[Tfug] scp transfers stalling

[John Gruenenfelder]

On Sat, Aug 02, 2008 at 12:48:58PM -0700, Robert Hunter wrote:
>On Fri, Aug 01, 2008 at 05:02:19PM -0700, John Gruenenfelder wrote:
>> A bit more about the network which may, perhaps, be relevant:
>
>Some cursory searching reveals quite a bit of discussion about stalled
>ssh sessions, and these discussions seem to fall into two categories.
>Stalled sessions due to inactivity, and stalled sessions during large

Yes, I also found a number of pages on the Net about these.  Most of them,
however, seem to be about SSH disconnects from inactivity.  I've experienced
this too on multiple servers; it seems to occur most often when using a
wireless link.  The *AliveInterval settings you mention below seem to fix that
issue (usually).

Those disconnects, though, are usually just that: disconnects.  You get a
message from your client that the connection was dropped.  The SSH "drops" I'm
getting don't actually disconnect.  It just goes out to lunch and never comes
back.  I have to give it a tilde-. combo to kill it manually.

>file transfers, although I know not if your problem falls into either
>of these categories.  Incidentally, the ssh_config and sshd_config
>manpages talk about various options (e.g.,
>{Client,Server}AliveInterval) that may have some relevance to your
>situation.

My bigger concern are these file transfer stalls.  However, they do not seem
to be strictly linked to large files.  That is, receiving five 500K files is
as likely to trigger the problem as is receiving one 20MB file.  And given the
bandwidth involved here, the stall can occur right away... like 3-4 seconds
into the transfer, or sometimes much further into the transfer of a large
file.

>Furthermore, have you tried running the server in debug mode?  To do
>this, I would launch an alternate instance of the server on a
>different port, so as not to interfere with remote access to the
>machine.  You may also want to do all this in a screen session, so
>that your session persists in the event you need to reconnect.

Ahhh... this is a good idea.  I've tried client in extra verbose mode without
any luck.  I'll try it with the server instead... maybe get some better info.
Thanks!

>You may also attempt to trace the session from the client with a
>protocol analyzer, like Wireshark.  Since you can somewhat reliably
>reproduce the stalling problem, it should be instructive to see what
>is happening to the connection at the packet level.

I had thought of this, but right now I am very remote to the machine in
question and I can't really run WireShark (when did Ethereal change its
name?).  But, I'll be back in Tucson in about a month.  If the problem
persists to that point, I'll try it then.


-- 
--John Gruenenfelder    Research Assistant, UMass Amherst student
                        Systems Manager, MKS Imaging Technology, LLC.
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