The Selective Repeat Protocol

The go-back-N protocol wastes bandwidth on retransmitted frames if the error rate is high.

Alternatively, in the selective repeat protocol, the receiver can buffer all received frames (up to some limit) and simply wait for the bad frame to be retransmitted.

• The sender's window size corresponds to the number of frames transmitted by not yet received - it varies, grows and shrinks, over time.
• The receiver's window size corresponds to the number of frames that the receiver is willing to receive - it too varies over time, and is always ≥ 1.