[OASIS Issue Tracker] (MQTT-204) Make $ topic separation normative

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    [ https://tools.oasis-open.org/issues/browse/MQTT-204?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=36925#comment-36925 ] 

Ken Borgendale commented on MQTT-204:
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I certainly was one of those who read the text in the subordinate section as being under the MAY in the superior section.  I agree that the text as it exists is a bit confusing.

The significant problem in defining this as a MUST is dealing with the MQTT version.  An MQTT connection has a version, but topic matching involves multiple connections: a publishing connection, a connection which creates a subscription, and a connection which consumes from a subscription.  These could be at differing MQTT versions.  If we agree that the normative language here is confined to cases where the involved connections are all at V3.1.1 then making this a strict rule is OK.  This is one of the few cases where conformance deals with multiple connections and therefore multiple possible versions.  

My preference is to say that an MQTT Server MAY define topics starting with $, and SHOULD NOT match such defined topics with a leading wildcard.  This would allow us to continue the current $SYS rules for V3.1 and honor the $ rule for V3.1.1.  The alternative is to define mixtures of connection versions as being outside the scope of this spec.

> Make $ topic separation normative
> ---------------------------------
>
>                 Key: MQTT-204
>                 URL: https://tools.oasis-open.org/issues/browse/MQTT-204
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: Improvement
>          Components: core
>            Reporter: Allan Stockdill-Mander 
>
> Section 4.7.2.1 of wd-20 states;
> A Topic Filter that starts with a wildcard character (# or +) does not match Topic Names that begin with a $ character
> This appears to be a normative statement regarding server behaviour without any accompanying normative language.
> While server implementations may choose whether or not to implement predefined $topics I thought we explicitly did want separate all topics starting with $ from the rest of the topic space.



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