Hypertext Transfer Protocol
The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model 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 and summarized in a simple document describing the behavior of a client and a server using the first HTTP protocol version  that was named 0.9.<ref name="HTTP/0.9-specifications"></ref> That first version of HTTP protocol soon evolved into a more elaborated version that was the first draft toward a far future version 1.0.<ref name="HTTP/1.0-first-unofficial-draft"></ref> Development of early HTTP Requests for Comments (RFCs) started a few years later and it 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 finalized and fully documented (as version 1.0) in 1996. It evolved (as version 1.1) in 1997 and then its specifications were updated  in 1999 and in 2014. Its secure variant named HTTPS is used by more than 76% of websites. HTTP/2 is a more efficient expression of HTTP's semantics "on the wire", and was published in 2015; it is used by more than 45% of websites; it is now supported by almost all web browsers (96% of users) 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; it is used by more than 20% of websites; it is now  supported by many web browsers (73% of users). HTTP/3 uses QUIC instead of TCP for the underlying transport protocol. Like HTTP/2, it does not obsolesce previous major versions of the protocol. Support for HTTP/3 was added to Cloudflare and Google Chrome first, and is also enabled in Firefox. Technical overview. HTTP functions as a request–response protocol in the client–server model. A web browser, for example, may be the "client" whereas a process, named web server, running on a computer hosting one or more websites 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, whenever 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. To allow intermediate HTTP nodes (proxy servers, web caches, etc.) to accomplish their functions, some of the HTTP headers (found in HTTP requests/responses) are managed hop-by-hop whereas other HTTP headers are managed end -to-end (managed only by the source client and by the target web server). 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, thus 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. In HTTP/1.0 a separate connection to the same server is made for every resource request. In HTTP/1.1 instead a TCP connection can be reused to make multiple resource requests (i.e. of HTML pages, frames, images, scripts, stylesheets, etc.). HTTP/1.1 communications therefore experience less latency as the establishment of TCP connections presents considerable overhead, specially under high traffic conditions. HTTP/2 is a revision of previous HTTP/1.1 in order to maintain the same client-server model and the same protocol methods but with these differences in  order: HTTP/2 communications therefore experience much less latency and, in most cases, even more speed than HTTP/1.1 communications. HTTP/3 is a revision of previous HTTP/2 in order to use QUIC + UDP transport protocols instead of TCP/IP connections also to slightly improve the average speed of communications and to avoid the occasional (very rare) problem of TCP/IP connection congestion that can temporarily block or slow down the data flow of all its streams (another form of ""head of line blocking""). 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 client user interface called web browser. Berners-Lee first proposed the "WorldWideWeb" project in 1989, now known as the World Wide Web. The first web server went live in 1990. The protocol used had only one method, namely GET, which would request a page from  a server. The response from the server was always an HTML page. HTTP data exchange. HTTP is a stateless application-level protocol and it requires a reliable network transport connection to exchange data between client and server. In HTTP implementations TCP/IP connections are used using well known ports (typically port 80 if connection is unencrypted or port 443 if connection is encrypted, see also List of TCP and UDP port numbers). In HTTP/2 a TCP/IP connection + multiple protocol channels are used. In HTTP/3 an application transport protocol QUIC + UDP is used. Request and response messages through  connections. Data is exchanged through a sequence of request–response messages which are exchanged by a session layer transport connection. An HTTP client initially tries to connect to a server establishing a connection (real or virtual). An HTTP(S) server listening on that port accepts the connection and then waits for a client's request message. The client sends its request to the server. Upon receiving the request, the server sends back an HTTP response message (header plus a body if it is required). The body of this message is typically the requested resource, although an error message or other information  may also be returned. At any time (for many reasons) client or server can close the connection. Closing a connection is usually advertised in advance by using one or more HTTP headers in the last request/response message sent to server or client. Persistent connections. In HTTP/0.9, the TCP/IP connection is always closed after server response has been sent, so it is never persistent. In HTTP/1.0, as stated in RFC 1945, the TCP/IP connection should always be closed by server after a response has been sent. In HTTP/1.1 a keep-alive-mechanism was officially introduced so that a  connection could be reused for more than one request/response. 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. HTTP/1.1 added also HTTP pipelining in order to further reduce lag time when using persistent connections by allowing clients to send multiple requests before waiting for each response. This optimization was never considered really safe because a few web servers  and many proxy servers, specially transparent proxy servers placed in Internet / Intranets between clients and servers, did not handled pipelined requests properly (they served only the first request discarding the others, they closed the connection because they saw more data after the first request or some proxies even returned responses out of order etc.). Besides this only HEAD and some GET requests (i.e. limited to real file requests and so with URLs without query string used as a command, etc.) could be pipelined in a safe and idempotent mode. After many years of struggling with the problems introduced by enabling  pipelining, this feature was first disabled and then removed from most browsers also because of the announced adoption of HTTP/2. HTTP/2 extended the usage of persistent connections by multiplexing many concurrent requests/responses through a single TCP/IP connection. HTTP/3 does not use TCP/IP connections but QUIC + UDP (see also: technical overview). 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. Above mechanism belong to HTTP protocol and it is managed by client and server HTTP software (if configured to require authentication before allowing client access to one or more web resources), not by web application that usually use a web application session. 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. HTTP application session. HTTP is a stateless protocol. A stateless protocol does not require the web server to retain information or status about each user for the duration of multiple requests. Some web applications need to manage user sessions, so they implement states, or server side sessions, using for instance HTTP cookies or hidden variables within web forms. To start an application user session, an interactive authentication via web  application login must be performed. To stop a user session a logout operation must be requested by user. These kind of operations do not use HTTP authentication but a custom managed web application authentication. HTTP/1.1 request messages. Request messages are sent by a client to a target server. Request syntax. A client sends "request messages" to the server, which consist of: GET /images/logo.png HTTP/1.1 Host: www.example.com In the HTTP/1.1 protocol, all header fields except codice_2 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") 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: PUT, DELETE, CONNECT, OPTIONS, and TRACE. 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, which 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 web servers are required to implement at least the GET and  HEAD methods, and all other methods are considered optional by the specification. Safe methods. A request method is "safe" if a request with that method has no intended effect on the server. The methods GET, HEAD, OPTIONS, and TRACE are defined as safe. In other words, safe methods are intended to be read-only. They do not include side effects though, such as appending request information to a log file or charging an advertising account, since they are not requested by the client, by definition. In contrast, the methods POST, PUT, DELETE, CONNECT, and PATCH are not safe. They may  modify the state of the server or have other effects such as sending an 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. Careless or deliberately irregular programming can allow GET requests to cause non-trivial changes on the server. This is discouraged because of the problems which can occur when web caching, search engines, and other automated  agents make unintended changes on the server. For example, a website might allow deletion of a resource through a URL such as "<nowiki>https://example.com/article/1234/delete</nowiki>", which, if arbitrarily fetched, even using GET, would simply delete the article. A properly coded website would require a DELETE or POST method for this action, which non-malicious bots would not make. 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. A request method is "idempotent" if multiple identical requests with that method have the same effect as a single such request. The methods PUT and DELETE, and safe methods are defined as idempotent. Safe methods are trivially idempotent, since they are intended to have no effect on the server whatsoever; the PUT and DELETE methods, meanwhile, are idempotent since successive identical requests will be ignored. A website might, for instance, set up a PUT endpoint to modify a user's recorded email address. If this endpoint  is configured correctly, any requests which ask to change a user's email address to the same email address which is already recorded—e.g. duplicate requests following a successful request—will have no effect. Similarly, a request to DELETE a certain user will have no effect if that user has already been deleted. In contrast, the methods POST, CONNECT, and PATCH are not necessarily idempotent, and therefore sending an identical POST request multiple times may further modify the state of the server or have further effects, such as sending multiple emails. In some cases this is the desired effect, but  in other cases it may occur accidentally. A user might, for example, inadvertently send multiple POST requests by clicking a button again if they were not given clear feedback that the first click was being processed. 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 or not 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. To do so against recommendations, however, may result in undesirable consequences, if a user agent assumes that repeating the same request is safe when it is not. Cacheable methods. A request method is "cacheable" if responses to requests with that method may be stored for future reuse. The methods GET, HEAD, and POST are defined as cacheable. In contrast, the methods PUT, DELETE, CONNECT, OPTIONS, TRACE, and PATCH are not cacheable . Request header fields. Request header fields allow the client to pass additional information beyond the request line, acting as request modifiers (similarly to the parameters of a procedure). They give information about the client, about the target resource, or about the expected handling of the request. HTTP/1.1 response messages. A response message is sent by a server to a client as a reply to its former request message. Response syntax. A server sends "response messages" to the client, which consist of: HTTP/1.1 200 OK Content-Type: text/html Response 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 response status code is a three-digit integer code representing the result of the server's attempt to understand and satisfy the client's corresponding request. The way the client handles the response depends primarily on the status code, and secondarily on the other response header fields. Clients may not understand all registered status codes but they must understand their class (given by the first digit of the status code) and treat an unrecognized  status code as being equivalent to the x00 status code of that class. 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. The first digit  of the status code defines its class: Response header fields. The response header fields allow the server to pass additional information beyond the status line, acting as response modifiers. They give information about the server or about further access to the target resource or related resources. Each response header field has a defined meaning which can be further refined by the semantics of the request method or response status code. HTTP/1.1 example of request / response transaction. Below is a sample HTTP transaction between an HTTP/1.1 client and an HTTP/1.1 server running on www.example.com, port 80. Client request. GET / HTTP/1.1 Host : www.example.com User-Agent: Mozilla/5.0 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8 Accept-Language: en-GB,en;q=0.5 Accept-Encoding: gzip, deflate, br Connection: keep-alive A client request (consisting in this case of the request line and a few headers that can be reduced to only the codice_9 header) is followed by a blank line, so that the request ends with a double end of line, each in the form of a carriage return followed by a line feed. The codice_9 header value 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> </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. codice_11 specifies the Internet media type of the data conveyed by the HTTP message, while codice_12 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 codice_13. 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 codice_14 is sent, it means that the web server will close the TCP connection immediately after the end of the transfer of this response. Most of the header lines are  optional but some are mandatory. When header codice_1 is missing in a response with an entity body then this should be considered an error in HTTP/1.0 but it may not be an error in HTTP/1.1 if header codice_16 is present. Chunked transfer encoding uses a chunk size of 0 to mark the end of the content. Some old implementations of HTTP/1.0 omitted the header codice_12 when the length of the body entity was not known at the beginning of the response and so the transfer of data to client continued until server closed the socket. A codice_18 can be used  to inform the client that the body entity part of the transmitted data is compressed by gzip algorithm. 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.