What is IPv4 ICMP and list three common ICMP message types used for diagnostics or error reporting.

• A. ICMP provides error reporting and diagnostics; common types include Echo Request, Echo Reply, Destination Unreachable.
• B. ICMP is used to deliver application data reliably with congestion control.
• C. ICMP is a transport protocol that guarantees in-order delivery.
• D. ICMP is used for DNS query responses.

Answer: (A)

ICMP, the Internet Control Message Protocol, is a helper protocol used by IP to report problems and support network diagnostics. It doesn’t carry user data or provide delivery guarantees like a transport protocol; instead, it sends control messages that tell devices why something couldn’t be delivered or provide information useful for troubleshooting.

Three common ICMP message types used for diagnostics or error reporting are Echo Request and Echo Reply, which together power the ping utility to check whether a host is reachable and to measure round-trip time; and Destination Unreachable, which is sent when a packet cannot be delivered for reasons such as no route, host unreachable, or port unreachable, signaling that delivery failed along the path.