frankenRFC723x_cache.txt   draft-ietf-httpbis-cache-12.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: April 5, 2021 J. Reschke, Ed.
greenbytes greenbytes
June 2014 October 2, 2020
Hypertext Transfer Protocol (HTTP/1.1): Caching HTTP Caching
draft-ietf-httpbis-cache-12
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.13.
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 April 5, 2021.
Copyright Notice Copyright Notice
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than English. than English.
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 1.3. Delta Seconds . . . . . . . . . . . . . . . . . . . . . . 6
2. Overview of Cache Operation .....................................5 2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 6
3. Storing Responses in Caches .....................................6 3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 7
3.1. Storing Incomplete Responses ...............................7 3.1. Storing Header and Trailer Fields . . . . . . . . . . . . 8
3.2. Storing Responses to Authenticated Requests ................7 3.2. Storing Incomplete Responses . . . . . . . . . . . . . . 9
3.3. Combining Partial Content ..................................8 3.3. Storing Responses to Authenticated Requests . . . . . . . 9
4. Constructing Responses from Caches ..............................8 3.4. Combining Partial Content . . . . . . . . . . . . . . . . 10
4.1. Calculating Secondary Keys with Vary .......................9 4. Constructing Responses from Caches . . . . . . . . . . . . . 10
4.2. Freshness .................................................11 4.1. Calculating Cache Keys with Vary . . . . . . . . . . . . 11
4.2.1. Calculating Freshness Lifetime .....................12 4.2. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2.2. Calculating Heuristic Freshness ....................13 4.2.1. Calculating Freshness Lifetime . . . . . . . . . . . 14
4.2.3. Calculating Age ....................................13 4.2.2. Calculating Heuristic Freshness . . . . . . . . . . . 14
4.2.4. Serving Stale Responses ............................15 4.2.3. Calculating Age . . . . . . . . . . . . . . . . . . . 15
4.3. Validation ................................................16 4.2.4. Serving Stale Responses . . . . . . . . . . . . . . . 16
4.3.1. Sending a Validation Request .......................16 4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . 17
4.3.2. Handling a Received Validation Request .............16 4.3.1. Sending a Validation Request . . . . . . . . . . . . 17
4.3.3. Handling a Validation Response .....................18 4.3.2. Handling a Received Validation Request . . . . . . . 18
4.3.4. Freshening Stored Responses upon Validation ........18 4.3.3. Handling a Validation Response . . . . . . . . . . . 19
4.3.5. Freshening Responses via HEAD ......................19 4.3.4. Freshening Stored Responses upon Validation . . . . . 20
4.4. Invalidation ..............................................20 4.3.5. Freshening Responses with HEAD . . . . . . . . . . . 21
5. Header Field Definitions .......................................21 4.4. Invalidation . . . . . . . . . . . . . . . . . . . . . . 21
5.1. Age .......................................................21 5. Field Definitions . . . . . . . . . . . . . . . . . . . . . . 22
5.2. Cache-Control .............................................21 5.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.2.1. Request Cache-Control Directives ...................22 5.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 23
5.2.2. Response Cache-Control Directives ..................24 5.2.1. Request Cache-Control Directives . . . . . . . . . . 24
5.2.3. Cache Control Extensions ...........................27 5.2.1.1. max-age . . . . . . . . . . . . . . . . . . . . . 24
5.3. Expires ...................................................28 5.2.1.2. max-stale . . . . . . . . . . . . . . . . . . . . 24
5.4. Pragma ....................................................29 5.2.1.3. min-fresh . . . . . . . . . . . . . . . . . . . . 25
5.5. Warning ...................................................29 5.2.1.4. no-cache . . . . . . . . . . . . . . . . . . . . 25
5.5.1. Warning: 110 - "Response is Stale" .................31 5.2.1.5. no-store . . . . . . . . . . . . . . . . . . . . 25
5.5.2. Warning: 111 - "Revalidation Failed" ...............31 5.2.1.6. no-transform . . . . . . . . . . . . . . . . . . 26
5.5.3. Warning: 112 - "Disconnected Operation" ............31 5.2.1.7. only-if-cached . . . . . . . . . . . . . . . . . 26
5.5.4. Warning: 113 - "Heuristic Expiration" ..............31 5.2.2. Response Cache-Control Directives . . . . . . . . . . 26
5.5.5. Warning: 199 - "Miscellaneous Warning" .............32 5.2.2.1. must-revalidate . . . . . . . . . . . . . . . . . 26
5.5.6. Warning: 214 - "Transformation Applied" ............32 5.2.2.2. must-understand . . . . . . . . . . . . . . . . . 27
5.5.7. Warning: 299 - "Miscellaneous Persistent Warning" ..32 5.2.2.3. no-cache . . . . . . . . . . . . . . . . . . . . 27
6. History Lists ..................................................32 5.2.2.4. no-store . . . . . . . . . . . . . . . . . . . . 28
7. IANA Considerations ............................................32 5.2.2.5. no-transform . . . . . . . . . . . . . . . . . . 28
7.1. Cache Directive Registry ..................................32 5.2.2.6. public . . . . . . . . . . . . . . . . . . . . . 28
7.1.1. Procedure ..........................................32 5.2.2.7. private . . . . . . . . . . . . . . . . . . . . . 28
7.1.2. Considerations for New Cache Control Directives ....33 5.2.2.8. proxy-revalidate . . . . . . . . . . . . . . . . 29
7.1.3. Registrations ......................................33 5.2.2.9. max-age . . . . . . . . . . . . . . . . . . . . . 29
7.2. Warn Code Registry ........................................34 5.2.2.10. s-maxage . . . . . . . . . . . . . . . . . . . . 30
7.2.1. Procedure ..........................................34 5.2.3. Cache Control Extensions . . . . . . . . . . . . . . 30
7.2.2. Registrations ......................................34 5.2.4. Cache Directive Registry . . . . . . . . . . . . . . 31
7.3. Header Field Registration .................................34 5.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 32
8. Security Considerations ........................................35 5.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . 33
9. Acknowledgments ................................................36 5.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 33
10. References ....................................................36 6. Relationship to Applications . . . . . . . . . . . . . . . . 33
10.1. Normative References .....................................36 7. Security Considerations . . . . . . . . . . . . . . . . . . . 34
10.2. Informative References ...................................37 7.1. Cache Poisoning . . . . . . . . . . . . . . . . . . . . . 34
Appendix A. Changes from RFC 2616 .................................38 7.2. Timing Attacks . . . . . . . . . . . . . . . . . . . . . 34
Appendix B. Imported ABNF .........................................39 7.3. Caching of Sensitive Information . . . . . . . . . . . . 35
Appendix C. Collected ABNF ........................................39 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
Index .............................................................41 8.1. Field Registration . . . . . . . . . . . . . . . . . . . 35
8.2. Cache Directive Registration . . . . . . . . . . . . . . 35
8.3. Warn Code Registry . . . . . . . . . . . . . . . . . . . 35
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
9.1. Normative References . . . . . . . . . . . . . . . . . . 35
9.2. Informative References . . . . . . . . . . . . . . . . . 36
Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 37
Appendix B. Changes from RFC 7234 . . . . . . . . . . . . . . . 37
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 38
C.1. Between RFC7234 and draft 00 . . . . . . . . . . . . . . 38
C.2. Since draft-ietf-httpbis-cache-00 . . . . . . . . . . . . 39
C.3. Since draft-ietf-httpbis-cache-01 . . . . . . . . . . . . 39
C.4. Since draft-ietf-httpbis-cache-02 . . . . . . . . . . . . 39
C.5. Since draft-ietf-httpbis-cache-03 . . . . . . . . . . . . 39
C.6. Since draft-ietf-httpbis-cache-04 . . . . . . . . . . . . 40
C.7. Since draft-ietf-httpbis-cache-05 . . . . . . . . . . . . 40
C.8. Since draft-ietf-httpbis-cache-06 . . . . . . . . . . . . 40
C.9. Since draft-ietf-httpbis-cache-07 . . . . . . . . . . . . 41
C.10. Since draft-ietf-httpbis-cache-08 . . . . . . . . . . . . 41
C.11. Since draft-ietf-httpbis-cache-09 . . . . . . . . . . . . 41
C.12. Since draft-ietf-httpbis-cache-10 . . . . . . . . . . . . 41
C.13. Since draft-ietf-httpbis-cache-11 . . . . . . . . . . . . 42
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42
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. HTTP is defined by a series of
documents that collectively form the HTTP/1.1 specification:
o "HTTP Semantics" [Semantics]
o "HTTP Caching" (this document)
o "HTTP/1.1 Messaging" [Messaging]
HTTP 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 subsystem
that controls storage, retrieval, and deletion of messages in it. A that controls storage, retrieval, and deletion of messages in it. A
cache stores cacheable responses in order to reduce the response time cache stores cacheable responses to reduce the response time and
and network bandwidth consumption on future, equivalent requests. network bandwidth consumption on future equivalent requests. Any
Any client or server MAY employ a cache, though a cache cannot be client or server MAY use a cache, though a server that is acting as a
used by a server that is acting as a tunnel. tunnel cannot.
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 HTTP caching's goal is significantly improving performance by reusing
performance by reusing a prior response message to satisfy a current a prior response message to satisfy a current request. A cache
request. A stored response is considered "fresh", as defined in considers a stored response "fresh", as defined in Section 4.2, if it
Section 4.2, if the response can be reused without "validation" can be reused without "validation" (checking with the origin server
(checking with the origin server to see if the cached response to see if the cached response remains valid for this request). A
remains valid for this request). A fresh response can therefore fresh response can therefore reduce both latency and network overhead
reduce both latency and network overhead each time it is reused. each time the cache reuses it. When a cached response is not fresh,
When a cached response is not fresh, it might still be reusable if it it might still be reusable if validation can freshen it (Section 4.3)
can be freshened by validation (Section 4.3) or if the origin is 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 [Semantics] 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].
It also uses a list extension, defined in Section 5.7.1 of
[Semantics], that allows for compact definition of comma-separated
lists using a '#' operator (similar to how the '*' operator indicates lists using a '#' operator (similar to how the '*' operator indicates
repetition). Appendix B describes rules imported from other repetition). Appendix A shows the collected grammar with all list
documents. Appendix C shows the collected grammar with all list
operators expanded to standard ABNF notation. operators expanded to standard ABNF notation.
Appendix B. Imported ABNF
The following core rules are included by reference, as defined in The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return), [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line
8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any
character). visible [USASCII] character).
The rules below are defined in [RFC7230]:
OWS = <OWS, see [RFC7230], Section 3.2.3>
field-name = <field-name, see [RFC7230], Section 3.2>
quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>
token = <token, see [RFC7230], Section 3.2.6>
port = <port, see [RFC7230], Section 2.7>
pseudonym = <pseudonym, see [RFC7230], Section 5.7.1>
uri-host = <uri-host, see [RFC7230], Section 2.7>
The rules below are defined in other parts: [Semantics] defines the following rules:
HTTP-date = <HTTP-date, see [RFC7231], Section 7.1.1.1> HTTP-date = <HTTP-date, see [Semantics], Section 5.7.7>
OWS = <OWS, see [Semantics], Section 5.7.3>
field-name = <field-name, see [Semantics], Section 5.4.3>
quoted-string = <quoted-string, see [Semantics], Section 5.7.4>
token = <token, see [Semantics], Section 5.7.2>
1.2.1. Delta Seconds 1.3. 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 | canned string if any overflow occurs, even if the calculations
are performed with an arithmetic type incapable of directly | are 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 ([Semantics]) while reducing the transfer of information already held
held in the cache. Although caching is an entirely OPTIONAL feature in the cache. Although caching is an entirely OPTIONAL feature of
of HTTP, it can be assumed that reusing a cached response is HTTP, it can be assumed that reusing a cached response is desirable
desirable and that such reuse is the default behavior when no and that such reuse is the default behavior when no requirement or
requirement or local configuration prevents it. Therefore, HTTP local configuration prevents it. Therefore, HTTP cache requirements
cache requirements are focused on preventing a cache from either are focused on preventing a cache from either storing a non-reusable
storing a non-reusable response or reusing a stored response response or reusing a stored response inappropriately, rather than
inappropriately, rather than mandating that caches always store and mandating that caches always store and reuse particular responses.
reuse particular responses.
Each cache entry consists of a cache key and one or more HTTP The base cache key comprises the request method and target URI used
responses corresponding to prior requests that used the same key. to retrieve the stored response; the method determines under which
The most common form of cache entry is a successful result of a circumstances that response can be used to satisfy a request.
retrieval request: i.e., a 200 (OK) response to a GET request, which However, many HTTP caches in common use today only cache GET
contains a representation of the resource identified by the request responses, and therefore only use the URI as the cache key,
target (Section 4.3.1 of [RFC7231]). However, it is also possible to forwarding other methods.
cache permanent 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.
The primary cache key consists of the request method and target URI. If a request target is subject to content negotiation, the cache
However, since HTTP caches in common use today are typically limited might store multiple responses for it. Caches differentiate these
to caching responses to GET, many caches simply decline other methods responses by incorporating values of the original request's selecting
and use only the URI as the primary cache key. header fields into the cache key as well, as per Section 4.1.
If a request target is subject to content negotiation, its cache Caches might incorporate additional material into the cache key. For
entry might consist of multiple stored responses, each differentiated example, user agent caches might include the referring site's
by a secondary key for the values of the original request's selecting identity, thereby "double keying" the cache to avoid some privacy
header fields (Section 4.1). 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 8.3.1 of [Semantics]).
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 o the request method is understood by the cache;
cacheable, and
o the response status code is understood by the cache, and o the response status code is final (see Section 14 of [Semantics]);
o the "no-store" cache directive (see Section 5.2) does not appear o if the response status code is 206 or 304, or the "must-
in request or response header fields, and understand" cache directive (see Section 5.2) is present: the
cache understands the response status code;
o the "private" response directive (see Section 5.2.2.6) does not o the "no-store" cache directive is not present in the response (see
appear in the response, if the cache is shared, and Section 5.2);
o the Authorization header field (see Section 4.2 of [RFC7235]) does o if the cache is shared: the "private" response directive is either
not appear in the request, if the cache is shared, unless the not present or allows a shared cache to store a modified response;
response explicitly allows it (see Section 3.2), and see Section 5.2.2.7);
o the response either: o if the cache is shared: the Authorization header field is not
present in the request (see Section 10.6.2 of [Semantics]) or a
response directive is present that explicitly allows shared
caching (see Section 3.3); and,
* contains an Expires header field (see Section 5.3), or o the response contains at least one of:
* contains a max-age response directive (see Section 5.2.2.8), or * a public response directive (see Section 5.2.2.6);
* contains a s-maxage response directive (see Section 5.2.2.9) * a private response directive, if the cache is not shared (see
and the cache is shared, or Section 5.2.2.7);
* contains a Cache Control Extension (see Section 5.2.3) that * an Expires header field (see Section 5.3);
allows it to be cached, or
* has a status code that is defined as cacheable by default (see * a max-age response directive (see Section 5.2.2.9);
Section 4.2.2), or
* contains a public response directive (see Section 5.2.2.5). * if the cache is shared, an s-maxage response directive (see
Section 5.2.2.10);
Note that any of the requirements listed above can be overridden by a * a Cache Control Extension that allows it to be cached (see
cache-control extension; see Section 5.2.3. Section 5.2.3); or,
* a status code that is defined as heuristically cacheable (see
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 Incomplete Responses 3.1. Storing Header and Trailer Fields
A response message is considered complete when all of the octets Caches MUST include all received header fields - including
indicated by the message framing ([RFC7230]) are received prior to unrecognised ones - when storing a response; this assures that new
the connection being closed. If the request method is GET, the HTTP header fields can be successfully deployed. However, the
response status code is 200 (OK), and the entire response header following exceptions are made:
section has been received, a cache MAY store an incomplete response
message body if the cache entry is recorded as incomplete. Likewise, o The Connection header field and fields whose names are listed in
a 206 (Partial Content) response MAY be stored as if it were an it are required by Section 13.1 of [Messaging] to be removed
incomplete 200 (OK) cache entry. However, a cache MUST NOT store before forwarding the message. This MAY be implemented by doing
incomplete or partial-content responses if it does not support the so before storage.
Range and Content-Range header fields or if it does not understand
the range units used in those fields. o 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 13.1 of [Messaging] for some examples.
o Header fields that are specific to a client's proxy configuration
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 10.7.1 of [Semantics]), Proxy-
Authentication-Info (Section 10.7.3 of [Semantics]), and Proxy-
Authorization (Section 10.7.2 of [Semantics]).
Caches MAY either store trailer fields separate from header fields,
or discard them. Caches MUST NOT combine trailer fields with header
fields.
3.2. Storing Incomplete Responses
If the request method is GET, the response status code is 200 (OK),
and the entire response header section has been received, a cache MAY
store a response body that is not complete (Section 3.3 of
[Semantics]) if the stored response is recorded as being incomplete.
Likewise, a 206 (Partial Content) response MAY be stored as if it
were an incomplete 200 (OK) response. However, a cache MUST NOT
store incomplete or partial-content responses if it does not support
the Range and Content-Range header fields or if it does not
understand the range units used in those fields.
A cache MAY complete a stored incomplete response by making a A cache MAY complete a stored incomplete response by making a
subsequent range request ([RFC7233]) and combining the successful subsequent range request (Section 13.2 of [Semantics]) and combining
response with the stored entry, as defined in Section 3.3. A cache the 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.
3.2. Storing Responses to Authenticated Requests 3.3. 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 10.6.2 of [Semantics]) to satisfy
subsequent request unless a cache directive that allows such any subsequent request unless the response contains a Cache-Control
responses to be stored is present in the response. field 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
In this specification, the following Cache-Control response of that 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 5.2.2.1), public (Section 5.2.2.6),
(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.
3.3. 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 13.2 of [Semantics]). 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 14.3.7.3 of [Semantics].
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 use the header fields provided in the new response, aside
from Content-Range, to replace all instances of the corresponding
o delete any Warning header fields in the stored response with header fields in the stored response.
warn-code 1xx (see Section 5.5);
o retain any Warning header fields in the stored response with
warn-code 2xx; and,
o use other header fields provided in the new response, aside from
Content-Range, to replace all instances of the corresponding
header fields in the stored response.
4. Constructing Responses from Caches 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 o The presented target URI (Section 6.1 of [Semantics]) and that of
that of the stored response match, and the stored response match, and
o the request method associated with the stored response allows it o 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) o 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
(Section 5.4), nor the no-cache cache directive (Section 5.2.1),
unless the stored response is successfully validated
(Section 4.3), and
o the stored response does not contain the no-cache cache directive o the stored response does not contain the no-cache cache directive
(Section 5.2.2.2), unless it is successfully validated (Section 5.2.2.3), unless it is successfully validated
(Section 4.3), and (Section 4.3), and
o the stored response is either: o 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 8.2.1 of [Semantics]) to the origin server; i.e., a cache is
not allowed to generate a reply to such a request before having not allowed to generate a reply to such a request before having
forwarded the request and having received a corresponding response. forwarded 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.
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 Vary
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 that has a Vary header field (Section 11.2.1 of
it MUST NOT use that response unless all of the selecting header [Semantics]), it MUST NOT use that response unless all the selecting
fields nominated by the Vary header field match in both the original header fields nominated by the Vary header field match in both the
request (i.e., that associated with the stored response), and the original request (i.e., that associated with the stored response),
presented request. and the presented request.
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 o adding or removing whitespace, where allowed in the header field's
syntax syntax
o combining multiple header fields with the same field name (see o combining multiple header fields with the same field name (see
Section 3.2 of [RFC7230]) Section 5.4.4 of [Semantics])
o normalizing both header field values in a way that is known to o 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 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 preferred responses; of the
remainder, the most recent response (as determined by the Date header remainder, the most recent response (as determined by the Date header
field) is used, as per Section 4. field) is used, as per Section 4.
Note that in practice, some resources might send the Vary header
field on responses inconsistently. 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 non-empty 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 freshness
lifetime. Conversely, a stale response is one where it has. 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
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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" in the cache, it can be used to satisfy
subsequent requests without contacting the origin server, thereby subsequent requests without contacting the origin server, thereby
improving efficiency. improving 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 5.2.2.9). 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).
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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 constrain or relax freshness calculations for the
corresponding response (Section 5.2.1). 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 o 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 day, week, and time-zone names case-
case-insensitively. insensitively.
o If a cache recipient's internal implementation of time has less o 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 o 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 o 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 or UTC 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 o 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, o If the max-age response directive (Section 5.2.2.9) is present,
use its value, or use its value, or
o If the Expires response header field (Section 5.3) is present, use o If the Expires response header field (Section 5.3) is present, use
its value minus the value of the Date response header field, or its value minus the value of the Date response header field, or
o Otherwise, no explicit expiration time is present in the response. o 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 not vulnerable to clock skew, since all
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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 header field
values (such as the Last-Modified time) to estimate a plausible values (such as the Last-Modified time) to estimate a plausible
expiration time. This specification does not provide specific expiration time. This specification does not provide specific
algorithms, but does impose worst-case constraints on their results. algorithms, but 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 14.1 of
6.1 of [RFC7231]), and those responses without explicit freshness [Semantics]), and those responses without explicit freshness that
that have been marked as explicitly cacheable (e.g., with a "public" have been marked as explicitly cacheable (e.g., with a "public"
response directive). response 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."
that is no more than some fraction of the interval since that time.
A typical setting of this fraction might be 10%.
When a heuristic is used to calculate freshness lifetime, a cache If the response has a Last-Modified header field (Section 7.9.2 of
SHOULD generate a Warning header field with a 113 warn-code (see [Semantics]), caches are encouraged to use a heuristic expiration
Section 5.5.4) in the response if its current_age is more than 24 value that is no more than some fraction of the interval since that
hours and such a warning is not already present. time. A typical setting of this fraction might be 10%.
Note: Section 13.9 of [RFC2616] prohibited caches from calculating | *Note:* Section 13.9 of [RFC2616] prohibited caches from
heuristic freshness for URIs with query components (i.e., those | calculating heuristic freshness for URIs with query components
containing '?'). In practice, this has not been widely | (i.e., those containing '?'). In practice, this has not been
implemented. Therefore, origin servers are encouraged to send | widely implemented. Therefore, origin servers are encouraged
explicit directives (e.g., Cache-Control: no-cache) if they wish | to send explicit directives (e.g., Cache-Control: no-cache) if
to preclude caching. | they wish to prevent 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_value
The term "age_value" denotes the value of the Age header field
(Section 5.1), in a form appropriate for arithmetic operation; or
0, if not available.
date_value Age calculation uses the following data:
The term "date_value" denotes the value of the Date header field, age_value The term "age_value" denotes the value of the Age header
in a form appropriate for arithmetic operations. See Section field (Section 5.1), in a form appropriate for arithmetic
7.1.1.2 of [RFC7231] for the definition of the Date header field, operation; or 0, if not available.
and for requirements regarding responses without it.
now date_value The term "date_value" denotes the value of the Date
header field, in a form appropriate for arithmetic operations.
See Section 9.2.2 of [Semantics] for the definition of the Date
header field, and for requirements regarding responses without it.
The term "now" means "the current value of the clock at the host now The term "now" means "the current value of the clock at the host
performing the calculation". A host ought to use NTP ([RFC5905]) performing the calculation". A host ought to use NTP ([RFC5905])
or some similar protocol to synchronize its clocks to Coordinated or some similar protocol to synchronize its clocks to Coordinated
Universal Time. Universal Time.
request_time request_time The current value of the clock at the host at the time
the request resulting in the stored response was made.
The current value of the clock at the host at the time the request
resulting in the stored response was made.
response_time
The current value of the clock at the host at the time the response_time The current value of the clock at the host at the time
response was received. 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 These are combined 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 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), (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 in which case the corrected_age_value MAY be used as the
corrected_initial_age. 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-store" or "no-cache"
cache directive, a "must-revalidate" cache-response-directive, or an cache 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 12.1 of [Semantics] in the forwarded
inbound server an opportunity to select a valid stored response to request to give the next inbound server an opportunity to select a
use, updating the stored metadata in the process, or to replace the valid stored response to use, updating the stored metadata in the
stored response(s) with a new response. This process is known as process, or to replace the stored response(s) with a new response.
"validating" or "revalidating" the stored response. This 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 cache key.
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 7.9.2 of [Semantics]), which can be used in an If-
If-Modified-Since header field for response validation, or in an Modified-Since header field for response validation, or in an If-
If-Unmodified-Since or If-Range header field for representation Unmodified-Since or If-Range header field for representation
selection (i.e., the client is referring specifically to a previously selection (i.e., the client is referring specifically to a previously
obtained representation with that timestamp). obtained 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 7.9.3 of [Semantics]). One or more entity-tags, indicating
or more stored responses, can be used in an If-None-Match header one or more stored responses, can be used in an If-None-Match header
field for response validation, or in an If-Match or If-Range header field for 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 one of
its stored 200 (OK) or 206 (Partial Content) responses, the cache its stored 200 (OK) or 206 (Partial Content) responses, the cache
SHOULD evaluate any applicable conditional header field preconditions SHOULD evaluate any applicable conditional header field preconditions
received in that request with respect to the corresponding validators received in that request with respect to the corresponding validators
contained within the selected response. A cache MUST NOT evaluate contained within the selected response. A cache MUST NOT evaluate
conditional header fields that are only applicable to an origin conditional header fields that only apply to an origin server, occur
server, found in a request with semantics that cannot be satisfied in a request with semantics that cannot be satisfied with a cached
with a cached response, or applied to a target resource for which it response, or occur in a request with a target resource for which it
has no stored responses; such preconditions are likely intended for has no stored responses; such preconditions are likely intended for
some other (inbound) server. 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, as
defined in Section 6 of [RFC7232]. The If-Match and defined in Section 12.3 of [Semantics]. The If-Match and If-
If-Unmodified-Since conditional header fields are not applicable to a Unmodified-Since conditional header fields are not applicable to a
cache. cache.
A request containing an If-None-Match header field (Section 3.2 of A request containing an If-None-Match header field (Section 12.1.2 of
[RFC7232]) indicates that the client wants to validate one or more of [Semantics]) indicates that the client wants to validate one or more
its own stored responses in comparison to whichever stored response of its own stored responses in comparison to whichever stored
is selected by the cache. If the field-value is "*", or if the response is selected by the cache. If the field value is "*", or if
field-value is a list of entity-tags and at least one of them matches the field value is a list of entity-tags and at least one of them
the entity-tag of the selected stored response, a cache recipient matches the entity-tag of the selected stored response, a cache
SHOULD generate a 304 (Not Modified) response (using the metadata of recipient SHOULD generate a 304 (Not Modified) response (using the
the selected stored response) instead of sending that stored metadata of the selected stored response) instead of sending that
response. stored response.
When a cache decides to revalidate its own stored responses for a When a cache decides to revalidate its own stored responses for a
request that contains an If-None-Match list of entity-tags, the cache request that contains an If-None-Match list of entity-tags, the cache
MAY combine the received list with a list of entity-tags from its own MAY combine the received list with a list of entity-tags from its own
stored set of responses (fresh or stale) and send the union of the stored set of responses (fresh or stale) and send the union of the
two lists as a replacement If-None-Match header field value in the two lists as a replacement If-None-Match header field value in the
forwarded request. If a stored response contains only partial forwarded request. If a stored response contains only partial
content, the cache MUST NOT include its entity-tag in the union content, the cache MUST NOT include its entity-tag in the union
unless the request is for a range that would be fully satisfied by unless the request is for a range that would be fully satisfied by
that partial stored response. If the response to the forwarded that partial stored response. If the response to the forwarded
request is 304 (Not Modified) and has an ETag header field value with request is 304 (Not Modified) and has an ETag field value with an
an entity-tag that is not in the client's list, the cache MUST entity-tag that is not in the client's list, the cache MUST generate
generate a 200 (OK) response for the client by reusing its a 200 (OK) response for the client by reusing its corresponding
corresponding stored response, as updated by the 304 response stored response, as updated by the 304 response metadata
metadata (Section 4.3.4). (Section 4.3.4).
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 12.1.3 of [Semantics])
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. A cache recipient SHOULD stored responses by modification date. A cache recipient SHOULD
generate a 304 (Not Modified) response (using the metadata of the generate a 304 (Not Modified) response (using the metadata of the
selected stored response) if one of the following cases is true: 1) selected stored response) if one of the following cases is true: 1)
the selected stored response has a Last-Modified field-value that is the selected stored response has a Last-Modified field value that is
earlier than or equal to the conditional timestamp; 2) no earlier than or equal to the conditional timestamp; 2) no Last-
Last-Modified field is present in the selected stored response, but Modified field is present in the selected stored response, but it has
it has a Date field-value that is earlier than or equal to the a Date field value that is earlier than or equal to the conditional
conditional timestamp; or, 3) neither Last-Modified nor Date is timestamp; or, 3) neither Last-Modified nor Date is present in the
present in the selected stored response, but the cache recorded it as selected stored response, but the cache recorded it as having been
having been received at a time earlier than or equal to the received at a time earlier than or equal to the conditional
conditional timestamp. 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 13.2 of [Semantics], also needs to evaluate a
header field (Section 3.2 of [RFC7233]) with respect to its selected received If-Range header field (Section 12.1.5 of [Semantics])
stored response. regarding its selected stored response.
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 o 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 o A full response (i.e., one with a payload body) indicates that
none of the stored responses nominated in the conditional request none of the stored responses nominated in the conditional request
is suitable. Instead, the cache MUST use the full response to is suitable. Instead, the cache MUST use the full response to
satisfy the request and MAY replace the stored response(s). satisfy the request and MAY replace the stored response(s).
o However, if a cache receives a 5xx (Server Error) response while o 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 MAY send a previously
stored response (see Section 4.2.4). stored response (see Section 4.2.4).
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 and already has
one or more stored 200 (OK) responses for the same cache key, the one or more stored 200 (OK) responses for the applicable cache key,
cache needs to identify which of the stored responses are updated by the cache needs to identify which (if any) are to be updated by the
this new response and then update the stored response(s) with the new new information provided, and then do so.
information provided in the 304 response.
The stored response to update is identified by using the first match The stored response(s) to update are identified by using the first
(if any) of the following: match (if any) of:
o If the new response contains a strong validator (see Section 2.1 o If the new response contains a strong validator (see Section 7.9.1
of [RFC7232]), then that strong validator identifies the selected of [Semantics]), then that strong validator identifies the
representation for update. All of the stored responses with the selected representation for update. All the stored responses with
same strong validator are selected. If none of the stored the same strong validator are identified for update. If none of
responses contain the same strong validator, then the cache MUST the stored responses contain the same strong validator, then the
NOT use the new response to update any stored responses. cache MUST NOT use the new response to update any stored
responses.
o If the new response contains a weak validator and that validator o If the new response contains a weak validator and that validator
corresponds to one of the cache's stored responses, then the most corresponds to one of the cache's stored responses, then the most
recent of those matching stored responses is selected for update. recent of those matching stored responses is identified for
update.
o If the new response does not include any form of validator (such o If the new response does not include any form of validator (such
as in the case where a client generates an If-Modified-Since as where a client generates an If-Modified-Since request from a
request from a source other than the Last-Modified response header source other than the Last-Modified response header field), and
field), and there is only one stored response, and that stored there is only one stored response, and that stored response also
response also lacks a validator, then that stored response is lacks a validator, then that stored response is identified for
selected for update. update.
If a stored response is selected for update, the cache MUST:
o delete any Warning header fields in the stored response with For each stored response identified for update, the cache MUST use
warn-code 1xx (see Section 5.5); the header fields provided in the 304 (Not Modified) response to
replace all instances of the corresponding header fields in the
stored response, with the following exceptions:
o retain any Warning header fields in the stored response with o The exceptions to header field storage in Section 3.1 also apply
warn-code 2xx; and, to header field updates.
o use other header fields provided in the 304 (Not Modified) o Caches MUST NOT update the following header fields: Content-
response to replace all instances of the corresponding header Encoding, Content-Length, Content-MD5 (Section 14.15 of
fields in the stored response. [RFC2616]), Content-Range, ETag.
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, except it lacks a body.
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 representation body is not
desired even if it has changed. desired 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 replace all instances of the corresponding header
o delete any Warning header fields in the stored response with fields in the stored response (subject to the exceptions in
warn-code 1xx (see Section 5.5); Section 4.3.4) and append new header fields to the stored response's
header section unless otherwise restricted by the Cache-Control
o retain any Warning header fields in the stored response with header field.
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 4.4. Invalidation
Because unsafe request methods (Section 4.2.1 of [RFC7231]) such as Because unsafe request methods (Section 8.2.1 of [Semantics]) such as
PUT, POST or DELETE have the potential for changing state on the PUT, POST or DELETE have the potential for changing state on the
origin server, intervening caches can use them to keep their contents origin server, intervening caches are required to invalidate stored
up to date. responses to keep their contents up to date. Invalidate means that
the cache will either remove all stored 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 sent in response to a
subsequent request.
A cache MUST invalidate the effective Request URI (Section 5.5 of Note that this does not guarantee that all appropriate responses are
[RFC7230]) as well as the URI(s) in the Location and Content-Location invalidated globally; a state-changing request would only invalidate
response header fields (if present) when a non-error status code is responses in the caches it travels through.
received in response to an unsafe request method.
A cache MUST invalidate the target URI (Section 6.1 of [Semantics])
and the URI(s) in the Location and Content-Location response header
fields (if present) when a non-error status code is received in
response to an unsafe request method.
However, a cache MUST NOT invalidate a URI from a Location or However, a cache MUST NOT invalidate a URI from a Location or
Content-Location response header field if the host part of that URI Content-Location response header field if the host part of that URI
differs from the host part in the effective request URI (Section 5.5 differs from the host part in the target URI (Section 6.1 of
of [RFC7230]). This helps prevent denial-of-service attacks. [Semantics]). This helps prevent denial-of-service attacks.
A cache MUST invalidate the effective request URI (Section 5.5 of A cache MUST invalidate the target URI (Section 6.1 of [Semantics])
[RFC7230]) when it receives a non-error response to a request with a when it receives a non-error response to a request with a method
method whose safety is unknown. whose safety is unknown.
Here, a "non-error response" is one with a 2xx (Successful) or 3xx Here, a "non-error response" is one with a 2xx (Successful) or 3xx
(Redirection) status code. "Invalidate" means that the cache will (Redirection) status code.
either remove all stored responses related to the effective request
URI or will mark these as "invalid" and in need of a mandatory
validation before they can be sent in response to a subsequent
request.
Note that this does not guarantee that all appropriate responses are 5. Field Definitions
invalidated. For example, a state-changing request might invalidate
responses in the caches it travels through, but relevant responses
still might be stored in other caches that it has not.
5. Header Field Definitions This section defines the syntax and semantics of HTTP fields related
to caching.
This section defines the syntax and semantics of HTTP/1.1 header --------------- ----------- ------
fields related to caching. Field Name Status Ref.
--------------- ----------- ------
Age standard 5.1
Cache-Control standard 5.2
Expires standard 5.3
Pragma standard 5.4
Warning obsoleted 5.5
--------------- ----------- ------
+-------------------+----------+----------+-------------+ Table 1
| Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+-------------+
| Age | http | standard | Section 5.1 |
| Cache-Control | http | standard | Section 5.2 |
| Expires | http | standard | Section 5.3 |
| Pragma | http | standard | Section 5.4 |
| Warning | http | standard | Section 5.5 |
+-------------------+----------+----------+-------------+
5.1. Age 5.1. Age
The "Age" header field conveys the sender's estimate of the amount of The "Age" header field conveys the sender's estimate of the time
time since the response was generated or successfully validated at since the response was generated or successfully validated at the
the origin server. Age values are calculated as specified in 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 1.3). A cache SHOULD consider a response to be
stale if an Age field is present and its value is invalid (i.e.,
contains a list or something other than a non-negative integer).
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, since the response might have been received from an
HTTP/1.0 cache that does not implement Age. 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 See Section 5.2.3 for information about how Cache-Control directives
defined in this section. See Section 5.2.3 for information about how defined elsewhere are handled.
Cache-Control directives defined elsewhere are handled.
Note: Some HTTP/1.0 caches might not implement Cache-Control. | *Note:* Some HTTP/1.0 caches might not implement Cache-Control.
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.
+------------------------+----------------------------------+ ------------------ ----------------------------------
| 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 5.2.2.9
| max-stale | Section 5.2.1.2 | max-stale Section 5.2.1.2
| min-fresh | Section 5.2.1.3 | min-fresh Section 5.2.1.3
| must-revalidate | Section 5.2.2.1 | must-revalidate Section 5.2.2.1
| no-cache | Section 5.2.1.4, Section 5.2.2.2 | must-understand Section 5.2.2.2
| no-store | Section 5.2.1.5, Section 5.2.2.3 | no-cache Section 5.2.1.4, Section 5.2.2.3
| no-transform | Section 5.2.1.6, Section 5.2.2.4 | no-store Section 5.2.1.5, Section 5.2.2.4
| only-if-cached | Section 5.2.1.7 | no-transform Section 5.2.1.6, Section 5.2.2.5
| private | Section 5.2.2.6 | only-if-cached Section 5.2.1.7
| proxy-revalidate | Section 5.2.2.7 | private Section 5.2.2.7
| public | Section 5.2.2.5 | proxy-revalidate Section 5.2.2.8
| s-maxage | Section 5.2.2.9 | public Section 5.2.2.6
| stale-if-error | [RFC5861], Section 4 | s-maxage Section 5.2.2.10
| stale-while-revalidate | [RFC5861], Section 3 | ------------------ ----------------------------------
+------------------------+----------------------------------+
Table 2
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 1.3)
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 1.3)
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 1.3)
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 ensuring
privacy. In particular, malicious or compromised caches might not privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might recognize or obey this directive, and communications networks might
be vulnerable to eavesdropping. 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 payload, as
payload, as defined in Section 5.7.2 of [RFC7230]. defined in Section 6.5 of [Semantics].
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. must-revalidate 5.2.2.1. 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 obey the must-revalidate directive; in particular, if a
cache cannot reach the origin server for any reason, it MUST generate cache is disconnected, the cache MUST generate a 504 (Gateway
a 504 (Gateway Timeout) response. Timeout) response rather than reuse the stale response.
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 on the representation could cause
in incorrect operation, such as a silently unexecuted financial incorrect operation, such as a silently unexecuted financial
transaction. transaction.
The must-revalidate directive also permits a shared cache to reuse a
response to a request containing an Authorization header field,
subject to the above requirement on revalidation (Section 3.3).
5.2.2.2. must-understand
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. A cache MUST NOT store a response
containing the must-understand directive if the cache does not
understand the response status code.
5.2.2.3. no-cache 5.2.2.3. 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. This allows an origin server to any other request without forwarding it for validation and receiving
prevent a cache from using it to satisfy a request without contacting a successful response; see Section 4.3.
it, even by caches that have been configured to send stale responses.
If the no-cache response directive specifies one or more field-names, This allows an origin server to prevent a cache from using the
then a cache MAY use the response to satisfy a subsequent request, response to satisfy a request without contacting it, even by caches
subject to any other restrictions on caching. However, any header that have been configured to send stale responses.
fields in the response that have the field-name(s) listed MUST NOT be
sent in the response to a subsequent request without successful
revalidation with the origin server. This allows an origin server to
prevent the re-use of certain header fields in a response, while
still allowing caching of the rest of the response.
The field-names given are not limited to the set of header fields The qualified form of no-cache response directive, with an argument
that lists one or more field names, indicates that a cache MAY use
the response to satisfy a subsequent request, subject to any other
restrictions on caching, if the listed header fields are excluded
from the subsequent response or the subsequent response has been
successfully revalidated with the origin server (updating or removing
those fields). This allows an origin server to prevent the re-use of
certain header fields in a response, while still allowing caching of
the rest of the response.
The field names given are not limited to the set of header fields
defined by this specification. Field names are case-insensitive. 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:* Although it has been back-ported to many implementations,
HTTP/1.0 caches will not recognize or obey this directive. Also, some HTTP/1.0 caches will not recognize or obey this directive.
no-cache response directives with field-names are often handled by Also, the qualified form of the directive is often handled by caches
caches as if an unqualified no-cache directive was received; i.e., as if an unqualified no-cache directive was received; i.e., the
the special handling for the qualified form is not widely special handling for the qualified form is not widely implemented.
implemented.
5.2.2.4. no-store 5.2.2.4. 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 ensuring
privacy. In particular, malicious or compromised caches might not privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might recognize or obey this directive, and communications networks might
be vulnerable to eavesdropping. be vulnerable to eavesdropping.
5.2.2.5. no-transform 5.2.2.5. 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]. payload, as defined in Section 6.5 of [Semantics].
5.2.2.6. public 5.2.2.6. 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 3.3).
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).
5.2.2.7. 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 fields are limited to a single user: a shared cache MUST NOT store
store the specified field-names(s), whereas it MAY store the the listed fields if they are present in the original response, but
remainder of the response message. MAY store the remainder of the response message without those 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 the
response can be stored; it cannot ensure the privacy of the message response can be stored; it cannot ensure the privacy of the message
content. Also, private response directives with field-names are content. Also, the qualified form of the directive is often handled
often handled by caches as if an unqualified private directive was by caches as if an unqualified private directive was received; i.e.,
received; i.e., the special handling for the qualified form is not the special handling for the qualified form is not widely
widely implemented. 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 5.2.2.1), 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 5.2.2.6), allowing the response to be
cached while requiring only a shared cache to revalidate when stale.
5.2.2.9. max-age 5.2.2.9. max-age
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.3)
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.
5.2.2.10. s-maxage 5.2.2.10. s-maxage
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.3)
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 3.3).
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 cache-extension tokens, each with an optional value. A cache
MUST ignore unrecognized cache directives. MUST ignore unrecognized 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
skipping to change at line 1318 skipping to change at page 31, line 28
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-extension 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-extension would ignore it and
adhere to the private directive. adhere to the private directive.
7.1.2. Considerations for New Cache Control Directives
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, o What it means for a directive to be specified multiple times,
o When the directive does not take an argument, what it means when o 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 o 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 o 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
7.1. 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>.
7.1.1. Procedure
A registration MUST include the following fields: A registration MUST include the following fields:
o Cache Directive Name o Cache Directive Name
o Pointer to specification text o 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" header field gives the date/time after which the
response is considered stale. See Section 4.2 for further discussion response is considered stale. See Section 4.2 for further discussion
of the freshness model. of the freshness model.
The presence of an Expires field does not imply that the original The presence of an Expires field does not imply that the original
resource will change or cease to exist at, before, or after that resource will change or cease to exist at, before, or after that
time. time.
The Expires value is an HTTP-date timestamp, as defined in Section The Expires value is an HTTP-date timestamp, as defined in
7.1.1.1 of [RFC7231]. Section 5.7.7 of [Semantics].
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 field with the max-age
directive (Section 5.2.2.8), a recipient MUST ignore the Expires directive (Section 5.2.2.9), a recipient MUST ignore the Expires
field. Likewise, if a response includes the s-maxage directive field. Likewise, if a response includes the s-maxage directive
(Section 5.2.2.9), a shared cache recipient MUST ignore the Expires (Section 5.2.2.10), a shared cache recipient MUST ignore the Expires
field. In both these cases, the value in Expires is only intended field. In both these cases, the value in Expires is only intended
for recipients that have not yet implemented the Cache-Control field. for recipients that have not yet implemented the Cache-Control field.
An origin server without a clock MUST NOT generate an Expires field An origin server without a clock MUST NOT generate an Expires field
unless its value represents a fixed time in the past (always expired) unless its value represents a fixed time in the past (always expired)
or its value has been associated with the resource by a system or or its value has been associated with the resource by a system or
user with a reliable clock. 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" header field was defined for HTTP/1.0 caches, so that
HTTP/1.0 caches, so that clients can specify a "no-cache" request clients could specify a "no-cache" request (as Cache-Control was not
that they will understand (as Cache-Control was not defined until 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
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.
The freshness model (Section 4.2) does not necessarily apply to Likewise, some Web browsers implement caching of images and other
history mechanisms. That is, a history mechanism can display a assets within a page view; they may or may not honor HTTP caching
previous representation even if it has expired. semantics.
This does not prohibit the history mechanism from telling the user The requirements in this specification do not necessarily apply to
that a view might be stale or from honoring cache directives (e.g., how applications use data after it is retrieved from a HTTP cache.
Cache-Control: no-store). That is, 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 application from taking HTTP 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).
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 messaging
[RFC7230] and semantics [RFC7231]. [Messaging] and semantics [Semantics].
Caches expose additional potential vulnerabilities, since the Caches expose additional potential vulnerabilities, since the
contents of the cache represent an attractive target for malicious contents of the cache represent an attractive target for malicious
exploitation. Because cache contents persist after an HTTP request exploitation. Because cache contents persist after an HTTP request
is complete, an attack on the cache can reveal information long after is complete, an attack on the cache can reveal information long after
a user believes that the information has been removed from the a user believes that the information has been removed from the
network. Therefore, cache contents need to be protected as sensitive network. Therefore, cache contents need to be protected as sensitive
information. information.
In particular, various attacks might be amplified by being stored in 7.1. Cache Poisoning
a shared cache; such "cache poisoning" attacks use the cache to
distribute a malicious payload to many clients, and are especially Various attacks might be amplified by being stored in a shared cache.
effective when an attacker can use implementation flaws, elevated Such "cache poisoning" attacks use the cache to distribute a
privileges, or other techniques to insert such a response into a malicious payload to many clients, and are especially effective when
cache. One common attack vector for cache poisoning is to exploit an attacker can use implementation flaws, elevated privileges, or
other techniques to insert such a response into a cache.
One common attack vector for cache poisoning is to exploit
differences in message parsing on proxies and in user agents; see 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 [Messaging] for the relevant requirements regarding
HTTP/1.1.
Likewise, implementation flaws (as well as misunderstanding of cache 7.2. Timing Attacks
operation) might lead to caching of sensitive information (e.g.,
authentication credentials) that is thought to be private, exposing
it to unauthorized parties.
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
Implementation and deployment flaws (as well as misunderstanding of
cache operation) might lead to caching of sensitive information
(e.g., authentication credentials) that is thought to be private,
exposing it to unauthorized parties.
Note that the Set-Cookie response header field [RFC6265] does not Note that the Set-Cookie response header field [RFC6265] 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. Header Field Registration
HTTP header fields are registered within the "Message Headers" 8.1. Field Registration
registry maintained at
<http://www.iana.org/assignments/message-headers/>.
This document defines the following HTTP header fields, so the Please update the "Hypertext Transfer Protocol (HTTP) Field Name
"Permanent Message Header Field Names" registry has been updated Registry" at <https://www.iana.org/assignments/http-fields> with the
accordingly (see [BCP90]). field names listed in the two tables of Section 5.
8.2. [Cache Directive] Registrations 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 of Section 5.2.
8.3. [Warn Code] Registrations 8.3. Warn Code Registry
The registry has been populated with the registrations below: 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
[Messaging]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1 Messaging", Work in Progress, Internet-
Draft, draft-ietf-httpbis-messaging-12, October 2, 2020,
<https://tools.ietf.org/html/draft-ietf-httpbis-messaging-
12>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008. Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer <https://www.rfc-editor.org/info/rfc5234>.
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, June 2014.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, RFC 7405, DOI 10.17487/RFC7405, December 2014,
June 2014. <https://www.rfc-editor.org/info/rfc7405>.
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
Protocol (HTTP/1.1): Conditional Requests", RFC 7232, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
June 2014. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., [Semantics]
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests", Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
RFC 7233, June 2014. Ed., "HTTP Semantics", Work in Progress, Internet-Draft,
draft-ietf-httpbis-semantics-12, October 2, 2020,
<https://tools.ietf.org/html/draft-ietf-httpbis-semantics-
12>.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [USASCII] American National Standards Institute, "Coded Character
Protocol (HTTP/1.1): Authentication", RFC 7235, June 2014. Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
9.2. Informative References 9.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., [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, [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011. DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. F. Reschke,
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 5.7.1.1 of [Semantics].
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 [Semantics], Section 5.7.7>
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 [Semantics], Section 5.7.3>
)
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 [Semantics], Section 5.4.3>
field-name = <field-name, see [RFC7230], Section 3.2> quoted-string = <quoted-string, see [Semantics], Section 5.7.4>
port = <port, see [RFC7230], Section 2.7> token = <token, see [Semantics], Section 5.7.2>
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 invalid and multiple Age header field values has been
clarified. (Section 5.1)
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)
uri-host = <uri-host, see [RFC7230], Section 2.7> The "public" and "private" cache directives were clarified, so that
they do not make responses reusable under any condition.
(Section 5.2.2)
warn-agent = ( uri-host [ ":" port ] ) / pseudonym The "must-understand" cache directive was introduced; caches are no
warn-code = 3DIGIT longer required to understand the semantics of new response status
warn-date = DQUOTE HTTP-date DQUOTE codes unless it is present. (Section 5.2.2.2)
warn-text = quoted-string
warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
]
Appendix B. Changes from RFC 2616 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)
[elided] Appendix C. Change Log
[*** Old Section 5.5 content deleted 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 o 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, o 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 o Adjust historical notes.
warning-value = warn-code SP warn-agent SP warn-text o Update links to sibling specifications.
[ SP warn-date ]
warn-code = 3DIGIT o 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 o 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 o 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 o 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 o 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., o 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 o 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.
+-----------+----------------------------------+---------------+ o 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" o 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. o 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" o 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 o In Section 3.3, 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 o 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" o 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 o 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" o 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 o 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" o In Section 3.3 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" o 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 o In Section 3.2, 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 o 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 o 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: o Refactored Section 7, and added a section on timing attacks
(<https://github.com/httpwg/http-core/issues/233>)
o Warn Code (3 digits) o 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 o In Section 3 and Section 5.2.2.2, 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 o 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>)
o Fix typo in Section 5.2.2.10 (<https://github.com/httpwg/http-
core/issues/264>)
o In Section 5.2.2.6 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>)
o In Section 3, distinguish between private with and without
qualifying headers (<https://github.com/httpwg/http-core/
issues/270>)
o In Section 4.1, clarify that any "*" as a member of Vary will
disable caching (<https://github.com/httpwg/http-core/issues/286>)
o 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
o Throughout, replace "effective request URI", "request-target" and
similar with "target URI" (<https://github.com/httpwg/http-core/
issues/259>)
o In Section 5.2.2.6 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>)
o In Section 3.2, move definition of "complete" into semantics
(<https://github.com/httpwg/http-core/issues/334>)
C.10. Since draft-ietf-httpbis-cache-08
o 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
o In Section 5.1, discuss handling of invalid and multiple Age
header field values (<https://github.com/httpwg/http-core/
issues/193>)
o Switch to xml2rfc v3 mode for draft generation
(<https://github.com/httpwg/http-core/issues/394>)
C.12. Since draft-ietf-httpbis-cache-10
o 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
o None.
Acknowledgments Acknowledgments
See Section 10 of [RFC7230]. See Appendix "Acknowledgments" of [Semantics].
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 Email: fielding@gbiv.com
URI: http://roy.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 M√ľnster
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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