• Traceroute program is helpful to trace or know the route from one host to another host.
• It also helpful to know the names and address of the routers used between the source and hosts.
• TTL (Time to live) expired and Port unreachable messages are received at the sending host which are executes the traceroute program.
TTL expired message:
• It is a warning message sent to the source in the traceroute program.
• When the router notices that the TTL has expired, it discards the datagram and sends the warning ICMP message of Type 11 Code 0 to the source.
• Warning message includes the name of the router and its IP address.
Port unreachable message:
• Host sends port unreachable ICMP message of Type 3 Code 3 to the source, when the datagram contains an unlikely port number.
• When the message reaches source, source knows to send additional probe packets.
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