Re: [AVTCORE] Duplication semantics drafts

Magnus Westerlund <magnus.westerlund@ericsson.com> Tue, 11 October 2011 12:31 UTC

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Date: Tue, 11 Oct 2011 14:31:01 +0200
From: Magnus Westerlund <magnus.westerlund@ericsson.com>
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To: "Ali C. Begen (abegen)" <abegen@cisco.com>
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Cc: "mmusic (E-mail)" <mmusic@ietf.org>, "avt@ietf.org" <avt@ietf.org>
Subject: Re: [AVTCORE] Duplication semantics drafts
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On 2011-10-11 12:46, Ali C. Begen (abegen) wrote:
>> From: Magnus Westerlund [mailto:magnus.westerlund@ericsson.com]
>> (as Individual)

>> I have looked at the redundancy-grouping draft and still think it fails
>> as RTP specifications for how duplication should be done in RTP, either
>> using SSRC multiplexing with two different SSRCs in the same RTP session
>> or for the alternative how having two different RTP sessions. This first
>> appears to be sufficiently specified, where my only comment is the
>> dependency on signalling to determine that this grouping is in place. I
>> do propose that an RTP/RTCP based complement is derived. I would
>> personally suggest that my own proposal for a new SDES item SRCNAME is
>> used but that should clearly be discussed.
> 
> Right, the ssrc muxing case is straightforward and easy to implement.
> Everything works fine without the need for defining new stuff except
> the grouping semantics. I am not sure why you think there is a problem
>  in the signaling.

Yes, it works as long as the application can accept to rely on the
signalling protocol. However, if one would like to to this in session
where one has a dynamic set of participants and it is large enough that
it isn't feasible to updated the signalling for each new participant
that joins the session and is a potential sender. Then it would be good
to have something that works on the RTP/RTCP level.


> 
> If duplication will be done across sessions (your second case), then
it gets complicated. And as I understand your claim is regular CNAME
falls short to define this relation. Is that correct? And your proposal
is SRCNAME, right?

Yes, the show case when CNAME falls short is for any case where an
end-point will have multiple media flows in the same session. So in this
case you have 2 SSRC in one session carrying the source (original)
stream and 2 SSRCs in the second carrying the duplicated data. As the
two source SSRC come from the same synchronization context they do carry
the same CNAME. Thus a receiver will have two sets of SSRCs that both
carry the same CNAME. Thus that can't be used to bind the correct set of
SSRCs together. Using the sequence numbers are prone to errors and can
easily fail. I don't like overriding the SSRC randomization and require
the duplicated streams to have the same SSRC as the original. It also
creates separate solutions for the two cases.

I therefore prefer to see a solution that can handle both cases and
targeted at this particular problem.

There will be a dedicated version of the SRCNAME draft submitted before
the deadline.

> 
>> For the case where multiple RTP sessions are used, it is still not clear
>> how a receiver determine which SSRC in each RTP session that are
>> related. And I would like to point out that CNAME is not sufficient.
>>
>> I still think we should consider splitting this into one AVTCORE
>> document discussing the RTP aspects of duplication, and another MMUSIC
>> document defining the grouping semantics.
> 
> This sounds like a good plan to me.

Good

Magnus Westerlund

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