Download Chapter 15 - The Application Layer

Chapter 15 – Application Layer
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The application layer is responsible for the following:
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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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Direct network applications
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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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Indirect network support
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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.
Domain Name System
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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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The domain name server
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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.
Application Layer Examples: File transfer protocol
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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.
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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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The End