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Re: [iccrg] When is congestion control safe and fair

Jonathan Morton <chromatix99@gmail.com> Thu, 17 December 2020 13:00 UTC

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From: Jonathan Morton <chromatix99@gmail.com>
In-Reply-To: <alpine.DEB.2.21.2012171248370.5844@hp8x-60.cs.helsinki.fi>
Date: Thu, 17 Dec 2020 15:00:06 +0200
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References: <alpine.DEB.2.21.2012171248370.5844@hp8x-60.cs.helsinki.fi>
To: Markku Kojo <kojo=40cs.helsinki.fi@dmarc.ietf.org>
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Subject: Re: [iccrg] When is congestion control safe and fair
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> On 17 Dec, 2020, at 1:55 pm, Markku Kojo <kojo=40cs.helsinki.fi@dmarc.ietf.org> wrote:
> 
> The proposed change would definitely improve loss recovery efficiency of the modified TCP sender but what would be the implications?

The major safeguard I think we'd have to insist on is that any further loss events during the retransmissions MUST be treated as indications of congestion and cause a reduction in cwnd.  Otherwise it looks possible for the recovery period to continue indefinitely at a constant cwnd and with a high loss rate, which seems dangerous.

Behaviourally, this looks similar to an ECN congestion event reducing cwnd to 20, followed by a brief period of application-limited inactivity, followed by data being available again to fill the cwnd.  Clearly this new traffic would respond to congestion signals as normal, and so should anything that looks like it.

 - Jonathan Morton

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