From owner-freebsd-net@freebsd.org  Fri Aug 28 19:14:27 2015
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Date: Fri, 28 Aug 2015 12:14:25 -0700
From: Chris Stankevitz <chris@stankevitz.com>
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To: freebsd-net@freebsd.org
Subject: Re: ssh over WAN: TCP window too small
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On 8/25/15 3:47 PM, Chris Stankevitz wrote:
> Can anyone explain my abysmally small TCP window?

So I believe this is the story:

1. openssh limits the size of some outgoing buffer to 65KB

2. openssh/HPN tries to improve on this by increasing the size of the 
outgoing buffer to match getsockopt(SO_SNDBUF)

3. When asked for the current SO_SNDBUF, FreeBSD 10.1 reports the high 
watermark of the outgoing buffer, not its capacity.

4. (2) is essentially a no-op because of (3).

5. openssh/HPN can be tricked into increasing its outgoing buffer by 
increasing sendspace/recvspace

My comments:

- (3) is not what I would expect -- perhaps the ssh/HPN folks would 
agree with me.  Shouldn't getsockopt(SO_SNDBUF) return the same value 
set by setsockopt(SO_SNDBUF)?

- I do not understand the mechanism by which (5) works.

Chris