A receiver acknowledges received data by sending acknowledgement (ACK) packets. If a sender receives an ACK packet for the data in its window, it removes that data from the window, because it has been successfully transmitted to the receiver.
TCP employs this mechanism for controlling of flow, so that a receiver can tell the sender, when it cannot process the data at the arriving rate. This mechanism also informs the sender that how much buffering space is available at the receiver’s end, in order to avoid overfilling of receiver’s buffer window.
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