实验3：OpenFlow协议分析实践

实验3：OpenFlow协议分析实践

（一）基本要求

  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。

img

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

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

控制器与交换机建立连接

源端口6633 -> 目的端口51732，从控制器到交换机，协议为openflow1.0

源端口51732 -> 目的端口6633，从交换机到控制器，协议为openflow1.5

FEATURES_REQUEST/FEATURES_REPLY

  • FEATURES_REQUEST

控制器请求交换器的特征信息

源端口6633 -> 目的端口21732，从控制器到交换机

  • FEATURES_REPLY

    源端口51732 -> 目的端口6633，从交换机到控制器

交换机通知控制器它的特征信息

SET_CONFIG

源端口6633 -> 目的端口51732，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

PORT_STATUS

源端口51732 -> 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

PACKET_IN

源端口51734 -> 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

PACKET_OUT

PACKET_OUT是从控制器向交换机发送的消息，源端口6633-> 目的端口51732，包含数据包发送命令的消息

控制器要求交换机按照所给出的action进行处理

  1. 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？
    由截图可以看出，交换机与控制器建立通信时是使用TCP协议

（二）进阶要求

  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。

    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，代码中ofp_header的变量和图片中的显示一一对应，分别由version,type,length和ID

    FEATURES_REQUEST

    可以看到格式与上述ofp_header结构体中数据相同

    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. */
      };

    SET_CONFIG

      /* 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. */
      };

可以看到源码与图中信息一一对应，其中包含ofp_header结构体

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;
      };

PACKET_IN

      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. */
      };

前面提到packetin分两种情况，所以有两段代码，抓到的是第二种，固定收到向控制器发送包

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.) */
      };

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];
      };

总结

这次作业比较直接，但增加了对OpenFlow在交换机和控制器间的交互过程的学习，第一次去看源代码，但好像没起到什么作用。看源代码时主要就是看其中定义的结构体，结构体中的信息和报文信息进行匹配，来一一对应。这次作业遇到的问题有一个，就是我没有抓到FLOW_MOD包，原因不清楚，但除了这个其他步骤都没有出问题，还有一个是PACKET_IN报文中的两种触发形式只有捕获到一种，但这个应该是正常的，因为和其他同学交流，大家也没有捕获到这个报文。希望今后可以更多学习到可编程的知识。

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原文地址：https://www.cnblogs.com/xiao-qingjiang/p/15336735.html