How Did I Get Here?

"My traceroute implementation uses a protocol called ICMP. ICMP was designed specifically for sending diagnostic information around the Internet, and, helpfully, almost every Internet-connected device speaks it. Interestingly, ICMP packets have a "TTL" (time to live) field. This isn't actually a "time" as implied by a name - it's a countdown! Every time a router forwards an ICMP packet along, it's supposed to decrement the TTL number."

"The first packet with a TTL of 1 will error on the first device it reaches, and so on, until we hopefully get an error back from every routing device that touched the packet. These error packets include diagnostic information like the IP address of the device that sent the error, allowing us to trace your packets' rough path across the Internet."

"How does ktr work? Let's start with a simplified explanation of Internet routing. Starting with the source device, each computer that handles a packet has to choose the best device to forward it to - I will explain how these routing decisions are made in a bit. Assuming everything works correctly, the packet will eventually reach a router that knows how to send it directly to its destination."

Each router that forwards a packet chooses the next device to send it toward the destination, ultimately reaching a router that can deliver it directly. ICMP is a diagnostic protocol implemented by most Internet-connected devices and includes a TTL field that routers decrement on forwarding. When TTL reaches zero, routers return an error message to the packet's source. Traceroute exploits this behavior by sending ICMP packets with increasing TTLs to provoke successive routers into returning errors that include their IP addresses. A server can run a traceroute to a visitor's IP on page load and present results without requiring JavaScript.