MQTT－新一代物联网协议

柔情只为你懂 2022-09-24 09:30 358阅读 0赞

## 一、MQTT - MQ Telemetry Transport ##

*  轻量级的 machine-to-machine 通信协议。
 *  publish/subscribe模式。
 *  基于TCP/IP。
 *  支持QoS。
 *  适合于低带宽、不可靠连接、嵌入式设备、CPU内存资源紧张。
 *  是一种比较不错的Android消息推送方案。
 *  FacebookMessenger采用了MQTT。
 *  MQTT有可能成为物联网的重要协议。

## 消息体 ##

![Image 1][]

![151002110437924.png][]

## MessageType ##

![151002431369685.png][]

![Image 1][]

CONNECT

TCP连接建立完毕后，Client向Server发出一个Request。

如果一段时间内接收不到Server的Response，则关闭socket，重新建立一个session连接。

如果一个ClientID已经与服务器连接，则持有同样ClientID的旧有连接必须由服务器关闭后，新建立才能建立。

CONNACK

Server发出Response响应。

0x00 Connection Accepted  
0x01 Connection Refused: unacceptable protocol version  
0x02 Connection Refused: identifier rejected  
0x03 Connection Refused: server unavailable  
0x04 Connection Refused: bad user name or password  
0x05 Connection Refused: not authorized

PUBLISH 发布消息

Client/Servier均可以进行PUBLISH。

publish message 应该包含一个TopicName(Subject/Channel)，即订阅关键词。

关于Topic通配符

/：用来表示层次，比如a/b，a/b/c。

\#：表示匹配>=0个层次，比如a/\#就匹配a/，a/b，a/b/c。

单独的一个\#表示匹配所有。

不允许 a\#和a/\#/c。

\+：表示匹配一个层次，例如a/+匹配a/b，a/c，不匹配a/b/c。

单独的一个+是允许的，a+不允许，a/+/b不允许

PUBACK 发布消息后的确认

QoS=1时，Server向Client发布该确认（Client收到确认后删除），订阅者向Server发布确认。

PUBREC / PUBREL / PUBCOMP

QoS=2时

1. Server->Client发布PUBREC（已收到）；

2. Client->Server发布PUBREL（已释放）；

3. Server->Client发布PUBCOMP（已完成），Client删除msg；

订阅者也会向Server发布类似过程确认。

PINGREQ / PINGRES 心跳

Client有责任发送KeepAliveTime时长告诉给Server。在一个时长内，发送PINGREQ，Server发送PINGRES确认。

Server在1.5个时长内未收到PINGREQ，就断开连接。

Client在1个时长内未收到PINGRES，断开连接。

一般来说，时长设置为几个分钟。最大18hours，0表示一直未断开。

## QoS ##

![Image 1][]

![151003150901190.png][]

QoS=0：最多一次，有可能重复或丢失。

QoS=1：至少一次，有可能重复。

Client\[Qos=1,DUP=0/\*重复次数\*/,MessageId=x\] --->PUBLISH--> Server收到后，存储Message，发布，删除，向Client回发PUBACK

Client收到PUBACK后，删除Message；如果未收到PUBACK，设置DUP++，重新发送，Server端重新发布，所以有可能重复发送消息。

QoS=2：只有一次，确保消息只到达一次（用于比较严格的计费系统）。

## Clean Session ##

如果为false(flag=0)，Client断开连接后，Server应该保存Client的订阅信息。

如果为true(flag=1)，表示Server应该立刻丢弃任何会话状态信息。

**二、协议初解**

先说一下整个协议的构造，整体上协议可拆分为：

**        固定头部+可变头部+消息体**

协议说白了就是对于双方通信的一个约定，比如传过来一段字符流，第1个字节表示什么，第2个字节表示什么。。。。一个约定。

所以在固定头部的构造如下：

![SouthEast][]

1、MessageType(0和15保留，共占4位)

public $operations=array(
             "MQTT_CONNECT"=>1,//请求连接
             "MQTT_CONNACK"=>2,//请求应答
             "MQTT_PUBLISH"=>3,//发布消息
             "MQTT_PUBACK"=>4,//发布应答
             "MQTT_PUBREC"=>5,//发布已接收，保证传递1
             "MQTT_PUBREL"=>6,//发布释放，保证传递2
             "MQTT_PUBCOMP"=>7,//发布完成，保证传递3
             "MQTT_SUBSCRIBE"=>8,//订阅请求
             "MQTT_SUBACK"=>9,//订阅应答
             "MQTT_UNSUBSCRIBE"=>10,//取消订阅
             "MQTT_UNSUBACK"=>11,//取消订阅应答
             "MQTT_PINGREQ"=>12,//ping请求
             "MQTT_PINGRESP"=>13,//ping响应
             "MQTT_DISCONNECT"=>14//断开连接
            );

2、DUP flag

其是用来在保证消息传输可靠的，如果设置为1，则在下面的变长头部里多加MessageId,并需要回复确认，保证消息传输完成，但不能用于检测消息重复发送。

3、Qos

主要用于PUBLISH（发布态）消息的，保证消息传递的次数。

00表示最多一次 即<=1

01表示至少一次  即>=1

10表示一次，即==1

11保留后用

4、Retain

主要用于PUBLISH(发布态)的消息，表示服务器要保留这次推送的信息，如果有新的订阅者出现，就把这消息推送给它。如果不设那么推送至当前订阅的就释放了。

5、固定头部的byte 2

是用来保存接下去的变长头部+消息体的总大小的。

但是不是并不是直接保存的，同样也是可以扩展的，其机制是，前7位用于保存长度，后一部用做标识。

我举个例了，即如果计算出后面的大小为0<length<=127的，正常保存

如果是127<length<16383的，则需要二个字节保存了，将第一个字节的最大的一位置1,表示未完。然后第二个字节继续存。

拿130来说，第一个字节存10000011,第二个字节存000000001，也就是0x83,0x01,把两个字节连起来看，第二个字节权重从2的8次开始。

同起可以加第3个字节，最多可以加至第4个字节。故MQTT协议最多可以实现*268 435 455 (0xFF, 0xFF, 0xFF, 0x7F)将近**256M**的数据。可谓能伸能缩。*

可变头部

![SouthEast 1][]

这个是可变头部的全貌。

1、首先最上面的8个字节是Protocol Name(编码名)，UTF编码的字符“MQIsdp”，头两个是编码名提长为6。

这里多说一些，接下去的协议多采用这种方式组合，即头两个字节表示下一部分的长，然后后面跟上内容。这里头两个字节长为6，下面跟6个字符“MQIsdp”。

2、Protocol Version，协议版本号，v3 也是固定的。

3、Connect Flag，连接标识，有点像固定头部的。8位分别代表不同的标志。第1个字节保留。

Clean Session,Will flag，Will Qos, Will Retain都是相对于CONNECT消息来说的。

**Clean Session:**0表示如果订阅的客户机断线了，那么要保存其要推送的消息，如果其重新连接时，则将这些消息推送。

1表示消除，表示客户机是第一次连接，消息所以以前的连接信息。

**Will Flag**，表示如果客户机在不是在发送DISCONNECT消息中断，比如IO错误等，将些置为1,要求重传。并且下且的WillQos和WillRetain也要设置，消息体中的Topic和MessageID也要设置，就是表示发生了错误，要重传。

**Will Qos**，在CONNECT非正常情况下设置，一般如果标识了WillFlag，那么这个位置也要标识。

**Will RETAIN**：同样在CONNECT中，如果标识了WillFlag,那么些位也一定要标识

usename flag和passwordflag，用来标识是否在消息体中传递用户和密码，只有标识了，消息体中的用户名和密码才用效，只标记密码而不标记用户名是不合法的。

4、Keep Alive，表示响应时间，如果这个时间内，连接或发送操作未完成，则断开tcp连接，表示离线。

**5、Connect Return Code**即通常于CONNACK消息中，表示返回的连接情况，我可以通过此检验连接情况。

![SouthEast 2][]

**6、Topic Name，**订阅消息标识，MQTT是基于订阅/发布的消息，那么这个就是消息订阅的标识，像新闻客户端里的订阅不同的栏目一样。用于区别消息的推送类别。

主要用于PUBLISH和SUBSCRIBE中。最大可支持**32767个**字符，即4个字节。

消息体（PayLoad）

只有3种消息有消息体**CONNECT，SUBSCRIBE，SUBACK**

**CONNECT**主要是客户机的ClientID，订阅的Topic和Message以及用户名和密码，其于变长头部中的will是对应的。

**SUBSCRIBE**是包含了一系列的要订阅的主题以及QOS。

**SUBACK**是用服务器对于SUBSCRIBE所申请的主题及QOS进行确认和回复。

而**PUBLISH**是消息体中则保存推送的消息，以二进制形式，当然这里的编辑可自定义。

**7、Message Identifier**

包含于***PUBLISH, PUBACK, PUBREC, PUBREL, PUBCOMP, SUBSCRIBE, SUBACK, UNSUBSCRIBE, UNSUBACK.***

*其为16位字符表示，用于在Qos为1或2时标识Message的，保证Message传输的可靠性。*

*至于具体的消息例子，我们在后面的代码中慢慢体现。*

## 三、[MQTT协议笔记之头部信息][MQTT] ##

## 前言 ##

MQTT（Message Queue Telemetry Transport）,遥测传输协议，提供订阅/发布模式，更为简约、轻量，易于使用，针对受限环境（带宽低、网络延迟高、网络通信不稳定），可以简单概括为物联网打造，官方总结特点如下：

1.使用发布/订阅消息模式，提供一对多的消息发布，解除应用程序耦合。
    2. 对负载内容屏蔽的消息传输。
    3. 使用 TCP/IP 提供网络连接。
    4. 有三种消息发布服务质量：
     “至多一次”，消息发布完全依赖底层 TCP/IP 网络。会发生消息丢失或重复。这一级别可用于如下情况，环境传感器数据，丢失一次读记录无所谓，因为不久后还会有第二次发送。
     “至少一次”，确保消息到达，但消息重复可能会发生。
     “只有一次”，确保消息到达一次。这一级别可用于如下情况，在计费系统中，消息重复或丢失会导致不正确的结果。
    5. 小型传输，开销很小（固定长度的头部是 2 字节），协议交换最小化，以降低网络流量。
    6. 使用 Last Will 和 Testament 特性通知有关各方客户端异常中断的机制。

MQTT 3.1协议在线版本： [http://public.dhe.ibm.com/software/dw/webservices/ws-mqtt/mqtt-v3r1.html][http_public.dhe.ibm.com_software_dw_webservices_ws-mqtt_mqtt-v3r1.html]

官方下载地址： [http://public.dhe.ibm.com/software/dw/webservices/ws-mqtt/MQTT\_V3.1\_Protocol\_Specific.pdf][http_public.dhe.ibm.com_software_dw_webservices_ws-mqtt_MQTT_V3.1_Protocol_Specific.pdf]

PDF版本，42页，不算多。

另外，目前MQTT大家都用在了手机推送，可能还有很多的使用方式，有待进一步的探索。

协议方面，以前曾简单实现过一点HTTP协议，基于HTTP上构建若干种通信管道的[socket.io][]协议，不过socket.io 0.9版本的协议才两三页而已。面对领域不同，自然解决的方式也不一样。

阅读完毕MQTT协议，有一个想法，其实可以基于MQTT协议，打造更加私有、精简（协议一些地方，略显多余）的传输协议，比如一个字节的传输开销。有时间，会详细说一下。

## 固定头部 ##

固定头部，使用两个字节，共16位：

### 第一个字节(byte 1) ###

消息类型(4-7)，使用4位二进制表示，可代表16种消息类型：

<table style="font-size:10pt; width:auto!important; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px; border-collapse:collapse; border-top-color:black; border-top-width:1px; border-top-style:solid; font-family:verdana,arial,helvetica,sans-serif; border-right-color:black; border-right-width:1px; border-right-style:solid; border-bottom-color:black; border-bottom-width:1px; border-bottom-style:solid; empty-cells:show; margin-left:3em; border-left-color:black; border-left-width:1px; border-left-style:solid; background-color:rgb(255,255,255)"> 
 <thead> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <th style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> Mnemonic</th> 
   <th align="center" style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> Enumeration</th> 
   <th style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Reserved</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Reserved</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> CONNECT</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Client request to connect to Server</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> CONNACK</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 2</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Connect Acknowledgment</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> PUBLISH</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 3</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Publish message</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> PUBACK</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 4</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Publish Acknowledgment</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> PUBREC</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 5</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Publish Received (assured delivery part 1)</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> PUBREL</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 6</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Publish Release (assured delivery part 2)</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> PUBCOMP</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 7</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Publish Complete (assured delivery part 3)</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> SUBSCRIBE</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 8</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Client Subscribe request</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> SUBACK</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 9</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Subscribe Acknowledgment</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> UNSUBSCRIBE</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 10</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Client Unsubscribe request</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> UNSUBACK</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 11</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Unsubscribe Acknowledgment</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> PINGREQ</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 12</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> PING Request</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> PINGRESP</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 13</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> PING Response</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> DISCONNECT</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 14</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Client is Disconnecting</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Reserved</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 15</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Reserved</td> 
  </tr> 
 </tbody> 
</table>

除去0和15位置属于保留待用，共14种消息事件类型。

### DUP flag（打开标志） ###

保证消息可靠传输，默认为0，只占用一个字节，表示第一次发送。不能用于检测消息重复发送等。只适用于客户端或服务器端尝试重发PUBLISH, PUBREL, SUBSCRIBE 或 UNSUBSCRIBE消息，注意需要满足以下条件：

当QoS > 0
     消息需要回复确认

此时，在可变头部需要包含消息ID。当值为1时，表示当前消息先前已经被传送过。

### QoS(Quality of Service,服务质量) ###

使用两个二进制表示PUBLISH类型消息：

<table style="width:auto!important; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px; border-collapse:collapse; font-size:15px; border-top-color:black; border-top-width:1px; border-top-style:solid; border-right-color:black; border-right-width:1px; border-right-style:solid; border-bottom-color:black; border-bottom-width:1px; border-bottom-style:solid; empty-cells:show; margin-left:3em; border-left-color:black; border-left-width:1px; border-left-style:solid"> 
 <thead> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <th style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> QoS value</th> 
   <th style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> bit 2</th> 
   <th style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> bit 1</th> 
   <th colspan="3" align="left" style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 至多一次</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 发完即丢弃</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> &lt;=1</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 至少一次</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 需要确认回复</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> &gt;=1</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 2</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 只有一次</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 需要确认回复</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> =1</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 3</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td colspan="3" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 待用，保留位置</td> 
  </tr> 
 </tbody> 
</table>

### RETAIN(保持) ###

仅针对PUBLISH消息。不同值，不同含义：

**1**：表示发送的消息需要一直持久保存（不受服务器重启影响），不但要发送给当前的订阅者，并且以后新来的订阅了此Topic name的订阅者会马上得到推送。

备注：新来乍到的订阅者，只会取出最新的一个RETAIN flag = 1的消息推送。

**0**：仅仅为当前订阅者推送此消息。

假如服务器收到一个空消息体(zero-length payload)、RETAIN = 1、已存在Topic name的PUBLISH消息，服务器可以删除掉对应的已被持久化的PUBLISH消息。

### 如何解析 ###

因为java使用有符号(最高位为符号位)数据表示，byte范围：-128-127。该字节的最高位（左边第一位），可能为1。若直接转换为byte类型，会出现负数，这是一个雷区。DataInputStream提供了int readUnsignedByte()读取方式，请注意。下面演示了，如何从一个字节中，获取到所有定义的信息，同时绕过雷区：

public static void main(String[] args) {
        byte publishFixHeader = 50;// 0 0 1 1 0 0 1 0
    
        doGetBit(publishFixHeader);
        int ori = 224;//1110000,DISCONNECT ,Message Type (14)
        byte flag = (byte) ori; //有符号byte 
        doGetBit(flag);
        doGetBit_v2(ori);
    }
    
    
    public static void doGetBit(byte flags) {
        boolean retain = (flags & 1) > 0;
        int qosLevel = (flags & 0x06) >> 1;
        boolean dupFlag = (flags & 8) > 0;
        int messageType = (flags >> 4) & 0x0f;
    
        System.out.format(
                "Message type:%d, DUP flag:%s, QoS level:%d, RETAIN:%s\n",
                messageType, dupFlag, qosLevel, retain);
    }
    
    public static void doGetBit_v2(int flags) {
        boolean retain = (flags & 1) > 0;
        int qosLevel = (flags & 0x06) >> 1;
        boolean dupFlag = (flags & 8) > 0;
        int messageType = flags >> 4;
    
        System.out.format(
                "Message type:%d, DUP flag:%s, QoS level:%d, RETAIN:%s\n",
                messageType, dupFlag, qosLevel, retain);
    }

### 处理Remaining Length（剩余长度） ###

在当前消息中剩余的byte(字节)数，包含可变头部和负荷(称之为内容/body，更为合适)。单个字节最大值：01111111，16进制：0x7F，10进制为127。单个字节为什么不能是11111111（0xFF）呢？因为MQTT协议规定，第八位（最高位）若为1，则表示还有后续字节存在。同时MQTT协议最多允许4个字节表示剩余长度。那么最大长度为：0xFF,0xFF,0xFF,0x7F，二进制表示为:11111111,11111111,11111111,01111111，十进制：268435455 byte=261120KB=256MB=0.25GB 四个字节之间值的范围：

如何换算成十进制呢 ? 使用java语言表示如下：

public static void main(String[] args) throws IOException {
        // 模拟客户端写入
       ByteArrayOutputStream arrayOutputStream = new ByteArrayOutputStream();
       DataOutputStream dataOutputStream = new DataOutputStream(arrayOutputStream);
       dataOutputStream.write(0xff);
       dataOutputStream.write(0xff);
       dataOutputStream.write(0xff);
       dataOutputStream.write(0x7f);
    
       InputStream arrayInputStream = new ByteArrayInputStream(arrayOutputStream.toByteArray());
    
        // 模拟服务器/客户端解析
       System. out.println( "result is " + bytes2Length(arrayInputStream));
    }
    
    /** * 转化字节为 int类型长度 * @param in * @return * @throws IOException */
    private static int bytes2Length(InputStream in) throws IOException {
        int multiplier = 1;
        int length = 0;
        int digit = 0;
        do {
            digit = in.read(); //一个字节的有符号或者无符号，转换转换为四个字节有符号 int类型
            length += (digit & 0x7f) * multiplier;
            multiplier *= 128;
       } while ((digit & 0x80) != 0);
    
        return length;
    }

一般最后一个字节小于127（01111111），和0x80（10000000）进行&操作，最终结果都为0，因此计算会终止。代理中间件和请求者，中间传递的是字节流Stream，自然要从流中读取，逐一解析出来。

那么如何将int类型长度解析为不确定的字节值呢？

public static void main(String[] args) throws IOException {
        // 模拟服务器/客户端写入
       ByteArrayOutputStream arrayOutputStream = new ByteArrayOutputStream();
       DataOutputStream dataOutputStream = new DataOutputStream(
                 arrayOutputStream);
    
        // 模拟服务器/客户端解析
        length2Bytes(dataOutputStream, 128);
    }
    
    /** * int类型长度解析为1-4个字节 * @param out * @param length * @throws IOException */
    private static void length2Bytes(OutputStream out, int length) throws IOException {
        int val = length;
        do {
             int digit = val % 128;
            val = val / 128;
             if (val > 0)
                 digit = digit | 0x80;
    
            out.write(digit);
       } while (val > 0);
    }

digit对val求模，最大值可能是127，一旦127 | 10000000 = 11111111 = 0xff = 255 请注意：剩余长度，只在固定头部中，无论是一个字节，还是四个字节，不能被算作可变头部中。

## 可变头部 ##

固定头部仅定义了消息类型和一些标志位，一些消息的元数据，需要放入可变头部中。可变头部内容字节长度 + Playload/负荷字节长度 = 剩余长度，这个是需要牢记的。可变头部，包含了协议名称，版本号，连接标志，用户授权，心跳时间等内容，这部分和后面要讲到的CONNECT消息类型，有重复，暂时略过。

### Playload/消息体/负荷 ###

消息体主要是为配合固定/可变头部命令（比如CONNECT可变头部User name标记若为1则需要在消息体中附加用户名称字符串）而存在。

CONNECT/SUBSCRIBE/SUBACK/PUBLISH等消息有消息体。PUBLISH的消息体以二进制形式对待。

请记住，MQTT协议只允许在PUBLISH类型消息体中使用自定义特性，在固定/可变头部想加入自定义私有特性，就免了吧。这也是为了协议免于流于形式，变得很分裂也为了兼顾现有客户端等。比如支持压缩等，那就可以在Playload中定义数据支持，在应用中进行读取处理。

这部分会在后面详细论述。

### 消息标识符/消息ID ###

固定头中的QoS level标志值为1或2时才会在：PUBLISH，PUBACK，PUBREC，PUBREL，PUBCOMP，SUBSCRIBE，SUBACK，UNSUBSCRIBE，UNSUBACK等消息的可变头中出现。

一个16位无符号位的short类型值（值不能为 0，0做保留作为无效的消息ID），仅仅要求在一个特定方向（服务器发往客户端为一个方向，客户端发送到服务器端为另一个方向）的通信消息中必须唯一。比如客户端发往服务器，有可能存在服务器发往客户端会同时存在重复，但不碍事。

可变头部中，需要两个字节的顺序是MSB(Most Significant Bit) LSB(Last/Least Significant Bit)，翻译成中文就是，最高有效位，最低有效位。最高有效位在最低有效位左边/上面，表示这是一个大端字节/网络字节序，符合人的阅读习惯，高位在最左边。

但凡如此表示的，都可以视为一个16位无符号short类型整数，两个字节表示。在JAVA中处理比较简单：

DataInputStream.readUnsignedShort

或者

in.read() * 0xFF + in.read();

最大长度可为： 65535

### UTF-8编码 ###

有关字符串，MQTT采用的是修改版的UTF-8编码，一般形式为如下，需要牢记：

比如AVA，使用writeUTF()方法写入一串文字“OTWP”，头两个字节为一个完整的无符号数字，代表字符串字节长度，后面四个字节才是字符串真正的长度，共六个字节：

<table style="font-size:10pt; width:800px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px; border-collapse:collapse; border-top-color:black; border-top-width:1px; border-top-style:solid; font-family:verdana,arial,helvetica,sans-serif; border-right-color:black; border-right-width:1px; border-right-style:solid; border-bottom-color:black; border-bottom-width:1px; border-bottom-style:solid; empty-cells:show; margin-left:3em; border-left-color:black; border-left-width:1px; border-left-style:solid; background-color:rgb(255,255,255)"> 
 <thead> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <th style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> bit</th> 
   <th align="center" style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> 7</th> 
   <th align="center" style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> 6</th> 
   <th align="center" style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> 5</th> 
   <th align="center" style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> 4</th> 
   <th align="center" style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> 3</th> 
   <th align="center" style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> 2</th> 
   <th align="center" style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> 1</th> 
   <th align="center" style="font-weight:bold; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; background-color:rgb(222,222,255)"> 0</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> byte 1</td> 
   <td colspan="8" align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Message Length MSB (0x00)</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> &nbsp;</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> byte 2</td> 
   <td colspan="8" align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> Message Length LSB (0x04)</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> &nbsp;</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> byte 3</td> 
   <td colspan="8" align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 'O' (0x4F)</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> &nbsp;</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> byte 4</td> 
   <td colspan="8" align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 'T' (0x54)</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> &nbsp;</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> byte 5</td> 
   <td colspan="8" align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 'W' (0x57)</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> &nbsp;</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:white; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> byte 6</td> 
   <td colspan="8" align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 'P' (0x50)</td> 
  </tr> 
  <tr style="border-top-width:1px; border-top-style:solid; border-top-color:rgb(204,204,204); background-color:rgb(248,248,248); margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px"> 
   <td style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> &nbsp;</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 1</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
   <td align="center" style="font-size:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:black; border-right-color:black; border-bottom-color:black; border-left-color:black; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px"> 0</td> 
  </tr> 
 </tbody> 
</table>

这点，在程序中，可不用单独处理默认，直接使用**readUTF()**方法，可自动省去了处理字符串长度的麻烦。当然，可以手动读取字符串：

// 模拟写入
    dataOutputStream.writeUTF( "abcd");// 2 + 4 = 6 byte
    ......
    // 模拟读取 
    int decodedLength = dataInputStream.readUnsignedShort();//2 byte
    byte[] decodedString = new byte[decodedLength]; // 4 bytes
    dataInputStream.read(decodedString);
    String target = new String(decodedString, "UTF-8");

等同于：

String target = dataInputStream.readUTF();

MQTT无论是可变头部还是消息体中，只要是字符串部分，都是采用了修改版的UTF-8编码，读取和写入，借助DataInputStream/DataOutputStream的帮助，一行语句，略去了手动处理的麻烦。

## 小结 ##

总之，掌握固定头部的QoS level、RETAIN标记、可变头部的Connect flags作用和意义，对总体理解MQTT作用很大。

# 四、MQTT协议笔记之连接和心跳 #

时间 2014-02-09 13:38:42   [BlogJava-技术区][BlogJava-]

原文   [http://www.blogjava.net/yongboy/archive/2014/02/09/409630.html][http_www.blogjava.net_yongboy_archive_2014_02_09_409630.html]

主题  [MQTT][MQTT 1]

## 前言 ##

本篇会把连接(CONNECT)、心跳(PINGREQ/PINGRESP)、确认(CONNACK)、断开连接(DISCONNECT)和在一起。

## CONNECT ##

像前面所说，MQTT有关字符串部分采用的修改版的UTF-8编码，CONNECT可变头部中协议名称、消息体都是采用修改版的UTF-8编码。前面基本上可变头部内容不多，下面是一个较为完整的CONNECT消息结构：

<table style=""> 
 <thead> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <th colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.5em; text-align:left; vertical-align:bottom; border-top-width:1px; border-top-style:solid; border-top-color:black; font-weight:bold; border-left-width:1px; border-left-style:solid; border-left-color:black; background-color:rgb(222,222,255); border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Description</th> 
   <th align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.5em; text-align:left; vertical-align:bottom; border-top-width:1px; border-top-style:solid; border-top-color:black; font-weight:bold; border-left-width:1px; border-left-style:solid; border-left-color:black; background-color:rgb(222,222,255); border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 7</th> 
   <th align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.5em; text-align:left; vertical-align:bottom; border-top-width:1px; border-top-style:solid; border-top-color:black; font-weight:bold; border-left-width:1px; border-left-style:solid; border-left-color:black; background-color:rgb(222,222,255); border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 6</th> 
   <th align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.5em; text-align:left; vertical-align:bottom; border-top-width:1px; border-top-style:solid; border-top-color:black; font-weight:bold; border-left-width:1px; border-left-style:solid; border-left-color:black; background-color:rgb(222,222,255); border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 5</th> 
   <th align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.5em; text-align:left; vertical-align:bottom; border-top-width:1px; border-top-style:solid; border-top-color:black; font-weight:bold; border-left-width:1px; border-left-style:solid; border-left-color:black; background-color:rgb(222,222,255); border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 4</th> 
   <th align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.5em; text-align:left; vertical-align:bottom; border-top-width:1px; border-top-style:solid; border-top-color:black; font-weight:bold; border-left-width:1px; border-left-style:solid; border-left-color:black; background-color:rgb(222,222,255); border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 3</th> 
   <th align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.5em; text-align:left; vertical-align:bottom; border-top-width:1px; border-top-style:solid; border-top-color:black; font-weight:bold; border-left-width:1px; border-left-style:solid; border-left-color:black; background-color:rgb(222,222,255); border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 2</th> 
   <th align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.5em; text-align:left; vertical-align:bottom; border-top-width:1px; border-top-style:solid; border-top-color:black; font-weight:bold; border-left-width:1px; border-left-style:solid; border-left-color:black; background-color:rgb(222,222,255); border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</th> 
   <th align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.5em; text-align:left; vertical-align:bottom; border-top-width:1px; border-top-style:solid; border-top-color:black; font-weight:bold; border-left-width:1px; border-left-style:solid; border-left-color:black; background-color:rgb(222,222,255); border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="10" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> <span style="text-indent:0px"><strong>Fixed header/固定头部</strong></span></td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td align="center" colspan="4" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Message Type(1)</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> DUP flag</td> 
   <td align="center" colspan="2" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> QoS level</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> RETAIN</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 
    <div style="font-size:16px; line-height:1.7em">
      byte 1 
    </div> </td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> x</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> x</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> x</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> x</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> byte 2</td> 
   <td align="center" colspan="10" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Remaining Length</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="10" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> <span style="text-indent:0px"><strong>Variable header</strong></span>&nbsp;/可变头部</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="10" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Protocol Name</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> byte 1</td> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Length MSB (0)</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> byte 2</td> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Length LSB (6)</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> byte 3</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 'M'</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> byte 4</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 'Q'</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> byte 5</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 'I'</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> byte 6</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 's'</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> byte 7</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 'd'</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> byte 8</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 'p'</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="10" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Protocol Version Number</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> byte 9</td> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Version (3)</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="10" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Connect Flags</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> User Name Flag</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Password Flag</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Will Retain</td> 
   <td align="center" colspan="2" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Will QoS</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Will Flag</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Clean Session</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Reserved</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="2" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 
    <div style="font-size:16px; line-height:1.7em">
      byte 10 
    </div> </td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> x</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="10" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Keep Alive timer</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> byte 11</td> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Keep Alive MSB (0)</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> byte 12</td> 
   <td colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> Keep Alive LSB (10)</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 1</td> 
   <td align="center" colspan="1" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> 0</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="10" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> <span style="text-indent:0px"><strong>Playload/消息体</strong></span></td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="10" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> <p style="margin-top:0px; margin-right:0px; margin-bottom:0.75em; margin-left:0px; font-size:16px; line-height:1.7em; text-indent:1em"> Client Identifier(客户端ID)</p> 1-23个字符长度，客户端到服务器的全局唯一标志，如果客户端ID超出23个字符长度，服务器需要返回码为2，标识符被拒绝响应的CONNACK消息。&nbsp;<br> 处理QoS级别1和2的消息ID中，可以使用到。&nbsp;<br> 必填项。</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="10" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> <p style="margin-top:0px; margin-right:0px; margin-bottom:0.75em; margin-left:0px; font-size:16px; line-height:1.7em; text-indent:1em"> Will Topic</p> Will Flag值为1，这里便是Will Topic的内容。QoS级别通过Will QoS字段定义，RETAIN值通过Will RETAIN标识，都定义在可变头里面。</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="10" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> <p style="margin-top:0px; margin-right:0px; margin-bottom:0.75em; margin-left:0px; font-size:16px; line-height:1.7em; text-indent:1em"> Will Message</p> Will Flag若设为1，这里便是Will Message定义消息的内容，对应的主题为Will Topic。如果客户端意外的断开触发服务器PUBLISH此消息。&nbsp;<br> 长度有可能为0。&nbsp;<br> 在CONNECT消息中的Will Message是UTF-8编码的，当被服务器发布时则作为二进制的消息体。</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="10" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> <p style="margin-top:0px; margin-right:0px; margin-bottom:0.75em; margin-left:0px; font-size:16px; line-height:1.7em; text-indent:1em"> User Name</p> 如果设置User Name标识，可以在此读取用户名称。一般可用于身份验证。协议建议用户名为不多于12个字符，不是必须。</td> 
  </tr> 
  <tr style="font-size:14px; line-height:1.3em"> 
   <td colspan="10" style="padding-top:5px; padding-right:5px; padding-bottom:5px; padding-left:5px; line-height:1.3em; text-align:left; vertical-align:top; border-top-width:1px; border-top-style:solid; border-top-color:black; border-left-width:1px; border-left-style:solid; border-left-color:black; font-size:14px; border-right-width:1px; border-bottom-width:1px; border-right-style:solid; border-bottom-style:solid; border-right-color:black; border-bottom-color:black"> <p style="margin-top:0px; margin-right:0px; margin-bottom:0.75em; margin-left:0px; font-size:16px; line-height:1.7em; text-indent:1em"> Password</p> 如果设置Password标识，便可读取用户密码。建议密码为12个字符或者更少，但不是必须。</td> 
  </tr> 
 </tbody> 
</table>

### 可变头部 ###

协议名称和协议版本都是固定的。

#### 连接标志(Connect Flags) ####

一个字节表示，除了第1位是保留未使用，其它7位都具有不同含义。

业务上很重要，对消息总体流程影响很大，需要牢记。

Clean Session

0，表示如果订阅的客户机断线了，要保存为其要推送的消息（QoS为1和QoS为2），若其重新连接时，需将这些消息推送（若客户端长时间不连接，需要设置一个过期值）。 1，断线服务器即清理相关信息，重新连接上来之后，会再次订阅。

Will Flag

定义了客户端（没有主动发送DISCONNECT消息）出现网络异常导致连接中断的情况下，服务器需要做的一些措施。

简而言之，就是客户端预先定义好，在自己异常断开的情况下，所留下的最后遗愿（Last Will），也称之为遗嘱（Testament）。 这个遗嘱就是一个由客户端预先定义好的主题和对应消息，附加在CONNECT的可变头部中，在客户端连接出现异常的情况下，由服务器主动发布此消息。

只有在Will Flag位为1时，Will Qos和Will Retain才会被读取，此时消息体Playload中要出现Will Topic和Will Message具体内容，否则，Will QoS和Will Retain值会被忽略掉。

Will Qos

两位表示，和PUBLISH消息固定头部的QoS level含义一样。这里先掠过，到PUBLISH消息再回过头来看看，会更明白些。

若标识了Will Flag值为1，那么Will QoS就会生效，否则会被忽略掉。

Will RETAIN

如果设置Will Flag，Will Retain标志就是有效的，否则它将被忽略。

当客户端意外断开服务器发布其Will Message之后，服务器是否应该继续保存。这个属性和PUBLISH固定头部的RETAIN标志含义一样，这里先掠过。

User name 和 password Flag：

用于授权，两者要么为0要么为1，否则都是无效。都为0，表示客户端可自由连接/订阅，都为1，表示连接/订阅需要授权。

#### Playload/消息体 ####

消息体定义的消息顺序（如上表所示），约定俗成，不得更改，否则将可能引起混乱。

若Will Flag值为0，那么在Playload中，Client Identifer后面就不会存在Will Topic和Will Message内容。

若User Name和Password都为0，意味着Playload/消息体中，找不到User Name和password的值，就算有，也是无效。标志决定着是否读取与否。

心跳时间(Keep Alive timer)

以秒为单位，定义服务器端从客户端接收消息的最大时间间隔。一般应用服务会在业务层次检测客户端网络是否连接，不是TCP/IP协议层面的心跳机制(比如开启SOCKET的SO\_KEEPALIVE选项)。 一般来讲，在一个心跳间隔内，客户端发送一个PINGREQ消息到服务器，服务器返回PINGRESP消息，完成一次心跳交互，继而等待下一轮。若客户端没有收到心跳反馈，会关闭掉TCP/IP端口连接，离线。 16位两个字节，可看做一个无符号的short类型值。最大值，2^16-1 = 65535秒 = 18小时。最小值可以为0，表示客户端不断开。一般设为几分钟，比如微信心跳周期为300秒。

Will Message编码

Will Message在CONNECT Payload/息体中，使用UTF-8编码。假设内容为“abcd”，大概如下：

有一点需要记住，PUBLISH的Payload/消息体中以二进制编码保存。

某刻客户端异常关闭触发服务器会PUBLISH此消息。那么服务器会直接把byte3-byte6之间字符取出，保存为二进制，附加到PUBLISH消息体中，大概存储如下：

另外，MQTT 3.1协议对Will message的说明很容易引起误解，3.1.1草案已经得到修正。