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User Datagram Protocol (UDP) is a connectionless transport layer protocol used when the overhead of a connection isn’t required. After UDP has placed a packet on the network (via the IP protocol), it forgets about it. UDP doesn’t guarantee that the packet arrives at its destination. Most applications that use UDP simply wait for any replies expected as a result of packets sent via UDP. If a reply doesn’t arrive within a certain period of time, the application either sends the packet again or gives up.

Probably the best-known application layer protocol that uses UDP is the Domain Name System (DNS). When an application needs to access a domain name (such as wiley.com), DNS sends a UDP packet to a DNS server to look up the domain. When the server finds the domain, it returns the domain’s IP address in another UDP packet.