Re: [tsvwg] RTT-independence

Jonathan Morton <chromatix99@gmail.com> Tue, 11 February 2020 11:54 UTC

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From: Jonathan Morton <chromatix99@gmail.com>
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Date: Tue, 11 Feb 2020 13:53:54 +0200
Cc: Bob Briscoe <ietf@bobbriscoe.net>, gorry@erg.abdn.ac.uk, ingemar.s.johansson@ericsson.com, tsvwg@ietf.org, tsvwg-chairs@ietf.org
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Subject: Re: [tsvwg] RTT-independence
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> 15ms comes from the amplitude of the sawteeth of typical traffic at typical Internet bottlenecks over typical paths. 

For this to be true, as a peak-to-peak measure, while maintaining 100% throughput, the baseline RTT would have to be 15ms (for basic Reno) or 35ms (for CUBIC as implemented in Linux).  This does not square with other research which has established typical Internet path RTTs in the region of 80ms.

NB: Codel has an even lower queue target of 5ms by default, but this is for controlling the *standing* queue, not peak nor average queue depth.  Some reduction in link utilisation is accepted in the design, in service of keeping the queue short.

In any case, from a scientific perspective, when testing a claim that a given system is less RTT-dependent than the status quo, it is of course necessary to choose a variety of test conditions which are capable of exposing this property.  Tests to date actually suggest *greater* RTT dependence of the L4S system compared to conventional TCPs with conventional AQM, largely because of the moderating effect (with a very short baseline RTT) of keeping a modest standing queue which L4S specifically avoids.

It is this evidence which the L4S team must defend against on this particular claim.  I think Sebastian is asking important questions here.

 - Jonathan Morton