/æk/
n. “The TCP nod that says ‘I got it.’”
ACK, short for Acknowledgment, is a flag in the TCP (Transmission Control Protocol) header that confirms receipt of data or the successful completion of a step in the TCP handshake. Whenever a device receives data, it sends an ACK packet back to the sender to indicate that the information was received correctly, helping ensure reliable, ordered communication.
Key characteristics of ACK include:
- Data Confirmation: Signals that the receiving device successfully got the transmitted segment.
- Part of TCP Handshake: Forms the final step in the three-way handshake: SYN → SYN-ACK → ACK.
- Flow Control & Reliability: Works with sequence numbers to manage packet order and retransmission if packets are lost.
- Lightweight: ACK packets often carry no payload; they serve purely as confirmation signals.
A conceptual example in the TCP handshake:
Client → Server: SYN (start connection)
Server → Client: SYN-ACK (acknowledges SYN)
Client → Server: ACK (confirms connection established)Another example for data transfer:
Server sends segment #101 → Client
Client → Server: ACK (acknowledges receipt of segment #101)Conceptually, ACK is like nodding during a conversation — it tells the sender, “I heard you and it’s all good,” which prevents miscommunication and ensures smooth delivery.
In essence, ACK is a fundamental TCP mechanism that provides reliability, sequencing, and confirmation, forming the backbone of dependable communication over networks.