Ack

ACK is an abbreviation for acknowledgement or acknowledge, used in telecommunications and computer networks to indicate that a message or data unit has been received and processed. An ACK is typically sent by a receiver to confirm successful reception, enabling the sender to proceed or to retransmit only if an ACK is not received within a timeout.

In TCP, the ACK concept is formalized as a flag in the TCP header. Each data segment carries a sequence number, and the receiver replies with an ACK containing the next expected sequence number. This enables reliable data transfer, allows for cumulative acknowledgments, and supports options such as selective acknowledgments (SACK). The ACK mechanism is central to the TCP three-way handshake, where a final ACK completes the connection establishment.

Other protocols, including HDLC, PPP, and various link-layer technologies, also use explicit ACK frames to confirm reception. In many networks, ACKs can be piggybacked onto outgoing data frames to reduce overhead, and some systems employ delayed acknowledgments to improve efficiency.

Not all protocols provide or require ACKs. Connectionless protocols like UDP do not guarantee delivery; reliability in such contexts is typically provided by higher-layer protocols or applications that implement their own ACK/NACK schemes. In messaging systems and data stores, an ACK may signify that a message has been consumed or processed, allowing producers or brokers to advance offsets or reclaim resources.

The term ACK derives from the word acknowledgement, and in practice it serves to manage reliability, flow control, and error recovery across diverse communication technologies.