three way handshake examples

Three-Way Handshake: Top Examples & Guide

By Amanda DavisApril 20, 2026

The three-way handshake is a fundamental concept in computer networking, particularly in the context of Transmission Control Protocol (TCP). It's a process that ensures reliable communication between two hosts by establishing a connection and synchronizing their sequence numbers. Let's delve into the intricacies of the three-way handshake, its importance, and explore some practical examples.

Understanding the Three-Way Handshake

The three-way handshake is a crucial part of the TCP connection establishment process. It involves the following steps:

Step 1 (SYN): The client sends a SYN packet to the server, initiating the handshake. This packet contains an initial sequence number (ISN).
Step 2 (SYN-ACK): The server responds with a SYN-ACK packet, acknowledging the received SYN packet and sending its own ISN.
Step 3 (ACK): The client sends an ACK packet, acknowledging the received SYN-ACK packet. This final ACK packet confirms that both the client and server have agreed on the initial sequence numbers, and the connection is established.

Why is the Three-Way Handshake Important?

The three-way handshake serves several critical purposes:

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**Reliable Connection Establishment:** It ensures that both the client and server agree to establish a connection.
**Sequence Number Synchronization:** It synchronizes the sequence numbers between the client and server, enabling reliable data transfer.
**Preventing Unwanted Connections:** It helps prevent unwanted connections by ensuring that the server only accepts connections from willing clients.

Three-Way Handshake Examples

Example 1: Establishing a Web Connection

When you navigate to a website, your browser (the client) initiates a three-way handshake with the web server. Here's how it works:

Packet Type	Sender	Receiver	Purpose
SYN	Browser	Web Server	Initiates the connection
SYN-ACK	Web Server	Browser	Acknowledges the SYN and sends its ISN
ACK	Browser	Web Server	Acknowledges the SYN-ACK and confirms the connection

Example 2: SSH Connection

When you connect to a remote server using SSH, a three-way handshake is also involved. In this case, the SSH client initiates the handshake with the SSH server. The process is similar to the web connection example, with the following key differences:

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The SYN packet contains additional SSH-specific information.
The SYN-ACK and ACK packets also contain SSH-specific data.

Example 3: TCP Port Scanning

In the context of network security, the three-way handshake is also used in TCP port scanning. A port scanner sends a SYN packet to a target host, expecting a SYN-ACK response if the port is open. If no response is received, the port is considered closed or filtered. This process is known as a half-open scan or stealth scan.