写一段基于RYU的pox算法的文件

以下是一个基于RYU的POX算法的示例文件：

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_0
from ryu.lib.packet import packet, ethernet
from ryu.lib.packet.ethernet import ETH_TYPE_IP
from ryu.lib.packet.ipv4 import ipv4

class SimpleSwitch(app_manager.RyuApp):
    OFP_VERSIONS = [ofproto_v1_0.OFP_VERSION]

    def __init__(self, *args, **kwargs):
        super(SimpleSwitch, self).__init__(*args, **kwargs)
        self.mac_to_port = {}

    @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.in_port

        pkt = packet.Packet(msg.data)
        eth = pkt.get_protocol(ethernet.ethernet)

        if eth.ethertype == ETH_TYPE_IP:
            ip = pkt.get_protocol(ipv4.ipv4)
            src_ip = ip.src
            dst_ip = ip.dst

            # Add flow entry to forward the packet to the correct port
            actions = [parser.OFPActionOutput(self.mac_to_port[dst_ip])]
            match = parser.OFPMatch(dl_type=ETH_TYPE_IP, nw_src=src_ip, nw_dst=dst_ip)
            self.add_flow(datapath, 1, match, actions)

            # Send packet out the correct port
            out = parser.OFPPacketOut(
                datapath=datapath,
                buffer_id=msg.buffer_id,
                in_port=in_port,
                actions=actions,
                data=msg.data
            )
            datapath.send_msg(out)

    def add_flow(self, datapath, priority, match, actions):
        ofproto = datapath.ofproto
        parser = datapath.ofproto_parser

        inst = [parser.OFPInstructionActions(ofproto.OFPIT_APPLY_ACTIONS, actions)]
        mod = parser.OFPFlowMod(
            datapath=datapath,
            priority=priority,
            match=match,
            instructions=inst
        )
        datapath.send_msg(mod)

    @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 default flow entry to send all packets to controller
        match = parser.OFPMatch()
        actions = [parser.OFPActionOutput(ofproto.OFPP_CONTROLLER, ofproto.OFPCML_NO_BUFFER)]
        self.add_flow(datapath, 0, match, actions)

    @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']

        pkt = packet.Packet(msg.data)
        eth = pkt.get_protocol(ethernet.ethernet)

        dst = eth.dst
        src = eth.src

        dpid = datapath.id
        self.mac_to_port.setdefault(dpid, {})

        # Learn the MAC address to avoid flooding 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 entry to forward the packet to the correct port
        if out_port != ofproto.OFPP_FLOOD:
            match = parser.OFPMatch(in_port=in_port, dl_dst=dst)
            self.add_flow(datapath, 1, match, actions)

        # Send packet out the correct port
        out = parser.OFPPacketOut(
            datapath=datapath,
            buffer_id=msg.buffer_id,
            in_port=in_port,
            actions=actions,
            data=msg.data
        )
        datapath.send_msg(out)

此文件实现了一个简单的交换机应用程序，其目的是学习MAC地址并使用流表转发数据包。它还添加了一个特定于IP地址的流表，以便可以根据源IP地址和目标IP地址转发数据包。