GATE CS 2018 Q13 — Concept of Layering: OSI and TCP/IP Protocol Stacks

We can determine the correct match by analyzing the standard header field lengths for each protocol:

• P. UDP Header's Port Number: Both source and destination port numbers in the UDP header are 16 bits (2 bytes) long.
  $\implies \text{P} \to \text{IV}$
• Q. Ethernet MAC Address: A physical MAC address consists of 6 bytes, which is $6 \times 8 = \mathbf{48\text{ bits}}$.
  $\implies \text{Q} \to \text{I}$
• R. IPv6 Next Header: The "Next Header" field in the IPv6 header specifies the type of the next header (similar to the Protocol field in IPv4) and is 8 bits (1 byte) long.
  $\implies \text{R} \to \text{II}$
• S. TCP Header's Sequence Number: The sequence number field in the TCP header is 32 bits (4 bytes) long to allow a large range of sequence numbers.
  $\implies \text{S} \to \text{III}$

Combining these matches, we get:

$$\text{P-IV, Q-I, R-II, S-III}$$

Hence, the correct option is C.