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rfc:rfc2705

Network Working Group M. Arango Request for Comments: 2705 RSL COM Category: Informational A. Dugan

                                                            I. Elliott
                                                 Level3 Communications
                                                            C. Huitema
                                                             Telcordia
                                                            S. Pickett
                                                     Vertical Networks
                                                          October 1999
               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 (1999).  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
 developments in the IETF Megaco Working Group and ITF-T SG16 who are
 currently working on a potential successor to this protocol.

Abstract

 This document describes an application programming interface and a
 corresponding protocol (MGCP) for controlling Voice over IP (VoIP)
 Gateways from external call control elements. MGCP assumes a call
 control architecture where the call control "intelligence" is outside
 the gateways and handled by external call control elements.
 The document is structured in 6 main sections:
  • The introduction presents the basic assumptions and the relation

to other protocols such as H.323, RTSP, SAP or SIP.

Arango, et al. Informational [Page 1] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

  • The interface section presents a conceptual overview of the MGCP,

presenting the naming conventions, the usage of the session

    description protocol SDP, and the procedures that compose MGCP:
    Notifications Request, Notification, Create Connection, Modify
    Connection, Delete Connection, AuditEndpoint, AuditConnection and
    RestartInProgress.
  • The protocol description section presents the MGCP encodings,

which are based on simple text formats, and the transmission

    procedure over UDP.
  • The security section presents the security requirement of MGCP,

and its usage of IP security services (IPSEC).

  • The event packages section provides an initial definition of

packages and event names.

  • The description of the changes made in combining SGCP 1.1 and IPDC

to create MGCP 1.0.

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
 2.  Media Gateway Control Interface ...........................  9
    2.1.  Model and naming conventions. ........................ 10
       2.1.1.  Types of endpoints .............................. 10
          2.1.1.1.  Digital channel (DS0) ...................... 11
          2.1.1.2.  Analog line ................................ 11
          2.1.1.3.  Annoucement server access point ............ 12
          2.1.1.4.  Interactive Voice Response access point .... 12
          2.1.1.5.  Conference bridge access point ............. 13
          2.1.1.6.  Packet relay ............................... 13
          2.1.1.7.  Wiretap access point ....................... 14
          2.1.1.8.  ATM "trunk side" interface. ................ 14
       2.1.2.  Endpoint identifiers ............................ 15
       2.1.3.  Calls and connections ........................... 17
          2.1.3.1.  Names of calls ............................. 20
          2.1.3.2.  Names of connections ....................... 20
          2.1.3.3.  Management of resources, attributes of ..... 20
          2.1.3.4.  Special case of local connections .......... 23
       2.1.4.  Names of Call Agents and other entities ......... 23
       2.1.5.  Digit maps ...................................... 24
       2.1.6.  Names of events ................................. 26
    2.2.  Usage of SDP ......................................... 29
    2.3.  Gateway Control Commands ............................. 30

Arango, et al. Informational [Page 2] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

       2.3.1.  EndpointConfiguration ........................... 32
       2.3.2.  NotificationRequest ............................. 33
       2.3.3.  CreateConnection ................................ 38
       2.3.4.  ModifyConnection ................................ 44
       2.3.5.  DeleteConnection (from the Call Agent) .......... 46
       2.3.6.  DeleteConnection (from the VoIP gateway) ........ 51
       2.3.7.  DeleteConnection (multiple connections, from the  51
       2.3.8.  Audit Endpoint .................................. 52
       2.3.9.  Audit Connection ................................ 55
       2.3.10.  Restart in progress ............................ 56
    2.4.  Return codes and error codes. ........................ 58
    2.5.  Reason Codes ......................................... 61
 3.  Media Gateway Control Protocol ............................ 61
    3.1.  General description .................................. 62
    3.2.  Command Header ....................................... 62
       3.2.1.  Command line .................................... 62
          3.2.1.1.  Coding of the requested verb ............... 63
          3.2.1.2.  Transaction Identifiers .................... 63
          3.2.1.3.  Coding of the endpoint identifiers and ..... 64
          3.2.1.4.  Coding of the protocol version ............. 65
       3.2.2.  Parameter lines ................................. 65
          3.2.2.1.  Response Acknowledgement ................... 68
          3.2.2.2.  Local connection options ................... 68
          3.2.2.3.  Capabilities ............................... 70
          3.2.2.4.  Connection parameters ...................... 71
          3.2.2.5.  Reason Codes ............................... 72
          3.2.2.6.  Connection mode ............................ 73
          3.2.2.7.  Coding of event names ...................... 73
          3.2.2.8.  RequestedEvents ............................ 74
          3.2.2.9.  SignalRequests ............................. 76
          3.2.2.10.  ObservedEvent ............................. 76
          3.2.2.11.  RequestedInfo ............................. 76
          3.2.2.12.  QuarantineHandling ........................ 77
          3.2.2.13.  DetectEvents .............................. 77
          3.2.2.14.  EventStates ............................... 77
          3.2.2.15.  RestartMethod ............................. 78
          3.2.2.16.  Bearer Information ........................ 78
    3.3.  Format of response headers ........................... 78
    3.4.  Formal syntax description of the protocol ............ 81
    3.5.  Encoding of the session description .................. 86
       3.5.1.  Usage of SDP for an audio service ............... 86
       3.5.2.  Usage of SDP in a network access service ........ 87
       3.5.3.  Usage of SDP for ATM connections ................ 90
       3.5.4.  Usage of SDP for local connections .............. 91
    3.6.  Transmission over UDP ................................ 91
       3.6.1.  Providing the At-Most-Once functionality ........ 91
       3.6.2.  Transaction identifiers and three ways handshake. 92
       3.6.3.  Computing retransmission timers ................. 93

Arango, et al. Informational [Page 3] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

       3.6.4.  Piggy backing ................................... 94
       3.6.5.  Provisional responses ........................... 94
 4.  States, failover and race conditions. ..................... 95
    4.1.  Basic Asumptions ..................................... 95
    4.2.  Security, Retransmission, and Detection of Lost ...... 96
    4.3.  Race conditions ...................................... 99
       4.3.1.  Quarantine list ................................. 99
       4.3.2.  Explicit detection ..............................103
       4.3.3.  Ordering of commands, and treatment of disorder .104
       4.3.4.  Fighting the restart avalanche ..................105
       4.3.5.  Disconnected Endpoints ..........................107
 1.   A "disconnected" timer is initialized to a random value, .107
 2.   The gateway then waits for either the end of this timer, .107
 3.   When the "disconnected" timer elapses, when a command is .107
 4.   If the "disconnected" procedure still left the endpoint ..107
 5.  Security requirements .....................................108
    5.1.  Protection of media connections ......................109
 6.  Event packages and end point types ........................109
    6.1.  Basic packages .......................................110
       6.1.1.  Generic Media Package ...........................110
       6.1.2.  DTMF package ....................................112
       6.1.3.  MF Package ......................................113
       6.1.4.  Trunk Package ...................................114
       6.1.5.  Line Package ....................................116
       6.1.6.  Handset emulation package .......................119
       6.1.7.  RTP Package .....................................120
       6.1.8.  Network Access Server Package ...................121
       6.1.9.  Announcement Server Package .....................122
       6.1.10.  Script Package .................................122
    6.2.  Basic endpoint types and profiles ....................123
 7.  Versions and compatibility ................................124
    7.1.  Differences between version 1.0 and draft 0.5 ........124
    7.2.  Differences between draft-04 and draft-05 ............125
    7.3.  Differences between draft-03 and draft-04 ............125
    7.4.  Differences between draft-02 and draft-03 ............125
    7.5.  Differences between draft-01 and draft-02 ............126
    7.6.  The making of MGCP from IPDC and SGCP ................126
    7.7.  Changes between MGCP and initial versions of SGCP ....126
 8.  Security Considerations ...................................128
 9.  Acknowledgements ..........................................128
 10. References ................................................129
 11. Authors' Addresses ........................................130
 12. Appendix A: Proposed "MoveConnection" command .............132
    12.1.  Proposed syntax modification ........................133
 13. Full Copyright Statement ..................................134

Arango, et al. Informational [Page 4] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

1. Introduction

 This document describes an abstract application programming interface
 and a corresponding protocol (MGCP) for controlling Telephony
 Gateways from external call control elements called media gateway
 controllers or call agents. A telephony gateway is a network element
 that provides conversion between the audio signals carried on
 telephone circuits and data packets carried over the Internet or over
 other packet networks.  Example of gateways are:
  • Trunking gateways, that interface between the telephone network

and a Voice over IP network. Such gateways typically manage a

    large number of digital circuits.
  • Voice over ATM gateways, which operate much the same way as voice

over IP trunking gateways, except that they interface to an ATM

    network.
  • Residential gateways, that provide a traditional analog (RJ11)

interface to a Voice over IP network. Examples of residential

    gateways include cable modem/cable set-top boxes, xDSL devices,
    broad-band wireless devices
  • Access gateways, that provide a traditional analog (RJ11) or

digital PBX interface to a Voice over IP network. Examples of

    access gateways include small-scale voice over IP gateways.
  • Business gateways, that provide a traditional digital PBX

interface or an integrated "soft PBX" interface to a Voice over IP

    network.
  • Network Access Servers, that can attach a "modem" to a telephone

circuit and provide data access to the Internet. We expect that,

    in the future, the same gateways will combine Voice over IP
    services and Network Access services.
  • Circuit switches, or packet switches, which can offer a control

interface to an external call control element.

 MGCP assumes a call control architecture where the call control
 "intelligence" is outside the gateways and handled by external call
 control elements. The MGCP assumes that these call control elements,
 or Call Agents, will synchronize with each other to send coherent
 commands to the gateways under their control. MGCP does not define a
 mechanism for synchronizing Call Agents. MGCP 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

Arango, et al. Informational [Page 5] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 only specify 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 or sinks of data and
 could be physical or virtual. Examples of physical endpoints are:
  • An interface on a gateway that terminates a trunk connected to a

PSTN switch (e.g., Class 5, Class 4, etc.). A gateway that

    terminates trunks is called a trunk gateway.
  • An interface on a gateway that terminates an analog POTS

connection to a phone, key system, PBX, etc. A gateway that

    terminates residential POTS lines (to phones) is called a
    residential gateway.
 An example of a virtual endpoint is an audio source in an audio-
 content server. Creation of physical endpoints requires hardware
 installation, while creation of virtual endpoints can be done by
 software.
 Connections may be either point to point or multipoint. A point to
 point connection is an association between two endpoints with the
 purpose of transmitting data between these endpoints. Once this
 association is established for both endpoints, data transfer between
 these endpoints can take place. A multipoint connection is
 established by connecting the endpoint to a multipoint session.
 Connections can be established over several types of bearer networks:
  • Transmission of audio packets using RTP and UDP over a TCP/IP

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.

Arango, et al. Informational [Page 6] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

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 "signalling
 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 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.

Arango, et al. Informational [Page 7] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

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),
  • the Session Initiation Protocol (SIP),
  • 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:

Arango, et al. Informational [Page 8] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

  _____________________________________________________________________
 | Functional|  Phone     |  Terminating    |  IETF conforming systems|
 | Plane     |  switch    |  Entity         |                         |
 |___________|____________|_________________|_________________________|
 | Signaling |  Signaling |  Call agent     |  Signaling exchanges    |
 | Plane     |  exchanges |                 |  with the call agent    |
 |           |  through   |                 |  through SAP, SIP or    |
 |           |  SS7/ISUP  |                 |  RTSP.                  |
 |___________|____________|_________________|_________________________|
 |           |            |                 |  Negotiation of session |
 |           |            |                 |  description parameters |
 |           |            |                 |  through SDP (telephony |
 |           |            |                 |  gateway terminated but |
 |           |            |                 |  passed via the call    |
 |           |            |                 |  agent to and from the  |
 |           |            |                 |  IETF conforming system)|
 |___________|____________|_________________|_________________________|
 |           |            |  Internal       |                         |
 |           |            |  synchronization|                         |
 |           |            |  through MGCP   |                         |
 |___________|____________|_________________|_________________________|
 | Bearer    |  Connection|  Telephony      |  Transmission of VoIP   |
 | Data      |  through   |  gateways       |  data using RTP,        |
 | Transport |  high speed|                 |  directly between the   |
 | Plane     |  trunk     |                 |  remote IP end system   |
 |           |  groups    |                 |  and the gateway.       |
 |___________|____________|_________________|_________________________|
 The SDP standard has a pivotal status in this architecture. We will
 see in the following description that we also use it to carry session
 descriptions in MGCP.

1.3. Definitions

 Trunk: A communication channel between two switching systems. E.g., a
 DS0 on a T1 or E1 line.

2. Media Gateway Control Interface

 The interface functions provide for connection control and endpoint
 control. Both use the same system model and the same naming
 conventions.

Arango, et al. Informational [Page 9] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

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 several 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
 packaging.  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 functionalities. Our analysis, so far, has led us to
 isolate the following basic endpoint types:
  • Digital channel (DS0),
  • Analog line,
  • Annoucement server access point,
  • Interactive Voice Response access point,
  • Conference bridge access point,
  • Packet relay,
  • Wiretap access point,
  • ATM "trunk side" interface.
 In this section, we will develop the expected behavior of such end
 points.
 This list is not limitative.  There may be other types of endpoints
 defined in the future, for example test endpoint that could be used
 to check network quality, or frame-relay endpoints that could be used
 to managed audio channels multiplexed over a frame-relay virtual
 circuit.

Arango, et al. Informational [Page 10] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

2.1.1.1. Digital channel (DS0)

 Digital channels provide an 8Khz*8bit 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 or mu-law, using either the complete set of 8 bits or
 only 7 of these bits, into audio packets.  When the media gateway
 also supports a 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 support is a characteristic of the
 gateway, and may in fact vary according with the allocation of
 resource within the gateway.
 In some cases, digital channels are used to carry signalling.  This
 is the case for example of SS7 "F" links, or ISDN "D" channels.
 Media gateways that support these signalling functions shall be able
 to send and receive the signalling packets to and from a call agent,
 using the "back haul" procedures defined by the SIGTRAN working group
 of the IETF.  Digital channels are sometimes used in conjunction with
 channel associated signalling, such as "MF R2".  Media gateways that
 support these signalling functions shall be able to detect and
 produce the corresponding signals, such as for example "wink" or "A",
 according to the event signalling 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.

Arango, et al. Informational [Page 11] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

                                       +-------
                      +---------------+|
            (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 support is a characteristic of
 the gateway, and may in fact vary according with the allocation of
 resource within the gateway.  A typical gateway should however be
 able to support two or three connections per endpoint, in order to
 provide services such as "call waiting" or "three ways calling".

2.1.1.3. Annoucement server access point

 An announcement server endpoint provides acces 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 signalling and reporting procedures
 defined in MGCP.
           +----------------------+
           | Announcement endpoint| -------- Connection
           +----------------------+
 A given announcement endpoint is not supposed 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 oneway, or "half
 duplex" -- the announcement server is not expected to listen the
 audio signals from the connection.

2.1.1.4. Interactive Voice Response access point

 An Interactive Voice Response (IVR) endpoint provides acces to an IVR
 service. Under requests from the call agent, the IVR server will
 "play" announcements and tones, and will "listen" to responses from
 the user.  The requests from the call agent will follow the event
 signalling and reporting procedures defined in MGCP.

Arango, et al. Informational [Page 12] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

                    +-------------+
                    | IVR endpoint| -------- Connection
                    +-------------+
 A given IVR endpoint is not supposed 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 support is a characteristic of the
 gateway, and may in fact vary according with the allocation of
 resource 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
                +---------------------+ |
                                        +-------

Arango, et al. Informational [Page 13] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

2.1.1.7. Wiretap access point

 A wiretap access point provides access to a wiretap service,
 providing either a recording or a life playback of a connection.
                +-----------------+
                | Wiretap endpoint| -------- Connection
                +-----------------+
 A given wiretap endpoint is not supposed to support more than one
 connection at a time. If several connections were established to the
 same endpoint, then the recording or playback would mix the audio
 signals received on this connections.
 Connections to an wiretap endpoint are typically oneway, or "half
 duplex" -- the wiretap server is not expected to signal its presence
 in a call.

2.1.1.8. 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 support is a characteristic of the
 gateway, and may in fact vary according with the allocation of
 resource within the gateway.

Arango, et al. Informational [Page 14] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

2.1.2. Endpoint identifiers

 Endpoints 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,
 The syntax of the local name depends on the type of endpoint being
 named. However, the local name for each of these types is naturally
 hierarchical, beginning with a term which identifies the physical
 gateway containing the given endpoint and ending in a term which
 specifies the individual endpoint concerned. With this in mind,  the
 following rules for construction and interpretation of the Entity
 Name field for these entity types MUST be supported:
 1) The individual terms of the naming path MUST be separated by a
    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 delimitors ("/", "@"), characters used for
    wildcarding ("*", "$") and white spaces.
 3) Wild-carding is represented either by an asterisk ("*") or a
    dollar sign ("$") for the terms of the naming path which are to be
    wild-carded. Thus, if the full naming path looks like
           term1/term2/term3
    then the Entity Name field looks like this depending on which
    terms are wild-carded:
  • /term2/term3 if term1 is wild-carded

term1/*/term3 if term2 is wild-carded

           term1/term2/* if term3 is wild-carded
           term1/*/* if term2 and term3 are wild-carded,
            etc.
    In each of these examples a dollar sign could have appeared
    instead of an asterisk.

Arango, et al. Informational [Page 15] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 4) A term represented by an asterisk is to be interpreted as: "use
    ALL values of this term known within the scope of the Media
    Gateway".  A term represented by a dollar sign is to be
    interpreted as: "use ANY ONE value of this term known within the
    scope of the Media Gateway".  The description of a specific
    command may add further criteria for selection within the general
    rules given here.
 If the Media Gateway controls multiple physical gateways, the first
 term of the naming MUST identify the physical gateway containing the
 desired entity.  If the Media Gateway controls only a single physical
 gateway, the first term of the naming string MAY identify that
 physical gateway, depending on local practice.  A local name that is
 composed of only a wildcard character refers to either all (*) or any
 ($) endpoints within the media gateway.
 In the case of trunking gateways, endpoints are trunk circuits
 linking a gateway to a telephone switch. These circuits are typically
 grouped into a digital multiplex, that is connected to the gateway by
 a physical interface. Such circuits are named in three contexts:
  • In the ISUP protocol, trunks are grouped into trunk groups,

identified by the SS7 point codes of the switches that the group

    connects. Circuits within a trunk group are identified by a
    circuit number (CIC in ISUP).
  • In the gateway configuration files, physical interfaces are

typically identified by the name of the interface, an arbitrary

    text string. When the interface multiplexes several circuits,
    individual circuits are typically identified by a circuit number.
  • In MGCP, the endpoints are identified by an endpoint identifier.
 The Call Agents use configuration databases to map ranges of circuit
 numbers within an ISUP trunk group to corresponding ranges of
 circuits in a multiplex connected to a gateway through a physical
 interface. The gateway will be identified, in MGCP, by a domain name.
 The local name will be structured to encode both the name of the
 physical interface, for example X35V3+A4, and the circuit number
 within the multiplex connected to the interface, for example 13. The
 circuit number will be separated from the name of the interface by a
 fraction bar, as in:
      X35V3+A4/13

Arango, et al. Informational [Page 16] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 Other types of endpoints will use different conventions. For example,
 in gateways were physical interfaces by construction only control one
 circuit, the circuit number will be omitted. The exact syntax of such
 names should be specified in the corresponding server specification.

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 end points will establish two connections (C1 and
 C2):
               +---+                            +---+
 (channel1) ===|EP1|--(C1)--...        ...(C2)--|EP2|===(channel2)
               +---+                            +---+
 Each connection will be designated locally by a 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 respond 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
    packetization 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 respond to the command
    by providing its own "session description."
 3) The call agent 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
 the different call agents, these two call agents shall exchange
 information through a call-agent to call-agent signalling protocol,
 in order to synchronize the creation of the connection on the two
 endpoints.

Arango, et al. Informational [Page 17] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 Once 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 compression algorithm 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 to the gateway a
 "delete connection" command.  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:

Arango, et al. Informational [Page 18] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

           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

Arango, et al. Informational [Page 19] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

2.1.3.1. Names of calls

 One of the attributes of each connection is the "call identifier."
 Calls are identified by unique identifiers, independent of the
 underlying platforms or agents. These identifiers are created by the
 Call Agent. They are treated in MGCP as unstructured octet strings.
 Call identifiers are expected to be unique within the system, or at a
 minimum, unique within the collection of Call Agents that control the
 same gateways. 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 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. They are treated in MGCP as unstructured
 octet strings.  The gateway should make sure that a proper waiting
 period, at least 3 minutes, elapses between the end of a connection
 that used this 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.)

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 set of parameters to the gateway:
 1) The local directives instruct the gateway on the choice of
    resources that should be used for a connection,

Arango, et al. Informational [Page 20] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 2) When available, the "session description" provided by the other
    end of the connection.
 The local directives specify such parameters as the mode of the
 connection (e.g. send only, 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.) For each of these parameters, the call agent can either
 specify a value, a range of value, or no value at all.  This allow
 various implementations to implement various level 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
 let the gateway choose the detailed values.
 Based on the value of the local directives, the gateway will
 determine the resources allocated 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 resource have been allocated, the gateway will compose a
 "session description" that describes the way it intends to 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 algorithm, it can provide a list of these
 accepted algorithms.

Arango, et al. Informational [Page 21] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

               Local Directives
              (from call agent 1)
                      |
                      V
               +-------------+
               | resources   |
               | allocation  |
               | (gateway 1) |
               +-------------+
                 |         |
                 V         |
               Local       |
            Parameters     V
                 |      Session
                 |    Description               Local Directives
                 |         |                   (from call agent 2)
                 |         +---> Transmission----+      |
                 |                (CA to CA)     |      |
                 |                               V      V
                 |                           +-------------+
                 |                           | resources   |
                 |                           | allocation  |
                 |                           | (gateway 2) |
                 |                           +-------------+
                 |                               |      |
                 |                               |      V
                 |                               |    Local
                 |                               |  Parameters
                 |                            Session
                 |                          Description
                 |         +---- Transmission<---+
                 |         |      (CA to CA)
                 V         V
               +-------------+
               | modification|
               | (gateway 1) |
               +-------------+
                 |
                 V
               Local
            Parameters
  1. - Information flow: local directives & session descriptions –

Arango, et al. Informational [Page 22] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

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 trunk line to a wiretap device,
  • Connecting a call to an Interactive Voice-Response unit,
  • Connecting a call to a Conferencing unit,
  • Routing a call from on 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 name 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.
 Reliability can be improved by the following precautions:
  • Entities such as endpoints or Call Agents are identified by their

domain name, not their network addresses. Several addresses can be

    associated with a domain name. If a command or a response cannot
    be forwarded to one of the network addresses, implementations
    should retry the transmission using another address.
  • Entities may move to another platform. The association between a

logical name (domain name) and the actual platform are kept in the

    domain name service. Call Agents and Gateways should keep track of
    the time-to-live of the record they read from the DNS. They should
    query the DNS to refresh the information if the time to live has
    expired.

Arango, et al. Informational [Page 23] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 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, and when the endpoint needs to send a
 command to the Call Agent, it MUST send the command to the current
 "notified entity" for which endpoint(s) the command pertains. 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 reboots. If the "notified entity" for an endpoint is empty
 or has not been set explicitly, the "notified entity" will then
 default to the source address of the last connection handling command
 or notification request received for the endpoint. Auditing will thus
 not change the "notified entity."

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 may also be used with
 trunking gateways and access gateways alike, to collect the access
 codes, credit card numbers and other numbers requested by call
 control services.
 An alternative procedure is for the gateway to notify the Call Agent
 of the dialed digits, as soon as they are dialed. However, such a
 procedure generates a large number of interactions. It is preferable
 to accumulate the dialed numbers in a buffer, and to transmit them in
 a single 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:

Arango, et al. Informational [Page 24] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

      _______________________________________________________
     |  0                     |  Local operator             |
     |  00                    |  Long distance operator     |
     |  xxxx                  |  Local extension number     |
     |  8xxxxxxx              |  Local number               |
     |  #xxxxxxx              |  Shortcut to local number at|
     |                        |  other corporate sites      |
     |  *xx                   |  Star services              |
     |  91xxxxxxxxxx          |  Long distance number       |
     |  9011 + up to 15 digits|  International number       |
     |________________________|_____________________________|
 The solution to this problem is to load the gateway with a digit map
 that correspond to the dial plan. This digit map is expressed using a
 syntax derived from the Unix system command, egrep. For example, the
 dial plan described above results in the following digit map:
    (0T| 00T|[1-7]xxx|8xxxxxxx|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)
 The formal syntax of the digit map is described by the DigitMap rule
 in the formal syntax description of the protocol (section 3.4).  A
 Digit-Map, according to this syntax, is defined either by a "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 regular expression. A gateway that detects digits, letters or
 timers will:
 1) Add the event parameter code as a token to the end of an internal
    state variable called the "current dial string"
 2) Apply the current dial string to the digit map table, attempting a
    match to each regular expression in the Digit Map in lexical order
 3) If the result is under-qualified (partially matches at least one
    entry in the digit map), do nothing further.
 If the result matches, or is over-qualified (i.e. no further digits
 could possibly produce a match), send the current digit string 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 occur when the dial string does not match any
 of the alternative. Unexpected timers, for example, can cause
 "impossible matches."  Both perfect matches and impossible matches
 trigger notification of the accumulated digits.
 Digit maps are provided to the gateway by the Call Agent, whenever
 the Call Agent instructs the gateway to listen for digits.

Arango, et al. Informational [Page 25] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

2.1.6. Names of events

 The concept of events and signals is central to MGCP. A Call Agent
 may ask to be notified about certain events occurring in an endpoint,
 e.g.  off-hook events, and a call agent may request certain signals
 to be applied to an endpoint, e.g. dial-tone.
 Events and signals are grouped in packages within which they share
 the same namespace which we will refer to as event names in the
 following.  Packages are groupings of the events and signals
 supported by a particular type of endpoint. For instance, one package
 may support a certain group of events and signals for analog access
 lines, and another package may support another group of events and
 signals for video lines. One or more packages may exist for a given
 endpoint-type.
 Event names are case insensitive and are composed of two logical
 parts, a package name and an event name. Both names are strings of
 letters, hyphens and digits, with the restriction that hyphens shall
 never be the first or last characters in a name. Package or event
 names are not case sensitive - values such as "hu", "Hu", "HU" or
 "hU" should be considered equal.
 Examples of package names are "D" (DTMF), "M" (MF), "T" (Trunk) or
 "L" (Line). Examples of event names can be "hu" (off hook or "hang-
 up" transition), "hf" (flash hook) or "0" (the digit zero).
 In textual representations, the package name, when present, is
 separated from the event name by a slash ("/").  The package name is
 in fact optional. Each endpoint-type has a default package associated
 with it, and if the package name is excluded from the event name, the
 default package name for that endpoint-type is assumed. For example,
 for an analog access line, the following two event names are equal:
 l/dl dial-tone in the line package for an analog access line.
 dl   dial-tone in the line package (default) for an analog access
      line.
 This document defines a basic set of package names and event names.
 Additional package names and event names can be registered with the
 IANA. A package definition shall define the name of the package, and
 the definition of each event belonging to the package. The event
 definition shall include the precise name of the event (i.e., the
 code used in MGCP), a plain text definition of the event, and, when
 appropriate, the precise definition of the corresponding signals, for
 example the exact frequencies of audio signal such as dial tones or
 DTMF tones.

Arango, et al. Informational [Page 26] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 In addition, implementers can gain experience by using experimental
 packages. The names of experimental packages must start with the two
 characters "x-"; the IANA shall not register package names that start
 with these characters.
 Digits, or letters, are supported in many packages, notably "DTMF"
 and "MF". 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 represent the keys that a user punched on a dial.
 In addition, the letter "X" can be used to represent all digits, and
 the sign "$" can be used in wildcard notations. The need to easily
 express the digit strings has a consequence on the form of event
 names:
   An event name that does not denote a digit should always contain at
   least one character that is neither a digit, nor one of the letters
   A, B, C, D, T or X. (Such names should not contain the special
   signs "*", "#", "/" or "$".)
 A Call Agent may often have to ask a gateway to detect a group of
 events. Two conventions can be used to denote such groups:
  • The wildcard convention can be used to detect any event belonging

to a package, or a given event in many packages, or event any

    event in any package supported by the gateway.
  • The regular expression Range notation can be used to detect a

range of digits.

 The 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 names "*" or "*/all" denote all events supported by the
   gate way.
 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:

Arango, et al. Informational [Page 27] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

   Use the letter "x" to denote "any letter or digit."
   Use the notation "[0-9#]" to denote the digits 0 to 9 and the pound
   sign.
 In some cases, Call Agents will request the gateway to generate or
 detect events on connections rather than on the end point itself.
 For example, gateways may be asked to provide a ringback tone on a
 connection.  When an event shall be applied on a connection, the name
 of the connection is added to the name of the event, using an "at"
 sign (@) as a delimiter, as in:
   G/rt@0A3F58
 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. An example of this convention could be:
   R/qa@*
 The wildcard character "$" can be used to denote "the current
 connection." It should only be used by the call agent, when the event
 notification request is "encapsulated" within a command creation or
 modification command. When this convention is used, the gateway will
 generate or detect the event on the connection that is currently
 being created or modified. An example of this convention is:
   G/rt@$
 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 connections.
 Events and signals are described in packages. The package description
 must provide, for each events, the following informations:
  • The description of the event and its purpose, which should mean

the actual signal that is generated by the client (i.e., xx ms FSK

    tone) as well as the resulting user observed result (i.e., MW
    light on/off).
  • The detailed characteristics of the event, such as for example

frequencies and amplitude of audio signals, modulations and

    repetitions,

Arango, et al. Informational [Page 28] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

  • The typical and maximum duration of the event.
 Signals are divided into different types depending on their behavior:
  • On/off (OO) Once applied, these signals last forever until they

are turned off. This may happen either as the result of an event

    or a new SignalRequests (see later).
  • Time-out (TO) Once applied, these signals last until they are

either turned off (by an event or SignalRequests) or a signal

    specific period of time has elapsed. Depending on package
    specifications, a signal that times out may generate an "operation
    complete" event.
  • Brief (BR) The duration of these signals is so short, that they

stop on their own. If an event occurs the signal will not stop,

    however if a new SignalRequests is applied, the signal will stop.
    (Note: this point should be debated.  One could make a case that
    events such as strings of DTMF digits should in fact be allowed to
    complete.)
    TO signals are normally used to alert the endpoints' users, to
    signal them that they are expected to perform a specific action,
    such as hang down the phone (ringing). Transmission of these
    signals should typically be interrupted as soon as the first of
    the requested events has been produced.
    Package descriptions should describe, for all signals, their type
    (OO, TO, BR). They should also describe the maximum duration of
    the TO signals.

2.2. Usage of SDP

 The Call Agent uses the MGCP to provision the gateways 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.
 SDP allows for description of multimedia conferences. This version
 limits SDP usage to the setting of audio circuits and data access
 circuits.  The initial session descriptions contain the description
 of exactly one media, of type "audio" for audio connections, "nas"
 for data access.

Arango, et al. Informational [Page 29] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

2.3. Gateway Control Commands

 This section describes the commands of the MGCP. The service consists
 of connection handling and endpoint handling commands. There are 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 to a previously established connection.

  • The Call Agent can use the DeleteConnection command to delete an

existing connection. The DeleteConnection command may also be used

    by a gateway to indicate that a connection can no longer be
    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 are generally desirable, e.g.
    information about the status of the gateway. 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 the gateway, or a group of endpoints managed by

    the gateway, is being taken out of service or is being placed back
    in service.
 These services allow a controller (normally, the Call Agent) to
 instruct a gateway on the creation of connections that terminate in
 an "endpoint" attached to the gateway, and to be informed about
 events occurring at the endpoint. An endpoint may be for example:

Arango, et al. Informational [Page 30] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

  • A specific trunk circuit, within a trunk group terminating in a

gateway,

  • A specific announcement handled by an announcement server.
 Connections are grouped into "calls". Several connections, that may
 or may not belong to the same call, can terminate in the same
 endpoint .  Each connection is qualified by a "mode" parameter, which
 can be set to "send only" (sendonly), "receive only" (recvonly),
 "send/receive" (sendrecv), "conference" (confrnce), "data",
 "inactive" (inactive), "loopback", "continuity test" (conttest),
 "network loop back" (netwloop) or "network continuity test"
 (netwtest).
 The handling of the audio signals received on these connections is
 determined by the mode parameters:
  • Audio signals received in data packets through connections in

"receive", "conference" or "send/receive" mode are mixed and sent

    to the endpoint.
  • Audio signals originating from the endpoint are transmitted over

all the connections whose mode is "send", "conference" or

    "send/receive."
  • In addition to being sent to the endpoint, audio signals received

in data packets through connections in "conference" mode are

    replicated to all the other connections whose mode is
    "conference."
 The "loopback" and "continuity test" modes are used during
 maintenance and continuity test operations. There are two flavors of
 continuity test, one specified by ITU and one used in the US. In the
 first case, the test is a loopback test. The originating switch will
 send a tone (the go tone) on the bearer circuit and expect the
 terminating switch to loopback the circuit. If the originating switch
 sees the same tone returned (the return tone), the COT has passed. If
 not, the COT has failed. In the second case, the go and return tones
 are different. The originating switch sends a certain go tone. The
 terminating switch detects the go tone, it asserts a different return
 tone in the backwards direction. When the originating switch detects
 the return tone, the COT is passed. If the originating switch never
 detects the return tone, the COT has failed.
 If the mode is set to "loopback", the gateway is expected to return
 the incoming signal from the endpoint back into that same endpoint.
 This procedure will be used, typically, for testing the continuity of
 trunk circuits according to the ITU specifications.

Arango, et al. Informational [Page 31] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 If the mode is set to "continuity test", the gateway is informed that
 the other end of the circuit has initiated a continuity test
 procedure according to the GR specification. The gateway will place
 the circuit in the transponder mode required for dual-tone continuity
 tests.
 If the mode is set to "network loopback", the audio signals received
 from the connection will be echoed back on the same connection.
 If the mode is set to "network continuity test", the gateway will
 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.

2.3.1. EndpointConfiguration

 The EndpointConfiguration commands are 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 other will use A-law.  The
 Call Agent will 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
         <-- EndpointConfiguration( EndpointId,
                                    BearerInformation)
 EndpointId is the name for the endpoint in the gateway where
 EndpointConfiguration executes, as defined in section 2.1.1.  The
 "any of" wildcard convention shall not be used.  If the "all of"
 wildcard convention is used, the command applies to all the endpoint
 whose name matches the wildcard.
 BearerInformation is a parameter defining the coding of the data
 received from the line side.  These information is encoded as a list
 of sub-parameters.  The only sub-parameter defined in this version of
 the specification is the encoding method, whose values can be set to
 "A-law" and "mu-law".
 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.

Arango, et al. Informational [Page 32] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

2.3.2. NotificationRequest

 The NotificationRequest commands are used to request the gateway to
 send notifications upon the occurrence of specified events in an
 endpoint.  For example, a notification may be requested for when a
 gateway detects that an endpoint is receiving tones associated with
 fax communication.  The entity receiving this notification may decide
 to use a different type of encoding method in the connections bound
 to this endpoint.
     ReturnCode
     <-- NotificationRequest( EndpointId,
                              [NotifiedEntity,]
                              [RequestedEvents,]
                              RequestIdentifier,
                              [DigitMap,]
                              [SignalRequests,]
                              [QuarantineHandling,]
                              [DetectEvents,]
                              [encapsulated EndpointConfiguration])
 EndpointId is the name for the endpoint in the gateway where
 NotificationRequest executes, as defined in section 2.1.1.
 NotifiedEntity is an optional parameter that specifies where the
 notifications should be sent. When this parameter is absent, the
 notifications should be sent to the originator of the
 NotificationRequest.
 RequestIdentifier is used to correlate this request with the
 notifications that it triggers.
 RequestedEvents is a list of events that the gateway is requested to
 detect and report. Such events include, for example, fax tones,
 continuity tones, or on-hook transition.  To each event is associated
 an action, 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,

Arango, et al. Informational [Page 33] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

  • process the Embedded Notification Request,
  • Ignore the event.
 Some actions can be combined.  In particular:
  • The "swap audio" action can be combined with "Notify",

"Accumulate" and "Ignore."

  • The "keep signal active" action can be combined with "Notify",

"Accumulate", "Accumulate according to Digit Map", "Ignore" and

    "Embedded Notification Request."
  • The "Embedded Notification Request" can be combined with

"Accumulate" and with "Keep signals active." It can also be

    combined with Notify, if the gateway is allowed to issue several
    Notify commands in response to a single Notification request.
 In addition to the requestedEvents parameter specified in the
 command, some profiles of MGCP have introduced the concept of
 "persistent events." According to such profiles, the persistent event
 list is configured in the endpoint, by means outside the scope of
 MGCP. The basic MGCP specification does not specify any persistent
 event.
 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. 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 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 then 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 "Ignore off-hook" was
 specified, and a new request without any off-hook instructions were
 received, the default "Notify off-hook" operation then would be
 restored. A given event MUST NOT appear more than once in a
 RequestedEvents.

Arango, et al. Informational [Page 34] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 The gateway will detect the union of the persistent events and the
 requested events. If an event is not specified in either list, it
 will be ignored.
 The Swap Audio action can be used when a gateway handles more than
 one active connection on an endpoint. This will be the case for
 three-way calling, call waiting, and possibly other feature
 scenarios. In order to avoid the round-trip to the Call Agent when
 just changing which 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 function swap audio,
 which selects the "next" connection in a round robin fashion. If
 there is only one connection, this action is effectively a no-op.
 If signal(s) are desired to start when an event being looked for
 occurs, the "Embedded NotificationRequest" action can be used. The
 embedded NotificationRequest may include a new list of
 RequestedEvents, 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,
 and RequestIdentifier. When the "Embedded NotificationRequest" is
 activated, the "current dial string" will be cleared; the list of
 observed events and the quarantine buffer will be unaffected.
 MGCP implementations shall be able to support at least one level of
 embedding.  An embedded NotificationRequest that respects this
 limitation shall not contain another Embedded NotificationRequest.
 DigitMap is an optional parameter that allows the Call Agent to
 provision the gateways with a digit map according to which digits
 will be accumulated. If this optional parameter is absent, the
 previously defined value is retained. This parameter must be defined,
 either explicitly or through a previous command, if the
 RequestedEvent parameters contain an 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 be accumulated may
 be found in between the list of digits.
 SignalRequests is a parameter that contains the set of signals that
 the gateway is asked to apply to the endpoint, such as, for example
 ringing, or continuity tones. Signals are identified by their name,
 which is an event name, and may be qualified by parameters.

Arango, et al. Informational [Page 35] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 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 event
 request ask to look for an "off-hook" event, the ringing shall stop
 as soon as the gateway detect an off hook event. The formal
 definition is that the generation of all "Time Out" signals shall
 stop as soon as one of the requested events is detected, unless the
 "Keep signals active" action is associated to the specified event.
 The specific definition of actions that are requested via these
 SignalRequests, such as the duration of and frequency of a DTMF
 digit, is out side the scope of MGCP. This definition may vary from
 location to location and hence from gateway to gateway.
 The RequestedEvents and SignalRequests 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 event packages
 that are supported by that end point.  Each package specifies a list
 of events and actions 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 shall return
 an error. When the event name is not qualified by a package name, the
 default package name for the end point is assumed.  If the event name
 is not registered in this default package, the gateway shall return
 an error.
 The Call Agent can send a NotificationRequest whose requested signal
 list is empty. It will do so for example when tone generation should
 stop.
 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:
  • 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 exactly one.)
 When the parameter is absent, the default value is assumed.

Arango, et al. Informational [Page 36] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 We should note that the quarantine-handling parameter also governs
 the handling of events that were detected but not yet notified when
 the command is received.
 DetectEvents is an optional parameter that specifies a list of events
 that the gateway is requested to detect during the quarantine period.
 When this parameter is absent, the events that should be detected in
 the quarantine period are those listed in the last received
 DetectEvents list.  In addition, the gateway should also detect the
 events specified in the request list, including those for which the
 "ignore" action is specified.
 Some events and signals, such as the in-line ringback or the quality
 alert, are performed or detected on connections terminating in the
 end point rather than on the endpoint itself.  The structure of the
 event names allow the Call Agent to specify the connection (or
 connections) on which the events should be performed or detected.
 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 inserted
 after 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. .NH 3 Notifications
 Notifications are sent via the Notify command and are sent by the
 gateway when the observed events occur.
             ReturnCode
             <-- Notify( EndpointId,
                         [NotifiedEntity,]
                         RequestIdentifier,
                         ObservedEvents)
 EndpointId is the name for the endpoint in the gateway which is
 issuing the Notify command, as defined in section 2.1.1. The
 identifier should be a fully qualified endpoint identifier, including
 the domain name of the gateway.  The local part of the name shall not
 use the wildcard convention.

Arango, et al. Informational [Page 37] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 NotifiedEntity is an optional parameter that identifies the entity to
 which the notifications is sent. This parameter is equal to the last
 received value of the NotifiedEntity parameter.  The parameter is
 absent if there was no such parameter in the triggering request. The
 notification is sent to the "current notified entity" or, if no such
 entity was ever specified, to the address from which the request was
 received.
 RequestIdentifier is 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.
 ObservedEvents is a list of events that the gateway detected. A
 single notification may report a list of events that will be reported
 in the order in which they were detected. The list may only contain
 the 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 detection or provided a final
 match in the digit map.
 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.

2.3.3. CreateConnection

 This command is used to create a connection between two endpoints.
          ReturnCode,
          ConnectionId,
          [SpecificEndPointId,]
          [LocalConnectionDescriptor,]
          [SecondEndPointId,]
          [SecondConnectionId]
          <--- CreateConnection(CallId,
                                EndpointId,
                                [NotifiedEntity,]
                                [LocalConnectionOptions,]
                                Mode,
                                [{RemoteConnectionDescriptor |
                                  SecondEndpointId}, ]
                                [Encapsulated NotificationRequest,]
                                [Encapsulated EndpointConfiguration])

Arango, et al. Informational [Page 38] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 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 globally unique parameter that identifies the call (or
 session) to which this connection belongs. Connections that belong to
 the same call share the same call-id. The call-id can be used to
 identify calls for reporting and accounting purposes. 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 "anyone"
 wildcard convention. If the endpoint is underspecified, the endpoint
 identifier will be assigned by the gateway and its complete value
 returned in the SpecificEndPointId parameter of the response.
 The NotifiedEntity is an optional parameter that specifies where the
 Notify or DeleteConnection commands should be sent. If the parameter
 is absent, the Notify or DeleteConnection commands should be sent to
 the last received Notified Entity, or to originator of the
 CreateConnection command if no Notified Entity was ever received for
 the end point.
 LocalConnectionOptions is a parameter used by the Call Agent to
 direct the handling of the connection by the gateway.  The fields
 contained in LocalConnectionOptions are the following:
  • Encoding Method,
  • Packetization period,
  • Bandwidth,
  • Type of Service,
  • Usage of echo cancellation,
  • Usage of silence suppression or voice activity detection,
  • Usage of signal level adaptation and noise level reduction, or

"gain control."

  • Usage of reservation service,
  • Usage of RTP security,

Arango, et al. Informational [Page 39] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

  • Type of network used to carry the connection.
 This set of field can be completed by vendor specific optional or
 mandatory extensions. The encoding of the first three fields, when
 they are present, will be compatible with the SDP and RTP profiles:
  • The encoding method shall be specified by using one or several

valid encoding names, as defined in the RTP AV Profile or

    registered with the IANA.
  • The packetization period is encoded as either the length of time

in milliseconds represented by the media in a packet, as specified

    in the "ptime" parameter of SDP, or as a range value, specifying
    both the minimum and maximum acceptable packetization periods.
  • The bandwidth is encoded as either a single value or a range,

expressed as an integer number of kilobit per seconds.

 For each of the first three fields, the Call Agent has three options:
  • It may state exactly one value, which the gateway will then use

for the connection,

  • It may provide a loose specification, such as a list of allowed

encoding methods or a range of packetization periods,

  • It may simply provide a bandwidth indication, leaving the choice

of encoding method and packetization period to the gateway.

 The bandwidth specification shall not contradict the specification of
 encoding methods and packetization period. If an encoding method is
 specified, then the gateway is authorized to use it, even if it
 results in the usage of a larger bandwidth than specified.
 The LocalConnectionOptions parameter may be absent in the case of a
 data call.
 The Type of Service specifies the class of service that will be used
 for the connection. When the connection is transmitted over an IP
 network, the parameters encodes the 8-bit type of service value
 parameter of the IP header. When the Type of Service is not
 specified, the gateway shall use a default or configured value.
 The gateways can be instructed to perform a reservation, for example
 using RSVP, on a given connection.  When a reservation is needed, the
 call agent will specify the reservation profile that should be used,
 which is either "controlled load" or "guaranteed service."  The

Arango, et al. Informational [Page 40] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 absence of reservation can be indicated by asking for the "best
 effort" service, which is the default value of this parameter. When
 reservation has been asked on a connection, the gateway will:
  • start emitting RSVP "PATH" messages if the connection is in

"send-only", "send-receive", "conference", "network loop back" or

    "network continuity test" mode (if a remote connection descriptor
    has been received,)
  • start emitting RSVP "RESV" messages as soon as it receives "PATH"

messages if the connection is in "receive-only", "send-receive",

    "conference", "network loop back" or "network continuity test"
    mode.
 The RSVP filters will be deduced from the characteristics of the
 connection. The RSVP resource profiles will be deduced from the
 connection's bandwidth and packetization period.
 By default, the telephony gateways always perform echo cancellation.
 However, it is necessary, for some calls, to turn off these
 operations.  The echo cancellation parameter can have two values,
 "on" (when the echo cancellation is requested) and "off" (when it is
 turned off.)
 The telephony gateways may perform gain control, in order to adapt
 the level of the signal.  However, it is necessary, for example for
 modem calls, to turn off this function.  The gain control parameter
 may either be specified as "automatic", or as an explicit number of
 decibels of gain.  The default is to not perform gain control, which
 is equivalent to specifying a gain of 0 decibels.
 The telephony gateways may perform 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 turned off.) The
 default is "off."
 The Call agent can request the gateway to enable encryption of the
 audio Packets.  It does so by providing an key specification, as
 specified in RFC 2327. By default, encryption is not used.
 The Call Agent may instruct the gateway to prepare the connection on
 a specified type of network.  The type of network is encoded as in
 the "connection-field" parameter of the SDP standard.  Possible
 values are IN (Internet), ATM and LOCAL. The parameter is optional;
 if absent, the network is determined by the type of gateway.

Arango, et al. Informational [Page 41] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 RemoteConnectionDescriptor is the connection descriptor for the
 remote side of a connection, on the other side of the IP network. It
 includes the same fields as in the LocalConnectionDescriptor, i.e.
 the fields that describe a session according to the SDP standard.
 This parameter may have a null value when the information for the
 remote end is not known yet. This occurs because the entity that
 builds a connection starts by sending a CreateConnection to one of
 the two gateways involved in it. For the first CreateConnection
 issued, there is no information available about the other side of the
 connection. This information may be provided later via a
 ModifyConnection call. In the case of data connections (mode=data),
 this parameter describes the characteristics of the data 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 "anyone"
 wildcard convention. If the secondendpoint is underspecified, the
 second endpoint identifier will be assigned by the gateway and its
 complete value returned in the SecondEndPointId parameter of the
 response.
 Mode indicates the mode of operation for this side of the connection.
 The mode are "send", "receive", "send/receive", "conference", "data",
 "inactive", "loopback", "continuity test", "network loop back" or
 "network continuity test." The expected handling of these modes is
 specified in the introduction of the "Gateway Handling Function"
 section. Some end points may not be capable of supporting all modes.
 If the command specifies a mode that the endpoint cannot support, and
 error shall be returned.
 The gateway returns a ConnectionId, that uniquely identifies the
 connection within one endpoint, and a LocalConnectionDescriptor,
 which is a session description that contains information about
 addresses and RTP ports, as defined in SDP. The
 LocalConnectionDescriptor is not returned in the case of data
 connections. The SpecificEndPointId is an optional parameter that
 identifies the responding endpoint. It can be used when the
 EndpointId argument referred to a "any of" wildcard name. When a
 SpecificEndPointId is returned, the Call Agent should use it as the
 EndpointId value is successive commands referring to this call.

Arango, et al. Informational [Page 42] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 When a SecondEndpointId is specified, the command really creates two
 connections that can be manipulated separately through
 ModifyConnection and DeleteConnection commands.  The response to the
 creation provides a SecondConnectionId parameter that identifies the
 second connection.
 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 should accept the

voice signals and transmit them through the endpoint.

  • If the mode was set to Inactive, Loopback, Continuity Test, the

gateway should refuse the voice signals.

  • If the mode was set to Network Loopback or Network Continuity

Test, the gateway should perform the expected echo or Response.

 Note that the mode values SendReceive, Conference, Data and SendOnly
 don't make sense in this situation. They should be treated as errors,
 and the command should be rejected (Error code 517).
 The command may optionally contain an encapsulated Notification
 Request command, in which case a RequestIdentifier parameter will be
 present, as well as, optionally, the RequestedEvents DigitMap,
 SignalRequests, QuarantineHandling and DetectEvents parameters. The
 encapsulated NotificationRequest is executed simultaneously with the
 creation of the connection. For example, when the Call Agent wants to
 initiate a call to an residential gateway, it should:
  • ask the residential gateway to prepare a connection, in order to

be sure that the user can start speaking as soon as the phone goes

    off hook,
  • ask the residential gateway to start ringing,
  • ask the residential gateway to notify the Call Agent when the

phone goes off-hook.

Arango, et al. Informational [Page 43] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 This can be accomplished in a single CreateConnection command, by
 also transmitting the RequestedEvent parameters for the off hook
 event, and the SignalRequest parameter for the ringing signal.
 When these parameters are present, the creation and the
 NotificationRequests should be synchronized, which means that
 bothshould be accepted, or both refused. In our example, the
 CreateConnection may be refused if the gateway does not have
 sufficient resources, or cannot get adequate resources from the local
 network access, and the off-hook Notification-Request can be refused
 in the glare condition, if the user is already off-hook. In this
 example, the phone should not ring if the connection cannot be
 established, and the connection should not be established if the user
 is already off hook.
 The NotifiedEntity parameter, if present, applies to both the
 CreateConnection and the NotificationRequest command. It 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 inserted
 after 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.
 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.

2.3.4. ModifyConnection

 This command is used to modify the characteristics of a gateway's
 "view" of a connection. This "view" of the call includes both the
 local connection descriptors as well as the remote connection
 descriptor.

Arango, et al. Informational [Page 44] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

    ReturnCode,
    [LocalConnectionDescriptor]
     <--- 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 is returned by the
 CreateConnection function, as part of the local connection
 descriptor. It uniquely identifies the connection within the context
 of the endpoint.
 The EndpointId should be a fully qualified endpoint identifier.  The
 local name shall not use the wildcard convention.
 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.

  • Activate or deactivate the connection, by changing the value of

the Mode parameter. This can occur at any time during the

    connection, with arbitrary parameter values.
  • Change the sending parameters of the connection, for example by

switching to a different coding scheme, changing the packetization

    period, or modifying the handling of echo cancellation.
 Connections can only be activated if the RemoteConnectionDescriptor
 has been provided to the gateway. The receive only mode, however, can
 be activated without the provision of this descriptor.
 The command will only return a LocalConnectionDescriptor if the local
 connection parameters, such as RTP ports, were modified. (Usage of
 this feature is actually for further study.)
 The command may optionally contain an encapsulated Notification
 Request command, in which case a RequestIdentifier parameter will be
 present, as well as, optionnally, the RequestedEvents DigitMap,
 SignalRequests, QuarantineHandling and DetectEvents parameters. The

Arango, et al. Informational [Page 45] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 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 RequestedEvent parameters, for the on hook
 event, and an empty SignalRequest parameter, to stop the provision of
 ringing tones.
 When these parameters are present, the modification and the
 NotificationRequests should be synchronized, which means that both
 should be accepted, or both refused.  The NotifiedEntity parameter,
 if present, applies to both the ModifyConnection and the
 NotificationRequest command.
 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 inserted
 after 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.

2.3.5. DeleteConnection (from the Call Agent)

 This command is used to terminate a connection. As a side effect, it
 collects statistics on the execution of the connection.
      ReturnCode,
      Connection-parameters
      <-- DeleteConnection(CallId,
                           EndpointId,
                           ConnectionId,
                           [Encapsulated NotificationRequest,]
                           [Encapsulated EndpointConfiguration])
 The endpoint identifier, in this form of the DeleteConnection
 command, shall be fully qualified.  Wildcard conventions shall not be
 used.

Arango, et al. Informational [Page 46] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 In the general case where a connection has two ends, this command has
 to be sent to both gateways involved in the connection. Some
 connections, however, may use IP multicast. In this case, they can be
 deleted individually.
 After the connection has been deleted, any loopback that has been
 requested for the connection should be cancelled. When all
 connections to an endpoint have been deleted, that endpoint should be
 placed in inactive mode.
 In response to the DeleteConnection command, the gateway returns a
 list of parameters that describe the status of the connection. These
 parameters are:
 Number of packets sent:
 The total number of RTP data packets transmitted by the sender since
 starting transmission on this connection. The count is not reset if
 the sender changes its synchronization source identifier (SSRC, as
 defined in RTP), for example as a result of a Modify command. The
 value is zero if the connection was set in "receive only" mode.
 Number of octets sent:
 The total number of payload octets (i.e., not including header or
 padding) transmitted in RTP data packets by the sender since starting
 transmission on this connection. The count is not reset if the sender
 changes its SSRC identifier, for example as a result of a
 ModifyConnection command. The value is zero if the connection was set
 in "receive only" mode.
 Number of packets received:
 The total number of RTP data packets received by the sender since
 starting reception on this connection. The count includes packets
 received from different SSRC, if the sender used several values. The
 value is zero if the connection was set in "send only" mode.
 Number of octets received:
 The total number of payload octets (i.e., not including header or
 padding) transmitted in RTP data packets by the sender since starting
 transmission on this connection. The count includes packets received
 from different SSRC, if the sender used several values. The value is
 zero if the connection was set in "send only" mode.

Arango, et al. Informational [Page 47] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 Number of packets lost:
 The total number of RTP data packets that have been lost since the
 beginning of reception. This number is defined to be the number of
 packets expected less the number of packets actually received, where
 the number of packets received includes any which are late or
 duplicates.  The count includes packets received from different SSRC,
 if the sender used several values. Thus packets that arrive late are
 not counted as lost, and the loss may be negative if there are
 duplicates. The count includes packets received from different SSRC,
 if the sender used several values. The number of packets expected is
 defined to be the extended last sequence number received, as defined
 next, less the initial sequence number received. The count includes
 packets received from different SSRC, if the sender used several
 values. The value is zero if the connection was set in "send only"
 mode. This parameter is omitted if the connection was set in "data"
 mode.
 Interarrival jitter:
 An estimate of the statistical variance of the RTP data packet
 interarrival time measured in milliseconds and expressed as an
 unsigned integer. The interarrival jitter J is defined to be the mean
 deviation (smoothed absolute value) of the difference D in packet
 spacing at the receiver compared to the sender for a pair of packets.
 Detailed computation algorithms are found in RFC 1889. The count
 includes packets received from different SSRC, if the sender used
 several values. The value is zero if the connection was set in "send
 only" mode. This parameter is omitted if the connection was set in
 "data" mode.
 Average transmission delay:
 An estimate of the network latency, expressed in milliseconds. This
 is the average value of the difference between the NTP timestamp
 indicated by the senders of the RTCP messages and the NTP timestamp
 of the receivers, measured when this messages are received. The
 average is obtained by summing all the estimates, then dividing by
 the number of RTCP messages that have been received. This parameter
 is omitted if the connection was set in "data" mode.
 When the gateway's clock is not synchronized by NTP, the latency
 value can be computed as one half of the round trip delay, as
 measured through RTCP.
 When the gateway cannot compute the one way delay or the round trip
 delay, the parameter conveys a null value.
 For a detailed definition of these variables, refer to RFC 1889.

Arango, et al. Informational [Page 48] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 When the connection was set up over an ATM network, the meaning of
 these parameters may change:
 Number of packets sent:  The total number of ATM cells transmitted
    since starting transmission on this connection.
 Number of octets sent:
    The total number of payload octets transmitted in ATM cells.
 Number of packets received:
    The total number of ATM cells received since starting reception on
    this connection.
 Number of octets received:
    The total number of payload octets received in ATM cells.
 Number of packets lost:
    Should be determined as the number of cell losts, or set to zero
    if the adaptation layer does not enable the gateway to assess
    losses.
 Interarrival jitter:
    Should be understood as the interarrival jitter between ATM cells.
 Average transmission delay:
    The gateway may not be able to assess this parameter over an ATM
    network.  It could simply report a null value.
 When the connection was set up over an LOCAL interconnect, the
 meaning of these parameters is defined as follows:
 Number of packets sent:
   Not significant.
 Number of octets sent:
   The total number of payload octets transmitted over the local
   connection.
 Number of packets received:
   Not significant.
 Number of octets received:
   The total number of payload octets received over the connection.
 Number of packets lost:
   Not significant.  A value of zero is assumed.

Arango, et al. Informational [Page 49] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 Interarrival jitter:
   Not significant.  A value of zero is assumed.
 Average transmission delay:
   Not significant.  A value of zero is assumed.
 The standard set of connection parameters can be extended by the
 creation of extension parameters.
 The command may optionally contain an encapsulated Notification
 Request command, in which case a RequestIdentifier parameter will be
 present, as well as, optionnally, the RequestedEvents DigitMap,
 SignalRequests, QuarantineHandling and DetectEvents parameters. The
 encapsulated NotificationRequest is executed simultaneously with the
 deletion of the connection. For example, when a user hang-up is
 notified, the gateway should be instructed to delete the connection
 and to start looking for an off hook event.
 This can be accomplished in a single DeleteConnection command, by
 also transmitting the RequestedEvent parameters, for the off hook
 event, and an empty SignalRequest parameter.
 When these parameters are present, the DeleteConnection and the
 NotificationRequests should be synchronized, which means that both
 should be accepted, or both refused.
 The command may carry an encapsulated EndpointConfiguration command,
 that will apply to the same endpoint.  When this command is present,
 the parameters of the EndpointConfiguration command are inserted
 after the normal parameters of the DeleteConnection with the
 exception of the EndpointId, which is not replicated. The
 EndpointConfiguration command may be encapsulated together with an
 encapsulated NotificationRequest command.
 The encapsulated EndpointConfiguration command shares the fate of the
 DeleteConnection command.  If the DeleteConnection is rejected, the
 EndpointConfiguration is not executed.
 ReturnCode is a parameter returned by the gateway. It indicates the
 outcome of the command and consists of an integer number optionally
 followed by commentary.

Arango, et al. Informational [Page 50] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

2.3.6. DeleteConnection (from the VoIP gateway)

 In some circumstances, a gateway may have to clear a connection, for
 example because it has lost the resource associated with the
 connection, or because it has detected that the endpoint no longer is
 capable or willing to send or receive voice. The gateway terminates
 the connection by using a variant of the DeleteConnection command:
          ReturnCode,
          <-- DeleteConnection( CallId,
                                EndpointId,
                                ConnectionId,
                                Reason-code,
                                Connection-parameters)
 In addition to the call, endpoint and connection identifiers, the
 gateway will also send the call's parameters that would have been
 returned to the Call Agent in response to a DeleteConnection command.
 The reason code indicates the cause of the disconnection.
 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.

2.3.7. DeleteConnection (multiple connections, from the Call Agent)

 A variation of the DeleteConnection function can be used by the Call
 Agent to delete multiple connections at the same time. The command
 can be used to delete all connections that relate to a Call for an
 endpoint:
          ReturnCode,
          <-- DeleteConnection( CallId,
                                EndpointId)
 It can also be used to delete all connections that terminate in a
 given endpoint:
          ReturnCode,
          <-- DeleteConnection( EndpointId)
 Finally, Call Agents can take advantage of the hierarchical naming
 structure of endoints to delete all the connections that belong to a
 group of endpoints.  In this case, the "local name" component of the
 EndpointID will be specified using the "all value" wildcarding
 convention. The "any value" convention shall not be used.  For
 example, if endpoints names are structured as the combination of a
 physical interface name and a circuit number, as in "X35V3+A4/13",

Arango, et al. Informational [Page 51] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 the Call Agent may replace the circuit number by a wild card
 character "*", as in "X35V3+A4/*".  This "wildcard" command instructs
 the gateway to delete all the connections that where attached to
 circuits connected to the physical interface "X35V3+A4".
 After the connections have been deleted, the endpoint should be
 placed in inactive mode. Any loopback that has been requested for the
 connections should be cancelled.
 This command does not return any individual statistics or call
 parameters.
 ReturnCode is a parameter returned by the gateway. It indicates the
 outcome of the command and consists of an integer number optionally
 followed by commentary.

2.3.8. Audit Endpoint

 The AuditEndPoint command can be used by the Call Agent to find out
 the status of a given endpoint.
            ReturnCode,
              EndPointIdList|{
              [RequestedEvents,]
              [DigitMap,]
              [SignalRequests,]
              [RequestIdentifier,]
              [NotifiedEntity,]
              [ConnectionIdentifiers,]
              [DetectEvents,]
              [ObservedEvents,]
              [EventStates,]
              [BearerInformation,]
              [RestartReason,]
              [RestartDelay,]
              [ReasonCode,]
              [Capabilities]}
                      <--- AuditEndPoint(EndpointId,
                                               [RequestedInfo])
 The EndpointId identifies the endpoint that is being audited. The
 "all of" wildcard convention can be used to start auditing of a group
 of endpoints. If this convention is used, the gateway should return
 the list of endpoint identifiers that match the wildcard in the
 EndPointIdList parameter. It shall not return any parameter specific
 to one of these endpoints.

Arango, et al. Informational [Page 52] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 When a non-wildcard EndpointId is specified, the (possibly empty)
 RequestedInfo parameter describes the information that is requested
 for the EndpointId specified. The following endpoint info can be
 audited with this command:
 RequestedEvents, DigitMap, SignalRequests, RequestIdentifier,
 NotifiedEntity, ConnectionIdentifiers, DetectEvents, ObservedEvents,
 EventStates, RestartReason, RestartDelay, ReasonCode, and
 Capabilities.
 The response will in turn include information about each of the items
 for which auditing info was requested:
  • RequestedEvents: The current value of RequestedEvents the endpoint

is using including the action associated with each event.

    Persistent events are included in the list.
  • DigitMap: the digit map the endpoint is currently using.
  • SignalRequests: A list of the; Time-Out signals that are currently

active, On/Off signals that are currently "on" for the endpoint

    (with or without parameter), and any pending Brief signals. Time-
    Out signals that have timed-out, and currently playing Brief
    signals are not included.
  • RequestIdentifier, the RequestIdentifier for the last Notification

Request received by this endpoint (includes NotificationRequest

    encapsulated in Connection handling primitives). If no
    notification request has been received, the value zero will be
    returned.
  • QuarantineHandling, the QuarantineHandling for the last

NotificationRequest received by this endpoint.

  • DetectEvents, the list of events that are currently detected in

quarantine mode.

  • NotifiedEntity, the current notified entity for the endpoint.
  • ConnectionIdentifiers, the list of ConnectionIdentifiers for all

connections that currently exist for the specified endpoint.

  • ObservedEvents: the current list of observed events for the

endpoint.

Arango, et al. Informational [Page 53] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

  • EventStates: For events that have auditable states associated with

them, the event corresponding to the state the endpoint is in,

    e.g., off-hook if the endpoint is off-hook. The definition of the
    individual events will state if the event in question has an
    auditable state associated with it.
  • BearerInformation: the value of the last received

BearerInformation parameter for this endpoint.

  • RestartReason: the value of the restart reason parameter in the

last RestartInProgress command issued by the endpoint, "restart"

    indicating a fully functional endpoint.
  • RestartDelay: the value of the restart delay parameter if a

RestartInProgress command was issued by the endpoint at the time

    of the response, or zero if the command would not include this
    parameter.
  • ReasonCode:the value of the Reason-Code parameter in the last

RestartInProgress or DeleteConnection command issued by the

    gateway for the endpoint, or the special value 000 if the
    endpoint's state is nominal.
  • The capabilities for the endpoint similar to the

LocalConnectionOptions parameter and including event packages and

    connection modes.  If there is a need to specify that some
    parameters, such as e.g., silence suppression, are only compatible
    with some
  • codecs, then the gateway will return several capability sets:
       Compression Algorithm: a list of supported codecs. The rest of
       the parameters will apply to all codecs specified in this list.
       Packetization Period: A single value or a range may be
       specified.
       Bandwidth: A single value or a range corresponding to the range
       for packetization periods may be specified (assuming no silence
       suppression).
       Echo Cancellation: Whether echo cancellation is supported or
       not.
       Silence Suppression: Whether silence suppression is supported
       or not.
       Type of Service: Whether type of service is supported or not.

Arango, et al. Informational [Page 54] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

       Event Packages: A list of event packages supported. The first
       event package in the list will be the default package.
       Modes: A list of supported connection modes.
 The Call Agent may then decide to use the AuditConnection command to
 obtain further information about the connections.
 If no info was requested and the EndpointId refers to a valid
 endpoint, the gateway simply returns a positive acknowledgement.
 If no NotifiedEntity has been specified in the last
 NotificationRequest, the notified entity defaults to the source
 address of the last NotificationRequest command received for this
 connection.
 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.

2.3.9. Audit Connection

 The AuditConnection command can be used by the Call Agent to retrieve
 the parameters attached to a connection:
            ReturnCode,
            [CallId,]
            [NotifiedEntity,]
            [LocalConnectionOptions,]
            [Mode,]
            [RemoteConnectionDescriptor,]
            [LocalConnectionDescriptor,]
            [ConnectionParameters]
                      <--- AuditConnection(EndpointId,
                                       ConnectionId,
                                       RequestedInfo)
 The EndpointId parameter specifies the endpoint that handles the
 connection. The wildcard conventions shall not be used.
 The ConnectionId parameter is the identifier of the audited
 connection, within the context of the specified endpoint.
 The (possibly empty) RequestedInfo describes the information that is
 requested for the ConnectionId within the EndpointId specified. The
 following connection info can be audited with this command:

Arango, et al. Informational [Page 55] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

    CallId, NotifiedEntity, LocalConnectionOptions, Mode,
    RemoteConnectionDescriptor, LocalConnectionDescriptor,
    ConnectionParameters
 The AuditConnectionResponse will in turn include information about
 each of the items auditing info was requested for:
  • CallId, the CallId for the call the connection belongs to.
  • NotifiedEntity, the current notified entity for the Connection.
  • LocalConnectionOptions, the LocalConnectionOptions that was

supplied for the connection.

  • Mode, the current mode of the connection.
  • RemoteConnectionDescriptor, the RemoteConnectionDescriptor that

was supplied to the gateway for the connection.

  • LocalConnectionDescriptor, the LocalConnectionDescriptor the gate-

way supplied for the connection.

  • ConnectionParameters, the current value of the connection

parameters for the connection.

 If no info was requested and the EndpointId is valid, the gateway
 simply checks that the connection exists, and if so returns a
 positive acknowledgement.
 If no NotifiedEntity has been specified for the connection, the
 notified entity defaults to the source address of the last connection
 handling command received for this connection.
 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.

2.3.10. Restart in progress

 The RestartInProgress command is used by the gateway to signal that
 An endpoint, or a group of endpoint, is taken in or out of service.
        ReturnCode,
        [NotifiedEntity]
              <------- RestartInProgress ( EndPointId,
                                           RestartMethod,
                                           [RestartDelay,]
                                           [Reason-code])

Arango, et al. Informational [Page 56] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 The EndPointId identifies the endpoint that are taken in or out of
 service.  The "all of" wildcard convention may be used to apply the
 command to a group of endpoint, such as for example all endpoints
 that are attached to a specified interface, or even all endpoints
 that are attached to a given gateway.  The "any of" wildcard
 convention shall not be used.
 The RestartMethod parameter specified the type of restart.  Three
 values have been defined:
  • A "graceful" restart method indicates that the specified endpoints

will Be taken out of service after the specified delay. The

    established connections are not yet affected, but the Call Agent
    should refrain to establish new connections, and should try to
    gracefully tear down the existing connections.
  • A "forced" restart method indicates that the specified endpoints

are taken abruptely out of service. The established connections,

    if any, are lost.
  • A "restart" method indicates that service will be restored on the

endpoints after the specified "restart delay." There are no

    connections that are currently established on the endpoints.
  • A "disconnected" method indicates that the endpoint has become

disconnected and is now trying to establish connectivity. The

    "restart delay" specifies the number of seconds the endpoint has
    been disconnected. Established connections are not affected.
  • A "cancel-graceful" method indicates that a gateway is canceling a

previously issued "graceful" restart command.

 The optional "restart delay" parameter is expressed as a number of
 seconds. If the number is absent, the delay value should be
 considered null.  In the case of the "graceful" method, a null delay
 indicates that the call agent should simply wait for the natural
 termination of the existing connections, without establishing new
 connections. The restart delay is always considered null in the case
 of the "forced" method.
 A restart delay of null for the "restart" method indicates that
 service has already been restored. This typically will occur after
 gateway startup/reboot.
 The optional reason code parameter the cause of the restart.

Arango, et al. Informational [Page 57] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 Gateways SHOULD send a "graceful" or "forced" RestartInProgress
 message as a courtesy to the Call Agent when they are taken out of
 service, e.g., by being shutdown, or taken out of service by a
 network management system, although the Call Agent cannot rely on
 always receiving such messages. Gateways MUST send a "restart"
 RestartInProgress message with a null delay to their Call Agent when
 they are back in service according to the restart procedure specified
 in Section 4.3.4 - Call Agents can rely on receiving this message.
 Also, gateways MUST send a "disconnected" RestartInProgress message
 to their current "notified entity" according to the "disconnected"
 procedure specified in Section 4.3.5.  The "restart delay" parameter
 MUST NOT be used with the "forced" restart method.
 The RestartInProgress message will be sent to the current notified
 entity for the EndpointId in question. It is expected that a default
 Call Agent, i.e., notified entity, has been provisioned for each
 endpoint so, after a reboot, the default Call Agent will be the
 notified entity for each endpoint. Gateways should take full
 advantage of wild- carding to minimize the number of
 RestartInProgress messages generated when multiple endpoints in a
 gateway restart and the endpoints are managed by the same Call Agent.
 ReturnCode is a parameter returned by the gateway. It indicates the
 outcome of the command and consists of an integer number optionally
 followed by commentary.
 A NotifiedEntity may additionally be returned with the response from
 the Call Agent:
  • If the response indicated success (return code 200 - transaction

executed), the restart procedure has completed, and the

    NotifiedEntity returned is the new "notified entity" for the
    endpoint(s).
  • If the response from the Call Agent indicated an error, the

restart procedure is not yet complete, and must therefore be

    initiated again. If a NotifiedEntity parameter was returned, it
    then specifies the new "notified entity" for the endpoint(s),
    which must consequently be used when retrying the restart
    procedure.

2.4. Return codes and error codes.

 All MGCP commands are acknowledged. The acknowledgment carries a
 return code, which indicates the status of the command. The return
 code is an integer number, for which four ranges of values have been
 defined:

Arango, et al. Informational [Page 58] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

  • values between 100 and 199 indicate a provisional response,
  • values between 200 and 299 indicate a successful completion,
  • values between 400 and 499 indicate a transient error,
  • values between 500 and 599 indicate a permanent error.
 The values that have been already defined are listed in the following
 list:
 100  The transaction is currently being executed.  An actual
      completion message will follow on later.
 200  The requested transaction was executed normally.
 250  The connection was deleted.
 400  The transaction could not be executed, due to a transient error.
 401  The phone is already off hook
 402  The phone is already on hook
 403  The transaction could not be executed, because the endpoint does
      not have sufficient resources at this time
 404  Insufficient bandwidth at this time
 500  The transaction could not be executed, because the endpoint is
      unknown.
 01   The transaction could not be executed, because the endpoint is
      not ready.
 502  The transaction could not be executed, because the endpoint does
      not have sufficient resources
 510  The transaction could not be executed, because a protocol error
      was detected.
 11   The transaction could not be executed, because the command
      contained an unrecognized extension.
 512  The transaction could not be executed, because the gateway is
      not equipped to detect one of the requested events.

Arango, et al. Informational [Page 59] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 513  The transaction could not be executed, because the gateway is
      not equipped to generate one of the requested signals.
 514  The transaction could not be executed, because the gateway
      cannot send the specified announcement.
 515  The transaction refers to an incorrect connection-id (may have
      been already deleted)
 516  The transaction refers to an unknown call-id.
 517  Unsupported or invalid mode.
 518  Unsupported or unknown package.
 519  Endpoint does not have a digit map.
 520  The transaction could not be executed, because the endpoint is
      "restarting".
 521  Endpoint redirected to another Call Agent.
 522  No such event or signal.
 523  Unknown action or illegal combination of actions
 524  Internal inconsistency in LocalConnectionOptions
 525  Unknown extension in LocalConnectionOptions
 526  Insufficient bandwidth
 527  Missing RemoteConnectionDescriptor
 528  Incompatible protocol version
 529  Internal hardware failure
 530  CAS signaling protocol error.
 531  failure of a grouping of trunks (e.g. facility failure).

Arango, et al. Informational [Page 60] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

2.5. Reason Codes

 Reason-codes are used by the gateway when deleting a connection to
 inform the Call Agent about the reason for deleting the connection.
 They may also be used in a RestartInProgress command, to inform the
 gateway of the Restart's reason. The reason code is an integer
 number, and the following values have been defined:
 000  Endpoint state is nominal. (This code is used only in response
      to audit requests.)
 900  Endpoint malfunctioning
 901  Endpoint taken out of service
 902  Loss of lower layer connectivity (e.g., downstream sync)

3. Media Gateway Control Protocol

 The MGCP implements the media gateway control interface as a set of
 transactions. The transactions are composed of a command and a
 mandatory response. There are eight types of command:
  • CreateConnection
  • ModifyConnection
  • DeleteConnection
  • NotificationRequest
  • Notify
  • AuditEndpoint
  • AuditConnection
  • RestartInProgress
 The first four commands are sent by the Call Agent to a gateway. The
 Notify command is sent by the gateway to the Call Agent. The gateway
 may also send a DeleteConnection as defined in 2.3.6. The Call Agent
 may send either of the Audit commands to the gateway.  The Gateway
 may send a RestartInProgress command to the Call Agent.

Arango, et al. Informational [Page 61] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

3.1. General description

 All commands are composed of a Command header, optionally followed by
 a session description.
 All responses are composed of a Response header, optionally followed
 by a session description.
 Headers and session descriptions are encoded as a set of text lines,
 separated by a carriage return and line feed character (or,
 optionnally, a single line-feed character). The headers are separated
 from the session description by an empty line.
 MGCP uses a transaction identifier to correlate commands and
 responses.  The transaction identifier is encoded as a component of
 the command header and repeated as a component of the response header
 (see section 3.2.1, 3.2.1.2 and 3.3).

3.2. Command Header

 The command header is composed of:
  • A command line, identifying the requested action or verb, the

transaction identifier, the endpoint towards which the action is

    requested, and the MGCP protocol version,
  • A set of parameter lines, composed of a parameter name followed by

a parameter value.

 Unless otherwise noted or dictated by other referenced standards,
 each component in the command header is case insensitive. This goes
 for verbs as well as parameters and values, and all comparisons MUST
 treat upper and lower case as well as combinations of these as being
 equal.

3.2.1. Command line

 The command line is composed of:
  • The name of the requested verb,
  • The identification of the transaction,
  • The name of the endpoint that should execute the command (in

notifications or restarts, the name of the endpoint that is

    issuing the command),
  • The protocol version.

Arango, et al. Informational [Page 62] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 These four items are encoded as strings of printable ASCII
 characters, separated by white spaces, i.e. the ASCII space (0x20) or
 tabulation (0x09) characters. It is recommended to use exactly one
 ASCII space separator.

3.2.1.1. Coding of the requested verb

 The verbs that can be requested are encoded as four letter upper or
 lower case ASCII codes (comparisons should be case insensitive) as
 defined in the following table:
                  ______________________________
                 | Verb                 |  Code|
                 |______________________|______|
                 | EndpointConfiguration|  EPCF|
                 | CreateConnection     |  CRCX|
                 | ModifyConnection     |  MDCX|
                 | DeleteConnection     |  DLCX|
                 | NotificationRequest  |  RQNT|
                 | Notify               |  NTFY|
                 | AuditEndpoint        |  AUEP|
                 | AuditConnection      |  AUCX|
                 | RestartInProgress    |  RSIP|
                 |______________________|______|
 The transaction identifier is encoded as a string of up to 9 decimal
 digits. In the command lines, it immediately follows the coding of
 the verb.
 New verbs may be defined in further versions of the protocol. It may
 be necessary, for experimentation purposes, to use new verbs before
 they are sanctioned in a published version of this protocol.
 Experimental verbs should be identified by a four letter code
 starting with the letter X, such as for example XPER.

3.2.1.2. Transaction Identifiers

 MGCP uses a transaction identifier to correlate commands and
 responses.  A gateway supports two separate transaction identifier
 name spaces:
   a transaction identifier name space for sending transactions, and
   a transaction identifier name space for receiving transactions.
 At a minimum, transaction identifiers for commands sent to a given
 gateway MUST be unique for the maximum lifetime of the transactions
 within the collection of Call Agents that control that gateway. Thus,

Arango, et al. Informational [Page 63] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 regardless of the sending Call Agent, gateways can always detect
 duplicate transactions by simply examining the transaction
 identifier. The coordination of these transaction identifiers between
 Call Agents is outside the scope of this specification though.
 Transaction identifiers for all commands sent from a given gateway
 MUST be unique for the maximum lifetime of the transactions
 regardless of which Call Agent the command is sent to. Thus, a Call
 Agent can always detect a duplicate transaction from a gateway by the
 combination of the domain-name of the endpoint and the transaction
 identifier.
 The transaction identifier is encoded as a string of up to nine
 decimal digits. In the command lines, it immediately follows the
 coding of the verb.
 Transaction identifiers have values between 1 and 999999999. An MGCP
 entity MUST NOT reuse a transaction identifier more quickly than
 three minutes after completion of the previous command in which the
 identifier was used.

3.2.1.3. Coding of the endpoint identifiers and entity names

 The endpoint identifiers and entity names are encoded as case
 insensitive e-mail addresses, as defined in RFC 821. In these
 addresses, the domain name identifies the system where the endpoint
 is attached, while the left side identifies a specific endpoint on
 that system.
 Examples of such addresses can be:
  ______________________________________________________________________
 | hrd4/56@gw23.example.net     |  Circuit number 56 in                |
 |                              |  interface "hrd4" of the Gateway 23  |
 |                              |  of the "Example" network            |
 | Call-agent@ca.example.net    |  Call Agent for the                  |
 |                              |  "example" network                   |
 | Busy-signal@ann12.example.net|  The "busy signal" virtual           |
 |                              |  endpoint in the announcement        |
 |                              |  server number 12.                   |
 |______________________________|______________________________________|
 The name of notified entities is expressed with the same syntax, with
 the possible addition of a port number as in:
   Call-agent@ca.example.net:5234

Arango, et al. Informational [Page 64] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 In case the port number is omitted, the default MGCP port (2427) will
 be used.

3.2.1.4. Coding of the protocol version

 The protocol version is coded as the key word MGCP followed by a
 white space and the version number, and optionally followed by a
 profile name.. The version number is composed of a major version,
 coded by a decimal number, a dot, and a minor version number, coded
 as a decimal number. The version described in this document is
 version 1.0.
 The profile name, if present, is represented by a white-space
 separated strings of  visible (printable) characters extending to the
 end of the line. Profile names may be defined for user communities
 who want to apply restrictions or other profiling to MGCP.
 In the initial messages, the version will be coded as:
      MGCP 1.0

3.2.2. Parameter lines

 Parameter lines are composed of a parameter name, which in most cases
 is composed of a single upper case character, followed by a colon, a
 white space and the parameter value. The parameter that can be
 present in commands are defined in the following table:

Arango, et al. Informational [Page 65] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

_ |Parameter name | Code| Parameter value | |||_| |ResponseAck | K | see description | |BearerInformation | B | see description | |CallId | C | Hexadecimal string, at most 32 chars.| |ConnectionId | I | Hexadecimal string, at most 32 chars.| |NotifiedEntity | N | An identifier, in RFC 821 format, | | | | composed of an arbitrary string and | | | | of the domain name of the requesting | | | | entity, possibly completed by a port | | | | number, as in: | | | | Call-agent@ca.example.net:5234 | |RequestIdentifier | X | Hexadecimal string, at most 32 chars.| |LocalConnectionOptions| L | See description | |Connection Mode | M | See description | |RequestedEvents | R | See description | |SignalRequests | S | See description | |DigitMap | D | A text encoding of a digit map | |ObservedEvents | O | See description | |ConnectionParameters | P | See description | |ReasonCode | E | An arbitrary character string | |SpecificEndpointID | Z | An identifier, in RFC 821 format, | | | | composed of an arbitrary string, | | | | followed by an "@" followed by the | | | | domain name of the gateway to which | | | | this endpoint is attached. | |Second Endpoint ID | Z2 | Endpoint Id. | |SecondConnectionId | I2 | Connection Id. | |RequestedInfo | F | See description | |QuarantineHandling | Q | See description | |DetectEvents | T | See Description | |RestartMethod | RM | See description | |RestartDelay | RD | A number of seconds, encoded as | | | | a decimal number | |EventStates | ES | See description | |Capabilities | A | See description | |||_| |RemoteConnection | RC | Session Description | |Descriptor | | | |LocalConnection | LC | Session Description | |Descriptor | | | |||_|

Arango, et al. Informational [Page 66] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 The parameters are not necessarily present in all commands. The
 following table provides the association between parameters and
 commands. The letter M stands for mandatory, O for optional and F for
 forbidden.
 ___________________________________________________________________
| Parameter name      |  EP|  CR|  MD|  DL|  RQ|  NT|  AU|  AU|  RS|
|                     |  CF|  CX|  CX|  CX|  NT|  FY|  EP|  CX|  IP|
|_____________________|____|____|____|____|____|____|____|____|____|
| ResponseAck         |  O |  O |  O |  O |  O |  O |  O |  O |  O |
| BearerInformation   |  M |  O |  O |  O |  O |  F |  F |  F |  F |
| CallId              |  F |  M |  M |  O |  F |  F |  F |  F |  F |
| ConnectionId        |  F |  F |  M |  O |  F |  F |  F |  M |  F |
| RequestIdentifier   |  F |  O+|  O+|  O+|  M |  M |  F |  F |  F |
| LocalConnection     |  F |  O |  O |  F |  F |  F |  F |  F |  F |
| Options             |    |    |    |    |    |    |    |    |    |
| Connection Mode     |  F |  M |  M |  F |  F |  F |  F |  F |  F |
| RequestedEvents     |  F |  O |  O |  O |  O*|  F |  F |  F |  F |
| SignalRequests      |  F |  O |  O |  O |  O*|  F |  F |  F |  F |
| NotifiedEntity      |  F |  O |  O |  O |  O |  O |  F |  F |  F |
| ReasonCode          |  F |  F |  F |  O |  F |  F |  F |  F |  O |
| ObservedEvents      |  F |  F |  F |  F |  F |  M |  F |  F |  F |
| DigitMap            |  F |  O |  O |  O |  O |  F |  F |  F |  F |
| Connection          |  F |  F |  F |  O |  F |  F |  F |  F |  F |
| parameters          |    |    |    |    |    |    |    |    |    |
| Specific Endpoint ID|  F |  F |  F |  F |  F |  F |  F |  F |  F |
| Second Endpoint ID  |  F |  O |  F |  F |  F |  F |  F |  F |  F |
| RequestedInfo       |  F |  F |  F |  F |  F |  F |  M |  M |  F |
| QuarantineHandling  |  F |  O |  O |  O |  O |  F |  F |  F |  F |
| DetectEvents        |  F |  O |  O |  O |  O |  F |  F |  F |  F |
| EventStates         |  F |  F |  F |  F |  F |  F |  F |  F |  F |
| RestartMethod       |  F |  F |  F |  F |  F |  F |  F |  F |  M |
| RestartDelay        |  F |  F |  F |  F |  F |  F |  F |  F |  O |
| SecondConnectionID  |  F |  F |  F |  F |  F |  F |  F |  F |  F |
| Capabilities        |  F |  F |  F |  F |  F |  F |  F |  F |  F |
|_____________________|____|____|____|____|____|____|____|____|____|
| RemoteConnection    |  F |  O |  O |  F |  F |  F |  F |  F |  F |
| Descriptor          |    |    |    |    |    |    |    |    |    |
| LocalConnection     |  F |  F |  F |  F |  F |  F |  F |  F |  F |
| Descriptor          |    |    |    |    |    |    |    |    |    |
|_____________________|____|____|____|____|____|____|____|____|____|
 Note (+) that the RequestIdentifier parameter is optional in
 connection creation, modification and deletion commands, but that it
 becomes mandatory if the command contains an encapsulated
 notification request.

Arango, et al. Informational [Page 67] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 Note (*) that the RequestedEvents and SignalRequests parameters are
 optional in the NotificationRequest. If these parameters are omitted,
 the corresponding lists will be considered empty.
 If implementers need to experiment with new parameters, for example
 when developing a new application of MGCP, they should identify these
 parameters by names that start with the string "X-" or "X+", such as
 for example:
           X-FlowerOfTheDay: Daisy
 Parameter names that start with "X+" are critical parameter
 extensions.  An MGCP entity that receives a critical parameter
 extension that it cannot understand should refuse to execute the
 command.  It should respond with an error code 511 (Unrecognized
 extension).
 Parameter names that start with "X-" are non critical parameter
 extensions. An MGCP entity that receives a non critical parameter
 extension that it cannot understand can safely ignore that parameter.

3.2.2.1. Response Acknowledgement

 The response acknowledgement attribute is used to managed the "at-
 most-once" facility described in the "transmission over UDP" section.
 It contains a comma separated list of "confirmed transaction-id
 ranges".
 Each "confirmed transaction-id ranges" is composed of either one
 decimal number, when the range includes exactly one transaction, or
 two decimal numbers separated by a single hyphen, describing the
 lower and higher transaction identifiers included in the range.
 An example of response acknowledgement is:
      K: 6234-6255, 6257, 19030-19044

3.2.2.2. Local connection options

 The local connection options describe the operational parameters that
 the Call Agent suggests to the gateway. These parameters are:
  • The packetization period in milliseconds, encoded as the keyword

"p", followed by a colon and a decimal number. If the Call Agent

    specifies a range of values, the range will be specified as two
    decimal numbers separated by an hyphen.

Arango, et al. Informational [Page 68] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

  • The preferred type of compression algorithm, encoded as the

keyword "a", followed by a colon and a character string. If the

    Call Agent specifies a list of values, these values will be
    separated by a semicolon.
  • The bandwidth in kilobits per second (1000 bits per second),

encoded as the keyword "b", followed by a colon and a decimal

    number. If the Call Agent specifies a range of values, the range
    will be specified as two decimal numbers separated by an hyphen.
  • The echo cancellation parameter, encoded as the keyword "e",

followed by a colon and the value "on" or "off".

  • The gain control parameter, encoded as the keyword "gc", followed

by a colon a value which can be either the keyword "auto" or a

    decimal number (positive or negative) representing the number of
    decibels of gain.
  • The silence suppression parameter, encoded as the keyword "s",

followed by a colon and the value "on" or "off".

  • The type of service parameter, encoded as the keyword "t",

followed by a colon and the value encoded as two hexadecimal

    digits.
  • The resource reservation parameter, encoded as the keyword "r",

followed by a colon and the value "g" (guaranteed service), "cl"

    (controlled load) or "be" (best effort).
  • The encryption key, encoded as the keyword "k" followed by a colon

and a key specification, as defined for the parameter "K" of SDP

    (RFC 2327).
  • The type of network, encoded as the keyword "nt" followed by a

colon and the type of network encoded as the keyword "IN", "ATM"

    or "LOCAL".
 Each of the parameters is optional. When several parameters are
 present, the values are separated by a comma.
 Examples of connection descriptors are:
           L: p:10, a:PCMU
           L: p:10, a:G726-32
           L: p:10-20, b:64
           L: b:32-64, e:off
 These set of attributes may be extended by extension attributes.

Arango, et al. Informational [Page 69] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 Extension attributes are composed of an attribute name, followed by a
 semi-colon and by an attribute value. The attribute name should start
 by the two characters "x+", for a mandatory extensions, or "x-", for
 a non mandatory extension.  If a gateway receives a mandatory
 extension attribute that it does not recognize, it should reject the
 command with an error code 525 (Unknown extension in
 LocalConnectionOptions).

3.2.2.3. Capabilities

 Capabilities inform the Call Agent about endpoints' capabilities when
 audited. The encoding of capabilities is based on the Local
 Connection Options encoding for the parameters that are common to
 both. In addition, capabilities can also contain a list of supported
 packages, and a list of supported modes.
 The parameters used are:

A list of supported codecs. The following parameters will apply to

    all codecs specified in this list.  If there is a need to specify
    that some parameters, such as e.g. silence suppression, are only
    compatible with some codecs, then the gateway will return several
    LocalConnectionOptions parameters, one for each set of codecs.
 Packetization Period:
    A range may be specified.
 Bandwidth:
    A range corresponding to the range for packetization periods may
    be specified (assuming no silence suppression). If absent, the
    values will be deduced from the codec type.
 Echo Cancellation:
    "on" if echo cancellation is supported for this codec, "off"
    otherwise. The default is support.
 Silence Suppression:
    "on" if silence suppression is supported for this codec, "off"
    otherwise. The default is support.
 Gain Control:
    "0" if gain control is not supported.  The default is support.
 Type of Service:
    The value "0" indicates no support for type of service, all other
    values indicate support for type of service. The default is
    support.

Arango, et al. Informational [Page 70] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 Resource Reservation:
    The parameter indicates the reservation services that are
    supported, in addition to best effort.  The value "g" is encoded
    when the gateway supports both the guaranteed and the controlled
    load service, "cl" when only the controlled load service is
    supported.  The default is "best effort."
 Encryption Key:
    Encoding any value indicates support for encryption.  Default is
    no support.
 Type of network:
    The keyword "nt", followed by a colon and a semicolon separated
    list of supported network types.  This parameter is optional.
 Event Packages
    The event packages supported by this endpoint encoded as the
    keyword "v", followed by a colon and a character string. If a list
    of values is specified, these values will be separated by a
    semicolon.  The first value specified will be the default package
    for that endpoint.
 Modes
    The modes supported by this endpoint encoded as the keyword "m",
    followed by a colon and a semicolon-separated list of supported
    connection modes for this endpoint.

3.2.2.4. Connection parameters

 Connection parameters are encoded as a string of type and value
 pairs, where the type is a either letter identifier of the parameter
 or an extension type, and the value a decimal integer. Types are
 separated from value by an `=' sign. Parameters are encoded from each
 other by a comma.

Arango, et al. Informational [Page 71] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 The connection parameter types are specified in the following table:
  __________________________________________________________________
 | Connection parameter|  Code|  Connection parameter              |
 | name                |      |  value                             |
 |_____________________|______|____________________________________|
 | Packets sent        |   PS |  The number of packets that        |
 |                     |      |  were sent on the connection.      |
 | Octets sent         |   OS |  The number of octets that         |
 |                     |      |  were sent on the connection.      |
 | Packets received    |   PR |  The number of packets that        |
 |                     |      |  were received on the connection.  |
 | Octets received     |   OR |  The number of octets that         |
 |                     |      |  were received on the connection.  |
 | Packets lost        |   PL |  The number of packets that        |
 |                     |      |  were not received on the          |
 |                     |      |  connection, as deduced from       |
 |                     |      |  gaps in the sequence number.      |
 | Jitter              |   JI |  The average inter-packet arrival  |
 |                     |      |  jitter, in milliseconds,          |
 |                     |      |  expressed as an integer number.   |
 | Latency             |   LA |  Average latency, in milliseconds, |
 |                     |      |  expressed as an integer number.   |
 |_____________________|______|____________________________________|
 Extension parameters names are composed of the string "X-" followed
 by a two letters extension parameter name.  Call agents that received
 unrecognized extensions shall silently ignore these extensions.
 An example of connection parameter encoding is:
       P: PS=1245, OS=62345, PR=0, OR=0, PL=0, JI=0, LA=48

3.2.2.5. Reason Codes

 Reason codes are three-digit numeric values. The reason code is
 optionally followed by a white space and commentary, e.g.:
    900 Endpoint malfunctioning
 A list of reason-codes can be found in Section 2.5.

Arango, et al. Informational [Page 72] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

3.2.2.6. Connection mode

 The connection mode describes the mode of operation of the
 connection.  The possible values are:
     ________________________________________________________
    | Mode       |  Meaning                                 |
    |____________|__________________________________________|
    | M: sendonly|  The gateway should only send packets    |
    | M: recvonly|  The gateway should only receive packets |
    | M: sendrecv|  The gateway should send                 |
    |            |  and receive packets                     |
    | M: confrnce|  The gateway should place                |
    |            |  the connection in conference mode       |
    | M: inactive|  The gateway should neither              |
    |            |  send nor receive packets                |
    | M: loopback|  The gateway should place                |
    |            |  the circuit in loopback mode.           |
    | M: conttest|  The gateway should place                |
    |            |  the circuit in test mode.               |
    | M: netwloop|  The gateway should place                |
    |            |  the connection in network loopback mode.|
    | M: netwtest|  The gateway should place                |
    |            |   the connection in network              |
    |            |   continuity test mode.                  |
    | M: data    |  The gateway should use the circuit      |
    |            |  for network access for data             |
    |            |  (e.g., PPP, SLIP, etc.).                |
    |____________|__________________________________________|

3.2.2.7. Coding of event names

 Event names are composed of an optional package name, separated by a
 slash (/) from the name of the actual event.  The event name can
 optionally be followed by an at sign (@) and the identifier of a
 connection on which the event should be observed. Event names are
 used in the RequestedEvents, SignalRequests and ObservedEvents
 parameter.
 Each signal has one of the following signal-types associated with:
 On/Off (OO), Time-out (TO), Brief (BR). (These signal types are
 specified in the package definitions, and are not present in the
 messages.)  On/Off signals can be parameterized with a "+" to turn
 the signal on, or a "-" to turn the signal off. If an on/off signal
 is not parameterized, the signal is turned on. Both of the following
 will turn the vmwi signal on:
    vmwi(+), vmwi

Arango, et al. Informational [Page 73] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 The following are valid examples of event names:
     ____________________________________________________________
    | L/hu        |   on-hook transition, in the line package   |
    | F/0         |   digit 0 in the MF package                 |
    | fh          |   Flash-hook, assuming that the line package|
    |             |   is a default package for the end point.   |
    | G/rt@0A3F58 |   Ring back signal on                       |
    |             |   connection "0A3F58".                      |
    |_____________|_____________________________________________|
 In addition, the range and wildcard notation of events can be used,
 instead of individual names, in the RequestedEvents and DetectEvents
 parameters. The star sign can be used to denote "all connections",
 and the dollar sign can be used to denote the "current" connection.
 The following are valid examples of such notations:
     __________________________________________________________
    | M/[0-9]   |   Digits 0 to 9 in the MF package           |
    | fh        |   Flash-hook, assuming that the line package|
    |           |   is a default package for the end point.   |
    | [0-9*#A-D]|   All digits and letters in the DTMF        |
    |           |   packages (default for endpoint).          |
    | T/$       |   All events in the trunk packages.         |
    | R/qa@*    |   The quality alert event in all            |
    |           |   connections                               |
    | R/rt@$    |   Ringback on current connection            |
    |___________|_____________________________________________|

3.2.2.8. RequestedEvents

 The RequestedEvent parameter provides the list of events that have
 been requested. The event codes are described in the previous
 section.
 Each event can be qualified by a requested action, or by a list of
 actions. The actions, when specified, are encoded as a list of
 keywords, enclosed in parenthesis and separated by commas. The codes
 for the various actions are:

Arango, et al. Informational [Page 74] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

              ______________________________________
             | Action                       |  Code|
             |______________________________|______|
             | Notify immediately           |  N   |
             | Accumulate                   |  A   |
             | Treat according to digit map |  D   |
             | Swap                         |  S   |
             | Ignore                       |  I   |
             | Keep Signal(s) active        |  K   |
             | Embedded Notification Request|  E   |
             |______________________________|______|
 When no action is specified, the default action is to notify the
 event.  This means that, for example, ft and ft(N) are equivalent.
 Events that are not listed are ignored.
 The digit-map action can only be specified for the digits, letters
 and interdigit timers in the MF and DTMF packages, or in other
 packages that would define the encoding of digits and timers.
 The requested list is encoded on a single line, with event/action
 groups separated by commas. Examples of RequestedEvents encoding are:
       R: hu(N), hf(S,N)
       R: hu(N), [0-9#T](D)
 In the case of the "enable" action, the embedded notification request
 parameters are encoded as a list of up to three parameter groups,
 separated by commas.  Each group start by a one letter identifier,
 followed by a list of parameters enclosed between parenthesis.  The
 first optional parameter group, identified by the letter "R", is the
 enabled value of the RequestedEvents parameter.  The second optional
 group, identified by the letter "S", is the enabled value of the
 SignalRequests parameter.  The third optional group, identified by
 the letter "D", is the enabled value of the DigitMap. (Note that some
 existing implementation may encode these three components in a
 different order.)
 If the RequestedEvents is not present, the parameter will be set to a
 null value.  If the SignalRequest is not present, the parameter will
 be set to a null value. If the DigitMap is absent, the current value
 should be used. The following are valid examples of embedded
 requests:
       R: hd(E(R([0-9#T](D),hu(N)),S(dl),D([0-9].[#T])))
       R: hd(E(R([0-9#T](D),hu(N)),S(dl)))

Arango, et al. Informational [Page 75] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

3.2.2.9. SignalRequests

 The SignalRequests parameter provides the name of the signals that
 have been requested. Each signal is identified by a name, as
 indicated in the previous section.
 Several signals, such as for example announcement or ADSI display,
 can be qualified by additional parameters:
  • the name and parameters of the announcement,
  • the string that should be displayed.
 These parameters will be encoded as a set of UTF8 character strings,
 spearated by comams and enclosed within parenthesis, as in:
    S: adsi("123456 Francois Gerard")
    S: ann(no-such-number, 1234567)
 When several signals are requested, their codes are separated by a
 comma, as in:
       S: asdi(123456 Your friend), rg

3.2.2.10. ObservedEvent

 The observed event parameters provides the list of events that have
 been observed. The event codes are the same as those used in the
 NotificationRequest. Events that have been accumulated according to
 the digit map may be grouped in a single string; they should be
 reported as lists of isolated events if other events where detected
 during the digit accumulation. Examples of observed actions are:
      O: L/hu
      O: 8295555T
      O: 8,2,9,5,5,L/hf,5,5,T
      O: L/hf, L/hf, L/hu

3.2.2.11. RequestedInfo

 The RequestedInfo parameter contains a comma separated list of
 parameter codes, as defined in the "Parameter lines" section.  For
 example, if one wants to audit the value of the NotifiedEntity,
 RequestIdentifier, RequestedEvents, SignalRequests, DigitMap,
 QuarantineHandling and DetectEvents parameters, The value of the
 RequestedInfo parameter will be:
       F:N,X,R,S,D,Q,T

Arango, et al. Informational [Page 76] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 The capabilities request, in the AuditEndPoint command, is encoded by
 the keyword "A", as in:
       F:A

3.2.2.12. QuarantineHandling

 The quarantine handling parameter contains a list of comma separated
 keywords:
  • The keyword "process" or "discard" to indicate the treatment of

quarantined events. If neither process or discard is present,

    process is assumed.
  • The keyword "step" or "loop" to indicate whether exactly at most

one notification is expected, or whether multiple notifications

    are allowed. If neither step or loop is present, step is assumed.
    The following values are valid examples:
             Q:loop
             Q:process
             Q:discard,loop

3.2.2.13. DetectEvents

 The DetectEvent parameter is encoded as a comma separated list of
 events, such as for example:
       T: hu,hd,hf,[0-9#*]
 It should be noted, that no actions can be associated with the
 events.

3.2.2.14. EventStates

 The EventStates parameter is encoded as a comma separated list of
 events, such as for example:
    ES: hu
 It should be noted, that no actions can be associated with the
 events.

Arango, et al. Informational [Page 77] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

3.2.2.15. RestartMethod

 The RestartMethod parameter is encoded as one of the keywords
 "graceful", "forced", "restart", "disconnected" or "cancel-graceful"
 as for example:
       RM:restart

3.2.2.16. Bearer Information

 The values of the bearer informations are encoded as a comma
 separated list of attributes, represented by an attribute name,
 separated by a colon from an attribute value.
 The only attribute that is defined is the "encoding" (code "e"),
 whose defined values are "A" (A-law) and "mu" (mu-law).
 An example of bearer information encoding is:
       B: e:mu

3.3. Format of response headers

 The response header is composed of a response line, optionally
 followed by headers that encode the response parameters.
 An example of response header could be:
       200 1203 OK
 The response line starts with the response code, which is a three
 digit numeric value. The code is followed by a white space, the
 transaction identifier, and an optional commentary preceded by a
 white space.
 The following table describe the parameters whose presence is
 mandatory or optional in a response header, as a function of the
 command that triggered the response. The letter M stands for
 mandatory, O for optional and F for forbidden.

Arango, et al. Informational [Page 78] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

  ___________________________________________________________________
 | Parameter name      |  EP|  CR|  MD|  DL|  RQ|  NT|  AU|  AU|  RS|
 |                     |  CF|  CX|  CX|  CX|  NT|  FY|  EP|  CX|  IP|
 |_____________________|____|____|____|____|____|____|____|____|____|
 | ResponseAck         |  F |  F |  F |  F |  F |  F |  F |  F |  F |
 | BearerInformation   |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | CallId              |  F |  F |  F |  F |  F |  F |  F |  O |  F |
 | ConnectionId        |  F |  O*|  F |  F |  F |  F |  F |  F |  F |
 | RequestIdentifier   |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | LocalConnection     |  F |  F |  F |  F |  F |  F |  O |  O |  F |
 | Options             |    |    |    |    |    |    |    |    |    |
 | Connection Mode     |  F |  F |  F |  F |  F |  F |  F |  O |  F |
 | RequestedEvents     |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | SignalRequests      |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | NotifiedEntity      |  F |  F |  F |  F |  F |  F |  F |  F |  O |
 | ReasonCode          |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | ObservedEvents      |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | DigitMap            |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | Connection          |  F |  F |  F |  O |  F |  F |  F |  O |  F |
 | Parameters          |    |    |    |    |    |    |    |    |    |
 | Specific Endpoint ID|  F |  O |  F |  F |  F |  F |  F |  F |  F |
 | RequestedInfo       |  F |  F |  F |  F |  F |  F |  F |  F |  F |
 | QuarantineHandling  |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | DetectEvents        |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | EventStates         |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | RestartMethod       |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | RestartDelay        |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | Capabilities        |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | SecondConnectionId  |  F |  O |  F |  F |  F |  F |  F |  F |  F |
 | SecondEndpointID    |  F |  O |  F |  F |  F |  F |  F |  F |  F |
 |_____________________|____|____|____|____|____|____|____|____|____|
 | LocalConnection     |  F |  M |  O |  F |  F |  F |  F |  O*|  F |
 | Descriptor          |    |    |    |    |    |    |    |    |    |
 | RemoteConnection    |  F |  F |  F |  F |  F |  F |  F |  O*|  F |
 | Descriptor          |    |    |    |    |    |    |    |    |    |
 |_____________________|____|____|____|____|____|____|____|____|____|
 In the case of a CreateConnection message, the response line is
 followed by a Connection-Id parameter. It may also be followed a
 Specific-Endpoint-Id parameter, if the creation request was sent to a
 wildcarded Endpoint-Id. The connection-Id parameter is marked as
 optional in the Table.  In fact, it is mandatory with all positive
 responses, when a connection was created, and forbidden when the
 response is negative, when no connection as created.
 In the case of a DeleteConnection message, the response line is
 followed by a Connection Parameters parameter, as defined in section
 3.2.2.2.

Arango, et al. Informational [Page 79] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

 A LocalConnectionDescriptor should be transmitted with a positive
 response (code 200) to a CreateConnection. It may be transmitted in
 response to a ModifyConnection command, if the modification resulted
 in a modification of the session parameters. The
 LocalConnectionDescriptor is encoded as a "session description," as
 defined in section 3.4. It is separated from the response header by
 an empty line.
 When several session descriptors are encoded in the same response,
 they are encoded one after each other, separated by an empty line.
 This is the case for example when the response to an audit connection
 request carries both a local session description and a remote session
 description, as in:
       200 1203 OK
       C: A3C47F21456789F0
       N: [128.96.41.12]
       L: p:10, a:PCMU;G726-32
       M: sendrecv
       P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27,LA=48
       v=0
       c=IN IP4 128.96.41.1
       m=audio 1296 RTP/AVP 0
       v=0
       c=IN IP4 128.96.63.25
       m=audio 1296 RTP/AVP 0 96
       a=rtpmap:96 G726-32/8000
 In this example, according to the SDP syntax, each description starts
 with a "version" line, (v=...).  The local description is always
 transmitted before the remote description. If a connection descriptor
 is requested, but it does not exist for the connection audited, that
 connection descriptor will appear with the SDP protocol version field
 only.

Arango, et al. Informational [Page 80] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

3.4. Formal syntax description of the protocol

 In this section, we provided a formal description of the protocol
 syntax, following the "Augmented BNF for Syntax Specifications"
 defined in RFC 2234.

MGCPMessage = MGCPCommand / MGCPResponse

MGCPCommand = MGCPCommandLine 0*(MGCPParameter) [EOL *SDPinformation]

MGCPCommandLine = MGCPVerb 1*(WSP) <transaction-id> 1*(WSP)

                      <endpointName> 1*(WSP) MGCPversion EOL

MGCPVerb = "EPCF" / "CRCX" / "MDCX" / "DLCX" / "RQNT"

       / "NTFY" / "AUEP" / "AUCX" / "RSIP" / extensionVerb

extensionVerb = "X" 3(ALPHA / DIGIT)

transaction-id = 1*9(DIGIT)

endpointName = localEndpointName "@" DomainName LocalEndpointName = LocalNamePart 0*("/" LocalNamePart) LocalNamePart = AnyName / AllName / NameString AnyName = "$" AllNames = "*" NameString = 1*(range-of-allowed-characters) DomainName = 1*256(ALPHA / DIGIT / "." / "-") ; as defined in RFC 821

MGCPversion = "MGCP" 1*(WSP) 1*(DIGIT) "." 1*(DIGIT)

            [1*(WSP) ProfileName]

ProfileName = 1*(range-of-allowed-characters)

MGCPParameter = ParameterValue EOL

ParameterValue = ("K" ":" 0*WSP <ResponseAck>) /

               ("B" ":" 0*WSP <BearerInformation>) /
               ("C" ":" 0*WSP <CallId>) /
               ("I" ":" 0*WSP <ConnectionId>) /
               ("N" ":" 0*WSP <NotifiedEntity>) /
               ("X" ":" 0*WSP <RequestIdentifier>) /
               ("L" ":" 0*WSP <LocalConnectionOptions>) /
               ("M" ":" 0*WSP <ConnectionMode>) /
               ("R" ":" 0*WSP <RequestedEvents>) /
               ("S" ":" 0*WSP <SignalRequests>) /
               ("D" ":" 0*WSP <DigitMap>) /
               ("O" ":" 0*WSP <ObservedEvents>) /
               ("P" ":" 0*WSP <ConnectionParameters>) /
               ("E" ":" 0*WSP <ReasonCode>) /

Arango, et al. Informational [Page 81] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999

               ("Z" ":" 0*WSP <SpecificEndpointID>) /
               ("Z2" ":" 0*WSP <SecondEndpointID>) /
               ("I2" ":" 0*WSP <SecondConnectionID>) /
               ("F" ":" 0*WSP <RequestedInfo>) /
               ("Q" ":" 0*WSP <QuarantineHandling>) /
               ("T" ":" 0*WSP <DetectEvents>) /
               ("RM" ":" 0*WSP <RestartMethod>) /
               ("RD" ":" 0*WSP <RestartDelay>) /
               ("A" ":" 0*WSP <Capabilities>) /
               ("ES" ":" 0*WSP <EventStates>) /
                   (extensionParameter ":" 0*WSP <parameterString>)

ResponseAck = confirmedTransactionIdRange

  • [ "," confirmedTransactionIdRange ]

confirmedTransactionIdRange = 1*9DIGIT [ "-" 1*9DIGIT ]

BearerInformation = BearerAttribute 0*("," 0*WSP BearerAttribute) BearerAttribute = ("e" ":" <BearerEncoding>) BearerEncoding = "A" / "mu"

CallId = 1*32(HEXDIG)

The audit request response may include a list of identifiers ConnectionId = 1*32(HEXDIG) 0*("," 1*32(HEXDIG)) SecondConnectionID = ConnectionId NotifiedEntity = [LocalName "@"] DomainName [":" portNumber] LocalName = 1*32(suitableCharacter) portNumber = 1*5(DIGIT) RequestIdentifier = 1*32(HEXDIG) LocalConnectionOptions = [ LocalOptionValue 0*(WSP) 0*("," 0*(WSP) LocalOptionValue 0*(WSP)) ] LocalOptionValue = ("p" ":" <packetizationPeriod> ) / ("a" ":" <compressionAlgorithm> ) / ("b" ":" <bandwidth> ) / ("e" ":" <echoCancellation> ) / ("gc" ":" <gainControl> ) / ("s" ":" <silenceSuppression> ) / ("t" ":" <typeOfService> ) / ("r" ":" <resourceReservation> ) / ("k" ":" <encryptionmethod>[":"<encryptionKey>]) / ("nt" ":" <typeOfNetwork> ) / (localOptionExtensionName ":" / localOptionExtensionValue) Arango, et al. Informational [Page 82] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 Capabilities = [ CapabilityValue 0*(WSP) 0*("," 0*(WSP) CapabilityValue 0*(WSP)) ] CapabilityValue = LocalOptionValue / ("v" ":" <supportedPackages>) / ("m" ":" <supportedModes> ) packetizationPeriod = 1*4(DIGIT)["-" 1*4(DIGIT)] compressionAlgorithm = algorithmName 0*(";" algorithmName) algorithmName = 1*32(SuitableCharacter) bandwidth = 1*4(DIGIT)["-" 1*4(DIGIT)] echoCancellation = "on" / "off" gainControl = "auto" / ["-"]1*4(DIGIT) silenceSuppression = "on" / "off" typeOfService = 2HEXDIG resourceReservation = "g" / "cl" / "be" ;encryption parameters are coded as in SDP (RFC 2327) encryptiondata = ( "clear" ":" <encryptionKey> ) / ( "base64" ":" <encodedEncryptionKey> ) / ( "uri" ":" <URItoObtainKey> ) / ( "prompt" ) ; defined in SDP, not usable in MGCP! encryptionKey = 1*(SuitableCharacter / SP) encodedEncryptionKey = 1*(ALPHA / DIGIT / "+" / "/" / "=") URItoObtainKey = 1*(SuitableCharacter) / quotedString typeOfNetwork = "IN" / "ATM" / "LOCAL" supportedModes= ConnectionMode 0*(";" ConnectionMode) supportedPackages = packageName 0*(";" packageName) localOptionExtensionName = "x" ("+"/"-") 1*32(SuitableCharacter) localOptionExtensionValue = 1*32(SuitableCharacter) / quotedString ConnectionMode = "sendonly" / "recvonly" / "sendrecv" / "confrnce" / "inactive" / "loopback" / "conttest" / "netwloop" / "netwtest" / "data" RequestedEvents = [requestedEvent 0*("," 0*(WSP) requestedEvent)] requestedEvent = eventName [ "(" requestedActions ")" ] eventName = [ (packageName / "*") "/" ] (eventId / "all" / eventRange) [ "@" (ConnectionId / "$" / "*") ] packageName = 1*(ALPHA / DIGIT / HYPHEN) eventId = 1*(SuitableCharacter) eventRange = "[" 1*(DIGIT / DTMFLetter / "*" / "#" / Arango, et al. Informational [Page 83] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 (DIGIT "-" DIGIT)/(DTMFLetter "-" DTMFLetter)) "]" requestedActions = requestedAction 0*("," 0*(WSP) requestedAction) requestedAction = "N" / "A" / "D" / "S" / "I" / "K" / "E" "(" EmbeddedRequest ")" EmbeddedRequest = ( "R" "(" EmbeddedRequestList ")" ["," "S" "(" EmbeddedSignalRequest ")" ] ["," "D" "(" EmbeddedDigitMap ")" ] ) / ( "S" "(" EmbeddedSignalRequest ")" ["," "D" "(" EmbeddedDigitMap ")" ] ) / ( "D" "(" EmbeddedDigitMap ")" ) EmbeddedRequestList = RequestedEvents EmbeddedSignalRequest = SignalRequests EmbeddedDigitMap = DigitMap SignalRequests = [ SignalRequest 0*("," 0*(WSP) SignalRequest ) ] SignalRequest = eventName [ "(" eventParameters ")" ] eventParameters = eventParameter 0*("," 0*(WSP) eventParameter) eventParameter = eventParameterString / quotedString eventParameterString = 1*(SuitableCharacter) DigitMap = DigitString / "(" DigitStringList ")" DigitStringList = DigitString 0*( "|" DigitString ) DigitString = 1*(DigitStringElement) DigitStringElement = DigitPosition ["."] DigitPosition = DigitMapLetter / DigitMapRange DigitMapLetter = DIGIT / "#" / "*" / "A" / "B" / "C" / "D" / "T" DigitMapRange = "x" / "[" 1*DigitLetter "]" DigitLetter ::= *1)

1)
DIGIT "-" DIGIT ) / DigitMapLetter) ObservedEvents = SignalRequests EventStates = SignalRequests ConnectionParameters = [ConnectionParameter
                      0*( "," 0*(WSP) ConnectionParameter )
ConnectionParameter = ( "PS" "=" packetsSent )
                  / ( "OS" "=" octetsSent )
                  / ( "PR" "=" packetsReceived )
                  / ( "OR" "=" octetsReceived )
                  / ( "PL" "=" packetsLost )
                  / ( "JI" "=" jitter )
                  / ( "LA" "=" averageLatency )
                  / ( ConnectionParameterExtensionName "="
                      ConnectionParameterExtensionValue )
packetsSent = 1*9(DIGIT) Arango, et al. Informational [Page 84] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 octetsSent = 1*9(DIGIT) packetsReceived = 1*9(DIGIT) octetsReceived = 1*9(DIGIT) packetsLost = 1*9(DIGIT) jitter = 1*9(DIGIT) averageLatency = 1*9(DIGIT) ConnectionParameterExtensionName = "X" "-" 2*ALPHA ConnectionParameterExtensionValue = 1*9(DIGIT) ReasonCode = 3DIGIT [SPACE 1*(%x20-7E)] SpecificEndpointID = endpointName SecondEndpointID = endpointName RequestedInfo = [infoCode 0*("," infoCode)] infoCode = "B" / "C" / "I" / "N" / "X" / "L" / "M" /
         "R" / "S" / "D" / "O" / "P" / "E" / "Z" /
         "Q" / "T" / "RC" / "LC" / "A" / "ES" / "RM" / "RD"
QuarantineHandling = loopControl / processControl /
            (loopControl "," processControl )
loopControl = "step" / "loop" processControl = "process" / "discard" DetectEvents = [eventName 0*("," eventName)] RestartMethod = "graceful" / "forced" / "restart" / "disconnected" RestartDelay = 1*6(DIGIT) extensionParameter = "X" ("-"/"+") 1*6(ALPHA / DIGIT) parameterString = 1*(%x20-7F) MGCPResponse = MGCPResponseLine 0*(MGCPParameter)
              [EOL *SDPinformation]
MGCPResponseLine = (<responseCode> 1*(WSP) <transaction-id>
                        [1*(WSP) <responseString>] EOL)
responseCode = 3DIGIT responseString = *(%x20-7E) SuitableCharacter= DIGIT / ALPHA / "+" / "-" / "_" / "&" /
                 "!" / "'" / "|" / "=" / "#" / "?" / "/" /
                 "." / "$" / "*" / ";" / "@" / "[" / "]" /
                 "^" / "`" / "{" / "}" / "~"
quotedString = DQUOTE visibleString Arango, et al. Informational [Page 85] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
               0*(quoteEscape visibleString) DQUOTE
quoteEscape = DQUOTE DQUOTE visibleString = (%x00-21 / %x23-FF) EOL = CRLF / LF SDPinformation = ;See RFC 2327 3.5. Encoding of the session description
 The session description is encoded in conformance with the session
 description protocol, SDP. MGCP implementations are expected to be
 fully capable of parsing any conformant SDP message, and should send
 session descriptions that strictly conform to the SDP standard. The
 usage of SDP actually depends on the type of session that is being,
 as specified in the "mode" parameter:
  • if the mode is set to "data", the session description describes
the configuration of a data access service.
  • if the mode is set to any other value, the session description is
for an audio service.
 For an audio service, the gateway will consider the information
 provided in SDP for the "audio" media. For a data service, the
 gateway will consider the information provided for the "network-
 access" media.
3.5.1. Usage of SDP for an audio service
 In a telephony gateway, we only have to describe sessions that use
 exactly one media, audio. The parameters of SDP that are relevant for
 the telephony application are:
    At the session description level:
  • The IP address of the remote gateway (in commands) or of the
local gateway (in responses), or multicast address of the audio
       conference, encoded as an SDP "connection data" parameter. This
       parameter specifies the IP address that will be used to
       exchange RTP packets.
    For the audio media:
  • Media description field (m) specifying the audio media, the
transport port used for receiving RTP packets by the remote
       gateway (commands) or by the local gateway (responses), the
Arango, et al. Informational [Page 86] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
       RTP/AVP transport, and the list of formats that the gateway
       will accept. This list should normally always include the code
       0 (reserved for PCMU).
  • Optionally, RTPMAP attributes that define the encoding of
dynamic audio formats,
  • Optionally, a packetization period (packet time) attribute
(Ptime) defining the duration of the packet,
  • Optionally, an attribute defining the type of connection
(sendonly, recvonly, sendrecv, inactive). Note that this
       attribute does not have a direct relation with the "Mode"
       parameter of MGCP.  In fact, the SDP type of connection will
       most of the time be set to "sendrecv", regardless of the value
       used by MGCP.  Other values will only be used rarely, for
       example in the case of information or announcement servers that
       need to establish one way connections.
  • The IP address of the remote gateway (in commands) or of the
local gateway (in responses), if it is not present at the
       session level.
    An example of SDP specification for an audio connection could be:
          v=0
          c=IN IP4 128.96.41.1
          m=audio 3456 RTP/AVP 0 96
          a=rtpmap:96 G726-32/8000
 There is a request, in some environments, to use the MGCP to
 negotiate connections that will use other transmission channels than
 RTP over UDP and IP. This will be detailed in an extension to this
 document.
3.5.2. Usage of SDP in a network access service
 The parameters of SDP that are relevant for a data network access
 application are:
    For the data media:
  • Media description field (m) specifying the network access
media, identified by the code "m=nas/xxxx", where "xxxx"
       describes the access control method that should be used for
       parametrizing the network access, as specified below. The field
       may also specify the port that should be used for contacting
       the server, as specified in the SDP syntax.
Arango, et al. Informational [Page 87] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
  • Connection address parameter (c=) specifying the address, or
the domain name, of the server that implement the access
       control method. This parameter may also be specified at the
       session level.
  • Optionally, a bearer type attribute (a=bearer:) describing the
type of data connection to be used, including the modem type.
  • Optionally, a framing type attribue (a=framing:) describing the
type of framing that will be used on the channel.
  • Optionally, attributes describing the called number
(a=dialed:), the number to which the call was delivered
       (a=called:) and the calling number (a=dialing:).
  • Optionally, attributes describing the range of addresses that
could be used by the dialup client on its LAN (a=subnet:).
  • Optionally, an encryption key, encoded as specified in the SDP
protocol(k=).
 The connection address shall be encoded as specified in the SDP
 standard. It will be used in conjunction with the port specified in
 the media line to access a server, whose type will one of:
     __________________________________________________________
    | Method name|  Method description                        |
    |____________|____________________________________________|
    | radius     |  Authentication according                  |
    |            |  to the Radius protocol.                   |
    | tacacs     |  Authentication according                  |
    |            |  to the TACACS+ protocol.                  |
    | diameter   |  Authentication according                  |
    |            |  to the Diameter protocol.                 |
    | l2tp       |  Level 2 tunneling protocol.               |
    |            |  The address and port are those of the LNS.|
    | login      |  Local login. (There is normally           |
    |            |  no server for that method.)               |
    | none       |  No authentication required.               |
    |            |  (The call was probably vetted             |
    |            |  by the Call Agent.)                       |
    |____________|____________________________________________|
 If needed, the gateway may use the key specified in the announcement
 to access the service. That key, in particular, may be used for the
 establishment of an L2TP tunnel.
Arango, et al. Informational [Page 88] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 The bearer attribute is composed of a bearer name and an optional
 extension.  The bearer type specifies the type of modulation (modem
 name) or, in the case of digital connections, the type of ISDN
 service (8 bits, 7 bits).  When an extension is present, it is
 separated from the bearer name by a single slash (/).  The valid
 values of the bearer attribute are defined in the following table:
  ____________________________________________________________________
 | Type of bearer description      |  Example of values              |
 |_________________________________|_________________________________|
 | ITU modem standard              |  V.32, V.34, V.90.              |
 | ITU modem standard qualified    |  v.90/3com,                     |
 | by a manufacturer name          |  v.90/rockwell,                 |
 |                                 |  v.90/xxx                       |
 | Well known modem types          |  X2, K56flex                    |
 | ISDN transparent access, 64 kbps|  ISDN64                         |
 | ISDN64 + V.110                  |  ISDN64/V.110                   |
 | ISDN64 + V.120                  |  ISDN64/V.120                   |
 | ISDN transparent access, 56 kbps|  ISDN56                         |
 | Informal identification         |  (Requires coordination between |
 |                                 |  the Call Agent and the gateway)|
 |_________________________________|_________________________________|
 The valid values of the framing attribute are defined in the
 following table:
           _________________________________________________
          | Type of framing description|  Example of values|
          |____________________________|___________________|
          | PPP, asynchronous framing  |  ppp-asynch       |
          | PPP, HDLC framing          |  ppp-hdlc         |
          | SLIP, asynchronous         |  slip             |
          | Asynchronous, no framing   |  asynch           |
          |____________________________|___________________|
 The network access authentication parameter provides instructions on
 the access control that should be exercized for the data call. This
 optional attribute is encoded as:
      "a=subnet:" <network type> <address type>
         <connection address> "/" <prefix length>
 Where the parameters "network type", "address type", and "connection
 address" are formatted as defined for the connection address
 parameter (c=) in SDP, and where the "prefix length" is a decimal
 representation of the number of bits in the prefix.
Arango, et al. Informational [Page 89] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 Examples of SDP announcement for the network access service could be:
       v=0
       m=nas/radius
       c=IN IP4 radius.example.net
       a=bearer:v.34
       a=framing:ppp-asynch
       a=dialed:18001234567
       a=called:12345678901
       a=dialing:12340567890
       v=0
       m=nas/none
       c=IN IP4 128.96.41.1
       a=subnet:IN IP4 123.45.67.64/26
       a=bearer:isdn64
       a=framing:ppp-sync
       a=dialed:18001234567
       a=dialing:2345678901
       v=0
       c=IN IP4 access.example.net
       m=nas/l2tp
       k=clear:some-shared-secret
       a=bearer:v.32
       a=framing:ppp-asynch
       a=dialed:18001234567
       a=dialing:2345678901
3.5.3. Usage of SDP for ATM connections
 The specification of the SDP payload for ATM connections will be
 described in a companion document, "Usage of MGCP to control Voice
 over ATM gateways." The following text is indicative.
 The SDP payload will specify:
  • That the connection is to be established over an ATM interface,
using the "c=" parameter of SDP to specify an address in the ATM
    family, the ATM addressing variant (NSAP, UNI, E.164) and the ATM
    address.
  • The "m=audio" parameter will specify the audio encoding and, if
needed, the VPI and VCI.
  • Additional attributes parameters (a=) will be used to specify the
ATM coding variants, such as the type of adaptation layer and the
    error correction or loss compenmsation algorithms.
Arango, et al. Informational [Page 90] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 An example of SDP payload for an ATM connection could be:
       v=0 c=ATM NSAP
       47.0091.8100.0000.0060.3e64.fd01.0060.3e64.fd01.fe m=audio
       5/1002 ATM/AVP PCMU a=connection_type:AAL2
3.5.4. Usage of SDP for local connections
 When MGCP is used to set up internal connections within a single
 gateway, the SDP format is used to encode the parameters of that
 connection.  The following parameters will be used:
  • The connection parameter (C=) will specify that the connection is
local, using the keyword "LOCAL" as network type space, the
    keyword "EPN" (endpoint name) as  address type, and the name of
    the endpoint as the connection-address.
  • The "m=audio" parameter will specify a port number, which will
always be set to 0, the type of protocol, always set to the
    keyword LOCAL, and the type of encoding, using the same
    conventions used for RTP (RTP payload numbers.) The type of
    encoding should normally be set to 0 (PCMU).
 An example of local SDP payload could be:
       v=0
       c=LOCAL EPN X35V3+A4/13
       m=audio 0 LOCAL 0
3.6. Transmission over UDP
 MGCP messages are transmitted over UDP. Commands are sent to one of
 the IP addresses defined in the DNS for the specified endpoint . The
 responses are sent back to the source address of the commands.
 When no port is specified for the endpoint, the commands should be
 sent:
  • by the Call Agents, to the default MGCP port for gateways, 2427.
  • by the Gateways, to the default MGCP port for Call Agents, 2727.
3.6.1. Providing the At-Most-Once functionality
 MGCP messages, being carried over UDP, may be subject to losses. In
 the absence of a timely response, commands are repeated. Most MGCP
 commands are not idempotent.  The state of the gateway would become
Arango, et al. Informational [Page 91] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 unpredictable if, for example, CreateConnection commands were
 executed several times.  The transmission procedures must thus
 provide an "At-Most-Once" functionality.
 MGCP entities are expected to keep in memory a list of the responses
 that they sent to recent transactions and a list of the transactions
 that are currently being executed. The transaction identifiers of
 incoming commands are compared to the transaction identifiers of the
 recent responses. If a match is found, the MGCP entity does not
 execute the transaction, but simply repeats the response. The
 remaining commands will be compared to the list of current
 transaction. If a match is found, the MGCP entity does not execute
 the transaction, which is simply ignored.
 The procedure use a long timer value, noted LONG-TIMER in the
 following.  The timer should be set larger than the maximum duration
 of a transaction, which should take into account the maximum number
 of repetitions, the maximum value of the repetition timer and the
 maximum propagation delay of a packet in the network.  A suggested
 value is 30 seconds.
 The copy of the responses can be destroyed either LONG-TIMER seconds
 after the response is issued, or when the gateway (or the call agent)
 receives a confirmation that the response has been received, through
 the "Response Acknowledgement attribute". For transactions that are
 acknowledged through this attribute, the gateway shall keep a copy of
 the transaction-id for LONG-TIMER seconds after the response is
 issued, in order to detect and ignore duplicate copies of the
 transaction request that could be produced by the network.
3.6.2. Transaction identifiers and three ways handshake
 Transaction identifiers are integer numbers in the range from 0 to
 999,999,999.  Call-agents may decide to use a specific number space
 for each of the gateways that they manage, or to use the same number
 space for all gateways that belong to some arbitrary group.  Call
 agents may decide to share the load of managing a large gateway
 between several independent processes.  These processes will share
 the same transaction number space.  There are multiple possible
 implementations of this sharing, such as having a centralized
 allocation of transaction identifiers, or pre-allocating non-
 overlapping ranges of identifiers to different processes.  The
 implementations must guarantee that unique transaction identifiers
 are allocated to all transactions that originate from a logical call
 agent, as defined in the "states, failover and race conditions"
 section. Gateways can simply detect duplicate transactions by looking
 at the transaction identifier only.
Arango, et al. Informational [Page 92] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 The Response Acknowledgement Attribute can be found in any command.
 It carries a set of "confirmed transaction-id ranges."
 MGCP gateways may choose to delete the copies of the responses to
 transactions whose id is included in "confirmed transaction-id
 ranges" received in the Response Confirmation messages. They should
 silently discard further commands from that Call Agent when the
 transaction-id falls within these ranges.
 The "confirmed transaction-id ranges" values shall not be used if
 more than LONG-TIMER seconds have elapsed since the gateway issued
 its last response to that call agent, or when a gateway resumes
 operation.  In this situation, commands should be accepted and
 processed, without any test on the transaction-id.
 Commands that carry the "Response Acknowledgement attribute" may be
 transmitted in disorder.  The gateway shall retain the union of the
 "confirmed transaction-id ranges" received in recent commands.
3.6.3. Computing retransmission timers
 It is the responsibility of the requesting entity to provide suitable
 time outs for all outstanding commands, and to retry commands when
 time outs have been exceeded. Furthermore, when repeated commands
 fail to be acknowledged, it is the responsibility of the requesting
 entity to seek redundant services and/or clear existing or pending
 connections.
 The specification purposely avoids specifying any value for the
 retransmission timers. These values are typically network dependent.
 The retransmission timers should normally estimate the timer by
 measuring the time spent between the sending of a command and the
 return of a response. One possibility is to use the algorithm
 implemented in TCP-IP, which uses two variables:
  • the average acknowledgement delay, AAD, estimated through an
exponentially smoothed average of the observed delays,
  • the average deviation, ADEV, estimated through an exponentially
smoothed average of the absolute value of the difference between
    the observed delay and the current average
 The retransmission timer, in TCP, is set to the sum of the average
 delay plus N times the average deviation. In MGCP, the maximum value
 of the timer should however be bounded, in order to guarantee that no
 repeated packet will be received by the gateways after LONG-TIMER
 seconds.  A suggested maximum value is 4 seconds.
Arango, et al. Informational [Page 93] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 After any retransmission, the MGCP entity should do the following:
  • It should double the estimated value of the average delay, AAD
  • It should compute a random value, uniformly distributed between
0.5 AAD and AAD
  • It should set the retransmission timer to the sum of that random
value and N times the average deviation.
 This procedure has two effects. Because it includes an exponentially
 increasing component, it will automatically slow down the stream of
 messages in case of congestion. Because it includes a random
 component, it will break the potential synchronization between
 notifications triggered by the same external event.
3.6.4. Piggy backing
 There are cases when a Call Agent will want to send several messages
 at the same time to the same gateways.  When several MGCP messages
 have to be sent in the same UDP packets, they should be separated by
 a line of text that contain a single dot, as in for example:
       200 2005 OK
       DLCX 1244 card23/21@trgw-7.example.net MGCP 1.0
       C: A3C47F21456789F0
       I: FDE234C8
 The piggy-backed messages should be processed exactly has if they had
 been received in several simultaneous messages.
3.6.5. Provisional responses
 Executing some transactions may require a long time. Long execution
 times may interact with the timer based retransmission procedure.
 This may result either in an inordinate number of retransmissions, or
 in timer values that become too long to be efficient.
 Gateways that can predict that a transaction will require a long
 execution time may send a provisional response, with response code
 100.  They should send this response if they receive a repetition of
 a transaction that is still being executed.
 MGCP entities that receive a provisional response shall switch to a
 longer repetition timer for that transaction.
Arango, et al. Informational [Page 94] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 4. States, failover and race conditions.
 In order to implement proper call signalling, the Call Agent must
 keep track of the state of the endpoint, and the gateway must make
 sure that events are properly notified to the call agent.  Special
 conditions exist when the gateway or the call agent are restarted:
 the gateway must be redirected to a new call agent during "failover"
 procedures, the call agent must take special action when the gateway
 is taken offline, or restarted.
4.1. Basic Asumptions
 The support of "failover" is based on the following assumptions:
  • Call Agents are identified by their domain name, not their network
addresses, and several addresses can be associated with a domain
    name.
  • An endpoint has one NotifiedEntity associated with it any given
point in time.
  • The NotifiedEntity is the last value of the "NotifiedEntity"
parameter received for this endpoint (including wild-carded end-
    point-names). If no explicit "NotifiedEntity" parameter has been
    received, the "NotifiedEntity" defaults to the provisioned
    NotifiedEntity value, or if no value was provisioned to the source
    address of the last command received for the endpoint,
  • Responses to commands are always sent to the source address of the
command, regardless of the NotifiedEntity.
  • When the "notified entity" refers to a domain name that resolves
to multiple IP- address, endpoints are capable of switching
    between different interfaces on the same  logical call agent,
    however they cannot switch to other (backup) call agent(s) on
    their own. A backup call agent can however instruct them to
    switch, either directly or indirectly.
  • If an entire call agent becomes unavailable, the endpoints managed
by that call agent will eventually become "disconnected". The only
    way for these endpoints to become connected again is either for
    the failed call agent to become available, or for a backup call
    agent to contact the affected endpoints.
  • When a backup call agent has taken over control of a group of
endpoints, it is assumed that the failed call agent will
    communicate and synchronize with the backup call agent in order to
Arango, et al. Informational [Page 95] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
    transfer control of the affected endpoints back to the original
    call agent (if that's even desired - maybe the failed call agent
    should simply become the backup call agent now).
 We should note that handover conflict resolution between separate
 CA's is not in place - we are relying strictly on the CA's knowing
 what they are doing and communicating with each other (although
 AuditEndpoint can be used to learn about the current NotifiedEntity).
4.2. Security, Retransmission, and Detection of Lost Associations:
 The media gateway control protocol is organized as a set of
 transactions, each of which is composed of a command and a response,
 commonly referred to as an acknowledgement.  The MGCP messages, being
 carried over UDP, may be subject to losses. In the absence of a
 timely response, commands are repeated. MGCP entities are expected to
 keep in memory a list of the responses that they sent to recent
 transactions, i.e. a list of all the responses they sent over the
 last LONG-TIMER seconds, and a list of the transactions that are
 currently being executed.
 The transaction identifiers of incoming commands are compared to the
 transaction identifiers of the recent responses. If a match is found,
 the MGCP entity does not execute the transaction, but simply repeats
 the response. The remaining commands will be compared to the list of
 current transaction. If a match is found, the MGCP entity does not
 execute the transaction, which is simply ignored - a response will be
 provided when the execution of the command is complete.
 The repetition mechanism is used to guard against four types of
 possible errors:
  • transmission errors, when for example a packet is lost due to
noise on a line or congestion in a queue,
  • component failure, when for example an interface to a call agent
becomes unavailable,
  • call agent failure, when for example an entire call agent becomes
unavailable,
  • failover, when a new call agent is "taking over" transparently.
 The elements should be able to derive from the past history an
 estimate of the packet loss rate due to transmission errors.  In a
 properly configured system, this loss rate should be kept very low,
 typically less than 1%.  If a call agent or a gateway has to repeat a
 message more than a few times, it is very legitimate to assume that
Arango, et al. Informational [Page 96] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 something else than a transmission error is occurring.  For example,
 given a loss rate of 1%, the probability that 5 consecutive
 transmission attempts fail is 1 in 100 billion, an event that should
 occur less than once every 10 days for a call agent that processes
 1,000 transactions per second. (Indeed, the number of repetition that
 is considered excessive should be a function of the prevailing packet
 loss rate.) We should note that the "suspicion threshold", which we
 will call "Max1", is normally lower than the "disconnection
 threshold", which should be set to a larger value.
    Command issued: N=0
            |
     transmission: N++
            |  +------------ retransmission: N++ -----------+
            |  |                                            |
            |  |       transmission                         |
            |  |  +---to new address -+<--------------------|--+
            |  |  |        N=0        |                     |  |
            V  V  V                   |                     |  |
      +-----------+                   |                     |  |
      | awaiting  |- new call agent ->+  +------------+     |  |
      |  response |--- timer elapsed --->| N > Max1 ? |-(no)+  |
      +-----------+ <----------+         +------------+     ^  |
            |   |              |               |            |  |
            |   +- wrong key? -+             (yes)          |  |
            |                                  |            |  |
    response received                    (if N=Max1,        |  |
            |                             or N=Max2         |  |
            |                             check DNS)        |  |
            v                                  |            |  |
          (end)                       +---------------+     |  |
                                      |more addresses?|(yes)|--+
                                      +---------------+     |
                                               |            |
                                             (no)           |
                                               |            |
                                         +------------+     |
                                         | N > Max2 ? |(no)-+
                                         +------------+
                                               |
                                             (yes)
                                               |
                                               v
                                        (disconnected)
 A classic retransmission algorithm would simply count the number of
 successive repetitions, and conclude that the association is broken
 after re-transmitting the packet an excessive number of times
Arango, et al. Informational [Page 97] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 (typically between 7 and 11 times.) In order to account for the
 possibility of an undetected or in-progress "failover", we modify the
 classic algorithm as follows:
  • We request that the gateway always checks for the presence of a
new call agent. It can be noticed either by
  1. receiving a valid multicast message announcing a failover, or
  1. receiving a command where the NotifiedEntity points to the new
call agent, or
  1. receiving a redirection response pointing to a new Call Agent.
    If a new call agent is detected, the gateway starts transmitting
    outstanding commands to that new agent.  Responses to commands are
    still transmitted to the source address of the command.
  • we request that if the number of repetitions for this Call Agent
is larger than "Max1", that the gateway actively queries the name
    server in order to detect the possible change of the call agent
    interfaces.
  • The gateway may have learned several IP addresses for the call
agent. If the number of repetitions is larger than "Max1" and
    lower than "Max2", and there are more interfaces that have not
    been tried, then the gateway should direct the retransmissions to
    alternate addresses.
  • If there are no more interfaces to try, and the number of
repetitions is Max2, then the gateway contacts the DNS one more
    time to see if any other interface should have become available.
    If not, the gateway is now disconnected.
 The procedure will maximize the chances of detecting an ongoing
 failover. It poses indeed two very specific problems, the potentially
 long delays of a timer based procedure and the risk of confusion
 caused by the use of cryptographic protections.
 In order to automatically adapt to network load, MGCP specifies
 exponentially increasing timers.  If the initial timer is set to 200
 milliseconds, the loss of a fifth retransmission will be detected
 after about 6 seconds.  This is probably an acceptable waiting delay
 to detect a failover.   The repetitions should continue after that
 delay not only in order to perhaps overcome a transient connectivity
 problem, but also in order to allow some more time for the execution
 of a failover - waiting a total delay of 30 seconds is probably
 acceptable.
Arango, et al. Informational [Page 98] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 It is however important that the maximum delay of retransmissions be
 bounded.  Prior to any retransmission, it is checked that the time
 elapsed since the sending of the initial datagram is no greater than
 T- MAX. If more than T-MAX time has elapsed, the endpoint becomes
 disconnected. The value T-MAX is related to the LONG-TIMER value: the
 LONG-TIMER value is obtained by adding to T-MAX the maximum
 propagation delay in the network.
 Another potential cause of connection failure would be the reception
 of a "wrong key" message, sent by a call agent that could not
 authenticate the command, presumably because it had lost the security
 parameters of the association.  Such messages are actually not
 authorized in IPSEC, and they should in fact not be taken at face
 value: an attacker could easily forge "wrong key" messages in order
 to precipitate the loss of a control connection.  The current
 algorithm ignores these messages, which translates into a strict
 reliance on timers.  The algorithm could in fact be improved, maybe
 by executing a check with the key server of the call agent after
 "Max1" repetitions.
4.3. Race conditions
 MGCP deals with race conditions through the notion of a "quarantine
 list" and through explicit detection of desynchronization.
 MGCP does not assume that the transport mechanism will maintain the
 order of command and responses.  This may cause race conditions, that
 may be obviated through a proper behavior of the call agent. (Note
 that some race conditions are inherent to distributed systems; they
 would still occur, even if the commands were transmitted in strict
 order.)
 In some cases, many gateways may decide to restart operation at the
 same time.  This may occur, for example, if an area loses power or
 transmission capability during an earthquake or an ice storm.  When
 power and transmission are reestablished, many gateways may decide to
 send "RestartInProgress" commands simultaneously, leading to very
 unstable operation.
4.3.1. Quarantine list
 MGCP controlled gateways will receive "notification requests" that
 ask them to watch for a list of "events."  The protocol elements that
 determine the handling of these events are the "Requested Events"
 list, the "Digit Map" and the "Detect Events" list.
Arango, et al. Informational [Page 99] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 When the endpoint is initialized, the requested events list and the
 digit map are empty.  After reception of a command, the gateway
 starts observing the endpoint for occurrences of the events mentioned
 in the list.
 The events are examined as they occur. The action that follows is
 determined by the "action" parameter associated to the event in the
 list of requested events, and also by the digit map.  The events that
 are defined as "accumulate" or "treat according to digit map" are
 accumulated in a list of events, the events that are marked as
 "treated according to the digit map" will additionally be accumulated
 in the dialed string. This will go on until one event is encountered
 that triggers a Notification to the "notified entity."
 The gateway, at this point, will transmit the notification command
 and will place the endpoint in a "notification" state. As long as the
 endpoint is in this notification state, the events that are to be
 detected on the endpoint are stored in a "quarantine" buffer for
 later processing.  The events are, in a sense, "quarantined." All
 events that are specified by the union of the RequestedEvents
 parameter and the most recently received DetectEvent parameter or, in
 the absence of the latter, all events that are referred to in the
 RequestedEvents, should be detected and quarantined, regardless of
 the action associated to the event.
 The endpoint exits the "notification state" when the acknowledgement
 of the Notify  command is received. The Notify command may be
 retransmitted in the "notification state", as specified in section
 3.5. When the endpoint exits the "notification state" it resets the
 list of observed events and the "current dial string" of the endpoint
 to a null value.
 Following that point, the behavior of the gateway depends on the
 value of The QuarantineHandling parameter in the notification
 request.  If the Call Agent specified that it expected at most one
 notification in response to the notification request command, then
 the gateway should simply keep on accumulating events in the
 quarantine list until it receives the next notification request
 command.
 If the gateway is authorized to send multiple successive Notify
 commands, it will proceed as follows.  When the gateway exits the
 "notification state", it resets the list of observed events and the
 "current dial string" of the endpoint to a null value and starts
 processing the list of quarantined events, using the already received
 list of requested events and digit map. When processing these events,
Arango, et al. Informational [Page 100] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 the gateway may encounter an event which requires a Notify command to
 be sent. If that is the case, the gateway can adopt one of the two
 following behaviors:
  • it can immediately transmit a Notify command that will report all
events that were accumulated in the list of observed events until
    the triggering event, included, leaving the unprocessed events in
    the quarantine list,
  • or it can attempt to empty the quarantined list and transmit a
single Notify command reporting several sets of events and
    possibly several dial strings. The dial string is reset to a null
    value after each triggering event. The events that follow the last
    triggering event are left in the quarantine list.
 If the gateway transmits a Notify command, the end point will remain
 in the "notification state" until the acknowledgement is received. If
 the gateway does not find a quarantined event that requests a Notify
 command, it places the end point in a normal state.  Events are then
 processed as they come, in exactly the same way as if a Notification
 Request command had just been received.
 A gateway may receive at any time a new Notification Request command
 for the end point. When a new notification request is received in the
 notification state, the gateway shall ensure that the pending
 notification is received by the Call Agent prior to a successful
 response to the new NotificationRequest. It does so by using the
 "piggy-backing" functionality of the protocol. The messages will then
 be sent in a single packetto the source of the new
 NotificationRequest, regardless of respectively the source and
 "notified entity" for the old and new command. The steps involved are
 the following:
 a) the gateway builds a message that carries in a single packet a
    repetition of the old pending Notify command and the
    acknowledgement of the new notification request.
 b) the endpoint is then taken out of the "notification state" without
    waiting for the acknowledgement of the notification command.
 c) a copy of the unacknowledged Notify command command is kept until
    an acknowledgement is received.  If a timer elapses, the
    notification will be repeated, in a packet that will also carry a
    repetition of the acknowledgement of the notification request.
Arango, et al. Informational [Page 101] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 d) if the acknowledgement is lost, the Call Agent will retransmit the
    Notification Request.  The gateway will reply to this repetition
    by retransmitting in a single packet the unacknowledged Notify and
    the acknowledgement of the notification request.
 e) if the gateway has to transmit a Notify before the previous Notify
    is acknowledged, it should construct a packet that piggybacks a
    repetition of the old Notify, a repetition of the acknowledgement
    of the last notification request and the new Notify.
 f) Gateways that cannot piggyback several packets in the same message
    should elect to leave the endpoint in the "notification" state as
    long as the last notification is not acknowledged.
 After receiving the Notification Request command, the requested
 events list and digit map (if a new one was provided) are replaced by
 the newly received parameters, and the list of observed events and
 accumulated dial string are reset to a null value.  The behavior is
 conditioned by the value of the QuarantineHandling parameter. The
 parameter may specify that quarantined events, or previously observed
 events, should be discarded, in which case they will be. If the
 parameter specifies that the quarantined events should be processed,
 the gateway will start processing the list of quarantined events or
 previously observed events, using the newly received list of
 requested events and digit map. When processing these events, the
 gateway may encounter an event which requires a Notify command to be
 sent. If that is the case, the gateway will immediately transmit a
 Notify command that will report all events that were accumulated in
 the list of observed events until the triggering event, included,
 leaving the unprocessed events in the quarantine buffer, and will
 enter the "notification state".
 A new notification request may be received while the gateway has
 accumulated events according to the previous notification requests,
 but has not yet detected a notification-triggering events.  The
 handling of not-yet-notified events is determined, as with the
 quarantined events, by the quarantine handling parameters:
  • If the quarantine-handling parameter specifies that quarantined
events shall be ignored, the observed event list is simply reset.
  • If the quarantine-handling parameter specifies that quarantined
events shall be processed, the observed event list is transferred
    to the quarantined event list.  The observed event list is then
    reset, and the quarantined event list is processed.
Arango, et al. Informational [Page 102] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 Call Agents SHOULD provide the response to a successful Notify
 message and the new NotificationRequest in the same datagram using
 the piggy-backing mechanism.
4.3.2. Explicit detection
 A key element of the state of several endpoints is the position of
 the hook. A race condition may occur when the user decides to go
 off-hook before the Call Agent has the time to ask the gateway to
 notify an off hook event (the "glare" condition well known in
 telephony), or if the user goes on-hook before the Call Agent has the
 time to request the event's notification.
 To avoid this race condition, the gateway should check the condition
 of the endpoint before acknowledging a NotificationRequest. It should
 return an error:
 1- If the gateway is requested to notify an "off hook" transition
    while the phone is already off hook,
 2- If the gateway is requested to notify an "on hook" or "flash hook"
    condition while the phone is already on hook.
 It should be noted, that the condition check is performed at the time
 the notification request is received, where as the actual event that
 caused the current condition may have either been reported, or
 ignored earlier, or it may currently be quarantined.
 The other state variables of the gateway, such as the list of
 RequestedEvent or list of requested signals, are entirely replaced
 after each successful NotificationRequest, which prevents any long
 term discrepancy between the Call Agent and the gateway.
 When a NotificationRequest is unsuccessful, whether it is included in
 a connection-handling command or not, the gateway will simply
 continue as if the command had never been received. As all other
 transactions, the NotificationRequest should operate as an atomic
 transaction, thus any changes initiated as a result of the command
 should be reverted.
 Another race condition may occur when a Notify is issued shortly
 before the reception by the gateway of a NotificationRequest. The
 RequestIdentifier is used to correlate Notify commands with
 NotificationRequest commands.
Arango, et al. Informational [Page 103] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 4.3.3. Ordering of commands, and treatment of disorder
 MGCP does not mandate that the underlying transport protocol
 guarantees the sequencing of commands sent to a gateway or an
 endpoint.  This property tends to maximize the timeliness of actions,
 but it has a few draw backs.  For example:
  • Notify commands may be delayed and arrive to the call agent after
the transmission of a new Notification Request command,
  • If a new NotificationRequest is transmitted before a previous one
is acknowledged, there is no guarantee that the previous one will
    not be received in second position.
 Call Agents that want to guarantee consistent operation of the end
 points can use the following rules:
 1) When a gateway handles several endpoints, commands pertaining to
    the different endpoints can be sent in parallel, for example
    following a model where each endpoint is controlled by its own
    process or its own thread.
 2) When several connections are created on the same endpoint,
    commands pertaining to different connections can be sent in
    parallel.
 3) On a given connection, there should normally be only one
    outstanding command (create or modify).  However, a
    DeleteConnection command can be issued at any time.  In
    consequence, a gateway may sometimes receive a ModifyConnection
    command that applies to a previously deleted connection.  Such
    commands should be ignored, and an error code should be returned.
 4) On a given endpoint, there should normally be only one outstanding
    NotificationRequest command at any time.  The RequestId parameter
    should be used to correlate Notify commands with the triggering
    notification request.
 5) In some cases, an implicitly or explicitly wildcarded
    DeleteConnection command that applies to a group of endpoints can
    step in front of a pending CreateConnection command.  The Call
    Agent should individually delete all connections whose completion
    was pending at the time of the global DeleteConnection command.
    Also, new CreateConnection commands for endpoints named by the
    wild-carding cannot be sent until the wild-carded DeleteConnection
    command is acknowledged.
Arango, et al. Informational [Page 104] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 6) When commands are embedded within each other, sequencing
    requirements for all commands must be adhered to. For example a
    Create Connection command with a Notification Request in it must
    adhere to the sequencing for CreateConnection and
    NotificationRequest at the same time.
 7) AuditEndpoint and AuditConnection is not subject to any
    sequencing.
 8) RestartInProgress must always be the first command sent by an
    endpoint as defined by the restart procedure. Any other command or
    response must be delivered after this RestartInProgress command
    (piggy-backing allowed).
 9) When multiple messages are piggy-backed in a single packet, the
    messages are always processed in order.
 These rules do not affect the gateway, which should always respond to
 commands.
4.3.4. Fighting the restart avalanche
 Let's suppose that a large number of gateways are powered on
 simultaneously.  If they were to all initiate a RestartInProgress
 transaction, the call agent would very likely be swamped, leading to
 message losses and network congestion during the critical period of
 service restoration. In order to prevent such avalanches, the
 following behavior is suggested:
 1) When a gateway is powered on, it should initiate a restart timer
    to a random value, uniformly distributed between 0 and a maximum
    waiting delay (MWD). Care should be taken to avoid synchronicity
    of the random number generation between multiple gateways that
    would use the same algorithm.
 2) The gateway should then wait for either the end of this timer, the
    reception of a command from the call agent, or the detection of a
    local user activity, such as for example an off-hook transition on
    a residential gateway.
 3) When the timer elapses, when a command is received, or when an
    activity is detected, the gateway should initiate the restart
    procedure.
Arango, et al. Informational [Page 105] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 The restart procedure simply requires the endpoint to guarantee that
 the first message (command or response) that the Call Agent sees from
 this endpoint is a RestartInProgress message informing the Call Agent
 about the restart. The endpoint is free to take full advantage of
 piggy-backing to achieve this.
 It is expected that each endpoint in a gateway will have a
 provisionable Call Agent, i.e., "notified entity", to direct the
 initial restart message towards. When the collection of endpoints in
 a gateway is managed by more than one Call Agent, the above procedure
 must be performed for each collection of endpoints managed by a given
 Call Agent. The gateway MUST take full advantage of wild-carding to
 minimize the number of RestartInProgress messages generated when
 multiple endpoints in a gateway restart and the endpoints are managed
 by the same Call Agent.
 The value of MWD is a configuration parameter that depends on the
 type of the gateway. The following ]reasoning can be used to
 determine the value of this delay on residential gateways.
 Call agents are typically dimensioned to handle the peak hour traffic
 load, during which, in average, 10% of the lines will be busy,
 placing calls whose average duration is typically 3 minutes.  The
 processing of a call typically involves 5 to 6 MGCP transactions
 between each end point and the call agent.  This simple calculation
 shows that the call agent is expected to handle 5 to 6 transactions
 for each end point, every 30 minutes on average, or, to put it
 otherwise, about one transaction per end point every 5 to 6 minutes
 on average.  This suggest that a reasonable value of MWD for a
 residential gateway would be 10 to 12 minutes.  In the absence of
 explicit configuration, residential gateways should adopt a value of
 600 seconds for MWD.
 The same reasoning suggests that the value of MWD should be much
 shorter for trunking gateways or for business gateways, because they
 handle a large number of endpoints, and also because the usage rate
 of these endpoints is much higher than 10% during the peak busy hour,
 a typical value being 60%.  These endpoints, during the peak hour,
 are this expected to contribute about one transaction per minute to
 the call agent load. A reasonable algorithm is to make the value of
 MWD per "trunk" endpoint six times shorter than the MWD per
 residential gateway, and also inversely proportional to the number of
 endpoints that are being restarted. for example MWD should be set to
 2.5 seconds for a gateway that handles a T1 line, or to 60
 milliseconds for a gateway that handles a T3 line.
Arango, et al. Informational [Page 106] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 4.3.5. Disconnected Endpoints
 In addition to the restart procedure, gateways also have a
 "disconnected" procedure, which is initiated when an endpoint becomes
 "disconnected" as described in Section 3.4.2. It should here be
 noted, that endpoints can only become disconnected when they attempt
 to communicate with the Call Agent. The following steps are followed
 by an endpoint that becomes "disconnected":
 1. A "disconnected" timer is initialized to a random value, uniformly
    distributed between 0 and a provisionable "disconnected" initial
    waiting delay (Tdinit), e.g., 15 seconds.  Care MUST be taken to
    avoid synchronicity of the random number generation between
    multiple gateways and endpoints that would use the same algorithm.
 2. The gateway then waits for either the end of this timer, the
    reception of a command from the call agent, or the detection of a
    local user activity for the endpoint, such as for example an off-
    hook transition.
 3. When the "disconnected" timer elapses, when a command is received,
    or when a local user activity is detected, the gateway initiates
    the "disconnected" procedure for the endpoint. In the case of
    local user activity, a provisionable "disconnected" minimum
    waiting delay (Tdmin) must furthermore have elapsed since the
    gateway became disconnected or the last time it initiated the
    "disconnected" procedure in order to limit the rate at which the
    procedure is performed.
 4. If the "disconnected" procedure still left the endpoint
    disconnected, the "disconnected" timer is then doubled, subject to
    a provisionable "disconnected" maximum waiting delay (Tdmax),
    e.g., 600 seconds, and the gateway proceeds with step 2 again.
 The "disconnected" procedure is similar to the restart procedure in
 that it now simply states that the endpoint MUST send a
 RestartInProgress command to the Call Agent informing it that the
 endpoint was disconnected and furthermore guarantee that the first
 message (command or response) that the Call Agent now sees from this
 endpoint MUST be this RestartInProgress command. The endpoint MUST
 take full advantage of piggy-backing in achieving this. The Call
 Agent may then for instance decide to audit the endpoint, or simply
 clear all connections for the endpoint.
 This specification purposely does not specify any additional behavior
 for a disconnected endpoint. Vendors MAY for instance choose to
 provide silence, play reorder tone, or even enable a downloaded wav
 file to be played.
Arango, et al. Informational [Page 107] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 The default value for Tdinit is 15 seconds, the default value for
 Tdmin, is 15 seconds, and the default value for Tdmax is 600 seconds.
5. Security requirements
 If unauthorized entities could use the MGCP, they would be able to
 set-up unauthorized calls, or to interfere with authorized calls. We
 expect that MGCP messages will always be carried over secure Internet
 connections, as defined in the IP security architecture as defined in
 RFC 2401, using either the IP Authentication Header, defined in RFC
 2402, or the IP Encapsulating Security Payload, defined in RFC 2406.
 The complete MGCP protocol stack would thus include the following
 layers:
              ________________________________
             |              MGCP             |
             |_______________________________|
             |              UDP              |
             |_______________________________|
             |          IP security          |
             | (authentication or encryption)|
             |_______________________________|
             |              IP               |
             |_______________________________|
             |       transmission media      |
             |_______________________________|
 Adequate protection of the connections will be achieved if the
 gateways and the Call Agents only accept messages for which IP
 security provided an authentication service. An encryption service
 will provide additional protection against eavesdropping, thus
 forbidding third parties from monitoring the connections set up by a
 given endpoint
 The encryption service will also be requested if the session
 descriptions are used to carry session keys, as defined in SDP.
 These procedures do not necessarily protect against denial of service
 attacks by misbehaving gateways or misbehaving call agents. However,
 they will provide an identification of these misbehaving entities,
 which should then be deprived of their authorization through
 maintenance procedures.
Arango, et al. Informational [Page 108] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 5.1. Protection of media connections
 MGCP allows call agent to provide gateways with "session keys" that
 can be used to encrypt the audio messages, protecting against
 eavesdropping.
 A specific problem of packet networks is "uncontrolled barge-in."
 This attack can be performed by directing media packets to the IP
 address and UDP port used by a connection. If no protection is
 implemented, the packets will be decompressed and the signals will be
 played on the "line side".
 A basic protection against this attack is to only accept packets from
 known sources, checking for example that the IP source address and
 UDP source port match the values announced in the "remote session
 description."  But this has two inconveniences: it slows down
 connection establishment and it can be fooled by source spoofing:
  • To enable the address-based protection, the call agent must obtain
the remote session description of the e-gress gateway and pass it
    to the in-gress gateway.  This requires at least one network round
    trip, and leaves us with a dilemma: either allow the call to
    proceed without waiting for the round trip to complete, and risk
    for example "clipping" a remote announcement, or wait for the full
    round trip and settle for slower call-set-up procedures.
  • Source spoofing is only effective if the attacker can obtain valid
pairs of source destination addresses and ports, for example by
    listening to a fraction of the traffic. To fight source spoofing,
    one could try to control all access points to the network.  But
    this is in practice very hard to achieve.
 An alternative to checking the source address is to encrypt and
 authenticate the packets, using a secret key that is conveyed during
 the call set-up procedure. This will no slow down the call set-up,
 and provides strong protection against address spoofing.
6. Event packages and end point types
 This section provides an initial definition of packages and event
 names.  More packages can be defined in additional documents.
Arango, et al. Informational [Page 109] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 6.1. Basic packages
 The list of basic packages includes the following:
              _________________________________________
             | Package                      |   name  |
             |______________________________|_________|
             | Generic Media Package        |   G     |
             | DTMF package                 |   D     |
             | MF Package                   |   M     |
             | Trunk Package                |   T     |
             | Line Package                 |   L     |
             | Handset Package              |   H     |
             | RTP Package                  |   R     |
             | Network Access Server Package|   N     |
             | Announcement Server Package  |   A     |
             | Script Package               |   Script|
             |______________________________|_________|
 In the tables of events for each package, there are five columns:
    Symbol: the unique symbol used for the event
    Definition: a short description of the event
    R: an x appears in this column is the event can be Requested by
       the call agent.
    S: if nothing appears in this column for an event, then the event
       cannot be signaled on command by the call agent. Otherwise, the
       following symbols identify the type of event:
    OO On/Off signal.  The signal is turned on until commanded by the
       call agent to turn it off, and vice versa.
    TO Timeout signal.  The signal lasts for a given duration unless
       it is superseded by a new signal.
    BR Brief signal.  The event has a short, known duration.
    Duration: specifies the duration of TO signals.
Arango, et al. Informational [Page 110] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 6.1.1. Generic Media Package
 Package Name: G
 The generic media package group the events and signals that can be
 observed on several types of endpoints, such as trunking gateways,
 access gateways or residential gateways.
_____________________________________________________________________
| Symbol | Definition | R | S Duration | |||_|| | mt | Modem detected | x | | | ft | Fax tone detected | x | | | ld | Long duration connection | x | | | pat(###) | Pattern ### detected | x | OO | | rt | Ringback tone | | TO | | rbk(###) | ring back on connection | | TO 180 seconds | | cf | Confirm tone | | BR | | cg | Network Congestion tone | | TO | | it | Intercept tone | | OO | | pt | Preemption tone | | OO | | of | report failure | x | | |||_||
 The signals are defined as follows:
    The pattern definition can be used for specific algorithms such as
    answering machine detection, tone detection, and the like.
 Ring back tone (rt)
    an Audible Ring Tone, a combination of two AC tones with
    frequencies of 440 and 480 Hertz and levels of -19 dBm each, to
    give a combined level of -16 dBm.  The cadence for Audible Ring
    Tone is 2 seconds on followed by 4 seconds off. See GR- 506-CORE -
    LSSGR:  SIGNALING, Section 17.2.5.
 Ring back on connection
    A ring back tone, applied to the connection whose identifier is
    passed as a parameter.
 The "long duration connection" is detected when a connection has been
 established for more than 1 hour.
Arango, et al. Informational [Page 111] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 6.1.2. DTMF package
 Package name: D
  _______________________________________________________________
 | Symbol |   Definition              |   R |   S      Duration |
 |________|___________________________|_____|___________________|
 | 0      |   DTMF 0                  |   x |   BR              |
 | 1      |   DTMF 1                  |   x |   BR              |
 | 2      |   DTMF 2                  |   x |   BR              |
 | 3      |   DTMF 3                  |   x |   BR              |
 | 4      |   DTMF 4                  |   x |   BR              |
 | 5      |   DTMF 5                  |   x |   BR              |
 | 6      |   DTMF 6                  |   x |   BR              |
 | 7      |   DTMF 7                  |   x |   BR              |
 | 8      |   DTMF 8                  |   x |   BR              |
 | 9      |   DTMF 9                  |   x |   BR              |
 | #      |   DTMF #                  |   x |   BR              |
 | *      |   DTMF *                  |   x |   BR              |
 | A      |   DTMF A                  |   x |   BR              |
 | B      |   DTMF B                  |   x |   BR              |
 | C      |   DTMF C                  |   x |   BR              |
 | D      |   DTMF D                  |   x |   BR              |
 | L      |   long duration indicator |   x |          2 seconds|
 | X      |   Wildcard, match         |   x |                   |
 |        |   any digit 0-9           |     |                   |
 | T      |   Interdigit timer        |   x |          4 seconds|
 | of     |   report failure          |   x |                   |
 |________|___________________________|_____|___________________|
 The "interdigit timer" T is a digit input timer that can be used in
 two ways:
  • When timer T is used with a digit map, the timer is not started
until the first digit is entered, and the timer is restarted after
    each new digit is entered until either a digit map match or
    mismatch occurs. In this case, timer T functions as an inter-digit
    timer.
  • When timer T is used without a digit map, the timer is started
immediately and simply cancelled (but not restarted) as soon as a
    digit is entered. In this case, timer T can be used as an
    interdigit timer when overlap sending is used.
    When used with a digit map, timer T takes on one of two values,
    T(partial) or T(critical). When at least one more digit is
    required for the digit string to match any of the patterns in the
    digit map, timer T takes on the value T(partial), corresponding to
Arango, et al. Informational [Page 112] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
    partial dial timing. If a timer is all that is required to produce
    a match, timer T takes on the value T(critical) corresponding to
    critical timing. When timer T is used without a digit map, timer T
    takes on the value T(critical).  The default value for T(partial)
    is 16 seconds and the default value for T(critical) is 4 seconds.
    The provisioning process may alter both of these.
    The "long duration indicator" is observed when a DTMF signal is
    produced for a duration larger than two seconds.  In this case,
    the gateway will detect two successive events: first, when the
    signal has been recognized, the DTMF signal, and then, 2 seconds
    later, the long duration signal.
6.1.3. MF Package
    Package Name: M
     ________________________________________________________
    | Symbol |   Definition       |   R |   S      Duration |
    |________|____________________|_____|___________________|
    | 0      |   MF 0             |   x |   BR              |
    | 1      |   MF 1             |   x |   BR              |
    | 2      |   MF 2             |   x |   BR              |
    | 3      |   MF 3             |   x |   BR              |
    | 4      |   MF 4             |   x |   BR              |
    | 5      |   MF 5             |   x |   BR              |
    | 6      |   MF 6             |   x |   BR              |
    | 7      |   MF 7             |   x |   BR              |
    | 8      |   MF 8             |   x |   BR              |
    | 9      |   MF 9             |   x |   BR              |
    | X      |   Wildcard, match  |   x |                   |
    |        |   any digit 0-9    |     |                   |
    | T      |   Interdigit timer |   x |          4 seconds|
    | K0     |   MF K0 or KP      |   x |   BR              |
    | K1     |   MF K1            |   x |   BR              |
    | K2     |   MF K2            |   x |   BR              |
    | S0     |   MF S0 or ST      |   x |   BR              |
    | S1     |   MF S1            |   x |   BR              |
    | S2     |   MF S2            |   x |   BR              |
    | S3     |   MF S3            |   x |   BR              |
    | wk     |   Wink             |   x |   BR              |
    | wko    |   Wink off         |   x |   BR              |
    | is     |   Incoming seizure |   x |   OO              |
    | rs     |   Return seizure   |   x |   OO              |
    | us     |   Unseize circuit  |   x |   OO              |
    | of     |   report failure   |   x |                   |
    |________|____________________|_____|___________________|
Arango, et al. Informational [Page 113] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 The definition of the MF package events is as follows:
 Wink
    A transition from unseized to seized to unseized trunk states
    within a specified period.  Typical seizure period is 100-350
    msec.)
 Incoming seizure
    Incoming indication of call attempt.
 Return seizure:
    Seizure in response to outgoing seizure.
 Unseize circuit:
    Unseizure of a circuit at the end of a call.
 Wink off:
    A signal used in operator services trunks.  A transition from
    seized to unseized to seized trunk states within a specified
    period of 100-350 ms. (To be checked)
6.1.4. Trunk Package
 Package Name: T
 _____________________________________________________________________
| Symbol |   Definition                   |   R |   S      Duration  |
|________|________________________________|_____|____________________|
| co1    |   Continuity tone (single tone,|   x |   OO               |
|        |   or return tone)              |     |                    |
| co2    |   Continuity test (go tone,    |   x |   OO               |
|        |   in dual tone procedures)     |     |                    |
| lb     |   Loopback                     |     |   OO               |
| om     |   Old Milliwatt Tone (1000 Hz) |   x |   OO               |
| nm     |   New Milliwatt Tone (1004 Hz) |   x |   OO               |
| tl     |   Test Line                    |   x |   OO               |
| zz     |   No circuit                   |   x |   OO               |
| as     |   Answer Supervision           |   x |   OO               |
| ro     |   Reorder Tone                 |   x |   TO     30 seconds|
| of     |   report failure               |   x |                    |
| bl     |   Blocking                     |     |   OO               |
|________|________________________________|_____|____________________|
 The definition of the trunk package signal events is as follows:
 Continuity Tone (co1):
    A tone at 2010 + or - 30 Hz.
Arango, et al. Informational [Page 114] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 Continuity Test (co2):
    A tone at the 1780 + or - 30 Hz.
 Milliwatt Tones:
    Old Milliwatt Tone (1000 Hz), New Milliwatt Tone (1004 Hz)
 Line Test:
    105 Test Line test progress tone (2225 Hz + or - 25 Hz at -10 dBm0
    + or -- 0.5dB).
 No circuit:
    (that annoying tri-tone, low to high)
 Answer Supervision:
 Reorder Tone:
    Reorder tone is a combination of two AC tones with frequencies of
    480 and 620 Hertz and levels of -24 dBm each, to give a combined
    level of -21 dBm.  The cadence for Station Busy Tone is 0.25
    seconds on followed by 0.25 seconds off, repeating continuously.
    See GR-506-CORE - LSSGR: SIGNALING, Section 17.2.7.
 Blocking:
    The call agent can place the circuit in a blocked state by
    applying the "bl(+)" signal to the endpoint.  It can unblock it by
    applying the "bl(-)" signal.
 The continuity tones are used when the call agent wants to initiate a
 continuity test. There are two types of tests, single tone and dual
 tone. The Call agent is expected to know, through provisioning
 information, which test should be applied to a given endpoint. For
 example, the call agent that wants to initiate a single frequency
 test will send to the gateway a command of the form:
       RQNT 1234 epx-t1/17@tgw2.example.net
       X: AB123FE0
       S: co1
       R: co1
 If it wanted instead to initiate a dual-tone test, it would send the
 command:
       RQNT 1234 epx-t1/17@tgw2.example.net
       X: AB123FE0
       S: co2
       R: co1
Arango, et al. Informational [Page 115] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 The gateway would send the requested signal, and in both cases would
 look for the return of the 2010 Hz tone (co1).  When it detects that
 tone, it will send the corresponding  notification.
 The tones are of type OO: the gateway will keep sending them until it
 receives a new notification request.
6.1.5. Line Package
 Package Name: L
Symbol Definition R S Duration
_|_|_
adsi(string) adsi display BR
vmwi visual message OO
waiting indicator
hd Off hook transition x
hu On hook transition x
hf Flash hook x
aw Answer tone x OO
bz Busy tone TO 30 seconds
ci(ti,nu,na) Caller-id BR
wt Call Waiting tone TO 30 seconds
wt1, wt2, Alternative call
wt3, wt4 waiting tones
dl Dial tone TO 16 seconds
mwi Message waiting ind. TO 16 seconds
nbz Network busy x OO
(fast cycle busy)
ro Reorder tone TO 30 seconds
rg Ringing TO 180 seconds
r0, r1, r2, Distinctive ringing TO 180 seconds
r3, r4, r5,
r6 or r7
rs Ringsplash BR
p Prompt tone x BR
e Error tone x BR
sl Stutter dialtone TO 16 seconds
v Alerting Tone OO
y Recorder Warning Tone OO
sit SIT tone
z Calling Card Service Tone OO
oc Report on completion x
ot Off hook warning tone TO indefinite
s(###) Distinctive tone pattern x BR
of report failure x
_|_|_
Arango, et al. Informational [Page 116] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 The definition of the tones is as follows:
 Dial tone:
    A combined 350 + 440 Hz tone.
 Visual Message Waiting Indicator
    The transmission of the VMWI messages will conform to the
    requirements in Section 2.3.2, "On-hook Data Transmission Not
    Associated with Ringing" in TR-H-000030 and the CPE guidelines in
    SR-TSV-002476. VMWI messages will only be sent from the SPCS when
    the line is idle. If new messages arrive while the line is busy,
    the VMWI indicator message will be delayed until the line goes
    back to the idle state. The CA should periodically refresh the
    CPE's visual indicator. See TR-NWT-001401 - Visual Message Waiting
    Indicator Generic Requirements; and GR- 30-CORE - Voiceband Data
    Transmission Interface.
 Message waiting Indicator
    See GR-506-CORE, 17.2.3.
 Alerting Tone:
    a 440 Hz Tone of 2 second duration followed by 1/2 second of tone
    every 10 seconds.
 Ring splash
    Ringsplash, also known as "Reminder ring" is a burst of ringing
    that may be applied to the physical forwarding line (when idle) to
    indicate that a call has been forwarded and to remind the user
    that a CF subfeature is active.  In the US, it is defined to be a
    0.5(-0,+0.1) second burst of power ringing. See TR-TSY-000586 -
    Call Forwarding Subfeatures.
 Call waiting tone
    Call Waiting tone is defined in GR-506-CORE, 14.2. Call Waiting
    feature is defined in TR-TSY-000571. By defining "wt" as a TO
    signal you are really defining the feature which seems wrong to me
    (given the spirit of MGCP), hence the definition of "wt" as a BR
    signal in ECS, per GR-506-CORE. Also, it turns out that there is
    actually four different call waiting tone patterns (see GR-506-
    CORE, 14.2) so we have wt1, wt2, wt3, wt4.
 Caller Id (ci(time, number, name)):
    The caller-id event carries three parameters, the time of the
    call, the calling number and the calling name. Each of the three
    fields are optional, however each of the commas will always be
    included.  See TR-NWT-001188, GR-30-CORE, and TR-NWT-000031.
Arango, et al. Informational [Page 117] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 Recorder Warning Tone:
    1400 Hz of Tone of 0.5 second duration every 15 seconds.
 SIT tone:
    used for indicating a line is out of service.
 Calling Card Service Tone:
    60 ms of 941 + 1477 Hz and 940 ms of 350 + 440 Hz (dial tone),
    decaying exponentially with a time constant of 200 ms.
 Distinctive tone pattern:
    where ### is any number between 000 and 999, inclusive.  Can be
    used for distinctive ringing, customized dial tone, etc.
 Report on completion
    The report on completion event is detected when the gateway was
    asked to perform one or several signals of type TO on the
    endpoint, and when these signals were completed without being
    stopped by the detection of a requested event such as off-hook
    transition or dialed digit.  The completion report may carry as
    parameter the name of the signal that came to the end of its live
    time, as in:
          O: L/oc(L/dl)
 Ring back on connection
    A ring back tone, applied to the connection wghose identifier is
    passed as a parameter.
 We should note that many of these definitions vary from country to
 country.  The frequencies listed above are the one in use in North
 America.  There is a need to accommodate different tone sets in
 different countries, and there is still an ongoing debate on the best
 way to meet that requirement:
  • One solution is to define different event packages specifying for
example the German dialtone as "L-DE/DL".
  • Another solution is to use a management interface to specify on an
endpoint basis which frequency shall be associated to what tone. Arango, et al. Informational [Page 118] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 6.1.6. Handset emulation package
 Package Name: H
Symbol Definition R S Duration
_|_|_
adsi(string) adsi display x BR
tdd
vmwi
hd Off hook transition x OO
hu On hook transition x OO
hf Flash hook x BR
aw Answer tone x OO
bz Busy tone x OO
wt Call Waiting tone x TO 30 seconds
dl Dial tone (350 + 440 Hz) x TO 120 seconds
nbz Network busy x OO
(fast cycle busy)
rg Ringing x TO 30 seconds
r0, r1, r2, Distinctive ringing x TO 30 seconds
r3, r4, r5,
r6 or r7
p Prompt tone x BR
e Error tone x BR
sdl Stutter dialtone x TO 16 seconds
v Alerting Tone x OO
y Recorder Warning Tone x OO
t SIT tone x
z Calling Card Service Tone x OO
oc Report on completion x
ot Off hook warning tone x OO
s(###) Distinctive tone pattern x BR
of report failure x
_|_|_
 The handset emulation package is an extension of the line package, to
 be used when the gateway is capable of emulating a handset.  The
 difference with the line package is that events such as "off hook"
 can be signalled as well as detected.
Arango, et al. Informational [Page 119] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 6.1.7. RTP Package
 Package Name: R
  ____________________________________________________________________
 | Symbol  |   Definition                   |   R |   S      Duration|
 |_________|________________________________|_____|__________________|
 | UC      |   Used codec changed           |   x |                  |
 | SR(###) |   Sampling rate changed        |   x |                  |
 | JI(###) |   Jitter buffer size changed   |   x |                  |
 | PL(###) |   Packet loss exceeded         |   x |                  |
 | qa      |   Quality alert                |   x |                  |
 | co1     |   Continuity tone (single tone,|   x |   OO             |
 |         |   or return tone)              |     |                  |
 | co2     |   Continuity test (go tone,    |   x |   OO             |
 |         |  in dual tone procedures)      |     |                  |
 | of      |   report failure               |   x |                  |
 |_________|________________________________|_____|__________________|
 Codec Changed:
    Codec changed to hexadecimal codec number enclosed in parenthesis,
    as in UC(15), to indicate the codec was changed to PCM mu-law.
    Codec Numbers are specified in RFC 1890, or in a new definition of
    the audio profiles for RTP that replaces this RFC.  Some
    implementations of media gateways may not allow the codec to be
    changed upon command from the call agent.  codec changed to codec
    hexadecimal ##.
 Sampling Rate Changed:
    Sampling rate changed to decimal number in milliseconds enclosed
    in parenthesis, as in SR(20), to indicate the sampling rate was
    changed to 20 milliseconds.  Some implementations of media
    gateways may not allow the sampling rate to be changed upon
    command from a call agent.
 Jitter Buffer Size Changed:
    When the media gateway has the ability to automatically adjust the
    depth of the jitter buffer for received RTP streams, it is useful
    for the media gateway controller to receive notification that the
    media gateway has automatically increased its jitter buffer size
    to accomodate increased or decreased variability in network
    latency.  The syntax for requesting notification is "JI", which
    tells the media gateway that the controller wants notification of
    any jitter buffer size changes.  The syntax for notification from
    the media gateway to the controller is "JI(####)", where the ####
    is the new size of the jitter buffer, in milliseconds.
Arango, et al. Informational [Page 120] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 Packet Loss Exceeded:
    Packet loss rate exceed the threshold of the specified decimal
    number of packets per 100,000 packets, where the packet loss
    number is contained in parenthesis.  For example, PL(10) indicates
    packets are being dropped at a rate of 1 in 10,000 packets.
 Quality alert
    The packet loss rate or the combination of delay and jitter exceed
    a specified quality threshold.
 The continuity tones are the same as those defined in the Trunk
 package.  They can be use in conjunction with the Network LoopBack or
 Network Continuity Test modes to test the continuity of an RTP
 circuit.
 The "operation failure" code can be used to report problems such as
 the loss of underlying connectivity.  The observed event can include
 as parameter the reason code of the failure.
6.1.8. Network Access Server Package
 Package Name: N
     ____________________________________________________________
    | Symbol |   Definition             |   R |   S     Duration|
    |________|__________________________|_____|_________________|
    | pa     |  Packet arrival          |  x  |                 |
    | cbk    |  Call back request       |  x  |                 |
    | cl     |  Carrier lost            |  x  |                 |
    | au     |   Authorization succeeded|  x  |                 |
    | ax     |   Authorization denied   |  x  |                 |
    | of     |   Report failure         |  x  |                 |
    |________|__________________________|_____|_________________|
 The packet arrival event is used to notify that at least one packet
 was recently sent to an Internet address that is observed by an
 endpoint.  The event report includes the Internet address, in
 standard ASCII encoding, between parenthesis:
       O: pa(192.96.41.1)
 The call back event is used to notify that a call back has been
 requested during the initial phase of a data connection. The event
 report includes the identification of the user that should be called
 back, between parenthesis:
       O: cbk(user25)
Arango, et al. Informational [Page 121] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 6.1.9. Announcement Server Package
 Package Name: A
  ___________________________________________________________________
 | Symbol         |   Definition           |   R |   S      Duration|
 |________________|________________________|_____|__________________|
 | ann(url,parms) |   Play an announcement |     |   TO     variable|
 | oc             |   Report on completion |   x |                  |
 | of             |   Report failure       |   x |                  |
 |________________|________________________|_____|__________________|
 The announcement action is qualified by an URL name and by a set of
 initial parameters as in for example:
       S: ann(http://scripts.example.net/all-lines-busy.au)
 The "operation complete" event will be detected when the announcement
 is played out. If the announcement cannot be played out, an operation
 failure event can be returned.  The failure may be explained by a
 commentary, as in:
       O: A/of(file not found)
6.1.10. Script Package
 Package Name: Script
  ______________________________________________________________
 | Symbol    |   Definition           |   R |   S  |   Duration|
 |___________|________________________|_____|______|___________|
 | java(url) |   Load a java script   |     |   TO |   variable|
 | perl(url) |   Load a perl script   |     |   TO |   variable|
 | tcl(url)  |   Load a TCL script    |     |   TO |   variable|
 | xml(url)  |   Load an XML script   |     |   TO |   variable|
 | oc        |   Report on completion |   x |      |           |
 | of        |   Report failure       |   x |      |           |
 |___________|________________________|_____|______|___________|
 The "language" action define is qualified by an URL name and by a set
 of initial parameters as in for example:
       S: script/java(http://scripts.example.net/credit-
          card.java,long,1234)
Arango, et al. Informational [Page 122] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 The current definition defines keywords for the most common
 languages.  More languages may be defined in further version of this
 documents.  For each language, an API specification will describe how
 the scripts can issue local "notificationRequest" commands, and
 receive the corresponding notifications.
 The script produces an output which consists of one or several text
 string, separated by commas.  The text string are reported as a
 commentary in the report on completion, as in for example:
       O: script/oc(21223456794567,9738234567)
 The failure report may also return a string, as in:
       O: script/oc(21223456794567,9738234567)
 The definition of the script environment and the specific actions in
 that environment are for further study.
6.2. Basic endpoint types and profiles
 We define the following basic endpoint types and profiles:
  • Trunk gateway (ISUP)
  • Trunk gateway (MF)
  • Network Access Server (NAS)
  • Combined NAS/VOIP gateway
  • Access Gateway
  • Residential Gateway
  • Announcement servers
Arango, et al. Informational [Page 123] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 These gateways are supposed to implement the following packages
     ___________________________________________________________
    | Gateway                    |   Supported packages        |
    |____________________________|_____________________________|
    | Trunk gateway (ISUP)       |   GM, DTMF, TK, RTP         |
    | Trunk gateway (MF)         |   GM, MF, DTMF, TK, RTP     |
    | Network Access Server (NAS)|   GM, MF, TK, NAS           |
    | Combined NAS/VOIP gateway  |   GM, MF, DTMF, TK, NAS, RTP|
    | Access Gateway (VOIP)      |   GM, DTMF, MF, RTP         |
    | Access Gateway (VOIP+NAS)  |   GM, DTMF, MF, NAS, RTP    |
    | Residential Gateway        |   GM, DTMF, Line, RTP       |
    | Announcement Server        |   ANN, RTP                  |
    |____________________________|_____________________________|
 Advanced announcement servers may also support the Script package.
 Advanced trunking servers may support the ANN package, the Script
 package, and in some cases the Line and Handset package as well.
7. Versions and compatibility 7.1. Differences between version 1.0 and draft 0.5
 Draft 0-5 was issued in February 1999, as the last update of draft
 version 0.1. Version 1.0 benefits from implementation experience, and
 also aligns as much as possible with the CableLabs' NCS project. The
 main differences between the February draft and version 1.0 are:
  • Specified more clearly that the encoding of three
LocalConnectionOptions parameters, Encoding Method, Packetization
    Period and Bandwidth, shall follow the conventions laid out in
    SDP.
  • Specified how the quarantine handling parameter governs the
handling of detected but not yet specified events.
  • Specified that unexpected timers or digits should trigger
transmission of the dialed string.
  • Removed the digit map syntax description from section 2.1.5 (it
was redundant with section 3.4.)
  • Corrected miscellaneous bugs in the formal syntax description.
  • Aligned specification of commands with the CableLabs NCS
specification. This mostly affects the AuditEndpoint and Arango, et al. Informational [Page 124] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
    RestartInProgress commands.
  • Aligned the handling of retransmission with the CableLabs NCS
specification.
  • Added the provisional response return code and corresponding
behavior description.
  • Added an optional reason code parameter to restart in progress.
  • Added the possibility to audit the restart method, restart delay
and reason code. 7.2. Differences between draft-04 and draft-05
 Differences are minor: corrected the copyright statement, and
 corrected a bug in the formal description.
7.3. Differences between draft-03 and draft-04
 Draft 04 corrects a number of minor editing mistakes that were
 pointed out during the review of draft 03, issued on February 1.
7.4. Differences between draft-02 and draft-03
 The main differences between draft-02, issued in January 22 1998, and
 draft 03 are:
  • Introduced a discussion on endpoint types,
  • Introduced a discussion of the connection set-up procedure, and of
the role of connection parameters,
  • Introduced a notation of the connection identifier within event
names,
  • Documented the extension procedure for the LocalConnectionOptions
parameter and for the ConnectionParameters parameter,
  • Introduced a three-way handshake procedure, using a ResponseAck
parameter, in order to allow gateways to delete copies of old
    responses without waiting for a 30 seconds timer,
  • Expanded the security section to include a discussion of
"uncontrolled barge-in."
  • Propsed a "create two connections" command, as an appendix.
Arango, et al. Informational [Page 125] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 7.5. Differences between draft-01 and draft-02
 The main differences between draft-01, issued in November 1998, and
 draft 02 are:
  • Added an ABNF description of the protocol.
  • Specification of an EndpointConfiguration command,
  • Addition of a "two endpoints" mode in the create connection
command,
  • Modification of the package wildcards from "$/$" to "*/all" at the
Request of early implementors,
  • Revision of some package definitions to better align with external
specifications.
  • Addition of a specification for the handling of "failover."
  • Revision of the section on race conditions.
7.6. The making of MGCP from IPDC and SGCP
 MGCP version 0.1 results from the fusion of the SGCP and IPDC
 proposals.
7.7. Changes between MGCP and initial versions of SGCP
 MGCP version 0.1 (which subsumes SGCP version 1.2) introduces the
 following changes from SGCP version 1.1:
  • Protocol name changed to MGCP.
  • Introduce a formal wildcarding structure in the name of endpoints,
inspired from IPDC, and detailed the usage of wildcard names in
    each operation.
  • Naming scheme for events, introducing a package structure inspired
from IPDC.
  • New operations for audit endpoint, audit connection (requested by
the Cablelabs) and restart (inspired from IPDC).
  • New parameter to control the behavior of the notification request.
  • Improved text on the detection and handling of race conditions.
Arango, et al. Informational [Page 126] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
  • Syntax modification for event reporting, to incorporate package
names.
  • Definition of basic event packages (inspired from IPDC).
  • Incorporation of mandatory and optional extension parameters,
inspired by IPDC.
 SGCP version 1.1 introduces the following changes from version SGCP
    1.0:
  • Extension parameters (X-??:)
  • Error Code 511 (Unrecognized extension).
  • All event codes can be used in RequestEvent, SignalRequest and
ObservedEvent parameters.
  • Error Code 512 (Not equipped to detect requested event).
  • Error Code 513 (Not equipped to generate requested signal).
  • Error Code 514 (Unrecognized announcement).
  • Specific Endpoint-ID can be returned in creation commands.
  • Changed the code for the ASDI display from "ad" to "asdi" to avoid
conflict with the digits A and D.
  • Changed the code for the answer tone from "at" to "aw" to avoid
conflict with the digit A and the timer mark T
  • Changed the code for the busy tone from "bt" to "bz" to avoid
conflict with the digit B and the timer mark T
  • Specified that the continuity tone value is "co" (CT was
incorrectly used in several instances; CT conflicts with .)
  • Changed the code for the dial tone from "dt" to "dl" to avoid
conflict with the digit D and the timer mark T
  • Added a code point for announcement requests.
  • Added a code point for the "wink" event.
  • Set the "octet received" code in the "Connection Parameters" to
"OR" (was set to RO, but then "OR" was used throughout all
    examples.)
Arango, et al. Informational [Page 127] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
  • Added a "data" mode.
  • Added a description of SDP parameters for the network access mode
(NAS).
  • Added four flow diagrams for the network access mode.
  • Incorporated numerous editing suggestions to make the description
easier to understand. In particular, cleared the confusion between
    requests, queries, functions and commands.
  • Defined the continuity test mode as specifying a dual-tone
transponder, while the loopback mode can be used for a single tone
    test.
  • Added event code "OC", operation completed.
  • Added the specification of the "quarantine list", which clarifies
the expected handling of events and notifications.
  • Added the specification of a "wildcard delete" operation.
8. Security Considerations
 Security issues are discussed in section 5.
9. Acknowledgements
 We want to thank here the many reviewers who provided us with advice
 on the design of SGCP and then MGCP, notably Flemming Andreasen,
 Sankar Ardhanari, Francois Berard, David Auerbach, Bob Biskner, David
 Bukovinsky, Jerry Kamitses, Oren Kudevitzki, Barry Hoffner, Troy
 Morley, Dave Oran, Jeff Orwick, John Pickens, Lou Rubin, Chip Sharp,
 Paul Sijben, Kurt Steinbrenner, Joe Stone and Stuart Wray.
 The version 0.1 of MGCP is heavily inspired by the "Internet Protocol
 Device Control" (IPDC) designed by the Technical Advisory Committee
 set up by Level 3 Communications.  Whole sets of text have been
 retrieved from the IP Connection Control protocol, IP Media Control
 protocol, and IP Device Management.  The authors wish to acknowledge
 the contribution to these protocols made by Ilya Akramovich, Bob
 Bell, Dan Brendes, Peter Chung, John Clark, Russ Dehlinger, Andrew
 Dugan, Isaac Elliott, Cary FitzGerald, Jan Gronski, Tom Hess, Geoff
 Jordan, Tony Lam, Shawn Lewis, Dave Mazik, Alan Mikhak, Pete
 O'Connell, Scott Pickett, Shyamal Prasad, Eric Presworsky, Paul
 Richards, Dale Skran, Louise Spergel, David Sprague, Raj Srinivasan,
 Tom Taylor and Michael Thomas.
Arango, et al. Informational [Page 128] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 10. References
  • Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
A Transport Protocol for Real-Time Applications", RFC 1889,
    January 1996.
  • Schulzrinne, H., "RTP Profile for Audio and Video Conferences with
Minimal Control", RFC 1890, January 1996.
  • Handley, M and V. Jacobson, "SDP: Session Description Protocol",
RFC 2327, April 1998.
  • Handley, M., "SAP - Session Announcement Protocol", Work in
Progress.
  • Handley, M., Schulzrinne, H. and E. Schooler, "Session Initiation
Protocol (SIP)", RFC 2543, March 1999.
  • Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
  • ITU-T, Recommendation Q.761, "FUNCTIONAL DESCRIPTION OF THE ISDN
USER PART OF SIGNALLING SYSTEM No. 7", (Malaga-Torremolinos, 1984;
    modified at Helsinki, 1993)
  • ITU-T, Recommendation Q.762, "GENERAL FUNCTION OF MESSAGES AND
SIGNALS OF THE ISDN USER PART OF SIGNALLING SYSTEM No. 7",
    (MalagaTorremolinos, 1984; modified at Helsinki, 1993)
  • ITU-T, Recommendation H.323 (02/98), "PACKET-BASED MULTIMEDIA
COMMUNICATIONS SYSTEMS."
  • ITU-T, Recommendation H.225, "Call Signaling Protocols and Media
Stream Packetization for Packet Based Multimedia Communications
    Systems."
  • ITU-T, Recommendation H.245 (02/98), "CONTROL PROTOCOL FOR
MULTIMEDIA COMMUNICATION."
  • Kent, S. and R. Atkinson, "Security Architecture for the Internet
Protocol", RFC 2401, November 1998.
  • Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
  • Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998. Arango, et al. Informational [Page 129] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
  • Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997. 11. Authors' Addresses
 Mauricio Arango
 RSL COM Latin America
 6300 N.W. 5th Way, Suite 100
 Ft. Lauderdale, FL 33309
 Phone: (954) 492-0913
 EMail: marango@rslcom.com
 Andrew Dugan
 Level3 Communications
 1450 Infinite Drive
 Louisville, CO 80027
 Phone: (303)926 3123
 EMail: andrew.dugan@l3.com
 Isaac Elliott
 Level3 Communications
 1450 Infinite Drive
 Louisville, CO 80027
 Phone: (303)926 3123
 EMail: ike.elliott@l3.com
Arango, et al. Informational [Page 130] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 Christian Huitema
 Telcordia Technologies
 MCC 1J236B
 445 South Street
 Morristown, NJ 07960
 U.S.A.
 Phone: +1 973-829-4266
 EMail: huitema@research.telcordia.com
 Scott Pickett
 Vertical Networks
 1148 East Arques Ave
 Sunnyvale, CA 94086
 Phone: (408) 523-9700 extension 200
 EMail: ScottP@vertical.com
 Further information is available on the SGCP web site:
         http://www.argreenhouse.com/SGCP/
Arango, et al. Informational [Page 131] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 12. Appendix A: Proposed "MoveConnection" command
 It has been proposed to create a new command, that would move an
 existing connection from one endpoint to another, on the same
 gateway.  This command would be specially useful for handling certain
 call services, such as call forwarding between endpoints served by
 the same gateway.
       [SecondEndPointId,]
       [ConnectionId,]
       [LocalConnectionDescriptor]
        <--- ModifyConnection(CallId,
                              EndpointId,
                              ConnectionId,
                              SecondEndPointId,
                              [NotifiedEntity,]
                              [LocalConnectionOptions,]
                              [Mode,]
                              [RemoteConnectionDescriptor,]
                              [Encapsulated NotificationRequest,]
                              [Encapsulated EndpointConfiguration])
 The parameters used are the same as in the ModifyConnection command,
 with the addition of a SecondEndpointId that identifies the endpoint
 towards which the connection is moved.
 The EndpointId should be the fully qualified endpoint identifier of
 the endpoint on which the connection has been created. The local name
 shall not use the wildcard convention.
 The SecondEndpointId shall be the endpoint identifier of the endpoint
 towards which the connection has been created. The "any of" wildcard
 convention can be used, but not the "all of" convention.  If the
 SecondEndpointId parameter is unqualified, the gateway will choose a
 value, that will be returned to the call agent as a response
 parameter.
 The command will result in the "move" of the existing connection to
 the second endpoint.  Depending on gateway implementations, the
 connection identifier of the connection after the move may or may not
 be the same as the connection identifier before the move.  If it is
 not the same, the new value is returned as a response parameter.
 The intent of the command is to effect a local relocation of the
 connection, without having to modify such transmission parameters as
 IP addresses and port, and thus without forcing the call agent to
 signal the change of parameters to the remote gateway, at the other
Arango, et al. Informational [Page 132] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
 end of the connection.  However, gateway architectures may not always
 allow such transparent moves.  For example, some architectures could
 allow specific IP addresses to different boards that handles specific
 group of endpoints.  If for any reason the transmission parameters
 have to be changed as a result of the move, the new
 LocalConnectionDescriptor is returned as a response parameter.
 The LocalConnectionOptions, Mode, and RemoteConnectionDescriptor,
 when present, are applied after the move.
 The RequestedEvents, RequestIdentifier, DigitMap, SignalRequests,
 QuarantineHandling and DetectEvents parameters are optional.  They
 can be used by the Call Agent to transmit a NotificationRequest that
 is executed simultaneously with the move of the connection. When
 these parameters are present, the NotificationRequest applies to the
 second endpoint.
 When these parameters are present, the move and the
 NotificationRequests should be synchronized, which means that both
 should be accepted, or both refused.  The NotifiedEntity parameter,
 if present, applies to both the ModifyConnection and the
 NotificationRequest command.
 The command may carry an encapsulated EndpointConfiguration command,
 that will also apply to the second endpoint.  When this command is
 present, the parameters of the EndpointConfiguration command are
 inserted after the normal parameters of the MoveConnection with the
 exception of the SecondEndpointId, which is not replicated. The End-
 pointConfiguration command may be encapsulated together with an
 encapsulated NotificationRequest command.
 The encapsulated EndpointConfiguration command shares the fate of the
 MoveConnection command.  If the MoveConnection is rejected, the End-
 pointConfiguration is not executed.
12.1. Proposed syntax modification
 The only syntax modification necessary for the addition of the
 moveConnection command is the addition of the keyword MOVE to the
 authorized values in the MGCPVerb clause of the formal syntax.
Arango, et al. Informational [Page 133] RFC 2705 Media Gateway Control Protocol (MGCP) October 1999 13. Full Copyright Statement
 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
 Funding for the RFC Editor function is currently provided by the
 Internet Society.
Arango, et al. Informational [Page 134]
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