Business Data Communications and Networking - Chapter 2: Application Layer

Business Data Communications and Networking 8th Edition Jerry Fitzgerald and Alan DennisJohn Wiley & Sons, Inc Prof. M. UlemaManhattan CollegeComputer Information Systems1Copyright 2005 John Wiley & Sons, IncChapter 2Application Layer2Copyright 2005 John Wiley & Sons, IncOutlineApplication Architectures Host-Based, Client-Based, and Client-Server Architectures, Choosing ArchitecturesWorld Wide WebHow the Web Works, Inside an HTTP Request & HTTP ResponseElectronic Mail How E-Mail Works, Inside an SMTP PacketListserv, Attachments in MIMEOther ApplicationsFtp, Telnet, Instant Messaging, Videoconferencing3Copyright 2005 John Wiley & Sons, IncApplication Layer - IntroductionApplication LayerNetwork LayerTransport LayerApplications(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 commands4Copyright 2005 John Wiley & Sons, IncApplication ArchitecturesDetermined by how functions of application programs are spread among clients and servers Host-based ArchitecturesServer performs almost all functions Client-based architecturesClient performs most functions Client-server architecturesFunctions shared between client and server5Copyright 2005 John Wiley & Sons, IncHost-Based Architecturescaptures key strokes then sends them to the mainframedisplays information according to the server’s instructionsor a PC running a terminal emulation program 6Copyright 2005 John Wiley & Sons, IncProblems with Host-based Arch.Host becoming a bottleneckAll processing done by the host, which can severely limit network performanceUpgrades 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.7Copyright 2005 John Wiley & Sons, IncClient-Based ArchitecturesWas popular in 1980s with the widespread use of PCs, LANs, and programs such as spreadsheets and word processorsExample: Using a word processor on a PC and storing data (file) on a server8Copyright 2005 John Wiley & Sons, IncProblems with Client-Based Arch.Data MUST travel back and forth between server and clientExample: when the client program is making a database query, the ENTIRE database must travel to the client before the query can be processedResult in poor network performance9Copyright 2005 John Wiley & Sons, IncClient-Server ArchitecturesClient(PC)Server(PC, mini, mainframe)Presentation logicApplication logicData Access logicData StorageApplication logicmay reside on the client, server or be split up between the twoUsed by most networks today Example: Using a Web browser to get pages from 10Copyright 2005 John Wiley & Sons, IncClient-Server ArchitecturesAdvantagesMore efficient because of distributed processingAllow hardware and software from different vendors to be used togetherDisadvantagesDifficulty in getting software from different vendors to work together smoothly May require Middleware, a third category of software11Copyright 2005 John Wiley & Sons, IncMiddlewareExamples:Distributed Computing Environment (DCE)Common Object Request Broker Architecture (CORBA)Open Database Connectivity (ODBC)Middlewareclient application programsserver application programsa standard way of translating between software from different vendorsManages message transfers Insulates network changes from the clients ((e.g., adding a new server) 12Copyright 2005 John Wiley & Sons, IncMulti-tier ArchitecturesInvolve more than two computers in distributing application program logic 2-tier architecture (architectures discussed so far)3-tier architecture3 sets of computers involvedN-tier architecture more than three sets of computers used13Copyright 2005 John Wiley & Sons, Inc3-tier Architecture14Copyright 2005 John Wiley & Sons, IncN-tier Architecture15Copyright 2005 John Wiley & Sons, IncMulti-tier ArchitecturesAdvantagesBetter load balancing: More evenly distributed processing. (e.g., application logic distributed between several servers.)More scalable:Only servers experiencing high demand need be upgradedDisadvantagesHeavily loaded network: More distributed processing  more exchanges Difficult to program and test due to increased complexity16Copyright 2005 John Wiley & Sons, IncFat vs. Thin ClientsDepends on how much of the application logic resides on the clientFat client: (a.k.a., thick client)All or most of the application logicThin client: Little or no application logicBecoming 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)17Copyright 2005 John Wiley & Sons, IncThin-Client Example: Web ArchitectureClient(PC)Web Server(PC, mini, mainframe)Presentation logicApplication LogicData Access logicData Storage18Copyright 2005 John Wiley & Sons, IncCriteria for Choosing ArchitectureInfrastructure CostCost of servers, clients, and circuitsMainframes: very expensive; terminals, PCs: very inexpensiveDevelopment CostMainly cost of software developmentSoftware: expensive to develop; off-the-shelf software: inexpensiveScalability Ability to increase (or decrease) computing capacity as network demand changesMainframes: not scalable; PCs: highly scalable; Server Farms highly scalable19Copyright 2005 John Wiley & Sons, Inc Host-BasedClient-BasedClient-ServerCost of InfrastructureHighMediumLowCost of DevelopmentLowMediumHighScalabilityLowMediumHighChoosing an Architecture20Copyright 2005 John Wiley & Sons, IncApplicationsWorld Wide WebE-mailFile Transfer VideoconferencingInstant Messaging21Copyright 2005 John Wiley & Sons, IncWorld Wide WebTwo central ideas:Hypertext A document containing links to other documentsUniform Resource Locators (URLs)A formal way of identifying links to other documentsInvention 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 NetscapeCERN - Centre Européan pour Rechèrche NucleaireNCSA - National Center for Supercomputing Applications22Copyright 2005 John Wiley & Sons, IncHow the Web WorksHTTP ResponseHTTP RequestClient ComputerServer Computer HTTP - Hypertext Transfer ProtocolMain Web communications protocol:Clicking on a hyperlink or typing a URL into a browser starts a request-response cycleA request-response cycle: include multiple steps since web pages often contain embedded files, such as graphics, each requiring a separate response.23Copyright 2005 John Wiley & Sons, IncHTTP Request MessageRequest 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)requiredoptionaloptional(If the user types in the URL by themselves, then the referring page is blank.)24Copyright 2005 John Wiley & Sons, IncExample of an HTTP RequestGET HTTP/1.1Date: Mon 06 Aug 2001 17:35:46 GMTUser-Agent: Mozilla/6.0Referer: HeaderRequest LineWeb browser (code name for Netscape)Command URL HTTP versionURL that contained the link to the requested URLNote that this HTTP Request message has no “Body” part.GMT – Greenwich Mean Time25Copyright 2005 John Wiley & Sons, IncHTTP Response MessageResponse 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)optionaloptionalrequired26Copyright 2005 John Wiley & Sons, IncAllen R. Dennis Allen R. Dennis Welcome to the home page of Allen R. DennisDate: Mon 06 Aug 2001 17:35:46 GMT Server: NCSA/1.3 Location: http:// www.kelley.indiana.edu/adennis/home.htm Content-type: text/htmlExample of an HTTP ResponseResponse HeaderResponse BodyHTTP/1.1 200 OKResponse StatusAnother example of response status: HTTP/1.1 404 page not found)27Copyright 2005 John Wiley & Sons, IncHTML - Hypertext Markup LanguageA markup language used to format Web pages Also developed at CERN (initially for text files – a subset of SGML)TAGs embedded in HTML documents include information on how to format the fileExtensions to HTML needed to display multimedia filesXML - Extensible Markup Language A new markup language for data28Copyright 2005 John Wiley & Sons, IncElectronic MailHeavily used Internet applicationMuch, much faster than snail mail (regular mail)Extremely inexpensive (compared to $3-$10 per paper mail total avg. 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 call29Copyright 2005 John Wiley & Sons, IncE-mail StandardsSMTP - Simple Mail Transfer ProtocolMain e-mail standard forOriginating user agent and the mail transfer agent Between mail transfer agentsOriginally written to handle only text filesUsually used in two-tier client-server architecturesPost Office Protocol (POP) and Internet Mail Access Protocol (IMAP)Main protocols used between the receiver user agent and mail transfer agentMain difference: with IMAP, messages can be left at the server after downloading them to the clientOther competing standardsCommon Messaging Calls (CMC), X.40030Copyright 2005 John Wiley & Sons, IncTwo-Tier E-mail ArchitectureUser agents (also called e-mail clients)Run on client computers Send e-mail to e-mail serversDownload e-mail from mailboxes on those serversExamples: Eudora, Outlook, Netscape MessengerMail transfer agents (also called mail server)Used by e-mail serversSend e-mail between e-mail serversMaintain individual mailboxes.31Copyright 2005 John Wiley & Sons, IncHow SMTP WorksLANInternetLANwith e-mail client software Client computer Server computer withServer computer withe-mail server softwaree-mail server softwareSMTP packetSMTP packetSMTP packetSMTP packetIMAP or POP packet(“message transfer agent”)an e-mail message is sent as an SMTP packet to the local mail serverreads 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 boxcontacts the mail server which then downloads the message(“user agent”)Client computer32Copyright 2005 John Wiley & Sons, IncHost Based e-mail ArchitecturesAn old method used on UNIX (main-frame) based hostsSimilar to client-server architecture, exceptClient PC replaced by a terminal (or emulator)Sends all keystrokes to the serverDisplay characters received from the serverAll software resides on serverTakes client keystrokes and understand user’s commandsCreates SMTP packets and sends them to next mail server33Copyright 2005 John Wiley & Sons, IncThree-Tier Client-Server Arch.Client computer with Web BrowserServer computer with Web server softwareServer computer with email server softwareperforms the same functions as the mail server in the two-tier examplesends HTTP requests to the Web serversends HTTP responses to the Web clienttranslates the client’s HTTP requests into SMTP packets then send them to the Mail serverBest known example: Web based email (e.g., Hotmail)No need for an email user agent34Copyright 2005 John Wiley & Sons, IncWeb-based e-mailLANLANSMTP packetSMTP packetSMTP packetInternetClient computer withClient computer withServer computer withServer computer withServer computer withServer computer withWeb server softwareWeb server softwareemail server softwaree-mail server softwareIMAP orPOP packetIMAP packetHTTP requestHTTP requestHTTP responseHTTP responseWeb browserWeb browser35Copyright 2005 John Wiley & Sons, IncSMTP MessageHeader(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)36Copyright 2005 John Wiley & Sons, IncSample SMTP MessageFrom: “Alan Dennis;”TO: “Pat Someone” Date: Mon 06 Aug 2001 19:03:02 GMTSubject: Sample NoteMessage-Id: DATA: This is an example of an e-mail messageHeaderBodyNote that this SMTP message has no attachments.Unique ID used to keep track of messages.Text in “ “ are ignored37Copyright 2005 John Wiley & Sons, IncMIMEMultipurpose Internet Mail ExtensionA graphics capable mail transfer agent protocol (to send graphical information in addition to text) SMPT designed for text transfer onlyIncluded as part of an e-mail clientTranslates 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 formatOther Graphics capable mail agent protocols uuencode and binhex38Copyright 2005 John Wiley & Sons, IncListserv Discussion GroupsMailing 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 restrictedParts of listservListserv ProcessorProcess commands ( subscriptions, etc,)Listserv MailerReceive a message and resend it to everyone To subscribeSend an email to Listserv processor (address of the processor is different than the address of mailer)39Copyright 2005 John Wiley & Sons, IncFile Transfer Protocol (FTP)Enables sending and receiving files over the InternetRequires an application program on the client computer and a FTP server program on a server Commonly used today for uploading web pagesMany packages available using FTPWS-FTP (a graphical FTP software)FTP sitesClosed sitesRequires account name and passwordAnonymous sitesAccount name: anonymous; pwd: your email address40Copyright 2005 John Wiley & Sons, IncTelnetAllows one computer to log into other computersRemote login enabling full control of the hostRequires an application program on the client computer and a Telnet server program on a serverClient program emulates a “dumb” terminalMany packages available conforming TelnetEWANRequires account name and passwordAnonymous sitesAccount name: anonymous; pwd: your email address41Copyright 2005 John Wiley & Sons, IncInstant Messaging (IM)A client-server program that allows real-time typed messages to be exchangedClient needs an IM client softwareServer needs an IM server packageSome types allow voice and video packets to be sentLike a telephoneExamples include AOL, MSN, Yahoo and ICQTwo step process:Telling IM server that you are onlineChatting42Copyright 2005 John Wiley & Sons, IncHow Instant Messaging WorksLANLANLANIM packetIM packetIM packetIM packetInternetServer computer withIM client softwaree-mail client softwareClient computer withClient computer withwith IM server softwaresends 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 versaIf 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. 43Copyright 2005 John Wiley & Sons, IncVideoconferencingProvides real time transmission of video and audio signals among two or more locationsAllows people to meet at the same time in different locationsSaving money and time by not having to move people around (conference calling does the same thing)Traditionally involves 2 special purpose rooms with cameras and displaysDesktop videoconferencingLow cost application linking small video cameras and microphones together over the InternetNo need for special roomsExample: Net Meeting s/w on clients communicating through a videoconference server 44Copyright 2005 John Wiley & Sons, IncVideoconferencing StandardsProprietary early systems Common standards in use todayH.320Designed for room-to-room videoconferencing over high-speed phone linesH.323Family of standards designed for desktop videoconferencing and just simple audio conferencing over InternetMPEG-2Designed for faster connections such as LAN or privately owned WANs45Copyright 2005 John Wiley & Sons, IncWebcastingSpecial type of one-directional videoconferencingContent is sent from the server to usersProcessContent created by developer Downloaded as needed by the user Played by a plug-in to a Web browserNo standards for webcasting yetDefacto standards: products by RealNetworks, Microsoft and Apple46Copyright 2005 John Wiley & Sons, IncImplications for ManagementNetwork 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 applicationsDramatic increase in number and type of applicationsRapid growth in amount and type of trafficDifferent implication on network design and managementIncreased operating cost47Copyright 2005 John Wiley & Sons, IncCopyright 2005 John Wiley & Sons, Inc. 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