Re: [tsvwg] Related to "Non-L4S traffic abusing the L-queue" discussion during the interim

[Luca Muscariello <[e-mail]>]

Hi David

I'm just reading the draft and cannot comment on the code, but it would be
intuitive that a coupled queuing systems of two queues would decouple when
one of the two queues is empty.

Reading the details of the pseudo code in the draft, if lq is empty p'L
should be 0 and pL=pCL=kp' which may be !=0.

Additional text below the pseudo code provides details about a Linux
implementation that marks packet IFF queue length is >= 2 MTU. Which may
suggest that decoupling happens when the L queue is empty.

I did not check the code though.

I will read the document more carefully beginning of next week but I admit
this part is ambiguous.

Luca

On Fri, Feb 25, 2022, 19:05 Black, David <[e-mail]> wrote:

> Hi Neal,
>
>
>
> So, I saw that explanation – could someone check the "running code" to
> make sure that the coupling and marking occur even when the L queue is
> always empty?
>
>
>
> Thanks, --David
>
>
>
> *From:* Neal Cardwell <[e-mail]>
> *Sent:* Friday, February 25, 2022 12:58 PM
> *To:* Black, David
> *Cc:* De Schepper, Koen (Nokia - BE/Antwerp); tsvwg IETF list; Jonathan
> Morton; Bob Briscoe
> *Subject:* Re: [tsvwg] Related to "Non-L4S traffic abusing the L-queue"
> discussion during the interim
>
>
>
> [EXTERNAL EMAIL]
>
>
>
>
>
> On Fri, Feb 25, 2022 at 11:56 AM Black, David <[e-mail]>
> wrote:
>
> Koen,
>
>
>
> I'll observe that "traffic that is not responding at all to CE marks" is
> not necessary to achieve the reported results if the experimental setup "prevents
> the L queue from seeing any
>
> need to apply congestion signals, because it is always empty" as there
> would be no CE marks for the traffic in the L queue to respond to.
>
>
>
> I think the key part here is "if". :-) The assertion "prevents the L queue
> from seeing any need to apply congestion signals, because it is always
> empty" is from:
>
>   https://sce.dnsmgr.net/downloads/L4S-WGLC2-objection-details.pdf
> [sce.dnsmgr.net]
> <https://urldefense.com/v3/__https:/sce.dnsmgr.net/downloads/L4S-WGLC2-objection-details.pdf__;!!LpKI!2-aGvIgLG5_UcfoDdFo5NR6FegDlw6w7v97l7dT8g-Or7eBQNEgIITlBB4xE-wzo$>
>
> That assertion is inconsistent with the functioning of the Dual-Q
> algorithm, as described in:
>
>   https://www.ietf.org/id/draft-ietf-tsvwg-aqm-dualq-coupled-21.html
> [ietf.org]
> <https://urldefense.com/v3/__https:/www.ietf.org/id/draft-ietf-tsvwg-aqm-dualq-coupled-21.html__;!!LpKI!2-aGvIgLG5_UcfoDdFo5NR6FegDlw6w7v97l7dT8g-Or7eBQNEgIITlBB6qmUFY4$>
>
>
>
> As Bob noted: "in the scenario shown, although the L queue is indeed
> always empty, it will see a high level of congestion signals (~10% in this
> case) via the coupling."
>
> Here's Bob's e-mail for more context/details:
>
>   https://mailarchive.ietf.org/arch/msg/tsvwg/joFr3sfOrxxkYhWdYrO2rLlCNUw/
> [mailarchive.ietf.org]
> <https://urldefense.com/v3/__https:/mailarchive.ietf.org/arch/msg/tsvwg/joFr3sfOrxxkYhWdYrO2rLlCNUw/__;!!LpKI!2-aGvIgLG5_UcfoDdFo5NR6FegDlw6w7v97l7dT8g-Or7eBQNEgIITlBByEN3xHA$>
>
>
>
> thanks,
>
> neal
>
>
>
>
>
>
>
> Please give that further consideration.
>
>
>
> Thanks, --David (as an individual)
>
>
>
> *From:* tsvwg <[e-mail]> *On Behalf Of *De Schepper, Koen
> (Nokia - BE/Antwerp)
> *Sent:* Friday, February 25, 2022 4:29 AM
> *To:* tsvwg IETF list; Jonathan Morton
> *Subject:* Re: [tsvwg] Related to "Non-L4S traffic abusing the L-queue"
> discussion during the interim
>
>
>
> [EXTERNAL EMAIL]
>
> Hi Jonathan,
>
>
>
> Can you confirm that this test is done with “Cubic” traffic that is not
> responding at all to CE marks? So it is just like any other non-responding
> traffic (like UDP CBR). We don’t see any other way to explain your results.
>
>
>
> If so, we can/should remove this “issue” from the shepherd’s write-up, as
> such unresponsive flows will get the same throughput on any single-Q
> bottleneck with or without AQM (taildrop/PI2/PIE/CoDel/STEP/RED/…) with a
> latency that matches the AQM strategy.
>
>
>
> Koen.
>
>
>
>
>
> *From:* tsvwg <[e-mail]> *On Behalf Of *De Schepper, Koen
> (Nokia - BE/Antwerp)
> *Sent:* Thursday, February 17, 2022 7:01 PM
> *To:* tsvwg IETF list <[e-mail]>; Jonathan Morton <
> [e-mail]>
> *Subject:* [tsvwg] Related to "Non-L4S traffic abusing the L-queue"
> discussion during the interim
>
>
>
> Hi Jonathan,
>
>
>
> It seems that the following open issue identified by the chairs:
>
>
>
> Non-L4S traffic abusing the L-queue
>
> • ‘DualQ gives a large throughput bonus to L queue traffic, ie. a “fast
> lane”’
>
> • Is this a matter specific for DualQ that can be left for experimentation?
>
>
>
> is based on the following experiment you performed:
>
>
>
> >             simple two-flow competition test on a standard dumbbell
> topology,
>
> >             with the bottleneck running a DualQ qdisc into a 50Mbps
> shaper.
>
> >             Both flows were configured to use CUBIC congestion control
> with
>
> >             ECN negotiated, but one was additionally tweaked to set
> ECT(1)
>
> >             instead of ECT(0) on all data segments, and to pace its
> output at
>
> >             40Mbps. This latter measure prevents the L queue from seeing
> any
>
> >             need to apply congestion signals, because it is always
> empty.  These
>
> >             tweaks allowed that flow to use 80% of the link capacity,
> gaining a
>
> >             fourfold advantage over its competitor,
>
>
>
> If there is capacity seeking traffic in the Classic queue, then it is even
> desired that the L4S queue does not add extra marks. The L4S marks should
> come only from the Classic coupling.
>
> Before diving into details, can you first explain why in your experiment
> the coupling from the Classic Q has no effect on your paced and ECT(1)
> labeled Cubic flow?
>
>
>
> I would expect that this ECT(1) labeled Cubic flow would get even less
> throughput than the Classic Cubic flow, as the first gets the doubled
> coupled CE marking probability (eg 2*10% = 20%) for L4S flows instead of
> the squared CE marking probability (10%^2 = 1%) which ECT(0) traffic would
> get.
>
>
>
> Thanks,
>
> Koen.
>
>
>
>
>
>