ICMP fundamentals: which statements about Internet Control Message Protocol (ICMP) are correct?

Difficulty: Easy

Correct Answer: 3 and 4

Explanation:


Introduction / Context:
ICMP (Internet Control Message Protocol) is used by IP devices to send control and error messages, such as “destination unreachable” or “time exceeded.” Understanding what ICMP does and does not do is essential for interpreting ping/traceroute results and diagnosing Layer-3 problems.



Given Data / Assumptions:

  • Statement 1: “They acknowledge receipt of a TCP segment.”
  • Statement 2: “They guarantee datagram delivery.”
  • Statement 3: “They can provide hosts with information about network problems.”
  • Statement 4: “They are encapsulated within IP datagrams.”


Concept / Approach:
ICMP is a companion to IP for signaling errors and informational events. It is not a transport protocol and does not provide reliability guarantees or TCP-style acknowledgments. ICMP messages are carried inside IP packets (protocol number 1).



Step-by-Step Solution:

Evaluate #1: TCP segments are acknowledged by TCP (ACK flags), not by ICMP → false.Evaluate #2: ICMP does not guarantee delivery; IP remains best-effort → false.Evaluate #3: ICMP messages like destination-unreachable, time-exceeded inform hosts of issues → true.Evaluate #4: ICMP rides directly over IP (protocol 1), thus encapsulated in IP → true.


Verification / Alternative check:
Ping uses ICMP Echo Request/Reply encapsulated in IP. Traceroute observes ICMP Time Exceeded responses. Neither mechanism acknowledges TCP segments nor guarantees delivery.



Why Other Options Are Wrong:
1 only / 2 and 3 / 2, 3 and 4: Any option including #1 or #2 is invalid because ICMP neither acknowledges TCP nor provides reliability guarantees.



Common Pitfalls:
Confusing ICMP replies (Echo Reply) with TCP ACKs; assuming “control” implies reliability. ICMP itself may be filtered by firewalls, further undermining any notion of delivery guarantees.



Final Answer:
3 and 4

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