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A connectionless service is a concept in data communications used to transfer data at the transport layer (layer 4) of the OSI model. It describes the communication between two nodes or terminals in which data is sent from one node to the other without first ensuring that the destination is available and ready to receive the data. A session connection between the sender and the receiver is not required, the sender just starts sending the data. The message or datagram is sent without prior arrangement, which is less reliable but faster transaction than a connection-oriented service.
User Datagram Protocol (UDP) is a connectionless protocol, while Transmission Control Protocol (TCP) is a connection-oriented network protocol.
Connectionless service means that a terminal or node can send data packets to its destination without establishing a connection to the destination. This works because of error handling protocols, which allow for error correction like requesting retransmission. LANs are actually connectionless systems with each computer able to transmit data packets as soon as it can access the network.
The Internet is a large connectionless packet network in which all packet delivery is handled by Internet providers. TCP adds connection-oriented services in addition to IP when required. TCP can provide all the top level connection services required to ensure proper data delivery.