OSI Application layer support for Distributed Systems.

It is in the upper layers that OSI has been most innovative - specifying common services which had previously been performed on an ad hoc basis within applications. In the lower layers the services offer only a few 'service elements'. Usually you can connect, send data and disconnect, and not much else. In the Application Layer things are much more complex. There are service elements relating to specific applications such as file transfer, electronic mail and so on. There are also more general service elements which are used by several different sorts of application. These are of interest to the distributed system designer as they are the basic building blocks from which new applications can be built. We consider these below.