[aqm] ECN: was Control Theoretic Analyses of PI(E)

John Leslie <john@jlc.net> Mon, 26 January 2015 17:14 UTC

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Date: Mon, 26 Jan 2015 12:14:39 -0500
From: John Leslie <john@jlc.net>
To: Vishal Misra <misra@cs.columbia.edu>
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Subject: [aqm] ECN: was Control Theoretic Analyses of PI(E)
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Vishal Misra <misra@cs.columbia.edu> wrote:
> <snip>
> <http://dna-pubs.cs.columbia.edu/citation/paperfile/23/MisraInfocom01-AQM-Controller.pdf>
>...
> One thing to note though that hasn't changed all these years is if you
> look at Section VII.A of our PI paper linked above, the full benefits
> of AQM are realized in conjunction with ECN. On a bottlenecked link,
> if you reduce delay (by controlling it via a mechanims like PI(E) or
> CoDel), unless you have ECN implemented you will end up increasing
> loss rates which may not be a good thing.

   I wish we'd discuss ECN more here, and state some benefit of its use;
perhaps even discuss how to route-around its misuse along the path.

   We should (IMHO) note that it's many years since its use in congestion
control could possibly be "the same as packet drop" -- and by the nature
of AQM, packets need to be ECN-marked before anything must be dropped
due to buffer overflow.

   Obviously (IMHO), an ECN-capable packet which encounters buffer
overflow needs to be dropped: not held until it can be ECN-marked and
forwarded.

--
John Leslie <john@jlc.net>