Hypertext Transfer Protocol

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Hypertext Transfer Protocol (HTTP) is a method used to transfer or convey information on the World Wide Web. Its original purpose was to provide a way to publish and retrieve HTML pages.

Development of HTTP was coordinated by the World Wide Web Consortium and 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. The originating client, such as a web browser, spider, or other end-user tool, is referred to as the user agent. The destination 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.

An HTTP client initiates a request by establishing a Transmission Control Protocol (TCP) connection to a particular port on a remote 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.

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.

Resources to be accessed by HTTP are identified using Uniform Resource Identifiers (URIs) (or, more specifically, URLs) using the http: or https URI schemes.

Contents

[edit] Request message

The request message consists of the following:

  • Request line, such as GET /images/logo.gif HTTP/1.1, which requests the file logo.gif from the /images directory
  • Headers, such as Accept-Language: en
  • An empty line
  • An optional message body

The request line and headers must all end with CRLF (i.e. a carriage return followed by a line feed). The empty line must consist of only CRLF and no other whitespace.

In the HTTP/1.1 protocol, all headers except Host are optional.

[edit] Request methods

HTTP defines eight methods (sometimes referred to as "verbs") indicating the desired action to be performed on the identified resource.

HEAD
Asks for the response identical to the one that would correspond to a GET request, but without the response body. This is useful for retrieving meta-information written in response headers, without having to transport the entire content.
GET
Requests a representation of the specified resource. By far the most common method used on the Web today. Should not be used for operations that cause side-effects (using it for actions in web applications is a common mis-use). See 'safe methods' below.
POST
Submits data to be processed (e.g. from a HTML form) to the identified resource. The data is included in the body of the request.
PUT
Uploads a representation of the specified resource.
DELETE
Deletes the specified resource.
TRACE
Echoes back the received request, so that a client can see what intermediate servers are adding or changing in the request.
OPTIONS
Returns the HTTP methods that the server supports. This can be used to check the functionality of a web server.
CONNECT
For use with a proxy that can change to being an SSL tunnel.

HTTP servers are supposed to implement at least the GET and HEAD methods and, whenever possible, also the OPTIONS method.

[edit] Safe methods

Some methods (e.g. HEAD or GET) are defined as safe, which means they are intended only for information retrieval and should not change the state of the server (in other words, they should not have side effects). Unsafe methods (such as POST, PUT and DELETE) should be displayed to the user in a special way, typically as buttons rather than links, thus making the user aware of possible obligations (such as a button that causes a financial transaction).

Despite the required safety of GET requests, in practice they can cause changes on the server. For example, an HTML page may use a simple hyperlink to initiate deletion of a domain database record, thus causing a change of the server's state as a side-effect of a GET request. This is discouraged, because it can cause problems for Web caching, search engines and other automated agents, who can make unintended changes on the server.

[edit] Idempotent methods

Methods GET, HEAD, PUT and DELETE are defined to be idempotent, meaning that multiple identical requests should have the same effect as a single request. Methods OPTIONS and TRACE, being safe, are inherently idempotent.

[edit] HTTP versions

HTTP has evolved into multiple, mostly backwards-compatible protocol versions. RFC 2145 describes the use of HTTP version numbers. Basically, the client tells in the beginning of the request the version it uses, and the server uses the same or earlier version in the response.

0.9
Deprecated. Only supports one command, GET — which does not specify the HTTP version. Does not support headers. Since this version does not support POST, the client can't pass much information to the server.
HTTP/1.0
This is the first protocol revision to specify its version in communications and is still in wide use, especially by proxy servers. Allows persistent connections (alias keep-alive connections, more than one request-response per TCP/IP connection) when explicitly negotiated; however, this only works well when not using proxy servers.
HTTP/1.1
Current version; persistent connections enabled by default and works well with proxies. Also supports request pipelining, allowing multiple requests to be sent at the same time, allowing the server to prepare for the workload and potentially transfer the requested resources more quickly to the client.
HTTP/1.2
The initial 1995 working drafts of PEP — an Extension Mechanism for HTTP prepared by W3C and submitted to IETF were aiming to become a distinguishing feature of HTTP/1.2. In later PEP working drafts however the reference to HTTP/1.2 was removed. PEP later became subsumed by the experimental RFC 2774 — HTTP Extension Framework.

[edit] Status codes

See also: List of HTTP status codes

In HTTP/1.0 and since, the first line of the HTTP response is called the status line and includes a numeric status code (such as "404") and a textual reason phrase (such as "Not Found"). The way the user agent handles the response primarily depends on the code and secondarily on the response headers. Custom status codes can be used since, if the user agent encounters a code it does not recognize, it can use the first digit of the code to determine the general class of the response. [1]

Also, the standard reason phrases are only recommendations and can be replaced with "local equivalents" at the web developer's discretion. If the status code indicated a problem, the user agent might display the reason phrase to the user to provide further information about the nature of the problem. The standard also allows the user agent to attempt to interpret the reason phrase, though this might be unwise since the standard explicitly specifies that status codes are machine-readable and reason phrases are human-readable.

[edit] Persistent connections

Main article: HTTP persistent connections

In HTTP/0.9, the connection is closed after a single request/response pair. HTTP/1.0 also defaults to this behavior, but during preliminary work on HTTP/1.1, many implementations started honoring "keep-alive" mechanisms, where a connection could be reused for more than one request.

Such persistent connections improve lag perceptably, because the client does not need to re-negotiate the TCP connection after the first request has been sent.

While a special header -- Keep-Alive -- is required to use persistent connections in HTTP/1.0, this behavior became the default in HTTP/1.1. That version of the protocol also introduced chunked encoding to allow content on persistent connections to be streamed, rather than buffered, and HTTP pipelining, which allows clients to send some types of requests before the previous response has been received, further reducing lag.

[edit] HTTP session state

HTTP can occasionally pose problems for Web developers (Web Applications), because HTTP is stateless. The advantage of a stateless protocol is that hosts don't need to retain information about users between requests, but this forces the use of alternative methods for maintaining users' state, for example, when a host would like to customize content for a user who has visited before. The common method for solving this problem involves the use of sending and requesting cookies. Other methods include server side sessions, hidden variables (when current page is a form), and URL encoded parameters (such as index.php?userid=3).

[edit] Secure HTTP

There are currently two methods of establishing a secure HTTP connection: the https URI scheme and the HTTP 1.1 Upgrade header. The https URI scheme has been deprecated by RFC 2817, which introduced the Upgrade header; however, as browser support for the Upgrade header is nearly non-existent, the https URI scheme is still the dominant method of establishing a secure HTTP connection.

[edit] https URI Scheme

Main article: https

https: is a URI scheme syntactically identical to the http: scheme used for normal HTTP connections, but which signals the browser to use an added encryption layer of SSL/TLS to protect the traffic. SSL is especially suited for HTTP since it can provide some protection even if only one side of the communication is authenticated. In the case of HTTP transactions over the Internet, typically only the server side is authenticated.

[edit] HTTP 1.1 Upgrade Header

HTTP 1.1 introduced support for the Upgrade header. In the exchange, the client begins by making a clear-text request, which is later upgraded to TLS. Either the client or the server may request (or demand) that the connection be upgraded. The most common usage is a clear-text request by the client followed by a server demand to upgrade the connection, which looks like this:

Client:

GET /encrypted-area HTTP/1.1
Host: www.example.com

Server:

HTTP/1.1 426 Upgrade Required
Upgrade: TLS/1.0, HTTP/1.1
Connection: Upgrade

The server returns a 426 status-code because 400 level codes indicate a client failure (see List of HTTP status codes), which correctly alerts legacy clients that the failure was client-related.

The benefits of using this method for establishing a secure connection are:

  • that it removes messy and problematic redirection and URL rewriting on the server side, and
  • it reduces user confusion by providing a single way to access a particular resource.

[edit] Sample

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

Client request (followed by a blank line, so that request ends with a double newline, each in the form of a carriage return followed by a line feed):

GET /index.html HTTP/1.1
Host: www.example.com

The "Host" header distinguishes between various DNS names sharing a single IP address, allowing name-based virtual hosting. While optional in HTTP/1.0, it is mandatory in HTTP/1.1.

Server response (followed by a blank line and text of the requested page):

HTTP/1.1 200 OK
Date: Mon, 23 May 2005 22:38:34 GMT
Server: Apache/1.3.27 (Unix)  (Red-Hat/Linux)
Last-Modified: Wed, 08 Jan 2003 23:11:55 GMT
Etag: "3f80f-1b6-3e1cb03b"
Accept-Ranges: bytes
Content-Length: 438
Connection: close
Content-Type: text/html; charset=UTF-8

[edit] See also

[edit] External links

[edit] Specifications and references

[edit] Tutorials and tools

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