Re: [rtcweb] Pictures of congestion control on the Internet - which is more realistic?

John Leslie <> Wed, 23 April 2014 16:37 UTC

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Date: Wed, 23 Apr 2014 12:37:17 -0400
From: John Leslie <>
To: Dave Taht <>
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Subject: Re: [rtcweb] Pictures of congestion control on the Internet - which is more realistic?
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Dave Taht <> wrote:
> I'm not sure if we have a shared definition of "congestion".  Mine is
> network delays exceeding that of what human factors research notes
> as "perceptible" with about 20ms as the outer bound.

   I, OTOH, am quite sure we don't have a shared definition of congestion.

   My preferred definition is rate-of-arrival exceeding rate-of-forwarding.

   But, of course, you don't have to agree with mine -- "shared" merely
implies _some_ overlap of the sets of definitions we use...

   Dave's, I fear, isn't terribly useful. There is a base latency (in the
absence of any traffic) which exceeds 20 milliseconds for many paths
of interest.

   To tell truth, though, most users probably only notice the total delay,
not the components of it; so I think Dave's metric is useful -- it's just
not helpful as a measure of "congestion".

   I'd like to follow up a bit on Dave's metric, regardless.

   There's a lot of talk about "buffer-bloat" where I believe we all
agree the situation would be better with less buffering.

   But can we talk about the other end of that spectrum?

   Surely there is some minimal amount of buffering, for any particular
traffic, which is needed in order to maintain a reasonable flow rate.
Most likely, that minimal buffering varies with factors such as RTT. Do
we even have a shared concept of what those factors are?

   I'd like to see a thread on what is needed as a minimum buffering
for various kinds of traffic. I suspect that minimum can be fairly small
except for TCP transport of "large" files...


   Excellent as food for thought...

John Leslie <>