Framework for Transcoding with the Session Initiation Protocol (SIP)
RFC 5369

 
Document Type RFC - Informational (October 2008; No errata)
Last updated 2013-03-02
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IESG IESG state RFC 5369 (Informational)
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Network Working Group                                       G. Camarillo
Request for Comments: 5369                                      Ericsson
Category: Informational                                     October 2008

  Framework for Transcoding with the Session Initiation Protocol (SIP)

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.

Abstract

   This document defines a framework for transcoding with SIP.  This
   framework includes how to discover the need for transcoding services
   in a session and how to invoke those transcoding services.  Two
   models for transcoding services invocation are discussed: the
   conference bridge model and the third-party call control model.  Both
   models meet 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.  Discovery of the Need for Transcoding Services  . . . . . . . . 2
   3.  Transcoding Services Invocation . . . . . . . . . . . . . . . . 4
     3.1.  Third-Party Call Control Transcoding Model  . . . . . . . . 4
     3.2.  Conference Bridge Transcoding Model . . . . . . . . . . . . 6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   5.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . 8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     6.2.  Informative References  . . . . . . . . . . . . . . . . . . 9

Camarillo                    Informational                      [Page 1]
RFC 5369                 Transcoding Framework              October 2008

1.  Introduction

   Two user agents involved in a SIP [RFC3261] dialog may find it
   impossible to establish a media session due to a variety of
   incompatibilities.  Assuming that both user agents understand the
   same session description format (e.g., SDP [RFC4566]),
   incompatibilities can be found at the user agent level and at the
   user level.  At the user agent level, both terminals may not support
   any common codec or may not support common media types (e.g., a text-
   only terminal and an audio-only terminal).  At the user level, a deaf
   person will not understand anything said over an audio stream.

   In order to make communications possible in the presence of
   incompatibilities, user agents need to introduce intermediaries that
   provide transcoding services to a session.  From the SIP point of
   view, the introduction of a transcoder is done in the same way to
   resolve both user level and user agent level incompatibilities.  So,
   the invocation mechanisms described in this document are generally
   applicable to any type of incompatibility related to how the
   information that needs to be communicated is encoded.

      Furthermore, although this framework focuses on transcoding, the
      mechanisms described are applicable to media manipulation in
      general.  It would be possible to use them, for example, to invoke
      a server that simply increases the volume of an audio stream.

   This document does not describe media server discovery.  That is an
   orthogonal problem that one can address using user agent provisioning
   or other methods.

   The remainder of this document is organized as follows.  Section 2
   deals with the discovery of the need for transcoding services for a
   particular session.  Section 3 introduces the third-party call
   control and conference bridge transcoding invocation models, which
   are further described in Sections 3.1 and 3.2, respectively.  Both
   models meet the requirements regarding transcoding services
   invocation in RFC 3351 [RFC3351], which support deaf, hard of
   hearing, and speech-impaired individuals.

2.  Discovery of the Need for Transcoding Services

   According to the one-party consent model defined in RFC 3238
   [RFC3238], services that involve media manipulation invocation are
   best invoked by one of the endpoints involved in the communication,
   as opposed to being invoked by an intermediary in the network.
   Following this principle, one of the endpoints should be the one
   detecting that transcoding is needed for a particular session.

Camarillo                    Informational                      [Page 2]
RFC 5369                 Transcoding Framework              October 2008

   In order to decide whether or not transcoding is needed, a user agent
   needs to know the capabilities of the remote user agent.  A user
   agent acting as an offerer [RFC3264] typically obtains this knowledge
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