Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests
draft-ietf-httpbis-p4-conditional-23

Abstract

The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating state changes, request header fields for making preconditions on such state, and rules for constructing the responses to a conditional request when one or more preconditions evaluate to false.

Editorial Note (To be removed by RFC Editor)

Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at http://lists.w3.org/Archives/Public/ietf-http-wg/.

The current issues list is at http://tools.ietf.org/wg/httpbis/trac/report/3 and related documents (including fancy diffs) can be found at http://tools.ietf.org/wg/httpbis/.

The changes in this draft are summarized in Appendix D.4.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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Table of Contents

• 1. Introduction
• 1.1. Conformance and Error Handling
• 1.2. Syntax Notation
• 2. Validators
• 2.1. Weak versus Strong
• 2.2. Last-Modified
• 2.2.1. Generation
• 2.2.2. Comparison
• 2.3. ETag
• 2.3.1. Generation
• 2.3.2. Comparison
• 2.3.3. Example: Entity-tags Varying on Content-Negotiated Resources
• 2.4. When to Use Entity-tags and Last-Modified Dates
• 3. Precondition Header Fields
• 3.1. If-Match
• 3.2. If-None-Match
• 3.3. If-Modified-Since
• 3.4. If-Unmodified-Since
• 3.5. If-Range
• 4. Status Code Definitions
• 4.1. 304 Not Modified
• 4.2. 412 Precondition Failed
• 5. Evaluation and Precedence
• 6. IANA Considerations
• 6.1. Status Code Registration
• 6.2. Header Field Registration
• 7. Security Considerations
• 8. Acknowledgments
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Changes from RFC 2616
• Appendix B. Imported ABNF
• Appendix C. Collected ABNF
• Appendix D. Change Log (to be removed by RFC Editor before publication)
• D.1. Since draft-ietf-httpbis-p4-conditional-19
• D.2. Since draft-ietf-httpbis-p4-conditional-20
• D.3. Since draft-ietf-httpbis-p4-conditional-21
• D.4. Since draft-ietf-httpbis-p4-conditional-22
• Index
• Authors' Addresses

1. Introduction

Conditional requests are HTTP requests [Part2] that include one or more header fields indicating a precondition to be tested before applying the method semantics to the target resource. This document defines the HTTP/1.1 conditional request mechanisms in terms of the architecture, syntax notation, and conformance criteria defined in [Part1].

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 a "selected representation" (Section 3 of [Part2]) 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.

The conditional request preconditions defined by this specification are evaluated by comparing the validators provided in the conditional request header fields to the current validators for the selected representation in the order defined by Section 5.

1.1. Conformance and Error Handling

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].

Conformance criteria and considerations regarding error handling are defined in Section 2.5 of [Part1].

1.2. Syntax Notation

This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] with the list rule extension defined in Section 1.2 of [Part1]. Appendix B describes rules imported from other documents. Appendix C shows the collected ABNF with the list rule expanded.

2. Validators

This specification defines two forms of metadata that are commonly used to observe resource state and test for preconditions: modification dates (Section 2.2) and opaque entity tags (Section 2.3). 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.

2.1. Weak versus Strong

Validators come in two flavors: strong or weak. Weak validators are easy to generate but are far less useful for comparisons. Strong validators are ideal for comparisons but can be very difficult (and occasionally impossible) to generate efficiently. Rather than impose that all forms of resource adhere to the same strength of validator, HTTP exposes the type of validator in use and imposes restrictions on when weak validators can be used as preconditions.

A "strong validator" is representation metadata that changes value whenever a change occurs to the representation data that would be observable in the payload body of a 200 (OK) response to GET.

A strong validator might change for other reasons, such as when a semantically significant part of the representation metadata is changed (e.g., Content-Type), but it is in the best interests of the origin server to only change the value when it is necessary to invalidate the stored responses held by remote caches and authoring tools. A strong validator is unique across all representations of a given resource, such that no two representations of that resource can share the same validator unless their representation data is identical.

Cache entries might persist for arbitrarily long periods, regardless of expiration times. Thus, a cache might attempt to validate an entry using a validator that it obtained in the distant past. A strong validator is unique across all versions of all representations associated with a particular resource over time. However, there is no implication of uniqueness across representations of different resources (i.e., the same strong validator might be in use for representations of multiple resources at the same time and does not imply that those representations are equivalent).

There are a variety of strong validators used in practice. The best are based on strict revision control, wherein each change to a representation always results in a unique node name and revision identifier being assigned before the representation is made accessible to GET. A collision-resistant hash function applied to the representation data is also sufficient if the data is available prior to the response header fields being sent and the digest does not need to be recalculated every time a validation request is received. However, if a resource has distinct representations that differ only in their metadata, such as might occur with content negotiation over media types that happen to share the same data format, then the origin server SHOULD incorporate additional information in the validator to distinguish those representations.

In contrast, a "weak validator" is representation metadata that might not change for every change to the representation data. This weakness might be due to limitations in how the value is calculated, such as clock resolution or an inability to ensure uniqueness for all possible representations of the resource, or due to a desire by the resource owner to group representations by some self-determined set of equivalency rather than unique sequences of data. An origin server SHOULD change a weak entity-tag whenever it considers prior representations to be unacceptable as a substitute for the current representation. In other words, a weak entity-tag ought to change whenever the origin server wants caches to invalidate old responses.

For example, the representation of a weather report that changes in content every second, based on dynamic measurements, might be grouped into sets of equivalent representations (from the origin server's perspective) with the same weak validator in order to allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server load or weather quality). Likewise, a representation's modification time, if defined with only one-second resolution, might be a weak validator if it is possible for the representation to be modified twice during a single second and retrieved between those modifications.

Likewise, a validator is weak if it is shared by two or more representations of a given resource at the same time, unless those representations have identical representation data. For example, if the origin server sends the same validator for a representation with a gzip content coding applied as it does for a representation with no content coding, then that validator is weak. However, two simultaneous representations might share the same strong validator if they differ only in the representation metadata, such as when two different media types are available for the same representation data.

A "use" of a validator occurs when either a client generates a request and includes the validator in a precondition or when a server compares two validators. Weak validators are only usable in contexts that do not depend on exact equality of the representation data. Strong validators are usable and preferred for all conditional requests, including cache validation, partial content ranges, and "lost update" avoidance.

2.2. Last-Modified

The "Last-Modified" header field in a response provides a timestamp indicating the date and time at which the origin server believes the selected representation was last modified, as determined at the conclusion of handling the request.

  Last-Modified = HTTP-date

An example of its use is

  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT

2.2.1. Generation

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.

An origin server without a clock MUST NOT assign Last-Modified values to a response unless these values were associated with the resource by some other system or user with a reliable clock.

2.2.2. Comparison

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:

• The validator is being compared by an origin server to the actual current validator for the representation and,
• That origin server reliably knows that the associated representation did not change twice during the second covered by the presented validator.

or

• The validator is about to be used by a client in an If-Modified-Since [header.if-modified-since], If-Unmodified-Since [header.if-unmodified-since] header field, because the client has a cache entry, or If-Range for the associated representation, and
• That cache entry includes a Date value, which gives the time when the origin server sent the original response, and
• The presented Last-Modified time is at least 60 seconds before the Date value.

or

• The validator is being compared by an intermediate cache to the validator stored in its cache entry for the representation, and
• That cache entry includes a Date value, which gives the time when the origin server sent the original response, and
• The presented Last-Modified time is at least 60 seconds before the Date value.

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.

2.3. ETag

The "ETag" header field in a response provides the current entity-tag for the selected representation, as determined at the conclusion of handling the request. An entity-tag is an opaque validator for differentiating between multiple representations of the same resource, regardless of whether those multiple representations are due to resource state changes over time, content negotiation resulting in multiple representations being valid at the same time, or both. An entity-tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.

  ETag       = entity-tag

  entity-tag = [ weak ] opaque-tag
  weak       = %x57.2F ; "W/", case-sensitive
  opaque-tag = DQUOTE *etagc DQUOTE
  etagc      = %x21 / %x23-7E / obs-text
             ; VCHAR except double quotes, plus obs-text

• Note: Previously, opaque-tag was defined to be a quoted-string ([RFC2616], Section 3.11), thus some recipients might perform backslash unescaping. Servers therefore ought to avoid backslash characters in entity tags.

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.

Examples:

  ETag: "xyzzy"
  ETag: W/"xyzzy"
  ETag: ""

An entity-tag can be either a weak or strong validator, with strong being the default. If an origin server provides an entity-tag for a representation and the generation of that entity-tag does not satisfy all of the characteristics of a strong validator (Section 2.1), then the origin server MUST mark the entity-tag as weak by prefixing its opaque value with "W/" (case-sensitive).

2.3.1. Generation

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 collision-resistant 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.

2.3.2. Comparison

There are two entity-tag comparison functions, depending on whether the comparison context allows the use of weak validators or not:

• Strong comparison: two entity-tags are equivalent if both are not weak and their opaque-tags match character-by-character.
• Weak comparison: two entity-tags are equivalent if their opaque-tags match character-by-character, regardless of either or both being tagged as "weak".

The example below shows the results for a set of entity-tag pairs, and both the weak and strong comparison function results:

2.3.3. Example: Entity-tags Varying on Content-Negotiated Resources

Consider a resource that is subject to content negotiation (Section 3.4 of [Part2]), and where the representations sent in response to a GET request vary based on the Accept-Encoding request header field (Section 5.3.4 of [Part2]):

>> Request:

  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:

>> Response:

  HTTP/1.1 200 OK
  Date: Fri, 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:

>> Response:

  HTTP/1.1 200 OK
  Date: Fri, 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...

• Note: Content codings are a property of the representation, so therefore an entity-tag of an encoded representation has to be distinct from an unencoded representation to prevent conflicts during cache updates and range requests. In contrast, transfer codings (Section 4 of [Part1]) apply only during message transfer and do not require distinct entity-tags.

2.4. When to Use Entity-tags and Last-Modified Dates

We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types ought to be used, and for what purposes.

In 200 (OK) responses to GET or HEAD, an origin server:

• SHOULD send an entity-tag validator unless it is not feasible to generate one.
• MAY send a weak entity-tag instead of a strong entity-tag, if performance considerations support the use of weak entity-tags, or if it is unfeasible to send a strong entity-tag.
• SHOULD send a Last-Modified [header.last-modified] value if it is feasible to send one.

In other words, the preferred behavior for an origin server is to send both a strong entity-tag and a Last-Modified [header.last-modified] value in successful responses to a retrieval request.

A client:

• MUST use that entity-tag in any cache-conditional request (using If-Match [header.if-match] or If-None-Match [header.if-none-match]) if an entity-tag has been provided by the origin server.
• SHOULD use the Last-Modified [header.last-modified] value in non-subrange cache-conditional requests (using If-Modified-Since [header.if-modified-since]) if only a Last-Modified value has been provided by the origin server.
• MAY use the Last-Modified [header.last-modified] value in subrange cache-conditional requests (using If-Unmodified-Since [header.if-unmodified-since]) if only a Last-Modified value has been provided by an HTTP/1.0 origin server. The user agent SHOULD provide a way to disable this, in case of difficulty.
• SHOULD use both validators in cache-conditional requests if both an entity-tag and a Last-Modified [header.last-modified] value have been provided by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.

3. Precondition Header Fields

This section defines the syntax and semantics of HTTP/1.1 header fields for applying preconditions on requests. Section 5 defines when the preconditions are applied and the order of evaluation when more than one precondition is present.

3.1. If-Match

The "If-Match" header field can 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-Match = "*" / 1#entity-tag

The If-Match condition is met if and only if any of the entity-tags listed in the If-Match field value match the entity-tag of the selected representation using the weak comparison function (as per Section 2.3.2), or if "*" is given and any current representation exists for the target resource.

If the condition is met, the server MAY perform the request method.

Origin servers MUST NOT perform the requested method if the condition is not met; instead they MUST respond with the 412 (Precondition Failed) [status.412] status code.

Proxy servers using a cached response as the selected representation MUST NOT perform the requested method if the condition is not met; instead, they MUST forward the request towards the origin server.

Examples:

  If-Match: "xyzzy"
  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
  If-Match: *

3.2. If-None-Match

The "If-None-Match" header field can 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 (Not Modified) [status.304] response if at least one of those representations matches the selected representation.

If-None-Match can also be used with a value of "*" to prevent an unsafe request method (e.g., PUT) from inadvertently modifying an existing representation of the target resource when the client believes that the resource does not have a current representation (Section 4.2.1 of [Part2]). 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-None-Match = "*" / 1#entity-tag

The If-None-Match condition is met if and only if none of the entity-tags listed in the If-None-Match field value match the entity-tag of the selected representation using the weak comparison function (as per Section 2.3.2), or if "*" is given and no current representation exists for that resource.

If the condition is not met, 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.304] status code, including the cache-related header fields (particularly ETag [header.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.412] status code when the condition is not met.

If the condition is met, the server MAY perform the requested method and MUST ignore any If-Modified-Since [header.if-modified-since] header field(s) in the request. That is, if no entity-tags match, then the server MUST NOT send a 304 (Not Modified) [status.304] response.

Examples:

  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: *

3.3. If-Modified-Since

The "If-Modified-Since" header field can be used with GET or HEAD to make the 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.

  If-Modified-Since = HTTP-date

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:

1. If the request would normally result in anything other than a 200 (OK) status code, or if the passed If-Modified-Since date is invalid, the response is exactly the same as for a normal GET. A date that is later than the server's current time is invalid.
2. If the selected representation has been modified since the If-Modified-Since date, the response is exactly the same as for a normal GET.
3. If the selected representation has not been modified since a valid If-Modified-Since date, the server SHOULD send a 304 (Not Modified) [status.304] response.

The two purposes of this feature are to allow efficient updates of cached information, with a minimum amount of transaction overhead, and to limit the scope of a web traversal to resources that have recently changed.

When used for cache updates, a cache will typically use the value of the cached message's Last-Modified [header.last-modified] field to generate the field value of If-Modified-Since. This behavior is most interoperable for cases where clocks are poorly synchronized or when the server has chosen to only honor exact timestamp matches (due to a problem with Last-Modified dates that appear to go "back in time" when the origin server's clock is corrected or a representation is restored from an archived backup). However, caches occasionally generate the field value based on other data, such as the Date header field of the cached message or the local clock time that the message was received, particularly when the cached message does not contain a Last-Modified [header.last-modified] field.

When used for limiting the scope of retrieval to a recent time window, a user agent will generate an If-Modified-Since field value based on either its own local clock or a Date header field received from the server during a past run. Origin servers that choose an exact timestamp match based on the selected representation's Last-Modified [header.last-modified] field will not be able to help the user agent limit its data transfers to only those changed during the specified window.

• Note: If a client uses an arbitrary date in the If-Modified-Since header field instead of a date taken from a Last-Modified [header.last-modified] or Date header field from the origin server, the client ought to be aware that its date will be interpreted according to the server's understanding of time.

3.4. If-Unmodified-Since

The "If-Unmodified-Since" header field can 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.412] status code. If the selected representation has not been modified since the time specified in this field, the server MAY perform the request.

  If-Unmodified-Since = HTTP-date

An example of the field is:

  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT

A server MUST ignore the If-Unmodified-Since header field if the received value is not a valid HTTP-date.

3.5. If-Range

The "If-Range" header field provides a special conditional request mechanism that is similar to If-Match [header.if-match] and If-Unmodified-Since [header.if-unmodified-since] but specific to range requests. If-Range is defined in Section 3.2 of [Part5].

4. Status Code Definitions

4.1. 304 Not Modified

The 304 (Not Modified) status code indicates that a conditional GET or HEAD request has been received and would have resulted in a 200 (OK) response if it were not for the fact that the condition has evaluated to false. In other words, there is no need for the server to transfer a representation of the target resource because the request indicates that the client, which made the request conditional, already has a valid representation; the server is therefore redirecting the client to make use of that stored representation as if it were the payload of a 200 (OK) response.

The server generating a 304 response MUST generate any of the following header fields that would have been sent in a 200 (OK) response to the same request: Cache-Control, Content-Location, ETag [header.etag], Expires, and Vary.

Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations, a sender SHOULD NOT generate representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding cache updates (e.g., Last-Modified [header.last-modified] might be useful if the response does not have an ETag [header.etag] field).

Requirements on a cache that receives a 304 response are defined in Section 4.2.1 of [Part6]. If the conditional request originated with an outbound client, such as a user agent with its own cache sending a conditional GET to a shared proxy, then the proxy SHOULD forward the 304 response to that client.

A 304 response cannot contain a message-body; it is always terminated by the first empty line after the header fields.

4.2. 412 Precondition Failed

The 412 (Precondition Failed) 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.

5. Evaluation and Precedence

For each conditional request, a server MUST evaluate the request preconditions after it has successfully performed its normal request checks (i.e., just before it would perform the action associated with the request method). Preconditions are ignored if the server determines that an error or redirect response applies before they are evaluated. Otherwise, the evaluation depends on both the method semantics and the choice of conditional.

A conditional request header field that is designed specifically for cache validation, which includes If-None-Match [header.if-none-match] and If-Modified-Since [header.if-modified-since] when used in a GET or HEAD request, allows cached representations to be refreshed without repeatedly transferring data already held by the client. Evaluating to false is thus an indication that the client can continue to use its local copy of the selected representation, as indicated by the server generating a 304 (Not Modified) [status.304] response that includes only those header fields useful for refreshing the cached representation.

All other conditionals are intended to signal failure when the precondition evaluates to false. For example, an If-Match [header.if-match] conditional sent with a state-changing method (e.g., POST, PUT, DELETE) is intended to prevent the request from taking effect on the target resource if the resource state does not match the expected state. In other words, evaluating the condition to false means that the resource has been changed by some other client, perhaps by another user attempting to edit the same resource, and thus preventing the request from being applied saves the client from overwriting some other client's work. This result is indicated by the server generating a 412 (Precondition Failed) [status.412] response.

The conditional request header fields defined by this specification are ignored for request methods that never involve the selection or modification of a selected representation (e.g., CONNECT, OPTIONS, and TRACE). Other conditional request header fields, defined by extensions to HTTP, might place conditions on the state of the target resource in general, or on a group of resources. For instance, the If header field in WebDAV can make a request conditional on various aspects (such as locks) of multiple resources ([RFC4918], Section 10.4).

When more than one conditional request header field is present in a request, the order in which the fields are evaluated becomes important. In practice, the fields defined in this document are consistently implemented in a single, logical order, due to the fact that entity tags are presumed to be more accurate than date validators. For example, the only reason to send both If-Modified-Since [header.if-modified-since] and If-None-Match [header.if-none-match] in the same GET request is to support intermediary caches that might not have implemented If-None-Match [header.if-none-match], so it makes sense to ignore the If-Modified-Since [header.if-modified-since] when entity tags are understood and available for the selected representation.

The general rule of conditional precedence is that exact match conditions are evaluated before cache-validating conditions and, within that order, last-modified conditions are only evaluated if the corresponding entity tag condition is not present (or not applicable because the selected representation does not have an entity tag).

Specifically, the fields defined by this specification are evaluated as follows:

1. When If-Match [header.if-match] is present, evaluate it:
   • if true, continue to step [precedence3]
   • if false, respond 412 (Precondition Failed) [status.412]

2. When If-Match [header.if-match] is not present and If-Unmodified-Since [header.if-unmodified-since] is present, evaluate it:
   • if true, continue to step [precedence3]
   • if false, respond 412 (Precondition Failed) [status.412]

3. When If-None-Match [header.if-none-match] is present, evaluate it:
   • if true, continue to step [precedence5]
   • if false for GET/HEAD, respond 304 (Not Modified) [status.304]
   • if false for other methods, respond 412 (Precondition Failed) [status.412]

4. When the method is GET or HEAD, If-None-Match [header.if-none-match] is not present, and If-Modified-Since [header.if-modified-since] is present, evaluate it:
   • if true, continue to step [precedence5]
   • if false, respond 304 (Not Modified) [status.304]

5. When the method is GET and both Range and If-Range are present, evaluate If-Range:
   • if the validator matches and the Range specification is applicable to the selected representation, respond 206 (Partial Content) [Part5]

6. Otherwise,
   • all conditions are met, so perform the requested action and respond according to its success or failure.

6. IANA Considerations

6.1. Status Code Registration

The HTTP Status Code Registry located at http://www.iana.org/assignments/http-status-codes shall be updated with the registrations below:

  OWS           = <OWS, defined in [Part1], Section 3.2.3>
  obs-text      = <obs-text, defined in [Part1], Section 3.2.6>

  HTTP-date     = <HTTP-date, defined in [Part2], Section 7.1.1.1>

ETag = entity-tag

HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1>

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 3.2.3>

entity-tag = [ weak ] opaque-tag
etagc = "!" / %x23-7E ; '#'-'~'
 / obs-text

obs-text = <obs-text, defined in [Part1], Section 3.2.6>
opaque-tag = DQUOTE *etagc DQUOTE

weak = %x57.2F ; W/