Route Tracing using ICMP

• “traceroute” (“tracert”) tool:

  • send UDP packet to destination host
  • start with TTL = 1, send 3, bump TTL and repeat
  • each router generates ICMP time exceeded, with its source address (provides route)
  • host generates ICMP port unreachable for bad UDP port in probe packet

• May be erroneous for changing and asymmetric routes

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