Understanding TCP Connections: Three‑Way Handshake and Four‑Way Termination

TCP is a connection‑oriented protocol, meaning a communication channel must be established before data is sent, similar to a phone call where the receiver must answer before conversation can begin.

In contrast, UDP is connectionless; it sends data without prior negotiation, akin to sending a text message regardless of the recipient’s status.

Three‑way handshake:

First, the client sends a SYN packet (seq=100) and waits for the server’s response.

Second, the server replies with SYN+ACK (ACK=101) and its own SYN (seq=300).

Third, the client acknowledges with ACK=301, after which data exchange can start.

The handshake can be illustrated with a metaphor of a boy proposing to a girl, the girl responding positively, and the boy confirming the relationship.

Four‑way termination:

After the client finishes sending data, it sends a FIN packet.

The server acknowledges with ACK, then when ready, sends its own FIN.

The client acknowledges the server’s FIN with ACK, completing the connection closure.

This termination process can be likened to a breakup scenario, with each side sending and acknowledging termination messages.