FreeBSD Window updates

[David Malone]

I've got an example extract tcpdump of this at the end of the mail
- here 6 ACKs are sent, 5 of which are pure window updates and
several are 2us apart!

I think the easy option is to delete the code that generates explicit
window updates if the window moves by 2*MSS. We then should be doing
something similar to Linux. The other easy alternative would be to
add a sysclt that lets us generate an window update every N*MSS and
by default set it to something big, like 10 or 100. That should
effectively eliminate the updates during bulk data transfer, but
may still generate some window updates after a loss.

Normal ACKing for driving congestion control shouldn't be impacted
by either of these suggested changes.

	David.

1227622713.276609 172.16.2.2.5002 > 172.16.1.51.39077: . ack 144097745 win 40798 <nop,nop,timestamp 5365425 5103763> (DF)
1227622713.276611 172.16.2.2.5002 > 172.16.1.51.39077: . ack 144097745 win 40830 <nop,nop,timestamp 5365425 5103763> (DF)
1227622713.276613 172.16.2.2.5002 > 172.16.1.51.39077: . ack 144097745 win 40862 <nop,nop,timestamp 5365425 5103763> (DF)
1227622713.276615 172.16.2.2.5002 > 172.16.1.51.39077: . ack 144097745 win 40894 <nop,nop,timestamp 5365425 5103763> (DF)
1227622713.276852 172.16.2.2.5002 > 172.16.1.51.39077: . ack 144097745 win 40926 <nop,nop,timestamp 5365425 5103763> (DF)
1227622713.276855 172.16.2.2.5002 > 172.16.1.51.39077: . ack 144097745 win 40958 <nop,nop,timestamp 5365425 5103763> (DF)
1227622713.296585 172.16.2.2.5002 > 172.16.1.51.39077: . ack 144100641 win 40956 <nop,nop,timestamp 5365445 5103766> (DF)