nanog mailing list archives

Re: ECN


From: Lukas Tribus <lists () ltri eu>
Date: Wed, 13 Nov 2019 21:57:40 +0100

Hello,

On Wed, Nov 13, 2019 at 8:35 PM Saku Ytti <saku () ytti fi> wrote:

On Wed, 13 Nov 2019 at 18:27, Matt Corallo <nanog () 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.

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


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