Re: [tsvwg] Todays Meeting material for RTT-independence in TCP Prague

Sebastian Moeller <moeller0@gmx.de> Tue, 25 February 2020 18:21 UTC

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From: Sebastian Moeller <moeller0@gmx.de>
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Date: Tue, 25 Feb 2020 19:21:00 +0100
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To: "De Schepper, Koen (Nokia - BE/Antwerp)" <koen.de_schepper@nokia-bell-labs.com>
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Subject: Re: [tsvwg] Todays Meeting material for RTT-independence in TCP Prague
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Dear Koen,

the short answer to the question " why you keep on saying/repeating that DualQ is broken" is. Because the class isolation component of this AQM is considerably worse than the state of the art.

Please compare https://l4s.cablelabs.com/l4s-testing/key_plots/batch-l4s-s1-2-cubic-vs-prague-50Mbit-0ms_var.png (dualQ @ 0ms added RTT, throughput ratio ~1:8) with https://l4s.cablelabs.com/l4s-testing/key_plots/batch-l4s-s2-2-cubic-vs-prague-50Mbit-0ms_var.png (fq_codel @ 0ms added RTT, throughput ratio ~1:1). Note how dialQ performs catastrophically worse than fq_codel under otherwise similar conditions. With just two flows (one TCP Prague and one cubic), fq_codel here acts as a per-class-fair AQM, perfectly sharing the bottleneck bandwidth between the two flows/classes. That is what I consider the state of the art.
	I hope that if the L4S team deploys an AQM on the internet that this will not regress in comparison what is achievable and already out there. 
Before somebody is going to misunderstand the point I am making here. I am NOT proposing you use fq_codel or flow queueing in general, but I do propose that you use an isolation mechanisms between your two traffic classes that is at least as robust and reliable as fq_codel's. As far as I can tell this level of isolation is a solved problem and not getting close to that is something I consider to be a deficit or brokenness.





> On Feb 25, 2020, at 17:46, De Schepper, Koen (Nokia - BE/Antwerp) <koen.de_schepper@nokia-bell-labs.com> wrote:
> 
> Hi Sebastian,
> 
> What I showed in the demo is what you called below B).

	[SM] Thanks for clearing this up.

> It was a convenient play-example to easily show that we can fully control the RTT-function f(). It compensates the 15 ms extra latency that classic flows gets by needing a bigger queue. So f()=RTT+15ms, making both Prague and classic flows getting exactly the same rate if they have the same base RTT. It is not our recommended f(), it was just simple to show.

	[SM] Okay, as I said it is a nice hack to support the hypothesis that dualQ's isolation method is insufficiently strict. 

> 
> If you want A) you need the following f()=max(15ms, RTT), meaning that any flow behaves as a 15ms flow (if it's real RTT is not bigger than 15ms).


	[SM] It is not so much that I "want" A) but that you promised A), by calling this RTT-independence...

> We haven't tested RTT independence for flows with a larger real RTT than the target RTT.
> We'll leave that up to others to further test/improve the throughput for higher RTTs (which everyone seems to accept).
>  
> 
> The following plot shows for A) where f()=max(15ms, RTT), the throughput for different 2-flow RTT-mixes (similar as in the paper you referred to):
> https://l4steam.github.io/40Mbps%20RTT-mix%20Prague-RTT-indep.png

	[SM] Thanks, appreciated.

> As you can see on the left half, flows below 15ms become RTT independent (get the same rate), and on the right half, lower than 15ms RTT flows are limited pushing away higher RTT flows (100ms here) up to one comparable to a 15ms flow.

	[SM] Yes I see. Calling that "RTT-independence" is a bit of a stretch though, no? 

> 
> Our implementation has currently both implemented as an option plus one extra proposal function that Bob provided (gradual changing - with a limited RTT independent for the lower RTTs). More on that later.

	[SM] So don't get me wrong, according to your slides the requirement "Reduce RTT dependence" has the status "evaluat'n in progress" how about you share the data/slides that you already have (like the figure above)? That would allow a considerably more data-driven discussion. 


> 
> To be clear we don't propose B), rather something A)-like with a bit lower target RTT (5ms?) that still gives benefits for lower RTTs, but also limited like Bob proposed.

	[SM] My problem with this approach is not necessarily the fact that plugging the 15ms number from the non-LL-queues AQM at some other place to undo its damage. But the fact that this other place is completely outside the node that will actually run the L4S-aware AQM. It is fine IMHO to have endpoint protocols to work with heuristics and approximations to deal with the existing internet, but I really wonder whether the need to modify a not-deployed-yet and un-finished protocol to make up for avoidable design decisions for a not-deployed-yet and un-finished AQM might not indicate that something is off. 

> 
> Other possible solutions are:
> - have the Classic AQM target at 1ms too

	[SM] Which will work great for RTTs in the 10-50ms range but will cause utilization issues at higher yet still realistically common RTTs, the reason why I keep asking for a test with 5ms is that this with Codel and Pie works reasonably well ven for true RTTs in the >= 200ms range.

> - have a bigger coupling factor

	[SM] Will that actually solve the problem though? My intuition tells me that this will just shift the conditions around under which LL pummels non-LL traffic.

> - make classic TCP RTT independent in the higher RTT range

	[SM] Not a viable option, we need to work with the already deployed TCPs reasonably equitable. This was in jest, surely?

> - FQ

	[SM] Don't get carried away, in your case all you need is fairly distribute between two classes queues, you could still call this "fair" queueing if referring to fairness between classes but CQ, class queuing might also be a less contentious name for it,. IMHO that a limited two class strict fairness scheduler is the proper solution, but I do not claim that there are not better fitting solutions around that achieve a similar robust and reliable isolation between the two traffic classes L4S considers.

> - provide RTT info in the packet header

	[SM] Theoretically a nice idea, but will not help with the existing internet much.

> - ...
> but I don't think people will in general favor these... but if possible they are still usable.

	[SM] Well, the two class fair queueing option seems like a winner to me.

> 
> I don't understand why you keep on saying/repeating that DualQ is broken.

	[SM] If a new solution to an old problem falls well behind the current state of the art, I consider the design of the new solution in that specific dimension to be insufficient or defective.


> DualQ wants to reduce the latency for L4S, but it cannot do the same for Classic, because of limitations of Classic congestion control itself.

	[SM] Fair enough, but the way you implemented that feature is by also giving L4S a massive "bandwidth" advantage and that is not how you frame and sell the whole L4S idea in the first place.

> We don't make Classic traffic RTT dependent, it is already RTT dependent, and has been since the beginning of congestion control.

	[SM] And it will stay RTT dependent just as TCP Prague will retain at least a residual RTT dependence, since shorter control loops are nimbler than longer ones, and the only general available option (make all congestion controller behave as if behind the maximum possible RTT) is clearly not suited for anything but Gedankenexperimente.


> So my conclusion is that the problem is with TCP congestion control that is RTT dependent and Classic that is not happy with a short queue. How do you suggest to solve this other than making TCP less RTT dependent???

	[SM] Use something like DRR++ to schedule packets from the two queues you still use to separate 1/p-type traffic from 1/sqrt(p)-type traffic, Use what ever classifier you want* to steer packets in one of the queues, and instantiate your two differential marking regimes depending on the traffic's type, that should solve most of the issues right there. The scheduler will make sure both queues share the egress traffic equitable and the rest just stays as is in you L4S design, except you might be able to abandon the cute but only approximate coupling idea** and deduce each queue's marking rate directly from its occupancy level/sojourn time. 
	Now, this will solve the particular issue I a concerned about, backward compatibility and equitable sharing with existing traffic***. It might well fall short to meet some of your design goals and requirements, I would appreciate if you could describe how and why the proposed scheme would not work for L4S?

Regards
	Sebastian

*) Actually I would strongly recommend to use a DSCP for the required marking, at least in addition to what ever else you end up picking.
**) If coupling would solve the issue we would not have this discussion...
***) I note that until L4S actually drives 50% of internet traffic equitable sharing is already giving a strong bandwidth bias towards L4S at bottleneck links. That is something I am not happy with, but not something I have a better solution for that you will find acceptable.




> 
> Regards,
> Koen.
> 
> -----Original Message-----
> From: tsvwg <tsvwg-bounces@ietf.org> On Behalf Of Sebastian Moeller
> Sent: Friday, February 21, 2020 9:00 AM
> To: tsvwg IETF list <tsvwg@ietf.org>
> Subject: [tsvwg] Todays Meeting material for RTT-independence in TCP Prague
> 
> Dear All,
> 
> 
> after today's virtual meeting I am still pondering Koen's RTT-independence day presentation. The more I am thinking about this the more confused I get about what was actually achieved. 
> 
> Was it:
> A) true RTT independence between TCP Prague flows so flows of wildly differing RTTs will share an L4S AQM's LL queue fairly?
> 
> B) class RTT independence, that is adding the so far under-explained 15 ms target for L4S's non-LL queue to the internal RTT response generation in TCP Prague (which, let me be frank would be a gross hack and solving the right problem (dualQ's failure to meet its goals robustly and reliably) at the wrong position)?
> 
> C) all of the above?
> 
> I had a look at the slides, and all I see is B) and no data for A), and IIRC the demo also focused on B), dod I miss something. If you have data for A) please share with us, because B) alone is not well-described with the RTT-independence moniker.
> 
> Question: is it just me, or do others also get uneasy when a yet un-deployed transport protocol modification (TCP Prague) grows a magic +14.5ms constant somewhere in its innards to work-around the existence of another under-explained 15ms constant somewhere in the innards of another yet un-deployed AQM, INSTEAD of simply fixing said un-deployed AQM to not require such and ugly hack in the first place? Are all L4S compliant transports expected to grow the same ~15ms constant? 
>         What if in the future the dualQ AQM is superceded by something else, that for good justification* wants to implement a target of 5ms, do you envision all modified transport protocols to be changed?** 
> 
> 
> 	The fact of the matter is, the dual queue coupled AQM as currently implemented is broken, but I see
> 
> 
> The rationale why the magic f() would have been added to TCP Prague without the need to paper over dualQ's major failure was a bit thin in Koen's presentation, so please supply me with more reasons why this is a good idea and not simply the cheapest way to paper over dualQ brokeness without actual real engineering to fix the root cause?
> 
> Also, please show how these modifications make bandwidth sharing inside the LL-queue more equitable and significantly less RTT-dependent, ideally by using a similar mix of flows like in The Good, the Bad and the WiFi: Modern AQMs in a residential setting: T.Høiland-Jørgensen, P. Hurtig, A. Brunstrom: https://www.sciencedirect.com/science/article/pii/S1389128615002479, so that your results can be compared to figure 6. Until that point I will assume that increased RTT-independence is still aspirational.
> 
> Best Regards
> 	Sebastian
> 
> 
> 
> *) I note again, that the CODEL RFC has a section that gives some rational why 5ms is a reasonable target value for flows in the 20-200ms RTT range, and that the PIE proponents have not presented any clear study demonstrating that the chosen 15ms is optimal in any dimension, which would be interesting as DCSIS-PIE actually seems to default to 10ms...
> 
> 
> **) This is another sticking point, I have asked the L4S team repeatedly to use their test-bed (which should make testing different configurations a breeze), to measure between-class fairness and link-utilization between the LL- and the non-LL queues for short medium and long RTTs with the non-LL-queues target set to 5ms. 
>         And so far all I hear is something along the lines of, if that interests me, I could do my own tests. My interpretation is that either the test bed is far less flexible and easy to use, or there is the fear that the 5ms data would reveal something unpleasant?
> 
> 
>