This flag is set to

1

1

in a segment to acknowledge a segment that was received previously. This is an important part of the protocol. In other words, when a receiver wants to acknowledge some received data, it sends a TCP segment with the ACK flag and the acknowledgment number field appropriately set. This flag is also used in connection establishment and termination as we will see in more detail later.

Note: These acknowledgment messages can and often do contain data as well!