What are the common purposes of ICMP Echo Request and Echo Reply messages?

• A. They are used to measure TTL
• B. They are used by the ping utility to test reachability and measure round-trip time.
• C. They are used to broadcast to all hosts
• D. They are used to negotiate MTU

Answer: (B)

ICMP Echo Request and Echo Reply are the diagnostic messages that the ping utility uses to verify reachability and measure latency. When you send an Echo Request to a specific host, that host should reply with an Echo Reply. The time from sending the request to receiving the reply gives the round-trip time, which tells you how long a packet takes to travel to the destination and back. This pair is designed for checking that a host is reachable and for gauging response time.

These messages aren’t used to measure TTL directly, aren’t broadcast to all hosts, and aren’t involved in negotiating MTU. MTU discovery relies on other mechanisms and ICMP codes, while TTL is a field in the IP header that can influence hop count but isn’t the purpose of the Echo messages.