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The Hypertext Transfer Protocol
) is an application layer
protocol for distributed, collaborative, hypermedia
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.
Hypertext Transfer Protocol
HTTP/1 was first documented (as version 1.1) in 1997.
As of 2021, about 30% of websites
only support HTTP/1.
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 QUIC
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
beginning with the HTTP scheme and the WWW
domain name label
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
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.
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/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
after the page has been delivered. HTTP/1.1 communications therefore experience less latency
as the establishment of TCP connections presents considerable overhead.
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
. RFC 1945
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 RFC 2068
(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 RFC 2068
was officially released in January 1997. Improvements and updates to the HTTP/1.1 standard were released under RFC 2616
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 RFC 2616
- RFC 7230, HTTP/1.1: Message Syntax and Routing
- RFC 7231, HTTP/1.1: Semantics and Content
- RFC 7232, HTTP/1.1: Conditional Requests
- RFC 7233, HTTP/1.1: Range Requests
- RFC 7234, HTTP/1.1: Caching
- RFC 7235, HTTP/1.1: Authentication
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 "HTTP/1.1 200 OK", 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.
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
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 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.
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.
The client sends requests to the server and the server sends responses.
The request message consists of the following:
- a request line (e.g., GET /images/logo.png HTTP/1.1, which requests a resource called /images/logo.png from the server)
- request header fields (e.g., Accept-Language: en)
- an empty line
- an optional message body
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
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 RFC 1945
An HTTP/1.1 request made using telnet. The request
header section, and response body are highlighted.
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 RFC 5789
specified the PATCH
Method names are case sensitive.
This is in contrast to HTTP header field names which are case-insensitive.
The GET method requests a representation of the specified resource. Requests using GET should only retrieve data
and should have no other effect. (This is also true of some other HTTP methods.)
has published guidance principles on this distinction, saying, "Web application
design should be informed by the above principles, but also by the relevant limitations."
See safe methods
The HEAD method asks for a response identical to that of 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.POST
The POST method
requests that the server accept the entity enclosed in the request as a new subordinate of the web resource
identified by the URI. The data POSTed might be, for example, an annotation for existing resources; a message for a bulletin board, newsgroup, mailing list, or comment thread; a block of data that is the result of submitting a web form
to a data-handling process; or an item to add to a database.PUT
The PUT method requests that the enclosed entity be stored under the supplied URI
. If the URI refers to an already existing resource, it is modified; if the URI does not point to an existing resource, then the server can create the resource with that URI.DELETE
The DELETE method deletes the specified resource.TRACE
The TRACE method echoes the received request so that a client can see what (if any) changes or additions have been made by intermediate servers.OPTIONS
The OPTIONS method returns the HTTP methods that the server supports for the specified URL
. This can be used to check the functionality of a web server by requesting '*' instead of a specific resource.CONNECT
The CONNECT method converts the request connection to a transparent TCP/IP tunnel
, usually to facilitate SSL
-encrypted communication (HTTPS) through an unencrypted HTTP proxy
See HTTP CONNECT method
The PATCH method applies partial modifications to a resource.
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.
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 Acceleratorbeta
, 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.
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.
supports a proprietary "TRACK" method, which behaves similarly, and which is likewise recommended to be disabled.
Security of HTTP methods
The response message consists of the following:
- a status line which includes the status code and reason message (e.g., HTTP/1.1 200 OK, which indicates that the client's request succeeded)
- response header fields (e.g., Content-Type: text/html)
- an empty line
- an optional message body
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.
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:
- Informational 1XX
- Successful 2XX
- Redirection 3XX
- Client Error 4XX
- Server Error 5XX
Below is a sample conversation between an HTTP client and an HTTP server running on www.example.com
, port 80.
GET / HTTP/1.1Host: 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.)
HTTP/1.1 200 OKDate: Mon, 23 May 2005 22:38:34 GMTContent-Type: text/html; charset=UTF-8Content-Length: 155Last-Modified: Wed, 08 Jan 2003 23:11:55 GMTServer: Apache/22.214.171.124 (Unix) (Red-Hat/Linux)ETag: "3f80f-1b6-3e1cb03b"Accept-Ranges: bytesConnection: close<html>
<title>An Example Page</title>
<p>Hello World, this is a very simple HTML document.</p>
(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.
can be used to compress the transmitted data.
- The Gopher protocol is a content delivery protocol that was displaced by HTTP in the early 1990s.
- The SPDY protocol is an alternative to HTTP developed at Google, superseded by HTTP/2.
- The Gemini protocol is a Gopher inspired protocol who mandates privacy-related features.
- ^ a b c d Fielding, Roy T.; Gettys, James; Mogul, Jeffrey C.; Nielsen, Henrik Frystyk; Masinter, Larry; Leach, Paul J.; Berners-Lee, Tim (June 1999). Hypertext Transfer Protocol – HTTP/1.1. IETF. doi:10.17487/RFC2616. RFC 2616.
- ^ In RFC 2068. That specification was obsoleted by RFC 2616 in 1999, which was likewise replaced by RFC 7230 in 2014.
- ^ "Can I use... Support tables for HTML5, CSS3, etc". caniuse.com. Retrieved 2020-06-02.
- ^ "Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension". IETF. July 2014. RFC 7301.
- ^ Belshe, M.; Peon, R.; Thomson, M. "Hypertext Transfer Protocol Version 2, Use of TLS Features". Retrieved 2015-02-10.
- ^ Benjamin, David. "Using TLS 1.3 with HTTP/2". tools.ietf.org. Retrieved 2020-06-02. This lowers the barrier for deploying TLS 1.3, a major security improvement over TLS 1.2.
- ^ Bishop, Mike (February 2, 2021). "Hypertext Transfer Protocol Version 3 (HTTP/3)". tools.ietf.org. Retrieved 2021-04-07.
- ^ Cimpanu, Catalin. "HTTP-over-QUIC to be renamed HTTP/3 | ZDNet". ZDNet. Retrieved 2018-11-19.
- ^ Cimpanu, Catalin (26 September 2019). "Cloudflare, Google Chrome, and Firefox add HTTP/3 support". ZDNet. Retrieved 27 September 2019.
- ^ "HTTP/3: the past, the present, and the future". The Cloudflare Blog. 2019-09-26. Retrieved 2019-10-30.
- ^ "Firefox Nightly supports HTTP 3 - General - Cloudflare Community". 2019-11-19. Retrieved 2020-01-23.
- ^ "Overall Operation". RFC 2616. p. 12. sec. 1.4. doi:10.17487/RFC2616. RFC 2616.
- ^ "Classic HTTP Documents". W3.org. 1998-05-14. Retrieved 2010-08-01.
- ^ Berners-Lee, Tim. "HyperText Transfer Protocol". World Wide Web Consortium. Retrieved 31 August 2010.
- ^ Tim Berners-Lee. "The Original HTTP as defined in 1991". World Wide Web Consortium. Retrieved 24 July 2010.
- ^ Raggett, Dave. "Dave Raggett's Bio". World Wide Web Consortium. Retrieved 11 June 2010.
- ^ Raggett, Dave; Berners-Lee, Tim. "Hypertext Transfer Protocol Working Group". World Wide Web Consortium. Retrieved 29 September 2010.
- ^ Raggett, Dave. "HTTP WG Plans". World Wide Web Consortium. Retrieved 29 September 2010.
- ^ "HTTP/1.1". Webcom.com Glossary entry. Archived from the original on 2001-11-21. Retrieved 2009-05-29.
- ^ a b Fielding, Roy T.; Reschke, Julian F. (June 2014). Hypertext Transfer Protocol (HTTP/1.1): Authentication. IETF. doi:10.17487/RFC7235. RFC 7235.
- ^ a b "HTTP Message". RFC 2616. p. 31. sec. 4. doi:10.17487/RFC2616. RFC 2616.
- ^ "Apache Week. HTTP/1.1". 090502 apacheweek.com
- ^ Berners-Lee, Tim; Fielding, Roy T.; Nielsen, Henrik Frystyk. "Method Definitions". Hypertext Transfer Protocol – HTTP/1.0. IETF. pp. 30–32. sec. 8. doi:10.17487/RFC1945. RFC1945.
- ^ "Method Definitions". RFC 2616. pp. 51–57. sec. 9. doi:10.17487/RFC2616. RFC 2616.
- ^ "RFC-7210 section 3.1.1". Tools.ietf.org. Retrieved 2019-06-26.
- ^ "RFC-7231 section 4.1". Tools.ietf.org. Retrieved 2019-06-26.
- ^ "RFC-7230 section 3.2". Tools.ietf.org. Retrieved 2019-06-26.
- ^ Jacobs, Ian (2004). "URIs, Addressability, and the use of HTTP GET and POST". Technical Architecture Group finding. W3C. Retrieved 26 September 2010.
- ^ "POST". RFC 2616. p. 54. sec. 9.5. doi:10.17487/RFC2616. RFC 2616.
- ^ "PUT". RFC 2616. p. 55. sec. 9.6. doi:10.17487/RFC2616. RFC 2616.
- ^ "CONNECT". Hypertext Transfer Protocol – HTTP/1.1. IETF. June 1999. p. 57. sec. 9.9. doi:10.17487/RFC2616. RFC 2616. Retrieved 23 February 2014.
- ^ Khare, Rohit; Lawrence, Scott (May 2000). Upgrading to TLS Within HTTP/1.1. IETF. doi:10.17487/RFC2817. RFC 2817.
- ^ "Vulnerability Note VU#150227: HTTP proxy default configurations allow arbitrary TCP connections". US-CERT. 2002-05-17. Retrieved 2007-05-10.
- ^ Dusseault, Lisa; Snell, James M. (March 2010). PATCH Method for HTTP. IETF. doi:10.17487/RFC5789. RFC 5789.
- ^ "Method". RFC 2616. p. 36. sec. 5.1.1. doi:10.17487/RFC2616. RFC 2616.
- ^ a b Ediger, Brad (2007-12-21). Advanced Rails: Building Industrial-Strength Web Apps in Record Time. O'Reilly Media, Inc. p. 188. ISBN 978-0596519728. A common mistake is to use GET for an action that updates a resource. [...] This problem came into the Rails public eye in 2005, when the Google Web Accelerator was released.
- ^ Cantrell, Christian (2005-06-01). "What Have We Learned From the Google Web Accelerator?". Adobe Blogs. Adobe. Archived from the original on 2017-08-19. Retrieved 2018-11-19.
- ^ a b "Cross Site Tracing". OWASP. Retrieved 2016-06-22.
- ^ "Canonicalization and Text Defaults". RFC 2616. sec. 3.7.1. doi:10.17487/RFC2616. RFC 2616.
- ^ "Status-Line". RFC 2616. p. 39. sec. 6.1. doi:10.17487/RFC2616. RFC 2616.
- ^ Canavan, John (2001). Fundamentals of Networking Security. Norwood, MA: Artech House. pp. 82–83. ISBN 9781580531764.
- ^ Zalewski, Michal. "Browser Security Handbook". Retrieved 30 April 2015.
- ^ "Chromium Issue 4527: implement RFC 2817: Upgrading to TLS Within HTTP/1.1". Retrieved 30 April 2015.
- ^ "Mozilla Bug 276813 – [RFE] Support RFC 2817 / TLS Upgrade for HTTP 1.1". Retrieved 30 April 2015.
- ^ Luotonen, Ari; Franks, John (February 22, 1996). Byte Range Retrieval Extension to HTTP. IETF. I-D draft-ietf-http-range-retrieval-00.
- ^ Nottingham, Mark (October 2010). Web Linking. IETF. doi:10.17487/RFC5988. RFC5988.
- ^ "Hypertext Transfer Protocol Bis (httpbis) – Charter". IETF. 2012.
Last edited on 7 June 2021, at 18:52
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