Four-way handshake

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

A four-way handshake is the typical four-message TCP procedure for terminating a connection using FIN and ACK control flags.

The four-way handshake is the typical TCP procedure for terminating a connection after data exchange. Each host finishes sending by transmitting a segment with the FIN flag, and the other host acknowledges it with ACK. section 22.2.1

Remember the usual termination sequence:

• The first host sends FIN. section 22.2.1
• The other host replies with ACK. section 22.2.1
• The other host sends its own FIN. section 22.2.1
• The first host replies with ACK. section 22.2.1

TCP termination scenario

After PC1 and SRV1 establish a TCP connection and exchange data, PC1 sends FIN. SRV1 acknowledges with ACK, sends its own FIN, and PC1 responds with ACK. This produces FIN, ACK, FIN, ACK. section 22.2.1 section 22.3

Figure 22.6 TCP connection termination. After the connection establishment and data exchange, PC1 sends a segment with the FIN flag to terminate the connection. SRV1 replies with an ACK segment and then sends its own FIN segment, to which PC1 replies with an ACK segment.

Use the TCP termination diagram to follow the direction and order of the FIN and ACK segments. section 22.2.1

Unlike connection establishment, termination does not always use four separate messages. A host may combine ACK and FIN, producing FIN, ACK-FIN, ACK; the typical sequence remains the CCNA exam focus. section 22.2.1

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

How deep it goes

Shows the FIN, ACK, FIN, ACK sequence and notes combined flags, while deferring unusual termination details.

Examples

Uses PC1 and SRV1 to illustrate the four termination messages after TCP data exchange.

What it assumes

Assumes basic TCP communication and connection establishment before explaining termination after data exchange.

What you can do after

Understand TCP termination by recalling and identifying the typical FIN, ACK, FIN, ACK exchange.

How it connects

Connects TCP connection termination with TCP connection establishment, FIN, ACK, SYN, and the three-way handshake.

liveBookmanning.com

Read Acing the CCNA Exam: Volume 1 Fundamentals and Protocols on liveBook Get the book at manning.com

Also explained in

• 

  liveBookmanning.com
  Acing the CCNA Exam: Volume 2 Advanced Networking and SecurityA four-way handshake is a WPA key-establishment exchange between a wireless client and authenticator that generates encryption keys for data communication.

Compare how 2 books explain four-way handshake →

Related concepts

• pre-shared keyuses · A pre-shared key is secret keying material provisioned in advance to communicating parties for authentication or cryptographic operations.
• tcp connection terminationkind of · TCP connection termination is the process of closing a TCP connection after the necessary data has been exchanged.
• three-way handshakecontrasts with · A three-way handshake is the exchange of SYN, SYN/ACK, and ACK messages that establishes a TCP connection between endpoints.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 1 Fundamentals and Protocols. The section links point to the parts of the book it draws on.