ssh over WAN: TCP window too small

[Bjoern A. Zeeb]

> On 25 Aug 2015, at 22:47 , Chris Stankevitz <chris at stankevitz.com> wrote:
> 
> Hi,
> 
> # 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?

if you have the memory, try these sysctls:

kern.ipc.maxsockbuf=146800640
net.inet.tcp.recvbuf_max=67108864
net.inet.tcp.sendbuf_max=67108864