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

Jonathan Morton <chromatix99@gmail.com> Sun, 21 July 2019 16:00 UTC

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From: Jonathan Morton <chromatix99@gmail.com>
In-Reply-To: <22af0671-fdd0-0953-fc96-55b34beb0be9@bobbriscoe.net>
Date: Sun, 21 Jul 2019 12:00:30 -0400
Cc: Sebastian Moeller <moeller0@gmx.de>, "De Schepper, Koen (Nokia - BE/Antwerp)" <koen.de_schepper@nokia-bell-labs.com>, "Black, David" <David.Black@dell.com>, "ecn-sane@lists.bufferbloat.net" <ecn-sane@lists.bufferbloat.net>, "tsvwg@ietf.org" <tsvwg@ietf.org>, Dave Taht <dave@taht.net>
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To: Bob Briscoe <in@bobbriscoe.net>
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Subject: Re: [tsvwg] [Ecn-sane] Comments on L4S drafts
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> On 21 Jul, 2019, at 7:53 am, Bob Briscoe <in@bobbriscoe.net> wrote:
> 
> Both teams brought their testbeds, and as of yesterday evening, Koen and Pete Heist had put the two together and started the tests Jonathan proposed. Usual problems: latest Linux kernel being used has introduced a bug, so need to wind back. But progressing.
> 
> Nonetheless, altho it's included in the tests, I don't see the particular concern with this 'Cake' scenario. How can "L4S flows crowd out more reactive RFC3168 flows" in "an RFC3168-compliant FQ-AQM". Whenever it would be happening, FQ would prevent it.
> 
> To ensure we're not continually being blown into the weeds, I thought the /only/ concern was about RFC3168-compliant /single-queue/ AQMs.

I drew up a list of five network topologies to test, each with the SCE set of tests and tools, but using mostly L4S network components and focused on L4S performance and robustness.


1: L4S sender -> L4S middlebox (bottleneck) -> L4S receiver.

This is simply a sanity check to make sure the tools worked.  Actually we fell over even at this stage yesterday, because we discovered problems in the system Bob and Koen had brought along to demo.  These may or may not be improved today; we'll see.


2: L4S sender -> FQ-AQM middlebox (bottleneck) -> L4S middlebox -> L4S receiver.

This is the most favourable-to-L4S topology that incorporates a non-L4S component that we could easily come up with, and therefore .  Apparently the L4S folks are also relatively unfamiliar with Codel, which is now the most widely deployed AQM in the world, and this would help to validate that L4S transports respond reasonably to it.


3: L4S sender -> single-AQM middlebox (bottleneck) -> L4S middlebox -> L4S receiver.

This is the topology of most concern, and is obtained from topology 2 by simply changing a parameter on our middlebox.


4: L4S sender -> ECT(1) mangler -> L4S middlebox (bottleneck) -> L4S receiver.

Exploring what happens if an adversary tries to game the system.  We could also try an ECT(0) mangler or a Not-ECT mangler, in the same spirit.


5: L4S sender -> L4S middlebox (bottleneck 1) -> Dumb FIFO (bottleneck 2) -> FQ-AQM middlebox (bottleneck 3) -> L4S receiver.

This is Sebastian's scenario.  We did have some discussion yesterday about the propensity of existing senders to produce line-rate bursts occasionally, and the way these bursts could collect in *all* of the queues at successively decreasing bottlenecks.  This is a test which explores that scenario and measures its effects, and is highly relevant to best consumer practice on today's Internet.


Naturally, we have tried the equivalent of most of the above scenarios on our SCE testbed already.  The only one we haven't explicitly tried out is #5; I think we'd need to use all of Pete's APUs plus at least one of my machines to set it up, and we were too tired for that last night.

 - Jonathan Morton