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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.

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    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.

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    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

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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
    ed            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.

    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.

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       +---------+-------------------------------------------------+-------+
       | 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

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       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;

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       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

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       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.

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       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

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       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

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       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

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       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] |
       +-------------------+--------------------------+

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    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.

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       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

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       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

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       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.

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       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

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          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,

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         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

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       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.

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       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

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       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,

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       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

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          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.

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       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.

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    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

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       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.

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       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

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       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] |
       +-------------------+--------------------------+

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    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>.

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    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:

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       +-------+-------------------------------+----------------+
       | 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].

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    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.

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       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

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       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.

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    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

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       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]

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       [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.

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       [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.

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       [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.

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    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.

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       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

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       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)

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       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)

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       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>

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         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

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       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 )

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       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

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    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

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          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

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       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

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             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

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       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/

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