NIX OS. For a UNIX system to interact with a heterogeneous system running another OS, using remote procedure calls (RPC), the usual presentation layer format is XDR. XDR has its own set of data types and the application layer types will get converted to XDR format before transfer of data and the response data also needs to be converted to XDR. In a distributed system having heterogeneous systems, the presentation layer plays a key role to facilitate communication. 10.8 Application Layer This is the top most layer of the OSI model. Being the seventh lowest layer in the ISO/OSI model it is also referred to as L7. The application layer manages commu- nication between applications and handles issues like network transparency and resource allocation. In addition to providing network access as the key functional- ity, the application layer also offers services for user applications. Let us consider how the application layer offers services for user applications with an example of World Wide Web (www). The Web browser is an application that runs on a machine to display Web pages and it makes use of the services offered by a protocol that operates at the application layer. Viewing the vast collec- tion of hypertext-based files available on the Internet as well as providing quick and easy retrieval of these files using a browser is made possible using the application layer protocol called hyper text transfer protocol (HTTP). Some of the other popular application layer protocols include simple mail trans- fer protocol (SMTP) that gives electronic mail functionality to be implemented in applications through which all kinds of data can be sent to other users of the Internet as an electronic postal system and file transfer protocol (FTP) is used to transfer files between two machines. The network management protocol SNMP is Seven-Layer Communication Model also an application layer protocol as it provides interfaces for network management applications to collect network data. A program that implements the interfaces provided b