UDP is a transport-layer datagram protocol that sends and receives whole packets across the network. UDP is used for application-to-application programs between TCP/IP hosts. UDP does not offer a guarantee of datagram delivery or duplication protection. UDP does provide checksums for both the header and data portions of a datagram. However, applications that require reliable delivery of streams of data should use TCP. UDP is the default protocol for datagram sockets in the internet domain.
Unlike applications using TCP, UDP applications are usually connectionless. A UDP socket application can become connected by calling the connect() API. An unconnected socket can be used to communicate with many hosts; but a connected socket, because it has a dedicated destination, can exchange data with only one host at a time.
UDP is considered an unreliable protocol because it sends its data over the network without verification. Consequently, after a packet has been accepted by the UDP interface, neither the arrival of the packet nor the arrival order of the packet at the destination can be guaranteed.