Hypertext Transfer Protocol (HTTP) is a communications protocol used to transfer or convey information on intranets and the World Wide Web. Its original purpose was to provide a way to publish and retrieve hypertext pages. Development of HTTP was coordinated by the W3C (World Wide Web Consortium) and the IETF (Internet Engineering Task Force), culminating in the publication of a series of RFCs, most notably RFC 2616 (June 1999), which defines HTTP/1.1, the version of HTTP in common use.

HTTP is a request/response protocol between a client and a server. The client making an HTTP request - such as a web browser, spider, or other end-user tool - is referred to as the user agent. The responding server - which stores or creates resources such as HTML files and images - is called the origin server. In between the user agent and origin server may be several intermediaries, such as proxies, gateways, and tunnels. HTTP is not constrained to using TCP/IP and its supporting layers, although this is its most popular application on the Internet. Indeed HTTP can be "implemented on top of any other protocol on the Internet, or on other networks. HTTP only presumes a reliable transport; any protocol that provides such guarantees can be used."

Typically, an HTTP client initiates a request by establishing a Transmission Control Protocol (TCP) connection to a particular port on a host (port 80 by default; see List of TCP and UDP port numbers). An HTTP server listening on that port waits for the client to send a request message.

The overall Operation

On the internet, the communication takes place over a TCP/IP connection. This does not preclude this protocol being implemented over any other protocol on the internet or other networks. In these cases, the mapping of the HTTP request and response structures onto the transport data units of the protocol in question is outside the scope of this specification. It should not however be at all complicated.

The protocol is basically stateless, a transaction consisting of:

Connection
The establishment of a connection by the client to the server - when using TCP/IP port 80 is the well-known port, but other non-re servered ports may be specified in the URL;

Request
The sending, by the client, of a request message to the server;

Response
The sending, by the server, of a response to the client;

Close
The closing of the connection by either both parties

The format of the request and response parts is defined in this specification. Whilst header information defined in this specification is sent in ISO Latin-1 character set in CRLF terminated lines, object transmission in binary is possible.

Upon receiving the request, the server sends back a status line, such as "HTTP/1.1 200 OK", and a message of its own, the body of which is perhaps the requested file, an error message, or some other information.

When many sources of networked information are available to a reader, and when a discipline of reference between different sources exists, it is possible to rapidly follow references between units of information which are provided at different remote locations. As response times should ideally be of the order of 100ms in, for example, a hypertext jump, this requires a fast, stateless, information retrieval protocol.

Practical information systems require more functionality than simple retrieval, including search, front-end update and annotation. This protocol allows an open-ended set of methods to be used. It builds on the discipline of reference provided by the Universal

Resource Identifier (URI) as a name (URN, RFCxxxx) or address (URL, RFCxxxx) allows the object of the method to be specified. Reference is made to the Multipurpose Internet Mail Extensions (MIME, RFC1341) which are used to allow objects to be transmitted in an open variety of representations