Hypertext Transfer Protocol (HTTP)

Hypertext Transfer Protocol HTTP is a protocol for transmitting hypermedia documents, and especially for fetching resources such as HTML documents from a server. It is the foundation of any data exchange on the Web and it is a client-server protocol, which means requests are initiated by the recipient, usually the Web browser. A complete document is typically constructed from resources such as text content, layout instructions, images, videos, scripts, and more.

HTTP can also be used for other purposes, such as machine-to-machine communication, programmatic access to APIs, and more. (See CouchDB: Another JSON Document DataBase Management System)

HTTP follows a classical client-server model, with a client opening a connection to make a request, then waiting until it receives a response from the server.

Clients and servers communicate by exchanging individual messages (as opposed to a stream of data). The messages sent by the client are called requests and the messages sent by the server as an answer are called responses.

HTTP is a stateless protocol, meaning that the server does not keep any session data between two requests, although the later addition of cookies adds state to some client-server interactions.

HTTP Features

HTTP is simple

HTTP is generally designed to be human-readable, even with the added complexity introduced in HTTP/2 by encapsulating HTTP messages into frames. HTTP messages can be read and understood by humans, providing easier testing for developers, and reduced complexity for newcomers.

HTTP is extensible

Introduced in HTTP/1.0, HTTP headers make this protocol easy to extend and experiment with. New functionality can even be introduced by an agreement between a client and a server about a new header's semantics.

HTTP is stateless, but not sessionless

HTTP is stateless: there is no link between two requests being successively carried out on the same connection. This immediately has the prospect of being problematic for users attempting to interact with certain pages coherently, for example, using e-commerce shopping baskets.

Still, while the core of HTTP itself is stateless, HTTP cookies allow the use of stateful sessions. Using header extensibility, HTTP Cookies are added to the workflow, allowing session creation on each HTTP request to share the same context, or the same state.

HTTP Components

HTTP Messages

HTTP messages, as defined in HTTP/1.1 and earlier, are human-readable. In HTTP/2, these messages are embedded into a binary structure, a frame, allowing optimizations like compression of headers and multiplexing. Even if only part of the original HTTP message is sent in this version of HTTP, the semantics of each message is unchanged and the client reconstitutes (virtually) the original HTTP/1.1 request. It is therefore useful to comprehend HTTP/2 messages in the HTTP/1.1 format.

There are two types of HTTP messages, requests and responses, each with its own format.

Three Types of Elements

HTTP headers: Message headers are used to send metadata about a resource or a HTTP message, and to describe the behavior of the client or the server.
HTTP request methods: Request methods indicate the purpose of the request and what is expected if the request is successful. The most common methods are GET and POST for retrieving and sending data to servers, respectively, but there are other methods which serve different purposes.
HTTP response status codes: Response status codes indicate the outcome of a specific HTTP request. Responses are grouped in five classes: informational, successful, redirections, client errors, and server errors.

HTTP Flow

When a client wants to communicate with a server, either the final server or an intermediate proxy, it performs the following steps:

Open a TCP connection: The TCP connection is used to send a request, or several, and receive an answer. The client may open a new connection, reuse an existing connection, or open several TCP connections to the servers.

With TCP the default port, for an HTTP server on a computer, is port 80. Other ports can also be used, like 8000 or 8080. The URL of a page to fetch contains both the domain name, and the port number, though the latter can be omitted if it is 80.
Send an HTTP message: HTTP messages (before HTTP/2) are human-readable. With HTTP/2, these messages are encapsulated in frames, making them impossible to read directly, but the principle remains the same. For example:
```
HTTP
Copy to Clipboard
GET / HTTP/1.1
Host: developer.mozilla.org
Accept-Language: fr
```

Read the response sent by the server, such as:

HTTP

HTTP/1.1 200 OK
Date: Sat, 09 Oct 2010 14:28:02 GMT
Server: Apache
Last-Modified: Tue, 01 Dec 2009 20:18:22 GMT
ETag: "51142bc1-7449-479b075b2891b"
Accept-Ranges: bytes
Content-Length: 29769
Content-Type: text/html

<!doctype html>… (here come the 29769 bytes of the requested web page)

Close or reuse the connection for further requests.

HTTP headers

HTTP request methods

Requests consist of the following elements:

An HTTP method, usually a verb like GET, POST, or a noun like OPTIONS or HEAD that defines the operation the client wants to perform. Typically, a client wants to fetch a resource (using GET) or post the value of an HTML form (using POST), though more operations may be needed in other cases.
The path of the resource to fetch; the URL of the resource stripped from elements that are obvious from the context, for example without the protocol (http://), the domain (here, developer.mozilla.org), or the TCP port (here, 80).
The version of the HTTP protocol.
Optional headers that convey additional information for the servers.
A body, for some methods like POST, similar to those in responses, which contain the resource sent.

HTTP Responses

Responses consist of the following elements:

The version of the HTTP protocol they follow.
A status code, indicating if the request was successful or not, and why.
A status message, a non-authoritative short description of the status code.
HTTP headers, like those for requests.
Optionally, a body containing the fetched resource.