A couple of things:

1) The system which was failing to get packets through due to PMTU
   discovery was actually the InterNIC endpoint, not the NetBSD
   endpoint.  Users of InterNIC services obviously have no control
   over this, except in as much as they can whine at the InterNIC to
   fix it.

   This issue has been brought up before in other forums.  The correct
   answer is for the InterNIC to either disable PMTU discovery, or
   allow ICMP NEEDS FRAGMENT packets through their firewall.  They
   have yet to do either.

2) Fragmenting packets with the `don't fragment' bit set explicitly
   violates the IP spec.  This is not a reasonable solution.

Rather than breaking the protocol, perhaps we should be concluding
that this is just another aspect of the shoddy service we get from the
InterNIC.