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rfc:rfc7234

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.

Fielding, et al. Standards Track [Page 1] RFC 7234 HTTP/1.1 Caching June 2014

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

Fielding, et al. Standards Track [Page 2] RFC 7234 HTTP/1.1 Caching June 2014

         4.3.3. Handling a Validation Response .....................18
         4.3.4. Freshening Stored Responses upon Validation ........18
         4.3.5. Freshening Responses via HEAD ......................19
    4.4. Invalidation ..............................................20
 5. Header Field Definitions .......................................21
    5.1. Age .......................................................21
    5.2. Cache-Control .............................................21
         5.2.1. Request Cache-Control Directives ...................22
         5.2.2. Response Cache-Control Directives ..................24
         5.2.3. Cache Control Extensions ...........................27
    5.3. Expires ...................................................28
    5.4. Pragma ....................................................29
    5.5. Warning ...................................................29
         5.5.1. Warning: 110 - "Response is Stale" .................31
         5.5.2. Warning: 111 - "Revalidation Failed" ...............31
         5.5.3. Warning: 112 - "Disconnected Operation" ............31
         5.5.4. Warning: 113 - "Heuristic Expiration" ..............31
         5.5.5. Warning: 199 - "Miscellaneous Warning" .............32
         5.5.6. Warning: 214 - "Transformation Applied" ............32
         5.5.7. Warning: 299 - "Miscellaneous Persistent Warning" ..32
 6. History Lists ..................................................32
 7. IANA Considerations ............................................32
    7.1. Cache Directive Registry ..................................32
         7.1.1. Procedure ..........................................32
         7.1.2. Considerations for New Cache Control Directives ....33
         7.1.3. Registrations ......................................33
    7.2. Warn Code Registry ........................................34
         7.2.1. Procedure ..........................................34
         7.2.2. Registrations ......................................34
    7.3. Header Field Registration .................................34
 8. Security Considerations ........................................35
 9. Acknowledgments ................................................36
 10. References ....................................................36
    10.1. Normative References .....................................36
    10.2. Informative References ...................................37
 Appendix A. Changes from RFC 2616 .................................38
 Appendix B. Imported ABNF .........................................39
 Appendix C. Collected ABNF ........................................39
 Index .............................................................41

Fielding, et al. Standards Track [Page 3] RFC 7234 HTTP/1.1 Caching June 2014

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

Fielding, et al. Standards Track [Page 4] RFC 7234 HTTP/1.1 Caching June 2014

 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)),

Fielding, et al. Standards Track [Page 5] RFC 7234 HTTP/1.1 Caching June 2014

 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

Fielding, et al. Standards Track [Page 6] RFC 7234 HTTP/1.1 Caching June 2014

  • 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.

Fielding, et al. Standards Track [Page 7] RFC 7234 HTTP/1.1 Caching June 2014

 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

Fielding, et al. Standards Track [Page 8] RFC 7234 HTTP/1.1 Caching June 2014

 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

Fielding, et al. Standards Track [Page 9] RFC 7234 HTTP/1.1 Caching June 2014

 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.

Fielding, et al. Standards Track [Page 10] RFC 7234 HTTP/1.1 Caching June 2014

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).

Fielding, et al. Standards Track [Page 11] RFC 7234 HTTP/1.1 Caching June 2014

 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.

Fielding, et al. Standards Track [Page 12] RFC 7234 HTTP/1.1 Caching June 2014

 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

Fielding, et al. Standards Track [Page 13] RFC 7234 HTTP/1.1 Caching June 2014

 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.

Fielding, et al. Standards Track [Page 14] RFC 7234 HTTP/1.1 Caching June 2014

   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.

Fielding, et al. Standards Track [Page 15] RFC 7234 HTTP/1.1 Caching June 2014

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

Fielding, et al. Standards Track [Page 16] RFC 7234 HTTP/1.1 Caching June 2014

 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

Fielding, et al. Standards Track [Page 17] RFC 7234 HTTP/1.1 Caching June 2014

 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.

Fielding, et al. Standards Track [Page 19] RFC 7234 HTTP/1.1 Caching June 2014

 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.

Fielding, et al. Standards Track [Page 20] RFC 7234 HTTP/1.1 Caching June 2014

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.

Fielding, et al. Standards Track [Page 21] RFC 7234 HTTP/1.1 Caching June 2014

   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

Fielding, et al. Standards Track [Page 23] RFC 7234 HTTP/1.1 Caching June 2014

 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.

Fielding, et al. Standards Track [Page 26] RFC 7234 HTTP/1.1 Caching June 2014

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.

Fielding, et al. Standards Track [Page 27] RFC 7234 HTTP/1.1 Caching June 2014

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.

Fielding, et al. Standards Track [Page 28] RFC 7234 HTTP/1.1 Caching June 2014

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.

Fielding, et al. Standards Track [Page 31] RFC 7234 HTTP/1.1 Caching June 2014

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

Fielding, et al. Standards Track [Page 32] RFC 7234 HTTP/1.1 Caching June 2014

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

Fielding, et al. Standards Track [Page 33] RFC 7234 HTTP/1.1 Caching June 2014

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/>.

Fielding, et al. Standards Track [Page 34] RFC 7234 HTTP/1.1 Caching June 2014

 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.

Fielding, et al. Standards Track [Page 35] RFC 7234 HTTP/1.1 Caching June 2014

 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.

Fielding, et al. Standards Track [Page 36] RFC 7234 HTTP/1.1 Caching June 2014

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.

Fielding, et al. Standards Track [Page 37] RFC 7234 HTTP/1.1 Caching June 2014

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)

Fielding, et al. Standards Track [Page 38] RFC 7234 HTTP/1.1 Caching June 2014

 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>

Fielding, et al. Standards Track [Page 39] RFC 7234 HTTP/1.1 Caching June 2014

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
  ]

Fielding, et al. Standards Track [Page 40] RFC 7234 HTTP/1.1 Caching June 2014

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

Fielding, et al. Standards Track [Page 41] RFC 7234 HTTP/1.1 Caching June 2014

       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

Fielding, et al. Standards Track [Page 42] RFC 7234 HTTP/1.1 Caching June 2014

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/

Fielding, et al. Standards Track [Page 43]

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