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Main article: "List of HTTP header fields
GET
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.)[1] The "W3C has published guidance principles on this distinction, saying, ""Web application design should be informed by the above principles, but also by the relevant limitations."[13] See safe methods below.
HEAD
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.[14]
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.[15]
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
[16] 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.[17][18] See "HTTP CONNECT tunneling.
PATCH
The PATCH method applies partial modifications to a resource.[19]

All general-purpose HTTP servers are required to implement at least the GET and HEAD methods,[20] and, whenever possible, also the OPTIONS method.["citation needed]

Safe methods[edit]

Some of the methods (for example, HEAD, GET, 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, "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.

Idempotent methods and web applications[edit]

Methods PUT and DELETE are defined to be "idempotent, meaning that multiple identical requests should have the same effect as a single request (note that idempotence refers to the state of the system after the request has completed, so while the action the server takes (e.g. deleting a record) or the response code it returns may be different on subsequent requests, the system state will be the same every time["citation needed]). Methods GET, HEAD, OPTIONS and TRACE, being prescribed as safe, should also be idempotent, as HTTP is a "stateless protocol.[1]

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 isn't.

Security[edit]

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.[21] Microsoft "IIS supports a proprietary "TRACK" method, which behaves similarly, and which is likewise recommended to be disabled.[21]

Summary table[edit]

HTTP Method RFC Request Has Body Response Has Body Safe Idempotent Cacheable
GET RFC 7231 No Yes Yes Yes Yes
HEAD RFC 7231 No No Yes Yes Yes
POST RFC 7231 Yes Yes No No Yes
PUT RFC 7231 Yes Yes No Yes No
DELETE RFC 7231 No Yes No Yes No
CONNECT RFC 7231 Yes Yes No No No
OPTIONS RFC 7231 Optional Yes Yes Yes No
TRACE RFC 7231 No Yes Yes Yes No
PATCH RFC 5789 Yes Yes No No Yes

Status codes[edit]

List of HTTP 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 primarily depends on the code and secondarily on the other "response header fields. Custom status codes can be used since, 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.[22]

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 and Server Error 5XX.

Persistent connections[edit]

HTTP persistent connection

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[edit]

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.

Encrypted connections[edit]

The most popular way of establishing an encrypted HTTP connection is "HTTP Secure.[23] 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.[24][25][26]

Message format[edit]

The client and server communicate by sending plain-text ("ASCII) messages. The client sends requests to the server and the server sends responses.

Request message[edit]

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.[27] 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.[28]

Response message[edit]

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.[27] This strict requirement for <CR><LF> is relaxed somewhat within message bodies for consistent use of other system linebreaks such as <CR> or <LF> alone.[29]

Example session[edit]

Below is a sample conversation between an HTTP client and an HTTP server running on "www.example.com, port 80. As mentioned in the previous sections, all the data is sent in a plain-text ("ASCII) encoding, using a "two-byte CR LF ('\r\n') line ending at the end of each line.

Client request[edit]

GET /index.html 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.

Server response[edit]

HTTP/1.1 200 OK
Date: Mon, 23 May 2005 22:38:34 GMT
Content-Type: text/html; charset=UTF-8
Content-Encoding: UTF-8
Content-Length: 138
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>
  Hello World, this is a very simple HTML document.
</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[30] 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.

Similar protocols[edit]

The "Gopher protocol was a content delivery protocol that was displaced by HTTP in the early 1990s. The "SPDY protocol is an alternative to HTTP developed at "Google, it is superseded by the new HTTP protocol, "HTTP/2.

See also[edit]

Notes[edit]

  1. ^ 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. RFC 2616. https://tools.ietf.org/html/rfc2616. 
  2. ^ "Overall Operation". p. 12. sec. 1.4. RFC 2616. https://tools.ietf.org/html/rfc2616#section-1.4. 
  3. ^ Berners-Lee, Tim. "HyperText Transfer Protocol". "World Wide Web Consortium. Retrieved 31 August 2010. 
  4. ^ "Tim Berners-Lee. "The Original HTTP as defined in 1991". "World Wide Web Consortium. Retrieved 24 July 2010. 
  5. ^ Raggett, Dave. "Dave Raggett's Bio". "World Wide Web Consortium. Retrieved 11 June 2010. 
  6. ^ Raggett, Dave; Berners-Lee, Tim. "Hypertext Transfer Protocol Working Group". World Wide Web Consortium. Retrieved 29 September 2010. 
  7. ^ Raggett, Dave. "HTTP WG Plans". World Wide Web Consortium. Retrieved 29 September 2010. 
  8. ^ a b c Simon Spero. "Progress on HTTP-NG". "World Wide Web Consortium. Retrieved 11 June 2010. 
  9. ^ "HTTP/1.1". Webcom.com Glossary entry. Retrieved 2009-05-29. 
  10. ^ a b Fielding, Roy T.; Reschke, Julian F. (June 2014). Hypertext Transfer Protocol (HTTP/1.1): Authentication. "IETF. RFC 7235. https://tools.ietf.org/html/rfc7235. 
  11. ^ Berners-Lee, Tim; Fielding, Roy T.; Nielsen, Henrik Frystyk. "Method Definitions". Hypertext Transfer Protocol -- HTTP/1.0. "IETF. pp. 30-32. sec. 8. RFC 1945. https://tools.ietf.org/html/rfc1945#section-8. 
  12. ^ "Method Definitions". pp. 51-57. sec. 9. RFC 2616. https://tools.ietf.org/html/rfc2616#section-9. 
  13. ^ Jacobs, Ian (2004). "URIs, Addressability, and the use of HTTP GET and POST". Technical Architecture Group finding. W3C. Retrieved 26 September 2010. 
  14. ^ "POST". p. 54. sec. 9.5. RFC 2616. https://tools.ietf.org/html/rfc2616#section-9.5. 
  15. ^ "PUT". p. 55. sec. 9.6. RFC 2616. https://tools.ietf.org/html/rfc2616#section-9.6. 
  16. ^ "CONNECT". Hypertext Transfer Protocol -- HTTP/1.1. "IETF. June 1999. p. 57. sec. 9.9. RFC 2616. https://tools.ietf.org/html/rfc2616#section-9.9. Retrieved 23 February 2014. 
  17. ^ Khare, Rohit; Lawrence, Scott (May 2000). Upgrading to TLS Within HTTP/1.1. "IETF. RFC 2817. https://tools.ietf.org/html/rfc2817. 
  18. ^ "Vulnerability Note VU#150227: HTTP proxy default configurations allow arbitrary TCP connections". "US-CERT. 2002-05-17. Retrieved 2007-05-10. 
  19. ^ Dusseault, Lisa; Snell, James M. (March 2010). PATCH Method for HTTP. "IETF. RFC 5789. https://tools.ietf.org/html/rfc5789. 
  20. ^ "Method". p. 36. sec. 5.1.1. RFC 2616. https://tools.ietf.org/html/rfc2616#section-5.1.1. 
  21. ^ a b "Cross Site Tracing". "OWASP. Retrieved 2016-06-22. 
  22. ^ "Status-Line". p. 39. sec. 6.1. RFC 2616. https://tools.ietf.org/html/rfc2616#section-6.1. 
  23. ^ Canavan, John (2001). Fundamentals of Networking Security. Norwood, MA: Artech House. pp. 82–83. "ISBN "9781580531764. 
  24. ^ Zalewski, Michal. "Browser Security Handbook". Retrieved 30 April 2015. 
  25. ^ "Chromium Issue 4527: implement RFC 2817: Upgrading to TLS Within HTTP/1.1". Retrieved 30 April 2015. 
  26. ^ "Mozilla Bug 276813 - [RFE] Support RFC 2817 / TLS Upgrade for HTTP 1.1". Retrieved 30 April 2015. 
  27. ^ a b "HTTP Message". p. 31. sec. 4. RFC 2616. https://tools.ietf.org/html/rfc2616#section-4. 
  28. ^ "Apache Week. HTTP/1.1".  090502 apacheweek.com
  29. ^ "Canonicalization and Text Defaults". sec. 3.7.1. RFC 2616. https://tools.ietf.org/html/rfc2616#section-3.7.1. 
  30. ^ Luotonen, Ari; Franks, John (February 22, 1996). Byte Range Retrieval Extension to HTTP. "IETF. I-D draft-ietf-http-range-retrieval-00. https://tools.ietf.org/html/draft-ietf-http-range-retrieval-00. 
  31. ^ Nottingham, Mark (October 2010). Web Linking. "IETF. RFC 5988. https://tools.ietf.org/html/rfc5988. 
  32. ^ "Hypertext Transfer Protocol Bis (httpbis) – Charter". IETF. 2012. 

References[edit]

External links[edit]

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