draft-ietf-httpbis-p6-cache-26.txt   draft-ietf-httpbis-p6-cache-latest.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 10, 2014 J. Reschke, Ed. Expires: December 3, 2014 J. Reschke, Ed.
greenbytes greenbytes
February 6, 2014 June 2014
Hypertext Transfer Protocol (HTTP/1.1): Caching Hypertext Transfer Protocol (HTTP/1.1): Caching
draft-ietf-httpbis-p6-cache-26 draft-ietf-httpbis-p6-cache-latest
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is a stateless application- The Hypertext Transfer Protocol (HTTP) is a stateless application-
level protocol for distributed, collaborative, hypertext information level protocol for distributed, collaborative, hypertext information
systems. This document defines HTTP caches and the associated header systems. This document defines HTTP caches and the associated header
fields that control cache behavior or indicate cacheable response fields that control cache behavior or indicate cacheable response
messages. messages.
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.2. _This is a temporary document for the purpose of tracking the
editorial changes made during the AUTH48 (RFC publication) phase._
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 10, 2014. This Internet-Draft will expire on December 3, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 3, line 44 skipping to change at page 3, line 44
7.2.2. Registrations . . . . . . . . . . . . . . . . . . . . 33 7.2.2. Registrations . . . . . . . . . . . . . . . . . . . . 33
7.3. Header Field Registration . . . . . . . . . . . . . . . . 34 7.3. Header Field Registration . . . . . . . . . . . . . . . . 34
8. Security Considerations . . . . . . . . . . . . . . . . . . . 34 8. Security Considerations . . . . . . . . . . . . . . . . . . . 34
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
10.1. Normative References . . . . . . . . . . . . . . . . . . . 35 10.1. Normative References . . . . . . . . . . . . . . . . . . . 35
10.2. Informative References . . . . . . . . . . . . . . . . . . 36 10.2. Informative References . . . . . . . . . . . . . . . . . . 36
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 36 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 36
Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 38 Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 38
Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 38 Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 38
Appendix D. Change Log (to be removed by RFC Editor before Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
publication) . . . . . . . . . . . . . . . . . . . . 39
D.1. Since draft-ietf-httpbis-p6-cache-24 . . . . . . . . . . . 40
D.2. Since draft-ietf-httpbis-p6-cache-25 . . . . . . . . . . . 40
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction 1. Introduction
HTTP is typically used for distributed information systems, where HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. This performance can be improved by the use of response caches. This
document defines aspects of HTTP/1.1 related to caching and reusing document defines aspects of HTTP/1.1 related to caching and reusing
response messages. response messages.
An HTTP "cache" is a local store of response messages and the An HTTP "cache" is a local store of response messages and the
subsystem that controls storage, retrieval, and deletion of messages subsystem that controls storage, retrieval, and deletion of messages
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can be freshened by validation (Section 4.3) or if the origin is can be freshened by validation (Section 4.3) or if the origin is
unavailable (Section 4.2.4). unavailable (Section 4.2.4).
1.1. Conformance and Error Handling 1.1. Conformance and Error Handling
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
Conformance criteria and considerations regarding error handling are Conformance criteria and considerations regarding error handling are
defined in Section 2.5 of [Part1]. defined in Section 2.5 of [RFC7230].
1.2. Syntax Notation 1.2. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF) This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with a list extension, defined in Section 7 of notation of [RFC5234] with a list extension, defined in Section 7 of
[Part1], that allows for compact definition of comma-separated lists [RFC7230], that allows for compact definition of comma-separated
using a '#' operator (similar to how the '*' operator indicates lists using a '#' operator (similar to how the '*' operator indicates
repetition). Appendix B describes rules imported from other repetition). Appendix B describes rules imported from other
documents. Appendix C shows the collected grammar with all list documents. Appendix C shows the collected grammar with all list
operators expanded to standard ABNF notation. operators expanded to standard ABNF notation.
1.2.1. Delta Seconds 1.2.1. Delta Seconds
The delta-seconds rule specifies a non-negative integer, representing The delta-seconds rule specifies a non-negative integer, representing
time in seconds. time in seconds.
delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
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to be stored in binary form; an implementation could produce it as to be stored in binary form; an implementation could produce it as
a canned string if any overflow occurs, even if the calculations a canned string if any overflow occurs, even if the calculations
are performed with an arithmetic type incapable of directly are performed with an arithmetic type incapable of directly
representing that number. What matters here is that an overflow representing that number. What matters here is that an overflow
be detected and not treated as a negative value in later be detected and not treated as a negative value in later
calculations. calculations.
2. Overview of Cache Operation 2. Overview of Cache Operation
Proper cache operation preserves the semantics of HTTP transfers Proper cache operation preserves the semantics of HTTP transfers
([Part2]) while eliminating the transfer of information already held ([RFC7231]) while eliminating the transfer of information already
in the cache. Although caching is an entirely OPTIONAL feature of held in the cache. Although caching is an entirely OPTIONAL feature
HTTP, it can be assumed that reusing a cached response is desirable of HTTP, it can be assumed that reusing a cached response is
and that such reuse is the default behavior when no requirement or desirable and that such reuse is the default behavior when no
local configuration prevents it. Therefore, HTTP cache requirements requirement or local configuration prevents it. Therefore, HTTP
are focused on preventing a cache from either storing a non-reusable cache requirements are focused on preventing a cache from either
response or reusing a stored response inappropriately, rather than storing a non-reusable response or reusing a stored response
mandating that caches always store and reuse particular responses. 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 [RFC7231]). 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 primary "cache key" consists of the request method and target The primary "cache key" consists of the request method and target
URI. However, since HTTP caches in common use today are typically URI. However, since HTTP caches in common use today are typically
limited to caching responses to GET, many caches simply decline other limited to caching responses to GET, many caches simply decline other
methods and use only the URI as the primary cache key. methods and use only the URI as the primary cache key.
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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 5.2) does not appear o the "no-store" cache directive (see Section 5.2) does not appear
in request or response header fields, and in request or response header fields, and
o the "private" response directive (see Section 5.2.2.6) does not o the "private" response directive (see Section 5.2.2.6) does not
appear in the response, if the cache is shared, and appear in the response, if the cache is shared, and
o the Authorization header field (see Section 4.2 of [Part7]) does o the Authorization header field (see Section 4.2 of [RFC7235]) 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 5.3), or * contains an Expires header field (see Section 5.3), or
* contains a max-age response directive (see Section 5.2.2.8), or * contains a max-age response directive (see Section 5.2.2.8), or
* contains a s-maxage response directive (see Section 5.2.2.9) * contains a s-maxage response directive (see Section 5.2.2.9)
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caching-related behavior. caching-related behavior.
Note that, in normal operation, some caches will not store a response Note that, in normal operation, some caches will not store a response
that has neither a cache validator nor an explicit expiration time, that has neither a cache validator nor an explicit expiration time,
as such responses are not usually useful to store. However, caches as such responses are not usually useful to store. However, caches
are not prohibited from storing such responses. are not prohibited from storing such responses.
3.1. Storing Incomplete Responses 3.1. Storing Incomplete Responses
A response message is considered complete when all of the octets A response message is considered complete when all of the octets
indicated by the message framing ([Part1]) are received prior to the indicated by the message framing ([RFC7230]) are received prior to
connection being closed. If the request method is GET, the response the connection being closed. If the request method is GET, the
status code is 200 (OK), and the entire response header section has response status code is 200 (OK), and the entire response header
been received, a cache MAY store an incomplete response message body section has been received, a cache MAY store an incomplete response
if the cache entry is recorded as incomplete. Likewise, a 206 message body if the cache entry is recorded as incomplete. Likewise,
(Partial Content) response MAY be stored as if it were an incomplete a 206 (Partial Content) response MAY be stored as if it were an
200 (OK) cache entry. However, a cache MUST NOT store incomplete or incomplete 200 (OK) cache entry. However, a cache MUST NOT store
partial content responses if it does not support the Range and incomplete or partial-content responses if it does not support the
Content-Range header fields or if it does not understand the range Range and Content-Range header fields or if it does not understand
units used in those fields. the range units used in those fields.
A cache MAY complete a stored incomplete response by making a A cache MAY complete a stored incomplete response by making a
subsequent range request ([Part5]) and combining the successful subsequent range request ([RFC7233]) and combining the successful
response with the stored entry, as defined in Section 3.3. A cache response with the stored entry, as defined in Section 3.3. A cache
MUST NOT use an incomplete response to answer requests unless the MUST NOT use an incomplete response to answer requests unless the
response has been made complete or the request is partial and response has been made complete or the request is partial and
specifies a range that is wholly within the incomplete response. A specifies a range that is wholly within the incomplete response. A
cache MUST NOT send a partial response to a client without explicitly cache MUST NOT send a partial response to a client without explicitly
marking it as such using the 206 (Partial Content) status code. marking it as such using the 206 (Partial Content) status code.
3.2. Storing Responses to Authenticated Requests 3.2. Storing Responses to Authenticated Requests
A shared cache MUST NOT use a cached response to a request with an A shared cache MUST NOT use a cached response to a request with an
Authorization header field (Section 4.2 of [Part7]) to satisfy any Authorization header field (Section 4.2 of [RFC7235]) to satisfy any
subsequent request unless a cache directive that allows such subsequent request unless a cache directive that allows such
responses to be stored is present in the response. responses to be stored is present in the response.
In this specification, the following Cache-Control response In this specification, the following Cache-Control response
directives (Section 5.2.2) have such an effect: must-revalidate, directives (Section 5.2.2) have such an effect: must-revalidate,
public, s-maxage. public, and 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 4.2.4) by shared caches. In particular, a response with (Section 4.2.4) by shared caches. In particular, a response with
either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
satisfy a subsequent request without revalidating it on the origin satisfy a subsequent request without revalidating it on the origin
server. server.
3.3. Combining Partial Content 3.3. Combining Partial Content
A response might transfer only a partial representation if the A response might transfer only a partial representation if the
connection closed prematurely or if the request used one or more connection closed prematurely or if the request used one or more
Range specifiers ([Part5]). After several such transfers, a cache Range specifiers ([RFC7233]). After several such transfers, a cache
might have received several ranges of the same representation. A might have received several ranges of the same representation. A
cache MAY combine these ranges into a single stored response, and cache MAY combine these ranges into a single stored response, and
reuse that response to satisfy later requests, if they all share the reuse that response to satisfy later requests, if they all share the
same strong validator and the cache complies with the client same strong validator and the cache complies with the client
requirements in Section 4.3 of [Part5]. requirements in Section 4.3 of [RFC7233].
When combining the new response with one or more stored responses, a When combining the new response with one or more stored responses, a
cache MUST: cache MUST:
o delete any Warning header fields in the stored response with warn- o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 5.5); code 1xx (see Section 5.5);
o retain any Warning header fields in the stored response with warn- o retain any Warning header fields in the stored response with warn-
code 2xx; and, code 2xx; and,
o use other header fields provided in the new response, aside from o use other header fields provided in the new response, aside from
Content-Range, to replace all instances of the corresponding Content-Range, to replace all instances of the corresponding
header fields in the stored response. header fields in the stored response.
4. Constructing Responses from Caches 4. Constructing Responses from Caches
When presented with a request, a cache MUST NOT reuse a stored When presented with a request, a cache MUST NOT reuse a stored
response, unless: response, unless:
o The presented effective request URI (Section 5.5 of [Part1]) and o The presented effective request URI (Section 5.5 of [RFC7230]) 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 4.1), and match those presented (see Section 4.1), and
o the presented request does not contain the no-cache pragma o the presented request does not contain the no-cache pragma
(Section 5.4), nor the no-cache cache directive (Section 5.2.1), (Section 5.4), nor the no-cache cache directive (Section 5.2.1),
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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 5.2.3. cache-control extension; see Section 5.2.3.
When a stored response is used to satisfy a request without When a stored response is used to satisfy a request without
validation, a cache MUST generate an Age header field (Section 5.1), validation, a cache MUST generate an Age header field (Section 5.1),
replacing any present in the response with a value equal to the replacing any present in the response with a value equal to the
stored response's current_age; see Section 4.2.3. stored response's current_age; see Section 4.2.3.
A cache MUST write through requests with methods that are unsafe A cache MUST write through requests with methods that are unsafe
(Section 4.2.1 of [Part2]) to the origin server; i.e., a cache is not (Section 4.2.1 of [RFC7231]) to the origin server; i.e., a cache is
allowed to generate a reply to such a request before having forwarded not allowed to generate a reply to such a request before having
the request and having received a corresponding response. forwarded the request and having received a corresponding response.
Also, note that unsafe requests might invalidate already stored Also, note that unsafe requests might invalidate already-stored
responses; see Section 4.4. responses; see Section 4.4.
When more than one suitable response is stored, a cache MUST use the When more than one suitable response is stored, a cache MUST use the
most recent response (as determined by the Date header field). It most recent response (as determined by the Date header field). It
can also forward the request with "Cache-Control: max-age=0" or can also forward the request with "Cache-Control: max-age=0" or
"Cache-Control: no-cache" to disambiguate which response to use. "Cache-Control: no-cache" to disambiguate which response to use.
A cache that does not have a clock available MUST NOT use stored A cache that does not have a clock available MUST NOT use stored
responses without revalidating them upon every use. responses without revalidating them upon every use.
4.1. Calculating Secondary Keys with Vary 4.1. 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 [RFC7231]),
MUST NOT use that response unless all of the selecting header fields it MUST NOT use that response unless all of the selecting header
nominated by the Vary header field match in both the original request fields nominated by the Vary header field match in both the original
(i.e., that associated with the stored response), and the presented request (i.e., that associated with the stored response), and the
request. presented 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
the second request by applying any of the following: the second request by applying any of the following:
o adding or removing whitespace, where allowed in the header field's o adding or removing whitespace, where allowed in the header field's
syntax syntax
o combining multiple header fields with the same field name (see o combining multiple header fields with the same field name (see
Section 3.2 of [Part1]) Section 3.2 of [RFC7230])
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., reordering field values when order is not
significant; case-normalization, where values are defined to be significant; case-normalization, where values are defined to be
case-insensitive) case-insensitive)
If (after any normalization that might take place) a header field is If (after any normalization that might take place) a header field is
absent from a request, it can only match another request if it is absent from a request, it can only match another request if it is
also absent there. also absent there.
A Vary header field-value of "*" always fails to match. A Vary header field-value of "*" always fails to match.
The stored response with matching selecting header fields is known as The stored response with matching selecting header fields is known as
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freshness_lifetime is defined in Section 4.2.1; current_age is freshness_lifetime is defined in Section 4.2.1; current_age is
defined in Section 4.2.3. defined in Section 4.2.3.
Clients can send the max-age or min-fresh 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 5.2.1). corresponding response (Section 5.2.1).
When calculating freshness, to avoid common problems in date parsing: When calculating freshness, to avoid common problems in date parsing:
o Although all date formats are specified to be case-sensitive, a o Although all date formats are specified to be case-sensitive, a
cache recipient SHOULD match day, week, and timezone names case- cache recipient SHOULD match day, week, and time-zone names case-
insensitively. insensitively.
o If a cache recipient's internal implementation of time has less o If a cache recipient's internal implementation of time has less
resolution than the value of an HTTP-date, the recipient MUST resolution than the value of an HTTP-date, the recipient MUST
internally represent a parsed Expires date as the nearest time internally represent a parsed Expires date as the nearest time
equal to or earlier than the received value. equal to or earlier than the received value.
o A cache recipient MUST NOT allow local time zones to influence the o A cache recipient MUST NOT allow local time zones to influence the
calculation or comparison of an age or expiration time. calculation or comparison of an age or expiration time.
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other than GMT or UTC to be invalid for calculating expiration. other than GMT or UTC to be invalid for calculating expiration.
Note that freshness applies only to cache operation; it cannot be Note that freshness applies only to cache operation; it cannot be
used to force a user agent to refresh its display or reload a used to force a user agent to refresh its display or reload a
resource. See Section 6 for an explanation of the difference between resource. See Section 6 for an explanation of the difference between
caches and history mechanisms. caches and history mechanisms.
4.2.1. Calculating Freshness Lifetime 4.2.1. Calculating Freshness Lifetime
A cache can calculate the freshness lifetime (denoted as A cache can calculate the freshness lifetime (denoted as
freshness_lifetime) of a response by using the first match of: freshness_lifetime) of a response by using the first match of the
following:
o If the cache is shared and the s-maxage response directive o If the cache is shared and the s-maxage response directive
(Section 5.2.2.9) is present, use its value, or (Section 5.2.2.9) is present, use its value, or
o If the max-age response directive (Section 5.2.2.8) is present, o If the max-age response directive (Section 5.2.2.8) is present,
use its value, or use its value, or
o If the Expires response header field (Section 5.3) is present, use o If the Expires response header field (Section 5.3) is present, use
its value minus the value of the Date response header field, or its value minus the value of the Date response header field, or
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is not specified, employing algorithms that use other header field is not specified, employing algorithms that use other header field
values (such as the Last-Modified time) to estimate a plausible values (such as the Last-Modified time) to estimate a plausible
expiration time. This specification does not provide specific expiration time. This specification does not provide specific
algorithms, but does impose worst-case constraints on their results. algorithms, but does impose worst-case constraints on their results.
A cache MUST NOT use heuristics to determine freshness when an A cache MUST NOT use heuristics to determine freshness when an
explicit expiration time is present in the stored response. Because explicit expiration time is present in the stored response. Because
of the requirements in Section 3, this means that, effectively, of the requirements in Section 3, this means that, effectively,
heuristics can only be used on responses without explicit freshness heuristics can only be used on responses without explicit freshness
whose status codes are defined as cacheable by default (see Section whose status codes are defined as cacheable by default (see Section
6.1 of [Part2]), and those responses without explicit freshness that 6.1 of [RFC7231]), and those responses without explicit freshness
have been marked as explicitly cacheable (e.g., with a "public" that have been marked as explicitly cacheable (e.g., with a "public"
response directive). response directive).
If the response has a Last-Modified header field (Section 2.2 of If the response has a Last-Modified header field (Section 2.2 of
[Part4]), caches are encouraged to use a heuristic expiration value [RFC7232]), caches are encouraged to use a heuristic expiration value
that is no more than some fraction of the interval since that time. that is no more than some fraction of the interval since that time.
A typical setting of this fraction might be 10%. A typical setting of this fraction might be 10%.
When a heuristic is used to calculate freshness lifetime, a cache When a heuristic is used to calculate freshness lifetime, a cache
SHOULD generate a Warning header field with a 113 warn-code (see SHOULD generate a Warning header field with a 113 warn-code (see
Section 5.5.4) in the response if its current_age is more than 24 Section 5.5.4) in the response if its current_age is more than 24
hours and such a warning is not already present. hours and such a warning is not already present.
Note: Section 13.9 of [RFC2616] prohibited caches from calculating Note: Section 13.9 of [RFC2616] prohibited caches from calculating
heuristic freshness for URIs with query components (i.e., those heuristic freshness for URIs with query components (i.e., those
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"age_value" "age_value"
The term "age_value" denotes the value of the Age header field The term "age_value" denotes the value of the Age header field
(Section 5.1), in a form appropriate for arithmetic operation; or (Section 5.1), in a form appropriate for arithmetic operation; or
0, if not available. 0, if not available.
"date_value" "date_value"
The term "date_value" denotes the value of the Date header field, The term "date_value" denotes the value of the Date header field,
in a form appropriate for arithmetic operations. See Section in a form appropriate for arithmetic operations. See Section
7.1.1.2 of [Part2] for the definition of the Date header field, 7.1.1.2 of [RFC7231] for the definition of the Date header field,
and for requirements regarding responses without it. and for requirements regarding responses without it.
"now" "now"
The term "now" means "the current value of the clock at the host The term "now" means "the current value of the clock at the host
performing the calculation". A host ought to use NTP ([RFC5905]) performing the calculation". A host ought to use NTP ([RFC5905])
or some similar protocol to synchronize its clocks to Coordinated or some similar protocol to synchronize its clocks to Coordinated
Universal Time. Universal Time.
"request_time" "request_time"
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A cache SHOULD NOT generate a new Warning header field when A cache SHOULD NOT generate a new Warning header field when
forwarding a response that does not have an Age header field, even if forwarding a response that does not have an Age header field, even if
the response is already stale. A cache need not validate a response the response is already stale. A cache need not validate a response
that merely became stale in transit. that merely became stale in transit.
4.3. Validation 4.3. Validation
When a cache has one or more stored responses for a requested URI, When a cache has one or more stored responses for a requested URI,
but cannot serve any of them (e.g., because they are not fresh, or but cannot serve any of them (e.g., because they are not fresh, or
one cannot be selected; see Section 4.1), it can use the conditional one cannot be selected; see Section 4.1), it can use the conditional
request mechanism [Part4] in the forwarded request to give the next request mechanism [RFC7232] in the forwarded request to give the next
inbound server an opportunity to select a valid stored response to inbound server an opportunity to select a valid stored response to
use, updating the stored metadata in the process, or to replace the use, updating the stored metadata in the process, or to replace the
stored response(s) with a new response. This process is known as stored response(s) with a new response. This process is known as
"validating" or "revalidating" the stored response. "validating" or "revalidating" the stored response.
4.3.1. Sending a Validation Request 4.3.1. Sending a Validation Request
When sending a conditional request for cache validation, a cache When sending a conditional request for cache validation, a cache
sends one or more precondition header fields containing "validator" sends one or more precondition header fields containing "validator"
metadata from its stored response(s), which is then compared by metadata from its stored response(s), which is then compared by
recipients to determine whether a stored response is equivalent to a recipients to determine whether a stored response is equivalent to a
current representation of the resource. current representation of the resource.
One such validator is the timestamp given in a Last-Modified header One such validator is the timestamp given in a Last-Modified header
field (Section 2.2 of [Part4]), which can be used in an If-Modified- field (Section 2.2 of [RFC7232]), which can be used in an If-
Since header field for response validation, or in an If-Unmodified- Modified-Since header field for response validation, or in an If-
Since or If-Range header field for representation selection (i.e., Unmodified-Since or If-Range header field for representation
the client is referring specifically to a previously obtained selection (i.e., the client is referring specifically to a previously
representation with that timestamp). obtained representation with that timestamp).
Another validator is the entity-tag given in an ETag header field Another validator is the entity-tag given in an ETag header field
(Section 2.3 of [Part4]). One or more entity-tags, indicating one or (Section 2.3 of [RFC7232]). One or more entity-tags, indicating one
more stored responses, can be used in an If-None-Match header field or more stored responses, can be used in an If-None-Match header
for response validation, or in an If-Match or If-Range header field field for response validation, or in an If-Match or If-Range header
for representation selection (i.e., the client is referring field for representation selection (i.e., the client is referring
specifically to one or more previously obtained representations with specifically to one or more previously obtained representations with
the listed entity-tags). the listed entity-tags).
4.3.2. Handling a Received Validation Request 4.3.2. Handling a Received Validation Request
Each client in the request chain may have its own cache, so it is Each client in the request chain may have its own cache, so it is
common for a cache at an intermediary to receive conditional requests common for a cache at an intermediary to receive conditional requests
from other (outbound) caches. Likewise, some user agents make use of from other (outbound) caches. Likewise, some user agents make use of
conditional requests to limit data transfers to recently modified conditional requests to limit data transfers to recently modified
representations or to complete the transfer of a partially retrieved representations or to complete the transfer of a partially retrieved
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received in that request with respect to the corresponding validators received in that request with respect to the corresponding validators
contained within the selected response. A cache MUST NOT evaluate contained within the selected response. A cache MUST NOT evaluate
conditional header fields that are only applicable to an origin conditional header fields that are only applicable to an origin
server, found in a request with semantics that cannot be satisfied server, found in a request with semantics that cannot be satisfied
with a cached response, or applied to a target resource for which it with a cached response, or applied to a target resource for which it
has no stored responses; such preconditions are likely intended for has no stored responses; such preconditions are likely intended for
some other (inbound) server. some other (inbound) server.
The proper evaluation of conditional requests by a cache depends on The proper evaluation of conditional requests by a cache depends on
the received precondition header fields and their precedence, as the received precondition header fields and their precedence, as
defined in Section 6 of [Part4]. The If-Match and If-Unmodified- defined in Section 6 of [RFC7232]. The If-Match and If-Unmodified-
Since conditional header fields are not applicable to a cache. Since conditional header fields are not applicable to a cache.
A request containing an If-None-Match header field (Section 3.2 of A request containing an If-None-Match header field (Section 3.2 of
[Part4]) indicates that the client wants to validate one or more of [RFC7232]) indicates that the client wants to validate one or more of
its own stored responses in comparison to whichever stored response its own stored responses in comparison to whichever stored response
is selected by the cache. If the field-value is "*", or if the is selected by the cache. If the field-value is "*", or if the
field-value is a list of entity-tags and at least one of them match field-value is a list of entity-tags and at least one of them matches
the entity-tag of the selected stored response, a cache recipient the entity-tag of the selected stored response, a cache recipient
SHOULD generate a 304 (Not Modified) response (using the metadata of SHOULD generate a 304 (Not Modified) response (using the metadata of
the selected stored response) instead of sending that stored the selected stored response) instead of sending that stored
response. response.
When a cache decides to revalidate its own stored responses for a When a cache decides to revalidate its own stored responses for a
request that contains an If-None-Match list of entity-tags, the cache request that contains an If-None-Match list of entity-tags, the cache
MAY combine the received list with a list of entity-tags from its own MAY combine the received list with a list of entity-tags from its own
stored set of responses (fresh or stale) and send the union of the stored set of responses (fresh or stale) and send the union of the
two lists as a replacement If-None-Match header field value in the two lists as a replacement If-None-Match header field value in the
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content, the cache MUST NOT include its entity-tag in the union content, the cache MUST NOT include its entity-tag in the union
unless the request is for a range that would be fully satisfied by unless the request is for a range that would be fully satisfied by
that partial stored response. If the response to the forwarded that partial stored response. If the response to the forwarded
request is 304 (Not Modified) and has an ETag header field value with request is 304 (Not Modified) and has an ETag header field value with
an entity-tag that is not in the client's list, the cache MUST an entity-tag that is not in the client's list, the cache MUST
generate a 200 (OK) response for the client by reusing its generate a 200 (OK) response for the client by reusing its
corresponding stored response, as updated by the 304 response corresponding stored response, as updated by the 304 response
metadata (Section 4.3.4). metadata (Section 4.3.4).
If an If-None-Match header field is not present, a request containing If an If-None-Match header field is not present, a request containing
an If-Modified-Since header field (Section 3.3 of [Part4]) indicates an If-Modified-Since header field (Section 3.3 of [RFC7232])
that the client wants to validate one or more of its own stored indicates that the client wants to validate one or more of its own
responses by modification date. A cache recipient SHOULD generate a stored responses by modification date. A cache recipient SHOULD
304 (Not Modified) response (using the metadata of the selected generate a 304 (Not Modified) response (using the metadata of the
stored response) if one of the following cases is true: 1) the selected stored response) if one of the following cases is true: 1)
selected stored response has a Last-Modified field-value that is the selected stored response has a Last-Modified field-value that is
earlier than or equal to the conditional timestamp; 2) no Last- earlier than or equal to the conditional timestamp; 2) no Last-
Modified field is present in the selected stored response, but it has Modified field is present in the selected stored response, but it has
a Date field-value that is earlier than or equal to the conditional a Date field-value that is earlier than or equal to the conditional
timestamp; or, 3) neither Last-Modified nor Date is present in the timestamp; or, 3) neither Last-Modified nor Date is present in the
selected stored response, but the cache recorded it as having been selected stored response, but the cache recorded it as having been
received at a time earlier than or equal to the conditional received at a time earlier than or equal to the conditional
timestamp. timestamp.
A cache that implements partial responses to range requests, as A cache that implements partial responses to range requests, as
defined in [Part5], also needs to evaluate a received If-Range header defined in [RFC7233], also needs to evaluate a received If-Range
field (Section 3.2 of [Part5]) with respect to its selected stored header field (Section 3.2 of [RFC7233]) with respect to its selected
response. stored response.
4.3.3. Handling a Validation Response 4.3.3. Handling a Validation Response
Cache handling of a response to a conditional request 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 4.3.4. stored response can be updated and reused; see Section 4.3.4.
o A full response (i.e., one with a payload body) indicates that o A full response (i.e., one with a payload body) indicates that
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4.3.4. Freshening Stored Responses upon Validation 4.3.4. Freshening Stored Responses upon Validation
When a cache receives a 304 (Not Modified) response and already has When a cache receives a 304 (Not Modified) response and already has
one or more stored 200 (OK) responses for the 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 the following:
o If the new response contains a "strong validator" (see Section 2.1 o If the new response contains a "strong validator" (see Section 2.1
of [Part4]), then that strong validator identifies the selected of [RFC7232]), then that strong validator identifies the selected
representation for update. All of the stored responses with the representation for update. All of the stored responses with the
same strong validator are selected. If none of the stored same strong validator are selected. If none of the stored
responses contain the same strong validator, then the cache MUST responses contain the same strong validator, then the cache MUST
NOT use the new response to update any stored 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 for update. 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
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code 2xx; and, code 2xx; and,
o use other header fields provided in the HEAD response to replace o use other header fields provided in the HEAD response to replace
all instances of the corresponding header fields in the stored all instances of the corresponding header fields in the stored
response and append new header fields to the stored response's response and append new header fields to the stored response's
header section unless otherwise restricted by the Cache-Control header section unless otherwise restricted by the Cache-Control
header field. header field.
4.4. Invalidation 4.4. Invalidation
Because unsafe request methods (Section 4.2.1 of [Part2]) such as Because unsafe request methods (Section 4.2.1 of [RFC7231]) such as
PUT, POST or DELETE have the potential for changing state on the PUT, POST or DELETE have the potential for changing state on the
origin server, intervening caches can use them to keep their contents origin server, intervening caches can use them to keep their contents
up-to-date. up to date.
A cache MUST invalidate the effective Request URI (Section 5.5 of A cache MUST invalidate the effective Request URI (Section 5.5 of
[Part1]) as well as the URI(s) in the Location and Content-Location [RFC7230]) as well as the URI(s) in the Location and Content-Location
response header fields (if present) when a non-error status code is response header fields (if present) when a non-error status code is
received in response to an unsafe request method. received in response to an unsafe request method.
However, a cache MUST NOT invalidate a URI from a Location or However, a cache MUST NOT invalidate a URI from a Location or
Content-Location response header field if the host part of that URI Content-Location response header field if the host part of that URI
differs from the host part in the effective request URI (Section 5.5 differs from the host part in the effective request URI (Section 5.5
of [Part1]). This helps prevent denial of service attacks. of [RFC7230]). This helps prevent denial-of-service attacks.
A cache MUST invalidate the effective request URI (Section 5.5 of A cache MUST invalidate the effective request URI (Section 5.5 of
[Part1]) when it receives a non-error response to a request with a [RFC7230]) when it receives a non-error response to a request with a
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, a state-changing request might invalidate invalidated. For example, a state-changing request might invalidate
responses in the caches it travels through, but relevant responses responses in the caches it travels through, but relevant responses
still might be stored in other caches that it has not. still might be stored in other caches that it has not.
5. Header Field Definitions 5. Header Field Definitions
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Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.2.1)
The "max-age" request directive indicates that the client is The "max-age" request directive indicates that the client is
unwilling to accept a response whose age is greater than the unwilling to accept a response whose age is greater than the
specified number of seconds. Unless the max-stale request directive specified number of seconds. Unless the max-stale request directive
is also present, the client is not willing to accept a stale is also present, the client is not willing to accept a stale
response. response.
This directive uses the token form of the argument syntax; e.g., This directive uses the token form of the argument syntax: e.g.,
'max-age=5', not 'max-age="5"'. A sender SHOULD NOT generate the 'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the
quoted-string form. quoted-string form.
5.2.1.2. max-stale 5.2.1.2. max-stale
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.2.1)
The "max-stale" request directive indicates that the client is The "max-stale" request directive indicates that the client is
willing to accept a response that has exceeded its freshness willing to accept a response that has exceeded its freshness
lifetime. If max-stale is assigned a value, then the client is lifetime. If max-stale is assigned a value, then the client is
willing to accept a response that has exceeded its freshness lifetime willing to accept a response that has exceeded its freshness lifetime
by no more than the specified number of seconds. If no value is by no more than the specified number of seconds. If no value is
assigned to max-stale, then the client is willing to accept a stale assigned to max-stale, then the client is willing to accept a stale
response of any age. response of any age.
This directive uses the token form of the argument syntax; e.g., This directive uses the token form of the argument syntax: e.g.,
'max-stale=10', not 'max-stale="10"'. A sender SHOULD NOT generate 'max-stale=10' not 'max-stale="10"'. A sender SHOULD NOT generate
the quoted-string form. the quoted-string form.
5.2.1.3. min-fresh 5.2.1.3. min-fresh
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) 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.
This directive uses the token form of the argument syntax; e.g., This directive uses the token form of the argument syntax: e.g.,
'min-fresh=20', not 'min-fresh="20"'. A sender SHOULD NOT generate 'min-fresh=20' not 'min-fresh="20"'. A sender SHOULD NOT generate
the quoted-string form. the quoted-string form.
5.2.1.4. no-cache 5.2.1.4. no-cache
The "no-cache" request directive indicates that a cache MUST NOT use The "no-cache" request directive indicates that a cache MUST NOT use
a stored response to satisfy the request without successful a stored response to satisfy the request without successful
validation on the origin server. validation on the origin server.
5.2.1.5. no-store 5.2.1.5. no-store
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be vulnerable to eavesdropping. be vulnerable to eavesdropping.
Note that if a request containing this directive is satisfied from a Note that if a request containing this directive is satisfied from a
cache, the no-store request directive does not apply to the already cache, the no-store request directive does not apply to the already
stored response. stored response.
5.2.1.6. no-transform 5.2.1.6. no-transform
The "no-transform" request directive indicates that 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 [RFC7230].
5.2.1.7. only-if-cached 5.2.1.7. only-if-cached
The "only-if-cached" request directive indicates that the client only The "only-if-cached" request directive indicates that the client only
wishes to obtain a stored response. 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.
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This directive is NOT a reliable or sufficient mechanism for ensuring This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might recognize or obey this directive, and communications networks might
be vulnerable to eavesdropping. be vulnerable to eavesdropping.
5.2.2.4. no-transform 5.2.2.4. no-transform
The "no-transform" response directive indicates that an intermediary The "no-transform" response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT transform the (regardless of whether it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of [Part1]. payload, as defined in Section 5.7.2 of [RFC7230].
5.2.2.5. public 5.2.2.5. public
The "public" response directive indicates that any cache MAY store The "public" response directive indicates that any cache MAY store
the response, even if the response would normally be non-cacheable or the response, even if the response would normally be non-cacheable or
cacheable only within a private cache. (See Section 3.2 for cacheable only within a private cache. (See Section 3.2 for
additional details related to the use of public in response to a additional details related to the use of public in response to a
request containing Authorization, and Section 3 for details of how request containing Authorization, and Section 3 for details of how
public affects responses that would normally not be stored, due to public affects responses that would normally not be stored, due to
their status codes not being defined as cacheable by default; see their status codes not being defined as cacheable by default; see
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5.2.2.8. max-age 5.2.2.8. max-age
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.2.1)
The "max-age" response directive indicates that the response is to be The "max-age" response directive indicates that the response is to be
considered stale after its age is greater than the specified number considered stale after its age is greater than the specified number
of seconds. of seconds.
This directive uses the token form of the argument syntax; e.g., This directive uses the token form of the argument syntax: e.g.,
'max-age=5', not 'max-age="5"'. A sender SHOULD NOT generate the 'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the
quoted-string form. quoted-string form.
5.2.2.9. s-maxage 5.2.2.9. s-maxage
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.2.1)
The "s-maxage" response directive indicates that, in shared caches, The "s-maxage" response directive indicates that, in shared caches,
the maximum age specified by this directive overrides the maximum age the maximum age specified by this directive overrides the maximum age
specified by either the max-age directive or the Expires header specified by either the max-age directive or the Expires header
field. The s-maxage directive also implies the semantics of the field. The s-maxage directive also implies the semantics of the
proxy-revalidate response directive. proxy-revalidate response directive.
This directive uses the token form of the argument syntax; e.g., This directive uses the token form of the argument syntax: e.g.,
's-maxage=10', not 's-maxage="10"'. A sender SHOULD NOT generate the 's-maxage=10' not 's-maxage="10"'. A sender SHOULD NOT generate the
quoted-string form. quoted-string form.
5.2.3. Cache Control Extensions 5.2.3. Cache Control Extensions
The Cache-Control header field can be extended through the use of one The Cache-Control header field can be extended through the use of one
or more cache-extension tokens, each with an optional value. A cache or more cache-extension tokens, each with an optional value. A cache
MUST ignore unrecognized cache directives. MUST ignore unrecognized cache directives.
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
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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 4.2 for further discussion response is considered stale. See Section 4.2 for further discussion
of the freshness model. of the freshness model.
The presence of an Expires field does not imply that the original The presence of an Expires field does not imply that the original
resource will change or cease to exist at, before, or after that resource will change or cease to exist at, before, or after that
time. time.
The Expires value is an HTTP-date timestamp, as defined in Section The Expires value is an HTTP-date timestamp, as defined in Section
7.1.1.1 of [Part2]. 7.1.1.1 of [RFC7231].
Expires = HTTP-date Expires = HTTP-date
For example For example
Expires: Thu, 01 Dec 1994 16:00:00 GMT Expires: Thu, 01 Dec 1994 16:00:00 GMT
A cache recipient MUST interpret invalid date formats, especially the A cache recipient MUST interpret invalid date formats, especially the
value "0", as representing a time in the past (i.e., "already value "0", as representing a time in the past (i.e., "already
expired"). expired").
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they appear in the response. Senders that generate multiple Warning they appear in the response. Senders that generate multiple Warning
header fields are encouraged to order them with this user agent header fields are encouraged to order them with this user agent
behavior in mind. A sender that generates new Warning header fields behavior in mind. A sender that generates new Warning header fields
MUST append them after any existing Warning header fields. MUST append them after any existing Warning header fields.
Warnings are assigned three digit warn-codes. The first digit Warnings are assigned three digit warn-codes. The first digit
indicates whether the Warning is required to be deleted from a stored indicates whether the Warning is required to be deleted from a stored
response after validation: response after validation:
o 1xx warn-codes describe the freshness or validation status of the o 1xx warn-codes describe the freshness or validation status of the
response, and so MUST be deleted by a cache after validation. response, and so they MUST be deleted by a cache after validation.
They can only be generated by a cache when validating a cached They can only be generated by a cache when validating a cached
entry, and MUST NOT be generated in any other situation. entry, and MUST NOT be generated in any other situation.
o 2xx warn-codes describe some aspect of the representation that is o 2xx warn-codes describe some aspect of the representation that is
not rectified by a validation (for example, a lossy compression of not rectified by a validation (for example, a lossy compression of
the representation) and MUST NOT be deleted by a cache after the representation) and they MUST NOT be deleted by a cache after
validation, unless a full response is sent, in which case they validation, unless a full response is sent, in which case they
MUST be. MUST be.
If a sender generates one or more 1xx warn-codes in a message to be If a sender generates one or more 1xx warn-codes in a message to be
sent to a recipient known to implement only HTTP/1.0, the sender MUST sent to a recipient known to implement only HTTP/1.0, the sender MUST
include in each corresponding warning-value a warn-date that matches include in each corresponding warning-value a warn-date that matches
the Date header field in the message. For example: the Date header field in the message. For example:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Date: Sat, 25 Aug 2012 23:34:45 GMT Date: Sat, 25 Aug 2012 23:34:45 GMT
skipping to change at page 31, line 25 skipping to change at page 31, line 25
5.5.4. Warning: 113 - "Heuristic Expiration" 5.5.4. Warning: 113 - "Heuristic Expiration"
A cache SHOULD generate this if it heuristically chose a freshness A cache SHOULD generate this if it heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater than lifetime greater than 24 hours and the response's age is greater than
24 hours. 24 hours.
5.5.5. Warning: 199 - "Miscellaneous Warning" 5.5.5. Warning: 199 - "Miscellaneous Warning"
The warning text can include arbitrary information to be presented to The warning text can include arbitrary information to be presented to
a human user, or logged. A system receiving this warning MUST NOT a human user or logged. A system receiving this warning MUST NOT
take any automated action, besides presenting the warning to the take any automated action, besides presenting the warning to the
user. user.
5.5.6. Warning: 214 - "Transformation Applied" 5.5.6. Warning: 214 - "Transformation Applied"
MUST be added by a proxy if it applies any transformation to the This Warning code MUST be added by a proxy if it applies any
representation, such as changing the content-coding, media-type, or transformation to the representation, such as changing the content-
modifying the representation data, unless this Warning code already coding, media-type, or modifying the representation data, unless this
appears in the response. Warning code already appears in the response.
5.5.7. Warning: 299 - "Miscellaneous Persistent Warning" 5.5.7. Warning: 299 - "Miscellaneous Persistent Warning"
The warning text can include arbitrary information to be presented to The warning text can include arbitrary information to be presented to
a human user, or logged. A system receiving this warning MUST NOT a human user or logged. A system receiving this warning MUST NOT
take any automated action. take any automated action.
6. History Lists 6. 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 4.2) does not necessarily apply to The freshness model (Section 4.2) does not necessarily apply to
history mechanisms. I.e., a history mechanism can display a previous history mechanisms. That is, a history mechanism can display a
representation even if it has expired. previous 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).
7. IANA Considerations 7. IANA Considerations
7.1. Cache Directive Registry 7.1. Cache Directive Registry
The HTTP Cache Directive Registry defines the name space for the The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry"
cache directives. It will be created and maintained at (the defines the namespace for the cache directives. It has been created
suggested URI) and is now maintained at
<http://www.iana.org/assignments/http-cache-directives>. <http://www.iana.org/assignments/http-cache-directives>.
7.1.1. Procedure 7.1.1. Procedure
A registration MUST include the following fields: A registration MUST include the following fields:
o Cache Directive Name o Cache Directive Name
o Pointer to specification text o Pointer to specification text
Values to be added to this name space require IETF Review (see Values to be added to this namespace require IETF Review (see
[RFC5226], Section 4.1). [RFC5226], Section 4.1).
7.1.2. Considerations for New Cache Control Directives 7.1.2. Considerations for New Cache Control Directives
New extension directives ought to consider defining: New extension directives ought to consider defining:
o What it means for a directive to be specified multiple times, o What it means for a directive to be specified multiple times,
o When the directive does not take an argument, what it means when o When the directive does not take an argument, what it means when
an argument is present, an argument is present,
skipping to change at page 32, line 46 skipping to change at page 32, line 46
o When the directive requires an argument, what it means when it is o When the directive requires an argument, what it means when it is
missing, missing,
o Whether the directive is specific to requests, responses, or able o Whether the directive is specific to requests, responses, or able
to be used in either. to be used in either.
See also Section 5.2.3. See also Section 5.2.3.
7.1.3. Registrations 7.1.3. Registrations
The HTTP Cache Directive Registry shall be populated with the The registry has been populated with the registrations below:
registrations below:
+------------------------+----------------------------------+ +------------------------+----------------------------------+
| Cache Directive | Reference | | Cache Directive | Reference |
+------------------------+----------------------------------+ +------------------------+----------------------------------+
| max-age | Section 5.2.1.1, Section 5.2.2.8 | | max-age | Section 5.2.1.1, Section 5.2.2.8 |
| max-stale | Section 5.2.1.2 | | max-stale | Section 5.2.1.2 |
| min-fresh | Section 5.2.1.3 | | min-fresh | Section 5.2.1.3 |
| must-revalidate | Section 5.2.2.1 | | must-revalidate | Section 5.2.2.1 |
| no-cache | Section 5.2.1.4, Section 5.2.2.2 | | no-cache | Section 5.2.1.4, Section 5.2.2.2 |
| no-store | Section 5.2.1.5, Section 5.2.2.3 | | no-store | Section 5.2.1.5, Section 5.2.2.3 |
skipping to change at page 33, line 26 skipping to change at page 33, line 26
| private | Section 5.2.2.6 | | private | Section 5.2.2.6 |
| proxy-revalidate | Section 5.2.2.7 | | proxy-revalidate | Section 5.2.2.7 |
| public | Section 5.2.2.5 | | public | Section 5.2.2.5 |
| s-maxage | Section 5.2.2.9 | | s-maxage | Section 5.2.2.9 |
| stale-if-error | [RFC5861], Section 4 | | stale-if-error | [RFC5861], Section 4 |
| stale-while-revalidate | [RFC5861], Section 3 | | stale-while-revalidate | [RFC5861], Section 3 |
+------------------------+----------------------------------+ +------------------------+----------------------------------+
7.2. Warn Code Registry 7.2. Warn Code Registry
The HTTP Warn Code Registry defines the name space for warn codes. The "Hypertext Transfer Protocol (HTTP) Warn Codes" registry defines
It will be created and maintained at (the suggested URI) the namespace for warn codes. It has been created and is now
<http://www.iana.org/assignments/http-warn-codes>. maintained at <http://www.iana.org/assignments/http-warn-codes>.
7.2.1. Procedure 7.2.1. Procedure
A registration MUST include the following fields: A registration MUST include the following fields:
o Warn Code (3 digits) o Warn Code (3 digits)
o Short Description o Short Description
o Pointer to specification text o Pointer to specification text
Values to be added to this name space require IETF Review (see Values to be added to this namespace require IETF Review (see
[RFC5226], Section 4.1). [RFC5226], Section 4.1).
7.2.2. Registrations 7.2.2. Registrations
The HTTP Warn Code Registry shall be populated with the registrations The registry has been populated with the registrations below:
below:
+-----------+----------------------------------+---------------+ +-----------+----------------------------------+---------------+
| Warn Code | Short Description | Reference | | Warn Code | Short Description | Reference |
+-----------+----------------------------------+---------------+ +-----------+----------------------------------+---------------+
| 110 | Response is Stale | Section 5.5.1 | | 110 | Response is Stale | Section 5.5.1 |
| 111 | Revalidation Failed | Section 5.5.2 | | 111 | Revalidation Failed | Section 5.5.2 |
| 112 | Disconnected Operation | Section 5.5.3 | | 112 | Disconnected Operation | Section 5.5.3 |
| 113 | Heuristic Expiration | Section 5.5.4 | | 113 | Heuristic Expiration | Section 5.5.4 |
| 199 | Miscellaneous Warning | Section 5.5.5 | | 199 | Miscellaneous Warning | Section 5.5.5 |
| 214 | Transformation Applied | Section 5.5.6 | | 214 | Transformation Applied | Section 5.5.6 |
| 299 | Miscellaneous Persistent Warning | Section 5.5.7 | | 299 | Miscellaneous Persistent Warning | Section 5.5.7 |
+-----------+----------------------------------+---------------+ +-----------+----------------------------------+---------------+
7.3. Header Field Registration 7.3. Header Field Registration
HTTP header fields are registered within the Message Header Field HTTP header fields are registered within the "Message Headers"
Registry maintained at <http://www.iana.org/assignments/ registry maintained at
message-headers/message-header-index.html>. <http://www.iana.org/assignments/message-headers/>.
This document defines the following HTTP header fields, so their This document defines the following HTTP header fields, so the
associated registry entries shall be updated according to the "Permanent Message Header Field Names" registry has been updated
permanent registrations below (see [BCP90]): accordingly (see [BCP90]).
+-------------------+----------+----------+-------------+ +-------------------+----------+----------+-------------+
| Header Field Name | Protocol | Status | Reference | | Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+-------------+ +-------------------+----------+----------+-------------+
| Age | http | standard | Section 5.1 | | Age | http | standard | Section 5.1 |
| Cache-Control | http | standard | Section 5.2 | | Cache-Control | http | standard | Section 5.2 |
| Expires | http | standard | Section 5.3 | | Expires | http | standard | Section 5.3 |
| Pragma | http | standard | Section 5.4 | | Pragma | http | standard | Section 5.4 |
| Warning | http | standard | Section 5.5 | | Warning | http | standard | Section 5.5 |
+-------------------+----------+----------+-------------+ +-------------------+----------+----------+-------------+
The change controller is: "IETF (iesg@ietf.org) - Internet The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force". Engineering Task Force".
8. Security Considerations 8. Security Considerations
This section is meant to inform developers, information providers, This section is meant to inform developers, information providers,
and users of known security concerns specific to HTTP caching. More and users of known security concerns specific to HTTP caching. More
general security considerations are addressed in HTTP messaging general security considerations are addressed in HTTP messaging
[Part1] and semantics [Part2]. [RFC7230] and semantics [RFC7231].
Caches expose additional potential vulnerabilities, since the Caches expose additional potential vulnerabilities, since the
contents of the cache represent an attractive target for malicious contents of the cache represent an attractive target for malicious
exploitation. Because cache contents persist after an HTTP request exploitation. Because cache contents persist after an HTTP request
is complete, an attack on the cache can reveal information long after is complete, an attack on the cache can reveal information long after
a user believes that the information has been removed from the a user believes that the information has been removed from the
network. Therefore, cache contents need to be protected as sensitive network. Therefore, cache contents need to be protected as sensitive
information. information.
In particular, various attacks might be amplified by being stored in In particular, various attacks might be amplified by being stored in
a shared cache; such "cache poisoning" attacks use the cache to a shared cache; such "cache poisoning" attacks use the cache to
distribute a malicious payload to many clients, and are especially distribute a malicious payload to many clients, and are especially
effective when an attacker can use implementation flaws, elevated effective when an attacker can use implementation flaws, elevated
privileges, or other techniques to insert such a response into a privileges, or other techniques to insert such a response into a
cache. One common attack vector for cache poisoning is to exploit cache. One common attack vector for cache poisoning is to exploit
differences in message parsing on proxies and in user agents; see differences in message parsing on proxies and in user agents; see
Section 3.3.3 of [Part1] for the relevant requirements. Section 3.3.3 of [RFC7230] for the relevant requirements.
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.
Furthermore, the very use of a cache can bring about privacy Furthermore, the very use of a cache can bring about privacy
concerns. For example, if two users share a cache, and the first one concerns. For example, if two users share a cache, and the first one
browses to a site, the second may be able to detect that the other browses to a site, the second may be able to detect that the other
has been to that site, because the resources from it load more has been to that site, because the resources from it load more
quickly, thanks to the cache. quickly, thanks to the cache.
Note that the Set-Cookie response header field [RFC6265] does not Note that the Set-Cookie response header field [RFC6265] does not
inhibit caching; a cacheable response with a Set-Cookie header field inhibit caching; a cacheable response with a Set-Cookie header field
can be (and often is) used to satisfy subsequent requests to caches. can be (and often is) used to satisfy subsequent requests to caches.
Servers who wish to control caching of these responses are encouraged Servers who wish to control caching of these responses are encouraged
to emit appropriate Cache-Control response header fields. to emit appropriate Cache-Control response header fields.
9. Acknowledgments 9. Acknowledgments
See Section 10 of [Part1]. See Section 10 of [RFC7230].
10. References 10. References
10.1. Normative References 10.1. Normative References
[Part1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC7230] 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-26 (work in progress), RFC 7230, June 2014.
February 2014.
[Part2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7231] 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-26 (work in progress), draft-ietf-httpbis-p2-semantics-latest (work in progress),
February 2014. June 2014.
[Part4] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7232] 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-26 (work in progress), draft-ietf-httpbis-p4-conditional-latest (work in
February 2014. progress), June 2014.
[Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., [RFC7233] 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-26 (work in progress), draft-ietf-httpbis-p5-range-latest (work in progress),
February 2014. June 2014.
[Part7] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", Protocol (HTTP/1.1): Authentication",
draft-ietf-httpbis-p7-auth-26 (work in progress), draft-ietf-httpbis-p7-auth-latest (work in progress),
February 2014. June 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
10.2. Informative References 10.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration [BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864, Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004. September 2004.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
skipping to change at page 37, line 5 skipping to change at page 37, line 4
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011. April 2011.
Appendix A. Changes from RFC 2616 Appendix A. Changes from RFC 2616
The specification has been substantially rewritten for clarity. The specification has been substantially rewritten for clarity.
The conditions under which an authenticated response can be cached The conditions under which an authenticated response can be cached
have been clarified. (Section 3.2) have been clarified. (Section 3.2)
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. Caches are now allowed to calculate heuristic freshness with them. Caches are now allowed to calculate
heuristic freshness for URIs with query components. (Section 4.2.2) heuristic freshness for URIs with query components. (Section 4.2.2)
The algorithm for calculating age is now less conservative. Caches The algorithm for calculating age is now less conservative. Caches
are now required to handle dates with timezones as if they're are now required to handle dates with time zones as if they're
invalid, because it's not possible to accurately guess. invalid, because it's not possible to accurately guess.
(Section 4.2.3) (Section 4.2.3)
The Content-Location response header field is no longer used to The Content-Location response header field is no longer used to
determine the appropriate response to use when validating. determine the appropriate response to use when validating.
(Section 4.3) (Section 4.3)
The algorithm for selecting a cached negotiated response to use has The algorithm for selecting a cached negotiated response to use has
been clarified in several ways. In particular, it now explicitly been clarified in several ways. In particular, it now explicitly
allows header-specific canonicalization when processing selecting allows header-specific canonicalization when processing selecting
header fields. (Section 4.1) header fields. (Section 4.1)
Requirements regarding denial of service attack avoidance when Requirements regarding denial-of-service attack avoidance when
performing invalidation have been clarified. (Section 4.4) performing invalidation have been clarified. (Section 4.4)
Cache invalidation only occurs when a successful response is Cache invalidation only occurs when a successful response is
received. (Section 4.4) received. (Section 4.4)
Cache directives are explicitly defined to be case-insensitive. Cache directives are explicitly defined to be case-insensitive.
Handling of multiple instances of cache directives when only one is Handling of multiple instances of cache directives when only one is
expected is now defined. (Section 5.2) expected is now defined. (Section 5.2)
The "no-store" request directive doesn't apply to responses; i.e., a The "no-store" request directive doesn't apply to responses; i.e., a
cache can satisfy a request with no-store on it, and does not cache can satisfy a request with no-store on it and does not
invalidate it. (Section 5.2.1.5) invalidate it. (Section 5.2.1.5)
The qualified forms of the private and no-cache cache directives are The qualified forms of the private and no-cache cache directives are
noted to not be widely implemented; e.g., "private=foo" is noted to not be widely implemented; for example, "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 5.2.2) (Section 5.2.2)
The "no-cache" response directive's meaning has been clarified. The "no-cache" response directive's meaning has been clarified.
(Section 5.2.2.2) (Section 5.2.2.2)
The one-year limit on Expires header field values has been removed; The one-year limit on Expires header field values has been removed;
instead, the reasoning for using a sensible value is given. instead, the reasoning for using a sensible value is given.
(Section 5.3) (Section 5.3)
skipping to change at page 38, line 5 skipping to change at page 38, line 4
The "no-cache" response directive's meaning has been clarified. The "no-cache" response directive's meaning has been clarified.
(Section 5.2.2.2) (Section 5.2.2.2)
The one-year limit on Expires header field values has been removed; The one-year limit on Expires header field values has been removed;
instead, the reasoning for using a sensible value is given. instead, the reasoning for using a sensible value is given.
(Section 5.3) (Section 5.3)
The Pragma header field is now only defined for backwards The Pragma header field is now only defined for backwards
compatibility; future pragmas are deprecated. (Section 5.4) compatibility; future pragmas are deprecated. (Section 5.4)
Some requirements regarding production and processing of the Warning Some requirements regarding production and processing of the Warning
header fields have been relaxed, as it is not widely implemented. header fields have been relaxed, as it is not widely implemented.
Furthermore, the Warning header field no longer uses RFC 2047 Furthermore, the Warning header field no longer uses RFC 2047
encoding, nor allows multiple languages, as these aspects were not encoding, nor does it allow multiple languages, as these aspects were
implemented. (Section 5.5) not implemented. (Section 5.5)
This specification introduces the Cache Directive and Warn Code This specification introduces the Cache Directive and Warn Code
Registries, and defines considerations for new cache directives. Registries, and defines considerations for new cache directives.
(Section 7.1 and Section 7.2) (Section 7.1 and Section 7.2)
Appendix B. Imported ABNF Appendix B. Imported ABNF
The following core rules are included by reference, as defined in The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return), Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
character). character).
The rules below are defined in [Part1]: The rules below are defined in [RFC7230]:
OWS = <OWS, defined in [Part1], Section 3.2.3> OWS = <OWS, see [RFC7230], Section 3.2.3>
field-name = <field-name, defined in [Part1], Section 3.2> field-name = <field-name, see [RFC7230], Section 3.2>
quoted-string = <quoted-string, defined in [Part1], Section 3.2.6> quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>
token = <token, defined in [Part1], Section 3.2.6> token = <token, see [RFC7230], Section 3.2.6>
port = <port, defined in [Part1], Section 2.7> port = <port, see [RFC7230], Section 2.7>
pseudonym = <pseudonym, defined in [Part1], Section 5.7.1> pseudonym = <pseudonym, see [RFC7230], Section 5.7.1>
uri-host = <uri-host, defined in [Part1], Section 2.7> uri-host = <uri-host, see [RFC7230], 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, see [RFC7231], 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 In the collected ABNF below, list rules are expanded as per Section
1.2 of [Part1]. 1.2 of [RFC7230].
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, see [RFC7231], Section 7.1.1.1>
OWS = <OWS, defined in [Part1], Section 3.2.3> OWS = <OWS, see [RFC7230], Section 3.2.3>
Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
pragma-directive ] ) pragma-directive ] )
Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ] Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
) )
cache-directive = token [ "=" ( token / quoted-string ) ] cache-directive = token [ "=" ( token / quoted-string ) ]
delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
extension-pragma = token [ "=" ( token / quoted-string ) ] extension-pragma = token [ "=" ( token / quoted-string ) ]
field-name = <field-name, defined in [Part1], Section 3.2> field-name = <field-name, see [RFC7230], Section 3.2>
port = <port, defined in [Part1], Section 2.7> port = <port, see [RFC7230], Section 2.7>
pragma-directive = "no-cache" / extension-pragma pragma-directive = "no-cache" / extension-pragma
pseudonym = <pseudonym, defined in [Part1], Section 5.7.1> pseudonym = <pseudonym, see [RFC7230], Section 5.7.1>
quoted-string = <quoted-string, defined in [Part1], Section 3.2.6> quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>
token = <token, defined in [Part1], Section 3.2.6> token = <token, see [RFC7230], Section 3.2.6>
uri-host = <uri-host, defined in [Part1], Section 2.7> uri-host = <uri-host, see [RFC7230], Section 2.7>
warn-agent = ( uri-host [ ":" port ] ) / pseudonym warn-agent = ( uri-host [ ":" port ] ) / pseudonym
warn-code = 3DIGIT warn-code = 3DIGIT
warn-date = DQUOTE HTTP-date DQUOTE warn-date = DQUOTE HTTP-date DQUOTE
warn-text = quoted-string warn-text = quoted-string
warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
] ]
Appendix D. Change Log (to be removed by RFC Editor before publication)
Changes up to the IETF Last Call draft are summarized in <http://
trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-24#appendix-D>.
D.1. Since draft-ietf-httpbis-p6-cache-24
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/499>: "RFC 1305 ref
needs to be updated to 5905"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/500>: "dangling
reference to cacheable status codes"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/512>: "APPSDIR
review of draft-ietf-httpbis-p6-cache-24"
D.2. Since draft-ietf-httpbis-p6-cache-25
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/535>: "IESG ballot
on draft-ietf-httpbis-p6-cache-25"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/538>: "add
'stateless' to Abstract"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/542>: "improve
introduction of list rule"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/549>: "augment
security considerations with pointers to current research"
Index Index
1 1
110 (warn-code) 30 110 (warn-code) 30
111 (warn-code) 31 111 (warn-code) 31
112 (warn-code) 31 112 (warn-code) 31
113 (warn-code) 31 113 (warn-code) 31
199 (warn-code) 31 199 (warn-code) 31
2 2
214 (warn-code) 31 214 (warn-code) 31
299 (warn-code) 31 299 (warn-code) 31
A A
age 10 age 10
Age header field 20 Age header field 20
C C
cache 4 cache 4
cache entry 5 cache entry 5
cache key 5 cache key 5-6
Cache-Control header field 21 Cache-Control header field 21
D D
Disconnected Operation (warn-text) 31 Disconnected Operation (warn-text) 31
E E
Expires header field 27 Expires header field 27
explicit expiration time 10 explicit expiration time 10
F F
 End of changes. 102 change blocks. 
213 lines changed or deleted 173 lines changed or added

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