Subject: [MMUSIC] Further layered codec comments: negotiating layer sets

From: Jonathan Lennox <[email-masked]>
Date: Fri, 07 Jul 2006 13:27:10 -0400

Following up to my comments on draft-scheirl-mmusic-layered-codec-00, I
thought I'd raise a somewhat broader issue.

As I mentioned in my previous message, the mapping between media layers and
RTP sessions can potentially be many-to-many.  Therefore, there may need to
be a mechanism by which the endpoints of a layered session can negotiate
(through offer-answer) the layers they are willing to send and/or receive in
a media session.

One possibility would be for this to be codec-specific, but this doesn't
provide the granularity needed.  For instance, in the current version of the
H.264 SVC payload specification, a sender just declares its stream's layer
structure in a sprop-scalability-info parameter.  The only options an
answerer has is to accept or reject the stream; there's no way in the
current version of the H.264 SVC payload to negotiate a subset of the
layers.

Instead, I think it would be useful to have media layers described in SDP in
a codec-independent way.  Something like (straw-man syntax):

a=layer:1 max-resolution:320 240 max-framerate:15
a=layer:2 max-resolution:640 480 max-framerate:15 depend: 1
a=layer:3 max-resolution:640 480 max-framerate:30 depend: 1 2

where layer attributes can be mapped to various SDP m lines more-or-less
arbitrarily.

Thoughts?

--
Jonathan Lennox -- Layered Media
[email-masked]

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