"Windows Scaling heuristics" changes my TCP autotuning level ?

It is sometimes possible for Windows Vista/7 to change the TCP autotuning level on its own, even after it being user-configured. When that happens, the "netsh int tcp show global" command displays the following message:

** The above autotuninglevel setting is the result of Windows Scaling heuristics
overriding any local/policy configuration on at least one profile.

To prevent that behavior and turn off heuristics, before setting the TCP Receive Window auto-tuning level you should execute the following command:

netsh int tcp set heuristics disabled

Note this should be executed in elevated command prompt (as administrator). If the command is accepted by the OS you should see an "Ok." on a new line.

For more information and TCP tweaks, see our Vista/Windows 7 tweaking article.