Understanding TCP's Three-Way Handshake and Four-Way Termination

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  • 2Minutes
  • 28 Aug, 2024

In our daily lives, network communication is everywhere, and TCP (Transmission Control Protocol) is a key tool in ensuring the reliability of these communications. TCP’s three-way handshake and four-way termination are like the process of confirming the start and end of a conversation on the phone.

What is the TCP Three-Way Handshake?

Imagine you’re making a phone call to a friend. Before the conversation begins, you need to confirm that both of you are on the line, which is similar to the TCP three-way handshake process:

  1. First Handshake: You (the client) dial the number and say, “Hello, I want to talk.” (The client sends a SYN request to the server, indicating a desire to establish a connection.)

  2. Second Handshake: Your friend (the server) picks up the phone and says, “I received it, we can talk.” (The server responds with a SYN-ACK, confirming that the connection can be established.)

  3. Third Handshake: You say, “Okay, let’s start.” (The client sends an ACK, confirming the connection is established.)

With these three simple confirmations, both parties confirm each other’s presence and can safely begin transmitting data.

Why is the Three-Way Handshake Necessary?

The three-way handshake ensures that both parties have confirmed each other’s ability to receive data and prevents old connection requests from misleading the server. For example, if an old request is delayed in the network and reaches the server, the server might mistakenly believe it’s a new connection request. Through the three-way handshake, TCP can avoid such errors and ensure the reliability of the connection.

What is the TCP Four-Way Termination?

When your conversation ends, both parties also need to confirm the disconnection, which is like TCP’s four-way termination:

  1. First Termination: You (the client) say, “I’m done talking, ready to hang up.” (The client sends a FIN request, indicating a desire to disconnect.)

  2. Second Termination: Your friend (the server) responds, “Okay, I got it.” (The server sends an ACK, confirming receipt of the request.)

  3. Third Termination: Your friend (the server) says, “I’m also done talking, ready to hang up.” (The server sends a FIN request, indicating they are also ready to disconnect.)

  4. Fourth Termination: You say, “Okay, let’s both hang up.” (The client sends an ACK, confirming the disconnection.)

Through these four confirmations, TCP ensures a smooth disconnection, avoiding data loss.

Summary

TCP’s three-way handshake and four-way termination are designed to ensure the reliability of network communication. The three-way handshake confirms that the connection is securely established, and the four-way termination ensures that the disconnection is orderly. Understanding these steps can help us better grasp the basic principles of network communication.