Once it reaches at the destination address, Time exceeded ICMP message is NOT sent back to the source. Traceroute keeps on doing this, and record the IP address and name of every router until the UDP packets reach to the destination address.Again source device sends two more packets, in the same way, to get an average value of the round-trip time and again TTL gets zero when it reaches to the 2 nd router and response through ICMP error message time exceeds.And at last, the source receives ICMP error message sent by the router. In this case, as the packet is sent back and forth between the looping points, the TTL keeps getting decrement until it becomes zero. This could be possible when there is a routing loop. In typical condition, a network doesn’t have such a diameter that lead the TTL=0.
The TTL time exceeded ICMP message is sent after the TTL value of a UDP packet gets zero. Thus Traceroute makes a list of the router’s address and the time taken for the round-trip.
When the first router receives the UDP packet it reduces the TTL value by 1 (1-1=0) then drop the packet and sends an ICMP message “Time exceeded” to the source. Traceroute sends a UDP packet with a TTL = 1 from the source to destination. The asterisk (*): Denotes probe timeout which means that the router at that hop doesn’t respond to the packet received from the source used for the traceroute due to firewall filter. On the internet, before the data reaches its final destination, it goes through several routers and a hop occurs when an incoming packet is forwarded to the next router. Data packets pass through bridges, routers, and gateways as they travel between source and destination. Hop: A hop is one portion of the path between source and destination. TTL: The time-to-live value, also known as the hop limit, is a mechanism that limits the lifespan or lifetime of data in a computer or network. It uses the ICMP error-reporting messages –Destination Unreachable and Time exceeded. Traceroute sends a UDP packet to the destination by taking benefit of ICMP’s messages. It traces the path of a packet from the source machine to an Internet host such as by calculating the average time taken each hop. Traceroute or Tracert: It is a CUI based computer network diagnostic tools used in UNIX and Windows-like system respectively. In this Post, we are going to discuss working with traceroute using UDP/ICMP/TCP packets with the help of Wireshark.
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