What is the purpose of the TTL field within an IP packet header?

clears an unreachable route from the routing table after the invalid timer expires

prevents regular update messages from inappropriately reinstating a route that may have gone bad

removes an unreachable route from the routing table after the flush timer expires

limits the period of time or number of hops a packet can traverse through the network before it should be discarded

used to mark the route as unreachable in a routing update that is sent to other routers