ECN

[lists at ltri.eu (Lukas Tribus)]

Hello,

On Wed, Nov 13, 2019 at 8:35 PM Saku Ytti <saku at ytti.fi> wrote:
>
> On Wed, 13 Nov 2019 at 18:27, Matt Corallo <nanog 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.
>
> Not true. Hash result should indicate discreet flow, more importantly
> discreet flow should not result into two unique hash numbers. Using
> whole TOS byte breaks this promise and thus breaks ECMP.
>
> Platforms allow you to configure which  bytes are part of hash
> calculation, whole TOS byte should not be used as discreet flow SHOULD
> have unique ECN bits during congestion. Toke has diagnosed the problem
> correctly, solution is to remove TOS from ECMP hash calculation.

In fact I believe everything beyond the 5-tuple is just a bad idea to
base your hash on. Here are some examples (not quite as straight
forward than the TOS/ECN case here):

TTL:
https://mailman.nanog.org/pipermail/nanog/2018-September/096871.html

IPv6 flow label:
https://blog.apnic.net/2018/01/11/ipv6-flow-label-misuse-hashing/
https://pc.nanog.org/static/published/meetings/NANOG71/1531/20171003_Jaeggli_Lightning_Talk_Ipv6_v1.pdf
https://www.youtube.com/watch?v=b0CRjOpnT7w



Lukas