PMTU and Broken Servers

Hi Leo,

#The tunnel between the tunnelboxes is a lower (1480) MTU. Originally
#the user couldn't access some servers, turns out the firewall was
#filtering ICMP Can't Fragment messages, preventing PMTU from working
#in the server->user direction (tunnelbox1 would generate Can't
#Fragement, firewall would filter).

This is actually a more broadly present problem than you might think.
I talked about this in the context of a jumbo frames presentation
("Practical Issues Associated with 9K MTUs") I did for the February
NLANR/I2 Joint Techs in Miami; see: (PDF and PowerPoint versions provided)

#I find it slightly
#(emphasis on the slightly) that someone would turn on PMTU discovery,
#and then filter it out right in front of the boxes where they turned
#it on.

Different folks are probably driving the server network configuration and
the firewall/border router configuration process. Disconnect is not
inconceivable in that scenario by any means.

#This is a new problem to me, but I'm sure people have run into it
#before. Are the servers really that broken (PMTU enabled, ICMP
#Can't Fragement filtered)?

Yes, it is a huge issue potentially.


Joe St Sauver (
Univeristy of Oregon Computing Center