source: draft-ietf-httpbis/03/draft-ietf-httpbis-p6-cache-03.txt @ 1647

Network Working Group                                   R. Fielding, Ed.
Internet-Draft                                              Day Software
Obsoletes: 2616 (if approved)                                  J. Gettys
Intended status: Standards Track                    One Laptop per Child
Expires: December 19, 2008                                      J. Mogul
                                                                      HP
                                                              H. Frystyk
                                                               Microsoft
                                                             L. Masinter
                                                           Adobe Systems
                                                                P. Leach
                                                               Microsoft
                                                          T. Berners-Lee
                                                                 W3C/MIT
                                                           Y. Lafon, Ed.
                                                                     W3C
                                                         J. Reschke, Ed.
                                                              greenbytes
                                                           June 17, 2008


                       HTTP/1.1, part 6: Caching
                     draft-ietf-httpbis-p6-cache-03

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on December 19, 2008.



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Abstract

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypermedia 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).  The current issues list is
   at <http://www.tools.ietf.org/wg/httpbis/trac/report/11> and related
   documents (including fancy diffs) can be found at
   <http://www.tools.ietf.org/wg/httpbis/>.

   The changes in this draft are summarized in Appendix B.4.































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
     1.3.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  7
   2.  Notational Conventions and Generic Grammar . . . . . . . . . .  8
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Cache Correctness  . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Warnings . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.3.  Cache-control Mechanisms . . . . . . . . . . . . . . . . . 10
     3.4.  Explicit User Agent Warnings . . . . . . . . . . . . . . . 10
     3.5.  Exceptions to the Rules and Warnings . . . . . . . . . . . 11
     3.6.  Client-controlled Behavior . . . . . . . . . . . . . . . . 11
   4.  Expiration Model . . . . . . . . . . . . . . . . . . . . . . . 12
     4.1.  Server-Specified Expiration  . . . . . . . . . . . . . . . 12
     4.2.  Heuristic Expiration . . . . . . . . . . . . . . . . . . . 13
     4.3.  Age Calculations . . . . . . . . . . . . . . . . . . . . . 13
     4.4.  Expiration Calculations  . . . . . . . . . . . . . . . . . 15
     4.5.  Disambiguating Expiration Values . . . . . . . . . . . . . 16
     4.6.  Disambiguating Multiple Responses  . . . . . . . . . . . . 17
   5.  Validation Model . . . . . . . . . . . . . . . . . . . . . . . 17
   6.  Response Cacheability  . . . . . . . . . . . . . . . . . . . . 18
   7.  Constructing Responses From Caches . . . . . . . . . . . . . . 19
     7.1.  End-to-end and Hop-by-hop Headers  . . . . . . . . . . . . 19
     7.2.  Non-modifiable Headers . . . . . . . . . . . . . . . . . . 20
     7.3.  Combining Headers  . . . . . . . . . . . . . . . . . . . . 21
   8.  Caching Negotiated Responses . . . . . . . . . . . . . . . . . 22
   9.  Shared and Non-Shared Caches . . . . . . . . . . . . . . . . . 23
   10. Errors or Incomplete Response Cache Behavior . . . . . . . . . 24
   11. Side Effects of GET and HEAD . . . . . . . . . . . . . . . . . 24
   12. Invalidation After Updates or Deletions  . . . . . . . . . . . 24
   13. Write-Through Mandatory  . . . . . . . . . . . . . . . . . . . 25
   14. Cache Replacement  . . . . . . . . . . . . . . . . . . . . . . 26
   15. History Lists  . . . . . . . . . . . . . . . . . . . . . . . . 26
   16. Header Field Definitions . . . . . . . . . . . . . . . . . . . 27
     16.1. Age  . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     16.2. Cache-Control  . . . . . . . . . . . . . . . . . . . . . . 27
       16.2.1.  What is Cacheable . . . . . . . . . . . . . . . . . . 29
       16.2.2.  What May be Stored by Caches  . . . . . . . . . . . . 30
       16.2.3.  Modifications of the Basic Expiration Mechanism . . . 31
       16.2.4.  Cache Revalidation and Reload Controls  . . . . . . . 33
       16.2.5.  No-Transform Directive  . . . . . . . . . . . . . . . 35
       16.2.6.  Cache Control Extensions  . . . . . . . . . . . . . . 36
     16.3. Expires  . . . . . . . . . . . . . . . . . . . . . . . . . 37
     16.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     16.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     16.6. Warning  . . . . . . . . . . . . . . . . . . . . . . . . . 39



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   17. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 42
     17.1. Message Header Registration  . . . . . . . . . . . . . . . 42
   18. Security Considerations  . . . . . . . . . . . . . . . . . . . 42
   19. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 43
   20. References . . . . . . . . . . . . . . . . . . . . . . . . . . 43
     20.1. Normative References . . . . . . . . . . . . . . . . . . . 43
     20.2. Informative References . . . . . . . . . . . . . . . . . . 44
   Appendix A.  Compatibility with Previous Versions  . . . . . . . . 44
     A.1.  Changes from RFC 2068  . . . . . . . . . . . . . . . . . . 44
     A.2.  Changes from RFC 2616  . . . . . . . . . . . . . . . . . . 45
   Appendix B.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 45
     B.1.  Since RFC2616  . . . . . . . . . . . . . . . . . . . . . . 45
     B.2.  Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 45
     B.3.  Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 46
     B.4.  Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 46
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49
   Intellectual Property and Copyright Statements . . . . . . . . . . 52
































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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, and
   includes a number of elements intended to make caching work as well
   as possible.  Because these elements interact with each other, it is
   useful to describe the caching design of HTTP separately.  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 include a cache, though a cache cannot be used
   by a server that is acting as a tunnel.

   Caching would be useless if it did not significantly improve
   performance.  The goal of caching in HTTP/1.1 is to reuse a prior
   response message to satisfy a current request.  In some cases, the
   existing response can be reused without the need for a network
   request, reducing latency and network round-trips; we use an
   "expiration" mechanism for this purpose (see Section 4).  Even when a
   new request is required, it is often possible to reuse all or parts
   of the payload of a prior response to satisfy the request, thereby
   reducing network bandwidth usage; we use a "validation" mechanism for
   this purpose (see Section 5).

   A cache behaves in a "semantically transparent" manner, with respect
   to a particular response, when its use affects neither the requesting
   client nor the origin server, except to improve performance.  When a
   cache is semantically transparent, the client receives exactly the
   same response status and payload that it would have received had its
   request been handled directly by the origin server.

   In an ideal world, all interactions with an HTTP cache would be
   semantically transparent.  However, for some resources, semantic
   transparency is not always necessary and can be effectively traded
   for the sake of bandwidth scaling, disconnected operation, and high
   availability.  HTTP/1.1 allows origin servers, caches, and clients to
   explicitly reduce transparency when necessary.  However, because non-
   transparent operation may confuse non-expert users and might be
   incompatible with certain server applications (such as those for
   ordering merchandise), the protocol requires that transparency be
   relaxed




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   o  only by an explicit protocol-level request when relaxed by client
      or origin server

   o  only with an explicit warning to the end user when relaxed by
      cache or client

   Therefore, HTTP/1.1 provides these important elements:

   1.  Protocol features that provide full semantic transparency when
       this is required by all parties.

   2.  Protocol features that allow an origin server or user agent to
       explicitly request and control non-transparent operation.

   3.  Protocol features that allow a cache to attach warnings to
       responses that do not preserve the requested approximation of
       semantic transparency.

   A basic principle is that it must be possible for the clients to
   detect any potential relaxation of semantic transparency.

      Note: The server, cache, or client implementor might be faced with
      design decisions not explicitly discussed in this specification.
      If a decision might affect semantic transparency, the implementor
      ought to err on the side of maintaining transparency unless a
      careful and complete analysis shows significant benefits in
      breaking transparency.

1.2.  Terminology

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

   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 may be additional
      constraints on whether a cache can use the cached copy for a
      particular request.

   first-hand

      A response is first-hand if it comes directly and without
      unnecessary delay from the origin server, perhaps via one or more
      proxies.  A response is also first-hand if its validity has just
      been checked directly with the origin server.




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   explicit expiration time

      The time at which the origin server intends that an entity should
      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.

   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.

   validator

      A protocol element (e.g., an entity tag or a Last-Modified time)
      that is used to find out whether a cache entry is an equivalent
      copy of an entity.

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



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   compliant."


2.  Notational Conventions and Generic Grammar

   This specification uses the ABNF syntax defined in Section 2.1 of
   [Part1] and the core rules defined in Section 2.2 of [Part1]:
   [[abnf.dep: ABNF syntax and basic rules will be adopted from RFC
   5234, see <http://tools.ietf.org/wg/httpbis/trac/ticket/36>.]]

     DIGIT         = <DIGIT, defined in [Part1], Section 2.2>
     DQUOTE        = <DQUOTE, defined in [Part1], Section 2.2>
     SP            = <SP, defined in [Part1], Section 2.2>


     quoted-string = <quoted-string, defined in [Part1], Section 2.2>
     token         = <token, defined in [Part1], Section 2.2>

   The ABNF rules below are defined in other parts:

     field-name    = <field-name, defined in [Part1], Section 4.2>
     HTTP-date     = <HTTP-date, defined in [Part1], Section 3.3.1>
     port          = <port, defined in [Part1], Section 3.2.1>
     pseudonym     = <pseudonym, defined in [Part1], Section 8.9>
     uri-host      = <uri-host, defined in [Part1], Section 3.2.1>


3.  Overview

3.1.  Cache Correctness

   A correct cache MUST respond to a request with the most up-to-date
   response held by the cache that is appropriate to the request (see
   Sections 4.5, 4.6, and 14) which meets one of the following
   conditions:

   1.  It has been checked for equivalence with what the origin server
       would have returned by revalidating the response with the origin
       server (Section 5);

   2.  It is "fresh enough" (see Section 4).  In the default case, this
       means it meets the least restrictive freshness requirement of the
       client, origin server, and cache (see Section 16.2); if the
       origin server so specifies, it is the freshness requirement of
       the origin server alone.  If a stored response is not "fresh
       enough" by the most restrictive freshness requirement of both the
       client and the origin server, in carefully considered
       circumstances the cache MAY still return the response with the



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       appropriate Warning header (see Sections 3.5 and 16.6), unless
       such a response is prohibited (e.g., by a "no-store" cache-
       directive, or by a "no-cache" cache-request-directive; see
       Section 16.2).

   3.  It is an appropriate 304 (Not Modified), 305 (Use Proxy), or
       error (4xx or 5xx) response message.

   If the cache can not communicate with the origin server, then a
   correct cache SHOULD respond as above if the response can be
   correctly served from the cache; if not it MUST return an error or
   warning indicating that there was a communication failure.

   If a cache receives a 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 headers).  A cache
   SHOULD NOT attempt to revalidate a response simply because that
   response became stale in transit; this might lead to an infinite
   loop.  A user agent that receives a stale response without a Warning
   MAY display a warning indication to the user.

3.2.  Warnings

   Whenever a cache returns a response that is neither first-hand nor
   "fresh enough" (in the sense of condition 2 in Section 3.1), it MUST
   attach a warning to that effect, using a Warning general-header.  The
   Warning header and the currently defined warnings are described in
   Section 16.6.  The warning allows clients to take appropriate action.

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

   Warnings are assigned three digit warn-codes.  The first digit
   indicates whether the Warning MUST or MUST NOT be deleted from a
   stored cache entry after a successful revalidation:

   1xx  Warnings that describe the freshness or revalidation status of
      the response, and so MUST be deleted after a successful
      revalidation. 1xx warn-codes MAY be generated by a cache only when
      validating a cached entry.  It MUST NOT be generated by clients.

   2xx  Warnings that describe some aspect of the entity body or entity
      headers that is not rectified by a revalidation (for example, a
      lossy compression of the entity bodies) and which MUST NOT be
      deleted after a successful revalidation.



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   See Section 16.6 for the definitions of the codes themselves.

   HTTP/1.0 caches will cache all Warnings in responses, without
   deleting the ones in the first category.  Warnings in responses that
   are passed to HTTP/1.0 caches carry an extra warning-date field,
   which prevents a future HTTP/1.1 recipient from believing an
   erroneously cached Warning.

   Warnings also carry a warning text.  The text MAY be in any
   appropriate natural language (perhaps based on the client's Accept
   headers), and include an OPTIONAL indication of what character set is
   used.

   Multiple warnings MAY be attached to a response (either by the origin
   server or by a cache), including multiple warnings with the same code
   number.  For example, a server might provide the same warning with
   texts in both English and Basque.

   When multiple warnings are attached to a response, it might not be
   practical or reasonable to display all of them to the user.  This
   version of HTTP does not specify strict priority rules for deciding
   which warnings to display and in what order, but does suggest some
   heuristics.

3.3.  Cache-control Mechanisms

   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
   times and validators) are implicit directives to caches.  In some
   cases, a server or client might need to provide explicit directives
   to the HTTP caches.  We use the Cache-Control header for this
   purpose.

   The Cache-Control header allows a client or server to transmit a
   variety of directives in either requests or responses.  These
   directives typically override the default caching algorithms.  As a
   general rule, if there is any apparent conflict between header
   values, the most restrictive interpretation is applied (that is, the
   one that is most likely to preserve semantic transparency).  However,
   in some cases, cache-control directives are explicitly specified as
   weakening the approximation of semantic transparency (for example,
   "max-stale" or "public").

   The cache-control directives are described in detail in Section 16.2.

3.4.  Explicit User Agent Warnings

   Many user agents make it possible for users to override the basic
   caching mechanisms.  For example, the user agent might allow the user



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   to specify that cached entities (even explicitly stale ones) are
   never validated.  Or the user agent might habitually add "Cache-
   Control: max-stale=3600" to every request.  The user agent SHOULD NOT
   default to either non-transparent behavior, or behavior that results
   in abnormally ineffective caching, but MAY be explicitly configured
   to do so by an explicit action of the user.

   If the user has overridden the basic caching mechanisms, the user
   agent SHOULD explicitly indicate to the user whenever this results in
   the display of information that might not meet the server's
   transparency requirements (in particular, if the displayed entity is
   known to be stale).  Since the protocol normally allows the user
   agent to determine if responses are stale or not, this indication
   need only be displayed when this actually happens.  The indication
   need not be a dialog box; it could be an icon (for example, a picture
   of a rotting fish) or some other indicator.

   If the user has overridden the caching mechanisms in a way that would
   abnormally reduce the effectiveness of caches, the user agent SHOULD
   continually indicate this state to the user (for example, by a
   display of a picture of currency in flames) so that the user does not
   inadvertently consume excess resources or suffer from excessive
   latency.

3.5.  Exceptions to the Rules and Warnings

   In some cases, the operator of a cache MAY choose to configure it to
   return stale responses even when not requested by clients.  This
   decision ought not be made lightly, but may be necessary for reasons
   of availability or performance, especially when the cache is poorly
   connected to the origin server.  Whenever a cache returns a stale
   response, it MUST mark it as such (using a Warning header) enabling
   the client software to alert the user that there might be a potential
   problem.

   It also allows the user agent to take steps to obtain a first-hand or
   fresh response.  For this reason, a cache SHOULD NOT return a stale
   response if the client explicitly requests a first-hand or fresh one,
   unless it is impossible to comply for technical or policy reasons.

3.6.  Client-controlled Behavior

   While the origin server (and to a lesser extent, intermediate caches,
   by their contribution to the age of a response) are the primary
   source of expiration information, in some cases the client might need
   to control a cache's decision about whether to return a cached
   response without validating it.  Clients do this using several
   directives of the Cache-Control header.



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   A client's request MAY specify the maximum age it is willing to
   accept of an unvalidated response; specifying a value of zero forces
   the cache(s) to revalidate all responses.  A client MAY also specify
   the minimum time remaining before a response expires.  Both of these
   options increase constraints on the behavior of caches, and so cannot
   further relax the cache's approximation of semantic transparency.

   A client MAY also specify that it will accept stale responses, up to
   some maximum amount of staleness.  This loosens the constraints on
   the caches, and so might violate the origin server's specified
   constraints on semantic transparency, but might be necessary to
   support disconnected operation, or high availability in the face of
   poor connectivity.


4.  Expiration Model

4.1.  Server-Specified Expiration

   HTTP caching works best when caches can entirely avoid making
   requests to the origin server.  The primary mechanism for avoiding
   requests is for an origin server to provide an explicit expiration
   time in the future, indicating that a response MAY be used to satisfy
   subsequent requests.  In other words, a cache can return a fresh
   response without first contacting the server.

   Our expectation is that servers will assign future explicit
   expiration times to responses in the belief that the entity is not
   likely to change, in a semantically significant way, before the
   expiration time is reached.  This normally preserves semantic
   transparency, as long as the server's expiration times are carefully
   chosen.

   The expiration mechanism applies only to responses taken from a cache
   and not to first-hand responses forwarded immediately to the
   requesting client.

   If an origin server wishes to force a semantically transparent cache
   to validate every request, it MAY assign an explicit expiration time
   in the past.  This means that the response is always stale, and so
   the cache SHOULD validate it before using it for subsequent requests.
   See Section 16.2.4 for a more restrictive way to force revalidation.

   If an origin server wishes to force any HTTP/1.1 cache, no matter how
   it is configured, to validate every request, it SHOULD use the "must-
   revalidate" cache-control directive (see Section 16.2).

   Servers specify explicit expiration times using either the Expires



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   header, or the max-age directive of the Cache-Control header.

   An expiration time cannot be used to force a user agent to refresh
   its display or reload a resource; its semantics apply only to caching
   mechanisms, and such mechanisms need only check a resource's
   expiration status when a new request for that resource is initiated.
   See Section 15 for an explanation of the difference between caches
   and history mechanisms.

4.2.  Heuristic Expiration

   Since origin servers do not always provide explicit expiration times,
   HTTP caches typically assign heuristic expiration times, employing
   algorithms that use other header values (such as the Last-Modified
   time) to estimate a plausible expiration time.  The HTTP/1.1
   specification does not provide specific algorithms, but does impose
   worst-case constraints on their results.  Since heuristic expiration
   times might compromise semantic transparency, they ought to be used
   cautiously, and we encourage origin servers to provide explicit
   expiration times as much as possible.

4.3.  Age Calculations

   In order to know if a cached entry is fresh, a cache needs to know if
   its age exceeds its freshness lifetime.  We discuss how to calculate
   the latter in Section 4.4; this section describes how to calculate
   the age of a response or cache entry.

   In this discussion, we use the term "now" to mean "the current value
   of the clock at the host performing the calculation."  Hosts that use
   HTTP, but especially hosts running origin servers and caches, SHOULD
   use NTP [RFC1305] or some similar protocol to synchronize their
   clocks to a globally accurate time standard.

   HTTP/1.1 requires origin servers to send a Date header, if possible,
   with every response, giving the time at which the response was
   generated (see Section 8.3 of [Part1]).  We use the term "date_value"
   to denote the value of the Date header, in a form appropriate for
   arithmetic operations.

   HTTP/1.1 uses the Age response-header 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 revalidated 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



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   network paths.

   We use the term "age_value" to denote the value of the Age header, in
   a form appropriate for arithmetic operations.

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

   1.  now 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.  age_value, if all of the caches along the response path implement
       HTTP/1.1.

   Given that we have two independent ways to compute the age of a
   response when it is received, we can combine these as

       corrected_received_age = max(now - date_value, age_value)

   and as long as we have either nearly synchronized clocks or all-
   HTTP/1.1 paths, one gets a reliable (conservative) result.

   Because of network-imposed delays, some significant interval might
   pass between the time that a server generates a response and the time
   it is received at the next outbound cache or client.  If uncorrected,
   this delay could result in improperly low ages.

   Because the request that resulted in the returned Age value must have
   been initiated prior to that Age value's generation, we can correct
   for delays imposed by the network by recording the time at which the
   request was initiated.  Then, when an Age value is received, it MUST
   be interpreted relative to the time the request was initiated, not
   the time that the response was received.  This algorithm results in
   conservative behavior no matter how much delay is experienced.  So,
   we compute:

      corrected_initial_age = corrected_received_age
                            + (now - request_time)

   where "request_time" is the time (according to the local clock) when
   the request that elicited this response was sent.

   Summary of age calculation algorithm, when a cache receives a
   response:







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      /*
       * age_value
       *      is the value of Age: header received by the cache with
       *              this response.
       * date_value
       *      is the value of the origin server's Date: header
       * request_time
       *      is the (local) time when the cache made the request
       *              that resulted in this cached response
       * response_time
       *      is the (local) time when the cache received the
       *              response
       * now
       *      is the current (local) time
       */

      apparent_age = max(0, response_time - date_value);
      corrected_received_age = max(apparent_age, age_value);
      response_delay = response_time - request_time;
      corrected_initial_age = corrected_received_age + response_delay;
      resident_time = now - response_time;
      current_age   = corrected_initial_age + resident_time;

   The current_age of a cache entry is calculated by adding the amount
   of time (in seconds) since the cache entry was last validated by the
   origin server to the corrected_initial_age.  When a response is
   generated from a cache entry, the cache MUST include a single Age
   header field in the response with a value equal to the cache entry's
   current_age.

   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
   the lack of an Age header field in a response does not imply that the
   response is first-hand unless all caches along the request path are
   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
   the Age header field).

4.4.  Expiration Calculations

   In order to decide whether a response is fresh or stale, we need to
   compare its freshness lifetime to its age.  The age is calculated as
   described in Section 4.3; this section describes how to calculate the
   freshness lifetime, and to determine if a response has expired.  In
   the discussion below, the values can be represented in any form
   appropriate for arithmetic operations.

   We use the term "expires_value" to denote the value of the Expires
   header.  We use the term "max_age_value" to denote an appropriate



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   value of the number of seconds carried by the "max-age" directive of
   the Cache-Control header in a response (see Section 16.2.3).

   The max-age directive takes priority over Expires, so if max-age is
   present in a response, the calculation is simply:

      freshness_lifetime = max_age_value

   Otherwise, if Expires is present in the response, the calculation is:

      freshness_lifetime = expires_value - date_value

   Note that neither of these calculations is vulnerable to clock skew,
   since all of the information comes from the origin server.

   If none of Expires, Cache-Control: max-age, or Cache-Control:
   s-maxage (see Section 16.2.3) appears in the response, and the
   response does not include other restrictions on caching, the cache
   MAY compute a freshness lifetime using a heuristic.  The cache MUST
   attach Warning 113 to any response whose age is more than 24 hours if
   such warning has not already been added.

   Also, if the response does have a Last-Modified time, the heuristic
   expiration value SHOULD be no more than some fraction of the interval
   since that time.  A typical setting of this fraction might be 10%.

   The calculation to determine if a response has expired is quite
   simple:

      response_is_fresh = (freshness_lifetime > current_age)

4.5.  Disambiguating Expiration Values

   Because expiration values are assigned optimistically, it is possible
   for two caches to contain fresh values for the same resource that are
   different.

   If a client performing a retrieval receives a non-first-hand response
   for a request that was already fresh in its own cache, and the Date
   header in its existing cache entry is newer than the Date on the new
   response, then the client MAY ignore the response.  If so, it MAY
   retry the request with a "Cache-Control: max-age=0" directive (see
   Section 16.2), to force a check with the origin server.

   If a cache has two fresh responses for the same representation with
   different validators, it MUST use the one with the more recent Date
   header.  This situation might arise because the cache is pooling
   responses from other caches, or because a client has asked for a



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   reload or a revalidation of an apparently fresh cache entry.

4.6.  Disambiguating Multiple Responses

   Because a client might be receiving responses via multiple paths, so
   that some responses flow through one set of caches and other
   responses flow through a different set of caches, a client might
   receive responses in an order different from that in which the origin
   server sent them.  We would like the client to use the most recently
   generated response, even if older responses are still apparently
   fresh.

   Neither the entity tag nor the expiration value can impose an
   ordering on responses, since it is possible that a later response
   intentionally carries an earlier expiration time.  The Date values
   are ordered to a granularity of one second.

   When a client tries to revalidate a cache entry, and the response it
   receives contains a Date header that appears to be older than the one
   for the existing entry, then the client SHOULD repeat the request
   unconditionally, and include

       Cache-Control: max-age=0

   to force any intermediate caches to validate their copies directly
   with the origin server, or

       Cache-Control: no-cache

   to force any intermediate caches to obtain a new copy from the origin
   server.

   If the Date values are equal, then the client MAY use either response
   (or MAY, if it is being extremely prudent, request a new response).
   Servers MUST NOT depend on clients being able to choose
   deterministically between responses generated during the same second,
   if their expiration times overlap.


5.  Validation Model

   When a cache has a stale entry that it would like to use as a
   response to a client's request, it first has to check with the origin
   server (or possibly an intermediate cache with a fresh response) to
   see if its cached entry is still usable.  We call this "validating"
   the cache entry.

   HTTP's conditional request mechanism, defined in [Part4], is used to



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   avoid retransmitting the response payload when the cached entry is
   valid.  When a cached response includes one or more "cache
   validators," such as the field values of an ETag or Last-Modified
   header field, then a validating GET request SHOULD be made
   conditional to those field values.  The server checks the conditional
   request's validator against the current state of the requested
   resource and, if they match, the server responds with a 304 (Not
   Modified) status code to indicate that the cached response can be
   refreshed and reused without retransmitting the response payload.  If
   the validator does not match the current state of the requested
   resource, then the server returns a full response, including payload,
   so that the request can be satisfied and the cache entry supplanted
   without the need for an additional network round-trip.


6.  Response Cacheability

   Unless specifically constrained by a cache-control (Section 16.2)
   directive, a caching system MAY always store a successful response
   (see Section 10) as a cache entry, MAY return it without validation
   if it is fresh, and MAY return it after successful validation.  If
   there is neither a cache validator nor an explicit expiration time
   associated with a response, we do not expect it to be cached, but
   certain caches MAY violate this expectation (for example, when little
   or no network connectivity is available).  A client can usually
   detect that such a response was taken from a cache by comparing the
   Date header to the current time.

      Note: some HTTP/1.0 caches are known to violate this expectation
      without providing any Warning.

   However, in some cases it might be inappropriate for a cache to
   retain an entity, or to return it in response to a subsequent
   request.  This might be because absolute semantic transparency is
   deemed necessary by the service author, or because of security or
   privacy considerations.  Certain cache-control directives are
   therefore provided so that the server can indicate that certain
   resource entities, or portions thereof, are not to be cached
   regardless of other considerations.

   Note that Section 4.1 of [Part7] normally prevents a shared cache
   from saving and returning a response to a previous request if that
   request included an Authorization header.

   A response received with a status code of 200, 203, 206, 300, 301 or
   410 MAY be stored by a cache and used in reply to a subsequent
   request, subject to the expiration mechanism, unless a cache-control
   directive prohibits caching.  However, a cache that does not support



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   the Range and Content-Range headers MUST NOT cache 206 (Partial
   Content) responses.

   A response received with any other status code (e.g. status codes 302
   and 307) MUST NOT be returned in a reply to a subsequent request
   unless there are cache-control directives or another header(s) that
   explicitly allow it.  For example, these include the following: an
   Expires header (Section 16.3); a "max-age", "s-maxage", "must-
   revalidate", "proxy-revalidate", "public" or "private" cache-control
   directive (Section 16.2).


7.  Constructing Responses From Caches

   The purpose of an HTTP cache is to store information received in
   response to requests for use in responding to future requests.  In
   many cases, a cache simply returns the appropriate parts of a
   response to the requester.  However, if the cache holds a cache entry
   based on a previous response, it might have to combine parts of a new
   response with what is held in the cache entry.

7.1.  End-to-end and Hop-by-hop Headers

   For the purpose of defining the behavior of caches and non-caching
   proxies, we divide HTTP headers into two categories:

   o  End-to-end headers, which are transmitted to the ultimate
      recipient of a request or response.  End-to-end headers in
      responses MUST be stored as part of a cache entry and MUST be
      transmitted in any response formed from a cache entry.

   o  Hop-by-hop headers, which are meaningful only for a single
      transport-level connection, and are not stored by caches or
      forwarded by proxies.

   The following HTTP/1.1 headers are hop-by-hop headers:

   o  Connection

   o  Keep-Alive

   o  Proxy-Authenticate

   o  Proxy-Authorization

   o  TE





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   o  Trailer

   o  Transfer-Encoding

   o  Upgrade

   All other headers defined by HTTP/1.1 are end-to-end headers.

   Other hop-by-hop headers MUST be listed in a Connection header
   (Section 8.1 of [Part1]).

7.2.  Non-modifiable Headers

   Some features of HTTP/1.1, such as Digest Authentication, depend on
   the value of certain end-to-end headers.  A transparent proxy SHOULD
   NOT modify an end-to-end header unless the definition of that header
   requires or specifically allows that.

   A transparent proxy MUST NOT modify any of the following fields in a
   request or response, and it MUST NOT add any of these fields if not
   already present:

   o  Content-Location

   o  Content-MD5

   o  ETag

   o  Last-Modified

   A transparent proxy MUST NOT modify any of the following fields in a
   response:

   o  Expires

   but it MAY add any of these fields if not already present.  If an
   Expires header is added, it MUST be given a field-value identical to
   that of the Date header in that response.

   A proxy MUST NOT modify or add any of the following fields in a
   message that contains the no-transform cache-control directive, or in
   any request:

   o  Content-Encoding

   o  Content-Range





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   o  Content-Type

   A non-transparent proxy MAY modify or add these fields to a message
   that does not include no-transform, but if it does so, it MUST add a
   Warning 214 (Transformation applied) if one does not already appear
   in the message (see Section 16.6).

      Warning: unnecessary modification of end-to-end headers might
      cause authentication failures if stronger authentication
      mechanisms are introduced in later versions of HTTP.  Such
      authentication mechanisms MAY rely on the values of header fields
      not listed here.

   The Content-Length field of a request or response is added or deleted
   according to the rules in Section 4.4 of [Part1].  A transparent
   proxy MUST preserve the entity-length (Section 4.2.2 of [Part3]) of
   the entity-body, although it MAY change the transfer-length (Section
   4.4 of [Part1]).

7.3.  Combining Headers

   When a cache makes a validating request to a server, and the server
   provides a 304 (Not Modified) response or a 206 (Partial Content)
   response, the cache then constructs a response to send to the
   requesting client.

   If the status code is 304 (Not Modified), the cache uses the entity-
   body stored in the cache entry as the entity-body of this outgoing
   response.  If the status code is 206 (Partial Content) and the ETag
   or Last-Modified headers match exactly, the cache MAY combine the
   contents stored in the cache entry with the new contents received in
   the response and use the result as the entity-body of this outgoing
   response, (see Section 5 of [Part5]).

   The end-to-end headers stored in the cache entry are used for the
   constructed response, except that

   o  any stored Warning headers with warn-code 1xx (see Section 16.6)
      MUST be deleted from the cache entry and the forwarded response.

   o  any stored Warning headers with warn-code 2xx MUST be retained in
      the cache entry and the forwarded response.

   o  any end-to-end headers provided in the 304 or 206 response MUST
      replace the corresponding headers from the cache entry.

   Unless the cache decides to remove the cache entry, it MUST also
   replace the end-to-end headers stored with the cache entry with



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   corresponding headers received in the incoming response, except for
   Warning headers as described immediately above.  If a header field-
   name in the incoming response matches more than one header in the
   cache entry, all such old headers MUST be replaced.

   In other words, the set of end-to-end headers received in the
   incoming response overrides all corresponding end-to-end headers
   stored with the cache entry (except for stored Warning headers with
   warn-code 1xx, which are deleted even if not overridden).

      Note: this rule allows an origin server to use a 304 (Not
      Modified) or a 206 (Partial Content) response to update any header
      associated with a previous response for the same entity or sub-
      ranges thereof, although it might not always be meaningful or
      correct to do so.  This rule does not allow an origin server to
      use a 304 (Not Modified) or a 206 (Partial Content) response to
      entirely delete a header that it had provided with a previous
      response.


8.  Caching Negotiated Responses

   Use of server-driven content negotiation (Section 5.1 of [Part3]), as
   indicated by the presence of a Vary header field in a response,
   alters the conditions and procedure by which a cache can use the
   response for subsequent requests.  See Section 16.5 for use of the
   Vary header field by servers.

   A server SHOULD use the Vary header field to inform a cache of what
   request-header fields were used to select among multiple
   representations of a cacheable response subject to server-driven
   negotiation.  The set of header fields named by the Vary field value
   is known as the "selecting" request-headers.

   When the cache receives a subsequent request whose Request-URI
   specifies one or more cache entries including a Vary header field,
   the cache MUST NOT use such a cache entry to construct a response to
   the new request unless all of the selecting request-headers present
   in the new request match the corresponding stored request-headers in
   the original request.

   The selecting request-headers from two requests are defined to match
   if and only if the selecting request-headers in the first request can
   be transformed to the selecting request-headers in the second request
   by adding or removing linear white space (LWS) at places where this
   is allowed by the corresponding BNF, and/or combining multiple
   message-header fields with the same field name following the rules
   about message headers in Section 4.2 of [Part1].



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   A Vary header field-value of "*" always fails to match and subsequent
   requests on that resource can only be properly interpreted by the
   origin server.

   If the selecting request header fields for the cached entry do not
   match the selecting request header fields of the new request, then
   the cache MUST NOT use a cached entry to satisfy the request unless
   it first relays the new request to the origin server in a conditional
   request and the server responds with 304 (Not Modified), including an
   entity tag or Content-Location that indicates the entity to be used.

   If an entity tag was assigned to a cached representation, the
   forwarded request SHOULD be conditional and include the entity tags
   in an If-None-Match header field from all its cache entries for the
   resource.  This conveys to the server the set of entities currently
   held by the cache, so that if any one of these entities matches the
   requested entity, the server can use the ETag header field in its 304
   (Not Modified) response to tell the cache which entry is appropriate.
   If the entity-tag of the new response matches that of an existing
   entry, the new response SHOULD be used to update the header fields of
   the existing entry, and the result MUST be returned to the client.

   If any of the existing cache entries contains only partial content
   for the associated entity, its entity-tag SHOULD NOT be included in
   the If-None-Match header field unless the request is for a range that
   would be fully satisfied by that entry.

   If a cache receives a successful response whose Content-Location
   field matches that of an existing cache entry for the same Request-
   URI, whose entity-tag differs from that of the existing entry, and
   whose Date is more recent than that of the existing entry, the
   existing entry SHOULD NOT be returned in response to future requests
   and SHOULD be deleted from the cache.


9.  Shared and Non-Shared Caches

   For reasons of security and privacy, it is necessary to make a
   distinction between "shared" and "non-shared" caches.  A non-shared
   cache is one that is accessible only to a single user.  Accessibility
   in this case SHOULD be enforced by appropriate security mechanisms.
   All other caches are considered to be "shared."  Other sections of
   this specification place certain constraints on the operation of
   shared caches in order to prevent loss of privacy or failure of
   access controls.






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10.  Errors or Incomplete Response Cache Behavior

   A cache that receives an incomplete response (for example, with fewer
   bytes of data than specified in a Content-Length header) MAY store
   the response.  However, the cache MUST treat this as a partial
   response.  Partial responses MAY be combined as described in Section
   5 of [Part5]; the result might be a full response or might still be
   partial.  A cache MUST NOT return a partial response to a client
   without explicitly marking it as such, using the 206 (Partial
   Content) status code.  A cache MUST NOT return a partial response
   using a status code of 200 (OK).

   If a cache receives a 5xx response while attempting to revalidate an
   entry, 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 received response unless the cached entry
   includes the "must-revalidate" cache-control directive (see
   Section 16.2).


11.  Side Effects of GET and HEAD

   Unless the origin server explicitly prohibits the caching of their
   responses, the application of GET and HEAD methods to any resources
   SHOULD NOT have side effects that would lead to erroneous behavior if
   these responses are taken from a cache.  They MAY still have side
   effects, but a cache is not required to consider such side effects in
   its caching decisions.  Caches are always expected to observe an
   origin server's explicit restrictions on caching.

   We note one exception to this rule: since some applications have
   traditionally used GET and HEAD requests with URLs containing a query
   part to perform operations with significant side effects, caches MUST
   NOT treat responses to such URIs as fresh unless the server provides
   an explicit expiration time.  This specifically means that responses
   from HTTP/1.0 servers for such URIs SHOULD NOT be taken from a cache.
   See Section 8.1.1 of [Part2] for related information.


12.  Invalidation After Updates or Deletions

   The effect of certain methods performed on a resource at the origin
   server might cause one or more existing cache entries to become non-
   transparently invalid.  That is, although they might continue to be
   "fresh," they do not accurately reflect what the origin server would
   return for a new request on that resource.

   There is no way for HTTP to guarantee that all such cache entries are



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   marked invalid.  For example, the request that caused the change at
   the origin server might not have gone through the proxy where a cache
   entry is stored.  However, several rules help reduce the likelihood
   of erroneous behavior.

   In this section, the phrase "invalidate an entity" means that the
   cache will either remove all instances of that entity from its
   storage, or will mark these as "invalid" and in need of a mandatory
   revalidation before they can be returned in response to a subsequent
   request.

   Some HTTP methods MUST cause a cache to invalidate an entity.  This
   is either the entity referred to by the Request-URI, or by the
   Location or Content-Location headers (if present).  These methods
   are:

   o  PUT

   o  DELETE

   o  POST

   An invalidation based on the URI in a Location or Content-Location
   header MUST NOT be performed if the host part of that URI differs
   from the host part in the Request-URI.  This helps prevent denial of
   service attacks.

   A cache that passes through requests for methods it does not
   understand SHOULD invalidate any entities referred to by the Request-
   URI.


13.  Write-Through Mandatory

   All methods that might be expected to cause modifications to the
   origin server's resources MUST be written through to the origin
   server.  This currently includes all methods except for GET and HEAD.
   A cache MUST NOT reply to such a request from a client before having
   transmitted the request to the inbound server, and having received a
   corresponding response from the inbound server.  This does not
   prevent a proxy cache from sending a 100 (Continue) response before
   the inbound server has sent its final reply.

   The alternative (known as "write-back" or "copy-back" caching) is not
   allowed in HTTP/1.1, due to the difficulty of providing consistent
   updates and the problems arising from server, cache, or network
   failure prior to write-back.




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14.  Cache Replacement

   If a new cacheable (see Sections 16.2.2, 4.5, 4.6 and 10) response is
   received from a resource while any existing responses for the same
   resource are cached, the cache SHOULD use the new response to reply
   to the current request.  It MAY insert it into cache storage and MAY,
   if it meets all other requirements, use it to respond to any future
   requests that would previously have caused the old response to be
   returned.  If it inserts the new response into cache storage the
   rules in Section 7.3 apply.

      Note: a new response that has an older Date header value than
      existing cached responses is not cacheable.


15.  History Lists

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

   History mechanisms and caches are different.  In particular history
   mechanisms SHOULD NOT try to show a semantically transparent view of
   the current state of a resource.  Rather, a history mechanism is
   meant to show exactly what the user saw at the time when the resource
   was retrieved.

   By default, an expiration time does not apply to history mechanisms.
   If the entity is still in storage, a history mechanism SHOULD display
   it even if the entity has expired, unless the user has specifically
   configured the agent to refresh expired history documents.

   This is not to be construed to prohibit the history mechanism from
   telling the user that a view might be stale.

      Note: if history list mechanisms unnecessarily prevent users from
      viewing stale resources, this will tend to force service authors
      to avoid using HTTP expiration controls and cache controls when
      they would otherwise like to.  Service authors may consider it
      important that users not be presented with error messages or
      warning messages when they use navigation controls (such as BACK)
      to view previously fetched resources.  Even though sometimes such
      resources ought not be cached, or ought to expire quickly, user
      interface considerations may force service authors to resort to
      other means of preventing caching (e.g. "once-only" URLs) in order
      not to suffer the effects of improperly functioning history
      mechanisms.




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16.  Header Field Definitions

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

   For entity-header fields, both sender and recipient refer to either
   the client or the server, depending on who sends and who receives the
   entity.

16.1.  Age

   The Age response-header field conveys the sender's estimate of the
   amount of time since the response (or its revalidation) was generated
   at the origin server.  A cached response is "fresh" if its age does
   not exceed its freshness lifetime.  Age values are calculated as
   specified in Section 4.3.

     Age = "Age" ":" age-value
     age-value = delta-seconds

   Age values are non-negative decimal integers, representing time in
   seconds.

     delta-seconds  = 1*DIGIT

   If a cache receives a value larger than the largest positive integer
   it can represent, or if any of its age calculations overflows, it
   MUST transmit an Age header with a value of 2147483648 (2^31).  An
   HTTP/1.1 server that includes a cache MUST include an Age header
   field in every response generated from its own cache.  Caches SHOULD
   use an arithmetic type of at least 31 bits of range.

16.2.  Cache-Control

   The Cache-Control general-header field is used to specify directives
   that MUST be obeyed by all caching mechanisms along the request/
   response chain.  The directives specify behavior intended to prevent
   caches from adversely interfering with the request or response.
   These directives typically override the default caching algorithms.
   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.

      Note that HTTP/1.0 caches might not implement Cache-Control and
      might only implement Pragma: no-cache (see Section 16.4).

   Cache directives MUST be passed through by a proxy or gateway
   application, regardless of their significance to that application,



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   since the directives might be applicable to all recipients along the
   request/response chain.  It is not possible to specify a cache-
   directive for a specific cache.

     Cache-Control   = "Cache-Control" ":" 1#cache-directive

     cache-directive = cache-request-directive
        | cache-response-directive

     cache-request-directive =
          "no-cache"                          ; Section 16.2.1
        | "no-store"                          ; Section 16.2.2
        | "max-age" "=" delta-seconds         ; Section 16.2.3, 16.2.4
        | "max-stale" [ "=" delta-seconds ]   ; Section 16.2.3
        | "min-fresh" "=" delta-seconds       ; Section 16.2.3
        | "no-transform"                      ; Section 16.2.5
        | "only-if-cached"                    ; Section 16.2.4
        | cache-extension                     ; Section 16.2.6

     cache-response-directive =
          "public"                               ; Section 16.2.1
        | "private" [ "=" DQUOTE 1#field-name DQUOTE ] ; Section 16.2.1
        | "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ] ; Section 16.2.1
        | "no-store"                             ; Section 16.2.2
        | "no-transform"                         ; Section 16.2.5
        | "must-revalidate"                      ; Section 16.2.4
        | "proxy-revalidate"                     ; Section 16.2.4
        | "max-age" "=" delta-seconds            ; Section 16.2.3
        | "s-maxage" "=" delta-seconds           ; Section 16.2.3
        | cache-extension                        ; Section 16.2.6

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

   When a directive appears without any 1#field-name parameter, the
   directive applies to the entire request or response.  When such a
   directive appears with a 1#field-name parameter, it applies only to
   the named field or fields, and not to the rest of the request or
   response.  This mechanism supports extensibility; implementations of
   future versions of HTTP might apply these directives to header fields
   not defined in HTTP/1.1.

   The cache-control directives can be broken down into these general
   categories:

   o  Restrictions on what are cacheable; these may only be imposed by
      the origin server.





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   o  Restrictions on what may be stored by a cache; these may be
      imposed by either the origin server or the user agent.

   o  Modifications of the basic expiration mechanism; these may be
      imposed by either the origin server or the user agent.

   o  Controls over cache revalidation and reload; these may only be
      imposed by a user agent.

   o  Control over transformation of entities.

   o  Extensions to the caching system.

16.2.1.  What is Cacheable

   By default, a response is cacheable if the requirements of the
   request method, request header fields, and the response status
   indicate that it is cacheable.  Section 6 summarizes these defaults
   for cacheability.  The following Cache-Control response directives
   allow an origin server to override the default cacheability of a
   response:

   public

      Indicates that the response MAY be cached by any cache, even if it
      would normally be non-cacheable or cacheable only within a non-
      shared cache.  (See also Authorization, Section 4.1 of [Part7],
      for additional details.)

   private

      Indicates that all or part of the response message is intended for
      a single user and MUST NOT be cached by a shared cache.  This
      allows an origin server to state that the specified parts of the
      response are intended for only one user and are not a valid
      response for requests by other users.  A private (non-shared)
      cache MAY cache the response.

      Note: This usage of the word private only controls where the
      response may be cached, and cannot ensure the privacy of the
      message content.

   no-cache

      If the no-cache directive does not specify a field-name, then a
      cache MUST NOT use the response to satisfy a subsequent request
      without successful revalidation with the origin server.  This
      allows an origin server to prevent caching even by caches that



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      have been configured to return stale responses to client requests.

      If the no-cache directive does specify 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, the
      specified field-name(s) 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.

         Note: Most HTTP/1.0 caches will not recognize or obey this
         directive.

16.2.2.  What May be Stored by Caches

   no-store

      The purpose of the no-store directive is to prevent the
      inadvertent release or retention of sensitive information (for
      example, on backup tapes).  The no-store directive applies to the
      entire message, and MAY be sent either in a response or in a
      request.  If sent in a request, a cache MUST NOT store any part of
      either this request or any response to it.  If sent in a response,
      a cache MUST NOT store any part of either this response or the
      request that elicited it.  This directive applies to both non-
      shared 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.

      Even when this directive is associated with a response, users
      might explicitly store such a response outside of the caching
      system (e.g., with a "Save As" dialog).  History buffers MAY store
      such responses as part of their normal operation.

      The purpose of this directive is to meet the stated requirements
      of certain users and service authors who are concerned about
      accidental releases of information via unanticipated accesses to
      cache data structures.  While the use of this directive might
      improve privacy in some cases, we caution that it is NOT in any
      way a reliable or sufficient mechanism for ensuring privacy.  In
      particular, malicious or compromised caches might not recognize or
      obey this directive, and communications networks might be
      vulnerable to eavesdropping.





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16.2.3.  Modifications of the Basic Expiration Mechanism

   The expiration time of an entity MAY be specified by the origin
   server using the Expires header (see Section 16.3).  Alternatively,
   it MAY be specified using the max-age directive in a response.  When
   the max-age cache-control directive is present in a cached response,
   the response is stale if its current age is greater than the age
   value given (in seconds) at the time of a new request for that
   resource.  The max-age directive on a response implies that the
   response is cacheable (i.e., "public") unless some other, more
   restrictive cache directive is also present.

   If a response includes both an Expires header and a max-age
   directive, the max-age directive overrides the Expires header, even
   if the Expires header is more restrictive.  This rule allows an
   origin server to provide, for a given response, a longer expiration
   time to an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache.  This
   might be useful if certain HTTP/1.0 caches improperly calculate ages
   or expiration times, perhaps due to desynchronized clocks.

   Many HTTP/1.0 cache implementations will treat an Expires value that
   is less than or equal to the response Date value as being equivalent
   to the Cache-Control response directive "no-cache".  If an HTTP/1.1
   cache receives such a response, and the response does not include a
   Cache-Control header field, it SHOULD consider the response to be
   non-cacheable in order to retain compatibility with HTTP/1.0 servers.

      Note: An origin server might wish to use a relatively new HTTP
      cache control feature, such as the "private" directive, on a
      network including older caches that do not understand that
      feature.  The origin server will need to combine the new feature
      with an Expires field whose value is less than or equal to the
      Date value.  This will prevent older caches from improperly
      caching the response.

   s-maxage

      If a response includes an s-maxage directive, then for a shared
      cache (but not for a private cache), the maximum age specified by
      this directive overrides the maximum age specified by either the
      max-age directive or the Expires header.  The s-maxage directive
      also implies the semantics of the proxy-revalidate directive (see
      Section 16.2.4), i.e., that the shared cache must not use the
      entry after it becomes stale to respond to a subsequent request
      without first revalidating it with the origin server.  The
      s-maxage directive is always ignored by a private cache.

   Note that most older caches, not compliant with this specification,



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   do not implement any cache-control directives.  An origin server
   wishing to use a cache-control directive that restricts, but does not
   prevent, caching by an HTTP/1.1-compliant cache MAY exploit the
   requirement that the max-age directive overrides the Expires header,
   and the fact that pre-HTTP/1.1-compliant caches do not observe the
   max-age directive.

   Other directives allow a user agent to modify the basic expiration
   mechanism.  These directives MAY be specified on a request:

   max-age

      Indicates that the client is willing to accept a response whose
      age is no greater than the specified time in seconds.  Unless max-
      stale directive is also included, the client is not willing to
      accept a stale response.

   min-fresh

      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.

   max-stale

      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.

   If a cache returns a stale response, either because of a max-stale
   directive on a request, or because the cache is configured to
   override the expiration time of a response, the cache MUST attach a
   Warning header to the stale response, using Warning 110 (Response is
   stale).

   A cache MAY be configured to return stale responses without
   validation, but only if this does not conflict with any "MUST"-level
   requirements concerning cache validation (e.g., a "must-revalidate"
   cache-control directive).

   If both the new request and the cached entry include "max-age"
   directives, then the lesser of the two values is used for determining
   the freshness of the cached entry for that request.



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16.2.4.  Cache Revalidation and Reload Controls

   Sometimes a user agent might want or need to insist that a cache
   revalidate its cache entry with the origin server (and not just with
   the next cache along the path to the origin server), or to reload its
   cache entry from the origin server.  End-to-end revalidation might be
   necessary if either the cache or the origin server has overestimated
   the expiration time of the cached response.  End-to-end reload may be
   necessary if the cache entry has become corrupted for some reason.

   End-to-end revalidation may be requested either when the client does
   not have its own local cached copy, in which case we call it
   "unspecified end-to-end revalidation", or when the client does have a
   local cached copy, in which case we call it "specific end-to-end
   revalidation."

   The client can specify these three kinds of action using Cache-
   Control request directives:

   End-to-end reload

      The request includes a "no-cache" cache-control directive or, for
      compatibility with HTTP/1.0 clients, "Pragma: no-cache".  Field
      names MUST NOT be included with the no-cache directive in a
      request.  The server MUST NOT use a cached copy when responding to
      such a request.

   Specific end-to-end revalidation

      The request includes a "max-age=0" cache-control directive, which
      forces each cache along the path to the origin server to
      revalidate its own entry, if any, with the next cache or server.
      The initial request includes a cache-validating conditional with
      the client's current validator.

   Unspecified end-to-end revalidation

      The request includes "max-age=0" cache-control directive, which
      forces each cache along the path to the origin server to
      revalidate its own entry, if any, with the next cache or server.
      The initial request does not include a cache-validating
      conditional; the first cache along the path (if any) that holds a
      cache entry for this resource includes a cache-validating
      conditional with its current validator.

   max-age





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      When an intermediate cache is forced, by means of a max-age=0
      directive, to revalidate its own cache entry, and the client has
      supplied its own validator in the request, the supplied validator
      might differ from the validator currently stored with the cache
      entry.  In this case, the cache MAY use either validator in making
      its own request without affecting semantic transparency.

      However, the choice of validator might affect performance.  The
      best approach is for the intermediate cache to use its own
      validator when making its request.  If the server replies with 304
      (Not Modified), then the cache can return its now validated copy
      to the client with a 200 (OK) response.  If the server replies
      with a new entity and cache validator, however, the intermediate
      cache can compare the returned validator with the one provided in
      the client's request, using the strong comparison function.  If
      the client's validator is equal to the origin server's, then the
      intermediate cache simply returns 304 (Not Modified).  Otherwise,
      it returns the new entity with a 200 (OK) response.

      If a request includes the no-cache directive, it SHOULD NOT
      include min-fresh, max-stale, or max-age.

   only-if-cached

      In some cases, such as times of extremely poor network
      connectivity, a client may want a cache to return only those
      responses that it currently has stored, and not to reload or
      revalidate with the origin server.  To do this, the client may
      include the only-if-cached directive in a request.  If it receives
      this directive, a cache SHOULD either respond using a cached entry
      that is consistent with the other constraints of the request, or
      respond with a 504 (Gateway Timeout) status.  However, if a group
      of caches is being operated as a unified system with good internal
      connectivity, such a request MAY be forwarded within that group of
      caches.

   must-revalidate

      Because a cache MAY be configured to ignore a server's specified
      expiration time, and because a client request MAY include a max-
      stale directive (which has a similar effect), the protocol also
      includes a mechanism for the origin server to require revalidation
      of a cache entry on any subsequent use.  When the must-revalidate
      directive is present in a response received by a cache, that cache
      MUST NOT use the entry after it becomes stale to respond to a
      subsequent request without first revalidating it with the origin
      server.  (I.e., the cache MUST do an end-to-end revalidation every
      time, if, based solely on the origin server's Expires or max-age



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      value, the cached response is stale.)

      The must-revalidate directive is necessary to support reliable
      operation for certain protocol features.  In all circumstances an
      HTTP/1.1 cache MUST obey the must-revalidate directive; in
      particular, if the cache cannot reach the origin server for any
      reason, it MUST generate a 504 (Gateway Timeout) response.

      Servers SHOULD send the must-revalidate directive if and only if
      failure to revalidate a request on the entity could result in
      incorrect operation, such as a silently unexecuted financial
      transaction.  Recipients MUST NOT take any automated action that
      violates this directive, and MUST NOT automatically provide an
      unvalidated copy of the entity if revalidation fails.

      Although this is not recommended, user agents operating under
      severe connectivity constraints MAY violate this directive but, if
      so, MUST explicitly warn the user that an unvalidated response has
      been provided.  The warning MUST be provided on each unvalidated
      access, and SHOULD require explicit user confirmation.

   proxy-revalidate

      The proxy-revalidate directive has the same meaning as the must-
      revalidate directive, except that it does not apply to non-shared
      user agent caches.  It can be used on a response to an
      authenticated request to permit the user's cache to store and
      later return the response without needing to revalidate it (since
      it has already been authenticated once by that user), while still
      requiring proxies that service many users to revalidate each time
      (in order to make sure that each user has been authenticated).
      Note that such authenticated responses also need the public cache
      control directive in order to allow them to be cached at all.

16.2.5.  No-Transform Directive

   no-transform

      Implementors of intermediate caches (proxies) have found it useful
      to convert the media type of certain entity bodies.  A non-
      transparent proxy might, for example, convert between image
      formats in order to save cache space or to reduce the amount of
      traffic on a slow link.

      Serious operational problems occur, however, when these
      transformations are applied to entity bodies intended for certain
      kinds of applications.  For example, applications for medical
      imaging, scientific data analysis and those using end-to-end



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      authentication, all depend on receiving an entity body that is bit
      for bit identical to the original entity-body.

      Therefore, if a message includes the no-transform directive, an
      intermediate cache or proxy MUST NOT change those headers that are
      listed in Section 7.2 as being subject to the no-transform
      directive.  This implies that the cache or proxy MUST NOT change
      any aspect of the entity-body that is specified by these headers,
      including the value of the entity-body itself.

16.2.6.  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 assigned value.
   Informational extensions (those which do not require a change in
   cache behavior) MAY 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 which 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.

   For example, consider a hypothetical new response directive called
   community which acts as a modifier to the private directive.  We
   define this new directive to mean that, in addition to any non-shared
   cache, any cache which 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.

   Unrecognized cache-directives MUST be ignored; it is assumed that any
   cache-directive likely to be unrecognized by an HTTP/1.1 cache will



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   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).

16.3.  Expires

   The Expires entity-header field gives the date/time after which the
   response is considered stale.  A stale cache entry may not normally
   be returned by a cache (either a proxy cache or a user agent cache)
   unless it is first validated with the origin server (or with an
   intermediate cache that has a fresh copy of the entity).  See
   Section 4 for further discussion of the expiration 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 format is an absolute date and time as defined by HTTP-date in
   Section 3.3.1 of [Part1]; it MUST be sent in rfc1123-date format.

     Expires = "Expires" ":" HTTP-date

   An example of its use is

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

      Note: if a response includes a Cache-Control field with the max-
      age directive (see Section 16.2.3), that directive overrides the
      Expires field.

   HTTP/1.1 clients and caches MUST treat other invalid date formats,
   especially including the value "0", as in the past (i.e., "already
   expired").

   To mark a response as "already expired," an origin server sends an
   Expires date that is equal to the Date header value.  (See the rules
   for expiration calculations in Section 4.4.)

   To mark a response as "never expires," an origin server sends an
   Expires date approximately one year from the time the response is
   sent.  HTTP/1.1 servers SHOULD NOT send Expires dates more than one
   year in the future.

   The presence of an Expires header field with a date value of some
   time in the future on a response that otherwise would by default be
   non-cacheable indicates that the response is cacheable, unless
   indicated otherwise by a Cache-Control header field (Section 16.2).




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16.4.  Pragma

   The Pragma general-header field is used to include implementation-
   specific directives that might apply to any recipient along the
   request/response chain.  All pragma directives specify optional
   behavior from the viewpoint of the protocol; however, some systems
   MAY require that behavior be consistent with the directives.

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

   When the no-cache directive is present in a request message, an
   application SHOULD forward the request toward the origin server even
   if it has a cached copy of what is being requested.  This pragma
   directive has the same semantics as the no-cache cache-directive (see
   Section 16.2) and is defined here for backward compatibility with
   HTTP/1.0.  Clients SHOULD include both header fields when a no-cache
   request is sent to a server not known to be HTTP/1.1 compliant.

   Pragma directives MUST be passed through by a proxy or gateway
   application, 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 specify a pragma for a
   specific recipient; however, any pragma directive not relevant to a
   recipient SHOULD be ignored by that recipient.

   HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
   sent "Cache-Control: no-cache".  No new Pragma directives will be
   defined in HTTP.

      Note: because the meaning of "Pragma: no-cache" as a response-
      header field is not actually specified, it does not provide a
      reliable replacement for "Cache-Control: no-cache" in a response.

16.5.  Vary

   The Vary field value indicates the set of request-header fields that
   fully determines, while the response is fresh, whether a cache is
   permitted to use the response to reply to a subsequent request
   without revalidation.  For uncacheable or stale responses, the Vary
   field value advises the user agent about the criteria that were used
   to select the representation.  A Vary field value of "*" implies that
   a cache cannot determine from the request headers of a subsequent
   request whether this response is the appropriate representation.  See
   Section 8 for use of the Vary header field by caches.

     Vary  = "Vary" ":" ( "*" | 1#field-name )



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   An HTTP/1.1 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 consisting of a list of field-names signals that
   the representation selected for the response is based on a selection
   algorithm which considers ONLY the listed request-header field values
   in selecting the most appropriate representation.  A cache MAY assume
   that the same selection will be made for future requests with the
   same values for the listed field names, for the duration of time for
   which the response is fresh.

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

   A Vary field value of "*" signals that unspecified parameters not
   limited to the request-headers (e.g., the network address of the
   client), play a role in the selection of the response representation.
   The "*" value MUST NOT be generated by a proxy server; it may only be
   generated by an origin server.

16.6.  Warning

   The Warning general-header field is used to carry additional
   information about the status or transformation of a message which
   might not be reflected in the message.  This information is typically
   used to warn about a possible lack of semantic transparency from
   caching operations or transformations applied to the entity body of
   the message.

   Warning headers are sent with responses using:













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     Warning    = "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, for use in debugging
     warn-text  = quoted-string
     warn-date  = DQUOTE HTTP-date DQUOTE

   A response MAY carry more than one Warning header.

   The warn-text SHOULD be in a natural language and character set that
   is most likely to be intelligible to the human user receiving the
   response.  This decision MAY be based on any available knowledge,
   such as the location of the cache or user, the Accept-Language field
   in a request, the Content-Language field in a response, etc.  The
   default language is English and the default character set is ISO-
   8859-1 ([ISO-8859-1]).

   If a character set other than ISO-8859-1 is used, it MUST be encoded
   in the warn-text using the method described in [RFC2047].

   Warning headers can in general be applied to any message, however
   some specific warn-codes are specific to caches and can only be
   applied to response messages.  New Warning headers SHOULD be added
   after any existing Warning headers.  A cache MUST NOT delete any
   Warning header that it received with a message.  However, if a cache
   successfully validates a cache entry, it SHOULD remove any Warning
   headers previously attached to that entry except as specified for
   specific Warning codes.  It MUST then add any Warning headers
   received in the validating response.  In other words, Warning headers
   are those that would be attached to the most recent relevant
   response.

   When multiple Warning headers are attached to a response, the user
   agent ought to inform the user of as many of them as possible, in the
   order that they appear in the response.  If it is not possible to
   inform the user of all of the warnings, the user agent SHOULD follow
   these heuristics:

   o  Warnings that appear early in the response take priority over
      those appearing later in the response.

   o  Warnings in the user's preferred character set take priority over
      warnings in other character sets but with identical warn-codes and



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      warn-agents.

   Systems that generate multiple Warning headers SHOULD order them with
   this user agent behavior in mind.

   Requirements for the behavior of caches with respect to Warnings are
   stated in Section 3.2.

   This is a list of the currently-defined warn-codes, each with a
   recommended warn-text in English, and a description of its meaning.

   110 Response is stale

      MUST be included whenever the returned response is stale.

   111 Revalidation failed

      MUST be included if a cache returns a stale response because an
      attempt to revalidate the response failed, due to an inability to
      reach the server.

   112 Disconnected operation

      SHOULD be included if the cache is intentionally disconnected from
      the rest of the network for a period of time.

   113 Heuristic expiration

      MUST be included if the cache heuristically chose a freshness
      lifetime greater than 24 hours and the response's age is greater
      than 24 hours.

   199 Miscellaneous warning

      The warning text MAY 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 an intermediate cache or proxy if it applies any
      transformation changing the content-coding (as specified in the
      Content-Encoding header) or media-type (as specified in the
      Content-Type header) of the response, or the entity-body of the
      response, unless this Warning code already appears in the
      response.




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   299 Miscellaneous persistent warning

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

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

   If an implementation 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.  (This prevents
   bad consequences of naive caching of Warning header fields.)  If all
   of the warning-values are deleted for this reason, the Warning header
   MUST be deleted as well.


17.  IANA Considerations

17.1.  Message Header Registration

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

        +-------------------+----------+----------+--------------+
        | Header Field Name | Protocol | Status   | Reference    |
        +-------------------+----------+----------+--------------+
        | Age               | http     | standard | Section 16.1 |
        | Cache-Control     | http     | standard | Section 16.2 |
        | Expires           | http     | standard | Section 16.3 |
        | Pragma            | http     | standard | Section 16.4 |
        | Vary              | http     | standard | Section 16.5 |
        | Warning           | http     | standard | Section 16.6 |
        +-------------------+----------+----------+--------------+

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


18.  Security Considerations

   Caching proxies provide 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



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   long after a user believes that the information has been removed from
   the network.  Therefore, cache contents should be protected as
   sensitive information.


19.  Acknowledgments

   Much of the content and presentation of the caching design is due to
   suggestions and comments from individuals including: Shel Kaphan,
   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.


20.  References

20.1.  Normative References

   [ISO-8859-1]
              International Organization for Standardization,
              "Information technology -- 8-bit single-byte coded graphic
              character sets -- Part 1: Latin alphabet No. 1", ISO/
              IEC 8859-1:1998, 1998.

   [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-03
              (work in progress), June 2008.

   [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-03 (work in
              progress), June 2008.

   [Part3]    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 3: Message Payload
              and Content Negotiation", draft-ietf-httpbis-p3-payload-03
              (work in progress), June 2008.

   [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-03 (work in
              progress), June 2008.

   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,



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              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
              Partial Responses", draft-ietf-httpbis-p5-range-03 (work
              in progress), June 2008.

   [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-03 (work in progress),
              June 2008.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

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

20.2.  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.


Appendix A.  Compatibility with Previous Versions

A.1.  Changes from RFC 2068

   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
   was introduced to add this missing case.  (Sections 6, 16.2, 16.2.3)

   Transfer-coding and message lengths all interact in ways that
   required fixing exactly when chunked encoding is used (to allow for
   transfer encoding that may not be self delimiting); it was important
   to straighten out exactly how message lengths are computed.
   (Section 7.2, see also [Part1], [Part3] and [Part5])

   Proxies should be able to add Content-Length when appropriate.
   (Section 7.2)

   Range request responses would become very verbose if all meta-data



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   were always returned; by allowing the server to only send needed
   headers in a 206 response, this problem can be avoided.
   (Section 7.3)

   The Cache-Control: max-age directive was not properly defined for
   responses.  (Section 16.2.3)

   Warnings could be cached incorrectly, or not updated appropriately.
   (Section 3.2, 4.4, 7.2, 7.3, 16.2.3, and 16.6) Warning also needed to
   be a general header, as PUT or other methods may have need for it in
   requests.

A.2.  Changes from RFC 2616

   Clarify denial of service attack avoidance requirement.  (Section 12)


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

B.1.  Since RFC2616

   Extracted relevant partitions from [RFC2616].

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

   Closed issues:

   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer"
      (<http://purl.org/NET/http-errata#trailer-hop>)

   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12>:
      "Invalidation after Update or Delete"
      (<http://purl.org/NET/http-errata#invalidupd>)

   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative
      and Informative references"

   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48>: "Date
      reference typo"

   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49>:
      "Connection header text"

   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65>:
      "Informative references"

   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66>:
      "ISO-8859-1 Reference"



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   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative
      up-to-date references"

   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in
      13.2.2"

   Other changes:

   o  Use names of RFC4234 core rules DQUOTE and HTAB (work in progress
      on <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36>)

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

   Closed issues:

   o  <http://www3.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://www3.tools.ietf.org/wg/httpbis/trac/ticket/36>)

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

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

   Ongoing work on IANA Message Header Registration
   (<http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40>):

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


Index

   A
      age  7
      Age header  27

   C
      cache  5
      Cache Directives
         max-age  32-33
         max-stale  32
         min-fresh  32



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         must-revalidate  34
         no-cache  29
         no-store  30
         no-transform  35
         only-if-cached  34
         private  29
         proxy-revalidate  35
         public  29
         s-maxage  31
      Cache-Control header  27
      cacheable  6

   E
      Expires header  37
      explicit expiration time  7

   F
      first-hand  6
      fresh  7
      freshness lifetime  7

   G
      Grammar
         Age  27
         age-value  27
         Cache-Control  28
         cache-directive  28
         cache-extension  28
         cache-request-directive  28
         cache-response-directive  28
         delta-seconds  27
         Expires  37
         extension-pragma  38
         Pragma  38
         pragma-directive  38
         Vary  38
         warn-agent  40
         warn-code  40
         warn-date  40
         warn-text  40
         Warning  40
         warning-value  40

   H
      Headers
         Age  27
         Cache-Control  27
         Expires  37



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         Pragma  38
         Vary  38
         Warning  39
      heuristic expiration time  7

   M
      max-age
         Cache Directive  32-33
      max-stale
         Cache Directive  32
      min-fresh
         Cache Directive  32
      must-revalidate
         Cache Directive  34

   N
      no-cache
         Cache Directive  29
      no-store
         Cache Directive  30
      no-transform
         Cache Directive  35

   O
      only-if-cached
         Cache Directive  34

   P
      Pragma header  38
      private
         Cache Directive  29
      proxy-revalidate
         Cache Directive  35
      public
         Cache Directive  29

   S
      s-maxage
         Cache Directive  31
      semantically transparent  5
      stale  7

   V
      validator  7
      Vary header  38

   W
      Warning header  39



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Authors' Addresses

   Roy T. Fielding (editor)
   Day Software
   23 Corporate Plaza DR, Suite 280
   Newport Beach, CA  92660
   USA

   Phone: +1-949-706-5300
   Fax:   +1-949-706-5305
   Email: fielding@gbiv.com
   URI:   http://roy.gbiv.com/


   Jim Gettys
   One Laptop per Child
   21 Oak Knoll Road
   Carlisle, MA  01741
   USA

   Email: jg@laptop.org
   URI:   http://www.laptop.org/


   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










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   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


   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/












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   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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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


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   The IETF invites any interested party to bring to its attention any
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   ietf-ipr@ietf.org.











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