TCP 3-Way Handshake

include teardown

💻

Client

CLOSED

🖥️

Server

LISTEN

Press Connect to establish a TCP connection step by step.

Client → Server: SYN seq=100

Server → Client: SYN-ACK seq=300, ack=101

Client → Server: ACK seq=101, ack=301

Client → Server: DATA GET /index.html

Server → Client: DATA + ACK 200 OK (page)

Client → Server: FIN seq=520

Server → Client: ACK ack=521

Server → Client: FIN seq=940

Client → Server: ACK ack=941

TCP 3-Way Handshake — Interactive Connection Simulator

Before any data flows, TCP opens a connection with a three-way handshake. This simulator steps through SYN → SYN-ACK → ACK, shows the sequence and acknowledgement numbers, tracks each side's TCP state, and then closes the connection with the FIN/ACK teardown.

What you can do in this simulation

Step through the SYN, SYN-ACK, and ACK messages that open a TCP connection
See the sequence and acknowledgement numbers, and why ACK = received seq + 1
Watch each side move through TCP states (CLOSED → SYN_SENT → ESTABLISHED)
Include the graceful teardown (FIN / ACK) to close the connection
Understand why three steps are needed to confirm both directions are ready

Concepts covered

TCP handshake · SYN · ACK · sequence numbers · connection setup · TCP state machine · FIN teardown

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