Datagram must be self-contained without reliance on earlier exchanges because there is no connection of fixed duration between the two communicating points.
Datagram service routes datagrams without first creating a predetermined path. Datagram service is therefore considered connectionless. There is also no consideration given to the order in which it and other datagrams are sent or received. In fact, many datagrams in the same group can travel along different paths before reaching the same destination
User Datagram Protocol (UDP) is run by a datagram service in the Internet Protocol which is an entirely connectionless, best effort, unreliable, message delivery service.