In transmit flow control, sliding window is a variable-duration window that allows a sender to transmit a specified number of Data units before an acknowledgement is received or before a specified event occurs.

An example of a sliding window in packet transmission (telecommunications) is one in which, after the sender fails to receive an acknowledgement for the first transmitted packet, the sender slides the window, i.e. resets the window, and sends a second packet. This process is repeated for the specified number of times before the sender interrupts transmission. Sliding window is sometimes (loosely) called acknowledgement delay period .

For example, supposing a fixed window size of m packet, a sender may send out packets [n ldots (n+m-1)] before receiving any acknowledgement. If acknowledgement arrives from the receiver for packet n, then the range (window) of unacknowledged packets slides to [(n+1) ldots (n+m)], and the sender is able to send out packet (n+m). In some way, sliding signifies a FIFO operation, trimming the range at one end, extending it at the other end.

The purpose of the sliding window is to increase , the number of packets transmitted out-of-order, and memory usage.

In practice, protocols often adapt the window size to the link s speed and actual Saturation (telecommunications) or congestion.

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