Sep 172008
 

A few days ago, we saw how to Enable/Disable Path MTU Discovery (PMTU Discovery) from the registry. Now, we shall go further and see how to Enable or Disable Path MTU Blackhole router detection in Windows 2008/Vista/2003/XP/2000 from Windows Registry. Path MTU Blackhole router detection is enabled by default so as the Path MTU Discovery.

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.

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Feb 192008
 

Maximum Transmission Unit (MTU) in simple words is the maximum IP packet size in bytes, that can be transmitted over the underlying network. While most of time this never has to be changed from the default values there are instances when you need to alter the MTU sizes to fix certain network performance issues. A best and classic example would be with a Windows Exchange Server being accessed over an Internet VPN setup. This in most of the cases demonstrate the problem with MTUs and related issues.

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