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.
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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:
Value Name: TcpMaxDataRetransmissions
Data Type: REG_DWORD - Number
Valid Range: 0 - 0xFFFFFFFF
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:
Value Name: TCPInitialRtt
Data Type: REG_DWORD
Valid Range: 300-65535 (milliseconds in decimal)
Default: 0xBB8 (3000 milliseconds expressed in hexadecimal)
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:
Value Name: InitialRttData Type: REG_DWORDValid Range: 0-65535 (decimal)Default: 0xBB8 (3000 decimal)
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.
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