ICMP

What ICMP is

The Internet Control Message Protocol carries status and error messages between hosts and routers. It does not transport application data; it tells the sender of a packet what happened to it. A router that cannot find a route replies with "destination unreachable". A router that drops a packet because its TTL ran out replies with "time exceeded". A host that receives an echo request replies with an echo reply. IPv6 has its own version, ICMPv6, which also handles neighbor discovery and router advertisements and cannot be blocked without breaking the protocol.

Where you meet it

• Ping sends ICMP echo requests and measures the time until the echo reply returns. That round trip is latency, and the variation between replies is jitter.
• Traceroute collects the time-exceeded messages that each router sends back, one hop at a time.
• Path MTU discovery depends on "fragmentation needed" messages. Networks that block all ICMP cause large packets to vanish silently, which shows up as websites that load headers but hang on the body.
• Port unreachable replies are how UDP-based tools learn that nothing is listening.

Why it matters

ICMP is the reason simple network tests work at all, and the reason some of them do not. Many hosts and firewalls drop echo requests, so a host that does not answer ping may still be up and serving a website. Many routers rate-limit their ICMP replies, so a slow or missing hop in a traceroute can be an artifact of the router's policy rather than a real delay. Reading these tools correctly means knowing which answers are missing by policy and which by failure.

For proxy users, ICMP is a quiet source of information. Echo replies carry the responder's starting TTL, which hints at its operating system, and the round-trip time to a supposedly local address that turns out to be 150 milliseconds away suggests a relay in another region.

How to check

The ping tool sends echo requests from our measurement node to any host and reports minimum, average, and maximum round-trip time, packet loss, and jitter. The traceroute tool shows the hop-by-hop path built from time-exceeded replies. Both run from our node rather than from your browser, since browsers cannot send ICMP.

A common misconception

Blocking ICMP does not make a host invisible or safer; it only hides it from ping. The services on it remain reachable, and the loss of error messages makes genuine problems harder to diagnose.