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
（一）基本要求
- 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 |
- 配置好主机 IP 地址导出
- 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 60450 端口
交换机 60450 端口（我最高能支持 OpenFlow 1.5）---> 控制器 6633 端口
于是双方建立连接，并使用 OpenFlow 1.0
-
Features Request / Set Config
控制器 6633 端口（我需要你的特征信息）---> 交换机 60450 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 60450 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 60450 端口（这是我的特征信息，请查收）---> 控制器 6633 端口
-
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 60450 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
-
Flow_mod
控制器通过 6633 端口向交换机 60450 端口下发流表项，指导数据的转发处理
控制器通过 6633 端口向交换机 60452 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 60450 端口
-
流程图
- 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？
通过抓取的报文分析可知使用的是TCP协议
（二）进阶要求
- 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; }; -
FEATURES_REQUEST
-
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. */ };头部信息 ofp_header header 加上 flags 和 miss_send_len
-
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. */ }; -
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
- 交换机查找流表，发现没有匹配条目时
enum ofp_packet_in_reason { OFPR_NO_MATCH, /* No matching flow. */ OFPR_ACTION /* Action explicitly output to controller. */ };- 有匹配条目但是对应的 action 是 OUTPUT=CONTROLLER 时
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
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]; }; -
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.) */ };
（三）个人总结
- 这次实验难度适中，建立拓扑、设置 IP地址都是前面学习过的知识。
- 学习使用了抓包工具 wireshark，通过过滤器过滤报文。
- 通过报文与源码之间的比照，可以清楚地知道报文各个字段的构成。
- 对 SDN 的数据平面与控制平面的解耦合有了更进一步的了解。
- 实验中遇到的困难
- 一开始过滤报文时，找不到交换机发出的 hello 报文；后来修改了几次过滤条件找到了交换机发出的 hello 报文。
- 找不到 Flow_mod 报文，通过命令行发出 ping 命令来发出 Flow_mod 报文。
- Port_Status 报文，通过修改保存的拓扑 py 文件内的主机与交换机的连接端口实现。