[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=36872#comment-36872 ] 

Peter Niblett commented on MQTT-204:
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We discussed this extensively in MQTT-16. My understanding is the same as Al's, namely we decided that a topic name with a leading $ is never matched by a topic filter with a leading + or #, regardless of whether the server has defined any $xxx topics or not. 

I think the reason it doesn't have a MUST in it was simply to make it look better as an English statement. However we need to clarify this as the fact that Section 4.2.7 starts off with a MAY and this statement is included in an apparently subordinate 4.2.7.1 makes it look as if it might only apply in cases where the server has defined $xxx topics.  

Incidentally this shows one of the problems of "hanging paragraphs" (see mqtt-142). If the bit saying the server MAY define $xxx was in 4.2.7.1 and the bit about subscription matching was in 4.2.7.2 then it would be more obvious that they are independent

> 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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