HTTP Semantics

This specification uses three rules to denote the use of linear whitespace: OWS (optional whitespace), RWS (required whitespace), and BWS ("bad" whitespace).¶

The OWS rule is used where zero or more linear whitespace octets might appear. For protocol elements where optional whitespace is preferred to improve readability, a sender SHOULD generate the optional whitespace as a single SP; otherwise, a sender SHOULD NOT generate optional whitespace except as needed to white out invalid or unwanted protocol elements during in-place message filtering.¶

The RWS rule is used when at least one linear whitespace octet is required to separate field tokens. A sender SHOULD generate RWS as a single SP.¶

OWS and RWS have the same semantics as a single SP. Any content known to be defined as OWS or RWS MAY be replaced with a single SP before interpreting it or forwarding the message downstream.¶

The BWS rule is used where the grammar allows optional whitespace only for historical reasons. A sender MUST NOT generate BWS in messages. A recipient MUST parse for such bad whitespace and remove it before interpreting the protocol element.¶

BWS has no semantics. Any content known to be defined as BWS MAY be removed before interpreting it or forwarding the message downstream.¶

  OWS            = *( SP / HTAB )
                 ; optional whitespace
  RWS            = 1*( SP / HTAB )
                 ; required whitespace
  BWS            = OWS
                 ; "bad" whitespace

The "http" URI scheme is hereby defined for minting identifiers within the hierarchical namespace governed by a potential HTTP origin server listening for TCP ([RFC0793]) connections on a given port.¶

  http-URI = "http" "://" authority path-abempty [ "?" query ]

The origin server for an "http" URI is identified by the authority component, which includes a host identifier and optional port number ([RFC3986], Section 3.2.2). If the port subcomponent is empty or not given, TCP port 80 (the reserved port for WWW services) is the default. The origin determines who has the right to respond authoritatively to requests that target the identified resource, as defined in Section 5.4.1.¶

A sender MUST NOT generate an "http" URI with an empty host identifier. A recipient that processes such a URI reference MUST reject it as invalid.¶

The hierarchical path component and optional query component identify the target resource within that origin server's name space.¶

The "https" URI scheme is hereby defined for minting identifiers within the hierarchical namespace governed by a potential origin server listening for TCP connections on a given port and capable of establishing a TLS ([RFC8446]) connection that has been secured for HTTP communication. In this context, "secured" specifically means that the server has been authenticated as acting on behalf of the identified authority and all HTTP communication with that server has been protected for confidentiality and integrity through the use of strong encryption.¶

  https-URI = "https" "://" authority path-abempty [ "?" query ]

The origin server for an "https" URI is identified by the authority component, which includes a host identifier and optional port number ([RFC3986], Section 3.2.2). If the port subcomponent is empty or not given, TCP port 443 (the reserved port for HTTP over TLS) is the default. The origin determines who has the right to respond authoritatively to requests that target the identified resource, as defined in Section 5.4.2.¶

A sender MUST NOT generate an "https" URI with an empty host identifier. A recipient that processes such a URI reference MUST reject it as invalid.¶

The hierarchical path component and optional query component identify the target resource within that origin server's name space.¶

A client MUST ensure that its HTTP requests for an "https" resource are secured, prior to being communicated, and that it only accepts secured responses to those requests.¶

Resources made available via the "https" scheme have no shared identity with the "http" scheme. They are distinct origins with separate namespaces. However, an extension to HTTP that is defined to apply to all origins with the same host, such as the Cookie protocol [RFC6265], can allow information set by one service to impact communication with other services within a matching group of host domains.¶

Since the "http" and "https" schemes conform to the URI generic syntax, such URIs are normalized and compared according to the algorithm defined in Section 6 of [RFC3986], using the defaults described above for each scheme.¶

If the port is equal to the default port for a scheme, the normal form is to omit the port subcomponent. When not being used in absolute form as the request target of an OPTIONS request, an empty path component is equivalent to an absolute path of "/", so the normal form is to provide a path of "/" instead. The scheme and host are case-insensitive and normally provided in lowercase; all other components are compared in a case-sensitive manner. Characters other than those in the "reserved" set are equivalent to their percent-encoded octets: the normal form is to not encode them (see Sections 2.1 and 2.2 of [RFC3986]).¶

   http://example.com:80/~smith/home.html
   http://EXAMPLE.com/%7Esmith/home.html
   http://EXAMPLE.com:/%7esmith/home.html

The URI generic syntax for authority also includes a userinfo subcomponent ([RFC3986], Section 3.2.1) for including user authentication information in the URI. In that subcomponent, the use of the format "user:password" is deprecated.¶

Some implementations make use of the userinfo component for internal configuration of authentication information, such as within command invocation options, configuration files, or bookmark lists, even though such usage might expose a user identifier or password.¶

A sender MUST NOT generate the userinfo subcomponent (and its "@" delimiter) when an "http" or "https" URI reference is generated within a message as a request target or field value.¶

Before making use of an "http" or "https" URI reference received from an untrusted source, a recipient SHOULD parse for userinfo and treat its presence as an error; it is likely being used to obscure the authority for the sake of phishing attacks.¶

Fragment identifiers allow for indirect identification of a secondary resource, independent of the URI scheme, as defined in Section 3.5 of [RFC3986]. Some protocol elements that refer to a URI allow inclusion of a fragment, while others do not. They are distinguished by use of the ABNF rule for elements where fragment is allowed; otherwise, a specific rule that excludes fragments is used (see Section 5.1).¶

There is no limit on the introduction of new field names, each presumably defining new semantics.¶

New fields can be defined such that, when they are understood by a recipient, they might override or enhance the interpretation of previously defined fields, define preconditions on request evaluation, or refine the meaning of responses.¶

A proxy MUST forward unrecognized header fields unless the field name is listed in the Connection header field (Section 9.1 of [Messaging]) or the proxy is specifically configured to block, or otherwise transform, such fields. Other recipients SHOULD ignore unrecognized header and trailer fields. These requirements allow HTTP's functionality to be enhanced without requiring prior update of deployed intermediaries.¶

The "Hypertext Transfer Protocol (HTTP) Field Name Registry" defines the namespace for HTTP field names.¶

Any party can request registration of a HTTP field. See Section 4.7 for considerations to take into account when creating a new HTTP field.¶

The "Hypertext Transfer Protocol (HTTP) Field Name Registry" is located at "https://www.iana.org/assignments/http-fields/". Registration requests can be made by following the instructions located there or by sending an email to the "ietf-http-wg@ietf.org" mailing list.¶

Field names are registered on the advice of a Designated Expert (appointed by the IESG or their delegate). Fields with the status 'permanent' are Specification Required (using terminology from [RFC8126]).¶

• Field name: The requested field name. It MUST conform to the field-name syntax defined in Section 4.3, and SHOULD be restricted to just letters, digits, hyphen ('-') and underscore ('_') characters, with the first character being a letter.

• Status: "permanent" or "provisional"

• Specification document(s): Reference to the document that specifies the field, preferably including a URI that can be used to retrieve a copy of the document. An indication of the relevant section(s) can also be included, but is not required.

The Expert(s) can define additional fields to be collected in the registry, in consultation with the community.¶

Standards-defined names have a status of "permanent". Other names can also be registered as permanent, if the Expert(s) find that they are in use, in consultation with the community. Other names should be registered as "provisional".¶

Provisional entries can be removed by the Expert(s) if — in consultation with the community — the Expert(s) find that they are not in use. The Experts can change a provisional entry's status to permanent at any time.¶

Note that names can be registered by third parties (including the Expert(s)), if the Expert(s) determines that an unregistered name is widely deployed and not likely to be registered in a timely manner otherwise.¶

Many HTTP field values are defined using common syntax components, separated by whitespace or specific delimiting characters. Delimiters are chosen from the set of US-ASCII visual characters not allowed in a token (DQUOTE and "(),/:;<=>?@[\]{}").¶

  1#element => element *( OWS "," OWS element )

  #element => [ 1#element ]

  <n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element )

  #element => [ element ] *( OWS "," OWS [ element ] )

Note that because of the potential presence of empty list elements, the RFC 5234 ABNF cannot enforce the cardinality of list elements, and consequently all cases are mapped is if there was no cardinality specified.¶

  example-list      = 1#example-list-elmt
  example-list-elmt = token ; see Section 4.4.1.1

  "foo,bar"
  "foo ,bar,"
  "foo , ,bar,charlie"

  ""
  ","
  ",   ,"

Appendix A shows the collected ABNF for recipients after the list constructs have been expanded.¶

In some HTTP versions, additional metadata can be sent after the initial header section has been completed (during or after transmission of the payload body), such as a message integrity check, digital signature, or post-processing status. For example, the chunked coding in HTTP/1.1 allows a trailer section after the payload body (Section 7.1.2 of [Messaging]) which can contain trailer fields: field names and values that share the same syntax and namespace as header fields but that are received after the header section.¶

Trailer fields ought to be processed and stored separately from the fields in the header section to avoid contradicting message semantics known at the time the header section was complete. The presence or absence of certain header fields might impact choices made for the routing or processing of the message as a whole before the trailers are received; those choices cannot be unmade by the later discovery of trailer fields.¶

Many fields cannot be processed outside the header section because their evaluation is necessary prior to receiving the message body, such as those that describe message framing, routing, authentication, request modifiers, response controls, or payload format. A sender MUST NOT generate a trailer field unless the sender knows the corresponding header field name's definition permits the field to be sent in trailers.¶

Trailer fields can be difficult to process by intermediaries that forward messages from one protocol version to another. If the entire message can be buffered in transit, some intermediaries could merge trailer fields into the header section (as appropriate) before it is forwarded. However, in most cases, the trailers are simply discarded. A recipient MUST NOT merge a trailer field into a header section unless the recipient understands the corresponding header field definition and that definition explicitly permits and defines how trailer field values can be safely merged.¶

A client can send a TE header field indicating "trailers" is acceptable, as described in Section 7.4 of [Messaging], to inform the server that it will not discard trailer fields.¶

Because of the potential for trailer fields to be discarded in transit, a server SHOULD NOT generate trailer fields that it believes are necessary for the user agent to receive.¶

The "Trailer" header field provides a list of field names that the sender anticipates sending as trailer fields within that message. This allows a recipient to prepare for receipt of the indicated metadata before it starts processing the body.¶

  Trailer = 1#field-name

For example, a sender might indicate that a message integrity check will be computed as the payload is being streamed and provide the final signature as a trailer field. This allows a recipient to perform the same check on the fly as the payload data is received.¶

A sender that intends to generate one or more trailer fields in a message SHOULD generate a Trailer header field in the header section of that message to indicate which fields might be present in the trailers.¶

Although HTTP is independent of the transport protocol, the "http" scheme is specific to associating authority with whomever controls the origin server listening for TCP connections on the indicated port of whatever host is identified within the authority component. This is a very weak sense of authority because it depends on both client-specific name resolution mechanisms and communication that might not be secured from man-in-the-middle attacks. Nevertheless, it is a sufficient minimum for binding "http" identifiers to an origin server for consistent resolution within a trusted environment.¶

If the host identifier is provided as an IP address, the origin server is the listener (if any) on the indicated TCP port at that IP address. If host is a registered name, the registered name is an indirect identifier for use with a name resolution service, such as DNS, to find an address for an appropriate origin server.¶

When an "http" URI is used within a context that calls for access to the indicated resource, a client MAY attempt access by resolving the host identifier to an IP address, establishing a TCP connection to that address on the indicated port, and sending an HTTP request message to the server containing the URI's identifying data (Section 2 of [Messaging]).¶

If the server responds to such a request with a non-interim HTTP response message, as described in Section 9, then that response is considered an authoritative answer to the client's request.¶

Note, however, that the above is not the only means for obtaining an authoritative response, nor does it imply that an authoritative response is always necessary (see [Caching]). For example, the Alt-Svc header field [RFC7838] allows an origin server to identify other services that are also authoritative for that origin. Access to "http" identified resources might also be provided by protocols outside the scope of this document.¶

See Section 11.1 for security considerations related to establishing authority.¶

The "https" scheme associates authority based on the ability of a server to use a private key associated with a certificate that the client considers to be trustworthy for the identified host. If a server presents a certificate that verifiably applies to the host, along with proof that it controls the corresponding private key, then a client will accept a secured connection to that server as being authoritative for all origins with the same scheme and host.¶

A client is therefore relying upon a chain of trust, conveyed from some trust anchor (which is usually prearranged or configured), through a chain of certificates (e.g., [RFC5280]) to a final certificate that binds a private key to the host name of the origin. The handshake and certificate validation in Section 5.4.3 describe how that final certificate can be used to initiate a secured connection.¶

Note that the "https" scheme does not rely on TCP and the connected port number for associating authority, since both are outside the secured communication and thus cannot be trusted as definitive. Hence, the HTTP communication might take place over any channel that has been secured, as defined in Section 2.5.2, including protocols that don't use TCP. It is the origin's responsibility to ensure that any services provided with control over its certificate's private key are equally responsible for managing the corresponding "https" namespaces, or at least prepared to reject requests that appear to have been misdirected. Regardless, the origin's host and port value are passed within each HTTP request, identifying the target resource and distinguishing it from other namespaces that might be controlled by the same server.¶

In HTTP/1.1 and earlier, the only URIs for which a client will attribute authority to a server are those for which a TLS connection was specifically established toward the origin's host. Only the characteristics of the connection establishment and certificate are used. For a host that is a domain name, the client MUST include that name in the TLS server_name extension (if used) and MUST verify that the name also appears as either the CN field of the certificate subject or as a dNSName in the subjectAltName field of the certificate (see [RFC6125]). For a host that is an IP address, the client MUST verify that the address appears in the subjectAltName of the certificate.¶

In HTTP/2, a client will associate authority to all names that are present in the certificate. However, a client will only do that if it concludes that it could open a connection to the origin for that URI. In practice, a client will make a DNS query and see that it contains the same server IP address. A server sending the ORIGIN frame removes this restriction [RFC8336].¶

In addition to the client's verification, an origin server is responsible for verifying that requests it receives over a connection correspond to resources for which it actually wants to be the origin. If a network attacker causes connections for port N to be received at port Q, checking the effective request URI is necessary to ensure that the attacker can't cause "https://example.com:N/foo" to be replaced by "https://example.com:Q/foo" without consent. Likewise, a server might be unwilling to serve as the origin for some hosts even when they have the authority to do so.¶

When an "https" URI is used within a context that calls for access to the indicated resource, a client MAY attempt access by resolving the host identifier to an IP address, establishing a TCP connection to that address on the indicated port, securing the connection end-to-end by successfully initiating TLS over TCP with confidentiality and integrity protection, and sending an HTTP request message to the server over that secured connection containing the URI's identifying data (Section 2 of [Messaging]).¶

If the server responds to such a request with a non-interim HTTP response message, as described in Section 9, then that response is considered an authoritative answer to the client's request.¶

Note, however, that the above is not the only means for obtaining an authoritative response, nor does it imply that an authoritative response is always necessary (see [Caching]).¶

Conceptually, HTTP/TLS is very simple. Simply use HTTP over TLS precisely as you would use HTTP over TCP.¶

The agent acting as the HTTP client should also act as the TLS client. It should initiate a connection to the server on the appropriate port and then send the TLS ClientHello to begin the TLS handshake. When the TLS handshake has finished. The client may then initiate the first HTTP request. All HTTP data MUST be sent as TLS "application data". Normal HTTP behavior, including retained connections should be followed.¶

The "Via" header field indicates the presence of intermediate protocols and recipients between the user agent and the server (on requests) or between the origin server and the client (on responses), similar to the "Received" header field in email (Section 3.6.7 of [RFC5322]). Via can be used for tracking message forwards, avoiding request loops, and identifying the protocol capabilities of senders along the request/response chain.¶

  Via = 1#( received-protocol RWS received-by [ RWS comment ] )

  received-protocol = [ protocol-name "/" ] protocol-version
                      ; see [Messaging], Section 9.9
  received-by       = pseudonym [ ":" port ]
  pseudonym         = token

Each member of the Via field value represents a proxy or gateway that has forwarded the message. Each intermediary appends its own information about how the message was received, such that the end result is ordered according to the sequence of forwarding recipients.¶

A proxy MUST send an appropriate Via header field, as described below, in each message that it forwards. An HTTP-to-HTTP gateway MUST send an appropriate Via header field in each inbound request message and MAY send a Via header field in forwarded response messages.¶

For each intermediary, the received-protocol indicates the protocol and protocol version used by the upstream sender of the message. Hence, the Via field value records the advertised protocol capabilities of the request/response chain such that they remain visible to downstream recipients; this can be useful for determining what backwards-incompatible features might be safe to use in response, or within a later request, as described in Section 3.5. For brevity, the protocol-name is omitted when the received protocol is HTTP.¶

The received-by portion is normally the host and optional port number of a recipient server or client that subsequently forwarded the message. However, if the real host is considered to be sensitive information, a sender MAY replace it with a pseudonym. If a port is not provided, a recipient MAY interpret that as meaning it was received on the default TCP port, if any, for the received-protocol.¶

A sender MAY generate comments to identify the software of each recipient, analogous to the User-Agent and Server header fields. However, comments in Via are optional, and a recipient MAY remove them prior to forwarding the message.¶

  Via: 1.0 fred, 1.1 p.example.net

An intermediary used as a portal through a network firewall SHOULD NOT forward the names and ports of hosts within the firewall region unless it is explicitly enabled to do so. If not enabled, such an intermediary SHOULD replace each received-by host of any host behind the firewall by an appropriate pseudonym for that host.¶

An intermediary MAY combine an ordered subsequence of Via header field list members into a single member if the entries have identical received-protocol values. For example,¶

  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy

could be collapsed to¶

  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

A sender SHOULD NOT combine multiple list members unless they are all under the same organizational control and the hosts have already been replaced by pseudonyms. A sender MUST NOT combine members that have different received-protocol values.¶

Some intermediaries include features for transforming messages and their payloads. A proxy might, for example, convert between image formats in order to save cache space or to reduce the amount of traffic on a slow link. However, operational problems might occur when these transformations are applied to payloads intended for critical applications, such as medical imaging or scientific data analysis, particularly when integrity checks or digital signatures are used to ensure that the payload received is identical to the original.¶

An HTTP-to-HTTP proxy is called a "transforming proxy" if it is designed or configured to modify messages in a semantically meaningful way (i.e., modifications, beyond those required by normal HTTP processing, that change the message in a way that would be significant to the original sender or potentially significant to downstream recipients). For example, a transforming proxy might be acting as a shared annotation server (modifying responses to include references to a local annotation database), a malware filter, a format transcoder, or a privacy filter. Such transformations are presumed to be desired by whichever client (or client organization) selected the proxy.¶

If a proxy receives a request-target with a host name that is not a fully qualified domain name, it MAY add its own domain to the host name it received when forwarding the request. A proxy MUST NOT change the host name if the request-target contains a fully qualified domain name.¶

A proxy MUST NOT modify the "absolute-path" and "query" parts of the received request-target when forwarding it to the next inbound server, except as noted above to replace an empty path with "/" or "*".¶

A proxy MAY modify the message body through application or removal of a transfer coding (Section 7 of [Messaging]).¶

A proxy MUST NOT transform the payload (Section 6.3) of a message that contains a no-transform cache-control response directive (Section 5.2 of [Caching]).¶

A proxy MAY transform the payload of a message that does not contain a no-transform cache-control directive. A proxy that transforms the payload of a 200 (OK) response can inform downstream recipients that a transformation has been applied by changing the response status code to 203 (Non-Authoritative Information) (Section 9.3.4).¶

A proxy SHOULD NOT modify header fields that provide information about the endpoints of the communication chain, the resource state, or the selected representation (other than the payload) unless the field's definition specifically allows such modification or the modification is deemed necessary for privacy or security.¶

HTTP uses media types [RFC2046] in the Content-Type (Section 6.2.1) and Accept (Section 8.4.2) 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 and subtype tokens are case-insensitive.¶

The type/subtype MAY be followed by semicolon-delimited parameters (Section 4.4.1.4) in the form of name=value pairs. 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. Parameter values might or might not be case-sensitive, depending on the semantics of the parameter name.¶

  text/html;charset=utf-8
  Text/HTML;Charset="utf-8"
  text/html; charset="utf-8"
  text/html;charset=UTF-8

Media types ought to be registered with IANA according to the procedures defined in [BCP13].¶

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 codings are case-insensitive and ought to be registered within the "HTTP Content Coding Registry", as defined in Section 6.1.2.4 ¶

Content-coding values are used in the Accept-Encoding (Section 8.4.4) and Content-Encoding (Section 6.2.2) header fields.¶

A language tag, as defined in [RFC5646], identifies a natural language spoken, written, or otherwise conveyed by human beings for communication of information to other human beings. Computer languages are explicitly excluded.¶

HTTP uses language tags within the Accept-Language and Content-Language header fields. Accept-Language uses the broader language-range production defined in Section 8.4.5, whereas Content-Language uses the language-tag production defined below.¶

  language-tag = <Language-Tag, see [RFC5646], Section 2.1>

  fr, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN

See [RFC5646] for further information.¶

Representation data can be partitioned into subranges when there are addressable structural units inherent to that data's content coding or media type. For example, octet (a.k.a., byte) boundaries are a structural unit common to all representation data, allowing partitions of the data to be identified as a range of bytes at some offset from the start or end of that data.¶

This general notion of a "range unit" is used in the Accept-Ranges (Section 10.4.1) response header field to advertise support for range requests, the Range (Section 8.3) request header field to delineate the parts of a representation that are requested, and the Content-Range (Section 6.3.4) payload header field to describe which part of a representation is being transferred.¶

  range-unit       = token

All range unit names are case-insensitive and ought to be registered within the "HTTP Range Unit Registry", as defined in Section 6.1.4.4 ¶

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 6.1.1. An example of the field is¶

  Content-Type: text/html; charset=ISO-8859-4

A sender that generates a message containing a payload body SHOULD generate a Content-Type header field in that message unless the intended media type of the enclosed representation is unknown to the sender. If a Content-Type header field is not present, the recipient MAY either assume a media type of "application/octet-stream" ([RFC2046], Section 4.5.1) or examine the data to determine its type.¶

In practice, resource owners do not always properly configure their origin server to provide the correct Content-Type for a given representation. Some user agents examine a payload's content and, in certain cases, override the received type (for example, see [Sniffing]). This "MIME sniffing" risks drawing incorrect conclusions about the data, which might expose the user to additional security risks (e.g., "privilege escalation"). Furthermore, it is impossible to determine the sender's intended processing model 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 to disable such sniffing.¶

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 6.1 of [Messaging]), 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.¶

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 6.1.3. 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.¶

[rfc.comment.2: The "Content-Length" header field indicates the number of data octets (body length) for the representation. In some cases, Content-Length is used to define or estimate message framing.]¶

  Content-Length = 1*DIGIT

An example is¶

  Content-Length: 3495

A sender MUST NOT send a Content-Length header field in any message that contains a Transfer-Encoding header field.¶

A user agent SHOULD send a Content-Length in a request message when no Transfer-Encoding is sent and the request method defines a meaning for an enclosed payload body. For example, a Content-Length header field is normally sent in a POST request even when the value is 0 (indicating an empty payload body). A user agent SHOULD NOT send a Content-Length header field when the request message does not contain a payload body and the method semantics do not anticipate such a body.¶

A server MAY send a Content-Length header field in a response to a HEAD request (Section 7.3.2); a server MUST NOT send Content-Length in such a response unless its field value equals the decimal number of octets that would have been sent in the payload body of a response if the same request had used the GET method.¶

A server MAY send a Content-Length header field in a 304 (Not Modified) response to a conditional GET request (Section 9.4.5); a server MUST NOT send Content-Length in such a response unless its field value equals the decimal number of octets that would have been sent in the payload body of a 200 (OK) response to the same request.¶

A server MUST NOT send a Content-Length header field in any response with a status code of 1xx (Informational) or 204 (No Content). A server MUST NOT send a Content-Length header field in any 2xx (Successful) response to a CONNECT request (Section 7.3.6).¶

Aside from the cases defined above, in the absence of Transfer-Encoding, an origin server SHOULD send a Content-Length header field when the payload body size is known prior to sending the complete header section. This will allow downstream recipients to measure transfer progress, know when a received message is complete, and potentially reuse the connection for additional requests.¶

Any Content-Length field value greater than or equal to zero is valid. Since there is no predefined limit to the length of a payload, a recipient MUST anticipate potentially large decimal numerals and prevent parsing errors due to integer conversion overflows (Section 11.5).¶

If a message is received that has multiple Content-Length header fields with field values consisting of the same decimal value, or a single Content-Length header field with a field value containing a list of identical decimal values (e.g., "Content-Length: 42, 42"), indicating that duplicate Content-Length header fields have been generated or combined by an upstream message processor, then the recipient MUST either reject the message as invalid or replace the duplicated field values with a single valid Content-Length field containing that decimal value prior to determining the message body length or forwarding the message.¶

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). It is representation metadata. It has the same syntax and semantics as the header field of the same name defined for MIME body parts in Section 4 of [RFC2557]. However, its appearance in an HTTP message has some special implications for HTTP recipients.¶

If Content-Location is included in a 2xx (Successful) response message and its value refers (after conversion to absolute form) to a URI that is the same as the effective request URI, then the recipient MAY consider the payload to be a current representation of that resource at the time indicated by the message origination date. For a GET (Section 7.3.1) or HEAD (Section 7.3.2) 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 7.3.4) or POST (Section 7.3.3), 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.¶

• 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.
• 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.
• 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 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 7.3.4) 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 7.3.3) 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 7.3.1) represents the current state of the target resource, as observed at the time of the message origination date (Section 10.1.1.2), 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.¶

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.¶

• 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.
• Otherwise, the payload is unidentified.

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).
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 payload body contains the data of a request or response. This is distinct from the message body (e.g., Section 6 of [Messaging]), which is how the payload body is transferred "on the wire", and might be encoded, depending on the HTTP version in use.¶

It is also distinct from a request or response's representation data (Section 6.1), which can be inferred from protocol operation, rather than necessarily appearing "on the wire."¶

The presence of a payload body in a request depends on whether the request method used defines semantics for it.¶

The presence of a payload body in a response depends on both the request method to which it is responding and the response status code (Section 9).¶

Responses to the HEAD request method (Section 7.3.2) never include a payload body because the associated response header fields indicate only what their values would have been if the request method had been GET (Section 7.3.1).¶

2xx (Successful) responses to a CONNECT request method (Section 7.3.6) switch the connection to tunnel mode instead of having a payload body.¶

All 1xx (Informational), 204 (No Content), and 304 (Not Modified) responses do not include a payload body.¶

All other responses do include a payload body, although that body might be of zero length.¶

The "Content-Range" header field is sent in a single part 206 (Partial Content) response to indicate the partial range of the selected representation enclosed as the message payload, sent in each part of a multipart 206 response to indicate the range enclosed within each body part, and sent in 416 (Range Not Satisfiable) responses to provide information about the selected representation.¶

  Content-Range       = range-unit SP
                        ( range-resp / unsatisfied-range )

  range-resp          = incl-range "/" ( complete-length / "*" )
  incl-range          = first-pos "-" last-pos
  unsatisfied-range   = "*/" complete-length

  complete-length     = 1*DIGIT

If a 206 (Partial Content) response contains a Content-Range header field with a range unit (Section 6.1.4) that the recipient does not understand, the recipient MUST NOT attempt to recombine it with a stored representation. A proxy that receives such a message SHOULD forward it downstream.¶

For byte ranges, a sender SHOULD indicate the complete length of the representation from which the range has been extracted, unless the complete length is unknown or difficult to determine. An asterisk character ("*") in place of the complete-length indicates that the representation length was unknown when the header field was generated.¶

  Content-Range: bytes 42-1233/1234

  Content-Range: bytes 42-1233/*

A Content-Range field value is invalid if it contains a range-resp that has a last-pos value less than its first-pos value, or a complete-length value less than or equal to its last-pos value. The recipient of an invalid Content-Range MUST NOT attempt to recombine the received content with a stored representation.¶

  Content-Range: bytes */1234

The complete-length in a 416 response indicates the current length of the selected representation.¶

The Content-Range header field has no meaning for status codes that do not explicitly describe its semantic. For this specification, only the 206 (Partial Content) and 416 (Range Not Satisfiable) status codes describe a meaning for Content-Range.¶

• The first 500 bytes:

  Content-Range: bytes 0-499/1234
  

• The second 500 bytes:

  Content-Range: bytes 500-999/1234
  

• All except for the first 500 bytes:

  Content-Range: bytes 500-1233/1234
  

• The last 500 bytes:

  Content-Range: bytes 734-1233/1234
  

When a 206 (Partial Content) response message includes the content of multiple ranges, they are transmitted as body parts in a multipart message body ([RFC2046], Section 5.1) with the media type of "multipart/byteranges".¶

The multipart/byteranges media type includes one or more body parts, each with its own Content-Type and Content-Range fields. The required boundary parameter specifies the boundary string used to separate each body part.¶

1. Additional CRLFs might precede the first boundary string in the body.
2. Although [RFC2046] permits the boundary string to be quoted, some existing implementations handle a quoted boundary string incorrectly.
3. A number of clients and servers were coded to an early draft of the byteranges specification that used a media type of multipart/x-byteranges, which is almost (but not quite) compatible with this type.

HTTP/1.1 206 Partial Content
Date: Tue, 14 Nov 1995 06:25:24 GMT
Last-Modified: Tue, 14 July 04:58:08 GMT
Content-Length: 2331785
Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES

--THIS_STRING_SEPARATES
Content-Type: video/example
Content-Range: exampleunit 1.2-4.3/25

...the first range...
--THIS_STRING_SEPARATES
Content-Type: video/example
Content-Range: exampleunit 11.2-14.3/25

...the second range
--THIS_STRING_SEPARATES--

The following information serves as the registration form for the multipart/byteranges media type.¶

Type name:

multipart

Subtype name:

byteranges

Required parameters:

boundary

Optional parameters:

N/A

Encoding considerations:

only "7bit", "8bit", or "binary" are permitted

Security considerations:

see Section 11

Interoperability considerations:

N/A

Published specification:

This specification (see Section 6.3.5).

Applications that use this media type:

HTTP components supporting multiple ranges in a single request.

Fragment identifier considerations:

N/A

Additional information:

Deprecated alias names for this type:

N/A

Magic number(s):

N/A

File extension(s):

N/A

Macintosh file type code(s):

N/A

Person and email address to contact for further information:

See Authors' Addresses section.

Intended usage:

COMMON

Restrictions on usage:

N/A

Author:

See Authors' Addresses section.

Change controller:

IESG

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 8.4 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.¶

• 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?);
• 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;
• It complicates the implementation of an origin server and the algorithms for generating responses to a request; and,
• 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 10.1.4) 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.¶

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.¶

A client SHOULD NOT automatically retry a request with a non-idempotent method unless it has some means to know that the request semantics are actually idempotent, regardless of the method, or some means to detect that the original request was never applied.¶

For example, a user agent that knows (through design or configuration) that a POST request to a given resource is safe can repeat that request automatically. Likewise, a user agent designed specifically to operate on a version control repository might be able to recover from partial failure conditions by checking the target resource revision(s) after a failed connection, reverting or fixing any changes that were partially applied, and then automatically retrying the requests that failed.¶

Some clients use weaker signals to initiate automatic retries. For example, when a POST request is sent, but the underlying transport connection is closed before any part of the response is received. Although this is commonly implemented, it is not recommended.¶

A proxy MUST NOT automatically retry non-idempotent requests. A client SHOULD NOT automatically retry a failed automatic retry.¶

For a cache to store and use a response, the associated method needs to explicitly allow caching, and detail under what conditions a response can be used to satisfy subsequent requests; a method definition which does not do so cannot be cached. For additional requirements see [Caching].¶

This specification defines caching semantics for GET, HEAD, and POST, 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.¶

The GET method is specifically intended to reflect the quality of "sameness" identified by the request URI as if it were referenced as an ordinary hypertext link.¶

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 11.3 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 (Section 8.3).¶

A client SHOULD NOT generate a body in a GET request. A payload received in a GET request has no defined semantics, cannot alter the meaning or target of the request, and might lead some implementations to reject the request and close the connection because of its potential as a request smuggling attack (Section 11.2 of [Messaging]).¶

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 [Caching]). A cache that receives a payload in a GET request is likely to ignore that payload and cache regardless of the payload contents.¶

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 6.3) 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 [Caching]). A HEAD response might also have an effect on previously cached responses to GET; see Section 4.3.5 of [Caching].¶

• Providing a block of data, such as the fields entered into an HTML form, to a data-handling process;
• Posting a message to a bulletin board, newsgroup, mailing list, blog, or similar group of articles;
• Creating a new resource that has yet to be identified by the origin server; and
• 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 10.1.2) 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 [Caching]) and a Content-Location header field that has the same value as the POST's effective request URI (Section 6.2.5). A cached POST response can be reused to satisfy a later GET or HEAD request, but not a POST request, since POST is required to be written through to the origin server, because it is unsafe; see Section 4 of [Caching].¶

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 and trailer 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.¶

1. reconfigure the target resource to reflect the new media type;
2. transform the PUT representation to a format consistent with that of the resource before saving it as the new resource state; or,
3. 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 10.2), 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 8.2).¶

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 6.3.4), since the payload is likely to be partial content that has been mistakenly PUT as a full representation. Partial content updates are possible by targeting a separately identified resource with state that overlaps a portion of the larger resource, or by using a different method that has been specifically defined for partial updates (for example, the PATCH method defined in [RFC5789]).¶

Responses to the PUT method are not cacheable. If a successful PUT request passes through a cache that has one or more stored responses for the effective request URI, those stored responses will be invalidated (see Section 4.4 of [Caching]).¶

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 client SHOULD NOT generate a body in a DELETE request. A payload received in a DELETE request has no defined semantics, cannot alter the meaning or target of the request, and might lead some implementations to reject the request.¶

Responses to the DELETE method are not cacheable. If a successful 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 [Caching]).¶

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, [RFC8446]).¶

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 3.2 of [Messaging]); 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 3.2 of [Messaging]) 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 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 client MAY send a Max-Forwards header field in an OPTIONS request to target a specific recipient in the request chain (see Section 8.1.2). 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. Note that this specification does not define any use for such a payload.¶

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 10.1 of [Messaging]). 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 8.1.2).¶

A client MUST NOT generate 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 Section 8.5 or cookies [RFC6265] in a TRACE request. The final recipient of the request SHOULD exclude any request 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) 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.¶

The "Hypertext Transfer Protocol (HTTP) Method Registry", maintained by IANA at <https://www.iana.org/assignments/http-methods>, registers method names.¶

• Method Name (see Section 7)
• Safe ("yes" or "no", see Section 7.2.1)
• Idempotent ("yes" or "no", see Section 7.2.2)
• Pointer to specification text

Values to be added to this namespace require IETF Review (see [RFC8126], Section 4.8).¶

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 6 of [Messaging]) 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 7.2.1), idempotent (Section 7.2.2), cacheable (Section 7.2.3), 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 8.2) 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 (Section 8.3), it ought to document this, too.¶

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.¶

• A client MUST NOT generate a 100-continue expectation in a request that does not include a message body.
• 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.
• 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.
• 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).

• A server that receives a 100-continue expectation in an HTTP/1.0 request MUST ignore that expectation.
• 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.
• 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.
• A server that responds with a final status code before reading the entire request payload body SHOULD indicate whether it intends to close the connection (see Section 9.7 of [Messaging]) or continue reading the payload body.

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.¶

The "Max-Forwards" header field provides a mechanism with the TRACE (Section 7.3.8) and OPTIONS (Section 7.3.7) 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.¶

Except when excluded below, a recipient cache or origin server MUST evaluate received request preconditions after it has successfully performed its normal request checks and just before it would perform the action associated with the request method. A server MUST ignore all received preconditions if its response to the same request without those conditions would have been a status code other than a 2xx (Successful) or 412 (Precondition Failed). In other words, redirects and failures take precedence over the evaluation of preconditions in conditional requests.¶

A server that is not the origin server for the target resource and cannot act as a cache for requests on the target resource MUST NOT evaluate the conditional request header fields defined by this specification, and it MUST forward them if the request is forwarded, since the generating client intends that they be evaluated by a server that can provide a current representation. Likewise, a server MUST ignore the conditional request header fields defined by this specification when received with a request method that does not involve the selection or modification of a selected representation, such as CONNECT, OPTIONS, or TRACE.¶

Note that protocol extensions can modify the conditions under which revalidation is triggered. For example, the "immutable" cache directive (defined by [RFC8246]) instructs caches to forgo revalidation of fresh responses even when requested by the client.¶

Conditional request header fields that are defined by extensions to HTTP might place conditions on all recipients, on the state of the target resource in general, or on a group of resources. For instance, the "If" header field in WebDAV can make a request conditional on various aspects of multiple resources, such as locks, if the recipient understands and implements that field ([RFC4918], Section 10.4).¶

Although conditional request header fields are defined as being usable with the HEAD method (to keep HEAD's semantics consistent with those of GET), there is no point in sending a conditional HEAD because a successful response is around the same size as a 304 (Not Modified) response and more useful than a 412 (Precondition Failed) response.¶

When more than one conditional request header field is present in a request, the order in which the fields are evaluated becomes important. In practice, the fields defined in this document are consistently implemented in a single, logical order, since "lost update" preconditions have more strict requirements than cache validation, a validated cache is more efficient than a partial response, and entity tags are presumed to be more accurate than date validators.¶

1. When recipient is the origin server and If-Match is present, evaluate the If-Match precondition:

   • if true, continue to step 3
   • if false, respond 412 (Precondition Failed) unless it can be determined that the state-changing request has already succeeded (see Section 8.2.3)

2. When recipient is the origin server, If-Match is not present, and If-Unmodified-Since is present, evaluate the If-Unmodified-Since precondition:

   • if true, continue to step 3
   • if false, respond 412 (Precondition Failed) unless it can be determined that the state-changing request has already succeeded (see Section 8.2.6)

3. When If-None-Match is present, evaluate the If-None-Match precondition:

   • if true, continue to step 5
   • if false for GET/HEAD, respond 304 (Not Modified)
   • if false for other methods, respond 412 (Precondition Failed)

4. When the method is GET or HEAD, If-None-Match is not present, and If-Modified-Since is present, evaluate the If-Modified-Since precondition:

   • if true, continue to step 5
   • if false, respond 304 (Not Modified)

5. When the method is GET and both Range and If-Range are present, evaluate the If-Range precondition:

   • if the validator matches and the Range specification is applicable to the selected representation, respond 206 (Partial Content)

6. Otherwise,

   • all conditions are met, so perform the requested action and respond according to its success or failure.

Any extension to HTTP/1.1 that defines additional conditional request header fields ought to define its own expectations regarding the order for evaluating such fields in relation to those defined in this document and other conditionals that might be found in practice.¶

The "If-Match" header field makes the request method conditional on the recipient origin server either having at least one current representation of the target resource, when the field value is "*", or having a current representation of the target resource that has an entity-tag matching a member of the list of entity-tags provided in the field value.¶

An origin server MUST use the strong comparison function when comparing entity-tags for If-Match (Section 10.2.3.2), since the client intends this precondition to prevent the method from being applied if there have been any changes to the representation data.¶

  If-Match = "*" / 1#entity-tag

  If-Match: "xyzzy"
  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
  If-Match: *

If-Match is most often used with state-changing methods (e.g., POST, PUT, DELETE) to prevent accidental overwrites when multiple user agents might be acting in parallel on the same resource (i.e., to prevent the "lost update" problem). It can also be used with safe methods to abort a request if the selected representation does not match one already stored (or partially stored) from a prior request.¶

An origin server that receives an If-Match header field MUST evaluate the condition prior to performing the method (Section 8.2.1). If the field value is "*", the condition is false if the origin server does not have a current representation for the target resource. If the field value is a list of entity-tags, the condition is false if none of the listed tags match the entity-tag of the selected representation.¶

An origin server MUST NOT perform the requested method if a received If-Match condition evaluates to false; instead, the origin server MUST respond with either a) the 412 (Precondition Failed) status code or b) one of the 2xx (Successful) status codes if the origin server has verified that a state change is being requested and the final state is already reflected in the current state of the target resource (i.e., the change requested by the user agent has already succeeded, but the user agent might not be aware of it, perhaps because the prior response was lost or a compatible change was made by some other user agent). In the latter case, the origin server MUST NOT send a validator header field in the response unless it can verify that the request is a duplicate of an immediately prior change made by the same user agent.¶

The If-Match header field can be ignored by caches and intermediaries because it is not applicable to a stored response.¶

The "If-None-Match" header field makes the request method conditional on a recipient cache or origin server either not having any current representation of the target resource, when the field value is "*", or having a selected representation with an entity-tag that does not match any of those listed in the field value.¶

A recipient MUST use the weak comparison function when comparing entity-tags for If-None-Match (Section 10.2.3.2), since weak entity-tags can be used for cache validation even if there have been changes to the representation data.¶

  If-None-Match = "*" / 1#entity-tag

  If-None-Match: "xyzzy"
  If-None-Match: W/"xyzzy"
  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
  If-None-Match: *

If-None-Match is primarily used in conditional GET requests to enable efficient updates of cached information with a minimum amount of transaction overhead. When a client desires to update one or more stored responses that have entity-tags, the client SHOULD generate an If-None-Match header field containing a list of those entity-tags when making a GET request; this allows recipient servers to send a 304 (Not Modified) response to indicate when one of those stored responses matches the selected representation.¶

If-None-Match can also be used with a value of "*" to prevent an unsafe request method (e.g., PUT) from inadvertently modifying an existing representation of the target resource when the client believes that the resource does not have a current representation (Section 7.2.1). This is a variation on the "lost update" problem that might arise if more than one client attempts to create an initial representation for the target resource.¶

An origin server that receives an If-None-Match header field MUST evaluate the condition prior to performing the method (Section 8.2.1). If the field value is "*", the condition is false if the origin server has a current representation for the target resource. If the field value is a list of entity-tags, the condition is false if one of the listed tags match the entity-tag of the selected representation.¶

An origin server MUST NOT perform the requested method if the condition evaluates to false; instead, the origin server MUST respond with either a) the 304 (Not Modified) status code if the request method is GET or HEAD or b) the 412 (Precondition Failed) status code for all other request methods.¶

Requirements on cache handling of a received If-None-Match header field are defined in Section 4.3.2 of [Caching].¶

The "If-Modified-Since" header field makes a GET or HEAD request method conditional on the selected representation's modification date being more recent than the date provided in the field value. Transfer of the selected representation's data is avoided if that data has not changed.¶

  If-Modified-Since = HTTP-date

  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT

A recipient MUST ignore If-Modified-Since if the request contains an If-None-Match header field; the condition in If-None-Match is considered to be a more accurate replacement for the condition in If-Modified-Since, and the two are only combined for the sake of interoperating with older intermediaries that might not implement If-None-Match.¶

A recipient MUST ignore the If-Modified-Since header field if the received field value is not a valid HTTP-date, or if the request method is neither GET nor HEAD.¶

A recipient MUST interpret an If-Modified-Since field value's timestamp in terms of the origin server's clock.¶

If-Modified-Since is typically used for two distinct purposes: 1) to allow efficient updates of a cached representation that does not have an entity-tag and 2) to limit the scope of a web traversal to resources that have recently changed.¶

When used for cache updates, a cache will typically use the value of the cached message's Last-Modified field to generate the field value of If-Modified-Since. This behavior is most interoperable for cases where clocks are poorly synchronized or when the server has chosen to only honor exact timestamp matches (due to a problem with Last-Modified dates that appear to go "back in time" when the origin server's clock is corrected or a representation is restored from an archived backup). However, caches occasionally generate the field value based on other data, such as the Date header field of the cached message or the local clock time that the message was received, particularly when the cached message does not contain a Last-Modified field.¶

When used for limiting the scope of retrieval to a recent time window, a user agent will generate an If-Modified-Since field value based on either its own local clock or a Date header field received from the server in a prior response. Origin servers that choose an exact timestamp match based on the selected representation's Last-Modified field will not be able to help the user agent limit its data transfers to only those changed during the specified window.¶

An origin server that receives an If-Modified-Since header field SHOULD evaluate the condition prior to performing the method (Section 8.2.1). The origin server SHOULD NOT perform the requested method if the selected representation's last modification date is earlier than or equal to the date provided in the field value; instead, the origin server SHOULD generate a 304 (Not Modified) response, including only those metadata that are useful for identifying or updating a previously cached response.¶

Requirements on cache handling of a received If-Modified-Since header field are defined in Section 4.3.2 of [Caching].¶

The "If-Unmodified-Since" header field makes the request method conditional on the selected representation's last modification date being earlier than or equal to the date provided in the field value. This field accomplishes the same purpose as If-Match for cases where the user agent does not have an entity-tag for the representation.¶

  If-Unmodified-Since = HTTP-date

  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT

A recipient MUST ignore If-Unmodified-Since if the request contains an If-Match header field; the condition in If-Match is considered to be a more accurate replacement for the condition in If-Unmodified-Since, and the two are only combined for the sake of interoperating with older intermediaries that might not implement If-Match.¶

A recipient MUST ignore the If-Unmodified-Since header field if the received field value is not a valid HTTP-date.¶

A recipient MUST interpret an If-Unmodified-Since field value's timestamp in terms of the origin server's clock.¶

If-Unmodified-Since is most often used with state-changing methods (e.g., POST, PUT, DELETE) to prevent accidental overwrites when multiple user agents might be acting in parallel on a resource that does not supply entity-tags with its representations (i.e., to prevent the "lost update" problem). It can also be used with safe methods to abort a request if the selected representation does not match one already stored (or partially stored) from a prior request.¶

An origin server that receives an If-Unmodified-Since header field MUST evaluate the condition prior to performing the method (Section 8.2.1). The origin server MUST NOT perform the requested method if the selected representation's last modification date is more recent than the date provided in the field value; instead the origin server MUST respond with either a) the 412 (Precondition Failed) status code or b) one of the 2xx (Successful) status codes if the origin server has verified that a state change is being requested and the final state is already reflected in the current state of the target resource (i.e., the change requested by the user agent has already succeeded, but the user agent might not be aware of that because the prior response message was lost or a compatible change was made by some other user agent). In the latter case, the origin server MUST NOT send a validator header field in the response unless it can verify that the request is a duplicate of an immediately prior change made by the same user agent.¶

The If-Unmodified-Since header field can be ignored by caches and intermediaries because it is not applicable to a stored response.¶

The "If-Range" header field provides a special conditional request mechanism that is similar to the If-Match and If-Unmodified-Since header fields but that instructs the recipient to ignore the Range header field if the validator doesn't match, resulting in transfer of the new selected representation instead of a 412 (Precondition Failed) response.¶

If a client has a partial copy of a representation and wishes to have an up-to-date copy of the entire representation, it could use the Range header field with a conditional GET (using either or both of If-Unmodified-Since and If-Match.) However, if the precondition fails because the representation has been modified, the client would then have to make a second request to obtain the entire current representation.¶

The "If-Range" header field allows a client to "short-circuit" the second request. Informally, its meaning is as follows: if the representation is unchanged, send me the part(s) that I am requesting in Range; otherwise, send me the entire representation.¶

  If-Range = entity-tag / HTTP-date

A client MUST NOT generate an If-Range header field in a request that does not contain a Range header field. A server MUST ignore an If-Range header field received in a request that does not contain a Range header field. An origin server MUST ignore an If-Range header field received in a request for a target resource that does not support Range requests.¶

A client MUST NOT generate an If-Range header field containing an entity-tag that is marked as weak. A client MUST NOT generate an If-Range header field containing an HTTP-date unless the client has no entity-tag for the corresponding representation and the date is a strong validator in the sense defined by Section 10.2.2.2.¶

A server that evaluates an If-Range precondition MUST use the strong comparison function when comparing entity-tags (Section 10.2.3.2) and MUST evaluate the condition as false if an HTTP-date validator is provided that is not a strong validator in the sense defined by Section 10.2.2.2. A valid entity-tag can be distinguished from a valid HTTP-date by examining the first two characters for a DQUOTE.¶

If the validator given in the If-Range header field matches the current validator for the selected representation of the target resource, then the server SHOULD process the Range header field as requested. If the validator does not match, the server MUST ignore the Range header field. Note that this comparison by exact match, including when the validator is an HTTP-date, differs from the "earlier than or equal to" comparison used when evaluating an If-Unmodified-Since conditional.¶

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 their preferences regarding response media types. For example, 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 8.4.1), 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.¶

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 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, */*

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

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 its preferences for charsets in textual response content. For example, 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 6.1.1.1. 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 8.4.1. 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.¶

Note: Accept-Charset is deprecated because UTF-8 has become nearly ubiquitous and sending a detailed list of user-preferred charsets wastes bandwidth, increases latency, and makes passive fingerprinting far too easy (Section 11.11). Most general-purpose user agents do not send Accept-Charset, unless specifically configured to do so.¶

The "Accept-Encoding" header field can be used by user agents to indicate their preferences regarding response content-codings (Section 6.1.2). 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 8.4.1. 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

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 value, then it is acceptable unless it is accompanied by a qvalue of 0. (As defined in Section 8.4.1, 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 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.¶

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 6.1.3.¶

  Accept-Language = 1#( language-range [ weight ] )
  language-range  = 
            <language-range, see [RFC4647], Section 2.1>

Each language-range can be given an associated quality value representing an estimate of the user's preference for the languages specified by that range, as defined in Section 8.4.1. 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".¶

Note that some recipients treat the order in which language tags are listed as an indication of descending priority, particularly for tags that are assigned equal quality values (no value is the same as q=1). However, this behavior cannot be relied upon. For consistency and to maximize interoperability, many user agents assign each language tag a unique quality value while also listing them in order of decreasing quality. Additional discussion of language priority lists can be found in Section 2.3 of [RFC4647].¶

For matching, Section 3 of [RFC4647] defines several matching schemes. Implementations can offer the most appropriate matching scheme for their requirements. The "Basic Filtering" scheme ([RFC4647], Section 3.3.1) is identical to the matching scheme that was previously defined for HTTP in Section 14.4 of [RFC2616].¶

It might be contrary to the privacy expectations of the user to send an Accept-Language header field with the complete linguistic preferences of the user in every request (Section 11.11).¶

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.¶

HTTP provides a simple challenge-response authentication framework that can be used by a server to challenge a client request and by a client to provide authentication information. It uses a case-insensitive token as a means to identify the authentication scheme, followed by additional information necessary for achieving authentication via that scheme. The latter can be either a comma-separated list of parameters or a single sequence of characters capable of holding base64-encoded information.¶

Authentication parameters are name=value pairs, where the name token is matched case-insensitively, and each parameter name MUST only occur once per challenge.¶

  auth-scheme    = token

  auth-param     = token BWS "=" BWS ( token / quoted-string )

  token68        = 1*( ALPHA / DIGIT /
                       "-" / "." / "_" / "~" / "+" / "/" ) *"="

The token68 syntax allows the 66 unreserved URI characters ([RFC3986]), plus a few others, so that it can hold a base64, base64url (URL and filename safe alphabet), base32, or base16 (hex) encoding, with or without padding, but excluding whitespace ([RFC4648]).¶

A 401 (Unauthorized) response message is used by an origin server to challenge the authorization of a user agent, including a WWW-Authenticate header field containing at least one challenge applicable to the requested resource.¶

A 407 (Proxy Authentication Required) response message is used by a proxy to challenge the authorization of a client, including a Proxy-Authenticate header field containing at least one challenge applicable to the proxy for the requested resource.¶

  challenge   = auth-scheme [ 1*SP ( token68 / #auth-param ) ]

A user agent that wishes to authenticate itself with an origin server — usually, but not necessarily, after receiving a 401 (Unauthorized) — can do so by including an Authorization header field with the request.¶

A client that wishes to authenticate itself with a proxy — usually, but not necessarily, after receiving a 407 (Proxy Authentication Required) — can do so by including a Proxy-Authorization header field with the request.¶

Both the Authorization field value and the Proxy-Authorization field value contain the client's credentials for the realm of the resource being requested, based upon a challenge received in a response (possibly at some point in the past). When creating their values, the user agent ought to do so by selecting the challenge with what it considers to be the most secure auth-scheme that it understands, obtaining credentials from the user as appropriate. Transmission of credentials within header field values implies significant security considerations regarding the confidentiality of the underlying connection, as described in Section 11.14.1.¶

  credentials = auth-scheme [ 1*SP ( token68 / #auth-param ) ]

Upon receipt of a request for a protected resource that omits credentials, contains invalid credentials (e.g., a bad password) or partial credentials (e.g., when the authentication scheme requires more than one round trip), an origin server SHOULD send a 401 (Unauthorized) response that contains a WWW-Authenticate header field with at least one (possibly new) challenge applicable to the requested resource.¶

Likewise, upon receipt of a request that omits proxy credentials or contains invalid or partial proxy credentials, a proxy that requires authentication SHOULD generate a 407 (Proxy Authentication Required) response that contains a Proxy-Authenticate header field with at least one (possibly new) challenge applicable to the proxy.¶

A server that receives valid credentials that are not adequate to gain access ought to respond with the 403 (Forbidden) status code (Section 9.5.4).¶

HTTP does not restrict applications to this simple challenge-response framework for access authentication. Additional mechanisms can be used, such as authentication at the transport level or via message encapsulation, and with additional header fields specifying authentication information. However, such additional mechanisms are not defined by this specification.¶

The "realm" authentication parameter is reserved for use by authentication schemes that wish to indicate a scope of protection.¶

A protection space is defined by the canonical root URI (the scheme and authority components of the effective request URI; see Section 5.5) of the server being accessed, in combination with the realm value if present. These realms allow the protected resources on a server to be partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database. The realm value is a string, generally assigned by the origin server, that can have additional semantics specific to the authentication scheme. Note that a response can have multiple challenges with the same auth-scheme but with different realms.¶

The protection space determines the domain over which credentials can be automatically applied. If a prior request has been authorized, the user agent MAY reuse the same credentials for all other requests within that protection space for a period of time determined by the authentication scheme, parameters, and/or user preferences (such as a configurable inactivity timeout). Unless specifically allowed by the authentication scheme, a single protection space cannot extend outside the scope of its server.¶

For historical reasons, a sender MUST only generate the quoted-string syntax. Recipients might have to support both token and quoted-string syntax for maximum interoperability with existing clients that have been accepting both notations for a long time.¶

The "Authorization" header field allows a user agent to authenticate itself with an origin server — usually, but not necessarily, after receiving a 401 (Unauthorized) response. Its value consists of credentials containing the authentication information of the user agent for the realm of the resource being requested.¶

  Authorization = credentials

If a request is authenticated and a realm specified, the same credentials are presumed to be valid for all other requests within this realm (assuming that the authentication scheme itself does not require otherwise, such as credentials that vary according to a challenge value or using synchronized clocks).¶

A proxy forwarding a request MUST NOT modify any Authorization fields in that request. See Section 3.2 of [Caching] for details of and requirements pertaining to handling of the Authorization field by HTTP caches.¶

The "Proxy-Authorization" header field allows the client to identify itself (or its user) to a proxy that requires authentication. Its value consists of credentials containing the authentication information of the client for the proxy and/or realm of the resource being requested.¶

  Proxy-Authorization = credentials

Unlike Authorization, the Proxy-Authorization header field applies only to the next inbound proxy that demanded authentication using the Proxy-Authenticate field. When multiple proxies are used in a chain, the Proxy-Authorization header field is consumed by the first inbound proxy that was expecting to receive credentials. A proxy MAY relay the credentials from the client request to the next proxy if that is the mechanism by which the proxies cooperatively authenticate a given request.¶

Aside from the general framework, this document does not specify any authentication schemes. New and existing authentication schemes are specified independently and ought to be registered within the "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry". For example, the "basic" and "digest" authentication schemes are defined by RFC 7617 and RFC 7616, respectively.¶

  From    = mailbox
  
  mailbox = <mailbox, see [RFC5322], Section 3.4>

  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], 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.¶

  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 11.8 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 4.4.1.3), 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).¶

  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 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 8.1.1. 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 9.9 of [Messaging]), 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.¶

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 heuristically cacheable; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [Caching]).¶

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 10.2 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). 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 [Caching]), which has the advantage of being applicable to responses with any status code.¶

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

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 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 heuristically cacheable; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [Caching]).¶

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 206 (Partial Content) status code indicates that the server is successfully fulfilling a range request for the target resource by transferring one or more parts of the selected representation.¶

When a 206 response is generated, the server MUST generate the following header fields, in addition to those required in the subsections below, if the field would have been sent in a 200 (OK) response to the same request: Date, Cache-Control, ETag, Expires, Content-Location, and Vary.¶

If a 206 is generated in response to a request with an If-Range header field, the sender SHOULD NOT generate other representation header fields beyond those required, because the client is understood to already have a prior response containing those header fields. Otherwise, the sender MUST generate all of the representation header fields that would have been sent in a 200 (OK) response to the same request.¶

A 206 response is heuristically cacheable; i.e., unless otherwise indicated by explicit cache controls (see Section 4.2.2 of [Caching]).¶

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 6.4).¶

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 heuristically cacheable; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [Caching]).¶

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).¶

A 301 response is heuristically cacheable; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [Caching]).¶

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).¶

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 304 (Not Modified) status code indicates that a conditional GET or HEAD request has been received and would have resulted in a 200 (OK) response if it were not for the fact that the condition evaluated to false. In other words, there is no need for the server to transfer a representation of the target resource because the request indicates that the client, which made the request conditional, already has a valid representation; the server is therefore redirecting the client to make use of that stored representation as if it were the payload of a 200 (OK) response.¶

The server generating a 304 response MUST generate any of the following header fields that would have been sent in a 200 (OK) response to the same request: Cache-Control, Content-Location, Date, ETag, Expires, and Vary.¶

Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations, a sender SHOULD NOT generate representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding cache updates (e.g., Last-Modified might be useful if the response does not have an ETag field).¶

Requirements on a cache that receives a 304 response are defined in Section 4.3.4 of [Caching]. If the conditional request originated with an outbound client, such as a user agent with its own cache sending a conditional GET to a shared proxy, then the proxy SHOULD forward the 304 response to that client.¶

A 304 response cannot contain a message-body; it is always terminated by the first empty line after the header fields.¶

The 305 (Use Proxy) status code was defined in a previous version of this specification and is now deprecated (Appendix B of [RFC7231]).¶

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).¶

The 308 (Permanent Redirect) 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).¶

A 308 response is heuristically cacheable; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [Caching]).¶

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 401 (Unauthorized) status code indicates that the request has not been applied because it lacks valid authentication credentials for the target resource. The server generating a 401 response MUST send a WWW-Authenticate header field (Section 10.3.1) containing at least one challenge applicable to the target resource.¶

If the request included authentication credentials, then the 401 response indicates that authorization has been refused for those credentials. The user agent MAY repeat the request with a new or replaced Authorization header field (Section 8.5.3). If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user agent SHOULD present the enclosed representation to the user, since it usually contains relevant diagnostic information.¶

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 fulfill 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 heuristically cacheable; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [Caching]).¶

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 heuristically cacheable; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [Caching]).¶

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 8.4), 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 9.4.1.¶

The 407 (Proxy Authentication Required) status code is similar to 401 (Unauthorized), but it indicates that the client needs to authenticate itself in order to use a proxy. The proxy MUST send a Proxy-Authenticate header field (Section 10.3.2) containing a challenge applicable to that proxy for the target resource. The client MAY repeat the request with a new or replaced Proxy-Authorization header field (Section 8.5.4).¶

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 9.1 of [Messaging]) 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 heuristically cacheable; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [Caching]).¶

The 411 (Length Required) status code indicates that the server refuses to accept the request without a defined Content-Length (Section 6.2.4). 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 412 (Precondition Failed) status code indicates that one or more conditions given in the request header fields evaluated to false when tested on the server. This response status code allows the client to place preconditions on the current resource state (its current representations and metadata) and, thus, prevent the request method from being applied if the target resource is in an unexpected state.¶

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 3.2 of [Messaging]) 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 heuristically cacheable; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [Caching]).¶

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 416 (Range Not Satisfiable) status code indicates that none of the ranges in the request's Range header field (Section 8.3) overlap the current extent of the selected representation or that the set of ranges requested has been rejected due to invalid ranges or an excessive request of small or overlapping ranges.¶

For byte ranges, failing to overlap the current extent means that the first-pos of all of the range-spec values were greater than or equal to the current length of the selected representation. When this status code is generated in response to a byte-range request, the sender SHOULD generate a Content-Range header field specifying the current length of the selected representation (Section 6.3.4).¶

HTTP/1.1 416 Range Not Satisfiable
Date: Fri, 20 Jan 2012 15:41:54 GMT
Content-Range: bytes */47022

The 417 (Expectation Failed) status code indicates that the expectation given in the request's Expect header field (Section 8.1.1) could not be met by at least one of the inbound servers.¶

[RFC2324] was an April 1 RFC that lampooned the various ways HTTP was abused; one such abuse was the definition of an application-specific 418 status code. In the intervening years, this status code has been widely implemented as an "Easter Egg", and therefore is effectively consumed by this use.¶

Therefore, the 418 status code is reserved in the IANA HTTP Status Code Registry. This indicates that the status code cannot be assigned to other applications currently. If future circumstances require its use (e.g., exhaustion of 4NN status codes), it can be re-assigned to another use.¶

The 422 (Unprocessable Payload) status code indicates that the server understands the content type of the request payload (hence a 415 (Unsupported Media Type) status code is inappropriate), and the syntax of the request payload is correct, but was unable to process the contained instructions. For example, this status code can be sent if an XML request payload contains well-formed (i.e., syntactically correct), but semantically erroneous XML instructions.¶

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 9.9 of [Messaging]).¶

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 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 heuristically cacheable; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [Caching]).¶

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 10.1.3) to suggest an appropriate amount of time for the client to wait before retrying the request.¶

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 3.5, 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 "Hypertext Transfer Protocol (HTTP) Status Code Registry", maintained by IANA at <https://www.iana.org/assignments/http-status-codes>, registers status code numbers.¶

• Status Code (3 digits)
• Short Description
• Pointer to specification text

Values to be added to the HTTP status code namespace require IETF Review (see [RFC8126], Section 4.8).¶

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 9. 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 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 [Caching] 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 6.3.2).¶

The "Location" header field is used in some responses to refer to a specific resource in relation to the response. The type of relationship is defined by the combination of request method and status code semantics.¶

  Location = URI-reference

The field value consists of a single URI-reference. When it has the form of a relative reference ([RFC3986], Section 4.2), the final value is computed by resolving it against the effective request URI ([RFC3986], Section 5).¶

For 201 (Created) responses, the Location value refers to the primary resource created by the request. For 3xx (Redirection) responses, the Location value refers to the preferred target resource for automatically redirecting the request.¶

If the Location value provided in a 3xx (Redirection) response does not have a fragment component, a user agent MUST process the redirection as if the value inherits the fragment component of the URI reference used to generate the request target (i.e., the redirection inherits the original reference's fragment, if any).¶

  Location: /People.html#tim

which suggests that the user agent redirect to "http://www.example.org/People.html#tim"¶

  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.¶

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 list of field 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.¶

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 [Caching]). 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 8.4) 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 8.5.3). Likewise, an origin server might use Cache-Control response directives (Section 5.2 of [Caching]) to supplant Vary if it considers the variance less significant than the performance cost of Vary's impact on caching.¶

Validators come in two flavors: strong or weak. Weak validators are easy to generate but are far less useful for comparisons. Strong validators are ideal for comparisons but can be very difficult (and occasionally impossible) to generate efficiently. Rather than impose that all forms of resource adhere to the same strength of validator, HTTP exposes the type of validator in use and imposes restrictions on when weak validators can be used as preconditions.¶

A "strong validator" is representation metadata that changes value whenever a change occurs to the representation data that would be observable in the payload body of a 200 (OK) response to GET.¶

A strong validator might change for reasons other than a change to the representation data, such as when a semantically significant part of the representation metadata is changed (e.g., Content-Type), but it is in the best interests of the origin server to only change the value when it is necessary to invalidate the stored responses held by remote caches and authoring tools.¶

Cache entries might persist for arbitrarily long periods, regardless of expiration times. Thus, a cache might attempt to validate an entry using a validator that it obtained in the distant past. A strong validator is unique across all versions of all representations associated with a particular resource over time. However, there is no implication of uniqueness across representations of different resources (i.e., the same strong validator might be in use for representations of multiple resources at the same time and does not imply that those representations are equivalent).¶

There are a variety of strong validators used in practice. The best are based on strict revision control, wherein each change to a representation always results in a unique node name and revision identifier being assigned before the representation is made accessible to GET. A collision-resistant hash function applied to the representation data is also sufficient if the data is available prior to the response header fields being sent and the digest does not need to be recalculated every time a validation request is received. However, if a resource has distinct representations that differ only in their metadata, such as might occur with content negotiation over media types that happen to share the same data format, then the origin server needs to incorporate additional information in the validator to distinguish those representations.¶

In contrast, a "weak validator" is representation metadata that might not change for every change to the representation data. This weakness might be due to limitations in how the value is calculated, such as clock resolution, an inability to ensure uniqueness for all possible representations of the resource, or a desire of the resource owner to group representations by some self-determined set of equivalency rather than unique sequences of data. An origin server SHOULD change a weak entity-tag whenever it considers prior representations to be unacceptable as a substitute for the current representation. In other words, a weak entity-tag ought to change whenever the origin server wants caches to invalidate old responses.¶

For example, the representation of a weather report that changes in content every second, based on dynamic measurements, might be grouped into sets of equivalent representations (from the origin server's perspective) with the same weak validator in order to allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server load or weather quality). Likewise, a representation's modification time, if defined with only one-second resolution, might be a weak validator if it is possible for the representation to be modified twice during a single second and retrieved between those modifications.¶

Likewise, a validator is weak if it is shared by two or more representations of a given resource at the same time, unless those representations have identical representation data. For example, if the origin server sends the same validator for a representation with a gzip content coding applied as it does for a representation with no content coding, then that validator is weak. However, two simultaneous representations might share the same strong validator if they differ only in the representation metadata, such as when two different media types are available for the same representation data.¶

Strong validators are usable for all conditional requests, including cache validation, partial content ranges, and "lost update" avoidance. Weak validators are only usable when the client does not require exact equality with previously obtained representation data, such as when validating a cache entry or limiting a web traversal to recent changes.¶

The "Last-Modified" header field in a response provides a timestamp indicating the date and time at which the origin server believes the selected representation was last modified, as determined at the conclusion of handling the request.¶

  Last-Modified = HTTP-date

An example of its use is¶

  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT

The "ETag" field in a response provides the current entity-tag for the selected representation, as determined at the conclusion of handling the request. An entity-tag is an opaque validator for differentiating between multiple representations of the same resource, regardless of whether those multiple representations are due to resource state changes over time, content negotiation resulting in multiple representations being valid at the same time, or both. An entity-tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.¶

  ETag       = entity-tag

  entity-tag = [ weak ] opaque-tag
  weak       = %s"W/"
  opaque-tag = DQUOTE *etagc DQUOTE
  etagc      = %x21 / %x23-7E / obs-text
             ; VCHAR except double quotes, plus obs-text

An entity-tag can be more reliable for validation than a modification date in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are not consistently maintained.¶

  ETag: "xyzzy"
  ETag: W/"xyzzy"
  ETag: ""

An entity-tag can be either a weak or strong validator, with strong being the default. If an origin server provides an entity-tag for a representation and the generation of that entity-tag does not satisfy all of the characteristics of a strong validator (Section 10.2.1), then the origin server MUST mark the entity-tag as weak by prefixing its opaque value with "W/" (case-sensitive).¶

A sender MAY send the Etag field in a trailer section (see Section 4.6). However, since trailers are often ignored, it is preferable to send Etag as a header field unless the entity-tag is generated while sending the message body.¶

• SHOULD send an entity-tag validator unless it is not feasible to generate one.
• MAY send a weak entity-tag instead of a strong entity-tag, if performance considerations support the use of weak entity-tags, or if it is unfeasible to send a strong entity-tag.
• SHOULD send a Last-Modified value if it is feasible to send one.

In other words, the preferred behavior for an origin server is to send both a strong entity-tag and a Last-Modified value in successful responses to a retrieval request.¶

• MUST send that entity-tag in any cache validation request (using If-Match or If-None-Match) if an entity-tag has been provided by the origin server.
• SHOULD send the Last-Modified value in non-subrange cache validation requests (using If-Modified-Since) if only a Last-Modified value has been provided by the origin server.
• MAY send the Last-Modified value in subrange cache validation requests (using If-Unmodified-Since) if only a Last-Modified value has been provided by an HTTP/1.0 origin server. The user agent SHOULD provide a way to disable this, in case of difficulty.
• SHOULD send both validators in cache validation requests if both an entity-tag and a Last-Modified value have been provided by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.

The "WWW-Authenticate" header field indicates the authentication scheme(s) and parameters applicable to the target resource.¶

  WWW-Authenticate = 1#challenge

A server generating a 401 (Unauthorized) response MUST send a WWW-Authenticate header field containing at least one challenge. A server MAY generate a WWW-Authenticate header field in other response messages to indicate that supplying credentials (or different credentials) might affect the response.¶

A proxy forwarding a response MUST NOT modify any WWW-Authenticate fields in that response.¶

User agents are advised to take special care in parsing the field value, as it might contain more than one challenge, and each challenge can contain a comma-separated list of authentication parameters. Furthermore, the header field itself can occur multiple times.¶

  WWW-Authenticate: Newauth realm="apps", type=1,
                    title="Login to \"apps\"", Basic realm="simple"

This header field contains two challenges; one for the "Newauth" scheme with a realm value of "apps", and two additional parameters "type" and "title", and another one for the "Basic" scheme with a realm value of "simple".¶

The "Proxy-Authenticate" header field consists of at least one challenge that indicates the authentication scheme(s) and parameters applicable to the proxy for this effective request URI (Section 5.5). A proxy MUST send at least one Proxy-Authenticate header field in each 407 (Proxy Authentication Required) response that it generates.¶

  Proxy-Authenticate = 1#challenge

Unlike WWW-Authenticate, the Proxy-Authenticate header field applies only to the next outbound client on the response chain. This is because only the client that chose a given proxy is likely to have the credentials necessary for authentication. However, when multiple proxies are used within the same administrative domain, such as office and regional caching proxies within a large corporate network, it is common for credentials to be generated by the user agent and passed through the hierarchy until consumed. Hence, in such a configuration, it will appear as if Proxy-Authenticate is being forwarded because each proxy will send the same challenge set.¶

Note that the parsing considerations for WWW-Authenticate apply to this header field as well; see Section 10.3.1 for details.¶

HTTP authentication schemes can use the Authentication-Info response header field to communicate information after the client's authentication credentials have been accepted. This information can include a finalization message from the server (e.g., it can contain the server authentication).¶

The field value is a list of parameters (name/value pairs), using the "auth-param" syntax defined in Section 8.5.1. This specification only describes the generic format; authentication schemes using Authentication-Info will define the individual parameters. The "Digest" Authentication Scheme, for instance, defines multiple parameters in Section 3.5 of [RFC7616].¶

  Authentication-Info = #auth-param

The Authentication-Info header field can be used in any HTTP response, independently of request method and status code. Its semantics are defined by the authentication scheme indicated by the Authorization header field (Section 8.5.3) of the corresponding request.¶

A proxy forwarding a response is not allowed to modify the field value in any way.¶

Authentication-Info can be used inside trailers (Section 7.1.2 of [Messaging]) when the authentication scheme explicitly allows this.¶

  Proxy-Authentication-Info = #auth-param

However, unlike Authentication-Info, the Proxy-Authentication-Info header field applies only to the next outbound client on the response chain. This is because only the client that chose a given proxy is likely to have the credentials necessary for authentication. However, when multiple proxies are used within the same administrative domain, such as office and regional caching proxies within a large corporate network, it is common for credentials to be generated by the user agent and passed through the hierarchy until consumed. Hence, in such a configuration, it will appear as if Proxy-Authentication-Info is being forwarded because each proxy will send the same field value.¶

The "Accept-Ranges" header field allows a server to indicate that it supports range requests for the target resource.¶

  Accept-Ranges     = acceptable-ranges
  acceptable-ranges = 1#range-unit / "none"

An origin server that supports byte-range requests for a given target resource MAY send¶

  Accept-Ranges: bytes

to indicate what range units are supported. A client MAY generate range requests without having received this header field for the resource involved. Range units are defined in Section 6.1.4.¶

A server that does not support any kind of range request for the target resource MAY send¶

  Accept-Ranges: none

to advise the client not to attempt a range request.¶

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

  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 4.4.1.3), 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 8.6.3.¶

  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 HTTP authentication framework does not define a single mechanism for maintaining the confidentiality of credentials; instead, each authentication scheme defines how the credentials are encoded prior to transmission. While this provides flexibility for the development of future authentication schemes, it is inadequate for the protection of existing schemes that provide no confidentiality on their own, or that do not sufficiently protect against replay attacks. Furthermore, if the server expects credentials that are specific to each individual user, the exchange of those credentials will have the effect of identifying that user even if the content within credentials remains confidential.¶

HTTP depends on the security properties of the underlying transport- or session-level connection to provide confidential transmission of fields. In other words, if a server limits access to authenticated users using this framework, the server needs to ensure that the connection is properly secured in accordance with the nature of the authentication scheme used. For example, services that depend on individual user authentication often require a connection to be secured with TLS ("Transport Layer Security", [RFC8446]) prior to exchanging any credentials.¶

Existing HTTP clients and user agents typically retain authentication information indefinitely. HTTP does not provide a mechanism for the origin server to direct clients to discard these cached credentials, since the protocol has no awareness of how credentials are obtained or managed by the user agent. The mechanisms for expiring or revoking credentials can be specified as part of an authentication scheme definition.¶

• Clients that have been idle for an extended period, following which the server might wish to cause the client to re-prompt the user for credentials.
• Applications that include a session termination indication (such as a "logout" or "commit" button on a page) after which the server side of the application "knows" that there is no further reason for the client to retain the credentials.

User agents that cache credentials are encouraged to provide a readily accessible mechanism for discarding cached credentials under user control.¶

Authentication schemes that solely rely on the "realm" mechanism for establishing a protection space will expose credentials to all resources on an origin server. Clients that have successfully made authenticated requests with a resource can use the same authentication credentials for other resources on the same origin server. This makes it possible for a different resource to harvest authentication credentials for other resources.¶

This is of particular concern when an origin server hosts resources for multiple parties under the same canonical root URI (Section 8.5.2). Possible mitigation strategies include restricting direct access to authentication credentials (i.e., not making the content of the Authorization request header field available), and separating protection spaces by using a different host name (or port number) for each party.¶

Adding information to responses that are sent over an unencrypted channel can affect security and privacy. The presence of the Authentication-Info and Proxy-Authentication-Info header fields alone indicates that HTTP authentication is in use. Additional information could be exposed by the contents of the authentication-scheme specific parameters; this will have to be considered in the definitions of these schemes.¶