NAK (protocol message)

The NAK (or NACK; standing for "Negative-Acknowledgment") protocol message is sent in many communications protocols to negatively acknowledge or reject a previously received message, or to indicate some kind of error.

Many protocols are acknowledgement (ACK)-based, meaning that they positively acknowledge receipt of messages. Transmission Control Protocol (TCP) is an example of an ACK-based protocol.

Other protocols are NAK-based, meaning that they only respond to messages if there is a problem. Examples include most reliable multicast protocols which send a NAK when the receiver detects missing packets.

Still other protocols make use of both NAKs and ACKs. Bisync and Adaptive Link Rate (for Energy-Efficient Ethernet) is an example.

A special case of the NAK protocol message is the negative-acknowledge character.