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。

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：
  控制器与交换机互相发送HELLO消息，可以看到控制器OpenFlow版本为1.0，交换机OpenFlow版本为1.5，依据向下兼容，使用OpenFlow1.0。
  （1）控制器6633端口->交换机55844端口：

（2）交换机55844端口->控制器6633端口：

• FEATURES_REQUEST/SET_CONFIG:
  （1）控制器6633端口（我需要你的特征信息）->交换机55844端口：
  

（2）控制器6633端口（请按照我给你的config flag和max bytes of packet进行配置）->交换机55844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：
  

• FEATURES_REPLY:
  交换机55844端口（这是我的特征信息，请查收）->控制器6633端口：
  

• PACKET_IN：
  （1）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
  （2）交换机55846（查找的是后面的，所以交换机端口与前面的55844端口略有差异）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，可以看到交换机没有发现自己要匹配的流表：Reason:
No matching flow (table-miss flow entry) (0)，所以要询问控制器如何处理。

• FLOW_MOD：
  分析抓取的flow_mod数据包，控制器通过6633端口向交换机55846端口下发流表项，指导数据的转发处理
  

• PACKET_OUT：
  控制器6633端口（请按照我给你的action进行处理）-> 交换机55846端口：输出到交换机的0端口（选择截取的内容有点问题，没有显示端口号）
  

• 综上可知，流程图为：
  

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？
对wireshark的抓包进行分析，可以看出交换器与控制器建立通信时使用的是TCP协议（Transmission Control Protocol)。

（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:
  
  OpenFlow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;
};

可以看到对应了HELLO报文的四个参数

• EATURES_REQUEST:
  
  可以看到格式与上述ofp_header结构体中数据相同

• SET_CONFIG:
  
  OpenFlow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:
  
  OpenFlow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:
  
  OpenFlow源码：

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:
  
  OpenFlow源码：

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

• FLOW_MOD:
  
  OpenFlow源码：

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];
};
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. */
};

• PACKET_OUT：
  
  OpenFlow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.) */
};

四、个人总结

• 实验难度：较简单

• 遇到困难及解决办法：
  1、一开始对于先开启抓包再构建拓扑这句话不是很能理解，先开启抓包是要开启到什么程度呢？是要抓包以后再构建拓扑，还是打开拓扑呢？所以便一 一尝试。首先是先构建完拓扑，打开以后再抓包，自然是不行的，找不到hallo，然后是开启抓包再运行拓扑，所幸上次实验已经了解了需要先pinggall，但是还是不行，最后抓包点完any以后再打开之前的拓扑就可以了。
  2、源码太多了，看着别人的源码再一一对照，倒也是找到了，在想有没有一种更快的方法可以进行源码的查找。
  3、后面步骤有一些图片截图的不太好，过两天想重新截取一下，发现重新抓包以后，交换机的端口发生了变化，所以也就直接用之前截取的图片了。

• 个人感想：
  总的来说，这次实验还是比较简单的，对照老师的PPT找就完事了，不是很费事，就是截图得截取好多，但也还行。自己在代码这块还是得抓紧学习，毕竟没什么基础，争取早日能勉强看懂。