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ACK flag

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

The ACK flag is a TCP control bit indicating that the acknowledgment-number field is valid for acknowledging received data or control messages.

The ACK flag is a TCP control bit that indicates the acknowledgment-number field is valid. When set to 1, it confirms received data or control messages and supports TCP's reliable, connection-oriented communication. section 22.2.1

The ACK flag appears in several important TCP exchanges:

  • During the three-way handshake, the server responds with SYN-ACK, and the client completes establishment with ACK. section 22.2.1 section 22.3
  • During data exchange, receivers acknowledge segments by using an acknowledgment number that identifies the next sequence number expected. section 22.3
  • During connection termination, hosts acknowledge FIN segments with ACK segments, commonly producing a FIN-ACK-FIN-ACK exchange. section 22.2.1 section 22.3
  • If an expected acknowledgment does not arrive before the retransmission timeout, the sender retransmits the segment. section 22.2.1 section 22.3

Three-way handshake

PC1 sends a TCP segment with SYN. SRV1 replies with SYN and ACK. PC1 then sends ACK, completing connection establishment before data exchange begins.

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes
Explains the flag's purpose, handshake and termination exchanges, acknowledgment numbers, and retransmission behavior.
Examples
Uses PC1 and SRV1 handshake messages plus FIN exchanges and lost-segment retransmission scenarios.
What it assumes
Assumes basic TCP segment awareness while defining flags, sequence numbers, acknowledgment numbers, and connection phases.
What you can do after
Evaluate TCP traces by identifying ACK's role in establishment, delivery confirmation, termination, and retransmission.
How it connects
Connects ACK with TCP, SYN, FIN, the three-way handshake, sequence numbers, acknowledgment numbers, retransmission, and flow control.
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Related concepts

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