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

    [ http://tools.oasis-open.org/issues/browse/MQTT-102?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=35898#action_35898 ] 

Peter Niblett commented on MQTT-102:

I suggest we have to choose between one of three approaches here

1. Remove the words "or duplication", from page 1, but somewhere in the spec add a note that if the publisher uses QoS 1 then a subscriber might receive duplicated messages, regardless of what maxQos they requested. This includes the case where they ask for Max QoS 0 and the messages are delivered as QoS 0.

2. Remove the words "or duplication"  and add some other strengthening words to say that when delivering a message via QoS 0 a broker should discard messages rather than risk sending one that might possibly be a duplicate of one it had sent earlier (this is where the idea of discarding incoming QoS1 messages that have the DUP set came from).

3. Leave the words "or duplication" and change the description to "unreliable". We could strengthen this somewhat by adding a statement sayingr that messages must never be resent on a single hop (client to server or server to client). 

Approach 3 means there's a clear gradient of improvement as you move from QoS 0 through the QoS 2.  On the other hand there's a nice symmetry between the At Most/At Least/Exactly Once formulation (and spec's like WS-ReliableMessaging also use these terms)

Also in all cases where the publisher choses to use QoS 0 you do in fact get At Most Once delivery (it's only in the QoS 1 -> QoS 0 downgrade scenarion that duplicates can occur). So there might be some merit in continuing to refer to QoS 0 as "At most once".

Approach 1 in effect means that the Publisher of the message has the right to decide whether duplicates can happen, and therefore the responsibility to use that right wisely.

> 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".
> At risk of complicating things, I would like to add a related point to be considered alongside this issue, and that is the requirement on the use of the DUP flag in QoS 1.  In WD14 line 363 there's a statement that says
> "If Dup -0 then the flow is the first occasion that the client or server has attempted to send the MQTT PUBISH (sic) packet" If DUP 1 then this indicates that the flow might be a re-delivery of an earlier packet".  
> It has been pointed out that a client that always sets DUP 1 would be compliant with this statement, which I am sure was not our intention. I think the statement should be reworded in normative language to say that you MUST set DUP 0 on the first occasion that a packet ID is used, and DUP 1 if you have attempted to send a message with that packet ID previously.. (the precise wording of this would need some work) 

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