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.16.
This document defines the HTTP/1.1 conditional request mechanisms, including both response metadata that can be used to indicate or observe changes to resource state and request header fields that specify preconditions to be checked before performing the action given by the request method. Conditional GET requests are the most efficient mechanism for HTTP cache updates [Part6]. Conditionals can also be applied to state-changing methods, such as PUT and DELETE, to prevent the "lost update" problem: one client accidentally overwriting the work of another client that has been acting in parallel.¶
Conditional request preconditions are based on the state of the target resource as a whole (its current value set) or the state as observed in a previously obtained representation (one value in that set). A resource might have multiple current representations, each with its own observable state. The conditional request mechanisms assume that the mapping of requests to corresponding representations will be consistent over time if the server intends to take advantage of conditionals. Regardless, if the mapping is inconsistent and the server is unable to select the appropriate representation, then no harm will result when the precondition evaluates to false.¶
We use the term "selected representation" to refer to the current representation of the target resource that would have been selected in a successful response if the same request had used the method GET and had excluded all of the conditional request header fields. The conditional request preconditions are evaluated by comparing the values provided in the request header fields to the current metadata for the selected representation.¶
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:¶
This specification defines two forms of metadata that are commonly used to observe resource state and test for preconditions: modification dates and opaque entity tags. Additional metadata that reflects resource state has been defined by various extensions of HTTP, such as WebDAV [RFC4918], that are beyond the scope of this specification. A resource metadata value is referred to as a "validator" when it is used within a precondition.¶
The "Last-Modified" header field indicates the date and time at which the origin server believes the selected representation was last modified.¶
An example of its use is¶
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
Origin servers SHOULD send Last-Modified for any selected representation for which a last modification date can be reasonably and consistently determined, since its use in conditional requests and evaluating cache freshness ([Part6]) results in a substantial reduction of HTTP traffic on the Internet and can be a significant factor in improving service scalability and reliability.¶
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 SHOULD obtain the Last-Modified value of the representation as close as possible to the time that it generates the Date field-value for 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.¶
An origin server with a clock MUST NOT send a Last-Modified date that is later than the server's time of message origination (Date). If the last modification time is derived from implementation-specific metadata that evaluates to some time in the future, according to the origin server's clock, then the origin server MUST replace that value with the message origination date. This prevents a future modification date from having an adverse impact on cache validation.¶
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.¶
The ETag header field provides the current entity-tag for the selected representation. An entity-tag is an opaque validator for differentiating between multiple representations of the same resource, regardless of whether those multiple representations are due to resource state changes over time, content negotiation resulting in multiple representations being valid at the same time, or both. An entity-tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.¶
ETag = entity-tag entity-tag = [ weak ] opaque-tag weak = %x57.2F ; "W/", case-sensitive opaque-tag = quoted-string
An entity-tag can be more reliable for validation than a modification date in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are not consistently maintained.¶
ETag: "xyzzy" ETag: W/"xyzzy" ETag: ""
The principle behind entity-tags is that only the service author knows the implementation of a resource well enough to select the most accurate and efficient validation mechanism for that resource, and that any such mechanism can be mapped to a simple sequence of octets for easy comparison. Since the value is opaque, there is no need for the client to be aware of how each entity-tag is constructed.¶
For example, a resource that has implementation-specific versioning applied to all changes might use an internal revision number, perhaps combined with a variance identifier for content negotiation, to accurately differentiate between representations. Other implementations might use a stored hash of representation content, a combination of various filesystem attributes, or a modification timestamp that has sub-second resolution.¶
Origin servers SHOULD send ETag for any selected representation for which detection of changes can be reasonably and consistently determined, since the entity-tag's use in conditional requests and evaluating cache freshness ([Part6]) can result in a substantial reduction of HTTP network traffic and can be a significant factor in improving service scalability and reliability.¶
Since both origin servers and caches will compare two validators to decide if they indicate 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 that validator a "strong validator". One example of a strong validator is an integer that is incremented in stable storage every time a representation is changed.¶
However, there might be cases when a server prefers to change the validator only when it desires cached representations to be invalidated. For example, the representation of a weather report that changes in content every second, based on dynamic measurements, might be grouped into sets of equivalent representations (from the origin server's perspective) in order to allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server load or weather quality). A validator that does not always change when the representation changes is a "weak validator".¶
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 equivalent representations (where this notion of equivalence is entirely governed by the origin server and beyond the scope of this specification).¶
An entity-tag is normally a strong validator, but the protocol provides a mechanism to tag an entity-tag as "weak". ¶
A strong entity-tag MUST change whenever the associated representation changes in any way. A weak entity-tag SHOULD change whenever the origin server considers prior representations to be unacceptable as a substitute for the current representation. In other words, a weak entity tag SHOULD change whenever the origin server wants caches to invalidate old responses.¶
A "strong entity-tag" MAY be shared by two representations of a resource only if they are equivalent by octet equality.¶
A "weak entity-tag", indicated by the "W/" prefix, MAY be shared by two representations of a resource. A weak entity-tag can only be used for weak comparison.¶
Cache entries might persist for arbitrarily long periods, regardless of expiration times. Thus, a cache might attempt to validate an entry using a validator that it obtained in the distant past. A strong entity-tag MUST be unique across all versions of all representations associated with a particular resource over time. However, there is no implication of uniqueness across entity-tags of different resources (i.e., the same entity-tag value might be in use for representations of multiple resources at the same time and does not imply that those representations are equivalent).¶
There are two entity-tag comparison functions, depending on whether the comparison context allows the use of weak validators or not: ¶
A "use" of a validator is either when a client generates a request and includes the validator in a precondition, 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.¶
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|
An entity-tag is strong unless it is explicitly tagged as weak.¶
Consider a resource that is subject to content negotiation (Section 5 of [Part3]), and where the representations returned upon a GET request vary based on the Accept-Encoding request header field (Section 6.3 of [Part3]):¶
GET /index HTTP/1.1 Host: www.example.com Accept-Encoding: gzip
In this case, the response might or might not use the gzip content coding. If it does not, the response might look like:¶
HTTP/1.1 200 OK Date: Thu, 26 Mar 2010 00:05:00 GMT ETag: "123-a" Content-Length: 70 Vary: Accept-Encoding Content-Type: text/plain Hello World! Hello World! Hello World! Hello World! Hello World!
An alternative representation that does use gzip content coding would be:¶
HTTP/1.1 200 OK Date: Thu, 26 Mar 2010 00:05:00 GMT ETag: "123-b" Content-Length: 43 Vary: Accept-Encoding Content-Type: text/plain Content-Encoding: gzip ...binary data...
This section defines the syntax and semantics of HTTP/1.1 header fields for applying preconditions on requests.¶
The "If-Match" header field MAY be used to make a request method conditional on the current existence or value of an entity-tag for one or more representations of the target resource. If-Match is generally useful for resource update requests, such as PUT requests, as a means for protecting against accidental overwrites when multiple clients are acting in parallel on the same resource (i.e., the "lost update" problem). An If-Match field-value of "*" places the precondition on the existence of any current representation for the target resource.¶
If any of the entity-tags listed in the If-Match field value match (as per Section 2.2.3) the entity-tag of the selected representation for the target resource, or if "*" is given and any current representation exists for the target resource, then the server MAY perform the request method as if the If-Match header field was not present.¶
If none of the entity-tags match, or if "*" is given and no current representation exists, the server MUST NOT perform the requested method. Instead, the server MUST respond with the 412 (Precondition Failed) status code.¶
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.¶
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-None-Match" header field MAY be used to make a request method conditional on not matching any of the current entity-tag values for representations of the target resource. If-None-Match is primarily used in conditional GET requests to enable efficient updates of cached information with a minimum amount of transaction overhead. A client that has one or more representations previously obtained from the target resource can send If-None-Match with a list of the associated entity-tags in the hope of receiving a 304 response if at least one of those representations matches the selected representation.¶
If-None-Match MAY also be used with a value of "*" to prevent an unsafe request method (e.g., PUT) from inadvertently modifying an existing representation of the target resource when the client believes that the resource does not have a current representation. This is a variation on the "lost update" problem that might arise if more than one client attempts to create an initial representation for the target resource.¶
If any of the entity-tags listed in the If-None-Match field-value match (as per Section 2.2.3) the entity-tag of the selected representation, or if "*" is given and any current representation exists for that resource, then the server MUST NOT perform the requested method. Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 (Not Modified) status code, including the cache-related header fields (particularly ETag) of the selected representation that has a matching entity-tag. 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 2.2.4 for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.)¶
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-Modified-Since" header field MAY be used to make a request method conditional by modification date: if the selected representation has not been modified since the time specified in this field, then do not perform the request 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 selected 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-Unmodified-Since" header field MAY be used to make a request method conditional by modification date: if the selected representation has been modified since the time specified in this field, then the server MUST NOT perform the requested operation and MUST instead respond with the 412 (Precondition Failed) status code. If the selected representation has not been modified since the time specified in this field, the server SHOULD perform the request method as if the If-Unmodified-Since header field were not present.¶
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 MUST be 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 304 status code indicates that a conditional GET request has been received and would have resulted in a 200 (OK) response if it were not for the fact that the condition has evaluated to false. In other words, there is no need for the server to transfer a representation of the target resource because the client's request indicates that it already has a valid representation, as indicated by the 304 response header fields, and is therefore redirecting the client to make use of that stored representation as if it were the payload of a 200 response. 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 the recipient of a 304 response does not have a cached representation corresponding to the entity-tag indicated by the 304 response, then the recipient MUST NOT use the 304 to update its own cache. If this 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, the recipient MUST disregard the 304 response and repeat the request without any preconditions.¶
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 412 status code indicates that one or more preconditions given in the request header fields evaluated to false when tested on the server. This response code allows the client to place preconditions on the current resource state (its current representations and metadata) and thus prevent the request method from being applied if the target resource is in an unexpected state.¶
The 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 (firstname.lastname@example.org) - Internet Engineering Task Force".¶
ETag = entity-tag HTTP-date = <HTTP-date, defined in [Part1], Section 6.1> If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS entity-tag ] ) ) If-Modified-Since = HTTP-date If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS entity-tag ] ) ) If-Unmodified-Since = HTTP-date Last-Modified = 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: ¶
Closed issues: ¶