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Layer 7- Application Layer

Layer 7- Application Layer. Application Layer Protocols. Application Layer Protocols & Functionality . Learning Objectives. Describe how the functions of the three upper OSI model layers provide network services to end user applications.

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Layer 7- Application Layer

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  1. Layer 7- Application Layer Application Layer Protocols

  2. Application Layer Protocols & Functionality Learning Objectives • Describe how the functions of the three upper OSI model layers provide network services to end user applications. • Describe how the TCP/IP Application Layer protocols provide the services specified by the upper layers of the OSI model. • Define how people use the Application Layer to communicate across the information network. • Describe the function of well-known TCP/IP applications, such as the World Wide Web and email, and their related services (HTTP, DNS, SMB, DHCP, SMTP/POP, and Telnet).

  3. Applications – The Interface Between Human and Data Networks • The Application layer is the top layer of both the OSI and TCP/IP models • Application layer protocols are used to exchange data between programs running on the source and destination hosts. • There are many Application layer protocols and new protocols are always being developed.

  4. OSI TCP/IP Application: Layer 7 Interface to operating System Application Presentation: Layer 6 Coding, encryption, compression Session: Layer 5 Create and maintain dialogues between applications

  5. Well Known Application Protocols APPLICATION • File Transfer: • File Transfer Protocol (FTP) • Trivial File Transfer Protocol (TFTP) Transfer to Application Virtual Terminal Session,Compression & Encryption • Email: • Simple Mail Transfer Protocol (SMTP) • Post Office Protocol 3 (POP3) Establish, send, close session, Authenticators, Master / Slave TRANSPORT • Web Browsing: • Hyper Text Transfer Protocol (HTTP) INTERNET • Network Management: • Simple Network Management • Protocol (SNMP) LOCAL NETWORK N I C • Name Resolution: • Domain Name Service (DNS)

  6. Client / Server Processes • Application layer protocols describe the format of the requests and responses between clients and servers Host A Host B Host C FTP Server Download Data flowing from a server to a client is known as download.

  7. Client / Server Processes • In a client/server network, the server runs a service, or process, sometimes called a server daemon, typically running in the background. • Daemons are described as "listening" for a request from a client, because they are programmed to respond whenever the server receives a request for the service provided by the daemon. • When a daemon "hears" a request from a client, it exchanges appropriate messages with the client, as required by its protocol, and proceeds to send the requested data to the client in the proper format.

  8. Peer-to-Peer Networks • In a peer-to-peer network, two or more computers are connected via a network and can share resources (i.e. printers and files) without having a dedicated server. • Every connected end device (known as a peer) can function as either a server or a client. One computer might assume the role of server for one transaction while simultaneously serving as a client for another. • The roles of client and server are set on a per request basis.

  9. Application Layer Services

  10. WWW and HTTP • The World Wide Web (WWW) is a repository of information stored on web pages, linked together from points all over the world. • Web pages are written in a language called Hypertext Mark-Up Language (HTML), and stored on web servers. • To retrieve an HTML web page, a client/server protocol called Hyper Text Transfer Protocol (HTTP) is used. • HTML web pages are displayed on a users PC by web browser software clients.

  11. Web Browsing Processes • Web browser uses HTTP to request a particular web-page from a web server. • Web server responds, using HTTP to send the HTML web page page to the web browser. • The web browser formats the web page for display on the client PC. Client Web Server HTML WWW Page Web Browser HTTP Request HTML WWW Page

  12. Hyper Text Mark-up Language (HTML) • Language used for creating web pages. • Mark-up language formats a web page independently from the process that created it – provides a standard way for web browsers to interpret web pages. • Uses only ASCII characters for both the main web page text and the formatting instructions.

  13. Hyper Text Transfer Protocol (HTTP) • HTTP is used mainly to access data on the WWW. • Functions like a combination of FTP and SMTP. • Used to transfer files using TCP and well-known port 80 – there is no control connection required. • Web browser utilises an HTTP client, while a web server runs an HTTP server. • The HTML data transferred by HTTP is not readable by the user – it has to be interpreted by a web-browser.

  14. Request Request Response Response Client – Web Browser WWW Server HTML Web Page • Web Page Transfer Phases: • Connection Establishment – Client makes connection to TCP port 80 on the web server. Server commences the connection phase. • HTTP Transfer – Server transfers HTML web page using HTTP • Connection Termination – After web page is transferred successfully, the client terminates the connection. HTTP Server HTTP Client Data (80) TRANSPORT INTERNET LOCALNETWORK

  15. Uniform Resource Locater (URL) • The navigation of web pages is achieved using locators called Uniform Resource Locators (URLs). • These allow a user to access sites without using IP addresses. • The URL is a standard for specifying any kind of information on the Internet, and defines four things: protocol, host computer, port and path.

  16. Uniform Resource Locater (URL) URLs are presented in a standard format: Method :// Host : Port / Path • Method – the protocol used to retrieve data (usually HTTP) • Host – alias of the web server where the data is located. Normally prefixed with ‘WWW’ signifying a web server. • Port – Transport layer port that the web server is using – not a mandatory field, as port 80 is used by default. • Path – the location of the data on the web server – the ‘/’ indicates directories and subdirectories.

  17. Simple Name Resolution • Host Table: • Phill - 192.168.1.100 • Lisa - 192.168.1.101 • Bazil - 192.168.1.102 • Host Table: • Phill - 192.168.1.100 • Lisa - 192.168.1.101 • Bazil - 192.168.1.102 • Host Table: • Phill - 192.168.1.100 • Lisa - 192.168.1.101 • Bazil - 192.168.1.102 Phill 192.168.1.100 Lisa 192.168.1.101 Bazil 192.168.1.102 Name Resolution can be achieved using a host table, mapping all the host names in a network to their respective IP addresses All host tables need to be changed every time a new PC is added to the network- this can be time consuming on a large network Switch

  18. Name Resolution Using DNS DNS Server: 192.168.1.103 DNS Server: 192.168.1.103 DNS Server: 192.168.1.103 What is IP Address of Bazil? Phill 192.168.1.100 Lisa 192.168.1.101 Bazil 192.168.1.102 DNS Client DNS Client DNS Client • A single DNS server holds the host table. • Client DNS services request host/IP address mappings from the server. DNS Server 192.168.1.103 • Host Table: • Phill - 192.168.1.100 • Lisa - 192.168.1.101 • Bazil - 192.168.1.102 Bazil = 192.168.1.103 Switch

  19. Conclusions • Today we looked at • how the functions of the three upper OSI model layers provide network services to end user applications • Protocol used by WWW

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