HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2616 (if approved)                                  J. Gettys
Intended status: Standards Track                          Alcatel-Lucent
Expires: September 15, October 20, 2011                                       J. Mogul
                                                                      HP
                                                              H. Frystyk
                                                               Microsoft
                                                             L. Masinter
                                                                   Adobe
                                                                P. Leach
                                                               Microsoft
                                                          T. Berners-Lee
                                                                 W3C/MIT
                                                           Y. Lafon, Ed.
                                                                     W3C
                                                         J. Reschke, Ed.
                                                              greenbytes
                                                          March 14,
                                                          April 18, 2011

                 HTTP/1.1, part 4: Conditional Requests
                  draft-ietf-httpbis-p4-conditional-13
                  draft-ietf-httpbis-p4-conditional-14

Abstract

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypermedia information
   systems.  HTTP has been in use by the World Wide Web global
   information initiative since 1990.  This document is Part 4 of the
   seven-part specification that defines the protocol referred to as
   "HTTP/1.1" and, taken together, obsoletes RFC 2616.  Part 4 defines
   request header fields for indicating conditional requests and the
   rules for constructing responses to those requests.

Editorial Note (To be removed by RFC Editor)

   Discussion of this draft should take place on the HTTPBIS working
   group mailing list (ietf-http-wg@w3.org). (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 C.14. C.15.

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."

   This Internet-Draft will expire on September 15, October 20, 2011.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  5
       1.2.1.  Core Rules  6
   2.  Resource State Metadata (Validators) . . . . . . . . . . . . .  6
     2.1.  Last-Modified  . . . . . . . . .  6
       1.2.2.  ABNF Rules defined in other Parts of the
               Specification . . . . . . . . . . . . .  6
       2.1.1.  Generation . . . . . . .  6
   2.  Entity-Tags . . . . . . . . . . . . . . .  6
       2.1.2.  Comparison . . . . . . . . . .  6
     2.1.  Example: Entity-tags varying on Content-Negotiated
           Resources . . . . . . . . . . . .  7
     2.2.  ETag . . . . . . . . . . . .  7
   3.  Status Code Definitions . . . . . . . . . . . . . . .  8
       2.2.1.  Generation . . . .  8
     3.1.  304 Not Modified . . . . . . . . . . . . . . . . . .  9
       2.2.2.  Weak versus Strong . . .  8
     3.2.  412 Precondition Failed . . . . . . . . . . . . . . .  9
       2.2.3.  Comparison . .  8
   4.  Weak and Strong Validators . . . . . . . . . . . . . . . . . .  9
   5. . . 11
       2.2.4.  Rules for When to Use Entity-tags and
               Last-Modified Dates  . . 11
   6.  Header Field Definitions . . . . . . . . . . . . . . . 11
       2.2.5.  Example: Entity-tags varying on Content-Negotiated
               Resources  . . . . . . . . . . 13
     6.1.  ETag . . . . . . . . . . . . 13
   3.  Precondition Header Fields . . . . . . . . . . . . . . . 13
     6.2. . . . 14
     3.1.  If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.3.  If-Modified-Since
     3.2.  If-None-Match  . . . . . . . . . . . . . . . . . . . . 15
     6.4.  If-None-Match . . 15
     3.3.  If-Modified-Since  . . . . . . . . . . . . . . . . . . . . 17
     6.5. 16
     3.4.  If-Unmodified-Since  . . . . . . . . . . . . . . . . . . . 18
     6.6.  Last-Modified
     3.5.  If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 18
   4.  Status Code Definitions  . . . . . . . . . . . . . . . . . . . 18
     4.1.  304 Not Modified . . . . . . . . . . . . . . . . . . . . . 18
     4.2.  412 Precondition Failed  . . . . . . . . . . . . . . . . . 19
   7.
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
     7.1.
     5.1.  Status Code Registration . . . . . . . . . . . . . . . . . 19
     7.2.
     5.2.  Header Field Registration  . . . . . . . . . . . . . . . . 20
   8.
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   9.
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 20
   10.
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     10.1.
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     10.2.
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 21
   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 22
   Appendix C.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 22
     C.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 22
     C.2.  Since draft-ietf-httpbis-p4-conditional-00 . . . . . . . . 23 22
     C.3.  Since draft-ietf-httpbis-p4-conditional-01 . . . . . . . . 23
     C.4.  Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 23
     C.5.  Since draft-ietf-httpbis-p4-conditional-03 . . . . . . . . 23
     C.6.  Since draft-ietf-httpbis-p4-conditional-04 . . . . . . . . 24 23
     C.7.  Since draft-ietf-httpbis-p4-conditional-05 . . . . . . . . 24
     C.8.  Since draft-ietf-httpbis-p4-conditional-06 . . . . . . . . 24
     C.9.  Since draft-ietf-httpbis-p4-conditional-07 . . . . . . . . 24
     C.10. Since draft-ietf-httpbis-p4-conditional-08 . . . . . . . . 24
     C.11. Since draft-ietf-httpbis-p4-conditional-09 . . . . . . . . 24
     C.12. Since draft-ietf-httpbis-p4-conditional-10 . . . . . . . . 25 24
     C.13. Since draft-ietf-httpbis-p4-conditional-11 . . . . . . . . 25
     C.14. Since draft-ietf-httpbis-p4-conditional-12 . . . . . . . . 25
     C.15. Since draft-ietf-httpbis-p4-conditional-13 . . . . . . . . 25
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

1.  Introduction

   This document defines the HTTP/1.1 conditional request mechanisms,
   including both response metadata for indicating
   potential that can be used to indicate or
   observe changes to payload content, including modification time
   stamps and opaque entity-tags, resource state and the HTTP conditional request
   mechanisms header fields that allow
   specify preconditions to be placed on a checked before performing the action
   given by the request method.  Conditional GET requests allow for efficient cache updates.  Other
   conditional request methods are used to protect against overwriting
   or misunderstanding 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 has been changed
   unbeknownst the mapping of requests to corresponding representations
   will be consistent over time if the requesting client.

   This document is currently disorganized in order server intends to minimize take advantage
   of conditionals.  Regardless, if the
   changes between drafts mapping is inconsistent and enable reviewers the
   server is unable to see select the smaller errata
   changes.  A future draft appropriate representation, then no
   harm will reorganize result when the sections precondition evaluates to better
   reflect false.

   We use the content.  In particular, term "selected representation" to refer to the sections on current
   representation of the target resource
   metadata will be discussed first that would have been selected
   in a successful response if the same request had used the method GET
   and then followed by each had excluded all of the conditional request header field, concluding with a definition of
   precedence and fields.  The
   conditional request preconditions are evaluated by comparing the expectation of ordering strong validator checks
   before weak validator checks.  It is likely that more content from
   [Part6] will migrate
   values provided in the request header fields to this part, where appropriate.  The the current
   mess reflects how widely dispersed these topics and associated
   requirements had become in [RFC2616]. metadata
   for the selected representation.

1.1.  Requirements

   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].

   An implementation is not compliant if it fails to satisfy one or more
   of the "MUST" or "REQUIRED" level requirements for the protocols it
   implements.  An implementation that satisfies all the "MUST" or
   "REQUIRED" level and all the "SHOULD" level requirements for its
   protocols is said to be "unconditionally compliant"; one that
   satisfies all the "MUST" level requirements but not all the "SHOULD"
   level requirements for its protocols is said to be "conditionally
   compliant".

1.2.  Syntax Notation

   This specification uses the ABNF syntax defined in Section 1.2 of
   [Part1] (which extends the syntax defined in [RFC5234] with a list
   rule).  Appendix B shows the collected ABNF, with the list rule
   expanded.

   The following core rules are included by reference, as defined in
   [RFC5234], Appendix 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), VCHAR (any visible USASCII character),
   and WSP (whitespace).

1.2.1.  Core Rules

   The core ABNF rules below are defined in Section 1.2.2 of [Part1]: other parts:

     quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
     OWS           = <OWS, defined in [Part1], Section 1.2.2>

1.2.2.  ABNF Rules defined in other Parts of the Specification

   The ABNF rules below are defined in other parts:
     HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>

2.  Entity-Tags

   Entity-tags  Resource State Metadata (Validators)

   This specification defines two forms of metadata that are commonly
   used to observe resource state and test for comparing two or more representations of the
   same resource.  HTTP/1.1 uses entity-tags in the ETag (Section 6.1),
   If-Match (Section 6.2), If-None-Match (Section 6.4), and If-Range
   (Section 5.3 of [Part5]) header fields.  The definition of how they
   are used preconditions:
   modification dates and compared as cache validators is in Section 4.  An
   entity-tag consists of an opaque quoted string, possibly prefixed by
   a weakness indicator.

     entity-tag = [ weak ] opaque-tag
     weak       = %x57.2F ; "W/", case-sensitive
     opaque-tag = quoted-string

   A "strong entity-tag" MAY be shared entity tags.  Additional metadata that
   reflects resource state has been defined by two representations various extensions of a
   resource only if they
   HTTP, such as WebDAV [RFC4918], that are equivalent by octet equality.

   A "weak entity-tag", indicated by beyond the "W/" prefix, MAY be shared by
   two representations scope of a this
   specification.  A resource only if metadata value is referred to as a
   "validator" when it is used within a precondition.

2.1.  Last-Modified

   The "Last-Modified" header field indicates the representations are
   equivalent date and could be substituted for each other with no
   significant change in semantics.  A weak entity-tag can only be used
   for weak comparison. time at which
   the origin server believes the selected representation was last
   modified.

     Last-Modified = HTTP-date

   An entity-tag MUST be unique across all versions example of all
   representations associated with its use is

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

2.1.1.  Generation

   Origin servers SHOULD send Last-Modified for any selected
   representation for which a particular resource.  A given
   entity-tag value MAY last modification date can be used for representations obtained by requests
   on different URIs.  The reasonably
   and consistently determined, since its use of the same entity-tag value in
   conjunction with representations obtained by conditional requests on different
   URIs does not imply the equivalence
   and evaluating cache freshness ([Part6]) results in a substantial
   reduction of those representations.

2.1.  Example: Entity-tags varying HTTP traffic on Content-Negotiated Resources

   Consider 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 subject
   determined for any given resource is an implementation detail beyond
   the scope of this specification.  What matters to content negotiation (Section 5 HTTP is how
   recipients of [Part3]), and where the representations returned upon a GET
   request vary based on the Accept-Encoding request Last-Modified header field
   (Section 6.3 of [Part3]):

   >> Request:

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

   In this case, the response might or might not can use its value to
   make conditional requests and test 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! validity of locally cached
   responses.

   An alternative origin server SHOULD obtain the Last-Modified value of the
   representation as close as possible to the time 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...

      Note: Content codings are a property of it generates the representation, so
      therefore
   Date field-value for its response.  This allows a recipient to make
   an entity-tag accurate assessment of an encoded representation must be
      distinct from an unencoded the representation's modification time,
   especially if the representation to prevent conflicts
      during cache updates and range requests.  In contrast, transfer
      codings (Section 6.2 of [Part1]) apply only during message
      transfer and do not require distinct entity-tags.

3.  Status Code Definitions

3.1.  304 Not Modified

   If the client has performed a conditional GET request and access is
   allowed, but changes near the document has not been modified, time that the
   response is generated.

   An origin server SHOULD
   respond with this status code.  The 304 response a clock MUST NOT contain send a
   message-body, and thus Last-Modified date that
   is always terminated by the first empty line
   after later than the header fields.

   A 304 response MUST include a Date header field (Section 9.3 of
   [Part1]) unless its omission is required by Section 9.3.1 server's time of [Part1]. message origination (Date).  If a 200 response
   the last modification time is derived from implementation-specific
   metadata that evaluates to some time in the same request would have included any of future, according to the
   header fields Cache-Control, Content-Location, ETag, Expires, Last-
   Modified, or Vary,
   origin server's clock, then those same header fields the origin server MUST be sent replace that value
   with the message origination date.  This prevents a future
   modification date from having an adverse impact on cache validation.

2.1.2.  Comparison

   A Last-Modified time, when used as a validator in a
   304 response.

   Since request, is
   implicitly weak unless it is possible to deduce that it is strong,
   using the goal of a 304 response following rules:

   o  The validator is being compared by an origin server to minimize information transfer
   when the recipient already has one or more cached representations, actual
      current validator for the response SHOULD NOT include representation metadata other than and,

   o  That origin server reliably knows that the above listed fields unless said metadata exists for the purpose
   of guiding cache updates (e.g., future HTTP extensions).

   If a 304 response includes an entity-tag that indicates a associated
      representation did not currently cached, then change twice during the recipient MUST NOT use second covered by
      the 304 presented validator.

   or

   o  The validator is about to update its own cache.  If that conditional request
   originated with be used by a client in an outbound client, such as If-Modified-
      Since or If-Unmodified-Since header field, because the client has
      a user agent with its own cache sending a conditional GET to entry for the associated representation, and

   o  That cache entry includes a shared proxy, then Date value, which gives the 304
   response MAY be forwarded to time when
      the outbound client.  Otherwise,
   disregard origin server sent the response original response, and repeat the request without

   o  The presented Last-Modified time is at least 60 seconds before the
   conditional.

   If a
      Date value.

   or

   o  The validator is being compared by an intermediate cache uses a received 304 response to update a cache entry, the
      validator stored in its cache MUST update 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 reflect any new field its Last-Modified time.  The arbitrary 60-
   second limit guards against the possibility that the Date and Last-
   Modified values given in are generated from different clocks, or at somewhat
   different times during the preparation of the response.

3.2.  412 Precondition Failed  An
   implementation MAY use a value larger than 60 seconds, if it is
   believed that 60 seconds is too short.

2.2.  ETag

   The precondition given ETag header field provides the current entity-tag for the
   selected representation.  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 one 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 = quoted-string

   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: ""

2.2.1.  Generation

   The principle behind entity-tags is that only the header fields evaluated 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 false when it was tested on a simple sequence of octets
   for easy comparison.  Since the server.  This response code allows value is opaque, there is no need for
   the client to place preconditions on the current be aware of how each entity-tag is constructed.

   For example, a resource metadata
   (header field data) and thus prevent the requested method from being that has implementation-specific versioning
   applied to all changes might use an internal revision number, perhaps
   combined with a resource other than variance identifier for content negotiation, to
   accurately differentiate between representations.  Other
   implementations might use a stored 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 one intended.

4.  Weak 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.

2.2.2.  Weak versus Strong Validators

   Since both origin servers and caches will compare two validators to
   decide if they represent indicate the same or different representations, one
   normally would expect that if the representation (including both
   representation header fields and representation body) changes in any
   way, then the associated validator would change as well.  If this is
   true, then we call this validator a "strong validator".

   However, there might be cases when a server prefers to change the
   validator only on semantically significant changes, and not when
   insignificant aspects of the representation change.  A validator that
   does not always change when the representation changes is a "weak
   validator".

   An entity-tag is normally a strong validator, but the protocol
   provides a mechanism to tag an entity-tag as "weak".  One can think
   of a strong validator as one that changes whenever the sequence of
   bits in a representation changes, while a weak value changes whenever
   the meaning of a representation changes.  Alternatively, one can
   think of a strong validator as part of an identifier for a specific
   representation, whereas a weak validator is part of an identifier for call that validator a set of semantically equivalent representations.

      Note: "strong validator".  One example
   of a strong validator is an integer that is incremented in stable
   storage every time a representation is changed.

      A representation's modification time, if defined with only one-
      second resolution, could be a weak validator, since it is possible
      that the representation might be modified twice during a single
      second.

      Support for weak validators is optional.

   However, weak validators
      allow for more efficient caching of equivalent objects; for
      example, a hit counter on a site is probably good enough if it is
      updated every few days or weeks, and any value during that period
      is likely "good enough" to there might be equivalent.

   A "use" of a validator is either cases when a client generates a request
   and includes server prefers to change the
   validator in a validating header field, or when a
   server compares two validators.

   Strong validators are usable in any context.  Weak validators are only usable in contexts that do not depend on exact equality of a
   representation. when it desires cached representations to be
   invalidated.  For example, either kind is usable for a normal
   conditional GET.  However, only a strong validator is usable for a
   sub-range retrieval, since otherwise the client might end up with an
   internally inconsistent representation.

   Clients MUST NOT use weak validators in range requests ([Part5]).

   The only function representation of a weather report
   that HTTP/1.1 defines on validators is comparison.
   There are two validator comparison functions, depending changes in content every second, based on whether
   the comparison context allows the use dynamic measurements,
   might be grouped into sets of weak validators or not:

   o  The strong comparison function: equivalent representations (from the
   origin server's perspective) in order to be considered equal,
      both opaque-tags MUST be identical character-by-character, and
      both MUST NOT be weak.

   o  The weak comparison function: in order allow cached representations
   to be considered equal,
      both opaque-tags MUST be identical character-by-character, but
      either valid for a reasonable period of time (perhaps adjusted
   dynamically based on server load or both weather quality).  A validator
   that does not always change when the representation changes is a
   "weak validator".

   One can think of them MAY be tagged a strong validator as "weak" without affecting
      the result.

   The example below shows the results part of an identifier for a
   specific representation, whereas a weak validator is part of an
   identifier for a set of entity-tag pairs,
   and both equivalent representations (where this notion
   of equivalence is entirely governed by the weak origin server 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           |
   +--------+--------+-------------------+-----------------+ beyond
   the scope of this specification).

   An entity-tag is normally a strong unless it is explicitly tagged as weak.
   Section 2 gives validator, but the syntax for entity-tags. protocol
   provides a mechanism to tag an entity-tag as "weak".

      A Last-Modified representation's modification time, when used as if defined with only one-
      second resolution, could be a validator in weak validator, since it is possible
      that the representation might be modified twice during a request, single
      second.

      Support for weak validators is
   implicitly optional.  However, weak unless validators
      allow for more efficient caching of equivalent objects; for
      example, a hit counter on a site is probably good enough if it is possible to deduce
      updated every few days or weeks, and any value during that it period
      is strong,
   using likely "good enough" to be equivalent.

   A strong entity-tag MUST change whenever the associated
   representation changes in any way.  A weak entity-tag SHOULD change
   whenever the following rules:

   o  The validator is being compared by an origin server considers prior representations to be
   unacceptable as a substitute for the actual current validator for representation.  In
   other words, a weak entity tag SHOULD change whenever 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 wants caches to invalidate old responses.

   A "strong entity-tag" MAY be used shared by two representations of a client in an If-Modified-
      Since or If-Unmodified-Since header field, because
   resource only if they are equivalent by octet equality.

   A "weak entity-tag", indicated by the client has "W/" prefix, MAY be shared by
   two representations of a cache entry resource.  A weak entity-tag can only be
   used for the associated representation, and

   o  That weak comparison.

   Cache entries might persist for arbitrarily long periods, regardless
   of expiration times.  Thus, a cache might attempt to validate an
   entry includes using 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 that it obtained in the distant past.  A
   strong entity-tag MUST be unique across all versions of all
   representations associated with a particular resource over time.
   However, there is being compared by an intermediate cache to no implication of uniqueness across entity-tags of
   different resources (i.e., the
      validator stored in its cache entry same entity-tag value might be in use
   for representations of multiple resources at the representation, and

   o  That cache entry includes a Date value, which gives the same 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 does
   not imply that if those representations are equivalent).

2.2.3.  Comparison

   There are two different responses were
   sent by the origin server during entity-tag comparison functions, depending on whether
   the same second, but both had comparison context allows the
   same Last-Modified time, then at least one use of those responses would
   have a Date value equal to its Last-Modified time. weak validators or not:

   o  The arbitrary 60-
   second limit guards against the possibility that the Date strong comparison function: in order to be considered equal,
      both opaque-tags MUST be identical character-by-character, and Last-
   Modified values are generated from different clocks,
      both MUST NOT be weak.

   o  The weak comparison function: in order to be considered equal,
      both opaque-tags MUST be identical character-by-character, but
      either or at somewhat
   different times during the preparation both of the response.  An
   implementation them MAY use be tagged as "weak" without affecting
      the result.

   A "use" of a value larger than 60 seconds, if it is
   believed that 60 seconds validator is too short.

   If either when a client wishes to perform generates a sub-range retrieval on request
   and includes the validator in a value for
   which it has only precondition, or when a Last-Modified time and no opaque validator, it
   MAY do this server
   compares two validators.

   Strong validators are usable in any context.  Weak validators are
   only if the Last-Modified time usable in contexts that do not depend on exact equality of a
   representation.  For example, either kind is strong in the sense
   described here.

   A cache or origin server receiving usable for a normal
   conditional range request
   ([Part5]) MUST use GET.

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

   These rules allow HTTP/1.1 caches and clients to safely perform sub-
   range retrievals on values that have been obtained from HTTP/1.0
   servers.

5. 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           |
   +--------+--------+-------------------+-----------------+

   An entity-tag is strong unless it is explicitly tagged as weak.

2.2.4.  Rules for 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.

   HTTP/1.1 origin servers:

   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,
      unless the risk of a breakdown in semantic transparency that could
      result from using this date in an If-Modified-Since header field
      would lead to serious problems. one.

   In other words, the preferred behavior for an HTTP/1.1 origin server
   is to send both a strong entity-tag and a Last-Modified value.

   In order to be legitimate, a strong entity-tag MUST change whenever
   the associated representation changes in any way.  A weak entity-tag
   SHOULD change whenever the associated representation changes in a
   semantically significant way.

      Note: In order to provide semantically transparent caching, an
      origin server must avoid reusing a specific strong entity-tag
      value for two different representations, or reusing a specific
      weak entity-tag value for two semantically different
      representations.  Cache entries might persist for arbitrarily long
      periods, regardless of expiration times, so it might be
      inappropriate to expect that a cache will never again attempt to
      validate an entry using a validator that it obtained at some point
      in the past. value.

   HTTP/1.1 clients:

   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.

   An HTTP/1.1 origin server, upon receiving a conditional request that
   includes both a Last-Modified date (e.g., in an If-Modified-Since or
   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
   in an If-Match, If-None-Match, or If-Range header field) as cache
   validators, MUST NOT return a response status code of 304 (Not
   Modified) unless doing so is consistent with all of the conditional
   header fields in the request.

   An HTTP/1.1 caching proxy, upon receiving a conditional request that
   includes both a Last-Modified date and one or more entity-tags as
   cache validators, MUST NOT return a locally cached response to the
   client unless that cached response is consistent with all of the
   conditional header fields in the request.

      Note: The general principle behind these rules is that HTTP/1.1
      servers and clients ought to transmit as much non-redundant
      information as is available in their responses and requests.
      HTTP/1.1 systems receiving this information will make the most
      conservative assumptions about the validators they receive.

      HTTP/1.0 clients and caches might ignore entity-tags.  Generally,
      last-modified values received or used by these systems will
      support transparent and efficient caching, and so HTTP/1.1 origin
      servers should provide Last-Modified values.  In those rare cases
      where the use of a Last-Modified value as a validator by an
      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
      origin servers should not provide one.

6.  Header Field Definitions

   This section defines the syntax and semantics of HTTP/1.1 header
   fields related to conditional requests.

6.1.  ETag

   The "ETag" header field provides the current value of the entity-tag
   (see Section 2) for one representation of the target resource.  An
   entity-tag is intended for use as

2.2.5.  Example: Entity-tags varying on Content-Negotiated Resources

   Consider a resource-local identifier for
   differentiating between representations of the same resource that
   vary over time or via content negotiation (see Section 4).

     ETag   = "ETag" ":" OWS ETag-v
     ETag-v = entity-tag

   Examples:

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

   An entity-tag provides an "opaque" cache validator that allows for
   more reliable validation than modification dates in situations where
   it is inconvenient subject to store modification dates, content negotiation (Section 5
   of [Part3]), and where the one-second
   resolution representations returned upon a GET
   request vary based on the Accept-Encoding request header field
   (Section 6.3 of HTTP date values is not sufficient, or where [Part3]):

   >> Request:

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

   In this case, the origin
   server wishes to avoid certain paradoxes that response might arise from 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...

      Note: Content codings are a property of modification dates.

   The principle behind entity-tags is that only the service author
   knows the semantics representation, so
      therefore an entity-tag of a resource well enough to select an
   appropriate encoded representation must be
      distinct from an unencoded representation to prevent conflicts
      during cache validation mechanism, updates and the specification of any
   validator comparison function more complex than byte-equality would
   open up a can range requests.  In contrast, transfer
      codings (Section 6.2 of worms.  Thus, comparisons [Part1]) apply only during message
      transfer and do not require distinct entity-tags.

3.  Precondition Header Fields

   This section defines the syntax and semantics of any other HTTP/1.1 header
   fields
   (except Last-Modified, for compatibility with HTTP/1.0) are never
   used for purposes of validating a cache entry.

6.2. applying preconditions on requests.

3.1.  If-Match

   The "If-Match" header field is MAY be used to make a request method
   conditional.  A client that has
   conditional on the current existence or value of an entity-tag for
   one or more representations
   previously obtained from the resource can verify that one of those
   representations the target resource.  If-Match is current by including
   generally useful for resource update requests, such as PUT requests,
   as a list of their associated
   entity-tags 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 header field.

   This allows efficient updates of cached information with a minimum
   amount of transaction overhead.  It is also used when updating
   resources, to prevent inadvertent modification field-value of "*" places the wrong version
   of a resource.  As a special case,
   precondition on the value "*" matches existence of any current representation of for the
   target resource.

     If-Match = "If-Match" ":" OWS If-Match-v
     If-Match-v = "*" / 1#entity-tag

   If any of the entity-tags listed in the If-Match field value match
   (as per Section 2.2.3) the entity-tag of the selected representation
   that would have been returned in the response to a similar GET
   request (without
   for the If-Match header field) on that target resource, or if "*" is given and any current
   representation exists for that the target resource, then the server MAY
   perform the requested request method as if the If-Match header field did was not exist.
   present.

   If none of the entity-tags match, or if "*" is given and no current
   representation exists, the server MUST NOT perform the requested
   method, and MUST return a 412 (Precondition Failed) response.  This
   behavior is most useful when
   method.  Instead, the client wants to prevent an updating
   request method, such as PUT, from modifying a resource that has
   changed since server MUST respond with the client last retrieved it. 412 (Precondition
   Failed) status code.

   If the request would, without the If-Match header field, result in
   anything other than a 2xx or 412 status code, then the If-Match
   header field MUST be ignored.

   The meaning of "If-Match: *" is that the request method SHOULD be
   performed if the representation selected by the origin server (or by
   a cache, possibly using the Vary mechanism, see Section 3.5 of
   [Part6]) exists, and MUST NOT be performed if the representation does
   not exist.

   A request intended to update a resource (e.g., a PUT) MAY include an
   If-Match header field to signal that the request method MUST NOT be
   applied if the representation corresponding to the If-Match value (a
   single entity-tag) is no longer a representation of that resource.
   This allows the user to indicate that they do not wish the request to
   be successful if the resource has been changed without their
   knowledge.

   Examples:

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

   The result of a request having both an If-Match header field and
   either an If-None-Match or an If-Modified-Since header fields is
   undefined by this specification.

6.3.  If-Modified-Since

   The "If-Modified-Since" header field is used to make a request method
   conditional by date: if the representation that would have been
   transferred in a 200 response to a GET request has not been modified
   since the time specified in this field, then do not perform the
   method; instead, respond as detailed below.

     If-Modified-Since   = "If-Modified-Since" ":" OWS
                           If-Modified-Since-v
     If-Modified-Since-v = 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 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 *

   The result in anything other than of a 200
       (OK) status code, request having both an If-Match header field and
   either an If-None-Match or if the passed an If-Modified-Since date is
       invalid, the response header fields is exactly the same as for
   undefined by this specification.

3.2.  If-None-Match

   The "If-None-Match" header field MAY be used to make a normal GET.  A
       date which is later than request method
   conditional on not matching any of the server's current time is invalid.

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

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

   The purpose
   representations of this feature the target resource.  If-None-Match is primarily
   used in conditional GET requests to allow enable efficient updates of
   cached information with a minimum amount of transaction overhead.

      Note: The Range header field modifies the meaning of If-Modified-
      Since; see Section 5.4 of [Part5] for full details.

      Note: If-Modified-Since times are interpreted by the server, whose
      clock might not be synchronized with the client.

      Note: When handling an If-Modified-Since header field, some
      servers will use an exact date comparison function, rather than a
      less-than function, for deciding whether to send a 304 (Not
      Modified) response.  To get best results when sending an If-
      Modified-Since header field for cache validation, clients are
      advised to use the exact date string received in a previous Last-
      Modified header field whenever possible.

      Note: If a  A
   client uses an arbitrary date 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 If-Modified-Since
      header field instead hope of receiving a date taken from 304 response if at
   least one of those representations matches the Last-Modified header
      field for selected
   representation.

   If-None-Match MAY also be used with a value of "*" to prevent an
   unsafe request method (e.g., PUT) from inadvertently modifying an
   existing representation of the same request, target resource when the client needs to be aware
   believes that this
      date the resource does not have a current representation.
   This is interpreted in a variation on the server's understanding of time.
      Unsynchronized clocks and rounding problems, due "lost update" problem that might arise if
   more than one client attempts to create an initial representation for
   the different
      encodings target resource.

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

   If any of time between the client and server, are concerns.
      This includes entity-tags listed in the possibility If-None-Match field-value
   match (as per Section 2.2.3) the entity-tag of race conditions the selected
   representation, or if "*" is given and any current representation
   exists for that resource, then the document
      has changed between server MUST NOT perform the time it was first
   requested and method.  Instead, if the If-
      Modified-Since date 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 subsequent request, and matching entity-tag.  For all
   other request methods, the possibility server MUST respond with a 412
   (Precondition Failed) status code.

   If none of clock-skew-related problems the entity-tags match, then the server MAY perform the
   requested method as if the If-None-Match header field did not exist,
   but MUST also ignore any If-Modified-Since header field(s) in the
   request.  That is, if no entity-tags match, then the If-Modified-Since date is
      derived from server MUST NOT
   return a 304 (Not Modified) response.

   If the client's clock request would, without correction to the server's
      clock.  Corrections If-None-Match header field, result
   in anything other than a 2xx or 304 status code, then the If-None-
   Match header field MUST be ignored.  (See Section 2.2.4 for different time bases between client and a
   discussion of server are at best approximate due to network latency. behavior when both If-Modified-Since and If-
   None-Match appear in the same request.)

   Examples:

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

   The result of a request having both an If-Modified-Since If-None-Match header field and
   either an If-Match or an If-Unmodified-Since header fields is
   undefined by this specification.

6.4.  If-None-Match

3.3.  If-Modified-Since

   The "If-None-Match" "If-Modified-Since" header field is MAY be used to make a request
   method
   conditional.  A client that conditional by modification date: if the selected
   representation has one or more representations
   previously obtained from not been modified since the resource can verify that none of those
   representations is current by including a list of their associated
   entity-tags time specified in this
   field, then do not perform the If-None-Match header field.

   This allows efficient updates of cached information with a minimum
   amount of transaction overhead.  It is also used to prevent a 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 (e.g., PUT) from inadvertently modifying with an existing resource
   when the client believes If-Modified-Since header field and no Range
   header field requests that the resource does not exist.

   As a special case, the value "*" matches any current selected representation
   of be transferred
   only if it has been modified since the resource.

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

   If any of date given by the entity-tags match If-Modified-
   Since header field.  The algorithm for determining this includes the entity-tag of
   following cases:

   1.  If the representation
   that request would have been returned normally result in the response to anything other than a similar GET
   request (without the If-None-Match header field) on that resource, 200
       (OK) status code, or if "*" the passed If-Modified-Since date is given and any current representation exists
       invalid, the response is exactly the same as for that
   resource, then a normal GET.  A
       date which is later than the server MUST NOT perform server's current time is invalid.

   2.  If the requested method,
   unless required to do so because selected representation has been modified since the resource's modification date
   fails to match that supplied in an If-Modified-Since header field in If-
       Modified-Since date, the request.  Instead, if response is exactly the request method was GET or HEAD, 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 respond with return a 304 (Not
       Modified) response, including
   the cache-related header fields (particularly ETag) response.

   The purpose of one this feature is to allow efficient updates of the
   representations that matched.  For all other request methods, the
   server MUST respond cached
   information with a 412 (Precondition Failed) status code.

   If none minimum amount of the entity-tags match, then the server MAY perform the
   requested method as if the If-None-Match transaction overhead.

      Note: The Range header field did modifies the meaning of If-Modified-
      Since; see Section 5.4 of [Part5] for full details.

      Note: If-Modified-Since times are interpreted by the server, whose
      clock might not exist,
   but MUST also ignore any be synchronized with the client.

      Note: When handling an If-Modified-Since header field(s) in the
   request.  That is, if no entity-tags match, then the server MUST NOT
   return field, some
      servers will use an exact date comparison function, rather than a
      less-than function, for deciding whether to send a 304 (Not
      Modified) response.

   If the request would, without  To get best results when sending an If-
      Modified-Since header field for cache validation, clients are
      advised to use the If-None-Match exact date string received in a previous Last-
      Modified header field, result field whenever possible.

      Note: If a client uses an arbitrary date in anything other than the If-Modified-Since
      header field instead of a 2xx or 304 status code, then date taken from the If-None-
   Match Last-Modified header
      field MUST be ignored.  (See Section 5 for a discussion the same request, the client needs to be aware that this
      date is interpreted in the server's understanding of server behavior when both If-Modified-Since time.
      Unsynchronized clocks and If-None-Match
   appear in rounding problems, due to the same request.)

   The meaning different
      encodings of "If-None-Match: *" is that time between the request method MUST NOT
   be performed client and server, are concerns.
      This includes the possibility of race conditions if the representation selected by document
      has changed between the origin server (or
   by a cache, possibly using time it was first requested and the Vary mechanism, see Section 3.5 If-
      Modified-Since date of
   [Part6]) exists, a subsequent request, and SHOULD be performed the possibility
      of clock-skew-related problems if the representation does
   not exist.  This feature If-Modified-Since date is intended
      derived from the client's clock without correction to be useful in preventing races the server's
      clock.  Corrections for different time bases between PUT operations.

   Examples:

     If-None-Match: "xyzzy"
     If-None-Match: W/"xyzzy"
     If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
     If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
     If-None-Match: * client and
      server are at best approximate due to network latency.

   The result of a request having both an If-None-Match If-Modified-Since header field
   and either an If-Match or an If-Unmodified-Since header fields is
   undefined by this specification.

6.5.

3.4.  If-Unmodified-Since

   The "If-Unmodified-Since" header field is MAY be used to make a request
   method conditional.  If conditional by modification date: if the selected
   representation that would have been
   transferred in a 200 response to a GET request on the same resource has not been modified since the time specified in this
   field, then the server SHOULD MUST NOT perform the requested operation as if and
   MUST instead respond with the If-
   Unmodified-Since header field were not present. 412 (Precondition Failed) status code.
   If the selected representation has not been modified since the time
   specified time, in this field, the server MUST NOT SHOULD perform the requested operation, and MUST return a
   412 (Precondition Failed). request method
   as if the If-Unmodified-Since header field were not present.

     If-Unmodified-Since   = "If-Unmodified-Since" ":" OWS
                             If-Unmodified-Since-v
     If-Unmodified-Since-v = HTTP-date

   An example of the field is:

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

   If the request normally (i.e., without the If-Unmodified-Since header
   field) would result in anything other than a 2xx or 412 status code,
   the If-Unmodified-Since header field SHOULD be ignored.

   If the specified date is invalid, the header field is MUST be ignored.

   The result of a request having both an If-Unmodified-Since header
   field and either an If-None-Match or an If-Modified-Since header
   fields is undefined by this specification.

6.6.  Last-Modified

3.5.  If-Range

   The "Last-Modified" If-Range header field indicates the date and time at which
   the origin server believes the representation was last modified.

     Last-Modified   = "Last-Modified" ":" OWS Last-Modified-v
     Last-Modified-v = HTTP-date

   An example of its use provides a special conditional request
   mechanism that is

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

   A representation similar to If-Match and If-Unmodified-Since but
   specific to HTTP range requests.  If-Range is typically the sum defined in Section 5.3
   of many parts behind the
   resource interface. [Part5].

4.  Status Code Definitions

4.1.  304 Not Modified

   The last-modified time would usually be the most
   recent time 304 status code indicates that any of those parts a conditional GET request has been
   received and would have resulted in a 200 (OK) response if it were changed.  How
   not for the fact that value the condition has evaluated to false.  In other
   words, there is
   determined no need for any given resource is an implementation detail beyond the scope of this specification.  What matters server to HTTP is how
   recipients transfer a representation
   of the Last-Modified target resource because the client's request indicates that it
   already has a valid representation, as indicated by the 304 response
   header field can use its value fields, and is therefore redirecting the client to make conditional requests and test use of
   that stored representation as if it were the validity payload of locally cached
   responses.

   An origin server a 200
   response.  The 304 response MUST NOT send contain a Last-Modified date which message-body, and thus
   is later
   than always terminated by the server's time first empty line after the header fields.

   A 304 response MUST include a Date header field (Section 9.3 of message origination.  In such cases, where
   [Part1]) unless its omission is required by Section 9.3.1 of [Part1].
   If a 200 response to the resource's last modification same request would indicate some time have included any of the
   header fields Cache-Control, Content-Location, ETag, Expires, Last-
   Modified, or Vary, then those same header fields MUST be sent in a
   304 response.

   Since the
   future, goal of a 304 response is to minimize information transfer
   when the server MUST replace that date with recipient already has one or more cached representations,
   the message
   origination date.

   An origin server response SHOULD obtain NOT include representation metadata other than
   the Last-Modified value above listed fields unless said metadata exists for the purpose
   of guiding cache updates (e.g., future HTTP extensions).

   If the recipient of a 304 response does not have a cached
   representation as close as possible corresponding to the time that it generates entity-tag indicated by the 304
   response, then the
   Date value of its response.  This allows a recipient MUST NOT use the 304 to make update its own
   cache.  If this conditional request originated with an
   accurate assessment of outbound
   client, such as a user agent with its own cache sending a conditional
   GET to a shared proxy, then the representation's modification time,
   especially if 304 response MAY be forwarded to the representation changes near
   outbound client.  Otherwise, the time that recipient MUST disregard the 304
   response is generated.

   HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.

   The Last-Modified header field value is often used as and repeat the request without any preconditions.

   If a cache
   validator.  In simple terms, uses a received 304 response to update a cache entry, the
   cache MUST update the entry is considered to be valid
   if reflect any new field values given in
   the representation has not been modified since response.

4.2.  412 Precondition Failed

   The 412 status code indicates that one or more preconditions given in
   the Last-Modified
   value.

7. 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.  IANA Considerations

7.1.

5.1.  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 3.1 4.1 |
   | 412   | Precondition Failed | Section 3.2 4.2 |
   +-------+---------------------+-------------+

7.2.

5.2.  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 [RFC3864]):

   +---------------------+----------+----------+-------------+
   | Header Field Name   | Protocol | Status   | Reference   |
   +---------------------+----------+----------+-------------+
   | ETag                | http     | standard | Section 6.1 2.2 |
   | If-Match            | http     | standard | Section 6.2 3.1 |
   | If-Modified-Since   | http     | standard | Section 6.3 3.3 |
   | If-None-Match       | http     | standard | Section 6.4 3.2 |
   | If-Unmodified-Since | http     | standard | Section 6.5 3.4 |
   | Last-Modified       | http     | standard | Section 6.6 2.1 |
   +---------------------+----------+----------+-------------+

   The change controller is: "IETF (iesg@ietf.org) - Internet
   Engineering Task Force".

8.

6.  Security Considerations

   No additional security considerations have been identified beyond
   those applicable to HTTP in general [Part1].

9.

7.  Acknowledgments

10.

8.  References

10.1.

8.1.  Normative References

   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
              and Message Parsing", draft-ietf-httpbis-p1-messaging-13 draft-ietf-httpbis-p1-messaging-14
              (work in progress), March April 2011.

   [Part3]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
              and Content Negotiation", draft-ietf-httpbis-p3-payload-13 draft-ietf-httpbis-p3-payload-14
              (work in progress), March April 2011.

   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
              Partial Responses", draft-ietf-httpbis-p5-range-13 draft-ietf-httpbis-p5-range-14 (work
              in progress), March April 2011.

   [Part6]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part
              6: Caching", draft-ietf-httpbis-p6-cache-13 draft-ietf-httpbis-p6-cache-14 (work in
              progress), March April 2011.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

10.2.

8.2.  Informative References

   [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.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

Appendix A.  Changes from RFC 2616

   Allow weak entity-tags in all requests except range requests
   (Sections 4 2.2.2 and 6.4). 3.2).

   Change ABNF productions for header fields to only define the field
   value.  (Section 3)

Appendix B.  Collected ABNF

   ETag = "ETag:" OWS ETag-v
   ETag-v = entity-tag

   HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>

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

   Last-Modified = "Last-Modified:" OWS Last-Modified-v
   Last-Modified-v = HTTP-date

   OWS = <OWS, defined in [Part1], Section 1.2.2>

   entity-tag = [ weak ] opaque-tag

   opaque-tag = quoted-string

   quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>

   weak = %x57.2F ; W/

   ABNF diagnostics:

   ; ETag defined but not used
   ; If-Match defined but not used
   ; If-Modified-Since defined but not used
   ; If-None-Match defined but not used
   ; If-Unmodified-Since defined but not used
   ; Last-Modified defined but not used

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

C.1.  Since RFC 2616

   Extracted relevant partitions from [RFC2616].

C.2.  Since draft-ietf-httpbis-p4-conditional-00

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
      Informative references"

   Other changes:

   o  Move definitions of 304 and 412 condition codes from Part2.

C.3.  Since draft-ietf-httpbis-p4-conditional-01

   Ongoing work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

   o  Add explicit references to BNF syntax and rules imported from
      other parts of the specification.

C.4.  Since draft-ietf-httpbis-p4-conditional-02

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on
      non-GET requests"

   Ongoing work on IANA Message Header Field Registration
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):

   o  Reference RFC 3984, and update header field registrations for
      header fields defined in this document.

C.5.  Since draft-ietf-httpbis-p4-conditional-03

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/71>: "Examples for
      ETag matching"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/124>: "'entity
      value' undefined"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/126>: "bogus 2068
      Date header reference"

C.6.  Since draft-ietf-httpbis-p4-conditional-04

   Ongoing work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

   o  Use "/" instead of "|" for alternatives.

   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
      whitespace ("OWS") and required whitespace ("RWS").

   o  Rewrite ABNFs to spell out whitespace rules, factor out header
      field value format definitions.

C.7.  Since draft-ietf-httpbis-p4-conditional-05

   Final work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

   o  Add appendix containing collected and expanded ABNF, reorganize
      ABNF introduction.

C.8.  Since draft-ietf-httpbis-p4-conditional-06

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/153>: "case-
      sensitivity of etag weakness indicator"

C.9.  Since draft-ietf-httpbis-p4-conditional-07

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on
      non-GET requests" (If-Match still was defined to require strong
      matching)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/198>: "move IANA
      registrations for optional status codes"

C.10.  Since draft-ietf-httpbis-p4-conditional-08

   No significant changes.

C.11.  Since draft-ietf-httpbis-p4-conditional-09

   No significant changes.

C.12.  Since draft-ietf-httpbis-p4-conditional-10

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/69>: "Clarify
      'Requested Variant'"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
      entity / representation / variant terminology"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
      removing the 'changes from 2068' sections"

C.13.  Since draft-ietf-httpbis-p4-conditional-11

   None.

C.14.  Since draft-ietf-httpbis-p4-conditional-12

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header
      Classification"

C.15.  Since draft-ietf-httpbis-p4-conditional-13

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/89>: "If-* and
      entities"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/101>: "Definition of
      validator weakness"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
      ABNFs for header fields"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/269>: "ETags and
      Quotes"

Index

   3
      304 Not Modified (status code)  8  18

   4
      412 Precondition Failed (status code)  8  19

   E
      ETag header field  13  8

   G
      Grammar
         entity-tag  6  8
         ETag  13
         ETag-v  13  8
         If-Match  14
         If-Match-v  14
         If-Modified-Since  15
         If-Modified-Since-v  15  16
         If-None-Match  17
         If-None-Match-v  17  15
         If-Unmodified-Since  18
         If-Unmodified-Since-v  18
         Last-Modified  19
         Last-Modified-v  19
         opaque-tag  6
         opaque-tag  8
         weak  6  8

   H
      Header Fields
         ETag  13  8
         If-Match  14
         If-Modified-Since  15  16
         If-None-Match  17  15
         If-Unmodified-Since  18
         Last-Modified  19  6

   I
      If-Match header field  14
      If-Modified-Since header field  15  16
      If-None-Match header field  17  15
      If-Unmodified-Since header field  18

   L
      Last-Modified header field  19  6

   M
      metadata  6

   S
      selected representation  5
      Status Codes
         304 Not Modified  8  18
         412 Precondition Failed  8  19

   V
      validator  6

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/
   Jim Gettys
   Alcatel-Lucent Bell Labs
   21 Oak Knoll Road
   Carlisle, MA  01741
   USA

   EMail: jg@freedesktop.org
   URI:   http://gettys.wordpress.com/

   Jeffrey C. Mogul
   Hewlett-Packard Company
   HP Labs, Large Scale Systems Group
   1501 Page Mill Road, MS 1177
   Palo Alto, CA  94304
   USA

   EMail: JeffMogul@acm.org

   Henrik Frystyk Nielsen
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052
   USA

   EMail: henrikn@microsoft.com

   Larry Masinter
   Adobe Systems Incorporated
   345 Park Ave
   San Jose, CA  95110
   USA

   EMail: LMM@acm.org
   URI:   http://larry.masinter.net/

   Paul J. Leach
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052

   EMail: paulle@microsoft.com
   Tim Berners-Lee
   World Wide Web Consortium
   MIT Computer Science and Artificial Intelligence Laboratory
   The Stata Center, Building 32
   32 Vassar Street
   Cambridge, MA  02139
   USA

   EMail: timbl@w3.org
   URI:   http://www.w3.org/People/Berners-Lee/

   Yves Lafon (editor)
   World Wide Web Consortium
   W3C / ERCIM
   2004, rte des Lucioles
   Sophia-Antipolis, AM  06902
   France

   EMail: ylafon@w3.org
   URI:   http://www.raubacapeu.net/people/yves/

   Julian F. Reschke (editor)
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155
   Germany

   Phone: +49 251 2807760
   Fax:   +49 251 2807761
   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/