Why would a client not send a FIN?

TCP Stream in a tcp dump off a remote network segment showed proper handshake and conversation up to the 8s mark followed by a 20s delay where no packets were sent from either client or server. Server send s开发者_开发知识库ent FIN,ACK followed shortly by client's ACK and then RST packets. Why wouldn't the client send a FIN? And is FIN,ACK the proper response for a session timeout from the server's side?

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The RST means that the client had already closed the connection.

The FIN means that the server closed the connection. It's one reasonable response to a timeout.