<protocol> (UDP) Internet standard network layer, transport layer and session layer protocols which provide simple but unreliable datagram services. UDP is defined in STD 6, RFC 768. It adds a checksum and additional process-to-process addressing information [to what?]. UDP is a connectionless protocol which, like TCP, is layered on top of IP.
UDP neither guarantees delivery nor does it require a connection. As a result it is lightweight and efficient, but all error processing and retransmission must be taken care of by the application program.
Unix manual page: udp(4).
[Postel, Jon, User Datagram Protocol, RFC 768, Network Information Center, SRI International, Menlo Park, Calif., August 1980].