Overview Now that you have seen what happens to data packets as they travel through the presentation layer, its time to look at the last layer in which data packets travel through before reaching its final destination. The last layer or Layer 7 of the OSI model is referred to the application layer. The application layer is the closest to you as an end-user, when you are interacting with software applications, such as sending and receiving e-mail over a network. In this chapter, you will see how the application layer deals with data packets from client-server applications, domain name services, and network applications. Additionally, in order to gain a better understanding of the functions of the application layer, you will see what happens to data packets by examining examples of the following network applications: Client-Server Redirectors Domain Name System E-mail Telnet FTP HTTP 15.1 Basics of the Application Layer 15.1.1 Application processes Instructor Note The purpose of this target indicator is to relate common application layer protocols to the protocols of other layers. This provides an overview of the chain of processes invoked by a particular application layer request.. In the context of the OSI reference model, the application layer (Layer 7) supports the communicating component of an application. The application layer is responsible for the following: identifying and establishing the availability of intended communication partners synchronizing cooperating applications establishing agreement on procedures for error recovery control of data integrity The application layer is the OSI layer closest to the end system, and determines whether sufficient resources exist for communication between systems. Without the application layer, there would be no network communication support. The application layer does not provide services to any other OSI layer; however, it does provide services to application processes lying outside the scope of the OSI model. Examples of such application processes include spreadsheet programs, word processing programs, and banking terminal programs. Additionally, the application layer provides a direct interface for the rest of the OSI model by using network applications (e.g. WWW, e-mail, FTP, Telnet), or an indirect interface by using standalone applications (e.g. word processors, spreadsheets, presentation managers) with a network redirector. Web Links The Application Layer of OSI 15.1 Basics of the Application Layer 15.1.2 Direct network applications Instructor Note The purpose of this target indicator is to introduce the concept of direct network applications. Most direct network applications use the client-server model. The same model which evolved for LANs -- client and server -- applies to the WAN knows as the Internet. When you request an URL from a browser, you are enacting the client server model. As an activity, have the students download some file of interest and ask them to consider what is visible and what is not visible as the download proceeds. Heighten their awareness of all the back-and-forth communication that is involved in a simple download. Web browsers are compared to TV remote controls. Most applications that work in a networked environment are classified as client-server applications. These applications, such as FTP, web browsers, and e-mail, all have two components, which allow them to function - the client side, and the server side. The client side is located on the local computer and is the requestor of the services. The server side is located on a remote computer and provides services in response to the clients requests. A client-server application works by constantly repeating the following looped routine: client-request, server-response; client-request, server-response; etc. For example, a web browser accesses a web page by requesting a uniform resource locator (URL), or web address, on a remote web server. After it locates the URL, the web server that is identified by that URL responds to the request. Then, based on the information received from the web server, the client can request more information from the same web server, or can access another web page from a different web server. The World Wide Web, Netscape Navigator, and Internet Explorer, are probably the most commonly used network applications. An easy way to understand a Web browser is to compare it to a television remote control. A remote control gives you the ability to directly control a TVs functions: volume, channels, brightness, etc. For the remote control to function properly, you do not need to understand how the remote control functions electronically. The same is true of a Web browser, in that the browser gives you the ability to navigate through the Web by clicking on hyperlinks. For the Web browser to function properly, it is not necessary for you to understand how the lower layer OSI protocols work and interact. Web Links Client/Server Frequently Asked Questions 15.1 Basics of the Application Layer 15.1.3 Indirect network support Instructor Note The purpose of this target indicator is to give more details about the client-server model as applied to LANs. Server file storage and server print operations are detailed. The notion of indirect network application support, using redirector protocols, is introduced. Within a LAN environment, indirect-application network support is a client-server function. If a client wants to save a file from a word processor to a network server, the redirector enables the word processing application to become a network client. Redirector is a protocol that works with computer operating systems and network clients instead of specific application programs. Examples of redirectors are: Apple File Protocol NetBIOS Extended User Interface (NetBEUI) Novell IPX/SPX protocols Network File System (NSF) of the TCP/IP protocol suite The redirector process is as follows:The client requests that the network file server allow the data file to be stored. The server responds by saving the file to its disk, or by rejecting the client's request. If the client requests that the network print server allow the data file to be printed by a remote (network) printer, the server processes the request by printing the file on one of its print devices, or by rejecting the request. Redirector allows a network administrator to assign remote resources to logical names on the local client. When you select one of these logical names to perform an operation such as saving a file, or printing a file, the network redirector sends the selected file to the proper remote resource on the network for processing. If the resource is on a local computer, the redirector ignores the request and allows the local operating system to process the request. The advantage of using a network redirector on a local client is that the applications on the client never have to recognize the network. In addition, the application that requests service is located on the local computer and the redirector reroutes the request to the proper network resource, while the application treats it as a local request.Redirectors expand the capabilities of non-network software. They also allow users to share documents, templates, databases, printers, and many other resource types, without having to use special application software.Networking has had a great influence on the development of programs like word processors, spreadsheets, presentation managers, database programs, graphics, and productivity software. Many of these software packages are now network-integrated or network-aware and have the capabilities of launching integrated Web browsers or Internet tools, and to publish their output to HTML for easy Web integration. Web Links Redirectors 15.1 Basics of the Application Layer 15.1.4 Making and breaking a connection Instructor Note The purpose of this target indicator is to show the common cycle -- make a connection, breaking a connection, between client and server, that underlies all Web Page requests. It is important to note that in each of the previous examples the connection to the server was maintained only long enough to process the transaction. In the Web example, the connection was maintained just long enough to download the current Web page. In the printer example, the connection was maintained just long enough to send the document to the print server. After the processing was completed, the connection was broken and had to be reestablished for the next processing request to take place. This is one of the two ways that communication processing takes place. - Later in this chapter, you will learn about the second method in which communication processing takes place. This is illustrated by the Telnet and FTP examples, which establish a connection to the server, and maintains that connection until all processing has been performed. The client computer terminates the connection when the user determines that he/she has finished. All communication activity falls into one of these two categories. In the next section, you will learn about the Domain Name System, which is supported by the application layer processes.
