Fragmentation and Performance Issues with PPTP Connections

Performance degradation may occur when you use Point-to-Point Tunneling Protocol (PPTP) connections.

To resolve this problem, obtain the latest service pack for Windows NT 4.0 or Windows NT Server 4.0, Terminal Server Edition. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

This hotfix does the following:

• Corrects an issue in TCP/IP that occasionally generates packets larger than 1,500 bytes.
• Corrects a resource allocation issue in TCP/IP that causes the stack (under heavy load) to drop forwarded packets instead of allocating more memory.
• Changes the MTU for IP connections over NDISWan down to 1,400. This helps to keep from fragmenting when you are connecting with PPTP directly across a LAN.

If your PPTP connection is exhibiting any of these types of behavior, please obtain the following fix or wait for the next Windows NT service pack.

NOTE: Ndiswan.sys is the 40-bit version and Ndiswand.sys is the 128-bit version of this file. If you are using 128-bit encryption, you will need to install Ndiswand.sys and rename it to Ndiswan.sys to retain your encryption.

After you apply the fix, shut down and restart your computer.

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

For information about how to edit the registry, view the "Changing Keys And Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it.

Registry Change

The registry change below can be used to increase performance on a reliable network. If the network is unreliable or congested, this setting should be set to the default value; otherwise, set this value to 8.

NOTE: This registry change is not a requirement for the hotfix itself, but should be used to gain performance under reliable conditions.

1. Run Registry Editor (Regedt32.exe).
2. Go to the following key:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Raspptpe \Parameters\Configurati
   
   NOTE: The above registry key is one path; it has been wrapped for readability.
3. Click Add Value on the Edit menu.
4. Add the following value:
   Value Name: RxPacketWindow
   Data Type: REG_DWORD
   Data: 2 (default)
   
   Set the data value to 8.
5. Click OK and then quit Registry Editor.
6. Shut down and restart Windows NT.

Microsoft has confirmed that this is a problem in Windows NT 4.0 and Windows NT Server 4.0, Terminal Server Edition. This problem was first corrected in Windows NT 4.0 Service Pack 4.0 and Windows NT Server 4.0, Terminal Server Edition Service Pack 4.