Re: [rtcweb] Plan A, respun

Paul Kyzivat <pkyzivat@alum.mit.edu> Wed, 08 May 2013 20:14 UTC

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Date: Wed, 08 May 2013 16:14:45 -0400
From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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To: Roni Even <ron.even.tlv@gmail.com>
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Cc: rtcweb@ietf.org
Subject: Re: [rtcweb] Plan A, respun
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On 5/7/13 6:16 PM, Roni Even wrote:
> Hi Paul,
> The problem in the CLUE case in the example you gave is that the receiver
> will get an RTP stream with a new SSRC not in the SDP  to the same port and
> will need to know how to map it to one of the media capture replacing the
> previous one.

My assumption was that each m-line would correspond to a capture, and 
according to what Adam specified, it would have to be demuxed by payload 
type.

And also, according to Adam's assumptions, whatever SSRCs are received 
would all be assumed to be part of the same capture.

So it works in a limited way, without a header extension, used PT demux. 
But its limited by the number of PTs.

> I think that this mode can work in plan B as an additional case, multiple
> RTP streams in an m-line but the SSRC or mapping to media capture is done
> via RTP header extension. For plan A it works only if it will allow multiple
> RTP streams based on the same m-line.  The assumption in both cases is that
> both sides identify that they understand this specific usage, for example,
> by support of the specific header extension

Either scheme can be *made* to work for the clue dynamic capture case 
using a header extension. But we have to work out the details of how.

	Thanks,
	Paul

> Roni
>
> -----Original Message-----
> From: rtcweb-bounces@ietf.org [mailto:rtcweb-bounces@ietf.org] On Behalf Of
> Paul Kyzivat
> Sent: 08 May, 2013 12:10 AM
> To: Adam Roach
> Cc: rtcweb@ietf.org
> Subject: Re: [rtcweb] Plan A, respun
>
> On 5/7/13 4:00 PM, Adam Roach wrote:
>> On 5/7/13 14:29, Paul Kyzivat wrote:
>>> If I bundle an m-line without a=ssrc, and with unique PTs, do you
>>> consider it ok if there are multiple "anonymous" flows each complying
>>> to the attributes of the m-line?
>>
>> I presume, by "multiple anonymous flows," you mean multiple series of
>> packets with the same PT, but with different SSRC values.
>>
>>> Or do you assume that all packets with that PT are treated as one
>>> flow, switching from one SSRC to another as they are received?
>>
>> In the example you gave, all of those packets are defined to be
>> associated with the same media line. Looking at basic principles, the
>> intention here is that the semantics would be identical to non-webrtc,
>> non-bundle clients receiving multiple SSRCs on the same port.
>> Admittedly, those semantics have never been crystal clear, but my
>> experience is that most systems would use this approach to send
>> multiple streams that are semantically the same "thing" and should be
>> rendered as one "thing" (e.g., window, speaker, etc).
>>
>> Specifically, we are *not* trying to allow the situation in which (1)
>> the SDP has no SSRC for the m-lines in a bundle group; (2) the party
>> who generated the SDP receives multiple streams (distinguished,
>> presumably, by SSRC) with the same PT, and (3) the recipient then
>> deduces that there should be two different rendering outputs (e.g.,
> windows) as a result.
>> That is specifically and intentionally one of the behaviors we removed
>> from the Orlando proposal, since it severely dilutes the value of
>> preserving existing SDP semantics.
>>
>> Does that answer the question you're answering?
>
> Yes, that exactly answers the question I was asking.
>
> It fits a clue case, of a "dynamic switched capture", where a single logical
> flow (capture) may be conveyed via unknown SSRCs.
>
> But it doesn't cope well with a case where there a several of those, because
> each will require distinct PTs, and that may not work.
>
> To adequately cover that, I think we need the potential to signal a
> *different* demux algorithm, instead of PT or SSRC. (We are assuming a new
> RTP header extension).
>
> Bottom line, instead of saying the demux is by ssrc if present, and
> otherwise by PT, I suggest that we explicitly signal the demux mechanism,
> with options being at least those, plus another one once we get it defined.
>
> Otherwise I think what you are proposing can work for CLUE.
>
> 	Thanks,
> 	Paul
>
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