GATE CS 2007 Q20 — Application Layer Protocols: DNS, SMTP, HTTP, FTP, Email

To determine which protocol uses UDP (User Datagram Protocol) as its primary transport protocol, let us analyze each option:

• HTTP (Hypertext Transfer Protocol): Requires reliable, ordered delivery of web resources. Therefore, it uses TCP (typically on port 80 or 443).
• Telnet: Used for interactive remote command-line sessions where every keystroke must be reliably transmitted in order. Thus, it uses TCP (port 23).
• DNS (Domain Name System): Designed for quick, single-request/single-response queries to resolve domain names to IP addresses. To minimize latency and overhead, it primarily uses UDP (port 53). (Note: DNS can fall back to TCP for zone transfers or when responses exceed 512 bytes, but UDP is its primary transport protocol for standard queries.)
• SMTP (Simple Mail Transfer Protocol): Used for email transmission, which demands error-free and reliable delivery of messages. Hence, it uses TCP (port 25).

Therefore, DNS is the correct answer.