We are working on adding MQTT v5.0 support to wolfMQTT.

Some of the new MQTT 5 features include:

AUTH packet type to submit authentication method/data information after connect. CONNACK packets now include a reason code to better describe connect failures. DISCONNECT now supports server to client. Packets can include optional key/value properties. New data type for UTF-8 string pairs. No retry for QoS 1 and 2 packets (let assumed TCP handle retry). Passwords can be provided without a username

The new specification can be found here: http://docs.oasis-open.org/mqtt/mqtt/v5.0/mqtt-v5.0.html

Support for these new features will be release in the next few weeks as wolfMQTT v1.0.