Transcript World Wide Web

The Internet
• Computer network: collection of autonomous
computers that can exchange information.
• Collection of interconnected networks is called
internet.
– LAN: (local area network)
– MAN: (metropolitan area network)
– WAN: (wide area network)
• Intranet: a local network.
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The Internet
• The Internet is the largest internet in the world.
These include commercial (.com or .co),
university (.ac or .edu) and other research
networks (.org, .net) and military (.mil)
networks and span many different physical
networks around the world with various
protocols, mainly the Internet Protocol.
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The Internet
History
• 1969: ARPAnet (US government)
• 1973: First email message
• 1980: CSNET (NSF National Science Foundation)
• 1980: BITNET (IBM mainframes)
• 1986: join CSNET + ARPAnet
• 1987: join CSNET + BITNET
• 1991: Tim Berners-Lee (CERN) announced WWW
(Gopher is released)
• 1993: Public awareness of Internet ( Mosaic is
released)
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The Internet
• Applications
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The World Wide Web (HTTP)
Electronic mail
Usenet (newsgroups)
Remote login (Telnet)
File transfer (FTP)
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Client-server
• Client- server
A common form of distributed system in which
software is split between server tasks and client
tasks. A client sends requests to a server,
according to some protocol, asking for
information or action, and the server responds.
• Protocol
A set of formal rules describing how to transmit
data, especially across a network.
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Client-server
Advantages
• Centralised storage
• Server is a powerful
machine, efficient
processing
Disadvantages
• “Eggs in one basket”
• Increasing load
Thin client
vs.
Reduce server load and network communication
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TCP/IP
• Internet Protocol (IP)
A connectionless, non-guaranteed best-effort
packet switching protocol.
– Packet: unit of data sent across a network.
– Connectionless: Packets sent between two hosts
may take different routes.
Application (HTTP)
Host-to-Host (TCP)
Internet (IP)
Physical
•Transmission Control Protocol (TCP)
Reliable connection-oriented (fragment, no error)
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World Wide Web
• An architectural framework for accessing
linked document spread out over the Internet.
• 1989 at CERN (European Center for Nuclear
Research) – Tim Berners-Lee.
• Mosaic (1994) – Netscape (1995)
• CERN + M.I.T  W3C (1994)
• Languages used (HTML, Java, ….)
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Client-server Web
The Client side
Network
Client
connection
•Browser
•Helper application
•Plug in
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World Wide Web
•Documents (web pages)
•Links (hypertext)
Client-server Web
The server side
Network
connection
Server
•Server name
•IP address
•Web document
•Listen to port 80
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Client-server Web
• User type the URL
• Browser ask DNS for IP
address (host name)
• Client connect to port 80 of IP address
•GET (file name)
•Client release connection
•Browser display the file
•Browser get and display images
Host Name
IP address
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•Receive request
•Send the file
DNS
Uniform Resource Locator
• A standard way of specifying the location of an
object on the Internet. URLs are the form of
address used on the World-Wide Web.
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http://www.w3.org/default.html
http://www.w3.org/default.html#Introduction
ftp://wuarchive.wustl.edu/mirrors/msdos/graphics/gifkit.zip
mailto:[email protected]
news.uni-mannheim.de
telnet://dra.com
• Access Protocol://server-name/path-name/filename#fragment identifier
• Mailbox@location
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Domain Name System(DNS)
• A general-purpose distributed, replicated, data query
service used on Internet for translating hostnames into
Internet addresses
• Hostname is an ASCII string, e.g. “java.sun.com"
which, consists of a local part (java) and a domain
name (sun.com)
• IP address is a 32-bit host address defined by the
Internet Protocol (IP). It is usually represented in
dotted decimal notation (e.g. 128.121.4.5). The address
can be split into a network number (or network
address) and a host number unique to each host on the
network and sometimes also a subnet address
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Domain Name System(DNS)
• The browser requests the IP address of a server
from the local DNS
• DNS contains resources records (server name, IP
address, type) for each server registered with it
• The domain name space hierarchy of the WWW
Com
Org
UK
co
ac
Edu
Jo
com
Leeds umist
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edu
just
uop
HTTP
• Hypertext Transfer Protocol
– A client-server, request/response protocol used
on the WWW to exchange HTML documents
– An application-level protocol for distributed,
collaborative, hypermedia information systems
– Communication usually takes place over
TCP/IP connections
– Client request (GET)+ file name+ protocol
version+ Request Header Fields
– Server respond (MIME header + data)
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MIME
• Multipurpose Internet Mail Extensions (MIME)
– A standard for multi-part, multimedia electronic
mail messages and World-Wide Web hypertext
documents on the Internet
– MIME provides the ability to transfer non-textual
data, such as graphics and audio
– Contains description and type of the data transferred
text/html
image/jpeg
video/mpeg
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Electronic Mail
• Messages automatically passed from one
computer user to another, often through
computer networks and/or via modems over
telephone lines
• Message:
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Envelope (destination address, priority, security)
Header (From, Date, Subject)
Body (message text)
Signature
Attachment (MIME)
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Electronic Mail
• Functions of an email reader (GUI to interact
with the email system)
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Composition
Transfer (forward, reply, reply to ALL)
Display messages
Report the delivery
Manipulate messages (Mark, Folders, follow up)
• Originally implemented as FTP
• Uses number of protocols: SMTP, POP3, IMAP
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Email Protocols
• SMTP: handles outgoing email between client
and server.
• POP3 (Post Office Protocol): handles incoming
email.
– Append new emails to mailbox (text file)
– Retrieve emails from the mailbox to read on client
• IMAP (Internet Mail Access Protocol)
– Read, organize and handle emails on the server
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SMTP
• Simple Mail Transfer Protocol
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Send: mailbox_name@server_name
TCP connection to the sender server
Server reply “ready message”
Client sends “sender name & recipient name”
If recipient exist => sever sends go ahead message
Email daemon accept message & copy to mailbox
•Email Daemon
•Listen to port 25
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File Transfer Protocol (FTP)
• A client-server protocol which allows a user on
one computer to transfer files to and from
another computer over a TCP/IP network
• User interface and command is based on Unix
and not user friendly
• You need FTP server, username and password
• From the Start menu, Choose Run and Type:
ftp rohan.sdsu.edu
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User (rohan.sdsu.edu:(none)): anonymous
331 Guest login ok, send your complete e-mail address as password.
Password:
230- Welcome to the San Diego State University anonymous ftp archive.
230- Local time in San Diego, California is Fri Feb 9 11:40:57 2001.
230- You are user 2 on this system, out of a possible 100
230- This server can automatically tar a directory and it's contents.
230- Simply add the appropriate sufix to the requested directory (.tar)
230- The tar'd directory can also be gziped by adding .gz (.tar.gz)
230- Some ftp clients have problems with these messages. Shut off verbose
230- messages by entering a "-" at the beggining of your password.
230 Guest login ok, access restrictions apply.
ftp> ls
200 PORT command successful.
150 Opening ASCII mode data connection for file list.
FTP.faq
ls-lR.Z
226 Transfer complete.
•18 bytes received in 0.00 seconds
(18000.00(601211)
Kbytes/sec)
Internet Programming