Q: How long do certificates last?
A: We use AWS certificates, they last 30 years
Q: Which MQTT version does things5 use?
A: We use version 3.1.1 and 5. You can find more about this topic here
Q: Do we need to exchange a message upon MQTT channel opening?
A: There is no need to exchange further informations
Q: How does keep-alive work?
A: When the client establishes a connection to the broker, the client communicates a time interval in seconds to the broker. This interval defines the maximum length of time that the broker and client may not communicate with each other.
If there is no client-server communication for 1.5x of the client's keep-alive time, the client is disconnected. You can take a look at the reference documentation available at AWS IoT docs