Hypertext Transfer Protocol

The Hypertext Transfer Protocol (HTTP) is an application layer protocol for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, where hypertext documents include hyperlinks to other resources that the user can easily access, for example by a mouse click or by tapping the screen in a web browser.
Development of HTTP was initiated by Tim Berners-Lee at CERN in 1989. Development of early HTTP Requests for Comments (RFCs) was a coordinated effort by the Internet Engineering Task Force (IETF) and the World Wide Web Consortium (W3C), with work later moving to the IETF.
HTTP/1 was first documented (as version 1.1) in 1997. As of 2021, about 30% of websites only support HTTP/1. 
HTTP/2 is a more efficient expression of HTTP's semantics "on the wire", and was published in 2015, and is used by over 50% of websites; it is now supported by virtually all web browsers and major web servers over Transport Layer Security (TLS) using an Application-Layer Protocol Negotiation (ALPN) extension where TLS 1.2 or newer is required.
HTTP/3 is the proposed successor to HTTP/2, and 2/3rd of web browser users (both on desktop and mobile) can already use HTTP/3, on the 19.0% of websites that already support it; it uses UDP instead of TCP for the underlying transport protocol. Like HTTP/2, it does not obsolete previous major versions of the protocol. Support for HTTP/3 was added to Cloudflare and Google Chrome in September 2019 (since enabled by default), and can be enabled in the stable versions of Firefox and Safari.
Technical overview.
HTTP functions as a request–response protocol in the client–server computing model. A web browser, for example, may be the "client" and an application running on a computer hosting a website may be the "server". The client submits an HTTP "request" message to the server. The server, which provides "resources" such as HTML files and other content, or performs other functions on behalf of the client, returns a "response" message to the client. The response contains completion status information about the request and may also contain requested content in its message body.
A web browser is an example of a "user agent" (UA). Other types of user agent include the indexing software used by search providers (web crawlers), voice browsers, mobile apps, and other software that accesses, consumes, or displays web content.
HTTP is designed to permit intermediate network elements to improve or enable communications between clients and servers. High-traffic websites often benefit from web cache servers that deliver content on behalf of upstream servers to improve response time. Web browsers cache previously accessed web resources and reuse them, when possible, to reduce network traffic. HTTP proxy servers at private network boundaries can facilitate communication for clients without a globally routable address, by relaying messages with external servers.
HTTP is an application layer protocol designed within the framework of the Internet protocol suite. Its definition presumes an underlying and reliable transport layer protocol, and Transmission Control Protocol (TCP) is commonly used. However, HTTP can be adapted to use unreliable protocols such as the User Datagram Protocol (UDP), for example in HTTPU and Simple Service Discovery Protocol (SSDP).
HTTP resources are identified and located on the network by Uniform Resource Locators (URLs), using the Uniform Resource Identifiers (URI's) schemes "http" and "https". As defined in , URIs are encoded as hyperlinks in HTML documents, so as to form interlinked hypertext documents.
HTTP/1.1 is a revision of the original HTTP (HTTP/1.0). In HTTP/1.0 a separate connection to the same server is made for every resource request. HTTP/1.1 can reuse a connection multiple times to download images, scripts, stylesheets, "etc" after the page has been delivered. HTTP/1.1 communications therefore experience less latency as the establishment of TCP connections presents considerable overhead.
History.
The term hypertext was coined by Ted Nelson in 1965 in the Xanadu Project, which was in turn inspired by Vannevar Bush's 1930s vision of the microfilm-based information retrieval and management "memex" system described in his 1945 essay "As We May Think". Tim Berners-Lee and his team at CERN are credited with inventing the original HTTP, along with HTML and the associated technology for a web server and a text-based web browser. Berners-Lee first proposed the "WorldWideWeb" project in 1989—now known as the World Wide Web. The first version of the protocol had only one method, namely GET, which would request a page from a server. The response from the server was always an HTML page.
The first documented version of HTTP was HTTP V0.9 (1991). Dave Raggett led the HTTP Working Group (HTTP WG) in 1995 and wanted to expand the protocol with extended operations, extended negotiation, richer meta-information, tied with a security protocol which became more efficient by adding additional methods and header fields. officially introduced and recognized HTTP V1.0 in 1996.
The HTTP WG planned to publish new standards in December 1995 and the support for pre-standard HTTP/1.1 based on the then developing (called HTTP-NG) was rapidly adopted by the major browser developers in early 1996. End-user adoption of the new browsers was rapid. In March 1996, one web hosting company reported that over 40% of browsers in use on the Internet were HTTP 1.1 compliant. That same web hosting company reported that by June 1996, 65% of all browsers accessing their servers were HTTP/1.1 compliant. The HTTP/1.1 standard as defined in was officially released in January 1997. Improvements and updates to the HTTP/1.1 standard were released under in June 1999.
In 2007, the HTTP Working Group was formed, in part, to revise and clarify the HTTP/1.1 specification. In June 2014, the WG released an updated six-part specification obsoleting :
HTTP/2 was published as in May 2015.
HTTP session.
An HTTP session is a sequence of network request–response transactions. An HTTP client initiates a request by establishing a Transmission Control Protocol (TCP) connection to a particular port on a server (typically port 80, occasionally port 8080; see List of TCP and UDP port numbers). An HTTP server listening on that port waits for a client's request message. Upon receiving the request, the server sends back a status line, such as "", and a message of its own. The body of this message is typically the requested resource, although an error message or other information may also be returned.
Persistent connections.
In HTTP/0.9 and 1.0, the connection is closed after a single request/response pair. In HTTP/1.1 a keep-alive-mechanism was introduced, where a connection could be reused for more than one request. Such "persistent connections" reduce request latency perceptibly because the client does not need to re-negotiate the TCP 3-Way-Handshake connection after the first request has been sent. Another positive side effect is that, in general, the connection becomes faster with time due to TCP's slow-start-mechanism.
Version 1.1 of the protocol also made bandwidth optimization improvements to HTTP/1.0. For example, HTTP/1.1 introduced chunked transfer encoding to allow content on persistent connections to be streamed rather than buffered. HTTP pipelining further reduces lag time, allowing clients to send multiple requests before waiting for each response. Another addition to the protocol was byte serving, where a server transmits just the portion of a resource explicitly requested by a client.
HTTP session state.
HTTP is a stateless protocol. A stateless protocol does not require the HTTP server to retain information or status about each user for the duration of multiple requests. However, some web applications implement states or server side sessions using for instance HTTP cookies or hidden variables within web forms.
HTTP authentication.
HTTP provides multiple authentication schemes such as basic access authentication and digest access authentication which operate via a challenge–response mechanism whereby the server identifies and issues a challenge before serving the requested content.
HTTP provides a general framework for access control and authentication, via an extensible set of challenge–response authentication schemes, which can be used by a server to challenge a client request and by a client to provide authentication information.
Authentication realms.
The HTTP Authentication specification also provides an arbitrary, implementation-specific construct for further dividing resources common to a given root URI. The realm value string, if present, is combined with the canonical root URI to form the protection space component of the challenge. This in effect allows the server to define separate authentication scopes under one root URI.
Message format.
The client sends requests to the server and the server sends responses.
Request message.
The request message consists of the following:
The request line and other header fields must each end with <CR><LF> (that is, a carriage return character followed by a line feed character). The empty line must consist of only <CR><LF> and no other whitespace. In the HTTP/1.1 protocol, all header fields except "Host" are optional.
A request line containing only the path name is accepted by servers to maintain compatibility with HTTP clients before the HTTP/1.0 specification in .
Request methods.
HTTP defines methods (sometimes referred to as "verbs", but nowhere in the specification does it mention "verb", nor is OPTIONS or HEAD a verb) to indicate the desired action to be performed on the identified resource. What this resource represents, whether pre-existing data or data that is generated dynamically, depends on the implementation of the server. Often, the resource corresponds to a file or the output of an executable residing on the server. The HTTP/1.0 specification defined the GET, HEAD and POST methods and the HTTP/1.1 specification added five new methods: OPTIONS, PUT, DELETE, TRACE and CONNECT. By being specified in these documents, their semantics are well-known and can be depended on. Any client can use any method and the server can be configured to support any combination of methods. If a method is unknown to an intermediate, it will be treated as an unsafe and non-idempotent method. There is no limit to the number of methods that can be defined and this allows for future methods to be specified without breaking existing infrastructure. For example, WebDAV defined seven new methods and specified the PATCH method.
Method names are case sensitive. This is in contrast to HTTP header field names which are case-insensitive.
All general-purpose HTTP servers are required to implement at least the GET and HEAD methods, and all other methods are considered optional by the specification.
Safe methods.
Some of the methods (for example, GET, HEAD, OPTIONS and TRACE) are, by convention, 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, beyond relatively harmless effects such as logging, web caching, the serving of banner advertisements or incrementing a web counter. Making arbitrary GET requests without regard to the context of the application's state should therefore be considered safe. However, this is not mandated by the standard, and it is explicitly acknowledged that it cannot be guaranteed.
By contrast, methods such as POST, PUT, DELETE and PATCH are intended for actions that may cause side effects either on the server, or external side effects such as financial transactions or transmission of email. Such methods are therefore not usually used by conforming web robots or web crawlers; some that do not conform tend to make requests without regard to context or consequences.
Despite the prescribed safety of "GET" requests, in practice their handling by the server is not technically limited in any way. Therefore, careless or deliberate programming can cause non-trivial changes on the server. This is discouraged, because it can cause problems for web caching, search engines and other automated agents, which can make unintended changes on the server. For example, a website might allow deletion of a resource through a URL such as "https://example.com/article/1234/delete", which, if arbitrarily fetched, even using "GET", would simply delete the article.
One example of this occurring in practice was during the short-lived Google Web Accelerator beta, which prefetched arbitrary URLs on the page a user was viewing, causing records to be automatically altered or deleted "en masse". The beta was suspended only weeks after its first release, following widespread criticism.
Idempotent methods and web applications.
Methods PUT and DELETE are defined to be idempotent, meaning that multiple identical requests should have the same effect as a single request. Methods GET, HEAD, OPTIONS and TRACE, being prescribed as safe, should also be idempotent, as HTTP is a stateless protocol.
In contrast, the POST method is not necessarily idempotent, and therefore sending an identical POST request multiple times may further affect state or cause further side effects (such as financial transactions). In some cases this may be desirable, but in other cases this could be due to an accident, such as when a user does not realize that their action will result in sending another request, or they did not receive adequate feedback that their first request was successful. While web browsers may show alert dialog boxes to warn users in some cases where reloading a page may re-submit a POST request, it is generally up to the web application to handle cases where a POST request should not be submitted more than once.
Note that whether a method is idempotent is not enforced by the protocol or web server. It is perfectly possible to write a web application in which (for example) a database insert or other non-idempotent action is triggered by a GET or other request. Ignoring this recommendation, however, may result in undesirable consequences, if a user agent assumes that repeating the same request is safe when it is not.
Security.
The TRACE method can be used as part of a class of attacks known as cross-site tracing; for that reason, common security advice is for it to be disabled in the server configuration. Microsoft IIS supports a proprietary "TRACK" method, which behaves similarly, and which is likewise recommended to be disabled.
Response message.
The response message consists of the following:
The status line and other header fields must all end with <CR><LF>. The empty line must consist of only <CR><LF> and no other whitespace. This strict requirement for <CR><LF> is relaxed somewhat within message bodies for consistent use of other system line breaks such as <CR> or <LF> alone.
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 depends primarily on the code, and secondarily on the other response header fields. Custom status codes can be used, for 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.
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. HTTP status code is primarily divided into five groups for better explanation of request and responses between client and server as named:
Encrypted connections.
The most popular way of establishing an encrypted HTTP connection is HTTPS. Two other methods for establishing an encrypted HTTP connection also exist: Secure Hypertext Transfer Protocol, and using the HTTP/1.1 Upgrade header to specify an upgrade to TLS. Browser support for these two is, however, nearly non-existent.
Example session.
Below is a sample conversation between an HTTP client and an HTTP server running on www.example.com, port 80.
Client request.
GET / HTTP/1.1
Host: www.example.com
A client request (consisting in this case of the request line and only one header field) is followed by a blank line, so that the request ends with a double newline, each in the form of a carriage return followed by a line feed. The "Host" field 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. (A "/" (slash) will usually fetch a /index.html file if there is one.)
Server response.
HTTP/1.1 200 OK
Date: Mon, 23 May 2005 22:38:34 GMT
Content-Type: text/html; charset=UTF-8
Content-Length: 155
Last-Modified: Wed, 08 Jan 2003 23:11:55 GMT
Server: Apache/1.3.3.7 (Unix) (Red-Hat/Linux)
ETag: "3f80f-1b6-3e1cb03b"
Accept-Ranges: bytes
Connection: close
<html>
 <head>
 <title>An Example Page</title>
 </head>
 <body>
 <p>Hello World, this is a very simple HTML document.</p>
 </body>
</html>
The ETag (entity tag) header field is used to determine if a cached version of the requested resource is identical to the current version of the resource on the server. "Content-Type" specifies the Internet media type of the data conveyed by the HTTP message, while "Content-Length" indicates its length in bytes. The HTTP/1.1 webserver publishes its ability to respond to requests for certain byte ranges of the document by setting the field "Accept-Ranges: bytes". This is useful, if the client needs to have only certain portions of a resource sent by the server, which is called byte serving. When "Connection: close" is sent, it means that the web server will close the TCP connection immediately after the transfer of this response.
Most of the header lines are optional. When "Content-Length" is missing the length is determined in other ways. Chunked transfer encoding uses a chunk size of 0 to mark the end of the content. "Identity" encoding without "Content-Length" reads content until the socket is closed.
A "Content-Encoding" like "gzip" can be used to compress the transmitted data.