[OASIS Issue Tracker] (MQTT-555) Clarification of broker behaviour when client REGISTERs a PREDEFINED topic

[OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org>]

    [ https://issues.oasis-open.org/browse/MQTT-555?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=81982#comment-81982 ] 

Ian Craggs commented on MQTT-555:
---------------------------------

Miroslav's comment:

_This means that the predefined alias can be known by the gateway and not by the client. Iâm not sure that this is a good concept. I always considered_
_predefined aliases to be a read-only map of aliases known by both parties before a connection is established._

The easiest way to fix this is to change this behavior:

*If the client initiates a REGISTER against a topic which is known by the gateway to have a predefined topicId associated with it, the gateway will specify its topicIdType to be predefined and set the topicId value to match this in the REGACK. The client can then choose to update its registry of predefined topicIds if it so wishes. If there are no predefined topicIds, the gateway will pass back a NORMAL alias topicIdType.*

so that it is an error to try to register a predefined topic alias. The REGACK should be returned with an error code and the topicidType of predefined.

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> Clarification of broker behaviour when client REGISTERs a PREDEFINED topic
> --------------------------------------------------------------------------
>
>                 Key: MQTT-555
>                 URL: https://issues.oasis-open.org/browse/MQTT-555
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: Improvement
>          Components: MQTT-SN
>    Affects Versions: MQTT-SN-1.2
>            Reporter: Simon Johnson [X]
>            Assignee: Ian Craggs
>            Priority: Minor
>              Labels: Implementation-notes
>
> If a client issues a âREGISTER âagainst the topic that happens to also be predefined, should  this be allowed? If so, does this yield a normalId in the REGACK, in which case the broker must use an order of precedence when selecting the topic id scheme to use in deliveries to this client.



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