NACKs

NACK stands for negative acknowledgment, a signal used in data communication to indicate that one or more previously sent data units were not received correctly or were not received at all. Unlike a positive acknowledgment (ACK), which confirms successful receipt, a NACK requests retransmission or informs the sender of a failure. NACKs are used in various layers and protocols across networks and storage systems.

NACK signaling can be explicit, where the receiver sends a dedicated NACK message, or implicit, where the sender infers failure from missing or corrupted acknowledgments (for example, a missing ACK within a timeout). Some systems also use a combination, such as selective acknowledgments alongside periodic ACKs, to improve efficiency.

In multimedia and real-time communications, NACKs are common in RTP/RTCP feedback (Generic NACK and related messages) to request retransmission of lost packets. In low- and high-level protocols such as I2C, a NACK is a specific response from the receiver indicating the last received byte or a failed transfer. NACK-based ARQ schemes are contrasted with cumulative ACK schemes and are tailored to minimize retransmissions in lossy or high-latency environments.

The use of NACKs involves trade-offs between signaling overhead, latency, and reliability. Compatibility with existing protocol designs and the ability to perform selective retransmissions are important considerations when implementing NACKs.

See also: Acknowledgment (ACK), Selective acknowledgment (SACK), Negative acknowledgment in RTP.