HTTP Semantics

The "http" URI scheme is hereby defined for minting identifiers within the hierarchical namespace governed by a potential HTTP origin server listening for TCP ([TCP]) 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 ([URI], Section 3.2.2) and optional port number ([URI], Section 3.2.3). 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 4.3.2.

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 namespace.

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 ([TLS13]) 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 confidentiality and integrity protection that is acceptable to both client and server.

  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 ([URI], Section 3.2.2) and optional port number ([URI], Section 3.2.3). 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 4.3.3.

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 namespace.

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. Note that the definition of what cryptographic mechanisms are acceptable to client and server are usually negotiated and can change over time.

Resources made available via the "https" scheme have no shared identity with the "http" scheme. They are distinct origins with separate namespaces. However, extensions to HTTP that are defined as applying to all origins with the same host, such as the Cookie protocol [COOKIE], allow information set by one service to impact communication with other services within a matching group of host domains. Such extensions ought to be designed with great care to prevent information obtained from a secured connection being inadvertently exchanged within an unsecured context.

URIs with an "http" or "https" scheme are normalized and compared according to the methods defined in Section 6 of [URI], using the defaults described above for each scheme.

HTTP does not require the use of a specific method for determining equivalence. For example, a cache key might be compared as a simple string, after syntax-based normalization, or after scheme-based normalization.

• 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 as the 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 [URI]).

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

Two HTTP URIs that are equivalent after normalization (using any method) can be assumed to identify the same resource, and any HTTP component MAY perform normalization. As a result, distinct resources SHOULD NOT be identified by HTTP URIs that are equivalent after normalization (using any method defined in Section 6.2 of [URI]).

The URI generic syntax for authority also includes a userinfo subcomponent ([URI], 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 target URI 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 [URI]. 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.

The origin for a given URI is the triple of scheme, host, and port after normalizing the scheme and host to lowercase and normalizing the port to remove any leading zeros. If port is elided from the URI, the default port for that scheme is used. For example, the URI

   https://Example.Com/happy.js

would have the origin

   { "https", "example.com", "443" }

   https://example.com:443

Each origin defines its own namespace and controls how identifiers within that namespace are mapped to resources. In turn, how the origin responds to valid requests, consistently over time, determines the semantics that users will associate with a URI, and the usefulness of those semantics is what ultimately transforms these mechanisms into a resource for users to reference and access in the future.

Two origins are distinct if they differ in scheme, host, or port. Even when it can be verified that the same entity controls two distinct origins, the two namespaces under those origins are distinct unless explicitly aliased by a server authoritative for that origin.

Origin is also used within HTML and related Web protocols, beyond the scope of this document, as described in [RFC6454].

Although HTTP is independent of the transport protocol, the "http" scheme (Section 4.2.1) 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 an on-path attacker. 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 over that connection an HTTP request message containing a request target that matches the client's target URI (Section 7.1).

If the server responds to such a request with a non-interim HTTP response message, as described in Section 15, 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 [ALTSVC] 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.

The "https" scheme (Section 4.2.2) associates authority based on the ability of a server to use the private key corresponding to a certificate that the client considers to be trustworthy for the identified origin server. The client usually relies upon a chain of trust, conveyed from some prearranged or configured trust anchor, to deem a certificate trustworthy (Section 4.3.4).

In HTTP/1.1 and earlier, a client will only attribute authority to a server when they are communicating over a successfully established and secured connection specifically to that URI origin's host. The connection establishment and certificate verification are used as proof of authority.

In HTTP/2 and HTTP/3, a client will attribute authority to a server when they are communicating over a successfully established and secured connection if the URI origin's host matches any of the hosts present in the server's certificate and the client believes that it could open a connection to that host for that URI. In practice, a client will make a DNS query to check that the origin's host contains the same server IP address as the established connection. This restriction can be removed by the origin server sending an equivalent ORIGIN frame [RFC8336].

The request target's host and port value are passed within each HTTP request, identifying the origin and distinguishing it from other namespaces that might be controlled by the same server (Section 7.2). 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 (Section 7.4).

An origin server might be unwilling to process requests for certain target URIs even when they have the authority to do so. For example, when a host operates distinct services on different ports (e.g., 443 and 8000), checking the target URI at the origin server is necessary (even after the connection has been secured) because a network attacker might cause connections for one port to be received at some other port. Failing to check the target URI might allow such an attacker to replace a response to one target URI (e.g., "https://example.com/foo") with a seemingly authoritative response from the other port (e.g., "https://example.com:8000/foo").

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 4.2.2, including protocols that don't use TCP.

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 over that connection an HTTP request message containing a request target that matches the client's target URI (Section 7.1).

If the server responds to such a request with a non-interim HTTP response message, as described in Section 15, 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]).

To establish a secured connection to dereference a URI, a client MUST verify that the service's identity is an acceptable match for the URI's origin server. Certificate verification is used to prevent server impersonation by an on-path attacker or by an attacker that controls name resolution. This process requires that a client be configured with a set of trust anchors.

In general, a client MUST verify the service identity using the verification process defined in Section 6 of [RFC6125]. The client MUST construct a reference identity from the service's host: if the host is a literal IP address (Section 4.3.5), the reference identity is an IP-ID, otherwise the host is a name and the reference identity is a DNS-ID.

A reference identity of type CN-ID MUST NOT be used by clients. As noted in Section 6.2.1 of [RFC6125], a reference identity of type CN-ID might be used by older clients.

A client might be specially configured to accept an alternative form of server identity verification. For example, a client might be connecting to a server whose address and hostname are dynamic, with an expectation that the service will present a specific certificate (or a certificate matching some externally defined reference identity) rather than one matching the target URI's origin.

In special cases, it might be appropriate for a client to simply ignore the server's identity, but it must be understood that this leaves a connection open to active attack.

If the certificate is not valid for the target URI's origin, a user agent MUST either obtain confirmation from the user before proceeding (see Section 3.5) or terminate the connection with a bad certificate error. Automated clients MUST log the error to an appropriate audit log (if available) and SHOULD terminate the connection (with a bad certificate error). Automated clients MAY provide a configuration setting that disables this check, but MUST provide a setting which enables it.

A server that is identified using an IP address literal in the "host" field of an "https" URI has a reference identity of type IP-ID. An IP version 4 address uses the "IPv4address" ABNF rule, and an IP version 6 address uses the "IP-literal" production with the "IPv6address" option; see Section 3.2.2 of [URI]. A reference identity of IP-ID contains the decoded bytes of the IP address.

An IP version 4 address is 4 octets, and an IP version 6 address is 16 octets. Use of IP-ID is not defined for any other IP version. The iPAddress choice in the certificate subjectAltName extension does not explicitly include the IP version and so relies on the length of the address to distinguish versions; see Section 4.2.1.6 of [RFC5280].

A reference identity of type IP-ID matches if the address is identical to an iPAddress value of the subjectAltName extension of the certificate.

A #rule extension to the ABNF rules of [RFC5234] is used to improve readability in the definitions of some list-based field values.

A construct "#" is defined, similar to "*", for defining comma-delimited lists of elements. The full form is "<n>#<m>element" indicating at least <n> and at most <m> elements, each separated by a single comma (",") and optional whitespace (OWS, defined in Section 5.6.3).

Tokens are short textual identifiers that do not include whitespace or delimiters.

  token          = 1*tchar

  tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
                 / DIGIT / ALPHA
                 ; any VCHAR, except delimiters

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 "(),/:;<=>?@[\]{}").

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 overwrite 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

A string of text is parsed as a single value if it is quoted using double-quote marks.

  quoted-string  = DQUOTE *( qdtext / quoted-pair ) DQUOTE
  qdtext         = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text

The backslash octet ("\") can be used as a single-octet quoting mechanism within quoted-string and comment constructs. Recipients that process the value of a quoted-string MUST handle a quoted-pair as if it were replaced by the octet following the backslash.

  quoted-pair    = "\" ( HTAB / SP / VCHAR / obs-text )

A sender SHOULD NOT generate a quoted-pair in a quoted-string except where necessary to quote DQUOTE and backslash octets occurring within that string. A sender SHOULD NOT generate a quoted-pair in a comment except where necessary to quote parentheses ["(" and ")"] and backslash octets occurring within that comment.

Comments can be included in some HTTP fields by surrounding the comment text with parentheses. Comments are only allowed in fields containing "comment" as part of their field value definition.

  comment        = "(" *( ctext / quoted-pair / comment ) ")"
  ctext          = HTAB / SP / %x21-27 / %x2A-5B / %x5D-7E / obs-text

Parameters are instances of name/value pairs; they are often used in field values as a common syntax for appending auxiliary information to an item. Each parameter is usually delimited by an immediately preceding semicolon.

  parameters      = *( OWS ";" OWS [ parameter ] )
  parameter       = parameter-name "=" parameter-value
  parameter-name  = token
  parameter-value = ( token / quoted-string )

Parameter names are case-insensitive. Parameter values might or might not be case-sensitive, depending on the semantics of the parameter name. Examples of parameters and some equivalent forms can be seen in media types (Section 8.3.1) and the Accept header field (Section 12.5.1).

A parameter value that matches the token production can be transmitted either as a token or within a quoted-string. The quoted and unquoted values are equivalent.

Prior to 1995, there were three different formats commonly used by servers to communicate timestamps. For compatibility with old implementations, all three are defined here. The preferred format is a fixed-length and single-zone subset of the date and time specification used by the Internet Message Format [RFC5322].

  HTTP-date    = IMF-fixdate / obs-date

An example of the preferred format is

Sun, 06 Nov 1994 08:49:37 GMT    ; IMF-fixdate

Examples of the two obsolete formats are

Sunday, 06-Nov-94 08:49:37 GMT   ; obsolete RFC 850 format
Sun Nov  6 08:49:37 1994         ; ANSI C's asctime() format

A recipient that parses a timestamp value in an HTTP field MUST accept all three HTTP-date formats. When a sender generates a field that contains one or more timestamps defined as HTTP-date, the sender MUST generate those timestamps in the IMF-fixdate format.

An HTTP-date value represents time as an instance of Coordinated Universal Time (UTC). The first two formats indicate UTC by the three-letter abbreviation for Greenwich Mean Time, "GMT", a predecessor of the UTC name; values in the asctime format are assumed to be in UTC.

A clock is an implementation capable of providing a reasonable approximation of the current instant in UTC. A clock implementation ought to use NTP ([RFC5905]), or some similar protocol, to synchronize with UTC.

  IMF-fixdate  = day-name "," SP date1 SP time-of-day SP GMT
  ; fixed length/zone/capitalization subset of the format
  ; see Section 3.3 of [RFC5322]

  day-name     = %s"Mon" / %s"Tue" / %s"Wed"
               / %s"Thu" / %s"Fri" / %s"Sat" / %s"Sun"

  date1        = day SP month SP year
               ; e.g., 02 Jun 1982

  day          = 2DIGIT
  month        = %s"Jan" / %s"Feb" / %s"Mar" / %s"Apr"
               / %s"May" / %s"Jun" / %s"Jul" / %s"Aug"
               / %s"Sep" / %s"Oct" / %s"Nov" / %s"Dec"
  year         = 4DIGIT

  GMT          = %s"GMT"

  time-of-day  = hour ":" minute ":" second
               ; 00:00:00 - 23:59:60 (leap second)

  hour         = 2DIGIT
  minute       = 2DIGIT
  second       = 2DIGIT

  obs-date     = rfc850-date / asctime-date

  rfc850-date  = day-name-l "," SP date2 SP time-of-day SP GMT
  date2        = day "-" month "-" 2DIGIT
               ; e.g., 02-Jun-82

  day-name-l   = %s"Monday" / %s"Tuesday" / %s"Wednesday"
               / %s"Thursday" / %s"Friday" / %s"Saturday"
               / %s"Sunday"

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

HTTP-date is case sensitive. Note that Section 4.2 of [CACHING] relaxes this for cache recipients.

A sender MUST NOT generate additional whitespace in an HTTP-date beyond that specifically included as SP in the grammar. The semantics of day-name, day, month, year, and time-of-day are the same as those defined for the Internet Message Format constructs with the corresponding name ([RFC5322], Section 3.3).

Recipients of a timestamp value in rfc850-date format, which uses a two-digit year, MUST interpret a timestamp that appears to be more than 50 years in the future as representing the most recent year in the past that had the same last two digits.

Recipients of timestamp values are encouraged to be robust in parsing timestamps unless otherwise restricted by the field definition. For example, messages are occasionally forwarded over HTTP from a non-HTTP source that might generate any of the date and time specifications defined by the Internet Message Format.

The purpose of content in a request is defined by the method semantics (Section 9).

For example, a representation in the content of a PUT request (Section 9.3.4) represents the desired state of the target resource after the request is successfully applied, whereas a representation in the content of a POST request (Section 9.3.3) represents information to be processed by the target resource.

In a response, the content's purpose is defined by the request method, response status code (Section 15), and response fields describing that content. For example, the content of a 200 (OK) response to GET (Section 9.3.1) represents the current state of the target resource, as observed at the time of the message origination date (Section 6.6.1), whereas the content 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.

The content of a 206 (Partial Content) response to GET contains either a single part of the selected representation or a multipart message body containing multiple parts of that representation, as described in Section 15.3.7.

Response messages with an error status code usually contain content that represents the error condition, such that the content describes the error state and what steps are suggested for resolving it.

Responses to the HEAD request method (Section 9.3.2) never include content; the associated response header fields indicate only what their values would have been if the request method had been GET (Section 9.3.1).

2xx (Successful) responses to a CONNECT request method (Section 9.3.6) switch the connection to tunnel mode instead of having content.

All 1xx (Informational), 204 (No Content), and 304 (Not Modified) responses do not include content.

All other responses do include content, although that content might be of zero length.

When a complete or partial representation is transferred as message content, it is often desirable for the sender to supply, or the recipient to determine, an identifier for a resource corresponding to that specific representation. For example, a client making a GET request on a resource for "the current weather report" might want an identifier specific to the content returned (e.g., "weather report for Laguna Beach at 20210720T1711"). This can be useful for sharing or bookmarking content from resources that are expected to have changing representations over time.

• If the request has a Content-Location header field, then the sender asserts that the content 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 content is unidentified by HTTP, but a more specific identifier might be supplied within the content itself.

1. If the request method is HEAD or the response status code is 204 (No Content) or 304 (Not Modified), there is no content in the response.
2. If the request method is GET and the response status code is 200 (OK), the content is a representation of the target resource (Section 7.1).
3. If the request method is GET and the response status code is 203 (Non-Authoritative Information), the content is a potentially modified or enhanced representation of the target resource as provided by an intermediary.
4. If the request method is GET and the response status code is 206 (Partial Content), the content is one or more parts of a representation of the target resource.
5. If the response has a Content-Location header field and its field value is a reference to the same URI as the target URI, the content is a representation of the target resource.
6. If the response has a Content-Location header field and its field value is a reference to a URI different from the target URI, then the sender asserts that the content 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).
7. Otherwise, the content is unidentified by HTTP, but a more specific identifier might be supplied within the content itself.

A trailer section is only possible when supported by the version of HTTP in use and enabled by an explicit framing mechanism. For example, the chunked transfer coding in HTTP/1.1 allows a trailer section to be sent after the content (Section 7.1.2 of [HTTP/1.1]).

Many fields cannot be processed outside the header section because their evaluation is necessary prior to receiving the content, such as those that describe message framing, routing, authentication, request modifiers, response controls, or content 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.

The presence of the keyword "trailers" in the TE header field (Section 10.1.4) of a request indicates that the client is willing to accept trailer fields, on behalf of itself and any downstream clients. For requests from an intermediary, this implies that all downstream clients are willing to accept trailer fields in the forwarded response. Note that the presence of "trailers" does not mean that the client(s) will process any particular trailer field in the response; only that the trailer section(s) will not be dropped by any of the clients.

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 (Section 6.6.2) can be sent to indicate fields likely to be sent in the trailer section, which allows recipients to prepare for their receipt before processing the content. For example, this could be useful if a field name indicates that a dynamic checksum should be calculated as the content is received and then immediately checked upon receipt of the trailer field value.

Like header fields, trailer fields with the same name are processed in the order received; multiple trailer field lines with the same name have the equivalent semantics as appending the multiple values as a list of members. Trailer fields that might be generated more than once during a message MUST be defined as a list-based field even if each member value is only processed once per field line received.

At the end of a message, a recipient MAY treat the set of received trailer fields as a data structure of name/value pairs, similar to (but separate from) the header fields. Additional processing expectations, if any, can be defined within the field specification for a field intended for use in trailers.

The "Date" header field represents the date and time at which the message was originated, having the same semantics as the Origination Date Field (orig-date) defined in Section 3.6.1 of [RFC5322]. The field value is an HTTP-date, as defined in Section 5.6.7.

  Date = HTTP-date

An example is

Date: Tue, 15 Nov 1994 08:12:31 GMT

A sender that generates a Date header field SHOULD generate its field value as the best available approximation of the date and time of message generation. In theory, the date ought to represent the moment just before generating the message content. In practice, a sender can generate the date value at any time during message origination.

An origin server with a clock (as defined in Section 5.6.7) MUST generate a Date header field in all 2xx (Successful), 3xx (Redirection), and 4xx (Client Error) responses, and MAY generate a Date header field in 1xx (Informational) and 5xx (Server Error) responses.

An origin server without a clock MUST NOT generate a Date header field.

A recipient with a clock that receives a response message without a Date header field MUST record the time it was received and append a corresponding Date header field to the message's header section if it is cached or forwarded downstream.

A recipient with a clock that receives a response with an invalid Date header field value MAY replace that value with the time that response was received.

A user agent MAY send a Date header field in a request, though generally will not do so unless it is believed to convey useful information to the server. For example, custom applications of HTTP might convey a Date if the server is expected to adjust its interpretation of the user's request based on differences between the user agent and server clocks.

The "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 content.

  Trailer = #field-name

For example, a sender might indicate that a signature will be computed as the content 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 it receives the content.

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.

If an intermediary discards the trailer section in transit, the Trailer field could provide a hint of what metadata was lost, though there is no guarantee that a sender of Trailer will always follow through by sending the named fields.

If the client has a cache [CACHING] and the request can be satisfied by it, then the request is usually directed there first.

If the request is not satisfied by a cache, then a typical client will check its configuration to determine whether a proxy is to be used to satisfy the request. Proxy configuration is implementation-dependent, but is often based on URI prefix matching, selective authority matching, or both, and the proxy itself is usually identified by an "http" or "https" URI.

If an "http" or "https" proxy is applicable, the client connects inbound by establishing (or reusing) a connection to that proxy and then sending it an HTTP request message containing a request target that matches the client's target URI.

If no proxy is applicable, a typical client will invoke a handler routine (specific to the target URI's scheme) to obtain access to the identified resource. How that is accomplished is dependent on the target URI scheme and defined by its associated specification.

Section 4.3.2 defines how to obtain access to an "http" resource by establishing (or reusing) an inbound connection to the identified origin server and then sending it an HTTP request message containing a request target that matches the client's target URI.

Section 4.3.3 defines how to obtain access to an "https" resource by establishing (or reusing) an inbound secured connection to an origin server that is authoritative for the identified origin and then sending it an HTTP request message containing a request target that matches the client's target URI.

The "Connection" header field allows the sender to list desired control options for the current connection.

  Connection        = #connection-option
  connection-option = token

Connection options are case-insensitive.

When a field aside from Connection is used to supply control information for or about the current connection, the sender MUST list the corresponding field name within the Connection header field. Note that some versions of HTTP prohibit the use of fields for such information, and therefore do not allow the Connection field.

Intermediaries MUST parse a received Connection header field before a message is forwarded and, for each connection-option in this field, remove any header or trailer field(s) from the message with the same name as the connection-option, and then remove the Connection header field itself (or replace it with the intermediary's own control options for the forwarded message).

Hence, the Connection header field provides a declarative way of distinguishing fields that are only intended for the immediate recipient ("hop-by-hop") from those fields that are intended for all recipients on the chain ("end-to-end"), enabling the message to be self-descriptive and allowing future connection-specific extensions to be deployed without fear that they will be blindly forwarded by older intermediaries.

• Proxy-Connection (Appendix C.2.2 of [HTTP/1.1])
• Keep-Alive (Section 19.7.1 of [RFC2068])
• TE (Section 10.1.4)
• Transfer-Encoding (Section 6.1 of [HTTP/1.1])
• Upgrade (Section 7.8)

A sender MUST NOT send a connection option corresponding to a field that is intended for all recipients of the content. For example, Cache-Control is never appropriate as a connection option (Section 5.2 of [CACHING]).

Connection options do not always correspond to a field present in the message, since a connection-specific field might not be needed if there are no parameters associated with a connection option. In contrast, a connection-specific field received without a corresponding connection option usually indicates that the field has been improperly forwarded by an intermediary and ought to be ignored by the recipient.

When defining a new connection option that does not correspond to a field, specification authors ought to reserve the corresponding field name anyway in order to avoid later collisions. Such reserved field names are registered in the "Hypertext Transfer Protocol (HTTP) Field Name Registry" (Section 16.3.1).

The "Max-Forwards" header field provides a mechanism with the TRACE (Section 9.3.8) and OPTIONS (Section 9.3.7) request methods to limit the number of times that the request is forwarded by proxies. This can be useful when the client is attempting to trace a request that appears to be failing or looping mid-chain.

  Max-Forwards = 1*DIGIT

The Max-Forwards value is a decimal integer indicating the remaining number of times this request message can be forwarded.

Each intermediary that receives a TRACE or OPTIONS request containing a Max-Forwards header field MUST check and update its value prior to forwarding the request. If the received value is zero (0), the intermediary MUST NOT forward the request; instead, the intermediary MUST respond as the final recipient. If the received Max-Forwards value is greater than zero, the intermediary MUST generate an updated Max-Forwards field in the forwarded message with a field value that is the lesser of a) the received value decremented by one (1) or b) the recipient's maximum supported value for Max-Forwards.

A recipient MAY ignore a Max-Forwards header field received with any other request methods.

The "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 = #( received-protocol RWS received-by [ RWS comment ] )

  received-protocol = [ protocol-name "/" ] protocol-version
                    ; see Section 7.8
  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 2.5. 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 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.

HTTP uses media types [RFC2046] in the Content-Type (Section 8.3) and Accept (Section 12.5.1) 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 the message context.

  media-type = type "/" subtype parameters
  type       = token
  subtype    = token

The type and subtype tokens are case-insensitive.

The type/subtype MAY be followed by semicolon-delimited parameters (Section 5.6.6) 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].

HTTP uses charset names to indicate or negotiate the character encoding scheme ([RFC6365], Section 2) of a textual representation. In the fields defined by this document, charset names appear either in parameters (Content-Type), or, for Accept-Encoding, in the form of a plain token. In both cases, charset names are matched case-insensitively.

Charset names ought to be registered in the IANA "Character Sets" registry (https://www.iana.org/assignments/character-sets) according to the procedures defined in Section 2 of [RFC2978].

MIME provides for a number of "multipart" types — encapsulations of one or more representations within a single message body. All multipart types share a common syntax, as defined in Section 5.1.1 of [RFC2046], and include a boundary parameter as part of the media type value. The message body is itself a protocol element; a sender MUST generate only CRLF to represent line breaks between body parts.

HTTP message framing does not use the multipart boundary as an indicator of message body length, though it might be used by implementations that generate or process the content. For example, the "multipart/form-data" type is often used for carrying form data in a request, as described in [RFC7578], and the "multipart/byteranges" type is defined by this specification for use in some 206 (Partial Content) responses (see Section 15.3.7).

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 described in Section 16.6

Content-coding values are used in the Accept-Encoding (Section 12.5.3) and Content-Encoding (Section 8.4) 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 12.5.4, 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.

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 content 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 (e.g., 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 8.8.1), 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 6.5). 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 content.

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 cause the server to fail. 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 target 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 can repeat a POST request automatically if it knows (through design or configuration) that the request is safe for that resource. 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 take a riskier approach and attempt to guess when an automatic retry is possible. For example, a client might automatically retry a POST request if the underlying transport connection closed before any part of a response is received, particularly if an idle persistent connection was used.

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 to detail under what conditions a response can be used to satisfy subsequent requests; a method definition that 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. A successful response reflects the quality of "sameness" identified by the target URI (Section 1.2.2 of [URI]). Hence, retrieving identifiable information via HTTP is usually performed by making a GET request on an identifier associated with the potential for providing that information in a 200 (OK) response.

GET is the primary mechanism of information retrieval and the focus of almost all performance optimizations. Applications that produce a URI for each important resource can benefit from those optimizations while enabling their reuse by other applications, creating a network effect that promotes further expansion of the Web.

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 17.3 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 resource identifier corresponds to an implementation and how that implementation manages to select and send a current representation of the target resource.

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 14.2).

Although request message framing is independent of the method used, content received in a GET request has no generally 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 [HTTP/1.1]). A client SHOULD NOT generate content in a GET request unless it is made directly to an origin server that has previously indicated, in or out of band, that such a request has a purpose and will be adequately supported. An origin server SHOULD NOT rely on private agreements to receive content, since participants in HTTP communication are often unaware of intermediaries along the request chain.

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]).

When information retrieval is performed with a mechanism that constructs a target URI from user-provided information, such as the query fields of a form using GET, potentially sensitive data might be provided that would not be appropriate for disclosure within a URI (see Section 17.9). In some cases, the data can be filtered or transformed such that it would not reveal such information. In others, particularly when there is no benefit from caching a response, using the POST method (Section 9.3.3) instead of GET can transmit such information in the request content rather than within the target URI.

The HEAD method is identical to GET except that the server MUST NOT send content in the response. HEAD is used to obtain metadata about the selected representation without transferring its representation data, often for the sake of testing hypertext links or finding recent modifications.

The server SHOULD send the same header fields in response to a HEAD request as it would have sent if the request method had been GET. However, a server MAY omit header fields for which a value is determined only while generating the content. For example, some servers buffer a dynamic response to GET until a minimum amount of data is generated so that they can more efficiently delimit small responses or make late decisions with regard to content selection. Such a response to GET might contain Content-Length and Vary fields, for example, that are not generated within a HEAD response. These minor inconsistencies are considered preferable to generating and discarding the content for a HEAD request, since HEAD is usually requested for the sake of efficiency.

Although request message framing is independent of the method used, content received in a HEAD request has no generally 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 [HTTP/1.1]). A client SHOULD NOT generate content in a HEAD request unless it is made directly to an origin server that has previously indicated, in or out of band, that such a request has a purpose and will be adequately supported. An origin server SHOULD NOT rely on private agreements to receive content, since participants in HTTP communication are often unaware of intermediaries along the request chain.

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 affect 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 could 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 10.2.2) 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 target URI (Section 8.7). A cached POST response can be reused to satisfy a later GET or HEAD request. In contrast, a POST request cannot be satisfied by a cached POST response because POST is potentially 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 content. 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 verify that the PUT representation is consistent with its configured constraints for the target resource. For example, if an origin server determines a resource's representation metadata based on the URI, then the origin server needs to ensure that the content received in a successful PUT request is consistent with that metadata. 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.

This extends to how header and trailer fields are stored; while common header fields like Content-Type will typically be stored and returned upon subsequent GET requests, header and trailer field handling is specific to the resource that received the request. As a result, an origin server SHOULD ignore unrecognized header and trailer fields received in a PUT request (i.e., not save them as part of the resource state).

An origin server MUST NOT send a validator field (Section 8.8), 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 content (i.e., the resource's new representation data is identical to the content 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 it sent (and retains in memory) is the result of the PUT, and thus it doesn't need to be retrieved again from the origin server. The new validator(s) received in the response can be used for future conditional requests in order to prevent accidental overwrites (Section 13.1).

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.

Some origin servers support use of the Content-Range header field (Section 14.4) as a request modifier to perform a partial PUT, as described in Section 14.5.

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 target 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.

Although request message framing is independent of the method used, content received in a DELETE request has no generally 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 [HTTP/1.1]). A client SHOULD NOT generate content in a DELETE request unless it is made directly to an origin server that has previously indicated, in or out of band, that such a request has a purpose and will be adequately supported. An origin server SHOULD NOT rely on private agreements to receive content, since participants in HTTP communication are often unaware of intermediaries along the request chain.

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 target 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 data, 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, [TLS13]).

CONNECT uses a special form of request target, unique to this method, consisting of only the host and port number of the tunnel destination, separated by a colon. There is no default port; a client MUST send the port number even if the CONNECT request is based on a URI reference that contains an authority component with an elided port (Section 4.1). For example,

CONNECT server.example.com:80 HTTP/1.1
Host: server.example.com

A server MUST reject a CONNECT request that targets an empty or invalid port number, typically by responding with a 400 (Bad Request) status code.

Because CONNECT changes the request/response nature of an HTTP connection, specific HTTP versions might have different ways of mapping its semantics into the protocol's wire format.

CONNECT is intended for use in requests to a proxy. The recipient can establish a tunnel either by directly connecting to the server identified by the request target or, if configured to use another proxy, by forwarding the CONNECT request to the next inbound proxy. An origin server MAY accept a CONNECT request, but most origin servers do not implement CONNECT.

Any 2xx (Successful) response indicates that the sender (and all inbound proxies) will switch to tunnel mode immediately after the response header section; data received after that header section 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.

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:443 HTTP/1.1
Host: server.example.com:443
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 "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 list 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 CONNECT request message does not have content. The interpretation of data sent after the header section of the CONNECT request message is specific to the version of HTTP in use.

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 7.1) 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 content, 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 7.6.2). 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 content MUST send a valid Content-Type header field describing the representation media type. Note that this specification does not define any use for such content.

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 content of a 200 (OK) response. The "message/http" format (Section 10.1 of [HTTP/1.1]) is one way to do so. 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 7.6.2).

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 11) or cookies [COOKIE] 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 content.

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 7.6.3) 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 content in a TRACE request.

Responses to the TRACE method are not cacheable.

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.

  Expect =      #expectation
  expectation = token [ "=" ( token / quoted-string ) parameters ]

The Expect field value is case-insensitive.

The only expectation defined by this specification is "100-continue" (with no defined parameters).

A server that receives an Expect field value containing a member 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 (presumably large) content in this request and wishes to receive a 100 (Continue) interim response if the method, target URI, 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 content before actually doing so, which can improve efficiency when the data 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 content.
• A client that will wait for a 100 (Continue) response before sending the request content 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 content 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 content.
• 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 content for the corresponding request, or if the framing indicates that there is no content.
• A server that sends a 100 (Continue) response MUST ultimately send a final status code, once it receives and processes the request content, unless the connection is closed prematurely.
• A server that responds with a final status code before reading the entire request content SHOULD indicate whether it intends to close the connection (e.g., see Section 9.6 of [HTTP/1.1]) or continue reading the request content.

• an immediate response with a final status code, if that status can be determined by examining just the method, target URI, and header fields, or
• an immediate 100 (Continue) response to encourage the client to send the request content.

The origin server MUST NOT wait for the content before sending the 100 (Continue) response.

• send an immediate response with a final status code, if that status can be determined by examining just the method, target URI, and header fields, or
• forward 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 content.

  From    = mailbox

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

From: spider-admin@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 its value is expected to be visible to anyone receiving or observing the request and is often recorded within logfiles and error reports without any expectation of privacy.

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 [URI], if any, when generating the Referer field value.

  Referer = absolute-URI / partial-URI

The field value is either an absolute-URI or a partial-URI. In the latter case (Section 4), the referenced URI is relative to the target URI ([URI], Section 5).

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 header field or send it with a value of "about:blank".

The Referer header field value need not convey the full URI of the referring resource; a user agent MAY truncate parts other than the referring origin.

The Referer header 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 SHOULD NOT send a Referer header field if the referring resource was accessed with a secure protocol and the request target has an origin differing from that of the referring resource, unless the referring resource explicitly allows Referer to be sent. A user agent MUST NOT send a Referer header field in an unsecured HTTP request if the referring resource was accessed with a secure protocol. See Section 17.9 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 target URI.

The "TE" header field describes capabilities of the client with regard to transfer codings and trailer sections.

As described in Section 6.5, a TE field with a "trailers" member sent in a request indicates that the client will not discard trailer fields.

TE is also used within HTTP/1.1 to advise servers about which transfer codings the client is able to accept in a response. As of publication, only HTTP/1.1 uses transfer codings (see Section 7 of [HTTP/1.1]).

The TE field value is a list of members, with each member (aside from "trailers") consisting of a transfer coding name token with an optional weight indicating the client's relative preference for that transfer coding (Section 12.4.2) and optional parameters for that transfer coding.

  TE                 = #t-codings
  t-codings          = "trailers" / ( transfer-coding [ weight ] )
  transfer-coding    = token *( OWS ";" OWS transfer-parameter )
  transfer-parameter = token BWS "=" BWS ( token / quoted-string )

A sender of TE MUST also send a "TE" connection option within the Connection header field (Section 7.6.1) to inform intermediaries not to forward this field.

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 header 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 5.6.5), 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 header 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 "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 header 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 "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 ([URI], Section 4.2), the final value is computed by resolving it against the target URI ([URI], 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 target URI (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 Retry-After field value can be either an HTTP-date or a number of seconds to delay after receiving the response.

  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 "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 header field in its responses.

  Server = product *( RWS ( product / comment ) )

The Server header field value consists of one or more product identifiers, each followed by zero or more comments (Section 5.6.5), 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 10.1.5.

Server: CERN/3.0 libwww/2.17

An origin server SHOULD NOT generate a Server header 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 "WWW-Authenticate" response header field indicates the authentication scheme(s) and parameters applicable to the target resource.

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

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

Some user agents do not recognize this form, however. As a result, sending a WWW-Authenticate field value with more than one member on the same field line might not be interoperable.

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 header fields in that request. See Section 3.5 of [CACHING] for details of and requirements pertaining to handling of the Authorization header field by HTTP caches.

HTTP authentication schemes can use the "Authentication-Info" response 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 11.3. 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 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 11.6.2) of the corresponding request.

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

Authentication-Info can be sent as a trailer field (Section 6.5) when the authentication scheme explicitly allows this.

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 request. A proxy MUST send at least one Proxy-Authenticate header field in each 407 (Proxy Authentication Required) response that it generates.

  Proxy-Authenticate = #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 11.6.1 for details.

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 header 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.

  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.

Proxy-Authentication-Info can be sent as a trailer field (Section 6.5) when the authentication scheme explicitly allows this.

For each of the content negotiation fields, a request that does not contain the field implies that the sender has no preference on that dimension of negotiation.

If a content negotiation header field is present in a request and none of the available representations for the response can be considered acceptable according to it, the origin server can either honor the header field by sending a 406 (Not Acceptable) response or disregard the header field by treating the response as if it is not subject to content negotiation for that request header field. This does not imply, however, that the client will be able to use the representation.

The content negotiation fields defined by this specification 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.

Most of these header fields, where indicated, define a wildcard value ("*") to select unspecified values. If no wildcard is present, values that are not explicitly mentioned in the field are considered unacceptable. Within Vary, the wildcard value means that the variance is unlimited.

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.

When sent by a server in a response, Accept provides information about which content types are preferred in the content of a subsequent request to the same resource.

  Accept = #( media-range [ weight ] )

  media-range    = ( "*/*"
                     / ( type "/" "*" )
                     / ( type "/" subtype )
                   ) parameters

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 optional applicable media type parameters (e.g., charset), followed by an optional "q" parameter for indicating a relative weight (Section 12.4.2).

Previous specifications allowed additional extension parameters to appear after the weight parameter. The accept extension grammar (accept-params, accept-ext) has been removed because it had a complicated definition, was not being used in practice, and is more easily deployed through new header fields. Senders using weights SHOULD send "q" last (after all media-range parameters). Recipients SHOULD process any parameter named "q" as weight, regardless of parameter ordering.

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/plain;q=0.7, text/plain;format=flowed,
       text/plain;format=fixed;q=0.4, */*;q=0.5

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 = #( ( token / "*" ) [ weight ] )

Charset names are defined in Section 8.3.2. A user agent MAY associate a quality value with each charset to indicate the user's relative preference for that charset, as defined in Section 12.4.2. An example is

Accept-Charset: iso-8859-5, unicode-1-1;q=0.8

The special value "*", if present in the Accept-Charset header field, matches every charset that is not mentioned elsewhere in the field.

The "Accept-Encoding" header field can be used to indicate preferences regarding the use of content codings (Section 8.4.1).

When sent by a user agent in a request, Accept-Encoding indicates the content codings acceptable in a response.

When sent by a server in a response, Accept-Encoding provides information about which content codings are preferred in the content of a subsequent request to the same resource.

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 (weight) representing the preference for that encoding, as defined in Section 12.4.2. The asterisk "*" symbol in an Accept-Encoding field matches any available content coding not explicitly listed in the field.

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 header 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 header 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 12.4.2, a qvalue of 0 means "not acceptable".)

A representation could be encoded with multiple content codings. However, most content codings are alternative ways to accomplish the same purpose (e.g., data compression). When selecting between multiple content codings that have the same purpose, 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 a non-empty 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 unless the identity coding is indicated as unacceptable.

When the Accept-Encoding header field is present in a response, it indicates what content codings the resource was willing to accept in the associated request. The field value is evaluated the same way as in a request.

Note that this information is specific to the associated request; the set of supported encodings might be different for other resources on the same server and could change over time or depend on other aspects of the request (such as the request method).

Servers that fail a request due to an unsupported content coding ought to respond with a 415 (Unsupported Media Type) status and include an Accept-Encoding header field in that response, allowing clients to distinguish between issues related to content codings and media types. In order to avoid confusion with issues related to media types, servers that fail a request with a 415 status for reasons unrelated to content codings MUST NOT include the Accept-Encoding header field.

The most common use of Accept-Encoding is in responses with a 415 (Unsupported Media Type) status code, in response to optimistic use of a content coding by clients. However, the header field can also be used to indicate to clients that content codings are supported in order to optimize future interactions. For example, a resource might include it in a 2xx (Successful) response when the request content was big enough to justify use of a compression coding but the client failed do so.

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 8.5.1.

  Accept-Language = #( 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 12.4.2. 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 17.13).

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.

The "Vary" header field in a response describes what parts of a request message, aside from the method and target URI, might have influenced the origin server's process for selecting the content of this response.

  Vary = #( "*" / field-name )

A Vary field value is either the wildcard member "*" or a list of request field names, known as the selecting header fields, that might have had a role in selecting the representation for this response. Potential selecting header fields are not limited to fields defined by this specification.

A list containing the member "*" signals that other aspects of the request might have played a role in selecting the response representation, possibly including aspects 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 "*" in a Vary field value.

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 header 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 header fields as the original request (Section 4.1 of [CACHING]) or reuse of the response has been validated by the origin server. 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 was subject to content negotiation (Section 12) and a different representation might be sent in a subsequent request if other values are provided in the listed header fields (proactive negotiation).

An origin server SHOULD generate a Vary header field on a cacheable response when it wishes that response to be selectively reused for subsequent requests. Generally, that is the case when the response content has been tailored to better fit the preferences expressed by those selecting header fields, such as when an origin server has selected the response's language based on the request's Accept-Language header field.

Vary might be elided when an origin server considers variance in content selection to be less significant than Vary's performance impact on caching, particularly when reuse is already limited by cache response directives (Section 5.2 of [CACHING]).

There is no need to send the Authorization field name in Vary because reuse of that response for a different user is prohibited by the field definition (Section 11.6.2). Likewise, if the response content has been selected or influenced by network region, but the origin server wants the cached response to be reused even if recipients move from one region to another, then there is no need for the origin server to indicate such variance in Vary.

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 8.8.3.2), 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 = "*" / #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). In general, it can be used with any method that involves the selection or modification of a representation to abort the request if the selected representation's current entity tag is not a member within the If-Match field value.

When an origin server receives a request that selects a representation and that request includes an If-Match header field, the origin server MUST evaluate the If-Match condition per Section 13.2 prior to performing the method.

1. If the field value is "*", the condition is true if the origin server has a current representation for the target resource.
2. If the field value is a list of entity tags, the condition is true if any of the listed tags match the entity tag of the selected representation.
3. Otherwise, the condition is false.

An origin server that evaluates an If-Match condition MUST NOT perform the requested method if the condition evaluates to false. Instead, the origin server MAY indicate that the conditional request failed by responding with a 412 (Precondition Failed) status code. Alternatively, if the request is a state-changing operation that appears to have already been applied to the selected representation, the origin server MAY respond with a 2xx (Successful) status code (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 an equivalent change was made by some other user agent).

Allowing an origin server to send a success response when a change request appears to have already been applied is more efficient for many authoring use cases, but comes with some risk if multiple user agents are making change requests that are very similar but not cooperative. For example, multiple user agents writing to a common resource as a semaphore (e.g., a nonatomic increment) are likely to collide and potentially lose important state transitions. For those kinds of resources, an origin server is better off being stringent in sending 412 for every failed precondition on an unsafe method. In other cases, excluding the ETag field from a success response might encourage the user agent to perform a GET as its next request to eliminate confusion about the resource's current state.

A client MAY send an If-Match header field in a GET request to indicate that it would prefer a 412 (Precondition Failed) response if the selected representation does not match. However, this is only useful in range requests (Section 14) for completing a previously received partial representation when there is no desire for a new representation. If-Range (Section 13.1.5) is better suited for range requests when the client prefers to receive a new representation.

A cache or intermediary MAY ignore If-Match because its interoperability features are only necessary for an origin server.

Note that an If-Match header field with a list value containing "*" and other values (including other instances of "*") is syntactically invalid (therefore not allowed to be generated) and furthermore is unlikely to be interoperable.

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 8.8.3.2), since weak entity tags can be used for cache validation even if there have been changes to the representation data.

  If-None-Match = "*" / #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 9.2.1). 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.

When an origin server receives a request that selects a representation and that request includes an If-None-Match header field, the origin server MUST evaluate the If-None-Match condition per Section 13.2 prior to performing the method.

1. If the field value is "*", the condition is false if the origin server has a current representation for the target resource.
2. If the field value is a list of entity tags, the condition is false if one of the listed tags matches the entity tag of the selected representation.
3. Otherwise, the condition is true.

An origin server that evaluates an If-None-Match condition 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].

Note that an If-None-Match header field with a list value containing "*" and other values (including other instances of "*") is syntactically invalid (therefore not allowed to be generated) and furthermore is unlikely to be interoperable.

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, the field value has more than one member, or if the request method is neither GET nor HEAD.

A recipient MUST ignore the If-Modified-Since header field if the resource does not have a modification date available.

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 header 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 clock time at which the message was received, particularly when the cached message does not contain a Last-Modified header 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 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 header field will not be able to help the user agent limit its data transfers to only those changed during the specified window.

When an origin server receives a request that selects a representation and that request includes an If-Modified-Since header field without an If-None-Match header field, the origin server SHOULD evaluate the If-Modified-Since condition per Section 13.2 prior to performing the method.

1. If the selected representation's last modification date is earlier or equal to the date provided in the field value, the condition is false.
2. Otherwise, the condition is true.

An origin server that evaluates an If-Modified-Since condition SHOULD NOT perform the requested method if the condition evaluates to false; 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 (including when the field value appears to be a list of dates).

A recipient MUST ignore the If-Unmodified-Since header field if the resource does not have a modification date available.

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). In general, it can be used with any method that involves the selection or modification of a representation to abort the request if the selected representation's last modification date has changed since the date provided in the If-Unmodified-Since field value.

When an origin server receives a request that selects a representation and that request includes an If-Unmodified-Since header field without an If-Match header field, the origin server MUST evaluate the If-Unmodified-Since condition per Section 13.2 prior to performing the method.

1. If the selected representation's last modification date is earlier than or equal to the date provided in the field value, the condition is true.
2. Otherwise, the condition is false.

An origin server that evaluates an If-Unmodified-Since condition MUST NOT perform the requested method if the condition evaluates to false. Instead, the origin server MAY indicate that the conditional request failed by responding with a 412 (Precondition Failed) status code. Alternatively, if the request is a state-changing operation that appears to have already been applied to the selected representation, the origin server MAY respond with a 2xx (Successful) status code (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 an equivalent change was made by some other user agent).

Allowing an origin server to send a success response when a change request appears to have already been applied is more efficient for many authoring use cases, but comes with some risk if multiple user agents are making change requests that are very similar but not cooperative. In those cases, an origin server is better off being stringent in sending 412 for every failed precondition on an unsafe method.

A client MAY send an If-Unmodified-Since header field in a GET request to indicate that it would prefer a 412 (Precondition Failed) response if the selected representation has been modified. However, this is only useful in range requests (Section 14) for completing a previously received partial representation when there is no desire for a new representation. If-Range (Section 13.1.5) is better suited for range requests when the client prefers to receive a new representation.

A cache or intermediary MAY ignore If-Unmodified-Since because its interoperability features are only necessary for an origin server.

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 valid entity-tag can be distinguished from a valid HTTP-date by examining the first three characters for a DQUOTE.

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 8.8.2.2.

A server that receives an If-Range header field on a Range request MUST evaluate the condition per Section 13.2 prior to performing the method.

1. If the HTTP-date validator provided is not a strong validator in the sense defined by Section 8.8.2.2, the condition is false.
2. If the HTTP-date validator provided exactly matches the Last-Modified field value for the selected representation, the condition is true.
3. Otherwise, the condition is false.

1. If the entity-tag validator provided exactly matches the ETag field value for the selected representation using the strong comparison function (Section 8.8.3.2), the condition is true.
2. Otherwise, the condition is false.

A recipient of an If-Range header field MUST ignore the Range header field if the If-Range condition evaluates to false. Otherwise, the recipient SHOULD process the Range header field as requested.

Note that the If-Range comparison is by exact match, including when the validator is an HTTP-date, and so it differs from the "earlier than or equal to" comparison used when evaluating an If-Unmodified-Since conditional.

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 process the request content (if any) or 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, prior to processing the request content, would have been a status code other than a 2xx (Successful) or 412 (Precondition Failed). In other words, redirects and failures that can be detected before significant processing occurs take precedence over the evaluation of preconditions.

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 preconditions are evaluated or the consequences of their evaluation. For example, the immutable cache directive (defined by [RFC8246]) instructs caches to forgo forwarding conditional requests when they hold a fresh response.

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 13.1.1)

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 13.1.4)

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 true and the Range is applicable to the selected representation, respond 206 (Partial Content)
   • otherwise, ignore the Range header field and respond 200 (OK)

6. Otherwise,

   • perform the requested method and respond according to its success or failure.

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

Ranges are expressed in terms of a range unit paired with a set of range specifiers. The range unit name determines what kinds of range-spec are applicable to its own specifiers. Hence, the following grammar is generic: each range unit is expected to specify requirements on when int-range, suffix-range, and other-range are allowed.

A range request can specify a single range or a set of ranges within a single representation.

  ranges-specifier = range-unit "=" range-set
  range-set        = 1#range-spec
  range-spec       = int-range
                   / suffix-range
                   / other-range

An int-range is a range expressed as two non-negative integers or as one non-negative integer through to the end of the representation data. The range unit specifies what the integers mean (e.g., they might indicate unit offsets from the beginning, inclusive numbered parts, etc.).

  int-range     = first-pos "-" [ last-pos ]
  first-pos     = 1*DIGIT
  last-pos      = 1*DIGIT

An int-range is invalid if the last-pos value is present and less than the first-pos.

A suffix-range is a range expressed as a suffix of the representation data with the provided non-negative integer maximum length (in range units). In other words, the last N units of the representation data.

  suffix-range  = "-" suffix-length
  suffix-length = 1*DIGIT

To provide for extensibility, the other-range rule is a mostly unconstrained grammar that allows application-specific or future range units to define additional range specifiers.

  other-range   = 1*( %x21-2B / %x2D-7E )
                ; 1*(VCHAR excluding comma)

A ranges-specifier is invalid if it contains any range-spec that is invalid or undefined for the indicated range-unit.

A valid ranges-specifier is satisfiable if it contains at least one range-spec that is satisfiable, as defined by the indicated range-unit. Otherwise, the ranges-specifier is unsatisfiable.

The "bytes" range unit is used to express subranges of a representation data's octet sequence. Each byte range is expressed as an integer range at some offset, relative to either the beginning (int-range) or end (suffix-range) of the representation data. Byte ranges do not use the other-range specifier.

The first-pos value in a bytes int-range gives the offset of the first byte in a range. The last-pos value gives the offset of the last byte in the range; that is, the byte positions specified are inclusive. Byte offsets start at zero.

If the representation data has a content coding applied, each byte range is calculated with respect to the encoded sequence of bytes, not the sequence of underlying bytes that would be obtained after decoding.

• The first 500 bytes (byte offsets 0-499, inclusive):

  bytes=0-499
  

• The second 500 bytes (byte offsets 500-999, inclusive):

  bytes=500-999
  

A client can limit the number of bytes requested without knowing the size of the selected representation. If the last-pos value is absent, or if the value is greater than or equal to the current length of the representation data, the byte range is interpreted as the remainder of the representation (i.e., the server replaces the value of last-pos with a value that is one less than the current length of the selected representation).

A client can refer to the last N bytes (N > 0) of the selected representation using a suffix-range. If the selected representation is shorter than the specified suffix-length, the entire representation is used.

• The final 500 bytes (byte offsets 9500-9999, inclusive):

  bytes=-500
  

  Or:

  bytes=9500-
  

• The first and last bytes only (bytes 0 and 9999):

  bytes=0-0,-1
  

• The first, middle, and last 1000 bytes:

  bytes= 0-999, 4500-5499, -1000
  

• Other valid (but not canonical) specifications of the second 500 bytes (byte offsets 500-999, inclusive):

  bytes=500-600,601-999
  bytes=500-700,601-999
  

• an int-range with a first-pos that is less than the current length of the selected representation or
• a suffix-range with a non-zero suffix-length.

When a selected representation has zero length, the only satisfiable form of range-spec in a GET request is a suffix-range with a non-zero suffix-length.

In the byte-range syntax, first-pos, last-pos, and suffix-length are expressed as decimal number of octets. Since there is no predefined limit to the length of content, recipients MUST anticipate potentially large decimal numerals and prevent parsing errors due to integer conversion overflows.

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 content, as described in Section 10.1.1. 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 7.8), 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 in effect after this 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.

Aside from responses to CONNECT, a 200 response is expected to contain message content unless the message framing explicitly indicates that the content has zero length. If some aspect of the request indicates a preference for no content upon success, the origin server ought to send a 204 (No Content) response instead. For CONNECT, there is no content 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]).

In 200 responses to GET or HEAD, an origin server SHOULD send any available validator fields (Section 8.8) for the selected representation, with both a strong entity tag and a Last-Modified date being preferred.

In 200 responses to state-changing methods, any validator fields (Section 8.8) sent in the response convey the current validators for the new representation formed as a result of successfully applying the request semantics. Note that the PUT method (Section 9.3.4) has additional requirements that might preclude sending such validators.

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 header field is received, by the target URI.

The 201 response content typically describes and links to the resource(s) created. Any validator fields (Section 8.8) sent in the response convey the current validators for a new representation created by the request. Note that the PUT method (Section 9.3.4) has additional requirements that might preclude sending such validators.

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 content has been modified from that of the origin server's 200 (OK) response by a transforming proxy (Section 7.7). 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).

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 content. 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 end of the header section; it cannot contain content or trailers.

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 content in a 205 response.

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.

A server that supports range requests (Section 14) will usually attempt to satisfy all of the requested ranges, since sending less data will likely result in another client request for the remainder. However, a server might want to send only a subset of the data requested for reasons of its own, such as temporary unavailability, cache efficiency, load balancing, etc. Since a 206 response is self-descriptive, the client can still understand a response that only partially satisfies its range request.

A client MUST inspect a 206 response's Content-Type and Content-Range field(s) to determine what parts are enclosed and whether additional requests are needed.

A server that generates a 206 response 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.

A Content-Length header field present in a 206 response indicates the number of octets in the content of this message, which is usually not the complete length of the selected representation. Each Content-Range header field includes information about the selected representation's complete length.

A sender that generates a 206 response to a request with an If-Range header field SHOULD NOT generate other representation header fields beyond those required because the client already has a prior response containing those header fields. Otherwise, a 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 12).

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 content 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 content. 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. The server is suggesting that a user agent with link-editing capability can permanently replace references to the target URI with one of the new references sent by the server. However, this suggestion is usually ignored unless the user agent is actively editing references (e.g., engaged in authoring content), the connection is secured, and the origin server is a trusted authority for the content being edited.

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 content 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 target 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 content 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 target 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 different resource, since doing so provides the information corresponding to the POST response as a resource that can be separately identified, bookmarked, and cached.

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 content of a 200 (OK) response.

• Content-Location, Date, ETag, and Vary
• Cache-Control and Expires (see [CACHING])

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 is terminated by the end of the header section; it cannot contain content or trailers.

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 target 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 content 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. The server is suggesting that a user agent with link-editing capability can permanently replace references to the target URI with one of the new references sent by the server. However, this suggestion is usually ignored unless the user agent is actively editing references (e.g., engaged in authoring content), the connection is secured, and the origin server is a trusted authority for the content being edited.

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 content 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 11.6.1) 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 11.6.2). 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 content (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 12.1), and the server is unwilling to supply a default representation.

The server SHOULD generate content 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 15.4.1.

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 for this request. The proxy MUST send a Proxy-Authenticate header field (Section 11.7.1) containing a challenge applicable to that proxy for the request. The client MAY repeat the request with a new or replaced Proxy-Authorization header field (Section 11.7.2).

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.

If the client has an outstanding request in transit, it MAY repeat that request. If the current connection is not usable (e.g., as it would be in HTTP/1.1 because request delimitation is lost), a new connection will be used.

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 content 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 8.6). The client MAY repeat the request if it adds a valid Content-Length header field containing the length of the request content.

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 (Section 13). 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 (Content Too Large) status code indicates that the server is refusing to process a request because the request content is larger than the server is willing or able to process. The server MAY terminate the request, if the protocol version in use allows it; otherwise, the server MAY close the connection.

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 target URI 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 an infinite loop 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 content 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.

If the problem was caused by an unsupported content coding, the Accept-Encoding response header field (Section 12.5.3) ought to be used to indicate which (if any) content codings would have been accepted in the request.

On the other hand, if the cause was an unsupported media type, the Accept response header field (Section 12.5.1) can be used to indicate which media types would have been accepted in the request.

The 416 (Range Not Satisfiable) status code indicates that the set of ranges in the request's Range header field (Section 14.2) has been rejected either because none of the requested ranges are satisfiable or because the client has requested an excessive number of small or overlapping ranges (a potential denial of service attack).

Each range unit defines what is required for its own range sets to be satisfiable. For example, Section 14.1.2 defines what makes a bytes range set satisfiable.

A server that generates a 416 response to a byte-range request SHOULD generate a Content-Range header field specifying the current length of the selected representation (Section 14.4).

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 10.1.1) 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, which has been deployed as a joke often enough for the code to be unusable for any future 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 421 (Misdirected Request) status code indicates that the request was directed at a server that is unable or unwilling to produce an authoritative response for the target URI. An origin server (or gateway acting on behalf of the origin server) sends 421 to reject a target URI that does not match an origin for which the server has been configured (Section 4.3.1) or does not match the connection context over which the request was received (Section 7.4).

A client that receives a 421 (Misdirected Request) response MAY retry the request, whether or not the request method is idempotent, over a different connection, such as a fresh connection specific to the target resource's origin, or via an alternative service [ALTSVC].

A proxy MUST NOT generate a 421 response.

The 422 (Unprocessable Content) status code indicates that the server understands the content type of the request content (hence a 415 (Unsupported Media Type) status code is inappropriate), and the syntax of the request content is correct, but it was unable to process the contained instructions. For example, this status code can be sent if an XML request content 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 7.8).

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 10.2.3) 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 2.5, 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) Method Registry", maintained by IANA at https://www.iana.org/assignments/http-methods, registers method names.

• Method Name (see Section 9)
• Safe ("yes" or "no", see Section 9.2.1)
• Idempotent ("yes" or "no", see Section 9.2.2)
• 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) 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 content on either the request or the response message. Definitions of new methods can specify that only a zero-length content is allowed by requiring a Content-Length header field with a value of "0".

Likewise, new methods cannot use the special host:port and asterisk forms of request target that are allowed for CONNECT and OPTIONS, respectively (Section 7.1). A full URI in absolute form is needed for the target URI, which means either the request target needs to be sent in absolute form or the target URI will be reconstructed from the request context in the same way it is for other methods.

A new method definition needs to indicate whether it is safe (Section 9.2.1), idempotent (Section 9.2.2), cacheable (Section 9.2.3), what semantics are to be associated with the request content (if any), 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 13.1) 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 14.2), it ought to document this, too.

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 15. To allow existing parsers to process the response message, new status codes cannot disallow content, although they can mandate a zero-length content.

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).

By default, a status code applies only to the request corresponding to the response it occurs within. If a status code applies to a larger scope of applicability — for example, all requests to the resource in question or all requests to a server — this must be explicitly specified. When doing so, it should be noted that not all clients can be expected to consistently apply a larger scope because they might not understand the new status code.

The definition of a new final status code ought to specify whether or not it is heuristically cacheable. Note that any response with a final status code can be cached if the response has explicit freshness information. A status code defined as heuristically cacheable is allowed to be cached without explicit freshness information. Likewise, the definition of a status code can place constraints upon cache behavior if the must-understand cache directive is used. See [CACHING] for more information.

Finally, the definition of a new status code ought to indicate whether the content has any implied association with an identified resource (Section 6.4.2).

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

Any party can request registration of an HTTP field. See Section 16.3.2 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@w3.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 ([RFC8126], Section 4.6).

Field name:

The requested field name. It MUST conform to the field-name syntax defined in Section 5.1, and it SHOULD be restricted to just letters, digits, and hyphen ('-') characters, with the first character being a letter.

Status:

"permanent", "provisional", "deprecated", or "obsoleted".

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. Optional but encouraged for provisional registrations. An indication of the relevant section(s) can also be included, but is not required.

Comments:

Additional information, such as about reserved entries.

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) finds 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 expert(s) 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.

HTTP header and trailer fields are a widely used extension point for the protocol. While they can be used in an ad hoc fashion, fields that are intended for wider use need to be carefully documented to ensure interoperability.

• Under what conditions the field can be used; e.g., only in responses or requests, in all messages, only on responses to a particular request method, etc.
• Whether the field semantics are further refined by their context, such as their use with certain request methods or status codes.
• The scope of applicability for the information conveyed. By default, fields apply only to the message they are associated with, but some response fields are designed to apply to all representations of a resource, the resource itself, or an even broader scope. Specifications that expand the scope of a response field will need to carefully consider issues such as content negotiation, the time period of applicability, and (in some cases) multi-tenant server deployments.
• Under what conditions intermediaries are allowed to insert, delete, or modify the field's value.
• If the field is allowable in trailers; by default, it will not be (see Section 6.5.1).
• Whether it is appropriate or even required to list the field name in the Connection header field (i.e., if the field is to be hop-by-hop; see Section 7.6.1).
• Whether the field introduces any additional security considerations, such as disclosure of privacy-related data.

• If it is appropriate to list the field name in a Vary response header field (e.g., when the request header field is used by an origin server's content selection algorithm; see Section 12.5.5).
• If the field is intended to be stored when received in a PUT request (see Section 9.3.4).
• If the field ought to be removed when automatically redirecting a request due to security concerns (see Section 15.4).

The "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry" defines the namespace for the authentication schemes in challenges and credentials. It is maintained at https://www.iana.org/assignments/http-authschemes.

• Authentication Scheme Name
• Pointer to specification text
• Notes (optional)

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

• HTTP authentication is presumed to be stateless: all of the information necessary to authenticate a request MUST be provided in the request, rather than be dependent on the server remembering prior requests. Authentication based on, or bound to, the underlying connection is outside the scope of this specification and inherently flawed unless steps are taken to ensure that the connection cannot be used by any party other than the authenticated user (see Section 3.3).

• The authentication parameter "realm" is reserved for defining protection spaces as described in Section 11.5. New schemes MUST NOT use it in a way incompatible with that definition.

• The "token68" notation was introduced for compatibility with existing authentication schemes and can only be used once per challenge or credential. Thus, new schemes ought to use the auth-param syntax instead, because otherwise future extensions will be impossible.

• The parsing of challenges and credentials is defined by this specification and cannot be modified by new authentication schemes. When the auth-param syntax is used, all parameters ought to support both token and quoted-string syntax, and syntactical constraints ought to be defined on the field value after parsing (i.e., quoted-string processing). This is necessary so that recipients can use a generic parser that applies to all authentication schemes.

  Note: The fact that the value syntax for the "realm" parameter is restricted to quoted-string was a bad design choice not to be repeated for new parameters.

• Definitions of new schemes ought to define the treatment of unknown extension parameters. In general, a "must-ignore" rule is preferable to a "must-understand" rule, because otherwise it will be hard to introduce new parameters in the presence of legacy recipients. Furthermore, it's good to describe the policy for defining new parameters (such as "update the specification" or "use this registry").

• Authentication schemes need to document whether they are usable in origin-server authentication (i.e., using WWW-Authenticate), and/or proxy authentication (i.e., using Proxy-Authenticate).

• The credentials carried in an Authorization header field are specific to the user agent and, therefore, have the same effect on HTTP caches as the "private" cache response directive (Section 5.2.2.7 of [CACHING]), within the scope of the request in which they appear.

  Therefore, new authentication schemes that choose not to carry credentials in the Authorization header field (e.g., using a newly defined header field) will need to explicitly disallow caching, by mandating the use of cache response directives (e.g., "private").

• Schemes using Authentication-Info, Proxy-Authentication-Info, or any other authentication related response header field need to consider and document the related security considerations (see Section 17.16.4).

The "HTTP Range Unit Registry" defines the namespace for the range unit names and refers to their corresponding specifications. It is maintained at https://www.iana.org/assignments/http-parameters.

• Name
• Description
• Pointer to specification text

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

Other range units, such as format-specific boundaries like pages, sections, records, rows, or time, are potentially usable in HTTP for application-specific purposes, but are not commonly used in practice. Implementors of alternative range units ought to consider how they would work with content codings and general-purpose intermediaries.

The "HTTP Content Coding Registry", maintained by IANA at https://www.iana.org/assignments/http-parameters/, registers content-coding names.

• Name
• Description
• Pointer to specification text

Names of content codings MUST NOT overlap with names of transfer codings (per the "HTTP Transfer Coding Registry" located at https://www.iana.org/assignments/http-parameters/) unless the encoding transformation is identical (as is the case for the compression codings defined in Section 8.4.1).

Values to be added to this namespace require IETF Review (see Section 4.8 of [RFC8126]) and MUST conform to the purpose of content coding defined in Section 8.4.1.

New content codings ought to be self-descriptive whenever possible, with optional parameters discoverable within the coding format itself, rather than rely on external metadata that might be lost during transit.

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. Services that depend on individual user authentication require a secured connection prior to exchanging credentials (Section 4.2.2).

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 origin (Section 11.5). 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.