What Is a Three-Way Handshake?

Written by Coursera Staff • Updated on

Learn what a three-way handshake is and why it’s important in network communications.

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Key takeaways

A three-way handshake is a three-step process in networking that establishes a transmission control protocol (TCP) connection on an internet network.

  • A three-way handshake involves the following three steps: synchronize (SYN), synchronize-acknowledge (SYN-ACK), and acknowledge (ACK).

  • A three-way handshake creates a connection between two computers for the passing of information, but a four-way handshake terminates that connection with a pair of finish (FIN) and ACK messages.

  • You can view a three-way handshake as important because it’s a key mechanism for creating a secure connection over a TCP/IP network.

Explore the intricacies of the three-way handshake, how it’s performed, and why it’s essential in establishing a reliable connection for network communications. If you’re ready to begin developing your skill set in this field, enroll in the Google IT Support Professional Certificate. In as little as three months, you can learn about computer networking, IT infrastructure, version control, systems administration, and more.

What is a three-way handshake in networking?

In networking, a three-way handshake is a process that's used to initiate a connection in a transmission control protocol/internet protocol (TCP/IP) network. TCP is responsible for ensuring data is delivered correctly between computers on an internet network. It’s similar to a handshake between two people: one person extends their hand, the other shakes it, and then the first person acknowledges and retracts their hand.

Explain three-way handshake protocol: What is the correct sequence of the three-way handshake?

It involves the following three steps: synchronize (SYN), synchronize-acknowledge (SYN-ACK), and acknowledge (ACK).

Here's how it works:

1. SYN: The initiating computer (or active client) sends a synchronize sequence number (SYN) packet to the receiving computer (usually a server). The SYN packet value is set to an arbitrary number (e.g., 100) to “ask” if any open connections are available.

2. SYN-ACK: If the receiving computer (also known as a passive client) has open ports that can accept the connection, it sends back a synchronize-acknowledge (SYN-ACK) packet to the initiating computer. The packet includes two numbers: the receiving computer’s own SYN, which can be any arbitrary number as well (e.g., 200), and the ACK number, which is the initiating computer’s SYN plus one (e.g., 101). 

3. ACK: The initiating computer (active client) then sends an acknowledge sequence number (ACK) packet back to the receiving computer. This ACK packet is an acknowledgement of receipt of the SYN-ACK packet. The packet value is set to the receiving computer’s SYN (sent in step two) plus one again (e.g., 201). With this final step, the connection is established, and data transmission can begin.

These steps acknowledge the connection parameters on each side, stabilize the connection, and verify the serial numbers of each device. Each side must acknowledge a sequence number that is one larger than the one sent because the acknowledgement field relies on the next sequence number expected, which in turn acknowledges all of the earlier sequence numbers.

What is a four-way handshake?

Where the three-way handshake creates a connection allowing two computers to seamlessly pass information back and forth, a four-way handshake terminates that connection with a pair of finish (FIN) and ACK messages. 

Why is a TCP three-way handshake important?

The three-way handshake is a cornerstone of the TCP protocol because it establishes the connection parameters and reliable communication between two computers before actual data transmission begins. Much of our online activity uses three-way handshakes. It's a key mechanism for creating a secure connection over a TCP/IP network. For example, when you open a web browser and search for something on the internet, a three-way handshake connects the client-facing web browser with the internet server. Without proper synchronization, malicious actors such as black hat hackers could establish connections to vulnerable systems.

You can learn about TCP connections, OSI models, and how devices communicate from the experts at Microsoft by enrolling in the online course Introduction to Secure Networking.

Read more: Network Certification: 6 Options for Your IT Career

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