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

Jonathan Morton <chromatix99@gmail.com> Thu, 13 October 2022 11:54 UTC

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From: Jonathan Morton <chromatix99@gmail.com>
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Date: Thu, 13 Oct 2022 14:54:01 +0300
Cc: Markku Kojo <kojo=40cs.helsinki.fi@dmarc.ietf.org>, Yoshifumi Nishida <nsd.ietf@gmail.com>, "tcpm@ietf.org Extensions" <tcpm@ietf.org>, tcpm-chairs <tcpm-chairs@ietf.org>
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To: Ian Swett <ianswett@google.com>
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Subject: Re: [tcpm] finalizing CUBIC draft (chairs' view)
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> On 13 Oct, 2022, at 1:47 pm, Ian Swett <ianswett@google.com> wrote:
> 
> IMO the question should be whether deploying Cubic has or will cause measurable harm on the public internet vs Reno.  I have never seen evidence of this because I think it's so rare it's in practice unmeasurable.

You're probably correct that the effect is not significant in practice.  However, there is a debate on that point and data has been requested to support that debate, so it is fortuitous that we are in a position to supply that data in the near future.

> Unfairness is only a problem if it's causing latency or drops to other flows (aka externalities).  If you're running a very unfair congestion controller on a dedicated link and it's the only flow, there's no harm.  If the flow is constantly app-limited or never exits slow start (the vast majority of flows), convergence in congestion avoidance is irrelevant.  How it exits slow start actually ends up mattering more, and Cubic recommends Hystart++.

A confounding factor in this analysis is that the potential bad behaviour is generally associated with dynamic conditions, which occur only transiently, rather than steady state conditions which may occur for long periods of observation.  Transient conditions are notoriously difficult to measure and reason about "in the wild", but much easier to induce and measure in the lab, from which we can then extrapolate to likely real-world conditions.

But you are perfectly correct that it is the externalities that matter most.  That is why I mentioned using the "harm metric" to help quantify them.  Under lab conditions it is straightforward to collect the appropriate comparative data from which harm metrics can be computed accurately.

 - Jonathan Morton