draft-ietf-httpbis-p6-cache-23.txt   draft-ietf-httpbis-p6-cache-24.txt 
HTTPbis Working Group R. Fielding, Ed. HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Adobe Internet-Draft Adobe
Obsoletes: 2616 (if approved) M. Nottingham, Ed. Obsoletes: 2616 (if approved) M. Nottingham, Ed.
Intended status: Standards Track Akamai Intended status: Standards Track Akamai
Expires: January 16, 2014 J. Reschke, Ed. Expires: March 29, 2014 J. Reschke, Ed.
greenbytes greenbytes
July 15, 2013 September 25, 2013
Hypertext Transfer Protocol (HTTP/1.1): Caching Hypertext Transfer Protocol (HTTP/1.1): Caching
draft-ietf-httpbis-p6-cache-23 draft-ietf-httpbis-p6-cache-24
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypertext information protocol for distributed, collaborative, hypertext information
systems. This document defines requirements on HTTP caches and the systems. This document defines requirements on HTTP caches and the
associated header fields that control cache behavior or indicate associated header fields that control cache behavior or indicate
cacheable response messages. cacheable response messages.
Editorial Note (To be removed by RFC Editor) Editorial Note (To be removed by RFC Editor)
Discussion of this draft takes place on the HTTPBIS working group Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at mailing list (ietf-http-wg@w3.org), which is archived at
<http://lists.w3.org/Archives/Public/ietf-http-wg/>. <http://lists.w3.org/Archives/Public/ietf-http-wg/>.
The current issues list is at The current issues list is at
<http://tools.ietf.org/wg/httpbis/trac/report/3> and related <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
documents (including fancy diffs) can be found at documents (including fancy diffs) can be found at
<http://tools.ietf.org/wg/httpbis/>. <http://tools.ietf.org/wg/httpbis/>.
The changes in this draft are summarized in Appendix D.4. The changes in this draft are summarized in Appendix D.5.
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 http://datatracker.ietf.org/drafts/current/. Drafts is at http://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 January 16, 2014. This Internet-Draft will expire on March 29, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2013 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 Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 45 skipping to change at page 2, line 45
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conformance and Error Handling . . . . . . . . . . . . . . 4 1.1. Conformance and Error Handling . . . . . . . . . . . . . . 4
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4
1.2.1. Delta Seconds . . . . . . . . . . . . . . . . . . . . 5 1.2.1. Delta Seconds . . . . . . . . . . . . . . . . . . . . 5
2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 5 2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 5
3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 6 3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 6
3.1. Storing Incomplete Responses . . . . . . . . . . . . . . . 7 3.1. Storing Incomplete Responses . . . . . . . . . . . . . . . 7
3.2. Storing Responses to Authenticated Requests . . . . . . . 7 3.2. Storing Responses to Authenticated Requests . . . . . . . 7
3.3. Combining Partial Content . . . . . . . . . . . . . . . . 7 3.3. Combining Partial Content . . . . . . . . . . . . . . . . 7
4. Constructing Responses from Caches . . . . . . . . . . . . . . 8 4. Constructing Responses from Caches . . . . . . . . . . . . . . 8
4.1. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 9 4.1. Calculating Secondary Keys with Vary . . . . . . . . . . . 9
4.1.1. Calculating Freshness Lifetime . . . . . . . . . . . . 11 4.2. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1.2. Calculating Heuristic Freshness . . . . . . . . . . . 11 4.2.1. Calculating Freshness Lifetime . . . . . . . . . . . . 12
4.1.3. Calculating Age . . . . . . . . . . . . . . . . . . . 12 4.2.2. Calculating Heuristic Freshness . . . . . . . . . . . 12
4.1.4. Serving Stale Responses . . . . . . . . . . . . . . . 13 4.2.3. Calculating Age . . . . . . . . . . . . . . . . . . . 13
4.2. Validation . . . . . . . . . . . . . . . . . . . . . . . . 14 4.2.4. Serving Stale Responses . . . . . . . . . . . . . . . 14
4.2.1. Freshening Stored Responses upon Validation . . . . . 15 4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . . 15
4.3.1. Sending a Validation Request . . . . . . . . . . . . . 15
4.3. Calculating Secondary Keys with Vary . . . . . . . . . . . 16 4.3.2. Handling a Received Validation Request . . . . . . . . 16
5. Updating Caches with HEAD Responses . . . . . . . . . . . . . 17 4.3.3. Handling a Validation Response . . . . . . . . . . . . 17
6. Request Methods that Invalidate . . . . . . . . . . . . . . . 17 4.3.4. Freshening Stored Responses upon Validation . . . . . 18
7. Header Field Definitions . . . . . . . . . . . . . . . . . . . 18 4.3.5. Freshening Responses via HEAD . . . . . . . . . . . . 18
7.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.4. Invalidation . . . . . . . . . . . . . . . . . . . . . . . 19
7.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 18 5. Header Field Definitions . . . . . . . . . . . . . . . . . . . 20
7.2.1. Request Cache-Control Directives . . . . . . . . . . . 19 5.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.2.2. Response Cache-Control Directives . . . . . . . . . . 21 5.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 20
7.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 24 5.2.1. Request Cache-Control Directives . . . . . . . . . . . 21
7.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.2.2. Response Cache-Control Directives . . . . . . . . . . 23
7.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 26 5.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 26
7.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 27 5.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.5.1. 110 Response is Stale . . . . . . . . . . . . . . . . 28 5.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.5.2. 111 Revalidation Failed . . . . . . . . . . . . . . . 28 5.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 29
7.5.3. 112 Disconnected Operation . . . . . . . . . . . . . . 28 5.5.1. Warning: 110 - "Response is Stale" . . . . . . . . . . 30
7.5.4. 113 Heuristic Expiration . . . . . . . . . . . . . . . 29 5.5.2. Warning: 111 - "Revalidation Failed" . . . . . . . . . 31
7.5.5. 199 Miscellaneous Warning . . . . . . . . . . . . . . 29 5.5.3. Warning: 112 - "Disconnected Operation" . . . . . . . 31
7.5.6. 214 Transformation Applied . . . . . . . . . . . . . . 29 5.5.4. Warning: 113 - "Heuristic Expiration" . . . . . . . . 31
7.5.7. 299 Miscellaneous Persistent Warning . . . . . . . . . 29 5.5.5. Warning: 199 - "Miscellaneous Warning" . . . . . . . . 31
7.5.8. Warn Code Extensions . . . . . . . . . . . . . . . . . 29 5.5.6. Warning: 214 - "Transformation Applied" . . . . . . . 31
8. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 29 5.5.7. Warning: 299 - "Miscellaneous Persistent Warning" . . 31
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 6. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 31
9.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 30 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
9.1.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 30 7.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 32
9.1.2. Considerations for New Cache Control Directives . . . 30 7.1.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 32
9.1.3. Registrations . . . . . . . . . . . . . . . . . . . . 30 7.1.2. Considerations for New Cache Control Directives . . . 32
9.2. Warn Code Registry . . . . . . . . . . . . . . . . . . . . 31 7.1.3. Registrations . . . . . . . . . . . . . . . . . . . . 32
9.2.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 31 7.2. Warn Code Registry . . . . . . . . . . . . . . . . . . . . 33
9.2.2. Registrations . . . . . . . . . . . . . . . . . . . . 31 7.2.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 33
9.3. Header Field Registration . . . . . . . . . . . . . . . . 32 7.2.2. Registrations . . . . . . . . . . . . . . . . . . . . 33
10. Security Considerations . . . . . . . . . . . . . . . . . . . 32 7.3. Header Field Registration . . . . . . . . . . . . . . . . 34
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33 8. Security Considerations . . . . . . . . . . . . . . . . . . . 34
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35
12.1. Normative References . . . . . . . . . . . . . . . . . . . 33 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
12.2. Informative References . . . . . . . . . . . . . . . . . . 34 10.1. Normative References . . . . . . . . . . . . . . . . . . . 35
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 34 10.2. Informative References . . . . . . . . . . . . . . . . . . 36
Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 36 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 36
Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 36 Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 38
Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 38
Appendix D. Change Log (to be removed by RFC Editor before Appendix D. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 37 publication) . . . . . . . . . . . . . . . . . . . . 39
D.1. Since draft-ietf-httpbis-p6-cache-19 . . . . . . . . . . . 38 D.1. Since draft-ietf-httpbis-p6-cache-19 . . . . . . . . . . . 40
D.2. Since draft-ietf-httpbis-p6-cache-20 . . . . . . . . . . . 38 D.2. Since draft-ietf-httpbis-p6-cache-20 . . . . . . . . . . . 40
D.3. Since draft-ietf-httpbis-p6-cache-21 . . . . . . . . . . . 39 D.3. Since draft-ietf-httpbis-p6-cache-21 . . . . . . . . . . . 41
D.4. Since draft-ietf-httpbis-p6-cache-22 . . . . . . . . . . . 39 D.4. Since draft-ietf-httpbis-p6-cache-22 . . . . . . . . . . . 41
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 D.5. Since draft-ietf-httpbis-p6-cache-23 . . . . . . . . . . . 41
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction 1. Introduction
HTTP is typically used for distributed information systems, where HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. This performance can be improved by the use of response caches. This
document defines aspects of HTTP/1.1 related to caching and reusing document defines aspects of HTTP/1.1 related to caching and reusing
response messages. response messages.
An HTTP cache is a local store of response messages and the subsystem An HTTP cache is a local store of response messages and the subsystem
that controls storage, retrieval, and deletion of messages in it. A that controls storage, retrieval, and deletion of messages in it. A
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used by a server that is acting as a tunnel. used by a server that is acting as a tunnel.
A shared cache is a cache that stores responses to be reused by more A shared cache is a cache that stores responses to be reused 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. dedicated to a single user.
The goal of caching in HTTP/1.1 is to significantly improve The goal of caching in HTTP/1.1 is to significantly improve
performance by reusing a prior response message to satisfy a current performance by reusing a prior response message to satisfy a current
request. A stored response is considered "fresh", as defined in request. A stored response is considered "fresh", as defined in
Section 4.1, if the response can be reused without "validation" Section 4.2, if the response can be reused without "validation"
(checking with the origin server to see if the cached response (checking with the origin server to see if the cached response
remains valid for this request). A fresh response can therefore remains valid for this request). A fresh response can therefore
reduce both latency and network overhead each time it is reused. reduce both latency and network overhead each time it is reused.
When a cached response is not fresh, it might still be reusable if it When a cached response is not fresh, it might still be reusable if it
can be freshened by validation (Section 4.2) or if the origin is can be freshened by validation (Section 4.3) or if the origin is
unavailable (Section 4.1.4). unavailable (Section 4.2.4).
1.1. Conformance and Error Handling 1.1. Conformance and Error Handling
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
Conformance criteria and considerations regarding error handling are Conformance criteria and considerations regarding error handling are
defined in Section 2.5 of [Part1]. defined in Section 2.5 of [Part1].
1.2. Syntax Notation 1.2. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF) This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with the list rule extension defined in Section notation of [RFC5234] with the list rule extension defined in Section
1.2 of [Part1]. Appendix B describes rules imported from other 7 of [Part1]. Appendix B describes rules imported from other
documents. Appendix C shows the collected ABNF with the list rule documents. Appendix C shows the collected ABNF with the list rule
expanded. expanded.
1.2.1. Delta Seconds 1.2.1. 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
If a cache receives a delta-seconds value larger than the largest If a cache receives a delta-seconds value larger than the largest
positive integer it can represent, or if any of its subsequent positive integer it can represent, or if any of its subsequent
calculations overflows, it MUST consider the value to be 2147483648 calculations overflows, the cache MUST consider the value to be
(2^31). Recipients parsing a delta-seconds value MUST use an 2147483648 (2^31). A recipient parsing a delta-seconds value MUST
arithmetic type of at least 31 bits of range, and senders MUST NOT use an arithmetic type of at least 31 bits of range, and a sender
generate delta-seconds with a value greater than 2147483648. MUST NOT generate delta-seconds with a value greater than 2147483648.
2. Overview of Cache Operation 2. Overview of Cache Operation
Proper cache operation preserves the semantics of HTTP transfers Proper cache operation preserves the semantics of HTTP transfers
([Part2]) while eliminating the transfer of information already held ([Part2]) while eliminating 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, we assume that reusing the cached response is desirable and HTTP, we assume that reusing the cached response is desirable and
that such reuse is the default behavior when no requirement or local that such reuse is the default behavior when no requirement or local
configuration prevents it. Therefore, HTTP cache requirements are configuration prevents it. Therefore, HTTP cache requirements are
focused on preventing a cache from either storing a non-reusable focused on preventing a cache from either storing a non-reusable
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and defines something suitable for use as a cache key. and defines something suitable for use as a cache key.
The primary cache key consists of the request method and target URI. The primary cache key consists of the request method and target URI.
However, since HTTP caches in common use today are typically limited However, since HTTP caches in common use today are typically limited
to caching responses to GET, many caches simply decline other methods to caching responses to GET, many caches simply decline other methods
and use only the URI as the primary cache key. and use only the URI as the primary cache key.
If a request target is subject to content negotiation, its cache If a request target is subject to content negotiation, its cache
entry might consist of multiple stored responses, each differentiated entry might consist of multiple stored responses, each differentiated
by a secondary key for the values of the original request's selecting by a secondary key for the values of the original request's selecting
header fields (Section 4.3). header fields (Section 4.1).
3. Storing Responses in Caches 3. Storing Responses in Caches
A cache MUST NOT store a response to any request, unless: A cache MUST NOT store a response to any request, unless:
o The request method is understood by the cache and defined as being o The request method is understood by the cache and defined as being
cacheable, and cacheable, and
o the response status code is understood by the cache, and o the response status code is understood by the cache, and
o the "no-store" cache directive (see Section 7.2) does not appear o the "no-store" cache directive (see Section 5.2) does not appear
in request or response header fields, and in request or response header fields, and
o the "private" cache response directive (see Section 7.2.2.6) does o the "private" cache response directive (see Section 5.2.2.6) does
not appear in the response, if the cache is shared, and not appear in the response, if the cache is shared, and
o the Authorization header field (see Section 4.1 of [Part7]) does o the Authorization header field (see Section 4.1 of [Part7]) does
not appear in the request, if the cache is shared, unless the not appear in the request, if the cache is shared, unless the
response explicitly allows it (see Section 3.2), and response explicitly allows it (see Section 3.2), and
o the response either: o the response either:
* contains an Expires header field (see Section 7.3), or * contains an Expires header field (see Section 5.3), or
* contains a max-age response cache directive (see * contains a max-age response cache directive (see
Section 7.2.2.8), or Section 5.2.2.8), or
* contains a s-maxage response cache directive (see * contains a s-maxage response cache directive (see
Section 7.2.2.9) and the cache is shared, or Section 5.2.2.9) and the cache is shared, or
* contains a Cache Control Extension (see Section 7.2.3) that * contains a Cache Control Extension (see Section 5.2.3) that
allows it to be cached, or allows it to be cached, or
* has a status code that is defined as cacheable (see * has a status code that is defined as cacheable (see
Section 4.1.2), or Section 4.2.2), or
* contains a public response cache directive (see * contains a public response cache directive (see
Section 7.2.2.5). Section 5.2.2.5).
Note that any of the requirements listed above can be overridden by a Note that any of the requirements listed above can be overridden by a
cache-control extension; see Section 7.2.3. cache-control extension; see Section 5.2.3.
In this context, a cache has "understood" a request method or a In this context, a cache has "understood" a request method or a
response status code if it recognizes it and implements all specified response status code if it recognizes it and implements all specified
caching-related behavior. caching-related behavior.
Note that, in normal operation, some caches will not store a response Note that, in normal operation, some caches will not store a response
that has neither a cache validator nor an explicit expiration time, that has neither a cache validator nor an explicit expiration time,
as such responses are not usually useful to store. However, caches as such responses are not usually useful to store. However, caches
are not prohibited from storing such responses. are not prohibited from storing such responses.
3.1. Storing Incomplete Responses 3.1. Storing Incomplete Responses
A response message is considered complete when all of the octets A response message is considered complete when all of the octets
indicated by the message framing ([Part1]) are received prior to the indicated by the message framing ([Part1]) are received prior to the
connection being closed. If the request is GET, the response status connection being closed. If the request method is GET, the response
is 200 (OK), and the entire response header block has been received, status code is 200 (OK), and the entire response header section has
a cache MAY store an incomplete response message body if the cache been received, a cache MAY store an incomplete response message body
entry is recorded as incomplete. Likewise, a 206 (Partial Content) if the cache entry is recorded as incomplete. Likewise, a 206
response MAY be stored as if it were an incomplete 200 (OK) cache (Partial Content) response MAY be stored as if it were an incomplete
entry. However, a cache MUST NOT store incomplete or partial content 200 (OK) cache entry. However, a cache MUST NOT store incomplete or
responses if it does not support the Range and Content-Range header partial content responses if it does not support the Range and
fields or if it does not understand the range units used in those Content-Range header fields or if it does not understand the range
fields. units used in those fields.
A cache MAY complete a stored incomplete response by making a A cache MAY complete a stored incomplete response by making a
subsequent range request ([Part5]) and combining the successful subsequent range request ([Part5]) and combining the successful
response with the stored entry, as defined in Section 3.3. A cache response with the stored entry, as defined in Section 3.3. A cache
MUST NOT use an incomplete response to answer requests unless the MUST NOT use an incomplete response to answer requests unless the
response has been made complete or the request is partial and response has been made complete or the request is partial and
specifies a range that is wholly within the incomplete response. A specifies a range that is wholly within the incomplete response. A
cache MUST NOT send a partial response to a client without explicitly cache MUST NOT send a partial response to a client without explicitly
marking it as such using the 206 (Partial Content) status code. marking it as such using the 206 (Partial Content) status code.
3.2. Storing Responses to Authenticated Requests 3.2. Storing Responses to Authenticated Requests
A shared cache MUST NOT use a cached response to a request with an A shared cache MUST NOT use a cached response to a request with an
Authorization header field (Section 4.1 of [Part7]) to satisfy any Authorization header field (Section 4.1 of [Part7]) to satisfy any
subsequent request unless a cache directive that allows such subsequent request unless a cache directive that allows such
responses to be stored is present in the response. responses to be stored is present in the response.
In this specification, the following Cache-Control response In this specification, the following Cache-Control response
directives (Section 7.2.2) have such an effect: must-revalidate, directives (Section 5.2.2) have such an effect: must-revalidate,
public, s-maxage. public, s-maxage.
Note that cached responses that contain the "must-revalidate" and/or Note that cached responses that contain the "must-revalidate" and/or
"s-maxage" response directives are not allowed to be served stale "s-maxage" response directives are not allowed to be served stale
(Section 4.1.4) by shared caches. In particular, a response with (Section 4.2.4) by shared caches. In particular, a response with
either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
satisfy a subsequent request without revalidating it on the origin satisfy a subsequent request without revalidating it on the origin
server. server.
3.3. Combining Partial Content 3.3. 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 ([Part5]). After several such transfers, a cache Range specifiers ([Part5]). After several such transfers, a cache
might have received several ranges of the same representation. A might have received several ranges of the same representation. A
cache MAY combine these ranges into a single stored response, and cache MAY combine these ranges into a single stored response, and
reuse that response to satisfy later requests, if they all share the reuse that response to satisfy later requests, if they all share the
same strong validator and the cache complies with the client same strong validator and the cache complies with the client
requirements in Section 4.3 of [Part5]. requirements in Section 4.3 of [Part5].
When combining the new response with one or more stored responses, a When combining the new response with one or more stored responses, a
cache MUST: cache MUST:
o delete any Warning header fields in the stored response with warn- o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 7.5); code 1xx (see Section 5.5);
o retain any Warning header fields in the stored response with warn- o retain any Warning header fields in the stored response with warn-
code 2xx; and, code 2xx; and,
o use other header fields provided in the new response, aside from o use other header fields provided in the new response, aside from
Content-Range, to replace all instances of the corresponding Content-Range, to replace all instances of the corresponding
header fields in the stored response. header fields in the stored response.
4. Constructing Responses from Caches 4. Constructing Responses from Caches
When presented with a request, a cache MUST NOT reuse a stored When presented with a request, a cache MUST NOT reuse a stored
response, unless: response, unless:
o The presented effective request URI (Section 5.5 of [Part1]) and o The presented effective request URI (Section 5.5 of [Part1]) and
that of the stored response match, and that of 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 4.3), and match those presented (see Section 4.1), and
o the presented request does not contain the no-cache pragma o the presented request does not contain the no-cache pragma
(Section 7.4), nor the no-cache cache directive (Section 7.2.1), (Section 5.4), nor the no-cache cache directive (Section 5.2.1),
unless the stored response is successfully validated unless the stored response is successfully validated
(Section 4.2), and (Section 4.3), and
o the stored response does not contain the no-cache cache directive o the stored response does not contain the no-cache cache directive
(Section 7.2.2.2), unless it is successfully validated (Section 5.2.2.2), unless it is successfully validated
(Section 4.2), and (Section 4.3), and
o the stored response is either: o the stored response is either:
* fresh (see Section 4.1), or * fresh (see Section 4.2), or
* allowed to be served stale (see Section 4.1.4), or * allowed to be served stale (see Section 4.2.4), or
* successfully validated (see Section 4.2). * successfully validated (see Section 4.3).
Note that any of the requirements listed above can be overridden by a Note that any of the requirements listed above can be overridden by a
cache-control extension; see Section 7.2.3. cache-control extension; 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 7.1), 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 4.1.3. stored response's current_age; see Section 4.2.3.
A cache MUST write through requests with methods that are unsafe A cache MUST write through requests with methods that are unsafe
(Section 4.2.1 of [Part2]) to the origin server; i.e., a cache is not (Section 4.2.1 of [Part2]) to the origin server; i.e., a cache is not
allowed to generate a reply to such a request before having forwarded allowed to generate a reply to such a request before having forwarded
the request and having received a corresponding response. the request and having received a corresponding response.
Also, note that unsafe requests might invalidate already stored Also, note that unsafe requests might invalidate already stored
responses; see Section 6. responses; see Section 4.4.
When more than one suitable response is stored, a cache MUST use the When more than one suitable response is stored, a cache MUST use the
most recent response (as determined by the Date header field). It most recent response (as determined by the Date header field). It
can also forward the request with "Cache-Control: max-age=0" or can also forward the request with "Cache-Control: max-age=0" or
"Cache-Control: no-cache" to disambiguate which response to use. "Cache-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. Freshness 4.1. Calculating Secondary Keys with Vary
When a cache receives a request that can be satisfied by a stored
response that has a Vary header field (Section 7.1.4 of [Part2]), it
MUST NOT use that response unless all of the selecting header fields
nominated by the Vary header field match in both the original request
(i.e., that associated with the stored response), and the presented
request.
The selecting header fields from two requests are defined to match if
and only if those in the first request can be transformed to those in
the second request by applying any of the following:
o adding or removing whitespace, where allowed in the header field's
syntax
o combining multiple header fields with the same field name (see
Section 3.2 of [Part1])
o normalizing both header field values in a way that is known to
have identical semantics, according to the header field's
specification (e.g., re-ordering field values when order is not
significant; case-normalization, where values are defined to be
case-insensitive)
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
also absent there.
A Vary header field-value of "*" always fails to match.
The stored response with matching selecting header fields is known as
the selected response.
If multiple selected responses are available (potentially including
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
doing so (e.g., qvalues on Accept and similar request header fields),
that mechanism MAY be used to select preferred responses; of the
remainder, the most recent response (as determined by the Date header
field) is used, as per Section 4.
If no selected response is available, the cache cannot satisfy the
presented request. Typically, it is forwarded to the origin server
in a (possibly conditional; see Section 4.3) request.
4.2. Freshness
A fresh response is one whose age has not yet exceeded its freshness A fresh response is one whose age has not yet exceeded its freshness
lifetime. Conversely, a stale response is one where it has. lifetime. Conversely, a stale response is one where it has.
A response's freshness lifetime is the length of time between its A response's freshness lifetime is the length of time between its
generation by the origin server and its expiration time. An explicit generation by the origin server and its expiration time. An explicit
expiration time is the time at which the origin server intends that a expiration time is the time at which the origin server intends that a
stored response can no longer be used by a cache without further stored response can no longer be used by a cache without further
validation, whereas a heuristic expiration time is assigned by a validation, whereas a heuristic expiration time is assigned by a
cache when no explicit expiriation time is available. 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 7.3) or the max-age response either the Expires header field (Section 5.3) or the max-age response
cache directive (Section 7.2.2.8). Generally, origin servers will cache directive (Section 5.2.2.8). Generally, origin servers will
assign future explicit expiration times to responses in the belief assign future explicit expiration times to responses in the belief
that the representation is not likely to change in a semantically that the representation is not likely to change in a semantically
significant way before the expiration time is reached. significant way before the expiration time is reached.
If an origin server wishes to force a cache to validate every If an origin server wishes to force a cache to validate every
request, it can assign an explicit expiration time in the past to request, it can assign an explicit expiration time in the past to
indicate that the response is already stale. Compliant caches will indicate that the response is already stale. Compliant caches will
normally validate a stale cached response before reusing it for normally validate a stale cached response before reusing it for
subsequent requests (see Section 4.1.4). subsequent requests (see Section 4.2.4).
Since origin servers do not always provide explicit expiration times, Since origin servers do not always provide explicit expiration times,
caches are also allowed to use a heuristic to determine an expiration caches are also allowed to use a heuristic to determine an expiration
time under certain circumstances (see Section 4.1.2). time under certain circumstances (see Section 4.2.2).
The calculation to determine if a response is fresh is: The calculation to determine if a response is fresh is:
response_is_fresh = (freshness_lifetime > current_age) response_is_fresh = (freshness_lifetime > current_age)
freshness_lifetime is defined in Section 4.1.1; current_age is freshness_lifetime is defined in Section 4.2.1; current_age is
defined in Section 4.1.3. defined in Section 4.2.3.
Clients can send the max-age or min-fresh cache directives in a Clients can send the max-age or min-fresh cache directives in a
request to constrain or relax freshness calculations for the request to constrain or relax freshness calculations for the
corresponding response (Section 7.2.1). corresponding response (Section 5.2.1).
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, o Although all date formats are specified to be case-sensitive, a
cache recipients SHOULD match day, week and timezone names case- cache recipient SHOULD match day, week, and timezone names 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 Cache recipients 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 Cache recipients SHOULD consider a date with a zone abbreviation o A cache recipient SHOULD consider a date with a zone abbreviation
other than "GMT" to be invalid for calculating expiration. other than GMT or UTC to be invalid for calculating expiration.
Note that freshness applies only to cache operation; it cannot be Note that freshness applies only to cache operation; it cannot be
used to force a user agent to refresh its display or reload a used to force a user agent to refresh its display or reload a
resource. See Section 8 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.
4.1.1. 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:
o If the cache is shared and the s-maxage response cache directive o If the cache is shared and the s-maxage response cache directive
(Section 7.2.2.9) is present, use its value, or (Section 5.2.2.9) is present, use its value, or
o If the max-age response cache directive (Section 7.2.2.8) is o If the max-age response cache directive (Section 5.2.2.8) is
present, use its value, or present, use its value, or
o If the Expires response header field (Section 7.3) is present, use o If the Expires response header field (Section 5.3) is present, use
its value minus the value of the Date response header field, or its value minus the value of the Date response header field, or
o Otherwise, no explicit expiration time is present in the response. o Otherwise, no explicit expiration time is present in the response.
A heuristic freshness lifetime might be applicable; see A heuristic freshness lifetime might be applicable; see
Section 4.1.2. 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 fields, multiple Cache-Control: max-age
directives), the directive's value is considered invalid. Caches are directives), the directive's value is considered invalid. Caches are
encouraged to consider responses that have invalid freshness encouraged to consider responses that have invalid freshness
information to be stale. information to be stale.
4.1.2. 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 header field
values (such as the Last-Modified time) to estimate a plausible values (such as the Last-Modified time) to estimate a plausible
expiration time. This specification does not provide specific expiration time. This specification does not provide specific
algorithms, but does impose worst-case constraints on their results. algorithms, but does impose worst-case constraints on their results.
A cache MUST NOT use heuristics to determine freshness when an A cache MUST NOT use heuristics to determine freshness when an
explicit expiration time is present in the stored response. Because explicit expiration time is present in the stored response. Because
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whose status codes are defined as cacheable, and responses without whose status codes are defined as cacheable, and responses without
explicit freshness that have been marked as explicitly cacheable explicit freshness that have been marked as explicitly cacheable
(e.g., with a "public" response cache directive). (e.g., with a "public" response cache directive).
If the response has a Last-Modified header field (Section 2.2 of If the response has a Last-Modified header field (Section 2.2 of
[Part4]), caches are encouraged to use a heuristic expiration value [Part4]), caches are encouraged to use a heuristic expiration value
that is no more than some fraction of the interval since that time. that is no more than some fraction of the interval since that time.
A typical setting of this fraction might be 10%. A typical setting of this fraction might be 10%.
When a heuristic is used to calculate freshness lifetime, a cache When a heuristic is used to calculate freshness lifetime, a cache
SHOULD attach a Warning header field with a 113 warn-code to the SHOULD generate a Warning header field with a 113 warn-code (see
response if its current_age is more than 24 hours and such a warning Section 5.5.4) in the response if its current_age is more than 24
is not already present. hours and such a warning is not already present.
Note: Section 13.9 of [RFC2616] prohibited caches from calculating Note: Section 13.9 of [RFC2616] prohibited caches from calculating
heuristic freshness for URIs with query components (i.e., those heuristic freshness for URIs with query components (i.e., those
containing '?'). In practice, this has not been widely containing '?'). In practice, this has not been widely
implemented. Therefore, origin servers are encouraged to send implemented. Therefore, origin servers are encouraged to send
explicit directives (e.g., Cache-Control: no-cache) if they wish explicit directives (e.g., Cache-Control: no-cache) if they wish
to preclude caching. to preclude caching.
4.1.3. Calculating Age 4.2.3. Calculating Age
The Age header field is used to convey an estimated age of the The Age header field is used to convey an estimated age of the
response message when obtained from a cache. The Age field value is response message when obtained from a cache. The Age field value is
the cache's estimate of the number of seconds since the response was the cache's estimate of the number of seconds since the response was
generated or validated by the origin server. In essence, the Age generated or validated by the origin server. In essence, the Age
value is the sum of the time that the response has been resident in value is 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 each of the caches along the path from the origin server, plus the
amount of time it has been in transit along network paths. amount of time it has been in transit along network paths.
The following data is used for the age calculation: The following data is used for the age calculation:
age_value age_value
The term "age_value" denotes the value of the Age header field The term "age_value" denotes the value of the Age header field
(Section 7.1), in a form appropriate for arithmetic operation; or (Section 5.1), in a form appropriate for arithmetic operation; or
0, if not available. 0, if not available.
date_value date_value
The term "date_value" denotes the value of the Date header field, The term "date_value" denotes the value of the Date header field,
in a form appropriate for arithmetic operations. See Section in a form appropriate for arithmetic operations. See Section
7.1.1.2 of [Part2] for the definition of the Date header field, 7.1.1.2 of [Part2] for the definition of the Date header field,
and for requirements regarding responses without it. and for requirements regarding responses without it.
now now
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in which case the corrected_age_value MAY be used as the in which case the corrected_age_value MAY be used as the
corrected_initial_age. corrected_initial_age.
The current_age of a stored response can then be calculated by adding The current_age of a stored response can then be calculated by adding
the amount of time (in seconds) since the stored response was last the amount of time (in seconds) since the stored response was last
validated by the origin server to the corrected_initial_age. validated by the origin server to the corrected_initial_age.
resident_time = now - response_time; resident_time = now - response_time;
current_age = corrected_initial_age + resident_time; current_age = corrected_initial_age + resident_time;
4.1.4. 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 4.1. according to the calculations in Section 4.2.
A cache MUST NOT generate a stale response if it is prohibited by an A cache MUST NOT generate a stale response if it is prohibited by an
explicit in-protocol directive (e.g., by a "no-store" or "no-cache" explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
cache directive, a "must-revalidate" cache-response-directive, or an cache directive, a "must-revalidate" cache-response-directive, or an
applicable "s-maxage" or "proxy-revalidate" cache-response-directive; applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
see Section 7.2.2). see Section 5.2.2).
A cache MUST NOT send stale responses unless it is disconnected A cache MUST NOT send stale responses unless it is disconnected
(i.e., it cannot contact the origin server or otherwise find a (i.e., it cannot contact the origin server or otherwise find a
forward path) or doing so is explicitly allowed (e.g., by the max- forward path) or doing so is explicitly allowed (e.g., by the max-
stale request directive; see Section 7.2.1). stale request directive; see Section 5.2.1).
A cache SHOULD append a Warning header field with the 110 warn-code A cache SHOULD generate a Warning header field with the 110 warn-code
(see Section 7.5) to stale responses. Likewise, a cache SHOULD add (see Section 5.5.1) in stale responses. Likewise, a cache SHOULD
the 112 warn-code to stale responses if the cache is disconnected. generate a 112 warn-code (see Section 5.5.3) in stale responses if
the cache is disconnected.
Note that if a cache receives a first-hand response (one where the A cache SHOULD NOT generate a new Warning header field when
freshness model is not in use; i.e., its age is 0, whether it is an forwarding a response that does not have an Age header field, even if
entire response, or a 304 (Not Modified) response) that it would the response is already stale. A cache need not validate a response
normally forward to the requesting client, and the received response that merely became stale in transit.
is no longer fresh, the cache MAY forward it to the requesting client
without adding a new Warning (but without removing any existing
Warning header fields). A cache ought not attempt to validate a
response simply because that response became stale in transit.
4.2. 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 4.3), it can use the conditional one cannot be selected; see Section 4.1), it can use the conditional
request mechanism [Part4] in the forwarded request to give the origin request mechanism [Part4] in the forwarded request to give the next
server an opportunity to both select a valid stored response to be inbound server an opportunity to select a valid stored response to
used, and to update it. This process is known as "validating" or use, updating the stored metadata in the process, or to replace the
"revalidating" the stored response. stored response(s) with a new response. This process is known as
"validating" or "revalidating" the stored response.
When sending such a conditional request, a cache adds a validator (or 4.3.1. Sending a Validation Request
more than one), that is used to find out whether a stored response is
an equivalent copy of a current representation of the resource.
One such validator is the If-Modified-Since header field, whose value When sending a conditional request for cache validation, a cache
is that of the Last-Modified header field from the selected (see sends one or more precondition header fields containing validator
Section 4.3) stored response, if available. metadata from its stored response(s), which is then compared by
recipients to determine whether a stored response is equivalent to a
current representation of the resource.
Another is the If-None-Match header field, whose value is that of the One such validator is the timestamp given in a Last-Modified header
ETag header field(s) from relevant responses stored for the primary field (Section 2.2 of [Part4]), which can be used in an If-Modified-
cache key, if present. However, if any of the stored responses Since header field for response validation, or in an If-Unmodified-
contains only partial content, the cache ought not include its Since or If-Range header field for representation selection (i.e.,
entity-tag in the If-None-Match header field unless the request is the client is referring specifically to a previously obtained
for a range that would be fully satisfied by that stored response. representation with that timestamp).
Another validator is the entity-tag given in an ETag header field
(Section 2.3 of [Part4]). One or more entity-tags, indicating 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 representation selection (i.e., the client is referring
specifically to one or more previously obtained representations with
the listed entity-tags).
4.3.2. Handling a Received Validation Request
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
from other (outbound) caches. Likewise, some user agents make use of
conditional requests to limit data transfers to recently modified
representations or to complete the transfer of a partially retrieved
representation.
If a cache receives a request that can be satisfied by reusing one of
its stored 200 (OK) or 206 (Partial Content) responses, the cache
SHOULD evaluate any applicable conditional header field preconditions
received in that request with respect to the corresponding validators
contained within the selected response. A cache MUST NOT evaluate
conditional header fields that are only applicable to an origin
server, found in a request with semantics that cannot be satisfied
with a cached response, or applied to a target resource for which it
has no stored responses; such preconditions are likely intended for
some other (inbound) server.
The proper evaluation of conditional requests by a cache depends on
the received precondition header fields and their precedence, as
defined in Section 6 of [Part4]. The If-Match and If-Unmodified-
Since conditional header fields are not applicable to a cache.
A request containing an If-None-Match header field (Section 3.2 of
[Part4]) indicates that the client wants to validate one or more of
its own stored responses in comparison to whichever stored 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 match
the entity-tag of the selected stored response, a cache recipient
SHOULD generate a 304 (Not Modified) response (using the metadata of
the selected stored response) instead of sending that stored
response.
When a cache decides to revalidate its own stored responses for a
request that contains an If-None-Match list of entity-tags, the cache
MAY combine the received list with a list of entity-tags from its own
stored set of responses (fresh or stale) and send the union of the
two lists as a replacement If-None-Match header field value in the
forwarded request. If a stored response contains only partial
content, the cache MUST NOT include its entity-tag in the union
unless the request is for a range that would be fully satisfied by
that partial stored response. If the response to the forwarded
request is 304 (Not Modified) and has an ETag header field value with
an entity-tag that is not in the client's list, the cache MUST
generate a 200 (OK) response for the client by reusing its
corresponding stored response, as updated by the 304 response
metadata (Section 4.3.4).
If an If-None-Match header field is not present, a request containing
an If-Modified-Since header field (Section 3.3 of [Part4]) indicates
that the client wants to validate one or more of its own stored
responses by modification date. A cache recipient SHOULD generate a
304 (Not Modified) response (using the metadata of the selected
stored response) if one of the following cases is true: 1) the
selected stored response has a Last-Modified field-value that is
earlier than or equal to the conditional timestamp; 2) no Last-
Modified field is present in the selected stored response, but it has
a Date field-value that is earlier than or equal to the conditional
timestamp; or, 3) neither Last-Modified nor Date is present in the
selected stored response, but the cache recorded it as having been
received at a time earlier than or equal to the conditional
timestamp.
A cache that implements partial responses to range requests, as
defined in [Part5], also needs to evaluate a received If-Range header
field (Section 3.2 of [Part5]) with respect to its selected stored
response.
4.3.3. Handling a Validation Response
Cache handling of a response to a conditional request is dependent Cache handling of a response to a conditional request is dependent
upon its status code: upon 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 4.2.1. 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 can use the full response to is suitable. Instead, the cache MUST use the full response to
satisfy the request and MAY replace the stored response(s). satisfy the request and MAY replace the stored response(s).
o However, if a cache receives a 5xx (Server Error) response while o However, if a cache receives a 5xx (Server Error) response while
attempting to validate a response, it can either forward this attempting to validate a response, it can either forward this
response to the requesting client, or act as if the server failed response to the requesting client, or act as if the server failed
to respond. In the latter case, it can send a previously stored to respond. In the latter case, the cache MAY send a previously
response (see Section 4.1.4). stored response (see Section 4.2.4).
4.2.1. Freshening Stored Responses upon Validation 4.3.4. Freshening Stored Responses upon Validation
When a cache receives a 304 (Not Modified) response and already has When a cache receives a 304 (Not Modified) response and already has
one or more stored 200 (OK) responses for the same cache key, the one or more stored 200 (OK) responses for the same cache key, the
cache needs to identify which of the stored responses are updated by cache needs to identify which of the stored responses are updated by
this new response and then update the stored response(s) with the new this new response and then update the stored response(s) with the new
information provided in the 304 response. information provided in the 304 response.
The stored response to update is identified by using the first match The stored response to update is identified by using the first match
(if any) of: (if any) of:
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o If the new response does not include any form of validator (such o If the new response does not include any form of validator (such
as in the case where a client generates an If-Modified-Since as in the case where a client generates an If-Modified-Since
request from a source other than the Last-Modified response header request from a source other than the Last-Modified response header
field), and there is only one stored response, and that stored field), and there is only one stored response, and that stored
response also lacks a validator, then that stored response is response also lacks a validator, then that stored response is
selected for update. selected for update.
If a stored response is selected for update, the cache MUST: If a stored response is selected for update, the cache MUST:
o delete any Warning header fields in the stored response with warn- o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 7.5); code 1xx (see Section 5.5);
o retain any Warning header fields in the stored response with warn- o retain any Warning header fields in the stored response with warn-
code 2xx; and, code 2xx; and,
o use other header fields provided in the 304 (Not Modified) o use other header fields provided in the 304 (Not Modified)
response to replace all instances of the corresponding header response to replace all instances of the corresponding header
fields in the stored response. fields in the stored response.
4.3. Calculating Secondary Keys with Vary 4.3.5. Freshening Responses via HEAD
When a cache receives a request that can be satisfied by a stored
response that has a Vary header field (Section 7.1.4 of [Part2]), it
MUST NOT use that response unless all of the selecting header fields
nominated by the Vary header field match in both the original request
(i.e., that associated with the stored response), and the presented
request.
The selecting header fields from two requests are defined to match if
and only if those in the first request can be transformed to those in
the second request by applying any of the following:
o adding or removing whitespace, where allowed in the header field's
syntax
o combining multiple header fields with the same field name (see
Section 3.2 of [Part1])
o normalizing both header field values in a way that is known to
have identical semantics, according to the header field's
specification (e.g., re-ordering field values when order is not
significant; case-normalization, where values are defined to be
case-insensitive)
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
also absent there.
A Vary header field-value of "*" always fails to match.
The stored response with matching selecting header fields is known as
the selected response.
If multiple selected responses are available (potentially including
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
doing so (e.g., qvalues on Accept and similar request header fields),
that mechanism MAY be used to select preferred responses; of the
remainder, the most recent response (as determined by the Date header
field) is used, as per Section 4.
If no selected response is available, the cache cannot satisfy the
presented request. Typically, it is forwarded to the origin server
in a (possibly conditional; see Section 4.2) request.
5. Updating Caches with HEAD Responses
A response to the HEAD method is identical to what an equivalent A response to the HEAD method is identical to what an equivalent
request made with a GET would have been, except it lacks a body. request made with a GET would have been, except it lacks a body.
This property of HEAD responses is used to both invalidate and update This property of HEAD responses can be used to invalidate or update a
cached GET responses. cached GET response if the more efficient conditional GET request
mechanism is not available (due to no validators being present in the
stored response) or if transmission of the representation body is not
desired even if it has changed.
If one or more stored GET responses can be selected (as per When a cache makes an inbound HEAD request for a given request target
Section 4.3) for a HEAD request, and the Content-Length, ETag or and receives a 200 (OK) response, the cache SHOULD update or
Last-Modified value of a HEAD response differs from that in a invalidate each of its stored GET responses that could have been
selected GET response, the cache MUST consider that selected response selected for that request (see Section 4.1).
to be stale.
If the Content-Length, ETag and Last-Modified values of a HEAD For each of the stored responses that could have been selected, if
response (when present) are the same as that in a selected GET the stored response and HEAD response have matching values for any
response (as per Section 4.3), the cache SHOULD update the remaining received validator fields (ETag and Last-Modified) and, if the HEAD
header fields in the stored response using the following rules: response has a Content-Length header field, the value of Content-
Length matches that of the stored response, the cache SHOULD update
the stored response a described below; otherwise, the cache SHOULD
consider the stored response to be stale.
If a cache updates a stored response with the metadata provided in a
HEAD response, the cache MUST:
o delete any Warning header fields in the stored response with warn- o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 7.5); code 1xx (see Section 5.5);
o retain any Warning header fields in the stored response with warn- o retain any Warning header fields in the stored response with warn-
code 2xx; and, code 2xx; and,
o use other header fields provided in the response to replace all o use other header fields provided in the HEAD response to replace
instances of the corresponding header fields in the stored all instances of the corresponding header fields in the stored
response. response and append new header fields to the stored response's
header section unless otherwise restricted by the Cache-Control
header field.
6. Request Methods that Invalidate 4.4. Invalidation
Because unsafe request methods (Section 4.2.1 of [Part2]) such as Because unsafe request methods (Section 4.2.1 of [Part2]) such as
PUT, POST or DELETE have the potential for changing state on the PUT, POST or DELETE have the potential for changing state on the
origin server, intervening caches can use them to keep their contents origin server, intervening caches can use them to keep their contents
up-to-date. up-to-date.
A cache MUST invalidate the effective Request URI (Section 5.5 of A cache MUST invalidate the effective Request URI (Section 5.5 of
[Part1]) as well as the URI(s) in the Location and Content-Location [Part1]) as well as the URI(s) in the Location and Content-Location
response header fields (if present) when a non-error response to a response header fields (if present) when a non-error status code is
request with an unsafe method is received. received in response to an unsafe request method.
However, a cache MUST NOT invalidate a URI from a Location or However, a cache MUST NOT invalidate a URI from a Location or
Content-Location response header field if the host part of that URI Content-Location response header field if the host part of that URI
differs from the host part in the effective request URI (Section 5.5 differs from the host part in the effective request URI (Section 5.5
of [Part1]). This helps prevent denial of service attacks. of [Part1]). This helps prevent denial of service attacks.
A cache MUST invalidate the effective request URI (Section 5.5 of A cache MUST invalidate the effective request URI (Section 5.5 of
[Part1]) when it receives a non-error response to a request with a [Part1]) when it receives a non-error response to a request with a
method whose safety is unknown. method whose safety is unknown.
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either remove all stored responses related to the effective request either remove all stored responses related to the effective request
URI, or will mark these as "invalid" and in need of a mandatory URI, or will mark these as "invalid" and in need of a mandatory
validation before they can be sent in response to a subsequent validation before they can be sent in response to a subsequent
request. request.
Note that this does not guarantee that all appropriate responses are Note that this does not guarantee that all appropriate responses are
invalidated. For example, a state-changing request might invalidate invalidated. For example, a state-changing request might invalidate
responses in the caches it travels through, but relevant responses responses in the caches it travels through, but relevant responses
still might be stored in other caches that it has not. still might be stored in other caches that it has not.
7. Header Field Definitions 5. Header Field Definitions
This section defines the syntax and semantics of HTTP/1.1 header This section defines the syntax and semantics of HTTP/1.1 header
fields related to caching. fields related to caching.
7.1. Age 5.1. Age
The "Age" header field conveys the sender's estimate of the amount of The "Age" header field conveys the sender's estimate of the amount of
time since the response was generated or successfully validated at time since the response was generated or successfully validated at
the origin server. Age values are calculated as specified in the origin server. Age values are calculated as specified in
Section 4.1.3. Section 4.2.3.
Age = delta-seconds Age = delta-seconds
Age field-values are non-negative integers, representing time in The Age field-value is a non-negative integer, representing time in
seconds (see Section 1.2.1). seconds (see Section 1.2.1).
The presence of an Age header field in a response implies that a The presence of an Age header field implies that the response was not
response is not first-hand. However, the converse is not true, since generated or validated by the origin server for this request.
HTTP/1.0 caches might not implement the Age header field. However, lack of an Age header field does not imply the origin was
contacted, since the response might have been received from an
HTTP/1.0 cache that does not implement Age.
7.2. Cache-Control 5.2. Cache-Control
The "Cache-Control" header field is used to specify directives for The "Cache-Control" header field is used to specify directives for
caches along the request/response chain. Such cache directives are caches along the request/response chain. Such cache directives are
unidirectional in that the presence of a directive in a request does unidirectional in that the presence of a directive in a request does
not imply that the same directive is to be given in the response. not imply that the same directive is to be given in the response.
A cache MUST obey the requirements of the Cache-Control directives A cache MUST obey the requirements of the Cache-Control directives
defined in this section. See Section 7.2.3 for information about how defined in this section. See Section 5.2.3 for information about how
Cache-Control directives defined elsewhere are handled. Cache-Control directives defined elsewhere are handled.
Note: Some HTTP/1.0 caches might not implement Cache-Control. Note: Some HTTP/1.0 caches might not implement Cache-Control.
A proxy, whether or not it implements a cache, MUST pass cache A proxy, whether or not it implements a cache, MUST pass cache
directives through in forwarded messages, regardless of their directives through in forwarded messages, regardless of their
significance to that application, since the directives might be significance to that application, since the directives might be
applicable to all recipients along the request/response chain. It is applicable to all recipients along the request/response chain. It is
not possible to target a directive to a specific cache. not possible to target a directive to a specific cache.
Cache directives are identified by a token, to be compared case- Cache directives are identified by a token, to be compared case-
insensitively, and have an optional argument, that can use both token insensitively, and have an optional argument, that can use both token
and quoted-string syntax. For the directives defined below that and quoted-string syntax. For the directives defined below that
define arguments, recipients ought to accept both forms, even if one define arguments, recipients ought to accept both forms, even if one
is documented to be preferred. For any directive not defined by this is documented to be preferred. For any directive not defined by this
specification, recipients MUST accept both forms. specification, a recipient MUST accept both forms.
Cache-Control = 1#cache-directive Cache-Control = 1#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.
7.2.1. Request Cache-Control Directives 5.2.1. Request Cache-Control Directives
7.2.1.1. max-age 5.2.1.1. max-age
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.2.1)
The "max-age" request directive indicates that the client is The "max-age" request directive indicates that the client is
unwilling to accept a response whose age is greater than the unwilling to accept a response whose age is greater than the
specified number of seconds. Unless the max-stale request directive specified number of seconds. Unless the max-stale request directive
is also present, the client is not willing to accept a stale is also present, the client is not willing to accept a stale
response. response.
Note: This directive uses the token form of the argument syntax; Note: This directive uses the token form of the argument syntax;
e.g., 'max-age=5', not 'max-age="5"'. Senders SHOULD NOT use the e.g., 'max-age=5', not 'max-age="5"'. A sender SHOULD NOT generate
quoted-string form. the quoted-string form.
7.2.1.2. max-stale 5.2.1.2. max-stale
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.2.1)
The "max-stale" request directive indicates that the client is The "max-stale" request directive indicates that the client is
willing to accept a response that has exceeded its freshness willing to accept a response that has exceeded its freshness
lifetime. If max-stale is assigned a value, then the client is lifetime. If max-stale is assigned a value, then the client is
willing to accept a response that has exceeded its freshness lifetime willing to accept a response that has exceeded its freshness lifetime
by no more than the specified number of seconds. If no value is by no more than the specified number of seconds. If no value is
assigned to max-stale, then the client is willing to accept a stale assigned to max-stale, then the client is willing to accept a stale
response of any age. response of any age.
Note: This directive uses the token form of the argument syntax; Note: This directive uses the token form of the argument syntax;
e.g., 'max-stale=10', not 'max-stale="10"'. Senders SHOULD NOT use e.g., 'max-stale=10', not 'max-stale="10"'. A sender SHOULD NOT
the quoted-string form. generate the quoted-string form.
7.2.1.3. min-fresh 5.2.1.3. min-fresh
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.2.1)
The "min-fresh" request directive indicates that the client is The "min-fresh" request directive indicates that the client is
willing to accept a response whose freshness lifetime is no less than willing to accept a response whose freshness lifetime is no less than
its current age plus the specified time in seconds. That is, the its current age plus the specified time in seconds. That is, the
client wants a response that will still be fresh for at least the client wants a response that will still be fresh for at least the
specified number of seconds. specified number of seconds.
Note: This directive uses the token form of the argument syntax; Note: This directive uses the token form of the argument syntax;
e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders SHOULD NOT use e.g., 'min-fresh=20', not 'min-fresh="20"'. A sender SHOULD NOT
the quoted-string form. generate the quoted-string form.
7.2.1.4. no-cache 5.2.1.4. no-cache
The "no-cache" request directive indicates that a cache MUST NOT use The "no-cache" request directive indicates that a cache MUST NOT use
a stored response to satisfy the request without successful a stored response to satisfy the request without successful
validation on the origin server. validation on the origin server.
7.2.1.5. no-store 5.2.1.5. no-store
The "no-store" request directive indicates that a cache MUST NOT The "no-store" request directive indicates that a cache MUST NOT
store any part of either this request or any response to it. This store any part of either this request or any response to it. This
directive applies to both private and shared caches. "MUST NOT directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a best- store the information in non-volatile storage, and MUST make a best-
effort attempt to remove the information from volatile storage as effort attempt to remove the information from volatile storage as
promptly as possible after forwarding it. promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for ensuring This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might recognize or obey this directive, and communications networks might
be vulnerable to eavesdropping. be vulnerable to eavesdropping.
Note that if a request containing this directive is satisfied from a Note that if a request containing this directive is satisfied from a
cache, the no-store request directive does not apply to the already cache, the no-store request directive does not apply to the already
stored response. stored response.
7.2.1.6. no-transform 5.2.1.6. no-transform
The "no-transform" request directive indicates that an intermediary The "no-transform" request directive indicates that an intermediary
(whether or not it implements a cache) MUST NOT transform the (whether or not it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of [Part1]. payload, as defined in Section 5.7.2 of [Part1].
7.2.1.7. only-if-cached 5.2.1.7. only-if-cached
The "only-if-cached" request directive indicates that the client only The "only-if-cached" request directive indicates that the client only
wishes to obtain a stored response. If it receives this directive, a wishes to obtain a stored response. If it receives this directive, a
cache SHOULD either respond using a stored response that is cache SHOULD either respond using a stored response that is
consistent with the other constraints of the request, or respond with consistent with the other constraints of the request, or respond with
a 504 (Gateway Timeout) status code. If a group of caches is being a 504 (Gateway Timeout) status code. If a group of caches is being
operated as a unified system with good internal connectivity, a operated as a unified system with good internal connectivity, a
member cache MAY forward such a request within that group of caches. member cache MAY forward such a request within that group of caches.
7.2.2. Response Cache-Control Directives 5.2.2. Response Cache-Control Directives
7.2.2.1. must-revalidate 5.2.2.1. must-revalidate
The "must-revalidate" response directive indicates that once it has The "must-revalidate" response directive indicates that once it has
become stale, a cache MUST NOT use the response to satisfy subsequent become stale, a cache MUST NOT use the response to satisfy subsequent
requests without successful validation on the origin server. requests without successful validation on the origin server.
The must-revalidate directive is necessary to support reliable The must-revalidate directive is necessary to support reliable
operation for certain protocol features. In all circumstances a operation for certain protocol features. In all circumstances a
cache MUST obey the must-revalidate directive; in particular, if a cache MUST obey the must-revalidate directive; in particular, if a
cache cannot reach the origin server for any reason, it MUST generate cache cannot reach the origin server for any reason, it MUST generate
a 504 (Gateway Timeout) response. a 504 (Gateway Timeout) response.
The must-revalidate directive ought to be used by servers if and only The must-revalidate directive ought to be used by servers if and only
if failure to validate a request on the representation could result if failure to validate a request on the representation could result
in incorrect operation, such as a silently unexecuted financial in incorrect operation, such as a silently unexecuted financial
transaction. transaction.
7.2.2.2. no-cache 5.2.2.2. no-cache
Argument syntax: Argument syntax:
#field-name #field-name
The "no-cache" response directive indicates that the response MUST The "no-cache" response directive indicates that the response MUST
NOT be used to satisfy a subsequent request without successful NOT be used to satisfy a subsequent request without successful
validation on the origin server. This allows an origin server to validation on the origin server. This allows an origin server to
prevent a cache from using it to satisfy a request without contacting prevent a cache from using it to satisfy a request without contacting
it, even by caches that have been configured to send stale responses. it, even by caches that have been configured to send stale responses.
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defined by this specification. Field names are case-insensitive. defined by this specification. Field names are case-insensitive.
Note: Although it has been back-ported to many implementations, some Note: Although it has been back-ported to many implementations, some
HTTP/1.0 caches will not recognize or obey this directive. Also, no- HTTP/1.0 caches will not recognize or obey this directive. Also, no-
cache response directives with field-names are often handled by cache response directives with field-names are often handled by
caches as if an unqualified no-cache directive was received; i.e., caches as if an unqualified no-cache directive was received; i.e.,
the special handling for the qualified form is not widely the special handling for the qualified form is not widely
implemented. implemented.
Note: This directive uses the quoted-string form of the argument Note: This directive uses the quoted-string form of the argument
syntax. Senders SHOULD NOT use the token form (even if quoting syntax. A sender SHOULD NOT generate the token form (even if quoting
appears not to be needed for single-entry lists). appears not to be needed for single-entry lists).
7.2.2.3. no-store 5.2.2.3. no-store
The "no-store" response directive indicates that a cache MUST NOT The "no-store" response directive indicates that a cache MUST NOT
store any part of either the immediate request or response. This store any part of either the immediate request or response. This
directive applies to both private and shared caches. "MUST NOT directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a best- store the information in non-volatile storage, and MUST make a best-
effort attempt to remove the information from volatile storage as effort attempt to remove the information from volatile storage as
promptly as possible after forwarding it. promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for 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.
7.2.2.4. no-transform 5.2.2.4. no-transform
The "no-transform" response directive indicates that an intermediary The "no-transform" response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT transform the (regardless of whether it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of [Part1]. payload, as defined in Section 5.7.2 of [Part1].
7.2.2.5. public 5.2.2.5. public
The "public" response directive indicates that any cache MAY store The "public" response directive indicates that any cache MAY store
the response, even if the response would normally be non-cacheable or the response, even if the response would normally be non-cacheable or
cacheable only within a non-shared cache. (See Section 3.2 for cacheable only within a private cache. (See Section 3.2 for
additional details related to the use of public in response to a additional details related to the use of public in response to a
request containing Authorization, and Section 3 for details of how request containing Authorization, and Section 3 for details of how
public affects responses that would normally not be stored, due to public affects responses that would normally not be stored, due to
their status codes not being defined as cacheable.) their status codes not being defined as cacheable.)
7.2.2.6. private 5.2.2.6. private
Argument syntax: Argument syntax:
#field-name #field-name
The "private" response directive indicates that the response message The "private" response directive indicates that the response message
is intended for a single user and MUST NOT be stored by a shared is intended for a single user and MUST NOT be stored by a shared
cache. A private cache MAY store the response and reuse it for later cache. A private cache MAY store the response and reuse it for later
requests, even if the response would normally be non-cacheable. requests, even if the response would normally be non-cacheable.
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defined by this specification. Field names are case-insensitive. defined by this specification. Field names are case-insensitive.
Note: This usage of the word "private" only controls where the Note: This usage of the word "private" only controls where the
response can be stored; it cannot ensure the privacy of the message response can be stored; it cannot ensure the privacy of the message
content. Also, private response directives with field-names are content. Also, private response directives with field-names are
often handled by caches as if an unqualified private directive was often handled by caches as if an unqualified private directive was
received; i.e., the special handling for the qualified form is not received; i.e., the special handling for the qualified form is not
widely implemented. widely implemented.
Note: This directive uses the quoted-string form of the argument Note: This directive uses the quoted-string form of the argument
syntax. Senders SHOULD NOT use the token form (even if quoting syntax. A sender SHOULD NOT generate the token form (even if quoting
appears not to be needed for single-entry lists). appears not to be needed for single-entry lists).
7.2.2.7. proxy-revalidate 5.2.2.7. proxy-revalidate
The "proxy-revalidate" response directive has the same meaning as the The "proxy-revalidate" response directive has the same meaning as the
must-revalidate response directive, except that it does not apply to must-revalidate response directive, except that it does not apply to
private caches. private caches.
7.2.2.8. max-age 5.2.2.8. max-age
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.2.1)
The "max-age" response directive indicates that the response is to be The "max-age" response directive indicates that the response is to be
considered stale after its age is greater than the specified number considered stale after its age is greater than the specified number
of seconds. of seconds.
Note: This directive uses the token form of the argument syntax; Note: This directive uses the token form of the argument syntax;
e.g., 'max-age=5', not 'max-age="5"'. Senders SHOULD NOT use the e.g., 'max-age=5', not 'max-age="5"'. A sender SHOULD NOT generate
quoted-string form. the quoted-string form.
7.2.2.9. s-maxage 5.2.2.9. s-maxage
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.2.1)
The "s-maxage" response directive indicates that, in shared caches, The "s-maxage" response directive indicates that, in shared caches,
the maximum age specified by this directive overrides the maximum age the maximum age specified by this directive overrides the maximum age
specified by either the max-age directive or the Expires header specified by either the max-age directive or the Expires header
field. The s-maxage directive also implies the semantics of the field. The s-maxage directive also implies the semantics of the
proxy-revalidate response directive. proxy-revalidate response directive.
Note: This directive uses the token form of the argument syntax; Note: This directive uses the token form of the argument syntax;
e.g., 's-maxage=10', not 's-maxage="10"'. Senders SHOULD NOT use the e.g., 's-maxage=10', not 's-maxage="10"'. A sender SHOULD NOT
quoted-string form. generate the quoted-string form.
7.2.3. 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. or more cache-extension tokens, each with an optional value.
Informational extensions (those that do not require a change in cache Informational extensions (those that do not require a change in cache
behavior) can be added without changing the semantics of other behavior) can be added without changing the semantics of other
directives. Behavioral extensions are designed to work by acting as directives. Behavioral extensions are designed to work by acting as
modifiers to the existing base of cache directives. modifiers to the existing base of cache directives.
Both the new directive and the standard directive are supplied, such Both the new directive and the standard directive are supplied, such
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does not understand the community cache-extension, since it will also does not understand the community cache-extension, since it will also
see and understand the private directive and thus default to the safe see and understand the private directive and thus default to the safe
behavior. behavior.
A cache MUST ignore unrecognized cache directives; it is assumed that A cache MUST ignore unrecognized cache directives; it is assumed that
any cache directive likely to be unrecognized by an HTTP/1.1 cache any cache directive likely to be unrecognized by an HTTP/1.1 cache
will be combined with standard directives (or the response's default will be combined with standard directives (or the response's default
cacheability) such that the cache behavior will remain minimally cacheability) such that the cache behavior will remain minimally
correct even if the cache does not understand the extension(s). correct even if the cache does not understand the extension(s).
7.3. Expires 5.3. Expires
The "Expires" header field gives the date/time after which the The "Expires" header field gives the date/time after which the
response is considered stale. See Section 4.1 for further discussion response is considered stale. See Section 4.2 for further discussion
of the freshness model. of the freshness model.
The presence of an Expires field does not imply that the original The presence of an Expires field does not imply that the original
resource will change or cease to exist at, before, or after that resource will change or cease to exist at, before, or after that
time. time.
The Expires value is an HTTP-date timestamp, as defined in Section The Expires value is an HTTP-date timestamp, as defined in Section
7.1.1.1 of [Part2]. 7.1.1.1 of [Part2].
Expires = HTTP-date Expires = HTTP-date
For example For example
Expires: Thu, 01 Dec 1994 16:00:00 GMT Expires: Thu, 01 Dec 1994 16:00:00 GMT
A cache recipient MUST interpret invalid date formats, especially the A cache recipient MUST interpret invalid date formats, especially the
value "0", as representing a time in the past (i.e., "already value "0", as representing a time in the past (i.e., "already
expired"). expired").
If a response includes a Cache-Control field with the max-age If a response includes a Cache-Control field with the max-age
directive (Section 7.2.2.8), a recipient MUST ignore the Expires directive (Section 5.2.2.8), a recipient MUST ignore the Expires
field. Likewise, if a response includes the s-maxage directive field. Likewise, if a response includes the s-maxage directive
(Section 7.2.2.9), a shared cache recipient MUST ignore the Expires (Section 5.2.2.9), a shared cache recipient MUST ignore the Expires
field. In both these cases, the value in Expires is only intended field. In both these cases, the value in Expires is only intended
for recipients that have not yet implemented the Cache-Control field. for recipients that have not yet implemented the Cache-Control field.
An origin server without a clock MUST NOT generate an Expires field An origin server without a clock MUST NOT generate an Expires field
unless its value represents a fixed time in the past (always expired) unless its value represents a fixed time in the past (always expired)
or its value has been associated with the resource by a system or or its value has been associated with the resource by a system or
user with a reliable clock. user with a reliable clock.
Historically, HTTP required the Expires field-value to be no more Historically, HTTP required the Expires field-value to be no more
than a year in the future. While longer freshness lifetimes are no than a year in the future. While longer freshness lifetimes are no
longer prohibited, extremely large values have been demonstrated to longer prohibited, extremely large values have been demonstrated to
cause problems (e.g., clock overflows due to use of 32-bit integers cause problems (e.g., clock overflows due to use of 32-bit integers
for time values), and many caches will evict a response far sooner for time values), and many caches will evict a response far sooner
than that. than that.
7.4. Pragma 5.4. Pragma
The "Pragma" header field allows backwards compatibility with The "Pragma" header field allows backwards compatibility with
HTTP/1.0 caches, so that clients can specify a "no-cache" request HTTP/1.0 caches, so that clients can specify a "no-cache" request
that they will understand (as Cache-Control was not defined until that they will understand (as Cache-Control was not defined until
HTTP/1.1). When the Cache-Control header field is also present and HTTP/1.1). When the Cache-Control header field is also present and
understood in a request, Pragma is ignored. understood in a request, Pragma is ignored.
In HTTP/1.0, Pragma was defined as an extensible field for In HTTP/1.0, Pragma was defined as an extensible field for
implementation-specified directives for recipients. This implementation-specified directives for recipients. This
specification deprecates such extensions to improve interoperability. specification deprecates such extensions to improve interoperability.
Pragma = 1#pragma-directive Pragma = 1#pragma-directive
pragma-directive = "no-cache" / extension-pragma pragma-directive = "no-cache" / extension-pragma
extension-pragma = token [ "=" ( token / quoted-string ) ] extension-pragma = token [ "=" ( token / quoted-string ) ]
When the Cache-Control header field is not present in a request, When the Cache-Control header field is not present in a request,
caches MUST consider the no-cache request pragma-directive as having caches MUST consider the no-cache request pragma-directive as having
the same effect as if "Cache-Control: no-cache" were present (see the same effect as if "Cache-Control: no-cache" were present (see
Section 7.2.1). Section 5.2.1).
When sending a no-cache request, a client ought to include both the When sending a no-cache request, a client ought to include both the
pragma and cache-control directives, unless Cache-Control: no-cache pragma and cache-control directives, unless Cache-Control: no-cache
is purposefully omitted to target other Cache-Control response is purposefully omitted to target other Cache-Control response
directives at HTTP/1.1 caches. For example: directives at HTTP/1.1 caches. For example:
GET / HTTP/1.1 GET / HTTP/1.1
Host: www.example.com Host: www.example.com
Cache-Control: max-age=30 Cache-Control: max-age=30
Pragma: no-cache Pragma: no-cache
will constrain HTTP/1.1 caches to serve a response no older than 30 will constrain HTTP/1.1 caches to serve a response no older than 30
seconds, while precluding implementations that do not understand seconds, while precluding implementations that do not understand
Cache-Control from serving a cached response. Cache-Control from serving a cached response.
Note: Because the meaning of "Pragma: no-cache" in responses is Note: Because the meaning of "Pragma: no-cache" in responses is
not specified, it does not provide a reliable replacement for not specified, it does not provide a reliable replacement for
"Cache-Control: no-cache" in them. "Cache-Control: no-cache" in them.
7.5. Warning 5.5. Warning
The "Warning" header field is used to carry additional information The "Warning" header field is 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 message. This information is typically used to warn reflected in the status code. This information is typically used to
about possible incorrectness introduced by caching operations or warn about possible incorrectness introduced by caching operations or
transformations applied to the payload of the message. transformations applied to the payload of the message.
Warnings can be used for other purposes, both cache-related and Warnings can be used for other purposes, both cache-related and
otherwise. The use of a warning, rather than an error status code, otherwise. The use of a warning, rather than an error status code,
distinguishes these responses from true failures. distinguishes these responses from true failures.
Warning header fields can in general be applied to any message, Warning header fields can in general be applied to any message,
however some warn-codes are specific to caches and can only be however some warn-codes are specific to caches and can only be
applied to response messages. applied to response messages.
Warning = 1#warning-value Warning = 1#warning-value
warning-value = warn-code SP warn-agent SP warn-text warning-value = warn-code SP warn-agent SP warn-text
[SP warn-date] [ SP warn-date ]
warn-code = 3DIGIT warn-code = 3DIGIT
warn-agent = ( uri-host [ ":" port ] ) / pseudonym warn-agent = ( uri-host [ ":" port ] ) / pseudonym
; the name or pseudonym of the server adding ; the name or pseudonym of the server adding
; the Warning header field, for use in debugging ; the Warning header field, for use in debugging
; a single "-" is recommended when agent unknown
warn-text = quoted-string warn-text = quoted-string
warn-date = DQUOTE HTTP-date DQUOTE warn-date = DQUOTE HTTP-date DQUOTE
Multiple warnings can be attached to a response (either by the origin Multiple warnings can be generated in a response (either by the
server or by a cache), including multiple warnings with the same code origin server or by a cache), including multiple warnings with the
number, only differing in warn-text. same warn-code number that only differ in warn-text.
When this occurs, the user agent SHOULD inform the user of as many of
them as possible, in the order that they appear in the response.
Systems that generate multiple Warning header fields are encouraged A user agent that receives one or more Warning header fields SHOULD
to order them with this user agent behavior in mind. New Warning inform the user of as many of them as possible, in the order that
header fields are added after any existing Warning header fields. they appear in the response. Senders that generate multiple Warning
header fields are encouraged to order them with this user agent
behavior in mind. A sender that generates new Warning header fields
MUST append them after any existing Warning header fields.
Warnings are assigned three digit warn-codes. The first digit Warnings are assigned three digit warn-codes. The first digit
indicates whether the Warning is required to be deleted from a stored indicates whether the Warning is required to be deleted from a stored
response after validation: response after validation:
o 1xx Warnings describe the freshness or validation status of the o 1xx warn-codes describe the freshness or validation status of the
response, and so MUST be deleted by a cache after validation. response, and so MUST be deleted by a cache after validation.
They can only be generated by a cache when validating a cached They can only be generated by a cache when validating a cached
entry, and MUST NOT be generated in any other situation. entry, and MUST NOT be generated in any other situation.
o 2xx Warnings describe some aspect of the representation that is o 2xx warn-codes describe some aspect of the representation that is
not rectified by a validation (for example, a lossy compression of not rectified by a validation (for example, a lossy compression of
the representation) and MUST NOT be deleted by a cache after the representation) and MUST NOT be deleted by a cache after
validation, unless a full response is sent, in which case they validation, unless a full response is sent, in which case they
MUST be. MUST be.
If an implementation sends a message with one or more Warning header If a sender generates one or more 1xx warn-codes in a message to be
fields to a receiver whose version is HTTP/1.0 or lower, then the sent to a recipient known to implement only HTTP/1.0, the sender MUST
sender MUST include in each warning-value a warn-date that matches include in each corresponding warning-value a warn-date that matches
the Date header field in the message. the Date header field in the message. For example:
If a system receives a message with a warning-value that includes a HTTP/1.1 200 OK
warn-date, and that warn-date is different from the Date value in the Date: Sat, 25 Aug 2012 23:34:45 GMT
response, then that warning-value MUST be deleted from the message Warning: 112 - "network down" "Sat, 25 Aug 2012 23:34:45 GMT"
before storing, forwarding, or using it. (preventing the consequences
of naive caching of Warning header fields.) If all of the warning- If a recipient that uses, evaluates, or displays Warning header
values are deleted for this reason, the Warning header field MUST be fields receives a warn-date that is different from the Date value in
deleted as well. the same message, the recipient MUST exclude the warning-value
containing that warn-date before storing, forwarding, or using the
message. This allows recipients to exclude warning-values that were
improperly retained after a cache validation. If all of the warning-
values are excluded, the recipient MUST exclude the Warning header
field as well.
The following warn-codes are defined by this specification, each with The following warn-codes are defined by this specification, each with
a recommended warn-text in English, and a description of its meaning. a recommended warn-text in English, and a description of its meaning.
The procedure for defining additional warn codes is described in
Section 7.2.1.
7.5.1. 110 Response is Stale 5.5.1. Warning: 110 - "Response is Stale"
A cache SHOULD generate this whenever the sent response is stale. A cache SHOULD generate this whenever the sent response is stale.
7.5.2. 111 Revalidation Failed 5.5.2. Warning: 111 - "Revalidation Failed"
A cache SHOULD generate this when sending a stale response because an A cache SHOULD generate this when sending a stale response because an
attempt to validate the response failed, due to an inability to reach attempt to validate the response failed, due to an inability to reach
the server. the server.
7.5.3. 112 Disconnected Operation 5.5.3. Warning: 112 - "Disconnected Operation"
A cache SHOULD generate this if it is intentionally disconnected from A cache SHOULD generate this if it is intentionally disconnected from
the rest of the network for a period of time. the rest of the network for a period of time.
7.5.4. 113 Heuristic Expiration 5.5.4. Warning: 113 - "Heuristic Expiration"
A cache SHOULD generate this if it heuristically chose a freshness A cache SHOULD generate this if it heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater than lifetime greater than 24 hours and the response's age is greater than
24 hours. 24 hours.
7.5.5. 199 Miscellaneous Warning 5.5.5. Warning: 199 - "Miscellaneous Warning"
The warning text can include arbitrary information to be presented to The warning text can include arbitrary information to be presented to
a human user, or logged. A system receiving this warning MUST NOT a human user, or logged. A system receiving this warning MUST NOT
take any automated action, besides presenting the warning to the take any automated action, besides presenting the warning to the
user. user.
7.5.6. 214 Transformation Applied 5.5.6. Warning: 214 - "Transformation Applied"
MUST be added by a proxy if it applies any transformation to the MUST be added by a proxy if it applies any transformation to the
representation, such as changing the content-coding, media-type, or representation, such as changing the content-coding, media-type, or
modifying the representation data, unless this Warning code already modifying the representation data, unless this Warning code already
appears in the response. appears in the response.
7.5.7. 299 Miscellaneous Persistent Warning 5.5.7. Warning: 299 - "Miscellaneous Persistent Warning"
The warning text can include arbitrary information to be presented to The warning text can include arbitrary information to be presented to
a human user, or logged. A system receiving this warning MUST NOT a human user, or logged. A system receiving this warning MUST NOT
take any automated action. take any automated action.
7.5.8. Warn Code Extensions 6. History Lists
Extension warn codes can be defined; see Section 9.2.1 for details.
8. History Lists
User agents often have history mechanisms, such as "Back" buttons and User agents often have history mechanisms, such as "Back" buttons and
history lists, that can be used to redisplay a representation history lists, that can be used to redisplay a representation
retrieved earlier in a session. retrieved earlier in a session.
The freshness model (Section 4.1) does not necessarily apply to The freshness model (Section 4.2) does not necessarily apply to
history mechanisms. I.e., a history mechanism can display a previous history mechanisms. I.e., a history mechanism can display a previous
representation even if it has expired. representation even if it has expired.
This does not prohibit the history mechanism from telling the user This does not prohibit the history mechanism from telling the user
that a view might be stale, or from honoring cache directives (e.g., that a view might be stale, or from honoring cache directives (e.g.,
Cache-Control: no-store). Cache-Control: no-store).
9. IANA Considerations 7. IANA Considerations
9.1. Cache Directive Registry
7.1. Cache Directive Registry
The HTTP Cache Directive Registry defines the name space for the The HTTP Cache Directive Registry defines the name space for the
cache directives. It will be created and maintained at cache directives. It will be created and maintained at
<http://www.iana.org/assignments/http-cache-directives>. <http://www.iana.org/assignments/http-cache-directives>.
9.1.1. Procedure 7.1.1. Procedure
A registration MUST include the following fields: A registration MUST include the following fields:
o Cache Directive Name o Cache Directive Name
o Pointer to specification text o Pointer to specification text
Values to be added to this name space require IETF Review (see Values to be added to this name space require IETF Review (see
[RFC5226], Section 4.1). [RFC5226], Section 4.1).
9.1.2. Considerations for New Cache Control Directives 7.1.2. Considerations for New Cache Control Directives
New extension directives ought to consider defining: New extension directives ought to consider defining:
o What it means for a directive to be specified multiple times, o What it means for a directive to be specified multiple times,
o When the directive does not take an argument, what it means when o When the directive does not take an argument, what it means when
an argument is present, an argument is present,
o When the directive requires an argument, what it means when it is o When the directive requires an argument, what it means when it is
missing, missing,
o Whether the directive is specific to requests, responses, or able o Whether the directive is specific to requests, responses, or able
to be used in either. to be used in either.
See also Section 7.2.3. See also Section 5.2.3.
9.1.3. Registrations 7.1.3. Registrations
The HTTP Cache Directive Registry shall be populated with the The HTTP Cache Directive Registry shall be populated with the
registrations below: registrations below:
+------------------------+----------------------------------+ +------------------------+----------------------------------+
| Cache Directive | Reference | | Cache Directive | Reference |
+------------------------+----------------------------------+ +------------------------+----------------------------------+
| max-age | Section 7.2.1.1, Section 7.2.2.8 | | max-age | Section 5.2.1.1, Section 5.2.2.8 |
| max-stale | Section 7.2.1.2 | | max-stale | Section 5.2.1.2 |
| min-fresh | Section 7.2.1.3 | | min-fresh | Section 5.2.1.3 |
| must-revalidate | Section 7.2.2.1 | | must-revalidate | Section 5.2.2.1 |
| no-cache | Section 7.2.1.4, Section 7.2.2.2 | | no-cache | Section 5.2.1.4, Section 5.2.2.2 |
| no-store | Section 7.2.1.5, Section 7.2.2.3 | | no-store | Section 5.2.1.5, Section 5.2.2.3 |
| no-transform | Section 7.2.1.6, Section 7.2.2.4 | | no-transform | Section 5.2.1.6, Section 5.2.2.4 |
| only-if-cached | Section 7.2.1.7 | | only-if-cached | Section 5.2.1.7 |
| private | Section 7.2.2.6 | | private | Section 5.2.2.6 |
| proxy-revalidate | Section 7.2.2.7 | | proxy-revalidate | Section 5.2.2.7 |
| public | Section 7.2.2.5 | | public | Section 5.2.2.5 |
| s-maxage | Section 7.2.2.9 | | s-maxage | Section 5.2.2.9 |
| stale-if-error | [RFC5861], Section 4 | | stale-if-error | [RFC5861], Section 4 |
| stale-while-revalidate | [RFC5861], Section 3 | | stale-while-revalidate | [RFC5861], Section 3 |
+------------------------+----------------------------------+ +------------------------+----------------------------------+
9.2. Warn Code Registry 7.2. Warn Code Registry
The HTTP Warn Code Registry defines the name space for warn codes. The HTTP Warn Code Registry defines the name space for warn codes.
It will be created and maintained at It will be created and maintained at
<http://www.iana.org/assignments/http-warn-codes>. <http://www.iana.org/assignments/http-warn-codes>.
9.2.1. Procedure 7.2.1. Procedure
A registration MUST include the following fields: A registration MUST include the following fields:
o Warn Code (3 digits) o Warn Code (3 digits)
o Short Description o Short Description
o Pointer to specification text o Pointer to specification text
Values to be added to this name space require IETF Review (see Values to be added to this name space require IETF Review (see
[RFC5226], Section 4.1). [RFC5226], Section 4.1).
9.2.2. Registrations 7.2.2. Registrations
The HTTP Warn Code Registry shall be populated with the registrations The HTTP Warn Code Registry shall be populated with the registrations
below: below:
+-----------+----------------------------------+---------------+ +-----------+----------------------------------+---------------+
| Warn Code | Short Description | Reference | | Warn Code | Short Description | Reference |
+-----------+----------------------------------+---------------+ +-----------+----------------------------------+---------------+
| 110 | Response is Stale | Section 7.5.1 | | 110 | Response is Stale | Section 5.5.1 |
| 111 | Revalidation Failed | Section 7.5.2 | | 111 | Revalidation Failed | Section 5.5.2 |
| 112 | Disconnected Operation | Section 7.5.3 | | 112 | Disconnected Operation | Section 5.5.3 |
| 113 | Heuristic Expiration | Section 7.5.4 | | 113 | Heuristic Expiration | Section 5.5.4 |
| 199 | Miscellaneous Warning | Section 7.5.5 | | 199 | Miscellaneous Warning | Section 5.5.5 |
| 214 | Transformation Applied | Section 7.5.6 | | 214 | Transformation Applied | Section 5.5.6 |
| 299 | Miscellaneous Persistent Warning | Section 7.5.7 | | 299 | Miscellaneous Persistent Warning | Section 5.5.7 |
+-----------+----------------------------------+---------------+ +-----------+----------------------------------+---------------+
9.3. Header Field Registration 7.3. Header Field Registration
HTTP header fields are registered within the Message Header Field HTTP header fields are registered within the Message Header Field
Registry maintained at <http://www.iana.org/assignments/ Registry maintained at <http://www.iana.org/assignments/
message-headers/message-header-index.html>. message-headers/message-header-index.html>.
This document defines the following HTTP header fields, so their This document defines the following HTTP header fields, so their
associated registry entries shall be updated according to the associated registry entries shall be updated according to the
permanent registrations below (see [BCP90]): permanent registrations below (see [BCP90]):
+-------------------+----------+----------+-------------+ +-------------------+----------+----------+-------------+
| Header Field Name | Protocol | Status | Reference | | Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+-------------+ +-------------------+----------+----------+-------------+
| Age | http | standard | Section 7.1 | | Age | http | standard | Section 5.1 |
| Cache-Control | http | standard | Section 7.2 | | Cache-Control | http | standard | Section 5.2 |
| Expires | http | standard | Section 7.3 | | Expires | http | standard | Section 5.3 |
| Pragma | http | standard | Section 7.4 | | Pragma | http | standard | Section 5.4 |
| Warning | http | standard | Section 7.5 | | Warning | http | standard | Section 5.5 |
+-------------------+----------+----------+-------------+ +-------------------+----------+----------+-------------+
The change controller is: "IETF (iesg@ietf.org) - Internet The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force". Engineering Task Force".
10. Security Considerations 8. 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/1.1 caching. and users of known security concerns specific to HTTP/1.1 caching.
More general security considerations are addressed in HTTP messaging More general security considerations are addressed in HTTP messaging
[Part1] and semantics [Part2]. [Part1] and semantics [Part2].
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
skipping to change at page 33, line 28 skipping to change at page 35, line 28
operation) might lead to caching of sensitive information (e.g., operation) might lead to caching of sensitive information (e.g.,
authentication credentials) that is thought to be private, exposing authentication credentials) that is thought to be private, exposing
it to unauthorized parties. it to unauthorized parties.
Note that the Set-Cookie response header field [RFC6265] does not Note that the Set-Cookie response header field [RFC6265] does not
inhibit caching; a cacheable response with a Set-Cookie header field inhibit caching; a cacheable response with a Set-Cookie header field
can be (and often is) used to satisfy subsequent requests to caches. can be (and often is) used to satisfy subsequent requests to caches.
Servers who wish to control caching of these responses are encouraged Servers who wish to control caching of these responses are encouraged
to emit appropriate Cache-Control response header fields. to emit appropriate Cache-Control response header fields.
11. Acknowledgments 9. Acknowledgments
See Section 9 of [Part1]. See Section 10 of [Part1].
12. References 10. References
12.1. Normative References 10.1. Normative References
[Part1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [Part1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing", Protocol (HTTP/1.1): Message Syntax and Routing",
draft-ietf-httpbis-p1-messaging-23 (work in progress), draft-ietf-httpbis-p1-messaging-24 (work in progress),
July 2013. September 2013.
[Part2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [Part2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", Protocol (HTTP/1.1): Semantics and Content",
draft-ietf-httpbis-p2-semantics-23 (work in progress), draft-ietf-httpbis-p2-semantics-24 (work in progress),
July 2013. September 2013.
[Part4] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [Part4] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", Protocol (HTTP/1.1): Conditional Requests",
draft-ietf-httpbis-p4-conditional-23 (work in progress), draft-ietf-httpbis-p4-conditional-24 (work in progress),
July 2013. September 2013.
[Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., [Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests", "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
draft-ietf-httpbis-p5-range-23 (work in progress), draft-ietf-httpbis-p5-range-24 (work in progress),
July 2013. September 2013.
[Part7] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [Part7] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", Protocol (HTTP/1.1): Authentication",
draft-ietf-httpbis-p7-auth-23 (work in progress), draft-ietf-httpbis-p7-auth-24 (work in progress),
July 2013. September 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008. Specifications: ABNF", STD 68, RFC 5234, January 2008.
12.2. Informative References 10.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration [BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864, Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004. September 2004.
[RFC1305] Mills, D., "Network Time Protocol (Version 3) [RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation", RFC 1305, March 1992. Specification, Implementation", RFC 1305, March 1992.
[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
skipping to change at page 34, line 44 skipping to change at page 36, line 44
May 2008. May 2008.
[RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale [RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale
Content", RFC 5861, April 2010. Content", RFC 5861, April 2010.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011. April 2011.
Appendix A. Changes from RFC 2616 Appendix A. Changes from RFC 2616
Caching-related text has been substantially rewritten for clarity. The specification has been substantially rewritten for clarity.
The algorithm for calculating age is now less conservative. The conditions under which an authenticated response can be cached
(Section 4.1.3) have been clarified. (Section 3.2)
Caches are now required to handle dates with timezones as if they're New status codes can now define that caches are allowed to use
heuristic freshness with them. Caches are now allowed to calculate
heuristic freshness for URIs with query components. (Section 4.2.2)
The algorithm for calculating age is now less conservative. Caches
are now required to handle dates with timezones as if they're
invalid, because it's not possible to accurately guess. invalid, because it's not possible to accurately guess.
(Section 4.1.3) (Section 4.2.3)
The Content-Location response header field is no longer used to The Content-Location response header field is no longer used to
determine the appropriate response to use when validating. determine the appropriate response to use when validating.
(Section 4.2) (Section 4.3)
The algorithm for selecting a cached negotiated response to use has The algorithm for selecting a cached negotiated response to use has
been clarified in several ways. In particular, it now explicitly been clarified in several ways. In particular, it now explicitly
allows header-specific canonicalization when processing selecting allows header-specific canonicalization when processing selecting
header fields. (Section 4.3) header fields. (Section 4.1)
Requirements regarding denial of service attack avoidance when Requirements regarding denial of service attack avoidance when
performing invalidation have been clarified. (Section 6) performing invalidation have been clarified. (Section 4.4)
Cache invalidation only occurs when a successful response is Cache invalidation only occurs when a successful response is
received. (Section 6) received. (Section 4.4)
The conditions under which an authenticated response can be cached
have been clarified. (Section 3.2)
The one-year limit on Expires header field values has been removed;
instead, the reasoning for using a sensible value is given.
(Section 7.3)
The Pragma header field is now only defined for backwards
compatibility; future pragmas are deprecated. (Section 7.4)
Cache directives are explicitly defined to be case-insensitive. Cache directives are explicitly defined to be case-insensitive.
(Section 7.2)
Handling of multiple instances of cache directives when only one is Handling of multiple instances of cache directives when only one is
expected is now defined. (Section 7.2) expected is now defined. (Section 5.2)
The "no-store" cache request directive doesn't apply to responses;
i.e., a cache can satisfy a request with no-store on it, and does not
invalidate it. (Section 5.2.1.5)
The qualified forms of the private and no-cache cache directives are The qualified forms of the private and no-cache cache directives are
noted to not be widely implemented; e.g., "private=foo" is noted to not be widely implemented; e.g., "private=foo" is
interpreted by many caches as simply "private". Additionally, the interpreted by many caches as simply "private". Additionally, the
meaning of the qualified form of no-cache has been clarified. meaning of the qualified form of no-cache has been clarified.
(Section 7.2.2) (Section 5.2.2)
The "no-store" cache request directive doesn't apply to responses;
i.e., a cache can satisfy a request with no-store on it, and does not
invalidate it. (Section 7.2.1.5)
The "no-cache" response cache directive's meaning has been clarified. The "no-cache" response cache directive's meaning has been clarified.
(Section 7.2.2.2) (Section 5.2.2.2)
New status codes can now define that caches are allowed to use The one-year limit on Expires header field values has been removed;
heuristic freshness with them. (Section 4.1.2) instead, the reasoning for using a sensible value is given.
(Section 5.3)
Caches are now allow to calculate heuristic freshness for URLs with The Pragma header field is now only defined for backwards
query components. (Section 4.1.2) compatibility; future pragmas are deprecated. (Section 5.4)
Some requirements regarding production of the Warning header fields Some requirements regarding production and processing of the Warning
have been relaxed, as it is not widely implemented. Furthermore, the header fields have been relaxed, as it is not widely implemented.
Warning header field no longer uses RFC 2047 encoding, nor allows Furthermore, the Warning header field no longer uses RFC 2047
multiple languages, as these aspects were not implemented. encoding, nor allows multiple languages, as these aspects were not
(Section 7.5) implemented. (Section 5.5)
This specification introduces the Cache Directive and Warn Code This specification introduces the Cache Directive and Warn Code
Registries, and defines considerations for new cache directives. Registries, and defines considerations for new cache directives.
(Section 7.2.3 and Section 7.5.8) (Section 7.1 and Section 7.2)
Appendix B. Imported ABNF Appendix B. Imported ABNF
The following core rules are included by reference, as defined in The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return), Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
character). character).
skipping to change at page 39, line 39 skipping to change at page 41, line 39
o <http://tools.ietf.org/wg/httpbis/trac/ticket/464>: "placement of o <http://tools.ietf.org/wg/httpbis/trac/ticket/464>: "placement of
extension point considerations" extension point considerations"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/469>: "Editorial o <http://tools.ietf.org/wg/httpbis/trac/ticket/469>: "Editorial
notes for p6" notes for p6"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/471>: "Vary and o <http://tools.ietf.org/wg/httpbis/trac/ticket/471>: "Vary and
future requests" future requests"
D.5. Since draft-ietf-httpbis-p6-cache-23
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/486>: "Requiring
proxies to process warn-date"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/496>: "add Warning
header field examples"
Index Index
1 1
110 Response is Stale (warn code) 28 110 (warn-code) 30
111 Revalidation Failed (warn code) 28 111 (warn-code) 31
112 Disconnected Operation (warn code) 28 112 (warn-code) 31
113 Heuristic Expiration (warn code) 29 113 (warn-code) 31
199 Miscellaneous Warning (warn code) 29 199 (warn-code) 31
2 2
214 Transformation Applied (warn code) 29 214 (warn-code) 31
299 Miscellaneous Persistent Warning (warn code) 29 299 (warn-code) 31
A A
age 9 age 10
Age header field 18 Age header field 20
C C
cache 4 cache 4
cache entry 5 cache entry 5
cache key 5 cache key 5
Cache-Control header field 18 Cache-Control header field 20
D
Disconnected Operation (warn-text) 31
E E
Expires header field 25 Expires header field 27
explicit expiration time 9 explicit expiration time 10
F F
first-hand 14 fresh 10
fresh 9 freshness lifetime 10
freshness lifetime 9
G G
Grammar Grammar
Age 18 Age 20
Cache-Control 19 Cache-Control 21
cache-directive 19 cache-directive 21
delta-seconds 5 delta-seconds 5
Expires 25 Expires 27
extension-pragma 26 extension-pragma 28
Pragma 26 Pragma 28
pragma-directive 26 pragma-directive 28
warn-agent 27 warn-agent 29
warn-code 27 warn-code 29
warn-date 27 warn-date 29
warn-text 27 warn-text 29
Warning 27 Warning 29
warning-value 27 warning-value 29
H H
heuristic expiration time 9 Heuristic Expiration (warn-text) 31
heuristic expiration time 10
M M
max-age (cache directive) 19, 24 max-age (cache directive) 21, 26
max-stale (cache directive) 19 max-stale (cache directive) 21
min-fresh (cache directive) 20 min-fresh (cache directive) 22
must-revalidate (cache directive) 21 Miscellaneous Persistent Warning (warn-text) 31
Miscellaneous Warning (warn-text) 31
must-revalidate (cache directive) 23
N N
no-cache (cache directive) 20-21 no-cache (cache directive) 22-23
no-store (cache directive) 20, 22 no-store (cache directive) 22, 24
no-transform (cache directive) 21-22 no-transform (cache directive) 23-24
O O
only-if-cached (cache directive) 21 only-if-cached (cache directive) 23
P P
Pragma header field 26 Pragma header field 28
private (cache directive) 23 private (cache directive) 25
private cache 4 private cache 4
proxy-revalidate (cache directive) 23 proxy-revalidate (cache directive) 25
public (cache directive) 23 public (cache directive) 25
R
Response is Stale (warn-text) 30
Revalidation Failed (warn-text) 31
S S
s-maxage (cache directive) 24 s-maxage (cache directive) 26
shared cache 4 shared cache 4
stale 9 stale 10
strong validator 15 strong validator 18
T
Transformation Applied (warn-text) 31
V V
validator 14 validator 15
W W
Warning header field 27 Warning header field 29
Authors' Addresses Authors' Addresses
Roy T. Fielding (editor) Roy T. Fielding (editor)
Adobe Systems Incorporated Adobe Systems Incorporated
345 Park Ave 345 Park Ave
San Jose, CA 95110 San Jose, CA 95110
USA USA
EMail: fielding@gbiv.com EMail: fielding@gbiv.com
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