Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content

If the selection of the best representation for a response is made by an algorithm located at the server, it is called proactive negotiation. Selection is based on the available representations of the response (the dimensions over which it can vary; e.g., language, content-coding, etc.) and the contents of particular header fields in the request message or on other information pertaining to the request (such as the network address of the client).¶

Proactive negotiation is advantageous when the algorithm for selecting from among the available representations is difficult to describe to the user agent, or when the server desires to send its "best guess" to the client along with the first response (hoping to avoid the round-trip delay of a subsequent request if the "best guess" is good enough for the user). In order to improve the server's guess, the user agent MAY include request header fields (Accept, Accept-Language, Accept-Encoding, etc.) which describe its preferences for such a response.¶

Proactive negotiation has disadvantages: ¶

1. It is impossible for the server to accurately determine what might be "best" for any given user, since that would require complete knowledge of both the capabilities of the user agent and the intended use for the response (e.g., does the user want to view it on screen or print it on paper?).
2. Having the user agent describe its capabilities in every request can be both very inefficient (given that only a small percentage of responses have multiple representations) and a potential violation of the user's privacy.
3. It complicates the implementation of an origin server and the algorithms for generating responses to a request.
4. It might limit a public cache's ability to use the same response for multiple user's requests.

Proactive negotiation allows the user agent to specify its preferences, but it cannot expect responses to always honor them. For example, the origin server might not implement proactive negotiation, or it might decide that sending a response that doesn't conform to them is better than sending a 406 (Not Acceptable) response.¶

HTTP/1.1 includes the following header fields for enabling proactive negotiation through description of user agent capabilities and user preferences: Accept (Section 6.3.2), Accept-Charset (Section 6.3.3), Accept-Encoding (Section 6.3.4), Accept-Language (Section 6.3.5), and User-Agent (Section 6.5.3). However, an origin server is not limited to these dimensions and MAY vary the response based on any aspect of the request, including aspects of the connection (e.g., IP address) or information within extension header fields not defined by this specification.¶

The Vary header field (Section 8.2.1) can be used to express the parameters the server uses to select a representation that is subject to proactive negotiation.¶

With reactive negotiation, selection of the best representation for a response is performed by the user agent after receiving an initial response from the origin server. Selection is based on a list of the available representations of the response included within the header fields or body of the initial response, with each representation identified by its own URI. Selection from among the representations can be performed automatically (if the user agent is capable of doing so) or manually by the user selecting from a generated (possibly hypertext) menu.¶

Reactive negotiation is advantageous when the response would vary over commonly-used dimensions (such as type, language, or encoding), when the origin server is unable to determine a user agent's capabilities from examining the request, and generally when public caches are used to distribute server load and reduce network usage.¶

Reactive negotiation suffers from the disadvantage of needing a second request to obtain the best alternate representation. This second request is only efficient when caching is used. In addition, this specification does not define any mechanism for supporting automatic selection, though it also does not prevent any such mechanism from being developed as an extension and used within HTTP/1.1.¶

This specification defines the 300 (Multiple Choices) and 406 (Not Acceptable) status codes for enabling reactive negotiation when the server is unwilling or unable to provide a varying response using proactive negotiation.¶

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, not entirely read-only, or which causes side-effects while invoking a safe method. What is important, however, is that the client did not request that additional behavior and cannot be held accountable for it. For example, most servers append request information to access log files at the completion of every response, regardless of the method, and that is considered safe even though the log storage might become full and crash the server. Likewise, a safe request initiated by selecting an advertisement on the Web will often have the side-effect of charging an advertising account.¶

The GET, HEAD, OPTIONS, and TRACE request methods are defined to be safe.¶

The purpose of distinguishing between safe and unsafe methods is to allow automated retrieval processes (spiders) and cache performance optimization (pre-fetching) to work without fear of causing harm. In addition, it allows a user agent to apply appropriate constraints on the automated use of unsafe methods when processing potentially untrusted content.¶

A user agent SHOULD distinguish between safe and unsafe methods when presenting potential actions to a user, such that the user can be made aware of an unsafe action before it is requested.¶

When a resource is constructed such that parameters within the effective request URI have the effect of selecting an action, it is the resource owner's responsibility to ensure that the action is consistent with the request method semantics. For example, it is common for Web-based content editing software to use actions within query parameters, such as "page?do=delete". If the purpose of such a resource is to perform an unsafe action, then the resource 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.¶

Request methods are considered "idempotent" if the intended effect of multiple identical requests is the same as for a single request. PUT, DELETE, and all 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 it can establish a new connection and retry the idempotent request because it knows that repeating the request will have the same effect even if the original request succeeded. Note, however, that repeated failures would indicate a problem within the server.¶

Request methods are considered "cacheable" if it is possible and useful to answer a current client request with a stored response from a prior request. GET and HEAD are defined to be cacheable. In general, safe methods that do not depend on a current or authoritative response are cacheable, though the overwhelming majority of caches only support GET and HEAD. HTTP requirements for cache behavior and cacheable responses are defined in [Part6].¶

The GET method requests transfer of a current representation of the target resource.¶

If the target resource is a data-producing process, it is the produced data which shall be returned as the representation in the response and not the source text of the process, unless that text happens to be the output of the process.¶

The semantics of the GET method change to a "conditional GET" if the request message includes an If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match, or If-Range header field ([Part4]). A conditional GET requests that the representation be transferred only under the circumstances described by the conditional header field(s). The conditional GET request is intended to reduce unnecessary network usage by allowing cached representations to be refreshed without requiring multiple requests or transferring data already held by the client.¶

The semantics of the GET method change to a "partial GET" if the request message includes a Range header field ([Part5]). A partial GET requests that only part of the representation be transferred, as described in Section 5.4 of [Part5]. The partial GET request is intended to reduce unnecessary network usage by allowing partially-retrieved representations to be completed without transferring data already held by the client.¶

A payload within a GET request message has no defined semantics; sending a payload body on a GET request might cause some existing implementations to reject the request.¶

The response to a GET request is cacheable and MAY be used to satisfy subsequent GET and HEAD requests (see [Part6]).¶

See Section 10.2 for security considerations when used for forms.¶

The HEAD method is identical to GET except that the server MUST NOT return a message body in the response. The metadata contained in the HTTP header fields in response to a HEAD request SHOULD be identical to the information sent in response to a GET request. This method can be used for obtaining metadata about the representation implied by the request without transferring the representation data. This method is often used for testing hypertext links for validity, accessibility, and recent modification.¶

The response to a HEAD request is cacheable and MAY be used to satisfy a subsequent HEAD request. It also has potential side effects on previously stored responses to GET; see Section 5 of [Part6].¶

A payload within a HEAD request message has no defined semantics; sending a payload body on a HEAD request might cause some existing implementations to reject the request.¶

The POST method requests that the origin server accept the representation enclosed in the request as data to be processed by the target resource. POST is designed to allow a uniform method to cover the following functions: ¶

• Annotation of existing resources;
• Posting a message to a bulletin board, newsgroup, mailing list, or similar group of articles;
• Providing a block of data, such as the result of submitting a form, to a data-handling process;
• Extending a database through an append operation.

The actual function performed by the POST method is determined by the server and is usually dependent on the effective request URI.¶

The action performed by the POST method might not result in a resource that can be identified by a URI. In this case, either 200 (OK) or 204 (No Content) is the appropriate response status code, depending on whether or not the response includes a representation that describes the result.¶

If a resource has been created on the origin server, the response SHOULD be 201 (Created) and contain a representation which describes the status of the request and refers to the new resource, and a Location header field (see Section 8.1.2).¶

Responses to POST requests are only cacheable when they include explicit freshness information (see Section 4.1.1 of [Part6]). A cached POST response with a Content-Location header field (see Section 3.1.4.2) whose value is the effective Request URI MAY be used to satisfy subsequent GET and HEAD (not POST) requests.¶

Note that POST caching is not widely implemented. However, the 303 (See Other) response can be used to direct the user agent to retrieve a cacheable representation of the resource.¶

The PUT method requests that the state of the target resource be created or replaced with the state defined by the representation enclosed in the request message payload. A successful PUT of a given representation would suggest that a subsequent GET on that same target resource will result in an equivalent representation being returned 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 either a 200 (OK) or 204 (No Content) response SHOULD be sent to indicate successful completion of the request.¶

Unrecognized header fields SHOULD be ignored (i.e., not saved as part of the resource state).¶

An origin server SHOULD verify that the PUT representation is consistent with any constraints which the server has for the target resource that cannot or will not be changed by the PUT. This is particularly important when the origin server uses internal configuration information related to the URI in order to set the values for representation metadata on GET responses. When a PUT representation is inconsistent with the target resource, the origin server SHOULD either make them consistent, by transforming the representation or changing the resource configuration, or respond with an appropriate error message containing sufficient information to explain why the representation is unsuitable. The 409 (Conflict) or 415 (Unsupported Media Type) status codes are suggested, with the latter being specific to constraints on Content-Type values.¶

For example, if the target resource is configured to always have a Content-Type of "text/html" and the representation being PUT has a Content-Type of "image/jpeg", then the origin server SHOULD do one of: ¶

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.¶

The fundamental difference between the POST and PUT methods is highlighted by the different intent for the target resource. The target resource in a POST request is intended to handle the enclosed representation as a data-accepting process, such as for a gateway to some other protocol or a document that accepts annotations. In contrast, the target resource in a PUT request is intended to take the enclosed representation as a new or replacement value. 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 what target resource is desired. A service that is intended to select 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 a 301 (Moved Permanently) 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 MAY have side-effects on other resources. For example, an article might have a URI for identifying "the current version" (a resource) which is separate from the URIs identifying each particular version (different resources that at one point shared the same state as the current version resource). A successful PUT request on "the current version" URI might therefore create a new version resource in addition to changing the state of the target resource, and might also cause links to be added between the related resources.¶

An origin server SHOULD reject any PUT request that contains a Content-Range header field (Section 5.2 of [Part5]), since it might be misinterpreted as partial content (or might be partial content that is being mistakenly PUT as a full representation). Partial content updates are possible by targeting a separately identified resource with state that overlaps a portion of the larger resource, or by using a different method that has been specifically defined for partial updates (for example, the PATCH method defined in [RFC5789]).¶

Responses to the PUT method are not cacheable. If a PUT request passes through a cache that has one or more stored responses for the effective request URI, those stored responses will be invalidated (see Section 6 of [Part6]).¶

The DELETE method requests that the origin server delete the target resource. This method MAY be overridden by human intervention (or other means) on the origin server. The client cannot be guaranteed that the operation has been carried out, even if the status code returned from the origin server indicates that the action has been completed successfully. However, the server SHOULD NOT indicate success unless, at the time the response is given, it intends to delete the resource or move it to an inaccessible location.¶

A successful response SHOULD be 200 (OK) if the response includes a representation describing the status, 202 (Accepted) if the action has not yet been enacted, or 204 (No Content) if the action has been enacted but the response does not include a representation.¶

A payload within a DELETE request message has no defined semantics; sending a payload body on a DELETE request might cause some existing implementations to reject the request.¶

Responses to the DELETE method are not cacheable. If a DELETE request passes through a cache that has one or more stored responses for the effective request URI, those stored responses will be invalidated (see Section 6 of [Part6]).¶

The CONNECT method requests that the proxy establish a tunnel to the request-target and, if successful, thereafter restrict its behavior to blind forwarding of packets until the connection is closed.¶

When using CONNECT, the request-target MUST use the authority form (Section 5.3 of [Part1]); i.e., the request-target consists of only the host name and port number of the tunnel destination, separated by a colon. For example,¶

Any 2xx (Successful) response to a CONNECT request indicates that the proxy has established a connection to the requested host and port, and has switched to tunneling the current connection to that server connection. The tunneled data from the server begins immediately after the blank line that concludes the successful response's header block.¶

A server SHOULD NOT send any Transfer-Encoding or Content-Length header fields in a successful response. A client MUST ignore any Content-Length or Transfer-Encoding header fields received in a successful response.¶

Any response other than a successful response indicates that the tunnel has not yet been formed and that the connection remains governed by HTTP.¶

A payload within a CONNECT request message has no defined semantics; sending a payload body on a CONNECT request might cause some existing implementations to reject the request.¶

Similar to a pipelined HTTP/1.1 request, data to be tunneled from client to server MAY be sent immediately after the request (before a response is received). The usual caveats also apply: data can be discarded if the eventual response is negative, and the connection can be reset with no response if more than one TCP segment is outstanding.¶

It might be the case that the proxy itself can only reach the requested origin server through another proxy. In this case, the first proxy SHOULD make a CONNECT request of that next proxy, requesting a tunnel to the authority. A proxy MUST NOT respond with any 2xx status code unless it has either a direct or tunnel connection established to the authority.¶

If at any point either one of the peers gets disconnected, any outstanding data that came from that peer will be passed to the other one, and after that also the other connection will be terminated by the proxy. If there is outstanding data to that peer undelivered, that data will be discarded.¶

An origin server which receives a CONNECT request for itself MAY respond with a 2xx status code to indicate that a connection is established. However, most origin servers do not implement CONNECT.¶

The OPTIONS method requests information about the communication options available on the request/response chain identified by the effective request URI. 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 or initiating a resource retrieval.¶

Responses to the OPTIONS method are not cacheable.¶

If the OPTIONS request includes a payload, then the media type MUST be indicated by a Content-Type field. Although this specification does not define any use for such a body, future extensions to HTTP might use the OPTIONS body to make more detailed queries on the server.¶

If the request-target (Section 5.3 of [Part1]) is an asterisk ("*"), the OPTIONS request is intended to apply 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 only to the options that are available when communicating with that resource.¶

A 200 (OK) response SHOULD include any header fields that indicate optional features implemented by the server and applicable to that resource (e.g., Allow), possibly including extensions not defined by this specification. The response payload, if any, SHOULD also include information about the communication options. The format for such a payload is not defined by this specification, but might be defined by future extensions to HTTP. Content negotiation MAY be used to select the appropriate representation. If no payload body is included, the response MUST include a Content-Length field with a field-value of "0".¶

The Max-Forwards header field MAY be used to target a specific proxy in the request chain (see Section 6.1.1). If no Max-Forwards field is present in the request, then the forwarded request MUST NOT include a Max-Forwards field.¶

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 back to the client as the message body of a 200 (OK) response. The final recipient is either the origin server or the first proxy to receive a Max-Forwards value of zero (0) in the request (see Section 6.1.1). A TRACE request MUST NOT include a message body.¶

TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing or diagnostic information. The value of the Via header field (Section 5.7 of [Part1]) 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.¶

If the request is valid, the response SHOULD have a Content-Type of "message/http" (see Section 7.3.1 of [Part1]) and contain a message body that encloses a copy of the entire request message. Responses to the TRACE method are not cacheable.¶

The "Max-Forwards" header field provides a mechanism with the TRACE (Section 5.3.8) and OPTIONS (Section 5.3.7) 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 which appears to be failing or looping mid-chain.¶

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

Each recipient of 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 recipient MUST NOT forward the request; instead, it MUST respond as the final recipient. If the received Max-Forwards value is greater than zero, then the forwarded message MUST contain an updated Max-Forwards field with a value decremented by one (1).¶

The Max-Forwards header field MAY be ignored for all other request methods.¶

The "Expect" header field is used to indicate that particular server behaviors are required by the client.¶

If all received Expect header field(s) are syntactically valid but contain an expectation that the recipient does not understand or cannot comply with, the recipient MUST respond with a 417 (Expectation Failed) status code. A recipient of a syntactically invalid Expectation header field MUST respond with a 4xx status code other than 417.¶

100-continue ¶

• The "100-continue" expectation is defined below. It does not support any expect-params.

Comparison is case-insensitive for names (expect-name), and case-sensitive for values (expect-value).¶

The Expect mechanism is hop-by-hop: the above requirements apply to any server, including proxies. However, the Expect header field itself is end-to-end; it MUST be forwarded if the request is forwarded.¶

Many older HTTP/1.0 and HTTP/1.1 applications do not understand the Expect header field.¶

Many of the request header fields for proactive content negotiation use a common parameter, named "q" (case-insensitive), to assign a relative "weight" to the preference for that associated kind of content. This weight is referred to as a "quality value" (or "qvalue") because the same parameter name is often used within server configurations to assign a weight to the relative quality of the various representations that can be selected for a resource.¶

The weight is normalized to a real number in the range 0 through 1, where 0.001 is the least preferred and 1 is the most preferred; a value of 0 means "not acceptable". If no "q" parameter is present, the default weight is 1.¶

A sender of qvalue MUST NOT generate more than three digits after the decimal point. User configuration of these values ought to be limited in the same fashion.¶

The "Accept" header field can be used by user agents to specify response media types that are acceptable. Accept header fields can be used to indicate that the request is specifically limited to a small set of desired types, as in the case of a request for an in-line image.¶

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 MAY include media type parameters that are applicable to that range.¶

Each media-range MAY be followed by one or more accept-params, beginning with the "q" parameter for indicating a relative weight, as defined in Section 6.3.1. The first "q" parameter (if any) separates the media-range parameter(s) from the accept-params.¶

The example¶

SHOULD be interpreted as "I prefer audio/basic, but send me any audio type if it is the best available after an 80% mark-down in quality".¶

A request without any Accept header field implies that the user agent will accept any media type in response. If an Accept header field is present in a request and none of the available representations for the response have a media type that is listed as acceptable, the origin server MAY either honor the Accept header field by sending a 406 (Not Acceptable) response or disregard the Accept header field by treating the response as if it is not subject to content negotiation.¶

A more elaborate example is¶

Verbally, this would be interpreted as "text/html and text/x-c are the 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,¶

have the following precedence: ¶

1. text/plain;format=flowed
2. text/plain
3. text/*
4. */*

The media type quality factor associated with a given type is determined by finding the media range with the highest precedence which matches that type. For example,¶

Note: A user agent might be provided with a default set of quality values for certain media ranges. However, unless the user agent is a closed system which cannot interact with other rendering agents, this default set ought to be configurable by the user.¶

The "Accept-Charset" header field can be used by user agents to indicate what character encodings are acceptable in a response payload. This field allows clients capable of understanding more comprehensive or special-purpose character encodings to signal that capability to a server which is capable of representing documents in those character encodings.¶

Character encoding values (a.k.a., charsets) are described in Section 3.1.1.2. Each charset MAY be given an associated quality value which represents the user's preference for that charset, as defined in Section 6.3.1. An example is¶

The special value "*", if present in the Accept-Charset field, matches every character encoding which is not mentioned elsewhere in the Accept-Charset field. If no "*" is present in an Accept-Charset field, then any character encodings not explicitly mentioned in the field are considered "not acceptable" to the client.¶

A request without any Accept-Charset header field implies that the user agent will accept any character encoding in response.¶

If an Accept-Charset header field is present in a request and none of the available representations for the response have a character encoding that is listed as acceptable, the origin server MAY either honor the Accept-Charset header field by sending a 406 (Not Acceptable) response or disregard the Accept-Charset header field by treating the response as if it is not subject to content negotiation.¶

The "Accept-Encoding" header field can be used by user agents to indicate what response content-codings (Section 3.1.2.1) are acceptable in the response. An "identity" token is used as a synonym for "no encoding" in order to communicate when no encoding is preferred.¶

Each codings value MAY be given an associated quality value which represents the preference for that encoding, as defined in Section 6.3.1.¶

For example,¶

A server tests whether a content-coding for a given representation is acceptable, according to an Accept-Encoding field, using these rules: ¶

1. The special "*" symbol in an Accept-Encoding field matches any available content-coding not explicitly listed in the header field.
2. If the representation has no content-coding, then it is acceptable by default unless specifically excluded by the Accept-Encoding field stating either "identity;q=0" or "*;q=0" without a more specific entry for "identity".
3. If the representation's content-coding is one of the content-codings listed in the Accept-Encoding field, then it is acceptable unless it is accompanied by a qvalue of 0. (As defined in Section 6.3.1, a qvalue of 0 means "not acceptable".)
4. If multiple content-codings are acceptable, then the acceptable content-coding with the highest non-zero qvalue is preferred.

An Accept-Encoding header field with a combined field-value that is empty implies that the user agent does not want any content-coding in response. If an Accept-Encoding header field is present in a request and none of the available representations for the response have a content-coding that is listed as acceptable, the origin server SHOULD send a response without any content-coding.¶

A request without an Accept-Encoding header field implies that the user agent will accept any content-coding in response.¶

The "Accept-Language" header field can be used by user agents to indicate the set of natural languages that are preferred in the response. Language tags are defined in Section 3.1.3.1.¶

Each language-range can be given an associated quality value which represents an estimate of the user's preference for the languages specified by that range, as defined in Section 6.3.1. For example,¶

would mean: "I prefer Danish, but will accept British English and other types of English". (see also 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.¶

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. For a discussion of this issue, see Section 10.5.¶

As intelligibility is highly dependent on the individual user, it is recommended that client applications make the choice of linguistic preference available to the user. If the choice is not made available, then the Accept-Language header field MUST NOT be given in the request.¶

This header field MAY be used for logging purposes and as a means for identifying the source of invalid or unwanted requests. It SHOULD NOT be used as an insecure form of access protection. The interpretation of this field is that the request is being performed on behalf of the person given, who accepts responsibility for the method performed. In particular, robot agents SHOULD include this header field so that the person responsible for running the robot can be contacted if problems occur on the receiving end.¶

The Internet e-mail address in this field MAY be separate from the Internet host which issued the request. For example, when a request is passed through a proxy the original issuer's address SHOULD be used.¶

The client SHOULD NOT send the From header field without the user's approval, as it might conflict with the user's privacy interests or their site's security policy. It is strongly recommended that the user be able to disable, enable, and modify the value of this field at any time prior to a request.¶

The "Referer" [sic] header field allows the client to specify the URI of the resource from which the target URI was obtained (the "referrer", although the header field is misspelled.).¶

The Referer header field allows servers to generate lists of back-links to resources for interest, logging, optimized caching, etc. It also allows obsolete or mistyped links to be traced for maintenance. Some servers use Referer as a means of controlling where they allow links from (so-called "deep linking"), but legitimate requests do not always contain a Referer header field.¶

If the target URI was obtained from a source that does not have its own URI (e.g., input from the user keyboard), the Referer field MUST either be sent with the value "about:blank", or not be sent at all. Note that this requirement does not apply to sources with non-HTTP URIs (e.g., FTP).¶

If the field value is a relative URI, it SHOULD be interpreted relative to the effective request URI. The URI MUST NOT include a fragment. See Section 10.2 for security considerations.¶

The "User-Agent" header field contains information about the user agent originating the request. User agents SHOULD include this field with requests.¶

Typically, it is used for statistical purposes, the tracing of protocol violations, and tailoring responses to avoid particular user agent limitations.¶

The field can contain multiple product tokens (Section 4) and comments (Section 3.2 of [Part1]) identifying the agent and its significant subproducts. By convention, the product tokens are listed in order of their significance for identifying the application.¶

Because this field is usually sent on every request a user agent makes, implementations are encouraged not to include needlessly fine-grained detail, and to limit (or even prohibit) the addition of subproducts by third parties. Overly long and detailed User-Agent field values make requests larger and can also be used to identify ("fingerprint") the user against their wishes.¶

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. Finally, they are encouraged not to use comments to identify products; doing so makes the field value more difficult to parse.¶

The client SHOULD continue with its request. This interim response is used to inform the client that the initial part of the request has been received and has not yet been rejected by the server. The client SHOULD continue by sending the remainder of the request or, if the request has already been completed, ignore this response. The server MUST send a final response after the request has been completed. See Section 6.1.2.1 for detailed discussion of the use and handling of this status code.¶

The server understands and is willing to comply with the client's request, via the Upgrade message header field (Section 6.3 of [Part1]), for a change in the application protocol being used on this connection. The server will switch protocols to those defined by the response's Upgrade header field immediately after the empty line which terminates the 101 response.¶

The protocol SHOULD be switched only when it is advantageous to do so. For example, switching to a newer version of HTTP is advantageous over older versions, and switching to a real-time, synchronous protocol might be advantageous when delivering resources that use such features.¶

The request has succeeded. The payload returned with the response is dependent on the method used in the request, for example: ¶

GET

a representation of the target resource is sent in the response;

HEAD

the same representation as GET, except without the message body;

POST

a representation describing or containing the result of the action;

TRACE

a representation containing the request message as received by the end server.

Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to determine freshness for 200 responses.¶

The request has been fulfilled and has resulted in one or more new resources being created.¶

Newly created resources are typically linked to from the response payload, with the most relevant URI also being carried in the Location header field. If the newly created resource's URI is the same as the Effective Request URI, this information can be omitted (e.g., in the case of a response to a PUT request).¶

The origin server MUST create the resource(s) before returning the 201 status code. If the action cannot be carried out immediately, the server SHOULD respond with 202 (Accepted) response instead.¶

A 201 response MAY contain an ETag response header field indicating the current value of the entity-tag for the representation of the resource identified by the Location header field or, in case the Location header field was omitted, by the Effective Request URI (see Section 2.3 of [Part4]).¶

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 for re-sending a status code from an asynchronous operation such as this.¶

The 202 response is intentionally non-committal. 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 returned with this response SHOULD include an indication of the request's current status and either a pointer to a status monitor or some estimate of when the user can expect the request to be fulfilled.¶

The representation in the response has been transformed or otherwise modified by a transforming proxy (Section 2.3 of [Part1]). Note that the behavior of transforming intermediaries is controlled by the no-transform Cache-Control directive (Section 7.2 of [Part6]).¶

This status code is only appropriate when the response status code would have been 200 (OK) otherwise. When the status code before transformation would have been different, the 214 Transformation Applied warn-code (Section 7.5 of [Part6]) is appropriate.¶

Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to determine freshness for 203 responses.¶

The 204 (No Content) status code indicates that the server has successfully fulfilled the request and that there is no additional content to return in the response payload body. Metadata in the response header fields refer to the target resource and its current representation after the requested action.¶

For example, if a 204 status code is received in response to a PUT request and the response contains an ETag header field, then the PUT was successful and the ETag field-value contains the entity-tag for the new representation of that target resource.¶

The 204 response allows a server to indicate that the action has been successfully applied to the target resource while implying that the user agent SHOULD NOT 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 the 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.¶

The 204 response MUST NOT include a message body, and thus is always terminated by the first empty line after the header fields.¶

The server has fulfilled the request and the user agent SHOULD reset the document view which caused the request to be sent. This response is primarily intended to allow input for actions to take place via user input, followed by a clearing of the form in which the input is given so that the user can easily initiate another input action.¶

The message body included with the response MUST be empty. Note that receivers still need to parse the response according to the algorithm defined in Section 3.3 of [Part1].¶

The target resource has more than one representation, each with its own specific location, and reactive negotiation information (Section 3.4) is being provided so that the user (or user agent) can select a preferred representation by redirecting its request to that location.¶

Unless it was a HEAD request, the response SHOULD include a representation containing a list of representation metadata and location(s) from which the user or user agent can choose the one most appropriate. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice MAY be performed automatically. However, this specification does not define any standard for such automatic selection.¶

If the server has a preferred choice of representation, it SHOULD include the specific URI for that representation in the Location field; user agents MAY use the Location field value for automatic redirection.¶

Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to determine freshness for 300 responses.¶

The target resource has been assigned a new permanent URI and any future references to this resource SHOULD use one of the returned URIs. Clients with link editing capabilities ought to automatically re-link references to the effective request URI to one or more of the new references returned by the server, where possible.¶

Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to determine freshness for 301 responses.¶

The new permanent URI SHOULD be given by the Location field in the response. A response payload can contain a short hypertext note with a hyperlink to the new URI(s).¶

The target resource resides temporarily under a different URI. Since the redirection might be altered on occasion, the client SHOULD continue to use the effective request URI for future requests.¶

The temporary URI SHOULD be given by the Location field in the response. A response payload can contain a short hypertext note with a hyperlink to the new URI(s).¶

The 303 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, that is intended to provide an indirect response to the original request. In order to satisfy the original request, a user agent SHOULD perform a retrieval request using the Location URI (a GET or HEAD request if using HTTP), which can itself be redirected further, and present the eventual result as an answer to the original request. Note that the new URI in the Location header field is not considered equivalent to the effective request URI.¶

This status code is generally applicable to any HTTP method. It is primarily used to allow the output of a POST action to redirect the user agent to a selected resource, since doing so provides the information corresponding to the POST response in a form that can be separately identified, bookmarked, and cached independent of the original request.¶

A 303 response to a GET request indicates that the requested resource does not have a representation of its own that can be transferred by the server over HTTP. The Location URI indicates a resource that is descriptive of the target resource, such that the follow-on representation might be useful to recipients without implying that it adequately represents the 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 and thus entirely determined by the URI owner(s).¶

Except for responses to a HEAD request, the representation of a 303 response SHOULD contain a short hypertext note with a hyperlink to the Location URI.¶

The 305 status code was defined in a previous version of this specification (see Appendix C), and is now deprecated.¶

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

The target resource resides temporarily under a different URI. Since the redirection can change over time, the client SHOULD continue to use the effective request URI for future requests.¶

The temporary URI SHOULD be given by the Location field in the response. A response payload can contain a short hypertext note with a hyperlink to the new URI(s).¶

The server cannot or will not process the request, due to a client error (e.g., malformed syntax).¶

This code is reserved for future use.¶

The server understood the request, but refuses to authorize it. Providing different user authentication credentials might be successful, but any credentials that were provided in the request are insufficient. The request SHOULD NOT be repeated with the same credentials.¶

If the request method was not HEAD and the server wishes to make public why the request has not been fulfilled, it SHOULD describe the reason for the refusal in the representation. If the server does not wish to make this information available to the client, the status code 404 (Not Found) MAY be used instead.¶

The server has not found anything matching the effective request URI. No indication is given of whether the condition is temporary or permanent. The 410 (Gone) status code SHOULD be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address. This status code is commonly used when the server does not wish to reveal exactly why the request has been refused, or when no other response is applicable.¶

The method specified in the request-line is not allowed for the target resource. The response MUST include an Allow header field containing a list of valid methods for the requested resource.¶

The resource identified by the request is only capable of generating response representations which have content characteristics not acceptable according to the Accept and Accept-* header fields sent in the request.¶

Unless it was a HEAD request, the response SHOULD include a representation containing a list of available representation characteristics and location(s) from which the user or user agent can choose the one most appropriate. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice MAY be performed automatically. However, this specification does not define any standard for such automatic selection.¶

If the response could be unacceptable, a user agent SHOULD temporarily stop receipt of more data and query the user for a decision on further actions.¶

The client did not produce a request within the time that the server was prepared to wait. The client MAY repeat the request without modifications at any later time.¶

The request could not be completed due to a conflict with the current state of the resource. This code is only allowed in situations where it is expected that the user might be able to resolve the conflict and resubmit the request. The payload SHOULD include enough information for the user to recognize the source of the conflict. Ideally, the response representation would include enough information for the user or user agent to fix the problem; however, that might not be possible and is not required.¶

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 which conflict with those made by an earlier (third-party) request, the server might use the 409 response to indicate that it can't complete the request. In this case, the response representation would likely contain a list of the differences between the two versions.¶

The target resource is no longer available at the server and no forwarding address is known. This condition is expected to be considered permanent. Clients with link editing capabilities SHOULD delete references to the effective request URI after user approval. If the server does not know, or has no facility to determine, whether or not the condition is permanent, the status code 404 (Not Found) SHOULD 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 working at the 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.¶

Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to determine freshness for 410 responses.¶

The server refuses to accept the request without a defined Content-Length. The client MAY repeat the request if it adds a valid Content-Length header field containing the length of the message body in the request message.¶

The server is refusing to process a request because the request representation is larger than the server is willing or able to process. The server MAY close the connection to prevent the client from continuing the request.¶

If the condition is temporary, the server SHOULD include a Retry-After header field to indicate that it is temporary and after what time the client MAY try again.¶

The server is refusing to service the request because the effective request 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 a URI "black hole" of redirection (e.g., a redirected URI prefix that points to a suffix of itself), or when the server is under attack by a client attempting to exploit security holes present in some servers using fixed-length buffers for reading or manipulating the request-target.¶

The server is refusing to service the request because the request payload is in a format not supported by this request method on the target resource.¶

The expectation given in an Expect header field (see Section 6.1.2) could not be met by this server, or, if the server is a proxy, the server has unambiguous evidence that the request could not be met by the next-hop server.¶

The request can not be completed without a prior protocol upgrade. This response MUST include an Upgrade header field (Section 6.3 of [Part1]) specifying the required protocols.¶

Example:

HTTP/1.1 426 Upgrade Required
Upgrade: HTTP/3.0
Connection: Upgrade
Content-Length: 53
Content-Type: text/plain

This service requires use of the HTTP/3.0 protocol.

The server SHOULD include a message body in the 426 response which indicates in human readable form the reason for the error and describes any alternative courses which might be available to the user.¶

The server encountered an unexpected condition which prevented it from fulfilling the request.¶

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.¶

The server, while acting as a gateway or proxy, received an invalid response from the upstream server it accessed in attempting to fulfill the request.¶

The server is currently unable to handle the request due to a temporary overloading or maintenance of the server.¶

The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay MAY be indicated in a Retry-After header field (Section 8.1.3). If no Retry-After is given, the client SHOULD handle the response as it would for a 500 (Internal Server Error) response.¶

The server, while acting as a gateway or proxy, did not receive a timely response from the upstream server specified by the URI (e.g., HTTP, FTP, LDAP) or some other auxiliary server (e.g., DNS) it needed to access in attempting to complete the request.¶

The server does not support, or refuses to support, the protocol version that was used in the request message. The server is indicating that it is unable or unwilling to complete the request using the same major version as the client, as described in Section 2.6 of [Part1], other than with this error message. The response SHOULD contain a representation describing why that version is not supported and what other protocols are supported by that server.¶

The "Location" header field MAY be sent in responses to refer to a specific resource in accordance with the semantics of the status code.¶

For 201 (Created) responses, the Location is the URI of the new resource which was created by the request. For 3xx (Redirection) responses, the location SHOULD indicate the server's preferred URI for automatic redirection to the resource.¶

The field value consists of a single URI-reference. When it has the form of a relative reference ([RFC3986], Section 4.2), the final value is computed by resolving it against the effective request URI ([RFC3986], Section 5). If the original URI, as navigated to by the user agent, did contain a fragment identifier, and the final value does not, then the original URI's fragment identifier is added to the final value.¶

For example, the original URI "http://www.example.org/~tim", combined with a field value given as:

  Location: /pub/WWW/People.html#tim

would result in a final value of "http://www.example.org/pub/WWW/People.html#tim"

An original URI "http://www.example.org/index.html#larry", combined with a field value given as:

  Location: http://www.example.net/index.html

would result in a final value of "http://www.example.net/index.html#larry", preserving the original fragment identifier.

There are circumstances in which a fragment identifier in a Location URI would not be appropriate. For instance, when it appears in a 201 (Created) response, where the Location header field specifies the URI for the entire created resource.¶

The header "Retry-After" field can be used with a 503 (Service Unavailable) response to indicate how long the service is expected to be unavailable to the requesting client. This field MAY also be used with any 3xx (Redirection) response to indicate the minimum time the user-agent is asked to wait before issuing the redirected request.¶

The value of this field can be either an HTTP-date or an integer number of seconds (in decimal) after the time of the response.¶

Time spans are non-negative decimal integers, representing time in seconds.¶

Two examples of its use are¶

In the latter example, the delay is 2 minutes.¶

The "Vary" header field conveys the set of header fields that were used to select the representation.¶

Caches use this information as part of determining whether a stored response can be used to satisfy a given request (Section 4.3 of [Part6]).¶

In uncacheable or stale responses, the Vary field value advises the user agent about the criteria that were used to select the representation.¶

The set of header fields named by the Vary field value is known as the selecting header fields.¶

A server SHOULD include a Vary header field with any cacheable response that is subject to proactive negotiation. Doing so allows a cache to properly interpret future requests on that resource and informs the user agent about the presence of negotiation on that resource. A server MAY include a Vary header field with a non-cacheable response that is subject to proactive negotiation, since this might provide the user agent with useful information about the dimensions over which the response varies at the time of the response.¶

A Vary field value of "*" signals that unspecified parameters not limited to the header fields (e.g., the network address of the client), play a role in the selection of the response representation; therefore, a cache cannot determine whether this response is appropriate. A proxy MUST NOT generate the "*" value.¶

The field-names given are not limited to the set of standard header fields defined by this specification. Field names are case-insensitive.¶

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.¶

The actual set of allowed methods is defined by the origin server at the time of each request.¶

A proxy MUST NOT modify the Allow header field — it does not need to understand all the methods specified in order to handle them according to the generic message handling rules.¶

The "Server" header field contains information about the software used by the origin server to handle the request.¶

The field can contain multiple product tokens (Section 4) and comments (Section 3.2 of [Part1]) identifying the server and any significant subproducts. The product tokens are listed in order of their significance for identifying the application.¶

If the response is being forwarded through a proxy, the proxy application MUST NOT modify the Server header field. Instead, it MUST include a Via field (as described in Section 5.7 of [Part1]).¶

HTTP method registrations MUST include the following fields: ¶

• Method Name (see Section 5)
• Safe ("yes" or "no", see Section 5.2.1)
• Idempotent ("yes" or "no", see Section 5.2.2)
• Pointer to specification text

Values to be added to this name space require IETF Review (see [RFC5226], Section 4.1).¶

Standardized methods are generic; that is, they are potentially applicable to any resource, not just one particular media type, kind of resource, or application. As such, it is preferred that new methods be registered in a document that isn't specific to a single application or data format, since orthogonal technologies deserve orthogonal specification.¶

Since message parsing (Section 3.3 of [Part1]) needs to be independent of method semantics (aside from responses to HEAD), definitions of new methods cannot change the parsing algorithm or prohibit the presence of a message body on either the request or the response message. Definitions of new methods can specify that only a zero-length message body is allowed by requiring a Content-Length header field with a value of "0".¶

New method definitions need to indicate whether they are safe (Section 5.2.1), idempotent (Section 5.2.2), cacheable (Section 5.2.3), and what semantics are to be associated with the payload body if any is present in the request. If a method is cacheable, the method definition ought to describe how, and under what conditions, a cache can store a response and use it to satisfy a subsequent request.¶

Values to be added to the HTTP status code name space require IETF Review (see [RFC5226], Section 4.1).¶

When it is necessary to express semantics for a response that are not defined by current status codes, a new status code can be registered. HTTP status codes are generic; they are potentially applicable to any resource, not just one particular media type, "type" of resource, or application. 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 7. To allow existing parsers to properly handle them, new status codes cannot disallow a payload, although they can mandate a zero-length payload body.¶

A definition for a new status code ought to explain the request conditions that produce 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 and what fields can modify the semantics). A response that can transfer a payload ought to specify expected cache behavior (e.g., cacheability and freshness criteria, as described in [Part6]) and whether the payload has any implied association with an identified resource (Section 3.1.4.1).¶

Header fields are key:value pairs that can be used to communicate data about the message, its payload, the target resource, or the connection (i.e., control data). See Section 3.2 of [Part1] for a general definition of header field syntax in HTTP messages.¶

The requirements for header field names are defined in Section 4.1 of [RFC3864]. Authors of specifications defining new fields are advised to keep the name as short as practical, and not to prefix them with "X-" if they are to be registered (either immediately or in the future).¶

New header field values typically have their syntax defined using ABNF ([RFC5234]), using the extension defined in Appendix B of [Part1] as necessary, and are usually constrained to the range of ASCII characters. Header fields needing a greater range of characters can use an encoding such as the one defined in [RFC5987].¶

Because commas (",") are used as a generic delimiter between field-values, they need to be treated with care if they are allowed in the field-value's payload. Typically, components that might contain a comma are protected with double-quotes using the quoted-string ABNF production (Section 3.2.4 of [Part1]).¶

Note that double-quote delimiters almost always are used with the quoted-string production; using a different syntax inside double-quotes will likely cause unnecessary confusion.¶

Many header fields use a format including (case-insensitively) named parameters (for instance, Content-Type, defined in Section 3.1.1.5). Allowing both unquoted (token) and quoted (quoted-string) syntax for the parameter value enables recipients to use existing parser components. When allowing both forms, the meaning of a parameter value ought to be independent of the syntax used for it (for an example, see the notes on parameter handling for media types in Section 3.1.1.1).¶

Authors of specifications defining new header fields are advised to consider documenting: ¶

• Whether the field is a single value, or whether it can be a list (delimited by commas; see Section 3.2 of [Part1]).

  If it does not use the list syntax, document how to treat messages where the header field occurs multiple times (a sensible default would be to ignore the header field, but this might not always be the right choice).

  Note that intermediaries and software libraries might combine multiple header field instances into a single one, despite the header field not allowing this. A robust format enables recipients to discover these situations (good example: "Content-Type", as the comma can only appear inside quoted strings; bad example: "Location", as a comma can occur inside a URI).

• Under what conditions the header field can be used; e.g., only in responses or requests, in all messages, only on responses to a particular request method.

• Whether it is appropriate to list the field-name in the Connection header field (i.e., if the header field is to be hop-by-hop, see Section 6.1 of [Part1]).

• Under what conditions intermediaries are allowed to modify the header field's value, insert or delete it.

• How the header field might interact with caching (see [Part6]).

• Whether the header field is useful or allowable in trailers (see Section 4.1 of [Part1]).

• Whether the header field ought to be preserved across redirects.

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

Content Coding registrations MUST include the following fields: ¶

• Name
• Description
• Pointer to specification text

Names of content codings MUST NOT overlap with names of transfer codings (Section 4 of [Part1]), unless the encoding transformation is identical (as is the case for the compression codings defined in Section 4.2 of [Part1]).¶

Values to be added to this name space require IETF Review (see Section 4.1 of [RFC5226]), and MUST conform to the purpose of content coding defined in this section.¶