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.However,IP masquerading is implemented in certain routers and is available for severalsoftware platforms.7.6 The Network ComputerAn alternative approach to solving the problem of a mobile workforce is not to tryand cope with the addressing issues of users' machines being moved around theorganization, but to provide fixed workstations around the organization that allowany user to access their own data and applications from anywhere.This is theprinciple of the network computer.In general terms, a network computer is usually a microprocessor-based systemwith:Local memory (NVRAM)434 TCP/IP Tutorial and Technical Overview No disk (neither hard drive nor diskette)Some form of network connectionTerminal emulation capabilitiesIP-based protocols, such as: TCP FTP Telnet NFSA Web browserA Java Virtual Machine (see 8.5, Java on page 449)In short, a network computer can be thought of as a smart terminal.All of the software required by a network computer, including the operating system,is stored on a server in the network and downloaded on demand when the networkcomputer is powered on or when the user activates new functions.The network computer stores only sufficient code in its own local NVRAM (the bootmonitor program) to be able to make a BOOTP or DHCP connection to a server, inorder to be able to download its operating system kernel.Download of theremainder of the operating system is then carried out using TFTP or NFS,depending upon implementation and configuration.Typically, having powered on a network computer, a user is presented with a logonpanel.On completion of a successful logon, the user is presented with apersonalized desktop with his or her own set of application icons.Selection of anyof those icons causes an application to be downloaded from a server.(Severaldifferent servers may be accessed.)The network computer relies on TCP/IP protocols for all of its connectivity.As wellas using NFS or TFTP to download applications, the applications themselves, ifthey require communication, will use TCP/IP.SNA emulators, for example, useTN3270 or TN5250 as appropriate.Although the network computer is not, typically, based on an Intel processor, it isable to access Windows applications on a Windows Terminal Server or CitrixSystems Metaframe, by loading an ICA (independent console architecture) client.In this case, processing of the Windows application is carried out on the NT serveritself and only screen changes are delivered to the client.The network computermay run applications remotely on a UNIX system, in a similar manner, bydownloading an X Window server.Having all the network computer software stored on a server allows for centralizedmanagement and simplified updates of operating system and application code.IBM's implementation of the network computer, the IBM Network Station, may bebooted, and managed from, Windows NT, AIX, OS/400, OS/390 and OS/2 servers.For further information on the IBM Network Station, please refer to the followingIBM Redbooks:SG24-4954 S/390 IBM Network Station - Getting StartedChapter 7.Dynamic IP, Mobile IP and Network Computers 435 SG24-2153 AS/400 IBM Network Station - Getting StartedSG24-2127 IBM Network Station and the Windows NT PlatformSG24-2016 IBM Network Station - RS/6000 NotebookYou may also refer to the IBM Network Station home page at:http://www.pc.ibm.com/networkstation/7.7 ReferencesThe following RFCs contain detailed information on dynamic and mobile IP:RFC 951 Bootstrap ProtocolRFC 1542 Clarifications and Extensions for the Bootstrap ProtocolRFC 2131 Dynamic Host Configuration ProtocolRFC 2132 DHCP Options and BOOTP Vendor ExtensionsRFC 1995 Incremental Zone Transfer in DNSRFC 1996 Prompt Notification of Zone TransferRFC 2065 DNS Security ExtensionsRFC 2136 Dynamic Updates in the Domain Name SystemRFC 2137 Secure DNS Dynamic UpdatesRFC 2002 IP Mobility Support436 TCP/IP Tutorial and Technical Overview Chapter 8.Internet Protocols and ApplicationsThis chapter introduces some of the protocols and applications that have made thetask of using the Internet both easier and very popular over the past couple ofyears.In fact, World Wide Web traffic, which is mostly HTTP, has outrun FTP asusing the most bandwidth of all protocols across the Internet.Modern computeroperating systems provide Web browser applications by default, some even provideWeb servers, thus making it ever easier for end users and businesses to exploreand exploit the vast capabilities of worldwide networked computing.8.1 The World Wide Web (WWW)The World Wide Web is a global hypertext system that was initially developed in1989 by Tim Berners Lee at the European Laboratory for Particle Physics, CERN inSwitzerland to facilitate an easy way of sharing and editing research documentsamong a geographically dispersed group of scientists.In 1993 the Web started to grow rapidly which was mainly due to the NationalCenter for Supercomputing Applications (NCSA) developing a Web browserprogram called Mosaic, an X Windows-based application.This application providedthe first graphical user interface to the Web and made browsing more convenient.Today there are Web browsers and servers available for nearly all platforms.Youcan get them either from an FTP site for free or buy a licensed copy.The rapidgrowth in popularity of the Web is due to the flexible way people can navigatethrough worldwide resources in the Internet and retrieve them.The number of Web servers is also increasing rapidly and the traffic over port 80,which is the well-known port for HTTP Web servers, on the NSF backbone has hada phenomenal rate of growth too.The NSFNET, however, was converted back to aprivate research network in 1995; therefore comprehensive statistics of backbonetraffic are not as easily available anymore, if they are at all.8.1.1 Web BrowsersGenerally, a browser is referred to as an application that provides access to a Webserver.Depending on the implementation, browser capabilities and hencestructures may vary.A Web browser, at a minimum, consists of an HTMLinterpreter and HTTP client which is used to access HTML Web pages.Besidesthis basic requirement, many browsers also support FTP, NNTP, e-mail (POP andSMTP clients), among other features, with an easy-to-manage graphical interface.Figure 251 on page 438 illustrates a basic Web browser structure.© Copyright IBM Corp.1989, 1998 437 GraphicalUserInterfaceDriverHTML OptionalInterpreter InterpreterControllerHTTPTorClientrDispatcheranspoFTPrClienttLayeMailrClient3376E\3376F8O7Figure 251.Structure of a Web BrowserAs with many other Internet facilities, the Web uses a client/server processingmodel.The Web browser is the client component.Examples of Web browsersinclude Mosaic, Netscape Navigator, Microsoft Internet Explorer, or Sun HotJavabrowser.Web browsers are responsible for formatting and displaying information,interacting with the user, and invoking external functions, such as telnet, or externalviewers for data types that Web browsers do not directly support.Web browsershave become the "universal client" for the GUI workstation environment, in muchthe same way that the ability to emulate popular terminals such as the DEC VT100or IBM 3270 allows connectivity and access to character-based applications on awide variety of computers.Web browsers are widely available for all popular GUIworkstation platforms and are inexpensive
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