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Subject: [OASIS Issue Tracker] (MQTT-269) MQTT-SN Feature: Topic Registration


    [ https://issues.oasis-open.org/browse/MQTT-269?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=61783#comment-61783 ] 

Andrew Banks commented on MQTT-269:
-----------------------------------

The following is a simple mechanism, discussed at the January 2016 face to face meeting.
It avoids repeating the same Topic Name in successive PUBLISH Packets by interpreting a zero length
Topic Name to mean the same Topic Name as was used with the previous PUBLISH packet
flowing on the TCP connection. The zero length UTF-8 String  representing the Toipc Name consists
of two zero bytes only. 

So...

4.7.3 Topic semantic and usage
The following rules apply to Topic Names and Topic Filters:
o	All Topic Names and Topic Filters MUST be at least one character long [MQTT-4.7.3-1]

Becomes...

4.7.3 Topic semantic and usage
The following rules apply to Topic Names and Topic Filters:
o	All Topic Filters MUST be at least one character long [MQTT-4.7.3-1]
o       If a Topic Name containing zero characters is received, then the most recent Topic Name
        received containing at least one character, on the same TCP connection is used in its place.
o       It is a protocol error to send only PUBLISH packets containing Topic Names containing
        zero characters on a TCP connection.

Non normative comment.
The sender can avoid repeatedly sending the same Topic Name by sending a zero length Topic Name in its 
place in the second and susequent PUBLISH packets of a sequence with the same Tiopic Name.
The receiver is reqired to cache the last non zero length Topic Name it receives in case 
it receives a zero length Topic Name next.

> MQTT-SN Feature: Topic Registration
> -----------------------------------
>
>                 Key: MQTT-269
>                 URL: https://issues.oasis-open.org/browse/MQTT-269
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: Improvement
>          Components: futures
>            Reporter: Ian Craggs
>
> MQTT-SN allows topic names to be registered with the server (and client) so that a topic id only has to be sent on subsequent packets.  This can reduce the data flowing on the wire and is consistent with the goals of MQTT.  It would however complicate the packet flows.



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