Detecting route failure
# each node broadcast a hello message periodically
# if a node does not receive a hello message from a neighbour in the last period it means the neighbour has moved out of range
# it purges all entries in its routing table for which the next hop is this unreachable neighbour
# it also send error message to all active neighbours (for which the next hop is this unreachable neighbour) so they can purge these routes as well from their routing table
# these active neighbours relay the error message to their active neighbours and so on