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。