[29177] in North American Network Operators' Group
Re: PMTU-D: remember, your load balancer is broken
daemon@ATHENA.MIT.EDU (Bennett Todd)
Wed Jun 14 11:46:32 2000
Date: Wed, 14 Jun 2000 11:43:36 -0400
From: Bennett Todd <bet@rahul.net>
To: Marc Slemko <marcs@znep.com>
Cc: Valdis.Kletnieks@vt.edu, nanog@merit.edu
Message-ID: <20000614114336.J2762@rahul.net>
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In-Reply-To: <Pine.BSF.4.20.0006132135340.572-100000@alive.znep.com>; from marcs@znep.com on Tue, Jun 13, 2000 at 10:36:08PM -0600
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2000-06-14-00:36:08 Marc Slemko:
> > b) If you're a webserver or something else providing service Out
> > There to random users, just nail the MTU at 1500, which will
> > work for any Ethernet/PPP/SLIP out there. And if you're load
> > balancing to geographically disparate servers, then your users
> > are probably Out There, with an MTU almost guaranteed to be 1500.
>=20
> Except that, technically, you are not permitted to just blindly send=20
> segments of such size. Well, you can but systems in the middle don't=20
> have to handle them. No?
No? I thought traffic only failed to flow when PMTU discovery was
attempted (dont-fragment bit set on first packet) but the needed
ICMP to make it work was being blocked. If you don't even try to do
PMTU, then people who have paths where middle links have MTUs
smaller than the smallest of the two end-points' MTUs will just have
to fragment. And as long as they're rare, that shouldn't be much
problem, no?
-Bennett
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