rfc7232 Varnish source references

Internet Engineering Task Force (IETF)                  R. Fielding, Ed.

Request for Comments: 7232                                         Adobe

Obsoletes: 2616                                          J. Reschke, Ed.

Category: Standards Track                                     greenbytes

ISSN: 2070-1721                                                June 2014

      Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests

Abstract

   The Hypertext Transfer Protocol (HTTP) is a stateless 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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force

   (IETF).  It represents the consensus of the IETF community.  It has

   received public review and has been approved for publication by the

   Internet Engineering Steering Group (IESG).  Further information on

   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,

   and how to provide feedback on it may be obtained at

   http://www.rfc-editor.org/info/rfc7232.

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RFC 7232              HTTP/1.1 Conditional Requests            June 2014

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the

   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal

   Provisions Relating to IETF Documents

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   publication of this document.  Please review these documents

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   include Simplified BSD License text as described in Section 4.e of

   the Trust Legal Provisions and are provided without warranty as

   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF

   Contributions published or made publicly available before November

   10, 2008.  The person(s) controlling the copyright in some of this

   material may not have granted the IETF Trust the right to allow

   modifications of such material outside the IETF Standards Process.

   Without obtaining an adequate license from the person(s) controlling

   the copyright in such materials, this document may not be modified

   outside the IETF Standards Process, and derivative works of it may

   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 ............................................4

   2. Validators ......................................................5

      2.1. Weak versus Strong .........................................5

      2.2. Last-Modified ..............................................7

           2.2.1. Generation ..........................................7

           2.2.2. Comparison ..........................................8

      2.3. ETag .......................................................9

           2.3.1. Generation .........................................10

           2.3.2. Comparison .........................................10

           2.3.3. Example: Entity-Tags Varying on

                  Content-Negotiated Resources .......................11

      2.4. When to Use Entity-Tags and Last-Modified Dates ...........12

   3. Precondition Header Fields .....................................13

      3.1. If-Match ..................................................13

      3.2. If-None-Match .............................................14

      3.3. If-Modified-Since .........................................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. Evaluation .....................................................19

   6. Precedence .....................................................20

   7. IANA Considerations ............................................22

      7.1. Status Code Registration ..................................22

      7.2. Header Field Registration .................................22

   8. Security Considerations ........................................22

   9. Acknowledgments ................................................23

   10. References ....................................................24

      10.1. Normative References .....................................24

      10.2. Informative References ...................................24

   Appendix A. Changes from RFC 2616 .................................25

   Appendix B. Imported ABNF .........................................25

   Appendix C. Collected ABNF ........................................26

   Index .............................................................27

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1.  Introduction

   Conditional requests are HTTP requests [RFC7231] that include one or

   more header fields indicating a precondition to be tested before

   applying the method semantics to the target resource.  This document

   defines the HTTP/1.1 conditional request mechanisms in terms of the

   architecture, syntax notation, and conformance criteria defined in

   [RFC7230].

   Conditional GET requests are the most efficient mechanism for HTTP

   cache updates [RFC7234].  Conditionals can also be applied to

   state-changing methods, such as PUT and DELETE, to prevent the "lost

   update" problem: one client accidentally overwriting the work of

   another client that has been acting in parallel.

   Conditional request preconditions are based on the state of the

   target resource as a whole (its current value set) or the state as

   observed in a previously obtained representation (one value in that

   set).  A resource might have multiple current representations, each

   with its own observable state.  The conditional request mechanisms

   assume that the mapping of requests to a "selected representation"

   (Section 3 of [RFC7231]) will be consistent over time if the server

   intends to take advantage of conditionals.  Regardless, if the

   mapping is inconsistent and the server is unable to select the

   appropriate representation, then no harm will result when the

   precondition evaluates to false.

   The conditional request preconditions defined by this specification

   (Section 3) are evaluated when applicable to the recipient

   (Section 5) according to their order of precedence (Section 6).

1.1.  Conformance and Error Handling

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

   document are to be interpreted as described in [RFC2119].

   Conformance criteria and considerations regarding error handling are

   defined in Section 2.5 of [RFC7230].

1.2.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)

   notation of [RFC5234] with a list extension, defined in Section 7 of

   [RFC7230], that allows for compact definition of comma-separated

   lists using a '#' operator (similar to how the '*' operator indicates

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   repetition).  Appendix B describes rules imported from other

   documents.  Appendix C shows the collected grammar with all list

   operators expanded to standard ABNF notation.

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 Web Distributed

   Authoring and Versioning (WebDAV, [RFC4918]), that are beyond the

   scope of this specification.  A resource metadata value is referred

   to as a "validator" when it is used within a precondition.

2.1.  Weak versus Strong

   Validators come in two flavors: strong or weak.  Weak validators are

   easy to generate but are far less useful for comparisons.  Strong

   validators are ideal for comparisons but can be very difficult (and

   occasionally impossible) to generate efficiently.  Rather than impose

   that all forms of resource adhere to the same strength of validator,

   HTTP exposes the type of validator in use and imposes restrictions on

   when weak validators can be used as preconditions.

   A "strong validator" is representation metadata that changes value

   whenever a change occurs to the representation data that would be

   observable in the payload body of a 200 (OK) response to GET.

   A strong validator might change for reasons other than a change to

   the representation data, 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.

   Cache entries might persist for arbitrarily long periods, regardless

   of expiration times.  Thus, a cache might attempt to validate an

   entry using a validator that it obtained in the distant past.  A

   strong validator is unique across all versions of all representations

   associated with a particular resource over time.  However, there is

   no implication of uniqueness across representations of different

   resources (i.e., the same strong validator might be in use for

   representations of multiple resources at the same time and does not

   imply that those representations are equivalent).

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   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 needs to incorporate additional

   information in the validator to distinguish those representations.

   In contrast, a "weak validator" is representation metadata that might

   not change for every change to the representation data.  This

   weakness might be due to limitations in how the value is calculated,

   such as clock resolution, an inability to ensure uniqueness for all

   possible representations of the resource, or a desire of the resource

   owner to group representations by some self-determined set of

   equivalency rather than unique sequences of data.  An origin server

   SHOULD change a weak entity-tag whenever it considers prior

   representations to be unacceptable as a substitute for the current

   representation.  In other words, a weak entity-tag ought to change

   whenever the origin server wants caches to invalidate old responses.

   For example, the representation of a weather report that changes in

   content every second, based on dynamic measurements, might be grouped

   into sets of equivalent representations (from the origin server's

   perspective) with the same weak validator in order to allow cached

   representations to be valid for a reasonable period of time (perhaps

   adjusted dynamically based on server load or weather quality).

   Likewise, a representation's modification time, if defined with only

   one-second resolution, might be a weak validator if it is possible

   for the representation to be modified twice during a single second

   and retrieved between those modifications.

   Likewise, a validator is weak if it is shared by two or more

   representations of a given resource at the same time, unless those

   representations have identical representation data.  For example, if

   the origin server sends the same validator for a representation with

   a gzip content coding applied as it does for a representation with no

   content coding, then that validator is weak.  However, two

   simultaneous representations might share the same strong validator if

   they differ only in the representation metadata, such as when two

   different media types are available for the same representation data.

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   Strong validators are usable for all conditional requests, including

   cache validation, partial content ranges, and "lost update"

   avoidance.  Weak validators are only usable when the client does not

   require exact equality with previously obtained representation data,

   such as when validating a cache entry or limiting a web traversal to

   recent changes.

2.2.  Last-Modified

   The "Last-Modified" header field in a response provides a timestamp

   indicating the date and time at which the origin server believes the

   selected representation was last modified, as determined at the

   conclusion of handling the request.

     Last-Modified = HTTP-date

   An example of its use is

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

2.2.1.  Generation

   An origin server 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 ([RFC7234]) 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

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   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

      the presented validator.

   or

   o  The validator is about to be used by a client in an

      If-Modified-Since, If-Unmodified-Since, or If-Range header field,

      because the client has a cache entry 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

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   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 in a response provides the current entity-tag

   for the selected representation, as determined at the conclusion of

   handling the request.  An entity-tag is an opaque validator for

   differentiating between multiple representations of the same

   resource, regardless of whether those multiple representations are

   due to resource state changes over time, content negotiation

   resulting in multiple representations being valid at the same time,

   or both.  An entity-tag consists of an opaque quoted string, possibly

   prefixed by a weakness indicator.

     ETag       = entity-tag

     entity-tag = [ weak ] opaque-tag

     weak       = %x57.2F ; "W/", case-sensitive

     opaque-tag = DQUOTE *etagc DQUOTE

     etagc      = %x21 / %x23-7E / obs-text

                ; VCHAR except double quotes, plus obs-text

      Note: Previously, opaque-tag was defined to be a quoted-string

      ([RFC2616], Section 3.11); thus, some recipients might perform

      backslash unescaping.  Servers therefore ought to avoid backslash

      characters in entity tags.

   An entity-tag can be more reliable for validation than a modification

   date in situations where it is inconvenient to store modification

   dates, where the one-second resolution of HTTP date values is not

   sufficient, or where modification dates are not consistently

   maintained.

   Examples:

     ETag: "xyzzy"

     ETag: W/"xyzzy"

     ETag: ""

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   An entity-tag can be either a weak or strong validator, with strong

   being the default.  If an origin server provides an entity-tag for a

   representation and the generation of that entity-tag does not satisfy

   all of the characteristics of a strong validator (Section 2.1), then

   the origin server MUST mark the entity-tag as weak by prefixing its

   opaque value with "W/" (case-sensitive).

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 file attributes, or

   a modification timestamp that has sub-second resolution.

   An origin server SHOULD send an 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 ([RFC7234]) 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

   or not the comparison context allows the use of weak validators:

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   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

   3.4 of [RFC7231]), and where the representations sent in response to

   a GET request vary based on the Accept-Encoding request header field

   (Section 5.3.4 of [RFC7231]):

   >> 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: Fri, 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!

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   An alternative representation that does use gzip content coding would

   be:

   >> Response:

     HTTP/1.1 200 OK

     Date: Fri, 26 Mar 2010 00:05:00 GMT

     ETag: "123-b"

     Content-Length: 43

     Vary: Accept-Encoding

     Content-Type: text/plain

     Content-Encoding: gzip

     ...binary data...

      Note: Content codings are a property of the representation data,

      so a strong entity-tag for a content-encoded representation has to

      be distinct from the entity tag of an unencoded representation to

      prevent potential conflicts during cache updates and range

      requests.  In contrast, transfer codings (Section 4 of [RFC7230])

      apply only during message transfer and do not result in distinct

      entity-tags.

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

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

   o  SHOULD send an entity-tag validator unless it is not feasible to

      generate one.

   o  MAY send a weak entity-tag instead of a strong entity-tag, if

      performance considerations support the use of weak entity-tags, or

      if it is unfeasible to send a strong entity-tag.

   o  SHOULD send a Last-Modified value if it is feasible to send one.

   In other words, the preferred behavior for an origin server is to

   send both a strong entity-tag and a Last-Modified value in successful

   responses to a retrieval request.

   A client:

   o  MUST send that entity-tag in any cache validation request (using

      If-Match or If-None-Match) if an entity-tag has been provided by

      the origin server.

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   o  SHOULD send the Last-Modified value in non-subrange cache

      validation requests (using If-Modified-Since) if only a

      Last-Modified value has been provided by the origin server.

   o  MAY send the Last-Modified value in subrange cache validation

      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 send both validators in cache validation 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.

3.  Precondition Header Fields

   This section defines the syntax and semantics of HTTP/1.1 header

   fields for applying preconditions on requests.  Section 5 defines

   when the preconditions are applied.  Section 6 defines the order of

   evaluation when more than one precondition is present.

3.1.  If-Match

   The "If-Match" header field makes the request method conditional on

   the recipient origin server either having at least one current

   representation of the target resource, when the field-value is "*",

   or having a current representation of the target resource that has an

   entity-tag matching a member of the list of entity-tags provided in

   the field-value.

   An origin server MUST use the strong comparison function when

   comparing entity-tags for If-Match (Section 2.3.2), since the client

   intends this precondition to prevent the method from being applied if

   there have been any changes to the representation data.

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

   Examples:

     If-Match: "xyzzy"

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

     If-Match: *

   If-Match is most often used with state-changing methods (e.g., POST,

   PUT, DELETE) to prevent accidental overwrites when multiple user

   agents might be acting in parallel on the same resource (i.e., to

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   prevent the "lost update" problem).  It can also be used with safe

   methods to abort a request if the selected representation does not

   match one already stored (or partially stored) from a prior request.

   An origin server that receives an If-Match header field MUST evaluate

   the condition prior to performing the method (Section 5).  If the

   field-value is "*", the condition is false if the origin server does

   not have a current representation for the target resource.  If the

   field-value is a list of entity-tags, the condition is false if none

   of the listed tags match the entity-tag of the selected

   representation.

   An origin server MUST NOT perform the requested method if a received

   If-Match condition evaluates to false; instead, the origin server

   MUST respond with either a) the 412 (Precondition Failed) status code

   or b) one of the 2xx (Successful) status codes if the origin server

   has verified that a state change is being requested and the final

   state is already reflected in the current state of the target

   resource (i.e., the change requested by the user agent has already

   succeeded, but the user agent might not be aware of it, perhaps

   because the prior response was lost or a compatible change was made

   by some other user agent).  In the latter case, the origin server

   MUST NOT send a validator header field in the response unless it can

   verify that the request is a duplicate of an immediately prior change

   made by the same user agent.

   The If-Match header field can be ignored by caches and intermediaries

   because it is not applicable to a stored response.

3.2.  If-None-Match

   The "If-None-Match" header field makes the request method conditional

   on a recipient cache or origin server either not having any current

   representation of the target resource, when the field-value is "*",

   or having a selected representation with an entity-tag that does not

   match any of those listed in the field-value.

   A recipient MUST use the weak comparison function when comparing

   entity-tags for If-None-Match (Section 2.3.2), since weak entity-tags

   can be used for cache validation even if there have been changes to

   the representation data.

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

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   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: *

   If-None-Match is primarily used in conditional GET requests to enable

   efficient updates of cached information with a minimum amount of

   transaction overhead.  When a client desires to update one or more

   stored responses that have entity-tags, the client SHOULD generate an

   If-None-Match header field containing a list of those entity-tags

   when making a GET request; this allows recipient servers to send a

   304 (Not Modified) response to indicate when one of those stored

   responses 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

   (Section 4.2.1 of [RFC7231]).  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.

   An origin server that receives an If-None-Match header field MUST

   evaluate the condition prior to performing the method (Section 5).

   If the field-value is "*", the condition is false if the origin

   server has a current representation for the target resource.  If the

   field-value is a list of entity-tags, the condition is false if one

   of the listed tags match the entity-tag of the selected

   representation.

   An origin server MUST NOT perform the requested method if the

   condition evaluates to false; instead, the origin server MUST respond

   with either a) the 304 (Not Modified) status code if the request

   method is GET or HEAD or b) the 412 (Precondition Failed) status code

   for all other request methods.

   Requirements on cache handling of a received If-None-Match header

   field are defined in Section 4.3.2 of [RFC7234].

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3.3.  If-Modified-Since

   The "If-Modified-Since" header field makes a GET or HEAD request

   method conditional on the selected representation's modification date

   being more recent than the date provided in the field-value.

   Transfer of the selected representation's data is avoided if that

   data has not changed.

     If-Modified-Since = HTTP-date

   An example of the field is:

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

   A recipient MUST ignore If-Modified-Since if the request contains an

   If-None-Match header field; the condition in If-None-Match is

   considered to be a more accurate replacement for the condition in

   If-Modified-Since, and the two are only combined for the sake of

   interoperating with older intermediaries that might not implement

   If-None-Match.

   method is neither GET nor HEAD.

   A recipient MUST interpret an If-Modified-Since field-value's

   timestamp in terms of the origin server's clock.

   If-Modified-Since is typically used for two distinct purposes: 1) to

   allow efficient updates of a cached representation that does not have

   an entity-tag and 2) to limit the scope of a web traversal to

   resources that have recently changed.

   When used for cache updates, a cache will typically use the value of

   the cached message's Last-Modified field to generate the field value

   of If-Modified-Since.  This behavior is most interoperable for cases

   where clocks are poorly synchronized or when the server has chosen to

   only honor exact timestamp matches (due to a problem with

   Last-Modified dates that appear to go "back in time" when the origin

   server's clock is corrected or a representation is restored from an

   archived backup).  However, caches occasionally generate the field

   value based on other data, such as the Date header field of the

   cached message or the local clock time that the message was received,

   particularly when the cached message does not contain a Last-Modified

   field.

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   When used for limiting the scope of retrieval to a recent time

   window, a user agent will generate an If-Modified-Since field value

   based on either its own local clock or a Date header field received

   from the server in a prior response.  Origin servers that choose an

   exact timestamp match based on the selected representation's

   Last-Modified field will not be able to help the user agent limit its

   data transfers to only those changed during the specified window.

   An origin server that receives an If-Modified-Since header field

   SHOULD evaluate the condition prior to performing the method

   (Section 5).  The origin server SHOULD NOT perform the requested

   method if the selected representation's last modification date is

   earlier than or equal to the date provided in the field-value;

   instead, the origin server SHOULD generate a 304 (Not Modified)

   response, including only those metadata that are useful for

   identifying or updating a previously cached response.

   Requirements on cache handling of a received If-Modified-Since header

   field are defined in Section 4.3.2 of [RFC7234].

3.4.  If-Unmodified-Since

   The "If-Unmodified-Since" header field makes the request method

   conditional on the selected representation's last modification date

   being earlier than or equal to the date provided in the field-value.

   This field accomplishes the same purpose as If-Match for cases where

   the user agent does not have an entity-tag for the representation.

     If-Unmodified-Since = HTTP-date

   An example of the field is:

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

   A recipient MUST ignore If-Unmodified-Since if the request contains

   an If-Match header field; the condition in If-Match is considered to

   be a more accurate replacement for the condition in

   If-Unmodified-Since, and the two are only combined for the sake of

   interoperating with older intermediaries that might not implement

   If-Match.

   A recipient MUST ignore the If-Unmodified-Since header field if the

   received field-value is not a valid HTTP-date.

   A recipient MUST interpret an If-Unmodified-Since field-value's

   timestamp in terms of the origin server's clock.

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   If-Unmodified-Since is most often used with state-changing methods

   (e.g., POST, PUT, DELETE) to prevent accidental overwrites when

   multiple user agents might be acting in parallel on a resource that

   does not supply entity-tags with its representations (i.e., to

   prevent the "lost update" problem).  It can also be used with safe

   methods to abort a request if the selected representation does not

   match one already stored (or partially stored) from a prior request.

   An origin server that receives an If-Unmodified-Since header field

   MUST evaluate the condition prior to performing the method

   (Section 5).  The origin server MUST NOT perform the requested method

   if the selected representation's last modification date is more

   recent than the date provided in the field-value; instead the origin

   server MUST respond with either a) the 412 (Precondition Failed)

   status code or b) one of the 2xx (Successful) status codes if the

   origin server has verified that a state change is being requested and

   the final state is already reflected in the current state of the

   target resource (i.e., the change requested by the user agent has

   already succeeded, but the user agent might not be aware of that

   because the prior response message was lost or a compatible change

   was made by some other user agent).  In the latter case, the origin

   server MUST NOT send a validator header field in the response unless

   it can verify that the request is a duplicate of an immediately prior

   change made by the same user agent.

   The If-Unmodified-Since header field can be ignored by caches and

   intermediaries because it is not applicable to a stored response.

3.5.  If-Range

   The "If-Range" header field provides a special conditional request

   mechanism that is similar to the If-Match and If-Unmodified-Since

   header fields but that instructs the recipient to ignore the Range

   header field if the validator doesn't match, resulting in transfer of

   the new selected representation instead of a 412 (Precondition

   Failed) response.  If-Range is defined in Section 3.2 of [RFC7233].

4.  Status Code Definitions

4.1.  304 Not Modified

   The 304 (Not Modified) status code indicates that a conditional GET

   or HEAD request has been received and would have resulted in a 200

   (OK) response if it were not for the fact that the condition

   evaluated to false.  In other words, there is no need for the server

   to transfer a representation of the target resource because the

   request indicates that the client, which made the request

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   conditional, already has a valid representation; the server is

   therefore redirecting the client to make use of that stored

   representation as if it were the payload of a 200 (OK) response.

   The server generating a 304 response MUST generate any of the

   following header fields that would have been sent in a 200 (OK)

   response to the same request: Cache-Control, Content-Location, Date,

   ETag, Expires, and Vary.

   Since the goal of a 304 response is to minimize information transfer

   when the recipient already has one or more cached representations, a

   sender SHOULD NOT generate representation metadata other than the

   above listed fields unless said metadata exists for the purpose of

   guiding cache updates (e.g., Last-Modified might be useful if the

   response does not have an ETag field).

   Requirements on a cache that receives a 304 response are defined in

   Section 4.3.4 of [RFC7234].  If the conditional request originated

   with an outbound client, such as a user agent with its own cache

   sending a conditional GET to a shared proxy, then the proxy SHOULD

   forward the 304 response to that client.

   A 304 response cannot contain a message-body; it is always terminated

   by the first empty line after the header fields.

4.2.  412 Precondition Failed

   The 412 (Precondition Failed) status code indicates that one or more

   conditions given in the request header fields evaluated to false when

   tested on the server.  This response code allows the client to place

   preconditions on the current resource state (its current

   representations and metadata) and, thus, prevent the request method

   from being applied if the target resource is in an unexpected state.

5.  Evaluation

   Except when excluded below, a recipient cache or origin server MUST

   evaluate received request preconditions after it has successfully

   performed its normal request checks and just before it would perform

   the action associated with the request method.  A server MUST ignore

   all received preconditions if its response to the same request

   without those conditions would have been a status code other than a

   2xx (Successful) or 412 (Precondition Failed).  In other words,

   redirects and failures take precedence over the evaluation of

   preconditions in conditional requests.

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   A server that is not the origin server for the target resource and

   cannot act as a cache for requests on the target resource MUST NOT

   evaluate the conditional request header fields defined by this

   specification, and it MUST forward them if the request is forwarded,

   since the generating client intends that they be evaluated by a

   server that can provide a current representation.  Likewise, a server

   MUST ignore the conditional request header fields defined by this

   specification when received with a request method that does not

   involve the selection or modification of a selected representation,

   such as CONNECT, OPTIONS, or TRACE.

   Conditional request header fields that are defined by extensions to

   HTTP might place conditions on all recipients, on the state of the

   target resource in general, or on a group of resources.  For

   instance, the "If" header field in WebDAV can make a request

   conditional on various aspects of multiple resources, such as locks,

   if the recipient understands and implements that field ([RFC4918],

   Section 10.4).

   Although conditional request header fields are defined as being

   usable with the HEAD method (to keep HEAD's semantics consistent with

   those of GET), there is no point in sending a conditional HEAD

   because a successful response is around the same size as a 304 (Not

   Modified) response and more useful than a 412 (Precondition Failed)

   response.

6.  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, since "lost

   update" preconditions have more strict requirements than cache

   validation, a validated cache is more efficient than a partial

   response, and entity tags are presumed to be more accurate than date

   validators.

   A recipient cache or origin server MUST evaluate the request

   preconditions defined by this specification in the following order:

   1.  When recipient is the origin server and If-Match is present,

       evaluate the If-Match precondition:

       *  if true, continue to step 3

       *  if false, respond 412 (Precondition Failed) unless it can be

          determined that the state-changing request has already

          succeeded (see Section 3.1)

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   2.  When recipient is the origin server, If-Match is not present, and

       If-Unmodified-Since is present, evaluate the If-Unmodified-Since

       precondition:

       *  if true, continue to step 3

       *  if false, respond 412 (Precondition Failed) unless it can be

          determined that the state-changing request has already

          succeeded (see Section 3.4)

   3.  When If-None-Match is present, evaluate the If-None-Match

       precondition:

       *  if true, continue to step 5

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

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

   4.  When the method is GET or HEAD, If-None-Match is not present, and

       If-Modified-Since is present, evaluate the If-Modified-Since

       precondition:

       *  if true, continue to step 5

       *  if false, respond 304 (Not Modified)

   5.  When the method is GET and both Range and If-Range are present,

       evaluate the If-Range precondition:

       *  if the validator matches and the Range specification is

          applicable to the selected representation, respond 206

          (Partial Content) [RFC7233]

   6.  Otherwise,

       *  all conditions are met, so perform the requested action and

          respond according to its success or failure.

   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.

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7.  IANA Considerations

7.1.  Status Code Registration

   The "Hypertext Transfer Protocol (HTTP) Status Code Registry" located

   at <http://www.iana.org/assignments/http-status-codes> has been

   updated with the registrations below:

   +-------+---------------------+-------------+

   | Value | Description         | Reference   |

   +-------+---------------------+-------------+

   | 304   | Not Modified        | Section 4.1 |

   | 412   | Precondition Failed | Section 4.2 |

   +-------+---------------------+-------------+

7.2.  Header Field Registration

   HTTP header fields are registered within the "Message Headers"

   registry maintained at

   <http://www.iana.org/assignments/message-headers/>.

   This document defines the following HTTP header fields, so their

   associated registry entries have been updated according to the

   permanent registrations below (see [BCP90]):

   +---------------------+----------+----------+-------------+

   | Header Field Name   | Protocol | Status   | Reference   |

   +---------------------+----------+----------+-------------+

   | ETag                | http     | standard | Section 2.3 |

   | If-Match            | http     | standard | Section 3.1 |

   | If-Modified-Since   | http     | standard | Section 3.3 |

   | If-None-Match       | http     | standard | Section 3.2 |

   | If-Unmodified-Since | http     | standard | Section 3.4 |

   | Last-Modified       | http     | standard | Section 2.2 |

   +---------------------+----------+----------+-------------+

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

   Engineering Task Force".

8.  Security Considerations

   This section is meant to inform developers, information providers,

   and users of known security concerns specific to the HTTP conditional

   request mechanisms.  More general security considerations are

   addressed in HTTP "Message Syntax and Routing" [RFC7230] and

   "Semantics and Content" [RFC7231].

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   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.

   An entity-tag can be abused in ways that create privacy risks.  For

   example, a site might deliberately construct a semantically invalid

   entity-tag that is unique to the user or user agent, send it in a

   cacheable response with a long freshness time, and then read that

   entity-tag in later conditional requests as a means of re-identifying

   that user or user agent.  Such an identifying tag would become a

   persistent identifier for as long as the user agent retained the

   original cache entry.  User agents that cache representations ought

   to ensure that the cache is cleared or replaced whenever the user

   performs privacy-maintaining actions, such as clearing stored cookies

   or changing to a private browsing mode.

9.  Acknowledgments

   See Section 10 of [RFC7230].

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10.  References

10.1.  Normative References

   [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.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer

              Protocol (HTTP/1.1): Message Syntax and Routing",

              RFC 7230, June 2014.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer

              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,

              June 2014.

   [RFC7233]  Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,

              "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",

              RFC 7233, June 2014.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,

              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",

              RFC 7234, June 2014.

10.2.  Informative References

   [BCP90]    Klyne, G., Nottingham, M., and J. Mogul, "Registration

              Procedures for Message Header Fields", BCP 90, RFC 3864,

              September 2004.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,

              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext

              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed

              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

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Appendix A.  Changes from RFC 2616

   The definition of validator weakness has been expanded and clarified.

   (Section 2.1)

   Weak entity-tags are now allowed in all requests except range

   requests.  (Sections 2.1 and 3.2)

   The ETag header field ABNF has been changed to not use quoted-string,

   thus avoiding escaping issues.  (Section 2.3)

   ETag is defined to provide an entity tag for the selected

   representation, thereby clarifying what it applies to in various

   situations (such as a PUT response).  (Section 2.3)

   The precedence for evaluation of conditional requests has been

   defined.  (Section 6)

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 [RFC7230]:

     OWS           = <OWS, see [RFC7230], Section 3.2.3>

     obs-text      = <obs-text, see [RFC7230], Section 3.2.6>

   The rules below are defined in other parts:

     HTTP-date     = <HTTP-date, see [RFC7231], Section 7.1.1.1>

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Appendix C.  Collected ABNF

   In the collected ABNF below, list rules are expanded as per Section

   1.2 of [RFC7230].

   ETag = entity-tag

   HTTP-date = <HTTP-date, see [RFC7231], Section 7.1.1.1>

   If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS

    entity-tag ] ) )

   If-Modified-Since = HTTP-date

   If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS

    entity-tag ] ) )

   If-Unmodified-Since = HTTP-date

   Last-Modified = HTTP-date

   OWS = <OWS, see [RFC7230], Section 3.2.3>

   entity-tag = [ weak ] opaque-tag

   etagc = "!" / %x23-7E ; '#'-'~'

    / obs-text

   obs-text = <obs-text, see [RFC7230], Section 3.2.6>

   opaque-tag = DQUOTE *etagc DQUOTE

   weak = %x57.2F ; W/

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Index

   3

      304 Not Modified (status code)  19

   4

      412 Precondition Failed (status code)  18

   E

      ETag header field  9

   G

      Grammar

         entity-tag  9

         ETag  9

         etagc  9

         If-Match  13

         If-Modified-Since  15

         If-None-Match  14

         If-Unmodified-Since  17

         Last-Modified  7

         opaque-tag  9

         weak  9

   I

      If-Match header field  13

      If-Modified-Since header field  16

      If-None-Match header field  14

      If-Unmodified-Since header field  17

   L

      Last-Modified header field  7

   M

      metadata  5

   S

      selected representation  4

   V

      validator  5

         strong  5

         weak  5

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Authors' Addresses

   Roy T. Fielding (editor)

   Adobe Systems Incorporated

   345 Park Ave

   San Jose, CA  95110

   USA

   EMail: fielding@gbiv.com

   URI:   http://roy.gbiv.com/

   Julian F. Reschke (editor)

   greenbytes GmbH

   Hafenweg 16

   Muenster, NW  48155

   Germany

   EMail: julian.reschke@greenbytes.de

   URI:   http://greenbytes.de/tech/webdav/

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