MQTT v5.0 introduces a new AUTH mechanism. This allows MQTT to bind with various authentication mechanisms such as SASL within the CONNECT / CONNACK exchange.
In its current form the Client is permitted to flow an Auth Packet for re-authenication at any point. There are a few potential issues with this approach:
1. Implementations might exploit the AUTH flow for application data and control.
2. Only the Client can initiate the re-authentication. In many cases the Server is likely to coordinate Clients to refresh keys.
3. It is likely that existing deployments simply use DISCONNECT to coordinate re-authentication and this might lead to little uptake on re-auth.
There are benefits to the current approach, for example in reducing bandwidth.