This illustration uses the familiar structure of a sender and receiver positioned side-by-side with timelines extending vertically down beneath them. Here, the sender is designated "Host A" and the receiver is designated "Host B."

The interaction between hosts begins at the topmost portion of Host A's timeline, at a point labeled "User types 'C.'" A blue arrow, labeled "Seq=42, ACK=79, data='C', extends diagonally downward to the right, to a point on Host B's timeline labeled "Host ACKs receipt of 'C', echoes back 'C'. An arrow, labeled "Seq=79, ACK=43, data='C', extends diagonally to the left, to a point on Host A's timeline labeled "Host ACKs receipt of echoed 'C'. An arrow labeled "Seq=43, ACK=80," extends diagonally to Host B's timeline.