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

Colin Perkins <> Tue, 26 August 2014 15:53 UTC

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Date: Tue, 26 Aug 2014 16:53:34 +0100
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Subject: Re: [Dart] Treatment of RTCP (was Re: Colin Perkins comments - WGLC: draft-ietf-dart-dscp-rtp-02)
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On 26 Aug 2014, at 16:39, Black, David <> wrote:

>>> 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?
> <editor hat off>
> Sure, when there's a notion of priority or importance.  There won't always
> be one (e.g., is CS2 higher priority than AF2x?  That depends ...).  OTOH,
> we don't need to say much here, e.g., the class selector codepoints (CSx)
> are ordered, and adding a pointer to draft-ietf-tsvwg-rtcweb-qos will be
> helpful.
> </editor hat off>
> Turning to the other RTCP issue, [F] on multi-stream optimization, I wonder
> whether we inadvertently framed that issue backwards.  Given the above, a
> single RTCP report on multiple RTP streams that use rather different DSCPs
> may not result in representative RTT values for all of the streams, because
> the report has to be sent with one DSCP.  If that outcome is a problem,
> one should send separate RTCP reports (duh!). 

Each reporting SSRC sends a separate RTCP report block for each SSRC it receives, so this is not a problem. The question is what marking an SSRC uses for the RTCP packets it sends, given that it uses several different markings for the RTP media packets it sends. 

> That statement seems valuable to make and can be made with a citation of
> RFC 3530, as opposed to the multi-stream optimization draft.
> Harald and Colin - what do you think?

The discussion of the multi-stream-optimisation draft is a separate issue, and is correct as it is.

Colin Perkins