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IP Packet Sizing - MTU

How to find max packet size for DSL, Cable, etc. by nocum
Posted: 3 Jun 02 (Edited 11 Jun 02)

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 û 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 û 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.

As of June 11, 2002 I have witnessed extreme variations in the MTU with my DSL connection.  It appears that it is due to the routing of my packets.  I have elected to adopt 576 as my MTU as a "safe harbor" setting as a result.  This FAQ will updated to reflect the resulting performance.

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

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 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 ûf ûl 1492 Your Internet Provider's URL 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 to set the Maximum Transmission Unit (MTU) manually"  for specific instructions on how to modify your PC to use this value.

AuthorÆs Note:  This document represents a best effort on the part of the author to provide assistance in a ôhow-toö desert.  It is submitted on the basis of ôIt worked for me.ö  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.  If this FAQ helps one professional keep his job or a client, then it has been worth the time.  I wasn't so lucky.

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