Windows Vista MTU for Wireless (English Version)

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Scenario

On Wednesday January 30th, I networked a Laptop PC running Windows Vista Home Premium Edition, with a Desktop PC running Windows XP Pro SP2 through a DSL Router with a 4-port integrated Switch and Wireless capability. The Desktop PC is connected to the Router with Ethernet Category 5 cable and the Laptop PC is connected Wirelessly to the DSL Router. Using the Router as Gateway, both machines share access to the Internet. They also offer File Sharing services to each other in a Peer-to-Peer (Workgroup) manner. The XP PC can “see” the files shared from the Vista PC as it should be, but when I tried to access the XP PC shared files from the Vista PC, I observed the following phenomenon.

Symptoms

I was able to browse quick and easy folders with four or five items in them (no matter if the items were exclusively files, folders or mix), but when I tried to browse folders with a great number of subfolders and files in them, after a delay of 30 seconds to a minute or so, I received the message “The specified network name is no longer available”.

Change from Wireless to Cable

Then I disconnected the Vista PC from the Wireless Network and plugged it directly to a Switch Port in the Router using a Category 5 Ethernet cable just like the one I used to wire the XP PC, and noticed that the previous described situation ceased to exist. In plain English, access to all XP shares performs with expected speed, no matter how many files or subfolders each share has, and with no error messages displayed.

Try another Laptop PC with other Windows Version

The next step was to try with a Laptop PC which would be connected wirelessly but with another version of windows. So, I tried with a Laptop running XP. Guess what? Everything worked fine.

Other changes that didn’t have any effect at all

Nothing happened when I changed the Wireless Channel on the Router, or Encryption Type from WPA to WEP and vice-versa, or the number of encryption bits (64bit, 128bit), or even changing the Router Firmware, and so on. These changes could make sense if the above issue had a “stable attitude”. Like if I couldn’t “see” any files or folders at all, or couldn’t copy files, or I was “Denied Access” while setting User Accounts and Share-NTFS Permissions properly, etc.

Theoretical Explanation

The cause of this unusual behavior in the current situation is a number called MTU (Maximum Transmission Unit). Theoretically speaking, going down (or up) in the TCP/IP Stack, the Link-Layer is responsible for setting this number to the correct value. Practically, it is the network adapter driver who sets the correct number. So, in accordance to the network medium (or network topology in other terms), like say Ethernet, FDDI, Token Ring etc., there is a default MTU value which must be met or network communication could become unreliable, unstable (like our case), poor performing or impossible.

Solution Procedure

In the Vista PC open a command prompt window with administrative privileges. That is right-click the Command Prompt icon (which in Windows Vista resides in the Accessories folder) and select Run As Administrator. Then type in the following command

netsh interface ipv4 show subinterfaces

Windows Vista displays all network interfaces with their respective values in columns starting with MTU. Now, notice that “Wireless Network Connection” has a MTU value of 1500. “Local Area Connection” has also 1500. But, 1500 is the default MTU value for Ethernet not Wireless. Wireless should be 1492 not 1500 because it is a different network medium.

So to set the correct MTU, we issue the following command

netsh interface ipv4 set subinterface “Wireless Network Connection” mtu=1492 store=persistent

Be Careful: Inside the ” ” you must type the exact name of the wireless network interface.

After this, we “run” once again the command

netsh interface ipv4 show subinterfaces

so to confirm that the change is done. We will be able to see Wireless MTU value 1492 instead of 1500 and…

… file browsing from Windows Vista through the wireless connection now works like a beauty.

Further Information

Information about default MTU values for different network mediums or topologies concerning Microsoft Windows Systems you can find at Microsoft Official Support Knowledge Base Article 314496:

http://support.microsoft.com/kb/314496