Re: [Dart] draft-dart-dscp-rtp - way forward

Harald Alvestrand <harald@alvestrand.no> Fri, 29 August 2014 19:29 UTC

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Date: Fri, 29 Aug 2014 21:29:14 +0200
From: Harald Alvestrand <harald@alvestrand.no>
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To: Colin Perkins <csp@csperkins.org>
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Cc: Ben Campbell <ben@nostrum.com>, "Black, David" <david.black@emc.com>, "dart@ietf.org" <dart@ietf.org>
Subject: Re: [Dart] draft-dart-dscp-rtp - way forward
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On 08/29/2014 06:04 PM, Colin Perkins wrote:
> On 29 Aug 2014, at 16:52, Harald Alvestrand <harald@alvestrand.no>; wrote:
>> On 08/29/2014 05:14 PM, Black, David wrote:
>>> Harald,
>>>
>>> I’m about to submit a -05 with the indication that the single DSCP recommendation for SCTP and DCCP may be revised.  The RTCP multi-stream optimisation text will still be in there with Colin's clarification about "received" streams.  I’m about to vanish for about 3 weeks, but could put in a revised -06 over the weekend if you can quickly convince Colin.
>> Explicitly pinging Colin - Colin, are you arguing that the sentence
>>
>>    RTCP multi-stream reporting optimizations for an RTP session
>>    [I-D.ietf-avtcore-rtp-multi-stream-optimisation] assume that the RTP
>>    streams involved experience the same packet loss behavior.  This
>>    mechanism is highly inappropriate when the RTP streams involved use
>>    different PHBs, even if those PHBs differ solely in drop precedence.
>>
>> should stay in the draft?
> I was, but thinking again, I’m not so sure.
>
>> I think this recommendation is wrong.
>>
>> I can't find anything in your latest messages that speak to this particular point.
>> You're one of the authors of -multi-stream, so you should be able to speak clearly to the point.
>>
>> Can you clarify?
> If I have several SSRCs, and receive several media streams, then provided each of my SSRCs sees the exact same quality for each received stream, then  I can use the multi-stream-optimisation to reduce the number of RTCP cross reports I send. The multi-stream-optimisation draft says that already, and it’s not clear that the DART drafts needs to say anything further on the topic.
>
> Whether I use the same DSCP for all RTCP reports I send is, I think, orthogonal to whether I use the multi-stream-optimisation. The dart draft should possibly say that, but I’m not sure that’s the sentence we have above.
>
>
I read the sentence above as saying flatly and unconditionally "don't 
use multi-stream-optimization when the RTP streams have different PHBs" 
- which means that if I want to use one PHB for audio and another PHB 
for video, I can't use multi-stream-optimization.

I'd be sad if that was the case, *especially* if we can't figure out any 
reason to make that recommendation.