Introduction Networks Lecture 15: Internet The Internet and the - PDF document

Introduction • Networks Lecture 15: – Internet The Internet and the Web – Organisation of networks – Peer-2-peer networks Computer Literacy 1 • Applications Thursday 27/10/2005 – The web – File-sharing ARPANET ARPANET • Aim – Military communications network – Robust to damage • Result – Packet-switching – All computers equally able to communicate – The internet! University network Computers as connectors • University LAN • Different networks can use different packet-switching methods – EdLAN • Edinburgh & Stirling MAN – EaStMAN • Bridge • UK education WAN – Connects networks of the same type – JANET • Gateway – SuperJANET5 – Connects networks of different type 1

Computers as connectors • Different networks can use different packet-switching methods • Routers & Switches – Decide path of packets • Repeaters – Boost, resend packets Intranet and Extranet • Intranet – Network with access restricted to members of an organisation • Extranet – Network outside intranet – Access restricted to authorised users – E.g. business clients Client-server model Client-server model: the Web • Server machine • The Web: method of communication using internet – Stores data – Or has access to data • Client is the Web browser • Client machine • Server stores Web pages – Runs application • Communication requires – Application requests data from server – Protocol • Server finds data and sends it to the – Addresses client – Shared language 2

The Web URL: Web page address • 3 standards • Protocol method – HTTP: data exchange protocol – Usually http, also https (secure http) – URL: address – HTML: shared language • Server name • Browser – www.inf.ed.ac.uk – Sends out URL through HTTP – Displays HTML data • Web page file path • Server – /teaching/courses/cl1 – Receives URLs through HTTP – Finds pages and sends back Web Cache Client-side • Copies of visited Web pages put in a cache • Client-side Web cache • Advantages – On client computer – Faster – Or ISP server – Frees Internet bandwidth • Example client-side Web application • Implementation – Java applets – HTTP: methods for expressing whether a page can be copied – Cache size limit on your computer Server-side Web proxy • Cache on Web server • Connections to a page are usually direct – Recently visited pages put in cache • Web proxy: Mediating connection to the – Reduces time to retrieve frequently visited page pages • Used to manage access • Speed of Web services not just – From a network to the Internet connection speed! – From outside the network to the intranet • Example server-side Web application • Can be a cache: faster access – CGI scripts 3

Cookies Peer-to-peer networks • Web page stores information on your • Network where each network node is a client and a server computer – No dedicated servers • Used to – P2P overlaid on Internet structure – Remember login details • Cost – Shopping baskets – Use internet infrastructure – Observe your use of web site • Reliability • Security problem? – Not affected by server downtime Bandwidth P2P Scalability P2P Routing • As the number of users and files grow • Napster – Routing becomes time consuming – Centralised file lists • But – Client server searching – P2P file distribution – Total bandwidth increases • Gnutella, FastTrack – Not limited by access to servers – Kazaa P2P network “rewards” high – Distributed file lists bandwidth users: supernodes – P2P searching + file distribution P2P Security P2P applications • Have to be careful what files P2P • Payment schemes (PayPal) software can access www.paypal.com/html/gartner-020102.html • VOIP, e.g. Skype www.skype.com • P2P software could observe your Web movements (“spyware”) • Spam detection razor.sourceforge.net/ • Freenet, GNUnet, AntsP2P… • Distributed computing: GIMPS prime number search www.mersenne.org/prime – Protect user and file identity – Slower • Instant Messenger – Safe? For who? • etc 4

Key Points • Network organisation – Bridges, Gateways, Routers, Repeaters – Client-server model – Peer-2-peer networks • Applications – How organisation affects implementation – Advantages and disadvantages: scalability, speed, security 5