The Application Layer is the highest layer of the OSI architecture, the purpose of which is to serve as a window between correspondent application processes so that they may exchange information on the open environment. The progams which use the Application Layer are known as Application Processes. A user program may interface directly with the Presentation Layer in which case it must include protocol modules to initialise communication with peer application processes, establish the appropriate Presentation context and transfer files or messages. Alternatively, the user process may include available modules which suppport commonly required application related services. Such modules are referred to as Application Specific Elements and are defined as an integrated set of functions which together provide one or more application related communication capabilities. These capabilities are defined by a protocol in a very similar manner to that used to specify a service provided by the layer below. The protocol may specify direct use of presentation layer services and/or those provided by other ASEs. The grouping of a user process (or application entity), relevant ASEs and interfaces between them is known as an application process.
Application Layer protocols are classified into Common Application
Specific Elements (CASE) and Specific Application Specific Elements
(SASE). CASE elements are commonly required by user elements and SASEs,
whereas SASEs are only included in an application process when their particular
service is specifically required. For more information on SASEs Group Xs project
can be consulted (LINK).
There are four CASEs currently defined. These are...