In a packet-switched network, connectionless mode transmission is transmission in which each packet is prepended with a header containing a destination address sufficient to permit the independent delivery of the packet without the aid of additional instructions.

A packet transmitted in a connectionless mode is frequently called a datagram.

In connection-oriented communication the stations about to exchange data first need to declare towards each other that they want to do so. This is called "establishing a connection". A connection is also defined sometimes as a logical relationship between the peers exchanging data.

An advantage of the connectionless mode over the connection-oriented is that it allows for multicast and broadcast operations, which may save network resources when the same data needs to be transmitted to several recepients. 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, such as the Transport Layer of the Open Systems Interconnection--Reference Model.)

Another drawback of the connectionless mode is that no optimisations 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 remember information, avoiding the re-computation for every frame. Or the network components could 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 sevaral layers of the Open Systems Interconnection--Reference Model:

• at the transport layer (e.g. TCP is a connection-oriented, UDP is a connectionless transport protocol)
• at the network layer
• at the data link layer: According to the IEEE 802 family of standards 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-oritented data link layer, others (like IP) do not. (After the appearance of APPN SNA could operate on a connectionless Data Link Service as well.)

Source: older version from Federal Standard 1037C