Microsoft KB Archive/183229

= RAS Uses Fixed TCP/IP MTU Size =

Article ID: 183229

Article Last Modified on 10/31/2006

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APPLIES TO


 * Microsoft Windows NT Server 4.0 Standard Edition
 * Microsoft Windows NT Workstation 4.0 Developer Edition
 * Microsoft Windows NT Server 4.0 Enterprise Edition
 * Microsoft Routing and Remote Access Service Update for Windows NT Server 4.0

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This article was previously published under Q183229



IMPORTANT: This article contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, click the following article number to view the article in the Microsoft Knowledge Base:

256986 Description of the Microsoft Windows Registry



SYMPTOMS
When you run a program over a Remote Access Service (RAS) connection, you may encounter one of the following symptoms:
 * You are able to download data normally but you are not always able to upload data successfully.
 * Your Internet mail or news reading programs may time out or stop responding (hang) when you attempt to send new messages.



CAUSE
This behavior can occur because Windows NT RAS uses a fixed size Maximum Transmission Unit (MTU) of 1500.



RESOLUTION
WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

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

152734 How to Obtain the Latest Windows NT 4.0 Service Pack

NOTE: Your computer must run Windows NT 4.0 Service Pack 3 before you apply this fix. This hotfix can also be applied to the Routing and Remote Access Service Upgrade.

This hotfix also includes the fix referred to in the following article in the Microsoft Knowledge Base:

163251 STOP 0xA Due to Buffer Overflow in Ndiswan.sys

There are two new registry keys to address this issue, one that allows you to edit the MTU for a RAS connection and another that allows you to edit the MTU for a Point-to-Point Tunneling Protocol (PPTP) connection. To edit the registry:  Start Registry Editor (Regedt32.exe or Regedit.exe) and locate the following registry key:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\NdisWan\Parameters

 Create the following two new data values using the following information:

Value name: IPMTU

Data type : REG_DWORD

Data range: 1 - 1500 (default is 1500)

Value name: TunnelMTU

Data type : REG_DWORD

Data range: 1 - 1500 (default is 1400)

NOTE: Setting these values to the smallest MTU of your data path will prevent fragmentation. Restart the computer.



STATUS
Microsoft has confirmed this to be a problem in Windows NT version 4.0. This problem was first corrected in Windows NT 4.0 Service Pack 4.



MORE INFORMATION
Using the EnablePMTUDiscovery and/or EnablePMTUBHDetect registry values will have no effect because the TCP/IP MTU for a RAS connection is a fixed value.

For additional information on TCP/IP, please see the following white paper available on the Microsoft ftp server:

File Name: Tcpipimp2.doc

Location: ftp://ftp.microsoft.com/bussys/winnt/winnt-docs/papers/

Title: "Microsoft Windows NT 3.5/3.51/4.0: TCP/IP Implementation Details TCP/IP Protocol Stack and Services, Version 2.0"

Additional query words: rras steelhead

Keywords: kbbug kbfix KB183229

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