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rfc:rfc7231

Internet Engineering Task Force (IETF) R. Fielding, Ed. Request for Comments: 7231 Adobe Obsoletes: 2616 J. Reschke, Ed. Updates: 2817 greenbytes Category: Standards Track June 2014 ISSN: 2070-1721

   Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content

Abstract

 The Hypertext Transfer Protocol (HTTP) is a stateless application-
 level protocol for distributed, collaborative, hypertext information
 systems.  This document defines the semantics of HTTP/1.1 messages,
 as expressed by request methods, request header fields, response
 status codes, and response header fields, along with the payload of
 messages (metadata and body content) and mechanisms for content
 negotiation.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7231.

Fielding & Reschke Standards Track [Page 1] RFC 7231 HTTP/1.1 Semantics and Content June 2014

Copyright Notice

 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Fielding & Reschke Standards Track [Page 2] RFC 7231 HTTP/1.1 Semantics and Content June 2014

Table of Contents

 1. Introduction ....................................................6
    1.1. Conformance and Error Handling .............................6
    1.2. Syntax Notation ............................................6
 2. Resources .......................................................7
 3. Representations .................................................7
    3.1. Representation Metadata ....................................8
         3.1.1. Processing Representation Data ......................8
         3.1.2. Encoding for Compression or Integrity ..............11
         3.1.3. Audience Language ..................................13
         3.1.4. Identification .....................................14
    3.2. Representation Data .......................................17
    3.3. Payload Semantics .........................................17
    3.4. Content Negotiation .......................................18
         3.4.1. Proactive Negotiation ..............................19
         3.4.2. Reactive Negotiation ...............................20
 4. Request Methods ................................................21
    4.1. Overview ..................................................21
    4.2. Common Method Properties ..................................22
         4.2.1. Safe Methods .......................................22
         4.2.2. Idempotent Methods .................................23
         4.2.3. Cacheable Methods ..................................24
    4.3. Method Definitions ........................................24
         4.3.1. GET ................................................24
         4.3.2. HEAD ...............................................25
         4.3.3. POST ...............................................25
         4.3.4. PUT ................................................26
         4.3.5. DELETE .............................................29
         4.3.6. CONNECT ............................................30
         4.3.7. OPTIONS ............................................31
         4.3.8. TRACE ..............................................32
 5. Request Header Fields ..........................................33
    5.1. Controls ..................................................33
         5.1.1. Expect .............................................34
         5.1.2. Max-Forwards .......................................36
    5.2. Conditionals ..............................................36
    5.3. Content Negotiation .......................................37
         5.3.1. Quality Values .....................................37
         5.3.2. Accept .............................................38
         5.3.3. Accept-Charset .....................................40
         5.3.4. Accept-Encoding ....................................41
         5.3.5. Accept-Language ....................................42
    5.4. Authentication Credentials ................................44
    5.5. Request Context ...........................................44
         5.5.1. From ...............................................44
         5.5.2. Referer ............................................45
         5.5.3. User-Agent .........................................46

Fielding & Reschke Standards Track [Page 3] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 6. Response Status Codes ..........................................47
    6.1. Overview of Status Codes ..................................48
    6.2. Informational 1xx .........................................50
         6.2.1. 100 Continue .......................................50
         6.2.2. 101 Switching Protocols ............................50
    6.3. Successful 2xx ............................................51
         6.3.1. 200 OK .............................................51
         6.3.2. 201 Created ........................................52
         6.3.3. 202 Accepted .......................................52
         6.3.4. 203 Non-Authoritative Information ..................52
         6.3.5. 204 No Content .....................................53
         6.3.6. 205 Reset Content ..................................53
    6.4. Redirection 3xx ...........................................54
         6.4.1. 300 Multiple Choices ...............................55
         6.4.2. 301 Moved Permanently ..............................56
         6.4.3. 302 Found ..........................................56
         6.4.4. 303 See Other ......................................57
         6.4.5. 305 Use Proxy ......................................58
         6.4.6. 306 (Unused) .......................................58
         6.4.7. 307 Temporary Redirect .............................58
    6.5. Client Error 4xx ..........................................58
         6.5.1. 400 Bad Request ....................................58
         6.5.2. 402 Payment Required ...............................59
         6.5.3. 403 Forbidden ......................................59
         6.5.4. 404 Not Found ......................................59
         6.5.5. 405 Method Not Allowed .............................59
         6.5.6. 406 Not Acceptable .................................60
         6.5.7. 408 Request Timeout ................................60
         6.5.8. 409 Conflict .......................................60
         6.5.9. 410 Gone ...........................................60
         6.5.10. 411 Length Required ...............................61
         6.5.11. 413 Payload Too Large .............................61
         6.5.12. 414 URI Too Long ..................................61
         6.5.13. 415 Unsupported Media Type ........................62
         6.5.14. 417 Expectation Failed ............................62
         6.5.15. 426 Upgrade Required ..............................62
    6.6. Server Error 5xx ..........................................62
         6.6.1. 500 Internal Server Error ..........................63
         6.6.2. 501 Not Implemented ................................63
         6.6.3. 502 Bad Gateway ....................................63
         6.6.4. 503 Service Unavailable ............................63
         6.6.5. 504 Gateway Timeout ................................63
         6.6.6. 505 HTTP Version Not Supported .....................64
 7. Response Header Fields .........................................64
    7.1. Control Data ..............................................64

ed 7.1.1. Origination Date ……………………………..65

         7.1.2. Location ...........................................68
         7.1.3. Retry-After ........................................69

Fielding & Reschke Standards Track [Page 4] RFC 7231 HTTP/1.1 Semantics and Content June 2014

         7.1.4. Vary ...............................................70
    7.2. Validator Header Fields ...................................71
    7.3. Authentication Challenges .................................72
    7.4. Response Context ..........................................72
         7.4.1. Allow ..............................................72
         7.4.2. Server .............................................73
 8. IANA Considerations ............................................73
    8.1. Method Registry ...........................................73
         8.1.1. Procedure ..........................................74
         8.1.2. Considerations for New Methods .....................74
         8.1.3. Registrations ......................................75
    8.2. Status Code Registry ......................................75
         8.2.1. Procedure ..........................................75
         8.2.2. Considerations for New Status Codes ................76
         8.2.3. Registrations ......................................76
    8.3. Header Field Registry .....................................77
         8.3.1. Considerations for New Header Fields ...............78
         8.3.2. Registrations ......................................80
    8.4. Content Coding Registry ...................................81
         8.4.1. Procedure ..........................................81
         8.4.2. Registrations ......................................81
 9. Security Considerations ........................................81
    9.1. Attacks Based on File and Path Names ......................82
    9.2. Attacks Based on Command, Code, or Query Injection ........82
    9.3. Disclosure of Personal Information ........................83
    9.4. Disclosure of Sensitive Information in URIs ...............83
    9.5. Disclosure of Fragment after Redirects ....................84
    9.6. Disclosure of Product Information .........................84
    9.7. Browser Fingerprinting ....................................84
 10. Acknowledgments ...............................................85
 11. References ....................................................85
    11.1. Normative References .....................................85
    11.2. Informative References ...................................86
 Appendix A. Differences between HTTP and MIME .....................89
    A.1. MIME-Version ..............................................89
    A.2. Conversion to Canonical Form ..............................89
    A.3. Conversion of Date Formats ................................90
    A.4. Conversion of Content-Encoding ............................90
    A.5. Conversion of Content-Transfer-Encoding ...................90
    A.6. MHTML and Line Length Limitations .........................90
 Appendix B. Changes from RFC 2616 .................................91
 Appendix C. Imported ABNF .........................................93
 Appendix D. Collected ABNF ........................................94
 Index .............................................................97

Fielding & Reschke Standards Track [Page 5] RFC 7231 HTTP/1.1 Semantics and Content June 2014

1. Introduction

 Each Hypertext Transfer Protocol (HTTP) message is either a request
 or a response.  A server listens on a connection for a request,
 parses each message received, interprets the message semantics in
 relation to the identified request target, and responds to that
 request with one or more response messages.  A client constructs
 request messages to communicate specific intentions, examines
 received responses to see if the intentions were carried out, and
 determines how to interpret the results.  This document defines
 HTTP/1.1 request and response semantics in terms of the architecture
 defined in [RFC7230].
 HTTP provides a uniform interface for interacting with a resource
 (Section 2), regardless of its type, nature, or implementation, via
 the manipulation and transfer of representations (Section 3).
 HTTP semantics include the intentions defined by each request method
 (Section 4), extensions to those semantics that might be described in
 request header fields (Section 5), the meaning of status codes to
 indicate a machine-readable response (Section 6), and the meaning of
 other control data and resource metadata that might be given in
 response header fields (Section 7).
 This document also defines representation metadata that describe how
 a payload is intended to be interpreted by a recipient, the request
 header fields that might influence content selection, and the various
 selection algorithms that are collectively referred to as "content
 negotiation" (Section 3.4).

1.1. Conformance and Error Handling

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].
 Conformance criteria and considerations regarding error handling are
 defined in Section 2.5 of [RFC7230].

1.2. Syntax Notation

 This specification uses the Augmented Backus-Naur Form (ABNF)
 notation of [RFC5234] with a list extension, defined in Section 7 of
 [RFC7230], that allows for compact definition of comma-separated
 lists using a '#' operator (similar to how the '*' operator indicates
 repetition).  Appendix C describes rules imported from other
 documents.  Appendix D shows the collected grammar with all list
 operators expanded to standard ABNF notation.

Fielding & Reschke Standards Track [Page 6] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 This specification uses the terms "character", "character encoding
 scheme", "charset", and "protocol element" as they are defined in
 [RFC6365].

2. Resources

 The target of an HTTP request is called a "resource".  HTTP does not
 limit the nature of a resource; it merely defines an interface that
 might be used to interact with resources.  Each resource is
 identified by a Uniform Resource Identifier (URI), as described in
 Section 2.7 of [RFC7230].
 When a client constructs an HTTP/1.1 request message, it sends the
 target URI in one of various forms, as defined in (Section 5.3 of
 [RFC7230]).  When a request is received, the server reconstructs an
 effective request URI for the target resource (Section 5.5 of
 [RFC7230]).
 One design goal of HTTP is to separate resource identification from
 request semantics, which is made possible by vesting the request
 semantics in the request method (Section 4) and a few
 request-modifying header fields (Section 5).  If there is a conflict
 between the method semantics and any semantic implied by the URI
 itself, as described in Section 4.2.1, the method semantics take
 precedence.

3. Representations

 Considering that a resource could be anything, and that the uniform
 interface provided by HTTP is similar to a window through which one
 can observe and act upon such a thing only through the communication
 of messages to some independent actor on the other side, an
 abstraction is needed to represent ("take the place of") the current
 or desired state of that thing in our communications.  That
 abstraction is called a representation [REST].
 For the purposes of HTTP, a "representation" is information that is
 intended to reflect a past, current, or desired state of a given
 resource, in a format that can be readily communicated via the
 protocol, and that consists of a set of representation metadata and a
 potentially unbounded stream of representation data.
 An origin server might be provided with, or be capable of generating,
 multiple representations that are each intended to reflect the
 current state of a target resource.  In such cases, some algorithm is
 used by the origin server to select one of those representations as
 most applicable to a given request, usually based on content
 negotiation.  This "selected representation" is used to provide the

Fielding & Reschke Standards Track [Page 7] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 data and metadata for evaluating conditional requests [RFC7232] and
 constructing the payload for 200 (OK) and 304 (Not Modified)
 responses to GET (Section 4.3.1).

3.1. Representation Metadata

 Representation header fields provide metadata about the
 representation.  When a message includes a payload body, the
 representation header fields describe how to interpret the
 representation data enclosed in the payload body.  In a response to a
 HEAD request, the representation header fields describe the
 representation data that would have been enclosed in the payload body
 if the same request had been a GET.
 The following header fields convey representation metadata:
 +-------------------+-----------------+
 | Header Field Name | Defined in...   |
 +-------------------+-----------------+
 | Content-Type      | Section 3.1.1.5 |
 | Content-Encoding  | Section 3.1.2.2 |
 | Content-Language  | Section 3.1.3.2 |
 | Content-Location  | Section 3.1.4.2 |
 +-------------------+-----------------+

3.1.1. Processing Representation Data

3.1.1.1. Media Type

 HTTP uses Internet media types [RFC2046] in the Content-Type
 (Section 3.1.1.5) and Accept (Section 5.3.2) header fields in order
 to provide open and extensible data typing and type negotiation.
 Media types define both a data format and various processing models:
 how to process that data in accordance with each context in which it
 is received.
   media-type = type "/" subtype *( OWS ";" OWS parameter )
   type       = token
   subtype    = token
 The type/subtype MAY be followed by parameters in the form of
 name=value pairs.
   parameter      = token "=" ( token / quoted-string )

Fielding & Reschke Standards Track [Page 8] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 The type, subtype, and parameter name tokens are case-insensitive.
 Parameter values might or might not be case-sensitive, depending on
 the semantics of the parameter name.  The presence or absence of a
 parameter might be significant to the processing of a media-type,
 depending on its definition within the media type registry.
 A parameter value that matches the token production can be
 transmitted either as a token or within a quoted-string.  The quoted
 and unquoted values are equivalent.  For example, the following
 examples are all equivalent, but the first is preferred for
 consistency:
   text/html;charset=utf-8
   text/html;charset=UTF-8
   Text/HTML;Charset="utf-8"
   text/html; charset="utf-8"
 Internet media types ought to be registered with IANA according to
 the procedures defined in [BCP13].
    Note: Unlike some similar constructs in other header fields, media
    type parameters do not allow whitespace (even "bad" whitespace)
    around the "=" character.

3.1.1.2. Charset

 HTTP uses charset names to indicate or negotiate the character
 encoding scheme of a textual representation [RFC6365].  A charset is
 identified by a case-insensitive token.
   charset = token
 Charset names ought to be registered in the IANA "Character Sets"
 registry (<http://www.iana.org/assignments/character-sets>) according
 to the procedures defined in [RFC2978].

3.1.1.3. Canonicalization and Text Defaults

 Internet media types are registered with a canonical form in order to
 be interoperable among systems with varying native encoding formats.
 Representations selected or transferred via HTTP ought to be in
 canonical form, for many of the same reasons described by the
 Multipurpose Internet Mail Extensions (MIME) [RFC2045].  However, the
 performance characteristics of email deployments (i.e., store and
 forward messages to peers) are significantly different from those
 common to HTTP and the Web (server-based information services).
 Furthermore, MIME's constraints for the sake of compatibility with
 older mail transfer protocols do not apply to HTTP (see Appendix A).

Fielding & Reschke Standards Track [Page 9] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 MIME's canonical form requires that media subtypes of the "text" type
 use CRLF as the text line break.  HTTP allows the transfer of text
 media with plain CR or LF alone representing a line break, when such
 line breaks are consistent for an entire representation.  An HTTP
 sender MAY generate, and a recipient MUST be able to parse, line
 breaks in text media that consist of CRLF, bare CR, or bare LF.  In
 addition, text media in HTTP is not limited to charsets that use
 octets 13 and 10 for CR and LF, respectively.  This flexibility
 regarding line breaks applies only to text within a representation
 that has been assigned a "text" media type; it does not apply to
 "multipart" types or HTTP elements outside the payload body (e.g.,
 header fields).
 If a representation is encoded with a content-coding, the underlying
 data ought to be in a form defined above prior to being encoded.

3.1.1.4. Multipart Types

 MIME provides for a number of "multipart" types -- encapsulations of
 one or more representations within a single message body.  All
 multipart types share a common syntax, as defined in Section 5.1.1 of
 [RFC2046], and include a boundary parameter as part of the media type
 value.  The message body is itself a protocol element; a sender MUST
 generate only CRLF to represent line breaks between body parts.
 HTTP message framing does not use the multipart boundary as an
 indicator of message body length, though it might be used by
 implementations that generate or process the payload.  For example,
 the "multipart/form-data" type is often used for carrying form data
 in a request, as described in [RFC2388], and the "multipart/
 byteranges" type is defined by this specification for use in some 206
 (Partial Content) responses [RFC7233].

3.1.1.5. Content-Type

 The "Content-Type" header field indicates the media type of the
 associated representation: either the representation enclosed in the
 message payload or the selected representation, as determined by the
 message semantics.  The indicated media type defines both the data
 format and how that data is intended to be processed by a recipient,
 within the scope of the received message semantics, after any content
 codings indicated by Content-Encoding are decoded.
   Content-Type = media-type

Fielding & Reschke Standards Track [Page 10] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 Media types are defined in Section 3.1.1.1.  An example of the field
 is
   Content-Type: text/html; charset=ISO-8859-4
 A sender that generates a message containing a payload body SHOULD
 generate a Content-Type header field in that message unless the
 intended media type of the enclosed representation is unknown to the
 sender.  If a Content-Type header field is not present, the recipient
 MAY either assume a media type of "application/octet-stream"
 ([RFC2046], Section 4.5.1) or examine the data to determine its type.
 In practice, resource owners do not always properly configure their
 origin server to provide the correct Content-Type for a given
 representation, with the result that some clients will examine a
 payload's content and override the specified type.  Clients that do
 so risk drawing incorrect conclusions, which might expose additional
 security risks (e.g., "privilege escalation").  Furthermore, it is
 impossible to determine the sender's intent by examining the data
 format: many data formats match multiple media types that differ only
 in processing semantics.  Implementers are encouraged to provide a
 means of disabling such "content sniffing" when it is used.

3.1.2. Encoding for Compression or Integrity

3.1.2.1. Content Codings

 Content coding values indicate an encoding transformation that has
 been or can be applied to a representation.  Content codings are
 primarily used to allow a representation to be compressed or
 otherwise usefully transformed without losing the identity of its
 underlying media type and without loss of information.  Frequently,
 the representation is stored in coded form, transmitted directly, and
 only decoded by the final recipient.
   content-coding   = token
 All content-coding values are case-insensitive and ought to be
 registered within the "HTTP Content Coding Registry", as defined in
 Section 8.4.  They are used in the Accept-Encoding (Section 5.3.4)
 and Content-Encoding (Section 3.1.2.2) header fields.

Fielding & Reschke Standards Track [Page 11] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 The following content-coding values are defined by this
 specification:
    compress (and x-compress): See Section 4.2.1 of [RFC7230].
    deflate: See Section 4.2.2 of [RFC7230].
    gzip (and x-gzip): See Section 4.2.3 of [RFC7230].

3.1.2.2. Content-Encoding

 The "Content-Encoding" header field indicates what content codings
 have been applied to the representation, beyond those inherent in the
 media type, and thus what decoding mechanisms have to be applied in
 order to obtain data in the media type referenced by the Content-Type
 header field.  Content-Encoding is primarily used to allow a
 representation's data to be compressed without losing the identity of
 its underlying media type.
   Content-Encoding = 1#content-coding
 An example of its use is
   Content-Encoding: gzip
 If one or more encodings have been applied to a representation, the
 sender that applied the encodings MUST generate a Content-Encoding
 header field that lists the content codings in the order in which
 they were applied.  Additional information about the encoding
 parameters can be provided by other header fields not defined by this
 specification.
 Unlike Transfer-Encoding (Section 3.3.1 of [RFC7230]), the codings
 listed in Content-Encoding are a characteristic of the
 representation; the representation is defined in terms of the coded
 form, and all other metadata about the representation is about the
 coded form unless otherwise noted in the metadata definition.
 Typically, the representation is only decoded just prior to rendering
 or analogous usage.
 If the media type includes an inherent encoding, such as a data
 format that is always compressed, then that encoding would not be
 restated in Content-Encoding even if it happens to be the same
 algorithm as one of the content codings.  Such a content coding would
 only be listed if, for some bizarre reason, it is applied a second
 time to form the representation.  Likewise, an origin server might
 choose to publish the same data as multiple representations that
 differ only in whether the coding is defined as part of Content-Type

Fielding & Reschke Standards Track [Page 12] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 or Content-Encoding, since some user agents will behave differently
 in their handling of each response (e.g., open a "Save as ..." dialog
 instead of automatic decompression and rendering of content).
 An origin server MAY respond with a status code of 415 (Unsupported
 Media Type) if a representation in the request message has a content
 coding that is not acceptable.

3.1.3. Audience Language

3.1.3.1. Language Tags

 A language tag, as defined in [RFC5646], identifies a natural
 language spoken, written, or otherwise conveyed by human beings for
 communication of information to other human beings.  Computer
 languages are explicitly excluded.
 HTTP uses language tags within the Accept-Language and
 Content-Language header fields.  Accept-Language uses the broader
 language-range production defined in Section 5.3.5, whereas
 Content-Language uses the language-tag production defined below.
   language-tag = <Language-Tag, see [RFC5646], Section 2.1>
 A language tag is a sequence of one or more case-insensitive subtags,
 each separated by a hyphen character ("-", %x2D).  In most cases, a
 language tag consists of a primary language subtag that identifies a
 broad family of related languages (e.g., "en" = English), which is
 optionally followed by a series of subtags that refine or narrow that
 language's range (e.g., "en-CA" = the variety of English as
 communicated in Canada).  Whitespace is not allowed within a language
 tag.  Example tags include:
   fr, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
 See [RFC5646] for further information.

3.1.3.2. Content-Language

 The "Content-Language" header field describes the natural language(s)
 of the intended audience for the representation.  Note that this
 might not be equivalent to all the languages used within the
 representation.
   Content-Language = 1#language-tag

Fielding & Reschke Standards Track [Page 13] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 Language tags are defined in Section 3.1.3.1.  The primary purpose of
 Content-Language is to allow a user to identify and differentiate
 representations according to the users' own preferred language.
 Thus, if the content is intended only for a Danish-literate audience,
 the appropriate field is
   Content-Language: da
 If no Content-Language is specified, the default is that the content
 is intended for all language audiences.  This might mean that the
 sender does not consider it to be specific to any natural language,
 or that the sender does not know for which language it is intended.
 Multiple languages MAY be listed for content that is intended for
 multiple audiences.  For example, a rendition of the "Treaty of
 Waitangi", presented simultaneously in the original Maori and English
 versions, would call for
   Content-Language: mi, en
 However, just because multiple languages are present within a
 representation does not mean that it is intended for multiple
 linguistic audiences.  An example would be a beginner's language
 primer, such as "A First Lesson in Latin", which is clearly intended
 to be used by an English-literate audience.  In this case, the
 Content-Language would properly only include "en".
 Content-Language MAY be applied to any media type -- it is not
 limited to textual documents.

3.1.4. Identification

3.1.4.1. Identifying a Representation

 When a complete or partial representation is transferred in a message
 payload, it is often desirable for the sender to supply, or the
 recipient to determine, an identifier for a resource corresponding to
 that representation.
 For a request message:
 o  If the request has a Content-Location header field, then the
    sender asserts that the payload is a representation of the
    resource identified by the Content-Location field-value.  However,
    such an assertion cannot be trusted unless it can be verified by
    other means (not defined by this specification).  The information
    might still be useful for revision history links.

Fielding & Reschke Standards Track [Page 14] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 o  Otherwise, the payload is unidentified.
 For a response message, the following rules are applied in order
 until a match is found:
 1.  If the request method is GET or HEAD and the response status code
     is 200 (OK), 204 (No Content), 206 (Partial Content), or 304 (Not
     Modified), the payload is a representation of the resource
     identified by the effective request URI (Section 5.5 of
     [RFC7230]).
 2.  If the request method is GET or HEAD and the response status code
     is 203 (Non-Authoritative Information), the payload is a
     potentially modified or enhanced representation of the target
     resource as provided by an intermediary.
 3.  If the response has a Content-Location header field and its
     field-value is a reference to the same URI as the effective
     request URI, the payload is a representation of the resource
     identified by the effective request URI.
 4.  If the response has a Content-Location header field and its
     field-value is a reference to a URI different from the effective
     request URI, then the sender asserts that the payload is a
     representation of the resource identified by the Content-Location
     field-value.  However, such an assertion cannot be trusted unless
     it can be verified by other means (not defined by this
     specification).
 5.  Otherwise, the payload is unidentified.

3.1.4.2. Content-Location

 The "Content-Location" header field references a URI that can be used
 as an identifier for a specific resource corresponding to the
 representation in this message's payload.  In other words, if one
 were to perform a GET request on this URI at the time of this
 message's generation, then a 200 (OK) response would contain the same
 representation that is enclosed as payload in this message.
   Content-Location = absolute-URI / partial-URI
 The Content-Location value is not a replacement for the effective
 Request URI (Section 5.5 of [RFC7230]).  It is representation
 metadata.  It has the same syntax and semantics as the header field
 of the same name defined for MIME body parts in Section 4 of
 [RFC2557].  However, its appearance in an HTTP message has some
 special implications for HTTP recipients.

Fielding & Reschke Standards Track [Page 15] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 If Content-Location is included in a 2xx (Successful) response
 message and its value refers (after conversion to absolute form) to a
 URI that is the same as the effective request URI, then the recipient
 MAY consider the payload to be a current representation of that
 resource at the time indicated by the message origination date.  For
 a GET (Section 4.3.1) or HEAD (Section 4.3.2) request, this is the
 same as the default semantics when no Content-Location is provided by
 the server.  For a state-changing request like PUT (Section 4.3.4) or
 POST (Section 4.3.3), it implies that the server's response contains
 the new representation of that resource, thereby distinguishing it
 from representations that might only report about the action (e.g.,
 "It worked!").  This allows authoring applications to update their
 local copies without the need for a subsequent GET request.
 If Content-Location is included in a 2xx (Successful) response
 message and its field-value refers to a URI that differs from the
 effective request URI, then the origin server claims that the URI is
 an identifier for a different resource corresponding to the enclosed
 representation.  Such a claim can only be trusted if both identifiers
 share the same resource owner, which cannot be programmatically
 determined via HTTP.
 o  For a response to a GET or HEAD request, this is an indication
    that the effective request URI refers to a resource that is
    subject to content negotiation and the Content-Location
    field-value is a more specific identifier for the selected
    representation.
 o  For a 201 (Created) response to a state-changing method, a
    Content-Location field-value that is identical to the Location
    field-value indicates that this payload is a current
    representation of the newly created resource.
 o  Otherwise, such a Content-Location indicates that this payload is
    a representation reporting on the requested action's status and
    that the same report is available (for future access with GET) at
    the given URI.  For example, a purchase transaction made via a
    POST request might include a receipt document as the payload of
    the 200 (OK) response; the Content-Location field-value provides
    an identifier for retrieving a copy of that same receipt in the
    future.
 A user agent that sends Content-Location in a request message is
 stating that its value refers to where the user agent originally
 obtained the content of the enclosed representation (prior to any
 modifications made by that user agent).  In other words, the user
 agent is providing a back link to the source of the original
 representation.

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 An origin server that receives a Content-Location field in a request
 message MUST treat the information as transitory request context
 rather than as metadata to be saved verbatim as part of the
 representation.  An origin server MAY use that context to guide in
 processing the request or to save it for other uses, such as within
 source links or versioning metadata.  However, an origin server MUST
 NOT use such context information to alter the request semantics.
 For example, if a client makes a PUT request on a negotiated resource
 and the origin server accepts that PUT (without redirection), then
 the new state of that resource is expected to be consistent with the
 one representation supplied in that PUT; the Content-Location cannot
 be used as a form of reverse content selection identifier to update
 only one of the negotiated representations.  If the user agent had
 wanted the latter semantics, it would have applied the PUT directly
 to the Content-Location URI.

3.2. Representation Data

 The representation data associated with an HTTP message is either
 provided as the payload body of the message or referred to by the
 message semantics and the effective request URI.  The representation
 data is in a format and encoding defined by the representation
 metadata header fields.
 The data type of the representation data is determined via the header
 fields Content-Type and Content-Encoding.  These define a two-layer,
 ordered encoding model:
   representation-data := Content-Encoding( Content-Type( bits ) )

3.3. Payload Semantics

 Some HTTP messages transfer a complete or partial representation as
 the message "payload".  In some cases, a payload might contain only
 the associated representation's header fields (e.g., responses to
 HEAD) or only some part(s) of the representation data (e.g., the 206
 (Partial Content) status code).
 The purpose of a payload in a request is defined by the method
 semantics.  For example, a representation in the payload of a PUT
 request (Section 4.3.4) represents the desired state of the target
 resource if the request is successfully applied, whereas a
 representation in the payload of a POST request (Section 4.3.3)
 represents information to be processed by the target resource.

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 In a response, the payload's purpose is defined by both the request
 method and the response status code.  For example, the payload of a
 200 (OK) response to GET (Section 4.3.1) represents the current state
 of the target resource, as observed at the time of the message
 origination date (Section 7.1.1.2), whereas the payload of the same
 status code in a response to POST might represent either the
 processing result or the new state of the target resource after
 applying the processing.  Response messages with an error status code
 usually contain a payload that represents the error condition, such
 that it describes the error state and what next steps are suggested
 for resolving it.
 Header fields that specifically describe the payload, rather than the
 associated representation, are referred to as "payload header
 fields".  Payload header fields are defined in other parts of this
 specification, due to their impact on message parsing.
 +-------------------+----------------------------+
 | Header Field Name | Defined in...              |
 +-------------------+----------------------------+
 | Content-Length    | Section 3.3.2 of [RFC7230] |
 | Content-Range     | Section 4.2 of [RFC7233]   |
 | Trailer           | Section 4.4 of [RFC7230]   |
 | Transfer-Encoding | Section 3.3.1 of [RFC7230] |
 +-------------------+----------------------------+

3.4. Content Negotiation

 When responses convey payload information, whether indicating a
 success or an error, the origin server often has different ways of
 representing that information; for example, in different formats,
 languages, or encodings.  Likewise, different users or user agents
 might have differing capabilities, characteristics, or preferences
 that could influence which representation, among those available,
 would be best to deliver.  For this reason, HTTP provides mechanisms
 for content negotiation.
 This specification defines two patterns of content negotiation that
 can be made visible within the protocol: "proactive", where the
 server selects the representation based upon the user agent's stated
 preferences, and "reactive" negotiation, where the server provides a
 list of representations for the user agent to choose from.  Other
 patterns of content negotiation include "conditional content", where
 the representation consists of multiple parts that are selectively
 rendered based on user agent parameters, "active content", where the
 representation contains a script that makes additional (more
 specific) requests based on the user agent characteristics, and
 "Transparent Content Negotiation" ([RFC2295]), where content

Fielding & Reschke Standards Track [Page 18] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 selection is performed by an intermediary.  These patterns are not
 mutually exclusive, and each has trade-offs in applicability and
 practicality.
 Note that, in all cases, HTTP is not aware of the resource semantics.
 The consistency with which an origin server responds to requests,
 over time and over the varying dimensions of content negotiation, and
 thus the "sameness" of a resource's observed representations over
 time, is determined entirely by whatever entity or algorithm selects
 or generates those responses.  HTTP pays no attention to the man
 behind the curtain.

3.4.1. Proactive Negotiation

 When content negotiation preferences are sent by the user agent in a
 request to encourage an algorithm located at the server to select the
 preferred representation, it is called proactive negotiation (a.k.a.,
 server-driven negotiation).  Selection is based on the available
 representations for a response (the dimensions over which it might
 vary, such as language, content-coding, etc.) compared to various
 information supplied in the request, including both the explicit
 negotiation fields of Section 5.3 and implicit characteristics, such
 as the client's network address or parts of the User-Agent field.
 Proactive negotiation is advantageous when the algorithm for
 selecting from among the available representations is difficult to
 describe to a user agent, or when the server desires to send its
 "best guess" to the user agent along with the first response (hoping
 to avoid the round trip delay of a subsequent request if the "best
 guess" is good enough for the user).  In order to improve the
 server's guess, a user agent MAY send request header fields that
 describe its preferences.
 Proactive negotiation has serious disadvantages:
 o  It is impossible for the server to accurately determine what might
    be "best" for any given user, since that would require complete
    knowledge of both the capabilities of the user agent and the
    intended use for the response (e.g., does the user want to view it
    on screen or print it on paper?);
 o  Having the user agent describe its capabilities in every request
    can be both very inefficient (given that only a small percentage
    of responses have multiple representations) and a potential risk
    to the user's privacy;
 o  It complicates the implementation of an origin server and the
    algorithms for generating responses to a request; and,

Fielding & Reschke Standards Track [Page 19] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 o  It limits the reusability of responses for shared caching.
 A user agent cannot rely on proactive negotiation preferences being
 consistently honored, since the origin server might not implement
 proactive negotiation for the requested resource or might decide that
 sending a response that doesn't conform to the user agent's
 preferences is better than sending a 406 (Not Acceptable) response.
 A Vary header field (Section 7.1.4) is often sent in a response
 subject to proactive negotiation to indicate what parts of the
 request information were used in the selection algorithm.

3.4.2. Reactive Negotiation

 With reactive negotiation (a.k.a., agent-driven negotiation),
 selection of the best response representation (regardless of the
 status code) is performed by the user agent after receiving an
 initial response from the origin server that contains a list of
 resources for alternative representations.  If the user agent is not
 satisfied by the initial response representation, it can perform a
 GET request on one or more of the alternative resources, selected
 based on metadata included in the list, to obtain a different form of
 representation for that response.  Selection of alternatives might be
 performed automatically by the user agent or manually by the user
 selecting from a generated (possibly hypertext) menu.
 Note that the above refers to representations of the response, in
 general, not representations of the resource.  The alternative
 representations are only considered representations of the target
 resource if the response in which those alternatives are provided has
 the semantics of being a representation of the target resource (e.g.,
 a 200 (OK) response to a GET request) or has the semantics of
 providing links to alternative representations for the target
 resource (e.g., a 300 (Multiple Choices) response to a GET request).
 A server might choose not to send an initial representation, other
 than the list of alternatives, and thereby indicate that reactive
 negotiation by the user agent is preferred.  For example, the
 alternatives listed in responses with the 300 (Multiple Choices) and
 406 (Not Acceptable) status codes include information about the
 available representations so that the user or user agent can react by
 making a selection.
 Reactive negotiation is advantageous when the response would vary
 over commonly used dimensions (such as type, language, or encoding),
 when the origin server is unable to determine a user agent's
 capabilities from examining the request, and generally when public
 caches are used to distribute server load and reduce network usage.

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 Reactive negotiation suffers from the disadvantages of transmitting a
 list of alternatives to the user agent, which degrades user-perceived
 latency if transmitted in the header section, and needing a second
 request to obtain an alternate representation.  Furthermore, this
 specification does not define a mechanism for supporting automatic
 selection, though it does not prevent such a mechanism from being
 developed as an extension.

4. Request Methods

4.1. Overview

 The request method token is the primary source of request semantics;
 it indicates the purpose for which the client has made this request
 and what is expected by the client as a successful result.
 The request method's semantics might be further specialized by the
 semantics of some header fields when present in a request (Section 5)
 if those additional semantics do not conflict with the method.  For
 example, a client can send conditional request header fields
 (Section 5.2) to make the requested action conditional on the current
 state of the target resource ([RFC7232]).
   method = token
 HTTP was originally designed to be usable as an interface to
 distributed object systems.  The request method was envisioned as
 applying semantics to a target resource in much the same way as
 invoking a defined method on an identified object would apply
 semantics.  The method token is case-sensitive because it might be
 used as a gateway to object-based systems with case-sensitive method
 names.
 Unlike distributed objects, the standardized request methods in HTTP
 are not resource-specific, since uniform interfaces provide for
 better visibility and reuse in network-based systems [REST].  Once
 defined, a standardized method ought to have the same semantics when
 applied to any resource, though each resource determines for itself
 whether those semantics are implemented or allowed.
 This specification defines a number of standardized methods that are
 commonly used in HTTP, as outlined by the following table.  By
 convention, standardized methods are defined in all-uppercase
 US-ASCII letters.

Fielding & Reschke Standards Track [Page 21] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 +---------+-------------------------------------------------+-------+
 | Method  | Description                                     | Sec.  |
 +---------+-------------------------------------------------+-------+
 | GET     | Transfer a current representation of the target | 4.3.1 |
 |         | resource.                                       |       |
 | HEAD    | Same as GET, but only transfer the status line  | 4.3.2 |
 |         | and header section.                             |       |
 | POST    | Perform resource-specific processing on the     | 4.3.3 |
 |         | request payload.                                |       |
 | PUT     | Replace all current representations of the      | 4.3.4 |
 |         | target resource with the request payload.       |       |
 | DELETE  | Remove all current representations of the       | 4.3.5 |
 |         | target resource.                                |       |
 | CONNECT | Establish a tunnel to the server identified by  | 4.3.6 |
 |         | the target resource.                            |       |
 | OPTIONS | Describe the communication options for the      | 4.3.7 |
 |         | target resource.                                |       |
 | TRACE   | Perform a message loop-back test along the path | 4.3.8 |
 |         | to the target resource.                         |       |
 +---------+-------------------------------------------------+-------+
 All general-purpose servers MUST support the methods GET and HEAD.
 All other methods are OPTIONAL.
 Additional methods, outside the scope of this specification, have
 been standardized for use in HTTP.  All such methods ought to be
 registered within the "Hypertext Transfer Protocol (HTTP) Method
 Registry" maintained by IANA, as defined in Section 8.1.
 The set of methods allowed by a target resource can be listed in an
 Allow header field (Section 7.4.1).  However, the set of allowed
 methods can change dynamically.  When a request method is received
 that is unrecognized or not implemented by an origin server, the
 origin server SHOULD respond with the 501 (Not Implemented) status
 code.  When a request method is received that is known by an origin
 server but not allowed for the target resource, the origin server
 SHOULD respond with the 405 (Method Not Allowed) status code.

4.2. Common Method Properties

4.2.1. Safe Methods

 Request methods are considered "safe" if their defined semantics are
 essentially read-only; i.e., the client does not request, and does
 not expect, any state change on the origin server as a result of
 applying a safe method to a target resource.  Likewise, reasonable
 use of a safe method is not expected to cause any harm, loss of
 property, or unusual burden on the origin server.

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 This definition of safe methods does not prevent an implementation
 from including behavior that is potentially harmful, that is not
 entirely read-only, or that causes side effects while invoking a safe
 method.  What is important, however, is that the client did not
 request that additional behavior and cannot be held accountable for
 it.  For example, most servers append request information to access
 log files at the completion of every response, regardless of the
 method, and that is considered safe even though the log storage might
 become full and crash the server.  Likewise, a safe request initiated
 by selecting an advertisement on the Web will often have the side
 effect of charging an advertising account.
 Of the request methods defined by this specification, the GET, HEAD,
 OPTIONS, and TRACE methods are defined to be safe.
 The purpose of distinguishing between safe and unsafe methods is to
 allow automated retrieval processes (spiders) and cache performance
 optimization (pre-fetching) to work without fear of causing harm.  In
 addition, it allows a user agent to apply appropriate constraints on
 the automated use of unsafe methods when processing potentially
 untrusted content.
 A user agent SHOULD distinguish between safe and unsafe methods when
 presenting potential actions to a user, such that the user can be
 made aware of an unsafe action before it is requested.
 When a resource is constructed such that parameters within the
 effective request URI have the effect of selecting an action, it is
 the resource owner's responsibility to ensure that the action is
 consistent with the request method semantics.  For example, it is
 common for Web-based content editing software to use actions within
 query parameters, such as "page?do=delete".  If the purpose of such a
 resource is to perform an unsafe action, then the resource owner MUST
 disable or disallow that action when it is accessed using a safe
 request method.  Failure to do so will result in unfortunate side
 effects when automated processes perform a GET on every URI reference
 for the sake of link maintenance, pre-fetching, building a search
 index, etc.

4.2.2. Idempotent Methods

 A request method is considered "idempotent" if the intended effect on
 the server of multiple identical requests with that method is the
 same as the effect for a single such request.  Of the request methods
 defined by this specification, PUT, DELETE, and safe request methods
 are idempotent.

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 Like the definition of safe, the idempotent property only applies to
 what has been requested by the user; a server is free to log each
 request separately, retain a revision control history, or implement
 other non-idempotent side effects for each idempotent request.
 Idempotent methods are distinguished because the request can be
 repeated automatically if a communication failure occurs before the
 client is able to read the server's response.  For example, if a
 client sends a PUT request and the underlying connection is closed
 before any response is received, then the client can establish a new
 connection and retry the idempotent request.  It knows that repeating
 the request will have the same intended effect, even if the original
 request succeeded, though the response might differ.

4.2.3. Cacheable Methods

 Request methods can be defined as "cacheable" to indicate that
 responses to them are allowed to be stored for future reuse; for
 specific requirements see [RFC7234].  In general, safe methods that
 do not depend on a current or authoritative response are defined as
 cacheable; this specification defines GET, HEAD, and POST as
 cacheable, although the overwhelming majority of cache
 implementations only support GET and HEAD.

4.3. Method Definitions

4.3.1. GET

 The GET method requests transfer of a current selected representation
 for the target resource.  GET is the primary mechanism of information
 retrieval and the focus of almost all performance optimizations.
 Hence, when people speak of retrieving some identifiable information
 via HTTP, they are generally referring to making a GET request.
 It is tempting to think of resource identifiers as remote file system
 pathnames and of representations as being a copy of the contents of
 such files.  In fact, that is how many resources are implemented (see
 Section 9.1 for related security considerations).  However, there are
 no such limitations in practice.  The HTTP interface for a resource
 is just as likely to be implemented as a tree of content objects, a
 programmatic view on various database records, or a gateway to other
 information systems.  Even when the URI mapping mechanism is tied to
 a file system, an origin server might be configured to execute the
 files with the request as input and send the output as the
 representation rather than transfer the files directly.  Regardless,
 only the origin server needs to know how each of its resource

Fielding & Reschke Standards Track [Page 24] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 identifiers corresponds to an implementation and how each
 implementation manages to select and send a current representation of
 the target resource in a response to GET.
 A client can alter the semantics of GET to be a "range request",
 requesting transfer of only some part(s) of the selected
 representation, by sending a Range header field in the request
 ([RFC7233]).
 A payload within a GET request message has no defined semantics;
 sending a payload body on a GET request might cause some existing
 implementations to reject the request.
 The response to a GET request is cacheable; a cache MAY use it to
 satisfy subsequent GET and HEAD requests unless otherwise indicated
 by the Cache-Control header field (Section 5.2 of [RFC7234]).

4.3.2. HEAD

 The HEAD method is identical to GET except that the server MUST NOT
 send a message body in the response (i.e., the response terminates at
 the end of the header section).  The server SHOULD send the same
 header fields in response to a HEAD request as it would have sent if
 the request had been a GET, except that the payload header fields
 (Section 3.3) MAY be omitted.  This method can be used for obtaining
 metadata about the selected representation without transferring the
 representation data and is often used for testing hypertext links for
 validity, accessibility, and recent modification.
 A payload within a HEAD request message has no defined semantics;
 sending a payload body on a HEAD request might cause some existing
 implementations to reject the request.
 The response to a HEAD request is cacheable; a cache MAY use it to
 satisfy subsequent HEAD requests unless otherwise indicated by the
 Cache-Control header field (Section 5.2 of [RFC7234]).  A HEAD
 response might also have an effect on previously cached responses to
 GET; see Section 4.3.5 of [RFC7234].

4.3.3. POST

 The POST method requests that the target resource process the
 representation enclosed in the request according to the resource's
 own specific semantics.  For example, POST is used for the following
 functions (among others):
 o  Providing a block of data, such as the fields entered into an HTML
    form, to a data-handling process;

Fielding & Reschke Standards Track [Page 25] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 o  Posting a message to a bulletin board, newsgroup, mailing list,
    blog, or similar group of articles;
 o  Creating a new resource that has yet to be identified by the
    origin server; and
 o  Appending data to a resource's existing representation(s).
 An origin server indicates response semantics by choosing an
 appropriate status code depending on the result of processing the
 POST request; almost all of the status codes defined by this
 specification might be received in a response to POST (the exceptions
 being 206 (Partial Content), 304 (Not Modified), and 416 (Range Not
 Satisfiable)).
 If one or more resources has been created on the origin server as a
 result of successfully processing a POST request, the origin server
 SHOULD send a 201 (Created) response containing a Location header
 field that provides an identifier for the primary resource created
 (Section 7.1.2) and a representation that describes the status of the
 request while referring to the new resource(s).
 Responses to POST requests are only cacheable when they include
 explicit freshness information (see Section 4.2.1 of [RFC7234]).
 However, POST caching is not widely implemented.  For cases where an
 origin server wishes the client to be able to cache the result of a
 POST in a way that can be reused by a later GET, the origin server
 MAY send a 200 (OK) response containing the result and a
 Content-Location header field that has the same value as the POST's
 effective request URI (Section 3.1.4.2).
 If the result of processing a POST would be equivalent to a
 representation of an existing resource, an origin server MAY redirect
 the user agent to that resource by sending a 303 (See Other) response
 with the existing resource's identifier in the Location field.  This
 has the benefits of providing the user agent a resource identifier
 and transferring the representation via a method more amenable to
 shared caching, though at the cost of an extra request if the user
 agent does not already have the representation cached.

4.3.4. PUT

 The PUT method requests that the state of the target resource be
 created or replaced with the state defined by the representation
 enclosed in the request message payload.  A successful PUT of a given
 representation would suggest that a subsequent GET on that same
 target resource will result in an equivalent representation being
 sent in a 200 (OK) response.  However, there is no guarantee that

Fielding & Reschke Standards Track [Page 26] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 such a state change will be observable, since the target resource
 might be acted upon by other user agents in parallel, or might be
 subject to dynamic processing by the origin server, before any
 subsequent GET is received.  A successful response only implies that
 the user agent's intent was achieved at the time of its processing by
 the origin server.
 If the target resource does not have a current representation and the
 PUT successfully creates one, then the origin server MUST inform the
 user agent by sending a 201 (Created) response.  If the target
 resource does have a current representation and that representation
 is successfully modified in accordance with the state of the enclosed
 representation, then the origin server MUST send either a 200 (OK) or
 a 204 (No Content) response to indicate successful completion of the
 request.
 An origin server SHOULD ignore unrecognized header fields received in
 a PUT request (i.e., do not save them as part of the resource state).
 An origin server SHOULD verify that the PUT representation is
 consistent with any constraints the server has for the target
 resource that cannot or will not be changed by the PUT.  This is
 particularly important when the origin server uses internal
 configuration information related to the URI in order to set the
 values for representation metadata on GET responses.  When a PUT
 representation is inconsistent with the target resource, the origin
 server SHOULD either make them consistent, by transforming the
 representation or changing the resource configuration, or respond
 with an appropriate error message containing sufficient information
 to explain why the representation is unsuitable.  The 409 (Conflict)
 or 415 (Unsupported Media Type) status codes are suggested, with the
 latter being specific to constraints on Content-Type values.
 For example, if the target resource is configured to always have a
 Content-Type of "text/html" and the representation being PUT has a
 Content-Type of "image/jpeg", the origin server ought to do one of:
 a.  reconfigure the target resource to reflect the new media type;
 b.  transform the PUT representation to a format consistent with that
     of the resource before saving it as the new resource state; or,
 c.  reject the request with a 415 (Unsupported Media Type) response
     indicating that the target resource is limited to "text/html",
     perhaps including a link to a different resource that would be a
     suitable target for the new representation.

Fielding & Reschke Standards Track [Page 27] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 HTTP does not define exactly how a PUT method affects the state of an
 origin server beyond what can be expressed by the intent of the user
 agent request and the semantics of the origin server response.  It
 does not define what a resource might be, in any sense of that word,
 beyond the interface provided via HTTP.  It does not define how
 resource state is "stored", nor how such storage might change as a
 result of a change in resource state, nor how the origin server
 translates resource state into representations.  Generally speaking,
 all implementation details behind the resource interface are
 intentionally hidden by the server.
 An origin server MUST NOT send a validator header field
 (Section 7.2), such as an ETag or Last-Modified field, in a
 successful response to PUT unless the request's representation data
 was saved without any transformation applied to the body (i.e., the
 resource's new representation data is identical to the representation
 data received in the PUT request) and the validator field value
 reflects the new representation.  This requirement allows a user
 agent to know when the representation body it has in memory remains
 current as a result of the PUT, thus not in need of being retrieved
 again from the origin server, and that the new validator(s) received
 in the response can be used for future conditional requests in order
 to prevent accidental overwrites (Section 5.2).
 The fundamental difference between the POST and PUT methods is
 highlighted by the different intent for the enclosed representation.
 The target resource in a POST request is intended to handle the
 enclosed representation according to the resource's own semantics,
 whereas the enclosed representation in a PUT request is defined as
 replacing the state of the target resource.  Hence, the intent of PUT
 is idempotent and visible to intermediaries, even though the exact
 effect is only known by the origin server.
 Proper interpretation of a PUT request presumes that the user agent
 knows which target resource is desired.  A service that selects a
 proper URI on behalf of the client, after receiving a state-changing
 request, SHOULD be implemented using the POST method rather than PUT.
 If the origin server will not make the requested PUT state change to
 the target resource and instead wishes to have it applied to a
 different resource, such as when the resource has been moved to a
 different URI, then the origin server MUST send an appropriate 3xx
 (Redirection) response; the user agent MAY then make its own decision
 regarding whether or not to redirect the request.
 A PUT request applied to the target resource can have side effects on
 other resources.  For example, an article might have a URI for
 identifying "the current version" (a resource) that is separate from
 the URIs identifying each particular version (different resources

Fielding & Reschke Standards Track [Page 28] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 that at one point shared the same state as the current version
 resource).  A successful PUT request on "the current version" URI
 might therefore create a new version resource in addition to changing
 the state of the target resource, and might also cause links to be
 added between the related resources.
 An origin server that allows PUT on a given target resource MUST send
 a 400 (Bad Request) response to a PUT request that contains a
 Content-Range header field (Section 4.2 of [RFC7233]), since the
 payload is likely to be partial content that has been mistakenly PUT
 as a full representation.  Partial content updates are possible by
 targeting a separately identified resource with state that overlaps a
 portion of the larger resource, or by using a different method that
 has been specifically defined for partial updates (for example, the
 PATCH method defined in [RFC5789]).
 Responses to the PUT method are not cacheable.  If a successful PUT
 request passes through a cache that has one or more stored responses
 for the effective request URI, those stored responses will be
 invalidated (see Section 4.4 of [RFC7234]).

4.3.5. DELETE

 The DELETE method requests that the origin server remove the
 association between the target resource and its current
 functionality.  In effect, this method is similar to the rm command
 in UNIX: it expresses a deletion operation on the URI mapping of the
 origin server rather than an expectation that the previously
 associated information be deleted.
 If the target resource has one or more current representations, they
 might or might not be destroyed by the origin server, and the
 associated storage might or might not be reclaimed, depending
 entirely on the nature of the resource and its implementation by the
 origin server (which are beyond the scope of this specification).
 Likewise, other implementation aspects of a resource might need to be
 deactivated or archived as a result of a DELETE, such as database or
 gateway connections.  In general, it is assumed that the origin
 server will only allow DELETE on resources for which it has a
 prescribed mechanism for accomplishing the deletion.
 Relatively few resources allow the DELETE method -- its primary use
 is for remote authoring environments, where the user has some
 direction regarding its effect.  For example, a resource that was
 previously created using a PUT request, or identified via the
 Location header field after a 201 (Created) response to a POST
 request, might allow a corresponding DELETE request to undo those
 actions.  Similarly, custom user agent implementations that implement

Fielding & Reschke Standards Track [Page 29] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 an authoring function, such as revision control clients using HTTP
 for remote operations, might use DELETE based on an assumption that
 the server's URI space has been crafted to correspond to a version
 repository.
 If a DELETE method is successfully applied, the origin server SHOULD
 send a 202 (Accepted) status code if the action will likely succeed
 but has not yet been enacted, a 204 (No Content) status code if the
 action has been enacted and no further information is to be supplied,
 or a 200 (OK) status code if the action has been enacted and the
 response message includes a representation describing the status.
 A payload within a DELETE request message has no defined semantics;
 sending a payload body on a DELETE request might cause some existing
 implementations to reject the request.
 Responses to the DELETE method are not cacheable.  If a DELETE
 request passes through a cache that has one or more stored responses
 for the effective request URI, those stored responses will be
 invalidated (see Section 4.4 of [RFC7234]).

4.3.6. CONNECT

 The CONNECT method requests that the recipient establish a tunnel to
 the destination origin server identified by the request-target and,
 if successful, thereafter restrict its behavior to blind forwarding
 of packets, in both directions, until the tunnel is closed.  Tunnels
 are commonly used to create an end-to-end virtual connection, through
 one or more proxies, which can then be secured using TLS (Transport
 Layer Security, [RFC5246]).
 CONNECT is intended only for use in requests to a proxy.  An origin
 server that receives a CONNECT request for itself MAY respond with a
 2xx (Successful) status code to indicate that a connection is
 established.  However, most origin servers do not implement CONNECT.
 A client sending a CONNECT request MUST send the authority form of
 request-target (Section 5.3 of [RFC7230]); i.e., the request-target
 consists of only the host name and port number of the tunnel
 destination, separated by a colon.  For example,
   CONNECT server.example.com:80 HTTP/1.1
   Host: server.example.com:80
 The recipient proxy can establish a tunnel either by directly
 connecting to the request-target or, if configured to use another
 proxy, by forwarding the CONNECT request to the next inbound proxy.
 Any 2xx (Successful) response indicates that the sender (and all

Fielding & Reschke Standards Track [Page 30] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 inbound proxies) will switch to tunnel mode immediately after the
 blank line that concludes the successful response's header section;
 data received after that blank line is from the server identified by
 the request-target.  Any response other than a successful response
 indicates that the tunnel has not yet been formed and that the
 connection remains governed by HTTP.
 A tunnel is closed when a tunnel intermediary detects that either
 side has closed its connection: the intermediary MUST attempt to send
 any outstanding data that came from the closed side to the other
 side, close both connections, and then discard any remaining data
 left undelivered.
 Proxy authentication might be used to establish the authority to
 create a tunnel.  For example,
   CONNECT server.example.com:80 HTTP/1.1
   Host: server.example.com:80
   Proxy-Authorization: basic aGVsbG86d29ybGQ=
 There are significant risks in establishing a tunnel to arbitrary
 servers, particularly when the destination is a well-known or
 reserved TCP port that is not intended for Web traffic.  For example,
 a CONNECT to a request-target of "example.com:25" would suggest that
 the proxy connect to the reserved port for SMTP traffic; if allowed,
 that could trick the proxy into relaying spam email.  Proxies that
 support CONNECT SHOULD restrict its use to a limited set of known
 ports or a configurable whitelist of safe request targets.
 A server MUST NOT send any Transfer-Encoding or Content-Length header
 fields in a 2xx (Successful) response to CONNECT.  A client MUST
 ignore any Content-Length or Transfer-Encoding header fields received
 in a successful response to CONNECT.
 A payload within a CONNECT request message has no defined semantics;
 sending a payload body on a CONNECT request might cause some existing
 implementations to reject the request.
 Responses to the CONNECT method are not cacheable.

4.3.7. OPTIONS

 The OPTIONS method requests information about the communication
 options available for the target resource, at either the origin
 server or an intervening intermediary.  This method allows a client
 to determine the options and/or requirements associated with a
 resource, or the capabilities of a server, without implying a
 resource action.

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 An OPTIONS request with an asterisk ("*") as the request-target
 (Section 5.3 of [RFC7230]) applies to the server in general rather
 than to a specific resource.  Since a server's communication options
 typically depend on the resource, the "*" request is only useful as a
 "ping" or "no-op" type of method; it does nothing beyond allowing the
 client to test the capabilities of the server.  For example, this can
 be used to test a proxy for HTTP/1.1 conformance (or lack thereof).
 If the request-target is not an asterisk, the OPTIONS request applies
 to the options that are available when communicating with the target
 resource.
 A server generating a successful response to OPTIONS SHOULD send any
 header fields that might indicate optional features implemented by
 the server and applicable to the target resource (e.g., Allow),
 including potential extensions not defined by this specification.
 The response payload, if any, might also describe the communication
 options in a machine or human-readable representation.  A standard
 format for such a representation is not defined by this
 specification, but might be defined by future extensions to HTTP.  A
 server MUST generate a Content-Length field with a value of "0" if no
 payload body is to be sent in the response.
 A client MAY send a Max-Forwards header field in an OPTIONS request
 to target a specific recipient in the request chain (see
 Section 5.1.2).  A proxy MUST NOT generate a Max-Forwards header
 field while forwarding a request unless that request was received
 with a Max-Forwards field.
 A client that generates an OPTIONS request containing a payload body
 MUST send a valid Content-Type header field describing the
 representation media type.  Although this specification does not
 define any use for such a payload, future extensions to HTTP might
 use the OPTIONS body to make more detailed queries about the target
 resource.
 Responses to the OPTIONS method are not cacheable.

4.3.8. TRACE

 The TRACE method requests a remote, application-level loop-back of
 the request message.  The final recipient of the request SHOULD
 reflect the message received, excluding some fields described below,
 back to the client as the message body of a 200 (OK) response with a
 Content-Type of "message/http" (Section 8.3.1 of [RFC7230]).  The
 final recipient is either the origin server or the first server to
 receive a Max-Forwards value of zero (0) in the request
 (Section 5.1.2).

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 A client MUST NOT generate header fields in a TRACE request
 containing sensitive data that might be disclosed by the response.
 For example, it would be foolish for a user agent to send stored user
 credentials [RFC7235] or cookies [RFC6265] in a TRACE request.  The
 final recipient of the request SHOULD exclude any request header
 fields that are likely to contain sensitive data when that recipient
 generates the response body.
 TRACE allows the client to see what is being received at the other
 end of the request chain and use that data for testing or diagnostic
 information.  The value of the Via header field (Section 5.7.1 of
 [RFC7230]) is of particular interest, since it acts as a trace of the
 request chain.  Use of the Max-Forwards header field allows the
 client to limit the length of the request chain, which is useful for
 testing a chain of proxies forwarding messages in an infinite loop.
 A client MUST NOT send a message body in a TRACE request.
 Responses to the TRACE method are not cacheable.

5. Request Header Fields

 A client sends request header fields to provide more information
 about the request context, make the request conditional based on the
 target resource state, suggest preferred formats for the response,
 supply authentication credentials, or modify the expected request
 processing.  These fields act as request modifiers, similar to the
 parameters on a programming language method invocation.

5.1. Controls

 Controls are request header fields that direct specific handling of
 the request.
 +-------------------+--------------------------+
 | Header Field Name | Defined in...            |
 +-------------------+--------------------------+
 | Cache-Control     | Section 5.2 of [RFC7234] |
 | Expect            | Section 5.1.1            |
 | Host              | Section 5.4 of [RFC7230] |
 | Max-Forwards      | Section 5.1.2            |
 | Pragma            | Section 5.4 of [RFC7234] |
 | Range             | Section 3.1 of [RFC7233] |
 | TE                | Section 4.3 of [RFC7230] |
 +-------------------+--------------------------+

Fielding & Reschke Standards Track [Page 33] RFC 7231 HTTP/1.1 Semantics and Content June 2014

5.1.1. Expect

 The "Expect" header field in a request indicates a certain set of
 behaviors (expectations) that need to be supported by the server in
 order to properly handle this request.  The only such expectation
 defined by this specification is 100-continue.
   Expect  = "100-continue"
 The Expect field-value is case-insensitive.
 A server that receives an Expect field-value other than 100-continue
 MAY respond with a 417 (Expectation Failed) status code to indicate
 that the unexpected expectation cannot be met.
 A 100-continue expectation informs recipients that the client is
 about to send a (presumably large) message body in this request and
 wishes to receive a 100 (Continue) interim response if the
 request-line and header fields are not sufficient to cause an
 immediate success, redirect, or error response.  This allows the
 client to wait for an indication that it is worthwhile to send the
 message body before actually doing so, which can improve efficiency
 when the message body is huge or when the client anticipates that an
 error is likely (e.g., when sending a state-changing method, for the
 first time, without previously verified authentication credentials).
 For example, a request that begins with
   PUT /somewhere/fun HTTP/1.1
   Host: origin.example.com
   Content-Type: video/h264
   Content-Length: 1234567890987
   Expect: 100-continue
 allows the origin server to immediately respond with an error
 message, such as 401 (Unauthorized) or 405 (Method Not Allowed),
 before the client starts filling the pipes with an unnecessary data
 transfer.
 Requirements for clients:
 o  A client MUST NOT generate a 100-continue expectation in a request
    that does not include a message body.
 o  A client that will wait for a 100 (Continue) response before
    sending the request message body MUST send an Expect header field
    containing a 100-continue expectation.

Fielding & Reschke Standards Track [Page 34] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 o  A client that sends a 100-continue expectation is not required to
    wait for any specific length of time; such a client MAY proceed to
    send the message body even if it has not yet received a response.
    Furthermore, since 100 (Continue) responses cannot be sent through
    an HTTP/1.0 intermediary, such a client SHOULD NOT wait for an
    indefinite period before sending the message body.
 o  A client that receives a 417 (Expectation Failed) status code in
    response to a request containing a 100-continue expectation SHOULD
    repeat that request without a 100-continue expectation, since the
    417 response merely indicates that the response chain does not
    support expectations (e.g., it passes through an HTTP/1.0 server).
 Requirements for servers:
 o  A server that receives a 100-continue expectation in an HTTP/1.0
    request MUST ignore that expectation.
 o  A server MAY omit sending a 100 (Continue) response if it has
    already received some or all of the message body for the
    corresponding request, or if the framing indicates that there is
    no message body.
 o  A server that sends a 100 (Continue) response MUST ultimately send
    a final status code, once the message body is received and
    processed, unless the connection is closed prematurely.
 o  A server that responds with a final status code before reading the
    entire message body SHOULD indicate in that response whether it
    intends to close the connection or continue reading and discarding
    the request message (see Section 6.6 of [RFC7230]).
 An origin server MUST, upon receiving an HTTP/1.1 (or later)
 request-line and a complete header section that contains a
 100-continue expectation and indicates a request message body will
 follow, either send an immediate response with a final status code,
 if that status can be determined by examining just the request-line
 and header fields, or send an immediate 100 (Continue) response to
 encourage the client to send the request's message body.  The origin
 server MUST NOT wait for the message body before sending the 100
 (Continue) response.
 A proxy MUST, upon receiving an HTTP/1.1 (or later) request-line and
 a complete header section that contains a 100-continue expectation
 and indicates a request message body will follow, either send an
 immediate response with a final status code, if that status can be
 determined by examining just the request-line and header fields, or
 begin forwarding the request toward the origin server by sending a

Fielding & Reschke Standards Track [Page 35] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 corresponding request-line and header section to the next inbound
 server.  If the proxy believes (from configuration or past
 interaction) that the next inbound server only supports HTTP/1.0, the
 proxy MAY generate an immediate 100 (Continue) response to encourage
 the client to begin sending the message body.
    Note: The Expect header field was added after the original
    publication of HTTP/1.1 [RFC2068] as both the means to request an
    interim 100 (Continue) response and the general mechanism for
    indicating must-understand extensions.  However, the extension
    mechanism has not been used by clients and the must-understand
    requirements have not been implemented by many servers, rendering
    the extension mechanism useless.  This specification has removed
    the extension mechanism in order to simplify the definition and
    processing of 100-continue.

5.1.2. Max-Forwards

 The "Max-Forwards" header field provides a mechanism with the TRACE
 (Section 4.3.8) and OPTIONS (Section 4.3.7) request methods to limit
 the number of times that the request is forwarded by proxies.  This
 can be useful when the client is attempting to trace a request that
 appears to be failing or looping mid-chain.
   Max-Forwards = 1*DIGIT
 The Max-Forwards value is a decimal integer indicating the remaining
 number of times this request message can be forwarded.
 Each intermediary that receives a TRACE or OPTIONS request containing
 a Max-Forwards header field MUST check and update its value prior to
 forwarding the request.  If the received value is zero (0), the
 intermediary MUST NOT forward the request; instead, the intermediary
 MUST respond as the final recipient.  If the received Max-Forwards
 value is greater than zero, the intermediary MUST generate an updated
 Max-Forwards field in the forwarded message with a field-value that
 is the lesser of a) the received value decremented by one (1) or b)
 the recipient's maximum supported value for Max-Forwards.
 A recipient MAY ignore a Max-Forwards header field received with any
 other request methods.

5.2. Conditionals

 The HTTP conditional request header fields [RFC7232] allow a client
 to place a precondition on the state of the target resource, so that
 the action corresponding to the method semantics will not be applied
 if the precondition evaluates to false.  Each precondition defined by

Fielding & Reschke Standards Track [Page 36] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 this specification consists of a comparison between a set of
 validators obtained from prior representations of the target resource
 to the current state of validators for the selected representation
 (Section 7.2).  Hence, these preconditions evaluate whether the state
 of the target resource has changed since a given state known by the
 client.  The effect of such an evaluation depends on the method
 semantics and choice of conditional, as defined in Section 5 of
 [RFC7232].
 +---------------------+--------------------------+
 | Header Field Name   | Defined in...            |
 +---------------------+--------------------------+
 | If-Match            | Section 3.1 of [RFC7232] |
 | If-None-Match       | Section 3.2 of [RFC7232] |
 | If-Modified-Since   | Section 3.3 of [RFC7232] |
 | If-Unmodified-Since | Section 3.4 of [RFC7232] |
 | If-Range            | Section 3.2 of [RFC7233] |
 +---------------------+--------------------------+

5.3. Content Negotiation

 The following request header fields are sent by a user agent to
 engage in proactive negotiation of the response content, as defined
 in Section 3.4.1.  The preferences sent in these fields apply to any
 content in the response, including representations of the target
 resource, representations of error or processing status, and
 potentially even the miscellaneous text strings that might appear
 within the protocol.
 +-------------------+---------------+
 | Header Field Name | Defined in... |
 +-------------------+---------------+
 | Accept            | Section 5.3.2 |
 | Accept-Charset    | Section 5.3.3 |
 | Accept-Encoding   | Section 5.3.4 |
 | Accept-Language   | Section 5.3.5 |
 +-------------------+---------------+

5.3.1. Quality Values

 Many of the request header fields for proactive negotiation use a
 common parameter, named "q" (case-insensitive), to assign a relative
 "weight" to the preference for that associated kind of content.  This
 weight is referred to as a "quality value" (or "qvalue") because the
 same parameter name is often used within server configurations to
 assign a weight to the relative quality of the various
 representations that can be selected for a resource.

Fielding & Reschke Standards Track [Page 37] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 The weight is normalized to a real number in the range 0 through 1,
 where 0.001 is the least preferred and 1 is the most preferred; a
 value of 0 means "not acceptable".  If no "q" parameter is present,
 the default weight is 1.
   weight = OWS ";" OWS "q=" qvalue
   qvalue = ( "0" [ "." 0*3DIGIT ] )
          / ( "1" [ "." 0*3("0") ] )
 A sender of qvalue MUST NOT generate more than three digits after the
 decimal point.  User configuration of these values ought to be
 limited in the same fashion.

5.3.2. Accept

 The "Accept" header field can be used by user agents to specify
 response media types that are acceptable.  Accept header fields can
 be used to indicate that the request is specifically limited to a
 small set of desired types, as in the case of a request for an
 in-line image.
   Accept = #( media-range [ accept-params ] )
   media-range    = ( "*/*"
                    / ( type "/" "*" )
                    / ( type "/" subtype )
                    ) *( OWS ";" OWS parameter )
   accept-params  = weight *( accept-ext )
   accept-ext = OWS ";" OWS token [ "=" ( token / quoted-string ) ]
 The asterisk "*" character is used to group media types into ranges,
 with "*/*" indicating all media types and "type/*" indicating all
 subtypes of that type.  The media-range can include media type
 parameters that are applicable to that range.
 Each media-range might be followed by zero or more applicable media
 type parameters (e.g., charset), an optional "q" parameter for
 indicating a relative weight (Section 5.3.1), and then zero or more
 extension parameters.  The "q" parameter is necessary if any
 extensions (accept-ext) are present, since it acts as a separator
 between the two parameter sets.
    Note: Use of the "q" parameter name to separate media type
    parameters from Accept extension parameters is due to historical
    practice.  Although this prevents any media type parameter named
    "q" from being used with a media range, such an event is believed
    to be unlikely given the lack of any "q" parameters in the IANA

Fielding & Reschke Standards Track [Page 38] RFC 7231 HTTP/1.1 Semantics and Content June 2014

    media type registry and the rare usage of any media type
    parameters in Accept.  Future media types are discouraged from
    registering any parameter named "q".
 The example
   Accept: audio/*; q=0.2, audio/basic
 is interpreted as "I prefer audio/basic, but send me any audio type
 if it is the best available after an 80% markdown in quality".
 A request without any Accept header field implies that the user agent
 will accept any media type in response.  If the header field is
 present in a request and none of the available representations for
 the response have a media type that is listed as acceptable, the
 origin server can either honor the header field by sending a 406 (Not
 Acceptable) response or disregard the header field by treating the
 response as if it is not subject to content negotiation.
 A more elaborate example is
   Accept: text/plain; q=0.5, text/html,
           text/x-dvi; q=0.8, text/x-c
 Verbally, this would be interpreted as "text/html and text/x-c are
 the equally preferred media types, but if they do not exist, then
 send the text/x-dvi representation, and if that does not exist, send
 the text/plain representation".
 Media ranges can be overridden by more specific media ranges or
 specific media types.  If more than one media range applies to a
 given type, the most specific reference has precedence.  For example,
   Accept: text/*, text/plain, text/plain;format=flowed, */*
 have the following precedence:
 1.  text/plain;format=flowed
 2.  text/plain
 3.  text/*
 4.  */*
 The media type quality factor associated with a given type is
 determined by finding the media range with the highest precedence
 that matches the type.  For example,

Fielding & Reschke Standards Track [Page 39] RFC 7231 HTTP/1.1 Semantics and Content June 2014

   Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
           text/html;level=2;q=0.4, */*;q=0.5
 would cause the following values to be associated:
 +-------------------+---------------+
 | Media Type        | Quality Value |
 +-------------------+---------------+
 | text/html;level=1 | 1             |
 | text/html         | 0.7           |
 | text/plain        | 0.3           |
 | image/jpeg        | 0.5           |
 | text/html;level=2 | 0.4           |
 | text/html;level=3 | 0.7           |
 +-------------------+---------------+
 Note: A user agent might be provided with a default set of quality
 values for certain media ranges.  However, unless the user agent is a
 closed system that cannot interact with other rendering agents, this
 default set ought to be configurable by the user.

5.3.3. Accept-Charset

 The "Accept-Charset" header field can be sent by a user agent to
 indicate what charsets are acceptable in textual response content.
 This field allows user agents capable of understanding more
 comprehensive or special-purpose charsets to signal that capability
 to an origin server that is capable of representing information in
 those charsets.
   Accept-Charset = 1#( ( charset / "*" ) [ weight ] )
 Charset names are defined in Section 3.1.1.2.  A user agent MAY
 associate a quality value with each charset to indicate the user's
 relative preference for that charset, as defined in Section 5.3.1.
 An example is
   Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
 The special value "*", if present in the Accept-Charset field,
 matches every charset that is not mentioned elsewhere in the
 Accept-Charset field.  If no "*" is present in an Accept-Charset
 field, then any charsets not explicitly mentioned in the field are
 considered "not acceptable" to the client.
 A request without any Accept-Charset header field implies that the
 user agent will accept any charset in response.  Most general-purpose
 user agents do not send Accept-Charset, unless specifically

Fielding & Reschke Standards Track [Page 40] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 configured to do so, because a detailed list of supported charsets
 makes it easier for a server to identify an individual by virtue of
 the user agent's request characteristics (Section 9.7).
 If an Accept-Charset header field is present in a request and none of
 the available representations for the response has a charset that is
 listed as acceptable, the origin server can either honor the header
 field, by sending a 406 (Not Acceptable) response, or disregard the
 header field by treating the resource as if it is not subject to
 content negotiation.

5.3.4. Accept-Encoding

 The "Accept-Encoding" header field can be used by user agents to
 indicate what response content-codings (Section 3.1.2.1) are
 acceptable in the response.  An "identity" token is used as a synonym
 for "no encoding" in order to communicate when no encoding is
 preferred.
   Accept-Encoding  = #( codings [ weight ] )
   codings          = content-coding / "identity" / "*"
 Each codings value MAY be given an associated quality value
 representing the preference for that encoding, as defined in
 Section 5.3.1.  The asterisk "*" symbol in an Accept-Encoding field
 matches any available content-coding not explicitly listed in the
 header field.
 For example,
   Accept-Encoding: compress, gzip
   Accept-Encoding:
   Accept-Encoding: *
   Accept-Encoding: compress;q=0.5, gzip;q=1.0
   Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
 A request without an Accept-Encoding header field implies that the
 user agent has no preferences regarding content-codings.  Although
 this allows the server to use any content-coding in a response, it
 does not imply that the user agent will be able to correctly process
 all encodings.
 A server tests whether a content-coding for a given representation is
 acceptable using these rules:
 1.  If no Accept-Encoding field is in the request, any content-coding
     is considered acceptable by the user agent.

Fielding & Reschke Standards Track [Page 41] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 2.  If the representation has no content-coding, then it is
     acceptable by default unless specifically excluded by the
     Accept-Encoding field stating either "identity;q=0" or "*;q=0"
     without a more specific entry for "identity".
 3.  If the representation's content-coding is one of the
     content-codings listed in the Accept-Encoding field, then it is
     acceptable unless it is accompanied by a qvalue of 0.  (As
     defined in Section 5.3.1, a qvalue of 0 means "not acceptable".)
 4.  If multiple content-codings are acceptable, then the acceptable
     content-coding with the highest non-zero qvalue is preferred.
 An Accept-Encoding header field with a combined field-value that is
 empty implies that the user agent does not want any content-coding in
 response.  If an Accept-Encoding header field is present in a request
 and none of the available representations for the response have a
 content-coding that is listed as acceptable, the origin server SHOULD
 send a response without any content-coding.
    Note: Most HTTP/1.0 applications do not recognize or obey qvalues
    associated with content-codings.  This means that qvalues might
    not work and are not permitted with x-gzip or x-compress.

5.3.5. Accept-Language

 The "Accept-Language" header field can be used by user agents to
 indicate the set of natural languages that are preferred in the
 response.  Language tags are defined in Section 3.1.3.1.
   Accept-Language = 1#( language-range [ weight ] )
   language-range  =
             <language-range, see [RFC4647], Section 2.1>
 Each language-range can be given an associated quality value
 representing an estimate of the user's preference for the languages
 specified by that range, as defined in Section 5.3.1.  For example,
   Accept-Language: da, en-gb;q=0.8, en;q=0.7
 would mean: "I prefer Danish, but will accept British English and
 other types of English".
 A request without any Accept-Language header field implies that the
 user agent will accept any language in response.  If the header field
 is present in a request and none of the available representations for
 the response have a matching language tag, the origin server can
 either disregard the header field by treating the response as if it

Fielding & Reschke Standards Track [Page 42] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 is not subject to content negotiation or honor the header field by
 sending a 406 (Not Acceptable) response.  However, the latter is not
 encouraged, as doing so can prevent users from accessing content that
 they might be able to use (with translation software, for example).
 Note that some recipients treat the order in which language tags are
 listed as an indication of descending priority, particularly for tags
 that are assigned equal quality values (no value is the same as q=1).
 However, this behavior cannot be relied upon.  For consistency and to
 maximize interoperability, many user agents assign each language tag
 a unique quality value while also listing them in order of decreasing
 quality.  Additional discussion of language priority lists can be
 found in Section 2.3 of [RFC4647].
 For matching, Section 3 of [RFC4647] defines several matching
 schemes.  Implementations can offer the most appropriate matching
 scheme for their requirements.  The "Basic Filtering" scheme
 ([RFC4647], Section 3.3.1) is identical to the matching scheme that
 was previously defined for HTTP in Section 14.4 of [RFC2616].
 It might be contrary to the privacy expectations of the user to send
 an Accept-Language header field with the complete linguistic
 preferences of the user in every request (Section 9.7).
 Since intelligibility is highly dependent on the individual user,
 user agents need to allow user control over the linguistic preference
 (either through configuration of the user agent itself or by
 defaulting to a user controllable system setting).  A user agent that
 does not provide such control to the user MUST NOT send an
 Accept-Language header field.
    Note: User agents ought to provide guidance to users when setting
    a preference, since users are rarely familiar with the details of
    language matching as described above.  For example, users might
    assume that on selecting "en-gb", they will be served any kind of
    English document if British English is not available.  A user
    agent might suggest, in such a case, to add "en" to the list for
    better matching behavior.

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5.4. Authentication Credentials

 Two header fields are used for carrying authentication credentials,
 as defined in [RFC7235].  Note that various custom mechanisms for
 user authentication use the Cookie header field for this purpose, as
 defined in [RFC6265].
 +---------------------+--------------------------+
 | Header Field Name   | Defined in...            |
 +---------------------+--------------------------+
 | Authorization       | Section 4.2 of [RFC7235] |
 | Proxy-Authorization | Section 4.4 of [RFC7235] |
 +---------------------+--------------------------+

5.5. Request Context

 The following request header fields provide additional information
 about the request context, including information about the user, user
 agent, and resource behind the request.
 +-------------------+---------------+
 | Header Field Name | Defined in... |
 +-------------------+---------------+
 | From              | Section 5.5.1 |
 | Referer           | Section 5.5.2 |
 | User-Agent        | Section 5.5.3 |
 +-------------------+---------------+

5.5.1. From

 The "From" header field contains an Internet email address for a
 human user who controls the requesting user agent.  The address ought
 to be machine-usable, as defined by "mailbox" in Section 3.4 of
 [RFC5322]:
   From    = mailbox
   mailbox = <mailbox, see [RFC5322], Section 3.4>
 An example is:
   From: webmaster@example.org
 The From header field is rarely sent by non-robotic user agents.  A
 user agent SHOULD NOT send a From header field without explicit
 configuration by the user, since that might conflict with the user's
 privacy interests or their site's security policy.

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 A robotic user agent SHOULD send a valid From header field so that
 the person responsible for running the robot can be contacted if
 problems occur on servers, such as if the robot is sending excessive,
 unwanted, or invalid requests.
 A server SHOULD NOT use the From header field for access control or
 authentication, since most recipients will assume that the field
 value is public information.

5.5.2. Referer

 The "Referer" [sic] header field allows the user agent to specify a
 URI reference for the resource from which the target URI was obtained
 (i.e., the "referrer", though the field name is misspelled).  A user
 agent MUST NOT include the fragment and userinfo components of the
 URI reference [RFC3986], if any, when generating the Referer field
 value.
   Referer = absolute-URI / partial-URI
 The Referer header field allows servers to generate back-links to
 other resources for simple analytics, logging, optimized caching,
 etc.  It also allows obsolete or mistyped links to be found for
 maintenance.  Some servers use the Referer header field as a means of
 denying links from other sites (so-called "deep linking") or
 restricting cross-site request forgery (CSRF), but not all requests
 contain it.
 Example:
   Referer: http://www.example.org/hypertext/Overview.html
 If the target URI was obtained from a source that does not have its
 own URI (e.g., input from the user keyboard, or an entry within the
 user's bookmarks/favorites), the user agent MUST either exclude the
 Referer field or send it with a value of "about:blank".
 The Referer field has the potential to reveal information about the
 request context or browsing history of the user, which is a privacy
 concern if the referring resource's identifier reveals personal
 information (such as an account name) or a resource that is supposed
 to be confidential (such as behind a firewall or internal to a
 secured service).  Most general-purpose user agents do not send the
 Referer header field when the referring resource is a local "file" or
 "data" URI.  A user agent MUST NOT send a Referer header field in an
 unsecured HTTP request if the referring page was received with a
 secure protocol.  See Section 9.4 for additional security
 considerations.

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 Some intermediaries have been known to indiscriminately remove
 Referer header fields from outgoing requests.  This has the
 unfortunate side effect of interfering with protection against CSRF
 attacks, which can be far more harmful to their users.
 Intermediaries and user agent extensions that wish to limit
 information disclosure in Referer ought to restrict their changes to
 specific edits, such as replacing internal domain names with
 pseudonyms or truncating the query and/or path components.  An
 intermediary SHOULD NOT modify or delete the Referer header field
 when the field value shares the same scheme and host as the request
 target.

5.5.3. User-Agent

 The "User-Agent" header field contains information about the user
 agent originating the request, which is often used by servers to help
 identify the scope of reported interoperability problems, to work
 around or tailor responses to avoid particular user agent
 limitations, and for analytics regarding browser or operating system
 use.  A user agent SHOULD send a User-Agent field in each request
 unless specifically configured not to do so.
   User-Agent = product *( RWS ( product / comment ) )
 The User-Agent field-value consists of one or more product
 identifiers, each followed by zero or more comments (Section 3.2 of
 [RFC7230]), which together identify the user agent software and its
 significant subproducts.  By convention, the product identifiers are
 listed in decreasing order of their significance for identifying the
 user agent software.  Each product identifier consists of a name and
 optional version.
   product         = token ["/" product-version]
   product-version = token
 A sender SHOULD limit generated product identifiers to what is
 necessary to identify the product; a sender MUST NOT generate
 advertising or other nonessential information within the product
 identifier.  A sender SHOULD NOT generate information in
 product-version that is not a version identifier (i.e., successive
 versions of the same product name ought to differ only in the
 product-version portion of the product identifier).
 Example:
   User-Agent: CERN-LineMode/2.15 libwww/2.17b3

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 A user agent SHOULD NOT generate a User-Agent field containing
 needlessly fine-grained detail and SHOULD limit the addition of
 subproducts by third parties.  Overly long and detailed User-Agent
 field values increase request latency and the risk of a user being
 identified against their wishes ("fingerprinting").
 Likewise, implementations are encouraged not to use the product
 tokens of other implementations in order to declare compatibility
 with them, as this circumvents the purpose of the field.  If a user
 agent masquerades as a different user agent, recipients can assume
 that the user intentionally desires to see responses tailored for
 that identified user agent, even if they might not work as well for
 the actual user agent being used.

6. Response Status Codes

 The status-code element is a three-digit integer code giving the
 result of the attempt to understand and satisfy the request.
 HTTP status codes are extensible.  HTTP clients are not required to
 understand the meaning of all registered status codes, though such
 understanding is obviously desirable.  However, a client MUST
 understand the class of any status code, as indicated by the first
 digit, and treat an unrecognized status code as being equivalent to
 the x00 status code of that class, with the exception that a
 recipient MUST NOT cache a response with an unrecognized status code.
 For example, if an unrecognized status code of 471 is received by a
 client, the client can assume that there was something wrong with its
 request and treat the response as if it had received a 400 (Bad
 Request) status code.  The response message will usually contain a
 representation that explains the status.
 The first digit of the status-code defines the class of response.
 The last two digits do not have any categorization role.  There are
 five values for the first digit:
 o  1xx (Informational): The request was received, continuing process
 o  2xx (Successful): The request was successfully received,
    understood, and accepted
 o  3xx (Redirection): Further action needs to be taken in order to
    complete the request
 o  4xx (Client Error): The request contains bad syntax or cannot be
    fulfilled

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 o  5xx (Server Error): The server failed to fulfill an apparently
    valid request

6.1. Overview of Status Codes

 The status codes listed below are defined in this specification,
 Section 4 of [RFC7232], Section 4 of [RFC7233], and Section 3 of
 [RFC7235].  The reason phrases listed here are only recommendations
 -- they can be replaced by local equivalents without affecting the
 protocol.
 Responses with status codes that are defined as cacheable by default
 (e.g., 200, 203, 204, 206, 300, 301, 404, 405, 410, 414, and 501 in
 this specification) can be reused by a cache with heuristic
 expiration unless otherwise indicated by the method definition or
 explicit cache controls [RFC7234]; all other status codes are not
 cacheable by default.

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 +------+-------------------------------+--------------------------+
 | Code | Reason-Phrase                 | Defined in...            |
 +------+-------------------------------+--------------------------+
 | 100  | Continue                      | Section 6.2.1            |
 | 101  | Switching Protocols           | Section 6.2.2            |
 | 200  | OK                            | Section 6.3.1            |
 | 201  | Created                       | Section 6.3.2            |
 | 202  | Accepted                      | Section 6.3.3            |
 | 203  | Non-Authoritative Information | Section 6.3.4            |
 | 204  | No Content                    | Section 6.3.5            |
 | 205  | Reset Content                 | Section 6.3.6            |
 | 206  | Partial Content               | Section 4.1 of [RFC7233] |
 | 300  | Multiple Choices              | Section 6.4.1            |
 | 301  | Moved Permanently             | Section 6.4.2            |
 | 302  | Found                         | Section 6.4.3            |
 | 303  | See Other                     | Section 6.4.4            |
 | 304  | Not Modified                  | Section 4.1 of [RFC7232] |
 | 305  | Use Proxy                     | Section 6.4.5            |
 | 307  | Temporary Redirect            | Section 6.4.7            |
 | 400  | Bad Request                   | Section 6.5.1            |
 | 401  | Unauthorized                  | Section 3.1 of [RFC7235] |
 | 402  | Payment Required              | Section 6.5.2            |
 | 403  | Forbidden                     | Section 6.5.3            |
 | 404  | Not Found                     | Section 6.5.4            |
 | 405  | Method Not Allowed            | Section 6.5.5            |
 | 406  | Not Acceptable                | Section 6.5.6            |
 | 407  | Proxy Authentication Required | Section 3.2 of [RFC7235] |
 | 408  | Request Timeout               | Section 6.5.7            |
 | 409  | Conflict                      | Section 6.5.8            |
 | 410  | Gone                          | Section 6.5.9            |
 | 411  | Length Required               | Section 6.5.10           |
 | 412  | Precondition Failed           | Section 4.2 of [RFC7232] |
 | 413  | Payload Too Large             | Section 6.5.11           |
 | 414  | URI Too Long                  | Section 6.5.12           |
 | 415  | Unsupported Media Type        | Section 6.5.13           |
 | 416  | Range Not Satisfiable         | Section 4.4 of [RFC7233] |
 | 417  | Expectation Failed            | Section 6.5.14           |
 | 426  | Upgrade Required              | Section 6.5.15           |
 | 500  | Internal Server Error         | Section 6.6.1            |
 | 501  | Not Implemented               | Section 6.6.2            |
 | 502  | Bad Gateway                   | Section 6.6.3            |
 | 503  | Service Unavailable           | Section 6.6.4            |
 | 504  | Gateway Timeout               | Section 6.6.5            |
 | 505  | HTTP Version Not Supported    | Section 6.6.6            |
 +------+-------------------------------+--------------------------+

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 Note that this list is not exhaustive -- it does not include
 extension status codes defined in other specifications.  The complete
 list of status codes is maintained by IANA.  See Section 8.2 for
 details.

6.2. Informational 1xx

 The 1xx (Informational) class of status code indicates an interim
 response for communicating connection status or request progress
 prior to completing the requested action and sending a final
 response. 1xx responses are terminated by the first empty line after
 the status-line (the empty line signaling the end of the header
 section).  Since HTTP/1.0 did not define any 1xx status codes, a
 server MUST NOT send a 1xx response to an HTTP/1.0 client.
 A client MUST be able to parse one or more 1xx responses received
 prior to a final response, even if the client does not expect one.  A
 user agent MAY ignore unexpected 1xx responses.
 A proxy MUST forward 1xx responses unless the proxy itself requested
 the generation of the 1xx response.  For example, if a proxy adds an
 "Expect: 100-continue" field when it forwards a request, then it need
 not forward the corresponding 100 (Continue) response(s).

6.2.1. 100 Continue

 The 100 (Continue) status code indicates that the initial part of a
 request has been received and has not yet been rejected by the
 server.  The server intends to send a final response after the
 request has been fully received and acted upon.
 When the request contains an Expect header field that includes a
 100-continue expectation, the 100 response indicates that the server
 wishes to receive the request payload body, as described in
 Section 5.1.1.  The client ought to continue sending the request and
 discard the 100 response.
 If the request did not contain an Expect header field containing the
 100-continue expectation, the client can simply discard this interim
 response.

6.2.2. 101 Switching Protocols

 The 101 (Switching Protocols) status code indicates that the server
 understands and is willing to comply with the client's request, via
 the Upgrade header field (Section 6.7 of [RFC7230]), for a change in
 the application protocol being used on this connection.  The server

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 MUST generate an Upgrade header field in the response that indicates
 which protocol(s) will be switched to immediately after the empty
 line that terminates the 101 response.
 It is assumed that the server will only agree to switch protocols
 when it is advantageous to do so.  For example, switching to a newer
 version of HTTP might be advantageous over older versions, and
 switching to a real-time, synchronous protocol might be advantageous
 when delivering resources that use such features.

6.3. Successful 2xx

 The 2xx (Successful) class of status code indicates that the client's
 request was successfully received, understood, and accepted.

6.3.1. 200 OK

 The 200 (OK) status code indicates that the request has succeeded.
 The payload sent in a 200 response depends on the request method.
 For the methods defined by this specification, the intended meaning
 of the payload can be summarized as:
 GET  a representation of the target resource;
 HEAD  the same representation as GET, but without the representation
    data;
 POST  a representation of the status of, or results obtained from,
    the action;
 PUT, DELETE  a representation of the status of the action;
 OPTIONS  a representation of the communications options;
 TRACE  a representation of the request message as received by the end
    server.
 Aside from responses to CONNECT, a 200 response always has a payload,
 though an origin server MAY generate a payload body of zero length.
 If no payload is desired, an origin server ought to send 204 (No
 Content) instead.  For CONNECT, no payload is allowed because the
 successful result is a tunnel, which begins immediately after the 200
 response header section.
 A 200 response is cacheable by default; i.e., unless otherwise
 indicated by the method definition or explicit cache controls (see
 Section 4.2.2 of [RFC7234]).

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6.3.2. 201 Created

 The 201 (Created) status code indicates that the request has been
 fulfilled and has resulted in one or more new resources being
 created.  The primary resource created by the request is identified
 by either a Location header field in the response or, if no Location
 field is received, by the effective request URI.
 The 201 response payload typically describes and links to the
 resource(s) created.  See Section 7.2 for a discussion of the meaning
 and purpose of validator header fields, such as ETag and
 Last-Modified, in a 201 response.

6.3.3. 202 Accepted

 The 202 (Accepted) status code indicates that the request has been
 accepted for processing, but the processing has not been completed.
 The request might or might not eventually be acted upon, as it might
 be disallowed when processing actually takes place.  There is no
 facility in HTTP for re-sending a status code from an asynchronous
 operation.
 The 202 response is intentionally noncommittal.  Its purpose is to
 allow a server to accept a request for some other process (perhaps a
 batch-oriented process that is only run once per day) without
 requiring that the user agent's connection to the server persist
 until the process is completed.  The representation sent with this
 response ought to describe the request's current status and point to
 (or embed) a status monitor that can provide the user with an
 estimate of when the request will be fulfilled.

6.3.4. 203 Non-Authoritative Information

 The 203 (Non-Authoritative Information) status code indicates that
 the request was successful but the enclosed payload has been modified
 from that of the origin server's 200 (OK) response by a transforming
 proxy (Section 5.7.2 of [RFC7230]).  This status code allows the
 proxy to notify recipients when a transformation has been applied,
 since that knowledge might impact later decisions regarding the
 content.  For example, future cache validation requests for the
 content might only be applicable along the same request path (through
 the same proxies).
 The 203 response is similar to the Warning code of 214 Transformation
 Applied (Section 5.5 of [RFC7234]), which has the advantage of being
 applicable to responses with any status code.

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 A 203 response is cacheable by default; i.e., unless otherwise
 indicated by the method definition or explicit cache controls (see
 Section 4.2.2 of [RFC7234]).

6.3.5. 204 No Content

 The 204 (No Content) status code indicates that the server has
 successfully fulfilled the request and that there is no additional
 content to send in the response payload body.  Metadata in the
 response header fields refer to the target resource and its selected
 representation after the requested action was applied.
 For example, if a 204 status code is received in response to a PUT
 request and the response contains an ETag header field, then the PUT
 was successful and the ETag field-value contains the entity-tag for
 the new representation of that target resource.
 The 204 response allows a server to indicate that the action has been
 successfully applied to the target resource, while implying that the
 user agent does not need to traverse away from its current "document
 view" (if any).  The server assumes that the user agent will provide
 some indication of the success to its user, in accord with its own
 interface, and apply any new or updated metadata in the response to
 its active representation.
 For example, a 204 status code is commonly used with document editing
 interfaces corresponding to a "save" action, such that the document
 being saved remains available to the user for editing.  It is also
 frequently used with interfaces that expect automated data transfers
 to be prevalent, such as within distributed version control systems.
 A 204 response is terminated by the first empty line after the header
 fields because it cannot contain a message body.
 A 204 response is cacheable by default; i.e., unless otherwise
 indicated by the method definition or explicit cache controls (see
 Section 4.2.2 of [RFC7234]).

6.3.6. 205 Reset Content

 The 205 (Reset Content) status code indicates that the server has
 fulfilled the request and desires that the user agent reset the
 "document view", which caused the request to be sent, to its original
 state as received from the origin server.
 This response is intended to support a common data entry use case
 where the user receives content that supports data entry (a form,
 notepad, canvas, etc.), enters or manipulates data in that space,

Fielding & Reschke Standards Track [Page 53] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 causes the entered data to be submitted in a request, and then the
 data entry mechanism is reset for the next entry so that the user can
 easily initiate another input action.
 Since the 205 status code implies that no additional content will be
 provided, a server MUST NOT generate a payload in a 205 response.  In
 other words, a server MUST do one of the following for a 205
 response: a) indicate a zero-length body for the response by
 including a Content-Length header field with a value of 0; b)
 indicate a zero-length payload for the response by including a
 Transfer-Encoding header field with a value of chunked and a message
 body consisting of a single chunk of zero-length; or, c) close the
 connection immediately after sending the blank line terminating the
 header section.

6.4. Redirection 3xx

 The 3xx (Redirection) class of status code indicates that further
 action needs to be taken by the user agent in order to fulfill the
 request.  If a Location header field (Section 7.1.2) is provided, the
 user agent MAY automatically redirect its request to the URI
 referenced by the Location field value, even if the specific status
 code is not understood.  Automatic redirection needs to done with
 care for methods not known to be safe, as defined in Section 4.2.1,
 since the user might not wish to redirect an unsafe request.
 There are several types of redirects:
 1.  Redirects that indicate the resource might be available at a
     different URI, as provided by the Location field, as in the
     status codes 301 (Moved Permanently), 302 (Found), and 307
     (Temporary Redirect).
 2.  Redirection that offers a choice of matching resources, each
     capable of representing the original request target, as in the
     300 (Multiple Choices) status code.
 3.  Redirection to a different resource, identified by the Location
     field, that can represent an indirect response to the request, as
     in the 303 (See Other) status code.
 4.  Redirection to a previously cached result, as in the 304 (Not
     Modified) status code.
    Note: In HTTP/1.0, the status codes 301 (Moved Permanently) and
    302 (Found) were defined for the first type of redirect
    ([RFC1945], Section 9.3).  Early user agents split on whether the
    method applied to the redirect target would be the same as the

Fielding & Reschke Standards Track [Page 54] RFC 7231 HTTP/1.1 Semantics and Content June 2014

    original request or would be rewritten as GET.  Although HTTP
    originally defined the former semantics for 301 and 302 (to match
    its original implementation at CERN), and defined 303 (See Other)
    to match the latter semantics, prevailing practice gradually
    converged on the latter semantics for 301 and 302 as well.  The
    first revision of HTTP/1.1 added 307 (Temporary Redirect) to
    indicate the former semantics without being impacted by divergent
    practice.  Over 10 years later, most user agents still do method
    rewriting for 301 and 302; therefore, this specification makes
    that behavior conformant when the original request is POST.
 A client SHOULD detect and intervene in cyclical redirections (i.e.,
 "infinite" redirection loops).
    Note: An earlier version of this specification recommended a
    maximum of five redirections ([RFC2068], Section 10.3).  Content
    developers need to be aware that some clients might implement such
    a fixed limitation.

6.4.1. 300 Multiple Choices

 The 300 (Multiple Choices) status code indicates that the target
 resource has more than one representation, each with its own more
 specific identifier, and information about the alternatives is being
 provided so that the user (or user agent) can select a preferred
 representation by redirecting its request to one or more of those
 identifiers.  In other words, the server desires that the user agent
 engage in reactive negotiation to select the most appropriate
 representation(s) for its needs (Section 3.4).
 If the server has a preferred choice, the server SHOULD generate a
 Location header field containing a preferred choice's URI reference.
 The user agent MAY use the Location field value for automatic
 redirection.
 For request methods other than HEAD, the server SHOULD generate a
 payload in the 300 response containing a list of representation
 metadata and URI reference(s) from which the user or user agent can
 choose the one most preferred.  The user agent MAY make a selection
 from that list automatically if it understands the provided media
 type.  A specific format for automatic selection is not defined by
 this specification because HTTP tries to remain orthogonal to the
 definition of its payloads.  In practice, the representation is
 provided in some easily parsed format believed to be acceptable to
 the user agent, as determined by shared design or content
 negotiation, or in some commonly accepted hypertext format.

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 A 300 response is cacheable by default; i.e., unless otherwise
 indicated by the method definition or explicit cache controls (see
 Section 4.2.2 of [RFC7234]).
    Note: The original proposal for the 300 status code defined the
    URI header field as providing a list of alternative
    representations, such that it would be usable for 200, 300, and
    406 responses and be transferred in responses to the HEAD method.
    However, lack of deployment and disagreement over syntax led to
    both URI and Alternates (a subsequent proposal) being dropped from
    this specification.  It is possible to communicate the list using
    a set of Link header fields [RFC5988], each with a relationship of
    "alternate", though deployment is a chicken-and-egg problem.

6.4.2. 301 Moved Permanently

 The 301 (Moved Permanently) status code indicates that the target
 resource has been assigned a new permanent URI and any future
 references to this resource ought to use one of the enclosed URIs.
 Clients with link-editing capabilities ought to automatically re-link
 references to the effective request URI to one or more of the new
 references sent by the server, where possible.
 The server SHOULD generate a Location header field in the response
 containing a preferred URI reference for the new permanent URI.  The
 user agent MAY use the Location field value for automatic
 redirection.  The server's response payload usually contains a short
 hypertext note with a hyperlink to the new URI(s).
    Note: For historical reasons, a user agent MAY change the request
    method from POST to GET for the subsequent request.  If this
    behavior is undesired, the 307 (Temporary Redirect) status code
    can be used instead.
 A 301 response is cacheable by default; i.e., unless otherwise
 indicated by the method definition or explicit cache controls (see
 Section 4.2.2 of [RFC7234]).

6.4.3. 302 Found

 The 302 (Found) status code indicates that the target resource
 resides temporarily under a different URI.  Since the redirection
 might be altered on occasion, the client ought to continue to use the
 effective request URI for future requests.

Fielding & Reschke Standards Track [Page 56] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 The server SHOULD generate a Location header field in the response
 containing a URI reference for the different URI.  The user agent MAY
 use the Location field value for automatic redirection.  The server's
 response payload usually contains a short hypertext note with a
 hyperlink to the different URI(s).
    Note: For historical reasons, a user agent MAY change the request
    method from POST to GET for the subsequent request.  If this
    behavior is undesired, the 307 (Temporary Redirect) status code
    can be used instead.

6.4.4. 303 See Other

 The 303 (See Other) status code indicates that the server is
 redirecting the user agent to a different resource, as indicated by a
 URI in the Location header field, which is intended to provide an
 indirect response to the original request.  A user agent can perform
 a retrieval request targeting that URI (a GET or HEAD request if
 using HTTP), which might also be redirected, and present the eventual
 result as an answer to the original request.  Note that the new URI
 in the Location header field is not considered equivalent to the
 effective request URI.
 This status code is applicable to any HTTP method.  It is primarily
 used to allow the output of a POST action to redirect the user agent
 to a selected resource, since doing so provides the information
 corresponding to the POST response in a form that can be separately
 identified, bookmarked, and cached, independent of the original
 request.
 A 303 response to a GET request indicates that the origin server does
 not have a representation of the target resource that can be
 transferred by the server over HTTP.  However, the Location field
 value refers to a resource that is descriptive of the target
 resource, such that making a retrieval request on that other resource
 might result in a representation that is useful to recipients without
 implying that it represents the original target resource.  Note that
 answers to the questions of what can be represented, what
 representations are adequate, and what might be a useful description
 are outside the scope of HTTP.
 Except for responses to a HEAD request, the representation of a 303
 response ought to contain a short hypertext note with a hyperlink to
 the same URI reference provided in the Location header field.

Fielding & Reschke Standards Track [Page 57] RFC 7231 HTTP/1.1 Semantics and Content June 2014

6.4.5. 305 Use Proxy

 The 305 (Use Proxy) status code was defined in a previous version of
 this specification and is now deprecated (Appendix B).

6.4.6. 306 (Unused)

 The 306 status code was defined in a previous version of this
 specification, is no longer used, and the code is reserved.

6.4.7. 307 Temporary Redirect

 The 307 (Temporary Redirect) status code indicates that the target
 resource resides temporarily under a different URI and the user agent
 MUST NOT change the request method if it performs an automatic
 redirection to that URI.  Since the redirection can change over time,
 the client ought to continue using the original effective request URI
 for future requests.
 The server SHOULD generate a Location header field in the response
 containing a URI reference for the different URI.  The user agent MAY
 use the Location field value for automatic redirection.  The server's
 response payload usually contains a short hypertext note with a
 hyperlink to the different URI(s).
    Note: This status code is similar to 302 (Found), except that it
    does not allow changing the request method from POST to GET.  This
    specification defines no equivalent counterpart for 301 (Moved
    Permanently) ([RFC7238], however, defines the status code 308
    (Permanent Redirect) for this purpose).

6.5. Client Error 4xx

 The 4xx (Client Error) class of status code indicates that the client
 seems to have erred.  Except when responding to a HEAD request, the
 server SHOULD send a representation containing an explanation of the
 error situation, and whether it is a temporary or permanent
 condition.  These status codes are applicable to any request method.
 User agents SHOULD display any included representation to the user.

6.5.1. 400 Bad Request

 The 400 (Bad Request) status code indicates that the server cannot or
 will not process the request due to something that is perceived to be
 a client error (e.g., malformed request syntax, invalid request
 message framing, or deceptive request routing).

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6.5.2. 402 Payment Required

 The 402 (Payment Required) status code is reserved for future use.

6.5.3. 403 Forbidden

 The 403 (Forbidden) status code indicates that the server understood
 the request but refuses to authorize it.  A server that wishes to
 make public why the request has been forbidden can describe that
 reason in the response payload (if any).
 If authentication credentials were provided in the request, the
 server considers them insufficient to grant access.  The client
 SHOULD NOT automatically repeat the request with the same
 credentials.  The client MAY repeat the request with new or different
 credentials.  However, a request might be forbidden for reasons
 unrelated to the credentials.
 An origin server that wishes to "hide" the current existence of a
 forbidden target resource MAY instead respond with a status code of
 404 (Not Found).

6.5.4. 404 Not Found

 The 404 (Not Found) status code indicates that the origin server did
 not find a current representation for the target resource or is not
 willing to disclose that one exists.  A 404 status code does not
 indicate whether this lack of representation is temporary or
 permanent; the 410 (Gone) status code is preferred over 404 if the
 origin server knows, presumably through some configurable means, that
 the condition is likely to be permanent.
 A 404 response is cacheable by default; i.e., unless otherwise
 indicated by the method definition or explicit cache controls (see
 Section 4.2.2 of [RFC7234]).

6.5.5. 405 Method Not Allowed

 The 405 (Method Not Allowed) status code indicates that the method
 received in the request-line is known by the origin server but not
 supported by the target resource.  The origin server MUST generate an
 Allow header field in a 405 response containing a list of the target
 resource's currently supported methods.
 A 405 response is cacheable by default; i.e., unless otherwise
 indicated by the method definition or explicit cache controls (see
 Section 4.2.2 of [RFC7234]).

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6.5.6. 406 Not Acceptable

 The 406 (Not Acceptable) status code indicates that the target
 resource does not have a current representation that would be
 acceptable to the user agent, according to the proactive negotiation
 header fields received in the request (Section 5.3), and the server
 is unwilling to supply a default representation.
 The server SHOULD generate a payload containing a list of available
 representation characteristics and corresponding resource identifiers
 from which the user or user agent can choose the one most
 appropriate.  A user agent MAY automatically select the most
 appropriate choice from that list.  However, this specification does
 not define any standard for such automatic selection, as described in
 Section 6.4.1.

6.5.7. 408 Request Timeout

 The 408 (Request Timeout) status code indicates that the server did
 not receive a complete request message within the time that it was
 prepared to wait.  A server SHOULD send the "close" connection option
 (Section 6.1 of [RFC7230]) in the response, since 408 implies that
 the server has decided to close the connection rather than continue
 waiting.  If the client has an outstanding request in transit, the
 client MAY repeat that request on a new connection.

6.5.8. 409 Conflict

 The 409 (Conflict) status code indicates that the request could not
 be completed due to a conflict with the current state of the target
 resource.  This code is used in situations where the user might be
 able to resolve the conflict and resubmit the request.  The server
 SHOULD generate a payload that includes enough information for a user
 to recognize the source of the conflict.
 Conflicts are most likely to occur in response to a PUT request.  For
 example, if versioning were being used and the representation being
 PUT included changes to a resource that conflict with those made by
 an earlier (third-party) request, the origin server might use a 409
 response to indicate that it can't complete the request.  In this
 case, the response representation would likely contain information
 useful for merging the differences based on the revision history.

6.5.9. 410 Gone

 The 410 (Gone) status code indicates that access to the target
 resource is no longer available at the origin server and that this
 condition is likely to be permanent.  If the origin server does not

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 know, or has no facility to determine, whether or not the condition
 is permanent, the status code 404 (Not Found) ought to be used
 instead.
 The 410 response is primarily intended to assist the task of web
 maintenance by notifying the recipient that the resource is
 intentionally unavailable and that the server owners desire that
 remote links to that resource be removed.  Such an event is common
 for limited-time, promotional services and for resources belonging to
 individuals no longer associated with the origin server's site.  It
 is not necessary to mark all permanently unavailable resources as
 "gone" or to keep the mark for any length of time -- that is left to
 the discretion of the server owner.
 A 410 response is cacheable by default; i.e., unless otherwise
 indicated by the method definition or explicit cache controls (see
 Section 4.2.2 of [RFC7234]).

6.5.10. 411 Length Required

 The 411 (Length Required) status code indicates that the server
 refuses to accept the request without a defined Content-Length
 (Section 3.3.2 of [RFC7230]).  The client MAY repeat the request if
 it adds a valid Content-Length header field containing the length of
 the message body in the request message.

6.5.11. 413 Payload Too Large

 The 413 (Payload Too Large) status code indicates that the server is
 refusing to process a request because the request payload is larger
 than the server is willing or able to process.  The server MAY close
 the connection to prevent the client from continuing the request.
 If the condition is temporary, the server SHOULD generate a
 Retry-After header field to indicate that it is temporary and after
 what time the client MAY try again.

6.5.12. 414 URI Too Long

 The 414 (URI Too Long) status code indicates that the server is
 refusing to service the request because the request-target (Section
 5.3 of [RFC7230]) is longer than the server is willing to interpret.
 This rare condition is only likely to occur when a client has
 improperly converted a POST request to a GET request with long query
 information, when the client has descended into a "black hole" of
 redirection (e.g., a redirected URI prefix that points to a suffix of
 itself) or when the server is under attack by a client attempting to
 exploit potential security holes.

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 A 414 response is cacheable by default; i.e., unless otherwise
 indicated by the method definition or explicit cache controls (see
 Section 4.2.2 of [RFC7234]).

6.5.13. 415 Unsupported Media Type

 The 415 (Unsupported Media Type) status code indicates that the
 origin server is refusing to service the request because the payload
 is in a format not supported by this method on the target resource.
 The format problem might be due to the request's indicated
 Content-Type or Content-Encoding, or as a result of inspecting the
 data directly.

6.5.14. 417 Expectation Failed

 The 417 (Expectation Failed) status code indicates that the
 expectation given in the request's Expect header field
 (Section 5.1.1) could not be met by at least one of the inbound
 servers.

6.5.15. 426 Upgrade Required

 The 426 (Upgrade Required) status code indicates that the server
 refuses to perform the request using the current protocol but might
 be willing to do so after the client upgrades to a different
 protocol.  The server MUST send an Upgrade header field in a 426
 response to indicate the required protocol(s) (Section 6.7 of
 [RFC7230]).
 Example:
   HTTP/1.1 426 Upgrade Required
   Upgrade: HTTP/3.0
   Connection: Upgrade
   Content-Length: 53
   Content-Type: text/plain
   This service requires use of the HTTP/3.0 protocol.

6.6. Server Error 5xx

 The 5xx (Server Error) class of status code indicates that the server
 is aware that it has erred or is incapable of performing the
 requested method.  Except when responding to a HEAD request, the
 server SHOULD send a representation containing an explanation of the
 error situation, and whether it is a temporary or permanent

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 condition.  A user agent SHOULD display any included representation
 to the user.  These response codes are applicable to any request
 method.

6.6.1. 500 Internal Server Error

 The 500 (Internal Server Error) status code indicates that the server
 encountered an unexpected condition that prevented it from fulfilling
 the request.

6.6.2. 501 Not Implemented

 The 501 (Not Implemented) status code indicates that the server does
 not support the functionality required to fulfill the request.  This
 is the appropriate response when the server does not recognize the
 request method and is not capable of supporting it for any resource.
 A 501 response is cacheable by default; i.e., unless otherwise
 indicated by the method definition or explicit cache controls (see
 Section 4.2.2 of [RFC7234]).

6.6.3. 502 Bad Gateway

 The 502 (Bad Gateway) status code indicates that the server, while
 acting as a gateway or proxy, received an invalid response from an
 inbound server it accessed while attempting to fulfill the request.

6.6.4. 503 Service Unavailable

 The 503 (Service Unavailable) status code indicates that the server
 is currently unable to handle the request due to a temporary overload
 or scheduled maintenance, which will likely be alleviated after some
 delay.  The server MAY send a Retry-After header field
 (Section 7.1.3) to suggest an appropriate amount of time for the
 client to wait before retrying the request.
    Note: The existence of the 503 status code does not imply that a
    server has to use it when becoming overloaded.  Some servers might
    simply refuse the connection.

6.6.5. 504 Gateway Timeout

 The 504 (Gateway Timeout) status code indicates that the server,
 while acting as a gateway or proxy, did not receive a timely response
 from an upstream server it needed to access in order to complete the
 request.

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6.6.6. 505 HTTP Version Not Supported

 The 505 (HTTP Version Not Supported) status code indicates that the
 server does not support, or refuses to support, the major version of
 HTTP that was used in the request message.  The server is indicating
 that it is unable or unwilling to complete the request using the same
 major version as the client, as described in Section 2.6 of
 [RFC7230], other than with this error message.  The server SHOULD
 generate a representation for the 505 response that describes why
 that version is not supported and what other protocols are supported
 by that server.

7. Response Header Fields

 The response header fields allow the server to pass additional
 information about the response beyond what is placed in the
 status-line.  These header fields give information about the server,
 about further access to the target resource, or about related
 resources.
 Although each response header field has a defined meaning, in
 general, the precise semantics might be further refined by the
 semantics of the request method and/or response status code.

7.1. Control Data

 Response header fields can supply control data that supplements the
 status code, directs caching, or instructs the client where to go
 next.
 +-------------------+--------------------------+
 | Header Field Name | Defined in...            |
 +-------------------+--------------------------+
 | Age               | Section 5.1 of [RFC7234] |
 | Cache-Control     | Section 5.2 of [RFC7234] |
 | Expires           | Section 5.3 of [RFC7234] |
 | Date              | Section 7.1.1.2          |
 | Location          | Section 7.1.2            |
 | Retry-After       | Section 7.1.3            |
 | Vary              | Section 7.1.4            |
 | Warning           | Section 5.5 of [RFC7234] |
 +-------------------+--------------------------+

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7.1.1. Origination Date

7.1.1.1. Date/Time Formats

 Prior to 1995, there were three different formats commonly used by
 servers to communicate timestamps.  For compatibility with old
 implementations, all three are defined here.  The preferred format is
 a fixed-length and single-zone subset of the date and time
 specification used by the Internet Message Format [RFC5322].
   HTTP-date    = IMF-fixdate / obs-date
 An example of the preferred format is
   Sun, 06 Nov 1994 08:49:37 GMT    ; IMF-fixdate
 Examples of the two obsolete formats are
   Sunday, 06-Nov-94 08:49:37 GMT   ; obsolete RFC 850 format
   Sun Nov  6 08:49:37 1994         ; ANSI C's asctime() format
 A recipient that parses a timestamp value in an HTTP header field
 MUST accept all three HTTP-date formats.  When a sender generates a
 header field that contains one or more timestamps defined as
 HTTP-date, the sender MUST generate those timestamps in the
 IMF-fixdate format.
 An HTTP-date value represents time as an instance of Coordinated
 Universal Time (UTC).  The first two formats indicate UTC by the
 three-letter abbreviation for Greenwich Mean Time, "GMT", a
 predecessor of the UTC name; values in the asctime format are assumed
 to be in UTC.  A sender that generates HTTP-date values from a local
 clock ought to use NTP ([RFC5905]) or some similar protocol to
 synchronize its clock to UTC.
 Preferred format:
   IMF-fixdate  = day-name "," SP date1 SP time-of-day SP GMT
   ; fixed length/zone/capitalization subset of the format
   ; see Section 3.3 of [RFC5322]
   day-name     = %x4D.6F.6E ; "Mon", case-sensitive
                / %x54.75.65 ; "Tue", case-sensitive
                / %x57.65.64 ; "Wed", case-sensitive
                / %x54.68.75 ; "Thu", case-sensitive
                / %x46.72.69 ; "Fri", case-sensitive
                / %x53.61.74 ; "Sat", case-sensitive
                / %x53.75.6E ; "Sun", case-sensitive

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   date1        = day SP month SP year
                ; e.g., 02 Jun 1982
   day          = 2DIGIT
   month        = %x4A.61.6E ; "Jan", case-sensitive
                / %x46.65.62 ; "Feb", case-sensitive
                / %x4D.61.72 ; "Mar", case-sensitive
                / %x41.70.72 ; "Apr", case-sensitive
                / %x4D.61.79 ; "May", case-sensitive
                / %x4A.75.6E ; "Jun", case-sensitive
                / %x4A.75.6C ; "Jul", case-sensitive
                / %x41.75.67 ; "Aug", case-sensitive
                / %x53.65.70 ; "Sep", case-sensitive
                / %x4F.63.74 ; "Oct", case-sensitive
                / %x4E.6F.76 ; "Nov", case-sensitive
                / %x44.65.63 ; "Dec", case-sensitive
   year         = 4DIGIT
   GMT          = %x47.4D.54 ; "GMT", case-sensitive
   time-of-day  = hour ":" minute ":" second
                ; 00:00:00 - 23:59:60 (leap second)
   hour         = 2DIGIT
   minute       = 2DIGIT
   second       = 2DIGIT
 Obsolete formats:
   obs-date     = rfc850-date / asctime-date
   rfc850-date  = day-name-l "," SP date2 SP time-of-day SP GMT
   date2        = day "-" month "-" 2DIGIT
                ; e.g., 02-Jun-82
   day-name-l   = %x4D.6F.6E.64.61.79    ; "Monday", case-sensitive
          / %x54.75.65.73.64.61.79       ; "Tuesday", case-sensitive
          / %x57.65.64.6E.65.73.64.61.79 ; "Wednesday", case-sensitive
          / %x54.68.75.72.73.64.61.79    ; "Thursday", case-sensitive
          / %x46.72.69.64.61.79          ; "Friday", case-sensitive
          / %x53.61.74.75.72.64.61.79    ; "Saturday", case-sensitive
          / %x53.75.6E.64.61.79          ; "Sunday", case-sensitive
   asctime-date = day-name SP date3 SP time-of-day SP year
   date3        = month SP ( 2DIGIT / ( SP 1DIGIT ))
                ; e.g., Jun  2

Fielding & Reschke Standards Track [Page 66] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 HTTP-date is case sensitive.  A sender MUST NOT generate additional
 whitespace in an HTTP-date beyond that specifically included as SP in
 the grammar.  The semantics of day-name, day, month, year, and
 time-of-day are the same as those defined for the Internet Message
 Format constructs with the corresponding name ([RFC5322], Section
 3.3).
 Recipients of a timestamp value in rfc850-date format, which uses a
 two-digit year, MUST interpret a timestamp that appears to be more
 than 50 years in the future as representing the most recent year in
 the past that had the same last two digits.
 Recipients of timestamp values are encouraged to be robust in parsing
 timestamps unless otherwise restricted by the field definition.  For
 example, messages are occasionally forwarded over HTTP from a
 non-HTTP source that might generate any of the date and time
 specifications defined by the Internet Message Format.
    Note: HTTP requirements for the date/time stamp format apply only
    to their usage within the protocol stream.  Implementations are
    not required to use these formats for user presentation, request
    logging, etc.

7.1.1.2. Date

 The "Date" header field represents the date and time at which the
 message was originated, having the same semantics as the Origination
 Date Field (orig-date) defined in Section 3.6.1 of [RFC5322].  The
 field value is an HTTP-date, as defined in Section 7.1.1.1.
   Date = HTTP-date
 An example is
   Date: Tue, 15 Nov 1994 08:12:31 GMT
 When a Date header field is generated, the sender SHOULD generate its
 field value as the best available approximation of the date and time
 of message generation.  In theory, the date ought to represent the
 moment just before the payload is generated.  In practice, the date
 can be generated at any time during message origination.
 An origin server MUST NOT send a Date header field if it does not
 have a clock capable of providing a reasonable approximation of the
 current instance in Coordinated Universal Time.  An origin server MAY
 send a Date header field if the response is in the 1xx
 (Informational) or 5xx (Server Error) class of status codes.  An
 origin server MUST send a Date header field in all other cases.

Fielding & Reschke Standards Track [Page 67] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 A recipient with a clock that receives a response message without a
 Date header field MUST record the time it was received and append a
 corresponding Date header field to the message's header section if it
 is cached or forwarded downstream.
 A user agent MAY send a Date header field in a request, though
 generally will not do so unless it is believed to convey useful
 information to the server.  For example, custom applications of HTTP
 might convey a Date if the server is expected to adjust its
 interpretation of the user's request based on differences between the
 user agent and server clocks.

7.1.2. Location

 The "Location" header field is used in some responses to refer to a
 specific resource in relation to the response.  The type of
 relationship is defined by the combination of request method and
 status code semantics.
   Location = URI-reference
 The field value consists of a single URI-reference.  When it has the
 form of a relative reference ([RFC3986], Section 4.2), the final
 value is computed by resolving it against the effective request URI
 ([RFC3986], Section 5).
 For 201 (Created) responses, the Location value refers to the primary
 resource created by the request.  For 3xx (Redirection) responses,
 the Location value refers to the preferred target resource for
 automatically redirecting the request.
 If the Location value provided in a 3xx (Redirection) response does
 not have a fragment component, a user agent MUST process the
 redirection as if the value inherits the fragment component of the
 URI reference used to generate the request target (i.e., the
 redirection inherits the original reference's fragment, if any).
 For example, a GET request generated for the URI reference
 "http://www.example.org/~tim" might result in a 303 (See Other)
 response containing the header field:
   Location: /People.html#tim
 which suggests that the user agent redirect to
 "http://www.example.org/People.html#tim"

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 Likewise, a GET request generated for the URI reference
 "http://www.example.org/index.html#larry" might result in a 301
 (Moved Permanently) response containing the header field:
   Location: http://www.example.net/index.html
 which suggests that the user agent redirect to
 "http://www.example.net/index.html#larry", preserving the original
 fragment identifier.
 There are circumstances in which a fragment identifier in a Location
 value would not be appropriate.  For example, the Location header
 field in a 201 (Created) response is supposed to provide a URI that
 is specific to the created resource.
    Note: Some recipients attempt to recover from Location fields that
    are not valid URI references.  This specification does not mandate
    or define such processing, but does allow it for the sake of
    robustness.
    Note: The Content-Location header field (Section 3.1.4.2) differs
    from Location in that the Content-Location refers to the most
    specific resource corresponding to the enclosed representation.
    It is therefore possible for a response to contain both the
    Location and Content-Location header fields.

7.1.3. Retry-After

 Servers send the "Retry-After" header field to indicate how long the
 user agent ought to wait before making a follow-up request.  When
 sent with a 503 (Service Unavailable) response, Retry-After indicates
 how long the service is expected to be unavailable to the client.
 When sent with any 3xx (Redirection) response, Retry-After indicates
 the minimum time that the user agent is asked to wait before issuing
 the redirected request.
 The value of this field can be either an HTTP-date or a number of
 seconds to delay after the response is received.
   Retry-After = HTTP-date / delay-seconds
 A delay-seconds value is a non-negative decimal integer, representing
 time in seconds.
   delay-seconds  = 1*DIGIT

Fielding & Reschke Standards Track [Page 69] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 Two examples of its use are
   Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
   Retry-After: 120
 In the latter example, the delay is 2 minutes.

7.1.4. Vary

 The "Vary" header field in a response describes what parts of a
 request message, aside from the method, Host header field, and
 request target, might influence the origin server's process for
 selecting and representing this response.  The value consists of
 either a single asterisk ("*") or a list of header field names
 (case-insensitive).
   Vary = "*" / 1#field-name
 A Vary field value of "*" signals that anything about the request
 might play a role in selecting the response representation, possibly
 including elements outside the message syntax (e.g., the client's
 network address).  A recipient will not be able to determine whether
 this response is appropriate for a later request without forwarding
 the request to the origin server.  A proxy MUST NOT generate a Vary
 field with a "*" value.
 A Vary field value consisting of a comma-separated list of names
 indicates that the named request header fields, known as the
 selecting header fields, might have a role in selecting the
 representation.  The potential selecting header fields are not
 limited to those defined by this specification.
 For example, a response that contains
   Vary: accept-encoding, accept-language
 indicates that the origin server might have used the request's
 Accept-Encoding and Accept-Language fields (or lack thereof) as
 determining factors while choosing the content for this response.
 An origin server might send Vary with a list of fields for two
 purposes:
 1.  To inform cache recipients that they MUST NOT use this response
     to satisfy a later request unless the later request has the same
     values for the listed fields as the original request (Section 4.1
     of [RFC7234]).  In other words, Vary expands the cache key
     required to match a new request to the stored cache entry.

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 2.  To inform user agent recipients that this response is subject to
     content negotiation (Section 5.3) and that a different
     representation might be sent in a subsequent request if
     additional parameters are provided in the listed header fields
     (proactive negotiation).
 An origin server SHOULD send a Vary header field when its algorithm
 for selecting a representation varies based on aspects of the request
 message other than the method and request target, unless the variance
 cannot be crossed or the origin server has been deliberately
 configured to prevent cache transparency.  For example, there is no
 need to send the Authorization field name in Vary because reuse
 across users is constrained by the field definition (Section 4.2 of
 [RFC7235]).  Likewise, an origin server might use Cache-Control
 directives (Section 5.2 of [RFC7234]) to supplant Vary if it
 considers the variance less significant than the performance cost of
 Vary's impact on caching.

7.2. Validator Header Fields

 Validator header fields convey metadata about the selected
 representation (Section 3).  In responses to safe requests, validator
 fields describe the selected representation chosen by the origin
 server while handling the response.  Note that, depending on the
 status code semantics, the selected representation for a given
 response is not necessarily the same as the representation enclosed
 as response payload.
 In a successful response to a state-changing request, validator
 fields describe the new representation that has replaced the prior
 selected representation as a result of processing the request.
 For example, an ETag header field in a 201 (Created) response
 communicates the entity-tag of the newly created resource's
 representation, so that it can be used in later conditional requests
 to prevent the "lost update" problem [RFC7232].
 +-------------------+--------------------------+
 | Header Field Name | Defined in...            |
 +-------------------+--------------------------+
 | ETag              | Section 2.3 of [RFC7232] |
 | Last-Modified     | Section 2.2 of [RFC7232] |
 +-------------------+--------------------------+

Fielding & Reschke Standards Track [Page 71] RFC 7231 HTTP/1.1 Semantics and Content June 2014

7.3. Authentication Challenges

 Authentication challenges indicate what mechanisms are available for
 the client to provide authentication credentials in future requests.
 +--------------------+--------------------------+
 | Header Field Name  | Defined in...            |
 +--------------------+--------------------------+
 | WWW-Authenticate   | Section 4.1 of [RFC7235] |
 | Proxy-Authenticate | Section 4.3 of [RFC7235] |
 +--------------------+--------------------------+

7.4. Response Context

 The remaining response header fields provide more information about
 the target resource for potential use in later requests.
 +-------------------+--------------------------+
 | Header Field Name | Defined in...            |
 +-------------------+--------------------------+
 | Accept-Ranges     | Section 2.3 of [RFC7233] |
 | Allow             | Section 7.4.1            |
 | Server            | Section 7.4.2            |
 +-------------------+--------------------------+

7.4.1. Allow

 The "Allow" header field lists the set of methods advertised as
 supported by the target resource.  The purpose of this field is
 strictly to inform the recipient of valid request methods associated
 with the resource.
   Allow = #method
 Example of use:
   Allow: GET, HEAD, PUT
 The actual set of allowed methods is defined by the origin server at
 the time of each request.  An origin server MUST generate an Allow
 field in a 405 (Method Not Allowed) response and MAY do so in any
 other response.  An empty Allow field value indicates that the
 resource allows no methods, which might occur in a 405 response if
 the resource has been temporarily disabled by configuration.
 A proxy MUST NOT modify the Allow header field -- it does not need to
 understand all of the indicated methods in order to handle them
 according to the generic message handling rules.

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7.4.2. Server

 The "Server" header field contains information about the software
 used by the origin server to handle the request, which is often used
 by clients to help identify the scope of reported interoperability
 problems, to work around or tailor requests to avoid particular
 server limitations, and for analytics regarding server or operating
 system use.  An origin server MAY generate a Server field in its
 responses.
   Server = product *( RWS ( product / comment ) )
 The Server field-value consists of one or more product identifiers,
 each followed by zero or more comments (Section 3.2 of [RFC7230]),
 which together identify the origin server software and its
 significant subproducts.  By convention, the product identifiers are
 listed in decreasing order of their significance for identifying the
 origin server software.  Each product identifier consists of a name
 and optional version, as defined in Section 5.5.3.
 Example:
   Server: CERN/3.0 libwww/2.17
 An origin server SHOULD NOT generate a Server field containing
 needlessly fine-grained detail and SHOULD limit the addition of
 subproducts by third parties.  Overly long and detailed Server field
 values increase response latency and potentially reveal internal
 implementation details that might make it (slightly) easier for
 attackers to find and exploit known security holes.

8. IANA Considerations

8.1. Method Registry

 The "Hypertext Transfer Protocol (HTTP) Method Registry" defines the
 namespace for the request method token (Section 4).  The method
 registry has been created and is now maintained at
 <http://www.iana.org/assignments/http-methods>.

Fielding & Reschke Standards Track [Page 73] RFC 7231 HTTP/1.1 Semantics and Content June 2014

8.1.1. Procedure

 HTTP method registrations MUST include the following fields:
 o  Method Name (see Section 4)
 o  Safe ("yes" or "no", see Section 4.2.1)
 o  Idempotent ("yes" or "no", see Section 4.2.2)
 o  Pointer to specification text
 Values to be added to this namespace require IETF Review (see
 [RFC5226], Section 4.1).

8.1.2. Considerations for New Methods

 Standardized methods are generic; that is, they are potentially
 applicable to any resource, not just one particular media type, kind
 of resource, or application.  As such, it is preferred that new
 methods be registered in a document that isn't specific to a single
 application or data format, since orthogonal technologies deserve
 orthogonal specification.
 Since message parsing (Section 3.3 of [RFC7230]) needs to be
 independent of method semantics (aside from responses to HEAD),
 definitions of new methods cannot change the parsing algorithm or
 prohibit the presence of a message body on either the request or the
 response message.  Definitions of new methods can specify that only a
 zero-length message body is allowed by requiring a Content-Length
 header field with a value of "0".
 A new method definition needs to indicate whether it is safe
 (Section 4.2.1), idempotent (Section 4.2.2), cacheable
 (Section 4.2.3), what semantics are to be associated with the payload
 body if any is present in the request and what refinements the method
 makes to header field or status code semantics.  If the new method is
 cacheable, its definition ought to describe how, and under what
 conditions, a cache can store a response and use it to satisfy a
 subsequent request.  The new method ought to describe whether it can
 be made conditional (Section 5.2) and, if so, how a server responds
 when the condition is false.  Likewise, if the new method might have
 some use for partial response semantics ([RFC7233]), it ought to
 document this, too.
    Note: Avoid defining a method name that starts with "M-", since
    that prefix might be misinterpreted as having the semantics
    assigned to it by [RFC2774].

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8.1.3. Registrations

 The "Hypertext Transfer Protocol (HTTP) Method Registry" has been
 populated with the registrations below:
 +---------+------+------------+---------------+
 | Method  | Safe | Idempotent | Reference     |
 +---------+------+------------+---------------+
 | CONNECT | no   | no         | Section 4.3.6 |
 | DELETE  | no   | yes        | Section 4.3.5 |
 | GET     | yes  | yes        | Section 4.3.1 |
 | HEAD    | yes  | yes        | Section 4.3.2 |
 | OPTIONS | yes  | yes        | Section 4.3.7 |
 | POST    | no   | no         | Section 4.3.3 |
 | PUT     | no   | yes        | Section 4.3.4 |
 | TRACE   | yes  | yes        | Section 4.3.8 |
 +---------+------+------------+---------------+

8.2. Status Code Registry

 The "Hypertext Transfer Protocol (HTTP) Status Code Registry" defines
 the namespace for the response status-code token (Section 6).  The
 status code registry is maintained at
 <http://www.iana.org/assignments/http-status-codes>.
 This section replaces the registration procedure for HTTP Status
 Codes previously defined in Section 7.1 of [RFC2817].

8.2.1. Procedure

 A registration MUST include the following fields:
 o  Status Code (3 digits)
 o  Short Description
 o  Pointer to specification text
 Values to be added to the HTTP status code namespace require IETF
 Review (see [RFC5226], Section 4.1).

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8.2.2. Considerations for New Status Codes

 When it is necessary to express semantics for a response that are not
 defined by current status codes, a new status code can be registered.
 Status codes are generic; they are potentially applicable to any
 resource, not just one particular media type, kind of resource, or
 application of HTTP.  As such, it is preferred that new status codes
 be registered in a document that isn't specific to a single
 application.
 New status codes are required to fall under one of the categories
 defined in Section 6.  To allow existing parsers to process the
 response message, new status codes cannot disallow a payload,
 although they can mandate a zero-length payload body.
 Proposals for new status codes that are not yet widely deployed ought
 to avoid allocating a specific number for the code until there is
 clear consensus that it will be registered; instead, early drafts can
 use a notation such as "4NN", or "3N0" .. "3N9", to indicate the
 class of the proposed status code(s) without consuming a number
 prematurely.
 The definition of a new status code ought to explain the request
 conditions that would cause a response containing that status code
 (e.g., combinations of request header fields and/or method(s)) along
 with any dependencies on response header fields (e.g., what fields
 are required, what fields can modify the semantics, and what header
 field semantics are further refined when used with the new status
 code).
 The definition of a new status code ought to specify whether or not
 it is cacheable.  Note that all status codes can be cached if the
 response they occur in has explicit freshness information; however,
 status codes that are defined as being cacheable are allowed to be
 cached without explicit freshness information.  Likewise, the
 definition of a status code can place constraints upon cache
 behavior.  See [RFC7234] for more information.
 Finally, the definition of a new status code ought to indicate
 whether the payload has any implied association with an identified
 resource (Section 3.1.4.1).

8.2.3. Registrations

 The status code registry has been updated with the registrations
 below:

Fielding & Reschke Standards Track [Page 76] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 +-------+-------------------------------+----------------+
 | Value | Description                   | Reference      |
 +-------+-------------------------------+----------------+
 | 100   | Continue                      | Section 6.2.1  |
 | 101   | Switching Protocols           | Section 6.2.2  |
 | 200   | OK                            | Section 6.3.1  |
 | 201   | Created                       | Section 6.3.2  |
 | 202   | Accepted                      | Section 6.3.3  |
 | 203   | Non-Authoritative Information | Section 6.3.4  |
 | 204   | No Content                    | Section 6.3.5  |
 | 205   | Reset Content                 | Section 6.3.6  |
 | 300   | Multiple Choices              | Section 6.4.1  |
 | 301   | Moved Permanently             | Section 6.4.2  |
 | 302   | Found                         | Section 6.4.3  |
 | 303   | See Other                     | Section 6.4.4  |
 | 305   | Use Proxy                     | Section 6.4.5  |
 | 306   | (Unused)                      | Section 6.4.6  |
 | 307   | Temporary Redirect            | Section 6.4.7  |
 | 400   | Bad Request                   | Section 6.5.1  |
 | 402   | Payment Required              | Section 6.5.2  |
 | 403   | Forbidden                     | Section 6.5.3  |
 | 404   | Not Found                     | Section 6.5.4  |
 | 405   | Method Not Allowed            | Section 6.5.5  |
 | 406   | Not Acceptable                | Section 6.5.6  |
 | 408   | Request Timeout               | Section 6.5.7  |
 | 409   | Conflict                      | Section 6.5.8  |
 | 410   | Gone                          | Section 6.5.9  |
 | 411   | Length Required               | Section 6.5.10 |
 | 413   | Payload Too Large             | Section 6.5.11 |
 | 414   | URI Too Long                  | Section 6.5.12 |
 | 415   | Unsupported Media Type        | Section 6.5.13 |
 | 417   | Expectation Failed            | Section 6.5.14 |
 | 426   | Upgrade Required              | Section 6.5.15 |
 | 500   | Internal Server Error         | Section 6.6.1  |
 | 501   | Not Implemented               | Section 6.6.2  |
 | 502   | Bad Gateway                   | Section 6.6.3  |
 | 503   | Service Unavailable           | Section 6.6.4  |
 | 504   | Gateway Timeout               | Section 6.6.5  |
 | 505   | HTTP Version Not Supported    | Section 6.6.6  |
 +-------+-------------------------------+----------------+

8.3. Header Field Registry

 HTTP header fields are registered within the "Message Headers"
 registry located at
 <http://www.iana.org/assignments/message-headers>, as defined by
 [BCP90].

Fielding & Reschke Standards Track [Page 77] RFC 7231 HTTP/1.1 Semantics and Content June 2014

8.3.1. Considerations for New Header Fields

 Header fields are key:value pairs that can be used to communicate
 data about the message, its payload, the target resource, or the
 connection (i.e., control data).  See Section 3.2 of [RFC7230] for a
 general definition of header field syntax in HTTP messages.
 The requirements for header field names are defined in [BCP90].
 Authors of specifications defining new fields are advised to keep the
 name as short as practical and not to prefix the name with "X-"
 unless the header field will never be used on the Internet.  (The
 "X-" prefix idiom has been extensively misused in practice; it was
 intended to only be used as a mechanism for avoiding name collisions
 inside proprietary software or intranet processing, since the prefix
 would ensure that private names never collide with a newly registered
 Internet name; see [BCP178] for further information).
 New header field values typically have their syntax defined using
 ABNF ([RFC5234]), using the extension defined in Section 7 of
 [RFC7230] as necessary, and are usually constrained to the range of
 US-ASCII characters.  Header fields needing a greater range of
 characters can use an encoding such as the one defined in [RFC5987].
 Leading and trailing whitespace in raw field values is removed upon
 field parsing (Section 3.2.4 of [RFC7230]).  Field definitions where
 leading or trailing whitespace in values is significant will have to
 use a container syntax such as quoted-string (Section 3.2.6 of
 [RFC7230]).
 Because commas (",") are used as a generic delimiter between
 field-values, they need to be treated with care if they are allowed
 in the field-value.  Typically, components that might contain a comma
 are protected with double-quotes using the quoted-string ABNF
 production.
 For example, a textual date and a URI (either of which might contain
 a comma) could be safely carried in field-values like these:
   Example-URI-Field: "http://example.com/a.html,foo",
                      "http://without-a-comma.example.com/"
   Example-Date-Field: "Sat, 04 May 1996", "Wed, 14 Sep 2005"
 Note that double-quote delimiters almost always are used with the
 quoted-string production; using a different syntax inside
 double-quotes will likely cause unnecessary confusion.

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 Many header fields use a format including (case-insensitively) named
 parameters (for instance, Content-Type, defined in Section 3.1.1.5).
 Allowing both unquoted (token) and quoted (quoted-string) syntax for
 the parameter value enables recipients to use existing parser
 components.  When allowing both forms, the meaning of a parameter
 value ought to be independent of the syntax used for it (for an
 example, see the notes on parameter handling for media types in
 Section 3.1.1.1).
 Authors of specifications defining new header fields are advised to
 consider documenting:
 o  Whether the field is a single value or whether it can be a list
    (delimited by commas; see Section 3.2 of [RFC7230]).
    If it does not use the list syntax, document how to treat messages
    where the field occurs multiple times (a sensible default would be
    to ignore the field, but this might not always be the right
    choice).
    Note that intermediaries and software libraries might combine
    multiple header field instances into a single one, despite the
    field's definition not allowing the list syntax.  A robust format
    enables recipients to discover these situations (good example:
    "Content-Type", as the comma can only appear inside quoted
    strings; bad example: "Location", as a comma can occur inside a
    URI).
 o  Under what conditions the header field can be used; e.g., only in
    responses or requests, in all messages, only on responses to a
    particular request method, etc.
 o  Whether the field should be stored by origin servers that
    understand it upon a PUT request.
 o  Whether the field semantics are further refined by the context,
    such as by existing request methods or status codes.
 o  Whether it is appropriate to list the field-name in the Connection
    header field (i.e., if the header field is to be hop-by-hop; see
    Section 6.1 of [RFC7230]).
 o  Under what conditions intermediaries are allowed to insert,
    delete, or modify the field's value.

Fielding & Reschke Standards Track [Page 79] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 o  Whether it is appropriate to list the field-name in a Vary
    response header field (e.g., when the request header field is used
    by an origin server's content selection algorithm; see
    Section 7.1.4).
 o  Whether the header field is useful or allowable in trailers (see
    Section 4.1 of [RFC7230]).
 o  Whether the header field ought to be preserved across redirects.
 o  Whether it introduces any additional security considerations, such
    as disclosure of privacy-related data.

8.3.2. Registrations

 The "Message Headers" registry has been updated with the following
 permanent registrations:
 +-------------------+----------+----------+-----------------+
 | Header Field Name | Protocol | Status   | Reference       |
 +-------------------+----------+----------+-----------------+
 | Accept            | http     | standard | Section 5.3.2   |
 | Accept-Charset    | http     | standard | Section 5.3.3   |
 | Accept-Encoding   | http     | standard | Section 5.3.4   |
 | Accept-Language   | http     | standard | Section 5.3.5   |
 | Allow             | http     | standard | Section 7.4.1   |
 | Content-Encoding  | http     | standard | Section 3.1.2.2 |
 | Content-Language  | http     | standard | Section 3.1.3.2 |
 | Content-Location  | http     | standard | Section 3.1.4.2 |
 | Content-Type      | http     | standard | Section 3.1.1.5 |
 | Date              | http     | standard | Section 7.1.1.2 |
 | Expect            | http     | standard | Section 5.1.1   |
 | From              | http     | standard | Section 5.5.1   |
 | Location          | http     | standard | Section 7.1.2   |
 | Max-Forwards      | http     | standard | Section 5.1.2   |
 | MIME-Version      | http     | standard | Appendix A.1    |
 | Referer           | http     | standard | Section 5.5.2   |
 | Retry-After       | http     | standard | Section 7.1.3   |
 | Server            | http     | standard | Section 7.4.2   |
 | User-Agent        | http     | standard | Section 5.5.3   |
 | Vary              | http     | standard | Section 7.1.4   |
 +-------------------+----------+----------+-----------------+
 The change controller for the above registrations is: "IETF
 (iesg@ietf.org) - Internet Engineering Task Force".

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8.4. Content Coding Registry

 The "HTTP Content Coding Registry" defines the namespace for content
 coding names (Section 4.2 of [RFC7230]).  The content coding registry
 is maintained at <http://www.iana.org/assignments/http-parameters>.

8.4.1. Procedure

 Content coding registrations MUST include the following fields:
 o  Name
 o  Description
 o  Pointer to specification text
 Names of content codings MUST NOT overlap with names of transfer
 codings (Section 4 of [RFC7230]), unless the encoding transformation
 is identical (as is the case for the compression codings defined in
 Section 4.2 of [RFC7230]).
 Values to be added to this namespace require IETF Review (see Section
 4.1 of [RFC5226]) and MUST conform to the purpose of content coding
 defined in this section.

8.4.2. Registrations

 The "HTTP Content Coding Registry" has been updated with the
 registrations below:
 +----------+----------------------------------------+---------------+
 | Name     | Description                            | Reference     |
 +----------+----------------------------------------+---------------+
 | identity | Reserved (synonym for "no encoding" in | Section 5.3.4 |
 |          | Accept-Encoding)                       |               |
 +----------+----------------------------------------+---------------+

9. Security Considerations

 This section is meant to inform developers, information providers,
 and users of known security concerns relevant to HTTP semantics and
 its use for transferring information over the Internet.
 Considerations related to message syntax, parsing, and routing are
 discussed in Section 9 of [RFC7230].
 The list of considerations below is not exhaustive.  Most security
 concerns related to HTTP semantics are about securing server-side
 applications (code behind the HTTP interface), securing user agent

Fielding & Reschke Standards Track [Page 81] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 processing of payloads received via HTTP, or secure use of the
 Internet in general, rather than security of the protocol.  Various
 organizations maintain topical information and links to current
 research on Web application security (e.g., [OWASP]).

9.1. Attacks Based on File and Path Names

 Origin servers frequently make use of their local file system to
 manage the mapping from effective request URI to resource
 representations.  Most file systems are not designed to protect
 against malicious file or path names.  Therefore, an origin server
 needs to avoid accessing names that have a special significance to
 the system when mapping the request target to files, folders, or
 directories.
 For example, UNIX, Microsoft Windows, and other operating systems use
 ".." as a path component to indicate a directory level above the
 current one, and they use specially named paths or file names to send
 data to system devices.  Similar naming conventions might exist
 within other types of storage systems.  Likewise, local storage
 systems have an annoying tendency to prefer user-friendliness over
 security when handling invalid or unexpected characters,
 recomposition of decomposed characters, and case-normalization of
 case-insensitive names.
 Attacks based on such special names tend to focus on either denial-
 of-service (e.g., telling the server to read from a COM port) or
 disclosure of configuration and source files that are not meant to be
 served.

9.2. Attacks Based on Command, Code, or Query Injection

 Origin servers often use parameters within the URI as a means of
 identifying system services, selecting database entries, or choosing
 a data source.  However, data received in a request cannot be
 trusted.  An attacker could construct any of the request data
 elements (method, request-target, header fields, or body) to contain
 data that might be misinterpreted as a command, code, or query when
 passed through a command invocation, language interpreter, or
 database interface.
 For example, SQL injection is a common attack wherein additional
 query language is inserted within some part of the request-target or
 header fields (e.g., Host, Referer, etc.).  If the received data is
 used directly within a SELECT statement, the query language might be
 interpreted as a database command instead of a simple string value.
 This type of implementation vulnerability is extremely common, in
 spite of being easy to prevent.

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 In general, resource implementations ought to avoid use of request
 data in contexts that are processed or interpreted as instructions.
 Parameters ought to be compared to fixed strings and acted upon as a
 result of that comparison, rather than passed through an interface
 that is not prepared for untrusted data.  Received data that isn't
 based on fixed parameters ought to be carefully filtered or encoded
 to avoid being misinterpreted.
 Similar considerations apply to request data when it is stored and
 later processed, such as within log files, monitoring tools, or when
 included within a data format that allows embedded scripts.

9.3. Disclosure of Personal Information

 Clients are often privy to large amounts of personal information,
 including both information provided by the user to interact with
 resources (e.g., the user's name, location, mail address, passwords,
 encryption keys, etc.) and information about the user's browsing
 activity over time (e.g., history, bookmarks, etc.).  Implementations
 need to prevent unintentional disclosure of personal information.

9.4. Disclosure of Sensitive Information in URIs

 URIs are intended to be shared, not secured, even when they identify
 secure resources.  URIs are often shown on displays, added to
 templates when a page is printed, and stored in a variety of
 unprotected bookmark lists.  It is therefore unwise to include
 information within a URI that is sensitive, personally identifiable,
 or a risk to disclose.
 Authors of services ought to avoid GET-based forms for the submission
 of sensitive data because that data will be placed in the
 request-target.  Many existing servers, proxies, and user agents log
 or display the request-target in places where it might be visible to
 third parties.  Such services ought to use POST-based form submission
 instead.
 Since the Referer header field tells a target site about the context
 that resulted in a request, it has the potential to reveal
 information about the user's immediate browsing history and any
 personal information that might be found in the referring resource's
 URI.  Limitations on the Referer header field are described in
 Section 5.5.2 to address some of its security considerations.

Fielding & Reschke Standards Track [Page 83] RFC 7231 HTTP/1.1 Semantics and Content June 2014

9.5. Disclosure of Fragment after Redirects

 Although fragment identifiers used within URI references are not sent
 in requests, implementers ought to be aware that they will be visible
 to the user agent and any extensions or scripts running as a result
 of the response.  In particular, when a redirect occurs and the
 original request's fragment identifier is inherited by the new
 reference in Location (Section 7.1.2), this might have the effect of
 disclosing one site's fragment to another site.  If the first site
 uses personal information in fragments, it ought to ensure that
 redirects to other sites include a (possibly empty) fragment
 component in order to block that inheritance.

9.6. Disclosure of Product Information

 The User-Agent (Section 5.5.3), Via (Section 5.7.1 of [RFC7230]), and
 Server (Section 7.4.2) header fields often reveal information about
 the respective sender's software systems.  In theory, this can make
 it easier for an attacker to exploit known security holes; in
 practice, attackers tend to try all potential holes regardless of the
 apparent software versions being used.
 Proxies that serve as a portal through a network firewall ought to
 take special precautions regarding the transfer of header information
 that might identify hosts behind the firewall.  The Via header field
 allows intermediaries to replace sensitive machine names with
 pseudonyms.

9.7. Browser Fingerprinting

 Browser fingerprinting is a set of techniques for identifying a
 specific user agent over time through its unique set of
 characteristics.  These characteristics might include information
 related to its TCP behavior, feature capabilities, and scripting
 environment, though of particular interest here is the set of unique
 characteristics that might be communicated via HTTP.  Fingerprinting
 is considered a privacy concern because it enables tracking of a user
 agent's behavior over time without the corresponding controls that
 the user might have over other forms of data collection (e.g.,
 cookies).  Many general-purpose user agents (i.e., Web browsers) have
 taken steps to reduce their fingerprints.
 There are a number of request header fields that might reveal
 information to servers that is sufficiently unique to enable
 fingerprinting.  The From header field is the most obvious, though it
 is expected that From will only be sent when self-identification is
 desired by the user.  Likewise, Cookie header fields are deliberately

Fielding & Reschke Standards Track [Page 84] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 designed to enable re-identification, so fingerprinting concerns only
 apply to situations where cookies are disabled or restricted by the
 user agent's configuration.
 The User-Agent header field might contain enough information to
 uniquely identify a specific device, usually when combined with other
 characteristics, particularly if the user agent sends excessive
 details about the user's system or extensions.  However, the source
 of unique information that is least expected by users is proactive
 negotiation (Section 5.3), including the Accept, Accept-Charset,
 Accept-Encoding, and Accept-Language header fields.
 In addition to the fingerprinting concern, detailed use of the
 Accept-Language header field can reveal information the user might
 consider to be of a private nature.  For example, understanding a
 given language set might be strongly correlated to membership in a
 particular ethnic group.  An approach that limits such loss of
 privacy would be for a user agent to omit the sending of
 Accept-Language except for sites that have been whitelisted, perhaps
 via interaction after detecting a Vary header field that indicates
 language negotiation might be useful.
 In environments where proxies are used to enhance privacy, user
 agents ought to be conservative in sending proactive negotiation
 header fields.  General-purpose user agents that provide a high
 degree of header field configurability ought to inform users about
 the loss of privacy that might result if too much detail is provided.
 As an extreme privacy measure, proxies could filter the proactive
 negotiation header fields in relayed requests.

10. Acknowledgments

 See Section 10 of [RFC7230].

11. References

11.1. Normative References

 [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part One: Format of Internet Message
            Bodies", RFC 2045, November 1996.
 [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part Two: Media Types", RFC 2046,
            November 1996.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.

Fielding & Reschke Standards Track [Page 85] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66,
            RFC 3986, January 2005.
 [RFC4647]  Phillips, A., Ed. and M. Davis, Ed., "Matching of Language
            Tags", BCP 47, RFC 4647, September 2006.
 [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
            Languages", BCP 47, RFC 5646, September 2009.
 [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
            Internationalization in the IETF", BCP 166, RFC 6365,
            September 2011.
 [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
            Protocol (HTTP/1.1): Message Syntax and Routing",
            RFC 7230, June 2014.
 [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
            Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
            June 2014.
 [RFC7233]  Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
            "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
            RFC 7233, June 2014.
 [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
            Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
            RFC 7234, June 2014.
 [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
            Protocol (HTTP/1.1): Authentication", RFC 7235, June 2014.

11.2. Informative References

 [BCP13]    Freed, N., Klensin, J., and T. Hansen, "Media Type
            Specifications and Registration Procedures", BCP 13,
            RFC 6838, January 2013.
 [BCP178]   Saint-Andre, P., Crocker, D., and M. Nottingham,
            "Deprecating the "X-" Prefix and Similar Constructs in
            Application Protocols", BCP 178, RFC 6648, June 2012.

Fielding & Reschke Standards Track [Page 86] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 [BCP90]    Klyne, G., Nottingham, M., and J. Mogul, "Registration
            Procedures for Message Header Fields", BCP 90, RFC 3864,
            September 2004.
 [OWASP]    van der Stock, A., Ed., "A Guide to Building Secure Web
            Applications and Web Services", The Open Web Application
            Security Project (OWASP) 2.0.1, July 2005,
            <https://www.owasp.org/>.
 [REST]     Fielding, R., "Architectural Styles and the Design of
            Network-based Software Architectures",
            Doctoral Dissertation, University of California, Irvine,
            September 2000,
            <http://roy.gbiv.com/pubs/dissertation/top.htm>.
 [RFC1945]  Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
            Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
 [RFC2049]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part Five: Conformance Criteria and
            Examples", RFC 2049, November 1996.
 [RFC2068]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
            Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
            RFC 2068, January 1997.
 [RFC2295]  Holtman, K. and A. Mutz, "Transparent Content Negotiation
            in HTTP", RFC 2295, March 1998.
 [RFC2388]  Masinter, L., "Returning Values from Forms:  multipart/
            form-data", RFC 2388, August 1998.
 [RFC2557]  Palme, F., Hopmann, A., Shelness, N., and E. Stefferud,
            "MIME Encapsulation of Aggregate Documents, such as HTML
            (MHTML)", RFC 2557, March 1999.
 [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
            Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
            Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
 [RFC2774]  Frystyk, H., Leach, P., and S. Lawrence, "An HTTP
            Extension Framework", RFC 2774, February 2000.
 [RFC2817]  Khare, R. and S. Lawrence, "Upgrading to TLS Within
            HTTP/1.1", RFC 2817, May 2000.
 [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
            Procedures", BCP 19, RFC 2978, October 2000.

Fielding & Reschke Standards Track [Page 87] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246, August 2008.
 [RFC5322]  Resnick, P., "Internet Message Format", RFC 5322,
            October 2008.
 [RFC5789]  Dusseault, L. and J. Snell, "PATCH Method for HTTP",
            RFC 5789, March 2010.
 [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
            "Network Time Protocol Version 4: Protocol and Algorithms
            Specification", RFC 5905, June 2010.
 [RFC5987]  Reschke, J., "Character Set and Language Encoding for
            Hypertext Transfer Protocol (HTTP) Header Field
            Parameters", RFC 5987, August 2010.
 [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.
 [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
            April 2011.
 [RFC6266]  Reschke, J., "Use of the Content-Disposition Header Field
            in the Hypertext Transfer Protocol (HTTP)", RFC 6266,
            June 2011.
 [RFC7238]  Reschke, J., "The Hypertext Transfer Protocol (HTTP)
            Status Code 308 (Permanent Redirect)", RFC 7238,
            June 2014.

Fielding & Reschke Standards Track [Page 88] RFC 7231 HTTP/1.1 Semantics and Content June 2014

Appendix A. Differences between HTTP and MIME

 HTTP/1.1 uses many of the constructs defined for the Internet Message
 Format [RFC5322] and the Multipurpose Internet Mail Extensions (MIME)
 [RFC2045] to allow a message body to be transmitted in an open
 variety of representations and with extensible header fields.
 However, RFC 2045 is focused only on email; applications of HTTP have
 many characteristics that differ from email; hence, HTTP has features
 that differ from MIME.  These differences were carefully chosen to
 optimize performance over binary connections, to allow greater
 freedom in the use of new media types, to make date comparisons
 easier, and to acknowledge the practice of some early HTTP servers
 and clients.
 This appendix describes specific areas where HTTP differs from MIME.
 Proxies and gateways to and from strict MIME environments need to be
 aware of these differences and provide the appropriate conversions
 where necessary.

A.1. MIME-Version

 HTTP is not a MIME-compliant protocol.  However, messages can include
 a single MIME-Version header field to indicate what version of the
 MIME protocol was used to construct the message.  Use of the
 MIME-Version header field indicates that the message is in full
 conformance with the MIME protocol (as defined in [RFC2045]).
 Senders are responsible for ensuring full conformance (where
 possible) when exporting HTTP messages to strict MIME environments.

A.2. Conversion to Canonical Form

 MIME requires that an Internet mail body part be converted to
 canonical form prior to being transferred, as described in Section 4
 of [RFC2049].  Section 3.1.1.3 of this document describes the forms
 allowed for subtypes of the "text" media type when transmitted over
 HTTP.  [RFC2046] requires that content with a type of "text"
 represent line breaks as CRLF and forbids the use of CR or LF outside
 of line break sequences.  HTTP allows CRLF, bare CR, and bare LF to
 indicate a line break within text content.
 A proxy or gateway from HTTP to a strict MIME environment ought to
 translate all line breaks within the text media types described in
 Section 3.1.1.3 of this document to the RFC 2049 canonical form of
 CRLF.  Note, however, this might be complicated by the presence of a
 Content-Encoding and by the fact that HTTP allows the use of some
 charsets that do not use octets 13 and 10 to represent CR and LF,
 respectively.

Fielding & Reschke Standards Track [Page 89] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 Conversion will break any cryptographic checksums applied to the
 original content unless the original content is already in canonical
 form.  Therefore, the canonical form is recommended for any content
 that uses such checksums in HTTP.

A.3. Conversion of Date Formats

 HTTP/1.1 uses a restricted set of date formats (Section 7.1.1.1) to
 simplify the process of date comparison.  Proxies and gateways from
 other protocols ought to ensure that any Date header field present in
 a message conforms to one of the HTTP/1.1 formats and rewrite the
 date if necessary.

A.4. Conversion of Content-Encoding

 MIME does not include any concept equivalent to HTTP/1.1's
 Content-Encoding header field.  Since this acts as a modifier on the
 media type, proxies and gateways from HTTP to MIME-compliant
 protocols ought to either change the value of the Content-Type header
 field or decode the representation before forwarding the message.
 (Some experimental applications of Content-Type for Internet mail
 have used a media-type parameter of ";conversions=<content-coding>"
 to perform a function equivalent to Content-Encoding.  However, this
 parameter is not part of the MIME standards).

A.5. Conversion of Content-Transfer-Encoding

 HTTP does not use the Content-Transfer-Encoding field of MIME.
 Proxies and gateways from MIME-compliant protocols to HTTP need to
 remove any Content-Transfer-Encoding prior to delivering the response
 message to an HTTP client.
 Proxies and gateways from HTTP to MIME-compliant protocols are
 responsible for ensuring that the message is in the correct format
 and encoding for safe transport on that protocol, where "safe
 transport" is defined by the limitations of the protocol being used.
 Such a proxy or gateway ought to transform and label the data with an
 appropriate Content-Transfer-Encoding if doing so will improve the
 likelihood of safe transport over the destination protocol.

A.6. MHTML and Line Length Limitations

 HTTP implementations that share code with MHTML [RFC2557]
 implementations need to be aware of MIME line length limitations.
 Since HTTP does not have this limitation, HTTP does not fold long
 lines.  MHTML messages being transported by HTTP follow all
 conventions of MHTML, including line length limitations and folding,
 canonicalization, etc., since HTTP transfers message-bodies as

Fielding & Reschke Standards Track [Page 90] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 payload and, aside from the "multipart/byteranges" type (Appendix A
 of [RFC7233]), does not interpret the content or any MIME header
 lines that might be contained therein.

Appendix B. Changes from RFC 2616

 The primary changes in this revision have been editorial in nature:
 extracting the messaging syntax and partitioning HTTP semantics into
 separate documents for the core features, conditional requests,
 partial requests, caching, and authentication.  The conformance
 language has been revised to clearly target requirements and the
 terminology has been improved to distinguish payload from
 representations and representations from resources.
 A new requirement has been added that semantics embedded in a URI be
 disabled when those semantics are inconsistent with the request
 method, since this is a common cause of interoperability failure.
 (Section 2)
 An algorithm has been added for determining if a payload is
 associated with a specific identifier.  (Section 3.1.4.1)
 The default charset of ISO-8859-1 for text media types has been
 removed; the default is now whatever the media type definition says.
 Likewise, special treatment of ISO-8859-1 has been removed from the
 Accept-Charset header field.  (Section 3.1.1.3 and Section 5.3.3)
 The definition of Content-Location has been changed to no longer
 affect the base URI for resolving relative URI references, due to
 poor implementation support and the undesirable effect of potentially
 breaking relative links in content-negotiated resources.
 (Section 3.1.4.2)
 To be consistent with the method-neutral parsing algorithm of
 [RFC7230], the definition of GET has been relaxed so that requests
 can have a body, even though a body has no meaning for GET.
 (Section 4.3.1)
 Servers are no longer required to handle all Content-* header fields
 and use of Content-Range has been explicitly banned in PUT requests.
 (Section 4.3.4)
 Definition of the CONNECT method has been moved from [RFC2817] to
 this specification.  (Section 4.3.6)
 The OPTIONS and TRACE request methods have been defined as being
 safe.  (Section 4.3.7 and Section 4.3.8)

Fielding & Reschke Standards Track [Page 91] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 The Expect header field's extension mechanism has been removed due to
 widely-deployed broken implementations.  (Section 5.1.1)
 The Max-Forwards header field has been restricted to the OPTIONS and
 TRACE methods; previously, extension methods could have used it as
 well.  (Section 5.1.2)
 The "about:blank" URI has been suggested as a value for the Referer
 header field when no referring URI is applicable, which distinguishes
 that case from others where the Referer field is not sent or has been
 removed.  (Section 5.5.2)
 The following status codes are now cacheable (that is, they can be
 stored and reused by a cache without explicit freshness information
 present): 204, 404, 405, 414, 501.  (Section 6)
 The 201 (Created) status description has been changed to allow for
 the possibility that more than one resource has been created.
 (Section 6.3.2)
 The definition of 203 (Non-Authoritative Information) has been
 broadened to include cases of payload transformations as well.
 (Section 6.3.4)
 The set of request methods that are safe to automatically redirect is
 no longer closed; user agents are able to make that determination
 based upon the request method semantics.  The redirect status codes
 301, 302, and 307 no longer have normative requirements on response
 payloads and user interaction.  (Section 6.4)
 The status codes 301 and 302 have been changed to allow user agents
 to rewrite the method from POST to GET.  (Sections 6.4.2 and 6.4.3)
 The description of the 303 (See Other) status code has been changed
 to allow it to be cached if explicit freshness information is given,
 and a specific definition has been added for a 303 response to GET.
 (Section 6.4.4)
 The 305 (Use Proxy) status code has been deprecated due to security
 concerns regarding in-band configuration of a proxy.  (Section 6.4.5)
 The 400 (Bad Request) status code has been relaxed so that it isn't
 limited to syntax errors.  (Section 6.5.1)
 The 426 (Upgrade Required) status code has been incorporated from
 [RFC2817].  (Section 6.5.15)

Fielding & Reschke Standards Track [Page 92] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 The target of requirements on HTTP-date and the Date header field
 have been reduced to those systems generating the date, rather than
 all systems sending a date.  (Section 7.1.1)
 The syntax of the Location header field has been changed to allow all
 URI references, including relative references and fragments, along
 with some clarifications as to when use of fragments would not be
 appropriate.  (Section 7.1.2)
 Allow has been reclassified as a response header field, removing the
 option to specify it in a PUT request.  Requirements relating to the
 content of Allow have been relaxed; correspondingly, clients are not
 required to always trust its value.  (Section 7.4.1)
 A Method Registry has been defined.  (Section 8.1)
 The Status Code Registry has been redefined by this specification;
 previously, it was defined in Section 7.1 of [RFC2817].
 (Section 8.2)
 Registration of content codings has been changed to require IETF
 Review.  (Section 8.4)
 The Content-Disposition header field has been removed since it is now
 defined by [RFC6266].
 The Content-MD5 header field has been removed because it was
 inconsistently implemented with respect to partial responses.

Appendix C. Imported ABNF

 The following core rules are included by reference, as defined in
 Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
 CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
 quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF
 (line feed), OCTET (any 8-bit sequence of data), SP (space), and
 VCHAR (any visible US-ASCII character).
 The rules below are defined in [RFC7230]:
   BWS           = <BWS, see [RFC7230], Section 3.2.3>
   OWS           = <OWS, see [RFC7230], Section 3.2.3>
   RWS           = <RWS, see [RFC7230], Section 3.2.3>
   URI-reference = <URI-reference, see [RFC7230], Section 2.7>
   absolute-URI  = <absolute-URI, see [RFC7230], Section 2.7>
   comment       = <comment, see [RFC7230], Section 3.2.6>
   field-name    = <comment, see [RFC7230], Section 3.2>
   partial-URI   = <partial-URI, see [RFC7230], Section 2.7>

Fielding & Reschke Standards Track [Page 93] RFC 7231 HTTP/1.1 Semantics and Content June 2014

   quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>
   token         = <token, see [RFC7230], Section 3.2.6>

Appendix D. Collected ABNF

 In the collected ABNF below, list rules are expanded as per Section
 1.2 of [RFC7230].
 Accept = [ ( "," / ( media-range [ accept-params ] ) ) *( OWS "," [
  OWS ( media-range [ accept-params ] ) ] ) ]
 Accept-Charset = *( "," OWS ) ( ( charset / "*" ) [ weight ] ) *( OWS
  "," [ OWS ( ( charset / "*" ) [ weight ] ) ] )
 Accept-Encoding = [ ( "," / ( codings [ weight ] ) ) *( OWS "," [ OWS
  ( codings [ weight ] ) ] ) ]
 Accept-Language = *( "," OWS ) ( language-range [ weight ] ) *( OWS
  "," [ OWS ( language-range [ weight ] ) ] )
 Allow = [ ( "," / method ) *( OWS "," [ OWS method ] ) ]
 BWS = <BWS, see [RFC7230], Section 3.2.3>
 Content-Encoding = *( "," OWS ) content-coding *( OWS "," [ OWS
  content-coding ] )
 Content-Language = *( "," OWS ) language-tag *( OWS "," [ OWS
  language-tag ] )
 Content-Location = absolute-URI / partial-URI
 Content-Type = media-type
 Date = HTTP-date
 Expect = "100-continue"
 From = mailbox
 GMT = %x47.4D.54 ; GMT
 HTTP-date = IMF-fixdate / obs-date
 IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT
 Location = URI-reference
 Max-Forwards = 1*DIGIT
 OWS = <OWS, see [RFC7230], Section 3.2.3>
 RWS = <RWS, see [RFC7230], Section 3.2.3>
 Referer = absolute-URI / partial-URI
 Retry-After = HTTP-date / delay-seconds

Fielding & Reschke Standards Track [Page 94] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 Server = product *( RWS ( product / comment ) )
 URI-reference = <URI-reference, see [RFC7230], Section 2.7>
 User-Agent = product *( RWS ( product / comment ) )
 Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
  ) )
 absolute-URI = <absolute-URI, see [RFC7230], Section 2.7>
 accept-ext = OWS ";" OWS token [ "=" ( token / quoted-string ) ]
 accept-params = weight *accept-ext
 asctime-date = day-name SP date3 SP time-of-day SP year
 charset = token
 codings = content-coding / "identity" / "*"
 comment = <comment, see [RFC7230], Section 3.2.6>
 content-coding = token
 date1 = day SP month SP year
 date2 = day "-" month "-" 2DIGIT
 date3 = month SP ( 2DIGIT / ( SP DIGIT ) )
 day = 2DIGIT
 day-name = %x4D.6F.6E ; Mon
  / %x54.75.65 ; Tue
  / %x57.65.64 ; Wed
  / %x54.68.75 ; Thu
  / %x46.72.69 ; Fri
  / %x53.61.74 ; Sat
  / %x53.75.6E ; Sun
 day-name-l = %x4D.6F.6E.64.61.79 ; Monday
  / %x54.75.65.73.64.61.79 ; Tuesday
  / %x57.65.64.6E.65.73.64.61.79 ; Wednesday
  / %x54.68.75.72.73.64.61.79 ; Thursday
  / %x46.72.69.64.61.79 ; Friday
  / %x53.61.74.75.72.64.61.79 ; Saturday
  / %x53.75.6E.64.61.79 ; Sunday
 delay-seconds = 1*DIGIT
 field-name = <comment, see [RFC7230], Section 3.2>
 hour = 2DIGIT
 language-range = <language-range, see [RFC4647], Section 2.1>
 language-tag = <Language-Tag, see [RFC5646], Section 2.1>
 mailbox = <mailbox, see [RFC5322], Section 3.4>
 media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS
  ";" OWS parameter )

Fielding & Reschke Standards Track [Page 95] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 media-type = type "/" subtype *( OWS ";" OWS parameter )
 method = token
 minute = 2DIGIT
 month = %x4A.61.6E ; Jan
  / %x46.65.62 ; Feb
  / %x4D.61.72 ; Mar
  / %x41.70.72 ; Apr
  / %x4D.61.79 ; May
  / %x4A.75.6E ; Jun
  / %x4A.75.6C ; Jul
  / %x41.75.67 ; Aug
  / %x53.65.70 ; Sep
  / %x4F.63.74 ; Oct
  / %x4E.6F.76 ; Nov
  / %x44.65.63 ; Dec
 obs-date = rfc850-date / asctime-date
 parameter = token "=" ( token / quoted-string )
 partial-URI = <partial-URI, see [RFC7230], Section 2.7>
 product = token [ "/" product-version ]
 product-version = token
 quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>
 qvalue = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
 rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT
 second = 2DIGIT
 subtype = token
 time-of-day = hour ":" minute ":" second
 token = <token, see [RFC7230], Section 3.2.6>
 type = token
 weight = OWS ";" OWS "q=" qvalue
 year = 4DIGIT

Fielding & Reschke Standards Track [Page 96] RFC 7231 HTTP/1.1 Semantics and Content June 2014

Index

 1
    1xx Informational (status code class)  50
 2
    2xx Successful (status code class)  51
 3
    3xx Redirection (status code class)  54
 4
    4xx Client Error (status code class)  58
 5
    5xx Server Error (status code class)  62
 1
    100 Continue (status code)  50
    100-continue (expect value)  34
    101 Switching Protocols (status code)  50
 2
    200 OK (status code)  51
    201 Created (status code)  52
    202 Accepted (status code)  52
    203 Non-Authoritative Information (status code)  52
    204 No Content (status code)  53
    205 Reset Content (status code)  53
 3
    300 Multiple Choices (status code)  55
    301 Moved Permanently (status code)  56
    302 Found (status code)  56
    303 See Other (status code)  57
    305 Use Proxy (status code)  58
    306 (Unused) (status code)  58
    307 Temporary Redirect (status code)  58
 4
    400 Bad Request (status code)  58
    402 Payment Required (status code)  59
    403 Forbidden (status code)  59
    404 Not Found (status code)  59
    405 Method Not Allowed (status code)  59
    406 Not Acceptable (status code)  59
    408 Request Timeout (status code)  60
    409 Conflict (status code)  60

Fielding & Reschke Standards Track [Page 97] RFC 7231 HTTP/1.1 Semantics and Content June 2014

    410 Gone (status code)  60
    411 Length Required (status code)  61
    413 Payload Too Large (status code)  61
    414 URI Too Long (status code)  61
    415 Unsupported Media Type (status code)  62
    417 Expectation Failed (status code)  62
    426 Upgrade Required (status code)  62
 5
    500 Internal Server Error (status code)  63
    501 Not Implemented (status code)  63
    502 Bad Gateway (status code)  63
    503 Service Unavailable (status code)  63
    504 Gateway Timeout (status code)  63
    505 HTTP Version Not Supported (status code)  64
 A
    Accept header field  38
    Accept-Charset header field  40
    Accept-Encoding header field  41
    Accept-Language header field  42
    Allow header field  72
 C
    cacheable  24
    compress (content coding)  11
    conditional request  36
    CONNECT method  30
    content coding  11
    content negotiation  6
    Content-Encoding header field  12
    Content-Language header field  13
    Content-Location header field  15
    Content-Transfer-Encoding header field  89
    Content-Type header field  10
 D
    Date header field  67
    deflate (content coding)  11
    DELETE method  29
 E
    Expect header field  34
 F
    From header field  44

Fielding & Reschke Standards Track [Page 98] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 G
    GET method  24
    Grammar
       Accept  38
       Accept-Charset  40
       Accept-Encoding  41
       accept-ext  38
       Accept-Language  42
       accept-params  38
       Allow  72
       asctime-date  66
       charset  9
       codings  41
       content-coding  11
       Content-Encoding  12
       Content-Language  13
       Content-Location  15
       Content-Type  10
       Date  67
       date1  65
       day  65
       day-name  65
       day-name-l  65
       delay-seconds  69
       Expect  34
       From  44
       GMT  65
       hour  65
       HTTP-date  65
       IMF-fixdate  65
       language-range  42
       language-tag  13
       Location  68
       Max-Forwards  36
       media-range  38
       media-type  8
       method  21
       minute  65
       month  65
       obs-date  66
       parameter  8
       product  46
       product-version  46
       qvalue  38
       Referer  45
       Retry-After  69
       rfc850-date  66
       second  65

Fielding & Reschke Standards Track [Page 99] RFC 7231 HTTP/1.1 Semantics and Content June 2014

       Server  73
       subtype  8
       time-of-day  65
       type  8
       User-Agent  46
       Vary  70
       weight  38
       year  65
    gzip (content coding)  11
 H
    HEAD method  25
 I
    idempotent  23
 L
    Location header field  68
 M
    Max-Forwards header field  36
    MIME-Version header field  89
 O
    OPTIONS method  31
 P
    payload  17
    POST method  25
    PUT method  26
 R
    Referer header field  45
    representation  7
    Retry-After header field  69
 S
    safe  22
    selected representation  7, 71
    Server header field  73
    Status Codes Classes
       1xx Informational  50
       2xx Successful  51
       3xx Redirection  54
       4xx Client Error  58
       5xx Server Error  62

Fielding & Reschke Standards Track [Page 100] RFC 7231 HTTP/1.1 Semantics and Content June 2014

 T
    TRACE method  32
 U
    User-Agent header field  46
 V
    Vary header field  70
 X
    x-compress (content coding)  11
    x-gzip (content coding)  11

Authors' Addresses

 Roy T. Fielding (editor)
 Adobe Systems Incorporated
 345 Park Ave
 San Jose, CA  95110
 USA
 EMail: fielding@gbiv.com
 URI:   http://roy.gbiv.com/
 Julian F. Reschke (editor)
 greenbytes GmbH
 Hafenweg 16
 Muenster, NW  48155
 Germany
 EMail: julian.reschke@greenbytes.de
 URI:   http://greenbytes.de/tech/webdav/

Fielding & Reschke Standards Track [Page 101]

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