/næk/
n. “The TCP or protocol signal that says ‘Nope, try again.’”
NAK, short for Negative Acknowledgment, is a control signal used in networking and communication protocols to indicate that a data packet or message was not received correctly or could not be processed. Unlike ACK, which confirms successful receipt, NAK tells the sender that the transmitted data needs to be retransmitted.
Key characteristics of NAK include:
- Error Detection: Used in conjunction with checksums or error-detection codes to identify corrupted or missing data.
- Retransmission Trigger: Signals the sender to resend the affected data segment.
- Protocol Dependent: While TCP typically uses ACKs with sequence numbers and retransmission timers rather than explicit NAKs, many other protocols (like serial communication, XMODEM, or some industrial protocols) use NAK directly.
- Lightweight Signal: NAK packets usually contain minimal information, just enough to indicate which data segment failed.
A conceptual example in a simple data transfer protocol:
Sender → Receiver: Data packet #5
Receiver → Sender: NAK (packet #5 corrupted)
Sender → Receiver: Resends Data packet #5Conceptually, NAK is like holding up a hand during a conversation and saying, “That message didn’t come through — repeat it.” It ensures that communication remains accurate despite errors or corruption.
In essence, NAK is a critical mechanism in error-sensitive communication protocols, enabling detection of failures and prompting corrective retransmission to maintain data integrity.