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Q183229: RAS Uses Fixed TCP/IP MTU Size

Article: Q183229
Product(s): Microsoft Windows NT
Version(s): 4.0
Operating System(s): 
Keyword(s): _IK12469
Last Modified: 13-JUN-2002

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The information in this article applies to:

- Microsoft Windows NT Server version 4.0 
- Microsoft Windows NT Workstation version 4.0 
- Microsoft Windows NT Server, Enterprise Edition version 4.0 
- Microsoft Routing and Remote Access Service Update for Windows NT Server version 4.0 
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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:

  Q256986 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:

  Q152734 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:

  Q163251 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:

1. Start Registry Editor (Regedt32.exe or Regedit.exe) and locate the following
  registry key:

  HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\NdisWan\Parameters

2. 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.

3. 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

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Keywords          : _IK12469 
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT400search kbWinNTSsearch kbWinNTSEntSearch kbWinNTSEnt400 kbWinNTS400search kbWinNTS400 kbAudDeveloper kbRRASNTSearch kbRRASNT400
Version           : :4.0
Issue type        : kbbug
Solution Type     : kbfix

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