draft-ietf-httpbis-cache-19.txt   draft-ietf-httpbis-cache-latest.txt 
HTTP Working Group R. Fielding, Ed. HTTP Working Group R. Fielding, Ed.
Internet-Draft Adobe Internet-Draft Adobe
Obsoletes: 7234 (if approved) M. Nottingham, Ed. Obsoletes: 7234 (if approved) M. Nottingham, Ed.
Intended status: Standards Track Fastly Intended status: Standards Track Fastly
Expires: March 14, 2022 J. Reschke, Ed. Expires: November 22, 2025 J. Reschke, Ed.
greenbytes greenbytes
September 10, 2021 May 21, 2025
HTTP Caching HTTP Caching
draft-ietf-httpbis-cache-19 draft-ietf-httpbis-cache-latest
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. This document obsoletes RFC 7234.
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This note is to be removed before publishing as an RFC. This note is to be removed before publishing as an RFC.
Discussion of this draft takes place on the HTTP working group Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at mailing list (ietf-http-wg@w3.org), which is archived at
<https://lists.w3.org/Archives/Public/ietf-http-wg/>. <https://lists.w3.org/Archives/Public/ietf-http-wg/>.
Working Group information can be found at <https://httpwg.org/>; Working Group information can be found at <https://httpwg.org/>;
source code and issues list for this draft can be found at source code and issues list for this draft can be found at
<https://github.com/httpwg/http-core>. <https://github.com/httpwg/http-core>.
The changes in this draft are summarized in Appendix C.20. The changes in this draft are summarized in Appendix C.1.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 14, 2022. This Internet-Draft will expire on November 22, 2025.
Copyright Notice Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as extracted from this document must include Revised BSD License text as
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provided without warranty as described in the Revised BSD License. provided without warranty as described in the Revised BSD License.
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not be created outside the IETF Standards Process, except to format not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other it for publication as an RFC or to translate it into languages other
than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 5 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 5
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 5 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 5
1.2.1. Imported Rules . . . . . . . . . . . . . . . . . . . 5 1.2.1. Imported Rules . . . . . . . . . . . . . . . . . . . 5
1.2.2. Delta Seconds . . . . . . . . . . . . . . . . . . . . 6 1.2.2. Delta Seconds . . . . . . . . . . . . . . . . . . . . 5
2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 6 2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 6
3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 7 3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 7
3.1. Storing Header and Trailer Fields . . . . . . . . . . . . 8 3.1. Storing Header and Trailer Fields . . . . . . . . . . . . 8
3.2. Updating Stored Header Fields . . . . . . . . . . . . . . 9 3.2. Updating Stored Header Fields . . . . . . . . . . . . . . 9
3.3. Storing Incomplete Responses . . . . . . . . . . . . . . 10 3.3. Storing Incomplete Responses . . . . . . . . . . . . . . 10
3.4. Combining Partial Content . . . . . . . . . . . . . . . . 11 3.4. Combining Partial Content . . . . . . . . . . . . . . . . 10
3.5. Storing Responses to Authenticated Requests . . . . . . . 11 3.5. Storing Responses to Authenticated Requests . . . . . . . 11
4. Constructing Responses from Caches . . . . . . . . . . . . . 11 4. Constructing Responses from Caches . . . . . . . . . . . . . 11
4.1. Calculating Cache Keys with the Vary Header Field . . . . 12 4.1. Calculating Cache Keys with the Vary Header Field . . . . 12
4.2. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 14 4.2. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2.1. Calculating Freshness Lifetime . . . . . . . . . . . 15 4.2.1. Calculating Freshness Lifetime . . . . . . . . . . . 15
4.2.2. Calculating Heuristic Freshness . . . . . . . . . . . 16 4.2.2. Calculating Heuristic Freshness . . . . . . . . . . . 16
4.2.3. Calculating Age . . . . . . . . . . . . . . . . . . . 17 4.2.3. Calculating Age . . . . . . . . . . . . . . . . . . . 16
4.2.4. Serving Stale Responses . . . . . . . . . . . . . . . 18 4.2.4. Serving Stale Responses . . . . . . . . . . . . . . . 18
4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . 18 4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . 18
4.3.1. Sending a Validation Request . . . . . . . . . . . . 19 4.3.1. Sending a Validation Request . . . . . . . . . . . . 18
4.3.2. Handling a Received Validation Request . . . . . . . 20 4.3.2. Handling a Received Validation Request . . . . . . . 20
4.3.3. Handling a Validation Response . . . . . . . . . . . 21 4.3.3. Handling a Validation Response . . . . . . . . . . . 21
4.3.4. Freshening Stored Responses upon Validation . . . . . 22 4.3.4. Freshening Stored Responses upon Validation . . . . . 21
4.3.5. Freshening Responses with HEAD . . . . . . . . . . . 22 4.3.5. Freshening Responses with HEAD . . . . . . . . . . . 22
4.4. Invalidating Stored Responses . . . . . . . . . . . . . . 23 4.4. Invalidating Stored Responses . . . . . . . . . . . . . . 23
5. Field Definitions . . . . . . . . . . . . . . . . . . . . . . 24 5. Field Definitions . . . . . . . . . . . . . . . . . . . . . . 24
5.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 5.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 24 5.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 24
5.2.1. Request Cache-Control Directives . . . . . . . . . . 25 5.2.1. Request Directives . . . . . . . . . . . . . . . . . 25
5.2.1.1. max-age . . . . . . . . . . . . . . . . . . . . . 25 5.2.1.1. max-age . . . . . . . . . . . . . . . . . . . . . 25
5.2.1.2. max-stale . . . . . . . . . . . . . . . . . . . . 25 5.2.1.2. max-stale . . . . . . . . . . . . . . . . . . . . 25
5.2.1.3. min-fresh . . . . . . . . . . . . . . . . . . . . 26 5.2.1.3. min-fresh . . . . . . . . . . . . . . . . . . . . 26
5.2.1.4. no-cache . . . . . . . . . . . . . . . . . . . . 26 5.2.1.4. no-cache . . . . . . . . . . . . . . . . . . . . 26
5.2.1.5. no-store . . . . . . . . . . . . . . . . . . . . 26 5.2.1.5. no-store . . . . . . . . . . . . . . . . . . . . 26
5.2.1.6. no-transform . . . . . . . . . . . . . . . . . . 27 5.2.1.6. no-transform . . . . . . . . . . . . . . . . . . 27
5.2.1.7. only-if-cached . . . . . . . . . . . . . . . . . 27 5.2.1.7. only-if-cached . . . . . . . . . . . . . . . . . 27
5.2.2. Response Cache-Control Directives . . . . . . . . . . 27 5.2.2. Response Directives . . . . . . . . . . . . . . . . . 27
5.2.2.1. max-age . . . . . . . . . . . . . . . . . . . . . 27 5.2.2.1. max-age . . . . . . . . . . . . . . . . . . . . . 27
5.2.2.2. must-revalidate . . . . . . . . . . . . . . . . . 27 5.2.2.2. must-revalidate . . . . . . . . . . . . . . . . . 27
5.2.2.3. must-understand . . . . . . . . . . . . . . . . . 28 5.2.2.3. must-understand . . . . . . . . . . . . . . . . . 28
5.2.2.4. no-cache . . . . . . . . . . . . . . . . . . . . 28 5.2.2.4. no-cache . . . . . . . . . . . . . . . . . . . . 28
5.2.2.5. no-store . . . . . . . . . . . . . . . . . . . . 29 5.2.2.5. no-store . . . . . . . . . . . . . . . . . . . . 29
5.2.2.6. no-transform . . . . . . . . . . . . . . . . . . 29 5.2.2.6. no-transform . . . . . . . . . . . . . . . . . . 29
5.2.2.7. private . . . . . . . . . . . . . . . . . . . . . 29 5.2.2.7. private . . . . . . . . . . . . . . . . . . . . . 29
5.2.2.8. proxy-revalidate . . . . . . . . . . . . . . . . 30 5.2.2.8. proxy-revalidate . . . . . . . . . . . . . . . . 30
5.2.2.9. public . . . . . . . . . . . . . . . . . . . . . 30 5.2.2.9. public . . . . . . . . . . . . . . . . . . . . . 30
5.2.2.10. s-maxage . . . . . . . . . . . . . . . . . . . . 31 5.2.2.10. s-maxage . . . . . . . . . . . . . . . . . . . . 31
5.2.3. Cache Control Extensions . . . . . . . . . . . . . . 31 5.2.3. Extension Directives . . . . . . . . . . . . . . . . 31
5.2.4. Cache Directive Registry . . . . . . . . . . . . . . 32 5.2.4. Cache Directive Registry . . . . . . . . . . . . . . 32
5.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 32 5.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . 33 5.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 34 5.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 34
6. Relationship to Applications and Other Caches . . . . . . . . 34 6. Relationship to Applications and Other Caches . . . . . . . . 34
7. Security Considerations . . . . . . . . . . . . . . . . . . . 35 7. Security Considerations . . . . . . . . . . . . . . . . . . . 35
7.1. Cache Poisoning . . . . . . . . . . . . . . . . . . . . . 35 7.1. Cache Poisoning . . . . . . . . . . . . . . . . . . . . . 35
7.2. Timing Attacks . . . . . . . . . . . . . . . . . . . . . 35 7.2. Timing Attacks . . . . . . . . . . . . . . . . . . . . . 35
7.3. Caching of Sensitive Information . . . . . . . . . . . . 36 7.3. Caching of Sensitive Information . . . . . . . . . . . . 36
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
8.1. Field Name Registration . . . . . . . . . . . . . . . . . 36 8.1. Field Name Registration . . . . . . . . . . . . . . . . . 36
8.2. Cache Directive Registration . . . . . . . . . . . . . . 37 8.2. Cache Directive Registration . . . . . . . . . . . . . . 36
8.3. Warn Code Registry . . . . . . . . . . . . . . . . . . . 37 8.3. Warn Code Registry . . . . . . . . . . . . . . . . . . . 37
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 37 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 37
9.1. Normative References . . . . . . . . . . . . . . . . . . 37 9.1. Normative References . . . . . . . . . . . . . . . . . . 37
9.2. Informative References . . . . . . . . . . . . . . . . . 38 9.2. Informative References . . . . . . . . . . . . . . . . . 38
Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 39 Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 38
Appendix B. Changes from RFC 7234 . . . . . . . . . . . . . . . 39 Appendix B. Changes from RFC 7234 . . . . . . . . . . . . . . . 39
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 40 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 40
C.1. Between RFC7234 and draft 00 . . . . . . . . . . . . . . 40 C.1. Since draft-ietf-httpbis-cache-19 . . . . . . . . . . . . 40
C.2. Since draft-ietf-httpbis-cache-00 . . . . . . . . . . . . 40 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 40
C.3. Since draft-ietf-httpbis-cache-01 . . . . . . . . . . . . 41 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
C.4. Since draft-ietf-httpbis-cache-02 . . . . . . . . . . . . 41 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42
C.5. Since draft-ietf-httpbis-cache-03 . . . . . . . . . . . . 41
C.6. Since draft-ietf-httpbis-cache-04 . . . . . . . . . . . . 42
C.7. Since draft-ietf-httpbis-cache-05 . . . . . . . . . . . . 42
C.8. Since draft-ietf-httpbis-cache-06 . . . . . . . . . . . . 42
C.9. Since draft-ietf-httpbis-cache-07 . . . . . . . . . . . . 43
C.10. Since draft-ietf-httpbis-cache-08 . . . . . . . . . . . . 43
C.11. Since draft-ietf-httpbis-cache-09 . . . . . . . . . . . . 43
C.12. Since draft-ietf-httpbis-cache-10 . . . . . . . . . . . . 43
C.13. Since draft-ietf-httpbis-cache-11 . . . . . . . . . . . . 43
C.14. Since draft-ietf-httpbis-cache-12 . . . . . . . . . . . . 43
C.15. Since draft-ietf-httpbis-cache-13 . . . . . . . . . . . . 45
C.16. Since draft-ietf-httpbis-cache-14 . . . . . . . . . . . . 45
C.17. Since draft-ietf-httpbis-cache-15 . . . . . . . . . . . . 46
C.18. Since draft-ietf-httpbis-cache-16 . . . . . . . . . . . . 46
C.19. Since draft-ietf-httpbis-cache-17 . . . . . . . . . . . . 46
C.20. Since draft-ietf-httpbis-cache-18 . . . . . . . . . . . . 46
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 46
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 49
1. Introduction 1. Introduction
The Hypertext Transfer Protocol (HTTP) is a stateless application- The Hypertext Transfer Protocol (HTTP) is a stateless application-
level request/response protocol that uses extensible semantics and level request/response protocol that uses extensible semantics and
self-descriptive messages for flexible interaction with network-based self-descriptive messages for flexible interaction with network-based
hypertext information systems. It is typically used for distributed hypertext information systems. It is typically used for distributed
information systems, where the use of response caches can improve information systems, where the use of response caches can improve
performance. This document defines aspects of HTTP related to performance. This document defines aspects of HTTP related to
caching and reusing response messages. caching and reusing response messages.
An HTTP _cache_ is a local store of response messages and the An HTTP "cache" is a local store of response messages and the
subsystem that controls storage, retrieval, and deletion of messages subsystem that controls storage, retrieval, and deletion of messages
in it. A cache stores cacheable responses to reduce the response in it. A cache stores cacheable responses to reduce the response
time and network bandwidth consumption on future equivalent requests. time and network bandwidth consumption on future equivalent requests.
Any client or server MAY use a cache, though not when acting as a Any client or server MAY use a cache, though not when acting as a
tunnel (Section 3.7 of [HTTP]). tunnel (Section 3.7 of [HTTP]).
A _shared cache_ is a cache that stores responses for reuse 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 HTTP caching is significantly improving performance by The goal of HTTP caching is significantly improving performance by
reusing a prior response message to satisfy a current request. A reusing a prior response message to satisfy a current request. A
cache considers a stored response "fresh", as defined in Section 4.2, cache considers a stored response "fresh", as defined in Section 4.2,
if it can be reused without "validation" (checking with the origin if it can be reused without "validation" (checking with the origin
server to see if the cached response remains valid for this request). server to see if the cached response remains valid for this request).
A fresh response can therefore reduce both latency and network A fresh response can therefore reduce both latency and network
overhead each time the cache reuses it. When a cached response is overhead each time the cache reuses it. When a cached response is
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considerations regarding error handling. 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], extended with the notation for case- notation of [RFC5234], extended with the notation for case-
sensitivity in strings defined in [RFC7405]. sensitivity in strings defined in [RFC7405].
It also uses a list extension, defined in Section 5.6.1 of [HTTP], It also uses a list extension, defined in Section 5.6.1 of [HTTP],
that allows for compact definition of comma-separated lists using a that allows for compact definition of comma-separated lists using a
'#' operator (similar to how the '*' operator indicates repetition). "#" operator (similar to how the "*" operator indicates repetition).
Appendix A shows the collected grammar with all list operators Appendix A shows the collected grammar with all list operators
expanded to standard ABNF notation. expanded to standard ABNF notation.
1.2.1. Imported Rules 1.2.1. Imported Rules
The following core rule is included by reference, as defined in The following core rule is included by reference, as defined in
[RFC5234], Appendix B.1: DIGIT (decimal 0-9). [RFC5234], Appendix B.1: DIGIT (decimal 0-9).
[HTTP] defines the following rules: [HTTP] defines the following rules:
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concepts of HTTP. concepts of HTTP.
Although caching is an entirely OPTIONAL feature of HTTP, it can be Although caching is an entirely OPTIONAL feature of HTTP, it can be
assumed that reusing a cached response is desirable and that such assumed that reusing a cached response is desirable and that such
reuse is the default behavior when no requirement or local reuse is the default behavior when no requirement or local
configuration prevents it. Therefore, HTTP cache requirements are configuration prevents it. Therefore, HTTP cache requirements are
focused on preventing a cache from either storing a non-reusable focused on preventing a cache from either storing a non-reusable
response or reusing a stored response inappropriately, rather than response or reusing a stored response inappropriately, rather than
mandating that caches always store and reuse particular responses. mandating that caches always store and reuse particular responses.
The _cache key_ is the information a cache uses to choose a response The "cache key" is the information a cache uses to choose a response
and is composed from, at a minimum, the request method and target URI and is composed from, at a minimum, the request method and target URI
used to retrieve the stored response; the method determines under used to retrieve the stored response; the method determines under
which circumstances that response can be used to satisfy a subsequent which circumstances that response can be used to satisfy a subsequent
request. However, many HTTP caches in common use today only cache request. However, many HTTP caches in common use today only cache
GET responses, and therefore only use the URI as the cache key, GET responses and therefore only use the URI as the cache key.
forwarding other methods.
A cache might store multiple responses for a request target that is A cache might store multiple responses for a request target that is
subject to content negotiation. Caches differentiate these responses subject to content negotiation. Caches differentiate these responses
by incorporating some of the original request's header fields into by incorporating some of the original request's header fields into
the cache key as well, using information in the Vary response header the cache key as well, using information in the Vary response header
field, as per Section 4.1. field, as per Section 4.1.
Caches might incorporate additional material into the cache key. For Caches might incorporate additional material into the cache key. For
example, user agent caches might include the referring site's example, user agent caches might include the referring site's
identity, thereby "double keying" the cache to avoid some privacy identity, thereby "double keying" the cache to avoid some privacy
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Most commonly, caches store the successful result of a retrieval Most commonly, caches store the successful result of a retrieval
request: i.e., a 200 (OK) response to a GET request, which contains a request: i.e., a 200 (OK) response to a GET request, which contains a
representation of the target resource (Section 9.3.1 of [HTTP]). representation of the target resource (Section 9.3.1 of [HTTP]).
However, it is also possible to store redirects, negative results However, it is also possible to store redirects, negative results
(e.g., 404 (Not Found)), incomplete results (e.g., 206 (Partial (e.g., 404 (Not Found)), incomplete results (e.g., 206 (Partial
Content)), and responses to methods other than GET if the method's Content)), and responses to methods other than GET if the method's
definition allows such caching and defines something suitable for use definition allows such caching and defines something suitable for use
as a cache key. as a cache key.
A cache is _disconnected_ when it cannot contact the origin server or A cache is "disconnected" when it cannot contact the origin server or
otherwise find a forward path for a request. A disconnected cache otherwise find a forward path for a request. A disconnected cache
can serve stale responses in some circumstances (Section 4.2.4). 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 a request unless: A cache MUST NOT store a response to a request unless:
o the request method is understood by the cache; o the request method is understood by the cache;
o the response status code is final (see Section 15 of [HTTP]); o the response status code is final (see Section 15 of [HTTP]);
o if the response status code is 206 or 304, or the "must- o if the response status code is 206 or 304, or the must-understand
understand" cache directive (see Section 5.2.2.3) is present: the cache directive (see Section 5.2.2.3) is present: the cache
cache understands the response status code; understands the response status code;
o the "no-store" cache directive is not present in the response (see o the no-store cache directive is not present in the response (see
Section 5.2.2.5); Section 5.2.2.5);
o if the cache is shared: the "private" response directive is either o if the cache is shared: the private response directive is either
not present or allows a shared cache to store a modified response; not present or allows a shared cache to store a modified response;
see Section 5.2.2.7); see Section 5.2.2.7);
o if the cache is shared: the Authorization header field is not o if the cache is shared: the Authorization header field is not
present in the request (see Section 11.6.2 of [HTTP]) or a present in the request (see Section 11.6.2 of [HTTP]) or a
response directive is present that explicitly allows shared response directive is present that explicitly allows shared
caching (see Section 3.5); and, caching (see Section 3.5); and
o the response contains at least one of: o the response contains at least one of the following:
* a public response directive (see Section 5.2.2.9); * a public response directive (see Section 5.2.2.9);
* a private response directive, if the cache is not shared (see * a private response directive, if the cache is not shared (see
Section 5.2.2.7); Section 5.2.2.7);
* an Expires header field (see Section 5.3); * an Expires header field (see Section 5.3);
* a max-age response directive (see Section 5.2.2.1); * a max-age response directive (see Section 5.2.2.1);
* if the cache is shared: an s-maxage response directive (see * if the cache is shared: an s-maxage response directive (see
Section 5.2.2.10); Section 5.2.2.10);
* a Cache Control Extension that allows it to be cached (see * a cache extension that allows it to be cached (see
Section 5.2.3); or, Section 5.2.3); or
* a status code that is defined as heuristically cacheable (see * a status code that is defined as heuristically cacheable (see
Section 4.2.2). Section 4.2.2).
Note that a cache-control extension can override any of the Note that a cache extension can override any of the requirements
requirements listed; see Section 5.2.3. listed; see Section 5.2.3.
In this context, a cache has "understood" a request method or a In this context, a cache has "understood" a request method or a
response status code if it recognizes it and implements all specified response status code if it recognizes it and implements all specified
caching-related behavior. caching-related behavior.
Note that, in normal operation, some caches will not store a response Note that, in normal operation, some caches will not store a response
that has neither a cache validator nor an explicit expiration time, that has neither a cache validator nor an explicit expiration time,
as such responses are not usually useful to store. However, caches as such responses are not usually useful to store. However, caches
are not prohibited from storing such responses. are not prohibited from storing such responses.
3.1. Storing Header and Trailer Fields 3.1. Storing Header and Trailer Fields
Caches MUST include all received response header fields -- including Caches MUST include all received response header fields -- including
unrecognised ones -- when storing a response; this assures that new unrecognized ones -- when storing a response; this assures that new
HTTP header fields can be successfully deployed. However, the HTTP header fields can be successfully deployed. However, the
following exceptions are made: following exceptions are made:
o The Connection header field and fields whose names are listed in o The Connection header field and fields whose names are listed in
it are required by Section 7.6.1 of [HTTP] to be removed before it are required by Section 7.6.1 of [HTTP] to be removed before
forwarding the message. This MAY be implemented by doing so forwarding the message. This MAY be implemented by doing so
before storage. before storage.
o Likewise, some fields' semantics require them to be removed before o Likewise, some fields' semantics require them to be removed before
forwarding the message, and this MAY be implemented by doing so forwarding the message, and this MAY be implemented by doing so
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forwarding the message, and this MAY be implemented by doing so forwarding the message, and this MAY be implemented by doing so
before storage; see Section 7.6.1 of [HTTP] for some examples. before storage; see Section 7.6.1 of [HTTP] for some examples.
o The no-cache (Section 5.2.2.4) and private (Section 5.2.2.7) cache o The no-cache (Section 5.2.2.4) and private (Section 5.2.2.7) cache
directives can have arguments that prevent storage of header directives can have arguments that prevent storage of header
fields by all caches and shared caches, respectively. fields by all caches and shared caches, respectively.
o Header fields that are specific to the proxy that a cache uses o Header fields that are specific to the proxy that a cache uses
when forwarding a request MUST NOT be stored, unless the cache when forwarding a request MUST NOT be stored, unless the cache
incorporates the identity of the proxy into the cache key. incorporates the identity of the proxy into the cache key.
Effectively, this is limited to Proxy-Authenticate (Section 11.7.1 Effectively, this is limited to Proxy-Authenticate (Section 11.7.1
of [HTTP]), Proxy-Authentication-Info (Section 11.7.3 of [HTTP]), of [HTTP]), Proxy-Authentication-Info (Section 11.7.3 of [HTTP]),
and Proxy-Authorization (Section 11.7.2 of [HTTP]). and Proxy-Authorization (Section 11.7.2 of [HTTP]).
Caches MAY either store trailer fields separate from header fields, Caches MAY either store trailer fields separate from header fields or
or discard them. Caches MUST NOT combine trailer fields with header discard them. Caches MUST NOT combine trailer fields with header
fields. fields.
3.2. Updating Stored Header Fields 3.2. Updating Stored Header Fields
Caches are required to update a stored response's header fields from Caches are required to update a stored response's header fields from
another (typically newer) response in several situations; for another (typically newer) response in several situations; for
example, see Section 3.4, Section 4.3.4 and Section 4.3.5. example, see Sections 3.4, 4.3.4, and 4.3.5.
When doing so, the cache MUST add each header field in the provided When doing so, the cache MUST add each header field in the provided
response to the stored response, replacing field values that are response to the stored response, replacing field values that are
already present, with the following exceptions: already present, with the following exceptions:
o Header fields excepted from storage in Section 3.1, o Header fields excepted from storage in Section 3.1,
o Header fields that the cache's stored response depends upon, as o Header fields that the cache's stored response depends upon, as
described below, described below,
o Header fields that are automatically processed and removed by the o Header fields that are automatically processed and removed by the
recipient, as described below, and recipient, as described below, and
o The Content-Length header field. o The Content-Length header field.
In some cases, caches (especially in user agents) store the results In some cases, caches (especially in user agents) store the results
of processing the received response, rather than the response itself, of processing the received response, rather than the response itself,
and updating header fields that affect that processing can result in and updating header fields that affect that processing can result in
inconsistent behavior and security issues. Caches in this situation inconsistent behavior and security issues. Caches in this situation
MAY omit these header fields from updating stored responses on an MAY omit these header fields from updating stored responses on an
exceptional basis, but SHOULD limit such omission to those fields exceptional basis but SHOULD limit such omission to those fields
necessary to assure integrity of the stored response. necessary to assure integrity of the stored response.
For example, a browser might decode the content coding of a response For example, a browser might decode the content coding of a response
while it is being received, creating a disconnect between the data it while it is being received, creating a disconnect between the data it
has stored and the response's original metadata. Updating that has stored and the response's original metadata. Updating that
stored metadata with a different Content-Encoding header field would stored metadata with a different Content-Encoding header field would
be problematic. Likewise, a browser might store a post-parse HTML be problematic. Likewise, a browser might store a post-parse HTML
tree, rather than the content received in the response; updating the tree rather than the content received in the response; updating the
Content-Type header field would not be workable in this case, because Content-Type header field would not be workable in this case because
any assumptions about the format made in parsing would now be any assumptions about the format made in parsing would now be
invalid. invalid.
Furthermore, some fields are automatically processed and removed by Furthermore, some fields are automatically processed and removed by
the HTTP implementation; for example, the Content-Range header field. the HTTP implementation, such as the Content-Range header field.
Implementations MAY automatically omit such header fields from Implementations MAY automatically omit such header fields from
updates, even when the processing does not actually occur. updates, even when the processing does not actually occur.
Note that the Content-* prefix is not a signal that a header field is Note that the Content-* prefix is not a signal that a header field is
omitted from update; it is a convention for MIME header fields, not omitted from update; it is a convention for MIME header fields, not
HTTP. HTTP.
3.3. Storing Incomplete Responses 3.3. Storing Incomplete Responses
If the request method is GET, the response status code is 200 (OK), 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 and the entire response header section has been received, a cache MAY
store a response body that is not complete (Section 3.4 of [HTTP]) if store a response that is not complete (Section 6.1 of [HTTP])
the stored response is recorded as being incomplete. Likewise, a 206 provided that the stored response is recorded as being incomplete.
(Partial Content) response MAY be stored as if it were an incomplete Likewise, a 206 (Partial Content) response MAY be stored as if it
200 (OK) response. However, a cache MUST NOT store incomplete or were an incomplete 200 (OK) response. However, a cache MUST NOT
partial-content responses if it does not support the Range and store incomplete or partial-content responses if it does not support
Content-Range header fields or if it does not understand the range the Range and Content-Range header fields or if it does not
units used in those fields. 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 (Section 14.2 of [HTTP]) and combining the subsequent range request (Section 14.2 of [HTTP]) and combining the
successful response with the stored response, as defined in successful response with the stored response, as defined in
Section 3.4. A cache MUST NOT use an incomplete response to answer Section 3.4. A cache MUST NOT use an incomplete response to answer
requests unless the response has been made complete, or the request requests unless the response has been made complete, or the request
is partial and specifies a range wholly within the incomplete is partial and specifies a range wholly within the incomplete
response. A cache MUST NOT send a partial response to a client response. A cache MUST NOT send a partial response to a client
without explicitly marking it using the 206 (Partial Content) status without explicitly marking it using the 206 (Partial Content) status
code. code.
skipping to change at page 11, line 26 skipping to change at page 11, line 11
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 update the stored response header fields using the header cache MUST update the stored response header fields using the header
fields provided in the new response, as per Section 3.2. fields provided in the new response, as per Section 3.2.
3.5. Storing Responses to Authenticated Requests 3.5. Storing Responses to Authenticated Requests
A shared cache MUST NOT use a cached response to a request with an A shared cache MUST NOT use a cached response to a request with an
Authorization header field (Section 11.6.2 of [HTTP]) to satisfy any Authorization header field (Section 11.6.2 of [HTTP]) to satisfy any
subsequent request unless the response contains a Cache-Control field subsequent request unless the response contains a Cache-Control field
with a response directive (Section 5.2.2) that allows it to be stored with a response directive (Section 5.2.2) that allows it to be stored
by a shared cache and the cache conforms to the requirements of that by a shared cache, and the cache conforms to the requirements of that
directive for that response. directive for that response.
In this specification, the following response directives have such an In this specification, the following response directives have such an
effect: must-revalidate (Section 5.2.2.2), public (Section 5.2.2.9), effect: must-revalidate (Section 5.2.2.2), public (Section 5.2.2.9),
and s-maxage (Section 5.2.2.10). and s-maxage (Section 5.2.2.10).
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 target URI (Section 7.1 of [HTTP]) and that of the o the presented target URI (Section 7.1 of [HTTP]) and that of the
stored response match, and 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 request header fields nominated by the stored response (if any) o request 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 stored response does not contain the no-cache cache directive o the stored response does not contain the no-cache directive
(Section 5.2.2.4), unless it is successfully validated (Section 5.2.2.4), unless it is successfully validated
(Section 4.3), and (Section 4.3), and
o the stored response is either: o the stored response is one of the following:
* 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 a cache-control extension can override any of the Note that a cache extension can override any of the requirements
requirements listed; see Section 5.2.3. 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 9.2.1 of [HTTP]) to the origin server; i.e., a cache is not (Section 9.2.1 of [HTTP]) to the origin server; i.e., a cache is not
allowed to generate a reply to such a request before having forwarded allowed to generate a reply to such a request before having forwarded
the request and having received a corresponding response. the request and having received a corresponding response.
Also, note that unsafe requests might invalidate already-stored Also, note that unsafe requests might invalidate already-stored
responses; see Section 4.4. responses; see Section 4.4.
A response that is stored or storable can be used to satisfy multiple A cache can use a response that is stored or storable to satisfy
requests, provided that it is allowed to reuse that response for the multiple requests, provided that it is allowed to reuse that response
requests in question. This enables caches to _collapse requests_ -- for the requests in question. This enables a cache to "collapse
or combine multiple incoming requests into a single forward request requests" -- or combine multiple incoming requests into a single
upon a cache miss -- thereby reducing load on the origin server and forward request upon a cache miss -- thereby reducing load on the
network. However, note that if the response returned is not able to origin server and network. Note, however, that if the cache cannot
be used for some or all of the collapsed requests, additional latency use the returned response for some or all of the collapsed requests,
might be introduced, because they will need to be forwarded to be it will need to forward the requests in order to satisfy them,
satisfied. potentially introducing additional latency.
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 one (as determined by the Date header field). It can most recent one (as determined by the Date header field). It can
also forward the request with "Cache-Control: max-age=0" or "Cache- also forward the request with "Cache-Control: max-age=0" or "Cache-
Control: no-cache" to disambiguate which response to use. Control: no-cache" to disambiguate which response to use.
A cache without a clock (Section 5.6.7 of [HTTP]) MUST revalidate A cache without a clock (Section 5.6.7 of [HTTP]) MUST revalidate
stored responses upon every use. stored responses upon every use.
4.1. Calculating Cache Keys with the Vary Header Field 4.1. Calculating Cache Keys with the Vary Header Field
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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 and that stored response contains a Vary header field response and that stored response contains a Vary header field
(Section 12.5.5 of [HTTP]), the cache MUST NOT use that stored (Section 12.5.5 of [HTTP]), the cache MUST NOT use that stored
response without revalidation unless all the presented request header response without revalidation unless all the presented request header
fields nominated by that Vary field value match those fields in the fields nominated by that Vary field value match those fields in the
original request (i.e., the request that caused the cached response original request (i.e., the request that caused the cached response
to be stored). to be stored).
The header fields from two requests are defined to match if and only The header fields from two requests are defined to match if and only
if those in the first request can be transformed to those in the if those in the first request can be transformed to those in the
second request by applying any of: second request by applying any of the following:
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 field lines with the same field name o combining multiple header field lines with the same field name
(see Section 5.2 of [HTTP]) (see Section 5.2 of [HTTP])
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
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cache SHOULD choose the most recent (see Section 4.2.3) stored cache SHOULD choose the most recent (see Section 4.2.3) stored
response with a valid Vary field value. response with a valid Vary field value.
If no stored response matches, the cache cannot satisfy the presented If no stored response matches, the cache cannot satisfy the presented
request. Typically, the request is forwarded to the origin server, request. Typically, the request is forwarded to the origin server,
potentially with preconditions added to describe what responses the potentially with preconditions added to describe what responses the
cache has already stored (Section 4.3). cache has already stored (Section 4.3).
4.2. Freshness 4.2. Freshness
A _fresh_ response is one whose age has not yet exceeded its A "fresh" response is one whose age has not yet exceeded its
freshness lifetime. Conversely, a _stale_ response is one where it freshness lifetime. Conversely, a "stale" response is one where it
has. has.
A response's _freshness lifetime_ is the length of time between its A response's "freshness lifetime" is the length of time between its
generation by the origin server and its expiration time. An generation by the origin server and its expiration time. An
_explicit expiration time_ is the time at which the origin server "explicit expiration time" is the time at which the origin server
intends that a stored response can no longer be used by a cache intends that a stored response can no longer be used by a cache
without further validation, whereas a _heuristic expiration time_ is without further validation, whereas a "heuristic expiration time" is
assigned by a cache when no explicit expiration time is available. assigned by a cache when no explicit expiration time is available.
A response's _age_ is the time that has passed since it was generated A response's "age" is the time that has passed since it was generated
by, or successfully validated with, the origin server. by, or successfully validated with, the origin server.
When a response is fresh, it can be used to satisfy subsequent When a response is fresh, it can be used to satisfy subsequent
requests without contacting the origin server, thereby improving requests without contacting the origin server, thereby improving
efficiency. efficiency.
The primary mechanism for determining freshness is for an origin The primary mechanism for determining freshness is for an origin
server to provide an explicit expiration time in the future, using server to provide an explicit expiration time in the future, using
either the Expires header field (Section 5.3) or the max-age response either the Expires header field (Section 5.3) or the max-age response
directive (Section 5.2.2.1). Generally, origin servers will assign directive (Section 5.2.2.1). Generally, origin servers will assign
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other than "GMT" to be invalid for calculating expiration. other than "GMT" to be invalid for calculating expiration.
Note that freshness applies only to cache operation; it cannot be Note that freshness applies only to cache operation; it cannot be
used to force a user agent to refresh its display or reload a used to force a user agent to refresh its display or reload a
resource. See Section 6 for an explanation of the difference between resource. See Section 6 for an explanation of the difference between
caches and history mechanisms. caches and history mechanisms.
4.2.1. Calculating Freshness Lifetime 4.2.1. Calculating Freshness Lifetime
A cache can calculate the freshness lifetime (denoted as A cache can calculate the freshness lifetime (denoted as
freshness_lifetime) of a response by using the first match of: freshness_lifetime) of a response by evaluating the following rules
and using the first match:
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 5.2.2.10) 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 5.2.2.1) is present, o If the max-age response directive (Section 5.2.2.1) is present,
use its value, or use its value, or
o If the Expires response header field (Section 5.3) is present, use o If the Expires response header field (Section 5.3) is present, use
its value minus the value of the Date response header field (using its value minus the value of the Date response header field (using
the time the message was received if it is not present, as per the time the message was received if it is not present, as per
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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 intended to reduce clock skew by using Note that this calculation is intended to reduce clock skew by using
the clock information provided by the origin server whenever the clock information provided by the origin server whenever
possible. possible.
When there is more than one value present for a given directive When there is more than one value present for a given directive
(e.g., two Expires header field lines or multiple Cache-Control: max- (e.g., two Expires header field lines or multiple Cache-Control: max-
age directives), either the first occurrence should be used, or the age directives), either the first occurrence should be used or the
response should be considered stale. If directives conflict (e.g., response should be considered stale. If directives conflict (e.g.,
both max-age and no-cache are present), the most restrictive both max-age and no-cache are present), the most restrictive
directive should be honored. Caches are encouraged to consider directive should be honored. Caches are encouraged to consider
responses that have invalid freshness information (e.g., a max-age responses that have invalid freshness information (e.g., a max-age
directive with non-integer content) to be stale. directive with non-integer content) to be stale.
4.2.2. Calculating Heuristic Freshness 4.2.2. Calculating Heuristic Freshness
Since origin servers do not always provide explicit expiration times, Since origin servers do not always provide explicit expiration times,
a cache MAY assign a heuristic expiration time when an explicit time a cache MAY assign a heuristic expiration time when an explicit time
is not specified, employing algorithms that use other field values is not specified, employing algorithms that use other field values
(such as the Last-Modified time) to estimate a plausible expiration (such as the Last-Modified time) to estimate a plausible expiration
time. This specification does not provide specific algorithms, but time. This specification does not provide specific algorithms, but
does impose worst-case constraints on their results. it 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 heuristics can only of the requirements in Section 3, heuristics can only be used on
be used on responses without explicit freshness whose status codes responses without explicit freshness whose status codes are defined
are defined as _heuristically cacheable_ (e.g., see Section 15.1 of as "heuristically cacheable" (e.g., see Section 15.1 of [HTTP]) and
[HTTP]), and those responses without explicit freshness that have on responses without explicit freshness that have been marked as
been marked as explicitly cacheable (e.g., with a "public" response explicitly cacheable (e.g., with a public response directive).
directive).
Note that in previous specifications heuristically cacheable response Note that in previous specifications, heuristically cacheable
status codes were called "cacheable by default." response status codes were called "cacheable by default".
If the response has a Last-Modified header field (Section 8.8.2 of If the response has a Last-Modified header field (Section 8.8.2 of
[HTTP]), caches are encouraged to use a heuristic expiration value [HTTP]), caches are encouraged to use a heuristic expiration value
that is no more than some fraction of the interval since that time. that is no more than some fraction of the interval since that time.
A typical setting of this fraction might be 10%. A typical setting of this fraction might be 10%.
| *Note:* Section 13.9 of [RFC2616] prohibited caches from | *Note:* A previous version of the HTTP specification
| calculating heuristic freshness for URIs with query components | (Section 13.9 of [RFC2616]) prohibited caches from calculating
| (i.e., those containing '?'). In practice, this has not been | heuristic freshness for URIs with query components (i.e., those
| widely implemented. Therefore, origin servers are encouraged | containing "?"). In practice, this has not been widely
| to send explicit directives (e.g., Cache-Control: no-cache) if | implemented. Therefore, origin servers are encouraged to send
| they wish to prevent caching. | explicit directives (e.g., Cache-Control: no-cache) if 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 origin server the cache's estimate of the number of seconds since the origin server
generated or validated the response. The Age value is therefore the generated or validated the response. The Age value is therefore the
sum of the time that the response has been resident in each of the sum of the time that the response has been resident in each of the
caches along the path from the origin server, plus the time it has caches along the path from the origin server, plus the time it has
been in transit along network paths. been in transit along network paths.
Age calculation uses the following data: Age calculation uses the following data:
_age_value_ The term "age_value" denotes the value of the Age header "age_value"
field (Section 5.1), in a form appropriate for arithmetic The term "age_value" denotes the value of the Age header field
operation; or 0, if not available. (Section 5.1), in a form appropriate for arithmetic operation; or
0, if not available.
_date_value_ The term "date_value" denotes the value of the Date "date_value"
header field, in a form appropriate for arithmetic operations. The term "date_value" denotes the value of the Date header field,
See Section 6.6.1 of [HTTP] for the definition of the Date header in a form appropriate for arithmetic operations. See
field, and for requirements regarding responses without it. Section 6.6.1 of [HTTP] for the definition of the Date header
field and for requirements regarding responses without it.
_now_ The term "now" means the current value of this "now"
implementation's clock (Section 5.6.7 of [HTTP]). The term "now" means the current value of this implementation's
clock (Section 5.6.7 of [HTTP]).
_request_time_ The value of the clock at the time of the request "request_time"
that resulted in the stored response. The value of the clock at the time of the request that resulted in
the stored response.
_response_time_ The value of the clock at the time the response was "response_time"
received. The value of the clock at the time the response was received.
A response's age can be calculated in two entirely independent ways: A response's age can be calculated in two entirely independent ways:
1. the "apparent_age": response_time minus date_value, if the 1. the "apparent_age": response_time minus date_value, if the
implementation's clock is reasonably well synchronized to the implementation's clock is reasonably well synchronized to the
origin server's clock. If the result is negative, the result is origin server's clock. If the result is negative, the result is
replaced by zero. replaced by 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 or greater. A cache MUST response path implement HTTP/1.1 or greater. A cache MUST
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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-cache" cache explicit in-protocol directive (e.g., by a no-cache response
directive, a "must-revalidate" cache-response-directive, or an directive, a must-revalidate response directive, or an applicable
applicable "s-maxage" or "proxy-revalidate" cache-response-directive; s-maxage or proxy-revalidate response directive; see Section 5.2.2).
see Section 5.2.2).
A cache MUST NOT generate a stale response unless it is disconnected A cache MUST NOT generate a stale response unless it is disconnected
or doing so is explicitly permitted by the client or origin server or doing so is explicitly permitted by the client or origin server
(e.g., by the max-stale request directive in Section 5.2.1, by (e.g., by the max-stale request directive in Section 5.2.1, extension
extension directives such as those defined in [RFC5861], or by directives such as those defined in [RFC5861], or configuration in
configuration in accordance with an out-of-band contract). accordance with an out-of-band contract).
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 chosen; see Section 4.1), it can use the conditional one cannot be chosen; see Section 4.1), it can use the conditional
request mechanism (Section 13.1 of [HTTP]) in the forwarded request request mechanism (Section 13 of [HTTP]) in the forwarded request to
to give the next inbound server an opportunity to choose a valid give the next inbound server an opportunity to choose a valid stored
stored response to use, updating the stored metadata in the process, response to use, updating the stored metadata in the process, or to
or to replace the stored response(s) with a new response. This replace the stored response(s) with a new response. This process is
process is known as _validating_ or _revalidating_ the stored known as "validating" or "revalidating" the stored response.
response.
4.3.1. Sending a Validation Request 4.3.1. Sending a Validation Request
When generating a conditional request for validation, a cache starts When generating a conditional request for validation, a cache either
with either a request it is attempting to satisfy, or -- if it is starts with a request it is attempting to satisfy or -- if it is
initiating the request independently -- it synthesises a request initiating the request independently -- synthesizes a request using a
using a stored response by copying the method, target URI, and stored response by copying the method, target URI, and request header
request header fields identified by the Vary header field fields identified by the Vary header field (Section 4.1).
(Section 4.1).
It then updates that request with one or more precondition header It then updates that request with one or more precondition header
fields. These contain validator metadata sourced from stored fields. These contain validator metadata sourced from a stored
response(s) that have the same URI. Typically, this will include response(s) that has the same URI. Typically, this will include only
only those stored responses(s) that have the same cache key, although the stored response(s) that has the same cache key, although a cache
a cache is allowed to validate a response that it cannot choose with is allowed to validate a response that it cannot choose with the
the request header fields it is sending (see Section 4.1). request header fields it is sending (see Section 4.1).
The precondition header fields are then compared by recipients to The precondition header fields are then compared by recipients to
determine whether any stored response is equivalent to a current determine whether any stored response is equivalent to a current
representation of the resource. 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 8.8.2 of [HTTP]), which can be used in an If-Modified- field (Section 8.8.2 of [HTTP]), which can be used in an If-Modified-
Since header field for response validation, or in an If-Unmodified- Since header field for response validation, or in an If-Unmodified-
Since or If-Range header field for representation selection (i.e., Since or If-Range header field for representation selection (i.e.,
the client is referring specifically to a previously obtained the client is referring specifically to a previously obtained
representation with that timestamp). representation with that timestamp).
Another validator is the entity-tag given in an ETag field Another validator is the entity tag given in an ETag field
(Section 8.8.3 of [HTTP]). One or more entity-tags, indicating one (Section 8.8.3 of [HTTP]). One or more entity tags, indicating one
or more stored responses, can be used in an If-None-Match header or more stored responses, can be used in an If-None-Match header
field for response validation, or in an If-Match or If-Range header field for response validation, or in an If-Match or If-Range header
field for representation selection (i.e., the client is referring field for representation selection (i.e., the client is referring
specifically to one or more previously obtained representations with specifically to one or more previously obtained representations with
the listed entity-tags). the listed entity tags).
When generating a conditional request for validation, a cache: When generating a conditional request for validation, a cache:
o MUST send the relevant entity-tags (using If-Match, If-None-Match, o MUST send the relevant entity tags (using If-Match, If-None-Match,
or If-Range) if the entity-tags were provided in the stored or If-Range) if the entity tags were provided in the stored
response(s) being validated. response(s) being validated.
o SHOULD send the Last-Modified value (using If-Modified-Since) if o SHOULD send the Last-Modified value (using If-Modified-Since) if
the request is not for a subrange, a single stored response is the request is not for a subrange, a single stored response is
being validated, and that response contains a Last-Modified value. being validated, and that response contains a Last-Modified value.
o MAY send the Last-Modified value (using If-Unmodified-Since or If- o MAY send the Last-Modified value (using If-Unmodified-Since or If-
Range) if the request is for a subrange, a single stored response Range) if the request is for a subrange, a single stored response
is being validated, and that response contains only a Last- is being validated, and that response contains only a Last-
Modified value (not an entity-tag). Modified value (not an entity tag).
In most cases, both validators are generated in cache validation In most cases, both validators are generated in cache validation
requests, even when entity-tags are clearly superior, to allow old requests, even when entity tags are clearly superior, to allow old
intermediaries that do not understand entity-tag preconditions to intermediaries that do not understand entity tag preconditions to
respond appropriately. respond appropriately.
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.
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field is present, the time that the stored response was received) to field is present, the time that the stored response was received) to
evaluate the conditional. evaluate the conditional.
A cache that implements partial responses to range requests, as A cache that implements partial responses to range requests, as
defined in Section 14.2 of [HTTP], also needs to evaluate a received defined in Section 14.2 of [HTTP], also needs to evaluate a received
If-Range header field (Section 13.1.5 of [HTTP]) with respect to the If-Range header field (Section 13.1.5 of [HTTP]) with respect to the
cache's chosen response. cache's chosen response.
When a cache decides to forward a request to revalidate its own When a cache decides to forward a request to revalidate its own
stored responses for a request that contains an If-None-Match list of stored responses for a request that contains an If-None-Match list of
entity-tags, the cache MAY combine the received list with a list of entity tags, the cache MAY combine the received list with a list of
entity-tags from its own stored set of responses (fresh or stale) and entity tags from its own stored set of responses (fresh or stale) and
send the union of the two lists as a replacement If-None-Match header send the union of the two lists as a replacement If-None-Match header
field value in the forwarded request. If a stored response contains field value in the forwarded request. If a stored response contains
only partial content, the cache MUST NOT include its entity-tag in only partial content, the cache MUST NOT include its entity tag in
the union unless the request is for a range that would be fully the union unless the request is for a range that would be fully
satisfied by that partial stored response. If the response to the satisfied by that partial stored response. If the response to the
forwarded request is 304 (Not Modified) and has an ETag field value forwarded request is 304 (Not Modified) and has an ETag field value
with an entity-tag that is not in the client's list, the cache MUST with an entity tag that is not in the client's list, the cache MUST
generate a 200 (OK) response for the client by reusing its generate a 200 (OK) response for the client by reusing its
corresponding stored response, as updated by the 304 response corresponding stored response, as updated by the 304 response
metadata (Section 4.3.4). metadata (Section 4.3.4).
4.3.3. Handling a Validation Response 4.3.3. Handling a Validation Response
Cache handling of a response to a conditional request depends upon Cache handling of a response to a conditional request depends 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 containing content) indicates that none o A full response (i.e., one containing content) indicates that none
of the stored responses nominated in the conditional request is of the stored responses nominated in the conditional request are
suitable. Instead, the cache MUST use the full response to suitable. Instead, the cache MUST use the full response to
satisfy the request. The cache MAY store such a full response, satisfy the request. The cache MAY store such a full response,
subject to its constraints (see Section 3). subject to its constraints (see Section 3).
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 can send a previously to respond. In the latter case, the cache can send a previously
stored response, subject to its constraints on doing so (see stored response, subject to its constraints on doing so (see
Section 4.2.4), or retry the validation request. Section 4.2.4), or retry the validation request.
4.3.4. Freshening Stored Responses upon Validation 4.3.4. Freshening Stored Responses upon Validation
When a cache receives a 304 (Not Modified) response, it needs to When a cache receives a 304 (Not Modified) response, it needs to
identify stored responses that are suitable for updating with the new identify stored responses that are suitable for updating with the new
information provided, and then do so. information provided, and then do so.
The initial set of stored responses to update are those that could The initial set of stored responses to update are those that could
have been chosen for that request -- i.e., those that meet the have been chosen for that request -- i.e., those that meet the
requirements in Section 4, except the last requirement to be fresh, requirements in Section 4, except the last requirement to be fresh,
able to be served stale or just validated. able to be served stale, or just validated.
Then, that initial set of stored response(s) is further filtered by Then, that initial set of stored responses is further filtered by the
the first match of: first match of:
o If the new response contains one or more _strong validators_ (see o If the new response contains one or more "strong validators" (see
Section 8.8.1 of [HTTP]), then each of those strong validators Section 8.8.1 of [HTTP]), then each of those strong validators
identify a selected representation for update. All the stored identifies a selected representation for update. All the stored
responses in the initial set with one of those same strong responses in the initial set with one of those same strong
validators are identified for update. If none of the initial set validators are identified for update. If none of the initial set
contain at least one of the same strong validators, then the cache contains at least one of the same strong validators, then the
MUST 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 no strong validators but does contain o If the new response contains no strong validators but does contain
one or more _weak validators_, and those validators correspond to one or more "weak validators", and those validators correspond to
one of the initial set's stored responses, then the most recent of one of the initial set's stored responses, then the most recent of
those matching stored responses is identified for update. 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 where a client generates an If-Modified-Since request from a as where a client generates an If-Modified-Since request from a
source other than the Last-Modified response header field), and source other than the Last-Modified response header field), and
there is only one stored response in the initial set, and that there is only one stored response in the initial set, and that
stored response also lacks a validator, then that stored response stored response also lacks a validator, then that stored response
is identified for update. is identified for update.
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the stored response as described below; otherwise, the cache SHOULD the stored response as described below; otherwise, the cache 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 use the header fields provided in the HEAD response, the cache MUST use the header fields provided in the
HEAD response to update the stored response (see Section 3.2). HEAD response to update the stored response (see Section 3.2).
4.4. Invalidating Stored Responses 4.4. Invalidating Stored Responses
Because unsafe request methods (Section 9.2.1 of [HTTP]) such as PUT, Because unsafe request methods (Section 9.2.1 of [HTTP]) such as PUT,
POST or DELETE have the potential for changing state on the origin POST, or DELETE have the potential for changing state on the origin
server, intervening caches are required to invalidate stored server, intervening caches are required to invalidate stored
responses to keep their contents up to date. responses to keep their contents up to date.
A cache MUST invalidate the target URI (Section 7.1 of [HTTP]) when A cache MUST invalidate the target URI (Section 7.1 of [HTTP]) when
it receives a non-error status code in response to an unsafe request it receives a non-error status code in response to an unsafe request
method (including methods whose safety is unknown). method (including methods whose safety is unknown).
A cache MAY invalidate other URIs when it receives a non-error status A cache MAY invalidate other URIs when it receives a non-error status
code in response to an unsafe request method (including methods whose code in response to an unsafe request method (including methods whose
safety is unknown). In particular, the URI(s) in the Location and safety is unknown). In particular, the URI(s) in the Location and
Content-Location response header fields (if present) are candidates Content-Location response header fields (if present) are candidates
for invalidation; other URIs might be discovered through mechanisms for invalidation; other URIs might be discovered through mechanisms
not specified in this document. However, a cache MUST NOT trigger an not specified in this document. However, a cache MUST NOT trigger an
invalidation under these conditions if the origin (Section 4.3.1 of invalidation under these conditions if the origin (Section 4.3.1 of
[HTTP]) of the URI to be invalidated differs from that of the target [HTTP]) of the URI to be invalidated differs from that of the target
URI (Section 7.1 of [HTTP]). This helps prevent denial-of-service URI (Section 7.1 of [HTTP]). This helps prevent denial-of-service
attacks. attacks.
_Invalidate_ means that the cache will either remove all stored "Invalidate" means that the cache will either remove all stored
responses whose target URI matches the given URI, or will mark them responses whose target URI matches the given URI or mark them as
as "invalid" and in need of a mandatory validation before they can be "invalid" and in need of a mandatory validation before they can be
sent in response to a subsequent request. sent in response to a subsequent request.
A "non-error response" is one with a 2xx (Successful) or 3xx A "non-error response" is one with a 2xx (Successful) or 3xx
(Redirection) status code. (Redirection) status code.
Note that this does not guarantee that all appropriate responses are Note that this does not guarantee that all appropriate responses are
invalidated globally; a state-changing request would only invalidate invalidated globally; a state-changing request would only invalidate
responses in the caches it travels through. responses in the caches it travels through.
5. Field Definitions 5. Field Definitions
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negative integer), a cache SHOULD ignore the field. negative integer), a cache SHOULD ignore the field.
The presence of an Age header field implies that the response was not The presence of an Age header field implies that the response was not
generated or validated by the origin server for this request. generated or validated by the origin server for this request.
However, lack of an Age header field does not imply the origin was However, lack of an Age header field does not imply the origin was
contacted. contacted.
5.2. Cache-Control 5.2. Cache-Control
The "Cache-Control" header field is used to list 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. 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 present in the response, or to not imply that the same directive is present or copied in the
be repeated in it. response.
See Section 5.2.3 for information about how Cache-Control directives See Section 5.2.3 for information about how Cache-Control directives
defined elsewhere are handled. defined elsewhere are handled.
A proxy, whether or not it implements a cache, MUST pass cache A proxy, whether or not it implements a cache, MUST pass cache
directives through in forwarded messages, regardless of their directives through in forwarded messages, regardless of their
significance to that application, since the directives might apply to significance to that application, since the directives might apply to
all recipients along the request/response chain. It is not possible all recipients along the request/response chain. It is not possible
to target a directive to a specific cache. to target a directive to a specific cache.
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define arguments, recipients ought to accept both forms, even if a define arguments, recipients ought to accept both forms, even if a
specific form is required for generation. specific form is required for generation.
Cache-Control = #cache-directive Cache-Control = #cache-directive
cache-directive = token [ "=" ( token / quoted-string ) ] cache-directive = token [ "=" ( token / quoted-string ) ]
For the cache directives defined below, no argument is defined (nor For the cache directives defined below, no argument is defined (nor
allowed) unless stated otherwise. allowed) unless stated otherwise.
5.2.1. Request Cache-Control Directives 5.2.1. Request Directives
This section defines cache request directives. They are advisory; This section defines cache request directives. They are advisory;
caches MAY implement them, but are not required to. 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 1.2.2) delta-seconds (see Section 1.2.2)
The "max-age" request directive indicates that the client prefers a The max-age request directive indicates that the client prefers a
response whose age is less than or equal to the specified number of response whose age is less than or equal to the specified number of
seconds. Unless the max-stale request directive is also present, the seconds. Unless the max-stale request directive is also present, the
client does not wish to receive a stale response. client does not wish to receive a stale 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 MUST 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 1.2.2) delta-seconds (see Section 1.2.2)
The "max-stale" request directive indicates that the client will The max-stale request directive indicates that the client will accept
accept a response that has exceeded its freshness lifetime. If a a response that has exceeded its freshness lifetime. If a value is
value is present, then the client is willing to accept a response present, then the client is willing to accept a response that has
that has exceeded its freshness lifetime by no more than the exceeded its freshness lifetime by no more than the specified number
specified number of seconds. If no value is assigned to max-stale, of seconds. If no value is assigned to max-stale, then the client
then the client will accept a stale response of any age. will accept a stale 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 MUST NOT generate the 'max-stale=10' not 'max-stale="10"'. A sender MUST NOT generate 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 1.2.2) delta-seconds (see Section 1.2.2)
The "min-fresh" request directive indicates that the client prefers a The min-fresh request directive indicates that the client prefers a
response whose freshness lifetime is no less than its current age response whose freshness lifetime is no less than its current age
plus the specified time in seconds. That is, the client wants a plus the specified time in seconds. That is, the client wants a
response that will still be fresh for at least the specified number response that will still be fresh for at least 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.,
'min-fresh=20' not 'min-fresh="20"'. A sender MUST NOT generate the 'min-fresh=20' not 'min-fresh="20"'. A sender MUST NOT generate 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 the client prefers The no-cache request directive indicates that the client prefers a
stored response not be used 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
store any part of either this request or any response to it. This 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 best- store the information in non-volatile storage and MUST make a best-
effort attempt to remove the information from volatile storage as effort attempt to remove the information from volatile storage as
promptly as possible after forwarding it. promptly as possible after forwarding it.
This directive is _not_ a reliable or sufficient mechanism for This directive is not a reliable or sufficient mechanism for ensuring
ensuring privacy. In particular, malicious or compromised caches privacy. In particular, malicious or compromised caches might not
might not recognize or obey this directive, and communications recognize or obey this directive, and communications networks might
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 the client is The no-transform request directive indicates that the client is
asking for intermediaries to avoid transforming the content, as asking for intermediaries to avoid transforming the content, as
defined in Section 7.7 of [HTTP]. defined in Section 7.7 of [HTTP].
5.2.1.7. only-if-cached 5.2.1.7. only-if-cached
The "only-if-cached" request directive indicates that the client only The only-if-cached request directive indicates that the client only
wishes to obtain a stored response. Caches that honor this request wishes to obtain a stored response. Caches that honor this request
directive SHOULD, upon receiving it, either respond using a stored directive SHOULD, upon receiving it, respond with either a stored
response consistent with the other constraints of the request, or response consistent with the other constraints of the request or a
respond with a 504 (Gateway Timeout) status code. 504 (Gateway Timeout) status code.
5.2.2. Response Cache-Control Directives 5.2.2. Response Directives
This section defines cache response directives. A cache MUST obey This section defines cache response directives. A cache MUST obey
the Cache-Control directives defined in this section. the Cache-Control directives defined in this section.
5.2.2.1. max-age 5.2.2.1. max-age
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.2) delta-seconds (see Section 1.2.2)
The "max-age" response directive indicates that the response is to be The max-age response directive indicates that the response is to be
considered stale after its age is greater than the specified number considered stale after its age is greater than the specified number
of seconds. of seconds.
This directive uses the token form of the argument syntax: e.g., This directive uses the token form of the argument syntax: e.g.,
'max-age=5' not 'max-age="5"'. A sender MUST NOT generate the 'max-age=5' not 'max-age="5"'. A sender MUST NOT generate the
quoted-string form. quoted-string form.
5.2.2.2. must-revalidate 5.2.2.2. must-revalidate
The "must-revalidate" response directive indicates that once the The must-revalidate response directive indicates that once the
response has become stale, a cache MUST NOT reuse that response to response has become stale, a cache MUST NOT reuse that response to
satisfy another request until it has been successfully validated by satisfy another request until it has been successfully validated by
the origin, as defined by Section 4.3. 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 NOT ignore the must-revalidate directive; in particular, cache MUST NOT ignore the must-revalidate directive; in particular,
if a cache is disconnected, the cache MUST generate an error response if a cache is disconnected, the cache MUST generate an error response
rather than reuse the stale response. The generated status code rather than reuse the stale response. The generated status code
SHOULD be 504 (Gateway Timeout) unless another error status code is SHOULD be 504 (Gateway Timeout) unless another error status code is
more applicable. more applicable.
The must-revalidate directive ought to be used by servers if and only The must-revalidate directive ought to be used by servers if and only
if failure to validate a request could cause incorrect operation, if failure to validate a request could cause incorrect operation,
such as a silently unexecuted financial transaction. such as a silently unexecuted financial transaction.
The must-revalidate directive also permits a shared cache to reuse a The must-revalidate directive also permits a shared cache to reuse a
response to a request containing an Authorization header field response to a request containing an Authorization header field
(Section 11.6.2 of [HTTP]), subject to the above requirement on (Section 11.6.2 of [HTTP]), subject to the above requirement on
revalidation (Section 3.5). revalidation (Section 3.5).
5.2.2.3. must-understand 5.2.2.3. must-understand
The "must-understand" response directive limits caching of the The must-understand response directive limits caching of the response
response to a cache that understands and conforms to the requirements to a cache that understands and conforms to the requirements for that
for that response's status code. response's status code.
Responses containing "must-understand" SHOULD also contain the "no- A response that contains the must-understand directive SHOULD also
store" directive; caches that implement "must-understand" SHOULD contain the no-store directive. When a cache that implements the
ignore the "no-store" directive in responses that contain both must-understand directive receives a response that includes it, the
directives and a status code that the cache understands and conforms cache SHOULD ignore the no-store directive if it understands and
to any related caching requirements. implements the status code's caching requirements.
5.2.2.4. no-cache 5.2.2.4. no-cache
Argument syntax: Argument syntax:
#field-name #field-name
The "no-cache" response directive, in its unqualified form (without The no-cache response directive, in its unqualified form (without an
an argument), indicates that the response MUST NOT be used to satisfy argument), indicates that the response MUST NOT be used to satisfy
any other request without forwarding it for validation and receiving any other request without forwarding it for validation and receiving
a successful response; see Section 4.3. a successful response; see Section 4.3.
This allows an origin server to prevent a cache from using the This allows an origin server to prevent a cache from using the
response to satisfy a request without contacting it, even by caches response to satisfy a request without contacting it, even by caches
that have been configured to send stale responses. that have been configured to send stale responses.
The qualified form of no-cache response directive, with an argument The qualified form of the no-cache response directive, with an
that lists one or more field names, indicates that a cache MAY use argument that lists one or more field names, indicates that a cache
the response to satisfy a subsequent request, subject to any other MAY use the response to satisfy a subsequent request, subject to any
restrictions on caching, if the listed header fields are excluded other restrictions on caching, if the listed header fields are
from the subsequent response or the subsequent response has been excluded from the subsequent response or the subsequent response has
successfully revalidated with the origin server (updating or removing been successfully revalidated with the origin server (updating or
those fields). This allows an origin server to prevent the re-use of removing those fields). This allows an origin server to prevent the
certain header fields in a response, while still allowing caching of reuse of certain header fields in a response, while still allowing
the rest of the response. caching of the rest of the response.
The field names given are not limited to the set of header fields The field names given are not limited to the set of header fields
defined by this specification. Field names are case-insensitive. defined by this specification. Field names are case-insensitive.
This directive uses the quoted-string form of the argument syntax. A This directive uses the quoted-string form of the argument syntax. A
sender SHOULD NOT generate the token form (even if quoting appears sender SHOULD NOT generate the token form (even if quoting appears
not to be needed for single-entry lists). not to be needed for single-entry lists).
| *Note:* The qualified form of the directive is often handled by | *Note:* The qualified form of the directive is often handled by
| caches as if an unqualified no-cache directive was received; | caches as if an unqualified no-cache directive was received;
| i.e., the special handling for the qualified form is not widely | that is, the special handling for the qualified form is not
| implemented. | widely implemented.
5.2.2.5. no-store 5.2.2.5. no-store
The "no-store" response directive indicates that a cache MUST NOT The no-store response directive indicates that a cache MUST NOT store
store any part of either the immediate request or response, and MUST any part of either the immediate request or the response and MUST NOT
NOT use the response to satisfy any other request. use the response to satisfy any other request.
This directive applies to both private and shared caches. "MUST NOT 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 best- store the information in non-volatile storage and MUST make a best-
effort attempt to remove the information from volatile storage as effort attempt to remove the information from volatile storage as
promptly as possible after forwarding it. promptly as possible after forwarding it.
This directive is _not_ a reliable or sufficient mechanism for This directive is not a reliable or sufficient mechanism for ensuring
ensuring privacy. In particular, malicious or compromised caches privacy. In particular, malicious or compromised caches might not
might not recognize or obey this directive, and communications recognize or obey this directive, and communications networks might
networks might be vulnerable to eavesdropping. be vulnerable to eavesdropping.
Note that the "must-understand" cache directive overrides "no-store" Note that the must-understand cache directive overrides no-store in
in certain circumstances; see Section 5.2.2.3. certain circumstances; see Section 5.2.2.3.
5.2.2.6. no-transform 5.2.2.6. no-transform
The "no-transform" response directive indicates that an intermediary The no-transform response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT transform the (regardless of whether it implements a cache) MUST NOT transform the
content, as defined in Section 7.7 of [HTTP]. content, as defined in Section 7.7 of [HTTP].
5.2.2.7. private 5.2.2.7. private
Argument syntax: Argument syntax:
#field-name #field-name
The unqualified "private" response directive indicates that a shared The unqualified private response directive indicates that a shared
cache MUST NOT store the response (i.e., the response is intended for cache MUST NOT store the response (i.e., the response is intended for
a single user). It also indicates that a private cache MAY store the a single user). It also indicates that a private cache MAY store the
response, subject the constraints defined in Section 3, even if the response, subject to the constraints defined in Section 3, even if
response would not otherwise be heuristically cacheable by a private the response would not otherwise be heuristically cacheable by a
cache. private cache.
If a qualified private response directive is present, with an If a qualified private response directive is present, with an
argument that lists one or more field names, then only the listed argument that lists one or more field names, then only the listed
header fields are limited to a single user: a shared cache MUST NOT header fields are limited to a single user: a shared cache MUST NOT
store the listed header fields if they are present in the original store the listed header fields if they are present in the original
response, but MAY store the remainder of the response message without response but MAY store the remainder of the response message without
those header fields, subject the constraints defined in Section 3. those header fields, subject the constraints defined in Section 3.
The field names given are not limited to the set of header fields The field names given are not limited to the set of header fields
defined by this specification. Field names are case-insensitive. defined by this specification. Field names are case-insensitive.
This directive uses the quoted-string form of the argument syntax. A This directive uses the quoted-string form of the argument syntax. A
sender SHOULD NOT generate the token form (even if quoting appears sender SHOULD NOT generate the token form (even if quoting appears
not to be needed for single-entry lists). not to be needed for single-entry lists).
| *Note:* This usage of the word "private" only controls where | *Note:* This usage of the word "private" only controls where
| the response can be stored; it cannot ensure the privacy of the | the response can be stored; it cannot ensure the privacy of the
| message content. Also, the qualified form of the directive is | message content. Also, the qualified form of the directive is
| often handled by caches as if an unqualified private directive | often handled by caches as if an unqualified private directive
| was received; i.e., the special handling for the qualified form | was received; that is, the special handling for the qualified
| is not widely implemented. | form is not widely implemented.
5.2.2.8. proxy-revalidate 5.2.2.8. proxy-revalidate
The "proxy-revalidate" response directive indicates that once the The proxy-revalidate response directive indicates that once the
response has become stale, a shared cache MUST NOT reuse that response has become stale, a shared cache MUST NOT reuse that
response to satisfy another request until it has been successfully response to satisfy another request until it has been successfully
validated by the origin, as defined by Section 4.3. This is validated by the origin, as defined by Section 4.3. This is
analogous to must-revalidate (Section 5.2.2.2), except that proxy- analogous to must-revalidate (Section 5.2.2.2), except that proxy-
revalidate does not apply to private caches. revalidate does not apply to private caches.
Note that "proxy-revalidate" on its own does not imply that a Note that proxy-revalidate on its own does not imply that a response
response is cacheable. For example, it might be combined with the is cacheable. For example, it might be combined with the public
public directive (Section 5.2.2.9), allowing the response to be directive (Section 5.2.2.9), allowing the response to be cached while
cached while requiring only a shared cache to revalidate when stale. requiring only a shared cache to revalidate when stale.
5.2.2.9. public 5.2.2.9. public
The "public" response directive indicates that a cache MAY store the The public response directive indicates that a cache MAY store the
response even if it would otherwise be prohibited, subject to the response even if it would otherwise be prohibited, subject to the
constraints defined in Section 3. In other words, public explicitly constraints defined in Section 3. In other words, public explicitly
marks the response as cacheable. For example, public permits a marks the response as cacheable. For example, public permits a
shared cache to reuse a response to a request containing an shared cache to reuse a response to a request containing an
Authorization header field (Section 3.5). Authorization header field (Section 3.5).
Note that it is unnecessary to add the public directive to a response Note that it is unnecessary to add the public directive to a response
that is already cacheable according to Section 3. that is already cacheable according to Section 3.
If a response with the public directive has no explicit freshness If a response with the public directive has no explicit freshness
information, it is heuristically cacheable (Section 4.2.2). information, it is heuristically cacheable (Section 4.2.2).
5.2.2.10. s-maxage 5.2.2.10. s-maxage
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.2) delta-seconds (see Section 1.2.2)
The "s-maxage" response directive indicates that, for a shared cache, 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. field.
The s-maxage directive incorporates the proxy-revalidate The s-maxage directive incorporates the semantics of the proxy-
(Section 5.2.2.8) response directive's semantics for a shared cache. revalidate response directive (Section 5.2.2.8) for a shared cache.
A shared cache MUST NOT reuse a stale response with s-maxage to A shared cache MUST NOT reuse a stale response with s-maxage to
satisfy another request until it has been successfully validated by satisfy another request until it has been successfully validated by
the origin, as defined by Section 4.3. This directive also permits a the origin, as defined by Section 4.3. This directive also permits a
shared cache to reuse a response to a request containing an shared cache to reuse a response to a request containing an
Authorization header field, subject to the above requirements on Authorization header field, subject to the above requirements on
maximum age and revalidation (Section 3.5). maximum age and revalidation (Section 3.5).
This directive uses the token form of the argument syntax: e.g., This directive uses the token form of the argument syntax: e.g.,
's-maxage=10' not 's-maxage="10"'. A sender MUST 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. Extension Directives
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 extension cache directives. A cache MUST ignore unrecognized or more extension cache directives. A cache MUST ignore unrecognized
cache directives. cache directives.
Informational extensions (those that do not require a change in cache Informational extensions (those that do not require a change in cache
behavior) can be added without changing the semantics of other behavior) can be added without changing the semantics of other
directives. directives.
Behavioral extensions are designed to work by acting as modifiers to Behavioral extensions are designed to work by acting as modifiers to
the existing base of cache directives. Both the new directive and the existing base of cache directives. Both the new directive and
the old directive are supplied, such that applications that do not the old directive are supplied, such that applications that do not
understand the new directive will default to the behavior specified understand the new directive will default to the behavior specified
by the old directive, and those that understand the new directive by the old directive, and those that understand the new directive
will recognize it as modifying the requirements associated with the will recognize it as modifying the requirements associated with the
old directive. In this way, extensions to the existing cache-control old directive. In this way, extensions to the existing cache
directives can be made without breaking deployed caches. directives can be made without breaking deployed caches.
For example, consider a hypothetical new response directive called For example, consider a hypothetical new response directive called
"community" that acts as a modifier to the private directive: in "community" that acts as a modifier to the private directive: in
addition to private caches, any cache that is shared only by members addition to private caches, only a cache that is shared by members of
of the named community is allowed to cache the response. An origin the named community is allowed to cache the response. An origin
server wishing to allow the UCI community to use an otherwise private server wishing to allow the UCI community to use an otherwise private
response in their shared cache(s) could do so by including response in their shared cache(s) could do so by including
Cache-Control: private, community="UCI" Cache-Control: private, community="UCI"
A cache that recognizes such a community cache directive could A cache that recognizes such a community cache directive could
broaden its behavior in accordance with that extension. A cache that broaden its behavior in accordance with that extension. A cache that
does not recognize the community cache directive would ignore it and does not recognize the community cache directive would ignore it and
adhere to the private directive. adhere to the private directive.
New extension directives ought to consider defining: New extension directives ought to consider defining:
o What it means for a directive to be specified multiple times, 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, and
o Whether the directive is specific to requests, responses, or able o Whether the directive is specific to requests, specific to
to be used in either. responses, or able to be used in either.
5.2.4. Cache Directive Registry 5.2.4. Cache Directive Registry
The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry" The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry"
defines the namespace for the cache directives. It has been created defines the namespace for the cache directives. It has been created
and is now maintained at <https://www.iana.org/assignments/http- and is now maintained at <https://www.iana.org/assignments/http-
cache-directives>. cache-directives>.
A registration MUST include the following fields: A registration MUST include the following fields:
skipping to change at page 34, line 43 skipping to change at page 34, line 43
cached data in other ways beyond its freshness lifetime. cached data in other ways beyond its freshness lifetime.
This specification does not prohibit the application from taking HTTP This specification does not prohibit the application from taking HTTP
caching into account; for example, a history mechanism might tell the 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., user that a view is stale, or it might honor cache directives (e.g.,
Cache-Control: no-store). Cache-Control: no-store).
However, when an application caches data and does not make this However, when an application caches data and does not make this
apparent to or easily controllable by the user, it is strongly apparent to or easily controllable by the user, it is strongly
encouraged to define its operation with respect to HTTP cache encouraged to define its operation with respect to HTTP cache
directives, so as not to surprise authors who expect caching directives so as not to surprise authors who expect caching semantics
semantics to be honoured. For example, while it might be reasonable to be honored. For example, while it might be reasonable to define
to define an application cache "above" HTTP that allows a response an application cache "above" HTTP that allows a response containing
containing Cache-Control: no-store to be reused for requests that are Cache-Control: no-store to be reused for requests that are directly
directly related to the request that fetched it (such as those related to the request that fetched it (such as those created during
created during the same page load), it would likely be surprising and the same page load), it would likely be surprising and confusing to
confusing to users and authors if it were allowed to be reused for users and authors if it were allowed to be reused for requests
requests unrelated in any way to the one from which it was obtained. unrelated in any way to the one from which it was obtained.
7. Security Considerations 7. Security Considerations
This section is meant to inform developers, information providers, This section is meant to inform developers, information providers,
and users of known security concerns specific to HTTP caching. More and users of known security concerns specific to HTTP caching. More
general security considerations are addressed in "HTTP/1.1" general security considerations are addressed in "HTTP/1.1"
(Section 11 of [HTTP/1.1]) and "HTTP Semantics" (Section 17 of (Section 11 of [HTTP/1.1]) and "HTTP Semantics" (Section 17 of
[HTTP]). [HTTP]).
Caches expose an additional attack surface, since the contents of the Caches expose an additional attack surface because the contents of
cache represent an attractive target for malicious exploitation. the cache represent an attractive target for malicious exploitation.
Because cache contents persist after an HTTP request is complete, an Since cache contents persist after an HTTP request is complete, an
attack on the cache can reveal information long after a user believes attack on the cache can reveal information long after a user believes
that the information has been removed from the network. Therefore, that the information has been removed from the network. Therefore,
cache contents need to be protected as sensitive information. cache contents need to be protected as sensitive information.
In particular, because private caches are restricted to a single In particular, because private caches are restricted to a single
user, they can be used to reconstruct a user's activity. As a user, they can be used to reconstruct a user's activity. As a
result, it is important for user agents to allow end users to control result, it is important for user agents to allow end users to control
them; for example, allowing stored responses to be removed for some them, for example, by allowing stored responses to be removed for
or all origin servers. some or all origin servers.
7.1. Cache Poisoning 7.1. Cache Poisoning
Storing a malicious payload in a cache can extend the reach of an Storing malicious content in a cache can extend the reach of an
attacker to affect multiple users. Such "cache poisoning" attacks attacker to affect multiple users. Such "cache poisoning" attacks
happen when an attacker uses implementation flaws, elevated happen when an attacker uses implementation flaws, elevated
privileges, or other techniques to insert a response into a cache. privileges, or other techniques to insert a response into a cache.
This is especially effective when shared caches are used to This is especially effective when shared caches are used to
distribute malicious content to many clients. distribute malicious content to many clients.
One common attack vector for cache poisoning is to exploit One common attack vector for cache poisoning is to exploit
differences in message parsing on proxies and in user agents; see differences in message parsing on proxies and in user agents; see
Section 6.3 of [HTTP/1.1] for the relevant requirements regarding Section 6.3 of [HTTP/1.1] for the relevant requirements regarding
HTTP/1.1. HTTP/1.1.
7.2. Timing Attacks 7.2. Timing Attacks
Because one of the primary uses of a cache is to optimise Because one of the primary uses of a cache is to optimize
performance, its use can "leak" information about what resources have performance, its use can "leak" information about which resources
been previously requested. have been previously requested.
For example, if a user visits a site and their browser caches some of 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 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 attempt to load responses it knows exist on the first site. If they
load quickly, it can be assumed that the user has visited that site, load quickly, it can be assumed that the user has visited that site,
or even a specific page on it. or even a specific page on it.
Such "timing attacks" can be mitigated by adding more information to 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 cache key, such as the identity of the referring site (to prevent
the attack described above). This is sometimes called "double the attack described above). This is sometimes called "double
keying." keying".
7.3. Caching of Sensitive Information 7.3. Caching of Sensitive Information
Implementation and deployment flaws (as well as misunderstanding of Implementation and deployment flaws (often led to by the
cache operation) might lead to caching of sensitive information misunderstanding of cache operation) might lead to the caching of
(e.g., authentication credentials) that is thought to be private, sensitive information (e.g., authentication credentials) that is
exposing it to unauthorized parties. thought to be private, exposing it to unauthorized parties.
Note that the Set-Cookie response header field [COOKIE] does not Note that the Set-Cookie response header field [COOKIE] does not
inhibit caching; a cacheable response with a Set-Cookie header field inhibit caching; a cacheable response with a Set-Cookie header field
can be (and often is) used to satisfy subsequent requests to caches. can be (and often is) used to satisfy subsequent requests to caches.
Servers who wish to control caching of these responses are encouraged Servers that wish to control caching of these responses are
to emit appropriate Cache-Control response header fields. encouraged to emit appropriate Cache-Control response header fields.
8. IANA Considerations 8. IANA Considerations
The change controller for the following registrations is: "IETF The change controller for the following registrations is: "IETF
(iesg@ietf.org) - Internet Engineering Task Force". (iesg@ietf.org) - Internet Engineering Task Force".
8.1. Field Name Registration 8.1. Field Name Registration
First, introduce the new "Hypertext Transfer Protocol (HTTP) Field IANA has updated the "Hypertext Transfer Protocol (HTTP) Field Name
Name Registry" at <https://www.iana.org/assignments/http-fields> as Registry" at <https://www.iana.org/assignments/http-fields>, as
described in Section 18.4 of [HTTP]. described in Section 18.4 of [HTTP], with the field names listed in
the table below:
Then, please update the registry with the field names listed in the
table below:
+---------------+-----------+------+----------+ +---------------+------------+---------+----------+
| Field Name | Status | Ref. | Comments | | Field Name | Status | Section | Comments |
+---------------+-----------+------+----------+ +---------------+------------+---------+----------+
| Age | standard | 5.1 | | | Age | permanent | 5.1 | |
| Cache-Control | standard | 5.2 | | | Cache-Control | permanent | 5.2 | |
| Expires | standard | 5.3 | | | Expires | permanent | 5.3 | |
| Pragma | standard | 5.4 | | | Pragma | deprecated | 5.4 | |
| Warning | obsoleted | 5.5 | | | Warning | obsoleted | 5.5 | |
+---------------+-----------+------+----------+ +---------------+------------+---------+----------+
Table 1 Table 1
8.2. Cache Directive Registration 8.2. Cache Directive Registration
Please update the "Hypertext Transfer Protocol (HTTP) Cache Directive IANA has updated the "Hypertext Transfer Protocol (HTTP) Cache
Registry" at <https://www.iana.org/assignments/http-cache-directives> Directive Registry" at <https://www.iana.org/assignments/http-cache-
with the registration procedure of Section 5.2.4 and the cache directives> with the registration procedure per Section 5.2.4 and the
directive names summarized in the table below. cache directive names summarized in the table below.
+------------------+----------------------------------+ +------------------+------------------+
| Cache Directive | Reference | | Cache Directive | Section |
+------------------+----------------------------------+ +------------------+------------------+
| max-age | Section 5.2.1.1, Section 5.2.2.1 | | max-age | 5.2.1.1, 5.2.2.1 |
| max-stale | Section 5.2.1.2 | | max-stale | 5.2.1.2 |
| min-fresh | Section 5.2.1.3 | | min-fresh | 5.2.1.3 |
| must-revalidate | Section 5.2.2.2 | | must-revalidate | 5.2.2.2 |
| must-understand | Section 5.2.2.3 | | must-understand | 5.2.2.3 |
| no-cache | Section 5.2.1.4, Section 5.2.2.4 | | no-cache | 5.2.1.4, 5.2.2.4 |
| no-store | Section 5.2.1.5, Section 5.2.2.5 | | no-store | 5.2.1.5, 5.2.2.5 |
| no-transform | Section 5.2.1.6, Section 5.2.2.6 | | no-transform | 5.2.1.6, 5.2.2.6 |
| only-if-cached | Section 5.2.1.7 | | only-if-cached | 5.2.1.7 |
| private | Section 5.2.2.7 | | private | 5.2.2.7 |
| proxy-revalidate | Section 5.2.2.8 | | proxy-revalidate | 5.2.2.8 |
| public | Section 5.2.2.9 | | public | 5.2.2.9 |
| s-maxage | Section 5.2.2.10 | | s-maxage | 5.2.2.10 |
+------------------+----------------------------------+ +------------------+------------------+
Table 2 Table 2
8.3. Warn Code Registry 8.3. Warn Code Registry
Please add a note to the "Hypertext Transfer Protocol (HTTP) Warn IANA has added the following note to the "Hypertext Transfer Protocol
Codes" registry at <https://www.iana.org/assignments/http-warn-codes> (HTTP) Warn Codes" registry at <https://www.iana.org/assignments/
to the effect that Warning is obsoleted. http-warn-codes> stating that "Warning" has been obsoleted:
| The Warning header field (and the warn codes that it uses) has
| been obsoleted for HTTP per [RFC9111].
9. References 9. References
9.1. Normative References 9.1. Normative References
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, [HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", Work in Progress, Internet-Draft, Ed., "HTTP Semantics", Work in Progress, Internet-Draft,
draft-ietf-httpbis-semantics-latest, September 2021, draft-ietf-httpbis-semantics-latest, May 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis- <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
semantics-latest>. semantics-latest>.
[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, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <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, Specifications: ABNF", STD 68, RFC 5234,
skipping to change at page 38, line 26 skipping to change at page 38, line 21
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References 9.2. Informative References
[COOKIE] Barth, A., "HTTP State Management Mechanism", RFC 6265, [COOKIE] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011, DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>. <https://www.rfc-editor.org/info/rfc6265>.
[HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, [HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1", Work in Progress, Internet-Draft, draft- Ed., "HTTP/1.1", Work in Progress, Internet-Draft, draft-
ietf-httpbis-messaging-latest, September 2021, ietf-httpbis-messaging-latest, May 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis- <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
messaging-latest>. messaging-latest>.
[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, Transfer Protocol -- HTTP/1.1", RFC 2616,
DOI 10.17487/RFC2616, June 1999, DOI 10.17487/RFC2616, June 1999,
<https://www.rfc-editor.org/info/rfc2616>. <https://www.rfc-editor.org/info/rfc2616>.
[RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale [RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale
Content", RFC 5861, DOI 10.17487/RFC5861, April 2010, Content", RFC 5861, DOI 10.17487/RFC5861, May 2010,
<https://www.rfc-editor.org/info/rfc5861>. <https://www.rfc-editor.org/info/rfc5861>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. F. Reschke, [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP): Caching", Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014, RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>. <https://www.rfc-editor.org/info/rfc7234>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26, Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017, RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>. <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 In the collected ABNF below, list rules are expanded per
Section 5.6.1 of [HTTP]. Section 5.6.1 of [HTTP].
Age = delta-seconds Age = delta-seconds
Cache-Control = [ cache-directive *( OWS "," OWS cache-directive ) ] Cache-Control = [ cache-directive *( OWS "," OWS cache-directive ) ]
Expires = HTTP-date Expires = HTTP-date
HTTP-date = <HTTP-date, see [HTTP], Section 5.6.7> HTTP-date = <HTTP-date, see [HTTP], Section 5.6.7>
skipping to change at page 39, line 36 skipping to change at page 39, line 31
quoted-string = <quoted-string, see [HTTP], Section 5.6.4> quoted-string = <quoted-string, see [HTTP], Section 5.6.4>
token = <token, see [HTTP], Section 5.6.2> token = <token, see [HTTP], Section 5.6.2>
Appendix B. Changes from RFC 7234 Appendix B. Changes from RFC 7234
Handling of duplicate and conflicting cache directives has been Handling of duplicate and conflicting cache directives has been
clarified. (Section 4.2.1) clarified. (Section 4.2.1)
Cache invalidation of the URIs in the Location and Content-Location Cache invalidation of the URIs in the Location and Content-Location
header fields is no longer required, but still allowed. header fields is no longer required but is still allowed.
(Section 4.4) (Section 4.4)
Cache invalidation of the URIs in the Location and Content-Location Cache invalidation of the URIs in the Location and Content-Location
header fields is disallowed when the origin is different; previously, header fields is disallowed when the origin is different; previously,
it was the host. (Section 4.4) it was the host. (Section 4.4)
Handling invalid and multiple Age header field values has been Handling invalid and multiple Age header field values has been
clarified. (Section 5.1) clarified. (Section 5.1)
Some cache directives defined by this specification now have stronger Some cache directives defined by this specification now have stronger
prohibitions against generating the quoted form of their values, prohibitions against generating the quoted form of their values,
since this has been found to create interoperability problems. since this has been found to create interoperability problems.
Consumers of extension cache directives are no longer required to Consumers of extension cache directives are no longer required to
accept both token and quoted-string forms, but they still need to accept both token and quoted-string forms, but they still need to
parse them properly for unknown extensions. (Section 5.2) parse them properly for unknown extensions. (Section 5.2)
The "public" and "private" cache directives were clarified, so that
they do not make responses reusable under any condition.
(Section 5.2.2)
The "must-understand" cache directive was introduced; caches are no The public and private cache directives were clarified, so that they
do not make responses reusable under any condition. (Section 5.2.2)
The must-understand cache directive was introduced; caches are no
longer required to understand the semantics of new response status longer required to understand the semantics of new response status
codes unless it is present. (Section 5.2.2.3) codes unless it is present. (Section 5.2.2.3)
The Warning response header was obsoleted. Much of the information The Warning response header was obsoleted. Much of the information
supported by Warning could be gleaned by examining the response, and supported by Warning could be gleaned by examining the response, and
the remaining warn-codes -- although potentially useful -- were the remaining information -- although potentially useful -- was
entirely advisory. In practice, Warning was not added by caches or entirely advisory. In practice, Warning was not added by caches or
intermediaries. (Section 5.5) intermediaries. (Section 5.5)
Appendix C. Change Log Appendix C. Change Log
This section is to be removed before publishing as an RFC. This section is to be removed before publishing as an RFC.
C.1. Between RFC7234 and draft 00 See <https://www.ietf.org/archive/id/draft-ietf-httpbis-cache-
19.html#appendix-C> for changes up to version 19 of this document.
The changes were purely editorial:
o Change boilerplate and abstract to indicate the "draft" status,
and update references to ancestor specifications.
o Remove version "1.1" from document title, indicating that this
specification applies to all HTTP versions.
o Adjust historical notes.
o Update links to sibling specifications.
o Replace sections listing changes from RFC 2616 by new empty
sections referring to RFC 723x.
o Remove acknowledgements specific to RFC 723x.
o Move "Acknowledgements" to the very end and make them unnumbered.
C.2. Since draft-ietf-httpbis-cache-00
The changes are purely editorial:
o Moved all extensibility tips, registration procedures, and
registry tables from the IANA considerations to normative
sections, reducing the IANA considerations to just instructions
that will be removed prior to publication as an RFC.
C.3. Since draft-ietf-httpbis-cache-01
o Cite RFC 8126 instead of RFC 5226 (<https://github.com/httpwg/
http-core/issues/75>)
o In Section 5.4, misleading statement about the relation between
Pragma and Cache-Control (<https://github.com/httpwg/http-core/
issues/92>, <https://www.rfc-editor.org/errata/eid4674>)
C.4. Since draft-ietf-httpbis-cache-02
o In Section 3, explain that only final responses are cacheable
(<https://github.com/httpwg/http-core/issues/29>)
o In Section 5.2.2, clarify what responses various directives apply
to (<https://github.com/httpwg/http-core/issues/52>)
o In Section 4.3.1, clarify the source of validators in conditional
requests (<https://github.com/httpwg/http-core/issues/110>)
o Revise Section 6 to apply to more than just History Lists
(<https://github.com/httpwg/http-core/issues/126>)
o In Section 5.5, deprecated "Warning" header field
(<https://github.com/httpwg/http-core/issues/139>)
o In Section 3.5, remove a spurious note
(<https://github.com/httpwg/http-core/issues/141>)
C.5. Since draft-ietf-httpbis-cache-03
o In Section 2, define what a disconnected cache is
(<https://github.com/httpwg/http-core/issues/5>)
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>)
o in Section 4.2.4, mention stale-while-revalidate and stale-if-
error (<https://github.com/httpwg/http-core/issues/122>)
o Remove requirements around cache request directives
(<https://github.com/httpwg/http-core/issues/129>)
o Deprecate Pragma (<https://github.com/httpwg/http-core/
issues/140>)
o In Section 3.5 and Section 5.2.2, note effect of some directives
on authenticated requests (<https://github.com/httpwg/http-core/
issues/161>)
C.6. Since draft-ietf-httpbis-cache-04
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>)
C.7. Since draft-ietf-httpbis-cache-05
o In Section 3.3, clarify how weakly framed content is considered
for purposes of completeness (<https://github.com/httpwg/http-
core/issues/25>)
o Throughout, describe Vary and cache key operations more clearly
(<https://github.com/httpwg/http-core/issues/28>)
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>)
o Refactored Section 7, and added a section on timing attacks
(<https://github.com/httpwg/http-core/issues/233>)
o Changed "cacheable by default" to "heuristically cacheable"
throughout (<https://github.com/httpwg/http-core/issues/242>)
C.8. Since draft-ietf-httpbis-cache-06
o In Section 3 and Section 5.2.2.3, change response cacheability to
only require understanding the response status code if the must-
understand cache directive is present (<https://github.com/httpwg/
http-core/issues/120>)
o Change requirements for handling different forms of cache
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 5.2.2.9 and Section 5.2.2.7, clarify "private" and
"public" so that they do not override all other cache directives
(<https://github.com/httpwg/http-core/issues/268>)
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 5.2.2.9 and Section 5.2.2.7, make it clear that these
directives do not ignore other requirements for caching
(<https://github.com/httpwg/http-core/issues/320>)
o In Section 3.3, move definition of "complete" into semantics
(<https://github.com/httpwg/http-core/issues/334>)
C.10. Since draft-ietf-httpbis-cache-08
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.
C.14. Since draft-ietf-httpbis-cache-12
o In Section 4.2.4, remove 'no-store', as it won't be in cache in
the first place (<https://github.com/httpwg/http-core/issues/447>,
<https://www.rfc-editor.org/errata/eid6279>)
o In Section 3.1, make it clear that only response headers need be
stored (<https://github.com/httpwg/http-core/issues/457>)
o Rewrote "Updating Stored Header Fields" Section 3.2
(<https://github.com/httpwg/http-core/issues/458>)
o In Section 4.2.1 clarify how to handle invalid and conflicting
directives (<https://github.com/httpwg/http-core/issues/460>)
o In Section 4.3.3, mention retry of failed validation requests
(<https://github.com/httpwg/http-core/issues/462>)
o In Section 4.3.3, clarify requirement on storing a full response
to a conditional request (<https://github.com/httpwg/http-core/
issues/463>)
o In Section 5.1, clarify error handling
(<https://github.com/httpwg/http-core/issues/471>)
o In Section 4.2, remove spurious "UTC" (<https://github.com/httpwg/
http-core/issues/472>)
o In Section 4.2, correct the date-related rule names to consider
case-insensitive (<https://github.com/httpwg/http-core/
issues/473>)
o In Section 6, strengthen recommendation for application caches to
pay attention to cache directives (<https://github.com/httpwg/
http-core/issues/474>)
o In Section 4, mention collapsed requests
(<https://github.com/httpwg/http-core/issues/475>)
o In Section 4.4, relax requirements on Content-Location and
Location invalidation (<https://github.com/httpwg/http-core/
issues/478>)
o In Section 4.3.4, refine the exceptions to update on a 304
(<https://github.com/httpwg/http-core/issues/488>)
o Moved table of Cache-Control directives into Section 8.2
(<https://github.com/httpwg/http-core/issues/506>)
o In Section 1.2, remove unused core ABNF rules
(<https://github.com/httpwg/http-core/issues/529>)
o Changed to using "payload data" when defining requirements about
the data being conveyed within a message, instead of the terms
"payload body" or "response body" or "representation body", since
they often get confused with the HTTP/1.1 message body (which
includes transfer coding) (<https://github.com/httpwg/http-core/
issues/553>)
C.15. Since draft-ietf-httpbis-cache-13
o In Section 5.2.2.2, clarify requirements around generating an
error response (<https://github.com/httpwg/http-core/issues/608>)
o Changed to using "content" instead of "payload" or "payload data"
to avoid confusion with the payload of version-specific messaging
frames (<https://github.com/httpwg/http-core/issues/654>)
o In Section 4.3.4, clarify how multiple validators are handled
(<https://github.com/httpwg/http-core/issues/659>)
o In Section 4.2.3, Section 5.2, and Section 5.2.2.4, remove notes
about very old HTTP/1.0 behaviours (<https://github.com/httpwg/
http-core/issues/660>)
o In Section 5.2.2.3, modify operation to be more backwards-
compatible with existing implementations
(<https://github.com/httpwg/http-core/issues/661>)
C.16. Since draft-ietf-httpbis-cache-14
o Fix subsection ordering in Section 5.2.2
(<https://github.com/httpwg/http-core/issues/674>)
o In Section 2, define what a cache key is
(<https://github.com/httpwg/http-core/issues/728>)
o In Section 3.1, clarify what cache proxy headers apply to
(<https://github.com/httpwg/http-core/issues/729>)
o In Section 7.1, cache poisoning can affect private caches too
(<https://github.com/httpwg/http-core/issues/730>)
o In Section 5.1, adjust handling of invalid values to match most
deployed caches (<https://github.com/httpwg/http-core/issues/778>)
o In Section 5.3, mention parsing requirement relaxation
(<https://github.com/httpwg/http-core/issues/779>)
C.17. Since draft-ietf-httpbis-cache-15
o In Section 4.3.1, tune description of relation between cache keys
and validators (<https://github.com/httpwg/http-core/issues/832>)
C.18. Since draft-ietf-httpbis-cache-16
This draft addresses mostly editorial issues raised during or past
IETF Last Call; see <https://github.com/httpwg/http-core/
issues?q=label%3Acaching+created%3A%3E2021-05-26> for a summary.
Furthermore:
o Addressed Genart last call review comments
(<https://github.com/httpwg/http-core/issues/847>)
o In Section 4.3.4, clarify that only selectable responses are
updated (<https://github.com/httpwg/http-core/issues/839>)
C.19. Since draft-ietf-httpbis-cache-17
o Made reference to [HTTP/1.1] informative only
(<https://github.com/httpwg/http-core/issues/911>)
o Move cache-related aspects of validator use from [HTTP] into
Section 4.3.1 (<https://github.com/httpwg/http-core/issues/933>)
o Use term "clock" defined in Section 6.6.1 of [HTTP] throughout
(<https://github.com/httpwg/http-core/issues/953>)
o Throughout, disambiguate "selected representation" and "selected C.1. Since draft-ietf-httpbis-cache-19
response" (now "chosen response") (<https://github.com/httpwg/
http-core/issues/958>)
C.20. Since draft-ietf-httpbis-cache-18 This (unpublished) draft contains changes that were made after draft
19 was approved by the IESG. Most changes are editorial only.
o None. o None.
Acknowledgements Acknowledgements
See Appendix "Acknowledgements" of [HTTP]. See Appendix "Acknowledgements" of [HTTP], which applies to this
document as well.
Index Index
A C E F G H M N O P S V W A C E F G H M N O P S V W
A A
age Section 4.2 age Section 4.2
Age header field *_Section 5.1_* Age header field *_Section 5.1_*
skipping to change at page 49, line 17 skipping to change at page 42, line 46
Roy T. Fielding (editor) Roy T. Fielding (editor)
Adobe Adobe
345 Park Ave 345 Park Ave
San Jose, CA 95110 San Jose, CA 95110
United States of America United States of America
Email: fielding@gbiv.com Email: fielding@gbiv.com
URI: https://roy.gbiv.com/ URI: https://roy.gbiv.com/
Mark Nottingham (editor) Mark Nottingham (editor)
Fastly Fastly
Prahran VIC Prahran
Australia Australia
Email: mnot@mnot.net Email: mnot@mnot.net
URI: https://www.mnot.net/ URI: https://www.mnot.net/
Julian Reschke (editor) Julian Reschke (editor)
greenbytes GmbH greenbytes GmbH
Hafenweg 16 Hafenweg 16
48155 Münster 48155 Münster
Germany Germany
Email: julian.reschke@greenbytes.de Email: julian.reschke@greenbytes.de
URI: https://greenbytes.de/tech/webdav/ URI: https://greenbytes.de/tech/webdav/
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