Hypertext Transfer Protocol HTTP

1
Hypertext Transfer Protocol (HTTP)

• HTTP is an application-level protocol for
  document retrieval used by the World-Wide Web.
• A client requests a document to be retrieved.
• A server provides the requested document.
• A document is identified by a URL (Uniform
  Resource Locator).
• HTTP is simple and versatile
• generic -- it can be used for various kinds of
  data.
• object-oriented -- it supports different kinds of
  commands.
• stateless -- the server does not need to worry
  about the state of the client.
• For the latest information, visit
  http//www.w3.org.

2
Examples of HTTP URLs

• http//www.engr.orst.edu
• The server retrieves file index.html in the
  root Web directory on host www.engr.orst.edu.
• http//www.wngr.orst.edu/minoura
• The server retrieves file index.html in
  directory minoura/public_html on host
  www.engr.orst.edu.
• http//flip.engr.orst.edu34567/test.html
• The client sends a GET command to a TCP
  socket connected to port 34567.
• http//www.engr.orst.edu/cgi-bin/cgiProg
• The server activates program cgiProg in the
  cgi-bin directory.
• http www.engr.orst.edu/cgi-bin/cgiwrap/minoura/t
  est.pl
• The server activates program test.pl in
• minoura/public_html/cgi-bin.

3
Example of HTTP Request

• Activate TypeServer on, e.g., flip.engr.orst.edu
  as
• java TypeServer 23456
• server socket opened on port 23456
• Provide URL http//flip.engr.orst.edu23456/test.h
  tml
• to the Netscape browser.
• Then TypeServer will produce the following
  output.
• server connected from client
• from client GET /test.html HTTP/1.0
• from client Connection Keep-Alive
• from client User-Agent Mozilla/3.01Gold
• (X11 I SunOS 5.5.1 sun4u)
• from client Host flip.engr.orst.edu23456
• from client Accept image/gif, image/x-xbitmap,
• image/jpeg, image/pjpeg, /
• from client

4
Server Response

• telnet www.engr.orst.edu 80
• Trying 207.98.118.10...
• Connected to www.engr.orst.edu.
• Escape character is ''.
• GET /minoura/
• lt!-- This file was created in June 1996 --gt
• lthtmlgt
• ltheadgt
• lttitlegtToshimi Minoura's Home Pagelt/titlegt
• lt/headgt
• ltbodygt
• lth2gt Toshimi Minoura lt/h2gt
• . . .
• lthrgt
• lt/bodygt

5
HTTP Methods

• GET
• To retrieve the data identified by the URL.
• HEAD
• To retrieve the header information of the
  entity identified by the URL.
• POST
• To request the server to accept the data
  enclosed in the request.
• PUT
• To store the enclosed data under the URL
  specified.
• DELETE
• To delete the entity specified by the URL.
• OPTIONS
• To obtain information on available
  communication options.

6
Head Method

• telnet www.engr.orst.edu 80
• Trying 207.98.118.10...
• Connected to www.engr.orst.edu.
• Escape character is ''.
• HEAD /minoura/ HTTP/1.0
• HTTP/1.0 200 Document follows
• Date Tue, 19 Nov 1996 063947 GMT
• Server NCSA/1.5
• Content-type text/html
• Last-modified Sat, 28 Sep 1996 184559 GMT
• Content-length 4507
• Connection closed by foreign host.

7
Request Syntax

• Request Request-Line
• ( general-header request-header
  entity-header )
• CRLF
• message-body
• Request-Line Method SP Request-URI SP
  HTTP-Version CRLF
• Method "OPTIONS"
• "GET"
• "HEAD"
• "POST"
• "PUT"
• "DELETE"
• "TRACE"
• extension-method
• message-body entity-body
• ltentity-body encoded as per
  Transfer-Encodinggt

8
Request Header

• request-header Accept
• Accept-Charset
• Accept-Encoding
• Accept-Language
• Authorization
• From
• Host
• If-Modified-Since
• If-Match
• If-None-Match
• If-Range
• If-Unmodified-Since
• Max-Forwards
• Proxy-Authorization
• Range
• Referer
• User-Agent

9
Request Header Fields

• Accept type/subtype
• This field lists acceptable MIME
  content-types.
• Accept-Language language
• This field lists the languages preferred by
  the browser.
• Frommail_address
• This field contains the mail address of the
  user accessing the server.
• If-Modified-Since date_time
• The requested document need be sent only if
  it has been changed
• since the indicated date and time.
• Referer URL
• This field gives the URL of the document
  from which the request has originated.
• User-Agent program/version
• This field provides information about the
  client software.

10
Response Syntax

• Response Status-Line
• ( general-header
• response-header
• entity-header )
• CRLF
• message-body
• message-body entity-body
• lt encoded entity-bodygt
• entity-body OCTET

11
Status Code

• Status-Line HTTP-Version SP Status-Code SP
  Reason-Phrase CRLF
• Status Code Categories
• 1xx -- information
• 2xx -- successful
• 3xx -- continue
• 4xx -- client error
• 5xx -- server error
• Status-Code "100" Continue
• "101" Switching Protocols
• "200" OK
• "201" Created
• "202" Accepted
• "203" Non-Authoritative
• . . .

12
Status Code (contd)

• Status-Code "100" Continue
• . . .
• "300" Multiple
  Choices
• "301" Moved
  Permanently
• " 302" Moved
  Temporarily
• "400" Bad
  Request
• "401"
  Unauthorized
• "402" Payment
  Required
• "403" Forbidden
• "404" Not Found
• "405" Method Not
  Allowed
• "406" Not
  Acceptable
• "500" Internal
  Server Error
• "501" Not
  Implemented
• "502" Bad
  Gateway
• "503" Service
  Unavailable
• "504" Gateway
  Time-out
• "505" HTTP
  Version not supported

13
Example of Status Code

• telnet www.cs.orst.edu 80
• Trying 207.98.118.10...
• Connected to www.cs.orst.edu.
• Escape character is ''.
• DELETE /hello.txt HTTP/1.0
• HTTP/1.0 404 Not Found
• Date Tue, 19 Nov 1996 064522 GMT
• Server NCSA/1.5
• Content-type text/html
• ltHEADgtltTITLEgt404 Not Foundlt/TITLEgtlt/HEADgt
• ltBODYgtltH1gt404 Not Foundlt/H1gt
• The requested URL /hello.txt was not found on
  this server.
• lt/BODYgt
• Connection closed by foreign host.

14
Response Header and Entity Header

• Response Header and Entity Header
• response-header Age
• Location
• Proxy-Authenticate
• Public
• Retry-After
• Server
• Vary
• Warning
• WWW-Authenticate
• entity-header Allow
• Content-Base
• Content-Encoding
• Content-Language
• Content-Length
• Content-Location
• Content-MD5
• Content-Range

15
Accept Field

• The Accept field in the request-header can be
  used to specify
• acceptable media types.
• Accept "Accept" ""
• ( media-range accept-params
  )
• media-range ( "/"
• ( type "/" "" )
• ( type "/" subtype )
• ) ( "" parameter )
• accept-params "" "q" "" qvalue (
  accept-extension )
• accept-extension "" token "" ( token
  quoted-string )

16
Examples of Accept Fields

• Accept audio/ q0.2, audio/basic
• I prefer audio/basic. If it does not exist,
  send me any audio type after an 80 mark-down in
  quality.
• Accept text/plain q0.5, text/html,
• text/x-dvi q0.8, text/x-c
• The preferred media types are text/html and
  text/x-c.
• If they do not exist, then send the text/x-dvi
  entity.
• If that does not exist, send the text/plain
  entity.

17
Summary

• The HTTP protocol is used to retrieve a sequence
  of bytes, which is often an HTML file.
• It is a stateless protocol following a simple
  request/response paradigm.
• A success or failure of the result is indicated
  by a status code.
• It is possible to specify the properties of the
  document to be retrieved, for example, the
  encoding method of the image, the language ot the
  text, etc.