Why TCP Uses a Three‑Way Handshake and a Four‑Step Teardown

1. What is TCP

TCP (Transmission Control Protocol) guarantees reliable data delivery: the data will reach the receiver, arrive in order, not be duplicated, and remain intact.

SYN (synchronization flag): indicates a request to establish a connection.

ACK (acknowledgment flag): confirms receipt of a segment.

ISN (initial sequence number): a random number for each connection to prevent attacks.

ack (acknowledgment number): the next expected byte = received sequence number + 1.

2. Three‑Way Handshake

Step 1 – Client → Server: SYN=1, seq=x. Client state changes from CLOSED to SYN_SENT; server state changes from LISTEN to SYN_RCVD. The client tells the server it wants to open a connection with initial sequence x.

Step 2 – Server → Client: SYN=1, ACK=1, ack=x+1, seq=y. Both client and server move to ESTABLISHED. The server acknowledges the client’s SYN (x+1) and supplies its own initial sequence y.

Step 3 – Client → Server: ACK=1, ack=y+1, seq=x+1. Server moves from SYN_RCVD to ESTABLISHED. The client acknowledges the server’s sequence, completing the connection.

3. Four‑Step Teardown

Step 1 – Client → Server: FIN=1, seq=u. Client state: ESTABLISHED → FIN_WAIT_1. The client indicates it has no more data to send.

Step 2 – Server → Client: ACK=1, ack=u+1, seq=v. Client state: FIN_WAIT_1 → FIN_WAIT_2; Server state: ESTABLISHED → CLOSE_WAIT. The server acknowledges the client’s FIN but may still have data to send.

Step 3 – Server → Client: FIN=1, ACK=1, ack=u+1, seq=w. Server state: CLOSE_WAIT → LAST_ACK. The server signals that it has finished sending data.

Step 4 – Client → Server: ACK=1, ack=w+1, seq=u+1. Client state: FIN_WAIT_2 → TIME_WAIT; Server state: LAST_ACK → CLOSED. The client acknowledges the server’s FIN and then waits 2 MSL before closing.

4. Why Three Handshakes?

Prevents a stale connection request from creating a false connection and wasting resources.

Ensures both sides can confirm that the other’s send/receive capabilities are functional.

5. Why Four Steps?

TCP is full‑duplex; each direction’s sending channel must be closed independently.

The server may still have data to send after receiving the client’s FIN, so it must ACK the FIN first, then send its own FIN after finishing transmission.

6. Role of TIME_WAIT

Guarantees the final ACK reaches the peer; if lost, the server will retransmit its FIN after timeout.

Prevents old duplicate segments from being misinterpreted by a new connection, avoiding data corruption.