Basic WWW Technologies

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Basic WWW Technologies

• 2.1 Web Documents.2.2 Resource Identifiers URI,
  URL, and URN.2.3 Protocols.

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Quiz

• What happens when a url entered in browser?

• DNS Look upwww.pku.edu.cn -gt 162.105.129.12
• TCP connection
• Send HTTP requestGET /
• Receive HTML document
• Render HTML document

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What Is the World Wide Web?

• The world wide web (web) is a network of
  information resources. The web relies on three
  mechanisms to make these resources readily
  available to the widest possible audience
• 1. A uniform naming scheme for locating resources
  on the web (e.g., URIs).
• 2. Protocols, for access to named resources over
  the web (e.g., HTTP).
• 3. Hypertext, for easy navigation among resources
  (e.g., HTML).

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Internet vs. Web

• Internet
• Internet is a more general term
• Includes physical aspect of underlying networks
  and mechanisms such as email, FTP, HTTP
• Web
• Associated with information stored on the
  Internet
• Refers to a broader class of networks, i.e. Web
  of English Literature
• Both Internet and web are networks

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Essential Components of WWW

• Resources
• Conceptual mappings to concrete or abstract
  entities, which do not change in the short term
• ex ICS website (web pages and other kinds of
  files)
• Resource identifiers (hyperlinks)
• Strings of characters represent generalized
  addresses that may contain instructions for
  accessing the identified resource
• http//www.ics.uci.edu is used to identify the
  ICS homepage
• Transfer protocols
• Conventions that regulate the communication
  between a browser (web user agent) and a server

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Standard Generalized Markup Language (SGML)

• Based on GML (generalized markup language),
  developed by IBM in the 1960s
• An international standard (ISO 88791986) defines
  how descriptive markup should be embedded in a
  document
• Gave birth to the extensible markup language
  (XML), W3C recommendation in 1998

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SGML Components

• SGML documents have three parts
• Declaration specifies which characters and
  delimiters may appear in the application
• DTD/ style sheet defines the syntax of markup
  constructs
• Document instance actual text (with the tag) of
  the documents
• More info could be found http//www.W3.Org/markup
  /SGML

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DTD Example One

• lt!ELEMENT UL - - (LI)gt
• ELEMENT is a keyword that introduces a new
  element type unordered list (UL)
• The two hyphens indicate that both the start tag
  ltULgt and the end tag lt/ULgt for this element type
  are required
• Any text between the two tags is treated as a
  list item (LI)

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DTD Example Two

• lt!ELEMENT IMG - O EMPTYgt
• The element type being declared is IMG
• The hyphen and the following "O" indicate that
  the end tag can be omitted
• Together with the content model "EMPTY", this is
  strengthened to the rule that the end tag must be
  omitted. (no closing tag)

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HTML Background

• HTML was originally developed by Tim Berners-Lee
  while at CERN, and popularized by the Mosaic
  browser developed at NCSA.
• The Web depends on Web page authors and vendors
  sharing the same conventions for HTML. This has
  motivated joint work on specifications for HTML.
• HTML standards are organized by W3C
  http//www.w3.org/MarkUp/

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HTML Functionalities

• HTML gives authors the means to
• Publish online documents with headings, text,
  tables, lists, photos, etc
• Include spread-sheets, video clips, sound clips,
  and other applications directly in their
  documents
• Link information via hypertext links, at the
  click of a button
• Design forms for conducting transactions with
  remote services, for use in searching for
  information, making reservations, ordering
  products, etc

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HTML Versions

• HTML 4.01 is a revision of the HTML 4.0
  Recommendation first released on 18th December
  1997.
• HTML 4.01 Specification
• http//www.w3.org/TR/1999/REC-html401-1
  9991224/html40.txt
• HTML 4.0 was first released as a W3C
  Recommendation on 18 December 1997
• HTML 3.2 was W3C's first Recommendation for HTML
  which represented the consensus on HTML features
  for 1996
• HTML 2.0 (RFC 1866) was developed by the IETF's
  HTML Working Group, which set the standard for
  core HTML features based upon current practice in
  1994.

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Sample Webpage
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Sample Webpage HTML Structure

• ltHTMLgt
• ltHEADgt
• ltTITLEgtThe title of the webpagelt/TITLEgt
  lt/HEADgt
• ltBODYgt ltPgtBody of the webpage
• lt/BODYgt
• lt/HTMLgt

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HTML Structure

• An HTML document is divided into a head section
  (here, between ltHEADgt and lt/HEADgt) and a body
  (here, between ltBODYgt and lt/BODYgt)
• The title of the document appears in the head
  (along with other information about the document)
• The content of the document appears in the body.
  The body in this example contains just one
  paragraph, marked up with ltPgt

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HTML Hyperlink

• lta href"relations/alumni"gtalumnilt/agt
• A link is a connection from one Web resource to
  another
• It has two ends, called anchors, and a direction
• Starts at the "source" anchor and points to the
  "destination" anchor, which may be any Web
  resource (e.g., an image, a video clip, a sound
  bite, a program, an HTML document)

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Resource Identifiers

• URI Uniform Resource Identifiers
• URL Uniform Resource Locators
• URN Uniform Resource Names

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Introduction to URIs

• Every resource available on the Web has an
  address that may be encoded by a URI
• URIs typically consist of three pieces
• The naming scheme of the mechanism used to access
  the resource. (HTTP, FTP)
• The name of the machine hosting the resource
• The name of the resource itself, given as a path

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URI Example

• http//www.w3.org/TR
• There is a document available via the HTTP
  protocol
• Residing on the machines hosting www.w3.org
• Accessible via the path "/TR"

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Protocols

• Describe how messages are encoded and exchanged
• Different Layering Architectures
• ISO OSI 7-Layer Architecture
• TCP/IP 4-Layer Architecture

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ISO OSI Layering Architecture
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ISOs Design Principles

• A layer should be created where a different level
  of abstraction is needed
• Each layer should perform a well-defined function
• The layer boundaries should be chosen to minimize
  information flow across the interfaces
• The number of layers should be large enough that
  distinct functions need not be thrown together in
  the same layer, and small enough that the
  architecture does not become unwieldy

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TCP/IP Layering Architecture
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TCP/IP Layering Architecture

• A simplified model, provides the end-to-end
  reliable connection
• The network layer
• Hosts drop packages into this layer, layer routes
  towards destination
• Only promise Try my best
• The transport layer
• Reliable byte-oriented stream

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Hypertext Transfer Protocol (HTTP)

• A connection-oriented protocol (TCP) used to
  carry WWW traffic between a browser and a server
• One of the transport layer protocol supported by
  Internet
• HTTP communication is established via a TCP
  connection and server port 80

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GET Method in HTTP
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Domain Name System

• DNS (domain name service) mapping from domain
  names to IP address
• IPv4
• IPv4 was initially deployed January 1st. 1983 and
  is still the most commonly used version.
• 32 bit address, a string of 4 decimal numbers
  separated by dot, range from 0.0.0.0 to
  255.255.255.255.
• IPv6
• Revision of IPv4 with 128 bit address

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Top Level Domains (TLD)

• Top level domain names, .com, .edu, .gov and ISO
  3166 country codes
• There are three types of top-level domains
• Generic domains were created for use by the
  Internet public
• Country code domains were created to be used by
  individual country
• The .arpa domain Address and Routing Parameter
  Area domain is designated to be used exclusively
  for Internet-infrastructure purposes

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Hierarchical architecture

• www.pku.edu.cn

cn
edu

pku
tsinghua

www
bbs

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Summary

• Web
• URL
• HTML
• TCP/IP
• HTTP
• DNS