A machine receives an IPv4 datagram. The protocol field of the IPv4 header has…
2025
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)
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Correct answer: D
Answer: Routing Information Protocol (RIP) is NOT a possible candidate for the IPv4 protocol field.
Key idea: the IPv4 protocol field identifies the next-level protocol by an assigned protocol number. Some protocols are carried directly over IP and therefore have their own protocol numbers; other protocols are carried inside transport protocols such as UDP or TCP.
Internet Control Message Protocol (ICMP) is carried directly over IP and uses IPv4 protocol number 1, so it can appear in the protocol field.
Internet Group Management Protocol (IGMP) is carried directly over IP and uses IPv4 protocol number 2, so it can appear in the protocol field.
Open Shortest Path First (OSPF) is carried directly over IP and uses IPv4 protocol number 89, so it can appear in the protocol field.
Routing Information Protocol (RIP) is transported inside UDP (uses UDP port 520). The IPv4 protocol field would therefore indicate UDP (protocol number 17), not RIP itself, so RIP cannot be the protocol number in the IPv4 header.
Therefore, the correct answer is Routing Information Protocol (RIP).
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