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MTU (Maximum Transfer Unit)
MTU is the maximum packet size (in bytes) that can be transported reliably across any particular network; IP Ethernet in this case. The maximum size of an IP Ethernet packet is 1500, but overhead like IP, TCP, and PPPoE must also be taken into account. The wrong MTU will actually prevent you from accessing some web sites or FTP sites. It may even cause you to not authenticate properly on mail servers or secure web sites. There are too many variables to be able to estimate what your optimum MTU should be, but there is an test that can determine your correct MTU. Although this simple test is accurate for testing end points, many users may find that a lower MTU may be better for their particular circumstances.
•Due to additional complications, VPNs require a different MTU. Please refer to the VPN MTU FAQ.
•If you have a network with multiple PCs every computer should be set up with the same MTU. Additionally, some PCs may use several Network Adapters or a VPN client adapter on one PC so you must verify you are changing the Network Adapter associated with your broadband service or VPN client.
• The built in PPPoE client for Windows XP uses an MTU that is set to 1480. The MTU For more information please reference this XP MTU article. This only applies if you are running the built in XP PPPoE client!
• Windows Vista has an auto-tuning function that automatically adjusts your settings, so tweaking is not possible. You can disable auto-tuning for troubleshooting connectivity issues. Click here to find out how.
Although the Broadband Reports Tweak Test offers very good general information, it may give erroneous or misleading MTU recommendations and its information should not to used. Since it simply looks at whether you use PPPoA or PPPoE, it can not give accurate, individualized MTU analysis. To guarantee the proper MTU customers should use the following procedure:
You can download DrTCP here (see example below) or find a similar registry editing application to easily change and adjust your MTU. Remember, you must change the MTU on the correct network adapter (if you have more than one) and you must reboot your machine in order for the settings to take place.
RWIN (Receive Window)
The TCP Receive Window can be thought of as the main data gate keeper to your computer. It sets the limits on the amount of data (in bytes) that can be received and buffered before it must send an acknowledgement to the sender. If the number is too low you will tie up bandwidth with frequent and unnecessary acknowledgements. Too high of an RWIN will create a slowdown if any packets need to be retransmitted. Helpful hint: When you increase your bandwidth you can also increase your RWIN.
There are mathematical calculations that can be used to obtain your optimum RWIN, but they are complicated and rely on variables that can easily change. I suggest using real-life testing with some guidelines from the Tweak Test:
Record the "MSS Requested" number (usually 1452 or 1460 due to default settings) shown in the left column of the results. Note: All RWIN values should be an even multiple of your MSS.
Use the following recommendations as a basic starting point for RWIN settings based on your service.
FastAccess Extreme 3.0 and 6.0 = 63888
FastAccess Ultra = 52272
FastAccess Lite = 52272
Change your RWIN to the value recommended above using DrTCP or similar registry editing application. Remember to change the RWIN on the correct network adapter (if you have more than one) and reboot your machine.
Retest your download speeds and record any changes.
Retest again using a higher RWIN value until you find your optimum speeds. When trying different RWIN values make sure you use EVEN multiples of your MSS.
For example: A standard FastAccess DSL connection could use:
x 34 Even Multiple
The next higher even multiple is 24 so the RWIN value would be:
x 36 Even Multiple
Changing the MTU and RWIN values
You can download DrTCP here or any similar registry editing application to change your MTU and RWIN. An example of changing the MTU using DrTCP is shown below.
Note: There may be more than one network adapter showing in the pull down menu for your PC. You must make sure you change the MTU and RWIN on the correct network adapter associated with your broadband connection or VPN client. You must also reboot your computer in order for the new settings to take place. Additionally, if you have a network with more than one PC, all computers should be tweaked.
Andy Houtz DSL
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