[OpenAFS] Network becomes terribly slow when cache manager flushes updates over xDSL

[Harald Barth]

> The standard is ICMP WOULD FRAGMENT. 

We have tried years ago. MTU detection through ICMP is not feasible on
networks you do not control.

> People who block that don't deserve functional apps. Don't cater to them.

Unfortunately the people who block are not the ones who take the hit.
The amount of igonrance and incompetence from network and firewall
admins in this field is bigger than our support could deal with. We
can just not afford it because the end user experience is that "PDC
(our site) does not work" and our users give us bad marks and take
their money and go somewhere else for their calculations.

Fortunately AFS has client and server on both sides of the path
through the network, so it is in a better position to find out about
max unfragmeneted MTU in between than ICMP from one end only.

Harald.