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IETF RFC 4574
The Session Description Protocol (SDP) Label Attribute
Last modified on Tuesday, August 29th, 2006
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Network Working Group O. Levin
Request for Comments: 4574 Microsoft Corporation
Category: Standards Track G. Camarillo
Ericsson
August 2006
The Session Description Protocol (SDP) Label 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 © The Internet Society (2006).
Abstract
This document defines a new Session Description Protocol (SDP)
media-level attribute: "label". The "label" attribute carries a
pointer to a media stream in the context of an arbitrary network
application that uses SDP. The sender of the SDP document can attach
the "label" attribute to a particular media stream or streams. The
application can then use the provided pointer to refer to each
particular media stream in its context.
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................2
3. Motivation for the New label Attribute ..........................2
4. The Label Attribute .............................................3
5. The Label Attribute in the Offer/Answer Model ...................4
6. Example .........................................................4
7. Security Considerations .........................................4
8. IANA Considerations .............................................5
9. Acknowledgements ................................................5
10. References .....................................................6
10.1. Normative References ......................................6
10.2. Informative References ....................................6
Levin & Camarillo Standards Track PAGE 1 
RFC 4574 SDP Label Attribute August 2006
1. Introduction
SDP is being used by a variety of distributed-over-the-network
applications. These applications deal with multiple sessions being
described by SDP [4] and serving multiple users or services in the
context of a single application instance. Applications of this kind
need a means to identify a particular media stream across multiple
SDP descriptions exchanged with different users.
The XCON framework is an example of a centralized conference
architecture that uses SDP according to the offer/answer mechanism
defined in [3] to establish media streams with each of the conference
participants. Additionally, XCON identifies the need to uniquely
identify a media stream in terms of its role in a conference
regardless of its media type, transport protocol, and media format.
This can be accomplished by using an external document that points to
the appropriate media stream and provides information (e.g., the
media stream's role in the conference) about it. The SIP Event
Package for Conference State [7] defines and uses a concrete format
for such external documents.
This specification defines the SDP [4] "label" media-level attribute,
which provides a pointer to a media stream that is described by an
'm' line in an SDP session description.
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 [1] and indicate requirement levels for
compliant implementations.
3. Motivation for the New label Attribute
Even though SDP and its extensions already provide a few ways to
refer to a media stream, none of them is appropriate to be used in
the context of external documents that may be created before the
session description itself and need to be handled by automata.
The 'i' SDP attribute, defined in RFC 2327 [4], can be used to label
media streams. Nevertheless, values of the 'i' attribute are
intended for human users and not for automata.
Levin & Camarillo Standards Track PAGE 2
RFC 4574 SDP Label Attribute August 2006
The 'mid' SDP attribute, defined in RFC 3388 [6], can be used to
identify media streams as well. Nevertheless, the scope of 'mid' is
too limited to be used by applications dealing with multiple SDP
sessions. This is because values of the 'mid' attribute are
meaningful in the context of a single SDP session, not in the context
of a broader application (e.g., a multiparty application).
Another way of referring to a media stream is by using the order of
the 'm' line in the SDP session document (e.g., the 5th media stream
in the session description). This is the mechanism used in the
offer/answer model [3].
The problem with this mechanism is that it can only be used to refer
to media streams in session descriptions that exist already. There
are scenarios where a static document needs to refer, using a
pointer, to a media stream that will be negotiated by SDP means and
created in the future. When the media stream is eventually created,
the application needs to label the media stream so that the pointer
in the static document points to the proper media stream in the
session description.
4. The Label Attribute
This specification defines a new media-level value attribute:
'label'. Its formatting in SDP is described by the following ABNF
[2]:
label-attribute = "a=label:" pointer
pointer = token
token = 1*(token-char)
token-char = %x21 / %x23-27 / %x2A-2B / %x2D-2E / %x30-39
/ %x41-5A / %x5E-7E
The token-char and token elements are defined in [4] but included
here to provide support for the implementor of this SDP feature.
The 'label' attribute contains a token that is defined by an
application and is used in its context. The new attribute can be
attached to 'm' lines in multiple SDP documents allowing the
application to logically group the media streams across SDP sessions
when necessary.
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RFC 4574 SDP Label Attribute August 2006
5. The Label Attribute in the Offer/Answer Model
This specification does not define a means to discover whether or not
the peer endpoint understands the 'label' attribute because 'label'
values are informative only at the offer/answer model level.
At the offer/answer level, it means that the fact that an offer does
not contain label attributes does not imply that the answer should
not have them. It also means that the fact that an offer contains
label attributes does not imply that the answer should have them too.
In addition to the basic offer/answer rule above, applications that
use 'label' as a pointer to media streams MUST specify its usage
constraints. For example, such applications MAY mandate support for
'label'. In this case, the application will define means for
negotiation of the 'label' attribute support as a part of its
specification.
6. Example
The following is an example of an SDP session description that uses
the 'label' attribute:
v=0
o=bob 280744730 28977631 IN IP4 host.example.com
s=
i=A Seminar on the session description protocol
c=IN IP4 192.0.2.2
t=0 0
m=audio 6886 RTP/AVP 0
a=label:1
m=audio 22334 RTP/AVP 0
a=label:2
7. Security Considerations
An attacker may attempt to add, modify, or remove 'label' attributes
from a session description. This could result in an application
behaving in a non-desirable way. So, it is strongly RECOMMENDED that
integrity protection be applied to the SDP session descriptions. For
session descriptions carried in SIP [5], S/MIME is the natural choice
to provide such end-to-end integrity protection, as described in RFC
3261 [5]. Other applications MAY use a different form of integrity
protection.
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RFC 4574 SDP Label Attribute August 2006
8. IANA Considerations
The IANA has registered the following new SDP attribute:
Contact name: Orit Levin oritl@microsoft.com.
Attribute name: "label".
Type of attribute: Media level.
Subject to charset: Not.
Purpose of attribute: The 'label' attribute associates a media
stream with a label. This label allows the media stream to be
referenced by external documents.
Allowed attribute values: A token.
9. Acknowledgements
Robert Sparks, Adam Roach, and Rohan Mahy provided useful comments on
this document.
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RFC 4574 SDP Label Attribute August 2006
10. References
10.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[3] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[4] Handley, M., Jacobson, V. and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
10.2. Informative References
[5] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[6] Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne,
"Grouping of Media Lines in the Session Description Protocol
(SDP)", RFC 3388, December 2002.
[7] Rosenberg, J., Schulzrinne, H., and O. Levin, "A Session
Initiation Protocol (SIP) Event Package for Conference State",
RFC 4575, August 2006.
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RFC 4574 SDP Label Attribute August 2006
Authors' Addresses
Orit Levin
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
EMail: oritl@microsoft.com
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
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RFC 4574 SDP Label Attribute August 2006
Full Copyright Statement
Copyright © The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
Levin & Camarillo Standards Track PAGE 8
The Session Description Protocol (SDP) Label Attribute
RFC TOTAL SIZE: 13484 bytes
PUBLICATION DATE: Tuesday, August 29th, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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