on PR 12790, I posted a simple patch for TCP PMTUD blackhole detection.
	if there's no ack for a while, the patch will do the following:
	- turn off path MTU discovery for IPv4
	- uses minimum MTU (1280) for IPv6
	the behavior will remedy path MTU discovery blackhole, if our side
	is the sender of bigger packet.  it does not solve problems when
	the other side is sending bigger packet and choking with the lack
	of icmp too big messages.
	see RFC2923 section 2.1 for the exact description of the issue.

	not sure if we should do more tricky thing, like
	- gradually decrease packet size until we get an ACK
	- try to re-do path MTU discovery.

	i have no environment to test, so i need your help.

	http://www.NetBSD.org/cgi-bin/query-pr-single.pl?number=12790
	(note: the patch has a typo in tcp_output.c - paren mismatch)

itojun