source: draft-ietf-httpbis/20/draft-ietf-httpbis-p4-conditional-20.txt @ 1807

HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2616 (if approved)                              Y. Lafon, Ed.
Intended status: Standards Track                                     W3C
Expires: January 17, 2013                                J. Reschke, Ed.
                                                              greenbytes
                                                           July 16, 2012


                 HTTP/1.1, part 4: Conditional Requests
                  draft-ietf-httpbis-p4-conditional-20

Abstract

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

Editorial Note (To be removed by RFC Editor)

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

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

   The changes in this draft are summarized in Appendix D.1.

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



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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 17, 2013.

Copyright Notice

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   document authors.  All rights reserved.

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   Without obtaining an adequate license from the person(s) controlling
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.





















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Conformance and Error Handling . . . . . . . . . . . . . .  4
     1.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  5
   2.  Validators . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Weak versus Strong . . . . . . . . . . . . . . . . . . . .  6
     2.2.  Last-Modified  . . . . . . . . . . . . . . . . . . . . . .  7
       2.2.1.  Generation . . . . . . . . . . . . . . . . . . . . . .  8
       2.2.2.  Comparison . . . . . . . . . . . . . . . . . . . . . .  8
     2.3.  ETag . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
       2.3.1.  Generation . . . . . . . . . . . . . . . . . . . . . . 10
       2.3.2.  Comparison . . . . . . . . . . . . . . . . . . . . . . 11
       2.3.3.  Example: Entity-tags varying on Content-Negotiated
               Resources  . . . . . . . . . . . . . . . . . . . . . . 11
     2.4.  Rules for When to Use Entity-tags and Last-Modified
           Dates  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   3.  Precondition Header Fields . . . . . . . . . . . . . . . . . . 14
     3.1.  If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14
     3.2.  If-None-Match  . . . . . . . . . . . . . . . . . . . . . . 15
     3.3.  If-Modified-Since  . . . . . . . . . . . . . . . . . . . . 16
     3.4.  If-Unmodified-Since  . . . . . . . . . . . . . . . . . . . 17
     3.5.  If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 18
   4.  Status Code Definitions  . . . . . . . . . . . . . . . . . . . 18
     4.1.  304 Not Modified . . . . . . . . . . . . . . . . . . . . . 18
     4.2.  412 Precondition Failed  . . . . . . . . . . . . . . . . . 19
   5.  Precedence . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
     6.1.  Status Code Registration . . . . . . . . . . . . . . . . . 20
     6.2.  Header Field Registration  . . . . . . . . . . . . . . . . 21
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 22
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 22
   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 22
   Appendix B.  Imported ABNF . . . . . . . . . . . . . . . . . . . . 23
   Appendix C.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 23
   Appendix D.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 24
     D.1.  Since draft-ietf-httpbis-p4-conditional-19 . . . . . . . . 24
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24









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1.  Introduction

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

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

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

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

   This specification targets conformance criteria according to the role
   of a participant in HTTP communication.  Hence, HTTP requirements are
   placed on senders, recipients, clients, servers, user agents,
   intermediaries, origin servers, proxies, gateways, or caches,
   depending on what behavior is being constrained by the requirement.
   See Section 2 of [Part1] for definitions of these terms.



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   The verb "generate" is used instead of "send" where a requirement
   differentiates between creating a protocol element and merely
   forwarding a received element downstream.

   An implementation is considered conformant if it complies with all of
   the requirements associated with the roles it partakes in HTTP.  Note
   that SHOULD-level requirements are relevant here, unless one of the
   documented exceptions is applicable.

   This document also uses ABNF to define valid protocol elements
   (Section 1.2).  In addition to the prose requirements placed upon
   them, senders MUST NOT generate protocol elements that do not match
   the grammar defined by the ABNF rules for those protocol elements
   that are applicable to the sender's role.  If a received protocol
   element is processed, the recipient MUST be able to parse any value
   that would match the ABNF rules for that protocol element, excluding
   only those rules not applicable to the recipient's role.

   Unless noted otherwise, a recipient MAY attempt to recover a usable
   protocol element from an invalid construct.  HTTP does not define
   specific error handling mechanisms except when they have a direct
   impact on security, since different applications of the protocol
   require different error handling strategies.  For example, a Web
   browser might wish to transparently recover from a response where the
   Location header field doesn't parse according to the ABNF, whereas a
   systems control client might consider any form of error recovery to
   be dangerous.

1.2.  Syntax Notation

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

2.  Validators

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





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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 (OK) 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
   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 collision-resistant hash function applied to
   the representation data is also sufficient if the data is available
   prior to the response header fields being sent and the digest does
   not need to be recalculated every time a validation request is
   received.  However, if a resource has distinct representations that
   differ only in their metadata, such as might occur with content
   negotiation over media types that happen to share the same data
   format, then the origin server SHOULD incorporate additional



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   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.  An origin server SHOULD change a weak entity-tag whenever it
   considers prior representations to be unacceptable as a substitute
   for the current representation.  In other words, a weak entity-tag
   ought to change whenever the origin server wants caches to invalidate
   old responses.

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

   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





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2.2.1.  Generation

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

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

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

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

   An origin server without a clock MUST NOT assign Last-Modified values
   to a response unless these values were associated with the resource
   by some other system or user with a reliable clock.

2.2.2.  Comparison

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

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

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



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      the presented validator.

   or

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

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

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

   or

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

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

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

   This method relies on the fact that if two different responses were
   sent by the origin server during the same second, but both had the
   same Last-Modified time, then at least one of those responses would
   have a Date value equal to its Last-Modified time.  The arbitrary 60-
   second limit guards against the possibility that the Date and Last-
   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.






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     ETag       = entity-tag

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

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

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

   Examples:

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

   An entity-tag can be either a weak or strong validator, with strong
   being the default.  If an origin server provides an entity-tag for a
   representation and the generation of that entity-tag does not satisfy
   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).

2.3.1.  Generation

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

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



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   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           |
   | "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 8
   of [Part2]), and where the representations returned upon a GET
   request vary based on the Accept-Encoding request header field
   (Section 9.3 of [Part2]):

   >> Request:

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




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

   >> Response:

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

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

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

   >> Response:

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

     ...binary data...

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

2.4.  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:




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   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.

   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



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      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 still ought to provide Last-Modified values.

3.  Precondition Header Fields

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

3.1.  If-Match

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

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

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

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

   If the condition is met, the server MAY perform the request method as
   if the If-Match header field was not present.

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

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




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

   Examples:

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

3.2.  If-None-Match

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

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

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

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

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

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




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   If the condition is met, 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 (Successful) or 304 (Not Modified)
   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: *

3.3.  If-Modified-Since

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

     If-Modified-Since = HTTP-date

   An example of the field is:

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

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

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



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   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-
      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.

3.4.  If-Unmodified-Since

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

     If-Unmodified-Since = HTTP-date




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   An example of the field is:

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

   If a request normally (i.e., in absence of the If-Unmodified-Since
   header field) would result in anything other than a 2xx (Successful)
   or 412 (Precondition Failed) status code, the If-Unmodified-Since
   header field SHOULD be ignored.

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

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 9.10 of
   [Part2]) unless the origin server does not have a clock that can
   provide a reasonable approximation of the current time.  If a 200
   (OK) response to the same request would have included any of the
   header fields Cache-Control, Content-Location, ETag, Expires, 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



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   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 that
   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.

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.  Precedence

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

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

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

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

       *  if true, continue to step 3

       *  if false, respond 412 (Precondition Failed)




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

       *  if true, continue to step 3

       *  if false, respond 412 (Precondition Failed)

   3.  When the method is GET and both Range and If-Range are present,
       evaluate it:

       *  if the validator matches, respond 206 (Partial Content)

       *  if the validator does not match, respond 200 (OK)

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

       *  if true, all conditions are met

       *  if false for GET/HEAD, respond 304 (Not Modified)

       *  if false for other methods, respond 412 (Precondition Failed)

   5.  When the method is GET or HEAD, If-None-Match is not present, and
       If-Modified-Since is present, evaluate it:

       *  if true, all conditions are met

       *  if false, respond 304 (Not Modified)

   Any extension to HTTP/1.1 that defines additional conditional request
   header fields ought to define its own expectations regarding the
   order for evaluating such fields in relation to those defined in this
   document and other conditionals that might be found in practice.

6.  IANA Considerations

6.1.  Status Code Registration

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










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

6.2.  Header Field Registration

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

7.  Security Considerations

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

   The validators defined by this specification are not intended to
   ensure the validity of a representation, guard against malicious
   changes, or detect man-in-the-middle attacks.  At best, they enable
   more efficient cache updates and optimistic concurrent writes when
   all participants are behaving nicely.  At worst, the conditions will
   fail and the client will receive a response that is no more harmful
   than an HTTP exchange without conditional requests.

8.  Acknowledgments

   See Section 9 of [Part1].

9.  References






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9.1.  Normative References

   [Part1]    Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "HTTP/1.1, part 1: Message Routing and Syntax"",
              draft-ietf-httpbis-p1-messaging-20 (work in progress),
              July 2012.

   [Part2]    Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "HTTP/1.1, part 2: Semantics and Payloads",
              draft-ietf-httpbis-p2-semantics-20 (work in progress),
              July 2012.

   [Part5]    Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "HTTP/1.1, part 5: Range Requests",
              draft-ietf-httpbis-p5-range-20 (work in progress),
              July 2012.

   [Part6]    Fielding, R., Ed., Lafon, Y., Ed., Nottingham, M., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 6: Caching",
              draft-ietf-httpbis-p6-cache-20 (work in progress),
              July 2012.

   [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.

9.2.  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 2.1 and 3.2).

   Change ETag header field ABNF not to use quoted-string, thus avoiding
   escaping issues.  (Section 2.3)



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

Appendix B.  Imported ABNF

   The following core rules are included by reference, as defined in
   Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
   CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
   8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
   character).

   The rules below are defined in [Part1]:

     OWS           = <OWS, defined in [Part1], Section 3.2.1>
     obs-text      = <obs-text, defined in [Part1], Section 3.2.4>

   The rules below are defined in other parts:

     HTTP-date     = <HTTP-date, defined in [Part2], Section 5.1>

Appendix C.  Collected ABNF

   ETag = entity-tag

   HTTP-date = <HTTP-date, defined in [Part2], Section 5.1>

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

   Last-Modified = HTTP-date

   OWS = <OWS, defined in [Part1], Section 3.2.1>

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

   obs-text = <obs-text, defined in [Part1], Section 3.2.4>
   opaque-tag = DQUOTE *etagc DQUOTE

   weak = %x57.2F ; W/





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

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

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

   Closed issues:

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

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

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

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

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

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

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

Index

   3
      304 Not Modified (status code)  18

   4
      412 Precondition Failed (status code)  19

   E
      ETag header field  9

   G
      Grammar
         entity-tag  10
         ETag  10
         etagc  10
         If-Match  14
         If-Modified-Since  16



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         If-None-Match  15
         If-Unmodified-Since  17
         Last-Modified  7
         opaque-tag  10
         weak  10

   H
      Header Fields
         ETag  9
         If-Match  14
         If-Modified-Since  16
         If-None-Match  15
         If-Unmodified-Since  17
         Last-Modified  7

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

   L
      Last-Modified header field  7

   M
      metadata  5

   S
      selected representation  4
      Status Codes
         304 Not Modified  18
         412 Precondition Failed  19

   V
      validator  5
         strong  6
         weak  6














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Authors' Addresses

   Roy T. Fielding (editor)
   Adobe Systems Incorporated
   345 Park Ave
   San Jose, CA  95110
   USA

   EMail: fielding@gbiv.com
   URI:   http://roy.gbiv.com/


   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

   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/




















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