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RFC 3525


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Gateway Control Protocol Version 1

Part 1 of 7, p. 1 to 26
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Obsoleted by:    5125
Obsoletes:    3015


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Network Working Group                                          C. Groves
Request for Comments: 3525                                   M. Pantaleo
Obsoletes: 3015                                              LM Ericsson
Category: Standards Track                                    T. Anderson
                                                              Consultant
                                                               T. Taylor
                                                         Nortel Networks
                                                                 Editors
                                                               June 2003


                  Gateway Control Protocol Version 1

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document defines the protocol used between elements of a
   physically decomposed multimedia gateway, i.e., a Media Gateway and a
   Media Gateway Controller.  The protocol presented in this document
   meets the requirements for a media gateway control protocol as
   presented in RFC 2805.

   This document replaces RFC 3015.  It is the result of continued
   cooperation between the IETF Megaco Working Group and ITU-T Study
   Group 16.  It incorporates the original text of RFC 3015, modified by
   corrections and clarifications discussed on the Megaco
   E-mail list and incorporated into the Study Group 16 Implementor's
   Guide for Recommendation H.248.  The present version of this document
   underwent  ITU-T Last Call as Recommendation H.248 Amendment 1.
   Because of ITU-T renumbering, it was published by the ITU-T as
   Recommendation H.248.1 (03/2002), Gateway Control Protocol Version 1.

   Users of this specification are advised to consult the H.248 Sub-
   series Implementors' Guide at http://www.itu.int/itudoc/itu-
   t/com16/implgd for additional corrections and clarifications.

Page 2 
Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Table of Contents

   1 Scope.........................................................5
     1.1 Changes From RFC 3015.....................................5
     1.2 Differences From ITU-T Recommendation H.248.1 (03/2002)...5
   2 References....................................................6
     2.1 Normative references......................................6
     2.2 Informative references....................................9
   3 Definitions..................................................10
   4 Abbreviations................................................11
   5 Conventions..................................................12
   6 Connection model.............................................13
     6.1 Contexts.................................................16
     6.2 Terminations.............................................17
       6.2.1 Termination dynamics.................................21
       6.2.2 TerminationIDs.......................................21
       6.2.3 Packages.............................................22
       6.2.4 Termination properties and descriptors...............23
       6.2.5 Root Termination.....................................25
   7 Commands.....................................................26
     7.1 Descriptors..............................................27
       7.1.1 Specifying parameters................................27
       7.1.2 Modem descriptor.....................................28
       7.1.3 Multiplex descriptor.................................28
       7.1.4 Media descriptor.....................................29
       7.1.5 TerminationState descriptor..........................29
       7.1.6 Stream descriptor....................................30
       7.1.7 LocalControl descriptor..............................31
       7.1.8 Local and Remote descriptors.........................32
       7.1.9 Events descriptor....................................35
       7.1.10 EventBuffer descriptor..............................38
       7.1.11 Signals descriptor..................................38
       7.1.12 Audit descriptor....................................40
       7.1.13 ServiceChange descriptor............................41
       7.1.14 DigitMap descriptor.................................41
       7.1.15 Statistics descriptor...............................46
       7.1.16 Packages descriptor.................................47
       7.1.17 ObservedEvents descriptor...........................47
       7.1.18 Topology descriptor.................................47
       7.1.19 Error Descriptor....................................50
     7.2 Command Application Programming Interface................50
       7.2.1 Add..................................................51

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       7.2.2 Modify...............................................52
       7.2.3 Subtract.............................................53
       7.2.4 Move.................................................55
       7.2.5 AuditValue...........................................56
       7.2.6 AuditCapabilities....................................59
       7.2.7 Notify...............................................60
       7.2.8 ServiceChange........................................61
       7.2.9 Manipulating and Auditing Context Attributes.........65
       7.2.10 Generic Command Syntax..............................66
     7.3 Command Error Codes......................................66
   8 Transactions.................................................66
     8.1 Common parameters........................................68
       8.1.1 Transaction Identifiers..............................68
       8.1.2 Context Identifiers..................................68
     8.2 Transaction Application Programming Interface............69
       8.2.1 TransactionRequest...................................69
       8.2.2 TransactionReply.....................................69
       8.2.3 TransactionPending...................................71
     8.3 Messages.................................................72
   9 Transport....................................................72
     9.1 Ordering of Commands.....................................73
     9.2 Protection against Restart Avalanche.....................74
   10 Security Considerations.....................................75
     10.1 Protection of Protocol Connections......................75
     10.2 Interim AH scheme.......................................76
     10.3 Protection of Media Connections.........................77
   11 MG-MGC Control Interface....................................78
     11.1 Multiple Virtual MGs....................................78
     11.2 Cold start..............................................79
     11.3 Negotiation of protocol version.........................79
     11.4 Failure of a MG.........................................80
     11.5 Failure of an MGC.......................................81
   12 Package definition..........................................82
     12.1 Guidelines for defining packages........................82
       12.1.1 Package.............................................83
       12.1.2 Properties..........................................84
       12.1.3 Events..............................................85
       12.1.4 Signals.............................................85
       12.1.5 Statistics..........................................86
       12.1.6 Procedures..........................................86
     12.2 Guidelines to defining Parameters to Events and Signals.86
     12.3 Lists...................................................87
     12.4 Identifiers.............................................87
     12.5 Package registration....................................88
   13 IANA Considerations.........................................88
     13.1 Packages................................................88
     13.2 Error codes.............................................89
     13.3 ServiceChange reasons...................................89

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   ANNEX A  Binary encoding of the protocol.......................90
     A.1 Coding of wildcards......................................90
     A.2 ASN.1 syntax specification...............................92
     A.3 Digit maps and path names...............................111
   ANNEX B Text encoding of the protocol.........................113
     B.1 Coding of wildcards.....................................113
     B.2 ABNF specification......................................113
     B.3 Hexadecimal octet coding................................127
     B.4 Hexadecimal octet sequence..............................127
   ANNEX C Tags for media stream properties......................128
     C.1 General media attributes................................128
     C.2 Mux properties..........................................130
     C.3 General bearer properties...............................130
     C.4 General ATM properties..................................130
     C.5 Frame Relay.............................................134
     C.6 IP......................................................134
     C.7 ATM AAL2................................................134
     C.8 ATM AAL1................................................136
     C.9 Bearer capabilities.....................................137
     C.10 AAL5 properties........................................147
     C.11 SDP equivalents........................................148
     C.12 H.245..................................................149
   ANNEX D Transport over IP.....................................150
     D.1 Transport over IP/UDP using Application Level Framing ..150
       D.1.1 Providing At-Most-Once functionality................150
       D.1.2 Transaction identifiers and three-way handshake.....151
       D.1.3 Computing retransmission timers.....................152
       D.1.4 Provisional responses...............................153
       D.1.5 Repeating Requests, Responses and Acknowledgements..153
     D.2 Using TCP...............................................155
       D.2.1 Providing the At-Most-Once functionality............155
       D.2.2 Transaction identifiers and three-way handshake.....155
       D.2.3 Computing retransmission timers.....................156
       D.2.4 Provisional responses...............................156
       D.2.5 Ordering of commands................................156
   ANNEX E  Basic packages.......................................157
     E.1 Generic.................................................157
     E.2 Base Root Package.......................................159
     E.3 Tone Generator Package..................................161
     E.4 Tone Detection Package..................................163
     E.5 Basic DTMF Generator Package............................166
     E.6 DTMF detection Package..................................167
     E.7 Call Progress Tones Generator Package...................169
     E.8 Call Progress Tones Detection Package...................171
     E.9 Analog Line Supervision Package.........................172
     E.10 Basic Continuity Package...............................175
     E.11 Network Package........................................178
     E.12 RTP Package............................................180

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     E.13 TDM Circuit Package....................................182
   APPENDIX I EXAMPLE CALL FLOWS (INFORMATIVE)...................184
     A.1 Residential Gateway to Residential Gateway Call.........184
       A.1.1 Programming Residential GW Analog Line Terminations
             for Idle Behavior...................................184
       A.1.2 Collecting Originator Digits and Initiating
             Termination.........................................186
   APPENDIX II  Changes From RFC 3015............................195
   Intellectual Property Rights..................................210
   Acknowledgments...............................................211
   Authors' Addresses............................................212
   Full Copyright Statement......................................213

1  Scope

   The present document, which is identical to the published version of
   ITU-T Recommendation H.248.1 (03/2002) except as noted below, defines
   the protocols used between elements of a physically decomposed
   multimedia gateway.  There are no functional differences from a
   system view between a decomposed gateway, with distributed sub-
   components potentially on more than one physical device, and a
   monolithic gateway such as described in ITU-T Recommendation H.246.
   This document does not define how gateways, multipoint control units
   or interactive voice response units (IVRs) work.  Instead it creates
   a general framework that is suitable for these applications.

   Packet network interfaces may include IP, ATM or possibly others.
   The interfaces will support a variety of Switched Circuit Network
   (SCN) signalling systems, including tone signalling, ISDN, ISUP, QSIG
   and GSM.  National variants of these signalling systems will be
   supported where applicable.

1.1 Changes From RFC 3015

   The differences between this document and RFC 3015 are documented in
   Appendix II.

1.2 Differences From ITU-T Recommendation H.248.1 (03/2002)

   This document differs from the corresponding ITU-T publication in the
   following respects:

   -  Added IETF front matter in place of the corresponding ITU-T
      material.

   -  The ITU-T summary is too H.323-specific and has been omitted.

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   -  The IETF conventions have been stated as governing this document.
      As discussed in section 5 below, this gives slightly greater
      strength to "should" requirements.

   -  The Scope section (just above) has been edited slightly to suit
      its IETF context.

   -  Added normative references to RFCs 2026 and 2119.

   -  Figures 4, 5, and 6 show the centre of the context for greater
      clarity.  Also added Figure 6a showing an important additional
      example.

   -  Added a paragraph in section 7.1.18 which was approved in the
      Implementor's Guide but lost inadvertently in the ITU-T approved
      version.

   -  This document incorporates corrections to the informative examples
      in Appendix I which also appear in H.248.1 version 2, but which
      were not picked up in H.248.1 (03/2002).

   -  This document includes a new Appendix II listing all the changes
      from RFC 3015.

   -  This document includes an Acknowledgements section listing the
      authors of RFC 3015 but also many other people who contributed to
      the development of the Megaco/H.248.x protocol.

   -  Moved the Intellectual Property declaration to its usual place in
      an IETF document and added a reference to declarations on the IETF
      web site.

2  References

   The following ITU-T Recommendations and other references contain
   provisions which, through reference in this text, constitute
   provisions of this RFC.  At the time of publication, the editions
   indicated were valid.  All Recommendations and other references are
   subject to revision; all users of this RFC are therefore encouraged
   to investigate the possibility of applying the most recent edition of
   the Recommendations and other references listed below.  A list of the
   currently valid ITU-T Recommendations is regularly published.

2.1   Normative references

   -  ITU-T Recommendation H.225.0 (1999), Call signalling protocols and
      media stream packetization for packet-based multimedia
      communication systems.

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   -  ITU-T Recommendation H.235 (1998), Security and encryption for
      H-Series (H.323 and other H.245-based) multimedia terminals.

   -  ITU-T Recommendation H.245 (1998), Control protocol for multimedia
      communication.

   -  ITU-T Recommendation H.246 (1998), Interworking of H-series
      multimedia terminals with H-series multimedia terminals and
      voice/voiceband terminals on GSTN and ISDN.

   -  ITU-T Recommendation H.248.8 (2002), H.248 Error Codes and Service
      Change Reasons.

   -  ITU-T Recommendation H.323 (1999), Packet-based multimedia
      communication systems.

   -  ITU-T Recommendation I.363.1 (1996), B-ISDN ATM adaptation layer
      (AAL) specification: Type 1 AAL.

   -  ITU-T Recommendation I.363.2 (1997), B-ISDN ATM adaptation layer
      (AAL) specification: Type 2 AAL.

   -  ITU-T Recommendation I.363.5 (1996), B-ISDN ATM adaptation layer
      (AAL) specification: Type 5 AAL.

   -  ITU-T Recommendation I.366.1 (1998), Segmentation and Reassembly
      Service Specific Convergence Sublayer for the AAL type 2.

   -  ITU-T Recommendation I.366.2 (1999), AAL type 2 service specific
      convergence sublayer for trunking.

   -  ITU-T Recommendation I.371 (2000), Traffic control and congestion
      control in B-ISDN.

   -  ITU-T Recommendation Q.763 (1999), Signalling System No. 7 - ISDN
      user part formats and codes.

   -  ITU-T Recommendation Q.765.5 (2001), Application transport
      mechanism - Bearer independent call control (BICC).

   -  ITU-T Recommendation Q.931 (1998), ISDN user-network interface
      layer 3 specification for basic call control.

   -  ITU-T Recommendation Q.2630.1 (1999), AAL type 2 signalling
      protocol (Capability Set 1).

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   -  ITU-T Recommendation Q.2931 (1995), Digital Subscriber Signalling
      System No. 2 (DSS2) - User-Network Interface (UNI) - Layer 3
      specification for basic call/connection control.

   -  ITU-T Recommendation Q.2941.1 (1997), Digital Subscriber
      Signalling System No. 2 - Generic identifier transport.

   -  ITU-T Recommendation Q.2961.1 (1995), Additional signalling
      capabilities to support traffic parameters for the tagging option
      and the sustainable call rate parameter set.

   -  ITU-T Recommendation Q.2961.2 (1997), Additional traffic
      parameters: Support of ATM transfer capability in the broadband
      bearer capability information element.

   -  ITU-T Recommendation Q.2965.1 (1999), Digital subscriber
      signalling system No. 2 - Support of Quality of Service classes.

   -  ITU-T Recommendation Q.2965.2 (1999), Digital subscriber
      signalling system No. 2 - Signalling of individual Quality of
      Service parameters.

   -  ITU-T Recommendation V.76 (1996), Generic multiplexer using V.42
      LAPM-based procedures.

   -  ITU-T Recommendation X.213 (1995), Information technology - Open
      Systems Interconnection - Network service definition plus
      Amendment 1 (1997), Addition of the Internet protocol address
      format identifier.

   -  ITU-T Recommendation X.680 (1997), Information technology -
      Abstract Syntax Notation One (ASN.1): Specification of basic
      notation.

   -  ITU-T Recommendation X.690 (1997), Information Technology - ASN.1
      Encoding Rules: Specification of Basic Encoding Rules (BER),
      Canonical Encoding Rules (CER) and Distinguished Encoding Rules
      (DER).

   -  ATM Forum (1996), ATM User-Network Interface (UNI) Signalling
      Specification - Version 4.0.

   [RFC 1006] Rose, M. and D. Cass, "ISO Transport Service on top of the
              TCP, Version 3", STD 35, RFC 1006, May 1987.

   [RFC 2026] Brander, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

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   [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC 2234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.

   [RFC 2327] Handley, M. and V. Jacobson, "SDP: Session Description
              Protocol", RFC 2327, April 1998.

   [RFC 2402] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
              2402, November 1998.

   [RFC 2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
              Payload (ESP)", RFC 2406, November 1998.

2.2   Informative references

   -  ITU-T Recommendation E.180/Q.35 (1998), Technical characteristics
      of tones for the telephone service.

   -  CCITT Recommendation G.711 (1988), Pulse Code Modulation (PCM) of
      voice frequencies.

   -  ITU-T Recommendation H.221 (1999), Frame structure for a 64 to
      1920 kbit/s channel in audiovisual teleservices.

   -  ITU  T Recommendation H.223 (1996), Multiplexing protocol for low
      bit rate multimedia communication.

   -  ITU-T Recommendation H.226 (1998), Channel aggregation protocol
      for multilink operation on circuit-switched networks

   -  ITU-T Recommendation Q.724 (1998), Signalling procedures.

   -  ITU-T Recommendation Q.764 (1999), Signalling system No. 7 - ISDN
      user part signalling procedures.

   -  ITU-T Recommendation Q.1902.4 (2001), Bearer independent call
      control protocol - Basic call procedures.

   [RFC 768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC 791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

   [RFC 793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

Top      ToC       Page 10 
   [RFC 1661] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)", STD
              51, RFC 1661, July 1994.

   [RFC 1889] Schulzrinne, H., Casner, S., Frederick, R. and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", RFC 1889, January 1996.

   [RFC 1890] Schulzrinne, H. and G. Fokus, "RTP Profile for Audio and
              Video Conferences with Minimal Control",  RFC 1890,
              January 1996.

   [RFC 2401] Kent, S. and R. Atkinson, "Security Architecture for the
              Internet Protocol", RFC 2401, November 1998.

   [RFC 2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC 2543] Handley, M., Schulzrinne, H., Schooler, E. and J.
              Rosenberg, "SIP: Session Initiation Protocol", RFC 2543,
              March 1999.

   [RFC 2805] Greene, N., Ramalho, M. and B. Rosen, "Media Gateway
              Control Protocol Architecture and Requirements", RFC 2805,
              April 2000.

3  Definitions

   This document defines the following terms:

   Access gateway:
   A type of gateway that provides a User-Network Interface (UNI) such
   as ISDN.

   Descriptor:
   A syntactic element of the protocol that groups related properties.
   For instance, the properties of a media flow on the MG can be set by
   the MGC by including the appropriate descriptor in a command.

   Media Gateway (MG):
   The media gateway converts media provided in one type of network to
   the format required in another type of network.  For example, a MG
   could terminate bearer channels from a switched circuit network
   (e.g., DS0s) and media streams from a packet network (e.g., RTP
   streams in an IP network).  This gateway may be capable of processing
   audio, video and T.120 alone or in any combination, and will be
   capable of full duplex media translations.  The MG may also play
   audio/video messages and perform other IVR functions, or may perform
   media conferencing.

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   Media Gateway Controller (MGC):
   Controls the parts of the call state that pertain to connection
   control for media channels in a MG.

   Multipoint Control Unit (MCU):
   An entity that controls the setup and coordination of a multi-user
   conference that typically includes processing of audio, video and
   data.

   Residential gateway:
   A gateway that interworks an analogue line to a packet network.  A
   residential gateway typically contains one or two analogue lines and
   is located at the customer premises.

   SCN FAS signalling gateway:
   This function contains the SCN Signalling Interface that terminates
   SS7, ISDN or other signalling links where the call control channel
   and bearer channels are collocated in the same physical span.

   SCN NFAS signalling gateway:
   This function contains the SCN Signalling Interface that terminates
   SS7 or other signalling links where the call control channels are
   separated from bearer channels.

   Stream:
   Bidirectional media or control flow received/sent by a media gateway
   as part of a call or conference.

   Trunk:
   A communication channel between two switching systems such as a DS0
   on a T1 or E1 line.

   Trunking gateway:
   A gateway between SCN network and packet network that typically
   terminates a large number of digital circuits.

4  Abbreviations

   This RFC document uses the following abbreviations:

   ALF   Application Layer Framing

   ATM   Asynchronous Transfer Mode

   CAS   Channel Associated Signalling

   DTMF  Dual Tone Multi-Frequency

Top      ToC       Page 12 
   FAS   Facility Associated Signalling

   GSM   Global System for Mobile communications

   GW    GateWay

   IANA  Internet Assigned Numbers Authority (superseded by Internet
         Corporation for Assigned Names and Numbers - ICANN)

   IP    Internet Protocol

   ISUP  ISDN User Part

   IVR   Interactive Voice Response

   MG    Media Gateway

   MGC   Media Gateway Controller

   NFAS  Non-Facility Associated Signalling

   PRI   Primary Rate Interface

   PSTN  Public Switched Telephone Network

   QoS   Quality of Service

   RTP   Real-time Transport Protocol

   SCN   Switched Circuit Network

   SG    Signalling Gateway

   SS7   Signalling System No. 7

5  Conventions

   In the H.248.1 Recommendation, "SHALL" refers to a mandatory
   requirement, while "SHOULD" refers to a suggested but optional
   feature or procedure.  The term "MAY" refers to an optional course of
   action without expressing a preference.  Note that these definition
   are overridden in the present document by the RFC 2119 conventions
   stated at the beginning of this document.  RFC 2119 has a more
   precise definition of "should" than is provided by the ITU-T.

Top      ToC       Page 13 
6  Connection model

   The connection model for the protocol describes the logical entities,
   or objects, within the Media Gateway that can be controlled by the
   Media Gateway Controller.  The main abstractions used in the
   connection model are Terminations and Contexts.

   A Termination sources and/or sinks one or more streams.  In a
   multimedia conference, a Termination can be multimedia and sources or
   sinks multiple media streams.  The media stream parameters, as well
   as modem, and bearer parameters are encapsulated within the
   Termination.

   A Context is an association between a collection of Terminations.
   There is a special type of Context, the null Context, which contains
   all Terminations that are not associated to any other Termination.
   For instance, in a decomposed access gateway, all idle lines are
   represented by Terminations in the null Context.

   Following is a graphical depiction of these concepts.  The diagram of
   Figure 1 gives several examples and is not meant to be an
   all-inclusive illustration.  The asterisk box in each of the Contexts
   represents the logical association of Terminations implied by the
   Context.

Top      ToC       Page 14 
         +------------------------------------------------------+
         |Media Gateway                                         |
         | +-------------------------------------------------+  |
         | |Context                          +-------------+ |  |
         | |                                 | Termination | |  |
         | |                                 |-------------| |  |
         | |  +-------------+             +->| SCN Bearer  |<---+->
         | |  | Termination |   +-----+   |  |   Channel   | |  |
         | |  |-------------|   |     |---+  +-------------+ |  |
       <-+--->| RTP Stream  |---|  *  |                      |  |
         | |  |             |   |     |---+  +-------------+ |  |
         | |  +-------------+   +-----+   |  | Termination | |  |
         | |                              |  |-------------| |  |
         | |                              +->| SCN Bearer  |<---+->
         | |                                 |   Channel   | |  |
         | |                                 +-------------+ |  |
         | +-------------------------------------------------+  |
         |                                                      |
         |                                                      |
         |                    +------------------------------+  |
         |   (NULL Context)   |Context                       |  |
         |  +-------------+   |              +-------------+ |  |
         |  | Termination |   | +-----+      | Termination | |  |
         |  |-------------|   | |     |      |-------------| |  |
         |  | SCN Bearer  |   | |  *  |------| SCN Bearer  |<---+->
         |  |   Channel   |   | |     |      |   Channel   | |  |
         |  +-------------+   | +-----+      +-------------+ |  |
         |                    +------------------------------+  |
         |                                                      |
         |                                                      |
         | +-------------------------------------------------+  |
         | |Context                                          |  |
         | |  +-------------+                +-------------+ |  |
         | |  | Termination |   +-----+      | Termination | |  |
         | |  |-------------|   |     |      |-------------| |  |
       <-+--->| SCN Bearer  |---|  *  |------| SCN Bearer  |<---+->
         | |  |   Channel   |   |     |      |   Channel   | |  |
         | |  +-------------+   +-----+      +-------------+ |  |
         | +-------------------------------------------------+  |
         | ___________________________________________________  |
         +------------------------------------------------------+

            Figure 1: Examples of Megaco/H.248 Connection Model

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   The example in Figure 2 shows an example of one way to accomplish a
   call-waiting scenario in a decomposed access gateway, illustrating
   the relocation of a Termination between Contexts.  Terminations T1
   and T2 belong to Context C1 in a two-way audio call.  A second audio
   call is waiting for T1 from Termination T3.  T3 is alone in Context
   C2.  T1 accepts the call from T3, placing T2 on hold.  This action
   results in T1 moving into Context C2, as shown in Figure 3.

         +------------------------------------------------------+
         |Media Gateway                                         |
         | +-------------------------------------------------+  |
         | |Context C1                                       |  |
         | |  +-------------+                +-------------+ |  |
         | |  | Term. T2    |   +-----+      | Term. T1    | |  |
         | |  |-------------|   |     |      |-------------| |  |
       <-+--->| RTP Stream  |---|  *  |------| SCN Bearer  |<---+->
         | |  |             |   |     |      |   Channel   | |  |
         | |  +-------------+   +-----+      +-------------+ |  |
         | +-------------------------------------------------+  |
         |                                                      |
         | +-------------------------------------------------+  |
         | |Context C2                                       |  |
         | |                                 +-------------+ |  |
         | |                    +-----+      | Term. T3    | |  |
         | |                    |     |      |-------------| |  |
         | |                    |  *  |------| SCN Bearer  |<---+->
         | |                    |     |      |   Channel   | |  |
         | |                    +-----+      +-------------+ |  |
         | +-------------------------------------------------+  |
         +------------------------------------------------------+

     Figure 2: Example Call Waiting Scenario / Alerting Applied to T1

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         +------------------------------------------------------+
         |Media Gateway                                         |
         | +-------------------------------------------------+  |
         | |Context C1                                       |  |
         | |  +-------------+                                |  |
         | |  | Term. T2    |   +-----+                      |  |
         | |  |-------------|   |     |                      |  |
       <-+--->| RTP Stream  |---|  *  |                      |  |
         | |  |             |   |     |                      |  |
         | |  +-------------+   +-----+                      |  |
         | +-------------------------------------------------+  |
         |                                                      |
         | +-------------------------------------------------+  |
         | |Context C2                                       |  |
         | |  +-------------+                +-------------+ |  |
         | |  | Term. T1    |   +-----+      | Term. T3    | |  |
         | |  |-------------|   |     |      |-------------| |  |
       <-+--->| SCN Bearer  |---|  *  |------| SCN Bearer  |<---+->
         | |  |   Channel   |   |     |      |   Channel   | |  |
         | |  +-------------+   +-----+      +-------------+ |  |
         | +-------------------------------------------------+  |
         +------------------------------------------------------+

          Figure 3. Example Call Waiting Scenario / Answer by T1

6.1   Contexts

   A Context is an association between a number of Terminations.  The
   Context describes the topology (who hears/sees whom) and the media
   mixing and/or switching parameters if more than two Terminations are
   involved in the association.

   There is a special Context called the null Context.  It contains
   Terminations that are not associated to any other Termination.
   Terminations in the null Context can have their parameters examined
   or modified, and may have events detected on them.

   In general, an Add command is used to add Terminations to Contexts.
   If the MGC does not specify an existing Context to which the
   Termination is to be added, the MG creates a new Context.  A
   Termination may be removed from a Context with a Subtract command,
   and a Termination may be moved from one Context to another with a
   Move command.  A Termination SHALL exist in only one Context at a
   time.

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   The maximum number of Terminations in a Context is a MG property.
   Media gateways that offer only point-to-point connectivity might
   allow at most two Terminations per Context.  Media gateways that
   support multipoint conferences might allow three or more Terminations
   per Context.

6.1.1 Context attributes and descriptors

   The attributes of Contexts are:

   -  ContextID.

   -  The topology (who hears/sees whom).

      The topology of a Context describes the flow of media between the
      Terminations within a Context.  In contrast, the mode of a
      Termination (send/receive/...) describes the flow of the media at
      the ingress/egress of the media gateway.

   -  The priority is used for a Context in order to provide the MG with
      information about a certain precedence handling for a Context.
      The MGC can also use the priority to control autonomously the
      traffic precedence in the MG in a smooth way in certain
      situations (e.g., restart), when a lot of Contexts must be handled
      simultaneously.  Priority 0 is the lowest priority and a priority
      of 15 is the highest priority.

   -  An indicator for an emergency call is also provided to allow a
      preference handling in the MG.

6.1.2 Creating, deleting and modifying Contexts

   The protocol can be used to (implicitly) create Contexts and modify
   the parameter values of existing Contexts.  The protocol has commands
   to add Terminations to Contexts, subtract them from Contexts, and to
   move Terminations between Contexts.  Contexts are deleted implicitly
   when the last remaining Termination is subtracted or moved out.

6.2   Terminations

   A Termination is a logical entity on a MG that sources and/or sinks
   media and/or control streams.  A Termination is described by a number
   of characterizing Properties, which are grouped in a set of
   Descriptors that are included in commands.  Terminations have unique
   identities (TerminationIDs), assigned by the MG at the time of their
   creation.

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   Terminations representing physical entities have a semi-permanent
   existence.  For example, a Termination representing a TDM channel
   might exist for as long as it is provisioned in the gateway.
   Terminations representing ephemeral information flows, such as RTP
   flows, would usually exist only for the duration of their use.

   Ephemeral Terminations are created by means of an Add command.  They
   are destroyed by means of a Subtract command.  In contrast, when a
   physical Termination is Added to or Subtracted from a Context, it is
   taken from or to the null Context, respectively.

   Terminations may have signals applied to them (see 7.1.11).
   Terminations may be programmed to detect Events, the occurrence of
   which can trigger notification messages to the MGC, or action by the
   MG.  Statistics may be accumulated on a Termination.  Statistics are
   reported to the MGC upon request (by means of the AuditValue command,
   see 7.2.5) and when the Termination is taken out of the call it is
   in.

   Multimedia gateways may process multiplexed media streams.  For
   example, Recommendation H.221 describes a frame structure for
   multiple media streams multiplexed on a number of digital 64 kbit/s
   channels.  Such a case is handled in the connection model in the
   following way.  For every bearer channel that carries part of the
   multiplexed streams, there is a physical or ephemeral "bearer
   Termination".  The bearer Terminations that source/sink the digital
   channels are connected to a separate Termination called the
   "multiplexing Termination".  The multiplexing termination is an
   ephemeral termination representing a frame-oriented session.  The
   MultiplexDescriptor for this Termination describes the multiplex used
   (e.g., H.221 for an H.320 session) and indicates the order in which
   the contained digital channels are assembled into a frame.

   Multiplexing terminations may be cascades (e.g., H.226 multiplex of
   digital channels feeding into a H.223 multiplex supporting an H.324
   session).

   The individual media streams carried in the session are described by
   StreamDescriptors on the multiplexing Termination.  These media
   streams can be associated with streams sourced/sunk by Terminations
   in the Context other than the bearer Terminations supporting the
   multiplexing Termination.  Each bearer Termination supports only a
   single data stream.  These data streams do not appear explicitly as
   streams on the multiplexing Termination and they are hidden from the
   rest of the context.

   Figures 4, 5, 6, and 6a illustrate typical applications of the
   multiplexing termination and Multiplex Descriptor.

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                  +-----------------------------------+
                  | Context     +-------+             |
                 +----+         |       |             |
   Circuit 1 -|--| TC1|---------+ Tmux  |             |
              |  +----+ (Str 1) |       |  Audio    +-----+
              |   |             |       +-----*-----+     |-----
              |  +----+         | H.22x | Stream 1  |     |
   Circuit 2 -|--| TC2|---------+ multi-|           | TR1 |
              |  +----+ (Str 1) | plex  |           |(RTP)|
              |   |             |       |  Video    |     |
              |  +----+         |       +-----*-----+     |-----
   Circuit 3 -|--| TC3|---------+       | Stream 2  |     |
              /  +----+ (Str 1) |       |           +-----+
             /    |             +-------+             |
            /     +-----------------\-----------------+
   Audio, video, and control         \
   signals are carried in frames    Tmux is an ephemeral with two
   spanning the circuits.           explicit Stream Descriptors
                                    and a Multiplex Descriptor.

      Figure 4: Multiplexed Termination Scenario - Circuit to Packet
              (Asterisks * denote the centre of the context)

                    Context
                  +--------------------------------------+
                  |       +-------+        +-------+     |
                 +----+   |       |        |       |   +----+
   Circuit 1 ----| TC1|---+ Tmux1 |  Audio | Tmux2 +---| TC4|---
                 +----+   |       +---*----+       |   +----+
                  |       |       |  Str 1 |       |     |
                 +----+   | H.22x |        | H.22x |   +----+
   Circuit 2 ----| TC2|---+ multi-|        | multi-+---| TC5|---
                 +----+   | plex  |        | plex  |   +----+
                  |       |       |  Video |       |     |
                 +----+   |       +---*----+       |   +----+
   Circuit 3 ----| TC3|---+       |  Str 2 |       +---| TC6|---
                 +----+   |       |        |       |   +----+
                  |       +-------+        +-------+     |
                  +-----------------\-----/--------------+
                                     \   /
             Tmux1 and Tmux2 are ephemerals each with two
            explicit Stream Descriptors and a Multiplex Descriptor.

      Figure 5: Multiplexed Termination Scenario - Circuit to Circuit
              (Asterisks * denote the centre of the context)

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                  +-----------------------------------+
                  | Context     +-------+             |
                 +----+         |       |             |
   Circuit 1 -|--| TC1|---------+ Tmux  |             |
              |  +----+ (Str 1) |       |  Audio    +-----+
              |   |             |       +-----*-----+ TR1 |-----
              |  +----+         | H.22x | Stream 1  |(RTP)|
   Circuit 2 -|--| TC2|---------+ multi-|           +-----+
              |  +----+ (Str 1) | plex  |             |
              |   |             |       |  Video    +-----+
              |  +----+         |       +-----*-----+ TR2 |-----
   Circuit 3 -|--| TC3|---------+       | Stream 2  |(RTP)|
              /  +----+ (Str 1) |       |           +-----+
             /    |             +-------+             |
            /     +-----------------\-----------------+
   Audio, video, and control         \ Tmux is an ephemeral with two
   signals are carried in frames    explicit Stream Descriptors and
   spanning the circuits.           and a Multiplex Descriptor.

      Figure 6: Multiplexed Termination Scenario - Single to Multiple
                               Terminations
              (Asterisks * denote the centre of the context)

            Context
          +---------------------------------------------+
          |       +-------+       +-------+             |
   Cct 1 +----+   |       |       |       | Audio     +-----+
     ----| TC1|---+ Tmux1 |       | Tmux2 +-----*-----| TR1 |-----
         +----+   |       |       |       | Stream 1  |(RTP)|
          |       |       | Data  |       |           +-----+
   Cct 2 +----+   | H.226 +-------+ H.223 |             |
     ----| TC2|---+ multi-|(Str 1)| multi-| Control   +-----+
         +----+   | plex  |       | plex  +-----*-----+ Tctl|-----
          |       |       |       |       | Stream 3  +-----+
   Cct 3 +----+   |       |       |       |             |
     ----| TC3|---+       |       |       |           +-----+
         +----+   |       |       |       +-----*-----+ TR2 |-----
          |       +-------+       |       |  Video    |(RTP)|
          |                       +-------+ Stream 2  +-----+
          |                                             |
          +---------------------------------------------+
        Tmux1 has a Multiplex Descriptor and a single data stream.
        Tmux2 has a Multiplex Descriptor with a single bearer and
        three explicit Stream Descriptors.

    Figure 6a: Multiplexed Termination Scenario - Cascaded Multiplexes
              (Asterisks * denote the centre of the context)
     Note: this figure does not appear in Rec.  H.248.1

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   Terminations may be created which represent multiplexed bearers, such
   as an ATM AAL Type 2 bearer.  When a new multiplexed bearer is to be
   created, an ephemeral Termination is created in a Context established
   for this purpose.  When the Termination is subtracted, the
   multiplexed bearer is destroyed.

6.2.1 Termination dynamics

   The protocol can be used to create new Terminations and to modify
   property values of existing Terminations.  These modifications
   include the possibility of adding or removing events and/or signals.
   The Termination properties, and events and signals are described in
   the ensuing subclauses.  An MGC can only release/modify Terminations
   and the resources that the Termination represents which it has
   previously seized via, e.g., the Add command.

6.2.2 TerminationIDs

   Terminations are referenced by a TerminationID, which is an arbitrary
   schema chosen by the MG.

   TerminationIDs of physical Terminations are provisioned in the Media
   Gateway.  The TerminationIDs may be chosen to have structure.  For
   instance, a TerminationID may consist of trunk group and a trunk
   within the group.

   A wildcarding mechanism using two types of wildcards can be used with
   TerminationIDs.  The two wildcards are ALL and CHOOSE.  The former is
   used to address multiple Terminations at once, while the latter is
   used to indicate to a media gateway that it must select a Termination
   satisfying the partially specified TerminationID.  This allows, for
   instance, that a MGC instructs a MG to choose a circuit within a
   trunk group.

   When ALL is used in the TerminationID of a command, the effect is
   identical to repeating the command with each of the matching
   TerminationIDs.  The use of ALL does not address the ROOT
   termination.  Since each of these commands may generate a response,
   the size of the entire response may be large.  If individual
   responses are not required, a wildcard response may be requested.  In
   such a case, a single response is generated, which contains the UNION
   of all of the individual responses which otherwise would have been
   generated, with duplicate values suppressed.  For instance, given a
   Termination Ta with properties p1=a, p2=b and Termination Tb with

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   properties p2=c, p3=d, a UNION response would consist of a wildcarded
   TerminationId and the sequence of properties p1=a, p2=b,c and p3=d.
   Wildcard response may be particularly useful in the Audit commands.

   The encoding of the wildcarding mechanism is detailed in Annexes A
   and B.

6.2.3 Packages

   Different types of gateways may implement Terminations that have
   widely differing characteristics.  Variations in Terminations are
   accommodated in the protocol by allowing Terminations to have
   optional Properties, Events, Signals and Statistics implemented by
   MGs.

   In order to achieve MG/MGC interoperability, such options are grouped
   into Packages, and typically a Termination realizes a set of such
   Packages.  More information on definition of packages can be found in
   clause 12.  An MGC can audit a Termination to determine which
   Packages it realizes.

   Properties, Events, Signals and Statistics defined in Packages, as
   well as parameters to them, are referenced by identifiers (Ids).
   Identifiers are scoped.  For each package, PropertyIds, EventIds,
   SignalIds, StatisticsIds and ParameterIds have unique name spaces and
   the same identifier may be used in each of them.  Two PropertyIds in
   different packages may also have the same identifier, etc.

   To support a particular package the MG must support all properties,
   signals, events and statistics defined in a package.  It must also
   support all Signal and Event parameters.  The MG may support a subset
   of the values listed in a package for a particular Property or
   Parameter.

   When packages are extended, the properties, events, signals and
   statistics defined in the base package can be referred to using
   either the extended package name or the base package name.  For
   example, if Package A defines event e1, and Package B extends Package
   A, then B/e1 is an event for a termination implementing Package B. By
   definition, the MG MUST also implement the base Package, but it is
   optional to publish the base package as an allowed interface.  If it
   does publish  A, then A would be reported on the Package Descriptor
   in AuditValue as well as B, and event A/e1 would be available on a
   termination.  If the MG does not publish A, then only B/e1 would be
   available.  If published through AuditValue, A/e1 and B/e1 are the
   same event.

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   For improved interoperability and backward compatibility, an MG MAY
   publish all Packages supported by its Terminations, including base
   Packages from which extended Packages are derived.  An exception to
   this is in cases where the base packages are expressly "Designed to
   be extended only".

6.2.4 Termination properties and descriptors

   Terminations have properties.  The properties have unique
   PropertyIDs. Most properties have default values, which are
   explicitly defined in this protocol specification or in a package
   (see clause 12) or set by provisioning.  If not provisioned
   otherwise, the properties in all descriptors except TerminationState
   and LocalControl default to empty/"no value" when a Termination is
   first created or returned to the null Context.  The default contents
   of the two exceptions are described in 7.1.5 and 7.1.7.

   The provisioning of a property value in the MG will override any
   default value, be it supplied in this protocol specification or in a
   package.  Therefore if it is essential for the MGC to have full
   control over the property values of a Termination, it should supply
   explicit values when ADDing the Termination to a Context.
   Alternatively, for a physical Termination the MGC can determine any
   provisioned property values by auditing the Termination while it is
   in the NULL Context.

   There are a number of common properties for Terminations and
   properties specific to media streams.  The common properties are also
   called the Termination state properties.  For each media stream,
   there are local properties and properties of the received and
   transmitted flows.

   Properties not included in the base protocol are defined in Packages.
   These properties are referred to by a name consisting of the
   PackageName and a PropertyId.  Most properties have default values
   described in the Package description.  Properties may be read-only or
   read/write.  The possible values of a property may be audited, as can
   their current values.  For properties that are read/write, the MGC
   can set their values.  A property may be declared as "Global" which
   has a single value shared by all Terminations realizing the package.
   Related properties are grouped into descriptors for convenience.

   When a Termination is added to a Context, the value of its read/write
   properties can be set by including the appropriate descriptors as
   parameters to the Add command.  Similarly, a property of a
   Termination in a Context may have its value changed by the Modify
   command.

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   Properties may also have their values changed when a Termination is
   moved from one Context to another as a result of a Move command.  In
   some cases, descriptors are returned as output from a command.

   In general, if a Descriptor is completely omitted from one of the
   aforementioned Commands, the properties in that Descriptor retain
   their prior values for the Termination(s) upon which the Command
   acts.  On the other hand, if some read/write properties are omitted
   from a Descriptor in a Command (i.e., the Descriptor is only
   partially specified), those properties will be reset to their default
   values for the Termination(s) upon which the Command acts, unless the
   package specifies other behavior.  For more details, see clause 7.1
   dealing with the individual Descriptors.

   The following table lists all of the possible descriptors and their
   use.  Not all descriptors are legal as input or output parameters to
   every command.

   Descriptor name  Description

   Modem            Identifies modem type and properties when
                    applicable

   Mux              Describes multiplex type for multimedia
                    Terminations (e.g., H.221, H.223, H.225.0) and
                    Terminations forming the input mux

   Media            A list of media stream specifications (see 7.1.4)

   TerminationState Properties of a Termination (which can be defined
                    in Packages) that are not stream specific

   Stream           A list of remote/local/localControl descriptors for
                    a single stream

   Local            Contains properties that specify the media flows
                    that the MG receives from the remote entity.

   Remote           Contains properties that specify the media flows
                    that the MG sends to the remote entity.

   LocalControl     Contains properties (which can be defined in
                    packages) that are of interest between the MG and
                    the MGC.

   Events           Describes events to be detected by the MG and what
                    to do when an event is detected.

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   EventBuffer      Describes events to be detected by the MG when
                    Event Buffering is active.

   Signals          Describes signals (see 7.1.11) applied  to
                    Terminations.

   Audit            In Audit commands, identifies which information is
                    desired.

   Packages         In AuditValue, returns a list of Packages realized
                    by Termination.

   DigitMap         Defines patterns against which sequences of a
                    specified set of events are to be matched so they
                    can be reported as a group rather than singly.

   ServiceChange    In ServiceChange, what, why service change
                    occurred, etc.

   ObservedEvents   In Notify or AuditValue, report of events observed.

   Statistics       In Subtract and Audit, report of Statistics kept on
                    a Termination.

   Topology         Specifies flow directions between Terminations in a
                    Context.

   Error            Contains an error code and optionally error text;
                    it may occur in command replies and in Notify
                    requests.

6.2.5 Root Termination

   Occasionally, a command must refer to the entire gateway, rather than
   a Termination within it.  A special TerminationID, "Root" is reserved
   for this purpose.  Packages may be defined on Root.  Root thus may
   have properties, events and statistics (signals are not appropriate
   for root).  Accordingly, the root TerminationID may appear in:

   -  a Modify command - to change a property or set an event

   -  a Notify command - to report an event

   -  an AuditValue return - to examine the values of properties and
      statistics implemented on root

   -  an AuditCapability - to determine what properties of root are
      implemented

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   -  a ServiceChange - to declare the gateway in or out of service.

   Any other use of the root TerminationID is an error.  Error code
   410 - Incorrect identifier shall be returned in these cases.



(page 26 continued on part 2)

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