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Subject: [OASIS Issue Tracker] Created: (MQTT-102) What does QoS 0 mean? Should it be called "At Most Once"?

What does QoS 0 mean? Should it be called "At Most Once"?

                 Key: MQTT-102
                 URL: http://tools.oasis-open.org/issues/browse/MQTT-102
             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
          Issue Type: Bug
          Components: core
    Affects Versions: 3.1.1
            Reporter: Peter Niblett

WD 14 lines 34, 378 and 1254  all refer to QoS 0 as "At most once" (in contrast to QoS 1 which is "At least once", and QoS 2 "Exactly once").  Line 35 goes on to state that for QoS 0 "Message loss or duplication can occur".   However the description of QoS 0 section 4, line 1256 says "The message arrives at the receiver either once or not at all".

The inclusion of "duplication" in the statement in 35 causes two problems:

1. It looks like a mistake to refer to something as "At most once" and at the same time tell people that duplication can occur
2. It gives equal status to loss and duplication. Someone implementing (for example) a server or a bridge from another protocol can find themselves in a position where they have a QoS 0 message which might possibly have already been sent. What should they do?  Should they assume that consumers don't care about duplicates and so they should send it just in case, or can they say that consumers don't care about lost messages, so they should discard it ?

We need to clarify this ambiguity and decide what QoS 0 really means

a) It really does mean "At most once", in which case we remove the word duplication from 35. We should also add some normative text to say that a sender  should never knowingly send a QoS 0 message that might be a duplicate (i.e. it should err on the side of dropping a message rather than possibly sending a duplicate).

b) It really does mean "Duplicates and Gaps" (so taken to an absurd extreme a server that continually published message 1 ad infinitum would be compliant). In this case we should stop referring to it as "At most once".

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