Flow removed message doesn't work (mininet+ryu controller in ubuntu20.10)

[plushao666]

Flow removed message：When flow entries time out or are deleted, the switch notifies controller with this message. I write the following example to my ryu app. And the ryu controller can't detect the flow removed message from switches, when the flow entries timeout or are deleted by hand. Plz tell me what the problem is......

    @set_ev_cls(ofp_event.EventOFPFlowRemoved, MAIN_DISPATCHER)
    def flow_removed_handler(self, ev):
        msg = ev.msg
        dp = msg.datapath
        ofp = dp.ofproto

        if msg.reason == ofp.OFPRR_IDLE_TIMEOUT:
            reason = 'IDLE TIMEOUT'
        elif msg.reason == ofp.OFPRR_HARD_TIMEOUT:
            reason = 'HARD TIMEOUT'
        elif msg.reason == ofp.OFPRR_DELETE:
            reason = 'DELETE'
        elif msg.reason == ofp.OFPRR_GROUP_DELETE:
            reason = 'GROUP DELETE'
        else:
            reason = 'unknown'

        self.logger.debug('OFPFlowRemoved received: '
                          'cookie=%d priority=%d reason=%s table_id=%d '
                          'duration_sec=%d duration_nsec=%d '
                          'idle_timeout=%d hard_timeout=%d '
                          'packet_count=%d byte_count=%d match.fields=%s',
                          msg.cookie, msg.priority, reason, msg.table_id,
                          msg.duration_sec, msg.duration_nsec,
                          msg.idle_timeout, msg.hard_timeout,
                          msg.packet_count, msg.byte_count, msg.match)

[plushao666]

Because you should add flags when add a flow entry. mod = parser.OFPFlowMod(datapath=datapath, priority=0, match=match, instructions=inst, flags=ofproto.OFPFF_SEND_FLOW_REM)