The TCP window scale option is an option to increase the TCP receive window size above its maximum value of 65,535 bytes. This TCP option, along with several others, is defined in IETF RFC 1323 which deals with Long-Fat Networks, or LFN.
In fact, the throughput of a communication is limited by two windows: the congestion window and the receive window. The former tries not to exceed the capacity of the network (congestion control) and the latter tries not to exceed the capacity of the receiver to process data (flow control). The receiver may be overwhelmed by data if for example it is very busy (such as a Web server). Each TCP segment contains the current value of the receive window. If for example a sender receives an ack which acknowledges byte 4000 and specifies a receive window of 10000 (bytes), the sender will not send packets after byte 14000, even if the congestion window allows it.
Read more about TCP Window Scale Option: Theory, Possible Side Effects
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