In telecommunication, connectionless describes communication between two network end points in which a message can be sent from one end point to another without prior arrangement. The device at one end of the communication transmits data to the other, without first ensuring that the recipient is available and ready to receive the data. The device sending a message simply sends it addressed to the intended recipient. If there are problems with the transmission, it may be necessary to resend the data several times. The Internet Protocol (IP) and User Datagram Protocol (UDP) are connectionless protocols.
Connectionless protocols are usually described as stateless because the endpoints have no protocol-defined way to remember where they are in a "conversation" of message exchanges. The alternative to the connectionless approach uses connection-oriented protocols, which are sometimes described as stateful because they can keep track of a conversation.