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Web pages

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Title: Web pages

1
Web pages

2
ARPANET

Initial idea was by Defense Advanced Research
Project Agency (DARPA) project in late 1960s for
a national defense network
began a project to see whether several computers,
widely separated geographically, could be linked
together to enable users at a terminal on one
system to access the resources on another
computer.
Initial military concept provide access to
computers if some communications lines are
destroyed by building a network where data
communications traffic could dynamically adapt to
changing conditions.
data communications -sending messages reliably
from one computer to another- was the major
obstacle.
The initial ARPANET began in 1970 as a three-node
network linking BBN in Cambridge, Massachusetts,
with UCLA and SRI in California using 56 kilobit
lines.
Sites added until several hundred by mid-1970s

3
ARPANET communications

Communication between two computers was handled
via messages. A message was broken down into
fixed-length strings called packets, and the
packets were sent from computer to computer until
the original message was reassembled at the
receiving node.
To ensure that messages destined for another
computer arrived reliably, a formal communication
model -called a protocol -was developed. For the
ARPANET, this developed as the Transmission
Control Program/ Internet Protocol (TCP/IP).
TCP/IP was a low-level communication mechanism
that simply determined that a sequence of bytes
destined for a specific computer arrived there
uncorrupted. It was generally too complex for
users to use directly for accessing a computer.

4
User protocols

Telnet is a protocol that makes the sending
computer -the computer the user is actually
working on -behave like a terminal connected to
the distant computer. - user is connected to a
client computer, which acts like a terminal, and
the terminal program is communicating using the
telnet protocol to a distant host computer, which
is providing the server program.
SMTP is Simple Mail Transport Protocol. This
provides the basic e-mail (electronic mail) that
has become so ubiquitous today
FTP is File Transfer Protocol. One would invoke
the FTP client on a local machine, log onto the
distant server machine using the FTP protocol,
and then retrieve the desired documents from the
distant machine or send documents from the user's
machine to the distant machine.

5
Weaknesses in FTP

one had to know explicitly which machine to
access to retrieve the desired data.
One also had to have access to the files of that
machine to retrieve the information. The
anonymous login partially solved that.
One had to know exactly where on the file system
the desired information was.
Despite these weaknesses, FTP was the standard
file transmission mechanism for many years until
the web changed all that.

6
Birth of the Internet

In the mid-1980s, ARPA decided to stop supporting
the ARPANET. As a research activity, the concept
had been proved, and ARPA was not in the business
of providing what was becoming a commercial
service.
The U.S. National Science Foundation (NSF) took
over the backbone network in the United States
-the set of high-speed telephone lines that
provided the basic TCP/IP communications traffic
between host computers as a way to link
universities together.
The name of the network gradually evolved into
the Internet. NSF support stopped.
Attached to this backbone, local networks (a
state, a university, a large company) were added
until the Internet became this amorphous
collection of computers all continually
chattering to one another.
Commercial providers, now called Internet Service
Providers (ISP) established links to the Internet
so that individuals on their home computers could
use a modem to dial into their local ISP to be on
the Internet.

7
The World Wide Web

By the late 1980s, widespread interest to easily
transfer files. FTP was a cumbersome process.
Systems like gopher, archie, veronica developed
Physicists -principally Tim Berners-Lee at CERN
in Geneva -desired a mechanism to access and
transfer documents by computer that was simpler
than the standard FTP server.
They developed the concept of a semantic
description language. One server program would
display a document, and a client program, called
a browser, would read and understand the
displayed document. The power of their system was
that the displayed document contained pointers to
other documents called hypertext.
An earlier version of hypertext was Apple
Computer's HyperCard product for the Macintosh,
but the real power of the CERN development was to
allow hypertext links to documents that existed
on other computers connected to the Internet.

8
HTTP

The protocol developed was the HyperText
Transport Protocol (HTTP). Http an addition to
TELNET, FTP, and SMTP protocols discussed
earlier.
Release of the first MOSAIC browser in 1993 led
to rapid growth of the web.
Each pointer became known as a Uniform Resource
Locator (URL). Document location was reduced to
invoking a Web browser on your local machine,
typing in a URL for the document you wanted to
access,
connecting to a Web server on the distant machine
that contained the location of the typed in URL,
displaying the document obeying the HTTP
protocol.
HTML language based upon SGML - to be explained
later.

9
Web navigation
10
Prentice Hall example of navigation

1. The user types in the URL for the home page.
This URL consists of a Domain name
(www.cs.umd.edu) and a file on that
machine(users/mvz/pzbook).
2. The Web browser sends the domain name to one
of several special Internet machines called
Domain Name Servers (DNS). The DNS returns the
Internet Protocol address of the desired web
page.
3. The web browser sends the file name to the Web
server at IP address 128.8.128.80. A HTTP Daemon
(HTTPD) program on this machine is the main
interface between a web server and the Internet.
4. The Web server appends the name index.html
because the given file was a directory and not a
file.
5. The contents of the file are sent back to the
Web browser and displayed to the user.
6. If the user now clicks on the URL for
Prentice-Hall that appears on the Web page
(www.prenticehall.com), the process is repeated
and the Prentice-Hall server at IP address
63.69.110.94 is accessed and the appropriate Web
page is displayed.

11
Portals

To make navigation easier, certain Web sites
are now known as portals -entrance sites to the
WWW.
These sites have programs known as search
engines. A search engine is a query processor in
which you enter a question. The result of that
query is a list of WWW locations that answer the
question.
Search engines often operate as Web crawlers.
Beginning at one location, the Web crawler
follows all links on that Web page to find other
Web pages.

12
SGML

Structured General Markup Language is basis of
SGML
an unstructured sequence of characters
within the text can be SGML elements. The
semantics of elements are unspecified, but their
syntax is given.
elements are bracketed by a start-tag and an
end-tag notation.
ltzorkgt I am a zork lt/zorkgt
identifies I am a zork as the contents of the
zork element.
A report in SGML
ltreportgt
lttitle text lt/titlegt
ltauthor text lt/authorgt
ltabstract text lt/abstractgt
ltbody text lt/bodygt
lt/reportgt
SGML handles semantic content, not presentation