HTTP

HTTP (for HyperText Transfer Protocol) is the primary method used to communicate information on the World Wide Web.

Development of HTTP was coordinated by the World Wide Web Consortium and working groups of the Internet Engineering Task Force, culminating in the publication of a series of RFCs, most notably RFC 2616, which defines HTTP/1.1, the version of HTTP in common use today.

HTTP is a request/response protocol between clients and servers. An HTTP client, such as a web browser, typically initiates a request by establishing a TCP/IP connection to a particular port on a remote host. An HTTP server listening on that port waits for the client to send a request string, such as "GET / HTTP/1.1" (which would request the default page of that web server), optionally followed by an email-like MIME message which has a number of informational header strings that describe aspects of the request, followed by an optional body of arbitrary data. Upon receiving the request string (and message, if any), the server sends back a response string, such as "200 OK", and a message of its own, the body of which is perhaps the requested file, an error message, or some other information.

HTTP differs from other TCP-based protocols such as FTP, in that connections are usually terminated once a particular request (or related series of requests) has been completed. This design makes HTTP ideal for the World Wide Web, where pages regularly link to pages on other servers. It can occasionally pose problems for Web designers, as the lack of a persistent connection necessitates alternative methods of maintaining users' "state". Many of these methods involve the use of "cookies".

There is a secure version of HTTP called HTTPS that can use any given encryption method as long as it is understood by both sides.

Sample

Below is a sample conversation between an HTTP client and an HTTP server running on www.google.com, port 80.

Client request:

GET / HTTP/1.1
Host: www.google.com

(followed by a new line, in the form of a carriage return followed by a line feed.)

Server response:

HTTP/1.1 200 OK
Content-Length: 3059
Server: GWS/2.0
Date: Sat, 11 Jan 2003 02:44:04 GMT
Content-Type: text/html
Cache-control: private
Set-Cookie: PREF=ID=73d4aef52e57bae9:TM=1042253044:LM=1042253044:S=SMCc_HRPCQiqy
X9j; expires=Sun, 17-Jan-2038 19:14:07 GMT; path=/; domain=.google.com
Connection: keep-alive

(followed by a blank line and HTML text comprising the Google home page)

See also

• 404 error

External links

• Tim Berners-Lee's original 1992 Internet-Draft http://www.w3.org/Protocols/HTTP/HTTP2.html
• RFC 2616 - The current HTTP/1.1 specification
• HTTP/1.1 specification errata (http://skrb.org/ietf/http_errata.html)
• HTTP header viewer (http://analyze.forret.com)
• List of HTTP error codes (http://www.modemhelp.net/httperrors/httperrors.shtml)
• Command-line HTTP clients: cURL (http://curl.haxx.se/), Wget (http://www.gnu.org/software/wget/wget.html)

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