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How TCP/IP Sliding Window is configured on Win7 hosts?

Posted on 2014-01-31
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Last Modified: 2014-02-07
How to set the TCP window size in Windows 7?  Environment Variables or Registry Entry? How to set adjust parameters (like TCP window size) in Windows 7 to improve network performance over LAN & WAN?
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Question by:helpdesk_wlk
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Expert Comment

by:David Johnson, CD, MVP
ID: 39825629
Windows Vista and newer use it by default .. They call it auto-tuning
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Sasha Kranjac earned 2000 total points
ID: 39825672
TCP Window Size can be adjusted editing the Registry, and it works back from NT times, for all Operating Systems, not just for Windows 7. Well, it should work - I've tested it on NT 4, Windows Server 2003 and Windows 7. I'm pretty sure it would work on Windows Server 2012 R2 and Windows 8.1 too.

You can set TCP Window size either A) globally, for all interfaces or B) per interface.

A) Run Regedit and under following key

HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
add GlobalMaxTcpWindowSize REG-DWORD entry. Put any value between 0x0 and 0x3FFFFFFF.

B) Run Regedit and under following key:

HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\ interface-name
add TcpWindowSize REG-DWORD entry. Put any value between 0x0 and 0x3FFFFFFF.

A lot of factors affect transfer rate and each network should be fine tuned - there is no "recipe for all". Bit errors, round-trip time, network bandwidth, sender/receiver configuration and many other factors have impact on network performance.

To check TCP/IP stack configuration on Windows 7 type in command prompt:

netsh interface tcp show global
netsh interface tcp show heuristics
Take a look at the two lines:
Receive Window Auto-Tuning Level : normal. It means OS is using TCP/IP normal level of auto-tuning.
Windows Scaling Heuristics : enabled tells you auto-tuning is enabled.

Be very careful and calculate TCP window size to avoid network performance penalties.

Check these sites for more info: TCP thoughoutput calculator, Brad Hedlund's Blog and Neil Carpenter's Blog
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Author Comment

by:helpdesk_wlk
ID: 39841033
Thank you for your answers.
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LVL 6

Expert Comment

by:Sasha Kranjac
ID: 39841311
You're welcome!
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