ssh ethernet tunnel performance problem

[Guennadi Liakhovetski]

Hi Darren

Thanks for your reply.

On Fri, 10 May 2013, Darren Tucker wrote:

> On Thu, May 09, 2013 at 06:23:24PM +0200, Guennadi Liakhovetski wrote:
> > Hi
> > 
> > (I'm not subscribed, please, cc)
> > 
> > I've got an ethernet level ssh tunnel to a remote system. That system is 
> > running a terminal server, so, that tunnel is the only connection, I've 
> > got. The terminal server is connected via a serial and an ethernet cable 
> > to a test board and is configured to bridge that ethernet connection to my 
> > tunnel. I can then trigger that test board to start a TFTP or NFS data 
> > transfer over the tunnel from my system. The whole should look like this:
> > 
> > [my client] --- <internet> --- [terminal] === <serial cable> === [test ]
> >                                [ server ] ----- <ethernet> ----- [board]
> > 
> > (whereas I don't know whether the terminal server actually has 2 ethernet 
> > interfaces or only 1. I think, it should really have 2 at least, because 
> > actually it's serving 8 test boards, I'll try to clarify its 
> > configuration and post an update)
> > 
> > This works in principle, and from a "geographically close" location also 
> > runs reasonably quickly. Whereas from my half-world distance I'm getting 
> > like 1.5kBps tftp and a bit more with NFS.
> 
> Based on the version and presumably latency I'd guess the reason is
> the small default channel window size in older versions of OpenSSH.
> You fill the channel window up and since you have a high-latency are
> then blocked until it drains.  That'd explain why it works ok when
> you're close.
> 
> The channel window size used to be (I think) 64k but was changed to
> 2M several years ago.  You can check this by running ssh -vvv and look
> for this:
> 
> debug2: channel 0: rcvd adjust 2097152

I see it on the client side, which is much older than the server, so, it 
should be ok on the server too.

> (You should also see the same thing on the server side)
> 
> > The person, who's set up the 
> > server says, that he previously also had performance problems with an 
> > older Fedora distro, but with Fedora 18 it runs fast now. I've tried with 
> > Debian squeeze with a self-build 3.9.1 and (about 1-2 year old) Ubuntu 
> 
> You mean OpenSSH 3.9p1?  That's pretty old and before the channel window
> changes.  Can you reproduce the problem with a current version (6.2p1?)

No, sorry, I meant Linux kernel 3.9.1 :)

Do you think the problem can be somewhere below ssh - in the OS networking 
layer? Although I tested 3 OSes in the meantime, the latter one was Fedora 
18 - the newest - with the same result.

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/