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LESSON 2

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					Internet Fundamentals - II
The World Wide Web (www)
   The Web presents information through multimedia
    formats: graphics, sound, animation, and video.
   The Web uses several tools to provide a visual layout:
         Hypertext links
         Browser software
         Code structure
   The Web resembles an electronic library – each location or
    site is like a book.
   These books are created using Hypertext Markup
    Language (HTML).
   These materials, along with interactive objects such as
    Java, JavaScript, and VBScript adds functionality to web
    pages.
Internet Fundamentals

          What is the World Wide Web?
                – A collection of electronic documents stored on
                  servers around the world that are running on
                  the Internet
         Who develops standards for the WWW?
                – World Wide Web Consortium (W3C)
SERVICES OF INTERNET –E-mail,
FTP, Telnet, WWW
 Internet mail is (e-mail or electronic mail), much faster as
  compared to normal postal mail. One can also send software
  and certain forms of compressed digital image as an
  attachment.
 Long-distance computing was an original inspiration for
  development of ARPANET and does still provide a very
  useful service on Internet.
 File transfer service allows Internet users to access remote
  machines and retrieve programs, data or text.
E-Mail (Electronic Mail)
 E-mail or Electronic mail is a paperless method of sending
    messages, notes or letters from one person to another or even
    many people at the same time via Internet. E-mail is very fast
    compared to the normal post.
   Although e-mail is faster and cheaper, it has many of the
    components of regular mail. It allows you to compose note, get
    the address of the recipient and send it.
   Once the mail is received and read, it can be forwarded, replied.
   One can even store it for later use, or delete.
   In a-mail even the sender can request for delivery receipt and read
    receipt from the recipient.
Features of E-mail:
 One-to-one or one-to-many communications
 Instant communications
 Physical presence of recipient is not required
 Most inexpensive mail services, 24-hours a day and seven
  days a week
 Encourages informal communications
Components of an E-mail Address
 E-mail is based upon the concept of a recipient address.
 The email address provides all of the information required to
    get a message to the recipient from anywhere in the world.
   Consider the e-mail ID john@hotmail.com
   In the example above, “john” is the local part, which is the
    name of a mailbox on the destination computer, where finally
    the mail will be delivered.
   Hotmail is the mail server where the mailbox “john” exist,
   .com is the type of organization on net, which is hosting the
    mail server.
There are six main categories;
 com Commercial institutions or organization
 edu Educational institutions
 gov Government site
 mil Military site
 net Gateways and administrative hosts
 org Private organizations
Gopher
 Gopher is a protocol system, which in advance of the World
  Wide Web, allowed server based text files to be hierarchically
  organised and easily viewed by end users who accessed the
  server using Gopher applications on remote computers.
 Initially Gopher browsers could only display text-based files
  before developments such as HyperGopher, which were able
  to handle simple graphic formats.
 they were never used on a widespread basis as by this time
  the World Wide Web and its Hypertext Transfer Protocol
  (HTTP) were gaining in popularity, and had similar and more
  extensive functions.
Origin and need of Gopher
 The Gopher protocol and original Gopher viewer application were
  first developed at the University of Minnesota in the early 1990’s.
 The efforts were done to make use of the Internet to enable the
  simple sharing of documents with people who could be located in
  institutions on opposite sides of the country or even the world, so
  that similar / related pages would be easily accessible.
 The value of the Gopher system was enhanced by the development
  of two systems known as Veronica and Jughead which allowed a
  user to search across resources stored in Gopher file hierarchies
  on a global basis.
What happened to it?
 By the mid 1990s the World Wide Web was growing at a huge
  rate, and given that the Web’s Hypertext Transfer Protocol
  (HTTP) and its browser Mosaic could match the functions of
  the Gopher protocol and additionally offer added functions
  such as hyper linking from within HTML files which brought
  together related pages more efficiently than Gopher, there
  was no longer a compelling reason to choose the Gopher
  system.
What happened to it?
 Most of the files and databases that had been available on
  Gopher were converted into HTTP compatible formats and
  made available on the Web, though for the interested it is still
  possible to access the Gopher root directory at the University
  of Minnesota and a few other places, but the vast majority of
  the other Gopher servers on the Net have since gone offline.
FTP
 FTP stands for File Transfer Protocol. It is the standard
  Internet protocol for transferring files from one computer to
  another.
 FTP is part of the TCP/IP protocol suite. TCP/IP is the basic
  protocol that runs the whole Internet. Whether you are
  checking your email, visiting a web site or downloading files,
  you are using TCP/IP.
 Its sole function is to move a file from a server to a client
  (download) or from a client to a server (upload).
How does FTP work?
 FTP requires two computers, one running an FTP server, the
  other running an FTP client.
 The exchange is initiated by the client which logs in under an
  accepted user name and password. Once this occurs, a
  session is opened and stays open until closed by either the
  client or the server, or until it times out.
 While the session is open, the client may execute numerous
  FTP commands on the server. These include commands to
  change directories, list files, get files and put files.
     ftp: the file transfer protocol
                            FTP              file transfer
                                      FTP                             FTP
                            user      client                          server
                            interface
            user
            at host                     local file                             remote file
                                        system                                 system


       transfer file to/from remote host
       client/server model
            client: side that initiates transfer (either to/from remote)
            server: remote host
       ftp: RFC 959
       ftp server: port 21


15   2: Application Layer
     FTP – Process Model
     Client System                               Server System



         Data               Control                 Control           Data
         Transfer           Process                 Process           Transfer




       Operating System                            Operating System




                                      TCP/IP
                                      Internet




16   2: Application Layer
         ftp: separate control, data connections
                                If you are to design FTP, which protocol
                                would you use? Why?
                                •TCP
                                •UDP
                                                                  control connection
      ftp client contacts ftp server at port 21,                 port 21
        specifying TCP (or UDP?) as transport
        protocol
                                                                  data connection
      two parallel connections opened:                  FTP      port 20              FTP
          control: exchange commands, responses         client                        server
           between client, server.
           “out of band control”
          data: file data to/from server
             One data connection per file!




17       2: Application Layer
          ftp commands, responses
                                                  Sample return codes
Sample commands:
                                                   status code and phrase (as in http)
sent as ASCII text over control channel
                                                   331 Username OK,
 USER username
                                                    password required
 PASS password
                                                   125 data connection
 LIST return list of file in current directory     already open; transfer
 RETR filename retrieves (gets) file
                                                    starting
                                                   425 Can’t open data
 STOR filename stores (puts) file onto
                                                    connection
     remote host
                                                   452 Error writing file
 ABOR abort previous FTP command
 PORT n1,n2,n3,n4,n5,n6
     Client IP address(n1,n2,n3,n4),
     Port (n5*256 + n6)
 QUIT logoff from server



18        2: Application Layer
         HTTP vs. FTP
                                 HTTP          FTP


      # of connections              1           2


          persistent?           Yes (1.1)   data(no),
                                no (1.0),   control (yes)

      stateful or stateless?    Stateless   stateful


     in-band or out-of-band?    In-band
                                            out-of-band




19       2: Application Layer
     Anonymous FTP
       To provide access to public files.
       Supported by a server that allows anyone to login and
        use FTP to transfer files.
       To use anonymous FTP –
         Login name - anonymous
         Password – guest or complete e-mail address




20   2: Application Layer

				
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