Internet Control Message Protocol (ICMP)
ICMP is the network-layer protocol used to send error and diagnostic messages about IP delivery, best known for the "ping" test.
What is Internet Control Message Protocol (ICMP)?
Routers and hosts use ICMP to report problems, such as a destination being unreachable or a packet's time-to-live expiring. The ping tool sends ICMP echo requests and measures the replies, giving a quick check of reachability and round-trip latency; traceroute also relies on ICMP.