When a router cannot route a datagram or host cannot deliver a datagram, the datagram is discarded and the router or the host sends a ____________ message back to the source host that initiated the datagram.

Correct Answer: Destination unreachable
Router sends destination unreachable message if the destination is not found. Destination unreachable is type 3 error reporting message. It is invoked when the router can’t find a path to the intended destination to forward the packet through.