Re: [tsvwg] Deprecating RFC 3168 for future ECN experimentation
Gorry Fairhurst <gorry@erg.abdn.ac.uk> Tue, 30 March 2021 11:15 UTC
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To: Pete Heist <pete@heistp.net>
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From: Gorry Fairhurst <gorry@erg.abdn.ac.uk>
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Date: Tue, 30 Mar 2021 12:15:46 +0100
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Subject: Re: [tsvwg] Deprecating RFC 3168 for future ECN experimentation
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Please see below: On 30/03/2021 11:56, Pete Heist wrote: > ... > > On Mon, 2021-03-29 at 10:26 +0200, Bless, Roland (TM) wrote: >> Hi, >> >> On 27.03.21 at 15:41 Pete Heist wrote: >>> I agree overall. If we want to introduce a proposal that's >>> incompatible >>> with RFC3168, we should first make it historic. >> I just want to add that RFC 3168 is a proposed standard, whereas we >> want to introduce L4S as an _experiment_. All the recent work of the >> AQM WG would be also void and I thought that vendors are actually >> implementing this in home routers. I think that we must be really sure >> about RFC3168 (non-)deployment before deprecating RFC3168. Even RFC >> 8311 >> still stated: "Forwarding behavior as described in RFC 3168 remains the >> preferred approach for routers that are not involved in ECN >> experiments,". > Just to clarify, although I think it *would* be the right approach to > deprecate RFC3168 in order to redefine its semantics, I don't > personally support doing that. Its benefits to typical Internet traffic > may not be dramatic, but on the other hand it is fully compatible with > non-ECN traffic, and avoids some packet loss and retransmissions, while > serving as a reliable base to build upon. > I also think recent discussion suggests that there is enough support > for the integrity of RFC3168 that traffic from new proposals, if not > guarded by a DSCP, must be fully compatible with existing deployed 3168 > middleboxes, effectively including those with single queue AQMs. >>> Before we do that though, we should make sure that the current CE >>> is >>> not actually useful. Figure 5 in this paper suggests some benefit >>> to >>> two bits of signal as opposed to one: >>> http://buffer-workshop.stanford.edu/papers/paper34.pdf >>> >>> A second signal provides a harder backoff without packet loss, for >>> example during capacity changes or flow introductions. It wouldn't >>> be >>> ideal to deprecate RFC3168, only to find out that another bit of >>> signal >>> in line with CE, along with ABE in RFC8511, or something similarly >>> deployable with today's equipment, is still useful. >> I'm also in favor of having better (i.e., more fine grained) >> explicit congestion signals as end-systems may use them to >> come up with better decisions. I expressed this clearly in the >> ECT(1) input cs. output signal discussion. >> Now, if we come up >> with a DSCP as additional safeguard qualifier for L4S, >> which I support, then it makes sense to revisit the earlier >> WG decision and use ECT(1) as additional congestion signal. >>> It's also my position that we can't ignore existing RFC3168 >>> bottlenecks, not just for safety but also for performance. The recent >>> ISP study we did suggested RFC3168 AQMs may be present on ~10% of >>> Internet paths there. Prior to that we heard 5% elsewhere. Whatever >>> the >>> number is exactly, these AQMs do exist and mark in response to both >>> ECT(0) and ECT(1). If you introduce traffic that backs off much less >>> in >>> response to CE, the AQMs may operate sub-optimally, since they >>> weren't >>> designed with that kind of traffic in mind >>> (https://github.com/heistp/l4s-tests/#intra-flow-latency-spikes). >> In case home routers are using RFC 3168 and congestion is in >> downstream direction one would only be able to detect this by seeing >> ECE in the other direction. Maybe servers from large content providers >> could also provide data about this... > That's also what we saw in the ISP study at the gateway, that ECE is a > more reliable indicator of AQM deployment than CE: > > https://www.ietf.org/archive/id/draft-heist-tsvwg-ecn-deployment-observations-02.html#name-tcp-initiated-from-lan-to-w > > Pete Thanks, I think would agree on your summary of the "non dramatic" benefits for existing using the currently standardised RFC-3168 marking, and hopefully what you say is not far from the summary we arrived at in RFC 8087. I see arguments in recent discussions for retaining backwards compatibility with this and also arguments stressing other aspects. Gorry >> Regards, >> Roland >> >>> On Fri, 2021-03-26 at 13:01 -0400, Steven Blake wrote: >>>> A lot (not all) of the recent arguments revolve around the >>>> assumption >>>> by some that RFC 3168 ECN deployment barely exists in the Internet, >>>> and >>>> the few networks where it does can be safely ignored, or cleaned >>>> out, >>>> or be expected to take proactive measures to protect themselves, >>>> which >>>> may in practice require them to lobby their router vendors to spin >>>> patch releases to enable (some of) the mitigation measures detailed >>>> in >>>> -l4ops-02 Sec. 5. >>>> >>>> If that is the WG consensus, then I *strongly urge* the WG to do >>>> the >>>> following: >>>> >>>> 1. Push to move RFC 3168 ECN to Historic >>>> >>>> 2. Adopt the following "New ECN" signals for future ECN >>>> experimentation: >>>> >>>> - Not-ECT >>>> - ECT >>>> - CE-a >>>> - CE-b >>>> >>>> This second step would allow for two sets of experiments. The >>>> semantics >>>> of CE-a and CE-b for the first set of experiments would be as >>>> follows: >>>> >>>> - CE-a: "Decelerate" >>>> - CE-b: "Decelerate harder" (multiplicative decrease) >>>> >>>> The exact behavior elicited by the "Decelerate" signal would be the >>>> subject of investigation. Since we are certain that any remaining >>>> RFC >>>> 3168 deployments can be safely ignored, then ECT/CE-a/CE-b can be >>>> used >>>> as unambiguous signals to steer packets into a low-latency queue, >>>> if >>>> desired. >>>> >>>> The semantics of CE-a and CE-b for the second set of experiments >>>> would >>>> be as follows: >>>> >>>> - CE-a: "Decelerate" >>>> - CE-b: "Accelerate" >>>> >>>> An aggressive fraction (100%?) of CE-b marked packets traversing a >>>> queue not in "Accelerate" state would be re-marked to either CE-a >>>> or >>>> ECT. Any packet discard (or detection of high delay variation?) >>>> must >>>> disable the transport's "Accelerate" mechanism for some interval >>>> and >>>> should cause the transport to revert to "TCP-friendly" behavior for >>>> some (different?) interval. The exact behaviors of "Accelerate" and >>>> "Decelerate" signals would be the subject of investigation. Again, >>>> since we are certain that any remaining RFC 3168 deployments can be >>>> safely ignored, then ECT/CE-a/CE-b can be used as unambiguous >>>> signals >>>> to steer packets into a low-latency queue. >>>> >>>> The differences between these two sets of experiments hinge on >>>> whether >>>> there is more utility in an "Accelerate" signal coupled with a >>>> "Decelerate" signal, or with two separate levels of "Decelerate" >>>> signals. Since it is WG consensus that the RFC 3168 ECN experiment >>>> failed after two decades, we probably only get one more chance to >>>> get >>>> this right, so careful and exhaustive experimentation which >>>> explores >>>> the design space is in order. >>>> >>>> Obviously, both sets of experiments cannot be run simultaneously on >>>> intersecting parts of the Internet. I leave the options for safely >>>> isolating these experiments as an exercise for the reader. Since we >>>> are >>>> certain that any remaining RFC 3168 ECN deployments can be safely >>>> ignored, I suggest choosing bit assignments for the four signals >>>> that >>>> induce maximum pain in the obstinate minority that might still >>>> deploy >>>> RFC 3168 ECN. >>>> >>>> Now, *if it is not WG consensus* that any existing RFC 3168 ECN >>>> deployments can be safely ignored, then I *strongly urge* the WG >>>> *to >>>> not adopt* experimental proposals that place burden and/or risk on >>>> networks that have deployed it. >>>> >>>> >>>> TL;DR: Either RFC 3168 ECN exists in the Internet, or it doesn't. >>>> Decide, and act appropriately. >>>> >>>> >>>> Regards, >>>> >>>> // Steve >>>> >>>> >>>> >>>
- [tsvwg] Deprecating RFC 3168 for future ECN exper… Steven Blake
- Re: [tsvwg] Deprecating RFC 3168 for future ECN e… Jonathan Morton
- Re: [tsvwg] Deprecating RFC 3168 for future ECN e… Pete Heist
- Re: [tsvwg] Deprecating RFC 3168 for future ECN e… Ingemar Johansson S
- Re: [tsvwg] Deprecating RFC 3168 for future ECN e… Sebastian Moeller
- Re: [tsvwg] Deprecating RFC 3168 for future ECN e… Bless, Roland (TM)
- Re: [tsvwg] Deprecating RFC 3168 for future ECN e… Gorry Fairhurst
- Re: [tsvwg] Deprecating RFC 3168 for future ECN e… Pete Heist
- Re: [tsvwg] Deprecating RFC 3168 for future ECN e… Gorry Fairhurst
- Re: [tsvwg] Deprecating RFC 3168 for future ECN e… Pete Heist
- Re: [tsvwg] Deprecating RFC 3168 for future ECN e… daihuichen
- Re: [tsvwg] Deprecating RFC 3168 for future ECN e… Jonathan Morton
- Re: [tsvwg] Deprecating RFC 3168 for future ECN e… daihuichen
- Re: [tsvwg] Deprecating RFC 3168 for future ECN e… Rodney W. Grimes
- Re: [tsvwg] Deprecating RFC 3168 for future ECN e… daihuichen