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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=36056#action_36056 ] 

Rahul Gupta commented on MQTT-102:

Changes made in WD-17

1) "or duplication" is removed 

"At most once", where messages are delivered according to the best efforts of the operating environment. Message loss 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.

2)   Made separate paragraphs for two converse non normative comments

The recipient of a Control Packet that contains the Dup flag set to 1 cannot assume that it has seen an earlier copy of this packet. 

It is important to note that the Dup flag refers to the Control Packet itself and not to the Application Message that it contains. When using QoS 1, it is possible for a Client to receive a PUBLISH Packet with DUP set to 0 that contains a repetition of an Application Message that it received earlier, but with a different Packet Identifier. See section ý2.3.1 Packet Identifier.

3) Normative statements updated for SUBSCRIBE control packet

The SUBACK Packet sent by the Server to the Client MUST contain a return code for each Topic Filter/QoS pair. This return code MUST either show the maximum QoS that was granted for that Subscription or indicate that the subscription failed [MQTT-3.8.4-4]. 

The Server might grant a lower maximum QoS than the subscriber requested. The QoS of Payload messages sent in response to a Subscription MUST be the minimum of the QoS of the originally published message and the maximum QoS granted by the Server. The server is permitted to send duplicate copies of a message to a subscriber in the case where the original message was published with QoS 1 and the maximum QoS granted was QoS 0.[MQTT-3.8.4-5].

4) Non Normative comment updated

If a subscribing Client has been granted maximum QoS 1 for a particular Topic Filter, then a QoS 0 application message matching the filter is delivered to the Client at QoS 0. This means that at most one copy of the message is received by the Client. On the other hand a QoS 2 Message published to the same topic is downgraded by the Server to QoS 1 for delivery to the Client, so that Client might receive duplicate copies of the Message.

If the subscribing Client has been granted maximum QoS 0, then an application message originally published as QoS 2 might get lost on the hop to the Client, but the Server should never send a duplicate of that message. A QoS 1 message published to the same topic might either get lost or duplicated on its transmission to that Client. 

> 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
>            Assignee: Rahul Gupta
>             Fix For: 3.1.1
> 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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