MQTT Security

[Nowa Concordia]

Hello there,

1.) I'm just wondering how much over head MQTT overhead over plain MQTT will add in terms of numbers. 

2.) In regard can the payload encryption be justified over using TLS? If so what are the recommended available payload encryption methods we can think of in terms of security, complexity and performance given constrained devices are included in the system?

Further if anyone can tell me what exactly is happening with delivery tokens in QoS1/2 it'd be great help, what is the delivery token timeout and what exactly happens if a timeout occur due to a server failure or network lag?

Thank you.

-Nowa