Port unreachable and TTL (Time to Live) Expired messages are received at the sending host.
In cases when a packet reaches an end-host destined for a UDP port for which no host is associated, it generates an ICMP port unreachable in response. Also, an ICMP Port unreachable message can be generated in instances where the router could not find the server.
TTL expired on the other hand is used upon the execution of traceroute command. An ICMP message is sent out with a TTL value of 1 set in the IP header. The end host receiving a traceroute probe would send a TTL Expired ICMP when a single probe finally reaches it.