Traceroute

• Traceroute is an application that allows the user to determine a possible route between two hosts

• • Called tracert on Windows, originally developed by Van Jacobson

• The route is not measured directly

• • Instead, it is determined hop by hop with a series of packets with the IP TTL field increasing by one
  • • E.g., 1, 2, 3, …
    • Three tries per TTL value

• The packets also contain a UDP header with a randomly chosen destination port over 30000

• • Windows uses ICMP echo request rather than UDP

• Intermediate hops will respond with an ICMP “time exceeded” message

• The final destination will presumably respond with an ICMP “port unreachable” message