ICMP

networking

Stands for: Internet Control Message Protocol

The protocol IP uses for error and diagnostic messages, like ping.

Internet Control Message Protocol (RFC 792) carries reachability errors and diagnostics rather than user data. Ping and traceroute are built on it, and a firewall's handling of ICMP shapes how a network reports problems.

ICMP carries control and error messages for IP: unreachable destinations, time exceeded, and the echo request and reply that ping uses. It is not optional signalling that can be discarded without consequence.

Blocking it wholesale is one of the most common self-inflicted network faults. Path MTU discovery depends on the fragmentation-needed message, and filtering it produces connections that establish and then hang on large transfers. Traceroute depends on time-exceeded. The defensible policy is selective: rate-limit echo if you must, and permit the types that make IP function, because a firewall configured on the belief that ICMP is simply dangerous breaks things in ways that are hard to attribute back to the firewall.

Also known as: icmp

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