Application Layer

Application LayerApplication Layer

ApplicationsApplications

A program or group of programs A program or group of programs designed for end users. Software can be designed for end users. Software can be divided into two general classes: divided into two general classes: systems software and systems software and applications applications softwaresoftware. Systems software consists of . Systems software consists of low-level programs that interact with low-level programs that interact with the computer at a very basic level. This the computer at a very basic level. This includes operating systems, compilers, includes operating systems, compilers, and utilities for managing computer and utilities for managing computer resources. resources.

Applications…Applications…

In contrast, applications software (also In contrast, applications software (also called called end-userend-user programsprograms) includes ) includes database programs, word processors, database programs, word processors, and spreadsheets for example. and spreadsheets for example. Figuratively speaking, applications Figuratively speaking, applications software sits on top of systems software sits on top of systems software because it is unable to run software because it is unable to run without the operating system and without the operating system and system utilities. system utilities.

Application LayerApplication Layer

The The application layerapplication layer is the is the seventh level of the seven layer OSI seventh level of the seven layer OSI model. It’s the “highest layer” of the model. It’s the “highest layer” of the OSI model.OSI model.

The book and the course are The book and the course are organized and broken down by the organized and broken down by the OSI model! OSI model!

What is the OSI model?What is the OSI model?

OSI (Open Systems Interconnection) is OSI (Open Systems Interconnection) is a standard description or "reference a standard description or "reference model" for how messages should be model" for how messages should be transmitted between any two points transmitted between any two points in a telecommunication network. Its in a telecommunication network. Its purpose is to guide product purpose is to guide product implementers so that their products implementers so that their products will consistently work with other will consistently work with other products. products.

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OSIOSI

The reference model defines seven layers of The reference model defines seven layers of functions that take place at each end of a functions that take place at each end of a communication. Although OSI is not always communication. Although OSI is not always strictly adhered to in terms of keeping related strictly adhered to in terms of keeping related functions together in a well-defined layer, functions together in a well-defined layer, many if not most products involved in many if not most products involved in telecommunication make an attempt to telecommunication make an attempt to describe themselves in relation to the OSI describe themselves in relation to the OSI model. It is also valuable as a single model. It is also valuable as a single reference view of communication that reference view of communication that furnishes everyone a common ground for furnishes everyone a common ground for education and discussion .education and discussion .


OSIOSI

Developed by representatives of major Developed by representatives of major computer and telecommunication computer and telecommunication companies beginning in 1983, OSI was companies beginning in 1983, OSI was originally intended to be a detailed originally intended to be a detailed specification of interfaces. Instead, the specification of interfaces. Instead, the committee decided to establish a common committee decided to establish a common reference model for which others could reference model for which others could develop detailed interfaces, that in turn develop detailed interfaces, that in turn could become standards. OSI was officially could become standards. OSI was officially adopted as an international standard by the adopted as an international standard by the International Organization of Standards International Organization of Standards (ISO). (ISO).


OSIOSI

The main idea in OSI is that the The main idea in OSI is that the process of communication between process of communication between two end points in a two end points in a telecommunication network can be telecommunication network can be divided into layers, with each layer divided into layers, with each layer adding its own set of special, related adding its own set of special, related functions.functions.

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OSIOSI

Each communicating user or program is at Each communicating user or program is at a computer equipped with these seven a computer equipped with these seven layers of function. So, in a given message layers of function. So, in a given message between users, there will be a flow of data between users, there will be a flow of data through each layer at one end down through each layer at one end down through the layers in that computer and, through the layers in that computer and, at the other end, when the message at the other end, when the message arrives, another flow of data up through arrives, another flow of data up through the layers in the receiving computer and the layers in the receiving computer and ultimately to the end user or program. ultimately to the end user or program.


OSIOSI

The actual programming and hardware The actual programming and hardware that furnishes these seven layers of that furnishes these seven layers of function is usually a combination of the function is usually a combination of the computer operating system, applications computer operating system, applications (such as your Web browser), TCP/IP or (such as your Web browser), TCP/IP or alternative transport and network alternative transport and network protocols, and the software and hardware protocols, and the software and hardware that enable you to put a signal on one of that enable you to put a signal on one of the lines attached to your computer. the lines attached to your computer.


OSIOSI

OSI divides telecommunication into seven OSI divides telecommunication into seven layers. The layers are in two groups. The layers. The layers are in two groups. The upper four layers are used whenever a upper four layers are used whenever a message passes from or to a user. The lower message passes from or to a user. The lower three layers (up to the network layer) are three layers (up to the network layer) are used when any message passes through the used when any message passes through the host computer. Messages intended for this host computer. Messages intended for this computer pass to the upper layers. Messages computer pass to the upper layers. Messages destined for some other host are not passed destined for some other host are not passed up to the upper layers but are forwarded to up to the upper layers but are forwarded to another host. The seven layers are in the next another host. The seven layers are in the next seven slidesseven slides


This graphic is taken from The Abdus Salam International Centre for Theoretical Physics.

OSI: Layer 7 – Application OSI: Layer 7 – Application LayerLayer

This is the layer at which communication This is the layer at which communication partners are identified, quality of service is partners are identified, quality of service is identified, user authentication and privacy identified, user authentication and privacy are considered, and any constraints on data are considered, and any constraints on data syntax are identified. (This layer is syntax are identified. (This layer is notnot the the application itself, although some application itself, although some applications may perform application layer applications may perform application layer functions.) functions.)

Examples of Application Layer Protocols: Examples of Application Layer Protocols: HTTP, UDP, TCP, Real Time Streaming HTTP, UDP, TCP, Real Time Streaming Protocol (RTSP), SSLProtocol (RTSP), SSL

OSI: Layer 6 – Presentation OSI: Layer 6 – Presentation LayerLayer

This is a layer, usually part of an operating This is a layer, usually part of an operating system, that converts incoming and system, that converts incoming and outgoing data from one presentation outgoing data from one presentation format to another (for example, from a format to another (for example, from a text stream into a popup window with the text stream into a popup window with the newly arrived text). newly arrived text).

Responds to service requests from Responds to service requests from application layer and issues service application layer and issues service requests to the session layer.requests to the session layer.

OSI: Layer 5 – Session LayerOSI: Layer 5 – Session Layer

This layer sets up, coordinates, and This layer sets up, coordinates, and terminates conversations, terminates conversations, exchanges, and dialogs between the exchanges, and dialogs between the applications at each end. It deals applications at each end. It deals with session and connection with session and connection coordination. coordination.

OSI: Layer 4 – Transport OSI: Layer 4 – Transport LayerLayer

This layer manages the end-to-end This layer manages the end-to-end control (for example, determining control (for example, determining whether all packets have arrived) whether all packets have arrived) and error-checking. It ensures and error-checking. It ensures complete data transfer. complete data transfer.

OSI: Layer 3 – Network OSI: Layer 3 – Network LayerLayer

This layer handles the routing of the This layer handles the routing of the data (sending it in the right direction data (sending it in the right direction to the right destination on outgoing to the right destination on outgoing transmissions and receiving incoming transmissions and receiving incoming transmissions at the packet level). transmissions at the packet level). The network layer does routing and The network layer does routing and forwarding. forwarding.

OSI: Layer 2 – Data Link OSI: Layer 2 – Data Link LayerLayer

This layer provides synchronization This layer provides synchronization for the physical level and does bit-for the physical level and does bit-stuffing for strings of 1's in excess of stuffing for strings of 1's in excess of 5. It furnishes transmission protocol 5. It furnishes transmission protocol knowledge and management. knowledge and management.

OSI: Layer 1 – Physical OSI: Layer 1 – Physical LayerLayer

This layer conveys the bit stream This layer conveys the bit stream through the network at the electrical through the network at the electrical and mechanical level. It provides the and mechanical level. It provides the hardware means of sending and hardware means of sending and receiving data on a carrier. receiving data on a carrier.

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Application Layer ReviewApplication Layer Review

This layer supports application and end-user This layer supports application and end-user processes. Communication partners are processes. Communication partners are identified, quality of service is identified, identified, quality of service is identified, user authentication and privacy are user authentication and privacy are considered, and any constraints on data considered, and any constraints on data syntax are identified. Everything at this layer syntax are identified. Everything at this layer is application-specific. This layer provides is application-specific. This layer provides application services for file transfers, e-mail, application services for file transfers, e-mail, and other network software services. Telnet and other network software services. Telnet and FTP are applications that exist entirely and FTP are applications that exist entirely in the application level. Tiered application in the application level. Tiered application architectures are part of this layer. architectures are part of this layer.

Application Layer Protocols: In-Application Layer Protocols: In-Depth Review of HTTPDepth Review of HTTP

The HyperText Transfer Protocol, or The HyperText Transfer Protocol, or HTTPHTTP, , must be the most widely used Application must be the most widely used Application layer protocol in the world today. It forms layer protocol in the world today. It forms the basis of what most people understand the basis of what most people understand the Internet to be—the World Wide Web. the Internet to be—the World Wide Web. Its purpose is to provide a lightweight Its purpose is to provide a lightweight protocol for the retrieval of HyperText protocol for the retrieval of HyperText Markup Language (Markup Language (HTMLHTML) and other ) and other documents from Web sites throughout the documents from Web sites throughout the Internet. Each time you open a Web Internet. Each time you open a Web browser to surf the Internet, you are using browser to surf the Internet, you are using HTTPHTTP over over TCPTCP//IPIP. .


Example from web on basic HTTP page retrieval Example from web on basic HTTP page retrieval process:process:

The first important point to note is that a Web The first important point to note is that a Web page is typically made up of many dozens of page is typically made up of many dozens of objects, ranging from the objects, ranging from the HTMLHTML base through to base through to the images that are present on the page.the images that are present on the page.

The The HTMLHTML can be thought of as the template for can be thought of as the template for the page overall, instructing the browser on the the page overall, instructing the browser on the layout of the text, font sizes and colors, layout of the text, font sizes and colors, background color of the page, and which other background color of the page, and which other images need to be retrieved to make up the images need to be retrieved to make up the page. page.


Think of the process, taking place in the Think of the process, taking place in the following order:following order:

Client sends a request for the required page to Client sends a request for the required page to the Web server.the Web server.

The server analyzes the request and sends The server analyzes the request and sends back an acknowledgment to the client along back an acknowledgment to the client along with the with the HTMLHTML code required to make the code required to make the page. page.

The client will begin interpreting the The client will begin interpreting the HTMLHTML and and building the page. building the page.

The client, in subsequent requests, will The client, in subsequent requests, will retrieve any embedded objects, such as retrieve any embedded objects, such as images or other multimedia sources.images or other multimedia sources.


Once all elements of the page have Once all elements of the page have been retrieved, the client browser will been retrieved, the client browser will display the completed Web page. The display the completed Web page. The order and timing of the process order and timing of the process described previously depends largely described previously depends largely on which implementation of on which implementation of HTTPHTTP is is used—1.0 or 1.1—although all browsers used—1.0 or 1.1—although all browsers work in this way of request and work in this way of request and response. response.


The The URLURL is the most important piece of is the most important piece of information that the client browser includes information that the client browser includes in any GET request. The in any GET request. The URLURL is defined as is defined as being a combination of the host where the being a combination of the host where the site is located, the scheme used to retrieve site is located, the scheme used to retrieve the page, and the full path and filename. the page, and the full path and filename. Optionally, the Optionally, the URLURL may include information may include information such as the such as the TCPTCP port number to be used or port number to be used or a unique reference point within a larger a unique reference point within a larger page. page.

SecuritySecurity

Security of the application layer is Security of the application layer is critical. critical.

Review the “Guard the Application Review the “Guard the Application Layer” document.Layer” document.

Documents

Application Layer