Which of the following protocols uses only User Datagram Protocol (UDP)?

POP3 uses TCP on port 110, relying on TCP's reliable, ordered delivery to ensure email messages are fully and correctly retrieved from a mail server.

FTP uses TCP on port 21 for control commands and port 20 for data transfer, requiring TCP's connection-oriented reliability for file integrity.

ICMP operates directly at the network layer over IP as protocol number 1 and does not use either TCP or UDP as a transport, making it outside the scope of the question entirely.

TFTP (Trivial File Transfer Protocol) was intentionally designed to use only UDP on port 69, forgoing TCP's connection establishment and reliability overhead. This makes it suitable for simple use cases such as network booting (PXE) and transferring router configurations where minimal protocol complexity is required, as defined in RFC 1350.