With an understanding of sequence numbers and sliding window it is possible to quickly understand the Go-back-N protocol.
The receiver sends an ACK for each frame as it is received. If a corrupted frame, F(n), is received or F(n)is missing (the packet failed to arrive) then the receiver sends a special N.ACK(n) to request that the sender start retransmitting from frame F(n).
Selective repeat protocol
This protocol can be executed using one of two methods:
- The receiver sends only ACK's and the sender implicitly retransmits frames, or the receiver transmits ACK's to explicitly request a frame retransmission.
- The sender buffers out-of-order frames until the missing frames are retransmitted.
As is suggested by the name, the receiver can “selectively” request retransmission of a frame. When using ACK's only, the absence of an ACK triggers the retransmission.