Re: [Dart] draft-ietf-dart-dscp-rtp-02 - Section 5.3 RTCP multi-stream optimization text

Colin Perkins <csp@csperkins.org> Thu, 28 August 2014 11:02 UTC

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From: Colin Perkins <csp@csperkins.org>
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Subject: Re: [Dart] draft-ietf-dart-dscp-rtp-02 - Section 5.3 RTCP multi-stream optimization text
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David,

On 26 Aug 2014, at 17:27, Black, David <david.black@emc.com> wrote:
> <editor hat OFF>
> 
> Colin writes:
> 
>   The discussion of the multi-stream-optimisation draft is a separate
>   issue, and is correct as it is.
> 
> Looking closer, that discussion in the current DART draft currently says:
> 
>   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.

A quick follow-up to ensure this is clear: the multi-stream-optimisation cannot be used when reporting on received RTP streams where the different streams have different markings, since those streams are treated differently in the network. 

The marking I use for the RTP streams I send (and hence the marking I use for my RTCP packets) doesn’t influence whether I can use the multi-stream-optimisation, however.

Colin




> That's based on this text from the last paragraph of Section 1
> (Introduction) of the multi-stream optimization draft:
> 
>   This memo defines such an RTCP extension, RTCP Reporting Groups.
>   This extension is used to indicate the SSRCs that originate from the
>   same endpoint, and therefore have identical reception quality, hence
>   allowing the endpoints to suppress unnecessary RTCP reception quality
>   reports.
> 
> When different DSCPs are used for SSRCs (RTP streams) that originate
> from the same source, the statement that they "therefore have identical
> reception quality" is clearly incorrect, so it looks like the current
> DART draft text is correct, IMHO.
> 
> Harald?
> 
> </editor hat OFF>
> 
> Thanks,
> --David
> 
>> -----Original Message-----
>> From: Colin Perkins [mailto:csp@csperkins.org]
>> Sent: Tuesday, August 26, 2014 11:54 AM
>> To: Black, David
>> Cc: Ben Campbell; Paul E. Jones; dart@ietf.org; avt@ietf.org WG; draft-ietf-
>> dart-dscp-rtp.all@tools.ietf.org
>> Subject: Re: Treatment of RTCP (was Re: [Dart] Colin Perkins comments - WGLC:
>> draft-ietf-dart-dscp-rtp-02)
>> 
>> 
>> On 26 Aug 2014, at 16:39, Black, David <david.black@emc.com> 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
>> http://csperkins.org/
>> 
>> 
> 



-- 
Colin Perkins
http://csperkins.org/