The Session Initiation Protocol (SIP) Conference Bridge Transcoding Model
RFC 5370

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Network Working Group                                       G. Camarillo
Request for Comments: 5370                                      Ericsson
Category: Standards Track                                   October 2008

                 The Session Initiation Protocol (SIP)
                  Conference Bridge Transcoding Model

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.

Abstract

   This document describes how to invoke transcoding services using the
   conference bridge model.  This way of invocation meets the
   requirements for SIP regarding transcoding services invocation to
   support deaf, hard of hearing, and speech-impaired individuals.

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. Caller's Invocation .............................................3
      3.1. Procedures at the User Agent ...............................3
      3.2. Procedures at the Transcoder ...............................3
      3.3. Example ....................................................4
      3.4. Unsuccessful Session Establishment .........................6
   4. Callee's Invocation .............................................7
   5. Security Considerations .........................................7
   6. Contributors ....................................................8
   7. References ......................................................8
      7.1. Normative References .......................................8
      7.2. Informative References .....................................9

Camarillo                   Standards Track                     [Page 1]
RFC 5370              Conference Transcoding Model          October 2008

1.  Introduction

   RFC 5369 [RFC5369] describes how two SIP [RFC3261] UAs (User Agents)
   can discover incompatibilities that prevent them from establishing a
   session (e.g., lack of support for a common codec or for a common
   media type).  When such incompatibilities are found, the UAs need to
   invoke transcoding services to successfully establish the session.
   The transcoding framework introduces two models to invoke transcoding
   services: the 3pcc (third-party call control) model [RFC4117] and the
   conference bridge model.  This document specifies the conference
   bridge model.

   In the conference bridge model for transcoding invocation, a
   transcoding server that provides a particular transcoding service
   (e.g., speech-to-text) behaves as a B2BUA (Back-to-Back User Agent)
   between both UAs and is identified by a URI.  As shown in Figure 1,
   both UAs, A and B, exchange signalling and media with the transcoder
   T.  The UAs do not exchange any traffic (signalling or media)
   directly between them.

                  +-------+
                  |       |**
                  |   T   |  **
                  |       |\   **
                  +-------+ \\   **
                    ^   *     \\   **
                    |   *       \\   **
                    |   *         SIP  **
                   SIP  *           \\   **
                    |   *             \\   **
                    |   *               \\   **
                    v   *                 \    **
                  +-------+               +-------+
                  |       |               |       |
                  |   A   |               |   B   |
                  |       |               |       |
                  +-------+               +-------+

                   <-SIP-> Signalling
                   ******* Media

                  Figure 1: Conference bridge model

   Sections 3 and 4 specify how the caller A or the callee B,
   respectively, can use the conference bridge model to invoke
   transcoding services from T.

Camarillo                   Standards Track                     [Page 2]
RFC 5370              Conference Transcoding Model          October 2008

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [RFC2119], and indicate requirement
   levels for compliant implementations.

3.  Caller's Invocation

   User agent A needs to perform two operations to invoke transcoding
   services from T for a session between user agent A and user agent B.
   User agent A needs to establish a session with T and provide T with
   user agent B's URI so that T can generate an INVITE towards user
   agent B.

3.1.  Procedures at the User Agent