Subject: [AVT] Retry: Re: I-D Action:draft-lennox-avt-rtp-layered-encoding-timestamps-00.txt

From: Randell Jesup <[email-masked]>
Date: Wed, 18 Jun 2008 00:19:29 -0400

Originally sent on 12-June:

David R Oran <[email-masked]> writes:
>On Jun 12, 2008, at 1:08 PM, Colin Perkins wrote:
>> I have no particular objection to this proposal in itself, but I
>> believe that using the same SSRC across multiple layers is
>> undesirable, and when different SSRC values are used for each layer,
>> this proposal offers no benefit.
>>
>> While using the same SSRC for each layer is simpler for some case, it
>> makes multiplexing several layers onto the same session difficult
>> (the suggestion below to demultiplex based on the RTP payload type
>> doesn't work). Since managing multiple UDP ports is difficult, I
>> would expect that multiplexing several layers onto the same port,
>> with media-aware RTP translators dropping layers at network
>> boundaries to perform adaptation, to be a common use case, and not
>> something we should break.
>>
>I agree with Colin. I wonder if there might be a way to get the  
>desired behavior here by pre-signaling the SSRCs and indicating in the  
>SDP that the set of SSRCs of the layers share a media clock and hence  
>a timestamp domain, so you can synchronize without relying on the more  
>general RTCP mechanisms?

(Pre-)Signaling of SSRC values has a problem in that SSRCs are not static -
a collision can happen at any time.  

The simplifying assumption that layers have to share a single timebase, or
even more simplified, share timestamp values, doesn't have the same sort of
collision problem that SSRC sharing does.

Since I assume the layers are already known to be linked via the SDP, you
can make a simplifying assumption for the layers to share TS values and
timebase (or perhaps more accurately/appropriately, declare this to be the
case in the SDP).  That means that they're cross-synced, and can be all
synced to other RTP streams (and to NTP) with a single RTCP for any of
them.  (And this would apply whether they're on the same port or not.)

-- 
Randell Jesup, Worldgate (developers of the Ojo videophone), ex-Amiga OS team
[email-masked]
"The fetters imposed on liberty at home have ever been forged out of the weapons
provided for defence against real, pretended, or imaginary dangers from abroad."
		- James Madison, 4th US president (1751-1836)

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