ip_input.c defines IPMTUDISC to be 0 (disabled) for IP Path MTU Discovery.
Enabling it turns on some complex host route adding code in 
ip_icmp.c:icmp_mtudisc().  Is this all working properly and
just off by default to be conservative?  It was added way back in NetBSD 1.3.

(And simply turning it on for myself won't help me without a lot of testing, 
as the correct behavior depends upon diverse router behaviors.)

-david waitzman