RFC 9111: HTTP Caching

RFC 9111: HTTP Caching
RFC 9111
HTTP Caching
June 2022
Fielding, et al.
Standards Track
[Page]
Stream:
Internet Engineering Task Force (IETF)
RFC:
9111
STD:
98
Obsoletes:
7234
Category:
Standards Track
Published:
June 2022
ISSN:
2070-1721
Authors:
R. Fielding,
Ed.
Adobe
M. Nottingham,
Ed.
Fastly
J. Reschke,
Ed.
greenbytes
RFC 9111
HTTP Caching
Abstract
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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by
the Internet Engineering Steering Group (IESG). Further
information on Internet Standards is available in Section 2 of
RFC 7841.
Information about the current status of this document, any
errata, and how to provide feedback on it may be obtained at
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
respect to this document. Code Components extracted from this
document must include Revised BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Revised BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s)
controlling the copyright in such materials, this document may not
be modified outside the IETF Standards Process, and derivative
works of it may not be created outside the IETF Standards Process,
except to format it for publication as an RFC or to translate it
into languages other than English.
Table of Contents
1.
Introduction
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. It is typically used for distributed information systems, where
the use of response caches can improve performance. 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 to reduce the response time and
network bandwidth consumption on future equivalent requests. Any client or
server
MAY
use a cache, though not when acting as a tunnel (
Section 3.7
of [
HTTP
).
A "shared cache" is a cache that stores responses for reuse
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 HTTP caching is significantly improving performance
by reusing a prior response message to satisfy a current request.
A cache considers a stored response "fresh", as defined in
Section 4.2
, if it 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 the cache reuses it.
When a cached response is not fresh, it might still be reusable if validation
can freshen it (
Section 4.3
) or if the
origin is unavailable (
Section 4.2.4
).
This document obsoletes
RFC 7234
with the changes being summarized in
Appendix B
1.1.
Requirements Notation
The key words "
MUST
", "
MUST NOT
",
REQUIRED
", "
SHALL
", "
SHALL NOT
", "
SHOULD
", "
SHOULD NOT
",
RECOMMENDED
", "
NOT RECOMMENDED
",
MAY
", and "
OPTIONAL
" in this document are
to be interpreted as described in BCP 14
RFC2119
RFC8174
when, and only when, they appear in all capitals,
as shown here.
Section 2
of [
HTTP
defines conformance criteria and contains considerations regarding error handling.
1.2.
Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF) notation of
RFC5234
, extended with the notation for case-sensitivity
in strings defined in
RFC7405
It also uses a list extension, defined in
Section 5.6.1
of [
HTTP
that allows for compact definition of comma-separated lists using a "#"
operator (similar to how the "*" operator indicates repetition).
Appendix A
shows the collected grammar with all list
operators expanded to standard ABNF notation.
1.2.1.
Imported Rules
The following core rule is included by
reference, as defined in
RFC5234
],
Appendix B.1
DIGIT (decimal 0-9).
HTTP
defines the following rules:
HTTP-date =
OWS =
field-name =
quoted-string =
token =
1.2.2.
Delta Seconds
The delta-seconds rule specifies a non-negative integer, representing time
in seconds.
delta-seconds = 1*DIGIT
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 2147483648
(2
31
) or the greatest positive integer it can conveniently
represent.
Note:
The value 2147483648 is here for historical reasons,
represents infinity (over 68 years), and does not need to be stored in
binary form; an implementation could produce it as a 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.
2.
Overview of Cache Operation
Proper cache operation preserves the semantics of HTTP transfers
while reducing the transmission of information already held in the
cache. See
Section 3
of [
HTTP
for the general terminology and core concepts of HTTP.
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.
The "cache key" is the information a cache uses to choose a response and
is composed from, at a minimum, the request method and target
URI used to retrieve the stored response; the method determines under which
circumstances that response can be used to satisfy a subsequent request. However, many
HTTP caches in common use today only cache GET responses and therefore only
use the URI as the cache key.
A cache might store multiple responses for a request target that is
subject to content negotiation. Caches differentiate these responses
by incorporating some of the original request's header fields
into the cache key as well, using information in the Vary
response header field, as per
Section 4.1
Caches might incorporate additional material into the cache key.
For example, user agent caches might include the referring site's identity,
thereby "double keying" the cache to avoid some privacy risks (see
Section 7.2
).
Most commonly, caches store the successful result of a retrieval
request: i.e., a 200 (OK) response to a GET request, which
contains a representation of the target resource
Section 9.3.1
of [
HTTP
). However, it is also possible to store
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.
A cache is "disconnected" when it cannot contact the origin
server or otherwise find a forward path for a request. A
disconnected cache can serve stale responses in some circumstances (
Section 4.2.4
).
3.
Storing Responses in Caches
A cache
MUST NOT
store a response to a request unless:
the request method is understood by the cache;
the response status code is final (see
Section 15
of [
HTTP
);
if the response status code is 206 or 304, or the must-understand cache directive (see
Section 5.2.2.3
) is present: the cache understands the response status code;
the no-store cache directive is not present in the response
(see
Section 5.2.2.5
);
if the cache is shared: the private response directive is either not
present or allows a shared cache to store a modified response;
see
Section 5.2.2.7
);
if the cache is shared: the Authorization header field
is not present in the request
(see
Section 11.6.2
of [
HTTP
) or a
response directive is present that explicitly allows shared caching
(see
Section 3.5
);
and
the response contains at least one of the following:
a public response directive
(see
Section 5.2.2.9
);
a private response directive, if the cache is not shared
(see
Section 5.2.2.7
);
an
Expires
header field
(see
Section 5.3
);
a max-age response directive
(see
Section 5.2.2.1
);
if the cache is shared: an s-maxage response directive
(see
Section 5.2.2.10
);
a cache extension that allows it to be cached
(see
Section 5.2.3
); or
a status code that is defined as heuristically cacheable
(see
Section 4.2.2
).
Note that a cache extension can override any of the requirements
listed; see
Section 5.2.3
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.
3.1.
Storing Header and Trailer Fields
Caches
MUST
include all received response header fields -- including
unrecognized ones -- when storing a response; this assures that new HTTP
header fields can be successfully deployed. However, the following exceptions
are made:
The Connection header field and fields whose names are listed in it are
required by
Section 7.6.1
of [
HTTP
to be removed before
forwarding the message. This
MAY
be implemented by doing so
before storage.
Likewise, some fields' semantics require them to be removed
before forwarding the message, and this
MAY
be implemented by doing so
before storage; see
Section 7.6.1
of [
HTTP
for some
examples.
The no-cache (
Section 5.2.2.4
) and
private (
Section 5.2.2.7
) cache
directives can have arguments that prevent storage of header fields by all
caches and shared caches, respectively.
Header fields that are specific to the proxy that a cache uses when forwarding a request
MUST NOT
be stored, unless the cache incorporates the identity of the
proxy into the cache key. Effectively, this is limited to Proxy-Authenticate
Section 11.7.1
of [
HTTP
), Proxy-Authentication-Info
Section 11.7.3
of [
HTTP
), and Proxy-Authorization
Section 11.7.2
of [
HTTP
).
Caches
MAY
either store trailer fields separate from header fields or
discard them. Caches
MUST NOT
combine trailer fields with header fields.
3.2.
Updating Stored Header Fields
Caches are required to update a stored response's header fields from another
(typically newer) response in several situations; for example, see Sections
3.4
4.3.4
, and
4.3.5
When doing so, the cache
MUST
add each header field in the provided response
to the stored response, replacing field values that are already present,
with the following exceptions:
Header fields excepted from storage in
Section 3.1
Header fields that the cache's stored response depends upon, as described below,
Header fields that are automatically processed and removed by the recipient, as described below, and
The Content-Length header field.
In some cases, caches (especially in user agents) store the results of
processing the received response, rather than the response itself,
and updating header fields that affect that processing can result in
inconsistent behavior and security issues. Caches in this situation
MAY
omit these header fields from updating stored responses on an
exceptional basis but
SHOULD
limit such omission to those fields
necessary to assure integrity of the stored response.
For example, a browser might decode the content coding of a response
while it is being received, creating a disconnect between the data it has
stored and the response's original metadata.
Updating that stored metadata with a different Content-Encoding
header field would be problematic. Likewise, a browser might store a
post-parse HTML tree rather than the content received in
the response; updating the Content-Type header field would not be workable
in this case because any assumptions about the format made in parsing would
now be invalid.
Furthermore, some fields are automatically processed and removed by the
HTTP implementation, such as the Content-Range header field.
Implementations
MAY
automatically omit such header fields from updates,
even when the processing does not actually occur.
Note that the Content-* prefix is not a signal that a header field is omitted
from update; it is a convention for MIME header fields, not HTTP.
3.3.
Storing Incomplete Responses
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 a response that is not complete (
Section 6.1
of [
HTTP
) provided that the stored response
is recorded as being incomplete. Likewise, a 206 (Partial
Content) response
MAY
be stored as if it were an incomplete
200 (OK) response. 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 14.2
of [
HTTP
) and combining the successful response with the
stored response, as defined in
Section 3.4
. 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 wholly within the incomplete response. A cache
MUST NOT
send a partial response to a client without explicitly marking it
using the 206 (Partial Content) status code.
3.4.
Combining Partial Content
A response might transfer only a partial representation if the
connection closed prematurely or if the request used one or more Range
specifiers (
Section 14.2
of [
HTTP
). 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 15.3.7.3
of [
HTTP
When combining the new response with one or more stored responses, a cache
MUST
update the stored response header fields using the header fields
provided in the new response, as per
Section 3.2
3.5.
Storing Responses to Authenticated Requests
A shared cache
MUST NOT
use a cached response to a request with an
Authorization header field (
Section 11.6.2
of [
HTTP
) to
satisfy any subsequent request unless the response contains a
Cache-Control
field with a response directive
Section 5.2.2
) that allows it to be stored by
a shared cache, and the cache conforms to the requirements of that
directive for that response.
In this specification, the following response directives have such an effect:
must-revalidate (
Section 5.2.2.2
),
public (
Section 5.2.2.9
), and
s-maxage (
Section 5.2.2.10
).
4.
Constructing Responses from Caches
When presented with a request, a cache
MUST NOT
reuse a stored response
unless:
the presented target URI (
Section 7.1
of [
HTTP
) and
that of the stored response match, and
the request method associated with the stored response allows it to
be used for the presented request, and
request header fields nominated by the stored response (if any)
match those presented (see
Section 4.1
), and
the stored response does not contain the no-cache directive
Section 5.2.2.4
), unless it is
successfully validated (
Section 4.3
), and
the stored response is one of the following:
fresh (see
Section 4.2
), or
allowed to be served stale (see
Section 4.2.4
), or
successfully validated (see
Section 4.3
).
Note that a cache extension can override any of the requirements
listed; see
Section 5.2.3
When a stored response is used to satisfy a request without validation, a
cache
MUST
generate an
Age
header field (
Section 5.1
), replacing any present in the response with a value
equal to the stored response's current_age; see
Section 4.2.3
A cache
MUST
write through requests with methods that are unsafe
Section 9.2.1
of [
HTTP
) 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
Section 4.4
A cache can use a response that is stored or storable to satisfy
multiple requests, provided that it is allowed to reuse that response
for the requests in question. This enables a cache to "collapse
requests" -- or combine multiple incoming requests into a single forward
request upon a cache miss -- thereby reducing load on the origin server
and network. Note, however, that if the cache cannot use the returned
response for some or all of the collapsed requests, it will need to
forward the requests in order to satisfy them, potentially introducing
additional latency.
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 without a clock (
Section 5.6.7
of [
HTTP
MUST
revalidate
stored responses upon every use.
4.1.
Calculating Cache Keys with the Vary Header Field
When a cache receives a request that can be satisfied by a stored response
and that stored response contains a Vary header field
Section 12.5.5
of [
HTTP
),
the cache
MUST NOT
use that stored response without revalidation unless
all the presented request header fields nominated by that Vary field value
match those fields in the original request (i.e., the
request that caused the cached response to be stored).
The 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 field lines with the same field name
(see
Section 5.2
of [
HTTP
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 stored response with a Vary header field value containing
a member "*" always fails to match.
If multiple stored responses match,
the cache will need to choose one to use.
When a nominated request header field has a known mechanism for ranking preference
(e.g., qvalues on Accept and similar request header
fields), that mechanism
MAY
be used to choose a preferred response.
If such a mechanism is not available, or leads to equally preferred
responses, the most recent
response (as determined by the Date header field) is
chosen, as
per
Section 4
Some resources mistakenly omit the Vary header field from their default
response (i.e., the one sent when the request does not express any preferences),
with the effect of choosing it for subsequent requests to that resource
even when more preferable responses are available. When a cache has
multiple stored responses for a target URI and one or more omits the Vary
header field, the cache
SHOULD
choose the most recent
(see
Section 4.2.3
) stored response with a valid Vary
field value.
If no stored response matches, the cache cannot satisfy the presented
request. Typically, the request is forwarded to the origin server,
potentially with preconditions added to describe what responses the cache
has already stored (
Section 4.3
).
4.2.
Freshness
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, 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 (
Section 5.3
) or
the max-age response directive (
Section 5.2.2.1
). 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
Section 4.2.4
).
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
Section 4.2.2
).
The calculation to determine if a response is fresh is:
response_is_fresh = (freshness_lifetime > current_age)
freshness_lifetime is defined in
Section 4.2.1
; current_age is defined in
Section 4.2.3
Clients can send the max-age or min-fresh request directives (
Section 5.2.1
) to suggest limits on the 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 the field value
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" 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
Section 6
for an explanation of the difference between
caches and history mechanisms.
4.2.1.
Calculating Freshness Lifetime
A cache can calculate the freshness lifetime (denoted as
freshness_lifetime) of a response by evaluating the following rules and using the first match:
If the cache is shared and the s-maxage response directive
Section 5.2.2.10
) is present, use its value,
or
If the max-age response directive (
Section 5.2.2.1
) is present, use its value, or
If the
Expires
response header field
Section 5.3
) is present, use its value minus the
value of the Date response header field
(using the time the message was received if it is not present, as per
Section 6.6.1
of [
HTTP
), or
Otherwise, no explicit expiration time is present in the response. A
heuristic freshness lifetime might be applicable; see
Section 4.2.2
Note that this calculation is intended to reduce clock skew by using the
clock information provided by the origin server whenever possible.
When there is more than one value present for a given directive (e.g., two
Expires
header field lines or multiple Cache-Control: max-age
directives), either the first occurrence should be used or the response should
be considered stale. If directives conflict (e.g.,
both max-age and no-cache are present), the most restrictive directive should
be honored. Caches are encouraged to consider responses that have
invalid freshness information (e.g., a max-age directive with non-integer content) to
be stale.
4.2.2.
Calculating Heuristic Freshness
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 field values (such as
the Last-Modified time) to estimate a plausible expiration
time. This specification does not provide specific algorithms, but it 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
Section 3
heuristics can only be used on responses without explicit
freshness whose status codes are defined as "heuristically cacheable" (e.g., see
Section 15.1
of [
HTTP
) and on responses without
explicit freshness that have been marked as explicitly cacheable (e.g.,
with a public response directive).
Note that in previous specifications, heuristically cacheable response status
codes were called "cacheable by default".
If the response has a Last-Modified header field
Section 8.8.2
of [
HTTP
), 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:
A previous version of the HTTP specification
Section 13.9
of [
RFC2616
) prohibited caches
from calculating heuristic freshness for URIs with query components
(i.e., those containing "?"). In practice, this has not been widely
implemented. Therefore, origin servers are encouraged to send explicit
directives (e.g., Cache-Control: no-cache) if they wish to prevent
caching.
4.2.3.
Calculating Age
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 origin server generated
or validated the response. The Age value is therefore
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 time it
has been in transit along network paths.
Age calculation uses the following data:
"age_value"
The term "age_value" denotes the value of the
Age
header field (
Section 5.1
), 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 6.6.1
of [
HTTP
for the definition of the Date header
field and for requirements regarding responses without it.
"now"
The term "now" means the current value of this implementation's clock
Section 5.6.7
of [
HTTP
).
"request_time"
The value of the clock at the time of the request that
resulted in the stored response.
"response_time"
The value of the clock 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
implementation's
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.
apparent_age = max(0, response_time - date_value);

response_delay = response_time - request_time;
corrected_age_value = age_value + response_delay;
The corrected_age_value
MAY
be used as the corrected_initial_age. In
circumstances where very old cache implementations that might not correctly
insert
Age
are present, corrected_initial_age can be calculated
more conservatively as
corrected_initial_age = max(apparent_age, corrected_age_value);
The current_age of a stored response can then be calculated by adding the
time (in seconds) since the stored response was last validated by
the origin server to the corrected_initial_age.
resident_time = now - response_time;
current_age = corrected_initial_age + resident_time;
4.2.4.
Serving Stale Responses
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
Section 4.2
A cache
MUST NOT
generate a stale response if it is prohibited by an
explicit in-protocol directive (e.g., by a no-cache response
directive, a must-revalidate response directive, or an applicable
s-maxage or proxy-revalidate response directive; see
Section 5.2.2
).
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
Section 5.2.1
, extension directives such as those
defined in
RFC5861
, or configuration in accordance
with an out-of-band contract).
4.3.
Validation
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 chosen; see
Section 4.1
), it can use
the conditional request mechanism (
Section 13
of [
HTTP
) in the forwarded request to
give the next inbound server an opportunity to choose 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.
4.3.1.
Sending a Validation Request
When generating a conditional request for validation, a cache either starts with
a request it is attempting to satisfy or -- if it is initiating
the request independently -- synthesizes a request using a stored
response by copying the method, target URI, and request header fields
identified by the Vary header field (
Section 4.1
).
It then updates that request with one or more precondition header fields.
These contain validator metadata sourced from a stored response(s) that has
the same URI. Typically, this will include only the stored response(s) that
has the same cache key, although a cache is allowed to validate
a response that it cannot choose with the request header fields it is sending
(see
Section 4.1
).
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 8.8.2
of [
HTTP
), 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
field (
Section 8.8.3
of [
HTTP
). 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).
When generating a conditional request for validation, a cache:
MUST
send the relevant entity tags
(using If-Match, If-None-Match, or
If-Range) if the entity tags were provided in the
stored response(s) being validated.
SHOULD
send the Last-Modified value (using
If-Modified-Since) if the request is not for a subrange,
a single stored response is being validated, and that response
contains a Last-Modified value.
MAY
send the Last-Modified value (using
If-Unmodified-Since or If-Range) if
the request is for a subrange, a single stored response is being
validated, and that response contains only a Last-Modified value
(not an entity tag).
In most cases, both validators are generated in cache validation requests,
even when entity tags are clearly superior, to allow old intermediaries
that do not understand entity tag preconditions to respond appropriately.
4.3.2.
Handling a Received Validation Request
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 a stored
200 (OK) or 206 (Partial Content)
response, as per
Section 4
the cache
SHOULD
evaluate any applicable conditional header
field preconditions received in that request with respect to the
corresponding validators contained within the stored response.
A cache
MUST NOT
evaluate conditional header fields that only
apply to an origin server, occur in a request with semantics that
cannot be satisfied with a cached response, or occur in a request with 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. In summary, the If-Match and
If-Unmodified-Since conditional header fields are not
applicable to a cache, and If-None-Match takes precedence over
If-Modified-Since. See
Section 13.2.2
of [
HTTP
for
a complete specification of precondition precedence.
A request containing an If-None-Match header field (
Section 13.1.2
of [
HTTP
) indicates that the client wants to
validate one or more of its own stored responses in comparison to the
stored response chosen by the cache (as per
Section 4
).
If an If-None-Match header field is not present, a request
containing an If-Modified-Since header field
Section 13.1.3
of [
HTTP
) indicates that the client wants to validate
one or more of its own stored responses by modification date.
If a request contains an If-Modified-Since header field and
the Last-Modified header field is not present in a
stored response, a cache
SHOULD
use the stored response's
Date field value (or, if no Date field is present, the time
that the stored response was received) to evaluate the conditional.
A cache that implements partial responses to range requests, as defined in
Section 14.2
of [
HTTP
, also needs to evaluate a received
If-Range header field (
Section 13.1.5
of [
HTTP
with respect to the cache's chosen response.
When a cache decides to forward a request 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 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
Section 4.3.4
).
4.3.3.
Handling a Validation Response
Cache handling of a response to a conditional request depends upon its
status code:
A 304 (Not Modified) response status code indicates
that the stored response can be updated and reused; see
Section 4.3.4
A full response (i.e., one containing content) indicates that none
of the stored responses nominated in the conditional request are
suitable. Instead, the cache
MUST
use the full response to
satisfy the request. The cache
MAY
store such a full response,
subject to its constraints (see
Section 3
).
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 can send a
previously stored response, subject to its constraints on doing so (see
Section 4.2.4
), or retry the validation request.
4.3.4.
Freshening Stored Responses upon Validation
When a cache receives a 304 (Not Modified) response, it needs
to identify stored responses that are suitable for updating with the new information
provided, and then do so.
The initial set of stored responses to update are those that could have been chosen for
that request -- i.e., those that meet the requirements in
Section 4
, except the last requirement
to be fresh, able to be served stale, or just validated.
Then, that initial set of stored responses is further filtered by the first match of:
If the new response contains one or more "strong validators" (see
Section 8.8.1
of [
HTTP
), then each of those strong validators
identifies a selected representation for update. All the stored
responses in the initial set with one of those same strong validators are identified for update. If
none of the initial set contains at least one of the same strong validators, then the
cache
MUST NOT
use the new response to update any stored responses.
If the new response contains no strong validators but does contain
one or more "weak validators", and those
validators correspond to one of the initial set'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
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 in the initial set, and that stored response
also lacks a validator, then that stored response is identified for update.
For each stored response identified, the cache
MUST
update
its header fields with the header fields provided in the 304 (Not
Modified) response, as per
Section 3.2
4.3.5.
Freshening Responses with HEAD
A response to the HEAD method is identical to what an equivalent request
made with a GET would have been, without sending the content. 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 content is not desired even if it has
changed.
When a cache makes an inbound HEAD request for a target URI and
receives a 200 (OK) response, the cache
SHOULD
update or
invalidate each of its stored GET responses that could have been chosen
for that request (see
Section 4.1
).
For each of the stored responses that could have been chosen, 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 update the stored response (see
Section 3.2
).
4.4.
Invalidating Stored Responses
Because unsafe request methods (
Section 9.2.1
of [
HTTP
) such as PUT, POST, or DELETE
have the potential for changing state on the origin server, intervening
caches are required to invalidate stored responses to keep their contents up to date.
A cache
MUST
invalidate the target URI
Section 7.1
of [
HTTP
) when it receives a non-error status
code in response to
an unsafe request method (including methods whose safety is unknown).
A cache
MAY
invalidate other URIs when it receives a non-error status
code in response to an unsafe request method (including methods whose
safety is unknown).
In particular, the URI(s) in the
Location and Content-Location response header
fields (if present) are candidates for invalidation; other URIs might be
discovered through mechanisms not specified in this document.
However, a cache
MUST NOT
trigger an invalidation under these conditions
if the origin (
Section 4.3.1
of [
HTTP
) of the URI to be invalidated differs from that of the target URI
Section 7.1
of [
HTTP
). This helps prevent denial-of-service attacks.
"Invalidate" means that the cache will either remove all
stored responses whose target URI matches the given URI or mark them
as "invalid" and in need of a mandatory validation before they can be sent
in response to a subsequent request.
A "non-error response" is one with a 2xx (Successful)
or 3xx (Redirection) status code.
Note that this does not guarantee that all appropriate responses are
invalidated globally; a state-changing request would only invalidate
responses in the caches it travels through.
5.
Field Definitions
This section defines the syntax and semantics of HTTP fields
related to caching.
5.1.
Age
The "Age" response header field conveys the sender's estimate of the
time since the response was generated or successfully validated at the
origin server. Age values are calculated as specified in
Section 4.2.3
Age = delta-seconds
The Age field value is a non-negative integer, representing time in seconds
(see
Section 1.2.2
).
Although it is defined as a singleton header field, a cache encountering a
message with a list-based Age field value
SHOULD
use the
first member of the field value, discarding subsequent ones.
If the field value (after discarding additional members, as per above) is invalid
(e.g., it contains something other than a non-negative integer),
a cache
SHOULD
ignore the field.
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.
5.2.
Cache-Control
The "Cache-Control" header field is used to list directives for caches
along the request/response chain. Cache directives are unidirectional,
in that the presence of a directive in a request does not imply that the
same directive is present or copied in the response.
See
Section 5.2.3
for information about how
Cache-Control directives defined elsewhere are handled.
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 apply
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 a specific form is required for generation.
Cache-Control = #cache-directive

cache-directive = token [ "=" ( token / quoted-string ) ]
For the cache directives defined below, no argument is defined (nor allowed)
unless stated otherwise.
5.2.1.
Request Directives
This section defines cache request directives. They are advisory; caches
MAY
implement them, but are not required to.
5.2.1.1.
max-age
Argument syntax:
delta-seconds
(see
Section 1.2.2
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
MUST NOT
generate the
quoted-string form.
5.2.1.2.
max-stale
Argument syntax:
delta-seconds
(see
Section 1.2.2
The max-stale request directive indicates that the client will
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 will 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
MUST NOT
generate
the quoted-string form.
5.2.1.3.
min-fresh
Argument syntax:
delta-seconds
(see
Section 1.2.2
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
MUST NOT
generate
the quoted-string form.
5.2.1.4.
no-cache
The no-cache request directive indicates that the client prefers
a stored response not be used to satisfy the request without successful
validation on the origin server.
5.2.1.5.
no-store
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.
5.2.1.6.
no-transform
The no-transform request directive indicates that the client is asking
for intermediaries to avoid
transforming the content, as defined in
Section 7.7
of [
HTTP
5.2.1.7.
only-if-cached
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, respond with either a stored
response consistent with the other constraints of the request or
a 504 (Gateway Timeout) status code.
5.2.2.
Response Directives
This section defines cache response directives. A cache
MUST
obey the
Cache-Control directives defined in this section.
5.2.2.1.
max-age
Argument syntax:
delta-seconds
(see
Section 1.2.2
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
MUST NOT
generate the
quoted-string form.
5.2.2.2.
must-revalidate
The must-revalidate response directive indicates that once the response
has become stale, a cache
MUST NOT
reuse that response to satisfy
another request until it has been successfully validated by the origin, as
defined by
Section 4.3
The must-revalidate directive is necessary to support reliable operation
for certain protocol features. In all circumstances, a cache
MUST NOT
ignore
the must-revalidate directive; in particular, if a cache is disconnected,
the cache
MUST
generate an error response rather than reuse the stale response.
The generated status code
SHOULD
be 504 (Gateway Timeout)
unless another error status code is more applicable.
The must-revalidate directive ought to be used by servers if and only
if failure to validate a request could cause
incorrect operation, such as a silently unexecuted financial
transaction.
The must-revalidate directive also permits a shared cache to
reuse a response to a request containing an Authorization
header field (
Section 11.6.2
of [
HTTP
),
subject to the above requirement on revalidation
Section 3.5
).
5.2.2.3.
must-understand
The must-understand response directive limits caching of the response to
a cache that understands and conforms to the requirements for that
response's status code.
A response that contains the must-understand directive
SHOULD
also contain the no-store directive. When a cache that implements the
must-understand directive receives a response that includes it,
the cache
SHOULD
ignore the no-store directive if it
understands and implements the status code's caching requirements.
5.2.2.4.
no-cache
Argument syntax:
field-name
The no-cache response directive, in its unqualified form (without an
argument), indicates that the response
MUST NOT
be used to satisfy any
other request without forwarding it for validation and receiving a
successful response; see
Section 4.3
This allows an origin server to prevent a cache from using
the response to satisfy a request without contacting it, even by caches that have
been configured to send stale responses.
The qualified form of the no-cache response directive, with an argument that
lists one or more field names, indicates that a cache
MAY
use the
response to satisfy a subsequent request, subject to any other restrictions
on caching, if the listed header fields are excluded from the subsequent
response or the subsequent response has been successfully revalidated with
the origin server (updating or removing those fields).
This allows an origin server to prevent the reuse 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:
The
qualified form of the directive is often handled by caches as if an
unqualified no-cache directive was received; that is, the special handling
for the qualified form is not widely implemented.
5.2.2.5.
no-store
The no-store response directive indicates that a cache
MUST NOT
store
any part of either the immediate request or the 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.
Note that the must-understand cache directive overrides no-store in certain
circumstances; see
Section 5.2.2.3
5.2.2.6.
no-transform
The no-transform response directive indicates that an intermediary
(regardless of whether it implements a cache)
MUST NOT
transform the
content, as defined in
Section 7.7
of [
HTTP
5.2.2.7.
private
Argument syntax:
field-name
The unqualified private response directive indicates that
a shared cache
MUST NOT
store the response (i.e., the response is
intended for a single user).
It also indicates that a private cache
MAY
store the response, subject
to the constraints defined in
Section 3
, even if
the response would not otherwise be heuristically cacheable by a private
cache.
If a qualified private response directive is present, with an argument that
lists one or more field names, then only the listed header fields are limited to a
single user: a shared cache
MUST NOT
store the listed header fields if they
are present in the original response but
MAY
store the remainder of the
response message without those header fields, subject
the constraints defined in
Section 3
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, the qualified form of the directive is
often handled by caches as if an unqualified private directive
was received; that is, the special handling for the qualified form is not
widely implemented.
5.2.2.8.
proxy-revalidate
The proxy-revalidate response directive indicates that once the response
has become stale, a shared cache
MUST NOT
reuse that response to satisfy
another request until it has been successfully validated by the origin,
as defined by
Section 4.3
. This is analogous to
must-revalidate (
Section 5.2.2.2
),
except that proxy-revalidate does not apply to private caches.
Note that proxy-revalidate on its own does not imply that a response is
cacheable. For example, it might be combined with the public directive
Section 5.2.2.9
), allowing the response
to be cached while requiring only a shared cache to revalidate when stale.
5.2.2.9.
public
The public response directive indicates that a cache
MAY
store the
response even if it would otherwise be prohibited, subject to the
constraints defined in
Section 3
. In other words,
public explicitly marks the response as cacheable. For example,
public permits a shared cache to reuse a response to a request containing
an Authorization header field (
Section 3.5
).
Note that it is unnecessary to add the public directive to a response that
is already cacheable according to
Section 3
If a response with the public directive has no explicit freshness information,
it is heuristically cacheable (
Section 4.2.2
).
5.2.2.10.
s-maxage
Argument syntax:
delta-seconds
(see
Section 1.2.2
The s-maxage response directive indicates that, for a shared cache, 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 incorporates the
semantics of the proxy‑revalidate response directive (
Section 5.2.2.8
for a shared cache.
A shared cache
MUST NOT
reuse a stale response with s-maxage to satisfy
another request until it has been successfully validated by the origin, as
defined by
Section 4.3
This directive also permits a shared cache to reuse a response to a
request containing an Authorization header field, subject to the above
requirements on maximum age and revalidation
Section 3.5
).
This directive uses the token form of the argument syntax:
e.g., 's-maxage=10' not 's-maxage="10"'. A sender
MUST NOT
generate
the quoted-string form.
5.2.3.
Extension Directives
The Cache-Control header field can be extended through the use of one or
more extension cache directives.
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 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, only a cache that is shared 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
Cache-Control: private, community="UCI"
A cache that recognizes such a community cache directive could broaden its
behavior in accordance with that extension. A cache that does not
recognize the community cache directive 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, and
Whether the directive is specific to requests, specific to responses, or able
to be used in either.
5.2.4.
Cache Directive Registry
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 Name
Pointer to specification text
Values to be added to this namespace require IETF Review (see
RFC8126
],
Section 4.8
).
5.3.
Expires
The "Expires" response header field gives the date/time after which the
response is considered stale. See
Section 4.2
for
further discussion of the freshness model.
The presence of an Expires header field does not imply that the original resource
will change or cease to exist at, before, or after that time.
The Expires field value is an HTTP-date timestamp, as defined in
Section 5.6.7
of [
HTTP
See also
Section 4.2
for parsing requirements specific to caches.
Expires = HTTP-date
For example
Expires: Thu, 01 Dec 1994 16:00:00 GMT
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
header field with
the max-age directive (
Section 5.2.2.1
),
a recipient
MUST
ignore the Expires header field.
Likewise, if a response includes the s-maxage directive
Section 5.2.2.10
), a shared cache
recipient
MUST
ignore the Expires header field. In both these cases, the value
in Expires is only intended for recipients that have not yet implemented
the Cache-Control header field.
An origin server without a clock (
Section 5.6.7
of [
HTTP
MUST NOT
generate an Expires header 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 with
a 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.
5.4.
Pragma
The "Pragma" request 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.
5.5.
Warning
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
6.
Relationship to Applications and Other Caches
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 an HTTP cache. For example, 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 specification 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).
However, when an application caches data and does not make this
apparent to or easily controllable by the user, it is strongly encouraged to
define its operation with respect to HTTP cache directives so as
not to surprise authors who expect caching semantics
to be honored. For example, while it might be reasonable to define an
application cache "above" HTTP that allows a response containing
Cache-Control: no-store to be reused for requests that are directly related
to the request that fetched it (such as those created during the same page
load), it would likely be surprising and confusing to users and authors if it
were allowed to be reused for requests unrelated in any way to the one from
which it was obtained.
7.
Security Considerations
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/1.1"
Section 11
of [
HTTP/1.1
and "HTTP Semantics"
Section 17
of [
HTTP
).
Caches expose an additional attack surface because the contents of
the cache represent an attractive target for malicious exploitation.
Since 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, because private caches are restricted to a single user,
they can be used to reconstruct a user's activity. As a result, it is
important for user agents to allow end users to control them, for example,
by allowing stored responses to be removed for some or all origin servers.
7.1.
Cache Poisoning
Storing malicious content in a cache can extend the reach of an attacker
to affect multiple users. Such
"cache poisoning" attacks happen when an attacker uses
implementation flaws, elevated privileges, or other techniques to insert
a response into a cache. This is especially effective when shared caches
are used to distribute malicious content to many clients.
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 [
HTTP/1.1
for the relevant requirements regarding
HTTP/1.1.
7.2.
Timing Attacks
Because one of the primary uses of a cache is to optimize performance,
its use can "leak" information about which resources have been previously
requested.
For example, if a user visits a site and their browser caches some of its
responses and then navigates to a second site, that site can attempt to
load responses it knows exist on the first site. If they load
quickly, it can be assumed that the user has visited that site, or even
a specific page on it.
Such "timing attacks" can be mitigated by adding more information to the
cache key, such as the identity of the referring site (to prevent the
attack described above). This is sometimes called "double keying".
7.3.
Caching of Sensitive Information
Implementation and deployment flaws (often led to by the misunderstanding of cache
operation) might lead to the caching of sensitive information (e.g.,
authentication credentials) that is thought to be private, exposing it to
unauthorized parties.
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 that wish to control caching of these responses are encouraged to
emit appropriate Cache-Control response header fields.
8.
IANA Considerations
The change controller for the following registrations is:
"IETF (iesg@ietf.org) - Internet Engineering Task Force".
8.1.
Field Name Registration
IANA has updated the "Hypertext Transfer Protocol (HTTP) Field
Name Registry" at
as described in
Section 18.4
of [
HTTP
with the field names listed in the table below:
Table 1
Field Name
Status
Section
Comments
Age
permanent
5.1
Cache-Control
permanent
5.2
Expires
permanent
5.3
Pragma
deprecated
5.4
Warning
obsoleted
5.5
8.2.
Cache Directive Registration
IANA has updated the
"Hypertext Transfer Protocol (HTTP) Cache Directive Registry"
at
with the registration procedure per
Section 5.2.4
and the cache directive names summarized in the table below.
Table 2
Cache Directive
Section
max-age
5.2.1.1
5.2.2.1
max-stale
5.2.1.2
min-fresh
5.2.1.3
must-revalidate
5.2.2.2
must-understand
5.2.2.3
no-cache
5.2.1.4
5.2.2.4
no-store
5.2.1.5
5.2.2.5
no-transform
5.2.1.6
5.2.2.6
only-if-cached
5.2.1.7
private
5.2.2.7
proxy-revalidate
5.2.2.8
public
5.2.2.9
s-maxage
5.2.2.10
8.3.
Warn Code Registry
IANA has added the following note to the "Hypertext Transfer Protocol (HTTP) Warn Codes"
registry at
stating that "Warning" has been obsoleted:
The Warning header field (and the warn codes that it uses) has been obsoleted
for HTTP per [RFC9111].
9.
References
9.1.
Normative References
[HTTP]
Fielding, R., Ed.
Nottingham, M., Ed.
, and
J. Reschke, Ed.
"HTTP Semantics"
STD 97
RFC 9110
DOI 10.17487/RFC9110
June 2022
[RFC2119]
Bradner, S.
"Key words for use in RFCs to Indicate Requirement Levels"
BCP 14
RFC 2119
DOI 10.17487/RFC2119
March 1997
[RFC5234]
Crocker, D., Ed.
and
P. Overell
"Augmented BNF for Syntax Specifications: ABNF"
STD 68
RFC 5234
DOI 10.17487/RFC5234
January 2008
[RFC7405]
Kyzivat, P.
"Case-Sensitive String Support in ABNF"
RFC 7405
DOI 10.17487/RFC7405
December 2014
[RFC8174]
Leiba, B.
"Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"
BCP 14
RFC 8174
DOI 10.17487/RFC8174
May 2017
9.2.
Informative References
[COOKIE]
Barth, A.
"HTTP State Management Mechanism"
RFC 6265
DOI 10.17487/RFC6265
April 2011
[HTTP/1.1]
Fielding, R., Ed.
Nottingham, M., Ed.
, and
J. Reschke, Ed.
"HTTP/1.1"
STD 99
RFC 9112
DOI 10.17487/RFC9112
June 2022
[RFC2616]
Fielding, R.
Gettys, J.
Mogul, J.
Frystyk, H.
Masinter, L.
Leach, P.
, and
T. Berners-Lee
"Hypertext Transfer Protocol -- HTTP/1.1"
RFC 2616
DOI 10.17487/RFC2616
June 1999
[RFC5861]
Nottingham, M.
"HTTP Cache-Control Extensions for Stale Content"
RFC 5861
DOI 10.17487/RFC5861
May 2010
[RFC7234]
Fielding, R., Ed.
Nottingham, M., Ed.
, and
J. Reschke, Ed.
"Hypertext Transfer Protocol (HTTP/1.1): Caching"
RFC 7234
DOI 10.17487/RFC7234
June 2014
[RFC8126]
Cotton, M.
Leiba, B.
, and
T. Narten
"Guidelines for Writing an IANA Considerations Section in RFCs"
BCP 26
RFC 8126
DOI 10.17487/RFC8126
June 2017
Appendix A.
Collected ABNF
In the collected ABNF below, list rules are expanded per
Section 5.6.1
of [
HTTP
Age = delta-seconds

Cache-Control = [ cache-directive *( OWS "," OWS cache-directive ) ]

Expires = HTTP-date

HTTP-date =

OWS =

cache-directive = token [ "=" ( token / quoted-string ) ]

delta-seconds = 1*DIGIT

field-name =

quoted-string =

token =
Appendix B.
Changes from RFC 7234
Handling of duplicate and conflicting cache directives has been clarified.
Section 4.2.1
Cache invalidation of the URIs in the Location and Content-Location
header fields is no longer required but is still allowed.
Section 4.4
Cache invalidation of the URIs in the Location and Content-Location header fields is disallowed
when the origin is different; previously, it was the host.
Section 4.4
Handling invalid and multiple Age header field values has been clarified.
Section 5.1
Some cache directives defined by this specification now have stronger
prohibitions against generating the quoted form of their values, since
this has been found to create interoperability problems. Consumers of
extension cache directives are no longer required to accept both token and
quoted-string forms, but they still need to parse them properly for
unknown extensions.
Section 5.2
The public and private cache directives were clarified, so that they
do not make responses reusable under any condition.
Section 5.2.2
The must-understand cache directive was introduced; caches are no
longer required to understand the semantics of new response status codes
unless it is present.
Section 5.2.2.3
The Warning response header was obsoleted. Much of the information
supported by Warning could be gleaned by examining the response, and the
remaining information -- although potentially useful -- was entirely
advisory. In practice, Warning was not added by caches or intermediaries.
Section 5.5
Acknowledgements
See Appendix "Acknowledgements" of
HTTP
, which applies to this document as well.
Index
age
Section 4.2
Age header field
Section 5.1
cache
Section 1
cache key
Section 2
Section 2
Cache-Control header field
Section 5.2
collapsed requests
Section 4
Expires header field
Section 5.3
explicit expiration time
Section 4.2
Fields
Age
Section 5.1
Section 5.1
Cache-Control
Section 5.2
Expires
Section 5.3
Section 5.3
Pragma
Section 5.4
Section 5.4
Warning
Section 5.5
fresh
Section 4.2
freshness lifetime
Section 4.2
Grammar
Age
Section 5.1
Cache-Control
Section 5.2
DIGIT
Section 1.2
Expires
Section 5.3
cache-directive
Section 5.2
delta-seconds
Section 1.2.2
Header Fields
Age
Section 5.1
Section 5.1
Cache-Control
Section 5.2
Expires
Section 5.3
Section 5.3
Pragma
Section 5.4
Section 5.4
Warning
Section 5.5
heuristic expiration time
Section 4.2
heuristically cacheable
Section 4.2.2
max-age (cache directive)
Section 5.2.1.1
Section 5.2.2.1
max-stale (cache directive)
Section 5.2.1.2
min-fresh (cache directive)
Section 5.2.1.3
must-revalidate (cache directive)
Section 5.2.2.2
must-understand (cache directive)
Section 5.2.2.3
no-cache (cache directive)
Section 5.2.1.4
Section 5.2.2.4
no-store (cache directive)
Section 5.2.1.5
Section 5.2.2.5
no-transform (cache directive)
Section 5.2.1.6
Section 5.2.2.6
only-if-cached (cache directive)
Section 5.2.1.7
Pragma header field
Section 5.4
private (cache directive)
Section 5.2.2.7
private cache
Section 1
proxy-revalidate (cache directive)
Section 5.2.2.8
public (cache directive)
Section 5.2.2.9
s-maxage (cache directive)
Section 5.2.2.10
shared cache
Section 1
stale
Section 4.2
validator
Section 4.3.1
Warning header field
Section 5.5
Authors' Addresses
Roy T. Fielding (
editor
Adobe
345 Park Ave
San Jose, CA 95110
United States of America
Email:
fielding@gbiv.com
URI:
Mark Nottingham (
editor
Fastly
Prahran
Australia
Email:
mnot@mnot.net
URI:
Julian Reschke (
editor
greenbytes GmbH
Hafenweg 16
48155 Münster
Germany
Email:
julian.reschke@greenbytes.de
URI: