JSI Tip 5182. Windows XP issues VPN connection error 800, 'Unable to establish connection'?

When you try to establish a VPN connection, you receive:

Error 800: Unable to establish connection.

This error can be the result of having outdated firmware on a router.

To see if this is the problem, perform a network trace. If the router advertises a 0 window size in the TCP handshake on port 1723, update the firmware.