[Bloat] [Ecn-sane] [iccrg] Fwd: [tcpPrague] Implementation and experimentation of TCP Prague/L4S hackaton at IETF104

Bob Briscoe ietf at bobbriscoe.net
Wed Mar 20 19:29:28 EDT 2019


On 20/03/2019 19:04, Holland, Jake wrote:
> Hi Bob & Greg,
> I agree there has been a reasonably open conversation about the L4S
> work, and thanks for all your efforts to make it so.
> However, I think there's 2 senses in which "private" might be fair that
> I didn't see covered in your rebuttals (merging forks and including
> Greg's rebuttal by reference from here:
> https://lists.bufferbloat.net/pipermail/bloat/2019-March/009038.html )
> Please note:
> I don't consider these senses of "private" a disqualifying argument
> against the use of L4S, though I do consider them costs that should be
> taken into account (and of course opinions may differ here).
Thanks for engaging in this.
I do also intend to address one or two other recurring questions on the 
bloat list, but I have a lot on at the mo.

> With that said, I wondered whether either of you have any responses that
> speak to these points:
> 1. the L4S use of the ECT(1) codepoint can't be marked CE except by a
> patent-protected AQM scheduler.
> I understand that l4s-id suggests the possibility of an alternate
> scheme.  However, comparing the MUSTs of the section 5 requirements
> with the claims made by the patent seems to leave no room for an
> alternate that would not infringe the patent claims, unless I'm missing
> something?
> https://tools.ietf.org/html/draft-ietf-tsvwg-ecn-l4s-id-06#section-5
> https://patents.google.com/patent/US20170019343A1/en
1/ In 2016, I arranged for the hire of a patent attorney to undertake 
the unusual step of filing a third party observation with the European 
Patent Office. This went through Al-Lu's patent application claim by 
claim pointing to prior art and giving the patent examiner all the 
arguments to reject each claim. However, the examiner chose to take very 
little note of it, which was disappointing and costly for us. The main 
prior art is:
     Gibbens, R.J. & Kelly, F.P., "On Packet Marking at Priority 
Queues," IEEE Transactions on Automatic Control 47(6):1016--1020 (June 2002)
The guys named as inventors in AL-Lu's filing published a paper on PI2 
with me, in which we included a citation of this Gibbens paper as 
inspiration for the coupling. The Gibbens paper was already cited as 
background by other patents, so the EPO has it in their citation index.

The coupling was also based on my prior research with Mirja before I 
started working with the guys from Al-Lu in the RITE European 
Collaborative project. we had to go through a few rejections, but Mirja 
and I finally got this work published in 2014  - still before the 
priority date of the Al-Lu patent application:
     Kühlewind, M., Wagner, D.P., Espinosa, J.M.R. & Briscoe, B., "Using 
Data Center TCP (DCTCP) in the Internet," In: Proc. Third IEEE Globecom 
Workshop on Telecommunications Standards: From Research to Standards 
pp.583-588 (December 2014)

2/ The only claim that I could not find prior art for (in the original 
EU filing) was a very specific claim about using a square root for the 
coupling. The Linux implementation runs this the other way round so that 
it only has to do a squaring. So I figured we were safe from that.

However, until just now, I had not noticed that Al-Lu has 
retrospectively re-written the claims in the US patent and in the EU 
patent application to claim this the other way round - as a squaring. 
And to claim the two random number trick. Both restructuring to use a 
squaring and the two random number trick were definitely my ideas (while 
working for BT in a collaboration with Al-Lu). I have emails to prove 
this (from memory they were actually both in the same email). This is 
important, because a patent has to be about mechanism, not algorithm.

3/ This is a positive development. It means this patent is on very shaky 
legal ground. I have been trying to put pressure on Nokia to license 
this royalty free. But now I see what they have done, I am going to have 
to get a different type of legal advice.

> 2. the L4S use of the ECT(1) codepoint privileges the low latency use
> case.
> As Jonathan Morton pointed out, low latency is only one of several
> known use cases that would be worthwhile to identify to an AQM
> scheduler, which the L4S approach cannot be extended to handle:
> - Minimize Latency
> - Minimize Loss
> - Minimize Cost
> - Maximize Throughput
> https://lists.bufferbloat.net/pipermail/ecn-sane/2019-March/000066.html
> I understand that "Minimize Cost" is perhaps covered by LEPHB, and that
> operator tuning parameters for a dualq node can possibly allow for
> tuning between minimizing loss and latency, as mentioned in section
> 4.1 of aqm-dualq, but since the signaling is bundled together, it can
> only happen for one at a time, if I'm reading it right.
Not at all. There is a reason why it's called Low Latency, Low Loss, 
Scalable throughput (L4S).

The L4S definition of ECN marking addresses all four of these at once. 
Strictly it directly addresses all except minimize cost, but minimize 
cost can be built around the system. This was the subject of my PhD. I 
haven't described L4S in these terms, because most people are only 
interested in the latency. But this is the underlying reason for my 
obsession with ECN.

Frank Kelly predicted that queuing delay would be removed from the 
optimization as it was minimized. With L4S we've got very close to that.

ECN removes all congestion loss.

And the use of a inverse linear congestion controller gives the scalable 
throughput. I shall be touching on this in my talk for netdev tomorrow, 
but it's not really a subject for an implementation conference.

Minimize cost is something you do by combining the congestion signals 
across a network. So any AQM is part of that. And congestion controllers 
are the other part - they implicitly optimize cost, using the congestion 
signals as shadow prices. The square root in classic TCP distorts this, 
but DCTCP's inverse linear controller gives proportional fairness 
directly. Without a weight term in the congestion controller, there is 
not really an economic optimization, but that can be built onto a 
proportionally fair system and competition will gradually cause that to 
happen (or regulation as a proxy for competition). These are very long 
term processes though.

> But more importantly, the L4S usage couples the minimized latency use
> case to any possibility of getting a high fidelity explicit congestion
> signal, so the "maximize throughput" use case can't ever get it.
Eh? There's definitely a misunderstanding or a difference in terminology 
between us here. The whole point of using a congestion controller like 
DCTCP is so that flow rate can scale indefinitely with capacity. Van 
Jacobson actually noted that the original TCP was unscalable in a 
footnote to the tech report version of the SIGCOMM paper.

The high fidelity congestion signal of what we call scalable congestion 
controllers (like DCTCP) is inversely proportional to the window. So as 
window scales up, the congestion signal scales down, so that their 
product remains constant. That means the number of ECN marks per RTT is 
scale-invariant. So the control signal remains just as tight at any scale.



> Regards,
> Jake
> PS:
> If you get a chance, I'm still interested in seeing answers to my
> questions about deployment mitigations on the tsvwg list:
> https://mailarchive.ietf.org/arch/msg/tsvwg/TWOVpI-SvVsYVy0_U6K8R04eq3A
> I'm not surprised if it slipped by unnoticed, there have been a lot of
> emails on this.  But good answers to those questions would go a long way
> toward easing my concerns about the urgency on this discussion.
> PPS:
> These issues are a bit sideways to my technical reasons for preferring
> the SCE formulation of ECT(1), which have more to do with the confusing
> semantics and proliferation of corner cases it creates for CE and ECE.
> But I thought I’d ask about them since it seemed like maybe there was a
> disconnect here.
> *From: *Bob Briscoe <ietf at bobbriscoe.net>
> *Date: *2019-03-18 at 18:07
> *To: *"David P. Reed" <dpreed at deepplum.com>, Vint Cerf <vint at google.com>
> *Cc: *tsvwg IETF list <tsvwg at ietf.org>, bloat 
> <bloat at lists.bufferbloat.net>
> *Subject: *Re: [Bloat] [Ecn-sane] [iccrg] Fwd: [tcpPrague] 
> Implementation and experimentation of TCP Prague/L4S hackaton at IETF104
> David,
> On 17/03/2019 18:07, David P. Reed wrote:
>     Vint -
>     BBR is the end-to-end control logic that adjusts the source rate
>     to match the share of the bolttleneck link it should use.
>     It depends on getting reliable current congestion information via
>     packet drops and/or ECN.
>     So the proposal by these guys (not the cable guys) is an attempt
>     to improve the quality of the congestion signal inserted by the
>     router with the bottleneck outbound link.
> What do you mean 'not the cable guys'?
> This thread was reasonably civil until this intervention.
>     THe cable guys are trying to get a "private" field in the IP
>     header for their own use.
> There is nothing private about this codepoint, and there never has 
> been. Here's some data points:
> * The IP header codepoint in question (ECT(1) in the ECN field) was 
> proposed for use as an alternative ECN behaviour in July 2105 in the 
> IETF AQM WG and the IETF's transport area WG (which handles all ECN 
> matters).
> * A year later there followed a packed IETF BoF on the subject (after 
> 2 open Bar BoFs).
> * Long discussion ensued on the merits of different IP header field 
> combinations, on both these IETF lists, involving people active on 
> this list (bloat), including Dave Taht, who is acknowledged for his 
> contributions in the IETF draft.
> * That was when it was decided that ECT(1) was most appropriate.
> * The logic of the decision is written up in an appendix of 
> draft-ietf-ecn-l4s-id.
> * David Black, one of the co-chairs of the IETF's transport area WG 
> and co-author of both the original ECN and Diffserv RFCs, wrote 
> RFC8311 to lay out the process for reclaiming and reusing the 
> necessary codepoints.
> * This work and the process of freeing up codepoints has been very 
> visible at every IETF ever since, with multiple drafts to fix other 
> aspects of the protocols working their way through the IETF process in 
> multiple WGs (tsvwg, tcpm, trill, etc).
> * L4S has also been mentioned in IETF liaisons with the IEEE and 3GPP.
> Some history:
> * I had been researching the idea since 2012.
> * In fact my first presentation on it was scheduled directly after Van 
> Jacobson's first presentation of CoDel at the IETF in July 2012. VJ 
> overran by nearly 20mins leaving just 3 mins for my presentation.
> * Mirja Kuehlewind and I did early groundwork in 2013 and published a 
> paper
> * Then I (working for BT) brought the work into the EU RITE project 
> (Reducing Internet Transport Latency) with Simula, Alcatel-Lucent, etc.
> * By 2015 the two main L4S proponents were Koen De Schepper from 
> Alcatel Lucent and myself (I had just switched from BT to Simula), 
> along with Olga Bondarenko (now Albisser), a PhD student at Simula who 
> now works for Microsoft (on something else) and is still doing her PhD 
> part-time with Simula
>   o By that time, Al-Lu and Simula had a cool prototype running.
>   o This was demonstrated publicly for the first time in the IETF AQM 
> WG over DC+core+backhaul+DSL+home networks.
> https://riteproject.eu/dctth/#1511dispatchwg 
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__riteproject.eu_dctth_-231511dispatchwg&d=DwMDaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=bqnFROivDo_4iF8Z3R4DyNWKbbMeXr0LOgLnElT1Ook&m=zfPW2a1vuvsWS3Jyy_VK9_HR7vCG_9ICWuN2-7yuuPU&s=W5ZSTVXb4iSChTS8-sSOHWDszX3AitVf8Qwh-dXbqCY&e=>
> * In May 2016, L4S was demonstrated at MultiMediaSystems'16 with 
> /every/ packet from all the following simultaneous applications 
> achieving ~1ms queuing delay or less over a 40Mb/s Internet access 
> link (7ms base RTT):
>   o cloud-rendered remote presence in a racing car within a VR headset
>   o the interactive cloud-rendered video already linked above
>   o an online gaming benchmark
>   o HAS video streaming
>   o a number of bulk file downloads
>   o a heavy synthetic load of web browsing
> L4S has never been access-technology-specific. Indeed the cable 
> industry has been funding my work at the IETF to help encourage a 
> wider L4S ecosystem. There is nothing private to the cable industry in 
> this:
> * Al-Lu used DSL as a use-case, but L4S was relevant to all the access 
> technologies they supplied.
> * BT was obviously interested in DSL,
> * but BT's initial motivating use-case was to incrementally deploy the 
> low queuing delay of DCTCP over BT's data centre interconnect networks.
> * In Jul 2016 the open-source Linux repo for the Coupled AQM was 
> published, with a fully working version to be used and abused.
>    Now at: https://github.com/L4STeam/sch_dualpi2_upstream 
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_L4STeam_sch-5Fdualpi2-5Fupstream&d=DwMDaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=bqnFROivDo_4iF8Z3R4DyNWKbbMeXr0LOgLnElT1Ook&m=zfPW2a1vuvsWS3Jyy_VK9_HR7vCG_9ICWuN2-7yuuPU&s=IrFWAYZca2EEiXNTyliUfh3DgYYEyvabNTq8xYIQjBQ&e=>
> * Of course, DCTCP was already open-sourced in Linux and FreeBSD, as 
> well as available in Windows
> * In Jul 2016, the main IETF BoF on L4S was held:
>   o Ingemar Johansson from Ericsson was one of the proponents, focused 
> on using L4S in LTE
>   o along with Kevin Smith from Vodafone and
>   o Praveen Balasubramanian from Microsoft (who maintains the Windows 
> TCP stack, including DCTCP).
>   o Ingemar has since written an open-source L4S variant of the SCReAM 
> congestion controller for real-time media: 
> https://github.com/EricssonResearch/scream/ 
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_EricssonResearch_scream_&d=DwMDaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=bqnFROivDo_4iF8Z3R4DyNWKbbMeXr0LOgLnElT1Ook&m=zfPW2a1vuvsWS3Jyy_VK9_HR7vCG_9ICWuN2-7yuuPU&s=KudwyCeLp1jJbSm0Qv-Rm45UKacU0Q0rtT_Kca9Z2uA&e=>
>   o Mirja Kuehlewind of ETHZ (and now Ericsson) implemented the 
> necessary feedback in TCP https://github.com/mirjak/linux-accecn 
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_mirjak_linux-2Daccecn&d=DwMDaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=bqnFROivDo_4iF8Z3R4DyNWKbbMeXr0LOgLnElT1Ook&m=zfPW2a1vuvsWS3Jyy_VK9_HR7vCG_9ICWuN2-7yuuPU&s=8xmJipLHdxCtcbf-ZSYOZUWjzgNd8p0dF4XTOe-Lwxo&e=>
> * In summer 2017 CableLabs started work on Low Latency DOCSIS, and 
> hired me later in the year to help develop and specify it, along with 
> support for L4S
>   o In Jan 2019 the Low Latency DOCSIS spec was published and is now 
> being implemented.
>   o You can find the primary companies involved at the end of the 
> White Paper. 
> https://cablela.bs/low-latency-docsis-technology-overview-february-2019 
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__cablela.bs_low-2Dlatency-2Ddocsis-2Dtechnology-2Doverview-2Dfebruary-2D2019&d=DwMDaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=bqnFROivDo_4iF8Z3R4DyNWKbbMeXr0LOgLnElT1Ook&m=zfPW2a1vuvsWS3Jyy_VK9_HR7vCG_9ICWuN2-7yuuPU&s=rAKo34ElWnLOIk827MWT75KG3rrRmc6dM3UaTtC9VBc&e=>
>   o Operators:
>     Liberty Global
>     Charter
>     Rogers
>     Comcast
>     Shaw
>     Cox Communications
>    o Equipment vendors
>     ARRIS
>     Huawei
>     Broadcom
>     Intel
>     Casa
>     Nokia
>     Cisco
>     Videotron
> * Nicolas Kuhn of CNES has been assessing the use of L4S for satellite.
> * Magnus Westerlund of Ericsson with a team of others has written the 
> necessary ECN feedback into QUIC
> * L4S hardware is also being implemented for hi-speed switches at the 
> moment
>     (the developer wants to announce it themselves, so I have been 
> asked not to identify them).
> There's a lot more stuff been going on, but I've tried to pick out 
> highlights.
> All this is good healthy development of much lower latency for 
> Internet technology.
> I find it extremely disappointing that some people on this list are 
> taking such a negative attitude to the major development in their own 
> field that they seem not to have noticed since it first hit the 
> limelight in 2015.
> L4S has been open-sourced since 2016 so that everyone can develop it 
> and make it better...
> If I was in this position, having overlooked something important for 
> multiple years, I would certainly not condemn it while I was trying to 
> understand what it was and how it worked. Can I suggest everyone takes 
> a step back, and suspends judgement until they have understood the 
> potential, the goals and the depth of what they have missed. People 
> who know me, know that I am very careful with Internet architecture, 
> and particularly with balancing public policy against commercial 
> issues. Please presume respect unless proven otherwise.
> Best Regards
> Bob
> PS. Oh and BBR would be welcome to use the ECT(1) codepoint to get 
> into the L4S queue. But only if it can keep latency down below around 
> 1ms. Currently those ~15-25ms delay spikes would not pass muster. 
> Indeed, its delay is not consistently low enough between the spikes 
> either.
>     -----Original Message-----
>     From: "Vint Cerf" <vint at google.com> <mailto:vint at google.com>
>     Sent: Saturday, March 16, 2019 5:57pm
>     To: "Holland, Jake" <jholland at akamai.com> <mailto:jholland at akamai.com>
>     Cc: "Mikael Abrahamsson" <swmike at swm.pp.se>
>     <mailto:swmike at swm.pp.se>, "David P. Reed" <dpreed at deepplum.com>
>     <mailto:dpreed at deepplum.com>, "ecn-sane at lists.bufferbloat.net"
>     <mailto:ecn-sane at lists.bufferbloat.net>
>     <ecn-sane at lists.bufferbloat.net>
>     <mailto:ecn-sane at lists.bufferbloat.net>, "bloat"
>     <bloat at lists.bufferbloat.net> <mailto:bloat at lists.bufferbloat.net>
>     Subject: Re: [Ecn-sane] [Bloat] [iccrg] Fwd: [tcpPrague]
>     Implementation and experimentation of TCP Prague/L4S hackaton at
>     IETF104
>     where does BBR fit into all this?
>     v
>     On Sat, Mar 16, 2019 at 5:39 PM Holland, Jake <jholland at akamai.com
>     <mailto:jholland at akamai.com>> wrote:
>         On 2019-03-15, 11:37, "Mikael Abrahamsson" <swmike at swm.pp.se
>         <mailto:swmike at swm.pp.se>> wrote:
>             L4S has a much better possibility of actually getting
>         deployment into the
>             wider Internet packet-moving equipment than anything being
>         talked about
>             here. Same with PIE as opposed to FQ_CODEL. I know it's
>         might not be as
>             good, but it fits better into actual silicon and it's
>         being proposed by
>             people who actually have better channels into the people
>         setting hard
>             requirements.
>             I suggest you consider joining them instead of opposing them.
>         Hi Mikael,
>         I agree it makes sense that fq_anything has issues when you're
>         talking
>         about the OLT/CMTS/BNG/etc., and I believe it when you tell me PIE
>         makes better sense there.
>         But fq_x makes great sense and provides real value for the
>         uplink in a
>         home, small office, coffee shop, etc. (if you run the final
>         rate limit
>         on the home side of the access link.)  I'm thinking maybe
>         there's a
>         disconnect here driven by the different use cases for where
>         AQMs can go.
>         The thing is, each of these is the most likely congestion point at
>         different times, and it's worthwhile for each of them to be
>         able to
>         AQM (and mark packets) under congestion.
>         One of the several things that bothers me with L4S is that
>         I've seen
>         precious little concern over interfering with the ability for
>         another
>         different AQM in-path to mark packets, and because it changes the
>         semantics of CE, you can't have both working at the same time
>         unless
>         they both do L4S.
>         SCE needs a lot of details filled in, but it's so much cleaner
>         that it
>         seems to me there's reasonably obvious answers to all (or
>         almost all) of
>         those detail questions, and because the semantics are so much
>         cleaner,
>         it's much easier to tell it's non-harmful.
>         <aside regarding="non-harmful">
>         The point you raised in another thread about reordering is mostly
>         well-taken, and a good counterpoint to the claim "non-harmful
>         relative
>         to L4S".
>         To me it seems sad and dumb that switches ended up trying to make
>         ordering guarantees at cost of switching performance, because
>         if it's
>         useful to put ordering in the switch, then it must be equally
>         useful to
>         put it in the receiver's NIC or OS.
>         So why isn't it in all the receivers' NIC or OS (where it
>         would render
>         the switch's ordering efforts moot) instead of in all the
>         switches?
>         I'm guessing the answer is a competition trap for the switch
>         vendors,
>         plus "with ordering goes faster than without, when you
>         benchmark the
>         switch with typical load and current (non-RACK) receivers".
>         If that's the case, it seems like the drive for a competitive
>         advantage
>         caused deployment of a packet ordering workaround in the wrong
>         network
>         location(s), out of a pure misalignment of incentives.
>         RACK rates to fix that in the end, but a lot of damage is
>         already done,
>         and the L4S approach gives switches a flag that can double as
>         proof that
>         RACK is there on the receiver, so they can stop trying to
>         order those
>         packets.
>         So point granted, I understand and agree there's a cost to
>         abandoning
>         that advantage.
>         </aside>
>         But as you also said so well in another thread, this is
>         important.  ("The
>         last unicorn", IIRC.)  How much does it matter if there's a
>         feature that
>         has value today, but only until RACK is widely deployed?  If
>         you were
>         convinced RACK would roll out everywhere within 3 years and
>         SCE would
>         produce better results than L4S over the following 15 years,
>         would that
>         change your mind?
>         It would for me, and that's why I'd like to see SCE explored
>         before
>         making a call.  I think at its core, it provides the same
>         thing L4S does
>         (a high-fidelity explicit congestion signal for the sender),
>         but with
>         much cleaner semantics that can be incrementally added to
>         congestion
>         controls that people are already using.
>         Granted, it still remains to be seen whether SCE in practice
>         can match
>         the results of L4S, and L4S was here first.  But it seems to
>         me L4S comes
>         with some problems that have not yet been examined, and that
>         are nicely
>         dodged by a SCE-based approach.
>         If L4S really is as good as they seem to think, I could
>         imagine getting
>         behind it, but I don't think that's proven yet.  I'm not
>         certain, but
>         all the comparative analyses I remember seeing have been from
>         more or
>         less the same team, and I'm not convinced they don't have some
>         misaligned incentives of their own.
>         I understand a lot of work has gone into L4S, but this move to
>         jump it
>         from interesting experiment to de-facto standard without a
>         more critical
>         review that digs deeper into some of the potential deployment
>         problems
>         has me concerned.
>         If it really does turn out to be good enough to be permanent,
>         I'm not
>         opposed to it, but I'm just not convinced that it's
>         non-harmful, and my
>         default position is that the cleaner solution is going to be
>         better in
>         the long run, if they can do the same job.
>         It's not that I want it to be a fight, but I do want to end up
>         with the
>         best solution we can get.  We only have the one internet.
>         Just my 2c.
>         -Jake
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> ________________________________________________________________
> Bob Briscoehttp://bobbriscoe.net/  <https://urldefense.proofpoint.com/v2/url?u=http-3A__bobbriscoe.net_&d=DwMDaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=bqnFROivDo_4iF8Z3R4DyNWKbbMeXr0LOgLnElT1Ook&m=zfPW2a1vuvsWS3Jyy_VK9_HR7vCG_9ICWuN2-7yuuPU&s=wzv0H2d7H27kBtLtx2XWzBZkzJ_s0BmWpPnMn9B7Pc8&e=>

Bob Briscoe                               http://bobbriscoe.net/

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