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

（一）基本要求

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

1. 配置ip地址
   

2. 运行wireshark，选择any模式进行抓包，开启另一个终端，命令行运行031902241.py文件，运行pingall
   

3. 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（截图以其中一个交换机为例）

• OFPT_HELLO 源端口6633 -> 目的端口55224，从控制器到交换机
  

• OFPT_HELLO 源端口55224-> 目的端口6633，从交换机到控制器，此处协议为openflow1.5

  控制器与交换机建立连接，并使用OpenFlow 1.0
  

• OFPT_FEATURES_REQUEST 源端口6633 -> 目的端口55224，从控制器到交换机

  控制器请求交换器的特征信息
  

• OFPT_SET_CONFIG 源端口6633 -> 目的端口55224，从控制器到交换机

  控制器要求交换机按照所给出的信息进行配置
  

• OFPT_PORT_STATUS 源端口55224-> 目的端口6633，从交换机到控制器

  当交换机端口发生变化时，交换机告知控制器相应的端口状态
  

• OFPT_FEATURES_REPLY 源端口55224-> 目的端口6633，从交换机到控制器

  交换机告知控制器它的特征信息
  

• OFPT_PACKET_IN 源端口55224-> 目的端口6633，从交换机到控制器

  交换机告知控制器有数据包进来，请求控制器指示
  

• OFPT_PACKET_OUT 源端口6633 -> 目的端口55224，从控制器到交换机

  控制器要求交换机按照所给出的action进行处理
  

• OFPT_FLOW_MOD 源端口6633 -> 目的端口55224，从控制器到交换机

  控制器对交换机进行流表的添加、删除、变更等操作
  

4. 交互图
   

5. 交换机与控制器建立通信时是使用TCP协议还是UDP协议？

如图所示为（Transmission Control Protocol）TCP协议。

（二）进阶要求

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

1. HELLO

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;
};

可以看到对应了HELLO报文的四个参数

2. FEATURES_REQUEST
   

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. */
};

3. SET_CONFIG
   

   控制器下发的交换机配置数据结构体

/* Switch configuration. */
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. */
};

4. PORT_STATUS
   

/* A physical port has changed in the datapath */
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;
};

5. FEATURES_REPLY

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. */
};
/* Description of a physical port */
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. */
};

可以看到与图中信息一一对应，包括交换机物理端口的信息

6. PACKET_IN
   

PACKET_IN有两种情况：

交换机查找流表，发现没有匹配条目

enum ofp_packet_in_reason {
    OFPR_NO_MATCH,          /* No matching flow. */
    OFPR_ACTION             /* Action explicitly output to controller. */
};

有匹配条目,对应的action是OUTPUT=CONTROLLER,固定收到向控制器发送包

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. */
};

7. PACKET_OUT

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.) */
};

8. FLOW_MOD
   

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. */
};
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];
};

实验总结

（一） 出现的问题

1. 抓不到hello包

解决方法：需要先打开wireshark抓包再运行python文件

2. 通过过滤规则openflow_v1后找不到从交换机到控制器的hello包

解决方法：发现交换机版本为openflow1.5，选择过滤规则为openflow_v6或者不设置过滤规则，可以抓取到该hello包

（二）个人心得

本次实验我们通过抓取数据包、分析数据包学习了OpenFlow协议下控制器和交换机的交互过程。本次实验是验证性实验，难度不大，只要按部就班一步步做就好。通过抓包并查看每一个OpenFlow数据包的源端口、目的端口、类型和相关字段，我更进一步了解与掌握了OpenFlow协议。通过这次实验，我能够更加熟悉运用 wireshark 软件进行抓包，且让我对拓扑建立使用到的协议和数据交互流程有了更深的理解；更加熟悉了mininet的一些命令行操作；