The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 4 defines request header fields for indicating conditional requests and the rules for constructing responses to those requests.
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The changes in this draft are summarized in Appendix C.14.
This document defines HTTP/1.1 response metadata for indicating potential changes to payload content, including modification time stamps and opaque entity-tags, and the HTTP conditional request mechanisms that allow preconditions to be placed on a request method. Conditional GET requests allow for efficient cache updates. Other conditional request methods are used to protect against overwriting or misunderstanding the state of a resource that has been changed unbeknownst to the requesting client.¶
This document is currently disorganized in order to minimize the changes between drafts and enable reviewers to see the smaller errata changes. A future draft will reorganize the sections to better reflect the content. In particular, the sections on resource metadata will be discussed first and then followed by each conditional request header field, concluding with a definition of precedence and the expectation of ordering strong validator checks before weak validator checks. It is likely that more content from [Part6] will migrate to this part, where appropriate. The current mess reflects how widely dispersed these topics and associated requirements had become in [RFC2616].¶
An implementation is not compliant if it fails to satisfy one or more of the "MUST" or "REQUIRED" level requirements for the protocols it implements. An implementation that satisfies all the "MUST" or "REQUIRED" level and all the "SHOULD" level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the "MUST" level requirements but not all the "SHOULD" level requirements for its protocols is said to be "conditionally compliant".¶
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), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), VCHAR (any visible USASCII character), and WSP (whitespace).¶
The ABNF rules below are defined in other parts:¶
If the client has performed a conditional GET request and access is allowed, but the document has not been modified, the server SHOULD respond with this status code. The 304 response MUST NOT contain a message-body, and thus is always terminated by the first empty line after the header fields.¶
A 304 response MUST include a Date header field (Section 9.3 of [Part1]) unless its omission is required by Section 9.3.1 of [Part1]. If a 200 response to the same request would have included any of the header fields Cache-Control, Content-Location, ETag, Expires, Last-Modified, or Vary, then those same header fields MUST be sent in a 304 response.¶
Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations, the response SHOULD NOT include representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding cache updates (e.g., future HTTP extensions).¶
If a 304 response includes an entity-tag that indicates a representation not currently cached, then the recipient MUST NOT use the 304 to update its own cache. If that conditional request originated with an outbound client, such as a user agent with its own cache sending a conditional GET to a shared proxy, then the 304 response MAY be forwarded to the outbound client. Otherwise, disregard the response and repeat the request without the conditional.¶
If a cache uses a received 304 response to update a cache entry, the cache MUST update the entry to reflect any new field values given in the response.¶
The precondition given in one or more of the header fields evaluated to false when it was tested on the server. This response code allows the client to place preconditions on the current resource metadata (header field data) and thus prevent the requested method from being applied to a resource other than the one intended.¶
Since both origin servers and caches will compare two validators to decide if they represent the same or different representations, one normally would expect that if the representation (including both representation header fields and representation body) changes in any way, then the associated validator would change as well. If this is true, then we call this validator a "strong validator".¶
However, there might be cases when a server prefers to change the validator only on semantically significant changes, and not when insignificant aspects of the representation change. A validator that does not always change when the representation changes is a "weak validator".¶
An entity-tag is normally a strong validator, but the protocol provides a mechanism to tag an entity-tag as "weak". One can think of a strong validator as one that changes whenever the sequence of bits in a representation changes, while a weak value changes whenever the meaning of a representation changes. Alternatively, one can think of a strong validator as part of an identifier for a specific representation, whereas a weak validator is part of an identifier for a set of semantically equivalent representations. ¶
A "use" of a validator is either when a client generates a request and includes the validator in a validating header field, or when a server compares two validators.¶
Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality of a representation. For example, either kind is usable for a normal conditional GET. However, only a strong validator is usable for a sub-range retrieval, since otherwise the client might end up with an internally inconsistent representation.¶
The only function that HTTP/1.1 defines on validators is comparison. There are two validator comparison functions, depending on whether the comparison context allows the use of weak validators or not: ¶
The example below shows the results for a set of entity-tag pairs, and both the weak and strong comparison function results:¶
|ETag 1||ETag 2||Strong Comparison||Weak Comparison|
|W/"1"||W/"2"||no match||no match|
A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is strong, using the following rules: ¶
This method relies on the fact that if two different responses were sent by the origin server during the same second, but both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified values are generated from different clocks, or at somewhat different times during the preparation of the response. An implementation MAY use a value larger than 60 seconds, if it is believed that 60 seconds is too short.¶
If a client wishes to perform a sub-range retrieval on a value for which it has only a Last-Modified time and no opaque validator, it MAY do this only if the Last-Modified time is strong in the sense described here.¶
These rules allow HTTP/1.1 caches and clients to safely perform sub-range retrievals on values that have been obtained from HTTP/1.0 servers.¶
This section defines the syntax and semantics of HTTP/1.1 header fields related to conditional requests.¶
The "ETag" header field provides the current value of the entity-tag (see Section 2) for one representation of the target resource. An entity-tag is intended for use as a resource-local identifier for differentiating between representations of the same resource that vary over time or via content negotiation (see Section 4).¶
ETag: "xyzzy" ETag: W/"xyzzy" ETag: ""
An entity-tag provides an "opaque" cache validator that allows for more reliable validation than modification dates in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date values is not sufficient, or where the origin server wishes to avoid certain paradoxes that might arise from the use of modification dates.¶
The principle behind entity-tags is that only the service author knows the semantics of a resource well enough to select an appropriate cache validation mechanism, and the specification of any validator comparison function more complex than byte-equality would open up a can of worms. Thus, comparisons of any other header fields (except Last-Modified, for compatibility with HTTP/1.0) are never used for purposes of validating a cache entry.¶
The "If-Match" header field is used to make a request method conditional. A client that has one or more representations previously obtained from the resource can verify that one of those representations is current by including a list of their associated entity-tags in the If-Match header field.¶
This allows efficient updates of cached information with a minimum amount of transaction overhead. It is also used when updating resources, to prevent inadvertent modification of the wrong version of a resource. As a special case, the value "*" matches any current representation of the resource.¶
If any of the entity-tags match the entity-tag of the representation that would have been returned in the response to a similar GET request (without the If-Match header field) on that resource, or if "*" is given and any current representation exists for that resource, then the server MAY perform the requested method as if the If-Match header field did not exist.¶
If none of the entity-tags match, or if "*" is given and no current representation exists, the server MUST NOT perform the requested method, and MUST return a 412 (Precondition Failed) response. This behavior is most useful when the client wants to prevent an updating request method, such as PUT, from modifying a resource that has changed since the client last retrieved it.¶
If the request would, without the If-Match header field, result in anything other than a 2xx or 412 status code, then the If-Match header field MUST be ignored.¶
The meaning of "If-Match: *" is that the request method SHOULD be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see Section 3.5 of [Part6]) exists, and MUST NOT be performed if the representation does not exist.¶
A request intended to update a resource (e.g., a PUT) MAY include an If-Match header field to signal that the request method MUST NOT be applied if the representation corresponding to the If-Match value (a single entity-tag) is no longer a representation of that resource. This allows the user to indicate that they do not wish the request to be successful if the resource has been changed without their knowledge. Examples:¶
If-Match: "xyzzy" If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-Match: *
The result of a request having both an If-Match header field and either an If-None-Match or an If-Modified-Since header fields is undefined by this specification.¶
The "If-Modified-Since" header field is used to make a request method conditional by date: if the representation that would have been transferred in a 200 response to a GET request has not been modified since the time specified in this field, then do not perform the method; instead, respond as detailed below.¶
An example of the field is:¶
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A GET method with an If-Modified-Since header field and no Range header field requests that the representation be transferred only if it has been modified since the date given by the If-Modified-Since header field. The algorithm for determining this includes the following cases: ¶
The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. ¶
The result of a request having both an If-Modified-Since header field and either an If-Match or an If-Unmodified-Since header fields is undefined by this specification.¶
The "If-None-Match" header field is used to make a request method conditional. A client that has one or more representations previously obtained from the resource can verify that none of those representations is current by including a list of their associated entity-tags in the If-None-Match header field.¶
This allows efficient updates of cached information with a minimum amount of transaction overhead. It is also used to prevent a request method (e.g., PUT) from inadvertently modifying an existing resource when the client believes that the resource does not exist.¶
As a special case, the value "*" matches any current representation of the resource.¶
If any of the entity-tags match the entity-tag of the representation that would have been returned in the response to a similar GET request (without the If-None-Match header field) on that resource, or if "*" is given and any current representation exists for that resource, then the server MUST NOT perform the requested method, unless required to do so because the resource's modification date fails to match that supplied in an If-Modified-Since header field in the request. Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 (Not Modified) response, including the cache-related header fields (particularly ETag) of one of the representations that matched. For all other request methods, the server MUST respond with a 412 (Precondition Failed) status code.¶
If none of the entity-tags match, then the server MAY perform the requested method as if the If-None-Match header field did not exist, but MUST also ignore any If-Modified-Since header field(s) in the request. That is, if no entity-tags match, then the server MUST NOT return a 304 (Not Modified) response.¶
If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status code, then the If-None-Match header field MUST be ignored. (See Section 5 for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.)¶
The meaning of "If-None-Match: *" is that the request method MUST NOT be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see Section 3.5 of [Part6]) exists, and SHOULD be performed if the representation does not exist. This feature is intended to be useful in preventing races between PUT operations.¶
If-None-Match: "xyzzy" If-None-Match: W/"xyzzy" If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" If-None-Match: *
The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since header fields is undefined by this specification.¶
The "If-Unmodified-Since" header field is used to make a request method conditional. If the representation that would have been transferred in a 200 response to a GET request on the same resource has not been modified since the time specified in this field, the server SHOULD perform the requested operation as if the If-Unmodified-Since header field were not present.¶
If the representation has been modified since the specified time, the server MUST NOT perform the requested operation, and MUST return a 412 (Precondition Failed).¶
If-Unmodified-Since = "If-Unmodified-Since" ":" OWS If-Unmodified-Since-v If-Unmodified-Since-v = HTTP-date
An example of the field is:¶
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
If the request normally (i.e., without the If-Unmodified-Since header field) would result in anything other than a 2xx or 412 status code, the If-Unmodified-Since header field SHOULD be ignored.¶
If the specified date is invalid, the header field is ignored.¶
The result of a request having both an If-Unmodified-Since header field and either an If-None-Match or an If-Modified-Since header fields is undefined by this specification.¶
The "Last-Modified" header field indicates the date and time at which the origin server believes the representation was last modified.¶
An example of its use is¶
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
A representation is typically the sum of many parts behind the resource interface. The last-modified time would usually be the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can use its value to make conditional requests and test the validity of locally cached responses.¶
An origin server MUST NOT send a Last-Modified date which is later than the server's time of message origination. In such cases, where the resource's last modification would indicate some time in the future, the server MUST replace that date with the message origination date.¶
An origin server SHOULD obtain the Last-Modified value of the representation as close as possible to the time that it generates the Date value of its response. This allows a recipient to make an accurate assessment of the representation's modification time, especially if the representation changes near the time that the response is generated.¶
HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.¶
The Last-Modified header field value is often used as a cache validator. In simple terms, a cache entry is considered to be valid if the representation has not been modified since the Last-Modified value.¶
The Message Header Field Registry located at <http://www.iana.org/assignments/message-headers/message-header-index.html> shall be updated with the permanent registrations below (see [RFC3864]):¶
|Header Field Name||Protocol||Status||Reference|
The change controller is: "IETF (email@example.com) - Internet Engineering Task Force".¶
ETag = "ETag:" OWS ETag-v ETag-v = entity-tag HTTP-date = <HTTP-date, defined in [Part1], Section 6.1> If-Match = "If-Match:" OWS If-Match-v If-Match-v = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS entity-tag ] ) ) If-Modified-Since = "If-Modified-Since:" OWS If-Modified-Since-v If-Modified-Since-v = HTTP-date If-None-Match = "If-None-Match:" OWS If-None-Match-v If-None-Match-v = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS entity-tag ] ) ) If-Unmodified-Since = "If-Unmodified-Since:" OWS If-Unmodified-Since-v If-Unmodified-Since-v = HTTP-date Last-Modified = "Last-Modified:" OWS Last-Modified-v Last-Modified-v = HTTP-date OWS = <OWS, defined in [Part1], Section 1.2.2> entity-tag = [ weak ] opaque-tag opaque-tag = quoted-string quoted-string = <quoted-string, defined in [Part1], Section 1.2.2> weak = %x57.2F ; W/
; ETag defined but not used ; If-Match defined but not used ; If-Modified-Since defined but not used ; If-None-Match defined but not used ; If-Unmodified-Since defined but not used ; Last-Modified defined but not used
Closed issues: ¶
Other changes: ¶
Closed issues: ¶
No significant changes.¶
No significant changes.¶
Closed issues: ¶