Thanks @descartes for the explanations I’m getting there 
can you share the link to the source code please…i would like to check that section out to understand how it’s implemented.
also how would i know if the Gateway that is out or if it’s TTN that is not finding a matching session keys as described in the situation above?
because if it’s the gateway that is out than i wouldn’t need to rejoin and instead just sleep try at a later time. but if it’s TTN issue then i would need to force a rejoin so i would need a way to distinguish between those two situations. how do you all do that?
cheers,
Jay