IP Fragmentation

Mon Aug 25 10:56:34 UTC 2008

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."