On Wed, 29 Sep 1999 02:22:18 +0200 
 Feico Dillema <dillema@acm.org> wrote:

 > Just a question: why is Path MTU discovery switched off by
 > default in NetBSD. Is there a reason not to enable it 
 > by default with:
 > 
 > sysctl -w net.inet.ip.mtudisc=1
 > 
 > Just curious,

NetBSD does not yet detect ICMP Black Holes.  These Black Holes occur
when network administrators are allowed to configure firewalls without
adult supervision.

ICMP is required for Path MTU Discovery to work properly (it relies on
the Needs-Fragmentation ICMP message).  If you have a Black Hole, your
packets will never reach the other endpoint.

        -- Jason R. Thorpe <thorpej@nas.nasa.gov>