Traceroute: discovering the path with TTL

Objective

Send probes with TTL 1, 2, 3 …: every router that discards one reveals itself with an ICMP 'time exceeded'.

Procedure

1. Run: watch the probes (ping) die at R1, then R2, then R3; the 'TTL!' packets come back.
2. Open the Summary tab: the hop list with RTT per hop.

Theory

A router decrements the TTL; at zero it drops the packet and returns ICMP time-exceeded from its own address. By raising the TTL one at a time the sender learns each hop and its round-trip time. The destination finally answers with an echo reply.

Network blocks used

2 × Host / PC, 3 × Router

More Computer Networks Lab experiments

• TTL expiry and ICMP time-exceeded
• RTT and packet loss on a lossy link
• Packet size versus latency
• Queueing delay and loss on an overloaded link
• TCP congestion control (slow start and loss recovery)