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Article ID: 170359 - Last Review: September 18, 2013 - Revision: 8.1

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Summary

TCP starts a retransmission timer when each outbound segment is handed down to IP. If no acknowledgment has been received for the data in a given segment before the timer expires, the segment is retransmitted, up to the TcpMaxDataRetransmissions value. The default value for this parameter is 5.

The retransmission timer is initialized to three seconds when a TCP connection is established. However, it is adjusted on the fly to match the characteristics of the connection by using Smoothed Round Trip Time (SRTT) calculations as described in RFC793. The timer for a given segment is doubled after each retransmission of that segment. By using this algorithm, TCP tunes itself to the normal delay of a connection. TCP connections that are made over high-delay links take much longer to time out than those that are made over low-delay links.

By default, after the retransmission timer hits 240 seconds, it uses that value for retransmission of any segment that has to be retransmitted. This can cause long delays for a client to time-out on a slow link.

For more information about the latest service pack for Windows 2000, click the following article number to view the article in the Microsoft Knowledge Base:
260910  (http://support.microsoft.com/kb/260910/ ) How to obtain the latest Windows 2000 service pack

More information

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756  (http://support.microsoft.com/kb/322756/ ) How to back up and restore the registry in Windows

The TcpMaxDataRetransmissions registry value controls the number of times that TCP retransmits an individual data segment before it aborts the connection. This value is not configured by default, but it can be entered to change the default number of retries.

Change the following subkey in Windows 7, Windows 2008 R2, Windows 2008, Windows 2000, Windows Vista, Windows 2003, and Windows XP:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters
Value Name:  TcpMaxDataRetransmissions
Data Type:   REG_DWORD - Number
Valid Range: 0 - 0xFFFFFFFF
Default:     5
					
Description: This parameter controls the number of times TCP retransmits an individual data segment (non connect segment) before aborting the connection. The retransmission time-out is doubled with each successive retransmission on a connection. It is reset when responses resume. The base time-out value is dynamically determined by the measured round-trip time on the connection.

Windows provides a mechanism to control the initial retransmit time, and the retransmit time is then dynamically self-tuned. To change the initial retransmit time, modify the following registry values.

Change the following subkey in Windows 2003, Windows XP, and Windows 2000:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\ID for Adapter
Value Name:  TCPInitialRtt
Data Type:   REG_DWORD
Valid Range: 300-65535 (milliseconds in decimal)
Default:     0xBB8 (3000 milliseconds expressed in hexadecimal)
					
Description: This parameter controls the initial retransmission time-out that is used by TCP on each new connection. It applies to the connection request (SYN) and to the first data segments that is sent on each connection. For example, the value data of "5000 decimal" sets the initial retransmit time to five seconds.

NOTE: You can increase the value only for the initial time-out. Decreasing the value is not supported.

Change the following key in Windows NT 4.0:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters
Value Name:  InitialRttData Type:   REG_DWORDValid Range: 0-65535 (decimal)Default:     0xBB8 (3000 decimal)
Description: This parameter controls the initial retransmission time-out 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.

For example, the value data of "5000 decimal" sets the initial retransmit time to five seconds.

The Initial RTO in Windows Server 2008 R2 and Windows 7 is can be controlled by using the NetSH command by initialRTO.

For prerequisites and more information, click the following article number to view the article in the Microsoft Knowledge Base:
2472264  (http://support.microsoft.com/kb/2472264/ ) You cannot customize some TCP configurations by using the netsh command in Windows Server 2008 R2

For more information about retransmit time, click the following article numbers to view the articles in the Microsoft Knowledge Base:
232512  (http://support.microsoft.com/kb/232512/ ) TCP/IP may retransmit packets prematurely
223450  (http://support.microsoft.com/kb/223450/ ) TCP Initial retransmission timer adjustment added to Windows NT
For more information, search the web for "RFC 793 (Section 3.7) TCP Protocol Specification."

Applies to
  • Microsoft Windows 2000 Server
  • Microsoft Windows 2000 Advanced Server
  • Microsoft Windows 2000 Professional Edition
  • Microsoft Windows NT Workstation 4.0 Developer Edition
  • Microsoft Windows NT Server 4.0 Standard Edition
  • Windows 7 Professional
  • Windows 7 Ultimate
  • Windows 7 Enterprise
  • Windows Server 2008 R2 Datacenter
  • Windows Server 2008 R2 Enterprise
  • Windows Server 2008 R2 Service Pack 1
  • Windows Server 2008 Datacenter
  • Windows Server 2008 Enterprise
  • Windows Server 2008 Service Pack 2
  • Windows Vista Enterprise
  • Windows Vista Service Pack 1
  • Windows Vista Service Pack 2
  • Windows Vista Ultimate
  • Windows Vista Business
  • Microsoft Windows Server 2003 R2 Datacenter Edition (32-Bit x86)
  • Microsoft Windows Server 2003 R2 Datacenter x64 Edition
  • Microsoft Windows Server 2003 Service Pack 1
  • Microsoft Windows Server 2003 R2 Enterprise x64 Edition
  • Microsoft Windows Server 2003, Enterprise x64 Edition
  • Microsoft Windows Server 2003 Service Pack 2
  • Microsoft Windows XP Professional
  • Microsoft Windows XP Service Pack 3
  • Microsoft Windows XP Service Pack 1
  • Microsoft Windows XP Service Pack 2
Keywords: 
kbhotfixserver kbqfe kbinfo kbnetwork kbwin2000sp3fix KB170359
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