[OASIS Issue Tracker] (MQTT-566) Capabilities

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    [ https://issues.oasis-open.org/browse/MQTT-566?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=78139#comment-78139 ] 

Ian Craggs commented on MQTT-566:
---------------------------------

An alternative to a capabilities packet would be to add (optional) fields to the connect and connack packets and have suitable defaults.

On connect, the flags field could have a bit to add capabilities fields if they differed from the MQTT-SN defaults of 3, 128, 3 (TBD).  These would be:
 * max packet size (2 bytes) up to 65535
 * receive maximum (max inflight messages) 4 bits (up to 16) or 1 byte (up to 256)
 * topic alias maximum, 4 bits, up to 16

On connack:
 * max packet size (2 bytes)
 * receive maximum (max inflight messages) 4 bits, up to 16 or 1 byte (up to 256)? 
 * topic alias maximum 4 bits, up to 16
 * wildcards available, 1 bit
 * retained messages available, 1 bit
 * maximum QoS, 2 bits
Connack flags:
  * max packet size included (1 bit)
  * topic alias and receive max included (1 bit)
  * wildcards available (1 bit)
  * retained messages available (1 bit)
  * max QoS (2 bits)
  
What would be suitable defaults?  

> Capabilities
> ------------
>
>                 Key: MQTT-566
>                 URL: https://issues.oasis-open.org/browse/MQTT-566
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: New Feature
>          Components: MQTT-SN
>            Reporter: Simon Johnson
>            Assignee: Ian Craggs
>            Priority: Major
>
> A new packet to be able to assert (or request) the capabilities of a client or indeed a broker as part of a 2 way exchange.



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