BOOL __stdcall SetMTUDecrement ( DWORD dwMTUDecrement );
TRUE if call was successful,
System-wide MTU decrement value to set.
Some network applications need to insert additional headers into the packet. The examples of these applications are IP in IP tunneling, IPSEC based VPNs etc. This function sets the parameter in the registry key which the filter driver reads during its initialization and subtracts this value from the real MTU when MSTCP requests it from the network, so this API call requires a system reboot to take effect.