� � � � � Lecture 18: The Internet and the Web Computer Literacy 1h Thursday 4/11/2004 Lecture Overview Topics covered The origin of the Internet The Internet and Network organisation The Web: client-server model The Web: caches, proxies and cookies Peer-to-peer networks The aim is to understand how network organisation affects network applications. The Internet In the 1960s, when computers were operated using punch cards and magnetic tape, they were viewed mainly as number crunchers. Few people saw the potential of computers as communication devices. The U.S. military were nervous because of the cold war; in particular they were nervous that their communication networks could be knocked out by strategic attacks. They funded a research project to establish a small network of computers that could communicate with each other, even if some were destroyed. The result of the project was a computer network called ARPANET . An important principle of the network was that all the computers on the network were equal in their ability to communicate. There was no central authority controlling the flow of information, as any such central component would make the whole network vulnerable to attack. An important step in allowing the network to operate was the invention of the packet-switching model of information transfer, discussed in detail in Lecture 8 (title: Communications 1). You should recognise the acronym ARPANET, and not remember that it stands for Advanced Research Projects Agency NETwork. The ARPANET network was quickly developed into a network that connected many military and university sites. It was disbanded in 1990, but by this time its success had led to the establishment of many similarly connected networks, many LANs (local area networks) and WANs (wide area networks) (discussed in Lecture 8). As these networks grew in number and themselves became connected, the Internet emerged. Network organisation The Internet today is a global network of computer networks. Individual networks may be run by universities, governments, businesses, individual etc. The network of connected networks is known as the Internet. Edinburgh university has a local area network (LAN) called EdLAN. EdLAN is connected
� � � � � to a metropolitan area network (MAN) that serves communication between LANs in the Edinburgh and Stirling area, called EaStMAN. EaStMAN is itself connected to the Joint Academic Network (JANET) which provides high speed connections between all UK higher and further education organisations and research councils. All these networks are being developed to provide faster connections and greater coverage: activity on SuperJANET5 is underway, which will include the Spark initiative in Scotland, that connects schools. The Internet is growing faster than anyone can keep track. True to the spirit of ARPANET, there are no central controlling authorities on the Internet and it is globally self-regulated by volunteers, the IETF (Internet Engineering Task Force), and locally regulated by governments. Some networks have remained independent of the Internet: one is Fidonet, begun in 1984 and linking computers via a phone line based messaging system. Within the Internet, different networks can have different levels of access. Intranets are networks with access restricted to an organisation or group. This can be achieved through securities measures, such as only allowing computers with authorised IP addresses to pass data on the network. To understand how this method to works, and indeed to understand how Internet and network services work, it is helpful to have some terms that describe the structure of networks in general: Bridges connect LANs of the same type. Different LANs can have different packet-switching technologies, and bridges connect compatible networks. (More technically, they connect networks with common network layer addresses in the OSI 7-layer model of communications, see Lecture 14 notes). Gateways connect networks of different type, for instance a LAN and a WAN. Because of the short distances separating nodes of a LAN, data transmission rates are high. In contrast, data transfer rates in the more distant nodes of a WAN are slower because of the larger distances between the nodes. Gateways are devices, sometimes computers, that solve these problems. Routers are devices that decide where packets should go next on their journey in order to reach their destination in the network. This routing decision is most often achieved using software that implements routing protocols. Switches are devices that perform the same role as routers but using dedicated hardware: they are usually faster but less flexible. Repeaters receive signals (e.g. packets in a packet-switching network) and amplify them to ensure successful transmission. A convenient way to establish an intranet is to control the flow of information at the network gateway. An elaboration of the intranet concept is the extranet . Extranets are networks that allow access only to authorised users
� � � outside the organisation. The most common application is in e-commerce: the extranet allows access to selected databases and information for the customers of the business. The WEB The World Wide Web (the Web) is one method of communication on the Internet. The Web uses the client-server model of network applications. In this model, information is stored on a network device called a server. In order to access the information, an application is run on another machine in the network, the client machine. In the case of the Web, Web pages are stored on Web servers, and the applications used to display the pages (browsers) are run on the computer of the user, the client. This organisation is useful, because the specific demands of storing large amounts of data, receiving requests for data, finding and transmitting the data, and possibly performing computationally intensive operations on the data can be handled by dedicated machines. One disadvantage is that if there is a problem with the server, access to the information on the server is interrupted for all users. For the Web to work, it is essential that the Web clients and servers can communicate. Three standards ensure that this happens. They are: HTTP . Hypertext Transfer Protocol. Defines how the Web browser and web page server exchange information. URL. Uniform Resource Locator. Specifies a unique address for each web page. Each URL has three components: the protocol used (e.g. http), the server name, and the path to the file on the server. E.g. in the URL http://www.inf.ed.ac.uk/teaching/courses/cl1, http is the protocol used, www.inf.ed.ac.uk is the name of the server and /teaching/courses/cl1 is the path of the cl1 page (so the file is in the directory courses/, which is in the directory teaching/ on the server). HTML. Hypertext Markup Language. Method of encoding information so that it can be displayed by different devices. The Web was invented by Tim Berners-Lee, who now head the World Wide Web Consortium, who develop and regulate the three standards. The client-server model used by the Web means that it takes the same amount of time and resources to visit a Web page for a second time. An efficient way to make the second visit quicker is to make and store a copy of the page when you visit it . This is the idea behind a Web cache , the place where copies of visited Web pages are kept. HTTP has a set of rules which determine whether a page can be stored in a cache or not. Web caches can be either client-sided or server-sided. With a client-side cache , the copies are stored on the hard drive of the user, or on the computers of the ISP (Internet service provider). This provides faster access
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