MMUSIC J. Lennox
Internet-Draft Vidyo
Intended status: Standards Track July 6, 2009
Expires: January 7, 2010
A Session Description Protocol (SDP) Attribute for Maximum Media Source
Count Indication
draft-lennox-mmusic-sdp-max-sources-00
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Abstract
The Real-Time Transport Protocol (RTP) is a multi-point protocol,
supporting multiple simultaneous sending sources in an RTP session.
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However, many existing RTP endpoints cannot usefully receive more
than one simultaneous source. This document provides a Session
Description Protocol (SDP) attribute that allows endpoints to
indicate the maximum number of sources they can usefully receive.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The "max-sources" Media Attribute . . . . . . . . . . . . . . . 3
4. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 4
5. Formal Grammar . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7.1. New SDP Media-Level Attribute . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . . 6
Appendix A. Open issues . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
The Real-Time Transport Protocol (RTP) [RFC3550] is a multi-point
protocol. It supports multiple simultaneous media senders, and, as
documented in RTP Topologies [RFC5117], scales from two-party point-
to-point sessions to large, wide-area sessions.
However, many existing systems do not make use of this capability.
Especially for Voice-over-IP and multimedia conferencing systems
using SIP [RFC3261], many existing terminals will not decode more
than a single RTP stream at a time per RTP session. Indeed, many
deployed systems are known to misbehave badly upon receiving more
than one simultaneous RTP stream in an RTP session.
Thus, even if endpoints can receive, decode, and usefully present
multiple sources in an RTP session, there is no portable way for
their peers to know this and take advantage of it.
To remedy this situation, this document defines a new SDP attribute,
"max-sources", which specifies the maximum number of sources that an
endpoint in an RTP session can usefully process.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119] and
indicate requirement levels for compliant implementations.
3. The "max-sources" Media Attribute
This section defines a new SDP media-level attribute, "max-sources",
which indicates the maximum number of sources supported in an RTP
session.
a=max-sources:<source count>
a=max-sources:unlimited
The SDP media attribute "max-sources" indicates the maximum number of
RTP sources that an RTP receiver supports within an RTP session.
<source count> is the maximum number of sources that can usefully be
decoded, expressed as a positive decimal integer or the string
"unlimited". Thus, it indicates the maximum number of RTP senders
that should be present in the session, other than those that the SDP
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author itself will send.
The "max-sources" media attribute MAY be used for any RTP-based media
transport. It is not defined for other transports.
The "max-sources" attribute is only defined for SDP Offer/Answer
[RFC3264] messages, not declarative SDP. A receiver of a "max-
sources" attribute in an offer or answer SHOULD restrict the number
of sources that it transmits on the RTP session to the number of
sources mentioned in the SDP message.
The "max-sources" attribute does not support the value of "0". If no
sources are supported on a session, an appropriate SDP sendrecv
attribute (e.g. "sendonly") SHOULD be used instead.
The "max-sources" attribute SHOULD NOT be used with RTP sessions sent
over any-source multicast (those that RTP Topologies [RFC5117]
describes as "Topo-Multicast", or the multicast side of "Topo-
Translator"), because new participants cannot immediately determine
how many existing senders are present in the RTP session.
For the unicast portion of translator-based topologies, it is the
translator's responsibility to forward only the appropriate number of
sources along the unicast path.
Figure 1 in Section 5 gives a formal Augmented Backus-Naur Form
(ABNF) [RFC5234] grammar for the "max-sources" attribute.
The "max-sources" media attribute is not dependent on charset.
4. Backward Compatibility
As mentioned in the introduction, it is unclear with existing
endpoints whether they can usefully support multiple sources on an
RTP session. In most circumstances, therefore, existing endpoints
will default to sending only a single source per session. This will
be compatible with any requested value of "max-sources" even if the
receiver of the SDP does not support the attribute.
5. Formal Grammar
This section gives a formal Augmented Backus-Naur Form (ABNF)
[RFC5234] grammar for each the "max-sources" media attribute.
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max-sources-attr = "max-sources:" ( 1*DIGIT / "unlimited" )
attribute =/ max-sources-attr
Figure 1: Syntax of the "max-sources" media attribute
6. Security Considerations
All the security implications of RTP [RFC3550] and of SDP [RFC4566]
apply. Explicitly indicating the number of RTP sources supported in
an RTP media stream does not appear to add further security issues.
7. IANA Considerations
7.1. New SDP Media-Level Attribute
This document defines a new SDP media-level attribute, "max-sources".
This attribute should be registered by IANA under "Session
Description Protocol (SDP) Parameters" under "att-field (media level
only)". Its format is defined in Section 3
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
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8.2. Informative References
[RFC3261] 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.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
January 2008.
Appendix A. Open issues
o Does "max-sources" need to be defined for declarative SDP (e.g.
for RTSP)?
Author's Address
Jonathan Lennox
Vidyo, Inc.
433 Hackensack Avenue
Sixth Floor
Hackensack, NJ 07601
US
Email: jonathan@vidyo.com
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