Before developing a protocol for reliably communicating over such a channel,
first consider how people might deal with such a situation. Consider how you yourself might dictate a long message over the phone. In a typical scenario, the message
taker might say “OK” after each sentence has been heard, understood, and recorded.
If the message taker hears a garbled sentence, you’re asked to repeat the garbled
sentence. This message-dictation protocol uses both positive acknowledgments
(“OK”) and negative acknowledgments (“Please repeat that.”). These control messages allow the receiver to let the sender know what has been received correctly, and
what has been received in error and thus requires repeating. In a computer network
setting, reliable data transfer protocols based on such retransmission are known as
ARQ (Automatic Repeat reQuest) protocols.