[107112] in North American Network Operators' Group
Re: IP Fragmentation
daemon@ATHENA.MIT.EDU (Simon Leinen)
Tue Aug 26 17:40:28 2008
From: Simon Leinen <simon.leinen@switch.ch>
To: Sam Stickland <sam_mailinglists@spacething.org>
In-Reply-To: <48AC959A.7030203@spacething.org> (Sam Stickland's message of
"Wed, 20 Aug 2008 23:07:22 +0100")
Date: Tue, 26 Aug 2008 23:40:17 +0200
Cc: OPS Gurus <nanog@merit.edu>
Errors-To: nanog-bounces@nanog.org
Sam Stickland writes:
> Iljitsch van Beijnum wrote:
>> Yet all OSes have it enabled and there is no fallback to
>> fragmentation in PMTUD: if your system doesn't get the ICMP
>> messages, your session is dead in the water.
>>
> Windows Vista/2007 has black hole detection enabled by default. It's
> not massively elegant, but it will keep sessions up (falls back to
> 536 byte MTU).
> http://support.microsoft.com/kb/925280
Note that there's a new IETF specification (RFC 4821) for
("Packetization Layer") Path MTU discovery, which doesn't rely on ICMP
messages to work. If what I wrote here
http://kb.pert.geant2.net/PERTKB/PathMTU
is correct, this has been implemented in recent (>= 2.6.17) Linux
kernels. I don't know of any other OSes that have this yet - not that
they'd tell me (but they could go and edit the page above, that's why
it's a Wiki).
--
Simon.
