source: draft-ietf-httpbis/16/draft-ietf-httpbis-p4-conditional-16.txt @ 1650

HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2616 (if approved)                                  J. Gettys
Intended status: Standards Track                          Alcatel-Lucent
Expires: February 25, 2012                                      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
                                                         August 24, 2011


                 HTTP/1.1, part 4: Conditional Requests
                  draft-ietf-httpbis-p4-conditional-16

Abstract

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypertext 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), 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.17.



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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
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   This Internet-Draft will expire on February 25, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   than English.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  5



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     1.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  6
   2.  Validators . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Weak versus Strong . . . . . . . . . . . . . . . . . . . .  6
     2.2.  Last-Modified  . . . . . . . . . . . . . . . . . . . . . .  8
       2.2.1.  Generation . . . . . . . . . . . . . . . . . . . . . .  8
       2.2.2.  Comparison . . . . . . . . . . . . . . . . . . . . . .  9
     2.3.  ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
       2.3.1.  Generation . . . . . . . . . . . . . . . . . . . . . . 11
       2.3.2.  Comparison . . . . . . . . . . . . . . . . . . . . . . 11
       2.3.3.  Example: Entity-tags varying on Content-Negotiated
               Resources  . . . . . . . . . . . . . . . . . . . . . . 12
     2.4.  Rules for When to Use Entity-tags and Last-Modified
           Dates  . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   3.  Precondition Header Fields . . . . . . . . . . . . . . . . . . 14
     3.1.  If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14
     3.2.  If-None-Match  . . . . . . . . . . . . . . . . . . . . . . 15
     3.3.  If-Modified-Since  . . . . . . . . . . . . . . . . . . . . 17
     3.4.  If-Unmodified-Since  . . . . . . . . . . . . . . . . . . . 18
     3.5.  If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 19
   4.  Status Code Definitions  . . . . . . . . . . . . . . . . . . . 19
     4.1.  304 Not Modified . . . . . . . . . . . . . . . . . . . . . 19
     4.2.  412 Precondition Failed  . . . . . . . . . . . . . . . . . 20
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
     5.1.  Status Code Registration . . . . . . . . . . . . . . . . . 20
     5.2.  Header Field Registration  . . . . . . . . . . . . . . . . 20
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 22
   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 22
   Appendix C.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 23
     C.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 23
     C.2.  Since draft-ietf-httpbis-p4-conditional-00 . . . . . . . . 23
     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
     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 . . . . . . . . 25
     C.12. Since draft-ietf-httpbis-p4-conditional-10 . . . . . . . . 25
     C.13. Since draft-ietf-httpbis-p4-conditional-11 . . . . . . . . 25
     C.14. Since draft-ietf-httpbis-p4-conditional-12 . . . . . . . . 25



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     C.15. Since draft-ietf-httpbis-p4-conditional-13 . . . . . . . . 25
     C.16. Since draft-ietf-httpbis-p4-conditional-14 . . . . . . . . 26
     C.17. Since draft-ietf-httpbis-p4-conditional-15 . . . . . . . . 26
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26















































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1.  Introduction

   This document defines the HTTP/1.1 conditional request mechanisms,
   including both metadata for indicating/observing changes in resource
   representations and request header fields that specify preconditions
   on that metadata be checked before performing the request method.
   Conditional GET requests are the most efficient mechanism for HTTP
   cache updates [Part6].  Conditionals can also be applied to state-
   changing methods, such as PUT and DELETE, to prevent the "lost
   update" problem: one client accidentally overwriting the work of
   another client that has been acting in parallel.

   Conditional request preconditions are based on the state of the
   target resource as a whole (its current value set) or the state as
   observed in a previously obtained representation (one value in that
   set).  A resource might have multiple current representations, each
   with its own observable state.  The conditional request mechanisms
   assume that the mapping of requests to corresponding representations
   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.

   We use the term "selected representation" to refer to the current
   representation of the target resource that would have been selected
   in a successful response if the same request had used the method GET
   and had excluded all of the conditional request header fields.  The
   conditional request preconditions are evaluated by comparing the
   values provided in the request header fields to the current 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".






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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).

   The ABNF rules below are defined in [Part1]:

     OWS           = <OWS, defined in [Part1], Section 1.2.2>
     quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
     HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>

2.  Validators

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

2.1.  Weak versus Strong

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

   A "strong validator" is a representation metadata value that MUST be
   changed to a new, previously unused or guaranteed unique, value
   whenever a change occurs to the representation data such that a
   change would be observable in the payload body of a 200 response to
   GET.  A strong validator MAY be changed for other reasons, such as
   when a semantically significant part of the representation metadata
   is changed (e.g., Content-Type), but it is in the best interests of
   the origin server to only change the value when it is necessary to



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   invalidate the stored responses held by remote caches and authoring
   tools.  A strong validator MUST be unique across all representations
   of a given resource, such that no two representations of that
   resource share the same validator unless their payload body would be
   identical.

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

   There are a variety of strong validators used in practice.  The best
   are based on strict revision control, wherein each change to a
   representation always results in a unique node name and revision
   identifier being assigned before the representation is made
   accessible to GET.  A cryptographic 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 a server
   SHOULD incorporate additional information in the validator to
   distinguish those representations and avoid confusing cache behavior.

   In contrast, a "weak validator" is a representation metadata value
   that might not be changed 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.  A weak entity-tag SHOULD change whenever the origin server
   considers prior representations to be unacceptable as a substitute
   for the current representation.  In other words, a weak entity-tag
   SHOULD change whenever the origin server wants caches to invalidate
   old responses.

   For example, the representation of a weather report that changes in
   content every second, based on dynamic measurements, might be grouped
   into sets of equivalent representations (from the origin server's
   perspective) with the same weak validator in order to allow cached
   representations to be valid for a reasonable period of time (perhaps



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   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.

   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 a representation's payload
   body.  Strong validators are usable and preferred for all conditional
   requests, including cache validation, partial content ranges, and
   "lost update" avoidance.

2.2.  Last-Modified

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

     Last-Modified = HTTP-date

   An example of its use is

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

2.2.1.  Generation

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

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

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



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   especially if the representation changes near the time that the
   response is generated.

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

2.2.2.  Comparison

   A Last-Modified time, when used as a validator in a request, is
   implicitly weak unless it is possible to deduce that it is strong,
   using the following rules:

   o  The validator is being compared by an origin server to the actual
      current validator for the representation and,

   o  That origin server reliably knows that the associated
      representation did not change twice during the second covered by
      the presented validator.

   or

   o  The validator is about to be used by a client in an If-Modified-
      Since, If-Unmodified-Since header field, because the client has a
      cache entry, or If-Range for the associated representation, and

   o  That cache entry includes a Date value, which gives the time when
      the origin server sent the original response, and

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

   or

   o  The validator is being compared by an intermediate cache to the
      validator stored in its cache entry for the representation, and

   o  That cache entry includes a Date value, which gives the time when
      the origin server sent the original response, and

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

   This method relies on the fact that if two different responses were



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   sent by the origin server during the same second, but both had the
   same Last-Modified time, then at least one of those responses would
   have a Date value equal to its Last-Modified time.  The arbitrary 60-
   second limit guards against the possibility that the Date and Last-
   Modified values are generated from different clocks, or at somewhat
   different times during the preparation of the response.  An
   implementation MAY use a value larger than 60 seconds, if it is
   believed that 60 seconds is too short.

2.3.  ETag

   The ETag header field 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 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: ""

   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
   the requirements for a strong validator (Section 2.1), then that
   entity-tag MUST be marked as weak by prefixing its opaque value with
   "W/" (case-sensitive).







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2.3.1.  Generation

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

   For example, a resource that has implementation-specific versioning
   applied to all changes might use an internal revision number, perhaps
   combined with a variance identifier for content negotiation, to
   accurately differentiate between representations.  Other
   implementations might use a 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 entity-tag's use in conditional requests and
   evaluating cache freshness ([Part6]) can result in a substantial
   reduction of HTTP network traffic and can be a significant factor in
   improving service scalability and reliability.

2.3.2.  Comparison

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

   o  The strong comparison function: 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 to be considered equal,
      both opaque-tags MUST be identical character-by-character, but
      either or both of them MAY be tagged as "weak" without affecting
      the result.

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

   +--------+--------+-------------------+-----------------+
   | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
   +--------+--------+-------------------+-----------------+
   | W/"1"  | W/"1"  | no match          | match           |
   | W/"1"  | W/"2"  | no match          | no match        |
   | W/"1"  | "1"    | no match          | match           |




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   | "1"    | "1"    | match             | match           |
   +--------+--------+-------------------+-----------------+

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

   Consider a resource that is subject to content negotiation (Section 5
   of [Part3]), and where the representations returned upon a GET
   request vary based on the Accept-Encoding request 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 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:














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   >> 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 the representation, so
      therefore an entity-tag of an encoded representation must be
      distinct from an unencoded 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.

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.

   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.

   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.



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   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.

3.  Precondition Header Fields

   This section defines the syntax and semantics of HTTP/1.1 header
   fields for applying preconditions on requests.

3.1.  If-Match

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



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

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

   If any of the entity-tags listed in the If-Match field value match
   (as per Section 2.3.2) the entity-tag of the selected representation
   for the target resource, or if "*" is given and any current
   representation exists for the target resource, then the server MAY
   perform the request method as if the If-Match header field was not
   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.  Instead, the server MUST respond with the 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.

   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.

3.2.  If-None-Match

   The "If-None-Match" header field MAY be used to make a request method
   conditional on not matching any of the current entity-tag values for
   representations of the target resource.  If-None-Match is primarily
   used in conditional GET requests to enable efficient updates of
   cached information with a minimum amount of transaction overhead.  A
   client that has one or more representations previously obtained from
   the target resource can send If-None-Match with a list of the
   associated entity-tags in the hope of receiving a 304 response if at
   least one of those representations matches the selected
   representation.




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   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 target resource when the client
   believes that the resource does not have a current representation.
   This is a variation on the "lost update" problem that might arise if
   more than one client attempts to create an initial representation for
   the target resource.

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

   If any of the entity-tags listed in the If-None-Match field-value
   match (as per Section 2.3.2) the entity-tag of the selected
   representation, or if "*" is given and any current representation
   exists for that resource, then the server MUST NOT perform the
   requested method.  Instead, if the request method was GET or HEAD,
   the server SHOULD respond with a 304 (Not Modified) status code,
   including the cache-related header fields (particularly ETag) of the
   selected representation that has a matching entity-tag.  For all
   other request methods, the server MUST respond with a 412
   (Precondition Failed) status code.

   If none of 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 server MUST NOT
   return a 304 (Not Modified) response.

   If the request would, without the 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 2.4 for a
   discussion of server 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-None-Match header field and
   either an If-Match or an If-Unmodified-Since header fields is
   undefined by this specification.







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3.3.  If-Modified-Since

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

     If-Modified-Since = HTTP-date

   An example of the field is:

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

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

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

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

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

   The purpose of this feature is to allow 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-



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      Modified header field whenever possible.

      Note: If a client uses an arbitrary date in the If-Modified-Since
      header field instead of a date taken from the Last-Modified header
      field for the same request, the client needs to be aware that this
      date is interpreted in the server's understanding of time.
      Unsynchronized clocks and rounding problems, due to the different
      encodings of time between the client and server, are concerns.
      This includes the possibility of race conditions if the document
      has changed between the time it was first requested and the If-
      Modified-Since date of a subsequent request, and the possibility
      of clock-skew-related problems if the If-Modified-Since date is
      derived from the client's clock without correction to the server's
      clock.  Corrections for different time bases between client and
      server are at best approximate due to network latency.

   The result of a request having both an If-Modified-Since header field
   and either an If-Match or an If-Unmodified-Since header fields is
   undefined by this specification.

3.4.  If-Unmodified-Since

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

     If-Unmodified-Since = 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 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.





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3.5.  If-Range

   The If-Range header field provides a special conditional request
   mechanism that is similar to If-Match and If-Unmodified-Since but
   specific to HTTP range requests.  If-Range is defined in Section 5.3
   of [Part5].

4.  Status Code Definitions

4.1.  304 Not Modified

   The 304 status code indicates that a conditional GET request has been
   received and would have resulted in a 200 (OK) response if it were
   not for the fact that the condition has evaluated to false.  In other
   words, there is no need for the server to transfer a representation
   of the target resource because the client's request indicates that it
   already has a valid representation, as indicated by the 304 response
   header fields, and is therefore redirecting the client to make use of
   that stored representation as if it were the payload of a 200
   response.  The 304 response MUST NOT contain a message-body, and thus
   is always terminated by the first empty line after 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 of [Part1].
   If a 200 response to the same request would 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 goal of a 304 response is to minimize information transfer
   when the recipient already has one or more cached representations,
   the response SHOULD NOT include representation metadata other than
   the 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 corresponding to the entity-tag indicated by the 304
   response, then the recipient MUST NOT use the 304 to update its own
   cache.  If this 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 304 response MAY be forwarded to the
   outbound client.  Otherwise, the recipient MUST disregard the 304
   response and repeat the request without any preconditions.

   If a cache uses a received 304 response to update a cache entry, the
   cache MUST update the entry to reflect any new field values given in
   the response.




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4.2.  412 Precondition Failed

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

5.  IANA Considerations

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 4.1 |
   | 412   | Precondition Failed | Section 4.2 |
   +-------+---------------------+-------------+

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 2.3 |
   | If-Match            | http     | standard | Section 3.1 |
   | If-Modified-Since   | http     | standard | Section 3.3 |
   | If-None-Match       | http     | standard | Section 3.2 |
   | If-Unmodified-Since | http     | standard | Section 3.4 |
   | Last-Modified       | http     | standard | Section 2.2 |
   +---------------------+----------+----------+-------------+

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

6.  Security Considerations

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



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7.  Acknowledgments

   See Section 12 of [Part1].

8.  References

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-16
              (work in progress), August 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-16
              (work in progress), August 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-16 (work
              in progress), August 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-16 (work in
              progress), August 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.

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.




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   [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 2.1 and 3.2).

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

Appendix B.  Collected ABNF

   ETag = entity-tag

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

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

   Last-Modified = HTTP-date

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

   entity-tag = [ weak ] opaque-tag

   opaque-tag = quoted-string

   quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>

   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







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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"




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   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.






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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"








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C.16.  Since draft-ietf-httpbis-p4-conditional-14

   None.

C.17.  Since draft-ietf-httpbis-p4-conditional-15

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/304>: "If-Range
      should be listed when dicussing contexts where L-M can be
      considered strong"

Index

   3
      304 Not Modified (status code)  19

   4
      412 Precondition Failed (status code)  20

   E
      ETag header field  10

   G
      Grammar
         entity-tag  10
         ETag  10
         If-Match  15
         If-Modified-Since  17
         If-None-Match  16
         If-Unmodified-Since  18
         Last-Modified  8
         opaque-tag  10
         weak  10

   H
      Header Fields
         ETag  10
         If-Match  14
         If-Modified-Since  17
         If-None-Match  15
         If-Unmodified-Since  18
         Last-Modified  8

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



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      If-Unmodified-Since header field  18

   L
      Last-Modified header field  8

   M
      metadata  6

   S
      selected representation  5
      Status Codes
         304 Not Modified  19
         412 Precondition Failed  20

   V
      validator  6
         strong  6
         weak  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/












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   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/




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   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/






























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