Re: [MMUSIC] What is an m-line?

Martin Thomson <martin.thomson@gmail.com> Mon, 13 May 2013 20:45 UTC

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Date: Mon, 13 May 2013 13:45:24 -0700
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From: Martin Thomson <martin.thomson@gmail.com>
To: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Subject: Re: [MMUSIC] What is an m-line?
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On 13 May 2013 12:02, Paul Kyzivat <pkyzivat@alum.mit.edu> wrote:
> On 5/13/13 2:42 PM, Martin Thomson wrote:
>>   1. an m-line represents a single RTP session
>>      (or at least the part of the session that touches the client)
>>      This appears to be consistent with the Plan B approach
>
> Well, it was my impression that Plan B still intends to allow bundling in
> order to combine audio and video into a single RTP session. So in that case
> neither of the m-lines represents a single RTP session in full.

"In full" ain't gonna happen, ever :)  With RTP you are always the
blind man with his tiny piece of the elephant.  Hence, I think that
your (4) is a clearer description of what I intended to say.

> I offer another try:
>
> 4. an m-line represents one or more (zero or more?)
> sources/flows/tracks/captures (pick the name that suits you) that share a
> single RTP session and that all are consistent with the media description
> accompanying the m-line.

>>   2. an m-line represents the set of RTP streams that decode into a
>> single rendering, including layers, simulcast and FEC
>
> Hmm. I was thinking *this* was what Cullen intended by Plan A.
> But now that you bring it up, I thing there is some existing SDP syntax for
> simulcast/FEC/FID that might only work with (3). (But I don't have the
> details of that stuff in my head.)

That existing stuff is what nudged this into a (3).  a=depend and a
few other things make it hard to get all the streams onto the one
line.