Support for mqtt 5.0

[l0rb]

Is support for mqtt 5.0 planned? What are the main obstacles to overcome for this?

[Namoshek]

Depends on how you define planned. I would like to implement MQTT 5 eventually and I'm totally open to thorough and solid contributions, but I have no concrete plans to work on this feature right now. MQTT 5 has quite a lot of new features and also some breaking changes, so the implementation is not as straight forward as one might think. The following is a list of things I know need attention, but it might not be complete yet.

New features which need to be designed and implemented:

• TTL for published messages (could be a simple addition to publish(), but with retransmission at higher QoS there are edge cases)
• Additional key-value pairs as header of published messages
• Return codes for features not supported by the broker (in CONNACK) and client-side deactivation of affected features
• New additional AUTH message for complex authentication flows (bi-directional)
• Improved UTF-8 support and new UTF-8 data type
• Bi-directional DISCONNECT support (to allow brokers to gracefully close connections)
• Support for topic aliases in subscriptions to reduce the traffic between broker and client
• Maximum packet size can be configured and communicated by both, client and broker
• Flow control (allows to limit the send and receive rate)
• Last Will publish delay
• Server-side assigned client ids
• Client redirection (if a broker wants a client to connect to a different broker)

Breaking changes to consider:

• Reason codes in acknowledgements for improved error handling
• Replacement of the clean session with clean start and session expiry interval
• Retransmission of unacknowledged messages is forbidden and only allowed after reconnecting to the broker (new TCP connection)
• Usage of passwords without username is permitted (probably only validation)

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If you are interested to work on MQTT 5, I'm willing to support you. A good start would be some design ideas regarding changes to interfaces and classes, as well as a solid test suite to simplify development.