TCP and anycast (was Re: ECN)

Fri Nov 15 02:26:02 UTC 2019

On Thu, Nov 14, 2019 at 1:10 AM Bill Woodcock <woody at pch.net> wrote:
> > On Nov 14, 2019, at 7:39 AM, Anoop Ghanwani <anoop at alumni.duke.edu>
wrote:
> > RFC 7094 (https://tools.ietf.org/html/rfc7094) describes the pitfalls &
risks of using TCP with an anycast address.  It recognizes that there are
valid use cases for it, though.
> > Specifically, section 3.1 says this:
> >    Most stateful transport protocols (e.g., TCP), without modification,
do not understand the properties of anycast; hence, they will fail
> >    probabilistically, but possibly catastrophically, when using anycast
addresses in the presence of "normal" routing dynamics.
> >    This can lead  to a protocol working fine in, say, a test lab but
not in the global Internet.
> >
> > On Thu, Nov 14, 2019 at 12:25 AM Matt Corallo <nanog at 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),
>
> No. We have been doing anycast TCP for more than _thirty years_, most of
that time on a global scale, without operational problems.

Hi Bill,

Not to put to fine a point on it but Baldur and Toke's scenario in which
anycast tcp failed, the one which started this thread, should probably be
classed as an operational problem.

It is possible to build an anycast TCP that works. YOU have not done so.
And Cloudflare certainly has not.

Regards,
Bill Herrin

-- 
William Herrin
bill at herrin.us
https://bill.herrin.us/
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