PMTU broken

Matthew Luckie mjl at
Wed Apr 18 18:03:12 CEST 2012

> Here's the odd thing though - the first time I tested, I saw the
> response above. Successive tests have resulted in 'pmtud-toosmall',
> which indicates that the server didn't send any individual packets
> above 1280 bytes. I wonder if there are more than one server behind
> a balancer, and somewhat different behavior?

The first time the webserver is tested it does not know what the path
MTU to a particular client is, so it uses the MTU of the outbound
interface.  When the PMTU is lowered, the webserver caches that value
for subsequent connections for a short time.


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