


HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                              Day Software
Obsoletes: 2616 (if approved)                                  J. Gettys
Intended status: Standards Track                    One Laptop per Child
Expires: September 10, 2009                                     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
                                                           March 9, 2009


                       HTTP/1.1, part 6: Caching
                     draft-ietf-httpbis-p6-cache-06

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   The list of current Internet-Drafts can be accessed at
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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/11> 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.7.













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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  6
     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 . . . . . . . . . . . .  8
     2.3.  Freshness Model  . . . . . . . . . . . . . . . . . . . . .  9
       2.3.1.  Calculating Freshness Lifetime . . . . . . . . . . . . 10
       2.3.2.  Calculating Age  . . . . . . . . . . . . . . . . . . . 11
       2.3.3.  Serving Stale Responses  . . . . . . . . . . . . . . . 13
     2.4.  Validation Model . . . . . . . . . . . . . . . . . . . . . 13
     2.5.  Request Methods that Invalidate  . . . . . . . . . . . . . 14
     2.6.  Caching Negotiated Responses . . . . . . . . . . . . . . . 15
     2.7.  Combining Responses  . . . . . . . . . . . . . . . . . . . 16
   3.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 16
     3.1.  Age  . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     3.2.  Cache-Control  . . . . . . . . . . . . . . . . . . . . . . 17
       3.2.1.  Request Cache-Control Directives . . . . . . . . . . . 18
       3.2.2.  Response Cache-Control Directives  . . . . . . . . . . 20
       3.2.3.  Cache Control Extensions . . . . . . . . . . . . . . . 22
     3.3.  Expires  . . . . . . . . . . . . . . . . . . . . . . . . . 23
     3.4.  Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     3.5.  Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     3.6.  Warning  . . . . . . . . . . . . . . . . . . . . . . . . . 25
   4.  History Lists  . . . . . . . . . . . . . . . . . . . . . . . . 28
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
     5.1.  Message Header Registration  . . . . . . . . . . . . . . . 28
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 29
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 29
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 29
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 30
   Appendix A.  Compatibility with Previous Versions  . . . . . . . . 31
     A.1.  Changes from RFC 2068  . . . . . . . . . . . . . . . . . . 31
     A.2.  Changes from RFC 2616  . . . . . . . . . . . . . . . . . . 31
   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 31
   Appendix C.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 33
     C.1.  Since RFC2616  . . . . . . . . . . . . . . . . . . . . . . 33
     C.2.  Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 33



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     C.3.  Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 34
     C.4.  Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 34
     C.5.  Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 34
     C.6.  Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 35
     C.7.  Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 35
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38












































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1.  Introduction

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

1.1.  Purpose

   An HTTP cache is a local store of response messages and the subsystem
   that controls its message storage, retrieval, and deletion.  A cache
   stores cacheable responses in order to reduce the response time and
   network bandwidth consumption on future, equivalent requests.  Any
   client or server may 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.







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   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



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   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 4.2>
     HTTP-date     = <HTTP-date, defined in [Part1], Section 3.2.1>
     port          = <port, defined in [Part1], Section 2.1>
     pseudonym     = <pseudonym, defined in [Part1], Section 8.9>
     uri-host      = <uri-host, defined in [Part1], Section 2.1>


2.  Cache Operation

2.1.  Response Cacheability

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





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   o  The request method is defined as being cacheable, 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 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 "public"
      directive is present; see Section 3.2), and

   o  the cache understands partial responses, if the response is
      partial or incomplete (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)
   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 Request-URI and that of the stored response match
      (see [[anchor1: TBD]]), 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 2.6), and




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   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).

   [[anchor2: TODO: 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.
   [[anchor3: DISCUSS: 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
   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.

   [[anchor4: TODO: end-to-end and hop-by-hop headers, non-modifiable
   headers removed; re-spec in p1]]

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



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   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. [[anchor5: 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.

   [[anchor6: ISSUE: there are not requirements directly applying to
   cache-request-directives and freshness.]]

   Note that freshness applies only to cache operation; it cannot be
   used to force a user agent to refresh its display or reload a
   resource.  See Section 4 for an explanation of the difference between
   caches and history mechanisms.

2.3.1.  Calculating Freshness Lifetime

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

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

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





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   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,
      but a heuristic may be used; 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%.

   [[anchor7: REVIEW: took away HTTP/1.0 query string heuristic
   uncacheability.]]

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 term "age_value" denotes the value of the Age header, in a form
   appropriate for arithmetic operations.

   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]).  The term "date_value"
   denotes the value of the Date header, in a form appropriate for
   arithmetic operations.




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   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.

   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.

   These are combined as

       corrected_received_age = max(now - date_value, age_value)

   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.

      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.

   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.

   In summary:

     age_value     - Age header field-value received with the response
     date_value    - Date header field-value received with the response
     request_time  - local time when the cache made the request
                    resulting in the stored response
     response_time - local time when the cache received the response
     now           - 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;



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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.

   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

   Checking with the origin server to see if a stale or otherwise
   unusable cached response can be reused is called "validating" or
   "revalidating."  Doing so potentially avoids the overhead of
   retransmitting the response body when the stored response is valid.

   HTTP's conditional request mechanism [Part4] is used for this
   purpose.  When a stored response includes one or more validators,
   such as the field values of an ETag or Last-Modified header field,
   then a validating request SHOULD be made conditional to those field
   values.

   A 304 (Not Modified) response status code indicates that the stored
   response can be updated and reused; see Section 2.7.

   If instead the cache receives a full response (i.e., one with a
   response body), it is used to satisfy the request and replace the
   stored response. [[anchor8: Should there be a requirement here?]]



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   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 (which SHOULD
   include the 111 warn-code; see Section 3.6) unless the stored
   response includes the "must-revalidate" cache directive (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.

   The following HTTP methods MUST cause a cache to invalidate the
   Request-URI as well as the Location and Content-Location headers (if
   present):

   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.

   [[anchor9: TODO: "host part" needs to be specified better.]]

   A cache that passes through requests for methods it does not
   understand SHOULD invalidate the Request-URI.

   Here, "invalidate" means that the cache will either remove all stored
   responses related to the 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.

   [[anchor10: TODO: specify that only successful (2xx, 3xx?) responses
   invalidate.]]





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2.6.  Caching Negotiated Responses

   Use of server-driven content negotiation (Section 4.1 of [Part3])
   alters the conditions under which a cache can use the response for
   subsequent requests.

   When a cache receives a request that can be satisfied by a stored
   response that includes a Vary header field (Section 3.5), it MUST NOT
   use that response unless all of the selecting request-headers in the
   presented request match the corresponding stored request-headers from
   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 [[anchor11: [ref]]] at
   places where this is allowed by the corresponding ABNF, and/or
   combining multiple message-header fields with the same field name
   following the rules about message headers in Section 4.2 of [Part1].
   [[anchor12: DISCUSS: header-specific canonicalisation]]

   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.

   If no stored response matches, the cache MAY forward the presented
   request to the origin server in a conditional request, and SHOULD
   include all ETags stored with potentially suitable responses in an
   If-None-Match request header.  If the server responds with 304 (Not
   Modified) and includes an entity tag or Content-Location that
   indicates the entity to be used, that cached response MUST be used to
   satisfy the presented request, and SHOULD be used to update the
   corresponding stored response; see Section 2.7.

   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.

   If a cache receives a successful response whose Content-Location
   field matches that of an existing stored response for the same
   Request-URI, whose entity-tag differs from that of the existing
   stored response, and whose Date is more recent than that of the
   existing response, the existing response SHOULD NOT be returned in
   response to future requests and SHOULD be deleted from the
   cache.[[anchor13: DISCUSS: Not sure if this is necessary.]]





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2.7.  Combining Responses

   When a cache receives a 304 (Not Modified) response or a 206 (Partial
   Content) response, it needs to update the stored response with the
   new one, so that the updated response can be sent to the client.

   If the status code is 304 (Not Modified), the cache SHOULD use the
   stored entity-body as the updated entity-body.  If the status code is
   206 (Partial Content) and the ETag or Last-Modified headers match
   exactly, the cache MAY combine the stored entity-body in the stored
   response with the updated entity-body received in the response and
   use the result as the updated entity-body (see Section 4 of [Part5]).

   The stored response headers are used for 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 forwarded
      response.

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

   o  any headers provided in the 304 or 206 response MUST replace the
      corresponding headers from the stored response.

   A cache MUST also replace any stored headers with 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 stored
   response, all such old headers MUST be replaced.  It MAY store the
   combined entity-body.

   [[anchor14: ISSUE: 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.







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3.1.  Age

   The response-header field "Age" conveys the sender's estimate of the
   amount of time since the response (or its validation) was generated
   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 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 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 general-header field "Cache-Control" is used to specify
   directives that MUST be obeyed by all caches along the request/
   response chain.  The directives specify behavior intended to prevent
   caches from adversely interfering with the request or response.
   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 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.









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

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

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

3.2.1.  Request Cache-Control Directives

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

   no-cache

      The no-cache request directive indicates that a 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 max-stale directive is also included, the
      client is not willing to accept a stale response.




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   max-stale

      The max-stale request directive indicates that the client is
      willing to accept a response that has exceeded its expiration
      time.  If max-stale is assigned a value, then the client is
      willing to accept a response that has exceeded its expiration time
      by no more than the specified number of seconds.  If no value is
      assigned to max-stale, then the client is willing to accept a
      stale response of any age. [[anchor15: 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.

















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3.2.2.  Response Cache-Control Directives

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

   public

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

   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 assosicated
      with the listed response headers.  That is, the specified field-



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      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.

   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



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      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
      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"



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   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
   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).

3.3.  Expires

   The entity-header field "Expires" 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 3.2.1 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,
   especially including the value "0", as in the past (i.e., "already
   expired").

3.4.  Pragma

   The general-header field "Pragma" is used to include implementation-
   specific directives that might apply to any recipient along the
   request/response chain.  All pragma directives specify optional



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   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's value indicates the set of
   request-header fields that 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 2.6.

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

     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,



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   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 general-header field "Warning" 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,
   distinguish 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.

     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.  For example, a server might provide the same warning with



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   texts in both English and Basque.

   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.  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
      warn-agents.

   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 that describe the freshness or validation status of
      the response, and so MUST be deleted by caches after validation.
      They MUST NOT be generated by a cache except when validating a
      cached entry, and MUST NOT be generated by clients.

   o  2xx Warnings that 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.

   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 can 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].

   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.



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   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
   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.




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   299 Miscellaneous persistent warning

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


4.  History Lists

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

   History mechanisms and caches are different.  In particular history
   mechanisms SHOULD NOT try to show a correct 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.


5.  IANA Considerations

5.1.  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]):




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   +-------------------+----------+----------+-------------+
   | 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

   [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-06
              (work in progress), March 2009.



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   [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-06 (work in
              progress), March 2009.

   [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-06
              (work in progress), March 2009.

   [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-06 (work in
              progress), March 2009.

   [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-06 (work
              in progress), March 2009.

   [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-06 (work in progress),
              March 2009.

   [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.

   [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.



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   [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 2.1, 3.2).

   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. (see also
   [Part1], [Part3] and [Part5]) [[anchor18: This used to refer to the
   text about non-modifiable headers, and will have to be updated later
   on. --jre]]

   Proxies should be able to add Content-Length when appropriate.
   [[anchor19: This used to refer to the text about non-modifiable
   headers, and will have to be updated later on. --jre]]

   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 2.7)

   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, 2.7, 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

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


Appendix B.  Collected ABNF

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

   Cache-Control = "Cache-Control:" OWS Cache-Control-v



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   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 3.2.1>

   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 4.2>

   port = <port, defined in [Part1], Section 2.1>
   pragma-directive = "no-cache" / extension-pragma
   pseudonym = <pseudonym, defined in [Part1], Section 8.9>

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

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



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   uri-host = <uri-host, defined in [Part1], Section 2.1>

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



   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"





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

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

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

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

   Other changes:

   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"





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

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

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

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

   o  Rewrite ABNFs to spell out whitespace rules, factor out header
      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://trac.tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement
      of 13.5.1 and 13.5.2"

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

   o  <http://trac.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.


Index

   A
      age  6
      Age header  17

   C
      cache  5
      Cache Directives
         max-age  18, 21



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         max-stale  19
         min-fresh  19
         must-revalidate  21
         no-cache  18, 20
         no-store  18, 21
         no-transform  19, 22
         only-if-cached  19
         private  20
         proxy-revalidate  21
         public  20
         s-maxage  22
      Cache-Control header  17
      cacheable  5

   E
      Expires header  23
      explicit expiration time  5

   F
      first-hand  6
      fresh  6
      freshness lifetime  6

   G
      Grammar
         Age  17
         Age-v  17
         Cache-Control  18
         Cache-Control-v  18
         cache-extension  18
         cache-request-directive  18
         cache-response-directive  20
         delta-seconds  17
         Expires  23
         Expires-v  23
         extension-pragma  24
         Pragma  24
         pragma-directive  24
         Pragma-v  24
         Vary  24
         Vary-v  24
         warn-agent  25
         warn-code  25
         warn-date  25
         warn-text  25
         Warning  25
         Warning-v  25
         warning-value  25



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   H
      Headers
         Age  17
         Cache-Control  17
         Expires  23
         Pragma  23
         Vary  24
         Warning  25
      heuristic expiration time  5

   M
      max-age
         Cache Directive  18, 21
      max-stale
         Cache Directive  19
      min-fresh
         Cache Directive  19
      must-revalidate
         Cache Directive  21

   N
      no-cache
         Cache Directive  18, 20
      no-store
         Cache Directive  18, 21
      no-transform
         Cache Directive  19, 22

   O
      only-if-cached
         Cache Directive  19

   P
      Pragma header  23
      private
         Cache Directive  20
      proxy-revalidate
         Cache Directive  21
      public
         Cache Directive  20

   S
      s-maxage
         Cache Directive  22
      stale  6

   V
      validator  6



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      Vary header  24

   W
      Warning header  25


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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