Disable Windows Scaling heuristics
Windows Vista/7 has the ability to automatically change its own TCP Window auto-tuning behavior to a more conservative state regardless of any user settings. It is possible for Windows to override the autotuninlevel even after an user sets their custom TCP auto-tuning level. When that behavior occurs, 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 enforce any user-set TCP Window auto-tunning level, you should execute the following command:
netsh int tcp set heuristics disabledpossible settings are: disabled,enabled,default (sets to the Windows default state)
recommended: disabled (to retain user-set auto-tuning level)
Note this should be executed in elevated command prompt (with admin priviledges) before setting the autotuninlevel in next section. If the command is accepted by the OS you will see an "Ok." on a new line.
The corresponding Registry value (not necessary to edit if setting via netsh) is located in:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\Tcpip\Parameters
EnableWsd=0 (default: 1, recommended: 0)
TCP Auto-Tuning
To turn off the default RWIN auto tuning behavior, (in elevated command prompt) type:
netsh int tcp set global autotuninglevel=disabledThe default auto-tuning level is "normal", and the possible settings for the above command are:
disabled: uses a fixed value for the tcp receive window. Limits it to 64KB (limited at 65535).
highlyrestricted: allows the receive window to grow beyond its default value, very conservatively
restricted: somewhat restricted growth of the tcp receive window beyond its default value
normal: default value, allows the receive window to grow to accommodate most conditions
experimental: allows the receive window to grow to accommodate extreme scenarios (not recommended, it can degrade performance in common scenarios, only intended for research purposes. It enables RWIN values of over 16 MB)
Our recommendation: normal (unless you're experiencing problems).
If you're experiencing problems with your NAT router or SPI firewall, try the "restricted", "highlyrestricted", or even "disabled" state.
Notes:
- Reportedly, some older residential NAT routers with a SPI firewall may have problems with enabled tcp auto-tuning in it's "normal" state, resulting in slow speeds, packet loss, reduced network performance in general.
- auto-tuning also causes problems with really old routers that do not support TCP Windows scaling. See MSKB 935400
- netsh set commands take effect immediately after executing, there is no need to reboot.
- sometimes when using "normal" mode and long lasting connections (p2p software / torrents), tcp windows can get very large and consume too much resources, if you're experiencing problems try a more conservative (restricted) setting.
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