QoS
MQTT is designed to work well on unreliable networks and as such provides three levels of Quality of Service for different situations. These allow clients to specify the reliability they desire.
QoS Level 0:
The simplest level of QoS, it requires no acknowledgment from the client and the reliability will be the same as that of the underlying network layer, TCP/IP.
QoS Level 1:
This ensures that the message is delivered to the client at least once, but it could be delivered more than once. It relies on the client sending an ACK packet when it receives a packet. This is commonly used for guaranteed delivery however developers must make sure that their systems can handle duplicate packets.
QoS Level 2:
This is the least common QoS, and ensures that a message is delivered once and only once. This method requires an exchange of four packets, and will decrease performance of the broker. This level is also often left out of MQTT implementations due to its relative complexity. Make sure you check this before choosing a library or broker.
Взято с https://zoetrope.io/tech-blog/brief-practical-introduction-mqtt-protocol-and-its-application-iot
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