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HTTP - Hypertext Transfer Protocol

A Network Programming Lecture by Steven Choy

Overview: What is HTTP? - Structure of HTTP Transactions - What are HTTP Headers? - HTTP Request Structure - HTTP Request Method: GET - HTTP Request Method: POST - HTTP Response Structure - HTTP Status Codes - HTTP Headers in HTTP Requests - HTTP Headers in HTTP Responses

What is HTTP?

• HTTP stands for Hypertext Transfer Protocol.

• It's the network protocol used to deliver virtually all resources on the web.
• Resources on the web include all files and other data such as HTML files, image files, query results, or anything else.
• Usually, HTTP takes place through TCP/IP sockets.
• Knowing HTTP enables you to write Web browsers, Web servers, automatic page downloaders, link-checkers, and other useful tools.

Structure of HTTP Transactions

• HTTP uses the client-server model:
  • An HTTP client opens a connection and sends a request message to an HTTP server.
  • The server then returns a response message, usually containing the resource that was requested.

• The format of an HTTP (request or response) message is:

        <initial request/response line>
        Header1: value1
        Header2: value2
        Header3: value3

        <optional message body goes here, like file contents or query data;
         it can be many lines long, or even binary data>

What are HTTP Headers?

• HTTP headers are the core part of these HTTP requests and responses, and they carry information about the client browser, the requested page, the server and more.

Example HTTP Request

      GET /tutorials/other/top-20-mysql-best-practices/ HTTP/1.1
      Host: net.tutsplus.com
      User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5 (.NET CLR 3.5.30729)
      Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
      Accept-Language: en-us,en;q=0.5
      Accept-Encoding: gzip,deflate
      Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
      Keep-Alive: 300
      Connection: keep-alive
      Cookie: PHPSESSID=r2t5uvjq435r4q7ib3vtdjq120
      Pragma: no-cache
      Cache-Control: no-cache

• The first line is the Request Line which contains some basic info on the request.
• The rest are the HTTP headers.

Example HTTP Response

      HTTP/1.x 200 OK
      Transfer-Encoding: chunked
      Date: Sat, 28 Nov 2009 04:36:25 GMT
      Server: LiteSpeed
      Connection: close
      X-Powered-By: W3 Total Cache/0.8
      Pragma: public
      Expires: Sat, 28 Nov 2009 05:36:25 GMT
      Etag: "pub1259380237;gz"
      Cache-Control: max-age=3600, public
      Content-Type: text/html; charset=UTF-8
      Last-Modified: Sat, 28 Nov 2009 03:50:37 GMT
      X-Pingback: http://net.tutsplus.com/xmlrpc.php
      Content-Encoding: gzip
      Vary: Accept-Encoding, Cookie, User-Agent

• The first line is the Status Line
• It is followed by HTTP headers, until the blank line.
• After that, the content starts.

How to view HTTP Headers

• Your browser only shows you the content of a HTTP response and not the HTTP response headers.
• You need to use some browser addons or plugins, or other specific programs to view HTTP headers.

HTTP Request Structure

• Request line = request method + path + protocol
• HTTP headers: name-value pairs
• Request methods: indicates what kind of request this is; most common methods are GET, POST and HEAD.

HTTP Request Method: GET

• This is the main method used for retrieving html, images, JavaScript, CSS, etc.
• Most data that loads in your browser was requested using this method.
• Web forms can be set to use the method GET. Here is an example

      <form method="GET" action="foo.php">
        First Name: <input name="first_name" type="text"> <br />
        Last Name: <input name="last_name" type="text"> <br />
        <input type="submit" name="action" value="Submit" />
      </form>

• When that form is submitted, the HTTP request line is:

      GET /foo.php?first_name=John&last_name=Doe&action=Submit HTTP/1.1

HTTP Request Method: POST

• POST requests are most commonly sent by web forms.

      <form method="POST" action="foo.php">
        First Name: <input type="text" name="first_name" /> <br />
        Last Name: <input type="text" name="last_name" /> <br />
        <input type="submit" name="action" value="Submit" />
      </form>

• When that form is submitted, the HTTP request line is:

      POST /foo.php HTTP/1.1
      Host: localhost
      User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5 (.NET CLR 3.5.30729)
      Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
      Accept-Language: en-us,en;q=0.5
      Accept-Encoding: gzip,deflate
      Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
      Keep-Alive: 300
      Connection: keep-alive
      Referer: http://localhost/test.php
      Content-Type: application/x-www-form-urlencoded
      Content-Length: 43

      first_name=John&last_name=Doe&action=Submit

• Note the following points:
  • Content-Type and Content-Lenght headers have been added
  • All the data is in now sent after the HTTP headers.
  • POST method requests can also be made via AJAX, applications, cURL, etc.
  • All file upload forms are required to use the POST method.

HTTP Request Method: HEAD

• HEAD is similar to GET, except the server does not return the content in the HTTP response.

HTTP Response Structure

• Response line = protocol + status code
• HTTP headers: name-value pairs
• Most of the name-vale pairs in the HTTP headers are optional.

HTTP Status Codes

• The status code is a three-digit integer
• The first digit identifies the general category of response:

      1xx indicates an informational message only
      2xx indicates success of some kind (i.e. for successful requests)
      3xx redirects the client to another URL (i.e. for redirections)
      4xx indicates an error on the client's part (i.e. there was a problem with the request)
      5xx indicates an error on the server's part (i.e. there was a problem with the server)

• The most common status codes are:

         order allow,deny
         deny from 192.168.44.201
         deny from 172.16.7.92
         allow from all

HTTP Headers in HTTP Requests

• Host

• User-Agent

• Accept-Language

• Accept-Encoding

• If-Modified-Since

• Cookie

• Referer

• Authorization

HTTP Headers in HTTP Responses

• Cache-Control

          Cache-Control: max-age=3600, public

          Cache-Control: no-cache

• Content-Type

             Content-Type: text/html; charset=UTF-8

             Content-Type: image/gif

             Content-Type: application/pdf

• Content-Disposition

          Content-Disposition: attachment; filename="download.zip"

• Content-Length

• Etag

          Etag: "pub1259380237;gz"  

          If-None-Match: "pub1259380237;gz"

• Last-Modified

          Last-Modified: Sat, 28 Nov 2009 03:50:37 GMT

• Location

• Set-Cookie

      Set-Cookie: skin=noskin; path=/; domain=.amazon.com; expires=Sun, 29-Nov-2009 21:42:28 GMT
      Set-Cookie: session-id=120-7333518-8165026; path=/; domain=.amazon.com; expires=Sat Feb 27 08:00:00 2010 GMT

• WWW-Authenticate

          WWW-Authenticate: Basic realm="Restricted Area"

• Content-Encoding

References

• HTTP Headers for Dummies
• HTTP Made Really Easy
• HTTP/1.1: Status Code Definitions
• Quick reference to HTTP headers

Cool Tools

• Firefox addons - Live HTTP Headers
• View HTTP Request and Response Header
• HttpWatch - It is an HTTP viewer and debugger that integrates with IE and Firefox to provide seamless HTTP and HTTPS monitoring without leaving the browser window.

Thanks for Reading

If you would rather like to have this lecture note in printed format, please click the print action link in the top right corner.

If you find any problem in this lecture note, please feel free to tell Steven via steven@findaway.hk.