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:

The IPv4 protocol field specifies which transport or network-layer protocol is being used. OSPF operates directly over IP, while RIP uses UDP as its transport protocol, which means it is not directly identified by the IPv4 protocol field.

Answer 1

The Correct Option is D

Understanding the IPv4 Protocol Field:

• The protocol field in the IPv4 header specifies the protocol used in the payload of the datagram.
• Common values include ICMP (1), IGMP (2), OSPF (89), etc. This field helps routers and systems understand what type of data they are handling in the packet's payload.

Analysis of the given options:

• (A) ICMP – Uses protocol number 1 in the IPv4 header. ✅ Possible candidate.
• (B) IGMP – Uses protocol number 2 in the IPv4 header. ✅ Possible candidate.
• (C) OSPF – Uses protocol number 89 in the IPv4 header. ✅ Possible candidate.
• (D) RIP – NOT directly encapsulated in an IPv4 datagram. Instead, it operates over UDP (port 520). ❌ Not a candidate.

Since RIP is transported over UDP and not directly in an IPv4 datagram, the correct answer is:

(D) Routing Information Protocol (RIP)