It does when the split flows land in different anycast origin POPs.
Making a few assumptions from the traceroutes, the ECMP paths are sending
some packets to Hamburg and some to Denmark. Each POP may be getting
parts of what should be a single TCP stream, and I doubt they have
anything to cope with that (another assumption).
On Wed, 13 Nov 2019, Matt Corallo wrote:
Not ideal, sure, but if it’s only for the SYN (as you seem to indicate),
splitting the flow shouldn’t have material performance degradation?
On Nov 13, 2019, at 11:51, Toke Høiland-Jørgensen <t...@toke.dk> wrote:
On 13 November 2019 17:20:18 CET, Matt Corallo <netad...@as397444.net> wrote:
This sounds like a bug on Cloudflare’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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