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HTTP Protocol


HTTP Protocol Amanda Burrows HTTP Protocol The HTTP protocol is used to send HTML documents through the Internet. The HTTP protocol sends the HTML documents in ... – PowerPoint PPT presentation

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Title: HTTP Protocol

HTTP Protocol
  • Amanda Burrows

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.

  • 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

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

  • 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.

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.

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

  • 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

  • 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

  • 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.

  • 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.

  • 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
  • Extending a database through an append

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

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

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.

Example of connections
  • Imagine that you are browsing a Web page and
    have just clicked on a link whose URL is
    http// 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,
  • The browser will use the TCP/IP protocols to
    establish a connection across the Internet
    between your computer and
  • 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 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.

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

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.

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

Any Questions??