WebSocket协议探究（三）：MQTT子协议
一 复习和目标
1 复习
- Nodejs实现WebSocket服务器
- Netty实现WebSocket服务器(附带了源码分析)
- Js api实现WebSocket客户端
注：Nodejs使用的Socket.io模块实现，Netty本身对WebSocket有一定的支持，所以这两种实现都相对容易理解，大家自己可以使用自己喜欢的语言实现(参考Nodejs版本，即不需要考虑过多的情况)。
2 目标
- 使用WebSocket协议进行发送Mqtt消息
- 即Mqtt协议作为WebSocket协议的子协议进行通信
注1：WebSocket协议内容不多，但是远远不止这么少。但是学习不要完美主义(即学到一样东西就想着一开始就把这样东西学透)，学习要带着目的性。比如：我就是想弄懂为什么前端需要使用WebSocket协议才能发送Mqtt消息？WebSocket为什么需要HTTP协议来进行协议升级协商？HTTP协议为什么基于TCP协议？TCP协议是全双工的为什么HTTP协议不能全双工？等等......
注2：协议目前学习了MQTT、TCP、HTTP和WebSocket，并不是现在学完了，以后就可以不再去学习这些，而是说对于现在的学习目的已经达到了，不需要再花费过多的精力罢了。
二 WireShark抓包
1 WebSocket连接
- 请求升级协议：HTTP -> WebSocket(mqtt)
# 省略无关头部
Hypertext Transfer Protocol
GET /mqtt HTTP/1.1\r\n
Host: 103.61.37.192:8083\r\n
Sec-WebSocket-Version: 13\r\n
Sec-WebSocket-Protocol: mqtt\r\n # 子协议为mqtt
Sec-WebSocket-Extensions: permessage-deflate\r\n
Sec-WebSocket-Key: npQCPFw1Pc5G9vqjJyi90w==\r\n
Connection: keep-alive, Upgrade\r\n
Upgrade: websocket\r\n
\r\n
- 响应成功
# 省略无关头部
Hypertext Transfer Protocol
HTTP/1.1 101 Switching Protocols\r\n
Connection: Upgrade\r\n
Content-Length: 0\r\n
Sec-Websocket-Accept: biw/vsujgvKx8B0bfoDnjuxHMgA=\r\n
Sec-Websocket-Protocol: mqtt\r\n # 确认子协议为mqtt
Upgrade: websocket\r\n
\r\n
2 MQTT建立连接
- 客户端 -> 服务器：CONNECT
- 本来MQTT TCP连接可以通过分段或一次传输完成的WebSocket则分成了13次发送
# 发送了13个帧
192.168.1.46 103.61.37.192 WebSocket WebSocket Binary [FIN] [MASKED]
1... .... = Fin: True
.000 .... = Reserved: 0x0
.... 0010 = Opcode: Binary (2)
1... .... = Mask: True
.000 0001 = Payload length: 1
Masking-Key: 0c3cb539
Masked payload
1C
# 帧序号 数据
# 1 Data: 10 -> Header Flags:Connect
# 2 Data: 58 -> Msg Len:88
# 3 Data: 0004 -> Protocol Name Length: 4
# 4 Data: 4d515454 -> Protocol Name: MQTT
# 5 Data: 04 -> Version：v3.1.1
# 6 Data: c2
# 1... .... = User Name Flag: Set # 用户名
# .1.. .... = Password Flag: Set # 密码
# ..0. .... = Will Retain: Not Set # 遗嘱保留
# ...0 0... = Qos Level: 0
# .... .0.. = Will Flag: Not Set # 遗嘱
# .... ..1. = Clean Session Flag: Set # 清除会话
# .... ...0 = (Reserverd): Not Set # 固定为0
# 7 Data: 003c -> Keep Alive: 60秒
# 8 Data: 0011 -> Client ID Length: 17
# 9 Data: 6d 71 74 74 6a 73 5f 39 38 33 37 66 65 30 62 61 38 -> ClientId
# 10 Data: 000b -> Username Length
# 11 Data: 64656c696768742f776562 -> Username
# 12 Data: 002c -> Password Length
# 13 Data: 45396a4d58416c35523939596b48756350506c3377545468... -> Password
- 服务器 -> 客户端：CONNACK
WebSocket
1... .... = Fin: True
.000 .... = Reserved: 0x0
.... 0010 = Opcode: Binary (2)
0... .... = Mask: False
.000 0100 = Payload length: 4
Data (4 bytes)
Data: 20 02 00 00
# 20 -> Header Flags:CONNACK
# 02 -> Msg Len：2
# 00 -> Acknowledge Flags: 0x00 # 连接确认标志
# 00 -> Return Code: Connection Accepted (0) # 连接返回码
3 MQTT的PINGREQ和PINGRESP
- 客户端 -> 服务器：PINGREQ
WebSocket
1... .... = Fin: True
.000 .... = Reserved: 0x0
.... 0010 = Opcode: Binary (2)
1... .... = Mask: True
.000 0010 = Payload length: 2
Masking-Key: 0657364e
Data (2 bytes)
Data: c000
# C0 -> Header Flags: 0xC0 (Ping Request)
# 00 -> Msg Len: 0
- 服务器 -> 客户端 ：PINGRESP
WebSocket
1... .... = Fin: True
.000 .... = Reserved: 0x0
.... 0010 = Opcode: Binary (2)
0... .... = Mask: False
.000 0010 = Payload length: 2
Data (2 bytes)
Data: d000
# D0 -> Header Flags: 0xD0 (Ping Response)
# 00 -> Msg Len: 0
4 MQTT的SUBSCRIBE和SUBACK
- 客户端 -> 服务器：SUBSCRIBE
WebSocket
1... .... = Fin: True
.000 .... = Reserved: 0x0
.... 0010 = Opcode: Binary (2)
1... .... = Mask: True
.000 0001 = Payload length: 1
Masking-Key: d0f56152
Data (1 byte)
Data: 82
# 帧序号 数据
# 1 Data: 82 -> Header Flags: Subscribe Request
# 2 Data: 0b -> Msg Len:11
# 3 Data: a5b2 -> Message Identifier: a5b2
# 4 Data: 0006 -> Topic Length: 6
# 5 Data: 2f 57 6f 72 6c 64 -> Topic:/World
# 6 Data: 00 -> Requested QoS:0
- 服务器 -> 客户端 ：SUBACK
WebSocket
1... .... = Fin: True
.000 .... = Reserved: 0x0
.... 0010 = Opcode: Binary (2)
0... .... = Mask: False
.000 0101 = Payload length: 5
Data (5 bytes)
Data: 9003a5b200
# 90 -> Header Flags: Subscribe Ack
# 03 -> Msg Len: 3
# a5b2 -> Message Identifier:a5b2
# 00 -> 成功
5 MQTT的PUBLISH(Qos0)
# 帧序号 数据
# 1 Data: 30 -> Header Flags: Publish Message
# 2 Data: 0d -> Msg Len: 13
# 3 Data: 0006 -> Topic Length:6
# 4 Data: 2f 57 6f 72 6c 64 -> Topic:/World
# 5 Data: 68 65 6c 6c 6f -> Message:hello
6 MQTT的DISCONNECT
WebSocket
1... .... = Fin: True
.000 .... = Reserved: 0x0
.... 0010 = Opcode: Binary (2)
1... .... = Mask: True
.000 0010 = Payload length: 2
Masking-Key: d70088a5
Data (5 bytes)
Data: e000
# e0 -> Header Flags: Disconnect
# 00 -> Msg Len: 0
注1：MQTT报文格式请参考MQTT协议探究(一) MQTT协议探究（二）
注2：MQTT其他报文就不做贴出来了，请自行测试。
注3：协议的学习基本完成，后面结合Netty来加深对协议的复习和补充。
