window size

It is simply an advertisement of how much data (in bytes) the receiving device is willing to receive at any point in time.

The TCP window is the maximum number of bytes that can be sent before the ACK must be received.

If the network is unreliable, it’s better to keep the window small. This way you don’t have to retransmit as much data if there’s a problem. But if the network is reliable, then the window can be quite large.

TCP window scale option

increase the total TCP window size by providing a multiplier value.

Devices can change the window size dynamically, making it smaller when there’s congestion and bigger when things are clear.

in Windows Vista / Server 2008 and newer (enabled by default )
 * 64 kB ~> 16 MB

 
Linux

cat /proc/sys/net/ipv4/tcp_window_scaling
1

 * 與 tcp_rmem, tcp_wmem 有關