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

[Pete Heist]

> 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. :)

As another test, I took out the one-armed router and just tried from a client to a server, no VLANs. Same result. So, still stumped. Thank you for the help...

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