Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on changes: draft-ietf-avtcore-rtp-circuit-breakers-16

Michael Welzl <michawe@ifi.uio.no> Mon, 27 June 2016 20:52 UTC

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From: Michael Welzl <michawe@ifi.uio.no>
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To: "Black, David" <david.black@emc.com>
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Cc: "rtcweb@ietf.org" <rtcweb@ietf.org>, tsvwg <tsvwg@ietf.org>, IETF AVTCore WG <avt@ietf.org>, "De Schepper, Koen \(Nokia - BE\)" <koen.de_schepper@nokia-bell-labs.com>
Subject: Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on changes: draft-ietf-avtcore-rtp-circuit-breakers-16
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David,


> On 27. jun. 2016, at 22.09, Black, David <david.black@emc.com>; wrote:
> 
>> As long as an AQM is marking at the same rate as dropping
> 
> That's an interesting assumption - it should be true for AQMs vetted
> here in the past, but there are easy ways for it not to hold (e.g., if dropping
> or marking is based on queue occupancy, it is possible that dropping
> reduces queue occupancy in a fashion that marking does not).
> 
> For ECN "classic" (i.e., see RFC 3168) where ECN-CE markings are treated
> as drop-equivalent, that is for congestion control purposes, which is similar
> to, (but not the same as) the throughput estimation usage for the RTP circuit
> breaker.    I'll note that ECN "classic" was designed congestion control
> algorithms for react to ECN-CE marks once per RTT, independent of how
> many ECN-CE marks are observed in an RTT.
> 
> Gorry wrote:
> 
>>> in this context we should use ECN to drive a CC algorithm and we should be
>>> cautious to avoid requiring its use within a Circuit Breaker - optional
>>> use, if you understand how to interpret a reaction to many CE-marks as
>>> excessive congestion, are permitted.
> 
> Something like that may be workable, starting with a clear distinction between
> the use of ECN by CC (routine, active at all times) and ECN by a circuit
> breaker (monitors for evidence that things have gotten bad, only activated
> when things get bad).   This would baseline the RTP circuit breaker on drops
> and allow use of ECN as additional evidence of problems, in contrast to
> congestion control where ECN-CE is effectively treated as drop-equivalent.
> 
> I'm not quite sure how to specify "use of ECN as additional evidence" of
> "excessive congestion" as drop-equivalence is about the best we have
> for current guidance.

I fail to parse that sentence, so maybe I’m getting you wrong, but anyway I wonder: what’s even the point of this?
Why even bother considering CE-marks as information for a circuit breaker?

CE-marks may *not* indicate *excessive* congestion - and since you say “additional evidence”: I don’t think that a combination of loss and CE-marks makes this any better? CE-marks may be produced by a shallow queue, which can be rather “mild” congestion, at least in the light of what a circuit breaker should consider…

Cheers,
Michael