In packet switching networks, CL-mode or connectionless communication is a data transmission method in which each data packet carries information in a header record that contains a destination address sufficient to permit the independent delivery of the packet to its destination via the network.
Under connectionless communication between two network end points, a message can be sent from one end point to another without prior arrangement. The device at one end of the communication transmits data addressed to the other, without first ensuring that the recipient is available and ready to receive the data. Some protocols allow for error correction by requested retransmission. Internet Protocol (IP) and User Datagram Protocol (UDP) are connectionless protocols.
A packet transmitted in a connectionless mode is frequently called a datagram.
In connection-oriented communication the communicating peers must first establish a logical or physical data channel or connection in a dialog preceding the exchange of user data.
The connectionless communications has the advantage over connection-oriented communications in that it has low overhead. It also allows for multicast and broadcast operations, which may save even more network resources when the same data needs to be transmitted to several recipients. In contrast, a connection is always unicast (point-to-point).
Unfortunately, in connectionless mode transmission of a packet, the service provider usually cannot guarantee that there will be no loss, error insertion, misdelivery, duplication, or out-of-sequence delivery of the packet. (However, the risk of these hazards may be reduced by providing a reliable transmission service at a higher protocol layer of the OSI Reference Model.)
Another drawback of the connectionless mode is that no optimizations are possible when sending several frames between the same two peers. By establishing a connection at the beginning of such a data exchange the components (routers, bridges) along the network path would be able to pre-compute (and hence cache) routing-related information, avoiding re-computation for every packet. Network components could also reserve capacity for the transfer of the subsequent frames of e.g. a video download.
The distinction between connectionless and connection-oriented transmission may take place at several layers of the OSI Reference Model:
- At the Transport Layer: TCP is a connection-oriented transport protocol. UDP is connectionless.
- At the Network Layer.
- At the Data Link Layer: The IEEE 802.2 protocol at the Logical Link Control sublayer of the data link layer may provide both connectionless and connection-oriented services. In fact, some network protocols (such as SNA's Path Control in its early stages) require a connection-oriented data link layer. Others (like IP) do not. (After the appearance of APPN, SNA could operate on a connectionless data link service as well.)
Notable connectionless protocols 
- Hypertext Transfer Protocol
- IP (internet layer, can also be used for connections)
- Information Processing Systems - Open Systems Interconnection, "Transport Service Definition - Addendum 1: Connectionless-mode Transmission", International Organization for Standardization, International Standard 8072/AD 1, December 1986.