Application Layer Honolulu Community College Cisco Academy Training Center Semester 1 Version 2.1.1

Functions  identifying and establishing the availability of intended communication partners (including resources)  synchronizing cooperating applications  establishing agreement on procedures for error recovery  controlling data integrity

Acts as an Interface The application layer provides a direct interface for the rest of the OSI model by using network applications (such as WWW, , FTP, Telnet). The application layer provides an indirect interface by using standalone applications (such as word processors, spreadsheets, presentation managers) with a network redirector.

Client-Server  Most applications that work in a networked environment are classified as client-server applications. These applications have two components which allow them to function - the client side, and the server side. The client side is located on the local computer and is the requestor (redirector) of the services. The server side is located on a remote computer and provides services in response to the client’s requests.  FTP, Telnet, and web browsers are all examples of client-server software - they require both a client and a server.

Redirectors A redirector is a protocol that works with computer operating systems and network clients instead of specific application programs. A typical use of a redirector would be to save a word processor file on a network server (with the word processing application acting as the client). Examples of redirectors are:  Apple File Protocol  NetBIOS Extended User Interface (NetBEUI)  Novell IPX/SPX protocols  Network File System (NFS) of the TCP/IP protocol suite

Redirectors (cont.)  Enables data to be sent to a device that is not directly connected to a computer.  The application that requests a service is located on the local computer and the redirector reroutes the request to the proper network resource, while the application treats it as a local request.  Allow users to share documents, templates, databases, printers, and many other resource types, without having to use special application software  Redirectors are used, among other things, to ‘map’ network drives (such as F: denoting a network drive).

Connection to Server Connection is maintained just long enough to process the transaction (download the current Web page in this case). FTP and Telnet are different.

Domain Name System  Internet is built on a hierarchical addressing scheme.  A domain name is a string of characters and/or numbers, usually a name or abbreviation, that represents the numeric address of an Internet site.  Domain name server responds to requests from clients to translate a domain name into the associated IP address.

POP3 is the most commonly used protocol - it is an application layer protocol.

(cont.)  addresses consist of the recipent’s address and the postoffice address:  For the process to work, DNS servers are used to translate domain names to IP addresses.  is stored in a user’s mailbox until it is received - typically this amounts to a directory on a hard drive under a user’s UNIX account.  A password is required to retrieve the - this is typically the user’s UNIX account password.

Telnet Processing takes place on the remote computer. Telnet works mostly at the application, presentation, and session layers of the OSI model.

File Transfer Protocol Used to upload files to and download files from the Internet. Moves files from one computer to another by copying or moving files from server to client and client to server.

Hypertext Transfer Protocol  A Web browser presents data in multimedia formats on Web pages that use text, graphics, sound, and video.  The Web pages are created with HTML.  HTML directs a Web browser on a particular Web page to produce the appearance of the page in a specific manner.  In addition, HTML specifies locations for the placement of text, files, and objects that are to be transferred from the Web server to the Web browser

Hypertext Transfer Protocol (cont.)  Hyperlinks make the World Wide Web easy to navigate. A hyperlink is an object (word, phrase, or picture) on a Web page that, when clicked, transfers you to a new Web page.  The Web page contains (within its HTML code) an address location known as a Uniform Resource Locator (URL).

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