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Networking Fundamentals · Intermediate

TCP Connection Termination

The FIN and ACK exchange (usually four segments) that gracefully closes a TCP connection in each direction.

Definition

Because TCP is full-duplex, each side closes its half independently: one side sends FIN, the other ACKs, later sends its own FIN, and the first side ACKs. The side that closes first enters TIME_WAIT for twice the maximum segment lifetime. A connection can also be aborted immediately with RST.

Example

netstat showing many TIME_WAIT entries on a busy web server reflects normal connection closure.

Exam tip

Graceful close uses FIN; an abrupt reset uses RST, often seen when a port is closed or a firewall rejects traffic.

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