MMUSIC O. Levin
Internet-Draft Microsoft Corporation
Expires: December 28, 2004 G. Camarillo
Ericsson
June 29, 2004
The SDP (Session Description Protocol) Label Attribute
draft-levin-mmmusic-sdp-media-label-00
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Abstract
This document defines a new Session Description Protocol (SDP)
media-level attribute: "label". The "label" attribute carries a
pointer to an application layer media stream identifier 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 media streams. The application receiving
the SDP document can then associate the particular media stream with
its application semantics or role.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The Label Attribute . . . . . . . . . . . . . . . . . . . . . 4
4. The Label Attribute in the Offer/Answer Model . . . . . . . . 4
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1 Normative References . . . . . . . . . . . . . . . . . . . . 5
8.2 Informative References . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . 7
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1. Introduction
SDP is being used by a variety of distributed over the network
applications. These applications deal with multiple SDP sessions
serving multiple users or services in the context of a single
application instance. As such, applications of this kind have their
means to identify a particular media stream across multiple SDP
sessions.
XCON framework [6] 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 defines the means to uniquely
identify a media stream in terms of its role in a conference
regardless its media type, transport protocol, and media format. It
is necessary to convey a pointer to this application layer identifier
in SDP. As a result, the application can choose from the media
streams offered in the SDP based not only on their media
characteristics, but also according to their roles in the
application.
This specification defines the SDP [2] "label" media-level attribute,
which carries the pointer to the application layer media stream
identifier in the SDP document across the network.
Note that the "i" SDP attribute, defined in RFC 2327 [2], can be used
to label media streams as well. Nevertheless, the audiences for the
"i" and the "label" attributes are different. While "i" values are
presented to the users as is, "label" values are processed by
automata.
Note that the "mid" SDP attribute, defined in RFC 3388 [4], can be
used to identify media streams as well. Nevertheless, the scopes of
the "mid" and the "label" attributes are different. While "mid"
values are meaningful in the context of a single SDP session, "label"
values are meaningful in the context of an application (e.g., a
multiparty application).
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.
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3. The Label Attribute
This specification defines a new media-level value attribute:
"label". Its formatting in SDP is described by the following BNF:
label-attribute = "a=label:" pointer
pointer = token
The semantics of the "label" attribute MUST be defined by
applications that use SDP with "label". This value is the
application layer identifier for a media stream regardless of its
media type, transport protocol, or media format.
4. The Label Attribute in the Offer/Answer Model
This specification does not define means to discover whether or not
the remote endpoint understands the "label" attribute. We have
chosen not to provide such a mechanism within this specification
because "label" values are only informative at the Offer/Answer model
level.
To the Offer/Answer mechanism, 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 define both the
"label" application semantics and its usage constraints. For
example, the specifications of such applications MAY mandate support
for "label". In this case, the application MUST define means for
negotiation of the "label" attribute support as a part of the
application.
5. 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=
c=IN IP4 192.0.2.2
t=0 0
m=audio 6886 RTP/AVP 0
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a=label:1
m=audio 22334 RTP/AVP 0
a=label:2
6. IANA Considerations
Contact name: Orit Levin oritl@microsoft.com.
Attribute name: "label".
Type of attribute Media level.
Subject to charset: Not.
Purpose of attribute: "Label" attribute associates the media stream
with specific application semantics. During the SDP Offer-Answer
mechanism, it allows application to make smarter choices
especially among m-lines of the same media type. In a context of
a multiparty application, it allows the application to pick out an
offered media stream based on its role in the application as the
only available meaningful identifier beyond a single SDP session
(i.e., across the application participants).
Allowed attribute values: Any octet string.
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.
8. References
8.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[3] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
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Session Description Protocol (SDP)", RFC 3264, June 2002.
[4] Camarillo, G., Eriksson, G., Holler, J. and H. Schulzrinne,
"Grouping of Media Lines in the Session Description Protocol
(SDP)", RFC 3388, December 2002.
8.2 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] Koskelainen, P. and H. Khartabil, "Requirements for Conference
Policy Control Protocol", draft-ietf-xcon-cpcp-reqs-03 (work in
progress), April 2004.
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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