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rfc:rfc7232

Internet Engineering Task Force (IETF) R. Fielding, Ed. Request for Comments: 7232 Adobe Obsoletes: 2616 J. Reschke, Ed. Category: Standards Track greenbytes ISSN: 2070-1721 June 2014

    Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests

Abstract

 The Hypertext Transfer Protocol (HTTP) is a stateless 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.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7232.

Fielding & Reschke Standards Track [Page 1] RFC 7232 HTTP/1.1 Conditional Requests June 2014

Copyright Notice

 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Fielding & Reschke Standards Track [Page 2] RFC 7232 HTTP/1.1 Conditional Requests June 2014

Table of Contents

 1. Introduction ....................................................4
    1.1. Conformance and Error Handling .............................4
    1.2. Syntax Notation ............................................4
 2. Validators ......................................................5
    2.1. Weak versus Strong .........................................5
    2.2. Last-Modified ..............................................7
         2.2.1. Generation ..........................................7
         2.2.2. Comparison ..........................................8
    2.3. ETag .......................................................9
         2.3.1. Generation .........................................10
         2.3.2. Comparison .........................................10
         2.3.3. Example: Entity-Tags Varying on
                Content-Negotiated Resources .......................11
    2.4. When to Use Entity-Tags and Last-Modified Dates ...........12
 3. Precondition Header Fields .....................................13
    3.1. If-Match ..................................................13
    3.2. If-None-Match .............................................14
    3.3. If-Modified-Since .........................................16
    3.4. If-Unmodified-Since .......................................17
    3.5. If-Range ..................................................18
 4. Status Code Definitions ........................................18
    4.1. 304 Not Modified ..........................................18
    4.2. 412 Precondition Failed ...................................19
 5. Evaluation .....................................................19
 6. Precedence .....................................................20
 7. IANA Considerations ............................................22
    7.1. Status Code Registration ..................................22
    7.2. Header Field Registration .................................22
 8. Security Considerations ........................................22
 9. Acknowledgments ................................................23
 10. References ....................................................24
    10.1. Normative References .....................................24
    10.2. Informative References ...................................24
 Appendix A. Changes from RFC 2616 .................................25
 Appendix B. Imported ABNF .........................................25
 Appendix C. Collected ABNF ........................................26
 Index .............................................................27

Fielding & Reschke Standards Track [Page 3] RFC 7232 HTTP/1.1 Conditional Requests June 2014

1. Introduction

 Conditional requests are HTTP requests [RFC7231] 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
 [RFC7230].
 Conditional GET requests are the most efficient mechanism for HTTP
 cache updates [RFC7234].  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 [RFC7231]) 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
 (Section 3) are evaluated when applicable to the recipient
 (Section 5) according to their order of precedence (Section 6).

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 [RFC7230].

1.2. Syntax Notation

 This specification uses the Augmented Backus-Naur Form (ABNF)
 notation of [RFC5234] with a list extension, defined in Section 7 of
 [RFC7230], that allows for compact definition of comma-separated
 lists using a '#' operator (similar to how the '*' operator indicates

Fielding & Reschke Standards Track [Page 4] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 repetition).  Appendix B describes rules imported from other
 documents.  Appendix C shows the collected grammar with all list
 operators expanded to standard ABNF notation.

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 Web Distributed
 Authoring and Versioning (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 reasons other than a change to
 the representation data, 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.
 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).

Fielding & Reschke Standards Track [Page 5] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 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 needs to 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, an inability to ensure uniqueness for all
 possible representations of the resource, or a desire of 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.

Fielding & Reschke Standards Track [Page 6] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 Strong validators are usable for all conditional requests, including
 cache validation, partial content ranges, and "lost update"
 avoidance.  Weak validators are only usable when the client does not
 require exact equality with previously obtained representation data,
 such as when validating a cache entry or limiting a web traversal to
 recent changes.

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

 An origin server 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 ([RFC7234]) 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

Fielding & Reschke Standards Track [Page 7] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 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:
 o  The validator is being compared by an origin server to the actual
    current validator for the representation and,
 o  That origin server reliably knows that the associated
    representation did not change twice during the second covered by
    the presented validator.
 or
 o  The validator is about to be used by a client in an
    If-Modified-Since, If-Unmodified-Since, or If-Range header field,
    because the client has a cache entry for the associated
    representation, and
 o  That cache entry includes a Date value, which gives the time when
    the origin server sent the original response, and
 o  The presented Last-Modified time is at least 60 seconds before the
    Date value.
 or
 o  The validator is being compared by an intermediate cache to the
    validator stored in its cache entry for the representation, and
 o  That cache entry includes a Date value, which gives the time when
    the origin server sent the original response, and
 o  The presented Last-Modified time is at least 60 seconds before the
    Date value.

Fielding & Reschke Standards Track [Page 8] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 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: ""

Fielding & Reschke Standards Track [Page 9] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 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 file attributes, or
 a modification timestamp that has sub-second resolution.
 An origin server SHOULD send an 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 ([RFC7234]) 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
 or not the comparison context allows the use of weak validators:
 o  Strong comparison: two entity-tags are equivalent if both are not
    weak and their opaque-tags match character-by-character.
 o  Weak comparison: two entity-tags are equivalent if their
    opaque-tags match character-by-character, regardless of either or
    both being tagged as "weak".

Fielding & Reschke Standards Track [Page 10] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 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/"1"  | no match          | match           |
 | W/"1"  | W/"2"  | no match          | no match        |
 | W/"1"  | "1"    | no match          | match           |
 | "1"    | "1"    | match             | match           |
 +--------+--------+-------------------+-----------------+

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

 Consider a resource that is subject to content negotiation (Section
 3.4 of [RFC7231]), 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 [RFC7231]):
 >> 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!

Fielding & Reschke Standards Track [Page 11] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 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 data,
    so a strong entity-tag for a content-encoded representation has to
    be distinct from the entity tag of an unencoded representation to
    prevent potential conflicts during cache updates and range
    requests.  In contrast, transfer codings (Section 4 of [RFC7230])
    apply only during message transfer and do not result in distinct
    entity-tags.

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

 In 200 (OK) responses to GET or HEAD, an origin server:
 o  SHOULD send an entity-tag validator unless it is not feasible to
    generate one.
 o  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.
 o  SHOULD send a 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 value in successful
 responses to a retrieval request.
 A client:
 o  MUST send that entity-tag in any cache validation request (using
    If-Match or If-None-Match) if an entity-tag has been provided by
    the origin server.

Fielding & Reschke Standards Track [Page 12] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 o  SHOULD send the Last-Modified value in non-subrange cache
    validation requests (using If-Modified-Since) if only a
    Last-Modified value has been provided by the origin server.
 o  MAY send the Last-Modified value in subrange cache validation
    requests (using 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.
 o  SHOULD send both validators in cache validation requests if both
    an entity-tag and a 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.  Section 6 defines the order of
 evaluation when more than one precondition is present.

3.1. If-Match

 The "If-Match" header field makes the request method conditional on
 the recipient origin server either having at least one current
 representation of the target resource, when the field-value is "*",
 or having a current representation of the target resource that has an
 entity-tag matching a member of the list of entity-tags provided in
 the field-value.
 An origin server MUST use the strong comparison function when
 comparing entity-tags for If-Match (Section 2.3.2), since the client
 intends this precondition to prevent the method from being applied if
 there have been any changes to the representation data.
   If-Match = "*" / 1#entity-tag
 Examples:
   If-Match: "xyzzy"
   If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
   If-Match: *
 If-Match is most often used with state-changing methods (e.g., POST,
 PUT, DELETE) to prevent accidental overwrites when multiple user
 agents might be acting in parallel on the same resource (i.e., to

Fielding & Reschke Standards Track [Page 13] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 prevent the "lost update" problem).  It can also be used with safe
 methods to abort a request if the selected representation does not
 match one already stored (or partially stored) from a prior request.
 An origin server that receives an If-Match header field MUST evaluate
 the condition prior to performing the method (Section 5).  If the
 field-value is "*", the condition is false if the origin server does
 not have a current representation for the target resource.  If the
 field-value is a list of entity-tags, the condition is false if none
 of the listed tags match the entity-tag of the selected
 representation.
 An origin server MUST NOT perform the requested method if a received
 If-Match condition evaluates to false; instead, the origin server
 MUST respond with either a) the 412 (Precondition Failed) status code
 or b) one of the 2xx (Successful) status codes if the origin server
 has verified that a state change is being requested and the final
 state is already reflected in the current state of the target
 resource (i.e., the change requested by the user agent has already
 succeeded, but the user agent might not be aware of it, perhaps
 because the prior response was lost or a compatible change was made
 by some other user agent).  In the latter case, the origin server
 MUST NOT send a validator header field in the response unless it can
 verify that the request is a duplicate of an immediately prior change
 made by the same user agent.
 The If-Match header field can be ignored by caches and intermediaries
 because it is not applicable to a stored response.

3.2. If-None-Match

 The "If-None-Match" header field makes the request method conditional
 on a recipient cache or origin server either not having any current
 representation of the target resource, when the field-value is "*",
 or having a selected representation with an entity-tag that does not
 match any of those listed in the field-value.
 A recipient MUST use the weak comparison function when comparing
 entity-tags for If-None-Match (Section 2.3.2), since weak entity-tags
 can be used for cache validation even if there have been changes to
 the representation data.
   If-None-Match = "*" / 1#entity-tag

Fielding & Reschke Standards Track [Page 14] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 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: *
 If-None-Match is primarily used in conditional GET requests to enable
 efficient updates of cached information with a minimum amount of
 transaction overhead.  When a client desires to update one or more
 stored responses that have entity-tags, the client SHOULD generate an
 If-None-Match header field containing a list of those entity-tags
 when making a GET request; this allows recipient servers to send a
 304 (Not Modified) response to indicate when one of those stored
 responses 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 [RFC7231]).  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.
 An origin server that receives an If-None-Match header field MUST
 evaluate the condition prior to performing the method (Section 5).
 If the field-value is "*", the condition is false if the origin
 server has a current representation for the target resource.  If the
 field-value is a list of entity-tags, the condition is false if one
 of the listed tags match the entity-tag of the selected
 representation.
 An origin server MUST NOT perform the requested method if the
 condition evaluates to false; instead, the origin server MUST respond
 with either a) the 304 (Not Modified) status code if the request
 method is GET or HEAD or b) the 412 (Precondition Failed) status code
 for all other request methods.
 Requirements on cache handling of a received If-None-Match header
 field are defined in Section 4.3.2 of [RFC7234].

Fielding & Reschke Standards Track [Page 15] RFC 7232 HTTP/1.1 Conditional Requests June 2014

3.3. If-Modified-Since

 The "If-Modified-Since" header field makes a GET or HEAD request
 method conditional on the selected representation's modification date
 being more recent than the date provided in the field-value.
 Transfer of the selected representation's data is avoided if that
 data has not changed.
   If-Modified-Since = HTTP-date
 An example of the field is:
   If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
 A recipient MUST ignore If-Modified-Since if the request contains an
 If-None-Match header field; the condition in If-None-Match is
 considered to be a more accurate replacement for the condition in
 If-Modified-Since, and the two are only combined for the sake of
 interoperating with older intermediaries that might not implement
 If-None-Match.
 A recipient MUST ignore the If-Modified-Since header field if the
 received field-value is not a valid HTTP-date, or if the request
 method is neither GET nor HEAD.
 A recipient MUST interpret an If-Modified-Since field-value's
 timestamp in terms of the origin server's clock.
 If-Modified-Since is typically used for two distinct purposes: 1) to
 allow efficient updates of a cached representation that does not have
 an entity-tag and 2) 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 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
 field.

Fielding & Reschke Standards Track [Page 16] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 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 in a prior response.  Origin servers that choose an
 exact timestamp match based on the selected representation's
 Last-Modified field will not be able to help the user agent limit its
 data transfers to only those changed during the specified window.
 An origin server that receives an If-Modified-Since header field
 SHOULD evaluate the condition prior to performing the method
 (Section 5).  The origin server SHOULD NOT perform the requested
 method if the selected representation's last modification date is
 earlier than or equal to the date provided in the field-value;
 instead, the origin server SHOULD generate a 304 (Not Modified)
 response, including only those metadata that are useful for
 identifying or updating a previously cached response.
 Requirements on cache handling of a received If-Modified-Since header
 field are defined in Section 4.3.2 of [RFC7234].

3.4. If-Unmodified-Since

 The "If-Unmodified-Since" header field makes the request method
 conditional on the selected representation's last modification date
 being earlier than or equal to the date provided in the field-value.
 This field accomplishes the same purpose as If-Match for cases where
 the user agent does not have an entity-tag for the representation.
   If-Unmodified-Since = HTTP-date
 An example of the field is:
   If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
 A recipient MUST ignore If-Unmodified-Since if the request contains
 an If-Match header field; the condition in If-Match is considered to
 be a more accurate replacement for the condition in
 If-Unmodified-Since, and the two are only combined for the sake of
 interoperating with older intermediaries that might not implement
 If-Match.
 A recipient MUST ignore the If-Unmodified-Since header field if the
 received field-value is not a valid HTTP-date.
 A recipient MUST interpret an If-Unmodified-Since field-value's
 timestamp in terms of the origin server's clock.

Fielding & Reschke Standards Track [Page 17] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 If-Unmodified-Since is most often used with state-changing methods
 (e.g., POST, PUT, DELETE) to prevent accidental overwrites when
 multiple user agents might be acting in parallel on a resource that
 does not supply entity-tags with its representations (i.e., to
 prevent the "lost update" problem).  It can also be used with safe
 methods to abort a request if the selected representation does not
 match one already stored (or partially stored) from a prior request.
 An origin server that receives an If-Unmodified-Since header field
 MUST evaluate the condition prior to performing the method
 (Section 5).  The origin server MUST NOT perform the requested method
 if the selected representation's last modification date is more
 recent than the date provided in the field-value; instead the origin
 server MUST respond with either a) the 412 (Precondition Failed)
 status code or b) one of the 2xx (Successful) status codes if the
 origin server has verified that a state change is being requested and
 the final state is already reflected in the current state of the
 target resource (i.e., the change requested by the user agent has
 already succeeded, but the user agent might not be aware of that
 because the prior response message was lost or a compatible change
 was made by some other user agent).  In the latter case, the origin
 server MUST NOT send a validator header field in the response unless
 it can verify that the request is a duplicate of an immediately prior
 change made by the same user agent.
 The If-Unmodified-Since header field can be ignored by caches and
 intermediaries because it is not applicable to a stored response.

3.5. If-Range

 The "If-Range" header field provides a special conditional request
 mechanism that is similar to the If-Match and If-Unmodified-Since
 header fields but that instructs the recipient to ignore the Range
 header field if the validator doesn't match, resulting in transfer of
 the new selected representation instead of a 412 (Precondition
 Failed) response.  If-Range is defined in Section 3.2 of [RFC7233].

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
 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

Fielding & Reschke Standards Track [Page 18] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 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, Date,
 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 might be useful if the
 response does not have an ETag field).
 Requirements on a cache that receives a 304 response are defined in
 Section 4.3.4 of [RFC7234].  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
 conditions 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

 Except when excluded below, a recipient cache or origin server MUST
 evaluate received request preconditions after it has successfully
 performed its normal request checks and just before it would perform
 the action associated with the request method.  A server MUST ignore
 all received preconditions if its response to the same request
 without those conditions would have been a status code other than a
 2xx (Successful) or 412 (Precondition Failed).  In other words,
 redirects and failures take precedence over the evaluation of
 preconditions in conditional requests.

Fielding & Reschke Standards Track [Page 19] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 A server that is not the origin server for the target resource and
 cannot act as a cache for requests on the target resource MUST NOT
 evaluate the conditional request header fields defined by this
 specification, and it MUST forward them if the request is forwarded,
 since the generating client intends that they be evaluated by a
 server that can provide a current representation.  Likewise, a server
 MUST ignore the conditional request header fields defined by this
 specification when received with a request method that does not
 involve the selection or modification of a selected representation,
 such as CONNECT, OPTIONS, or TRACE.
 Conditional request header fields that are defined by extensions to
 HTTP might place conditions on all recipients, 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 of multiple resources, such as locks,
 if the recipient understands and implements that field ([RFC4918],
 Section 10.4).
 Although conditional request header fields are defined as being
 usable with the HEAD method (to keep HEAD's semantics consistent with
 those of GET), there is no point in sending a conditional HEAD
 because a successful response is around the same size as a 304 (Not
 Modified) response and more useful than a 412 (Precondition Failed)
 response.

6. Precedence

 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, since "lost
 update" preconditions have more strict requirements than cache
 validation, a validated cache is more efficient than a partial
 response, and entity tags are presumed to be more accurate than date
 validators.
 A recipient cache or origin server MUST evaluate the request
 preconditions defined by this specification in the following order:
 1.  When recipient is the origin server and If-Match is present,
     evaluate the If-Match precondition:
  • if true, continue to step 3
  • if false, respond 412 (Precondition Failed) unless it can be

determined that the state-changing request has already

        succeeded (see Section 3.1)

Fielding & Reschke Standards Track [Page 20] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 2.  When recipient is the origin server, If-Match is not present, and
     If-Unmodified-Since is present, evaluate the If-Unmodified-Since
     precondition:
  • if true, continue to step 3
  • if false, respond 412 (Precondition Failed) unless it can be

determined that the state-changing request has already

        succeeded (see Section 3.4)
 3.  When If-None-Match is present, evaluate the If-None-Match
     precondition:
  • if true, continue to step 5
  • if false for GET/HEAD, respond 304 (Not Modified)
  • if false for other methods, respond 412 (Precondition Failed)
 4.  When the method is GET or HEAD, If-None-Match is not present, and
     If-Modified-Since is present, evaluate the If-Modified-Since
     precondition:
  • if true, continue to step 5
  • if false, respond 304 (Not Modified)
 5.  When the method is GET and both Range and If-Range are present,
     evaluate the If-Range precondition:
  • if the validator matches and the Range specification is

applicable to the selected representation, respond 206

        (Partial Content) [RFC7233]
 6.  Otherwise,
  • all conditions are met, so perform the requested action and

respond according to its success or failure.

 Any extension to HTTP/1.1 that defines additional conditional request
 header fields ought to define its own expectations regarding the
 order for evaluating such fields in relation to those defined in this
 document and other conditionals that might be found in practice.

Fielding & Reschke Standards Track [Page 21] RFC 7232 HTTP/1.1 Conditional Requests June 2014

7. IANA Considerations

7.1. Status Code Registration

 The "Hypertext Transfer Protocol (HTTP) Status Code Registry" located
 at <http://www.iana.org/assignments/http-status-codes> has been
 updated with the registrations below:
 +-------+---------------------+-------------+
 | Value | Description         | Reference   |
 +-------+---------------------+-------------+
 | 304   | Not Modified        | Section 4.1 |
 | 412   | Precondition Failed | Section 4.2 |
 +-------+---------------------+-------------+

7.2. Header Field Registration

 HTTP header fields are registered within the "Message Headers"
 registry maintained at
 <http://www.iana.org/assignments/message-headers/>.
 This document defines the following HTTP header fields, so their
 associated registry entries have been updated according to the
 permanent registrations below (see [BCP90]):
 +---------------------+----------+----------+-------------+
 | Header Field Name   | Protocol | Status   | Reference   |
 +---------------------+----------+----------+-------------+
 | ETag                | http     | standard | Section 2.3 |
 | If-Match            | http     | standard | Section 3.1 |
 | If-Modified-Since   | http     | standard | Section 3.3 |
 | If-None-Match       | http     | standard | Section 3.2 |
 | If-Unmodified-Since | http     | standard | Section 3.4 |
 | Last-Modified       | http     | standard | Section 2.2 |
 +---------------------+----------+----------+-------------+
 The change controller is: "IETF (iesg@ietf.org) - Internet
 Engineering Task Force".

8. Security Considerations

 This section is meant to inform developers, information providers,
 and users of known security concerns specific to the HTTP conditional
 request mechanisms.  More general security considerations are
 addressed in HTTP "Message Syntax and Routing" [RFC7230] and
 "Semantics and Content" [RFC7231].

Fielding & Reschke Standards Track [Page 22] RFC 7232 HTTP/1.1 Conditional Requests June 2014

 The validators defined by this specification are not intended to
 ensure the validity of a representation, guard against malicious
 changes, or detect man-in-the-middle attacks.  At best, they enable
 more efficient cache updates and optimistic concurrent writes when
 all participants are behaving nicely.  At worst, the conditions will
 fail and the client will receive a response that is no more harmful
 than an HTTP exchange without conditional requests.
 An entity-tag can be abused in ways that create privacy risks.  For
 example, a site might deliberately construct a semantically invalid
 entity-tag that is unique to the user or user agent, send it in a
 cacheable response with a long freshness time, and then read that
 entity-tag in later conditional requests as a means of re-identifying
 that user or user agent.  Such an identifying tag would become a
 persistent identifier for as long as the user agent retained the
 original cache entry.  User agents that cache representations ought
 to ensure that the cache is cleared or replaced whenever the user
 performs privacy-maintaining actions, such as clearing stored cookies
 or changing to a private browsing mode.

9. Acknowledgments

 See Section 10 of [RFC7230].

Fielding & Reschke Standards Track [Page 23] RFC 7232 HTTP/1.1 Conditional Requests June 2014

10. References

10.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
            Protocol (HTTP/1.1): Message Syntax and Routing",
            RFC 7230, June 2014.
 [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
            Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
            June 2014.
 [RFC7233]  Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
            "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
            RFC 7233, June 2014.
 [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
            Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
            RFC 7234, June 2014.

10.2. Informative References

 [BCP90]    Klyne, G., Nottingham, M., and J. Mogul, "Registration
            Procedures for Message Header Fields", BCP 90, RFC 3864,
            September 2004.
 [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.
 [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
            Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

Fielding & Reschke Standards Track [Page 24] RFC 7232 HTTP/1.1 Conditional Requests June 2014

Appendix A. Changes from RFC 2616

 The definition of validator weakness has been expanded and clarified.
 (Section 2.1)
 Weak entity-tags are now allowed in all requests except range
 requests.  (Sections 2.1 and 3.2)
 The ETag header field ABNF has been changed to not use quoted-string,
 thus avoiding escaping issues.  (Section 2.3)
 ETag is defined to provide an entity tag for the selected
 representation, thereby clarifying what it applies to in various
 situations (such as a PUT response).  (Section 2.3)
 The precedence for evaluation of conditional requests has been
 defined.  (Section 6)

Appendix B. Imported ABNF

 The following core rules are included by reference, as defined in
 Appendix B.1 of [RFC5234]: 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), and VCHAR (any visible US-ASCII
 character).
 The rules below are defined in [RFC7230]:
   OWS           = <OWS, see [RFC7230], Section 3.2.3>
   obs-text      = <obs-text, see [RFC7230], Section 3.2.6>
 The rules below are defined in other parts:
   HTTP-date     = <HTTP-date, see [RFC7231], Section 7.1.1.1>

Fielding & Reschke Standards Track [Page 25] RFC 7232 HTTP/1.1 Conditional Requests June 2014

Appendix C. Collected ABNF

 In the collected ABNF below, list rules are expanded as per Section
 1.2 of [RFC7230].
 ETag = entity-tag
 HTTP-date = <HTTP-date, see [RFC7231], 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, see [RFC7230], Section 3.2.3>
 entity-tag = [ weak ] opaque-tag
 etagc = "!" / %x23-7E ; '#'-'~'
  / obs-text
 obs-text = <obs-text, see [RFC7230], Section 3.2.6>
 opaque-tag = DQUOTE *etagc DQUOTE
 weak = %x57.2F ; W/

Fielding & Reschke Standards Track [Page 26] RFC 7232 HTTP/1.1 Conditional Requests June 2014

Index

 3
    304 Not Modified (status code)  19
 4
    412 Precondition Failed (status code)  18
 E
    ETag header field  9
 G
    Grammar
       entity-tag  9
       ETag  9
       etagc  9
       If-Match  13
       If-Modified-Since  15
       If-None-Match  14
       If-Unmodified-Since  17
       Last-Modified  7
       opaque-tag  9
       weak  9
 I
    If-Match header field  13
    If-Modified-Since header field  16
    If-None-Match header field  14
    If-Unmodified-Since header field  17
 L
    Last-Modified header field  7
 M
    metadata  5
 S
    selected representation  4
 V
    validator  5
       strong  5
       weak  5

Fielding & Reschke Standards Track [Page 27] RFC 7232 HTTP/1.1 Conditional Requests June 2014

Authors' Addresses

 Roy T. Fielding (editor)
 Adobe Systems Incorporated
 345 Park Ave
 San Jose, CA  95110
 USA
 EMail: fielding@gbiv.com
 URI:   http://roy.gbiv.com/
 Julian F. Reschke (editor)
 greenbytes GmbH
 Hafenweg 16
 Muenster, NW  48155
 Germany
 EMail: julian.reschke@greenbytes.de
 URI:   http://greenbytes.de/tech/webdav/

Fielding & Reschke Standards Track [Page 28]

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