I am fining difficulties understanding trace route command how does trace route works from source to a destination?
how the routers reply to the source with their IP?

thank you


traceroute works by sending fake UDP datagrams to the destination (probes), starting with TTL set to 1 and increasing one by one. On each router/hop on the way, the packet TTL is decreased when it's forwarded. When TTL reaches 0 the packet is dropped.

The hop where the TTL ran out sends an ICMP Time Exceeded (=TTL expired) back to the source. The traceroute tool collects these ICMP messages and displays them in a table together with their return times and the ICMP packet source.

Note that ICMP packets have a very low processing and forwarding priority. It is quite possible that a hop drops the expired probe packet silently, without bothering with ICMP, or another hop on the way back drops the message due to congestion or overload.

  • Note that not all traceroute implementations use UDP. Also note that the destination host does not send an ICMP time exceeded.
    – hertitu
    Oct 29 '18 at 23:05
  • @hertitu Thx - yes, I was trying to keep it simple.
    – Zac67
    Oct 30 '18 at 7:21

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