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7/17/2026
How traceroute Really Works: TTL, ICMP Time-Exceeded, and Mapping a Path Hop by Hop

How traceroute Really Works: TTL, ICMP Time-Exceeded, and Mapping a Path Hop by Hop

Short summary

A hands-on Protocol Lab tutorial explaining how traceroute maps network paths by exploiting IP TTL decrements and ICMP time-exceeded messages. The lab builds a four-node topology (client, two routers, server) using containerlab and Docker, with packet captures showing each hop's ICMP reply. It references RFCs 791, 792, and 1812, and includes step-by-step deployment scripts for a 40-55 minute lab session.

  • •traceroute sends probes with increasing TTL values; each expired probe reveals a router hop
  • •Lab uses containerlab with four nodes to demonstrate ICMP time-exceeded replies in packet captures
  • •References RFCs 791, 792, and 1812 for protocol-level understanding

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