HOW TO fix VPN connection problems.
VPN connections differ in the protocol which they use to tunnel IP packets through the Internet. Prior to Windows 2000, all Microsoft VPNs used the Point-to-Point Tunnelling Protocol PPTP, but several other tunnelling protocols are now in use, including L2TP and IPSec.
NAT32 fully supports PPTP-based and L2TP-based VPN connections, but IPSec-based connections are only supported if the VPN Server accepts Pass Through Mode connections (which is often not the case).
Irrespective of the type of VPN connection, the following changes to the Microsoft TCP/IP stack on your machine take effect for the duration of the connection:
The following problem occurs in Windows 9X VPNs:
The mechanism works fine for IP packets, but it does not work for the GRE Packet type used to transfer data over PPTP VPNs.
A fix for this problem is to modify the IPMTU registry variable at the following location:
Locate each Net key which contains the string "Dial-Up Adapter" in its DriverDesc variable. Then set the IPMTU variable for each such adapter to 1400 bytes and reboot Windows.
The main symptoms of this problem are that Web pages fail to load completely, or that Outlook does not start correctly, or that Outlook starts but can send and receive only short Emails.
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