North American Network Operators Group
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Re: IP Fragmentation

  • From: Fernando Gont
  • Date: Mon Aug 25 06:36:10 2008
At 07:07 p.m. 20/08/2008, Sam Stickland 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

IPv4 minimum MTU is 68 bytes, not 536. 536 is the minimum fragment re-assembly buffer size. Falling back to 536-byte packets does not guarantee that sessions will be kept up.

Kind regards,

--
Fernando Gont
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