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How can I modify TCP retransmission timeout?

A. Service Pack 5 adds a new registry entry, InitialRtt, which allows the retransmission time to be modified. The range is 0 - 65535 milliseconds and can be set as follows:

  1. Start the registry editor (regedit.exe)
  2. Move to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
  3. From the Edit menu select New - DWORD value
  4. Enter a name of InitialRtt and press Enter
  5. Double click the new value and set to the number of milliseconds for the timeout, e.g. 5000 for 5 seconds (the old default was 3 seconds). Click OK
  6. Close the registry editor
  7. Restart the machine for the change to take effect

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.

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).

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