超文本传输​​协议（HTTP/1.1）：语义和内容

Each Hypertext Transfer Protocol (HTTP) message is either a request    or a response.  A server listens on a connection for a request,    parses each message received, interprets the message semantics in    relation to the identified request target, and responds to that    request with one or more response messages.  A client constructs    request messages to communicate specific intentions, examines    received responses to see if the intentions were carried out, and    determines how to interpret the results.  This document defines    HTTP/1.1 request and response semantics in terms of the architecture    defined in [[RFC7230](https://tools.ietf.org/html/rfc7230)].     HTTP provides a uniform interface for interacting with a resource    ([Section 2](about:blank#section-2)), regardless of its type, nature, or implementation, via    the manipulation and transfer of representations ([Section 3](about:blank#section-3)).     HTTP semantics include the intentions defined by each request method    ([Section 4](about:blank#section-4)), extensions to those semantics that might be described in    request header fields ([Section 5](about:blank#section-5)), the meaning of status codes to    indicate a machine-readable response ([Section 6](about:blank#section-6)), and the meaning of    other control data and resource metadata that might be given in    response header fields ([Section 7](about:blank#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](about:blank#section-3.4)).  

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](https://tools.ietf.org/html/rfc2119)].     Conformance criteria and considerations regarding error handling are    defined in [Section 2.5 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-2.5).  

This specification uses the Augmented Backus-Naur Form (ABNF)    notation of [[RFC5234](https://tools.ietf.org/html/rfc5234)] with a list extension, defined in [Section 7 of    [RFC7230]](https://tools.ietf.org/html/rfc7230#section-7), that allows for compact definition of comma-separated    lists using a '#' operator (similar to how the '\*' operator indicates    repetition).  [Appendix C](about:blank#appendix-C) describes rules imported from other    documents.  [Appendix D](about:blank#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](https://tools.ietf.org/html/rfc6365)].  

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]](https://tools.ietf.org/html/rfc7230#section-2.7).     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]](https://tools.ietf.org/html/rfc7230#section-5.3)).  When a request is received, the server reconstructs an    effective request URI for the target resource ([Section 5.5 of    [RFC7230]](https://tools.ietf.org/html/rfc7230#section-5.5)).     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](about:blank#section-4)) and a few    request-modifying header fields ([Section 5](about:blank#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](about:blank#section-4.2.1), the method semantics take    precedence.  

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](about:blank#ref-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](https://tools.ietf.org/html/rfc7232)] and    constructing the payload for 200 (OK) and 304 (Not Modified)    responses to GET ([Section 4.3.1](about:blank#section-4.3.1)).  

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

HTTP uses Internet media types [[RFC2046](https://tools.ietf.org/html/rfc2046)] in the Content-Type    ([Section 3.1.1.5](about:blank#section-3.1.1.5)) and Accept ([Section 5.3.2](about:blank#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.    Parameter values might or might not be case-sensitive, depending on    the semantics of the parameter name.  The presence or absence of a    parameter might be significant to the processing of a media-type,    depending on its definition within the media type registry.     A parameter value that matches the token production can be    transmitted either as a token or within a quoted-string.  The quoted    and unquoted values are equivalent.  For example, the following    examples are all equivalent, but the first is preferred for    consistency:       text/html;charset=utf-8      text/html;charset=UTF-8      Text/HTML;Charset="utf-8"      text/html; charset="utf-8"     Internet media types ought to be registered with IANA according to    the procedures defined in [[BCP13](about:blank#ref-BCP13)].        Note: Unlike some similar constructs in other header fields, media       type parameters do not allow whitespace (even "bad" whitespace)       around the "=" character.  

HTTP uses charset names to indicate or negotiate the character    encoding scheme of a textual representation [[RFC6365](https://tools.ietf.org/html/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](https://tools.ietf.org/html/rfc2978)].  

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

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]](https://tools.ietf.org/html/rfc2046#section-5.1.1), 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](https://tools.ietf.org/html/rfc2388)], and the "multipart/    byteranges" type is defined by this specification for use in some 206    (Partial Content) responses [[RFC7233](https://tools.ietf.org/html/rfc7233)].  

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](about:blank#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](https://tools.ietf.org/html/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.  

Content coding values indicate an encoding transformation that has    been or can be applied to a representation.  Content codings are    primarily used to allow a representation to be compressed or    otherwise usefully transformed without losing the identity of its    underlying media type and without loss of information.  Frequently,    the representation is stored in coded form, transmitted directly, and    only decoded by the final recipient.       content-coding   = token     All content-coding values are case-insensitive and ought to be    registered within the "HTTP Content Coding Registry", as defined in    [Section 8.4](about:blank#section-8.4).  They are used in the Accept-Encoding ([Section 5.3.4](about:blank#section-5.3.4))    and Content-Encoding ([Section 3.1.2.2](about:blank#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]](https://tools.ietf.org/html/rfc7230#section-4.2.1).        deflate: See [Section 4.2.2 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-4.2.2).        gzip (and x-gzip): See [Section 4.2.3 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-4.2.3).  

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]](https://tools.ietf.org/html/rfc7230#section-3.3.1)), 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.  

A language tag, as defined in [[RFC5646](https://tools.ietf.org/html/rfc5646)], identifies a natural    language spoken, written, or otherwise conveyed by human beings for    communication of information to other human beings.  Computer    languages are explicitly excluded.     HTTP uses language tags within the Accept-Language and    Content-Language header fields.  Accept-Language uses the broader    language-range production defined in [Section 5.3.5](about:blank#section-5.3.5), whereas    Content-Language uses the language-tag production defined below.       language-tag = <Language-Tag, see [[RFC5646], Section 2.1](https://tools.ietf.org/html/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](https://tools.ietf.org/html/rfc5646)] for further information.  

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

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]](https://tools.ietf.org/html/rfc7230#section-5.5)).     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.  

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]](https://tools.ietf.org/html/rfc7230#section-5.5)).  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]](https://tools.ietf.org/html/rfc2557#section-4).  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](about:blank#section-4.3.1)) or HEAD ([Section 4.3.2](about:blank#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](about:blank#section-4.3.4)) or    POST ([Section 4.3.3](about:blank#section-4.3.3)), it implies that the server's response contains    the new representation of that resource, thereby distinguishing it    from representations that might only report about the action (e.g.,    "It worked!").  This allows authoring applications to update their    local copies without the need for a subsequent GET request.     If Content-Location is included in a 2xx (Successful) response    message and its field-value refers to a URI that differs from the    effective request URI, then the origin server claims that the URI is    an identifier for a different resource corresponding to the enclosed    representation.  Such a claim can only be trusted if both identifiers    share the same resource owner, which cannot be programmatically    determined via HTTP.     o  For a response to a GET or HEAD request, this is an indication       that the effective request URI refers to a resource that is       subject to content negotiation and the Content-Location       field-value is a more specific identifier for the selected       representation.     o  For a 201 (Created) response to a state-changing method, a       Content-Location field-value that is identical to the Location       field-value indicates that this payload is a current       representation of the newly created resource.     o  Otherwise, such a Content-Location indicates that this payload is       a representation reporting on the requested action's status and       that the same report is available (for future access with GET) at       the given URI.  For example, a purchase transaction made via a       POST request might include a receipt document as the payload of       the 200 (OK) response; the Content-Location field-value provides       an identifier for retrieving a copy of that same receipt in the       future.     A user agent that sends Content-Location in a request message is    stating that its value refers to where the user agent originally    obtained the content of the enclosed representation (prior to any    modifications made by that user agent).  In other words, the user    agent is providing a back link to the source of the original    representation.      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.  

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 ) )  

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](about:blank#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](about:blank#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](about:blank#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](about:blank#section-7.1.1.2)), whereas the payload of the same    status code in a response to POST might represent either the    processing result or the new state of the target resource after    applying the processing.  Response messages with an error status code    usually contain a payload that represents the error condition, such    that it describes the error state and what next steps are suggested    for resolving it.     Header fields that specifically describe the payload, rather than the    associated representation, are referred to as "payload header    fields".  Payload header fields are defined in other parts of this    specification, due to their impact on message parsing.     +-------------------+----------------------------+    | Header Field Name | Defined in...              |    +-------------------+----------------------------+    | Content-Length    | [Section 3.3.2 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-3.3.2) |    | Content-Range     | [Section 4.2 of [RFC7233]](https://tools.ietf.org/html/rfc7233#section-4.2)   |    | Trailer           | [Section 4.4 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-4.4)   |    | Transfer-Encoding | [Section 3.3.1 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-3.3.1) |    +-------------------+----------------------------+  

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](https://tools.ietf.org/html/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.  

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](about:blank#section-5.3) and implicit characteristics, such    as the client's network address or parts of the User-Agent field.     Proactive negotiation is advantageous when the algorithm for    selecting from among the available representations is difficult to    describe to a user agent, or when the server desires to send its    "best guess" to the user agent along with the first response (hoping    to avoid the round trip delay of a subsequent request if the "best    guess" is good enough for the user).  In order to improve the    server's guess, a user agent MAY send request header fields that    describe its preferences.     Proactive negotiation has serious disadvantages:     o  It is impossible for the server to accurately determine what might       be "best" for any given user, since that would require complete       knowledge of both the capabilities of the user agent and the       intended use for the response (e.g., does the user want to view it       on screen or print it on paper?     o  Having the user agent describe its capabilities in every request       can be both very inefficient (given that only a small percentage       of responses have multiple representations) and a potential risk       to the user's privacy;     o  It complicates the implementation of an origin server and the       algorithms for generating responses to a request; and,      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](about:blank#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.  

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.  

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](about:blank#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](about:blank#section-5.2)) to make the requested action conditional on the current    state of the target resource ([[RFC7232](https://tools.ietf.org/html/rfc7232)]).       method = token     HTTP was originally designed to be usable as an interface to    distributed object systems.  The request method was envisioned as    applying semantics to a target resource in much the same way as    invoking a defined method on an identified object would apply    semantics.  The method token is case-sensitive because it might be    used as a gateway to object-based systems with case-sensitive method    names.     Unlike distributed objects, the standardized request methods in HTTP    are not resource-specific, since uniform interfaces provide for    better visibility and reuse in network-based systems [[REST](about:blank#ref-REST)].  Once    defined, a standardized method ought to have the same semantics when    applied to any resource, though each resource determines for itself    whether those semantics are implemented or allowed.     This specification defines a number of standardized methods that are    commonly used in HTTP, as outlined by the following table.  By    convention, standardized methods are defined in all-uppercase    US-ASCII letters.      +---------+-------------------------------------------------+-------+    | Method  | Description                                     | Sec.  |    +---------+-------------------------------------------------+-------+    | GET     | Transfer a current representation of the target | 4.3.1 |    |         | resource.                                       |       |    | HEAD    | Same as GET, but only transfer the status line  | 4.3.2 |    |         | and header section.                             |       |    | POST    | Perform resource-specific processing on the     | 4.3.3 |    |         | request payload.                                |       |    | PUT     | Replace all current representations of the      | 4.3.4 |    |         | target resource with the request payload.       |       |    | DELETE  | Remove all current representations of the       | 4.3.5 |    |         | target resource.                                |       |    | CONNECT | Establish a tunnel to the server identified by  | 4.3.6 |    |         | the target resource.                            |       |    | OPTIONS | Describe the communication options for the      | 4.3.7 |    |         | target resource.                                |       |    | TRACE   | Perform a message loop-back test along the path | 4.3.8 |    |         | to the target resource.                         |       |    +---------+-------------------------------------------------+-------+     All general-purpose servers MUST support the methods GET and HEAD.    All other methods are OPTIONAL.     Additional methods, outside the scope of this specification, have    been standardized for use in HTTP.  All such methods ought to be    registered within the "Hypertext Transfer Protocol (HTTP) Method    Registry" maintained by IANA, as defined in [Section 8.1](about:blank#section-8.1).     The set of methods allowed by a target resource can be listed in an    Allow header field ([Section 7.4.1](about:blank#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.  

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 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.  

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.  

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](https://tools.ietf.org/html/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.  

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](about:blank#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](https://tools.ietf.org/html/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]](https://tools.ietf.org/html/rfc7234#section-5.2)).  

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](about:blank#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]](https://tools.ietf.org/html/rfc7234#section-5.2)).  A HEAD    response might also have an effect on previously cached responses to    GET; see [Section 4.3.5 of [RFC7234]](https://tools.ietf.org/html/rfc7234#section-4.3.5).  

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](about:blank#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]](https://tools.ietf.org/html/rfc7234#section-4.2.1)).    However, POST caching is not widely implemented.  For cases where an    origin server wishes the client to be able to cache the result of a    POST in a way that can be reused by a later GET, the origin server    MAY send a 200 (OK) response containing the result and a    Content-Location header field that has the same value as the POST's    effective request URI ([Section 3.1.4.2](about:blank#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.  

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](about:blank#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](about:blank#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]](https://tools.ietf.org/html/rfc7233#section-4.2)), 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](https://tools.ietf.org/html/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]](https://tools.ietf.org/html/rfc7234#section-4.4)).  

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]](https://tools.ietf.org/html/rfc7234#section-4.4)).  

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](https://tools.ietf.org/html/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]](https://tools.ietf.org/html/rfc7230#section-5.3) 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.  

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]](https://tools.ietf.org/html/rfc7230#section-5.3)) 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](about:blank#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.  

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]](https://tools.ietf.org/html/rfc7230#section-8.3.1)).  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](about:blank#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](https://tools.ietf.org/html/rfc7235)] or cookies [[RFC6265](https://tools.ietf.org/html/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]](https://tools.ietf.org/html/rfc7230#section-5.7.1)) 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.  

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.  

Controls are request header fields that direct specific handling of    the request.     +-------------------+--------------------------+    | Header Field Name | Defined in...            |    +-------------------+--------------------------+    | Cache-Control     | [Section 5.2 of [RFC7234]](https://tools.ietf.org/html/rfc7234#section-5.2) |    | Expect            | [Section 5.1.1](about:blank#section-5.1.1)            |    | Host              | [Section 5.4 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-5.4) |    | Max-Forwards      | [Section 5.1.2](about:blank#section-5.1.2)            |    | Pragma            | [Section 5.4 of [RFC7234]](https://tools.ietf.org/html/rfc7234#section-5.4) |    | Range             | [Section 3.1 of [RFC7233]](https://tools.ietf.org/html/rfc7233#section-3.1) |    | TE                | [Section 4.3 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-4.3) |    +-------------------+--------------------------+   

The "Expect" header field in a request indicates a certain set of    behaviors (expectations) that need to be supported by the server in    order to properly handle this request.  The only such expectation    defined by this specification is 100-continue.       Expect  = "100-continue"     The Expect field-value is case-insensitive.     A server that receives an Expect field-value other than 100-continue    MAY respond with a 417 (Expectation Failed) status code to indicate    that the unexpected expectation cannot be met.     A 100-continue expectation informs recipients that the client is    about to send a (presumably large) message body in this request and    wishes to receive a 100 (Continue) interim response if the    request-line and header fields are not sufficient to cause an    immediate success, redirect, or error response.  This allows the    client to wait for an indication that it is worthwhile to send the    message body before actually doing so, which can improve efficiency    when the message body is huge or when the client anticipates that an    error is likely (e.g., when sending a state-changing method, for the    first time, without previously verified authentication credentials).     For example, a request that begins with       PUT /somewhere/fun HTTP/1.1      Host: origin.example.com      Content-Type: video/h264      Content-Length: 1234567890987      Expect: 100-continue      allows the origin server to immediately respond with an error    message, such as 401 (Unauthorized) or 405 (Method Not Allowed),    before the client starts filling the pipes with an unnecessary data    transfer.     Requirements for clients:     o  A client MUST NOT generate a 100-continue expectation in a request       that does not include a message body.     o  A client that will wait for a 100 (Continue) response before       sending the request message body MUST send an Expect header field       containing a 100-continue expectation.      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]](https://tools.ietf.org/html/rfc7230#section-6.6)).     An origin server MUST, upon receiving an HTTP/1.1 (or later)    request-line and a complete header section that contains a    100-continue expectation and indicates a request message body will    follow, either send an immediate response with a final status code,    if that status can be determined by examining just the request-line    and header fields, or send an immediate 100 (Continue) response to    encourage the client to send the request's message body.  The origin    server MUST NOT wait for the message body before sending the 100    (Continue) response.     A proxy MUST, upon receiving an HTTP/1.1 (or later) request-line and    a complete header section that contains a 100-continue expectation    and indicates a request message body will follow, either send an    immediate response with a final status code, if that status can be    determined by examining just the request-line and header fields, or    begin forwarding the request toward the origin server by sending a      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](https://tools.ietf.org/html/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.  

The "Max-Forwards" header field provides a mechanism with the TRACE    ([Section 4.3.8](about:blank#section-4.3.8)) and OPTIONS ([Section 4.3.7](about:blank#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.  

The HTTP conditional request header fields [[RFC7232](https://tools.ietf.org/html/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](about:blank#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]](https://tools.ietf.org/html/rfc7232#section-5).     +---------------------+--------------------------+    | Header Field Name   | Defined in...            |    +---------------------+--------------------------+    | If-Match            | [Section 3.1 of [RFC7232]](https://tools.ietf.org/html/rfc7232#section-3.1) |    | If-None-Match       | [Section 3.2 of [RFC7232]](https://tools.ietf.org/html/rfc7232#section-3.2) |    | If-Modified-Since   | [Section 3.3 of [RFC7232]](https://tools.ietf.org/html/rfc7232#section-3.3) |    | If-Unmodified-Since | [Section 3.4 of [RFC7232]](https://tools.ietf.org/html/rfc7232#section-3.4) |    | If-Range            | [Section 3.2 of [RFC7233]](https://tools.ietf.org/html/rfc7233#section-3.2) |    +---------------------+--------------------------+  

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](about:blank#section-3.4.1).  The preferences sent in these fields apply to any    content in the response, including representations of the target    resource, representations of error or processing status, and    potentially even the miscellaneous text strings that might appear    within the protocol.     +-------------------+---------------+    | Header Field Name | Defined in... |    +-------------------+---------------+    | Accept            | [Section 5.3.2](about:blank#section-5.3.2) |    | Accept-Charset    | [Section 5.3.3](about:blank#section-5.3.3) |    | Accept-Encoding   | [Section 5.3.4](about:blank#section-5.3.4) |    | Accept-Language   | [Section 5.3.5](about:blank#section-5.3.5) |    +-------------------+---------------+  

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.  

The "Accept" header field can be used by user agents to specify    response media types that are acceptable.  Accept header fields can    be used to indicate that the request is specifically limited to a    small set of desired types, as in the case of a request for an    in-line image.       Accept = #( media-range [ accept-params ] )       media-range    = ( "\*/\*"                       / ( type "/" "\*" )                       / ( type "/" subtype )                       ) \*( OWS ";" OWS parameter )      accept-params  = weight \*( accept-ext )      accept-ext = OWS ";" OWS token [ "=" ( token / quoted-string ) ]     The asterisk "\*" character is used to group media types into ranges,    with "\*/\*" indicating all media types and "type/\*" indicating all    subtypes of that type.  The media-range can include media type    parameters that are applicable to that range.     Each media-range might be followed by zero or more applicable media    type parameters (e.g., charset), an optional "q" parameter for    indicating a relative weight ([Section 5.3.1](about:blank#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.  

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](about:blank#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](about:blank#section-5.3.1).    An example is       Accept-Charset: iso-8859-5, unicode-1-1;q=0.8     The special value "\*", if present in the Accept-Charset field,    matches every charset that is not mentioned elsewhere in the    Accept-Charset field.  If no "\*" is present in an Accept-Charset    field, then any charsets not explicitly mentioned in the field are    considered "not acceptable" to the client.     A request without any Accept-Charset header field implies that the    user agent will accept any charset in response.  Most general-purpose    user agents do not send Accept-Charset, unless specifically      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](about:blank#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.  

The "Accept-Encoding" header field can be used by user agents to    indicate what response content-codings ([Section 3.1.2.1](about:blank#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](about:blank#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 field stating either "identity;q=0" or "\*;q=0"        without a more specific entry for "identity".     3.  If the representation's content-coding is one of the        content-codings listed in the Accept-Encoding field, then it is        acceptable unless it is accompanied by a qvalue of 0.  (As        defined in [Section 5.3.1](about:blank#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.  

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](about:blank#section-3.1.3.1).       Accept-Language = 1#( language-range [ weight ] )      language-range  =                <language-range, see [[RFC4647], Section 2.1](https://tools.ietf.org/html/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](about:blank#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]](https://tools.ietf.org/html/rfc4647#section-2.3).     For matching, [Section 3 of [RFC4647]](https://tools.ietf.org/html/rfc4647#section-3) defines several matching    schemes.  Implementations can offer the most appropriate matching    scheme for their requirements.  The "Basic Filtering" scheme    ([[RFC4647], Section 3.3.1](https://tools.ietf.org/html/rfc4647#section-3.3.1)) is identical to the matching scheme that    was previously defined for HTTP in [Section 14.4 of [RFC2616]](https://tools.ietf.org/html/rfc2616#section-14.4).     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](about:blank#section-9.7)).     Since intelligibility is highly dependent on the individual user,    user agents need to allow user control over the linguistic preference    (either through configuration of the user agent itself or by    defaulting to a user controllable system setting).  A user agent that    does not provide such control to the user MUST NOT send an    Accept-Language header field.        Note: User agents ought to provide guidance to users when setting       a preference, since users are rarely familiar with the details of       language matching as described above.  For example, users might       assume that on selecting "en-gb", they will be served any kind of       English document if British English is not available.  A user       agent might suggest, in such a case, to add "en" to the list for       better matching behavior.   

Two header fields are used for carrying authentication credentials,    as defined in [[RFC7235](https://tools.ietf.org/html/rfc7235)].  Note that various custom mechanisms for    user authentication use the Cookie header field for this purpose, as    defined in [[RFC6265](https://tools.ietf.org/html/rfc6265)].     +---------------------+--------------------------+    | Header Field Name   | Defined in...            |    +---------------------+--------------------------+    | Authorization       | [Section 4.2 of [RFC7235]](https://tools.ietf.org/html/rfc7235#section-4.2) |    | Proxy-Authorization | [Section 4.4 of [RFC7235]](https://tools.ietf.org/html/rfc7235#section-4.4) |    +---------------------+--------------------------+  

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

The "From" header field contains an Internet email address for a    human user who controls the requesting user agent.  The address ought    to be machine-usable, as defined by "mailbox" in [Section 3.4 of    [RFC5322]](https://tools.ietf.org/html/rfc5322#section-3.4):       From    = mailbox       mailbox = <mailbox, see [[RFC5322], Section 3.4](https://tools.ietf.org/html/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.  

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](https://tools.ietf.org/html/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](about:blank#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.  

The "User-Agent" header field contains information about the user    agent originating the request, which is often used by servers to help    identify the scope of reported interoperability problems, to work    around or tailor responses to avoid particular user agent    limitations, and for analytics regarding browser or operating system    use.  A user agent SHOULD send a User-Agent field in each request    unless specifically configured not to do so.       User-Agent = product \*( RWS ( product / comment ) )     The User-Agent field-value consists of one or more product    identifiers, each followed by zero or more comments ([Section 3.2 of    [RFC7230]](https://tools.ietf.org/html/rfc7230#section-3.2)), which together identify the user agent software and its    significant subproducts.  By convention, the product identifiers are    listed in decreasing order of their significance for identifying the    user agent software.  Each product identifier consists of a name and    optional version.       product         = token ["/" product-version]      product-version = token     A sender SHOULD limit generated product identifiers to what is    necessary to identify the product; a sender MUST NOT generate    advertising or other nonessential information within the product    identifier.  A sender SHOULD NOT generate information in    product-version that is not a version identifier (i.e., successive    versions of the same product name ought to differ only in the    product-version portion of the product identifier).     Example:       User-Agent: CERN-LineMode/2.15 libwww/2.17b3      A user agent SHOULD NOT generate a User-Agent field containing    needlessly fine-grained detail and SHOULD limit the addition of    subproducts by third parties.  Overly long and detailed User-Agent    field values increase request latency and the risk of a user being    identified against their wishes ("fingerprinting").     Likewise, implementations are encouraged not to use the product    tokens of other implementations in order to declare compatibility    with them, as this circumvents the purpose of the field.  If a user    agent masquerades as a different user agent, recipients can assume    that the user intentionally desires to see responses tailored for    that identified user agent, even if they might not work as well for    the actual user agent being used.  

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  

The status codes listed below are defined in this specification,    [Section 4 of [RFC7232]](https://tools.ietf.org/html/rfc7232#section-4), [Section 4 of [RFC7233]](https://tools.ietf.org/html/rfc7233#section-4), and [Section 3 of    [RFC7235]](https://tools.ietf.org/html/rfc7235#section-3).  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](https://tools.ietf.org/html/rfc7234)]; all other status codes are not    cacheable by default.      +------+-------------------------------+--------------------------+    | Code | Reason-Phrase                 | Defined in...            |    +------+-------------------------------+--------------------------+    | 100  | Continue                      | [Section 6.2.1](about:blank#section-6.2.1)            |    | 101  | Switching Protocols           | [Section 6.2.2](about:blank#section-6.2.2)            |    | 200  | OK                            | [Section 6.3.1](about:blank#section-6.3.1)            |    | 201  | Created                       | [Section 6.3.2](about:blank#section-6.3.2)            |    | 202  | Accepted                      | [Section 6.3.3](about:blank#section-6.3.3)            |    | 203  | Non-Authoritative Information | [Section 6.3.4](about:blank#section-6.3.4)            |    | 204  | No Content                    | [Section 6.3.5](about:blank#section-6.3.5)            |    | 205  | Reset Content                 | [Section 6.3.6](about:blank#section-6.3.6)            |    | 206  | Partial Content               | [Section 4.1 of [RFC7233]](https://tools.ietf.org/html/rfc7233#section-4.1) |    | 300  | Multiple Choices              | [Section 6.4.1](about:blank#section-6.4.1)            |    | 301  | Moved Permanently             | [Section 6.4.2](about:blank#section-6.4.2)            |    | 302  | Found                         | [Section 6.4.3](about:blank#section-6.4.3)            |    | 303  | See Other                     | [Section 6.4.4](about:blank#section-6.4.4)            |    | 304  | Not Modified                  | [Section 4.1 of [RFC7232]](https://tools.ietf.org/html/rfc7232#section-4.1) |    | 305  | Use Proxy                     | [Section 6.4.5](about:blank#section-6.4.5)            |    | 307  | Temporary Redirect            | [Section 6.4.7](about:blank#section-6.4.7)            |    | 400  | Bad Request                   | [Section 6.5.1](about:blank#section-6.5.1)            |    | 401  | Unauthorized                  | [Section 3.1 of [RFC7235]](https://tools.ietf.org/html/rfc7235#section-3.1) |    | 402  | Payment Required              | [Section 6.5.2](about:blank#section-6.5.2)            |    | 403  | Forbidden                     | [Section 6.5.3](about:blank#section-6.5.3)            |    | 404  | Not Found                     | [Section 6.5.4](about:blank#section-6.5.4)            |    | 405  | Method Not Allowed            | [Section 6.5.5](about:blank#section-6.5.5)            |    | 406  | Not Acceptable                | [Section 6.5.6](about:blank#section-6.5.6)            |    | 407  | Proxy Authentication Required | [Section 3.2 of [RFC7235]](https://tools.ietf.org/html/rfc7235#section-3.2) |    | 408  | Request Timeout               | [Section 6.5.7](about:blank#section-6.5.7)            |    | 409  | Conflict                      | [Section 6.5.8](about:blank#section-6.5.8)            |    | 410  | Gone                          | [Section 6.5.9](about:blank#section-6.5.9)            |    | 411  | Length Required               | [Section 6.5.10](about:blank#section-6.5.10)           |    | 412  | Precondition Failed           | [Section 4.2 of [RFC7232]](https://tools.ietf.org/html/rfc7232#section-4.2) |    | 413  | Payload Too Large             | [Section 6.5.11](about:blank#section-6.5.11)           |    | 414  | URI Too Long                  | [Section 6.5.12](about:blank#section-6.5.12)           |    | 415  | Unsupported Media Type        | [Section 6.5.13](about:blank#section-6.5.13)           |    | 416  | Range Not Satisfiable         | [Section 4.4 of [RFC7233]](https://tools.ietf.org/html/rfc7233#section-4.4) |    | 417  | Expectation Failed            | [Section 6.5.14](about:blank#section-6.5.14)           |    | 426  | Upgrade Required              | [Section 6.5.15](about:blank#section-6.5.15)           |    | 500  | Internal Server Error         | [Section 6.6.1](about:blank#section-6.6.1)            |    | 501  | Not Implemented               | [Section 6.6.2](about:blank#section-6.6.2)            |    | 502  | Bad Gateway                   | [Section 6.6.3](about:blank#section-6.6.3)            |    | 503  | Service Unavailable           | [Section 6.6.4](about:blank#section-6.6.4)            |    | 504  | Gateway Timeout               | [Section 6.6.5](about:blank#section-6.6.5)            |    | 505  | HTTP Version Not Supported    | [Section 6.6.6](about:blank#section-6.6.6)            |    +------+-------------------------------+--------------------------+      Note that this list is not exhaustive -- it does not include    extension status codes defined in other specifications.  The complete    list of status codes is maintained by IANA.  See [Section 8.2](about:blank#section-8.2) for    details.  

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

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](about:blank#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.  

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]](https://tools.ietf.org/html/rfc7230#section-6.7)), 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.  

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

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]](https://tools.ietf.org/html/rfc7234#section-4.2.2)).   

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](about:blank#section-7.2) for a discussion of the meaning    and purpose of validator header fields, such as ETag and    Last-Modified, in a 201 response.  

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.  

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]](https://tools.ietf.org/html/rfc7230#section-5.7.2)).  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]](https://tools.ietf.org/html/rfc7234#section-5.5)), 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]](https://tools.ietf.org/html/rfc7234#section-4.2.2)).  

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]](https://tools.ietf.org/html/rfc7234#section-4.2.2)).  

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.  

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](about:blank#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](about:blank#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](https://tools.ietf.org/html/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](https://tools.ietf.org/html/rfc2068#section-10.3)).  Content       developers need to be aware that some clients might implement such       a fixed limitation.  

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](about:blank#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]](https://tools.ietf.org/html/rfc7234#section-4.2.2)).        Note: The original proposal for the 300 status code defined the       URI header field as providing a list of alternative       representations, such that it would be usable for 200, 300, and       406 responses and be transferred in responses to the HEAD method.       However, lack of deployment and disagreement over syntax led to       both URI and Alternates (a subsequent proposal) being dropped from       this specification.  It is possible to communicate the list using       a set of Link header fields [[RFC5988](https://tools.ietf.org/html/rfc5988)], each with a relationship of       "alternate", though deployment is a chicken-and-egg problem.  

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]](https://tools.ietf.org/html/rfc7234#section-4.2.2)).  

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).        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.  

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.   

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

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

The 307 (Temporary Redirect) status code indicates that the target    resource resides temporarily under a different URI and the user agent    MUST NOT change the request method if it performs an automatic    redirection to that URI.  Since the redirection can change over time,    the client ought to continue using the original effective request URI    for future requests.     The server SHOULD generate a Location header field in the response    containing a URI reference for the different URI.  The user agent MAY    use the Location field value for automatic redirection.  The server's    response payload usually contains a short hypertext note with a    hyperlink to the different URI(s).        Note: This status code is similar to 302 (Found), except that it       does not allow changing the request method from POST to GET.  This       specification defines no equivalent counterpart for 301 (Moved       Permanently) ([[RFC7238](https://tools.ietf.org/html/rfc7238)], however, defines the status code 308       (Permanent Redirect) for this purpose).  

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.  

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).   

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

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).  

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]](https://tools.ietf.org/html/rfc7234#section-4.2.2)).  

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]](https://tools.ietf.org/html/rfc7234#section-4.2.2)).   

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](about:blank#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](about:blank#section-6.4.1).  

The 408 (Request Timeout) status code indicates that the server did    not receive a complete request message within the time that it was    prepared to wait.  A server SHOULD send the "close" connection option    ([Section 6.1 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-6.1)) 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.  

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.  

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]](https://tools.ietf.org/html/rfc7234#section-4.2.2)).  

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]](https://tools.ietf.org/html/rfc7230#section-3.3.2)).  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.  

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

The 414 (URI Too Long) status code indicates that the server is    refusing to service the request because the request-target ([Section](https://tools.ietf.org/html/rfc7230#section-5.3) [5.3 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-5.3)) 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]](https://tools.ietf.org/html/rfc7234#section-4.2.2)).  

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

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

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]](https://tools.ietf.org/html/rfc7230#section-6.7)).     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.  

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.  

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

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]](https://tools.ietf.org/html/rfc7234#section-4.2.2)).  

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.  

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](about:blank#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.  

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.   

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]](https://tools.ietf.org/html/rfc7230#section-2.6), 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.  

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

Response header fields can supply control data that supplements the    status code, directs caching, or instructs the client where to go    next.     +-------------------+--------------------------+    | Header Field Name | Defined in...            |    +-------------------+--------------------------+    | Age               | [Section 5.1 of [RFC7234]](https://tools.ietf.org/html/rfc7234#section-5.1) |    | Cache-Control     | [Section 5.2 of [RFC7234]](https://tools.ietf.org/html/rfc7234#section-5.2) |    | Expires           | [Section 5.3 of [RFC7234]](https://tools.ietf.org/html/rfc7234#section-5.3) |    | Date              | [Section 7.1.1.2](about:blank#section-7.1.1.2)          |    | Location          | [Section 7.1.2](about:blank#section-7.1.2)            |    | Retry-After       | [Section 7.1.3](about:blank#section-7.1.3)            |    | Vary              | [Section 7.1.4](about:blank#section-7.1.4)            |    | Warning           | [Section 5.5 of [RFC7234]](https://tools.ietf.org/html/rfc7234#section-5.5) |    +-------------------+--------------------------+   

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](https://tools.ietf.org/html/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](https://tools.ietf.org/html/rfc850) format      Sun Nov  6 08:49:37 1994         ; ANSI C's asctime() format     A recipient that parses a timestamp value in an HTTP header field    MUST accept all three HTTP-date formats.  When a sender generates a    header field that contains one or more timestamps defined as    HTTP-date, the sender MUST generate those timestamps in the    IMF-fixdate format.     An HTTP-date value represents time as an instance of Coordinated    Universal Time (UTC).  The first two formats indicate UTC by the    three-letter abbreviation for Greenwich Mean Time, "GMT", a    predecessor of the UTC name; values in the asctime format are assumed    to be in UTC.  A sender that generates HTTP-date values from a local    clock ought to use NTP ([[RFC5905](https://tools.ietf.org/html/rfc5905)]) or some similar protocol to    synchronize its clock to UTC.     Preferred format:       IMF-fixdate  = day-name "," SP date1 SP time-of-day SP GMT      ; fixed length/zone/capitalization subset of the format      ; see [Section 3.3 of [RFC5322]](https://tools.ietf.org/html/rfc5322#section-3.3)       day-name     = %x4D.6F.6E ; "Mon", case-sensitive                   / %x54.75.65 ; "Tue", case-sensitive                   / %x57.65.64 ; "Wed", case-sensitive                   / %x54.68.75 ; "Thu", case-sensitive                   / %x46.72.69 ; "Fri", case-sensitive                   / %x53.61.74 ; "Sat", case-sensitive                   / %x53.75.6E ; "Sun", case-sensitive        date1        = day SP month SP year                   ; e.g., 02 Jun 1982       day          = 2DIGIT      month        = %x4A.61.6E ; "Jan", case-sensitive                   / %x46.65.62 ; "Feb", case-sensitive                   / %x4D.61.72 ; "Mar", case-sensitive                   / %x41.70.72 ; "Apr", case-sensitive                   / %x4D.61.79 ; "May", case-sensitive                   / %x4A.75.6E ; "Jun", case-sensitive                   / %x4A.75.6C ; "Jul", case-sensitive                   / %x41.75.67 ; "Aug", case-sensitive                   / %x53.65.70 ; "Sep", case-sensitive                   / %x4F.63.74 ; "Oct", case-sensitive                   / %x4E.6F.76 ; "Nov", case-sensitive                   / %x44.65.63 ; "Dec", case-sensitive      year         = 4DIGIT       GMT          = %x47.4D.54 ; "GMT", case-sensitive       time-of-day  = hour ":" minute ":" second                   ; 00:00:00 - 23:59:60 (leap second)       hour         = 2DIGIT      minute       = 2DIGIT      second       = 2DIGIT     Obsolete formats:       obs-date     = [rfc850](https://tools.ietf.org/html/rfc850)-date / asctime-date       [rfc850](https://tools.ietf.org/html/rfc850)-date  = day-name-l "," SP date2 SP time-of-day SP GMT      date2        = day "-" month "-" 2DIGIT                   ; e.g., 02-Jun-82       day-name-l   = %x4D.6F.6E.64.61.79    ; "Monday", case-sensitive             / %x54.75.65.73.64.61.79       ; "Tuesday", case-sensitive             / %x57.65.64.6E.65.73.64.61.79 ; "Wednesday", case-sensitive             / %x54.68.75.72.73.64.61.79    ; "Thursday", case-sensitive             / %x46.72.69.64.61.79          ; "Friday", case-sensitive             / %x53.61.74.75.72.64.61.79    ; "Saturday", case-sensitive             / %x53.75.6E.64.61.79          ; "Sunday", case-sensitive        asctime-date = day-name SP date3 SP time-of-day SP year      date3        = month SP ( 2DIGIT / ( SP 1DIGIT ))                   ; e.g., Jun  2      HTTP-date is case sensitive.  A sender MUST NOT generate additional    whitespace in an HTTP-date beyond that specifically included as SP in    the grammar.  The semantics of day-name, day, month, year, and    time-of-day are the same as those defined for the Internet Message    Format constructs with the corresponding name ([[RFC5322](https://tools.ietf.org/html/rfc5322)], [Section](about:blank#section-3.3) [3.3](about:blank#section-3.3)).     Recipients of a timestamp value in [rfc850](https://tools.ietf.org/html/rfc850)-date format, which uses a    two-digit year, MUST interpret a timestamp that appears to be more    than 50 years in the future as representing the most recent year in    the past that had the same last two digits.     Recipients of timestamp values are encouraged to be robust in parsing    timestamps unless otherwise restricted by the field definition.  For    example, messages are occasionally forwarded over HTTP from a    non-HTTP source that might generate any of the date and time    specifications defined by the Internet Message Format.        Note: HTTP requirements for the date/time stamp format apply only       to their usage within the protocol stream.  Implementations are       not required to use these formats for user presentation, request       logging, etc.  

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]](https://tools.ietf.org/html/rfc5322#section-3.6.1).  The    field value is an HTTP-date, as defined in [Section 7.1.1.1](about:blank#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.  

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](https://tools.ietf.org/html/rfc3986#section-4.2)), the final    value is computed by resolving it against the effective request URI    ([[RFC3986], Section 5](https://tools.ietf.org/html/rfc3986#section-5)).     For 201 (Created) responses, the Location value refers to the primary    resource created by the request.  For 3xx (Redirection) responses,    the Location value refers to the preferred target resource for    automatically redirecting the request.     If the Location value provided in a 3xx (Redirection) response does    not have a fragment component, a user agent MUST process the    redirection as if the value inherits the fragment component of the    URI reference used to generate the request target (i.e., the    redirection inherits the original reference's fragment, if any).     For example, a GET request generated for the URI reference    "http://www.example.org/~tim" might result in a 303 (See Other)    response containing the header field:       Location: /People.html#tim     which suggests that the user agent redirect to    "http://www.example.org/People.html#tim"      Likewise, a GET request generated for the URI reference    "http://www.example.org/index.html#larry" might result in a 301    (Moved Permanently) response containing the header field:       Location: http://www.example.net/index.html     which suggests that the user agent redirect to    "http://www.example.net/index.html#larry", preserving the original    fragment identifier.     There are circumstances in which a fragment identifier in a Location    value would not be appropriate.  For example, the Location header    field in a 201 (Created) response is supposed to provide a URI that    is specific to the created resource.        Note: Some recipients attempt to recover from Location fields that       are not valid URI references.  This specification does not mandate       or define such processing, but does allow it for the sake of       robustness.        Note: The Content-Location header field ([Section 3.1.4.2](about:blank#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.  

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.  

The "Vary" header field in a response describes what parts of a    request message, aside from the method, Host header field, and    request target, might influence the origin server's process for    selecting and representing this response.  The value consists of    either a single asterisk ("\*") or a list of header field names    (case-insensitive).       Vary = "\*" / 1#field-name     A Vary field value of "\*" signals that anything about the request    might play a role in selecting the response representation, possibly    including elements outside the message syntax (e.g., the client's    network address).  A recipient will not be able to determine whether    this response is appropriate for a later request without forwarding    the request to the origin server.  A proxy MUST NOT generate a Vary    field with a "\*" value.     A Vary field value consisting of a comma-separated list of names    indicates that the named request header fields, known as the    selecting header fields, might have a role in selecting the    representation.  The potential selecting header fields are not    limited to those defined by this specification.     For example, a response that contains       Vary: accept-encoding, accept-language     indicates that the origin server might have used the request's    Accept-Encoding and Accept-Language fields (or lack thereof) as    determining factors while choosing the content for this response.     An origin server might send Vary with a list of fields for two    purposes:     1.  To inform cache recipients that they MUST NOT use this response        to satisfy a later request unless the later request has the same        values for the listed fields as the original request ([Section 4.1        of [RFC7234]](https://tools.ietf.org/html/rfc7234#section-4.1)).  In other words, Vary expands the cache key        required to match a new request to the stored cache entry.      2.  To inform user agent recipients that this response is subject to        content negotiation ([Section 5.3](about:blank#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]](https://tools.ietf.org/html/rfc7235#section-4.2)).  Likewise, an origin server might use Cache-Control    directives ([Section 5.2 of [RFC7234]](https://tools.ietf.org/html/rfc7234#section-5.2)) to supplant Vary if it    considers the variance less significant than the performance cost of    Vary's impact on caching.  

Validator header fields convey metadata about the selected    representation ([Section 3](about:blank#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](https://tools.ietf.org/html/rfc7232)].     +-------------------+--------------------------+    | Header Field Name | Defined in...            |    +-------------------+--------------------------+    | ETag              | [Section 2.3 of [RFC7232]](https://tools.ietf.org/html/rfc7232#section-2.3) |    | Last-Modified     | [Section 2.2 of [RFC7232]](https://tools.ietf.org/html/rfc7232#section-2.2) |    +-------------------+--------------------------+   

Authentication challenges indicate what mechanisms are available for    the client to provide authentication credentials in future requests.     +--------------------+--------------------------+    | Header Field Name  | Defined in...            |    +--------------------+--------------------------+    | WWW-Authenticate   | [Section 4.1 of [RFC7235]](https://tools.ietf.org/html/rfc7235#section-4.1) |    | Proxy-Authenticate | [Section 4.3 of [RFC7235]](https://tools.ietf.org/html/rfc7235#section-4.3) |    +--------------------+--------------------------+  

The remaining response header fields provide more information about    the target resource for potential use in later requests.     +-------------------+--------------------------+    | Header Field Name | Defined in...            |    +-------------------+--------------------------+    | Accept-Ranges     | [Section 2.3 of [RFC7233]](https://tools.ietf.org/html/rfc7233#section-2.3) |    | Allow             | [Section 7.4.1](about:blank#section-7.4.1)            |    | Server            | [Section 7.4.2](about:blank#section-7.4.2)            |    +-------------------+--------------------------+  

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.   

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]](https://tools.ietf.org/html/rfc7230#section-3.2)),    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](about:blank#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.  

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

HTTP method registrations MUST include the following fields:     o  Method Name (see [Section 4](about:blank#section-4))     o  Safe ("yes" or "no", see [Section 4.2.1](about:blank#section-4.2.1))     o  Idempotent ("yes" or "no", see [Section 4.2.2](about:blank#section-4.2.2))     o  Pointer to specification text     Values to be added to this namespace require IETF Review (see    [[RFC5226], Section 4.1](https://tools.ietf.org/html/rfc5226#section-4.1)).  

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]](https://tools.ietf.org/html/rfc7230#section-3.3)) 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](about:blank#section-4.2.1)), idempotent ([Section 4.2.2](about:blank#section-4.2.2)), cacheable    ([Section 4.2.3](about:blank#section-4.2.3)), what semantics are to be associated with the payload    body if any is present in the request and what refinements the method    makes to header field or status code semantics.  If the new method is    cacheable, its definition ought to describe how, and under what    conditions, a cache can store a response and use it to satisfy a    subsequent request.  The new method ought to describe whether it can    be made conditional ([Section 5.2](about:blank#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](https://tools.ietf.org/html/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](https://tools.ietf.org/html/rfc2774)].   

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](about:blank#section-4.3.6) |    | DELETE  | no   | yes        | [Section 4.3.5](about:blank#section-4.3.5) |    | GET     | yes  | yes        | [Section 4.3.1](about:blank#section-4.3.1) |    | HEAD    | yes  | yes        | [Section 4.3.2](about:blank#section-4.3.2) |    | OPTIONS | yes  | yes        | [Section 4.3.7](about:blank#section-4.3.7) |    | POST    | no   | no         | [Section 4.3.3](about:blank#section-4.3.3) |    | PUT     | no   | yes        | [Section 4.3.4](about:blank#section-4.3.4) |    | TRACE   | yes  | yes        | [Section 4.3.8](about:blank#section-4.3.8) |    +---------+------+------------+---------------+  

The "Hypertext Transfer Protocol (HTTP) Status Code Registry" defines    the namespace for the response status-code token ([Section 6](about:blank#section-6)).  The    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]](https://tools.ietf.org/html/rfc2817#section-7.1).  

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](https://tools.ietf.org/html/rfc5226#section-4.1)).   

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](about:blank#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](https://tools.ietf.org/html/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](about:blank#section-3.1.4.1)).  

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

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](about:blank#ref-BCP90)].   

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]](https://tools.ietf.org/html/rfc7230#section-3.2) for a    general definition of header field syntax in HTTP messages.     The requirements for header field names are defined in [[BCP90](about:blank#ref-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](about:blank#ref-BCP178)] for further information).     New header field values typically have their syntax defined using    ABNF ([[RFC5234](https://tools.ietf.org/html/rfc5234)]), using the extension defined in [Section 7 of    [RFC7230]](https://tools.ietf.org/html/rfc7230#section-7) as necessary, and are usually constrained to the range of    US-ASCII characters.  Header fields needing a greater range of    characters can use an encoding such as the one defined in [[RFC5987](https://tools.ietf.org/html/rfc5987)].     Leading and trailing whitespace in raw field values is removed upon    field parsing ([Section 3.2.4 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-3.2.4)).  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]](https://tools.ietf.org/html/rfc7230#section-3.2.6)).     Because commas (",") are used as a generic delimiter between    field-values, they need to be treated with care if they are allowed    in the field-value.  Typically, components that might contain a comma    are protected with double-quotes using the quoted-string ABNF    production.     For example, a textual date and a URI (either of which might contain    a comma) could be safely carried in field-values like these:       Example-URI-Field: "http://example.com/a.html,foo",                         "http://without-a-comma.example.com/"      Example-Date-Field: "Sat, 04 May 1996", "Wed, 14 Sep 2005"     Note that double-quote delimiters almost always are used with the    quoted-string production; using a different syntax inside    double-quotes will likely cause unnecessary confusion.      Many header fields use a format including (case-insensitively) named    parameters (for instance, Content-Type, defined in [Section 3.1.1.5](about:blank#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](about:blank#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]](https://tools.ietf.org/html/rfc7230#section-3.2)).        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]](https://tools.ietf.org/html/rfc7230#section-6.1)).     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](about:blank#section-7.1.4)).     o  Whether the header field is useful or allowable in trailers (see       [Section 4.1 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-4.1)).     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.  

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

The "HTTP Content Coding Registry" defines the namespace for content    coding names ([Section 4.2 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-4.2)).  The content coding registry    is maintained at <[http://www.iana.org/assignments/http-parameters](http://www.iana.org/assignments/http-parameters)>.  

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]](https://tools.ietf.org/html/rfc7230#section-4)), unless the encoding transformation    is identical (as is the case for the compression codings defined in    [Section 4.2 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-4.2)).     Values to be added to this namespace require IETF Review (see [Section](https://tools.ietf.org/html/rfc5226#section-4.1) [4.1 of [RFC5226]](https://tools.ietf.org/html/rfc5226#section-4.1)) and MUST conform to the purpose of content coding    defined in this section.  

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

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]](https://tools.ietf.org/html/rfc7230#section-9).     The list of considerations below is not exhaustive.  Most security    concerns related to HTTP semantics are about securing server-side    applications (code behind the HTTP interface), securing user agent      processing of payloads received via HTTP, or secure use of the    Internet in general, rather than security of the protocol.  Various    organizations maintain topical information and links to current    research on Web application security (e.g., [[OWASP](about:blank#ref-OWASP)]).  

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

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.  

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.  

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

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](about:blank#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.  

The User-Agent ([Section 5.5.3](about:blank#section-5.5.3)), Via ([Section 5.7.1 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-5.7.1)), and    Server ([Section 7.4.2](about:blank#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.  

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](about:blank#section-5.3)), including the Accept, Accept-Charset,    Accept-Encoding, and Accept-Language header fields.     In addition to the fingerprinting concern, detailed use of the    Accept-Language header field can reveal information the user might    consider to be of a private nature.  For example, understanding a    given language set might be strongly correlated to membership in a    particular ethnic group.  An approach that limits such loss of    privacy would be for a user agent to omit the sending of    Accept-Language except for sites that have been whitelisted, perhaps    via interaction after detecting a Vary header field that indicates    language negotiation might be useful.     In environments where proxies are used to enhance privacy, user    agents ought to be conservative in sending proactive negotiation    header fields.  General-purpose user agents that provide a high    degree of header field configurability ought to inform users about    the loss of privacy that might result if too much detail is provided.    As an extreme privacy measure, proxies could filter the proactive    negotiation header fields in relayed requests.  

See [Section 10 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-10).  

[[RFC2045]()]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail               Extensions (MIME) Part One: Format of Internet Message               Bodies", [RFC 2045](https://tools.ietf.org/html/rfc2045), November 1996.     [[RFC2046]()]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail               Extensions (MIME) Part Two: Media Types", [RFC 2046](https://tools.ietf.org/html/rfc2046),               November 1996.     [[RFC2119]()]  Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels", [BCP 14](https://tools.ietf.org/html/bcp14), [RFC 2119](https://tools.ietf.org/html/rfc2119), March 1997.      [[RFC3986]()]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform               Resource Identifier (URI): Generic Syntax", STD 66,               [RFC 3986](https://tools.ietf.org/html/rfc3986), January 2005.     [[RFC4647]()]  Phillips, A., Ed. and M. Davis, Ed., "Matching of Language               Tags", [BCP 47](https://tools.ietf.org/html/bcp47), [RFC 4647](https://tools.ietf.org/html/rfc4647), September 2006.     [[RFC5234]()]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax               Specifications: ABNF", STD 68, [RFC 5234](https://tools.ietf.org/html/rfc5234), January 2008.     [[RFC5646]()]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying               Languages", [BCP 47](https://tools.ietf.org/html/bcp47), [RFC 5646](https://tools.ietf.org/html/rfc5646), September 2009.     [[RFC6365]()]  Hoffman, P. and J. Klensin, "Terminology Used in               Internationalization in the IETF", [BCP 166](https://tools.ietf.org/html/bcp166), [RFC 6365](https://tools.ietf.org/html/rfc6365),               September 2011.     [[RFC7230]()]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer               Protocol (HTTP/1.1): Message Syntax and Routing",               [RFC 7230](https://tools.ietf.org/html/rfc7230), June 2014.     [[RFC7232]()]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer               Protocol (HTTP/1.1): Conditional Requests", [RFC 7232](https://tools.ietf.org/html/rfc7232),               June 2014.     [[RFC7233]()]  Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,               "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",               [RFC 7233](https://tools.ietf.org/html/rfc7233), June 2014.     [[RFC7234]()]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,               Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",               [RFC 7234](https://tools.ietf.org/html/rfc7234), June 2014.     [[RFC7235]()]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer               Protocol (HTTP/1.1): Authentication", [RFC 7235](https://tools.ietf.org/html/rfc7235), June 2014.  

[[BCP13]()]    Freed, N., Klensin, J., and T. Hansen, "Media Type               Specifications and Registration Procedures", [BCP 13](https://tools.ietf.org/html/bcp13),               [RFC 6838](https://tools.ietf.org/html/rfc6838), January 2013.     [[BCP178]()]   Saint-Andre, P., Crocker, D., and M. Nottingham,               "Deprecating the "X-" Prefix and Similar Constructs in               Application Protocols", [BCP 178](https://tools.ietf.org/html/bcp178), [RFC 6648](https://tools.ietf.org/html/rfc6648), June 2012.      [[BCP90]()]    Klyne, G., Nottingham, M., and J. Mogul, "Registration               Procedures for Message Header Fields", [BCP 90](https://tools.ietf.org/html/bcp90), [RFC 3864](https://tools.ietf.org/html/rfc3864),               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](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](https://tools.ietf.org/html/rfc1945), May 1996.     [[RFC2049]()]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail               Extensions (MIME) Part Five: Conformance Criteria and               Examples", [RFC 2049](https://tools.ietf.org/html/rfc2049), November 1996.     [[RFC2068]()]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.               Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",               [RFC 2068](https://tools.ietf.org/html/rfc2068), January 1997.     [[RFC2295]()]  Holtman, K. and A. Mutz, "Transparent Content Negotiation               in HTTP", [RFC 2295](https://tools.ietf.org/html/rfc2295), March 1998.     [[RFC2388]()]  Masinter, L., "Returning Values from Forms:  multipart/               form-data", [RFC 2388](https://tools.ietf.org/html/rfc2388), August 1998.     [[RFC2557]()]  Palme, F., Hopmann, A., Shelness, N., and E. Stefferud,               "MIME Encapsulation of Aggregate Documents, such as HTML               (MHTML)", [RFC 2557](https://tools.ietf.org/html/rfc2557), 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](https://tools.ietf.org/html/rfc2616), June 1999.     [[RFC2774]()]  Frystyk, H., Leach, P., and S. Lawrence, "An HTTP               Extension Framework", [RFC 2774](https://tools.ietf.org/html/rfc2774), February 2000.     [[RFC2817]()]  Khare, R. and S. Lawrence, "Upgrading to TLS Within               HTTP/1.1", [RFC 2817](https://tools.ietf.org/html/rfc2817), May 2000.     [[RFC2978]()]  Freed, N. and J. Postel, "IANA Charset Registration               Procedures", [BCP 19](https://tools.ietf.org/html/bcp19), [RFC 2978](https://tools.ietf.org/html/rfc2978), October 2000.      [[RFC5226]()]  Narten, T. and H. Alvestrand, "Guidelines for Writing an               IANA Considerations Section in RFCs", [BCP 26](https://tools.ietf.org/html/bcp26), [RFC 5226](https://tools.ietf.org/html/rfc5226),               May 2008.     [[RFC5246]()]  Dierks, T. and E. Rescorla, "The Transport Layer Security               (TLS) Protocol Version 1.2", [RFC 5246](https://tools.ietf.org/html/rfc5246), August 2008.     [[RFC5322]()]  Resnick, P., "Internet Message Format", [RFC 5322](https://tools.ietf.org/html/rfc5322),               October 2008.     [[RFC5789]()]  Dusseault, L. and J. Snell, "PATCH Method for HTTP",               [RFC 5789](https://tools.ietf.org/html/rfc5789), March 2010.     [[RFC5905]()]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,               "Network Time Protocol Version 4: Protocol and Algorithms               Specification", [RFC 5905](https://tools.ietf.org/html/rfc5905), June 2010.     [[RFC5987]()]  Reschke, J., "Character Set and Language Encoding for               Hypertext Transfer Protocol (HTTP) Header Field               Parameters", [RFC 5987](https://tools.ietf.org/html/rfc5987), August 2010.     [[RFC5988]()]  Nottingham, M., "Web Linking", [RFC 5988](https://tools.ietf.org/html/rfc5988), October 2010.     [[RFC6265]()]  Barth, A., "HTTP State Management Mechanism", [RFC 6265](https://tools.ietf.org/html/rfc6265),               April 2011.     [[RFC6266]()]  Reschke, J., "Use of the Content-Disposition Header Field               in the Hypertext Transfer Protocol (HTTP)", [RFC 6266](https://tools.ietf.org/html/rfc6266),               June 2011.     [[RFC7238]()]  Reschke, J., "The Hypertext Transfer Protocol (HTTP)               Status Code 308 (Permanent Redirect)", [RFC 7238](https://tools.ietf.org/html/rfc7238),               June 2014.   

HTTP/1.1 uses many of the constructs defined for the Internet Message    Format [[RFC5322](https://tools.ietf.org/html/rfc5322)] and the Multipurpose Internet Mail Extensions (MIME)    [[RFC2045](https://tools.ietf.org/html/rfc2045)] to allow a message body to be transmitted in an open    variety of representations and with extensible header fields.    However, [RFC 2045](https://tools.ietf.org/html/rfc2045) 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.  

HTTP is not a MIME-compliant protocol.  However, messages can include    a single MIME-Version header field to indicate what version of the    MIME protocol was used to construct the message.  Use of the    MIME-Version header field indicates that the message is in full    conformance with the MIME protocol (as defined in [[RFC2045](https://tools.ietf.org/html/rfc2045)]).    Senders are responsible for ensuring full conformance (where    possible) when exporting HTTP messages to strict MIME environments.  

MIME requires that an Internet mail body part be converted to    canonical form prior to being transferred, as described in [Section 4    of [RFC2049]](https://tools.ietf.org/html/rfc2049#section-4).  [Section 3.1.1.3](about:blank#section-3.1.1.3) of this document describes the forms    allowed for subtypes of the "text" media type when transmitted over    HTTP.  [[RFC2046](https://tools.ietf.org/html/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](about:blank#section-3.1.1.3) of this document to the [RFC 2049](https://tools.ietf.org/html/rfc2049) 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.  

HTTP/1.1 uses a restricted set of date formats ([Section 7.1.1.1](about:blank#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.  

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

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.  

HTTP implementations that share code with MHTML [[RFC2557](https://tools.ietf.org/html/rfc2557)]    implementations need to be aware of MIME line length limitations.    Since HTTP does not have this limitation, HTTP does not fold long    lines.  MHTML messages being transported by HTTP follow all    conventions of MHTML, including line length limitations and folding,    canonicalization, etc., since HTTP transfers message-bodies as      payload and, aside from the "multipart/byteranges" type (Appendix A    of [[RFC7233](https://tools.ietf.org/html/rfc7233)]), does not interpret the content or any MIME header    lines that might be contained therein.  

The primary changes in this revision have been editorial in nature:    extracting the messaging syntax and partitioning HTTP semantics into    separate documents for the core features, conditional requests,    partial requests, caching, and authentication.  The conformance    language has been revised to clearly target requirements and the    terminology has been improved to distinguish payload from    representations and representations from resources.     A new requirement has been added that semantics embedded in a URI be    disabled when those semantics are inconsistent with the request    method, since this is a common cause of interoperability failure.    ([Section 2](about:blank#section-2))     An algorithm has been added for determining if a payload is    associated with a specific identifier.  ([Section 3.1.4.1](about:blank#section-3.1.4.1))     The default charset of ISO-8859-1 for text media types has been    removed; the default is now whatever the media type definition says.    Likewise, special treatment of ISO-8859-1 has been removed from the    Accept-Charset header field.  ([Section 3.1.1.3](about:blank#section-3.1.1.3) and [Section 5.3.3](about:blank#section-5.3.3))     The definition of Content-Location has been changed to no longer    affect the base URI for resolving relative URI references, due to    poor implementation support and the undesirable effect of potentially    breaking relative links in content-negotiated resources.    ([Section 3.1.4.2](about:blank#section-3.1.4.2))     To be consistent with the method-neutral parsing algorithm of    [[RFC7230](https://tools.ietf.org/html/rfc7230)], the definition of GET has been relaxed so that requests    can have a body, even though a body has no meaning for GET.    ([Section 4.3.1](about:blank#section-4.3.1))     Servers are no longer required to handle all Content-\* header fields    and use of Content-Range has been explicitly banned in PUT requests.    ([Section 4.3.4](about:blank#section-4.3.4))     Definition of the CONNECT method has been moved from [[RFC2817](https://tools.ietf.org/html/rfc2817)] to    this specification.  ([Section 4.3.6](about:blank#section-4.3.6))     The OPTIONS and TRACE request methods have been defined as being    safe.  ([Section 4.3.7](about:blank#section-4.3.7) and [Section 4.3.8](about:blank#section-4.3.8))      The Expect header field's extension mechanism has been removed due to    widely-deployed broken implementations.  ([Section 5.1.1](about:blank#section-5.1.1))     The Max-Forwards header field has been restricted to the OPTIONS and    TRACE methods; previously, extension methods could have used it as    well.  ([Section 5.1.2](about:blank#section-5.1.2))     The "about:blank" URI has been suggested as a value for the Referer    header field when no referring URI is applicable, which distinguishes    that case from others where the Referer field is not sent or has been    removed.  ([Section 5.5.2](about:blank#section-5.5.2))     The following status codes are now cacheable (that is, they can be    stored and reused by a cache without explicit freshness information    present): 204, 404, 405, 414, 501.  ([Section 6](about:blank#section-6))     The 201 (Created) status description has been changed to allow for    the possibility that more than one resource has been created.    ([Section 6.3.2](about:blank#section-6.3.2))     The definition of 203 (Non-Authoritative Information) has been    broadened to include cases of payload transformations as well.    ([Section 6.3.4](about:blank#section-6.3.4))     The set of request methods that are safe to automatically redirect is    no longer closed; user agents are able to make that determination    based upon the request method semantics.  The redirect status codes    301, 302, and 307 no longer have normative requirements on response    payloads and user interaction.  ([Section 6.4](about:blank#section-6.4))     The status codes 301 and 302 have been changed to allow user agents    to rewrite the method from POST to GET.  (Sections [6.4.2](about:blank#section-6.4.2) and [6.4.3](about:blank#section-6.4.3))     The description of the 303 (See Other) status code has been changed    to allow it to be cached if explicit freshness information is given,    and a specific definition has been added for a 303 response to GET.    ([Section 6.4.4](about:blank#section-6.4.4))     The 305 (Use Proxy) status code has been deprecated due to security    concerns regarding in-band configuration of a proxy.  ([Section 6.4.5](about:blank#section-6.4.5))     The 400 (Bad Request) status code has been relaxed so that it isn't    limited to syntax errors.  ([Section 6.5.1](about:blank#section-6.5.1))     The 426 (Upgrade Required) status code has been incorporated from    [[RFC2817](https://tools.ietf.org/html/rfc2817)].  ([Section 6.5.15](about:blank#section-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.  ([Section 7.1.1](about:blank#section-7.1.1))     The syntax of the Location header field has been changed to allow all    URI references, including relative references and fragments, along    with some clarifications as to when use of fragments would not be    appropriate.  ([Section 7.1.2](about:blank#section-7.1.2))     Allow has been reclassified as a response header field, removing the    option to specify it in a PUT request.  Requirements relating to the    content of Allow have been relaxed; correspondingly, clients are not    required to always trust its value.  ([Section 7.4.1](about:blank#section-7.4.1))     A Method Registry has been defined.  ([Section 8.1](about:blank#section-8.1))     The Status Code Registry has been redefined by this specification;    previously, it was defined in [Section 7.1 of [RFC2817]](https://tools.ietf.org/html/rfc2817#section-7.1).    ([Section 8.2](about:blank#section-8.2))     Registration of content codings has been changed to require IETF    Review.  ([Section 8.4](about:blank#section-8.4))     The Content-Disposition header field has been removed since it is now    defined by [[RFC6266](https://tools.ietf.org/html/rfc6266)].     The Content-MD5 header field has been removed because it was    inconsistently implemented with respect to partial responses.  

The following core rules are included by reference, as defined in    [Appendix B.1 of [RFC5234]](https://tools.ietf.org/html/rfc5234#appendix-B.1): 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](https://tools.ietf.org/html/rfc7230)]:       BWS           = <BWS, see [[RFC7230], Section 3.2.3](https://tools.ietf.org/html/rfc7230#section-3.2.3)>      OWS           = <OWS, see [[RFC7230], Section 3.2.3](https://tools.ietf.org/html/rfc7230#section-3.2.3)>      RWS           = <RWS, see [[RFC7230], Section 3.2.3](https://tools.ietf.org/html/rfc7230#section-3.2.3)>      URI-reference = <URI-reference, see [[RFC7230], Section 2.7](https://tools.ietf.org/html/rfc7230#section-2.7)>      absolute-URI  = <absolute-URI, see [[RFC7230], Section 2.7](https://tools.ietf.org/html/rfc7230#section-2.7)>      comment       = <comment, see [[RFC7230], Section 3.2.6](https://tools.ietf.org/html/rfc7230#section-3.2.6)>      field-name    = <comment, see [[RFC7230], Section 3.2](https://tools.ietf.org/html/rfc7230#section-3.2)>      partial-URI   = <partial-URI, see [[RFC7230], Section 2.7](https://tools.ietf.org/html/rfc7230#section-2.7)>        quoted-string = <quoted-string, see [[RFC7230], Section 3.2.6](https://tools.ietf.org/html/rfc7230#section-3.2.6)>      token         = <token, see [[RFC7230], Section 3.2.6](https://tools.ietf.org/html/rfc7230#section-3.2.6)>  

In the collected ABNF below, list rules are expanded as per [Section](https://tools.ietf.org/html/rfc7230#section-1.2) [1.2 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-1.2).     Accept = [ ( "," / ( media-range [ accept-params ] ) ) \*( OWS "," [     OWS ( media-range [ accept-params ] ) ] ) ]    Accept-Charset = \*( "," OWS ) ( ( charset / "\*" ) [ weight ] ) \*( OWS     "," [ OWS ( ( charset / "\*" ) [ weight ] ) ] )    Accept-Encoding = [ ( "," / ( codings [ weight ] ) ) \*( OWS "," [ OWS     ( codings [ weight ] ) ] ) ]    Accept-Language = \*( "," OWS ) ( language-range [ weight ] ) \*( OWS     "," [ OWS ( language-range [ weight ] ) ] )    Allow = [ ( "," / method ) \*( OWS "," [ OWS method ] ) ]     BWS = <BWS, see [[RFC7230], Section 3.2.3](https://tools.ietf.org/html/rfc7230#section-3.2.3)>     Content-Encoding = \*( "," OWS ) content-coding \*( OWS "," [ OWS     content-coding ] )    Content-Language = \*( "," OWS ) language-tag \*( OWS "," [ OWS     language-tag ] )    Content-Location = absolute-URI / partial-URI    Content-Type = media-type     Date = HTTP-date     Expect = "100-continue"     From = mailbox     GMT = %x47.4D.54 ; GMT     HTTP-date = IMF-fixdate / obs-date     IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT     Location = URI-reference     Max-Forwards = 1\*DIGIT     OWS = <OWS, see [[RFC7230], Section 3.2.3](https://tools.ietf.org/html/rfc7230#section-3.2.3)>     RWS = <RWS, see [[RFC7230], Section 3.2.3](https://tools.ietf.org/html/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, see [[RFC7230], Section 2.7](https://tools.ietf.org/html/rfc7230#section-2.7)>    User-Agent = product \*( RWS ( product / comment ) )     Vary = "\*" / ( \*( "," OWS ) field-name \*( OWS "," [ OWS field-name ]     ) )     absolute-URI = <absolute-URI, see [[RFC7230], Section 2.7](https://tools.ietf.org/html/rfc7230#section-2.7)>    accept-ext = OWS ";" OWS token [ "=" ( token / quoted-string ) ]    accept-params = weight \*accept-ext    asctime-date = day-name SP date3 SP time-of-day SP year     charset = token    codings = content-coding / "identity" / "\*"    comment = <comment, see [[RFC7230], Section 3.2.6](https://tools.ietf.org/html/rfc7230#section-3.2.6)>    content-coding = token     date1 = day SP month SP year    date2 = day "-" month "-" 2DIGIT    date3 = month SP ( 2DIGIT / ( SP DIGIT ) )    day = 2DIGIT    day-name = %x4D.6F.6E ; Mon     / %x54.75.65 ; Tue     / %x57.65.64 ; Wed     / %x54.68.75 ; Thu     / %x46.72.69 ; Fri     / %x53.61.74 ; Sat     / %x53.75.6E ; Sun    day-name-l = %x4D.6F.6E.64.61.79 ; Monday     / %x54.75.65.73.64.61.79 ; Tuesday     / %x57.65.64.6E.65.73.64.61.79 ; Wednesday     / %x54.68.75.72.73.64.61.79 ; Thursday     / %x46.72.69.64.61.79 ; Friday     / %x53.61.74.75.72.64.61.79 ; Saturday     / %x53.75.6E.64.61.79 ; Sunday    delay-seconds = 1\*DIGIT     field-name = <comment, see [[RFC7230], Section 3.2](https://tools.ietf.org/html/rfc7230#section-3.2)>     hour = 2DIGIT     language-range = <language-range, see [[RFC4647], Section 2.1](https://tools.ietf.org/html/rfc4647#section-2.1)>    language-tag = <Language-Tag, see [[RFC5646], Section 2.1](https://tools.ietf.org/html/rfc5646#section-2.1)>     mailbox = <mailbox, see [[RFC5322], Section 3.4](https://tools.ietf.org/html/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](https://tools.ietf.org/html/rfc850)-date / asctime-date     parameter = token "=" ( token / quoted-string )    partial-URI = <partial-URI, see [[RFC7230], Section 2.7](https://tools.ietf.org/html/rfc7230#section-2.7)>    product = token [ "/" product-version ]    product-version = token    quoted-string = <quoted-string, see [[RFC7230], Section 3.2.6](https://tools.ietf.org/html/rfc7230#section-3.2.6)>    qvalue = ( "0" [ "." \*3DIGIT ] ) / ( "1" [ "." \*3"0" ] )     [rfc850](https://tools.ietf.org/html/rfc850)-date = day-name-l "," SP date2 SP time-of-day SP GMT     second = 2DIGIT    subtype = token     time-of-day = hour ":" minute ":" second    token = <token, see [[RFC7230], Section 3.2.6](https://tools.ietf.org/html/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  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](https://tools.ietf.org/html/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/](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/](http://greenbytes.de/tech/webdav/)   Fielding & Reschke           Standards Track                  [Page 101]