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Re: [MMUSIC] Simulcast and 5576 source attributes?

Jonathan Lennox <jonathan@vidyo.com> Mon, 13 November 2017 06:49 UTC

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From: Jonathan Lennox <jonathan@vidyo.com>
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Date: Mon, 13 Nov 2017 14:49:01 +0800
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Cc: mmusic <mmusic@ietf.org>
To: Bo Burman <bo.burman@ericsson.com>
References: <58277100-1D4C-44E0-B621-30972D8BBC41@vidyo.com> <qbfr28fwgrgcelwf0qc3rsl5.1510544803607@email.android.com>
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Subject: Re: [MMUSIC] Simulcast and 5576 source attributes?
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a=ssrc:<ssrc> fmtp:<pt> doesn’t constrain what payload types the SSRC can use; it just gives additional format parameters for that payload type as applied to that SSRC.  (The motivating use case was H.264 sprop- parameters.)  

> On Nov 13, 2017, at 11:49 AM, Bo Burman <bo.burman@ericsson.com> 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.
> 
> Cheers
> Bo 
> (as individual)
> 
> 
> -------- Original Message --------
> Subject: [MMUSIC] Simulcast and 5576 source attributes?
> From: Jonathan Lennox <jonathan@vidyo.com <mailto:jonathan@vidyo.com>>
> Date: 9 nov. 2017 15:25
> To: mmusic <mmusic@ietf.org <mailto: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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