[107069] in North American Network Operators' Group
Re: IP Fragmentation
daemon@ATHENA.MIT.EDU (Iljitsch van Beijnum)
Mon Aug 25 06:56:51 2008
From: Iljitsch van Beijnum <iljitsch@muada.com>
To: Fernando Gont <fernando@gont.com.ar>
In-Reply-To: <200808251035.m7PAZtXv003025@venus.xmundo.net>
Date: Mon, 25 Aug 2008 12:56:34 +0200
Cc: OPS Gurus <nanog@merit.edu>
Errors-To: nanog-bounces@nanog.org
On 25 aug 2008, at 12:27, Fernando Gont wrote:
>> http://support.microsoft.com/kb/925280
> IPv4 minimum MTU is 68 bytes,
That's kind of like "a human being can live without food for four to
six weeks". It's not a recommendation.
> 536 is the minimum fragment re-assembly buffer size. Falling back to
> 536-byte packets does not guarantee that sessions will be kept up.
But:
"PMTU black hole router detection is triggered on a TCP connection
when TCP starts retransmitting full-sized segments with the DF flag
set. TCP resets the PTMU for the connection to 536 bytes. Then, TCP
retransmits its segments when the DF flag is clear."
