Theory
The TCP window scale option is needed for efficient transfer of data when the bandwidth-delay product is greater than 64K. For instance, if a T1 transmission line of 1.5Mbits/second was used over a satellite link with a 513 millisecond round trip time (RTT), the bandwidth-delay product is (1500000 * 0.513) = 769,500 bits or 96,188 bytes. Using a maximum buffer size of 64K only allows the buffer to be filled to (65535 / 96188) = 68% of the theoretical maximum speed of 1.5Mbits/second, or 1.02 Mbit/s.
By using the window scale option, files can be transferred at nearly 1.5Mbit/second utilizing nearly all of the available bandwidth.
This option is also useful when sending large files greater than 64KB over slow networks.
By using the window scale option, the receive window size may be increased up to a maximum value of 1,073,725,440 bytes; almost 1 Gibibyte. This is done by specifying a one byte shift count in the header options field. The true receive window size is left shifted by the value in shift count. A maximum value of 14 may be used for the shift count value.
Read more about this topic: TCP Window Scale Option
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