Fragmentation and Performance Issues with PPTP Connections

• Microsoft Windows NT Workstation version 4.0
• Microsoft Windows NT Server version 4.0
• Microsoft Routing and Remote Access Service Update for Windows NT Server 4.0

IMPORTANT: This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs. For information about how to do this, view the "Restoring the Registry" Help topic in Regedit.exe or the "Restoring a Registry Key" Help topic in Regedt32.exe.

SYMPTOMS

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

RESOLUTION

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.

This fix should have the following time stamp(s):

   11/05/97  04:44p                57,360  Ndiswan.sys  (Intel)
   11/05/97  04:44p                57,744  Ndiswand.sys (Intel)
   10/22/97  01:26p               140,560  Tcpip.sys    (Intel)

   11/05/97  04:41p               103,248  Ndiswan.sys  (Alpha)
   11/05/97  04:41p               103,984  Ndiswand.sys (Alpha)
   10/22/97  01:23p               258,416  Tcpip.sys    (Alpha)

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\Configuration
   

   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.

STATUS

Microsoft has confirmed this to be a problem in Windows NT version 4.0. A supported fix is now available, but has not been fully regression tested and should be applied only to systems experiencing this specific problem. Unless you are severely impacted by this specific problem, Microsoft recommends that you wait for the next Service Pack that contains this fix. Contact Microsoft Technical Support for more information.