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FIN says: “I finished talking to you, but I’ll still listen to everything you have to say until you say that you’re done.”
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RST says: “There is no conversation. I won’t say anything and I won’t listen to anything you say.”
RST is useful if you have long lasting TCP connection with little traffic. If one of the computers is restarted, it forgets about the connection, and the other computer gets RST, as soon as it sends another packet.