Network Working Group                                          F. Cuervo
Request for Comments: 2885                                     N. Greene
Category: Standards Track                                Nortel Networks
                                                              C. Huitema
                                                   Microsoft Corporation
                                                               A. Rayhan
                                                         Nortel Networks
                                                                B. Rosen
                                                                 Marconi
                                                               J. Segers
                                                     Lucent Technologies
                                                             August 2000


                      Megaco Protocol version 0.8

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 (2000).  All Rights Reserved.

Abstract

   This document is common text with Recommendation H.248 as
   redetermined in Geneva, February 2000.  It must be read in
   conjunction with the Megaco Errata, RFC 2886.  A merged document
   presenting the Megaco protocol with the Errata incorporated will be
   available shortly.

   The protocol presented in this document meets the requirements for a
   media gateway control protocol as presented in RFC 2805.













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TABLE OF CONTENTS

   1. SCOPE..........................................................6
   2. REFERENCES.....................................................6
   2.1 Normative references..........................................6
   2.2 Informative references........................................8
   3. DEFINITIONS....................................................9
   4. ABBREVIATIONS.................................................10
   5. CONVENTIONS...................................................11
   6. CONNECTION MODEL..............................................11
   6.1 Contexts.....................................................14
        6.1.1 Context Attributes and Descriptors....................15
        6.1.2 Creating, Deleting and Modifying Contexts.............15
   6.2 Terminations.................................................15
        6.2.1 Termination Dynamics..................................16
        6.2.2 TerminationIDs........................................17
        6.2.3 Packages..............................................17
        6.2.4 Termination Properties and Descriptors................18
        6.2.5 Root Termination......................................20
   7. COMMANDS......................................................20
   7.1 Descriptors..................................................21
        7.1.1 Specifying Parameters.................................21
        7.1.2 Modem Descriptor......................................22
        7.1.3 Multiplex Descriptor..................................22
        7.1.4 Media Descriptor......................................23
        7.1.5 Termination State Descriptor..........................23
        7.1.6 Stream Descriptor.....................................24
        7.1.7 LocalControl Descriptor...............................24
        7.1.8 Local and Remote Descriptors..........................25
        7.1.9 Events Descriptor.....................................28
        7.1.10 EventBuffer Descriptor...............................31
        7.1.11 Signals Descriptor...................................31
        7.1.12 Audit Descriptor.....................................32
        7.1.13 ServiceChange Descriptor.............................33
        7.1.14 DigitMap Descriptor..................................33
        7.1.15 Statistics Descriptor................................38
        7.1.16 Packages Descriptor..................................39
        7.1.17 ObservedEvents Descriptor............................39
        7.1.18  Topology Descriptor.................................39
   7.2 Command Application Programming Interface....................42
        7.2.1 Add...................................................43
        7.2.2 Modify................................................44
        7.2.3 Subtract..............................................45
        7.2.4 Move..................................................46
        7.2.5 AuditValue............................................47
        7.2.6 AuditCapabilities.....................................48
        7.2.7 Notify................................................49
        7.2.8 ServiceChange.........................................50



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        7.2.9 Manipulating and Auditing Context Attributes..........54
        7.2.10 Generic Command Syntax...............................54
   7.3 Command Error Codes..........................................55
   8. TRANSACTIONS..................................................56
   8.1 Common Parameters............................................58
        8.1.1 Transaction Identifiers...............................58
        8.1.2 Context Identifiers...................................58
   8.2 Transaction Application Programming Interface................58
        8.2.1 TransactionRequest....................................59
        8.2.2 TransactionReply......................................59
        8.2.3 TransactionPending....................................60
   8.3 Messages.....................................................61
   9. TRANSPORT.....................................................61
   9.1 Ordering of Commands.........................................62
   9.2 Protection against Restart Avalanche.........................63
   10. SECURITY CONSIDERATIONS......................................64
   10.1 Protection of Protocol Connections..........................64
   10.2 Interim AH scheme...........................................65
   10.3 Protection of Media Connections.............................66
   11.  MG-MGC CONTROL INTERFACE....................................66
   11.1 Multiple Virtual MGs........................................67
   11.2 Cold Start..................................................68
   11.3 Negotiation of Protocol Version.............................68
   11.4 Failure of an MG............................................69
   11.5 Failure of an MGC...........................................69
   12. PACKAGE DEFINITION...........................................70
   12.1 Guidelines for defining packages............................71
        12.1.1 Package..............................................71
        12.1.2 Properties...........................................72
        12.1.3 Events...............................................72
        12.1.4 Signals..............................................73
        12.1.5 Statistics...........................................73
        12.1.6 Procedures...........................................73
   12.2 Guidelines to defining Properties, Statistics and Parameters
        to Events and Signals.......................................73
   12.3 Lists.......................................................74
   12.4 Identifiers.................................................74
   12.5 Package Registration........................................74
   13.  IANA CONSIDERATIONS.........................................74
   13.1 Packages....................................................74
   13.2 Error Codes.................................................75
   13.3 ServiceChange Reasons.......................................76
   ANNEX A: BINARY ENCODING OF THE PROTOCOL (NORMATIVE).............77
   A.1 Coding of wildcards..........................................77
   A.2 ASN.1 syntax specification...................................78
   A.3 Digit maps and path names....................................94
   ANNEX B TEXT ENCODING OF THE PROTOCOL (NORMATIVE)................95
   B.1 Coding of wildcards..........................................95



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   B.2 ABNF specification...........................................95
   ANNEX C TAGS FOR MEDIA STREAM PROPERTIES (NORMATIVE)............107
   C.1 General Media Attributes....................................107
   C.2 Mux Properties..............................................108
   C.3 General bearer properties...................................109
   C.4 General ATM properties......................................109
   C.5 Frame Relay.................................................112
   C.6 IP..........................................................113
   C.7 ATM AAL2....................................................113
   C.8 ATM AAL1....................................................114
   C.9 Bearer Capabilities.........................................116
   C.10 AAL5 Properties............................................123
   C.11 SDP Equivalents............................................124
   C.12 H.245......................................................124
   ANNEX D TRANSPORT OVER IP (NORMATIVE)...........................125
   D.1 Transport over IP/UDP using Application Level Framing.......125
        D.1.1 Providing At-Most-Once Functionality.................125
        D.1.2 Transaction identifiers and three-way handshake......126
                D.1.2.1 Transaction identifiers....................126
                D.1.2.2 Three-way handshake........................126
        D.1.3 Computing retransmission timers......................127
        D.1.4 Provisional responses................................128
        D.1.5 Repeating Requests, Responses and Acknowledgements...128
   D.2  using TCP..................................................130
           D.2.1 Providing the At-Most-Once functionality..........130
           D.2.2 Transaction identifiers and three way handshake...130
           D.2.3 Computing retransmission timers...................131
           D.2.4 Provisional responses.............................131
           D.2.5 Ordering of commands..............................131
   ANNEX E BASIC PACKAGES..........................................131
   E.1 Generic.....................................................131
        E.1.1 Properties...........................................132
        E.1.2 Events...............................................132
        E.1.3 Signals..............................................133
        E.1.4 Statistics...........................................133
   E.2 Base Root Package...........................................133
        E.2.1 Properties...........................................134
        E.2.2 Events...............................................135
        E.2.3 Signals..............................................135
        E.2.4 Statistics...........................................135
        E.2.5 Procedures...........................................135
   E.3 Tone Generator Package......................................135
        E.3.1 Properties...........................................135
        E.3.2 Events...............................................136
        E.3.3 Signals..............................................136
        E.3.4 Statistics...........................................136
        E.3.5 Procedures...........................................136
   E.4 Tone Detection Package......................................137



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        E.4.1 Properties...........................................137
        E.4.2 Events...............................................137
        E.4.3 Signals..............................................139
        E.4.4 Statistics...........................................139
        E.4.5 Procedures...........................................139
   E.5 Basic DTMF Generator Package................................140
        E.5.1 Properties...........................................140
        E.5.2 Events...............................................140
        E.5.3 Signals..............................................140
        E.5.4 Statistics...........................................141
        E.5.5 Procedures...........................................141
   E.6 DTMF detection Package......................................141
        E.6.1 Properties...........................................142
        E.6.2 Events...............................................142
        E.6.3 Signals..............................................143
        E.6.4 Statistics...........................................143
        E.6.5 Procedures...........................................143
   E.7 Call Progress Tones Generator Package.......................143
        E.7.1 Properties...........................................144
        E.7.2 Events...............................................144
        E.7.3 Signals..............................................144
        E.7.4 Statistics...........................................145
        E.7.5 Procedures...........................................145
   E.8 Call Progress Tones Detection Package.......................145
        E.8.1 Properties...........................................145
        E.8.2 Events...............................................145
        E.8.3 Signals..............................................145
        E.8.4 Statistics...........................................145
        E.8.5 Procedures...........................................146
   E.9 Analog Line Supervision Package.............................146
        E.9.1 Properties...........................................146
        E.9.2 Events...............................................146
        E.9.3 Signals..............................................147
        E.9.4 Statistics...........................................148
        E.9.5 Procedures...........................................148
   E.10 Basic Continuity Package...................................148
        E.10.1 Properties..........................................148
        E.10.2 Events..............................................148
        E.10.3 Signals.............................................149
        E.10.4 Statistics..........................................150
        E.10.5 Procedures..........................................150
   E.11 Network Package............................................150
        E.11.1 Properties..........................................150
        E.11.2 Events..............................................151
        E.11.3 Signals.............................................152
        E.11.4 Statistics..........................................152
        E.11.5 Procedures..........................................153
   E.12 RTP  Package...............................................153



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        E.12.1 Properties..........................................153
        E.12.2 Events..............................................153
        E.12.3 Signals.............................................153
        E.12.4 Statistics..........................................153
        E.12.5 Procedures..........................................154
   E.13 TDM Circuit Package........................................154
        E.13.1 Properties..........................................155
        E.13.2 Events..............................................155
        E.13.3 Signals.............................................155
        E.13.4 Statistics..........................................156
        E.13.5 Procedures..........................................156
   APPENDIX A EXAMPLE CALL FLOWS (INFORMATIVE).....................157
   A.1 Residential Gateway to Residential Gateway Call.............157
        A.1.1 Programming Residential GW Analog Line Terminations for
        Idle Behavior..............................................157
        A.1.2 Collecting Originator Digits and Initiating Termination
        ...........................................................159
   Authors' Addresses..............................................168
   Full Copyright Statement........................................170

1. SCOPE

   This document defines the protocol 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 H.246. This
   recommendation does not define how gateways, multipoint control units
   or integrated 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 SCN signalling systems,
   including tone signalling, ISDN, ISUP, QSIG, and GSM.  National
   variants of these signalling systems will be supported where
   applicable.

   The protocol definition in this document is common text with ITU-T
   Recommendation H.248.  It meets the requirements documented in RFC
   2805.

2. REFERENCES

2.1 Normative references

   ITU-T Recommendation H.225.0 (1998): "Call Signalling Protocols and
   Media Stream Packetization for Packet Based Multimedia Communications
   Systems".




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   ITU-T Recommendation H.235 (02/98): "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.323 (1998): "Packet Based Multimedia
   Communication Systems".

   ITU-T Recommendation I.363.1 (08/96), "B-ISDN ATM Adaptation Layer
   specification: Type 1 AAL".

   ITU-T Recommendation I.363.2 (09/97), "B-ISDN ATM Adaptation Layer
   specification: Type 2 AAL".

   ITU-T Recommendation I.363.5 (08/96), "B-ISDN ATM Adaptation Layer
   specification: Type 5 AAL".

   ITU-T Recommendation I.366.1 (06/98), "Segmentation and Reassembly
   Service Specific Convergence Sublayer for the AAL type 2".

   ITU-T Recommendation I.366.2 (02/99), "AAL type 2 service specific
   convergence sublayer for trunking".

   ITU-T Recommendation I.371 (08/96), "Traffic control and congestion
   control in B-ISDN".

   ITU-T Recommendation Q.763 (09/97), "Signalling System No. 7 - ISDN
   user part formats and codes".

   ITU-T Recommendation Q.765, "Signalling System No. 7 - Application
   transport mechanism".

   ITU-T Recommendation Q.931 (05/98): "Digital Subscriber Signalling
   System No.  1 (DSS 1) - 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)".

   ITU-T Recommendation Q.2931 (10/95), "Broadband Integrated Services
   Digital Network (B-ISDN) - Digital Subscriber Signalling System No.
   2 (DSS 2) - User-Network Interface (UNI) - Layer 3 specification for
   basic call/connection control".

   ITU-T Recommendation Q.2941.1 (09/97), "Digital Subscriber Signalling
   System No. 2 - Generic Identifier Transport".




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   ITU-T Recommendation Q.2961 (10/95), "Broadband integrated services
   digital network (B-ISDN) - Digital subscriber signalling system no.2
   (DSS 2) - additional traffic parameters".

   ITU-T Recommendation Q.2961.2 (06/97), "Digital subscriber signalling
   system No. 2 - Additional traffic parameters: Support of ATM transfer
   capability in the broadband bearer capability information element."

   ITU-T Recommendation X.213 (11/1995), "Information technology - Open
   System Interconnection - Network service definition plus Amendment 1
   (08/1997), Addition of the Internet protocol address format
   identifier".

   ITU-T Recommendation V.76 (08/96), "Generic multiplexer using V.42
   LAPM-based procedures".

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

   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".

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

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

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

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 (05/99),"Frame structure for a 64 to 1920
   kbit/s channel in audiovisual teleservices".



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

   ITU-T Recommendation Q.724 (1988): "Signalling procedures".

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

   Postel, J., "Internet protocol", STD 5, RFC 791, September 1981.

   Postel, J., "TRANSMISSION CONTROL PROTOCOL", STD 7, RFC 793,
   September 1981.

   Simpson, W., "The Point-to-Point Protocol", STD 51, RFC 1661, July
   1994.

   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.

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

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

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

   Greene, N., Ramalho, M. and B. Rosen, "Media Gateway control protocol
   architecture and requirements", RFC 2805, April 1999.

3. DEFINITIONS

   Access Gateway: A type of gateway that provides a User to 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



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   (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 performs other IVR functions, or may
   perform media conferencing.

   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 recommendation defines the following terms.

   ATM          Asynchronous Transfer Mode
   BRI          Basic Rate Interface
   CAS          Channel Associated Signalling
   DTMF         Dual Tone Multi-Frequency
   FAS          Facility Associated Signalling
   GW           GateWay
   IANA         Internet Assigned Numbers Authority
   IP           Internet Protocol
   ISUP         ISDN User Part



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   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 this 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.

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.














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          +------------------------------------------------------+
          |Media Gateway                                         |
          | +-------------------------------------------------+  |
          | |Context                          +-------------+ |  |
          | |                                 | Termination | |  |
          | |                                 |-------------| |  |
          | |  +-------------+             +->| SCN Bearer  |<---+->
          | |  | Termination |   +-----+   |  |   Channel   | |  |
          | |  |-------------|   |     |---+  +-------------+ |  |
        <-+--->| RTP Stream  |---|  *  |                      |  |
          | |  |             |   |     |---+  +-------------+ |  |
          | |  +-------------+   +-----+   |  | Termination | |  |
          | |                              |  |-------------| |  |
          | |                              +->| SCN Bearer  |<---+->
          | |                                 |   Channel   | |  |
          | |                                 +-------------+ |  |
          | +-------------------------------------------------+  |
          |                                                      |
          |                                                      |
          |                    +------------------------------+  |
          |                    |Context                       |  |
          |  +-------------+   |              +-------------+ |  |
          |  | Termination |   | +-----+      | Termination | |  |
          |  |-------------|   | |     |      |-------------| |  |
        <-+->| SCN Bearer  |   | |  *  |------| SCN Bearer  |<---+->
          |  |   Channel   |   | |     |      |   Channel   | |  |
          |  +-------------+   | +-----+      +-------------+ |  |
          |                    +------------------------------+  |
          |                                                      |
          |                                                      |
          | +-------------------------------------------------+  |
          | |Context                                          |  |
          | |  +-------------+                +-------------+ |  |
          | |  | Termination |   +-----+      | Termination | |  |
          | |  |-------------|   |     |      |-------------| |  |
        <-+--->| SCN Bearer  |---|  *  |------| SCN Bearer  |<---+->
          | |  |   Channel   |   |     |      |   Channel   | |  |
          | |  +-------------+   +-----+      +-------------+ |  |
          | +-------------------------------------------------+  |
          | ___________________________________________________  |
          +------------------------------------------------------+

                Figure 1: Example of H.248 Connection Model








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   Figure 1 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.

   The example below 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 below.

          +------------------------------------------------------+
          |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.

    .  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.

   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.



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   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.  Signals are MG
   generated media streams such as tones and announcements as well as
   line signals such as hookswitch.  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 section 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 Termination.  The Terminations that
   source/sink the digital channels are connected to a separate
   Termination called the multiplexing Termination. This Termination
   describes the multiplex used (e.g. how the H.221 frames are carried
   over the digital channels used).  The MuxDescriptor is used to this
   end.  If multiple media are carried, this Termination contains
   multiple StreamDescriptors. The media streams can be associated with
   streams sourced/sunk by other Terminations in the Context.

   Terminations may be created which represent multiplexed bearers, such
   as an ATM AAL2.  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 sections. 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.





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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.  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.
   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 a Termination realizes a set of such Packages.
   More information on definition of packages can be found in section
   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,



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   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.

6.2.4 Termination Properties and Descriptors

   Terminations have properties.  The properties have unique
   PropertyIDs.  Most properties have default values.  When a
   Termination is created, properties get their default values, unless
   the controller specifically sets a different value.  The default
   value of a property of a physical Termination can be changed by
   setting it to a different value when the Termination is in the null
   Context.  Every time such a Termination returns to the null Context,
   the values of its properties are reset to this default value.

   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.  Properties not mentioned in the
   command retain their prior values.  Similarly, a property of a
   Termination in a Context may have its value changed by the Modify
   command.  Properties not mentioned in the Modify command retain their
   prior values. 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.

   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.






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   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.
   EventBuffer               Describes events to be detected by the MG
                             when Event Buffering is active.
   Signals                   Describes signals and/or actions to be
                             applied (e.g. Busy Tone) to the
                             Terminations.
   Audit                     In Audit commands, identifies which
                             information is desired.
   Packages                  In AuditValue, returns a list of Packages
                             realized by Termination.
   DigitMap                  Instructions for handling DTMF tones at
                             the MG.
   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.









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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 and events (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
       implemented on root
    .  an AuditCapability - to determine what properties of root are
       implemented
    .  a ServiceChange - to declare the gateway in or out of service.

   Any other use of the root TerminationID is an error.

7. COMMANDS

   The protocol provides commands for manipulating the logical entities
   of the protocol connection model, Contexts and Terminations.
   Commands provide control at the finest level of granularity supported
   by the protocol.  For example, Commands exist to add Terminations to
   a Context, modify Terminations, subtract Terminations from a Context,
   and audit properties of Contexts or Terminations. Commands provide
   for complete control of the properties of Contexts and Terminations.
   This includes specifying which events a Termination is to report,
   which signals/actions are to be applied to a Termination and
   specifying the topology of a Context (who hears/sees whom).

   Most commands are for the specific use of the Media Gateway
   Controller as command initiator in controlling Media Gateways as
   command responders.  The exceptions are the Notify and ServiceChange
   commands: Notify is sent from Media Gateway to Media Gateway
   Controller, and ServiceChange may be sent by either entity.  Below is
   an overview of the commands; they are explained in more detail in
   section 7.2.

   1. Add. The Add command adds a termination to a context.  The Add
      command on the first Termination in a Context is used to create a
      Context.

   2. Modify. The Modify command modifies the properties, events and
      signals of a termination.






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   3. Subtract. The Subtract command disconnects a Termination from its
      Context and returns statistics on the Termination's participation
      in the Context.  The Subtract command on the last Termination in a
      Context deletes the Context.

   4. Move. The Move command atomically moves a Termination to another
      context.

   5. AuditValue. The AuditValue command returns the current state of
      properties, events,  signals and statistics of Terminations.

   6. AuditCapabilities. The AuditCapabilities command returns all the
      possible values for Termination properties, events and signals
      allowed by the Media Gateway.

   7. Notify. The Notify command allows the Media Gateway to inform the
      Media Gateway Controller of the occurrence of events in the Media
      Gateway.

   8. ServiceChange. The ServiceChange Command allows the Media Gateway
      to notify the Media Gateway Controller that a Termination or group
      of Terminations is about to be taken out of service or has just
      been returned to service.   ServiceChange is also used by the MG
      to announce its availability to an MGC (registration), and to
      notify the MGC of impending or completed restart of the MG.  The
      MGC may announce a handover to the MG by sending it a
      ServiceChange command.  The MGC may also use ServiceChange to
      instruct the MG to take a Termination or group of Terminations in
      or out of service.

   These commands are detailed in sections 7.2.1 through 7.2.8

7.1 Descriptors

   The parameters to a command are termed Descriptors. A Descriptor
   consists of a name and a list of items. Some items may have values.
   Many Commands share common Descriptors.  This subsection enumerates
   these Descriptors.  Descriptors may be returned as output from a
   command.  Parameters and parameter usage specific to a given Command
   type are described in the subsection that describes the Command.

7.1.1 Specifying Parameters

   Command parameters are structured into a number of descriptors. In
   general, the text format of descriptors is
   DescriptorName=<someID>{parm=value, parm=value_.}.





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   Parameters may be fully specified, over-specified or under-specified:

   1. Fully specified parameters have a single, unambiguous value that
      the command initiator is instructing the command responder to use
      for the specified parameter.

   2. Under-specified parameters, using the CHOOSE value, allow the
      command responder to choose any value it can support.

   3. Over-specified parameters have a list of potential values.  The
      list order specifies the command initiator's order of preference
      of selection.  The command responder chooses one value from the
      offered list and returns that value to the command initiator.

   Unspecified mandatory parameters (i.e. mandatory parameters not
   specified in a descriptor) result in the command responder retaining
   the previous value for that parameter.  Unspecified optional
   parameters result in the command responder using the default value of
   the parameter.  Whenever a parameter is underspecified or
   overspecified, the descriptor containing the value chosen by the
   responder is included as output from the command.

   Each command specifies the TerminationId the command operates on.
   This TerminationId may be "wildcarded".  When the TerminationId of a
   command is wildcarded, the effect shall be as if the command was
   repeated with each of the TerminationIds matched.

7.1.2 Modem Descriptor

   The Modem descriptor specifies the modem type and parameters, if any,
   required for use in e.g. H.324 and text conversation.  The descriptor
   includes the following modem types: V.18, V.22, V.22bis, V.32,
   V.32bis, V.34, V.90, V.91, Synchronous ISDN, and allows for
   extensions.  By default, no modem descriptor is present in a
   Termination.

7.1.3 Multiplex Descriptor

   In multimedia calls, a number of media streams are carried on a
   (possibly different) number of bearers.  The multiplex descriptor
   associates the media and the bearers. The descriptor includes the
   multiplex type:

    . H.221
    . H.223,
    . H.226,
    . V.76,
    . Possible Extensions



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   and a set of TerminationIDs representing the multiplexed inputs, in
   order.  For example:

       Mux = H.221{ MyT3/1/2, MyT3/2/13, MyT3/3/6, MyT3/21/22}

7.1.4 Media Descriptor

   The Media Descriptor specifies the parameters for all the media
   streams.  These parameters are structured into two descriptors, a
   Termination State Descriptor, which specifies the properties of a
   termination that are not stream dependent, and one or more Stream
   Descriptors each of which describes a single media stream.

   A stream is identified by a StreamID.  The StreamID is used to link
   the streams in a Context that belong together. Multiple streams
   exiting a termination shall be synchronized with each other.  Within
   the Stream Descriptor, there are up to three subsidiary descriptors,
   LocalControl, Local, and Remote. The relationship between these
   descriptors is thus:

   Media Descriptor
        TerminationStateDescriptor
        Stream Descriptor
                LocalControl Descriptor
                Local Descriptor
                Remote Descriptor

   As a convenience a LocalControl, Local, or Remote descriptor may be
   included in the Media Descriptor without an enclosing Stream
   descriptor.  In this case, the StreamID is assumed to be 1.

7.1.5 Termination State Descriptor

   The Termination State Descriptor contains the ServiceStates property,
   the EventBufferControl property and properties of a termination
   (defined in Packages) that are not stream specific.

   The ServiceStates property describes the overall state of the
   termination (not stream-specific).  A Termination can be in one of
   the following states: "test", "out of service", or "in service".  The
   "test" state indicates that the termination is being tested. The
   state "out of service" indicates that the termination cannot be used
   for traffic.  The state "in service" indicates that a termination can
   be used or is being used for normal traffic.  "in service" is the
   default state.






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   Values assigned to Properties may be simple values
   (integer/string/enumeration) or may be underspecified, where more
   than one value is supplied and the MG may make a choice:
    .  Alternative Values: multiple values in a list, one of which must
       be selected
    .  Ranges: minimum and maximum values, any value between min and max
       must be selected, boundary values included
    .  Greater Than/Less Than: value must be greater/less than specified
       value
    .  CHOOSE Wildcard: the MG chooses from the allowed values for the
       property

   The EventBufferControl property  specifies whether events are
   buffered following detection of an event in the Events Descriptor, or
   processed immediately.  See section 7.1.9 for details.

7.1.6 Stream Descriptor

   A Stream descriptor specifies the parameters of a single bi-
   directional stream.  These parameters are structured into three
   descriptors: one that contains termination properties specific to a
   stream and one each for local and remote flows. The Stream Descriptor
   includes a StreamID which identifies the stream.  Streams are created
   by specifying a new StreamID on one of the terminations in a Context.
   A stream is deleted by setting empty Local and Remote descriptors for
   the stream with ReserveGroup and ReserveValue in LocalControl set to
   "false" on all terminations in the context that previously supported
   that stream.

   StreamIDs are of local significance between MGC and MG and they are
   assigned by the MGC.  Within a context, StreamID is a means by which
   to indicate which media flows are interconnected:  streams with the
   same StreamID are connected.

   If a termination is moved from one context to another, the effect on
   the context to which the termination is moved is the same as in the
   case that a new termination were added with the same StreamIDs as the
   moved termination.

7.1.7 LocalControl Descriptor

   The LocalControl Descriptor contains the Mode property, the
   ReserveGroup and ReserveValue properties and properties of a
   termination (defined in Packages) that are stream specific, and are
   of interest between the MG and the MGC.  Values of properties may be
   underspecified as in section 7.1.1.





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   The allowed values for the mode property are send-only, receive-only,
   send/receive, inactive and loop-back.  "Send" and "receive" are with
   respect to the exterior of the context, so that, for example, a
   stream set to mode=sendonly does not pass received media into the
   context.  Signals and Events are not affected by mode.

   The boolean-valued Reserve properties, ReserveValue and ReserveGroup,
   of a Termination indicate what the MG is expected to do when it
   receives a  local and/or remote descriptor.

   If the value of a Reserve property is True, the MG SHALL reserve
   resources for all alternatives specified in the local and/or remote
   descriptors for which it currently has resources available.  It SHALL
   respond with the alternatives for which it reserves resources.  If it
   cannot not support any of the alternatives, it SHALL respond with a
   reply to the MGC that contains empty local and/or remote descriptors.

   If the value of a Reserve property is False, the MG SHALL choose one
   of the alternatives specified in the local descriptor (if present)
   and one of the alternatives specified in the remote descriptor (if
   present).  If the MG has not yet reserved resources to support the
   selected alternative, it SHALL reserve the resources.  If, on the
   other hand, it already reserved resources for the Termination
   addressed (because of a prior exchange with ReserveValue and/or
   ReserveGroup equal to True), it SHALL release any excess resources it
   reserved previously.  Finally, the MG shall send a reply to the MGC
   containing the alternatives for the local and/or remote descriptor
   that it selected.  If the MG does not have sufficient resources to
   support any of the alternatives specified, is SHALL respond with
   error 510 (insufficient resources).

   The default value of ReserveValue and ReserveGroup is False.

   A new setting of the LocalControl Descriptor completely replaces the
   previous setting of that descriptor in the MG.  Thus to retain
   information from the previous setting the MGC must include that
   information in the new setting.  If the MGC wishes to delete some
   information from the existing descriptor, it merely resends the
   descriptor (in a Modify command) with the unwanted information
   stripped out.

7.1.8 Local and Remote Descriptors

   The MGC uses Local and Remote descriptors to reserve and commit MG
   resources for media decoding and encoding for the given Stream(s) and
   Termination to which they apply.  The MG includes these descriptors
   in its response to indicate what it is actually prepared to support.
   The MG SHALL include additional properties and their values in its



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   response if these properties are mandatory yet not present in the
   requests made by the MGC (e.g., by specifying detailed video encoding
   parameters where the MGC only specified the payload type).

   Local refers to the media received by the MG and Remote refers to the
   media sent by the MG.

   When text encoding the protocol, the descriptors consist of session
   descriptions as defined in SDP (RFC2327).  In session descriptions
   sent from the MGC to the MG, the following exceptions to the syntax
   of RFC 2327 are allowed:

    .  the "s=", "t=" and "o=" lines are optional,
    .  the use of CHOOSE is allowed in place of a single parameter
       value, and
    .  the use of alternatives is allowed in place of a single parameter
       value.

   When multiple session descriptions are provided in one descriptor,
   the "v=" lines are required as delimiters; otherwise they are
   optional in session descriptions sent to the MG.  Implementations
   shall accept session descriptions that are fully conformant to
   RFC2327. When binary encoding the protocol the descriptor consists of
   groups of properties (tag-value pairs) as specified in Annex C.  Each
   such group may contain the parameters of a session description.

   Below, the semantics of the local and remote descriptors are
   specified in detail.  The specification consists of two parts.  The
   first part specifies the interpretation of the contents of the
   descriptor.  The