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?
Official answer and solution
- Official answer
- D
- From
- Official GATE 2025 answer key
Solution
Identify which of the listed protocols runs over UDP (application layer) rather than being encapsulated directly inside an IPv4 datagram.
The 8-bit Protocol field in an IPv4 header specifies the protocol encapsulated directly in the IPv4 payload:
- ICMP has IPv4 protocol number
1. - IGMP has IPv4 protocol number
2. - OSPF is encapsulated directly in IPv4 with protocol number
89. - RIP (Routing Information Protocol) is an application-layer protocol that runs on top of UDP (port 520). Therefore, an IPv4 datagram carrying a RIP message has
17(UDP) in its IPv4 header Protocol field, not RIP.
Option (D)
Confusing OSPF (which runs directly over IP, protocol 89) with RIP (which runs over UDP port 520).
The answer is the one in the official GATE answer key. The solution is this site's own explanation.
Source
- Paper
- GATE 2025 Computer Science and Information Technology - Session 2 (CS2)
- Question
- 18 of 65
- Section
- Computer Science and Information Technology
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