rfc7234 Varnish source references







Internet Engineering Task Force (IETF)                  R. Fielding, Ed.
Request for Comments: 7234                                         Adobe
Obsoletes: 2616                                       M. Nottingham, Ed.
Category: Standards Track                                         Akamai
ISSN: 2070-1721                                          J. Reschke, Ed.
                                                              greenbytes
                                                               June 2014


            Hypertext Transfer Protocol (HTTP/1.1): Caching

Abstract

   The Hypertext Transfer Protocol (HTTP) is a stateless application-
   level protocol for distributed, collaborative, hypertext information
   systems.  This document defines HTTP caches and the associated header
   fields that control cache behavior or indicate cacheable response
   messages.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7234.



















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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

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

Table of Contents

   1. Introduction ....................................................4
      1.1. Conformance and Error Handling .............................4
      1.2. Syntax Notation ............................................4
           1.2.1. Delta Seconds .......................................5
   2. Overview of Cache Operation .....................................5
   3. Storing Responses in Caches .....................................6
      3.1. Storing Incomplete Responses ...............................7
      3.2. Storing Responses to Authenticated Requests ................7
      3.3. Combining Partial Content ..................................8
   4. Constructing Responses from Caches ..............................8
      4.1. Calculating Secondary Keys with Vary .......................9
      4.2. Freshness .................................................11
           4.2.1. Calculating Freshness Lifetime .....................12
           4.2.2. Calculating Heuristic Freshness ....................13
           4.2.3. Calculating Age ....................................13
           4.2.4. Serving Stale Responses ............................15
      4.3. Validation ................................................16
           4.3.1. Sending a Validation Request .......................16
           4.3.2. Handling a Received Validation Request .............16



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117
118
119	4.3.3. Handling a Validation Response .....................18
120	4.3.4. Freshening Stored Responses upon Validation ........18
121	4.3.5. Freshening Responses via HEAD ......................19
122	4.4. Invalidation ..............................................20
123	5. Header Field Definitions .......................................21
124	5.1. Age .......................................................21
125	5.2. Cache-Control .............................................21
126	5.2.1. Request Cache-Control Directives ...................22
127	5.2.2. Response Cache-Control Directives ..................24
128	5.2.3. Cache Control Extensions ...........................27
129	5.3. Expires ...................................................28
130	5.4. Pragma ....................................................29
131	5.5. Warning ...................................................29
132	5.5.1. Warning: 110 - "Response is Stale" .................31
133	5.5.2. Warning: 111 - "Revalidation Failed" ...............31
134	5.5.3. Warning: 112 - "Disconnected Operation" ............31
135	5.5.4. Warning: 113 - "Heuristic Expiration" ..............31
136	5.5.5. Warning: 199 - "Miscellaneous Warning" .............32
137	5.5.6. Warning: 214 - "Transformation Applied" ............32
138	5.5.7. Warning: 299 - "Miscellaneous Persistent Warning" ..32
139	6. History Lists ..................................................32
140	7. IANA Considerations ............................................32
141	7.1. Cache Directive Registry ..................................32
142	7.1.1. Procedure ..........................................32
143	7.1.2. Considerations for New Cache Control Directives ....33
144	7.1.3. Registrations ......................................33
145	7.2. Warn Code Registry ........................................34
146	7.2.1. Procedure ..........................................34
147	7.2.2. Registrations ......................................34
148	7.3. Header Field Registration .................................34
149	8. Security Considerations ........................................35
150	9. Acknowledgments ................................................36
151	10. References ....................................................36
152	10.1. Normative References .....................................36
153	10.2. Informative References ...................................37
154	Appendix A. Changes from RFC 2616 .................................38
155	Appendix B. Imported ABNF .........................................39
156	Appendix C. Collected ABNF ........................................39
157	Index .............................................................41
158
159
160
161
162
163
164
165
166
167
168
169
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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.

   An HTTP cache is a local store of response messages and the subsystem
   that controls storage, retrieval, and deletion of messages in it.  A
   cache stores cacheable responses in order to reduce the response time
   and network bandwidth consumption on future, equivalent requests.
   Any client or server MAY employ a cache, though a cache cannot be
   used by a server that is acting as a tunnel.

   A shared cache is a cache that stores responses to be reused by more
   than one user; shared caches are usually (but not always) deployed as
   a part of an intermediary.  A private cache, in contrast, is
   dedicated to a single user; often, they are deployed as a component
   of a user agent.

   The goal of caching in HTTP/1.1 is to significantly improve
   performance by reusing a prior response message to satisfy a current
   request.  A stored response is considered "fresh", as defined in
   Section 4.2, if the response can be reused without "validation"
   (checking with the origin server to see if the cached response
   remains valid for this request).  A fresh response can therefore
   reduce both latency and network overhead each time it is reused.
   When a cached response is not fresh, it might still be reusable if it
   can be freshened by validation (Section 4.3) or if the origin is
   unavailable (Section 4.2.4).

1.1.  Conformance and Error Handling

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   Conformance criteria and considerations regarding error handling are
   defined in Section 2.5 of [RFC7230].

1.2.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234] with a list extension, defined in Section 7 of
   [RFC7230], that allows for compact definition of comma-separated
   lists using a '#' operator (similar to how the '*' operator indicates





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   repetition).  Appendix B describes rules imported from other
   documents.  Appendix C shows the collected grammar with all list
   operators expanded to standard ABNF notation.

1.2.1.  Delta Seconds

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

     delta-seconds  = 1*DIGIT

   A recipient parsing a delta-seconds value and converting it to binary
   form ought to use an arithmetic type of at least 31 bits of
   non-negative integer range.  If a cache receives a delta-seconds
   value greater than the greatest integer it can represent, or if any
   of its subsequent calculations overflows, the cache MUST consider the
   value to be either 2147483648 (2^31) or the greatest positive integer
   it can conveniently represent.

      Note: The value 2147483648 is here for historical reasons,
      effectively represents infinity (over 68 years), and does not need
      to be stored in binary form; an implementation could produce it as
      a canned string if any overflow occurs, even if the calculations
      are performed with an arithmetic type incapable of directly
      representing that number.  What matters here is that an overflow
      be detected and not treated as a negative value in later
      calculations.

2.  Overview of Cache Operation

   Proper cache operation preserves the semantics of HTTP transfers
   ([RFC7231]) while eliminating the transfer of information already
   held in the cache.  Although caching is an entirely OPTIONAL feature
   of HTTP, it can be assumed that reusing a cached response is
   desirable and that such reuse is the default behavior when no
   requirement or local configuration prevents it.  Therefore, HTTP
   cache requirements are focused on preventing a cache from either
   storing a non-reusable response or reusing a stored response
   inappropriately, rather than mandating that caches always store and
   reuse particular responses.

   Each cache entry consists of a cache key and one or more HTTP
   responses corresponding to prior requests that used the same key.
   The most common form of cache entry is a successful result of a
   retrieval request: i.e., a 200 (OK) response to a GET request, which
   contains a representation of the resource identified by the request
   target (Section 4.3.1 of [RFC7231]).  However, it is also possible to
   cache permanent redirects, negative results (e.g., 404 (Not Found)),



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   incomplete results (e.g., 206 (Partial Content)), and responses to
   methods other than GET if the method's definition allows such caching
   and defines something suitable for use as a cache key.

   The primary cache key consists of the request method and target URI.
   However, since HTTP caches in common use today are typically limited
   to caching responses to GET, many caches simply decline other methods
   and use only the URI as the primary cache key.

   If a request target is subject to content negotiation, its cache
   entry might consist of multiple stored responses, each differentiated
   by a secondary key for the values of the original request's selecting
   header fields (Section 4.1).

3.  Storing Responses in Caches

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

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

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

   o  the "no-store" cache directive (see Section 5.2) does not appear
      in request or response header fields, and

   o  the "private" response directive (see Section 5.2.2.6) does not
      appear in the response, if the cache is shared, and

   o  the Authorization header field (see Section 4.2 of [RFC7235]) does
      not appear in the request, if the cache is shared, unless the
      response explicitly allows it (see Section 3.2), and

   o  the response either:

      *  contains an Expires header field (see Section 5.3), or

      *  contains a max-age response directive (see Section 5.2.2.8), or

      *  contains a s-maxage response directive (see Section 5.2.2.9)
         and the cache is shared, or

      *  contains a Cache Control Extension (see Section 5.2.3) that
         allows it to be cached, or

      *  has a status code that is defined as cacheable by default (see
         Section 4.2.2), or




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      *  contains a public response directive (see Section 5.2.2.5).

   Note that any of the requirements listed above can be overridden by a
   cache-control extension; see Section 5.2.3.

   In this context, a cache has "understood" a request method or a
   response status code if it recognizes it and implements all specified
   caching-related behavior.

   Note that, in normal operation, some 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.

3.1.  Storing Incomplete Responses

   A response message is considered complete when all of the octets
   indicated by the message framing ([RFC7230]) are received prior to
   the connection being closed.  If the request method is GET, the
   response status code is 200 (OK), and the entire response header
   section has been received, a cache MAY store an incomplete response
   message body if the cache entry is recorded as incomplete.  Likewise,
   a 206 (Partial Content) response MAY be stored as if it were an
   incomplete 200 (OK) cache entry.  However, a cache MUST NOT store
   incomplete or partial-content responses if it does not support the
   Range and Content-Range header fields or if it does not understand
   the range units used in those fields.

   A cache MAY complete a stored incomplete response by making a
   subsequent range request ([RFC7233]) and combining the successful
   response with the stored entry, as defined in Section 3.3.  A cache
   MUST NOT use an incomplete response to answer requests unless the
   response has been made complete or the request is partial and
   specifies a range that is wholly within the incomplete response.  A
   cache MUST NOT send a partial response to a client without explicitly
   marking it as such using the 206 (Partial Content) status code.

3.2.  Storing Responses to Authenticated Requests

   A shared cache MUST NOT use a cached response to a request with an
   Authorization header field (Section 4.2 of [RFC7235]) 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 5.2.2) have such an effect: must-revalidate,
   public, and s-maxage.




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   Note that cached responses that contain the "must-revalidate" and/or
   "s-maxage" response directives are not allowed to be served stale
   (Section 4.2.4) by shared caches.  In particular, a response with
   either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
   satisfy a subsequent request without revalidating it on the origin
   server.

3.3.  Combining Partial Content

   A response might transfer only a partial representation if the
   connection closed prematurely or if the request used one or more
   Range specifiers ([RFC7233]).  After several such transfers, a cache
   might have received several ranges of the same representation.  A
   cache MAY combine these ranges into a single stored response, and
   reuse that response to satisfy later requests, if they all share the
   same strong validator and the cache complies with the client
   requirements in Section 4.3 of [RFC7233].

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

   o  delete any Warning header fields in the stored response with
      warn-code 1xx (see Section 5.5);

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

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

4.  Constructing Responses from Caches

   When presented with a request, a cache MUST NOT reuse a stored
   response, unless:

   o  The presented effective request URI (Section 5.5 of [RFC7230]) and
      that of the stored response match, and

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

   o  selecting header fields nominated by the stored response (if any)
      match those presented (see Section 4.1), and







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   o  the presented request does not contain the no-cache pragma
      (Section 5.4), nor the no-cache cache directive (Section 5.2.1),
      unless the stored response is successfully validated
      (Section 4.3), and

   o  the stored response does not contain the no-cache cache directive
      (Section 5.2.2.2), unless it is successfully validated
      (Section 4.3), and

   o  the stored response is either:

      *  fresh (see Section 4.2), or

      *  allowed to be served stale (see Section 4.2.4), or

      *  successfully validated (see Section 4.3).

   Note that any of the requirements listed above can be overridden by a
   cache-control extension; see Section 5.2.3.

   When a stored response is used to satisfy a request without
   validation, a cache MUST generate an Age header field (Section 5.1),
   replacing any present in the response with a value equal to the
   stored response's current_age; see Section 4.2.3.

   A cache MUST write through requests with methods that are unsafe
   (Section 4.2.1 of [RFC7231]) to the origin server; i.e., a cache is
   not allowed to generate a reply to such a request before having
   forwarded the request and having received a corresponding response.

   Also, note that unsafe requests might invalidate already-stored
   responses; see Section 4.4.

   When more than one suitable response is stored, a cache MUST use the
   most recent response (as determined by the Date header field).  It
   can also forward the request with "Cache-Control: max-age=0" or
   "Cache-Control: no-cache" to disambiguate which response to use.

   A cache that does not have a clock available MUST NOT use stored
   responses without revalidating them upon every use.

4.1.  Calculating Secondary Keys with Vary

   When a cache receives a request that can be satisfied by a stored
   response that has a Vary header field (Section 7.1.4 of [RFC7231]),
   it MUST NOT use that response unless all of the selecting header





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   fields nominated by the Vary header field match in both the original
   request (i.e., that associated with the stored response), and the
   presented request.

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

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

   o  combining multiple header fields with the same field name (see
      Section 3.2 of [RFC7230])

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

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

   A Vary header field-value of "*" always fails to match.

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

   If multiple selected responses are available (potentially including
   responses without a Vary header field), the cache will need to choose
   one to use.  When a selecting header field has a known mechanism for
   doing so (e.g., qvalues on Accept and similar request header fields),
   that mechanism MAY be used to select preferred responses; of the
   remainder, the most recent response (as determined by the Date header
   field) is used, as per Section 4.

   If no selected response is available, the cache cannot satisfy the
   presented request.  Typically, it is forwarded to the origin server
   in a (possibly conditional; see Section 4.3) request.











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4.2.  Freshness

   A fresh response is one whose age has not yet exceeded its freshness
   lifetime.  Conversely, a stale response is one where it has.

   A response's freshness lifetime is the length of time between its
   generation by the origin server and its expiration time.  An explicit
   expiration time is the time at which the origin server intends that a
   stored response can no longer be used by a cache without further
   validation, whereas a heuristic expiration time is assigned by a
   cache when no explicit expiration time is available.

   A response's age is the time that has passed since it was generated
   by, or successfully validated with, the origin server.

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

   The primary mechanism for determining freshness is for an origin
   server to provide an explicit expiration time in the future, using
   either the Expires header field (Section 5.3) or the max-age response
   directive (Section 5.2.2.8).  Generally, origin servers will assign
   future explicit expiration times to responses in the belief that the
   representation is not likely to change in a semantically significant
   way before the expiration time is reached.

   If an origin server wishes to force a cache to validate every
   request, it can assign an explicit expiration time in the past to
   indicate that the response is already stale.  Compliant caches will
   normally validate a stale cached response before reusing it for
   subsequent requests (see Section 4.2.4).

   Since origin servers do not always provide explicit expiration times,
   caches are also allowed to use a heuristic to determine an expiration
   time under certain circumstances (see Section 4.2.2).

   The calculation to determine if a response is fresh is:

      response_is_fresh = (freshness_lifetime > current_age)

   freshness_lifetime is defined in Section 4.2.1; current_age is
   defined in Section 4.2.3.

   Clients can send the max-age or min-fresh cache directives in a
   request to constrain or relax freshness calculations for the
   corresponding response (Section 5.2.1).




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   When calculating freshness, to avoid common problems in date parsing:

   o  Although all date formats are specified to be case-sensitive, a
      cache recipient SHOULD match day, week, and time-zone names
      case-insensitively.

   o  If a cache recipient's internal implementation of time has less
      resolution than the value of an HTTP-date, the recipient MUST
      internally represent a parsed Expires date as the nearest time
      equal to or earlier than the received value.

   o  A cache recipient MUST NOT allow local time zones to influence the
      calculation or comparison of an age or expiration time.

   o  A cache recipient SHOULD consider a date with a zone abbreviation
      other than GMT or UTC to be invalid for calculating expiration.

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

4.2.1.  Calculating Freshness Lifetime

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

   o  If the cache is shared and the s-maxage response directive
      (Section 5.2.2.9) is present, use its value, or

   o  If the max-age response directive (Section 5.2.2.8) is present,
      use its value, or

   o  If the Expires response header field (Section 5.3) is present, use
      its value minus the value of the Date response header field, or

   o  Otherwise, no explicit expiration time is present in the response.
      A heuristic freshness lifetime might be applicable; see
      Section 4.2.2.

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








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   When there is more than one value present for a given directive
   (e.g., two Expires header fields, multiple Cache-Control: max-age
   directives), the directive's value is considered invalid.  Caches are
   encouraged to consider responses that have invalid freshness
   information to be stale.

4.2.2.  Calculating Heuristic Freshness

   Since origin servers do not always provide explicit expiration times,
   a cache MAY assign a heuristic expiration time when an explicit time
   is not specified, employing algorithms that use other header field
   values (such as the Last-Modified time) to estimate a plausible
   expiration time.  This specification does not provide specific
   algorithms, but does impose worst-case constraints on their results.

   A cache MUST NOT use heuristics to determine freshness when an
   explicit expiration time is present in the stored response.  Because
   of the requirements in Section 3, this means that, effectively,
   heuristics can only be used on responses without explicit freshness
   whose status codes are defined as cacheable by default (see Section
   6.1 of [RFC7231]), and those responses without explicit freshness
   that have been marked as explicitly cacheable (e.g., with a "public"
   response directive).

   If the response has a Last-Modified header field (Section 2.2 of
   [RFC7232]), caches are encouraged to use a heuristic expiration value
   that is no more than some fraction of the interval since that time.
   A typical setting of this fraction might be 10%.

   When a heuristic is used to calculate freshness lifetime, a cache
   SHOULD generate a Warning header field with a 113 warn-code (see
   Section 5.5.4) in the response if its current_age is more than 24
   hours and such a warning is not already present.

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

4.2.3.  Calculating Age

   The Age header field is used to convey an estimated age of the
   response message when obtained from a cache.  The Age field value is
   the cache's estimate of the number of seconds since the response was
   generated or validated by the origin server.  In essence, the Age




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

   The following data is used for the age calculation:

   age_value

      The term "age_value" denotes the value of the Age header field
      (Section 5.1), in a form appropriate for arithmetic operation; or
      0, if not available.

   date_value

      The term "date_value" denotes the value of the Date header field,
      in a form appropriate for arithmetic operations.  See Section
      7.1.1.2 of [RFC7231] for the definition of the Date header field,
      and for requirements regarding responses without it.

   now

      The term "now" means "the current value of the clock at the host
      performing the calculation".  A host ought to use NTP ([RFC5905])
      or some similar protocol to synchronize its clocks to Coordinated
      Universal Time.

   request_time

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

   response_time

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

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

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

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




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     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);

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

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

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

4.2.4.  Serving Stale Responses

   A "stale" response is one that either has explicit expiry information
   or is allowed to have heuristic expiry calculated, but is not fresh
   according to the calculations in Section 4.2.

   A cache MUST NOT generate 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 5.2.2).

   A cache MUST NOT send stale responses unless it is disconnected
   (i.e., it cannot contact the origin server or otherwise find a
   forward path) or doing so is explicitly allowed (e.g., by the
   max-stale request directive; see Section 5.2.1).

   A cache SHOULD generate a Warning header field with the 110 warn-code
   (see Section 5.5.1) in stale responses.  Likewise, a cache SHOULD
   generate a 112 warn-code (see Section 5.5.3) in stale responses if
   the cache is disconnected.

   A cache SHOULD NOT generate a new Warning header field when
   forwarding a response that does not have an Age header field, even if
   the response is already stale.  A cache need not validate a response
   that merely became stale in transit.




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4.3.  Validation

   When a cache has one or more stored responses for a requested URI,
   but cannot serve any of them (e.g., because they are not fresh, or
   one cannot be selected; see Section 4.1), it can use the conditional
   request mechanism [RFC7232] in the forwarded request to give the next
   inbound server an opportunity to select a valid stored response to
   use, updating the stored metadata in the process, or to replace the
   stored response(s) with a new response.  This process is known as
   "validating" or "revalidating" the stored response.

4.3.1.  Sending a Validation Request

   When sending a conditional request for cache validation, a cache
   sends one or more precondition header fields containing validator
   metadata from its stored response(s), which is then compared by
   recipients to determine whether a stored response is equivalent to a
   current representation of the resource.

   One such validator is the timestamp given in a Last-Modified header
   field (Section 2.2 of [RFC7232]), which can be used in an
   If-Modified-Since header field for response validation, or in an
   If-Unmodified-Since or If-Range header field for representation
   selection (i.e., the client is referring specifically to a previously
   obtained representation with that timestamp).

   Another validator is the entity-tag given in an ETag header field
   (Section 2.3 of [RFC7232]).  One or more entity-tags, indicating one
   or more stored responses, can be used in an If-None-Match header
   field for response validation, or in an If-Match or If-Range header
   field for representation selection (i.e., the client is referring
   specifically to one or more previously obtained representations with
   the listed entity-tags).

4.3.2.  Handling a Received Validation Request

   Each client in the request chain may have its own cache, so it is
   common for a cache at an intermediary to receive conditional requests
   from other (outbound) caches.  Likewise, some user agents make use of
   conditional requests to limit data transfers to recently modified
   representations or to complete the transfer of a partially retrieved
   representation.

   If a cache receives a request that can be satisfied by reusing one of
   its stored 200 (OK) or 206 (Partial Content) responses, the cache
   SHOULD evaluate any applicable conditional header field preconditions
   received in that request with respect to the corresponding validators
   contained within the selected response.  A cache MUST NOT evaluate



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   conditional header fields that are only applicable to an origin
   server, found in a request with semantics that cannot be satisfied
   with a cached response, or applied to a target resource for which it
   has no stored responses; such preconditions are likely intended for
   some other (inbound) server.

   The proper evaluation of conditional requests by a cache depends on
   the received precondition header fields and their precedence, as
   defined in Section 6 of [RFC7232].  The If-Match and
   If-Unmodified-Since conditional header fields are not applicable to a
   cache.

   A request containing an If-None-Match header field (Section 3.2 of
   [RFC7232]) indicates that the client wants to validate one or more of
   its own stored responses in comparison to whichever stored response
   is selected by the cache.  If the field-value is "*", or if the
   field-value is a list of entity-tags and at least one of them matches
   the entity-tag of the selected stored response, a cache recipient
   SHOULD generate a 304 (Not Modified) response (using the metadata of
   the selected stored response) instead of sending that stored
   response.

   When a cache decides to revalidate its own stored responses for a
   request that contains an If-None-Match list of entity-tags, the cache
   MAY combine the received list with a list of entity-tags from its own
   stored set of responses (fresh or stale) and send the union of the
   two lists as a replacement If-None-Match header field value in the
   forwarded request.  If a stored response contains only partial
   content, the cache MUST NOT include its entity-tag in the union
   unless the request is for a range that would be fully satisfied by
   that partial stored response.  If the response to the forwarded
   request is 304 (Not Modified) and has an ETag header field value with
   an entity-tag that is not in the client's list, the cache MUST
   generate a 200 (OK) response for the client by reusing its
   corresponding stored response, as updated by the 304 response
   metadata (Section 4.3.4).

   If an If-None-Match header field is not present, a request containing
   an If-Modified-Since header field (Section 3.3 of [RFC7232])
   indicates that the client wants to validate one or more of its own
   stored responses by modification date.  A cache recipient SHOULD
   generate a 304 (Not Modified) response (using the metadata of the
   selected stored response) if one of the following cases is true: 1)
   the selected stored response has a Last-Modified field-value that is
   earlier than or equal to the conditional timestamp; 2) no
   Last-Modified field is present in the selected stored response, but
   it has a Date field-value that is earlier than or equal to the
   conditional timestamp; or, 3) neither Last-Modified nor Date is



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   present in the selected stored response, but the cache recorded it as
   having been received at a time earlier than or equal to the
   conditional timestamp.

   A cache that implements partial responses to range requests, as
   defined in [RFC7233], also needs to evaluate a received If-Range
   header field (Section 3.2 of [RFC7233]) with respect to its selected
   stored response.

4.3.3.  Handling a Validation Response

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

   o  A 304 (Not Modified) response status code indicates that the
      stored response can be updated and reused; see Section 4.3.4.

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

   o  However, if a cache receives a 5xx (Server Error) response while
      attempting to validate a response, it can either forward this
      response to the requesting client, or act as if the server failed
      to respond.  In the latter case, the cache MAY send a previously
      stored response (see Section 4.2.4).

4.3.4.  Freshening Stored Responses upon Validation

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

   The stored response to update is identified by using the first match
   (if any) of the following:

   o  If the new response contains a strong validator (see Section 2.1
      of [RFC7232]), then that strong validator identifies the selected
      representation for update.  All of the stored responses with the
      same strong validator are selected.  If none of the stored
      responses contain the same strong validator, then the cache MUST
      NOT use the new response to update any stored responses.






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   o  If the new response contains a weak validator and that validator
      corresponds to one of the cache's stored responses, then the most
      recent of those matching stored responses is selected for update.

   o  If the new response does not include any form of validator (such
      as in the case where a client generates an If-Modified-Since
      request from a source other than the Last-Modified response header
      field), and there is only one stored response, and that stored
      response also lacks a validator, then that stored response is
      selected for update.

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

   o  delete any Warning header fields in the stored response with
      warn-code 1xx (see Section 5.5);

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

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

4.3.5.  Freshening Responses via HEAD

   A response to the HEAD method is identical to what an equivalent
   request made with a GET would have been, except it lacks a body.
   This property of HEAD responses can be used to invalidate or update a
   cached GET response if the more efficient conditional GET request
   mechanism is not available (due to no validators being present in the
   stored response) or if transmission of the representation body is not
   desired even if it has changed.

   When a cache makes an inbound HEAD request for a given request target
   and receives a 200 (OK) response, the cache SHOULD update or
   invalidate each of its stored GET responses that could have been
   selected for that request (see Section 4.1).

   For each of the stored responses that could have been selected, if
   the stored response and HEAD response have matching values for any
   received validator fields (ETag and Last-Modified) and, if the HEAD
   response has a Content-Length header field, the value of
   Content-Length matches that of the stored response, the cache SHOULD
   update the stored response as described below; otherwise, the cache
   SHOULD consider the stored response to be stale.






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   If a cache updates a stored response with the metadata provided in a
   HEAD response, the cache MUST:

   o  delete any Warning header fields in the stored response with
      warn-code 1xx (see Section 5.5);

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

   o  use other header fields provided in the HEAD response to replace
      all instances of the corresponding header fields in the stored
      response and append new header fields to the stored response's
      header section unless otherwise restricted by the Cache-Control
      header field.

4.4.  Invalidation

   Because unsafe request methods (Section 4.2.1 of [RFC7231]) such as
   PUT, POST or DELETE have the potential for changing state on the
   origin server, intervening caches can use them to keep their contents
   up to date.

   A cache MUST invalidate the effective Request URI (Section 5.5 of
   [RFC7230]) as well as the URI(s) in the Location and Content-Location
   response header fields (if present) when a non-error status code is
   received in response to an unsafe request method.

   However, a cache MUST NOT invalidate a URI from a Location or
   Content-Location response header field if the host part of that URI
   differs from the host part in the effective request URI (Section 5.5
   of [RFC7230]).  This helps prevent denial-of-service attacks.

   A cache MUST invalidate the effective request URI (Section 5.5 of
   [RFC7230]) when it receives a non-error response to a request with a
   method whose safety is unknown.

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

   Note that this does not guarantee that all appropriate responses are
   invalidated.  For example, a state-changing request might invalidate
   responses in the caches it travels through, but relevant responses
   still might be stored in other caches that it has not.




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5.  Header Field Definitions

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

5.1.  Age

   The "Age" header field conveys the sender's estimate of the amount of
   time since the response was generated or successfully validated at
   the origin server.  Age values are calculated as specified in
   Section 4.2.3.

     Age = delta-seconds

   The Age field-value is a non-negative integer, representing time in
   seconds (see Section 1.2.1).

   The presence of an Age header field implies that the response was not
   generated or validated by the origin server for this request.
   However, lack of an Age header field does not imply the origin was
   contacted, since the response might have been received from an
   HTTP/1.0 cache that does not implement Age.

5.2.  Cache-Control

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

   A cache MUST obey the requirements of the Cache-Control directives
   defined in this section.  See Section 5.2.3 for information about how
   Cache-Control directives defined elsewhere are handled.

      Note: Some HTTP/1.0 caches might not implement Cache-Control.

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

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



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     Cache-Control   = 1#cache-directive

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

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

5.2.1.  Request Cache-Control Directives

5.2.1.1.  max-age

   Argument syntax:

      delta-seconds (see Section 1.2.1)

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

   This directive uses the token form of the argument syntax: e.g.,
   'max-age=5' not 'max-age="5"'.  A sender SHOULD NOT generate the
   quoted-string form.

5.2.1.2.  max-stale

   Argument syntax:

      delta-seconds (see Section 1.2.1)

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

   This directive uses the token form of the argument syntax: e.g.,
   'max-stale=10' not 'max-stale="10"'.  A sender SHOULD NOT generate
   the quoted-string form.

5.2.1.3.  min-fresh

   Argument syntax:

      delta-seconds (see Section 1.2.1)



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   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.

   This directive uses the token form of the argument syntax: e.g.,
   'min-fresh=20' not 'min-fresh="20"'.  A sender SHOULD NOT generate
   the quoted-string form.

5.2.1.4.  no-cache

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

5.2.1.5.  no-store

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

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

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

5.2.1.6.  no-transform

   The "no-transform" request directive indicates that an intermediary
   (whether or not it implements a cache) MUST NOT transform the
   payload, as defined in Section 5.7.2 of [RFC7230].

5.2.1.7.  only-if-cached

   The "only-if-cached" request directive indicates that the client only
   wishes to obtain a stored response.  If it receives this directive, a
   cache SHOULD either respond using a stored response that is
   consistent with the other constraints of the request, or respond with




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   a 504 (Gateway Timeout) status code.  If a group of caches is being
   operated as a unified system with good internal connectivity, a
   member cache MAY forward such a request within that group of caches.

5.2.2.  Response Cache-Control Directives

5.2.2.1.  must-revalidate

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

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

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

5.2.2.2.  no-cache

   Argument syntax:

      #field-name

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

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

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





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

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

5.2.2.3.  no-store

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

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

5.2.2.4.  no-transform

   The "no-transform" response directive indicates that an intermediary
   (regardless of whether it implements a cache) MUST NOT transform the
   payload, as defined in Section 5.7.2 of [RFC7230].

5.2.2.5.  public

   The "public" response directive indicates that any cache MAY store
   the response, even if the response would normally be non-cacheable or
   cacheable only within a private cache.  (See Section 3.2 for
   additional details related to the use of public in response to a
   request containing Authorization, and Section 3 for details of how
   public affects responses that would normally not be stored, due to
   their status codes not being defined as cacheable by default; see
   Section 4.2.2.)

5.2.2.6.  private

   Argument syntax:

      #field-name



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   The "private" response directive indicates that the response message
   is intended for a single user and MUST NOT be stored by a shared
   cache.  A private cache MAY store the response and reuse it for later
   requests, even if the response would normally be non-cacheable.

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

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

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

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

5.2.2.7.  proxy-revalidate

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

5.2.2.8.  max-age

   Argument syntax:

      delta-seconds (see Section 1.2.1)

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

   This directive uses the token form of the argument syntax: e.g.,
   'max-age=5' not 'max-age="5"'.  A sender SHOULD NOT generate the
   quoted-string form.







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5.2.2.9.  s-maxage

   Argument syntax:

      delta-seconds (see Section 1.2.1)

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

   This directive uses the token form of the argument syntax: e.g.,
   's-maxage=10' not 's-maxage="10"'.  A sender SHOULD NOT generate the
   quoted-string form.

5.2.3.  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.  A cache
   MUST ignore unrecognized cache directives.

   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 old directive are supplied, such that applications that do not
   understand the new directive will default to the behavior specified
   by the old directive, and those that understand the new directive
   will recognize it as modifying the requirements associated with the
   old directive.  In this way, extensions to the existing cache-control
   directives can be made without breaking deployed caches.

   For example, consider a hypothetical new response directive called
   "community" that acts as a modifier to the private directive: in
   addition to private caches, any cache that is shared only by members
   of the named community is allowed to cache the response.  An origin
   server wishing to allow the UCI community to use an otherwise private
   response in their shared cache(s) could do so by including

     Cache-Control: private, community="UCI"

   A cache that recognizes such a community cache-extension could
   broaden its behavior in accordance with that extension.  A cache that
   does not recognize the community cache-extension would ignore it and
   adhere to the private directive.



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5.3.  Expires

   The "Expires" header field gives the date/time after which the
   response is considered stale.  See Section 4.2 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 Expires value is an HTTP-date timestamp, as defined in Section
   7.1.1.1 of [RFC7231].

     Expires = HTTP-date

   For example

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

   A cache recipient MUST interpret invalid date formats, especially the
   value "0", as representing a time in the past (i.e., "already
   expired").

   If a response includes a Cache-Control field with the max-age
   directive (Section 5.2.2.8), a recipient MUST ignore the Expires
   field.  Likewise, if a response includes the s-maxage directive
   (Section 5.2.2.9), a shared cache recipient MUST ignore the Expires
   field.  In both these cases, the value in Expires is only intended
   for recipients that have not yet implemented the Cache-Control field.

   An origin server without a clock MUST NOT generate an Expires field
   unless its value represents a fixed time in the past (always expired)
   or its value has been associated with the resource by a system or
   user with a reliable clock.

   Historically, HTTP required the Expires field-value to be no more
   than a year in the future.  While longer freshness lifetimes are no
   longer prohibited, extremely large values have been demonstrated to
   cause problems (e.g., clock overflows due to use of 32-bit integers
   for time values), and many caches will evict a response far sooner
   than that.










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5.4.  Pragma

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

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

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

   When the Cache-Control header field is not present in a request,
   caches MUST consider the no-cache request pragma-directive as having
   the same effect as if "Cache-Control: no-cache" were present (see
   Section 5.2.1).

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

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

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

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

5.5.  Warning

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





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

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

     Warning       = 1#warning-value

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

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

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

   A user agent that receives one or more Warning header fields SHOULD
   inform the user of as many of them as possible, in the order that
   they appear in the response.  Senders that generate multiple Warning
   header fields are encouraged to order them with this user agent
   behavior in mind.  A sender that generates new Warning header fields
   MUST append them after any existing Warning header fields.

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

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

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





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   If a sender generates one or more 1xx warn-codes in a message to be
   sent to a recipient known to implement only HTTP/1.0, the sender MUST
   include in each corresponding warning-value a warn-date that matches
   the Date header field in the message.  For example:

     HTTP/1.1 200 OK
     Date: Sat, 25 Aug 2012 23:34:45 GMT
     Warning: 112 - "network down" "Sat, 25 Aug 2012 23:34:45 GMT"


   Warnings have accompanying warn-text that describes the error, e.g.,
   for logging.  It is advisory only, and its content does not affect
   interpretation of the warn-code.

   If a recipient that uses, evaluates, or displays Warning header
   fields receives a warn-date that is different from the Date value in
   the same message, the recipient MUST exclude the warning-value
   containing that warn-date before storing, forwarding, or using the
   message.  This allows recipients to exclude warning-values that were
   improperly retained after a cache validation.  If all of the
   warning-values are excluded, the recipient MUST exclude the Warning
   header field 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.
   The procedure for defining additional warn codes is described in
   Section 7.2.1.

5.5.1.  Warning: 110 - "Response is Stale"

   A cache SHOULD generate this whenever the sent response is stale.

5.5.2.  Warning: 111 - "Revalidation Failed"

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

5.5.3.  Warning: 112 - "Disconnected Operation"

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

5.5.4.  Warning: 113 - "Heuristic Expiration"

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



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5.5.5.  Warning: 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.

5.5.6.  Warning: 214 - "Transformation Applied"

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

5.5.7.  Warning: 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.

6.  History Lists

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

   The freshness model (Section 4.2) does not necessarily apply to
   history mechanisms.  That is, 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).

7.  IANA Considerations

7.1.  Cache Directive Registry

   The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry"
   defines the namespace for the cache directives.  It has been created
   and is now maintained at
   <http://www.iana.org/assignments/http-cache-directives>.

7.1.1.  Procedure

   A registration MUST include the following fields:

   o  Cache Directive Name



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   o  Pointer to specification text

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

7.1.2.  Considerations for New Cache Control Directives

   New extension directives ought to consider defining:

   o  What it means for a directive to be specified multiple times,

   o  When the directive does not take an argument, what it means when
      an argument is present,

   o  When the directive requires an argument, what it means when it is
      missing,

   o  Whether the directive is specific to requests, responses, or able
      to be used in either.

   See also Section 5.2.3.

7.1.3.  Registrations

   The registry has been populated with the registrations below:

   +------------------------+----------------------------------+
   | Cache Directive        | Reference                        |
   +------------------------+----------------------------------+
   | max-age                | Section 5.2.1.1, Section 5.2.2.8 |
   | max-stale              | Section 5.2.1.2                  |
   | min-fresh              | Section 5.2.1.3                  |
   | must-revalidate        | Section 5.2.2.1                  |
   | no-cache               | Section 5.2.1.4, Section 5.2.2.2 |
   | no-store               | Section 5.2.1.5, Section 5.2.2.3 |
   | no-transform           | Section 5.2.1.6, Section 5.2.2.4 |
   | only-if-cached         | Section 5.2.1.7                  |
   | private                | Section 5.2.2.6                  |
   | proxy-revalidate       | Section 5.2.2.7                  |
   | public                 | Section 5.2.2.5                  |
   | s-maxage               | Section 5.2.2.9                  |
   | stale-if-error         | [RFC5861], Section 4             |
   | stale-while-revalidate | [RFC5861], Section 3             |
   +------------------------+----------------------------------+







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7.2.  Warn Code Registry

   The "Hypertext Transfer Protocol (HTTP) Warn Codes" registry defines
   the namespace for warn codes.  It has been created and is now
   maintained at <http://www.iana.org/assignments/http-warn-codes>.

7.2.1.  Procedure

   A registration MUST include the following fields:

   o  Warn Code (3 digits)

   o  Short Description

   o  Pointer to specification text

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

7.2.2.  Registrations

   The registry has been populated with the registrations below:

   +-----------+----------------------------------+---------------+
   | Warn Code | Short Description                | Reference     |
   +-----------+----------------------------------+---------------+
   | 110       | Response is Stale                | Section 5.5.1 |
   | 111       | Revalidation Failed              | Section 5.5.2 |
   | 112       | Disconnected Operation           | Section 5.5.3 |
   | 113       | Heuristic Expiration             | Section 5.5.4 |
   | 199       | Miscellaneous Warning            | Section 5.5.5 |
   | 214       | Transformation Applied           | Section 5.5.6 |
   | 299       | Miscellaneous Persistent Warning | Section 5.5.7 |
   +-----------+----------------------------------+---------------+

7.3.  Header Field Registration

   HTTP header fields are registered within the "Message Headers"
   registry maintained at
   <http://www.iana.org/assignments/message-headers/>.











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   This document defines the following HTTP header fields, so the
   "Permanent Message Header Field Names" registry has been updated
   accordingly (see [BCP90]).

   +-------------------+----------+----------+-------------+
   | Header Field Name | Protocol | Status   | Reference   |
   +-------------------+----------+----------+-------------+
   | Age               | http     | standard | Section 5.1 |
   | Cache-Control     | http     | standard | Section 5.2 |
   | Expires           | http     | standard | Section 5.3 |
   | Pragma            | http     | standard | Section 5.4 |
   | Warning           | http     | standard | Section 5.5 |
   +-------------------+----------+----------+-------------+

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

8.  Security Considerations

   This section is meant to inform developers, information providers,
   and users of known security concerns specific to HTTP caching.  More
   general security considerations are addressed in HTTP messaging
   [RFC7230] and semantics [RFC7231].

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

   In particular, various attacks might be amplified by being stored in
   a shared cache; such "cache poisoning" attacks use the cache to
   distribute a malicious payload to many clients, and are especially
   effective when an attacker can use implementation flaws, elevated
   privileges, or other techniques to insert such a response into a
   cache.  One common attack vector for cache poisoning is to exploit
   differences in message parsing on proxies and in user agents; see
   Section 3.3.3 of [RFC7230] for the relevant requirements.

   Likewise, implementation flaws (as well as misunderstanding of cache
   operation) might lead to caching of sensitive information (e.g.,
   authentication credentials) that is thought to be private, exposing
   it to unauthorized parties.






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   Furthermore, the very use of a cache can bring about privacy
   concerns.  For example, if two users share a cache, and the first one
   browses to a site, the second may be able to detect that the other
   has been to that site, because the resources from it load more
   quickly, thanks to the cache.

   Note that the Set-Cookie response header field [RFC6265] does not
   inhibit caching; a cacheable response with a Set-Cookie header field
   can be (and often is) used to satisfy subsequent requests to caches.
   Servers who wish to control caching of these responses are encouraged
   to emit appropriate Cache-Control response header fields.

9.  Acknowledgments

   See Section 10 of [RFC7230].

10.  References

10.1.  Normative References

   [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.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, June 2014.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              June 2014.

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              June 2014.

   [RFC7233]  Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
              RFC 7233, June 2014.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235, June 2014.







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10.2.  Informative References

   [BCP90]    Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [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.

   [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.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, June 2010.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.




























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Appendix A.  Changes from RFC 2616

   The specification has been substantially rewritten for clarity.

   The conditions under which an authenticated response can be cached
   have been clarified.  (Section 3.2)

   New status codes can now define that caches are allowed to use
   heuristic freshness with them.  Caches are now allowed to calculate
   heuristic freshness for URIs with query components.  (Section 4.2.2)

   The algorithm for calculating age is now less conservative.  Caches
   are now required to handle dates with time zones as if they're
   invalid, because it's not possible to accurately guess.
   (Section 4.2.3)

   The Content-Location response header field is no longer used to
   determine the appropriate response to use when validating.
   (Section 4.3)

   The algorithm for selecting a cached negotiated response to use has
   been clarified in several ways.  In particular, it now explicitly
   allows header-specific canonicalization when processing selecting
   header fields.  (Section 4.1)

   Requirements regarding denial-of-service attack avoidance when
   performing invalidation have been clarified.  (Section 4.4)

   Cache invalidation only occurs when a successful response is
   received.  (Section 4.4)

   Cache directives are explicitly defined to be case-insensitive.
   Handling of multiple instances of cache directives when only one is
   expected is now defined.  (Section 5.2)

   The "no-store" request directive doesn't apply to responses; i.e., a
   cache can satisfy a request with no-store on it and does not
   invalidate it.  (Section 5.2.1.5)

   The qualified forms of the private and no-cache cache directives are
   noted to not be widely implemented; for example, "private=foo" is
   interpreted by many caches as simply "private".  Additionally, the
   meaning of the qualified form of no-cache has been clarified.
   (Section 5.2.2)

   The "no-cache" response directive's meaning has been clarified.
   (Section 5.2.2.2)




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   The one-year limit on Expires header field values has been removed;
   instead, the reasoning for using a sensible value is given.
   (Section 5.3)

   The Pragma header field is now only defined for backwards
   compatibility; future pragmas are deprecated.  (Section 5.4)

   Some requirements regarding production and processing of the Warning
   header fields have been relaxed, as it is not widely implemented.
   Furthermore, the Warning header field no longer uses RFC 2047
   encoding, nor does it allow multiple languages, as these aspects were
   not implemented.  (Section 5.5)

   This specification introduces the Cache Directive and Warn Code
   Registries, and defines considerations for new cache directives.
   (Section 7.1 and Section 7.2)

Appendix B.  Imported ABNF

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

   The rules below are defined in [RFC7230]:

     OWS           = <OWS, see [RFC7230], Section 3.2.3>
     field-name    = <field-name, see [RFC7230], Section 3.2>
     quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>
     token         = <token, see [RFC7230], Section 3.2.6>

     port          = <port, see [RFC7230], Section 2.7>
     pseudonym     = <pseudonym, see [RFC7230], Section 5.7.1>
     uri-host      = <uri-host, see [RFC7230], Section 2.7>

   The rules below are defined in other parts:

     HTTP-date     = <HTTP-date, see [RFC7231], Section 7.1.1.1>











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Appendix C.  Collected ABNF

   In the collected ABNF below, list rules are expanded as per Section
   1.2 of [RFC7230].

   Age = delta-seconds

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

   Expires = HTTP-date

   HTTP-date = <HTTP-date, see [RFC7231], Section 7.1.1.1>

   OWS = <OWS, see [RFC7230], Section 3.2.3>

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

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

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

   delta-seconds = 1*DIGIT

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

   field-name = <field-name, see [RFC7230], Section 3.2>

   port = <port, see [RFC7230], Section 2.7>
   pragma-directive = "no-cache" / extension-pragma
   pseudonym = <pseudonym, see [RFC7230], Section 5.7.1>

   quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>

   token = <token, see [RFC7230], Section 3.2.6>

   uri-host = <uri-host, see [RFC7230], Section 2.7>

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





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Index

   1
      110 (warn-code)  31
      111 (warn-code)  31
      112 (warn-code)  31
      113 (warn-code)  31
      199 (warn-code)  32

   2
      214 (warn-code)  32
      299 (warn-code)  32

   A
      age  11
      Age header field  21

   C
      cache  4
      cache entry  5
      cache key  5-6
      Cache-Control header field  21

   D
      Disconnected Operation (warn-text)  31

   E
      Expires header field  28
      explicit expiration time  11

   F
      fresh  11
      freshness lifetime  11

   G
      Grammar
         Age  21
         Cache-Control  22
         cache-directive  22
         delta-seconds  5
         Expires  28
         extension-pragma  29
         Pragma  29
         pragma-directive  29
         warn-agent  29
         warn-code  29
         warn-date  29
         warn-text  29



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         Warning  29
         warning-value  29

   H
      Heuristic Expiration (warn-text)  31
      heuristic expiration time  11
   M
      max-age (cache directive)  22, 26
      max-stale (cache directive)  22
      min-fresh (cache directive)  22
      Miscellaneous Persistent Warning (warn-text)  32
      Miscellaneous Warning (warn-text)  32
      must-revalidate (cache directive)  24

   N
      no-cache (cache directive)  23, 25
      no-store (cache directive)  23, 24
      no-transform (cache directive)  23, 25

   O
      only-if-cached (cache directive)  23

   P
      Pragma header field  29
      private (cache directive)  25
      private cache  4
      proxy-revalidate (cache directive)  26
      public (cache directive)  25

   R
      Response is Stale (warn-text)  30
      Revalidation Failed (warn-text)  31

   S
      s-maxage (cache directive)  27
      shared cache  4
      stale  11
      strong validator  18

   T
      Transformation Applied (warn-text)  32

   V
      validator  16

   W
      Warning header field  29




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Authors' Addresses

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

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


   Mark Nottingham (editor)
   Akamai

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


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

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
























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284	RFC 7234 HTTP/1.1 Caching June 2014
285
286
287	incomplete results (e.g., 206 (Partial Content)), and responses to
288	methods other than GET if the method's definition allows such caching
289	and defines something suitable for use as a cache key.
290
291	The primary cache key consists of the request method and target URI.
292	However, since HTTP caches in common use today are typically limited
293	to caching responses to GET, many caches simply decline other methods
294	and use only the URI as the primary cache key.
295
296	If a request target is subject to content negotiation, its cache
297	entry might consist of multiple stored responses, each differentiated
298	by a secondary key for the values of the original request's selecting
299	header fields (Section 4.1).
300
301	3. Storing Responses in Caches
302
303	A cache MUST NOT store a response to any request, unless:
304
305	o The request method is understood by the cache and defined as being
306	cacheable, and
307
308	o the response status code is understood by the cache, and
309
310	o the "no-store" cache directive (see Section 5.2) does not appear
311	in request or response header fields, and
312
313	o the "private" response directive (see Section 5.2.2.6) does not
314	appear in the response, if the cache is shared, and
315
316	o the Authorization header field (see Section 4.2 of [RFC7235]) does
317	not appear in the request, if the cache is shared, unless the
318	response explicitly allows it (see Section 3.2), and
319
320	o the response either:
321
322	* contains an Expires header field (see Section 5.3), or
323
324	* contains a max-age response directive (see Section 5.2.2.8), or
325
326	* contains a s-maxage response directive (see Section 5.2.2.9)
327	and the cache is shared, or
328
329	* contains a Cache Control Extension (see Section 5.2.3) that
330	allows it to be cached, or
331
332	* has a status code that is defined as cacheable by default (see
333	Section 4.2.2), or
334
335
336
337
338	Fielding, et al. Standards Track [Page 6]

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342
343	* contains a public response directive (see Section 5.2.2.5).
344
345	Note that any of the requirements listed above can be overridden by a
346	cache-control extension; see Section 5.2.3.
347
348	In this context, a cache has "understood" a request method or a
349	response status code if it recognizes it and implements all specified
350	caching-related behavior.
351
352	Note that, in normal operation, some caches will not store a response
353	that has neither a cache validator nor an explicit expiration time,
354	as such responses are not usually useful to store. However, caches
355	are not prohibited from storing such responses.
356
357	3.1. Storing Incomplete Responses
358
359	A response message is considered complete when all of the octets
360	indicated by the message framing ([RFC7230]) are received prior to
361	the connection being closed. If the request method is GET, the
362	response status code is 200 (OK), and the entire response header
363	section has been received, a cache MAY store an incomplete response
364	message body if the cache entry is recorded as incomplete. Likewise,
365	a 206 (Partial Content) response MAY be stored as if it were an
366	incomplete 200 (OK) cache entry. However, a cache MUST NOT store
367	incomplete or partial-content responses if it does not support the
368	Range and Content-Range header fields or if it does not understand
369	the range units used in those fields.
370
371	A cache MAY complete a stored incomplete response by making a
372	subsequent range request ([RFC7233]) and combining the successful
373	response with the stored entry, as defined in Section 3.3. A cache
374	MUST NOT use an incomplete response to answer requests unless the
375	response has been made complete or the request is partial and
376	specifies a range that is wholly within the incomplete response. A
377	cache MUST NOT send a partial response to a client without explicitly
378	marking it as such using the 206 (Partial Content) status code.
379
380	3.2. Storing Responses to Authenticated Requests
381
382	A shared cache MUST NOT use a cached response to a request with an
383	Authorization header field (Section 4.2 of [RFC7235]) to satisfy any
384	subsequent request unless a cache directive that allows such
385	responses to be stored is present in the response.
386
387	In this specification, the following Cache-Control response
388	directives (Section 5.2.2) have such an effect: must-revalidate,
389	public, and s-maxage.
390
391
392
393
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398
399	Note that cached responses that contain the "must-revalidate" and/or
400	"s-maxage" response directives are not allowed to be served stale
401	(Section 4.2.4) by shared caches. In particular, a response with
402	either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
403	satisfy a subsequent request without revalidating it on the origin
404	server.
405
406	3.3. Combining Partial Content
407
408	A response might transfer only a partial representation if the
409	connection closed prematurely or if the request used one or more
410	Range specifiers ([RFC7233]). After several such transfers, a cache
411	might have received several ranges of the same representation. A
412	cache MAY combine these ranges into a single stored response, and
413	reuse that response to satisfy later requests, if they all share the
414	same strong validator and the cache complies with the client
415	requirements in Section 4.3 of [RFC7233].
416
417	When combining the new response with one or more stored responses, a
418	cache MUST:
419
420	o delete any Warning header fields in the stored response with
421	warn-code 1xx (see Section 5.5);
422
423	o retain any Warning header fields in the stored response with
424	warn-code 2xx; and,
425
426	o use other header fields provided in the new response, aside from
427	Content-Range, to replace all instances of the corresponding
428	header fields in the stored response.
429
430	4. Constructing Responses from Caches
431
432	When presented with a request, a cache MUST NOT reuse a stored
433	response, unless:
434
435	o The presented effective request URI (Section 5.5 of [RFC7230]) and
436	that of the stored response match, and
437
438	o the request method associated with the stored response allows it
439	to be used for the presented request, and
440
441	o selecting header fields nominated by the stored response (if any)
442	match those presented (see Section 4.1), and
443
444
445
446
447
448
449
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452	RFC 7234 HTTP/1.1 Caching June 2014
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454
455	o the presented request does not contain the no-cache pragma
456	(Section 5.4), nor the no-cache cache directive (Section 5.2.1),
457	unless the stored response is successfully validated
458	(Section 4.3), and
459
460	o the stored response does not contain the no-cache cache directive
461	(Section 5.2.2.2), unless it is successfully validated
462	(Section 4.3), and
463
464	o the stored response is either:
465
466	* fresh (see Section 4.2), or
467
468	* allowed to be served stale (see Section 4.2.4), or
469
470	* successfully validated (see Section 4.3).
471
472	Note that any of the requirements listed above can be overridden by a
473	cache-control extension; see Section 5.2.3.
474
475	When a stored response is used to satisfy a request without
476	validation, a cache MUST generate an Age header field (Section 5.1),
477	replacing any present in the response with a value equal to the
478	stored response's current_age; see Section 4.2.3.
479
480	A cache MUST write through requests with methods that are unsafe
481	(Section 4.2.1 of [RFC7231]) to the origin server; i.e., a cache is
482	not allowed to generate a reply to such a request before having
483	forwarded the request and having received a corresponding response.
484
485	Also, note that unsafe requests might invalidate already-stored
486	responses; see Section 4.4.
487
488	When more than one suitable response is stored, a cache MUST use the
489	most recent response (as determined by the Date header field). It
490	can also forward the request with "Cache-Control: max-age=0" or
491	"Cache-Control: no-cache" to disambiguate which response to use.
492
493	A cache that does not have a clock available MUST NOT use stored
494	responses without revalidating them upon every use.
495
496	4.1. Calculating Secondary Keys with Vary
497
498	When a cache receives a request that can be satisfied by a stored
499	response that has a Vary header field (Section 7.1.4 of [RFC7231]),
500	it MUST NOT use that response unless all of the selecting header
501
502
503
504
505
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510
511	fields nominated by the Vary header field match in both the original
512	request (i.e., that associated with the stored response), and the
513	presented request.
514
515	The selecting header fields from two requests are defined to match if
516	and only if those in the first request can be transformed to those in
517	the second request by applying any of the following:
518
519	o adding or removing whitespace, where allowed in the header field's
520	syntax
521
522	o combining multiple header fields with the same field name (see
523	Section 3.2 of [RFC7230])
524
525	o normalizing both header field values in a way that is known to
526	have identical semantics, according to the header field's
527	specification (e.g., reordering field values when order is not
528	significant; case-normalization, where values are defined to be
529	case-insensitive)
530
531	If (after any normalization that might take place) a header field is
532	absent from a request, it can only match another request if it is
533	also absent there.
534
535	A Vary header field-value of "*" always fails to match.
536
537	The stored response with matching selecting header fields is known as
538	the selected response.
539
540	If multiple selected responses are available (potentially including
541	responses without a Vary header field), the cache will need to choose
542	one to use. When a selecting header field has a known mechanism for
543	doing so (e.g., qvalues on Accept and similar request header fields),
544	that mechanism MAY be used to select preferred responses; of the
545	remainder, the most recent response (as determined by the Date header
546	field) is used, as per Section 4.
547
548	If no selected response is available, the cache cannot satisfy the
549	presented request. Typically, it is forwarded to the origin server
550	in a (possibly conditional; see Section 4.3) request.
551
552
553
554
555
556
557
558
559
560
561
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1
2
3
4
5
6
7	Internet Engineering Task Force (IETF) R. Fielding, Ed.
8	Request for Comments: 7234 Adobe
9	Obsoletes: 2616 M. Nottingham, Ed.
10	Category: Standards Track Akamai
11	ISSN: 2070-1721 J. Reschke, Ed.
12	greenbytes
13	June 2014
14
15
16	Hypertext Transfer Protocol (HTTP/1.1): Caching
17
18	Abstract
19
20	The Hypertext Transfer Protocol (HTTP) is a stateless application-
21	level protocol for distributed, collaborative, hypertext information
22	systems. This document defines HTTP caches and the associated header
23	fields that control cache behavior or indicate cacheable response
24	messages.
25
26	Status of This Memo
27
28	This is an Internet Standards Track document.
29
30	This document is a product of the Internet Engineering Task Force
31	(IETF). It represents the consensus of the IETF community. It has
32	received public review and has been approved for publication by the
33	Internet Engineering Steering Group (IESG). Further information on
34	Internet Standards is available in Section 2 of RFC 5741.
35
36	Information about the current status of this document, any errata,
37	and how to provide feedback on it may be obtained at
38	http://www.rfc-editor.org/info/rfc7234.
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
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60	RFC 7234 HTTP/1.1 Caching June 2014
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62
63	Copyright Notice
64
65	Copyright (c) 2014 IETF Trust and the persons identified as the
66	document authors. All rights reserved.
67
68	This document is subject to BCP 78 and the IETF Trust's Legal
69	Provisions Relating to IETF Documents
70	(http://trustee.ietf.org/license-info) in effect on the date of
71	publication of this document. Please review these documents
72	carefully, as they describe your rights and restrictions with respect
73	to this document. Code Components extracted from this document must
74	include Simplified BSD License text as described in Section 4.e of
75	the Trust Legal Provisions and are provided without warranty as
76	described in the Simplified BSD License.
77
78	This document may contain material from IETF Documents or IETF
79	Contributions published or made publicly available before November
80	10, 2008. The person(s) controlling the copyright in some of this
81	material may not have granted the IETF Trust the right to allow
82	modifications of such material outside the IETF Standards Process.
83	Without obtaining an adequate license from the person(s) controlling
84	the copyright in such materials, this document may not be modified
85	outside the IETF Standards Process, and derivative works of it may
86	not be created outside the IETF Standards Process, except to format
87	it for publication as an RFC or to translate it into languages other
88	than English.
89
90	Table of Contents
91
92	1. Introduction ....................................................4
93	1.1. Conformance and Error Handling .............................4
94	1.2. Syntax Notation ............................................4
95	1.2.1. Delta Seconds .......................................5
96	2. Overview of Cache Operation .....................................5
97	3. Storing Responses in Caches .....................................6
98	3.1. Storing Incomplete Responses ...............................7
99	3.2. Storing Responses to Authenticated Requests ................7
100	3.3. Combining Partial Content ..................................8
101	4. Constructing Responses from Caches ..............................8
102	4.1. Calculating Secondary Keys with Vary .......................9
103	4.2. Freshness .................................................11
104	4.2.1. Calculating Freshness Lifetime .....................12
105	4.2.2. Calculating Heuristic Freshness ....................13
106	4.2.3. Calculating Age ....................................13
107	4.2.4. Serving Stale Responses ............................15
108	4.3. Validation ................................................16
109	4.3.1. Sending a Validation Request .......................16
110	4.3.2. Handling a Received Validation Request .............16
111
112
113
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174
175	1. Introduction
176
177	HTTP is typically used for distributed information systems, where
178	performance can be improved by the use of response caches. This
179	document defines aspects of HTTP/1.1 related to caching and reusing
180	response messages.
181
182	An HTTP cache is a local store of response messages and the subsystem
183	that controls storage, retrieval, and deletion of messages in it. A
184	cache stores cacheable responses in order to reduce the response time
185	and network bandwidth consumption on future, equivalent requests.
186	Any client or server MAY employ a cache, though a cache cannot be
187	used by a server that is acting as a tunnel.
188
189	A shared cache is a cache that stores responses to be reused by more
190	than one user; shared caches are usually (but not always) deployed as
191	a part of an intermediary. A private cache, in contrast, is
192	dedicated to a single user; often, they are deployed as a component
193	of a user agent.
194
195	The goal of caching in HTTP/1.1 is to significantly improve
196	performance by reusing a prior response message to satisfy a current
197	request. A stored response is considered "fresh", as defined in
198	Section 4.2, if the response can be reused without "validation"
199	(checking with the origin server to see if the cached response
200	remains valid for this request). A fresh response can therefore
201	reduce both latency and network overhead each time it is reused.
202	When a cached response is not fresh, it might still be reusable if it
203	can be freshened by validation (Section 4.3) or if the origin is
204	unavailable (Section 4.2.4).
205
206	1.1. Conformance and Error Handling
207
208	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
209	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
210	document are to be interpreted as described in [RFC2119].
211
212	Conformance criteria and considerations regarding error handling are
213	defined in Section 2.5 of [RFC7230].
214
215	1.2. Syntax Notation
216
217	This specification uses the Augmented Backus-Naur Form (ABNF)
218	notation of [RFC5234] with a list extension, defined in Section 7 of
219	[RFC7230], that allows for compact definition of comma-separated
220	lists using a '#' operator (similar to how the '*' operator indicates
221
222
223
224
225
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228	RFC 7234 HTTP/1.1 Caching June 2014
229
230
231	repetition). Appendix B describes rules imported from other
232	documents. Appendix C shows the collected grammar with all list
233	operators expanded to standard ABNF notation.
234
235	1.2.1. Delta Seconds
236
237	The delta-seconds rule specifies a non-negative integer, representing
238	time in seconds.
239
240	delta-seconds = 1*DIGIT
241
242	A recipient parsing a delta-seconds value and converting it to binary
243	form ought to use an arithmetic type of at least 31 bits of
244	non-negative integer range. If a cache receives a delta-seconds
245	value greater than the greatest integer it can represent, or if any
246	of its subsequent calculations overflows, the cache MUST consider the
247	value to be either 2147483648 (2^31) or the greatest positive integer
248	it can conveniently represent.
249
250	Note: The value 2147483648 is here for historical reasons,
251	effectively represents infinity (over 68 years), and does not need
252	to be stored in binary form; an implementation could produce it as
253	a canned string if any overflow occurs, even if the calculations
254	are performed with an arithmetic type incapable of directly
255	representing that number. What matters here is that an overflow
256	be detected and not treated as a negative value in later
257	calculations.
258
259	2. Overview of Cache Operation
260
261	Proper cache operation preserves the semantics of HTTP transfers
262	([RFC7231]) while eliminating the transfer of information already
263	held in the cache. Although caching is an entirely OPTIONAL feature
264	of HTTP, it can be assumed that reusing a cached response is
265	desirable and that such reuse is the default behavior when no
266	requirement or local configuration prevents it. Therefore, HTTP
267	cache requirements are focused on preventing a cache from either
268	storing a non-reusable response or reusing a stored response
269	inappropriately, rather than mandating that caches always store and
270	reuse particular responses.
271
272	Each cache entry consists of a cache key and one or more HTTP
273	responses corresponding to prior requests that used the same key.
274	The most common form of cache entry is a successful result of a
275	retrieval request: i.e., a 200 (OK) response to a GET request, which
276	contains a representation of the resource identified by the request
277	target (Section 4.3.1 of [RFC7231]). However, it is also possible to
278	cache permanent redirects, negative results (e.g., 404 (Not Found)),
279
280
281
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564	RFC 7234 HTTP/1.1 Caching June 2014
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566
567	4.2. Freshness
568
569	A fresh response is one whose age has not yet exceeded its freshness
570	lifetime. Conversely, a stale response is one where it has.
571
572	A response's freshness lifetime is the length of time between its
573	generation by the origin server and its expiration time. An explicit
574	expiration time is the time at which the origin server intends that a
575	stored response can no longer be used by a cache without further
576	validation, whereas a heuristic expiration time is assigned by a
577	cache when no explicit expiration time is available.
578
579	A response's age is the time that has passed since it was generated
580	by, or successfully validated with, the origin server.
581
582	When a response is "fresh" in the cache, it can be used to satisfy
583	subsequent requests without contacting the origin server, thereby
584	improving efficiency.
585
586	The primary mechanism for determining freshness is for an origin
587	server to provide an explicit expiration time in the future, using
588	either the Expires header field (Section 5.3) or the max-age response
589	directive (Section 5.2.2.8). Generally, origin servers will assign
590	future explicit expiration times to responses in the belief that the
591	representation is not likely to change in a semantically significant
592	way before the expiration time is reached.
593
594	If an origin server wishes to force a cache to validate every
595	request, it can assign an explicit expiration time in the past to
596	indicate that the response is already stale. Compliant caches will
597	normally validate a stale cached response before reusing it for
598	subsequent requests (see Section 4.2.4).
599
600	Since origin servers do not always provide explicit expiration times,
601	caches are also allowed to use a heuristic to determine an expiration
602	time under certain circumstances (see Section 4.2.2).
603
604	The calculation to determine if a response is fresh is:
605
606	response_is_fresh = (freshness_lifetime > current_age)
607
608	freshness_lifetime is defined in Section 4.2.1; current_age is
609	defined in Section 4.2.3.
610
611	Clients can send the max-age or min-fresh cache directives in a
612	request to constrain or relax freshness calculations for the
613	corresponding response (Section 5.2.1).
614
615
616
617
618	Fielding, et al. Standards Track [Page 11]

620	RFC 7234 HTTP/1.1 Caching June 2014
621
622
623	When calculating freshness, to avoid common problems in date parsing:
624
625	o Although all date formats are specified to be case-sensitive, a
626	cache recipient SHOULD match day, week, and time-zone names
627	case-insensitively.
628
629	o If a cache recipient's internal implementation of time has less
630	resolution than the value of an HTTP-date, the recipient MUST
631	internally represent a parsed Expires date as the nearest time
632	equal to or earlier than the received value.
633
634	o A cache recipient MUST NOT allow local time zones to influence the
635	calculation or comparison of an age or expiration time.
636
637	o A cache recipient SHOULD consider a date with a zone abbreviation
638	other than GMT or UTC to be invalid for calculating expiration.
639
640	Note that freshness applies only to cache operation; it cannot be
641	used to force a user agent to refresh its display or reload a
642	resource. See Section 6 for an explanation of the difference between
643	caches and history mechanisms.
644
645	4.2.1. Calculating Freshness Lifetime
646
647	A cache can calculate the freshness lifetime (denoted as
648	freshness_lifetime) of a response by using the first match of the
649	following:
650
651	o If the cache is shared and the s-maxage response directive
652	(Section 5.2.2.9) is present, use its value, or
653
654	o If the max-age response directive (Section 5.2.2.8) is present,
655	use its value, or
656
657	o If the Expires response header field (Section 5.3) is present, use
658	its value minus the value of the Date response header field, or
659
660	o Otherwise, no explicit expiration time is present in the response.
661	A heuristic freshness lifetime might be applicable; see
662	Section 4.2.2.
663
664	Note that this calculation is not vulnerable to clock skew, since all
665	of the information comes from the origin server.
666
667
668
669
670
671
672
673
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676	RFC 7234 HTTP/1.1 Caching June 2014
677
678
679	When there is more than one value present for a given directive
680	(e.g., two Expires header fields, multiple Cache-Control: max-age
681	directives), the directive's value is considered invalid. Caches are
682	encouraged to consider responses that have invalid freshness
683	information to be stale.
684
685	4.2.2. Calculating Heuristic Freshness
686
687	Since origin servers do not always provide explicit expiration times,
688	a cache MAY assign a heuristic expiration time when an explicit time
689	is not specified, employing algorithms that use other header field
690	values (such as the Last-Modified time) to estimate a plausible
691	expiration time. This specification does not provide specific
692	algorithms, but does impose worst-case constraints on their results.
693
694	A cache MUST NOT use heuristics to determine freshness when an
695	explicit expiration time is present in the stored response. Because
696	of the requirements in Section 3, this means that, effectively,
697	heuristics can only be used on responses without explicit freshness
698	whose status codes are defined as cacheable by default (see Section
699	6.1 of [RFC7231]), and those responses without explicit freshness
700	that have been marked as explicitly cacheable (e.g., with a "public"
701	response directive).
702
703	If the response has a Last-Modified header field (Section 2.2 of
704	[RFC7232]), caches are encouraged to use a heuristic expiration value
705	that is no more than some fraction of the interval since that time.
706	A typical setting of this fraction might be 10%.
707
708	When a heuristic is used to calculate freshness lifetime, a cache
709	SHOULD generate a Warning header field with a 113 warn-code (see
710	Section 5.5.4) in the response if its current_age is more than 24
711	hours and such a warning is not already present.
712
713	Note: Section 13.9 of [RFC2616] prohibited caches from calculating
714	heuristic freshness for URIs with query components (i.e., those
715	containing '?'). In practice, this has not been widely
716	implemented. Therefore, origin servers are encouraged to send
717	explicit directives (e.g., Cache-Control: no-cache) if they wish
718	to preclude caching.
719
720	4.2.3. Calculating Age
721
722	The Age header field is used to convey an estimated age of the
723	response message when obtained from a cache. The Age field value is
724	the cache's estimate of the number of seconds since the response was
725	generated or validated by the origin server. In essence, the Age
726
727
728
729
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732	RFC 7234 HTTP/1.1 Caching June 2014
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734
735	value is the sum of the time that the response has been resident in
736	each of the caches along the path from the origin server, plus the
737	amount of time it has been in transit along network paths.
738
739	The following data is used for the age calculation:
740
741	age_value
742
743	The term "age_value" denotes the value of the Age header field
744	(Section 5.1), in a form appropriate for arithmetic operation; or
745	0, if not available.
746
747	date_value
748
749	The term "date_value" denotes the value of the Date header field,
750	in a form appropriate for arithmetic operations. See Section
751	7.1.1.2 of [RFC7231] for the definition of the Date header field,
752	and for requirements regarding responses without it.
753
754	now
755
756	The term "now" means "the current value of the clock at the host
757	performing the calculation". A host ought to use NTP ([RFC5905])
758	or some similar protocol to synchronize its clocks to Coordinated
759	Universal Time.
760
761	request_time
762
763	The current value of the clock at the host at the time the request
764	resulting in the stored response was made.
765
766	response_time
767
768	The current value of the clock at the host at the time the
769	response was received.
770
771	A response's age can be calculated in two entirely independent ways:
772
773	1. the "apparent_age": response_time minus date_value, if the local
774	clock is reasonably well synchronized to the origin server's
775	clock. If the result is negative, the result is replaced by
776	zero.
777
778	2. the "corrected_age_value", if all of the caches along the
779	response path implement HTTP/1.1. A cache MUST interpret this
780	value relative to the time the request was initiated, not the
781	time that the response was received.
782
783
784
785
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788	RFC 7234 HTTP/1.1 Caching June 2014
789
790
791	apparent_age = max(0, response_time - date_value);
792
793	response_delay = response_time - request_time;
794	corrected_age_value = age_value + response_delay;
795
796	These are combined as
797
798	corrected_initial_age = max(apparent_age, corrected_age_value);
799
800	unless the cache is confident in the value of the Age header field
801	(e.g., because there are no HTTP/1.0 hops in the Via header field),
802	in which case the corrected_age_value MAY be used as the
803	corrected_initial_age.
804
805	The current_age of a stored response can then be calculated by adding
806	the amount of time (in seconds) since the stored response was last
807	validated by the origin server to the corrected_initial_age.
808
809	resident_time = now - response_time;
810	current_age = corrected_initial_age + resident_time;
811
812	4.2.4. Serving Stale Responses
813
814	A "stale" response is one that either has explicit expiry information
815	or is allowed to have heuristic expiry calculated, but is not fresh
816	according to the calculations in Section 4.2.
817
818	A cache MUST NOT generate a stale response if it is prohibited by an
819	explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
820	cache directive, a "must-revalidate" cache-response-directive, or an
821	applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
822	see Section 5.2.2).
823
824	A cache MUST NOT send stale responses unless it is disconnected
825	(i.e., it cannot contact the origin server or otherwise find a
826	forward path) or doing so is explicitly allowed (e.g., by the
827	max-stale request directive; see Section 5.2.1).
828
829	A cache SHOULD generate a Warning header field with the 110 warn-code
830	(see Section 5.5.1) in stale responses. Likewise, a cache SHOULD
831	generate a 112 warn-code (see Section 5.5.3) in stale responses if
832	the cache is disconnected.
833
834	A cache SHOULD NOT generate a new Warning header field when
835	forwarding a response that does not have an Age header field, even if
836	the response is already stale. A cache need not validate a response
837	that merely became stale in transit.
838
839
840
841
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844	RFC 7234 HTTP/1.1 Caching June 2014
845
846
847	4.3. Validation
848
849	When a cache has one or more stored responses for a requested URI,
850	but cannot serve any of them (e.g., because they are not fresh, or
851	one cannot be selected; see Section 4.1), it can use the conditional
852	request mechanism [RFC7232] in the forwarded request to give the next
853	inbound server an opportunity to select a valid stored response to
854	use, updating the stored metadata in the process, or to replace the
855	stored response(s) with a new response. This process is known as
856	"validating" or "revalidating" the stored response.
857
858	4.3.1. Sending a Validation Request
859
860	When sending a conditional request for cache validation, a cache
861	sends one or more precondition header fields containing validator
862	metadata from its stored response(s), which is then compared by
863	recipients to determine whether a stored response is equivalent to a
864	current representation of the resource.
865
866	One such validator is the timestamp given in a Last-Modified header
867	field (Section 2.2 of [RFC7232]), which can be used in an
868	If-Modified-Since header field for response validation, or in an
869	If-Unmodified-Since or If-Range header field for representation
870	selection (i.e., the client is referring specifically to a previously
871	obtained representation with that timestamp).
872
873	Another validator is the entity-tag given in an ETag header field
874	(Section 2.3 of [RFC7232]). One or more entity-tags, indicating one
875	or more stored responses, can be used in an If-None-Match header
876	field for response validation, or in an If-Match or If-Range header
877	field for representation selection (i.e., the client is referring
878	specifically to one or more previously obtained representations with
879	the listed entity-tags).
880
881	4.3.2. Handling a Received Validation Request
882
883	Each client in the request chain may have its own cache, so it is
884	common for a cache at an intermediary to receive conditional requests
885	from other (outbound) caches. Likewise, some user agents make use of
886	conditional requests to limit data transfers to recently modified
887	representations or to complete the transfer of a partially retrieved
888	representation.
889
890	If a cache receives a request that can be satisfied by reusing one of
891	its stored 200 (OK) or 206 (Partial Content) responses, the cache
892	SHOULD evaluate any applicable conditional header field preconditions
893	received in that request with respect to the corresponding validators
894	contained within the selected response. A cache MUST NOT evaluate
895
896
897
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900	RFC 7234 HTTP/1.1 Caching June 2014
901
902
903	conditional header fields that are only applicable to an origin
904	server, found in a request with semantics that cannot be satisfied
905	with a cached response, or applied to a target resource for which it
906	has no stored responses; such preconditions are likely intended for
907	some other (inbound) server.
908
909	The proper evaluation of conditional requests by a cache depends on
910	the received precondition header fields and their precedence, as
911	defined in Section 6 of [RFC7232]. The If-Match and
912	If-Unmodified-Since conditional header fields are not applicable to a
913	cache.
914
915	A request containing an If-None-Match header field (Section 3.2 of
916	[RFC7232]) indicates that the client wants to validate one or more of
917	its own stored responses in comparison to whichever stored response
918	is selected by the cache. If the field-value is "*", or if the
919	field-value is a list of entity-tags and at least one of them matches
920	the entity-tag of the selected stored response, a cache recipient
921	SHOULD generate a 304 (Not Modified) response (using the metadata of
922	the selected stored response) instead of sending that stored
923	response.
924
925	When a cache decides to revalidate its own stored responses for a
926	request that contains an If-None-Match list of entity-tags, the cache
927	MAY combine the received list with a list of entity-tags from its own
928	stored set of responses (fresh or stale) and send the union of the
929	two lists as a replacement If-None-Match header field value in the
930	forwarded request. If a stored response contains only partial
931	content, the cache MUST NOT include its entity-tag in the union
932	unless the request is for a range that would be fully satisfied by
933	that partial stored response. If the response to the forwarded
934	request is 304 (Not Modified) and has an ETag header field value with
935	an entity-tag that is not in the client's list, the cache MUST
936	generate a 200 (OK) response for the client by reusing its
937	corresponding stored response, as updated by the 304 response
938	metadata (Section 4.3.4).
939
940	If an If-None-Match header field is not present, a request containing
941	an If-Modified-Since header field (Section 3.3 of [RFC7232])
942	indicates that the client wants to validate one or more of its own
943	stored responses by modification date. A cache recipient SHOULD
944	generate a 304 (Not Modified) response (using the metadata of the
945	selected stored response) if one of the following cases is true: 1)
946	the selected stored response has a Last-Modified field-value that is
947	earlier than or equal to the conditional timestamp; 2) no
948	Last-Modified field is present in the selected stored response, but
949	it has a Date field-value that is earlier than or equal to the
950	conditional timestamp; or, 3) neither Last-Modified nor Date is
951
952
953
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956	RFC 7234 HTTP/1.1 Caching June 2014
957
958
959	present in the selected stored response, but the cache recorded it as
960	having been received at a time earlier than or equal to the
961	conditional timestamp.
962
963	A cache that implements partial responses to range requests, as
964	defined in [RFC7233], also needs to evaluate a received If-Range
965	header field (Section 3.2 of [RFC7233]) with respect to its selected
966	stored response.
967
968	4.3.3. Handling a Validation Response
969
970	Cache handling of a response to a conditional request is dependent
971	upon its status code:
972
973	o A 304 (Not Modified) response status code indicates that the
974	stored response can be updated and reused; see Section 4.3.4.
975
976	o A full response (i.e., one with a payload body) indicates that
977	none of the stored responses nominated in the conditional request
978	is suitable. Instead, the cache MUST use the full response to
979	satisfy the request and MAY replace the stored response(s).
980
981	o However, if a cache receives a 5xx (Server Error) response while
982	attempting to validate a response, it can either forward this
983	response to the requesting client, or act as if the server failed
984	to respond. In the latter case, the cache MAY send a previously
985	stored response (see Section 4.2.4).
986
987	4.3.4. Freshening Stored Responses upon Validation
988
989	When a cache receives a 304 (Not Modified) response and already has
990	one or more stored 200 (OK) responses for the same cache key, the
991	cache needs to identify which of the stored responses are updated by
992	this new response and then update the stored response(s) with the new
993	information provided in the 304 response.
994
995	The stored response to update is identified by using the first match
996	(if any) of the following:
997
998	o If the new response contains a strong validator (see Section 2.1
999	of [RFC7232]), then that strong validator identifies the selected
1000	representation for update. All of the stored responses with the
1001	same strong validator are selected. If none of the stored
1002	responses contain the same strong validator, then the cache MUST
1003	NOT use the new response to update any stored responses.
1004
1005
1006
1007
1008
1009
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1012	RFC 7234 HTTP/1.1 Caching June 2014
1013
1014
1015	o If the new response contains a weak validator and that validator
1016	corresponds to one of the cache's stored responses, then the most
1017	recent of those matching stored responses is selected for update.
1018
1019	o If the new response does not include any form of validator (such
1020	as in the case where a client generates an If-Modified-Since
1021	request from a source other than the Last-Modified response header
1022	field), and there is only one stored response, and that stored
1023	response also lacks a validator, then that stored response is
1024	selected for update.
1025
1026	If a stored response is selected for update, the cache MUST:
1027
1028	o delete any Warning header fields in the stored response with
1029	warn-code 1xx (see Section 5.5);
1030
1031	o retain any Warning header fields in the stored response with
1032	warn-code 2xx; and,
1033
1034	o use other header fields provided in the 304 (Not Modified)
1035	response to replace all instances of the corresponding header
1036	fields in the stored response.
1037
1038	4.3.5. Freshening Responses via HEAD
1039
1040	A response to the HEAD method is identical to what an equivalent
1041	request made with a GET would have been, except it lacks a body.
1042	This property of HEAD responses can be used to invalidate or update a
1043	cached GET response if the more efficient conditional GET request
1044	mechanism is not available (due to no validators being present in the
1045	stored response) or if transmission of the representation body is not
1046	desired even if it has changed.
1047
1048	When a cache makes an inbound HEAD request for a given request target
1049	and receives a 200 (OK) response, the cache SHOULD update or
1050	invalidate each of its stored GET responses that could have been
1051	selected for that request (see Section 4.1).
1052
1053	For each of the stored responses that could have been selected, if
1054	the stored response and HEAD response have matching values for any
1055	received validator fields (ETag and Last-Modified) and, if the HEAD
1056	response has a Content-Length header field, the value of
1057	Content-Length matches that of the stored response, the cache SHOULD
1058	update the stored response as described below; otherwise, the cache
1059	SHOULD consider the stored response to be stale.
1060
1061
1062
1063
1064
1065
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1068	RFC 7234 HTTP/1.1 Caching June 2014
1069
1070
1071	If a cache updates a stored response with the metadata provided in a
1072	HEAD response, the cache MUST:
1073
1074	o delete any Warning header fields in the stored response with
1075	warn-code 1xx (see Section 5.5);
1076
1077	o retain any Warning header fields in the stored response with
1078	warn-code 2xx; and,
1079
1080	o use other header fields provided in the HEAD response to replace
1081	all instances of the corresponding header fields in the stored
1082	response and append new header fields to the stored response's
1083	header section unless otherwise restricted by the Cache-Control
1084	header field.
1085
1086	4.4. Invalidation
1087
1088	Because unsafe request methods (Section 4.2.1 of [RFC7231]) such as
1089	PUT, POST or DELETE have the potential for changing state on the
1090	origin server, intervening caches can use them to keep their contents
1091	up to date.
1092
1093	A cache MUST invalidate the effective Request URI (Section 5.5 of
1094	[RFC7230]) as well as the URI(s) in the Location and Content-Location
1095	response header fields (if present) when a non-error status code is
1096	received in response to an unsafe request method.
1097
1098	However, a cache MUST NOT invalidate a URI from a Location or
1099	Content-Location response header field if the host part of that URI
1100	differs from the host part in the effective request URI (Section 5.5
1101	of [RFC7230]). This helps prevent denial-of-service attacks.
1102
1103	A cache MUST invalidate the effective request URI (Section 5.5 of
1104	[RFC7230]) when it receives a non-error response to a request with a
1105	method whose safety is unknown.
1106
1107	Here, a "non-error response" is one with a 2xx (Successful) or 3xx
1108	(Redirection) status code. "Invalidate" means that the cache will
1109	either remove all stored responses related to the effective request
1110	URI or will mark these as "invalid" and in need of a mandatory
1111	validation before they can be sent in response to a subsequent
1112	request.
1113
1114	Note that this does not guarantee that all appropriate responses are
1115	invalidated. For example, a state-changing request might invalidate
1116	responses in the caches it travels through, but relevant responses
1117	still might be stored in other caches that it has not.
1118
1119
1120
1121
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1125
1126
1127	5. Header Field Definitions
1128
1129	This section defines the syntax and semantics of HTTP/1.1 header
1130	fields related to caching.
1131
1132	5.1. Age
1133
1134	The "Age" header field conveys the sender's estimate of the amount of
1135	time since the response was generated or successfully validated at
1136	the origin server. Age values are calculated as specified in
1137	Section 4.2.3.
1138
1139	Age = delta-seconds
1140
1141	The Age field-value is a non-negative integer, representing time in
1142	seconds (see Section 1.2.1).
1143
1144	The presence of an Age header field implies that the response was not
1145	generated or validated by the origin server for this request.
1146	However, lack of an Age header field does not imply the origin was
1147	contacted, since the response might have been received from an
1148	HTTP/1.0 cache that does not implement Age.
1149
1150	5.2. Cache-Control
1151
1152	The "Cache-Control" header field is used to specify directives for
1153	caches along the request/response chain. Such cache directives are
1154	unidirectional in that the presence of a directive in a request does
1155	not imply that the same directive is to be given in the response.
1156
1157	A cache MUST obey the requirements of the Cache-Control directives
1158	defined in this section. See Section 5.2.3 for information about how
1159	Cache-Control directives defined elsewhere are handled.
1160
1161	Note: Some HTTP/1.0 caches might not implement Cache-Control.
1162
1163	A proxy, whether or not it implements a cache, MUST pass cache
1164	directives through in forwarded messages, regardless of their
1165	significance to that application, since the directives might be
1166	applicable to all recipients along the request/response chain. It is
1167	not possible to target a directive to a specific cache.
1168
1169	Cache directives are identified by a token, to be compared
1170	case-insensitively, and have an optional argument, that can use both
1171	token and quoted-string syntax. For the directives defined below
1172	that define arguments, recipients ought to accept both forms, even if
1173	one is documented to be preferred. For any directive not defined by
1174	this specification, a recipient MUST accept both forms.
1175
1176
1177
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1180	RFC 7234 HTTP/1.1 Caching June 2014
1181
1182
1183	Cache-Control = 1#cache-directive
1184
1185	cache-directive = token [ "=" ( token / quoted-string ) ]
1186
1187	For the cache directives defined below, no argument is defined (nor
1188	allowed) unless stated otherwise.
1189
1190	5.2.1. Request Cache-Control Directives
1191
1192	5.2.1.1. max-age
1193
1194	Argument syntax:
1195
1196	delta-seconds (see Section 1.2.1)
1197
1198	The "max-age" request directive indicates that the client is
1199	unwilling to accept a response whose age is greater than the
1200	specified number of seconds. Unless the max-stale request directive
1201	is also present, the client is not willing to accept a stale
1202	response.
1203
1204	This directive uses the token form of the argument syntax: e.g.,
1205	'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the
1206	quoted-string form.
1207
1208	5.2.1.2. max-stale
1209
1210	Argument syntax:
1211
1212	delta-seconds (see Section 1.2.1)
1213
1214	The "max-stale" request directive indicates that the client is
1215	willing to accept a response that has exceeded its freshness
1216	lifetime. If max-stale is assigned a value, then the client is
1217	willing to accept a response that has exceeded its freshness lifetime
1218	by no more than the specified number of seconds. If no value is
1219	assigned to max-stale, then the client is willing to accept a stale
1220	response of any age.
1221
1222	This directive uses the token form of the argument syntax: e.g.,
1223	'max-stale=10' not 'max-stale="10"'. A sender SHOULD NOT generate
1224	the quoted-string form.
1225
1226	5.2.1.3. min-fresh
1227
1228	Argument syntax:
1229
1230	delta-seconds (see Section 1.2.1)
1231
1232
1233
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1236	RFC 7234 HTTP/1.1 Caching June 2014
1237
1238
1239	The "min-fresh" request directive indicates that the client is
1240	willing to accept a response whose freshness lifetime is no less than
1241	its current age plus the specified time in seconds. That is, the
1242	client wants a response that will still be fresh for at least the
1243	specified number of seconds.
1244
1245	This directive uses the token form of the argument syntax: e.g.,
1246	'min-fresh=20' not 'min-fresh="20"'. A sender SHOULD NOT generate
1247	the quoted-string form.
1248
1249	5.2.1.4. no-cache
1250
1251	The "no-cache" request directive indicates that a cache MUST NOT use
1252	a stored response to satisfy the request without successful
1253	validation on the origin server.
1254
1255	5.2.1.5. no-store
1256
1257	The "no-store" request directive indicates that a cache MUST NOT
1258	store any part of either this request or any response to it. This
1259	directive applies to both private and shared caches. "MUST NOT
1260	store" in this context means that the cache MUST NOT intentionally
1261	store the information in non-volatile storage, and MUST make a
1262	best-effort attempt to remove the information from volatile storage
1263	as promptly as possible after forwarding it.
1264
1265	This directive is NOT a reliable or sufficient mechanism for ensuring
1266	privacy. In particular, malicious or compromised caches might not
1267	recognize or obey this directive, and communications networks might
1268	be vulnerable to eavesdropping.
1269
1270	Note that if a request containing this directive is satisfied from a
1271	cache, the no-store request directive does not apply to the already
1272	stored response.
1273
1274	5.2.1.6. no-transform
1275
1276	The "no-transform" request directive indicates that an intermediary
1277	(whether or not it implements a cache) MUST NOT transform the
1278	payload, as defined in Section 5.7.2 of [RFC7230].
1279
1280	5.2.1.7. only-if-cached
1281
1282	The "only-if-cached" request directive indicates that the client only
1283	wishes to obtain a stored response. If it receives this directive, a
1284	cache SHOULD either respond using a stored response that is
1285	consistent with the other constraints of the request, or respond with
1286
1287
1288
1289
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1292	RFC 7234 HTTP/1.1 Caching June 2014
1293
1294
1295	a 504 (Gateway Timeout) status code. If a group of caches is being
1296	operated as a unified system with good internal connectivity, a
1297	member cache MAY forward such a request within that group of caches.
1298
1299	5.2.2. Response Cache-Control Directives
1300
1301	5.2.2.1. must-revalidate
1302
1303	The "must-revalidate" response directive indicates that once it has
1304	become stale, a cache MUST NOT use the response to satisfy subsequent
1305	requests without successful validation on the origin server.
1306
1307	The must-revalidate directive is necessary to support reliable
1308	operation for certain protocol features. In all circumstances a
1309	cache MUST obey the must-revalidate directive; in particular, if a
1310	cache cannot reach the origin server for any reason, it MUST generate
1311	a 504 (Gateway Timeout) response.
1312
1313	The must-revalidate directive ought to be used by servers if and only
1314	if failure to validate a request on the representation could result
1315	in incorrect operation, such as a silently unexecuted financial
1316	transaction.
1317
1318	5.2.2.2. no-cache
1319
1320	Argument syntax:
1321
1322	#field-name
1323
1324	The "no-cache" response directive indicates that the response MUST
1325	NOT be used to satisfy a subsequent request without successful
1326	validation on the origin server. This allows an origin server to
1327	prevent a cache from using it to satisfy a request without contacting
1328	it, even by caches that have been configured to send stale responses.
1329
1330	If the no-cache response directive specifies one or more field-names,
1331	then a cache MAY use the response to satisfy a subsequent request,
1332	subject to any other restrictions on caching. However, any header
1333	fields in the response that have the field-name(s) listed MUST NOT be
1334	sent in the response to a subsequent request without successful
1335	revalidation with the origin server. This allows an origin server to
1336	prevent the re-use of certain header fields in a response, while
1337	still allowing caching of the rest of the response.
1338
1339	The field-names given are not limited to the set of header fields	.../cache/cache.h 698
1340	defined by this specification. Field names are case-insensitive.
1341
1342
1343
1344
1345
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1348	RFC 7234 HTTP/1.1 Caching June 2014
1349
1350
1351	This directive uses the quoted-string form of the argument syntax. A
1352	sender SHOULD NOT generate the token form (even if quoting appears
1353	not to be needed for single-entry lists).
1354
1355	Note: Although it has been back-ported to many implementations, some
1356	HTTP/1.0 caches will not recognize or obey this directive. Also,
1357	no-cache response directives with field-names are often handled by
1358	caches as if an unqualified no-cache directive was received; i.e.,
1359	the special handling for the qualified form is not widely
1360	implemented.
1361
1362	5.2.2.3. no-store
1363
1364	The "no-store" response directive indicates that a cache MUST NOT
1365	store any part of either the immediate request or response. This
1366	directive applies to both private and shared caches. "MUST NOT
1367	store" in this context means that the cache MUST NOT intentionally
1368	store the information in non-volatile storage, and MUST make a
1369	best-effort attempt to remove the information from volatile storage
1370	as promptly as possible after forwarding it.
1371
1372	This directive is NOT a reliable or sufficient mechanism for ensuring
1373	privacy. In particular, malicious or compromised caches might not
1374	recognize or obey this directive, and communications networks might
1375	be vulnerable to eavesdropping.
1376
1377	5.2.2.4. no-transform
1378
1379	The "no-transform" response directive indicates that an intermediary
1380	(regardless of whether it implements a cache) MUST NOT transform the
1381	payload, as defined in Section 5.7.2 of [RFC7230].
1382
1383	5.2.2.5. public
1384
1385	The "public" response directive indicates that any cache MAY store
1386	the response, even if the response would normally be non-cacheable or
1387	cacheable only within a private cache. (See Section 3.2 for
1388	additional details related to the use of public in response to a
1389	request containing Authorization, and Section 3 for details of how
1390	public affects responses that would normally not be stored, due to
1391	their status codes not being defined as cacheable by default; see
1392	Section 4.2.2.)
1393
1394	5.2.2.6. private
1395
1396	Argument syntax:
1397
1398	#field-name
1399
1400
1401
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1404	RFC 7234 HTTP/1.1 Caching June 2014
1405
1406
1407	The "private" response directive indicates that the response message
1408	is intended for a single user and MUST NOT be stored by a shared
1409	cache. A private cache MAY store the response and reuse it for later
1410	requests, even if the response would normally be non-cacheable.
1411
1412	If the private response directive specifies one or more field-names,
1413	this requirement is limited to the field-values associated with the
1414	listed response header fields. That is, a shared cache MUST NOT
1415	store the specified field-names(s), whereas it MAY store the
1416	remainder of the response message.
1417
1418	The field-names given are not limited to the set of header fields	.../cache/cache.h 699
1419	defined by this specification. Field names are case-insensitive.
1420
1421	This directive uses the quoted-string form of the argument syntax. A
1422	sender SHOULD NOT generate the token form (even if quoting appears
1423	not to be needed for single-entry lists).
1424
1425	Note: This usage of the word "private" only controls where the
1426	response can be stored; it cannot ensure the privacy of the message
1427	content. Also, private response directives with field-names are
1428	often handled by caches as if an unqualified private directive was
1429	received; i.e., the special handling for the qualified form is not
1430	widely implemented.
1431
1432	5.2.2.7. proxy-revalidate
1433
1434	The "proxy-revalidate" response directive has the same meaning as the
1435	must-revalidate response directive, except that it does not apply to
1436	private caches.
1437
1438	5.2.2.8. max-age
1439
1440	Argument syntax:
1441
1442	delta-seconds (see Section 1.2.1)
1443
1444	The "max-age" response directive indicates that the response is to be
1445	considered stale after its age is greater than the specified number
1446	of seconds.
1447
1448	This directive uses the token form of the argument syntax: e.g.,
1449	'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the
1450	quoted-string form.
1451
1452
1453
1454
1455
1456
1457
1458	Fielding, et al. Standards Track [Page 26]

1460	RFC 7234 HTTP/1.1 Caching June 2014
1461
1462
1463	5.2.2.9. s-maxage
1464
1465	Argument syntax:
1466
1467	delta-seconds (see Section 1.2.1)
1468
1469	The "s-maxage" response directive indicates that, in shared caches,
1470	the maximum age specified by this directive overrides the maximum age
1471	specified by either the max-age directive or the Expires header
1472	field. The s-maxage directive also implies the semantics of the
1473	proxy-revalidate response directive.
1474
1475	This directive uses the token form of the argument syntax: e.g.,
1476	's-maxage=10' not 's-maxage="10"'. A sender SHOULD NOT generate the
1477	quoted-string form.
1478
1479	5.2.3. Cache Control Extensions
1480
1481	The Cache-Control header field can be extended through the use of one
1482	or more cache-extension tokens, each with an optional value. A cache
1483	MUST ignore unrecognized cache directives.
1484
1485	Informational extensions (those that do not require a change in cache
1486	behavior) can be added without changing the semantics of other
1487	directives.
1488
1489	Behavioral extensions are designed to work by acting as modifiers to
1490	the existing base of cache directives. Both the new directive and
1491	the old directive are supplied, such that applications that do not
1492	understand the new directive will default to the behavior specified
1493	by the old directive, and those that understand the new directive
1494	will recognize it as modifying the requirements associated with the
1495	old directive. In this way, extensions to the existing cache-control
1496	directives can be made without breaking deployed caches.
1497
1498	For example, consider a hypothetical new response directive called
1499	"community" that acts as a modifier to the private directive: in
1500	addition to private caches, any cache that is shared only by members
1501	of the named community is allowed to cache the response. An origin
1502	server wishing to allow the UCI community to use an otherwise private
1503	response in their shared cache(s) could do so by including
1504
1505	Cache-Control: private, community="UCI"
1506
1507	A cache that recognizes such a community cache-extension could
1508	broaden its behavior in accordance with that extension. A cache that
1509	does not recognize the community cache-extension would ignore it and
1510	adhere to the private directive.
1511
1512
1513
1514	Fielding, et al. Standards Track [Page 27]

1516	RFC 7234 HTTP/1.1 Caching June 2014
1517
1518
1519	5.3. Expires
1520
1521	The "Expires" header field gives the date/time after which the
1522	response is considered stale. See Section 4.2 for further discussion
1523	of the freshness model.
1524
1525	The presence of an Expires field does not imply that the original
1526	resource will change or cease to exist at, before, or after that
1527	time.
1528
1529	The Expires value is an HTTP-date timestamp, as defined in Section
1530	7.1.1.1 of [RFC7231].
1531
1532	Expires = HTTP-date
1533
1534	For example
1535
1536	Expires: Thu, 01 Dec 1994 16:00:00 GMT
1537
1538	A cache recipient MUST interpret invalid date formats, especially the
1539	value "0", as representing a time in the past (i.e., "already
1540	expired").
1541
1542	If a response includes a Cache-Control field with the max-age
1543	directive (Section 5.2.2.8), a recipient MUST ignore the Expires
1544	field. Likewise, if a response includes the s-maxage directive
1545	(Section 5.2.2.9), a shared cache recipient MUST ignore the Expires
1546	field. In both these cases, the value in Expires is only intended
1547	for recipients that have not yet implemented the Cache-Control field.
1548
1549	An origin server without a clock MUST NOT generate an Expires field
1550	unless its value represents a fixed time in the past (always expired)
1551	or its value has been associated with the resource by a system or
1552	user with a reliable clock.
1553
1554	Historically, HTTP required the Expires field-value to be no more
1555	than a year in the future. While longer freshness lifetimes are no
1556	longer prohibited, extremely large values have been demonstrated to
1557	cause problems (e.g., clock overflows due to use of 32-bit integers
1558	for time values), and many caches will evict a response far sooner
1559	than that.
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570	Fielding, et al. Standards Track [Page 28]

1572	RFC 7234 HTTP/1.1 Caching June 2014
1573
1574
1575	5.4. Pragma
1576
1577	The "Pragma" header field allows backwards compatibility with
1578	HTTP/1.0 caches, so that clients can specify a "no-cache" request
1579	that they will understand (as Cache-Control was not defined until
1580	HTTP/1.1). When the Cache-Control header field is also present and
1581	understood in a request, Pragma is ignored.
1582
1583	In HTTP/1.0, Pragma was defined as an extensible field for
1584	implementation-specified directives for recipients. This
1585	specification deprecates such extensions to improve interoperability.
1586
1587	Pragma = 1#pragma-directive
1588	pragma-directive = "no-cache" / extension-pragma
1589	extension-pragma = token [ "=" ( token / quoted-string ) ]
1590
1591	When the Cache-Control header field is not present in a request,
1592	caches MUST consider the no-cache request pragma-directive as having
1593	the same effect as if "Cache-Control: no-cache" were present (see
1594	Section 5.2.1).
1595
1596	When sending a no-cache request, a client ought to include both the
1597	pragma and cache-control directives, unless Cache-Control: no-cache
1598	is purposefully omitted to target other Cache-Control response
1599	directives at HTTP/1.1 caches. For example:
1600
1601	GET / HTTP/1.1
1602	Host: www.example.com
1603	Cache-Control: max-age=30
1604	Pragma: no-cache
1605
1606	will constrain HTTP/1.1 caches to serve a response no older than 30
1607	seconds, while precluding implementations that do not understand
1608	Cache-Control from serving a cached response.
1609
1610	Note: Because the meaning of "Pragma: no-cache" in responses is
1611	not specified, it does not provide a reliable replacement for
1612	"Cache-Control: no-cache" in them.
1613
1614	5.5. Warning
1615
1616	The "Warning" header field is used to carry additional information
1617	about the status or transformation of a message that might not be
1618	reflected in the status code. This information is typically used to
1619	warn about possible incorrectness introduced by caching operations or
1620	transformations applied to the payload of the message.
1621
1622
1623
1624
1625
1626	Fielding, et al. Standards Track [Page 29]

1628	RFC 7234 HTTP/1.1 Caching June 2014
1629
1630
1631	Warnings can be used for other purposes, both cache-related and
1632	otherwise. The use of a warning, rather than an error status code,
1633	distinguishes these responses from true failures.
1634
1635	Warning header fields can in general be applied to any message,
1636	however some warn-codes are specific to caches and can only be
1637	applied to response messages.
1638
1639	Warning = 1#warning-value
1640
1641	warning-value = warn-code SP warn-agent SP warn-text
1642	[ SP warn-date ]
1643
1644	warn-code = 3DIGIT
1645	warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1646	; the name or pseudonym of the server adding
1647	; the Warning header field, for use in debugging
1648	; a single "-" is recommended when agent unknown
1649	warn-text = quoted-string
1650	warn-date = DQUOTE HTTP-date DQUOTE
1651
1652	Multiple warnings can be generated in a response (either by the
1653	origin server or by a cache), including multiple warnings with the
1654	same warn-code number that only differ in warn-text.
1655
1656	A user agent that receives one or more Warning header fields SHOULD
1657	inform the user of as many of them as possible, in the order that
1658	they appear in the response. Senders that generate multiple Warning
1659	header fields are encouraged to order them with this user agent
1660	behavior in mind. A sender that generates new Warning header fields
1661	MUST append them after any existing Warning header fields.
1662
1663	Warnings are assigned three digit warn-codes. The first digit
1664	indicates whether the Warning is required to be deleted from a stored
1665	response after validation:
1666
1667	o 1xx warn-codes describe the freshness or validation status of the
1668	response, and so they MUST be deleted by a cache after validation.
1669	They can only be generated by a cache when validating a cached
1670	entry, and MUST NOT be generated in any other situation.
1671
1672	o 2xx warn-codes describe some aspect of the representation that is
1673	not rectified by a validation (for example, a lossy compression of
1674	the representation) and they MUST NOT be deleted by a cache after
1675	validation, unless a full response is sent, in which case they
1676	MUST be.
1677
1678
1679
1680
1681
1682	Fielding, et al. Standards Track [Page 30]

1684	RFC 7234 HTTP/1.1 Caching June 2014
1685
1686
1687	If a sender generates one or more 1xx warn-codes in a message to be
1688	sent to a recipient known to implement only HTTP/1.0, the sender MUST
1689	include in each corresponding warning-value a warn-date that matches
1690	the Date header field in the message. For example:
1691
1692	HTTP/1.1 200 OK
1693	Date: Sat, 25 Aug 2012 23:34:45 GMT
1694	Warning: 112 - "network down" "Sat, 25 Aug 2012 23:34:45 GMT"
1695
1696
1697	Warnings have accompanying warn-text that describes the error, e.g.,
1698	for logging. It is advisory only, and its content does not affect
1699	interpretation of the warn-code.
1700
1701	If a recipient that uses, evaluates, or displays Warning header
1702	fields receives a warn-date that is different from the Date value in
1703	the same message, the recipient MUST exclude the warning-value
1704	containing that warn-date before storing, forwarding, or using the
1705	message. This allows recipients to exclude warning-values that were
1706	improperly retained after a cache validation. If all of the
1707	warning-values are excluded, the recipient MUST exclude the Warning
1708	header field as well.
1709
1710	The following warn-codes are defined by this specification, each with
1711	a recommended warn-text in English, and a description of its meaning.
1712	The procedure for defining additional warn codes is described in
1713	Section 7.2.1.
1714
1715	5.5.1. Warning: 110 - "Response is Stale"
1716
1717	A cache SHOULD generate this whenever the sent response is stale.
1718
1719	5.5.2. Warning: 111 - "Revalidation Failed"
1720
1721	A cache SHOULD generate this when sending a stale response because an
1722	attempt to validate the response failed, due to an inability to reach
1723	the server.
1724
1725	5.5.3. Warning: 112 - "Disconnected Operation"
1726
1727	A cache SHOULD generate this if it is intentionally disconnected from
1728	the rest of the network for a period of time.
1729
1730	5.5.4. Warning: 113 - "Heuristic Expiration"
1731
1732	A cache SHOULD generate this if it heuristically chose a freshness
1733	lifetime greater than 24 hours and the response's age is greater than
1734	24 hours.
1735
1736
1737
1738	Fielding, et al. Standards Track [Page 31]

1740	RFC 7234 HTTP/1.1 Caching June 2014
1741
1742
1743	5.5.5. Warning: 199 - "Miscellaneous Warning"
1744
1745	The warning text can include arbitrary information to be presented to
1746	a human user or logged. A system receiving this warning MUST NOT
1747	take any automated action, besides presenting the warning to the
1748	user.
1749
1750	5.5.6. Warning: 214 - "Transformation Applied"
1751
1752	This Warning code MUST be added by a proxy if it applies any
1753	transformation to the representation, such as changing the
1754	content-coding, media-type, or modifying the representation data,
1755	unless this Warning code already appears in the response.
1756
1757	5.5.7. Warning: 299 - "Miscellaneous Persistent Warning"
1758
1759	The warning text can include arbitrary information to be presented to
1760	a human user or logged. A system receiving this warning MUST NOT
1761	take any automated action.
1762
1763	6. History Lists
1764
1765	User agents often have history mechanisms, such as "Back" buttons and
1766	history lists, that can be used to redisplay a representation
1767	retrieved earlier in a session.
1768
1769	The freshness model (Section 4.2) does not necessarily apply to
1770	history mechanisms. That is, a history mechanism can display a
1771	previous representation even if it has expired.
1772
1773	This does not prohibit the history mechanism from telling the user
1774	that a view might be stale or from honoring cache directives (e.g.,
1775	Cache-Control: no-store).
1776
1777	7. IANA Considerations
1778
1779	7.1. Cache Directive Registry
1780
1781	The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry"
1782	defines the namespace for the cache directives. It has been created
1783	and is now maintained at
1784	<http://www.iana.org/assignments/http-cache-directives>.
1785
1786	7.1.1. Procedure
1787
1788	A registration MUST include the following fields:
1789
1790	o Cache Directive Name
1791
1792
1793
1794	Fielding, et al. Standards Track [Page 32]

1796	RFC 7234 HTTP/1.1 Caching June 2014
1797
1798
1799	o Pointer to specification text
1800
1801	Values to be added to this namespace require IETF Review (see
1802	[RFC5226], Section 4.1).
1803
1804	7.1.2. Considerations for New Cache Control Directives
1805
1806	New extension directives ought to consider defining:
1807
1808	o What it means for a directive to be specified multiple times,
1809
1810	o When the directive does not take an argument, what it means when
1811	an argument is present,
1812
1813	o When the directive requires an argument, what it means when it is
1814	missing,
1815
1816	o Whether the directive is specific to requests, responses, or able
1817	to be used in either.
1818
1819	See also Section 5.2.3.
1820
1821	7.1.3. Registrations
1822
1823	The registry has been populated with the registrations below:
1824
1825	+------------------------+----------------------------------+
1826	\| Cache Directive \| Reference \|
1827	+------------------------+----------------------------------+
1828	\| max-age \| Section 5.2.1.1, Section 5.2.2.8 \|
1829	\| max-stale \| Section 5.2.1.2 \|
1830	\| min-fresh \| Section 5.2.1.3 \|
1831	\| must-revalidate \| Section 5.2.2.1 \|
1832	\| no-cache \| Section 5.2.1.4, Section 5.2.2.2 \|
1833	\| no-store \| Section 5.2.1.5, Section 5.2.2.3 \|
1834	\| no-transform \| Section 5.2.1.6, Section 5.2.2.4 \|
1835	\| only-if-cached \| Section 5.2.1.7 \|
1836	\| private \| Section 5.2.2.6 \|
1837	\| proxy-revalidate \| Section 5.2.2.7 \|
1838	\| public \| Section 5.2.2.5 \|
1839	\| s-maxage \| Section 5.2.2.9 \|
1840	\| stale-if-error \| [RFC5861], Section 4 \|
1841	\| stale-while-revalidate \| [RFC5861], Section 3 \|
1842	+------------------------+----------------------------------+
1843
1844
1845
1846
1847
1848
1849
1850	Fielding, et al. Standards Track [Page 33]

1852	RFC 7234 HTTP/1.1 Caching June 2014
1853
1854
1855	7.2. Warn Code Registry
1856
1857	The "Hypertext Transfer Protocol (HTTP) Warn Codes" registry defines
1858	the namespace for warn codes. It has been created and is now
1859	maintained at <http://www.iana.org/assignments/http-warn-codes>.
1860
1861	7.2.1. Procedure
1862
1863	A registration MUST include the following fields:
1864
1865	o Warn Code (3 digits)
1866
1867	o Short Description
1868
1869	o Pointer to specification text
1870
1871	Values to be added to this namespace require IETF Review (see
1872	[RFC5226], Section 4.1).
1873
1874	7.2.2. Registrations
1875
1876	The registry has been populated with the registrations below:
1877
1878	+-----------+----------------------------------+---------------+
1879	\| Warn Code \| Short Description \| Reference \|
1880	+-----------+----------------------------------+---------------+
1881	\| 110 \| Response is Stale \| Section 5.5.1 \|
1882	\| 111 \| Revalidation Failed \| Section 5.5.2 \|
1883	\| 112 \| Disconnected Operation \| Section 5.5.3 \|
1884	\| 113 \| Heuristic Expiration \| Section 5.5.4 \|
1885	\| 199 \| Miscellaneous Warning \| Section 5.5.5 \|
1886	\| 214 \| Transformation Applied \| Section 5.5.6 \|
1887	\| 299 \| Miscellaneous Persistent Warning \| Section 5.5.7 \|
1888	+-----------+----------------------------------+---------------+
1889
1890	7.3. Header Field Registration
1891
1892	HTTP header fields are registered within the "Message Headers"
1893	registry maintained at
1894	<http://www.iana.org/assignments/message-headers/>.
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906	Fielding, et al. Standards Track [Page 34]

1908	RFC 7234 HTTP/1.1 Caching June 2014
1909
1910
1911	This document defines the following HTTP header fields, so the
1912	"Permanent Message Header Field Names" registry has been updated
1913	accordingly (see [BCP90]).
1914
1915	+-------------------+----------+----------+-------------+
1916	\| Header Field Name \| Protocol \| Status \| Reference \|
1917	+-------------------+----------+----------+-------------+
1918	\| Age \| http \| standard \| Section 5.1 \|
1919	\| Cache-Control \| http \| standard \| Section 5.2 \|
1920	\| Expires \| http \| standard \| Section 5.3 \|
1921	\| Pragma \| http \| standard \| Section 5.4 \|
1922	\| Warning \| http \| standard \| Section 5.5 \|
1923	+-------------------+----------+----------+-------------+
1924
1925	The change controller is: "IETF (iesg@ietf.org) - Internet
1926	Engineering Task Force".
1927
1928	8. Security Considerations
1929
1930	This section is meant to inform developers, information providers,
1931	and users of known security concerns specific to HTTP caching. More
1932	general security considerations are addressed in HTTP messaging
1933	[RFC7230] and semantics [RFC7231].
1934
1935	Caches expose additional potential vulnerabilities, since the
1936	contents of the cache represent an attractive target for malicious
1937	exploitation. Because cache contents persist after an HTTP request
1938	is complete, an attack on the cache can reveal information long after
1939	a user believes that the information has been removed from the
1940	network. Therefore, cache contents need to be protected as sensitive
1941	information.
1942
1943	In particular, various attacks might be amplified by being stored in
1944	a shared cache; such "cache poisoning" attacks use the cache to
1945	distribute a malicious payload to many clients, and are especially
1946	effective when an attacker can use implementation flaws, elevated
1947	privileges, or other techniques to insert such a response into a
1948	cache. One common attack vector for cache poisoning is to exploit
1949	differences in message parsing on proxies and in user agents; see
1950	Section 3.3.3 of [RFC7230] for the relevant requirements.
1951
1952	Likewise, implementation flaws (as well as misunderstanding of cache
1953	operation) might lead to caching of sensitive information (e.g.,
1954	authentication credentials) that is thought to be private, exposing
1955	it to unauthorized parties.
1956
1957
1958
1959
1960
1961
1962	Fielding, et al. Standards Track [Page 35]

1964	RFC 7234 HTTP/1.1 Caching June 2014
1965
1966
1967	Furthermore, the very use of a cache can bring about privacy
1968	concerns. For example, if two users share a cache, and the first one
1969	browses to a site, the second may be able to detect that the other
1970	has been to that site, because the resources from it load more
1971	quickly, thanks to the cache.
1972
1973	Note that the Set-Cookie response header field [RFC6265] does not
1974	inhibit caching; a cacheable response with a Set-Cookie header field
1975	can be (and often is) used to satisfy subsequent requests to caches.
1976	Servers who wish to control caching of these responses are encouraged
1977	to emit appropriate Cache-Control response header fields.
1978
1979	9. Acknowledgments
1980
1981	See Section 10 of [RFC7230].
1982
1983	10. References
1984
1985	10.1. Normative References
1986
1987	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
1988	Requirement Levels", BCP 14, RFC 2119, March 1997.
1989
1990	[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
1991	Specifications: ABNF", STD 68, RFC 5234, January 2008.
1992
1993	[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
1994	Protocol (HTTP/1.1): Message Syntax and Routing",
1995	RFC 7230, June 2014.
1996
1997	[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
1998	Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
1999	June 2014.
2000
2001	[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
2002	Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
2003	June 2014.
2004
2005	[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
2006	"Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
2007	RFC 7233, June 2014.
2008
2009	[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
2010	Protocol (HTTP/1.1): Authentication", RFC 7235, June 2014.
2011
2012
2013
2014
2015
2016
2017
2018	Fielding, et al. Standards Track [Page 36]