ssh over WAN: TCP window too small

[Mark Martinec]

Chris Stankevitz wrote:

> # cat /dev/urandom | ssh root at host 'cat > /dev/null'
> 
> I use the above ssh command over a high-BDP WAN link (80 ms @ 100 
> Mbps).
> tcpdump shows I am TCP window limited to 64 KBytes (yielding 5 Mbps).
> iperf with default options gets the window opened to 500 KBytes 
> (yielding 35 Mbps).
> 
> Both sides of the connection: FreeBSD 10.1 w/default sshd options
> (except I permit root login).  In particular, HPN is not disabled.
> 
> Can anyone explain my abysmally small TCP window?
> 
> Can anyone recommend some tools/tricks to figure out what in FreeBSD
> and/or base SSH is limiting the send/recv buffer and/or TCP window?

As an alternative to ssh for copying large files across high-BDP WAN 
links
consider sysutils/bbcp, optionally coupled with security/hpenc for 
encryption.

  Mark