The Session Description Protocol (SDP) Content Attribute
RFC 4796

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Network Working Group                                      J. Hautakorpi
Request for Comments: 4796                                  G. Camarillo
Category: Standards Track                                       Ericsson
                                                           February 2007

        The Session Description Protocol (SDP) Content Attribute

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 IETF Trust (2007).

Abstract

   This document defines a new Session Description Protocol (SDP) media-
   level attribute, 'content'.  The 'content' attribute defines the
   content of the media stream to a more detailed level than the media
   description line.  The sender of an SDP session description can
   attach the 'content' attribute to one or more media streams.  The
   receiving application can then treat each media stream differently
   (e.g., show it on a big or small screen) based on its content.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  2
   3.  Related Techniques . . . . . . . . . . . . . . . . . . . . . .  2
   4.  Motivation for the New Content Attribute . . . . . . . . . . .  3
   5.  The Content Attribute  . . . . . . . . . . . . . . . . . . . .  4
   6.  The Content Attribute in the Offer/Answer Model  . . . . . . .  5
   7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   8.  Operation with SMIL  . . . . . . . . . . . . . . . . . . . . .  7
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     12.1.  Normative References  . . . . . . . . . . . . . . . . . .  9
     12.2.  Informational References  . . . . . . . . . . . . . . . .  9

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RFC 4796                   Content Attribute               February 2007

1.  Introduction

   The Session Description Protocol (SDP) [1] is a protocol that is
   intended to describe multimedia sessions for the purposes of session
   announcement, session invitation, and other forms of multimedia
   session initiation.  One of the most typical use cases of SDP is
   where it is used with the Session Initiation Protocol (SIP) [5].

   There are situations where one application receives several similar
   media streams, which are described in an SDP session description.
   The media streams can be similar in the sense that their content
   cannot be distinguished just by examining their media description
   lines (e.g., two video streams).  The 'content' attribute is needed
   so that the receiving application can treat each media stream
   appropriately based on its content.

   This specification defines the SDP 'content' media-level attribute,
   which provides more information about the media stream than the 'm'
   line in an SDP session description.

   The main purpose of this specification is to allow applications to
   take automated actions based on the 'content' attributes.  However,
   this specification does not define those actions.  Consequently, two
   implementations can behave completely differently when receiving the
   same 'content' attribute.

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 [3], and indicate requirement levels
   for compliant implementations.

3.  Related Techniques

   The 'label' attribute [10] enables a sender to attach a pointer to a
   particular media stream.  The namespace of the 'label' attribute
   itself is unrestricted; so, in principle, it could also be used to
   convey information about the content of a media stream.  However, in
   practice, this is not possible because of the need for backward
   compatibility.  Existing implementations of the 'label' attribute
   already use values from that unrestricted namespace in an
   application-specific way.  So, it is not possible to reserve portions
   of the 'label' attribute's namespace without possible conflict with
   already used application-specific labels.

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RFC 4796                   Content Attribute               February 2007

   It is possible to assign semantics to a media stream with an external