Source: Microsoft Support
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This article describes how to disable or enable the transmission control protocol (TCP) window autotuning diagnostic tool in Vista. The autotuning diagnostic tool diagnoses and fixes problems related to autotuning. However, in some cases (for example, network performance testing) you may want to disable the diagnostic tool. This article describes the autotuning feature, the diagnostic tool, and how they help prevent problems.
The TCP receive window autotuning feature in Vista lets the operating system continually monitor conditions such as bandwidth, network delay, and application delay. The operating system can configure connections by scaling the TCP receive window to maximize the network performance based on these parameters. To determine the optimal size for the receive window, the receive window autotuning feature measures the product of the network delay and bandwidth, and also looks at the application retrieve-rates. Then, the receive window autotuning feature changes the receive window size of the ongoing transmission to take advantage of any unused bandwidth.
When the receive window autotuning feature is enabled, older routers, older firewalls, and older operating systems that are incompatible with the receive window autotuning feature may sometimes cause slow data transfer or a loss of connectivity between Vista clients. 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.
Note 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. In order to determine whether the issue is caused by faulty firewalls, Vista SP1 contains an autotuning diagnostic tool that determines whether a faulty device is in the path from your computer. If the diagnostic tool detects a faulty device on the network, it reduces the degree of optimization performed by the receive window autotuning feature. The user may not notice that the window autotuning feature was turned down and may continue to use the computer as before. However, the state of the computer is changed. In order to control the state of the autotuning feature, the user may want to disable the diagnostic tool. Doing this lets them control the state of the receive window and lets them know what throughput to expect.
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:
322756 How to back up and restore the registry in Windows
On Vista Service Pack 1 (SP1), the registry key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\ holds all the configurable parameters for these diagnostics. An update to these parameters requires a restart before they take effect. To disable or enable the window autotuning diagnostic tool, follow these steps:
1. Open Registry Editor.
2. Locate the following registry value: EnableWsd
3. Type REG_DWORD.
4. Enter 0 to disable the tool.
Note If the value is set to 0, any previous decision taken to disable or maintain window scaling will not be applied by TCP.
5. Enter 1 to enable the tool.
Note If the value is set to 1, the diagnostics are enabled, and the TCP maintains the state described earlier.
6. Exit Registry Editor.
7. Restart the computer.
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