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
Hautakorpi & Camarillo Standards Track [Page 1]
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
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