HTTP Working Group                                      R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 7234 (if approved)                         M. Nottingham, Ed.
Intended status: Standards Track                                  Fastly
Expires: January 3, April 21, 2019                                  J. Reschke, Ed.
                                                              greenbytes
                                                            July 2,
                                                        October 18, 2018

                              HTTP Caching
                      draft-ietf-httpbis-cache-02
                      draft-ietf-httpbis-cache-03

Abstract

   The Hypertext Transfer Protocol (HTTP) is a stateless application-
   level protocol for distributed, collaborative, hypertext information
   systems.  This document defines HTTP caches and the associated header
   fields that control cache behavior or indicate cacheable response
   messages.

   This document obsoletes RFC 7234.

Editorial Note

   This note is to be removed before publishing as an RFC.

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

   Working Group information can be found at <https://httpwg.org/>;
   source code and issues list for this draft can be found at
   <https://github.com/httpwg/http-core>.

   The changes in this draft are summarized in Appendix C.3. C.4.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on January 3, April 21, 2019.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Requirements Notation . . . . . . . . . . . . . . . . . .   5
     1.2.  Syntax Notation . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Delta Seconds . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Overview of Cache Operation . . . . . . . . . . . . . . . . .   6
   3.  Storing Responses in Caches . . . . . . . . . . . . . . . . .   7
     3.1.  Storing Incomplete Responses  . . . . . . . . . . . . . .   8
     3.2.  Storing Responses to Authenticated Requests . . . . . . .   8
     3.3.  Combining Partial Content . . . . . . . . . . . . . . . .   8
   4.  Constructing Responses from Caches  . . . . . . . . . . . . .   9
     4.1.  Calculating Secondary Keys with Vary  . . . . . . . . . .  10
     4.2.  Freshness . . . . . . . . . . . . . . . . . . . . . . . .  11
       4.2.1.  Calculating Freshness Lifetime  . . . . . . . . . . .  12
       4.2.2.  Calculating Heuristic Freshness . . . . . . . . . . .  13
       4.2.3.  Calculating Age . . . . . . . . . . . . . . . . . . .  14
       4.2.4.  Serving Stale Responses . . . . . . . . . . . . . . .  15
     4.3.  Validation  . . . . . . . . . . . . . . . . . . . . . . .  16  15
       4.3.1.  Sending a Validation Request  . . . . . . . . . . . .  16
       4.3.2.  Handling a Received Validation Request  . . . . . . .  17  16
       4.3.3.  Handling a Validation Response  . . . . . . . . . . .  18
       4.3.4.  Freshening Stored Responses upon Validation . . . . .  18
       4.3.5.  Freshening Responses via with HEAD  . . . . . . . . . . . .  19
     4.4.  Invalidation  . . . . . . . . . . . . . . . . . . . . . .  20
   5.  Header Field Definitions  . . . . . . . . . . . . . . . . . .  21  20
     5.1.  Age . . . . . . . . . . . . . . . . . . . . . . . . . . .  21
     5.2.  Cache-Control . . . . . . . . . . . . . . . . . . . . . .  22  21
       5.2.1.  Request Cache-Control Directives  . . . . . . . . . .  23
       5.2.2.  Response Cache-Control Directives  22
         5.2.1.1.  max-age . . . . . . . . . .  25
       5.2.3.  Cache Control Extensions . . . . . . . . . . .  22
         5.2.1.2.  max-stale . . .  28
       5.2.4.  Cache Directive Registry . . . . . . . . . . . . . .  29
     5.3.  Expires . . .  22
         5.2.1.3.  min-fresh . . . . . . . . . . . . . . . . . . . .  23
         5.2.1.4.  no-cache  . .  29
     5.4.  Pragma . . . . . . . . . . . . . . . . . .  23
         5.2.1.5.  no-store  . . . . . . .  30
     5.5.  Warning . . . . . . . . . . . . .  23
         5.2.1.6.  no-transform  . . . . . . . . . . . .  31
       5.5.1.  Warning: 110 - "Response is Stale" . . . . . .  24
         5.2.1.7.  only-if-cached  . . .  33
       5.5.2.  Warning: 111 - "Revalidation Failed" . . . . . . . .  33
       5.5.3.  Warning: 112 - "Disconnected Operation" . . . . . .  24
       5.2.2.  Response Cache-Control Directives .  33
       5.5.4.  Warning: 113 - "Heuristic Expiration" . . . . . . . .  34
       5.5.5.  Warning: 199 - "Miscellaneous Warning" .  24
         5.2.2.1.  must-revalidate . . . . . .  34
       5.5.6.  Warning: 214 - "Transformation Applied" . . . . . . .  34
       5.5.7.  Warning: 299 - "Miscellaneous Persistent Warning" . .  34
       5.5.8.  Warn Code Registry . .  24
         5.2.2.2.  no-cache  . . . . . . . . . . . . . . .  34
   6.  History Lists . . . . .  24
         5.2.2.3.  no-store  . . . . . . . . . . . . . . . . . . .  34
   7.  Security Considerations .  25
         5.2.2.4.  no-transform  . . . . . . . . . . . . . . . . . .  35
   8.  IANA Considerations  26
         5.2.2.5.  public  . . . . . . . . . . . . . . . . . . . . .  36
     8.1.  Header Field Registration  26
         5.2.2.6.  private . . . . . . . . . . . . . . . .  36
     8.2.  Cache Directive Registration . . . . .  26
         5.2.2.7.  proxy-revalidate  . . . . . . . . .  36
     8.3.  Warn Code Registration . . . . . . .  27
         5.2.2.8.  max-age . . . . . . . . . .  36
   9.  References . . . . . . . . . . .  27
         5.2.2.9.  s-maxage  . . . . . . . . . . . . . .  36
     9.1.  Normative References . . . . . .  27
       5.2.3.  Cache Control Extensions  . . . . . . . . . . . .  36
     9.2.  Informative References . .  27
       5.2.4.  Cache Directive Registry  . . . . . . . . . . . . . .  28
     5.3.  Expires .  37
   Appendix A.  Collected ABNF . . . . . . . . . . . . . . . . . . .  38
   Appendix B.  Changes from RFC 7234 . . . . .  29
     5.4.  Pragma  . . . . . . . . . .  39
   Appendix C.  Change Log . . . . . . . . . . . . . . .  30
     5.5.  Warning . . . . . .  39
     C.1.  Between RFC7234 and draft 00 . . . . . . . . . . . . . .  39
     C.2.  Since draft-ietf-httpbis-cache-00 . . . . .  30
   6.  Relationship to Applications  . . . . . . .  39
     C.3.  Since draft-ietf-httpbis-cache-01 . . . . . . . . .  31
   7.  Security Considerations . . .  39
   Index . . . . . . . . . . . . . . . .  31
   8.  IANA Considerations . . . . . . . . . . . . . .  40
   Acknowledgments . . . . . . .  32
     8.1.  Header Field Registration . . . . . . . . . . . . . . . .  32
     8.2.  Cache Directive Registration  . .  42
   Authors' Addresses . . . . . . . . . . . .  32
     8.3.  Warn Code Registry  . . . . . . . . . . .  42

1.  Introduction

   The Hypertext Transfer Protocol (HTTP) is a stateless application-
   level request/response protocol that uses extensible semantics and
   self-descriptive messages for flexible interaction with network-based
   hypertext information systems.  HTTP is defined by a series of
   documents that collectively form the HTTP/1.1 specification:

   o  "HTTP Semantics" [Semantics]

   o  "HTTP Caching" (this document)

   o  "HTTP/1.1 Messaging" [Messaging]

   HTTP is typically used for distributed information systems, where
   performance can be improved by the use of response caches.  This
   document defines aspects of HTTP related to caching and reusing
   response messages.

   An HTTP cache is a local store of response messages and the subsystem
   that controls storage, retrieval, and deletion of messages in it.  A
   cache stores cacheable responses in order to reduce the response time
   and network bandwidth consumption on future, equivalent requests.
   Any client or server MAY employ a cache, though a cache cannot be
   used by a server that is acting as a tunnel.

   A shared cache is a cache that stores responses to be reused by more
   than one user; shared caches are usually (but not always) deployed as
   a part of an intermediary.  A private cache, in contrast, is
   dedicated to a single user; often, they are deployed as a component
   of a user agent.

   The goal of caching in HTTP/1.1 is to significantly improve
   performance by reusing a prior response message to satisfy a current
   request.  A stored response is considered "fresh", as defined in
   Section 4.2, if the response can be reused without "validation"
   (checking with the origin server to see if the cached response
   remains valid for this request).  A fresh response can therefore
   reduce both latency and network overhead each time it is reused.
   When a cached response is not fresh, it might still be reusable if it
   can be freshened by validation (Section 4.3) or if the origin is
   unavailable (Section 4.2.4).

   This document obsoletes RFC 7234, with the changes being summarized
   in Appendix B.

1.1.  Requirements Notation

   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 3 of [Semantics].

1.2.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234] with a list extension, defined in Section 11 of
   [Semantics], that allows for compact definition of comma-separated
   lists using a '#' operator (similar to how the '*' operator indicates
   repetition).  Appendix A shows the collected grammar with all list
   operators expanded to standard ABNF notation.

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

   The rules below are defined in [Semantics]:

     HTTP-date     = <HTTP-date, see [Semantics], Section 10.1.1.1>
     OWS           = <OWS, see [Semantics], Section 4.3>
     field-name    = <field-name, see [Semantics], Section 4.2>
     quoted-string = <quoted-string, see [Semantics], Section 4.2.3>
     token         = <token, see [Semantics], Section 4.2.3>
     uri-host      = <host, see [RFC3986], Section 3.2.2>
     port          = <port, see [RFC3986], Section 3.2.3>
     pseudonym     = <pseudonym, see [Semantics], Section 5.6.1>

1.3.  Delta Seconds

   The delta-seconds rule specifies a non-negative integer, representing
   time in seconds.

     delta-seconds  = 1*DIGIT

   A recipient parsing a delta-seconds value and converting it to binary
   form ought to use an arithmetic type of at least 31 bits of non-
   negative integer range.  If a cache receives a delta-seconds value
   greater than the greatest integer it can represent, or if any of its
   subsequent calculations overflows, the cache MUST consider the value
   to be either 2147483648 (2^31) or the greatest positive integer it
   can conveniently represent.

      Note: The value 2147483648 is here for historical reasons,
      effectively represents infinity (over 68 years), and does not need
      to be stored in binary form; an implementation could produce it as
      a canned string if any overflow occurs, even if the calculations
      are performed with an arithmetic type incapable of directly
      representing that number.  What matters here is that an overflow
      be detected and not treated as a negative value in later
      calculations.

2.  Overview of Cache Operation

   Proper cache operation preserves the semantics of HTTP transfers
   ([Semantics]) while eliminating the transfer of information already
   held in the cache.  Although caching is an entirely OPTIONAL feature
   of HTTP, it can be assumed that reusing a cached response is
   desirable and that such reuse is the default behavior when no
   requirement or local configuration prevents it.  Therefore, HTTP
   cache requirements are focused on preventing a cache from either
   storing a non-reusable response or reusing a stored response
   inappropriately, rather than mandating that caches always store and
   reuse particular responses.

   Each cache entry consists of a cache key and one or more HTTP
   responses corresponding to prior requests that used the same key.
   The most common form of cache entry is a successful result of a
   retrieval request: i.e., a 200 (OK) response to a GET request, which
   contains a representation of the resource identified by the request
   target (Section 7.3.1 of [Semantics]).  However, it is also possible
   to cache permanent redirects, negative results (e.g., 404 (Not
   Found)), incomplete results (e.g., 206 (Partial Content)), and
   responses to methods other than GET if the method's definition allows
   such caching and defines something suitable for use as a cache key.

   The primary cache key consists of the request method and target URI.
   However, since HTTP caches in common use today are typically limited
   to caching responses to GET, many caches simply decline other methods
   and use only the URI as the primary cache key.

   If a request target is subject to content negotiation, its cache
   entry might consist of multiple stored responses, each differentiated
   by a secondary key for the values of the original request's selecting
   header fields (Section 4.1).

3.  Storing Responses in Caches

   A cache MUST NOT store a response to any request, unless:

   o  The request method is understood by the cache and defined as being
      cacheable, and

   o  the response status code is understood by the cache, and

   o  the "no-store" cache directive (see Section 5.2) does not appear
      in request or response header fields, and

   o  the "private" response directive (see Section 5.2.2.6) does not
      appear in the response, if the cache is shared, and

   o  the Authorization header field (see Section 8.5.3 of [Semantics])
      does not appear in the request, if the cache is shared, unless the
      response explicitly allows it (see Section 3.2), and

   o  the response either:

      *  contains an Expires header field (see Section 5.3), or

      *  contains a max-age response directive (see Section 5.2.2.8), or

      *  contains a s-maxage response directive (see Section 5.2.2.9)
         and the cache is shared, or

      *  contains a Cache Control Extension (see Section 5.2.3) that
         allows it to be cached, or

      *  has a status code that is defined as cacheable by default (see
         Section 4.2.2), or

      *  contains a public response directive (see Section 5.2.2.5).

   Note that any of the requirements listed above can be overridden by a
   cache-control extension; see Section 5.2.3.

   In this context, a cache has "understood" a request method or a
   response status code if it recognizes it and implements all specified
   caching-related behavior.

   Note that, in normal operation, some caches will not store a response
   that has neither a cache validator nor an explicit expiration time,
   as such responses are not usually useful to store.  However, caches
   are not prohibited from storing such responses.

3.1.  Storing Incomplete Responses

   A response message is considered complete when all of the octets
   indicated by the message framing ([Messaging]) are received prior to
   the connection being closed.  If the request method is GET, the
   response status code is 200 (OK), and the entire response header
   section has been received, a cache MAY store an incomplete response
   message body if the cache entry is recorded as incomplete.  Likewise,
   a 206 (Partial Content) response MAY be stored as if it were an
   incomplete 200 (OK) cache entry.  However, a cache MUST NOT store
   incomplete or partial-content responses if it does not support the
   Range and Content-Range header fields or if it does not understand
   the range units used in those fields.

   A cache MAY complete a stored incomplete response by making a
   subsequent range request (Section 8.3 of [Semantics]) and combining
   the successful response with the stored entry, as defined in
   Section 3.3.  A cache MUST NOT use an incomplete response to answer
   requests unless the response has been made complete or the request is
   partial and specifies a range that is wholly within the incomplete
   response.  A cache MUST NOT send a partial response to a client
   without explicitly marking it as such using the 206 (Partial Content)
   status code.

3.2.  Storing Responses to Authenticated Requests

   A shared cache MUST NOT use a cached response to a request with an
   Authorization header field (Section 8.5.3 of [Semantics]) to satisfy
   any subsequent request unless a cache directive that allows such
   responses to be stored is present in the response.

   In this specification, the following Cache-Control response
   directives (Section 5.2.2) have such an effect: must-revalidate,
   public, . . . . . . . .  32
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  32
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  33
   Appendix A.  Collected ABNF . . . . . . . . . . . . . . . . . . .  35
   Appendix B.  Changes from RFC 7234  . . . . . . . . . . . . . . .  35
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  36
     C.1.  Between RFC7234 and s-maxage.

   Note that cached responses that contain the "must-revalidate" and/or
   "s-maxage" response directives are not allowed to be served stale
   (Section 4.2.4) by shared caches.  In particular, a response with
   either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
   satisfy a subsequent request without revalidating it on the origin
   server.

3.3.  Combining Partial Content

   A response might transfer only a partial representation if the
   connection closed prematurely or if the request used one or more
   Range specifiers (Section 8.3 of [Semantics]).  After several such
   transfers, a cache might have received several ranges of the same
   representation.  A cache MAY combine these ranges into draft 00  . . . . . . . . . . . . . .  36
     C.2.  Since draft-ietf-httpbis-cache-00 . . . . . . . . . . . .  36
     C.3.  Since draft-ietf-httpbis-cache-01 . . . . . . . . . . . .  36
     C.4.  Since draft-ietf-httpbis-cache-02 . . . . . . . . . . . .  36
   Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  37
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  39
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

1.  Introduction

   The Hypertext Transfer Protocol (HTTP) is a single
   stored response, and reuse stateless application-
   level request/response protocol that response to satisfy later requests,
   if they all share the same strong validator uses extensible semantics and the cache complies
   with the client requirements in Section 9.3.7.3 of [Semantics].

   When combining the new response with one or more stored responses, a
   cache MUST:

   o  delete any Warning header fields in the stored response with warn-
      code 1xx (see Section 5.5);

   o  retain any Warning header fields in the stored response with warn-
      code 2xx; and,

   o  use other header fields provided in the new response, aside from
      Content-Range, to replace all instances of the corresponding
      header fields in the stored response.

4.  Constructing Responses from Caches

   When presented
   self-descriptive messages for flexible interaction with network-based
   hypertext information systems.  HTTP is defined by a request, a cache MUST NOT reuse a stored
   response, unless:

   o  The presented effective request URI (Section 5.3 series of [Semantics])
      and
   documents that of the stored response match, and

   o  the request method associated with the stored response allows it
      to be used for the presented request, and

   o  selecting header fields nominated by the stored response (if any)
      match those presented (see Section 4.1), and

   o  the presented request does not contain the no-cache pragma
      (Section 5.4), nor collectively form the no-cache cache directive (Section 5.2.1),
      unless HTTP/1.1 specification:

   o  "HTTP Semantics" [Semantics]

   o  "HTTP Caching" (this document)

   o  "HTTP/1.1 Messaging" [Messaging]

   HTTP is typically used for distributed information systems, where
   performance can be improved by the stored use of response is successfully validated
      (Section 4.3), caches.  This
   document defines aspects of HTTP related to caching and

   o  the stored reusing
   response does not contain the no-cache messages.

   An HTTP cache directive
      (Section 5.2.2.2), unless it is successfully validated
      (Section 4.3), a local store of response messages and

   o the stored response is either:

      *  fresh (see Section 4.2), or

      *  allowed to be served stale (see Section 4.2.4), or

      *  successfully validated (see Section 4.3).

   Note subsystem
   that any controls storage, retrieval, and deletion of messages in it.  A
   cache stores cacheable responses in order to reduce the requirements listed above can be overridden by response time
   and network bandwidth consumption on future, equivalent requests.
   Any client or server MAY employ a
   cache-control extension; see Section 5.2.3.

   When cache, though a stored response is cache cannot be
   used to satisfy by a request without
   validation, server that is acting as a tunnel.

   A shared cache MUST generate an Age header field (Section 5.1),
   replacing any present in the response with is a value equal to the
   stored response's current_age; see Section 4.2.3.

   A cache MUST write through requests with methods that stores responses to be reused by more
   than one user; shared caches are unsafe
   (Section 7.2.1 usually (but not always) deployed as
   a part of [Semantics]) an intermediary.  A private cache, in contrast, is
   dedicated to the origin server; i.e., a cache single user; often, they are deployed as a component
   of a user agent.

   The goal of caching in HTTP is
   not allowed to generate significantly improve performance
   by reusing a reply prior response message to such a request before having
   forwarded the request and having received satisfy a corresponding response.

   Also, note that unsafe requests might invalidate already-stored
   responses; see Section 4.4.

   When more than one suitable current request.  A
   stored response is stored, a cache MUST use considered "fresh", as defined in Section 4.2, if
   the
   most recent response (as determined by the Date header field).  It can also forward the request be reused without "validation" (checking with "Cache-Control: max-age=0" or
   "Cache-Control: no-cache" the
   origin server to disambiguate which see if the cached response to use. remains valid for this
   request).  A cache that does not have a clock available MUST NOT use stored
   responses without revalidating them upon every use.

4.1.  Calculating Secondary Keys with Vary fresh response can therefore reduce both latency and
   network overhead each time it is reused.  When a cache receives a request that cached response is
   not fresh, it might still be reusable if it can be satisfied freshened by
   validation (Section 4.3) or if the origin is unavailable
   (Section 4.2.4).

   This document obsoletes RFC 7234, with the changes being summarized
   in Appendix B.

1.1.  Requirements Notation

   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 3 of [Semantics].

1.2.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234] with a stored
   response that has a Vary header field (Section 10.1.4 list extension, defined in Section 11 of
   [Semantics]), it MUST NOT use
   [Semantics], that response unless all allows for compact definition of comma-separated
   lists using a '#' operator (similar to how the
   selecting header fields nominated by the Vary header field match in
   both '*' operator indicates
   repetition).  Appendix A shows the original request (i.e., that associated collected grammar with the stored
   response), and the presented request. all list
   operators expanded to standard ABNF notation.

   The selecting header fields from two requests following core rules are included by reference, as defined to match if
   and only if those in the first request can be transformed to those in
   the second request by applying any
   [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
   (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line
   feed), OCTET (any 8-bit sequence of the following:

   o  adding or removing whitespace, where allowed data), SP (space), and VCHAR (any
   visible [USASCII] character).

   The rules below are defined in the header field's
      syntax

   o  combining multiple header fields with the same field name (see [Semantics]:

     HTTP-date     = <HTTP-date, see [Semantics], Section 4.2 of [Semantics])

   o  normalizing both header field values 10.1.1.1>
     OWS           = <OWS, see [Semantics], Section 4.3>
     field-name    = <field-name, see [Semantics], Section 4.2>
     quoted-string = <quoted-string, see [Semantics], Section 4.2.3>
     token         = <token, see [Semantics], Section 4.2.3>
     uri-host      = <host, see [RFC3986], Section 3.2.2>
     port          = <port, see [RFC3986], Section 3.2.3>
     pseudonym     = <pseudonym, see [Semantics], Section 5.5.1>

1.3.  Delta Seconds

   The delta-seconds rule specifies a non-negative integer, representing
   time in seconds.

     delta-seconds  = 1*DIGIT

   A recipient parsing a way that is known to
      have identical semantics, according delta-seconds value and converting it to the header field's
      specification (e.g., reordering field values when order is not
      significant; case-normalization, where values are defined binary
   form ought to be
      case-insensitive) use an arithmetic type of at least 31 bits of non-
   negative integer range.  If (after any normalization that might take place) a header field is
   absent from cache receives a request, delta-seconds value
   greater than the greatest integer it can only match another request represent, or if it is
   also absent there.

   A Vary header field-value any of "*" always fails to match.

   The stored response with matching selecting header fields is known as
   the selected response.

   If multiple selected responses are available (potentially including
   responses without a Vary header field), its
   subsequent calculations overflows, the cache will need to choose
   one MUST consider the value
   to use.  When a selecting header field has a known mechanism for
   doing so (e.g., qvalues on Accept and similar request header fields),
   that mechanism MAY be used to select preferred responses; of the
   remainder, the most recent response (as determined by the Date header
   field) is used, as per Section 4.

   If no selected response is available, the cache cannot satisfy either 2147483648 (2^31) or the
   presented request.  Typically, greatest positive integer it
   can conveniently represent.

      Note: The value 2147483648 is forwarded to the origin server
   in a (possibly conditional; see Section 4.3) request.

4.2.  Freshness

   A fresh response is one whose age has not yet exceeded its freshness
   lifetime.  Conversely, a stale response is one where here for historical reasons,
      effectively represents infinity (over 68 years), and does not need
      to be stored in binary form; an implementation could produce it has.

   A response's freshness lifetime is as
      a canned string if any overflow occurs, even if the length calculations
      are performed with an arithmetic type incapable of time between its
   generation by the origin server and its expiration time.  An explicit
   expiration time directly
      representing that number.  What matters here is the time at which the origin server intends that a
   stored response can no longer an overflow
      be used by a cache without further
   validation, whereas a heuristic expiration time is assigned by detected and not treated as a negative value in later
      calculations.

2.  Overview of Cache Operation

   Proper cache when no explicit expiration time is available.

   A response's age is operation preserves the time that has passed since it was generated
   by, or successfully validated with, semantics of HTTP transfers
   ([Semantics]) while reducing the origin server.

   When a response is "fresh" transfer of information already held
   in the cache, cache.  Although caching is an entirely OPTIONAL feature of
   HTTP, it can be used to satisfy
   subsequent requests without contacting the origin server, thereby
   improving efficiency.

   The primary mechanism for determining freshness assumed that reusing a cached response is desirable
   and that such reuse is for an origin
   server to provide an explicit expiration time in the future, using default behavior when no requirement or
   local configuration prevents it.  Therefore, HTTP cache requirements
   are focused on preventing a cache from either the Expires header field (Section 5.3) storing a non-reusable
   response or the max-age reusing a stored response
   directive (Section 5.2.2.8).  Generally, origin servers will assign
   future explicit expiration times to inappropriately, rather than
   mandating that caches always store and reuse particular responses.

   Each cache entry consists of a cache key and one or more HTTP
   responses in the belief corresponding to prior requests that used the
   representation same key.
   The most common form of cache entry is not likely a successful result of a
   retrieval request: i.e., a 200 (OK) response to change in a semantically significant
   way before GET request, which
   contains a representation of the expiration time resource identified by the request
   target (Section 7.3.1 of [Semantics]).  However, it is reached.

   If an origin server wishes also possible
   to force a cache permanent redirects, negative results (e.g., 404 (Not
   Found)), incomplete results (e.g., 206 (Partial Content)), and
   responses to validate every
   request, it can assign an explicit expiration time in the past to
   indicate that methods other than GET if the response is already stale.  Compliant caches will
   normally validate a stale cached response before reusing it method's definition allows
   such caching and defines something suitable for
   subsequent requests (see Section 4.2.4).

   Since origin servers do not always provide explicit expiration times, use as a cache key.

   The primary cache key consists of the request method and target URI.
   However, since HTTP caches in common use today are also allowed typically limited
   to caching responses to GET, many caches simply decline other methods
   and use only the URI as the primary cache key.

   If a request target is subject to content negotiation, its cache
   entry might consist of multiple stored responses, each differentiated
   by a secondary key for the values of the original request's selecting
   header fields (Section 4.1).

3.  Storing Responses in Caches

   A cache MUST NOT store a heuristic response to determine an expiration
   time under certain circumstances (see Section 4.2.2). any request, unless:

   o  The calculation to determine if a response is fresh is:

      response_is_fresh = (freshness_lifetime > current_age)

   freshness_lifetime request method is understood by the cache and defined in Section 4.2.1; current_age as being
      cacheable, and

   o  the response status code is
   defined in final (see Section 4.2.3.

   Clients can send 9.3 of
      [Messaging]), and

   o  the max-age or min-fresh response status code is understood by the cache, and

   o  the "no-store" cache directives directive (see Section 5.2) does not appear
      in a request to constrain or relax freshness calculations for response header fields, and

   o  the
   corresponding "private" response (Section 5.2.1).

   When calculating freshness, to avoid common problems directive (see Section 5.2.2.6) does not
      appear in date parsing:

   o  Although all date formats are specified to be case-sensitive, a the response, if the cache recipient SHOULD match day, week, is shared, and time-zone names case-
      insensitively.

   o  If a cache recipient's internal implementation of time has less
      resolution than  the value Authorization header field (see Section 8.5.3 of an HTTP-date, [Semantics])
      does not appear in the recipient MUST
      internally represent a parsed Expires date as request, if the nearest time
      equal to or earlier than cache is shared, unless the received value.
      response explicitly allows it (see Section 3.2), and

   o  A cache recipient MUST NOT allow local time zones to influence  the
      calculation or comparison of response either:

      *  contains an age or expiration time.

   o  A cache recipient SHOULD consider Expires header field (see Section 5.3), or

      *  contains a date with max-age response directive (see Section 5.2.2.8), or

      *  contains a zone abbreviation
      other than GMT s-maxage response directive (see Section 5.2.2.9)
         and the cache is shared, or UTC to be invalid for calculating expiration.

   Note

      *  contains a Cache Control Extension (see Section 5.2.3) that freshness applies only to cache operation;
         allows it cannot be
   used to force be cached, or

      *  has a user agent to refresh its display status code that is defined as cacheable by default (see
         Section 4.2.2), or reload

      *  contains a
   resource.  See public response directive (see Section 6 for an explanation 5.2.2.5).

   Note that any of the difference between
   caches requirements listed above can be overridden by a
   cache-control extension; see Section 5.2.3.

   In this context, a cache has "understood" a request method or a
   response status code if it recognizes it and history mechanisms.

4.2.1.  Calculating Freshness Lifetime

   A implements all specified
   caching-related behavior.

   Note that, in normal operation, some caches will not store a response
   that has neither a cache can calculate the freshness lifetime (denoted validator nor an explicit expiration time,
   as
   freshness_lifetime) of a such responses are not usually useful to store.  However, caches
   are not prohibited from storing such responses.

3.1.  Storing Incomplete Responses

   A response message is considered complete when all of the octets
   indicated by using the first match of message framing ([Messaging]) are received prior to
   the
   following:

   o connection being closed.  If the request method is GET, the
   response status code is 200 (OK), and the entire response header
   section has been received, a cache MAY store an incomplete response
   message body if the cache entry is shared and the s-maxage recorded as incomplete.  Likewise,
   a 206 (Partial Content) response directive
      (Section 5.2.2.9) is present, use its value, MAY be stored as if it were an
   incomplete 200 (OK) cache entry.  However, a cache MUST NOT store
   incomplete or

   o  If partial-content responses if it does not support the max-age response directive (Section 5.2.2.8) is present,
      use its value,
   Range and Content-Range header fields or

   o  If if it does not understand
   the Expires range units used in those fields.

   A cache MAY complete a stored incomplete response header field by making a
   subsequent range request (Section 5.3) is present, use
      its value minus the value 8.3 of [Semantics]) and combining
   the Date successful response header field, or

   o  Otherwise, no explicit expiration time is present in with the response.
      A heuristic freshness lifetime might be applicable; see stored entry, as defined in
   Section 4.2.2.

   Note that this calculation is not vulnerable 3.3.  A cache MUST NOT use an incomplete response to clock skew, since all
   of answer
   requests unless the information comes from response has been made complete or the origin server.

   When there request is more than one value present for
   partial and specifies a given directive
   (e.g., two Expires header fields, multiple Cache-Control: max-age
   directives), the directive's value is considered invalid.  Caches are
   encouraged to consider responses range that have invalid freshness
   information to be stale.

4.2.2.  Calculating Heuristic Freshness

   Since origin servers do not always provide explicit expiration times,
   a cache MAY assign a heuristic expiration time when an explicit time is not specified, employing algorithms that use other header field
   values (such as wholly within the Last-Modified time) incomplete
   response.  A cache MUST NOT send a partial response to estimate a plausible
   expiration time.  This specification does not provide specific
   algorithms, but does impose worst-case constraints on their results. client
   without explicitly marking it as such using the 206 (Partial Content)
   status code.

3.2.  Storing Responses to Authenticated Requests

   A shared cache MUST NOT use heuristics a cached response to determine freshness when a request with an
   explicit expiration time
   Authorization header field (Section 8.5.3 of [Semantics]) to satisfy
   any subsequent request unless a cache directive that allows such
   responses to be stored is present in the stored response.  Because
   of the requirements in Section 3,

   In this means that, effectively,
   heuristics can only be used on responses without explicit freshness
   whose status codes are defined as cacheable by default (see
   Section 9.1 of [Semantics]), and those responses without explicit
   freshness that have been marked as explicitly cacheable (e.g., with a
   "public" response directive).

   If specification, the following Cache-Control response has
   directives (Section 5.2.2) have such an effect: must-revalidate,
   public, and s-maxage.

3.3.  Combining Partial Content

   A response might transfer only a Last-Modified header field partial representation if the
   connection closed prematurely or if the request used one or more
   Range specifiers (Section 10.2.2 8.3 of
   [Semantics]), caches are encouraged to use [Semantics]).  After several such
   transfers, a heuristic expiration
   value that is no more than some fraction cache might have received several ranges of the interval since that
   time. same
   representation.  A typical setting of this fraction might be 10%.

   When cache MAY combine these ranges into a heuristic is used single
   stored response, and reuse that response to calculate freshness lifetime, a satisfy later requests,
   if they all share the same strong validator and the cache
   SHOULD generate a Warning header field complies
   with a 113 warn-code (see
   Section 5.5.4) the client requirements in Section 9.3.7.3 of [Semantics].

   When combining the new response if its current_age is with one or more than 24
   hours and such stored responses, a warning is not already present.

      Note: Section 13.9 of [RFC2616] prohibited caches from calculating
      heuristic freshness for URIs with query components (i.e., those
      containing '?').  In practice, this has not been widely
      implemented.  Therefore, origin servers are encouraged to send
      explicit directives (e.g., Cache-Control: no-cache) if they wish
      to preclude caching.

4.2.3.  Calculating Age

   The Age
   cache MUST use the header field is used fields provided in the new response, aside
   from Content-Range, to convey an estimated age replace all instances of the
   response message when obtained corresponding
   header fields in the stored response.

4.  Constructing Responses from Caches

   When presented with a cache. request, a cache MUST NOT reuse a stored
   response, unless:

   o  The Age field value is
   the cache's estimate presented effective request URI (Section 5.3 of the number [Semantics])
      and that of seconds since the stored response was
   generated or validated by the origin server.  In essence, match, and

   o  the Age
   value is request method associated with the sum of stored response allows it
      to be used for the time that presented request, and

   o  selecting header fields nominated by the stored response has been resident in
   each of (if any)
      match those presented (see Section 4.1), and

   o  the caches along presented request does not contain the path from no-cache pragma
      (Section 5.4), nor the no-cache cache directive (Section 5.2.1),
      unless the stored response is successfully validated
      (Section 4.3), and

   o  the origin server, plus stored response does not contain the
   amount of time no-cache cache directive
      (Section 5.2.2.2), unless it has been in transit along network paths.

   The following data is used for the age calculation:

   age_value
      The term "age_value" denotes successfully validated
      (Section 4.3), and

   o  the value stored response is either:

      *  fresh (see Section 4.2), or

      *  allowed to be served stale (see Section 4.2.4), or

      *  successfully validated (see Section 4.3).

   Note that any of the requirements listed above can be overridden by a
   cache-control extension; see Section 5.2.3.

   When a stored response is used to satisfy a request without
   validation, a cache MUST generate an Age header field (Section 5.1),
   replacing any present in a form appropriate for arithmetic operation; or
      0, if not available.

   date_value
      The term "date_value" denotes the response with a value equal to the
   stored response's current_age; see Section 4.2.3.

   A cache MUST write through requests with methods that are unsafe
   (Section 7.2.1 of [Semantics]) to the Date header field,
      in origin server; i.e., a form appropriate for arithmetic operations.  See cache is
   not allowed to generate a reply to such a request before having
   forwarded the request and having received a corresponding response.

   Also, note that unsafe requests might invalidate already-stored
   responses; see Section 10.1.1.2 of [Semantics] for 4.4.

   When more than one suitable response is stored, a cache MUST use the definition of
   most recent response (as determined by the Date header field, and for requirements regarding field).  It
   can also forward the request with "Cache-Control: max-age=0" or
   "Cache-Control: no-cache" to disambiguate which response to use.

   A cache that does not have a clock available MUST NOT use stored
   responses without it.

   now
      The term "now" means "the current value revalidating them upon every use.

4.1.  Calculating Secondary Keys with Vary

   When a cache receives a request that can be satisfied by a stored
   response that has a Vary header field (Section 10.1.4 of the clock at the host
      performing the calculation".  A host ought to
   [Semantics]), it MUST NOT use NTP ([RFC5905])
      or some similar protocol to synchronize its clocks to Coordinated
      Universal Time.

   request_time
      The current value that response unless all of the clock at the host at
   selecting header fields nominated by the time Vary header field match in
   both the original request
      resulting in (i.e., that associated with the stored response was made.

   response_time
      The current value of the clock at the host at
   response), and the time presented request.

   The selecting header fields from two requests are defined to match if
   and only if those in the
      response was received.

   A response's age first request can be calculated transformed to those in two entirely independent ways:

   1.
   the "apparent_age": response_time minus date_value, if second request by applying any of the local
       clock is reasonably well synchronized to following:

   o  adding or removing whitespace, where allowed in the origin server's
       clock.  If header field's
      syntax

   o  combining multiple header fields with the result same field name (see
      Section 4.2 of [Semantics])

   o  normalizing both header field values in a way that is negative, known to
      have identical semantics, according to the result header field's
      specification (e.g., reordering field values when order is not
      significant; case-normalization, where values are defined to be
      case-insensitive)

   If (after any normalization that might take place) a header field is replaced by
       zero.

   2.  the "corrected_age_value",
   absent from a request, it can only match another request if all of the caches along the
       response path implement HTTP/1.1. it is
   also absent there.

   A cache MUST interpret this
       value relative Vary header field-value of "*" always fails to the time the request was initiated, not the
       time that the match.

   The stored response was received.

     apparent_age = max(0, response_time - date_value);

     response_delay = response_time - request_time;
     corrected_age_value = age_value + response_delay;

   These are combined as

     corrected_initial_age = max(apparent_age, corrected_age_value);

   unless the cache with matching selecting header fields is confident in known as
   the value of selected response.

   If multiple selected responses are available (potentially including
   responses without a Vary header field), the Age cache will need to choose
   one to use.  When a selecting header field has a known mechanism for
   doing so (e.g., because there are no HTTP/1.0 hops in the Via qvalues on Accept and similar request header field),
   in which case the corrected_age_value fields),
   that mechanism MAY be used as the
   corrected_initial_age.

   The current_age to select preferred responses; of a stored response can then be calculated by adding the amount of time (in seconds) since
   remainder, the stored most recent response was last
   validated (as determined by the origin server to the corrected_initial_age.

     resident_time = now - response_time;
     current_age = corrected_initial_age + resident_time;

4.2.4.  Serving Stale Responses

   A "stale" response Date header
   field) is one that either has explicit expiry information
   or used, as per Section 4.

   If no selected response is allowed to have heuristic expiry calculated, but available, the cache cannot satisfy the
   presented request.  Typically, it is not fresh
   according forwarded to the calculations origin server
   in a (possibly conditional; see Section 4.3) request.

4.2.  Freshness

   A cache MUST NOT generate fresh response is one whose age has not yet exceeded its freshness
   lifetime.  Conversely, a stale response if is one where it has.

   A response's freshness lifetime is prohibited the length of time between its
   generation by an the origin server and its expiration time.  An explicit in-protocol directive (e.g.,
   expiration time is the time at which the origin server intends that a
   stored response can no longer be used by a "no-store" or "no-cache" cache directive, without further
   validation, whereas a heuristic expiration time is assigned by a "must-revalidate" cache-response-directive, or an
   applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
   see Section 5.2.2).

   A
   cache MUST NOT send stale responses unless it when no explicit expiration time is disconnected
   (i.e., available.

   A response's age is the time that has passed since it cannot contact was generated
   by, or successfully validated with, the origin server or otherwise find server.

   When a
   forward path) or doing so response is explicitly allowed (e.g., by "fresh" in the max-
   stale request directive; see Section 5.2.1).

   A cache SHOULD generate a Warning cache, it can be used to satisfy
   subsequent requests without contacting the origin server, thereby
   improving efficiency.

   The primary mechanism for determining freshness is for an origin
   server to provide an explicit expiration time in the future, using
   either the Expires header field with (Section 5.3) or the 110 warn-code
   (see Section 5.5.1) max-age response
   directive (Section 5.2.2.8).  Generally, origin servers will assign
   future explicit expiration times to responses in stale responses.  Likewise, the belief that the
   representation is not likely to change in a semantically significant
   way before the expiration time is reached.

   If an origin server wishes to force a cache SHOULD
   generate a 112 warn-code (see Section 5.5.3) to validate every
   request, it can assign an explicit expiration time in stale responses if the cache is disconnected.

   A cache SHOULD NOT generate a new Warning header field when
   forwarding a response past to
   indicate that does not have an Age header field, even if the response is already stale.  A cache need not  Compliant caches will
   normally validate a response
   that merely became stale in transit.

4.3.  Validation

   When a cache has one or more stored responses cached response before reusing it for a requested URI,
   but cannot serve any of them (e.g., because they are not fresh, or
   one cannot be selected; see
   subsequent requests (see Section 4.1), it can 4.2.4).

   Since origin servers do not always provide explicit expiration times,
   caches are also allowed to use the conditional
   request mechanism a heuristic to determine an expiration
   time under certain circumstances (see Section 8.2 of [Semantics] 4.2.2).

   The calculation to determine if a response is fresh is:

      response_is_fresh = (freshness_lifetime > current_age)

   freshness_lifetime is defined in Section 4.2.1; current_age is
   defined in Section 4.2.3.

   Clients can send the forwarded max-age or min-fresh cache directives in a
   request to give constrain or relax freshness calculations for the next inbound server an opportunity to select a valid
   stored
   corresponding response (Section 5.2.1).

   When calculating freshness, to use, updating the stored metadata avoid common problems in the process,
   or date parsing:

   o  Although all date formats are specified to replace be case-sensitive, a
      cache recipient SHOULD match day, week, and time-zone names case-
      insensitively.

   o  If a cache recipient's internal implementation of time has less
      resolution than the value of an HTTP-date, the stored response(s) with recipient MUST
      internally represent a new response.  This
   process is known parsed Expires date as "validating" the nearest time
      equal to or "revalidating" earlier than the stored
   response.

4.3.1.  Sending received value.

   o  A cache recipient MUST NOT allow local time zones to influence the
      calculation or comparison of an age or expiration time.

   o  A cache recipient SHOULD consider a Validation Request

   When sending date with a conditional request zone abbreviation
      other than GMT or UTC to be invalid for calculating expiration.

   Note that freshness applies only to cache validation, a cache
   sends one or more precondition header fields containing validator
   metadata from its stored response(s), which is then compared by
   recipients operation; it cannot be
   used to determine whether force a stored response is equivalent user agent to refresh its display or reload a
   current representation
   resource.  See Section 6 for an explanation of the resource.

   One such validator is difference between
   caches and history mechanisms.

4.2.1.  Calculating Freshness Lifetime

   A cache can calculate the timestamp given in a Last-Modified header
   field (Section 10.2.2 freshness lifetime (denoted as
   freshness_lifetime) of [Semantics]), which can be used in an If-
   Modified-Since header field for a response validation, or in an If-
   Unmodified-Since or If-Range header field for representation
   selection (i.e., by using the client is referring specifically to a previously
   obtained representation with that timestamp).

   Another validator first match of the
   following:

   o  If the cache is shared and the entity-tag given in an ETag header field s-maxage response directive
      (Section 10.2.3 of [Semantics]).  One 5.2.2.9) is present, use its value, or more entity-tags, indicating
   one

   o  If the max-age response directive (Section 5.2.2.8) is present,
      use its value, or more stored responses, can be used in an If-None-Match

   o  If the Expires response header field for (Section 5.3) is present, use
      its value minus the value of the Date response validation, header field, or

   o  Otherwise, no explicit expiration time is present in an If-Match or If-Range header
   field for representation selection (i.e., the client response.
      A heuristic freshness lifetime might be applicable; see
      Section 4.2.2.

   Note that this calculation is referring
   specifically not vulnerable to one or more previously obtained representations with clock skew, since all
   of the listed entity-tags).

4.3.2.  Handling a Received Validation Request

   Each client in information comes from the request chain may have its own cache, so it origin server.

   When there is
   common more than one value present for a cache at an intermediary to receive conditional requests
   from other (outbound) caches.  Likewise, some user agents make use of
   conditional requests to limit data transfers given directive
   (e.g., two Expires header fields, multiple Cache-Control: max-age
   directives), the directive's value is considered invalid.  Caches are
   encouraged to recently modified
   representations or consider responses that have invalid freshness
   information to complete the transfer of a partially retrieved
   representation.

   If be stale.

4.2.2.  Calculating Heuristic Freshness

   Since origin servers do not always provide explicit expiration times,
   a cache receives MAY assign a request heuristic expiration time when an explicit time
   is not specified, employing algorithms that can be satisfied by reusing one of
   its stored 200 (OK) or 206 (Partial Content) responses, the cache
   SHOULD evaluate any applicable conditional use other header field preconditions
   received in that request with respect to the corresponding validators
   contained within
   values (such as the selected response. Last-Modified time) to estimate a plausible
   expiration time.  This specification does not provide specific
   algorithms, but does impose worst-case constraints on their results.

   A cache MUST NOT evaluate
   conditional header fields that are only applicable use heuristics to determine freshness when an origin
   server, found
   explicit expiration time is present in a request with semantics that cannot the stored response.  Because
   of the requirements in Section 3, this means that, effectively,
   heuristics can only be satisfied used on responses without explicit freshness
   whose status codes are defined as cacheable by default (see
   Section 9.1 of [Semantics]), and those responses without explicit
   freshness that have been marked as explicitly cacheable (e.g., with a cached response, or applied
   "public" response directive).

   If the response has a Last-Modified header field (Section 10.2.2 of
   [Semantics]), caches are encouraged to use a target resource for which it
   has heuristic expiration
   value that is no stored responses; such preconditions are likely intended for more than some other (inbound) server.

   The proper evaluation fraction of conditional requests by a cache depends on the received precondition header fields and their precedence, as
   defined in interval since that
   time.  A typical setting of this fraction might be 10%.

      Note: Section 8.2.2 13.9 of [Semantics]. [RFC2616] prohibited caches from calculating
      heuristic freshness for URIs with query components (i.e., those
      containing '?').  In practice, this has not been widely
      implemented.  Therefore, origin servers are encouraged to send
      explicit directives (e.g., Cache-Control: no-cache) if they wish
      to preclude caching.

4.2.3.  Calculating Age

   The If-Match and If-
   Unmodified-Since conditional Age header fields are not applicable field is used to convey an estimated age of the
   response message when obtained from a cache.

   A request containing an If-None-Match header  The Age field (Section 8.2.4 value is
   the cache's estimate of
   [Semantics]) indicates that the client wants to validate one or more number of its own stored responses in comparison to whichever stored seconds since the response is selected was
   generated or validated by the cache.  If origin server.  In essence, the field-value Age
   value is "*", or if the field-value is a list of entity-tags and at least one sum of them
   matches the entity-tag of time that the selected stored response, a cache
   recipient SHOULD generate a 304 (Not Modified) response (using the
   metadata of the selected stored response) instead of sending that
   stored response.

   When a cache decides to revalidate its own stored responses for a
   request that contains an If-None-Match list has been resident in
   each of entity-tags, the cache
   MAY combine caches along the received list with a list of entity-tags path from its own
   stored set of responses (fresh or stale) and send the union of the
   two lists as a replacement If-None-Match header field value in the
   forwarded request.  If a stored response contains only partial
   content, origin server, plus the cache MUST NOT include its entity-tag
   amount of time it has been in the union
   unless the request transit along network paths.

   The following data is used for a range that would be fully satisfied by
   that partial stored response.  If the response to age calculation:

   age_value
      The term "age_value" denotes the forwarded
   request is 304 (Not Modified) and has an ETag value of the Age header field value with
   an entity-tag that is not
      (Section 5.1), in a form appropriate for arithmetic operation; or
      0, if not available.

   date_value
      The term "date_value" denotes the client's list, value of the cache MUST
   generate Date header field,
      in a 200 (OK) response form appropriate for arithmetic operations.  See
      Section 10.1.1.2 of [Semantics] for the client by reusing its
   corresponding stored response, as updated by definition of the 304 response
   metadata (Section 4.3.4).

   If an If-None-Match header field is not present, a request containing
   an If-Modified-Since Date
      header field (Section 8.2.5 field, and for requirements regarding responses without it.

   now
      The term "now" means "the current value of [Semantics])
   indicates that the client wants clock at the host
      performing the calculation".  A host ought to validate one use NTP ([RFC5905])
      or more of some similar protocol to synchronize its own
   stored responses by modification date.  A cache recipient SHOULD
   generate a 304 (Not Modified) response (using the metadata clocks to Coordinated
      Universal Time.

   request_time
      The current value of the
   selected clock at the host at the time the request
      resulting in the stored response) if one response was made.

   response_time
      The current value of the following cases is true: 1) clock at the host at the time the selected stored
      response has a Last-Modified field-value that was received.

   A response's age can be calculated in two entirely independent ways:

   1.  the "apparent_age": response_time minus date_value, if the local
       clock is
   earlier than or equal reasonably well synchronized to the conditional timestamp; 2) no Last-
   Modified field is present in origin server's
       clock.  If the selected stored response, but it has
   a Date field-value that result is earlier than or equal to negative, the conditional
   timestamp; or, 3) neither Last-Modified nor Date result is present in replaced by
       zero.

   2.  the "corrected_age_value", if all of the
   selected stored response, but caches along the cache recorded it as having been
   received at a time earlier than
       response path implement HTTP/1.1 or equal to the conditional
   timestamp. greater.  A cache that implements partial responses MUST
       interpret this value relative to range requests, the time the request was
       initiated, not the time that the response was received.

     apparent_age = max(0, response_time - date_value);

     response_delay = response_time - request_time;
     corrected_age_value = age_value + response_delay;

   These are combined as
   defined

     corrected_initial_age = max(apparent_age, corrected_age_value);

   unless the cache is confident in Section 8.3 the value of [Semantics], also needs to evaluate a
   received If-Range the Age header field (Section 8.2.7 of [Semantics]) with
   respect to its selected stored response.

4.3.3.  Handling a Validation Response

   Cache handling
   (e.g., because there are no HTTP/1.0 hops in the Via header field),
   in which case the corrected_age_value MAY be used as the
   corrected_initial_age.

   The current_age of a response to a conditional request is dependent
   upon its status code:

   o  A 304 (Not Modified) response status code indicates that the stored response can then be updated and reused; see Section 4.3.4.

   o  A full response (i.e., one with a payload body) indicates that
      none calculated by adding
   the amount of time (in seconds) since the stored responses nominated in response was last
   validated by the conditional request origin server to the corrected_initial_age.

     resident_time = now - response_time;
     current_age = corrected_initial_age + resident_time;

4.2.4.  Serving Stale Responses

   A "stale" response is suitable.  Instead, one that either has explicit expiry information
   or is allowed to have heuristic expiry calculated, but is not fresh
   according to the calculations in Section 4.2.

   A cache MUST use the full NOT generate a stale response to
      satisfy the request and MAY replace the stored response(s).

   o  However, if it is prohibited by an
   explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
   cache receives a 5xx (Server Error) response while
      attempting to validate directive, a response, it can either forward this
      response to the requesting client, "must-revalidate" cache-response-directive, or act as if the server failed
      to respond.  In the latter case, the an
   applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
   see Section 5.2.2).

   A cache MAY MUST NOT send stale responses unless it is disconnected
   (i.e., it cannot contact the origin server or otherwise find a previously
      stored response (see
   forward path) or doing so is explicitly allowed (e.g., by the max-
   stale request directive; see Section 4.2.4).

4.3.4.  Freshening Stored Responses upon 5.2.1).

4.3.  Validation

   When a cache receives a 304 (Not Modified) response and already has one or more stored 200 (OK) responses for a requested URI,
   but cannot serve any of them (e.g., because they are not fresh, or
   one cannot be selected; see Section 4.1), it can use the same cache key, conditional
   request mechanism Section 8.2 of [Semantics] in the
   cache needs forwarded request
   to identify which of give the next inbound server an opportunity to select a valid
   stored responses are updated by
   this new response and then update to use, updating the stored metadata in the process,
   or to replace the stored response(s) with the a new
   information provided in the 304 response.

   The  This
   process is known as "validating" or "revalidating" the stored response
   response.

4.3.1.  Sending a Validation Request

   When generating a conditional request for validation, a cache starts
   with either a request it is attempting to update satisfy, or -- if it is identified by
   initiating the request independently -- it synthesises a request
   using a stored response by copying the first match
   (if any) of the following:

   o  If method, request-target, and
   request header fields used for identifying the new response contains a strong validator (see secondary cache key
   Section 10.2.1 of [Semantics]), 4.1.

   It then updates that strong validator
      identifies the selected representation for update.  All of the
      stored responses request with the same strong one or more precondition header
   fields.  These contain validator are selected.  If
      none of the metadata sourced from stored responses contain
   response(s) that have the same strong validator,
      then the cache MUST NOT use the new response key (both primary and secondary,
   as applicable).

   The precondition header fields are then compared by recipients to update
   determine whether any stored
      responses.

   o  If the new response contains is equivalent to a weak current
   representation of the resource.

   One such validator and is the timestamp given in a Last-Modified header
   field (Section 10.2.2 of [Semantics]), which can be used in an If-
   Modified-Since header field for response validation, or in an If-
   Unmodified-Since or If-Range header field for representation
   selection (i.e., the client is referring specifically to a previously
   obtained representation with that timestamp).

   Another validator
      corresponds to one of is the cache's entity-tag given in an ETag header field
   (Section 10.2.3 of [Semantics]).  One or more entity-tags, indicating
   one or more stored responses, then can be used in an If-None-Match header
   field for response validation, or in an If-Match or If-Range header
   field for representation selection (i.e., the most
      recent of those matching stored responses client is selected for update.

   o  If referring
   specifically to one or more previously obtained representations with
   the new response does not include any form of validator (such
      as listed entity-tags).

4.3.2.  Handling a Received Validation Request

   Each client in the case where request chain may have its own cache, so it is
   common for a client generates cache at an If-Modified-Since
      request intermediary to receive conditional requests
   from a source other than (outbound) caches.  Likewise, some user agents make use of
   conditional requests to limit data transfers to recently modified
   representations or to complete the Last-Modified response header
      field), and there is only one stored response, and that stored
      response also lacks transfer of a validator, then that stored response is
      selected for update. partially retrieved
   representation.

   If a cache receives a request that can be satisfied by reusing one of
   its stored response is selected for update, 200 (OK) or 206 (Partial Content) responses, the cache MUST:

   o  delete
   SHOULD evaluate any Warning applicable conditional header fields field preconditions
   received in the stored response that request with warn-
      code 1xx (see Section 5.5);

   o  retain any Warning respect to the corresponding validators
   contained within the selected response.  A cache MUST NOT evaluate
   conditional header fields that are only applicable to an origin
   server, found in the stored response a request with warn-
      code 2xx; and,

   o  use semantics that cannot be satisfied
   with a cached response, or applied to a target resource for which it
   has no stored responses; such preconditions are likely intended for
   some other (inbound) server.

   The proper evaluation of conditional requests by a cache depends on
   the received precondition header fields provided and their precedence, as
   defined in the 304 (Not Modified)
      response to replace all instances Section 8.2.2 of the corresponding [Semantics].  The If-Match and If-
   Unmodified-Since conditional header fields in the stored response.

4.3.5.  Freshening Responses via HEAD

   A response are not applicable to a
   cache.

   A request containing an If-None-Match header field (Section 8.2.4 of
   [Semantics]) indicates that the HEAD method is identical client wants to what an equivalent
   request made with a GET would have been, except it lacks a body.
   This property validate one or more
   of HEAD its own stored responses can be used in comparison to invalidate or update a
   cached GET whichever stored
   response if the more efficient conditional GET request
   mechanism is not available (due to no validators being present in selected by the
   stored response) cache.  If the field-value is "*", or if transmission of
   the representation body field-value is not
   desired even if it has changed.

   When a cache makes an inbound HEAD request for a given request target list of entity-tags and receives a 200 (OK) response, at least one of them
   matches the entity-tag of the selected stored response, a cache
   recipient SHOULD update or
   invalidate each generate a 304 (Not Modified) response (using the
   metadata of the selected stored response) instead of sending that
   stored response.

   When a cache decides to revalidate its own stored GET responses that could have been
   selected for that a
   request (see Section 4.1).

   For each that contains an If-None-Match list of entity-tags, the stored responses that could have been selected, if cache
   MAY combine the stored response and HEAD response have matching values for any received validator fields (ETag and Last-Modified) and, if the HEAD
   response has list with a Content-Length header field, the value of Content-
   Length matches that list of the entity-tags from its own
   stored response, set of responses (fresh or stale) and send the cache SHOULD update union of the stored response
   two lists as described below; otherwise, the cache SHOULD
   consider a replacement If-None-Match header field value in the stored response to be stale.
   forwarded request.  If a cache updates a stored response with the metadata provided in a
   HEAD response, contains only partial
   content, the cache MUST:

   o  delete any Warning header fields MUST NOT include its entity-tag in the stored response with warn-
      code 1xx (see Section 5.5);

   o  retain any Warning header fields in union
   unless the request is for a range that would be fully satisfied by
   that partial stored response with warn-
      code 2xx; and,

   o  use other header fields provided in response.  If the HEAD response to replace
      all instances of the corresponding forwarded
   request is 304 (Not Modified) and has an ETag header fields field value with
   an entity-tag that is not in the stored client's list, the cache MUST
   generate a 200 (OK) response and append new header fields to for the client by reusing its
   corresponding stored response's
      header section unless otherwise restricted response, as updated by the Cache-Control 304 response
   metadata (Section 4.3.4).

   If an If-None-Match header field.

4.4.  Invalidation

   Because unsafe field is not present, a request methods containing
   an If-Modified-Since header field (Section 7.2.1 8.2.5 of [Semantics]) such as
   PUT, POST or DELETE have the potential for changing state on
   indicates that the
   origin server, intervening caches can use them to keep their contents
   up client wants to validate one or more of its own
   stored responses by modification date.  A cache MUST invalidate recipient SHOULD
   generate a 304 (Not Modified) response (using the effective Request URI (Section 5.3 metadata of
   [Semantics]) as well as the URI(s)
   selected stored response) if one of the following cases is true: 1)
   the selected stored response has a Last-Modified field-value that is
   earlier than or equal to the conditional timestamp; 2) no Last-
   Modified field is present in the Location and Content-
   Location response header fields (if present) when selected stored response, but it has
   a non-error status
   code Date field-value that is received in response earlier than or equal to an unsafe request method.

   However, a the conditional
   timestamp; or, 3) neither Last-Modified nor Date is present in the
   selected stored response, but the cache MUST NOT invalidate a URI from recorded it as having been
   received at a Location time earlier than or
   Content-Location response header field if equal to the host part of conditional
   timestamp.

   A cache that URI
   differs from the host part implements partial responses to range requests, as
   defined in the effective request URI (Section 5.3 Section 8.3 of [Semantics]).  This helps prevent denial-of-service attacks.

   A cache MUST invalidate the effective request URI [Semantics], also needs to evaluate a
   received If-Range header field (Section 5.3 8.2.7 of [Semantics]) when it receives with
   respect to its selected stored response.

4.3.3.  Handling a Validation Response

   Cache handling of a non-error response to a conditional request with
   a method whose safety is unknown.

   Here, a "non-error response" is dependent
   upon its status code:

   o  A 304 (Not Modified) response status code indicates that the
      stored response can be updated and reused; see Section 4.3.4.

   o  A full response (i.e., one with a 2xx (Successful) or 3xx
   (Redirection) status code.  "Invalidate" means payload body) indicates that
      none of the cache will
   either remove all stored responses related nominated in the conditional request
      is suitable.  Instead, the cache MUST use the full response to
      satisfy the request and MAY replace the stored response(s).

   o  However, if a cache receives a 5xx (Server Error) response while
      attempting to validate a response, it can either forward this
      response to the effective request
   URI requesting client, or will mark these act as "invalid" and in need of if the server failed
      to respond.  In the latter case, the cache MAY send a mandatory
   validation before they can be sent in previously
      stored response to (see Section 4.2.4).

4.3.4.  Freshening Stored Responses upon Validation

   When a subsequent
   request.

   Note that this does not guarantee that all appropriate responses are
   invalidated.  For example, cache receives a state-changing request might invalidate 304 (Not Modified) response and already has
   one or more stored 200 (OK) responses in for the caches it travels through, but relevant responses
   still might applicable cache key,
   the cache needs to identify which (if any) are to be stored in other caches that it has not.

5.  Header Field Definitions

   This section defines updated by the syntax
   new information provided, and semantics of HTTP/1.1 header
   fields related to caching.

   +-------------------+----------+----------+--------------+
   | 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  |
   +-------------------+----------+----------+--------------+

5.1.  Age then do so.

   The "Age" header field conveys the sender's estimate of stored response(s) to update are identified by using the amount first
   match (if any) 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 following:

   o  If the new response contains a non-negative integer, representing time in
   seconds strong validator (see
      Section 1.3).

   The presence 10.2.1 of an Age header field implies [Semantics]), then that strong validator
      identifies the response was not
   generated or validated by selected representation for update.  All of the origin server
      stored responses with the same strong validator are identified for this request.
   However, lack
      update.  If none of an Age header field does not imply the origin was
   contacted, since stored responses contain the same strong
      validator, then the response might have been received from an
   HTTP/1.0 cache MUST NOT use the new response to update
      any stored responses.

   o  If the new response contains a weak validator and that validator
      corresponds to one of the cache's stored responses, then the most
      recent of those matching stored responses is identified for
      update.

   o  If the new response does not implement Age.

5.2.  Cache-Control

   The "Cache-Control" include any form of validator (such
      as in the case where a client generates an If-Modified-Since
      request from a source other than the Last-Modified response header field
      field), and there is used to specify directives only one stored response, and that stored
      response also lacks a validator, then that stored response is
      identified for
   caches along update.

   For each stored response identified for update, the request/response chain.  Such cache directives are
   unidirectional MUST use
   the header fields provided in that the presence 304 (Not Modified) response to
   replace all instances of the corresponding header fields in the
   stored response.

4.3.5.  Freshening Responses with HEAD

   A response to the HEAD method is identical to what an equivalent
   request made with a GET would have been, except it lacks a directive in body.
   This property of HEAD responses can be used to invalidate or update a request does
   not imply that
   cached GET response if the same directive more efficient conditional GET request
   mechanism is not available (due to be given no validators being present in the response.

   A cache MUST obey the requirements
   stored response) or if transmission 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 representation body is not
   desired even if it implements has changed.

   When a cache, MUST pass cache
   directives through in forwarded messages, regardless makes an inbound HEAD request for a given request target
   and receives a 200 (OK) response, the cache SHOULD update or
   invalidate each of their
   significance to its stored GET responses that application, since could have been
   selected for that request (see Section 4.1).

   For each of the directives might be
   applicable to all recipients along stored responses that could have been selected, if
   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, stored response and HEAD response have an optional argument, that can use both token matching values for any
   received validator fields (ETag and quoted-string syntax.  For Last-Modified) and, if the directives defined below HEAD
   response has a Content-Length header field, the value of Content-
   Length matches that
   define arguments, recipients ought to accept both forms, even if one
   is documented of the stored response, the cache SHOULD update
   the stored response as described below; otherwise, the cache SHOULD
   consider the stored response to be preferred.  For any directive not defined by this
   specification, stale.

   If a recipient MUST accept both forms.

     Cache-Control   = 1#cache-directive

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

   For cache updates a stored response with the metadata provided in a
   HEAD response, the cache directives defined below, no argument is defined (nor
   allowed) MUST use the header fields provided in the
   HEAD response to replace all instances of the corresponding header
   fields in the stored response and append new header fields to the
   stored response's header section unless stated otherwise.

   +------------------------+-----------------------------------+
   | 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              |
   +------------------------+-----------------------------------+

5.2.1.  Request otherwise restricted by the
   Cache-Control Directives

5.2.1.1.  max-age

   Argument syntax:

      delta-seconds (see Section 1.3)

   The "max-age" header field.

4.4.  Invalidation

   Because unsafe request directive indicates that methods (Section 7.2.1 of [Semantics]) such as
   PUT, POST or DELETE have the client is
   unwilling potential for changing state on the
   origin server, intervening caches can use them to accept a response whose age is greater than keep their contents
   up to date.

   A cache MUST invalidate the
   specified number effective Request URI (Section 5.3 of seconds.  Unless
   [Semantics]) as well as the max-stale request directive
   is also present, URI(s) in the client Location and Content-
   Location response header fields (if present) when a non-error status
   code is not willing received in response to accept an unsafe request method.

   However, a stale
   response.

   This directive uses cache MUST NOT invalidate a URI from a Location or
   Content-Location response header field if the token form host part of that URI
   differs from the argument syntax: e.g.,
   'max-age=5' not 'max-age="5"'.  A sender SHOULD NOT generate host part in the
   quoted-string form.

5.2.1.2.  max-stale

   Argument syntax:

      delta-seconds (see Section 1.3)

   The "max-stale" effective request directive indicates that URI (Section 5.3
   of [Semantics]).  This helps prevent denial-of-service attacks.

   A cache MUST invalidate the client is
   willing to accept effective request URI (Section 5.3 of
   [Semantics]) when it receives a non-error response that has exceeded its freshness
   lifetime.  If max-stale to a request with
   a method whose safety is assigned unknown.

   Here, a value, then the client "non-error response" is
   willing one with a 2xx (Successful) or 3xx
   (Redirection) status code.  "Invalidate" means that the cache will
   either remove all stored responses related to accept the effective request
   URI or will mark these as "invalid" and in need of a mandatory
   validation before they can be sent in response to a subsequent
   request.

   Note that this does not guarantee that all appropriate responses are
   invalidated.  For example, a state-changing request might invalidate
   responses in the caches it travels through, but relevant responses
   still might be stored in other caches that it has exceeded its freshness lifetime
   by no more than not.

5.  Header Field Definitions

   This section defines the specified number syntax and semantics of seconds.  If no value is
   assigned HTTP header fields
   related to max-stale, then caching.

   +-------------------+----------+-----------+--------------+
   | 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     | obsoleted | Section 5.5  |
   +-------------------+----------+-----------+--------------+

5.1.  Age

   The "Age" header field conveys the client is willing to accept a stale
   response sender's estimate of any age.

   This directive uses the token form amount of
   time since the argument syntax: e.g.,
   'max-stale=10' not 'max-stale="10"'.  A sender SHOULD NOT generate response was generated or successfully validated at
   the quoted-string form.

5.2.1.3.  min-fresh

   Argument syntax:

      delta-seconds (see origin server.  Age values are calculated as specified in
   Section 1.3) 4.2.3.

     Age = delta-seconds

   The "min-fresh" request directive indicates that the client Age field-value is
   willing to accept a response whose freshness lifetime is no less than
   its current age plus the specified non-negative integer, representing time in seconds.  That is,
   seconds (see Section 1.3).

   The presence of an Age header field implies that the
   client wants a response that will still be fresh for at least was not
   generated or validated by the
   specified number origin server for this request.
   However, lack of seconds.

   This directive uses an Age header field does not imply the token form of origin was
   contacted, since the argument syntax: e.g.,
   'min-fresh=20' response might have been received from an
   HTTP/1.0 cache that does not 'min-fresh="20"'.  A sender SHOULD NOT generate
   the quoted-string form.

5.2.1.4.  no-cache implement Age.

5.2.  Cache-Control

   The "no-cache" request directive indicates that a cache MUST NOT use
   a stored response "Cache-Control" header field is used to satisfy specify directives for
   caches along the request without successful
   validation on request/response chain.  Such cache directives are
   unidirectional in that the origin server.

5.2.1.5.  no-store

   The "no-store" request presence of a directive indicates that in a cache MUST NOT
   store any part of either this request or any response to it.  This does
   not imply that the same directive applies to both private and shared caches.  "MUST NOT
   store" is present in this context means that the response, or to
   be repeated in it.

   A cache MUST NOT intentionally
   store obey the information in non-volatile storage, and MUST make a best-
   effort attempt to remove requirements of the information from volatile storage as
   promptly as possible after forwarding it.

   This directive is NOT a reliable or sufficient mechanism Cache-Control directives
   defined in this section.  See Section 5.2.3 for ensuring
   privacy.  In particular, malicious or compromised information about how
   Cache-Control directives defined elsewhere are handled.

      Note: Some HTTP/1.0 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 implement Cache-Control.

   A proxy, whether or not it implements a cache) cache, MUST NOT transform the
   payload, as defined pass cache
   directives through in Section 5.6.2 forwarded messages, regardless of [Semantics].

5.2.1.7.  only-if-cached

   The "only-if-cached" request directive indicates their
   significance to that application, since the client only
   wishes directives might be
   applicable to obtain all recipients along the request/response chain.  It is
   not possible to target a stored response.  If it receives this directive, directive to a
   cache SHOULD either respond using specific cache.

   Cache directives are identified by a stored response token, to be compared case-
   insensitively, and have an optional argument, that is
   consistent with the other constraints of can use both token
   and quoted-string syntax.  For the request, or respond with
   a 504 (Gateway Timeout) status code.  If a group of caches directives defined below that
   define arguments, recipients ought to accept both forms, even if one
   is being
   operated as a unified system with good internal connectivity, documented to be preferred.  For any directive not defined by this
   specification, a
   member recipient MUST accept both forms.

     Cache-Control   = 1#cache-directive

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

   For the cache MAY forward such a request within that group of caches.

5.2.2.  Response directives defined below, no argument is defined (nor
   allowed) unless stated otherwise.

   +------------------------+-----------------------------------+
   | 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              |
   +------------------------+-----------------------------------+

5.2.1.  Request Cache-Control Directives

5.2.2.1.  must-revalidate

5.2.1.1.  max-age

   Argument syntax:

      delta-seconds (see Section 1.3)

   The "must-revalidate" response "max-age" request directive indicates that once it has
   become stale, a cache MUST NOT use the response client is
   unwilling to satisfy subsequent
   requests without successful validation on accept a response whose age is greater than the origin server.

   The must-revalidate
   specified number of seconds.  Unless the max-stale request directive
   is necessary also present, the client is not willing to support reliable
   operation for certain protocol features.  In all circumstances accept a
   cache MUST obey stale
   response.

   This directive uses the must-revalidate directive; in particular, if a
   cache cannot reach token form of the origin server for any reason, it MUST argument syntax: e.g.,
   'max-age=5' not 'max-age="5"'.  A sender SHOULD NOT 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
   quoted-string form.

5.2.1.2.  max-stale

   Argument syntax:

      #field-name

      delta-seconds (see Section 1.3)

   The "no-cache" response "max-stale" request 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 client is
   willing to satisfy accept a request without contacting
   it, even by caches response that have been configured to send stale responses. has exceeded its freshness
   lifetime.  If max-stale is assigned a value, then the no-cache client is
   willing to accept a response directive specifies one or that has exceeded its freshness lifetime
   by no more field-names, than the specified number of seconds.  If no value is
   assigned to max-stale, then a cache MAY use the response client is willing to satisfy accept a subsequent request,
   subject to any other restrictions on caching.  However, stale
   response of any header
   fields in age.

   This directive uses the response that have token form of the field-name(s) listed MUST argument syntax: e.g.,
   'max-stale=10' not 'max-stale="10"'.  A sender SHOULD NOT be
   sent in generate
   the response to a subsequent quoted-string form.

5.2.1.3.  min-fresh

   Argument syntax:

      delta-seconds (see Section 1.3)

   The "min-fresh" request without successful
   revalidation with directive indicates that the origin server.  This allows an origin server client is
   willing to
   prevent accept a response whose freshness lifetime is no less than
   its current age plus the re-use of certain header fields specified time in seconds.  That is, the
   client wants a response, while response that will still allowing caching of the rest of the response.

   The field-names given are not limited to be fresh for at least the set
   specified number of header fields
   defined by this specification.  Field names are case-insensitive. seconds.

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

5.2.1.4.  no-cache

   The "no-cache" request directive was received; i.e., indicates that a cache MUST NOT use
   a stored response to satisfy the special handling for request without successful
   validation on the qualified form is not widely
   implemented.

5.2.2.3. origin server.

5.2.1.5.  no-store

   The "no-store" response request directive indicates that a cache MUST NOT
   store any part of either the immediate this request or response. 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.

5.2.2.4.

   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" response request directive indicates that an intermediary
   (regardless of whether
   (whether or not it implements a cache) MUST NOT transform the
   payload, as defined in Section 5.6.2 5.5.2 of [Semantics].

5.2.2.5.  public

5.2.1.7.  only-if-cached

   The "public" response "only-if-cached" request directive indicates that any the client only
   wishes to obtain a stored response.  If it receives this directive, a
   cache MAY store SHOULD either respond using a stored response that is
   consistent with the response, even if other constraints of the response would normally be non-cacheable request, or
   cacheable only respond with
   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, the response MUST NOT be used to satisfy any other
   request without forwarding it for validation and receiving a private cache.  (See
   successful response; see Section 3.2 4.3.

   The must-revalidate directive is necessary to support reliable
   operation for
   additional details related 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 the use of public in response be used by servers if and only
   if failure to validate 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 on the representation could result
   in incorrect operation, such as cacheable by default; see
   Section 4.2.2.)

5.2.2.6.  private a silently unexecuted financial
   transaction.

5.2.2.2.  no-cache

   Argument syntax:

      #field-name

   The "private" "no-cache" response directive indicates that the response message
   is intended for a single user and MUST
   NOT be stored by used to satisfy any other request without forwarding it for
   validation and receiving a successful response; see Section 4.3.

   This allows an origin server to prevent a shared
   cache.  A private cache MAY store the response and reuse from using it for later
   requests, to
   satisfy a request without contacting it, even if the response would normally be non-cacheable. by caches that have
   been configured to send stale responses.

   If the private no-cache response directive specifies one or more field-names,
   this requirement is limited
   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 field-values associated with response that have the field-name(s) listed response header fields.  That is, a shared cache MUST NOT
   store be
   sent in the specified field-names(s), whereas it MAY store response to a subsequent request without successful
   revalidation with the
   remainder 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 response message. 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.

   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; needed for single-entry lists).

   Note: Although it cannot ensure the privacy of the message
   content. has been back-ported to many implementations, some
   HTTP/1.0 caches will not recognize or obey this directive.  Also, private no-
   cache response directives with field-names are often handled by
   caches as if an unqualified private no-cache 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 1.3)

   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.

5.2.2.9.  s-maxage

   Argument syntax:

      delta-seconds (see Section 1.3)

5.2.2.3.  no-store

   The "s-maxage" "no-store" 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 MUST NOT
   store any part of cache directives.  Both either the new directive immediate request or response, and MUST
   NOT use the old directive are supplied, such that applications that do not
   understand the new response to satisfy any other request.

   This directive will default applies to the behavior specified
   by the old directive, both private and those shared caches.  "MUST NOT
   store" in this context means that understand the new directive
   will recognize it as modifying cache MUST NOT intentionally
   store the requirements associated with information in non-volatile storage, and MUST make a best-
   effort attempt to remove the
   old directive. 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 way, extensions to the existing cache-control
   directives can directive, and communications networks might
   be made without breaking deployed caches.

   For example, consider a hypothetical new vulnerable to eavesdropping.

5.2.2.4.  no-transform

   The "no-transform" response directive called
   "community" indicates that acts as an intermediary
   (regardless of whether it implements a modifier to cache) MUST NOT transform the private directive:
   payload, as defined in
   addition to private caches, Section 5.5.2 of [Semantics].

5.2.2.5.  public

   The "public" response directive indicates that any cache that is shared only by members
   of MAY store
   the named community is allowed to cache response, even if the response.  An origin
   server wishing response would normally be non-cacheable or
   cacheable only within a private cache.  (See Section 3.2 for
   additional details related to allow the UCI community 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 use an otherwise private
   response in
   their shared cache(s) could do so status codes not being defined as cacheable by including

     Cache-Control: private, community="UCI"

   A cache default; see
   Section 4.2.2.)

5.2.2.6.  private

   Argument syntax:

      #field-name

   The "private" response directive indicates that recognizes such the response message
   is intended for a community cache-extension could
   broaden its behavior in accordance with that extension. single user and MUST NOT be stored by a shared
   cache.  A private cache that
   does not recognize MAY store the community cache-extension would ignore it response and
   adhere to the private directive.

   New extension directives ought to consider defining:

   o  What reuse it means for a directive to later
   requests, even if the response would normally be specified multiple times,

   o  When non-cacheable.

   If the private response directive does not take an argument, what it means when
      an argument specifies one or more field-names,
   this requirement is present,

   o  When limited to the directive requires an argument, what it means when field-values associated with the
   listed response header fields.  That is, a shared cache MUST NOT
   store the specified field-names(s), whereas it is
      missing,

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

5.2.4.  Cache Directive Registry
   remainder of the response message.

   The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry"
   defines field-names given are not limited to the namespace for set of header fields
   defined by this specification.  Field names are case-insensitive.

   This directive uses the cache directives.  It has been created
   and is now maintained at <https://www.iana.org/assignments/http-
   cache-directives>. quoted-string form of the argument syntax.  A registration MUST include
   sender SHOULD NOT generate the following fields:

   o  Cache Directive Name

   o  Pointer to specification text

   Values token form (even if quoting appears
   not to be added to this namespace require IETF Review (see
   [RFC8126], Section 4.8).

5.3.  Expires

   The "Expires" header field gives needed for single-entry lists).

   Note: This usage of the date/time after which word "private" only controls where the
   response is considered stale.  See Section 4.2 for further discussion
   of can be stored; it cannot ensure the freshness model.

   The presence privacy of the message
   content.  Also, private response directives with field-names are
   often handled by caches as if an Expires field does not imply that unqualified private directive was
   received; i.e., the original
   resource will change or cease to exist at, before, or after that
   time.

   The Expires value special handling for the qualified form is an HTTP-date timestamp, as defined in
   Section 10.1.1.1 of [Semantics].

     Expires = HTTP-date

   For example

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

   A cache recipient MUST interpret invalid date formats, especially not
   widely implemented.

5.2.2.7.  proxy-revalidate

   The "proxy-revalidate" response directive has the
   value "0", same meaning as representing a time in the past (i.e., "already
   expired").

   If a
   must-revalidate response includes a Cache-Control field with the directive, except that it does not apply to
   private caches.

5.2.2.8.  max-age

   Argument syntax:

      delta-seconds (see Section 1.3)

   The "max-age" response directive (Section 5.2.2.8), a recipient MUST ignore indicates that the Expires
   field.  Likewise, if a response includes is to be
   considered stale after its age is greater than the s-maxage specified number
   of seconds.

   This directive
   (Section 5.2.2.9), a shared cache recipient MUST ignore the Expires
   field.  In both these cases, uses the value in Expires is only intended
   for recipients that have not yet implemented token form of the Cache-Control field.

   An origin server without a clock MUST argument syntax: e.g.,
   'max-age=5' not 'max-age="5"'.  A sender SHOULD NOT generate an Expires field
   unless its value represents a fixed time the
   quoted-string form.

5.2.2.9.  s-maxage

   Argument syntax:

      delta-seconds (see Section 1.3)

   The "s-maxage" response directive indicates that, in shared caches,
   the past (always expired)
   or its value has been associated with maximum age specified by this directive overrides the resource maximum age
   specified by a system either the max-age directive or
   user with a reliable clock.

   Historically, HTTP required the Expires field-value to be no more
   than a year in header
   field.  The s-maxage directive also implies 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 semantics of 32-bit integers
   for time values), and many caches will evict a response far sooner
   than that.

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.
   proxy-revalidate response directive.

   This
   specification deprecates such extensions to improve interoperability.

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

   When 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 is 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 present in require a request,
   caches MUST consider change in cache
   behavior) can be added without changing the no-cache request pragma-directive semantics of other
   directives.

   Behavioral extensions are designed to work by acting as having modifiers to
   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 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 include both the
   pragma behavior specified
   by the old directive, and cache-control directives, unless Cache-Control: no-cache
   is purposefully omitted those that understand the new directive
   will recognize it as modifying the requirements associated with the
   old directive.  In this way, extensions to target other Cache-Control request the existing cache-control
   directives at HTTP/1.1 can be made without breaking deployed 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 example, consider a hypothetical new response no older than 30
   seconds, while precluding implementations directive called
   "community" that do not understand
   Cache-Control from serving acts as a cached response.

      Note: Because modifier to the meaning of "Pragma: no-cache" private directive: in responses
   addition to private caches, any cache that is
      not specified, it does not provide a reliable replacement for
      "Cache-Control: no-cache" in them.

5.5.  Warning

   The "Warning" header field shared only by members
   of the named community is used allowed to carry additional information
   about cache the status or transformation of 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 message community cache-extension could
   broaden its behavior in accordance with that might extension.  A cache that
   does not be
   reflected in recognize the status code.  This information is typically used to
   warn about possible incorrectness introduced by caching operations or
   transformations applied community cache-extension would ignore it and
   adhere to the payload of the message.

   Warnings can be used private directive.

   New extension directives ought to consider defining:

   o  What it means 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 directive 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
                     ; specified multiple times,

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

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

   o  Whether the Warning header field, for use in debugging
                     ; a single "-" directive is recommended when agent unknown
     warn-text  = quoted-string
     warn-date  = DQUOTE HTTP-date DQUOTE

   Multiple warnings can specific to requests, responses, or able
      to be generated used in a response (either by the
   origin server or by a cache), including multiple warnings with either.

5.2.4.  Cache Directive Registry

   The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry"
   defines the
   same warn-code number that only differ in warn-text. namespace for the cache directives.  It has been created
   and is now maintained at <https://www.iana.org/assignments/http-
   cache-directives>.

   A user agent that receives one or more Warning registration MUST include the following fields:

   o  Cache Directive Name

   o  Pointer to specification text
   Values to be added to this namespace require IETF Review (see
   [RFC8126], Section 4.8).

5.3.  Expires

   The "Expires" header fields SHOULD
   inform field gives the user of as many date/time after which the
   response is considered stale.  See Section 4.2 for further discussion
   of them as possible, in the order freshness model.

   The presence of an Expires field does not imply that
   they appear in the response.  Senders that generate multiple Warning
   header fields are encouraged original
   resource will change or cease to order them with this user agent
   behavior exist at, before, or after that
   time.

   The Expires value is an HTTP-date timestamp, as defined in mind.
   Section 10.1.1.1 of [Semantics].

     Expires = HTTP-date

   For example

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

   A sender that generates new Warning header fields cache recipient MUST append them after any existing Warning header fields.

   Warnings are assigned three digit warn-codes.  The first digit
   indicates whether interpret invalid date formats, especially the Warning is required to be deleted from
   value "0", as representing a stored
   response after validation:

   o  1xx warn-codes describe time in the freshness or validation status of past (i.e., "already
   expired").

   If a response includes a Cache-Control field with the
      response, and so they max-age
   directive (Section 5.2.2.8), a recipient MUST be deleted by ignore the Expires
   field.  Likewise, if a cache after validation.
      They can only be generated by response includes the s-maxage directive
   (Section 5.2.2.9), a shared cache when validating a cached
      entry, and recipient MUST NOT be generated in any other situation.

   o  2xx warn-codes describe some aspect of ignore the representation that Expires
   field.  In both these cases, the value in Expires is only intended
   for recipients that have not rectified by a validation (for example, a lossy compression of yet implemented the representation) and they Cache-Control field.

   An origin server without a clock MUST NOT be deleted by a cache after
      validation, generate an Expires field
   unless its value represents a full response is sent, fixed time in which case they
      MUST be.

   If the past (always expired)
   or its value has been associated with the resource by a sender generates one system or more 1xx warn-codes in
   user with a message reliable clock.

   Historically, HTTP required the Expires field-value to be
   sent to a recipient known to implement only HTTP/1.0, the sender MUST
   include in each corresponding warning-value no more
   than a warn-date that matches
   the Date header field year 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 future.  While longer freshness lifetimes are no
   longer prohibited, extremely large values have accompanying warn-text that describes the error, e.g., been demonstrated to
   cause problems (e.g., clock overflows due to use of 32-bit integers
   for logging.  It is advisory only, time values), and its content does not affect
   interpretation of the warn-code.

   If many caches will evict a recipient that uses, evaluates, or displays Warning response far sooner
   than that.

5.4.  Pragma

   The "Pragma" 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 field allows recipients to exclude warning-values backwards compatibility with
   HTTP/1.0 caches, so that were
   improperly retained after clients can specify a cache validation.  If all of the warning-
   values are excluded, the recipient MUST exclude "no-cache" request
   that they will understand (as Cache-Control was not defined until
   HTTP/1.1).  When the Warning Cache-Control header field as well.

   The following warn-codes are defined by this specification, each with
   a recommended warn-text in English, is also present and
   understood in a description of its meaning.
   The procedure request, Pragma is ignored.

   In HTTP/1.0, Pragma was defined as an extensible field for defining additional warn codes
   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 described not present in
   Section 5.5.8.

   +-----------+----------------------------------+----------------+
   | 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  |
   +-----------+----------------------------------+----------------+

5.5.1.  Warning: 110 - "Response is Stale"

   A cache SHOULD generate this whenever a request,
   caches MUST consider the sent response is stale.

5.5.2.  Warning: 111 - "Revalidation Failed"

   A cache SHOULD generate this when 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 stale response because an
   attempt no-cache request, a client ought to validate include both the response failed, due
   pragma and cache-control directives, unless Cache-Control: no-cache
   is purposefully omitted to an inability target other Cache-Control request
   directives at HTTP/1.1 or greater caches.  For example:

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

   will constrain HTTP/1.1 and greater caches to reach
   the server.

5.5.3.  Warning: 112 - "Disconnected Operation"

   A cache SHOULD generate this if it is intentionally disconnected serve a response no
   older than 30 seconds, while precluding implementations that do not
   understand Cache-Control from
   the rest of the network for serving a period cached response.

      Note: Because the meaning of time.

5.5.4.  Warning: 113 - "Heuristic Expiration"

   A cache SHOULD generate this if "Pragma: no-cache" in responses is
      not specified, it heuristically chose does not provide a freshness
   lifetime greater than 24 hours and the response's age is greater than
   24 hours.

5.5.5.  Warning: 199 - "Miscellaneous Warning" reliable replacement for
      "Cache-Control: no-cache" in them.

5.5.  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 "Warning" header field was used to carry additional information
   about the
   user.

5.5.6.  Warning: 214 - "Transformation Applied"

   This Warning code MUST be added by a proxy if it applies any status or transformation to of a message that might not be
   reflected in the representation, such status code.  This specification obsoletes it, as changing the content-
   coding, media-type, it
   is not widely generated 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 surfaced to
   a human user or logged.  A system receiving this warning MUST NOT
   take any automated action.

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

   A registration MUST include the following fields:

   o  Warn Code (3 digits)

   o  Short Description

   o  Pointer to specification text

   Values to information it
   carried can be added to this namespace require IETF Review (see
   [RFC8126], Section 4.8). gleaned from examining other header fields, such as
   Age.

6.  History Lists

   User agents  Relationship to Applications

   Applications using HTTP often specify additional forms of caching.
   For example, Web browsers often have history mechanisms, mechanisms such as
   "Back" buttons and
   history lists, that can be used to redisplay a representation
   retrieved earlier in a session.

   Likewise, some Web browsers implement caching of images and other
   assets within a page view; they may or may not honor HTTP caching
   semantics.

   The freshness model (Section 4.2) does requirements in this specification do not necessarily apply to
   history mechanisms.
   how applications use data after it is retrieved from a HTTP cache.
   That is, a history mechanism can display a previous representation
   even if it has expired. expired, and an application can use cached data in
   other ways beyond its freshness lifetime.

   This does not prohibit the application from taking HTTP caching into
   account; for example, a history mechanism from telling might tell the user that a
   view might be stale is stale, or from honoring it might honor cache directives (e.g.,
   Cache-Control: Cache-
   Control: no-store).

7.  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
   [Messaging] and semantics [Semantics].

   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 6.3 of [Messaging] 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.

   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.

8.  IANA Considerations

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

8.1.  Header Field Registration

   Please update the "Message Headers" registry of "Permanent Message
   Header Field Names" at <https://www.iana.org/assignments/message-
   headers> with the header field names listed in the table of
   Section 5.

8.2.  Cache Directive Registration

   Please update the "Hypertext Transfer Protocol (HTTP) Cache Directive
   Registry" at <https://www.iana.org/assignments/http-cache-directives>
   with the registration procedure of Section 5.2.4 and the cache
   directive names summarized in the table of Section 5.2.

8.3.  Warn Code Registration Registry

   Please update add a note to the "Hypertext Transfer Protocol (HTTP) Warn
   Codes" registry at <https://www.iana.org/assignments/http-warn-codes> with
   the registration procedure of Section 5.5.8 and the warn code values
   summarized in
   to the table of Section 5.5. effect that Warning is obsoleted.

9.  References

9.1.  Normative References

   [Messaging]
              Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP/1.1 Messaging", draft-ietf-httpbis-messaging-02 draft-ietf-httpbis-messaging-03
              (work in progress), July October 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [Semantics]
              Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", draft-ietf-httpbis-semantics-02 draft-ietf-httpbis-semantics-03
              (work in progress), July October 2018.

   [USASCII]  American National Standards Institute, "Coded Character
              Set -- 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

9.2.  Informative References

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616,
              DOI 10.17487/RFC2616, June 1999,
              <https://www.rfc-editor.org/info/rfc2616>.

   [RFC5861]  Nottingham, M., "HTTP Cache-Control Extensions for Stale
              Content", RFC 5861, DOI 10.17487/RFC5861, April 2010,
              <https://www.rfc-editor.org/info/rfc5861>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/info/rfc5905>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/info/rfc7234>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

Appendix A.  Collected ABNF

   In the collected ABNF below, list rules are expanded as per
   Section 11 of [Semantics].

   Age = delta-seconds

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

   Expires = HTTP-date

   HTTP-date = <HTTP-date, see [Semantics], Section 10.1.1.1>

   OWS = <OWS, see [Semantics], Section 4.3>

   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 [Semantics], Section 4.2>

   port = <port, see [RFC3986], Section 3.2.3>
   pragma-directive = "no-cache" / extension-pragma
   pseudonym = <pseudonym, see [Semantics], Section 5.6.1> 5.5.1>

   quoted-string = <quoted-string, see [Semantics], Section 4.2.3>

   token = <token, see [Semantics], Section 4.2.3>

   uri-host = <host, see [RFC3986], Section 3.2.2>

   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
    ]

Appendix B.  Changes from RFC 7234

   None yet.

   The Warning response header was obsoleted.  Much of the information
   supported by Warning could be gleaned by examining the response), and
   the remaining warn-codes -- although potentially useful -- were
   entirely advisory, and in practice were not added by caches or
   intermediaries.  (Section 5.5)

Appendix C.  Change Log

   This section is to be removed before publishing as an RFC.

C.1.  Between RFC7234 and draft 00

   The changes were purely editorial:

   o  Change boilerplate and abstract to indicate the "draft" status,
      and update references to ancestor specifications.

   o  Remove version "1.1" from document title, indicating that this
      specification applies to all HTTP versions.

   o  Adjust historical notes.

   o  Update links to sibling specifications.

   o  Replace sections listing changes from RFC 2616 by new empty
      sections referring to RFC 723x.

   o  Remove acknowledgements specific to RFC 723x.

   o  Move "Acknowledgements" to the very end and make them unnumbered.

C.2.  Since draft-ietf-httpbis-cache-00

   The changes are purely editorial:

   o  Moved all extensibility tips, registration procedures, and
      registry tables from the IANA considerations to normative
      sections, reducing the IANA considerations to just instructions
      that will be removed prior to publication as an RFC.

C.3.  Since draft-ietf-httpbis-cache-01

   o  Cite RFC 8126 instead of RFC 5226 (<https://github.com/httpwg/
      http-core/issues/75>)

   o  In Section 5.4, misleading statement about the relation between
      Pragma and Cache-Control (<https://github.com/httpwg/http-core/
      issues/92>, <https://www.rfc-editor.org/errata/eid4674>)

C.4.  Since draft-ietf-httpbis-cache-02

   o  In Section 3, explain that only final responses are cacheable
      (<https://github.com/httpwg/http-core/issues/29>)

   o  In Section 5.2.2, clarify what responses various directives apply
      to (<https://github.com/httpwg/http-core/issues/52>)

   o  In Section 4.3.1, clarify the source of validators in conditional
      requests (<https://github.com/httpwg/http-core/issues/110>)

   o  Revise Section 6 to apply to more than just History Lists
      (<https://github.com/httpwg/http-core/issues/126>)

   o  In Section 5.5, deprecated "Warning" header field
      (<https://github.com/httpwg/http-core/issues/139>)

   o  In Section 3.2, remove a spurious note
      (<https://github.com/httpwg/http-core/issues/141>)

Index

   1
      110 (warn-code)  33
      111 (warn-code)  33
      112 (warn-code)  33
      113 (warn-code)  34
      199 (warn-code)  34

   2
      214 (warn-code)  34
      299 (warn-code)  34

   A
      Age header field  21
      age  11

   C
      Cache-Control header field  22  21
      cache  4
      cache entry  6
      cache key  6

   D
      Disconnected Operation (warn-text)  33

   E
      Expires header field  29
      explicit expiration time  11

   F
      fresh  11
      freshness lifetime  11

   G
      Grammar
         Age  21
         ALPHA  5
         Cache-Control  22  21
         cache-directive  22  21
         CR  5
         CRLF  5
         CTL  5
         delta-seconds  5
         DIGIT  5
         DQUOTE  5
         Expires  30  29
         extension-pragma  31  30
         HEXDIG  5
         HTAB  5
         LF  5
         OCTET  5
         Pragma  31  30
         pragma-directive  31  30
         SP  5
         VCHAR  5
         warn-agent  32
         warn-code  32
         warn-date  32
         warn-text  32
         Warning  32
         warning-value  32

   H
      Heuristic Expiration (warn-text)  34
      heuristic expiration time  11

   M
      Miscellaneous Persistent Warning (warn-text)  34
      Miscellaneous Warning (warn-text)  34
      max-age (cache directive)  23, 28  22, 27
      max-stale (cache directive)  23  22
      min-fresh (cache directive)  24  23
      must-revalidate (cache directive)  25  24

   N
      no-cache (cache directive)  24-25  23-24
      no-store (cache directive)  24, 26  23, 25
      no-transform (cache directive)  25-26  24, 26

   O
      only-if-cached (cache directive)  25  24

   P
      Pragma header field  30
      private (cache directive)  27  26
      private cache  4
      proxy-revalidate (cache directive)  27
      public (cache directive)  27

   R
      Response is Stale (warn-text)  33
      Revalidation Failed (warn-text)  33  26

   S
      s-maxage (cache directive)  28  27
      shared cache  4
      stale  11
      strong validator  19

   T
      Transformation Applied (warn-text)  34  18

   V
      validator  16

   W
      Warning header field  31  30

Acknowledgments

   See Appendix "Acknowledgments" of [Semantics].

Authors' Addresses

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

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

   Mark Nottingham (editor)
   Fastly

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

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

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