Hi Jonathan,
much appreciated!
> On Aug 26, 2023, at 14:06, Jonathan Morton <chromati...@gmail.com> wrote:
>
>> On 26 Aug, 2023, at 2:48 pm, Sebastian Moeller via Ecn-sane
>> <ecn-s...@lists.bufferbloat.net> wrote:
>>
>> percentage of packets marked: 100 * (2346329 / 3259777) = 72%
>>
>> This seems like too high a marking rate to me. I would naively expect that a
>> flow on getting a mark scale back by its cwin by 20-50% and then slowly
>> increaer it again, so I expect the actual marking rate to be considerably
>> below 50% per flow...
>
>> My gut feeling is that these steam flows do not obey RFC3168 ECN (or
>> something wipes the CE marks my router sends upstream along the path)... but
>> without a good model what marking rate I should expect this is very
>> hand-wavy, so if anybody could help me out with an easy derivation of the
>> expected average marking rate I would be grateful.
>
> Yeah, that's definitely too much marking. We've actually seen this behaviour
> from Steam servers before, but they had fixed it at some point. Perhaps
> they've unfixed it again.
Hmm, I guess the next time around I will take a packet capture and see
whether I see ECE (expected) and especially CWR flags in these streams... my
side is a recent ubuntu (kernel from the 5.15 series I believe) with sysctl
configured to negotiate ECN...
>
> My best guess is that they're running an old version of BBR with ECN
> negotiation left on. BBRv1, at least, completely ignores ECE responses.
> Fortunately BBR itself does a good job of congestion control in the FQ
> environment which Cake provides, as you can tell by the fact that the queues
> never get full enough to trigger heavy dropping.
Yes, good point! Next time I see a big download I will take a packet capture to
look for additional markers of ECN activity... Sidenote, BBRv1 ignoring ECN and
not making sure to veto ECN negotiation really seems like a sub-optimal
coincidence...
>
> The CUBIC RFC offers an answer to your question:
>
> <Screenshot 2023-08-26 at 3.03.03 pm.png>
>
> Reading the table, for RTT of 100ms and throughput 100Mbps in a single flow,
> a "loss rate" (equivalent to a marking rate) of about 1 per 7000 packets is
> required. The formula can be rearranged to find a more general answer.
Thanks! Will look into this...
Regards
Sebastian
>
> - Jonathan Morton
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