wdiff rfc7231v1.txt rfc7231.txt

Internet Engineering Task Force (IETF) R. Fielding, Ed.
Request for Comments: 7231 Adobe
Obsoletes: 2616 J. Reschke, Ed.
Updates: 2817 greenbytes
Category: Standards Track May 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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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 ....................................................6
1.1. Conformance and Error Handling . . . . . . . . . . . . . 5 .............................6
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 6 ............................................6
2. Resources . . . . . . . . . . . . . . . . . . . . . . . . . . 6 .......................................................7
3. Representations . . . . . . . . . . . . . . . . . . . . . . . 6 .................................................7
3.1. Representation Metadata . . . . . . . . . . . . . . . . . 7 ....................................8
3.1.1. Processing Representation Data . . . . . . . . . . . 7 ......................8
3.1.2. Encoding for Compression or Integrity . . . . . . . . 10 ..............11
3.1.3. Audience Language . . . . . . . . . . . . . . . . . . 12 ..................................13
3.1.4. Identification . . . . . . . . . . . . . . . . . . . 13 .....................................14
3.2. Representation Data . . . . . . . . . . . . . . . . . . . 16 .......................................17
3.3. Payload Semantics . . . . . . . . . . . . . . . . . . . . 16 .........................................17
3.4. Content Negotiation . . . . . . . . . . . . . . . . . . . 17 .......................................18
3.4.1. Proactive Negotiation . . . . . . . . . . . . . . . . 18 ..............................19
3.4.2. Reactive Negotiation . . . . . . . . . . . . . . . . 19 ...............................20
4. Request Methods . . . . . . . . . . . . . . . . . . . . . . . 20 ................................................21
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 20 ..................................................21
4.2. Common Method Properties . . . . . . . . . . . . . . . . 21 ..................................22
4.2.1. Safe Methods . . . . . . . . . . . . . . . . . . . . 21 .......................................22
4.2.2. Idempotent Methods . . . . . . . . . . . . . . . . . 22 .................................23
4.2.3. Cacheable Methods . . . . . . . . . . . . . . . . . . 23 ..................................24
4.3. Method Definitions . . . . . . . . . . . . . . . . . . . 23 ........................................24
4.3.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 23 ................................................24
4.3.2. HEAD . . . . . . . . . . . . . . . . . . . . . . . . 24 ...............................................25
4.3.3. POST . . . . . . . . . . . . . . . . . . . . . . . . 24 ...............................................25
4.3.4. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 25 ................................................26
4.3.5. DELETE . . . . . . . . . . . . . . . . . . . . . . . 28 .............................................29
4.3.6. CONNECT . . . . . . . . . . . . . . . . . . . . . . . 29 ............................................30
4.3.7. OPTIONS . . . . . . . . . . . . . . . . . . . . . . . 30 ............................................31
4.3.8. TRACE . . . . . . . . . . . . . . . . . . . . . . . . 31 ..............................................32
5. Request Header Fields . . . . . . . . . . . . . . . . . . . . 32 ..........................................33
5.1. Controls . . . . . . . . . . . . . . . . . . . . . . . . 32 ..................................................33
5.1.1. Expect . . . . . . . . . . . . . . . . . . . . . . . 33 .............................................34
5.1.2. Max-Forwards . . . . . . . . . . . . . . . . . . . . 35 .......................................36
5.2. Conditionals . . . . . . . . . . . . . . . . . . . . . . 36 ..............................................36
5.3. Content Negotiation . . . . . . . . . . . . . . . . . . . 36 .......................................37
5.3.1. Quality Values . . . . . . . . . . . . . . . . . . . 37 .....................................37
5.3.2. Accept . . . . . . . . . . . . . . . . . . . . . . . 37 .............................................38
5.3.3. Accept-Charset . . . . . . . . . . . . . . . . . . . 39 .....................................40
5.3.4. Accept-Encoding . . . . . . . . . . . . . . . . . . . 40 ....................................41
5.3.5. Accept-Language . . . . . . . . . . . . . . . . . . . 42 ....................................42
5.4. Authentication Credentials . . . . . . . . . . . . . . . 43 ................................44
5.5. Request Context . . . . . . . . . . . . . . . . . . . . . 43 ...........................................44
5.5.1. From . . . . . . . . . . . . . . . . . . . . . . . . 43 ...............................................44
5.5.2. Referer . . . . . . . . . . . . . . . . . . . . . . . 44 ............................................45
5.5.3. User-Agent . . . . . . . . . . . . . . . . . . . . . 45 .........................................46
6. Response Status Codes . . . . . . . . . . . . . . . . . . . . 46 ..........................................47
6.1. Overview of Status Codes . . . . . . . . . . . . . . . . 47 ..................................48
6.2. Informational 1xx . . . . . . . . . . . . . . . . . . . . 49 .........................................50
6.2.1. 100 Continue . . . . . . . . . . . . . . . . . . . . 49 .......................................50
6.2.2. 101 Switching Protocols . . . . . . . . . . . . . . . 49 ............................50
6.3. Successful 2xx . . . . . . . . . . . . . . . . . . . . . 50 ............................................51
6.3.1. 200 OK . . . . . . . . . . . . . . . . . . . . . . . 50 .............................................51
6.3.2. 201 Created . . . . . . . . . . . . . . . . . . . . . 51 ........................................52
6.3.3. 202 Accepted . . . . . . . . . . . . . . . . . . . . 51 .......................................52
6.3.4. 203 Non-Authoritative Information . . . . . . . . . . 51 ..................52
6.3.5. 204 No Content . . . . . . . . . . . . . . . . . . . 52 .....................................53
6.3.6. 205 Reset Content . . . . . . . . . . . . . . . . . . 52 ..................................53
6.4. Redirection 3xx . . . . . . . . . . . . . . . . . . . . . 53 ...........................................54
6.4.1. 300 Multiple Choices . . . . . . . . . . . . . . . . 54 ...............................55
6.4.2. 301 Moved Permanently . . . . . . . . . . . . . . . . 55 ..............................56
6.4.3. 302 Found . . . . . . . . . . . . . . . . . . . . . . 55 ..........................................56
6.4.4. 303 See Other . . . . . . . . . . . . . . . . . . . . 56 ......................................57
6.4.5. 305 Use Proxy . . . . . . . . . . . . . . . . . . . . 56 ......................................58
6.4.6. 306 (Unused) . . . . . . . . . . . . . . . . . . . . 57 .......................................58
6.4.7. 307 Temporary Redirect . . . . . . . . . . . . . . . 57 .............................58
6.5. Client Error 4xx . . . . . . . . . . . . . . . . . . . . 57 ..........................................58
6.5.1. 400 Bad Request . . . . . . . . . . . . . . . . . . . 57 ....................................58
6.5.2. 402 Payment Required . . . . . . . . . . . . . . . . 57 ...............................59
6.5.3. 403 Forbidden . . . . . . . . . . . . . . . . . . . . 58 ......................................59
6.5.4. 404 Not Found . . . . . . . . . . . . . . . . . . . . 58 ......................................59
6.5.5. 405 Method Not Allowed . . . . . . . . . . . . . . . 58 .............................59
6.5.6. 406 Not Acceptable . . . . . . . . . . . . . . . . . 59 .................................60
6.5.7. 408 Request Timeout . . . . . . . . . . . . . . . . . 59 ................................60
6.5.8. 409 Conflict . . . . . . . . . . . . . . . . . . . . 59 .......................................60
6.5.9. 410 Gone . . . . . . . . . . . . . . . . . . . . . . 59 ...........................................60
6.5.10. 411 Length Required . . . . . . . . . . . . . . . . . 60 ...............................61
6.5.11. 413 Payload Too Large . . . . . . . . . . . . . . . . 60 .............................61
6.5.12. 414 URI Too Long . . . . . . . . . . . . . . . . . . 60 ..................................61
6.5.13. 415 Unsupported Media Type . . . . . . . . . . . . . 61 ........................62
6.5.14. 417 Expectation Failed . . . . . . . . . . . . . . . 61 ............................62
6.5.15. 426 Upgrade Required . . . . . . . . . . . . . . . . 61 ..............................62
6.6. Server Error 5xx . . . . . . . . . . . . . . . . . . . . 61 ..........................................62
6.6.1. 500 Internal Server Error . . . . . . . . . . . . . . 62 ..........................63
6.6.2. 501 Not Implemented . . . . . . . . . . . . . . . . . 62 ................................63
6.6.3. 502 Bad Gateway . . . . . . . . . . . . . . . . . . . 62 ....................................63
6.6.4. 503 Service Unavailable . . . . . . . . . . . . . . . 62 ............................63
6.6.5. 504 Gateway Timeout . . . . . . . . . . . . . . . . . 62 ................................63
6.6.6. 505 HTTP Version Not Supported . . . . . . . . . . . 63 .....................64
7. Response Header Fields . . . . . . . . . . . . . . . . . . . 63 .........................................64
7.1. Control Data . . . . . . . . . . . . . . . . . . . . . . 63 ..............................................64
7.1.1. Origination Date . . . . . . . . . . . . . . . . . . 63 ...................................65
7.1.2. Location . . . . . . . . . . . . . . . . . . . . . . 67 ...........................................68
7.1.3. Retry-After . . . . . . . . . . . . . . . . . . . . . 68 ........................................69
7.1.4. Vary . . . . . . . . . . . . . . . . . . . . . . . . 69 ...............................................70
7.2. Validator Header Fields . . . . . . . . . . . . . . . . . 70 ...................................71
7.3. Authentication Challenges . . . . . . . . . . . . . . . . 71 .................................72
7.4. Response Context . . . . . . . . . . . . . . . . . . . . 71 ..........................................72
7.4.1. Allow . . . . . . . . . . . . . . . . . . . . . . . . 71 ..............................................72
7.4.2. Server . . . . . . . . . . . . . . . . . . . . . . . 72 .............................................73
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 73 ............................................73
8.1. Method Registry . . . . . . . . . . . . . . . . . . . . . 73 ...........................................73
8.1.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 73 ..........................................74
8.1.2. Considerations for New Methods . . . . . . . . . . . 73 .....................74
8.1.3. Registrations . . . . . . . . . . . . . . . . . . . . 74 ......................................75
8.2. Status Code Registry . . . . . . . . . . . . . . . . . . 74 ......................................75
8.2.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 74 ..........................................75
8.2.2. Considerations for New Status Codes . . . . . . . . . 75 ................76
8.2.3. Registrations . . . . . . . . . . . . . . . . . . . . 76 ......................................76
8.3. Header Field Registry . . . . . . . . . . . . . . . . . . 77 .....................................77
8.3.1. Considerations for New Header Fields . . . . . . . . 77 ...............78
8.3.2. Registrations . . . . . . . . . . . . . . . . . . . . 79 ......................................80
8.4. Content Coding Registry . . . . . . . . . . . . . . . . . 80 ...................................81
8.4.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 80 ..........................................81
8.4.2. Registrations . . . . . . . . . . . . . . . . . . . . 80 ......................................81
9. Security Considerations . . . . . . . . . . . . . . . . . . . 80 ........................................81
9.1. Attacks Based on File and Path Names . . . . . . . . . . 81 ......................82
9.2. Attacks Based on Command, Code, or Query Injection . . . 81 ........82
9.3. Disclosure of Personal Information . . . . . . . . . . . 82 ........................83
9.4. Disclosure of Sensitive Information in URIs . . . . . . . 82 ...............83
9.5. Disclosure of Fragment after Redirects . . . . . . . . . 83 ....................84
9.6. Disclosure of Product Information . . . . . . . . . . . . 83 .........................84
9.7. Browser Fingerprinting . . . . . . . . . . . . . . . . . 83 ....................................84
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 84 ...............................................85
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 84 ....................................................85
11.1. Normative References . . . . . . . . . . . . . . . . . . 84 .....................................85
11.2. Informative References . . . . . . . . . . . . . . . . . 85 ...................................86
Appendix A. Differences between HTTP and MIME . . . . . . . . . 88 .....................89
A.1. MIME-Version . . . . . . . . . . . . . . . . . . . . . . 88 ..............................................89
A.2. Conversion to Canonical Form . . . . . . . . . . . . . . 88 ..............................89
A.3. Conversion of Date Formats . . . . . . . . . . . . . . . 89 ................................90
A.4. Conversion of Content-Encoding . . . . . . . . . . . . . 89 ..........................90
A.5. Conversion of Content-Transfer-Encoding . . . . . . . . . 89 .................90
A.6. MHTML and Line-Length Line Length Limitations . . . . . . . . . . . . 89 .........................90
Appendix B. Changes from RFC 2616 . . . . . . . . . . . . . . . 90 .................................91
Appendix C. Imported ABNF . . . . . . . . . . . . . . . . . . . 92 .........................................93
Appendix D. Collected ABNF . . . . . . . . . . . . . . . . . . . 93 ........................................94
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 .............................................................97

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
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.

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
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
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:

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 )
The type, subtype, and parameter name tokens are case insensitive. case-insensitive.
Parameter values might or might not be case sensitive, 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> (<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).

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 carriage return (CR) CR or line feed (LF) 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
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 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.

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
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
Content-Language header fields. Accept-Language uses the broader language-
range
language-range production defined in Section 5.3.5, whereas
Content-Language uses the language-tag production defined below.

language-tag = <Language-Tag, defined in 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
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 being used. 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.

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.

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
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.

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.

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.

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
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 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,
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.

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 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, 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 ASCII
US-ASCII letters.

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 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.

This definition of safe method 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.

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
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;
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
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
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.

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
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
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
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.

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).

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.

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. 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
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.

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
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
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
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].

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.

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.

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
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
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,
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
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
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.

2. If the representation has no content-coding, then it is
acceptable by default unless specifically excluded by the Accept-
Encoding
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
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, defined in 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
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
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.

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].

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.

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, machine-usable, as defined by "mailbox" in Section 3.4 of
[RFC5322]:

From = mailbox

mailbox = <mailbox, defined in 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.

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.

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 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 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
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
product-version that is not a version identifier (i.e., successive
versions of the same product name ought only to differ only in the
product-version portion of the product identifier).

Example:

User-Agent: CERN-LineMode/2.15 libwww/2.17b3
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 implemented. 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
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.

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. All 1xx responses consist of only the status-line and
optional header fields and, thus, are terminated by the first empty line at after
the status-line (the empty line signaling the end of the header section.
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
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]).

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,
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.

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,
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
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.

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 response 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.

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).

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.

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.

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]; ([RFC7238], however, it 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).

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]).

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 "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
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
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.

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
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
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.

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.
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.

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,
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
; defined in see Section 3.3 of [RFC5322]

day-name = %x4D.6F.6E ; "Mon", case sensitive case-sensitive
/ %x54.75.65 ; "Tue", case sensitive case-sensitive
/ %x57.65.64 ; "Wed", case sensitive case-sensitive
/ %x54.68.75 ; "Thu", case sensitive case-sensitive
/ %x46.72.69 ; "Fri", case sensitive case-sensitive
/ %x53.61.74 ; "Sat", case sensitive case-sensitive
/ %x53.75.6E ; "Sun", case sensitive case-sensitive
date1 = day SP month SP year
; e.g., 02 Jun 1982

day = 2DIGIT
month = %x4A.61.6E ; "Jan", case sensitive case-sensitive
/ %x46.65.62 ; "Feb", case sensitive case-sensitive
/ %x4D.61.72 ; "Mar", case sensitive case-sensitive
/ %x41.70.72 ; "Apr", case sensitive case-sensitive
/ %x4D.61.79 ; "May", case sensitive case-sensitive
/ %x4A.75.6E ; "Jun", case sensitive case-sensitive
/ %x4A.75.6C ; "Jul", case sensitive case-sensitive
/ %x41.75.67 ; "Aug", case sensitive case-sensitive
/ %x53.65.70 ; "Sep", case sensitive case-sensitive
/ %x4F.63.74 ; "Oct", case sensitive case-sensitive
/ %x4E.6F.76 ; "Nov", case sensitive case-sensitive
/ %x44.65.63 ; "Dec", case sensitive case-sensitive
year = 4DIGIT

GMT = %x47.4D.54 ; "GMT", case sensitive 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 case-sensitive
/ %x54.75.65.73.64.61.79 ; "Tuesday", case sensitive case-sensitive
/ %x57.65.64.6E.65.73.64.61.79 ; "Wednesday", case sensitive case-sensitive
/ %x54.68.75.72.73.64.61.79 ; "Thursday", case sensitive case-sensitive
/ %x46.72.69.64.61.79 ; "Friday", case sensitive case-sensitive
/ %x53.61.74.75.72.64.61.79 ; "Saturday", case sensitive case-sensitive
/ %x53.75.6E.64.61.79 ; "Sunday", case sensitive case-sensitive

asctime-date = day-name SP date3 SP time-of-day SP year
date3 = month SP ( 2DIGIT / ( SP 1DIGIT ))
; e.g., Jun 2
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
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
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.

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"
"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"
"http://www.example.org/People.html#tim"
Likewise, a GET request generated for the URI reference "http://
www.example.org/index.html#larry"
"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",
"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
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).
(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.

2. To inform user-agent 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].

7.3. Authentication Challenges

Authentication challenges indicate what mechanisms are available for
the client to provide authentication credentials in future requests.

7.4. Response Context

The remaining response header fields provide more information about
the target resource for potential use in later requests.

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.

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 "HTTP Method
Registry" method
registry has been created and is now maintained at <http://
www.iana.org/assignments/http-methods>.
<http://www.iana.org/assignments/http-methods>.

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), or cacheable
(Section 4.2.3). It needs to indicate 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].

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
"HTTP Status Codes"
status code registry is maintained at <http://www.iana.org/
assignments/http-status-codes>.
<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).

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 "HTTP Status Codes" status code registry has been updated with the registrations
below:

+-------+-------------------------------+-----------------+

8.3. Header Field Registry

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

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]) (implementing ([RFC5234]), using the extension defined in Section 7 of
[RFC7230] as necessary), necessary, and they are usually constrained to the range of ASCII
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,
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
double-quotes will likely cause unnecessary confusion.

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.

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:

+-------------------+----------+----------+------------------+

The change controller for the above registrations is: "IETF
(iesg@ietf.org) - Internet Engineering Task Force".

8.4. Content Coding Registry

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

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) or interface), securing user-agent user agent
processing of payloads received via HTTP. Secure HTTP, or secure use of the
Internet in general, rather than security of the protocol, might also be
related. 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. Implementers need to be aware that most Most file systems are not designed to protect
against malicious file or path
names and, thus, depend on the names. Therefore, an origin server
needs to avoid mapping to file
names, folders, or directories accessing names that have a special significance to
the
system. 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 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.

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.
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.

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
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
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.

[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, May June 2014.

[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232, May
June 2014.

[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
RFC 7233, May June 2014.

[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, May June 2014.

[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235, May 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.

[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/>.
<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",
HTTP/1.1", RFC 2817, May 2000.

[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000.

[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, May
June 2014.

Appendix A. Differences between HTTP and MIME

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

This appendix describes specific areas where HTTP differs from MIME.
Proxies and gateways to and from strict MIME environments need to be
aware of these differences and provide the appropriate conversions
where necessary.

A.1. MIME-Version

HTTP is not a MIME-compliant protocol. However, messages can include
a single MIME-Version header field to indicate what version of the
MIME protocol was used to construct the message. Use of the MIME-
Version
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.

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
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 Line Length Limitations

HTTP implementations that share code with MHTML [RFC2557]
implementations need to be aware of MIME line-length 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 line length limitations and folding,
canonicalization, etc., since HTTP transfers message-bodies as
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 failure.
(Section 2). 2)

An algorithm has been added for determining if a payload is
associated with a specific identifier identifier. (Section 3.1.4.1). 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. (Sections (Section 3.1.1.3 and 5.3.3.) 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 resources.
(Section 3.1.4.2). 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 GET.
(Section 4.3.1). 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 requests.
(Section 4.3.4). 4.3.4)

Definition of the CONNECT method has been moved from [RFC2817] to
this specification specification. (Section 4.3.6). 4.3.6)

The OPTIONS and TRACE request methods have been defined as being safe
safe. (Section 4.3.7 and Section 4.3.8). 4.3.8)
The Expect header field's extension mechanism has been removed due to
widely deployed
widely-deployed broken implementations implementations. (Section 5.1.1). 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
well. (Section 5.1.2). 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
removed. (Section 5.5.2). 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, 501 501. (Section 6). 6)

The 201 (Created) status description has been changed to allow for
the possibility that more than one resource has been created created.
(Section 6.3.2). 6.3.2)

The definition of 203 (Non-Authoritative Information) has been
broadened to include cases of payload transformations as well well.
(Section 6.3.4). 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 interaction. (Section 6.4). 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.) 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 GET.
(Section 6.4.4). 6.4.4)

The 305 (Use Proxy) status code has been deprecated due to security
concerns regarding in-band configuration of a proxy proxy. (Section 6.4.5). 6.4.5)

The 400 (Bad Request) status code has been relaxed so that it isn't
limited to syntax errors errors. (Section 6.5.1). 6.5.1)

The 426 (Upgrade Required) status code has been incorporated from
[RFC2817]
[RFC2817]. (Section 6.5.15). 6.5.15)
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 date. (Section 7.1.1). 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
appropriate. (Section 7.1.2). 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 value. (Section 7.4.1). 7.4.1)

A Method Registry has been defined defined. (Section 8.1). 8.1)

The Status Code Registry has been redefined by this specification;
previously, it was defined in Section 7.1 of [RFC2817] [RFC2817].
(Section 8.2). 8.2)

Registration of content codings has been changed to require IETF
Review
Review. (Section 8.4). 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, defined in see [RFC7230], Section 3.2.3>
OWS = <OWS, defined in see [RFC7230], Section 3.2.3>
RWS = <RWS, defined in see [RFC7230], Section 3.2.3>
URI-reference = <URI-reference, defined in see [RFC7230], Section 2.7>
absolute-URI = <absolute-URI, defined in see [RFC7230], Section 2.7>
comment = <comment, defined in see [RFC7230], Section 3.2.6>
field-name = <comment, defined in see [RFC7230], Section 3.2>
partial-URI = <partial-URI, defined in see [RFC7230], Section 2.7>
quoted-string = <quoted-string, defined in see [RFC7230], Section 3.2.6>
token = <token, defined in 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, defined in 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, defined in see [RFC7230], Section 3.2.3>

RWS = <RWS, defined in see [RFC7230], Section 3.2.3>
Referer = absolute-URI / partial-URI
Retry-After = HTTP-date / delay-seconds
Server = product *( RWS ( product / comment ) )

URI-reference = <URI-reference, defined in see [RFC7230], Section 2.7>
User-Agent = product *( RWS ( product / comment ) )

Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
) )

absolute-URI = <absolute-URI, defined in 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, defined in 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, defined in see [RFC7230], Section 3.2>

hour = 2DIGIT

language-range = <language-range, defined in see [RFC4647], Section 2.1>
language-tag = <Language-Tag, defined in see [RFC5646], Section 2.1>

mailbox = <mailbox, defined in see [RFC5322], Section 3.4>
media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS
";" OWS parameter )
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, defined in see [RFC7230], Section 2.7>
product = token [ "/" product-version ]
product-version = token
quoted-string = <quoted-string, defined in 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, defined in see [RFC7230], Section 3.2.6>
type = token

weight = OWS ";" OWS "q=" qvalue

year = 4DIGIT

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
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 5 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
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
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
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/