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RFC 3435 - Media Gateway Control Protocol (MGCP) Version 1.0
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RFC3435 - Media Gateway Control Protocol (MGCP) Version 1.0
Network Working Group F. Andreasen
Request for Comments: 3435 B. Foster
Obsoletes: 2705 Cisco Systems
Category: Informational January 2003
Media Gateway Control Protocol (MGCP)
Version 1.0
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
IESG Note
This document is being published for the information of the
community. It describes a protocol that is currently being deployed
in a number of products. Implementers should be aware of RFC 3015,
which was developed in the IETF Megaco Working Group and the ITU-T
SG16 and which is considered by the IETF and ITU-T to be the
standards-based (including reviewed security considerations) way to
meet the needs that MGCP was designed to address.
Abstract
This document describes an application programming interface and a
corresponding protocol (MGCP) which is used between elements of a
decomposed multimedia gateway. The decomposed multimedia gateway
consists of a Call Agent, which contains the call control
"intelligence", and a media gateway which contains the media
functions, e.g., conversion from TDM voice to Voice over IP.
Media gateways contain endpoints on which the Call Agent can create,
modify and delete connections in order to establish and control media
sessions with other multimedia endpoints. Also, the Call Agent can
instruct the endpoints to detect certain events and generate signals.
The endpoints automatically communicate changes in service state to
the Call Agent. Furthermore, the Call Agent can audit endpoints as
well as the connections on endpoints.
The basic and general MGCP protocol is defined in this document,
however most media gateways will need to implement one or more MGCP
packages, which define extensions to the protocol suitable for use
with specific types of media gateways. Such packages are defined in
separate documents.
Table of Contents
1. Introduction.................................................5
1.1 Relation with the H.323 Standards............................7
1.2 Relation with the IETF Standards.............................8
1.3 Definitions..................................................9
1.4 Conventions used in this Document............................9
2. Media Gateway Control Interface.............................10
2.1 Model and Naming Conventions................................10
2.1.1 Types of Endpoints..........................................10
2.1.2 Endpoint Identifiers........................................14
2.1.3 Calls and Connections.......................................16
2.1.4 Names of Call Agents and Other Entities.....................22
2.1.5 Digit Maps..................................................23
2.1.6 Packages....................................................26
2.1.7 Events and Signals..........................................28
2.2 Usage of SDP................................................33
2.3 Gateway Control Commands....................................33
2.3.1 Overview of Commands........................................33
2.3.2 EndpointConfiguration.......................................36
2.3.3 NotificationRequest.........................................37
2.3.4 Notify......................................................44
2.3.5 CreateConnection............................................46
2.3.6 ModifyConnection............................................52
2.3.7 DeleteConnection (from the Call Agent)......................54
2.3.8 DeleteConnection (from the gateway).........................58
2.3.9 DeleteConnection (multiple connections from the Call Agent) 59
2.3.10 AuditEndpoint...............................................60
2.3.11 AuditConnection.............................................65
2.3.12 RestartInProgress...........................................66
2.4 Return Codes and Error Codes................................69
2.5 Reason Codes................................................74
2.6 Use of Local Connection Options and Connection Descriptors..75
2.7 Resource Reservations.......................................77
3. Media Gateway Control Protocol..............................77
3.1 General Description.........................................78
3.2 Command Header..............................................79
3.2.1 Command Line................................................79
3.2.2 Parameter Lines.............................................82
3.3 Format of response headers.................................101
3.3.1 CreateConnection Response..................................104
3.3.2 ModifyConnection Response..................................105
3.3.3 DeleteConnection Response..................................106
3.3.4 NotificationRequest Response...............................106
3.3.5 Notify Response............................................106
3.3.6 AuditEndpoint Response.....................................106
3.3.7 AuditConnection Response...................................107
3.3.8 RestartInProgress Response.................................108
3.4 Encoding of the Session Description (SDP)..................108
3.4.1 Usage of SDP for an Audio Service..........................110
3.4.2 Usage of SDP for LOCAL Connections.........................110
3.5 Transmission over UDP......................................111
3.5.1 Providing the At-Most-Once Functionality...................112
3.5.2 Transaction Identifiers and Three Ways Handshake...........113
3.5.3 Computing Retransmission Timers............................114
3.5.4 Maximum Datagram Size, Fragmentation and Reassembly........115
3.5.5 Piggybacking...............................................116
3.5.6 Provisional Responses......................................117
4. States, Failover and Race Conditions.......................119
4.1 Failover Assumptions and Highlights........................119
4.2 Communicating with Gateways................................121
4.3 Retransmission, and Detection of Lost Associations:........122
4.4 Race Conditions............................................126
4.4.1 Quarantine List............................................127
4.4.2 Explicit Detection.........................................133
4.4.3 Transactional Semantics....................................134
4.4.4 Ordering of Commands, and Treatment of Misorder............135
4.4.5 Endpoint Service States....................................137
4.4.6 Fighting the Restart Avalanche.............................140
4.4.7 Disconnected Endpoints.....................................143
4.4.8 Load Control in General....................................146
5. Security Requirements......................................147
5.1 Protection of Media Connections............................148
6. Packages...................................................148
6.1 Actions....................................................150
6.2 BearerInformation..........................................150
6.3 ConnectionModes............................................151
6.4 ConnectionParameters.......................................151
6.5 DigitMapLetters............................................151
6.6 Events and Signals.........................................152
6.6.1 Default and Reserved Events................................155
6.7 ExtensionParameters........................................156
6.8 LocalConnectionOptions.....................................157
6.9 Reason Codes...............................................157
6.10 RestartMethods.............................................158
6.11 Return Codes...............................................158
7. Versions and Compatibility.................................158
7.1 Changes from RFC 2705......................................158
8. Security Considerations....................................164
9. Acknowledgments............................................164
10. References.................................................164
Appendix A: Formal Syntax Description of the Protocol.............167
Appendix B: Base Package..........................................175
B.1 Events.....................................................175
B.2 Extension Parameters.......................................176
B.2.1 PersistentEvents...........................................176
B.2.2 NotificationState..........................................177
B.3 Verbs......................................................177
Appendix C: IANA Considerations...................................179
C.1 New MGCP Package Sub-Registry..............................179
C.2 New MGCP Package...........................................179
C.3 New MGCP LocalConnectionOptions Sub-Registry...............179
Appendix D: Mode Interactions.....................................180
Appendix E: Endpoint Naming Conventions...........................182
E.1 Analog Access Line Endpoints...............................182
E.2 Digital Trunks.............................................182
E.3 Virtual Endpoints..........................................183
E.4 Media Gateway..............................................184
E.5 Range Wildcards............................................184
Appendix F: Example Command Encodings.............................185
F.1 NotificationRequest........................................185
F.2 Notify.....................................................186
F.3 CreateConnection...........................................186
F.4 ModifyConnection...........................................189
F.5 DeleteConnection (from the Call Agent).....................189
F.6 DeleteConnection (from the gateway)........................190
F.7 DeleteConnection (multiple connections
from the Call Agent).......................................190
F.8 AuditEndpoint..............................................191
F.9 AuditConnection............................................192
F.10 RestartInProgress..........................................193
Appendix G: Example Call Flows....................................194
G.1 Restart....................................................195
G.1.1 Residential Gateway Restart................................195
G.1.2 Call Agent Restart.........................................198
G.2 Connection Creation........................................200
G.2.1 Residential Gateway to Residential Gateway.................200
G.3 Connection Deletion........................................206
G.3.1 Residential Gateway to Residential Gateway.................206
Authors' Addresses................................................209
Full Copyright Statement..........................................210
1. Introduction
This document describes an abstract application programming interface
(MGCI) and a corresponding protocol (MGCP) for controlling media
gateways from external call control elements called media gateway
controllers or Call Agents. A media gateway is typically a network
element that provides conversion between the audio signals carried on
telephone circuits and data packets carried over the Internet or over
other packet networks. Examples of media gateways are:
* Trunking gateways, that interface between the telephone network and
a Voice over IP network. Such gateways typically manage a large
number of digital circuits.
* Voice over ATM gateways, which operate much the same way as voice
over IP trunking gateways, except that they interface to an ATM
network.
* Residential gateways, that provide a traditional analog (RJ11)
interface to a Voice over IP network. Examples of residential
gateways include cable modem/cable set-top boxes, xDSL devices, and
broad-band wireless devices.
* Access gateways, that provide a traditional analog (RJ11) or
digital PBX interface to a Voice over IP network. Examples of
access gateways include small-scale voice over IP gateways.
* Business gateways, that provide a traditional digital PBX interface
or an integrated "soft PBX" interface to a Voice over IP network.
* Network Access Servers, that can attach a "modem" to a telephone
circuit and provide data access to the Internet. We expect that in
the future, the same gateways will combine Voice over IP services
and Network Access services.
* Circuit switches, or packet switches, which can offer a control
interface to an external call control element.
MGCP assumes a call control architecture where the call control
"intelligence" is outside the gateways and handled by external call
control elements known as Call Agents. The MGCP assumes that these
call control elements, or Call Agents, will synchronize with each
other to send coherent commands and responses to the gateways under
their control. If this assumption is violated, inconsistent behavior
should be expected. MGCP does not define a mechanism for
synchronizing Call Agents. MGCP is, in essence, a master/slave
protocol, where the gateways are expected to execute commands sent by
the Call Agents. In consequence, this document specifies in great
detail the expected behavior of the gateways, but only specifies
those parts of a Call Agent implementation, such as timer management,
that are mandated for proper operation of the protocol.
MGCP assumes a connection model where the basic constructs are
endpoints and connections. Endpoints are sources and/or sinks of
data and can be physical or virtual. Examples of physical endpoints
are:
* An interface on a gateway that terminates a trunk connected to a
PSTN switch (e.g., Class 5, Class 4, etc.). A gateway that
terminates trunks is called a trunking gateway.
* An interface on a gateway that terminates an analog POTS connection
to a phone, key system, PBX, etc. A gateway that terminates
residential POTS lines (to phones) is called a residential gateway.
An example of a virtual endpoint is an audio source in an audio-
content server. Creation of physical endpoints requires hardware
installation, while creation of virtual endpoints can be done by
software.
Connections may be either point to point or multipoint. A point to
point connection is an association between two endpoints with the
purpose of transmitting data between these endpoints. Once this
association is established for both endpoints, data transfer between
these endpoints can take place. A multipoint connection is
established by connecting the endpoint to a multipoint session.
Connections can be established over several types of bearer networks,
for example:
* Transmission of audio packets using RTP and UDP over an IP network.
* Transmission of audio packets using AAL2, or another adaptation
layer, over an ATM network.
* Transmission of packets over an internal connection, for example
the TDM backplane or the interconnection bus of a gateway. This is
used, in particular, for "hairpin" connections, connections that
terminate in a gateway but are immediately rerouted over the
telephone network.
For point-to-point connections the endpoints of a connection could be
in separate gateways or in the same gateway.
1.1 Relation with the H.323 Standards
MGCP is designed as an internal protocol within a distributed system
that appears to the outside as a single VoIP gateway. This system is
composed of a Call Agent, that may or may not be distributed over
several computer platforms, and of a set of gateways, including at
least one "media gateway" that perform the conversion of media
signals between circuits and packets, and at least one "signaling
gateway" when connecting to an SS7 controlled network. In a typical
configuration, this distributed gateway system will interface on one
side with one or more telephony (i.e., circuit) switches, and on the
other side with H.323 conformant systems, as indicated in the
following table:
------------------------------------------------------------------
| Functional| Phone | Terminating | H.323 conformant |
| Plane | switch | Entity | systems |
|-----------|------------|-----------------|-----------------------|
| Signaling | Signaling | Call agent | Signaling exchanges |
| Plane | exchanges | | with the Call Agent |
| | through | | through H.225/RAS and|
| | SS7/ISUP | | H.225/Q.931. |
|-----------|------------|-----------------|-----------------------|
| | | | Possible negotiation |
| | | | of logical channels |
| | | | and transmission |
| | | | parameters through |
| | | | H.245 with the call |
| | | | agent. |
|-----------|------------|-----------------|-----------------------|
| | | Internal | |
| | | synchronization| |
| | | through MGCP | |
|-----------|------------|-----------------|-----------------------|
| Bearer | Connection| Telephony | Transmission of VoIP |
| Data | through | gateways | data using RTP |
| Transport | high speed| | directly between the |
| Plane | trunk | | H.323 station and the|
| | groups | | gateway. |
------------------------------------------------------------------
In the MGCP model, the gateways focus on the audio signal translation
function, while the Call Agent handles the call signaling and call
processing functions. As a consequence, the Call Agent implements
the "signaling" layers of the H.323 standard, and presents itself as
an "H.323 Gatekeeper" or as one or more "H.323 Endpoints" to the
H.323 systems.
1.2 Relation with the IETF Standards
While H.323 is the recognized standard for VoIP terminals, the IETF
has also produced specifications for other types of multi-media
applications. These other specifications include:
* the Session Description Protocol (SDP), RFC 2327
* the Session Announcement Protocol (SAP), RFC 2974
* the Session Initiation Protocol (SIP), RFC 3261
* the Real Time Streaming Protocol (RTSP), RFC 2326.
The latter three specifications are in fact alternative signaling
standards that allow for the transmission of a session description to
an interested party. SAP is used by multicast session managers to
distribute a multicast session description to a large group of
recipients, SIP is used to invite an individual user to take part in
a point-to-point or unicast session, RTSP is used to interface a
server that provides real time data. In all three cases, the session
description is described according to SDP; when audio is transmitted,
it is transmitted through the Real-time Transport Protocol, RTP.
The distributed gateway systems and MGCP will enable PSTN telephony
users to access sessions set up using SAP, SIP or RTSP. The Call
Agent provides for signaling conversion, according to the following
table:
------------------------------------------------------------------
| Functional| Phone | Terminating | IETF conforming systems|
| Plane | switch | Entity | |
|-----------|------------|---------------|-------------------------|
| Signaling | Signaling | Call agent | Signaling exchanges |
| Plane | exchanges | | with the Call Agent |
| | through | | through SAP, SIP or |
| | SS7/ISUP | | RTSP. |
|-----------|------------|---------------|-------------------------|
| | | | Negotiation of session |
| | | | description parameters |
| | | | through SDP (telephony |
| | | | gateway terminated but |
| | | | passed via the call |
| | | | agent to and from the |
| | | | IETF conforming system)|
|-----------|------------|---------------|-------------------------|
| | | Internal syn- | |
| | | chronization | |
| | | through MGCP | |
|-----------|------------|---------------|-------------------------|
| Bearer | Connection| Telephony | Transmission of VoIP |
| Data | through | gateways | data using RTP, |
| Transport | high speed| | directly between the |
| Plane | trunk | | remote IP end system |
| | groups | | and the gateway. |
------------------------------------------------------------------
The SDP standard has a pivotal status in this architecture. We will
see in the following description that we also use it to carry session
descriptions in MGCP.
1.3 Definitions
Trunk: A communication channel between two switching systems, e.g.,
a DS0 on a T1 or E1 line.
1.4 Conventions used in this Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED, "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14, RFC 2119 [2].
2. Media Gateway Control Interface
The interface functions provide for connection control and endpoint
control. Both use the same system model and the same naming
conventions.
2.1 Model and Naming Conventions
The MGCP assumes a connection model where the basic constructs are
endpoints and connections. Connections are grouped in calls. One or
more connections can belong to one call. Connections and calls are
set up at the initiative of one or more Call Agents.
2.1.1 Types of Endpoints
In the introduction, we presented several classes of gateways. Such
classifications, however, can be misleading. Manufacturers can
arbitrarily decide to provide several types of services in a single
package. A single product could well, for example, provide some
trunk connections to telephony switches, some primary rate
connections and some analog line interfaces, thus sharing the
characteristics of what we described in the introduction as
"trunking", "access" and "residential" gateways. MGCP does not make
assumptions about such groupings. We simply assume that media
gateways support collections of endpoints. The type of the endpoint
determines its functionality. Our analysis, so far, has led us to
isolate the following basic endpoint types:
* Digital channel (DS0),
* Analog line,
* Announcement server access point,
* Interactive Voice Response access point,
* Conference bridge access point,
* Packet relay,
* ATM "trunk side" interface.
In this section, we will describe the expected behavior of such
endpoints.
This list is not final. There may be other types of endpoints
defined in the future, for example test endpoints that could be used
to check network quality, or frame-relay endpoints that could be used
to manage audio channels multiplexed over a frame-relay virtual
circuit.
2.1.1.1 Digital Channel (DS0)
Digital channels provide a 64 Kbps service. Such channels are found
in trunk and ISDN interfaces. They are typically part of digital
multiplexes, such as T1, E1, T3 or E3 interfaces. Media gateways
that support such channels are capable of translating the digital
signals received on the channel, which may be encoded according to
A-law or mu-law, using either the complete set of 8 bits per sample
or only 7 of these bits, into audio packets. When the media gateway
also supports a Network Access Server (NAS) service, the gateway
shall be capable of receiving either audio-encoded data (modem
connection) or binary data (ISDN connection) and convert them into
data packets.
+-------
+------------+|
(channel) ===|DS0 endpoint| -------- Connections
+------------+|
+-------
Media gateways should be able to establish several connections
between the endpoint and the packet networks, or between the endpoint
and other endpoints in the same gateway. The signals originating
from these connections shall be mixed according to the connection
"mode", as specified later in this document. The precise number of
connections that an endpoint supports is a characteristic of the
gateway, and may in fact vary according to the allocation of
resources within the gateway.
In some cases, digital channels are used to carry signaling. This is
the case for example for SS7 "F" links, or ISDN "D" channels. Media
gateways that support these signaling functions shall be able to send
and receive the signaling packets to and from a Call Agent, using the
"backhaul" procedures defined by the SIGTRAN working group of the
IETF. Digital channels are sometimes used in conjunction with
channel associated signaling, such as "MF R2". Media gateways that
support these signaling functions shall be able to detect and produce
the corresponding signals, such as for example "wink" or "A",
according to the event signaling and reporting procedures defined in
MGCP.
2.1.1.2 Analog Line
Analog lines can be used either as a "client" interface, providing
service to a classic telephone unit, or as a "service" interface,
allowing the gateway to send and receive analog calls. When the
media gateway also supports a NAS service, the gateway shall be
capable of receiving audio-encoded data (modem connection) and
convert them into data packets.
+-------
+---------------+|
(line) ===|analog endpoint| -------- Connections
+---------------+|
+-------
Media gateways should be able to establish several connections
between the endpoint and the packet networks, or between the endpoint
and other endpoints in the same gateway. The audio signals
originating from these connections shall be mixed according to the
connection "mode", as specified later in this document. The precise
number of connections that an endpoint supports is a characteristic
of the gateway, and may in fact vary according to the allocation of
resources within the gateway. A typical gateway should however be
able to support two or three connections per endpoint, in order to
support services such as "call waiting" or "three way calling".
2.1.1.3 Announcement Server Access Point
An announcement server endpoint provides access to an announcement
service. Under requests from the Call Agent, the announcement server
will "play" a specified announcement. The requests from the Call
Agent will follow the event signaling and reporting procedures
defined in MGCP.
+----------------------+
| Announcement endpoint| -------- Connection
+----------------------+
A given announcement endpoint is not expected to support more than
one connection at a time. If several connections were established to
the same endpoint, then the same announcements would be played
simultaneously over all the connections.
Connections to an announcement server are typically one way, or "half
duplex" -- the announcement server is not expected to listen to the
audio signals from the connection.
2.1.1.4 Interactive Voice Response Access Point
An Interactive Voice Response (IVR) endpoint provides access to an
IVR service. Under requests from the Call Agent, the IVR server will
"play" announcements and tones, and will "listen" to responses, such
as DTMF input or voice messages, from the user. The requests from
the Call Agent will follow the event signaling and reporting
procedures defined in MGCP.
+-------------+
| IVR endpoint| -------- Connection
+-------------+
A given IVR endpoint is not expected to support more than one
connection at a time. If several connections were established to the
same endpoint, then the same tones and announcements would be played
simultaneously over all the connections.
2.1.1.5 Conference Bridge Access Point
A conference bridge endpoint is used to provide access to a specific
conference.
+-------
+--------------------------+|
|Conference bridge endpoint| -------- Connections
+--------------------------+|
+-------
Media gateways should be able to establish several connections
between the endpoint and the packet networks, or between the endpoint
and other endpoints in the same gateway. The signals originating
from these connections shall be mixed according to the connection
"mode", as specified later in this document. The precise number of
connections that an endpoint supports is a characteristic of the
gateway, and may in fact vary according to the allocation of
resources within the gateway.
2.1.1.6 Packet Relay
A packet relay endpoint is a specific form of conference bridge, that
typically only supports two connections. Packets relays can be found
in firewalls between a protected and an open network, or in
transcoding servers used to provide interoperation between
incompatible gateways, for example gateways that do not support
compatible compression algorithms, or gateways that operate over
different transmission networks such as IP and ATM.
+-------
+---------------------+ |
|Packet relay endpoint| 2 connections
+---------------------+ |
+-------
2.1.1.7 ATM "trunk side" Interface
ATM "trunk side" endpoints are typically found when one or several
ATM permanent virtual circuits are used as a replacement for the
classic "TDM" trunks linking switches. When ATM/AAL2 is used,
several trunks or channels are multiplexed on a single virtual
circuit; each of these trunks correspond to a single endpoint.
+-------
+------------------+|
(channel) = |ATM trunk endpoint| -------- Connections
+------------------+|
+-------
Media gateways should be able to establish several connections
between the endpoint and the packet networks, or between the endpoint
and other endpoints in the same gateway. The signals originating
from these connections shall be mixed according to the connection
"mode", as specified later in this document. The precise number of
connections that an endpoint supports is a characteristic of the
gateway, and may in fact vary according to the allocation of
resources within the gateway.
2.1.2 Endpoint Identifiers
Endpoint identifiers have two components that both are case-
insensitive:
* the domain name of the gateway that is managing the endpoint
* a local name within that gateway
Endpoint names are of the form:
local-endpoint-name@domain-name
where domain-name is an absolute domain-name as defined in RFC 1034
and includes a host portion, thus an example domain-name could be:
mygateway.whatever.net
Also, domain-name may be an IP-address of the form defined for domain
name in RFC 821, thus another example could be (see RFC 821 for
details):
[192.168.1.2]
Both IPv4 and IPv6 addresses can be specified, however use of IP
addresses as endpoint identifiers is generally discouraged.
Note that since the domain name portion is part of the endpoint
identifier, different forms or different values referring to the same
entity are not freely interchangeable. The most recently supplied
form and value MUST always be used.
The local endpoint name is case-insensitive. The syntax of the local
endpoint name is hierarchical, where the least specific component of
the name is the leftmost term, and the most specific component is the
rightmost term. The precise syntax depends on the type of endpoint
being named and MAY start with a term that identifies the endpoint
type. In any case, the local endpoint name MUST adhere to the
following naming rules:
1) The individual terms of the naming path MUST be separated by a
single slash ("/", ASCII 2F hex).
2) The individual terms are character strings composed of letters,
digits or other printable characters, with the exception of
characters used as delimiters ("/", "@"), characters used for
wildcarding ("*", "$") and white spaces.
3) Wild-carding is represented either by an asterisk ("*") or a
dollar sign ("$") for the terms of the naming path which are to be
wild-carded. Thus, if the full local endpoint name is of the
form:
term1/term2/term3
then the entity name field looks like this depending on which
terms are wild-carded:
*/term2/term3 if term1 is wild-carded
term1/*/term3 if term2 is wild-carded
term1/term2/* if term3 is wild-carded
term1/*/* if term2 and term3 are wild-carded, etc.
In each of these examples a dollar sign could have appeared
instead of an asterisk.
4) A term represented by an asterisk ("*") is to be interpreted as:
"use ALL values of this term known within the scope of the Media
Gateway". Unless specified otherwise, this refers to all
endpoints configured for service, regardless of their actual
service state, i.e., in-service or out-of-service.
5) A term represented by a dollar sign ("$") is to be interpreted as:
"use ANY ONE value of this term known within the scope of the
Media Gateway". Unless specified otherwise, this only refers to
endpoints that are in-service.
Furthermore, it is RECOMMENDED that Call Agents adhere to the
following:
* Wild-carding should only be done from the right, thus if a term is
wild-carded, then all terms to the right of that term should be
wild-carded as well.
* In cases where mixed dollar sign and asterisk wild-cards are used,
dollar-signs should only be used from the right, thus if a term had
a dollar sign wild-card, all terms to the right of that term should
also contain dollar sign wild-cards.
The description of a specific command may add further criteria for
selection within the general rules given above.
Note, that wild-cards may be applied to more than one term in which
case they shall be evaluated from left to right. For example, if we
have the endpoint names "a/1", "a/2", "b/1", and "b/2", then "$/*"
(which is not recommended) will evaluate to either "a/1, a/2", or
"b/1, b/2". However, "*/$" may evaluate to "a/1, b/1", "a/1, b/2",
"a/2, b/1", or "a/2, b/2". The use of mixed wild-cards in a command
is considered error prone and is consequently discouraged.
A local name that is composed of only a wildcard character refers to
either all (*) or any ($) endpoints within the media gateway.
2.1.3 Calls and Connections
Connections are created on the Call Agent on each endpoint that will
be involved in the "call". In the classic example of a connection
between two "DS0" endpoints (EP1 and EP2), the Call Agents
controlling the endpoints will establish two connections (C1 and C2):
+---+ +---+
(channel1) ===|EP1|--(C1)--... ...(C2)--|EP2|===(channel2)
+---+ +---+
Each connection will be designated locally by an endpoint unique
connection identifier, and will be characterized by connection
attributes.
When the two endpoints are located on gateways that are managed by
the same Call Agent, the creation is done via the three following
steps:
1) The Call Agent asks the first gateway to "create a connection" on
the first endpoint. The gateway allocates resources to that
connection, and responds to the command by providing a "session
description". The session description contains the information
necessary for a third party to send packets towards the newly
created connection, such as for example IP address, UDP port, and
codec parameters.
2) The Call Agent then asks the second gateway to "create a
connection" on the second endpoint. The command carries the
"session description" provided by the first gateway. The gateway
allocates resources to that connection, and responds to the
command by providing its own "session description".
3) The Call Agent then uses a "modify connection" command to provide
this second "session description" to the first endpoint. Once
this is done, communication can proceed in both directions.
When the two endpoints are located on gateways that are managed by
two different Call Agents, the Call Agents exchange information
through a Call-Agent to Call-Agent signaling protocol, e.g., SIP [7],
in order to synchronize the creation of the connection on the two
endpoints.
Once a connection has been established, the connection parameters can
be modified at any time by a "modify connection" command. The Call
Agent may for example instruct the gateway to change the codec used
on a connection, or to modify the IP address and UDP port to which
data should be sent, if a connection is "redirected".
The Call Agent removes a connection by sending a "delete connection"
command to the gateway. The gateway may also, under some
circumstances, inform a gateway that a connection could not be
sustained.
The following diagram provides a view of the states of a connection,
as seen from the gateway:
Create connection
received
|
V
+-------------------+
|resource allocation|-(failed)-+
+-------------------+ |
| (connection refused)
(successful)
|
v
+----------->+
| |
| +-------------------+
| | remote session |
| | description |----------(yes)--------+
| | available ? | |
| +-------------------+ |
| | |
| (no) |
| | |
| +-----------+ +------+
| +--->| half open |------> Delete <-------| open |<----------+
| | | (wait) | Connection |(wait)| |
| | +-----------+ received +------+ |
| | | | | |
| | Modify Connection | Modify Connection |
| | received | received |
| | | | | |
| | +--------------------+ | +--------------------+ |
| | |assess modification | | |assess modification | |
| | +--------------------+ | +--------------------+ |
| | | | | | | |
| |(failed) (successful) | (failed) (successful) |
| | | | | | | |
| +<---+ | | +-------------+-------+
| | |
+<-------------------+ |
|
+-----------------+
| Free connection |
| resources. |
| Report. |
+-----------------+
|
V
2.1.3.1 Names of Calls
One of the attributes of each connection is the "call identifier",
which as far as the MGCP protocol is concerned has little semantic
meaning, and is mainly retained for backwards compatibility.
Calls are identified by unique identifiers, independent of the
underlying platforms or agents. Call identifiers are hexadecimal
strings, which are created by the Call Agent. The maximum length of
call identifiers is 32 characters.
Call identifiers are expected to be unique within the system, or at a
minimum, unique within the collection of Call Agents that control the
same gateways. From the gateway's perspective, the Call identifier
is thus unique. When a Call Agent builds several connections that
pertain to the same call, either on the same gateway or in different
gateways, these connections that belong to the same call should share
the same call-id. This identifier can then be used by accounting or
management procedures, which are outside the scope of MGCP.
2.1.3.2 Names of Connections
Connection identifiers are created by the gateway when it is
requested to create a connection. They identify the connection
within the context of an endpoint. Connection identifiers are
treated in MGCP as hexadecimal strings. The gateway MUST make sure
that a proper waiting period, at least 3 minutes, elapses between the
end of a connection that used this identifier and its use in a new
connection for the same endpoint (gateways MAY decide to use
identifiers that are unique within the context of the gateway). The
maximum length of a connection identifier is 32 characters.
2.1.3.3 Management of Resources, Attributes of Connections
Many types of resources will be associated to a connection, such as
specific signal processing functions or packetization functions.
Generally, these resources fall in two categories:
1) Externally visible resources, that affect the format of "the bits
on the network" and must be communicated to the second endpoint
involved in the connection.
2) Internal resources, that determine which signal is being sent over
the connection and how the received signals are processed by the
endpoint.
The resources allocated to a connection, and more generally the
handling of the connection, are chosen by the gateway under
instructions from the Call Agent. The Call Agent will provide these
instructions by sending two sets of parameters to the gateway:
1) The local directives instruct the gateway on the choice of
resources that should be used for a connection,
2) When available, the "session description" provided by the other
end of the connection (referred to as the remote session
description).
The local directives specify such parameters as the mode of the
connection (e.g., send-only, or send-receive), preferred coding or
packetization methods, usage of echo cancellation or silence
suppression. (A detailed list can be found in the specification of
the LocalConnectionOptions parameter of the CreateConnection
command.) Depending on the parameter, the Call Agent MAY either
specify a value, a range of values, or no value at all. This allows
various implementations to implement various levels of control, from
a very tight control where the Call Agent specifies minute details of
the connection handling to a very loose control where the Call Agent
only specifies broad guidelines, such as the maximum bandwidth, and
lets the gateway choose the detailed values subject to the
guidelines.
Based on the value of the local directives, the gateway will
determine the resources to allocate to the connection. When this is
possible, the gateway will choose values that are in line with the
remote session description - but there is no absolute requirement
that the parameters be exactly the same.
Once the resources have been allocated, the gateway will compose a
"session description" that describes the way it intends to send and
receive packets. Note that the session description may in some cases
present a range of values. For example, if the gateway is ready to
accept one of several compression algorithms, it can provide a list
of these accepted algorithms.
Local Directives
(from Call Agent 1)
|
V
+-------------+
| resource |
| allocation |
| (gateway 1) |
+-------------+
| |
V |
Local |
Parameters V
| Session
| Description Local Directives
| | (from Call Agent 2)
| +---> Transmission----+ |
| (CA to CA) | |
| V V
| +-------------+
| | resource |
| | allocation |
| | (gateway 2) |
| +-------------+
| | |
| | V
| | Local
| | Parameters
| Session
| Description
| +---- Transmission<---+
| | (CA to CA)
V V
+-------------+
| modification|
| (gateway 1) |
+-------------+
|
V
Local
Parameters
-- Information flow: local directives & session descriptions --
2.1.3.4 Special Case of Local Connections
Large gateways include a large number of endpoints which are often of
different types. In some networks, we may often have to set-up
connections between endpoints that are located within the same
gateway. Examples of such connections may be:
* Connecting a call to an Interactive Voice-Response unit,
* Connecting a call to a Conferencing unit,
* Routing a call from one endpoint to another, something often
described as a "hairpin" connection.
Local connections are much simpler to establish than network
connections. In most cases, the connection will be established
through some local interconnecting device, such as for example a TDM
bus.
When two endpoints are managed by the same gateway, it is possible to
specify the connection in a single command that conveys the names of
the two endpoints that will be connected. The command is essentially
a "Create Connection" command which includes the name of the second
endpoint in lieu of the "remote session description".
2.1.4 Names of Call Agents and Other Entities
The media gateway control protocol has been designed to allow the
implementation of redundant Call Agents, for enhanced network
reliability. This means that there is no fixed binding between
entities and hardware platforms or network interfaces.
Call Agent names consist of two parts, similar to endpoint names.
Semantically, the local portion of the name does not exhibit any
internal structure. An example Call Agent name is:
ca1@ca.whatever.net
Note that both the local part and the domain name have to be
supplied. Nevertheless, implementations are encouraged to accept call
agent names consisting of only the domain name.
Reliability can be improved by using the following procedures:
* Entities such as endpoints or Call Agents are identified by their
domain name, not their network addresses. Several addresses can be
associated with a domain name. If a command or a response cannot
be forwarded to one of the network addresses, implementations MUST
retry the transmission using another address.
* Entities MAY move to another platform. The association between a
logical name (domain name) and the actual platform is kept in the
domain name service. Call Agents and Gateways MUST keep track of
the time-to-live of the record they read from the DNS. They MUST
query the DNS to refresh the information if the time to live has
expired.
In addition to the indirection provided by the use of domain names
and the DNS, the concept of "notified entity" is central to
reliability and fail-over in MGCP. The "notified entity" for an
endpoint is the Call Agent currently controlling that endpoint. At
any point in time, an endpoint has one, and only one, "notified
entity" associated with it. The "notified entity" determines where
the endpoint will send commands to; when the endpoint needs to send a
command to the Call Agent, it MUST send the command to its current
"notified entity". The "notified entity" however does not determine
where commands can be received from; any Call Agent can send commands
to the endpoint. Please refer to Section 5 for the relevant security
considerations.
Upon startup, the "notified entity" MUST be set to a provisioned
value. Most commands sent by the Call Agent include the ability to
explicitly name the "notified entity" through the use of a
"NotifiedEntity" parameter. The "notified entity" will stay the same
until either a new "NotifiedEntity" parameter is received or the
endpoint does a warm or cold (power-cycle) restart.
If a "NotifiedEntity" parameter is sent with an "empty" value, the
"notified entity" for the endpoint will be set to empty. If the
"notified entity" for an endpoint is empty or has not been set
explicitly (neither by a command nor by provisioning), the "notified
entity" will then default to the source address (i.e., IP address and
UDP port number) of the last successful non-audit command received
for the endpoint. Auditing will thus not change the "notified
entity". Use of an empty "NotifiedEntity" parameter value is
strongly discouraged as it is error prone and eliminates the DNS-
based fail-over and reliability mechanisms.
2.1.5 Digit Maps
The Call Agent can ask the gateway to collect digits dialed by the
user. This facility is intended to be used with residential gateways
to collect the numbers that a user dials; it can also be used with
trunking gateways and access gateways alike, to collect access codes,
credit card numbers and other numbers requested by call control
services.
One procedure is for the gateway to notify the Call Agent of each
individual dialed digit, as soon as they are dialed. However, such a
procedure generates a large number of interactions. It is preferable
to accumulate the dialed numbers in a buffer, and to transmit them in
a single message.
The problem with this accumulation approach, however, is that it is
hard for the gateway to predict how many numbers it needs to
accumulate before transmission. For example, using the phone on our
desk, we can dial the following numbers:
------------------------------------------------------
| 0 | Local operator |
| 00 | Long distance operator |
| xxxx | Local extension number |
| 8xxxxxxx | Local number |
| #xxxxxxx | Shortcut to local number at|
| | other corporate sites |
| *xx | Star services |
| 91xxxxxxxxxx | Long distance number |
| 9011 + up to 15 digits| International number |
------------------------------------------------------
The solution to this problem is to have the Call Agent load the
gateway with a digit map that may correspond to the dial plan. This
digit map is expressed using a syntax derived from the Unix system
command, egrep. For example, the dial plan described above results
in the following digit map:
(0T|00T|[1-7]xxx|8xxxxxxx|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)
The formal syntax of the digit map is described by the DigitMap rule
in the formal syntax description of the protocol (see Appendix A) -
support for basic digit map letters is REQUIRED while support for
extension digit map letters is OPTIONAL. A gateway receiving a digit
map with an extension digit map letter not supported SHOULD return
error code 537 (unknown digit map extension).
A digit map, according to this syntax, is defined either by a (case
insensitive) "string" or by a list of strings. Each string in the
list is an alternative numbering scheme, specified either as a set of
digits or timers, or as an expression over which the gateway will
attempt to find a shortest possible match. The following constructs
can be used in each numbering scheme:
* Digit: A digit from "0" to "9".
* Timer: The symbol "T" matching a timer expiry.
* DTMF: A digit, a timer, or one of the symbols "A", "B", "C",
"D", "#", or "*". Extensions may be defined.
* Wildcard: The symbol "x" which matches any digit ("0" to "9").
* Range: One or more DTMF symbols enclosed between square brackets
("[" and "]").
* Subrange: Two digits separated by hyphen ("-") which matches any
digit between and including the two. The subrange
construct can only be used inside a range construct,
i.e., between "[" and "]".
* Position: A period (".") which matches an arbitrary number,
including zero, of occurrences of the preceding
construct.
A gateway that detects events to be matched against a digit map MUST
do the following:
1) Add the event code as a token to the end of an internal state
variable for the endpoint called the "current dial string".
2) Apply the current dial string to the digit map table, attempting a
match to each expression in the digit map.
3) If the result is under-qualified (partially matches at least one
entry in the digit map and doesn't completely match another
entry), do nothing further.
If the result matches an entry, or is over-qualified (i.e., no
further digits could possibly produce a match), send the list of
accumulated events to the Call Agent. A match, in this
specification, can be either a "perfect match," exactly matching one
of the specified alternatives, or an impossible match, which occurs
when the dial string does not match any of the alternatives.
Unexpected timers, for example, can cause "impossible matches". Both
perfect matches and impossible matches trigger notification of the
accumulated digits (which may include other events - see Section
2.3.3).
The following example illustrates the above. Assume we have the
digit map:
(xxxxxxx|x11)
and a current dial string of "41". Given the input "1" the current
dial string becomes "411". We have a partial match with "xxxxxxx",
but a complete match with "x11", and hence we send "411" to the Call
Agent.
The following digit map example is more subtle:
(0[12].|00|1[12].1|2x.#)
Given the input "0", a match will occur immediately since position
(".") allows for zero occurrences of the preceding construct. The
input "00" can thus never be produced in this digit map.
Given the input "1", only a partial match exists. The input "12" is
also only a partial match, however both "11" and "121" are a match.
Given the input "2", a partial match exists. A partial match also
exists for the input "23", "234", "2345", etc. A full match does not
occur here until a "#" is generated, e.g., "2345#". The input "2#"
would also have been a match.
Note that digit maps simply define a way of matching sequences of
event codes against a grammar. Although digit maps as defined here
are for DTMF input, extension packages can also be defined so that
digit maps can be used for other types of input represented by event
codes that adhere to the digit map syntax already defined for these
event codes (e.g., "1" or "T"). Where such usage is envisioned, the
definition of the particular event(s) SHOULD explicitly state that in
the package definition.
Since digit maps are not bounded in size, it is RECOMMENDED that
gateways support digit maps up to at least 2048 bytes per endpoint.
2.1.6 Packages
MGCP is a modular and extensible protocol, however with extensibility
comes the need to manage, identify, and name the individual
extensions. This is achieved by the concept of packages, which are
simply well-defined groupings of extensions. For example, one
package may support a certain group of events and signals, e.g.,
off-hook and ringing, for analog access lines. Another package may
support another group of events and signals for analog access lines
or for another type of endpoint such as video. One or more packages
may be supported by a given endpoint.
MGCP allows the following types of extensions to be defined in a
package:
* BearerInformation
* LocalConnectionOptions
* ExtensionParameters
* ConnectionModes
* Events
* Signals
* Actions
* DigitMapLetters
* ConnectionParameters
* RestartMethods
* ReasonCodes
* Return codes
each of which will be explained in more detail below. The rules for
defining each of these extensions in a package are described in
Section 6, and the encoding and syntax are defined in Section 3 and
Appendix A.
With the exception of DigitMapLetters, a package defines a separate
name space for each type of extension by adding the package name as a
prefix to the extension, i.e.:
package-name/extension
Thus the package-name is followed by a slash ("/") and the name of
the extension.
An endpoint supporting one or more packages may define one of those
packages as the default package for the endpoint. Use of the package
name for events and signals in the default package for an endpoint is
OPTIONAL, however it is RECOMMENDED to always include the package
name. All other extensions, except DigitMapLetter, defined in the
package MUST include the package-name when referring to the
extension.
Package names are case insensitive strings of letters, hyphens and
digits, with the restriction that hyphens shall never be the first or
last character in a name. Examples of package names are "D", "T",
and "XYZ". Package names are not case sensitive - names such as
"XYZ", "xyz", and "xYz" are equal.
Package definitions will be provided in other documents and with
package names and extensions names registered with IANA. For more
details, refer to section 6.
Implementers can gain experience by using experimental packages. The
name of an experimental package MUST start with the two characters
"x-"; the IANA SHALL NOT register package names that start with these
characters, or the characters "x+", which are reserved. A gateway
that receives a command referring to an unsupported package MUST
return an error (error code 518 - unsupported package, is
RECOMMENDED).
2.1.7 Events and Signals
The concept of events and signals is central to MGCP. A Call Agent
may ask to be notified about certain events occurring in an endpoint
(e.g., off-hook events) by including the name of the event in a
RequestedEvents parameter (in a NotificationRequest command - see
Section 2.3.3).
A Call Agent may also request certain signals to be applied to an
endpoint (e.g., dial-tone) by supplying the name of the event in a
SignalRequests parameter.
Events and signals are grouped in packages, within which they share
the same name space which we will refer to as event names in the
following. Event names are case insensitive strings of letters,
hyphens and digits, with the restriction that hyphens SHALL NOT be
the first or last character in a name. Some event codes may need to
be parameterized with additional data, which is accomplished by
adding the parameters between a set of parentheses. Event names are
not case sensitive - values such as "hu", "Hu", "HU" or "hU" are
equal.
Examples of event names can be "hu" (off hook or "hang-up"
transition), "hf" (hook-flash) or "0" (the digit zero).
The package name is OPTIONAL for events in the default package for an
endpoint, however it is RECOMMENDED to always include the package
name. If the package name is excluded from the event name, the
default package name for that endpoint MUST be assumed. For example,
for an analog access line which has the line package ("L") as a
default with dial-tone ("dl") as one of the events in that package,
the following two event names are equal:
L/dl
and
dl
For any other non-default packages that are associated with that
endpoint, (such as the generic package for an analog access
endpoint-type for example), the package name MUST be included with
the event name. Again, unconditional inclusion of the package name
is RECOMMENDED.
Digits, or letters, are supported in some packages, notably "DTMF".
Digits and letters are defined by the rules "Digit" and "Letter" in
the definition of digit maps. This definition refers to the digits
(0 to 9), to the asterisk or star ("*") and orthotrope, number or
pound sign ("#"), and to the letters "A", "B", "C" and "D", as well
as the timer indication "T". These letters can be combined in "digit
string" that represents the keys that a user punched on a dial. In
addition, the letter "X" can be used to represent all digits (0 to
9). Also, extensions MAY define use of other letters. The need to
easily express the digit strings in earlier versions of the protocol
has a consequence on the form of event names:
An event name that does not denote a digit MUST always contain at
least one character that is neither a digit, nor one of the letters
A, B, C, D, T or X (such names also MUST NOT just contain the special
signs "*", or "#"). Event names consisting of more than one
character however may use any of the above.
A Call Agent may often have to ask a gateway to detect a group of
events. Two conventions can be used to denote such groups:
* The "*" and "all" wildcard conventions (see below) can be used to
detect any event belonging to a package, or a given event in many
packages, or any event in any package supported by the gateway.
* The regular expression Range notation can be used to detect a range
of digits.
The star sign (*) can be used as a wildcard instead of a package
name, and the keyword "all" can be used as a wildcard instead of an
event name:
* A name such as "foo/all" denotes all events in package "foo".
* A name such as "*/bar" denotes the event "bar" in any package
supported by the gateway.
* The name "*/all" denotes all events supported by the endpoint.
This specification purposely does not define any additional detail
for the "all packages" and "all events" wildcards. They provide
limited benefits, but introduce significant complexity along with the
potential for errors. Their use is consequently strongly
discouraged.
The Call Agent can ask a gateway to detect a set of digits or letters
either by individually describing those letters, or by using the
"range" notation defined in the syntax of digit strings. For
example, the Call Agent can:
* Use the letter "x" to denote" digits from 0 to 9.
* Use the notation "[0-9#]" to denote the digits 0 to 9 and the pound
sign.
The individual event codes are still defined in a package though
(e.g., the "DTMF" package).
Events can by default only be generated and detected on endpoints,
however events can be also be defined so they can be generated or
detected on connections rather than on the endpoint itself (see
Section 6.6). For example, gateways may be asked to provide a
ringback tone on a connection. When an event is to be applied on a
connection, the name of the connection MUST be added to the name of
the event, using an "at" sign (@) as a delimiter, as in:
G/rt@0A3F58
where "G" is the name of the package and "rt" is the name of the
event. Should the connection be deleted while an event or signal is
being detected or applied on it, that particular event detection or
signal generation simply stops. Depending on the signal, this may
generate a failure (see below).
The wildcard character "*" (star) can be used to denote "all
connections". When this convention is used, the gateway will
generate or detect the event on all the connections that are
connected to the endpoint. This applies to existing as well as
future connections created on the endpoint. An example of this
convention could be:
R/qa@*
where "R" is the name of the package and "qa" is the name of the
event.
When processing a command using the "all connections" wildcard, the
"*" wildcard character applies to all current and future connections
on the endpoint, however it will not be expanded. If a subsequent
command either explicitly (e.g., by auditing) or implicitly (e.g., by
persistence) refers to such an event, the "*" value will be used.
However, when the event is actually observed, that particular
occurrence of the event will include the name of the specific
connection it occurred on.
The wildcard character "$" can be used to denote "the current
connection". It can only be used by the Call Agent, when the event
notification request is "encapsulated" within a connection creation
or modification command. When this convention is used, the gateway
will generate or detect the event on the connection that is currently
being created or modified. An example of this convention is:
G/rt@$
When processing a command using the "current connection" wildcard,
the "$" wildcard character will be expanded to the value of the
current connection. If a subsequent command either explicitly (e.g.,
by auditing) or implicitly (e.g., by persistence) refers to such an
event, the expanded value will be used. In other words, the "current
connection" wildcard is expanded once, which is at the initial
processing of the command in which it was explicitly included.
The connection id, or a wildcard replacement, can be used in
conjunction with the "all packages" and "all events" conventions. For
example, the notation:
*/all@*
can be used to designate all events on all current and future
connections on the endpoint. However, as mentioned before, the use
of the "all packages" and "all events" wildcards are strongly
discouraged.
Signals are divided into different types depending on their behavior:
* On/off (OO): Once applied, these signals last until they are
turned off. This can only happen as the result of a reboot/restart
or a new SignalRequests where the signal is explicitly turned off
(see later). Signals of type OO are defined to be idempotent, thus
multiple requests to turn a given OO signal on (or off) are
perfectly valid and MUST NOT result in any errors. An On/Off
signal could be a visual message-waiting indicator (VMWI). Once
turned on, it MUST NOT be turned off until explicitly instructed to
by the Call Agent, or as a result of an endpoint restart, i.e.,
these signals will not turn off as a result of the detection of a
requested event.
* Time-out (TO): Once applied, these signals last until they are
either cancelled (by the occurrence of an event or by not being
included in a subsequent (possibly empty) list of signals), or a
signal-specific period of time has elapsed. A TO signal that times
out will generate an "operation complete" event. A TO signal could
be "ringback" timing out after 180 seconds. If an event occurs
prior to the 180 seconds, the signal will, by default, be stopped
(the "Keep signals active" action - see Section 2.3.3 - will
override this behavior). If the signal is not stopped, the signal
will time out, stop and generate an "operation complete" event,
about which the Call Agent may or may not have requested to be
notified. If the Call Agent has asked for the "operation complete"
event to be notified, the "operation complete" event sent to the
Call Agent SHALL include the name(s) of the signal(s) that timed
out (note that if parameters were passed to the signal, the
parameters will not be reported). If the signal was generated on a
connection, the name of the connection SHALL be included as
described above. Time-out signals have a default time-out value
defined for them, which MAY be altered by the provisioning process.
Also, the time-out period may be provided as a parameter to the
signal (see Section 3.2.2.4). A value of zero indicates that the
time-out period is infinite. A TO signal that fails after being
started, but before having generated an "operation complete" event
will generate an "operation failure" event which will include the
name of the signal that failed. Deletion of a connection with an
active TO signal will result in such a failure.
* Brief (BR): The duration of these signals is normally so short
that they stop on their own. If a signal stopping event occurs, or
a new SignalRequests is applied, a currently active BR signal will
not stop. However, any pending BR signals not yet applied MUST be
cancelled (a BR signal becomes pending if a NotificationRequest
includes a BR signal, and there is already an active BR signal). As
an example, a brief tone could be a DTMF digit. If the DTMF digit
"1" is currently being played, and a signal stopping event occurs,
the "1" would play to completion. If a request to play DTMF digit
"2" arrives before DTMF digit "1" finishes playing, DTMF digit "2"
would become pending.
Signal(s) generated on a connection MUST include the name of that
connection.
2.2 Usage of SDP
The Call Agent uses the MGCP to provide the endpoint with the
description of connection parameters such as IP addresses, UDP port
and RTP profiles. These descriptions will follow the conventions
delineated in the Session Description Protocol which is now an IETF
proposed standard, documented in RFC 2327.
2.3 Gateway Control Commands
2.3.1 Overview of Commands
This section describes the commands of the MGCP. The service
consists of connection handling and endpoint handling commands.
There are currently nine commands in the protocol:
* The Call Agent can issue an EndpointConfiguration command to a
gateway, instructing the gateway about the coding characteristics
expected by the "line-side" of the endpoint.
* The Call Agent can issue a NotificationRequest command to a
gateway, instructing the gateway to watch for specific events such
as hook actions or DTMF tones on a specified endpoint.
* The gateway will then use the Notify command to inform the Call
Agent when the requested events occur.
* The Call Agent can use the CreateConnection command to create a
connection that terminates in an "endpoint" inside the gateway.
* The Call Agent can use the ModifyConnection command to change the
parameters associated with a previously established connection.
* The Call Agent can use the DeleteConnection command to delete an
existing connection. The DeleteConnection command may also be used
by a gateway to indicate that a connection can no longer be
sustained.
* The Call Agent can use the AuditEndpoint and AuditConnection
commands to audit the status of an "endpoint" and any connections
associated with it. Network management beyond the capabilities
provided by these commands is generally desirable. Such
capabilities are expected to be supported by the use of the Simple
Network Management Protocol (SNMP) and definition of a MIB which is
outside the scope of this specification.
* The Gateway can use the RestartInProgress command to notify the
Call Agent that a group of endpoints managed by the gateway is
being taken out-of-service or is being placed back in-service.
These services allow a controller (normally, the Call Agent) to
instruct a gateway on the creation of connections that terminate in
an "endpoint" attached to the gateway, and to be informed about
events occurring at the endpoint. An endpoint may be for example:
* A specific trunk circuit, within a trunk group terminating in a
gateway,
* A specific announcement handled by an announcement server.
Connections are logically grouped into "calls" (the concept of a
"call" has however little semantic meaning in MGCP itself). Several
connections, that may or may not belong to the same call, can
terminate in the same endpoint. Each connection is qualified by a
"mode" parameter, which can be set to "send only" (sendonly),
"receive only" (recvonly), "send/receive" (sendrecv), "conference"
(confrnce), "inactive" (inactive), "loopback", "continuity test"
(conttest), "network loop back" (netwloop) or "network continuity
test" (netwtest).
Media generated by the endpoint is sent on connections whose mode is
either "send only", "send/receive", or "conference", unless the
endpoint has a connection in "loopback" or "continuity test" mode.
However, media generated by applying a signal to a connection is
always sent on the connection, regardless of the mode.
The handling of the media streams received on connections is
determined by the mode parameters:
* Media streams received through connections in "receive",
"conference" or "send/receive" mode are mixed and sent to the
endpoint, unless the endpoint has another connection in "loopback"
or "continuity test" mode.
* Media streams originating from the endpoint are transmitted over
all the connections whose mode is "send", "conference" or
"send/receive", unless the endpoint has another connection in
"loopback" or "continuity test" mode.
* In addition to being sent to the endpoint, a media stream received
through a connection in "conference" mode is forwarded to all the
other connections whose mode is "conference". This also applies
when the endpoint has a connection in "loopback" or "continuity
test" mode. The details of this forwarding, e.g., RTP translator
or mixer, is outside the scope of this document.
Note that in order to detect events on a connection, the connection
must by default be in one of the modes "receive", "conference",
"send/receive", "network loopback" or "network continuity test". The
event detection only applies to the incoming media. Connections in
"sendonly", "inactive", "loopback", or "continuity test" mode will
thus normally not detect any events, although requesting to do so is
not considered an error.
The "loopback" and "continuity test" modes are used during
maintenance and continuity test operations. An endpoint may have
more than one connection in either "loopback" or "continuity test"
mode. As long as there is one connection in that particular mode,
and no other connection on the endpoint is placed in a different
maintenance or test mode, the maintenance or test operation shall
continue undisturbed. There are two flavors of continuity test, one
specified by ITU and one used in the US. In the first case, the test
is a loopback test. The originating switch will send a tone (the go
tone) on the bearer circuit and expects the terminating switch to
loopback the tone. If the originating switch sees the same tone
returned (the return tone), the COT has passed. If not, the COT has
failed. In the second case, the go and return tones are different.
The originating switch sends a certain go tone. The terminating
switch detects the go tone, it asserts a different return tone in the
backwards direction. When the originating switch detects the return
tone, the COT is passed. If the originating switch never detects the
return tone, the COT has failed.
If the mode is set to "loopback", the gateway is expected to return
the incoming signal from the endpoint back into that same endpoint.
This procedure will be used, typically, for testing the continuity of
trunk circuits according to the ITU specifications. If the mode is
set to "continuity test", the gateway is informed that the other end
of the circuit has initiated a continuity test procedure according to
the GR specification (see [22]). The gateway will place the circuit
in the transponder mode required for dual-tone continuity tests.
If the mode is set to "network loopback", the audio signals received
from the connection will be echoed back on the same connection. The
media is not forwarded to the endpoint.
If the mode is set to "network continuity test", the gateway will
process the packets received from the connection according to the
transponder mode required for dual-tone continuity test, and send the
processed signal back on the connection. The media is not forwarded
to the endpoint. The "network continuity test" mode is included for
backwards compatibility only and use of it is discouraged.
2.3.2 EndpointConfiguration
The EndpointConfiguration command can be used to specify the encoding
of the signals that will be received by the endpoint. For example,
in certain international telephony configurations, some calls will
carry mu-law encoded audio signals, while others will use A-law. The
Call Agent can use the EndpointConfiguration command to pass this
information to the gateway. The configuration may vary on a call by
call basis, but can also be used in the absence of any connection.
ReturnCode,
[PackageList]
<-- EndpointConfiguration(EndpointId,
[BearerInformation])
EndpointId is the name of the endpoint(s) in the gateway where
EndpointConfiguration executes. The "any of" wildcard convention
MUST NOT be used. If the "all of" wildcard convention is used, the
command applies to all the endpoints whose name matches the wildcard.
BearerInformation is a parameter defining the coding of the data sent
to and received from the line side. The information is encoded as a
list of sub-parameters. The only sub-parameter defined in this
version of the specification is the bearer encoding, whose value can
be set to "A-law" or "mu-law". The set of sub-parameters may be
extended.
In order to allow for extensibility, while remaining backwards
compatible, the BearerInformation parameter is conditionally optional
based on the following conditions:
* if Extension Parameters (vendor, package or other) are not used,
the BearerInformation parameter is REQUIRED,
* otherwise, the BearerInformation parameter is OPTIONAL.
When omitted, BearerInformation MUST retain its current value.
ReturnCode is a parameter returned by the gateway. It indicates the
outcome of the command and consists of an integer number optionally
followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.3 NotificationRequest
The NotificationRequest command is used to request the gateway to
send notifications upon the occurrence of specified events in an
endpoint. For example, a notification may be requested for when a
gateway detects that an endpoint is receiving tones associated with
fax communication. The entity receiving this notification may then
decide to specify use of a different type of encoding method in the
connections bound to this endpoint and instruct the gateway
accordingly with a ModifyConnection Command.
ReturnCode,
[PackageList]
<-- NotificationRequest(EndpointId,
[NotifiedEntity,]
[RequestedEvents,]
RequestIdentifier,
[DigitMap,]
[SignalRequests,]
[QuarantineHandling,]
[DetectEvents,]
[encapsulated EndpointConfiguration])
EndpointId is the identifier for the endpoint(s) in the the gateway
where the NotificationRequest executes. The "any of" wildcard MUST
NOT be used.
NotifiedEntity is an optional parameter that specifies a new
"notified entity" for the endpoint.
RequestIdentifier is used to correlate this request with the
notifications that it triggers. It will be repeated in the
corresponding Notify command.
RequestedEvents is a list of events, possibly qualified by event
parameters (see Section 3.2.2.4), that the gateway is requested to
detect and report. Such events may include, for example, fax tones,
continuity tones, or on-hook transition. Unless otherwise specified,
events are detected on the endpoint, however some events can be
detected on a connection. A given event MUST NOT appear more than
once in a RequestedEvents. If the parameter is omitted, it defaults
to empty.
To each event is associated one or more actions, which can be:
* Notify the event immediately, together with the accumulated list of
observed events,
* Swap audio,
* Accumulate the event in an event buffer, but don't notify yet,
* Accumulate according to Digit Map,
* Keep Signal(s) active,
* Process the Embedded Notification Request,
* Ignore the event.
Support for Notify, Accumulate, Keep Signal(s) Active, Embedded
Notification Request, and Ignore is REQUIRED. Support for Accumulate
according to Digit Map is REQUIRED on any endpoint capable of
detecting DTMF. Support for any other action is OPTIONAL. The set
of actions can be extended.
A given action can by default be specified for any event, although
some actions will not make sense for all events. For example, an
off-hook event with the Accumulate according to Digit Map action is
valid, but will of course immediately trigger a digit map mismatch
when the off-hook event occurs. Needless to say, such practice is
discouraged.
Some actions can be combined as shown in the table below, where "Y"
means the two actions can be combined, and "N" means they cannot:
--------------------------------------------------------------
| | Notif | Swap | Accum | AccDi | KeSiA | EmbNo | Ignor |
|--------------------------------------------------------------|
| Notif | N | Y | N | N | Y | Y* | N |
| Swap | - | N | Y | N | N | N | Y |
| Accum | - | - | N | N | Y | Y | N |
| AccDi | - | - | - | N | Y | N | N |
| KeSiA | - | - | - | - | N | Y | Y |
| EmbNo | - | - | - | - | - | N | N |
| Ignor | - | - | - | - | - | - | N |
--------------------------------------------------------------
Note (*): The "Embedded Notification Request" can only be
combined with "Notify", if the gateway is allowed to issue more
than one Notify command per Notification request (see below and
Section 4.4.1).
If no action is specified, the Notify action will be applied. If one
or more actions are specified, only those actions apply. When two or
more actions are specified, each action MUST be combinable with all
the other actions as defined by the table above - the individual
actions are assumed to occur simultaneously.
If a client receives a request with an invalid or unsupported action
or an illegal combination of actions, it MUST return an error to the
Call Agent (error code 523 - unknown or illegal combination of
actions, is RECOMMENDED).
In addition to the RequestedEvents parameter specified in the
command, some MGCP packages may contain "persistent events" (this is
generally discouraged though - see Appendix B for an alternative).
Persistent events in a given package are always detected on an
endpoint that implements that package. If a persistent event is not
included in the list of RequestedEvents, and the event occurs, the
event will be detected anyway and processed like all other events, as
if the persistent event had been requested with a Notify action. A
NotificationRequest MUST still be in place for a persistent event to
trigger a Notify though. Thus, informally, persistent events can be
viewed as always being implicitly included in the list of
RequestedEvents with an action to Notify, although no glare
detection, etc., will be performed.
Non-persistent events are those events that need to be explicitly
included in the RequestedEvents list. The (possibly empty) list of
requested events completely replaces the previous list of requested
events. In addition to the persistent events, only the events
specified in the requested events list will be detected by the
endpoint. If a persistent event is included in the RequestedEvents
list, the action specified will replace the default action associated
with the event for the life of the RequestedEvents list, after which
the default action is restored. For example, if "off-hook"was a
persistent event, the "Ignore off-hook" action was specified, and a
new request without any off-hook instructions were received, the
default "Notify off-hook" operation would be restored.
The gateway will detect the union of the persistent events and the
requested events. If an event is not included in either list, it
will be ignored.
The Call Agent can send a NotificationRequest with an empty (or
omitted) RequestedEvents list to the gateway. The Call Agent can do
so, for example, to a gateway when it does not want to collect any
more DTMF digits. However, persistent events will still be detected
and notified.
The Swap Audio action can be used when a gateway handles more than
one connection on an endpoint. This will be the case for call
waiting, and possibly other feature scenarios. In order to avoid the
round-trip to the Call Agent when just changing which connection is
attached to the audio functions of the endpoint, the
NotificationRequest can map an event (usually hook flash, but could
be some other event) to a local swap audio function, which selects
the "next" connection in a round robin fashion. If there is only one
connection, this action is effectively a no-op. If there are more
than two connections, the order is undefined. If the endpoint has
exactly two connections, one of which is "inactive", the other of
which is in "send/receive" mode, then swap audio will attempt to make
the "send/receive" connection "inactive", and vice versa. This
specification intentionally does not provide any additional detail on
the swap audio action.
If signal(s) are desired to start when an event being looked for
occurs, the "Embedded NotificationRequest" action can be used. The
embedded NotificationRequest may include a new list of
RequestedEvents, SignalRequests and a new digit map as well. The
semantics of the embedded NotificationRequest is as if a new
NotificationRequest was just received with the same NotifiedEntity,
RequestIdentifier, QuarantineHandling and DetectEvents. When the
"Embedded NotificationRequest" is activated, the "current dial
string" will be cleared; however the list of observed events and the
quarantine buffer will be unaffected (if combined with a Notify, the
Notify will clear the list of observed events though - see Section
4.4.1). Note, that the Embedded NotificationRequest action does not
accumulate the triggering event, however it can be combined with the
Accumulate action to achieve that. If the Embedded
NotificationRequest fails, an Embedded NotificationRequest failure
event SHOULD be generated (see Appendix B).
MGCP implementations SHALL be able to support at least one level of
embedding. An embedded NotificationRequest that respects this
limitation MUST NOT contain another Embedded NotificationRequest.
DigitMap is an optional parameter that allows the Call Agent to
provision the endpoint with a digit map according to which digits
will be accumulated. If this optional parameter is absent, the
previously defined value is retained. This parameter MUST be
defined, either explicitly or through a previous command, if the
RequestedEvents parameter contains a request to "accumulate according
to the digit map". The collection of these digits will result in a
digit string. The digit string is initialized to a null string upon
reception of the NotificationRequest, so that a subsequent
notification only returns the digits that were collected after this
request. Digits that were accumulated according to the digit map are
reported as any other accumulated event, in the order in which they
occur. It is therefore possible that other events accumulated are
found in between the list of digits. If the gateway is requested to
"accumulate according to digit map" and the gateway currently does
not have a digit map for the endpoint in question, the gateway MUST
return an error (error code 519 - endpoint does not have a digit map,
is RECOMMENDED).
SignalRequests is an optional parameter that contains the set of
signals that the gateway is asked to apply. When omitted, it
defaults to empty. When multiple signals are specified, the signals
MUST be applied in parallel. Unless otherwise specified, signals are
applied to the endpoint. However some signals can be applied to a
connection. Signals are identified by their name, which is an event
name, and may be qualified by signal parameters (see Section
3.2.2.4). The following are examples of signals:
* Ringing,
* Busy tone,
* Call waiting tone,
* Off hook warning tone,
* Ringback tones on a connection.
Names and descriptions of signals are defined in the appropriate
package.
Signals are, by default, applied to endpoints. If a signal applied
to an endpoint results in the generation of a media stream (audio,
video, etc.), then by default the media stream MUST NOT be forwarded
on any connection associated with that endpoint, regardless of the
mode of the connection. For example, if a call-waiting tone is
applied to an endpoint involved in an active call, only the party
using the endpoint in question will hear the call-waiting tone.
However, individual signals may define a different behavior.
When a signal is applied to a connection that has received a
RemoteConnectionDescriptor, the media stream generated by that signal
will be forwarded on the connection regardless of the current mode of
the connection (including loopback and continuity test). If a
RemoteConnectionDescriptor has not been received, the gateway MUST
return an error (error code 527 - missing RemoteConnectionDescriptor,
is RECOMMENDED). Note that this restriction does not apply to
detecting events on a connection.
When a (possibly empty) list of signal(s) is supplied, this list
completely replaces the current list of active time-out signals.
Currently active time-out signals that are not provided in the new
list MUST be stopped and the new signal(s) provided will now become
active. Currently active time-out signals that are provided in the
new list of signals MUST remain active without interruption, thus the
timer for such time-out signals will not be affected. Consequently,
there is currently no way to restart the timer for a currently active
time-out signal without turning the signal off first. If the time-
out signal is parameterized, the original set of parameters MUST
remain in effect, regardless of what values are provided
subsequently. A given signal MUST NOT appear more than once in a
SignalRequests. Note that applying a signal S to an endpoint,
connection C1 and connection C2, constitutes three different and
independent signals.
The action triggered by the SignalRequests is synchronized with the
collection of events specified in the RequestedEvents parameter. For
example, if the NotificationRequest mandates "ringing" and the
RequestedEvents asks to look for an "off-hook" event, the ringing
SHALL stop as soon as the gateway detects an off-hook event. The
formal definition is that the generation of all "Time Out" signals
SHALL stop as soon as one of the requested events is detected, unless
the "Keep signals active" action is associated to the detected event.
The RequestedEvents and SignalRequests may refer to the same event
definitions. In one case, the gateway is asked to detect the
occurrence of the event, and in the other case it is asked to
generate it. The specific events and signals that a given endpoint
can detect or perform are determined by the list of packages that are
supported by that endpoint. Each package specifies a list of events
and signals that can be detected or performed. A gateway that is
requested to detect or perform an event belonging to a package that
is not supported by the specified endpoint MUST return an error
(error code 518 - unsupported or unknown package, is RECOMMENDED).
When the event name is not qualified by a package name, the default
package name for the endpoint is assumed. If the event name is not
registered in this default package, the gateway MUST return an error
(error code 522 - no such event or signal, is RECOMMENDED).
The Call Agent can send a NotificationRequest whose requested signal
list is empty. It will do so for example when a time-out signal(s)
should stop.
If signal(s) are desired to start as soon as a "looked-for" event
occurs, the "Embedded NotificationRequest" action can be used. The
embedded NotificationRequest may include a new list of
RequestedEvents, SignalRequests and a new Digit Map as well. The
embedded NotificationRequest action allows the Call Agent to set up a
"mini-script" to be processed by the gateway immediately following
the detection of the associated event. Any SignalRequests specified
in the embedded NotificationRequest will start immediately.
Considerable care must be taken to prevent discrepancies between the
Call Agent and the gateway. However, long-term discrepancies should
not occur as a new SignalRequests completely replaces the old list of
active time-out signals, and BR-type signals always stop on their
own. Limiting the number of On/Off-type signals is encouraged. It
is considered good practice for a Call Agent to occasionally turn on
all On/Off signals that should be on, and turn off all On/Off signals
that should be off.
The Ignore action can be used to ignore an event, e.g., to prevent a
persistent event from being notified. However, the synchronization
between the event and an active time-out signal will still occur by
default (e.g., a time-out dial-tone signal will stop when an off-hook
occurs even if off-hook was a requested event with action "Ignore").
To prevent this synchronization from happening, the "Keep Signal(s)
Active" action will have to be specified as well.
The optional QuarantineHandling parameter specifies the handling of
"quarantine" events, i.e., events that have been detected by the
gateway before the arrival of this NotificationRequest command, but
have not yet been notified to the Call Agent. The parameter provides
a set of handling options (see Section 4.4.1 for details):
* whether the quarantined events should be processed or discarded
(the default is to process them).
* whether the gateway is expected to generate at most one
notification (step by step), or multiple notifications (loop), in
response to this request (the default is at most one).
When the parameter is absent, the default value is assumed.
We should note that the quarantine-handling parameter also governs
the handling of events that were detected and processed but not yet
notified when the command is received.
DetectEvents is an optional parameter, possibly qualified by event
parameters, that specifies a list of events that the gateway is
requested to detect during the quarantine period. When this
parameter is absent, the events to be detected in the quarantine
period are those listed in the last received DetectEvents list. In
addition, the gateway will also detect persistent events and the
events specified in the RequestedEvents list, including those for
which the "ignore" action is specified.
Some events and signals, such as the in-line ringback or the quality
alert, are performed or detected on connections terminating in the
endpoint rather than on the endpoint itself. The structure of the
event names (see Section 2.1.7) allows the Call Agent to specify the
connection(s) on which the events should be performed or detected.
The NotificationRequest command may carry an encapsulated
EndpointConfiguration command, that will apply to the same
endpoint(s). When this command is present, the parameters of the
EndpointConfiguration command are included with the normal parameters
of the NotificationRequest, with the exception of the EndpointId,
which is not replicated.
The encapsulated EndpointConfiguration command shares the fate of the
NotificationRequest command. If the NotificationRequest is rejected,
the EndpointConfiguration is not executed.
ReturnCode is a parameter returned by the gateway. It indicates the
outcome of the command and consists of an integer number optionally
followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.4 Notify
Notifications with the observed events are sent by the gateway via
the Notify command when a triggering event occurs.
ReturnCode,
[PackageList]
<-- Notify(EndpointId,
[NotifiedEntity,]
RequestIdentifier,
ObservedEvents)
EndpointId is the name for the endpoint in the gateway which is
issuing the Notify command. The identifier MUST be a fully qualified
endpoint identifier, including the domain name of the gateway. The
local part of the name MUST NOT use any of the wildcard conventions.
NotifiedEntity is a parameter that identifies the entity which
requested the notification. This parameter is equal to the
NotifiedEntity parameter of the NotificationRequest that triggered
this notification. The parameter is absent if there was no such
parameter in the triggering request. Regardless of the value of the
NotifiedEntity parameter, the notification MUST be sent to the
current "notified entity" for the endpoint.
RequestIdentifier is a parameter that repeats the RequestIdentifier
parameter of the NotificationRequest that triggered this
notification. It is used to correlate this notification with the
request that triggered it. Persistent events will be viewed here as
if they had been included in the last NotificationRequest. An
implicit NotificationRequest MAY be in place right after restart -
the RequestIdentifier used for it will be zero ("0") - see Section
4.4.1 for details.
ObservedEvents is a list of events that the gateway detected and
accumulated. A single notification may report a list of events that
will be reported in the order in which they were detected (FIFO).
The list will only contain the identification of events that were
requested in the RequestedEvents parameter of the triggering
NotificationRequest. It will contain the events that were either
accumulated (but not notified) or treated according to digit map (but
no match yet), and the final event that triggered the notification or
provided a final match in the digit map. It should be noted that
digits MUST be added to the list of observed events as they are
accumulated, irrespective of whether they are accumulated according
to the digit map or not. For example, if a user enters the digits
"1234" and some event E is accumulated between the digits "3" and "4"
being entered, the list of observed events would be "1, 2, 3, E, 4".
Events that were detected on a connection SHALL include the name of
that connection as in "R/qa@0A3F58" (see Section 2.1.7).
If the list of ObservedEvents reaches the capacity of the endpoint,
an ObservedEvents Full event (see Appendix B) SHOULD be generated
(the endpoint shall ensure it has capacity to include this event in
the list of ObservedEvents). If the ObservedEvents Full event is not
used to trigger a Notify, event processing continues as before
(including digit map matching); however, the subsequent events will
not be included in the list of ObservedEvents.
ReturnCode is a parameter returned by the Call Agent. It indicates
the outcome of the command and consists of an integer number
optionally followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.5 CreateConnection
This command is used to create a connection between two endpoints.
ReturnCode,
[ConnectionId,]
[SpecificEndPointId,]
[LocalConnectionDescriptor,]
[SecondEndPointId,]
[SecondConnectionId,]
[PackageList]
<-- CreateConnection(CallId,
EndpointId,
[NotifiedEntity,]
[LocalConnectionOptions,]
Mode,
[{RemoteConnectionDescriptor |
SecondEndpointId}, ]
[Encapsulated NotificationRequest,]
[Encapsulated EndpointConfiguration])
A connection is defined by its endpoints. The input parameters in
CreateConnection provide the data necessary to build a gateway's
"view" of a connection.
CallId is a parameter that identifies the call (or session) to which
this connection belongs. This parameter SHOULD, at a minimum, be
unique within the collection of Call Agents that control the same
gateways. Connections that belong to the same call SHOULD share the
same call-id. The call-id has little semantic meaning in the
protocol; however it can be used to identify calls for reporting and
accounting purposes. It does not affect the handling of connections
by the gateway.
EndpointId is the identifier for the connection endpoint in the
gateway where CreateConnection executes. The EndpointId can be
fully-specified by assigning a value to the parameter EndpointId in
the function call or it may be under-specified by using the "any of"
wildcard convention. If the endpoint is underspecified, the endpoint
identifier SHALL be assigned by the gateway and its complete value
returned in the SpecificEndPointId parameter of the response. When
the "any of" wildcard is used, the endpoint assigned MUST be in-
service and MUST NOT already have any connections on it. If no such
endpoint is available, error code 410 (no endpoint available) SHOULD
be returned. The "all of" wildcard MUST NOT be used.
The NotifiedEntity is an optional parameter that specifies a new
"notified entity" for the endpoint.
LocalConnectionOptions is an optional structure used by the Call
Agent to direct the handling of the connection by the gateway. The
fields contained in a LocalConnectionOptions structure may include
one or more of the following (each field MUST NOT be supplied more
than once):
* Codec compression algorithm: One or more codecs, listed in order
of preference. For interoperability, it is RECOMMENDED to support
G.711 mu-law encoding ("PCMU"). See Section 2.6 for details on the
codec selection process.
* Packetization period: A single millisecond value or a range may be
specified. The packetization period SHOULD NOT contradict the
specification of the codec compression algorithm. If a codec is
specified that has a frame size which is inconsistent with the
packetization period, and that codec is selected, the gateway is
authorized to use a packetization period that is consistent with
the frame size even if it is different from that specified. In so
doing, the gateway SHOULD choose a non-zero packetization period as
close to that specified as possible. If a packetization period is
not specified, the endpoint SHOULD use the default packetization
period(s) for the codec(s) selected.
* Bandwidth: The allowable bandwidth, i.e., payload plus any header
overhead from the transport layer and up, e.g., IP, UDP, and RTP.
The bandwidth specification SHOULD NOT contradict the specification
of codec compression algorithm or packetization period. If a codec
is specified, then the gateway is authorized to use it, even if it
results in the usage of a larger bandwidth than specified. Any
discrepancy between the bandwidth and codec specification will not
be reported as an error.
* Type of Service: This indicates the class of service to be used
for this connection. When the Type of Service is not specified,
the gateway SHALL use a default value of zero unless provisioned
otherwise.
* Usage of echo cancellation: By default, the telephony gateways
always perform echo cancellation on the endpoint. However, it may
be necessary, for some calls, to turn off these operations. The
echo cancellation parameter can have two values, "on" (when the
echo cancellation is requested) and "off" (when it is turned off).
The parameter is optional. If the parameter is omitted when
creating a connection and there are no other connections on the
endpoint, the endpoint SHALL apply echo cancellation initially. If
the parameter is omitted when creating a connection and there are
existing connections on the endpoint, echo cancellation is
unchanged. The endpoint SHOULD subsequently enable or disable echo
cancellation when voiceband data is detected - see e.g., ITU-T
recommendation V.8, V.25, and G.168. Following termination of
voiceband data, the handling of echo cancellation SHALL then revert
to the current value of the echo cancellation parameter. It is
RECOMMENDED that echo cancellation handling is left to the gateway
rather than having this parameter specified by the Call Agent.
* Silence Suppression: The telephony gateways may perform voice
activity detection, and avoid sending packets during periods of
silence. However, it is necessary, for example for modem calls, to
turn off this detection. The silence suppression parameter can
have two values, "on" (when the detection is requested) and "off"
(when it is not requested). The default is "off" (unless
provisioned otherwise). Upon detecting voiceband data, the
endpoint SHOULD disable silence suppression. Following termination
of voiceband data, the handling of silence suppression SHALL then
revert to the current value of the silence suppression parameter.
* Gain Control: The telephony gateways may perform gain control on
the endpoint, in order to adapt the level of the signal. However,
it is necessary, for example for some modem calls, to turn off this
function. The gain control parameter may either be specified as
"automatic", or as an explicit number of decibels of gain. The
gain specified will be added to media sent out over the endpoint
(as opposed to the connection) and subtracted from media received
on the endpoint. The parameter is optional. When there are no
other connections on the endpoint, and the parameter is omitted,
the default is to not perform gain control (unless provisioned
otherwise), which is equivalent to specifying a gain of 0 decibels.
If there are other connections on the endpoint, and the parameter
is omitted, gain control is unchanged. Upon detecting voiceband
data, the endpoint SHOULD disable gain control if needed.
Following termination of voiceband data, the handling of gain
control SHALL then revert to the current value of the gain control
parameter. It should be noted, that handling of gain control is
normally best left to the gateway and hence use of this parameter
is NOT RECOMMENDED.
* RTP security: The Call agent can request the gateway to enable
encryption of the audio Packets. It does so by providing a key
specification, as specified in RFC 2327. By default, encryption is
not performed.
* Network Type: The Call Agent may instruct the gateway to prepare
the connection on a specified type of network. If absent, the
value is based on the network type of the gateway being used.
* Resource reservation: The Call Agent may instruct the gateway to
use network resource reservation for the connection. See Section
2.7 for details.
The Call Agent specifies the relevant fields it cares about in the
command and leaves the rest to the discretion of the gateway. For
those of the above parameters that were not explicitly included, the
gateway SHOULD use the default values if possible. For a detailed
list of local connection options included with this specification
refer to section 3.2.2.10. The set of local connection options can
be extended.
The Mode indicates the mode of operation for this side of the
connection. The basic modes are "send", "receive", "send/receive",
"conference", "inactive", "loopback", "continuity test", "network
loop back" and "network continuity test". The expected handling of
these modes is specified in the introduction of the "Gateway Control
Commands", Section 2.3. Note that signals applied to a connection do
not follow the connection mode. Some endpoints may not be capable of
supporting all modes. If the command specifies a mode that the
endpoint does not support, an error SHALL be returned (error 517 -
unsupported mode, is RECOMMENDED). Also, if a connection has not yet
received a RemoteConnectionDescriptor, an error MUST be returned if
the connection is attempted to be placed in any of the modes "send
only", "send/receive", "conference", "network loopback", "network
continuity test", or if a signal (as opposed to detecting an event)
is to be applied to the connection (error code 527 - missing
RemoteConnectionDescriptor, is RECOMMENDED). The set of modes can be
extended.
The gateway returns a ConnectionId, that uniquely identifies the
connection within the endpoint, and a LocalConnectionDescriptor,
which is a session description that contains information about the
connection, e.g., IP address and port for the media, as defined in
SDP.
The SpecificEndPointId is an optional parameter that identifies the
responding endpoint. It is returned when the EndpointId argument
referred to an "any of" wildcard name and the command succeeded.
When a SpecificEndPointId is returned, the Call Agent SHALL use it as
the EndpointId value in successive commands referring to this
connection.
The SecondEndpointId can be used instead of the
RemoteConnectionDescriptor to establish a connection between two
endpoints located on the same gateway. The connection is by
definition a local connection. The SecondEndpointId can be fully-
specified by assigning a value to the parameter SecondEndpointId in
the function call or it may be under-specified by using the "any of"
wildcard convention. If the SecondEndpointId is underspecified, the
second endpoint identifier will be assigned by the gateway and its
complete value returned in the SecondEndPointId parameter of the
response.
When a SecondEndpointId is specified, the command really creates two
connections that can be manipulated separately through
ModifyConnection and DeleteConnection commands. In addition to the
ConnectionId and LocalConnectionDescriptor for the first connection,
the response to the creation provides a SecondConnectionId parameter
that identifies the second connection. The second connection is
established in "send/receive" mode.
After receiving a "CreateConnection" request that did not include a
RemoteConnectionDescriptor parameter, a gateway is in an ambiguous
situation. Because it has exported a LocalConnectionDescriptor
parameter, it can potentially receive packets. Because it has not
yet received the RemoteConnectionDescriptor parameter of the other
gateway, it does not know whether the packets that it receives have
been authorized by the Call Agent. It must thus navigate between two
risks, i.e., clipping some important announcements or listening to
insane data. The behavior of the gateway is determined by the value
of the Mode parameter:
* If the mode was set to ReceiveOnly, the gateway MUST accept the
media and transmit them through the endpoint.
* If the mode was set to Inactive, Loopback, or Continuity Test, the
gateway MUST NOT transmit the media through to the endpoint.
Note that the mode values SendReceive, Conference, SendOnly, Network
Loopback and Network Continuity Test do not make sense in this
situation. They MUST be treated as errors, and the command MUST be
rejected (error code 527 - missing RemoteConnectionDescriptor, is
RECOMMENDED).
The command may optionally contain an encapsulated Notification
Request command, which applies to the EndpointId, in which case a
RequestIdentifier parameter MUST be present, as well as, optionally,
other parameters of the NotificationRequest with the exception of the
EndpointId, which is not replicated. The encapsulated
NotificationRequest is executed simultaneously with the creation of
the connection. For example, when the Call Agent wants to initiate a
call to a residential gateway, it could:
* ask the residential gateway to prepare a connection, in order to be
sure that the user can start speaking as soon as the phone goes off
hook,
* ask the residential gateway to start ringing,
* ask the residential gateway to notify the Call Agent when the phone
goes off-hook.
This can be accomplished in a single CreateConnection command, by
also transmitting the RequestedEvents parameters for the off-hook
event, and the SignalRequests parameter for the ringing signal.
When these parameters are present, the creation and the
NotificationRequest MUST be synchronized, which means that both MUST
be accepted, or both MUST be refused. In our example, the
CreateConnection may be refused if the gateway does not have
sufficient resources, or cannot get adequate resources from the local
network access, and the off-hook NotificationRequest can be refused
in the glare condition, if the user is already off-hook. In this
example, the phone must not ring if the connection cannot be
established, and the connection must not be established if the user
is already off-hook.
The NotifiedEntity parameter, if present, defines the new "notified
entity" for the endpoint.
The command may carry an encapsulated EndpointConfiguration command,
which applies to the EndpointId. When this command is present, the
parameters of the EndpointConfiguration command are included with the
normal parameters of the CreateConnection with the exception of the
EndpointId, which is not replicated. The EndpointConfiguration
command may be encapsulated together with an encapsulated
NotificationRequest command. Note that both of these apply to the
EndpointId only.
The encapsulated EndpointConfiguration command shares the fate of the
CreateConnection command. If the CreateConnection is rejected, the
EndpointConfiguration is not executed.
ReturnCode is a parameter returned by the gateway. It indicates the
outcome of the command and consists of an integer number optionally
followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.6 ModifyConnection
This command is used to modify the characteristics of a gateway's
"view" of a connection. This "view" of the call includes both the
local connection descriptor as well as the remote connection
descriptor.
ReturnCode,
[LocalConnectionDescriptor,]
[PackageList]
<-- ModifyConnection(CallId,
EndpointId,
ConnectionId,
[NotifiedEntity,]
[LocalConnectionOptions,]
[Mode,]
[RemoteConnectionDescriptor,]
[Encapsulated NotificationRequest,]
[Encapsulated EndpointConfiguration])
The parameters used are the same as in the CreateConnection command,
with the addition of a ConnectionId that identifies the connection
within the endpoint. This parameter was returned by the
CreateConnection command, in addition to the local connection
descriptor. It uniquely identifies the connection within the context
of the endpoint. The CallId used when the connection was created
MUST be included as well.
The EndpointId MUST be a fully qualified endpoint identifier. The
local name MUST NOT use the wildcard conventions.
The ModifyConnection command can be used to affect parameters of a
connection in the following ways:
* Provide information about the other end of the connection, through
the RemoteConnectionDescriptor. If the parameter is omitted, it
retains its current value.
* Activate or deactivate the connection, by changing the value of the
Mode parameter. This can occur at any time during the connection,
with arbitrary parameter values. If the parameter is omitted, it
retains its current value.
* Change the parameters of the connection through the
LocalConnectionOptions, for example by switching to a different
coding scheme, changing the packetization period, or modifying the
handling of echo cancellation. If one or more
LocalConnectionOptions parameters are omitted, then the gateway
SHOULD refrain from changing that parameter from its current value,
unless another parameter necessitating such a change is explicitly
provided. For example, a codec change might require a change in
silence suppression. Note that if a RemoteConnectionDescriptor is
supplied, then only the LocalConnectionOptions actually supplied
with the ModifyConnection command will affect the codec negotiation
(as described in Section 2.6).
Connections can only be fully activated if the
RemoteConnectionDescriptor has been provided to the gateway. The
receive-only mode, however, can be activated without the provision of
this descriptor.
The command will only return a LocalConnectionDescriptor if the local
connection parameters, such as RTP ports, were modified. Thus, if,
for example, only the mode of the connection is changed, a
LocalConnectionDescriptor will not be returned. Note however, that
inclusion of LocalConnectionOptions in the command is not a
prerequisite for local connection parameter changes to occur. If a
connection parameter is omitted, e.g., silence suppression, the old
value of that parameter will be retained if possible. If a parameter
change necessitates a change in one or more unspecified parameters,
the gateway is free to choose suitable values for the unspecified
parameters that must change. This can for instance happen if the
packetization period was not specified. If the new codec supported
the old packetization period, the value of this parameter would not
change, as a change would not be necessary. However, if it did not
support the old packetization period, it would choose a suitable
value.
The command may optionally contain an encapsulated Notification
Request command, in which case a RequestIdentifier parameter MUST be
present, as well as, optionally, other parameters of the
NotificationRequest with the exception of the EndpointId, which is
not replicated. The encapsulated NotificationRequest is executed
simultaneously with the modification of the connection. For example,
when a connection is accepted, the calling gateway should be
instructed to place the circuit in send-receive mode and to stop
providing ringing tones. This can be accomplished in a single
ModifyConnection command, by also transmitting the RequestedEvents
parameters, for the on-hook event, and an empty SignalRequests
parameter, to stop the provision of ringing tones.
When these parameters are present, the modification and the
NotificationRequest MUST be synchronized, which means that both MUST
be accepted, or both MUST be refused.
The NotifiedEntity parameter, if present, defines the new "notified
entity" for the endpoint.
The command may carry an encapsulated EndpointConfiguration command,
that will apply to the same endpoint. When this command is present,
the parameters of the EndpointConfiguration command are included with
the normal parameters of the ModifyConnection with the exception of
the EndpointId, which is not replicated. The EndpointConfiguration
command may be encapsulated together with an encapsulated
NotificationRequest command.
The encapsulated EndpointConfiguration command shares the fate of the
ModifyConnection command. If the ModifyConnection is rejected, the
EndpointConfiguration is not executed.
ReturnCode is a parameter returned by the gateway. It indicates the
outcome of the command and consists of an integer number optionally
followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.7 DeleteConnection (from the Call Agent)
This command is used to terminate