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

[Sebastian Moeller <[e-mail]>]

Hi Koen,

from your link:

"Only when non-responsive traffic is below the link capacity it can fully use that share, making the responsive flows share the rest of the capacity (as usual for any AQM on the Internet on a shared queue)." 


This pretty much confirms what I predicted, except the drop plots look incomplete, the highly interesting drops for Not-ECT UDP (for CoDel) seem to be missing (generally the labeling for figure 8 is imprecise, e.g. showing Drops ECT(1)-UDP in the first set PIE with ECT(0)-UDP). My hunch is that comparing drops ECT(1)-UDP in DualPi2 with the missing drops Not-ECT UDP in Codel, that in the latter we see much more drops than in the former, et voila, exploitable way to gain more throughput by marking traffic ECT(1)... as long as that traffic is reasonably paced and does not exceed the capacity of the L4S AQM (not an unlikely scenario even for capacity seeking traffic, my measly 100Mbps access link will not saturate the BNG uplink of my ISP or the link to the DSLAM)

Regards
	Sebastian


> On Feb 25, 2022, at 19:30, De Schepper, Koen (Nokia - BE/Antwerp) <[e-mail]> wrote:
> 
> Hi David,
>  
> To be sure, we re-did the overload tests recently, confirming the previous overload results. These results are available at: Overload results caused by non-responsive UDP traffic for PIE, DualPI2 and CoDel AQMs | l4steam.github.io
>  
> Specifically look at figure 8 at the end which shows that L4S traffic gets marks, up to 100% and appropriate drop if it reaches and exceeds the link capacity.
>  
> The test case of Jonathan is approximated by the 70Mbps non-responsive ECT(1) UDP traffic on a 100Mbps link on a DualPI2 (Prague+Cubic) test case. In Jonathan’s case it was 40Mbps on a 50Mbps link. We also evaluated in extreme when sending at 100Mbps non-responsive ECT(1) UDP traffic on a 100Mbps link, and even exceeding at 140Mbps and 200Mbps. You will see the results are as if it is on a Single Q PIE AQM. Note also that CoDel which never drops ECT packets, causes actually close to starvation and high tail-drop delay results as shown in figure 1, even with ECT(0). So I guess all the concerns about FQ_CoDel and tunnels/Hash-collisions are equally severe and not related to L4S alone (can just be exploited by ECT(0) traffic today already!!).
>  
> Koen.
>  
> From: Black, David <[e-mail]> 
> Sent: Friday, February 25, 2022 7:04 PM
> To: Neal Cardwell <[e-mail]>
> Cc: De Schepper, Koen (Nokia - BE/Antwerp) <[e-mail]>; tsvwg IETF list <[e-mail]>; Jonathan Morton <[e-mail]>; Bob Briscoe <[e-mail]>; Black, David <[e-mail]>
> Subject: RE: [tsvwg] Related to "Non-L4S traffic abusing the L-queue" discussion during the interim
>  
> 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]
> 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]
>  
> 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]
>  
> 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.