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Note for Web Technology And Its Application - WTATA by Ashutosh Jaiswal

  • Web Technology And Its Application - WTATA
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www.jntuworld.com 1. INTRODUCTION 1.1. INTERNET PRINCIPLES This page reviews the central concepts of software on the internet. It briefly explains:  TCP/IP  UDP  IP Addresses  domain names  the domain name system  ports  sockets  URL's 1.1. 1. TCP/IP The Internet is the network that connects computers all over the world. It works according to a set of agreed protocols. TCP (Transmission Control Protocol) and IP(Internet Protocol) are the most commonly-used protocols for using the Internet. (But there are others at lower levels.) The combination is simply known as TCP/IP. www.jntuworld.com

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www.jntuworld.com The Internet is a packet switching system. Any message is broken into packets that are transmitted independently across the interment (sometime by different routes). These packets are called datagrams. The route chosen for each datagram depends on the traffic at any point in time. Each datagram has a header of between 20 and 60 bytes, followed by the payload of up to 65,515 bytes of data. The header consists of, amongst other data: 1. the version number of the protocol in use 2. IP address of sender 3. IP address of destination TCP breaks down a message into packets. At the destination, it re-assembles packets into messages. It attaches a checksum to each packet. If the checksum doesn't match the computed checksum at the destination, the packet is re-transmitted. Thus TCP ensures reliable transmission of information. In summary, TCP: 1. provides re-transmission of lost data 2. delivery of data in the correct order 1.1.2. UDP Most applications use TCP. However, an example of a situation in which it is desirable to use a lower-level protocol is the case of audio streaming. If you want to download a sound file, it can take some time, even though it may be compressed. You have to wait (maybe some time) for the complete file to download before it can be played. An alternative is to listen to the sound as it is being downloaded - streaming. One of the most popular technologies is called RealAudio. RealAudi does not use TCP because of its overhead. The sound file is sent in IP packets using the UDP (User Datagram Protocol) instead of TCP. UDP is an unreliable protocol:  it doesn't guarantee that a packet will arrive www.jntuworld.com

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www.jntuworld.com  it doesn't guarantee that packets are in the right order UDP doesn't re-send a packet if it is missing or there is some other error, and it doesn't assemble packets into the correct order. But it is faster than TCP. In this application, losing a few bits of data is better than waiting for the re-transmission of some missing data. The application's major mission is to keep playing the sound without interruption. (In contrast, the main goal of a file transfer program is to transmit the data accurately.) The same mechanism is used with video streaming. UDP is a protocol at the same level as TCP, above the level of IP. 1.1.3. Domain name The domain name is the user-friendly equivalent of an IP address. Used because the numbers in an IP address are hard to remember and use. Also known as a host name. Example: shu.ac.uk Such a name starts with the most local part of the name and is followed by the most general. The whole name space is a tree, whose root has no name. The first level in the tree has com, org, edu, UK, etc. The parts of a domain name don't correspond to the parts of an IP address. Indeed domain names don't always have 4 parts - they can have 2, 5 or whatever. All applications that use an address should work whether an IP address or a domain name is used. In fact, a domain name is converted to an IP address before it is used. www.jntuworld.com

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www.jntuworld.com Exercise: Compare and contrast IP addresses with domain names. 1.1.4. Domain Name System A program, say a Web browser, that has a domain address usually needs to convert it into an IP address before making contact with the server. The domain name system (DNS) provides a mapping between IP addresses and domain names. All this information cannot be all in one place and so it is a distributed database. 1.1.5. Clients, Servers and Peers A network application usually involves a client and a server. Each is a process (an independently running program) running on a (different) computer. A server runs on a host and provides some particular service, e.g. e-mail, access to local Web pages. Thus a Web server is a server. A commonly-used web server program is called Apache. A client runs on a host but needs to connect with a sever on another host to accomplish its task. Usually, different clients are used for different tasks, e.g. Web browsing and e-mail. Thus a Web browser is a client. Some programs are not structured as clients and servers. For example a game, played across the internet by two or more players is a peer to peer relationship. Other examples: chat, internet phone, shared whiteboard. www.jntuworld.com

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