关于MQTT固定标头结构? [英] Regarding the MQTT fixed header structure?

问题描述

我正在尝试实现MQTT库.并且根据一个教程，MQTT消息的固定标头应该看起来像发布的图片中所示.

i am trying to implement a MQTT library. And according to a tutorial, the fixed header of the MQTT message should look like as shown in the posted picture.

也在同一教程中，我发现固定标头的编码方法写为:

Also in the same tutorial, i found that the encodeing method of the fixed header is written as:

mqtt.write((byte) ((retain ? 1 : 0) | qos << 1 | (dup ? 1 : 0) << 3 | type << 4));

我的问题是，根据发布的固定标头规范，字段retain，qos，dup和msg type应该具有1，2，1和4位.

My Question is, according to the fixed header specifications posted, the fields retain, qos, dup and the msg type should have 1, 2, 1 and 4 bits respectively.

为什么将msg type字段扩展到5位<< 4"，而将字段dup扩展到4 bits<< 3"?

why the msg type field is expanded upto 5 bits "<<4" and the field dup is expanded upto 4 bits "<<3" ?

Fixed_Header

推荐答案

为什么将msg类型字段扩展到5位"<<< 4"，并且将字段dup扩展到4位<<< 3"?

why the msg type field is expanded upto 5 bits "<<4" and the field dup is expanded upto 4 bits "<<3" ?

<<不会扩展"该字段，它会移动它离开.

<< doesn't "expand" the field, it shifts it left.

最初，我们的每个数字如下所示:

Initially, each of our numbers looks like this:

bit: |   7   |   6   |   5   |   4   |   3   |   2   |   1   |   0   |
======================================================================
                                     |------------- type ------------|
                                                             |- dup -|
                                                     |----- qos -----|
                                                             | retain|

假设每个数字都在该字段的有效范围内，则它们已经是正确的宽度.具体来说，type的值可以为0到15，因此(最多)4位宽； dup和retain必须为0或1，因此只有1位宽； qos的值可以是0到3，即2位宽.

Assuming that each of the numbers is in a valid range for that field, then they are already the correct width. Specifically, type can have a value from 0 to 15, so it is (up to) 4 bits wide; dup and retain must be 0 or 1 so are only 1 bit wide; qos can have a value from 0 to 3, which is 2 bits wide.

但是，它们在错误的位置.它们卡在低位(3,2,1,0)中.我们需要将它们向左移动以将它们放置到正确的位置:

But, they are in the wrong place. They are stuck down in the low bits (3,2,1,0). We need to shift them left to get them to the correct positions:

bit: |   7   |   6   |   5   |   4   |   3   |   2   |   1   |   0   |
======================================================================
     |----------- type<<4 -----------|                                
                                     | dup<<3|                        
                                             |---- qos<<1 ---|        
                                                             | retain|

例如，dup被左移3位，因为我们要在其下3个空格(实际上是值为0的位). retain不需要移动，因为它恰好位于正确的位置.

For example, dup is shifted left 3 bits because we want 3 spaces (actually bits of value 0) below it. retain does not need to be shifted because it happened to be in the correct place already.

一旦所有位都放置在正确的位置，它们就使用 ORed 一起 c20>给出该字节:

Once all the bits are in the correct place, they are ORed together using | to give this byte:

bit: |   7   |   6   |   5   |   4   |   3   |   2   |   1   |   0   |
======================================================================
     |----------- type<<4 -----------| dup<<3|---- qos<<1 ---| retain|

想要的是什么.

这篇关于关于MQTT固定标头结构?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！