In telecommunications, 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. As such there are more frequent problems with transmission than with connection-oriented protocols and it may be necessary to resend the data several times. Connectionless protocols are often disfavored by network administrators because it is much harder to filter malicious packets from a connectionless protocol using a firewall. The Internet Protocol (IP) and User Datagram Protocol (UDP) are connectionless protocols, but TCP/IP (the most common use of IP) is connection-oriented.
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.