Why?
Many Internet carriers/providers are using large headers on IP packets. When these packets are transmitted they are fragmented (1 becomes 2). This fragmentation has two end results – slower Internet speeds and VPN tunnel failures. Identifying and modifying your computer to create packets of the optimal size will have positive results in both regards. Following the instructions below will determine the maximum transmission unit (MTU) for your system. The default MTU in Windows is 1500 when speeds are over 128kbps (kilobits) where the MTU for dialup is 576 (re: below 128kbps). It is remotely possible your MTU will be forced to 576 by a router somewhere between your connection and your desired destination.
The whole concept behind broadband is speed, therefore anything that reduces speed is bad, well not acceptable. When your PC routes a packet larger than the MTU of your broadband provider and any router between, it is fragmented into two, and possibly more packets. This reduces speed and can play havoc with VPN connections – no connects, drops, etc. Determining and adjusting the MTU on your PC can provide you with the speed that you are paying for while eliminating a common problem with VPN connectivity and stability.
VPN Consideration:
When using a VPN Client to connect to a VPN host where both are using DSL and/or Cable Modems, you MUST run this test from both sides of the connection. Do this regardless of using the same DSL/Cable modem provider. Adjust the MTU on the Client side to the hosting VPN MTU value ONLY IF IT IS SMALLER. Changing the MTU on the newer Firewall/VPN appliances may require guidance from the manufacturer, but it must be done.
OS Covered:
Windows 95/98
Windows ME
Windows NT
Windows 2000
Preparation:
Make sure that you are connected to the Internet.
Open a Command Window in Windows 95/98/ME:
Click on Start > Run > type in "command" > Ok.
Open a Command Window in Windows NT/2000:
Click on Start > Run > type in "cmd" > Ok.
Testing to Determine the Optimal MTU:
In the command window type in:
PING 4.2.2.1 and press ENTER key
If you see "Request timed out" then your DNS is not functioning
or you are not connected to the Internet. Abandon this process
until you have it resolved!
PING –f –l 1492 Provider Site> and press ENTER key
[If doing VPN, use the IP address of the VPN Host in place of the Internet Provider Site above. Since it is unlikely that your route will be static through the Internet, you may want to repeat the test number of times at different times of the day.]
If you see "Packet needs to be fragmented but DF set." then
reduce the value by 8 until you no longer get this message. Once
you get an accepted value try adding 2 to the value and repeating
the test until the message above reappears. Retest at the last
accepted value to confirm. Write it down.
See in this forum the " How do I set the Maximum Transmission Unit (MTU) value in the workstation manually?" for specific instructions on how to modify your PC to use this value.
Author’s Note: This document represents a best effort on the part of the author to provide assistance in a “how-to” desert. It is submitted on the basis of “It worked for me.” It may not work for the reader. Use these guidelines at your own risk. If you are not technically adept, then you are advised to seek professional technical assistance.
"Whether you believe you can or you believe you cannot, either way you are right." Henry Ford
Many Internet carriers/providers are using large headers on IP packets. When these packets are transmitted they are fragmented (1 becomes 2). This fragmentation has two end results – slower Internet speeds and VPN tunnel failures. Identifying and modifying your computer to create packets of the optimal size will have positive results in both regards. Following the instructions below will determine the maximum transmission unit (MTU) for your system. The default MTU in Windows is 1500 when speeds are over 128kbps (kilobits) where the MTU for dialup is 576 (re: below 128kbps). It is remotely possible your MTU will be forced to 576 by a router somewhere between your connection and your desired destination.
The whole concept behind broadband is speed, therefore anything that reduces speed is bad, well not acceptable. When your PC routes a packet larger than the MTU of your broadband provider and any router between, it is fragmented into two, and possibly more packets. This reduces speed and can play havoc with VPN connections – no connects, drops, etc. Determining and adjusting the MTU on your PC can provide you with the speed that you are paying for while eliminating a common problem with VPN connectivity and stability.
VPN Consideration:
When using a VPN Client to connect to a VPN host where both are using DSL and/or Cable Modems, you MUST run this test from both sides of the connection. Do this regardless of using the same DSL/Cable modem provider. Adjust the MTU on the Client side to the hosting VPN MTU value ONLY IF IT IS SMALLER. Changing the MTU on the newer Firewall/VPN appliances may require guidance from the manufacturer, but it must be done.
OS Covered:
Windows 95/98
Windows ME
Windows NT
Windows 2000
Preparation:
Make sure that you are connected to the Internet.
Open a Command Window in Windows 95/98/ME:
Click on Start > Run > type in "command" > Ok.
Open a Command Window in Windows NT/2000:
Click on Start > Run > type in "cmd" > Ok.
Testing to Determine the Optimal MTU:
In the command window type in:
PING 4.2.2.1 and press ENTER key
If you see "Request timed out" then your DNS is not functioning
or you are not connected to the Internet. Abandon this process
until you have it resolved!
PING –f –l 1492 Provider Site> and press ENTER key
[If doing VPN, use the IP address of the VPN Host in place of the Internet Provider Site above. Since it is unlikely that your route will be static through the Internet, you may want to repeat the test number of times at different times of the day.]
If you see "Packet needs to be fragmented but DF set." then
reduce the value by 8 until you no longer get this message. Once
you get an accepted value try adding 2 to the value and repeating
the test until the message above reappears. Retest at the last
accepted value to confirm. Write it down.
See in this forum the " How do I set the Maximum Transmission Unit (MTU) value in the workstation manually?" for specific instructions on how to modify your PC to use this value.
Author’s Note: This document represents a best effort on the part of the author to provide assistance in a “how-to” desert. It is submitted on the basis of “It worked for me.” It may not work for the reader. Use these guidelines at your own risk. If you are not technically adept, then you are advised to seek professional technical assistance.
"Whether you believe you can or you believe you cannot, either way you are right." Henry Ford