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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 two-thirds 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.
A client sends request messages
to the server, which consist of:
- a request line, consisting of the case-sensitive request method, a space, the request target, another space, the protocol version, a carriage return, and a line feed (e.g. GET /images/logo.png HTTP/1.1);
- zero or more request header fields, each consisting of the case-insensitive field name, a colon, optional leading whitespace, the field value, and optional trailing whitespace (e.g. Accept-Language: en), and ending with a carriage return and a line feed;
- an empty line, consisting of a carriage return and a line feed;
- an optional message body.
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 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: PUT, DELETE, CONNECT, OPTIONS, and TRACE. 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 that the target resource transfers a representation of its state. GET requests 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 requests that the target resource transfers a representation of its state, like for a GET request, but without the representation data enclosed in the response body. This is useful for retrieving the representation metadata in the response header, without having to transfer the entire representation.
The POST method
requests that the target resource processes the representation enclosed in the request according to the semantics of the target resource. For example, it is used for posting a message to an Internet forum
, subscribing to a mailing list
, or completing an online shopping
The PUT method requests that the target resource creates or updates its state with the state defined by the representation enclosed in the request.
The DELETE method requests that the target resource deletes its state.
The OPTIONS method requests that the target resource transfers the HTTP methods that it supports. This can be used to check the functionality of a web server by requesting '*' instead of a specific resource.
The TRACE method requests that the target resource transfers the received request in the response body. That way a client can see what (if any) changes or additions have been made by intermediaries.
The PATCH method requests that the target resource modifies its state according to the partial update defined in the representation enclosed in the request.
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.
Properties of request 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 exclude 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. 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.
A request method is idempotent if multiple identical requests with that method have the same effect as the intended effect of a single such request. The methods PUT and DELETE, and safe methods are defined as idempotent.
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 an email
. 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.
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.
A server sends response messages
to the client, which consist of:
- a status line, consisting of the protocol version, a space, the response status code, another space, a possibly empty reason phrase, a carriage return, and a line feed (e.g. HTTP/1.1 200 OK);
- zero or more response header fields, each consisting of the case-insensitive field name, a colon, optional leading whitespace, the field value, and optional trailing whitespace (e.g. Content-Type: text/html), and ending with a carriage return and a line feed;
- an empty line, consisting of a carriage return and a line feed;
- an optional message body.
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
The first digit of the status code defines its class:
The request was received, continuing process.
The request was successfully received, understood, and accepted.
Further action needs to be taken in order to complete the request.
4XX (client error)
The request contains bad syntax or cannot be fulfilled.
5XX (server error)
The server failed to fulfill an apparently valid request.
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.
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/126.96.36.199 (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 which mandates privacy-related features.
- ^ a b c 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 "Message format". RFC 7230: HTTP/1.1 Message Syntax and Routing. p. 19. sec. 3. doi:10.17487/RFC7230. RFC 7230.
- ^ "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.
- ^ 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 24 June 2021, at 07:07
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