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HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2616 (if approved)                              Y. Lafon, Ed.
Intended status: Standards Track                                     W3C
Expires: January 17, 2013                             M. Nottingham, Ed.
                                                               Rackspace
                                                         J. Reschke, Ed.
                                                              greenbytes
                                                           July 16, 2012


                       HTTP/1.1, part 6: Caching
                     draft-ietf-httpbis-p6-cache-20

Abstract

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypertext information
   systems.  This document 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 takes 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
   <http://tools.ietf.org/wg/httpbis/>.

   The changes in this draft are summarized in Appendix D.1.

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



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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 17, 2013.

Copyright Notice

   Copyright (c) 2012 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Conformance and Error Handling . . . . . . . . . . . . . .  6
     1.4.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  7
       1.4.1.  Delta Seconds  . . . . . . . . . . . . . . . . . . . .  7
   2.  Overview of Cache Operation  . . . . . . . . . . . . . . . . .  7
   3.  Storing Responses in Caches  . . . . . . . . . . . . . . . . .  8
     3.1.  Storing Incomplete Responses . . . . . . . . . . . . . . .  9
     3.2.  Storing Responses to Authenticated Requests  . . . . . . .  9
   4.  Constructing Responses from Caches . . . . . . . . . . . . . . 10
     4.1.  Freshness Model  . . . . . . . . . . . . . . . . . . . . . 11
       4.1.1.  Calculating Freshness Lifetime . . . . . . . . . . . . 12
       4.1.2.  Calculating Heuristic Freshness  . . . . . . . . . . . 12
       4.1.3.  Calculating Age  . . . . . . . . . . . . . . . . . . . 13



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       4.1.4.  Serving Stale Responses  . . . . . . . . . . . . . . . 15
     4.2.  Validation Model . . . . . . . . . . . . . . . . . . . . . 16
       4.2.1.  Freshening Responses with 304 Not Modified . . . . . . 16
     4.3.  Using Negotiated Responses . . . . . . . . . . . . . . . . 17
     4.4.  Combining Partial Content  . . . . . . . . . . . . . . . . 18
   5.  Updating Caches with HEAD Responses  . . . . . . . . . . . . . 19
   6.  Request Methods that Invalidate  . . . . . . . . . . . . . . . 19
   7.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 20
     7.1.  Age  . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     7.2.  Cache-Control  . . . . . . . . . . . . . . . . . . . . . . 20
       7.2.1.  Request Cache-Control Directives . . . . . . . . . . . 21
       7.2.2.  Response Cache-Control Directives  . . . . . . . . . . 23
       7.2.3.  Cache Control Extensions . . . . . . . . . . . . . . . 26
     7.3.  Expires  . . . . . . . . . . . . . . . . . . . . . . . . . 28
     7.4.  Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     7.5.  Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     7.6.  Warning  . . . . . . . . . . . . . . . . . . . . . . . . . 30
       7.6.1.  110 Response is Stale  . . . . . . . . . . . . . . . . 31
       7.6.2.  111 Revalidation Failed  . . . . . . . . . . . . . . . 32
       7.6.3.  112 Disconnected Operation . . . . . . . . . . . . . . 32
       7.6.4.  113 Heuristic Expiration . . . . . . . . . . . . . . . 32
       7.6.5.  199 Miscellaneous Warning  . . . . . . . . . . . . . . 32
       7.6.6.  214 Transformation Applied . . . . . . . . . . . . . . 32
       7.6.7.  299 Miscellaneous Persistent Warning . . . . . . . . . 32
       7.6.8.  Warn Code Extensions . . . . . . . . . . . . . . . . . 32
   8.  History Lists  . . . . . . . . . . . . . . . . . . . . . . . . 33
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 33
     9.1.  Cache Directive Registry . . . . . . . . . . . . . . . . . 33
     9.2.  Warn Code Registry . . . . . . . . . . . . . . . . . . . . 34
     9.3.  Header Field Registration  . . . . . . . . . . . . . . . . 34
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 35
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 35
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 35
     12.2. Informative References . . . . . . . . . . . . . . . . . . 36
   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 36
   Appendix B.  Imported ABNF . . . . . . . . . . . . . . . . . . . . 37
   Appendix C.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 38
   Appendix D.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 39
     D.1.  Since draft-ietf-httpbis-p6-cache-19 . . . . . . . . . . . 39
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39









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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 4.1, 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 4.2) 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.  Conformance and Error Handling

   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].

   This specification targets conformance criteria according to the role
   of a participant in HTTP communication.  Hence, HTTP requirements are
   placed on senders, recipients, clients, servers, user agents,
   intermediaries, origin servers, proxies, gateways, or caches,
   depending on what behavior is being constrained by the requirement.
   See Section 2 of [Part1] for definitions of these terms.

   The verb "generate" is used instead of "send" where a requirement
   differentiates between creating a protocol element and merely
   forwarding a received element downstream.

   An implementation is considered conformant if it complies with all of
   the requirements associated with the roles it partakes in HTTP.  Note
   that SHOULD-level requirements are relevant here, unless one of the
   documented exceptions is applicable.

   This document also uses ABNF to define valid protocol elements
   (Section 1.4).  In addition to the prose requirements placed upon
   them, senders MUST NOT generate protocol elements that do not match
   the grammar defined by the ABNF rules for those protocol elements
   that are applicable to the sender's role.  If a received protocol
   element is processed, the recipient MUST be able to parse any value
   that would match the ABNF rules for that protocol element, excluding
   only those rules not applicable to the recipient's role.

   Unless noted otherwise, a recipient MAY attempt to recover a usable
   protocol element from an invalid construct.  HTTP does not define
   specific error handling mechanisms except when they have a direct
   impact on security, since different applications of the protocol
   require different error handling strategies.  For example, a Web
   browser might wish to transparently recover from a response where the
   Location header field doesn't parse according to the ABNF, whereas a
   systems control client might consider any form of error recovery to
   be dangerous.



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1.4.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234] with the list rule extension defined in Section
   1.2 of [Part1].  Appendix B describes rules imported from other
   documents.  Appendix C shows the collected ABNF with the list rule
   expanded.

1.4.1.  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 MUST 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.  Overview of 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 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, many implementations simply decline
   other methods and use only the URI as the key.




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   If a request target is subject to content negotiation, its cache
   entry might consist of multiple stored responses, each differentiated
   by a secondary key for the values of the original request's selecting
   header fields (Section 4.3).

3.  Storing Responses in Caches

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

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

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

   o  the "no-store" cache directive (see Section 7.2) does not appear
      in request or response header fields, and

   o  the "private" cache response directive (see Section 7.2.2.2) 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 3.2), and

   o  the response either:

      *  contains an Expires header field (see Section 7.3), or

      *  contains a max-age response cache directive (see
         Section 7.2.2.7), or

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

      *  contains a Cache Control Extension (see Section 7.2.3) that
         allows it to be cached, or

      *  has a status code that can be served with heuristic freshness
         (see Section 4.1.2).

   Note that any of the requirements listed above can be overridden by a
   cache-control extension; see Section 7.2.3.

   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.

   Note that, in normal operation, many caches will not store a response



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   that has neither a cache validator nor an explicit expiration time,
   as such responses are not usually useful to store.  However, caches
   are not prohibited from storing such responses.

3.1.  Storing Incomplete Responses

   A response message is considered complete when all of the octets
   indicated by the message framing ([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 4.4.  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 that is wholly within the incomplete response.  A
   cache MUST NOT send a partial response to a client without explicitly
   marking it as such using the 206 (Partial Content) status code.

3.2.  Storing Responses to Authenticated Requests

   A shared cache MUST NOT use a cached response to a request with an
   Authorization header field (Section 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 7.2.2) 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 4.1.4) by shared caches.  In particular, a response with
   either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
   satisfy a subsequent request without revalidating it on the origin
   server.







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4.  Constructing Responses from Caches

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

   o  The presented effective request URI (Section 5.5 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 4.3), and

   o  the presented request does not contain the no-cache pragma
      (Section 7.4), nor the no-cache cache directive (Section 7.2.1),
      unless the stored response is successfully validated
      (Section 4.2), and

   o  the stored response does not contain the no-cache cache directive
      (Section 7.2.2.3), unless it is successfully validated
      (Section 4.2), and

   o  the stored response is either:

      *  fresh (see Section 4.1), or

      *  allowed to be served stale (see Section 4.1.4), or

      *  successfully validated (see Section 4.2).

   Note that any of the requirements listed above can be overridden by a
   cache-control extension; see Section 7.2.3.

   When a stored response is used to satisfy a request without
   validation, a cache MUST include a single Age header field
   (Section 7.1) in the response with a value equal to the stored
   response's current_age; see Section 4.1.3.

   A cache MUST write through requests with methods that are unsafe
   (Section 2.1.1 of [Part2]) 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 6.

   When more than one suitable response is stored, a cache MUST use the



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   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.

4.1.  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 7.3) or the max-age response
   cache directive (Section 7.2.2.7).  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 the cached response before reusing it for
   subsequent requests (see Section 4.1.4).

   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 4.1.1; the current_age
   is defined in Section 4.1.3.

   Additionally, clients can influence freshness calculation -- either
   constraining it relaxing it -- by using the max-age and min-fresh
   request cache directives.  See Section 7.2.1 for details.




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

4.1.1.  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 7.2.2.8) is present, use its value, or

   o  If the max-age response cache directive (Section 7.2.2.7) is
      present, use its value, or

   o  If the Expires response header field (Section 7.3) 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 4.1.2.

   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), it is considered invalid.  Caches are encouraged to
   consider responses that have invalid freshness information to be
   stale.

4.1.2.  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 4 of [Part2]: 200 (OK), 203
   (Non-Authoritative Information), 206 (Partial Content), 300 (Multiple
   Choices), 301 (Moved Permanently) and 410 (Gone)), 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.




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   Also, if the response has a Last-Modified header field (Section 2.2
   of [Part4]), 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, servers are encouraged to send explicit
      directives (e.g., Cache-Control: no-cache) if they wish to
      preclude caching.

4.1.3.  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 7.1), 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
      response was generated.  The term "date_value" denotes the value
      of the Date header field, in a form appropriate for arithmetic
      operations.  See Section 9.10 of [Part2] 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.





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   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 SHOULD be combined as

     corrected_initial_age = max(apparent_age, corrected_age_value);

   unless the cache is confident in the value of the Age header field
   (e.g., because there are no HTTP/1.0 hops in the Via header field),
   in which case the corrected_age_value MAY be used as the
   corrected_initial_age.

   The current_age of a stored response can then be calculated by adding
   the amount of time (in seconds) since the stored response was last
   validated by the origin server to the corrected_initial_age.

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

   Additionally, to avoid common problems in date parsing:






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   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.

4.1.4.  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 4.1.

   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 7.2.2).

   A cache MUST 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 7.2.1).

   A cache SHOULD append a Warning header field with the 110 warn-code
   (see Section 7.6) to stale responses.  Likewise, a cache SHOULD add
   the 112 warn-code to stale responses if the cache is disconnected.

   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 can forward it to the requesting client without adding a
   new Warning (but without removing any existing Warning header
   fields).  A cache shouldn't attempt to validate a response simply



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   because that response became stale in transit.

4.2.  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 4.3), 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 adds an If-Modified-
   Since header field whose value is that of the Last-Modified header
   field from the selected (see Section 4.3) stored response, if
   available.

   Additionally, a cache can 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 shouldn't 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.

   Cache handling of a response to a conditional request is dependent
   upon its status code:

   o  A 304 (Not Modified) response status code indicates that the
      stored response can be updated and reused; see Section 4.2.1.

   o  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, the cache can use the full response to
      satisfy the request and MAY replace the stored response(s).

   o  However, if a cache receives a 5xx (Server Error) response while
      attempting to validate a response, it can either forward this
      response to the requesting client, or act as if the server failed
      to respond.  In the latter case, it can return a previously stored
      response (see Section 4.1.4).

4.2.1.  Freshening Responses with 304 Not Modified

   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.



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   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 7.6);

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

   o  use other header fields provided in the 304 (Not Modified)
      response to replace all instances of the corresponding header
      fields in the stored response.

4.3.  Using Negotiated Responses

   When a cache receives a request that can be satisfied by a stored
   response that has a Vary header field (Section 7.5), 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



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      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 4.

   If no selected response is available, the cache can forward the
   presented request to the origin server in a conditional request; see
   Section 4.2.

4.4.  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 ([Part5]).  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 4.2 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 7.6);

   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.






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5.  Updating Caches with HEAD Responses

   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 is used to both invalidate and update
   cached GET responses.

   If one or more stored GET responses can be selected (as per
   Section 4.3) for a HEAD request, and the Content-Length, ETag or
   Last-Modified value of a HEAD response differs from that in a
   selected GET response, the cache MUST consider that selected response
   to be stale.

   If the Content-Length, ETag and Last-Modified values of a HEAD
   response (when present) are the same as that in a selected GET
   response (as per Section 4.3), the cache SHOULD update the remaining
   header fields in the stored response using the following rules:

   o  delete any Warning header fields in the stored response with warn-
      code 1xx (see Section 7.6);

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

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

6.  Request Methods that Invalidate

   Because unsafe request methods (Section 2.1.1 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
   up-to-date.

   A cache MUST invalidate the effective Request URI (Section 5.5 of
   [Part1]) as well as the URI(s) in the Location and Content-Location
   response 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 response header field if the host part of that URI
   differs from the host part in the effective request URI (Section 5.5
   of [Part1]).  This helps prevent denial of service attacks.

   A cache MUST invalidate the effective request URI (Section 5.5 of
   [Part1]) when it receives a non-error response to a request with a
   method whose safety is unknown.



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

7.  Header Field Definitions

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

7.1.  Age

   The "Age" header field conveys the sender's estimate of the amount of
   time since the response was generated or successfully validated at
   the origin server.  Age values are calculated as specified in
   Section 4.1.3.

     Age = delta-seconds

   Age field-values are non-negative integers, representing time in
   seconds (see Section 1.4.1).

   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.

7.2.  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 7.2.3 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 7.4).

   A proxy, whether or not it implements a cache, MUST pass cache



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

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

     Cache-Control   = 1#cache-directive

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

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

7.2.1.  Request Cache-Control Directives

7.2.1.1.  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.

7.2.1.2.  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.






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7.2.1.3.  max-age

   Argument syntax:

      delta-seconds (see Section 1.4.1)

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

   Note: This directive uses the token form of the argument syntax;
   e.g., 'max-age=5', not 'max-age="5"'.  Senders SHOULD NOT use the
   quoted-string form.

7.2.1.4.  max-stale

   Argument syntax:

      delta-seconds (see Section 1.4.1)

   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.

   Note: This directive uses the token form of the argument syntax;
   e.g., 'max-stale=10', not 'max-stale="10"'.  Senders SHOULD NOT use
   the quoted-string form.

7.2.1.5.  min-fresh

   Argument syntax:

      delta-seconds (see Section 1.4.1)

   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.

   Note: This directive uses the token form of the argument syntax;
   e.g., 'min-fresh=20', not 'min-fresh="20"'.  Senders SHOULD NOT use



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   the quoted-string form.

7.2.1.6.  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.

7.2.1.7.  only-if-cached

   The "only-if-cached" request directive indicates that the client only
   wishes to obtain a stored response.  If it receives this directive, a
   cache SHOULD either respond using a stored response that is
   consistent with the other constraints of the request, or respond with
   a 504 (Gateway Timeout) status code.  If a group of caches is being
   operated as a unified system with good internal connectivity, a
   member cache MAY forward such a request within that group of caches.

7.2.2.  Response Cache-Control Directives

7.2.2.1.  public

   The "public" response directive indicates that a response whose
   associated request contains an 'Authentication' header MAY be stored
   (see Section 3.2).

7.2.2.2.  private

   Argument syntax:

      #field-name

   The "private" response directive indicates that the response message
   is intended for a single user and MUST NOT be stored by a shared
   cache.  A private cache MAY store the response.

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

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

   Note: This usage of the word "private" only controls where the



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   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.

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

7.2.2.3.  no-cache

   Argument syntax:

      #field-name

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

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

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

   Note: Many 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.

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






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7.2.2.4.  no-store

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

7.2.2.5.  must-revalidate

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

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

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

7.2.2.6.  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.

7.2.2.7.  max-age

   Argument syntax:

      delta-seconds (see Section 1.4.1)

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




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   Note: This directive uses the token form of the argument syntax;
   e.g., 'max-age=5', not 'max-age="5"'.  Senders SHOULD NOT use the
   quoted-string form.

7.2.2.8.  s-maxage

   Argument syntax:

      delta-seconds (see Section 1.4.1)

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

   Note: This directive uses the token form of the argument syntax;
   e.g., 's-maxage=10', not 's-maxage="10"'.  Senders SHOULD NOT use the
   quoted-string form.

7.2.2.9.  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.

7.2.3.  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 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 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).

   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.

   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 require IETF Review (see
   [RFC5226], Section 4.1).

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






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7.3.  Expires

   The "Expires" header field gives the date/time after which the
   response is considered stale.  See Section 4.1 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 5.1 of [Part2]; a sender MUST use the rfc1123-date format.

     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 7.2.2.7), that directive overrides the
      Expires field.  Likewise, the s-maxage directive (Section 7.2.2.8)
      overrides the Expires header fieldin 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 many caches will evict a response far sooner
   than that.  Therefore, senders ought not produce them.

   An origin server without a clock MUST NOT assign Expires values to a
   response unless these values were associated with the resource by a
   system or user with a reliable clock.  It MAY assign an Expires value
   that is known, at or before server configuration time, to be in the
   past (this allows "pre-expiration" of responses without storing
   separate Expires values for each resource).

7.4.  Pragma

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



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

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

   When the Cache-Control header field is not present in a request, the
   no-cache request pragma-directive MUST have the same effect on caches
   as if "Cache-Control: no-cache" were present (see Section 7.2.1).

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

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


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

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

7.5.  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 4.3.

   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.




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   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
   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.

7.6.  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




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   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 are encouraged
   to order them with this user agent behavior in mind.  New Warning
   header fields are added after any existing Warning header fields.

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

   o  1xx 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.

7.6.1.  110 Response is Stale

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






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7.6.2.  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.

7.6.3.  112 Disconnected Operation

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

7.6.4.  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.

7.6.5.  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.

7.6.6.  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.

7.6.7.  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.

7.6.8.  Warn Code Extensions

   The HTTP Warn Code Registry defines the name space for warn codes.

   A registration MUST include the following fields:

   o  Warn Code (3 digits)

   o  Short Description





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   o  Pointer to specification text

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

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

8.  History Lists

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

   The freshness model (Section 4.1) 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).

9.  IANA Considerations

9.1.  Cache Directive Registry

   The registration procedure for HTTP Cache Directives is defined by
   Section 7.2.3 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 7.2.1.3, Section 7.2.2.7 |
   | max-stale              | Section 7.2.1.4                  |
   | min-fresh              | Section 7.2.1.5                  |
   | must-revalidate        | Section 7.2.2.5                  |
   | no-cache               | Section 7.2.1.1, Section 7.2.2.3 |
   | no-store               | Section 7.2.1.2, Section 7.2.2.4 |
   | no-transform           | Section 7.2.1.6, Section 7.2.2.9 |
   | only-if-cached         | Section 7.2.1.7                  |
   | private                | Section 7.2.2.2                  |
   | proxy-revalidate       | Section 7.2.2.6                  |
   | public                 | Section 7.2.2.1                  |
   | s-maxage               | Section 7.2.2.8                  |
   | stale-if-error         | [RFC5861], Section 4             |
   | stale-while-revalidate | [RFC5861], Section 3             |
   +------------------------+----------------------------------+

9.2.  Warn Code Registry

   The registration procedure for HTTP Warn Codes is defined by
   Section 7.6.8 of this document.

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

   +-----------+----------------------------------+---------------+
   | Warn Code | Short Description                | Reference     |
   +-----------+----------------------------------+---------------+
   | 110       | Response is Stale                | Section 7.6.1 |
   | 111       | Revalidation Failed              | Section 7.6.2 |
   | 112       | Disconnected Operation           | Section 7.6.3 |
   | 113       | Heuristic Expiration             | Section 7.6.4 |
   | 199       | Miscellaneous Warning            | Section 7.6.5 |
   | 214       | Transformation Applied           | Section 7.6.6 |
   | 299       | Miscellaneous Persistent Warning | Section 7.6.7 |
   +-----------+----------------------------------+---------------+

9.3.  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]):






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   +-------------------+----------+----------+-------------+
   | Header Field Name | Protocol | Status   | Reference   |
   +-------------------+----------+----------+-------------+
   | Age               | http     | standard | Section 7.1 |
   | Cache-Control     | http     | standard | Section 7.2 |
   | Expires           | http     | standard | Section 7.3 |
   | Pragma            | http     | standard | Section 7.4 |
   | Vary              | http     | standard | Section 7.5 |
   | Warning           | http     | standard | Section 7.6 |
   +-------------------+----------+----------+-------------+

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

10.  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.

11.  Acknowledgments

   See Section 9 of [Part1].

12.  References

12.1.  Normative References

   [Part1]    Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "HTTP/1.1, part 1: Message Routing and Syntax"",
              draft-ietf-httpbis-p1-messaging-20 (work in progress),
              July 2012.

   [Part2]    Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "HTTP/1.1, part 2: Semantics and Payloads",
              draft-ietf-httpbis-p2-semantics-20 (work in progress),
              July 2012.

   [Part4]    Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "HTTP/1.1, part 4: Conditional Requests",
              draft-ietf-httpbis-p4-conditional-20 (work in progress),
              July 2012.

   [Part5]    Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,



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              "HTTP/1.1, part 5: Range Requests",
              draft-ietf-httpbis-p5-range-20 (work in progress),
              July 2012.

   [Part7]    Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "HTTP/1.1, part 7: Authentication",
              draft-ietf-httpbis-p7-auth-20 (work in progress),
              July 2012.

   [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.

12.2.  Informative References

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

   [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 4.1.3)

   Remove requirement to consider Content-Location in successful
   responses in order to determine the appropriate response to use.
   (Section 4.2)

   Clarify denial of service attack avoidance requirement.  (Section 6)

   Change ABNF productions for header fields to only define the field
   value.  (Section 7)



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   Do not mention RFC 2047 encoding and multiple languages in Warning
   header fields anymore, as these aspects never were implemented.
   (Section 7.6)

   Introduce Cache Directive and Warn Code Registries.  (Section 7.2.3
   and Section 7.6.8)

Appendix B.  Imported ABNF

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

   The rules below are defined in [Part1]:

     OWS           = <OWS, defined in [Part1], Section 3.2.1>
     field-name    = <field-name, defined in [Part1], Section 3.2>
     quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>
     token         = <token, defined in [Part1], Section 3.2.4>

     port          = <port, defined in [Part1], Section 2.8>
     pseudonym     = <pseudonym, defined in [Part1], Section 6.2>
     uri-host      = <uri-host, defined in [Part1], Section 2.8>

   The rules below are defined in other parts:

     HTTP-date     = <HTTP-date, defined in [Part2], Section 5.1>





















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Appendix C.  Collected ABNF

   Age = delta-seconds

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

   Expires = HTTP-date

   HTTP-date = <HTTP-date, defined in [Part2], Section 5.1>

   OWS = <OWS, defined in [Part1], Section 3.2.1>

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

   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 2.8>
   pragma-directive = "no-cache" / extension-pragma
   pseudonym = <pseudonym, defined in [Part1], Section 6.2>

   quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>

   token = <token, defined in [Part1], Section 3.2.4>

   uri-host = <uri-host, defined in [Part1], Section 2.8>

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

   Changes up to the first Working Group Last Call draft are summarized
   in <http://trac.tools.ietf.org/html/
   draft-ietf-httpbis-p6-cache-19#appendix-C>.

D.1.  Since draft-ietf-httpbis-p6-cache-19

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/307>: "untangle
      Cache-Control ABNF"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/353>: "Multiple
      values in Cache-Control header fields"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/355>: "Case
      sensitivity of header fields in CC values"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/356>: "Spurious
      'MAYs'"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/360>: "enhance
      considerations for new cache control directives"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/361>: "ABNF
      requirements for recipients"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/368>: "note
      introduction of new IANA registries as normative changes"

Index

   1
      110 Response is Stale (warn code)  31
      111 Revalidation Failed (warn code)  32
      112 Disconnected Operation (warn code)  32
      113 Heuristic Expiration (warn code)  32
      199 Miscellaneous Warning (warn code)  32

   2
      214 Transformation Applied (warn code)  32
      299 Miscellaneous Persistent Warning (warn code)  32

   A
      age  5
      Age header field  20




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   C
      cache  4
      Cache Directives
         max-age  22, 25
         max-stale  22
         min-fresh  22
         must-revalidate  25
         no-cache  21, 24
         no-store  21, 25
         no-transform  23, 26
         only-if-cached  23
         private  23
         proxy-revalidate  25
         public  23
         s-maxage  26
      cache entry  7
      cache key  7
      Cache-Control header field  20
      cacheable  4

   E
      Expires header field  28
      explicit expiration time  5

   F
      first-hand  5
      fresh  5
      freshness lifetime  5

   G
      Grammar
         Age  20
         Cache-Control  21
         cache-directive  21
         delta-seconds  7
         Expires  28
         extension-pragma  29
         Pragma  29
         pragma-directive  29
         Vary  29
         warn-agent  30
         warn-code  30
         warn-date  30
         warn-text  30
         Warning  30
         warning-value  30

   H



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      Header Fields
         Age  20
         Cache-Control  20
         Expires  28
         Pragma  28
         Vary  29
         Warning  30
      heuristic expiration time  5

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

   N
      no-cache
         Cache Directive  21, 24
      no-store
         Cache Directive  21, 25
      no-transform
         Cache Directive  23, 26

   O
      only-if-cached
         Cache Directive  23

   P
      Pragma header field  28
      private
         Cache Directive  23
      private cache  4
      proxy-revalidate
         Cache Directive  25
      public
         Cache Directive  23

   S
      s-maxage
         Cache Directive  26
      shared cache  4
      stale  5
      strong validator  6




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   V
      validator  5
         strong  6
      Vary header field  29

   W
      Warn Codes
         110 Response is Stale  31
         111 Revalidation Failed  32
         112 Disconnected Operation  32
         113 Heuristic Expiration  32
         199 Miscellaneous Warning  32
         214 Transformation Applied  32
         299 Miscellaneous Persistent Warning  32
      Warning header field  30

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/


   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)
   Rackspace

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







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

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











































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