CITS1231 Web Technologies. Client, Server, the Internet, and the Web

Transcription

1 CITS1231 Web Technologies Client, Server, the Internet, and the Web

2 Topic Outline How does the Internet work Client Server Architecture Connections Communications Protocols Addressing Routing One of the important Internet Applications: WWW 2

3 What are the main components here? Client Request Internet Returned Web Pages 3

4 What does a client do? Once wired, the user accesses the Web via software, called a browser (e.g. Firefox or Internet Explorer). Browsers locate and display information from the Web. Communication is by an agreed transmission language or protocol, eg. HTTP (HyperText Transfer Protocol). The user requests a Web page through the browser, which communicates this to the server. The browser waits for the Web page to be delivered, typically a text file containing HTML instructions. The intricate graphics and formatting results from the browser rendering that page in the format defined in the file. 4

5 What does a server do? The server s job is somewhat easier. The server is software running on a computer, and it responds to client requests for Web pages. The Web pages exist on its local file system. The server retrieves and then transmits the files to the client. 5

6 What does the cloud do? A network is a structure linking computers together for the purpose of sharing resources such as printers and files Users typically access a network through a computer called a host or node A computer that makes a service available to a network is called a server A computer or other device that requests services from a server is called a client One of the most common network structures is the clientserver architecture 6

7 The Largest Inter-connected network the Internet If the computers that make up a network are close together (within a single department or building), then the network is referred to as a local area network (LAN). A network that covers a wide area, such as several buildings or cities, is called a wide area network (WAN). The largest WAN in existence is the Internet. The Internet was called ARPANET, created by the Advanced Research Projects Agency (ARPA) and the U.S. Department of Defense for scientific and military communications. In 1969, it consists of four network nodes, connected by a phone line. 7

8 Physical Structure of the Internet Today the Internet has grown to include hundreds of millions of interconnected computers, cell phones, PDAs, televisions, and networks The physical structure of the Internet uses fiber-optic cables, satellites, phone lines, and other telecommunications media 8

9 Structure of the Internet 9

10 In summary, the Intenet is A network connects computers so they can communicate, exchange information, and share resources. The Internet is an infrastructure, in particular a global computer network, supporting data transmission. The Internet is a network of interconnected networks. If part of its infrastructure is destroyed, data can still flow (in principle) through the remaining networks. The Internet uses high-speed data lines (backbones) to carry data. Smaller networks connect to the backbone, enabling any user on any network to exchange data with any other user. 10

11 How the Internet works TCP/IP Addressing Schemes Domains and Sub-domains Routing Traffic Across the Internet 11

12 How the Internet works - Protocols Network communication is made possible only if computers speak a common language. The rules and procedures for controlling timing and data format are the protocols and they, signal another computer requesting communication. (client) identify the requesting computer. (server) transmit messages in blocks. (server) retransmit if messages fail to arrive. (server) detect errors and recovers. (client) signal transmission is complete. (client) terminate the connection. (server) 12

13 How the Internet Works TCP/IP Every computer and network on the Internet uses the same protocols - the Transmission Control Protocol/Internet Protocol, or TCP/IP. No matter what type of computer system you connect to the Internet, if it uses TCP/IP, it can exchange data with any other type of computer. TCP/IP was developed to tolerate unreliable sub-networks and the protocol guarantees proper transmission of data, since the physical network can t. For transmission not needing guarantees (even unreliable networks are very reliable) one can use User Datagram Protocol (UDP). Data transmitted by UDP arrive faster, with none of the error detection or correction overheads that are in TCP/IP. 13

14 TCP 23,578 12,333 14,132 17, IP IP IP IP 14

15 Router Corrupt TCP

16 How the Internet works - Addressing In order to communicate across the Internet, a computer must have a unique address. Every computer on the Internet has a unique numeric identifier, called an Internet Protocol (IP) address. Each IP address has four parts each part a number between 0 and 255. An IP address can have up to 12 digits and might look like: Our departmental computer (the mail server) has the IP address, , and no other machine in the world has this IP number. Otherwise you would never get your . Since we do not think in terms of long integers, host machines can have a symbolic name(s). For our machine, the symbolic name is mailhost.csse.uwa.edu.au, consisting of the host name (mailhost) followed by the domain name (csse.uwa.edu.au). 16

17 How the Internet works - Domains So in addition to an IP address, most Internet hosts or servers have a domain name address, using words. A domain name identifies the type of institution that owns the computer. An Internet server owned by IBM might have the domain name ibm.com. The domain name is itself made up of name levels so that, au is Australia, edu covers educational sites (within Australia), uwa is The University of Western Australia. Some enterprises have multiple servers, and identify them with subdomains, such as products.ibm.com. 17

18 Internet Domains 18

19 How the Internet works Domain Name Server As far as the Internet is concerned the symbolic machine names are eye candy for human consumption. IP addresses are necessary for computer communication. The IP addresses are provided by Domain Name Server (DNS) computers that map symbolic names to their IP. Computers on a network are designated DNS machines and they are responsible for providing the IP mapping and for the upkeep of the database as new machines and IP are added to the Internet. 19

20 How the Internet works - Alias A host machine must have one, single and unique IP address, but it can have any number of symbolic names. So the machine www is also known as and which are redirected to As an exercise visit

21 Routing Traffic Across the Internet Most computers connect to the Internet via a smaller network that is connected to the Internet backbone. The Internet includes thousands of host computers (servers), which provide data and services as requested by client systems. When you use the Internet, your client requests data from a host system. The request and data are broken into packets and travel across multiple networks before being reassembled at their destination. 21

22 22

23 An Example Say I have a 300 Kb file to transmit to Sydney. The software sending that file first splits it into reasonable size packets, say 15Kb each. Each packet has a header containing essential information like, destination IP address, source IP address, size of the transmission and packet position in the original file. But what path through the network does it take to get to its destination? 23

24 Dynamic routing For robust networks, a transmission protocol must find new routes to a destination as preferred routes fail. This is achieved by dynamic routing, where the routes are selected at the time of transmission, after considering current network conditions. Dynamic routing requires a network architecture devoid of critical sites, whose failure will bring down the entire network. That is, the network cannot be hierarchical. 24

25 Dynamic routing continued The Internet was designed on a lattice or graph, where there are a large number of widely distributed paths. The hosts performing routing duties are called routers, of which there are thousands on the Internet. Back to our example 25 The message packets need not travel in order, or follow the same path. The packets are sent to a router, which based on the destination address decides where to send the packets (usually to another router). At every point in time, the current router decides where to send the packets next. The packets are transmitted with the intermediate destinations assigned as they go.

26 Example Continued Note that if some packets are sent to a particular router, it may be possible that the subsequent packets are sent to a different router, if the original router becomes unavailable. Eventually all packets arrive at their destination, possibly out of order because they have taken different journeys. If packets fail to arrive or arrive corrupted, the destination host transmits a message to the original host to re-send the particular packets. Once all packets are gathered the original message can be reconstructed from the ordering contained in the packet headers. 26

27 Popular applications of the Internet Many users confuse the Internet with applications that work over the Internet. There are many such applications but the six listed below are amongst the most popular. News Telnet File Transfer Protocol (FTP) Internet Relay Chat (IRC) The World Wide Web 27

28 The History of the Word Wide Web Timothy Berners-Lee and other researchers at the CERN nuclear research facility near Geneva, Switzerland laid the foundations for the World Wide Web, or the Web, in 1989 They developed a system of interconnected hypertext documents that allowed their users to easily navigate from one topic to another Hypertext is a method of organizing information that gives the reader control over the order in which the information is presented 28

29 Hypertext Documents When you read a book, you follow a linear progression, reading one page after another With hypertext, you progress through pages in whatever way is best suited to you and your objectives Hypertext lets you skip from one topic to another 29

30 Linear versus hypertext documents 30

31 Hypertext Documents The key to hypertext is the use of hyperlinks (or links) which are the elements in a hypertext document that allow you to jump from one topic to another A link may point to another section of the same document, or to another document entirely A link can open a document on your computer, or through the Internet, a document on a computer anywhere in the world An entire collection of linked documents is referred to as a Web site The hypertext documents within a Web site are known as Web pages Individual pages can contain text, audio, video, and even programs that can be run remotely 31

32 In Summary, the World Wide Web is A distributed hypertext system Defined by a set of common communication protocols Accessible from a variety of platforms Resources on the Web about WWW and its protocols:

33 Pondering Moment What do I do when I surf the Web? If I were to create a website, what would be the purpose? provide a service sell a product present information on a topic make an announcement inform people about yourself create a forum that addresses a certain issue deliver news on a particular subject 33