source: draft-ietf-httpbis/10/draft-ietf-httpbis-p6-cache-10.txt @ 955

HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                              Day Software
Obsoletes: 2616 (if approved)                                  J. Gettys
Intended status: Standards Track                          Alcatel-Lucent
Expires: January 13, 2011                                       J. Mogul
                                                                      HP
                                                              H. Frystyk
                                                               Microsoft
                                                             L. Masinter
                                                           Adobe Systems
                                                                P. Leach
                                                               Microsoft
                                                          T. Berners-Lee
                                                                 W3C/MIT
                                                           Y. Lafon, Ed.
                                                                     W3C
                                                      M. Nottingham, Ed.

                                                         J. Reschke, Ed.
                                                              greenbytes
                                                           July 12, 2010


                       HTTP/1.1, part 6: Caching
                     draft-ietf-httpbis-p6-cache-10

Abstract

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypermedia information
   systems.  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://tools.ietf.org/wg/httpbis/trac/report/3> and related
   documents (including fancy diffs) can be found at
   <http://tools.ietf.org/wg/httpbis/>.

   The changes in this draft are summarized in Appendix C.11.

Status of This Memo




Fielding, et al.        Expires January 13, 2011                [Page 1]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 13, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  6



Fielding, et al.        Expires January 13, 2011                [Page 2]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


     1.4.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  7
       1.4.1.  Core Rules . . . . . . . . . . . . . . . . . . . . . .  7
       1.4.2.  ABNF Rules defined in other Parts of the
               Specification  . . . . . . . . . . . . . . . . . . . .  7
   2.  Cache Operation  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.1.  Response Cacheability  . . . . . . . . . . . . . . . . . .  7
       2.1.1.  Storing Partial and Incomplete Responses . . . . . . .  8
     2.2.  Constructing Responses from Caches . . . . . . . . . . . .  9
     2.3.  Freshness Model  . . . . . . . . . . . . . . . . . . . . . 10
       2.3.1.  Calculating Freshness Lifetime . . . . . . . . . . . . 11
       2.3.2.  Calculating Age  . . . . . . . . . . . . . . . . . . . 12
       2.3.3.  Serving Stale Responses  . . . . . . . . . . . . . . . 13
     2.4.  Validation Model . . . . . . . . . . . . . . . . . . . . . 14
     2.5.  Request Methods that Invalidate  . . . . . . . . . . . . . 14
     2.6.  Shared Caching of Authenticated Responses  . . . . . . . . 15
     2.7.  Caching Negotiated Responses . . . . . . . . . . . . . . . 16
     2.8.  Combining Responses  . . . . . . . . . . . . . . . . . . . 16
   3.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 17
     3.1.  Age  . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     3.2.  Cache-Control  . . . . . . . . . . . . . . . . . . . . . . 18
       3.2.1.  Request Cache-Control Directives . . . . . . . . . . . 19
       3.2.2.  Response Cache-Control Directives  . . . . . . . . . . 20
       3.2.3.  Cache Control Extensions . . . . . . . . . . . . . . . 23
     3.3.  Expires  . . . . . . . . . . . . . . . . . . . . . . . . . 24
     3.4.  Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     3.5.  Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     3.6.  Warning  . . . . . . . . . . . . . . . . . . . . . . . . . 26
   4.  History Lists  . . . . . . . . . . . . . . . . . . . . . . . . 29
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
     5.1.  Cache Directive Registry . . . . . . . . . . . . . . . . . 29
     5.2.  Message Header Registration  . . . . . . . . . . . . . . . 30
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 30
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 30
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 31
   Appendix A.  Compatibility with Previous Versions  . . . . . . . . 32
     A.1.  Changes from RFC 2068  . . . . . . . . . . . . . . . . . . 32
     A.2.  Changes from RFC 2616  . . . . . . . . . . . . . . . . . . 32
   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 32
   Appendix C.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 34
     C.1.  Since RFC2616  . . . . . . . . . . . . . . . . . . . . . . 34
     C.2.  Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 34
     C.3.  Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 35
     C.4.  Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 35
     C.5.  Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 35
     C.6.  Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 35



Fielding, et al.        Expires January 13, 2011                [Page 3]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


     C.7.  Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 36
     C.8.  Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 36
     C.9.  Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 36
     C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 37
     C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 37
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38













































Fielding, et al.        Expires January 13, 2011                [Page 4]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


1.  Introduction

   HTTP is typically used for distributed information systems, where
   performance can be improved by the use of response caches.  This
   document defines aspects of HTTP/1.1 related to caching and reusing
   response messages.

1.1.  Purpose

   An HTTP cache is a local store of response messages and the subsystem
   that controls its message storage, retrieval, and deletion.  A cache
   stores cacheable responses in order to reduce the response time and
   network bandwidth consumption on future, equivalent requests.  Any
   client or server may 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, a
   stored response can be reused without the need for a network request,
   reducing latency and network round-trips; a "freshness" mechanism is
   used for this purpose (see Section 2.3).  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; a "validation" mechanism is used for this purpose
   (see Section 2.4).

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 to satisfy
      a particular request.

   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.







Fielding, et al.        Expires January 13, 2011                [Page 5]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


   heuristic expiration time

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

   age

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

   first-hand

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

   freshness lifetime

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

   fresh

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

   stale

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

   validator

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

   shared cache

      A cache that is accessible to more than one user.  A non-shared
      cache is dedicated to a single user.

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



Fielding, et al.        Expires January 13, 2011                [Page 6]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


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

1.4.  Syntax Notation

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

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

1.4.1.  Core Rules

   The core rules below are defined in Section 1.2.2 of [Part1]:

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

1.4.2.  ABNF Rules defined in other Parts of the Specification

   The ABNF rules below are defined in other parts:

     field-name    = <field-name, defined in [Part1], Section 3.2>
     HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
     port          = <port, defined in [Part1], Section 2.6>
     pseudonym     = <pseudonym, defined in [Part1], Section 9.9>
     uri-host      = <uri-host, defined in [Part1], Section 2.6>

2.  Cache Operation

2.1.  Response Cacheability

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

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



Fielding, et al.        Expires January 13, 2011                [Page 7]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


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

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

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

   o  the "Authorization" header (see Section 3.1 of [Part7]) does not
      appear in the request, if the cache is shared, unless the response
      explicitly allows it (see Section 2.6), and

   o  the response either:

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

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

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

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

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

   In this context, a cache has "understood" a request method or a
   response status code if it recognises it and implements any cache-
   specific behaviour.  In particular, 206 Partial Content responses
   cannot be cached by an implementation that does not handle partial
   content (see Section 2.1.1).

   Note that in normal operation, most caches will not store a response
   that has neither a cache validator nor an explicit expiration time,
   as such responses are not usually useful to store.  However, caches
   are not prohibited from storing such responses.

2.1.1.  Storing Partial and Incomplete Responses

   A cache that receives an incomplete response (for example, with fewer
   bytes of data than specified in a Content-Length header) can store
   the response, but MUST treat it as a partial response [Part5].
   Partial responses can be combined as described in Section 4 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)



Fielding, et al.        Expires January 13, 2011                [Page 8]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


   status code.

   A cache that does not support the Range and Content-Range headers
   MUST NOT store incomplete or partial responses.

2.2.  Constructing Responses from Caches

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

   o  The presented Effective Request URI (Section 4.3 of [Part1]) and
      that of the stored response match, and

   o  the request method associated with the stored response allows it
      to be used for the presented request, and

   o  selecting request-headers nominated by the stored response (if
      any) match those presented (see Section 2.7), and

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

   o  the stored response is either:

      *  fresh (see Section 2.3), or

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

      *  successfully validated (see Section 2.4).

   [[TODO-method-cacheability: define method cacheability for GET, HEAD
   and POST in p2-semantics.]]

   When a stored response is used to satisfy a request, caches MUST
   include a single Age header field (Section 3.1) in the response with
   a value equal to the stored response's current_age; see
   Section 2.3.2.  [[DISCUSS-includes-validated: this currently includes
   successfully validated responses.]]

   Requests with methods that are unsafe (Section 7.1.1 of [Part2]) MUST
   be written through the cache to the origin server; i.e., a cache must
   not reply to such a request before having forwarded the request and
   having received a corresponding response.

   Also, note that unsafe requests might invalidate already stored
   responses; see Section 2.5.

   Caches MUST use the most recent response (as determined by the Date



Fielding, et al.        Expires January 13, 2011                [Page 9]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


   header) when more than one suitable response is stored.  They can
   also forward a request with "Cache-Control: max-age=0" or "Cache-
   Control: no-cache" to disambiguate which response to use.

2.3.  Freshness Model

   When a response is "fresh" in the cache, it can be used to satisfy
   subsequent requests without contacting the origin server, thereby
   improving efficiency.

   The primary mechanism for determining freshness is for an origin
   server to provide an explicit expiration time in the future, using
   either the Expires header (Section 3.3) or the max-age response cache
   directive (Section 3.2.2).  Generally, origin servers will assign
   future explicit expiration times to responses in the belief that the
   entity is not likely to change in a semantically significant way
   before the expiration time is reached.

   If an origin server wishes to force a cache to validate every
   request, it can assign an explicit expiration time in the past.  This
   means that the response is always stale, so that caches should
   validate it before using it for subsequent requests.  [[TODO-
   response-stale: This wording may cause confusion, because the
   response may still be served stale.]]

   Since origin servers do not always provide explicit expiration times,
   HTTP caches may also assign heuristic expiration times when they are
   not specified, 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.

   The calculation to determine if a response is fresh is:

      response_is_fresh = (freshness_lifetime > current_age)

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

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

   [[ISSUE-no-req-for-directives: there are not requirements directly
   applying to cache-request-directives and freshness.]]

   Note that freshness applies only to cache operation; it cannot be



Fielding, et al.        Expires January 13, 2011               [Page 10]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


   used to force a user agent to refresh its display or reload a
   resource.  See Section 4 for an explanation of the difference between
   caches and history mechanisms.

2.3.1.  Calculating Freshness Lifetime

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

   o  If the cache is shared and the s-maxage response cache directive
      (Section 3.2.2) is present, use its value, or

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

   o  If the Expires response header (Section 3.3) is present, use its
      value minus the value of the Date response header, or

   o  Otherwise, no explicit expiration time is present in the response.
      A heuristic freshness lifetime might be applicable; see
      Section 2.3.1.1.

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

2.3.1.1.  Calculating Heuristic Freshness

   If no explicit expiration time is present in a stored response that
   has a status code of 200, 203, 206, 300, 301 or 410, a heuristic
   expiration time can be calculated.  Heuristics MUST NOT be used for
   other response status codes.

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

   Also, if the response has a Last-Modified header (Section 6.6 of
   [Part4]), 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%.

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



Fielding, et al.        Expires January 13, 2011               [Page 11]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


2.3.2.  Calculating Age

   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 validated by the origin server.  In essence, the Age
   value is the sum of the time that the response has been resident in
   each of the caches along the path from the origin server, plus the
   amount of time it has been in transit along network paths.

   The following data is used for the age calculation:

   age_value

      The term "age_value" denotes the value of the Age header
      (Section 3.1), in a form appropriate for arithmetic operation; or
      0, if not available.

   date_value

      HTTP/1.1 requires origin servers to send a Date header, if
      possible, with every response, giving the time at which the
      response was generated.  The term "date_value" denotes the value
      of the Date header, in a form appropriate for arithmetic
      operations.  See Section 9.3 of [Part1] for the definition of the
      Date header, and for requirements regarding responses without a
      Date response header.

   now

      The term "now" means "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.

   request_time

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

   response_time

      The current value of the clock at the host at the time the
      response was received.

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




Fielding, et al.        Expires January 13, 2011               [Page 12]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


   1.  the "apparent_age": response_time minus date_value, if the local
       clock is reasonably well synchronized to the origin server's
       clock.  If the result is negative, the result is replaced by
       zero.

   2.  the "corrected_age_value", if all of the caches along the
       response path implement HTTP/1.1; note this value MUST be
       interpreted relative to the time the request was initiated, not
       the time that the response was received.


     apparent_age = max(0, response_time - date_value);

     response_delay = response_time - request_time;
     corrected_age_value = age_value + response_delay;

   These are combined as

     corrected_initial_age = max(apparent_age, corrected_age_value);

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

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

2.3.3.  Serving Stale Responses

   A "stale" response is one that either has explicit expiry information
   or is allowed to have heuristic expiry calculated, but is not fresh
   according to the calculations in Section 2.3.

   Caches MUST NOT return a stale response if it is prohibited by an
   explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
   cache directive, a "must-revalidate" cache-response-directive, or an
   applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
   see Section 3.2.2).

   Caches SHOULD NOT return stale responses unless they are disconnected
   (i.e., it cannot contact the origin server or otherwise find a
   forward path) or otherwise explicitly allowed (e.g., the max-stale
   request directive; see Section 3.2.1).

   Stale responses SHOULD have a Warning header with the 110 warn-code
   (see Section 3.6).  Likewise, the 112 warn-code SHOULD be sent on
   stale responses if the cache is disconnected.




Fielding, et al.        Expires January 13, 2011               [Page 13]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


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

2.4.  Validation Model

   When a cache has one or more stored responses for a requested URI,
   but cannot serve any of them (e.g., because they are not fresh, or
   one cannot be selected; see Section 2.7), it can use the conditional
   request mechanism [Part4] in the forwarded request to give the origin
   server an opportunity to both select a valid stored response to be
   used, and to update it.  This process is known as "validating" or
   "revalidating" the stored response.

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

   Additionally, the cache SHOULD add an If-None-Match header whose
   value is that of the ETag header(s) from all responses stored for the
   requested URI, if present.  However, if any of the stored responses
   contains only partial content, 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 stored response.

   A 304 (Not Modified) response status code indicates that the stored
   response can be updated and reused; see Section 2.8.

   A full response (i.e., one with a response body) indicates that none
   of the stored responses nominated in the conditional request is
   suitable.  Instead, the full response is used both to satisfy the
   request and replace the stored response.  [[TODO-req-missing: Should
   there be a requirement here?]]

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

2.5.  Request Methods that Invalidate

   Because unsafe methods (Section 7.1.1 of [Part2]) have the potential
   for changing state on the origin server, intervening caches can use
   them to keep their contents up-to-date.



Fielding, et al.        Expires January 13, 2011               [Page 14]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


   The following HTTP methods MUST cause a cache to invalidate the
   Effective Request URI (Section 4.3 of [Part1]) as well as the URI(s)
   in the Location and Content-Location headers (if present):

   o  PUT

   o  DELETE

   o  POST

   An invalidation based on a URI from a Location or Content-Location
   header MUST NOT be performed if the host part of that URI differs
   from the host part in the Effective Request URI (Section 4.3 of
   [Part1]).  This helps prevent denial of service attacks.

   [[TODO-def-host-part: "host part" needs to be specified better.]]

   A cache that passes through requests for methods it does not
   understand SHOULD invalidate the Effective Request URI (Section 4.3
   of [Part1]).

   Here, "invalidate" means that the cache will either remove all stored
   responses related to the Effective Request URI, or will mark these as
   "invalid" and in need of a mandatory validation before they can be
   returned in response to a subsequent request.

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

   [[TODO-spec-success-invalidate: specify that only successful (2xx,
   3xx?) responses invalidate.]]

2.6.  Shared Caching of Authenticated Responses

   Shared caches MUST NOT use a cached response to a request with an
   Authorization header (Section 3.1 of [Part7]) to satisfy any
   subsequent request unless a cache directive that allows such
   responses to be stored is present in the response.

   In this specification, the following Cache-Control response
   directives (Section 3.2.2) have such an effect: must-revalidate,
   public, s-maxage.

   Note that cached responses that contain the "must-revalidate" and/or
   "s-maxage" response directives are not allowed to be served stale
   (Section 2.3.3) by shared caches.  In particular, a response with



Fielding, et al.        Expires January 13, 2011               [Page 15]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


   either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
   satisfy a subsequent request without revalidating it on the origin
   server.

2.7.  Caching Negotiated Responses

   When a cache receives a request that can be satisfied by a stored
   response that has a Vary header field (Section 3.5), it MUST NOT use
   that response unless all of the selecting request-headers nominated
   by the Vary header match in both the original request (i.e., that
   associated with the stored response), and the presented request.

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

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

   o  combining multiple message-header fields with the same field name
      (see Section 3.2 of [Part1])

   o  normalizing both header values in a way that is known to have
      identical semantics, according to the header's specification
      (e.g., re-ordering field values when order is not significant;
      case-normalization, where values are defined to be case-
      insensitive)

   If (after any normalisation that may take place) a header field is
   absent from a request, it can only match another request if it is
   also absent there.

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

   The stored response with matching selecting request-headers is known
   as the selected response.

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

2.8.  Combining Responses

   When a cache receives a 304 (Not Modified) response or a 206 (Partial
   Content) response (in this section, the "new" response"), it needs to
   created an updated response by combining the stored response with the



Fielding, et al.        Expires January 13, 2011               [Page 16]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


   new one, so that the updated response can be used to satisfy the
   request.

   If the new response contains an ETag, it identifies the stored
   response to use.  [[TODO-mention-CL: may need language about Content-
   Location here]][[TODO-inm-mult-etags: cover case where INM with
   multiple etags was sent]]

   If the status code is 206 (partial content), both the stored and new
   responses MUST have validators, and those validators MUST match using
   the strong comparison function (see Section 4 of [Part4]).
   Otherwise, the responses MUST NOT be combined.

   The stored response headers are used as those of the updated
   response, except that

   o  any stored Warning headers with warn-code 1xx (see Section 3.6)
      MUST be deleted from the stored response and the updated response.

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

   o  any headers provided in the new response MUST replace the
      corresponding headers from the stored response.

   If a header field-name in the new response matches more than one
   header in the stored response, all such stored headers MUST be
   replaced.

   The updated response can [[TODO-is-req: requirement?]] be used to
   replace the stored response in cache.  In the case of a 206 response,
   the combined entity-body MAY be stored.

   [[ISSUE-how-head: discuss how to handle HEAD updates]]

3.  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.

3.1.  Age

   The "Age" response-header field conveys the sender's estimate of the
   amount of time since the response was generated or successfully



Fielding, et al.        Expires January 13, 2011               [Page 17]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


   validated at the origin server.  Age values are calculated as
   specified in Section 2.3.2.

     Age   = "Age" ":" OWS Age-v
     Age-v = delta-seconds

   Age field-values are non-negative 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 field-value of 2147483648 (2^31).
   Caches SHOULD use an arithmetic type of at least 31 bits of range.

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

3.2.  Cache-Control

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

   HTTP/1.1 caches MUST obey the requirements of the Cache-Control
   directives defined in this section.  See Section 3.2.3 for
   information about how Cache-Control directives defined elsewhere are
   handled.

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

   Cache 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 target a directive to
   a specific cache.

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

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




Fielding, et al.        Expires January 13, 2011               [Page 18]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


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

3.2.1.  Request Cache-Control Directives

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

   no-cache

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

   no-store

      The no-store request directive indicates that a cache MUST NOT
      store any part of either this request or any response to it.  This
      directive applies to both 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.

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

   max-age

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

   max-stale

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



Fielding, et al.        Expires January 13, 2011               [Page 19]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


      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.  [[TODO-staleness: of any staleness?
      --mnot]]

   min-fresh

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

   no-transform

      The no-transform request directive indicates that an intermediate
      cache or proxy MUST NOT change the Content-Encoding, Content-Range
      or Content-Type request headers, nor the request entity-body.

   only-if-cached

      The only-if-cached request directive indicates that the client
      only wishes to return a stored response.  If it receives this
      directive, a cache SHOULD either respond using a stored response
      that is consistent with the other constraints of the request, or
      respond with a 504 (Gateway Timeout) status.  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.

3.2.2.  Response Cache-Control Directives

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

   public

      The public response directive indicates that the response MAY be
      cached, even if it would normally be non-cacheable or cacheable



Fielding, et al.        Expires January 13, 2011               [Page 20]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


      only within a non-shared cache.  (See also Authorization, Section
      3.1 of [Part7], for additional details.)

   private

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

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

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

   no-cache

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

      If the no-cache response directive specifies one or more field-
      names, this requirement is limited to the field-values associated
      with the listed response headers.  That is, the specified field-
      name(s) MUST NOT be sent in the response to a subsequent request
      without successful validation on the origin server.  This allows
      an origin server to prevent the re-use of certain header fields in
      a response, while still allowing caching of the rest of the
      response.

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







Fielding, et al.        Expires January 13, 2011               [Page 21]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


   no-store

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

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

   must-revalidate

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

      The must-revalidate directive is necessary to support reliable
      operation for certain protocol features.  In all circumstances 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 validate a request on the entity could result in
      incorrect operation, such as a silently unexecuted financial
      transaction.

   proxy-revalidate

      The proxy-revalidate response directive has the same meaning as
      the must-revalidate response directive, except that it does not
      apply to non-shared caches.

   max-age

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

   s-maxage

      The s-maxage response directive indicates that, in shared caches,
      the maximum age specified by this directive overrides the maximum



Fielding, et al.        Expires January 13, 2011               [Page 22]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


      age specified by either the max-age directive or the Expires
      header.  The s-maxage directive also implies the semantics of the
      proxy-revalidate response directive.

   no-transform

      The no-transform response directive indicates that an intermediate
      cache or proxy MUST NOT change the Content-Encoding, Content-Range
      or Content-Type response headers, nor the response entity-body.

3.2.3.  Cache Control Extensions

   The Cache-Control header field can be extended through the use of one
   or more cache-extension tokens, each with an optional value.
   Informational extensions (those that do not require a change in cache
   behavior) can be added without changing the semantics of other
   directives.  Behavioral extensions are designed to work by acting as
   modifiers to the existing base of cache directives.  Both the new
   directive and the standard directive are supplied, such that
   applications that do not understand the new directive will default to
   the behavior specified by the standard directive, and those that
   understand the new directive will recognize it as modifying the
   requirements associated with the standard directive.  In this way,
   extensions to the cache-control directives can be made without
   requiring changes to the base protocol.

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

   For example, consider a hypothetical new response directive called
   "community" that acts as a modifier to the private directive.  We
   define this new directive to mean that, in addition to any non-shared
   cache, any cache that is shared only by members of the community
   named within its value may cache the response.  An origin server
   wishing to allow the UCI community to use an otherwise private
   response in their shared cache(s) could do so by including

     Cache-Control: private, community="UCI"

   A cache seeing this header field will act correctly even if the cache
   does not understand the community cache-extension, since it will also
   see and understand the private directive and thus default to the safe
   behavior.

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



Fielding, et al.        Expires January 13, 2011               [Page 23]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


   be combined with standard directives (or the response's default
   cacheability) such that the cache behavior will remain minimally
   correct even if the cache does not understand the extension(s).

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

   Registrations MUST include the following fields:

   o  Cache Directive Name

   o  Pointer to specification text

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

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

3.3.  Expires

   The "Expires" entity-header field gives the date/time after which the
   response is considered stale.  See Section 2.3 for further discussion
   of the freshness model.

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

   The field-value is an absolute date and time as defined by HTTP-date
   in Section 6.1 of [Part1]; it MUST be sent in rfc1123-date format.

     Expires   = "Expires" ":" OWS Expires-v
     Expires-v = HTTP-date

   For example

     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 3.2.2), that directive overrides the
      Expires field.  Likewise, the s-maxage directive overrides Expires
      in shared caches.

   HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in
   the future.

   HTTP/1.1 clients and caches MUST treat other invalid date formats,



Fielding, et al.        Expires January 13, 2011               [Page 24]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


   especially including the value "0", as in the past (i.e., "already
   expired").

3.4.  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" ":" OWS Pragma-v
     Pragma-v          = 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 response directive
   (see Section 3.2.2) 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.
   HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
   sent "Cache-Control: no-cache".

      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.

   This mechanism is deprecated; no new Pragma directives will be
   defined in HTTP.

3.5.  Vary

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

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

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




Fielding, et al.        Expires January 13, 2011               [Page 25]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


     Vary   = "Vary" ":" OWS Vary-v
     Vary-v = "*" / 1#field-name

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

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

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

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

3.6.  Warning

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

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

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








Fielding, et al.        Expires January 13, 2011               [Page 26]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


     Warning    = "Warning" ":" OWS Warning-v
     Warning-v  = 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

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

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

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

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

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

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

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

   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. (preventing the



Fielding, et al.        Expires January 13, 2011               [Page 27]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


   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.

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

   110 Response is stale

      SHOULD be included whenever the returned response is stale.

   111 Revalidation failed

      SHOULD be included if a cache returns a stale response because an
      attempt to validate 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

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

   214 Transformation applied

      MUST be added by 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.

   299 Miscellaneous persistent warning

      The warning text can include arbitrary information to be presented
      to a human user, or logged.  A system receiving this warning MUST



Fielding, et al.        Expires January 13, 2011               [Page 28]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


      NOT take any automated action.

4.  History Lists

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

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

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

5.  IANA Considerations

5.1.  Cache Directive Registry

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

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

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






Fielding, et al.        Expires January 13, 2011               [Page 29]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


5.2.  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 3.1 |
   | Cache-Control     | http     | standard | Section 3.2 |
   | Expires           | http     | standard | Section 3.3 |
   | Pragma            | http     | standard | Section 3.4 |
   | Vary              | http     | standard | Section 3.5 |
   | Warning           | http     | standard | Section 3.6 |
   +-------------------+----------+----------+-------------+

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

6.  Security Considerations

   Caches expose additional potential vulnerabilities, since the
   contents of the cache represent an attractive target for malicious
   exploitation.  Because cache contents persist after an HTTP request
   is complete, an attack on the cache can reveal information long after
   a user believes that the information has been removed from the
   network.  Therefore, cache contents should be protected as sensitive
   information.

7.  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.

8.  References

8.1.  Normative References

   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
              and Message Parsing", draft-ietf-httpbis-p1-messaging-10
              (work in progress), July 2010.

   [Part2]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,



Fielding, et al.        Expires January 13, 2011               [Page 30]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


              and J. Reschke, Ed., "HTTP/1.1, part 2: Message
              Semantics", draft-ietf-httpbis-p2-semantics-10 (work in
              progress), July 2010.

   [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-10 (work in
              progress), July 2010.

   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
              Partial Responses", draft-ietf-httpbis-p5-range-10 (work
              in progress), July 2010.

   [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-10 (work in progress),
              July 2010.

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

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

8.2.  Informative References

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

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

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

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

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




Fielding, et al.        Expires January 13, 2011               [Page 31]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


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 2.1, 3.2).

   Range request responses would become very verbose if all meta-data
   were always returned; by allowing the server to only send needed
   headers in a 206 response, this problem can be avoided.
   (Section 2.8)

   The Cache-Control: max-age directive was not properly defined for
   responses.  (Section 3.2.2)

   Warnings could be cached incorrectly, or not updated appropriately.
   (Section 2.3, 2.8, 3.2, and 3.6) 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

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

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

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

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

Appendix B.  Collected ABNF

   Age = "Age:" OWS Age-v
   Age-v = delta-seconds

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

   Expires = "Expires:" OWS Expires-v
   Expires-v = HTTP-date

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



Fielding, et al.        Expires January 13, 2011               [Page 32]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


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

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

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

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

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

   delta-seconds = 1*DIGIT

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

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

   port = <port, defined in [Part1], Section 2.6>
   pragma-directive = "no-cache" / extension-pragma
   pseudonym = <pseudonym, defined in [Part1], Section 9.9>

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

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

   uri-host = <uri-host, defined in [Part1], Section 2.6>

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




Fielding, et al.        Expires January 13, 2011               [Page 33]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


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

   ABNF diagnostics:

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

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

C.1.  Since RFC2616

   Extracted relevant partitions from [RFC2616].

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

   Closed issues:

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

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

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

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

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

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

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

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





Fielding, et al.        Expires January 13, 2011               [Page 34]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


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

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

   Closed issues:

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

   Other changes:

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

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

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

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

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

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

   Closed issues:

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

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

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

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

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




Fielding, et al.        Expires January 13, 2011               [Page 35]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


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

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

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

   Affected issues:

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

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

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

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

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

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

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

   Closed issues:

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

   Affected issues:

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

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

   Closed issues:

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

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



Fielding, et al.        Expires January 13, 2011               [Page 36]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


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

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

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

   Closed issues:

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

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

   Affected issues:

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

   Partly resolved issues:

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

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

   Closed issues:

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

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

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

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

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

   Partly resolved issues:




Fielding, et al.        Expires January 13, 2011               [Page 37]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


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

Index

   A
      age  6
      Age header  17

   C
      cache  5
      Cache Directives
         max-age  19, 22
         max-stale  19
         min-fresh  20
         must-revalidate  22
         no-cache  19, 21
         no-store  19, 22
         no-transform  20, 23
         only-if-cached  20
         private  21
         proxy-revalidate  22
         public  20
         s-maxage  22
      Cache-Control header  18
      cacheable  5

   E
      Expires header  24
      explicit expiration time  5

   F
      first-hand  6
      fresh  6
      freshness lifetime  6

   G
      Grammar
         Age  18
         Age-v  18
         Cache-Control  18
         Cache-Control-v  18
         cache-extension  18
         cache-request-directive  19
         cache-response-directive  20
         delta-seconds  18
         Expires  24
         Expires-v  24



Fielding, et al.        Expires January 13, 2011               [Page 38]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


         extension-pragma  25
         Pragma  25
         pragma-directive  25
         Pragma-v  25
         Vary  26
         Vary-v  26
         warn-agent  27
         warn-code  27
         warn-date  27
         warn-text  27
         Warning  27
         Warning-v  27
         warning-value  27

   H
      Headers
         Age  17
         Cache-Control  18
         Expires  24
         Pragma  25
         Vary  25
         Warning  26
      heuristic expiration time  5

   M
      max-age
         Cache Directive  19, 22
      max-stale
         Cache Directive  19
      min-fresh
         Cache Directive  20
      must-revalidate
         Cache Directive  22

   N
      no-cache
         Cache Directive  19, 21
      no-store
         Cache Directive  19, 22
      no-transform
         Cache Directive  20, 23

   O
      only-if-cached
         Cache Directive  20

   P
      Pragma header  25



Fielding, et al.        Expires January 13, 2011               [Page 39]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


      private
         Cache Directive  21
      proxy-revalidate
         Cache Directive  22
      public
         Cache Directive  20

   S
      s-maxage
         Cache Directive  22
      stale  6

   V
      validator  6
      Vary header  25

   W
      Warning header  26

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

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










Fielding, et al.        Expires January 13, 2011               [Page 40]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


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

   EMail: JeffMogul@acm.org


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

   EMail: henrikn@microsoft.com


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

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


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

   EMail: paulle@microsoft.com


   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/




Fielding, et al.        Expires January 13, 2011               [Page 41]

Internet-Draft              HTTP/1.1, Part 6                   July 2010


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

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


   Mark Nottingham (editor)

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


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

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
























Fielding, et al.        Expires January 13, 2011               [Page 42]