Sliding Window Protocol

A flow and error control technique that lets the sender transmit several frames before waiting for an acknowledgement, so the channel stays busy (unlike Stop-and-Wait). The sender and receiver each keep a window of allowed sequence numbers.

Working

1. Frames are numbered with $m$-bit sequence numbers (0 to $2^m - 1$).
2. The sender window holds the numbers of frames that may be sent or are awaiting ACK; it slides forward when ACKs arrive.
3. The receiver window holds the numbers of frames it can accept.
4. ACKs may be cumulative, and can be piggybacked on data frames.

Variants

	Go-Back-N ARQ	Selective Repeat ARQ
Sender window	Up to $2^m - 1$	Up to $2^{m-1}$
Receiver window	1	Same as sender
On error	Retransmit the lost frame and all after it	Retransmit only the lost frame
Out-of-order frames	Discarded	Buffered
Complexity	Low	High

Efficiency

where $N$ is the window size and $a = T_p / T_t$ (capped at 1). A larger window gives better utilization than Stop-and-Wait ($N=1$).