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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.