Application Layer

1. Application Layer

2. Applications • 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.

3. Applications… • In contrast, applications software (also called end-userprograms) 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.

4. Application Layer • 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!

5. What is the OSI model? • 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. • Whatis.com

6. OSI • 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 . • Whatis.com

7. OSI • 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). • Whatis.com

8. OSI • 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. • Whatis.com

9. OSI • 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. • Whatis.com

10. OSI • 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. • Whatis.com

11. OSI • 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 • Whatis.com

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

13. OSI: Layer 7 – Application Layer • 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

14. OSI: Layer 6 – Presentation Layer • 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.

15. OSI: Layer 5 – Session Layer • This layer sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. It deals with session and connection coordination.

16. OSI: Layer 4 – Transport Layer • This layer manages the end-to-end control (for example, determining whether all packets have arrived) and error-checking. It ensures complete data transfer.

17. OSI: Layer 3 – Network Layer • 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.

18. OSI: Layer 2 – Data Link Layer • 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.

19. OSI: Layer 1 – Physical Layer • 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.

20. Whatis.com

22. Application Layer Review • 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.

23. Application Layer Protocols: In-Depth Review of HTTP • 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.

24. Application Layer Protocols: In-Depth Review of HTTP • 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.

25. Application Layer Protocols: In-Depth Review of HTTP • Think of the process, taking place in the following order: • 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.

26. Application Layer Protocols: In-Depth Review of HTTP • 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.

27. Application Layer Protocols: In-Depth Review of HTTP • 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.

28. Security • Security of the application layer is critical. • Review the “Guard the Application Layer” document.