Setting your RWIN and MTU settings in Windows 2000 for maximum speed

Situation:

You get timeouts or no connection when browsing websites or generally connecting to other things.

Problem:

Win9X,  NT4,  XP,  and Windows2000 may not have the correct RWIN and MTU settings.

Solution:

Easy Way:     

Go to http://www.dslreports.com/tweeks,  run the test.  Note the recommendations.   Download the DRTCP.exe from http://www.dslreports.com/front/drtcp.html.  Run the program and input the recommendations.

For more detail on using drtcp, read my Using DRTCP document.

Hard Way:     

Using a windows PC,  ping -f -l packet_size IP_address.  The -f switch means don't fragment the packet.  Start with a packet size of 1500 and work your way backwarks to 0.  Generally,  1472 is the answer.  If your packets are fragmenting,  you'll get "Packet needs to be fragmented but DF set."  When you reach the correct packet size,  you'll see a normal ping response.  At this point, assuming DRTCP didn't work, you'll have to edit the registry and place a mtu entry under the correct interface.  My Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces.  On machines where the interface in question is not the first listed in the registry, drtcp will fail.  You can briefly look at the 3 or 4 interface entries and see which one has an IP assigned to it.  That's the one you want.

Using a linux pc, ping -s packet_size -M do IP_address.  Same procedure as above.

You can temporarily make this change with ifconfig eth0 mtu 1472

You can make these changes permanent by adding MTU=1472 to the /etc/sysconfig/network-scripts/ifcfg-eth0 file.  (or ifcfg-eth1, etc).

Also make sure your adminstator has not blocked ICMP packets at the firewall or from your workstation. This causes the MTU Path Discovery to fail.

References:

http://www.dslreports.com/faq/578

Last updated March 12, 2006.

Jeff Borders