RAS Uses Fixed TCP/IP MTU Size

ID: Q183229

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 & Remote Access Service Update for Windows NT Server version 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

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

To resolve this problem, obtain the latest service pack for Windows NT version 4.0. For additional information, please see the following article(s) in the Microsoft Knowledge Base:

Q152734 How to Obtain the Latest Windows NT 4.0 Service Pack

NOTE: Service Pack 3 must be applied to Windows NT 4.0 prior to applying 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

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. If you are running Windows NT, you should also update your Emergency Repair Disk (ERD).

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.

Start Registry Editor (Regedt32.exe or Regedit.exe) and locate the following registry key:
HKEY_LOCAL_MACHINE\System\CCS\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 from occurring.

And then 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

NOTE: 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 : ntras kbbug4.00 kbfix4.00 kbRAS nttcp NTSrvWkst
Version : winnt:4.0
Platform : winnt
Issue type : kbbug