ICMP

If a packet was sent and never arrived, how does the sender know what went wrong? “No response” is not enough. It cannot distinguish between a broken path, expired TTL, unreachable destination, or a destination port with nothing listening. IP deliberately does not do that kind of reporting. It forwards best effort and does not explain failure.

ICMP handles the layer where the network talks to the network about what happened. It is not a business-data protocol, but it carries error feedback, reachability probes, path exposure, and some control signals. Without it, many troubleshooting judgments collapse from “know where it broke” to “guess where it broke.”