text: from ryu.base import app_manager
  from ryu.controller import ofp_event
  from ryu.controller.handler import CONFIG_DISPATCHER, MAIN_DISPATCHER
  from ryu.controller.handler import set_ev_cls
  from ryu.ofproto import ofproto_v1_3
  from ryu.lib.packet import packet
  from ryu.lib.packet import ethernet
  
  
  class SimpleSwitch13(app_manager.RyuApp):
      OFP_VERSIONS = [ofproto_v1_3.OFP_VERSION]
  
      def __init__(self, *args, **kwargs):
          super(SimpleSwitch13, self).__init__(*args, **kwargs)
  
          # we want to remember the port mappings in this structure
          self.mac_to_port = {}
  
      @set_ev_cls(ofp_event.EventOFPSwitchFeatures, CONFIG_DISPATCHER)
      def switch_features_handler(self, ev):
          datapath = ev.msg.datapath
          ofproto = datapath.ofproto
          parser = datapath.ofproto_parser
  
          # install table-miss flow entry
          #
          # We specify NO BUFFER to max_len of the output action due to
          # OVS bug. At this moment, if we specify a lesser number, e.g.,
          # 128, OVS will send Packet-In with invalid buffer_id and
          # truncated packet data. In that case, we cannot output packets
          # correctly.  The bug has been fixed in OVS v2.1.0.
          match = parser.OFPMatch()
          actions = [parser.OFPActionOutput(ofproto.OFPP_CONTROLLER,
                                            ofproto.OFPCML_NO_BUFFER)]
          inst = [parser.OFPInstructionActions(ofproto.OFPIT_APPLY_ACTIONS,
                                               actions)]
          mod = parser.OFPFlowMod(datapath=datapath, priority=0,
                                  match=match, instructions=inst)
          datapath.send_msg(mod)
  
      @set_ev_cls(ofp_event.EventOFPPacketIn, MAIN_DISPATCHER)
      def _packet_in_handler(self, ev):
          msg = ev.msg
          datapath = msg.datapath
          ofproto = datapath.ofproto
          parser = datapath.ofproto_parser
          in_port = msg.match['in_port']
  
          dpid = datapath.id
  
          # if we have not seen this switch yet then we add it to our memory
          # with an empty port mapping
          self.mac_to_port.setdefault(dpid, {})
  
          # we need to disect the message payload
          pkt = packet.Packet(msg.data)
          eth = pkt.get_protocol(ethernet.ethernet)
  
          # from the ethernet frame we can extract the MAC addresses
          dst = eth.dst
          src = eth.src
  
          # learn a mac address to avoid FLOOD next time.
          self.mac_to_port[dpid][src] = in_port
  
  
          if dst in self.mac_to_port[dpid]:
              # if we already know the mac we can select the output port
              out_port = self.mac_to_port[dpid][dst]
              self.logger.info('Message for known {} at port {}'.format(src, out_port))
          else:
              # but if it can't be found we just flood the packet
              self.logger.info('Message for unknown {}'.format(src))
              out_port = ofproto.OFPP_FLOOD
  
          actions = [parser.OFPActionOutput(out_port)]
  
          data = None
          if msg.buffer_id == ofproto.OFP_NO_BUFFER:
              data = msg.data
  
          out = parser.OFPPacketOut(datapath=datapath, buffer_id=msg.buffer_id,
                                    in_port=in_port, actions=actions, data=data)
          datapath.send_msg(out)
  