tcp keepalive strangeness

[Marco S Hyman <marc@snafu.org>]

A while back I noted that when I use

 $ ssh -n mc-w xterm -geometry 80x36 &

the session will go away in about 2 hours of idle time.   Looking at
what goes on over the wire is interesting.   It also points to a keepalive
strangeness.   Note: both sides are running -current as of about 5 days
ago.

It takes about 3 seconds for the session to settle down after the session
is started.   7200 seconds later keepalive packets are sent by both sides.
However, the checksums on the keepalives a bad!

17:39:12.961140 mc.wireless.snafu.org.ssh > hana.wireless.snafu.org.43165:
  . [bad tcp cksum 6fd1!] ack 16581 win 17376 (DF) (ttl 64, id 29883)
17:39:13.263984 hana.wireless.snafu.org.43165 > mc.wireless.snafu.org.ssh:
  . [bad tcp cksum 6fd1!] ack 7918 win 17376 (ttl 64, id 21183)

Both sides are sending keepalives with bad checksums.

// marc