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[MMUSIC] Simulcast and 5576 source attributes?

Jonathan Lennox <jonathan@vidyo.com> Thu, 09 November 2017 22:25 UTC

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From: Jonathan Lennox <jonathan@vidyo.com>
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Date: Thu, 09 Nov 2017 17:25:23 -0500
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Subject: [MMUSIC] Simulcast and 5576 source attributes?
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The latest Simulcast draft (draft-ietf-mmusic-sdp-simulcast-10) has a paragraph at the end of Section 5.1 that says:

   It is possible to use source-specific signaling [RFC5576] with
   "a=simulcast", but it is only in certain cases possible to learn from
   that signaling which SSRC will belong to a particular simulcast
   stream.

However, there’s no other mention of 5576 in the document, and no registration of any 5576-style source-specific attributes.

What are the “certain cases”? It’s not clear to me there’s any way for the currently-registered 5576 attributes to tell you which SSRC corresponds to which simulcast stream, prior to receiving RID in RTCP or header extensions.  (Notably, there’s no rid source attribute.)

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