Re: [tcpm] FW: New Version Notification for draft-balasubramanian-tcpm-hystartplusplus-00.txt

Neal Cardwell <ncardwell@google.com> Tue, 09 July 2019 15:07 UTC

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From: Neal Cardwell <ncardwell@google.com>
Date: Tue, 9 Jul 2019 11:06:58 -0400
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To: Praveen Balasubramanian <pravb=40microsoft.com@dmarc.ietf.org>
Cc: "tcpm@ietf.org" <tcpm@ietf.org>, Matt Olson <maolson@microsoft.com>
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Subject: Re: [tcpm] FW: New Version Notification for draft-balasubramanian-tcpm-hystartplusplus-00.txt
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On Mon, Jul 8, 2019 at 7:11 PM Praveen Balasubramanian
<pravb=40microsoft.com@dmarc.ietf.org>; wrote:
>
> We submitted a draft for modified slow start using HyStart and Limited Slow Start. Feedback is welcome.

Thanks for posting this draft! (
https://tools.ietf.org/html/draft-balasubramanian-tcpm-hystartplusplus-00
)

This is a very nice, clear document, IMHO.

I was curious about a couple of items:

(1) In the draft the condition for exiting slow start is:

            Eta = clamp(MIN_ETA, currentRoundMinRTT / 8, MAX_ETA)
            if (currentRoundMinRTT >= (lastRoundMinRTT + Eta))

The use of currentRoundMinRTT/8 to compute Eta is counterintuitive to
me, and is different from both the Hystart paper and the Linux CUBIC
Hystart implementation from the CUBIC authors.

The Hystart paper used (modified pseudocode):

  if (currentRoundMinRTT> lastRoundMinRTT + clamp(lastRoundMinRTT / 16))

The Linux implementation from the Hystart author Sangtae Ha in 2008
used the approach (pseudocode):

  if (currentRoundMinRTT> lifetimeMinRTT + clamp(lifetimeMinRTT / 16))

Eric Dumazet changed the Linux approach in 2014 to use a different
constant scaling factor (also pseudocode):

  if (currentRoundMinRTT> lifetimeMinRTT + clamp(lifetimeMinRTT / 8))

I am curious about the rationale for:

(a) Using an RTT threshold for exiting slow-start that is based on
currentRoundMinRTT/8, rather than being purely a function of the RTT
values from previous rounds (as in the Hystart paper) or the lifetime
of the connection (as in Linux)?

(b) Using a function of lastRoundMinRTT and currentRoundMinRTT for the
exit threshold, rather than a function of the min RTT over the
lifetime of the connection ("lifetimeMinRTT" in my pseudocode, above)?
Is the rationale here partly to deal with fluctuations in RTT due to
cellular/wifi/DOCSIS paths with noisy RTTs?

Any background on the motivation would be interesting to hear, or
perhaps to include in future drafts.

(2) The Limited Slow-Start approach described in the draft seems to
boil down to increasing cwnd by roughly ssthresh/4 each round trip.
It does seem like that would be considerably faster than Reno or CUBIC
would normally grow in congestion avoidance, for the first few seconds
after exiting slow start. But it does seem to be essentially just a
large additive increase. And so it would seem that if CUBIC were
instead in its native CUBIC congestion avoidance instead of limited
slow-start, then CUBIC would eventually (e.g. after a few seconds)
tend to reach the rapid-growth convex portion of the cubic curve, and
eventually grow much faster (eventually reaching exponential growth of
1.5x per round trip). I wonder if it would be advantageous for CUBIC
connections exiting HyStart++ to use a  strategy of growing the cwnd
using the maximum of the schedule calculated using limited slow-start
and the schedule calculated using the normal CUBIC congestion
avoidance logic. That seems like it might have the potential to
mitigate performance issues for CUBIC connections that spuriously exit
slow-start too soon?

best regards,
neal