Is disabled by default. This specifies whether the stack will be able to detect Maximum Transmission Unit (MTU) routers that do not send back ICMP (Internet Message Protocol) fragmentation-needed messages. With this feature enabled, TCP will try to send segments without the Don’t Fragment bit set if several re-transmissions of a segment go unacknowledged. If the segment is acknowledged as a result, the MTU will be decreased and the Don’t Fragment bit will be set in future packets on the connection.
 

Enabling black hole detection increases the maximum number of re-transmissions performed for a given segment. Setting this parameter when it is not needed can cause performance degradation. However; if the router is not sending back the ICMP messages, then PMTU Discovery will not work. This can cause even greater performance degradation.