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Re: [tsvwg] new tests of L4S RTT fairness and intra-flow latency: defaults ready for testing

"De Schepper, Koen (Nokia - BE/Antwerp)" <koen.de_schepper@nokia-bell-labs.com> Wed, 18 November 2020 11:13 UTC

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From: "De Schepper, Koen (Nokia - BE/Antwerp)" <koen.de_schepper@nokia-bell-labs.com>
To: Jonathan Morton <chromatix99@gmail.com>, Sebastian Moeller <moeller0@gmx.de>
CC: tsvwg IETF list <tsvwg@ietf.org>
Thread-Topic: [tsvwg] new tests of L4S RTT fairness and intra-flow latency: defaults ready for testing
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Date: Wed, 18 Nov 2020 11:13:29 +0000
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Subject: Re: [tsvwg] new tests of L4S RTT fairness and intra-flow latency: defaults ready for testing
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Hi Jonathan, Sebastian,

Thanks for sharing these tests results again.

Indeed, a detailed study can be performed to define the optimal behavior. It probably will also depend on the applications using it. I think it is commonly accepted that it is beneficial to prioritize short flows and that we don't want to spoil the benefit of having a lower latency  (being able to respond faster) we only enabled the RTT fairness after a currently reasonable and easy to implement number of RTTs.  Improvements could be to do this more gradually, ... Also if applications stop sending data, and start again, it can be considered to restart the (gradual) RTT independence.

I guess the basic issue (too aggressive Short RTT flows due to the 0 to 1ms L4S queue latency target) is solved with this, and I encourage everyone to share data/experience, further study and define more optimal behavior. We can easily change this.

Regards,
Koen.



From: Jonathan Morton <chromatix99@gmail.com>
Sent: Wednesday, November 18, 2020 7:45 AM
To: Sebastian Moeller <moeller0@gmx.de>
Cc: De Schepper, Koen (Nokia - BE/Antwerp) <koen.de_schepper@nokia-bell-labs.com>; tsvwg IETF list <tsvwg@ietf.org>
Subject: Re: [tsvwg] new tests of L4S RTT fairness and intra-flow latency: defaults ready for testing

On 18 Nov, 2020, at 8:21 am, Sebastian Moeller <moeller0@gmx.de<mailto:moeller0@gmx.de>> wrote:

The changes that Oliver submitted to TCP Prague not only increase the fudge RTT to 25ms, but they also, as Koen mentioned, increase the time it takes for Prague to switch to the new fairness mode to 500 RTTs, or 5 seconds at 10ms RTT.
I really wonder how that affects fairness if the shallow queue is used by a bunch of short flows with 10ms RTT. As far as I can tell the consequences of this delayed engagement have not been described properly.

Yes, that engagement delay is clearly visible in a time-series plot.  I think it may even be more than 5 seconds in practice:
[cid:image001.png@01D6BD86.684F67B0]

 - Jonathan Morton

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