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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.
•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.
•Due to additional overhead, some VPNs may require an MTU that is much smaller than the results of the ping test. The best way to eliminate MTU as a possible source of the problem is to lower the MTU to 1200 and test the VPN connection. If it functions correctly at 1200 increase the MTU until the VPN stops working properly and use the largest size MTU that did work. Additionally, some VPNs create separate network adapters so be sure to select the correct adapter when changing MTU size.
• The built in PPPoE client for Windows XP uses an MTU that can not easily be changed or set manually. The MTU is set to 1480. For more information please reference this XP MTU article. This only applies if you are running the built in XP PPPoE client!
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:
Change your MTU using DrTCP (see example below) or any similar registry editing application. 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 that can be received before it must send an acknowledgement and broadcasts that 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.
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 (possibly 1452 or 1460) shown in the left column of the results. Note: All RWIN values should be an even multiple of your MSS.
Change your RWIN to the highest recommended setting (roughly 37,000) using DrTCP (see example below) 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 in order for the settings to take place.
Retest your download speeds and record any changes.
Retest again using the next higher or lower RWIN value until you find your optimum speeds. Note: When trying different RWIN values make sure you use even multiples of your MSS.
x 26 Even Multiple
The next lower RWIN value would be:
x 24 Even Multiple
Changing the MTU and RWIN values
You can download and use DrTCP or any similar registry editing application to change your MTU. An example of changing the MTU using DrTCP is shown below. (Figure 4)
Pictures by Andy Houtz
Note: If you have more than one network adapter on a PC you must change the MTU on the correct network adapter associated with your broadband connection. You must reboot your computer in order for the new MTU settings to take place. Additionally, if you have a network with more than one PC, all computers and the router must have the same MTU setting. Please reference the links below to learn how to change the MTU on some popular routers:
•MTU change on a Linksys Router
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