详解车载TBox的MQTT协议

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1.基本概念

MQTT全称为消息队列遥测传输协议，是基于TCP/IP之上的应用层协议，采用发布订阅的模式，由于非轮询或周期传输消息，减小了网络带宽，不同重要度的消息采用不同的QoS等级，保证重要消息不丢失。

           

MQTT协议本身占用资源很小，通常作为客户端部署于各种嵌入式系统内，包括各种传感器、执行器，即可完成消息的订阅和发布。

           

           

2.车上的应用

MQTT协议一般部署在可以联网的ECU内，实现远程控制、远程刷写、数据上传、远程诊断、OTA等功能。

           

           

远程控制

用户通过手机APP下发远程解门锁的请求，该请求以MQTT的topic发布到云服务器，云服务器根据topic名称，将该topic发布到车载终端内，车载终端订阅该topic，当收到该topic后，解析payload的内容，将解锁报文通过CAN总线发送到门窗控制器，并将解锁结果通过topic发布到云服务器，云服务器将topic发布到手机APP，用户即成功执行了远程解锁的动作。   

           

数据上传

车载终端接受车内ECU发送的CAN报文，解析所需的信号，按周期将信号打包后，通过指定的topic将信号上传到平台。

           

远程诊断

云平台通过指定的topic下发诊断指令，终端订阅到该topic，发起向车内ECU的诊断请求，例如读取ECU的故障码，收到诊断响应后，通过指定topic将诊断结果发布到云平台。

           

3.技术细节

介绍一下MQTT协议的报文格式、端口号、以及相关的概念。

           

报文格式

MQTT报文有固定报头、可变报头、有效载荷3个部分。

固定报头组成：

报文类型：位于固定报头的第一个字节的高4位（bit 7-4），用于标识MQTT报文的类型。MQTT协议定义了14种不同类型的报文，如CONNECT（连接请求）、CONNACK（连接确认）、PUBLISH（发布消息）、PUBACK（发布确认）等。

标志位：位于固定报头的第一个字节的低4位（bit 3-0），用于某些特定类型的报文，如PUBLISH报文中的DUP（重复分发标志）、QoS（服务质量等级）、RETAIN（保留标志）等。这些标志位的具体含义取决于报文类型。   

剩余长度：紧接着报文类型之后，表示当前报文剩余部分的字节数，包括可变报头和有效载荷。剩余长度字段使用变长编码方案，最小一个字节，最大四个字节。

           

可变报头组成：

可变报头存在于某些类型的报文中，如CONNECT、PUBLISH、SUBSCRIBE等，可变报头的内容根据报文类型的不同而不同，通常包含以下字段：

报文标识符：一个两字节的字段，用于标识MQTT报文，在需要确认的报文中，如PUBLISH（QoS > 0）、PUBACK、PUBREC、PUBREL、PUBCOMP、SUBSCRIBE、SUBACK、UNSUBSCRIBE、UNSUBACK等，报文标识符用于匹配请求和响应。

其他字段：根据报文类型的不同，可变报头还可能包含其他字段，如CONNECT报文中的协议名（Protocol Name）、协议级别（Protocol Level）、连接标志（Connect Flags）和保持连接（Keep Alive）等。

           

有效载荷：

包含客户端和服务器之间传输的应用数据，不是所有MQTT报文都包含有效载荷，只有那些需要传输数据的报文（如PUBLISH、CONNECT等）才包含有效载荷。

CONNECT报文的有效载荷：包含客户端标识符（Client Identifier）、遗嘱主题（Will Topic）、遗嘱消息（Will Message）、用户名（User Name）和密码（Password）等字段。

其他报文的有效载荷：对于其他类型的报文，如PUBLISH报文，有效载荷就是实际要传输的应用消息。

           

以CONNECT报文为例，其报文格式如下：

           

固定报头：0x10（报文类型CONNECT）+ 剩余长度（根据有效载荷长度计算）

可变报头：

协议名（Protocol Name）："MQTT"（UTF-8编码）

协议级别（Protocol Level）：0x04（表示MQTT 3.1.1版本）

连接标志（Connect Flags）：包含多个标志位，如会话清理标志、遗嘱标志、用户名标志、密码标志等

保持连接（Keep Alive）：两次发送数据的最大时间间隔（秒）   

有效载荷：客户端标识符（Client Identifier）+（可选）遗嘱主题（Will Topic）+（可选）遗嘱消息（Will Message）+（可选）用户名（User Name）+（可选）密码（Password）

           

4.Demo实操

借助MQTTX软件，我们简单实操一下。

           

连接配置：我们这里配置连接到MQTTX的测试服务器，当然，实际使用时，一般连接到我们自己的MQTT服务器。

连接配置         

           

发布消息：我们模拟发布一个解锁的topic，然后再订阅这个topic，实际应用中，我们连接到自己的MQTT服务器以后，发布topic消息，服务器会把该topic消息发送到车载终端。

发布解锁topic         

   

可承接汽车电子、嵌入式软件开发和测试相关的需求，包括C、Matlab、Python、C#、CANoe等。

开源项目链接：

基于UDS on CAN的Bootloader的刷写上位机：

https://github.com/sydyg/UDS-on-Bootloader.git

OSEKNM协议栈：

https://github.com/sydyg/OSEK_NM

嵌入式/汽车电子测试、开发学习课程链接：

https://mv0qxzpryx9.feishu.cn/wiki/H3JCwYIVGiEOKSk9VkQcxTJ9nRf?from=from_copylink

九二，汽车行业，域控软件架构师。
熟悉汽车电子和嵌入式开发测试✍。
跟大伙聊聊天、交朋友。
喜欢的朋友可以关注和赞赏。

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