Path maximum transmission unit (PMTU) discovery is the process of discovering the maximum size of packet that can be sent across the network between two hosts without fragmentation (that is, without the packet being broken into multiple frames during transmission). By discovering the path MTU (PMTU) and limiting TCP segments to this size, TCP can eliminate fragmentation at routers along the path that connects networks with different MTUs. Fragmentation adversely affects TCP throughput and network congestion.
Path MTU Discovery is enabled by default in Windows 2000/XP/2003/2008/Vista. However, there are instances like problems with DSL connections when you need to disable the Path MTU discovery this can be done from the Windows Registry.
This applies to Windows Server 2008, Windows Vista, Windows XP, Windows Server 2003 (if enabled) and Windows 2000.
The following procedure to edit the Windows Registry should help you disable the Path MTU Discovery.
1. Click Start - Search and type "regedit". This launches the Windows Registry.
2. In the registry, navigate to the following registry key
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters]
HKEY_LOCAL_MACHINE
\SYSTEM
\CurrentControlSet
\Services
\Tcpip
\Parameters
3. In the right pane, right-click and select New - DWORD and set its name as "EnablePMTUDiscovery" and set its value as "0". Setting "0" will disable Path MTU Discovery. This can impact performance as Windows will revert to an MTU of 576 bytes per packet for packets to destinations hosts that are not within the same subnet unless you manually set the MTU in your Windows.
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