GATE CS 2007 Q70 — Concept of Layering: OSI and TCP/IP Protocol Stacks

To find the correct match, let us analyze the function and operating layer of each protocol:

• P. SMTP (Simple Mail Transfer Protocol): Used for sending and receiving email messages across IP networks. It is an end-user process protocol and operates at the Application layer (1).
• Q. BGP (Border Gateway Protocol): An inter-domain routing protocol used to exchange routing and reachability information between autonomous systems on the Internet. Since its primary function is routing, it is associated with the Network layer (4).
• R. TCP (Transmission Control Protocol): A core protocol of the Internet protocol suite that provides reliable, ordered, and error-checked delivery of a stream of octets between applications. It operates at the Transport layer (2).
• S. PPP (Point-to-Point Protocol): A communication protocol used to establish a direct connection between two nodes over a physical link. It handles framing, link establishment, and error detection, operating at the Data link layer (3).

Matching these together:

• $\text{P} \rightarrow 1$
• $\text{Q} \rightarrow 4$
• $\text{R} \rightarrow 2$
• $\text{S} \rightarrow 3$

This corresponds to Option B: $\text{P - 1, Q - 4, R - 2, S - 3}$.