Subject: [rtcweb] Signalling for simulcast (Re: Support of video with different resolutions)

From: Harald Alvestrand <[email-masked]>
Date: Fri, 04 Jan 2013 12:32:16 +0100

On 01/01/2013 12:45 AM, Bernard Aboba wrote:
> Out of curiosity, is there a proposal for how to signal this in VP8? 
> (E.g. RFC 5888 + something akin to 
> draft-westerlund-avtcore-multistream-and-simulcast)
I don't think this signalling should be codec specific; one could even 
imagine simulcasting on 2 different codecs in some scenarios (mixes of 
hardware clients).
(Do we have simulcast, as opposed to SVC, signalling defined for any 
other codec?)

Off the top of my head, I'd use a=msid with a new semantic tag, since I 
don't believe in having different mechanisms for m-line-level and 
ssrc-level multiplexing.

Something like:

a=msid-semantic:SIM abcde
m=video
a=ssrc:12345 msid:abcde
a=ssrc:12377 msid:abcde

(if needed, use the "appdata" field of msid to indicate something about 
each stream)

The alternative (for ssrc multiplexing) is of course straight RFC 5576:

a=ssrc-group:SIM 12345 12377
m=video
... other declarations as appropriate

and for m-line level multiplexing, straight RFC 5888:

a=group:SIM 1 2
m=video
a=mid:1
m=video
a=mid:2

With the fast work we're making on other topic, it shouldn't take more 
than a year or so to agree on these choices.