The three Qualities of Service, as defined by the MQTT specification, are:
<ul>
    <li>"At most once", where messages are delivered according to the best efforts of
the underlying TCP/IP network. Message loss or duplication can occur. This
level could be used, for example, with ambient sensor data where it does not
matter if an individual reading is lost as the next one will be published soon
after.</li>
    <li>"At least once", where messages are assured to arrive but duplicates may
occur.</li>
    <li>"Exactly once", where message are assured to arrive exactly once. This level
could be used, for example, with billing systems where duplicate or lost
messages could lead to incorrect charges being applied.</li>
</ul>