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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:
Source: older version from Federal Standard 1037C
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