Re: [tcpm] finalizing CUBIC draft (chairs' view)

Jonathan Morton <chromatix99@gmail.com> Thu, 13 October 2022 10:09 UTC

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From: Jonathan Morton <chromatix99@gmail.com>
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Date: Thu, 13 Oct 2022 13:09:36 +0300
Cc: Yoshifumi Nishida <nsd.ietf@gmail.com>, "tcpm@ietf.org Extensions" <tcpm@ietf.org>, tcpm-chairs <tcpm-chairs@ietf.org>
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To: Markku Kojo <kojo=40cs.helsinki.fi@dmarc.ietf.org>
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Subject: Re: [tcpm] finalizing CUBIC draft (chairs' view)
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> On 11 Oct, 2022, at 1:13 pm, Markku Kojo <kojo=40cs.helsinki.fi@dmarc.ietf.org> wrote:
> 
> We cannot have evidence if nobody has done any measurements. The impact to other coexisting traffic cannot be observed without *measuring* it.

On this point, I have prepared a "neutered" version of Linux CUBIC called LINEAR, specifically to explore this topic.

Essentially LINEAR is CUBIC, but with the polynomial growth curve removed so that it always operates in Reno-compatible mode, and with the facility to be easily configured (currently at compile time) for different values of beta (and the corresponding value of alpha).  Other features such as HyStart are still present, as they are relevant to dynamic performance when exiting slow-start.

Currently we have the following variants which can be instantiated and intermixed freely:

- LINEAR-A with beta=0.5, simulating NewReno plus HyStart.

- LINEAR-B with beta=0.7, simulating the Reno-compatible mode of CUBIC.

- LINEAR-C with beta=0.85, the most aggressive value permitted by RFC-8511.

Other values could easily also be tried if desirable.  In each case the corresponding alpha value is computed as per the RFC-8312 formula.  The baseline code was taken from Linux 5.10, so might not reflect the very latest fixes.

Pete Heist and I expect to conduct various tests on their performance and interaction in the near future, exploring both static (steady-state congestion avoidance) and dynamic (exiting slow start, step changes in channel capacity, start-up and shut-down of competing flows) behaviour.  Various path characteristics, particularly baseline RTT and queue type, will undoubtedly be relevant for testing.  We should be able to use the "harm metric" to quantify effects on competing traffic, again using a free choice of congestion controls.

 - Jonathan Morton