why filtering ICMP can break PMTU-D

[Marc Slemko <marcs () znep com>]

I have had a few requests regarding just how ICMP messages not reaching
where they should (for whatever reason, including using private address
space that is filtered somewhere) can break path MTU discovery.

Anyone interested who doesn't understand it should take a look at the
brief description I have available at: 

        http://www.worldgate.com/~marcs/mtu/

which tries to be as simple as possible and explain it in a way that
almost anyone can understand.  Was originally made for referencing in
complaints to silly providers that break connectivity by broken filtering
and can't understand the problems.