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Path Maximum Transmission Unit (PMTU) Black Hole Router Detection
http://www.vistaarticles.com/articles/187/1/Path-Maximum-Transmission-Unit-%28PMTU%29-Black-Hole-Router-Detection
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Published on 12/9/2006
 
These changes are also included in Microsoft Windows Vista.

Path Maximum Transmission Unit (PMTU) Black Hole Router Detection

The following is a list of black hole router detection changes that are included in Windows Server 2003 SP2:

By default, black hole detection is turned on. 
Black hole probing is performed only for connections in established or advanced states. 
Black hole probing is performed only for connections on which there is no full-size segment acknowledgement. 
If black hole probing fails, the TCP Maximum Segment Size (MSS) of the connection is restored to what it used to be before probing. 
With black hole router detection turned off in versions of Windows that are earlier than Windows Server 2003 SP2, a connection times out approximately after 63 * RTT. In Windows Server 2003 SP2, connections time out after 71 * RTT.