NFS via VPN stuck after a certain amount of transfer

[Yusuke Shinyama]

Hello,

I'm testing NFS via VPN on openssh-4.3p2 and experienced
occational glitches.  When I tried to copy a huge directory,
sometimes the VPN connection was stuck and the tun interface
stopped responding.  The underlying network connection was still
alive.  When I quit the client and reconnect, it starts
working again.

More concretely, I made a connection from a Linux box
to a FreeBSD box via a cable modem. Both are running openssh-4.3p2
client/server and enabled for point-to-point tunneling.
Then I mounted the remote directory via NFS with tcp and intr option.
Everything worked fine so far. Then I tried to
"cp -Rp /mnt/remote/linux-2.4.31 /local".
After the directory is partially transferred, the cp process
stopped and ping to the remote host (via tun) didn't respond.
But this only happens occationaly. For smaller directories, 
the copy finishes without any problem.

I've also tried the same thing with -v -v option for the client
and seen the following output:

...
debug2: channel 1: window 64688 sent adjust 66384
debug2: channel 1: window 64936 sent adjust 66136
debug2: channel 1: window 64272 sent adjust 66800
debug2: channel 1: window 64100 sent adjust 66972
debug2: channel 1: rcvd adjust 247216

I noticed this "rcvd" number that is ocasinally displayed kept growing.
But I'm not sure if this is a problem of ssh/sshd or NFS.
How to diagnose this? I am happy to do further testing.

Thank you very much,
Yusuke