> Well the topic says it all:
> % netstat -p tcp
> <snip>
> 	        387 PMTUD blackholes detected
> I found this, but it means little to me..
> http://www.google.com/search?q=pmtud+netstat+netbsd
> 	[ 3rd link down ]

	if the node is performing PMTUD (DF bit is 1) and TCP retransmission
	timer went off, we try turning PMTUD off (DF bit will be 0 beyond that).

	/*
	 * If we are losing and we are trying path MTU discovery,
	 * try turning it off.  This will avoid black holes in
	 * the network which suppress or fail to send "packet
	 * too big" ICMP messages.  We should ideally do
	 * lots more sophisticated searching to find the right
	 * value here...
	 */

itojun