In a TCP session, what does the ACK flag signify?

The ACK flag in a TCP session signifies that data has been acknowledged. When a TCP segment is transmitted, the sender expects an acknowledgment from the recipient indicating that the data has been successfully received. This acknowledgment is crucial for ensuring reliable data transmission across the network.

The use of the ACK flag is part of TCP's connection-oriented protocol, where maintaining the integrity and order of data packets is essential. When the receiving device sends an acknowledgment (with the ACK flag set), it informs the sender that it has received all data up to the specified sequence number, which helps in managing retransmissions and flow control.

In contrast to the function of the ACK flag, other flags like the SYN flag are used to initiate a connection, the RST flag can indicate that a connection is being reset, or the FIN flag is utilized to terminate a session. Understanding these flags and their roles is fundamental in grasping how TCP maintains reliable communication between devices.