The document discusses User Datagram Protocol (UDP). It defines UDP as an unreliable, connectionless transport layer protocol used for process-to-process communication. It describes UDP's features like best-effort delivery, no acknowledgment or retransmission, and connectionless transmission. It also outlines the UDP header format, how UDP works, applications that use UDP like TFTP, and compares UDP to TCP in terms of reliability, acknowledgment, connection mode, and error checking.