Go-Back-N

Go-Back-N (GBN)

GBN permits the sender to transmit multiple packets without waiting for acknowledgments. The sender's packets are divided into four regions.

Sequence numbers in $[0, base - 1]$ identify sent, acknowledged packets; $[base, nextseqnum - 1]$ identifies sent, unacknowledged packets; $[nextseqnum, base + N - 1]$ identifies packets waiting to be sent; numbers at least $base + N$ are unavailable.

As the protocol runs, its window slides forward through sequence space, so GBN is also called a sliding-window protocol. The receiver's window is one, so it discards all out-of-order packets.

Cumulative Acknowledgments and One Timer

Here $ACK\ n$ means packet $n$ and every preceding packet have arrived in order. A later cumulative ACK advances the window even if an earlier ACK was lost. For out-of-order data, the receiver repeats the ACK for the last in-order packet.

The sender stores unacknowledged packets and maintains one timer for the oldest one. On timeout, it retransmits every packet from $base$ through $nextseqnum-1$ and restarts the timer. This notation differs from TCP's ACK number, which identifies the next expected byte.

Finite Sequence Space

A $k$-bit field provides $2^k$ sequence numbers. GBN requires $N\le 2^k-1$; otherwise, an old retransmission can be confused with new data after wraparound. Real implementations compare circular windows using modular arithmetic; this lesson uses increasing logical packet IDs for readable traces.