Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on changes: draft-ietf-avtcore-rtp-circuit-breakers-16
Michael Welzl <michawe@ifi.uio.no> Mon, 20 June 2016 16:35 UTC
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From: Michael Welzl <michawe@ifi.uio.no>
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Cc: "<gorry@erg.abdn.ac.uk> Fairhurst" <gorry@erg.abdn.ac.uk>, Magnus Westerlund <magnus.westerlund@ericsson.com>, tsvwg <tsvwg@ietf.org>, IETF AVTCore WG <avt@ietf.org>, Mirja Kühlewind <mirja.kuehlewind@tik.ee.ethz.ch>, "rtcweb@ietf.org" <rtcweb@ietf.org>, Colin Perkins <csp@csperkins.org>
Subject: Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on changes: draft-ietf-avtcore-rtp-circuit-breakers-16
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> On 20. jun. 2016, at 15.16, Black, David <david.black@emc.com> wrote: > >>> But I’m less concerned than David about eventually ignoring it for circuit >> breaker. >>> >> Agree. Loss is the measurement that a CB MUST respond to. > > Mumble. I would be ok with a clear discouragement for use of ECN-CE marks, accompanied by the sort of design rationale here, or even better, a clear statement that lost packets for the purpose of the RTP circuit breaker have to be actually lost without getting into whether or not ECN-CE marks are involved -i.e., the RTP circuit breaker is specified against actual drops as a network protection backstop. > > A related concern is that ECN marks may overstate equivalent loss behavior - a simplistic queue management discipline that marks every packet when the queue is over a threshold (NB: this class of marking behavior is NOT RECOMMENDED - a real AQM SHOULD be used) could yield a run of ECN-CE marks that would not cause a corresponding with a run of packet drops. This is among the reasons that TCP reacts to ECN-CE marks only once per RTT, and might be a reason to treat multiple ECN-CE marks in an RTT interval as not representing drops of all packets for the RTP circuit breaker's TCP-equivalent throughput calculation. I’m not sure we need such complicated logic to find a case where ECN marks are different from packet drops: Basically, they simply aren’t - even “real” AQMs marking isn’t exactly the same as a packet drop: the marks themselves inform you that an AQM did its job, and with modern AQMs like CoDel / PIE etc., you’re probably getting this from a shallow queue. Chances are that this is less than a BDP worth of queuing, which is our justification for recommending a different back-off behavior in draft-khademi-tsvwg-ecn-response-00 and draft-khademi-tcpm-alternativebackoff-ecn-00 So the point is not that AQMs would treat ECN marking and dropping differently - it’s that ECN indicates an AQM, and hence probably a shallow queue. With a drop, you just don’t know. Back to the CB, I think an AQM marking at a shallow queue (like e.g. CoDel) is indeed quite different from a “broken connection”. Cheers, Michael > > Thanks, --David > >> -----Original Message----- >> From: Gorry (erg) [mailto:gorry@erg.abdn.ac.uk] >> Sent: Saturday, June 18, 2016 2:23 AM >> To: Mirja Kühlewind >> Cc: Black, David; Magnus Westerlund; Colin Perkins; rtcweb@ietf.org; IETF >> AVTCore WG; tsvwg >> Subject: Re: [tsvwg] [rtcweb] [AVTCORE] WG Last Call on changes: draft-ietf- >> avtcore-rtp-circuit-breakers-16 >> >> I think we SHOULD NOT recommend to use ECN marks as inputs to a CB. See >> below: >> >>> On 17 Jun 2016, at 16:02, Mirja Kühlewind <mirja.kuehlewind@tik.ee.ethz.ch> >> wrote: >>> >>> +1 to not use normative language here. >>> >>> However, please note that having a high level of ECN-CE marks (without any >> losses) means that all packets were received correctly. This situation can even >> occurs without high delays (depending on the AQM used), which would just >> mean the services works perfectly. Therefore for me CE marks are a perfect input >> signal for a congestion control loop (where the AQM tell the sender to take action >> - whatever that means). >> >> We may in future figure out ways to do this to detect significant failure using a >> rate adaptive transport and ECN e.g. Observing 100% CE marks or something, for >> an RTP flow that is trying to send well below its peak rate decided by CC -- but I >> think this is speculating at an algorithm and adding details here is not a good idea. >> Especially as AQM continues to evolve. >> >>> But I’m less concerned than David about eventually ignoring it for circuit >> breaker. >>> >> Agree. Loss is the measurement that a CB MUST respond to. >> >>> In addition one point on something Magnus wrote earlier: >>> "If the implementation only have circuit breaker, i.e. no full fledged congestion >> controller and uses ECN, they can in worst case drive the buffer into the overload >> regime where it starts dropping packets. „ >>> >>> I’m not sure about this case. ECN is an input signal for congestion control. If you >> don’t use congestion control but only a circuit breaker, you should probably not >> enable ECN. At least it not clear to me why you would enable it, and it's definitely >> not conform to the ECN spec. Probably we should say something about this in the >> draft...? >>> >> Agree, enabling ECN without a responsive CC is going to lead to trouble. >> >>> Mirja >>> >> Gorry >> >>>> Am 17.06.2016 um 16:03 schrieb Black, David <david.black@emc.com>: >>>> >>>> Colin, >>>> >>>>>>> ... I view the current text as providing implementers with too much >>>>>>> latitude to ignore ECN-CE marks (e.g., because an implementer doesn't >>>>>>> want to think about this problem space in the first place). >>>>> >>>>> I agree, but the argument is that doing so is less harmful than deploying a >> circuit >>>>> breaker that triggers too often when ECN is used. >>>>> >>>>> I’m not sure I believe this argument, though, since it seems that any new >> AQM >>>>> that applies ECN marks much more often than at present will have to >> consider >>>>> backwards compatibility, to work with deployed TCP (e.g., draft-briscoe- >> tsvwg- >>>>> aqm-tcpm-rmcat-l4s-problem uses ECT(1) as a signal to use the new marking, >>>>> while existing implementations set ECT(0)). These compatibility mechanisms >>>>> would seem to prevent the issues with the circuit breaker too. >>>> >>>> That roughly matches my line of thinking, and I'll observe that the original >> DCTCP >>>> protocol design that used more aggressive ECN-CE marking was only safe for >>>> Controlled Environment deployments. See the TSVWG rfc5405bis draft for >> the >>>> definition of Controlled Environment, and ignore the fact that the rfc5405bis >>>> draft is a UDP draft - this definition is more broadly applicable. >>>> >>>> Going back over Section 7 in this avtcore draft, my views are: >>>> >>>> [A] None of these drafts justify a "MAY ignore" response to ECN-CE marks: >>>> - draft-khademi-tcpm-alternativebackoff-ecn >>>> - draft-ietf-rmcat-nada >>>> - draft-ietf-rmcat-scream-cc >>>> >>>> [B] In line with Colin's comment on the L4S draft, I think it's incumbent on >>>> the authors of draft-briscoe-aqm-dualq-coupled to figure out how that will >>>> coexist (or avoid) deployed TCP, and this avtcore draft ought not to be >>>> trying to prejudge what will be done there. >>>> >>>> So, I don't think the current text in Section 7 has justified the unfettered >>>> "implementations MAY ignore ECN-CE marks" text, as ignoring those marks >>>> is not consistent with any of the four cited drafts. >>>> >>>> In more detail, I think making changes to normative requirements here based >>>> on [B] is premature, and I would hope that the rmcat WG could be >> encouraged >>>> to consider the RTP circuit breaker in its congestion control drafts, as those CC >>>> mechanisms are related to the circuit breaker mechanism, hence likely >>>> to be in related areas of an RTP implementation. >>>> >>>> That leaves draft-khademi-tcpm-alternativebackoff-ecn, which TSVWG >>>> will be looking at in Berlin. If a normative statement about ECN-CE reaction >>>> is going to rest on that draft, then the reference to that draft should be >>>> normative. Something about doing that strikes me as premature ... >>>> >>>> I realize that we're trying to predict and accommodate the future, which >>>> is an imprecise undertaking at best. As an alternative to the current text, >>>> would it be reasonable to say (without any RFC 2119 keywords) that the >>>> best current guidance is still to treat ECN-CE marks as indicating drops, >>>> with a warning that there is a good possibility of this changing in the >>>> near future due to all of the work in progress cited in Section 7? >>>> >>>> Thanks, --David >>>> >>>>> -----Original Message----- >>>>> From: Colin Perkins [mailto:csp@csperkins.org] >>>>> Sent: Friday, June 17, 2016 6:14 AM >>>>> To: John Leslie; Black, David >>>>> Cc: rtcweb@ietf.org; IETF AVTCore WG; tsvwg >>>>> Subject: Re: [rtcweb] [AVTCORE] [tsvwg] WG Last Call on changes: draft-ietf- >>>>> avtcore-rtp-circuit-breakers-16 >>>>> >>>>> >>>>>> On 16 Jun 2016, at 23:25, John Leslie <john@jlc.net> wrote: >>>>>> >>>>>> Black, David <david.black@emc.com> wrote: >>>>>>> >>>>>>> ... I view the current text as providing implementers with too much >>>>>>> latitude to ignore ECN-CE marks (e.g., because an implementer doesn't >>>>>>> want to think about this problem space in the first place). >>>>> >>>>> I agree, but the argument is that doing so is less harmful than deploying a >> circuit >>>>> breaker that triggers too often when ECN is used. >>>>> >>>>> I’m not sure I believe this argument, though, since it seems that any new >> AQM >>>>> that applies ECN marks much more often than at present will have to >> consider >>>>> backwards compatibility, to work with deployed TCP (e.g., draft-briscoe- >> tsvwg- >>>>> aqm-tcpm-rmcat-l4s-problem uses ECT(1) as a signal to use the new marking, >>>>> while existing implementations set ECT(0)). These compatibility mechanisms >>>>> would seem to prevent the issues with the circuit breaker too. >>>>> >>>>>> Understand, we have at least two proposals to make ECN-CE more >> frequent >>>>>> than packet drop would be for non-ECN packets: possibly substantially >>>>>> more frequent. Unless both are killed off, ECN-CE will show up frequently >>>>>> enough that closing the flow on ECN-CE would kill too many connections. >>>>>> >>>>>> If you want circuit-breaking on such connections, there are two ways: >>>>>> 1. convince the forwarding nodes to drop packets if their queue exceeds >>>>>> design capacity; or >>>>>> 2. require the sender to send enough not-ECN-capable packets so that our >>>>>> receiver will see enough packet-drops when a circuit-breaker should >>>>>> activate. >>>>>> >>>>>> (I prefer the first option; but I wouldn't object to the second.) >>>>>> >>>>>> There really isn't any way for our circuit-breaker to know _how_much_ >>>>>> more frequent the ECN-CE marks may be. :^( >>>>> >>>>> This is a problem, both for the circuit breaker, and for the algorithms being >>>>> defined in RMCAT. We do need some understanding what the expected >> marking >>>>> rates are likely to be, so congestion control and circuit breakers can be >> defined. >>>>> >>>>>> We _will_ be sorry if we >>>>>> allot the same frequency of CE packets as packet-drops to trigger the >>>>>> circuit-breaker. >>>>>> >>>>>>> Could someone propose initial text to qualifies the current "MAY ignore" >>>>>>> statement? >>>>>> >>>>>> Essentially, for the second option, you might propose text to the >>>>>> effect of: >>>>>> ] >>>>>> ] If too many ECN-CE packets are received, the sender SHOULD send some >>>>>> ] not-ECN-capable packets to determine whether enough packets along the >>>>>> ] path are being dropped to justify activating our circuit-breaker. >>>>>> >>>>>> I’m not enthusiastic about adding that; but it would resolve the issue. >>>>> >>>>> I’m not convinced this would work. The circuit breaker is looking at long term >>>>> trends, and in order to have enough not-ECT packets to determine if it >> should >>>>> trigger, you’d essentially have to run without ECN for some seconds. >>>>> >>>>> -- >>>>> Colin Perkins >>>>> https://csperkins.org/ >>>> >>>> _______________________________________________ >>>> rtcweb mailing list >>>> rtcweb@ietf.org >>>> https://www.ietf.org/mailman/listinfo/rtcweb >
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… Michael Welzl
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… De Schepper, Koen (Nokia - BE)
- Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on ch… Colin Perkins
- Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on ch… Black, David
- Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on ch… Michael Welzl
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… Ruediger.Geib
- Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on ch… Colin Perkins
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… Colin Perkins
- Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on ch… Black, David
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… Fred Baker (fred)
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… John Leslie
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… Fred Baker (fred)
- Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on ch… Michael Welzl
- Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on ch… Colin Perkins
- Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on ch… Michael Welzl
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… Black, David
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… Michael Welzl
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… De Schepper, Koen (Nokia - BE)
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… gorry
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… De Schepper, Koen (Nokia - BE)
- Re: [AVTCORE] [rtcweb] WG Last Call on changes: d… Ben Campbell
- Re: [AVTCORE] [rtcweb] WG Last Call on changes: d… Magnus Westerlund
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… Michael Welzl
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… Black, David
- Re: [AVTCORE] [tsvwg] [rtcweb] WG Last Call on ch… Gorry (erg)
- Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on ch… Mirja Kühlewind
- Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on ch… Black, David
- Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on ch… Colin Perkins
- Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on ch… John Leslie
- Re: [AVTCORE] [tsvwg] WG Last Call on changes: dr… Black, David
- Re: [AVTCORE] [tsvwg] WG Last Call on changes: dr… Colin Perkins
- Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on ch… Michael Welzl
- Re: [AVTCORE] [tsvwg] WG Last Call on changes: dr… Black, David
- Re: [AVTCORE] [rtcweb] [tsvwg] WG Last Call on ch… John Leslie
- Re: [AVTCORE] [tsvwg] WG Last Call on changes: dr… Magnus Westerlund
- [AVTCORE] WG Last Call on changes: draft-ietf-avt… Magnus Westerlund
- Re: [AVTCORE] [tsvwg] WG Last Call on changes: dr… Black, David