[Slide]Application Layer

Copyright 2011 John Wiley & Sons, Inc2 - *Business Data Communications and Networking 11th Edition Jerry Fitzgerald and Alan Dennis

John Wiley & Sons, Inc Dwayne Whitten, D.B.AMays Business SchoolTexas A&M University

Copyright 2011 John Wiley & Sons, Inc

Copyright 2011 John Wiley & Sons, Inc2 - *Chapter 2

Application Layer


Copyright 2011 John Wiley & Sons, Inc2 - *Chapter 2 Outline2.1 - Introduction2.2 - Application Architectures Host-Based, Client-Based, Client-Server and Peer-to-Peer ArchitecturesChoosing Architectures2.3 - World Wide WebHow the Web WorksInside an HTTP Request & HTTP Response2.4 - Electronic Mail How E-Mail Works and Inside an SMTP PacketAttachments in MIME2.5 - Other ApplicationsTelnet, Instant Messaging, and Videoconferencing2.6 Implications for Management


Copyright 2011 John Wiley & Sons, Inc2 - *1.1 IntroductionApplication LayerNetwork LayerTransport LayerApplications(e.g., email, web, word processing)Application architectureThe way in which the functions of the application layer software are spread among the clients and servers on the network Functions of Application Layer 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 such as word processors, spreadsheets Presentation logic - Presentation of info to user & acceptance of user commands


Copyright 2011 John Wiley & Sons, Inc2 - *2.2 Application 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 server


Copyright 2011 John Wiley & Sons, Inc2 - *Host-based Architecture ProblemsHost becoming a bottleneckAll processing done by the host, which can severely limit network performanceHost upgrades typically expensive and lumpyAvailable upgrades require large scale and often costly jumps in processing and memoryNetwork demand grows more incrementally than does the host capacityMay see poor fit (too much or too little) between host performance and network demand


Copyright 2011 John Wiley & Sons, Inc2 - *Client-Based Architecture ProblemsData traffic 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 processedOften the large file sizes moving across the LAN can yield a poor result in network performance


Copyright 2011 John Wiley & Sons, Inc2 - *Client-Server ArchitecturesExample: Using a Web browser to obtain web pages uses logic balanced between the client and server Application logic can be on client and/or server


Copyright 2011 John Wiley & Sons, Inc2 - *Client-Server ArchitecturesAdvantagesMore efficient because of distributed processingAllow hardware and software from different vendors to be used togetherEnables cloud computingDisadvantagesDifficulty in getting software from different vendors to work together smoothly May require Middleware, a third category of software


Copyright 2011 John Wiley & Sons, Inc2 - *MiddlewareExamples of standards for Middleware: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)


Copyright 2011 John Wiley & Sons, Inc2 - *Multi-tier ArchitecturesInvolve more than two computers in distributing application program logic 2-tier architecture Uses clients and servers in a balance, very popular approach in simple LANs3-tier architecture3 sets of computers involvedN-tier architecture More than three sets of computers used, more typical across complex organizationsAllows load balancing across servers


Copyright 2011 John Wiley & Sons, Inc2 - *Multi-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 necessitates more data exchanges Difficult to program and test due to increased complexity


Copyright 2011 John Wiley & Sons, Inc2 - *Thin and Thick ClientsClassification depends on how much of the application logic resides on the client or serverThin client: Little or no application logic on clientBecoming 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)Thick client:All or most of the application logic resides on the client


Peer to Peer ArchitectureCopyright 2011 John Wiley & Sons, Inc2 - *All computers can serve as a client and a serverIncreased popularity in the last decade due to the rise of P2P services such as NapsterAdvantages:Data can be stored anywhere on the networkVery resilient to failureDisadvantages:Finding dataSecurity


Copyright 2011 John Wiley & Sons, Inc2 - *Criteria for Choosing ArchitectureInfrastructure CostCost of servers, clients, and circuitsMainframes: very expensive; terminals, PCs: inexpensiveDevelopment CostMainly cost of software developmentSoftware: expensive to develop; off-the-shelf software: inexpensiveScalability Ability to increase (or decrease) in computing capacity as network demand changesMainframes: not scalable; PCs: highly scalable


Copyright 2011 John Wiley & Sons, Inc2 - *2.3 World Wide WebWeb began with two innovative 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 - Conseil Europen pour la Rechrche Nuclaire (Berners-Lee, T. (2000) Weaving the Web. New York: HarperCollins. P. 4)NCSA - National Center for Supercomputing Applications


Copyright 2011 John Wiley & Sons, Inc2 - *HTTP 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, such as from a form)requiredoptionaloptional(If the user types in the URL by themselves, then the referring page is blank.)


Copyright 2011 John Wiley & Sons, Inc2 - *HTTP 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)optionaloptionalrequired


Copyright 2011 John Wiley & Sons, Inc2 - *HTML - Hypertext Markup LanguageA language used to create Web pages Also developed at CERN (initially for text files)Tags are embedded in HTML documents include information on how to format the fileExtensions to HTML needed to format multimedia filesXML - Extensible Markup Language A new markup language becoming popular


Copyright 2011 John Wiley & Sons, Inc2 - *E-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.400


Copyright 2011 John Wiley & Sons, Inc2 - *Two-Tier E-mail ArchitectureUser agent is another word for an e-mail client applicationRun on client computers Send e-mail to e-mail serversDownload e-mail from mailboxes on those serversExamples: Eudora, Outlook, Netscape MessengerMail transfer agent is another word for the mail server applicationUsed by e-mail serversSend e-mail between e-mail serversMaintain individual mailboxes.


Copyright 2011 John Wiley & Sons, Inc2 - *Host Based e-mail ArchitecturesAn old method used on UNIX based hostsSimilar to client-server architecture, exceptClient PC replaced by a terminal (or terminal emulator)Sends all keystrokes to the serverDisplay characters received from the serverAll software resides on the serverTakes client keystrokes and understand users commandsCreates SMTP packets and sends them to next mail serverUseful when traveling in locations with poor internet facilities


Copyright 2011 John Wiley & Sons, Inc2 - *MIMEMultipurpose Internet Mail ExtensionA graphics capable mail transfer agent protocol (to send graphical information in addition to text) SMTP was designed years ago for text transfer onlyMIME software is included 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) Receivers e-mail client then translates the MIME attachment from text back into graphical formatMIME example


Copyright 2011 John Wiley & Sons, Inc2 - *TelnetAllows one computer to log into another computerRemote login enabling full control of the hostRequires an application program on the client computer and a Telnet server program on the serverClient program emulates a dumb terminal off the serverMost popular Telnet software is PuTTYOpen sourceUses SSH encryption for securityRequires account name and passwordAnonymous sites similar to FTP approach


Copyright 2011 John Wiley & Sons, Inc2 - *Instant 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 and ICQTwo step process:Telling IM server that you are onlineChatting


Copyright 2011 John Wiley & Sons, Inc2 - *VideoconferencingProvides real time transmission of video and audio signals between two or more locationsAllows people to meet at the same time in different locationsSaves money and time by not having to move people around Typically involves matched 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 software on clients communicating through a common videoconference server


Copyright 2011 John Wiley & Sons, Inc2 - *Videoconferencing 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 WANs


Copyright 2011 John Wiley & Sons, Inc2 - *WebcastingSpecial type of uni-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


Copyright 2011 John Wiley & Sons, Inc2 - *2.6 Implications for ManagementNetwork may be used to provide a worry-free environment for applicationsNetwork is the critical infrastructure over which the wide variety of strategic applications enable an organization to compete in its environmentThe applications running on the network have the potential for changing the organizationDramatic increase in number and type of applicationsRapid growth in amount and type of networking traffic over timeDifferent implication on network design and managementIncreased operating cost for the network function


Copyright 2011 John Wiley & Sons, Inc2 - *Copyright 2011 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.


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[Slide]Application Layer