One document matched: draft-ietf-httpbis-p6-cache-00.xml
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<rfc obsoletes="2068, 2616" category="std" ipr="full3978" docName="draft-ietf-httpbis-p6-cache-00">
<front>
<title abbrev="HTTP/1.1, part 6">HTTP/1.1, part 6: Caching</title>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
<organization abbrev="Day Software">Day Software</organization>
<address>
<postal>
<street>23 Corporate Plaza DR, Suite 280</street>
<city>Newport Beach</city>
<region>CA</region>
<code>92660</code>
<country>USA</country>
</postal>
<phone>+1-949-706-5300</phone>
<facsimile>+1-949-706-5305</facsimile>
<email>fielding@gbiv.com</email>
<uri>http://roy.gbiv.com/</uri>
</address>
</author>
<author initials="J." surname="Gettys" fullname="Jim Gettys">
<organization>One Laptop per Child</organization>
<address>
<postal>
<street>21 Oak Knoll Road</street>
<city>Carlisle</city>
<region>MA</region>
<code>01741</code>
<country>USA</country>
</postal>
<email>jg@laptop.org</email>
<uri>http://www.laptop.org/</uri>
</address>
</author>
<author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
<organization abbrev="HP">Hewlett-Packard Company</organization>
<address>
<postal>
<street>HP Labs, Large Scale Systems Group</street>
<street>1501 Page Mill Road, MS 1177</street>
<city>Palo Alto</city>
<region>CA</region>
<code>94304</code>
<country>USA</country>
</postal>
<email>JeffMogul@acm.org</email>
</address>
</author>
<author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
<organization abbrev="Microsoft">Microsoft Corporation</organization>
<address>
<postal>
<street>1 Microsoft Way</street>
<city>Redmond</city>
<region>WA</region>
<code>98052</code>
<country>USA</country>
</postal>
<email>henrikn@microsoft.com</email>
</address>
</author>
<author initials="L." surname="Masinter" fullname="Larry Masinter">
<organization abbrev="Adobe Systems">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>LMM@acm.org</email>
<uri>http://larry.masinter.net/</uri>
</address>
</author>
<author initials="P." surname="Leach" fullname="Paul J. Leach">
<organization abbrev="Microsoft">Microsoft Corporation</organization>
<address>
<postal>
<street>1 Microsoft Way</street>
<city>Redmond</city>
<region>WA</region>
<code>98052</code>
</postal>
<email>paulle@microsoft.com</email>
</address>
</author>
<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
<organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
<address>
<postal>
<street>MIT Computer Science and Artificial Intelligence Laboratory</street>
<street>The Stata Center, Building 32</street>
<street>32 Vassar Street</street>
<city>Cambridge</city>
<region>MA</region>
<code>02139</code>
<country>USA</country>
</postal>
<email>timbl@w3.org</email>
<uri>http://www.w3.org/People/Berners-Lee/</uri>
</address>
</author>
<date month="December" year="2007"/>
<abstract>
<t>
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypermedia information
systems. HTTP has been in use by the World Wide Web global information
initiative since 1990. This document is Part 6 of the seven-part specification
that defines the protocol referred to as "HTTP/1.1" and, taken together,
obsoletes RFC 2616. Part 6 defines requirements on HTTP caches
and the associated header fields that control cache behavior or indicate
cacheable response messages.
</t>
</abstract>
<note title="Editorial Note (To be removed by RFC Editor)">
<t>
This version of the HTTP specification contains only minimal editorial
changes from <xref target="RFC2616"/> (abstract, introductory paragraph,
and authors' addresses). All other changes are due to partitioning the
original into seven mostly independent parts. The intent is for readers
of future drafts to able to use draft 00 as the basis for comparison
when the WG makes later changes to the specification text. This draft
will shortly be followed by draft 01 (containing the first round of changes
that have already been agreed to on the mailing list). There is no point in
reviewing this draft other than to verify that the partitioning has been
done correctly. Roy T. Fielding, Yves Lafon, and Julian Reschke
will be the editors after draft 00 is submitted.
</t>
<t>
Discussion of this draft should take place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org). The current issues list is
at <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/report/11"/>
and related documents (including fancy diffs) can be found at
<eref target="http://www3.tools.ietf.org/wg/httpbis/"/>.
</t>
</note>
</front>
<middle>
<section title="Introduction" anchor="introduction">
<t>
This document will define aspects of HTTP related to caching response
messages. Right now it only includes the extracted relevant sections
of <xref target="RFC2616">RFC 2616</xref> without edit.
</t>
<section title="Terminology" anchor="intro.terminology">
<t>
This specification uses a number of terms to refer to the roles
played by participants in, and objects of, the HTTP communication.
</t>
<t>
<iref item="cache"/>
cache
<list>
<t>
A program's 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 include a cache, though a cache
cannot be used by a server that is acting as a tunnel.
</t>
</list>
</t>
<t>
<iref item="cacheable"/>
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. The
rules for determining the cacheability of HTTP responses are
defined in <xref target="caching"/>. Even if a resource is cacheable, there may
be additional constraints on whether a cache can use the cached
copy for a particular request.
</t>
</list>
</t>
<t>
<iref item="first-hand"/>
first-hand
<list>
<t>
A response is first-hand if it comes directly and without
unnecessary delay from the origin server, perhaps via one or more
proxies. A response is also first-hand if its validity has just
been checked directly with the origin server.
</t>
</list>
</t>
<t>
<iref item="explicit expiration time"/>
explicit expiration time
<list>
<t>
The time at which the origin server intends that an entity should
no longer be returned by a cache without further validation.
</t>
</list>
</t>
<t>
<iref item="heuristic expiration time"/>
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"/>
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="freshness lifetime"/>
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"/>
fresh
<list>
<t>
A response is fresh if its age has not yet exceeded its freshness
lifetime.
</t>
</list>
</t>
<t>
<iref item="stale"/>
stale
<list>
<t>
A response is stale if its age has passed its freshness lifetime.
</t>
</list>
</t>
<t>
<iref item="semantically transparent"/>
semantically transparent
<list>
<t>
A cache behaves in a "semantically transparent" manner, with
respect to a particular response, when its use affects neither the
requesting client nor the origin server, except to improve
performance. When a cache is semantically transparent, the client
receives exactly the same response (except for hop-by-hop headers)
that it would have received had its request been handled directly
by the origin server.
</t>
</list>
</t>
<t>
<iref item="validator"/>
validator
<list>
<t>
A protocol element (e.g., an entity tag or a Last-Modified time)
that is used to find out whether a cache entry is an equivalent
copy of an entity.
</t>
</list>
</t>
</section>
<section title="Delta Seconds" anchor="delta.seconds">
<t>
Some HTTP header fields allow a time value to be specified as an
integer number of seconds, represented in decimal, after the time
that the message was received.
</t>
<figure><iref primary="true" item="Grammar" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
delta-seconds = 1*DIGIT
]]></artwork></figure>
</section>
</section>
<section title="Caching in HTTP" anchor="caching">
<section title="Overview" anchor="caching.overview">
<t>
HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. The
HTTP/1.1 protocol includes a number of elements intended to make
caching work as well as possible. Because these elements are
inextricable from other aspects of the protocol, and because they
interact with each other, it is useful to describe the basic caching
design of HTTP separately from the detailed descriptions of methods,
headers, response codes, etc.
</t>
<t>
Caching would be useless if it did not significantly improve
performance. The goal of caching in HTTP/1.1 is to eliminate the need
to send requests in many cases, and to eliminate the need to send
full responses in many other cases. The former reduces the number of
network round-trips required for many operations; we use an
"expiration" mechanism for this purpose (see <xref target="expiration.model"/>). The
latter reduces network bandwidth requirements; we use a "validation"
mechanism for this purpose (see <xref target="validation.model"/>).
</t>
<t>
Requirements for performance, availability, and disconnected
operation require us to be able to relax the goal of semantic
transparency. The HTTP/1.1 protocol allows origin servers, caches,
and clients to explicitly reduce transparency when necessary.
However, because non-transparent operation may confuse non-expert
users, and might be incompatible with certain server applications
(such as those for ordering merchandise), the protocol requires that
transparency be relaxed
<list style="symbols">
<t>only by an explicit protocol-level request when relaxed by
client or origin server</t>
<t>only with an explicit warning to the end user when relaxed by
cache or client</t>
</list>
</t>
<t>
Therefore, the HTTP/1.1 protocol provides these important elements:
<list style="numbers">
<t>Protocol features that provide full semantic transparency when
this is required by all parties.</t>
<t>Protocol features that allow an origin server or user agent to
explicitly request and control non-transparent operation.</t>
<t>Protocol features that allow a cache to attach warnings to
responses that do not preserve the requested approximation of
semantic transparency.</t>
</list>
</t>
<t>
A basic principle is that it must be possible for the clients to
detect any potential relaxation of semantic transparency.
<list><t>
Note: The server, cache, or client implementor might be faced with
design decisions not explicitly discussed in this specification.
If a decision might affect semantic transparency, the implementor
ought to err on the side of maintaining transparency unless a
careful and complete analysis shows significant benefits in
breaking transparency.
</t></list>
</t>
<section title="Cache Correctness" anchor="cache.correctness">
<t>
A correct cache MUST respond to a request with the most up-to-date
response held by the cache that is appropriate to the request (see
sections <xref target="disambiguating.expiration.values" format="counter"/>,
<xref target="disambiguating.multiple.responses" format="counter"/>,
and <xref target="cache.replacement" format="counter"/>) which meets one of the following
conditions:
<list style="numbers">
<t>It has been checked for equivalence with what the origin server
would have returned by revalidating the response with the
origin server (<xref target="validation.model"/>);</t>
<t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
this means it meets the least restrictive freshness requirement
of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
the origin server so specifies, it is the freshness requirement
of the origin server alone.
If a stored response is not "fresh enough" by the most
restrictive freshness requirement of both the client and the
origin server, in carefully considered circumstances the cache
MAY still return the response with the appropriate Warning
header (see section <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
and <xref target="header.warning" format="counter"/>), unless such a response
is prohibited (e.g., by a "no-store" cache-directive, or by a
"no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
<t>It is an appropriate 304 (Not Modified), 305 (Proxy Redirect),
or error (4xx or 5xx) response message.</t>
</list>
</t>
<t>
If the cache can not communicate with the origin server, then a
correct cache SHOULD respond as above if the response can be
correctly served from the cache; if not it MUST return an error or
warning indicating that there was a communication failure.
</t>
<t>
If a cache receives a 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 SHOULD forward it to the requesting client without adding a new
Warning (but without removing any existing Warning headers). A cache
SHOULD NOT attempt to revalidate a response simply because that
response became stale in transit; this might lead to an infinite
loop. A user agent that receives a stale response without a Warning
MAY display a warning indication to the user.
</t>
</section>
<section title="Warnings" anchor="warnings">
<t>
Whenever a cache returns a response that is neither first-hand nor
"fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
MUST attach a warning to that effect, using a Warning general-header.
The Warning header and the currently defined warnings are described
in <xref target="header.warning"/>. The warning allows clients to take appropriate
action.
</t>
<t>
Warnings MAY be used for other purposes, both cache-related and
otherwise. The use of a warning, rather than an error status code,
distinguish these responses from true failures.
</t>
<t>
Warnings are assigned three digit warn-codes. The first digit
indicates whether the Warning MUST or MUST NOT be deleted from a
stored cache entry after a successful revalidation:
</t>
<t>
<list style="hanging">
<t hangText="1xx">Warnings that describe the freshness or revalidation status of
the response, and so MUST be deleted after a successful
revalidation. 1XX warn-codes MAY be generated by a cache only when
validating a cached entry. It MUST NOT be generated by clients.</t>
<t hangText="2xx">Warnings that describe some aspect of the entity body or entity
headers that is not rectified by a revalidation (for example, a
lossy compression of the entity bodies) and which MUST NOT be
deleted after a successful revalidation.</t>
</list>
</t>
<t>
See <xref target="header.warning"/> for the definitions of the codes themselves.
</t>
<t>
HTTP/1.0 caches will cache all Warnings in responses, without
deleting the ones in the first category. Warnings in responses that
are passed to HTTP/1.0 caches carry an extra warning-date field,
which prevents a future HTTP/1.1 recipient from believing an
erroneously cached Warning.
</t>
<t>
Warnings also carry a warning text. The text MAY be in any
appropriate natural language (perhaps based on the client's Accept
headers), and include an OPTIONAL indication of what character set is
used.
</t>
<t>
Multiple warnings MAY be attached to a response (either by the origin
server or by a cache), including multiple warnings with the same code
number. For example, a server might provide the same warning with
texts in both English and Basque.
</t>
<t>
When multiple warnings are attached to a response, it might not be
practical or reasonable to display all of them to the user. This
version of HTTP does not specify strict priority rules for deciding
which warnings to display and in what order, but does suggest some
heuristics.
</t>
</section>
<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
<t>
The basic cache mechanisms in HTTP/1.1 (server-specified expiration
times and validators) are implicit directives to caches. In some
cases, a server or client might need to provide explicit directives
to the HTTP caches. We use the Cache-Control header for this purpose.
</t>
<t>
The Cache-Control header allows a client or server to transmit a
variety of directives in either requests or responses. These
directives typically override the default caching algorithms. As a
general rule, if there is any apparent conflict between header
values, the most restrictive interpretation is applied (that is, the
one that is most likely to preserve semantic transparency). However,
in some cases, cache-control directives are explicitly specified as
weakening the approximation of semantic transparency (for example,
"max-stale" or "public").
</t>
<t>
The cache-control directives are described in detail in <xref target="header.cache-control"/>.
</t>
</section>
<section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
<t>
Many user agents make it possible for users to override the basic
caching mechanisms. For example, the user agent might allow the user
to specify that cached entities (even explicitly stale ones) are
never validated. Or the user agent might habitually add "Cache-Control:
max-stale=3600" to every request. The user agent SHOULD NOT
default to either non-transparent behavior, or behavior that results
in abnormally ineffective caching, but MAY be explicitly configured
to do so by an explicit action of the user.
</t>
<t>
If the user has overridden the basic caching mechanisms, the user
agent SHOULD explicitly indicate to the user whenever this results in
the display of information that might not meet the server's
transparency requirements (in particular, if the displayed entity is
known to be stale). Since the protocol normally allows the user agent
to determine if responses are stale or not, this indication need only
be displayed when this actually happens. The indication need not be a
dialog box; it could be an icon (for example, a picture of a rotting
fish) or some other indicator.
</t>
<t>
If the user has overridden the caching mechanisms in a way that would
abnormally reduce the effectiveness of caches, the user agent SHOULD
continually indicate this state to the user (for example, by a
display of a picture of currency in flames) so that the user does not
inadvertently consume excess resources or suffer from excessive
latency.
</t>
</section>
<section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
<t>
In some cases, the operator of a cache MAY choose to configure it to
return stale responses even when not requested by clients. This
decision ought not be made lightly, but may be necessary for reasons
of availability or performance, especially when the cache is poorly
connected to the origin server. Whenever a cache returns a stale
response, it MUST mark it as such (using a Warning header) enabling
the client software to alert the user that there might be a potential
problem.
</t>
<t>
It also allows the user agent to take steps to obtain a first-hand or
fresh response. For this reason, a cache SHOULD NOT return a stale
response if the client explicitly requests a first-hand or fresh one,
unless it is impossible to comply for technical or policy reasons.
</t>
</section>
<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
<t>
While the origin server (and to a lesser extent, intermediate caches,
by their contribution to the age of a response) are the primary
source of expiration information, in some cases the client might need
to control a cache's decision about whether to return a cached
response without validating it. Clients do this using several
directives of the Cache-Control header.
</t>
<t>
A client's request MAY specify the maximum age it is willing to
accept of an unvalidated response; specifying a value of zero forces
the cache(s) to revalidate all responses. A client MAY also specify
the minimum time remaining before a response expires. Both of these
options increase constraints on the behavior of caches, and so cannot
further relax the cache's approximation of semantic transparency.
</t>
<t>
A client MAY also specify that it will accept stale responses, up to
some maximum amount of staleness. This loosens the constraints on the
caches, and so might violate the origin server's specified
constraints on semantic transparency, but might be necessary to
support disconnected operation, or high availability in the face of
poor connectivity.
</t>
</section>
</section>
<section title="Expiration Model" anchor="expiration.model">
<section title="Server-Specified Expiration" anchor="server-specified.expiration">
<t>
HTTP caching works best when caches can entirely avoid making
requests to the origin server. The primary mechanism for avoiding
requests is for an origin server to provide an explicit expiration
time in the future, indicating that a response MAY be used to satisfy
subsequent requests. In other words, a cache can return a fresh
response without first contacting the server.
</t>
<t>
Our expectation is that servers will assign future explicit
expiration times to responses in the belief that the entity is not
likely to change, in a semantically significant way, before the
expiration time is reached. This normally preserves semantic
transparency, as long as the server's expiration times are carefully
chosen.
</t>
<t>
The expiration mechanism applies only to responses taken from a cache
and not to first-hand responses forwarded immediately to the
requesting client.
</t>
<t>
If an origin server wishes to force a semantically transparent cache
to validate every request, it MAY assign an explicit expiration time
in the past. This means that the response is always stale, and so the
cache SHOULD validate it before using it for subsequent requests. See
<xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
</t>
<t>
If an origin server wishes to force any HTTP/1.1 cache, no matter how
it is configured, to validate every request, it SHOULD use the "must-revalidate"
cache-control directive (see <xref target="header.cache-control"/>).
</t>
<t>
Servers specify explicit expiration times using either the Expires
header, or the max-age directive of the Cache-Control header.
</t>
<t>
An expiration time cannot be used to force a user agent to refresh
its display or reload a resource; its semantics apply only to caching
mechanisms, and such mechanisms need only check a resource's
expiration status when a new request for that resource is initiated.
See <xref target="history.lists"/> for an explanation of the difference between caches
and history mechanisms.
</t>
</section>
<section title="Heuristic Expiration" anchor="heuristic.expiration">
<t>
Since origin servers do not always provide explicit expiration times,
HTTP caches typically assign heuristic expiration times, employing
algorithms that use other header values (such as the Last-Modified
time) to estimate a plausible expiration time. The HTTP/1.1
specification does not provide specific algorithms, but does impose
worst-case constraints on their results. Since heuristic expiration
times might compromise semantic transparency, they ought to used
cautiously, and we encourage origin servers to provide explicit
expiration times as much as possible.
</t>
</section>
<section title="Age Calculations" anchor="age.calculations">
<t>
In order to know if a cached entry is fresh, a cache needs to know if
its age exceeds its freshness lifetime. We discuss how to calculate
the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
the age of a response or cache entry.
</t>
<t>
In this discussion, we use the term "now" to mean "the current value
of the clock at the host performing the calculation." Hosts that use
HTTP, but especially hosts running origin servers and caches, SHOULD
use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
a globally accurate time standard.
</t>
<t>
HTTP/1.1 requires origin servers to send a Date header, if possible,
with every response, giving the time at which the response was
generated (see Section 8.3 of <xref target="Part1"/>). We use the term "date_value" to denote
the value of the Date header, in a form appropriate for arithmetic
operations.
</t>
<t>
HTTP/1.1 uses the Age response-header 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 revalidated by the origin server.
</t>
<t>
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>
We use the term "age_value" to denote the value of the Age header, in
a form appropriate for arithmetic operations.
</t>
<t>
A response's age can be calculated in two entirely independent ways:
<list style="numbers">
<t>now 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>age_value, if all of the caches along the response path
implement HTTP/1.1.</t>
</list>
</t>
<t>
Given that we have two independent ways to compute the age of a
response when it is received, we can combine these as
</t>
<figure><artwork type="code"><![CDATA[
corrected_received_age = max(now - date_value, age_value)
]]></artwork></figure>
<t>
and as long as we have either nearly synchronized clocks or all-HTTP/1.1
paths, one gets a reliable (conservative) result.
</t>
<t>
Because of network-imposed delays, some significant interval might
pass between the time that a server generates a response and the time
it is received at the next outbound cache or client. If uncorrected,
this delay could result in improperly low ages.
</t>
<t>
Because the request that resulted in the returned Age value must have
been initiated prior to that Age value's generation, we can correct
for delays imposed by the network by recording the time at which the
request was initiated. Then, when an Age value is received, it MUST
be interpreted relative to the time the request was initiated, not
the time that the response was received. This algorithm results in
conservative behavior no matter how much delay is experienced. So, we
compute:
</t>
<figure><artwork type="code"><![CDATA[
corrected_initial_age = corrected_received_age
+ (now - request_time)
]]></artwork></figure>
<t>
where "request_time" is the time (according to the local clock) when
the request that elicited this response was sent.
</t>
<t>
Summary of age calculation algorithm, when a cache receives a
response:
</t>
<figure><artwork type="code"><![CDATA[
/*
* age_value
* is the value of Age: header received by the cache with
* this response.
* date_value
* is the value of the origin server's Date: header
* request_time
* is the (local) time when the cache made the request
* that resulted in this cached response
* response_time
* is the (local) time when the cache received the
* response
* now
* is the current (local) time
*/
apparent_age = max(0, response_time - date_value);
corrected_received_age = max(apparent_age, age_value);
response_delay = response_time - request_time;
corrected_initial_age = corrected_received_age + response_delay;
resident_time = now - response_time;
current_age = corrected_initial_age + resident_time;
]]></artwork></figure>
<t>
The current_age of a cache entry is calculated by adding the amount
of time (in seconds) since the cache entry was last validated by the
origin server to the corrected_initial_age. When a response is
generated from a cache entry, the cache MUST include a single Age
header field in the response with a value equal to the cache entry's
current_age.
</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
the lack of an Age header field in a response does not imply that the
response is first-hand unless all caches along the request path are
compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
the Age header field).
</t>
</section>
<section title="Expiration Calculations" anchor="expiration.calculations">
<t>
In order to decide whether a response is fresh or stale, we need to
compare its freshness lifetime to its age. The age is calculated as
described in <xref target="age.calculations"/>; this section describes how to calculate
the freshness lifetime, and to determine if a response has expired.
In the discussion below, the values can be represented in any form
appropriate for arithmetic operations.
</t>
<t>
We use the term "expires_value" to denote the value of the Expires
header. We use the term "max_age_value" to denote an appropriate
value of the number of seconds carried by the "max-age" directive of
the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
</t>
<t>
The max-age directive takes priority over Expires, so if max-age is
present in a response, the calculation is simply:
</t>
<figure><artwork type="code"><![CDATA[
freshness_lifetime = max_age_value
]]></artwork></figure>
<t>
Otherwise, if Expires is present in the response, the calculation is:
</t>
<figure><artwork type="code"><![CDATA[
freshness_lifetime = expires_value - date_value
]]></artwork></figure>
<t>
Note that neither of these calculations is vulnerable to clock skew,
since all of the information comes from the origin server.
</t>
<t>
If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
(see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
does not include other restrictions on caching, the cache MAY compute
a freshness lifetime using a heuristic. The cache MUST attach Warning
113 to any response whose age is more than 24 hours if such warning
has not already been added.
</t>
<t>
Also, if the response does have a Last-Modified time, the heuristic
expiration value SHOULD be no more than some fraction of the interval
since that time. A typical setting of this fraction might be 10%.
</t>
<t>
The calculation to determine if a response has expired is quite
simple:
</t>
<figure><artwork type="code"><![CDATA[
response_is_fresh = (freshness_lifetime > current_age)
]]></artwork></figure>
</section>
<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
<t>
Because expiration values are assigned optimistically, it is possible
for two caches to contain fresh values for the same resource that are
different.
</t>
<t>
If a client performing a retrieval receives a non-first-hand response
for a request that was already fresh in its own cache, and the Date
header in its existing cache entry is newer than the Date on the new
response, then the client MAY ignore the response. If so, it MAY
retry the request with a "Cache-Control: max-age=0" directive (see
<xref target="header.cache-control"/>), to force a check with the origin server.
</t>
<t>
If a cache has two fresh responses for the same representation with
different validators, it MUST use the one with the more recent Date
header. This situation might arise because the cache is pooling
responses from other caches, or because a client has asked for a
reload or a revalidation of an apparently fresh cache entry.
</t>
</section>
<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
<t>
Because a client might be receiving responses via multiple paths, so
that some responses flow through one set of caches and other
responses flow through a different set of caches, a client might
receive responses in an order different from that in which the origin
server sent them. We would like the client to use the most recently
generated response, even if older responses are still apparently
fresh.
</t>
<t>
Neither the entity tag nor the expiration value can impose an
ordering on responses, since it is possible that a later response
intentionally carries an earlier expiration time. The Date values are
ordered to a granularity of one second.
</t>
<t>
When a client tries to revalidate a cache entry, and the response it
receives contains a Date header that appears to be older than the one
for the existing entry, then the client SHOULD repeat the request
unconditionally, and include
</t>
<figure><artwork type="example"><![CDATA[
Cache-Control: max-age=0
]]></artwork></figure>
<t>
to force any intermediate caches to validate their copies directly
with the origin server, or
</t>
<figure><artwork type="example"><![CDATA[
Cache-Control: no-cache
]]></artwork></figure>
<t>
to force any intermediate caches to obtain a new copy from the origin
server.
</t>
<t>
If the Date values are equal, then the client MAY use either response
(or MAY, if it is being extremely prudent, request a new response).
Servers MUST NOT depend on clients being able to choose
deterministically between responses generated during the same second,
if their expiration times overlap.
</t>
</section>
</section>
<section title="Validation Model" anchor="validation.model">
<t>
When a cache has a stale entry that it would like to use as a
response to a client's request, it first has to check with the origin
server (or possibly an intermediate cache with a fresh response) to
see if its cached entry is still usable. We call this "validating"
the cache entry. Since we do not want to have to pay the overhead of
retransmitting the full response if the cached entry is good, and we
do not want to pay the overhead of an extra round trip if the cached
entry is invalid, the HTTP/1.1 protocol supports the use of
conditional methods.
</t>
<t>
The key protocol features for supporting conditional methods are
those concerned with "cache validators." When an origin server
generates a full response, it attaches some sort of validator to it,
which is kept with the cache entry. When a client (user agent or
proxy cache) makes a conditional request for a resource for which it
has a cache entry, it includes the associated validator in the
request.
</t>
<t>
The server then checks that validator against the current validator
for the entity, and, if they match (see Section 4 of <xref target="Part4"/>), it responds
with a special status code (usually, 304 (Not Modified)) and no
entity-body. Otherwise, it returns a full response (including
entity-body). Thus, we avoid transmitting the full response if the
validator matches, and we avoid an extra round trip if it does not
match.
</t>
<t>
In HTTP/1.1, a conditional request looks exactly the same as a normal
request for the same resource, except that it carries a special
header (which includes the validator) that implicitly turns the
method (usually, GET) into a conditional.
</t>
<t>
The protocol includes both positive and negative senses of cache-validating
conditions. That is, it is possible to request either that
a method be performed if and only if a validator matches or if and
only if no validators match.
<list><t>
Note: a response that lacks a validator may still be cached, and
served from cache until it expires, unless this is explicitly
prohibited by a cache-control directive. However, a cache cannot
do a conditional retrieval if it does not have a validator for the
entity, which means it will not be refreshable after it expires.
</t></list>
</t>
<section title="Last-Modified Dates" anchor="last-modified.dates">
<t>
The Last-Modified entity-header field value is often used as a cache
validator. In simple terms, a cache entry is considered to be valid
if the entity has not been modified since the Last-Modified value.
</t>
</section>
<section title="Entity Tag Cache Validators" anchor="entity.tag.cache.validators">
<t>
The ETag response-header field value, an entity tag, provides for an
"opaque" cache validator. This might allow more reliable validation
in situations where it is inconvenient to store modification dates,
where the one-second resolution of HTTP date values is not
sufficient, or where the origin server wishes to avoid certain
paradoxes that might arise from the use of modification dates.
</t>
<t>
Entity Tags are described in Section 2 of <xref target="Part4"/>.
</t>
</section>
<section title="Non-validating Conditionals" anchor="non-validating.conditionals">
<t>
The principle behind entity tags is that only the service author
knows the semantics of a resource well enough to select an
appropriate cache validation mechanism, and the specification of any
validator comparison function more complex than byte-equality would
open up a can of worms. Thus, comparisons of any other headers
(except Last-Modified, for compatibility with HTTP/1.0) are never
used for purposes of validating a cache entry.
</t>
</section>
</section>
<section title="Response Cacheability" anchor="response.cacheability">
<t>
Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
directive, a caching system MAY always store a successful response
(see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, MAY return it without validation
if it is fresh, and MAY return it after successful validation. If
there is neither a cache validator nor an explicit expiration time
associated with a response, we do not expect it to be cached, but
certain caches MAY violate this expectation (for example, when little
or no network connectivity is available). A client can usually detect
that such a response was taken from a cache by comparing the Date
header to the current time.
<list><t>
Note: some HTTP/1.0 caches are known to violate this expectation
without providing any Warning.
</t></list>
</t>
<t>
However, in some cases it might be inappropriate for a cache to
retain an entity, or to return it in response to a subsequent
request. This might be because absolute semantic transparency is
deemed necessary by the service author, or because of security or
privacy considerations. Certain cache-control directives are
therefore provided so that the server can indicate that certain
resource entities, or portions thereof, are not to be cached
regardless of other considerations.
</t>
<t>
Note that Section 3.1 of <xref target="Part7"/> normally prevents a shared cache from saving
and returning a response to a previous request if that request
included an Authorization header.
</t>
<t>
A response received with a status code of 200, 203, 206, 300, 301 or
410 MAY be stored by a cache and used in reply to a subsequent
request, subject to the expiration mechanism, unless a cache-control
directive prohibits caching. However, a cache that does not support
the Range and Content-Range headers MUST NOT cache 206 (Partial
Content) responses.
</t>
<t>
A response received with any other status code (e.g. status codes 302
and 307) MUST NOT be returned in a reply to a subsequent request
unless there are cache-control directives or another header(s) that
explicitly allow it. For example, these include the following: an
Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage", "must-revalidate",
"proxy-revalidate", "public" or "private" cache-control
directive (<xref target="header.cache-control"/>).
</t>
</section>
<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
<t>
The purpose of an HTTP cache is to store information received in
response to requests for use in responding to future requests. In
many cases, a cache simply returns the appropriate parts of a
response to the requester. However, if the cache holds a cache entry
based on a previous response, it might have to combine parts of a new
response with what is held in the cache entry.
</t>
<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
<t>
For the purpose of defining the behavior of caches and non-caching
proxies, we divide HTTP headers into two categories:
<list style="symbols">
<t>End-to-end headers, which are transmitted to the ultimate
recipient of a request or response. End-to-end headers in
responses MUST be stored as part of a cache entry and MUST be
transmitted in any response formed from a cache entry.</t>
<t>Hop-by-hop headers, which are meaningful only for a single
transport-level connection, and are not stored by caches or
forwarded by proxies.</t>
</list>
</t>
<t>
The following HTTP/1.1 headers are hop-by-hop headers:
<list style="symbols">
<t>Connection</t>
<t>Keep-Alive</t>
<t>Proxy-Authenticate</t>
<t>Proxy-Authorization</t>
<t>TE</t>
<t>Trailers</t>
<t>Transfer-Encoding</t>
<t>Upgrade</t>
</list>
</t>
<t>
All other headers defined by HTTP/1.1 are end-to-end headers.
</t>
<t>
Other hop-by-hop headers MUST be listed in a Connection header,
(Section 8.1 of <xref target="Part1"/>) to be introduced into HTTP/1.1 (or later).
</t>
</section>
<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
<t>
Some features of the HTTP/1.1 protocol, such as Digest
Authentication, depend on the value of certain end-to-end headers. A
transparent proxy SHOULD NOT modify an end-to-end header unless the
definition of that header requires or specifically allows that.
</t>
<t>
A transparent proxy MUST NOT modify any of the following fields in a
request or response, and it MUST NOT add any of these fields if not
already present:
<list style="symbols">
<t>Content-Location</t>
<t>Content-MD5</t>
<t>ETag</t>
<t>Last-Modified</t>
</list>
</t>
<t>
A transparent proxy MUST NOT modify any of the following fields in a
response:
<list style="symbols">
<t>Expires</t>
</list>
</t>
<t>
but it MAY add any of these fields if not already present. If an
Expires header is added, it MUST be given a field-value identical to
that of the Date header in that response.
</t>
<t>
A proxy MUST NOT modify or add any of the following fields in a
message that contains the no-transform cache-control directive, or in
any request:
<list style="symbols">
<t>Content-Encoding</t>
<t>Content-Range</t>
<t>Content-Type</t>
</list>
</t>
<t>
A non-transparent proxy MAY modify or add these fields to a message
that does not include no-transform, but if it does so, it MUST add a
Warning 214 (Transformation applied) if one does not already appear
in the message (see <xref target="header.warning"/>).
<list><t>
Warning: unnecessary modification of end-to-end headers might
cause authentication failures if stronger authentication
mechanisms are introduced in later versions of HTTP. Such
authentication mechanisms MAY rely on the values of header fields
not listed here.
</t></list>
</t>
<t>
The Content-Length field of a request or response is added or deleted
according to the rules in Section 4.4 of <xref target="Part1"/>. A transparent proxy MUST
preserve the entity-length (Section 3.2.2 of <xref target="Part3"/>) of the entity-body,
although it MAY change the transfer-length (Section 4.4 of <xref target="Part1"/>).
</t>
</section>
<section title="Combining Headers" anchor="combining.headers">
<t>
When a cache makes a validating request to a server, and the server
provides a 304 (Not Modified) response or a 206 (Partial Content)
response, the cache then constructs a response to send to the
requesting client.
</t>
<t>
If the status code is 304 (Not Modified), the cache uses the entity-body
stored in the cache entry as the entity-body of this outgoing
response. If the status code is 206 (Partial Content) and the ETag or
Last-Modified headers match exactly, the cache MAY combine the
contents stored in the cache entry with the new contents received in
the response and use the result as the entity-body of this outgoing
response, (see Section 4 of <xref target="Part5"/>).
</t>
<t>
The end-to-end headers stored in the cache entry are used for the
constructed response, except that
<list style="symbols">
<t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
MUST be deleted from the cache entry and the forwarded response.</t>
<t>any stored Warning headers with warn-code 2xx MUST be retained
in the cache entry and the forwarded response.</t>
<t>any end-to-end headers provided in the 304 or 206 response MUST
replace the corresponding headers from the cache entry.</t>
</list>
</t>
<t>
Unless the cache decides to remove the cache entry, it MUST also
replace the end-to-end headers stored with the cache entry with
corresponding headers received in the incoming response, except for
Warning headers as described immediately above. If a header field-name
in the incoming response matches more than one header in the
cache entry, all such old headers MUST be replaced.
</t>
<t>
In other words, the set of end-to-end headers received in the
incoming response overrides all corresponding end-to-end headers
stored with the cache entry (except for stored Warning headers with
warn-code 1xx, which are deleted even if not overridden).
<list><t>
Note: this rule allows an origin server to use a 304 (Not
Modified) or a 206 (Partial Content) response to update any header
associated with a previous response for the same entity or sub-ranges
thereof, although it might not always be meaningful or
correct to do so. This rule does not allow an origin server to use
a 304 (Not Modified) or a 206 (Partial Content) response to
entirely delete a header that it had provided with a previous
response.
</t></list>
</t>
</section>
</section>
<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
<t>
Use of server-driven content negotiation (Section 4.1 of <xref target="Part3"/>), as indicated
by the presence of a Vary header field in a response, alters the
conditions and procedure by which a cache can use the response for
subsequent requests. See <xref target="header.vary"/> for use of the Vary header
field by servers.
</t>
<t>
A server SHOULD use the Vary header field to inform a cache of what
request-header fields were used to select among multiple
representations of a cacheable response subject to server-driven
negotiation. The set of header fields named by the Vary field value
is known as the "selecting" request-headers.
</t>
<t>
When the cache receives a subsequent request whose Request-URI
specifies one or more cache entries including a Vary header field,
the cache MUST NOT use such a cache entry to construct a response to
the new request unless all of the selecting request-headers present
in the new request match the corresponding stored request-headers in
the original request.
</t>
<t>
The selecting request-headers from two requests are defined to match
if and only if the selecting request-headers in the first request can
be transformed to the selecting request-headers in the second request
by adding or removing linear white space (LWS) at places where this
is allowed by the corresponding BNF, and/or combining multiple
message-header fields with the same field name following the rules
about message headers in Section 4.2 of <xref target="Part1"/>.
</t>
<t>
A Vary header field-value of "*" always fails to match and subsequent
requests on that resource can only be properly interpreted by the
origin server.
</t>
<t>
If the selecting request header fields for the cached entry do not
match the selecting request header fields of the new request, then
the cache MUST NOT use a cached entry to satisfy the request unless
it first relays the new request to the origin server in a conditional
request and the server responds with 304 (Not Modified), including an
entity tag or Content-Location that indicates the entity to be used.
</t>
<t>
If an entity tag was assigned to a cached representation, the
forwarded request SHOULD be conditional and include the entity tags
in an If-None-Match header field from all its cache entries for the
resource. This conveys to the server the set of entities currently
held by the cache, so that if any one of these entities matches the
requested entity, the server can use the ETag header field in its 304
(Not Modified) response to tell the cache which entry is appropriate.
If the entity-tag of the new response matches that of an existing
entry, the new response SHOULD be used to update the header fields of
the existing entry, and the result MUST be returned to the client.
</t>
<t>
If any of the existing cache entries contains only partial content
for the associated entity, its entity-tag SHOULD NOT be included in
the If-None-Match header field unless the request is for a range that
would be fully satisfied by that entry.
</t>
<t>
If a cache receives a successful response whose Content-Location
field matches that of an existing cache entry for the same Request-URI,
whose entity-tag differs from that of the existing entry, and
whose Date is more recent than that of the existing entry, the
existing entry SHOULD NOT be returned in response to future requests
and SHOULD be deleted from the cache.
</t>
</section>
<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
<t>
For reasons of security and privacy, it is necessary to make a
distinction between "shared" and "non-shared" caches. A non-shared
cache is one that is accessible only to a single user. Accessibility
in this case SHOULD be enforced by appropriate security mechanisms.
All other caches are considered to be "shared." Other sections of
this specification place certain constraints on the operation of
shared caches in order to prevent loss of privacy or failure of
access controls.
</t>
</section>
<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
<t>
A cache that receives an incomplete response (for example, with fewer
bytes of data than specified in a Content-Length header) MAY store
the response. However, the cache MUST treat this as a partial
response. Partial responses MAY be combined as described in Section 4 of <xref target="Part5"/>;
the result might be a full response or might still be
partial. A cache MUST NOT return a partial response to a client
without explicitly marking it as such, using the 206 (Partial
Content) status code. A cache MUST NOT return a partial response
using a status code of 200 (OK).
</t>
<t>
If a cache receives a 5xx response while attempting to revalidate an
entry, it MAY either forward this response to the requesting client,
or act as if the server failed to respond. In the latter case, it MAY
return a previously received response unless the cached entry
includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
</t>
</section>
<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
<t>
Unless the origin server explicitly prohibits the caching of their
responses, the application of GET and HEAD methods to any resources
SHOULD NOT have side effects that would lead to erroneous behavior if
these responses are taken from a cache. They MAY still have side
effects, but a cache is not required to consider such side effects in
its caching decisions. Caches are always expected to observe an
origin server's explicit restrictions on caching.
</t>
<t>
We note one exception to this rule: since some applications have
traditionally used GETs and HEADs with query URLs (those containing a
"?" in the rel_path part) to perform operations with significant side
effects, caches MUST NOT treat responses to such URIs as fresh unless
the server provides an explicit expiration time. This specifically
means that responses from HTTP/1.0 servers for such URIs SHOULD NOT
be taken from a cache. See Section 8.1.1 of <xref target="Part2"/> for related information.
</t>
</section>
<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
<t>
The effect of certain methods performed on a resource at the origin
server might cause one or more existing cache entries to become non-transparently
invalid. That is, although they might continue to be
"fresh," they do not accurately reflect what the origin server would
return for a new request on that resource.
</t>
<t>
There is no way for the HTTP protocol to guarantee that all such
cache entries are marked invalid. For example, the request that
caused the change at the origin server might not have gone through
the proxy where a cache entry is stored. However, several rules help
reduce the likelihood of erroneous behavior.
</t>
<t>
In this section, the phrase "invalidate an entity" means that the
cache will either remove all instances of that entity from its
storage, or will mark these as "invalid" and in need of a mandatory
revalidation before they can be returned in response to a subsequent
request.
</t>
<t>
Some HTTP methods MUST cause a cache to invalidate an entity. This is
either the entity referred to by the Request-URI, or by the Location
or Content-Location headers (if present). These methods are:
<list style="symbols">
<t>PUT</t>
<t>DELETE</t>
<t>POST</t>
</list>
</t>
<t>
In order to prevent denial of service attacks, an invalidation based
on the URI in a Location or Content-Location header MUST only be
performed if the host part is the same as in the Request-URI.
</t>
<t>
A cache that passes through requests for methods it does not
understand SHOULD invalidate any entities referred to by the
Request-URI.
</t>
</section>
<section title="Write-Through Mandatory" anchor="write-through.mandatory">
<t>
All methods that might be expected to cause modifications to the
origin server's resources MUST be written through to the origin
server. This currently includes all methods except for GET and HEAD.
A cache MUST NOT reply to such a request from a client before having
transmitted the request to the inbound server, and having received a
corresponding response from the inbound server. This does not prevent
a proxy cache from sending a 100 (Continue) response before the
inbound server has sent its final reply.
</t>
<t>
The alternative (known as "write-back" or "copy-back" caching) is not
allowed in HTTP/1.1, due to the difficulty of providing consistent
updates and the problems arising from server, cache, or network
failure prior to write-back.
</t>
</section>
<section title="Cache Replacement" anchor="cache.replacement">
<t>
If a new cacheable (see sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
<xref target="disambiguating.expiration.values" format="counter"/>,
<xref target="disambiguating.multiple.responses" format="counter"/>
and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
response is received from a resource while any existing responses for
the same resource are cached, the cache SHOULD use the new response
to reply to the current request. It MAY insert it into cache storage
and MAY, if it meets all other requirements, use it to respond to any
future requests that would previously have caused the old response to
be returned. If it inserts the new response into cache storage the
rules in <xref target="combining.headers"/> apply.
<list><t>
Note: a new response that has an older Date header value than
existing cached responses is not cacheable.
</t></list>
</t>
</section>
<section title="History Lists" anchor="history.lists">
<t>
User agents often have history mechanisms, such as "Back" buttons and
history lists, which can be used to redisplay an entity retrieved
earlier in a session.
</t>
<t>
History mechanisms and caches are different. In particular history
mechanisms SHOULD NOT try to show a semantically transparent view of
the current state of a resource. Rather, a history mechanism is meant
to show exactly what the user saw at the time when the resource was
retrieved.
</t>
<t>
By default, an expiration time does not apply to history mechanisms.
If the entity is still in storage, a history mechanism SHOULD display
it even if the entity has expired, unless the user has specifically
configured the agent to refresh expired history documents.
</t>
<t>
This is not to be construed to prohibit the history mechanism from
telling the user that a view might be stale.
<list><t>
Note: if history list mechanisms unnecessarily prevent users from
viewing stale resources, this will tend to force service authors
to avoid using HTTP expiration controls and cache controls when
they would otherwise like to. Service authors may consider it
important that users not be presented with error messages or
warning messages when they use navigation controls (such as BACK)
to view previously fetched resources. Even though sometimes such
resources ought not to cached, or ought to expire quickly, user
interface considerations may force service authors to resort to
other means of preventing caching (e.g. "once-only" URLs) in order
not to suffer the effects of improperly functioning history
mechanisms.
</t></list>
</t>
</section>
</section>
<section title="Header Field Definitions" anchor="header.fields">
<t>
This section defines the syntax and semantics of all standard
HTTP/1.1 header fields. For entity-header fields, both sender and
recipient refer to either the client or the server, depending on who
sends and who receives the entity.
</t>
<section title="Age" anchor="header.age">
<iref primary="true" item="Age header"/>
<iref primary="true" item="Headers" subitem="Age"/>
<t>
The Age response-header field conveys the sender's estimate of the
amount of time since the response (or its revalidation) was
generated at the origin server. A cached response is "fresh" if
its age does not exceed its freshness lifetime. Age values are
calculated as specified in <xref target="age.calculations"/>.
</t>
<figure><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/><artwork type="abnf2616"><![CDATA[
Age = "Age" ":" age-value
age-value = delta-seconds
]]></artwork></figure>
<t>
Age values are non-negative decimal integers, representing time in
seconds.
</t>
<t>
If a cache receives a value larger than the largest positive
integer it can represent, or if any of its age calculations
overflows, it MUST transmit an Age header with a value of
2147483648 (2^31). An HTTP/1.1 server that includes a cache MUST
include an Age header field in every response generated from its
own cache. Caches SHOULD use an arithmetic type of at least 31
bits of range.
</t>
</section>
<section title="Cache-Control" anchor="header.cache-control">
<iref primary="true" item="Cache-Control header"/>
<iref primary="true" item="Headers" subitem="Cache-Control"/>
<t>
The Cache-Control general-header field is used to specify directives
that MUST be obeyed by all caching mechanisms along the
request/response chain. The directives specify behavior intended to
prevent caches from adversely interfering with the request or
response. These directives typically override the default caching
algorithms. 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.
<list><t>
Note that 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>
Cache directives MUST be passed through by a proxy or gateway
application, 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 specify a cache-directive
for a specific cache.
</t>
<figure><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/><iref primary="true" item="Grammar" subitem="cache-request-directive"/><iref primary="true" item="Grammar" subitem="cache-response-directive"/><iref primary="true" item="Grammar" subitem="cache-extension"/><artwork type="abnf2616"><![CDATA[
Cache-Control = "Cache-Control" ":" 1#cache-directive
cache-directive = cache-request-directive
| cache-response-directive
cache-request-directive =
"no-cache" ; Section 3.2.1
| "no-store" ; Section 3.2.2
| "max-age" "=" delta-seconds ; Section 3.2.3, 3.2.4
| "max-stale" [ "=" delta-seconds ] ; Section 3.2.3
| "min-fresh" "=" delta-seconds ; Section 3.2.3
| "no-transform" ; Section 3.2.5
| "only-if-cached" ; Section 3.2.4
| cache-extension ; Section 3.2.6
cache-response-directive =
"public" ; Section 3.2.1
| "private" [ "=" <"> 1#field-name <"> ] ; Section 3.2.1
| "no-cache" [ "=" <"> 1#field-name <"> ]; Section 3.2.1
| "no-store" ; Section 3.2.2
| "no-transform" ; Section 3.2.5
| "must-revalidate" ; Section 3.2.4
| "proxy-revalidate" ; Section 3.2.4
| "max-age" "=" delta-seconds ; Section 3.2.3
| "s-maxage" "=" delta-seconds ; Section 3.2.3
| cache-extension ; Section 3.2.6
cache-extension = token [ "=" ( token | quoted-string ) ]
]]></artwork></figure>
<t>
When a directive appears without any 1#field-name parameter, the
directive applies to the entire request or response. When such a
directive appears with a 1#field-name parameter, it applies only to
the named field or fields, and not to the rest of the request or
response. This mechanism supports extensibility; implementations of
future versions of the HTTP protocol might apply these directives to
header fields not defined in HTTP/1.1.
</t>
<t>
The cache-control directives can be broken down into these general
categories:
<list style="symbols">
<t>Restrictions on what are cacheable; these may only be imposed by
the origin server.</t>
<t>Restrictions on what may be stored by a cache; these may be
imposed by either the origin server or the user agent.</t>
<t>Modifications of the basic expiration mechanism; these may be
imposed by either the origin server or the user agent.</t>
<t>Controls over cache revalidation and reload; these may only be
imposed by a user agent.</t>
<t>Control over transformation of entities.</t>
<t>Extensions to the caching system.</t>
</list>
</t>
<section title="What is Cacheable" anchor="what.is.cacheable">
<t>
By default, a response is cacheable if the requirements of the
request method, request header fields, and the response status
indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
for cacheability. The following Cache-Control response directives
allow an origin server to override the default cacheability of a
response:
</t>
<t>
<iref item="Cache Directives" subitem="public" primary="true"/>
<iref item="public" subitem="Cache Directive" primary="true"/>
public
<list><t>
Indicates that the response MAY be cached by any cache, even if it
would normally be non-cacheable or cacheable only within a non-shared
cache. (See also Authorization, Section 3.1 of <xref target="Part7"/>, for
additional details.)
</t></list>
</t>
<t>
<iref item="Cache Directives" subitem="private" primary="true"/>
<iref item="private" subitem="Cache Directive" primary="true"/>
private
<list><t>
Indicates that all or part of the response message is intended for
a single user and MUST NOT be cached by a shared cache. This
allows an origin server to state that the specified parts of the
response are intended for only one user and are not a valid
response for requests by other users. A private (non-shared) cache
MAY cache the response.
</t><t>
Note: This usage of the word private only controls where the
response may be cached, and cannot ensure the privacy of the
message content.
</t></list>
</t>
<t>
<iref item="Cache Directives" subitem="no-cache" primary="true"/>
<iref item="no-cache" subitem="Cache Directive" primary="true"/>
no-cache
<list><t>
If the no-cache directive does not specify a field-name, then a
cache MUST NOT use the response to satisfy a subsequent request
without successful revalidation with the origin server. This
allows an origin server to prevent caching even by caches that
have been configured to return stale responses to client requests.
</t><t>
If the no-cache directive does specify 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, the
specified field-name(s) 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.
<list><t>
Note: Most HTTP/1.0 caches will not recognize or obey this
directive.
</t></list>
</t></list>
</t>
</section>
<section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
<t>
<iref item="Cache Directives" subitem="no-store" primary="true"/>
<iref item="no-store" subitem="Cache Directive" primary="true"/>
no-store
<list><t>
The purpose of the no-store directive is to prevent the
inadvertent release or retention of sensitive information (for
example, on backup tapes). The no-store directive applies to the
entire message, and MAY be sent either in a response or in a
request. If sent in a request, a cache MUST NOT store any part of
either this request or any response to it. If sent in a response,
a cache MUST NOT store any part of either this response or the
request that elicited it. This directive applies to both non-shared
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>
Even when this directive is associated with a response, users
might explicitly store such a response outside of the caching
system (e.g., with a "Save As" dialog). History buffers MAY store
such responses as part of their normal operation.
</t><t>
The purpose of this directive is to meet the stated requirements
of certain users and service authors who are concerned about
accidental releases of information via unanticipated accesses to
cache data structures. While the use of this directive might
improve privacy in some cases, we caution that it is NOT in any
way a reliable or sufficient mechanism for ensuring privacy. In
particular, malicious or compromised caches might not recognize or
obey this directive, and communications networks might be
vulnerable to eavesdropping.
</t></list>
</t>
</section>
<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
<t>
The expiration time of an entity MAY be specified by the origin
server using the Expires header (see <xref target="header.expires"/>). Alternatively,
it MAY be specified using the max-age directive in a response. When
the max-age cache-control directive is present in a cached response,
the response is stale if its current age is greater than the age
value given (in seconds) at the time of a new request for that
resource. The max-age directive on a response implies that the
response is cacheable (i.e., "public") unless some other, more
restrictive cache directive is also present.
</t>
<t>
If a response includes both an Expires header and a max-age
directive, the max-age directive overrides the Expires header, even
if the Expires header is more restrictive. This rule allows an origin
server to provide, for a given response, a longer expiration time to
an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
useful if certain HTTP/1.0 caches improperly calculate ages or
expiration times, perhaps due to desynchronized clocks.
</t>
<t>
Many HTTP/1.0 cache implementations will treat an Expires value that
is less than or equal to the response Date value as being equivalent
to the Cache-Control response directive "no-cache". If an HTTP/1.1
cache receives such a response, and the response does not include a
Cache-Control header field, it SHOULD consider the response to be
non-cacheable in order to retain compatibility with HTTP/1.0 servers.
<list><t>
Note: An origin server might wish to use a relatively new HTTP
cache control feature, such as the "private" directive, on a
network including older caches that do not understand that
feature. The origin server will need to combine the new feature
with an Expires field whose value is less than or equal to the
Date value. This will prevent older caches from improperly
caching the response.
</t></list>
</t>
<t>
<iref item="Cache Directives" subitem="s-maxage" primary="true"/>
<iref item="s-maxage" subitem="Cache Directive" primary="true"/>
s-maxage
<list><t>
If a response includes an s-maxage directive, then for a shared
cache (but not for a private cache), the maximum age specified by
this directive overrides the maximum age specified by either the
max-age directive or the Expires header. The s-maxage directive
also implies the semantics of the proxy-revalidate directive (see
<xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
entry after it becomes stale to respond to a subsequent request
without first revalidating it with the origin server. The s-maxage
directive is always ignored by a private cache.
</t></list>
</t>
<t>
Note that most older caches, not compliant with this specification,
do not implement any cache-control directives. An origin server
wishing to use a cache-control directive that restricts, but does not
prevent, caching by an HTTP/1.1-compliant cache MAY exploit the
requirement that the max-age directive overrides the Expires header,
and the fact that pre-HTTP/1.1-compliant caches do not observe the
max-age directive.
</t>
<t>
Other directives allow a user agent to modify the basic expiration
mechanism. These directives MAY be specified on a request:
</t>
<t>
<iref item="Cache Directives" subitem="max-age" primary="true"/>
<iref item="max-age" subitem="Cache Directive" primary="true"/>
max-age
<list><t>
Indicates that the client is willing to accept a response whose
age is no greater than the specified time in seconds. Unless max-stale
directive is also included, the client is not willing to
accept a stale response.
</t></list>
</t>
<t>
<iref item="Cache Directives" subitem="min-fresh" primary="true"/>
<iref item="min-fresh" subitem="Cache Directive" primary="true"/>
min-fresh
<list><t>
Indicates that the client is willing to accept a response whose
freshness lifetime is no less than its current age plus the
specified time in seconds. That is, the client wants a response
that will still be fresh for at least the specified number of
seconds.
</t></list>
</t>
<t>
<iref item="Cache Directives" subitem="max-stale" primary="true"/>
<iref item="max-stale" subitem="Cache Directive" primary="true"/>
max-stale
<list><t>
Indicates that the client is willing to accept a response that has
exceeded its expiration time. If max-stale is assigned a value,
then the client is willing to accept a response that has exceeded
its expiration time by no more than the specified number of
seconds. If no value is assigned to max-stale, then the client is
willing to accept a stale response of any age.
</t></list>
</t>
<t>
If a cache returns a stale response, either because of a max-stale
directive on a request, or because the cache is configured to
override the expiration time of a response, the cache MUST attach a
Warning header to the stale response, using Warning 110 (Response is
stale).
</t>
<t>
A cache MAY be configured to return stale responses without
validation, but only if this does not conflict with any "MUST"-level
requirements concerning cache validation (e.g., a "must-revalidate"
cache-control directive).
</t>
<t>
If both the new request and the cached entry include "max-age"
directives, then the lesser of the two values is used for determining
the freshness of the cached entry for that request.
</t>
</section>
<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
<t>
Sometimes a user agent might want or need to insist that a cache
revalidate its cache entry with the origin server (and not just with
the next cache along the path to the origin server), or to reload its
cache entry from the origin server. End-to-end revalidation might be
necessary if either the cache or the origin server has overestimated
the expiration time of the cached response. End-to-end reload may be
necessary if the cache entry has become corrupted for some reason.
</t>
<t>
End-to-end revalidation may be requested either when the client does
not have its own local cached copy, in which case we call it
"unspecified end-to-end revalidation", or when the client does have a
local cached copy, in which case we call it "specific end-to-end
revalidation."
</t>
<t>
The client can specify these three kinds of action using Cache-Control
request directives:
</t>
<t>
End-to-end reload
<list><t>
The request includes a "no-cache" cache-control directive or, for
compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
names MUST NOT be included with the no-cache directive in a
request. The server MUST NOT use a cached copy when responding to
such a request.
</t></list>
</t>
<t>
Specific end-to-end revalidation
<list><t>
The request includes a "max-age=0" cache-control directive, which
forces each cache along the path to the origin server to
revalidate its own entry, if any, with the next cache or server.
The initial request includes a cache-validating conditional with
the client's current validator.
</t></list>
</t>
<t>
Unspecified end-to-end revalidation
<list><t>
The request includes "max-age=0" cache-control directive, which
forces each cache along the path to the origin server to
revalidate its own entry, if any, with the next cache or server.
The initial request does not include a cache-validating
conditional; the first cache along the path (if any) that holds a
cache entry for this resource includes a cache-validating
conditional with its current validator.
</t></list>
</t>
<t>
<iref item="Cache Directives" subitem="max-age" primary="true"/>
<iref item="max-age" subitem="Cache Directive" primary="true"/>
max-age
<list><t>
When an intermediate cache is forced, by means of a max-age=0
directive, to revalidate its own cache entry, and the client has
supplied its own validator in the request, the supplied validator
might differ from the validator currently stored with the cache
entry. In this case, the cache MAY use either validator in making
its own request without affecting semantic transparency.
</t><t>
However, the choice of validator might affect performance. The
best approach is for the intermediate cache to use its own
validator when making its request. If the server replies with 304
(Not Modified), then the cache can return its now validated copy
to the client with a 200 (OK) response. If the server replies with
a new entity and cache validator, however, the intermediate cache
can compare the returned validator with the one provided in the
client's request, using the strong comparison function. If the
client's validator is equal to the origin server's, then the
intermediate cache simply returns 304 (Not Modified). Otherwise,
it returns the new entity with a 200 (OK) response.
</t><t>
If a request includes the no-cache directive, it SHOULD NOT
include min-fresh, max-stale, or max-age.
</t></list>
</t>
<t>
<iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
<iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
only-if-cached
<list><t>
In some cases, such as times of extremely poor network
connectivity, a client may want a cache to return only those
responses that it currently has stored, and not to reload or
revalidate with the origin server. To do this, the client may
include the only-if-cached directive in a request. If it receives
this directive, a cache SHOULD either respond using a cached entry
that is consistent with the other constraints of the request, or
respond with a 504 (Gateway Timeout) status. However, if a group
of caches is being operated as a unified system with good internal
connectivity, such a request MAY be forwarded within that group of
caches.
</t></list>
</t>
<t>
<iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
<iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
must-revalidate
<list><t>
Because a cache MAY be configured to ignore a server's specified
expiration time, and because a client request MAY include a max-stale
directive (which has a similar effect), the protocol also
includes a mechanism for the origin server to require revalidation
of a cache entry on any subsequent use. When the must-revalidate
directive is present in a response received by a cache, that cache
MUST NOT use the entry after it becomes stale to respond to a
subsequent request without first revalidating it with the origin
server. (I.e., the cache MUST do an end-to-end revalidation every
time, if, based solely on the origin server's Expires or max-age
value, the cached response is stale.)
</t><t>
The must-revalidate directive is necessary to support reliable
operation for certain protocol features. In all circumstances an
HTTP/1.1 cache MUST obey the must-revalidate directive; in
particular, if the cache cannot reach the origin server for any
reason, it MUST generate a 504 (Gateway Timeout) response.
</t><t>
Servers SHOULD send the must-revalidate directive if and only if
failure to revalidate a request on the entity could result in
incorrect operation, such as a silently unexecuted financial
transaction. Recipients MUST NOT take any automated action that
violates this directive, and MUST NOT automatically provide an
unvalidated copy of the entity if revalidation fails.
</t><t>
Although this is not recommended, user agents operating under
severe connectivity constraints MAY violate this directive but, if
so, MUST explicitly warn the user that an unvalidated response has
been provided. The warning MUST be provided on each unvalidated
access, and SHOULD require explicit user confirmation.
</t></list>
</t>
<t>
<iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
<iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
proxy-revalidate
<list><t>
The proxy-revalidate directive has the same meaning as the must-revalidate
directive, except that it does not apply to non-shared
user agent caches. It can be used on a response to an
authenticated request to permit the user's cache to store and
later return the response without needing to revalidate it (since
it has already been authenticated once by that user), while still
requiring proxies that service many users to revalidate each time
(in order to make sure that each user has been authenticated).
Note that such authenticated responses also need the public cache
control directive in order to allow them to be cached at all.
</t></list>
</t>
</section>
<section title="No-Transform Directive" anchor="no-transform.directive">
<t>
<iref item="Cache Directives" subitem="no-transform" primary="true"/>
<iref item="no-transform" subitem="Cache Directive" primary="true"/>
no-transform
<list><t>
Implementors of intermediate caches (proxies) have found it useful
to convert the media type of certain entity bodies. A non-transparent
proxy might, for example, convert between image
formats in order to save cache space or to reduce the amount of
traffic on a slow link.
</t><t>
Serious operational problems occur, however, when these
transformations are applied to entity bodies intended for certain
kinds of applications. For example, applications for medical
imaging, scientific data analysis and those using end-to-end
authentication, all depend on receiving an entity body that is bit
for bit identical to the original entity-body.
</t><t>
Therefore, if a message includes the no-transform directive, an
intermediate cache or proxy MUST NOT change those headers that are
listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
directive. This implies that the cache or proxy MUST NOT change
any aspect of the entity-body that is specified by these headers,
including the value of the entity-body itself.
</t></list>
</t>
</section>
<section title="Cache Control Extensions" anchor="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 assigned value.
Informational extensions (those which do not require a change in
cache behavior) MAY 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 which 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 which acts as a modifier to the private directive. We
define this new directive to mean that, in addition to any non-shared
cache, any cache which is shared only by members of the community
named within its value may cache the response. An origin server
wishing to allow the UCI community to use an otherwise private
response in their shared cache(s) could do so by including
</t>
<figure><artwork type="example"><![CDATA[
Cache-Control: private, community="UCI"
]]></artwork></figure>
<t>
A cache seeing this header field will act correctly even if the cache
does not understand the community cache-extension, since it will also
see and understand the private directive and thus default to the safe
behavior.
</t>
<t>
Unrecognized cache-directives MUST be ignored; 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 title="Expires" anchor="header.expires">
<iref primary="true" item="Expires header"/>
<iref primary="true" item="Headers" subitem="Expires"/>
<t>
The Expires entity-header field gives the date/time after which the
response is considered stale. A stale cache entry may not normally be
returned by a cache (either a proxy cache or a user agent cache)
unless it is first validated with the origin server (or with an
intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
for further discussion of the expiration 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 format is an absolute date and time as defined by HTTP-date in
Section 3.3.1 of <xref target="Part1"/>; it MUST be in RFC 1123 date format:
</t>
<figure><iref primary="true" item="Grammar" subitem="Expires"/><artwork type="abnf2616"><![CDATA[
Expires = "Expires" ":" HTTP-date
]]></artwork></figure>
<t>
An example of its use is
</t>
<figure><artwork type="example"><![CDATA[
Expires: Thu, 01 Dec 1994 16:00:00 GMT
]]></artwork></figure>
<t>
<list><t>
Note: if a response includes a Cache-Control field with the max-age
directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
Expires field.
</t></list>
</t>
<t>
HTTP/1.1 clients and caches MUST treat other invalid date formats,
especially including the value "0", as in the past (i.e., "already
expired").
</t>
<t>
To mark a response as "already expired," an origin server sends an
Expires date that is equal to the Date header value. (See the rules
for expiration calculations in <xref target="expiration.calculations"/>.)
</t>
<t>
To mark a response as "never expires," an origin server sends an
Expires date approximately one year from the time the response is
sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one
year in the future.
</t>
<t>
The presence of an Expires header field with a date value of some
time in the future on a response that otherwise would by default be
non-cacheable indicates that the response is cacheable, unless
indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
</t>
</section>
<section title="Pragma" anchor="header.pragma">
<iref primary="true" item="Pragma header"/>
<iref primary="true" item="Headers" subitem="Pragma"/>
<t>
The Pragma general-header field is used to include implementation-specific
directives that might apply to any recipient along the
request/response chain. All pragma directives specify optional
behavior from the viewpoint of the protocol; however, some systems
MAY require that behavior be consistent with the directives.
</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 = "Pragma" ":" 1#pragma-directive
pragma-directive = "no-cache" | extension-pragma
extension-pragma = token [ "=" ( token | quoted-string ) ]
]]></artwork></figure>
<t>
When the no-cache directive is present in a request message, an
application SHOULD forward the request toward the origin server even
if it has a cached copy of what is being requested. This pragma
directive has the same semantics as the no-cache cache-directive (see
<xref target="header.cache-control"/>) and is defined here for backward compatibility with
HTTP/1.0. Clients SHOULD include both header fields when a no-cache
request is sent to a server not known to be HTTP/1.1 compliant.
</t>
<t>
Pragma directives MUST be passed through by a proxy or gateway
application, 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 specify a pragma for a
specific recipient; however, any pragma directive not relevant to a
recipient SHOULD be ignored by that recipient.
</t>
<t>
HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
sent "Cache-Control: no-cache". No new Pragma directives will be
defined in HTTP.
<list><t>
Note: because the meaning of "Pragma: no-cache as a response
header field is not actually specified, it does not provide a
reliable replacement for "Cache-Control: no-cache" in a response
</t></list>
</t>
</section>
<section title="Vary" anchor="header.vary">
<iref primary="true" item="Vary header"/>
<iref primary="true" item="Headers" subitem="Vary"/>
<t>
The Vary field value indicates the set of request-header fields that
fully determines, while the response is fresh, whether a cache is
permitted to use the response to reply to a subsequent request
without revalidation. For uncacheable or stale responses, the Vary
field value advises the user agent about the criteria that were used
to select the representation. A Vary field value of "*" implies that
a cache cannot determine from the request headers of a subsequent
request whether this response is the appropriate representation. See
<xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
</t>
<figure><iref primary="true" item="Grammar" subitem="Vary"/><artwork type="abnf2616"><![CDATA[
Vary = "Vary" ":" ( "*" | 1#field-name )
]]></artwork></figure>
<t>
An HTTP/1.1 server SHOULD include a Vary header field with any
cacheable response that is subject to server-driven negotiation.
Doing so allows a cache to properly interpret future requests on that
resource and informs the user agent about the presence of negotiation
on that resource. A server MAY include a Vary header field with a
non-cacheable response that is subject to server-driven negotiation,
since this might provide the user agent with useful information about
the dimensions over which the response varies at the time of the
response.
</t>
<t>
A Vary field value consisting of a list of field-names signals that
the representation selected for the response is based on a selection
algorithm which considers ONLY the listed request-header field values
in selecting the most appropriate representation. A cache MAY assume
that the same selection will be made for future requests with the
same values for the listed field names, for the duration of time for
which the response is fresh.
</t>
<t>
The field-names given are not limited to the set of standard
request-header fields defined by this specification. Field names are
case-insensitive.
</t>
<t>
A Vary field value of "*" signals that unspecified parameters not
limited to the request-headers (e.g., the network address of the
client), play a role in the selection of the response representation.
The "*" value MUST NOT be generated by a proxy server; it may only be
generated by an origin server.
</t>
</section>
<section title="Warning" anchor="header.warning">
<iref primary="true" item="Warning header"/>
<iref primary="true" item="Headers" subitem="Warning"/>
<t>
The Warning general-header field is used to carry additional
information about the status or transformation of a message which
might not be reflected in the message. This information is typically
used to warn about a possible lack of semantic transparency from
caching operations or transformations applied to the entity body of
the message.
</t>
<t>
Warning headers are sent with responses using:
</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 = "Warning" ":" 1#warning-value
warning-value = warn-code SP warn-agent SP warn-text
[SP warn-date]
warn-code = 3DIGIT
warn-agent = ( host [ ":" port ] ) | pseudonym
; the name or pseudonym of the server adding
; the Warning header, for use in debugging
warn-text = quoted-string
warn-date = <"> HTTP-date <">
]]></artwork></figure>
<t>
A response MAY carry more than one Warning header.
</t>
<t>
The warn-text SHOULD be in a natural language and character set that
is most likely to be intelligible to the human user receiving the
response. This decision MAY be based on any available knowledge, such
as the location of the cache or user, the Accept-Language field in a
request, the Content-Language field in a response, etc. The default
language is English and the default character set is ISO-8859-1.
</t>
<t>
If a character set other than ISO-8859-1 is used, it MUST be encoded
in the warn-text using the method described in RFC 2047 <xref target="RFC2047"/>.
</t>
<t>
Warning headers can in general be applied to any message, however
some specific warn-codes are specific to caches and can only be
applied to response messages. New Warning headers SHOULD be added
after any existing Warning headers. A cache MUST NOT delete any
Warning header that it received with a message. However, if a cache
successfully validates a cache entry, it SHOULD remove any Warning
headers previously attached to that entry except as specified for
specific Warning codes. It MUST then add any Warning headers received
in the validating response. In other words, Warning headers are those
that would be attached to the most recent relevant response.
</t>
<t>
When multiple Warning headers are attached to a response, the user
agent ought to inform the user of as many of them as possible, in the
order that they appear in the response. If it is not possible to
inform the user of all of the warnings, the user agent SHOULD follow
these heuristics:
<list style="symbols">
<t>Warnings that appear early in the response take priority over
those appearing later in the response.</t>
<t>Warnings in the user's preferred character set take priority
over warnings in other character sets but with identical warn-codes
and warn-agents.</t>
</list>
</t>
<t>
Systems that generate multiple Warning headers SHOULD order them with
this user agent behavior in mind.
</t>
<t>
Requirements for the behavior of caches with respect to Warnings are
stated in <xref target="warnings"/>.
</t>
<t>
This is a list of the currently-defined warn-codes, each with a
recommended warn-text in English, and a description of its meaning.
</t>
<t>
110 Response is stale
<list><t>
MUST be included whenever the returned response is stale.
</t></list>
</t>
<t>
111 Revalidation failed
<list><t>
MUST be included if a cache returns a stale response because an
attempt to revalidate the response failed, due to an inability to
reach the server.
</t></list>
</t>
<t>
112 Disconnected operation
<list><t>
SHOULD be included if the cache is intentionally disconnected from
the rest of the network for a period of time.
</t></list>
</t>
<t>
113 Heuristic expiration
<list><t>
MUST be included if the cache heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater
than 24 hours.
</t></list>
</t>
<t>
199 Miscellaneous warning
<list><t>
The warning text MAY 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></list>
</t>
<t>
214 Transformation applied
<list><t>
MUST be added by an intermediate cache or proxy if it applies any
transformation changing the content-coding (as specified in the
Content-Encoding header) or media-type (as specified in the
Content-Type header) of the response, or the entity-body of the
response, unless this Warning code already appears in the response.
</t></list>
</t>
<t>
299 Miscellaneous persistent warning
<list><t>
The warning text MAY include arbitrary information to be presented
to a human user, or logged. A system receiving this warning MUST NOT
take any automated action.
</t></list>
</t>
<t>
If an implementation sends a message with one or more Warning headers
whose version is HTTP/1.0 or lower, then the sender MUST include in
each warning-value a warn-date that matches the date in the response.
</t>
<t>
If an implementation 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. (This prevents
bad consequences of naive caching of Warning header fields.) If all
of the warning-values are deleted for this reason, the Warning header
MUST be deleted as well.
</t>
</section>
</section>
<section title="IANA Considerations" anchor="IANA.considerations">
<t>
TBD.
</t>
</section>
<section title="Security Considerations" anchor="security.considerations">
<t>
Caching proxies provide 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 should be protected as
sensitive information.
</t>
</section>
<section title="Acknowledgments" anchor="ack">
<t>
Much of the content and presentation of the caching design is due to
suggestions and comments from individuals including: Shel Kaphan,
Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
</t>
<t>
Based on an XML translation of RFC 2616 by Julian Reschke.
</t>
</section>
</middle>
<back>
<references>
<reference anchor="Part1">
<front>
<title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
<organization abbrev="Day Software">Day Software</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author initials="J." surname="Gettys" fullname="Jim Gettys">
<organization>One Laptop per Child</organization>
<address><email>jg@laptop.org</email></address>
</author>
<author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
<organization abbrev="HP">Hewlett-Packard Company</organization>
<address><email>JeffMogul@acm.org</email></address>
</author>
<author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
<organization abbrev="Microsoft">Microsoft Corporation</organization>
<address><email>henrikn@microsoft.com</email></address>
</author>
<author initials="L." surname="Masinter" fullname="Larry Masinter">
<organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
<address><email>LMM@acm.org</email></address>
</author>
<author initials="P." surname="Leach" fullname="Paul J. Leach">
<organization abbrev="Microsoft">Microsoft Corporation</organization>
<address><email>paulle@microsoft.com</email></address>
</author>
<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
<organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
<address><email>timbl@w3.org</email></address>
</author>
<date month="December" year="2007"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-00"/>
</reference>
<reference anchor="Part2">
<front>
<title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
<organization abbrev="Day Software">Day Software</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author initials="J." surname="Gettys" fullname="Jim Gettys">
<organization>One Laptop per Child</organization>
<address><email>jg@laptop.org</email></address>
</author>
<author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
<organization abbrev="HP">Hewlett-Packard Company</organization>
<address><email>JeffMogul@acm.org</email></address>
</author>
<author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
<organization abbrev="Microsoft">Microsoft Corporation</organization>
<address><email>henrikn@microsoft.com</email></address>
</author>
<author initials="L." surname="Masinter" fullname="Larry Masinter">
<organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
<address><email>LMM@acm.org</email></address>
</author>
<author initials="P." surname="Leach" fullname="Paul J. Leach">
<organization abbrev="Microsoft">Microsoft Corporation</organization>
<address><email>paulle@microsoft.com</email></address>
</author>
<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
<organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
<address><email>timbl@w3.org</email></address>
</author>
<date month="December" year="2007"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-00"/>
</reference>
<reference anchor="Part3">
<front>
<title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
<organization abbrev="Day Software">Day Software</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author initials="J." surname="Gettys" fullname="Jim Gettys">
<organization>One Laptop per Child</organization>
<address><email>jg@laptop.org</email></address>
</author>
<author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
<organization abbrev="HP">Hewlett-Packard Company</organization>
<address><email>JeffMogul@acm.org</email></address>
</author>
<author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
<organization abbrev="Microsoft">Microsoft Corporation</organization>
<address><email>henrikn@microsoft.com</email></address>
</author>
<author initials="L." surname="Masinter" fullname="Larry Masinter">
<organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
<address><email>LMM@acm.org</email></address>
</author>
<author initials="P." surname="Leach" fullname="Paul J. Leach">
<organization abbrev="Microsoft">Microsoft Corporation</organization>
<address><email>paulle@microsoft.com</email></address>
</author>
<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
<organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
<address><email>timbl@w3.org</email></address>
</author>
<date month="December" year="2007"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-00"/>
</reference>
<reference anchor="Part4">
<front>
<title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
<organization abbrev="Day Software">Day Software</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author initials="J." surname="Gettys" fullname="Jim Gettys">
<organization>One Laptop per Child</organization>
<address><email>jg@laptop.org</email></address>
</author>
<author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
<organization abbrev="HP">Hewlett-Packard Company</organization>
<address><email>JeffMogul@acm.org</email></address>
</author>
<author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
<organization abbrev="Microsoft">Microsoft Corporation</organization>
<address><email>henrikn@microsoft.com</email></address>
</author>
<author initials="L." surname="Masinter" fullname="Larry Masinter">
<organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
<address><email>LMM@acm.org</email></address>
</author>
<author initials="P." surname="Leach" fullname="Paul J. Leach">
<organization abbrev="Microsoft">Microsoft Corporation</organization>
<address><email>paulle@microsoft.com</email></address>
</author>
<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
<organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
<address><email>timbl@w3.org</email></address>
</author>
<date month="December" year="2007"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-00"/>
</reference>
<reference anchor="Part5">
<front>
<title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
<organization abbrev="Day Software">Day Software</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author initials="J." surname="Gettys" fullname="Jim Gettys">
<organization>One Laptop per Child</organization>
<address><email>jg@laptop.org</email></address>
</author>
<author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
<organization abbrev="HP">Hewlett-Packard Company</organization>
<address><email>JeffMogul@acm.org</email></address>
</author>
<author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
<organization abbrev="Microsoft">Microsoft Corporation</organization>
<address><email>henrikn@microsoft.com</email></address>
</author>
<author initials="L." surname="Masinter" fullname="Larry Masinter">
<organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
<address><email>LMM@acm.org</email></address>
</author>
<author initials="P." surname="Leach" fullname="Paul J. Leach">
<organization abbrev="Microsoft">Microsoft Corporation</organization>
<address><email>paulle@microsoft.com</email></address>
</author>
<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
<organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
<address><email>timbl@w3.org</email></address>
</author>
<date month="December" year="2007"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-00"/>
</reference>
<reference anchor="Part7">
<front>
<title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
<organization abbrev="Day Software">Day Software</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author initials="J." surname="Gettys" fullname="Jim Gettys">
<organization>One Laptop per Child</organization>
<address><email>jg@laptop.org</email></address>
</author>
<author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
<organization abbrev="HP">Hewlett-Packard Company</organization>
<address><email>JeffMogul@acm.org</email></address>
</author>
<author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
<organization abbrev="Microsoft">Microsoft Corporation</organization>
<address><email>henrikn@microsoft.com</email></address>
</author>
<author initials="L." surname="Masinter" fullname="Larry Masinter">
<organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
<address><email>LMM@acm.org</email></address>
</author>
<author initials="P." surname="Leach" fullname="Paul J. Leach">
<organization abbrev="Microsoft">Microsoft Corporation</organization>
<address><email>paulle@microsoft.com</email></address>
</author>
<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
<organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
<address><email>timbl@w3.org</email></address>
</author>
<date month="December" year="2007"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-00"/>
</reference>
<reference anchor="RFC2616">
<front>
<title>Hypertext Transfer Protocol -- HTTP/1.1</title>
<author initials="R." surname="Fielding" fullname="R. Fielding">
<organization>University of California, Irvine</organization>
<address><email>fielding@ics.uci.edu</email></address>
</author>
<author initials="J." surname="Gettys" fullname="J. Gettys">
<organization>W3C</organization>
<address><email>jg@w3.org</email></address>
</author>
<author initials="J." surname="Mogul" fullname="J. Mogul">
<organization>Compaq Computer Corporation</organization>
<address><email>mogul@wrl.dec.com</email></address>
</author>
<author initials="H." surname="Frystyk" fullname="H. Frystyk">
<organization>MIT Laboratory for Computer Science</organization>
<address><email>frystyk@w3.org</email></address>
</author>
<author initials="L." surname="Masinter" fullname="L. Masinter">
<organization>Xerox Corporation</organization>
<address><email>masinter@parc.xerox.com</email></address>
</author>
<author initials="P." surname="Leach" fullname="P. Leach">
<organization>Microsoft Corporation</organization>
<address><email>paulle@microsoft.com</email></address>
</author>
<author initials="T." surname="Berners-Lee" fullname="T. 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="RFC2047">
<front>
<title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
<author initials="K." surname="Moore" fullname="Keith Moore">
<organization>University of Tennessee</organization>
<address>
<postal>
<street>107 Ayres Hall</street>
<street>Knoxville TN 37996-1301</street></postal>
<email>moore@cs.utk.edu</email></address></author>
<date month="November" year="1996"/>
<area>Applications</area>
<keyword>Amercian Standard Code for Information Interchange</keyword>
<keyword>mail</keyword>
<keyword>multipurpose internet mail extensions</keyword>
</front>
<seriesInfo name="RFC" value="2047"/>
</reference>
<reference anchor="RFC1305">
<front>
<title>Network Time Protocol (Version 3) Specification, Implementation</title>
<author initials="D." surname="Mills" fullname="David L. Mills">
<organization>University of Delaware, Electrical Engineering Department</organization>
<address>
<postal>
<street/>
<city>Newark</city>
<region>DE</region>
<code>19716</code>
<country>US</country></postal>
<phone>+1 302 451 8247</phone>
<email>mills@udel.edu</email></address></author>
<date month="March" year="1992"/>
<abstract>
<t>This document describes the Network Time Protocol (NTP), specifies its normal structure and summarizes information useful for its implementation. NTP provides the mechanisms to synchronize time and coordinate time distribution in a large, diverse internet operating at rates from mundane to lightwave. It uses a returnable-time design in which a distributed subnet of time servers operating in a self-organizing, hierarchical-master-slave configuration synchronizes local clocks within the subnet and to national time standards via wire or radio. The servers can also redistribute reference time via local routing algorithms and time daemons.</t></abstract></front>
<seriesInfo name="RFC" value="1305"/>
</reference>
</references>
<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
<t>
A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
<xref target="header.cache-control" format="counter"/>,
<xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
</t>
<t>
The Cache-Control: max-age directive was not properly defined for
responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
</t>
<t>
Warnings could be cached incorrectly, or not updated appropriately.
(Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
<xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
and <xref target="header.warning" format="counter"/>) Warning
also needed to be a general header, as PUT or other methods may have
need for it in requests.
</t>
</section>
</back>
</rfc>| PAFTECH AB 2003-2026 | 2026-04-23 20:33:45 |