The acknowledgement frame (ACK) is used in data transmission for flow control in an end-to-end connection. As a positive acknowledgement, the ACK frame is transmitted to confirm the receipt of one or more data packets.
The ACK flag or frame is used in various protocols such as the TCP protocol or Fibre Channel( FC). For example, as a bit indicator in the control flag field of the TCP header. A set flag indicates that the acknowledgement number entered in the acknowledgement field is relevant.