Setting The MTU In Windows Vista

Long ago, when I first heard of the dynamic tuning in the Windows Vista TCP/IP stack, I envisioned the manual setting of the static MTU to be a thing of the past. Unfortunately, I was wrong - the MTU is still a fixed setting, in Vista.

The legendary tool, DrTCP, which is used by everybody to change the MTU in Windows 2000 and XP, doesn't work under Windows Vista. Fortunately, Microsoft now allows us to adjust the MTU using the "netsh" command. As other commands in Windows Vista, you run "netsh" using the command window, in Administrative mode.

To see what interfaces you have on your computer, type

netsh interface ipv4 show subinterfaces

To change the MTU, type

netsh interface ipv4 set subinterface "Local Area Connection" mtu=nnnn store=persistent

where

Local Area Connection is the name of the network connection on your computer, from the list obtained above.
nnnn is the desired value for MTU.

Reboot after making the change.

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Determining The MTU To A Single Server

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Internet Connectivity Problems Caused By The MTU Setting

The messages sent and received between your computer, and the Internet web servers that you're accessing, may go thru dozens of networks. The Internet is, by design, dynamic. The networks that you use, to access any server, may change within seconds.

Any one of those networks might have a restriction on the maximum message (packet) size that it will accept. Each computer has a setting, called the Maximum Transmission Unit (MTU), which controls how large it may make any packet. The larger your packets, the fewer packets required for sending or receiving a web page, but the greater chance any network have a problem with your packet size.

Setting the MTU on your computer can be a double edged sword.

• If you make the MTU too large, some networks will split (fragment) your packets. Some servers may have a problem with fragmented packets, causing the dreaded "Server not available..." error, or other symptoms.
  
• If you make the MTU too small, your computer will send and receive small packets. You'll be able to access any server, thru any network, but a web page will require too many packets. The speed that your web pages download will make you think you're not connected at all.
  

If you have a problem accessing some websites (or running some programs like email or IM), but not others, or if this problem seems to come and go, you may have an MTU setting problem. The best known examples of this problem are those with dial-up or PPPoE sevice, or those using ICS. An MTU issue can affect anybody, though, and different people (computers) will, almost certainly, be affected differently, with different web sites being unreachable at different times.

Here are several articles, of differing technical detail and level, discussing the issue further, and offering ways to diagnose and correct it. Read through each one, until you find one that explains the issue to your liking. This list, like each of the ones that I create, is alphabetised.

• Allen Smith: ICS Web-browsing Problem: the Real Solution.
• Annoyances: Can't access some websites while using a shared PPPoE connection.
• Microsoft: (KB319661): Connectivity Problems on ICS Clients When You Use a PPPoE Connection on a Windows XP ICS Host.
• Microsoft: (KB314825): How to Troubleshoot Black Hole Router Issues.
• NetHeaven: Sites With Broken / Working Path MTU Discovery
• Nitecruzr Dot Net: The MTU Setting Problem - Why Is It So Obscure?

When you get ready to adjust the MTU setting, make it easy on yourself. Download DrTCP, from DSLReports, and use it to make the changes for you. Simply copy the downloaded file into any convenient folder, and run it from there.

You'll be changing the "MaxMTU" or "MTU" value under Adapter Settings. If you have multiple network adapters, be sure to choose the one that provides the Internet service. For instructions about what values to change MTU to, see the articles linked above. Read all 6, and pick the one that you're most comfortable with.

Note: An MTU problem can be confused with, or masked by, a DNS problem, or LSP / Winsock Corruption. If you're here after trying the above procedures, unsuccessfully, consider each of the latter possibilities.

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