Which application layer protocol can operate over both UDP and TCP?

Which application layer protocol can operate over both UDP and TCP?

• A. DNS
• B. DHCP
• C. FTP
• D. HTTP

Answer: (A)

The key idea is that the Domain Name System (DNS) is designed to work over either UDP or TCP at the transport layer. For most name lookups, DNS uses UDP because it’s fast and lightweight—queries fit in a single UDP datagram, so responses come back quickly. But when the response is too large to fit in a single UDP packet, or when reliability is required (such as zone transfers between DNS servers), DNS switches to TCP. This dual transport capability—fast UDP for ordinary queries and TCP for larger or more critical transfers—makes DNS the protocol that can operate over both UDP and TCP. In contrast, DHCP relies on UDP exclusively, while FTP and HTTP run over TCP by design.

The key idea is that the Domain Name System (DNS) is designed to work over either UDP or TCP at the transport layer. For most name lookups, DNS uses UDP because it’s fast and lightweight—queries fit in a single UDP datagram, so responses come back quickly. But when the response is too large to fit in a single UDP packet, or when reliability is required (such as zone transfers between DNS servers), DNS switches to TCP. This dual transport capability—fast UDP for ordinary queries and TCP for larger or more critical transfers—makes DNS the protocol that can operate over both UDP and TCP. In contrast, DHCP relies on UDP exclusively, while FTP and HTTP run over TCP by design.