Why Does TCP Require a Three‑Way Handshake and a Four‑Way Teardown?

Three‑Way Handshake

TCP ensures reliable data transfer by establishing a connection through three sequential steps.

First Handshake

The client sends a SYN segment (SYN=1, ACK=0, seq=x) to the server and enters the SYN‑SENT state.

SYN=1, ACK=0 indicates a connection request.

x is the initial sequence number; a SYN segment consumes one sequence number.

Second Handshake

Upon receiving the request, the server replies with SYN=1, ACK=1, seq=y, ack=x+1 and moves to the SYN‑RCVD state.

SYN=1, ACK=1 marks the server’s acceptance.

y is the server’s initial sequence number.

ack=x+1 tells the client the next expected byte.

Third Handshake

The client acknowledges the server’s response with ACK=1, seq=x+1, ack=y+1, entering ESTABLISHED; the server also moves to ESTABLISHED, completing the connection.

Why Three Handshakes?

The extra step prevents stale connection requests from being mistakenly accepted, which could waste server resources if a client’s request is lost and later arrives after the client has timed out.

Four‑Way Teardown

Closing a TCP connection requires four steps, reflecting its bidirectional nature.

First Teardown

Side A, having finished sending data, sends a FIN=1, seq=u segment and enters FIN‑WAIT‑1.

FIN=1 signals a release request.

seq=u (u‑1 is the last byte sent).

Second Teardown

Side B acknowledges with ACK=1, seq=v, ack=u+1, entering CLOSE‑WAIT, then sends its own FIN=1, seq=v, ack=u+1.

ACK=1 confirms receipt.

v‑1 is B’s last byte.

ack=u+1 expects the next byte from A.

Third Teardown

After sending all data, B sends FIN=1, ACK=1, seq=w, ack=u+1 and moves to LAST‑ACK.

Fourth Teardown

A acknowledges with ACK=1, enters TIME‑WAIT for 2 MSL to ensure B receives the final ACK; after this period, A closes, and B follows suit, releasing both TCBs.

Why TIME‑WAIT?

TIME‑WAIT guarantees that B can retransmit its final FIN if the ACK is lost; without this wait, B might never close properly.