Connectionless mode transmission
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In a packet-switched network, connectionless mode transmission is a 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 sometimes defined as a logical relationship between the peers exchanging data.
An advantage of connectionless mode over connection-oriented mode is that it has a low data 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 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 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.)
