Re: Anycast provider for SMTP?

[William Herrin <bill () herrin us>]

On Mon, Jun 15, 2015 at 2:13 PM, Bill Woodcock <woody () pch net> wrote:
> Or you could skip the MX records, and just put both US and European
> SMTP servers on the same IP address, which would save a lot of
> steps and simplify the system, but leave you with the _very_
> occasional corner-case of someone equal-path-length load-balancing
> traffic to you such that half of one TCP session goes to Europe, and
> half the the US.  That’s a bogeyman that scares a lot of people into not
> using anycast for TCP services, particularly long-lived ones, but it’s a
> theoretical problem rather than an actually-observed-in-the-wild problem.
> But since it scares people, it’s probably safer just doing the DNS
> anycast, rather than SMTP anycast, to avoid startling the
> easily-upset out there.  :-)

If I had a dollar for every system that's collapsed from a known but
previously "theoretical" problem... It's only theoretical until a VIP
can't connect. Deploy a system without covering the corner cases and
your comeuppance is assured.

Okay, granted you can probably cover your corner case here with a
priority 20 MX that leads to a unicast address on one of the two
servers. SMTP can let the rare fellow with the bisected packet flow
gracefully fall back.

Nevertheless, I think you've offered some really bad advice here Bill.
Hijackers killing the passengers was a bogeyman too. If you just kept
calm and cooperated, you lived through it. Until you didn't, and
allowed yourself to be an instrument in killing thousands on the
ground as a bonus. Sometimes the math offers really bad advice.


On Mon, Jun 15, 2015 at 2:28 PM, Nick Hilliard <nick () foobar org> wrote:
> On 15/06/2015 19:09, William Herrin wrote:
> > Anycast + TCP = much pain, for reasons which should be obvious.
>
> This was presented at some conference or other a couple of years ago:
> https://www.nanog.org/meetings/nanog37/presentations/matt.levine.pdf

Thought the comment on page 22 was apropos: their plan is to be dead
before future change catches up with them.

Regards,
Bill Herrin

-- 
William Herrin ................ herrin () dirtside com  bill () herrin us
Owner, Dirtside Systems ......... Web: <http://www.dirtside.com/>