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eChapter 15 – Application Layer

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 Controlling data integrity

The application layer does not provide services to any other OSI layer. 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

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eDirect network applications

Most applications that work in a networked environment are classified as client-server applications.

A client is defined as a requester of services and a server is defined as the provider of services. A single machine can be both a client and a server depending on the software configuration.

Examples are: web browsers , e-mail, FTP, …

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eIndirect network support

The redirector is a protocol that works with computer operating systems and network clients instead of specific application programs. It allows using the network storage devices as if they were locally attached.

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eDomain Name System

In order to associate the contents of the site with its address, a domain naming system was developed.

A domain is a group of computers that are associated by their geographical location or their business type.

A domain name is a string of characters and/or numbers, usually a name or abbreviation, that represents the numeric address of an Internet site.

Top-level domain names examples:

•.za – South Africa

•.us - United States

•.uk - United Kingdom

.edu - educational sites

.com - commercial sites

.gov - government sites

.org - non-profit sites

.net - network service

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eThe domain name server

The Domain Name System server (DNS) is a device on a network. It responds to requests from clients to translate a domain name into the associated IP address.

The DNS system is set up in a hierarchy that creates different levels of DNS servers.

If a local DNS server is able to translate a domain name into its associated IP address, it does so, and returns the result to the client. If it cannot translate the address, it passes the request up to the next higher-level DNS server on the system, which then tries to translate the address. This process repeats itself until the domain name has been translated, or the top-level DNS server has been reached.

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eApplication Layer Examples: File transfer protocol

The main purpose of file transfer protocol (FTP) is to transfer files from one computer to another by copying and moving files from servers to clients, and from clients to servers.

When you copy files from a server, FTP establishes a data link between the computers, across which the data is transferred. Data transfer can occur in ASCII mode or in binary mode. These two modes determine how the data file is to be transferred between the stations.

After the file transfer has ended, the data connection terminates automatically. After you have completed the entire session of copying and moving files, you may log off, thus closing the command link, and ending the session.

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e The End