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?

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.