A. By default, NT will use a Maximum Transmission Unit (MTU) (packet size) over the path to a remote host of 576. Problems can arise if the data is sent over routes etc that cannot handle data of this size and the packets get fragmented.

The parameter EnablePMTUDiscovery set to 1 forces NT to discover the maximum MTU of all connections that are not on the local subnet. To change this perform the following:

  1. Start the registry editor (regedit.exe)
  2. Move to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
  3. From the Edit menu select New-DWord value
  4. Enter a name of EnablePMTUDiscovery and press enter
  5. Double click on this new value and set to 1 then click OK
  6. Close the registry editor and reboot the machine.

By discovering the Path MTU and limiting TCP segments to this size, TCP can eliminate fragmentation at routers along the path that connect networks with different MTUs. Fragmentation adversely affects TCP throughput and network congestion.