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.
The book and the course are organized
and broken down by 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.
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OSI
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The reference model defines seven layers of
functions that take place at each end of a
communication. Although OSI is not always
strictly adhered to in terms of keeping related
functions together in a well-defined layer, many
if not most products involved in
telecommunication make an attempt to describe
themselves in relation to the OSI model. It is
also valuable as a single reference view of
communication that furnishes everyone a
common ground for education and discussion .
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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).
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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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OSI
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OSI divides telecommunication into seven layers.
The layers are in two groups. The upper four
layers are used whenever a message passes
from or to a user. The lower three layers (up to
the network layer) are used when any message
passes through the host computer. Messages
intended for this computer pass to the upper
layers. Messages destined for some other host
are not passed up to the upper layers but are
forwarded to another host. The seven layers are
in the next seven slides
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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
OSI: Layer 6 – Presentation Layer
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This is a layer, usually part of an operating
system, that converts incoming and outgoing
data from one presentation format to another
(for example, from a text stream into a popup
window with the newly arrived text).
Responds to service requests from application
layer and issues service requests to the session
layer.
OSI: Layer 5 – Session Layer
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This layer sets up, coordinates, and
terminates conversations, exchanges, and
dialogs between the applications at each
end. It deals with session and connection
coordination.
OSI: Layer 4 – Transport Layer
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This layer manages the end-to-end control
(for example, determining whether all
packets have arrived) and error-checking.
It ensures complete data transfer.
OSI: Layer 3 – Network Layer
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This layer handles the routing of the data
(sending it in the right direction to the
right destination on outgoing
transmissions and receiving incoming
transmissions at the packet level). The
network layer does routing and
forwarding.
OSI: Layer 2 – Data Link Layer
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This layer provides synchronization for the
physical level and does bit-stuffing for
strings of 1's in excess of 5. It furnishes
transmission protocol knowledge and
management.
OSI: Layer 1 – Physical Layer
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This layer conveys the bit stream through
the network at the electrical and
mechanical level. It provides the hardware
means of sending and receiving data on a
carrier.
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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.
Review the “Guard the Application Layer”
document.