Re: [tcpm] On Sender Control of Delayed Acks in TCP

[Carles Gomez Montenegro <[e-mail]>]

Hi Neal,

>> > On Wed, Apr 29, 2020 at 1:03 PM Scheffenegger, Richard
>> <[e-mail]>
>> > wrote:
>> >> Eliciting an ACK under certain circumstances, for a timely
>> continuation
>> >> of the data transmission / growth of the congestion window is a known
>> >> method to reduce network delay.
>> >>
>> >> E.g. Linux has been using the CWR flag for the purpose of sending out
>> an
>> >> immediate ACK by the receiver, since there is an increased chance of
>> a
>> >> very small cwnd when the sender had just reduced it's congestion
>> window.
>> >>
>> >> https://lore.kernel.org/patchwork/patch/970486/
>> >>
>> >> and we also found latency improvements doing this when running
>> >> ECN-enabled sessions
>> >>
>> >> https://reviews.freebsd.org/D22670
>> >
>> > Yes, agreed.
>> >
>> > IMHO an explicit ACK-pull mechanism would be very nice for the cwnd<=1
>> > case.
>>
>> (While this question is about the solution space, I'm anyway curious...)
>> In the context of datacenter networks, would you have any suggestion or
>> preference regarding the solutions that have been mentioned so far?
>>
>> Or perhaps any particular requirement for a potential solution?
>
> Among the solutions outlined in
>   https://tools.ietf.org/html/draft-gomez-tcpm-delack-suppr-reqs-00
> my first preference would be the AKP option, since that approach
> avoids burning a precious flag bit.
>
> The AKP option may be stripped by some middleboxes, but (a) as a
> performance optimization, that should be acceptable, and (b) for the
> datacenter case (where cwnd=1 is a motivating use case) this should
> not be a concern.
>
> IMHO a flag bit makes sense for a small signal that a sender might
> want to send frequently or at a high rate, but senders should not be
> trying to force immediate ACKs frequently.

Thanks a lot for the very useful feedback.

In the next draft update, we will expand the discussion of potential
solutions accordingly.

Cheers,

Carles


> best,
> neal