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E-Commerce is taking over the traditional commerce practices. It is of special concern for the IT students. Following are the key points of these Lecture Slides : Uniform Resource Locators, Contain Pointers, Mechanisms, Locating Pages, Solution, Resource Locator, Worldwide Name, Effectively Serves, Protocol, Machine
Typology: Slides
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pointers to other pages
, we will
see how those pointers are implemented • When the web was first created, it was apparent that havingone page point to another required
mechanisms for naming
and locating pages
. In particular there were 3 questions that had to be answered before a selected page could be displayed:^ –^ What is the page called?^ –^ Where is the page located?^ –^ How can the page be accessed?
-^ Many sites have certain shortcuts for file names built in.For example,
-^ Thus the author’s home page can be reached at^ http://www.cs.vu.nl/~ast/
-^ To make a piece of text clickable the page writer must provide2 items of information:^ –^
The clickable text to be displayed, and – The URL of the page to go to if the text is selected
-^ When the text is selected, the browser looks up the hostname using DNS. Now armed with the host’s IP address, thebrowser then establishes a TCP connection to the host. Overthat connection it sends the file name using the specifiedprotocol. Next, back comes the page.
Name^
Used for^
Example
http^
Hypertext^
http://www.cs.vu.nl/~ast/
ftp^
File Transfer Protocol
ftp://ftp.cs.vu.nl/pub
file^
Local file^
file:///usr/Suzanne/prog.c
news^
News group
news:comp.os.minix
news^
News article
News:[email protected]
gopher^
Gopher^
gopher://gopher.tc.umn.edu/11/Libraries
mailto^
Sending email
mailto:[email protected]
telnet^
Remote login
telnet://www.w3.org:
-^ The basic idea of MIME is to define encoding rules for non-ASCII messages. MIME defines 5 message headers:^ Header
Meaning
MIME-Version
Identifies the MIME version
Content-Description
Human readable string telling what is the message
Content-ID
Unique identifier
Content-Transfer-Encoding
How the body is wrapped for the transmission
Content-Type
Nature of the message
Header^
Subtype
Meaning
Text^
PlainRichtext
Unformatted textText including simple formatting
Image^
GifJpeg
Still picture in GIF formatStill picture in JPEG format
Audio^
Basic^
Audible sound
Video^
Mpeg^
Movie in MPEG format
Application
Octet-streamPostscript
An uninterpreted byte sequenceA printable document in PostScript
Message
Rfc822PartialExternal-body
A MIME RFC 822 messageMessage has been split for transmissionMessage must be fetched over the net
Multipart
MixedAlternativeParallelDigest
Independent partsSame message in different formatsParts must be viewed simultaneouslyEach part is a complete RFC 822 message
-^ A request is just a GET line, naming the page desired and the HTTPprotocol version:
GET /hypertext/WWW/TheProject.html HTTP/1.
-^ The response is just the raw page, headers, and MIME information •^ For example, because HTTP is an ASCII protocol, it is easy for aperson at aterminal (opposed to a browser) to direcly talk to Web servers. All that is aneeded is a TCP connection to port 80 on the server. The simplest way toget such connection is the Telnet program:
Client: Telnet www.w3.org 80Trying 18.23.0.23Connected to www.w3.orgClient: GET /hypertext/WWW/TheProject.html HTTP/1.1Server: HTTP/1.1 200 Document followsServer: MIME-Version: 1.0Server: Server: CERN/3.0Server: Content-Type: text/htmlServer: Content-Length: 8247Server: The World Wide Web Consortium (W3C) Server: …
HTML – HyperText Markup Language HTML
is a^
markup language
, a language for
describing
how documents are to be formatted
The term “markup” comes from the old dayswhen copyeditors acutally marked up documentsto tell the printer (in those days a human being)which fonts to use, and so on. Markup languages thus contain
explicit
commands for formatting
. For example, in HTML,
means start boldface mode, and
means leave boldface mode.
-^ HTTP and HTML are
constantly evolving
. When Mosaic was
the only browser, the language it interpreted, HTML 1.0,was de facto standard. • When new browsers came along, there was a need for aformal Internet standard, so the HTML 2.0 standard wasproduced. Next, HTML 3.0 was created as a research effortto add many new features to HTML 2.0, including tables,toolbars, mathematical formulas, advanced style sheets(for defining page layout and the meaning of symbols), etc.
^ A proper Web page consists of a head andbody enclosed by and
tags
(formatting commands), although mostbrowsers do not complain if these tags aremissing. The head is bracketed by tags, and the body is bracketed by tags The commands inside the tags are called directives
. Most HTML tags have this format, that is, to mark the beginningof something and to mark its end.
