實驗6：開源控制器實踐——RYU

安裝截圖

拓撲視覺化

tcpdump檢視

h1 ping h2

h1 ping h3

可以看到均為洪泛轉發

檢視控制器流表，如下圖：

看到沒有流表，而使用pox的hub模組則會看到流表，如下圖：

所以可以看到二者都是洪泛轉發，但是不同之處在於POX是直接向交換機下發流表，而Ryu是在每個 Packet In 事件之後，向交換機下發動作。

進階要求

simple_switch_13.py程式碼註釋

# Copyright (C) 2011 Nippon Telegraph and Telephone Corporation.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
# implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# 引入包
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
from ryu.lib.packet import ether_types


class SimpleSwitch13(app_manager.RyuApp):
    # 定義openflow版本
    OFP_VERSIONS = [ofproto_v1_3.OFP_VERSION]

    def __init__(self, *args, **kwargs):
        super(SimpleSwitch13, self).__init__(*args, **kwargs)
        # 定義儲存mac地址到埠的一個對映
        self.mac_to_port = {}

    # 處理EventOFPSwitchFeatures事件
    @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)]
        self.add_flow(datapath, 0, match, actions)

    # 新增流表函式
    def add_flow(self, datapath, priority, match, actions, buffer_id=None):
        # 獲取交換機資訊
        ofproto = datapath.ofproto
        parser = datapath.ofproto_parser

        # 對action進行包裝
        inst = [parser.OFPInstructionActions(ofproto.OFPIT_APPLY_ACTIONS,
                                             actions)]
        # 判斷是否有buffer_id，生成mod物件
        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)
        # 傳送mod
        datapath.send_msg(mod)

    # 處理 packet in 事件
    @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)
        # 獲取包資訊，交換機資訊，協議等等
        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]

        # 忽略LLDP型別
        if eth.ethertype == ether_types.ETH_TYPE_LLDP:
            # ignore lldp packet
            return

        # 獲取源埠，目的埠
        dst = eth.dst
        src = eth.src

        dpid = format(datapath.id, "d").zfill(16)
        self.mac_to_port.setdefault(dpid, {})

        self.logger.info("packet in %s %s %s %s", dpid, src, dst, in_port)

        # 學習包的源地址，和交換機上的入埠繫結
        # learn a mac address to avoid FLOOD next time.
        self.mac_to_port[dpid][src] = in_port

        # 檢視是否已經學習過該目的mac地址
        if dst in self.mac_to_port[dpid]:
            out_port = self.mac_to_port[dpid][dst]
        # 否則進行洪泛
        else:
            out_port = ofproto.OFPP_FLOOD

        actions = [parser.OFPActionOutput(out_port)]

        # 下發流表處理後續包，不再觸發 packet in 事件
        # 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, eth_src=src)
            # 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)
        # 傳送流表
        datapath.send_msg(out)
 

程式碼當中的mac_to_port的作用是什麼？

儲存mac地址到交換機埠的對映，為交換機自學習功能提供資料結構進行 mac-埠 的儲存

simple_switch和simple_switch_13在dpid的輸出上有何不同？

simple_switch的dpid賦值：dpid = datapath.id

simple_switch_13的dpid賦值：dpid = format(datapath.id, "d").zfill(16)

在python console進行測試，可以看到在simple_switch直接獲取的id，在simple_switch_13中，會在前端加上0將其填充至16位

相比simple_switch，simple_switch_13增加的switch_feature_handler實現了什麼功能？

實現交換機以特性應答訊息響應特性請求，可檢視文件

https://ryu.readthedocs.io/en/latest/ofproto_v1_3_ref.html#ryu.ofproto.ofproto_v1_3_parser.OFPSwitchFeatures

simple_switch_13是如何實現流規則下發的？

在接收到packetin事件後，首先獲取包學習，交換機資訊，乙太網資訊，協議資訊等。如果乙太網型別是LLDP型別，則不予處理。如果不是，則獲取源埠目的埠，以及交換機id，先學習源地址對應的交換機的入埠，再檢視是否已經學習目的mac地址，如果沒有則進行洪泛轉發。如果學習過該mac地址，則檢視是否有buffer_id，如果有的話，則在新增流動作時加上buffer_id，向交換機發送流表。

總結

本次實驗難度較難，主要在於對openflow協議的理解，以及對Ryu原始碼的熟悉程度。在實驗過程中，遇到如下問題：

• 在用Ryu的L2Switch模組下發流表時，看到洪泛現像，但是在交換機上沒有看到流表，在請教老師之後才知道，這才是Ryu與POX之間的差別
• 在分析simple_switch.py和simple_switch_13.py原始碼時，遇到困難，不理解函式的作用，在檢視官方文件，以及搜尋相關資料之後，對原始碼的理解相對透徹了些

這次實驗相比上次難度更大，對原始碼分析和對openflow協議的理解有一定的要求，但是做完實驗後感受到收穫頗多，學習到了更多的知識。