How can you inspect the path to a destination?

Packet lifetime

TTL puts a limit on forwarding.

In an IPv4 packet, TTL (Time To Live) is a counter carried in the header. Each router decreases it before forwarding the packet. When the value reaches zero, the router discards the packet and may send an expiry message back to the sender.

This prevents a packet caught in a routing loop from circulating forever. Traceroute deliberately sends probes with small TTL values so one router at a time reaches zero and reveals an intermediate hop.

Reveal one hop at a time

A path is observed through controlled probes.

Traceroute varies the packet's hop limit, called TTL (Time To Live), so different routers may return an expiry message. The tool records the responding address and the measured delay for each step.

Different probes can take different paths. Routers may also rate-limit or ignore diagnostic replies while continuing to forward ordinary traffic.

Interpret the evidence

A slow hop is not automatically the bottleneck.

Traceroute measures the time for a diagnostic reply to return from each hop. A router may answer those control messages slowly or rate-limit them while forwarding ordinary packets normally.

Look for a delay pattern that continues through later hops and reaches the destination. A high value at one intermediate hop that disappears on the next line is weaker evidence than a sustained increase. Compare repeated probes and application-level measurements before naming a bottleneck.

Build the model

Four traceroute clues