rfc2705.txt

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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.






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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



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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



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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






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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



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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.