Re: [tsvwg] Comments on L4S drafts

Ingemar Johansson S <> Sat, 08 June 2019 19:46 UTC

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From: Ingemar Johansson S <>
To: "Holland, Jake" <>, Bob Briscoe <>
CC: "" <>, Ingemar Johansson S <>
Thread-Topic: [tsvwg] Comments on L4S drafts
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Date: Sat, 8 Jun 2019 19:46:26 +0000
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Subject: Re: [tsvwg] Comments on L4S drafts
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+1 on Bob's comment
"The aim here is to use the last unicorn in the world (ECT(1)) to the full.
If we don't make delay extremely low and extremely consistent we'll have
wasted it. So we must focus on 99th percentile delay"
I (and others) have tried to get ECN support in 3GPP access earlier. This is
in some cases made complicated by how the 3GPP protocol layers are defined,
the IP header is for instance ciphered or authenticated below the PDCP
layer, which makes I poses additional challenges if packets are queued up on
the RLC layer.
Up until now it has been quite a challenge to make ECN happen, I believe
that part of the reason has been that ECN is not judged to give a large
enough gain. 
L4S changes all this (I hope and believe), as it makes it possible to reach
very low queue delay, especially for NR. Besides this, L4S has the nice
property that it has potential to allow for faster rate increase when link
capacity increases. The latter is a welcome component when 5G access is to
be used to its full extent as the combined use of carrier aggregation, dual
connectivity, mix of low and high frequency band transmission etc., can make
the link throughput vary more than what is seen today with e.g. LTE. 
I see many applications that can benefit greatly from L4S, besides AR/VR,
there is also an increased interest in the deployment of remote control
capabilities for vehicles such as cars, trucks and drones, all of which
require low latency video streaming.
My bottomline is that I believe L4S provides with a clear benefit that is
large enough to be more widely accepted in 3GPP. SCE is as I see it more
like something that is just a minor enhancement to ECN and is therefore much
harder to sell in to 3GPP.   


> ----------------------------------------------------------------------
> Message: 1
> Date: Fri, 7 Jun 2019 19:07:53 +0100
> From: Bob Briscoe <>
> To: "Holland, Jake" <>om>, ""
> 	<>
> Cc: "" <>
> Subject: Re: [tsvwg] Comments on L4S drafts
> Message-ID: <>
> Content-Type: text/plain; charset="utf-8"; Format="flowed"
> Thanks Jake,
> I'll address each of your questions inline. But I notice that I need to
lay down
> some context first.
> The problem boils down to deployment incentives. The introduction of fine-
> grained congestion control requires changes to sender, receiver and at
least the
> bottleneck link before it is effective. ECN deployment faced the same
> deployment problem. So we tried hard to learn from it.
> Faced with a 3-part deployment, no single party makes a move unless they
> that the potential gain is worth the effort and that /all/ the other parts
> client, network) are strongly likely to make the same judgement {Note 1}.
> The effort isn't just the coding, it's all the hassle dealing with
> consequences of making the change, e.g. the risk of people's Internet
> being taken out by a middlebox black-holing the new protocol. High risk of
> cost/effort needs very high gain.
> So the improvement has to be remarkable. Not just incremental, but
> enough to enable applications that are not even possible otherwise.
> The aim here is to use the last unicorn in the world (ECT(1)) to the full.
If we
> don't make delay extremely low and extremely consistent we'll have wasted
> So we must focus on 99th percentile delay (and more 9s if you want to take
> longer to measure it). Now, inline...
> On 05/06/2019 01:01, Holland, Jake wrote:
> > Hi Bob,
> >
> > I have a few comments and questions on draft-ietf-tsvwg-ecn-l4s-id-06
> > and draft-ietf-tsvwg-l4s-arch-03.
> >
> > I've been re-reading these with an eye toward whether it would be
> > feasible to make L4S compatible with SCE[1] by using ECN capability
> > alone as the dualq classifier (roughly as described in Appendix B.3 of
> > l4s-id), and using ECT(1) to indicate a sub-loss congestion signal,
> > assuming some reasonable mechanism for reflecting the ECT(1) signals
> > to sender (such as AccECN in TCP, or even just reflecting each SCE
> > signal in the NS bit from receiver, if AccECN is un-negotiated).
> >
> > I'm trying to understand the impact this approach would have on the
> > overall L4S architecture, and I thought I'd write out some of the
> > comments and questions that taking this angle on a review has left me
> > with.
> >
> > This approach of course would require some minor updates to DCTCP or
> > other CCs that hope to make use of the sub-loss signal, but the
> > changes seem relatively straightforward (I believe there's a
> > preliminary implementation that was able to achieve similarly reduced
> > RTT in lab) and the idea of course comes with some tradeoffs--I've
> > tried to articulate the key ones I noticed below, which I think are
> > mostly already stated in the l4s drafts, but I thought I'd ask your
> > opinion of whether you agree with this interpretation of what these
> > tradeoffs would look like, or there are other important points you'd
like to
> mention for consideration.
> May I give this proposal a name for brevity: ECN-DualQ-SCE (which sort-of
> represents ECN as the input classifier into 1 of 2 queues and SCE as the
> from that queue).
> >
> >
> > 1.
> > Of course, I understand using SCE-style signaling with ECT capability as
> > the dualq classifier would come with a cost that where there's classic
> > behavior at endpoints, the low latency queue would routinely get some
> > queue-building, until there's pretty wide deployment of scalable
> > and feedback for the congestion signals at the endpoints.
> >
> > This is a downside for the proposal, but of course even under this
> > there's the gains described in Section 5.2 of l4s-arch:
> >     "State-of-the-art AQMs:  AQMs such as PIE and fq_CoDel give a
> >        significant reduction in queuing delay relative to no AQM at
> Indeed, herein lies the problem. Imagine you are trying to convince a
> network operator to start a major project to tender for a new low
> latency technology then deploy it across their access network. You tell
> them it will also depend on:
> * servers/CDNs deploying new OS code.
> * and clients deploying new OS code.
> Then you tell them that, until /most/ servers deploy, and /most/ clients
> deploy (maybe a decade?), the low latency queue will routinely add as
> much queue delay as we can already get (without clients and servers
> changing)....
> One day, you continue, if all the other servers and clients passing
> traffic through that box get upgraded, it will be cool. Until that day,
> a gamer in augmented reality gets stunningly low delay,... except every
> time her daughter in the bedroom looks at a mate's facebook page or
> watches a YouTube clip.
> Is the network operator really going to take all those risks for jam
> tomorrow (=? maybe a decade)? I really don't think so.
> Then we'll have burned the last unicorn to routinely get what we've
> already got.
>   * Incremental deployment means, as you deploy the new capability, old
>     traffic continues to work, while new traffic gets the new service.
>   * As you say, with ECN-DualQ-SCE, new traffic only gets the new
>     service if there's no old traffic there. That's not only incremental
>     deployment; that's also ineffective deployment.
> >
> > On top of that, the same pressures that l4s-arch describes that should
> > cause rapid rollout of L4S should for the same reasons cause rapid
> > of the endpoint capabilities, especially if the network capability is
> > there.
> I'm afraid there are not the same pressures to cause rapid roll-out at
> all, cos it's flakey now, jam tomorrow. (Actually ECN-DualQ-SCE has a
> much greater problem - complete starvation of SCE flows - but we'll come
> on to that in Q4.)
> I want to say at this point, that I really appreciate all the effort
> you've been putting in, trying to find common ground.
> In trying to find a compromise, you've taken the fire that is really
> aimed at the inadequacy of underlying SCE protocol - for anything other
> than FQ. If the primary SCE proponents had attempted to articulate a way
> to use SCE in a single queue or a dual queue, as you have, that would
> have taken my fire.
> >
> > But regardless, the queue-building from classic ECN-capable endpoints
> > only get 1 congestion signal per RTT is what I understand as the main
> > downside of the tradeoff if we try to use ECN-capability as the dualq
> > classifier.  Does that match your understanding?
> This is indeed a major concern of mine (not as major as the starvation
> of SCE explained under Q4, but we'll come to that).
> Fine-grained (DCTCP-like) and coarse-grained (Cubic-like) congestion
> controls need to be isolated, but I don't see how, unless their packets
> are tagged for separate queues. Without a specific fine/coarse
> identifier, we're left with having to re-use other identifiers:
>   * You've tried to use ECN vs Not-ECN. But that still lumps two large
>     incompatible groups (fine ECN and coarse ECN) together.
>   * The only alternative that would serve this purpose is the flow
>     identifier at layer-4, because it isolates everything from
>     everything else. FQ is where SCE started, and that seems to be as
>     far as it can go.
> Should we burn the last unicorn for a capability needed on
> "carrier-scale" boxes, but which requires FQ to work? Perhaps yes if
> there was no alternative. But there is: L4S.
> That brings us neatly to the outstanding issues with L4S...
> >
> >
> > 2.
> > I ended up confused about how falling back works, and I didn't see it
> > spelled out anywhere.  I had assumed it was a persistent state-change
> > for the sender for the rest of the flow lifetime after detecting a
> > condition that required it, but then I saw some text that seemed to
> > indicate it might be temporary? From section 4.3 in l4s-id:
> >     "Note that a scalable congestion control is not expected to change
> >        to setting ECT(0) while it temporarily falls back to coexist with
> >        Reno ."
> >
> > Can you clarify whether the fall-back is meant to be temporary or not,
> > and whether I missed a more complete explanation of how it's supposed to
> > work?
> Firstly, as has been made clear in our latest talk/paper at Linux netdev
> and in my latest iccrg talk, currently TCP Prague only includes
> fall-back to Reno on loss. It does not do fall-back on classic ECN
> marking (yet). We're still working on RTT-independence and scaling to
> very low RTT (sub-MSS window) first.
> Fall-back on loss is definitely very temporary: it does one large
> Reno-style window halving on a loss (ignoring any other losses in that
> RTT as Reno does), then immediately continues with DCTCP-style
> congestion avoidance driven by all the ECN marks (not just one per-RTT).
> For classic ECN AQM detection, we only have initial design ideas.
> Olivier posted his design ideas here:
>  ???
> 863d9bcb726f-
> 062d85b471dea4c1&q=1&
> prague%2Fissues%2F2
> I want to keep it simple (see response to Q4 about false negatives).
> Fall-back would be temporary, but last longer than for loss - until the
> flow next goes idle. Here's the simplest that I think might work:
>  ??? Starting X RTTs after first CE mark;??? // allows end of Slow Start
> to stabilize
>  ??? if (srtt > (min_rtt + Y) || rttvar > Z) {fallback()};
> Where X,Y&Z are TBD, dependent on experiments, but say X=5-6 RTT,
> Y=4-5ms & Z=dunno_without_measuring. The min_rtt could be taken only
> since the previous start-up or idle period (or perhaps the previous
> two). An idle would have to be defined as >3-4 RTT, to allow any
> self-induced queue to drain.
> The whole of L4S is experimental track. So others might take different
> approaches (e.g. BBRv2) and I'm sure our approach will evolve, which is
> why the requirement is worded liberally (it has to cover real-time, etc.
> not just TCP).
> >
> >
> > 3.
> > I also was a little confused on the implementation status of the
> > logic.  I was looking through some of the various links I could find,
> > I think these 2 are the main ones to consider? (from
> >
> 863d9bcb726f-
> db345c174f8a010a&q=1&
> de ):
> > -
> 863d9bcb726f-
> 36bf3624b9b0808f&q=1&
> ualpi2_upstream
> > -
> 863d9bcb726f-
> 67813c5bf9b5fa59&q=1&
> prague
> >
> > It looks like the prague_fallback_to_ca case so far only happens when
> > AccECN is not negotiated, right?
> That's not the same sort of fall-back. That's fall-back because without
> AccECN there's only one ECN feedback signal per RTT, so it falls back to
> the configured classic congestion controller for the whole connection.
> Which controller depends on the parameter prague_ca_fallback which
> defaults to cubic.
> As said above, fall-back on classic ECN has not yet been implemented in
> TCP Prague. Of the 3 things left on our list, it's the last 'cos we're
> waiting to see the results of measurements from a CDN, to see if there
> are any single queue classic ECN AQMs out there. If there aren't we
> would not plan to implement this requirement until there were. Whether
> others do is up to them of course.
> >
> > To me, the logic for when to do this (especially for rtt changes) seems
> > fairly complicated and easy to get wrong, especially if it's meant to be
> > temporary for the flow, or if needs to deal with things like network
> > changes unrelated to the bottleneck, or variations in rtt due an
> > being a mobile device, or on wi-fi.
> >
> > Which brings me to:
> >
> >
> > *4.
> > (* I think this is the biggest point driving me to ask about this.)
> >
> > I'm pretty worried about mis-categorizing CE marking from classic AQM
> > algorithms as L4S-style markings, when using ECT(1) as the dualq
> > classifier.
> >
> > I did see this issue addressed in the l4s drafts, but reviewing it
> > left me a little confused, so I thought I'd ask about a point I
> > noticed for clarification:
> >
> >  From section 6.3.3 of l4s-arch:
> >     "an L4S sender will have to
> >     fall back to a classic ('TCP-Friendly') behaviour if it detects that
> >     ECN marking is accompanied by greater queuing delay or greater delay
> >     variation than would be expected with L4S"
> >
> >  From the abstract in l4s-arch:
> >     "In
> >     extensive testing the new L4S service keeps average queuing delay
> >     under a millisecond for _all_ applications even under very heavy
> >     load"
> >
> > My reading of these seems to suggest that if the sender can observe
> > a variance or increase of more than 1 millisecond of rtt, it should fall
> > back to classic ECN?
> >
> > I'm not sure yet how to square that with Section A.1.4 of l4s-id:
> >     "An increase in queuing delay or in delay variation would be
> >     a tell-tale sign, but it is not yet clear where a line would be
> >     between the two behaviours."
> >
> > Is the discrepancy here because the extensive testing (also mentioned in
> > the abstract of l4s-arch) was mainly in controlled environments, but the
> > internet is expected to introduce extra non-bottleneck delays even where
> > a dualq is present at the bottleneck, such as those from wi-fi, mobile
> > networks, and path changes?
> No, it's simply 'cos there is no implementation of this requirement yet.
> >
> > Regardless, this seems to me like a worrisome gap in the spec, because
> > the claim that dualq will get deployed and enabled quickly and widely is
> > correct, it means this will be a common scenario in
> > wherever there's existing classic AQMs deployed, especially since in CPE
> > devices the existing AQMs are generally configured to have a lower
> > bandwidth limit than the subscriber limit, so they'll (deliberately) be
> > the bottleneck whenever the upstream access network isn't overly
> > congested.
> I believe FQ-CoDel is the only AQM in CPE that I know of that supports
> classic ECN. In this case, an L4S-ECN congestion controller cannot
> starve a Cubic-ECN or Reno-ECN flow, cos the FQ scheduler controls their
> capacity shares.
> The only other CPE AQM I am aware of is DOCSIS-PIE, which doesn't
> support ECN.
> If the IETF assigns the ECT(1) codepoint to L4S, then it would be
> extremely easy to modify FQ-Codel to set a very shallow ECN threshold in
> any queue where at least one ECT(1) codepoint had been detected. This
> would work fine with highly transient flow queues.
> >
> > I guess if it's really a 1-2 ms variance threshold to fall back, that
> > would probably address the safety concern, but it seems like it would
> > have a lot of false positives, and unnecessarily fall back on a lot of
> > flows.
> >
> > But worse, if there's some (not yet specified?) logic that tries to
> > those false positives by relaxing a simple very-few-ms threshold, it
> > like there's a high likelihood of logic that produces false negatives
> > undetected.
> >
> > If that's the case, to me it seems like it will remain a significant
> > even while TCP Prague has been deployed for quite a long time at a
> > as long as different endpoint and AQM implementations roll out randomly
> > behind different network conditions, for the various endpoints that end
> > up connected with the sender.
> I am less worried about this. I would be comfortable erring on the side
> of reducing false positives at the expense of false negatives.
> Nonetheless, this position depends on what we find in measurement studies.
> * If we find no single-queue AQMs that do ECN-marking, it's a
> non-problem {Note 2}.
> * If such AQMs exist but are rare, they are likely to be in specific
> operator's networks, so there would be operator-specific ways to address
> such problems. E.g. if a CDN wanted to deploy the L4S experiment on its
> caches for that network, in collaboration with the network operator it
> could set a local-use DSCP instead of using ECT(1). That would still not
> deal with L4S traffic to/from the Internet, but the probability that
> different types of long-running flows coincide is low anyway, so the
> probability that different types of flows that are both long-running and
> non-CDN will coincide must surely be tiny.
> >
> > It also seems to me there's a high likelihood of causing unsafe non-
> > responsive sender conditions in some of the cases where this kind of
> > negative happens in any kind of systematic way.
> This overstates the problem. There is no unresponsiveness. Even when two
> long-running flows coincide, an L4S flow does not actually starve a
> classic (e.g. Reno-ECN) TCP flow. They come to a balance that can be
> highly unequal in high BDP links, but never starvation or
> unresponsiveness. Indeed, as the link's BDP gets smaller, or the more
> flows there are, the more DCTCP & Reno-ECN tend to equality.
> >
> > By contrast, as I understand it an SCE-based approach wouldn't need the
> > same kind of fallback state-change logic for the flow, since any CE
> > indicate a RFC 3168-style multiplicative decrease, and only ECT(1) would
> > indicate sub-loss congestion.
> I'm afraid you understand it wrong.
> With the ECN-DualQ-SCE approach, any flows where the receiver does not
> feed back SCE (ECT(1)) markings starve any SCE (DCTCP-like) flows in the
> same bottleneck.
> Similarly, any Reno-ECN or Cubic-ECN senders (i.e. without the logic to
> understand SCE) starve the SCE (DCTCP-like) flows in the same
> ECN-DualQ-SCE bottleneck.
> And here, starve actually means starve. Not just come to a highly
> unbalanced equilibrium, but completely starve.
> This is because a Cubic-ECN flow will keep pushing the queue up to the
> point where it emits CE markings, because it doesn't understand and
> therefore ignores the SCE markings. One queue can only have one length.
> So, because the Cubic flow(s) have pushed the queue past the shallower
> point where it starts to emit SCE markings, all packets not marked CE
> will be marked SCE.
> For example, say Cubic flow(s) induce a fairly normal 0.5% CE marking
> (or 0.5% drop for non-ECN flows). Then there will be 99.5% SCE marking.
> Then, the DCTCP-like flows designed to understand SCE will keep reducing
> in response to this saturated SCE marking and the Cubic flows will fill
> the space they leave and starve them.
> We did experiments to try to minimize this starvation, with two AQMs in
> one queue where one type of CC ignores the signals from the lower
> threshold back in 2012. See:
>  ???
> 863d9bcb726f-
> 4c3e071522a975f6&q=1&
> CTCP-Internet
> This led us to realize we would have to use at least two queues.
> >
> > This is one of the big advantages of the SCE-based approach in my mind,
> > since there's no chance of mis-classifying the meaning of a CE mark and
> > no need for a state change for how the sender handles the ECT backoff
> > or sets the ECT markings.  (It just goes back to treating any CE as
> > style loss equivalent, and SCE as a sub-loss signal.)
> >
> > Since an SCE-based approach would avoid this problem nicely, I consider
> > the reduced risk of false negatives (and unresponsive flows) here one of
> > important gains, to be weighed against the key downside mentioned in
> comment
> > #1.
> I hope you can see now that the ECN-DualQ-SCE approach suffers from the
> same problem as you are concerned about with L4S. Except the difference
> is it's not in 'legacy' non-SCE queues, but in the queue implementing
> SCE marking itself.
> Unless one separates non-SCE traffic into a different queue, it starves
> SCE traffic.
> >
> >
> > 5.
> > Something similar comes up again in some other places, for instance:
> >
> > from A.1.4 in l4s-id:
> (it's A.1.1.)
> >     "Description: A scalable congestion control needs to distinguish the
> >     packets it sends from those sent by classic congestion controls.
> >
> >     Motivation: It needs to be possible for a network node to classify
> >     L4S packets without flow state into a queue that applies an L4S ECN
> >     marking behaviour and isolates L4S packets from the queuing delay of
> >     classic packets."
> >
> > Listing this as a requirement seems to prioritize enabling the gains of
> > L4S ahead of avoiding the dangers of L4S flows failing to back off in
> > presence of possibly-miscategorized CE markings, if I'm reading it
> > I guess Appendix A says these "requirements" are non-normative, but I'm
> > little concerned that framing it as a requirement instead of a design
> > choice with a tradeoff in its consequences is misleading here, and
> > pushes toward a less safe choice.
> As I hope you can now see from the last part of answer #4 that, if you
> try to classify ECN flows with fine-grained (DCTCP-like) and coarse
> (Cubic-like) congestion controls into the same queue (whether L4S or SCE
> marking), the Cubic-like congestion controls ruin it.
> So I think this requirement stands. I've made a note-to-self to add the
> text: "To avoid having to use per-flow classification..." though.
> >
> >
> > 6.
> > If queuing from classic ECN-capable flows is the main issue with using
> > ECT as the dualq classifier, do you think it would still be possible to
> > get the queuing delay down to a max of ~20-40ms right away for
> > endpoints in networks that deploy this kind of dualq, and then hopefully
> > see it drop further to ~1-5ms as more endpoints get updated with AccECN
> > some kind of ECT(1) feedback and a scalable congestion controller that
> > can respond to SCE-style marking?
> Technically yes, but realistically no.
> What I mean is, as I said from the start, if you remove the feature that
> deploying the L4S DualQ Coupled AQM gives very low and consistently very
> low latency straight away, then operators will lose interest in
> deploying it.
> > Or is it your position that the additional gains from the ~1ms queueing
> > that should be achievable from the beginning by using ECT(1) (in
> > where enough of the key entities upgrade) are worth the risks?
> Well, I'd say "probably worth the risks", cos we're waiting for
> measurements to get a feel for whether any of the CE markings seen by
> the tests Apple reported in 2016-2017 are from single queue ECN AQMs.
> See
> implementing-the-prague-requirements-in-tcp-for-l4s-01#page=11
> >
> > (And if so, do you happen to have a pointer to any presentations or
> > that made a quantitative comparison of the benefits from those 2
> > I don't recall any offhand, but there's a lot of papers...)
> Latest results here (actually no different from results we reported in
> 2015 - all the changes to the code since have been non-performance
> "DUALPI2 - Low Latency, Low Loss and Scalable (L4S) AQM" Olga Albisser
> (Simula), Koen De Schepper (Nokia Bell-Labs), Bob Briscoe (Independent),
> Olivier Tilmans (Nokia Bell-Labs) and Henrik Steen (Simula), in Proc.
> Netdev 0x13
> <
> 863d9bcb726f-
> 10f0079ea0780acc&q=1&
> Fsession.html%3Ftalk-DUALPI2-AQM> (Mar 2019).
> The paper via the netdev link shows qdelay, utilization, completion time
> efficiency, etc with the most extreme traffic load we use (2
> long-running flows plus 5X Web flows per sec, where X is each link rate
> in Mb/s, e.g. 600 flows/sec over the 120Mb/s link), for a full range of
> link rates, round trip times, etc.
> The plots are pretty crammed, so if you'd prefer one example qdelay
> cumulative distribution function for the same extreme traffic load, see
> here:
> implementing-the-prague-requirements-in-tcp-for-l4s-01#page=22
> If you want results from a range of less-extreme traffic models, just ask.
> Bob
> >
> >
> > Best regards,
> > Jake
> >
> >
> {Note 1}: Or different server, client and network operators all agree to
> deploy, but let's assume that would be a bonus and not rely on it.
> {Note 2}: Even where there are no single-queue AQMs now, there might be
> a concern that some could be enabled in future. Given study after study
> since ECN was first standardized (2001) have detected hardly any CE
> marks on the Internet until FQ-CoDel was deployed about 15 years later,
> the chance of those AQMs being turned on now is surely vanishing.
> --
> ________________________________________________________________
> Bob Briscoe
> 5c69c9b4-00e35dee-863d9bcb726f-
> a188c700e74cbf8e&q=1&
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