MMUSIC Working Group F. Andreasen
Internet-Draft Cisco Systems
Document: draft-andreasen-mmusic-sdp-simcap-04.txt August 2001
Category: Informational
SDP Simple Capability Negotiation
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
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1. Abstract
This document defines a set of Session Description Protocol (SDP)
attributes that enables 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 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
"next generation SDP" (SDPng) [4,5] that supports both session
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description and capability negotiation. SDPng is not anticipated to
be backwards compatible with SDP and work on SDPng is currently in
the early stages. However, several other protocols, e.g. SIP [6] and
MGCP [7], use SDP and are likely to continue doing so for the
foreseeable future. Nevertheless, in many cases these signaling
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 cannot 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 these
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. 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 MUST begin 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. Note, that by definition, the capability set MUST
include capability descriptions for all the media formats listed in
the media lines ("m=").
The individual capability descriptions in a capability set may be
provided contiguously or they may be scattered throughout the
session description. The first capability description however MUST
follow immediately after the sequence number.
The sequence number is on the form:
a=sqn: <sqn-num>
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where <sqn-num> is an integer between 0 and 255 (both included). The
initial sequence number MUST be 0 (zero) and it MUST be incremented
by 1 modulo 256 with each new capability set issued by the endpoint.
Receivers however may not necessarily see all capability sets issued
and hence MUST NOT reject a capability set due to gaps in sequence
numbers. The sequence number MUST either be provided as a session-
level or media-level attribute, however 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 description
refers to a send and receive capability by default. When generating
a capability set, the capability number MUST 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. Receivers of a capability set however MUST NOT reject
capability descriptions due to gaps in the capability numbers. The
capability number provides a convenient handle within the context of
the capability set (as referenced by the sequence number) which may
be used to reference a particular capability by means outside of
this specification.
A capability description can 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 for a
capability description SHOULD immediately follow the "cdsc" line
they refer to. Nevertheless, the capability description includes all
capability parameter lines until the next capability description
("cdsc") or media ("m=") line in the session description.
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
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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.
Capability descriptions may be provided at both the session-level
and media-level. A capability description provided at the session-
level applies to all the media streams of the indicated media type
in the session description. A capability description provided at the
media-level only applies to that particular media stream (regardless
of media type). If a capability description with media type X is
provided at the session-level, and there are no media streams of
type X in the session description, then it is undefined which of the
media streams the capability description applies to (except if there
is only one media stream). It is therefore RECOMMENDED, that such
capabilities are provided at the media-level instead.
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:
* PCMU encoding for the audio media stream,
* telephone events 0-16 and 32-35,
* T.38 fax relay using udp or tcp (see [8]).
Note, that the first capability number specified is 1, whereas the
next is 4 since three media formats were included in the first
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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.
Below, we show another example of the simple capability negotiation
mechanism, this time with multiple media streams:
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
a=sqn: 0
a=cdsc: 1 audio RTP/AVP 0 18
m=video 3458 RTP/AVP 31
a=cdsc: 3 video RTP/AVP 31 34
The sender of this session description is currently prepared to send
and receive G.729 audio and H.261 video. The sender is furthermore
capable of supporting:
* PCMU encoding for the audio media stream,
* H.263 for the video media stream.
Note, that the first capability number specified is 1, whereas the
next is 3 since two media formats were included in the first
capability description. Also note, that the sequence number applies
to the entire capability set, i.e. both audio and video, and hence
is only supplied once. Finally, note that the media formats 18 and
31 are listed in both the media lines and the capability set as
required. The above session description could equally well have been
supplied as follows:
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
a=sqn: 0
a=cdsc: 1 audio RTP/AVP 0 18
a=cdsc: 3 video RTP/AVP 31 34
m=audio 3456 RTP/AVP 18
m=video 3458 RTP/AVP 31
i.e., with the capability set provided at the session-level.
5. Security Considerations
The addition of the simple capability negotiation attributes to SDP
is not believed to cause security issues beyond those discussed in
RFC 2327.
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6. IANA Considerations
This document defines the following new SDP parameters of type "att-
field" (attribute names):
Attribute name: sqn
Long form name: Sequence number.
Type of attribute: Session-level and media-level.
Subject to charset: No.
Purpose: Capability set numbering.
Appropriate values: See Section 4.
Attribute name: cdsc
Long form name: Capability description.
Type of attribute: Session-level and media-level.
Subject to charset: No.
Purpose: Describe capabilities in a capability set.
Appropriate values: See Section 4.
Attribute name: cpar
Long form name: Capability parameter line.
Type of attribute: Session-level and media-level.
Subject to charset: No.
Purpose: Provide capability description parameters.
Appropriate values: See Section 4.
Attribute name: cparmin
Long form name: Minimum capability parameter line.
Type of attribute: Session-level and media-level.
Subject to charset: No.
Purpose: Provide minimum capability description
parameters.
Appropriate values: See Section 4.
Attribute name: cparmax
Long form name: Maximum capability parameter line.
Type of attribute: Session-level and media-level.
Subject to charset: No.
Purpose: Provide maximum capability description
parameters.
Appropriate values: See Section 4.
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, Curcio, "Requirements for Session
Description and Capability Negotiation", Internet-Draft, Internet
Engineering Task Force, April 2001. Work in Progress.
5 Kutscher, Ott, Borman, "Session Description and Capability
Negotiation", Internet-Draft, Internet Engineering Task Force,
April 2001. Work in Progress.
6 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.
7 Arango, M., Dugan, A., Elliott, I., Huitema, C. and S. Pickett,
"Media Gateway Control Protocol (MGCP) Version 1.0", RFC 2705,
October 1999.
8 ITU-T Recommendation T.38 Annex D, "SIP/SDP Call Establishment
Procedures".
8. Acknowledgments
This work draws upon the ongoing work on SDPng in the IETF MMUSIC
Working Group; in particular [4]. Furthermore this work was inspired
by the CableLabs PacketCable project. The author would like to
recognize and thank Joerg Ott, who provided many detailed comments
and suggestions to improve this specification. Orit Levin and Tom
Taylor provided valuable feedback 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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