
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 parame-
ter, 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 should accept the
voice signals and transmit them through the endpoint.
* If the mode was set to Inactive, Loopback, Continuity Test, the
gateway should refuse the voice signals.
* If the mode was set to Network Loopback or Network Continuity Test,
the gateway should perform the expected echo or Response.
Note that the mode values SendReceive, Conference, Data and SendOnly
don't make sense in this situation. They should be treated as
ReceiveOnly and Inactive.
The RequestedEvents, RequestIdentifier, DigitMap, SignalRequests,
QuarantineHandling and DetectEvents parameters are optional. They can
be used by the Call Agent to transmit a NotificationRequest that is exe-
cuted simultaneously with the creation of the connection. For example,
when the Call Agent wants to initiate a call to an residential gateway,
it should:
* 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
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goes off-hook.
This can be accomplished in a single CreateConnection command, by also
transmitting the RequestedEvent parameters for the off hook event, and
the SignalRequest parameter for the ringing signal.
When these parameters are present, the creation and the NotificationRe-
quests should be synchronized, which means that both should be accepted,
or both 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 Notification-
Request can be refused in the glare condition, if the user is already
off-hook. In this example, the phone should not ring if the connection
cannot be established, and the connection should not be established if
the user is already off hook.
The NotifiedEntity parameter, if present, applies to both the CreateCon-
nection and the NotificationRequest command.
2.3.4. 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 connec-
tion descriptors as well as the remote connection descriptor.
[LocalConnectionDescriptor]
<--- ModifyConnection(CallId,
EndpointId,
ConnectionId,
NotifiedEntity,
LocalConnectionOptions,
Mode,
RemoteConnectionDescriptor,
RequestedEvents,
RequestIdentifier,
DigitMap,
SignalRequests,
QuarantineHandling,
DetectEvents)
The parameters used are the same as in the CreateConnection command,
with the addition of a ConnectionId that identifies the connection
within the call. This parameter is returned by the CreateConnection
function, as part of the local connection descriptor. It uniquely iden-
tifies the connection within the context of the endpoint.
The EndpointId should be a fully qualified endpoint name. The local
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name shall not use the wildcard convention.
The ModifyConnection command can be used to affect parameters of a con-
nection in the following ways:
* Provide information on the other end of the connection, through the
RemoteConnectionDescriptor.
* 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.
* Change the sending parameters of the connection, for example by
switching to a different coding scheme, changing the packetization
period, or modifying the handling of echo cancellation.
Connections can only be 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. (Usage of this
feature is actually for further study.)
The RequestedEvents, RequestIdentifier, DigitMap, SignalRequests,
QuarantineHandling and DetectEvents parameters are optional. They can
be used by the Call Agent to transmit a NotificationRequest that is exe-
cuted simultaneously with the modification of the connection. For exam-
ple, when a connection is accepted, the calling gateway should be
instructed to place the circuit in send-receive mode and to stop provid-
ing ringing tones.
This can be accomplished in a single ModifyConnection command, by also
transmitting the RequestedEvent parameters, for the on hook event, and
an empty SignalRequest parameter, to stop the provision of ringing
tones.
When these parameters are present, the modification and the Notifica-
tionRequests should be synchronized, which means that both should be
accepted, or both refused. The NotifiedEntity parameter, if present,
applies to both the ModifyConnection and the NotificationRequest com-
mand.
2.3.5. 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.
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Connection-parameters <-- DeleteConnection(CallId,
EndpointId,
ConnectionId,
NotifiedEntity,
RequestedEvents,
RequestIdentifier,
DigitMap,
SignalRequests,
QuarantineHandling,
DetectEvents)
The endpoint identifier, in this form of the DeleteConnection command,
shall be fully qualified. Wildcard conventions shall not be used.
In the general case where a connection has two ends, this command has to
be sent to both gateways involved in the connection. Some connections,
however, may use IP multicast. In this case, they can be deleted indivi-
dually.
After the connection has been deleted, the endpoint should be placed in
inactive mode. Any loopback that has been requested for the connection
should be cancelled.
In response to the DeleteConnection command, the gateway returns a list
of parameters that describe the status of the connection. These parame-
ters are:
Number of packets sent:
The total number of RTP data packets transmitted by the sender since
starting transmission on this connection. 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 Modify command. The value is zero
if the connection was set in "receive only" mode.
Number of octets sent:
The total number of payload octets (i.e., not including header or pad-
ding) transmitted in RTP data packets by the sender since starting
transmission on this connection. The count is not reset if the sender
changes its SSRC identifier, for example as a result of a ModifyConnec-
tion command. The value is zero if the connection was set in "receive
only" mode.
Number of packets received:
The total number of RTP data packets received by the sender since
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starting reception on this connection. The count includes packets
received from different SSRC, if the sender used several values. The
value is zero if the connection was set in "send only" mode.
Number of octets received:
The total number of payload octets (i.e., not including header or pad-
ding) transmitted in RTP data packets by the sender since starting
transmission on this connection. The count includes packets received
from different SSRC, if the sender used several values. The value is
zero if the connection was set in "send only" mode.
Number of packets lost:
The total number of RTP data packets that have been lost since the
beginning of reception. This number is defined to be the number of pack-
ets expected less the number of packets actually received, where the
number of packets received includes any which are late or duplicates.
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 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 ini-
tial 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 set in "send only" mode. This parameter is omitted if
the connection was set in "data" mode.
Interarrival jitter:
An estimate of the statistical variance of the RTP data packet interar-
rival time measured in milliseconds and expressed as an unsigned
integer. 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 computa-
tion 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 set in "send only" mode. This param-
eter is omitted if the connection was set in "data" mode.
Average transmission delay:
An estimate of the network latency, expressed in milliseconds. 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 this messages are received. The average is
obtained by summing all the estimates, then dividing by the number of
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Internet draft Media Gateway Control Protocol (MGCP) October 27, 1998
RTCP messages that have been received. This parameter is omitted if the
connection was set in "data" mode.
For a detailed definition of these variables, refer to RFC 1889.
When the connection was set up over an ATM network, the meaning of these
parameters may change:
Number of packets sent:
The total number of ATM cells transmitted since starting transmis-
sion on this connection.
Number of octets sent:
The total number of payload octets transmitted in ATM cells.
Number of packets received:
The total number of ATM cells received since starting reception on
this connection.
Number of octets received:
The total number of payload octets received in ATM cells.
Number of packets lost:
Should be determined as the number of cell losts, or set to zero if
the adaptation layer does not enable the gateway to assess losses.
Interarrival jitter:
Should be understood as the interarrival jitter between ATM cells.
Average transmission delay:
The gateway may not be able to assess this parameter over an ATM
network. It could simply report a null value.
When the connection was set up over an LOCAL interconnect, the meaning
of these parameters is defined as follow:
Number of packets sent:
Not significant.
Number of octets sent:
The total number of payload octets transmitted over the local con-
nection.
Number of packets received:
Not significant.
Number of octets received:
The total number of payload octets received over the connection.
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Internet draft Media Gateway Control Protocol (MGCP) October 27, 1998
Number of packets lost:
Not significant. A value of zero is assumed.
Interarrival jitter:
Not significant. A value of zero is assumed.
Average transmission delay:
Not significant. A value of zero is assumed.
The NotifiedEntity, RequestedEvents, RequestIdentifier, DigitMap, Sig-
nalRequests, QuarantineHandling and DetectEvents parameters are
optional. They can be used by the Call Agent to transmit a Notification-
Request that is executed simultaneously with the deletion of the connec-
tion. For example, when a user hangs up is accepted, 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 RequestedEvent parameters, for the off hook event, and
an empty SignalRequest parameter.
When these parameters are present, the DeleteConnection and the Notifi-
cationRequests should be synchronized, which means that both should be
accepted, or both refused.
2.3.6. DeleteConnection (from the VoIP gateway)
In some circumstances, a gateway may have to clear a connection, for
example because it has lost the resource associated with the connection,
or because it has detected that the endpoint no longer is capable or
willing to send or receive voice. The gateway terminates the connection
by using a variant of the DeleteConnection command:
DeleteConnection( CallId,
EndpointId,
ConnectionId,
Reason-code,
Connection-parameters)
In addition to the call, endpoint and connection identifiers, the gate-
way will also send the call's parameters that would have been returned
to the Call Agent in response to a DeleteConnection command. The reason
code indicates the cause of the disconnection.
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2.3.7. 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. The command can
be used to delete all connections that relate to a Call for an endpoint:
DeleteConnection( CallId,
EndpointId)
It can also be used to delete all connections that terminate in a given
endpoint:
DeleteConnection( EndpointId)
Finally, Call Agents can take advantage of the hierarchical naming
structure of endoints 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 value" wildcarding conven-
tion. The "any value" convention shall not be used. For example, if
endpoints names are structured as the combination of a physical inter-
face name and a circuit number, as in "X35V3+A4/13", the Call Agent may
replace the circuit number by a 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 the connections have been deleted, the endpoint should be placed
in inactive mode. Any loopback that has been requested for the connec-
tions should be cancelled.
This command does not return any individual statistics or call parame-
ters.
2.3.8. Audit Endpoint
The AuditEndPoint command can be used by the Call Agent to find out the
status of a given endpoint.
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[EndPointIdList,]
[NotifiedEntity,]
[RequestedEvents,]
[DigitMap,]
[SignalRequests,]
[RequestIdentifier,]
[NotifiedEntity,]
[ConnectionIdentifiers,]
[DetectEvents,]
[LocalConnectionOptions,]
[SupportedModes]
<--- AuditEndPoint(EndpointId,
RequestedInfo)
The EndpointId identifies the endpoint that is being audited. The "all
of" wildcard convention can be used to start auditing of a group of end-
points. If this convention is used, the gateway should return the list
of endpoint identifiers that match the wildcard in the EndPointIdList
parameter. It shall not return any parameter specific to one of these
endpoints.
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, Noti-
fiedEntity, ConnectionIdentifiers, DetectEvents, and Capabilities.
The response will in turn include information about each of the items
for which auditing info was requested:
* RequestedEvents, the list of events that the endpoint is currently
looking for.
* DigitMap, the digit map the endpoint is currently using.
* SignalRequests, the list of the signals that are currently being
applied to the endpoint
* RequestIdentifier, the RequestIdentifier for the last Notification-
Request received by this endpoint (includes NotificationRequest
embedded in Connection handling primitives).
* NotifiedEntity, the current notified entity for any active Notifi-
cationRequest (including embedded NotificationRequest).
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* ConnectionIdentifiers, the list of ConnectionIdentifiers for all
connections that currently exist for the endpoint specified.
* DetectEvents, the list of events that are currently detected in
quarantine mode.
* Capabilities requests a list of LocalConnectionOptions, the parame-
ter values such as compression algorithms, packetization period,
connection networks that the gateway is ready to support for that
endpoint, as defined in the "CreateConnection" section. In addi-
tion, the option can also be used to encode the event packages that
the endpoint supports, and the list of connection modes that the
gateway is ready to support for that endpoint.
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,
the gateway simply returns a positive acknowledgement.
If no NotifiedEntity has been specified in the last NotificationRequest,
the notified entity defaults to the source address of the last Notifica-
tionRequest command received for this connection.
2.3.9. Audit Connection
The AuditConnection command can be used by the Call Agent to retrieve
the parameters attached to a connection:
[CallId,]
[NotifiedEntity,]
[LocalConnectionOptions,]
[Mode,]
[RemoteConnectionDescriptor,]
[LocalConnectionDescriptor,]
[ConnectionParameters]
<--- AuditConnection(EndpointId,
ConnectionId,
RequestedInfo)
The EndpointId parameter specifies the endpoint that handles the connec-
tion. 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
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requested for the ConnectionId within the EndpointId specified. The fol-
lowing connection info can be audited with this command:
CallId, NotifiedEntity, LocalConnectionOptions, Mode, RemoteConnec-
tionDescriptor, LocalConnectionDescriptor, ConnectionParameters
The AuditConnectionResponse 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.
* LocalConnectionOptions, the LocalConnectionOptions that was sup-
plied for the connection.
* Mode, the current mode of the connection.
* RemoteConnectionDescriptor, the RemoteConnectionDescriptor that was
supplied to the gateway for the connection.
* LocalConnectionDescriptor, the LocalConnectionDescriptor the gate-
way supplied for the connection.
* ConnectionParameters, the current value of the connection parame-
ters for the connection.
If no info was requested and the EndpointId is valid, the gateway simply
checks that the connection exists, and if so returns a positive ack-
nowledgement.
If no NotifiedEntity has been specified for the connection, the notified
entity defaults to the source address of the last connection handling
command received for this connection.
2.3.10. Restart in progress
The RestartInProgress command is used by the gateway to signal that An
endpoint, or a group of endpoint, is taken in or out of service.
[NotifiedEntity]
<------- RestartInProgress ( EndPointId,
RestartMethod,
RestartDelay)
The EndPointId identifies the endpoint that are taken in or out of ser-
vice. The "all of" wildcard convention may be used to apply the command
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to a group of endpoint, 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 specified the type of restart. Three values
have been defined:
* A "graceful" restart method indicates that the specified endpoints
will Be taken out of service after the specified delay. The esta-
blished connections are not yet affected, but the Call Agent should
refrain to establish new connections, and should try to gracefully
tear down the existing connections.
* A "forced" restart method indicates that the specified endpoints
are taken abruptely 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." There are no connec-
tions that are currently established on the endpoints.
The optional "restart delay" parameter is expressed as a number of
seconds. If the number is absent, the delay value should be considered
null. 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" method.
2.4. Race conditions
In order to implement proper call signalling, 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. MGCP deals with race
conditions through the notion of a "quarantine list" and through expli-
cit detections of desynchronizations.
MGCP does not assume that the transport mechanism will maintain the
order of comamnd and responses. This may cause race conditions, that
may be obviated through a proper behavior of the call agent.
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 trasnmis-
sion capability during an earthquake or an ice storm. When power and
transmission are reestablished, many gateways may decide to send "Res-
tartInProgress" commands in synchronicity, leading to very unstable
operation.
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2.4.1. Quarantine list
MGCP controlled gateways will receive "notification requests" that ask
them to watch for a list of "events." The two protocol elements that
determine the handling of these events are the "Requested Events" list
and the "Digit Map."
When the endpoint is initialized, the requested events list and the
digit map are empty. After reception of a command, the gateway starts
observing the endpoint for occurences of the event mentioned in the
list.
The events are examined as they occur. The action that follows is deter-
mined by the "action" parameter associated to the event in the list of
requested events, and also by the digit map. The events that are
defined as "accumulate" are accumulated in a list of events, the events
that are marked as treated according to the digit map will be accumu-
lated in the dialed string. This will go on until one event is encoun-
tered that triggers a Notification to the "notified entity."
The gateway, at this point, will transmit the notification command and
will place the endpoint in a "notification" state. As long as the end-
point is in this notification state, the events that may occur on the
endpoint are accumulated, but not transmitted, even if they had normally
triggered a notification. The events are, in a sense, "quarantined." All
events that are specified in the DetectEventList, or, in the absence of
that list, all events that are refered to in the RequestEventList,
should be detected and quarantined, regardless of the action associated
to the event.
The endpoint exits the notification state when the acknowledgement of
the notification command is received. During that period, the notifica-
tion command may be retransmitted, as specified in section 3.5.
Following that point, the behavior of the gateway depends on the value
of The QuarantineHandling parameter in the notification request. If the
Call Agent specified that it expected at most one notification in
response to the notification request command, then the gateway should
simply keep on accumulating events in the quarantine list until it
receives the next notification request command.
If the gateway is authorized to send multiple successive notifications,
It will proceed as follow. When the gateway receives the acknowledge-
ment, it resets 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 requires a Notify
command to be sent. If that is the case, the gateway can adopt one of
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the two following behaviors:
* it can immediately transmit a Notify command that will report all
events that where accumulated until the triggering event, included,
leaving the unprocessed events in the quarantine list,
* or it can attempt to empty the quarantined list and transmit a sin-
gle Notify command reporting several sets of events and possibly
several dial strings. The dial string is reset to a null value
after each triggering event that satisfies the DigitMap. The events
that follow the last triggering event are left in the quarantine
list.
If the gateway transmits a Notify command, the end point will remain in
the notification state until the acknowledgement is received. If the
gateway does not find a quarantined event that requests a notification,
it places the end point 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 end point. When a new notification request is received in the notif-
ication state, the endpoint is taken out of the notification state
without waiting for the acknowledgement of the notification command, and
the pending notification command will not be repeated.
After receiving the Notification Request command, the requested events
list and digit map are replaced by the newly received parameters, and
the accumulated dial string is reset to a null value. The behavior is
conditioned by the value of the QuarantineHandling parameter. The
parameter may specify that quarantine events should be discarded, in
which case they will be. If the parameter specifies that the event
should be processed, the gateway will start processing the list of
quarantined events, using the newly received list of requested events
and digit map. 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 where accumulated until the triggering event,
included, leaving the unprocessed events in the quarantine list, and
will enter the Notification state.
2.4.2. 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
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event's notification.
To avoid this race condition, the gateway should check the condition of
the endpoint before acknowledging a NotificationRequest. It should
return an error:
1- If the gateway is requested to notify an "off hook" transition
while the phone is already off hook,
2- If the gateway is requested to notify an "on hook" or "flash hook"
condition while the phone is already on hook.
The other state variables of the gateway, such as the list of
RequestedEvent 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, the list of RequestedEvents
and requested signals are emptied. They must be reinstated by a new
request.
Another race condition may occur when a notification is issued shortly
before the reception by the gateway of a NotificationRequest. The
RequestIdentifier is used to correlate Notify commands with Notifica-
tionRequest commands.
2.4.3. Ordering of commands, and treatment of disorder
MGCP does not mandate that the underlying transport protocol guarantees
the sequencing of commands send to a gateway or an endpoint. This pro-
perty tends to maximize the timeliness of actions, but it has a few draw
backs. For example:
* Notification commands may be delayed and arrive to 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 in second position.
Call Agents that want to guarantee consistent operation of the end
points 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 fol-
lowing a model where each endpoint is controlled by its own process
or its own thread.
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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 outstand-
ing command (create or modify). However, a DeleteConnection com-
mand can be issued at any time. In consequence, a gateway may
sometime receive a ModifyConnection command that applies to a pre-
viously deleted connection. Such commands should be ignored, and
an error code should be returned.
4) On a given endpoint, there should normally be only one outstanding
NotificationRequest command at any time. The RequestId parameter
should be used to correlate notification comamnds with the trigger-
ing notification request.
5) In some cases, a "global" DeleteConnection command that applies to
a group of endpoints can step in front of a pending CreateConnec-
tion command. The Call Agent should individually delete all con-
nections whose completion was pending at the time of the global
DeleteConnection command.
These rules do not affect the gateway, which should always respond to
commands.
2.4.4. Fighting the restart avalanche
Let's suppose that a large number of gateways are powered on simultane-
ously. 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 restora-
tion. In order to prevent such avalanches, the following behavior is
suggested:
1) When a gateway is powered on, it should initiate a restart timer to
a random value, uniformly distributed between 0 and a maximum wait-
ing 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 should 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 should initiate the restart pro-
cedure.
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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 between each end
point and the call agent. This simple calculation shows that the call
agent is expected to handle 5 to 6 transactions for each end point,
every 30 minutes on average, or, to put it otherwise, about one transac-
tion per end point 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 360 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 end-
points is much higher than 10% during the peak busy hour, a typical
value being 60%. These endpoints, during the peak hour, are this
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.
2.5. 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 three ranges of values have been defined:
* 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.
The values that have been already defined are listed in the following
table:
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_________________________________________________________
| Code| Meaning |
|_____|__________________________________________________|
| 200 | The requested transaction was executed normally.|
| 250 | The connection was deleted. |
| 400 | The transaction could not be executed, |
| | due to a transient error. |
| 401 | The phone is already off hook |
| 402 | The phone is already on hook |
| 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. |
| 502 | The transaction could not be executed, |
| | because the endpoint does not have |
| | sufficient resources |
| 510 | The transaction could not be executed, |
| | because a protocol error was detected. |
| 511 | The transaction could not be executed, |
| | because the command contained an |
| | unrecognized extension. |
| 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. |
| 517 | Unsupported or invalid mode. |
| 518 | Unsupported or unknown package. |
| 519 | Gateway does not have a digit map. |
| 520 | The transaction could not be executed, |
| | because the endpoint is "restarting". |
|_____|__________________________________________________|
3. Media Gateway Control Protocol
The 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 eight types of command:
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* CreateConnection
* ModifyConnection
* DeleteConnection
* NotificationRequest
* Notify
* AuditEndpoint
* AuditConnection
* RestartInProgress
The first four 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 2.3.6. The Call Agent may
send either of the Audit commands to the gateway. 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
a session description.
Headers and session descriptions are encoded as a set of text lines,
separated by a line feed character. The headers are separated from the
session description 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 section
3.2.1, 3.2.1.2 and 3.3).
Transaction identifiers have values between 1 and 999999999. An MGCP
entity shall not reuse a transaction identifier sooner than 3 minutes
after completion of the previous command in which the identifier was
used.
3.2. Command Header
The command header is composed of:
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* A command line, identifying the requested action or verb, the end-
point towards which the action is requested, and the MGCP protocol
version,
* A set of parameter lines, composed of a parameter name followed by
a parameter value.
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 that should execute the command (in notif-
ications, the name of the endpoint that is issuing the notifica-
tion),
* 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.
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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 should be case insensitive) as
defined in the following table:
____________________________
| Verb | Code|
|____________________|______|
| 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 lines, 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 should be identified by a four letter code starting with the
letter X, such as for example XPER.
3.2.1.2. Coding of the endpoint names
The endpoint names are encoded as e-mail addresses, as defined in RFC
821. In these addresses, the domain name identifies the system where the
endpoint is attached, while the left side identifies a specific endpoint
on that system.
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Examples of such addresses can be:
____________________________________________________________________
| 123456@gw23.whatever.net | Circuit number 123456 in |
| | the Gateway 23 of the "Whatever" |
| | network |
| Call-agent@ca.whatever.net | Call Agent for the |
| | "whatever" network |
| Busy-signal@ann12.whatever.net| The "busy signal" virtual |
| | endpoint in the announcement |
| | server number 12. |
|_______________________________|___________________________________|
The name of notified entities is expressed with the same syntax, with
the possible addition of a port number as in:
Call-agent@ca.whatever.net:5234
3.2.1.3. Coding of the protocol version
The protocol version is coded as the key word 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 as a decimal
number. The version described in this document is version 1.2.
The profile name, if present, is represented by a string of letters and
digits. 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 0.1
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3.2.2. Parameter lines
Parameter lines are composed of a parameter name, which in most cases is
composed of a single upper case character, followed by a colon, a white
space and the parameter value. The parameter that can be present in com-
mands are defined in the following table:
_______________________________________________________________________
|Parameter name | Code| Parameter value |
|______________________|______|_______________________________________|
|CallId | C | Hexadecimal string, |
| | | at most 32 characters |
|ConnectionId | I | Hexadecimal string, |
| | | at most 32 characters |
|NotifiedEntity | N | An identifier, in RFC 821 format, |
| | | composed of an arbitrary string and |
| | | of the domain name of the requesting |
| | | entity, possibly completed by a port |
| | | number, as in: |
| | | Call-agent@ca.whatever.net:5234 |
|RequestIdentifier | X | Hexadecimal string, |
| | | at most 32 characters |
|LocalConnectionOptions| L | See |
|description | | |
|Connection Mode | M | See description |
|RequestedEvents | R | See description |
|SignalRequests | S | See description |
|DigitMap | D | A text encoding of a digit map |
|ObservedEvents | O | See description |
|ConnectionParameters | P | See description |
|ReasonCode | E | An arbitrary character string |
|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. |
|RequestedInfo | F | See description |
|QuarantineHandling | Q | See description |
|DetectEvents | T | See Description |
|RestartMethod | RM | See description |
|RestartDelay | RD | A number of seconds, encoded as |
| | | a decimal number |
|______________________|______|_______________________________________|
The parameters are not necessarily present in all commands. The follow-
ing table provides the association between parameters and commands. The
letter M stands for mandatory, O for optional and F for forbidden.
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_________________________________________________________________________
|Parameter name | CRCX| MDCX| DLCX| RQNT| NTFY| AUED| AUCX|
|______________________|______|______|______|______|______|______|______|
|CallId | M | M | O | F | F | F | F |
|ConnectionId | F | M | O | F | F | F | M |
|RequestIdentifier | O | O | O | M | M | F | F |
|LocalConnectionOptions| O | O | F | F | F | F | F |
|Connection Mode | M | M | F | F | F | F | F |
|RequestedEvents | O | O | O | O* | F | F | F |
|SignalRequests | O | O | O | O* | F | F | F |
|NotifiedEntity | O | O | O | O | O | F | F |
|ReasonCode | F | F | O | F | F | F | F |
|ObservedEvents | F | F | F | F | M | F | F |
|DigitMap | O | O | O | O | F | F | F |
|Connection parameters | F | F | O | F | F | F | F |
|Specific Endpoint ID | F | F | F | F | F | F | F |
|RequestedInfo | F | F | F | F | F | M | M |
|QuarantineHandling | O | O | O | O | O | F | F |
|DetectEvents | O | O | O | O | O | F | F |
|RestartMethod | F | F | F | F | F | F | F |
|RestartDelay | F | F | F | F | F | F | F |
|______________________|______|______|______|______|______|______|______|
Note (*) that the RequestedEvents and SignalRequests parameters are
optional in the NotificationRequest. If these parameters are omitted,
the corresponding lists will be considered empty.
If implementers need to experiment with new parameters, for example when
developing a new application of MGCP, they should identify these parame-
ters by names that start with the string "X-" or "X+", such as for exam-
ple:
X-FlowerOfTheDay: Daisy
Parameter names that start with "X+" are mandatory parameter extensions.
An MGCP entity that receives a mandatory parameter extension that it
cannot understand should refuse to execute the command. It should
respond with an error code 511 (Unrecognized extension).
Parameter names that start with "X-" are non critical parameter exten-
sions. An MGCP entity that receives a mandatory parameter extension that
it cannot understand can safely ignore that parameter.
3.2.2.1. Local connection options
The local connection options describe the operational parameters that
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the Call Agent suggests to the gateway. These parameters are:
* 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 an hyphen.
* The preferred type of compression algorithm, encoded as the keyword
"a", followed by a character string. If the Call Agent specifies a
list of values, these values will be separated by a semicolon.
* 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 an hyphen.
* The echo cancellation parameter, encoded as the keyword "e", fol-
lowed by a colon and the value "on" or "off".
* The gain control parameter, encoded as the keyword "gc", followed
by a colon 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", fol-
lowed by a colon and the value "on" or "off".
* The type of service parameter, encoded as the keyword "t", followed
by a colon and the value encoded as two hexadecimal digits.
* 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" of 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", "ATM" or
"LOCAL".
Each of the parameters is optional. When several parameters are present,
the values are separated by a comma.
Examples of connection descriptors are:
L: p:10, a:G.711
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Internet draft Media Gateway Control Protocol (MGCP) October 27, 1998
L: p:10, a:G.711;G.726-32
L: p:10-20, b: 64
L: b:32-64, e:off
LocalConnectionOptions may furthermore be used by the embedded client to
inform the call agent about its capabilities when audited. In that case,
parameters will have conventional values that are related to capabili-
ties rather than actual connections, and may also contain a list of sup-
ported packages, and a list of supported modes:
*
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
LocalConnectionOptions 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 for this codec, "off" other-
wise. The default is support.
Silence Suppression:
"on" if silence suppression is supported for this codec, "off" oth-
erwise. The default is support.
Gain Control:
"0" if gain control is not supported. 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 sup-
port.
Resource Reservation:
The parameter indicates the reservation services that are sup-
ported, 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.
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The default is "best effort."
Encryption Key:
Encoding any value indicates support for encryption. Default is no
support.
Type of network:
The keyword "nt", followed by a colon and a semicolon separated
list of supported network types. This parameter is optional.
Event Packages
The event packages supported by this endpoint encoded as the key-
word "v", followed by a colon and a character string. If a list of
values is specified, these values will be separated by a semicolon.
The first value specified will be the default package for that end-
point.
Modes
A comma-separated list of supported connection modes for this end-
point.
3.2.2.2. Connection parameters
Connection parameters are encoded as a string of type and value pairs,
where the type is a two letter identifier of the parameter, and the
value a decimal integer. Types are separated from value by an `=' sign.
Parameters are encoded from each other by a comma.
The connection parameter types are specified in the following table:
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__________________________________________________________________
| 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 not received on the |
| | | connection, as deduced from |
| | | gaps in the 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. |
|_____________________|______|____________________________________|
An example of connection parameter encoding is:
P: PS=1245, OS=62345, PR=0, OR=0, PL=0, JI=0, LA=48
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3.2.2.3. Connection mode
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. |
| M: data | The gateway should use the circuit |
| | for network access for data |
| | (e.g., PPP, SLIP, etc.). |
|____________|__________________________________________|
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. Event names are used in
the RequestedEvents, SignalRequests and ObservedEvents parameter. The
following are valid examples of event names:
_____________________________________________________
| L/hu | on-hook transition, in the line package |
| F/0 | digit 0 in the MF package |
| fh | Flash-hook, assuming that the line package|
| | is a default package for the end point. |
|______|_____________________________________________|
In addition, the range and wildcard notation of events can be used,
instead of individual names, in the RequestedEvents and DetectEvents
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parameters. The following are valid examples of such notations:
__________________________________________________________
| M/[0-9] | Digits 0 to 9 in the MF package |
| fh | Flash-hook, assuming that the line package|
| | is a default package for the end point. |
| [0-9*#A-D]| All digits and letters in the DTMF |
| | packages (default for endpoint). |
| T/$ | All events in teh trunk packages. |
|___________|_____________________________________________|
3.2.2.5. RequestedEvents
The RequestedEvent parameter provides the list of events that have been
requested. The event codes are described in the previous section.
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 vari-
ous 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.
The digit-map action can only be specified for the digits, letters and
interdigit timers in the MF and DTMF packages.
The requested list is encoded on a single line, with event/action groups
separated by commas. Examples of RequestedEvents encoding are:
R: hu(N), hf(S,N)
R: hu(N), [0-9#T](D)
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In the case of the "enable" action, the embedded notification request
parameters are encoded as a list of three parameter groups, delimitated
by an opening and closing parenthesis, and separated by semicolons.
R: hd(E([0-9#T](D),hu(N);dl;[0-9].[#T]))
The first parameter group is the enabled value of the RequestedEvents
parameter, followed by the enabled value of the SignalRequests parame-
ter, optionally followed by the enabled value of the DigitMap. This
last parameter is optional. If it is absent, the current value should be
used. The following are valid examples of embedded requests:
R: hd(E([0-9#T](D),hu(N);dl;[0-9].[#T]))
R: hd(E([0-9#T](D),hu(N);dl)
3.2.2.6. SignalRequests
The SignalRequests parameter provides the name of the signals that have
been requested. Each signal is identified by a name, as indicated in the
previous section.
Several signals, such as for example announcement or ADSI display, can
be qualified by additional parameters:
* the name and parameters of the announcement,
* the string that should be displayed.
These parameters will be encoded as a set of UTF8 character strings,
spearated by comams and enclosed within parenthesis, as in:
S: adsi("123456 Francois Gerard")
S: ann(no-such-number, 1234567)
When several signals are requested, their codes are separated by a
comma, as in:
S: asdi(123456 Your friend), rg
3.2.2.7. ObservedEvent
The observed event parameters provides the list of events that have been
observed. The event codes are the same as those used in the Notifica-
tionRequest. Events that have been accumulated according to the digit
map are grouped in a single string. Examples of observed actions are:
O: hu
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Internet draft Media Gateway Control Protocol (MGCP) October 27, 1998
O: 8295555T
O: hf, hf, hu
The packet arrival event is used to notify that at least one packet was
recently sent to an Internet address that is observed by an endpoint.
The event report includes the Internet address, in standard ASCII encod-
ing, between parenthesis:
O: pa(192.96.41.1)
The call back event is used to notify that a call back has been
requested during the initial phase of a data connection. The event
report includes the identification of the user that should be called
back, between parenthesis:
O: cbk(user25)
3.2.2.8. RequestedInfo
The RequestedInfo parameter contains a comma separated list of parameter
codes, as defined in the "Parameter lines" section. For example, if one
wants to audit the value of the NotifiedEntity, RequestIdentifier,
RequestedEvents, SignalRequests, DigitMap, QuarantineHandling and Detec-
tEvents parameters, The value of the RequestedInfo parameter will be:
F:N,X,R,S,D,Q,T
The capabilities request, in the AuditEndPoint command, is encoded by
the keyword "A", as in:
F:A
3.2.2.9. QuarantineHandling
The quarantine handling parameter contains a list of comma separated
keywords:
* The keyword "process" or "discard" to indicate the treatment of
quarantined events. If neither process or discard is present, dis-
card is assumed.
* The keyword "step" or "loop" to indicate whether exactly at most
one notification is expected, or whether multiple notifications are
allowed. If neither step or loop is present, step is assumed. The
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Internet draft Media Gateway Control Protocol (MGCP) October 27, 1998
following values are valid examples:
Q:loop
Q:process
Q:discard,loop
3.2.2.10. DetectEvents
The DetectEvent parameter is encoded as a comma separated list of
events, such as for example:
T: hu,hd,hf,[0-9#*]
3.2.2.11. RestartMethod
The RestartMethod parameter is encoded as one of the keywords "grace-
ful", "forced" or "restart", as for example:
RM:restart
3.3. Format of response headers
The response header is composed of a response line, optionally followed
by headers that encode the response parameters.
The response line starts with the response code, which is a three digit
numeric value. The code is followed by a white space, the transaction
identifier, and an optional commentary.
In the case of a CreateConnection message, the response line is followed
by a Connection-Id parameter. It may also be followed a Specific-
Endpoint-Id parameter, if the creation request was sent to a generic
Endpoint-Id.
In the case of a DeleteConnection message, the response line is followed
by a Connection Parameters parameter, as defined in section 3.2.2.2.
A LocalConnectionDescriptor should be transmitted with a positive
response (code 200) to a CreateConnection. It may 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.
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3.4. Encoding of the session description
The session description is encoded in conformance with the session
description protocol, SDP. MGCP implementations are expected to be fully
capable of parsing any conformant SDP message, and should send session
descriptions that strictly conform to the SDP standard. The usage of SDP
actually depends on the type of session that is being, as specified in
the "mode" parameter:
* if the mode is set to "data", the session description describes the
configuration of a data access service.
* if the mode is set to any other value, the session description is
for an audio service.
For an audio service, the gateway will consider the information provided
in SDP for the "audio" media. For a data service, the gateway will con-
sider the information provided for the "network-access" media.
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 parameters of SDP that are relevant for
the telephony application are:
At the session description level:
* The IP address of the remote gateway (in commands) or of the
local gateway (in responses), or multicast address of the
audio conference, encoded as an SDP "connection data" parame-
ter. This parameter specifies the IP address that will be used
to exchange RTP packets.
For the audio media:
* Media description field (m) specifying the audio media, the
transport port used for receiving RTP packets by the remote
gateway (commands) or by the local gateway (responses) , the
RTP/AVP transport, and the list of formats that the gateway
will accept. This list should normally always include the code
0 (reserved for G.711).
* Optionally, RTPMAP attributes that define the encoding of
dynamic audio formats,
* Optionally, a packetization period (packet time) attribute
(Ptime) defining the duration of the packet,
* Optionally, an attribute defining the type of connection
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Internet draft Media Gateway Control Protocol (MGCP) October 27, 1998
(sendonly, recvonly, sendrecv, inactive)
* The IP address of the remote gateway (in commands) or of the
local gateway (in responses), if it is not present at the ses-
sion level.
There is a request, in some environments, to use the MGCP to negotiate
connections that will use other transmission channels than RTP over UDP
and IP. This will be detailed in an extension to this document.
3.4.2. Usage of SDP in a network access service
The parameters of SDP that are relevant for a data network access appli-
cation are:
For the data media:
* Media description field (m) specifying the network access
media, identified by the code "m=nas/xxxx", where "xxxx"
describes the access control method that should be used for
parametrizing the network access, as specified below. The
field may also specify the port that should be used for con-
tacting the server, as specified in the SDP syntax.
* Connection address parameter (c=) specifying the address, or
the domain name, of the server that implement the access con-
trol method. This parameter may also be specified at the ses-
sion level.
* Optionally, a bearer type attribute (a=bearer:) describing the
type of data connection to be used.
* Optionally, a framing type attribue (a=framing:) describing
the type of framing that will be used on the channel.
* Optionally, attributes describing the called number
(a=dialed:), the number to which the call was delivered
(a=called:) and the calling number (a=dialing:).
* Optionally, attributes describing the range of addresses that
could be used by the dialup client on its LAN (a=subnet:).
* Optionally, an encryption key, encoded as specified in the SDP
protocol (k=).
The connection address shall be encoded as specified in the SDP stan-
dard. It will be used in conjunction with the port specified in the
media line to access a server, whose type will one of:
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__________________________________________________________
| Method name| Method description |
|____________|____________________________________________|
| radius | Authentication according |
| | to the Radius protocol. |
| tacacs | Authentication according |
| | to the TACACS+ protocol. |
| diameter | Authentication according |
| | to the Diameter protocol. |
| l2tp | Level 2 tunneling protocol. |
| | The address and port are those of the LNS.|
| login | Local login. (There is normally |
| | no server for that method.) |
| none | No authentication required. |
| | (The call was probably vetted |
| | by the Call Agent.) |
|____________|____________________________________________|
If needed, the gateway may use the key specified in the announcement to
access the service. That key, in particular, may be used for the estab-
lishment of an L2TP tunnel.
The valid values of the bearer attribute are defined in the following
table:
____________________________________________________________________
| Type of bearer description | Example of values |
|_________________________________|_________________________________|
| ITU modem standard | V.32, V.34, V.90. |
| ISDN transparent access, 64 kbps| ISDN64 |
| ISDN64 + V.110 | ISDN64/V.110 |
| ISDN64 + V.120 | ISDN64/V.120 |
| ISDN transparent access, 56 kbps| ISDN56 |
| Informal identification | (Requires coordination between |
| | the Call Agent and the gateway)|
|_________________________________|_________________________________|
The valid values of the framing attribute are defined in the following
table:
_________________________________________________
Type of framing description Example of values
_________________________________________________
PPP, asynchronous framing ppp-asynch
PPP, HDLC framing ppp-hdlc
SLIP, asynchronous slip
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| Asynchronous, no framing | asynch |
|____________________________|___________________|
The network access authentication parameter provides instructions on the
access control that should be exercized for the data call. This optional
attribute is encoded as:
"a=subnet:" <network type> <address type>
<connection address> "/" <prefix length>
Where the parameters "network type", "address type", and "connection
address" are formatted as defined for the connection address parameter
(c=) in SDP, and where the "prefix length" is a decimal representation
of the number of bits in the prefix.
Examples of SDP announcement for the network access service could be:
v=0
m=nas/radius
c=IN IP4 radius.example.net
a=bearer:v.34
a=framing:ppp-asynch
a=dialed:18001234567
a=called:12345678901
a=dialing:12340567890
v=0
m=nas/none
c=IN IP4 128.96.41.1
a=subnet:IN IP4 123.45.67.64/26
a=bearer:isdn64
a=framing:ppp-sync
a=dialed:18001234567
a=dialing:2345678901
v=0
c=IN IP4 access.example.net
m=nas/l2tp
k=clear:some-shared-secret
a=bearer:v.32
a=framing:ppp-asynch
a=dialed:18001234567
a=dialing:2345678901
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3.4.3. Usage of SDP for ATM connections
The specification of the SDP payload for ATM connections will be
described in a companion document, "Usage of MGCP to control Voice over
ATM gateways." The following text is indicative.
The SDP payload will specify:
* That the connection is to be established over an ATM interface,
using the "c=" parameter of SDP to specify an address in the ATM
family, the ATM addressing variant (NSAP, UNI, E.164) and the ATM
address.
* The "m=audio" parameter will specify the audio encoding and, if
needed, the VPI and VCI.
* Additional attributes parameters (a=) will be used to specify the
ATM coding variants, such as the type of adaptation layer and the
error correction or loss compenmsation algorithms.
An example of SDP payload for an ATM connection could be:
v=0
c=ATM NSAP 47.0091.8100.0000.0060.3e64.fd01.0060.3e64.fd01.fe
m=audio 5/1002 ATM/AVP G.711u
a=connection_type:AAL2
3.4.4. Usage of SDP for local connections
When MGCP is used to set up internal connections within a single gate-
way, the SDP format is used to encode the parameters of that connection.
The following parameters will be used:
* The connection parameter (C=) will specify that the connection is
local, using the keyword "LOCAL" as network type space, the keyword
"EPN" (endpoint name) as address type, and the name of the end-
point 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 RTP (RTP payload numbers.) The type of encoding should normally
be set to 0 (G.711).
An example of local SDP payload could be:
v=0
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c=LOCAL EPN X35V3+A4/13
m=audio 0 LOCAL 0
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 of the commands.
When no port is specified for the endpoint, the commands should be sent
to the default MGCP port, 2427.
MGCP messages, being carried over UDP, may be subject to losses. In the
absence of a timely response, commands are repeated. MGCP entities are
expected to 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 30 seconds, and a list of the transactions that are currently being
executed. 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. The remaining commands will be compared to the
list of current transaction. If a match is found, the MGCP entity does
not execute the transaction, which is simply ignored.
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 connections.
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 a
response. 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
The retransmission timer, in TCP, is set to the sum of the average delay
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Internet draft Media Gateway Control Protocol (MGCP) October 27, 1998
plus N times the average deviation.
After the any retransmission, the MGCP entity should do the following:
* It should double the estimated value of the average delay, AAD
* It should compute a random value, uniformly distributed between 0.5
AAD and AAD
* It should set the retransmission timer to the sum of that random
value and N times the average deviation.
This procedure has two effects. Because it includes an exponentially
increasing component, it will automatically slow down the stream of mes-
sages in case of congestion. Because it includes a random component, it
will break the potential synchronization between notifications triggered
by the same external event.
3.5.1. Piggy backing
There are cases when a Call Agent will want to send several messages at
the same time to the same gateways. When several MGCP messages have to
be sent in the same UDP packets, they should be separated by a line of
text that contain a single dot, as in for example:
200 2005 OK
DLCX 1244 card23/21@trgw-7.whatever.net SGCP 1.1
C: A3C47F21456789F0
I: FDE234C8
The piggy-backed messages should be processed exactly has if they had
been received in several simultaneous messages.
4. Security requirements
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 connec-
tions, as defined in the IP security architecture as defined in RFC
1825, using either the IP Authentication Header, defined in RFC 1826, or
the IP Encapsulating Security Payload, defined in RFC 1827. The complete
MGCP protocol stack would thus include the following layers:
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________________________________
| 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 descrip-
tions are used to carry session keys, as defined in SDP.
These procedure 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 pro-
cedures.
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5. Event packages and end point types
This section provides an initial definition of packages and event names.
More packages can be defined in additional documents.
5.1. Basic packages
The list of basic packages includes the following:
_________________________________________
| Package | name |
|______________________________|_________|
| Generic Media Package | G |
| DTMF package | D |
| MF Package | M |
| Trunk Package | T |
| Line Package | L |
| Handset Package | H |
| RTP Package | R |
| Netwark Access Server Package| N |
| Announcement Server Package | A |
| Script Package | Script|
|______________________________|_________|
In the tables of events for each package, there are five columns:
Symbol: the unique symbol used for the event
Definition: a short description of the event
R: an x appears in this column is the event can be Requested by
the call agent.
S: if nothing appears in this column for an event, then the event
cannot be signaled on command by the call agent. Otherwise,
the following symbols identify the type of event:
OO On/Off signal. The signal is turned on until commanded
by the call agent to turn it off, or until the event is
superseded by a new signal.
TO Timeout signal. The signal lasts for a given duration
unless it is superseded by a new signal.
BR Brief signal. The event has a short, known duration.
Duration: specifies the duration of TO signals.
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5.1.1. Generic Media Package
Package Name: G
The generic media package group the events and signals that can be
observed on several types of endpoints, such as trunking gateways,
access gateways or residential gateways.
_________________________________________________________________
| Symbol | Definition | R | S Duration|
|__________|____________________________|_____|__________________|
| mt | Modem detected | x | |
| ft | Fax tone detected | x | |
| ld | Long duration connection | x | |
| pat(###) | Pattern ### detected | x | OO |
| rt | Ringback tone | | OO |
| cf | Confirm tone | | OO |
| cg | Network Congestion tone | | OO |
| it | Intercept tone | | OO |
| pt | Preemption tone | | OO |
| of | report failure | x | |
|__________|____________________________|_____|__________________|
The pattern definition can be used for specific algorithms such as
answering machine detection, tone detection, and the like.
The "long duration connection" is detected when a connection has been
established for more than 1 hour.
5.1.2. DTMF package
Package name: D
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_______________________________________________________________
| Symbol | Definition | R | S Duration |
|________|___________________________|_____|___________________|
| 0 | DTMF 0 | x | BR |
| 1 | DTMF 1 | x | BR |
| 2 | DTMF 2 | x | BR |
| 3 | DTMF 3 | x | BR |
| 4 | DTMF 4 | x | BR |
| 5 | DTMF 5 | x | BR |
| 6 | DTMF 6 | x | BR |
| 7 | DTMF 7 | x | BR |
| 8 | DTMF 8 | x | BR |
| 9 | DTMF 9 | x | BR |
| # | DTMF # | x | BR |
| * | DTMF * | x | BR |
| A | DTMF A | x | BR |
| B | DTMF B | x | BR |
| C | DTMF C | x | BR |
| D | DTMF D | x | BR |
| L | long duration indicator | x | 2 seconds|
| X | Wildcard, match | x | |
| | any digit 0-9 | | |
| T | Interdigit timer | x | 4 seconds|
| of | report failure | x | |
|________|___________________________|_____|___________________|
The "long duration indicator" is observed when a DTMF signal is produced
for a duration larger than two seconds. In this case, the gateway will
detect two successive events: first, when the signal has been recog-
nized, the DTMF signal, and then, 2 seconds later, the long duration
signal.
5.1.3. MF Package
Package Name: M
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________________________________________________________
| Symbol | Definition | R | S Duration |
|________|____________________|_____|___________________|
| 0 | MF 0 | x | BR |
| 1 | MF 1 | x | BR |
| 2 | MF 2 | x | BR |
| 3 | MF 3 | x | BR |
| 4 | MF 4 | x | BR |
| 5 | MF 5 | x | BR |
| 6 | MF 6 | x | BR |
| 7 | MF 7 | x | BR |
| 8 | MF 8 | x | BR |
| 9 | MF 9 | x | BR |
| X | Wildcard, match | x | |
| | any digit 0-9 | | |
| T | Interdigit timer | x | 4 seconds|
| K0 | MF K0 or KP | x | BR |
| K1 | MF K1 | x | BR |
| K2 | MF K2 | x | BR |
| S0 | MF S0 or ST | x | BR |
| S1 | MF S1 | x | BR |
| S2 | MF S2 | x | BR |
| S3 | MF S3 | x | BR |
| wk | Wink | x | BR |
| wko | Wink off | x | BR |
| is | Incoming seizure | x | OO |
| rs | Return seizure | x | OO |
| us | Unseize circuit | x | OO |
| of | report failure | x | |
|________|____________________|_____|___________________|
The definition of the MF package events is as follow:
Wink
A transition from unseized to seized to unseized trunk states
within a specified period. Typical seizure period is 100-350
msec.)
Incoming seizure
Incoming indication of call attempt.
Return seizure:
Seizure in response to outgoing seizure.
Unseize circuit:
Unseizure of a circuit at the end of a call.
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Wink off:
A signal used in operator services trunks. A transition from
seized to unseized to seized trunk states within a specified period
of 100-350 ms. (To be checked)
5.1.4. Trunk Package
Package Name: T
___________________________________________________________________
| Symbol | Definition | R | S Duration|
|________|________________________________|_____|__________________|
| co1 | Continuity tone (single tone,| x | OO |
| | or return tone) | | |
| co2 | Continuity test (go tone, | x | OO |
| | in dual tone procedures) | | |
| lb | Loopback | | OO |
| om | Old Milliwatt Tone (1000 Hz) | x | OO |
| nm | New Milliwatt Tone (1004 Hz) | x | OO |
| tl | Test Line | x | OO |
| zz | No circuit | x | OO |
| as | Answer Supervision | x | OO |
| ro | Reorder Tone | x | OO |
| of | report failure | x | |
|________|________________________________|_____|__________________|
The definition of the trunk package signal events is as follow:
A tone at 2010 + or - 30 Hz.
Continuity Test (co2):
A tone at the 1780 + or - 30 Hz.
Milliwatt Tones:
Old Milliwatt Tone (1000 Hz), New Milliwatt Tone (1004 Hz)
Line Test:
105 Test Line test progress tone (2225 Hz + or - 25 Hz at -10 dBm0
+ or -- 0.5dB).
No circuit:
(that annoying tri-tone, low to high)
Answer Supervision:
Reorder Tone:
(120 Impulses per minute tone).
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The continuity tones are used when the call agent wants to initiate a
continuity test. There are two types of tests, single tone and dual
tone. The Call agent is expected to know, through provisioning informa-
tion, which test should be applied to a given endpoint. For example, the
call agent that wants to initiate a single frequency test will send to
the gateway a command of the form:
RQNT 1234 epx-t1/17@tgw2.example.net
X: AB123FE0
S: co1
R: co1
If it wanted instead to initiate a dual-tone test, it would send the
command:
RQNT 1234 epx-t1/17@tgw2.example.net
X: AB123FE0
S: co2
R: co1
The gateway would send the requested signal, and in both cases would
look for the return of the 2010 Hz tone (co1). When it detects that
tone, it will send the corresponding notification.
The tones are of type OO: the gateway will keep sending them until it
receives a new notification request.
5.1.5. Line Package
Package Name: L
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__________________________________________________________________________
|Symbol | Definition | R | S Duration |
|_____________|______________________________|_____|_____________________|
|adsi(string) | adsi display | | BR |
|hd | Off hook transition | x | |
|hu | On hook transition | x | |
|hf | Flash hook | x | |
|aw | Answer tone | x | OO |
|bz | Busy tone | | OO |
|wt | Call Waiting tone | | TO 30 seconds |
|dl | Dial tone (350 + 440 Hz) | | TO 120 seconds|
|nbz | Network busy | x | OO |
| | (fast cycle busy) | | |
|rg | Ringing | | TO 30 seconds |
|r0, r1, r2, | Distinctive ringing | | TO 30 seconds |
|r3, r4, r5, | | | |
|r6 or r7 | | | |
|p | Prompt tone | x | BR |
|e | Error tone | x | BR |
|sdl | Stutter dialtone | | BR |
|v | Alerting Tone | | OO |
|y | Recorder Warning Tone | | OO |
|t | SIT tone | | |
|z | Calling Card Service Tone | | OO |
|oc | Report on completion | x | |
|ot | Off hook warning tone | | OO |
|s(###) | Distinctive tone pattern | x | BR |
|of | report failure | x | |
|_____________|______________________________|_____|_____________________|
The definition of the tones is as follow:
Alerting Tone:
a 440 Hz Tone of 2 second duration followed by 1/2 second of tone
every 10 seconds.
Recorder Warning Tone:
1400 Hz of Tone of 0.5 second duration every 15 seconds.
SIT tone:
used for indicating a line is out of service.
Calling Card Service Tone:
60 ms of 941 + 1477 Hz and 940 ms of 350 + 440 Hz (dial tone),
decaying exponentially with a time constant of 200 ms.
Distinctive tone pattern:
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where ### is any number between 000 and 999, inclusive. Can be
used for distinctive ringing, customized dial tone, etc.
Report on completion
The report on completion event is detected when the gateway was
asked to perform one or several signals of type TO on the endpoint,
and when these signals were completed without being stopped by the
detection of a requested event such as off-hook transition or
dialed digit. The completion report may carry as parameter the
name of the signal that came to the end of its live time, as in:
O: L/oc(L/dl)
5.1.6. Handset emulation package
Package Name: H
__________________________________________________________________________
|Symbol | Definition | R | S Duration |
|_____________|______________________________|_____|_____________________|
|adsi(string) | adsi display | x | BR |
|hd | Off hook transition | x | OO |
|hu | On hook transition | x | OO |
|hf | Flash hook | x | BR |
|aw | Answer tone | x | OO |
|bz | Busy tone | x | OO |
|wt | Call Waiting tone | x | TO 30 seconds |
|dl | Dial tone (350 + 440 Hz) | x | TO 120 seconds|
|nbz | Network busy | x | OO |
| | (fast cycle busy) | | |
|rg | Ringing | x | TO 30 seconds |
|r0, r1, r2, | Distinctive ringing | x | TO 30 seconds |
|r3, r4, r5, | | | |
|r6 or r7 | | | |
|p | Prompt tone | x | BR |
|e | Error tone | x | BR |
|sdl | Stutter dialtone | x | BR |
|v | Alerting Tone | x | OO |
|y | Recorder Warning Tone | x | OO |
|t | SIT tone | x | |
|z | Calling Card Service Tone | x | OO |
|oc | Report on completion | x | |
|ot | Off hook warning tone | x | OO |
|s(###) | Distinctive tone pattern | x | BR |
|of | report failure | x | |
|_____________|______________________________|_____|_____________________|
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The handset emulation package is an extension of the line package, to be
used when the gateway is capable of emulating a handset. The difference
with the line pacake is that events such as "off hook" can be signalled
as well as detected.
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5.1.7. RTP Package
Package Name: R
_________________________________________________________________
| Symbol | Definition | R | S Duration|
|_________|______________________________|_____|_________________|
| UC | Used codec changed | x | |
| SR(###) | Sampling rate changed | x | |
| JI(###) | Jitter buffer size changed | x | |
| PL(###) | Packet loss exceeded | x | |
| qa | Quality alert | x | |
| of | report failure | x | |
|_________|______________________________|_____|_________________|
Codec Changed:
Codec changed to hexadecimal codec number enclosed in parenthesis,
as in UC(15), to indicate the codec was changed to PCM mu-law.
Codec Numbers are specified in RFC 1890, or in a new definition of
the audio profiles for RTP that replaces this RFC. Some implemen-
tations of media gateways may not allow the codec to be changed
upon command from the call agent. codec changed to codec hexade-
cimal ##.
Sampling Rate Changed:
Sampling rate changed to decimal number in milliseconds enclosed in
parenthesis, as in SR(20), to indicate the sampling rate was
changed to 20 milliseconds. Some implementations of media gateways
may not allow the sampling rate to be changed upon command from a
call agent.
Packet Loss Exceeded:
Packet loss rate exceed the threshold of the specified decimal
number of packets per 100,000 packets, where the packet loss number
is contained in parenthesis. For example, PL(10) indicates packets
are being dropped at a rate of 1 in 10,000 packets.
Quality alert
The packet loss rate or the comination of delay and jitter exceed a
specified quality threshold.
5.1.8. Netwark Access Server Package
Package Name: N
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____________________________________________________________
| Symbol | Definition | R | S Duration|
|________|__________________________|_____|_________________|
| pa | Packet arrival | x | |
| cbk | Call back request | x | |
| cl | Carrier lost | x | |
| au | Authorization succeeded| x | |
| ax | Authorization denied | x | |
| of | Report failure | x | |
|________|__________________________|_____|_________________|
5.1.9. Announcement Server Package
Package Name: A
___________________________________________________________________
| Symbol | Definition | R | S Duration|
|________________|________________________|_____|__________________|
| ann(url,parms) | Play an announcement | | TO variable|
| oc | Report on completion | x | |
| of | Report failure | x | |
|________________|________________________|_____|__________________|
The announcement action is qualified by an URL name and by a set of ini-
tial parameters as in for example:
S: ann(http://scripts.example.net/all-lines-busy.au)
The "operation complete" event will be detected when the announcement is
played out. If the announcement cannot be played out, an operation
failure event can be returned. The failure may be explained by a com-
mentary, as in:
O: A/of(file not found)
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5.1.10. Script Package
Package Name: Script
______________________________________________________________
| Symbol | Definition | R | S | Duration|
|___________|________________________|_____|______|___________|
| java(url) | Load a java script | | TO | variable|
| perl(url) | Load a perl script | | TO | variable|
| tcl(url) | Load a TCL script | | TO | variable|
| xml(url) | Load an XML script | | TO | variable|
| oc | Report on completion | x | | |
| of | Report failure | x | | |
|___________|________________________|_____|______|___________|
The "language" action define is qualified by an URL name and by a set of
initial parameters as in for example:
S: script/java(http://scripts.example.net/credit-card.java,long,1234)
The current definition defines keywords for the most common languages.
More languages may be defined in further version of this documents. For
each language, an API specification will describe how the scripts can
issue local "notificationRequest" commands, and receive the correspond-
ing notifications.
The script produces an output which consists of one or several text
string, separated by commas. The text string are reported as a commen-
tary in the report on completion, as in for example:
O: script/oc(21223456794567,9738234567)
The failure report may also return a string, as in:
O: script/oc(21223456794567,9738234567)
The definition of the script environment and the specific actions in
that environment are for further study.
5.2. Basic endpoint types and profiles
We define the following basic endpoint types and profiles:
* Trunk gateway (ISUP)
* Trunk gateway (MF)
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* Network Access Server (NAS)
* Combined NAS/VOIP gateway
* Access Gateway
* Residential Gateway
* Announcement servers
These gateways are supposed to implement the following packages
___________________________________________________________
| Gateway | Supported packages |
|____________________________|_____________________________|
| Trunk gateway (ISUP) | GM, DTMF, TK, RTP |
| Trunk gateway (MF) | GM, MF, DTMF, TK, RTP |
| Network Access Server (NAS)| GM, MF, TK, NAS |
| Combined NAS/VOIP gateway | GM, MF, DTMF, TK, NAS, RTP|
| Access Gateway (VOIP) | GM, DTMF, MF, RTP |
| Access Gateway (VOIP+NAS) | GM, DTMF, MF, NAS, RTP |
| Residential Gateway | GM, DTMF, Line, RTP |
| Announcement Server | ANN, RTP |
|____________________________|_____________________________|
Advanced announcement servers may also support the Script package.
Advanced trunking servers may support the ANN package, the Script pack-
age, and in some cases the Line and Handset package as well.
6. Versions and compatibility
MGCP version 0.1 results from the fusion of the SGCP and IPDC proposals.
6.1. Changes between MGCP and initial versions of SGCP
MGCP version 0.1 (which subsumes SGCP version 1.2) introduces the fol-
lowing changes from SGCP version 1.1:
* Protocol name changed to MGCP.
* Introduce a formal wildcarding structure in the name of endpoints,
inspired from IPDC, and detailed the usage of wildcard names in
each operation.
* Naming scheme for events, introducing a package structure inspired
from IPDC.
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* New operations for audit endpoint, audit connection (requested by
the Cablelabs) and restart (inspired from IPDC).
* New parameter to control the behavior of the notification request.
* Improved text on the detection and handling of race conditions.
* Syntax modification for event reporting, to incorporate package
names.
* Definition of basic event packages (inspired from IPDC).
* Incorporation of mandatory and optional extension parameters,
inspired by IPDC.
SGCP version 1.1 introduces the following changes from version SGCP 1.0:
* Extension parameters (X-??:)
* Error Code 511 (Unrecognized extension).
* All event codes can be used in RequestEvent, SignalRequest and
ObservedEvent parameters.
* Error Code 512 (Not equipped to detect requested event).
* Error Code 513 (Not equipped to generate requested signal).
* Error Code 514 (Unrecognized announcement).
* Specific Endpoint-ID can be returned in creation commands.
* Changed the code for the ASDI display from "ad" to "asdi" to avoid
conflict with the digits A and D.
* Changed the code for the answer tone from "at" to "aw" to avoid
conflict with the digit A and the timer mark T
* Changed the code for the busy tone from "bt" to "bz" to avoid con-
flict with the digit B and the timer mark T
* Specified that the continuity tone value is "co" (CT was
incorrectly used in several instances; CT conflicts with .)
* Changed the code for the dial tone from "dt" to "dl" to avoid con-
flict with the digit D and the timer mark T
* Added a code point for announcement requests.
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* Added a code point for the "wink" event.
* Set the "octet received" code in the "Connection Parameters" to
"OR" (was set to RO, but then "OR" was used throughout all exam-
ples.)
* Added a "data" mode.
* Added a description of SDP parameters for the network access mode
(NAS).
* Added four flow diagrams for the network access mode.
* Incorporated numerous editing suggestions to make the description
easier to understand. In particular, cleared the confusion between
requests, queries, functions and commands.
* Defined the continuity test mode as specifying a dual-tone tran-
sponder, while the loopback mode can be used for a single tone
test.
* Added event code "OC", operation completed.
* Added the specification of the "quarantine list", which clarifies
the expected handling of events and notifications.
* Added the specification of a "wildcard delete" operation.
7. Acknowledgements
We want to thank here the many reviewers who provided us with advice on
the design of SGCP, notably Flemming Andreasen, David Auerbach, Bob
Biskner, David Bukovinsky, Barry Hoffner, Dave Oran, Jeff Orwick, John
Pickens, Lou Rubin, Chip Sharp, Kurt Steinbrenner and Joe Stone.
The version 0.1 of MGCP is 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 have been 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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8. References
* Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A
Transport Protocol for Real-Time Applications", RFC 1889, January
1996.
* Schulzrinne, H., "RTP Profile for Audio and Video Conferences with
Minimal Control", RFC 1890, January 1996
* Handley, M, Jacobson, V., "SDP: Session Description Protocol", RFC
2327, April 1998.
* Handley, M., "SAP - Session Announcement Protocol", Work in Pro-
gress.
* Handley, M., Schooler, E., and H. Schulzrinne, "Session Initiation
Protocol (SIP)", Work in Progress.
* Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
* ITU-T, Recommendation Q.761, "FUNCTIONAL DESCRIPTION OF THE ISDN
USER PART OF SIGNALLING SYSTEM No. 7", (Malaga-Torremolinos, 1984;
modified at Helsinki, 1993)
* ITU-T, Recommendation Q.762, "GENERAL FUNCTION OF MESSAGES AND SIG-
NALS OF THE ISDN USER PART OF SIGNALLING SYSTEM No. 7", (Malaga-
Torremolinos, 1984; modified at Helsinki, 1993)
* ITU-T, Recommendation H.323, "VISUAL TELEPHONE SYSTEMS AND EQUIP-
MENT FOR LOCAL AREA NETWORKS WHICH PROVIDE A NON-GUARANTEED QUALITY
OF SERVICE."
* ITU-T, Recommendation H.225, "Call Signaling Protocols and Media
Stream Packetization for Packet Based Multimedia Communications
Systems."
* ITU-T, Recommendation H.245, "LINE TRANSMISSION OF NON-TELEPHONE
SIGNALS."
* Atkinson, R., "Security Architecture for the Internet Protocol."
RFC 1825, August 1995.
* Atkinson, R., "IP Authentication Header." RFC 1826, August 1995.
* Atkinson, R., "IP Encapsulating Security Payload (ESP)." RFC 1827,
August 1995.
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9. Authors' Addresses
Mauricio Arango
RSL COM Latin America
6300 N.W. 5th Way, Suite 100
Ft. Lauderdale, FL 33309
Phone: (954) 492-0913
Email: marango@rslcom.com
Andrew Dugan
Level3 Communications
1450 Infinite Drive
Louisville, CO 80027
Phone: (303)926 3123
Email: andrew.dugan@l3.com
Isaac Elliott
Level3 Communications
1450 Infinite Drive
Louisville, CO 80027
Phone: (303)926 3123
Email: ike.elliott@l3.com
Christian Huitema
Bellcore
MCC 1J236B
445 South Street
Morristown, NJ 07960
U.S.A.
Phone: +1 973-829-4266
EMail: huitema@bellcore.com
Scott Pickett
Vertical Networks
1148 East Arques Ave
Sunnyvale, CA 94086
Phone: (408) 523-9700 extension 200
Email: ScottP@vertical.com
Further information is available on the SGCP web site:
http://sgcp.bellcore.com
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Internet draft Media Gateway Control Protocol (MGCP) October 27, 1998
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