ECN

[owen at delong.com (Owen DeLong)]

Like it or not (and I really donâ??t), the majority of modern CDNs are using TCP over Anycast.

Itâ??s ugly and itâ??s prone to problems like this. Itâ??s nice to see a customer with know-how actually publicizing and digging into the problem.

Until now, I believe an unknown number of customers have been suffering in silence or relegated to the ISPs â??We canâ??t reproduce you problemâ?? bin without resolution.

Iâ??ve had lots of discussions on the subject and the usual end result is â??Itâ??s too hard to measure or quantify and thereâ??s no visible contingent of impacted usersâ??.

Now we at least have one visible impacted user.

Owen


> On Nov 13, 2019, at 09:19 , Anoop Ghanwani <anoop at alumni.duke.edu> wrote:
> 
> Not to condone what cloudflare is doing, but...
> 
> An ECN connection will have different bits on various packets for the duration of the connection -- pure ACKs (ACKs not piggybacking on data) will have the ECN bits as 00b, while all other packets will have either 01b, 10b (when no congestion was experienced) or 11b (when congestion was experienced).  So using the ECN bits as part of the hash would affect performance throughout the life of the connection.
> 
> On Wed, Nov 13, 2019 at 9:00 AM Matt Corallo <nanog at as397444.net <mailto:nanog at as397444.net>> wrote:
> Not ideal, sure, but if itâ??s only for the SYN (as you seem to indicate), splitting the flow shouldnâ??t have material performance degradation? 
> 
> > On Nov 13, 2019, at 11:51, Toke Høiland-Jørgensen <toke at toke.dk <mailto:toke at toke.dk>> wrote:
> > 
> > 
> > 
> >> On 13 November 2019 17:20:18 CET, Matt Corallo <netadmin at as397444.net <mailto:netadmin at as397444.net>> wrote:
> >> This sounds like a bug on Cloudflareâ??s end (cause trying to do anycast
> >> TCP is... out of spec to say the least), not a bug in ECN/ECMP.
> > 
> > Even without anycast, an ECMP shouldn't hash on the ECN bits. Doing so will split the flow over multiple paths; avoiding that is the whole point of doing the flow-based hashing in the first place.
> > 
> > Anycast "only" turns a potential degradation of TCP performance into a hard failure... :)
> > 
> > -Toke
> 

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