One document matched: draft-ietf-httpbis-p6-cache-21.xml
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<rfc category="std" docName="draft-ietf-httpbis-p6-cache-21" 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="October" year="2012" day="4"/>
<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.20"/>.
</t>
</note>
</front>
<middle>
<section anchor="caching" title="Introduction">
<t>
HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. This document
defines aspects of HTTP/1.1 related to caching and reusing response
messages.
</t>
<section anchor="intro.purpose" title="Purpose">
<iref item="cache"/>
<t>
An HTTP cache is a local store of response messages and the
subsystem that controls its message storage, retrieval, and deletion. A
cache stores cacheable responses in order to reduce the response time and
network bandwidth consumption on future, equivalent requests. Any client or
server MAY employ a cache, though a cache cannot be used by a server that
is acting as a tunnel.
</t>
<t>
The goal of caching in HTTP/1.1 is to significantly improve performance
by reusing a prior response message to satisfy a current request.
A stored response is considered "fresh", as defined in
<xref target="expiration.model"/>, if the response can be reused without
"validation" (checking with the origin server to see if the cached response
remains valid for this request). A fresh cache response can therefore
reduce both latency and network transfers each time it is reused.
When a cached response is not fresh, it might still be reusable if it can
be freshened by validation (<xref target="validation.model"/>) or if the
origin is unavailable.
</t>
</section>
<section anchor="intro.terminology" title="Terminology">
<t>
This specification uses a number of terms to refer to the roles played by
participants in, and objects of, HTTP caching.
</t>
<t>
<iref item="cache"/>
<?rfc needLines="4"?>cache
<list>
<t>A conformant implementation of a HTTP cache. Note that this implies
an HTTP/1.1 cache; this specification does not define conformance
for HTTP/1.0 caches.</t>
</list>
</t>
<t anchor="shared.and.non-shared.caches">
<iref item="shared cache"/>
<?rfc needLines="4"?>shared cache
<list>
<t>A cache that stores responses to be reused by more than one user;
usually (but not always) deployed as part of an intermediary.</t>
</list>
</t>
<t>
<iref item="private cache"/>
<?rfc needLines="4"?>private cache
<list>
<t>A cache that is dedicated to a single user.</t>
</list>
</t>
<t>
<iref item="cacheable"/>
<?rfc needLines="4"?>cacheable
<list>
<t>A response is cacheable if a cache is allowed to store a copy of the
response message for use in answering subsequent requests. Even when a
response is cacheable, there might be additional constraints on whether
a cache can use the stored copy to satisfy a particular request.</t>
</list>
</t>
<t>
<iref item="explicit expiration time"/>
<?rfc needLines="4"?>explicit expiration time
<list>
<t>The time at which the origin server intends that a representation
no longer be returned by a cache without further validation.</t>
</list>
</t>
<t>
<iref item="heuristic expiration time"/>
<?rfc needLines="4"?>heuristic expiration time
<list>
<t>An expiration time assigned by a cache when no explicit expiration
time is available.</t>
</list>
</t>
<t>
<iref item="age"/>
<?rfc needLines="4"?>age
<list>
<t>The age of a response is the time since it was sent by, or
successfully validated with, the origin server.</t>
</list>
</t>
<t>
<iref item="first-hand"/>
<?rfc needLines="4"?>first-hand
<list>
<t>A response is first-hand if the freshness model is not in use; i.e.,
its age is 0.</t>
</list>
</t>
<t>
<iref item="freshness lifetime"/>
<?rfc needLines="4"?>freshness lifetime
<list>
<t>The length of time between the generation of a response and its
expiration time.</t>
</list>
</t>
<t>
<iref item="fresh"/>
<?rfc needLines="4"?>fresh
<list>
<t>A response is fresh if its age has not yet exceeded its freshness
lifetime.</t>
</list>
</t>
<t>
<iref item="stale"/>
<?rfc needLines="4"?>stale
<list>
<t>A response is stale if its age has passed its freshness lifetime
(either explicit or heuristic).</t>
</list>
</t>
<t>
<iref item="validator"/>
<?rfc needLines="4"?>validator
<list>
<t>A protocol element (e.g., an entity-tag or a Last-Modified
time) that is used to find out whether a stored response is an equivalent
copy of a representation. See Section 2.1 of <xref target="Part4"/>.</t>
</list>
</t>
<t>
<iref item="strong validator"/>
<iref item="validator" subitem="strong"/>
<?rfc needLines="4"?>strong validator
<list>
<t>A validator that is defined by the origin server such that its
current value will change if the representation data changes; i.e.,
an entity-tag that is not marked as weak (Section 2.3 of <xref target="Part4"/>) or,
if no entity-tag is provided, a Last-Modified value
that is strong in the sense defined by Section 2.2.2 of <xref target="Part4"/>.</t>
</list>
</t>
</section>
<section title="Conformance and Error Handling" anchor="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 an implementation receives a delta-seconds value larger than the largest
positive integer it can represent, or if any of its subsequent calculations
overflows, it MUST consider the value to be 2147483648
(2^31). Recipients parsing a delta-seconds value MUST use
an arithmetic type of at least 31 bits of range, and senders MUST NOT
send delta-seconds with a value greater than 2147483648.
</t>
</section>
</section>
</section>
<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 locally-desired
configuration prevents it. Therefore, HTTP cache requirements are focused
on preventing a cache from either storing a non-reusable response or
reusing a stored response inappropriately.
</t>
<t>
Each cache entry consists of a cache key and one or more
HTTP responses corresponding to prior requests that used the same key. The
most common form of cache entry is a successful result of a retrieval
request: i.e., a 200 (OK) response containing a
representation of the resource identified by the request target. However,
it is also possible to cache negative results (e.g., 404 (Not
Found), incomplete results (e.g., 206 (Partial
Content)), and responses to methods other than GET if the method's
definition allows such caching and defines something suitable for use as a
cache key.
</t>
<t>
The default cache key consists of the request method and
target URI. However, since HTTP caches in common use today are typically
limited to caching responses to GET, many implementations simply decline
other methods and use only the URI as the key.
</t>
<t>
If a request target is subject to content negotiation, its cache entry
might consist of multiple stored responses, each differentiated by a
secondary key for the values of the original request's selecting header
fields (<xref target="caching.negotiated.responses"/>).
</t>
</section>
<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 and the cache is
shared, or</t>
<t>contains a Cache Control Extension (see <xref target="cache.control.extensions"/>) that allows it to be cached,
or</t>
<t>has a status code that can be served with heuristic freshness
(see <xref target="heuristic.freshness"/>).</t>
</list>
</t>
</list>
</t>
<t>
Note that any of the requirements listed above can be overridden by a
cache-control extension; see <xref target="cache.control.extensions"/>.
</t>
<t>
In this context, a cache has "understood" a request method or a response
status code if it recognizes it and implements any cache-specific
behavior.
</t>
<t>
Note that, in normal operation, many 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>
<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
<t>
For a presented request, a cache MUST NOT return a stored response,
unless:
<list style="symbols">
<t>The presented effective request URI (Section 5.5 of <xref target="Part1"/>) and
that of the stored response match, and</t>
<t>the request method associated with the stored response allows it to
be used for the presented request, and</t>
<t>selecting header fields nominated by the stored response (if any)
match those presented (see <xref target="caching.negotiated.responses"/>), and</t>
<t>the presented request does not contain the no-cache pragma (<xref target="header.pragma"/>), nor the no-cache cache directive (<xref target="cache-request-directive"/>), unless the stored response is
successfully validated (<xref target="validation.model"/>), and</t>
<t>the stored response does not contain the no-cache cache directive
(<xref target="cache-response-directive.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 include a single <xref target="header.age" format="none">Age</xref> header field
(<xref target="header.age"/>) in the response with a value equal to the
stored response's current_age; see <xref target="age.calculations"/>.
</t>
<t>
A cache MUST write through requests with methods that are unsafe
(Section 5.2.1 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 a request with "Cache-Control: max-age=0" or
"Cache-Control: no-cache" to disambiguate which response to use.
</t>
<t>
A cache that does not have a clock available MUST NOT use stored
responses without revalidating them on every use. A cache, especially a
shared cache, SHOULD use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its clock with a reliable external
standard.
</t>
<section anchor="expiration.model" title="Freshness Model">
<t>
When a response is "fresh" in the cache, it can be used to satisfy
subsequent requests without contacting the origin server, thereby improving
efficiency.
</t>
<t>
The primary mechanism for determining freshness is for an origin server to
provide an explicit expiration time in the future, using either the
<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 the
cached response before reusing it for subsequent requests (see <xref target="serving.stale.responses"/>).
</t>
<t>
Since origin servers do not always provide explicit expiration times, a
cache MAY assign a heuristic expiration time when an explicit time is not
specified, employing algorithms that use other header field values (such as
the Last-Modified time) to estimate a plausible expiration
time. This specification does not provide specific algorithms, but does
impose worst-case constraints on their results.
</t>
<figure>
<preamble>
The calculation to determine if a response is fresh is:
</preamble>
<artwork type="code"><![CDATA[
response_is_fresh = (freshness_lifetime > current_age)
]]></artwork>
</figure>
<t>
The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>; the current_age is defined in
<xref target="age.calculations"/>.
</t>
<t>
Additionally, clients can influence freshness calculation — either
constraining it relaxing it — by using the max-age and min-fresh
request cache directives. See <xref target="cache-request-directive"/>
for details.
</t>
<t>
Note that freshness applies only to cache operation; it cannot be used to
force a user agent to refresh its display or reload a resource. See <xref target="history.lists"/> for an explanation of the difference between
caches and history mechanisms.
</t>
<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
<t>
A cache can calculate the freshness lifetime (denoted as
freshness_lifetime) of a response by using the first match of:
<list style="symbols">
<t>If the cache is shared and the s-maxage response cache directive
(<xref target="cache-response-directive.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), it 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>
If no explicit expiration time is present in a stored response that has a
status code whose definition allows heuristic freshness to be used
(including the following in Section 7 of <xref target="Part2"/>: 200 (OK),
203 (Non-Authoritative Information), 206 (Partial
Content), 300 (Multiple Choices), 301 (Moved
Permanently) and 410 (Gone)), a cache MAY
calculate a heuristic expiration time. A cache MUST NOT use heuristics to
determine freshness for responses with status codes that do not explicitly
allow it.
</t>
<t>
When a heuristic is used to calculate freshness lifetime, a cache SHOULD
attach a <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>
Also, if the response has a Last-Modified header field
(Section 2.2 of <xref target="Part4"/>), caches are encouraged to use a heuristic
expiration value that is no more than some fraction of the interval since
that time. A typical setting of this fraction might be 10%.
</t>
<t><list>
<t>
Note: 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, 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>
HTTP/1.1 uses the <xref target="header.age" format="none">Age</xref> header field to convey the estimated
age of the response message when obtained from a cache. The Age field value
is the cache's estimate of the amount of time since the response was
generated or validated by the origin server. In essence, the Age value is
the sum of the time that the response has been resident in each of the
caches along the path from the origin server, plus the amount of time it
has been in transit along network paths.
</t>
<t>
The following data is used for the age calculation:
</t>
<t>
<?rfc needLines="4"?>age_value
<list>
<t>
The term "age_value" denotes the value of the <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>
HTTP/1.1 requires origin servers to send a Date header
field, if possible, with every response, giving the time at which the
response was generated. The term "date_value" denotes the value of
the Date header field, in a form appropriate for arithmetic
operations. See Section 8.1.1.2 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 cache SHOULD use NTP (<xref target="RFC1305"/>) or some similar protocol to synchronize its
clocks to a globally accurate time standard.
</t>
</list>
</t>
<t>
<?rfc needLines="4"?>request_time
<list>
<t>
The current value of the clock at the host at the time the request
resulting in the stored response was made.
</t>
</list>
</t>
<t>
<?rfc needLines="4"?>response_time
<list>
<t>
The current value of the clock at the host at the time the response
was received.
</t>
</list>
</t>
<t>
A response's age can be calculated in two entirely independent ways:
<list style="numbers">
<t>the "apparent_age": response_time minus date_value, if the local
clock is reasonably well synchronized to the origin server's clock. If
the result is negative, the result is replaced by zero.</t>
<t>the "corrected_age_value", if all of the caches along the response
path implement HTTP/1.1. A cache MUST interpret this value relative
to the time the request was initiated, not the time that the response
was received.</t>
</list>
</t>
<figure>
<artwork type="code"><![CDATA[
apparent_age = max(0, response_time - date_value);
response_delay = response_time - request_time;
corrected_age_value = age_value + response_delay;
]]></artwork>
</figure>
<figure>
<preamble>These SHOULD be combined as</preamble>
<artwork type="code"><![CDATA[
corrected_initial_age = max(apparent_age, corrected_age_value);
]]></artwork></figure>
<t>
unless the cache is confident in the value of the <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>
<t>
Additionally, to avoid common problems in date parsing:
</t>
<t>
<list style="symbols">
<t>Recipients SHOULD assume that an RFC-850 date
which appears to be more than 50 years in the future is in fact
in the past (this helps solve the "year 2000" problem).</t>
<t>Although all date formats are specified to be case-sensitive,
recipients SHOULD match day, week and timezone names
case-insensitively.</t>
<t>An implementation MAY internally represent a parsed
<xref target="header.expires" format="none">Expires</xref> date as earlier than the proper value, but
MUST NOT internally represent a parsed Expires date as later than the
proper value.</t>
<t>Recipients MUST perform all expiration-related calculations in GMT.
The local time zone MUST NOT influence the calculation or comparison
of an age or expiration time.</t>
<t>Caches SHOULD consider dates with time zones other than "GMT"
invalid.</t>
</list>
</t>
</section>
<section anchor="serving.stale.responses" title="Serving Stale Responses">
<t>
A "stale" response is one that either has explicit expiry information or is
allowed to have heuristic expiry calculated, but is not fresh according to
the calculations in <xref target="expiration.model"/>.
</t>
<t>
A cache MUST NOT return a stale response if it is prohibited by an
explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
directive, a "must-revalidate" cache-response-directive, or an applicable
"s-maxage" or "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
</t>
<t>
A cache MUST NOT return stale responses unless it is disconnected
(i.e., it cannot contact the origin server or otherwise find a forward
path) or doing so is explicitly allowed (e.g., by the max-stale request
directive; see <xref target="cache-request-directive"/>).
</t>
<t>
A cache SHOULD append a <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>
<t>
If a cache receives a first-hand response (either an entire response, or a
304 (Not Modified) response) that it would normally forward
to the requesting client, and the received response is no longer fresh, the
cache can forward it to the requesting client without adding a new
<xref target="header.warning" format="none">Warning</xref> (but without removing any existing Warning header
fields). A cache shouldn't attempt to validate a response simply because
that response became stale in transit.
</t>
</section>
</section>
<section anchor="validation.model" title="Validation Model">
<t>
When a cache has one or more stored responses for a requested URI, but
cannot serve any of them (e.g., because they are not fresh, or one cannot
be selected; see <xref target="caching.negotiated.responses"/>), it can use
the conditional request mechanism <xref target="Part4"/> in the forwarded request to
give the origin server an opportunity to both select a valid stored
response to be used, and to update it. This process is known as
"validating" or "revalidating" the stored response.
</t>
<t>
When sending such a conditional request, a cache adds an
If-Modified-Since header field whose value is that of the
Last-Modified header field from the selected
(see <xref target="caching.negotiated.responses"/>) stored response, if
available.
</t>
<t>
Additionally, a cache can add an If-None-Match header field
whose value is that of the ETag header field(s) from all
responses stored for the requested URI, if present. However, if any of the
stored responses contains only partial content, the cache shouldn't
include its entity-tag in the If-None-Match header field unless the request
is for a range that would be fully satisfied by that stored response.
</t>
<t>Cache handling of a response to a conditional request is dependent upon its
status code:</t>
<t>
<list style="symbols">
<t>
A 304 (Not Modified) response status code indicates
that the stored response can be updated and reused; see <xref target="freshening.responses"/>.
</t>
<t>
A full response (i.e., one with a 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 return a
previously stored response (see <xref target="serving.stale.responses"/>).
</t>
</list>
</t>
<section anchor="freshening.responses" title="Freshening Responses with 304 Not Modified">
<t>
When a cache receives a 304 (Not Modified) response and
already has one or more stored 200 (OK) responses for the
same cache key, the cache needs to identify which of the stored responses
are updated by this new response and then update the stored response(s)
with the new information provided in the 304 response.
<list style="symbols">
<t>
If the new response contains a strong validator, then that strong
validator identifies the selected representation. All of the stored
responses with the same strong validator are selected.
If none of the stored responses contain the same strong validator, then
this new response corresponds to a new selected representation and
MUST NOT update the existing stored responses.
</t>
<t>
If the new response contains a weak validator and that validator
corresponds to one of the cache's stored responses, then the most
recent of those matching stored responses is selected.
</t>
<t>
If the new response does not include any form of validator, there is
only one stored response, and that stored response also lacks a
validator, then that stored response is selected.
</t>
</list>
</t>
<t>
If a stored response is selected for update, the cache MUST:
<list style="symbols">
<t>delete any <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="Using Negotiated Responses">
<t>
When a cache receives a request that can be satisfied by a stored response
that has a Vary header field (Section 8.2.1 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, and
subsequent requests to that resource can only be properly interpreted by the
origin server.
</t>
<t>
The stored response with matching selecting header fields is known as the
selected response.
</t>
<t>
If multiple selected responses are available, the most recent response
(as determined by the Date header field) is used; see <xref target="constructing.responses.from.caches"/>.
</t>
<t>
If no selected response is available, the cache can forward the presented
request to the origin server in a conditional request; see <xref target="validation.model"/>.
</t>
</section>
<section anchor="combining.responses" title="Combining Partial Content">
<t>
A response might transfer only a partial representation if the
connection closed prematurely or if the request used one or more Range
specifiers (<xref target="Part5"/>). After several such transfers, a cache might have
received several ranges of the same representation. A cache MAY combine
these ranges into a single stored response, and reuse that response to
satisfy later requests, if they all share the same strong validator and
the cache complies with the client requirements in Section 4.2 of <xref target="Part5"/>.
</t>
<t>
When combining the new response with one or more stored responses, a
cache MUST:
<list style="symbols">
<t>delete any <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="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 5.2.1 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 returned in response to a subsequent request.
</t>
<t>
Note that this does not guarantee that all appropriate responses are
invalidated. For example, the request that caused the change at the origin
server might not have gone through the cache where a response is stored.
</t>
</section>
<section anchor="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: HTTP/1.0 caches might not implement Cache-Control and
might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
</t>
</list></t>
<t>
A proxy, whether or not it implements a cache, MUST pass cache directives
through in forwarded messages, regardless of their
significance to that application, since the directives might be applicable
to all recipients along the request/response chain. It is not possible to
target a directive to a specific cache.
</t>
<t>
Cache directives are identified by a token, to be compared case-insensitively,
and have an optional argument, 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)
otherwise stated otherwise.
</t>
<section title="Request Cache-Control Directives" anchor="cache-request-directive">
<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="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 expiration time. If max-stale
is assigned a value, then the client is willing to accept a response
that has exceeded its expiration time by no more than the specified
number of seconds. If no value is assigned to max-stale, then the client
is willing to accept a stale response of any age.
</t>
<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-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 change the
Content-Encoding, Content-Range or
Content-Type request header fields, nor the request
representation.
</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="public" anchor="cache-response-directive.only-if-cached">
<iref item="public (cache directive)" primary="true"/>
<t>
The "public" response directive indicates that a response whose
associated request contains an 'Authentication' header MAY be
stored (see <xref target="caching.authenticated.responses"/>).
</t>
</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.
</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 standard 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 implementations 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="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 return stale responses.
</t>
<t>
If the no-cache response directive specifies one or more field-names,
then a cache MAY use the response to satisfy a subsequent request,
subject to any other restrictions on caching. However, any header fields
in the response that have the field-name(s) listed MUST NOT be sent
in the response to a subsequent request without successful revalidation
with the origin server. This allows an origin server to prevent the
re-use of certain header fields in a response, while still allowing
caching of the rest of the response.
</t>
<t>
The field-names given are not limited to the set of standard header
fields defined by this specification. Field names are case-insensitive.
</t>
<t>
Note: Many HTTP/1.0 caches will not recognize or obey
this directive. Also, no-cache response directives with field-names are
often handled by implementations as if an unqualified no-cache directive
was received; i.e., the special handling for the qualified form is not
widely implemented.
</t>
<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="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="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 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 change the
Content-Encoding, Content-Range or
Content-Type response header fields, nor the response
representation.
</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. Informational
extensions (those that do not require a change in cache behavior) can be
added without changing the semantics of other directives. Behavioral
extensions are designed to work by acting as modifiers to the existing base
of cache directives. Both the new directive and the standard directive are
supplied, such that applications that do not understand the new directive
will default to the behavior specified by the standard directive, and those
that understand the new directive will recognize it as modifying the
requirements associated with the standard directive. In this way,
extensions to the cache-control directives can be made without requiring
changes to the base protocol.
</t>
<t>
This extension mechanism depends on an HTTP cache obeying all of the
cache-control directives defined for its native HTTP-version, obeying
certain extensions, and ignoring all directives that it does not
understand.
</t>
<t>
For example, consider a hypothetical new response directive called
"community" that acts as a modifier to the private directive. We define
this new directive to mean that, in addition to any private cache, any
cache that is shared only by members of the community named within its
value 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>
<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>
</list>
</t>
<t>
The HTTP Cache Directive Registry defines the name space for the cache
directives.
</t>
<t>
A registration MUST include the following fields:
<list style="symbols">
<t>Cache Directive Name</t>
<t>Pointer to specification text</t>
</list>
</t>
<t>
Values to be added to this name space require IETF Review (see <xref target="RFC5226"/>, Section 4.1).
</t>
<t>
The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
</t>
</section>
</section>
<section anchor="header.expires" title="Expires">
<iref item="Expires header field" primary="true"/>
<t>
The "Expires" header field gives the date/time after which the
response is considered stale. See <xref target="expiration.model"/> for
further discussion of the freshness model.
</t>
<t>
The presence of an Expires field does not imply that the original resource
will change or cease to exist at, before, or after that time.
</t>
<t>
The field-value is an absolute date and time as defined by HTTP-date in
Section 8.1.1.1 of <xref target="Part2"/>; a sender MUST use the rfc1123-date format.
</t>
<figure><iref primary="true" item="Grammar" subitem="Expires"/><artwork type="abnf2616"><![CDATA[
Expires = HTTP-date
]]></artwork></figure>
<figure>
<preamble>For example</preamble>
<artwork type="example"><![CDATA[
Expires: Thu, 01 Dec 1994 16:00:00 GMT
]]></artwork></figure>
<t>
A cache MUST treat other invalid date formats,
especially including the value "0", as in the past (i.e., "already
expired").
</t>
<t><list>
<t>
Note: If a response includes a <xref target="header.cache-control" format="none">Cache-Control</xref> field with
the max-age directive (see <xref target="cache-response-directive.max-age"/>),
that directive overrides the Expires field. Likewise, the s-maxage
directive (<xref target="cache-response-directive.s-maxage"/>) overrides
the <xref target="header.expires" format="none">Expires</xref> header fieldin shared caches.
</t>
</list></t>
<t>
Historically, HTTP required the Expires field-value to be no more than a
year in the future. While longer freshness lifetimes are no longer
prohibited, extremely large values have been demonstrated to cause
problems (e.g., clock overflows due to use of 32-bit integers for
time values), and many caches will evict a response far sooner than
that. Therefore, senders ought not produce them.
</t>
<t>
An origin server without a clock MUST NOT assign Expires
values to a response unless these values were associated
with the resource by a system or user with a reliable clock. It MAY
assign an Expires value that is known, at or before server
configuration time, to be in the past (this allows "pre-expiration"
of responses without storing separate Expires values for each
resource).
</t>
</section>
<section anchor="header.pragma" title="Pragma">
<iref item="Pragma header field" primary="true"/>
<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, the no-cache request pragma-directive MUST have the same effect
on caches as if "Cache-Control: no-cache" were present (see <xref target="cache-request-directive"/>).
</t>
<t>
When sending a no-cache request, a client ought to include both the pragma
and cache-control directives, unless Cache-Control: no-cache is
purposefully omitted to target other <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 returned, in which case they MUST be.</t>
</list>
</t>
<t>
If an implementation sends a message with one or more Warning header fields
to a receiver whose version is HTTP/1.0 or lower, then the sender MUST
include in each warning-value a warn-date that matches the
Date header field in the message.
</t>
<t>
If a system receives a message with a warning-value that includes a
warn-date, and that warn-date is different from the Date
value in the response, then that warning-value MUST be deleted from the
message before storing, forwarding, or using it. (preventing the
consequences of naive caching of Warning header fields.) If all of the
warning-values are deleted for this reason, the Warning header field MUST
be deleted as well.
</t>
<t>
The following warn-codes are defined by this specification, each with a
recommended warn-text in English, and a description of its meaning.
</t>
<section title="110 Response is Stale" anchor="warn.110">
<iref primary="true" item="110 Response is Stale (warn code)"/>
<t>
A cache SHOULD include this whenever the returned response is stale.
</t>
</section>
<section title="111 Revalidation Failed" anchor="warn.111">
<iref primary="true" item="111 Revalidation Failed (warn code)"/>
<t>
A cache SHOULD include this when returning a stale response because an
attempt to validate the response failed, due to an inability to reach
the server.
</t>
</section>
<section title="112 Disconnected Operation" anchor="warn.112">
<iref primary="true" item="112 Disconnected Operation (warn code)"/>
<t>
A cache SHOULD include this if it is intentionally disconnected from
the rest of the network for a period of time.
</t>
</section>
<section title="113 Heuristic Expiration" anchor="warn.113">
<iref primary="true" item="113 Heuristic Expiration (warn code)"/>
<t>
A cache SHOULD include this if it heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater than 24
hours.
</t>
</section>
<section title="199 Miscellaneous Warning" anchor="warn.199">
<iref primary="true" item="199 Miscellaneous Warning (warn code)"/>
<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>
The HTTP Warn Code Registry defines the name space for warn codes.
</t>
<t>
A registration MUST include the following fields:
<list style="symbols">
<t>Warn Code (3 digits)</t>
<t>Short Description</t>
<t>Pointer to specification text</t>
</list>
</t>
<t>
Values to be added to this name space require IETF Review (see <xref target="RFC5226"/>, Section 4.1).
</t>
<t>
The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-warn-codes"/>.
</t>
</section>
</section>
</section>
<section anchor="history.lists" title="History Lists">
<t>
User agents often have history mechanisms, such as "Back" buttons and
history lists, that can be used to redisplay a representation retrieved
earlier in a session.
</t>
<t>
The freshness model (<xref target="expiration.model"/>) does not
necessarily apply to history mechanisms. I.e., a history mechanism can
display a previous representation even if it has expired.
</t>
<t>
This does not prohibit the history mechanism from telling the user that a
view might be stale, or from honoring cache directives (e.g.,
Cache-Control: no-store).
</t>
</section>
<section anchor="IANA.considerations" title="IANA Considerations">
<section title="Cache Directive Registry" anchor="cache.directive.registration">
<t>
The registration procedure for HTTP Cache Directives is defined by <xref target="cache.control.extensions"/> of this document.
</t>
<t>
The HTTP Cache Directive Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
populated with the registrations below:
</t>
<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
<texttable align="left" suppress-title="true" anchor="iana.cache.directive.registration.table">
<ttcol>Cache Directive</ttcol>
<ttcol>Reference</ttcol>
<c>max-age</c>
<c>
<xref target="cache-request-directive.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.only-if-cached"/>
</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 title="Warn Code Registry" anchor="warn.code.registration">
<t>
The registration procedure for HTTP Warn Codes is defined by <xref target="warn.code.extensions"/> of this document.
</t>
<t>
The HTTP Warn Code Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
populated with the registrations below:
</t>
<!--AUTOGENERATED FROM extract-warn-code-defs.xslt, do not edit manually-->
<texttable align="left" suppress-title="true" anchor="iana.warn.code.registration.table">
<ttcol>Warn Code</ttcol>
<ttcol>Short Description</ttcol>
<ttcol>Reference</ttcol>
<c>110</c>
<c>Response is Stale</c>
<c>
<xref target="warn.110"/>
</c>
<c>111</c>
<c>Revalidation Failed</c>
<c>
<xref target="warn.111"/>
</c>
<c>112</c>
<c>Disconnected Operation</c>
<c>
<xref target="warn.112"/>
</c>
<c>113</c>
<c>Heuristic Expiration</c>
<c>
<xref target="warn.113"/>
</c>
<c>199</c>
<c>Miscellaneous Warning</c>
<c>
<xref target="warn.199"/>
</c>
<c>214</c>
<c>Transformation Applied</c>
<c>
<xref target="warn.214"/>
</c>
<c>299</c>
<c>Miscellaneous Persistent Warning</c>
<c>
<xref target="warn.299"/>
</c>
</texttable>
<!--(END)-->
</section>
<section title="Header Field Registration" anchor="header.field.registration">
<t>
The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>
shall be updated with the permanent registrations below (see <xref target="RFC3864"/>):
</t>
<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
<ttcol>Header Field Name</ttcol>
<ttcol>Protocol</ttcol>
<ttcol>Status</ttcol>
<ttcol>Reference</ttcol>
<c>Age</c>
<c>http</c>
<c>standard</c>
<c>
<xref target="header.age"/>
</c>
<c>Cache-Control</c>
<c>http</c>
<c>standard</c>
<c>
<xref target="header.cache-control"/>
</c>
<c>Expires</c>
<c>http</c>
<c>standard</c>
<c>
<xref target="header.expires"/>
</c>
<c>Pragma</c>
<c>http</c>
<c>standard</c>
<c>
<xref target="header.pragma"/>
</c>
<c>Warning</c>
<c>http</c>
<c>standard</c>
<c>
<xref target="header.warning"/>
</c>
</texttable>
<!--(END)-->
<t>
The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
Force".
</t>
</section>
</section>
<section anchor="security.considerations" title="Security Considerations">
<t>
Caches expose additional potential vulnerabilities, since the contents of
the cache represent an attractive target for malicious exploitation.
Because cache contents persist after an HTTP request is complete, an attack
on the cache can reveal information long after a user believes that the
information has been removed from the network. Therefore, cache contents
need to be protected as sensitive information.
</t>
<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
unauthorised parties.
</t>
<t>
Note that the Set-Cookie response header <xref target="RFC6265"/> does not
inhibit caching; a cacheable response with a Set-Cookie header 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 headers.
</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="October" year="2012"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-21"/>
</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="October" year="2012"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-21"/>
</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="October" year="2012"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-21"/>
</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="October" year="2012"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-21"/>
</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="October" year="2012"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-21"/>
</reference>
<reference anchor="RFC2119">
<front>
<title>Key words for use in RFCs to Indicate Requirement Levels</title>
<author fullname="Scott Bradner" initials="S." surname="Bradner">
<organization>Harvard University</organization>
<address><email>sob@harvard.edu</email></address>
</author>
<date month="March" year="1997"/>
</front>
<seriesInfo name="BCP" value="14"/>
<seriesInfo name="RFC" value="2119"/>
</reference>
<reference anchor="RFC5234">
<front>
<title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
<author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
<organization>Brandenburg InternetWorking</organization>
<address>
<email>dcrocker@bbiw.net</email>
</address>
</author>
<author initials="P." surname="Overell" fullname="Paul Overell">
<organization>THUS plc.</organization>
<address>
<email>paul.overell@thus.net</email>
</address>
</author>
<date month="January" year="2008"/>
</front>
<seriesInfo name="STD" value="68"/>
<seriesInfo name="RFC" value="5234"/>
</reference>
</references>
<references title="Informative References">
<reference anchor="RFC1305">
<front>
<title>Network Time Protocol (Version 3) Specification, Implementation</title>
<author fullname="David L. Mills" initials="D." surname="Mills">
<organization>University of Delaware, Electrical Engineering Department</organization>
<address><email>mills@udel.edu</email></address>
</author>
<date month="March" year="1992"/>
</front>
<seriesInfo name="RFC" value="1305"/>
</reference>
<reference anchor="RFC2616">
<front>
<title>Hypertext Transfer Protocol -- HTTP/1.1</title>
<author fullname="R. Fielding" initials="R." surname="Fielding">
<organization>University of California, Irvine</organization>
<address><email>fielding@ics.uci.edu</email></address>
</author>
<author fullname="J. Gettys" initials="J." surname="Gettys">
<organization>W3C</organization>
<address><email>jg@w3.org</email></address>
</author>
<author fullname="J. Mogul" initials="J." surname="Mogul">
<organization>Compaq Computer Corporation</organization>
<address><email>mogul@wrl.dec.com</email></address>
</author>
<author fullname="H. Frystyk" initials="H." surname="Frystyk">
<organization>MIT Laboratory for Computer Science</organization>
<address><email>frystyk@w3.org</email></address>
</author>
<author fullname="L. Masinter" initials="L." surname="Masinter">
<organization>Xerox Corporation</organization>
<address><email>masinter@parc.xerox.com</email></address>
</author>
<author fullname="P. Leach" initials="P." surname="Leach">
<organization>Microsoft Corporation</organization>
<address><email>paulle@microsoft.com</email></address>
</author>
<author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
<organization>W3C</organization>
<address><email>timbl@w3.org</email></address>
</author>
<date month="June" year="1999"/>
</front>
<seriesInfo name="RFC" value="2616"/>
</reference>
<reference anchor="RFC3864">
<front>
<title>Registration Procedures for Message Header Fields</title>
<author fullname="G. Klyne" initials="G." surname="Klyne">
<organization>Nine by Nine</organization>
<address><email>GK-IETF@ninebynine.org</email></address>
</author>
<author fullname="M. Nottingham" initials="M." surname="Nottingham">
<organization>BEA Systems</organization>
<address><email>mnot@pobox.com</email></address>
</author>
<author fullname="J. Mogul" initials="J." surname="Mogul">
<organization>HP Labs</organization>
<address><email>JeffMogul@acm.org</email></address>
</author>
<date month="September" year="2004"/>
</front>
<seriesInfo name="BCP" value="90"/>
<seriesInfo name="RFC" value="3864"/>
</reference>
<reference anchor="RFC5226">
<front>
<title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
<author initials="T." surname="Narten" fullname="T. Narten">
<organization>IBM</organization>
<address><email>narten@us.ibm.com</email></address>
</author>
<author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
<organization>Google</organization>
<address><email>Harald@Alvestrand.no</email></address>
</author>
<date year="2008" month="May"/>
</front>
<seriesInfo name="BCP" value="26"/>
<seriesInfo name="RFC" value="5226"/>
</reference>
<reference anchor="RFC5861">
<front>
<title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
<author initials="M." surname="Nottingham" fullname="Mark Nottingham">
<organization>Yahoo! Inc.</organization>
<address><email>mnot@yahoo-inc.com</email></address>
</author>
<date month="April" year="2010"/>
</front>
<seriesInfo name="RFC" value="5861"/>
</reference>
<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>
Make the specified age calculation algorithm less conservative.
(<xref target="age.calculations"/>)
</t>
<t>
Remove requirement to consider "Content-Location" in successful
responses in order to determine the appropriate response to use.
(<xref target="validation.model"/>)
</t>
<t>
Clarify denial of service attack avoidance requirement.
(<xref target="invalidation.after.updates.or.deletions"/>)
</t>
<t>
Do not mention RFC 2047 encoding and multiple languages in "<xref target="header.warning" format="none">Warning</xref>"
header fields anymore, as these aspects never were implemented.
(<xref target="header.warning"/>)
</t>
<t>
Introduce Cache Directive and Warn Code Registries.
(<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 3.2.1>
field-name = <field-name, defined in [Part1], Section 3.2>
quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>
token = <token, defined in [Part1], Section 3.2.4>
port = <port, defined in [Part1], Section 2.7>
pseudonym = <pseudonym, defined in [Part1], Section 5.7>
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 8.1.1.1>
]]></artwork></figure>
</section>
<section title="Collected ABNF" anchor="collected.abnf">
<figure>
<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
Age = delta-seconds
Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
cache-directive ] )
Expires = HTTP-date
HTTP-date = <HTTP-date, defined in [Part2], Section 8.1.1.1>
OWS = <OWS, defined in [Part1], Section 3.2.1>
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>
quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>
token = <token, defined in [Part1], Section 3.2.4>
uri-host = <uri-host, defined in [Part1], Section 2.7>
warn-agent = ( uri-host [ ":" port ] ) / pseudonym
warn-code = 3DIGIT
warn-date = DQUOTE HTTP-date DQUOTE
warn-text = quoted-string
warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
]
]]></artwork>
</figure>
</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>
</back>
</rfc>| PAFTECH AB 2003-2026 | 2026-04-23 10:03:23 |