RAS Uses Fixed TCP/IP MTU Size

This article was previously published under Q183229
This article has been archived. It is offered "as is" and will no longer be updated.

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
When you run a program over a Remote Access Service (RAS) connection, youmay 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 stopresponding (hang) when you attempt to send new messages.
This behavior can occur because Windows NT RAS uses a fixed size MaximumTransmission Unit (MTU) of 1500.
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 articlein 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 allowsyou to edit the MTU for a RAS connection and another that allows you toedit 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 thefollowing registry key:
  2. Create the following two new data values using the followinginformation:
    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.
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.
Using the EnablePMTUDiscovery and/or EnablePMTUBHDetect registry values will have no effect because the TCP/IP MTU for a RAS connection is afixed value.

For additional information on TCP/IP, please see the following white paperavailable 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 ImplementationDetails TCP/IP Protocol Stack and Services, Version 2.0"
rras steelhead

Article ID: 183229 - Last Review: 02/21/2014 00:28:58 - Revision: 1.1

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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