TCP Handshake Basics

This poster summarizes the TCP connection lifecycle: the three-way handshake used to establish a connection, the meaning of common TCP control flags, and the four-way handshake usually used to close a connection.

1. TCP Three-Way Handshake

TCP uses a three-way handshake to establish a connection before data transmission.

Steps

1. SYN
   The client sends a request to the server:
   "I want to connect."

2. SYN-ACK
   The server replies with SYN + ACK:

   • ACK: "I received your request."
   • SYN: "I am also ready to establish the connection."

3. ACK
   The client replies:
   "I received your confirmation."

After these three steps, the TCP connection is established.

2. TCP Four-Way Handshake

TCP usually uses a four-way handshake to close a connection.

Steps

1. FIN
   Client says:
   "I am done sending data. I want to close."

2. ACK
   Server says:
   "I received your close request."

3. FIN
   Server says:
   "I am also done. I want to close."

4. ACK
   Client says:
   "I received your close confirmation."

After these steps, the TCP connection is closed.

3. Key Terms

SYN

SYN means synchronize.

It is used to start a TCP connection.

ACK

ACK means acknowledgment.

It means: "I received your message."

FIN

FIN means finish.

It is used when one side wants to close the TCP connection.

4. Simple Summary

• Three-way handshake = open a TCP connection
• Four-way handshake = close a TCP connection
• SYN = start
• ACK = received
• FIN = finish / close