Acknowledgement (data networks)
computing signal confirming receipt of a message
In data networking, telecommunications, and computer buses, an acknowledgement (ACK) or acknowledgment is a signal that is passed between communicating processes, computers, or devices to signify acknowledgement, or receipt of message, as part of a communications protocol. Correspondingly a negative-acknowledgement (NAK or NACK) is a signal that is sent to reject a previously received message or to indicate some kind of error. Acknowledgments and negative acknowledgments inform a sender of the receiver's state so that it can adjust its own state accordingly.
Acknowledgment signal types
The ASCII code point for ACK is 0x06 (binary 0000 0110). By convention, a receiving device sends an ACK to indicate it successfully received a message. ASCII also provides a NAK code point (0x15, binary 0001 0101), which can be used to indicate the receiving device cannot, or will not, comply with the message. Unicode provides visible symbols for these ASCII characters, U+2406 (␆) and U+2415 (␕).
ACK and NAK symbols may also take the form of single bits or bit fields depending on the protocol data link layer definition or even as a dedicated wire at the physical layer.
Protocol usage
Many protocols are acknowledgement-based, meaning that they positively acknowledge receipt of messages. The Internet's Transmission Control Protocol (TCP) is an example of an acknowledgement-based protocol.
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