One document matched: draft-ietf-httpbis-p6-cache-19.xml
<?xml version="1.0" encoding="UTF-8"?>
<!--
This XML document is the output of clean-for-DTD.xslt; a tool that strips
extensions to RFC2629(bis) from documents for processing with xml2rfc.
-->
<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
<?rfc toc="yes" ?>
<?rfc symrefs="yes" ?>
<?rfc sortrefs="yes" ?>
<?rfc compact="yes"?>
<?rfc subcompact="no" ?>
<?rfc linkmailto="no" ?>
<?rfc editing="no" ?>
<?rfc comments="yes"?>
<?rfc inline="yes"?>
<?rfc rfcedstyle="yes"?>
<!DOCTYPE rfc
PUBLIC "" "rfc2629.dtd">
<rfc category="std" docName="draft-ietf-httpbis-p6-cache-19" ipr="pre5378Trust200902" obsoletes="2616">
<front>
<title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
<author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address>
<postal>
<street>345 Park Ave</street>
<city>San Jose</city>
<region>CA</region>
<code>95110</code>
<country>USA</country>
</postal>
<email>fielding@gbiv.com</email>
<uri>http://roy.gbiv.com/</uri>
</address>
</author>
<author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
<organization abbrev="W3C">World Wide Web Consortium</organization>
<address>
<postal>
<street>W3C / ERCIM</street>
<street>2004, rte des Lucioles</street>
<city>Sophia-Antipolis</city>
<region>AM</region>
<code>06902</code>
<country>France</country>
</postal>
<email>ylafon@w3.org</email>
<uri>http://www.raubacapeu.net/people/yves/</uri>
</address>
</author>
<author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
<organization>Rackspace</organization>
<address>
<email>mnot@mnot.net</email>
<uri>http://www.mnot.net/</uri>
</address>
</author>
<author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address>
<postal>
<street>Hafenweg 16</street>
<city>Muenster</city><region>NW</region><code>48155</code>
<country>Germany</country>
</postal>
<phone>+49 251 2807760</phone>
<facsimile>+49 251 2807761</facsimile>
<email>julian.reschke@greenbytes.de</email>
<uri>http://greenbytes.de/tech/webdav/</uri>
</address>
</author>
<date month="March" year="2012" day="12"/>
<workgroup>HTTPbis Working Group</workgroup>
<abstract>
<t>
The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
distributed, collaborative, hypertext information systems. HTTP has been in
use by the World Wide Web global information initiative since 1990. This
document is Part 6 of the seven-part specification that defines the protocol
referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
</t>
<t>
Part 6 defines requirements on HTTP caches and the associated header fields
that control cache behavior or indicate cacheable response messages.
</t>
</abstract>
<note title="Editorial Note (To be removed by RFC Editor)">
<t>
Discussion of this draft should take place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at
<eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
</t>
<t>
The current issues list is at
<eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
documents (including fancy diffs) can be found at
<eref target="http://tools.ietf.org/wg/httpbis/"/>.
</t>
<t>
The changes in this draft are summarized in <xref target="changes.since.18"/>.
</t>
</note>
</front>
<middle>
<section anchor="caching" title="Introduction">
<t>
HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. This document
defines aspects of HTTP/1.1 related to caching and reusing response
messages.
</t>
<section anchor="intro.purpose" title="Purpose">
<iref item="cache"/>
<t>
An HTTP cache is a local store of response messages and the
subsystem that controls its message storage, retrieval, and deletion. A
cache stores cacheable responses in order to reduce the response time and
network bandwidth consumption on future, equivalent requests. Any client or
server MAY employ a cache, though a cache cannot be used by a server that
is acting as a tunnel.
</t>
<t>
The goal of caching in HTTP/1.1 is to significantly improve performance
by reusing a prior response message to satisfy a current request.
A stored response is considered "fresh", as defined in
<xref target="expiration.model"/>, if the response can be reused without
"validation" (checking with the origin server to see if the cached response
remains valid for this request). A fresh cache response can therefore
reduce both latency and network transfers each time it is reused.
When a cached response is not fresh, it might still be reusable if it can
be freshened by validation (<xref target="validation.model"/>) or if the
origin is unavailable.
</t>
</section>
<section anchor="intro.terminology" title="Terminology">
<t>
This specification uses a number of terms to refer to the roles played by
participants in, and objects of, HTTP caching.
</t>
<t>
<iref item="cache"/>
<?rfc needLines="4"?>cache
<list>
<t>A conformant implementation of a HTTP cache. Note that this implies
an HTTP/1.1 cache; this specification does not define conformance
for HTTP/1.0 caches.</t>
</list>
</t>
<t anchor="shared.and.non-shared.caches">
<iref item="shared cache"/>
<?rfc needLines="4"?>shared cache
<list>
<t>A cache that stores responses to be reused by more than one user;
usually (but not always) deployed as part of an intermediary.</t>
</list>
</t>
<t>
<iref item="private cache"/>
<?rfc needLines="4"?>private cache
<list>
<t>A cache that is dedicated to a single user.</t>
</list>
</t>
<t>
<iref item="cacheable"/>
<?rfc needLines="4"?>cacheable
<list>
<t>A response is cacheable if a cache is allowed to store a copy of the
response message for use in answering subsequent requests. Even when a
response is cacheable, there might be additional constraints on whether
a cache can use the stored copy to satisfy a particular request.</t>
</list>
</t>
<t>
<iref item="explicit expiration time"/>
<?rfc needLines="4"?>explicit expiration time
<list>
<t>The time at which the origin server intends that a representation
no longer be returned by a cache without further validation.</t>
</list>
</t>
<t>
<iref item="heuristic expiration time"/>
<?rfc needLines="4"?>heuristic expiration time
<list>
<t>An expiration time assigned by a cache when no explicit expiration
time is available.</t>
</list>
</t>
<t>
<iref item="age"/>
<?rfc needLines="4"?>age
<list>
<t>The age of a response is the time since it was sent by, or
successfully validated with, the origin server.</t>
</list>
</t>
<t>
<iref item="first-hand"/>
<?rfc needLines="4"?>first-hand
<list>
<t>A response is first-hand if the freshness model is not in use; i.e.,
its age is 0.</t>
</list>
</t>
<t>
<iref item="freshness lifetime"/>
<?rfc needLines="4"?>freshness lifetime
<list>
<t>The length of time between the generation of a response and its
expiration time.</t>
</list>
</t>
<t>
<iref item="fresh"/>
<?rfc needLines="4"?>fresh
<list>
<t>A response is fresh if its age has not yet exceeded its freshness
lifetime.</t>
</list>
</t>
<t>
<iref item="stale"/>
<?rfc needLines="4"?>stale
<list>
<t>A response is stale if its age has passed its freshness lifetime
(either explicit or heuristic).</t>
</list>
</t>
<t>
<iref item="validator"/>
<?rfc needLines="4"?>validator
<list>
<t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
is used to find out whether a stored response is an equivalent copy of
a representation. See Section 2.1 of <xref target="Part4"/>.</t>
</list>
</t>
<t>
<iref item="strong validator"/>
<iref item="validator" subitem="strong"/>
<?rfc needLines="4"?>strong validator
<list>
<t>A validator that is defined by the origin server such that its
current value will change if the representation body changes; i.e.,
an entity-tag that is not marked as weak (Section 2.3 of <xref target="Part4"/>) or,
if no entity-tag is provided, a Last-Modified value that is strong
in the sense defined by Section 2.2.2 of <xref target="Part4"/>.</t>
</list>
</t>
</section>
<section title="Conformance and Error Handling" anchor="intro.conformance.and.error.handling">
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref target="RFC2119"/>.
</t>
<t>
This document defines conformance criteria for several roles in HTTP
communication, including Senders, Recipients, Clients, Servers, User-Agents,
Origin Servers, Intermediaries, Proxies and Gateways. See Section 2 of <xref target="Part1"/>
for definitions of these terms.
</t>
<t>
An implementation is considered conformant if it complies with all of the
requirements associated with its role(s). Note that SHOULD-level requirements
are relevant here, unless one of the documented exceptions is applicable.
</t>
<t>
This document also uses ABNF to define valid protocol elements
(<xref target="notation"/>). In addition to the prose requirements placed
upon them, Senders MUST NOT generate protocol elements that are invalid.
</t>
<t>
Unless noted otherwise, Recipients MAY take steps to recover a usable
protocol element from an invalid construct. However, HTTP does not define
specific error handling mechanisms, except in cases where it has direct
impact on security. This is because different uses of the protocol require
different error handling strategies; for example, a Web browser may wish to
transparently recover from a response where the Location header field
doesn't parse according to the ABNF, whereby in a systems control protocol
using HTTP, this type of error recovery could lead to dangerous consequences.
</t>
</section>
<section title="Syntax Notation" anchor="notation">
<t>
This specification uses the Augmented Backus-Naur Form (ABNF) notation
of <xref target="RFC5234"/> with the list rule extension defined in
Section 1.2 of <xref target="Part1"/>. <xref target="collected.abnf"/> shows the collected ABNF
with the list rule expanded.
</t>
<t>
The following core rules are included by reference, as defined in <xref target="RFC5234"/>, Appendix B.1: ALPHA (letters), CR (carriage
return), 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 8-bit
sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
</t>
<section title="Core Rules" anchor="core.rules">
<t>
The core rules below are defined in <xref target="Part1"/>:
</t>
<figure><artwork type="abnf2616"><![CDATA[
OWS = <OWS, defined in [Part1], Section 3.2.1>
quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>
token = <token, defined in [Part1], Section 3.2.4>
]]></artwork></figure>
</section>
<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
<t>
The ABNF rules below are defined in other parts:
</t>
<figure><artwork type="abnf2616"><![CDATA[
field-name = <field-name, defined in [Part1], Section 3.2>
HTTP-date = <HTTP-date, defined in [Part2], Section 8>
port = <port, defined in [Part1], Section 2.7>
pseudonym = <pseudonym, defined in [Part1], Section 6.2>
uri-host = <uri-host, defined in [Part1], Section 2.7>
]]></artwork></figure>
</section>
</section>
<section title="Delta Seconds" anchor="delta-seconds">
<t>
The delta-seconds rule specifies a non-negative integer, representing time
in seconds.
</t>
<figure><iref item="Grammar" primary="true" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
delta-seconds = 1*DIGIT
]]></artwork></figure>
<t>
If an implementation receives a delta-seconds value larger than the largest
positive integer it can represent, or if any of its subsequent calculations
overflows, it MUST consider the value to be 2147483648 (2^31).
Recipients parsing a delta-seconds value MUST use an arithmetic type of
at least 31 bits of range, and senders MUST NOT send delta-seconds with a
value greater than 2147483648.
</t>
</section>
</section>
<section anchor="caching.overview" title="Cache Operation">
<iref item="cache entry"/>
<iref item="cache key"/>
<t>
Proper cache operation preserves the semantics of HTTP transfers
(<xref target="Part2"/>) while eliminating the transfer of information already held
in the cache. Although caching is an entirely OPTIONAL feature of HTTP,
we assume that reusing the cached response is desirable and that such
reuse is the default behavior when no requirement or locally-desired
configuration prevents it. Therefore, HTTP cache requirements are focused
on preventing a cache from either storing a non-reusable response or
reusing a stored response inappropriately.
</t>
<t>
Each cache entry consists of a cache key and one or more
HTTP responses corresponding to prior requests that used the same key.
The most common form of cache entry is a successful result of a retrieval
request: i.e., a 200 (OK) response containing a representation of the
resource identified by the request target. However, it is also possible
to cache negative results (e.g., 404 not found), incomplete results
(e.g., 206 partial content), and responses to safe methods other than
GET if the method's definition allows such caching and defines something
suitable for use as a cache key.
</t>
<t>
The default cache key consists of the request method and
target URI. However, since HTTP caches in common use today are typically
limited to caching responses to GET, most implementations simply decline
other methods and use only the URI as the key.
</t>
<t>
If a request target is subject to content negotiation, its cache entry
might consist of multiple stored responses, each differentiated by a
secondary key for the values of the original request's selecting header
fields (<xref target="caching.negotiated.responses"/>).
</t>
<section anchor="response.cacheability" title="Response Cacheability">
<t>
A cache MUST NOT store a response to any request, unless:
<list style="symbols">
<t>The request method is understood by the cache and defined as being
cacheable, and</t>
<t>the response status code is understood by the cache, and</t>
<t>the "no-store" cache directive (see <xref target="header.cache-control"/>) does not appear in request or response
header fields, and</t>
<t>the "private" cache response directive (see <xref target="cache-response-directive"/>) does not appear in the response, if
the cache is shared, and</t>
<t>the "Authorization" header field (see Section 4.1 of <xref target="Part7"/>) does not
appear in the request, if the cache is shared, unless the response
explicitly allows it (see <xref target="caching.authenticated.responses"/>), and</t>
<t>the response either:
<list style="symbols">
<t>contains an Expires header field (see <xref target="header.expires"/>), or</t>
<t>contains a max-age response cache directive (see <xref target="cache-response-directive"/>), or</t>
<t>contains a s-maxage response cache directive and the cache is
shared, or</t>
<t>contains a Cache Control Extension (see <xref target="cache.control.extensions"/>) that allows it to be cached,
or</t>
<t>has a status code that can be served with heuristic freshness
(see <xref target="heuristic.freshness"/>).</t>
</list>
</t>
</list>
</t>
<t>
Note that any of the requirements listed above can be overridden by a
cache-control extension; see <xref target="cache.control.extensions"/>.
</t>
<t>
In this context, a cache has "understood" a request method or a response
status code if it recognizes it and implements any cache-specific
behavior.
</t>
<t>
Note that, in normal operation, most caches will not store a response that
has neither a cache validator nor an explicit expiration time, as such
responses are not usually useful to store. However, caches are not
prohibited from storing such responses.
</t>
<t>
A response message is considered complete when all of the octets
indicated by the message framing (<xref target="Part1"/>) are received
prior to the connection being closed.
If the request is GET, the response status is 200 (OK), and the entire
response header block has been received, a cache MAY store an incomplete
response message body if the cache entry is recorded as incomplete.
Likewise, a 206 (Partial Content) response MAY be stored as if it were
an incomplete 200 (OK) cache entry. However, a cache MUST NOT store
incomplete or partial content responses if it does not support the Range
and Content-Range header fields or if it does not understand the
range units used in those fields.
</t>
<t>
A cache MAY complete a stored incomplete response by making a subsequent
range request (<xref target="Part5"/>) and combining the successful response with the
stored entry, as defined in <xref target="combining.responses"/>.
A cache MUST NOT use an incomplete response to answer requests
unless the response has been made complete or the request is partial and
specifies a range that is wholly within the incomplete response.
A cache MUST NOT send a partial response to a client without explicitly
marking it as such using the 206 (Partial Content) status code.
</t>
</section>
<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
<t>
For a presented request, a cache MUST NOT return a stored response,
unless:
<list style="symbols">
<t>The presented effective request URI (Section 5.5 of <xref target="Part1"/>) and
that of the stored response match, and</t>
<t>the request method associated with the stored response allows it to
be used for the presented request, and</t>
<t>selecting header fields nominated by the stored response (if any)
match those presented (see <xref target="caching.negotiated.responses"/>), and</t>
<t>the presented request does not contain the no-cache pragma (<xref target="header.pragma"/>), nor the no-cache cache directive (<xref target="cache-request-directive"/>), unless the stored response is
successfully validated (<xref target="validation.model"/>), and</t>
<t>the stored response does not contain the no-cache cache directive
(<xref target="cache-response-directive"/>), unless it is successfully
validated (<xref target="validation.model"/>), and</t>
<t>the stored response is either:
<list style="symbols">
<t>fresh (see <xref target="expiration.model"/>), or</t>
<t>allowed to be served stale (see <xref target="serving.stale.responses"/>), or</t>
<t>successfully validated (see <xref target="validation.model"/>).</t>
</list>
</t>
</list>
</t>
<t>
Note that any of the requirements listed above can be overridden by a
cache-control extension; see <xref target="cache.control.extensions"/>.
</t>
<t>
When a stored response is used to satisfy a request without validation,
a cache MUST include a single Age header field (<xref target="header.age"/>) in the response with a value equal to the stored response's
current_age; see <xref target="age.calculations"/>.
</t>
<t>
A cache MUST write through requests with methods that are unsafe
(Section 6.1.1 of <xref target="Part2"/>) to the origin server; i.e., a cache must not generate
a reply to such a request before having forwarded the request and having
received a corresponding response.
</t>
<t>
Also, note that unsafe requests might invalidate already stored responses;
see <xref target="invalidation.after.updates.or.deletions"/>.
</t>
<t>
When more than one suitable response is stored, a cache MUST use the
most recent response (as determined by the Date header field). It can also
forward a request with "Cache-Control: max-age=0" or "Cache-Control:
no-cache" to disambiguate which response to use.
</t>
<t>
A cache that does not have a clock available MUST NOT use stored responses
without revalidating them on every use. A cache, especially a shared
cache, SHOULD use a mechanism, such as NTP <xref target="RFC1305"/>, to
synchronize its clock with a reliable external standard.
</t>
</section>
<section anchor="expiration.model" title="Freshness Model">
<t>
When a response is "fresh" in the cache, it can be used to satisfy
subsequent requests without contacting the origin server, thereby improving
efficiency.
</t>
<t>
The primary mechanism for determining freshness is for an origin server to
provide an explicit expiration time in the future, using either the Expires
header field (<xref target="header.expires"/>) or the max-age response cache
directive (<xref target="cache-response-directive"/>). Generally, origin
servers will assign future explicit expiration times to responses in the
belief that the representation is not likely to change in a semantically
significant way before the expiration time is reached.
</t>
<t>
If an origin server wishes to force a cache to validate every request, it
can assign an explicit expiration time in the past to indicate that the
response is already stale. Compliant caches will normally validate the
cached response before reusing it for subsequent requests (see <xref target="serving.stale.responses"/>).
</t>
<t>
Since origin servers do not always provide explicit expiration times,
a cache MAY assign a heuristic expiration time when an explicit time is not
specified, employing algorithms that use other header field values (such as the
Last-Modified time) to estimate a plausible expiration time. This
specification does not provide specific algorithms, but does impose
worst-case constraints on their results.
</t>
<figure>
<preamble>
The calculation to determine if a response is fresh is:
</preamble>
<artwork type="code"><![CDATA[
response_is_fresh = (freshness_lifetime > current_age)
]]></artwork>
</figure>
<t>
The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>; the current_age is defined in
<xref target="age.calculations"/>.
</t>
<t>
Additionally, clients can influence freshness calculation -- either
constraining it relaxing it -- by using the max-age and min-fresh
request cache directives. See <xref target="cache-request-directive"/>
for details.
</t>
<t>
Note that freshness applies only to cache operation; it cannot be used to
force a user agent to refresh its display or reload a resource. See <xref target="history.lists"/> for an explanation of the difference between
caches and history mechanisms.
</t>
<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
<t>
A cache can calculate the freshness lifetime (denoted as
freshness_lifetime) of a response by using the first match of:
<list style="symbols">
<t>If the cache is shared and the s-maxage response cache directive
(<xref target="cache-response-directive"/>) is present, use its value,
or</t>
<t>If the max-age response cache directive (<xref target="cache-response-directive"/>) is present, use its value, or</t>
<t>If the Expires response header field (<xref target="header.expires"/>) is
present, use its value minus the value of the Date response header field,
or</t>
<t>Otherwise, no explicit expiration time is present in the response. A
heuristic freshness lifetime might be applicable; see <xref target="heuristic.freshness"/>.</t>
</list>
</t>
<t>
Note that this calculation is not vulnerable to clock skew, since all of
the information comes from the origin server.
</t>
<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
<t>
If no explicit expiration time is present in a stored response that has a
status code whose definition allows heuristic freshness to be used
(including the following in Section 7 of <xref target="Part2"/>: 200, 203, 206, 300, 301 and
410), a cache MAY calculate a heuristic expiration time. A cache MUST NOT
use heuristics to determine freshness for responses with status codes that do
not explicitly allow it.
</t>
<t>
When a heuristic is used to calculate freshness lifetime, a cache
SHOULD attach a Warning header field with a 113 warn-code to the response if
its current_age is more than 24 hours and such a warning is not already
present.
</t>
<t>
Also, if the response has a Last-Modified header field
(Section 2.2 of <xref target="Part4"/>), caches are encouraged to use a heuristic
expiration value that is no more than some fraction of the interval since
that time. A typical setting of this fraction might be 10%.
</t>
<t><list>
<t>
Note: RFC 2616 (<xref target="RFC2616"/>, Section 13.9) required that caches do not calculate heuristic
freshness for URIs with query components (i.e., those containing '?').
In practice, this has not been widely implemented. Therefore, servers
are encouraged to send explicit directives (e.g., Cache-Control:
no-cache) if they wish to preclude caching.
</t>
</list></t>
</section>
</section>
<section anchor="age.calculations" title="Calculating Age">
<t>
HTTP/1.1 uses the Age header field to convey the estimated age of the
response message when obtained from a cache. The Age field value is the
cache's estimate of the amount of time since the response was generated or
validated by the origin server. In essence, the Age value is the sum of the
time that the response has been resident in each of the caches along the
path from the origin server, plus the amount of time it has been in transit
along network paths.
</t>
<t>
The following data is used for the age calculation:
</t>
<t>
<?rfc needLines="4"?>age_value
<list>
<t>
The term "age_value" denotes the value of the Age header field (<xref target="header.age"/>), in a form appropriate for arithmetic
operation; or 0, if not available.
</t>
</list>
</t>
<t>
<?rfc needLines="4"?>date_value
<list>
<t>
HTTP/1.1 requires origin servers to send a Date header field, if possible,
with every response, giving the time at which the response was
generated. The term "date_value" denotes the value of the Date
header field, in a form appropriate for arithmetic operations. See
Section 10.2 of <xref target="Part2"/> for the definition of the Date header field, and for
requirements regarding responses without it.
</t>
</list>
</t>
<t>
<?rfc needLines="4"?>now
<list>
<t>
The term "now" means "the current value of the clock at the host
performing the calculation". A cache SHOULD use NTP (<xref target="RFC1305"/>) or some similar protocol to synchronize its
clocks to a globally accurate time standard.
</t>
</list>
</t>
<t>
<?rfc needLines="4"?>request_time
<list>
<t>
The current value of the clock at the host at the time the request
resulting in the stored response was made.
</t>
</list>
</t>
<t>
<?rfc needLines="4"?>response_time
<list>
<t>
The current value of the clock at the host at the time the response
was received.
</t>
</list>
</t>
<t>
A response's age can be calculated in two entirely independent ways:
<list style="numbers">
<t>the "apparent_age": response_time minus date_value, if the local
clock is reasonably well synchronized to the origin server's clock. If
the result is negative, the result is replaced by zero.</t>
<t>the "corrected_age_value", if all of the caches along the response
path implement HTTP/1.1. A cache MUST interpret this value relative
to the time the request was initiated, not the time that the response
was received.</t>
</list>
</t>
<figure>
<artwork type="code"><![CDATA[
apparent_age = max(0, response_time - date_value);
response_delay = response_time - request_time;
corrected_age_value = age_value + response_delay;
]]></artwork>
</figure>
<figure>
<preamble>These SHOULD be combined as</preamble>
<artwork type="code"><![CDATA[
corrected_initial_age = max(apparent_age, corrected_age_value);
]]></artwork></figure>
<t>
unless the cache is confident in the value of the Age header (e.g., because
there are no HTTP/1.0 hops in the Via header), in which case the
corrected_age_value MAY be used as the corrected_initial_age.</t>
<t>
The current_age of a stored response can then be calculated by adding the
amount of time (in seconds) since the stored response was last validated by
the origin server to the corrected_initial_age.
</t>
<figure><artwork type="code"><![CDATA[
resident_time = now - response_time;
current_age = corrected_initial_age + resident_time;
]]></artwork></figure>
<t>
Additionally, to avoid common problems in date parsing:
</t>
<t>
<list style="symbols">
<t>HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date
which appears to be more than 50 years in the future is in fact
in the past (this helps solve the "year 2000" problem).</t>
<t>Although all date formats are specified to be case-sensitive,
recipients SHOULD match day, week and timezone names
case-insensitively.</t>
<t>An HTTP/1.1 implementation MAY internally represent a parsed
Expires date as earlier than the proper value, but MUST NOT
internally represent a parsed Expires date as later than the
proper value.</t>
<t>All expiration-related calculations MUST be done in GMT. The
local time zone MUST NOT influence the calculation or comparison
of an age or expiration time.</t>
<t>If an HTTP header field incorrectly carries a date value with a time
zone other than GMT, it MUST be converted into GMT using the
most conservative possible conversion.</t>
</list>
</t>
</section>
<section anchor="serving.stale.responses" title="Serving Stale Responses">
<t>
A "stale" response is one that either has explicit expiry information or is
allowed to have heuristic expiry calculated, but is not fresh according to
the calculations in <xref target="expiration.model"/>.
</t>
<t>
A cache MUST NOT return a stale response if it is prohibited by an
explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
directive, a "must-revalidate" cache-response-directive, or an applicable
"s-maxage" or "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
</t>
<t>
A cache MUST NOT return stale responses unless it is disconnected
(i.e., it cannot contact the origin server or otherwise find a forward
path) or doing so is explicitly allowed (e.g., by the max-stale request
directive; see <xref target="cache-request-directive"/>).
</t>
<t>
A cache SHOULD append a Warning header field with the 110 warn-code (see
<xref target="header.warning"/>) to stale responses. Likewise, a cache
SHOULD add the 112 warn-code to stale responses if the cache is
disconnected.
</t>
<t>
If a cache receives a first-hand response (either an entire response, or a
304 (Not Modified) response) that it would normally forward to the
requesting client, and the received response is no longer fresh, the cache
can forward it to the requesting client without adding a new Warning (but
without removing any existing Warning header fields). A cache shouldn't
attempt to validate a response simply because that response became stale in
transit.
</t>
</section>
</section>
<section anchor="validation.model" title="Validation Model">
<t>
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 one cannot
be selected; see <xref target="caching.negotiated.responses"/>), it can use
the conditional request mechanism <xref target="Part4"/> in the forwarded request to
give the origin server an opportunity to both select a valid stored
response to be used, and to update it. This process is known as
"validating" or "revalidating" the stored response.
</t>
<t>
When sending such a conditional request, a cache adds an If-Modified-Since
header field whose value is that of the Last-Modified header field from the
selected (see <xref target="caching.negotiated.responses"/>) stored
response, if available.
</t>
<t>
Additionally, a cache can add an If-None-Match header field whose value is
that of the ETag header field(s) from all responses stored for the
requested URI, if present. However, if any of the stored responses contains
only partial content, the cache shouldn't include its entity-tag in the
If-None-Match header field unless the request is for a range that would be
fully satisfied by that stored response.
</t>
<t>Cache handling of a response to a conditional request is dependent upon its status code:</t>
<t>
<list style="symbols">
<t>
A 304 (Not Modified) response status code indicates that the stored
response can be updated and reused; see <xref target="freshening.responses"/>.
</t>
<t>
A full response (i.e., one with a response body) indicates that none
of the stored responses nominated in the conditional request is
suitable. Instead, the cache can use the full response to
satisfy the request and MAY replace the stored response(s).
</t>
<t>
However, if a cache receives a 5xx response while attempting to
validate a response, it can either forward this response to the
requesting client, or act as if the server failed to respond. In the
latter case, it can return a previously stored response (see <xref target="serving.stale.responses"/>).
</t>
</list>
</t>
<section anchor="freshening.responses" title="Freshening Responses with 304 Not Modified">
<t>
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 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 information provided in
the 304 response.
<list style="symbols">
<t>
If the new response contains a strong validator, then that strong
validator identifies the selected representation. All of the stored
responses with the same strong validator are selected.
If none of the stored responses contain the same strong validator, then
this new response corresponds to a new selected representation and
MUST NOT update the existing stored responses.
</t>
<t>
If the new response contains a weak validator and that validator
corresponds to one of the cache's stored responses, then the most
recent of those matching stored responses is selected.
</t>
<t>
If the new response does not include any form of validator, there is
only one stored response, and that stored response also lacks a
validator, then that stored response is selected.
</t>
</list>
</t>
<t>
If a stored response is selected for update, the cache MUST:
<list style="symbols">
<t>delete any Warning header fields in the stored response with
warn-code 1xx (see <xref target="header.warning"/>);</t>
<t>retain any Warning header fields in the stored response with
warn-code 2xx; and,</t>
<t>use other header fields provided in the 304 response to replace
all instances of the corresponding header fields in the stored
response.</t>
</list>
</t>
</section>
</section>
<section anchor="head.effects" title="Updating Caches with HEAD Responses">
<t>
A response to the HEAD method is identical to what an equivalent request
made with a GET would have been, except it lacks a body. This property
of HEAD responses is used to both invalidate and update cached GET
responses.
</t>
<t>
If one or more stored GET responses can be selected (as per <xref target="caching.negotiated.responses"/>) for a HEAD request, and the
Content-Length, ETag or Last-Modified value of a HEAD response differs from
that in a selected GET response, the cache MUST consider that selected
response to be stale.
</t>
<t>
If the Content-Length, ETag and Last-Modified values of a HEAD response
(when present) are the same as that in a selected GET response (as per
<xref target="caching.negotiated.responses"/>), the cache SHOULD update the
remaining headers in the stored response using the following rules:
<list style="symbols">
<t>delete any Warning header fields in the stored response with
warn-code 1xx (see <xref target="header.warning"/>);</t>
<t>retain any Warning header fields in the stored response with
warn-code 2xx; and,</t>
<t>use other header fields provided in the response to replace
all instances of the corresponding header fields in the stored
response.</t>
</list>
</t>
</section>
<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
<t>
Because unsafe request methods (Section 6.1.1 of <xref target="Part2"/>) such as PUT, POST or DELETE
have the potential for changing state on the origin server, intervening
caches can use them to keep their contents up-to-date.
</t>
<t>
A cache MUST invalidate the effective Request URI
(Section 5.5 of <xref target="Part1"/>) as well as the URI(s) in the Location
and Content-Location response header fields (if present) when a non-error
response to a request with an unsafe method is received.
</t>
<t>
However, a cache MUST NOT invalidate a URI from a Location or
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 of <xref target="Part1"/>).
This helps prevent denial of service attacks.
</t>
<t>
A cache MUST invalidate the effective request URI
(Section 5.5 of <xref target="Part1"/>) when it receives a non-error response
to a request with a method whose safety is unknown.
</t>
<t>
Here, a "non-error response" is one with a 2xx or 3xx status code.
"Invalidate" means that the cache will either remove all stored
responses related to the effective request URI, or will mark these as
"invalid" and in need of a mandatory validation before they can be returned
in response to a subsequent request.
</t>
<t>
Note that this does not guarantee that all appropriate responses are
invalidated. For example, the request that caused the change at the origin
server might not have gone through the cache where a response is stored.
</t>
</section>
<section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
<t>
A shared cache MUST NOT use a cached response to a request with an
Authorization header field (Section 4.1 of <xref target="Part7"/>) to satisfy any subsequent
request unless a cache directive that allows such responses to be stored is
present in the response.
</t>
<t>
In this specification, the following Cache-Control response directives
(<xref target="cache-response-directive"/>) have such an effect:
must-revalidate, public, s-maxage.
</t>
<t>
Note that cached responses that contain the "must-revalidate" and/or
"s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a
response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
used to satisfy a subsequent request without revalidating it on the origin
server.
</t>
</section>
<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
<t>
When a cache receives a request that can be satisfied by a stored response
that has a Vary header field (<xref target="header.vary"/>), it MUST NOT
use that response unless all of the selecting header fields nominated by
the Vary header field match in both the original request (i.e., that associated
with the stored response), and the presented request.
</t>
<t>
The selecting header fields from two requests are defined to match if and
only if those in the first request can be transformed to those in the
second request by applying any of the following:
<list style="symbols">
<t>
adding or removing whitespace, where allowed in the header field's syntax
</t>
<t>
combining multiple header fields with the same field name
(see Section 3.2 of <xref target="Part1"/>)
</t>
<t>
normalizing both header field values in a way that is known to have
identical semantics, according to the header field's specification (e.g.,
re-ordering field values when order is not significant;
case-normalization, where values are defined to be case-insensitive)
</t>
</list>
</t>
<t>
If (after any normalization that might take place) a header field is absent
from a request, it can only match another request if it is also absent
there.
</t>
<t>
A Vary header field-value of "*" always fails to match, and subsequent
requests to that resource can only be properly interpreted by the origin
server.
</t>
<t>
The stored response with matching selecting header fields is known as the
selected response.
</t>
<t>
If multiple selected responses are available, the most recent response
(as determined by the Date header field) is used; see <xref target="constructing.responses.from.caches"/>.
</t>
<t>
If no selected response is available, the cache can forward the presented
request to the origin server in a conditional request; see <xref target="validation.model"/>.
</t>
</section>
<section anchor="combining.responses" title="Combining Partial Content">
<t>
A response might transfer only a partial representation if the
connection closed prematurely or if the request used one or more Range
specifiers (<xref target="Part5"/>). After several such transfers, a cache might have
received several ranges of the same representation. A cache MAY combine
these ranges into a single stored response, and reuse that response to
satisfy later requests, if they all share the same strong validator and
the cache complies with the client requirements in Section 4.2 of <xref target="Part5"/>.
</t>
<t>
When combining the new response with one or more stored responses, a
cache MUST:
<list style="symbols">
<t>delete any Warning header fields in the stored response with
warn-code 1xx (see <xref target="header.warning"/>);</t>
<t>retain any Warning header fields in the stored response with
warn-code 2xx; and,</t>
<t>use other header fields provided in the new response, aside
from Content-Range, to replace all instances of the corresponding
header fields in the stored response.</t>
</list>
</t>
</section>
</section>
<section anchor="header.field.definitions" title="Header Field Definitions">
<t>
This section defines the syntax and semantics of HTTP/1.1 header fields
related to caching.
</t>
<section anchor="header.age" title="Age">
<iref item="Age header field" primary="true"/>
<iref item="Header Fields" primary="true" subitem="Age"/>
<t>
The "Age" header field conveys the sender's estimate of the amount
of time since the response was generated or successfully validated at the
origin server. Age values are calculated as specified in <xref target="age.calculations"/>.
</t>
<figure><iref primary="true" item="Grammar" subitem="Age"/><artwork type="abnf2616"><![CDATA[
Age = delta-seconds
]]></artwork></figure>
<t>
Age field-values are non-negative integers, representing time in seconds
(see <xref target="delta-seconds"/>).
</t>
<t>
The presence of an Age header field in a response implies that a response
is not first-hand. However, the converse is not true, since HTTP/1.0 caches
might not implement the Age header field.
</t>
</section>
<section anchor="header.cache-control" title="Cache-Control">
<iref item="Cache-Control header field" primary="true"/>
<iref item="Header Fields" primary="true" subitem="Cache-Control"/>
<t>
The "Cache-Control" header field is used to specify directives for
caches along the request/response chain. Such cache directives are
unidirectional in that the presence of a directive in a request does not
imply that the same directive is to be given in the response.
</t>
<t>
A cache MUST obey the requirements of the Cache-Control
directives defined in this section. See <xref target="cache.control.extensions"/> for information about how Cache-Control
directives defined elsewhere are handled.
</t>
<t><list>
<t>
Note: HTTP/1.0 caches might not implement Cache-Control and
might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
</t>
</list></t>
<t>
A proxy, whether or not it implements a cache, MUST pass cache directives
through in forwarded messages, regardless of their
significance to that application, since the directives might be applicable
to all recipients along the request/response chain. It is not possible to
target a directive to a specific cache.
</t>
<t>Cache directives are identified by a token, to be compared case-insensitively, and have an optional argument.</t>
<figure><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-extension"/><artwork type="abnf2616"><![CDATA[
Cache-Control = 1#cache-directive
cache-directive = cache-request-directive
/ cache-response-directive
cache-extension = token [ "=" ( token / quoted-string ) ]
]]></artwork></figure>
<section anchor="cache-request-directive" title="Request Cache-Control Directives">
<figure><iref item="Grammar" primary="true" subitem="cache-request-directive"/><artwork type="abnf2616"><![CDATA[
cache-request-directive =
"no-cache"
/ "no-store"
/ "max-age" "=" delta-seconds
/ "max-stale" [ "=" delta-seconds ]
/ "min-fresh" "=" delta-seconds
/ "no-transform"
/ "only-if-cached"
/ cache-extension
]]></artwork></figure>
<t>
<?rfc needLines="4"?>no-cache
<iref item="Cache Directives" primary="true" subitem="no-cache"/>
<iref item="no-cache" primary="true" subitem="Cache Directive"/>
<list>
<t>The no-cache request directive indicates that a cache MUST NOT
use a stored response to satisfy the request without successful
validation on the origin server.</t>
</list>
</t>
<t>
<?rfc needLines="4"?>no-store
<iref item="Cache Directives" primary="true" subitem="no-store"/>
<iref item="no-store" primary="true" subitem="Cache Directive"/>
<list>
<t>The no-store request directive indicates that a cache MUST NOT
store any part of either this request or any response to it. This
directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a
best-effort attempt to remove the information from volatile storage as
promptly as possible after forwarding it.</t>
<t>This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might be
vulnerable to eavesdropping.</t>
<t>Note that if a request containing this directive is satisfied from a
cache, the no-store request directive does not apply to the already
stored response.</t>
</list>
</t>
<t>
<?rfc needLines="4"?>max-age
<iref item="Cache Directives" primary="true" subitem="max-age"/>
<iref item="max-age" primary="true" subitem="Cache Directive"/>
<list>
<t>The max-age request directive indicates that the client is unwilling to
accept a response whose age is greater than the specified number of
seconds. Unless the max-stale request directive is also present, the
client is not willing to accept a stale response.</t>
</list>
</t>
<t>
<?rfc needLines="4"?>max-stale
<iref item="Cache Directives" primary="true" subitem="max-stale"/>
<iref item="max-stale" primary="true" subitem="Cache Directive"/>
<list>
<t>The max-stale request directive indicates that the client is willing
to accept a response that has exceeded its expiration time. If max-stale
is assigned a value, then the client is willing to accept a response
that has exceeded its expiration time 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 response of any age.</t>
</list>
</t>
<t>
<?rfc needLines="4"?>min-fresh
<iref item="Cache Directives" primary="true" subitem="min-fresh"/>
<iref item="min-fresh" primary="true" subitem="Cache Directive"/>
<list>
<t>The min-fresh request directive indicates that the client is willing
to accept a response whose freshness lifetime is no less than 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 specified
number of seconds.</t>
</list>
</t>
<t>
<?rfc needLines="4"?>no-transform
<iref item="Cache Directives" primary="true" subitem="no-transform"/>
<iref item="no-transform" primary="true" subitem="Cache Directive"/>
<list>
<t>The no-transform request directive indicates that an intermediary
(whether or not it implements a cache) MUST NOT change the
Content-Encoding, Content-Range or Content-Type request header fields,
nor the request representation.</t>
</list>
</t>
<t>
<?rfc needLines="4"?>only-if-cached
<iref item="Cache Directives" primary="true" subitem="only-if-cached"/>
<iref item="only-if-cached" primary="true" subitem="Cache Directive"/>
<list>
<t>The only-if-cached request directive indicates that the client only
wishes to obtain a stored response. If it receives this directive, a
cache SHOULD either respond using a stored response that is consistent
with the other constraints of the request, or respond with a 504
(Gateway Timeout) status code. If a group of caches is being operated as
a unified system with good internal connectivity, a member cache MAY
forward such a request within that group of caches.</t>
</list>
</t>
</section>
<section anchor="cache-response-directive" title="Response Cache-Control Directives">
<figure><iref item="Grammar" primary="true" subitem="cache-response-directive"/><artwork type="abnf2616"><![CDATA[
cache-response-directive =
"public"
/ "private" [ "=" DQUOTE 1#field-name DQUOTE ]
/ "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
/ "no-store"
/ "no-transform"
/ "must-revalidate"
/ "proxy-revalidate"
/ "max-age" "=" delta-seconds
/ "s-maxage" "=" delta-seconds
/ cache-extension
]]></artwork></figure>
<t>
<?rfc needLines="4"?>public
<iref item="Cache Directives" primary="true" subitem="public"/>
<iref item="public" primary="true" subitem="Cache Directive"/>
<list>
<t>The public response directive indicates that a response whose
associated request contains an 'Authentication' header MAY be
stored (see <xref target="caching.authenticated.responses"/>).</t>
</list>
</t>
<t>
<?rfc needLines="4"?>private
<iref item="Cache Directives" primary="true" subitem="private"/>
<iref item="private" primary="true" subitem="Cache Directive"/>
<list>
<t>The private response directive indicates that the response message is
intended for a single user and MUST NOT be stored by a shared cache. A
private cache MAY store the response.</t>
<t>If the private response directive specifies one or more field-names,
this requirement is limited to the field-values associated with the
listed response header fields. That is, a shared cache MUST NOT store
the specified field-names(s), whereas it MAY store the remainder of the
response message.</t>
<t> Note: This usage of the word "private" only controls
where the response can be stored; it cannot ensure the privacy of the
message content. Also, private response directives with field-names are
often handled by implementations as if an unqualified private directive
was received; i.e., the special handling for the qualified form is not
widely implemented.</t>
</list>
</t>
<t>
<?rfc needLines="4"?>no-cache
<iref item="Cache Directives" primary="true" subitem="no-cache"/>
<iref item="no-cache" primary="true" subitem="Cache Directive"/>
<list>
<t>The no-cache response directive indicates that the response MUST NOT
be used to satisfy a subsequent request without successful validation on
the origin server. This allows an origin server to prevent a cache from
using it to satisfy a request without contacting it, even by caches that
have been configured to return stale responses.</t>
<t>If the no-cache response directive specifies one or more field-names,
then a cache MAY use the response to satisfy a subsequent request,
subject to any other restrictions on caching. However, any header fields
in the response that have the field-name(s) listed MUST NOT be sent
in the response to a subsequent request without successful revalidation
with the origin server. This allows an origin server to prevent the
re-use of certain header fields in a response, while still allowing
caching of the rest of the response.</t>
<t> Note: Most HTTP/1.0 caches will not recognize or obey
this directive. Also, no-cache response directives with field-names are
often handled by implementations as if an unqualified no-cache directive
was received; i.e., the special handling for the qualified form is not
widely implemented.</t>
</list>
</t>
<t>
<?rfc needLines="4"?>no-store
<iref item="Cache Directives" primary="true" subitem="no-store"/>
<iref item="no-store" primary="true" subitem="Cache Directive"/>
<list>
<t>The no-store response directive indicates that a cache MUST NOT
store any part of either the immediate request or response. This
directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a
best-effort attempt to remove the information from volatile storage as
promptly as possible after forwarding it.</t>
<t>This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might be
vulnerable to eavesdropping.</t>
</list>
</t>
<t>
<?rfc needLines="4"?>must-revalidate
<iref item="Cache Directives" primary="true" subitem="must-revalidate"/>
<iref item="must-revalidate" primary="true" subitem="Cache Directive"/>
<list>
<t>The must-revalidate response directive indicates that once it has
become stale, a cache MUST NOT use the response to satisfy subsequent
requests without successful validation on the origin server.</t>
<t>The must-revalidate directive is necessary to support reliable
operation for certain protocol features. In all circumstances a
cache MUST obey the must-revalidate directive; in particular,
if a cache cannot reach the origin server for any reason, it MUST
generate a 504 (Gateway Timeout) response.</t>
<t>The must-revalidate directive ought to be used by servers if and only
if failure to validate a request on the representation could result in
incorrect operation, such as a silently unexecuted financial
transaction.</t>
</list>
</t>
<t>
<?rfc needLines="4"?>proxy-revalidate
<iref item="Cache Directives" primary="true" subitem="proxy-revalidate"/>
<iref item="proxy-revalidate" primary="true" subitem="Cache Directive"/>
<list>
<t>The proxy-revalidate response directive has the same meaning as the
must-revalidate response directive, except that it does not apply to
private caches.</t>
</list>
</t>
<t>
<?rfc needLines="4"?>max-age
<iref item="Cache Directives" primary="true" subitem="max-age"/>
<iref item="max-age" primary="true" subitem="Cache Directive"/>
<list>
<t>The max-age response directive indicates that the response is to be
considered stale after its age is greater than the specified number of
seconds.</t>
</list>
</t>
<t>
<?rfc needLines="4"?>s-maxage
<iref item="Cache Directives" primary="true" subitem="s-maxage"/>
<iref item="s-maxage" primary="true" subitem="Cache Directive"/>
<list>
<t>The s-maxage response directive indicates that, in shared caches, the
maximum age specified by this directive overrides the maximum age
specified by either the max-age directive or the Expires header field. The
s-maxage directive also implies the semantics of the proxy-revalidate
response directive.</t>
</list>
</t>
<t>
<?rfc needLines="4"?>no-transform
<iref item="Cache Directives" primary="true" subitem="no-transform"/>
<iref item="no-transform" primary="true" subitem="Cache Directive"/>
<list>
<t>The no-transform response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT change the
Content-Encoding, Content-Range or Content-Type response header fields,
nor the response representation.</t>
</list>
</t>
</section>
<section anchor="cache.control.extensions" title="Cache Control Extensions">
<t>
The Cache-Control header field can be extended through the use of one or
more cache-extension tokens, each with an optional value. Informational
extensions (those that do not require a change in cache behavior) can be
added without changing the semantics of other directives. Behavioral
extensions are designed to work by acting as modifiers to the existing base
of cache directives. Both the new directive and the standard directive are
supplied, such that applications that do not understand the new directive
will default to the behavior specified by the standard directive, and those
that understand the new directive will recognize it as modifying the
requirements associated with the standard directive. In this way,
extensions to the cache-control directives can be made without requiring
changes to the base protocol.
</t>
<t>
This extension mechanism depends on an HTTP cache obeying all of the
cache-control directives defined for its native HTTP-version, obeying
certain extensions, and ignoring all directives that it does not
understand.
</t>
<t>
For example, consider a hypothetical new response directive called
"community" that acts as a modifier to the private directive. We define
this new directive to mean that, in addition to any private cache, any
cache that is shared only by members of the community named within its
value may cache the response. An origin server wishing to allow the UCI
community to use an otherwise private response in their shared cache(s)
could do so by including
</t>
<figure><artwork type="example"><![CDATA[
Cache-Control: private, community="UCI"
]]></artwork></figure>
<t>
A cache seeing this header field will act correctly even if the cache does
not understand the community cache-extension, since it will also see and
understand the private directive and thus default to the safe behavior.
</t>
<t>
A cache MUST ignore unrecognized cache directives; it is assumed that any
cache directive likely to be unrecognized by an HTTP/1.1 cache will be
combined with standard directives (or the response's default cacheability)
such that the cache behavior will remain minimally correct even if the
cache does not understand the extension(s).
</t>
<t>
The HTTP Cache Directive Registry defines the name space for the cache
directives.
</t>
<t>
A registration MUST include the following fields:
<list style="symbols">
<t>Cache Directive Name</t>
<t>Pointer to specification text</t>
</list>
</t>
<t>
Values to be added to this name space require IETF Review (see <xref target="RFC5226"/>, Section 4.1).
</t>
<t>
The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
</t>
</section>
</section>
<section anchor="header.expires" title="Expires">
<iref item="Expires header field" primary="true"/>
<iref item="Header Fields" primary="true" subitem="Expires"/>
<t>
The "Expires" header field gives the date/time after which the
response is considered stale. See <xref target="expiration.model"/> for
further discussion of the freshness model.
</t>
<t>
The presence of an Expires field does not imply that the original resource
will change or cease to exist at, before, or after that time.
</t>
<t>
The field-value is an absolute date and time as defined by HTTP-date in
Section 8 of <xref target="Part2"/>; a sender MUST use the rfc1123-date format.
</t>
<figure><iref primary="true" item="Grammar" subitem="Expires"/><artwork type="abnf2616"><![CDATA[
Expires = HTTP-date
]]></artwork></figure>
<figure>
<preamble>For example</preamble>
<artwork type="example"><![CDATA[
Expires: Thu, 01 Dec 1994 16:00:00 GMT
]]></artwork></figure>
<t>
A cache MUST treat other invalid date formats,
especially including the value "0", as in the past (i.e., "already
expired").
</t>
<t><list>
<t>
Note: If a response includes a Cache-Control field with the
max-age directive (see <xref target="cache-response-directive"/>),
that directive overrides the Expires field. Likewise, the s-maxage
directive overrides Expires in shared caches.
</t>
</list></t>
<t>
Historically, HTTP required the Expires field-value to be no more than a
year in the future. While longer freshness lifetimes are no longer
prohibited, extremely large values have been demonstrated to cause
problems (e.g., clock overflows due to use of 32-bit integers for
time values), and most caches will evict a response far sooner than
that. Therefore, senders ought not produce them.
</t>
<t>
An origin server without a clock MUST NOT assign Expires
values to a response unless these values were associated
with the resource by a system or user with a reliable clock. It MAY
assign an Expires value that is known, at or before server
configuration time, to be in the past (this allows "pre-expiration"
of responses without storing separate Expires values for each
resource).
</t>
</section>
<section anchor="header.pragma" title="Pragma">
<iref item="Pragma header field" primary="true"/>
<iref item="Header Fields" primary="true" subitem="Pragma"/>
<t>
The "Pragma" header field allows backwards compatibility with HTTP/1.0
caches, so that clients can specify a "no-cache" request that they will
understand (as Cache-Control was not defined until HTTP/1.1). When the
Cache-Control header is also present and understood in a request, Pragma is
ignored.
</t>
<t>
In HTTP/1.0, Pragma was defined as an extensible field for
implementation-specified directives for recipients. This specification
deprecates such extensions to improve interoperability.
</t>
<figure><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/><artwork type="abnf2616"><![CDATA[
Pragma = 1#pragma-directive
pragma-directive = "no-cache" / extension-pragma
extension-pragma = token [ "=" ( token / quoted-string ) ]
]]></artwork></figure>
<t>
When the Cache-Control header is not present in a request, the no-cache
request pragma-directive MUST have the same effect on caches as if
"Cache-Control: no-cache" were present (see <xref target="cache-request-directive"/>).
</t>
<t>
When sending a no-cache request, a client ought to include both the pragma
and cache-control directives, unless Cache-Control: no-cache is
purposefully omitted to target other Cache-Control response directives at
HTTP/1.1 caches. For example:
</t>
<figure>
<artwork type="message/http; msgtype="response""><![CDATA[
GET / HTTP/1.1
Host: www.example.com
Cache-Control: max-age=30
Pragma: no-cache
]]></artwork>
</figure>
<t>
will constrain HTTP/1.1 caches to serve a response no older than 30
seconds, while precluding implementations that do not understand
Cache-Control from serving a cached response.
</t>
<t><list>
<t>
Note: Because the meaning of "Pragma: no-cache" in responses is not
specified, it does not provide a reliable replacement for
"Cache-Control: no-cache" in them.
</t>
</list></t>
</section>
<section anchor="header.vary" title="Vary">
<iref item="Vary header field" primary="true"/>
<iref item="Header Fields" primary="true" subitem="Vary"/>
<t>
The "Vary" header field conveys the set of header fields
that were used to select the representation.
</t>
<t>
Caches use this information, in part, to determine whether a stored
response can be used to satisfy a given request; see <xref target="caching.negotiated.responses"/>. determines, while the response is
fresh, whether a cache is permitted to use the response to reply to a
subsequent request without validation; see <xref target="caching.negotiated.responses"/>.
</t>
<t>
In uncacheable or stale responses, the Vary field value advises the user
agent about the criteria that were used to select the representation.
</t>
<figure><iref primary="true" item="Grammar" subitem="Vary"/><artwork type="abnf2616"><![CDATA[
Vary = "*" / 1#field-name
]]></artwork></figure>
<t>
The set of header fields named by the Vary field value is known as the
selecting header fields.
</t>
<t>
A server SHOULD include a Vary header field with any cacheable response
that is subject to server-driven negotiation. Doing so allows a cache to
properly interpret future requests on that resource and informs the user
agent about the presence of negotiation on that resource. A server MAY
include a Vary header field with a non-cacheable response that is subject
to server-driven negotiation, since this might provide the user agent with
useful information about the dimensions over which the response varies at
the time of the response.
</t>
<t>
A Vary field value of "*" signals that unspecified parameters not limited
to the header fields (e.g., the network address of the client), play a
role in the selection of the response representation; therefore, a cache
cannot determine whether this response is appropriate. A proxy MUST NOT
generate the "*" value.
</t>
<t>
The field-names given are not limited to the set of standard header
fields defined by this specification. Field names are case-insensitive.
</t>
</section>
<section anchor="header.warning" title="Warning">
<iref item="Warning header field" primary="true"/>
<iref item="Header Fields" primary="true" subitem="Warning"/>
<t>
The "Warning" header field is used to carry additional information
about the status or transformation of a message that might not be reflected
in the message. This information is typically used to warn about possible
incorrectness introduced by caching operations or transformations applied
to the payload of the message.
</t>
<t>
Warnings can be used for other purposes, both cache-related and otherwise.
The use of a warning, rather than an error status code, distinguishes these
responses from true failures.
</t>
<t>
Warning header fields can in general be applied to any message, however some
warn-codes are specific to caches and can only be applied to response
messages.
</t>
<figure><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/><artwork type="abnf2616"><![CDATA[
Warning = 1#warning-value
warning-value = warn-code SP warn-agent SP warn-text
[SP warn-date]
warn-code = 3DIGIT
warn-agent = ( uri-host [ ":" port ] ) / pseudonym
; the name or pseudonym of the server adding
; the Warning header field, for use in debugging
warn-text = quoted-string
warn-date = DQUOTE HTTP-date DQUOTE
]]></artwork></figure>
<t>
Multiple warnings can be attached to a response (either by the origin
server or by a cache), including multiple warnings with the same code
number, only differing in warn-text.
</t>
<t>
When this occurs, the user agent SHOULD inform the user of as many of
them as possible, in the order that they appear in the response.
</t>
<t>
Systems that generate multiple Warning header fields are encouraged to
order them with this user agent behavior in mind. New Warning header fields
are added after any existing Warning headers fields.
</t>
<t>
Warnings are assigned three digit warn-codes. The first digit indicates
whether the Warning is required to be deleted from a stored response after
validation:
<list style="symbols">
<t>1xx Warnings describe the freshness or validation status of the
response, and so MUST be deleted by a cache after validation. They can
only be generated by a cache when validating a cached entry, and
MUST NOT be generated in any other situation.</t>
<t>2xx Warnings describe some aspect of the representation that is not
rectified by a validation (for example, a lossy compression of the
representation) and MUST NOT be deleted by a cache after validation,
unless a full response is returned, in which case they MUST be.</t>
</list>
</t>
<t>
If an implementation sends a message with one or more Warning header fields to a
receiver whose version is HTTP/1.0 or lower, then the sender MUST include
in each warning-value a warn-date that matches the Date header field in the
message.
</t>
<t>
If a system receives a message with a warning-value that includes
a warn-date, and that warn-date is different from the Date value in the
response, then that warning-value MUST be deleted from the message before
storing, forwarding, or using it. (preventing the consequences of naive
caching of Warning header fields.) If all of the warning-values are deleted
for this reason, the Warning header field MUST be deleted as well.
</t>
<t>
The following warn-codes are defined by this specification, each with a
recommended warn-text in English, and a description of its meaning.
</t>
<section title="110 Response is Stale" anchor="warn.110">
<iref primary="true" item="110 Response is Stale (warn code)"/>
<iref primary="true" item="Warn Codes" subitem="110 Response is Stale"/>
<t>
A cache SHOULD include this whenever the returned response is stale.
</t>
</section>
<section title="111 Revalidation Failed" anchor="warn.111">
<iref primary="true" item="111 Revalidation Failed (warn code)"/>
<iref primary="true" item="Warn Codes" subitem="111 Revalidation Failed"/>
<t>
A cache SHOULD include this when returning a stale response because an
attempt to validate the response failed, due to an inability to reach
the server.
</t>
</section>
<section title="112 Disconnected Operation" anchor="warn.112">
<iref primary="true" item="112 Disconnected Operation (warn code)"/>
<iref primary="true" item="Warn Codes" subitem="112 Disconnected Operation"/>
<t>
A cache SHOULD include this if it is intentionally disconnected from
the rest of the network for a period of time.
</t>
</section>
<section title="113 Heuristic Expiration" anchor="warn.113">
<iref primary="true" item="113 Heuristic Expiration (warn code)"/>
<iref primary="true" item="Warn Codes" subitem="113 Heuristic Expiration"/>
<t>
A cache SHOULD include this if it heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater than 24
hours.
</t>
</section>
<section title="199 Miscellaneous Warning" anchor="warn.199">
<iref primary="true" item="199 Miscellaneous Warning (warn code)"/>
<iref primary="true" item="Warn Codes" subitem="199 Miscellaneous Warning"/>
<t>
The warning text can include arbitrary information to be presented to
a human user, or logged. A system receiving this warning MUST NOT take
any automated action, besides presenting the warning to the user.
</t>
</section>
<section title="214 Transformation Applied" anchor="warn.214">
<iref primary="true" item="214 Transformation Applied (warn code)"/>
<iref primary="true" item="Warn Codes" subitem="214 Transformation Applied"/>
<t>
MUST be added by a proxy if it applies any transformation to the
representation, such as changing the content-coding, media-type, or
modifying the representation data, unless this Warning code already appears
in the response.
</t>
</section>
<section title="299 Miscellaneous Persistent Warning" anchor="warn.299">
<iref primary="true" item="299 Miscellaneous Persistent Warning (warn code)"/>
<iref primary="true" item="Warn Codes" subitem="299 Miscellaneous Persistent Warning"/>
<t>
The warning text can include arbitrary information to be presented to
a human user, or logged. A system receiving this warning MUST NOT take
any automated action.
</t>
</section>
<section title="Warn Code Extensions" anchor="warn.code.extensions">
<t>
The HTTP Warn Code Registry defines the name space for warn codes.
</t>
<t>
A registration MUST include the following fields:
<list style="symbols">
<t>Warn Code (3 digits)</t>
<t>Short Description</t>
<t>Pointer to specification text</t>
</list>
</t>
<t>
Values to be added to this name space require IETF Review (see <xref target="RFC5226"/>, Section 4.1).
</t>
<t>
The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-warn-codes"/>.
</t>
</section>
</section>
</section>
<section anchor="history.lists" title="History Lists">
<t>
User agents often have history mechanisms, such as "Back" buttons and
history lists, that can be used to redisplay a representation retrieved
earlier in a session.
</t>
<t>
The freshness model (<xref target="expiration.model"/>) does not
necessarily apply to history mechanisms. I.e., a history mechanism can
display a previous representation even if it has expired.
</t>
<t>
This does not prohibit the history mechanism from telling the user that a
view might be stale, or from honoring cache directives (e.g.,
Cache-Control: no-store).
</t>
</section>
<section anchor="IANA.considerations" title="IANA Considerations">
<section title="Cache Directive Registry" anchor="cache.directive.registration">
<t>
The registration procedure for HTTP Cache Directives is defined by <xref target="cache.control.extensions"/> of this document.
</t>
<t>
The HTTP Cache Directive Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
populated with the registrations below:
</t>
<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
<texttable align="left" suppress-title="true" anchor="iana.cache.directive.registration.table">
<ttcol>Cache Directive</ttcol>
<ttcol>Reference</ttcol>
<c>max-age</c>
<c>
<xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
</c>
<c>max-stale</c>
<c>
<xref target="cache-request-directive"/>
</c>
<c>min-fresh</c>
<c>
<xref target="cache-request-directive"/>
</c>
<c>must-revalidate</c>
<c>
<xref target="cache-response-directive"/>
</c>
<c>no-cache</c>
<c>
<xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
</c>
<c>no-store</c>
<c>
<xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
</c>
<c>no-transform</c>
<c>
<xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
</c>
<c>only-if-cached</c>
<c>
<xref target="cache-request-directive"/>
</c>
<c>private</c>
<c>
<xref target="cache-response-directive"/>
</c>
<c>proxy-revalidate</c>
<c>
<xref target="cache-response-directive"/>
</c>
<c>public</c>
<c>
<xref target="cache-response-directive"/>
</c>
<c>s-maxage</c>
<c>
<xref target="cache-response-directive"/>
</c>
<c>stale-if-error</c>
<c>
<xref target="RFC5861"/>, Section 4
</c>
<c>stale-while-revalidate</c>
<c>
<xref target="RFC5861"/>, Section 3
</c>
</texttable>
<!--(END)-->
</section>
<section title="Warn Code Registry" anchor="warn.code.registration">
<t>
The registration procedure for HTTP Warn Codes is defined by <xref target="warn.code.extensions"/> of this document.
</t>
<t>
The HTTP Warn Code Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
populated with the registrations below:
</t>
<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
<ttcol>Warn Code</ttcol>
<ttcol>Short Description</ttcol>
<ttcol>Reference</ttcol>
<c>110</c>
<c>Response is Stale</c>
<c>
<xref target="warn.110"/>
</c>
<c>111</c>
<c>Revalidation Failed</c>
<c>
<xref target="warn.111"/>
</c>
<c>112</c>
<c>Disconnected Operation</c>
<c>
<xref target="warn.112"/>
</c>
<c>113</c>
<c>Heuristic Expiration</c>
<c>
<xref target="warn.113"/>
</c>
<c>199</c>
<c>Miscellaneous Warning</c>
<c>
<xref target="warn.199"/>
</c>
<c>214</c>
<c>Transformation Applied</c>
<c>
<xref target="warn.214"/>
</c>
<c>299</c>
<c>Miscellaneous Persistent Warning</c>
<c>
<xref target="warn.299"/>
</c>
</texttable>
<!--(END)-->
</section>
<section title="Header Field Registration" anchor="header.field.registration">
<t>
The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>
shall be updated with the permanent registrations below (see <xref target="RFC3864"/>):
</t>
<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
<ttcol>Header Field Name</ttcol>
<ttcol>Protocol</ttcol>
<ttcol>Status</ttcol>
<ttcol>Reference</ttcol>
<c>Age</c>
<c>http</c>
<c>standard</c>
<c>
<xref target="header.age"/>
</c>
<c>Cache-Control</c>
<c>http</c>
<c>standard</c>
<c>
<xref target="header.cache-control"/>
</c>
<c>Expires</c>
<c>http</c>
<c>standard</c>
<c>
<xref target="header.expires"/>
</c>
<c>Pragma</c>
<c>http</c>
<c>standard</c>
<c>
<xref target="header.pragma"/>
</c>
<c>Vary</c>
<c>http</c>
<c>standard</c>
<c>
<xref target="header.vary"/>
</c>
<c>Warning</c>
<c>http</c>
<c>standard</c>
<c>
<xref target="header.warning"/>
</c>
</texttable>
<!--(END)-->
<t>
The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
Force".
</t>
</section>
</section>
<section anchor="security.considerations" title="Security Considerations">
<t>
Caches expose additional potential vulnerabilities, since the contents of
the cache represent an attractive target for malicious exploitation.
Because cache contents persist after an HTTP request is complete, an attack
on the cache can reveal information long after a user believes that the
information has been removed from the network. Therefore, cache contents
need to be protected as sensitive information.
</t>
</section>
<section title="Acknowledgments" anchor="acks">
<t>
See Section 9 of <xref target="Part1"/>.
</t>
</section>
</middle>
<back>
<references title="Normative References">
<reference anchor="Part1">
<front>
<title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
<author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
<organization abbrev="W3C">World Wide Web Consortium</organization>
<address><email>ylafon@w3.org</email></address>
</author>
<author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address><email>julian.reschke@greenbytes.de</email></address>
</author>
<date month="March" year="2012"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-19"/>
</reference>
<reference anchor="Part2">
<front>
<title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
<author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
<organization abbrev="W3C">World Wide Web Consortium</organization>
<address><email>ylafon@w3.org</email></address>
</author>
<author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address><email>julian.reschke@greenbytes.de</email></address>
</author>
<date month="March" year="2012"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-19"/>
</reference>
<reference anchor="Part4">
<front>
<title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
<author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
<organization abbrev="W3C">World Wide Web Consortium</organization>
<address><email>ylafon@w3.org</email></address>
</author>
<author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address><email>julian.reschke@greenbytes.de</email></address>
</author>
<date month="March" year="2012"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-19"/>
</reference>
<reference anchor="Part5">
<front>
<title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
<author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
<organization abbrev="W3C">World Wide Web Consortium</organization>
<address><email>ylafon@w3.org</email></address>
</author>
<author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address><email>julian.reschke@greenbytes.de</email></address>
</author>
<date month="March" year="2012"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-19"/>
</reference>
<reference anchor="Part7">
<front>
<title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
<author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
<organization abbrev="W3C">World Wide Web Consortium</organization>
<address><email>ylafon@w3.org</email></address>
</author>
<author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address><email>julian.reschke@greenbytes.de</email></address>
</author>
<date month="March" year="2012"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-19"/>
</reference>
<reference anchor="RFC2119">
<front>
<title>Key words for use in RFCs to Indicate Requirement Levels</title>
<author fullname="Scott Bradner" initials="S." surname="Bradner">
<organization>Harvard University</organization>
<address><email>sob@harvard.edu</email></address>
</author>
<date month="March" year="1997"/>
</front>
<seriesInfo name="BCP" value="14"/>
<seriesInfo name="RFC" value="2119"/>
</reference>
<reference anchor="RFC5234">
<front>
<title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
<author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
<organization>Brandenburg InternetWorking</organization>
<address>
<email>dcrocker@bbiw.net</email>
</address>
</author>
<author initials="P." surname="Overell" fullname="Paul Overell">
<organization>THUS plc.</organization>
<address>
<email>paul.overell@thus.net</email>
</address>
</author>
<date month="January" year="2008"/>
</front>
<seriesInfo name="STD" value="68"/>
<seriesInfo name="RFC" value="5234"/>
</reference>
</references>
<references title="Informative References">
<reference anchor="RFC1305">
<front>
<title>Network Time Protocol (Version 3) Specification, Implementation</title>
<author fullname="David L. Mills" initials="D." surname="Mills">
<organization>University of Delaware, Electrical Engineering Department</organization>
<address><email>mills@udel.edu</email></address>
</author>
<date month="March" year="1992"/>
</front>
<seriesInfo name="RFC" value="1305"/>
</reference>
<reference anchor="RFC2616">
<front>
<title>Hypertext Transfer Protocol -- HTTP/1.1</title>
<author fullname="R. Fielding" initials="R." surname="Fielding">
<organization>University of California, Irvine</organization>
<address><email>fielding@ics.uci.edu</email></address>
</author>
<author fullname="J. Gettys" initials="J." surname="Gettys">
<organization>W3C</organization>
<address><email>jg@w3.org</email></address>
</author>
<author fullname="J. Mogul" initials="J." surname="Mogul">
<organization>Compaq Computer Corporation</organization>
<address><email>mogul@wrl.dec.com</email></address>
</author>
<author fullname="H. Frystyk" initials="H." surname="Frystyk">
<organization>MIT Laboratory for Computer Science</organization>
<address><email>frystyk@w3.org</email></address>
</author>
<author fullname="L. Masinter" initials="L." surname="Masinter">
<organization>Xerox Corporation</organization>
<address><email>masinter@parc.xerox.com</email></address>
</author>
<author fullname="P. Leach" initials="P." surname="Leach">
<organization>Microsoft Corporation</organization>
<address><email>paulle@microsoft.com</email></address>
</author>
<author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
<organization>W3C</organization>
<address><email>timbl@w3.org</email></address>
</author>
<date month="June" year="1999"/>
</front>
<seriesInfo name="RFC" value="2616"/>
</reference>
<reference anchor="RFC3864">
<front>
<title>Registration Procedures for Message Header Fields</title>
<author fullname="G. Klyne" initials="G." surname="Klyne">
<organization>Nine by Nine</organization>
<address><email>GK-IETF@ninebynine.org</email></address>
</author>
<author fullname="M. Nottingham" initials="M." surname="Nottingham">
<organization>BEA Systems</organization>
<address><email>mnot@pobox.com</email></address>
</author>
<author fullname="J. Mogul" initials="J." surname="Mogul">
<organization>HP Labs</organization>
<address><email>JeffMogul@acm.org</email></address>
</author>
<date month="September" year="2004"/>
</front>
<seriesInfo name="BCP" value="90"/>
<seriesInfo name="RFC" value="3864"/>
</reference>
<reference anchor="RFC5226">
<front>
<title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
<author initials="T." surname="Narten" fullname="T. Narten">
<organization>IBM</organization>
<address><email>narten@us.ibm.com</email></address>
</author>
<author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
<organization>Google</organization>
<address><email>Harald@Alvestrand.no</email></address>
</author>
<date year="2008" month="May"/>
</front>
<seriesInfo name="BCP" value="26"/>
<seriesInfo name="RFC" value="5226"/>
</reference>
<reference anchor="RFC5861">
<front>
<title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
<author initials="M." surname="Nottingham" fullname="Mark Nottingham">
<organization>Yahoo! Inc.</organization>
<address><email>mnot@yahoo-inc.com</email></address>
</author>
<date month="April" year="2010"/>
</front>
<seriesInfo name="RFC" value="5861"/>
</reference>
</references>
<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
<t>
Make the specified age calculation algorithm less conservative.
(<xref target="age.calculations"/>)
</t>
<t>
Remove requirement to consider Content-Location in successful responses
in order to determine the appropriate response to use.
(<xref target="validation.model"/>)
</t>
<t>
Clarify denial of service attack avoidance requirement.
(<xref target="invalidation.after.updates.or.deletions"/>)
</t>
<t>
Change ABNF productions for header fields to only define the field value.
(<xref target="header.field.definitions"/>)
</t>
<t>
Do not mention RFC 2047 encoding and multiple languages in Warning header fields
anymore, as these aspects never were implemented.
(<xref target="header.warning"/>)
</t>
</section>
<section title="Collected ABNF" anchor="collected.abnf">
<figure>
<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
Age = delta-seconds
Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
cache-directive ] )
Expires = HTTP-date
HTTP-date = <HTTP-date, defined in [Part2], Section 8>
OWS = <OWS, defined in [Part1], Section 3.2.1>
Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
pragma-directive ] )
Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
) )
Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
)
cache-directive = cache-request-directive / cache-response-directive
cache-extension = token [ "=" ( token / quoted-string ) ]
cache-request-directive = "no-cache" / "no-store" / ( "max-age="
delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
"min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
cache-extension
cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
"no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
"must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
) / ( "s-maxage=" delta-seconds ) / cache-extension
delta-seconds = 1*DIGIT
extension-pragma = token [ "=" ( token / quoted-string ) ]
field-name = <field-name, defined in [Part1], Section 3.2>
port = <port, defined in [Part1], Section 2.7>
pragma-directive = "no-cache" / extension-pragma
pseudonym = <pseudonym, defined in [Part1], Section 6.2>
quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>
token = <token, defined in [Part1], Section 3.2.4>
uri-host = <uri-host, defined in [Part1], Section 2.7>
warn-agent = ( uri-host [ ":" port ] ) / pseudonym
warn-code = 3DIGIT
warn-date = DQUOTE HTTP-date DQUOTE
warn-text = quoted-string
warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
]
]]></artwork>
</figure>
<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
; Age defined but not used
; Cache-Control defined but not used
; Expires defined but not used
; Pragma defined but not used
; Vary defined but not used
; Warning defined but not used
]]></artwork></figure></section>
<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
<section title="Since RFC 2616">
<t>Extracted relevant partitions from <xref target="RFC2616"/>.</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-00">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/12"/>: "Invalidation after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd"/>)</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>: "Normative and Informative references"</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>: "Date reference typo"</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>: "Connection header text"</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>: "Informative references"</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>: "ISO-8859-1 Reference"</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>: "Normative up-to-date references"</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>: "typo in 13.2.2"</t>
</list>
</t>
<t>
Other changes:
<list style="symbols">
<t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-01">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>: "rel_path not used"</t>
</list>
</t>
<t>
Other changes:
<list style="symbols">
<t>Get rid of duplicate BNF rule names ("host" -> "uri-host") (work in progress
on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)</t>
<t>Add explicit references to BNF syntax and rules imported from other parts of the
specification.</t>
</list>
</t>
</section>
<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
<t>
Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
<list style="symbols">
<t>Reference RFC 3984, and update header field registrations for header fields defined in this
document.</t>
</list>
</t>
</section>
<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>: "Vary header classification"</t>
</list>
</t>
</section>
<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
<t>
Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
<list style="symbols">
<t>
Use "/" instead of "|" for alternatives.
</t>
<t>
Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
whitespace ("OWS") and required whitespace ("RWS").
</t>
<t>
Rewrite ABNFs to spell out whitespace rules, factor out
header field value format definitions.
</t>
</list>
</t>
</section>
<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
<t>
This is a total rewrite of this part of the specification.
</t>
<t>
Affected issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>: "Definition of 1xx Warn-Codes"</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>: "Placement of 13.5.1 and 13.5.2"</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138"/>: "The role of Warning and Semantic Transparency in Caching"</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139"/>: "Methods and Caching"</t>
</list>
</t>
<t>
In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
<list style="symbols">
<t>
Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
</t>
</list>
</t>
</section>
<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
"base for numeric protocol elements"
</t>
</list>
</t>
<t>
Affected issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
"Vary and non-existant headers"
</t>
</list>
</t>
</section>
<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>:
"Definition of 1xx Warn-Codes"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
"Content-Location on 304 responses"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169"/>:
"private and no-cache CC directives with headers"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
"RFC2047 and warn-text"
</t>
</list>
</t>
</section>
<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147"/>:
"serving negotiated responses from cache: header-specific canonicalization"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197"/>:
"Effect of CC directives on history lists"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291"/>:
"Cache Extensions can override no-store, etc."
</t>
</list>
</t>
<t>
Affected issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
Status codes and caching
</t>
</list>
</t>
<t>
Partly resolved issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
"Placement of 13.5.1 and 13.5.2"
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29"/>:
"Age calculation"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168"/>:
"Clarify differences between / requirements for request and response CC directives"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174"/>:
"Caching authenticated responses"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208"/>:
"IANA registry for cache-control directives"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211"/>:
"Heuristic caching of URLs with query components"
</t>
</list>
</t>
<t>
Partly resolved issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
"Term for the requested resource's URI"
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
"Clarify entity / representation / variant terminology"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
"consider removing the 'changes from 2068' sections"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
"Allowing heuristic caching for new status codes"
</t>
<t>
Clean up TODOs and prose in "Combining Responses."
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
"Text about clock requirement for caches belongs in p6"
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
"Header Classification"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/268"/>:
"Clarify 'public'"
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-13" anchor="changes.since.13">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
"untangle ABNFs for header fields"
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-14" anchor="changes.since.14">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/38"/>:
"Mismatch Vary"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/235"/>:
"Cache Invalidation only happens upon successful responses"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/282"/>:
"Recommend minimum sizes for protocol elements"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/289"/>:
"Proxies don't 'understand' methods"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291"/>:
"Cache Extensions can override no-store, etc."
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/292"/>:
"Pragma"
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-15" anchor="changes.since.15">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/290"/>:
"Motivate one-year limit for Expires"
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-16" anchor="changes.since.16">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/186"/>:
"Document HTTP's error-handling philosophy"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/317"/>:
"Cache-Control directive case sensitivity"
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-17" anchor="changes.since.17">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/293"/>:
"Interaction of request and response Cache-Control"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/212"/>:
"Refining age for 1.1 proxy chains"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/274"/>:
"warn-code registry"
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-18" anchor="changes.since.18">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/227"/>:
"Combining HEAD responses"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/337"/>:
"Field names in cache-control header arguments"
</t>
</list>
</t>
</section>
</section>
</back>
</rfc>| PAFTECH AB 2003-2026 | 2026-04-23 14:17:53 |