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Auto Tune Receive Buffer 
If this option is enabled the Auto Tuning of the TCP Receive Window feature from Windows is used.
Default: Enabled

Receive Buffer Size
Sets the size of the socket receive buffer (SO_RCVBUF). This value directly affects the TCP window size.
Default: 128 KB 

Auto Tune Send Buffer Size
Windows 7: If CTCP [6] is not enabled the user defined socket buffer size is used. If CTCP is enabled the Windows TCP stack automatically adjusts the sender's congestion window.
Windows 8 and higher: CTCP is enabled by default. The TCP stack automatically optimizes the congestion window. 
Default: Enabled

Send Buffer Size
Sets the size of the socket send buffer (SO_SNDBUF). This value directly affects the TCP window size. 
Default: 128 KB

References

1. Technet: TCP Receive Window Auto-Tuning
2. MSDN: TCP Receive Window Size and Window Scaling (Applies to Windows XP and Windows 2003)
3. MSDN: Very technical article about Auto Tuning on Windows Vista and higher
4. MSDN: setsockopt Function (Windows)
5. RFC1323: TCP Extensions for High Performance (TCP Window Scaling, SACK, etc)
6. KB: Enable CTCP