Re: [tcpm] hystart++

Thanks, Randall. That's a nice analysis, and I agree that your proposal
sounds like a nice improvement.

This nice analysis brings to mind another potential (related) issue that
might cause spurious oscillations: after Hystart++ exits slow-start, if
there are application-limited periods where the amount of data in flight
falls below the BDP (e.g. flow goes idle and then restarts), then we would
expect the currentRoundMinRTT to be very low, triggering a resumption of
slow-start in the current logic, even if the Hystart++ exit of slow-start
was correct.

It seems that the connection should not use a data ACK's RTT sample to
estimate whether Hystart++ exit was spurious unless the amount of data in
flight at the time that data was sent was "sufficiently large". How to
define "sufficiently large" may be a little tricky (the cwnd is probably
not a good gauge for "sufficiently large" since it is probably somewhere
out beyond twice the BDP in CSS).

One option would be to track the maximum number of packets delivered in a
round of slow-start (slowStartMaxDeliveredPackets), and only check for
spurious Hystart++ exits if the most recent round of CSS has at least
slowStartMaxDeliveredPackets delivered. Something like:

- if (currentRoundDeliveredPackets >= slowStartMaxDeliveredPackets &&
      currentRoundMinRTT < cssBaselineMinRtt)

         o  cssBaselineMinRtt = infinity

         o  resume slow start including HyStart++

best regards,
neal

On Thu, Dec 2, 2021 at 1:46 PM Randall Stewart <rrs=
40netflix.com@dmarc.ietf.org> wrote:

> Greetings all:
>
> I have implemented hystart++ off of the draft-03 for FreeBSD
> (both newreno and cubic) and I have noted one thing that I think
> is a bug in the spec:
>
> Draft-03 currently says:
>
>
>
>         o  if (currentRoundMinRTT >= (lastRoundMinRTT + RttThresh))
>
>            +  cssBaselineMinRtt = currentRoundMinRTT
>
>            +  exit slow start and enter CSS
>
>
> <And>
>
>
>
>      -  if (currentRoundMinRTT < cssBaselineMinRtt)
>
>         o  cssBaselineMinRtt = infinity
>
>         o  resume slow start including HyStart++
>
>
>
> But notice that the threshold for entering CSS is (lastRoundMinRtt +
> RttThresh) and
> the exiting of CSS back to Slow Start is (currentRoundMinRTT <
> cssBaseLineMinRTT), which can
> under the right circumstances set up an oscillation. In the case I
> observed I saw lastRoundMinRtt set
> to like 63ms and currentRoundMinRtt set at 115ms. And the next RTT in was
> 114ms.. so out of
> CSS into Slow Start, but the next measurement (114ms again) pushes you
> back into CSS since
> the lastRoundMinRTT + RttThresh is around 63ms or so.
>
> I would think what you really want here is to set cssBaseLineMinRTT to
> (lastRoundMinRtt + RttThresh).
>
> I.e. on entry:
>
>
>         o  if (currentRoundMinRTT >= (lastRoundMinRTT + RttThresh))
>
>            +  cssBaselineMinRtt = (lastRoundMinRtt + RttThresh)
>
>            +  exit slow start and enter CSS
>
> R
>
> ------
> Randall Stewart
> rrs@netflix.com
>
>
>
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