[OASIS Issue Tracker] (MQTT-553) Clarify whether clients should maintain normal topicIds during sleep state

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    [ https://issues.oasis-open.org/browse/MQTT-553?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=77325#comment-77325 ] 

Ian Craggs commented on MQTT-553:
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Also, from the MQTT 5.0 specification:

_The Topic Alias mappings used by the Client and Server are independent from each other. Thus, when a Client sends a PUBLISH containing a Topic Alias value of 1 to a Server and the Server sends a PUBLISH with a Topic Alias value of 1 to that Client they will in general be referring to different Topics._

I suggest it should be the same for MQTT-SN.

> Clarify whether clients should maintain normal topicIds during sleep state
> --------------------------------------------------------------------------
>
>                 Key: MQTT-553
>                 URL: https://issues.oasis-open.org/browse/MQTT-553
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: Improvement
>          Components: MQTT-SN
>    Affects Versions: MQTT-SN-1.2
>            Reporter: Simon Johnson
>            Assignee: Ian Craggs
>            Priority: Minor
>              Labels: Implementation-notes
>
> Whether or not a client maintains its normal topicId registry whilst in a sleeping state is a fundamental implementation decision, and one that effects the entire client lifecycle and broker interactions upon waking. I do not believe this to be a trivial implementation detail or broker "feature".
> Should, for example, the broker have to re-issue REGISTER messages for every unconfirmed topic during a wake up phase, only for them to be cleared again upon receipt of the PINGRESP.
> Mandating clients maintain the registry across sleeps, means low powered devices can not truly power down where they have only volatile space to write their registry's. Flushing the registry means more message interaction upon wake. 
> Neither are small overheads. Perhaps there is scope for different sleep states?



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