[Cake] dual-src/dsthost unfairness, only with bi-directional traffic

[Toke Høiland-Jørgensen]

Pete Heist <pete at heistp.net> writes:

>> On Jan 3, 2019, at 12:03 PM, Toke Høiland-Jørgensen <toke at toke.dk> wrote:
>> 
>>> Jon, is there anything I can check by instrumenting the code somewhere
>>> specific?
>> 
>> Is there any way you could test with a bulk UDP flow? I'm wondering
>> whether this is a second-order effect where TCP ACKs are limited in a
>> way that cause the imbalance? Are you using ACK compression?
>
>
> Not using ack-filter, if that’s what’s meant by ACK compression. I
> thought about the TCP ACK traffic, but would be very surprised if that
> amount of ACK traffic could cause that large of an imbalance, although
> it’s worth trying to find out.
>
> I tried iperf3 in UDP mode, but cake is treating these flows
> aggressively. I get the impression that cake penalizes flows heavily
> that do not respond to congestion control signals. If I pit one 8 TCP
> flows against a single UDP flow at 40mbit, the UDP flow goes into a
> death spiral with increasing drops over time (iperf3 output attached).
>
> I’m not sure there’d be any way I can test fairness with iperf3 in UDP
> mode. We’d need something that has some congestion control feedback,
> right? Otherwise, I don’t think there are any rates I can choose to
> both reach saturation and not be severely punished. And if it has
> congestion control feedback, it has the ACK-like traffic we’re trying
> to avoid for the test. :)

Try setting cake to 'interplanetary' - that should basically turn off
the AQM dropping...

-Toke