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HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2616 (if approved)                            J. Reschke, Ed.
Intended status: Standards Track                              greenbytes
Expires: August 27, 2013                               February 23, 2013


      Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests
                  draft-ietf-httpbis-p4-conditional-22

Abstract

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

Editorial Note (To be removed by RFC Editor)

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

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

   The changes in this draft are summarized in Appendix D.3.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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   This Internet-Draft will expire on August 27, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
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   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  . . . . . . . . . . . . . . . . . . . . 15
     3.4.  If-Unmodified-Since  . . . . . . . . . . . . . . . . . . . 16
     3.5.  If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 16
   4.  Status Code Definitions  . . . . . . . . . . . . . . . . . . . 17
     4.1.  304 Not Modified . . . . . . . . . . . . . . . . . . . . . 17
     4.2.  412 Precondition Failed  . . . . . . . . . . . . . . . . . 17
   5.  Evaluation and Precedence  . . . . . . . . . . . . . . . . . . 17
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
     6.1.  Status Code Registration . . . . . . . . . . . . . . . . . 20
     6.2.  Header Field Registration  . . . . . . . . . . . . . . . . 20
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 22
   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 22
   Appendix B.  Imported ABNF . . . . . . . . . . . . . . . . . . . . 22
   Appendix C.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 23
   Appendix D.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 23
     D.1.  Since draft-ietf-httpbis-p4-conditional-19 . . . . . . . . 23
     D.2.  Since draft-ietf-httpbis-p4-conditional-20 . . . . . . . . 24
     D.3.  Since draft-ietf-httpbis-p4-conditional-21 . . . . . . . . 24
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25








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1.  Introduction

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

   Conditional GET requests are the most efficient mechanism for HTTP
   cache updates [Part6].  Conditionals can also be applied to state-
   changing methods, such as PUT and DELETE, to prevent the "lost
   update" problem: one client accidentally overwriting the work of
   another client that has been acting in parallel.

   Conditional request preconditions are based on the state of the
   target resource as a whole (its current value set) or the state as
   observed in a previously obtained representation (one value in that
   set).  A resource might have multiple current representations, each
   with its own observable state.  The conditional request mechanisms
   assume that the mapping of requests to a "selected representation"
   (Section 3 of [Part2]) will be consistent over time if the server
   intends to take advantage of conditionals.  Regardless, if the
   mapping is inconsistent and the server is unable to select the
   appropriate representation, then no harm will result when the
   precondition evaluates to false.

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

1.1.  Conformance and Error Handling

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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

1.2.  Syntax Notation

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



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2.  Validators

   This specification defines two forms of metadata that are commonly
   used to observe resource state and test for preconditions:
   modification dates (Section 2.2) and opaque entity tags
   (Section 2.3).  Additional metadata that reflects resource state has
   been defined by various extensions of HTTP, such as WebDAV [RFC4918],
   that are beyond the scope of this specification.  A resource metadata
   value is referred to as a "validator" when it is used within a
   precondition.

2.1.  Weak versus Strong

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

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

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

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

   There are a variety of strong validators used in practice.  The best
   are based on strict revision control, wherein each change to a
   representation always results in a unique node name and revision



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   identifier being assigned before the representation is made
   accessible to GET.  A collision-resistant hash function applied to
   the representation data is also sufficient if the data is available
   prior to the response header fields being sent and the digest does
   not need to be recalculated every time a validation request is
   received.  However, if a resource has distinct representations that
   differ only in their metadata, such as might occur with content
   negotiation over media types that happen to share the same data
   format, then the origin server SHOULD incorporate additional
   information in the validator to distinguish those representations and
   avoid confusing cache behavior.

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

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

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

   A "use" of a validator occurs when either a client generates a
   request and includes the validator in a precondition or when a server
   compares two validators.  Weak validators are only usable in contexts



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   that do not depend on exact equality of the representation data.
   Strong validators are usable and preferred for all conditional
   requests, including cache validation, partial content ranges, and
   "lost update" avoidance.

2.2.  Last-Modified

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

     Last-Modified = HTTP-date

   An example of its use is

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

2.2.1.  Generation

   Origin servers SHOULD send Last-Modified for any selected
   representation for which a last modification date can be reasonably
   and consistently determined, since its use in conditional requests
   and evaluating cache freshness ([Part6]) results in a substantial
   reduction of HTTP traffic on the Internet and can be a significant
   factor in improving service scalability and reliability.

   A representation is typically the sum of many parts behind the
   resource interface.  The last-modified time would usually be the most
   recent time that any of those parts were changed.  How that value is
   determined for any given resource is an implementation detail beyond
   the scope of this specification.  What matters to HTTP is how
   recipients of the Last-Modified header field can use its value to
   make conditional requests and test the validity of locally cached
   responses.

   An origin server SHOULD obtain the Last-Modified value of the
   representation as close as possible to the time that it generates the
   Date field value for its response.  This allows a recipient to make
   an accurate assessment of the representation's modification time,
   especially if the representation changes near the time that the
   response is generated.

   An origin server with a clock MUST NOT send a Last-Modified date that
   is later than the server's time of message origination (Date).  If
   the last modification time is derived from implementation-specific
   metadata that evaluates to some time in the future, according to the
   origin server's clock, then the origin server MUST replace that value



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   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 header field, because the client has a
      cache entry, or If-Range 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.

   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-



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   Modified values are generated from different clocks, or at somewhat
   different times during the preparation of the response.  An
   implementation MAY use a value larger than 60 seconds, if it is
   believed that 60 seconds is too short.

2.3.  ETag

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

     ETag       = entity-tag

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

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

   An entity-tag can be more reliable for validation than a modification
   date in situations where it is inconvenient to store modification
   dates, where the one-second resolution of HTTP date values is not
   sufficient, or where modification dates are not consistently
   maintained.

   Examples:

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

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



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2.3.1.  Generation

   The principle behind entity-tags is that only the service author
   knows the implementation of a resource well enough to select the most
   accurate and efficient validation mechanism for that resource, and
   that any such mechanism can be mapped to a simple sequence of octets
   for easy comparison.  Since the value is opaque, there is no need for
   the client to be aware of how each entity-tag is constructed.

   For example, a resource that has implementation-specific versioning
   applied to all changes might use an internal revision number, perhaps
   combined with a variance identifier for content negotiation, to
   accurately differentiate between representations.  Other
   implementations might use a collision-resistant hash of
   representation content, a combination of various filesystem
   attributes, or a modification timestamp that has sub-second
   resolution.

   Origin servers SHOULD send ETag for any selected representation for
   which detection of changes can be reasonably and consistently
   determined, since the entity-tag's use in conditional requests and
   evaluating cache freshness ([Part6]) can result in a substantial
   reduction of HTTP network traffic and can be a significant factor in
   improving service scalability and reliability.

2.3.2.  Comparison

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

   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".

   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           |
   +--------+--------+-------------------+-----------------+



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

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

   >> Request:

     GET /index HTTP/1.1
     Host: www.example.com
     Accept-Encoding: gzip


   In this case, the response might or might not use the gzip content
   coding.  If it does not, the response might look like:

   >> Response:

     HTTP/1.1 200 OK
     Date: Thu, 26 Mar 2010 00:05:00 GMT
     ETag: "123-a"
     Content-Length: 70
     Vary: Accept-Encoding
     Content-Type: text/plain

     Hello World!
     Hello World!
     Hello World!
     Hello World!
     Hello World!

   An alternative representation that does use gzip content coding would
   be:

   >> Response:

     HTTP/1.1 200 OK
     Date: Thu, 26 Mar 2010 00:05:00 GMT
     ETag: "123-b"
     Content-Length: 43
     Vary: Accept-Encoding
     Content-Type: text/plain
     Content-Encoding: gzip

     ...binary data...





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

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

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

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

   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 use that entity-tag in any cache-conditional request (using
      If-Match or If-None-Match) if an entity-tag has been provided by
      the origin server.

   o  SHOULD use the Last-Modified value in non-subrange cache-
      conditional requests (using If-Modified-Since) if only a Last-
      Modified value has been provided by the origin server.

   o  MAY use the Last-Modified value in subrange cache-conditional
      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 use both validators in cache-conditional 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.




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3.  Precondition Header Fields

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

3.1.  If-Match

   The "If-Match" header field can be used to make a request method
   conditional on the current existence or value of an entity-tag for
   one or more representations of the target resource.

   If-Match is generally useful for resource update requests, such as
   PUT requests, as a means for protecting against accidental overwrites
   when multiple clients are acting in parallel on the same resource
   (i.e., the "lost update" problem).  An If-Match field-value of "*"
   places the precondition on the existence of any current
   representation for the target resource.

     If-Match = "*" / 1#entity-tag

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

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

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

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

   Examples:

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








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3.2.  If-None-Match

   The "If-None-Match" header field can be used to make a request method
   conditional on not matching any of the current entity-tag values for
   representations of the target resource.

   If-None-Match is primarily used in conditional GET requests to enable
   efficient updates of cached information with a minimum amount of
   transaction overhead.  A client that has one or more representations
   previously obtained from the target resource can send If-None-Match
   with a list of the associated entity-tags in the hope of receiving a
   304 (Not Modified) response if at least one of those representations
   matches the selected representation.

   If-None-Match can also be used with a value of "*" to prevent an
   unsafe request method (e.g., PUT) from inadvertently modifying an
   existing representation of the target resource when the client
   believes that the resource does not have a current representation.
   This is a variation on the "lost update" problem that might arise if
   more than one client attempts to create an initial representation for
   the target resource.

     If-None-Match = "*" / 1#entity-tag

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

   If the condition is not met, the server MUST NOT perform the
   requested method.  Instead, if the request method was GET or HEAD,
   the server SHOULD respond with a 304 (Not Modified) status code,
   including the cache-related header fields (particularly ETag) of the
   selected representation that has a matching entity-tag.  For all
   other request methods, the server MUST respond with a 412
   (Precondition Failed) status code when the condition is not met.

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

   Examples:







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     If-None-Match: "xyzzy"
     If-None-Match: W/"xyzzy"
     If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
     If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
     If-None-Match: *

3.3.  If-Modified-Since

   The "If-Modified-Since" header field can be used with GET or HEAD to
   make the method conditional by modification date: if the selected
   representation has not been modified since the time specified in this
   field, then do not perform the request method; instead, respond as
   detailed below.

     If-Modified-Since = HTTP-date

   An example of the field is:

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

   A GET method with an If-Modified-Since header field and no Range
   header field requests that the selected representation be transferred
   only if it has been modified since the date given by the If-Modified-
   Since header field.  The algorithm for determining this includes the
   following cases:

   1.  If the request would normally result in anything other than a 200
       (OK) status code, or if the passed If-Modified-Since date is
       invalid, the response is exactly the same as for a normal GET.  A
       date that is later than the server's current time is invalid.

   2.  If the selected representation has been modified since the If-
       Modified-Since date, the response is exactly the same as for a
       normal GET.

   3.  If the selected representation has not been modified since a
       valid If-Modified-Since date, the server SHOULD send a 304 (Not
       Modified) response.

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

   When used for cache updates, a cache will typically use the value of
   the cached message's Last-Modified 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



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   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.

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

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

3.4.  If-Unmodified-Since

   The "If-Unmodified-Since" header field can be used to make a request
   method conditional by modification date: if the selected
   representation has been modified since the time specified in this
   field, then the server MUST NOT perform the requested operation and
   MUST instead respond with the 412 (Precondition Failed) status code.
   If the selected representation has not been modified since the time
   specified in this field, the server MAY perform the request.

     If-Unmodified-Since = HTTP-date

   An example of the field is:

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

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

3.5.  If-Range

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



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4.  Status Code Definitions

4.1.  304 Not Modified

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

   The server generating a 304 response MUST generate any of the
   following header fields that would have been sent in a 200 (OK)
   response to the same request: Cache-Control, Content-Location, ETag,
   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 4.2.1 of [Part6].  If the conditional request originated with
   an outbound client, such as a user agent with its own cache sending a
   conditional GET to a shared proxy, then the proxy SHOULD forward the
   304 response to that client.

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

4.2.  412 Precondition Failed

   The 412 (Precondition Failed) status code indicates that one or more
   preconditions given in the request header fields evaluated to false
   when tested on the server.  This response code allows the client to
   place preconditions on the current resource state (its current
   representations and metadata) and thus prevent the request method
   from being applied if the target resource is in an unexpected state.

5.  Evaluation and Precedence

   For each conditional request, a server MUST evaluate the request
   preconditions after it has successfully performed its normal request



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   checks (i.e., just before it would perform the action associated with
   the request method).  Preconditions are ignored if the server
   determines that an error or redirect response applies before they are
   evaluated.  Otherwise, the evaluation depends on both the method
   semantics and the choice of conditional.

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

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

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

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



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

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

   1.  When If-Match is present, evaluate it:

       *  if true, continue to step 3

       *  if false, respond 412 (Precondition Failed)

   2.  When If-Match is not present and If-Unmodified-Since is present,
       evaluate it:

       *  if true, continue to step 3

       *  if false, respond 412 (Precondition Failed)

   3.  When If-None-Match is present, evaluate it:

       *  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 it:

       *  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 If-Range:

       *  if the validator matches and the Range specification is
          applicable to the selected representation, respond 206
          (Partial Content) [Part5]

   6.  Otherwise,

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



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   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.

6.  IANA Considerations

6.1.  Status Code Registration

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

   +-------+---------------------+-------------+
   | Value | Description         | Reference   |
   +-------+---------------------+-------------+
   | 304   | Not Modified        | Section 4.1 |
   | 412   | Precondition Failed | Section 4.2 |
   +-------+---------------------+-------------+

6.2.  Header Field Registration

   The Message Header Field Registry located at <http://www.iana.org/
   assignments/message-headers/message-header-index.html> shall be
   updated with the permanent registrations below (see [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".

7.  Security Considerations

   This section is meant to inform developers, information providers,
   and users of known security concerns specific to the HTTP/1.1
   conditional request mechanisms.  More general security considerations
   are addressed in HTTP messaging [Part1] and semantics [Part2].

   The validators defined by this specification are not intended to



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   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.

8.  Acknowledgments

   See Section 9 of [Part1].

9.  References

9.1.  Normative References

   [Part1]    Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              draft-ietf-httpbis-p1-messaging-22 (work in progress),
              February 2013.

   [Part2]    Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content",
              draft-ietf-httpbis-p2-semantics-22 (work in progress),
              February 2013.

   [Part5]    Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
              draft-ietf-httpbis-p5-range-22 (work in progress),
              February 2013.

   [Part6]    Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              draft-ietf-httpbis-p6-cache-22 (work in progress),
              February 2013.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate



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              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.

9.2.  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.

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 5)

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 [Part1]:

     OWS           = <OWS, defined in [Part1], Section 3.2.3>



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

   The rules below are defined in other parts:

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

Appendix C.  Collected ABNF

   ETag = entity-tag

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

   If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
    entity-tag ] ) )
   If-Modified-Since = HTTP-date
   If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
    entity-tag ] ) )
   If-Unmodified-Since = HTTP-date

   Last-Modified = HTTP-date

   OWS = <OWS, defined in [Part1], Section 3.2.3>

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

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

   weak = %x57.2F ; W/

Appendix D.  Change Log (to be removed by RFC Editor before publication)

   Changes up to the first Working Group Last Call draft are summarized
   in <http://tools.ietf.org/html/
   draft-ietf-httpbis-p4-conditional-19#appendix-C>.

D.1.  Since draft-ietf-httpbis-p4-conditional-19

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/241>: "Need to
      clarify eval order/interaction of conditional headers"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/345>: "Required
      headers on 304 and 206"




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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/350>: "Optionality
      of Conditional Request Support"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/354>: "ETags and
      Conditional Requests"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/361>: "ABNF
      requirements for recipients"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/363>: "Rare cases"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/365>: "Conditional
      Request Security Considerations"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/371>: "If-Modified-
      Since lacks definition for method != GET"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/372>: "refactor
      conditional header field descriptions"

D.2.  Since draft-ietf-httpbis-p4-conditional-20

   o  Conformance criteria and considerations regarding error handling
      are now defined in Part 1.

D.3.  Since draft-ietf-httpbis-p4-conditional-21

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/96>: "Conditional
      GET text"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/350>: "Optionality
      of Conditional Request Support"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/384>: "unclear prose
      in definition of 304"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/401>: "ETags and
      Conneg"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/402>: "Comparison
      function for If-Match and If-None-Match"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/406>: "304 without
      validator"





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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/427>: "If-Match and
      428"

Index

   3
      304 Not Modified (status code)  17

   4
      412 Precondition Failed (status code)  17

   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  16
         Last-Modified  7
         opaque-tag  9
         weak  9

   I
      If-Match header field  13
      If-Modified-Since header field  15
      If-None-Match header field  14
      If-Unmodified-Since header field  16

   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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