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Versions: (draft-fielding-httpbis-http-cache) 00 01 02 03 04 05 06 07 08 09 10 11 12

HTTP Working Group                                      R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 7234 (if approved)                         M. Nottingham, Ed.
Intended status: Standards Track                                  Fastly
Expires: April 5, 2021                                   J. Reschke, Ed.
                                                              greenbytes
                                                         October 2, 2020


                              HTTP Caching
                      draft-ietf-httpbis-cache-12

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.13.

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/.






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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 April 5, 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Requirements Notation . . . . . . . . . . . . . . . . . .   5
     1.2.  Syntax Notation . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Delta Seconds . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Overview of Cache Operation . . . . . . . . . . . . . . . . .   6
   3.  Storing Responses in Caches . . . . . . . . . . . . . . . . .   7
     3.1.  Storing Header and Trailer Fields . . . . . . . . . . . .   8
     3.2.  Storing Incomplete Responses  . . . . . . . . . . . . . .   9
     3.3.  Storing Responses to Authenticated Requests . . . . . . .   9
     3.4.  Combining Partial Content . . . . . . . . . . . . . . . .  10
   4.  Constructing Responses from Caches  . . . . . . . . . . . . .  10
     4.1.  Calculating Cache Keys with Vary  . . . . . . . . . . . .  11
     4.2.  Freshness . . . . . . . . . . . . . . . . . . . . . . . .  12



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       4.2.1.  Calculating Freshness Lifetime  . . . . . . . . . . .  14
       4.2.2.  Calculating Heuristic Freshness . . . . . . . . . . .  14
       4.2.3.  Calculating Age . . . . . . . . . . . . . . . . . . .  15
       4.2.4.  Serving Stale Responses . . . . . . . . . . . . . . .  16
     4.3.  Validation  . . . . . . . . . . . . . . . . . . . . . . .  17
       4.3.1.  Sending a Validation Request  . . . . . . . . . . . .  17
       4.3.2.  Handling a Received Validation Request  . . . . . . .  18
       4.3.3.  Handling a Validation Response  . . . . . . . . . . .  19
       4.3.4.  Freshening Stored Responses upon Validation . . . . .  20
       4.3.5.  Freshening Responses with HEAD  . . . . . . . . . . .  21
     4.4.  Invalidation  . . . . . . . . . . . . . . . . . . . . . .  21
   5.  Field Definitions . . . . . . . . . . . . . . . . . . . . . .  22
     5.1.  Age . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
     5.2.  Cache-Control . . . . . . . . . . . . . . . . . . . . . .  23
       5.2.1.  Request Cache-Control Directives  . . . . . . . . . .  24
         5.2.1.1.  max-age . . . . . . . . . . . . . . . . . . . . .  24
         5.2.1.2.  max-stale . . . . . . . . . . . . . . . . . . . .  24
         5.2.1.3.  min-fresh . . . . . . . . . . . . . . . . . . . .  25
         5.2.1.4.  no-cache  . . . . . . . . . . . . . . . . . . . .  25
         5.2.1.5.  no-store  . . . . . . . . . . . . . . . . . . . .  25
         5.2.1.6.  no-transform  . . . . . . . . . . . . . . . . . .  26
         5.2.1.7.  only-if-cached  . . . . . . . . . . . . . . . . .  26
       5.2.2.  Response Cache-Control Directives . . . . . . . . . .  26
         5.2.2.1.  must-revalidate . . . . . . . . . . . . . . . . .  26
         5.2.2.2.  must-understand . . . . . . . . . . . . . . . . .  27
         5.2.2.3.  no-cache  . . . . . . . . . . . . . . . . . . . .  27
         5.2.2.4.  no-store  . . . . . . . . . . . . . . . . . . . .  28
         5.2.2.5.  no-transform  . . . . . . . . . . . . . . . . . .  28
         5.2.2.6.  public  . . . . . . . . . . . . . . . . . . . . .  28
         5.2.2.7.  private . . . . . . . . . . . . . . . . . . . . .  28
         5.2.2.8.  proxy-revalidate  . . . . . . . . . . . . . . . .  29
         5.2.2.9.  max-age . . . . . . . . . . . . . . . . . . . . .  29
         5.2.2.10. s-maxage  . . . . . . . . . . . . . . . . . . . .  30
       5.2.3.  Cache Control Extensions  . . . . . . . . . . . . . .  30
       5.2.4.  Cache Directive Registry  . . . . . . . . . . . . . .  31
     5.3.  Expires . . . . . . . . . . . . . . . . . . . . . . . . .  32
     5.4.  Pragma  . . . . . . . . . . . . . . . . . . . . . . . . .  33
     5.5.  Warning . . . . . . . . . . . . . . . . . . . . . . . . .  33
   6.  Relationship to Applications  . . . . . . . . . . . . . . . .  33
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  34
     7.1.  Cache Poisoning . . . . . . . . . . . . . . . . . . . . .  34
     7.2.  Timing Attacks  . . . . . . . . . . . . . . . . . . . . .  34
     7.3.  Caching of Sensitive Information  . . . . . . . . . . . .  35
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  35
     8.1.  Field Registration  . . . . . . . . . . . . . . . . . . .  35
     8.2.  Cache Directive Registration  . . . . . . . . . . . . . .  35
     8.3.  Warn Code Registry  . . . . . . . . . . . . . . . . . . .  35
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  35



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     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  35
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  36
   Appendix A.  Collected ABNF . . . . . . . . . . . . . . . . . . .  37
   Appendix B.  Changes from RFC 7234  . . . . . . . . . . . . . . .  37
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  38
     C.1.  Between RFC7234 and draft 00  . . . . . . . . . . . . . .  38
     C.2.  Since draft-ietf-httpbis-cache-00 . . . . . . . . . . . .  39
     C.3.  Since draft-ietf-httpbis-cache-01 . . . . . . . . . . . .  39
     C.4.  Since draft-ietf-httpbis-cache-02 . . . . . . . . . . . .  39
     C.5.  Since draft-ietf-httpbis-cache-03 . . . . . . . . . . . .  39
     C.6.  Since draft-ietf-httpbis-cache-04 . . . . . . . . . . . .  40
     C.7.  Since draft-ietf-httpbis-cache-05 . . . . . . . . . . . .  40
     C.8.  Since draft-ietf-httpbis-cache-06 . . . . . . . . . . . .  40
     C.9.  Since draft-ietf-httpbis-cache-07 . . . . . . . . . . . .  41
     C.10. Since draft-ietf-httpbis-cache-08 . . . . . . . . . . . .  41
     C.11. Since draft-ietf-httpbis-cache-09 . . . . . . . . . . . .  41
     C.12. Since draft-ietf-httpbis-cache-10 . . . . . . . . . . . .  41
     C.13. Since draft-ietf-httpbis-cache-11 . . . . . . . . . . . .  42
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  42
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  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 the
   use of response caches can improve performance.  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 to reduce the response time and
   network bandwidth consumption on future equivalent requests.  Any
   client or server MAY use a cache, though a server that is acting as a
   tunnel cannot.





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   A shared cache is a cache that stores responses for reuse 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.

   HTTP caching's goal is significantly improving performance by reusing
   a prior response message to satisfy a current request.  A cache
   considers a stored response "fresh", as defined in Section 4.2, if it
   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 the cache reuses it.  When a cached response is not fresh,
   it might still be reusable if validation can freshen it (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", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Section 2 of [Semantics] defines conformance criteria and contains
   considerations regarding error handling.

1.2.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234], extended with the notation for case-
   sensitivity in strings defined in [RFC7405].

   It also uses a list extension, defined in Section 5.7.1 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).



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   [Semantics] defines the following rules:

     HTTP-date     = <HTTP-date, see [Semantics], Section 5.7.7>
     OWS           = <OWS, see [Semantics], Section 5.7.3>
     field-name    = <field-name, see [Semantics], Section 5.4.3>
     quoted-string = <quoted-string, see [Semantics], Section 5.7.4>
     token         = <token, see [Semantics], Section 5.7.2>

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 2147483648 (2^31) or the greatest positive integer it can
   conveniently represent.

      |  *Note:* The value 2147483648 is here for historical reasons,
      |  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.








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   The base cache key comprises the request method and target URI used
   to retrieve the stored response; the method determines under which
   circumstances that response can be used to satisfy a request.
   However, many HTTP caches in common use today only cache GET
   responses, and therefore only use the URI as the cache key,
   forwarding other methods.

   If a request target is subject to content negotiation, the cache
   might store multiple responses for it.  Caches differentiate these
   responses by incorporating values of the original request's selecting
   header fields into the cache key as well, as per Section 4.1.

   Caches might incorporate additional material into the cache key.  For
   example, user agent caches might include the referring site's
   identity, thereby "double keying" the cache to avoid some privacy
   risks (see Section 7.2).

   Most commonly, caches store the successful result of a retrieval
   request: i.e., a 200 (OK) response to a GET request, which contains a
   representation of the target resource (Section 8.3.1 of [Semantics]).
   However, it is also possible to store 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.

   A cache is disconnected when it cannot contact the origin server or
   otherwise find a forward path for a request.  A disconnected cache
   can serve stale responses in some circumstances (Section 4.2.4).

3.  Storing Responses in Caches

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

   o  the request method is understood by the cache;

   o  the response status code is final (see Section 14 of [Semantics]);

   o  if the response status code is 206 or 304, or the "must-
      understand" cache directive (see Section 5.2) is present: the
      cache understands the response status code;

   o  the "no-store" cache directive is not present in the response (see
      Section 5.2);

   o  if the cache is shared: the "private" response directive is either
      not present or allows a shared cache to store a modified response;
      see Section 5.2.2.7);



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   o  if the cache is shared: the Authorization header field is not
      present in the request (see Section 10.6.2 of [Semantics]) or a
      response directive is present that explicitly allows shared
      caching (see Section 3.3); and,

   o  the response contains at least one of:

      *  a public response directive (see Section 5.2.2.6);

      *  a private response directive, if the cache is not shared (see
         Section 5.2.2.7);

      *  an Expires header field (see Section 5.3);

      *  a max-age response directive (see Section 5.2.2.9);

      *  if the cache is shared, an s-maxage response directive (see
         Section 5.2.2.10);

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

      *  a status code that is defined as heuristically cacheable (see
         Section 4.2.2).

   Note that a cache-control extension can override any of the
   requirements listed; 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 Header and Trailer Fields

   Caches MUST include all received header fields - including
   unrecognised ones - when storing a response; this assures that new
   HTTP header fields can be successfully deployed.  However, the
   following exceptions are made:

   o  The Connection header field and fields whose names are listed in
      it are required by Section 13.1 of [Messaging] to be removed
      before forwarding the message.  This MAY be implemented by doing
      so before storage.



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   o  Likewise, some fields' semantics require them to be removed before
      forwarding the message, and this MAY be implemented by doing so
      before storage; see Section 13.1 of [Messaging] for some examples.

   o  Header fields that are specific to a client's proxy configuration
      MUST NOT be stored, unless the cache incorporates the identity of
      the proxy into the cache key.  Effectively, this is limited to
      Proxy-Authenticate (Section 10.7.1 of [Semantics]), Proxy-
      Authentication-Info (Section 10.7.3 of [Semantics]), and Proxy-
      Authorization (Section 10.7.2 of [Semantics]).

   Caches MAY either store trailer fields separate from header fields,
   or discard them.  Caches MUST NOT combine trailer fields with header
   fields.

3.2.  Storing Incomplete Responses

   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 a response body that is not complete (Section 3.3 of
   [Semantics]) if the stored response is recorded as being incomplete.
   Likewise, a 206 (Partial Content) response MAY be stored as if it
   were an incomplete 200 (OK) response.  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 13.2 of [Semantics]) and combining
   the successful response with the stored response, as defined in
   Section 3.4.  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 wholly within the incomplete
   response.  A cache MUST NOT send a partial response to a client
   without explicitly marking it using the 206 (Partial Content) status
   code.

3.3.  Storing Responses to Authenticated Requests

   A shared cache MUST NOT use a cached response to a request with an
   Authorization header field (Section 10.6.2 of [Semantics]) to satisfy
   any subsequent request unless the response contains a Cache-Control
   field with a response directive (Section 5.2.2) that allows it to be
   stored by a shared cache and the cache conforms to the requirements
   of that directive for that response.






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   In this specification, the following response directives have such an
   effect: must-revalidate (Section 5.2.2.1), public (Section 5.2.2.6),
   and s-maxage (Section 5.2.2.10).

3.4.  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 13.2 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,
   if they all share the same strong validator and the cache complies
   with the client requirements in Section 14.3.7.3 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 target URI (Section 6.1 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 stored response does not contain the no-cache cache directive
      (Section 5.2.2.3), 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 a cache-control extension can override any of the
   requirements listed; see Section 5.2.3.



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   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.

   A cache MUST write through requests with methods that are unsafe
   (Section 8.2.1 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 one (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 Cache Keys with Vary

   When a cache receives a request that can be satisfied by a stored
   response that has a Vary header field (Section 11.2.1 of
   [Semantics]), it MUST NOT use that response unless all 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:

   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 5.4.4 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)






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   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 containing a member "*" 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), 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.

   Note that in practice, some resources might send the Vary header
   field on responses inconsistently.  When a cache has multiple
   responses for a target URI, and one or more omits the Vary header
   field, it SHOULD use the most recent non-empty value available to
   select an appropriate response for the request.

   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.





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   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 5.2.2.9).  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).

   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 request directives
   (Section 5.2.1) to constrain or relax freshness calculations for the
   corresponding response.  However, caches are not required to honor
   them.

   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.





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   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:

   o  If the cache is shared and the s-maxage response directive
      (Section 5.2.2.10) is present, use its value, or

   o  If the max-age response directive (Section 5.2.2.9) is present,
      use its value, or

   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.











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   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 heuristics can only
   be used on responses without explicit freshness whose status codes
   are defined as "heuristically cacheable" (e.g., see Section 14.1 of
   [Semantics]), and those responses without explicit freshness that
   have been marked as explicitly cacheable (e.g., with a "public"
   response directive).

   Note that in previous specifications heuristically cacheable response
   status codes were called "cacheable by default."

   If the response has a Last-Modified header field (Section 7.9.2 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 prevent caching.

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 origin server
   generated or validated the response.  The Age value is therefore 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 time it has
   been in transit along network paths.

   Age calculation uses the following data:

   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 9.2.2 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





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      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.

     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 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.



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   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 generate a stale response unless it is disconnected
   or doing so is explicitly permitted by the client or origin server
   (e.g., by the max-stale request directive in Section 5.2.1, by
   extension directives such as those defined in [RFC5861], or by
   configuration in accordance with an out-of-band contract).

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 12.1 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 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, target URI, and request
   header fields identified by the Vary header field 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.

   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 7.9.2 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).





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   Another validator is the entity-tag given in an ETag field
   (Section 7.9.3 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
   received in that request with respect to the corresponding validators
   contained within the selected response.  A cache MUST NOT evaluate
   conditional header fields that only apply to an origin server, occur
   in a request with semantics that cannot be satisfied with a cached
   response, or occur in a request with 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 12.3 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 12.1.2 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



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   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 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 12.1.3 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
   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 13.2 of [Semantics], also needs to evaluate a
   received If-Range header field (Section 12.1.5 of [Semantics])
   regarding its selected stored response.

4.3.3.  Handling a Validation Response

   Cache handling of a response to a conditional request depends 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).








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   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:

   o  If the new response contains a strong validator (see Section 7.9.1
      of [Semantics]), then that strong validator identifies the
      selected representation for update.  All 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.

   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 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, with the following exceptions:

   o  The exceptions to header field storage in Section 3.1 also apply
      to header field updates.

   o  Caches MUST NOT update the following header fields: Content-
      Encoding, Content-Length, Content-MD5 (Section 14.15 of
      [RFC2616]), Content-Range, ETag.




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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 target URI 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 (subject to the exceptions in
   Section 4.3.4) and append new header fields to the stored response's
   header section unless otherwise restricted by the Cache-Control
   header field.

4.4.  Invalidation

   Because unsafe request methods (Section 8.2.1 of [Semantics]) such as
   PUT, POST or DELETE have the potential for changing state on the
   origin server, intervening caches are required to invalidate stored
   responses to keep their contents up to date.  Invalidate means that
   the cache will either remove all stored responses whose target URI
   matches the given URI, or will mark them 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 globally; a state-changing request would only invalidate
   responses in the caches it travels through.






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   A cache MUST invalidate the target URI (Section 6.1 of [Semantics])
   and 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 target URI (Section 6.1 of
   [Semantics]).  This helps prevent denial-of-service attacks.

   A cache MUST invalidate the target URI (Section 6.1 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.

5.  Field Definitions

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

    --------------- ----------- ------
     Field Name      Status      Ref.
    --------------- ----------- ------
     Age             standard    5.1
     Cache-Control   standard    5.2
     Expires         standard    5.3
     Pragma          standard    5.4
     Warning         obsoleted   5.5
    --------------- ----------- ------

                 Table 1

5.1.  Age

   The "Age" header field conveys the sender's estimate of the 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).  A cache SHOULD consider a response to be
   stale if an Age field is present and its value is invalid (i.e.,
   contains a list or something other than a non-negative integer).




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   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 list 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.

   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 apply 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 a
   specific form is required for generation.

     Cache-Control   = #cache-directive

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

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














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    ------------------ ----------------------------------
     Cache Directive    Reference
    ------------------ ----------------------------------
     max-age            Section 5.2.1.1, Section 5.2.2.9
     max-stale          Section 5.2.1.2
     min-fresh          Section 5.2.1.3
     must-revalidate    Section 5.2.2.1
     must-understand    Section 5.2.2.2
     no-cache           Section 5.2.1.4, Section 5.2.2.3
     no-store           Section 5.2.1.5, Section 5.2.2.4
     no-transform       Section 5.2.1.6, Section 5.2.2.5
     only-if-cached     Section 5.2.1.7
     private            Section 5.2.2.7
     proxy-revalidate   Section 5.2.2.8
     public             Section 5.2.2.6
     s-maxage           Section 5.2.2.10
    ------------------ ----------------------------------

                           Table 2

5.2.1.  Request Cache-Control Directives

   This section defines cache request directives.  They are advisory;
   caches MAY implement them, but are not required to.

5.2.1.1.  max-age

   Argument syntax:

      delta-seconds (see Section 1.3)

   The "max-age" request directive indicates that the client prefers a
   response whose age is less than or equal to the specified number of
   seconds.  Unless the max-stale request directive is also present, the
   client does not wish to receive a stale response.

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

5.2.1.2.  max-stale

   Argument syntax:

      delta-seconds (see Section 1.3)






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   The "max-stale" request directive indicates that the client will
   accept a response that has exceeded its freshness lifetime.  If a
   value is present, 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 will 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 MUST 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 prefers 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 MUST NOT generate the
   quoted-string form.

5.2.1.4.  no-cache

   The "no-cache" request directive indicates that the client prefers
   stored response not be used 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 the client is
   asking for intermediaries to avoid transforming the payload, as
   defined in Section 6.5 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.  Caches that honor this request
   directive SHOULD, upon receiving it, either respond using a stored
   response consistent with the other constraints of the request, or
   respond with a 504 (Gateway Timeout) status code.

5.2.2.  Response Cache-Control Directives

   This section defines cache response directives.  A cache MUST obey
   the Cache-Control directives defined in this section.

5.2.2.1.  must-revalidate

   The "must-revalidate" response directive indicates that once the
   response has become stale, a cache MUST NOT reuse that response to
   satisfy another request until it has been successfully validated by
   the origin, as defined by 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 is disconnected, the cache MUST generate a 504 (Gateway
   Timeout) response rather than reuse the stale response.

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

   The must-revalidate directive also permits a shared cache to reuse a
   response to a request containing an Authorization header field,
   subject to the above requirement on revalidation (Section 3.3).







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5.2.2.2.  must-understand

   The "must-understand" response directive limits caching of the
   response to a cache that understands and conforms to the requirements
   for that response's status code.  A cache MUST NOT store a response
   containing the must-understand directive if the cache does not
   understand the response status code.

5.2.2.3.  no-cache

   Argument syntax:

      #field-name

   The "no-cache" response directive, in its unqualified form (without
   an argument), 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 the
   response to satisfy a request without contacting it, even by caches
   that have been configured to send stale responses.

   The qualified form of no-cache response directive, with an argument
   that lists one or more field names, indicates that a cache MAY use
   the response to satisfy a subsequent request, subject to any other
   restrictions on caching, if the listed header fields are excluded
   from the subsequent response or the subsequent response has been
   successfully revalidated with the origin server (updating or removing
   those fields).  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, the qualified form of the directive is 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.






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5.2.2.4.  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.

5.2.2.5.  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 6.5 of [Semantics].

5.2.2.6.  public

   The "public" response directive indicates that a cache MAY store the
   response even if it would otherwise be prohibited, subject to the
   constraints defined in Section 3.  In other words, public explicitly
   marks the response as cacheable.  For example, public permits a
   shared cache to reuse a response to a request containing an
   Authorization header field (Section 3.3).

   Note that it is unnecessary to add the public directive to a response
   that is already cacheable according to Section 3.

   If a response with the public directive has no explicit freshness
   information, it is heuristically cacheable (Section 4.2.2).

5.2.2.7.  private

   Argument syntax:

      #field-name








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   The unqualified "private" response directive indicates that a shared
   cache MUST NOT store the response (i.e., the response is intended for
   a single user).  It also indicates that a private cache MAY store the
   response, subject the constraints defined in Section 3, even if the
   response would not otherwise be heuristically cacheable by a private
   cache.

   If a qualified private response directive is present, with an
   argument that lists one or more field names, then only the listed
   fields are limited to a single user: a shared cache MUST NOT store
   the listed fields if they are present in the original response, but
   MAY store the remainder of the response message without those fields,
   subject the constraints defined in Section 3.

   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, the qualified form of the directive is often handled
   by caches as if an unqualified private directive was received; i.e.,
   the special handling for the qualified form is not widely
   implemented.

5.2.2.8.  proxy-revalidate

   The "proxy-revalidate" response directive indicates that once the
   response has become stale, a shared cache MUST NOT reuse that
   response to satisfy another request until it has been successfully
   validated by the origin, as defined by Section 4.3.  This is
   analogous to must-revalidate (Section 5.2.2.1), except that proxy-
   revalidate does not apply to private caches.

   Note that "proxy-revalidate" on its own does not imply that a
   response is cacheable.  For example, it might be combined with the
   public directive (Section 5.2.2.6), allowing the response to be
   cached while requiring only a shared cache to revalidate when stale.

5.2.2.9.  max-age

   Argument syntax:

      delta-seconds (see Section 1.3)




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   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 MUST NOT generate the
   quoted-string form.

5.2.2.10.  s-maxage

   Argument syntax:

      delta-seconds (see Section 1.3)

   The "s-maxage" response directive indicates that, for a shared cache,
   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 incorporates the proxy-revalidate
   (Section 5.2.2.8) response directive's semantics for a shared cache.
   A shared cache MUST NOT reuse a stale response with s-maxage to
   satisfy another request until it has been successfully validated by
   the origin, as defined by Section 4.3.  This directive also permits a
   shared cache to reuse a response to a request containing an
   Authorization header field, subject to the above requirements on
   maximum age and revalidation (Section 3.3).

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










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

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

     Cache-Control: private, community="UCI"

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

   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 5.7.7 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 5.2.2.9), a recipient MUST ignore the Expires
   field.  Likewise, if a response includes the s-maxage directive
   (Section 5.2.2.10), 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 was defined for HTTP/1.0 caches, so that
   clients could specify a "no-cache" request (as Cache-Control was not
   defined until HTTP/1.1).

   However, support for Cache-Control is now widespread.  As a result,
   this specification deprecates Pragma.

      |  *Note:* Because the meaning of "Pragma: no-cache" in responses
      |  was never 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.

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).








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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.

7.1.  Cache Poisoning

   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 regarding
   HTTP/1.1.

7.2.  Timing Attacks

   Because one of the primary uses of a cache is to optimise
   performance, its use can "leak" information about what resources have
   been previously requested.

   For example, if a user visits a site and their browser caches some of
   its responses, and then navigates to a second site, that site can
   attempt to load responses it knows exists on the first site.  If they
   load quickly, it can be assumed that the user has visited that site,
   or even a specific page on it.

   Such "timing attacks" can be mitigated by adding more information to
   the cache key, such as the identity of the referring site (to prevent
   the attack described above).  This is sometimes called "double
   keying."







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7.3.  Caching of Sensitive Information

   Implementation and deployment 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.

   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.  Field Registration

   Please update the "Hypertext Transfer Protocol (HTTP) Field Name
   Registry" at <https://www.iana.org/assignments/http-fields> with the
   field names listed in the two tables 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", Work in Progress, Internet-
              Draft, draft-ietf-httpbis-messaging-12, October 2, 2020,
              <https://tools.ietf.org/html/draft-ietf-httpbis-messaging-
              12>.




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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>.

   [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>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [Semantics]
              Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", Work in Progress, Internet-Draft,
              draft-ietf-httpbis-semantics-12, October 2, 2020,
              <https://tools.ietf.org/html/draft-ietf-httpbis-semantics-
              12>.

   [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>.






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   [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. F. 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 5.7.1.1 of [Semantics].

   Age = delta-seconds

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

   Expires = HTTP-date

   HTTP-date = <HTTP-date, see [Semantics], Section 5.7.7>

   OWS = <OWS, see [Semantics], Section 5.7.3>

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

   delta-seconds = 1*DIGIT

   field-name = <field-name, see [Semantics], Section 5.4.3>

   quoted-string = <quoted-string, see [Semantics], Section 5.7.4>

   token = <token, see [Semantics], Section 5.7.2>

Appendix B.  Changes from RFC 7234

   Handling invalid and multiple Age header field values has been
   clarified.  (Section 5.1)








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   Some cache directives defined by this specification now have stronger
   prohibitions against generating the quoted form of their values,
   since this has been found to create interoperability problems.
   Consumers of extension cache directives are no longer required to
   accept both token and quoted-string forms, but they still need to
   parse them properly for unknown extensions.  (Section 5.2)

   The "public" and "private" cache directives were clarified, so that
   they do not make responses reusable under any condition.
   (Section 5.2.2)

   The "must-understand" cache directive was introduced; caches are no
   longer required to understand the semantics of new response status
   codes unless it is present.  (Section 5.2.2.2)

   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.  In practice, Warning was 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.






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

C.5.  Since draft-ietf-httpbis-cache-03

   o  In Section 2, define what a disconnected cache is
      (<https://github.com/httpwg/http-core/issues/5>)

   o  In Section 4, clarify language around how to select a response
      when more than one matches (<https://github.com/httpwg/http-core/
      issues/23>)





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   o  in Section 4.2.4, mention stale-while-revalidate and stale-if-
      error (<https://github.com/httpwg/http-core/issues/122>)

   o  Remove requirements around cache request directives
      (<https://github.com/httpwg/http-core/issues/129>)

   o  Deprecate Pragma (<https://github.com/httpwg/http-core/
      issues/140>)

   o  In Section 3.3 and Section 5.2.2, note effect of some directives
      on authenticated requests (<https://github.com/httpwg/http-core/
      issues/161>)

C.6.  Since draft-ietf-httpbis-cache-04

   o  In Section 5.2, remove the registrations for stale-if-error and
      stale-while-revalidate which happened in RFC 7234
      (<https://github.com/httpwg/http-core/issues/207>)

C.7.  Since draft-ietf-httpbis-cache-05

   o  In Section 3.2, clarify how weakly framed content is considered
      for purposes of completeness (<https://github.com/httpwg/http-
      core/issues/25>)

   o  Throughout, describe Vary and cache key operations more clearly
      (<https://github.com/httpwg/http-core/issues/28>)

   o  In Section 3, remove concept of "cacheable methods" in favor of
      prose (<https://github.com/httpwg/http-core/issues/54>,
      <https://www.rfc-editor.org/errata/eid5300>)

   o  Refactored Section 7, and added a section on timing attacks
      (<https://github.com/httpwg/http-core/issues/233>)

   o  Changed "cacheable by default" to "heuristically cacheable"
      throughout (<https://github.com/httpwg/http-core/issues/242>)

C.8.  Since draft-ietf-httpbis-cache-06

   o  In Section 3 and Section 5.2.2.2, change response cacheability to
      only require understanding the response status code if the must-
      understand cache directive is present (<https://github.com/httpwg/
      http-core/issues/120>)

   o  Change requirements for handling different forms of cache
      directives in Section 5.2 (<https://github.com/httpwg/http-core/
      issues/128>)



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   o  Fix typo in Section 5.2.2.10 (<https://github.com/httpwg/http-
      core/issues/264>)

   o  In Section 5.2.2.6 and Section 5.2.2.7, clarify "private" and
      "public" so that they do not override all other cache directives
      (<https://github.com/httpwg/http-core/issues/268>)

   o  In Section 3, distinguish between private with and without
      qualifying headers (<https://github.com/httpwg/http-core/
      issues/270>)

   o  In Section 4.1, clarify that any "*" as a member of Vary will
      disable caching (<https://github.com/httpwg/http-core/issues/286>)

   o  In Section 1.1, reference RFC 8174 as well
      (<https://github.com/httpwg/http-core/issues/303>)

C.9.  Since draft-ietf-httpbis-cache-07

   o  Throughout, replace "effective request URI", "request-target" and
      similar with "target URI" (<https://github.com/httpwg/http-core/
      issues/259>)

   o  In Section 5.2.2.6 and Section 5.2.2.7, make it clear that these
      directives do not ignore other requirements for caching
      (<https://github.com/httpwg/http-core/issues/320>)

   o  In Section 3.2, move definition of "complete" into semantics
      (<https://github.com/httpwg/http-core/issues/334>)

C.10.  Since draft-ietf-httpbis-cache-08

   o  Appendix A now uses the sender variant of the "#" list expansion
      (<https://github.com/httpwg/http-core/issues/192>)

C.11.  Since draft-ietf-httpbis-cache-09

   o  In Section 5.1, discuss handling of invalid and multiple Age
      header field values (<https://github.com/httpwg/http-core/
      issues/193>)

   o  Switch to xml2rfc v3 mode for draft generation
      (<https://github.com/httpwg/http-core/issues/394>)

C.12.  Since draft-ietf-httpbis-cache-10

   o  In Section 5.2 (Cache-Control), adjust ABNF to allow empty lists
      (<https://github.com/httpwg/http-core/issues/210>)



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C.13.  Since draft-ietf-httpbis-cache-11

   o  None.

Acknowledgments

   See Appendix "Acknowledgments" of [Semantics].

Authors' Addresses

   Roy T. Fielding (editor)
   Adobe
   345 Park Ave
   San Jose, CA 95110
   United States of America

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


   Mark Nottingham (editor)
   Fastly
   Prahran VIC
   Australia

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


   Julian Reschke (editor)
   greenbytes GmbH
   Hafenweg 16
   48155 M√ľnster
   Germany

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














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