Description of the Receive Window Auto-Tuning feature for HTTP traffic on Windows Vista-based computers

Windows Vista includes the Receive Window Auto-Tuning (http://technet.microsoft.com/en-us/magazine/2007.01.cableguy.aspx) feature which improves performance for programs that receive TCP data over a network. However, this feature is disabled by default for programs that use the Windows HTTP Services (WinHTTP) (http://msdn.microsoft.com/en-us/library/aa382925(VS.85).aspx) interface. Some examples of programs that use WinHTTP include Automatic Updates, Windows Update, Remote Desktop Connection, Windows Explorer (network file copy), and Sharepoint (WebDAV).

If you enable Receive Window Auto-Tuning for WinHTTP traffic, data transfers over the network may be more efficient. However, in some cases you might experience slower data transfers or loss of connectivity if your network uses an older router and firewall that does not support this feature. For example, when you use Windows Internet Explorer to access applications that are hosted in Microsoft Office SharePoint Server, the HTTP traffic may slow down. This occurs because certain routers do not support the Receive Window Auto-Tuning feature.

Note Receive Window Auto-Tuning is not available for programs that use the Windows Internet (WinINet) (http://msdn.microsoft.com/en-us/library/aa385331(VS.85).aspx) application programming interface (API) for HTTP requests instead of WinHTTP. Some examples of programs that use WinINet for HTTP traffic include Internet Explorer, Outlook, and Outlook Express.

This article describes the following:

• How the Receive Window Auto-Tuning feature improves data transfer
• How to enable the Receive Window Auto-Tuning feature for WinHTTP traffic
• Issues that may occur after you enable the Receive Window Auto-Tuning feature for WinHTTP traffic
• How to disable the Receive Window Auto-Tuning feature

The Receive Window Auto-Tuning feature lets the operating system continually monitor routing conditions such as bandwidth, network delay, and application delay. Therefore, the operating system can configure connections by scaling the TCP receive window to maximize the network performance. To determine the optimal receive window size, the Receive Window Auto-Tuning feature measures the products that delay bandwidth and the application retrieve rates. Then, the Receive Window Auto-Tuning feature adapts the receive window size of the ongoing transmission to take advantage of any unused bandwidth.

Prerequisites

You must be running Windows Vista Service Pack 1 or have hotfix 939006  (http://support.microsoft.com/kb/939006/LN/ ) installed to enable auto-tuning for WinHTTP.

To have us enable auto-tuning for you, go to the "Fix it for me" section. If you would rather enable auto-tuning yourself, go to the "Let me fix it myself" section.

Fix it for me

To enable auto-tuning automatically, click the Fix this problem link. Then, click Run in the File Download dialog box, and follow the steps in this wizard.

Fix this problem
Microsoft Fix it 50097

Note this wizard may be in English only; however, the automatic fix also works for other language versions of Windows.

Note If you are not on the computer that has the problem, you can save the automatic fix to a flash drive or to a CD, and then you can run it on the computer that has the problem.

Now go to the "Did this fix the problem?" section.

Let me fix it myself

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base: To enable the Receive Window Auto-Tuning feature for HTTP traffic on Windows Vista-based client computers, you must edit the registry. To do this, follow these steps:

1. Click Start, type regedit in the Start Search box, and then press ENTER.
2. Locate and then right-click the following registry subkey:
   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Internet Settings\WinHttp
3. Point to New, and then click DWORD Value.
4. Type TcpAutotuning, and then press ENTER.
5. Right-click TcpAutotuning, and then click Modify.
6. In the Value data box, type 1, and then click OK.
7. Exit Registry Editor.
8. Restart the computer.

Did this fix the problem?

• Check whether the problem is fixed. If the problem is fixed, you are finished with this article. If the problem is not fixed, you can contact support (http://support.microsoft.com/contactus) .
• We would appreciate your feedback. To provide feedback or to report any issues with this solution, please leave a comment on the "Fix it for me" blog (http://blogs.technet.com/fixit4me/) , or send us an e-mail (mailto:fixit4me@microsoft.com?Subject=KB947239 - EnableTCP auto-tuning) message.

The Receive Window Auto-Tuning feature is enabled for HTTP traffic if the TcpAutotuning registry entry is set to 1. The Receive Window Auto-Tuning feature is not enabled for HTTP traffic if the TcpAutotuning registry entry does not exist or if it is set to a value that is not 1.

When the Receive Window Auto-Tuning feature is enabled for HTTP traffic, older routers, older firewalls, and older operating systems that are incompatible with the Receive Window Auto-Tuning feature may sometimes cause slow data transfer or a loss of connectivity. When this occurs, users may experience slow performance. Or, the applications may crash. These older devices do not comply with the RFC 1323 standard. Some device manufacturers provide software that works around the hardware limitations. Contact the device manufacturer to determine whether this kind of software is available.

If the incompatible devices are outside your organization, and you cannot change the devices, this issue will remain. Therefore, you may have to disable the Receive Window Auto-Tuning feature for HTTP traffic.

To have us disable auto-tuning for you, go to the "Fix it for me" section. If you would rather disable auto-tuning yourself, go to the "Let me fix it myself" section.

Fix it for me

To disable auto-tuning automatically, click the Fix this problem link. Then, click Run in the File Download dialog box, and follow the steps in this wizard.

Fix this problem
Microsoft Fix it 50056

Note this wizard may be in English only; however, the automatic fix also works for other language versions of Windows.

Note If you are not on the computer that has the problem, you can save the automatic fix to a flash drive or to a CD, and then you can run it on the computer that has the problem.

Now go to the "Did this fix the problem?" section.

Let me fix it myself

To disable the Receive Window Auto-Tuning feature for HTTP traffic, follow these steps:

1. Log on to the computer as a user who has administrative credentials.
2. Click Start, type runas /user: local_computer_name\administrator cmd in the Start Search box, and then press ENTER.
3. When you are prompted for the administrator account password, type the correct password, and then press ENTER.
4. At the command prompt, type the following command, and then press ENTER:
   netsh interface tcp set global autotuninglevel=disabled
5. Exit the Command Prompt window.
6. Restart the computer.

Now go to the "Did this fix the problem?" section.

Did this fix the problem?

Check whether the problem is fixed. If the problem is fixed, you are finished with this article. If the problem is not fixed, you can contact support (http://support.microsoft.com/contactus) .