VNC dies and requires reconnection every few minutes (Win NT - AT T Extranet - Cable Modem)

Chris Jaecker cjaecker "at"
Thu, 24 Jan 2002 11:43:42 +0000

A shot in the dark here, but it may be that you've got an MTU problem.

Both your client and server are connected to Ethernet so they're assuming 
1500 bytes of payload in the frame. But, you're going through some kind of 
security filter, which may be adding bytes to each packet.

If the packets are leaving as 1500 and the security process adds say 60 
bytes, then the payload is too large for the frame, so it get's fragmented, 
doubling the number of packets and putting the overhead of packet 
reassembly onto the client, and creating a congestive collapse at layer 3 
(because tcp will order a retransmit if it doesn't get it's acknowledgement 
in time). Hence the blank/dead screen.

To test for this try sending out pings with 1500 mtu and df set (client to 
server or vice versa). If it is the problem, try this:

The solution was to reduce the MTU of the VNC host. This can be done in the 
registry permanently with:


which should be set to 1400

The NT server can also be forced to discover the MTU of a path by setting:


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.

The other possibility is if you have any kind of NAT or PAT running and 
it's aging out the ports too quickly. I don't think this is likely unless 
the TCP "hello"s are too widely spaced. You'd fix this on the router.

Hope this is useful

Chris Jaecker
At 06:00  24/01/02 +0000, you wrote:
>Date: Wed, 23 Jan 2002 09:51:56 -0500
>From: mpatient "at"
>Subject: VNC dies and requires reconnection every few minutes (Win NT - AT 
>T Extranet - Cable Modem)
>When in a VNC viewer session, after a few minutes my viewer cursor detaches
>from my host cursor, indicating that I have lost the session.
>I am unable to type anything on the host display. My connection appears to
>still be on, and I can often start another viewer, type in the password, be
>connected, and resume mouseing and typing right where I left off. This
>happens frequently during most sessions.
> > The host is a PC running Windows NT and VNC is loaded during boot-up.
> > The viewer is a PC running Windows NT.
> > I am working through a cable modem connected to Time Warner Communications
>(Road Runner).
> > I am using AT&T's Extranet security interface.
> > I have 2 PCs connected to a Linksys 5-port Workgroup Hub, which is
>connected to the TWC Cable Modem
>Note: When VNC dies, I am not loosing the AT&T Extranet connection and AT&T
>technical support says "it's not them".
>??? Do anyone know about this behavior and what might be causing it ???
>Mike Patient
>Solution Engineer
> > *     Email : mpatient "at"
> > *     Phone:          407.805.1558
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