TCP Initial Retransmission Timer Adjustment Added to Windows NT 4.0

When you establish TCP connections between computers across high-delay networks, connection requests and data may be retransmitted unnecessarily. This can result in extra congestion and slow response.

The initial TCP retransmission timer was hard-coded to three seconds.

Windows NT Server or Workstation 4.0

To resolve this problem, obtain the latest service pack for Windows NT 4.0 or the individual software update. For information on obtaining the latest service pack, please go to:

• http://www.microsoft.com/downloads/details.aspx?familyid=E396D059-E402-46EF-B095-A74399E25737&displaylang=en/ (http://www.microsoft.com/Windows/ServicePacks/) -or-
  
  
• 152734  (http://support.microsoft.com/kb/152734/EN-US/ ) how to obtain the latest windows nt 4.0 service pack

For information on obtaining the individual software update, contact Microsoft Product Support Services. For a complete list of Microsoft Product Support Services phone numbers and information on support costs, please go to the following address on the World Wide Web:
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.

A new registry parameter has been added to TCP to allow the initial retransmission timer to be adjusted. This parameter may be useful for high-delay networks, where the round-trip time often exceeds three seconds.

This parameter controls the retransmission time-out of the initial TCP connection request (SYN) and of the first data sent on a connection. It is a global value, which applies to all interfaces in the computer.

To adjust the InitialRtt parameter, perform the following steps:

1. Start Registry Editor (Regedt32.exe).
2. Locate the following key in the registry:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
3. On the Edit menu, click Add Value, and then add the following registry value:
   Value Name: InitialRtt
   Data Type: REG_DWORD
   Value: 5000 decimal (for example, to set it to 5 seconds)
   Valid Range: 0-65535 (decimal)
   Default: 0xBB8 (3000 decimal)
   Description: This parameter controls the initial retransmission timeout used by TCP on each new connection. It applies to the connection request (SYN) and to the first data segment(s) sent on each connection.
4. Quit Registry Editor.
5. Restart the computer.

Windows NT Server 4.0, Terminal Server Edition

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

Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article. This problem was first corrected in Windows NT Server version 4.0, Terminal Server Edition Service Pack 6.

Care should be used when adjusting this value. Setting it to large values will dramatically increase the amount of time that it takes for a TCP connection attempt to fail, if the target IP address does not exist.

For instance, the default value is 3,000, or 3 seconds. By default, a connection request is retried 2 times. The total time-out is (3+6+12) seconds, or 21 seconds.

If this registry value is set to 6,000 (6 seconds), the total timeout will be (6+12+24) seconds, or 42 seconds. During this time, an application can appear to stop responding (hang).