ICMP is a network-layer protocol that reports feedback about problems processing IP packets — its common messages (in both ICMPv4 and ICMPv6) are host reachability, destination/service unreachable, and time exceeded.

ICMP (Internet Control Message Protocol):

A network layer protocol that provides feedback about issues related to IP packet processing.

Two Versions:

Common ICMP Messages (both versions):

• Host reachability - Test if a host is available
• Destination or Service Unreachable - Report delivery failures
• Time exceeded - TTL/Hop Limit reached zero

Security Note: ICMPv4 messages are not required and are often not allowed within a network for security reasons. Firewalls frequently block ICMP to prevent network reconnaissance.

Go deeper:

• RFC 792 — Internet Control Message Protocol — the original ICMP specification defining echo, destination unreachable, and time exceeded.
• Internet Control Message Protocol — Wikipedia — overview of ICMP's role, message types, and diagnostic uses (ping, traceroute).

The source sends an ICMP Echo Request; if the destination is available it returns an ICMP Echo Reply. No reply suggests the host is unreachable. This is the mechanism behind ping.

ICMP Echo Messages for Host Reachability:

ICMP Echo messages test whether a host is reachable on an IP network.

* The source sends an Echo Request; a reachable host answers with an Echo Reply, which is exactly what ping does. *

Process:

Source Host                    Destination Host
     │                              │
     │──── ICMP Echo Request ─────→│
     │                              │
     │←──── ICMP Echo Reply ───────│
     │                              │

Steps:

1. Local host sends an ICMP Echo Request to a destination
2. If the destination is available, it responds with an ICMP Echo Reply
3. If no reply is received, the destination may be unreachable

ICMP Message Types:

Key insight: This is the mechanism behind the ping command.

Go deeper:

• ICMP, Ping & Traceroute Explained — Sunny Classroom — animated walkthrough of how echo request/reply tests connectivity.
• Ping (networking utility) — Wikipedia — how echo messages measure round-trip time and reachability.