Hi,
I wrote in October with an issue regarding VNC connecting but "hanging at
the "Please wait, initial screen loading" prompt. Although it was suggested
this might be because of virus checking software, in my case the problem
was that I was connecting to the VNC host through a VPN tunnel.
My topology is as follows:
VNC
Viewer:---------CiscoRouter-------Internet--------CiscoRouter---------VNC
Host (NT 4.0 Server)
<-----------VPN Tunnel----------->
The VPN tunnel has an MTU of 1500 bytes, as do all interfaces on all
devices. However, because the VNC IP packets originated from the VNC viewer
and hosts were built for 1500 byte MTU, they must have been fragmented by
the Cisco Routers, in order to accommodate the IPSec headers for transport
through the Internet over the VPN tunnel (the access to the internet was
usually over ISDN, which defaults to a 1500 MTU).
The solution was to reduce the MTU of the VNC host. This can be done in the
registry permanently with:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\<adapter>\Tcpip\Paramet
ers\MTU
which should be set to 1400
The NT server can also be forced to discover the MTU of a path by setting:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters\Enable
PMTUBHDetect
which is a REG_DWORD value. Setting it to 1 forces the NT server to
discover the MTU of the path to the target system (in this case the VNC
viewer). If you set the value to 0, it forces the MTU down to 576, and
doesn't attempt to discover the MTU at all.
Diagnosing the problem in the first place was done by sending pings of
various sizes with the DF bit set to true.
Having made this change, I have had no issues with connectivity through
Internet VPN tunnels, and have run tunnel connections from Singapore and
Taiwan to Sydney.
Hope this will help others facing similar problems.
Chris Jaecker
______________________________________________________
Chris Jaecker, Director Illustrated Networks
tel: +61 (0)2 9418 4226 [EMAIL PROTECTED]
fax: +61 (0)2 9403 1188 www.in-training.net
______________________________________________________
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