I've been having a problem with a VPN client for months now where it would exit out of the connection very frequently complaining that the routing table could not be changed once the VPN was up. And routes were being added, but I had no idea why. Not only that, but they were useless host routes for random computers I was talking to on the internet that pointed them to my default router, where they were routed to anyway. I finally did some in-depth research and came to the conclusion that those routes were being added as a result of the OS receiving ICMP Fragmentation Needed packets. I reduced my MTU to an absurdly small size (500) and I've now had the VPN up for hundreds of times longer than I ever have before.

I want to know if it seems reasonable for the OS (WinXP Pro and Home) to add host routes when it receives an ICMP Fragmentation Needed packet. Other than my VPN client being anal, it's pretty innocuous to add redundant routes, but I don't see any reason it should be doing it at all. Maybe it keeps MTU sizes for particular hosts in the routing table? If so, it doesn't show it to me in any routing table UI I know of.

Anyway, if anyone has any feedback on this, I'd love to hear it.