[Codel] [Cake] Proposing COBALT

[Andrew McGregor]

There are undoubtedly DCTCP-like ECN responses widely deployed, since
that is the default behaviour in Windows Server (gated on RTT in some
versions).  But also, ECN bleaching exists, as do servers with ECN
response turned off even though they negotiate ECN.  It would be good
to know some specifics as to which site, whose DC they're hosted in,
etc.

Also, do you have fallback behaviour such that an ECN-unresponsive
flow eventually sees drops?  I think that will be essential.

On Sat, Jun 4, 2016 at 5:34 AM, Jonathan Morton <chromatix99 at gmail.com> wrote:
>
>> On 3 Jun, 2016, at 22:09, Noah Causin <n0manletter at gmail.com> wrote:
>>
>> Was the issue, where the drops and marks did not seem to occur, resolved?
>
> Examination of packet dumps obtained under controlled conditions showed that marking and dropping *did* occur as normal, and I got a normal response from a local machine sending through a virtual delay line.  My Internet connection is such that extremely short RTTs never occur.
>
> However, it seems that some Internet servers I use often do not respond as much as they should to ECN marking, resulting in excessively long queues despite a relatively small number of flows.
>
> It rather reminds me of the symptoms one would expect to see if DCTCP found its way onto the public Internet.  And these are very popular servers with an extremely large userbase.  However it’s also possible that the ECN information is somehow disappearing en route.
>
> I plan to investigate in more detail once COBALT is up and running, with behaviour I can reason about more intuitively than the “evolved Codel” Cake has been using up to now.  With COBALT integrated into Cake, I’ll also be able to directly track the number of unresponsive flows.
>
> Part of that investigation may be to enquire as to whether DCTCP is in fact in use.  If so, the TCP Prague people should be brought into the loop, as this would constitute evidence that Codel can’t control DCTCP via ECN under practical Internet conditions.
>
>  - Jonathan Morton
>
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