Go-Back-N ARQ is a sliding-window protocol used for error and flow control, where a sender can transmit multiple frames before needing an acknowledgment — up to a window size N — but the receiver only accepts frames in strict order.

How it works

• The sender maintains a window of size N, allowing up to N unacknowledged frames to be in transit at once.
• Frames are numbered using sequence numbers modulo 2^m (m = bits used for numbering), and the window size must satisfy N <= 2^m - 1.
• The receiver uses a single variable to track the next expected frame. It discards any out-of-order frame — even if it arrived correctly — and re-sends an ACK for the last correctly received in-order frame (cumulative ACK).
• If the sender's timer for the oldest unacknowledged frame expires, it retransmits every frame from that point onward — not just the lost one. This is the "go back N" behavior.

Example

Sender window (N=4): frames 0,1,2,3 sent
Frame 1 is lost in transit
Receiver gets: 0 (accept, ACK1) -> 2 (discard, out of order) -> 3 (discard, out of order)
Sender's timer for frame 1 expires -> resends frames 1,2,3

Trade-off

Go-Back-N is simple to implement (receiver needs no buffering for out-of-order frames) but wastes bandwidth on retransmission, since even correctly-received frames after a lost one get discarded and re-sent. This is what Selective Repeat improves on, at the cost of a more complex receiver.