Re: [rtcweb] A problem with both A and B

Paul Kyzivat <pkyzivat@alum.mit.edu> Wed, 22 May 2013 15:31 UTC

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Date: Wed, 22 May 2013 11:31:18 -0400
From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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To: Martin Thomson <martin.thomson@gmail.com>
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Cc: "rtcweb@ietf.org" <rtcweb@ietf.org>
Subject: Re: [rtcweb] A problem with both A and B
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On 5/21/13 6:02 PM, Martin Thomson wrote:
> This is an excellent summary.
>
> On 21 May 2013 13:13, Paul Kyzivat <pkyzivat@alum.mit.edu> wrote:
>> AFAIK there is wide agreement that an RTP *session* can carry multiple RTP
>> streams.
>>
>> An SDP O/A exchange negotiating an RTP m-line pair establishes an RTP
>> session. There is disagreement whether it is permissible for the resulting
>> RTP session to carry multiple RTP streams.
>
> Nitpick:
>
> As I understand it, there's a disagreement whether O/A allows for
> creating multiple streams *in each negotiated direction*... as
> determined by a=sendrecv/sendonly/recvonly/inactive.

> In the Plan A view, there are 0 or 1 streams in the each "negotiated
> open" direction.  The SSRC might change over time, but it maps to the
> same rendering.

So your nit is that for sendrecv there might be *two* (one each way) 
rather than just one?

Sure, *for BUNDLE* I agree with that.

I don't agree that is the agreed meaning of an unbundled m-line.

> In the Plan B view, there are 0 or more streams in the each
> "negotiated open" direction.  I believe that the assumption is that
> every SSRC is a different rendering (in the absence of a=ssrc:...
> previous-ssrc:...).

Or some other attributes that affect it.

>> The specs are ambiguous on this
>> point. Clearly there are well identified cases where they do, and we aren't
>> likely to rule those incorrect. Its also clear that some of these cases
>> start out sending a single SSRC, and then later "add" another. (It may
>> actually be a substitution.)
>