实验任务一
试验任务二
OpenFlow协议中交换机与控制器的消息交互过程如下图所示
Hello
首先，控制器与交互及互相发送 Hello 消息。
Features Request
其次，OpenFlow 连接建立之后,控制器需要获得交换机的特性信息，因此控制器向交换机发送 Features Request 消息查询交换机特性.
Features Reply
交换机在收到控制器发出的 Features Request 消息后，将自己的特性告诉给控制器，返回 Features Request 消息.
Set config
知道了交换机的特性之后就要配置交换机了。
Packet-in
有两种情况会触发交换机向控制器发送 Packet_in 消息
- 1.数据包在交换机中匹配不到流表，则向controller发送Packet_in消息
- 2.数据包在流表中有匹配的条目，但是其中所指示的 action 列表中包含转发给控制器的动作（Output = CONTROLLER）
(注：该图是因为匹配不到流表，属于第一种)
Flow-Mod / Packet-out
当控制器收到 Packet-in 消息时有两种响应的方式:
- Flow-Mod：控制器收到 Packet‐in 消息后，可以发送 Flow‐Mod 消息向交换机下发一个流表项。
- Packet-out：与Flow-Mod不同的是，控制器不会下发流表，而是直接告诉交换机该如何做。
（Flow-Mod）
（Packet_out）
实验任务三
Q:交换机与控制器建立通信时是使用TCP协议还是UDP协议？TCP协议
进阶任务
OFPT_Hello
Hello包定义了一个header，header数据结构如下：
struct ofp_header {
uint8_t version; /* 版本号 */
uint8_t type; /* Openflow 协议类型. */
uint16_t length; /* Openflow 数据包包头. */
uint32_t xid; /* 数据包编号. */
};
struct ofp_hello {
struct ofp_header header;
};
OFPT_ERROR
如果连接失败，会发送一个error包,error类型如下。
enum ofp_error_type {
OFPET_HELLO_FAILED, /* Hello protocol failed. */
OFPET_BAD_REQUEST, /* Request was not understood. */
OFPET_BAD_ACTION, /* Error in action description. */
OFPET_FLOW_MOD_FAILED, /* Problem modifying flow entry. */
OFPET_PORT_MOD_FAILED, /* Port mod request failed. */
OFPET_QUEUE_OP_FAILED /* Queue operation failed. */
};
OFPT_FEATURES
OFPT_FEATURES 主要是请求交换机的特性，而交换机的特性数据结构定义如下
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. */
};
OFPT_PACKET_IN
OFPT_PACKET_IN产生的原因有两种，一种是没匹配到流表，另一种是匹配到了，动作是转发的控制器，代码如下
enum ofp_packet_in_reason {
OFPR_NO_MATCH, /* No matching flow. */
OFPR_ACTION /* Action explicitly output to controller. */
};
OFPT_PACKET_IN的数据格式如下：
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. */
};
OFPT_PACKET_OUT
packet_in事件之后，一般会触发两类事件，packet_out和flow_mod。两者都是指导交换机如何处理数据包，区别是是否下发流表项。packet_out数据结构如下。
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.) */
};
OFPT_FLOW_MOD
Flow-Mod：控制器收到 Packet‐in 消息后，可以发送 Flow‐Mod 消息向交换机下发一个流表项。指导交换机转发数据包，其数据格式如下：
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. */
};
OFP_ASSERT(sizeof(struct ofp_flow_mod) == 72);
/* Why was this flow removed? */
enum ofp_flow_removed_reason {
OFPRR_IDLE_TIMEOUT, /* Flow idle time exceeded idle_timeout. */
OFPRR_HARD_TIMEOUT, /* Time exceeded hard_timeout. */
OFPRR_DELETE /* Evicted by a DELETE flow mod. */
};
个人心得
源码阅读其实有很多内容可写，如果把整个流程写出来，就太麻烦了。所以这边就非常简单写了一些OpenFlow主要消息类型对应的数据结构定义。还有交互过程也应该详细一些的。不过不太符合作业简要的要求，就没写太多。