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RFC 7234

 
 
 

Hypertext Transfer Protocol (HTTP/1.1): Caching

Part 2 of 2, p. 21 to 43
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5.  Header Field Definitions

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

5.1.  Age

   The "Age" header field conveys the sender's estimate of the amount of
   time since the response was generated or successfully validated at
   the origin server.  Age values are calculated as specified in
   Section 4.2.3.

     Age = delta-seconds

   The Age field-value is a non-negative integer, representing time in
   seconds (see Section 1.2.1).

   The presence of an Age header field implies that the response was not
   generated or validated by the origin server for this request.
   However, lack of an Age header field does not imply the origin was
   contacted, since the response might have been received from an
   HTTP/1.0 cache that does not implement Age.

5.2.  Cache-Control

   The "Cache-Control" header field is used to specify directives for
   caches along the request/response chain.  Such cache directives are
   unidirectional in that the presence of a directive in a request does
   not imply that the same directive is to be given in the response.

   A cache MUST obey the requirements of the Cache-Control directives
   defined in this section.  See Section 5.2.3 for information about how
   Cache-Control directives defined elsewhere are handled.

      Note: Some HTTP/1.0 caches might not implement Cache-Control.

   A proxy, whether or not it implements a cache, MUST pass cache
   directives through in forwarded messages, regardless of their
   significance to that application, since the directives might be
   applicable to all recipients along the request/response chain.  It is
   not possible to target a directive to a specific cache.

   Cache directives are identified by a token, to be compared
   case-insensitively, and have an optional argument, that can use both
   token and quoted-string syntax.  For the directives defined below
   that define arguments, recipients ought to accept both forms, even if
   one is documented to be preferred.  For any directive not defined by
   this specification, a recipient MUST accept both forms.

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     Cache-Control   = 1#cache-directive

     cache-directive = token [ "=" ( token / quoted-string ) ]

   For the cache directives defined below, no argument is defined (nor
   allowed) unless stated otherwise.

5.2.1.  Request Cache-Control Directives

5.2.1.1.  max-age

   Argument syntax:

      delta-seconds (see Section 1.2.1)

   The "max-age" request directive indicates that the client is
   unwilling to accept a response whose age is greater than the
   specified number of seconds.  Unless the max-stale request directive
   is also present, the client is not willing to accept a stale
   response.

   This directive uses the token form of the argument syntax: e.g.,
   'max-age=5' not 'max-age="5"'.  A sender SHOULD NOT generate the
   quoted-string form.

5.2.1.2.  max-stale

   Argument syntax:

      delta-seconds (see Section 1.2.1)

   The "max-stale" request directive indicates that the client is
   willing to accept a response that has exceeded its freshness
   lifetime.  If max-stale is assigned a value, then the client is
   willing to accept a response that has exceeded its freshness lifetime
   by no more than the specified number of seconds.  If no value is
   assigned to max-stale, then the client is willing to accept a stale
   response of any age.

   This directive uses the token form of the argument syntax: e.g.,
   'max-stale=10' not 'max-stale="10"'.  A sender SHOULD NOT generate
   the quoted-string form.

5.2.1.3.  min-fresh

   Argument syntax:

      delta-seconds (see Section 1.2.1)

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   The "min-fresh" request directive indicates that the client is
   willing to accept a response whose freshness lifetime is no less than
   its current age plus the specified time in seconds.  That is, the
   client wants a response that will still be fresh for at least the
   specified number of seconds.

   This directive uses the token form of the argument syntax: e.g.,
   'min-fresh=20' not 'min-fresh="20"'.  A sender SHOULD NOT generate
   the quoted-string form.

5.2.1.4.  no-cache

   The "no-cache" request directive indicates that a cache MUST NOT use
   a stored response to satisfy the request without successful
   validation on the origin server.

5.2.1.5.  no-store

   The "no-store" request directive indicates that a cache MUST NOT
   store any part of either this request or any response to it.  This
   directive applies to both private and shared caches.  "MUST NOT
   store" in this context means that the cache MUST NOT intentionally
   store the information in non-volatile storage, and MUST make a
   best-effort attempt to remove the information from volatile storage
   as promptly as possible after forwarding it.

   This directive is NOT a reliable or sufficient mechanism for ensuring
   privacy.  In particular, malicious or compromised caches might not
   recognize or obey this directive, and communications networks might
   be vulnerable to eavesdropping.

   Note that if a request containing this directive is satisfied from a
   cache, the no-store request directive does not apply to the already
   stored response.

5.2.1.6.  no-transform

   The "no-transform" request directive indicates that an intermediary
   (whether or not it implements a cache) MUST NOT transform the
   payload, as defined in Section 5.7.2 of [RFC7230].

5.2.1.7.  only-if-cached

   The "only-if-cached" request directive indicates that the client only
   wishes to obtain a stored response.  If it receives this directive, a
   cache SHOULD either respond using a stored response that is
   consistent with the other constraints of the request, or respond with

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   a 504 (Gateway Timeout) status code.  If a group of caches is being
   operated as a unified system with good internal connectivity, a
   member cache MAY forward such a request within that group of caches.

5.2.2.  Response Cache-Control Directives

5.2.2.1.  must-revalidate

   The "must-revalidate" response directive indicates that once it has
   become stale, a cache MUST NOT use the response to satisfy subsequent
   requests without successful validation on the origin server.

   The must-revalidate directive is necessary to support reliable
   operation for certain protocol features.  In all circumstances a
   cache MUST obey the must-revalidate directive; in particular, if a
   cache cannot reach the origin server for any reason, it MUST generate
   a 504 (Gateway Timeout) response.

   The must-revalidate directive ought to be used by servers if and only
   if failure to validate a request on the representation could result
   in incorrect operation, such as a silently unexecuted financial
   transaction.

5.2.2.2.  no-cache

   Argument syntax:

      #field-name

   The "no-cache" response directive indicates that the response MUST
   NOT be used to satisfy a subsequent request without successful
   validation on the origin server.  This allows an origin server to
   prevent a cache from using it to satisfy a request without contacting
   it, even by caches that have been configured to send stale responses.

   If the no-cache response directive specifies one or more field-names,
   then a cache MAY use the response to satisfy a subsequent request,
   subject to any other restrictions on caching.  However, any header
   fields in the response that have the field-name(s) listed MUST NOT be
   sent in the response to a subsequent request without successful
   revalidation with the origin server.  This allows an origin server to
   prevent the re-use of certain header fields in a response, while
   still allowing caching of the rest of the response.

   The field-names given are not limited to the set of header fields
   defined by this specification.  Field names are case-insensitive.

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   This directive uses the quoted-string form of the argument syntax.  A
   sender SHOULD NOT generate the token form (even if quoting appears
   not to be needed for single-entry lists).

   Note: Although it has been back-ported to many implementations, some
   HTTP/1.0 caches will not recognize or obey this directive.  Also,
   no-cache response directives with field-names are often handled by
   caches as if an unqualified no-cache directive was received; i.e.,
   the special handling for the qualified form is not widely
   implemented.

5.2.2.3.  no-store

   The "no-store" response directive indicates that a cache MUST NOT
   store any part of either the immediate request or response.  This
   directive applies to both private and shared caches.  "MUST NOT
   store" in this context means that the cache MUST NOT intentionally
   store the information in non-volatile storage, and MUST make a
   best-effort attempt to remove the information from volatile storage
   as promptly as possible after forwarding it.

   This directive is NOT a reliable or sufficient mechanism for ensuring
   privacy.  In particular, malicious or compromised caches might not
   recognize or obey this directive, and communications networks might
   be vulnerable to eavesdropping.

5.2.2.4.  no-transform

   The "no-transform" response directive indicates that an intermediary
   (regardless of whether it implements a cache) MUST NOT transform the
   payload, as defined in Section 5.7.2 of [RFC7230].

5.2.2.5.  public

   The "public" response directive indicates that any cache MAY store
   the response, even if the response would normally be non-cacheable or
   cacheable only within a private cache.  (See Section 3.2 for
   additional details related to the use of public in response to a
   request containing Authorization, and Section 3 for details of how
   public affects responses that would normally not be stored, due to
   their status codes not being defined as cacheable by default; see
   Section 4.2.2.)

5.2.2.6.  private

   Argument syntax:

      #field-name

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   The "private" response directive indicates that the response message
   is intended for a single user and MUST NOT be stored by a shared
   cache.  A private cache MAY store the response and reuse it for later
   requests, even if the response would normally be non-cacheable.

   If the private response directive specifies one or more field-names,
   this requirement is limited to the field-values associated with the
   listed response header fields.  That is, a shared cache MUST NOT
   store the specified field-names(s), whereas it MAY store the
   remainder of the response message.

   The field-names given are not limited to the set of header fields
   defined by this specification.  Field names are case-insensitive.

   This directive uses the quoted-string form of the argument syntax.  A
   sender SHOULD NOT generate the token form (even if quoting appears
   not to be needed for single-entry lists).

   Note: This usage of the word "private" only controls where the
   response can be stored; it cannot ensure the privacy of the message
   content.  Also, private response directives with field-names are
   often handled by caches as if an unqualified private directive was
   received; i.e., the special handling for the qualified form is not
   widely implemented.

5.2.2.7.  proxy-revalidate

   The "proxy-revalidate" response directive has the same meaning as the
   must-revalidate response directive, except that it does not apply to
   private caches.

5.2.2.8.  max-age

   Argument syntax:

      delta-seconds (see Section 1.2.1)

   The "max-age" response directive indicates that the response is to be
   considered stale after its age is greater than the specified number
   of seconds.

   This directive uses the token form of the argument syntax: e.g.,
   'max-age=5' not 'max-age="5"'.  A sender SHOULD NOT generate the
   quoted-string form.

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5.2.2.9.  s-maxage

   Argument syntax:

      delta-seconds (see Section 1.2.1)

   The "s-maxage" response directive indicates that, in shared caches,
   the maximum age specified by this directive overrides the maximum age
   specified by either the max-age directive or the Expires header
   field.  The s-maxage directive also implies the semantics of the
   proxy-revalidate response directive.

   This directive uses the token form of the argument syntax: e.g.,
   's-maxage=10' not 's-maxage="10"'.  A sender SHOULD NOT generate the
   quoted-string form.

5.2.3.  Cache Control Extensions

   The Cache-Control header field can be extended through the use of one
   or more cache-extension tokens, each with an optional value.  A cache
   MUST ignore unrecognized cache directives.

   Informational extensions (those that do not require a change in cache
   behavior) can be added without changing the semantics of other
   directives.

   Behavioral extensions are designed to work by acting as modifiers to
   the existing base of cache directives.  Both the new directive and
   the old directive are supplied, such that applications that do not
   understand the new directive will default to the behavior specified
   by the old directive, and those that understand the new directive
   will recognize it as modifying the requirements associated with the
   old directive.  In this way, extensions to the existing cache-control
   directives can be made without breaking deployed caches.

   For example, consider a hypothetical new response directive called
   "community" that acts as a modifier to the private directive: in
   addition to private caches, any cache that is shared only by members
   of the named community is allowed to cache the response.  An origin
   server wishing to allow the UCI community to use an otherwise private
   response in their shared cache(s) could do so by including

     Cache-Control: private, community="UCI"

   A cache that recognizes such a community cache-extension could
   broaden its behavior in accordance with that extension.  A cache that
   does not recognize the community cache-extension would ignore it and
   adhere to the private directive.

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5.3.  Expires

   The "Expires" header field gives the date/time after which the
   response is considered stale.  See Section 4.2 for further discussion
   of the freshness model.

   The presence of an Expires field does not imply that the original
   resource will change or cease to exist at, before, or after that
   time.

   The Expires value is an HTTP-date timestamp, as defined in Section
   7.1.1.1 of [RFC7231].

     Expires = HTTP-date

   For example

     Expires: Thu, 01 Dec 1994 16:00:00 GMT

   A cache recipient MUST interpret invalid date formats, especially the
   value "0", as representing a time in the past (i.e., "already
   expired").

   If a response includes a Cache-Control field with the max-age
   directive (Section 5.2.2.8), a recipient MUST ignore the Expires
   field.  Likewise, if a response includes the s-maxage directive
   (Section 5.2.2.9), a shared cache recipient MUST ignore the Expires
   field.  In both these cases, the value in Expires is only intended
   for recipients that have not yet implemented the Cache-Control field.

   An origin server without a clock MUST NOT generate an Expires field
   unless its value represents a fixed time in the past (always expired)
   or its value has been associated with the resource by a system or
   user with a reliable clock.

   Historically, HTTP required the Expires field-value to be no more
   than a year in the future.  While longer freshness lifetimes are no
   longer prohibited, extremely large values have been demonstrated to
   cause problems (e.g., clock overflows due to use of 32-bit integers
   for time values), and many caches will evict a response far sooner
   than that.

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5.4.  Pragma

   The "Pragma" header field allows backwards compatibility with
   HTTP/1.0 caches, so that clients can specify a "no-cache" request
   that they will understand (as Cache-Control was not defined until
   HTTP/1.1).  When the Cache-Control header field is also present and
   understood in a request, Pragma is ignored.

   In HTTP/1.0, Pragma was defined as an extensible field for
   implementation-specified directives for recipients.  This
   specification deprecates such extensions to improve interoperability.

     Pragma           = 1#pragma-directive
     pragma-directive = "no-cache" / extension-pragma
     extension-pragma = token [ "=" ( token / quoted-string ) ]

   When the Cache-Control header field is not present in a request,
   caches MUST consider the no-cache request pragma-directive as having
   the same effect as if "Cache-Control: no-cache" were present (see
   Section 5.2.1).

   When sending a no-cache request, a client ought to include both the
   pragma and cache-control directives, unless Cache-Control: no-cache
   is purposefully omitted to target other Cache-Control response
   directives at HTTP/1.1 caches.  For example:

     GET / HTTP/1.1
     Host: www.example.com
     Cache-Control: max-age=30
     Pragma: no-cache

   will constrain HTTP/1.1 caches to serve a response no older than 30
   seconds, while precluding implementations that do not understand
   Cache-Control from serving a cached response.

      Note: Because the meaning of "Pragma: no-cache" in responses is
      not specified, it does not provide a reliable replacement for
      "Cache-Control: no-cache" in them.

5.5.  Warning

   The "Warning" header field is used to carry additional information
   about the status or transformation of a message that might not be
   reflected in the status code.  This information is typically used to
   warn about possible incorrectness introduced by caching operations or
   transformations applied to the payload of the message.

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   Warnings can be used for other purposes, both cache-related and
   otherwise.  The use of a warning, rather than an error status code,
   distinguishes these responses from true failures.

   Warning header fields can in general be applied to any message,
   however some warn-codes are specific to caches and can only be
   applied to response messages.

     Warning       = 1#warning-value

     warning-value = warn-code SP warn-agent SP warn-text
                                           [ SP warn-date ]

     warn-code  = 3DIGIT
     warn-agent = ( uri-host [ ":" port ] ) / pseudonym
                     ; the name or pseudonym of the server adding
                     ; the Warning header field, for use in debugging
                     ; a single "-" is recommended when agent unknown
     warn-text  = quoted-string
     warn-date  = DQUOTE HTTP-date DQUOTE

   Multiple warnings can be generated in a response (either by the
   origin server or by a cache), including multiple warnings with the
   same warn-code number that only differ in warn-text.

   A user agent that receives one or more Warning header fields SHOULD
   inform the user of as many of them as possible, in the order that
   they appear in the response.  Senders that generate multiple Warning
   header fields are encouraged to order them with this user agent
   behavior in mind.  A sender that generates new Warning header fields
   MUST append them after any existing Warning header fields.

   Warnings are assigned three digit warn-codes.  The first digit
   indicates whether the Warning is required to be deleted from a stored
   response after validation:

   o  1xx warn-codes describe the freshness or validation status of the
      response, and so they MUST be deleted by a cache after validation.
      They can only be generated by a cache when validating a cached
      entry, and MUST NOT be generated in any other situation.

   o  2xx warn-codes describe some aspect of the representation that is
      not rectified by a validation (for example, a lossy compression of
      the representation) and they MUST NOT be deleted by a cache after
      validation, unless a full response is sent, in which case they
      MUST be.

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   If a sender generates one or more 1xx warn-codes in a message to be
   sent to a recipient known to implement only HTTP/1.0, the sender MUST
   include in each corresponding warning-value a warn-date that matches
   the Date header field in the message.  For example:

     HTTP/1.1 200 OK
     Date: Sat, 25 Aug 2012 23:34:45 GMT
     Warning: 112 - "network down" "Sat, 25 Aug 2012 23:34:45 GMT"


   Warnings have accompanying warn-text that describes the error, e.g.,
   for logging.  It is advisory only, and its content does not affect
   interpretation of the warn-code.

   If a recipient that uses, evaluates, or displays Warning header
   fields receives a warn-date that is different from the Date value in
   the same message, the recipient MUST exclude the warning-value
   containing that warn-date before storing, forwarding, or using the
   message.  This allows recipients to exclude warning-values that were
   improperly retained after a cache validation.  If all of the
   warning-values are excluded, the recipient MUST exclude the Warning
   header field as well.

   The following warn-codes are defined by this specification, each with
   a recommended warn-text in English, and a description of its meaning.
   The procedure for defining additional warn codes is described in
   Section 7.2.1.

5.5.1.  Warning: 110 - "Response is Stale"

   A cache SHOULD generate this whenever the sent response is stale.

5.5.2.  Warning: 111 - "Revalidation Failed"

   A cache SHOULD generate this when sending a stale response because an
   attempt to validate the response failed, due to an inability to reach
   the server.

5.5.3.  Warning: 112 - "Disconnected Operation"

   A cache SHOULD generate this if it is intentionally disconnected from
   the rest of the network for a period of time.

5.5.4.  Warning: 113 - "Heuristic Expiration"

   A cache SHOULD generate this if it heuristically chose a freshness
   lifetime greater than 24 hours and the response's age is greater than
   24 hours.

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5.5.5.  Warning: 199 - "Miscellaneous Warning"

   The warning text can include arbitrary information to be presented to
   a human user or logged.  A system receiving this warning MUST NOT
   take any automated action, besides presenting the warning to the
   user.

5.5.6.  Warning: 214 - "Transformation Applied"

   This Warning code MUST be added by a proxy if it applies any
   transformation to the representation, such as changing the
   content-coding, media-type, or modifying the representation data,
   unless this Warning code already appears in the response.

5.5.7.  Warning: 299 - "Miscellaneous Persistent Warning"

   The warning text can include arbitrary information to be presented to
   a human user or logged.  A system receiving this warning MUST NOT
   take any automated action.

6.  History Lists

   User agents often have history mechanisms, such as "Back" buttons and
   history lists, that can be used to redisplay a representation
   retrieved earlier in a session.

   The freshness model (Section 4.2) does not necessarily apply to
   history mechanisms.  That is, a history mechanism can display a
   previous representation even if it has expired.

   This does not prohibit the history mechanism from telling the user
   that a view might be stale or from honoring cache directives (e.g.,
   Cache-Control: no-store).

7.  IANA Considerations

7.1.  Cache Directive Registry

   The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry"
   defines the namespace for the cache directives.  It has been created
   and is now maintained at
   <http://www.iana.org/assignments/http-cache-directives>.

7.1.1.  Procedure

   A registration MUST include the following fields:

   o  Cache Directive Name

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   o  Pointer to specification text

   Values to be added to this namespace require IETF Review (see
   [RFC5226], Section 4.1).

7.1.2.  Considerations for New Cache Control Directives

   New extension directives ought to consider defining:

   o  What it means for a directive to be specified multiple times,

   o  When the directive does not take an argument, what it means when
      an argument is present,

   o  When the directive requires an argument, what it means when it is
      missing,

   o  Whether the directive is specific to requests, responses, or able
      to be used in either.

   See also Section 5.2.3.

7.1.3.  Registrations

   The registry has been populated with the registrations below:

   +------------------------+----------------------------------+
   | Cache Directive        | Reference                        |
   +------------------------+----------------------------------+
   | max-age                | Section 5.2.1.1, Section 5.2.2.8 |
   | max-stale              | Section 5.2.1.2                  |
   | min-fresh              | Section 5.2.1.3                  |
   | must-revalidate        | Section 5.2.2.1                  |
   | no-cache               | Section 5.2.1.4, Section 5.2.2.2 |
   | no-store               | Section 5.2.1.5, Section 5.2.2.3 |
   | no-transform           | Section 5.2.1.6, Section 5.2.2.4 |
   | only-if-cached         | Section 5.2.1.7                  |
   | private                | Section 5.2.2.6                  |
   | proxy-revalidate       | Section 5.2.2.7                  |
   | public                 | Section 5.2.2.5                  |
   | s-maxage               | Section 5.2.2.9                  |
   | stale-if-error         | [RFC5861], Section 4             |
   | stale-while-revalidate | [RFC5861], Section 3             |
   +------------------------+----------------------------------+

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7.2.  Warn Code Registry

   The "Hypertext Transfer Protocol (HTTP) Warn Codes" registry defines
   the namespace for warn codes.  It has been created and is now
   maintained at <http://www.iana.org/assignments/http-warn-codes>.

7.2.1.  Procedure

   A registration MUST include the following fields:

   o  Warn Code (3 digits)

   o  Short Description

   o  Pointer to specification text

   Values to be added to this namespace require IETF Review (see
   [RFC5226], Section 4.1).

7.2.2.  Registrations

   The registry has been populated with the registrations below:

   +-----------+----------------------------------+---------------+
   | Warn Code | Short Description                | Reference     |
   +-----------+----------------------------------+---------------+
   | 110       | Response is Stale                | Section 5.5.1 |
   | 111       | Revalidation Failed              | Section 5.5.2 |
   | 112       | Disconnected Operation           | Section 5.5.3 |
   | 113       | Heuristic Expiration             | Section 5.5.4 |
   | 199       | Miscellaneous Warning            | Section 5.5.5 |
   | 214       | Transformation Applied           | Section 5.5.6 |
   | 299       | Miscellaneous Persistent Warning | Section 5.5.7 |
   +-----------+----------------------------------+---------------+

7.3.  Header Field Registration

   HTTP header fields are registered within the "Message Headers"
   registry maintained at
   <http://www.iana.org/assignments/message-headers/>.

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   This document defines the following HTTP header fields, so the
   "Permanent Message Header Field Names" registry has been updated
   accordingly (see [BCP90]).

   +-------------------+----------+----------+-------------+
   | Header Field Name | Protocol | Status   | Reference   |
   +-------------------+----------+----------+-------------+
   | Age               | http     | standard | Section 5.1 |
   | Cache-Control     | http     | standard | Section 5.2 |
   | Expires           | http     | standard | Section 5.3 |
   | Pragma            | http     | standard | Section 5.4 |
   | Warning           | http     | standard | Section 5.5 |
   +-------------------+----------+----------+-------------+

   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 HTTP caching.  More
   general security considerations are addressed in HTTP messaging
   [RFC7230] and semantics [RFC7231].

   Caches expose additional potential vulnerabilities, since the
   contents of the cache represent an attractive target for malicious
   exploitation.  Because cache contents persist after an HTTP request
   is complete, an attack on the cache can reveal information long after
   a user believes that the information has been removed from the
   network.  Therefore, cache contents need to be protected as sensitive
   information.

   In particular, various attacks might be amplified by being stored in
   a shared cache; such "cache poisoning" attacks use the cache to
   distribute a malicious payload to many clients, and are especially
   effective when an attacker can use implementation flaws, elevated
   privileges, or other techniques to insert such a response into a
   cache.  One common attack vector for cache poisoning is to exploit
   differences in message parsing on proxies and in user agents; see
   Section 3.3.3 of [RFC7230] for the relevant requirements.

   Likewise, implementation flaws (as well as misunderstanding of cache
   operation) might lead to caching of sensitive information (e.g.,
   authentication credentials) that is thought to be private, exposing
   it to unauthorized parties.

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   Furthermore, the very use of a cache can bring about privacy
   concerns.  For example, if two users share a cache, and the first one
   browses to a site, the second may be able to detect that the other
   has been to that site, because the resources from it load more
   quickly, thanks to the cache.

   Note that the Set-Cookie response header field [RFC6265] does not
   inhibit caching; a cacheable response with a Set-Cookie header field
   can be (and often is) used to satisfy subsequent requests to caches.
   Servers who wish to control caching of these responses are encouraged
   to emit appropriate Cache-Control response header fields.

9.  Acknowledgments

   See Section 10 of [RFC7230].

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.

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              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.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235, June 2014.

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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.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5861]  Nottingham, M., "HTTP Cache-Control Extensions for Stale
              Content", RFC 5861, April 2010.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, June 2010.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.

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Appendix A.  Changes from RFC 2616

   The specification has been substantially rewritten for clarity.

   The conditions under which an authenticated response can be cached
   have been clarified.  (Section 3.2)

   New status codes can now define that caches are allowed to use
   heuristic freshness with them.  Caches are now allowed to calculate
   heuristic freshness for URIs with query components.  (Section 4.2.2)

   The algorithm for calculating age is now less conservative.  Caches
   are now required to handle dates with time zones as if they're
   invalid, because it's not possible to accurately guess.
   (Section 4.2.3)

   The Content-Location response header field is no longer used to
   determine the appropriate response to use when validating.
   (Section 4.3)

   The algorithm for selecting a cached negotiated response to use has
   been clarified in several ways.  In particular, it now explicitly
   allows header-specific canonicalization when processing selecting
   header fields.  (Section 4.1)

   Requirements regarding denial-of-service attack avoidance when
   performing invalidation have been clarified.  (Section 4.4)

   Cache invalidation only occurs when a successful response is
   received.  (Section 4.4)

   Cache directives are explicitly defined to be case-insensitive.
   Handling of multiple instances of cache directives when only one is
   expected is now defined.  (Section 5.2)

   The "no-store" request directive doesn't apply to responses; i.e., a
   cache can satisfy a request with no-store on it and does not
   invalidate it.  (Section 5.2.1.5)

   The qualified forms of the private and no-cache cache directives are
   noted to not be widely implemented; for example, "private=foo" is
   interpreted by many caches as simply "private".  Additionally, the
   meaning of the qualified form of no-cache has been clarified.
   (Section 5.2.2)

   The "no-cache" response directive's meaning has been clarified.
   (Section 5.2.2.2)

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   The one-year limit on Expires header field values has been removed;
   instead, the reasoning for using a sensible value is given.
   (Section 5.3)

   The Pragma header field is now only defined for backwards
   compatibility; future pragmas are deprecated.  (Section 5.4)

   Some requirements regarding production and processing of the Warning
   header fields have been relaxed, as it is not widely implemented.
   Furthermore, the Warning header field no longer uses RFC 2047
   encoding, nor does it allow multiple languages, as these aspects were
   not implemented.  (Section 5.5)

   This specification introduces the Cache Directive and Warn Code
   Registries, and defines considerations for new cache directives.
   (Section 7.1 and Section 7.2)

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>
     field-name    = <field-name, see [RFC7230], Section 3.2>
     quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>
     token         = <token, see [RFC7230], Section 3.2.6>

     port          = <port, see [RFC7230], Section 2.7>
     pseudonym     = <pseudonym, see [RFC7230], Section 5.7.1>
     uri-host      = <uri-host, see [RFC7230], Section 2.7>

   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].

   Age = delta-seconds

   Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
    cache-directive ] )

   Expires = HTTP-date

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

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

   Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
    pragma-directive ] )

   Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
    )

   cache-directive = token [ "=" ( token / quoted-string ) ]

   delta-seconds = 1*DIGIT

   extension-pragma = token [ "=" ( token / quoted-string ) ]

   field-name = <field-name, see [RFC7230], Section 3.2>

   port = <port, see [RFC7230], Section 2.7>
   pragma-directive = "no-cache" / extension-pragma
   pseudonym = <pseudonym, see [RFC7230], Section 5.7.1>

   quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>

   token = <token, see [RFC7230], Section 3.2.6>

   uri-host = <uri-host, see [RFC7230], Section 2.7>

   warn-agent = ( uri-host [ ":" port ] ) / pseudonym
   warn-code = 3DIGIT
   warn-date = DQUOTE HTTP-date DQUOTE
   warn-text = quoted-string
   warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
    ]

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Index

   1
      110 (warn-code)  31
      111 (warn-code)  31
      112 (warn-code)  31
      113 (warn-code)  31
      199 (warn-code)  32

   2
      214 (warn-code)  32
      299 (warn-code)  32

   A
      age  11
      Age header field  21

   C
      cache  4
      cache entry  5
      cache key  5-6
      Cache-Control header field  21

   D
      Disconnected Operation (warn-text)  31

   E
      Expires header field  28
      explicit expiration time  11

   F
      fresh  11
      freshness lifetime  11

   G
      Grammar
         Age  21
         Cache-Control  22
         cache-directive  22
         delta-seconds  5
         Expires  28
         extension-pragma  29
         Pragma  29
         pragma-directive  29
         warn-agent  29
         warn-code  29
         warn-date  29
         warn-text  29

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         Warning  29
         warning-value  29

   H
      Heuristic Expiration (warn-text)  31
      heuristic expiration time  11
   M
      max-age (cache directive)  22, 26
      max-stale (cache directive)  22
      min-fresh (cache directive)  22
      Miscellaneous Persistent Warning (warn-text)  32
      Miscellaneous Warning (warn-text)  32
      must-revalidate (cache directive)  24

   N
      no-cache (cache directive)  23, 25
      no-store (cache directive)  23, 24
      no-transform (cache directive)  23, 25

   O
      only-if-cached (cache directive)  23

   P
      Pragma header field  29
      private (cache directive)  25
      private cache  4
      proxy-revalidate (cache directive)  26
      public (cache directive)  25

   R
      Response is Stale (warn-text)  30
      Revalidation Failed (warn-text)  31

   S
      s-maxage (cache directive)  27
      shared cache  4
      stale  11
      strong validator  18

   T
      Transformation Applied (warn-text)  32

   V
      validator  16

   W
      Warning header field  29

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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/


   Mark Nottingham (editor)
   Akamai

   EMail: mnot@mnot.net
   URI:   http://www.mnot.net/


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

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