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Re: [MMUSIC] Simulcast and 5576 source attributes?

Paul Kyzivat <pkyzivat@alum.mit.edu> Mon, 13 November 2017 17:40 UTC

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References: <58277100-1D4C-44E0-B621-30972D8BBC41@vidyo.com> <qbfr28fwgrgcelwf0qc3rsl5.1510544803607@email.android.com>
From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Date: Mon, 13 Nov 2017 12:40:49 -0500
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Subject: Re: [MMUSIC] Simulcast and 5576 source attributes?
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On 11/12/17 10:49 PM, Bo Burman wrote:
> One "certain case" is when using a 1:1 mapping of rid-id and RTP PT, in 
> combination with a 1:1 mapping of SSRC and RTP PT. This would e.g. be:
> a=rid:<rid-id1> pt=<pt1>
> a=ssrc:<ssrc1> fmtp:<pt1>
> a=rid:<rid-id2> pt=<pt2>
> a=ssrc:<ssrc2> fmtp:<pt2>
> a=simulcast:send 1,2
> 
> This way, ssrc1 can be known to use rid-id1 and ssrc2 rid-id2, via 
> knowledge what RTP PT they use. Therefore, which simulcast stream uses 
> which SSRC is also known.

I'm scratching my head trying to figure out how any of this would be 
helpful in practice. I'm coming up dry. Anything that simply allows you 
to figure out something in the limited case where the other side uses a 
particular subset of features in coordination can't be relied upon 
because you don't know that the situation will arise when you need it.

	Thanks,
	Paul

> Cheers
> Bo
> (as individual)
> 
> 
> -------- Original Message --------
> Subject: [MMUSIC] Simulcast and 5576 source attributes?
> From: Jonathan Lennox <jonathan@vidyo.com>
> Date: 9 nov. 2017 15:25
> To: mmusic <mmusic@ietf.org>
> 
> 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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