Connectionless refers to network protocols in which a host (i.e., computer) can send a message without prior arrangement, i.e., without establishing a connection, also referred to as a session, with the recipient.
A protocol is a mutually agreed-upon format for doing something, such as communicating between computers. Connection-oriented protocols require that the hosts use a preliminary protocol to establish an end-to-end connection prior to any data being sent. They are sometimes termed reliable network services, because they guarantee that data will reach its destination. Transmission control protocol (TCP) is a connection-oriented protocol.
In connectionless protocols, however, the transmitting host merely puts the message into the network without first ensuring that the recipient is available and ready to receive the data, and a best effort is made to deliver it. Packets comprising the message might take different routes, as each is independent of the other. There is no guarantee that they will arrive and that they will arrive without errors, without duplication, without delays, and in sequence. Also, there is no acknowledgment if they do arrive, and there is no retransmission if they do not arrive.
An advantage of the connectionless communication as compared with connection-oriented communication is that it allows for broadcast and multicast operations, which can conserve bandwidth when the same data needs to be transmitted to multiple recipients. A broadcast is the transmission of a single message to all hosts on the network simultaneously. A multicast message is one that is transmitted to selected multiple hosts which have joined the appropriate multicast group. In contrast, a connection is always unicast, i.e., between two hosts.
Connectionless protocols are usually described as stateless, because the end points have no protocol-defined way to remember where they are in a conversation of message exchanges. Likewise, connection-oriented protocols, which are sometimes described as stateful, because the hosts can keep track of a conversation.
A packet transmitted in a connectionless mode is often called a datagram.
Created October 15, 2005.