''Linux (and most other recent OS's) use a clever system called 'path-MTU-discovery'; Linux starts any new connection only sending very small packets and gradually makes them bigger. At the same time it sets the 'don't fragment' flag on all the packets. If a link somewhere is using a too-small MTU and wants to fragment them it's not allowed to, so it sends back an ICMP message 'Need to fragment, but don't fragment is set'. When we get this back, we know what's the largest packet we can use on that connection without it getting fragmented, which happens to be the most efficient size..'' 

  

 ''The reason I bothered to tell you all this is because some IDIOT admins (particularly BANKS!) ignorantly block all ICMP without fully understanding why ICMP is an important part of IP, and/or without thinking of the consequences. I just wanted to vent that, OK? :-) - zcat ''