SuSE Linux Enterprise Server OpenSSH 5.1p1 nagle issue?

Jeremy Guthrie jeremy.guthrie at
Thu Oct 18 06:14:27 EST 2012

Correction, when looking at debug, I see:
debug2: channel 0: request shell confirm 1
debug2: fd 3 setting TCP_NODELAY
debug2: callback done

Nagle isn't our issue.

Any other ideas on what might be causing the different behavior out of 

On 10/17/12 2:11 PM, Jeremy Guthrie wrote:
> I have a system in place where it appears that TCP will make a massive 
> change in behavior mid-stream with existing SSH sessions. We noticed 
> the issue first with an application using an SSH forward.  However, we 
> were able to rule that out by generating the same TCP characteristics 
> by having a perl script dump text out to a terminal simulating a large 
> data flow from the far end(ssh server) back to us(ssh client).
> The issue manifests roughly as follows:
> 1.  Generate a bunch of terminal output(500k)
> 2.  Sleep 15 seconds
> 3.  Go back to step 1
> After repeating steps 1-3 for some random amount of time(sometimes 3 
> minutes, sometimes 50+), the SSH server will go from streaming the 
> output back to the client @ 4-4.5 mbps(normal-behavior.png), down to 
> 30-40kbps(bad-behavior.png).  Most of the time, SSH stays in this 
> 30-40kbps state for as long as their is data in the TCP queue.  ie.  
> during peaks, netstat will show the queue having 90-100k of data 
> waiting to be transmitted.
> We think that Nagle may be taking effect randomly for some reason.  
> When I 'strace -f ssh user at hostname', I don't see the TCP_NODELAY flag 
> being set so that could certainly be true.  I look in the ssh docs and 
> I don't see anything about NoDelay but there use to be something 
> according to O'Reilly docs.  When I examine the source code, it looks 
> like setting the TCP_NODELAY is some kind of default.
> The odd thing is that I have hundreds of boxes running this same 
> release of software and no one else is exhibiting this issue.
> Does anyone have any ideas?
> -- 
> *Jeremy Guthrie*

*Jeremy Guthrie*
Technical Architect - Orchestration | *CDW*
5520 Research Park | Madison, WI 53711
Phone: 608.298.1061 | Fax: 608.288.3007 | NOC: 608.298.1102 | Toll Free: 

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