isakmpd and icmp dependencies

[Todd Boyer <TBoyer@smcteam.com>]

How dependant is isakmpd on being able to echo icmp responses from peers?
My ISP just locked down all icmp types.  No echo's, no responses from any
subscriber in their loop (in the process they opened up port 80, real smart)
Isakmpd still negotiates and nails up routes, expect nothing.  No response
back from peers after the link is established. My thought is if packets need
to be fragmented, without the proper icmp response, the peer waits
indefinitely for instructions on how top handle the request.  Is my thought
process correct? Any way around this problem? Maybe setting the max MTU to a
lower value?  Thanks