rfc2805.txt

RFC 的详细文档！

Network Working Group                                          N. Greene
Request for Comments: 2805                               Nortel Networks
Category: Informational                                       M. Ramalho
                                                           Cisco Systems
                                                                B. Rosen
                                                                 Marconi
                                                              April 2000


      Media Gateway Control Protocol Architecture and Requirements

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document describes protocol requirements for the Media Gateway
   Control Protocol between a Media Gateway Controller and a Media
   Gateway.

























Greene, et al.               Informational                      [Page 1]

RFC 2805            MG Control Protocol Requirements          April 2000


Table of Contents

   1.  Introduction ..............................................  3
   2.  Terminology ...............................................  3
   3.  Definitions ...............................................  3
   4.  Specific functions assumed within the MG ..................  5
   5.  Per-Call Requirements .....................................  6
      5.1.  Resource Reservation .................................  6
      5.2.  Connection Requirements ..............................  7
      5.3.  Media Transformations ................................  8
      5.4.  Signal/Event Processing and Scripting ................  9
      5.5.  QoS/CoS .............................................. 10
      5.6.  Test Support ......................................... 11
      5.7.  Accounting ........................................... 11
      5.8.  Signalling Control ................................... 11
   6.  Resource Control .......................................... 12
      6.1.  Resource Status Management ........................... 12
      6.2.  Resource Assignment .................................. 13
   7.  Operational/Management Requirements ....................... 13
      7.1.  Assurance of Control/Connectivity .................... 13
      7.2.  Error Control ........................................ 14
      7.3.  MIB Requirements ..................................... 15
   8.  General Protocol Requirements ............................. 15
      8.1.  MG-MGC Association Requirements ...................... 16
      8.2.  Performance Requirements ............................. 17
   9.  Transport ................................................. 17
      9.1.  Assumptions made for underlying network .............. 17
      9.2.  Transport Requirements ............................... 18
   10.  Security Requirements .................................... 18
   11.  Requirements specific to particular bearer types ......... 19
      11.1.  Media-specific Bearer types ......................... 20
         11.1.1.  Requirements for TDM PSTN (Circuit) ............ 20
         11.1.2.  Packet Bearer type ............................. 22
         11.1.3.  Bearer type requirements for ATM ............... 23
      11.2.  Application-Specific Requirements ................... 26
         11.2.1.  Trunking Gateway ............................... 26
         11.2.2.  Access Gateway ................................. 27
         11.2.3.  Trunking/Access Gateway with fax ports ......... 27
         11.2.4.  Trunking/Access Gateway with text telephone .... 28
         11.2.5.  Network Access Server .......................... 29
         11.2.6.  Restricted Capability Gateway .................. 30
         11.2.7.  Multimedia Gateway ............................. 31
         11.2.8.  Audio Resource Function ........................ 32
         11.2.9. Multipoint Control Units ........................ 42
   12.  References ............................................... 43
   13.  Acknowledgements ......................................... 43
   14.  Authors' Addresses ....................................... 44
   15.  Full Copyright Statement ................................. 45



Greene, et al.               Informational                      [Page 2]

RFC 2805            MG Control Protocol Requirements          April 2000


1.  Introduction

   This document describes requirements to be placed on the Media
   Gateway Control Protocol. When the word protocol is used on its own
   in this document it implicitly means the Media Gateway Control
   Protocol.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
   indicate requirement levels for the protocol.

3.  Definitions

   *    Connection

   Under the control of a Media Gateway Controller (MGC), the Media
   Gateway (MG) realizes connections. In this document, connections are
   associations of resources hosted by the MG. They typically involve
   two terminations, but may involve more.

   *    Line or Loop

   An analogue or digital access connection from a user terminal which
   carries user media content and telephony access signalling (DP, DTMF,
   BRI, proprietary business set).

   *    Media Gateway (MG) function

   A Media Gateway (MG) function provides the media mapping and/or
   transcoding functions between potentially dissimilar networks, one of
   which is presumed to be a packet, frame or cell network.  For
   example, an MG might terminate switched circuit network (SCN)
   facilities (trunks, loops), packetize the media stream, if it is not
   already packetized, and deliver packetized traffic to a packet
   network.  It would perform these functions in the reverse order for
   media streams flowing from the packet network to the SCN.

   Media Gateways are not limited to SCN <-> packet/frame/cell
   functions: A conference bridge with all packet interfaces could be an
   MG, as well as an (IVR) interactive voice recognition unit, an audio
   resource function, or a voice recognition system with a cell
   interface.






Greene, et al.               Informational                      [Page 3]

RFC 2805            MG Control Protocol Requirements          April 2000


   *    Media Gateway unit (MG-unit)

   An MG-unit is a physical entity that contains an MG function and may
   also contain other functions, e.g. an SG function.

   *    Media Gateway Controller (MGC) function

   A Media Gateway Controller (MGC) function controls a MG.

   *    Media Resource

   Examples of media resources are codecs, announcements, tones, and
   modems, interactive voice response (IVR) units, bridges, etc.

   *    Signaling Gateway (SG) function

   An SG function receives/sends SCN native signalling at the edge of a
   data network. For example the SG function may relay, translate or
   terminate SS7 signaling in an SS7-Internet Gateway. The SG function
   may also be co-resident with the MG function to process SCN
   signalling associated with line or trunk terminations controlled by
   the MG, such as the "D" channel of an ISDN PRI trunk.

   *    Termination

   A termination is a point of entry and/or exit of media flows relative
   to the MG. When an MG is asked to connect two or more terminations,
   it understands how the flows entering and leaving each termination
   are related to each other.

   Terminations are, for instance, DS0's, ATM VCs and RTP ports. Another
   word for this is bearer point.

   *    Trunk

   An analog or digital connection from a circuit switch which carries
   user media content and may carry telephony signalling (MF, R2, etc.).
   Digital trunks may be transported and may appear at the Media Gateway
   as channels within a framed bit stream, or as an ATM cell stream.
   Trunks are typically provisioned in groups, each member of which
   provides equivalent routing and service.

   *    Type of Bearer

   A Type of Bearer definition provides the detailed requirements for
   its particular application/bearer type. A particular class of Media
   Gateway, for example, would support a particular set of Bearer types.




Greene, et al.               Informational                      [Page 4]

RFC 2805            MG Control Protocol Requirements          April 2000


4.  Specific functions assumed within the MG

   This section provides an environment for the definition of the
   general Media Gateway Control Protocol requirements.

   MGs can be architected in many different ways depending where the
   media conversions and transcoding (if required) are performed, the
   level of programmability of resources, how conferences are supported,
   and how associated signalling is treated. The functions assumed to be
   within the MG must not be biased towards a particular architecture.

   For instance, announcements in a MG could be provided by media
   resources or by the bearer point resource or termination itself.
   Further, this difference must not be visible to MGC: The MGC must be
   able to issue the identical request to two different implementations
   and achieve the identical functionality.

   Depending on the application of the MG (e.g., trunking, residential),
   some functions listed below will be more prominent than others, and
   in some cases, functions may even disappear.

   Although media adaptation is the essence of the MG, it is not
   necessary for it to be involved every time. An MG may join two
   terminations/resources of the same type (i.e., the MG behaves as a
   switch). The required media conversion depends on the media type
   supported by the resources being joined together.

   In addition to media adaptation function, resources have a number of
   unique properties, for instance:

   *    certain types of resources have associated signalling
        capabilities (e.g., PRI signalling, DTMF),

   *    some resources perform maintenance functions (e.g., continuity
        tests),

   *    the MGC needs to know the state changes of resources (e.g., a
        trunk group going out of service),

   *    the MG retains some control over the allocation and control of
        some resources (e.g., resource name space: RTP port numbers).

   Therefore, an MG realizes point-to-point connections and conferences,
   and supports several resource functions. These functions include
   media conversion, resource allocation and management, and event
   notifications.  Handling termination associated signalling is either
   done using event notifications, or is handled by the signalling
   backhaul part of a MG-unit (i.e. NOT directly handled by the MG).



Greene, et al.               Informational                      [Page 5]

RFC 2805            MG Control Protocol Requirements          April 2000


   MGs must also support some level of system related functions, such as
   establishing and maintaining some kind of MG-MGC association. This is
   essential for MGC redundancy, fail-over and resource sharing.

   Therefore, an MG is assumed to contain these functions:

   *    Reservation and release, of resources

   *    Ability to provide state of resources

   *    Maintenance of resources - It must be possible to make
        maintenance operations independent of other termination
        functions, for instance, some maintenance states should not
        affect the resources associated with that resource . Examples of
        maintenance functions are loopbacks and continuity tests.

   *    Connection management, including connection state.

   *    Media processing, using media resources: these provide services
        such as transcoding, conferencing, interactive voice recognition
        units, audio resource function units. Media resources may or may
        not be directly part of other resources.

   *    Incoming digit analysis for terminations, interpretation of
        scripts for terminations

   *    Event detection and signal insertion for per-channel signalling

   *    Ability to configure signalling backhauls (for example, a
        Sigtran backhaul)

   *    Management of the association between the MGC and MG, or between
        the MGC and MG resources.

5.  Per-Call Requirements

5.1.  Resource Reservation

   The protocol must:

   a.   Support reservation of bearer terminations and media resources
        for use by a particular call and support their subsequent
        release (which may be implicit or explicit).

   b.   Allow release in a single exchange of messages, of all resources
        associated with a particular set of connectivity and/or
        associations between a given number terminations.




Greene, et al.               Informational                      [Page 6]

RFC 2805            MG Control Protocol Requirements          April 2000


   c.   The MG is not required (or allowed) by the protocol to maintain
        a sense of future time: a reservation remains in effect until
        explicitly released by the MGC.

5.2.  Connection Requirements

   The protocol must:

   a.   Support connections involving packet and circuit bearer
        terminations in any combination, including "hairpin" connections
        (connections between two circuit connections within the same