[Vpn-help] SSH connection hang with beta 2

Matthew Grooms mgrooms at shrew.net
Tue May 8 00:59:01 CDT 2007

Tai-hwa Liang wrote:
> On Sun, 6 May 2007, Matthew Grooms wrote:
> [...]
>> With the mss clamping enabled on the firewall, you shouldn't 
>> experience any hangs at all due to fragmentation issues with tcp 
>> communications. I will do some testing and try to reproduce the issue.
>   Thank you.


I ran a battery of tests with all the combinations I could think of to 
reproduce the problem but was unsuccessful. What operating system are 
you using on the client side? Are you performing the tests from a cable 
or dsl modem over the internet? If so, is there a firewall/router being 
used between the client and the gateway?

My gut feeling is that esp packet fragments are not getting back to the 
client for re-assembly. Using a "find /" over an ssh connection makes me 
think that the bulk of the traffic would be emitted from the server 
destined to the ssh client. There wouldn't be much sent back to the 
server with the exception of the ssh protocol window adjustments. I 
don't think that kind of packet would ever get big enough to warrant 
fragmentation. Maybe we can look at some packet dumps to determine for 
sure in what direction the tcp stall is happening.



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