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Versions: (draft-fielding-httpbis-http-cache) 00 01 02 03 04 05

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


                              HTTP Caching
                      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.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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 21, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
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   Without obtaining an adequate license from the person(s) controlling
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   than English.

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



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       4.2.3.  Calculating Age . . . . . . . . . . . . . . . . . . .  14
       4.2.4.  Serving Stale Responses . . . . . . . . . . . . . . .  15
     4.3.  Validation  . . . . . . . . . . . . . . . . . . . . . . .  15
       4.3.1.  Sending a Validation Request  . . . . . . . . . . . .  16
       4.3.2.  Handling a Received Validation Request  . . . . . . .  16
       4.3.3.  Handling a Validation Response  . . . . . . . . . . .  18
       4.3.4.  Freshening Stored Responses upon Validation . . . . .  18
       4.3.5.  Freshening Responses with HEAD  . . . . . . . . . . .  19
     4.4.  Invalidation  . . . . . . . . . . . . . . . . . . . . . .  20
   5.  Header Field Definitions  . . . . . . . . . . . . . . . . . .  20
     5.1.  Age . . . . . . . . . . . . . . . . . . . . . . . . . . .  21
     5.2.  Cache-Control . . . . . . . . . . . . . . . . . . . . . .  21
       5.2.1.  Request Cache-Control Directives  . . . . . . . . . .  22
         5.2.1.1.  max-age . . . . . . . . . . . . . . . . . . . . .  22
         5.2.1.2.  max-stale . . . . . . . . . . . . . . . . . . . .  22
         5.2.1.3.  min-fresh . . . . . . . . . . . . . . . . . . . .  23
         5.2.1.4.  no-cache  . . . . . . . . . . . . . . . . . . . .  23
         5.2.1.5.  no-store  . . . . . . . . . . . . . . . . . . . .  23
         5.2.1.6.  no-transform  . . . . . . . . . . . . . . . . . .  24
         5.2.1.7.  only-if-cached  . . . . . . . . . . . . . . . . .  24
       5.2.2.  Response Cache-Control Directives . . . . . . . . . .  24
         5.2.2.1.  must-revalidate . . . . . . . . . . . . . . . . .  24
         5.2.2.2.  no-cache  . . . . . . . . . . . . . . . . . . . .  24
         5.2.2.3.  no-store  . . . . . . . . . . . . . . . . . . . .  25
         5.2.2.4.  no-transform  . . . . . . . . . . . . . . . . . .  26
         5.2.2.5.  public  . . . . . . . . . . . . . . . . . . . . .  26
         5.2.2.6.  private . . . . . . . . . . . . . . . . . . . . .  26
         5.2.2.7.  proxy-revalidate  . . . . . . . . . . . . . . . .  27
         5.2.2.8.  max-age . . . . . . . . . . . . . . . . . . . . .  27
         5.2.2.9.  s-maxage  . . . . . . . . . . . . . . . . . . . .  27
       5.2.3.  Cache Control Extensions  . . . . . . . . . . . . . .  27
       5.2.4.  Cache Directive Registry  . . . . . . . . . . . . . .  28
     5.3.  Expires . . . . . . . . . . . . . . . . . . . . . . . . .  29
     5.4.  Pragma  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     5.5.  Warning . . . . . . . . . . . . . . . . . . . . . . . . .  30
   6.  Relationship to Applications  . . . . . . . . . . . . . . . .  31
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  31
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  32
     8.1.  Header Field Registration . . . . . . . . . . . . . . . .  32
     8.2.  Cache Directive Registration  . . . . . . . . . . . . . .  32
     8.3.  Warn Code Registry  . . . . . . . . . . . . . . . . . . .  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 draft 00  . . . . . . . . . . . . . .  36



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     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 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 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




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   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 5.5.1>

1.3.  Delta Seconds

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

     delta-seconds  = 1*DIGIT



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   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 reducing 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




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   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 final (see Section 9.3 of
      [Messaging]), 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.



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   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, and s-maxage.

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 a single
   stored response, and reuse that response to satisfy later requests,



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   if they all share the same strong validator 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 use the 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 with a request, a cache MUST NOT reuse a stored
   response, unless:

   o  The presented effective request URI (Section 5.3 of [Semantics])
      and 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 the no-cache cache directive (Section 5.2.1),
      unless the stored response is successfully validated
      (Section 4.3), and

   o  the stored response does not contain the no-cache cache directive
      (Section 5.2.2.2), unless it is successfully validated
      (Section 4.3), 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 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 the response with a value equal to the
   stored response's current_age; see Section 4.2.3.





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   A cache MUST write through requests with methods that are unsafe
   (Section 7.2.1 of [Semantics]) to the origin server; i.e., a 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 4.4.

   When more than one suitable response is stored, a cache MUST use the
   most recent response (as determined by the Date header 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 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
   [Semantics]), it MUST NOT use that response unless all of the
   selecting header fields nominated by the Vary header field match in
   both the original request (i.e., that associated with the stored
   response), and the presented request.

   The selecting header fields from two requests are defined to match if
   and only if those in the first request can be transformed to those in
   the second request by applying any of the following:

   o  adding or removing whitespace, where allowed in the header field's
      syntax

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

   o  normalizing both header field values in a way that is known to
      have identical semantics, according to the 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
   absent from a request, it can only match another request if it is
   also absent there.

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





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   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), the cache will need to choose
   one 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 the
   presented request.  Typically, it 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 it has.

   A response's freshness lifetime is the length of time between its
   generation by the origin server and its expiration time.  An explicit
   expiration time is the time at which the origin server intends that a
   stored response can no longer be used by a cache without further
   validation, whereas a heuristic expiration time is assigned by a
   cache when no explicit expiration time is available.

   A response's age is the time that has passed since it was generated
   by, or successfully validated with, the origin server.

   When a response is "fresh" in the 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 (Section 5.3) or the max-age response
   directive (Section 5.2.2.8).  Generally, origin servers will assign
   future explicit expiration times to responses in 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 to validate every
   request, it can assign an explicit expiration time in the past to
   indicate that the response is already stale.  Compliant caches will
   normally validate a stale cached response before reusing it for
   subsequent requests (see Section 4.2.4).




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   Since origin servers do not always provide explicit expiration times,
   caches are also allowed to use a heuristic to determine an expiration
   time under certain circumstances (see Section 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 max-age or min-fresh cache directives in a
   request to constrain or relax freshness calculations for the
   corresponding response (Section 5.2.1).

   When calculating freshness, to avoid common problems in date parsing:

   o  Although all date formats are specified to 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 recipient MUST
      internally represent a parsed Expires date as the nearest time
      equal to or earlier than the 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 date with a zone abbreviation
      other than GMT or UTC to be invalid for calculating expiration.

   Note that freshness applies only to cache operation; it cannot be
   used to force a user agent to refresh its display or reload a
   resource.  See Section 6 for an explanation of the difference between
   caches and history mechanisms.

4.2.1.  Calculating Freshness Lifetime

   A cache can calculate the freshness lifetime (denoted as
   freshness_lifetime) of a response by using the first match of the
   following:

   o  If the cache is shared and the s-maxage response directive
      (Section 5.2.2.9) is present, use its value, or

   o  If the max-age response directive (Section 5.2.2.8) is present,
      use its value, or



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

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

   Note that this calculation is not vulnerable to clock skew, since all
   of the information comes from the origin server.

   When there is more than one value present for 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 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 the 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 use heuristics to determine freshness when an
   explicit expiration time is present in the stored response.  Because
   of the requirements in Section 3, 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 the response has a Last-Modified header field (Section 10.2.2 of
   [Semantics]), caches are encouraged to use a heuristic expiration
   value that is no more than some fraction of the interval since that
   time.  A typical setting of this fraction might be 10%.

      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.






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4.2.3.  Calculating Age

   The Age header field is used to convey an estimated age of the
   response message when obtained from a cache.  The Age field value is
   the cache's estimate of the number of seconds since the response was
   generated or validated by the origin server.  In essence, the Age
   value is the sum of the time that the response has been resident in
   each of the caches along the path from the origin server, plus the
   amount of time it has been in transit along network paths.

   The following data is used for the age calculation:

   age_value
      The term "age_value" denotes the value of the Age header field
      (Section 5.1), in a form appropriate for arithmetic operation; or
      0, if not available.

   date_value
      The term "date_value" denotes the value of the Date header field,
      in a form appropriate for arithmetic operations.  See
      Section 10.1.1.2 of [Semantics] for the definition of the Date
      header field, and for requirements regarding responses without it.

   now
      The term "now" means "the current value of the clock at the host
      performing the calculation".  A host ought to use NTP ([RFC5905])
      or some similar protocol to synchronize its clocks to Coordinated
      Universal Time.

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

   response_time
      The current value of the clock at the host at the time the
      response 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 reasonably well synchronized to the origin server's
       clock.  If the result is negative, the result is replaced by
       zero.

   2.  the "corrected_age_value", if all of the caches along the
       response path implement HTTP/1.1 or greater.  A cache MUST
       interpret this value relative to the time the request was
       initiated, not the time that the response was received.



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     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 is confident in the value of the Age header field
   (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 stored response can then be calculated by adding
   the amount of time (in seconds) since the stored response was last
   validated 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 is 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 NOT generate a stale response if it is prohibited by an
   explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
   cache directive, 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 is disconnected
   (i.e., it cannot contact the origin server or otherwise find a
   forward path) or doing so is explicitly allowed (e.g., by the max-
   stale request directive; see Section 5.2.1).

4.3.  Validation

   When a cache has one or more stored 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 conditional
   request mechanism Section 8.2 of [Semantics] in the forwarded request
   to give the next inbound server an opportunity to select a valid
   stored response to use, updating the stored metadata in the process,
   or to replace the stored response(s) with a new response.  This




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   process is known as "validating" or "revalidating" the stored
   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 satisfy, or -- if it is
   initiating the request independently -- it synthesises a request
   using a stored response by copying the method, request-target, and
   request header fields used for identifying the secondary cache key
   Section 4.1.

   It then updates that request with one or more precondition header
   fields.  These contain validator metadata sourced from stored
   response(s) that have the same cache key (both primary and secondary,
   as applicable).

   The precondition header fields are then compared by recipients to
   determine whether any stored response is equivalent to a current
   representation of the resource.

   One such validator 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 is the 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, 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 client is referring
   specifically to one or more previously obtained representations with
   the listed entity-tags).

4.3.2.  Handling a Received Validation Request

   Each client in the request chain may have its own cache, so it is
   common 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 to recently modified
   representations or to complete the transfer of a partially retrieved
   representation.

   If a cache receives a request that can be satisfied by reusing one of
   its stored 200 (OK) or 206 (Partial Content) responses, the cache
   SHOULD evaluate any applicable conditional header field preconditions



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   received in that request with 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 a request with 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 and their precedence, as
   defined in Section 8.2.2 of [Semantics].  The If-Match and If-
   Unmodified-Since conditional header fields are not applicable to a
   cache.

   A request containing an If-None-Match header field (Section 8.2.4 of
   [Semantics]) indicates that the client wants to validate one or more
   of its own stored responses in comparison to whichever stored
   response is selected by the cache.  If the field-value is "*", or if
   the field-value is a list of entity-tags and at least one of them
   matches the entity-tag of 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 of entity-tags, the cache
   MAY combine the received list with a list of entity-tags 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, the cache MUST NOT include its entity-tag in the union
   unless the request is for a range that would be fully satisfied by
   that partial stored response.  If the response to the forwarded
   request is 304 (Not Modified) and has an ETag header field value with
   an entity-tag that is not in the client's list, the cache MUST
   generate a 200 (OK) response for the client by reusing its
   corresponding stored response, as updated by 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 header field (Section 8.2.5 of [Semantics])
   indicates that the client wants to validate one or more of its own
   stored responses by modification date.  A cache recipient SHOULD
   generate a 304 (Not Modified) response (using the metadata of the
   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 selected stored response, but it has



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   a Date field-value that is earlier than or equal to the conditional
   timestamp; or, 3) neither Last-Modified nor Date is present in the
   selected stored response, but the cache recorded it as having been
   received at a time earlier than or equal to the conditional
   timestamp.

   A cache that implements partial responses to range requests, as
   defined in Section 8.3 of [Semantics], also needs to evaluate a
   received If-Range header field (Section 8.2.7 of [Semantics]) with
   respect to its selected stored response.

4.3.3.  Handling a Validation Response

   Cache handling 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 be updated and reused; see Section 4.3.4.

   o  A full response (i.e., one with a payload body) indicates that
      none of the stored responses 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 requesting client, or act as if the server failed
      to respond.  In the latter case, the cache MAY send a previously
      stored response (see Section 4.2.4).

4.3.4.  Freshening Stored Responses upon Validation

   When a cache receives a 304 (Not Modified) response and already has
   one or more stored 200 (OK) responses for the applicable cache key,
   the cache needs to identify which (if any) are to be updated by the
   new information provided, and then do so.

   The stored response(s) to update are identified by using the first
   match (if any) of the following:

   o  If the new response contains a strong validator (see
      Section 10.2.1 of [Semantics]), then that strong validator
      identifies the selected representation for update.  All of the
      stored responses with the same strong validator are identified for
      update.  If none of the stored responses contain the same strong
      validator, then the cache MUST NOT use the new response to update
      any stored responses.




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   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 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), and there is only one stored response, and that stored
      response also lacks a validator, then that stored response is
      identified for update.

   For each stored response identified for update, the cache MUST use
   the header fields provided in the 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 body.
   This property of HEAD responses can be used to invalidate or update a
   cached GET response if the more efficient conditional GET request
   mechanism is not available (due to no validators being present in the
   stored response) or if transmission of the representation body is not
   desired even if it has changed.

   When a cache makes an inbound HEAD request for a given request target
   and receives a 200 (OK) response, the cache SHOULD update or
   invalidate each of its stored GET responses that could have been
   selected for that request (see Section 4.1).

   For each of the stored responses that could have been selected, if
   the stored response and HEAD response have matching values for any
   received validator fields (ETag and Last-Modified) and, if the HEAD
   response has a Content-Length header field, the value of Content-
   Length matches that of the stored response, the cache SHOULD update
   the stored response as described below; otherwise, the cache SHOULD
   consider the stored response to be stale.

   If a cache updates a stored response with the metadata provided in a
   HEAD response, the cache 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 otherwise restricted by the
   Cache-Control header field.





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4.4.  Invalidation

   Because unsafe request methods (Section 7.2.1 of [Semantics]) such as
   PUT, POST or DELETE have the potential for changing state on the
   origin server, intervening caches can use them to keep their contents
   up to date.

   A cache MUST invalidate the effective Request URI (Section 5.3 of
   [Semantics]) as well as the URI(s) in the Location and Content-
   Location response header fields (if present) when a non-error status
   code is received in response to an unsafe request method.

   However, a cache MUST NOT invalidate a URI from a Location or
   Content-Location response header field if the host part of that URI
   differs from the host part in the effective request URI (Section 5.3
   of [Semantics]).  This helps prevent denial-of-service attacks.

   A cache MUST invalidate the effective request URI (Section 5.3 of
   [Semantics]) when it receives a non-error response to a request with
   a method whose safety is unknown.

   Here, a "non-error response" is one with a 2xx (Successful) or 3xx
   (Redirection) status code.  "Invalidate" means that the cache will
   either remove all stored responses related to 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 not.

5.  Header Field Definitions

   This section defines the syntax and semantics of HTTP 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     | obsoleted | Section 5.5  |
   +-------------------+----------+-----------+--------------+




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5.1.  Age

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

     Age = delta-seconds

   The Age field-value is a non-negative integer, representing time in
   seconds (see Section 1.3).

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

5.2.  Cache-Control

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

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

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

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

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

     Cache-Control   = 1#cache-directive

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



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   For the cache 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

5.2.1.1.  max-age

   Argument syntax:

      delta-seconds (see Section 1.3)

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

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

5.2.1.2.  max-stale

   Argument syntax:

      delta-seconds (see Section 1.3)

   The "max-stale" request directive indicates that the client is
   willing to accept a response that has exceeded its freshness



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

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

5.2.1.3.  min-fresh

   Argument syntax:

      delta-seconds (see Section 1.3)

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

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

5.2.1.4.  no-cache

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

5.2.1.5.  no-store

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

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





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

5.2.1.6.  no-transform

   The "no-transform" request directive indicates that an intermediary
   (whether or not it implements a cache) MUST NOT transform the
   payload, as defined in Section 5.5.2 of [Semantics].

5.2.1.7.  only-if-cached

   The "only-if-cached" request directive indicates that the client only
   wishes to obtain a stored response.  If it receives this directive, a
   cache SHOULD either respond using a stored response that is
   consistent with the other constraints of the request, or respond with
   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
   successful response; see Section 4.3.

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

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

5.2.2.2.  no-cache

   Argument syntax:

      #field-name






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   The "no-cache" response directive indicates that the response MUST
   NOT be 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 cache from using it to
   satisfy a request without contacting it, even by caches that have
   been configured to send stale responses.

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

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

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

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

5.2.2.3.  no-store

   The "no-store" response directive indicates that a cache MUST NOT
   store any part of either the immediate request or response, and MUST
   NOT use the response to satisfy any other request.

   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.




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5.2.2.4.  no-transform

   The "no-transform" response directive indicates that an intermediary
   (regardless of whether it implements a cache) MUST NOT transform the
   payload, as defined in Section 5.5.2 of [Semantics].

5.2.2.5.  public

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

5.2.2.6.  private

   Argument syntax:

      #field-name

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

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

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

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

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




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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)

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

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

5.2.3.  Cache Control Extensions

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

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

   Behavioral extensions are designed to work by acting as modifiers to
   the existing base of cache directives.  Both the new directive and



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

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

     Cache-Control: private, community="UCI"

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

   New extension directives ought to consider defining:

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

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

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

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

5.2.4.  Cache Directive Registry

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

   A registration MUST include the following fields:

   o  Cache Directive Name

   o  Pointer to specification text





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

5.3.  Expires

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

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

   The Expires value is an HTTP-date timestamp, as defined in
   Section 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 the
   value "0", as representing a time in the past (i.e., "already
   expired").

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

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

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







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5.4.  Pragma

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

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

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

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

   When sending a no-cache request, a client ought to include both the
   pragma and cache-control directives, unless Cache-Control: no-cache
   is purposefully omitted to target other Cache-Control 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 serve a response no
   older than 30 seconds, while precluding implementations that do not
   understand Cache-Control from serving a cached response.

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

5.5.  Warning

   The "Warning" header field was used to carry additional information
   about the status or transformation of a message that might not be
   reflected in the status code.  This specification obsoletes it, as it
   is not widely generated or surfaced to users.  The information it
   carried can be gleaned from examining other header fields, such as
   Age.



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6.  Relationship to Applications

   Applications using HTTP often specify additional forms of caching.
   For example, Web browsers often have history mechanisms such as
   "Back" buttons 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 requirements in this specification do not necessarily apply to
   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, 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 might tell the user that a
   view is stale, or it might honor cache directives (e.g., 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.,



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   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 Registry

   Please add a note to the "Hypertext Transfer Protocol (HTTP) Warn
   Codes" registry at <https://www.iana.org/assignments/http-warn-codes>
   to the 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-03
              (work in progress), October 2018.



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   [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-03
              (work in progress), 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>.







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   [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>.










































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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 ] )

   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 5.5.1>

   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>

Appendix B.  Changes from RFC 7234

   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)






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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>)




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   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

   A
      Age header field  21
      age  11

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

   E
      Expires header field  29
      explicit expiration time  11

   F
      fresh  11
      freshness lifetime  11

   G
      Grammar
         Age  21
         ALPHA  5
         Cache-Control  21
         cache-directive  21
         CR  5
         CRLF  5
         CTL  5
         delta-seconds  5
         DIGIT  5
         DQUOTE  5
         Expires  29



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         extension-pragma  30
         HEXDIG  5
         HTAB  5
         LF  5
         OCTET  5
         Pragma  30
         pragma-directive  30
         SP  5
         VCHAR  5

   H
      heuristic expiration time  11

   M
      max-age (cache directive)  22, 27
      max-stale (cache directive)  22
      min-fresh (cache directive)  23
      must-revalidate (cache directive)  24

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

   O
      only-if-cached (cache directive)  24

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

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

   V
      validator  16

   W
      Warning header field  30






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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/




















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