Which of the following protocols uses both TCP and UDP?

DNS, or Domain Name System, is unique in the way it utilizes both TCP and UDP protocols. Primarily, DNS queries and responses are sent using UDP due to its low overhead and faster processing, making it suitable for the quick requests and replies that DNS typically handles—such as resolving domain names into IP addresses.

However, in scenarios where the response data size exceeds the limits of a typical UDP packet or when reliability is critical, DNS can also use TCP. This dual protocol approach facilitates not only faster lookups but also provides a reliable mechanism for larger data transfers, such as zone transfers between DNS servers. This capability to operate over both TCP and UDP is a distinct characteristic of DNS, setting it apart from protocols that strictly utilize one type of transport layer protocol.

FTP, SMTP, and Telnet each operate exclusively over TCP, focusing on the reliability and ordered delivery that TCP offers for their respective services. Thus, while those protocols can manage connections and data integrity effectively within the constraints of TCP, DNS’s dual utilization highlights its adaptability and importance in network communications.