Subject: Re: SDPng/SDP: video configuration negotiation,

From: Markus Buchhorn <[email-masked]>
Date: Fri, 18 Aug 2000 10:10:41 +1000

<<<<
Thomas wrote:
------------
It points out that the format of the image (such as CIF, QCIF..) is a
determining parameter for some video codecs (such as H.261). 
But as I look to RFC 2327 (SDP) and RFC 2038 (RTP Profile for Audio and
Video ...) , I can't imagine how such a parameter is currently negotiated
in SDP. 
<<<<
-------------

It isn't. SDP is currently one-way/informational only - I advertise a
stream/source/whatever as having these characteristics, you can use it or
ignore it. If I want to be friendly I might advertise one or more different
versions of the same stream and the end user can decide. If you're
multicasting in a huge crowd you don't want everybody to negotiate with
everybody one-to-one. So yes, you can end up with a group where everyone
sends their own choice of video encoding, and you have to have to decode
everybody else's choices or not see them.

In a small group, or just point-to-point, you may want to negotiate
features, and that is one of the things SDPng is looking at. 

Hmm - interesting problem - create/join a large group and have a
distributed election to decide what encodings are used....

Cheers,
	Markus

Markus Buchhorn,  Advanced Computational Systems CRC     | Ph: +61 2 62798810
email: [email-masked], snail: ACSys, RSISE Bldg,|Fax: +61 2 62798602
Australian National University, Canberra 0200, Australia |Mobile: 0417 281429

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From: Christoph Reichert <[email-masked]>
Date: Fri, 18 Aug 2000 15:02:01 +0200

On Fri, Aug 18, 2000 at 10:10:41AM +1000, Markus Buchhorn wrote:

[...]

> distributed election to decide what encodings are used....

The basic approach can be rather simple:
Everybody multicasts its set of codecs when joining the group, and
everybody intersects these sets.

The question is whether SDPng shall support both, a very detailed
point-to-point negotiation (preferences, number of channels, optional features
and so on) and a negotiation mechanism for large groups (say, some hundreds
of participants).

On the other hand, point-to-point is just a group with only two
participants.

BTW, does anybody think that UDP ports need to be negotiated ?
Eg. 500 participants try to join a multicast session, some cannot participate
as the port is already in use by some other application...

cheers, chris