[Ecn-sane] [tsvwg] Comments on L4S drafts

[Jonathan Morton]

> On 5 Jul, 2019, at 9:46 am, De Schepper, Koen (Nokia - BE/Antwerp) <koen.de_schepper at nokia-bell-labs.com> wrote:
> 
>>> 2: DualQ can be defeated by an adversary, destroying its ability to isolate L4S traffic.
> 
> Before jumping to another point, let's close down your original issue. Since you didn't mention, I assume that you agree with the following, right?
> 
>        "You cannot defeat a DualQ" (at least no more than a single Q)

I consider forcibly degrading DualQ to single-queue mode to be a defeat.  However…

>>> But that's exactly the problem.  Single queue AQM does not isolate L4S traffic from "classic" traffic, so the latter suffers from the former's relative aggression in the face of AQM activity.
> 
> With L4S a single queue can differentiate between Classic and L4S traffic. That's why it knows exactly how to treat the traffic. For Non-ECT and ECT(0) square the probability, and for ECT(1) don't square, and it works exactly like a DualQ, but then without the latency isolation. Both types get the same throughput, AND delay. See the PI2 paper, which is exactly about a single Q.

Okay, this is an important point: the real assertion is not that DualQ itself is needed for L4S to be safe on the Internet, but for differential AQM treatment to be present at the bottleneck.  Defeating DualQ only destroys L4S' latency advantage over "classic" traffic.  We might actually be making progress here!

> I agree you cannot isolate in a single Q, and this is why L4S is better than SCE, because it tells the AQM what to do, even if it has a single Q. SCE needs isolation, L4S not.

Devil's advocate time.  What if, instead of providing differential treatment WRT CE marking, PI2 instead applied both marking strategies simultaneously - the higher rate using SCE, and the lower rate using CE?  Classic traffic would see only the latter; L4S could use the former.

> We tried years ago similar things like needed for SCE, and found that it can't work. For throughput fairness you need the squared relation between the 2 signals, but with SCE, you need to apply both signals in parallel, because you don't know the sender type. 

Yes, that's exactly what we do - and it does work.

> 	- So either the sender needs to ignore CE if it gets SCE, or ignore SCE if you get CE. The first is dangerous if you have multiple bottlenecks, and the second is defeating the purpose of SCE. Any other combination leads to unfairness (double response).

This is a false dichotomy.  We quickly realised both of those options were unacceptable, and sought a third way.

SCE senders apply a reduced CE response when also responding to parallel SCE feedback, roughly in line with ABE, on the grounds that responding to SCE does some of the necessary reduction already.  The reduced response is still a Multiplicative Decrease, so it fits with normal TCP congestion control principles.

> 	- you separate the signals in queue dept, first applying SCE and later CE, as you originally proposed, but that results in starvation for SCE.

Yes, although this approach gives the best performance for SCE when used with flow isolation, or when all flows are known to be SCE-aware.  So we apply this strategy in those cases, and move the SCE marking function up to overlap CE marking specifically for single queues.

It has been suggested that single queue AQMs are rare in any case, but this approach covers that corner case.

> Add on top that SCE makes it impossible to use DualQ, as you cannot differentiate the traffic types.

SCE is designed around not *needing* to differentiate the traffic types.  Single queues have known disadvantages, and SCE doesn't worsen them.

Meanwhile, we have proposed LFQ to cover the DualQ use case.  I'd be interested in hearing a principled critique of it.

 - Jonathan Morton