Hypertext Transfer Protocol (HTTP/1.1): CachingAdobe Systems Incorporated345 Park AveSan JoseCA95110USAfielding@gbiv.comhttp://roy.gbiv.com/Akamaimnot@mnot.nethttp://www.mnot.net/greenbytes GmbHHafenweg 16MuensterNW48155Germanyjulian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/
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HTTPbis Working GroupHypertext Transfer ProtocolHTTPHTTP Caching
The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for
distributed, collaborative, hypertext information systems. This document
defines HTTP caches and the associated header fields
that control cache behavior or indicate cacheable response messages.
HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. This document
defines aspects of HTTP/1.1 related to caching and reusing response
messages.
An HTTP cache is a local store of response messages and the
subsystem that controls storage, retrieval, and deletion of messages in it.
A cache stores cacheable responses in order to reduce the response time and
network bandwidth consumption on future, equivalent requests. Any client or
server MAY employ a cache, though a cache cannot be used by a server that
is acting as a tunnel.
A shared cache is a cache that stores responses to be reused
by more than one user; shared caches are usually (but not always) deployed
as a part of an intermediary. A private cache, in contrast,
is dedicated to a single user; often, they are deployed as a component of
a user agent.
The goal of caching in HTTP/1.1 is to significantly improve performance
by reusing a prior response message to satisfy a current request.
A stored response is considered "fresh", as defined in
, if the response can be reused without
"validation" (checking with the origin server to see if the cached response
remains valid for this request). A fresh response can therefore
reduce both latency and network overhead each time it is reused.
When a cached response is not fresh, it might still be reusable if it can
be freshened by validation () or if the
origin is unavailable ().
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in .
Conformance criteria and considerations regarding error handling
are defined in Section 2.5 of .
This specification uses the Augmented Backus-Naur Form (ABNF) notation of
with a list extension, defined in
Section 7 of , that allows for compact definition of
comma-separated lists using a '#' operator (similar to how the '*' operator
indicates repetition).
describes rules imported from
other documents.
shows the collected grammar with all list
operators expanded to standard ABNF notation.
The delta-seconds rule specifies a non-negative integer, representing time
in seconds.
A recipient parsing a delta-seconds value and converting it to binary form
ought to use an arithmetic type of at least 31 bits of non-negative integer
range.
If a cache receives a delta-seconds value greater than the greatest integer
it can represent, or if any of its subsequent calculations overflows,
the cache MUST consider the value to be either 2147483648
(2^31) or the greatest positive integer it can conveniently
represent.
Note: The value 2147483648 is here for historical reasons, effectively
represents infinity (over 68 years), and does not need to be stored in
binary form; an implementation could produce it as a canned string if
any overflow occurs, even if the calculations are performed with an
arithmetic type incapable of directly representing that number.
What matters here is that an overflow be detected and not treated as a
negative value in later calculations.
Proper cache operation preserves the semantics of HTTP transfers
() while eliminating the transfer of information already held
in the cache. Although caching is an entirely OPTIONAL feature of HTTP,
it can be assumed that reusing a cached response is desirable and that such
reuse is the default behavior when no requirement or local
configuration prevents it. Therefore, HTTP cache requirements are focused
on preventing a cache from either storing a non-reusable response or
reusing a stored response inappropriately, rather than mandating that
caches always store and reuse particular responses.
Each cache entry consists of a cache key and one or more
HTTP responses corresponding to prior requests that used the same key. The
most common form of cache entry is a successful result of a retrieval
request: i.e., a 200 (OK) response to a GET request, which
contains a representation of the resource identified by the request target
(Section 4.3.1 of ). However, it is also possible to cache permanent redirects,
negative results (e.g., 404 (Not Found)),
incomplete results (e.g., 206 (Partial Content)), and
responses to methods other than GET if the method's definition allows such
caching and defines something suitable for use as a cache key.
The primary cache key consists of the request method and
target URI. However, since HTTP caches in common use today are typically
limited to caching responses to GET, many caches simply decline
other methods and use only the URI as the primary cache key.
If a request target is subject to content negotiation, its cache entry
might consist of multiple stored responses, each differentiated by a
secondary key for the values of the original request's selecting header
fields ().
A cache MUST NOT store a response to any request, unless:
The request method is understood by the cache and defined as being
cacheable, andthe response status code is understood by the cache, andthe "no-store" cache directive (see ) does not appear in request or response
header fields, andthe "private" response directive (see ) does not appear in the
response, if the cache is shared, andthe Authorization header field (see
Section 4.2 of ) does not appear in the request, if the cache is
shared, unless the response explicitly allows it (see ), andthe response either:
contains an Expires header field (see
), orcontains a max-age response directive (see ), orcontains a s-maxage response directive (see ) and the cache is
shared, orcontains a Cache Control Extension (see ) that allows it to be cached,
orhas a status code that is defined as cacheable by default
(see ), orcontains a public response directive (see ).
Note that any of the requirements listed above can be overridden by a
cache-control extension; see .
In this context, a cache has "understood" a request method or a response
status code if it recognizes it and implements all specified
caching-related behavior.
Note that, in normal operation, some caches will not store a response that
has neither a cache validator nor an explicit expiration time, as such
responses are not usually useful to store. However, caches are not
prohibited from storing such responses.
A response message is considered complete when all of the octets indicated
by the message framing () are received prior to the connection
being closed. If the request method is GET, the response status code is
200 (OK), and the entire response header section has been received, a
cache MAY store an incomplete response message body if the cache entry is
recorded as incomplete. Likewise, a 206 (Partial Content)
response MAY be stored as if it were an incomplete 200
(OK) cache entry. However, a cache MUST NOT store incomplete or
partial-content responses if it does not support the Range
and Content-Range header fields or if it does not understand
the range units used in those fields.
A cache MAY complete a stored incomplete response by making a subsequent
range request () and combining the successful response with the
stored entry, as defined in . A cache
MUST NOT use an incomplete response to answer requests unless the
response has been made complete or the request is partial and specifies a
range that is wholly within the incomplete response. A cache MUST NOT
send a partial response to a client without explicitly marking it as such
using the 206 (Partial Content) status code.
A shared cache MUST NOT use a cached response to a request with an
Authorization header field (Section 4.2 of ) to
satisfy any subsequent request unless a cache directive that allows such
responses to be stored is present in the response.
In this specification, the following Cache-Control response
directives () have such an effect:
must-revalidate, public, and s-maxage.
Note that cached responses that contain the "must-revalidate" and/or
"s-maxage" response directives are not allowed to be served stale () by shared caches. In particular, a
response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
used to satisfy a subsequent request without revalidating it on the origin
server.
A response might transfer only a partial representation if the
connection closed prematurely or if the request used one or more Range
specifiers (). After several such transfers, a cache might have
received several ranges of the same representation. A cache MAY combine
these ranges into a single stored response, and reuse that response to
satisfy later requests, if they all share the same strong validator and
the cache complies with the client requirements in Section 4.3 of .
When combining the new response with one or more stored responses, a
cache MUST:
delete any Warning header fields in the stored response
with warn-code 1xx (see );retain any Warning header fields in the stored response
with warn-code 2xx; and,use other header fields provided in the new response, aside
from Content-Range, to replace all instances of the
corresponding header fields in the stored response.
When presented with a request, a cache MUST NOT reuse a stored response,
unless:
The presented effective request URI (Section 5.5 of ) and
that of the stored response match, andthe request method associated with the stored response allows it to
be used for the presented request, andselecting header fields nominated by the stored response (if any)
match those presented (see ), andthe presented request does not contain the no-cache pragma (), nor the no-cache cache directive (), unless the stored response is
successfully validated (), andthe stored response does not contain the no-cache cache directive
(), unless it is
successfully validated (), andthe stored response is either:
fresh (see ), orallowed to be served stale (see ), orsuccessfully validated (see ).
Note that any of the requirements listed above can be overridden by a
cache-control extension; see .
When a stored response is used to satisfy a request without validation, a
cache MUST generate an Age header field (), replacing any present in the response with a value
equal to the stored response's current_age; see .
A cache MUST write through requests with methods that are unsafe
(Section 4.2.1 of ) to the origin server; i.e., a cache is not allowed to
generate a reply to such a request before having forwarded the request and
having received a corresponding response.
Also, note that unsafe requests might invalidate already-stored responses;
see .
When more than one suitable response is stored, a cache MUST use the
most recent response (as determined by the Date header
field). It can also forward the request with "Cache-Control: max-age=0" or
"Cache-Control: no-cache" to disambiguate which response to use.
A cache that does not have a clock available MUST NOT use stored
responses without revalidating them upon every use.
When a cache receives a request that can be satisfied by a stored response
that has a Vary header field (Section 7.1.4 of ),
it MUST NOT use that response unless all of the selecting header fields
nominated by the Vary header field match in both the original request
(i.e., that associated with the stored response), and the presented
request.
The selecting header fields from two requests are defined to match if and
only if those in the first request can be transformed to those in the
second request by applying any of the following:
adding or removing whitespace, where allowed in the header field's
syntax
combining multiple header fields with the same field name
(see Section 3.2 of )
normalizing both header field values in a way that is known to have
identical semantics, according to the header field's specification
(e.g., reordering field values when order is not significant;
case-normalization, where values are defined to be case-insensitive)
If (after any normalization that might take place) a header field is absent
from a request, it can only match another request if it is also absent
there.
A Vary header field-value of "*" always fails to match.
The stored response with matching selecting header fields is known as the
selected response.
If multiple selected responses are available (potentially including
responses without a Vary header field), the cache will need to choose one to use.
When a selecting header field has a known mechanism for doing so (e.g., qvalues on
Accept and similar request header fields), that mechanism MAY be
used to select preferred responses; of the remainder, the most recent
response (as determined by the Date header field) is used, as
per .
If no selected response is available, the cache cannot satisfy the
presented request. Typically, it is forwarded to the origin server
in a (possibly conditional; see ) request.
A fresh response is one whose age has not yet exceeded its
freshness lifetime. Conversely, a stale
response is one where it has.
A response's freshness lifetime is the length of time
between its generation by the origin server and its expiration time. An
explicit expiration time is the time at which the origin
server intends that a stored response can no longer be used by a cache
without further validation, whereas a heuristic expiration
time is assigned by a cache when no explicit expiration time is
available.
A response's age is the time that has passed since it was
generated by, or successfully validated with, the origin server.
When a response is "fresh" in the cache, it can be used to satisfy
subsequent requests without contacting the origin server, thereby improving
efficiency.
The primary mechanism for determining freshness is for an origin server to
provide an explicit expiration time in the future, using either the
Expires header field () or
the max-age response directive (). Generally, origin servers
will assign future explicit expiration times to responses in the belief
that the representation is not likely to change in a semantically
significant way before the expiration time is reached.
If an origin server wishes to force a cache to validate every request, it
can assign an explicit expiration time in the past to indicate that the
response is already stale. Compliant caches will normally validate a stale
cached response before reusing it for subsequent requests (see ).
Since origin servers do not always provide explicit expiration times,
caches are also allowed to use a heuristic to determine an expiration time
under certain circumstances (see ).
freshness_lifetime is defined in ; current_age is defined in
.
Clients can send the max-age or min-fresh cache directives in a request to
constrain or relax freshness calculations for the corresponding response
().
When calculating freshness, to avoid common problems in date parsing:
Although all date formats are specified to be case-sensitive,
a cache recipient SHOULD match day, week, and time-zone names
case-insensitively.If a cache recipient's internal implementation of time has less
resolution than the value of an HTTP-date, the recipient MUST
internally represent a parsed Expires date as the
nearest time equal to or earlier than the received value.A cache recipient MUST NOT allow local time zones to influence the
calculation or comparison of an age or expiration time.A cache recipient SHOULD consider a date with a zone abbreviation
other than GMT or UTC to be invalid for calculating expiration.
Note that freshness applies only to cache operation; it cannot be used to
force a user agent to refresh its display or reload a resource. See for an explanation of the difference between
caches and history mechanisms.
A cache can calculate the freshness lifetime (denoted as
freshness_lifetime) of a response by using the first match of the following:
If the cache is shared and the s-maxage response directive
() is present, use its value,
orIf the max-age response directive () is present, use its value, orIf the Expires response header field
() is present, use its value minus the
value of the Date response header field, orOtherwise, no explicit expiration time is present in the response. A
heuristic freshness lifetime might be applicable; see .
Note that this calculation is not vulnerable to clock skew, since all of
the information comes from the origin server.
When there is more than one value present for a given directive (e.g., two
Expires header fields, multiple Cache-Control: max-age
directives), the directive's value is considered invalid. Caches are
encouraged to consider responses that have invalid freshness information to
be stale.
Since origin servers do not always provide explicit expiration times, a
cache MAY assign a heuristic expiration time when an explicit time is not
specified, employing algorithms that use other header field values (such as
the Last-Modified time) to estimate a plausible expiration
time. This specification does not provide specific algorithms, but does
impose worst-case constraints on their results.
A cache MUST NOT use heuristics to determine freshness when an explicit
expiration time is present in the stored response. Because of the
requirements in , this means that,
effectively, heuristics can only be used on responses without explicit
freshness whose status codes are defined as cacheable by default (see
Section 6.1 of ), and those responses without explicit freshness that have
been marked as explicitly cacheable (e.g., with a "public" response
directive).
If the response has a Last-Modified header field
(Section 2.2 of ), caches are encouraged to use a heuristic
expiration value that is no more than some fraction of the interval since
that time. A typical setting of this fraction might be 10%.
When a heuristic is used to calculate freshness lifetime, a cache SHOULD
generate a Warning header field with a 113 warn-code (see
) in the response if its current_age is more than
24 hours and such a warning is not already present.
Note: Section 13.9 of prohibited caches
from calculating heuristic freshness for URIs with query components
(i.e., those containing '?'). In practice, this has not been widely
implemented. Therefore, origin servers are encouraged to send explicit
directives (e.g., Cache-Control: no-cache) if they wish to preclude
caching.
The Age header field is used to convey an estimated
age of the response message when obtained from a cache. The Age field value
is the cache's estimate of the number of seconds since the response was
generated or validated by the origin server. In essence, the Age value is
the sum of the time that the response has been resident in each of the
caches along the path from the origin server, plus the amount of time it
has been in transit along network paths.
The following data is used for the age calculation:
age_value
The term "age_value" denotes the value of the Age
header field (), in a form appropriate for
arithmetic operation; or 0, if not available.
date_value
The term "date_value" denotes the value of
the Date header field, in a form appropriate for arithmetic
operations. See Section 7.1.1.2 of for the definition of the Date header
field, and for requirements regarding responses without it.
now
The term "now" means "the current value of the clock at the host
performing the calculation". A host ought to use NTP () or some similar protocol to synchronize its
clocks to Coordinated Universal Time.
request_time
The current value of the clock at the host at the time the request
resulting in the stored response was made.
response_time
The current value of the clock at the host at the time the response
was received.
A response's age can be calculated in two entirely independent ways:
the "apparent_age": response_time minus date_value, if the local
clock is reasonably well synchronized to the origin server's clock. If
the result is negative, the result is replaced by zero.the "corrected_age_value", if all of the caches along the response
path implement HTTP/1.1. A cache MUST interpret this value relative
to the time the request was initiated, not the time that the response
was received.
unless the cache is confident in the value of the Age header
field (e.g., because there are no HTTP/1.0 hops in the Via
header field), in which case the corrected_age_value MAY be used as the
corrected_initial_age.
The current_age of a stored response can then be calculated by adding the
amount of time (in seconds) since the stored response was last validated by
the origin server to the corrected_initial_age.
A "stale" response is one that either has explicit expiry information or is
allowed to have heuristic expiry calculated, but is not fresh according to
the calculations in .
A cache MUST NOT generate a stale response if it is prohibited by an
explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
directive, a "must-revalidate" cache-response-directive, or an applicable
"s-maxage" or "proxy-revalidate" cache-response-directive; see ).
A cache MUST NOT send stale responses unless it is disconnected
(i.e., it cannot contact the origin server or otherwise find a forward
path) or doing so is explicitly allowed (e.g., by the max-stale request
directive; see ).
A cache SHOULD generate a Warning header field with the
110 warn-code (see ) in stale responses.
Likewise, a cache SHOULD generate a 112 warn-code (see
) in stale responses if the cache is disconnected.
A cache SHOULD NOT generate a new Warning header field
when forwarding a response that does not have an Age header
field, even if the response is already stale. A cache need not validate
a response that merely became stale in transit.
When a cache has one or more stored responses for a requested URI, but
cannot serve any of them (e.g., because they are not fresh, or one cannot
be selected; see ), it can use
the conditional request mechanism in the forwarded request to
give the next inbound server an opportunity to select a valid stored
response to use, updating the stored metadata in the process, or to replace
the stored response(s) with a new response. This process is known as
"validating" or "revalidating" the stored response.
When sending a conditional request for cache validation, a cache sends one
or more precondition header fields containing validator
metadata from its stored response(s), which is then compared by recipients
to determine whether a stored response is equivalent to a current
representation of the resource.
One such validator is the timestamp given in a Last-Modified
header field (Section 2.2 of ), which can be used in an
If-Modified-Since header field for response validation, or
in an If-Unmodified-Since or If-Range header
field for representation selection (i.e., the client is referring
specifically to a previously obtained representation with that timestamp).
Another validator is the entity-tag given in an ETag header
field (Section 2.3 of ). One or more entity-tags, indicating one or more
stored responses, can be used in an If-None-Match header
field for response validation, or in an If-Match or
If-Range header field for representation selection (i.e.,
the client is referring specifically to one or more previously obtained
representations with the listed entity-tags).
Each client in the request chain may have its own cache, so it is common
for a cache at an intermediary to receive conditional requests from other
(outbound) caches. Likewise, some user agents make use of conditional
requests to limit data transfers to recently modified representations or to
complete the transfer of a partially retrieved representation.
If a cache receives a request that can be satisfied by reusing one of
its stored 200 (OK) or 206 (Partial Content)
responses, the cache SHOULD evaluate any applicable conditional header
field preconditions received in that request with respect to the
corresponding validators contained within the selected response.
A cache MUST NOT evaluate conditional header fields that are only
applicable to an origin server, found in a request with semantics that
cannot be satisfied with a cached response, or applied to a target resource
for which it has no stored responses; such preconditions are likely
intended for some other (inbound) server.
The proper evaluation of conditional requests by a cache depends on the
received precondition header fields and their precedence, as defined in
Section 6 of . The If-Match and
If-Unmodified-Since conditional header fields are not
applicable to a cache.
A request containing an If-None-Match header field
(Section 3.2 of ) indicates that the client wants to validate one
or more of its own stored responses in comparison to whichever stored
response is selected by the cache.
If the field-value is "*", or if the field-value is a list of entity-tags
and at least one of them matches the entity-tag of the selected stored
response, a cache recipient SHOULD generate a
304 (Not Modified) response (using the metadata of the
selected stored response) instead of sending that stored response.
When a cache decides to revalidate its own stored responses for a
request that contains an If-None-Match list of entity-tags,
the cache MAY combine the received list with a list of entity-tags
from its own stored set of responses (fresh or stale) and send the union of
the two lists as a replacement If-None-Match header
field value in the forwarded request.
If a stored response contains only partial content, the
cache MUST NOT include its entity-tag in the union unless the request is
for a range that would be fully satisfied by that partial stored response.
If the response to the forwarded request is
304 (Not Modified) and has an ETag header field value with
an entity-tag that is not in the client's list, the cache MUST
generate a 200 (OK) response for the client by reusing its
corresponding stored response, as updated by the 304 response metadata
().
If an If-None-Match header field is not present, a request
containing an If-Modified-Since header field
(Section 3.3 of ) indicates that the client wants to validate
one or more of its own stored responses by modification date.
A cache recipient SHOULD generate a 304 (Not Modified)
response (using the metadata of the selected stored response) if one of the
following cases is true:
1) the selected stored response has a Last-Modified
field-value that is earlier than or equal to the conditional timestamp;
2) no Last-Modified field is present in the selected stored
response, but it has a Date field-value that is earlier than
or equal to the conditional timestamp; or,
3) neither Last-Modified nor Date is present
in the selected stored response, but the cache recorded it as having been
received at a time earlier than or equal to the conditional timestamp.
A cache that implements partial responses to range requests, as defined in
, also needs to evaluate a received If-Range header
field (Section 3.2 of ) with respect to its selected stored response.
Cache handling of a response to a conditional request is dependent upon its
status code:
A 304 (Not Modified) response status code indicates
that the stored response can be updated and reused; see .
A full response (i.e., one with a payload body) indicates that none
of the stored responses nominated in the conditional request is
suitable. Instead, the cache MUST use the full response to
satisfy the request and MAY replace the stored response(s).
However, if a cache receives a 5xx (Server Error)
response while attempting to validate a response, it can either
forward this response to the requesting client, or act as if the
server failed to respond. In the latter case, the cache MAY send a
previously stored response (see ).
When a cache receives a 304 (Not Modified) response and
already has one or more stored 200 (OK) responses for the
same cache key, the cache needs to identify which of the stored responses
are updated by this new response and then update the stored response(s)
with the new information provided in the 304 response.
The stored response to update is identified by using the first match (if
any) of the following:
If the new response contains a strong validator (see
Section 2.1 of ), then that strong validator identifies the selected
representation for update. All of the stored responses with the same
strong validator are selected. If none of the stored responses contain the
same strong validator, then the cache MUST NOT use the new response to
update any stored responses.
If the new response contains a weak validator and that validator
corresponds to one of the cache's stored responses, then the most
recent of those matching stored responses is selected for update.
If the new response does not include any form of validator (such as in
the case where a client generates an If-Modified-Since request from a
source other than the Last-Modified response header field), and there is
only one stored response, and that stored response also lacks a
validator, then that stored response is selected for update.
If a stored response is selected for update, the cache MUST:
delete any Warning header fields in the stored response
with warn-code 1xx (see );retain any Warning header fields in the stored response
with warn-code 2xx; and,use other header fields provided in the 304 (Not Modified)
response to replace all instances of the corresponding header
fields in the stored response.
A response to the HEAD method is identical to what an equivalent request
made with a GET would have been, except it lacks a body. This property
of HEAD responses can be used to invalidate or update a cached GET
response if the more efficient conditional GET request mechanism is not
available (due to no validators being present in the stored response) or
if transmission of the representation body is not desired even if it has
changed.
When a cache makes an inbound HEAD request for a given request target and
receives a 200 (OK) response, the cache SHOULD update or
invalidate each of its stored GET responses that could have been selected
for that request (see ).
For each of the stored responses that could have been selected, if the
stored response and HEAD response have matching values for any received
validator fields (ETag and Last-Modified)
and, if the HEAD response has a Content-Length header field,
the value of Content-Length matches that of the stored
response, the cache SHOULD update the stored response as described below;
otherwise, the cache SHOULD consider the stored response to be stale.
If a cache updates a stored response with the metadata provided in a HEAD
response, the cache MUST:
delete any Warning header fields in the stored response
with warn-code 1xx (see );retain any Warning header fields in the stored response
with warn-code 2xx; and,use other header fields provided in the HEAD response to replace all
instances of the corresponding header fields in the stored response
and append new header fields to the stored response's header section
unless otherwise restricted by the Cache-Control
header field.
Because unsafe request methods (Section 4.2.1 of ) such as PUT, POST or DELETE
have the potential for changing state on the origin server, intervening
caches can use them to keep their contents up to date.
A cache MUST invalidate the effective Request URI
(Section 5.5 of ) as well as the URI(s) in the
Location and Content-Location response header
fields (if present) when a non-error status code is received in response to
an unsafe request method.
However, a cache MUST NOT invalidate a URI from a Location
or Content-Location response header field if the host part of
that URI differs from the host part in the effective request URI
(Section 5.5 of ). This helps prevent denial-of-service attacks.
A cache MUST invalidate the effective request URI
(Section 5.5 of ) when it receives a non-error response
to a request with a method whose safety is unknown.
Here, a "non-error response" is one with a 2xx (Successful)
or 3xx (Redirection) status code. "Invalidate" means that
the cache will either remove all stored responses related to the effective
request URI or will mark these as "invalid" and in need of a mandatory
validation before they can be sent in response to a subsequent request.
Note that this does not guarantee that all appropriate responses are
invalidated. For example, a state-changing request might invalidate
responses in the caches it travels through, but relevant responses still
might be stored in other caches that it has not.
This section defines the syntax and semantics of HTTP/1.1 header fields
related to caching.
The "Age" header field conveys the sender's estimate of the amount
of time since the response was generated or successfully validated at the
origin server. Age values are calculated as specified in .
The Age field-value is a non-negative integer, representing time in seconds
(see ).
The presence of an Age header field implies that the response was not
generated or validated by the origin server for this request. However,
lack of an Age header field does not imply the origin was contacted, since
the response might have been received from an HTTP/1.0 cache that does not
implement Age.
The "Cache-Control" header field is used to specify directives for
caches along the request/response chain. Such cache directives are
unidirectional in that the presence of a directive in a request does not
imply that the same directive is to be given in the response.
A cache MUST obey the requirements of the Cache-Control
directives defined in this section. See for information about how Cache-Control
directives defined elsewhere are handled.
Note: Some HTTP/1.0 caches might not implement Cache-Control.
A proxy, whether or not it implements a cache, MUST pass cache directives
through in forwarded messages, regardless of their
significance to that application, since the directives might be applicable
to all recipients along the request/response chain. It is not possible to
target a directive to a specific cache.
Cache directives are identified by a token, to be compared case-insensitively,
and have an optional argument, that can use both token and quoted-string
syntax. For the directives defined below that define arguments, recipients
ought to accept both forms, even if one is documented to be preferred. For
any directive not defined by this specification, a recipient MUST accept
both forms.
For the cache directives defined below, no argument is defined (nor allowed)
unless stated otherwise.
Argument syntax:
delta-seconds (see )
The "max-age" request directive indicates that the client is unwilling
to accept a response whose age is greater than the specified number of
seconds. Unless the max-stale request directive is also present, the client
is not willing to accept a stale response.
This directive uses the token form of the argument syntax:
e.g., 'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the
quoted-string form.
Argument syntax:
delta-seconds (see )
The "max-stale" request directive indicates that the client is willing
to accept a response that has exceeded its freshness lifetime. If max-stale
is assigned a value, then the client is willing to accept a response
that has exceeded its freshness lifetime by no more than the specified
number of seconds. If no value is assigned to max-stale, then the client
is willing to accept a stale response of any age.
This directive uses the token form of the argument syntax:
e.g., 'max-stale=10' not 'max-stale="10"'. A sender SHOULD NOT generate
the quoted-string form.
Argument syntax:
delta-seconds (see )
The "min-fresh" request directive indicates that the client is willing
to accept a response whose freshness lifetime is no less than its
current age plus the specified time in seconds. That is, the client
wants a response that will still be fresh for at least the specified
number of seconds.
This directive uses the token form of the argument syntax:
e.g., 'min-fresh=20' not 'min-fresh="20"'. A sender SHOULD NOT generate
the quoted-string form.
The "no-cache" request directive indicates that a cache MUST NOT
use a stored response to satisfy the request without successful
validation on the origin server.
The "no-store" request directive indicates that a cache MUST NOT
store any part of either this request or any response to it. This
directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a
best-effort attempt to remove the information from volatile storage as
promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might be
vulnerable to eavesdropping.
Note that if a request containing this directive is satisfied from a
cache, the no-store request directive does not apply to the already
stored response.
The "no-transform" request directive indicates that an intermediary
(whether or not it implements a cache) MUST NOT transform the payload,
as defined in Section 5.7.2 of .
The "only-if-cached" request directive indicates that the client only wishes
to obtain a stored response. If it receives this directive, a cache SHOULD
either respond using a stored response that is consistent with the other
constraints of the request, or respond with a 504 (Gateway
Timeout) status code. If a group of caches is being operated as a
unified system with good internal connectivity, a member cache MAY
forward such a request within that group of caches.
The "must-revalidate" response directive indicates that once it has
become stale, a cache MUST NOT use the response to satisfy subsequent
requests without successful validation on the origin server.
The must-revalidate directive is necessary to support reliable
operation for certain protocol features. In all circumstances a
cache MUST obey the must-revalidate directive; in particular,
if a cache cannot reach the origin server for any reason, it MUST
generate a 504 (Gateway Timeout) response.
The must-revalidate directive ought to be used by servers if and only
if failure to validate a request on the representation could result in
incorrect operation, such as a silently unexecuted financial
transaction.
Argument syntax:
#field-name
The "no-cache" response directive indicates that the response MUST NOT
be used to satisfy a subsequent request without successful validation on
the origin server. This allows an origin server to prevent a cache from
using it to satisfy a request without contacting it, even by caches that
have been configured to send stale responses.
If the no-cache response directive specifies one or more field-names,
then a cache MAY use the response to satisfy a subsequent request,
subject to any other restrictions on caching. However, any header fields
in the response that have the field-name(s) listed MUST NOT be sent
in the response to a subsequent request without successful revalidation
with the origin server. This allows an origin server to prevent the
re-use of certain header fields in a response, while still allowing
caching of the rest of the response.
The field-names given are not limited to the set of header
fields defined by this specification. Field names are case-insensitive.
This directive uses the quoted-string form of the argument syntax.
A sender SHOULD NOT generate the token form (even if quoting appears not
to be needed for single-entry lists).
Note: Although it has been back-ported to many implementations, some
HTTP/1.0 caches will not recognize or obey this directive. Also, no-cache
response directives with field-names are often handled by caches
as if an unqualified no-cache directive was received; i.e., the special
handling for the qualified form is not widely implemented.
The "no-store" response directive indicates that a cache MUST NOT
store any part of either the immediate request or response. This
directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a
best-effort attempt to remove the information from volatile storage as
promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might be
vulnerable to eavesdropping.
The "no-transform" response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of .
The "public" response directive indicates that any cache MAY store the
response, even if the response would normally be non-cacheable or cacheable
only within a private cache. (See for additional details related to
the use of public in response to a request containing
Authorization, and for
details of how public affects responses that would normally not be stored,
due to their status codes not being defined as cacheable by default; see
.)
Argument syntax:
#field-name
The "private" response directive indicates that the response message is
intended for a single user and MUST NOT be stored by a shared cache. A
private cache MAY store the response and reuse it for later requests,
even if the response would normally be non-cacheable.
If the private response directive specifies one or more field-names,
this requirement is limited to the field-values associated with the
listed response header fields. That is, a shared cache MUST NOT store
the specified field-names(s), whereas it MAY store the remainder of the
response message.
The field-names given are not limited to the set of header
fields defined by this specification. Field names are case-insensitive.
This directive uses the quoted-string form of the argument syntax.
A sender SHOULD NOT generate the token form (even if quoting appears not
to be needed for single-entry lists).
Note: This usage of the word "private" only controls
where the response can be stored; it cannot ensure the privacy of the
message content. Also, private response directives with field-names are
often handled by caches as if an unqualified private directive
was received; i.e., the special handling for the qualified form is not
widely implemented.
The "proxy-revalidate" response directive has the same meaning as the
must-revalidate response directive, except that it does not apply to
private caches.
Argument syntax:
delta-seconds (see )
The "max-age" response directive indicates that the response is to be
considered stale after its age is greater than the specified number of
seconds.
This directive uses the token form of the argument syntax:
e.g., 'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the
quoted-string form.
Argument syntax:
delta-seconds (see )
The "s-maxage" response directive indicates that, in shared caches, the
maximum age specified by this directive overrides the maximum age
specified by either the max-age directive or the Expires
header field. The s-maxage directive also implies the semantics of the
proxy-revalidate response directive.
This directive uses the token form of the argument syntax:
e.g., 's-maxage=10' not 's-maxage="10"'. A sender SHOULD NOT generate
the quoted-string form.
The Cache-Control header field can be extended through the use of one or
more cache-extension tokens, each with an optional value.
A cache MUST ignore unrecognized cache directives.
Informational extensions (those that do not require a change in cache
behavior) can be added without changing the semantics of other directives.
Behavioral extensions are designed to work by acting as modifiers to the
existing base of cache directives.
Both the new directive and the old directive are supplied, such that
applications that do not understand the new directive will default to the
behavior specified by the old directive, and those that understand the
new directive will recognize it as modifying the requirements associated
with the old directive. In this way, extensions to the existing
cache-control directives can be made without breaking deployed caches.
For example, consider a hypothetical new response directive called
"community" that acts as a modifier to the private directive: in addition
to private caches, any cache that is shared only by members of the named
community is allowed to cache the response. An origin server wishing to
allow the UCI community to use an otherwise private response in their
shared cache(s) could do so by including
A cache that recognizes such a community cache-extension could broaden its
behavior in accordance with that extension. A cache that does not
recognize the community cache-extension would ignore it and adhere to the
private directive.
The "Expires" header field gives the date/time after which the
response is considered stale. See for
further discussion of the freshness model.
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.
The Expires value is an HTTP-date timestamp, as defined in Section 7.1.1.1 of .
A cache recipient MUST interpret invalid date formats, especially the
value "0", as representing a time in the past (i.e., "already expired").
If a response includes a Cache-Control field with
the max-age directive (),
a recipient MUST ignore the Expires field.
Likewise, if a response includes the s-maxage directive
(), a shared cache
recipient MUST ignore the Expires field. In both these cases, the value
in Expires is only intended for recipients that have not yet implemented
the Cache-Control field.
An origin server without a clock MUST NOT generate an Expires field
unless its value represents a fixed time in the past (always expired)
or its value has been associated with the resource by a system or user
with a reliable clock.
Historically, HTTP required the Expires field-value to be no more than a
year in the future. While longer freshness lifetimes are no longer
prohibited, extremely large values have been demonstrated to cause
problems (e.g., clock overflows due to use of 32-bit integers for
time values), and many caches will evict a response far sooner than
that.
The "Pragma" header field allows backwards compatibility with HTTP/1.0
caches, so that clients can specify a "no-cache" request that they will
understand (as Cache-Control was not defined until HTTP/1.1).
When the Cache-Control header field is also present and understood in a
request, Pragma is ignored.
In HTTP/1.0, Pragma was defined as an extensible field for
implementation-specified directives for recipients. This specification
deprecates such extensions to improve interoperability.
When the Cache-Control header field is not present in a
request, caches MUST consider the no-cache request pragma-directive as
having the same effect as if "Cache-Control: no-cache" were present (see
).
When sending a no-cache request, a client ought to include both the pragma
and cache-control directives, unless Cache-Control: no-cache is
purposefully omitted to target other Cache-Control response
directives at HTTP/1.1 caches. For example:
will constrain HTTP/1.1 caches to serve a response no older than 30
seconds, while precluding implementations that do not understand
Cache-Control from serving a cached response.
Note: Because the meaning of "Pragma: no-cache" in responses is not
specified, it does not provide a reliable replacement for
"Cache-Control: no-cache" in them.
The "Warning" header field is used to carry additional information
about the status or transformation of a message that might not be reflected
in the status code. This information is typically used to warn about possible
incorrectness introduced by caching operations or transformations applied
to the payload of the message.
Warnings can be used for other purposes, both cache-related and otherwise.
The use of a warning, rather than an error status code, distinguishes these
responses from true failures.
Warning header fields can in general be applied to any message, however some
warn-codes are specific to caches and can only be applied to response
messages.
Multiple warnings can be generated in a response (either by the origin
server or by a cache), including multiple warnings with the same warn-code
number that only differ in warn-text.
A user agent that receives one or more Warning header fields SHOULD
inform the user of as many of them as possible, in the order that they
appear in the response. Senders that generate multiple Warning header
fields are encouraged to order them with this user agent behavior in mind.
A sender that generates new Warning header fields MUST append them after
any existing Warning header fields.
Warnings are assigned three digit warn-codes. The first digit indicates
whether the Warning is required to be deleted from a stored response after
validation:
1xx warn-codes describe the freshness or validation status of the
response, and so they MUST be deleted by a cache after validation. They can
only be generated by a cache when validating a cached entry, and
MUST NOT be generated in any other situation.2xx warn-codes describe some aspect of the representation that is not
rectified by a validation (for example, a lossy compression of the
representation) and they MUST NOT be deleted by a cache after validation,
unless a full response is sent, in which case they MUST be.
If a sender generates one or more 1xx warn-codes in a message to be
sent to a recipient known to implement only HTTP/1.0, the sender MUST
include in each corresponding warning-value a warn-date that matches the
Date header field in the message. For example:
Warnings have accompanying warn-text that describes the error, e.g., for
logging. It is advisory only, and its content does not affect interpretation
of the warn-code.
If a recipient that uses, evaluates, or displays Warning header fields
receives a warn-date that is different from the Date value
in the same message, the recipient MUST exclude the warning-value
containing that warn-date before storing, forwarding, or using the message.
This allows recipients to exclude warning-values that were improperly
retained after a cache validation.
If all of the warning-values are excluded, the recipient MUST exclude
the Warning header field as well.
The following warn-codes are defined by this specification, each with a
recommended warn-text in English, and a description of its meaning.
The procedure for defining additional warn codes is described in
.
A cache SHOULD generate this whenever the sent response is stale.
A cache SHOULD generate this when sending a stale response because an
attempt to validate the response failed, due to an inability to reach
the server.
A cache SHOULD generate this if it is intentionally disconnected from
the rest of the network for a period of time.
A cache SHOULD generate this if it heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater than 24
hours.
The warning text can include arbitrary information to be presented to
a human user or logged. A system receiving this warning MUST NOT take
any automated action, besides presenting the warning to the user.
This Warning code MUST be added by a proxy if it applies any transformation to the
representation, such as changing the content-coding, media-type, or
modifying the representation data, unless this Warning code already appears
in the response.
The warning text can include arbitrary information to be presented to
a human user or logged. A system receiving this warning MUST NOT take
any automated action.
User agents often have history mechanisms, such as "Back" buttons and
history lists, that can be used to redisplay a representation retrieved
earlier in a session.
The freshness model () does not
necessarily apply to history mechanisms. That is, a history mechanism can
display a previous representation even if it has expired.
This does not prohibit the history mechanism from telling the user that a
view might be stale or from honoring cache directives (e.g.,
Cache-Control: no-store).
The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry" defines the namespace for the
cache directives. It has been created and is now maintained at
<http://www.iana.org/assignments/http-cache-directives>.
A registration MUST include the following fields:
Cache Directive NamePointer to specification text
Values to be added to this namespace require IETF Review (see , Section 4.1).
New extension directives ought to consider defining:
What it means for a directive to be specified multiple times,When the directive does not take an argument, what it means when an
argument is present,When the directive requires an argument, what it means when it is
missing,Whether the directive is specific to requests, responses, or able
to be used in either.
See also .
The registry has been populated with the registrations below:
Cache DirectiveReferencemax-age, max-stalemin-freshmust-revalidateno-cache, no-store, no-transform, only-if-cachedprivateproxy-revalidatepublics-maxagestale-if-error, Section 4
stale-while-revalidate, Section 3
The "Hypertext Transfer Protocol (HTTP) Warn Codes" registry defines the namespace for warn codes.
It has been created and is now maintained at
<http://www.iana.org/assignments/http-warn-codes>.
A registration MUST include the following fields:
Warn Code (3 digits)Short DescriptionPointer to specification text
Values to be added to this namespace require IETF Review (see , Section 4.1).
The registry has been populated with the registrations below:
Warn CodeShort DescriptionReference110Response is Stale111Revalidation Failed112Disconnected Operation113Heuristic Expiration199Miscellaneous Warning214Transformation Applied299Miscellaneous Persistent Warning
HTTP header fields are registered within the "Message Headers" registry
maintained at
<http://www.iana.org/assignments/message-headers/>.
This document defines the following HTTP header fields, so the
"Permanent Message Header Field Names" registry has been updated
accordingly (see ).
Header Field NameProtocolStatusReferenceAgehttpstandardCache-ControlhttpstandardExpireshttpstandardPragmahttpstandardWarninghttpstandard
The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
Force".
This section is meant to inform developers, information providers, and
users of known security concerns specific to HTTP caching.
More general security considerations are addressed in HTTP messaging
and semantics .
Caches expose additional potential vulnerabilities, since the contents of
the cache represent an attractive target for malicious exploitation.
Because cache contents persist after an HTTP request is complete, an attack
on the cache can reveal information long after a user believes that the
information has been removed from the network. Therefore, cache contents
need to be protected as sensitive information.
In particular, various attacks might be amplified by being stored in a
shared cache; such "cache poisoning" attacks use the cache to distribute a
malicious payload to many clients, and are especially effective when an
attacker can use implementation flaws, elevated privileges, or other
techniques to insert such a response into a cache. One common attack vector
for cache poisoning is to exploit differences in message parsing on proxies
and in user agents; see Section 3.3.3 of for the relevant requirements.
Likewise, implementation flaws (as well as misunderstanding of cache
operation) might lead to caching of sensitive information (e.g.,
authentication credentials) that is thought to be private, exposing it to
unauthorized parties.
Furthermore, the very use of a cache can bring about privacy concerns. For
example, if two users share a cache, and the first one browses to a site,
the second may be able to detect that the other has been to that site,
because the resources from it load more quickly, thanks to the cache.
Note that the Set-Cookie response header field does not
inhibit caching; a cacheable response with a Set-Cookie header field can be (and
often is) used to satisfy subsequent requests to caches. Servers who wish
to control caching of these responses are encouraged to emit appropriate
Cache-Control response header fields.
See Section 10 of .
Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): Semantics and ContentAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): Conditional RequestsAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): Range RequestsAdobe Systems Incorporatedfielding@gbiv.comWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): AuthenticationAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deKey words for use in RFCs to Indicate Requirement LevelsHarvard Universitysob@harvard.eduAugmented BNF for Syntax Specifications: ABNFBrandenburg InternetWorkingdcrocker@bbiw.netTHUS plc.paul.overell@thus.netHypertext Transfer Protocol -- HTTP/1.1University of California, Irvinefielding@ics.uci.eduW3Cjg@w3.orgCompaq Computer Corporationmogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgXerox Corporationmasinter@parc.xerox.comMicrosoft Corporationpaulle@microsoft.comW3Ctimbl@w3.orgRegistration Procedures for Message Header FieldsNine by NineGK-IETF@ninebynine.orgBEA Systemsmnot@pobox.comHP LabsJeffMogul@acm.orgGuidelines for Writing an IANA Considerations Section in RFCsIBMnarten@us.ibm.comGoogleHarald@Alvestrand.noHTTP Cache-Control Extensions for Stale ContentYahoo! Inc.mnot@yahoo-inc.comNetwork Time Protocol Version 4: Protocol and Algorithms SpecificationHTTP State Management Mechanism
University of California, Berkeley
abarth@eecs.berkeley.edu
The specification has been substantially rewritten for clarity.
The conditions under which an authenticated response can be cached have been
clarified.
()
New status codes can now define that caches are allowed to use heuristic
freshness with them.
Caches are now allowed to calculate heuristic freshness for URIs with query
components.
()
The algorithm for calculating age is now less conservative.
Caches are now required to handle dates with time zones as if they're
invalid, because it's not possible to accurately guess.
()
The Content-Location response header field is no longer used
to determine the appropriate response to use when validating.
()
The algorithm for selecting a cached negotiated response to use has been
clarified in several ways. In particular, it now explicitly allows
header-specific canonicalization when processing selecting header fields.
()
Requirements regarding denial-of-service attack avoidance when performing
invalidation have been clarified.
()
Cache invalidation only occurs when a successful response is received.
()
Cache directives are explicitly defined to be case-insensitive.
Handling of multiple instances of cache directives when only one is
expected is now defined.
()
The "no-store" request directive doesn't apply to responses; i.e.,
a cache can satisfy a request with no-store on it and does not invalidate
it.
()
The qualified forms of the private and no-cache cache directives are noted
to not be widely implemented; for example, "private=foo" is interpreted by many
caches as simply "private". Additionally, the meaning of the qualified
form of no-cache has been clarified.
()
The "no-cache" response directive's meaning has been clarified.
()
The one-year limit on Expires header field values has been removed; instead,
the reasoning for using a sensible value is given.
()
The Pragma header field is now only defined for backwards compatibility;
future pragmas are deprecated.
()
Some requirements regarding production and processing of the Warning
header fields have been relaxed, as it is not widely implemented.
Furthermore, the Warning header field no longer uses RFC 2047 encoding,
nor does it allow multiple languages, as these aspects were not implemented.
()
This specification introduces the Cache Directive and Warn Code Registries,
and defines considerations for new cache directives.
( and )
The following core rules are included by reference, as defined in Appendix B.1 of : ALPHA (letters), CR (carriage
return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
The rules below are defined in :
The rules below are defined in other parts:
In the collected ABNF below, list rules are expanded as per Section 1.2 of .