Windows Vista includes the Receive Window Auto-Tuning feature. This feature improves performance when you access resources on a network. By default, this feature is enabled for non-HTTP traffic. If you enable this feature for HTTP traffic, HTTP traffic may be more efficient.

However, potential risks exist after you enable the Receive Window Auto-Tuning feature for HTTP traffic. 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.

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 HTTP traffic on Windows Vista-based client computers
•	Issues that may occur after you enable the Receive Window Auto-Tuning feature for HTTP traffic
•	How to disable the Receive Window Auto-Tuning feature

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How the Receive Window Auto-Tuning feature improves data transfer

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.

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How to enable the Receive Window Auto-Tuning feature for HTTP traffic

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, 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\InternetSettings
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.

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.

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Issues that may occur

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 stop responding. 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.

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How to disable the Receive Window Auto-Tuning feature

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.	Type the following command at the command prompt, and then press ENTER: netsh interface tcp set global autotuninglevel=disabled
5.	Exit the Command Prompt window.
6.	Restart the computer.

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