HTTP Protocol

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HTTP Protocol

• Amanda Burrows

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HTTP Protocol

• The HTTP protocol is used to send HTML documents
  through the Internet. The HTTP protocol sends
  the HTML documents in packets, using TCP/IP.
  With each packet, the HTTP protocol attaches a
  header, which contains information such as the
  name and location of the page being requested,
  the name and IP address of the remote server that
  contains the Web page, the IP address of the
  local client, the HTTP version number, and the
  URL of the referring page. This information is
  referred to as the server variables.

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Information

• The Hypertext Transfer Protocol (HTTP) is an
  application-level protocol with the lightness and
  speed necessary for distributed, collaborative,
  hypermedia information systems. HTTP has been in
  use by the World-Wide Web global information
  initiative since 1990.
• HTTP version 1.0 is a stateless protocol
• HTTP 1.1 is stateful

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Improvements in HTTP 1.1

• Requests include a Host MIME header so that one
  web server can easily serve different sites at
  different URLs.
• Servers and browsers can exchange compressed
  files and particular byte ranges of a document,
  both of which can decrease network traffic.
• HTTP 1.1 is designed to work much better with
  proxy servers
• HTTP 1.1 is a strict superset of HTTP 1.0, so
  HTTP 1.1 web servers have no trouble interacting
  with older browsers that speak only HTTP 1.0

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HTTP

• HTTP, the Hypertext Transfer Protocol, is the
  standard protocol for communication between web
  browsers and web servers. HTTP specifies how a
  client and server establish a connection, how the
  client requests data from the server, how the
  server responds to that request, and finally how
  the connection is closed. HTTP connections use
  the TCP/IP protocol for data transfer.

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Step 1

• HTTP 1.0 is the currently accepted version of
  the protocol. It uses MIME to encode data. The
  basic protocol defines a sequence of four steps
  for each request from a client to the server
• Making the connection. The client establishes
  a TCP connection to the server, on port 80 by
  default other ports may be specified in the URL.

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Step 2

• Making a request. The client sends a
  message to the server requesting the page at a
  specified URL. The format of this request is
  typically something like
• GET /index.html HTTP 1.0

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Request-Line

• The Request-Line begins with a method token,
  followed by the Request-URI and the protocol
  version, and ending with CRLF. The elements are
  separated by SP characters. No CR or LF are
  allowed except in the final CRLF sequence.
• Request-Line Method SP Request-URI SP
  HTTP-Version CRLF
• Note that the difference between a
  Simple-Request and the Request-Line of a
  Full-Request is the presence of the HTTP-Version
  field and the availability of methods other than
  GET.

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Method

• The Method token indicates the method to be
  performed on the resource identified by the
  Request-URI. The method is case-sensitive.
• Method "GET"
• "HEAD"
• "POST"
• extension-method
• extension-method token
• The list of methods acceptable by a specific
  resource can change dynamically the client is
  notified through the return code of the response
  if a method is not allowed on a resource. Servers
  should return the status code 501 (not
  implemented) if the method is unrecognized or not
  implemented.

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GET

• The GET method means retrieve whatever
  information is identified by the Request-URI. If
  the Request-URI refers to a data-producing
  process, it is the produced data which shall be
  returned as the entity in the response and not
  the source text of the process, unless that text
  happens to be the output of the process.
• The semantics of the GET method changes to a
  "conditional GET" if the request message includes
  an If-Modified-Since header field. A conditional
  GET method requests that the identified resource
  be transferred only if it has been modified since
  the date given by the If-Modified-Since header.
• The conditional GET method is intended to reduce
  network usage by allowing cached entities to be
  refreshed without requiring multiple requests or
  transferring unnecessary data.

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Head

• The HEAD method is identical to GET except
  that the server must not return any Entity-Body
  in the response. The metainformation contained in
  the HTTP headers in response to a HEAD request
  should be identical to the information sent in
  response to a GET request. This method can be
  used for obtaining metainformation about the
  resource identified by the Request-URI without
  transferring the Entity-Body itself. This method
  is often used for testing hypertext links for
  validity, accessibility, and recent modification.
  
• There is no "conditional HEAD" request
  analogous to the conditional GET. If an
  If-Modified-Since header field is included with a
  HEAD request, it should be ignored.

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Post

• The POST method is used to request that the
  destination server accept the entity enclosed in
  the request as a new subordinate of the resource
  identified by the Request-URI in the
  Request-Line. POST is designed to allow a uniform
  method to cover the following functions
• Annotation of existing resources
• Posting a message to a bulletin board,
  newsgroup, mailing list, or similar group
  of articles
• Providing a block of data, such as the result
  of submitting a form, to a data-handling
  process
• Extending a database through an append
  operation.

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Step 3

• The response. The server sends a response
  to the client. The response begins with a
  response code, followed by MIME header
  information, then a blank line, then the
  requested document or an error message. Assuming
  the requested file is found, a typical response
  looks like this

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Step 3 (cont)

• HTTP 1.0 200 OK
• Server NCSA/1.4.2
• MIME-version 1.0
• Content-type text/html
• Content-length 107
• lthtmlgt
• ltHeadgt
• ltTitlegt
• A Sample HTML file
• lt/Titlegt
• lt/Headgt
• ltbodygt
• The rest of the document goes here
• lt/bodygt
• lt/htmlgt

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Step 4

• Closing the connection. Either client or
  the server or both close the connection. Thus, a
  separate network connection is used for each
  request. If the client reconnects, the server
  retains no memory of past requests is called
  stateless in contrast, a stateful protocol such
  as FTP can process many requests before the
  connection is closed. The lack of state is both
  a strength and a weakness of HTTP.

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Example of connections

• Imagine that you are browsing a Web page and
  have just clicked on a link whose URL is
  http//www.cob.mnsu.edu/faculty.html. The
  following sequence of events will take place to
  let you access that page
• Your Web browser will determine the URL
  associated with the link and will extract the
  name of the machine to which it must connect in
  this case, www.cob.mnsu.edu.
• The browser will use the TCP/IP protocols to
  establish a connection across the Internet
  between your computer and www.cob.mnsu.edu.
• When the connection between these two machines
  has been established, your browser will send a
  special HTTP message called GET, which indicates
  that it wants the destination machine to retrieve
  a page. The GET command contains the name of the
  desired page, in this case faculty.html.
• The remote machine www.cob.mnsu.edu locates the
  file name in the GET message, reads it, copies
  it, and returns the copy to your browser, again
  using TCP/IP and the Internet.
• Your browser receives the page and displays its
  contents on your screen.

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Multipurpose Internet Mail Extensions

• MIME is an open standard for sending multipart,
  multimedia data through Internet email.
• Originally intended for email
• Content types are classified at two levels a
  type and a subtype

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HTTP communication

• Most HTTP communication is initiated by a user
  agent and consists of a request to be applied to
  a resource on some original server.
• A more complicated situation occurs when one or
  more intermediaries are present in the
  request/response chain.

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Three common forms

• Proxy is a forwarding agend, receiving requests
  for a URI in its absolute form, rewriting all or
  parts of the message, and forwarding the
  reformatted request toward the server identified
  by the URI
• Gateway is a receiving agent, acting as a layer
  above some other server and, if necessary,
  translating the requests to the underlying
  servers protocol
• Tunnel acts as a relay point between two
  connections without changing the messages

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Any Questions??