Sometimes change is good, sometimes it's bad, and sometimes it's
a mixed bag. Such was the case recently when a customer switched
a remote office from a 56K line to using a Kerio® VPN over Internet connections. The 56K line
was slow and expensive, so the switch had immediate benefits. Unfortunately
a problem also appeared immediately: an Okidata printer couldn't
print more than six or seven pages at a time.
The printer was set up as a shared printer hanging off a PC. As
they had a spare HPJetDirect printserver hanging about, my first
thought was to throw that in, but nothing changed: after six or seven
pages, the printer would still stop.
My next thought was that this might be a MTU problem. I've seen that now and then
with VPN's and also with
printers, so it was worth a shot. As there were other branch
offices not experiencing this problem, I felt the problem had to
be at the office with the troublesome printer, so I went there to
You can set the MTU of a Windows NIC with a registry key: it's down under
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces. The MTU key may not be there, but you can add it to the correct card under
interfaces. An easier way is to download "DR TCP" from
and use that to make your tweaks (thanks to Jeff Wadlow from Kerio for
pointing that out). I started experimenting, but nothing changed:
the printer would still stop when sent a large enough job.
However, while experimenting, I noticed a curious thing: if
I pressed the Select button on the printer to set it off-line, and
then pressed it again to set it back on-line, printing would
immediately resume with no loss of data. This told me that there was
nothing wrong with data flowing through the VPN, and indeed the Kerio
logs had already confirmed that: the VPN logs showed no dropped packets
The Okidata printer has a 128K buffer. That's more than six or seven
pages, but it might be close to its high water mark: the flow
control signal gets sent before the buffer is absolutely full to
avoid lost data. So what it now looked like to me was that the
printer was sending a flow control "stop" signal but the other
end was never seeing a "start" after the buffer was bled down. When
I cycled the Select switch, a "start" was being sent, and the printing
would resume. Probably the 56K connection was slow enough that
the buffer never would fill up, so this was never observed
Now the question is "whose fault is it?". It's most likely the
printer, but it could be the print server. As having a spare printer
isn't a bad idea, I suggested trying that first. They've ordered one,
so we'll know soon.
The other complaint at this office was that telnet connections
would disconnect after ten or fifteen minutes of inactivity. My
suspicion there was a low timeout setting in their emulation
software because TCP timeouts would ordinarily be much
longer than that. However, I wasn't able to observe this behavior on site: I waited
45 minutes and there was no problem. I noticed that their app displays
the current time in its menus; I asked the people on site to
note what it says as they walk up to the machine: that will tell
me how long it actually takes to lose the connection and may give
a clue as to what is actually happening.
Turns out it was about 40 minutes; I must have noted the time
wrong and been just under. The 40 minutes is expected, Kerio
says: "WinRoute will discard TCP packets after forty minutes of inactivity.
If this is the problem then this problem might be solved by either
increasing the TCP timeout or by activating a keep alive feature in
the telnet client so that the inactivity timeout is repeatedly reset."
Kerio support provided this:
1. Stop the WinRoute engine
2. Go to C:\Program Files\Kerio\WinRoute Firewall
3. Edit the winroute.cfg file with notepad
4. Search for DefaultTcpTimeout
5. Set DefaultTcpTimeout to a higher value
6. Save the winroute.cfg file
7. Restart the WinRoute engine
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