Any transport layer protocol that relies on broadcast packets instead of directed packets. Connectionless protocols cannot guarantee delivery of packets. Instead, reliability of packets is handled by the application itself or some higher layer of the protocol stack.
An example of a connectionless protocol is the User Datagram Protocol (UDP), which is part of the TCP/IP protocol suite. UDP provides connectionless services for delivering small packets of information commonly called datagrams. Connectionless protocols can only offer “best-effort” delivery and cannot guarantee that packets will arrive in the correct order or even at all.
In Microsoft Windows NT, connectionless protocols such as UDP are used in services such as the NetBIOS name service and the Simple Network Management Protocol (SNMP) service.