< draft-ietf-httpbis-p6-cache-22.txt   draft-ietf-httpbis-p6-cache-23.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: August 27, 2013 J. Reschke, Ed. Expires: January 16, 2014 J. Reschke, Ed.
greenbytes greenbytes
February 23, 2013 July 15, 2013
Hypertext Transfer Protocol (HTTP/1.1): Caching Hypertext Transfer Protocol (HTTP/1.1): Caching
draft-ietf-httpbis-p6-cache-22 draft-ietf-httpbis-p6-cache-23
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.3. The changes in this draft are summarized in Appendix D.4.
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 August 27, 2013. This Internet-Draft will expire on January 16, 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.
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outside the IETF Standards Process, and derivative works of it may outside the IETF Standards Process, and derivative works of it may
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than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Conformance and Error Handling . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4
1.3. Conformance and Error Handling . . . . . . . . . . . . . . 6 1.2.1. Delta Seconds . . . . . . . . . . . . . . . . . . . . 5
1.4. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 6 2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 5
1.4.1. Delta Seconds . . . . . . . . . . . . . . . . . . . . 6 3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 6
2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 6 3.1. Storing Incomplete Responses . . . . . . . . . . . . . . . 7
3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 7 3.2. Storing Responses to Authenticated Requests . . . . . . . 7
3.1. Storing Incomplete Responses . . . . . . . . . . . . . . . 8 3.3. Combining Partial Content . . . . . . . . . . . . . . . . 7
3.2. Storing Responses to Authenticated Requests . . . . . . . 9 4. Constructing Responses from Caches . . . . . . . . . . . . . . 8
4. Constructing Responses from Caches . . . . . . . . . . . . . . 9 4.1. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Freshness Model . . . . . . . . . . . . . . . . . . . . . 10
4.1.1. Calculating Freshness Lifetime . . . . . . . . . . . . 11 4.1.1. Calculating Freshness Lifetime . . . . . . . . . . . . 11
4.1.2. Calculating Heuristic Freshness . . . . . . . . . . . 12 4.1.2. Calculating Heuristic Freshness . . . . . . . . . . . 11
4.1.3. Calculating Age . . . . . . . . . . . . . . . . . . . 12 4.1.3. Calculating Age . . . . . . . . . . . . . . . . . . . 12
4.1.4. Serving Stale Responses . . . . . . . . . . . . . . . 14 4.1.4. Serving Stale Responses . . . . . . . . . . . . . . . 13
4.2. Validation Model . . . . . . . . . . . . . . . . . . . . . 15 4.2. Validation . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2.1. Freshening Responses with 304 Not Modified . . . . . . 16 4.2.1. Freshening Stored Responses upon Validation . . . . . 15
4.3. Using Negotiated Responses . . . . . . . . . . . . . . . . 17
4.4. Combining Partial Content . . . . . . . . . . . . . . . . 17 4.3. Calculating Secondary Keys with Vary . . . . . . . . . . . 16
5. Updating Caches with HEAD Responses . . . . . . . . . . . . . 18 5. Updating Caches with HEAD Responses . . . . . . . . . . . . . 17
6. Request Methods that Invalidate . . . . . . . . . . . . . . . 19 6. Request Methods that Invalidate . . . . . . . . . . . . . . . 17
7. Header Field Definitions . . . . . . . . . . . . . . . . . . . 19 7. Header Field Definitions . . . . . . . . . . . . . . . . . . . 18
7.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 7.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 20 7.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 18
7.2.1. Request Cache-Control Directives . . . . . . . . . . . 20 7.2.1. Request Cache-Control Directives . . . . . . . . . . . 19
7.2.2. Response Cache-Control Directives . . . . . . . . . . 22 7.2.2. Response Cache-Control Directives . . . . . . . . . . 21
7.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 26 7.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 24
7.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 27 7.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 25
7.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 28 7.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 29 7.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.5.1. 110 Response is Stale . . . . . . . . . . . . . . . . 30 7.5.1. 110 Response is Stale . . . . . . . . . . . . . . . . 28
7.5.2. 111 Revalidation Failed . . . . . . . . . . . . . . . 30 7.5.2. 111 Revalidation Failed . . . . . . . . . . . . . . . 28
7.5.3. 112 Disconnected Operation . . . . . . . . . . . . . . 30 7.5.3. 112 Disconnected Operation . . . . . . . . . . . . . . 28
7.5.4. 113 Heuristic Expiration . . . . . . . . . . . . . . . 30 7.5.4. 113 Heuristic Expiration . . . . . . . . . . . . . . . 29
7.5.5. 199 Miscellaneous Warning . . . . . . . . . . . . . . 31 7.5.5. 199 Miscellaneous Warning . . . . . . . . . . . . . . 29
7.5.6. 214 Transformation Applied . . . . . . . . . . . . . . 31 7.5.6. 214 Transformation Applied . . . . . . . . . . . . . . 29
7.5.7. 299 Miscellaneous Persistent Warning . . . . . . . . . 31 7.5.7. 299 Miscellaneous Persistent Warning . . . . . . . . . 29
7.5.8. Warn Code Extensions . . . . . . . . . . . . . . . . . 31 7.5.8. Warn Code Extensions . . . . . . . . . . . . . . . . . 29
8. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 31 8. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 29
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
9.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 32 9.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 30
9.2. Warn Code Registry . . . . . . . . . . . . . . . . . . . . 32 9.1.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 30
9.3. Header Field Registration . . . . . . . . . . . . . . . . 33 9.1.2. Considerations for New Cache Control Directives . . . 30
10. Security Considerations . . . . . . . . . . . . . . . . . . . 33 9.1.3. Registrations . . . . . . . . . . . . . . . . . . . . 30
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34 9.2. Warn Code Registry . . . . . . . . . . . . . . . . . . . . 31
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34 9.2.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 31
12.1. Normative References . . . . . . . . . . . . . . . . . . . 34 9.2.2. Registrations . . . . . . . . . . . . . . . . . . . . 31
12.2. Informative References . . . . . . . . . . . . . . . . . . 35 9.3. Header Field Registration . . . . . . . . . . . . . . . . 32
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 35 10. Security Considerations . . . . . . . . . . . . . . . . . . . 32
Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 37 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33
Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 38 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
12.1. Normative References . . . . . . . . . . . . . . . . . . . 33
12.2. Informative References . . . . . . . . . . . . . . . . . . 34
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 34
Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 36
Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 36
Appendix D. Change Log (to be removed by RFC Editor before Appendix D. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 39 publication) . . . . . . . . . . . . . . . . . . . . 37
D.1. Since draft-ietf-httpbis-p6-cache-19 . . . . . . . . . . . 39 D.1. Since draft-ietf-httpbis-p6-cache-19 . . . . . . . . . . . 38
D.2. Since draft-ietf-httpbis-p6-cache-20 . . . . . . . . . . . 39 D.2. Since draft-ietf-httpbis-p6-cache-20 . . . . . . . . . . . 38
D.3. Since draft-ietf-httpbis-p6-cache-21 . . . . . . . . . . . 40 D.3. Since draft-ietf-httpbis-p6-cache-21 . . . . . . . . . . . 39
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 D.4. Since draft-ietf-httpbis-p6-cache-22 . . . . . . . . . . . 39
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
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.
1.1. Purpose
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 its message storage, retrieval, and deletion. A cache that controls storage, retrieval, and deletion of messages in it. A
stores cacheable responses in order to reduce the response time and cache stores cacheable responses in order to reduce the response time
network bandwidth consumption on future, equivalent requests. Any and network bandwidth consumption on future, equivalent requests.
client or server MAY employ a cache, though a cache cannot be used by Any client or server MAY employ a cache, though a cache cannot be
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
than one user; shared caches are usually (but not always) deployed as
a part of an intermediary. A private cache, in contrast, is
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 4.1, 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 cache response can remains valid for this request). A fresh response can therefore
therefore reduce both latency and network transfers each time it is reduce both latency and network overhead each time it is reused.
reused. When a cached response is not fresh, it might still be When a cached response is not fresh, it might still be reusable if it
reusable if it can be freshened by validation (Section 4.2) or if the can be freshened by validation (Section 4.2) or if the origin is
origin is unavailable. unavailable (Section 4.1.4).
1.2. Terminology
This specification uses a number of terms to refer to the roles
played by participants in, and objects of, HTTP caching.
cache
A conformant implementation of an HTTP cache. Note that this
implies an HTTP/1.1 cache; this specification does not define
conformance for HTTP/1.0 caches.
shared cache
A cache that stores responses to be reused by more than one user;
usually (but not always) deployed as part of an intermediary.
private cache
A cache that is dedicated to a single user.
cacheable
A response is cacheable if a cache is allowed to store a copy of
the response message for use in answering subsequent requests.
Even when a response is cacheable, there might be additional
constraints on whether a cache can use the stored copy to satisfy
a particular request.
explicit expiration time
The time at which the origin server intends that a stored response
no longer be used by a cache without further validation.
heuristic expiration time
An expiration time assigned by a cache when no explicit expiration
time is available.
age
The age of a response is the time since it was sent by, or
successfully validated with, the origin server.
first-hand
A response is first-hand if the freshness model is not in use;
i.e., its age is 0.
freshness lifetime
The length of time between the generation of a response and its
expiration time.
fresh
A response is fresh if its age has not yet exceeded its freshness
lifetime.
stale
A response is stale if its age has passed its freshness lifetime
(either explicit or heuristic).
validator
A protocol element (e.g., an entity-tag or a Last-Modified time)
that is used to find out whether a stored response is an
equivalent copy of a representation. See Section 2.1 of [Part4].
strong validator
A validator that is defined by the origin server such that its
current value will change if the representation data changes;
i.e., an entity-tag that is not marked as weak (Section 2.3 of
[Part4]) or, if no entity-tag is provided, a Last-Modified value
that is strong in the sense defined by Section 2.2.2 of [Part4].
1.3. 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].
1.4. 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 1.2 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.
1.4.1. 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 an implementation receives a delta-seconds value larger than the If a cache receives a delta-seconds value larger than the largest
largest positive integer it can represent, or if any of its positive integer it can represent, or if any of its subsequent
subsequent calculations overflows, it MUST consider the value to be calculations overflows, it MUST consider the value to be 2147483648
2147483648 (2^31). Recipients parsing a delta-seconds value MUST use (2^31). Recipients parsing a delta-seconds value MUST use an
an arithmetic type of at least 31 bits of range, and senders MUST NOT arithmetic type of at least 31 bits of range, and senders MUST NOT
send delta-seconds with a value greater than 2147483648. 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 that such reuse is the default behavior when no requirement or local
locally-desired configuration prevents it. Therefore, HTTP cache configuration prevents it. Therefore, HTTP cache requirements are
requirements are focused on preventing a cache from either storing a focused on preventing a cache from either storing a non-reusable
non-reusable response or reusing a stored response inappropriately. response or reusing a stored response inappropriately, rather than
mandating that caches always store and reuse particular responses.
Each cache entry consists of a cache key and one or more HTTP Each cache entry consists of a cache key and one or more HTTP
responses corresponding to prior requests that used the same key. responses corresponding to prior requests that used the same key.
The most common form of cache entry is a successful result of a The most common form of cache entry is a successful result of a
retrieval request: i.e., a 200 (OK) response to a GET request, which retrieval request: i.e., a 200 (OK) response to a GET request, which
contains a representation of the resource identified by the request contains a representation of the resource identified by the request
target (Section 4.3.1 of [Part2]). However, it is also possible to target (Section 4.3.1 of [Part2]). However, it is also possible to
cache permanent redirects, negative results (e.g., 404 (Not Found)), cache permanent redirects, negative results (e.g., 404 (Not Found)),
incomplete results (e.g., 206 (Partial Content)), and responses to incomplete results (e.g., 206 (Partial Content)), and responses to
methods other than GET if the method's definition allows such caching methods other than GET if the method's definition allows such caching
and defines something suitable for use as a cache key. and defines something suitable for use as a cache key.
The default 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 implementations simply decline to caching responses to GET, many caches simply decline other methods
other methods and use only the URI as the 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 4.3).
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 7.2) 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 7.2.2.2) does o the "private" cache response directive (see Section 7.2.2.6) 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 7.3), or
* contains a max-age response cache directive (see * contains a max-age response cache directive (see
Section 7.2.2.7), or Section 7.2.2.8), or
* contains a s-maxage response cache directive and the cache is * contains a s-maxage response cache directive (see
shared, or Section 7.2.2.9) and the cache is shared, or
* contains a Cache Control Extension (see Section 7.2.3) that * contains a Cache Control Extension (see Section 7.2.3) 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 4.1.2), or
* contains a public response cache directive (see * contains a public response cache directive (see
Section 7.2.2.1). Section 7.2.2.5).
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 7.2.3.
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 any cache- response status code if it recognizes it and implements all specified
specific behavior. caching-related behavior.
Note that, in normal operation, many 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 is GET, the response status
is 200 (OK), and the entire response header block has been received, is 200 (OK), and the entire response header block has been received,
a cache MAY store an incomplete response message body if the cache a cache MAY store an incomplete response message body if the cache
entry is recorded as incomplete. Likewise, a 206 (Partial Content) entry is recorded as incomplete. Likewise, a 206 (Partial Content)
response MAY be stored as if it were an incomplete 200 (OK) cache response MAY be stored as if it were an incomplete 200 (OK) cache
entry. However, a cache MUST NOT store incomplete or partial content entry. However, a cache MUST NOT store incomplete or partial content
responses if it does not support the Range and Content-Range header responses if it does not support the Range and Content-Range header
fields or if it does not understand the range units used in those fields or if it does not understand the range units used in those
fields. 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 4.4. 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
skipping to change at page 9, line 23 skipping to change at page 7, line 47
directives (Section 7.2.2) have such an effect: must-revalidate, directives (Section 7.2.2) 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 4.1.4) 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
A response might transfer only a partial representation if the
connection closed prematurely or if the request used one or more
Range specifiers ([Part5]). After several such transfers, a cache
might have received several ranges of the same representation. A
cache MAY combine these ranges into a single stored response, and
reuse that response to satisfy later requests, if they all share the
same strong validator and the cache complies with the client
requirements in Section 4.3 of [Part5].
When combining the new response with one or more stored responses, a
cache MUST:
o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 7.5);
o retain any Warning header fields in the stored response with warn-
code 2xx; and,
o use other header fields provided in the new response, aside from
Content-Range, to replace all instances of the corresponding
header fields in the stored response.
4. Constructing Responses from Caches 4. Constructing Responses from Caches
For a presented request, a cache MUST NOT send a stored response, When presented with a request, a cache MUST NOT reuse a stored
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 4.3), 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 7.4), nor the no-cache cache directive (Section 7.2.1),
unless the stored response is successfully validated unless the stored response is successfully validated
(Section 4.2), and (Section 4.2), 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 7.2.2.3), unless it is successfully validated (Section 7.2.2.2), unless it is successfully validated
(Section 4.2), and (Section 4.2), and
o the stored response is either: o the stored response is either:
* fresh (see Section 4.1), or * fresh (see Section 4.1), or
* allowed to be served stale (see Section 4.1.4), or * allowed to be served stale (see Section 4.1.4), or
* successfully validated (see Section 4.2). * successfully validated (see Section 4.2).
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 7.2.3.
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 send a single Age header field (Section 7.1) validation, a cache MUST generate an Age header field (Section 7.1),
in the response with a value equal to the stored response's replacing any present in the response with a value equal to the
current_age; see Section 4.1.3. stored response's current_age; see Section 4.1.3.
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 6.
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 a request with "Cache-Control: max-age=0" or "Cache- can also forward the request with "Cache-Control: max-age=0" or
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 on every use. responses without revalidating them upon every use.
4.1. Freshness Model 4.1. Freshness
A fresh response is one whose age has not yet exceeded its freshness
lifetime. Conversely, a stale response is one where it has.
A response's freshness lifetime is the length of time between its
generation by the origin server and its expiration time. An explicit
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
validation, whereas a heuristic expiration time is assigned by a
cache when no explicit expiriation time is available.
A response's age is the time that has passed since it was generated
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 7.3) or the max-age response
cache directive (Section 7.2.2.7). Generally, origin servers will cache directive (Section 7.2.2.8). 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 4.1.4).
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 4.1.2).
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)
The freshness_lifetime is defined in Section 4.1.1; the current_age freshness_lifetime is defined in Section 4.1.1; current_age is
is defined in Section 4.1.3. defined in Section 4.1.3.
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 7.2.1).
When calculating freshness, to avoid common problems in date parsing:
o Although all date formats are specified to be case-sensitive,
cache recipients SHOULD match day, week and timezone names case-
insensitively.
o If a cache recipient's internal implementation of time has less
resolution than the value of an HTTP-date, the recipient MUST
internally represent a parsed Expires date as the nearest time
equal to or earlier than the received value.
o Cache recipients MUST NOT allow local time zones to influence the
calculation or comparison of an age or expiration time.
o Cache recipients SHOULD consider a date with a zone abbreviation
other than "GMT" to be invalid for calculating expiration.
Note that freshness applies only to cache operation; it cannot be Note that freshness applies only to cache operation; it cannot be
used to force a user agent to refresh its display or reload a used to force a user agent to refresh its display or reload a
resource. See Section 8 for an explanation of the difference between resource. See Section 8 for an explanation of the difference between
caches and history mechanisms. caches and history mechanisms.
4.1.1. Calculating Freshness Lifetime 4.1.1. 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 7.2.2.8) is present, use its value, or (Section 7.2.2.9) is present, use its value, or
o If the max-age response cache directive (Section 7.2.2.7) is o If the max-age response cache directive (Section 7.2.2.8) 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 7.3) 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 4.1.2.
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), it is considered invalid. Caches are encouraged to directives), the directive's value is considered invalid. Caches are
consider responses that have invalid freshness information to be encouraged to consider responses that have invalid freshness
stale. information to be stale.
4.1.2. Calculating Heuristic Freshness 4.1.2. 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.
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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 attach a Warning header field with a 113 warn-code to the
response if its current_age is more than 24 hours and such a warning response if its current_age is more than 24 hours and such a warning
is not already present. 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, servers are encouraged to send explicit implemented. Therefore, origin servers are encouraged to send
directives (e.g., Cache-Control: no-cache) if they wish to explicit directives (e.g., Cache-Control: no-cache) if they wish
preclude caching. to preclude caching.
4.1.3. Calculating Age 4.1.3. 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.
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2. the "corrected_age_value", if all of the caches along the 2. the "corrected_age_value", if all of the caches along the
response path implement HTTP/1.1. A cache MUST interpret this response path implement HTTP/1.1. A cache MUST interpret this
value relative to the time the request was initiated, not the value relative to the time the request was initiated, not the
time that the response was received. time that the response was received.
apparent_age = max(0, response_time - date_value); apparent_age = max(0, response_time - date_value);
response_delay = response_time - request_time; response_delay = response_time - request_time;
corrected_age_value = age_value + response_delay; corrected_age_value = age_value + response_delay;
These SHOULD be combined as These are combined as
corrected_initial_age = max(apparent_age, corrected_age_value); corrected_initial_age = max(apparent_age, corrected_age_value);
unless the cache is confident in the value of the Age header field unless the cache is confident in the value of the Age header field
(e.g., because there are no HTTP/1.0 hops in the Via header field), (e.g., because there are no HTTP/1.0 hops in the Via header field),
in which case the corrected_age_value MAY be used as the in which case the corrected_age_value MAY be used as the
corrected_initial_age. corrected_initial_age.
The current_age of a stored response can then be calculated by adding The current_age of a stored response can then be calculated by adding
the amount of time (in seconds) since the stored response was last the 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;
Additionally, to avoid common problems in date parsing:
o Although all date formats are specified to be case-sensitive,
cache recipients SHOULD match day, week and timezone names case-
insensitively.
o If a cache recipient's internal implementation of time has less
resolution than the value of an HTTP-date, the recipient MUST
internally represent a parsed Expires date as the nearest time
equal to or earlier than the received value.
o Cache recipients MUST NOT allow local time zones to influence the
calculation or comparison of an age or expiration time.
o Cache recipients SHOULD consider a date with a zone abbreviation
other than "GMT" to be invalid for calculating expiration.
4.1.4. Serving Stale Responses 4.1.4. 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 4.1.
A cache MUST NOT send 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 7.2.2).
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 7.2.1).
A cache SHOULD append a Warning header field with the 110 warn-code A cache SHOULD append a Warning header field with the 110 warn-code
(see Section 7.5) to stale responses. Likewise, a cache SHOULD add (see Section 7.5) to stale responses. Likewise, a cache SHOULD add
the 112 warn-code to stale responses if the cache is disconnected. the 112 warn-code to stale responses if the cache is disconnected.
If a cache receives a first-hand response (either an entire response, Note that if a cache receives a first-hand response (one where the
or a 304 (Not Modified) response) that it would normally forward to freshness model is not in use; i.e., its age is 0, whether it is an
the requesting client, and the received response is no longer fresh, entire response, or a 304 (Not Modified) response) that it would
the cache can forward it to the requesting client without adding a normally forward to the requesting client, and the received response
new Warning (but without removing any existing Warning header is no longer fresh, the cache MAY forward it to the requesting client
fields). A cache shouldn't attempt to validate a response simply without adding a new Warning (but without removing any existing
because that response became stale in transit. Warning header fields). A cache ought not attempt to validate a
response simply because that response became stale in transit.
4.2. Validation Model 4.2. 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 4.3), 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 origin
server an opportunity to both select a valid stored response to be server an opportunity to both select a valid stored response to be
used, and to update it. This process is known as "validating" or used, and to update it. This process is known as "validating" or
"revalidating" the stored response. "revalidating" the stored response.
When sending such a conditional request, a cache adds an If-Modified- When sending such a conditional request, a cache adds a validator (or
Since header field whose value is that of the Last-Modified header more than one), that is used to find out whether a stored response is
field from the selected (see Section 4.3) stored response, if an equivalent copy of a current representation of the resource.
available.
Additionally, a cache can add an If-None-Match header field whose One such validator is the If-Modified-Since header field, whose value
value is that of the ETag header field(s) from all responses stored is that of the Last-Modified header field from the selected (see
for the requested URI, if present. However, if any of the stored Section 4.3) stored response, if available.
responses contains only partial content, the cache shouldn't include
its entity-tag in the If-None-Match header field unless the request Another is the If-None-Match header field, whose value is that of the
is for a range that would be fully satisfied by that stored response. ETag header field(s) from relevant responses stored for the primary
cache key, if present. However, if any of the stored responses
contains only partial content, the cache ought not include its
entity-tag in the If-None-Match header field unless the request is
for a range that would be fully satisfied by that stored 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 4.2.1.
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 can 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, it can send a previously stored
response (see Section 4.1.4). response (see Section 4.1.4).
4.2.1. Freshening Responses with 304 Not Modified 4.2.1. 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:
o If the new response contains a strong validator, then that strong o If the new response contains a strong validator (see Section 2.1
validator identifies the selected representation. All of the of [Part4]), then that strong validator identifies the selected
stored responses with the same strong validator are selected. If representation for update. All of the stored responses with the
none of the stored responses contain the same strong validator, same strong validator are selected. If none of the stored
then the new response MUST NOT be used to update any stored responses contain the same strong validator, then the cache MUST
responses. NOT use the new response to update any stored responses.
o If the new response contains a weak validator and that validator o If the new response contains a weak validator and that validator
corresponds to one of the cache's stored responses, then the most corresponds to one of the cache's stored responses, then the most
recent of those matching stored responses is selected. recent of those matching stored responses is selected for update.
o If the new response does not include any form of validator (such o If the new response does not include any form of validator (such
as in the case where a client generates an If-Modified-Since as 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. 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 7.5);
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. Using Negotiated Responses 4.3. Calculating Secondary Keys with Vary
When a cache receives a request that can be satisfied by a stored When a cache receives a request that can be satisfied by a stored
response that has a Vary header field (Section 7.1.4 of [Part2]), it 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 MUST NOT use that response unless all of the selecting header fields
nominated by the Vary header field match in both the original request nominated by the Vary header field match in both the original request
(i.e., that associated with the stored response), and the presented (i.e., that associated with the stored response), and the presented
request. request.
The selecting header fields from two requests are defined to match if The selecting header fields from two requests are defined to match if
and only if those in the first request can be transformed to those in and only if those in the first request can be transformed to those in
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o normalizing both header field values in a way that is known to o normalizing both header field values in a way that is known to
have identical semantics, according to the header field's have identical semantics, according to the header field's
specification (e.g., re-ordering field values when order is not specification (e.g., re-ordering field values when order is not
significant; case-normalization, where values are defined to be significant; case-normalization, where values are defined to be
case-insensitive) case-insensitive)
If (after any normalization that might take place) a header field is If (after any normalization that might take place) a header field is
absent from a request, it can only match another request if it is absent from a request, it can only match another request if it is
also absent there. also absent there.
A Vary header field-value of "*" always fails to match, and A Vary header field-value of "*" always fails to match.
subsequent requests to that resource can only be properly interpreted
by the origin server.
The stored response with matching selecting header fields is known as The stored response with matching selecting header fields is known as
the selected response. the selected response.
If multiple selected responses are available, the most recent If multiple selected responses are available (potentially including
response (as determined by the Date header field) is used; see responses without a Vary header field), the cache will need to choose
Section 4. 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 If no selected response is available, the cache cannot satisfy the
presented request. Typically, it is forwarded to the origin server presented request. Typically, it is forwarded to the origin server
in a (possibly conditional; see Section 4.2) request. in a (possibly conditional; see Section 4.2) request.
4.4. Combining Partial Content
A response might transfer only a partial representation if the
connection closed prematurely or if the request used one or more
Range specifiers ([Part5]). After several such transfers, a cache
might have received several ranges of the same representation. A
cache MAY combine these ranges into a single stored response, and
reuse that response to satisfy later requests, if they all share the
same strong validator and the cache complies with the client
requirements in Section 4.3 of [Part5].
When combining the new response with one or more stored responses, a
cache MUST:
o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 7.5);
o retain any Warning header fields in the stored response with warn-
code 2xx; and,
o use other header fields provided in the new response, aside from
Content-Range, to replace all instances of the corresponding
header fields in the stored response.
5. Updating Caches with HEAD Responses 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 is used to both invalidate and update
cached GET responses. cached GET responses.
If one or more stored GET responses can be selected (as per If one or more stored GET responses can be selected (as per
Section 4.3) for a HEAD request, and the Content-Length, ETag or Section 4.3) for a HEAD request, and the Content-Length, ETag or
Last-Modified value of a HEAD response differs from that in a Last-Modified value of a HEAD response differs from that in a
skipping to change at page 19, line 34 skipping to change at page 18, line 17
method whose safety is unknown. method whose safety is unknown.
Here, a "non-error response" is one with a 2xx (Successful) or 3xx Here, a "non-error response" is one with a 2xx (Successful) or 3xx
(Redirection) status code. "Invalidate" means that the cache will (Redirection) status code. "Invalidate" means that the cache will
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, the request that caused the change at the invalidated. For example, a state-changing request might invalidate
origin server might not have gone through the cache where a response responses in the caches it travels through, but relevant responses
is stored. still might be stored in other caches that it has not.
7. Header Field Definitions 7. 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 7.1. 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 4.1.3.
Age = delta-seconds Age = delta-seconds
Age field-values are non-negative integers, representing time in Age field-values are non-negative integers, representing time in
seconds (see Section 1.4.1). 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 in a response implies that a
response is not first-hand. However, the converse is not true, since response is not first-hand. However, the converse is not true, since
HTTP/1.0 caches might not implement the Age header field. HTTP/1.0 caches might not implement the Age header field.
7.2. Cache-Control 7.2. 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 7.2.3 for information about how
Cache-Control directives defined elsewhere are handled. Cache-Control directives defined elsewhere are handled.
Note: HTTP/1.0 caches might not implement Cache-Control and might Note: Some HTTP/1.0 caches might not implement Cache-Control.
only implement Pragma: no-cache (see Section 7.4).
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, recipients 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) otherwise stated otherwise. allowed) unless stated otherwise.
7.2.1. Request Cache-Control Directives 7.2.1. Request Cache-Control Directives
7.2.1.1. no-cache 7.2.1.1. max-age
The "no-cache" request directive indicates that a cache MUST NOT use
a stored response to satisfy the request without successful
validation on the origin server.
7.2.1.2. no-store
The "no-store" request directive indicates that a cache MUST NOT
store any part of either this request or any response to it. This
directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a best-
effort attempt to remove the information from volatile storage as
promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might
be vulnerable to eavesdropping.
Note that if a request containing this directive is satisfied from a
cache, the no-store request directive does not apply to the already
stored response.
7.2.1.3. max-age
Argument syntax: Argument syntax:
delta-seconds (see Section 1.4.1) 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"'. Senders SHOULD NOT use the
quoted-string form. quoted-string form.
7.2.1.4. max-stale 7.2.1.2. max-stale
Argument syntax: Argument syntax:
delta-seconds (see Section 1.4.1) 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 expiration time. willing to accept a response that has exceeded its freshness
If max-stale is assigned a value, then the client is willing to lifetime. If max-stale is assigned a value, then the client is
accept a response that has exceeded its expiration time by no more willing to accept a response that has exceeded its freshness lifetime
than the specified number of seconds. If no value is assigned to by no more than the specified number of seconds. If no value is
max-stale, then the client is willing to accept a stale response of assigned to max-stale, then the client is willing to accept a stale
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"'. Senders SHOULD NOT use
the quoted-string form. the quoted-string form.
7.2.1.5. min-fresh 7.2.1.3. min-fresh
Argument syntax: Argument syntax:
delta-seconds (see Section 1.4.1) 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"'. Senders SHOULD NOT use
the quoted-string form. the quoted-string form.
7.2.1.4. no-cache
The "no-cache" request directive indicates that a cache MUST NOT use
a stored response to satisfy the request without successful
validation on the origin server.
7.2.1.5. no-store
The "no-store" request directive indicates that a cache MUST NOT
store any part of either this request or any response to it. This
directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a best-
effort attempt to remove the information from volatile storage as
promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might
be vulnerable to eavesdropping.
Note that if a request containing this directive is satisfied from a
cache, the no-store request directive does not apply to the already
stored response.
7.2.1.6. no-transform 7.2.1.6. 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 7.2.1.7. 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 7.2.2. Response Cache-Control Directives
7.2.2.1. public 7.2.2.1. must-revalidate
The "public" response directive indicates that any cache MAY store
the response, even if the response would normally be non-cacheable or
cacheable only within a non-shared cache. (See Section 3.2 for
additional details related to the use of public in response to a
request containing Authorization, and Section 3 for details of how
public affects responses that would normally not be stored, due to
their status codes not being defined as cacheable.)
7.2.2.2. private
Argument syntax:
#field-name
The "private" response directive indicates that the response message
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
requests, even if the response would normally be non-cacheable.
If the private response directive specifies one or more field-names,
this requirement is limited to the field-values associated with the
listed response header fields. That is, a shared cache MUST NOT
store the specified field-names(s), whereas it MAY store the
remainder of the response message.
The field-names given are not limited to the set of standard header The "must-revalidate" response directive indicates that once it has
fields defined by this specification. Field names are case- become stale, a cache MUST NOT use the response to satisfy subsequent
insensitive. requests without successful validation on the origin server.
Note: This usage of the word "private" only controls where the The must-revalidate directive is necessary to support reliable
response can be stored; it cannot ensure the privacy of the message operation for certain protocol features. In all circumstances a
content. Also, private response directives with field-names are cache MUST obey the must-revalidate directive; in particular, if a
often handled by implementations as if an unqualified private cache cannot reach the origin server for any reason, it MUST generate
directive was received; i.e., the special handling for the qualified a 504 (Gateway Timeout) response.
form is not widely implemented.
Note: This directive uses the quoted-string form of the argument The must-revalidate directive ought to be used by servers if and only
syntax. Senders SHOULD NOT use the token form (even if quoting if failure to validate a request on the representation could result
appears not to be needed for single-entry lists). in incorrect operation, such as a silently unexecuted financial
transaction.
7.2.2.3. no-cache 7.2.2.2. 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.
If the no-cache response directive specifies one or more field-names, If the no-cache response directive specifies one or more field-names,
then a cache MAY use the response to satisfy a subsequent request, then a cache MAY use the response to satisfy a subsequent request,
subject to any other restrictions on caching. However, any header subject to any other restrictions on caching. However, any header
fields in the response that have the field-name(s) listed MUST NOT be fields in the response that have the field-name(s) listed MUST NOT be
sent in the response to a subsequent request without successful sent in the response to a subsequent request without successful
revalidation with the origin server. This allows an origin server to revalidation with the origin server. This allows an origin server to
prevent the re-use of certain header fields in a response, while prevent the re-use of certain header fields in a response, while
still allowing caching of the rest of the response. still allowing caching of the rest of the response.
The field-names given are not limited to the set of standard header The field-names given are not limited to the set of header fields
fields defined by this specification. Field names are case- defined by this specification. Field names are case-insensitive.
insensitive.
Note: Many HTTP/1.0 caches will not recognize or obey this directive. Note: Although it has been back-ported to many implementations, some
Also, no-cache response directives with field-names are often handled HTTP/1.0 caches will not recognize or obey this directive. Also, no-
by implementations as if an unqualified no-cache directive was cache response directives with field-names are often handled by
received; i.e., the special handling for the qualified form is not caches as if an unqualified no-cache directive was received; i.e.,
widely implemented. the special handling for the qualified form is not 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. Senders SHOULD NOT use the token form (even if quoting
appears not to be needed for single-entry lists). appears not to be needed for single-entry lists).
7.2.2.4. no-store 7.2.2.3. 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.
7.2.2.5. must-revalidate 7.2.2.4. no-transform
The "must-revalidate" response directive indicates that once it has The "no-transform" response directive indicates that an intermediary
become stale, a cache MUST NOT use the response to satisfy subsequent (regardless of whether it implements a cache) MUST NOT transform the
requests without successful validation on the origin server. payload, as defined in Section 5.7.2 of [Part1].
The must-revalidate directive is necessary to support reliable 7.2.2.5. public
operation for certain protocol features. In all circumstances a
cache MUST obey the must-revalidate directive; in particular, if a
cache cannot reach the origin server for any reason, it MUST generate
a 504 (Gateway Timeout) response.
The must-revalidate directive ought to be used by servers if and only The "public" response directive indicates that any cache MAY store
if failure to validate a request on the representation could result the response, even if the response would normally be non-cacheable or
in incorrect operation, such as a silently unexecuted financial cacheable only within a non-shared cache. (See Section 3.2 for
transaction. additional details related to the use of public in response to a
request containing Authorization, and Section 3 for details of how
public affects responses that would normally not be stored, due to
their status codes not being defined as cacheable.)
7.2.2.6. proxy-revalidate 7.2.2.6. private
Argument syntax:
#field-name
The "private" response directive indicates that the response message
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
requests, even if the response would normally be non-cacheable.
If the private response directive specifies one or more field-names,
this requirement is limited to the field-values associated with the
listed response header fields. That is, a shared cache MUST NOT
store the specified field-names(s), whereas it MAY store the
remainder of the response message.
The field-names given are not limited to the set of header fields
defined by this specification. Field names are case-insensitive.
Note: This usage of the word "private" only controls where the
response can be stored; it cannot ensure the privacy of the message
content. Also, private response directives with field-names are
often handled by caches as if an unqualified private directive was
received; i.e., the special handling for the qualified form is not
widely implemented.
Note: This directive uses the quoted-string form of the argument
syntax. Senders SHOULD NOT use the token form (even if quoting
appears not to be needed for single-entry lists).
7.2.2.7. 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.
7.2.2.7. max-age 7.2.2.8. max-age
Argument syntax: Argument syntax:
delta-seconds (see Section 1.4.1) 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"'. Senders SHOULD NOT use the
quoted-string form. quoted-string form.
7.2.2.8. s-maxage 7.2.2.9. s-maxage
Argument syntax: Argument syntax:
delta-seconds (see Section 1.4.1) 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"'. Senders SHOULD NOT use the
quoted-string form. quoted-string form.
7.2.2.9. no-transform
The "no-transform" response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of [Part1].
7.2.3. Cache Control Extensions 7.2.3. 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. Both the new modifiers to the existing base of cache directives.
directive and the standard directive are supplied, such that
applications that do not understand the new directive will default to Both the new directive and the standard directive are supplied, such
the behavior specified by the standard directive, and those that that applications that do not understand the new directive will
understand the new directive will recognize it as modifying the default to the behavior specified by the standard directive, and
requirements associated with the standard directive. In this way, those that understand the new directive will recognize it as
extensions to the cache-control directives can be made without modifying the requirements associated with the standard directive.
requiring changes to the base protocol. In this way, extensions to the cache-control directives can be made
without requiring changes to the base protocol.
This extension mechanism depends on an HTTP cache obeying all of the This extension mechanism depends on an HTTP cache obeying all of the
cache-control directives defined for its native HTTP-version, obeying cache-control directives defined for its native HTTP-version, obeying
certain extensions, and ignoring all directives that it does not certain extensions, and ignoring all directives that it does not
understand. understand.
For example, consider a hypothetical new response directive called For example, consider a hypothetical new response directive called
"community" that acts as a modifier to the private directive. We "community" that acts as a modifier to the private directive. We
define this new directive to mean that, in addition to any private define this new directive to mean that, in addition to any private
cache, any cache that is shared only by members of the community cache, any cache that is shared only by members of the community
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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).
New extension directives ought to consider defining:
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
an argument is present,
o When the directive requires an argument, what it means when it is
missing.
The HTTP Cache Directive Registry defines the name space for the
cache directives.
A registration MUST include the following fields:
o Cache Directive Name
o Pointer to specification text
Values to be added to this name space require IETF Review (see
[RFC5226], Section 4.1).
The registry itself is maintained at
<http://www.iana.org/assignments/http-cache-directives>.
7.3. Expires 7.3. 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 4.1 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.
skipping to change at page 27, line 47 skipping to change at page 26, line 6
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 7.2.2.7), a recipient MUST ignore the Expires directive (Section 7.2.2.8), 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 7.2.2.8), a shared cache recipient MUST ignore the Expires (Section 7.2.2.9), 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
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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, the When the Cache-Control header field is not present in a request,
no-cache request pragma-directive MUST have the same effect on caches caches MUST consider the no-cache request pragma-directive as having
as if "Cache-Control: no-cache" were present (see Section 7.2.1). the same effect as if "Cache-Control: no-cache" were present (see
Section 7.2.1).
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
skipping to change at page 31, line 27 skipping to change at page 29, line 33
appears in the response. appears in the response.
7.5.7. 299 Miscellaneous Persistent Warning 7.5.7. 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 7.5.8. Warn Code Extensions
The HTTP Warn Code Registry defines the name space for warn codes. Extension warn codes can be defined; see Section 9.2.1 for details.
A registration MUST include the following fields:
o Warn Code (3 digits)
o Short Description
o Pointer to specification text
Values to be added to this name space require IETF Review (see
[RFC5226], Section 4.1).
The registry itself is maintained at
<http://www.iana.org/assignments/http-warn-codes>.
8. History Lists 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 4.1) 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.
skipping to change at page 32, line 10 skipping to change at page 30, line 4
The freshness model (Section 4.1) does not necessarily apply to The freshness model (Section 4.1) 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 9. IANA Considerations
9.1. Cache Directive Registry 9.1. Cache Directive Registry
The registration procedure for HTTP Cache Directives is defined by The HTTP Cache Directive Registry defines the name space for the
Section 7.2.3 of this document. cache directives. It will be created and maintained at
<http://www.iana.org/assignments/http-cache-directives>.
The HTTP Cache Directive Registry shall be created at 9.1.1. Procedure
<http://www.iana.org/assignments/http-cache-directives> and be
populated with the registrations below: A registration MUST include the following fields:
o Cache Directive Name
o Pointer to specification text
Values to be added to this name space require IETF Review (see
[RFC5226], Section 4.1).
9.1.2. Considerations for New Cache Control Directives
New extension directives ought to consider defining:
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
an argument is present,
o When the directive requires an argument, what it means when it is
missing,
o Whether the directive is specific to requests, responses, or able
to be used in either.
See also Section 7.2.3.
9.1.3. Registrations
The HTTP Cache Directive Registry shall be populated with the
registrations below:
+------------------------+----------------------------------+ +------------------------+----------------------------------+
| Cache Directive | Reference | | Cache Directive | Reference |
+------------------------+----------------------------------+ +------------------------+----------------------------------+
| max-age | Section 7.2.1.3, Section 7.2.2.7 | | max-age | Section 7.2.1.1, Section 7.2.2.8 |
| max-stale | Section 7.2.1.4 | | max-stale | Section 7.2.1.2 |
| min-fresh | Section 7.2.1.5 | | min-fresh | Section 7.2.1.3 |
| must-revalidate | Section 7.2.2.5 | | must-revalidate | Section 7.2.2.1 |
| no-cache | Section 7.2.1.1, Section 7.2.2.3 | | no-cache | Section 7.2.1.4, Section 7.2.2.2 |
| no-store | Section 7.2.1.2, Section 7.2.2.4 | | no-store | Section 7.2.1.5, Section 7.2.2.3 |
| no-transform | Section 7.2.1.6, Section 7.2.2.9 | | no-transform | Section 7.2.1.6, Section 7.2.2.4 |
| only-if-cached | Section 7.2.1.7 | | only-if-cached | Section 7.2.1.7 |
| private | Section 7.2.2.2 | | private | Section 7.2.2.6 |
| proxy-revalidate | Section 7.2.2.6 | | proxy-revalidate | Section 7.2.2.7 |
| public | Section 7.2.2.1 | | public | Section 7.2.2.5 |
| s-maxage | Section 7.2.2.8 | | s-maxage | Section 7.2.2.9 |
| 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 9.2. Warn Code Registry
The registration procedure for HTTP Warn Codes is defined by The HTTP Warn Code Registry defines the name space for warn codes.
Section 7.5.8 of this document. It will be created and maintained at
<http://www.iana.org/assignments/http-warn-codes>.
The HTTP Warn Code Registry shall be created at 9.2.1. Procedure
<http://www.iana.org/assignments/http-cache-directives> and be
populated with the registrations below: A registration MUST include the following fields:
o Warn Code (3 digits)
o Short Description
o Pointer to specification text
Values to be added to this name space require IETF Review (see
[RFC5226], Section 4.1).
9.2.2. Registrations
The HTTP Warn Code Registry shall be populated with the registrations
below:
+-----------+----------------------------------+---------------+ +-----------+----------------------------------+---------------+
| Warn Code | Short Description | Reference | | Warn Code | Short Description | Reference |
+-----------+----------------------------------+---------------+ +-----------+----------------------------------+---------------+
| 110 | Response is Stale | Section 7.5.1 | | 110 | Response is Stale | Section 7.5.1 |
| 111 | Revalidation Failed | Section 7.5.2 | | 111 | Revalidation Failed | Section 7.5.2 |
| 112 | Disconnected Operation | Section 7.5.3 | | 112 | Disconnected Operation | Section 7.5.3 |
| 113 | Heuristic Expiration | Section 7.5.4 | | 113 | Heuristic Expiration | Section 7.5.4 |
| 199 | Miscellaneous Warning | Section 7.5.5 | | 199 | Miscellaneous Warning | Section 7.5.5 |
| 214 | Transformation Applied | Section 7.5.6 | | 214 | Transformation Applied | Section 7.5.6 |
| 299 | Miscellaneous Persistent Warning | Section 7.5.7 | | 299 | Miscellaneous Persistent Warning | Section 7.5.7 |
+-----------+----------------------------------+---------------+ +-----------+----------------------------------+---------------+
9.3. Header Field Registration 9.3. Header Field Registration
The Message Header Field Registry located at <http://www.iana.org/ HTTP header fields are registered within the Message Header Field
assignments/message-headers/message-header-index.html> shall be Registry maintained at <http://www.iana.org/assignments/
updated with the permanent registrations below (see [BCP90]): message-headers/message-header-index.html>.
This document defines the following HTTP header fields, so their
associated registry entries shall be updated according to the
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 7.1 |
| Cache-Control | http | standard | Section 7.2 | | Cache-Control | http | standard | Section 7.2 |
| Expires | http | standard | Section 7.3 | | Expires | http | standard | Section 7.3 |
| Pragma | http | standard | Section 7.4 | | Pragma | http | standard | Section 7.4 |
| Warning | http | standard | Section 7.5 | | Warning | http | standard | Section 7.5 |
+-------------------+----------+----------+-------------+ +-------------------+----------+----------+-------------+
skipping to change at page 34, line 18 skipping to change at page 33, line 22
Implementation flaws might allow attackers to insert content into a Implementation flaws might allow attackers to insert content into a
cache ("cache poisoning"), leading to compromise of clients that cache ("cache poisoning"), leading to compromise of clients that
trust that content. Because of their nature, these attacks are trust that content. Because of their nature, these attacks are
difficult to mitigate. difficult to mitigate.
Likewise, implementation flaws (as well as misunderstanding of cache Likewise, implementation flaws (as well as misunderstanding of cache
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 [RFC6265] does not inhibit Note that the Set-Cookie response header field [RFC6265] does not
caching; a cacheable response with a Set-Cookie header can be (and inhibit caching; a cacheable response with a Set-Cookie header field
often is) used to satisfy subsequent requests to caches. Servers who can be (and often is) used to satisfy subsequent requests to caches.
wish to control caching of these responses are encouraged to emit Servers who wish to control caching of these responses are encouraged
appropriate Cache-Control response headers. to emit appropriate Cache-Control response header fields.
11. Acknowledgments 11. Acknowledgments
See Section 9 of [Part1]. See Section 9 of [Part1].
12. References 12. References
12.1. Normative References 12.1. 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-22 (work in progress), draft-ietf-httpbis-p1-messaging-23 (work in progress),
February 2013. July 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-22 (work in progress), draft-ietf-httpbis-p2-semantics-23 (work in progress),
February 2013. July 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-22 (work in progress), draft-ietf-httpbis-p4-conditional-23 (work in progress),
February 2013. July 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-22 (work in progress), draft-ietf-httpbis-p5-range-23 (work in progress),
February 2013. July 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-22 (work in progress), draft-ietf-httpbis-p7-auth-23 (work in progress),
February 2013. July 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 12.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration [BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
skipping to change at page 36, line 39 skipping to change at page 35, line 43
expected is now defined. (Section 7.2) expected is now defined. (Section 7.2)
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 7.2.2)
The "no-store" cache request directive doesn't apply to responses; 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 i.e., a cache can satisfy a request with no-store on it, and does not
invalidate it. (Section 7.2.1.2) 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 7.2.2.3) (Section 7.2.2.2)
New status codes can now define that caches are allowed to use New status codes can now define that caches are allowed to use
heuristic freshness with them. (Section 4.1.2) heuristic freshness with them. (Section 4.1.2)
Caches are now allow to calculate heuristic freshness for URLs with Caches are now allow to calculate heuristic freshness for URLs with
query components. (Section 4.1.2) query components. (Section 4.1.2)
Some requirements regarding production of the Warning header have Some requirements regarding production of the Warning header fields
been relaxed, as it is not widely implemented. (Section 7.5) have been relaxed, as it is not widely implemented. Furthermore, the
The Warning header field no longer uses RFC 2047 encoding, nor allows Warning header field no longer uses RFC 2047 encoding, nor allows
multiple languages, as these aspects were not implemented. multiple languages, as these aspects were not implemented.
(Section 7.5) (Section 7.5)
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 7.2.3 and Section 7.5.8)
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
skipping to change at page 38, line 7 skipping to change at page 36, line 42
port = <port, defined in [Part1], Section 2.7> port = <port, defined in [Part1], Section 2.7>
pseudonym = <pseudonym, defined in [Part1], Section 5.7.1> pseudonym = <pseudonym, defined in [Part1], Section 5.7.1>
uri-host = <uri-host, defined in [Part1], Section 2.7> uri-host = <uri-host, defined in [Part1], Section 2.7>
The rules below are defined in other parts: The rules below are defined in other parts:
HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1> HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1>
Appendix C. Collected ABNF Appendix C. Collected ABNF
In the collected ABNF below, list rules are expanded as per Section
1.2 of [Part1].
Age = delta-seconds Age = delta-seconds
Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
cache-directive ] ) cache-directive ] )
Expires = HTTP-date Expires = HTTP-date
HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1> HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1>
OWS = <OWS, defined in [Part1], Section 3.2.3> OWS = <OWS, defined in [Part1], Section 3.2.3>
skipping to change at page 40, line 23 skipping to change at page 39, line 20
heuristic caching for new status codes" heuristic caching for new status codes"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/406>: "304 without o <http://tools.ietf.org/wg/httpbis/trac/ticket/406>: "304 without
validator" validator"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/418>: "No-Transform" o <http://tools.ietf.org/wg/httpbis/trac/ticket/418>: "No-Transform"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/430>: "Revert prior o <http://tools.ietf.org/wg/httpbis/trac/ticket/430>: "Revert prior
change to the meaning of the public cache response directive. change to the meaning of the public cache response directive.
D.4. Since draft-ietf-httpbis-p6-cache-22
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/436>: "explain list
expansion in ABNF appendices"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/453>: "Returning the
freshest response"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/464>: "placement of
extension point considerations"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/469>: "Editorial
notes for p6"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/471>: "Vary and
future requests"
Index Index
1 1
110 Response is Stale (warn code) 30 110 Response is Stale (warn code) 28
111 Revalidation Failed (warn code) 30 111 Revalidation Failed (warn code) 28
112 Disconnected Operation (warn code) 30 112 Disconnected Operation (warn code) 28
113 Heuristic Expiration (warn code) 30 113 Heuristic Expiration (warn code) 29
199 Miscellaneous Warning (warn code) 31 199 Miscellaneous Warning (warn code) 29
2 2
214 Transformation Applied (warn code) 31 214 Transformation Applied (warn code) 29
299 Miscellaneous Persistent Warning (warn code) 31 299 Miscellaneous Persistent Warning (warn code) 29
A A
age 5 age 9
Age header field 19 Age header field 18
C C
cache 4 cache 4
cache entry 6 cache entry 5
cache key 6 cache key 5
Cache-Control header field 20 Cache-Control header field 18
cacheable 4
E E
Expires header field 27 Expires header field 25
explicit expiration time 5 explicit expiration time 9
F F
first-hand 5 first-hand 14
fresh 5 fresh 9
freshness lifetime 5 freshness lifetime 9
G G
Grammar Grammar
Age 19 Age 18
Cache-Control 20 Cache-Control 19
cache-directive 20 cache-directive 19
delta-seconds 6 delta-seconds 5
Expires 27 Expires 25
extension-pragma 28 extension-pragma 26
Pragma 28 Pragma 26
pragma-directive 28 pragma-directive 26
warn-agent 29 warn-agent 27
warn-code 29 warn-code 27
warn-date 29 warn-date 27
warn-text 29 warn-text 27
Warning 29 Warning 27
warning-value 29 warning-value 27
H H
heuristic expiration time 5 heuristic expiration time 9
M M
max-age (cache directive) 21, 25 max-age (cache directive) 19, 24
max-stale (cache directive) 21 max-stale (cache directive) 19
min-fresh (cache directive) 22 min-fresh (cache directive) 20
must-revalidate (cache directive) 24 must-revalidate (cache directive) 21
N N
no-cache (cache directive) 20, 23 no-cache (cache directive) 20-21
no-store (cache directive) 21, 24 no-store (cache directive) 20, 22
no-transform (cache directive) 22, 25 no-transform (cache directive) 21-22
O O
only-if-cached (cache directive) 22 only-if-cached (cache directive) 21
P P
Pragma header field 28 Pragma header field 26
private (cache directive) 23 private (cache directive) 23
private cache 4 private cache 4
proxy-revalidate (cache directive) 25 proxy-revalidate (cache directive) 23
public (cache directive) 22 public (cache directive) 23
S S
s-maxage (cache directive) 25 s-maxage (cache directive) 24
shared cache 4 shared cache 4
stale 5 stale 9
strong validator 6 strong validator 15
V V
validator 5 validator 14
strong 6
W W
Warning header field 29 Warning header field 27
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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