Internet Control Message Protocol

The Internet Control Message Protocol (ICMP) is a network layer that IP devices, like protocol routers, use to send error messages and operational status updates, like if a requested service or destination host can't be reached. It powers diagnostic tools like ping and traceroute.

[2] It's often used on network devices, like routers and can also be used in distributed denial-of-service (DDoS) attacks.

ICMP doesn't normally carry application data like HTTP or FTP.