frankenRFC723x_cache.txt   draft-ietf-httpbis-cache-17.txt 
Internet Engineering Task Force (IETF) R. Fielding, Ed. HTTP Working Group R. Fielding, Ed.
Request for Comments: 7234 Adobe Internet-Draft Adobe
Obsoletes: 2616 M. Nottingham, Ed. Obsoletes: 7234 (if approved) M. Nottingham, Ed.
Category: Standards Track Akamai Intended status: Standards Track Fastly
ISSN: 2070-1721 J. Reschke, Ed. Expires: 27 January 2022 J. Reschke, Ed.
greenbytes greenbytes
June 2014 26 July 2021
Hypertext Transfer Protocol (HTTP/1.1): Caching HTTP Caching
draft-ietf-httpbis-cache-17
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is a stateless application- The Hypertext Transfer Protocol (HTTP) is a stateless application-
level protocol for distributed, collaborative, hypertext information level protocol for distributed, collaborative, hypertext information
systems. This document defines HTTP caches and the associated header systems. This document defines HTTP caches and the associated header
fields that control cache behavior or indicate cacheable response fields that control cache behavior or indicate cacheable response
messages. messages.
This document obsoletes RFC 7234.
Editorial Note Editorial Note
This note is not in the original RFC. This note is to be removed before publishing as an RFC.
The purpose of this document is to produce diffs that show just the Discussion of this draft takes place on the HTTP working group
changes from text in the original RFCs that were input for http-core. mailing list (ietf-http-wg@w3.org), which is archived at
Hence, the frankenRFC documents show all of the original text (including <https://lists.w3.org/Archives/Public/ietf-http-wg/>.
stuff that has been deleted) plus some new text [in brackets] to anchor
context, rearranged to minimize the resulting diffs when compared to the
most recently published version of draft-ietf-httpbis-cache.
After this document is updated to match any reorg changes in the latest Working Group information can be found at <https://httpwg.org/>;
version, the franken diffs are saved and published in this directory as source code and issues list for this draft can be found at
diff_cache_frfc_to_NN.html (where NN is the I-D draft revision) <https://github.com/httpwg/http-core>.
The changes in this draft are summarized in Appendix C.18.
Status of This Memo Status of This Memo
This is an Internet Standards Track document. This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
This document is a product of the Internet Engineering Task Force Internet-Drafts are working documents of the Internet Engineering
(IETF). It represents the consensus of the IETF community. It has Task Force (IETF). Note that other groups may also distribute
received public review and has been approved for publication by the working documents as Internet-Drafts. The list of current Internet-
Internet Engineering Steering Group (IESG). Further information on Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, Internet-Drafts are draft documents valid for a maximum of six months
and how to provide feedback on it may be obtained at and may be updated, replaced, or obsoleted by other documents at any
http://www.rfc-editor.org/info/rfc7234. time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 27 January 2022.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction ....................................................4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conformance and Error Handling .............................4 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 5
1.2. Syntax Notation ............................................4 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 5
1.2.1. Delta Seconds .......................................5 1.2.1. Imported Rules . . . . . . . . . . . . . . . . . . . 5
2. Overview of Cache Operation .....................................5 1.2.2. Delta Seconds . . . . . . . . . . . . . . . . . . . . 6
3. Storing Responses in Caches .....................................6 2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 6
3.1. Storing Incomplete Responses ...............................7 3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 7
3.2. Storing Responses to Authenticated Requests ................7 3.1. Storing Header and Trailer Fields . . . . . . . . . . . . 8
3.3. Combining Partial Content ..................................8 3.2. Updating Stored Header Fields . . . . . . . . . . . . . . 9
4. Constructing Responses from Caches ..............................8 3.3. Storing Incomplete Responses . . . . . . . . . . . . . . 10
4.1. Calculating Secondary Keys with Vary .......................9 3.4. Combining Partial Content . . . . . . . . . . . . . . . . 11
4.2. Freshness .................................................11 3.5. Storing Responses to Authenticated Requests . . . . . . . 11
4.2.1. Calculating Freshness Lifetime .....................12 4. Constructing Responses from Caches . . . . . . . . . . . . . 11
4.2.2. Calculating Heuristic Freshness ....................13 4.1. Calculating Cache Keys with the Vary Header Field . . . . 13
4.2.3. Calculating Age ....................................13 4.2. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2.4. Serving Stale Responses ............................15 4.2.1. Calculating Freshness Lifetime . . . . . . . . . . . 15
4.3. Validation ................................................16 4.2.2. Calculating Heuristic Freshness . . . . . . . . . . . 16
4.3.1. Sending a Validation Request .......................16 4.2.3. Calculating Age . . . . . . . . . . . . . . . . . . . 17
4.3.2. Handling a Received Validation Request .............16 4.2.4. Serving Stale Responses . . . . . . . . . . . . . . . 18
4.3.3. Handling a Validation Response .....................18 4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . 19
4.3.4. Freshening Stored Responses upon Validation ........18 4.3.1. Sending a Validation Request . . . . . . . . . . . . 19
4.3.5. Freshening Responses via HEAD ......................19 4.3.2. Handling a Received Validation Request . . . . . . . 20
4.4. Invalidation ..............................................20 4.3.3. Handling a Validation Response . . . . . . . . . . . 21
5. Header Field Definitions .......................................21 4.3.4. Freshening Stored Responses upon Validation . . . . . 21
5.1. Age .......................................................21 4.3.5. Freshening Responses with HEAD . . . . . . . . . . . 22
5.2. Cache-Control .............................................21 4.4. Invalidating Stored Responses . . . . . . . . . . . . . . 23
5.2.1. Request Cache-Control Directives ...................22 5. Field Definitions . . . . . . . . . . . . . . . . . . . . . . 24
5.2.2. Response Cache-Control Directives ..................24 5.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.2.3. Cache Control Extensions ...........................27 5.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 24
5.3. Expires ...................................................28 5.2.1. Request Cache-Control Directives . . . . . . . . . . 25
5.4. Pragma ....................................................29 5.2.1.1. max-age . . . . . . . . . . . . . . . . . . . . . 25
5.5. Warning ...................................................29 5.2.1.2. max-stale . . . . . . . . . . . . . . . . . . . . 25
5.5.1. Warning: 110 - "Response is Stale" .................31 5.2.1.3. min-fresh . . . . . . . . . . . . . . . . . . . . 26
5.5.2. Warning: 111 - "Revalidation Failed" ...............31 5.2.1.4. no-cache . . . . . . . . . . . . . . . . . . . . 26
5.5.3. Warning: 112 - "Disconnected Operation" ............31 5.2.1.5. no-store . . . . . . . . . . . . . . . . . . . . 26
5.5.4. Warning: 113 - "Heuristic Expiration" ..............31 5.2.1.6. no-transform . . . . . . . . . . . . . . . . . . 27
5.5.5. Warning: 199 - "Miscellaneous Warning" .............32 5.2.1.7. only-if-cached . . . . . . . . . . . . . . . . . 27
5.5.6. Warning: 214 - "Transformation Applied" ............32 5.2.2. Response Cache-Control Directives . . . . . . . . . . 27
5.5.7. Warning: 299 - "Miscellaneous Persistent Warning" ..32 5.2.2.1. max-age . . . . . . . . . . . . . . . . . . . . . 27
6. History Lists ..................................................32 5.2.2.2. must-revalidate . . . . . . . . . . . . . . . . . 27
7. IANA Considerations ............................................32 5.2.2.3. must-understand . . . . . . . . . . . . . . . . . 28
7.1. Cache Directive Registry ..................................32 5.2.2.4. no-cache . . . . . . . . . . . . . . . . . . . . 28
7.1.1. Procedure ..........................................32 5.2.2.5. no-store . . . . . . . . . . . . . . . . . . . . 29
7.1.2. Considerations for New Cache Control Directives ....33 5.2.2.6. no-transform . . . . . . . . . . . . . . . . . . 29
7.1.3. Registrations ......................................33 5.2.2.7. private . . . . . . . . . . . . . . . . . . . . . 29
7.2. Warn Code Registry ........................................34 5.2.2.8. proxy-revalidate . . . . . . . . . . . . . . . . 30
7.2.1. Procedure ..........................................34 5.2.2.9. public . . . . . . . . . . . . . . . . . . . . . 30
7.2.2. Registrations ......................................34 5.2.2.10. s-maxage . . . . . . . . . . . . . . . . . . . . 31
7.3. Header Field Registration .................................34 5.2.3. Cache Control Extensions . . . . . . . . . . . . . . 31
8. Security Considerations ........................................35 5.2.4. Cache Directive Registry . . . . . . . . . . . . . . 32
9. Acknowledgments ................................................36 5.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 32
10. References ....................................................36 5.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . 33
10.1. Normative References .....................................36 5.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 34
10.2. Informative References ...................................37 6. Relationship to Applications and Other Caches . . . . . . . . 34
Appendix A. Changes from RFC 2616 .................................38 7. Security Considerations . . . . . . . . . . . . . . . . . . . 35
Appendix B. Imported ABNF .........................................39 7.1. Cache Poisoning . . . . . . . . . . . . . . . . . . . . . 35
Appendix C. Collected ABNF ........................................39 7.2. Timing Attacks . . . . . . . . . . . . . . . . . . . . . 35
Index .............................................................41 7.3. Caching of Sensitive Information . . . . . . . . . . . . 36
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
8.1. Field Name Registration . . . . . . . . . . . . . . . . . 36
8.2. Cache Directive Registration . . . . . . . . . . . . . . 37
8.3. Warn Code Registry . . . . . . . . . . . . . . . . . . . 37
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 37
9.1. Normative References . . . . . . . . . . . . . . . . . . 37
9.2. Informative References . . . . . . . . . . . . . . . . . 38
Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 39
Appendix B. Changes from RFC 7234 . . . . . . . . . . . . . . . 39
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 40
C.1. Between RFC7234 and draft 00 . . . . . . . . . . . . . . 40
C.2. Since draft-ietf-httpbis-cache-00 . . . . . . . . . . . . 41
C.3. Since draft-ietf-httpbis-cache-01 . . . . . . . . . . . . 41
C.4. Since draft-ietf-httpbis-cache-02 . . . . . . . . . . . . 41
C.5. Since draft-ietf-httpbis-cache-03 . . . . . . . . . . . . 41
C.6. Since draft-ietf-httpbis-cache-04 . . . . . . . . . . . . 42
C.7. Since draft-ietf-httpbis-cache-05 . . . . . . . . . . . . 42
C.8. Since draft-ietf-httpbis-cache-06 . . . . . . . . . . . . 42
C.9. Since draft-ietf-httpbis-cache-07 . . . . . . . . . . . . 43
C.10. Since draft-ietf-httpbis-cache-08 . . . . . . . . . . . . 43
C.11. Since draft-ietf-httpbis-cache-09 . . . . . . . . . . . . 43
C.12. Since draft-ietf-httpbis-cache-10 . . . . . . . . . . . . 44
C.13. Since draft-ietf-httpbis-cache-11 . . . . . . . . . . . . 44
C.14. Since draft-ietf-httpbis-cache-12 . . . . . . . . . . . . 44
C.15. Since draft-ietf-httpbis-cache-13 . . . . . . . . . . . . 45
C.16. Since draft-ietf-httpbis-cache-14 . . . . . . . . . . . . 45
C.17. Since draft-ietf-httpbis-cache-15 . . . . . . . . . . . . 46
C.18. Since draft-ietf-httpbis-cache-16 . . . . . . . . . . . . 46
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 46
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48
1. Introduction 1. Introduction
HTTP is typically used for distributed information systems, where The Hypertext Transfer Protocol (HTTP) is a stateless application-
performance can be improved by the use of response caches. This level request/response protocol that uses extensible semantics and
document defines aspects of HTTP/1.1 related to caching and reusing self-descriptive messages for flexible interaction with network-based
response messages. hypertext information systems. It is typically used for distributed
information systems, where the use of response caches can improve
performance. This document defines aspects of HTTP related to
caching and reusing response messages.
An HTTP cache is a local store of response messages and the subsystem An HTTP _cache_ is a local store of response messages and the
that controls storage, retrieval, and deletion of messages in it. A subsystem that controls storage, retrieval, and deletion of messages
cache stores cacheable responses in order to reduce the response time in it. A cache stores cacheable responses to reduce the response
and network bandwidth consumption on future, equivalent requests. time and network bandwidth consumption on future equivalent requests.
Any client or server MAY employ a cache, though a cache cannot be Any client or server MAY use a cache, though not when acting as a
used by a server that is acting as a tunnel. tunnel (Section 3.7 of [HTTP]).
A shared cache is a cache that stores responses to be reused by more A _shared cache_ is a cache that stores responses for reuse by more
than one user; shared caches are usually (but not always) deployed as than one user; shared caches are usually (but not always) deployed as
a part of an intermediary. A private cache, in contrast, is a part of an intermediary. A _private cache_, in contrast, is
dedicated to a single user; often, they are deployed as a component dedicated to a single user; often, they are deployed as a component
of a user agent. of a user agent.
The goal of caching in HTTP/1.1 is to significantly improve The goal of HTTP caching is significantly improving performance by
performance by reusing a prior response message to satisfy a current reusing a prior response message to satisfy a current request. A
request. A stored response is considered "fresh", as defined in cache considers a stored response "fresh", as defined in Section 4.2,
Section 4.2, if the response can be reused without "validation" if it can be reused without "validation" (checking with the origin
(checking with the origin server to see if the cached response server to see if the cached response remains valid for this request).
remains valid for this request). A fresh response can therefore A fresh response can therefore reduce both latency and network
reduce both latency and network overhead each time it is reused. overhead each time the cache reuses it. When a cached response is
When a cached response is not fresh, it might still be reusable if it not fresh, it might still be reusable if validation can freshen it
can be freshened by validation (Section 4.3) or if the origin is (Section 4.3) or if the origin is unavailable (Section 4.2.4).
unavailable (Section 4.2.4).
1.1. Conformance and Error Handling This document obsoletes RFC 7234, with the changes being summarized
in Appendix B.
1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in [RFC2119]. "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Conformance criteria and considerations regarding error handling are Section 2 of [HTTP] defines conformance criteria and contains
defined in Section 2.5 of [RFC7230]. considerations regarding error handling.
1.2. Syntax Notation 1.2. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF) This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with a list extension, defined in Section 7 of notation of [RFC5234], extended with the notation for case-
[RFC7230], that allows for compact definition of comma-separated sensitivity in strings defined in [RFC7405].
lists using a '#' operator (similar to how the '*' operator indicates
repetition). Appendix B describes rules imported from other
documents. Appendix C shows the collected grammar with all list
operators expanded to standard ABNF notation.
The following core rules are included by reference, as defined in It also uses a list extension, defined in Section 5.6.1 of [HTTP],
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return), that allows for compact definition of comma-separated lists using a
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double '#' operator (similar to how the '*' operator indicates repetition).
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any Appendix A shows the collected grammar with all list operators
8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII expanded to standard ABNF notation.
character).
The rules below are defined in other parts: 1.2.1. Imported Rules
port = <port, see [RFC7230], Section 2.7> The following core rule is included by reference, as defined in
pseudonym = <pseudonym, see [RFC7230], Section 5.7.1> [RFC5234], Appendix B.1: DIGIT (decimal 0-9).
uri-host = <uri-host, see [RFC7230], Section 2.7>
The rules below are defined in [RFC7230]: [HTTP] defines the following rules:
HTTP-date = <HTTP-date, see [RFC7231], Section 7.1.1.1> HTTP-date = <HTTP-date, see [HTTP], Section 5.6.7>
OWS = <OWS, see [RFC7230], Section 3.2.3> OWS = <OWS, see [HTTP], Section 5.6.3>
field-name = <field-name, see [RFC7230], Section 3.2> field-name = <field-name, see [HTTP], Section 5.1>
quoted-string = <quoted-string, see [RFC7230], Section 3.2.6> quoted-string = <quoted-string, see [HTTP], Section 5.6.4>
token = <token, see [RFC7230], Section 3.2.6> token = <token, see [HTTP], Section 5.6.2>
1.3. Delta Seconds 1.2.2. Delta Seconds
The delta-seconds rule specifies a non-negative integer, representing The delta-seconds rule specifies a non-negative integer, representing
time in seconds. time in seconds.
delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
A recipient parsing a delta-seconds value and converting it to binary 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 form ought to use an arithmetic type of at least 31 bits of non-
non-negative integer range. If a cache receives a delta-seconds negative integer range. If a cache receives a delta-seconds value
value greater than the greatest integer it can represent, or if any greater than the greatest integer it can represent, or if any of its
of its subsequent calculations overflows, the cache MUST consider the subsequent calculations overflows, the cache MUST consider the value
value to be either 2147483648 (2^31) or the greatest positive integer to be 2147483648 (2^31) or the greatest positive integer it can
it can conveniently represent. conveniently represent.
Note: The value 2147483648 is here for historical reasons, | *Note:* The value 2147483648 is here for historical reasons,
effectively represents infinity (over 68 years), and does not need | represents infinity (over 68 years), and does not need to be
to be stored in binary form; an implementation could produce it as | stored in binary form; an implementation could produce it as a
a canned string if any overflow occurs, even if the calculations | string if any overflow occurs, even if the calculations are
are performed with an arithmetic type incapable of directly | performed with an arithmetic type incapable of directly
representing that number. What matters here is that an overflow | representing that number. What matters here is that an
be detected and not treated as a negative value in later | overflow be detected and not treated as a negative value in
calculations. | later calculations.
2. Overview of Cache Operation 2. Overview of Cache Operation
Proper cache operation preserves the semantics of HTTP transfers Proper cache operation preserves the semantics of HTTP transfers
([RFC7231]) while eliminating the transfer of information already while reducing the transmission of information already held in the
held in the cache. Although caching is an entirely OPTIONAL feature cache. See Section 3 of [HTTP] for the general terminology and core
of HTTP, it can be assumed that reusing a cached response is concepts of HTTP.
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.
The primary cache key consists of the request method and target URI. Although caching is an entirely OPTIONAL feature of HTTP, it can be
However, since HTTP caches in common use today are typically limited assumed that reusing a cached response is desirable and that such
to caching responses to GET, many caches simply decline other methods reuse is the default behavior when no requirement or local
and use only the URI as the primary cache key. 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.
If a request target is subject to content negotiation, its cache The _cache key_ is the information a cache uses to select a response
entry might consist of multiple stored responses, each differentiated and is composed from, at a minimum, the request method and target URI
by a secondary key for the values of the original request's selecting used to retrieve the stored response; the method determines under
header fields (Section 4.1). which circumstances that response can be used to satisfy a subsequent
request. However, many HTTP caches in common use today only cache
GET responses, and therefore only use the URI as the cache key,
forwarding other methods.
Each cache entry consists of a cache key and one or more HTTP If a request target is subject to content negotiation, the cache
responses corresponding to prior requests that used the same key. might store multiple responses for it. Caches differentiate these
The most common form of cache entry is a successful result of a responses by incorporating values of the original request's selecting
retrieval request: i.e., a 200 (OK) response to a GET request, which header fields into the cache key as well, using information in the
contains a representation of the resource identified by the request Vary response header field, as per Section 4.1.
target (Section 4.3.1 of [RFC7231]). However, it is also possible to
cache permanent redirects, negative results (e.g., 404 (Not Found)), Caches might incorporate additional material into the cache key. For
incomplete results (e.g., 206 (Partial Content)), and responses to example, user agent caches might include the referring site's
methods other than GET if the method's definition allows such caching identity, thereby "double keying" the cache to avoid some privacy
and defines something suitable for use as a cache key. risks (see Section 7.2).
Most commonly, caches store the successful result of a retrieval
request: i.e., a 200 (OK) response to a GET request, which contains a
representation of the target resource (Section 9.3.1 of [HTTP]).
However, it is also possible to store redirects, negative results
(e.g., 404 (Not Found)), 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.
A cache is _disconnected_ when it cannot contact the origin server or
otherwise find a forward path for a request. A disconnected cache
can serve stale responses in some circumstances (Section 4.2.4).
3. Storing Responses in Caches 3. Storing Responses in Caches
A cache MUST NOT store a response to any request, unless: A cache MUST NOT store a response to a request unless:
o The request method is understood by the cache and defined as being * the request method is understood by the cache;
cacheable, and
o the response status code is understood by the cache, and * the response status code is final (see Section 15 of [HTTP]);
[new] * if the response status code is 206 or 304, or the "must-
understand" cache directive (see Section 5.2.2.3) is present: the
cache understands the response status code;
o the "no-store" cache directive (see Section 5.2) does not appear * the "no-store" cache directive is not present in the response (see
in request or response header fields, and Section 5.2.2.5);
o the "private" response directive (see Section 5.2.2.6) does not * if the cache is shared: the "private" response directive is either
appear in the response, if the cache is shared, and not present or allows a shared cache to store a modified response;
see Section 5.2.2.7);
o the Authorization header field (see Section 4.2 of [RFC7235]) does * if the cache is shared: the Authorization header field is not
not appear in the request, if the cache is shared, unless the present in the request (see Section 11.6.2 of [HTTP]) or a
response explicitly allows it (see Section 3.2), and response directive is present that explicitly allows shared
caching (see Section 3.5); and,
o the response either: * the response contains at least one of:
* contains an Expires header field (see Section 5.3), or - a public response directive (see Section 5.2.2.9);
* contains a max-age response directive (see Section 5.2.2.8), or - a private response directive, if the cache is not shared (see
Section 5.2.2.7);
* contains a s-maxage response directive (see Section 5.2.2.9) - an Expires header field (see Section 5.3);
and the cache is shared, or
* contains a Cache Control Extension (see Section 5.2.3) that - a max-age response directive (see Section 5.2.2.1);
allows it to be cached, or
* has a status code that is defined as cacheable by default (see - if the cache is shared: an s-maxage response directive (see
Section 4.2.2), or Section 5.2.2.10);
* contains a public response directive (see Section 5.2.2.5). - a Cache Control Extension that allows it to be cached (see
Section 5.2.3); or,
Note that any of the requirements listed above can be overridden by a - a status code that is defined as heuristically cacheable (see
cache-control extension; see Section 5.2.3. Section 4.2.2).
Note that a cache-control extension can override any of the
requirements listed; see Section 5.2.3.
In this context, a cache has "understood" a request method or a In this context, a cache has "understood" a request method or a
response status code if it recognizes it and implements all specified response status code if it recognizes it and implements all specified
caching-related behavior. caching-related behavior.
Note that, in normal operation, some caches will not store a response Note that, in normal operation, some caches will not store a response
that has neither a cache validator nor an explicit expiration time, that has neither a cache validator nor an explicit expiration time,
as such responses are not usually useful to store. However, caches as such responses are not usually useful to store. However, caches
are not prohibited from storing such responses. are not prohibited from storing such responses.
3.1. Storing Header and Trailer Fields 3.1. Storing Header and Trailer Fields
[new] Caches MUST include all received response header fields - including
unrecognised ones - when storing a response; this assures that new
[new] HTTP header fields can be successfully deployed. However, the
following exceptions are made:
[new] * The Connection header field and fields whose names are listed in
it are required by Section 7.6.1 of [HTTP] to be removed before
forwarding the message. This MAY be implemented by doing so
before storage.
[new] * Likewise, some fields' semantics require them to be removed before
forwarding the message, and this MAY be implemented by doing so
before storage; see Section 7.6.1 of [HTTP] for some examples.
[new] * The no-cache (Section 5.2.2.4) and private (Section 5.2.2.7) cache
directives can have arguments that prevent storage of header
fields by all caches and shared caches, respectively.
[new] * Header fields that are specific to the proxy that a cache uses
when forwarding a request MUST NOT be stored, unless the cache
incorporates the identity of the proxy into the cache key.
Effectively, this is limited to Proxy-Authenticate (Section 11.7.1
of [HTTP]), Proxy-Authentication-Info (Section 11.7.3 of [HTTP]),
and Proxy-Authorization (Section 11.7.2 of [HTTP]).
[new] Caches MAY either store trailer fields separate from header fields,
or discard them. Caches MUST NOT combine trailer fields with header
fields.
3.2. Updating Stored Header Fields 3.2. Updating Stored Header Fields
[new] Caches are required to update a stored response's header fields from
another (typically newer) response in several situations; for
example, see Section 3.4, Section 4.3.4 and Section 4.3.5.
[new] When doing so, the cache MUST add each header field in the provided
response to the stored response, replacing field values that are
already present, with the following exceptions:
[new] * Header fields excepted from storage in Section 3.1,
[new] * Header fields that the cache's stored response depends upon, as
described below,
[new] * Header fields that are automatically processed and removed by the
recipient, as described below, and
[new] * The Content-Length header field.
[new] In some cases, caches (especially in user agents) store the results
of processing the received response, rather than the response itself,
and updating header fields that affect that processing can result in
inconsistent behavior and security issues. Caches in this situation
MAY omit these header fields from updating stored responses on an
exceptional basis, but SHOULD limit such omission to those fields
necessary to assure integrity of the stored response.
[new] For example, a browser might decode the content coding of a response
while it is being received, creating a disconnect between the data it
has stored and the response's original metadata. Updating that
stored metadata with a different Content-Encoding header field would
be problematic. Likewise, a browser might store a post-parse HTML
tree, rather than the content received in the response; updating the
Content-Type header field would not be workable in this case, because
any assumptions about the format made in parsing would now be
invalid.
[new] Furthermore, some fields are automatically processed and removed by
the HTTP implementation; for example, the Content-Range header field.
Implementations MAY automatically omit such header fields from
updates, even when the processing does not actually occur.
[new] Note that the Content-* prefix is not a signal that a header field is
omitted from update; it is a convention for MIME header fields, not
HTTP.
3.3. Storing Incomplete Responses 3.3. Storing Incomplete Responses
A response message is considered complete when all of the octets If the request method is GET, the response status code is 200 (OK),
indicated by the message framing ([RFC7230]) are received prior to and the entire response header section has been received, a cache MAY
the connection being closed. If the request method is GET, the store a response body that is not complete (Section 3.4 of [HTTP]) if
response status code is 200 (OK), and the entire response header the stored response is recorded as being incomplete. Likewise, a 206
section has been received, a cache MAY store an incomplete response (Partial Content) response MAY be stored as if it were an incomplete
message body if the cache entry is recorded as incomplete. Likewise, 200 (OK) response. However, a cache MUST NOT store incomplete or
a 206 (Partial Content) response MAY be stored as if it were an partial-content responses if it does not support the Range and
incomplete 200 (OK) cache entry. However, a cache MUST NOT store Content-Range header fields or if it does not understand the range
incomplete or partial-content responses if it does not support the units used in those fields.
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 A cache MAY complete a stored incomplete response by making a
subsequent range request ([RFC7233]) and combining the successful subsequent range request (Section 14.2 of [HTTP]) and combining the
response with the stored entry, as defined in Section 3.3. A cache successful response with the stored response, as defined in
MUST NOT use an incomplete response to answer requests unless the Section 3.4. A cache MUST NOT use an incomplete response to answer
response has been made complete or the request is partial and requests unless the response has been made complete, or the request
specifies a range that is wholly within the incomplete response. A is partial and specifies a range wholly within the incomplete
cache MUST NOT send a partial response to a client without explicitly response. A cache MUST NOT send a partial response to a client
marking it as such using the 206 (Partial Content) status code. without explicitly marking it using the 206 (Partial Content) status
code.
3.4. Combining Partial Content 3.4. Combining Partial Content
A response might transfer only a partial representation if the A response might transfer only a partial representation if the
connection closed prematurely or if the request used one or more connection closed prematurely or if the request used one or more
Range specifiers ([RFC7233]). After several such transfers, a cache Range specifiers (Section 14.2 of [HTTP]). After several such
might have received several ranges of the same representation. A transfers, a cache might have received several ranges of the same
cache MAY combine these ranges into a single stored response, and representation. A cache MAY combine these ranges into a single
reuse that response to satisfy later requests, if they all share the stored response, and reuse that response to satisfy later requests,
same strong validator and the cache complies with the client if they all share the same strong validator and the cache complies
requirements in Section 4.3 of [RFC7233]. with the client requirements in Section 15.3.7.3 of [HTTP].
When combining the new response with one or more stored responses, a When combining the new response with one or more stored responses, a
cache MUST: cache MUST update the stored response header fields using the header
fields provided in the new response, as per Section 3.2.
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.
3.5. Storing Responses to Authenticated Requests 3.5. Storing Responses to Authenticated Requests
A shared cache MUST NOT use a cached response to a request with an 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 Authorization header field (Section 11.6.2 of [HTTP]) to satisfy any
subsequent request unless a cache directive that allows such subsequent request unless the response contains a Cache-Control field
responses to be stored is present in the response. with a response directive (Section 5.2.2) that allows it to be stored
by a shared cache and the cache conforms to the requirements of that
In this specification, the following Cache-Control response directive for that response.
directives (Section 5.2.2) have such an effect: must-revalidate,
public, and s-maxage.
Note that cached responses that contain the "must-revalidate" and/or In this specification, the following response directives have such an
"s-maxage" response directives are not allowed to be served stale effect: must-revalidate (Section 5.2.2.2), public (Section 5.2.2.9),
(Section 4.2.4) by shared caches. In particular, a response with and s-maxage (Section 5.2.2.10).
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.
4. Constructing Responses from Caches 4. Constructing Responses from Caches
When presented with a request, a cache MUST NOT reuse a stored When presented with a request, a cache MUST NOT reuse a stored
response, unless: response unless:
o The presented effective request URI (Section 5.5 of [RFC7230]) and * The presented target URI (Section 7.1 of [HTTP]) and that of the
that of the stored response match, and stored response match, and
o the request method associated with the stored response allows it * the request method associated with the stored response allows it
to be used for the presented request, and to be used for the presented request, and
o selecting header fields nominated by the stored response (if any) * selecting header fields nominated by the stored response (if any)
match those presented (see Section 4.1), and match those presented (see Section 4.1), and
o the presented request does not contain the no-cache pragma * the stored response does not contain the no-cache cache directive
(Section 5.4), nor the no-cache cache directive (Section 5.2.1), (Section 5.2.2.4), unless it is successfully validated
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 (Section 4.3), and
o the stored response is either: * the stored response is either:
* fresh (see Section 4.2), or - fresh (see Section 4.2), or
* allowed to be served stale (see Section 4.2.4), or - allowed to be served stale (see Section 4.2.4), or
* successfully validated (see Section 4.3). - successfully validated (see Section 4.3).
Note that any of the requirements listed above can be overridden by a Note that a cache-control extension can override any of the
cache-control extension; see Section 5.2.3. requirements listed; see Section 5.2.3.
When a stored response is used to satisfy a request without When a stored response is used to satisfy a request without
validation, a cache MUST generate an Age header field (Section 5.1), validation, a cache MUST generate an Age header field (Section 5.1),
replacing any present in the response with a value equal to the replacing any present in the response with a value equal to the
stored response's current_age; see Section 4.2.3. stored response's current_age; see Section 4.2.3.
A cache MUST write through requests with methods that are unsafe 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 (Section 9.2.1 of [HTTP]) to the origin server; i.e., a cache is not
not allowed to generate a reply to such a request before having allowed to generate a reply to such a request before having forwarded
forwarded the request and having received a corresponding response. the request and having received a corresponding response.
Also, note that unsafe requests might invalidate already-stored Also, note that unsafe requests might invalidate already-stored
responses; see Section 4.4. responses; see Section 4.4.
A response that is stored or storable can be used to satisfy multiple
requests, provided that it is allowed to reuse that response for the
requests in question. This enables caches to _collapse requests_ -
or combine multiple incoming requests into a single forward request
upon a cache miss - thereby reducing load on the origin server and
network. However, note that if the response returned is not able to
be used for some or all of the collapsed requests, additional latency
might be introduced, because they will need to be forwarded to be
satisfied.
When more than one suitable response is stored, a cache MUST use the When more than one suitable response is stored, a cache MUST use the
most recent response (as determined by the Date header field). It most recent one (as determined by the Date header field). It can
can also forward the request with "Cache-Control: max-age=0" or also forward the request with "Cache-Control: max-age=0" or "Cache-
"Cache-Control: no-cache" to disambiguate which response to use. Control: no-cache" to disambiguate which response to use.
A cache that does not have a clock available MUST NOT use stored A cache that does not have a clock available MUST NOT use stored
responses without revalidating them upon every use. responses without revalidating them upon every use.
4.1. Calculating Secondary Keys with Vary 4.1. Calculating Cache Keys with the Vary Header Field
When a cache receives a request that can be satisfied by a stored 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]), response and that stored response contains a Vary header field
it MUST NOT use that response unless all of the selecting header (Section 12.5.5 of [HTTP]), the cache MUST NOT use that stored
fields nominated by the Vary header field match in both the original response without revalidation unless all the selecting header fields
request (i.e., that associated with the stored response), and the (nominated by that Vary field value) in the present request match
presented request. those fields in the original request (i.e., the request that caused
the cached response to be stored).
The selecting header fields from two requests are defined to match if 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 and only if those in the first request can be transformed to those in
the second request by applying any of the following: the second request by applying any of:
o adding or removing whitespace, where allowed in the header field's * adding or removing whitespace, where allowed in the header field's
syntax syntax
o combining multiple header fields with the same field name (see * combining multiple header field lines with the same field name
Section 3.2 of [RFC7230]) (see Section 5.2 of [HTTP])
o normalizing both header field values in a way that is known to * normalizing both header field values in a way that is known to
have identical semantics, according to the header field's have identical semantics, according to the header field's
specification (e.g., reordering field values when order is not specification (e.g., reordering field values when order is not
significant; case-normalization, where values are defined to be significant; case-normalization, where values are defined to be
case-insensitive) case-insensitive)
If (after any normalization that might take place) a header field is 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 absent from a request, it can only match another request if it is
also absent there. also absent there.
A Vary header field-value of "*" always fails to match. A stored response with a Vary header field value containing a member
"*" always fails to match.
The stored response with matching selecting header fields is known as The stored response with matching selecting header fields is known as
the selected response. the _selected response_.
If multiple selected responses are available (potentially including If multiple selected responses are available (potentially including
responses without a Vary header field), the cache will need to choose 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 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), doing so (e.g., qvalues on Accept and similar request header fields),
that mechanism MAY be used to select preferred responses; of the that mechanism MAY be used to select a preferred response. If such a
remainder, the most recent response (as determined by the Date header mechanism is not available, or leads to equally preferred responses,
field) is used, as per Section 4. the most recent response (as determined by the Date header field) is
used, as per Section 4.
Some resources mistakenly omit the Vary header field from their
default response (i.e., the one sent when no more preferable response
is available), with the effect of selecting it for requests to that
resource even when more preferable responses are available. When a
cache has multiple responses for a target URI and one or more omits
the Vary header field, it SHOULD use the most recent (see
Section 4.2.3) valid Vary field value available to select an
appropriate response for the request.
If no selected response is available, the cache cannot satisfy the If no selected response is available, the cache cannot satisfy the
presented request. Typically, it is forwarded to the origin server presented request. Typically, it is forwarded to the origin server
in a (possibly conditional; see Section 4.3) request. in a (possibly conditional; see Section 4.3) request.
4.2. Freshness 4.2. Freshness
A fresh response is one whose age has not yet exceeded its freshness A _fresh_ response is one whose age has not yet exceeded its
lifetime. Conversely, a stale response is one where it has. freshness lifetime. Conversely, a _stale_ response is one where it
has.
A response's freshness lifetime is the length of time between its A response's _freshness lifetime_ is the length of time between its
generation by the origin server and its expiration time. An explicit generation by the origin server and its expiration time. An
expiration time is the time at which the origin server intends that a _explicit expiration time_ is the time at which the origin server
stored response can no longer be used by a cache without further intends that a stored response can no longer be used by a cache
validation, whereas a heuristic expiration time is assigned by a without further validation, whereas a _heuristic expiration time_ is
cache when no explicit expiration time is available. 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 A response's _age_ is the time that has passed since it was generated
by, or successfully validated with, the origin server. by, or successfully validated with, the origin server.
When a response is "fresh" in the cache, it can be used to satisfy When a response is fresh, it can be used to satisfy subsequent
subsequent requests without contacting the origin server, thereby requests without contacting the origin server, thereby improving
improving efficiency. efficiency.
The primary mechanism for determining freshness is for an origin The primary mechanism for determining freshness is for an origin
server to provide an explicit expiration time in the future, using server to provide an explicit expiration time in the future, using
either the Expires header field (Section 5.3) or the max-age response either the Expires header field (Section 5.3) or the max-age response
directive (Section 5.2.2.8). Generally, origin servers will assign directive (Section 5.2.2.1). Generally, origin servers will assign
future explicit expiration times to responses in the belief that the future explicit expiration times to responses in the belief that the
representation is not likely to change in a semantically significant representation is not likely to change in a semantically significant
way before the expiration time is reached. way before the expiration time is reached.
If an origin server wishes to force a cache to validate every If an origin server wishes to force a cache to validate every
request, it can assign an explicit expiration time in the past to request, it can assign an explicit expiration time in the past to
indicate that the response is already stale. Compliant caches will indicate that the response is already stale. Compliant caches will
normally validate a stale cached response before reusing it for normally validate a stale cached response before reusing it for
subsequent requests (see Section 4.2.4). subsequent requests (see Section 4.2.4).
skipping to change at line 564 skipping to change at page 15, line 12
caches are also allowed to use a heuristic to determine an expiration caches are also allowed to use a heuristic to determine an expiration
time under certain circumstances (see Section 4.2.2). time under certain circumstances (see Section 4.2.2).
The calculation to determine if a response is fresh is: The calculation to determine if a response is fresh is:
response_is_fresh = (freshness_lifetime > current_age) response_is_fresh = (freshness_lifetime > current_age)
freshness_lifetime is defined in Section 4.2.1; current_age is freshness_lifetime is defined in Section 4.2.1; current_age is
defined in Section 4.2.3. defined in Section 4.2.3.
Clients can send the max-age or min-fresh cache directives in a Clients can send the max-age or min-fresh request directives
request to constrain or relax freshness calculations for the (Section 5.2.1) to suggest limits on the freshness calculations for
corresponding response (Section 5.2.1). the corresponding response. However, caches are not required to
honor them.
When calculating freshness, to avoid common problems in date parsing: When calculating freshness, to avoid common problems in date parsing:
o Although all date formats are specified to be case-sensitive, a * Although all date formats are specified to be case-sensitive, a
cache recipient SHOULD match day, week, and time-zone names cache recipient SHOULD match the field value case-insensitively.
case-insensitively.
o If a cache recipient's internal implementation of time has less * If a cache recipient's internal implementation of time has less
resolution than the value of an HTTP-date, the recipient MUST resolution than the value of an HTTP-date, the recipient MUST
internally represent a parsed Expires date as the nearest time internally represent a parsed Expires date as the nearest time
equal to or earlier than the received value. equal to or earlier than the received value.
o A cache recipient MUST NOT allow local time zones to influence the * A cache recipient MUST NOT allow local time zones to influence the
calculation or comparison of an age or expiration time. calculation or comparison of an age or expiration time.
o A cache recipient SHOULD consider a date with a zone abbreviation * A cache recipient SHOULD consider a date with a zone abbreviation
other than GMT or UTC to be invalid for calculating expiration. other than "GMT" to be invalid for calculating expiration.
Note that freshness applies only to cache operation; it cannot be Note that freshness applies only to cache operation; it cannot be
used to force a user agent to refresh its display or reload a used to force a user agent to refresh its display or reload a
resource. See Section 6 for an explanation of the difference between resource. See Section 6 for an explanation of the difference between
caches and history mechanisms. caches and history mechanisms.
4.2.1. Calculating Freshness Lifetime 4.2.1. Calculating Freshness Lifetime
A cache can calculate the freshness lifetime (denoted as A cache can calculate the freshness lifetime (denoted as
freshness_lifetime) of a response by using the first match of the freshness_lifetime) of a response by using the first match of:
following:
o If the cache is shared and the s-maxage response directive * If the cache is shared and the s-maxage response directive
(Section 5.2.2.9) is present, use its value, or (Section 5.2.2.10) is present, use its value, or
o If the max-age response directive (Section 5.2.2.8) is present, * If the max-age response directive (Section 5.2.2.1) is present,
use its value, or use its value, or
o If the Expires response header field (Section 5.3) is present, use * If the Expires response header field (Section 5.3) is present, use
its value minus the value of the Date response header field, or its value minus the value of the Date response header field (using
the time the message was received if it is not present, as per
Section 10.2.2 of [HTTP]), or
o Otherwise, no explicit expiration time is present in the response. * Otherwise, no explicit expiration time is present in the response.
A heuristic freshness lifetime might be applicable; see A heuristic freshness lifetime might be applicable; see
Section 4.2.2. Section 4.2.2.
Note that this calculation is not vulnerable to clock skew, since all Note that this calculation is intended to reduce clock skew by using
of the information comes from the origin server. the clock information provided by the origin server whenever
possible.
When there is more than one value present for a given directive When there is more than one value present for a given directive
(e.g., two Expires header fields, multiple Cache-Control: max-age (e.g., two Expires header field lines or multiple Cache-Control: max-
directives), the directive's value is considered invalid. Caches are age directives), either the first occurrence should be used, or the
encouraged to consider responses that have invalid freshness response should be considered stale. If directives conflict (e.g.,
information to be stale. both max-age and no-cache are present), the most restrictive
directive should be honored. Caches are encouraged to consider
responses that have invalid freshness information (e.g., a max-age
directive with non-integer content) to be stale.
4.2.2. Calculating Heuristic Freshness 4.2.2. Calculating Heuristic Freshness
Since origin servers do not always provide explicit expiration times, Since origin servers do not always provide explicit expiration times,
a cache MAY assign a heuristic expiration time when an explicit time a cache MAY assign a heuristic expiration time when an explicit time
is not specified, employing algorithms that use other header field is not specified, employing algorithms that use other field values
values (such as the Last-Modified time) to estimate a plausible (such as the Last-Modified time) to estimate a plausible expiration
expiration time. This specification does not provide specific time. This specification does not provide specific algorithms, but
algorithms, but does impose worst-case constraints on their results. does impose worst-case constraints on their results.
A cache MUST NOT use heuristics to determine freshness when an A cache MUST NOT use heuristics to determine freshness when an
explicit expiration time is present in the stored response. Because explicit expiration time is present in the stored response. Because
of the requirements in Section 3, this means that, effectively, of the requirements in Section 3, this means that heuristics can only
heuristics can only be used on responses without explicit freshness be used on responses without explicit freshness whose status codes
whose status codes are defined as cacheable by default (see Section are defined as _heuristically cacheable_ (e.g., see Section 15.1 of
6.1 of [RFC7231]), and those responses without explicit freshness [HTTP]), and those responses without explicit freshness that have
that have been marked as explicitly cacheable (e.g., with a "public" been marked as explicitly cacheable (e.g., with a "public" response
response directive). directive).
If the response has a Last-Modified header field (Section 2.2 of Note that in previous specifications heuristically cacheable response
[RFC7232]), caches are encouraged to use a heuristic expiration value status codes were called "cacheable by default."
If the response has a Last-Modified header field (Section 8.8.2 of
[HTTP]), caches are encouraged to use a heuristic expiration value
that is no more than some fraction of the interval since that time. that is no more than some fraction of the interval since that time.
A typical setting of this fraction might be 10%. A typical setting of this fraction might be 10%.
When a heuristic is used to calculate freshness lifetime, a cache | *Note:* Section 13.9 of [RFC2616] prohibited caches from
SHOULD generate a Warning header field with a 113 warn-code (see | calculating heuristic freshness for URIs with query components
Section 5.5.4) in the response if its current_age is more than 24 | (i.e., those containing '?'). In practice, this has not been
hours and such a warning is not already present. | widely implemented. Therefore, origin servers are encouraged
| to send explicit directives (e.g., Cache-Control: no-cache) if
Note: Section 13.9 of [RFC2616] prohibited caches from calculating | they wish to prevent caching.
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 4.2.3. Calculating Age
The Age header field is used to convey an estimated age of the 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 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 the cache's estimate of the number of seconds since the origin server
generated or validated by the origin server. In essence, the Age generated or validated the response. The Age value is therefore the
value is the sum of the time that the response has been resident in sum of the time that the response has been resident in each of the
each of the caches along the path from the origin server, plus the caches along the path from the origin server, plus the time it has
amount of time it has been in transit along network paths. been in transit along network paths.
The following data is used for the age calculation: Age calculation uses the following data:
_age_value_ The term "age_value" denotes the value of the Age header _age_value_ The term "age_value" denotes the value of the Age header
field (Section 5.1), in a form appropriate for arithmetic field (Section 5.1), in a form appropriate for arithmetic
operation; or 0, if not available. operation; or 0, if not available.
_date_value_ The term "date_value" denotes the value of the Date _date_value_ The term "date_value" denotes the value of the Date
header field, in a form appropriate for arithmetic operations. header field, in a form appropriate for arithmetic operations.
See Section 7.1.1.2 of [RFC7231] for the definition of the Date See Section 10.2.2 of [HTTP] for the definition of the Date header
header field, and for requirements regarding responses without it. field, and for requirements regarding responses without it.
_now_ The term "now" means "the current value of the clock at the _now_ The term "now" means "the current value of the clock at the
host performing the calculation". A host ought to use NTP host performing the calculation". A host ought to use NTP
([RFC5905]) or some similar protocol to synchronize its clocks to ([RFC5905]) or some similar protocol to synchronize its clocks to
Coordinated Universal Time. Coordinated Universal Time.
_request_time_ The current value of the clock at the host at the _request_time_ The current value of the clock at the host at the
time the request resulting in the stored response was made. time the request resulting in the stored response was made.
_response_time_ The current value of the clock at the host at the _response_time_ The current value of the clock at the host at the
time the response was received. time the response was received.
A response's age can be calculated in two entirely independent ways: A response's age can be calculated in two entirely independent ways:
1. the "apparent_age": response_time minus date_value, if the local 1. the "apparent_age": response_time minus date_value, if the local
clock is reasonably well synchronized to the origin server's clock is reasonably well synchronized to the origin server's
clock. If the result is negative, the result is replaced by clock. If the result is negative, the result is replaced by
zero. zero.
2. the "corrected_age_value", if all of the caches along the 2. the "corrected_age_value", if all of the caches along the
response path implement HTTP/1.1. A cache MUST interpret this response path implement HTTP/1.1 or greater. A cache MUST
value relative to the time the request was initiated, not the interpret this value relative to the time the request was
time that the response was received. initiated, not the time that the response was received.
apparent_age = max(0, response_time - date_value); apparent_age = max(0, response_time - date_value);
response_delay = response_time - request_time; response_delay = response_time - request_time;
corrected_age_value = age_value + response_delay; corrected_age_value = age_value + response_delay;
These are combined as The corrected_age_value MAY be used as the corrected_initial_age. In
circumstances where very old cache implementations that might not
correctly insert Age are present, corrected_initial_age can be
calculated more conservatively as
corrected_initial_age = max(apparent_age, corrected_age_value); 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 current_age of a stored response can then be calculated by adding
the amount of time (in seconds) since the stored response was last the time (in seconds) since the stored response was last validated by
validated by the origin server to the corrected_initial_age. the origin server to the corrected_initial_age.
resident_time = now - response_time; resident_time = now - response_time;
current_age = corrected_initial_age + resident_time; current_age = corrected_initial_age + resident_time;
4.2.4. Serving Stale Responses 4.2.4. Serving Stale Responses
A "stale" response is one that either has explicit expiry information A "stale" response is one that either has explicit expiry information
or is allowed to have heuristic expiry calculated, but is not fresh or is allowed to have heuristic expiry calculated, but is not fresh
according to the calculations in Section 4.2. according to the calculations in Section 4.2.
A cache MUST NOT generate a stale response if it is prohibited by an 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" explicit in-protocol directive (e.g., by a "no-cache" cache
cache directive, a "must-revalidate" cache-response-directive, or an directive, a "must-revalidate" cache-response-directive, or an
applicable "s-maxage" or "proxy-revalidate" cache-response-directive; applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
see Section 5.2.2). see Section 5.2.2).
A cache MUST NOT send stale responses unless it is disconnected A cache MUST NOT generate a stale response unless it is disconnected
(i.e., it cannot contact the origin server or otherwise find a or doing so is explicitly permitted by the client or origin server
forward path) or doing so is explicitly allowed (e.g., by the (e.g., by the max-stale request directive in Section 5.2.1, by
max-stale request directive; see Section 5.2.1). extension directives such as those defined in [RFC5861], or by
configuration in accordance with an out-of-band contract).
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.
4.3. Validation 4.3. Validation
When a cache has one or more stored responses for a requested URI, 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 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 one cannot be selected; see Section 4.1), it can use the conditional
request mechanism [RFC7232] in the forwarded request to give the next request mechanism (Section 13.1 of [HTTP]) in the forwarded request
inbound server an opportunity to select a valid stored response to to give the next inbound server an opportunity to select a valid
use, updating the stored metadata in the process, or to replace the stored response to use, updating the stored metadata in the process,
stored response(s) with a new response. This process is known as or to replace the stored response(s) with a new response. This
"validating" or "revalidating" the stored response. process is known as _validating_ or _revalidating_ the stored
response.
4.3.1. Sending a Validation Request 4.3.1. Sending a Validation Request
When sending a conditional request for cache validation, a cache When generating a conditional request for validation, a cache starts
sends one or more precondition header fields containing validator with either a request it is attempting to satisfy, or - if it is
metadata from its stored response(s), which is then compared by initiating the request independently - it synthesises a request using
recipients to determine whether a stored response is equivalent to a a stored response by copying the method, target URI, and request
current representation of the resource. header fields identified by the Vary header field (Section 4.1).
It then updates that request with one or more precondition header
fields. These contain validator metadata sourced from stored
response(s) that have the same URI. Typically, this will include
only those stored responses(s) that have the same cache key, although
a cache is allowed to validate a response that it cannot select with
the request header fields it is sending (see Section 4.1).
The precondition header fields are then compared by recipients to
determine whether any stored response is equivalent to a current
representation of the resource.
One such validator is the timestamp given in a Last-Modified header One such validator is the timestamp given in a Last-Modified header
field (Section 2.2 of [RFC7232]), which can be used in an field (Section 8.8.2 of [HTTP]), which can be used in an If-Modified-
If-Modified-Since header field for response validation, or in an Since header field for response validation, or in an If-Unmodified-
If-Unmodified-Since or If-Range header field for representation Since or If-Range header field for representation selection (i.e.,
selection (i.e., the client is referring specifically to a previously the client is referring specifically to a previously obtained
obtained representation with that timestamp). representation with that timestamp).
Another validator is the entity-tag given in an ETag header field Another validator is the entity-tag given in an ETag field
(Section 2.3 of [RFC7232]). One or more entity-tags, indicating one (Section 8.8.3 of [HTTP]). One or more entity-tags, indicating one
or more stored responses, can be used in an If-None-Match header 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 response validation, or in an If-Match or If-Range header
field for representation selection (i.e., the client is referring field for representation selection (i.e., the client is referring
specifically to one or more previously obtained representations with specifically to one or more previously obtained representations with
the listed entity-tags). the listed entity-tags).
4.3.2. Handling a Received Validation Request 4.3.2. Handling a Received Validation Request
Each client in the request chain may have its own cache, so it is 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 common for a cache at an intermediary to receive conditional requests
from other (outbound) caches. Likewise, some user agents make use of from other (outbound) caches. Likewise, some user agents make use of
conditional requests to limit data transfers to recently modified conditional requests to limit data transfers to recently modified
representations or to complete the transfer of a partially retrieved representations or to complete the transfer of a partially retrieved
representation. representation.
If a cache receives a request that can be satisfied by reusing one of If a cache receives a request that can be satisfied by reusing a
its stored 200 (OK) or 206 (Partial Content) responses, the cache stored 200 (OK) or 206 (Partial Content) response, as per Section 4,
SHOULD evaluate any applicable conditional header field preconditions the cache SHOULD evaluate any applicable conditional header field
received in that request with respect to the corresponding validators preconditions received in that request with respect to the
contained within the selected response. corresponding validators contained within the selected response.
A cache MUST NOT evaluate conditional header fields that are only applicable A cache MUST NOT evaluate conditional header fields that only apply
to an origin server, found in a request with semantics that cannot be to an origin server, occur in a request with semantics that cannot be
satisfied with a cached response, or applied to a target satisfied with a cached response, or occur in a request with a target
resource for which it has no stored responses; such preconditions are resource for which it has no stored responses; such preconditions are
likely intended for some other (inbound) server. likely intended for some other (inbound) server.
The proper evaluation of conditional requests by a cache depends on The proper evaluation of conditional requests by a cache depends on
the received precondition header fields and their precedence, as the received precondition header fields and their precedence. In
defined in Section 6 of [RFC7232]. The If-Match and summary, the If-Match and If-Unmodified-Since conditional header
If-Unmodified-Since conditional header fields are not applicable to a fields are not applicable to a cache, and If-None-Match takes
cache. precedence over If-Modified-Since. See Section 13.2.2 of [HTTP] for
a complete specification of precondition precedence.
A request containing an If-None-Match header field (Section 3.2 of A request containing an If-None-Match header field (Section 13.1.2 of
[RFC7232]) indicates that the client wants to validate one or more of [HTTP]) indicates that the client wants to validate one or more of
its own stored responses in comparison to whichever stored response its own stored responses in comparison to the stored response
is selected by the cache. If the field-value is "*", or if the selected by the cache (as per Section 4).
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.
If an If-None-Match header field is not present, a request containing If an If-None-Match header field is not present, a request containing
an If-Modified-Since header field (Section 3.3 of [RFC7232]) an If-Modified-Since header field (Section 13.1.3 of [HTTP])
indicates that the client wants to validate one or more of its own indicates that the client wants to validate one or more of its own
stored responses by modification date. stored responses by modification date.
A cache recipient SHOULD If a request contains an If-Modified-Since header field and the Last-
generate a 304 (Not Modified) response (using the metadata of the Modified header field is not present in a selected stored response, a
selected stored response) if one of the following cases is true: 1) cache SHOULD use the stored response's Date field value (or, if no
the selected stored response has a Last-Modified field-value that is Date field is present, the time that the stored response was
earlier than or equal to the conditional timestamp; 2) no Last- received) to evaluate the conditional.
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
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 A cache that implements partial responses to range requests, as
defined in [RFC7233], also needs to evaluate a received If-Range defined in Section 14.2 of [HTTP], also needs to evaluate a received
header field (Section 3.2 of [RFC7233]) with respect to its selected If-Range header field (Section 13.1.5 of [HTTP]) regarding its
stored response. selected stored response.
When a cache decides to revalidate its own stored responses for a When a cache decides to forward a request to revalidate its own
request that contains an If-None-Match list of entity-tags, the cache stored responses for a request that contains an If-None-Match list of
MAY combine the received list with a list of entity-tags from its own entity-tags, the cache MAY combine the received list with a list of
stored set of responses (fresh or stale) and send the union of the entity-tags from its own stored set of responses (fresh or stale) and
two lists as a replacement If-None-Match header field value in the send the union of the two lists as a replacement If-None-Match header
forwarded request. If a stored response contains only partial field value in the forwarded request. If a stored response contains
content, the cache MUST NOT include its entity-tag in the union only partial content, the cache MUST NOT include its entity-tag in
unless the request is for a range that would be fully satisfied by the union unless the request is for a range that would be fully
that partial stored response. If the response to the forwarded satisfied by that partial stored response. If the response to the
request is 304 (Not Modified) and has an ETag header field value with forwarded request is 304 (Not Modified) and has an ETag field value
an entity-tag that is not in the client's list, the cache MUST 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 generate a 200 (OK) response for the client by reusing its
corresponding stored response, as updated by the 304 response corresponding stored response, as updated by the 304 response
metadata (Section 4.3.4). metadata (Section 4.3.4).
4.3.3. Handling a Validation Response 4.3.3. Handling a Validation Response
Cache handling of a response to a conditional request is dependent Cache handling of a response to a conditional request depends upon
upon its status code: its status code:
o A 304 (Not Modified) response status code indicates that the * A 304 (Not Modified) response status code indicates that the
stored response can be updated and reused; see Section 4.3.4. 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 * A full response (i.e., one containing content) indicates that none
none of the stored responses nominated in the conditional request of the stored responses nominated in the conditional request is
is suitable. Instead, the cache MUST use the full response to suitable. Instead, the cache MUST use the full response to
satisfy the request and MAY replace the stored response(s). satisfy the request. The cache MAY store such a full response,
subject to its constraints (see Section 3).
o However, if a cache receives a 5xx (Server Error) response while * However, if a cache receives a 5xx (Server Error) response while
attempting to validate a response, it can either forward this attempting to validate a response, it can either forward this
response to the requesting client, or act as if the server failed response to the requesting client, or act as if the server failed
to respond. In the latter case, the cache MAY send a previously to respond. In the latter case, the cache can send a previously
stored response (see Section 4.2.4). stored response, subject to its constraints on doing so (see
Section 4.2.4), or retry the validation request.
4.3.4. Freshening Stored Responses upon Validation 4.3.4. Freshening Stored Responses upon Validation
When a cache receives a 304 (Not Modified) response and already has When a cache receives a 304 (Not Modified) response, it needs to
one or more stored 200 (OK) responses for the same cache key, the identify stored responses that are suitable for updating with the new
cache needs to identify which of the stored responses are updated by information provided, and then do so.
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.
o If the new response contains a weak validator and that validator The initial set of stored responses to update are those that could
corresponds to one of the cache's stored responses, then the most have been selected for that request - i.e., those that meet the
recent of those matching stored responses is selected for update. requirements in Section 4, except the last requirement to be fresh,
able to be served stale or just validated.
o If the new response does not include any form of validator (such Then, that initial set of stored response(s) is further filtered by
as in the case where a client generates an If-Modified-Since the first match of:
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: * If the new response contains one or more _strong validators_ (see
Section 8.8.1 of [HTTP]), then each of those strong validators
identify a selected representation for update. All the stored
responses in the initial set with one of those same strong
validators are identified for update. If none of the initial set
contain at least one of the same strong validators, then the cache
MUST NOT use the new response to update any stored responses.
o delete any Warning header fields in the stored response with * If the new response contains no strong validators but does contain
warn-code 1xx (see Section 5.5); one or more _weak validators_, and those validators correspond to
one of the initial set's stored responses, then the most recent of
those matching stored responses is identified for update.
o retain any Warning header fields in the stored response with * If the new response does not include any form of validator (such
warn-code 2xx; and, as 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 in the initial set, and that
stored response also lacks a validator, then that stored response
is identified for update.
o use other header fields provided in the 304 (Not Modified) For each stored response identified, the cache MUST update its header
response to replace all instances of the corresponding header fields with the header fields provided in the 304 (Not Modified)
fields in the stored response. response, as per Section 3.2.
4.3.5. Freshening Responses via HEAD 4.3.5. Freshening Responses with HEAD
A response to the HEAD method is identical to what an equivalent 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. request made with a GET would have been, without sending the content.
This property of HEAD responses can be used to invalidate or update a This property of HEAD responses can be used to invalidate or update a
cached GET response if the more efficient conditional GET request cached GET response if the more efficient conditional GET request
mechanism is not available (due to no validators being present in the mechanism is not available (due to no validators being present in the
stored response) or if transmission of the representation body is not stored response) or if transmission of the content is not desired
desired even if it has changed. even if it has changed.
When a cache makes an inbound HEAD request for a given request target When a cache makes an inbound HEAD request for a target URI and
and receives a 200 (OK) response, the cache SHOULD update or receives a 200 (OK) response, the cache SHOULD update or invalidate
invalidate each of its stored GET responses that could have been each of its stored GET responses that could have been selected for
selected for that request (see Section 4.1). that request (see Section 4.1).
For each of the stored responses that could have been selected, if For each of the stored responses that could have been selected, if
the stored response and HEAD response have matching values for any the stored response and HEAD response have matching values for any
received validator fields (ETag and Last-Modified) and, if the HEAD received validator fields (ETag and Last-Modified) and, if the HEAD
response has a Content-Length header field, the value of response has a Content-Length header field, the value of Content-
Content-Length matches that of the stored response, the cache SHOULD Length matches that of the stored response, the cache SHOULD update
update the stored response as described below; otherwise, the cache the stored response as described below; otherwise, the cache SHOULD
SHOULD consider the stored response to be stale. consider the stored response to be stale.
If a cache updates a stored response with the metadata provided in a If a cache updates a stored response with the metadata provided in a
HEAD response, the cache MUST: HEAD response, the cache MUST use the header fields provided in the
HEAD response to update the stored response (see Section 3.2).
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 4.4. Invalidating Stored Responses
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 Because unsafe request methods (Section 9.2.1 of [HTTP]) such as PUT,
[RFC7230]) as well as the URI(s) in the Location and Content-Location POST or DELETE have the potential for changing state on the origin
response header fields (if present) when a non-error status code is server, intervening caches are required to invalidate stored
received in response to an unsafe request method. responses to keep their contents up to date.
However, a cache MUST NOT invalidate a URI from a Location or A cache MUST invalidate the target URI (Section 7.1 of [HTTP]) when
Content-Location response header field if the host part of that URI it receives a non-error status code in response to an unsafe request
differs from the host part in the effective request URI (Section 5.5 method (including methods whose safety is unknown).
of [RFC7230]). This helps prevent denial-of-service attacks.
A cache MUST invalidate the effective request URI (Section 5.5 of A cache MAY invalidate other URIs when it receives a non-error status
[RFC7230]) when it receives a non-error response to a request with a code in response to an unsafe request method (including methods whose
method whose safety is unknown. safety is unknown). In particular, the URI(s) in the Location and
Content-Location response header fields (if present) are candidates
for invalidation; other URIs might be discovered through mechanisms
not specified in this document. However, a cache MUST NOT trigger an
invalidation under these conditions if the origin (Section 4.3.1 of
[HTTP]) of the URI to be invalidated differs from that of the target
URI (Section 7.1 of [HTTP]). This helps prevent denial-of-service
attacks.
"Invalidate" means that the cache will either remove all stored _Invalidate_ means that the cache will either remove all stored
responses related to the effective request URI or will mark these responses whose target URI matches the given URI, or will mark them
as "invalid" and in need of a mandatory validation before they can be as "invalid" and in need of a mandatory validation before they can be
sent in response to a subsequent request. sent in response to a subsequent request.
Here, a "non-error response" is one with a 2xx (Successful) or 3xx A "non-error response" is one with a 2xx (Successful) or 3xx
(Redirection) status code. (Redirection) status code.
Note that this does not guarantee that all appropriate responses are Note that this does not guarantee that all appropriate responses are
invalidated. For example, a state-changing request might invalidate invalidated globally; a state-changing request would only invalidate
responses in the caches it travels through, but relevant responses responses in the caches it travels through.
still might be stored in other caches that it has not.
5. Header Field Definitions 5. Field Definitions
This section defines the syntax and semantics of HTTP/1.1 header This section defines the syntax and semantics of HTTP fields related
fields related to caching. to caching.
5.1. Age 5.1. Age
The "Age" header field conveys the sender's estimate of the amount of The "Age" response header field conveys the sender's estimate of the
time since the response was generated or successfully validated at time since the response was generated or successfully validated at
the origin server. Age values are calculated as specified in the origin server. Age values are calculated as specified in
Section 4.2.3. Section 4.2.3.
Age = delta-seconds Age = delta-seconds
The Age field-value is a non-negative integer, representing time in The Age field value is a non-negative integer, representing time in
seconds (see Section 1.2.1). seconds (see Section 1.2.2).
Although it is defined as a singleton header field, a cache
encountering a message with a list-based Age field value SHOULD use
the first member of the field value, discarding subsequent ones.
If the field value (after discarding additional members, as per
above) is invalid (e.g., it contains something other than a non-
negative integer), a cache SHOULD ignore the field.
The presence of an Age header field implies that the response was not The presence of an Age header field implies that the response was not
generated or validated by the origin server for this request. generated or validated by the origin server for this request.
However, lack of an Age header field does not imply the origin was However, lack of an Age header field does not imply the origin was
contacted, since the response might have been received from an contacted.
HTTP/1.0 cache that does not implement Age.
5.2. Cache-Control 5.2. Cache-Control
The "Cache-Control" header field is used to specify directives for The "Cache-Control" header field is used to list directives for
caches along the request/response chain. Such cache directives are caches along the request/response chain. Such cache directives are
unidirectional in that the presence of a directive in a request does 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. not imply that the same directive is present in the response, or to
be repeated in it.
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. See Section 5.2.3 for information about how Cache-Control directives
defined elsewhere are handled.
A proxy, whether or not it implements a cache, MUST pass cache A proxy, whether or not it implements a cache, MUST pass cache
directives through in forwarded messages, regardless of their directives through in forwarded messages, regardless of their
significance to that application, since the directives might be significance to that application, since the directives might apply to
applicable to all recipients along the request/response chain. It is all recipients along the request/response chain. It is not possible
not possible to target a directive to a specific cache. to target a directive to a specific cache.
Cache directives are identified by a token, to be compared Cache directives are identified by a token, to be compared case-
case-insensitively, and have an optional argument, that can use both insensitively, and have an optional argument that can use both token
token and quoted-string syntax. For the directives defined below and quoted-string syntax. For the directives defined below that
that define arguments, recipients ought to accept both forms, even if define arguments, recipients ought to accept both forms, even if a
one is documented to be preferred. For any directive not defined by specific form is required for generation.
this specification, a recipient MUST accept both forms.
Cache-Control = 1#cache-directive Cache-Control = #cache-directive
cache-directive = token [ "=" ( token / quoted-string ) ] cache-directive = token [ "=" ( token / quoted-string ) ]
For the cache directives defined below, no argument is defined (nor For the cache directives defined below, no argument is defined (nor
allowed) unless stated otherwise. allowed) unless stated otherwise.
5.2.1. Request Cache-Control Directives 5.2.1. Request Cache-Control Directives
This section defines cache request directives. They are advisory;
caches MAY implement them, but are not required to.
5.2.1.1. max-age 5.2.1.1. max-age
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.2.2)
The "max-age" request directive indicates that the client is The "max-age" request directive indicates that the client prefers a
unwilling to accept a response whose age is greater than the response whose age is less than or equal to the specified number of
specified number of seconds. Unless the max-stale request directive seconds. Unless the max-stale request directive is also present, the
is also present, the client is not willing to accept a stale client does not wish to receive a stale response.
response.
This directive uses the token form of the argument syntax: e.g., 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 'max-age=5' not 'max-age="5"'. A sender MUST NOT generate the
quoted-string form. quoted-string form.
5.2.1.2. max-stale 5.2.1.2. max-stale
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.2.2)
The "max-stale" request directive indicates that the client is The "max-stale" request directive indicates that the client will
willing to accept a response that has exceeded its freshness accept a response that has exceeded its freshness lifetime. If a
lifetime. If max-stale is assigned a value, then the client is value is present, then the client is willing to accept a response
willing to accept a response that has exceeded its freshness lifetime that has exceeded its freshness lifetime by no more than the
by no more than the specified number of seconds. If no value is specified number of seconds. If no value is assigned to max-stale,
assigned to max-stale, then the client is willing to accept a stale then the client will accept a stale response of any age.
response of any age.
This directive uses the token form of the argument syntax: e.g., This directive uses the token form of the argument syntax: e.g.,
'max-stale=10' not 'max-stale="10"'. A sender SHOULD NOT generate 'max-stale=10' not 'max-stale="10"'. A sender MUST NOT generate the
the quoted-string form. quoted-string form.
5.2.1.3. min-fresh 5.2.1.3. min-fresh
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.2.2)
The "min-fresh" request directive indicates that the client is The "min-fresh" request directive indicates that the client prefers a
willing to accept a response whose freshness lifetime is no less than response whose freshness lifetime is no less than its current age
its current age plus the specified time in seconds. That is, the plus the specified time in seconds. That is, the client wants a
client wants a response that will still be fresh for at least the response that will still be fresh for at least the specified number
specified number of seconds. of seconds.
This directive uses the token form of the argument syntax: e.g., This directive uses the token form of the argument syntax: e.g.,
'min-fresh=20' not 'min-fresh="20"'. A sender SHOULD NOT generate 'min-fresh=20' not 'min-fresh="20"'. A sender MUST NOT generate the
the quoted-string form. quoted-string form.
5.2.1.4. no-cache 5.2.1.4. no-cache
The "no-cache" request directive indicates that a cache MUST NOT use The "no-cache" request directive indicates that the client prefers
a stored response to satisfy the request without successful stored response not be used to satisfy the request without successful
validation on the origin server. validation on the origin server.
5.2.1.5. no-store 5.2.1.5. no-store
The "no-store" request directive indicates that a cache MUST NOT The "no-store" request directive indicates that a cache MUST NOT
store any part of either this request or any response to it. This store any part of either this request or any response to it. This
directive applies to both private and shared caches. "MUST NOT directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a store the information in non-volatile storage, and MUST make a best-
best-effort attempt to remove the information from volatile storage effort attempt to remove the information from volatile storage as
as promptly as possible after forwarding it. promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for ensuring This directive is _not_ a reliable or sufficient mechanism for
privacy. In particular, malicious or compromised caches might not ensuring privacy. In particular, malicious or compromised caches
recognize or obey this directive, and communications networks might might not recognize or obey this directive, and communications
be vulnerable to eavesdropping. networks might be vulnerable to eavesdropping.
Note that if a request containing this directive is satisfied from a Note that if a request containing this directive is satisfied from a
cache, the no-store request directive does not apply to the already cache, the no-store request directive does not apply to the already
stored response. stored response.
5.2.1.6. no-transform 5.2.1.6. no-transform
The "no-transform" request directive indicates that an intermediary The "no-transform" request directive indicates that the client is
(whether or not it implements a cache) MUST NOT transform the asking for intermediaries to avoid transforming the content, as
payload, as defined in Section 5.7.2 of [RFC7230]. defined in Section 7.7 of [HTTP].
5.2.1.7. only-if-cached 5.2.1.7. only-if-cached
The "only-if-cached" request directive indicates that the client only The "only-if-cached" request directive indicates that the client only
wishes to obtain a stored response. If it receives this directive, a wishes to obtain a stored response. Caches that honor this request
cache SHOULD either respond using a stored response that is directive SHOULD, upon receiving it, either respond using a stored
consistent with the other constraints of the request, or respond with response consistent with the other constraints of the request, or
a 504 (Gateway Timeout) status code. If a group of caches is being respond with a 504 (Gateway Timeout) status code.
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. Response Cache-Control Directives
This section defines cache response directives. A cache MUST obey
the Cache-Control directives defined in this section.
5.2.2.1. max-age 5.2.2.1. max-age
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.2.2)
The "max-age" response directive indicates that the response is to be The "max-age" response directive indicates that the response is to be
considered stale after its age is greater than the specified number considered stale after its age is greater than the specified number
of seconds. of seconds.
This directive uses the token form of the argument syntax: e.g., 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 'max-age=5' not 'max-age="5"'. A sender MUST NOT generate the
quoted-string form. quoted-string form.
5.2.2.2. must-revalidate 5.2.2.2. must-revalidate
The "must-revalidate" response directive indicates that once it has The "must-revalidate" response directive indicates that once the
become stale, a cache MUST NOT use the response to satisfy subsequent response has become stale, a cache MUST NOT reuse that response to
requests without successful validation on the origin server. satisfy another request until it has been successfully validated by
the origin, as defined by Section 4.3.
The must-revalidate directive is necessary to support reliable The must-revalidate directive is necessary to support reliable
operation for certain protocol features. In all circumstances a operation for certain protocol features. In all circumstances a
cache MUST obey the must-revalidate directive; in particular, if a cache MUST NOT ignore the must-revalidate directive; in particular,
cache cannot reach the origin server for any reason, it MUST generate if a cache is disconnected, the cache MUST generate an error response
a 504 (Gateway Timeout) response. rather than reuse the stale response. The generated status code
SHOULD be 504 (Gateway Timeout) unless another error status code is
more applicable.
The must-revalidate directive ought to be used by servers if and only The must-revalidate directive ought to be used by servers if and only
if failure to validate a request on the representation could result if failure to validate a request could cause incorrect operation,
in incorrect operation, such as a silently unexecuted financial such as a silently unexecuted financial transaction.
transaction.
[new] The must-revalidate directive also permits a shared cache to reuse a
response to a request containing an Authorization header field
(Section 11.6.2 of [HTTP]), subject to the above requirement on
revalidation (Section 3.5).
5.2.2.3. must-understand 5.2.2.3. must-understand
[new] The "must-understand" response directive limits caching of the
response to a cache that understands and conforms to the requirements
for that response's status code.
[new] Responses containing "must-understand" SHOULD also contain the "no-
store" directive; caches that implement "must-understand" SHOULD
ignore the "no-store" directive in responses that contain both
directives and a status code that the cache understands and conforms
to any related caching requirements.
5.2.2.4. no-cache 5.2.2.4. no-cache
Argument syntax: Argument syntax:
#field-name #field-name
The "no-cache" response directive indicates that the response MUST The "no-cache" response directive, in its unqualified form (without
NOT be used to satisfy a subsequent request without successful an argument), indicates that the response MUST NOT be used to satisfy
validation on the origin server. any other request without forwarding it for validation and receiving
a successful response; see Section 4.3.
This allows an origin server to prevent a cache from using it This allows an origin server to prevent a cache from using the
to satisfy a request without contacting it, even by caches response to satisfy a request without contacting it, even by caches
that have been configured to send stale responses. that have been configured to send stale responses.
If the no-cache response directive specifies one or more field-names, The qualified form of no-cache response directive, with an argument
then a cache MAY use the response to satisfy a subsequent request, that lists one or more field names, indicates that a cache MAY use
subject to any other restrictions on caching. However, any header the response to satisfy a subsequent request, subject to any other
fields in the response that have the field-name(s) listed MUST NOT be restrictions on caching, if the listed header fields are excluded
sent in the response to a subsequent request without successful from the subsequent response or the subsequent response has been
revalidation with the origin server. This allows an origin server to successfully revalidated with the origin server (updating or removing
prevent the re-use of certain header fields in a response, while those fields). This allows an origin server to prevent the re-use of
still allowing caching of the rest of the response. 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 The field names given are not limited to the set of header fields
defined by this specification. Field names are case-insensitive. defined by this specification. Field names are case-insensitive.
This directive uses the quoted-string form of the argument syntax. A This directive uses the quoted-string form of the argument syntax. A
sender SHOULD NOT generate the token form (even if quoting appears sender SHOULD NOT generate the token form (even if quoting appears
not to be needed for single-entry lists). not to be needed for single-entry lists).
Note: Although it has been back-ported to many implementations, some | *Note:* The qualified form of the directive is often handled by
HTTP/1.0 caches will not recognize or obey this directive. Also, | caches as if an unqualified no-cache directive was received;
no-cache response directives with field-names are often handled by | i.e., the special handling for the qualified form is not widely
caches as if an unqualified no-cache directive was received; i.e., | implemented.
the special handling for the qualified form is not widely
implemented.
5.2.2.5. no-store 5.2.2.5. no-store
The "no-store" response directive indicates that a cache MUST NOT The "no-store" response directive indicates that a cache MUST NOT
store any part of either the immediate request or response. This store any part of either the immediate request or response, and MUST
directive applies to both private and shared caches. "MUST NOT NOT use the response to satisfy any other request.
This directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a store the information in non-volatile storage, and MUST make a best-
best-effort attempt to remove the information from volatile storage effort attempt to remove the information from volatile storage as
as promptly as possible after forwarding it. promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for ensuring This directive is _not_ a reliable or sufficient mechanism for
privacy. In particular, malicious or compromised caches might not ensuring privacy. In particular, malicious or compromised caches
recognize or obey this directive, and communications networks might might not recognize or obey this directive, and communications
be vulnerable to eavesdropping. networks might be vulnerable to eavesdropping.
Note that the "must-understand" cache directive overrides "no-store"
in certain circumstances; see Section 5.2.2.3.
5.2.2.6. no-transform 5.2.2.6. no-transform
The "no-transform" response directive indicates that an intermediary The "no-transform" response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT transform the (regardless of whether it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of [RFC7230]. content, as defined in Section 7.7 of [HTTP].
5.2.2.7. private 5.2.2.7. private
Argument syntax: Argument syntax:
#field-name #field-name
The "private" response directive indicates that the response message The unqualified "private" response directive indicates that a shared
is intended for a single user and MUST NOT be stored by a shared cache MUST NOT store the response (i.e., the response is intended for
cache. A private cache MAY store the response and reuse it for later a single user). It also indicates that a private cache MAY store the
requests, even if the response would normally be non-cacheable. response, subject the constraints defined in Section 3, even if the
response would not otherwise be heuristically cacheable by a private
cache.
If the private response directive specifies one or more field-names, If a qualified private response directive is present, with an
this requirement is limited to the field-values associated with the argument that lists one or more field names, then only the listed
listed response header fields. That is, a shared cache MUST NOT header fields are limited to a single user: a shared cache MUST NOT
store the specified field-names(s), whereas it MAY store the store the listed header fields if they are present in the original
remainder of the response message. response, but MAY store the remainder of the response message without
those header fields, subject the constraints defined in Section 3.
The field-names given are not limited to the set of header fields The field names given are not limited to the set of header fields
defined by this specification. Field names are case-insensitive. defined by this specification. Field names are case-insensitive.
This directive uses the quoted-string form of the argument syntax. A This directive uses the quoted-string form of the argument syntax. A
sender SHOULD NOT generate the token form (even if quoting appears sender SHOULD NOT generate the token form (even if quoting appears
not to be needed for single-entry lists). not to be needed for single-entry lists).
Note: This usage of the word "private" only controls where the | *Note:* This usage of the word "private" only controls where
response can be stored; it cannot ensure the privacy of the message | the response can be stored; it cannot ensure the privacy of the
content. Also, private response directives with field-names are | message content. Also, the qualified form of the directive is
often handled by caches as if an unqualified private directive was | often handled by caches as if an unqualified private directive
received; i.e., the special handling for the qualified form is not | was received; i.e., the special handling for the qualified form
widely implemented. | is not widely implemented.
5.2.2.8. proxy-revalidate 5.2.2.8. proxy-revalidate
The "proxy-revalidate" response directive has the same meaning as the The "proxy-revalidate" response directive indicates that once the
must-revalidate response directive, except that it does not apply to response has become stale, a shared cache MUST NOT reuse that
private caches. response to satisfy another request until it has been successfully
validated by the origin, as defined by Section 4.3. This is
analogous to must-revalidate (Section 5.2.2.2), except that proxy-
revalidate does not apply to private caches.
Note that "proxy-revalidate" on its own does not imply that a
response is cacheable. For example, it might be combined with the
public directive (Section 5.2.2.9), allowing the response to be
cached while requiring only a shared cache to revalidate when stale.
5.2.2.9. public 5.2.2.9. public
The "public" response directive indicates that any cache MAY store The "public" response directive indicates that a cache MAY store the
the response, even if the response would normally be non-cacheable or response even if it would otherwise be prohibited, subject to the
cacheable only within a private cache. (See Section 3.2 for constraints defined in Section 3. In other words, public explicitly
additional details related to the use of public in response to a marks the response as cacheable. For example, public permits a
request containing Authorization, and Section 3 for details of how shared cache to reuse a response to a request containing an
public affects responses that would normally not be stored, due to Authorization header field (Section 3.5).
their status codes not being defined as cacheable by default; see
Section 4.2.2.) Note that it is unnecessary to add the public directive to a response
that is already cacheable according to Section 3.
If a response with the public directive has no explicit freshness
information, it is heuristically cacheable (Section 4.2.2).
5.2.2.10. s-maxage 5.2.2.10. s-maxage
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.2.2)
The "s-maxage" response directive indicates that, in shared caches, The "s-maxage" response directive indicates that, for a shared cache,
the maximum age specified by this directive overrides the maximum age the maximum age specified by this directive overrides the maximum age
specified by either the max-age directive or the Expires header specified by either the max-age directive or the Expires header
field. The s-maxage directive also implies the semantics of the field.
proxy-revalidate response directive.
The s-maxage directive incorporates the proxy-revalidate
(Section 5.2.2.8) response directive's semantics for a shared cache.
A shared cache MUST NOT reuse a stale response with s-maxage to
satisfy another request until it has been successfully validated by
the origin, as defined by Section 4.3. This directive also permits a
shared cache to reuse a response to a request containing an
Authorization header field, subject to the above requirements on
maximum age and revalidation (Section 3.5).
This directive uses the token form of the argument syntax: e.g., 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 's-maxage=10' not 's-maxage="10"'. A sender MUST NOT generate the
quoted-string form. quoted-string form.
5.2.3. Cache Control Extensions 5.2.3. Cache Control Extensions
The Cache-Control header field can be extended through the use of one 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 or more extension cache directives. A cache MUST ignore unrecognized
MUST ignore unrecognized cache directives. cache directives.
Informational extensions (those that do not require a change in cache Informational extensions (those that do not require a change in cache
behavior) can be added without changing the semantics of other behavior) can be added without changing the semantics of other
directives. directives.
Behavioral extensions are designed to work by acting as modifiers to Behavioral extensions are designed to work by acting as modifiers to
the existing base of cache directives. Both the new directive and the existing base of cache directives. Both the new directive and
the old directive are supplied, such that applications that do not the old directive are supplied, such that applications that do not
understand the new directive will default to the behavior specified understand the new directive will default to the behavior specified
by the old directive, and those that understand the new directive by the old directive, and those that understand the new directive
skipping to change at line 1323 skipping to change at page 32, line 12
old directive. In this way, extensions to the existing cache-control old directive. In this way, extensions to the existing cache-control
directives can be made without breaking deployed caches. directives can be made without breaking deployed caches.
For example, consider a hypothetical new response directive called For example, consider a hypothetical new response directive called
"community" that acts as a modifier to the private directive: in "community" that acts as a modifier to the private directive: in
addition to private caches, any cache that is shared only by members addition to private caches, any cache that is shared only by members
of the named community is allowed to cache the response. An origin of the named community is allowed to cache the response. An origin
server wishing to allow the UCI community to use an otherwise private server wishing to allow the UCI community to use an otherwise private
response in their shared cache(s) could do so by including response in their shared cache(s) could do so by including
Cache-Control: private, community="UCI" Cache-Control: private, community="UCI"
A cache that recognizes such a community cache-extension could A cache that recognizes such a community cache directive could
broaden its behavior in accordance with that extension. A cache that broaden its behavior in accordance with that extension. A cache that
does not recognize the community cache-extension would ignore it and does not recognize the community cache directive would ignore it and
adhere to the private directive. adhere to the private directive.
New extension directives ought to consider defining: New extension directives ought to consider defining:
o What it means for a directive to be specified multiple times, * What it means for a directive to be specified multiple times,
o When the directive does not take an argument, what it means when * When the directive does not take an argument, what it means when
an argument is present, an argument is present,
o When the directive requires an argument, what it means when it is * When the directive requires an argument, what it means when it is
missing, missing,
o Whether the directive is specific to requests, responses, or able * Whether the directive is specific to requests, responses, or able
to be used in either. to be used in either.
See also Section 5.2.3.
5.2.4. Cache Directive Registry 5.2.4. Cache Directive Registry
The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry" The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry"
defines the namespace for the cache directives. It has been created defines the namespace for the cache directives. It has been created
and is now maintained at and is now maintained at <https://www.iana.org/assignments/http-
<http://www.iana.org/assignments/http-cache-directives>. cache-directives>.
A registration MUST include the following fields: A registration MUST include the following fields:
o Cache Directive Name * Cache Directive Name
o Pointer to specification text * Pointer to specification text
Values to be added to this namespace require IETF Review (see Values to be added to this namespace require IETF Review (see
[RFC5226], Section 4.1). [RFC8126], Section 4.8).
5.3. Expires 5.3. Expires
The "Expires" header field gives the date/time after which the The "Expires" response header field gives the date/time after which
response is considered stale. See Section 4.2 for further discussion the response is considered stale. See Section 4.2 for further
of the freshness model. discussion of the freshness model.
The presence of an Expires field does not imply that the original The presence of an Expires header field does not imply that the
resource will change or cease to exist at, before, or after that original resource will change or cease to exist at, before, or after
time. that time.
The Expires value is an HTTP-date timestamp, as defined in Section The Expires field value is an HTTP-date timestamp, as defined in
7.1.1.1 of [RFC7231]. Section 5.6.7 of [HTTP]. See also Section 4.2 for parsing
requirements specific to caches.
Expires = HTTP-date Expires = HTTP-date
For example For example
Expires: Thu, 01 Dec 1994 16:00:00 GMT Expires: Thu, 01 Dec 1994 16:00:00 GMT
A cache recipient MUST interpret invalid date formats, especially the A cache recipient MUST interpret invalid date formats, especially the
value "0", as representing a time in the past (i.e., "already value "0", as representing a time in the past (i.e., "already
expired"). expired").
If a response includes a Cache-Control field with the max-age If a response includes a Cache-Control header field with the max-age
directive (Section 5.2.2.8), a recipient MUST ignore the Expires directive (Section 5.2.2.1), a recipient MUST ignore the Expires
field. Likewise, if a response includes the s-maxage directive header field. Likewise, if a response includes the s-maxage
(Section 5.2.2.9), a shared cache recipient MUST ignore the Expires directive (Section 5.2.2.10), a shared cache recipient MUST ignore
field. In both these cases, the value in Expires is only intended the Expires header field. In both these cases, the value in Expires
for recipients that have not yet implemented the Cache-Control field. is only intended for recipients that have not yet implemented the
Cache-Control header field.
An origin server without a clock MUST NOT generate an Expires field An origin server without a clock MUST NOT generate an Expires header
unless its value represents a fixed time in the past (always expired) field unless its value represents a fixed time in the past (always
or its value has been associated with the resource by a system or expired) or its value has been associated with the resource by a
user with a reliable clock. system or user with a reliable clock.
Historically, HTTP required the Expires field-value to be no more Historically, HTTP required the Expires field value to be no more
than a year in the future. While longer freshness lifetimes are no than a year in the future. While longer freshness lifetimes are no
longer prohibited, extremely large values have been demonstrated to longer prohibited, extremely large values have been demonstrated to
cause problems (e.g., clock overflows due to use of 32-bit integers 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 for time values), and many caches will evict a response far sooner
than that. than that.
5.4. Pragma 5.4. Pragma
The "Pragma" header field allows backwards compatibility with The "Pragma" request header field was defined for HTTP/1.0 caches, so
HTTP/1.0 caches, so that clients can specify a "no-cache" request that clients could specify a "no-cache" request (as Cache-Control was
that they will understand (as Cache-Control was not defined until not defined until HTTP/1.1).
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 However, support for Cache-Control is now widespread. As a result,
seconds, while precluding implementations that do not understand this specification deprecates Pragma.
Cache-Control from serving a cached response.
Note: Because the meaning of "Pragma: no-cache" in responses is | *Note:* Because the meaning of "Pragma: no-cache" in responses
not specified, it does not provide a reliable replacement for | was never specified, it does not provide a reliable replacement
"Cache-Control: no-cache" in them. | for "Cache-Control: no-cache" in them.
5.5. Warning 5.5. Warning
The "Warning" header field is used to carry additional information The "Warning" header field was used to carry additional information
about the status or transformation of a message that might not be about the status or transformation of a message that might not be
reflected in the status code. This information is typically used to reflected in the status code. This specification obsoletes it, as it
warn about possible incorrectness introduced by caching operations or is not widely generated or surfaced to users. The information it
transformations applied to the payload of the message. carried can be gleaned from examining other header fields, such as
Age.
[... deleted sections moved to bottom ...]
6. History Lists 6. Relationship to Applications and Other Caches
User agents often have history mechanisms, such as "Back" buttons and Applications using HTTP often specify additional forms of caching.
history lists, that can be used to redisplay a representation For example, Web browsers often have history mechanisms such as
"Back" buttons that can be used to redisplay a representation
retrieved earlier in a session. retrieved earlier in a session.
[new] Likewise, some Web browsers implement caching of images and other
assets within a page view; they may or may not honor HTTP caching
semantics.
The freshness model (Section 4.2) does not necessarily apply to The requirements in this specification do not necessarily apply to
history mechanisms. That is, a history mechanism can display a how applications use data after it is retrieved from an HTTP cache.
previous representation even if it has expired. For example, a history mechanism can display a previous
representation even if it has expired, and an application can use
cached data in other ways beyond its freshness lifetime.
This does not prohibit the history mechanism from telling the user This specification does not prohibit the application from taking HTTP
that a view might be stale or from honoring cache directives (e.g., caching into account; for example, a history mechanism might tell the
user that a view is stale, or it might honor cache directives (e.g.,
Cache-Control: no-store). Cache-Control: no-store).
However, when an application caches data and does not make this
apparent to or easily controllable by the user, it is strongly
encouraged to define its operation with respect to HTTP cache
directives, so as not to surprise authors who expect caching
semantics to be honoured. For example, while it might be reasonable
to define an application cache "above" HTTP that allows a response
containing Cache-Control: no-store to be reused for requests that are
directly related to the request that fetched it (such as those
created during the same page load), it would likely be surprising and
confusing to users and authors if it were allowed to be reused for
requests unrelated in any way to the one from which it was obtained.
7. Security Considerations 7. Security Considerations
This section is meant to inform developers, information providers, This section is meant to inform developers, information providers,
and users of known security concerns specific to HTTP caching. More and users of known security concerns specific to HTTP caching. More
general security considerations are addressed in HTTP messaging general security considerations are addressed in "HTTP/1.1"
[RFC7230] and semantics [RFC7231]. (Section 11 of [HTTP/1.1]) and "HTTP Semantics" (Section 17 of
[HTTP]).
Caches expose additional potential vulnerabilities, since the Caches expose an additional attack surface, since the contents of the
contents of the cache represent an attractive target for malicious cache represent an attractive target for malicious exploitation.
exploitation. Because cache contents persist after an HTTP request Because cache contents persist after an HTTP request is complete, an
is complete, an attack on the cache can reveal information long after attack on the cache can reveal information long after a user believes
a user believes that the information has been removed from the that the information has been removed from the network. Therefore,
network. Therefore, cache contents need to be protected as sensitive cache contents need to be protected as sensitive information.
information.
In particular, because private caches are restricted to a single
user, they can be used to reconstruct a user's activity. As a
result, is important for user agents to allow end users to control
them; for example, allowing stored responses to be removed for some
or all origin servers.
7.1. Cache Poisoning 7.1. Cache Poisoning
In particular, various attacks might be amplified by being stored in Storing a malicious payload in a cache can extend the reach of an
a shared cache; such "cache poisoning" attacks use the cache to attacker to affect multiple users. Such "cache poisoning" attacks
distribute a malicious payload to many clients, and are especially happen when an attacker uses implementation flaws, elevated
effective when an attacker can use implementation flaws, elevated privileges, or other techniques to insert a response into a cache.
privileges, or other techniques to insert such a response into a This is especially effective when shared caches are used to
cache. distribute malicious content to many clients.
One common attack vector for cache poisoning is to exploit One common attack vector for cache poisoning is to exploit
differences in message parsing on proxies and in user agents; see differences in message parsing on proxies and in user agents; see
Section 3.3.3 of [RFC7230] for the relevant requirements. Section 6.3 of [HTTP/1.1] for the relevant requirements regarding
HTTP/1.1.
7.2. Timing Attacks 7.2. Timing Attacks
Furthermore, the very use of a cache can bring about privacy Because one of the primary uses of a cache is to optimise
concerns. For example, if two users share a cache, and the first one performance, its use can "leak" information about what resources have
browses to a site, the second may be able to detect that the other been previously requested.
has been to that site, because the resources from it load more
quickly, thanks to the cache. For example, if a user visits a site and their browser caches some of
its responses, and then navigates to a second site, that site can
attempt to load responses it knows exists on the first site. If they
load quickly, it can be assumed that the user has visited that site,
or even a specific page on it.
Such "timing attacks" can be mitigated by adding more information to
the cache key, such as the identity of the referring site (to prevent
the attack described above). This is sometimes called "double
keying."
7.3. Caching of Sensitive Information 7.3. Caching of Sensitive Information
Likewise, implementation flaws (as well as misunderstanding of cache Implementation and deployment flaws (as well as misunderstanding of
operation) might lead to caching of sensitive information (e.g., cache operation) might lead to caching of sensitive information
authentication credentials) that is thought to be private, exposing (e.g., authentication credentials) that is thought to be private,
it to unauthorized parties. exposing it to unauthorized parties.
Note that the Set-Cookie response header field [RFC6265] does not Note that the Set-Cookie response header field [COOKIE] does not
inhibit caching; a cacheable response with a Set-Cookie header field inhibit caching; a cacheable response with a Set-Cookie header field
can be (and often is) used to satisfy subsequent requests to caches. can be (and often is) used to satisfy subsequent requests to caches.
Servers who wish to control caching of these responses are encouraged Servers who wish to control caching of these responses are encouraged
to emit appropriate Cache-Control response header fields. to emit appropriate Cache-Control response header fields.
8. IANA Considerations 8. IANA Considerations
The change controller is: "IETF (iesg@ietf.org) - Internet The change controller for the following registrations is: "IETF
Engineering Task Force". (iesg@ietf.org) - Internet Engineering Task Force".
8.1. Field Name Registration 8.1. Field Name Registration
HTTP header fields are registered within the "Message Headers" First, introduce the new "Hypertext Transfer Protocol (HTTP) Field
registry maintained at Name Registry" at <https://www.iana.org/assignments/http-fields> as
<http://www.iana.org/assignments/message-headers/>. described in Section 18.4 of [HTTP].
This document defines the following HTTP header fields, so the Then, please update the registry with the field names listed in the
"Permanent Message Header Field Names" registry has been updated table below:
accordingly (see [BCP90]).
+-------------------+----------+----------+-------------+ +===============+===========+======+==========+
| Header Field Name | Protocol | Status | Reference | | Field Name | Status | Ref. | Comments |
+-------------------+----------+----------+-------------+ +===============+===========+======+==========+
| Age | http | standard | Section 5.1 | | Age | standard | 5.1 | |
| Cache-Control | http | standard | Section 5.2 | +---------------+-----------+------+----------+
| Expires | http | standard | Section 5.3 | | Cache-Control | standard | 5.2 | |
| Pragma | http | standard | Section 5.4 | +---------------+-----------+------+----------+
| Warning | http | standard | Section 5.5 | | Expires | standard | 5.3 | |
+-------------------+----------+----------+-------------+ +---------------+-----------+------+----------+
| Pragma | standard | 5.4 | |
+---------------+-----------+------+----------+
| Warning | obsoleted | 5.5 | |
+---------------+-----------+------+----------+
Table 1
8.2. Cache Directive Registration 8.2. Cache Directive Registration
The registry has been populated with the registrations below: Please update the "Hypertext Transfer Protocol (HTTP) Cache Directive
Registry" at <https://www.iana.org/assignments/http-cache-directives>
with the registration procedure of Section 5.2.4 and the cache
directive names summarized in the table below.
+------------------------+----------------------------------+ +==================+==================================+
| Cache Directive | Reference | | Cache Directive | Reference |
+------------------------+----------------------------------+ +==================+==================================+
| max-age | Section 5.2.1.1, Section 5.2.2.8 | | max-age | Section 5.2.1.1, Section 5.2.2.1 |
| max-stale | Section 5.2.1.2 | +------------------+----------------------------------+
| min-fresh | Section 5.2.1.3 | | max-stale | Section 5.2.1.2 |
| must-revalidate | Section 5.2.2.1 | +------------------+----------------------------------+
| no-cache | Section 5.2.1.4, Section 5.2.2.2 | | min-fresh | Section 5.2.1.3 |
| no-store | Section 5.2.1.5, Section 5.2.2.3 | +------------------+----------------------------------+
| no-transform | Section 5.2.1.6, Section 5.2.2.4 | | must-revalidate | Section 5.2.2.2 |
| only-if-cached | Section 5.2.1.7 | +------------------+----------------------------------+
| private | Section 5.2.2.6 | | must-understand | Section 5.2.2.3 |
| proxy-revalidate | Section 5.2.2.7 | +------------------+----------------------------------+
| public | Section 5.2.2.5 | | no-cache | Section 5.2.1.4, Section 5.2.2.4 |
| s-maxage | Section 5.2.2.9 | +------------------+----------------------------------+
| stale-if-error | [RFC5861], Section 4 | | no-store | Section 5.2.1.5, Section 5.2.2.5 |
| stale-while-revalidate | [RFC5861], Section 3 | +------------------+----------------------------------+
+------------------------+----------------------------------+ | no-transform | Section 5.2.1.6, Section 5.2.2.6 |
+------------------+----------------------------------+
| only-if-cached | Section 5.2.1.7 |
+------------------+----------------------------------+
| private | Section 5.2.2.7 |
+------------------+----------------------------------+
| proxy-revalidate | Section 5.2.2.8 |
+------------------+----------------------------------+
| public | Section 5.2.2.9 |
+------------------+----------------------------------+
| s-maxage | Section 5.2.2.10 |
+------------------+----------------------------------+
8.3. [Warn Code] Registrations Table 2
The registry has been populated with the registrations below: 8.3. Warn Code Registry
Please add a note to the "Hypertext Transfer Protocol (HTTP) Warn
Codes" registry at <https://www.iana.org/assignments/http-warn-codes>
to the effect that Warning is obsoleted.
9. References 9. References
9.1. Normative References 9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Requirement Levels", BCP 14, RFC 2119, March 1997. Ed., "HTTP Semantics", Work in Progress, Internet-Draft,
draft-ietf-httpbis-semantics-17, 26 July 2021,
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
Specifications: ABNF", STD 68, RFC 5234, January 2008. semantics-17>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Protocol (HTTP/1.1): Message Syntax and Routing", Ed., "HTTP/1.1", Work in Progress, Internet-Draft, draft-
RFC 7230, June 2014. ietf-httpbis-messaging-17, 26 July 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
messaging-17>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, Requirement Levels", BCP 14, RFC 2119,
June 2014. DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Protocol (HTTP/1.1): Conditional Requests", RFC 7232, Specifications: ABNF", STD 68, RFC 5234,
June 2014. DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests", RFC 7405, DOI 10.17487/RFC7405, December 2014,
RFC 7233, June 2014. <https://www.rfc-editor.org/info/rfc7405>.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
Protocol (HTTP/1.1): Authentication", RFC 7235, June 2014. 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References 9.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration [COOKIE] Barth, A., "HTTP State Management Mechanism", RFC 6265,
Procedures for Message Header Fields", BCP 90, RFC 3864, DOI 10.17487/RFC6265, April 2011,
September 2004. <https://www.rfc-editor.org/info/rfc6265>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. Transfer Protocol -- HTTP/1.1", RFC 2616,
DOI 10.17487/RFC2616, June 1999,
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an <https://www.rfc-editor.org/info/rfc2616>.
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale [RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale
Content", RFC 5861, April 2010. Content", RFC 5861, DOI 10.17487/RFC5861, April 2010,
<https://www.rfc-editor.org/info/rfc5861>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms "Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010. Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. F. Reschke,
April 2011. Ed., "Hypertext Transfer Protocol (HTTP): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
Appendix A. Collected ABNF Appendix A. Collected ABNF
In the collected ABNF below, list rules are expanded as per Section In the collected ABNF below, list rules are expanded as per
1.2 of [RFC7230]. Section 5.6.1.1 of [HTTP].
Age = delta-seconds Age = delta-seconds
Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS Cache-Control = [ cache-directive *( OWS "," OWS cache-directive ) ]
cache-directive ] )
Expires = HTTP-date Expires = HTTP-date
HTTP-date = <HTTP-date, see [RFC7231], Section 7.1.1.1> HTTP-date = <HTTP-date, see [HTTP], Section 5.6.7>
OWS = <OWS, see [RFC7230], Section 3.2.3>
Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
pragma-directive ] )
Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ] OWS = <OWS, see [HTTP], Section 5.6.3>
)
cache-directive = token [ "=" ( token / quoted-string ) ] cache-directive = token [ "=" ( token / quoted-string ) ]
delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
extension-pragma = token [ "=" ( token / quoted-string ) ] field-name = <field-name, see [HTTP], Section 5.1>
field-name = <field-name, see [RFC7230], Section 3.2> quoted-string = <quoted-string, see [HTTP], Section 5.6.4>
port = <port, see [RFC7230], Section 2.7> token = <token, see [HTTP], Section 5.6.2>
pragma-directive = "no-cache" / extension-pragma
pseudonym = <pseudonym, see [RFC7230], Section 5.7.1>
quoted-string = <quoted-string, see [RFC7230], Section 3.2.6> Appendix B. Changes from RFC 7234
token = <token, see [RFC7230], Section 3.2.6> Handling of duplicate and conflicting cache directives has been
clarified. (Section 4.2.1)
uri-host = <uri-host, see [RFC7230], Section 2.7> Cache invalidation of the URIs in the Location and Content-Location
header fields is no longer required, but still allowed.
(Section 4.4)
Cache invalidation of the URIs in the Location and Content-Location
header fields is disallowed when the origin is different; previously,
it was the host. (Section 4.4)
warn-agent = ( uri-host [ ":" port ] ) / pseudonym Handling invalid and multiple Age header field values has been
warn-code = 3DIGIT clarified. (Section 5.1)
warn-date = DQUOTE HTTP-date DQUOTE
warn-text = quoted-string
warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
]
Appendix B. Changes from RFC 2616 Some cache directives defined by this specification now have stronger
prohibitions against generating the quoted form of their values,
since this has been found to create interoperability problems.
Consumers of extension cache directives are no longer required to
accept both token and quoted-string forms, but they still need to
parse them properly for unknown extensions. (Section 5.2)
[elided] The "public" and "private" cache directives were clarified, so that
they do not make responses reusable under any condition.
(Section 5.2.2)
The "must-understand" cache directive was introduced; caches are no
longer required to understand the semantics of new response status
codes unless it is present. (Section 5.2.2.3)
The Warning response header was obsoleted. Much of the information
supported by Warning could be gleaned by examining the response, and
the remaining warn-codes - although potentially useful - were
entirely advisory. In practice, Warning was not added by caches or
intermediaries. (Section 5.5)
Appendix C. Change Log Appendix C. Change Log
[*** Old deleted Section 5.5 content moved here to preserve the diff ***] This section is to be removed before publishing as an RFC.
5.5. Warning C.1. Between RFC7234 and draft 00
The "Warning" header field is used to carry additional information The changes were purely editorial:
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.
Warnings can be used for other purposes, both cache-related and * Change boilerplate and abstract to indicate the "draft" status,
otherwise. The use of a warning, rather than an error status code, and update references to ancestor specifications.
distinguishes these responses from true failures.
Warning header fields can in general be applied to any message, * Remove version "1.1" from document title, indicating that this
however some warn-codes are specific to caches and can only be specification applies to all HTTP versions.
applied to response messages.
Warning = 1#warning-value * Adjust historical notes.
warning-value = warn-code SP warn-agent SP warn-text * Update links to sibling specifications.
[ SP warn-date ]
warn-code = 3DIGIT * Replace sections listing changes from RFC 2616 by new empty
warn-agent = ( uri-host [ ":" port ] ) / pseudonym sections referring to RFC 723x.
; 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 * Remove acknowledgements specific to RFC 723x.
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 * Move "Acknowledgements" to the very end and make them unnumbered.
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 C.2. Since draft-ietf-httpbis-cache-00
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 The changes are purely editorial:
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 * Moved all extensibility tips, registration procedures, and
not rectified by a validation (for example, a lossy compression of registry tables from the IANA considerations to normative
the representation) and they MUST NOT be deleted by a cache after sections, reducing the IANA considerations to just instructions
validation, unless a full response is sent, in which case they that will be removed prior to publication as an RFC.
MUST be.
If a sender generates one or more 1xx warn-codes in a message to be C.3. Since draft-ietf-httpbis-cache-01
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 * Cite RFC 8126 instead of RFC 5226 (<https://github.com/httpwg/
Date: Sat, 25 Aug 2012 23:34:45 GMT http-core/issues/75>)
Warning: 112 - "network down" "Sat, 25 Aug 2012 23:34:45 GMT"
Warnings have accompanying warn-text that describes the error, e.g., * In Section 5.4, misleading statement about the relation between
for logging. It is advisory only, and its content does not affect Pragma and Cache-Control (<https://github.com/httpwg/http-core/
interpretation of the warn-code. issues/92>, <https://www.rfc-editor.org/errata/eid4674>)
If a recipient that uses, evaluates, or displays Warning header C.4. Since draft-ietf-httpbis-cache-02
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 * In Section 3, explain that only final responses are cacheable
a recommended warn-text in English, and a description of its meaning. (<https://github.com/httpwg/http-core/issues/29>)
The procedure for defining additional warn codes is described in
Section 7.2.1.
+-----------+----------------------------------+---------------+ * In Section 5.2.2, clarify what responses various directives apply
| Warn Code | Short Description | Reference | to (<https://github.com/httpwg/http-core/issues/52>)
+-----------+----------------------------------+---------------+
| 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 |
+-----------+----------------------------------+---------------+
5.5.1. Warning: 110 - "Response is Stale" * In Section 4.3.1, clarify the source of validators in conditional
requests (<https://github.com/httpwg/http-core/issues/110>)
A cache SHOULD generate this whenever the sent response is stale. * Revise Section 6 to apply to more than just History Lists
(<https://github.com/httpwg/http-core/issues/126>)
5.5.2. Warning: 111 - "Revalidation Failed" * In Section 5.5, deprecated "Warning" header field
(<https://github.com/httpwg/http-core/issues/139>)
A cache SHOULD generate this when sending a stale response because an * In Section 3.5, remove a spurious note
attempt to validate the response failed, due to an inability to reach (<https://github.com/httpwg/http-core/issues/141>)
the server.
5.5.3. Warning: 112 - "Disconnected Operation" C.5. Since draft-ietf-httpbis-cache-03
A cache SHOULD generate this if it is intentionally disconnected from * In Section 2, define what a disconnected cache is
the rest of the network for a period of time. (<https://github.com/httpwg/http-core/issues/5>)
5.5.4. Warning: 113 - "Heuristic Expiration" * In Section 4, clarify language around how to select a response
when more than one matches (<https://github.com/httpwg/http-core/
issues/23>)
A cache SHOULD generate this if it heuristically chose a freshness * in Section 4.2.4, mention stale-while-revalidate and stale-if-
lifetime greater than 24 hours and the response's age is greater than error (<https://github.com/httpwg/http-core/issues/122>)
24 hours.
5.5.5. Warning: 199 - "Miscellaneous Warning" * Remove requirements around cache request directives
(<https://github.com/httpwg/http-core/issues/129>)
The warning text can include arbitrary information to be presented to * Deprecate Pragma (<https://github.com/httpwg/http-core/
a human user or logged. A system receiving this warning MUST NOT issues/140>)
take any automated action, besides presenting the warning to the
user.
5.5.6. Warning: 214 - "Transformation Applied" * In Section 3.5 and Section 5.2.2, note effect of some directives
on authenticated requests (<https://github.com/httpwg/http-core/
issues/161>)
This Warning code MUST be added by a proxy if it applies any C.6. Since draft-ietf-httpbis-cache-04
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" * In Section 5.2, remove the registrations for stale-if-error and
stale-while-revalidate which happened in RFC 7234
(<https://github.com/httpwg/http-core/issues/207>)
The warning text can include arbitrary information to be presented to C.7. Since draft-ietf-httpbis-cache-05
a human user or logged. A system receiving this warning MUST NOT
take any automated action.
7.2. Warn Code Registry * In Section 3.3, clarify how weakly framed content is considered
for purposes of completeness (<https://github.com/httpwg/http-
core/issues/25>)
The "Hypertext Transfer Protocol (HTTP) Warn Codes" registry defines * Throughout, describe Vary and cache key operations more clearly
the namespace for warn codes. It has been created and is now (<https://github.com/httpwg/http-core/issues/28>)
maintained at <http://www.iana.org/assignments/http-warn-codes>.
7.2.1. Procedure * In Section 3, remove concept of "cacheable methods" in favor of
prose (<https://github.com/httpwg/http-core/issues/54>,
<https://www.rfc-editor.org/errata/eid5300>)
A registration MUST include the following fields: * Refactored Section 7, and added a section on timing attacks
(<https://github.com/httpwg/http-core/issues/233>)
o Warn Code (3 digits) * Changed "cacheable by default" to "heuristically cacheable"
throughout (<https://github.com/httpwg/http-core/issues/242>)
o Short Description C.8. Since draft-ietf-httpbis-cache-06
o Pointer to specification text * In Section 3 and Section 5.2.2.3, change response cacheability to
only require understanding the response status code if the must-
understand cache directive is present (<https://github.com/httpwg/
http-core/issues/120>)
Values to be added to this namespace require IETF Review (see * Change requirements for handling different forms of cache
[RFC5226], Section 4.1). directives in Section 5.2 (<https://github.com/httpwg/http-core/
issues/128>)
Acknowledgments * Fix typo in Section 5.2.2.10 (<https://github.com/httpwg/http-
core/issues/264>)
See Section 10 of [RFC7230]. * In Section 5.2.2.9 and Section 5.2.2.7, clarify "private" and
"public" so that they do not override all other cache directives
(<https://github.com/httpwg/http-core/issues/268>)
* In Section 3, distinguish between private with and without
qualifying headers (<https://github.com/httpwg/http-core/
issues/270>)
* In Section 4.1, clarify that any "*" as a member of Vary will
disable caching (<https://github.com/httpwg/http-core/issues/286>)
* In Section 1.1, reference RFC 8174 as well
(<https://github.com/httpwg/http-core/issues/303>)
C.9. Since draft-ietf-httpbis-cache-07
* Throughout, replace "effective request URI", "request-target" and
similar with "target URI" (<https://github.com/httpwg/http-core/
issues/259>)
* In Section 5.2.2.9 and Section 5.2.2.7, make it clear that these
directives do not ignore other requirements for caching
(<https://github.com/httpwg/http-core/issues/320>)
* In Section 3.3, move definition of "complete" into semantics
(<https://github.com/httpwg/http-core/issues/334>)
C.10. Since draft-ietf-httpbis-cache-08
* Appendix A now uses the sender variant of the "#" list expansion
(<https://github.com/httpwg/http-core/issues/192>)
C.11. Since draft-ietf-httpbis-cache-09
* In Section 5.1, discuss handling of invalid and multiple Age
header field values (<https://github.com/httpwg/http-core/
issues/193>)
* Switch to xml2rfc v3 mode for draft generation
(<https://github.com/httpwg/http-core/issues/394>)
C.12. Since draft-ietf-httpbis-cache-10
* In Section 5.2 (Cache-Control), adjust ABNF to allow empty lists
(<https://github.com/httpwg/http-core/issues/210>)
C.13. Since draft-ietf-httpbis-cache-11
* None.
C.14. Since draft-ietf-httpbis-cache-12
* In Section 4.2.4, remove 'no-store', as it won't be in cache in
the first place (<https://github.com/httpwg/http-core/issues/447>,
<https://www.rfc-editor.org/errata/eid6279>)
* In Section 3.1, make it clear that only response headers need be
stored (<https://github.com/httpwg/http-core/issues/457>)
* Rewrote "Updating Stored Header Fields" Section 3.2
(<https://github.com/httpwg/http-core/issues/458>)
* In Section 4.2.1 clarify how to handle invalid and conflicting
directives (<https://github.com/httpwg/http-core/issues/460>)
* In Section 4.3.3, mention retry of failed validation requests
(<https://github.com/httpwg/http-core/issues/462>)
* In Section 4.3.3, clarify requirement on storing a full response
to a conditional request (<https://github.com/httpwg/http-core/
issues/463>)
* In Section 5.1, clarify error handling
(<https://github.com/httpwg/http-core/issues/471>)
* In Section 4.2, remove spurious "UTC" (<https://github.com/httpwg/
http-core/issues/472>)
* In Section 4.2, correct the date-related rule names to consider
case-insensitive (<https://github.com/httpwg/http-core/
issues/473>)
* In Section 6, strengthen recommendation for application caches to
pay attention to cache directives (<https://github.com/httpwg/
http-core/issues/474>)
* In Section 4, mention collapsed requests
(<https://github.com/httpwg/http-core/issues/475>)
* In Section 4.4, relax requirements on Content-Location and
Location invalidation (<https://github.com/httpwg/http-core/
issues/478>)
* In Section 4.3.4, refine the exceptions to update on a 304
(<https://github.com/httpwg/http-core/issues/488>)
* Moved table of Cache-Control directives into Section 8.2
(<https://github.com/httpwg/http-core/issues/506>)
* In Section 1.2, remove unused core ABNF rules
(<https://github.com/httpwg/http-core/issues/529>)
* Changed to using "payload data" when defining requirements about
the data being conveyed within a message, instead of the terms
"payload body" or "response body" or "representation body", since
they often get confused with the HTTP/1.1 message body (which
includes transfer coding) (<https://github.com/httpwg/http-core/
issues/553>)
C.15. Since draft-ietf-httpbis-cache-13
* In Section 5.2.2.2, clarify requirements around generating an
error response (<https://github.com/httpwg/http-core/issues/608>)
* Changed to using "content" instead of "payload" or "payload data"
to avoid confusion with the payload of version-specific messaging
frames (<https://github.com/httpwg/http-core/issues/654>)
* In Section 4.3.4, clarify how multiple validators are handled
(<https://github.com/httpwg/http-core/issues/659>)
* In Section 4.2.3, Section 5.2, and Section 5.2.2.4, remove notes
about very old HTTP/1.0 behaviours (<https://github.com/httpwg/
http-core/issues/660>)
* In Section 5.2.2.3, modify operation to be more backwards-
compatible with existing implementations
(<https://github.com/httpwg/http-core/issues/661>)
C.16. Since draft-ietf-httpbis-cache-14
* Fix subsection ordering in Section 5.2.2
(<https://github.com/httpwg/http-core/issues/674>)
* In Section 2, define what a cache key is
(<https://github.com/httpwg/http-core/issues/728>)
* In Section 3.1, clarify what cache proxy headers apply to
(<https://github.com/httpwg/http-core/issues/729>)
* In Section 7.1, cache poisoning can affect private caches too
(<https://github.com/httpwg/http-core/issues/730>)
* In Section 5.1, adjust handling of invalid values to match most
deployed caches (<https://github.com/httpwg/http-core/issues/778>)
* In Section 5.3, mention parsing requirement relaxation
(<https://github.com/httpwg/http-core/issues/779>)
C.17. Since draft-ietf-httpbis-cache-15
* In Section 4.3.1, tune description of relation between cache keys
and validators (<https://github.com/httpwg/http-core/issues/832>)
C.18. Since draft-ietf-httpbis-cache-16
This draft addresses mostly editorial issues raised during or past
IETF Last Call; see <https://github.com/httpwg/http-core/
issues?q=label%3Acaching+created%3A%3E2021-05-26> for a summary.
Furthermore:
* Addressed Genart last call review comments
(<https://github.com/httpwg/http-core/issues/847>)
* In Section 4.3.4, clarify that only selectable responses are
updated (<https://github.com/httpwg/http-core/issues/839>)
Acknowledgements
See Appendix "Acknowledgements" of [HTTP].
Index
A C E F G H M N O P S V W
A
Age header field Section 5.1
age Section 4.2
C
Cache-Control header field Section 5.2
cache Section 1
cache key Section 2; Section 2
collapsed requests Section 4
E
Expires header field Section 5.3
explicit expiration time Section 4.2
F
Fields
Age Section 5.1; Section 5.1
Cache-Control Section 5.2
Expires Section 5.3; Section 5.3
Pragma Section 5.4; Section 5.4
Warning Section 5.5
fresh Section 4.2
freshness lifetime Section 4.2
G
Grammar
Age Section 5.1
Cache-Control Section 5.2
DIGIT Section 1.2
Expires Section 5.3
cache-directive Section 5.2
delta-seconds Section 1.2.2
H
Header Fields
Age Section 5.1; Section 5.1
Cache-Control Section 5.2
Expires Section 5.3; Section 5.3
Pragma Section 5.4; Section 5.4
Warning Section 5.5
heuristic expiration time Section 4.2
heuristically cacheable Section 4.2.2
M
max-age (cache directive) Section 5.2.1.1; Section 5.2.2.1
max-stale (cache directive) Section 5.2.1.2
min-fresh (cache directive) Section 5.2.1.3
must-revalidate (cache directive) Section 5.2.2.2
must-understand (cache directive) Section 5.2.2.3
N
no-cache (cache directive) Section 5.2.1.4; Section 5.2.2.4
no-store (cache directive) Section 5.2.1.5; Section 5.2.2.5
no-transform (cache directive) Section 5.2.1.6;
Section 5.2.2.6
O
only-if-cached (cache directive) Section 5.2.1.7
P
Pragma header field Section 5.4
private (cache directive) Section 5.2.2.7
private cache Section 1
proxy-revalidate (cache directive) Section 5.2.2.8
public (cache directive) Section 5.2.2.9
S
s-maxage (cache directive) Section 5.2.2.10
selected response Section 4.1
shared cache Section 1
stale Section 4.2
V
validator Section 4.3.1
W
Warning header field Section 5.5
Authors' Addresses Authors' Addresses
Roy T. Fielding (editor) Roy T. Fielding (editor)
Adobe Systems Incorporated Adobe
345 Park Ave 345 Park Ave
San Jose, CA 95110 San Jose, CA 95110
USA United States of America
EMail: fielding@gbiv.com
URI: http://roy.gbiv.com/
Email: fielding@gbiv.com
URI: https://roy.gbiv.com/
Mark Nottingham (editor) Mark Nottingham (editor)
Akamai Fastly
Prahran VIC
Australia
EMail: mnot@mnot.net Email: mnot@mnot.net
URI: http://www.mnot.net/ URI: https://www.mnot.net/
Julian F. Reschke (editor) Julian Reschke (editor)
greenbytes GmbH greenbytes GmbH
Hafenweg 16 Hafenweg 16
Muenster, NW 48155 48155 Münster
Germany Germany
EMail: julian.reschke@greenbytes.de Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/ URI: https://greenbytes.de/tech/webdav/
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