draft-ietf-httpbis-cache-12.txt   draft-ietf-httpbis-cache-13.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: April 5, 2021 J. Reschke, Ed. Expires: June 17, 2021 J. Reschke, Ed.
greenbytes greenbytes
October 2, 2020 December 14, 2020
HTTP Caching HTTP Caching
draft-ietf-httpbis-cache-12 draft-ietf-httpbis-cache-13
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.13. The changes in this draft are summarized in Appendix C.14.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 5, 2021. This Internet-Draft will expire on June 17, 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 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 1.3. Delta Seconds . . . . . . . . . . . . . . . . . . . . . . 5
2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 6 2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 6
3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 7 3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 7
3.1. Storing Header and Trailer Fields . . . . . . . . . . . . 8 3.1. Storing Header and Trailer Fields . . . . . . . . . . . . 8
3.2. Storing Incomplete Responses . . . . . . . . . . . . . . 9 3.2. Updating Stored Header Fields . . . . . . . . . . . . . . 9
3.3. Storing Responses to Authenticated Requests . . . . . . . 9 3.3. Storing Incomplete Responses . . . . . . . . . . . . . . 10
3.4. Combining Partial Content . . . . . . . . . . . . . . . . 10 3.4. Storing Responses to Authenticated Requests . . . . . . . 10
4. Constructing Responses from Caches . . . . . . . . . . . . . 10 3.5. Combining Partial Content . . . . . . . . . . . . . . . . 10
4.1. Calculating Cache Keys with Vary . . . . . . . . . . . . 11 4. Constructing Responses from Caches . . . . . . . . . . . . . 11
4.2. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 12 4.1. Calculating Cache Keys with Vary . . . . . . . . . . . . 12
4.2.1. Calculating Freshness Lifetime . . . . . . . . . . . 14 4.2. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2.2. Calculating Heuristic Freshness . . . . . . . . . . . 14 4.2.1. Calculating Freshness Lifetime . . . . . . . . . . . 15
4.2.3. Calculating Age . . . . . . . . . . . . . . . . . . . 15 4.2.2. Calculating Heuristic Freshness . . . . . . . . . . . 15
4.2.4. Serving Stale Responses . . . . . . . . . . . . . . . 16 4.2.3. Calculating Age . . . . . . . . . . . . . . . . . . . 16
4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . 17 4.2.4. Serving Stale Responses . . . . . . . . . . . . . . . 17
4.3.1. Sending a Validation Request . . . . . . . . . . . . 17 4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . 18
4.3.2. Handling a Received Validation Request . . . . . . . 18 4.3.1. Sending a Validation Request . . . . . . . . . . . . 18
4.3.3. Handling a Validation Response . . . . . . . . . . . 19 4.3.2. Handling a Received Validation Request . . . . . . . 19
4.3.4. Freshening Stored Responses upon Validation . . . . . 20 4.3.3. Handling a Validation Response . . . . . . . . . . . 20
4.3.5. Freshening Responses with HEAD . . . . . . . . . . . 21 4.3.4. Freshening Stored Responses upon Validation . . . . . 21
4.4. Invalidation . . . . . . . . . . . . . . . . . . . . . . 21 4.3.5. Freshening Responses with HEAD . . . . . . . . . . . 22
5. Field Definitions . . . . . . . . . . . . . . . . . . . . . . 22 4.4. Invalidating Stored Responses . . . . . . . . . . . . . . 22
5.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 5. Field Definitions . . . . . . . . . . . . . . . . . . . . . . 23
5.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 23 5.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 24
5.2.1. Request Cache-Control Directives . . . . . . . . . . 24 5.2.1. Request Cache-Control Directives . . . . . . . . . . 24
5.2.1.1. max-age . . . . . . . . . . . . . . . . . . . . . 24
5.2.1.2. max-stale . . . . . . . . . . . . . . . . . . . . 24
5.2.1.3. min-fresh . . . . . . . . . . . . . . . . . . . . 25
5.2.1.4. no-cache . . . . . . . . . . . . . . . . . . . . 25
5.2.1.5. no-store . . . . . . . . . . . . . . . . . . . . 25
5.2.1.6. no-transform . . . . . . . . . . . . . . . . . . 26
5.2.1.7. only-if-cached . . . . . . . . . . . . . . . . . 26
5.2.2. Response Cache-Control Directives . . . . . . . . . . 26 5.2.2. Response Cache-Control Directives . . . . . . . . . . 26
5.2.2.1. must-revalidate . . . . . . . . . . . . . . . . . 26
5.2.2.2. must-understand . . . . . . . . . . . . . . . . . 27
5.2.2.3. no-cache . . . . . . . . . . . . . . . . . . . . 27
5.2.2.4. no-store . . . . . . . . . . . . . . . . . . . . 28
5.2.2.5. no-transform . . . . . . . . . . . . . . . . . . 28
5.2.2.6. public . . . . . . . . . . . . . . . . . . . . . 28
5.2.2.7. private . . . . . . . . . . . . . . . . . . . . . 28
5.2.2.8. proxy-revalidate . . . . . . . . . . . . . . . . 29
5.2.2.9. max-age . . . . . . . . . . . . . . . . . . . . . 29
5.2.2.10. s-maxage . . . . . . . . . . . . . . . . . . . . 30
5.2.3. Cache Control Extensions . . . . . . . . . . . . . . 30 5.2.3. Cache Control Extensions . . . . . . . . . . . . . . 30
5.2.4. Cache Directive Registry . . . . . . . . . . . . . . 31 5.2.4. Cache Directive Registry . . . . . . . . . . . . . . 31
5.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 32 5.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . 33 5.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 33 5.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 33
6. Relationship to Applications . . . . . . . . . . . . . . . . 33 6. Relationship to Applications and Other Caches . . . . . . . . 33
7. Security Considerations . . . . . . . . . . . . . . . . . . . 34 7. Security Considerations . . . . . . . . . . . . . . . . . . . 34
7.1. Cache Poisoning . . . . . . . . . . . . . . . . . . . . . 34 7.1. Cache Poisoning . . . . . . . . . . . . . . . . . . . . . 34
7.2. Timing Attacks . . . . . . . . . . . . . . . . . . . . . 34 7.2. Timing Attacks . . . . . . . . . . . . . . . . . . . . . 35
7.3. Caching of Sensitive Information . . . . . . . . . . . . 35 7.3. Caching of Sensitive Information . . . . . . . . . . . . 35
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
8.1. Field Registration . . . . . . . . . . . . . . . . . . . 35 8.1. Field Name Registration . . . . . . . . . . . . . . . . . 35
8.2. Cache Directive Registration . . . . . . . . . . . . . . 35 8.2. Cache Directive Registration . . . . . . . . . . . . . . 36
8.3. Warn Code Registry . . . . . . . . . . . . . . . . . . . 35 8.3. Warn Code Registry . . . . . . . . . . . . . . . . . . . 36
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 35 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 36
9.1. Normative References . . . . . . . . . . . . . . . . . . 35 9.1. Normative References . . . . . . . . . . . . . . . . . . 36
9.2. Informative References . . . . . . . . . . . . . . . . . 36 9.2. Informative References . . . . . . . . . . . . . . . . . 37
Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 37 Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 38
Appendix B. Changes from RFC 7234 . . . . . . . . . . . . . . . 37 Appendix B. Changes from RFC 7234 . . . . . . . . . . . . . . . 38
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 38 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 39
C.1. Between RFC7234 and draft 00 . . . . . . . . . . . . . . 38 C.1. Between RFC7234 and draft 00 . . . . . . . . . . . . . . 39
C.2. Since draft-ietf-httpbis-cache-00 . . . . . . . . . . . . 39 C.2. Since draft-ietf-httpbis-cache-00 . . . . . . . . . . . . 40
C.3. Since draft-ietf-httpbis-cache-01 . . . . . . . . . . . . 39 C.3. Since draft-ietf-httpbis-cache-01 . . . . . . . . . . . . 40
C.4. Since draft-ietf-httpbis-cache-02 . . . . . . . . . . . . 39 C.4. Since draft-ietf-httpbis-cache-02 . . . . . . . . . . . . 40
C.5. Since draft-ietf-httpbis-cache-03 . . . . . . . . . . . . 39 C.5. Since draft-ietf-httpbis-cache-03 . . . . . . . . . . . . 40
C.6. Since draft-ietf-httpbis-cache-04 . . . . . . . . . . . . 40 C.6. Since draft-ietf-httpbis-cache-04 . . . . . . . . . . . . 41
C.7. Since draft-ietf-httpbis-cache-05 . . . . . . . . . . . . 40 C.7. Since draft-ietf-httpbis-cache-05 . . . . . . . . . . . . 41
C.8. Since draft-ietf-httpbis-cache-06 . . . . . . . . . . . . 40 C.8. Since draft-ietf-httpbis-cache-06 . . . . . . . . . . . . 41
C.9. Since draft-ietf-httpbis-cache-07 . . . . . . . . . . . . 41 C.9. Since draft-ietf-httpbis-cache-07 . . . . . . . . . . . . 42
C.10. Since draft-ietf-httpbis-cache-08 . . . . . . . . . . . . 41 C.10. Since draft-ietf-httpbis-cache-08 . . . . . . . . . . . . 42
C.11. Since draft-ietf-httpbis-cache-09 . . . . . . . . . . . . 41 C.11. Since draft-ietf-httpbis-cache-09 . . . . . . . . . . . . 42
C.12. Since draft-ietf-httpbis-cache-10 . . . . . . . . . . . . 41 C.12. Since draft-ietf-httpbis-cache-10 . . . . . . . . . . . . 43
C.13. Since draft-ietf-httpbis-cache-11 . . . . . . . . . . . . 42 C.13. Since draft-ietf-httpbis-cache-11 . . . . . . . . . . . . 43
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 42 C.14. Since draft-ietf-httpbis-cache-12 . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
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. HTTP is defined by a series of hypertext information systems. It is typically used for distributed
documents that collectively form the HTTP/1.1 specification: information systems, where the use of response caches can improve
performance. This document defines aspects of HTTP related to
o "HTTP Semantics" [Semantics] caching and reusing response messages.
o "HTTP Caching" (this document)
o "HTTP/1.1 Messaging" [Messaging]
HTTP is typically used for distributed information systems, where the
use of response caches can improve performance. This document
defines aspects of HTTP related to caching and reusing response
messages.
An HTTP cache is a local store of response messages and the subsystem An HTTP _cache_ is a local store of response messages and the
that controls storage, retrieval, and deletion of messages in it. A subsystem that controls storage, retrieval, and deletion of messages
cache stores cacheable responses to reduce the response time and in it. A cache stores cacheable responses to reduce the response
network bandwidth consumption on future equivalent requests. Any time and network bandwidth consumption on future equivalent requests.
client or server MAY use a cache, though a server that is acting as a Any client or server MAY use a cache, though a server that is acting
tunnel cannot. as a tunnel cannot.
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 HTTP caching's goal is significantly improving performance by reusing
a prior response message to satisfy a current request. A cache a prior response message to satisfy a current request. A cache
considers a stored response "fresh", as defined in Section 4.2, if it considers a stored response "fresh", as defined in Section 4.2, if it
can be reused without "validation" (checking with the origin server can be reused without "validation" (checking with the origin server
to see if the cached response remains valid for this request). A to see if the cached response remains valid for this request). A
fresh response can therefore reduce both latency and network overhead fresh response can therefore reduce both latency and network overhead
each time the cache reuses it. When a cached response is not fresh, each time the cache reuses it. When a cached response is not fresh,
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Section 2 of [Semantics] defines conformance criteria and contains Section 2 of [Semantics] 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 5.7.1 of It also uses a list extension, defined in Section 5.6.1 of
[Semantics], that allows for compact definition of comma-separated [Semantics], that allows for compact definition of comma-separated
lists using a '#' operator (similar to how the '*' operator indicates lists using a '#' operator (similar to how the '*' operator indicates
repetition). Appendix A shows the collected grammar with all list repetition). Appendix A shows the collected grammar with all list
operators expanded to standard ABNF notation. operators expanded to standard ABNF notation.
The following core rules are included by reference, as defined in The following core rule is included by reference, as defined in
[RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF [RFC5234], Appendix B.1: DIGIT (decimal 0-9).
(CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line
feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any
visible [USASCII] character).
[Semantics] defines the following rules: [Semantics] defines the following rules:
HTTP-date = <HTTP-date, see [Semantics], Section 5.7.7> HTTP-date = <HTTP-date, see [Semantics], Section 5.6.7>
OWS = <OWS, see [Semantics], Section 5.7.3> OWS = <OWS, see [Semantics], Section 5.6.3>
field-name = <field-name, see [Semantics], Section 5.4.3> field-name = <field-name, see [Semantics], Section 5.1>
quoted-string = <quoted-string, see [Semantics], Section 5.7.4> quoted-string = <quoted-string, see [Semantics], Section 5.6.4>
token = <token, see [Semantics], Section 5.7.2> token = <token, see [Semantics], Section 5.6.2>
1.3. Delta Seconds 1.3. Delta Seconds
The delta-seconds rule specifies a non-negative integer, representing The delta-seconds rule specifies a non-negative integer, representing
time in seconds. time in seconds.
delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
A recipient parsing a delta-seconds value and converting it to binary A recipient parsing a delta-seconds value and converting it to binary
form ought to use an arithmetic type of at least 31 bits of 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 | canned string if any overflow occurs, even if the calculations
| are performed with an arithmetic type incapable of directly | are performed with an arithmetic type incapable of directly
| representing that number. What matters here is that an | 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.
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Proper cache operation preserves the semantics of HTTP transfers Proper cache operation preserves the semantics of HTTP transfers
([Semantics]) while reducing the transfer of information already held ([Semantics]) while reducing the transfer of information already held
in the cache. Although caching is an entirely OPTIONAL feature of in the cache. Although caching is an entirely OPTIONAL feature of
HTTP, it can be assumed that reusing a cached response is desirable HTTP, it can be assumed that reusing a cached response is desirable
and that such reuse is the default behavior when no requirement or and that such reuse is the default behavior when no requirement or
local configuration prevents it. Therefore, HTTP cache requirements local configuration prevents it. Therefore, HTTP cache requirements
are focused on preventing a cache from either storing a non-reusable are focused on preventing a cache from either storing a non-reusable
response or reusing a stored response inappropriately, rather than response or reusing a stored response inappropriately, rather than
mandating that caches always store and reuse particular responses. mandating that caches always store and reuse particular responses.
The base cache key comprises the request method and target URI used The base _cache key_ comprises the request method and target URI used
to retrieve the stored response; the method determines under which to retrieve the stored response; the method determines under which
circumstances that response can be used to satisfy a request. circumstances that response can be used to satisfy a request.
However, many HTTP caches in common use today only cache GET However, many HTTP caches in common use today only cache GET
responses, and therefore only use the URI as the cache key, 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, as per Section 4.1. header fields into the cache key as well, 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 8.3.1 of [Semantics]). representation of the target resource (Section 9.3.1 of [Semantics]).
However, it is also possible to store redirects, negative results However, it is also possible to store redirects, negative results
(e.g., 404 (Not Found)), incomplete results (e.g., 206 (Partial (e.g., 404 (Not Found)), incomplete results (e.g., 206 (Partial
Content)), and responses to methods other than GET if the method's Content)), and responses to methods other than GET if the method's
definition allows such caching and defines something suitable for use definition allows such caching and defines something suitable for use
as a cache key. as a cache key.
A cache is disconnected when it cannot contact the origin server or A cache is _disconnected_ when it cannot contact the origin server or
otherwise find a forward path for a request. A disconnected cache otherwise find a forward path for a request. A disconnected cache
can serve stale responses in some circumstances (Section 4.2.4). can serve stale responses in some circumstances (Section 4.2.4).
3. Storing Responses in Caches 3. Storing Responses in Caches
A cache MUST NOT store a response to a request unless: A cache MUST NOT store a response to a request unless:
o the request method is understood by the cache; o the request method is understood by the cache;
o the response status code is final (see Section 14 of [Semantics]); o the response status code is final (see Section 15 of [Semantics]);
o if the response status code is 206 or 304, or the "must- o if the response status code is 206 or 304, or the "must-
understand" cache directive (see Section 5.2) is present: the understand" cache directive (see Section 5.2) is present: the
cache understands the response status code; cache understands the response status code;
o the "no-store" cache directive is not present in the response (see o the "no-store" cache directive is not present in the response (see
Section 5.2); Section 5.2);
o if the cache is shared: the "private" response directive is either o if the cache is shared: the "private" response directive is either
not present or allows a shared cache to store a modified response; not present or allows a shared cache to store a modified response;
see Section 5.2.2.7); see Section 5.2.2.7);
o if the cache is shared: the Authorization header field is not o if the cache is shared: the Authorization header field is not
present in the request (see Section 10.6.2 of [Semantics]) or a present in the request (see Section 11.6.2 of [Semantics]) or a
response directive is present that explicitly allows shared response directive is present that explicitly allows shared
caching (see Section 3.3); and, caching (see Section 3.4); and,
o the response contains at least one of: o the response contains at least one of:
* a public response directive (see Section 5.2.2.6); * a public response directive (see Section 5.2.2.6);
* a private response directive, if the cache is not shared (see * a private response directive, if the cache is not shared (see
Section 5.2.2.7); Section 5.2.2.7);
* an Expires header field (see Section 5.3); * an Expires header field (see Section 5.3);
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response status code if it recognizes it and implements all specified response status code if it recognizes it and implements all specified
caching-related behavior. caching-related behavior.
Note that, in normal operation, some caches will not store a response Note that, in normal operation, some caches will not store a response
that has neither a cache validator nor an explicit expiration time, that has neither a cache validator nor an explicit expiration time,
as such responses are not usually useful to store. However, caches as such responses are not usually useful to store. However, caches
are not prohibited from storing such responses. are not prohibited from storing such responses.
3.1. Storing Header and Trailer Fields 3.1. Storing Header and Trailer Fields
Caches MUST include all received 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:
o The Connection header field and fields whose names are listed in o The Connection header field and fields whose names are listed in
it are required by Section 13.1 of [Messaging] to be removed it are required by Section 7.6.1 of [Semantics] to be removed
before forwarding the message. This MAY be implemented by doing before forwarding the message. This MAY be implemented by doing
so before storage. so before storage.
o Likewise, some fields' semantics require them to be removed before o Likewise, some fields' semantics require them to be removed before
forwarding the message, and this MAY be implemented by doing so forwarding the message, and this MAY be implemented by doing so
before storage; see Section 13.1 of [Messaging] for some examples. before storage; see Section 7.6.1 of [Semantics] for some
examples.
o The no-cache (Section 5.2.2.3) and private (Section 5.2.2.7) cache
directives can have arguments that prevent storage of header
fields by all caches and shared caches, respectively.
o Header fields that are specific to a client's proxy configuration o Header fields that are specific to a client's proxy configuration
MUST NOT be stored, unless the cache incorporates the identity of MUST NOT be stored, unless the cache incorporates the identity of
the proxy into the cache key. Effectively, this is limited to the proxy into the cache key. Effectively, this is limited to
Proxy-Authenticate (Section 10.7.1 of [Semantics]), Proxy- Proxy-Authenticate (Section 11.7.1 of [Semantics]), Proxy-
Authentication-Info (Section 10.7.3 of [Semantics]), and Proxy- Authentication-Info (Section 11.7.3 of [Semantics]), and Proxy-
Authorization (Section 10.7.2 of [Semantics]). Authorization (Section 11.7.2 of [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. Storing Incomplete Responses 3.2. Updating Stored Header Fields
Caches are required to update a stored response's header fields from
another (typically newer) response in several situations; for
example, see Section 3.5, Section 4.3.4 and Section 4.3.5.
When doing so, the cache MUST add each header field in the provided
response to the stored response, replacing field values that are
already present, with the following exceptions:
o Header fields excepted from storage in Section 3.1,
o Header fields that the cache's stored representation of the
response depends upon, as described below,
o Header fields that are automatically processed and removed by the
recipient, as described below, and
o The Content-Length header field.
In some cases, caches (especially in user agents) store processed
representations of the received response, rather than the response
itself, and updating header fields that affect that processing can
result in inconsistent behavior and security issues. Caches in this
situation MAY omit these header fields from updating stored
representations on an exceptional basis, but SHOULD limit such
omission to those fields necessary to assure integrity of the stored
representation.
For example, a browser might decode the content coding of a response
payload while it is being received, creating a disconnect between the
data it has stored and the response payload's original metadata.
Updating that stored metadata with a different Content-Encoding
header field would be problematic. Likewise, a browser might store a
post-parse tree representation of HTML, rather than the payload
received in the response; updating the Content-Type header field
would not be workable in this case, because any assumptions about the
format made in parsing would now be invalid.
Furthermore, some fields are automatically processed and removed by
the HTTP implementation; for example, the Content-Range header field.
Implementations MAY automatically omit such header fields from
updates, even when the processing does not actually occur.
Note that the Content-* prefix is not a signal that a header field is
omitted from update; it is a convention for MIME header fields, not
HTTP.
3.3. Storing Incomplete Responses
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 3.3 of store a response body that is not complete (Section 3.3 of
[Semantics]) if the stored response is recorded as being incomplete. [Semantics]) if the stored response is recorded as being incomplete.
Likewise, a 206 (Partial Content) response MAY be stored as if it Likewise, a 206 (Partial Content) response MAY be stored as if it
were an incomplete 200 (OK) response. However, a cache MUST NOT were an incomplete 200 (OK) response. However, a cache MUST NOT
store incomplete or partial-content responses if it does not support store incomplete or partial-content responses if it does not support
the Range and Content-Range header fields or if it does not the Range and Content-Range header fields or if it does not
understand the range units used in those fields. understand the range units used in those fields.
A cache MAY complete a stored incomplete response by making a A cache MAY complete a stored incomplete response by making a
subsequent range request (Section 13.2 of [Semantics]) and combining subsequent range request (Section 14.2 of [Semantics]) and combining
the successful response with the stored response, as defined in the successful response with the stored response, as defined in
Section 3.4. A cache MUST NOT use an incomplete response to answer Section 3.5. 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.
3.3. Storing Responses to Authenticated Requests 3.4. 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 10.6.2 of [Semantics]) to satisfy Authorization header field (Section 11.6.2 of [Semantics]) to satisfy
any subsequent request unless the response contains a Cache-Control any subsequent request unless the response contains a Cache-Control
field with a response directive (Section 5.2.2) that allows it to be field with a response directive (Section 5.2.2) that allows it to be
stored by a shared cache and the cache conforms to the requirements stored by a shared cache and the cache conforms to the requirements
of that directive for that response. of that 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 5.2.2.1), public (Section 5.2.2.6), effect: must-revalidate (Section 5.2.2.1), public (Section 5.2.2.6),
and s-maxage (Section 5.2.2.10). and s-maxage (Section 5.2.2.10).
3.4. Combining Partial Content 3.5. 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 13.2 of [Semantics]). After several such Range specifiers (Section 14.2 of [Semantics]). 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 14.3.7.3 of [Semantics]. with the client requirements in Section 15.3.7.3 of [Semantics].
When combining the new response with one or more stored responses, a When combining the new response with one or more stored responses, a
cache MUST use the header fields provided in the new response, aside cache MUST update the stored response header fields using the header
from Content-Range, to replace all instances of the corresponding fields provided in the new response, as per Section 3.2.
header fields in the stored response.
4. Constructing Responses from Caches 4. Constructing Responses from Caches
When presented with a request, a cache MUST NOT reuse a stored When presented with a request, a cache MUST NOT reuse a stored
response, unless: response, unless:
o The presented target URI (Section 6.1 of [Semantics]) and that of o The presented target URI (Section 7.1 of [Semantics]) and that of
the stored response match, and the stored response match, and
o the request method associated with the stored response allows it o the request method associated with the stored response allows it
to be used for the presented request, and to be used for the presented request, and
o selecting header fields nominated by the stored response (if any) o selecting header fields nominated by the stored response (if any)
match those presented (see Section 4.1), and match those presented (see Section 4.1), and
o the stored response does not contain the no-cache cache directive o the stored response does not contain the no-cache cache directive
(Section 5.2.2.3), unless it is successfully validated (Section 5.2.2.3), unless it is successfully validated
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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 8.2.1 of [Semantics]) to the origin server; i.e., a cache is (Section 9.2.1 of [Semantics]) to the origin server; i.e., a cache is
not allowed to generate a reply to such a request before having not allowed to generate a reply to such a request before having
forwarded the request and having received a corresponding response. forwarded the request and having received a corresponding response.
Also, note that unsafe requests might invalidate already-stored Also, note that unsafe requests might invalidate already-stored
responses; see Section 4.4. responses; see Section 4.4.
A response that is stored or storable can be used to satisfy multiple
requests, provided that it is allowed to reuse that response for the
requests in question. This enables caches to "collapse" multiple
incoming requests into a single forward request upon a cache miss,
thereby reducing load on the origin server and network. However,
note that if the response returned is not able to be used for some or
all of the collapsed requests, additional latency might be
introduced, because they will need to be forwarded to be satisfied.
When more than one suitable response is stored, a cache MUST use the When more than one suitable response is stored, a cache MUST use the
most recent 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 Vary
When a cache receives a request that can be satisfied by a stored When a cache receives a request that can be satisfied by a stored
response that has a Vary header field (Section 11.2.1 of response that has a Vary header field (Section 12.5.5 of
[Semantics]), it MUST NOT use that response unless all the selecting [Semantics]), it MUST NOT use that response unless all the selecting
header fields nominated by the Vary header field match in both the header fields nominated by the Vary header field match in both the
original request (i.e., that associated with the stored response), original request (i.e., that associated with the stored response),
and the presented request. and the presented request.
The selecting header fields from two requests are defined to match if The selecting header field values from two requests are defined to
and only if those in the first request can be transformed to those in match if and only if those in the first request can be transformed to
the second request by applying any of: those in the second request by applying any of:
o adding or removing whitespace, where allowed in the header field's o adding or removing whitespace, where allowed in the header field's
syntax syntax
o combining multiple header fields with the same field name (see o combining multiple header field lines with the same field name
Section 5.4.4 of [Semantics]) (see Section 5.2 of [Semantics])
o normalizing both header field values in a way that is known to o normalizing both header field values in a way that is known to
have identical semantics, according to the header field's have identical semantics, according to the header field's
specification (e.g., reordering field values when order is not specification (e.g., reordering field values when order is not
significant; case-normalization, where values are defined to be significant; case-normalization, where values are defined to be
case-insensitive) case-insensitive)
If (after any normalization that might take place) a header field is If (after any normalization that might take place) a header field is
absent from a request, it can only match another request if it is absent from a request, it can only match another request if it is
also absent there. also absent there.
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A Vary header field value containing a member "*" always fails to A Vary header field value containing a member "*" always fails to
match. match.
The stored response with matching selecting header fields is known as The stored response with matching selecting header fields is known as
the selected response. the selected response.
If multiple selected responses are available (potentially including If multiple selected responses are available (potentially including
responses without a Vary header field), the cache will need to choose responses without a Vary header field), the cache will need to choose
one to use. When a selecting header field has a known mechanism for one to use. When a selecting header field has a known mechanism for
doing so (e.g., qvalues on Accept and similar request header fields), doing so (e.g., qvalues on Accept and similar request header fields),
that mechanism MAY be used to select preferred responses; of the that mechanism MAY be used to select a preferred response. If such a
remainder, the most recent response (as determined by the Date header mechanism is not available, or leads to equally preferred responses,
field) is used, as per Section 4. the most recent response (as determined by the Date header field) is
used, as per Section 4.
Note that in practice, some resources might send the Vary header Some resources mistakenly omit the Vary header field from their
field on responses inconsistently. When a cache has multiple default response (i.e., the one sent when no more preferable response
responses for a target URI, and one or more omits the Vary header is available), selecting it for requests to that resource even when
field, it SHOULD use the most recent non-empty value available to more preferable responses are available. When a cache has multiple
select an appropriate response for the request. responses for a target URI and one or more omits the Vary header
field, it SHOULD use the most recent (see Section 4.2.3) valid Vary
field value available to select an appropriate response for the
request.
If no selected response is available, the cache cannot satisfy the If no selected response is available, the cache cannot satisfy the
presented request. Typically, it is forwarded to the origin server presented request. Typically, it is forwarded to the origin server
in a (possibly conditional; see Section 4.3) request. in a (possibly conditional; see Section 4.3) request.
4.2. Freshness 4.2. Freshness
A fresh response is one whose age has not yet exceeded its freshness A _fresh_ response is one whose age has not yet exceeded its
lifetime. Conversely, a stale response is one where it has. freshness lifetime. Conversely, a _stale_ response is one where it
has.
A response's freshness lifetime is the length of time between its A response's _freshness lifetime_ is the length of time between its
generation by the origin server and its expiration time. An explicit generation by the origin server and its expiration time. An
expiration time is the time at which the origin server intends that a _explicit expiration time_ is the time at which the origin server
stored response can no longer be used by a cache without further intends that a stored response can no longer be used by a cache
validation, whereas a heuristic expiration time is assigned by a without further validation, whereas a _heuristic expiration time_ is
cache when no explicit expiration time is available. assigned by a cache when no explicit expiration time is available.
A response's age is the time that has passed since it was generated A response's _age_ is the time that has passed since it was generated
by, or successfully validated with, the origin server. by, or successfully validated with, the origin server.
When a response is "fresh" in the cache, it can be used to satisfy When a response is "fresh" in the cache, it can be used to satisfy
subsequent requests without contacting the origin server, thereby subsequent requests without contacting the origin server, thereby
improving efficiency. improving efficiency.
The primary mechanism for determining freshness is for an origin The primary mechanism for determining freshness is for an origin
server to provide an explicit expiration time in the future, using server to provide an explicit expiration time in the future, using
either the Expires header field (Section 5.3) or the max-age response either the Expires header field (Section 5.3) or the max-age response
directive (Section 5.2.2.9). Generally, origin servers will assign directive (Section 5.2.2.9). Generally, origin servers will assign
skipping to change at page 13, line 38 skipping to change at page 14, line 34
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 constrain or relax freshness calculations for the
corresponding response. However, caches are not required to honor corresponding response. However, caches are not required to honor
them. them.
When calculating freshness, to avoid common problems in date parsing: When calculating freshness, to avoid common problems in date parsing:
o Although all date formats are specified to be case-sensitive, a o Although all date formats are specified to be case-sensitive, a
cache recipient SHOULD match day, week, and time-zone names case- cache recipient SHOULD match the field value case-insensitively.
insensitively.
o If a cache recipient's internal implementation of time has less o If a cache recipient's internal implementation of time has less
resolution than the value of an HTTP-date, the recipient MUST resolution than the value of an HTTP-date, the recipient MUST
internally represent a parsed Expires date as the nearest time internally represent a parsed Expires date as the nearest time
equal to or earlier than the received value. equal to or earlier than the received value.
o A cache recipient MUST NOT allow local time zones to influence the o A cache recipient MUST NOT allow local time zones to influence the
calculation or comparison of an age or expiration time. calculation or comparison of an age or expiration time.
o A cache recipient SHOULD consider a date with a zone abbreviation o A cache recipient SHOULD consider a date with a zone abbreviation
other than GMT or UTC to be invalid for calculating expiration. other than "GMT" to be invalid for calculating expiration.
Note that freshness applies only to cache operation; it cannot be Note that freshness applies only to cache operation; it cannot be
used to force a user agent to refresh its display or reload a used to force a user agent to refresh its display or reload a
resource. See Section 6 for an explanation of the difference between resource. See Section 6 for an explanation of the difference between
caches and history mechanisms. caches and history mechanisms.
4.2.1. Calculating Freshness Lifetime 4.2.1. Calculating Freshness Lifetime
A cache can calculate the freshness lifetime (denoted as A cache can calculate the freshness lifetime (denoted as
freshness_lifetime) of a response by using the first match of: freshness_lifetime) of a response by using the first match of:
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its value minus the value of the Date response header field, or its value minus the value of the Date response header field, or
o Otherwise, no explicit expiration time is present in the response. o Otherwise, no explicit expiration time is present in the response.
A heuristic freshness lifetime might be applicable; see A heuristic freshness lifetime might be applicable; see
Section 4.2.2. Section 4.2.2.
Note that this calculation is not vulnerable to clock skew, since all Note that this calculation is not vulnerable to clock skew, since all
of the information comes from the origin server. of the information comes from the origin server.
When there is more than one value present for a given directive When there is more than one value present for a given directive
(e.g., two Expires header fields, multiple Cache-Control: max-age (e.g., two Expires header field lines or multiple Cache-Control: max-
directives), the directive's value is considered invalid. Caches are age directives), either the first occurrence should be used, or the
encouraged to consider responses that have invalid freshness response should be considered stale. If directives conflict (e.g.,
information to be stale. both max-age and no-cache are present), the most restrictive
directive should be honored. Caches are encouraged to consider
responses that have invalid freshness information (e.g., a max-age
directive with non-integer content) to be stale.
4.2.2. Calculating Heuristic Freshness 4.2.2. Calculating Heuristic Freshness
Since origin servers do not always provide explicit expiration times, Since origin servers do not always provide explicit expiration times,
a cache MAY assign a heuristic expiration time when an explicit time a cache MAY assign a heuristic expiration time when an explicit time
is not specified, employing algorithms that use other header field is not specified, employing algorithms that use other field values
values (such as the Last-Modified time) to estimate a plausible (such as the Last-Modified time) to estimate a plausible expiration
expiration time. This specification does not provide specific time. This specification does not provide specific algorithms, but
algorithms, but does impose worst-case constraints on their results. does impose worst-case constraints on their results.
A cache MUST NOT use heuristics to determine freshness when an A cache MUST NOT use heuristics to determine freshness when an
explicit expiration time is present in the stored response. Because explicit expiration time is present in the stored response. Because
of the requirements in Section 3, this means that 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 14.1 of are defined as "_heuristically cacheable_" (e.g., see Section 15.1 of
[Semantics]), and those responses without explicit freshness that [Semantics]), and those responses without explicit freshness that
have been marked as explicitly cacheable (e.g., with a "public" have been marked as explicitly cacheable (e.g., with a "public"
response directive). response directive).
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 7.9.2 of If the response has a Last-Modified header field (Section 8.9.2 of
[Semantics]), caches are encouraged to use a heuristic expiration [Semantics]), caches are encouraged to use a heuristic expiration
value that is no more than some fraction of the interval since that value that is no more than some fraction of the interval since that
time. A typical setting of this fraction might be 10%. time. 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.
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The Age header field is used to convey an estimated age of the The Age header field is used to convey an estimated age of the
response message when obtained from a cache. The Age field value is response message when obtained from a cache. The Age field value is
the cache's estimate of the number of seconds since the origin server the cache's estimate of the number of seconds since the origin server
generated or validated the response. The Age value is therefore the generated or validated the response. The Age value is therefore the
sum of the time that the response has been resident in each of the sum of the time that the response has been resident in each of the
caches along the path from the origin server, plus the time it has caches along the path from the origin server, plus the time it has
been in transit along network paths. been in transit along network paths.
Age calculation uses the following data: Age calculation uses the following data:
age_value The term "age_value" denotes the value of the Age header _age_value_ 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 9.2.2 of [Semantics] for the definition of the Date See Section 10.2.2 of [Semantics] for the definition of the Date
header field, and for requirements regarding responses without it. header field, and for requirements regarding responses without it.
now The term "now" means "the current value of the clock at the host _now_ The term "now" means "the current value of the clock at the
performing the calculation". A host ought to use NTP ([RFC5905]) host performing the calculation". A host ought to use NTP
or some similar protocol to synchronize its clocks to Coordinated ([RFC5905]) or some similar protocol to synchronize its clocks to
Universal Time. Coordinated Universal Time.
request_time The current value of the clock at the host at the time _request_time_ The current value of the clock at the host at the
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 time _response_time_ The current value of the clock at the host at the
the response was received. time the response was received.
A response's age can be calculated in two entirely independent ways: A response's age can be calculated in two entirely independent ways:
1. the "apparent_age": response_time minus date_value, if the local 1. the "apparent_age": response_time minus date_value, if the local
clock is reasonably well synchronized to the origin server's clock is reasonably well synchronized to the origin server's
clock. If the result is negative, the result is replaced by clock. If the result is negative, the result is replaced by
zero. zero.
2. the "corrected_age_value", if all of the caches along the 2. the "corrected_age_value", if all of the caches along the
response path implement HTTP/1.1 or greater. A cache MUST response path implement HTTP/1.1 or greater. A cache MUST
skipping to change at page 17, line 6 skipping to change at page 17, line 43
resident_time = now - response_time; resident_time = now - response_time;
current_age = corrected_initial_age + resident_time; current_age = corrected_initial_age + resident_time;
4.2.4. Serving Stale Responses 4.2.4. Serving Stale Responses
A "stale" response is one that either has explicit expiry information A "stale" response is one that either has explicit expiry information
or is allowed to have heuristic expiry calculated, but is not fresh or is allowed to have heuristic expiry calculated, but is not fresh
according to the calculations in Section 4.2. according to the calculations in Section 4.2.
A cache MUST NOT generate a stale response if it is prohibited by an A cache MUST NOT generate a stale response if it is prohibited by an
explicit in-protocol directive (e.g., by a "no-store" or "no-cache" explicit in-protocol directive (e.g., by a "no-cache" cache
cache directive, a "must-revalidate" cache-response-directive, or an directive, a "must-revalidate" cache-response-directive, or an
applicable "s-maxage" or "proxy-revalidate" cache-response-directive; applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
see Section 5.2.2). see Section 5.2.2).
A cache MUST NOT generate a stale response unless it is disconnected A cache MUST NOT generate a stale response unless it is disconnected
or doing so is explicitly permitted by the client or origin server or doing so is explicitly permitted by the client or origin server
(e.g., by the max-stale request directive in Section 5.2.1, by (e.g., by the max-stale request directive in Section 5.2.1, 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 12.1 of [Semantics] in the forwarded request mechanism (Section 13.1 of [Semantics]) in the forwarded
request to give the next inbound server an opportunity to select a request to give the next inbound server an opportunity to select a
valid stored response to use, updating the stored metadata in the valid stored response to use, updating the stored metadata in the
process, or to replace the stored response(s) with a new response. process, or to replace the stored response(s) with a new response.
This process is known as "validating" or "revalidating" the stored This process is known as "_validating_" or "_revalidating_" the
response. stored 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 cache key. response(s) that have the same cache key.
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 7.9.2 of [Semantics]), which can be used in an If- field (Section 8.9.2 of [Semantics]), which can be used in an If-
Modified-Since header field for response validation, or in an If- Modified-Since header field for response validation, or in an If-
Unmodified-Since or If-Range header field for representation Unmodified-Since or If-Range header field for representation
selection (i.e., the client is referring specifically to a previously selection (i.e., the client is referring specifically to a previously
obtained representation with that timestamp). obtained representation with that timestamp).
Another validator is the entity-tag given in an ETag field Another validator is the entity-tag given in an ETag field
(Section 7.9.3 of [Semantics]). One or more entity-tags, indicating (Section 8.9.3 of [Semantics]). One or more entity-tags, indicating
one or more stored responses, can be used in an If-None-Match header one or more stored responses, can be used in an If-None-Match header
field for response validation, or in an If-Match or If-Range header field for response validation, or in an If-Match or If-Range header
field for representation selection (i.e., the client is referring field for representation selection (i.e., the client is referring
specifically to one or more previously obtained representations with specifically to one or more previously obtained representations with
the listed entity-tags). the listed entity-tags).
4.3.2. Handling a Received Validation Request 4.3.2. Handling a Received Validation Request
Each client in the request chain may have its own cache, so it is Each client in the request chain may have its own cache, so it is
common for a cache at an intermediary to receive conditional requests common for a cache at an intermediary to receive conditional requests
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received in that request with respect to the corresponding validators received in that request with respect to the corresponding validators
contained within the selected response. A cache MUST NOT evaluate contained within the selected response. A cache MUST NOT evaluate
conditional header fields that only apply to an origin server, occur conditional header fields that only apply to an origin server, occur
in a request with semantics that cannot be satisfied with a cached in a request with semantics that cannot be satisfied with a cached
response, or occur in a request with a target resource for which it response, or occur in a request with a target resource for which it
has no stored responses; such preconditions are likely intended for has no stored responses; such preconditions are likely intended for
some other (inbound) server. some other (inbound) server.
The proper evaluation of conditional requests by a cache depends on The proper evaluation of conditional requests by a cache depends on
the received precondition header fields and their precedence, as the received precondition header fields and their precedence, as
defined in Section 12.3 of [Semantics]. The If-Match and If- defined in Section 13.3 of [Semantics]. The If-Match and If-
Unmodified-Since conditional header fields are not applicable to a Unmodified-Since conditional header fields are not applicable to a
cache. cache.
A request containing an If-None-Match header field (Section 12.1.2 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 [Semantics]) indicates that the client wants to validate one or more
of its own stored responses in comparison to whichever stored of its own stored responses in comparison to whichever stored
response is selected by the cache. If the field value is "*", or if response is selected by the cache. If the field value is "*", or if
the field value is a list of entity-tags and at least one of them the field value is a list of entity-tags and at least one of them
matches the entity-tag of the selected stored response, a cache matches the entity-tag of the selected stored response, a cache
recipient SHOULD generate a 304 (Not Modified) response (using the recipient SHOULD generate a 304 (Not Modified) response (using the
metadata of the selected stored response) instead of sending that metadata of the selected stored response) instead of sending that
stored response. stored response.
When a cache decides to revalidate its own stored responses for a When a cache decides to revalidate its own stored responses for a
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content, the cache MUST NOT include its entity-tag in the union content, the cache MUST NOT include its entity-tag in the union
unless the request is for a range that would be fully satisfied by unless the request is for a range that would be fully satisfied by
that partial stored response. If the response to the forwarded that partial stored response. If the response to the forwarded
request is 304 (Not Modified) and has an ETag field value with an request is 304 (Not Modified) and has an ETag field value with an
entity-tag that is not in the client's list, the cache MUST generate entity-tag that is not in the client's list, the cache MUST generate
a 200 (OK) response for the client by reusing its corresponding a 200 (OK) response for the client by reusing its corresponding
stored response, as updated by the 304 response metadata stored response, as updated by the 304 response metadata
(Section 4.3.4). (Section 4.3.4).
If an If-None-Match header field is not present, a request containing If an If-None-Match header field is not present, a request containing
an If-Modified-Since header field (Section 12.1.3 of [Semantics]) an If-Modified-Since header field (Section 13.1.3 of [Semantics])
indicates that the client wants to validate one or more of its own indicates that the client wants to validate one or more of its own
stored responses by modification date. A cache recipient SHOULD stored responses by modification date. A cache recipient SHOULD
generate a 304 (Not Modified) response (using the metadata of the generate a 304 (Not Modified) response (using the metadata of the
selected stored response) if one of the following cases is true: 1) selected stored response) if one of the following cases is true:
the selected stored response has a Last-Modified field value that is
earlier than or equal to the conditional timestamp; 2) no Last- 1. the selected stored response has a Last-Modified field value that
Modified field is present in the selected stored response, but it has is earlier than or equal to the conditional timestamp;
a Date field value that is earlier than or equal to the conditional
timestamp; or, 3) neither Last-Modified nor Date is present in the 2. no Last-Modified header field is present in the selected stored
selected stored response, but the cache recorded it as having been response, but it has a Date field value that is earlier than or
received at a time earlier than or equal to the conditional equal to the conditional timestamp; or,
timestamp.
3. neither Last-Modified nor Date is present in the selected stored
response, but the cache recorded it as having been received at a
time earlier than or equal to the conditional timestamp.
A cache that implements partial responses to range requests, as A cache that implements partial responses to range requests, as
defined in Section 13.2 of [Semantics], also needs to evaluate a defined in Section 14.2 of [Semantics], also needs to evaluate a
received If-Range header field (Section 12.1.5 of [Semantics]) received If-Range header field (Section 13.1.5 of [Semantics])
regarding its selected stored response. regarding its selected stored response.
4.3.3. Handling a Validation Response 4.3.3. Handling a Validation Response
Cache handling of a response to a conditional request depends upon Cache handling of a response to a conditional request depends upon
its status code: its status code:
o A 304 (Not Modified) response status code indicates that the o A 304 (Not Modified) response status code indicates that the
stored response can be updated and reused; see Section 4.3.4. stored response can be updated and reused; see Section 4.3.4.
o A full response (i.e., one with a payload body) indicates that o A full response (i.e., one with a payload body) indicates that
none of the stored responses nominated in the conditional request none of the stored responses nominated in the conditional request
is suitable. Instead, the cache MUST use the full response to is suitable. Instead, the cache MUST use the full response to
satisfy the request and MAY replace the stored response(s). satisfy the request. The cache MAY store such a full response,
subject to its constraints (see Section 3).
o However, if a cache receives a 5xx (Server Error) response while o However, if a cache receives a 5xx (Server Error) response while
attempting to validate a response, it can either forward this attempting to validate a response, it can either forward this
response to the requesting client, or act as if the server failed response to the requesting client, or act as if the server failed
to respond. In the latter case, the cache MAY send a previously to respond. In the latter case, the cache can send a previously
stored response (see Section 4.2.4). stored response, subject to its constraints on doing so (see
Section 4.2.4), or retry the validation request.
4.3.4. Freshening Stored Responses upon Validation 4.3.4. Freshening Stored Responses upon Validation
When a cache receives a 304 (Not Modified) response and already has When a cache receives a 304 (Not Modified) response and already has
one or more stored 200 (OK) responses for the applicable cache key, one or more stored 200 (OK) responses for the applicable cache key,
the cache needs to identify which (if any) are to be updated by the the cache needs to identify which (if any) are to be updated by the
new 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 stored response(s) to update are identified by using the first
match (if any) of: match (if any) of:
o If the new response contains a strong validator (see Section 7.9.1 o If the new response contains a _strong validator_ (see
of [Semantics]), then that strong validator identifies the Section 8.9.1 of [Semantics]), then that strong validator
selected representation for update. All the stored responses with identifies the selected representation for update. All the stored
the same strong validator are identified for update. If none of responses with the same strong validator are identified for
the stored responses contain the same strong validator, then the update. If none of the stored responses contain the same strong
cache MUST NOT use the new response to update any stored validator, then the cache MUST NOT use the new response to update
responses. any stored responses.
o If the new response contains a weak validator and that validator o If the new response contains a _weak validator_ and that validator
corresponds to one of the cache's stored responses, then the most corresponds to one of the cache's stored responses, then the most
recent of those matching stored responses is identified for recent of those matching stored responses is identified for
update. update.
o If the new response does not include any form of validator (such o If the new response does not include any form of validator (such
as where a client generates an If-Modified-Since request from a as where a client generates an If-Modified-Since request from a
source other than the Last-Modified response header field), and source other than the Last-Modified response header field), and
there is only one stored response, and that stored response also there is only one stored response, and that stored response also
lacks a validator, then that stored response is identified for lacks a validator, then that stored response is identified for
update. update.
For each stored response identified for update, the cache MUST use For each stored response identified, the cache MUST update its header
the header fields provided in the 304 (Not Modified) response to fields with the header fields provided in the 304 (Not Modified)
replace all instances of the corresponding header fields in the response, as per Section 3.2.
stored response, with the following exceptions:
o The exceptions to header field storage in Section 3.1 also apply
to header field updates.
o Caches MUST NOT update the following header fields: Content-
Encoding, Content-Length, Content-MD5 (Section 14.15 of
[RFC2616]), Content-Range, ETag.
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, except it lacks a body. request made with a GET would have been, except it lacks payload
This property of HEAD responses can be used to invalidate or update a data. This property of HEAD responses can be used to invalidate or
cached GET response if the more efficient conditional GET request update a cached GET response if the more efficient conditional GET
mechanism is not available (due to no validators being present in the request mechanism is not available (due to no validators being
stored response) or if transmission of the representation body is not present in the stored response) or if transmission of the payload
desired even if it has changed. data is not desired even if it has changed.
When a cache makes an inbound HEAD request for a target URI and When a cache makes an inbound HEAD request for a target URI and
receives a 200 (OK) response, the cache SHOULD update or invalidate receives a 200 (OK) response, the cache SHOULD update or invalidate
each of its stored GET responses that could have been selected for each of its stored GET responses that could have been selected for
that request (see Section 4.1). that request (see Section 4.1).
For each of the stored responses that could have been selected, if For each of the stored responses that could have been selected, if
the stored response and HEAD response have matching values for any the stored response and HEAD response have matching values for any
received validator fields (ETag and Last-Modified) and, if the HEAD received validator fields (ETag and Last-Modified) and, if the HEAD
response has a Content-Length header field, the value of Content- response has a Content-Length header field, the value of Content-
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 replace all instances of the corresponding header HEAD response to update the stored response (see Section 3.2).
fields in the stored response (subject to the exceptions in
Section 4.3.4) and append new header fields to the stored response's
header section unless otherwise restricted by the Cache-Control
header field.
4.4. Invalidation 4.4. Invalidating Stored Responses
Because unsafe request methods (Section 8.2.1 of [Semantics]) such as Because unsafe request methods (Section 9.2.1 of [Semantics]) such as
PUT, POST or DELETE have the potential for changing state on the PUT, POST or DELETE have the potential for changing state on the
origin server, intervening caches are required to invalidate stored origin server, intervening caches are required to invalidate stored
responses to keep their contents up to date. Invalidate means that responses to keep their contents up to date.
the cache will either remove all stored responses whose target URI
matches the given URI, or will mark them as "invalid" and in need of
a mandatory validation before they can be sent in response to a
subsequent request.
Note that this does not guarantee that all appropriate responses are
invalidated globally; a state-changing request would only invalidate
responses in the caches it travels through.
A cache MUST invalidate the target URI (Section 6.1 of [Semantics]) A cache MUST invalidate the target URI (Section 7.1 of [Semantics])
and the URI(s) in the Location and Content-Location response header when a non-error status code is received in response to an unsafe
fields (if present) when a non-error status code is received in request method (including methods whose safety is unknown).
response to an unsafe request method.
However, a cache MUST NOT invalidate a URI from a Location or A cache MAY invalidate other URIs when a non-error status code is
Content-Location response header field if the host part of that URI received in response to an unsafe request method (including methods
differs from the host part in the target URI (Section 6.1 of whose safety is unknown). In particular, the URI(s) in the Location
[Semantics]). This helps prevent denial-of-service attacks. and Content-Location response header fields (if present) are
candidates for invalidation; other URIs might be discovered through
mechanisms not specified in this document. However, a cache MUST NOT
trigger an invalidation under these conditions if the origin
(Section 4.3.1 of [Semantics]) of the URI to be invalidated differs
from that of the target URI (Section 7.1 of [Semantics]). This helps
prevent denial-of-service attacks.
A cache MUST invalidate the target URI (Section 6.1 of [Semantics]) _Invalidate_ means that the cache will either remove all stored
when it receives a non-error response to a request with a method responses whose target URI matches the given URI, or will mark them
whose safety is unknown. as "invalid" and in need of a mandatory validation before they can be
sent in response to a subsequent request.
Here, a "non-error response" is one with a 2xx (Successful) or 3xx A "non-error response" is one with a 2xx (Successful) or 3xx
(Redirection) status code. (Redirection) status code.
Note that this does not guarantee that all appropriate responses are
invalidated globally; a state-changing request would only invalidate
responses in the caches it travels through.
5. Field Definitions 5. Field Definitions
This section defines the syntax and semantics of HTTP fields related This section defines the syntax and semantics of HTTP fields related
to caching. to caching.
--------------- ----------- ------
Field Name Status Ref.
--------------- ----------- ------
Age standard 5.1
Cache-Control standard 5.2
Expires standard 5.3
Pragma standard 5.4
Warning obsoleted 5.5
--------------- ----------- ------
Table 1
5.1. Age 5.1. Age
The "Age" header field conveys the sender's estimate of the time The "Age" response header field conveys the sender's estimate of the
since the response was generated or successfully validated at the time since the response was generated or successfully validated at
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). A cache SHOULD consider a response to be seconds (see Section 1.3).
stale if an Age field is present and its value is invalid (i.e.,
contains a list or something other than a non-negative integer). Although it is defined as a singleton header field, a cache
encountering a message with multiple Age field lines SHOULD use the
first field line, discarding subsequent ones.
If the field value (after discarding additional lines, as per above)
is invalid (e.g., it contains a list or something other than a non-
negative integer), a cache SHOULD consider the response to be stale.
The presence of an Age header field implies that the response was not The presence of an Age header field implies that the response was not
generated or validated by the origin server for this request. generated or validated by the origin server for this request.
However, lack of an Age header field does not imply the origin was However, lack of an Age header field does not imply the origin was
contacted, since the response might have been received from an contacted, since the response might have been received from an
HTTP/1.0 cache that does not implement Age. HTTP/1.0 cache that does not implement Age.
5.2. Cache-Control 5.2. Cache-Control
The "Cache-Control" header field is used to list directives for The "Cache-Control" header field is used to list directives for
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define arguments, recipients ought to accept both forms, even if a define arguments, recipients ought to accept both forms, even if a
specific form is required for generation. specific form is required for generation.
Cache-Control = #cache-directive Cache-Control = #cache-directive
cache-directive = token [ "=" ( token / quoted-string ) ] cache-directive = token [ "=" ( token / quoted-string ) ]
For the cache directives defined below, no argument is defined (nor For the cache directives defined below, no argument is defined (nor
allowed) unless stated otherwise. allowed) unless stated otherwise.
------------------ ----------------------------------
Cache Directive Reference
------------------ ----------------------------------
max-age Section 5.2.1.1, Section 5.2.2.9
max-stale Section 5.2.1.2
min-fresh Section 5.2.1.3
must-revalidate Section 5.2.2.1
must-understand Section 5.2.2.2
no-cache Section 5.2.1.4, Section 5.2.2.3
no-store Section 5.2.1.5, Section 5.2.2.4
no-transform Section 5.2.1.6, Section 5.2.2.5
only-if-cached Section 5.2.1.7
private Section 5.2.2.7
proxy-revalidate Section 5.2.2.8
public Section 5.2.2.6
s-maxage Section 5.2.2.10
------------------ ----------------------------------
Table 2
5.2.1. Request Cache-Control Directives 5.2.1. Request Cache-Control Directives
This section defines cache request directives. They are advisory; 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 1.3)
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be vulnerable to eavesdropping. be vulnerable to eavesdropping.
Note that if a request containing this directive is satisfied from a Note that if a request containing this directive is satisfied from a
cache, the no-store request directive does not apply to the already cache, the no-store request directive does not apply to the already
stored response. stored response.
5.2.1.6. no-transform 5.2.1.6. no-transform
The "no-transform" request directive indicates that the client is The "no-transform" request directive indicates that the client is
asking for intermediaries to avoid transforming the payload, as asking for intermediaries to avoid transforming the payload, as
defined in Section 6.5 of [Semantics]. defined in Section 7.7 of [Semantics].
5.2.1.7. only-if-cached 5.2.1.7. only-if-cached
The "only-if-cached" request directive indicates that the client only The "only-if-cached" request directive indicates that the client only
wishes to obtain a stored response. 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
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cache MUST obey the must-revalidate directive; in particular, if a cache MUST obey the must-revalidate directive; in particular, if a
cache is disconnected, the cache MUST generate a 504 (Gateway cache is disconnected, the cache MUST generate a 504 (Gateway
Timeout) response rather than reuse the stale response. Timeout) response rather than reuse the stale response.
The must-revalidate directive ought to be used by servers if and only The must-revalidate directive ought to be used by servers if and only
if failure to validate a request on the representation could cause if failure to validate a request on the representation could cause
incorrect operation, such as a silently unexecuted financial incorrect operation, such as a silently unexecuted financial
transaction. transaction.
The must-revalidate directive also permits a shared cache to reuse a 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
subject to the above requirement on revalidation (Section 3.3). (Section 11.6.2 of [Semantics]), subject to the above requirement on
revalidation (Section 3.4).
5.2.2.2. must-understand 5.2.2.2. 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. A cache MUST NOT store a response for that response's status code. A cache MUST NOT store a response
containing the must-understand directive if the cache does not containing the must-understand directive if the cache does not
understand the response status code. understand the response status code.
5.2.2.3. no-cache 5.2.2.3. no-cache
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certain header fields in a response, while still allowing caching of certain header fields in a response, while still allowing caching of
the rest of the response. the rest of the response.
The field names given are not limited to the set of header fields The field names given are not limited to the set of header fields
defined by this specification. Field names are case-insensitive. defined by this specification. Field names are case-insensitive.
This directive uses the quoted-string form of the argument syntax. A This directive uses the quoted-string form of the argument syntax. A
sender SHOULD NOT generate the token form (even if quoting appears sender SHOULD NOT generate the token form (even if quoting appears
not to be needed for single-entry lists). not to be needed for single-entry lists).
*Note:* Although it has been back-ported to many implementations, | *Note:* Although it has been back-ported to many
some HTTP/1.0 caches will not recognize or obey this directive. | implementations, some HTTP/1.0 caches will not recognize or
Also, the qualified form of the directive is often handled by caches | obey this directive. Also, the qualified form of the directive
as if an unqualified no-cache directive was received; i.e., the | is often handled by caches as if an unqualified no-cache
special handling for the qualified form is not widely implemented. | directive was received; i.e., the special handling for the
| qualified form is not widely implemented.
5.2.2.4. no-store 5.2.2.4. no-store
The "no-store" response directive indicates that a cache MUST NOT The "no-store" response directive indicates that a cache MUST NOT
store any part of either the immediate request or response, and MUST store any part of either the immediate request or response, and MUST
NOT use the response to satisfy any other request. NOT use the response to satisfy any other request.
This directive applies to both private and shared caches. "MUST NOT This directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a best- store the information in non-volatile storage, and MUST make a best-
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This directive is NOT a reliable or sufficient mechanism for ensuring This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might recognize or obey this directive, and communications networks might
be vulnerable to eavesdropping. be vulnerable to eavesdropping.
5.2.2.5. no-transform 5.2.2.5. no-transform
The "no-transform" response directive indicates that an intermediary The "no-transform" response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT transform the (regardless of whether it implements a cache) MUST NOT transform the
payload, as defined in Section 6.5 of [Semantics]. payload, as defined in Section 7.7 of [Semantics].
5.2.2.6. public 5.2.2.6. public
The "public" response directive indicates that a cache MAY store the The "public" response directive indicates that a cache MAY store the
response even if it would otherwise be prohibited, subject to the response even if it would otherwise be prohibited, subject to the
constraints defined in Section 3. In other words, public explicitly constraints defined in Section 3. In other words, public explicitly
marks the response as cacheable. For example, public permits a marks the response as cacheable. For example, public permits a
shared cache to reuse a response to a request containing an shared cache to reuse a response to a request containing an
Authorization header field (Section 3.3). Authorization header field (Section 3.4).
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).
5.2.2.7. private 5.2.2.7. private
Argument syntax: Argument syntax:
skipping to change at page 29, line 14 skipping to change at page 29, line 23
The unqualified "private" response directive indicates that a shared The unqualified "private" response directive indicates that a shared
cache MUST NOT store the response (i.e., the response is intended for cache MUST NOT store the response (i.e., the response is intended for
a single user). It also indicates that a private cache MAY store the a single user). It also indicates that a private cache MAY store the
response, subject the constraints defined in Section 3, even if the response, subject the constraints defined in Section 3, even if the
response would not otherwise be heuristically cacheable by a private response would not otherwise be heuristically cacheable by a private
cache. cache.
If a qualified private response directive is present, with an If a qualified private response directive is present, with an
argument that lists one or more field names, then only the listed argument that lists one or more field names, then only the listed
fields are limited to a single user: a shared cache MUST NOT store header fields are limited to a single user: a shared cache MUST NOT
the listed fields if they are present in the original response, but store the listed header fields if they are present in the original
MAY store the remainder of the response message without those fields, response, but MAY store the remainder of the response message without
subject the constraints defined in Section 3. those header fields, subject the constraints defined in Section 3.
The field names given are not limited to the set of header fields The field names given are not limited to the set of header fields
defined by this specification. Field names are case-insensitive. defined by this specification. Field names are case-insensitive.
This directive uses the quoted-string form of the argument syntax. A This directive uses the quoted-string form of the argument syntax. A
sender SHOULD NOT generate the token form (even if quoting appears sender SHOULD NOT generate the token form (even if quoting appears
not to be needed for single-entry lists). not to be needed for single-entry lists).
*Note:* This usage of the word "private" only controls where the | *Note:* This usage of the word "private" only controls where
response can be stored; it cannot ensure the privacy of the message | the response can be stored; it cannot ensure the privacy of the
content. Also, the qualified form of the directive is often handled | message content. Also, the qualified form of the directive is
by caches as if an unqualified private directive was received; i.e., | often handled by caches as if an unqualified private directive
the special handling for the qualified form is not widely | was received; i.e., the special handling for the qualified form
implemented. | is not widely implemented.
5.2.2.8. proxy-revalidate 5.2.2.8. proxy-revalidate
The "proxy-revalidate" response directive indicates that once the The "proxy-revalidate" response directive indicates that once the
response has become stale, a shared cache MUST NOT reuse that response has become stale, a shared cache MUST NOT reuse that
response to satisfy another request until it has been successfully response to satisfy another request until it has been successfully
validated by the origin, as defined by Section 4.3. This is validated by the origin, as defined by Section 4.3. This is
analogous to must-revalidate (Section 5.2.2.1), except that proxy- analogous to must-revalidate (Section 5.2.2.1), except that proxy-
revalidate does not apply to private caches. revalidate does not apply to private caches.
skipping to change at page 30, line 31 skipping to change at page 30, line 42
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
the origin, as defined by Section 4.3. This directive also permits a the origin, as defined by Section 4.3. This directive also permits a
shared cache to reuse a response to a request containing an shared cache to reuse a response to a request containing an
Authorization header field, subject to the above requirements on Authorization header field, subject to the above requirements on
maximum age and revalidation (Section 3.3). maximum age and revalidation (Section 3.4).
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 cache-extension tokens, each with an optional value. A cache
MUST ignore unrecognized cache directives. MUST ignore unrecognized cache directives.
skipping to change at page 32, line 4 skipping to change at page 32, line 8
The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry" The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry"
defines the namespace for the cache directives. It has been created defines the namespace for the cache directives. It has been created
and is now maintained at <https://www.iana.org/assignments/http- and is now maintained at <https://www.iana.org/assignments/http-
cache-directives>. cache-directives>.
A registration MUST include the following fields: A registration MUST include the following fields:
o Cache Directive Name o Cache Directive Name
o Pointer to specification text o Pointer to specification text
Values to be added to this namespace require IETF Review (see Values to be added to this namespace require IETF Review (see
[RFC8126], Section 4.8). [RFC8126], Section 4.8).
5.3. Expires 5.3. Expires
The "Expires" header field gives the date/time after which the The "Expires" response header field gives the date/time after which
response is considered stale. See Section 4.2 for further discussion the response is considered stale. See Section 4.2 for further
of the freshness model. discussion of the freshness model.
The presence of an Expires field does not imply that the original The presence of an Expires header field does not imply that the
resource will change or cease to exist at, before, or after that original resource will change or cease to exist at, before, or after
time. that time.
The Expires value is an HTTP-date timestamp, as defined in The Expires field value is an HTTP-date timestamp, as defined in
Section 5.7.7 of [Semantics]. Section 5.6.7 of [Semantics].
Expires = HTTP-date Expires = HTTP-date
For example For example
Expires: Thu, 01 Dec 1994 16:00:00 GMT Expires: Thu, 01 Dec 1994 16:00:00 GMT
A cache recipient MUST interpret invalid date formats, especially the A cache recipient MUST interpret invalid date formats, especially the
value "0", as representing a time in the past (i.e., "already value "0", as representing a time in the past (i.e., "already
expired"). expired").
If a response includes a Cache-Control field with the max-age If a response includes a Cache-Control header field with the max-age
directive (Section 5.2.2.9), a recipient MUST ignore the Expires directive (Section 5.2.2.9), a recipient MUST ignore the Expires
field. Likewise, if a response includes the s-maxage directive header field. Likewise, if a response includes the s-maxage
(Section 5.2.2.10), a shared cache recipient MUST ignore the Expires directive (Section 5.2.2.10), a shared cache recipient MUST ignore
field. In both these cases, the value in Expires is only intended the Expires header field. In both these cases, the value in Expires
for recipients that have not yet implemented the Cache-Control field. is only intended for recipients that have not yet implemented the
Cache-Control header field.
An origin server without a clock MUST NOT generate an Expires field An origin server without a clock MUST NOT generate an Expires header
unless its value represents a fixed time in the past (always expired) field unless its value represents a fixed time in the past (always
or its value has been associated with the resource by a system or expired) or its value has been associated with the resource by a
user with a reliable clock. system or user with a reliable clock.
Historically, HTTP required the Expires field value to be no more Historically, HTTP required the Expires field value to be no more
than a year in the future. While longer freshness lifetimes are no than a year in the future. While longer freshness lifetimes are no
longer prohibited, extremely large values have been demonstrated to longer prohibited, extremely large values have been demonstrated to
cause problems (e.g., clock overflows due to use of 32-bit integers cause problems (e.g., clock overflows due to use of 32-bit integers
for time values), and many caches will evict a response far sooner for time values), and many caches will evict a response far sooner
than that. than that.
5.4. Pragma 5.4. Pragma
The "Pragma" header field was defined for HTTP/1.0 caches, so that The "Pragma" request header field was defined for HTTP/1.0 caches, so
clients could specify a "no-cache" request (as Cache-Control was not that clients could specify a "no-cache" request (as Cache-Control was
defined until HTTP/1.1). not defined until HTTP/1.1).
However, support for Cache-Control is now widespread. As a result, However, support for Cache-Control is now widespread. As a result,
this specification deprecates Pragma. this specification deprecates Pragma.
| *Note:* Because the meaning of "Pragma: no-cache" in responses | *Note:* Because the meaning of "Pragma: no-cache" in responses
| was never specified, it does not provide a reliable replacement | was never specified, it does not provide a reliable replacement
| for "Cache-Control: no-cache" in them. | for "Cache-Control: no-cache" in them.
5.5. Warning 5.5. Warning
The "Warning" header field was used to carry additional information The "Warning" header field was used to carry additional information
about the status or transformation of a message that might not be about the status or transformation of a message that might not be
reflected in the status code. This specification obsoletes it, as it reflected in the status code. This specification obsoletes it, as it
is not widely generated or surfaced to users. The information it is not widely generated or surfaced to users. The information it
carried can be gleaned from examining other header fields, such as carried can be gleaned from examining other header fields, such as
Age. Age.
6. Relationship to Applications 6. Relationship to Applications and Other Caches
Applications using HTTP often specify additional forms of caching. Applications using HTTP often specify additional forms of caching.
For example, Web browsers often have history mechanisms such as For example, Web browsers often have history mechanisms such as
"Back" buttons that can be used to redisplay a representation "Back" buttons that can be used to redisplay a representation
retrieved earlier in a session. retrieved earlier in a session.
Likewise, some Web browsers implement caching of images and other Likewise, some Web browsers implement caching of images and other
assets within a page view; they may or may not honor HTTP caching assets within a page view; they may or may not honor HTTP caching
semantics. semantics.
The requirements in this specification do not necessarily apply to The requirements in this specification do not necessarily apply to
how applications use data after it is retrieved from a HTTP cache. how applications use data after it is retrieved from a HTTP cache.
That is, a history mechanism can display a previous representation For example, a history mechanism can display a previous
even if it has expired, and an application can use cached data in representation even if it has expired, and an application can use
other ways beyond its freshness lifetime. cached data in other ways beyond its freshness lifetime.
This does not prohibit the application from taking HTTP caching into This specification does not prohibit the application from taking HTTP
account; for example, a history mechanism might tell the user that a caching into account; for example, a history mechanism might tell the
view is stale, or it might honor cache directives (e.g., Cache- user that a view is stale, or it might honor cache directives (e.g.,
Control: no-store). Cache-Control: no-store).
However, when an application caches data and does not make this
apparent to or easily controllable by the user, it is strongly
encouraged to define its operation with respect to HTTP cache
directives, so as not to surprise authors who expect caching
semantics to be honoured. For example, while it might be reasonable
to define an application cache "above" HTTP that allows a response
containing Cache-Control: no-store to be reused for requests that are
directly related to the request that fetched it (such as those
created during the same page load), it would likely be surprising and
confusing to users and authors if it were allowed to be reused for
requests unrelated in any way to the one from which it was obtained.
7. Security Considerations 7. Security Considerations
This section is meant to inform developers, information providers, This section is meant to inform developers, information providers,
and users of known security concerns specific to HTTP caching. More and users of known security concerns specific to HTTP caching. More
general security considerations are addressed in HTTP messaging general security considerations are addressed in "HTTP/1.1"
[Messaging] and semantics [Semantics]. (Section 11 of [Messaging]) and "HTTP Semantics" (Section 17 of
[Semantics]).
Caches expose additional potential vulnerabilities, since the Caches expose additional potential vulnerabilities, since the
contents of the cache represent an attractive target for malicious contents of the cache represent an attractive target for malicious
exploitation. Because cache contents persist after an HTTP request exploitation. Because cache contents persist after an HTTP request
is complete, an attack on the cache can reveal information long after is complete, an attack on the cache can reveal information long after
a user believes that the information has been removed from the a user believes that the information has been removed from the
network. Therefore, cache contents need to be protected as sensitive network. Therefore, cache contents need to be protected as sensitive
information. information.
7.1. Cache Poisoning 7.1. Cache Poisoning
skipping to change at page 35, line 23 skipping to change at page 35, line 40
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 Registration 8.1. Field Name Registration
Please update the "Hypertext Transfer Protocol (HTTP) Field Name First, introduce the new "Hypertext Transfer Protocol (HTTP) Field
Registry" at <https://www.iana.org/assignments/http-fields> with the Name Registry" at <https://www.iana.org/assignments/http-fields> as
field names listed in the two tables of Section 5. described in Section 18.4 of [Semantics].
Then, please update the registry with the field names listed in the
table below:
--------------- ----------- ------ ----------
Field Name Status Ref. Comments
--------------- ----------- ------ ----------
Age standard 5.1
Cache-Control standard 5.2
Expires standard 5.3
Pragma standard 5.4
Warning obsoleted 5.5
--------------- ----------- ------ ----------
Table 1
8.2. Cache Directive Registration 8.2. Cache Directive Registration
Please update the "Hypertext Transfer Protocol (HTTP) Cache Directive Please update the "Hypertext Transfer Protocol (HTTP) Cache Directive
Registry" at <https://www.iana.org/assignments/http-cache-directives> Registry" at <https://www.iana.org/assignments/http-cache-directives>
with the registration procedure of Section 5.2.4 and the cache with the registration procedure of Section 5.2.4 and the cache
directive names summarized in the table of Section 5.2. directive names summarized in the table below.
------------------ ----------------------------------
Cache Directive Reference
------------------ ----------------------------------
max-age Section 5.2.1.1, Section 5.2.2.9
max-stale Section 5.2.1.2
min-fresh Section 5.2.1.3
must-revalidate Section 5.2.2.1
must-understand Section 5.2.2.2
no-cache Section 5.2.1.4, Section 5.2.2.3
no-store Section 5.2.1.5, Section 5.2.2.4
no-transform Section 5.2.1.6, Section 5.2.2.5
only-if-cached Section 5.2.1.7
private Section 5.2.2.7
proxy-revalidate Section 5.2.2.8
public Section 5.2.2.6
s-maxage Section 5.2.2.10
------------------ ----------------------------------
Table 2
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] [Messaging]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1 Messaging", Work in Progress, Internet- Ed., "HTTP/1.1", Work in Progress, Internet-Draft, draft-
Draft, draft-ietf-httpbis-messaging-12, October 2, 2020, ietf-httpbis-messaging-13, December 14, 2020,
<https://tools.ietf.org/html/draft-ietf-httpbis-messaging- <https://tools.ietf.org/html/draft-ietf-httpbis-messaging-
12>. 13>.
[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>.
skipping to change at page 36, line 26 skipping to change at page 37, line 33
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] [Semantics]
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, Ed., "HTTP Semantics", Work in Progress, Internet-Draft,
draft-ietf-httpbis-semantics-12, October 2, 2020, draft-ietf-httpbis-semantics-13, December 14, 2020,
<https://tools.ietf.org/html/draft-ietf-httpbis-semantics- <https://tools.ietf.org/html/draft-ietf-httpbis-semantics-
12>. 13>.
[USASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
9.2. Informative References 9.2. Informative References
[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
skipping to change at page 37, line 22 skipping to change at page 38, line 22
<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 5.7.1.1 of [Semantics]. Section 5.6.1.1 of [Semantics].
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 5.7.7> HTTP-date = <HTTP-date, see [Semantics], Section 5.6.7>
OWS = <OWS, see [Semantics], Section 5.7.3> OWS = <OWS, see [Semantics], 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 5.4.3> field-name = <field-name, see [Semantics], Section 5.1>
quoted-string = <quoted-string, see [Semantics], Section 5.7.4> quoted-string = <quoted-string, see [Semantics], Section 5.6.4>
token = <token, see [Semantics], Section 5.7.2> token = <token, see [Semantics], 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
clarified. (Section 4.2.1)
Cache invalidation of the URIs in the Location and Content-Location
header fields is no longer required, but still allowed.
(Section 4.4)
Cache invalidation of the URIs in the Location and Content-Location
header fields is disallowed when the origin is different; previously,
it was the host. (Section 4.4)
Handling invalid and multiple Age header field values has been Handling invalid and multiple Age header field values has been
clarified. (Section 5.1) clarified. (Section 5.1)
Some cache directives defined by this specification now have stronger Some cache directives defined by this specification now have stronger
prohibitions against generating the quoted form of their values, prohibitions against generating the quoted form of their values,
since this has been found to create interoperability problems. since this has been found to create interoperability problems.
Consumers of extension cache directives are no longer required to Consumers of extension cache directives are no longer required to
accept both token and quoted-string forms, but they still need to accept both token and quoted-string forms, but they still need to
parse them properly for unknown extensions. (Section 5.2) parse them properly for unknown extensions. (Section 5.2)
The "public" and "private" cache directives were clarified, so that The "public" and "private" cache directives were clarified, so that
they do not make responses reusable under any condition. they do not make responses reusable under any condition.
(Section 5.2.2) (Section 5.2.2)
skipping to change at page 39, line 40 skipping to change at page 40, line 44
o In Section 4.3.1, clarify the source of validators in conditional o In Section 4.3.1, clarify the source of validators in conditional
requests (<https://github.com/httpwg/http-core/issues/110>) requests (<https://github.com/httpwg/http-core/issues/110>)
o Revise Section 6 to apply to more than just History Lists o Revise Section 6 to apply to more than just History Lists
(<https://github.com/httpwg/http-core/issues/126>) (<https://github.com/httpwg/http-core/issues/126>)
o In Section 5.5, deprecated "Warning" header field o In Section 5.5, deprecated "Warning" header field
(<https://github.com/httpwg/http-core/issues/139>) (<https://github.com/httpwg/http-core/issues/139>)
o In Section 3.3, remove a spurious note o In Section 3.4, remove a spurious note
(<https://github.com/httpwg/http-core/issues/141>) (<https://github.com/httpwg/http-core/issues/141>)
C.5. Since draft-ietf-httpbis-cache-03 C.5. Since draft-ietf-httpbis-cache-03
o In Section 2, define what a disconnected cache is o In Section 2, define what a disconnected cache is
(<https://github.com/httpwg/http-core/issues/5>) (<https://github.com/httpwg/http-core/issues/5>)
o In Section 4, clarify language around how to select a response o In Section 4, clarify language around how to select a response
when more than one matches (<https://github.com/httpwg/http-core/ when more than one matches (<https://github.com/httpwg/http-core/
issues/23>) issues/23>)
o in Section 4.2.4, mention stale-while-revalidate and stale-if- o in Section 4.2.4, mention stale-while-revalidate and stale-if-
error (<https://github.com/httpwg/http-core/issues/122>) error (<https://github.com/httpwg/http-core/issues/122>)
o Remove requirements around cache request directives o Remove requirements around cache request directives
(<https://github.com/httpwg/http-core/issues/129>) (<https://github.com/httpwg/http-core/issues/129>)
o Deprecate Pragma (<https://github.com/httpwg/http-core/ o Deprecate Pragma (<https://github.com/httpwg/http-core/
issues/140>) issues/140>)
o In Section 3.3 and Section 5.2.2, note effect of some directives o In Section 3.4 and Section 5.2.2, note effect of some directives
on authenticated requests (<https://github.com/httpwg/http-core/ on authenticated requests (<https://github.com/httpwg/http-core/
issues/161>) issues/161>)
C.6. Since draft-ietf-httpbis-cache-04 C.6. Since draft-ietf-httpbis-cache-04
o In Section 5.2, remove the registrations for stale-if-error and o In Section 5.2, remove the registrations for stale-if-error and
stale-while-revalidate which happened in RFC 7234 stale-while-revalidate which happened in RFC 7234
(<https://github.com/httpwg/http-core/issues/207>) (<https://github.com/httpwg/http-core/issues/207>)
C.7. Since draft-ietf-httpbis-cache-05 C.7. Since draft-ietf-httpbis-cache-05
o In Section 3.2, clarify how weakly framed content is considered o In Section 3.3, clarify how weakly framed content is considered
for purposes of completeness (<https://github.com/httpwg/http- for purposes of completeness (<https://github.com/httpwg/http-
core/issues/25>) core/issues/25>)
o Throughout, describe Vary and cache key operations more clearly o Throughout, describe Vary and cache key operations more clearly
(<https://github.com/httpwg/http-core/issues/28>) (<https://github.com/httpwg/http-core/issues/28>)
o In Section 3, remove concept of "cacheable methods" in favor of o In Section 3, remove concept of "cacheable methods" in favor of
prose (<https://github.com/httpwg/http-core/issues/54>, prose (<https://github.com/httpwg/http-core/issues/54>,
<https://www.rfc-editor.org/errata/eid5300>) <https://www.rfc-editor.org/errata/eid5300>)
skipping to change at page 41, line 32 skipping to change at page 42, line 36
C.9. Since draft-ietf-httpbis-cache-07 C.9. Since draft-ietf-httpbis-cache-07
o Throughout, replace "effective request URI", "request-target" and o Throughout, replace "effective request URI", "request-target" and
similar with "target URI" (<https://github.com/httpwg/http-core/ similar with "target URI" (<https://github.com/httpwg/http-core/
issues/259>) issues/259>)
o In Section 5.2.2.6 and Section 5.2.2.7, make it clear that these o In Section 5.2.2.6 and Section 5.2.2.7, make it clear that these
directives do not ignore other requirements for caching directives do not ignore other requirements for caching
(<https://github.com/httpwg/http-core/issues/320>) (<https://github.com/httpwg/http-core/issues/320>)
o In Section 3.2, move definition of "complete" into semantics o In Section 3.3, move definition of "complete" into semantics
(<https://github.com/httpwg/http-core/issues/334>) (<https://github.com/httpwg/http-core/issues/334>)
C.10. Since draft-ietf-httpbis-cache-08 C.10. Since draft-ietf-httpbis-cache-08
o Appendix A now uses the sender variant of the "#" list expansion o Appendix A now uses the sender variant of the "#" list expansion
(<https://github.com/httpwg/http-core/issues/192>) (<https://github.com/httpwg/http-core/issues/192>)
C.11. Since draft-ietf-httpbis-cache-09 C.11. Since draft-ietf-httpbis-cache-09
o In Section 5.1, discuss handling of invalid and multiple Age o In Section 5.1, discuss handling of invalid and multiple Age
skipping to change at page 42, line 9 skipping to change at page 43, line 14
C.12. Since draft-ietf-httpbis-cache-10 C.12. Since draft-ietf-httpbis-cache-10
o In Section 5.2 (Cache-Control), adjust ABNF to allow empty lists o In Section 5.2 (Cache-Control), adjust ABNF to allow empty lists
(<https://github.com/httpwg/http-core/issues/210>) (<https://github.com/httpwg/http-core/issues/210>)
C.13. Since draft-ietf-httpbis-cache-11 C.13. Since draft-ietf-httpbis-cache-11
o None. o None.
C.14. Since draft-ietf-httpbis-cache-12
o In Section 4.2.4, remove 'no-store', as it won't be in cache in
the first place (<https://github.com/httpwg/http-core/issues/447>)
o In Section 3.1, make it clear that only response headers need be
stored (<https://github.com/httpwg/http-core/issues/457>)
o Rewrote "Updating Stored Header Fields" Section 3.2
(<https://github.com/httpwg/http-core/issues/458>)
o In Section 4.2.1 clarify how to handle invalid and conflicting
directives (<https://github.com/httpwg/http-core/issues/460>)
o In Section 4.3.3, mention retry of failed validation requests
(<https://github.com/httpwg/http-core/issues/462>)
o In Section 4.3.3, clarify requirement on storing a full response
to a conditional request (<https://github.com/httpwg/http-core/
issues/463>)
o In Section 5.1, clarify error handling
(<https://github.com/httpwg/http-core/issues/471>)
o In Section 4.2, remove spurious "UTC" (<https://github.com/httpwg/
http-core/issues/472>)
o In Section 4.2, correct the date-related rule names to consider
case-insensitive (<https://github.com/httpwg/http-core/
issues/473>)
o In Section 6, strengthen recommendation for application caches to
pay attention to cache directives (<https://github.com/httpwg/
http-core/issues/474>)
o In Section 4, mention collapsed requests
(<https://github.com/httpwg/http-core/issues/475>)
o In Section 4.4, relax requirements on Content-Location and
Location invalidation (<https://github.com/httpwg/http-core/
issues/478>)
o In Section 4.3.4, refine the exceptions to update on a 304
(<https://github.com/httpwg/http-core/issues/488>)
o Moved table of Cache-Control directives into Section 8.2
(<https://github.com/httpwg/http-core/issues/506>)
o In Section 1.2, remove unused core ABNF rules
(<https://github.com/httpwg/http-core/issues/529>)
o Changed to using "payload data" when defining requirements about
the data being conveyed within a message, instead of the terms
"payload body" or "response body" or "representation body", since
they often get confused with the HTTP/1.1 message body (which
includes transfer coding) (<https://github.com/httpwg/http-core/
issues/553>)
Acknowledgments Acknowledgments
See Appendix "Acknowledgments" of [Semantics]. See Appendix "Acknowledgments" of [Semantics].
Authors' Addresses Authors' Addresses
Roy T. Fielding (editor) Roy T. Fielding (editor)
Adobe Adobe
345 Park Ave 345 Park Ave
San Jose, CA 95110 San Jose, CA 95110
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