HTTP/1.1, part 2: Message Semantics

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶

This document defines conformance criteria for several roles in HTTP communication, including Senders, Recipients, Clients, Servers, User-Agents, Origin Servers, Intermediaries, Proxies and Gateways. See Section 2 of [Part1] for definitions of these terms.¶

An implementation is considered conformant if it complies with all of the requirements associated with its role(s). Note that SHOULD-level requirements are relevant here, unless one of the documented exceptions is applicable.¶

This document also uses ABNF to define valid protocol elements (Section 1.2). In addition to the prose requirements placed upon them, Senders MUST NOT generate protocol elements that are invalid.¶

Unless noted otherwise, Recipients MAY take steps to recover a usable protocol element from an invalid construct. However, HTTP does not define specific error handling mechanisms, except in cases where it has direct impact on security. This is because different uses of the protocol require different error handling strategies; for example, a Web browser may wish to transparently recover from a response where the Location header field doesn't parse according to the ABNF, whereby in a systems control protocol using HTTP, this type of error recovery could lead to dangerous consequences.¶

This specification uses the ABNF syntax defined in Section 1.2 of [Part1] (which extends the syntax defined in [RFC5234] with a list rule). Appendix B shows the collected ABNF, with the list rule expanded.¶

The following core rules are included by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII character).¶

The methods listed below are defined in Section 6.¶

Note that this list is not exhaustive — it does not include request methods defined in other specifications.¶

The HTTP Method Registry defines the name space for the Method token in the Request line of an HTTP request.¶

Registrations MUST include the following fields: ¶

• Method Name (see Section 2)
• Safe ("yes" or "no", see Section 6.1.1)
• Pointer to specification text

Values to be added to this name space are subject to IETF review ([RFC5226], Section 4.1).¶

The registry itself is maintained at <http://www.iana.org/assignments/http-methods>.¶

New header fields are registered using the procedures described in [RFC3864].¶

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 extensions defined in Section 1.2.1 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 3.2.3 of [Part1]).¶

Many header fields use a format including (case-insensitively) named parameters (for instance, Content-Type, defined in Section 6.8 of [Part3]). 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 2.3 of [Part3]).¶

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 (i.e., if the header is to be hop-by-hop, see Section 8.1 of [Part1]).

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

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

• Whether the header field is useful or allowable in trailers (see Section 5.1.1 of [Part1]).

• Whether the header field should be preserved across redirects.

The request header fields allow the client to pass additional information about the request, and about the client itself, to the server. These fields act as request modifiers, with semantics equivalent to the parameters on a programming language method invocation.¶

The response header fields allow the server to pass additional information about the response which cannot be placed in the Status-Line. These header fields give information about the server and about further access to the target resource (Section 4.3 of [Part1]).¶

The status codes listed below are defined in Section 7 of this specification, Section 4 of [Part4], Section 3 of [Part5], and Section 3 of [Part7]. The reason phrases listed here are only recommendations — they can be replaced by local equivalents without affecting the protocol.¶

Note that this list is not exhaustive — it does not include extension status codes defined in other specifications.¶

The HTTP Status Code Registry defines the name space for the Status-Code token in the Status-Line of an HTTP response.¶

Values to be added to this name space are subject to IETF review ([RFC5226], Section 4.1).¶

The registry itself is maintained at <http://www.iana.org/assignments/http-status-codes>.¶

It is sometimes necessary to determine an identifier for the resource associated with a representation.¶

An HTTP request representation, when present, is always associated with an anonymous (i.e., unidentified) resource.¶

1. If the response status code is 200 or 203 and the request method was GET, the response payload is a representation of the target resource.
2. If the response status code is 204, 206, or 304 and the request method was GET or HEAD, the response payload is a partial representation of the target resource.
3. If the response has a Content-Location header field, and that URI is the same as the effective request URI, the response payload is a representation of the target resource.
4. If the response has a Content-Location header field, and that URI is not the same as the effective request URI, then the response asserts that its payload is a representation of the resource identified by the Content-Location URI. However, such an assertion cannot be trusted unless it can be verified by other means (not defined by HTTP).
5. Otherwise, the response is a representation of an anonymous (i.e., unidentified) resource.

[TODO-req-uri: The comparison function is going to have to be defined somewhere, because we already need to compare URIs for things like cache invalidation.] ¶

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 message-body (as indicated by the presence of Content-Length or Transfer-Encoding), 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 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 compliance (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 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 body, if any, SHOULD also include information about the communication options. The format for such a body is not defined by this specification, but might be defined by future extensions to HTTP. Content negotiation MAY be used to select the appropriate response format. If no response 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 9.6). If no Max-Forwards field is present in the request, then the forwarded request MUST NOT include a Max-Forwards field.¶

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. 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. 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.¶

Bodies on GET requests have no defined semantics. Note that sending a 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 11.2 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 body. 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; see [Part6]. It also MAY be used to update a previously cached representation from that resource; if the new field values indicate that the cached representation differs from the current representation (as would be indicated by a change in Content-Length, ETag or Last-Modified), then the cache MUST treat the cache entry as stale.¶

Bodies on HEAD requests have no defined semantics. Note that sending a 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 9.5).¶

Responses to POST requests are only cacheable when they include explicit freshness information (see Section 2.3.1 of [Part6]). A cached POST response with a Content-Location header field (see Section 6.7 of [Part3]) whose value is the effective Request URI MAY be used to satisfy subsequent GET and HEAD 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 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: (a) reconfigure the target resource to reflect the new media type; (b) transform the PUT representation to a format consistent with that of the resource before saving it as the new resource state; or, (c) reject the request with a 415 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, 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 2.5 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 an 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.¶

Bodies on DELETE requests have no defined semantics. Note that sending a 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 2.5 of [Part6]).¶

The TRACE method requests a remote, application-layer 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 9.6). 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 8.8 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 9.3.1 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 CONNECT method requests that the proxy establish a tunnel to the request-target and then 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 3.1.1.2 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,¶

Other HTTP mechanisms can be used normally with the CONNECT method — except end-to-end protocol Upgrade requests, since the tunnel must be established first.¶

Bodies on CONNECT requests have no defined semantics. Note that sending a body on a CONNECT request might cause some existing implementations to reject the request.¶

Like any other pipelined HTTP/1.1 request, data to be tunnel may be sent immediately after the blank line. The usual caveats also apply: data may be discarded if the eventual response is negative, and the connection may be reset with no response if more than one TCP segment is outstanding.¶

This class of status code indicates a provisional response, consisting only of the Status-Line and optional header fields, and is terminated by an empty line. There are no required header fields for this class of status code. Since HTTP/1.0 did not define any 1xx status codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client except under experimental conditions.¶

A client MUST be prepared to accept one or more 1xx status responses prior to a regular response, even if the client does not expect a 100 (Continue) status message. Unexpected 1xx status responses MAY be ignored by a user agent.¶

Proxies MUST forward 1xx responses, unless the connection between the proxy and its client has been closed, or unless the proxy itself requested the generation of the 1xx response. (For example, if a proxy adds a "Expect: 100-continue" field when it forwards a request, then it need not forward the corresponding 100 (Continue) response(s).)¶

This class of status code indicates that the client's request was successfully received, understood, and accepted.¶

This class of status code indicates that further action needs to be taken by the user agent in order to fulfill the request. If the required action involves a subsequent HTTP request, it MAY be carried out by the user agent without interaction with the user if and only if the method used in the second request is known to be "safe", as defined in Section 6.1.1.¶

There are several types of redirects: ¶

1. Redirects of the request to another URI, either temporarily or permanently. The new URI is specified in the Location header field. In this specification, the status codes 301 (Moved Permanently), 302 (Found), and 307 (Temporary Redirect) fall under this category.

2. Redirection to a new location that represents an indirect response to the request, such as the result of a POST operation to be retrieved with a subsequent GET request. This is status code 303 (See Other).

3. Redirection offering a choice of matching resources for use by agent-driven content negotiation (Section 5.2 of [Part3]). This is status code 300 (Multiple Choices).

4. Other kinds of redirection, such as to a cached result (status code 304 (Not Modified)).

Clients SHOULD detect and intervene in cyclical redirections (i.e., "infinite" redirection loops).¶

The 4xx class of status code is intended for cases in which the client seems to have erred. Except when responding to a HEAD request, the server SHOULD include a representation containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. User agents SHOULD display any included representation to the user.¶

If the client is sending data, a server implementation using TCP SHOULD be careful to ensure that the client acknowledges receipt of the packet(s) containing the response, before the server closes the input connection. If the client continues sending data to the server after the close, the server's TCP stack will send a reset packet to the client, which might erase the client's unacknowledged input buffers before they can be read and interpreted by the HTTP application.¶

Response status codes beginning with the digit "5" indicate cases in which the server is aware that it has erred or is incapable of performing the request. Except when responding to a HEAD request, the server SHOULD include a representation containing an explanation of the error situation, and whether it is a temporary or permanent condition. User agents SHOULD display any included representation to the user. These response codes are applicable to any request method.¶

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 "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 8; it MUST be sent in rfc1123-date format.¶

An example is¶

Origin servers MUST include a Date header field in all responses, except in these cases: ¶

1. If the response status code is 100 (Continue) or 101 (Switching Protocols), the response MAY include a Date header field, at the server's option.
2. If the response status code conveys a server error, e.g., 500 (Internal Server Error) or 503 (Service Unavailable), and it is inconvenient or impossible to generate a valid Date.
3. If the server does not have a clock that can provide a reasonable approximation of the current time, its responses MUST NOT include a Date header field.

A received message that does not have a Date header field MUST be assigned one by the recipient if the message will be cached by that recipient.¶

Clients can use the Date header field as well; in order to keep request messages small, they are advised not to include it when it doesn't convey any useful information (as it is usually the case for requests that do not contain a payload).¶

The HTTP-date sent in a Date header field SHOULD NOT represent a date and time subsequent to the generation of the message. It SHOULD represent the best available approximation of the date and time of message generation, unless the implementation has no means of generating a reasonably accurate date and time. In theory, the date ought to represent the moment just before the payload is generated. In practice, the date can be generated at any time during the message origination without affecting its semantic value.¶

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

A server that does not understand or is unable to comply with any of the expectation values in the Expect field of a request MUST respond with appropriate error status code. The server MUST respond with a 417 (Expectation Failed) status code if any of the expectations cannot be met or, if there are other problems with the request, some other 4xx status code.¶

This header field is defined with extensible syntax to allow for future extensions. If a server receives a request containing an Expect field that includes an expectation-extension that it does not support, it MUST respond with a 417 (Expectation Failed) status code.¶

Comparison of expectation values is case-insensitive for unquoted tokens (including the 100-continue token), and is case-sensitive for quoted-string expectation-extensions.¶

The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST return a 417 (Expectation Failed) status code if it receives a request with an expectation that it cannot meet. 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.¶

See Section 6.2.3 of [Part1] for the use of the 100 (Continue) status code.¶

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 "Location" header field is used to identify a newly created resource, or to redirect the recipient to a different location for completion of the request.¶

For 201 (Created) responses, the Location is the URI of the new resource which was created by the request. For 3xx 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).¶

Examples are:

  Location: http://www.example.org/pub/WWW/People.html#tim

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 "Max-Forwards" header field provides a mechanism with the TRACE (Section 6.8) and OPTIONS (Section 6.2) 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 in 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 "Referer" [sic] header field allows the client to specify the URI of the resource from which the effective request 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 effective request 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 11.2 for security considerations.¶

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 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 "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 5.2 of [Part1]) 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 8.8 of [Part1]).¶

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 5.2 of [Part1]) 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 registration procedure for HTTP request methods is defined by Section 2.2 of this document.¶

The registration procedure for HTTP Status Codes — previously defined in Section 7.1 of [RFC2817] — is now defined by Section 4.2 of this document.¶

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

Like any generic data transfer protocol, HTTP cannot regulate the content of the data that is transferred, nor is there any a priori method of determining the sensitivity of any particular piece of information within the context of any given request. Therefore, applications SHOULD supply as much control over this information as possible to the provider of that information. Four header fields are worth special mention in this context: Server, Via, Referer and From.¶

Revealing the specific software version of the server might allow the server machine to become more vulnerable to attacks against software that is known to contain security holes. Implementors SHOULD make the Server header field a configurable option.¶

Proxies which serve as a portal through a network firewall SHOULD take special precautions regarding the transfer of header information that identifies the hosts behind the firewall. In particular, they SHOULD remove, or replace with sanitized versions, any Via fields generated behind the firewall.¶

The Referer header field allows reading patterns to be studied and reverse links drawn. Although it can be very useful, its power can be abused if user details are not separated from the information contained in the Referer. Even when the personal information has been removed, the Referer header field might indicate a private document's URI whose publication would be inappropriate.¶

The information sent in the From field might conflict with the user's privacy interests or their site's security policy, and hence it SHOULD NOT be transmitted without the user being able to disable, enable, and modify the contents of the field. The user MUST be able to set the contents of this field within a user preference or application defaults configuration.¶

We suggest, though do not require, that a convenient toggle interface be provided for the user to enable or disable the sending of From and Referer information.¶

The User-Agent (Section 9.10) or Server (Section 9.9) header fields can sometimes be used to determine that a specific client or server have a particular security hole which might be exploited. Unfortunately, this same information is often used for other valuable purposes for which HTTP currently has no better mechanism.¶

Furthermore, the User-Agent header field may contain enough entropy to be used, possibly in conjunction with other material, to uniquely identify the user.¶

Some request methods, like TRACE (Section 6.8), expose information that was sent in request header fields within the body of their response. Clients SHOULD be careful with sensitive information, like Cookies, Authorization credentials, and other header fields that might be used to collect data from the client.¶

Because the source of a link might be private information or might reveal an otherwise private information source, it is strongly recommended that the user be able to select whether or not the Referer field is sent. For example, a browser client could have a toggle switch for browsing openly/anonymously, which would respectively enable/disable the sending of Referer and From information.¶

Clients SHOULD NOT include a Referer header field in a (non-secure) HTTP request if the referring page was transferred with a secure protocol.¶

Authors of services SHOULD NOT use GET-based forms for the submission of sensitive data because that data will be placed in the request-target. Many existing servers, proxies, and user agents log or display the request-target in places where it might be visible to third parties. Such services can use POST-based form submission instead.¶

If a single server supports multiple organizations that do not trust one another, then it MUST check the values of Location and Content-Location header fields in responses that are generated under control of said organizations to make sure that they do not attempt to invalidate resources over which they have no authority.¶

Since tunneled data is opaque to the proxy, there are additional risks to tunneling to other well-known or reserved ports. A HTTP client CONNECTing to port 25 could relay spam via SMTP, for example. As such, proxies SHOULD restrict CONNECT access to a small number of known ports.¶

13.1. Normative References

[Part1]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 1: URIs, Connections, and Message Parsing”, Internet-Draft draft-ietf-httpbis-p1-messaging-17 (work in progress), October 2011.

[Part3]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 3: Message Payload and Content Negotiation”, Internet-Draft draft-ietf-httpbis-p3-payload-17 (work in progress), October 2011.

[Part4]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 4: Conditional Requests”, Internet-Draft draft-ietf-httpbis-p4-conditional-17 (work in progress), October 2011.

[Part5]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 5: Range Requests and Partial Responses”, Internet-Draft draft-ietf-httpbis-p5-range-17 (work in progress), October 2011.

[Part6]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “HTTP/1.1, part 6: Caching”, Internet-Draft draft-ietf-httpbis-p6-cache-17 (work in progress), October 2011.

[Part7]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 7: Authentication”, Internet-Draft draft-ietf-httpbis-p7-auth-17 (work in progress), October 2011.

[RFC2119]

Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.

[RFC3986]

Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax”, STD 66, RFC 3986, January 2005.

[RFC5234]

Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, January 2008.

13.2. Informative References

[RFC1123]

Braden, R., “Requirements for Internet Hosts - Application and Support”, STD 3, RFC 1123, October 1989.

[RFC1945]

Berners-Lee, T., Fielding, R., and H. Nielsen, “Hypertext Transfer Protocol -- HTTP/1.0”, RFC 1945, May 1996.

[RFC2068]

Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2068, January 1997.

[RFC2616]

Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2616, June 1999.

[RFC2817]

Khare, R. and S. Lawrence, “Upgrading to TLS Within HTTP/1.1”, RFC 2817, May 2000.

[RFC3864]

Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004.

[RFC5226]

Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, May 2008.

[RFC5322]

Resnick, P., “Internet Message Format”, RFC 5322, October 2008.

[RFC5789]

Dusseault, L. and J. Snell, “PATCH Method for HTTP”, RFC 5789, March 2010.

[RFC5987]

Reschke, J., “Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters”, RFC 5987, August 2010.

Extracted relevant partitions from [RFC2616].¶

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/5>: "Via is a MUST" (<http://purl.org/NET/http-errata#via-must>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/6>: "Fragments allowed in Location" (<http://purl.org/NET/http-errata#location-fragments>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/10>: "Safe Methods vs Redirection" (<http://purl.org/NET/http-errata#saferedirect>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/17>: "Revise description of the POST method" (<http://purl.org/NET/http-errata#post>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and Informative references"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/42>: "RFC2606 Compliance"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative references"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/84>: "Redundant cross-references"

Other changes: ¶

• Move definitions of 304 and 412 condition codes to [Part4]

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/21>: "PUT side effects"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/91>: "Duplicate Host header requirements"

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>): ¶

• Move "Product Tokens" section (back) into Part 1, as "token" is used in the definition of the Upgrade header field.
• Add explicit references to BNF syntax and rules imported from other parts of the specification.
• Copy definition of delta-seconds from Part6 instead of referencing it.

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/24>: "Requiring Allow in 405 responses"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/59>: "Status Code Registry"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/61>: "Redirection vs. Location"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/70>: "Cacheability of 303 response"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/76>: "305 Use Proxy"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/105>: "Classification for Allow header"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/112>: "PUT - 'store under' vs 'store at'"

Ongoing work on IANA Message Header Field Registration (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>): ¶

• Reference RFC 3984, and update header field registrations for headers defined in this document.

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>): ¶

• Replace string literals when the string really is case-sensitive (method).

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/98>: "OPTIONS request bodies"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/119>: "Description of CONNECT should refer to RFC2817"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/125>: "Location Content-Location reference request/response mixup"

Ongoing work on Method Registry (<http://tools.ietf.org/wg/httpbis/trac/ticket/72>): ¶

• Added initial proposal for registration process, plus initial content (non-HTTP/1.1 methods to be added by a separate specification).

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/103>: "Content-*"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/132>: "RFC 2822 is updated by RFC 5322"

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>): ¶

• Use "/" instead of "|" for alternatives.
• Introduce new ABNF rules for "bad" whitespace ("BWS"), optional whitespace ("OWS") and required whitespace ("RWS").
• Rewrite ABNFs to spell out whitespace rules, factor out header field value format definitions.

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/94>: "Reason-Phrase BNF"

Final work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>): ¶

• Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/144>: "Clarify when Referer is sent"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/164>: "status codes vs methods"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/170>: "Do not require "updates" relation for specs that register status codes or method names"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/27>: "Idempotency"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/33>: "TRACE security considerations"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/110>: "Clarify rules for determining what entities a response carries"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/140>: "update note citing RFC 1945 and 2068"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/182>: "update note about redirect limit"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/191>: "Location header ABNF should use 'URI'"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/192>: "fragments in Location vs status 303"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/198>: "move IANA registrations for optional status codes"

Partly resolved issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/171>: "Are OPTIONS and TRACE safe?"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/10>: "Safe Methods vs Redirection" (we missed the introduction to the 3xx status codes when fixing this previously)

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/43>: "Fragment combination / precedence during redirects"

Partly resolved issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/185>: "Location header payload handling"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the requested resource's URI"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/69>: "Clarify 'Requested Variant'"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify entity / representation / variant terminology"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods and Caching"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/190>: "OPTIONS vs Max-Forwards"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/199>: "Status codes and caching"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider removing the 'changes from 2068' sections"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/229>: "Considerations for new status codes"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/230>: "Considerations for new methods"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/232>: "User-Agent guidelines" (relating to the 'User-Agent' header field)

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/43>: "Fragment combination / precedence during redirects" (added warning about having a fragid on the redirect may cause inconvenience in some cases)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/79>: "Content-* vs. PUT"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/88>: "205 Bodies"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/102>: "Understanding Content-* on non-PUT requests"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/103>: "Content-*"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/104>: "Header type defaulting"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/112>: "PUT - 'store under' vs 'store at'"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/137>: "duplicate ABNF for Reason-Phrase"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/180>: "Note special status of Content-* prefix in header registration procedures"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/203>: "Max-Forwards vs extension methods"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/213>: "What is the value space of HTTP status codes?" (actually fixed in draft-ietf-httpbis-p2-semantics-11)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header Classification"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/225>: "PUT side effect: invalidation or just stale?"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/226>: "proxies not supporting certain methods"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/239>: "Migrate CONNECT from RFC2817 to p2"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/240>: "Migrate Upgrade details from RFC2817"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/267>: "clarify PUT semantics'"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/275>: "duplicate ABNF for 'Method'"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle ABNFs for header fields"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle ABNFs for header fields"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/251>: "message-body in CONNECT request"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/255>: "Clarify status code for rate limiting"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/294>: "clarify 403 forbidden"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/296>: "Clarify 203 Non-Authoritative Information"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/298>: "update default reason phrase for 413"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/285>: "Strength of requirements on Accept re: 406"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/303>: "400 response isn't generic"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/160>: "Redirects and non-GET methods"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/186>: "Document HTTP's error-handling philosophy"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/231>: "Considerations for new headers"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/310>: "clarify 303 redirect on HEAD"