Question

A machine receives an IPv4 datagram. The protocol field of the IPv4 header has the protocol number of a protocol X. Which ONE of the following is NOT a possible candidate for X?

• A. Internet Control Message Protocol (ICMP)
• B. Internet Group Management Protocol (IGMP)
• C. Open Shortest Path First (OSPF)
• D. Routing Information Protocol (RIP)

Hint:

Remember: The IPv4 protocol field contains only protocols that run directly over IP (e.g., ICMP, IGMP, OSPF). Application or transport-layer protocols like RIP-over-UDP do not appear here.

Answer 1

The Correct Option is D

Step 1: Understanding the IPv4 protocol field.
In the IPv4 header, the 8-bit Protocol field specifies the protocol used in the payload of the IP packet. It directly identifies protocols that operate at the network layer or are carried directly over IP.
Step 2: Recall relevant protocol numbers.
Some standard IPv4 protocol numbers are:
- ICMP = 1
- IGMP = 2
- OSPF = 89
These are directly identified in the IPv4 protocol field.
Step 3: Analyzing the options.
(A) ICMP: Operates directly over IP and has a valid protocol number → Possible candidate.
(B) IGMP: Also operates directly over IP and has its own protocol number → Possible candidate.
(C) OSPF: Is a routing protocol that runs directly over IP (protocol number 89) → Possible candidate.
(D) RIP: Incorrect option. RIP does not run directly over IP; it runs over UDP (usually port 520). Therefore, it cannot appear in the IPv4 protocol field.
Step 4: Conclusion.
Since RIP is encapsulated inside UDP and not carried directly by IP, it cannot be the value of the IPv4 protocol field. Hence, the correct answer is (D).