# 3.7 PUBCOMP – 发布完成（QoS 2，第三步）

PUBCOMP报文是对PUBREL报文的响应。它是QoS 2等级协议交换的第四个也是最后一个报文。

## 3.7.1 固定报头

### 图例 3.18 – PUBCOMP报文固定报头

| **Bit** | **7**          | **6** | **5** | **4** | **3** | **2** | **1** | **0** |
| ------- | -------------- | ----- | ----- | ----- | ----- | ----- | ----- | ----- |
| byte 1  | MQTT控制报文类型 (7) | 保留位   |       |       |       |       |       |       |
|         | 0              | 1     | 1     | 1     | 0     | 0     | 0     | 0     |
| byte 2  | 剩余长度 (2)       |       |       |       |       |       |       |       |
|         | 0              | 0     | 0     | 0     | 0     | 0     | 1     | 0     |

**剩余长度字段**

表示可变报头的长度。对PUBCOMP报文这个值等于2。

## 3.7.2 可变报头

可变报头包含与等待确认的PUBREL报文相同的报文标识符。

### 图例 3.19 – PUBCOMP报文可变报头

| **Bit** | **7**     | **6** | **5** | **4** | **3** | **2** | **1** | **0** |
| ------- | --------- | ----- | ----- | ----- | ----- | ----- | ----- | ----- |
| byte 1  | 报文标识符 MSB |       |       |       |       |       |       |       |
| byte 2  | 报文标识符 LSB |       |       |       |       |       |       |       |

## 3.7.3 有效载荷

PUBCOMP报文没有有效载荷。

## 3.7.4 动作

完整的描述见4.3.3节。

## 第三章目录 MQTT控制报文

* [3.0 Contents – MQTT控制报文](/mqtt/03-controlpackets.md)
* [3.1 CONNECT – 连接服务端](/mqtt/03-controlpackets/0301-connect.md)
* [3.2 CONNACK – 确认连接请求](/mqtt/03-controlpackets/0302-connack.md)
* [3.3 PUBLISH – 发布消息](/mqtt/03-controlpackets/0303-publish.md)
* [3.4 PUBACK –发布确认](/mqtt/03-controlpackets/0304-puback.md)
* [3.5 PUBREC – 发布收到（QoS 2，第一步）](/mqtt/03-controlpackets/0305-pubrec.md)
* [3.6 PUBREL – 发布释放（QoS 2，第二步）](/mqtt/03-controlpackets/0306-pubrel.md)
* [3.7 PUBCOMP – 发布完成（QoS 2，第三步）](/mqtt/03-controlpackets/0307-pubcomp.md)
* [3.8 SUBSCRIBE - 订阅主题](/mqtt/03-controlpackets/0308-subscribe.md)
* [3.9 SUBACK – 订阅确认](/mqtt/03-controlpackets/0309-suback.md)
* [3.10 UNSUBSCRIBE –取消订阅](/mqtt/03-controlpackets/0310-unsubscribe.md)
* [3.11 UNSUBACK – 取消订阅确认](/mqtt/03-controlpackets/0311-unsuback.md)
* [3.12 PINGREQ – 心跳请求](/mqtt/03-controlpackets/0312-pingreq.md)
* [3.13 PINGRESP – 心跳响应](/mqtt/03-controlpackets/0313-pingresp.md)
* [3.14 DISCONNECT –断开连接](/mqtt/03-controlpackets/0314-disconnect.md)

## 项目主页

* [MQTT协议中文版](https://github.com/mcxiaoke/mqtt)


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