MMUSIC Working Group F. Andreasen
Internet-Draft Cisco Systems
Document: draft-andreasen-mmusic-sdp-simcap-01.txt March 2001
Category: Informational
SDP Simple Capability Negotiation
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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1. Abstract
This document defines a set of Session Description Protocol (SDP)
attributes that allow SDP to provide a minimal and backwards
compatible capability negotiation mechanism. The mechanism can be
used as a simple and limited solution to the general capability
negotiation problem being addressed by ongoing work on the next
generation of SDP, also known as SDPng.
2. Conventions used in this document
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 [2].
3. Introduction
The Session Description Protocol (SDP) [3] describes multimedia
sessions for the purposes of session announcement, session
invitation, and other forms of multimedia session initiation. SDP
was not intended to provide capability negotiation, however as the
need for this has become increasingly important, work has begun on a
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"next generation SDP" (SDPng) [4] that supports both session
description and capability negotiation. SDPng is not anticipated to
be backwards compatible with SDP and work on SDPng is currently only
in the requirements phase. However, several other protocols, e.g.
SIP [5] and MGCP [6], use SDP, and are likely to continue doing so
for the foreseeable future. Nevertheless, in many cases these
protocols have an urgent need for some limited form of capability
negotiation.
For example, an endpoint may support G.711 audio (over RTP) as well
as T.38 fax relay (over UDP or TCP). Unless the endpoint is willing
to support two media streams at the same time, this can not
currently be expressed in SDP. Another example involves support for
multiple codecs. An endpoint indicates this by including all the
codecs in the "m=" line in the session description. However, the
endpoint thereby also commits to simultaneous support for each of
those codecs. In practice, DSP memory and processing power
limitations may not make this feasible.
As noted in [4], the problem with SDP is, that media descriptions
are used to describe session parameters as well as capabilities
without a clear distinction between the two.
In this document, we define a minimal and backwards compatible
capability negotiation feature in SDP by defining a set of new SDP
attributes that satisfy the requirements in [7]. It should be noted,
that the mechanism is not intended to solve the general capability
negotiation problem targeted by SDPng. It is merely intended as a
simple and limited solution to the most urgent problems facing
current users of SDP.
4. Simple Capability Negotiation Attributes
The SDP Simple Capability Negotiation (simcap) is defined by a set
of SDP attributes. Together, these attributes form a capability set
which describes the media capabilities of the endpoint.
The capability set begins with a single sequence number followed by
one or more capability descriptions listing all media formats the
endpoint is currently able and willing to support. A subsequent
request to use one of these media formats is however not guaranteed
to succeed, e.g. due to limited DSP processing power, or bandwidth
constraints.
The sequence number is on the form
a=sqn: <sqn-num>
where <sqn-num> is an integer between 0 and 255 (both included). The
initial sequence number is 0 and increments by 1 modulo 256 with
each new capability set from the endpoint. The sequence number may
either be provided as a session- or media-level attribute, however
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there MUST NOT be more than one occurrence of the sequence number in
the session description.
Each capability description in the capability set is on the form:
a=cdsc: <cap-num> <media> <transport> <fmt list>
where <cap-num> is an integer between 1 and 255 (both included)
identifying the capability, and <media>, <transport>, and <fmt
list> are defined as in the SDP "m=" line. The capability number
should start with 1 in the first capability description, and be
incremented by the number of capabilities in the <fmt list> for each
subsequent capability description.
A capability description may include one or more capability
parameter lines on the form:
a=cpar: <cap-par>
a=cparmin: <cap-par>
a=cparmax: <cap-par>
where <cap-par> is either bandwidth information ("b=") or an
attribute ("a=") in its full '<type>=<value>' form (see [3]). A
capability parameter line provides additional parameters for the
preceding "cdsc" attribute line. Capability parameter lines MUST
immediately follow the "cdsc" line they refer to, thus a capability
description ends at the first non "cpar", "cparmin" or "cparmax"
line that follows the "cdsc" attribute line.
The "cpar" attribute should normally be used when parameter values
are to be specified. A capability description may contain zero, one,
or more "cpar" attribute lines describing either the same or
different parameters. Describing the same parameter more than once
can be used to specify alternatives.
Where a minimum numerical value is to be specified, the "cparmin"
attribute should be used. There may be zero, one, or more "cparmin"
attribute lines in a capability description, however a given
parameter MUST NOT be described by a "cparmin" attribute more than
once.
Where a maximum numerical value is to be specified, the "cparmax"
attribute should be used. There may be zero, one, or more "cparmax"
attribute lines in a capability description, however a given
parameter MUST NOT be described by a "cparmax" attribute more than
once.
Ranges of numerical values can be expressed by using a "cparmin" and
a "cparmax" attribute for a given parameter. It follows from the
previous rules, that only a single range can be specified for a
given parameter.
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Capability descriptions may be provided at both the session- and
media-level. A capability description provided at the session-level
applies to all the media streams in the session description, where
as a capability description provided at the media-level only applies
to that particular media stream. It is RECOMMENDED that the first
capability description follows immediately after the sequence
number.
Below we show an example session description using the above simple
capability negotiation mechanism:
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 18 96
a=rtpmap:96 telephone-event
a=fmtp:96 0-15,32-35
a=sqn: 0
a=cdsc: 1 audio RTP/AVP 0 18 96
a=cpar: a=fmtp:96 0-16,32-35
a=cdsc: 4 image udptl t38
a=cdsc: 5 image tcp t38
The sender of this session description is currently prepared to send
and receive G.729 audio as well as telephone-events 0-15 and 32-35.
The sender is furthermore capable of supporting:
* media streams using PCMU encoding
* telephone events 0-16 and 32-35
* T.38 fax relay using udp or tcp (see [9])
Note, that the first capability number specified is 1, where as the
next is 4 since three media formats were included in the first
capability description. Also note, that the rtpmap for payload type
96 was not included in the capability description, as it was already
specified for the media (m=) line.
6. Security Considerations
The addition of the simple capability negotiation attributes to SDP
is not believed to affect security.
7. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
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2 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
3 M. Handley and V. Jacobson, "SDP: session description protocol,"
Request for Comments (Proposed Standard) 2327, Internet
Engineering Task Force, Apr. 1998.
4 Kutscher, Ott, Bormann, "Requirements for Session Description and
Capability Negotiation", draft-kutscher-mmusic-sdpng-req-00.txt,
July 14, 2000
5 M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
session initiation protocol," Request for Comments (Proposed
Standard) 2543, Internet Engineering Task Force, Mar. 1999.
6 Arango, M., Dugan, A., Elliott, I., Huitema, C. and S. Pickett,
"Media Gateway Control Protocol (MGCP) Version 1.0", RFC 2705,
October 1999.
7 F. Andreasen, "SDP Simple Capability Negotiation Requirements",
draft-andreasen-mmusic-sdp-simcap-reqts-00.txt, February 2001
8 J. Ott, J. Kutscher, C. Bormann, "Capability description for
group cooperation", draft-ott-mmusic-cap-00.txt, June 1999
9 PROPOSED T.38 AMENDMENT รป REC. T.38 ANNEX D, Geneva, 2-10
February, 2000, (available from
ftp://standards.nortelnetworks.com/itu_to_ietf/SG8/February00/Dra
ft_T38_Annex_D.txt)
10 Beser, B., "Codec Capabilities Attribute for SDP", Internet
Draft, draft-beser-mmusic-capabilities-00.txt, March 2000.
8. Acknowledgments
This work draws upon the ongoing work on SDPng; in particular [4].
Furthermore, this work was inspired by [8] and the CableLabs
PacketCable project. Related work can be found in [10] as well.
9. Author's Addresses
Flemming Andreasen
Cisco Systems
499 Thornall Street, 8th floor
Edison, NJ
Email: fandreas@cisco.com
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