[52756] in North American Network Operators' Group
Re: Broken PMTU (was: Who does source address validation? (was Re:what's
daemon@ATHENA.MIT.EDU (Tony Rall)
Thu Oct 10 18:07:38 2002
In-Reply-To: <20021010004821.F85622-100000@sequoia.muada.com>
To: <nanog@merit.edu>
From: Tony Rall <trall@almaden.ibm.com>
Date: Thu, 10 Oct 2002 15:07:01 -0700
Errors-To: owner-nanog-outgoing@merit.edu
On Thursday, 2002-10-10 at 00:55 ZE2, Iljitsch van Beijnum
<iljitsch@muada.com> wrote:
> You can also get around this by making the first hop the one with the
> lowest MTU. This is no fun for ethernet-connected stuff, but for dial-up
> this is easy. Then this box will announce a smaller TCP MSS when the
> connection is established and there aren't any problems.
Traffic consists of more than tcp; setting your mtu low might get your tcp
traffic delivered but won't help inbound traffic using other protocols.
Mtu discrepancies must be dealt with in at least one of the following ways
if you don't want it to lead to fatally dropped packets:
1. Fragmentation must work. This applies to systems that don't use PMTUD
or use blackhole detection. (Some folks think it a good "security"
practice to drop fragments! Some nat boxes don't know what to do with
fragments when they arrive out of order - especially a non-initial
fragment before the first.)
2. PMTUD must work.
3. PMTUD blackhole detection must be used with operable fragmentation. (If
you have to fallback to this you're likely to suffer significant
performance hits.)
Tony Rall
