实验3：OpenFlow协议分析实践

一、实验目的

  • 能够运用 wireshark 对 OpenFlow 协议数据交互过程进行抓包
  • 能够借助包解析工具，分析与解释 OpenFlow协议的数据包交互过程与机制
     

二、实验环境

  • 下载虚拟机软件Oracle VisualBox
  • 在虚拟机中安装Ubuntu 20.04 Desktop amd64，并完整安装Mininet
     

三、实验要求

3.1 基本要求

3.1.1 搭建下图所示拓扑，完成相关 IP 配置，并实现主机与主机之间的 IP 通信。用抓包软件获取控制器与交换机之间的通信数据包。


 

主机 IP地址
h1 192.168.0.101/24
h2 192.168.0.102/24
h3 192.168.0.103/24
h4 192.168.0.104/24
 
  • 在Mininet可视化工具中搭建目标拓扑，并保存为py文件

     
  • 代码如下：
#!/usr/bin/env python

from mininet.net import Mininet
from mininet.node import Controller, RemoteController, OVSController
from mininet.node import CPULimitedHost, Host, Node
from mininet.node import OVSKernelSwitch, UserSwitch
from mininet.node import IVSSwitch
from mininet.cli import CLI
from mininet.log import setLogLevel, info
from mininet.link import TCLink, Intf
from subprocess import call

def myNetwork():

    net = Mininet( topo=None,
                   build=False,
                   ipBase='192.168.0.0/24')

    info( '*** Adding controller\n' )
    c0=net.addController(name='c0',
                      controller=Controller,
                      protocol='tcp',
                      port=6633)

    info( '*** Add switches\n')
    s1 = net.addSwitch('s1', cls=OVSKernelSwitch)
    s2 = net.addSwitch('s2', cls=OVSKernelSwitch)

    info( '*** Add hosts\n')
    h1 = net.addHost('h1', cls=Host, ip='192.168.0.101/24', defaultRoute=None)
    h2 = net.addHost('h2', cls=Host, ip='192.168.0.102/24', defaultRoute=None)
    h3 = net.addHost('h3', cls=Host, ip='192.168.0.103/24', defaultRoute=None)
    h4 = net.addHost('h4', cls=Host, ip='192.168.0.104/24', defaultRoute=None)

    info( '*** Add links\n')
    net.addLink(h1, s1)
    net.addLink(h3, s1)
    net.addLink(s1, s2)
    net.addLink(s2, h4)
    net.addLink(h2, s2)

    info( '*** Starting network\n')
    net.build()
    info( '*** Starting controllers\n')
    for controller in net.controllers:
        controller.start()

    info( '*** Starting switches\n')
    net.get('s1').start([c0])
    net.get('s2').start([c0])

    info( '*** Post configure switches and hosts\n')

    CLI(net)
    net.stop()

if __name__ == '__main__':
    setLogLevel( 'info' )
    myNetwork()
 

3.1.2 查看抓包结果，分析OpenFlow协议中交换机与控制器的消息交互过程，画出相关交互图或流程图。

  • Hello
    • 控制器6633端口（我最高能支持OpenFlow 1.0）---> 交换机44026端口

       
    • 交换机44026端口（我最高能支持OpenFlow 1.5) ---> 控制器6633端口

       
    • 于是双方建立连接，并使用OpenFlow 1.0
       
  • Features Request
    • 控制器6633端口（我需要你的特征信息） ---> 交换机44026端口

       
    • 控制器6633端口（请按照我给你的 flag 和 max bytes of packet 进行配置） --->
      交换机44026端口
      • flag：指示交换机如何处理 IP 分片数据包
      • max bytes of packet：当交换机无法处理到达的数据包时，向控制器发送如何处理的最大字节数，本实验中控制器发送的值是0x0080，即128字节。

         
  • Port_Status
    • 当交换机端口发生变化时，告知控制器相应的端口状态

       
  • Features Reply
    • 交换机44026端口（这是我的特征信息，请查收）--- 控制器6633端口

       
  • Packet_in
    • 有两种情况：
      • 交换机查找流表，发现没有匹配条目时
      • 有匹配条目但是对应的action是OUTPUT=CONTROLLER时
    • 交换机44026端口（有数据包进来，请指示）--- 控制器6633端口

       
    • 分析抓取的数据包，可以发现是因为交换机发现此时自己并没有匹配的流表（Reason: No matching flow (table-miss flow entry) (0)），所以要问控制器如何处理
       
  • Flow_mod
    • 分析抓取的flow_mod数据包，控制器通过6633端口向交换机44026端口、交换机44028端口下发流表项，指导数据的转发处理

       

       
  • Packet_out
    • 控制器6633端口（请按照我给你的action进行处理） ---> 交换机44026端口

       
    • 告诉输出到交换机的65531端口
       
  • 流程图

     

3.1.3 回答问题：交换机与控制器建立通信时是使用TCP协议还是UDP协议？

答：在wireshark的抓包工具中，可以看到交换器与控制器建立通信时是使用TCP协议。
 

3.2 进阶要求

3.2.1 将抓包结果对照OpenFlow源码，了解OpenFlow主要消息类型对应的数据结构定义。

  • 相关数据结构可在openflow安装目录openflow/include/openflow当中的openflow.h头文件中查询到。
     
  • Hello

     
    • OpenFlow源码：
struct ofp_header {
    uint8_t version;    /* OFP_VERSION. */
    uint8_t type;       /* One of the OFPT_ constants. */
    uint16_t length;    /* Length including this ofp_header. */
    uint32_t xid;       /* Transaction id associated with this packet.
                           Replies use the same id as was in the request
                           to facilitate pairing. */
};

struct ofp_hello {
    struct ofp_header header;
};
  • 可以看到结构体中的四个参数对应上图中的四个数据
  • version：OFP协议版本，type：OFPT的常数之一，length：OFP头的长度，xid：与此包相关的交互ID。回复时使用与请求中相同的ID，以方便配对。
     
  • FEATURES_REQUEST

     
    • 可以看到与Hello中的数据格式相同
       
  • Set Conig

     
    • OpenFlow源码：
struct ofp_switch_config {
    struct ofp_header header;
    uint16_t flags;             /* OFPC_* flags. */
    uint16_t miss_send_len;     /* Max bytes of new flow that datapath should
                                   send to the controller. */
};

 

  • flag：指示交换机如何处理 IP 分片数据包，max bytes of packet：当交换机无法处理到达的数据包时，向控制器发送如何处理的最大字节数，本实验中控制器发送的值是0x0080，即128字节。
     
  • Port_Status
    • OpenFlow源码：
struct ofp_port_status {
    struct ofp_header header;
    uint8_t reason;          /* One of OFPPR_*. */
    uint8_t pad[7];          /* Align to 64-bits. */
    struct ofp_phy_port desc;
};

 

  • Features Reply

     
    • OpenFlow源码：
struct ofp_phy_port {
    uint16_t port_no;
    uint8_t hw_addr[OFP_ETH_ALEN];
    char name[OFP_MAX_PORT_NAME_LEN]; /* Null-terminated */

    uint32_t config;        /* Bitmap of OFPPC_* flags. */
    uint32_t state;         /* Bitmap of OFPPS_* flags. */

    /* Bitmaps of OFPPF_* that describe features.  All bits zeroed if
     * unsupported or unavailable. */
    uint32_t curr;          /* Current features. */
    uint32_t advertised;    /* Features being advertised by the port. */
    uint32_t supported;     /* Features supported by the port. */
    uint32_t peer;          /* Features advertised by peer. */
};

struct ofp_switch_features {
    struct ofp_header header;
    uint64_t datapath_id;   /* Datapath unique ID.  The lower 48-bits are for
                               a MAC address, while the upper 16-bits are
                               implementer-defined. */

    uint32_t n_buffers;     /* Max packets buffered at once. */

    uint8_t n_tables;       /* Number of tables supported by datapath. */
    uint8_t pad[3];         /* Align to 64-bits. */

    /* Features. */
    uint32_t capabilities;  /* Bitmap of support "ofp_capabilities". */
    uint32_t actions;       /* Bitmap of supported "ofp_action_type"s. */

    /* Port info.*/
    struct ofp_phy_port ports[0];  /* Port definitions.  The number of ports
                                      is inferred from the length field in
                                      the header. */
};

 

  • datapath_id：唯一标识符，n_buffers：交换机缓冲区可以缓存的最大数据包个数，n_tables：流表数量，pad：可以理解为填充值，capabilities：支持的特殊功能，actions：支持的动作，port data：物理端口描述列表。
     
  • Packet_in

     
    • OpenFlow源码：
#没有匹配到
enum ofp_packet_in_reason {
    OFPR_NO_MATCH,          /* No matching flow. */
    OFPR_ACTION             /* Action explicitly output to controller. */
};

#向控制器发送包
struct ofp_packet_in {
    struct ofp_header header;
    uint32_t buffer_id;     /* ID assigned by datapath. */
    uint16_t total_len;     /* Full length of frame. */
    uint16_t in_port;       /* Port on which frame was received. */
    uint8_t reason;         /* Reason packet is being sent (one of OFPR_*) */
    uint8_t pad;
    uint8_t data[0];        /* Ethernet frame, halfway through 32-bit word,
                               so the IP header is 32-bit aligned.  The
                               amount of data is inferred from the length
                               field in the header.  Because of padding,
                               offsetof(struct ofp_packet_in, data) ==
                               sizeof(struct ofp_packet_in) - 2. */
};

 

  • Flow_mod

     
    • OpenFlow源码：
struct ofp_match {
    uint32_t wildcards;        /* Wildcard fields. */
    uint16_t in_port;          /* Input switch port. */
    uint8_t dl_src[OFP_ETH_ALEN]; /* Ethernet source address. */
    uint8_t dl_dst[OFP_ETH_ALEN]; /* Ethernet destination address. */
    uint16_t dl_vlan;          /* Input VLAN id. */
    uint8_t dl_vlan_pcp;       /* Input VLAN priority. */
    uint8_t pad1[1];           /* Align to 64-bits */
    uint16_t dl_type;          /* Ethernet frame type. */
    uint8_t nw_tos;            /* IP ToS (actually DSCP field, 6 bits). */
    uint8_t nw_proto;          /* IP protocol or lower 8 bits of
                                * ARP opcode. */
    uint8_t pad2[2];           /* Align to 64-bits */
    uint32_t nw_src;           /* IP source address. */
    uint32_t nw_dst;           /* IP destination address. */
    uint16_t tp_src;           /* TCP/UDP source port. */
    uint16_t tp_dst;           /* TCP/UDP destination port. */
};

struct ofp_flow_mod {
    struct ofp_header header;
    struct ofp_match match;      /* Fields to match */
    uint64_t cookie;             /* Opaque controller-issued identifier. */

    /* Flow actions. */
    uint16_t command;             /* One of OFPFC_*. */
    uint16_t idle_timeout;        /* Idle time before discarding (seconds). */
    uint16_t hard_timeout;        /* Max time before discarding (seconds). */
    uint16_t priority;            /* Priority level of flow entry. */
    uint32_t buffer_id;           /* Buffered packet to apply to (or -1).
                                     Not meaningful for OFPFC_DELETE*. */
    uint16_t out_port;            /* For OFPFC_DELETE* commands, require
                                     matching entries to include this as an
                                     output port.  A value of OFPP_NONE
                                     indicates no restriction. */
    uint16_t flags;               /* One of OFPFF_*. */
    struct ofp_action_header actions[0]; /* The action length is inferred
                                            from the length field in the
                                            header. */
};

 

  • Packet_out

     
    • OpenFlow源码：
struct ofp_action_header {
    uint16_t type;                  /* One of OFPAT_*. */
    uint16_t len;                   /* Length of action, including this
                                       header.  This is the length of action,
                                       including any padding to make it
                                       64-bit aligned. */
    uint8_t pad[4];
};

struct ofp_packet_out {
    struct ofp_header header;
    uint32_t buffer_id;           /* ID assigned by datapath (-1 if none). */
    uint16_t in_port;             /* Packet's input port (OFPP_NONE if none). */
    uint16_t actions_len;         /* Size of action array in bytes. */
    struct ofp_action_header actions[0]; /* Actions. */
    /* uint8_t data[0]; */        /* Packet data.  The length is inferred
                                     from the length field in the header.
                                     (Only meaningful if buffer_id == -1.) */
};
 

四、总结

  • 实验难度：适中
  • 实验过程遇到的困难及解决办法：
    • 在进行基本要求中的查看抓包结果，分析OpenFlow协议中交换机与控制器的消息交互过程时，忘了先开启抓包再运行拓扑，弄好后发现根本找不到flow_mod信号，于是发现要先ping一下主机，才能看到flow_mod。
    • 刚开始找不到交换机对控制器发送的Hello信号，后来发现它支持最高的版本为OpenFlow 1.5，于是要用openflow_v6来过滤。
    • 在完成进阶要求的时候，1000行的源码看的真的很辛苦，找的很累，后来就直接把源码复制到word，用word的查找功能，效率就快了很多！
  • 个人感想：
    • 这次实验感觉就是不需要自己思考什么东西，最主要的就是用眼睛去看，去理解。
    • 能够熟练运用 wireshark 对 OpenFlow 协议数据交互过程进行抓包。
    • 学会借助包解析工具，分析与解释 OpenFlow协议的数据包交互过程与机制。
    • OpenFlow的源码看的真累，有的还是看不懂，但看代码的话能确实能更加直观的理解OpenFlow协议。
posted @ 2021-09-22 23:44  雀食蟀  阅读(517)  评论(1)  编辑  收藏  举报