Date: Thu, 27 Aug 2015 09:14:31 -0400 From: Kurt Lidl <lidl@pix.net> To: freebsd-net@freebsd.org Subject: Re: ssh over WAN: TCP window too small Message-ID: <55DF0D37.5060003@pix.net> In-Reply-To: <55DCF080.7080208@stankevitz.com> References: <55DCF080.7080208@stankevitz.com>
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Chris Stankevitz 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? I know this response is a little late to the party, but... I spent a bit of time last year tuning my FreeBSD 10.1 host to be able to transfer a bunch of data between the east coast of the US and the west cost. My WAN link was more like 70ms @ 75 Mbps, so not too different than yours. The other end of the connection was also a FreeBSD 10.1 host. I have the following in my /etc/sysctl.conf - and I get pretty much all 75Mbps when I scp or rsync a file: # tcp options for long-haul speedups kern.ipc.maxsockbuf=4194304 # (2 * default 2097152) net.inet.tcp.mssdflt=1448 # (default 576) net.inet.tcp.sendbuf_max=4194304 # (2 * default 2097152) net.inet.tcp.recvbuf_max=4194304 # (2 * default 2097152) net.inet.tcp.syncache.rexmtlimit=1 # (default 3) net.inet.tcp.recvspace=262144 # (4 * default 65,536) net.inet.tcp.sendspace=262144 # (4 * default 65,536) net.inet.tcp.sendbuf_inc=65536 # (8 * default 8192) net.inet.tcp.recvbuf_inc=131072 # (8 * default 16384) One thing that was noticed - it can take a really, really, really long time for the TCP window to open up the whole way with the default net.inet.tcp.sendbuf_inc setting! -Kurt
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