new_contents: 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 SimpleSwitch(app_manager.RyuApp):
      OFP_VERSIONS = [ofproto_v1_3.OFP_VERSION]
  
      def __init__(self, *args, **kwargs):
          super(SimpleSwitch, self).__init__(*args, **kwargs)
          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
  
          match = parser.OFPMatch()
          actions = [parser.OFPActionOutput(ofproto.OFPP_CONTROLLER,
                                            ofproto.OFPCML_NO_BUFFER)]
          self.add_flow(datapath, 0, match, actions)
  
      def add_flow(self, datapath, priority, match, actions, buffer_id=None):
          ofproto = datapath.ofproto
          parser = datapath.ofproto_parser
  
          inst = [parser.OFPInstructionActions(ofproto.OFPIT_APPLY_ACTIONS,
                                               actions)]
          if buffer_id:
              mod = parser.OFPFlowMod(datapath=datapath, buffer_id=buffer_id,
                                      priority=priority, match=match,
                                      instructions=inst)
          else:
              mod = parser.OFPFlowMod(datapath=datapath, priority=priority,
                                      match=match, instructions=inst)
          datapath.send_msg(mod)
  
      @set_ev_cls(ofp_event.EventOFPPacketIn, MAIN_DISPATCHER)
      def _packet_in_handler(self, ev):
          # If you hit this you might want to increase
          # the "miss_send_length" of your switch
          if ev.msg.msg_len < ev.msg.total_len:
              self.logger.debug("packet truncated: only %s of %s bytes",
                                ev.msg.msg_len, ev.msg.total_len)
          msg = ev.msg
          datapath = msg.datapath
          ofproto = datapath.ofproto
          parser = datapath.ofproto_parser
          in_port = msg.match['in_port']
  
          pkt = packet.Packet(msg.data)
          eth = pkt.get_protocols(ethernet.ethernet)[0]
  
          dst = eth.dst
          src = eth.src
  
          dpid = datapath.id
          self.mac_to_port.setdefault(dpid, {})
  
          self.logger.info("packet in %s %s %s %s", dpid, src, dst, in_port)
  
          # 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]:
              out_port = self.mac_to_port[dpid][dst]
          else:
              out_port = ofproto.OFPP_FLOOD
  
          actions = [parser.OFPActionOutput(out_port)]
  
          # install a flow to avoid packet_in next time
          if out_port != ofproto.OFPP_FLOOD:
              match = parser.OFPMatch(in_port=in_port, eth_dst=dst)
              # verify if we have a valid buffer_id, if yes avoid to send both
              # flow_mod & packet_out
              if msg.buffer_id != ofproto.OFP_NO_BUFFER:
                  self.add_flow(datapath, 1, match, actions, msg.buffer_id)
                  return
              else:
                  self.add_flow(datapath, 1, match, actions)
          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)
  
subject: Add simple switch