实验3：OpenFlow协议分析实践

（一）基本要求
1./home/用户名/学号/lab3/目录下的拓扑文件


2.wireshark抓包的结果截图和对应的文字说明

• hello
  控制器6633端口（我最高能支持OpenFlow 1.0） ---> 交换机59728端口
  
  交换机59728端口（我最高能支持OpenFlow 1.0） ---> 控制器6633端口
  
• Set Conig
  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59728端口
  
• Features Request
  控制器6633端口（我需要你的特征信息） ---> 交换机59728端口
  
• Features Reply
  交换机59728端口（这是我的特征信息，请查收） ---> 控制器6633端口
  
• 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。
  
• Packet_in
  • 有两种情况：
  •交换机查找流表，发现没有匹配条目时
  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
  • 有匹配条目但是对应的action是OUTPUT=CONTROLLER时
  交换机59728端口（有数据包进来，请指示）--- 控制器6633端口
  
• Packet_out
  控制器6633端口（请按照我给你的action进行处理） ---> 交换机59728端口
  
• Flow_mod
  分析抓取的flow_mod数据包，控制器通过6633端口向交换机33640端口下发流表项，指导数据的转发处理
  
  3.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
  
  4.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：TCP协议
  （二）进阶要求
  将抓包基础要求第2步的抓包结果对照OpenFlow源码，了解OpenFlow主要消息类型对应的数据结构定义。
• hello
  

/* Header on all OpenFlow packets. */
struct ofp_header {
    uint8_t version;   //版本号 /* OFP_VERSION. */
    uint8_t type;      //消息类型 /* One of the OFPT_ constants. */
    uint16_t length;   //长度 /* Length including this ofp_header. */
    uint32_t xid;      //id /* Transaction id associated with this packet.
                           Replies use the same id as was in the request
                           to facilitate pairing. */
};
OFP_ASSERT(sizeof(struct ofp_header) == 8);

/* OFPT_HELLO.  This message has an empty body, but implementations must
 * ignore any data included in the body, to allow for future extensions. */
struct ofp_hello {
    struct ofp_header header;
};

• Set Conig
  

enum ofp_config_flags {
    /* Handling of IP fragments. */
    OFPC_FRAG_NORMAL   = 0,  /* No special handling for fragments. */
    OFPC_FRAG_DROP     = 1,  /* Drop fragments. */
    OFPC_FRAG_REASM    = 2,  /* Reassemble (only if OFPC_IP_REASM set). */
    OFPC_FRAG_MASK     = 3
};
//flag不同的值代表不同的处理方式
/* Switch configuration. */
struct ofp_switch_config {
    struct ofp_header header;
    uint16_t flags;        //用来指示交换机如何处理 IP 分片数据包     /* OFPC_* flags. */
    uint16_t miss_send_len;  //用来指示当一个交换机无法处理的数据包到达时，将数据包发给控制器的最大字节数。   /* Max bytes of new flow that datapath should
                                   send to the controller. */
};
OFP_ASSERT(sizeof(struct ofp_switch_config) == 12);

• Features Reply
  

/* Switch features. */
struct ofp_switch_features {
    struct ofp_header header;
    uint64_t datapath_id;   //唯一标识ID号/* Datapath unique ID.  The lower 48-bits are for
                               a MAC address, while the upper 16-bits are
                               implementer-defined. */

    uint32_t n_buffers;    //缓冲区可缓存的最大数据包个数 /* Max packets buffered at once. */

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

    /* Features. */
    uint32_t capabilities;  //支持的特殊功能 /* Bitmap of support "ofp_capabilities". */
    uint32_t actions;       //支持的动作 /* Bitmap of supported "ofp_action_type"s. */

    /* Port info.*/
    struct ofp_phy_port ports[0]; //物理端口描述列表 /* Port definitions.  The number of ports
                                      is inferred from the length field in
                                      the header. */
};
OFP_ASSERT(sizeof(struct ofp_switch_features) == 32);

• 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. */
};

//该参数与hello报文结构相同

• Port_Status
  

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;
};
OFP_ASSERT(sizeof(struct ofp_port_status) == 64);
//这个是当交换机端口发生变化时，告知控制器相应的端口状态
//发生变化包括增加、删除、修改物理端口，则需发送port status来告知

• Packet_in
  

/* Why is this packet being sent to the controller? */
enum ofp_packet_in_reason {
    OFPR_NO_MATCH,          /* No matching flow. */
    OFPR_ACTION             /* Action explicitly output to controller. */
};
//第一行是没有匹配的条目，第二行是action列表中包含转发给控制器的动作。
//两种情况：1.交换机查找流表，发现没有匹配条目,但是这种包没有抓到过2.有匹配条目,对应的action是OUTPUT=CONTROLLER,固定收到向控制器发送包（分别是这上下两段代码）
/* Packet received on port (datapath -> controller). */
struct ofp_packet_in {
    struct ofp_header header;
    uint32_t buffer_id;   //Packet-in消息所携带的数据包在交换机缓存区中的ID  /* ID assigned by datapath. */
    uint16_t total_len;    //data字段的长度 /* Full length of frame. */
    uint16_t in_port;      //数据包进入交换机时的端口号 /* Port on which frame was received. */
    uint8_t reason;        //发送Packet-in消息的原因 /* Reason packet is being sent (one of OFPR_*) */
    uint8_t pad;
    uint8_t data[0];       //携带的数据包 /* 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. */
};
OFP_ASSERT(sizeof(struct ofp_packet_in) == 20);

• Packet_out
  

/* Send packet (controller -> datapath). */
struct ofp_packet_out {
    struct ofp_header header;
    uint32_t buffer_id;       //交换机缓存区id，如果为-1则指定的为packet-out消息携带的data字段    /* ID assigned by datapath (-1 if none). */
    uint16_t in_port;          //如果buffer_id为‐1，并且action列表中指定了Output=TABLE的动作,in_port将作为data段数据包的额外匹配信息进行流表查询  
 /* Packet's input port (OFPP_NONE if none). */
    uint16_t actions_len;      //action列表的长度，可以用来区分actions和data段   /* Size of action array in bytes. */
    struct ofp_action_header actions[0]; //动作列表 /* Actions. */
    /* uint8_t data[0]; */   // 数据缓存区，可以存储一个以太网帧，可选    /* Packet data.  The length is inferred
                                     from the length field in the header.
                                     (Only meaningful if buffer_id == -1.) */
};
OFP_ASSERT(sizeof(struct ofp_packet_out) == 16);

• Flow_mod
  

/* Flow setup and teardown (controller -> datapath). */
struct ofp_flow_mod {
    struct ofp_header header;
    struct ofp_match match;    //流表的匹配域  /* Fields to match */
    uint64_t cookie;           // 流表项标识符 /* Opaque controller-issued identifier. */

    /* Flow actions. */
    uint16_t command;          //可以是ADD,DELETE,DELETE-STRICT,MODIFY,MODIFY-STRICT   /* One of OFPFC_*. */
    uint16_t idle_timeout;      //空闲超时时间  /* Idle time before discarding (seconds). */
    uint16_t hard_timeout;       //最大生存时间 /* Max time before discarding (seconds). */
    uint16_t priority;          //优先级，优先级高的流表项优先匹配  /* Priority level of flow entry. */
    uint32_t buffer_id;      //缓存区ID ,用于指定缓存区中的一个数据包按这个消息的action列表处理     /* Buffered packet to apply to (or -1).
                                     Not meaningful for OFPFC_DELETE*. */
    uint16_t out_port;       //如果这条消息是用于删除流表则需要提供额外的匹配参数     /* 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;           //标志位，可以用来指示流表删除后是否发送flow‐removed消息，添加流表时是否检查流表重复项，添加的流表项是否为应急流表项。    /* One of OFPFF_*. */
    struct ofp_action_header actions[0]; //action列表/* The action length is inferred
                                            from the length field in the
                                            header. */
};
OFP_ASSERT(sizeof(struct ofp_flow_mod) == 72);

（三）个人总结

• 一定要先开启抓包再构建拓扑，不然wireshark抓包抓不到hello包
• 通过抓包了解openflow协议中交换机与控制器的消息交互过程与机制
• 不懂的记得到网络上去了解相关的信息，与同学多沟通自己遇到的问题，别一个人死磕
• 进阶通过抓包和openflow源码相对照可了解主要的消息类型的结构定义