Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests
RFC 7232

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.

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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
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   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
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   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.

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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

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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

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   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).

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   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.

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   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

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   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.

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   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: ""

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   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".

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   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!

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   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.

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   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

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   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

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   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].

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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.

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   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.

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   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

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   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.

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   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)

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   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.

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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].

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   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].

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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.

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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>

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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/

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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

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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/

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