Check the TCP/IP state
To check the current status of the Vista TCP/IP tweakable parameters, in elevated command prompt type the following command:
netsh int tcp show global
You will be presented with something like the following:
The settings, as well as their default and recommended state are explained below. The two most important tweakable parameters are “Auto-Tuning Level” and “Congestion Control Provider”.
When checking the TCP state with the “netsh int tcp show global” command, it is also possible to see the following message below all those parameters:
** The above autotuninglevel setting is the result of Windows Scaling heuristics overriding any local/policy configuration on at least one profile.
It is displayed when the “Receive Window Auto-Tuning Level” is not explicitly set, or if the system deemed it necessary to make a change because of user prompted “repairing” of your network connection, for example.
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 disabled
possible 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.
TCP Auto-Tuning
To turn off the default RWIN auto tuning behavior, (in elevated command prompt) type:
netsh int tcp set global autotuninglevel=disabled
The 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.
QoS Reserved Bandwidth
As with Windows XP, nework adapters have a “QoS Packet Scheduler” enabled by default, which reserves 20% of bandwidth by default for QoS applications that request priority traffic. Note this only has effect in the presence of running QoS applications that request priority traffic. Registry value is undocumented for the Vista version of Windows. To customize this setting, in the Windows Registry:
HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\Psched
NonBestEffortLimit=0 (DWORD, not present in the registry by default. Recommended: 0 , possible values between 0 and 100) – indicates the percentage value of reserved bandwidth for QoS applications. Set to 0 to disable.
Notes: This tweak applies only to Windows versions that have Qos Packet Scheduler enabled. It will ONLY have effect in the presense of running QoSapplications.
Gaming Tweak – Disable Nagle’s algorithm
The tweak below allows for tweaking or disabling Nagle’s alogrithm. Disabling ”nagling” allows for very small packets to be transferred immediately without delay. Note that disabling Nagle’s algorithm is only recommended for some games, and it may have negative impact on file transfers/throughput. The dafault state (Nagling enabled) improves performance by allowing several small packets to be combined together into a single, larger packet for more efficient transmission. While this improves overall performance and reduces TCP/IP overhead, it may briefly delay transmission of smaller packets. Keep in mind that disabling Nagle’s algorithm may have some negative effect on file transfers, and can only help reduce delay in some games. To implement this tweak, in the registry editor (Start>Run>regedit) find:
This setting configures the maximum number of outstanding ACKs in Windows XP/2003/Vista/2008:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\
Interfaces\{NIC-id}
There will be multiple NIC interfaces listed there, for example: {1660430C-B14A-4AC2-8F83-B653E83E8297}. Find the correct one with your IP address listed. Under this {NIC-id} key, create a new DWORD value:
TcpAckFrequency=1 (DWORD value, 1=disable, 2=default, 2-n=send ACKs if outstanding ACKs before timed interval. Setting not present by default).
For gaming performance, recommended is 1 (disable). For pure throughput and data streaming, you can experiment with values over 2. If you try larger values, just make sure TcpAckFrequency*MTU is less than RWIN, since the sender may stop sending data if RWIN fills witout acknowledgement.
Also, find the following key (if present):
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSMQ\Parameters
Add a new DWORD value:
TCPNoDelay=1 (DWORD value, 0 to enable Nagle’s algorithm, 1 to disable, not present by default)
To configure the ACK interval timeout (only has effect if nagling is enabled), find the following key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\
Interfaces\{NIC-id}
TcpDelAckTicks=0 (DWORD value, default=2, 0=disable nagling, 1-6=100-600 ms). Note you can also set this to 1 to reduce the nagle effect from the default of 200ms without disabling it.
Notes:
Reportedly, the above gaming tweak (disabling nagle’s algorithm) can reduce WoW (World of Warcraft) latency by almost half!
XP/2003 needs hotfix or SP2 for it to work (MS KB 815230)
Vista needs hotfix or SP1 for it to work (MS KB 935458)
SG Vista TCP/IP Patch – automatic optimization
For user convenience, we also provide a quick way to apply all optimal values as recommended above using our SG Vista TCP/IP Patch. It allows for tweaking all the above netsh settings and registry values in one simple step (with the exception of the “gaming tweak” section). The patch also provides for easily reverting the settings to their Windows default values. To apply, save to your desktop and run as administrator (right-click -> run as administrator). Click Y when prompted to apply settings.
See Also
Windows Vista tcpip.sys connection limit patch for Event ID 4226 – removing the limit on half-open TCP connections.
References
Windows Server 2008 Network Shell (Netsh) Technical Reference
Microsoft KB951037
RFC 2581
Wikipedia: Nagle’s algorithm
Technet: TCPNoDelay
MS KB 311833
MS KB 328890
MS KB 321098
MS KB 321169
MS KB 951037 – TCP Chimney Offload, Receive Side Scaling, and Network DMA in Windows Server 2008
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