MQTT is based on TCP/IP which guarantees data delivery but messages can be lost if a TCP connection breaks down. Every communication protocol must ensure a quality of service (QoS) and MQTT defines three levels for message delivery on top of TCP. From level 0 to level 2, the server increases the effort to ensure that messages are delivered to subscribers but with higher QoS there are greater bandwidth consumption and latency. Quality of Service levels are the following:

• QoS Level 0 (At most once):  In this case MQTT doesn’t add any feature on TCP so that this level is equal to TCP “best-effort”.  A message can arrive to destination or not
• QoS Level 1 (At least once):  This level guarantees that the message arrives to subscribers but there may be duplicates (a client can receive the same message one or more times)
• QoS Level 2 (Exactly once):  In this case a message arrives exactly once to the subscribers. In this case there is the maximum overhead and the broker need to store messages locally