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: Identify the purpose of the IPv4 Protocol field. 
The IPv4 header contains an 8-bit field called the Protocol field.
This field indicates which protocol is encapsulated inside the IP packet payload.
Only protocols that run directly over IP can be represented in this field.

Step 2: Recall common protocol numbers used in IPv4.
Some well-known protocols that are carried directly by IP include:

• ICMP – protocol number 1
• IGMP – protocol number 2
• OSPF – protocol number 89

Each of these protocols can therefore appear as a valid value in the IPv4 Protocol field.

Step 3: Examine each option.
(A) ICMP: Communicates directly over IP, so it can be identified by the Protocol field.
(B) IGMP: Also runs directly over IP and is assigned its own protocol number.
(C) OSPF: A routing protocol that is encapsulated directly within IP packets.
(D) RIP: This protocol does not run directly over IP. Instead, it is carried inside UDP packets, typically using port 520.

Because RIP is encapsulated within UDP, the IPv4 Protocol field would contain the value for UDP—not RIP itself.

Step 4: Final conclusion.
The protocol that cannot appear as a value in the IPv4 Protocol field is:

\[ \boxed{\text{RIP}} \]