MQTT

QOS Study

`What is qos`

Qos means "quality of service". It defined whether is message is stable. Different with DDS (qos 7 levels), mqtt has 3 levels which are 0, 1, 2.

qos0: at least 0 time. In brief, just send, do not care whether received.
qos1: at least once.
qos2: confirm exactly once.

`Pre-preparation`

It is better to use mqtt first to better understand what it could do for us.

1. Install an mqtt broker on your local pc or on your cloud server. Emqx or mosquitto are both good choices.
1. And then you could install mosquitto-clients (sudo apt install mosquitto-clients on ubuntu) to test the communication between subscriber mosquitto_sub and publisher mosquitto_pub. If you want to develop some demos by yourself there is a project that you could refer.

`Understand qos by wireshark`

The qos could be defined both on sub and pub side. So the different combinations are as below.

P(QOS0)、S(QOS0)
P(QOS0)、S(QOS1)
P(QOS0)、S(QOS2)
P(QOS1)、S(QOS0)
P(QOS1)、S(QOS1)
P(QOS1)、S(QOS2)
P(QOS2)、S(QOS0)
P(QOS2)、S(QOS1)
P(QOS2)、S(QOS2)

But as I test, if( S(QOS) > P(QOS) ) { S(QOS) = P(QOS) ;}. So actually, there are only 6 combinations.

P(QOS0)、S(QOS0)
P(QOS1)、S(QOS0)
P(QOS1)、S(QOS1)
P(QOS2)、S(QOS0)
P(QOS2)、S(QOS1)
P(QOS2)、S(QOS2)

Lets use wireshark to check what happens.

	Subscriber	Publisher	Broker
ip	10.30.64.129	10.30.64.129	103.39.213.201

P(QOS0)、S(QOS2) == P(QOS0)、S(QOS1) == P(QOS0)、S(QOS0)

p0s0
p0s1
p0s2

P(QOS1)、S(QOS2) == P(QOS1)、P(QOS1)

p1s0
p1s1
p1s2

P(QOS2)

p2s0
p2s1
p2s2

Reference

link1

FengD / mqtt_qos_wireshark_test