One document matched: draft-ietf-httpbis-p6-cache-23.xml
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<rfc category="std" docName="draft-ietf-httpbis-p6-cache-23" ipr="pre5378Trust200902" obsoletes="2616">
<front>
<title abbrev="HTTP/1.1 Caching">Hypertext Transfer Protocol (HTTP/1.1): 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="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
<organization>Akamai</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>
<email>julian.reschke@greenbytes.de</email>
<uri>http://greenbytes.de/tech/webdav/</uri>
</address>
</author>
<date month="July" year="2013" day="15"/>
<workgroup>HTTPbis Working Group</workgroup>
<abstract>
<t>
The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
distributed, collaborative, hypertext information systems. This document
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 takes 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.22"/>.
</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>
<iref item="cache"/>
<t>
An HTTP cache is a local store of response messages and the
subsystem that controls storage, retrieval, and deletion of messages in it.
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>
<iref item="shared cache"/>
<iref item="private cache"/>
<t anchor="shared.and.non-shared.caches">
A shared cache is a cache that stores responses to be reused
by more than one user; shared caches are usually (but not always) deployed
as a part of an intermediary. A private cache, in contrast,
is dedicated to a single user.
</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 response can therefore
reduce both latency and network overhead each time it is reused.
When a cached response is not fresh, it might still be reusable if it can
be freshened by validation (<xref target="validation.model"/>) or if the
origin is unavailable (<xref target="serving.stale.responses"/>).
</t>
<section title="Conformance and Error Handling" anchor="conformance">
<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>
Conformance criteria and considerations regarding error handling
are defined in Section 2.5 of <xref target="Part1"/>.
</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="imported.abnf"/> describes rules imported from
other documents. <xref target="collected.abnf"/> shows the collected ABNF
with the list rule expanded.
</t>
<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 a cache 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
generate delta-seconds with a value greater than 2147483648.
</t>
</section>
</section>
</section>
<section anchor="caching.overview" title="Overview of 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 local
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, rather than mandating that
caches always store and reuse particular responses.
</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 to a GET request, which
contains a representation of the resource identified by the request target
(Section 4.3.1 of <xref target="Part2"/>). However, it is also possible to cache permanent redirects,
negative results (e.g., 404 (Not Found)),
incomplete results (e.g., 206 (Partial Content)), and
responses to methods other than GET if the method's definition allows such
caching and defines something suitable for use as a cache key.
</t>
<iref item="cache key"/>
<t>
The primary 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, many caches simply decline
other methods and use only the URI as the primary cache 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>
<section anchor="response.cacheability" title="Storing Responses in Caches">
<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.private"/>) 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 <xref target="header.expires" format="none">Expires</xref> header field (see
<xref target="header.expires"/>), or</t>
<t>contains a max-age response cache directive (see <xref target="cache-response-directive.max-age"/>), or</t>
<t>contains a s-maxage response cache directive (see <xref target="cache-response-directive.s-maxage"/>) 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 is defined as cacheable
(see <xref target="heuristic.freshness"/>), or</t>
<t>contains a public response cache directive (see <xref target="cache-response-directive.public"/>).</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 all specified
caching-related behavior.
</t>
<t>
Note that, in normal operation, some 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>
<section anchor="incomplete.responses" title="Storing Incomplete Responses">
<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="caching.authenticated.responses" title="Storing Responses to Authenticated Requests">
<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 <xref target="header.cache-control" format="none">Cache-Control</xref> 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="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 4.3 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 <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
with warn-code 1xx (see <xref target="header.warning"/>);</t>
<t>retain any <xref target="header.warning" format="none">Warning</xref> 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="constructing.responses.from.caches" title="Constructing Responses from Caches">
<t>
When presented with a request, a cache MUST NOT reuse 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.no-cache"/>), 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 generate an <xref target="header.age" format="none">Age</xref> header field (<xref target="header.age"/>), replacing any present 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 4.2.1 of <xref target="Part2"/>) to the origin server; i.e., a cache is not allowed to
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 the 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 upon every use.
</t>
<section anchor="expiration.model" title="Freshness">
<iref item="fresh"/>
<iref item="stale"/>
<t>
A fresh response is one whose age has not yet exceeded its
freshness lifetime. Conversely, a stale
response is one where it has.
</t>
<iref item="freshness lifetime"/>
<iref item="explicit expiration time"/>
<iref item="heuristic expiration time"/>
<t>
A response's freshness lifetime is the length of time
between its generation by the origin server and its expiration time. An
explicit expiration time is the time at which the origin
server intends that a stored response can no longer be used by a cache
without further validation, whereas a heuristic expiration
time is assigned by a cache when no explicit expiriation time is
available.
</t>
<iref item="age"/>
<t>
A response's age is the time that has passed since it was
generated by, or successfully validated with, the origin server.
</t>
<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
<xref target="header.expires" format="none">Expires</xref> header field (<xref target="header.expires"/>) or
the max-age response cache directive (<xref target="cache-response-directive.max-age"/>). 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 a stale
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,
caches are also allowed to use a heuristic to determine an expiration time
under certain circumstances (see <xref target="heuristic.freshness"/>).
</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>
freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>; current_age is defined in
<xref target="age.calculations"/>.
</t>
<t>
Clients can send the max-age or min-fresh cache directives in a request to
constrain or relax freshness calculations for the corresponding response
(<xref target="cache-request-directive"/>).
</t>
<t>
When calculating freshness, to avoid common problems in date parsing:
</t>
<t>
<list style="symbols">
<t>Although all date formats are specified to be case-sensitive,
cache recipients SHOULD match day, week and timezone names
case-insensitively.</t>
<t>If a cache recipient's internal implementation of time has less
resolution than the value of an HTTP-date, the recipient MUST
internally represent a parsed <xref target="header.expires" format="none">Expires</xref> date as the
nearest time equal to or earlier than the received value.</t>
<t>Cache recipients MUST NOT allow local time zones to influence the
calculation or comparison of an age or expiration time.</t>
<t>Cache recipients SHOULD consider a date with a zone abbreviation
other than "GMT" to be invalid for calculating expiration.</t>
</list>
</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.s-maxage"/>) is present, use its value,
or</t>
<t>If the max-age response cache directive (<xref target="cache-response-directive.max-age"/>) is present, use its value, or</t>
<t>If the <xref target="header.expires" format="none">Expires</xref> 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>
<t>
When there is more than one value present for a given directive (e.g., two
<xref target="header.expires" format="none">Expires</xref> header fields, multiple Cache-Control: max-age
directives), the directive's value is considered invalid. Caches are
encouraged to consider responses that have invalid freshness information to
be stale.
</t>
</section>
<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
<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>
<t>
A cache MUST NOT use heuristics to determine freshness when an explicit
expiration time is present in the stored response. Because of the
requirements in <xref target="response.cacheability"/>, this means that,
effectively, heuristics can only be used on responses without explicit
freshness whose status codes are defined as cacheable, and responses
without explicit freshness that have been marked as explicitly cacheable
(e.g., with a "public" response cache directive).
</t>
<t>
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>
When a heuristic is used to calculate freshness lifetime, a cache SHOULD
attach a <xref target="header.warning" format="none">Warning</xref> 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><list>
<t>
Note: Section 13.9 of <xref target="RFC2616"/> prohibited caches
from calculating heuristic freshness for URIs with query components
(i.e., those containing '?'). In practice, this has not been widely
implemented. Therefore, origin servers are encouraged to send explicit
directives (e.g., Cache-Control: no-cache) if they wish to preclude
caching.
</t>
</list></t>
</section>
<section anchor="age.calculations" title="Calculating Age">
<t>
The <xref target="header.age" format="none">Age</xref> header field is used to convey an estimated
age of the response message when obtained from a cache. The Age field value
is the cache's estimate of the number of seconds since the response was
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 <xref target="header.age" format="none">Age</xref>
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>
The term "date_value" denotes the value of
the Date header field, in a form appropriate for arithmetic
operations. See Section 7.1.1.2 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 host ought to use NTP (<xref target="RFC1305"/>) or some similar protocol to synchronize its
clocks to Coordinated Universal Time.
</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 are 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 <xref target="header.age" format="none">Age</xref> header
field (e.g., because there are no HTTP/1.0 hops in the Via
header field), in which case the corrected_age_value MAY be used as the
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>
</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 generate 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 send 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 <xref target="header.warning" format="none">Warning</xref> 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>
<iref item="first-hand"/>
<t>
Note that if a cache receives a first-hand response (one
where the freshness model is not in use; i.e., its age is 0, whether it is
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 MAY forward it to the requesting
client without adding a new <xref target="header.warning" format="none">Warning</xref> (but without removing
any existing Warning header fields). A cache ought not attempt to validate
a response simply because that response became stale in transit.
</t>
</section>
</section>
<section anchor="validation.model" title="Validation">
<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>
<iref item="validator"/>
<t>
When sending such a conditional request, a cache adds a
validator (or more than one), that is used to find
out whether a stored response is an equivalent copy of a current
representation of the resource.
</t>
<t>
One such validator is the 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>
Another is the If-None-Match header field,
whose value is that of the ETag header field(s) from
relevant responses stored for the primary cache key, if present. However,
if any of the stored responses contains only partial content, the cache
ought not include its entity-tag in the If-None-Match header field unless
the request is for a range that would be fully satisfied by that stored
response.
</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 payload 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 (Server Error)
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 send a
previously stored response (see <xref target="serving.stale.responses"/>).
</t>
</list>
</t>
<section anchor="freshening.responses" title="Freshening Stored Responses upon Validation">
<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.
</t>
<iref item="strong validator"/>
<t>
The stored response to update is identified by using the first match (if
any) of:
<list style="symbols">
<t>
If the new response contains a strong validator (see
Section 2.1 of <xref target="Part4"/>), then that strong validator identifies the selected
representation for update. All of the stored responses with the same
strong validator are selected. If none of the stored responses contain the
same strong validator, then the cache MUST NOT use the new response to
update any 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 for update.
</t>
<t>
If the new response does not include any form of validator (such as in
the case where a client generates an If-Modified-Since request from a
source other than the Last-Modified response header field), and there is
only one stored response, and that stored response also lacks a
validator, then that stored response is selected for update.
</t>
</list>
</t>
<t>
If a stored response is selected for update, the cache MUST:
<list style="symbols">
<t>delete any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
with warn-code 1xx (see <xref target="header.warning"/>);</t>
<t>retain any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
with warn-code 2xx; and,</t>
<t>use other header fields provided in the 304 (Not Modified)
response to replace all instances of the corresponding header
fields in the stored response.</t>
</list>
</t>
</section>
</section>
<section anchor="caching.negotiated.responses" title="Calculating Secondary Keys with Vary">
<t>
When a cache receives a request that can be satisfied by a stored response
that has a Vary header field (Section 7.1.4 of <xref target="Part2"/>),
it MUST NOT use that response unless all of the selecting header fields
nominated by the Vary header field match in both the original request
(i.e., that associated with the stored response), and the presented
request.
</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.
</t>
<t>
The stored response with matching selecting header fields is known as the
selected response.
</t>
<t>
If multiple selected responses are available (potentially including
responses without a Vary header field), the cache will need to choose one to use.
When a selecting header field has a known mechanism for doing so (e.g., qvalues on
Accept and similar request header fields), that mechanism MAY be
used to select preferred responses; of the remainder, the most recent
response (as determined by the Date header field) is used, as
per <xref target="constructing.responses.from.caches"/>.
</t>
<t>
If no selected response is available, the cache cannot satisfy the
presented request. Typically, it is forwarded to the origin server
in a (possibly conditional; see <xref target="validation.model"/>) request.
</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 header fields in the stored response using the following
rules:
<list style="symbols">
<t>delete any <xref target="header.warning" format="none">Warning</xref> header fields in the stored response
with warn-code 1xx (see <xref target="header.warning"/>);</t>
<t>retain any <xref target="header.warning" format="none">Warning</xref> 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 4.2.1 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 (Successful)
or 3xx (Redirection) 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 sent in response to a subsequent request.
</t>
<t>
Note that this does not guarantee that all appropriate responses are
invalidated. For example, a state-changing request might invalidate
responses in the caches it travels through, but relevant responses still
might be stored in other caches that it has not.</t>
</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"/>
<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"/>
<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: Some HTTP/1.0 caches might not implement Cache-Control.
</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, that can use both token and quoted-string
syntax. For the directives defined below that define arguments, recipients
ought to accept both forms, even if one is documented to be preferred. For
any directive not defined by this specification, recipients MUST accept
both forms.
</t>
<figure><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/><artwork type="abnf2616"><![CDATA[
Cache-Control = 1#cache-directive
cache-directive = token [ "=" ( token / quoted-string ) ]
]]></artwork></figure>
<t>
For the cache directives defined below, no argument is defined (nor allowed)
unless stated otherwise.
</t>
<section title="Request Cache-Control Directives" anchor="cache-request-directive">
<section title="max-age" anchor="cache-request-directive.max-age">
<iref item="max-age (cache directive)" primary="true"/>
<t>
Argument syntax:
<list>
<t>
<xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
</t>
</list>
</t>
<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>
<t>
Note: This directive uses the token form of the argument syntax;
e.g., 'max-age=5', not 'max-age="5"'. Senders SHOULD NOT use the
quoted-string form.
</t>
</section>
<section title="max-stale" anchor="cache-request-directive.max-stale">
<iref item="max-stale (cache directive)" primary="true"/>
<t>
Argument syntax:
<list>
<t>
<xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
</t>
</list>
</t>
<t>
The "max-stale" request directive indicates that the client is willing
to accept a response that has exceeded its freshness lifetime. If max-stale
is assigned a value, then the client is willing to accept a response
that has exceeded its freshness lifetime 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>
<t>
Note: This directive uses the token form of the argument syntax;
e.g., 'max-stale=10', not 'max-stale="10"'. Senders SHOULD NOT use the
quoted-string form.
</t>
</section>
<section title="min-fresh" anchor="cache-request-directive.min-fresh">
<iref item="min-fresh (cache directive)" primary="true"/>
<t>
Argument syntax:
<list>
<t>
<xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
</t>
</list>
</t>
<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>
<t>
Note: This directive uses the token form of the argument syntax;
e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders SHOULD NOT use the
quoted-string form.
</t>
</section>
<section title="no-cache" anchor="cache-request-directive.no-cache">
<iref item="no-cache (cache directive)" primary="true"/>
<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>
</section>
<section title="no-store" anchor="cache-request-directive.no-store">
<iref item="no-store (cache directive)" primary="true"/>
<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>
</section>
<section title="no-transform" anchor="cache-request-directive.no-transform">
<iref item="no-transform (cache directive)" primary="true"/>
<t>
The "no-transform" request directive indicates that an intermediary
(whether or not it implements a cache) MUST NOT transform the payload,
as defined in Section 5.7.2 of <xref target="Part1"/>.
</t>
</section>
<section title="only-if-cached" anchor="cache-request-directive.only-if-cached">
<iref item="only-if-cached (cache directive)" primary="true"/>
<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>
</section>
</section>
<section anchor="cache-response-directive" title="Response Cache-Control Directives">
<section title="must-revalidate" anchor="cache-response-directive.must-revalidate">
<iref item="must-revalidate (cache directive)" primary="true"/>
<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>
</section>
<section title="no-cache" anchor="cache-response-directive.no-cache">
<iref item="no-cache (cache directive)" primary="true"/>
<t>
Argument syntax:
<list>
<t>
#<xref target="imported.abnf" format="none">field-name</xref>
</t>
</list>
</t>
<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 send 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>
The field-names given are not limited to the set of header
fields defined by this specification. Field names are case-insensitive.
</t>
<t>
Note: Although it has been back-ported to many implementations, some
HTTP/1.0 caches will not recognize or obey this directive. Also, no-cache
response directives with field-names are often handled by caches
as if an unqualified no-cache directive was received; i.e., the special
handling for the qualified form is not widely implemented.
</t>
<t>
Note: This directive uses the quoted-string form of the argument syntax.
Senders SHOULD NOT use the token form (even if quoting appears not to be
needed for single-entry lists).
</t>
</section>
<section title="no-store" anchor="cache-response-directive.no-store">
<iref item="no-store (cache directive)" primary="true"/>
<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>
</section>
<section title="no-transform" anchor="cache-response-directive.no-transform">
<iref item="no-transform (cache directive)" primary="true"/>
<t>
The "no-transform" response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of <xref target="Part1"/>.
</t>
</section>
<section title="public" anchor="cache-response-directive.public">
<iref item="public (cache directive)" primary="true"/>
<t>
The "public" response directive indicates that any cache MAY store the
response, even if the response would normally be non-cacheable or cacheable
only within a non-shared cache. (See <xref target="caching.authenticated.responses"/> for additional details related
to the use of public in response to a request containing
Authorization, and <xref target="response.cacheability"/>
for details of how public affects responses that would normally not be
stored, due to their status codes not being defined as cacheable.)
</t>
</section>
<section title="private" anchor="cache-response-directive.private">
<iref item="private (cache directive)" primary="true"/>
<t>
Argument syntax:
<list>
<t>
#<xref target="imported.abnf" format="none">field-name</xref>
</t>
</list>
</t>
<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 and reuse it for later requests,
even if the response would normally be non-cacheable.
</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>
The field-names given are not limited to the set of header
fields defined by this specification. Field names are case-insensitive.
</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 caches as if an unqualified private directive
was received; i.e., the special handling for the qualified form is not
widely implemented.
</t>
<t>
Note: This directive uses the quoted-string form of the argument syntax.
Senders SHOULD NOT use the token form (even if quoting appears not to be
needed for single-entry lists).
</t>
</section>
<section title="proxy-revalidate" anchor="cache-response-directive.proxy-revalidate">
<iref item="proxy-revalidate (cache directive)" primary="true"/>
<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>
</section>
<section title="max-age" anchor="cache-response-directive.max-age">
<iref item="max-age (cache directive)" primary="true"/>
<t>
Argument syntax:
<list>
<t>
<xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
</t>
</list>
</t>
<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>
<t>
Note: This directive uses the token form of the argument syntax;
e.g., 'max-age=5', not 'max-age="5"'. Senders SHOULD NOT use the
quoted-string form.
</t>
</section>
<section title="s-maxage" anchor="cache-response-directive.s-maxage">
<iref item="s-maxage (cache directive)" primary="true"/>
<t>
Argument syntax:
<list>
<t>
<xref target="delta-seconds" format="none">delta-seconds</xref> (see <xref target="delta-seconds"/>)
</t>
</list>
</t>
<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 <xref target="header.expires" format="none">Expires</xref>
header field. The s-maxage directive also implies the semantics of the
proxy-revalidate response directive.
</t>
<t>
Note: This directive uses the token form of the argument syntax;
e.g., 's-maxage=10', not 's-maxage="10"'. Senders SHOULD NOT use the
quoted-string form.
</t>
</section>
</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.
</t>
<t>
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.
</t>
<t>
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 is allowed to 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>
</section>
</section>
<section anchor="header.expires" title="Expires">
<iref item="Expires header field" primary="true"/>
<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 Expires value is an HTTP-date timestamp, as defined in Section 7.1.1.1 of <xref target="Part2"/>.
</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 recipient MUST interpret invalid date formats, especially the
value "0", as representing a time in the past (i.e., "already expired").
</t>
<t>
If a response includes a <xref target="header.cache-control" format="none">Cache-Control</xref> field with
the max-age directive (<xref target="cache-response-directive.max-age"/>),
a recipient MUST ignore the Expires field.
Likewise, if a response includes the s-maxage directive
(<xref target="cache-response-directive.s-maxage"/>), a shared cache
recipient MUST ignore the Expires field. In both these cases, the value
in Expires is only intended for recipients that have not yet implemented
the Cache-Control field.
</t>
<t>
An origin server without a clock MUST NOT generate an Expires field
unless its value represents a fixed time in the past (always expired)
or its value has been associated with the resource by a system or user
with a reliable clock.
</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 many caches will evict a response far sooner than
that.
</t>
</section>
<section anchor="header.pragma" title="Pragma">
<iref item="Pragma header field" primary="true"/>
<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 <xref target="header.cache-control" format="none">Cache-Control</xref> was not defined until HTTP/1.1).
When the Cache-Control header field 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 <xref target="header.cache-control" format="none">Cache-Control</xref> header field is not present in a
request, caches MUST consider the no-cache request pragma-directive as
having the same effect 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 <xref target="header.cache-control" format="none">Cache-Control</xref> 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
<xref target="header.cache-control" format="none">Cache-Control</xref> 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.warning" title="Warning">
<iref item="Warning header field" primary="true"/>
<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 header 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 sent, 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)"/>
<t>
A cache SHOULD generate this whenever the sent response is stale.
</t>
</section>
<section title="111 Revalidation Failed" anchor="warn.111">
<iref primary="true" item="111 Revalidation Failed (warn code)"/>
<t>
A cache SHOULD generate this when sending 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)"/>
<t>
A cache SHOULD generate 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)"/>
<t>
A cache SHOULD generate 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)"/>
<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)"/>
<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)"/>
<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>
Extension warn codes can be defined; see <xref target="warn.code.registry.procedure"/>
for details.
</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 title="IANA Considerations" anchor="iana.considerations">
<section title="Cache Directive Registry" anchor="cache.directive.registry">
<t>
The HTTP Cache Directive Registry defines the name space for the
cache directives. It will be created and maintained at
<eref target="http://www.iana.org/assignments/http-cache-directives"/>.
</t>
<section title="Procedure" anchor="cache.directive.registry.procedure">
<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>
</section>
<section title="Considerations for New Cache Control Directives" anchor="cache.directive.considerations">
<t>
New extension directives ought to consider defining:
</t>
<t>
<list style="symbols">
<t>What it means for a directive to be specified multiple times,</t>
<t>When the directive does not take an argument, what it means when an
argument is present,</t>
<t>When the directive requires an argument, what it means when it is
missing,</t>
<t>Whether the directive is specific to requests, responses, or able
to be used in either.</t>
</list>
</t>
<t>
See also <xref target="cache.control.extensions"/>.
</t>
</section>
<section title="Registrations" anchor="cache.directive.registration">
<t>
The HTTP Cache Directive Registry shall 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.max-age"/>, <xref target="cache-response-directive.max-age"/>
</c>
<c>max-stale</c>
<c>
<xref target="cache-request-directive.max-stale"/>
</c>
<c>min-fresh</c>
<c>
<xref target="cache-request-directive.min-fresh"/>
</c>
<c>must-revalidate</c>
<c>
<xref target="cache-response-directive.must-revalidate"/>
</c>
<c>no-cache</c>
<c>
<xref target="cache-request-directive.no-cache"/>, <xref target="cache-response-directive.no-cache"/>
</c>
<c>no-store</c>
<c>
<xref target="cache-request-directive.no-store"/>, <xref target="cache-response-directive.no-store"/>
</c>
<c>no-transform</c>
<c>
<xref target="cache-request-directive.no-transform"/>, <xref target="cache-response-directive.no-transform"/>
</c>
<c>only-if-cached</c>
<c>
<xref target="cache-request-directive.only-if-cached"/>
</c>
<c>private</c>
<c>
<xref target="cache-response-directive.private"/>
</c>
<c>proxy-revalidate</c>
<c>
<xref target="cache-response-directive.proxy-revalidate"/>
</c>
<c>public</c>
<c>
<xref target="cache-response-directive.public"/>
</c>
<c>s-maxage</c>
<c>
<xref target="cache-response-directive.s-maxage"/>
</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>
<section title="Warn Code Registry" anchor="warn.code.registry">
<t>
The HTTP Warn Code Registry defines the name space for warn codes.
It will be created and maintained at
<eref target="http://www.iana.org/assignments/http-warn-codes"/>.
</t>
<section title="Procedure" anchor="warn.code.registry.procedure">
<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>
</section>
<section title="Registrations" anchor="warn.code.registration">
<t>
The HTTP Warn Code Registry shall 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>
<section title="Header Field Registration" anchor="header.field.registration">
<t>
HTTP header fields are registered within the Message Header Field Registry
maintained at
<eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
</t>
<t>
This document defines the following HTTP header fields, so their
associated registry entries shall be updated according to the permanent
registrations below (see <xref target="BCP90"/>):
</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>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>
This section is meant to inform developers, information providers, and
users of known security concerns specific to HTTP/1.1 caching.
More general security considerations are addressed in HTTP messaging
<xref target="Part1"/> and semantics <xref target="Part2"/>.
</t>
<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>
<t>
Furthermore, the very use of a cache can bring about privacy concerns. For
example, if two users share a cache, and the first one browses to a site,
the second may be able to detect that the other has been to that site,
because the resources from it load more quickly, thanks to the cache.
</t>
<t>
Implementation flaws might allow attackers to insert content into a cache
("cache poisoning"), leading to compromise of clients that trust that
content. Because of their nature, these attacks are difficult to mitigate.
</t>
<t>
Likewise, implementation flaws (as well as misunderstanding of cache
operation) might lead to caching of sensitive information (e.g.,
authentication credentials) that is thought to be private, exposing it to
unauthorized parties.
</t>
<t>
Note that the Set-Cookie response header field <xref target="RFC6265"/> does not
inhibit caching; a cacheable response with a Set-Cookie header field can be (and
often is) used to satisfy subsequent requests to caches. Servers who wish
to control caching of these responses are encouraged to emit appropriate
Cache-Control response header fields.
</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>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</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="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="July" year="2013"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-23"/>
</reference>
<reference anchor="Part2">
<front>
<title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</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="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="July" year="2013"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-23"/>
</reference>
<reference anchor="Part4">
<front>
<title>Hypertext Transfer Protocol (HTTP/1.1): 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="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="July" year="2013"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-23"/>
</reference>
<reference anchor="Part5">
<front>
<title>Hypertext Transfer Protocol (HTTP/1.1): Range 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="July" year="2013"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-23"/>
</reference>
<reference anchor="Part7">
<front>
<title>Hypertext Transfer Protocol (HTTP/1.1): 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="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="July" year="2013"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-23"/>
</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="BCP90">
<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>
<reference anchor="RFC6265">
<front>
<title>HTTP State Management Mechanism</title>
<author initials="A." surname="Barth" fullname="Adam Barth">
<organization abbrev="U.C. Berkeley">
University of California, Berkeley
</organization>
<address><email>abarth@eecs.berkeley.edu</email></address>
</author>
<date year="2011" month="April"/>
</front>
<seriesInfo name="RFC" value="6265"/>
</reference>
</references>
<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
<t>
Caching-related text has been substantially rewritten for clarity.
</t>
<t>
The algorithm for calculating age is now less conservative.
(<xref target="age.calculations"/>)
</t>
<t>
Caches are now required to handle dates with timezones as if they're
invalid, because it's not possible to accurately guess.
(<xref target="age.calculations"/>)
</t>
<t>
The Content-Location response header field is no longer used
to determine the appropriate response to use when validating.
(<xref target="validation.model"/>)
</t>
<t>
The algorithm for selecting a cached negotiated response to use has been
clarified in several ways. In particular, it now explicitly allows
header-specific canonicalization when processing selecting header fields.
(<xref target="caching.negotiated.responses"/>)
</t>
<t>
Requirements regarding denial of service attack avoidance when performing
invalidation have been clarified.
(<xref target="invalidation.after.updates.or.deletions"/>)
</t>
<t>
Cache invalidation only occurs when a successful response is received.
(<xref target="invalidation.after.updates.or.deletions"/>)
</t>
<t>
The conditions under which an authenticated response can be cached have been
clarified.
(<xref target="caching.authenticated.responses"/>)
</t>
<t>
The one-year limit on Expires header field values has been removed; instead,
the reasoning for using a sensible value is given.
(<xref target="header.expires"/>)
</t>
<t>
The Pragma header field is now only defined for backwards compatibility;
future pragmas are deprecated.
(<xref target="header.pragma"/>)
</t>
<t>
Cache directives are explicitly defined to be case-insensitive.
(<xref target="header.cache-control"/>)
</t>
<t>
Handling of multiple instances of cache directives when only one is
expected is now defined.
(<xref target="header.cache-control"/>)
</t>
<t>
The qualified forms of the private and no-cache cache directives are noted
to not be widely implemented; e.g., "private=foo" is interpreted by many
caches as simply "private". Additionally, the meaning of the qualified
form of no-cache has been clarified.
(<xref target="cache-response-directive"/>)
</t>
<t>
The "no-store" cache request directive doesn't apply to responses; i.e.,
a cache can satisfy a request with no-store on it, and does not invalidate
it.
(<xref target="cache-request-directive.no-store"/>)
</t>
<t>
The "no-cache" response cache directive's meaning has been clarified.
(<xref target="cache-response-directive.no-cache"/>)
</t>
<t>
New status codes can now define that caches are allowed to use heuristic
freshness with them.
(<xref target="heuristic.freshness"/>)
</t>
<t>
Caches are now allow to calculate heuristic freshness for URLs with query
components.
(<xref target="heuristic.freshness"/>)
</t>
<t>
Some requirements regarding production of the <xref target="header.warning" format="none">Warning</xref>
header fields have been relaxed, as it is not widely implemented.
Furthermore, the <xref target="header.warning" format="none">Warning</xref> header field no longer uses RFC 2047 encoding,
nor allows multiple languages, as these aspects were not implemented.
(<xref target="header.warning"/>)
</t>
<t>
This specification introduces the Cache Directive and Warn Code Registries,
and defines considerations for new cache directives.
(<xref target="cache.control.extensions"/> and <xref target="warn.code.extensions"/>)
</t>
</section>
<section title="Imported ABNF" anchor="imported.abnf">
<t>
The following core rules are included by reference, as defined in Appendix B.1 of <xref target="RFC5234"/>: 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>
<t>
The rules below are defined in <xref target="Part1"/>:
</t>
<figure><artwork type="abnf2616"><![CDATA[
OWS = <OWS, defined in [Part1], Section 3.2.3>
field-name = <field-name, defined in [Part1], Section 3.2>
quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
token = <token, defined in [Part1], Section 3.2.6>
port = <port, defined in [Part1], Section 2.7>
pseudonym = <pseudonym, defined in [Part1], Section 5.7.1>
uri-host = <uri-host, defined in [Part1], Section 2.7>
]]></artwork></figure>
<t>
The rules below are defined in other parts:
</t>
<figure><artwork type="abnf2616"><![CDATA[
HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1>
]]></artwork></figure>
</section>
<section title="Collected ABNF" anchor="collected.abnf">
<t>
In the collected ABNF below, list rules are expanded as per Section 1.2 of <xref target="Part1"/>.
</t><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 7.1.1.1>
OWS = <OWS, defined in [Part1], Section 3.2.3>
Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
pragma-directive ] )
Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
)
cache-directive = token [ "=" ( token / quoted-string ) ]
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 5.7.1>
quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
token = <token, defined in [Part1], Section 3.2.6>
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>
</section>
<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
<t>
Changes up to the first Working Group Last Call draft are summarized
in <eref target="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-19#appendix-C"/>.
</t>
<section title="Since draft-ietf-httpbis-p6-cache-19" anchor="changes.since.19">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/307"/>:
"untangle Cache-Control ABNF"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/353"/>:
"Multiple values in Cache-Control header fields"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/355"/>:
"Case sensitivity of header fields in CC values"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/356"/>:
"Spurious 'MAYs'"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/360"/>:
"enhance considerations for new cache control directives"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
"ABNF requirements for recipients"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/368"/>:
"note introduction of new IANA registries as normative changes"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/373"/>:
"broken prose in description of 'Vary'"
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-20" anchor="changes.since.20">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/375"/>:
"'Most Conservative'"
</t>
</list>
</t>
<t>
Other changes:
<list style="symbols">
<t>
Conformance criteria and considerations regarding error handling are
now defined in Part 1.
</t>
<t>
Move definition of "Vary" header field into Part 2.
</t>
<t>
Add security considerations with respect to cache poisoning and
the "Set-Cookie" header field.
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-21" anchor="changes.since.21">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
"Allowing heuristic caching for new status codes"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/406"/>:
"304 without validator"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/418"/>:
"No-Transform"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/430"/>:
"Revert prior change to the meaning of the public cache response
directive.
</t>
</list>
</t>
</section>
<section title="Since draft-ietf-httpbis-p6-cache-22" anchor="changes.since.22">
<t>
Closed issues:
<list style="symbols">
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/436"/>:
"explain list expansion in ABNF appendices"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/453"/>:
"Returning the freshest response"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/464"/>:
"placement of extension point considerations"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/469"/>:
"Editorial notes for p6"
</t>
<t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/471"/>:
"Vary and future requests"
</t>
</list>
</t>
</section>
</section>
</back>
</rfc>| PAFTECH AB 2003-2026 | 2026-04-23 20:33:56 |