Internet Engineering Task Force (IETF) R. Fielding, Ed.
Request for Comments: 9111 Adobe
STD: 98 M. Nottingham, Ed.
Obsoletes: 7234 Fastly
Category: Standards Track J. Reschke, Ed.
ISSN: 2070-1721 greenbytes
June 2022
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
https://www.rfc-editor.org/info/rfc9111.
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Table of Contents
1. Introduction
1.1. Requirements Notation
1.2. Syntax Notation
1.2.1. Imported Rules
1.2.2. Delta Seconds
2. Overview of Cache Operation
3. Storing Responses in Caches
3.1. Storing Header and Trailer Fields
3.2. Updating Stored Header Fields
3.3. Storing Incomplete Responses
3.4. Combining Partial Content
3.5. Storing Responses to Authenticated Requests
4. Constructing Responses from Caches
4.1. Calculating Cache Keys with the Vary Header Field
4.2. Freshness
4.2.1. Calculating Freshness Lifetime
4.2.2. Calculating Heuristic Freshness
4.2.3. Calculating Age
4.2.4. Serving Stale Responses
4.3. Validation
4.3.1. Sending a Validation Request
4.3.2. Handling a Received Validation Request
4.3.3. Handling a Validation Response
4.3.4. Freshening Stored Responses upon Validation
4.3.5. Freshening Responses with HEAD
4.4. Invalidating Stored Responses
5. Field Definitions
5.1. Age
5.2. Cache-Control
5.2.1. Request Directives
5.2.1.1. max-age
5.2.1.2. max-stale
5.2.1.3. min-fresh
5.2.1.4. no-cache
5.2.1.5. no-store
5.2.1.6. no-transform
5.2.1.7. only-if-cached
5.2.2. Response Directives
5.2.2.1. max-age
5.2.2.2. must-revalidate
5.2.2.3. must-understand
5.2.2.4. no-cache
5.2.2.5. no-store
5.2.2.6. no-transform
5.2.2.7. private
5.2.2.8. proxy-revalidate
5.2.2.9. public
5.2.2.10. s-maxage
5.2.3. Extension Directives
5.2.4. Cache Directive Registry
5.3. Expires
5.4. Pragma
5.5. Warning
6. Relationship to Applications and Other Caches
7. Security Considerations
7.1. Cache Poisoning
7.2. Timing Attacks
7.3. Caching of Sensitive Information
8. IANA Considerations
8.1. Field Name Registration
8.2. Cache Directive Registration
8.3. Warn Code Registry
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Collected ABNF
Appendix B. Changes from RFC 7234
Acknowledgements
Index
Authors' Addresses
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 = <HTTP-date, see [HTTP], Section 5.6.7>
OWS = <OWS, see [HTTP], Section 5.6.3>
field-name = <field-name, see [HTTP], Section 5.1>
quoted-string = <quoted-string, see [HTTP], Section 5.6.4>
token = <token, see [HTTP], Section 5.6.2>
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:
1. 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.
2. 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 <https://www.iana.org/assignments/http-
cache-directives>.
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 [COOKIE] 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 <https://www.iana.org/assignments/http-fields>, as
described in Section 18.4 of [HTTP], with the field names listed in
the table below:
+===============+============+=========+==========+
| 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 | |
+---------------+------------+---------+----------+
Table 1
8.2. Cache Directive Registration
IANA has updated the "Hypertext Transfer Protocol (HTTP) Cache
Directive Registry" at <https://www.iana.org/assignments/http-cache-
directives> with the registration procedure per Section 5.2.4 and the
cache directive names summarized in the table below.
+==================+==================+
| 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 |
+------------------+------------------+
Table 2
8.3. Warn Code Registry
IANA has added the following note to the "Hypertext Transfer Protocol
(HTTP) Warn Codes" registry at <https://www.iana.org/assignments/
http-warn-codes> 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,
<https://www.rfc-editor.org/info/rfc9110>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[COOKIE] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[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, <https://www.rfc-editor.org/info/rfc9112>.
[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,
<https://www.rfc-editor.org/info/rfc2616>.
[RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale
Content", RFC 5861, DOI 10.17487/RFC5861, May 2010,
<https://www.rfc-editor.org/info/rfc5861>.
[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,
<https://www.rfc-editor.org/info/rfc7234>.
[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,
<https://www.rfc-editor.org/info/rfc8126>.
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 = <HTTP-date, see [HTTP], Section 5.6.7>
OWS = <OWS, see [HTTP], Section 5.6.3>
cache-directive = token [ "=" ( token / quoted-string ) ]
delta-seconds = 1*DIGIT
field-name = <field-name, see [HTTP], Section 5.1>
quoted-string = <quoted-string, see [HTTP], Section 5.6.4>
token = <token, see [HTTP], Section 5.6.2>
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
A C E F G H M N O P S V W
A
age Section 4.2
Age header field *_Section 5.1_*
C
cache Section 1
cache key Section 2; Section 2
Cache-Control header field *_Section 5.2_*
collapsed requests Section 4
E
Expires header field *_Section 5.3_*
explicit expiration time Section 4.2
F
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
G
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_*
H
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
M
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_*
N
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_*
O
only-if-cached (cache directive) *_Section 5.2.1.7_*
P
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
s-maxage (cache directive) *_Section 5.2.2.10_*
shared cache Section 1
stale Section 4.2
V
validator Section 4.3.1
W
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: https://roy.gbiv.com/
Mark Nottingham (editor)
Fastly
Prahran
Australia
Email: mnot@mnot.net
URI: https://www.mnot.net/
Julian Reschke (editor)
greenbytes GmbH
Hafenweg 16
48155 Münster
Germany
Email: julian.reschke@greenbytes.de
URI: https://greenbytes.de/tech/webdav/
