North American Network Operators Group

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Re: IP Fragmentation

  • From: Sam Stickland
  • Date: Wed Aug 20 18:07:30 2008

Iljitsch van Beijnum wrote:
On 20 aug 2008, at 20:04, [email protected] wrote:

Hypothetically true. Unfortunately, enough places do bozo firewalling and drop
the ICMP Frag Needed packets to severely limit the utility of PMTU Discovery.

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

Sam