Both sockets and transport-level interface provide two modes of service: connection-mode and connectionless-mode.
Connection-mode service is circuit-oriented. This service enables the transmission of data over an established connection in a reliable, sequenced manner. This service also provides an identification procedure that avoids the overhead of address resolution and transmission during the data transfer phase. This service is attractive for applications that require relatively long-lived, datastream-oriented interactions.
Connectionless-mode service is message-oriented and supports data transfer in self-contained units with no logical relationship required among multiple units. A single service request passes all the information required to deliver a unit of data from the sender to the transport provider. This service request includes the destination address and the data to be delivered. Connectionless-mode service is attractive for applications that involve short-term interactions that do not require guaranteed, in-sequence delivery of data. Connectionless transports are generally unreliable.