Transcoding Services Invocation in the Session Initiation Protocol (SIP) Using Third Party Call Control (3pcc)
RFC 4117

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Network Working Group                                       G. Camarillo
Request for Comments: 4117                                      Ericsson
Category: Informational                                        E. Burger
                                                              Brooktrout
                                                          H. Schulzrinne
                                                     Columbia University
                                                             A. van Wijk
                                                                 Viataal
                                                               June 2005

                  Transcoding Services Invocation in
                 the Session Initiation Protocol (SIP)
                 Using Third Party Call Control (3pcc)

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 (2005).

Abstract

   This document describes how to invoke transcoding services using
   Session Initiation Protocol (SIP) and third party call control.  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. General Overview ................................................2
   3. Third Party Call Control Flows ..................................2
      3.1. Terminology ................................................3
      3.2. Callee's Invocation ........................................3
      3.3. Caller's Invocation ........................................8
      3.4. Receiving the Original Stream ..............................8
      3.5. Transcoding Services in Parallel ..........................10
      3.6. Multiple Transcoding Services in Series ...................14
   4. Security Considerations ........................................16
   5. Normative References ...........................................17
   6. Informative References .........................................17

Camarillo, et al.            Informational                      [Page 1]
RFC 4117                3pcc Transcoding in SIP                June 2005

1.  Introduction

   The framework for transcoding with SIP [4] describes how two SIP [1]
   UAs (User Agents) can discover incompatibilities that prevent them
   from establishing a session (e.g., lack of support for a common codec
   or common media type).  When such incompatibilities are found, the
   UAs need to invoke transcoding services to successfully establish the
   session.  3pcc (third party call control) [2] is one way to perform
   such invocation.

2.  General Overview

   In the 3pcc model for transcoding invocation, a transcoding server
   that provides a particular transcoding service (e.g., speech-to-text)
   is identified by a URI.  A UA that wishes to invoke that service
   sends an INVITE request to that URI establishing a number of media
   streams.  The way the transcoder manipulates and manages the contents
   of those media streams (e.g., the text received over the text stream
   is transformed into speech and sent over the audio stream) is service
   specific.

   All the call flows in this document use SDP.  The same call flows
   could be used with another session description protocol that provides
   similar session description capabilities.

3.  Third Party Call Control Flows

   Given two UAs (A and B) and a transcoding server (T), the invocation
   of a transcoding service consists of establishing two sessions; A-T
   and T-B.  How these sessions are established depends on which party,
   the caller (A) or the callee (B), invokes the transcoding services.
   Section 3.2 deals with callee invocation and Section 3.3 deals with
   caller invocation.

   In all our 3pcc flows we have followed the general principle that a
   200 (OK) response from the transcoding service has to be received
   before contacting the callee.  This tries to ensure that the
   transcoding service will be available when the callee accepts the
   session.

   Still, the transcoding service does not know the exact type of
   transcoding it will be performing until the callee accepts the
   session.  So, there is always the chance of failing to provide
   transcoding services after the callee has accepted the session.  A
   system with more stringent requirements could use preconditions to
   avoid this situation.  When preconditions are used, the callee is not
   alerted until everything is ready for the session.

Camarillo, et al.            Informational                      [Page 2]
RFC 4117                3pcc Transcoding in SIP                June 2005

3.1.  Terminology

   All the flows in this document follow the naming convention below: