Application layer

Outline Application Architectures Host-Based, Client-Based, and Client-Server Architectures, Choosing Architectures World Wide Web How the Web Works, Inside an HTTP Request & HTTP Response Electronic Mail How E-Mail Works, Inside an SMTP Packet Listserv, Attachments in MIME Other Applications Ftp, Telnet, Instant Messaging, Videoconferencing Copyright 2005 John Wiley & Sons, Inc 2 -

Application Layer - Introduction Copyright 2005 John Wiley & Sons, Inc 2 - Application Layer Network Layer Transport Layer Applications (e.g., email, web, word processing) Functions of Applications Data storage Storing of data generated by programs (e.g., files, records) Data access logic Processing required to access stored data (e.g., SQL) Application logic Business logic Presentation logic Presentation of info to user and acceptance of user commands

Application Architectures Determined by how functions of application programs are spread among clients and servers Host-based Architectures Server performs almost all functions Client-based architectures Client performs most functions Client-server architectures Functions shared between client and server Copyright 2005 John Wiley & Sons, Inc 2 -

Host-Based Architectures Copyright 2005 John Wiley & Sons, Inc 2 - captures key strokes then sends them to the mainframe displays information according to the server’s instructions or a PC running a terminal emulation program

Problems with Host-based Arch. Host becoming a bottleneck All processing done by the host, which can severely limit network performance Upgrades typically expensive and “lumpy” Available upgrades require big jumps in processing and memory  $$$ Network demand grows more incrementally, so this often means a poor fit (too much or too little) between host performance and network demand. Copyright 2005 John Wiley & Sons, Inc 2 -

Client-Based Architectures Copyright 2005 John Wiley & Sons, Inc 2 - Was popular in 1980s with the widespread use of PCs, LANs, and programs such as spreadsheets and word processors Example: Using a word processing on a PC and storing data (file) on a server

Problems with Client-Based Arch. Data MUST travel back and forth between server and client Example: when the client program is making a database query, the ENTIRE database must travel to the client before the query can be processed Result in poor network performance Copyright 2005 John Wiley & Sons, Inc 2 -

Client-Server Architectures Copyright 2005 John Wiley & Sons, Inc 2 - Client (PC) Server (PC, mini, mainframe) Presentation logic Application logic Data Access logic Data Storage Application logic may reside on the client, server or be split up between the two Used by most networks today Example: Using a Web browser to get pages from

Client-Server Architectures Advantages More efficient because of distributed processing Allow hardware and software from different vendors to be used together Disadvantages Difficulty in getting software from different vendors to work together smoothly  May require Middleware, a third category of software Copyright 2005 John Wiley & Sons, Inc 2 -

Middleware Examples: Distributed Computing Environment (DCE) Common Object Request Broker Architecture (CORBA) Open Database Connectivity (ODBC) Copyright 2005 John Wiley & Sons, Inc 2 - Middleware client application programs server application programs a standard way of translating between software from different vendors Manages message transfers Insulates network changes from the clients ((e.g., adding a new server)

Multi-tier Architectures Involve more than two computers in distributing application program logic 2-tier architecture (architectures discussed so far) 3-tier architecture 3 sets of computers involved N-tier architecture more than three sets of computers used Copyright 2005 John Wiley & Sons, Inc 2 -

Multi-tier Architectures Advantages Better load balancing: More evenly distributed processing. (e.g., application logic distributed between several servers.) More scalable: Only servers experiencing high demand need be upgraded Disadvantages Heavily loaded network: More distributed processing  more exchanges Difficult to program and test due to increased complexity Copyright 2005 John Wiley & Sons, Inc 2 -

Fat vs. Thin Clients Depends on how much of the application logic resides on the client Fat client: (a.k.a., thick client) All or most of the application logic Thin client: Little or no application logic Becoming popular because easier to manage, (only the server application logic generally needs to be updated) The best example: World Wide Web architecture (uses a two-tier, thin client architecture) Copyright 2005 John Wiley & Sons, Inc 2 -

Criteria for Choosing Architecture Infrastructure Cost Cost of servers, clients, and circuits Mainframes: very expensive; terminals, PCs: very inexpensive Development Cost Mainly cost of software development Software: expensive to develop; off-the-shelf software: inexpensive Scalability Ability to increase (or decrease) in computing capacity as network demand changes Mainframes: not scalable; PCs: highly scalable Copyright 2005 John Wiley & Sons, Inc 2 -

Choosing an Architecture Copyright 2005 John Wiley & Sons, Inc 2 - Host-Based Client-Based Client-Server Cost of Infrastructure High Medium Low Cost of Development Low Medium High Scalability Low Medium High

World Wide Web Two central ideas: Hypertext A document containing links to other documents Uniform Resource Locators (URLs) A formal way of identifying links to other documents Invention of WWW (1989) By Tim Berners-Lee at CERN in Switzerland First graphical browser, Mosaic, (1993) By Marc Andressen at NCSA in USA; later founded Netscape Copyright 2005 John Wiley & Sons, Inc 2 - CERN - Centre Européan pour Rechèrche Nucleaire NCSA - National Center for Supercomputing Applications

How the Web Works Copyright 2005 John Wiley & Sons, Inc 2 - HTTP Response HTTP Request Client Computer Server Computer HTTP - Hypertext Transfer Protocol Main Web communications protocol: Clicking on a hyperlink or typing a URL into a browser starts a request-response cycle A request-response cycle: include multiple steps since web pages often contain embedded files, such as graphics, each requiring a separate response.

HTTP Request Message Copyright 2005 John Wiley & Sons, Inc 2 - Request line ( command, URL, HTTP version number) Request header ( information on the browser, date, and the referring page ) Request body (information sent to the server, e.g., from a form) required optional optional (If the user types in the URL by themselves, then the referring page is blank.)

Example of an HTTP Request Copyright 2005 John Wiley & Sons, Inc 2 - GET http://www.kelley.indiana.edu/ardennis/home.htm HTTP/1.1 Date: Mon 06 Aug 2001 17:35:46 GMT User-Agent: Mozilla/6.0 Referer: http://www.indiana.edu/~aisdept/faculty.htm Request Header Request Line Web browser (code name for Netscape) Command URL HTTP version URL that contained the link to the requested URL Note that this HTTP Request message has no “Body” part. GMT – Greenwich Mean Time

HTTP Response Message Copyright 2005 John Wiley & Sons, Inc 2 - Response status ( http version number, status code, reason) Response header ( information on the server, date, URL of the page retrieved, format used ) Response body (requested web page) optional optional required

Example of an HTTP Response Copyright 2005 John Wiley & Sons, Inc 2 - <html> <head> <title>Allen R. Dennis</title> </head> <body> <H2> Allen R. Dennis </H2> <P>Welcome to the home page of Allen R. Dennis</P> </body> </html> Date: Mon 06 Aug 2001 17:35:46 GMT Server: NCSA/1.3 Location: http:// www.kelley.indiana.edu/adennis/home.htm Content-type: text/html Response Header Response Body HTTP/1.1 200 OK Response Status Another example of response status: HTTP/1.1 404 page not found)

HTML - Hypertext Markup Language A protocol used to format Web pages Also developed at CERN (initially for text files) TAGs embedded in HTML documents include information on how to format the file Extensions to HTML needed to format multimedia files XML - Extensible Markup Language A new markup language becoming popular Copyright 2005 John Wiley & Sons, Inc 2 -

Electronic Mail Heavily used Internet application Much, much faster than snail mail (regular mail) Extremely inexpensive (compared to $3-$10 per paper mail cost) Includes preparation, paper, postage, etc, Can substitute for other forms of communication, such as telephone calls Eliminates “telephone tag” E-mail users can answer at his/her convenience, instead of time of call Copyright 2005 John Wiley & Sons, Inc 2 -

E-mail Standards SMTP - Simple Mail Transfer Protocol Main e-mail standard for Originating user agent and the mail transfer agent Between mail transfer agents Originally written to handle only text files Usually used in two-tier client-server architectures Post Office Protocol (POP) and Internet Mail Access Protocol (IMAP) Main protocols used between the receiver user agent and mail transfer agent Main difference: with IMAP, messages can be left at the server after downloading them to the client Other competing standards Common Messaging Calls (CMC), X.400 Copyright 2005 John Wiley & Sons, Inc 2 -

Two-Tier E-mail Architecture User agents (also called e-mail clients) Run on client computers Send e-mail to e-mail servers Download e-mail from mailboxes on those servers Examples: Eudora, Outlook, Netscape Messenger Mail transfer agents (also called mail server) Used by e-mail servers Send e-mail between e-mail servers Maintain individual mailboxes. Copyright 2005 John Wiley & Sons, Inc 2 -

How SMTP Works Copyright 2005 John Wiley & Sons, Inc 2 - LAN Internet LAN with e-mail client software Client computer Server computer with Server computer with e-mail server software e-mail server software SMTP packet SMTP packet SMTP packet SMTP packet IMAP or POP packet (“message transfer agent”) an e-mail message is sent as an SMTP packet to the local mail server reads the packet’s destination address and sends it over the Internet to the receiver’s mail server. stores the message in the receiver’s mail box contacts the mail server which then downloads the message (“user agent”) Client computer

Host Based e-mail Architectures An old method used on UNIX based hosts Similar to client-server architecture, except Client PC replaced by a terminal (or emulator) Sends all keystrokes to the server Display characters received from the server All software reside on server Takes client keystrokes and understand user’s commands Creates SMTP packets and sends them to next mail server Copyright 2005 John Wiley & Sons, Inc 2 -

Three-Tier Client-Server Arch. Copyright 2005 John Wiley & Sons, Inc 2 - Client computer with Web Browser Server computer with Web server software Server computer with email server software performs the same functions as the mail server in the two-tier example sends HTTP requests to the Web server sends HTTP responses to the Web client translates the client’s HTTP requests into SMTP packets then send them to the Mail server Best known example: Web based email (e.g., Hotmail) No need for an email user agent

Web-based e-mail Copyright 2005 John Wiley & Sons, Inc 2 - LAN LAN SMTP packet SMTP packet SMTP packet Internet Client computer with Client computer with Server computer with Server computer with Server computer with Web server software Web server software e-mail server software IMAP or POP packet IMAP packet HTTP request HTTP request HTTP response HTTP response Web browser Web browser Server computer with email server software

SMTP Message Copyright 2005 John Wiley & Sons, Inc 2 - Header ( source and destination addresses, date, subject, and other information about the e-mail message) Body ( message itself) Attachments (additional files included along with the e-mail message )

Sample SMTP Message From: “Alan Dennis;”<adennis@indiana.edu> TO: “Pat Someone” <someone@somewhere.com> Date: Mon 06 Aug 2001 19:03:02 GMT Subject: Sample Note Message-Id: <4.1.20000623164823.009f5e80@IMAP.IU.EDU> DATA: This is an example of an e-mail message Copyright 2005 John Wiley & Sons, Inc 2 - Header Body Note that this SMTP message has no attachments. Unique ID used to keep track of messages. Text in “ “ are ignored

MIME Multipurpose Internet Mail Extension A graphics capable mail transfer agent protocol (to send graphical information in addition to text) SMPT designed for text transfer only Included as part of an e-mail client Translates graphical information into text allowing the graphic to be sent as part of an SMTP message (as a special attachment) Receiver’s e-mail client then translates the MIME attachment from text back into graphical format Other Graphics capable mail agent protocols uuencode and binhex Copyright 2005 John Wiley & Sons, Inc 2 -

Listserv Discussion Groups Mailing lists of users who join to discuss some special topic (e.g., cooking, typing, networking) Some permit any member to post messages, some are more restricted Parts of listserv Listserv Processor Process commands ( subscriptions, etc,) Listserv Mailer Receive a message and resend it to everyone To subscribe Send an email to Listserv processor (address of the processor is different than the address of mailer) Copyright 2005 John Wiley & Sons, Inc 2 -

File Transfer Protocol (FTP) Enables sending and receiving files over the Internet Requires an application program on the client computer and a FTP server program on a server Commonly used today for uploading web pages Many packages available using FTP WS-FTP (a graphical FTP software) FTP sites Closed sites Requires account name and password Anonymous sites Account name: anonymous ; pwd: your email address Copyright 2005 John Wiley & Sons, Inc 2 -

Telnet Allows one computer to log into other computers Remote login enabling full control of the host Requires an application program on the client computer and a Telnet server program on a server Client program emulates a “dumb” terminal Many packages available conforming Telnet EWAN Requires account name and password Anonymous sites Account name: anonymous ; pwd: your email address Copyright 2005 John Wiley & Sons, Inc 2 -

Instant Messaging (IM) A client-server program that allows real-time typed messages to be exchanged Client needs an IM client software Server needs an IM server package Some types allow voice and video packets to be sent Like a telephone Examples include AOL and ICQ Two step process: Telling IM server that you are online Chatting Copyright 2005 John Wiley & Sons, Inc 2 -

How Instant Messaging Works Copyright 2005 John Wiley & Sons, Inc 2 - LAN LAN LAN IM packet IM packet IM packet IM packet Internet Server computer with IM client software e-mail client software Client computer with Client computer with with IM server software sends a request to the IM server telling it that you are online. If a friend connects, the IM server sends a packet to your IM client and vice versa If a chat session has more than two clients, multiple packets are sent by the IM server. IM servers can also relay information to other IM servers. When you type some text, your IM client sends the text in a packet to the IM server which relays it to your friend.

Videoconferencing Provides real time transmission of video and audio signals among two or more locations Allows people to meet at the same time in different locations Saving money and time by not having to move people around (conference calling does the same thing) Typically involves 2 special purpose rooms with cameras and displays Desktop videoconferencing Low cost application linking small video cameras and microphones together over the Internet No need for special rooms Example: Net Meeting sw on clients communicating through a videoconference server Copyright 2005 John Wiley & Sons, Inc 2 -

Videoconferencing Standards Proprietary early systems Common standards in use today H.320 Designed for room-to-room videoconferencing over high-speed phone lines H.323 Family of standards designed for desktop videoconferencing and just simple audio conferencing over Internet MPEG-2 Designed for faster connections such as LAN or privately owned WANs Copyright 2005 John Wiley & Sons, Inc 2 -

Webcasting Special type of one-directional videoconferencing Content is sent from the server to users Process Content created by developer Downloaded as needed by the user Played by a plug-in to a Web browser No standards for webcasting yet Defacto standards: products by RealNetworks Copyright 2005 John Wiley & Sons, Inc 2 -

Implications for Management Network must be used to provide a worry-free environment for applications Network should not change the way an organization operates; application should! Network should enable wide variety of applications Dramatic increase in number and type of applications Rapid growth in amount and type of traffic Different implication on network design and management Increased operating cost Copyright 2005 John Wiley & Sons, Inc 2 -

Copyright 2005 John Wiley & Sons, Inc. All rights reserved. Reproduction or translation of this work beyond that permitted in section 117 of the 1976 United States Copyright Act without express permission of the copyright owner is unlawful. Request for further information should be addressed to the Permissions Department, John Wiley & Sons, Inc. The purchaser may make back-up copies for his/her own use only and not for distribution or resale. The Publisher assumes no responsibility for errors, omissions, or damages caused by the use of these programs or from the use of the information herein. Copyright 2005 John Wiley & Sons, Inc 2 -

Application layer

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