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‘Internet & WWW Notes, init I:, ‘Internet;, , jThe Internet is the global system of interconnected computer networks that use the Intemet, Protocol suite (TCP/IP) to link devices worldwide. It is a network of networks that consists of, Private, public, academic, business, and government networks of local to global scope, linked by, 8 broad array of electronic, wireless, and optical networking technologies. The Internet carries a, vast range of information resources and services, such as the inter-linked hypertext documents, jand applications of the World Wide Web (WWW), electronic mail, telephony, and file sharing., , ‘The origins of the Internet date back to research commissioned by the federal government of the, [United States in the 1960s to build robust, fault-tolerant communication with computer, networks. The primary precursor network, the ARPANET, initially served as a backbone for, ‘interconnection of regional academic and military networks in the 1980s, The funding of, ithe National Science Foundation Network as a new backbone in the 1980s, as well as private, funding for other commercial extensions, led to worldwide participation in the development of, |new networking technologies, and the merger of many networks. The linking of commercial, |networks and enterprises by the early 1990s marked the beginning of the transition to the modern, ‘Internet, and generated a sustained exponential growth as generations of institutional, personal,, ‘and mobile computers were connected to the network. Although the Internet was widely used, ‘by academia since the 1980s, the commercialization incorporated its services and technologies, linto virtually every aspect of modem life., , Most traditional communications media, including telephony, radio, television, paper mail and, , newspapers are reshaped, redefined, or even bypassed by the Internet, giving birth to new, ‘services such as email, Internet telephony, Internet television, online music, digital newspapers,, jand video streaming websites. Newspaper, book, and other print publishing are adapting, to website technology, or are reshaped into blogging, web feeds and online news aggregators., The Internet has enabled and accelerated new forms of personal interactions through instant, \messaging, Internet forums, and social networking. Online shopping has grown exponentially, both for major retailers and small businesses and entrepreneurs, as it enables firms to extend their, “brick and mortar" presence to serve a larger market or even sell goods and services entirely, lonline. Business-to-business and financial services on the Intemet affect supply chains across, lentire industries., , ‘The Internet has no centralized governance in either technological implementation or policies for, laccess and usage; cach constituent network sets its own policies." Only the overreaching, ‘definitions of the two principal name spaces in the Internet, the Internet Protocol address (IP, laddress) space and the Domain Name System (DNS), are directed by a maintainer organization,, jthe Internet Corporation for Assigned Names and Numbers (ICANN). The technical, junderpinning and standardization of the core protocols is an activity of the Internet Engineering, jTask Force (IETF), a non-profit organization of loosely affiliated international participants that, janyone may associate with by contributing technical expertise. In November 2006, the internet, , was included on USA Today's list of New Seven Wonders., , |, , |, , |, , Page 1 of 1S

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|, |, \, |, \History;, , ‘Research into packet switching, one of the fundamental Internet technologies started in the early, , 1960s in the work of Paul Baran, and packet switched networks such as the NPL, ‘network by Donald Davies, ARPANET, Tymnet, the Merit Network, Telenet,, and CYCLADES, were developed in the late 1960s and 1970s using a variety of protocols. The, jARPANET project led to the development of protocols for internetworking, by which multiple, separate networks could be joined into a network of networks. ARPANET development began, with two nctwork nodes which were interconnected between the Network Measurement Center, at the University of California, Los Angeles (UCLA) Henry Samucli School of Engineering and, Applied Science directed by Leonard Kleinrock, and the NLS system at SRI International (SRI), by Douglas Engelbart in Menlo Park, California, on 29 October 1969. The third site was the, \Culler-Fried Interactive Mathematics Center at the University of Califomia, Santa Barbara,, ‘followed by the University of Utah Graphics Department, In an early sign of future growth,, fifteen sites were connected to the young ARPANET by the end of 1971, These early years were, documented in the 1972 film Computer Networks: The Heralds of Resource Sharing, , ‘Early international collaborations on the ARPANET were rare. European developers were, \concemed with developing the X.25 networks. Notable exceptions were the Norwegian Seismic, Array (NORSAR) in June 1973, followed in 1973 by Sweden with satellite links to, the Tanum Earth Station and Peter T. Kirstein’s research group in the United Kingdom, initially, at the Institute of Computer Science, University of London and later at University College, London. In December 1974, RFC 675 (Specification of Internet Transmission Control Program),, ‘by Vinton Cerf, Yogen Dalal, and Carl Sunshine, used the term internet as a shorthand, ‘for internetworking and later RFCs repeated this use. Access to the ARPANET was expanded in, ‘1981 when the National Science Foundation(NSF) funded the Computer Science, Network (CSNET). In 1982, the Internet Protocol Suite(TCP/IP) was standardized, which, permitted worldwide proliferation of interconnected networks., , ‘ and could access Usenet newsgroups and FTP files), the first HTTP server software(later known, 'as CERN httpd), the first web server, and the first Web pages that described the project itself. In, {1991 the Commercial Internet eXchange was founded, allowing PSInet to communicate with the, other commercial networks CERFnet and Alternet, Since 1995 the Internet has tremendously, ‘impacted culture and commerce, including the rise of near instant communication by, email, instant messaging, telephony (Voice over Internet Protocol or VoIP), two-way interactive, ‘video calls, and the World Wide Web! with its discussion forums, blogs, social networking,, ‘and online shopping sites. Increasing amounts of data are transmitted at higher and higher speeds, lover fiber optic networks operating at 1-Gbit/s, 10-Gbiv’s, or more., , ‘Client-Server:, , 1, , (The client-server model is a distributed application structure that partitions tasks or workloads, {between the providers of a resource or service, called servers, and service requesters,, called clients. Often clients and servers communicate over a computer network on separate, ‘hardware, but both client and server may reside in the same system. A server host runs one or, — server programs which share their resources with clients. A client does not share any of its, , Page 2 0f 15, , |, 1, \

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|, |, , Tesources, but requests a server's content or service function. Clients therefore initiate, communication sessions with servers which await incoming requests, Examples of computer, applications that use the client-server model are Email, network printing, and the World Wide, Web The client-server characteristic describes the telationship of cooperating programs in an, application. The server component provides a function or Service to one or many clients, which, initiate requests for such services, Servers are classified by the services they provide. For, example, a web server serves web pages and a file server serves computer files. A shared, resource may be any of the server computer's software and electronic components,, ‘from programs and data to processors and storage devices. The sharing of resources of a server, constitutes a service,, , Whether a computer is a client, a server, or both, is determined by the nature of the application, Ithat requires the service functions. For example, a single computer can run web server and file, Server software at the same time to serve different data to clients making different kinds of, Fequests. Client software can also communicate with server software within the same, \computer. Communication between servers, such as to synchronize data, is sometimes, (Called inter-server or server-to-server communication., , ‘In general, a service is an abstraction of computer resources and a client does not have to, be concemed with how the server performs while fulfilling the request and delivering the, Tesponse. The client only has to understand the Fesponse based on the well-known application, Protocol, i.e, the content and the formatting of the data for the fequested service, , Clients and servers exchange messages in a request-response messaging pattem. The client, sends a request, and the server retums a response. This exchange of messages is an example, of inter-process communication. To communicate, the computers must have a common language,, and they must follow rules so that both the client and the server know what to expect, The, tanguage and rules of communication are defined in a communications protocol, All client-server, Protocols operate in the application layer. The application layer protocol defines the basic, ‘pattems of the dialogue. To formalize the data exchange even further, the server may implement, an application programming interface (API). The API is an abstraction layer for accessing a, Service. By restricting communication to a specific content format, it facilitates parsing. By, abstracting access, it facilitates cross-platform data exchange., , \A server may receive requests from many distinct clients in a short period of time. A computer, ian only perform a limited number of tasks at any moment, and relies on a scheduling system to, Prioritize incoming requests from clients to accommodate them. To prevent abuse and, ‘maximize availability, server software may limit the availability to clients. Denial of service, ‘attacks are designed to exploit a server's obligation to process requests by overloading it with, ‘excessive request rates,, , i:, , | i, , | Page 3 of 15

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TCP\P:, , The Internet protocol suite is the conceptual model and set of communications protocols used, ‘on the Internet and similar computer networks. It is commonly known as TCP/IP because the, foundational protocols in the suite are the Transmission Control Protocol (TCP) and the Internet, Protocol (IP). It is occasionally known as the Department of Defense (DoD) model because the, development of the networking method was funded by the United States Department of, Defense through DARPA., , The Internet protocol suite provides end-to-end data communication specifying how data should, be packetized, addressed, transmitted, routed, and received. This functionality is organized into, four abstraction layers, which classify all related protocols according to the scope of networking, involved. From lowest to highest, the layers are the link layer, containing communication, ‘methods for data that remains within a single network segment (link), the intemct layer,, ‘providing internetworking between independent networks; the transport layer, handling host-to{host communication; and the application layer, providing process-to-process data exchange for, applications., , ‘Technical standards specifying the Internet protocol suite and many of its constituent protocols, jare maintained by the Internet Engineering Task Force (IETF), The Internet protocol suite, , predates the OS] model, a more comprehensive reference framework for general networking, systems., , The Internet protocol suite resulted from research and development conducted by the Defense, Advanced Research Projects Agency (DARPA) in the late 1960s. After initiating the, pioneering ARPANET in 1969, DARPA started work on a number of other data transmission, technologies. In 1972, Robert E. Kahn joined the DARPA Information Processing Technology, \Office, where he worked on both satellite packet networks and ground-based radio packet, networks, and recognized the value of being able to communicate across both. In the spring of, ‘1973, Vinton Cerf, the developer of the existing ARPANET Network Control Program (NCP), , 1, joined Kahn to work on open-architecture interconnection models with the goal of, \designing the next protocol generation for the ARPANET., , By the summer of 1973, Kahn and Cerf had worked out a fundamental reformulation, in which, the differences between local network protocols were hidden by using a common internetwork, protocol, and, instead of the network being responsible for reliability, as in the ARPANET, this, function was delegated to the hosts, Cerf credits Hubert Zimmermann and Louis Pouzin,, designer of the CYCLADESnetwork, with important influences on this design. The protocol was, ‘implemented as the Transmission Control Program, first published in 1974., , {nitially, the TCP managed both datagram transmissions and routing, but as the protocol grew,, other researchers recommended a division of functionality into protocol layers. Advocates, included Jonathan Postel of the University of Southern California's Information Sciences, Institute, who edited the Request for Comments (RFCs), the technical and strategic document, iSeries that has both documented and catalyzed Internet development. Postel stated, "We are, ‘screwing up in our design of Internet protocols by violating the principle of, layering.” Encapsulation of different mechanisms was intended to create an environment where, the upper layers could access only what was needed from the lower layers. A monolithic design, , Page 4of 15

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would be inflexible and lead to scalability issues. The Transmission Control Program was split, into two distinct protocols, the Transmission Control Protocol and the Internet Protocol., , The design of the network included the Fecognition that it should provide only the functions of, efficiently transmitting and routing traffic between end nodes and that all other intelligence, should be located at the edge of the network, in the end nodes. This design is known as the endto-end principle. Using this design, it became possible to connect almost any network to the, ARPANET, irrespective of the local characteristics, thereby solving Kahn's initial, intemetworking problem. One popular expression is that TCP/IP, the eventual product of Cerf, and Kahn's work, can run over “two tin cans and a string.” Years later, as a joke, the IP over, Avian Carriers formal protocol specification was created and successfully tested., , A computer cailed a router is provided with an interface to each network. It forwards network, packets back and forth between them. Originally a router was called gateway, but the term was, changed to avoid confusion with other types of gateways, , An early architectural document, RFC 1122, emphasizes architectural principles over, , layering. RFC 1122, titled Host Requirements, is structured in paragraphs referring to layers, but, , the document refers to many other architectural principles not emphasizing layering. It loosely, , defines a four-layer model, with the layers having names, not numbers, as follows:, Application layer, , The application layer is the scope within which applications create user data and, communicate this data to other applications on another or the same host. The, applications, or processes, make use of the services provided by the underlying, lower, layers, especially the Transport Layer which provides reliable or unreliable pipes to other, processes. The communications partners are characterized by the application architecture,, such as the client-server model and peer-to-peer networking. This is the layer in which, all higher level protocols, such as SMTP, FTP, SSH, HTTP, operate. Processes are, ’ addressed via ports which essentially represent services., Transport layer, The transport layer performs host-to-host communications on either the same or different, hosts and on either the local network or remote networks separated by routers. It provides, a channel for the communication needs of applications. UDP is the basic transport layer, protocol, providing an unreliable datagram service. The Transmission Control Protocol, provides flow-control, connection establishment, and reliable transmission of data., Internet layer, The internet layer exchanges datagrams across network boundaries. It provides a uniform, networking interface that hides the actual topology (layout) of the underlying network, connections. It is therefore also referred to as the layer that establishes internetworking., Indeed, it defines and establishes the Internet. This layer defines the addressing and, routing structures used for the TCP/IP protocol suite. The primary protocol in this scope, is the Internet Protocol, which defines IP addresses, Its function in routing is to transport, datagrams to the next IP router that has the connectivity to a network closer to the final, data destination., Link layer, The link layer defines the networking methods within the scope of the local network link, on which hosts communicate without intervening routers. This layer includes the, , Page S of 15