ACK
ACKnowledgement

At the time ofcommunicating between the computer, the affirmativereply which is sent to the transmissionoriginalcomputer from the computer ahead transmitting. When data transfer ends normally and the like, it is transmitted. It has the meaning "just the data of part receiving just", there is also a signal, "Selective ACK (s)", it is used for the occasion where requires the retransmission just of the data which fails in communication.