Frame-counter out of sync management

ChirpStack Network Server
1

Hello,
I’m working around the out synchronization management and I’d like to know what’s the best to do in the following situations:

server detects a FcntUp < to the expected

server detects a FcntUp > to the expected

device detects a FcntDown > to the expected

device detects a FcntDown < to the expected

As for the specification I know if a device or server detetcs a Fcnt > to the expected it updates the Fcnt to that received provided that the delta is inside a predetermined interval (Fcnt_GAP)

But what to do if the received Fcnt is lower than the expected?

I’d like to avoid a dead lock (imagine to ave nodes working in ABP in a remote installation), but the specification doesn’t suggests any solution expecially with respect security.

2

Frame-counter values less than expected must always be dropped, as the authenticity can’t be proved. E.g. it could be a replay attack.