Re: [tsvwg] L4S status: #17 Interaction w/ FQ AQMs

"Bless, Roland (TM)" <roland.bless@kit.edu> Mon, 11 November 2019 09:34 UTC

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To: "Tilmans, Olivier (Nokia - BE/Antwerp)" <olivier.tilmans@nokia-bell-labs.com>, Sebastian Moeller <moeller0@gmx.de>
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From: "Bless, Roland (TM)" <roland.bless@kit.edu>
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Organization: Institute of Telematics, Karlsruhe Institute of Technology
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Date: Mon, 11 Nov 2019 10:34:02 +0100
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Subject: Re: [tsvwg] L4S status: #17 Interaction w/ FQ AQMs
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Hi Olivier,

Am 11.11.19 um 01:38 schrieb Tilmans, Olivier (Nokia - BE/Antwerp):
> Given that flows have different overall RTTs, but are subject to the same marking
> probabilities (albeit tuned to their reponse, i.e., linear or multiplicative), you
> instead need to look at their cwnd--which must be equal[1]. Unfortunately, eye-balling

Not sure that I understand what you mean here. Flows with different RTTs
typically need different CWnds to achieve sufficient throughput, but
their share of queued data at the bottleneck needs to be the same in
case you want flow rate fairness between them. This is, however,
different from requiring their CWnds to be equal...

> this from flent graph is not super convenient--tracing it, e.g., using eBPF, is
> however straightforward.


> [1] The 0ms case will likely yield a smaller than expected cwnd for L4S, cause by
> 10% WRR protection kicking it and artificially increasing classic's cwnd (as 1ms
> vs 15ms would yield a 15x throughput difference if it was reno).
> 
>> I now wonder what happens if you pair a short RTT
>> (say 10ms) prague flow (simulating traffic from a nearby data center) with a
>> cubic flow with a typical internet RTT (say 80ms). My prediction is that
>> prague will dominate cubic similarly to the 0ms RTT. I guess this is coming
>> from the default 1/16 scheduling weight for the normal queue, do you agree?
> 
> You can disable the WRR entirely (i.e., set it to 0%) and the results will stay
> the same (except for the 0ms case...).

Regards,
 Roland