A PMTU black hole router is a routing device which does not return ICMP Destination Unreachable messages when it needs to fragment an IPv4 packet that has the Don’t Fragment (DF) flag set. TCP depends on receiving these messages to perform PMTU discovery. With this feature enabled, TCP tries to send segments without the Don’t Fragment flag set if several retransmissions of a segment go unacknowledged. If the segment is then acknowledged, the TCP maximum segment size (MSS) for the connection is decreased and the Don’t Fragment flag is set for subsequent segments. Enabling PMTU black hole detection increases the maximum number of retransmissions that are performed for a given segment.
This procedure applies to Windows Server 2008, Windows Vista, Windows XP, Windows Server 2003 and Windows 2000.
The following procedure to edit the Windows Registry should help you disable the Path MTU Blackhole detection (this is enabled by default and you may not find the key discussed below).
1. Click Start - Search and type “regedit”. This launches the WIndows Registry.
2. In the registry, navigate to the following registry key
3. In the right pane, right-click and select New – DWORD (32-bit value) and set its name as “EnablePMTUBHDetect” and set its value as “0″. Setting “0″ will disable Path MTU blackhole router detection. Should you like to enable this later at a later time, set it “1″ or simply delete the this key.