Transcript Application Layer

Application Layer
Applications
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A program or group of programs designed for
end users.
Software can be divided into two general
classes: systems software and applications
software.
Systems software consists of low-level programs
that interact with the computer at a very basic
level.
This includes operating systems, compilers, and
utilities for managing computer resources.
Applications…
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In contrast, applications software (also
called end-user programs) includes
database programs, word processors, and
spreadsheets for example.
Figuratively speaking, applications
software sits on top of systems software
because it is unable to run without the
operating system and system utilities.
Application Layer
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The application layer is the seventh
level of the seven layer OSI model. It’s the
“highest layer” of the OSI model.
What is the OSI model?
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OSI (Open Systems Interconnection) is a
standard description or "reference model"
for how messages should be transmitted
between any two points in a
telecommunication network.
Its purpose is to guide product
implementers so that their products will
consistently work with other products.
OSI
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Developed by representatives of major computer
and telecommunication companies beginning in
1983, OSI was originally intended to be a
detailed specification of interfaces.
Instead, the committee decided to establish a
common reference model for which others could
develop detailed interfaces, that in turn could
become standards.
OSI was officially adopted as an international
standard by the International Organization of
Standards (ISO).
OSI
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The main idea in OSI is that the process of
communication between two end points in
a telecommunication network can be
divided into layers, with each layer adding
its own set of special, related functions.
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OSI
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Each communicating user or program is at a
computer equipped with these seven layers of
function.
So, in a given message between users, there will
be a flow of data through each layer at one end
down through the layers in that computer and,
at the other end, when the message arrives,
another flow of data up through the layers in the
receiving computer and ultimately to the end
user or program.
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OSI
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The actual programming and hardware that
furnishes these seven layers of function is
usually a combination of the computer operating
system, applications (such as your Web
browser), TCP/IP or alternative transport and
network protocols, and the software and
hardware that enable you to put a signal on one
of the lines attached to your computer.
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This graphic is taken from The Abdus Salam International Centre for Theoretical Physics.
OSI: Layer 7 – Application Layer
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This is the layer at which communication
partners are identified, quality of service is
identified, user authentication and privacy are
considered, and any constraints on data syntax
are identified.
This layer is not the application itself, although
some applications may perform application layer
functions.
Examples of Application Layer Protocols: HTTP,
UDP, TCP, Real Time Streaming Protocol (RTSP),
SSL
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Application Layer Review
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This layer supports application and end-user processes.
Communication partners are identified, quality of service
is identified, user authentication and privacy are
considered, and any constraints on data syntax are
identified.
Everything at this layer is application-specific.
This layer provides application services for file transfers,
e-mail, and other network software services.
Telnet and FTP are applications that exist entirely in the
application level.
Tiered application architectures are part of this layer.
Application Layer Protocols: InDepth Review of HTTP
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The HyperText Transfer Protocol, or HTTP, must be the
most widely used Application layer protocol in the world
today.
It forms the basis of what most people understand the
Internet to be—the World Wide Web.
Its purpose is to provide a lightweight protocol for the
retrieval of HyperText Markup Language (HTML) and
other documents from Web sites throughout the
Internet.
Each time you open a Web browser to surf the Internet,
you are using HTTP over TCP/IP.
Application Layer Protocols: InDepth Review of HTTP
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Example from web on basic HTTP page retrieval process:
The first important point to note is that a Web page is
typically made up of many dozens of objects, ranging
from the HTML base through to the images that are
present on the page.
The HTML can be thought of as the template for the
page overall, instructing the browser on the layout of the
text, font sizes and colors, background color of the page,
and which other images need to be retrieved to make up
the page.
Application Layer Protocols: InDepth Review of HTTP
Think of the process, taking place in the
following order:
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Client sends a request for the required page to the
Web server.
The server analyzes the request and sends back an
acknowledgment to the client along with the HTML
code required to make the page.
The client will begin interpreting the HTML and
building the page.
The client, in subsequent requests, will retrieve any
embedded objects, such as images or other
multimedia sources.
Application Layer Protocols: InDepth Review of HTTP
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Once all elements of the page have been
retrieved, the client browser will display
the completed Web page.
The order and timing of the process
described previously depends largely on
which implementation of HTTP is used—
1.0 or 1.1—although all browsers work in
this way of request and response.
Application Layer Protocols: InDepth Review of HTTP
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The URL is the most important piece of
information that the client browser includes in
any GET request.
The URL is defined as being a combination of
the host where the site is located, the scheme
used to retrieve the page, and the full path and
filename.
Optionally, the URL may include information
such as the TCP port number to be used or a
unique reference point within a larger page.
Security
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Security of the application layer is critical.