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.

Vista’s new TCP/IP stack offers plenty of improvements, but unfortunately doesn’t always play well with older hardware that isn’t aware of the newer features it uses. Steve Riley on Security mentions that this can be a problem when trying to use wireless networking with a router that doesn’t understand the updated ‘Windows Tuning’ feature. He suggests this is more common with routers used for public internet connections, such as at a hotel. The fix is to disable autotuning, which happens to be the same fix from tip 25 which dealt with threaded network applications. Keep reading →

I‘ve seen mentioned in a few places that applications that use multiple network threads (BitTorrent clients, download managers, etc) don’t download files as quickly in Vista as they do in Windows XP. Most seem to attribute the difference to the new AutoTuning function in the TCP/IP stack. Keep reading →