A connectionless communication service is a communication service model in which data packets are simply transmitted from end-system to end-system without any form of error detection (except perhaps for simple checksumming). In the Internet, a connectionless service will rely on the “best-effort” datagram delivery service provided by the Internet Protocol (IP) itself. There is no connection set-up or tear-down overhead, which means that connectionless service is likely to be somewhat more efficient than a connection-oriented service. Also, as there is no error detection or flow control, the data can probably be sent quickly which may be important for some time-critical applications such as streaming multimedia or voice-over-IP. Conversely, as there is no flow control data may be transmitted more quickly than the network can deliver it, or the receiving application can process it; and without congestion control datagrams can be lost and the sender will not be asked to resend them. Thus it is important that a connectionless service is only used when some data loss can be tolerated.

The User Datagram Protocol (UDP) is the connectionless communication service that is used on the Internet. UDP can be used to implement a connection-oriented service, but it will require the error-correction protocols to be built into the application layer.

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