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How Does Traceroute Work: A Step-by-Step Breakdown

Traceroute is a network diagnostic tool used to track the path packets take from a source device to a destination across an IP network, helping identify routing paths and any potential delays or failures.

Here’s how traceroute works:

  1. ICMP and TTL (Time-To-Live): Traceroute sends packets with an initial TTL value of 1. The first router the packet encounters decrements the TTL by 1, causing it to reach zero. When the TTL hits zero, the router discards the packet and sends back an ICMP "TTL expired" message to the source. This helps the source router document the identity (IP address) of the responding router as part of the path.
  2. Incrementing TTL: Traceroute then increases the TTL by 1 for each subsequent packet. The second router will forward the packet to the next hop until the TTL expires, at which point it also sends a TTL expired message back to the source. The process repeats, each time documenting the responding routers and the path the packet follows.
  3. Tracking Response Times: Traceroute also measures the time it takes for each router to return the TTL expired message, giving insight into the round-trip time (RTT) for each hop. This helps identify latency at each step along the way.
  4. Completion at Destination: When the packet reaches the final destination, instead of sending a "TTL expired" message, the destination device sends an ICMP "port unreachable" message back to the source. This happens because traceroute typically sends packets to a high, unused UDP port number (above 30,000) that no services are actively listening on. Since no application is using this port at the destination, the destination host returns a "port unreachable" message. This response signifies to the source that the trace is complete, as the packet has successfully reached the destination.

The "port unreachable" message confirms that the destination device has been reached, and it marks the end of the traceroute process. This behavior allows traceroute to conclude that the path has been fully traced, with all intermediate hops documented.

 


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