How does traceroute work? Now how does traceroute make sure that the packet follows the same path that a previous (with ttl - 1) probe packet went in?

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  • Nov 17th, 2005

See traceroute works using ICMP packets. First source sends an ICMP packet with Time to Live (TTL) field as 1 to the destination address. Now intermediate router receives the packet and sees that TTL field has expired, so it sends a ICMP TTL expired reply. Now the source machine again sends the ICMP packet with TTL field as 2. This time second intermediate router replies. This process is repeated till destination is reached. That way the source can get the entire route upto destination.


  • Oct 16th, 2007

tracert is a network tool used to determine the route taken by packets across the network.Tracert works by increasing the TTL value of each successive batch of packets sent. The first 3 packets have a TTL of 1 (i.e. they make a single hop). The next 3 packets have a TTL of 2, and so on.When a packet passes through a host, the host decrements the TTL value by one and forwards the packet to the next host. When the packet with a TTL of 1 reaches a host, the host discards the packet and send an ICMP TYPE 11 (ICMP time exceeded) packet to the sender. The tracert uses these returning packets to create a list of hosts that the packets have traversed en route to the destination. The 3 timestamp values returned for each host along the path are the latency values (in ms) for each packet in the batch. It a packet does not return within the expected timeout period, an asterisk is printed.Tracert may not list the real hosts. It only indicates that the first host is one hop away, the second host at 2 hops etc.Thanks.

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