[Dart] Treatment of RTCP (was Re: Colin Perkins comments - WGLC: draft-ietf-dart-dscp-rtp-02)

Ben Campbell <ben@nostrum.com> Tue, 26 August 2014 14:16 UTC

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From: Ben Campbell <ben@nostrum.com>
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Subject: [Dart] Treatment of RTCP (was Re: Colin Perkins comments - WGLC: draft-ietf-dart-dscp-rtp-02)
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On Aug 26, 2014, at 5:17 AM, Colin Perkins <csp@csperkins.org> wrote:

>>>> 
>>>> Getting feedback to the sender is important, though. In an ideal world, I would argue that RTCP packets should be marked with whatever DSCP value will deliver RTCP packets in the most expedient way. Since we don't have an ideal world, I don't know which DSCP value that would be.
>>> 
>>> Would the starting positions of "getting feedback is important, even of not for RTT estimates" and "we need RTCP for RTT estimates" likely land on the same guidance for DSCP values?
>>> 
>>> The argument to send RTCP packets in the most expedient way sounds reasonable. I don't know if we need to recommend a particular DSCP, since we already have quite a bit of text on how DSCPs might (or might not) map into some predictable PHB treatment.
>> 
>> Good question and valid point.  Nowhere in the document do we recommend the use of a particular DSCP value for any particular thing, and we should not recommend a particular value for RTCP in this document.  I'm just not sure what statements should be made.
>> 
>> I suspect we can all agree that RTCP information is important.  It's just the DSCP-related guidance that goes with that that is challenging.
> 
> I don’t agree that RTCP information should be sent as higher priority than the media. Ideally, it should be sent with the same priority as the media, so it can be used to sample the RTT. This RTT sample is independent of RMCAT. It’s in base RTP specification, and so is something we need to support to the extent possible. 
> 
> Since not all the media sent by a single SSRC has the same marking, my suggestion would be that each SSRC mark the RTCP packets it sends with one of the same code points as it uses to mark the media. Since RTCP is somewhat important, it would make sense for each SSRC to mark the RTCP packets it sends using the highest priority code point it uses to mark the RTP media packets it sends.

That makes sense to me. Paul, and others, do you agree with that last paragraph?

/Ben