From owner-freebsd-net@freebsd.org  Tue Aug 25 23:11:38 2015
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Subject: Re: ssh over WAN: TCP window too small
From: "Bjoern A. Zeeb" <bzeeb-lists@lists.zabbadoz.net>
In-Reply-To: <55DCF080.7080208@stankevitz.com>
Date: Tue, 25 Aug 2015 23:11:06 +0000
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To: Chris Stankevitz <chris@stankevitz.com>
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> On 25 Aug 2015, at 22:47 , Chris Stankevitz <chris@stankevitz.com> =
wrote:
>=20
> Hi,
>=20
> # cat /dev/urandom | ssh root@host 'cat > /dev/null'
>=20
> 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).
>=20
> Both sides of the connection: FreeBSD 10.1 w/default sshd options =
(except I permit root login).  In particular, HPN is not disabled.
>=20
> Can anyone explain my abysmally small TCP window?
>=20
> 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=3D146800640
net.inet.tcp.recvbuf_max=3D67108864
net.inet.tcp.sendbuf_max=3D67108864=