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Hypertext Transfer Protocol (HTTP) is an application-layer protocol for transmitting hypermedia documents, such as HTML. It was designed for communication between web browsers and web servers, but it can also be used for other purposes. HTTP follows a classical client-server model, with a client opening a connection to make a request, then waiting until it receives a response. HTTP is a stateless protocol, meaning that the server does not keep any data (state) between two requests. Though often based on a TCP/IP layer, it can be used on any reliable transport layer; that is, a protocol that doesn't lose messages silently, such as UDP.

Tutorials

Learn how to use HTTP with guides and tutorials.

Overview of HTTP
The basic features of the client-server protocol: what it can do and its intended uses.
HTTP Cache
Caching is very important for fast Web sites. This article describes different methods of caching and how to use HTTP Headers to control them.
HTTP Cookies
How cookies work is defined by RFC 6265. When serving an HTTP request, a server can send a Set-Cookie HTTP header with the response. The client then returns the cookie's value with every request to the same server in the form of a Cookie request header. The cookie can also be set to expire on a certain date, or restricted to a specific domain and path.
HTTP Access Control (CORS)
Cross-site HTTP requests are HTTP requests for resources from a different domain than the domain of the resource making the request. For instance, an HTML page from Domain A (http://domaina.example/) makes a request for an image on Domain B (http://domainb.foo/image.jpg) via the img element. Web pages today very commonly load cross-site resources, including CSS stylesheets, images, scripts, and other resources. CORS allows web developers to control how their site reacts to cross-site requests.
Evolution of HTTP
A brief description of the changes between the early versions of HTTP, to the modern HTTP/2 and beyond.
Mozilla web security guidelines
A collection of tips to help operational teams with creating secure web applications.
HTTP Messages
Describes the type and structure of the different kind of messages of HTTP/1.x and HTTP/2.
A typical HTTP session
Shows and explains the flow of a usual HTTP session.
Connection management in HTTP/1.x
Describes the three connection management models available in HTTP/1.x, their strengths, and their weaknesses.

Reference

Browse through detailed HTTP reference documentation.

HTTP Headers
HTTP message headers are used to describe a resource, or the behavior of the server or the client. Custom proprietary headers can be added using the X- prefix; others in an IANA registry, whose original content was defined in RFC 4229. IANA also maintains a registry of proposed new HTTP message headers.
HTTP Request Methods
The different operations that can be done with HTTP: GET, POST, and also less common requests like OPTIONS, DELETE, or TRACE.
HTTP Status Response Codes
HTTP response codes indicate whether a specific HTTP request has been successfully completed. Responses are grouped in five classes: informational responses, successful responses, redirections, client errors, and servers errors.
CSP directives
The Content-Security-Policy response header fields allows web site administrators to control resources the user agent is allowed to load for a given page. With a few exceptions, policies mostly involve specifying server origins and script endpoints.

Tools & resources

Helpful tools and resources for understanding and debugging HTTP.

Firefox Developer Tools
Network monitor
Mozilla Observatory

A project designed to help developers, system administrators, and security professionals configure their sites safely and securely.

RedBot
Tools to check your cache-related headers
How Browsers Work
A very comprehensive article on browser internals and request flow through HTTP protocol. A MUST-READ for any web developer.