traceroute was written by Van Jacobson in 1987, and works by deliberately exploiting a feature of IP packets — the Time To Live field, which decrements at each router hop and triggers an ICMP “time exceeded” error when it hits zero. By sending packets with incrementing TTL values, traceroute tricks each successive router into announcing itself. The whole thing is, technically, an elegant abuse of error handling. Van Jacobson also invented TCP congestion control, tcpdump, and several other foundational pieces of internet infrastructure, apparently in his spare time. The output of traceroute — a neat ladder of routers and latencies — was magical in 1987 and remains, for anyone who stops to think about it, a small cartographic miracle.
