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

• 1.能够运用 wireshark 对 OpenFlow 协议数据交互过程进行抓包；

• 2.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

• 1.下载虚拟机软件Oracle VisualBox；

• 2.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

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

• 在终端输入sudo ./../mininet/examples/miniedit.py搭建拓扑

• 配置网段
  

• 配置IP地址
  

• 保存py文件到lab3

• 测试连通性
  

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。

（1）Hello

• 控制器6633端口（我最高能支持OpenFlow 1.0） ---> 交换机48528端口
  

• 交换机48528端口（我最高能支持OpenFlow 1.3）--- 控制器6633端口

• 于是双方建立连接，并使用OpenFlow 1.0

（2）Features Request / Set Conig

• 控制器6633端口（我需要你的特征信息） ---> 交换机48528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48528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。
    

（3）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。
  

（4）Features Reply

• 交换机48528端口（这是我的特征信息，请查收）--- 控制器6633端口

（5）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48528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

（6）Flow_mod

• 分析抓取的flow_mod数据包，控制器通过6633端口向交换机48528端口、交换机48530端口下发流表项，指导数据的转发处理

• 分析抓取的flow_mod数据包，控制器通过6633端口向交换机48528端口、交换机48530端口下发流表项，指导数据的转发处理

（7）Packet_out

• 控制器6633端口（请按照我给你的action进行处理） ---> 交换机48528端口
  

（8）交互图

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？

使用了TCP协议，抓包截图与0031902627.py源码中均能体现

(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。

(1)Hello

• 对应

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. */
};
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;
};

(2)Features Request

• 与Hello中的数据格式相同
  

(3)Set Conig

• 对应源代码

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. */
};
OFP_ASSERT(sizeof(struct ofp_switch_config) == 12);

(4)Port_Status

• 对应源代码

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;
};
OFP_ASSERT(sizeof(struct ofp_port_status) == 64);

(5)Features Reply

• 对应源代码

/* 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. */
};
/* Switch features. */
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. */
};

(6)Packet_in

• 对应源代码

/* 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. */
};
 
/* Packet received on port (datapath -> 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. */
};

(7)Flow_mod

• 对应源代码

/* Fields to match against flows */
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. */
};
/* Flow setup and teardown (controller -> datapath). */
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. */
};

(8)Packet_out

• 对应源代码

/* Action header that is common to all actions.  The length includes the
 * header and any padding used to make the action 64-bit aligned.
 * NB: The length of an action *must* always be a multiple of eight. */
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];
};
OFP_ASSERT(sizeof(struct ofp_action_header) == 8);
 
/* Send packet (controller -> datapath). */
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、个人总结

• 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！
  • 打开抓包工具进入any后，没有hello的包，只看到的requst和reply的包，找了很久，重新打开也没用，后面发现需要先打开抓包工具，再打开mininet，不然会丢失前面的包。这次试验让我更加熟悉了拓扑构建工具和wireshark抓包工具的使用。

• 个人感想：

  • 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。