A connectionless service, Connectionless Network Service (CLNS), operates on a datagram basis and is characterized by Best Effort Delivery. In connectionless communication, only a single block of information with a complete source address and destination address is transmitted from one subsystem to another.
In connectionless service, the service provider does not provide any guarantee that the datagrams will reach the recipient at all or that they will reach the recipient in the correct order. In addition, no service priorities are available with the connectionless service. Connectionless service is located at the network layer and the transport layer of the OSI reference model. In this mode of operation, which is typically used in local area networks( LAN), no prior check is made to determine whether the line path to the desired station is also available.
As an alternative to connectionless services, there are connection-oriented services, the Connection Oriented Network Services( CONS), in which a connection is established between the stations before the data is transferred.