[Cake] Query on ACK

Dave Taht dave.taht at gmail.com
Fri May 8 02:50:38 EDT 2020

On Thu, May 7, 2020 at 11:36 PM Avakash bhat <avakash261 at gmail.com> wrote:
> Ok thanks so much for the clarifications.
> That cleared it up quite a bit.

I note that there was something really subtle that could have been
done to improve cake's ack handling, and for all I know
it actually happened in the final codebase.

so, please, go forth and duplicate the existing implementation, and
ignore me, cause looking at this hairy code gives me a

anyway, to try and describe what I thought I saw an interaction with
the scheduler back in the day.

The ack-filter runs, deleting all but one packet from the ack queue,
and delivers that.
the scheduler runs, serves a bunch of other flows, then returns to the
ack queue, which has accumulated a couple more packets,
the ack-filter runs, deleting all but one packet from the ack queue,
and delivers that, but doesn't exhaust its qauntum
but now that flow is in the "fast" queue, and we service just a few
other flows, and return to it, delete a couple, service one... and
stay stuck in the fast queue.

better, I thought, was once the ack filter exceeded the quantum of
packets for that flow in that drr round, even if it only delivered one
that it should always return it to the bulk queue, because tons more
packets would arrive in the interval between servicing
all the rest of the flows, thus more of which could be safely removed,
while maintaining a steadier clock for tcp.

I've already seen cake remove over 25% of all ack packets with no harm
to the other flows. So for all I know (and I'd have to
look) it's already doing it this way.

> Thanks,
> Avakash Bhat
> On Thu, May 7, 2020 at 12:37 PM Sebastian Moeller <moeller0 at gmx.de> wrote:
>> I think that you will remove all redundant Backs in one go considerably advancing the new ACK in the queue. And more importantly, in most relevant modes cake will apply one queue per flow stochastically, so almost all packet's in a reverse ACK flow will be ACK with identical 5-tupel....
>> On 7 May 2020 08:44:59 CEST, Avakash bhat <avakash261 at gmail.com> wrote:
>>> Thanks for the quick response. I also had a followup question.
>>> If the ack filter adds the new ack to the tail of the queue after removing an ack from the queue, won't it be starving the ack?
>>> The replaced ack was much ahead in the queue than the ack we replaced at the tail right?
>>> Thanks,
>>> Avakash Bhat
>> --
>> Sent from my Android device with K-9 Mail. Please excuse my brevity.
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Make Music, Not War

Dave Täht
CTO, TekLibre, LLC
Tel: 1-831-435-0729

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