IP Fragmentation

Simon Leinen simon.leinen at switch.ch
Tue Aug 26 16:40:17 CDT 2008

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


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).

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