1. SDP Simple Capability Negotiation (SDP Simcap)
draft-andreasen-mmusic-sdp-simcap-00.txt
49th IETF - December 13, 2000
Flemming Andreasen

2. Situation
SDP currently used for two things without a clear distinction:
Describe media parameters for the session
Describe capabilites
Capability description limited to media parameters leads to problems with, e.g.
Different media types
Multiple codecs
Real problems today.

3. Multiple Media Types
A single media line can only express a single media type.
Two “m=“ lines implies simultaneous support of two separate media streams.
Example (audio and T.38 fax relay): m=audio 3456 RTP/AVP 0 m=image 3458 udptl t38
Similar issue with upcoming modem relay standards, and possibly other.

4. Multiple Codecs
Supported codecs listed in media line implies commitment to use any of the codecs.
But DSP’s have limited resources, especially memory:
Real-time codec switch on the fly problematic.
Thus cannot list all possible codecs in media line:
Limits initial negotiation ability as well as mid-call changes.

5. SDPng
SDPng will take care of the previous problems and much more, but:
SDPng not backwards compatible with SDP
SDPng is work in progress
Existing base of existing SDP users which is unlikely to go away any time soon:
SIP, MGCP, Megaco, RTSP, etc.
Need a simple and limited solution we can use with SDP:
SDP Simcap proposal to address this

6. SDP Simcap
Simple and limited backwards compatible SDP capability negotiation - general capability negotiation problem addressed by SDPng.
How simple and how limited ?
Proposed minimum requirements in draft:
Each capability description independent
Alternative values supported
Capability does not imply commitment (no reserve)
Capability syntax follows media description (simple)
Further discussion needed

7. Syntax Define additional SDP attributes.
Sequence number
Capability
Capability description
Capability parameter(s)
Example a=sqn: a=cdsc: 1 audio RTP/AVP a=cpar: a=fmtp: , a=cdsc: 4 image udptl t38

8. Next Steps Soliciting input and comments: Working Group Item ?
Look further at scope and requirements first
Working Group Item ?
Informational ?