Applications: World Wide Web(HTTP)

1
Chapter 28

• Applications World Wide Web(HTTP)
• Guoying Yang
• Liangqi Guo
• Song Ye

2
Introduction

• World Wide Web(WWW)
• The primary protocol used to transfer a Web page
  from a server to a Web browser.

3
Importance of The Web

• History
• During the early of the internet, FTP data
  transfers accounted for one third of Internet
  traffic.
• By 1995, Web traffic became the largest
  consumer of Internet backbone bandwidth.
• More people know about and use the Web.
• Most companies have Web sites to de business.

4
Architectural Components

• Web pages the Web consists of a large set of
  documents that are accessible over the Internet.
• Each Web page is classified as a hypermedia
  document.
• Suffix media indicate that a document can
  contain items other than text.
• Prefix hyper a document can contain
  selectable links that refer to other, related
  documents.

5
Architectural Components

• Web browser consists of an application that a
  user invokes to access and display a Web page.
• Web server obtain a copy of the specified page,
  response the clients request.
• HyperText Markup Language(HTML)
• Tags give guidelines for display. Some tags
  come in pairs that apply to all items between the
  pair.
• For example ltcentergt lt/centergt

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Uniform Resource Locators

• Uniform Resource Locator(URL)
• Each Web page is assigned a unique
  name(URL).
• A URL follows http scheme has the following
  form
• http// hostname port / path parameters ?
  query
• port an optional protocol port number .
• path a string that identifies one
  particular document on the server.
• parameters an optional string supplied be
  the client.
• ?query an optional string used when the
  browser send a question.

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Uniform Resource Locators

• The absolute form of a URL
• http//www.cs.purdue.edu/people/comer
• The relative URL
• Communication has been established with a
  specific server.
• Omits the address of the server.
• For example only the string /people/comer/
  is need to specify the document named by the
  absolute URL above.

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An Example Document

• ltHTMLgt
• The author of this text is
• ltA HREF
• http//nas.cl.uh.edu/perkin
  s
• gt TCP/IP lt/Agt
• ltHTMLgt

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Hypertext Transfer Protocol

• What is HTTP?
• The protocol used for communication between a
  browser and a Web server or between intermediate
  machines and Web servers.
• HTTP has following characteristics

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Hypertext Transfer Protocol

• Application Level.
• Request/Response.
• Stateless.
• Bi-Directional Transfer.
• Capability Negotiation.
• Support For Caching.
• Support For Intermediaries.

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HTTP GET Request

• The browser sends a GET request to which a
  server responds by sending the requested item.
• Browser
• Sends GET command followed by a URL and an
  HTTP version number.
• Examples
• GET http//www.cs.purdue.edu/people/comer/
  HTTP/1.1
• GET /people/comer/ HTTP/1.0
• Server
• Responds by sending a copy of the page.

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Some Useful URLs

• http//www.w3.org/Protocols/
• http//www.w3.org/Protocols/rfc2616/rfc2616.html
• http//www.w3.org/Library/Examples/
• http//www.w3.org/Library/User/Applications.html

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Error Messages

• When a Web server receives an illegal request,
  it usually generates error messages in valid
  HTML.
• The browser will display the error message
  like this

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Error Messages Code and Meaning

• 400 Wrong request syntax.
• 401 Authorization required. A list of allowed
  authorization scheme will also be sent.
• 402 No Chargeto field on the request for a paid
  service.
• 403 Forbidden resource
• 404 The server cannot find the URL requested.
• 405 Accessing the resource using a method not
  allowed.
• 406 Resource type incompatible with the client.
• 410 Resource no longer available and no
  forwarding information exist.
• 500 The server has encountered an internal error
  and cannot continue with the request.
• 501 The server does not support the method of a
  legal request.
• 502 Secondary server does not return a valid
  response.
• 503 The service is unavailable, because the
  server is too busy.
• 504 Secondary server takes too long to respond.

15
Persistent Connections And Lengths

• What is persistent Connections?
• Once a client opens a TCP connection to a
  particular server, the client leaves the
  connection in place during multiple request and
  response. When either a client or server is ready
  to close the connection, it informs the other
  side, and the connection is closed.
• Advantage Reduced overhead.
• Disadvantage Need to identify the beginning and
  end of each item sent over the connection.

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Data Length And Program Output

• To allow a TCP connection to persist through
  multiple requests and responses. HTP sends a
  length before each response. Thus, to provide for
  dynamic Web pages, The HTTP standard specifies
  that if the server does not know the length of an
  item, the server can inform the browser(client)
  that it will close the connection after
  transmitting the item.

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Length Encoding and header

• HTTP borrows the format from e-mail,use 822
  format and MIME extension.
• Example KEYWORD information
• content--length 34
• content--Language en
• content--Encoding ascii
• Other header
• Connection close (used when server dont
  know the length )

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Negotiation

• In addition to specifying details about an item
  being sent ,HTTP use header to permit client and
  server to negotiate capabilities.
• Capabilities include connection
• representation
• control
• content
• Two basic type of Negotiation
• Server--driven(server select)
• Agent--driven ( 2 steps )

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ContinueAgent driven

• agent driven 2 steps
• 1. First browser sent request to server to
  ask what is available, the server return a
  list of possibilities
• 2. Browser select one of the possibilities
  and sent a second request to obtain the item
• Advantage browser have full control about the
  choice
• Disadvantageselect one possibility ,sent two
  request

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Accept header

• Accept headerbrowser use this to specify which
  media or representations are acceptable
• Example
• Accept text/html , text/plain q0.5 ,
  text/x-dvi q0.8
• q is preference level
• variety of Accept header
• Accept-Encoding
• Accept-charset
• Accept-Language

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Conditonal request

• HTTP allow a sender to make a request conditional
• Example
• If-Modified-Since Sat,01 Jan 2000 050001
  GMT
• (avoid get item older than Jan 1,2000.)

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Proxy server

• A local server which is configured to cache
  copies of web page of original source .
• Advantage
• 1 . Decrease latency
• 2 . Reduce load of server
• To guarantee correctness ,HTTP includes explicit
  support for proxy server
• 1.How proxy handle each request
• 2. How header should be interpreted
• 3. How browser negotiate with a proxy

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Caching

• The goal of caching is improve efficiency
  reduce both latency and network traffic
• How long should a item be kept in cache?
• HTTP allows a server to control caching in two
  way
• 1. Server specify caching details
• 2. HTTP allow browser to force REVALIDATION

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SUMMARY

• The World wild web consists of hypermedia
  document stored on a set of web server and
  accessed by browser. Each document is assigned a
  URL that uniquely identifies it .
• A browser and server use HTTP to communicate HTTP
  is an application-level protocol with explicit
  support for negotiation, proxy server,caching
  ,and persistent connection.