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 00:20:45 -0400

Colin Perkins <[email-masked]> writes:
>>> 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.
>
>See draft-ietf-mmusic-sdp-source-attributes-01.txt - there are  mechanisms
>to avoid and resolve collisions with signalled SSRCs.

Having read that, it looks useful for this sort of thing (assuming these
layers are all on the same port... which I'm not sure will be valid).
However, even assuming this draft is accepted as currently written, it adds
a bunch of bookkeeping to RTP handling and a bunch more interactions
between the RTP layer and the SDP/signalling layer that don't exist today.

I question how many UAs that don't depend on a spec that mandates that
draft will ever implement any of it.  I also worry if various
midboxes/translators/SBCs/whatever (see the topology RFC) will cause
issues.

Now, with the layered streams we may be talking about a case where a higher
level (the layering) could mandate (or strongly encourage) use of that
draft.  However, since this requires a number of API changes between 
signalling and RTP layers in applications, which will significantly
increase the cost/delay in implementing this.

My question is: do the SSRC's *have* to be pre-signalled here (via that
draft or otherwise)?  Or can (as part of the layering) the layers (which
already must be linked somehow) be defined as sharing timestamp bases/etc
without requiring use of a draft that adds a bunch of RTP-layer changes.

-- 
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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