How to Modify the TCP/IP Maximum Retransmission Timeout

ID: Q170359

The information in this article applies to:

Microsoft Windows NT Workstation version 4.0
Microsoft Windows NT Server version 4.0
Microsoft Windows 2000 Professional
Microsoft Windows 2000 Server
Microsoft Windows 2000 Advanced Server

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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, then the segment is retransmitted, up to the TcpMaxDataRetransmissions times. 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 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. Using this algorithm, TCP tunes itself to the normal delay of a connection. TCP connections over high-delay links will take much longer to time out than those over low- delay links.

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

MORE INFORMATION

Windows NT provides a mechanism to control the initial retransmit time, and then the retransmit time is self-tuning. 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).
To change the initial retransmit time, modify the following values in the following registry key:

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 timeout is doubled with each successive retransmission on a connection. It is reset when responses resume. The base timeout value is dynamically determined by the measured round-trip time on the connection.

Value Name:  InitialRtt
Data Type:   REG_DWORD
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.

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

For additional information about retransmit time, click the article numbers below to view the articles in the Microsoft Knowledge Base:

Q232512 TCP/IP may Retransmit Packets Prematurely

Q223450 TCP Initial Retransmission Timer Adjustment Added to Windows NT

For additional information, search the Web for RFC 793 (Section 3.7) TCP Protocol Specification.

Additional query words: lpr lpd lpdsvc tcpip print

Keywords : kbnetwork nttcp NTSrvWkst
Version : WINDOWS:2000; winnt:4.0
Platform : WINDOWS winnt
Issue type :