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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 RFC 7234

HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2616 (if approved)                                  J. Gettys
Intended status: Standards Track                          Alcatel-Lucent
Expires: February 25, 2012                                      J. Mogul
                                                                      HP
                                                              H. Frystyk
                                                               Microsoft
                                                             L. Masinter
                                                                   Adobe
                                                                P. Leach
                                                               Microsoft
                                                          T. Berners-Lee
                                                                 W3C/MIT
                                                           Y. Lafon, Ed.
                                                                     W3C
                                                      M. Nottingham, Ed.

                                                         J. Reschke, Ed.
                                                              greenbytes
                                                         August 24, 2011


                       HTTP/1.1, part 6: Caching
                     draft-ietf-httpbis-p6-cache-16

Abstract

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypertext information
   systems.  HTTP has been in use by the World Wide Web global
   information initiative since 1990.  This document is Part 6 of the
   seven-part specification that defines the protocol referred to as
   "HTTP/1.1" and, taken together, obsoletes RFC 2616.

   Part 6 defines requirements on HTTP caches and the associated header
   fields that control cache behavior or indicate cacheable response
   messages.

Editorial Note (To be removed by RFC Editor)

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

   The current issues list is at
   <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
   documents (including fancy diffs) can be found at



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   <http://tools.ietf.org/wg/httpbis/>.

   The changes in this draft are summarized in Appendix C.17.

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 http://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
   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 February 25, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
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   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  7
     1.4.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  7
       1.4.1.  Core Rules . . . . . . . . . . . . . . . . . . . . . .  7
       1.4.2.  ABNF Rules defined in other Parts of the
               Specification  . . . . . . . . . . . . . . . . . . . .  8
     1.5.  Delta Seconds  . . . . . . . . . . . . . . . . . . . . . .  8
   2.  Cache Operation  . . . . . . . . . . . . . . . . . . . . . . .  8
     2.1.  Response Cacheability  . . . . . . . . . . . . . . . . . .  9
     2.2.  Constructing Responses from Caches . . . . . . . . . . . . 10
     2.3.  Freshness Model  . . . . . . . . . . . . . . . . . . . . . 11
       2.3.1.  Calculating Freshness Lifetime . . . . . . . . . . . . 12
       2.3.2.  Calculating Age  . . . . . . . . . . . . . . . . . . . 13
       2.3.3.  Serving Stale Responses  . . . . . . . . . . . . . . . 15
     2.4.  Validation Model . . . . . . . . . . . . . . . . . . . . . 16
     2.5.  Request Methods that Invalidate  . . . . . . . . . . . . . 16
     2.6.  Shared Caching of Authenticated Responses  . . . . . . . . 17
     2.7.  Caching Negotiated Responses . . . . . . . . . . . . . . . 18
     2.8.  Combining Partial Content  . . . . . . . . . . . . . . . . 18
     2.9.  Freshening Responses . . . . . . . . . . . . . . . . . . . 19
   3.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 20
     3.1.  Age  . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     3.2.  Cache-Control  . . . . . . . . . . . . . . . . . . . . . . 20
       3.2.1.  Request Cache-Control Directives . . . . . . . . . . . 21
       3.2.2.  Response Cache-Control Directives  . . . . . . . . . . 23
       3.2.3.  Cache Control Extensions . . . . . . . . . . . . . . . 25
     3.3.  Expires  . . . . . . . . . . . . . . . . . . . . . . . . . 26
     3.4.  Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     3.5.  Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     3.6.  Warning  . . . . . . . . . . . . . . . . . . . . . . . . . 29
   4.  History Lists  . . . . . . . . . . . . . . . . . . . . . . . . 31
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 31
     5.1.  Cache Directive Registry . . . . . . . . . . . . . . . . . 31
     5.2.  Header Field Registration  . . . . . . . . . . . . . . . . 32
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 32
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 33
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 33
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 33
   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 34
   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 34
   Appendix C.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 36
     C.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 36



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     C.2.  Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 36
     C.3.  Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 37
     C.4.  Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 37
     C.5.  Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 37
     C.6.  Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 37
     C.7.  Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 38
     C.8.  Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 38
     C.9.  Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 38
     C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 39
     C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 39
     C.12. Since draft-ietf-httpbis-p6-cache-10 . . . . . . . . . . . 40
     C.13. Since draft-ietf-httpbis-p6-cache-11 . . . . . . . . . . . 40
     C.14. Since draft-ietf-httpbis-p6-cache-12 . . . . . . . . . . . 40
     C.15. Since draft-ietf-httpbis-p6-cache-13 . . . . . . . . . . . 40
     C.16. Since draft-ietf-httpbis-p6-cache-14 . . . . . . . . . . . 40
     C.17. Since draft-ietf-httpbis-p6-cache-15 . . . . . . . . . . . 41
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41


































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1.  Introduction

   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/1.1 related to caching and reusing
   response messages.

1.1.  Purpose

   An HTTP cache is a local store of response messages and the subsystem
   that controls its message storage, retrieval, and deletion.  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.

   The goal of caching in HTTP/1.1 is to significantly improve
   performance by reusing a prior response message to satisfy a current
   request.  A stored response is considered "fresh", as defined in
   Section 2.3, 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 cache response can
   therefore reduce both latency and network transfers each time it is
   reused.  When a cached response is not fresh, it might still be
   reusable if it can be freshened by validation (Section 2.4) or if the
   origin is unavailable.

1.2.  Terminology

   This specification uses a number of terms to refer to the roles
   played by participants in, and objects of, HTTP caching.

   cache

      A conformant implementation of a HTTP cache.  Note that this
      implies an HTTP/1.1 cache; this specification does not define
      conformance for HTTP/1.0 caches.

   shared cache

      A cache that stores responses to be reused by more than one user;
      usually (but not always) deployed as part of an intermediary.

   private cache

      A cache that is dedicated to a single user.





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   cacheable

      A response is cacheable if a cache is allowed to store a copy of
      the response message for use in answering subsequent requests.
      Even when a response is cacheable, there might be additional
      constraints on whether a cache can use the stored copy to satisfy
      a particular request.

   explicit expiration time

      The time at which the origin server intends that a representation
      no longer be returned by a cache without further validation.

   heuristic expiration time

      An expiration time assigned by a cache when no explicit expiration
      time is available.

   age

      The age of a response is the time since it was sent by, or
      successfully validated with, the origin server.

   first-hand

      A response is first-hand if the freshness model is not in use;
      i.e., its age is 0.

   freshness lifetime

      The length of time between the generation of a response and its
      expiration time.

   fresh

      A response is fresh if its age has not yet exceeded its freshness
      lifetime.

   stale

      A response is stale if its age has passed its freshness lifetime
      (either explicit or heuristic).

   validator

      A protocol element (e.g., an entity-tag or a Last-Modified time)
      that is used to find out whether a stored response is an
      equivalent copy of a representation.  See Section 2.1 of [Part4].



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   strong validator

      A validator that is defined by the origin server such that its
      current value will change if the representation body changes;
      i.e., an entity-tag that is not marked as weak (Section 2.3 of
      [Part4]) or, if no entity-tag is provided, a Last-Modified value
      that is strong in the sense defined by Section 2.2.2 of [Part4].

1.3.  Requirements

   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].

   An implementation is not compliant if it fails to satisfy one or more
   of the "MUST" or "REQUIRED" level requirements for the protocols it
   implements.  An implementation that satisfies all the "MUST" or
   "REQUIRED" level and all the "SHOULD" level requirements for its
   protocols is said to be "unconditionally compliant"; one that
   satisfies all the "MUST" level requirements but not all the "SHOULD"
   level requirements for its protocols is said to be "conditionally
   compliant".

1.4.  Syntax Notation

   This specification uses the ABNF syntax defined in Section 1.2 of
   [Part1] (which extends the syntax defined in [RFC5234] with a list
   rule).  Appendix B shows the collected ABNF, with the list rule
   expanded.

   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), LF (line feed), OCTET (any 8-bit
   sequence of data), SP (space), VCHAR (any visible USASCII character),
   and WSP (whitespace).

1.4.1.  Core Rules

   The core rules below are defined in [Part1]:

     OWS           = <OWS, defined in [Part1], Section 1.2.2>
     quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
     token         = <token, defined in [Part1], Section 3.2.3>







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1.4.2.  ABNF Rules defined in other Parts of the Specification

   The ABNF rules below are defined in other parts:

     field-name    = <field-name, defined in [Part1], Section 3.2>
     HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
     port          = <port, defined in [Part1], Section 2.7>
     pseudonym     = <pseudonym, defined in [Part1], Section 9.9>
     uri-host      = <uri-host, defined in [Part1], Section 2.7>

1.5.  Delta Seconds

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

     delta-seconds  = 1*DIGIT

   If an implementation receives a delta-seconds value larger than the
   largest positive integer it can represent, or if any of its
   subsequent calculations overflows, it MUST consider the value to be
   2147483648 (2^31).  Recipients parsing a delta-seconds value SHOULD
   use an arithmetic type of at least 31 bits of range, and senders MUST
   NOT send delta-seconds with a value greater than 2147483648.

2.  Cache Operation

   Proper cache operation preserves the semantics of HTTP transfers
   ([Part2]) while eliminating the transfer of information already held
   in the cache.  Although caching is an entirely OPTIONAL feature of
   HTTP, we assume that reusing the cached response is desirable and
   that such reuse is the default behavior when no requirement or
   locally-desired 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.

   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 containing a
   representation of the resource identified by the request target.
   However, it is also possible to cache negative results (e.g., 404 not
   found), incomplete results (e.g., 206 partial content), and responses
   to safe methods other than GET if the method's definition allows such
   caching and defines something suitable for use as a cache key.

   The default 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, most implementations simply decline



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   other methods and use only the URI as the key.

   If a request target is subject to content negotiation, its cache
   entry might consist of multiple stored responses, each differentiated
   by a secondary key for the values of the original request's selecting
   header fields (Section 2.7).

2.1.  Response Cacheability

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

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

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

   o  the "no-store" cache directive (see Section 3.2) does not appear
      in request or response header fields, and

   o  the "private" cache response directive (see Section 3.2.2 does not
      appear in the response, if the cache is shared, and

   o  the "Authorization" header field (see Section 4.1 of [Part7]) does
      not appear in the request, if the cache is shared, unless the
      response explicitly allows it (see Section 2.6), and

   o  the response either:

      *  contains an Expires header field (see Section 3.3), or

      *  contains a max-age response cache directive (see
         Section 3.2.2), or

      *  contains a s-maxage response cache directive and the cache is
         shared, or

      *  contains a Cache Control Extension (see Section 3.2.3) that
         allows it to be cached, or

      *  has a status code that can be served with heuristic freshness
         (see Section 2.3.1.1).

   Note that any of the requirements listed above can be overridden by a
   cache-control extension; see Section 3.2.3.

   In this context, a cache has "understood" a request method or a
   response status code if it recognizes it and implements any cache-
   specific behavior.



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   Note that, in normal operation, most 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.

   A response message is considered complete when all of the octets
   indicated by the message framing ([Part1]) are received prior to the
   connection being closed.  If the request is GET, the response status
   is 200 (OK), and the entire response header block 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 ([Part5]) and combining the successful
   response with the stored entry, as defined in Section 2.8.  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.

2.2.  Constructing Responses from Caches

   For a presented request, a cache MUST NOT return a stored response,
   unless:

   o  The presented effective request URI (Section 4.3 of [Part1]) 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 2.7), and

   o  the presented request and stored response are free from directives
      that would prevent its use (see Section 3.2 and Section 3.4), and

   o  the stored response is either:

      *  fresh (see Section 2.3), or





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      *  allowed to be served stale (see Section 2.3.3), or

      *  successfully validated (see Section 2.4).

   Note that any of the requirements listed above can be overridden by a
   cache-control extension; see Section 3.2.3.

   When a stored response is used to satisfy a request without
   validation, a cache MUST include a single Age header field
   (Section 3.1) in the response with a value equal to the stored
   response's current_age; see Section 2.3.2.

   A cache MUST write through requests with methods that are unsafe
   (Section 7.1.1 of [Part2]) to the origin server; i.e., a cache must
   not 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 2.5.

   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 a 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 on every use.  A cache,
   especially a shared cache, SHOULD use a mechanism, such as NTP
   [RFC1305], to synchronize its clock with a reliable external
   standard.

2.3.  Freshness Model

   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 3.3) or the max-age response
   cache directive (Section 3.2.2).  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



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   normally validate the cached response before reusing it for
   subsequent requests (see Section 2.3.3).

   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.

   The calculation to determine if a response is fresh is:

      response_is_fresh = (freshness_lifetime > current_age)

   The freshness_lifetime is defined in Section 2.3.1; the current_age
   is defined in Section 2.3.2.

   Additionally, clients might need to influence freshness calculation.
   They can do this using several request cache directives, with the
   effect of either increasing or loosening constraints on freshness.
   See Section 3.2.1.

   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 4 for an explanation of the difference between
   caches and history mechanisms.

2.3.1.  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 cache directive
      (Section 3.2.2) is present, use its value, or

   o  If the max-age response cache directive (Section 3.2.2) is
      present, use its value, or

   o  If the Expires response header field (Section 3.3) 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 2.3.1.1.

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




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2.3.1.1.  Calculating Heuristic Freshness

   If no explicit expiration time is present in a stored response that
   has a status code whose definition allows heuristic freshness to be
   used (including the following in Section 8 of [Part2]: 200, 203, 206,
   300, 301 and 410), a cache MAY calculate a heuristic expiration time.
   A cache MUST NOT use heuristics to determine freshness for responses
   with status codes that do not explicitly allow it.

   When a heuristic is used to calculate freshness lifetime, a cache
   SHOULD attach a Warning header field with a 113 warn-code to the
   response if its current_age is more than 24 hours and such a warning
   is not already present.

   Also, if the response has a Last-Modified header field (Section 2.2
   of [Part4]), a cache SHOULD NOT use a heuristic expiration value that
   is more than some fraction of the interval since that time.  A
   typical setting of this fraction might be 10%.

      Note: RFC 2616 ([RFC2616], Section 13.9) required that caches do
      not calculate heuristic freshness for URIs with query components
      (i.e., those containing '?').  In practice, this has not been
      widely implemented.  Therefore, servers are encouraged to send
      explicit directives (e.g., Cache-Control: no-cache) if they wish
      to preclude caching.

2.3.2.  Calculating Age

   HTTP/1.1 uses the Age header field to convey the estimated age of the
   response message when obtained from a cache.  The Age field value is
   the cache's estimate of the amount of time 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 3.1), in a form appropriate for arithmetic operation; or
      0, if not available.

   date_value

      HTTP/1.1 requires origin servers to send a Date header field, if
      possible, with every response, giving the time at which the



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      response was generated.  The term "date_value" denotes the value
      of the Date header field, in a form appropriate for arithmetic
      operations.  See Section 9.3 of [Part1] 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 cache SHOULD use NTP ([RFC1305])
      or some similar protocol to synchronize its clocks to a globally
      accurate time standard.

   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.  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);

   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.



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

   Additional rules for requirements on parsing and encoding of dates
   and other potential problems with date encodings include:

   o  HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date
      which appears to be more than 50 years in the future is in fact in
      the past (this helps solve the "year 2000" problem).

   o  Although all date formats are specified to be case-sensitive,
      recipients SHOULD match day, week and timezone names case-
      insensitively.

   o  An HTTP/1.1 implementation MAY internally represent a parsed
      Expires date as earlier than the proper value, but MUST NOT
      internally represent a parsed Expires date as later than the
      proper value.

   o  All expiration-related calculations MUST be done in GMT.  The
      local time zone MUST NOT influence the calculation or comparison
      of an age or expiration time.

   o  If an HTTP header field incorrectly carries a date value with a
      time zone other than GMT, it MUST be converted into GMT using the
      most conservative possible conversion.

2.3.3.  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 2.3.

   A cache MUST NOT return 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 3.2.2).

   A cache SHOULD NOT return 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 3.2.1).

   A cache SHOULD append a Warning header field with the 110 warn-code
   (see Section 3.6) to stale responses.  Likewise, a cache SHOULD add
   the 112 warn-code to stale responses if the cache is disconnected.




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   If a cache receives a first-hand response (either an entire response,
   or a 304 (Not Modified) response) that it would normally forward to
   the requesting client, and the received response is no longer fresh,
   the cache SHOULD forward it to the requesting client without adding a
   new Warning (but without removing any existing Warning header
   fields).  A cache SHOULD NOT attempt to validate a response simply
   because that response became stale in transit.

2.4.  Validation Model

   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 2.7), it can use the conditional
   request mechanism [Part4] in the forwarded request to give the origin
   server an opportunity to both select a valid stored response to be
   used, and to update it.  This process is known as "validating" or
   "revalidating" the stored response.

   When sending such a conditional request, a cache SHOULD add an If-
   Modified-Since header field whose value is that of the Last-Modified
   header field from the selected (see Section 2.7) stored response, if
   available.

   Additionally, a cache SHOULD add an If-None-Match header field whose
   value is that of the ETag header field(s) from all responses stored
   for the requested URI, if present.  However, if any of the stored
   responses contains only partial content, the cache SHOULD NOT include
   its entity-tag in the If-None-Match header field unless the request
   is for a range that would be fully satisfied by that stored response.

   A 304 (Not Modified) response status code indicates that the stored
   response can be updated and reused; see Section 2.9.

   A full response (i.e., one with a response body) indicates that none
   of the stored responses nominated in the conditional request is
   suitable.  Instead, a cache SHOULD use the full response to satisfy
   the request and MAY replace the stored response(s).

   If a cache receives a 5xx response while attempting to validate a
   response, it MAY either forward this response to the requesting
   client, or act as if the server failed to respond.  In the latter
   case, it MAY return a previously stored response (see Section 2.3.3).

2.5.  Request Methods that Invalidate

   Because unsafe request methods (Section 7.1.1 of [Part2]) 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



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   up-to-date.

   A cache MUST invalidate the effective Request URI (Section 4.3 of
   [Part1]) as well as the URI(s) in the Location and Content-Location
   header fields (if present) when a non-error response to a request
   with an unsafe method is received.

   However, a cache MUST NOT invalidate a URI from a Location or
   Content-Location header field if the host part of that URI differs
   from the host part in the effective request URI (Section 4.3 of
   [Part1]).  This helps prevent denial of service attacks.

   A cache SHOULD invalidate the effective request URI (Section 4.3 of
   [Part1]) 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 or 3xx 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
   returned in response to a subsequent request.

   Note that this does not guarantee that all appropriate responses are
   invalidated.  For example, the request that caused the change at the
   origin server might not have gone through the cache where a response
   is stored.

2.6.  Shared Caching of Authenticated Responses

   A shared cache MUST NOT use a cached response to a request with an
   Authorization header field (Section 4.1 of [Part7]) 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 3.2.2) have such an effect: must-revalidate,
   public, s-maxage.

   Note that cached responses that contain the "must-revalidate" and/or
   "s-maxage" response directives are not allowed to be served stale
   (Section 2.3.3) by shared caches.  In particular, a response with
   either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
   satisfy a subsequent request without revalidating it on the origin
   server.







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2.7.  Caching Negotiated Responses

   When a cache receives a request that can be satisfied by a stored
   response that has a Vary header field (Section 3.5), 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 3.2 of [Part1])

   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., re-ordering 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, and
   subsequent requests to that resource can only be properly interpreted
   by the origin server.

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

   If multiple selected responses are available, the most recent
   response (as determined by the Date header field) is used; see
   Section 2.2.

   If no selected response is available, the cache MAY forward the
   presented request to the origin server in a conditional request; see
   Section 2.4.

2.8.  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 ([Part5]).  After several such transfers, a cache



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   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 4 of [Part5].

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

   o  delete any Warning header fields in the stored response with warn-
      code 1xx (see Section 3.6);

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

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

2.9.  Freshening Responses

   When a cache receives a 304 (Not Modified) response and already has
   one or more stored 200 (OK) responses for the same cache key, the
   cache needs to identify which of the stored responses are updated by
   this new response and then update the stored response(s) with the new
   information provided in the 304 response.

   o  If the new response contains a strong validator, then that strong
      validator identifies the selected representation.  All of the
      stored responses with the same strong validator are selected.  If
      none of the stored responses contain the same strong validator,
      then this new response corresponds to a new selected
      representation and MUST NOT update the existing 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 selected.

   o  If the new response does not include any form of validator, there
      is only one stored response, and that stored response also lacks a
      validator, then that stored response is selected.

   If a stored response is selected for update, the cache MUST:

   o  delete any Warning header fields in the stored response with warn-
      code 1xx (see Section 3.6);





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

   o  use other header fields provided in the 304 response to replace
      all instances of the corresponding header fields in the stored
      response.

3.  Header Field Definitions

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

3.1.  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 2.3.2.

     Age = delta-seconds

   Age field-values are non-negative integers, representing time in
   seconds (see Section 1.5).

   The presence of an Age header field in a response implies that a
   response is not first-hand.  However, the converse is not true, since
   HTTP/1.0 caches might not implement the Age header field.

3.2.  Cache-Control

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

   A cache MUST obey the requirements of the Cache-Control directives
   defined in this section.  See Section 3.2.3 for information about how
   Cache-Control directives defined elsewhere are handled.

      Note: HTTP/1.0 caches might not implement Cache-Control and might
      only implement Pragma: no-cache (see Section 3.4).

   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.




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     Cache-Control   = 1#cache-directive

     cache-directive = cache-request-directive
        / cache-response-directive

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

3.2.1.  Request Cache-Control Directives

     cache-request-directive =
          "no-cache"
        / "no-store"
        / "max-age" "=" delta-seconds
        / "max-stale" [ "=" delta-seconds ]
        / "min-fresh" "=" delta-seconds
        / "no-transform"
        / "only-if-cached"
        / cache-extension

   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.

   no-store

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

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

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

   max-age

      The max-age request directive indicates that the client is
      unwilling to accept a response whose age is greater than the



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      specified number of seconds.  Unless the max-stale request
      directive is also present, the client is not willing to accept a
      stale response.

   max-stale

      The max-stale request directive indicates that the client is
      willing to accept a response that has exceeded its expiration
      time.  If max-stale is assigned a value, then the client is
      willing to accept a response that has exceeded its expiration time
      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.

   min-fresh

      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.

   no-transform

      The no-transform request directive indicates that an intermediary
      (whether or not it implements a cache) MUST NOT change the
      Content-Encoding, Content-Range or Content-Type request header
      fields, nor the request representation.

   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.












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3.2.2.  Response Cache-Control Directives

     cache-response-directive =
          "public"
        / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
        / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
        / "no-store"
        / "no-transform"
        / "must-revalidate"
        / "proxy-revalidate"
        / "max-age" "=" delta-seconds
        / "s-maxage" "=" delta-seconds
        / cache-extension

   public

      The public response directive indicates that a response whose
      associated request contains an 'Authentication' header MAY be
      stored (see Section 2.6).

   private

      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.

      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.

      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 implementations as if an unqualified
      private directive was received; i.e., the special handling for the
      qualified form is not widely implemented.

   no-cache

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




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      If the no-cache 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 cache MUST NOT
      send the specified field-name(s) in the response to a subsequent
      request without successful validation on 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.

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

   no-store

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

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

   must-revalidate

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

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

      A server SHOULD send the must-revalidate directive if and only if
      failure to validate a request on the representation could result
      in incorrect operation, such as a silently unexecuted financial
      transaction.




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   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.

   max-age

      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.

   s-maxage

      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.

   no-transform

      The no-transform response directive indicates that an intermediary
      (regardless of whether it implements a cache) MUST NOT change the
      Content-Encoding, Content-Range or Content-Type response header
      fields, nor the response representation.

3.2.3.  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.
   Informational extensions (those that do not require a change in cache
   behavior) can be added without changing the semantics of other
   directives.  Behavioral extensions are designed to work by acting as
   modifiers to the existing base of cache directives.  Both the new
   directive and the standard directive are supplied, such that
   applications that do not understand the new directive will default to
   the behavior specified by the standard directive, and those that
   understand the new directive will recognize it as modifying the
   requirements associated with the standard directive.  In this way,
   extensions to the cache-control directives can be made without
   requiring changes to the base protocol.

   This extension mechanism depends on an HTTP cache obeying all of the
   cache-control directives defined for its native HTTP-version, obeying
   certain extensions, and ignoring all directives that it does not
   understand.




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   For example, consider a hypothetical new response directive called
   "community" that acts as a modifier to the private directive.  We
   define this new directive to mean that, in addition to any private
   cache, any cache that is shared only by members of the community
   named within its value may 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 seeing this header field will act correctly even if the cache
   does not understand the community cache-extension, since it will also
   see and understand the private directive and thus default to the safe
   behavior.

   A cache MUST ignore unrecognized cache directives; it is assumed that
   any cache directive likely to be unrecognized by an HTTP/1.1 cache
   will be combined with standard directives (or the response's default
   cacheability) such that the cache behavior will remain minimally
   correct even if the cache does not understand the extension(s).

   The HTTP Cache Directive Registry defines the name space for the
   cache directives.

   A registration MUST include the following fields:

   o  Cache Directive Name

   o  Pointer to specification text

   Values to be added to this name space are subject to IETF review
   ([RFC5226], Section 4.1).

   The registry itself is maintained at
   <http://www.iana.org/assignments/http-cache-directives>.

3.3.  Expires

   The "Expires" header field gives the date/time after which the
   response is considered stale.  See Section 2.3 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 field-value is an absolute date and time as defined by HTTP-date
   in Section 6.1 of [Part1]; a sender MUST use the rfc1123-date format.



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     Expires = HTTP-date

   For example

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

   A cache MUST treat other invalid date formats, especially including
   the value "0", as in the past (i.e., "already expired").

      Note: If a response includes a Cache-Control field with the max-
      age directive (see Section 3.2.2), that directive overrides the
      Expires field.  Likewise, the s-maxage directive overrides Expires
      in shared caches.

   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 most caches will evict a response far sooner
   than that.  Therefore, senders ought not produce them.

3.4.  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 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 is not present in a request, the no-
   cache request pragma-directive MUST have the same effect on caches as
   if "Cache-Control: no-cache" were present (see Section 3.2.1).

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






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


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

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

3.5.  Vary

   The "Vary" header field conveys the set of header fields that were
   used to select the representation.

   Caches use this information, in part, to determine whether a stored
   response can be used to satisfy a given request; see Section 2.7.
   determines, while the response is fresh, whether a cache is permitted
   to use the response to reply to a subsequent request without
   validation; see Section 2.7.

   In uncacheable or stale responses, the Vary field value advises the
   user agent about the criteria that were used to select the
   representation.

     Vary = "*" / 1#field-name

   The set of header fields named by the Vary field value is known as
   the selecting header fields.

   A server SHOULD include a Vary header field with any cacheable
   response that is subject to server-driven negotiation.  Doing so
   allows a cache to properly interpret future requests on that resource
   and informs the user agent about the presence of negotiation on that
   resource.  A server MAY include a Vary header field with a non-
   cacheable response that is subject to server-driven negotiation,
   since this might provide the user agent with useful information about
   the dimensions over which the response varies at the time of the
   response.

   A Vary field value of "*" signals that unspecified parameters not
   limited to the header fields (e.g., the network address of the
   client), play a role in the selection of the response representation;
   therefore, a cache cannot determine whether this response is



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   appropriate.  A proxy MUST NOT generate the "*" value.

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

3.6.  Warning

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

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

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

     Warning       = 1#warning-value

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

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

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

   When this occurs, the user agent SHOULD inform the user of as many of
   them as possible, in the order that they appear in the response.

   Systems that generate multiple Warning header fields SHOULD order
   them with this user agent behavior in mind.  New Warning header
   fields SHOULD be added after any existing Warning headers fields.

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



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

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

   If an implementation sends a message with one or more Warning header
   fields to a receiver whose version is HTTP/1.0 or lower, then the
   sender MUST include in each warning-value a warn-date that matches
   the Date header field in the message.

   If a system receives a message with a warning-value that includes a
   warn-date, and that warn-date is different from the Date value in the
   response, then that warning-value MUST be deleted from the message
   before storing, forwarding, or using it. (preventing the consequences
   of naive caching of Warning header fields.)  If all of the warning-
   values are deleted for this reason, the Warning header field MUST be
   deleted as well.

   The following warn-codes are defined by this specification, each with
   a recommended warn-text in English, and a description of its meaning.

   110 Response is stale

      A cache SHOULD include this whenever the returned response is
      stale.

   111 Revalidation failed

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

   112 Disconnected operation

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

   113 Heuristic expiration

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



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   199 Miscellaneous warning

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

   214 Transformation applied

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

   299 Miscellaneous persistent warning

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

4.  History Lists

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

   The freshness model (Section 2.3) does not necessarily apply to
   history mechanisms.  I.e., a history mechanism can display a previous
   representation even if it has expired.

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

5.  IANA Considerations

5.1.  Cache Directive Registry

   The registration procedure for HTTP Cache Directives is defined by
   Section 3.2.3 of this document.

   The HTTP Cache Directive Registry shall be created at
   <http://www.iana.org/assignments/http-cache-directives> and be
   populated with the registrations below:







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   +------------------------+------------------------------+
   | Cache Directive        | Reference                    |
   +------------------------+------------------------------+
   | max-age                | Section 3.2.1, Section 3.2.2 |
   | max-stale              | Section 3.2.1                |
   | min-fresh              | Section 3.2.1                |
   | must-revalidate        | Section 3.2.2                |
   | no-cache               | Section 3.2.1, Section 3.2.2 |
   | no-store               | Section 3.2.1, Section 3.2.2 |
   | no-transform           | Section 3.2.1, Section 3.2.2 |
   | only-if-cached         | Section 3.2.1                |
   | private                | Section 3.2.2                |
   | proxy-revalidate       | Section 3.2.2                |
   | public                 | Section 3.2.2                |
   | s-maxage               | Section 3.2.2                |
   | stale-if-error         | [RFC5861], Section 4         |
   | stale-while-revalidate | [RFC5861], Section 3         |
   +------------------------+------------------------------+

5.2.  Header Field Registration

   The Message Header Field Registry located at <http://www.iana.org/
   assignments/message-headers/message-header-index.html> shall be
   updated with the permanent registrations below (see [RFC3864]):

   +-------------------+----------+----------+-------------+
   | Header Field Name | Protocol | Status   | Reference   |
   +-------------------+----------+----------+-------------+
   | Age               | http     | standard | Section 3.1 |
   | Cache-Control     | http     | standard | Section 3.2 |
   | Expires           | http     | standard | Section 3.3 |
   | Pragma            | http     | standard | Section 3.4 |
   | Vary              | http     | standard | Section 3.5 |
   | Warning           | http     | standard | Section 3.6 |
   +-------------------+----------+----------+-------------+

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

6.  Security Considerations

   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.



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7.  Acknowledgments

   See Section 12 of [Part1].

8.  References

8.1.  Normative References

   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
              and Message Parsing", draft-ietf-httpbis-p1-messaging-16
              (work in progress), August 2011.

   [Part2]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 2: Message
              Semantics", draft-ietf-httpbis-p2-semantics-16 (work in
              progress), August 2011.

   [Part4]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional
              Requests", draft-ietf-httpbis-p4-conditional-16 (work in
              progress), August 2011.

   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
              Partial Responses", draft-ietf-httpbis-p5-range-16 (work
              in progress), August 2011.

   [Part7]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication",
              draft-ietf-httpbis-p7-auth-16 (work in progress),
              August 2011.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

8.2.  Informative References

   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
              Specification, Implementation", RFC 1305, March 1992.



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   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

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

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

Appendix A.  Changes from RFC 2616

   Make the specified age calculation algorithm less conservative.
   (Section 2.3.2)

   Remove requirement to consider Content-Location in successful
   responses in order to determine the appropriate response to use.
   (Section 2.4)

   Clarify denial of service attack avoidance requirement.
   (Section 2.5)

   Change ABNF productions for header fields to only define the field
   value.  (Section 3)

   Do not mention RFC 2047 encoding and multiple languages in Warning
   header fields anymore, as these aspects never were implemented.
   (Section 3.6)

Appendix B.  Collected ABNF

   Age = delta-seconds

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

   Expires = HTTP-date

   HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>

   OWS = <OWS, defined in [Part1], Section 1.2.2>





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

   Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
    ) )

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

   cache-directive = cache-request-directive / cache-response-directive
   cache-extension = token [ "=" ( token / quoted-string ) ]
   cache-request-directive = "no-cache" / "no-store" / ( "max-age="
    delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
    "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
    cache-extension
   cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
    OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
    "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
    field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
    "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
    ) / ( "s-maxage=" delta-seconds ) / cache-extension

   delta-seconds = 1*DIGIT

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

   field-name = <field-name, defined in [Part1], Section 3.2>

   port = <port, defined in [Part1], Section 2.7>
   pragma-directive = "no-cache" / extension-pragma
   pseudonym = <pseudonym, defined in [Part1], Section 9.9>

   quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>

   token = <token, defined in [Part1], Section 3.2.3>

   uri-host = <uri-host, defined in [Part1], Section 2.7>

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







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   ABNF diagnostics:

   ; Age defined but not used
   ; Cache-Control defined but not used
   ; Expires defined but not used
   ; Pragma defined but not used
   ; Vary defined but not used
   ; Warning defined but not used

Appendix C.  Change Log (to be removed by RFC Editor before publication)

C.1.  Since RFC 2616

   Extracted relevant partitions from [RFC2616].

C.2.  Since draft-ietf-httpbis-p6-cache-00

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer"
      (<http://purl.org/NET/http-errata#trailer-hop>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/12>: "Invalidation
      after Update or Delete"
      (<http://purl.org/NET/http-errata#invalidupd>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
      Informative references"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/48>: "Date reference
      typo"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/49>: "Connection
      header text"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative
      references"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/66>: "ISO-8859-1
      Reference"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative up-
      to-date references"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in
      13.2.2"

   Other changes:



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   o  Use names of RFC4234 core rules DQUOTE and HTAB (work in progress
      on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)

C.3.  Since draft-ietf-httpbis-p6-cache-01

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not
      used"

   Other changes:

   o  Get rid of duplicate BNF rule names ("host" -> "uri-host") (work
      in progress on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)

   o  Add explicit references to BNF syntax and rules imported from
      other parts of the specification.

C.4.  Since draft-ietf-httpbis-p6-cache-02

   Ongoing work on IANA Message Header Field Registration
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):

   o  Reference RFC 3984, and update header field registrations for
      header fields defined in this document.

C.5.  Since draft-ietf-httpbis-p6-cache-03

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/106>: "Vary header
      classification"

C.6.  Since draft-ietf-httpbis-p6-cache-04

   Ongoing work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

   o  Use "/" instead of "|" for alternatives.

   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
      whitespace ("OWS") and required whitespace ("RWS").

   o  Rewrite ABNFs to spell out whitespace rules, factor out header
      field value format definitions.






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C.7.  Since draft-ietf-httpbis-p6-cache-05

   This is a total rewrite of this part of the specification.

   Affected issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
      1xx Warn-Codes"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
      13.5.1 and 13.5.2"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/138>: "The role of
      Warning and Semantic Transparency in Caching"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods and
      Caching"

   In addition: Final work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

   o  Add appendix containing collected and expanded ABNF, reorganize
      ABNF introduction.

C.8.  Since draft-ietf-httpbis-p6-cache-06

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/161>: "base for
      numeric protocol elements"

   Affected issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/37>: "Vary and non-
      existant headers"

C.9.  Since draft-ietf-httpbis-p6-cache-07

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
      1xx Warn-Codes"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/167>: "Content-
      Location on 304 responses"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/169>: "private and
      no-cache CC directives with headers"



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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/187>: "RFC2047 and
      warn-text"

C.10.  Since draft-ietf-httpbis-p6-cache-08

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/147>: "serving
      negotiated responses from cache: header-specific canonicalization"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/197>: "Effect of CC
      directives on history lists"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/291>: "Cache
      Extensions can override no-store, etc."

   Affected issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/199>: Status codes
      and caching

   Partly resolved issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
      13.5.1 and 13.5.2"

C.11.  Since draft-ietf-httpbis-p6-cache-09

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/29>: "Age
      calculation"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/168>: "Clarify
      differences between / requirements for request and response CC
      directives"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/174>: "Caching
      authenticated responses"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/208>: "IANA registry
      for cache-control directives"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/211>: "Heuristic
      caching of URLs with query components"

   Partly resolved issues:




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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the
      requested resource's URI"

C.12.  Since draft-ietf-httpbis-p6-cache-10

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
      entity / representation / variant terminology"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
      removing the 'changes from 2068' sections"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
      heuristic caching for new status codes"

   o  Clean up TODOs and prose in "Combining Responses."

C.13.  Since draft-ietf-httpbis-p6-cache-11

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/204>: "Text about
      clock requirement for caches belongs in p6"

C.14.  Since draft-ietf-httpbis-p6-cache-12

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header
      Classification"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/268>: "Clarify
      'public'"

C.15.  Since draft-ietf-httpbis-p6-cache-13

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
      ABNFs for header fields"

C.16.  Since draft-ietf-httpbis-p6-cache-14

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/38>: "Mismatch Vary"




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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/235>: "Cache
      Invalidation only happens upon successful responses"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/282>: "Recommend
      minimum sizes for protocol elements"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/289>: "Proxies don't
      'understand' methods"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/291>: "Cache
      Extensions can override no-store, etc."

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/292>: "Pragma"

C.17.  Since draft-ietf-httpbis-p6-cache-15

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/290>: "Motivate one-
      year limit for Expires"

Index

   A
      age  6
      Age header field  20

   C
      cache  5
      Cache Directives
         max-age  21, 25
         max-stale  22
         min-fresh  22
         must-revalidate  24
         no-cache  21, 23
         no-store  21, 24
         no-transform  22, 25
         only-if-cached  22
         private  23
         proxy-revalidate  25
         public  23
         s-maxage  25
      cache entry  8
      cache key  8
      Cache-Control header field  20
      cacheable  5

   E



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      Expires header field  26
      explicit expiration time  6

   F
      first-hand  6
      fresh  6
      freshness lifetime  6

   G
      Grammar
         Age  20
         Cache-Control  21
         cache-extension  21
         cache-request-directive  21
         cache-response-directive  23
         delta-seconds  8
         Expires  27
         extension-pragma  27
         Pragma  27
         pragma-directive  27
         Vary  28
         warn-agent  29
         warn-code  29
         warn-date  29
         warn-text  29
         Warning  29
         warning-value  29

   H
      Header Fields
         Age  20
         Cache-Control  20
         Expires  26
         Pragma  27
         Vary  28
         Warning  29
      heuristic expiration time  6

   M
      max-age
         Cache Directive  21, 25
      max-stale
         Cache Directive  22
      min-fresh
         Cache Directive  22
      must-revalidate
         Cache Directive  24




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   N
      no-cache
         Cache Directive  21, 23
      no-store
         Cache Directive  21, 24
      no-transform
         Cache Directive  22, 25

   O
      only-if-cached
         Cache Directive  22

   P
      Pragma header field  27
      private
         Cache Directive  23
      private cache  5
      proxy-revalidate
         Cache Directive  25
      public
         Cache Directive  23

   S
      s-maxage
         Cache Directive  25
      shared cache  5
      stale  6
      strong validator  7

   V
      validator  6
         strong  7
      Vary header field  28

   W
      Warning header field  29

Authors' Addresses

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

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




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   Jim Gettys
   Alcatel-Lucent Bell Labs
   21 Oak Knoll Road
   Carlisle, MA  01741
   USA

   EMail: jg@freedesktop.org
   URI:   http://gettys.wordpress.com/


   Jeffrey C. Mogul
   Hewlett-Packard Company
   HP Labs, Large Scale Systems Group
   1501 Page Mill Road, MS 1177
   Palo Alto, CA  94304
   USA

   EMail: JeffMogul@acm.org


   Henrik Frystyk Nielsen
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052
   USA

   EMail: henrikn@microsoft.com


   Larry Masinter
   Adobe Systems Incorporated
   345 Park Ave
   San Jose, CA  95110
   USA

   EMail: LMM@acm.org
   URI:   http://larry.masinter.net/


   Paul J. Leach
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052

   EMail: paulle@microsoft.com






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   Tim Berners-Lee
   World Wide Web Consortium
   MIT Computer Science and Artificial Intelligence Laboratory
   The Stata Center, Building 32
   32 Vassar Street
   Cambridge, MA  02139
   USA

   EMail: timbl@w3.org
   URI:   http://www.w3.org/People/Berners-Lee/


   Yves Lafon (editor)
   World Wide Web Consortium
   W3C / ERCIM
   2004, rte des Lucioles
   Sophia-Antipolis, AM  06902
   France

   EMail: ylafon@w3.org
   URI:   http://www.raubacapeu.net/people/yves/


   Mark Nottingham (editor)

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


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

   Phone: +49 251 2807760
   Fax:   +49 251 2807761
   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/












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