[Ecn-sane] quick question

[Jonathan Morton]

> On 26 Aug, 2023, at 2:48 pm, Sebastian Moeller via Ecn-sane <ecn-sane at 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.

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.

The CUBIC RFC offers an answer to your question:



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.

 - Jonathan Morton
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