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RFC 7231

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

Part 1 of 5, p. 1 to 21
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Obsoletes:    2616
Updates:    2817


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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.

Page 2 
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.

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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

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   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
           7.1.1. Origination Date ...................................65
           7.1.2. Location ...........................................68
           7.1.3. Retry-After ........................................69

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           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

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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.

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   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

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   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 )

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   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).

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   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

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   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.

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   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

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   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

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   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.

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   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.

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   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

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   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,

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   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.



(page 21 continued on part 2)

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