Message From ______ to ______ Type: Unreachable
TTL: ______ Unreachable destination: _____
First subtract 1 from TTL.
If message is to you: A message to the above destination did not get through.
If TTL is 0, discard message.
If destination is in your table, forward message to next hop.
If destination unknown, DO NOT send back Unreachable Notification.