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Subject: [OASIS Issue Tracker] (MQTT-566) Capabilities packet

    [ https://issues.oasis-open.org/browse/MQTT-566?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=77334#comment-77334 ] 

Ian Craggs commented on MQTT-566:

These properties in MQTT 5.0 describe capabilities:
 * maximum packet size
 * topic alias maximum
 * maximum QoS (server only - returned on connack)
 * receive maximum
 * retain available
 * Wildcard Subscription Available
 * Subscription Identifier Available (no subscription identifiers in MQTT-SN)
 * Shared Subscription Available (no shared subscriptions in MQTT-SN)


I think the most important of these are:
 * receive maximum (default 65535)
 * max packet size (default - the MQTT limit)
 * topic alias maximum (default - 0)

and the others are a nice to have.

The defaults in MQTT 5.0 do not try to be good or average cases. In MQTT-SN we could have defaults which many (most?) applications would not need to change. For instance:
 * receive maximum 3
 * max packet size 128
 * topic alias maximum 3

then you only need to send a capability packet if you want to change the defaults.



> Capabilities packet
> -------------------
>                 Key: MQTT-566
>                 URL: https://issues.oasis-open.org/browse/MQTT-566
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: Task
>          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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