实验3：OpenFlow协议分析实践

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miniedit中构建好后存为topo.py，并在代码中更改主机IP地址
  
• 命令行搭建拓扑ping检测是否正确
  

3.2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55570端口
    
  • 交换机55570端口（我最高能支持OpenFlow 1.5) ---> 控制器6633端口
    
  • 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;
    };
  

• Features Request/Set Conig

  • 控制器6633端口（我需要你的特征信息） ---> 交换机55570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55570端口
    
  • 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. */
     };
  

• 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
    

  • 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

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

    /* 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. */
    };
    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. */
    };
  
  

• 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55570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
  • 源码

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

  • 前者是没成功匹配的，后者是收到向控制器发送的包

• 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
    
    
    • 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

  • 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
  • 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.) */
    };
  

• 流程图
  

3.3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：在使用wireshark抓包时，可以清楚看到是使用TCP协议。

总结

遇到问题及解决方法：

• 在筛选交换机向控制器传的Hello参数时，没能找到，直到筛选openflow_v6才成功找到。
• 找不到flow_mod参数，在将任意两主机ping通后再重新抓包，成功找到。

个人感想

这次实验难度对我来说中等。大部分的环节是抓包、筛选、看代码，由于在前几次实验中对大部分相关的实验步骤都熟悉了许多，这次实验做起来比较迅速。遇到的错误也能很快得到解决。最麻烦的一点其实是去openflow.h看源代码，不过也能运用一些类如word的工具来快速查找。最大的收获是对openflow协议的数据交互更加清晰，对wireshark的运用也更加熟练了。