draft-ietf-httpbis-cache-16.txt   draft-ietf-httpbis-cache-17.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: 28 November 2021 J. Reschke, Ed. Expires: 27 January 2022 J. Reschke, Ed.
greenbytes greenbytes
27 May 2021 26 July 2021
HTTP Caching HTTP Caching
draft-ietf-httpbis-cache-16 draft-ietf-httpbis-cache-17
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is a stateless application- The Hypertext Transfer Protocol (HTTP) is a stateless application-
level protocol for distributed, collaborative, hypertext information level protocol for distributed, collaborative, hypertext information
systems. This document defines HTTP caches and the associated header systems. This document defines HTTP caches and the associated header
fields that control cache behavior or indicate cacheable response fields that control cache behavior or indicate cacheable response
messages. messages.
This document obsoletes RFC 7234. This document obsoletes RFC 7234.
skipping to change at page 1, line 36 skipping to change at page 1, line 36
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.17. The changes in this draft are summarized in Appendix C.18.
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-
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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 28 November 2021. This Internet-Draft will expire on 27 January 2022.
Copyright Notice Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2021 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
Provisions Relating to IETF Documents (https://trustee.ietf.org/ Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document. license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
skipping to change at page 2, line 43 skipping to change at page 2, line 43
outside the IETF Standards Process, and derivative works of it may outside the IETF Standards Process, and derivative works of it may
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
1.3. Delta Seconds . . . . . . . . . . . . . . . . . . . . . . 6 1.2.1. Imported Rules . . . . . . . . . . . . . . . . . . . 5
1.2.2. Delta Seconds . . . . . . . . . . . . . . . . . . . . 6
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 . . . . . . . . . . . . . . . . 11
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 Vary . . . . . . . . . . . . 12 4.1. Calculating Cache Keys with the Vary Header Field . . . . 13
4.2. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 14 4.2. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 14
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 . . . . . . . . . . . . . . . . . . . 17
4.2.4. Serving Stale Responses . . . . . . . . . . . . . . . 18 4.2.4. Serving Stale Responses . . . . . . . . . . . . . . . 18
4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . 18 4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . 19
4.3.1. Sending a Validation Request . . . . . . . . . . . . 19 4.3.1. Sending a Validation Request . . . . . . . . . . . . 19
4.3.2. Handling a Received Validation Request . . . . . . . 19 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 . . . . . 21 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 . . . . . . . . . . . . . . 22 4.4. Invalidating Stored Responses . . . . . . . . . . . . . . 23
5. Field Definitions . . . . . . . . . . . . . . . . . . . . . . 23 5. Field Definitions . . . . . . . . . . . . . . . . . . . . . . 24
5.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 24 5.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 24
5.2.1. Request Cache-Control Directives . . . . . . . . . . 24 5.2.1. Request Cache-Control Directives . . . . . . . . . . 25
5.2.1.1. max-age . . . . . . . . . . . . . . . . . . . . . 24 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 . . . . . . . . . . . . . . . . . . . . 25 5.2.1.3. min-fresh . . . . . . . . . . . . . . . . . . . . 26
5.2.1.4. no-cache . . . . . . . . . . . . . . . . . . . . 25 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 . . . . . . . . . . . . . . . . . . 26 5.2.1.6. no-transform . . . . . . . . . . . . . . . . . . 27
5.2.1.7. only-if-cached . . . . . . . . . . . . . . . . . 26 5.2.1.7. only-if-cached . . . . . . . . . . . . . . . . . 27
5.2.2. Response Cache-Control Directives . . . . . . . . . . 26 5.2.2. Response Cache-Control Directives . . . . . . . . . . 27
5.2.2.1. max-age . . . . . . . . . . . . . . . . . . . . . 26 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 . . . . . . . . . . . . . . . . . 27 5.2.2.3. must-understand . . . . . . . . . . . . . . . . . 28
5.2.2.4. no-cache . . . . . . . . . . . . . . . . . . . . 27 5.2.2.4. no-cache . . . . . . . . . . . . . . . . . . . . 28
5.2.2.5. no-store . . . . . . . . . . . . . . . . . . . . 28 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 . . . . . . . . . . . . . . . . . . . . 30 5.2.2.10. s-maxage . . . . . . . . . . . . . . . . . . . . 31
5.2.3. Cache Control Extensions . . . . . . . . . . . . . . 31 5.2.3. Cache Control Extensions . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . . . 33 5.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 34
6. Relationship to Applications and Other Caches . . . . . . . . 33 6. Relationship to Applications and Other Caches . . . . . . . . 34
7. Security Considerations . . . . . . . . . . . . . . . . . . . 34 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 . . . . . . . . . . . . 35 7.3. Caching of Sensitive Information . . . . . . . . . . . . 36
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
8.1. Field Name Registration . . . . . . . . . . . . . . . . . 36 8.1. Field Name Registration . . . . . . . . . . . . . . . . . 36
8.2. Cache Directive Registration . . . . . . . . . . . . . . 36 8.2. Cache Directive Registration . . . . . . . . . . . . . . 37
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 . . . . . . . . . . . . . . . . . . . 39
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. Between RFC7234 and draft 00 . . . . . . . . . . . . . . 40
C.2. Since draft-ietf-httpbis-cache-00 . . . . . . . . . . . . 41 C.2. Since draft-ietf-httpbis-cache-00 . . . . . . . . . . . . 41
C.3. Since draft-ietf-httpbis-cache-01 . . . . . . . . . . . . 41 C.3. Since draft-ietf-httpbis-cache-01 . . . . . . . . . . . . 41
C.4. Since draft-ietf-httpbis-cache-02 . . . . . . . . . . . . 41 C.4. Since draft-ietf-httpbis-cache-02 . . . . . . . . . . . . 41
C.5. Since draft-ietf-httpbis-cache-03 . . . . . . . . . . . . 41 C.5. Since draft-ietf-httpbis-cache-03 . . . . . . . . . . . . 41
C.6. Since draft-ietf-httpbis-cache-04 . . . . . . . . . . . . 42 C.6. Since draft-ietf-httpbis-cache-04 . . . . . . . . . . . . 42
C.7. Since draft-ietf-httpbis-cache-05 . . . . . . . . . . . . 42 C.7. Since draft-ietf-httpbis-cache-05 . . . . . . . . . . . . 42
C.8. Since draft-ietf-httpbis-cache-06 . . . . . . . . . . . . 42 C.8. Since draft-ietf-httpbis-cache-06 . . . . . . . . . . . . 42
C.9. Since draft-ietf-httpbis-cache-07 . . . . . . . . . . . . 43 C.9. Since draft-ietf-httpbis-cache-07 . . . . . . . . . . . . 43
C.10. Since draft-ietf-httpbis-cache-08 . . . . . . . . . . . . 43 C.10. Since draft-ietf-httpbis-cache-08 . . . . . . . . . . . . 43
C.11. Since draft-ietf-httpbis-cache-09 . . . . . . . . . . . . 43 C.11. Since draft-ietf-httpbis-cache-09 . . . . . . . . . . . . 43
C.12. Since draft-ietf-httpbis-cache-10 . . . . . . . . . . . . 43 C.12. Since draft-ietf-httpbis-cache-10 . . . . . . . . . . . . 44
C.13. Since draft-ietf-httpbis-cache-11 . . . . . . . . . . . . 44 C.13. Since draft-ietf-httpbis-cache-11 . . . . . . . . . . . . 44
C.14. Since draft-ietf-httpbis-cache-12 . . . . . . . . . . . . 44 C.14. Since draft-ietf-httpbis-cache-12 . . . . . . . . . . . . 44
C.15. Since draft-ietf-httpbis-cache-13 . . . . . . . . . . . . 45 C.15. Since draft-ietf-httpbis-cache-13 . . . . . . . . . . . . 45
C.16. Since draft-ietf-httpbis-cache-14 . . . . . . . . . . . . 45 C.16. Since draft-ietf-httpbis-cache-14 . . . . . . . . . . . . 45
C.17. Since draft-ietf-httpbis-cache-15 . . . . . . . . . . . . 46 C.17. Since draft-ietf-httpbis-cache-15 . . . . . . . . . . . . 46
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 46 C.18. Since draft-ietf-httpbis-cache-16 . . . . . . . . . . . . 46
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 46
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48
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. 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.
HTTP caching's goal is significantly improving performance by reusing The goal of HTTP caching is significantly improving performance by
a prior response message to satisfy a current request. A cache reusing a prior response message to satisfy a current request. A
considers a stored response "fresh", as defined in Section 4.2, if it cache considers a stored response "fresh", as defined in Section 4.2,
can be reused without "validation" (checking with the origin server if it can be reused without "validation" (checking with the origin
to see if the cached response remains valid for this request). A server to see if the cached response remains valid for this request).
fresh response can therefore reduce both latency and network overhead A fresh response can therefore reduce both latency and network
each time the cache reuses it. When a cached response is not fresh, overhead each time the cache reuses it. When a cached response is
it might still be reusable if validation can freshen it (Section 4.3) not fresh, it might still be reusable if validation can freshen it
or if the origin is unavailable (Section 4.2.4). (Section 4.3) or if the origin is unavailable (Section 4.2.4).
This document obsoletes RFC 7234, with the changes being summarized This document obsoletes RFC 7234, with the changes being summarized
in Appendix B. in Appendix B.
1.1. Requirements Notation 1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
Section 2 of [Semantics] defines conformance criteria and contains Section 2 of [HTTP] defines conformance criteria and contains
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 It also uses a list extension, defined in Section 5.6.1 of [HTTP],
[Semantics], that allows for compact definition of comma-separated that allows for compact definition of comma-separated lists using a
lists using a '#' operator (similar to how the '*' operator indicates '#' operator (similar to how the '*' operator indicates repetition).
repetition). Appendix A shows the collected grammar with all list Appendix A shows the collected grammar with all list operators
operators expanded to standard ABNF notation. expanded to standard ABNF notation.
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).
[Semantics] defines the following rules: [HTTP] defines the following rules:
HTTP-date = <HTTP-date, see [Semantics], Section 5.6.7> HTTP-date = <HTTP-date, see [HTTP], Section 5.6.7>
OWS = <OWS, see [Semantics], Section 5.6.3> OWS = <OWS, see [HTTP], Section 5.6.3>
field-name = <field-name, see [Semantics], Section 5.1> field-name = <field-name, see [HTTP], Section 5.1>
quoted-string = <quoted-string, see [Semantics], Section 5.6.4> quoted-string = <quoted-string, see [HTTP], Section 5.6.4>
token = <token, see [Semantics], Section 5.6.2> token = <token, see [HTTP], Section 5.6.2>
1.3. Delta Seconds 1.2.2. Delta Seconds
The delta-seconds rule specifies a non-negative integer, representing The delta-seconds rule specifies a non-negative integer, representing
time in seconds. time in seconds.
delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
A recipient parsing a delta-seconds value and converting it to binary A recipient parsing a delta-seconds value and converting it to binary
form ought to use an arithmetic type of at least 31 bits of non- form ought to use an arithmetic type of at least 31 bits of non-
negative integer range. If a cache receives a delta-seconds value negative integer range. If a cache receives a delta-seconds value
greater than the greatest integer it can represent, or if any of its greater than the greatest integer it can represent, or if any of its
subsequent calculations overflows, the cache MUST consider the value subsequent calculations overflows, the cache MUST consider the value
to be 2147483648 (2^31) or the greatest positive integer it can to be 2147483648 (2^31) or the greatest positive integer it can
conveniently represent. conveniently represent.
| *Note:* The value 2147483648 is here for historical reasons, | *Note:* The value 2147483648 is here for historical reasons,
| represents infinity (over 68 years), and does not need to be | represents infinity (over 68 years), and does not need to be
| stored in binary form; an implementation could produce it as a | stored in binary form; an implementation could produce it as a
| canned string if any overflow occurs, even if the calculations | string if any overflow occurs, even if the calculations are
| are performed with an arithmetic type incapable of directly | performed with an arithmetic type incapable of directly
| representing that number. What matters here is that an | representing that number. What matters here is that an
| overflow be detected and not treated as a negative value in | overflow be detected and not treated as a negative value in
| later calculations. | later calculations.
2. Overview of Cache Operation 2. Overview of Cache Operation
Proper cache operation preserves the semantics of HTTP transfers Proper cache operation preserves the semantics of HTTP transfers
([Semantics]) while reducing the transmission of information already while reducing the transmission of information already held in the
held in the cache. Although caching is an entirely OPTIONAL feature cache. See Section 3 of [HTTP] for the general terminology and core
of HTTP, it can be assumed that reusing a cached response is concepts of HTTP.
desirable and that such reuse is the default behavior when no
requirement or local configuration prevents it. Therefore, HTTP Although caching is an entirely OPTIONAL feature of HTTP, it can be
cache requirements are focused on preventing a cache from either assumed that reusing a cached response is desirable and that such
storing a non-reusable response or reusing a stored response reuse is the default behavior when no requirement or local
inappropriately, rather than mandating that caches always store and configuration prevents it. Therefore, HTTP cache requirements are
reuse particular responses. focused on preventing a cache from either storing a non-reusable
response or reusing a stored response inappropriately, rather than
mandating that caches always store and reuse particular responses.
The _cache key_ is the information a cache uses to select a response The _cache key_ is the information a cache uses to select a response
and is comprised of, 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. forwarding other methods.
If a request target is subject to content negotiation, the cache If a request target is subject to content negotiation, the cache
might store multiple responses for it. Caches differentiate these might store multiple responses for it. Caches differentiate these
responses by incorporating values of the original request's selecting responses by incorporating values of the original request's selecting
header fields into the cache key as well, using information in the header fields into the cache key as well, using information in the
Vary response header field, as per Section 4.1. Vary response header 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
risks (see Section 7.2). risks (see Section 7.2).
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 [Semantics]). 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:
* the request method is understood by the cache; * the request method is understood by the cache;
* the response status code is final (see Section 15 of [Semantics]); * the response status code is final (see Section 15 of [HTTP]);
* if the response status code is 206 or 304, or the "must- * if the response status code is 206 or 304, or the "must-
understand" cache directive (see Section 5.2.2.3) is present: the understand" cache directive (see Section 5.2.2.3) is present: the
cache understands the response status code; cache understands the response status code;
* the "no-store" cache directive is not present in the response (see * the "no-store" cache directive is not present in the response (see
Section 5.2.2.5); Section 5.2.2.5);
* if the cache is shared: the "private" response directive is either * 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);
* if the cache is shared: the Authorization header field is not * if the cache is shared: the Authorization header field is not
present in the request (see Section 11.6.2 of [Semantics]) 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,
* the response contains at least one of: * the response contains at least one of:
- 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);
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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 unrecognised 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:
* The Connection header field and fields whose names are listed in * The Connection header field and fields whose names are listed in
it are required by Section 7.6.1 of [Semantics] to be removed it are required by Section 7.6.1 of [HTTP] to be removed before
before forwarding the message. This MAY be implemented by doing forwarding the message. This MAY be implemented by doing so
so before storage. before storage.
* Likewise, some fields' semantics require them to be removed before * 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
before storage; see Section 7.6.1 of [Semantics] for some before storage; see Section 7.6.1 of [HTTP] for some examples.
examples.
* The no-cache (Section 5.2.2.4) and private (Section 5.2.2.7) cache * 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.
* Header fields that are specific to the proxy that a cache uses * 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 [Semantics]), Proxy-Authentication-Info (Section 11.7.3 of of [HTTP]), Proxy-Authentication-Info (Section 11.7.3 of [HTTP]),
[Semantics]), and Proxy-Authorization (Section 11.7.2 of and Proxy-Authorization (Section 11.7.2 of [HTTP]).
[Semantics]).
Caches MAY either store trailer fields separate from header fields, Caches MAY either store trailer fields separate from header fields,
or discard them. Caches MUST NOT combine trailer fields with header or 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 Section 3.4, Section 4.3.4 and Section 4.3.5.
skipping to change at page 10, line 36 skipping to change at page 10, line 28
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 store a response body that is not complete (Section 3.4 of [HTTP]) if
[Semantics]) if the stored response is recorded as being incomplete. the stored response is recorded as being incomplete. Likewise, a 206
Likewise, a 206 (Partial Content) response MAY be stored as if it (Partial Content) response MAY be stored as if it were an incomplete
were an incomplete 200 (OK) response. However, a cache MUST NOT 200 (OK) response. However, a cache MUST NOT store incomplete or
store incomplete or partial-content responses if it does not support partial-content responses if it does not support the Range and
the Range and Content-Range header fields or if it does not Content-Range header fields or if it does not understand the range
understand the range units used in those fields. 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 [Semantics]) and combining subsequent range request (Section 14.2 of [HTTP]) and combining the
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.
3.4. Combining Partial Content 3.4. Combining Partial Content
A response might transfer only a partial representation if the A response might transfer only a partial representation if the
connection closed prematurely or if the request used one or more connection closed prematurely or if the request used one or more
Range specifiers (Section 14.2 of [Semantics]). After several such Range specifiers (Section 14.2 of [HTTP]). After several such
transfers, a cache might have received several ranges of the same transfers, a cache might have received several ranges of the same
representation. A cache MAY combine these ranges into a single representation. A cache MAY combine these ranges into a single
stored response, and reuse that response to satisfy later requests, stored response, and reuse that response to satisfy later requests,
if they all share the same strong validator and the cache complies if they all share the same strong validator and the cache complies
with the client requirements in Section 15.3.7.3 of [Semantics]. with the client requirements in Section 15.3.7.3 of [HTTP].
When combining the new response with one or more stored responses, a When combining the new response with one or more stored responses, a
cache MUST 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 [Semantics]) to satisfy Authorization header field (Section 11.6.2 of [HTTP]) to satisfy any
any subsequent request unless the response contains a Cache-Control subsequent request unless the response contains a Cache-Control field
field with a response directive (Section 5.2.2) that allows it to be with a response directive (Section 5.2.2) that allows it to be stored
stored by a shared cache and the cache conforms to the requirements by a shared cache and the cache conforms to the requirements of that
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:
* The presented target URI (Section 7.1 of [Semantics]) and that of * The presented target URI (Section 7.1 of [HTTP]) and that of the
the stored response match, and stored response match, and
* the request method associated with the stored response allows it * the request method associated with the stored response allows it
to be used for the presented request, and to be used for the presented request, and
* selecting header fields nominated by the stored response (if any) * selecting header fields nominated by the stored response (if any)
match those presented (see Section 4.1), and match those presented (see Section 4.1), and
* the stored response does not contain the no-cache cache directive * the stored response does not contain the no-cache 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
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Note that a cache-control extension can override any of the Note that a cache-control extension can override any of the
requirements listed; see Section 5.2.3. requirements listed; see Section 5.2.3.
When a stored response is used to satisfy a request without When a stored response is used to satisfy a request without
validation, a cache MUST generate an Age header field (Section 5.1), validation, a cache MUST generate an Age header field (Section 5.1),
replacing any present in the response with a value equal to the replacing any present in the response with a value equal to the
stored response's current_age; see Section 4.2.3. stored response's current_age; see Section 4.2.3.
A cache MUST write through requests with methods that are unsafe A cache MUST write through requests with methods that are unsafe
(Section 9.2.1 of [Semantics]) to the origin server; i.e., a cache is (Section 9.2.1 of [HTTP]) to the origin server; i.e., a cache is not
not allowed to generate a reply to such a request before having allowed to generate a reply to such a request before having forwarded
forwarded the request and having received a corresponding response. the request and having received a corresponding response.
Also, note that unsafe requests might invalidate already-stored Also, note that unsafe requests might invalidate already-stored
responses; see Section 4.4. responses; see Section 4.4.
A response that is stored or storable can be used to satisfy multiple A response that is stored or storable can be used to satisfy multiple
requests, provided that it is allowed to reuse that response for the requests, provided that it is allowed to reuse that response for the
requests in question. This enables caches to _collapse requests_ - requests in question. This enables caches to _collapse requests_ -
or combine multiple incoming requests into a single forward one upon or combine multiple incoming requests into a single forward request
a cache miss - thereby reducing load on the origin server and upon a cache miss - thereby reducing load on the origin server and
network. However, note that if the response returned is not able to network. However, note that if the response returned is not able to
be used for some or all of the collapsed requests, additional latency be used for some or all of the collapsed requests, additional latency
might be introduced, because they will need to be forwarded to be might be introduced, because they will need to be forwarded to be
satisfied. satisfied.
When more than one suitable response is stored, a cache MUST use the When more than one suitable response is stored, a cache MUST use the
most recent 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 that does not have a clock available MUST NOT use stored A cache that does not have a clock available MUST NOT use stored
responses without revalidating them upon every use. responses without revalidating them upon every use.
4.1. Calculating Cache Keys with Vary 4.1. Calculating Cache Keys with the Vary Header Field
When a cache receives a request that can be satisfied by a stored When a cache receives a request that can be satisfied by a stored
response that has a Vary header field (Section 12.5.5 of response and that stored response contains a Vary header field
[Semantics]), it MUST NOT use that response unless all the selecting (Section 12.5.5 of [HTTP]), the cache MUST NOT use that stored
header fields nominated by the Vary header field match in both the response without revalidation unless all the selecting header fields
original request (i.e., that associated with the stored response), (nominated by that Vary field value) in the present request match
and the presented request. those fields in the original request (i.e., the request that caused
the cached response to be stored).
The selecting header fields from two requests are defined to match if The selecting header fields from two requests are defined to match if
and only if those in the first request can be transformed to those in and only if those in the first request can be transformed to those in
the second request by applying any of: the second request by applying any of:
* adding or removing whitespace, where allowed in the header field's * adding or removing whitespace, where allowed in the header field's
syntax syntax
* combining multiple header field lines with the same field name * combining multiple header field lines with the same field name
(see Section 5.2 of [Semantics]) (see Section 5.2 of [HTTP])
* normalizing both header field values in a way that is known to * normalizing both header field values in a way that is known to
have identical semantics, according to the header field's have identical semantics, according to the header field's
specification (e.g., reordering field values when order is not specification (e.g., reordering field values when order is not
significant; case-normalization, where values are defined to be significant; case-normalization, where values are defined to be
case-insensitive) case-insensitive)
If (after any normalization that might take place) a header field is If (after any normalization that might take place) a header field is
absent from a request, it can only match another request if it is absent from a request, it can only match another request if it is
also absent there. also absent there.
A Vary header field value containing a member "*" always fails to A stored response with a Vary header field value containing a member
match. "*" always fails to match.
The stored response with matching selecting header fields is known as The stored response with matching selecting header fields is known as
the _selected response_. the _selected response_.
If multiple selected responses are available (potentially including If multiple selected responses are available (potentially including
responses without a Vary header field), the cache will need to choose responses without a Vary header field), the cache will need to choose
one to use. When a selecting header field has a known mechanism for one to use. When a selecting header field has a known mechanism for
doing so (e.g., qvalues on Accept and similar request header fields), doing so (e.g., qvalues on Accept and similar request header fields),
that mechanism MAY be used to select a preferred response. If such a that mechanism MAY be used to select a preferred response. If such a
mechanism is not available, or leads to equally preferred responses, mechanism is not available, or leads to equally preferred responses,
skipping to change at page 15, line 6 skipping to change at page 15, line 13
time under certain circumstances (see Section 4.2.2). time under certain circumstances (see Section 4.2.2).
The calculation to determine if a response is fresh is: The calculation to determine if a response is fresh is:
response_is_fresh = (freshness_lifetime > current_age) response_is_fresh = (freshness_lifetime > current_age)
freshness_lifetime is defined in Section 4.2.1; current_age is freshness_lifetime is defined in Section 4.2.1; current_age is
defined in Section 4.2.3. defined in Section 4.2.3.
Clients can send the max-age or min-fresh request directives Clients can send the max-age or min-fresh request directives
(Section 5.2.1) to constrain or relax freshness calculations for the (Section 5.2.1) to suggest limits on the freshness calculations for
corresponding response. However, caches are not required to honor the corresponding response. However, caches are not required to
them. honor them.
When calculating freshness, to avoid common problems in date parsing: When calculating freshness, to avoid common problems in date parsing:
* Although all date formats are specified to be case-sensitive, a * Although all date formats are specified to be case-sensitive, a
cache recipient SHOULD match the field value case-insensitively. cache recipient SHOULD match the field value case-insensitively.
* If a cache recipient's internal implementation of time has less * If a cache recipient's internal implementation of time has less
resolution than the value of an HTTP-date, the recipient MUST resolution than the value of an HTTP-date, the recipient MUST
internally represent a parsed Expires date as the nearest time internally represent a parsed Expires date as the nearest time
equal to or earlier than the received value. equal to or earlier than the received value.
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* If the cache is shared and the s-maxage response directive * If the cache is shared and the s-maxage response directive
(Section 5.2.2.10) is present, use its value, or (Section 5.2.2.10) is present, use its value, or
* If the max-age response directive (Section 5.2.2.1) is present, * If the max-age response directive (Section 5.2.2.1) is present,
use its value, or use its value, or
* If the Expires response header field (Section 5.3) is present, use * If the Expires response header field (Section 5.3) is present, use
its value minus the value of the Date response header field (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
Section 10.2.2 of [Semantics]), or Section 10.2.2 of [HTTP]), or
* Otherwise, no explicit expiration time is present in the response. * Otherwise, no explicit expiration time is present in the response.
A heuristic freshness lifetime might be applicable; see A heuristic freshness lifetime might be applicable; see
Section 4.2.2. Section 4.2.2.
Note that this calculation is not vulnerable to clock skew, since all Note that this calculation is intended to reduce clock skew by using
of the information comes from the origin server. the clock information provided by the origin server whenever
possible.
When there is more than one value present for a given directive When there is more than one value present for a given directive
(e.g., two Expires header 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.
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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. 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, this means that heuristics can only
be used on responses without explicit freshness whose status codes be used on responses without explicit freshness whose status codes
are defined as _heuristically cacheable_ (e.g., see Section 15.1 of are defined as _heuristically cacheable_ (e.g., see Section 15.1 of
[Semantics]), and those responses without explicit freshness that [HTTP]), and those responses without explicit freshness that have
have been marked as explicitly cacheable (e.g., with a "public" been marked as explicitly cacheable (e.g., with a "public" response
response directive). directive).
Note that in previous specifications heuristically cacheable response Note that in previous specifications heuristically cacheable response
status codes were called "cacheable by default." 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
[Semantics]), caches are encouraged to use a heuristic expiration [HTTP]), caches are encouraged to use a heuristic expiration value
value that is no more than some fraction of the interval since that that is no more than some fraction of the interval since that time.
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:* Section 13.9 of [RFC2616] prohibited caches from
| calculating heuristic freshness for URIs with query components | calculating heuristic freshness for URIs with query components
| (i.e., those containing '?'). In practice, this has not been | (i.e., those containing '?'). In practice, this has not been
| widely implemented. Therefore, origin servers are encouraged | widely implemented. Therefore, origin servers are encouraged
| to send explicit directives (e.g., Cache-Control: no-cache) if | to send explicit directives (e.g., Cache-Control: no-cache) if
| they wish to prevent caching. | they wish to prevent caching.
4.2.3. Calculating Age 4.2.3. Calculating Age
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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_ The term "age_value" denotes the value of the Age header
field (Section 5.1), in a form appropriate for arithmetic field (Section 5.1), in a form appropriate for arithmetic
operation; or 0, if not available. operation; or 0, if not available.
_date_value_ The term "date_value" denotes the value of the Date _date_value_ The term "date_value" denotes the value of the Date
header field, in a form appropriate for arithmetic operations. header field, in a form appropriate for arithmetic operations.
See Section 10.2.2 of [Semantics] for the definition of the Date See Section 10.2.2 of [HTTP] for the definition of the Date header
header field, and for requirements regarding responses without it. field, and for requirements regarding responses without it.
_now_ The term "now" means "the current value of the clock at the _now_ The term "now" means "the current value of the clock at the
host performing the calculation". A host ought to use NTP host performing the calculation". A host ought to use NTP
([RFC5905]) or some similar protocol to synchronize its clocks to ([RFC5905]) or some similar protocol to synchronize its clocks to
Coordinated Universal Time. Coordinated Universal Time.
_request_time_ The current value of the clock at the host at the _request_time_ The current value of the clock at the host at the
time the request resulting in the stored response was made. time the request resulting in the stored response was made.
_response_time_ The current value of the clock at the host at the _response_time_ The current value of the clock at the host at the
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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, by
extension directives such as those defined in [RFC5861], or by extension directives such as those defined in [RFC5861], or by
configuration in accordance with an out-of-band contract). configuration in 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 selected; see Section 4.1), it can use the conditional one cannot be selected; see Section 4.1), it can use the conditional
request mechanism (Section 13.1 of [Semantics]) in the forwarded request mechanism (Section 13.1 of [HTTP]) in the forwarded request
request to give the next inbound server an opportunity to select a to give the next inbound server an opportunity to select a valid
valid stored response to use, updating the stored metadata in the stored response to use, updating the stored metadata in the process,
process, or to replace the stored response(s) with a new response. or to replace the stored response(s) with a new response. This
This process is known as _validating_ or _revalidating_ the stored process is 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 starts
with either a request it is attempting to satisfy, or - if it is with either a request it is attempting to satisfy, or - if it is
initiating the request independently - it synthesises a request using initiating the request independently - it synthesises a request using
a stored response by copying the method, target URI, and request a stored response by copying the method, target URI, and request
header fields identified by the Vary header field (Section 4.1). header fields identified by the Vary header field (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 stored
response(s) that have the same URI. Typically, this will include response(s) that have the same URI. Typically, this will include
only those stored responses(s) that have the same cache key, although only those stored responses(s) that have the same cache key, although
a cache is allowed to validate a response that it cannot select with a cache is allowed to validate a response that it cannot select with
the request header fields it is sending. the 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 [Semantics]), which can be used in an If- field (Section 8.8.2 of [HTTP]), which can be used in an If-Modified-
Modified-Since header field for response validation, or in an If- Since header field for response validation, or in an If-Unmodified-
Unmodified-Since or If-Range header field for representation Since or If-Range header field for representation selection (i.e.,
selection (i.e., the client is referring specifically to a previously the client is referring specifically to a previously obtained
obtained representation with that timestamp). representation with that timestamp).
Another validator is the entity-tag given in an ETag field Another validator is the entity-tag given in an ETag field
(Section 8.8.3 of [Semantics]). One or more entity-tags, indicating (Section 8.8.3 of [HTTP]). One or more entity-tags, indicating one
one or more stored responses, can be used in an If-None-Match header or more stored responses, can be used in an If-None-Match header
field for response validation, or in an If-Match or If-Range header field for response validation, or in an If-Match or If-Range header
field for representation selection (i.e., the client is referring field for representation selection (i.e., the client is referring
specifically to one or more previously obtained representations with specifically to one or more previously obtained representations with
the listed entity-tags). the listed entity-tags).
4.3.2. Handling a Received Validation Request 4.3.2. Handling a Received Validation Request
Each client in the request chain may have its own cache, so it is Each client in the request chain may have its own cache, so it is
common for a cache at an intermediary to receive conditional requests common for a cache at an intermediary to receive conditional requests
from other (outbound) caches. Likewise, some user agents make use of from other (outbound) caches. Likewise, some user agents make use of
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A cache MUST NOT evaluate conditional header fields that only apply A cache MUST NOT evaluate conditional header fields that only apply
to an origin server, occur in a request with semantics that cannot be to an origin server, occur in a request with semantics that cannot be
satisfied with a cached response, or occur in a request with a target satisfied with a cached response, or occur in a request with a target
resource for which it has no stored responses; such preconditions are resource for which it has no stored responses; such preconditions are
likely intended for some other (inbound) server. likely intended for some other (inbound) server.
The proper evaluation of conditional requests by a cache depends on The proper evaluation of conditional requests by a cache depends on
the received precondition header fields and their precedence. In the received precondition header fields and their precedence. In
summary, the If-Match and If-Unmodified-Since conditional header summary, the If-Match and If-Unmodified-Since conditional header
fields are not applicable to a cache, and If-None-Match takes fields are not applicable to a cache, and If-None-Match takes
precedence over If-Modified-Since. See Section 13.2.2 of [Semantics] precedence over If-Modified-Since. See Section 13.2.2 of [HTTP] for
for a complete specification of precondition precedence. a complete specification of precondition precedence.
A request containing an If-None-Match header field (Section 13.1.2 of A request containing an If-None-Match header field (Section 13.1.2 of
[Semantics]) indicates that the client wants to validate one or more [HTTP]) indicates that the client wants to validate one or more of
of its own stored responses in comparison to the stored response its own stored responses in comparison to the stored response
selected by the cache (as per Section 4). selected by the cache (as per Section 4).
If an If-None-Match header field is not present, a request containing If an If-None-Match header field is not present, a request containing
an If-Modified-Since header field (Section 13.1.3 of [Semantics]) an If-Modified-Since header field (Section 13.1.3 of [HTTP])
indicates that the client wants to validate one or more of its own indicates that the client wants to validate one or more of its own
stored responses by modification date. stored responses by modification date.
If a request contains an If-Modified-Since header field and the Last- If a request contains an If-Modified-Since header field and the Last-
Modified header field is not present in a selected stored response, a Modified header field is not present in a selected stored response, a
cache SHOULD use the stored response's Date field value (or, if no cache SHOULD use the stored response's Date field value (or, if no
Date field is present, the time that the stored response was Date field is present, the time that the stored response was
received) to evaluate the conditional. received) to 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 [Semantics], also needs to evaluate a defined in Section 14.2 of [HTTP], also needs to evaluate a received
received If-Range header field (Section 13.1.5 of [Semantics]) If-Range header field (Section 13.1.5 of [HTTP]) regarding its
regarding its selected stored response. selected stored 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
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* However, if a cache receives a 5xx (Server Error) response while * However, if a cache receives a 5xx (Server Error) response while
attempting to validate a response, it can either forward this attempting to validate a response, it can either forward this
response to the requesting client, or act as if the server failed response to the requesting client, or act as if the server failed
to respond. In the latter case, the cache 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 and already has When a cache receives a 304 (Not Modified) response, it needs to
one or more stored 200 (OK) responses for the applicable request URI, identify stored responses that are suitable for updating with the new
the cache needs to identify which (if any) are to be updated by the information provided, and then do so.
new information provided, and then do so.
The stored response(s) to update are identified by using the first The initial set of stored responses to update are those that could
match (if any) of: have been selected for that request - i.e., those that meet the
requirements in Section 4, except the last requirement to be fresh,
able to be served stale or just validated.
Then, that initial set of stored response(s) is further filtered by
the first match of:
* If the new response contains one or more _strong validators_ (see * If the new response contains one or more _strong validators_ (see
Section 8.8.1 of [Semantics]), then each of those strong Section 8.8.1 of [HTTP]), then each of those strong validators
validators identify the selected representation for update. All identify a selected representation for update. All the stored
the stored responses with one of those same strong validators are responses in the initial set with one of those same strong
identified for update. If none of the stored responses contain at validators are identified for update. If none of the initial set
least one of the same strong validators, then the cache MUST NOT contain at least one of the same strong validators, then the cache
use the new response to update any stored responses. MUST NOT use the new response to update any stored responses.
* If the new response contains no strong validators but does contain * 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 cache's stored responses, then the most recent of those one of the initial set's stored responses, then the most recent of
matching stored responses is identified for update. those matching stored responses is identified for update.
* If the new response does not include any form of validator (such * 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, and that stored response also there is only one stored response in the initial set, and that
lacks a validator, then that stored response is identified for stored response also lacks a validator, then that stored response
update. is identified for update.
For each stored response identified, the cache MUST update its header For each stored response identified, the cache MUST update its header
fields with the header fields provided in the 304 (Not Modified) fields with the header fields provided in the 304 (Not Modified)
response, as per Section 3.2. response, as per Section 3.2.
4.3.5. Freshening Responses with HEAD 4.3.5. Freshening Responses with HEAD
A response to the HEAD method is identical to what an equivalent A response to the HEAD method is identical to what an equivalent
request made with a GET would have been, without sending the content. request made with a GET would have been, without sending the content.
This property of HEAD responses can be used to invalidate or update a This property of HEAD responses can be used to invalidate or update a
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Length matches that of the stored response, the cache SHOULD update Length matches that of the stored response, the cache SHOULD update
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 [Semantics]) such as Because unsafe request methods (Section 9.2.1 of [HTTP]) such as PUT,
PUT, POST or DELETE have the potential for changing state on the POST or DELETE have the potential for changing state on the origin
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 [Semantics]) A cache MUST invalidate the target URI (Section 7.1 of [HTTP]) when
when it receives a non-error status code in response to an unsafe it receives a non-error status code in response to an unsafe request
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
[Semantics]) of the URI to be invalidated differs from that of the [HTTP]) of the URI to be invalidated differs from that of the target
target URI (Section 7.1 of [Semantics]). This helps prevent denial- URI (Section 7.1 of [HTTP]). This helps prevent denial-of-service
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 will mark them
as "invalid" and in need of a mandatory validation before they can be as "invalid" and in need of a mandatory validation before they can be
sent in response to a subsequent request. sent in response to a subsequent request.
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
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5.1. Age 5.1. Age
The "Age" response header field conveys the sender's estimate of the The "Age" response header field conveys the sender's estimate of the
time since the response was generated or successfully validated at time since the response was generated or successfully validated at
the origin server. Age values are calculated as specified in the origin server. Age values are calculated as specified in
Section 4.2.3. Section 4.2.3.
Age = delta-seconds Age = delta-seconds
The Age field value is a non-negative integer, representing time in The Age field value is a non-negative integer, representing time in
seconds (see Section 1.3). seconds (see Section 1.2.2).
Although it is defined as a singleton header field, a cache Although it is defined as a singleton header field, a cache
encountering a message with a list-based Age field value SHOULD use encountering a message with a list-based Age field value SHOULD use
the first member of the field value, discarding subsequent ones. the first member of the field value, discarding subsequent ones.
If the field value (after discarding additional members, as per If the field value (after discarding additional members, as per
above) is invalid (e.g., it contains something other than a non- above) is invalid (e.g., it contains something other than a non-
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
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5.2.1. Request Cache-Control Directives 5.2.1. Request Cache-Control 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 1.3) 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 1.3) 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 a response that has exceeded its freshness lifetime. If a accept a response that has exceeded its freshness lifetime. If a
value is present, then the client is willing to accept a response value is present, then the client is willing to accept a response
that has exceeded its freshness lifetime by no more than the that has exceeded its freshness lifetime by no more than the
specified number of seconds. If no value is assigned to max-stale, specified number of seconds. If no value is assigned to max-stale,
then the client will accept a stale response of any age. then the client 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 1.3) 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.
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networks might be vulnerable to eavesdropping. networks might be vulnerable to eavesdropping.
Note that if a request containing this directive is satisfied from a Note that if a request containing this directive is satisfied from a
cache, the no-store request directive does not apply to the already cache, the no-store request directive does not apply to the already
stored response. stored response.
5.2.1.6. no-transform 5.2.1.6. no-transform
The "no-transform" request directive indicates that 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 [Semantics]. 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, either respond using a stored
response consistent with the other constraints of the request, or response consistent with the other constraints of the request, or
respond with a 504 (Gateway Timeout) status code. respond with a 504 (Gateway Timeout) status code.
5.2.2. Response Cache-Control Directives 5.2.2. Response Cache-Control 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 1.3) 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
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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 [Semantics]), 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 to a cache that understands and conforms to the requirements response to a cache that understands and conforms to the requirements
for that response's status code. for that response's status code.
Responses containing "must-understand" SHOULD also contain the "no- Responses containing "must-understand" SHOULD also contain the "no-
store" directive; caches that implement "must-understand" SHOULD store" directive; caches that implement "must-understand" SHOULD
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might not recognize or obey this directive, and communications might not recognize or obey this directive, and communications
networks might be vulnerable to eavesdropping. networks might 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 certain circumstances; see Section 5.2.2.3. in certain circumstances; see Section 5.2.2.3.
5.2.2.6. no-transform 5.2.2.6. no-transform
The "no-transform" response directive indicates that an intermediary The "no-transform" response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT transform the (regardless of whether it implements a cache) MUST NOT transform the
content, as defined in Section 7.7 of [Semantics]. 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
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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 1.3) 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 proxy-revalidate
(Section 5.2.2.8) response directive's semantics for a shared cache. (Section 5.2.2.8) response directive's semantics for a shared cache.
A shared cache MUST NOT reuse a stale response with s-maxage to 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
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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. Cache Control Extensions
The Cache-Control header field can be extended through the use of one The Cache-Control header field can be extended through the use of one
or more cache-extension tokens, each with an optional value. A cache or more extension cache directives. A cache MUST ignore unrecognized
MUST ignore unrecognized cache directives. cache directives.
Informational extensions (those that do not require a change in cache Informational extensions (those that do not require a change in cache
behavior) can be added without changing the semantics of other behavior) can be added without changing the semantics of other
directives. directives.
Behavioral extensions are designed to work by acting as modifiers to Behavioral extensions are designed to work by acting as modifiers to
the existing base of cache directives. Both the new directive and the existing base of cache directives. Both the new directive and
the old directive are supplied, such that applications that do not the old directive are supplied, such that applications that do not
understand the new directive will default to the behavior specified understand the new directive will default to the behavior specified
by the old directive, and those that understand the new directive by the old directive, and those that understand the new directive
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For example, consider a hypothetical new response directive called For example, consider a hypothetical new response directive called
"community" that acts as a modifier to the private directive: in "community" that acts as a modifier to the private directive: in
addition to private caches, any cache that is shared only by members addition to private caches, any cache that is shared only by members
of the named community is allowed to cache the response. An origin of the named community is allowed to cache the response. An origin
server wishing to allow the UCI community to use an otherwise private server wishing to allow the UCI community to use an otherwise private
response in their shared cache(s) could do so by including response in their shared cache(s) could do so by including
Cache-Control: private, community="UCI" Cache-Control: private, community="UCI"
A cache that recognizes such a community cache-extension could A cache that recognizes such a community cache directive could
broaden its behavior in accordance with that extension. A cache that broaden its behavior in accordance with that extension. A cache that
does not recognize the community cache-extension would ignore it and does not recognize the community cache directive would ignore it and
adhere to the private directive. adhere to the private directive.
New extension directives ought to consider defining: New extension directives ought to consider defining:
* What it means for a directive to be specified multiple times, * What it means for a directive to be specified multiple times,
* When the directive does not take an argument, what it means when * When the directive does not take an argument, what it means when
an argument is present, an argument is present,
* When the directive requires an argument, what it means when it is * When the directive requires an argument, what it means when it is
skipping to change at page 32, line 35 skipping to change at page 33, line 10
The "Expires" response header field gives the date/time after which The "Expires" response header field gives the date/time after which
the response is considered stale. See Section 4.2 for further the response is considered stale. See Section 4.2 for further
discussion of the freshness model. discussion of the freshness model.
The presence of an Expires header field does not imply that the The presence of an Expires header field does not imply that the
original resource will change or cease to exist at, before, or after original resource will change or cease to exist at, before, or after
that time. that time.
The Expires field value is an HTTP-date timestamp, as defined in The Expires field value is an HTTP-date timestamp, as defined in
Section 5.6.7 of [Semantics]. See also Section 4.2 for parsing Section 5.6.7 of [HTTP]. See also Section 4.2 for parsing
requirements specific to caches. requirements specific to caches.
Expires = HTTP-date Expires = HTTP-date
For example For example
Expires: Thu, 01 Dec 1994 16:00:00 GMT Expires: Thu, 01 Dec 1994 16:00:00 GMT
A cache recipient MUST interpret invalid date formats, especially the A cache recipient MUST interpret invalid date formats, especially the
value "0", as representing a time in the past (i.e., "already value "0", as representing a time in the past (i.e., "already
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directly related to the request that fetched it (such as those directly related to the request that fetched it (such as those
created during the same page load), it would likely be surprising and created during the same page load), it would likely be surprising and
confusing to users and authors if it were allowed to be reused for confusing to users and authors if it were allowed to be reused for
requests unrelated in any way to the one from which it was obtained. requests unrelated in any way to the one from which it was obtained.
7. Security Considerations 7. Security Considerations
This section is meant to inform developers, information providers, This section is meant to inform developers, information providers,
and users of known security concerns specific to HTTP caching. More and users of known security concerns specific to HTTP caching. More
general security considerations are addressed in "HTTP/1.1" general security considerations are addressed in "HTTP/1.1"
(Section 11 of [Messaging]) and "HTTP Semantics" (Section 17 of (Section 11 of [HTTP/1.1]) and "HTTP Semantics" (Section 17 of
[Semantics]). [HTTP]).
Caches expose additional potential vulnerabilities, since the Caches expose an additional attack surface, since the contents of the
contents of the cache represent an attractive target for malicious cache represent an attractive target for malicious exploitation.
exploitation. Because cache contents persist after an HTTP request Because cache contents persist after an HTTP request is complete, an
is complete, an attack on the cache can reveal information long after attack on the cache can reveal information long after a user believes
a user believes that the information has been removed from the that the information has been removed from the network. Therefore,
network. Therefore, cache contents need to be protected as sensitive cache contents need to be protected as sensitive information.
information.
In particular, because private caches are restricted to a single
user, they can be used to reconstruct a user's activity. As a
result, is important for user agents to allow end users to control
them; for example, allowing stored responses to be removed for some
or all origin servers.
7.1. Cache Poisoning 7.1. Cache Poisoning
Various attacks might be amplified by being stored in a cache. Such Storing a malicious payload in a cache can extend the reach of an
"cache poisoning" attacks happen when an attacker uses implementation attacker to affect multiple users. Such "cache poisoning" attacks
flaws, elevated privileges, or other techniques to insert a response happen when an attacker uses implementation flaws, elevated
into a cache. This is especially effective when shared caches are privileges, or other techniques to insert a response into a cache.
used to distribute malicious content to many clients. This is especially effective when shared caches are used to
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 [Messaging] 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 optimise
performance, its use can "leak" information about what resources have performance, its use can "leak" information about what resources have
been previously requested. 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
skipping to change at page 35, line 42 skipping to change at page 36, line 17
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 (as well as misunderstanding of
cache operation) might lead to caching of sensitive information cache operation) might lead to caching of sensitive information
(e.g., authentication credentials) that is thought to be private, (e.g., authentication credentials) that is thought to be private,
exposing it to unauthorized parties. exposing it to unauthorized parties.
Note that the Set-Cookie response header field [RFC6265] does not Note that the Set-Cookie response header field [COOKIE] does not
inhibit caching; a cacheable response with a Set-Cookie header field inhibit caching; a cacheable response with a Set-Cookie header field
can be (and often is) used to satisfy subsequent requests to caches. can be (and often is) used to satisfy subsequent requests to caches.
Servers who wish to control caching of these responses are encouraged Servers who wish to control caching of these responses are encouraged
to emit appropriate Cache-Control response header fields. to emit appropriate Cache-Control response header fields.
8. IANA Considerations 8. IANA Considerations
The change controller 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 First, introduce the new "Hypertext Transfer Protocol (HTTP) Field
Name Registry" at <https://www.iana.org/assignments/http-fields> as Name Registry" at <https://www.iana.org/assignments/http-fields> as
described in Section 18.4 of [Semantics]. described in Section 18.4 of [HTTP].
Then, please update the registry with the field names listed in the Then, please update the registry with the field names listed in the
table below: table below:
+===============+===========+======+==========+ +===============+===========+======+==========+
| Field Name | Status | Ref. | Comments | | Field Name | Status | Ref. | Comments |
+===============+===========+======+==========+ +===============+===========+======+==========+
| Age | standard | 5.1 | | | Age | standard | 5.1 | |
+---------------+-----------+------+----------+ +---------------+-----------+------+----------+
| Cache-Control | standard | 5.2 | | | Cache-Control | standard | 5.2 | |
skipping to change at page 37, line 47 skipping to change at page 38, line 5
8.3. Warn Code Registry 8.3. Warn Code Registry
Please add a note to the "Hypertext Transfer Protocol (HTTP) Warn Please add a note to the "Hypertext Transfer Protocol (HTTP) Warn
Codes" registry at <https://www.iana.org/assignments/http-warn-codes> Codes" registry at <https://www.iana.org/assignments/http-warn-codes>
to the effect that Warning is obsoleted. to the effect that Warning is obsoleted.
9. References 9. References
9.1. Normative References 9.1. Normative References
[Messaging] [HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", Work in Progress, Internet-Draft,
draft-ietf-httpbis-semantics-17, 26 July 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
semantics-17>.
[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-16, 27 May 2021, ietf-httpbis-messaging-17, 26 July 2021,
<https://tools.ietf.org/html/draft-ietf-httpbis-messaging- <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
16>. messaging-17>.
[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,
DOI 10.17487/RFC5234, January 2008, DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>. <https://www.rfc-editor.org/info/rfc5234>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014, RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>. <https://www.rfc-editor.org/info/rfc7405>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[Semantics]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", Work in Progress, Internet-Draft,
draft-ietf-httpbis-semantics-16, 27 May 2021,
<https://tools.ietf.org/html/draft-ietf-httpbis-semantics-
16>.
9.2. Informative References 9.2. Informative References
[COOKIE] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, 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, April 2010,
<https://www.rfc-editor.org/info/rfc5861>. <https://www.rfc-editor.org/info/rfc5861>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms "Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>. <https://www.rfc-editor.org/info/rfc5905>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. F. Reschke, [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. F. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP): Caching", Ed., "Hypertext Transfer Protocol (HTTP): 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 as per
Section 5.6.1.1 of [Semantics]. Section 5.6.1.1 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 [Semantics], Section 5.6.7> HTTP-date = <HTTP-date, see [HTTP], Section 5.6.7>
OWS = <OWS, see [Semantics], Section 5.6.3> OWS = <OWS, see [HTTP], Section 5.6.3>
cache-directive = token [ "=" ( token / quoted-string ) ] cache-directive = token [ "=" ( token / quoted-string ) ]
delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
field-name = <field-name, see [Semantics], Section 5.1> field-name = <field-name, see [HTTP], Section 5.1>
quoted-string = <quoted-string, see [Semantics], Section 5.6.4> quoted-string = <quoted-string, see [HTTP], Section 5.6.4>
token = <token, see [Semantics], 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 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)
skipping to change at page 46, line 19 skipping to change at page 46, line 22
deployed caches (<https://github.com/httpwg/http-core/issues/778>) deployed caches (<https://github.com/httpwg/http-core/issues/778>)
* In Section 5.3, mention parsing requirement relaxation * In Section 5.3, mention parsing requirement relaxation
(<https://github.com/httpwg/http-core/issues/779>) (<https://github.com/httpwg/http-core/issues/779>)
C.17. Since draft-ietf-httpbis-cache-15 C.17. Since draft-ietf-httpbis-cache-15
* In Section 4.3.1, tune description of relation between cache keys * In Section 4.3.1, tune description of relation between cache keys
and validators (<https://github.com/httpwg/http-core/issues/832>) and validators (<https://github.com/httpwg/http-core/issues/832>)
Acknowledgments C.18. Since draft-ietf-httpbis-cache-16
See Appendix "Acknowledgments" of [Semantics]. This draft addresses mostly editorial issues raised during or past
IETF Last Call; see <https://github.com/httpwg/http-core/
issues?q=label%3Acaching+created%3A%3E2021-05-26> for a summary.
Furthermore:
* Addressed Genart last call review comments
(<https://github.com/httpwg/http-core/issues/847>)
* In Section 4.3.4, clarify that only selectable responses are
updated (<https://github.com/httpwg/http-core/issues/839>)
Acknowledgements
See Appendix "Acknowledgements" of [HTTP].
Index 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 header field Section 5.1 Age header field Section 5.1
age Section 4.2 age Section 4.2
skipping to change at page 47, line 14 skipping to change at page 47, line 31
freshness lifetime Section 4.2 freshness lifetime Section 4.2
G G
Grammar Grammar
Age Section 5.1 Age Section 5.1
Cache-Control Section 5.2 Cache-Control Section 5.2
DIGIT Section 1.2 DIGIT Section 1.2
Expires Section 5.3 Expires Section 5.3
cache-directive Section 5.2 cache-directive Section 5.2
delta-seconds Section 1.3 delta-seconds Section 1.2.2
H H
Header Fields Header Fields
Age Section 5.1; Section 5.1 Age Section 5.1; Section 5.1
Cache-Control Section 5.2 Cache-Control Section 5.2
Expires Section 5.3; Section 5.3 Expires Section 5.3; Section 5.3
Pragma Section 5.4; Section 5.4 Pragma Section 5.4; Section 5.4
Warning Section 5.5 Warning Section 5.5
heuristic expiration time Section 4.2 heuristic expiration time Section 4.2
skipping to change at page 48, line 12 skipping to change at page 48, line 30
private cache Section 1 private cache Section 1
proxy-revalidate (cache directive) Section 5.2.2.8 proxy-revalidate (cache directive) Section 5.2.2.8
public (cache directive) Section 5.2.2.9 public (cache directive) Section 5.2.2.9
S S
s-maxage (cache directive) Section 5.2.2.10 s-maxage (cache directive) Section 5.2.2.10
selected response Section 4.1 selected response Section 4.1
shared cache Section 1 shared cache Section 1
stale Section 4.2 stale Section 4.2
strong validator Section 4.3.4
V V
validator Section 4.3.1 validator Section 4.3.1
W W
Warning header field Section 5.5 Warning header field Section 5.5
Authors' Addresses Authors' Addresses
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