- NAK (protocol message)
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The NAK (or NACK) 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 ACK-based, meaning that they positively acknowledge receipt of messages. 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.
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