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Three-way handshake

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

A three-way handshake is a three-message exchange used by TCP hosts to establish a connection before transmitting application data.

The three-way handshake establishes a TCP connection before application data is transmitted. It uses TCP header flags to coordinate both hosts. section 22.2.1

Follow the three-message sequence:

  • SYN: The initiating host requests a connection and synchronizes its initial sequence number. section 22.2.1 section 22.3
  • SYN-ACK: The receiving host acknowledges the request and sends its own synchronization information. section 22.2.1
  • ACK: The initiating host acknowledges the response; the TCP connection is ready for data exchange. section 22.2.1 section 22.3

PC1 connects to SRV1

PC1 sends a TCP segment with SYN, SRV1 replies with SYN-ACK, and PC1 returns ACK. These three establishment segments carry no application data; they prepare the connection for later exchange. section 22.2.1

The handshake adds time and processing overhead compared with UDP, which sends data without first establishing a connection. TCP trades that overhead for connection-oriented, reliable communication and flow control. section 22.2.3

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

How deep it goes
Explains the three messages, TCP flags, empty segments, and initial sequence numbers without detailing lower-level implementation.
Examples
Uses a PC1-to-SRV1 exchange and a SYN, SYN-ACK, ACK sequence diagram.
What you can do after
Evaluate/understand TCP connection setup and distinguish its startup overhead from UDP transmission.
How it connects
Connects TCP flags, sequence numbers, acknowledgment numbers, reliable communication, flow control, connection termination, and UDP.
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Related concepts

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