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
Ubuntu 20.04 Desktop amd64
三、实验要求
（一）基本要求
1.hello
控制器6633端口（我最高支持OpenFlow1.0）--->交换机57036端口
交换机57036端口（我最高支持OpenFlow1.5）--->控制器6633端口
2.Features Request/Set Config
控制器6633端口（我需要你的特征信息） ---> 交换机57036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57036端口
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57036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57036端口（有数据包进来，请指示）--- 控制器6633端口
6.Packet_out
控制器6633端口（请按照我给你的action进行处理） ---> 交换机57036端口
7.flow_mod
分析抓取的flow_mod数据包，控制器通过6633端口向交换机57036端口、交换机57038端口下发流表项，指导数据的转发处理
8.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
9.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？
是TCP协议，可以从报文信息中直接看出。
（二）进阶要求
1.hello
/* Header on all OpenFlow packets. */
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. */
};
//HELLO报文中有四个参数，对应版本号、消息类型、长度及ID
struct ofp_hello {
struct ofp_header header;
};
2.FEATURE REQUEST
/* Header on all OpenFlow packets. */
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. */
};
//feature_request的结构与hello一致
3.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. */
};
//set_config比header还多了两项
//flags:交换机如何处理IP分片数据包
//miss_send_len: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
4.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;
};
//在OpenFlow交换机中添加、删除或修改物理端口时，需要发送Port-Status 消息来通知OpenFlow 控制器
5.FEATURE_REPLY
/* 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. */
};
//除了header还包括唯一ID号、缓冲区可以缓存的最大数据包个数、流表数量、功能、动作、端口等
6.PACKET_IN
//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
enum ofp_packet_in_reason {
OFPR_NO_MATCH, /* No matching flow. */
OFPR_ACTION /* Action explicitly output to controller. */
};
// 2.有匹配条目,对应的action是OUTPUT=CONTROLLER,固定收到向控制器发送包
/* 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.PACKET_OUT
/* 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.) */
};
//包括动作列表、缓冲区ID等
8.FLOW_MOD
/* 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. */
};
//包括流表项标志符cookie，command代表五种操作，对应值分别为0-4，优先级等
四、个人总结
遇到的问题及解决
在运行拓扑python文件后，wireshark界面并没有抓到包，回头复习mininet相关内容时发现，是在建立拓扑图保存py文件时，点了save而没有选择Export level 2 Script。在重新创建正确文件后，实验内容就可以正常运行了。由于本次实验较简单，所以后续实验内容基本都没有遇到什么问题，值得一说的只有在跟着老师的pdf过滤openflow_v4后发现什么包也没有，同学讲解了一下这个需要看我们自己的机子他实际能支持的最高OpenFlow版本，不一定是在openflow_v4，是因人而异的，果然后来筛选了一下就在过滤openflow_v6之后找到了抓包信息，我最高支持OpenFlow1.5。
关于OpenFlow的学习
- hello：双方选取Hello消息中最低版本的协议作为通信协议如果有一方不支持Openflow协议版本，应发送Error消息后断开连接。如果双方Openflow版本可以兼容，则Openflow连接建立成功。
- OpenFlow交换机把传统网络中完全由交换机/路由器控制的报文转换为由交换机和控制器共同完成的数据转发操作，实现了数据转发与路由控制的分离。
- 控制器通过事先规定的Open Flow交换机中的流表进行数据转发。