source: draft-ietf-httpbis/latest/p4-conditional.xml @ 2293

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<front>

  <title abbrev="HTTP/1.1 Conditional Requests">Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>

  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
    <address>
      <postal>
        <street>345 Park Ave</street>
        <city>San Jose</city>
        <region>CA</region>
        <code>95110</code>
        <country>USA</country>
      </postal>
      <email>fielding@gbiv.com</email>
      <uri>http://roy.gbiv.com/</uri>
    </address>
  </author>

  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    <organization abbrev="greenbytes">greenbytes GmbH</organization>
    <address>
      <postal>
        <street>Hafenweg 16</street>
        <city>Muenster</city><region>NW</region><code>48155</code>
        <country>Germany</country>
      </postal>
      <email>julian.reschke@greenbytes.de</email>
      <uri>http://greenbytes.de/tech/webdav/</uri>
    </address>
  </author>

  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
  <workgroup>HTTPbis Working Group</workgroup>

<abstract>
<t>
   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.
</t>
</abstract>

<note title="Editorial Note (To be removed by RFC Editor)">
  <t>
    Discussion of this draft takes place on the HTTPBIS working group
    mailing list (ietf-http-wg@w3.org), which is archived at
    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
  </t>
  <t>
    The current issues list is at
    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
    documents (including fancy diffs) can be found at
    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
  </t>
  <t>
    The changes in this draft are summarized in <xref target="changes.since.22"/>.
  </t>
</note>
</front>

<middle>
<section title="Introduction" anchor="introduction">
<t>
   Conditional requests are HTTP requests <xref target="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
   <xref target="Part1"/>.
</t>
<t>
   Conditional GET requests are the most efficient mechanism for HTTP
   cache updates &caching;.  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.
</t>
<t><iref primary="true" item="selected representation"/>
   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" (&representation;)
   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.
</t>
<t>
   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 <xref target="precedence"/>.
</t>

<section title="Conformance and Error Handling" anchor="conformance">
<t>
   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 <xref target="RFC2119"/>.
</t>
<t>
   Conformance criteria and considerations regarding error handling
   are defined in &conformance;.
</t>
</section>

<section title="Syntax Notation" anchor="notation">
<t>
   This specification uses the Augmented Backus-Naur Form (ABNF) notation
   of <xref target="RFC5234"/> with the list rule extension defined in
   &notation;. <xref target="imported.abnf"/> describes rules imported from
   other documents. <xref target="collected.abnf"/> shows the collected ABNF
   with the list rule expanded.
</t>
</section>
</section>

<section title="Validators" anchor="validators">
   <iref primary="true" item="metadata"/>
   <iref primary="true" item="validator"/>
<t>
   This specification defines two forms of metadata that are commonly used
   to observe resource state and test for preconditions: modification dates
   (<xref target="header.last-modified"/>) and opaque entity tags
   (<xref target="header.etag"/>).  Additional metadata that reflects resource state
   has been defined by various extensions of HTTP, such as WebDAV
   <xref target="RFC4918"/>, that are beyond the scope of this specification.
   A resource metadata value is referred to as a "<x:dfn>validator</x:dfn>"
   when it is used within a precondition.
</t>

<section title="Weak versus Strong" anchor="weak.and.strong.validators">
   <iref primary="true" item="validator" subitem="weak"/>
   <iref primary="true" item="validator" subitem="strong"/>
<t>
   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.
</t>
<t>
   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 <x:ref>200 (OK)</x:ref> response to GET.
</t>
<t>   
   A strong validator might change for other reasons, such as when a
   semantically significant part of the representation metadata is changed
   (e.g., <x:ref>Content-Type</x:ref>), 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.
</t>
<t>
   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).
</t>
<t>
   There are a variety of strong validators used in practice.  The best are
   based on strict revision control, wherein each change to a representation
   always results in a unique node name and revision identifier being assigned
   before the representation is made accessible to GET.  A collision-resistant hash
   function applied to the representation data is also sufficient if the data
   is available prior to the response header fields being sent and the digest
   does not need to be recalculated every time a validation request is
   received.  However, if a resource has distinct representations that differ
   only in their metadata, such as might occur with content negotiation over
   media types that happen to share the same data format, then the origin
   server &SHOULD; incorporate additional information in the validator to
   distinguish those representations.
</t>
<t>
   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.
</t>
<t>
   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.
</t>
<t>
   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.
</t>
<t>
   A "use" of a validator occurs when either a client generates a request
   and includes the validator in a precondition or when a server
   compares two validators.
   Weak validators are only usable in contexts that do not depend on exact
   equality of the representation data.
   Strong validators are usable and preferred for all conditional requests,
   including cache validation, partial content ranges, and "lost update"
   avoidance.
</t>
</section>

<section title="Last-Modified" anchor="header.last-modified">
  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
  <x:anchor-alias value="Last-Modified"/>
<t>
   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.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
</artwork></figure>
<t>
   An example of its use is
</t>
<figure><artwork type="example">
  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
</artwork></figure>

<section title="Generation" anchor="lastmod.generation">
<t>
   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 (&caching;) results in a substantial
   reduction of HTTP traffic on the Internet and can be a significant
   factor in improving service scalability and reliability.
</t>
<t>
   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.
</t>
<t>
   An origin server &SHOULD; obtain the Last-Modified value of the
   representation as close as possible to the time that it generates the
   <x:ref>Date</x:ref> 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.
</t>
<t>
   An origin server with a clock &MUST-NOT; send a Last-Modified date
   that is later than the server's time of message origination (<x:ref>Date</x:ref>).
   If the last modification time is derived from implementation-specific
   metadata that evaluates to some time in the future, according to the
   origin server's clock, then the origin server &MUST; replace that
   value with the message origination date. This prevents a future
   modification date from having an adverse impact on cache validation.
</t>
<t>
   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.
</t>
</section>

<section title="Comparison" anchor="lastmod.comparison">
<t>
   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:
  <list style="symbols">
     <t>The validator is being compared by an origin server to the
        actual current validator for the representation and,</t>
     <t>That origin server reliably knows that the associated representation did
        not change twice during the second covered by the presented
        validator.</t>
  </list>
</t>
<t>
   or
  <list style="symbols">
     <t>The validator is about to be used by a client in an <x:ref>If-Modified-Since</x:ref>,
        <x:ref>If-Unmodified-Since</x:ref> header field, because the client has
        a cache entry, or <x:ref>If-Range</x:ref> for the associated
        representation, and</t>
     <t>That cache entry includes a <x:ref>Date</x:ref> value, which gives the
        time when the origin server sent the original response, and</t>
     <t>The presented Last-Modified time is at least 60 seconds before
        the Date value.</t>
  </list>
</t>
<t>
   or
  <list style="symbols">
     <t>The validator is being compared by an intermediate cache to the
        validator stored in its cache entry for the representation, and</t>
     <t>That cache entry includes a <x:ref>Date</x:ref> value, which gives the
        time when the origin server sent the original response, and</t>
     <t>The presented Last-Modified time is at least 60 seconds before
        the Date value.</t>
  </list>
</t>
<t>
   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 <x:ref>Date</x:ref> 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.
</t>
</section>
</section>

<section title="ETag" anchor="header.etag">
  <iref primary="true" item="ETag header field" x:for-anchor=""/>
  <x:anchor-alias value="ETag"/>
  <x:anchor-alias value="entity-tag"/>
  <x:anchor-alias value="opaque-tag"/>
  <x:anchor-alias value="weak"/>
  <x:anchor-alias value="etagc"/>
<t>
   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.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="entity-tag"/><iref primary="true" item="Grammar" subitem="weak"/><iref primary="true" item="Grammar" subitem="opaque-tag"/><iref primary="true" item="Grammar" subitem="etagc"/>
  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>

  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
  <x:ref>opaque-tag</x:ref> = <x:ref>DQUOTE</x:ref> *<x:ref>etagc</x:ref> <x:ref>DQUOTE</x:ref>
  <x:ref>etagc</x:ref>      = %x21 / %x23-7E / <x:ref>obs-text</x:ref>
             ; <x:ref>VCHAR</x:ref> except double quotes, plus obs-text
</artwork></figure>
<x:note>
  <t>
    &Note; Previously, opaque-tag was defined to be a quoted-string
    (<xref target="RFC2616" x:fmt="," x:sec="3.11"/>), thus some recipients
    might perform backslash unescaping. Servers therefore ought to avoid
    backslash characters in entity tags.
  </t>
</x:note>
<t>
   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.
</t>
<figure><preamble>
  Examples:
</preamble>
<artwork type="example">
  ETag: "xyzzy"
  ETag: W/"xyzzy"
  ETag: ""
</artwork></figure>
<t>
   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
   (<xref target="weak.and.strong.validators"/>), then the origin server
   &MUST; mark the entity-tag as weak by prefixing its opaque value
   with "W/" (case-sensitive).
</t>

<section title="Generation" anchor="entity.tag.generation">
<t>
   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.
</t>
<t>
   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.
</t>
<t>
   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 (&caching;) can result in a substantial
   reduction of HTTP network traffic and can be a significant factor in
   improving service scalability and reliability.
</t>
</section>

<section title="Comparison" anchor="entity.tag.comparison">
  <x:anchor-alias value="validator.comparison"/>
  <x:anchor-alias value="strong comparison"/>
  <x:anchor-alias value="weak comparison"/>
<t>
   There are two entity-tag comparison functions, depending
   on whether the comparison context allows the use of weak validators
   or not:
  <list style="symbols">
     <t><x:dfn>Strong comparison</x:dfn>: two entity-tags are equivalent if both
        are not weak and their opaque-tags match character-by-character.</t>
     <t><x:dfn>Weak comparison</x:dfn>: two entity-tags are equivalent if their opaque-tags
        match character-by-character, regardless of either or both
        being tagged as "weak".</t>
  </list>
</t>
<t>
   The example below shows the results for a set of entity-tag pairs,
   and both the weak and strong comparison function results:
</t>
<texttable align="left">
  <ttcol>ETag 1</ttcol>
  <ttcol>ETag 2</ttcol>
  <ttcol>Strong Comparison</ttcol>
  <ttcol>Weak Comparison</ttcol>

  <c>W/"1"</c>
  <c>W/"1"</c>
  <c>no match</c>
  <c>match</c>
  
  <c>W/"1"</c>
  <c>W/"2"</c>
  <c>no match</c>
  <c>no match</c>

  <c>W/"1"</c>
  <c>"1"</c>
  <c>no match</c>
  <c>match</c>

  <c>"1"</c>
  <c>"1"</c>
  <c>match</c>
  <c>match</c>
</texttable>
</section>

<section title="Example: Entity-tags Varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
<t>
   Consider a resource that is subject to content negotiation
   (&content-negotiation;), and where the representations sent in response to
   a GET request vary based on the <x:ref>Accept-Encoding</x:ref> request
   header field (&header-accept-encoding;):
</t>
<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
GET /index HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip

</artwork></figure>
<t>
   In this case, the response might or might not use the gzip content coding.
   If it does not, the response might look like:
</t>
<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT
ETag: "123-a"
Content-Length: <x:length-of target="exbody"/>
Vary: Accept-Encoding
Content-Type: text/plain

<x:span anchor="exbody">Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
</x:span></artwork></figure>
<t>
   An alternative representation that does use gzip content coding would be:
</t>
<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
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

<spanx>...binary data...</spanx></artwork></figure>
<x:note>
  <t>
    &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 (&transfer-codings;)
    apply only during message transfer and do not require distinct entity-tags.
  </t>
</x:note>
</section>
</section>

<section title="When to Use Entity-tags and Last-Modified Dates" anchor="when.to.use.entity.tags.and.last-modified.dates">
<t>
   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.
</t>
<t>
   In <x:ref>200 (OK)</x:ref> responses to GET or HEAD, an origin server:
  <list style="symbols">
     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
        generate one.</t>

     <t>&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.</t>

     <t>&SHOULD; send a <x:ref>Last-Modified</x:ref> value if it is feasible to
        send one.</t>
  </list>
</t>
<t>
   In other words, the preferred behavior for an origin server
   is to send both a strong entity-tag and a <x:ref>Last-Modified</x:ref>
   value in successful responses to a retrieval request.
</t>
<t>
   A client:
  <list style="symbols">
     <t>&MUST; use that entity-tag in any cache-conditional request (using
        <x:ref>If-Match</x:ref> or <x:ref>If-None-Match</x:ref>) if an
        entity-tag has been provided by the origin server.</t>

     <t>&SHOULD; use the <x:ref>Last-Modified</x:ref> value in non-subrange
        cache-conditional requests (using <x:ref>If-Modified-Since</x:ref>)
        if only a Last-Modified value has been provided by the origin server.</t>

     <t>&MAY; use the <x:ref>Last-Modified</x:ref> value in subrange
        cache-conditional requests (using <x:ref>If-Unmodified-Since</x:ref>)
        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.</t>

     <t>&SHOULD; use both validators in cache-conditional requests if both an
        entity-tag and a <x:ref>Last-Modified</x:ref> value have been provided
        by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
        respond appropriately.</t>
  </list>
</t>
</section>
</section>

<section title="Precondition Header Fields" anchor="header.field.definitions">
<t>
   This section defines the syntax and semantics of HTTP/1.1 header fields
   for applying preconditions on requests.
   <xref target="precedence"/> defines when the preconditions are applied and
   the order of evaluation when more than one precondition is present.
</t>

<section title="If-Match" anchor="header.if-match">
  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
  <x:anchor-alias value="If-Match"/>
<t>
   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.
</t>
<t>
   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.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
</artwork></figure>
<t>
   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 <xref
   target="entity.tag.comparison"/>), or if "*" is given and any current
   representation exists for the target resource.
</t>
<t>
   If the condition is met, the server &MAY; perform the request method.
</t>
<t>
   Origin servers &MUST-NOT; perform the requested method if the condition is
   not met; instead they &MUST; respond with the <x:ref>412 (Precondition
   Failed)</x:ref> status code.
</t>
<t>
   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.
</t>
<t>
   Examples:
</t>
<figure><artwork type="example">
  If-Match: "xyzzy"
  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
  If-Match: *
</artwork></figure>
</section>

<section title="If-None-Match" anchor="header.if-none-match">
  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
  <x:anchor-alias value="If-None-Match"/>
<t>
   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.
</t>
<t>
   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 <x:ref>304 (Not
   Modified)</x:ref> response if at least one of those representations matches
   the selected representation.
</t>
<t>
   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 (&safe-methods;).
   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.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
</artwork></figure>
<t>
   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 <xref
   target="entity.tag.comparison"/>), or if "*" is given and no current
   representation exists for that resource.
</t>
<t>
   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 <x:ref>304 (Not Modified)</x:ref> status code, including the
   cache-related header fields (particularly <x:ref>ETag</x:ref>) of the
   selected representation that has a matching entity-tag. For all other
   request methods, the server &MUST; respond with a <x:ref>412 (Precondition
   Failed)</x:ref> status code when the condition is not met.
</t>
<t>
   If the condition is met, the server &MAY; perform the requested method and
   &MUST; ignore any <x:ref>If-Modified-Since</x:ref> header field(s) in the
   request. That is, if no entity-tags match, then the server &MUST-NOT; send
   a <x:ref>304 (Not Modified)</x:ref> response.
</t>
<t>
   Examples:
</t>
<figure><artwork type="example">
  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: *
</artwork></figure>
</section>

<section title="If-Modified-Since" anchor="header.if-modified-since">
  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
  <x:anchor-alias value="If-Modified-Since"/>
<t>
   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.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
</artwork></figure>
<t>
   An example of the field is:
</t>
<figure><artwork type="example">
  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
</artwork></figure>
<t>
   A GET method with an If-Modified-Since header field and no <x:ref>Range</x:ref>
   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:
  <list style="numbers">
      <t>If the request would normally result in anything other than a
         <x:ref>200 (OK)</x:ref> 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.</t>

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

      <t>If the selected representation has not been modified since a valid
         If-Modified-Since date, the server &SHOULD; send a
         <x:ref>304 (Not Modified)</x:ref> response.</t>
  </list>
</t>
<t>
   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.
</t>
<t>
   When used for cache updates, a cache will typically use the value of the
   cached message's <x:ref>Last-Modified</x:ref> 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 <x:ref>Date</x:ref> 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 <x:ref>Last-Modified</x:ref> field.
</t>
<t>
   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 <x:ref>Date</x:ref> header field received from the
   server during a past run. Origin servers that choose an exact timestamp
   match based on the selected representation's <x:ref>Last-Modified</x:ref>
   field will not be able to help the user agent limit its data transfers to
   only those changed during the specified window.
</t>
<x:note>
  <t>
     &Note; If a client uses an arbitrary date in the If-Modified-Since
     header field instead of a date taken from a <x:ref>Last-Modified</x:ref>
     or <x:ref>Date</x:ref> 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.
  </t>
</x:note>
</section>

<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
  <x:anchor-alias value="If-Unmodified-Since"/>
<t>
   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 <x:ref>412 (Precondition Failed)</x:ref> status code.
   If the selected representation has not been modified since the time
   specified in this field, the server &MAY; perform the request.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
</artwork></figure>
<t>
   An example of the field is:
</t>
<figure><artwork type="example">
  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
</artwork></figure>
<t>
   A server &MUST; ignore the If-Unmodified-Since header field if the
   received value is not a valid HTTP-date.
</t>
</section>

<section title="If-Range" anchor="header.if-range">
<t>
   The "If-Range" header field provides a special conditional request
   mechanism that is similar to <x:ref>If-Match</x:ref> and
   <x:ref>If-Unmodified-Since</x:ref> but specific to range requests.
   If-Range is defined in &header-if-range;.
</t>
</section>

</section>

<section title="Status Code Definitions" anchor="status.code.definitions">
<section title="304 Not Modified" anchor="status.304">
  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
  <x:anchor-alias value="304"/>
  <x:anchor-alias value="304 (Not Modified)"/>
<t>
   The <x:dfn>304 (Not Modified)</x:dfn> status code indicates that a
   conditional GET or HEAD request has been
   received and would have resulted in a <x:ref>200 (OK)</x:ref> 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
   <x:ref>200 (OK)</x:ref> response.
</t>
<t>
   The server generating a 304 response &MUST; generate any of the following
   header fields that would have been sent in a <x:ref>200 (OK)</x:ref>
   response to the same request:
   <x:ref>Cache-Control</x:ref>,
   <x:ref>Content-Location</x:ref>,
   <x:ref>ETag</x:ref>,
   <x:ref>Expires</x:ref>, and
   <x:ref>Vary</x:ref>.
</t>
<t>
   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., <x:ref>Last-Modified</x:ref> might
   be useful if the response does not have an <x:ref>ETag</x:ref> field).
</t>
<t>
   Requirements on a cache that receives a 304 response are defined in
   &freshening-responses;. 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.
</t>
<t>
   A 304 response cannot contain a message-body; it is always
   terminated by the first empty line after the header fields.
</t>
</section>

<section title="412 Precondition Failed" anchor="status.412">
  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
  <x:anchor-alias value="412 (Precondition Failed)"/>
<t>
   The <x:dfn>412 (Precondition Failed)</x:dfn> 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.
</t>
</section>
</section>

<section title="Evaluation and Precedence" anchor="precedence">
<t>
   For each conditional request, a server &MUST; evaluate the request
   preconditions after it has successfully performed its normal request checks
   (i.e., just before it would perform the action associated with the request
   method). Preconditions are ignored if the server determines that an error
   or redirect response applies before they are evaluated. Otherwise, the
   evaluation depends on both the method semantics and the choice of
   conditional.
</t>
<t>
   A conditional request header field that is designed specifically for cache
   validation, which includes <x:ref>If-None-Match</x:ref> and
   <x:ref>If-Modified-Since</x:ref> 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
   <x:ref>304 (Not Modified)</x:ref> response that includes only those header
   fields useful for refreshing the cached representation.
</t>
<t>
   All other conditionals are intended to signal failure when the
   precondition evaluates to false. For example, an <x:ref>If-Match</x:ref>
   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 <x:ref>412 (Precondition Failed)</x:ref>
   response.
</t>
<t>
   The conditional request header fields defined by this specification are
   ignored for request methods that never involve the selection or
   modification of a <x:ref>selected representation</x:ref> (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
   (<xref target="RFC4918" x:fmt="," x:sec="10.4"/>).
</t>
<t>
   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 <x:ref>If-Modified-Since</x:ref> and <x:ref>If-None-Match</x:ref> in
   the same GET request is to support intermediary caches that might not have
   implemented <x:ref>If-None-Match</x:ref>, so it makes sense to ignore the
   <x:ref>If-Modified-Since</x:ref> when entity tags are understood and
   available for the selected representation.
</t>
<t>
   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).
</t>
<t>
   Specifically, the fields defined by this specification are evaluated
   as follows:
   <list style="numbers">
     <t anchor="precedence1">When <x:ref>If-Match</x:ref> is present, evaluate it:
       <list style="symbols">
         <t>if true, continue to step <xref target="precedence3" format="counter"/></t>
         <t>if false, respond <x:ref>412 (Precondition Failed)</x:ref></t>
       </list>
     </t>
     <t anchor="precedence2">When <x:ref>If-Match</x:ref> is not present and
        <x:ref>If-Unmodified-Since</x:ref> is present, evaluate it:
       <list style="symbols">
         <t>if true, continue to step <xref target="precedence3" format="counter"/></t>
         <t>if false, respond <x:ref>412 (Precondition Failed)</x:ref></t>
       </list>
     </t>
     <t anchor="precedence3">When <x:ref>If-None-Match</x:ref> is present, evaluate it:
       <list style="symbols">
         <t>if true, continue to step <xref target="precedence5" format="counter"/></t>
         <t>if false for GET/HEAD, respond <x:ref>304 (Not Modified)</x:ref></t>
         <t>if false for other methods, respond <x:ref>412 (Precondition Failed)</x:ref></t>
       </list>
     </t>
     <t anchor="precedence4">When the method is GET or HEAD,
        <x:ref>If-None-Match</x:ref> is not present, and
        <x:ref>If-Modified-Since</x:ref> is present, evaluate it:
       <list style="symbols">
         <t>if true, continue to step <xref target="precedence5" format="counter"/></t>
         <t>if false, respond <x:ref>304 (Not Modified)</x:ref></t>
       </list>
     </t>
     <t anchor="precedence5">When the method is GET and both <x:ref>Range</x:ref> and
        <x:ref>If-Range</x:ref> are present, evaluate If-Range:
       <list style="symbols">
         <t>if the validator matches and the Range specification is
            applicable to the selected representation, respond
            <x:ref>206 (Partial Content)</x:ref> <xref target="Part5"/></t>
       </list>
     </t>
     <t anchor="precedencelast">Otherwise,
       <list style="symbols">
         <t>all conditions are met, so perform the requested action and
            respond according to its success or failure.</t>
       </list>
     </t>
   </list>
</t>
<t>
   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.
</t>
</section>

<section title="IANA Considerations" anchor="IANA.considerations">

<section title="Status Code Registration" anchor="status.code.registration">
<t>
   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
   shall be updated with the registrations below:
</t>
<?BEGININC p4-conditional.iana-status-codes ?>
<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
   <ttcol>Value</ttcol>
   <ttcol>Description</ttcol>
   <ttcol>Reference</ttcol>
   <c>304</c>
   <c>Not Modified</c>
   <c>
      <xref target="status.304"/>
   </c>
   <c>412</c>
   <c>Precondition Failed</c>
   <c>
      <xref target="status.412"/>
   </c>
</texttable>
<!--(END)-->
<?ENDINC p4-conditional.iana-status-codes ?>
</section>

<section title="Header Field Registration" anchor="header.field.registration">
<t>
   HTTP header fields are registered within the Message Header Field Registry
   maintained at
   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
</t>
<t>
   This document defines the following HTTP header fields, so their
   associated registry entries shall be updated according to the permanent
   registrations below (see <xref target="BCP90"/>):
</t>
<?BEGININC p4-conditional.iana-headers ?>
<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
   <ttcol>Header Field Name</ttcol>
   <ttcol>Protocol</ttcol>
   <ttcol>Status</ttcol>
   <ttcol>Reference</ttcol>

   <c>ETag</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.etag"/>
   </c>
   <c>If-Match</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.if-match"/>
   </c>
   <c>If-Modified-Since</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.if-modified-since"/>
   </c>
   <c>If-None-Match</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.if-none-match"/>
   </c>
   <c>If-Unmodified-Since</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.if-unmodified-since"/>
   </c>
   <c>Last-Modified</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.last-modified"/>
   </c>
</texttable>
<!--(END)-->
<?ENDINC p4-conditional.iana-headers ?>
<t>
   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
</t>
</section>
</section>

<section title="Security Considerations" anchor="security.considerations">
<t>
   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 &messaging; and semantics &semantics;.
</t>
<t>
   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.
</t>
<t>
   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.
</t>
</section>

<section title="Acknowledgments" anchor="acks">
<t>
  See &acks;.
</t>
</section>
</middle>
<back>

<references title="Normative References">

<reference anchor="Part1">
  <front>
    <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
</reference>

<reference anchor="Part2">
  <front>
    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
  <x:source href="p2-semantics.xml" basename="p2-semantics">
    <x:defines>2xx</x:defines>
    <x:defines>2xx (Successful)</x:defines>
    <x:defines>200 (OK)</x:defines>
    <x:defines>Accept-Encoding</x:defines>
    <x:defines>Content-Location</x:defines>
    <x:defines>Content-Type</x:defines>
    <x:defines>Date</x:defines>
    <x:defines>Location</x:defines>
    <x:defines>Vary</x:defines>
    <x:defines>selected representation</x:defines>
  </x:source>
</reference>

<reference anchor="Part5">
  <front>
    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
  <x:source href="p5-range.xml" basename="p5-range">
    <x:defines>If-Range</x:defines>
    <x:defines>Range</x:defines>
    <x:defines>206 (Partial Content)</x:defines>
  </x:source>
</reference>

<reference anchor="Part6">
  <front>
    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
      <organization>Akamai</organization>
      <address><email>mnot@mnot.net</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
  <x:source href="p6-cache.xml" basename="p6-cache">
    <x:defines>Cache-Control</x:defines>
    <x:defines>Expires</x:defines>
  </x:source>
</reference>

<reference anchor="RFC2119">
  <front>
    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
    <author initials="S." surname="Bradner" fullname="Scott Bradner">
      <organization>Harvard University</organization>
      <address><email>sob@harvard.edu</email></address>
    </author>
    <date month="March" year="1997"/>
  </front>
  <seriesInfo name="BCP" value="14"/>
  <seriesInfo name="RFC" value="2119"/>
</reference>

<reference anchor="RFC5234">
  <front>
    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
      <organization>Brandenburg InternetWorking</organization>
      <address>
        <email>dcrocker@bbiw.net</email>
      </address>  
    </author>
    <author initials="P." surname="Overell" fullname="Paul Overell">
      <organization>THUS plc.</organization>
      <address>
        <email>paul.overell@thus.net</email>
      </address>
    </author>
    <date month="January" year="2008"/>
  </front>
  <seriesInfo name="STD" value="68"/>
  <seriesInfo name="RFC" value="5234"/>
</reference>

</references>

<references title="Informative References">

<reference anchor="RFC2616">
  <front>
    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
    <author initials="R." surname="Fielding" fullname="R. Fielding">
      <organization>University of California, Irvine</organization>
      <address><email>fielding@ics.uci.edu</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="J. Gettys">
      <organization>W3C</organization>
      <address><email>jg@w3.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="J. Mogul">
      <organization>Compaq Computer Corporation</organization>
      <address><email>mogul@wrl.dec.com</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
      <organization>MIT Laboratory for Computer Science</organization>
      <address><email>frystyk@w3.org</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="L. Masinter">
      <organization>Xerox Corporation</organization>
      <address><email>masinter@parc.xerox.com</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="P. Leach">
      <organization>Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
      <organization>W3C</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <date month="June" year="1999"/>
  </front>
  <seriesInfo name="RFC" value="2616"/>
</reference>

<reference anchor='BCP90'>
  <front>
    <title>Registration Procedures for Message Header Fields</title>
    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
      <organization>Nine by Nine</organization>
      <address><email>GK-IETF@ninebynine.org</email></address>
    </author>
    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
      <organization>BEA Systems</organization>
      <address><email>mnot@pobox.com</email></address>
    </author>
    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
      <organization>HP Labs</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <date year='2004' month='September' />
  </front>
  <seriesInfo name='BCP' value='90' />
  <seriesInfo name='RFC' value='3864' />
</reference>

<reference anchor='RFC4918'>
  <front>
    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
      <organization abbrev="CommerceNet">CommerceNet</organization>
      <address><email>ldusseault@commerce.net</email></address>
    </author>
    <date month="June" year="2007" />
  </front>
  <seriesInfo name='RFC' value='4918' />
</reference>
</references>

<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
<t>
  The definition of validator weakness has been expanded and clarified.
  (<xref target="weak.and.strong.validators" />)
</t>
<t>
  Weak entity-tags are now allowed in all requests except range requests
  (Sections <xref target="weak.and.strong.validators" format="counter"/> and
  <xref target="header.if-none-match" format="counter"/>).
</t>
<t>
  The <x:ref>ETag</x:ref> header field ABNF has been changed to not use
  quoted-string, thus avoiding escaping issues.
  (<xref target="header.etag" />)
</t>
<t>
  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).
  (<xref target="header.etag" />)
</t>
<t>
  The precedence for evaluation of conditional requests has been defined.
  (<xref target="precedence" />)
</t>
</section>

<section title="Imported ABNF" anchor="imported.abnf">
  <x:anchor-alias value="ALPHA"/>
  <x:anchor-alias value="CR"/>
  <x:anchor-alias value="DIGIT"/>
  <x:anchor-alias value="DQUOTE"/>
  <x:anchor-alias value="LF"/>
  <x:anchor-alias value="OCTET"/>
  <x:anchor-alias value="VCHAR"/>
  <x:anchor-alias value="core.rules"/>
  <x:anchor-alias value="obs-text"/>
  <x:anchor-alias value="OWS"/>
  <x:anchor-alias value="HTTP-date"/>
<t>
  The following core rules are included by
  reference, as defined in <xref target="RFC5234" x:fmt="of" x:sec="B.1"/>:
  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
  DIGIT (decimal 0-9), DQUOTE (double quote),
  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
  OCTET (any 8-bit sequence of data), SP (space), and
  VCHAR (any visible US-ASCII character).
</t>
<t>
  The rules below are defined in <xref target="Part1"/>:
</t>
<figure><artwork type="abnf2616">
  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &whitespace;&gt;
  <x:ref>obs-text</x:ref>      = &lt;obs-text, defined in &field-components;&gt;
</artwork></figure>
<t>
  The rules below are defined in other parts: 
</t>
<figure><artwork type="abnf2616">
  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
</artwork></figure>
</section> 

<?BEGININC p4-conditional.abnf-appendix ?>
<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
<t>
  In the collected ABNF below, list rules are expanded as per <xref target="Part1" x:rel="#notation"/>.
</t><figure>
<artwork type="abnf" name="p4-conditional.parsed-abnf">
<x:ref>ETag</x:ref> = entity-tag

<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 7.1.1.1&gt;

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

<x:ref>Last-Modified</x:ref> = HTTP-date

<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.3&gt;

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

<x:ref>obs-text</x:ref> = &lt;obs-text, defined in [Part1], Section 3.2.6&gt;
<x:ref>opaque-tag</x:ref> = DQUOTE *etagc DQUOTE

<x:ref>weak</x:ref> = %x57.2F ; W/
</artwork>
</figure>
</section>
<?ENDINC p4-conditional.abnf-appendix ?>

<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
<t>
  Changes up to the first Working Group Last Call draft are summarized
  in <eref target="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-19#appendix-C"/>.
</t>

<section title="Since draft-ietf-httpbis-p4-conditional-19" anchor="changes.since.19">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/241"/>:
      "Need to clarify eval order/interaction of conditional headers"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/345"/>:
      "Required headers on 304 and 206"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/350"/>:
      "Optionality of Conditional Request Support"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/354"/>:
      "ETags and Conditional Requests"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
      "ABNF requirements for recipients"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/363"/>:
      "Rare cases"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/365"/>:
      "Conditional Request Security Considerations"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/371"/>:
      "If-Modified-Since lacks definition for method != GET"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/372"/>:
      "refactor conditional header field descriptions"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p4-conditional-20" anchor="changes.since.20">
<t>
  <list style="symbols">
    <t>
      Conformance criteria and considerations regarding error handling are
      now defined in Part 1.
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p4-conditional-21" anchor="changes.since.21">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/96"/>:
      "Conditional GET text"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/350"/>:
      "Optionality of Conditional Request Support"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/384"/>:
      "unclear prose in definition of 304"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/401"/>:
      "ETags and Conneg"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/402"/>:
      "Comparison function for If-Match and If-None-Match"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/406"/>:
      "304 without validator"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/427"/>:
      "If-Match and 428"
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p4-conditional-22" anchor="changes.since.22">
<t>
  Closed issues:
  <list style="symbols">
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/436"/>:
      "explain list expansion in ABNF appendices"
    </t>
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/437"/>:
      "incorrect example dates"
    </t>
  </list>
</t>
<t>
  Partly resolved issues:
  <list style="symbols">
    <t>
      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/461"/>:
      "Editorial suggestions"
    </t>
  </list>
</t>
</section>
</section>

</back>
</rfc>