HTTP CachingAdobe345 Park AveSan JoseCA95110USAfielding@gbiv.comhttps://roy.gbiv.com/Fastlymnot@mnot.nethttps://www.mnot.net/greenbytes GmbHHafenweg 16MuensterNW48155Germanyjulian.reschke@greenbytes.dehttps://greenbytes.de/tech/webdav/
Applications and Real-Time
HTTP 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.
This document obsoletes RFC 7234.
This note is to be removed before publishing as an RFC.
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
.
Working Group information can be found at ;
source code and issues list for this draft can be found at
.
The changes in this draft are summarized in .
The Hypertext Transfer Protocol (HTTP) is a stateless application-level
request/response protocol that uses extensible semantics and
self-descriptive messages for flexible interaction with network-based
hypertext information systems. HTTP is defined by a series of documents
that collectively form the HTTP/1.1 specification:
"HTTP Semantics" "HTTP Caching" (this document)"HTTP/1.1 Messaging"
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 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 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 ().
This document obsoletes RFC 7234,
with the changes being summarized in .
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 3 of .
This specification uses the Augmented Backus-Naur Form (ABNF) notation of
, extended with the notation for case-sensitivity
in strings defined in .
It also uses a list extension, defined in Section 11 of ,
that allows for compact definition of comma-separated lists using a '#'
operator (similar to how the '*' operator indicates repetition). shows the collected grammar with all list
operators expanded to standard ABNF notation.
The following core rules are included by
reference, as defined in , Appendix B.1:
ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
OCTET (any 8-bit sequence of data), SP (space), and
VCHAR (any visible character).
The rules below are defined in :
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 reducing 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 7.3.1 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 is disconnected when it cannot contact the origin
server or otherwise find a forward path for a given request. A
disconnected cache can serve stale responses in some circumstances ().
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 final (see Section 9.3 of ), andthe response status code is understood by the cache, andthe "no-store" cache directive (see ) does not appear in the response,
andthe "private" response directive (see ) does not appear in the
response, if the cache is shared, andthe Authorization header field (see
Section 8.5.3 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 (Section 8.3 of ) 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 8.5.3 of ) to
satisfy any subsequent request unless a response directive that allows such
responses to be stored is present.
In this specification, the following Cache-Control response
directives () have such an effect:
must-revalidate, public, and s-maxage.
A response might transfer only a partial representation if the
connection closed prematurely or if the request used one or more Range
specifiers (Section 8.3 of ). 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 9.3.7.3 of .
When combining the new response with one or more stored responses, a cache
MUST use the 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 5.3 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 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 7.2.1 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 one (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 10.1.4 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 4.2 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 ).
The calculation to determine if a response is fresh is:
freshness_lifetime is defined in ; current_age is defined in
.
Clients can send the max-age or min-fresh request directives () to constrain or relax freshness
calculations for the corresponding response. However, caches are not
required to honor them.
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 9.1 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 10.2.2 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%.
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:
The term "age_value" denotes the value of the Age
header field (), in a form appropriate for
arithmetic operation; or 0, if not available.
The term "date_value" denotes the value of
the Date header field, in a form appropriate for arithmetic
operations. See Section 10.1.1.2 of for the definition of the Date header
field, and for requirements regarding responses without it.
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.
The current value of the clock at the host at the time the request
resulting in the stored response was made.
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 or greater. A cache MUST interpret this value
relative to the time the request was initiated, not the time that the
response was received.
These are combined as
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 generate a stale response unless it is disconnected
or doing so is explicitly permitted by the client or origin server
(e.g., by the max-stale request directive in , by extension directives such as those
defined in , or by configuration in accordance
with an out-of-band contract).
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 Section 8.2 of 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 generating a conditional request for validation, a cache starts with
either a request it is attempting to satisfy, or -- if it is initiating
the request independently -- it synthesises a request using a stored
response by copying the method, request-target, and request header fields
used for identifying the secondary cache key .
It then updates that request with one or more precondition header fields.
These contain validator metadata sourced from stored response(s) that have
the same cache key (both primary and secondary, as applicable).
The precondition header fields are then compared by recipients to
determine whether any 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 10.2.2 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 10.2.3 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 8.2.2 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 8.2.4 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 8.2.5 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
Section 8.3 of , also needs to evaluate a received If-Range header
field (Section 8.2.7 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
applicable cache key, the cache needs to identify which (if any) are to
be updated by the new information provided, and then do so.
The stored response(s) to update are identified by using the first match
(if any) of the following:
If the new response contains a strong validator (see
Section 10.2.1 of ), then that strong validator
identifies the selected representation for update. All of the stored
responses with the same strong validator are identified for update. 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 identified 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 identified for update.
For each stored response identified for update, the cache MUST use the
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 use the 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 7.2.1 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 5.3 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 5.3 of ). This helps prevent denial-of-service attacks.
A cache MUST invalidate the effective request URI
(Section 5.3 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 header fields
related to caching.
Header Field NameStatusReferenceAgestandardCache-ControlstandardExpiresstandardPragmastandardWarningobsoleted
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 present in the response, or to be repeated in it.
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.
Cache DirectiveReferencemax-age, max-stalemin-freshmust-revalidateno-cache, no-store, no-transform, only-if-cachedprivateproxy-revalidatepublics-maxage
This section defines cache request directives. They are advisory; caches
MAY implement them, but are not required to.
Argument syntax:
delta-seconds (see )
The "max-age" request directive indicates that the client prefers a
response whose age is less than or equal to the specified number of
seconds. Unless the max-stale request directive is also present, the
client does not wish to receive 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 a value is
present, 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 prefers 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 the client prefers
stored response not be used 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 the client is asking
for intermediares (whether or not they implement a cache) to avoid
transforming the payload, as defined in Section 5.5.2 of .
The "only-if-cached" request directive indicates that the client only
wishes to obtain a stored response. Caches that honor this request
directive SHOULD, upon receiving it, 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.
This section defines cache response directives. A cache MUST obey the
requirements of the Cache-Control directives defined in this section.
The "must-revalidate" response directive indicates that once it has become
stale, the response MUST NOT be used to satisfy any other request
without forwarding it for validation and receiving a successful response;
see .
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 is disconnected,
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.
The must-revalidate directive also has the effect of allowing a stored
response to be used to satisfy a request with an Authorization header
field; see .
Argument syntax:
#field-name
The "no-cache" response directive indicates that the response MUST NOT
be used to satisfy any other request without forwarding it for validation
and receiving a successful response; see .
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, and MUST NOT use
the response to satisfy any other request.
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 5.5.2 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.
The must-revalidate directive also has the effect of allowing a stored
response to be used to satisfy a request with an Authorization header
field; see .
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.
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.
The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry" defines the namespace for the
cache directives. It has been created and is now maintained at
.
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 4.8).
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 10.1.1.1 of .
For example
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 was defined for HTTP/1.0 caches, so that clients
could specify a "no-cache" request (as Cache-Control was
not defined until HTTP/1.1).
However, support for Cache-Control is now widespread. As a result, this
specification deprecates Pragma.
Note: Because the meaning of "Pragma: no-cache" in responses was never
specified, it does not provide a reliable replacement for
"Cache-Control: no-cache" in them.
The "Warning" header field was used to carry additional information
about the status or transformation of a message that might not be reflected
in the status code. This specification obsoletes it, as it is not widely
generated or surfaced to users. The information it carried can be gleaned
from examining other header fields, such as Age.
Applications using HTTP often specify additional forms of caching. For
example, Web browsers often have history mechanisms such as "Back" buttons
that can be used to redisplay a representation retrieved earlier in a
session.
Likewise, some Web browsers implement caching of images and other assets
within a page view; they may or may not honor HTTP caching semantics.
The requirements in this specification do not necessarily apply to how
applications use data after it is retrieved from a HTTP cache. That is, a
history mechanism can display a previous representation even if it has
expired, and an application can use cached data in other ways beyond its
freshness lifetime.
This does not prohibit the application from taking HTTP caching into
account; for example, a history mechanism might tell the user that a view
is stale, or it might honor cache directives (e.g., Cache-Control:
no-store).
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 6.3 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.
The change controller for the following registrations is:
"IETF (iesg@ietf.org) - Internet Engineering Task Force".
Please update the "Hypertext Transfer Protocol (HTTP) Header Field
Registry" registry at with the
header field names listed in the two tables of .
Please update the
"Hypertext Transfer Protocol (HTTP) Cache Directive Registry"
at
with the registration procedure of
and the cache directive names summarized in the table of
.
Please add a note to the "Hypertext Transfer Protocol (HTTP) Warn Codes"
registry at to the effect
that Warning is obsoleted.
HTTP/1.1 MessagingAdobe345 Park AveSan JoseCA95110USAfielding@gbiv.comhttps://roy.gbiv.com/Fastlymnot@mnot.nethttps://www.mnot.net/greenbytes GmbHHafenweg 16MuensterNW48155Germanyjulian.reschke@greenbytes.dehttps://greenbytes.de/tech/webdav/HTTP SemanticsAdobe345 Park AveSan JoseCA95110USAfielding@gbiv.comhttps://roy.gbiv.com/Fastlymnot@mnot.nethttps://www.mnot.net/greenbytes GmbHHafenweg 16MuensterNW48155Germanyjulian.reschke@greenbytes.dehttps://greenbytes.de/tech/webdav/Key words for use in RFCs to Indicate Requirement LevelsHarvard Universitysob@harvard.eduUniform Resource Identifier (URI): Generic SyntaxWorld Wide Web Consortiumtimbl@w3.orghttp://www.w3.org/People/Berners-Lee/Day Softwarefielding@gbiv.comhttp://roy.gbiv.com/AdobeLMM@acm.orghttp://larry.masinter.net/Augmented BNF for Syntax Specifications: ABNFBrandenburg InternetWorkingdcrocker@bbiw.netTHUS plc.paul.overell@thus.netCase-Sensitive String Support in ABNFpkyzivat@alum.mit.eduCoded Character Set -- 7-bit American Standard Code for Information InterchangeAmerican National Standards InstituteHypertext 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.orgHypertext Transfer Protocol (HTTP): CachingAdobefielding@gbiv.comAkamaimnot@mnot.netgreenbytes GmbHjulian.reschke@greenbytes.deHTTP 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.eduGuidelines for Writing an IANA Considerations Section in RFCsIn the collected ABNF below, list rules are expanded as per Section 11 of .
The Warning response header was obsoleted. Much of the information
supported by Warning could be gleaned by examining the response), and the
remaining warn-codes -- although potentially useful -- were entirely
advisory, and in practice were not added by caches or intermediaries.
()
This section is to be removed before publishing as an RFC.
The changes were purely editorial:
Change boilerplate and abstract to indicate the "draft" status, and update references to ancestor specifications.Remove version "1.1" from document title, indicating that this specification applies to all HTTP versions.Adjust historical notes.Update links to sibling specifications.Replace sections listing changes from RFC 2616 by new empty sections referring to RFC 723x.Remove acknowledgements specific to RFC 723x.Move "Acknowledgements" to the very end and make them unnumbered.
The changes are purely editorial:
Moved all extensibility tips, registration procedures, and registry
tables from the IANA considerations to normative sections, reducing the
IANA considerations to just instructions that will be removed prior to
publication as an RFC.Cite RFC 8126 instead of RFC 5226 ()In , misleading statement about the relation between Pragma and Cache-Control (, )In , explain that only final responses are cacheable ()In , clarify what responses various directives apply to ()In , clarify the source of validators in conditional requests ()Revise to apply to more than just History Lists ()In , deprecated "Warning" header field ()In , remove a spurious note ()In , define what a disconnected cache is ()In , clarify language around how to select a response when more than one matches ()in , mention stale-while-revalidate and stale-if-error ()Remove requirements around cache request directives ()Deprecate Pragma ()In and , note effect of some directives on authenticated requests ()In , remove the registrations for stale-if-error and stale-while-revalidate which happened in RFC 7234 ()
See Appendix "Acknowledgments" of .