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The window, checksum and urgent pointer are another important fields in TCP header. Each of these fields are 2 bytes long.
Window size the most important part in the TCP header. This fields is used by the receiver to indicate to the sender, the amouth of data that it can accept. So, this field is very important for efficient data transfer and flow control.
Window size uses byte as a metric. If it is mentined that window size is 60k, then it means that receiver accept 60 kilobyte data. When the data transmitted reaches window value then sender expect another window value from the receiver along with the acknowledgement for the window just received.
Window size is not a random value. It is calculated with some parameters like below: