Re: [Bloat] Bloat done correctly?

[Sebastian Moeller <moeller0@gmx.de>]

Hi Jonathan,

On June 12, 2015 9:14:02 PM GMT+02:00, Jonathan Morton <chromatix99@gmail.com> wrote:
>We have a test in Flent which tries to exercise this case: 50 flows in
>one
>direction and 1 in the other, all TCP. Where the 50 flows are on the
>narrow
>side of an asymmetric link, it is possible to see just what happens
>when
>there isn't enough bandwidth for the acks of the single opposing flow.
>
>What I see is that acks behave like an unresponsive flow in themselves,
>but
>one that is reasonably tolerant to loss (more so than to delay). On a
>standard AQM, the many flows end up yielding to the acks; on a
>flow-isolating AQM, the acks are restricted to a fair (1/51) share, but
>enough of them are dropped to (eventually) let the opposing flow get
>most
>of the available bandwidth on its side. But on an FQ without AQM, acks
>don't get dropped so they get delayed instead, and the opposing flow
>will
>be ack clocked to a limited bandwidth until the ack queue overflows.
>
>Cake ends up causing odd behaviour this way. I have a suspicion about
>why
>one of the weirder effects shows up - it has to get so aggressive about
>dropping acks that the count variable for that queue wraps around.
>Implementing saturating arithmetic there might help.
>
>There is a proposed TCP extension for ack congestion control, which
>allows
>the ack ratio to be varied in response to ack losses. This would be a
>cleaner way to achieve the same effect, and would allow enabling ECN on
>the
>acks, but it's highly experimental.

       This is reducing the ACK-rate to make losses less likely, but at the same time it makes a single loss more costly, so whether this is a win depends on whether the sparser ACK flow has a much higher probability to pass trough the congested link. I wonder what percentage of an ACK flow can be dropped without slowing the sender?

>
>- Jonathan Morton
>
>
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