RFC 7234 PROPOSED STANDARD HTTP · CACHING · PERFORMANCE
HTTP Caching: Hypertext Transfer Protocol (HTTP/1.1): Caching
How a copy of a web response is kept, when it may be reused, and how a server says when it must be checked again.
RFC EDITOR
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>.
BIBTEX
@misc{rfc7234,
series = {Request for Comments},
number = 7234,
howpublished = {RFC 7234},
publisher = {RFC Editor},
doi = {10.17487/RFC7234},
url = {https://www.rfc-editor.org/info/rfc7234},
author = {R. Fielding and M. Nottingham and J. Reschke},
title = {{Hypertext Transfer Protocol (HTTP/1.1): Caching}},
pagetotal = 43,
year = 2014,
month = jun,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Internet Engineering Task Force (IETF) R. Fielding, Ed.
Request for Comments: 7234 Adobe
Obsoletes: 2616 M. Nottingham, Ed.
Category: Standards Track Akamai
ISSN: 2070-1721 J. Reschke, Ed.
greenbytes
June 2014
Hypertext Transfer Protocol (HTTP/1.1): 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.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7234.
Copyright Notice
Copyright (c) 2014 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
(http://trustee.ietf.org/license-info) 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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified 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 ....................................................4
1.1. Conformance and Error Handling .............................4
1.2. Syntax Notation ............................................4
1.2.1. Delta Seconds .......................................5
2. Overview of Cache Operation .....................................5
3. Storing Responses in Caches .....................................6
3.1. Storing Incomplete Responses ...............................7
3.2. Storing Responses to Authenticated Requests ................7
3.3. Combining Partial Content ..................................8
4. Constructing Responses from Caches ..............................8
4.1. Calculating Secondary Keys with Vary .......................9
4.2. Freshness .................................................11
4.2.1. Calculating Freshness Lifetime .....................12
4.2.2. Calculating Heuristic Freshness ....................13
4.2.3. Calculating Age ....................................13
4.2.4. Serving Stale Responses ............................15
4.3. Validation ................................................16
4.3.1. Sending a Validation Request .......................16
4.3.2. Handling a Received Validation Request .............16
4.3.3. Handling a Validation Response .....................18
4.3.4. Freshening Stored Responses upon Validation ........18
4.3.5. Freshening Responses via HEAD ......................19
4.4. Invalidation ..............................................20
5. Header Field Definitions .......................................21
5.1. Age .......................................................21
5.2. Cache-Control .............................................21
5.2.1. Request Cache-Control Directives ...................22
5.2.2. Response Cache-Control Directives ..................24
5.2.3. Cache Control Extensions ...........................27
5.3. Expires ...................................................28
5.4. Pragma ....................................................29
5.5. Warning ...................................................29
5.5.1. Warning: 110 - "Response is Stale" .................31
5.5.2. Warning: 111 - "Revalidation Failed" ...............31
5.5.3. Warning: 112 - "Disconnected Operation" ............31
5.5.4. Warning: 113 - "Heuristic Expiration" ..............31
5.5.5. Warning: 199 - "Miscellaneous Warning" .............32
5.5.6. Warning: 214 - "Transformation Applied" ............32
5.5.7. Warning: 299 - "Miscellaneous Persistent Warning" ..32
6. History Lists ..................................................32
7. IANA Considerations ............................................32
7.1. Cache Directive Registry ..................................32
7.1.1. Procedure ..........................................32
7.1.2. Considerations for New Cache Control Directives ....33
7.1.3. Registrations ......................................33
7.2. Warn Code Registry ........................................34
7.2.1. Procedure ..........................................34
7.2.2. Registrations ......................................34
7.3. Header Field Registration .................................34
8. Security Considerations ........................................35
9. Acknowledgments ................................................36
10. References ....................................................36
10.1. Normative References .....................................36
10.2. Informative References ...................................37
Appendix A. Changes from RFC 2616 .................................38
Appendix B. Imported ABNF .........................................39
Appendix C. Collected ABNF ........................................39
Index .............................................................41
1. Introduction
HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. This
document defines aspects of HTTP/1.1 related to caching and reusing
response messages.
An HTTP cache is a local store of response messages and the subsystem
that controls storage, retrieval, and deletion of messages in it. A
cache stores cacheable responses in order to reduce the response time
and network bandwidth consumption on future, equivalent requests.
Any client or server MAY employ a cache, though a cache cannot be
used by a server that is acting as a tunnel.
A shared cache is a cache that stores responses to be reused by more
than one user; shared caches are usually (but not always) deployed as
a part of an intermediary. A private cache, in contrast, is
dedicated to a single user; often, they are deployed as a component
of a user agent.
The goal of caching in HTTP/1.1 is to significantly improve
performance by reusing a prior response message to satisfy a current
request. A stored response is considered "fresh", as defined in
Section 4.2, if the response can be reused without "validation"
(checking with the origin server to see if the cached response
remains valid for this request). A fresh response can therefore
reduce both latency and network overhead each time it is reused.
When a cached response is not fresh, it might still be reusable if it
can be freshened by validation (Section 4.3) or if the origin is
unavailable (Section 4.2.4).
1.1. Conformance and Error Handling
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Conformance criteria and considerations regarding error handling are
defined in Section 2.5 of [RFC7230].
1.2. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with a list extension, defined in Section 7 of
[RFC7230], that allows for compact definition of comma-separated
lists using a '#' operator (similar to how the '*' operator indicates
repetition). Appendix B describes rules imported from other
documents. Appendix C shows the collected grammar with all list
operators expanded to standard ABNF notation.
1.2.1. 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 either 2147483648 (2^31) or the greatest positive integer
it can conveniently represent.
Note: The value 2147483648 is here for historical reasons,
effectively represents infinity (over 68 years), and does not need
to be stored in binary form; an implementation could produce it as
a canned string if any overflow occurs, even if the calculations
are performed with an arithmetic type incapable of directly
representing that number. What matters here is that an overflow
be detected and not treated as a negative value in later
calculations.
2. Overview of Cache Operation
Proper cache operation preserves the semantics of HTTP transfers
([RFC7231]) while eliminating the transfer of information already
held in the cache. Although caching is an entirely OPTIONAL feature
of HTTP, it can be assumed that reusing a cached response is
desirable and that such reuse is the default behavior when no
requirement or local configuration prevents it. Therefore, HTTP
cache requirements are focused on preventing a cache from either
storing a non-reusable response or reusing a stored response
inappropriately, rather than mandating that caches always store and
reuse particular responses.
Each cache entry consists of a cache key and one or more HTTP
responses corresponding to prior requests that used the same key.
The most common form of cache entry is a successful result of a
retrieval request: i.e., a 200 (OK) response to a GET request, which
contains a representation of the resource identified by the request
target (Section 4.3.1 of [RFC7231]). However, it is also possible to
cache permanent redirects, negative results (e.g., 404 (Not Found)),
incomplete results (e.g., 206 (Partial Content)), and responses to
methods other than GET if the method's definition allows such caching
and defines something suitable for use as a cache key.
The primary cache key consists of the request method and target URI.
However, since HTTP caches in common use today are typically limited
to caching responses to GET, many caches simply decline other methods
and use only the URI as the primary cache key.
If a request target is subject to content negotiation, its cache
entry might consist of multiple stored responses, each differentiated
by a secondary key for the values of the original request's selecting
header fields (Section 4.1).
3. Storing Responses in Caches
A cache MUST NOT store a response to any request, unless:
o The request method is understood by the cache and defined as being
cacheable, and
o the response status code is understood by the cache, and
o the "no-store" cache directive (see Section 5.2) does not appear
in request or response header fields, and
o the "private" response directive (see Section 5.2.2.6) does not
appear in the response, if the cache is shared, and
o the Authorization header field (see Section 4.2 of [RFC7235]) does
not appear in the request, if the cache is shared, unless the
response explicitly allows it (see Section 3.2), and
o the response either:
* contains an Expires header field (see Section 5.3), or
* contains a max-age response directive (see Section 5.2.2.8), or
* contains a s-maxage response directive (see Section 5.2.2.9)
and the cache is shared, or
* contains a Cache Control Extension (see Section 5.2.3) that
allows it to be cached, or
* has a status code that is defined as cacheable by default (see
Section 4.2.2), or
* contains a public response directive (see Section 5.2.2.5).
Note that any of the requirements listed above can be overridden by a
cache-control extension; 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 Incomplete Responses
A response message is considered complete when all of the octets
indicated by the message framing ([RFC7230]) are received prior to
the connection being closed. If the request method is GET, the
response status code is 200 (OK), and the entire response header
section has been received, a cache MAY store an incomplete response
message body if the cache entry is recorded as incomplete. Likewise,
a 206 (Partial Content) response MAY be stored as if it were an
incomplete 200 (OK) cache entry. However, a cache MUST NOT store
incomplete or partial-content responses if it does not support the
Range and Content-Range header fields or if it does not understand
the range units used in those fields.
A cache MAY complete a stored incomplete response by making a
subsequent range request ([RFC7233]) and combining the successful
response with the stored entry, as defined in Section 3.3. A cache
MUST NOT use an incomplete response to answer requests unless the
response has been made complete or the request is partial and
specifies a range that is wholly within the incomplete response. A
cache MUST NOT send a partial response to a client without explicitly
marking it as such using the 206 (Partial Content) status code.
3.2. Storing Responses to Authenticated Requests
A shared cache MUST NOT use a cached response to a request with an
Authorization header field (Section 4.2 of [RFC7235]) to satisfy any
subsequent request unless a cache directive that allows such
responses to be stored is present in the response.
In this specification, the following Cache-Control response
directives (Section 5.2.2) have such an effect: must-revalidate,
public, and s-maxage.
Note that cached responses that contain the "must-revalidate" and/or
"s-maxage" response directives are not allowed to be served stale
(Section 4.2.4) by shared caches. In particular, a response with
either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
satisfy a subsequent request without revalidating it on the origin
server.
3.3. 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 ([RFC7233]). After several such transfers, a cache
might have received several ranges of the same representation. A
cache MAY combine these ranges into a single stored response, and
reuse that response to satisfy later requests, if they all share the
same strong validator and the cache complies with the client
requirements in Section 4.3 of [RFC7233].
When combining the new response with one or more stored responses, a
cache MUST:
o delete any Warning header fields in the stored response with
warn-code 1xx (see Section 5.5);
o retain any Warning header fields in the stored response with
warn-code 2xx; and,
o use other header fields provided in the new response, aside from
Content-Range, to replace all instances of the corresponding
header fields in the stored response.
4. Constructing Responses from Caches
When presented with a request, a cache MUST NOT reuse a stored
response, unless:
o The presented effective request URI (Section 5.5 of [RFC7230]) and
that of the stored response match, and
o the request method associated with the stored response allows it
to be used for the presented request, and
o selecting header fields nominated by the stored response (if any)
match those presented (see Section 4.1), and
o the presented request does not contain the no-cache pragma
(Section 5.4), nor the no-cache cache directive (Section 5.2.1),
unless the stored response is successfully validated
(Section 4.3), and
o the stored response does not contain the no-cache cache directive
(Section 5.2.2.2), unless it is successfully validated
(Section 4.3), and
o the stored response is either:
* 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 any of the requirements listed above can be overridden by a
cache-control extension; 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 4.2.1 of [RFC7231]) 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.
When more than one suitable response is stored, a cache MUST use the
most recent response (as determined by the Date header field). It
can also forward the request with "Cache-Control: max-age=0" or
"Cache-Control: no-cache" to disambiguate which response to use.
A cache that does not have a clock available MUST NOT use stored
responses without revalidating them upon every use.
4.1. Calculating Secondary Keys with Vary
When a cache receives a request that can be satisfied by a stored
response that has a Vary header field (Section 7.1.4 of [RFC7231]),
it MUST NOT use that response unless all of the selecting header
fields nominated by the Vary header field match in both the original
request (i.e., that associated with the stored response), and the
presented request.
The selecting header fields from two requests are defined to match if
and only if those in the first request can be transformed to those in
the second request by applying any of the following:
o adding or removing whitespace, where allowed in the header field's
syntax
o combining multiple header fields with the same field name (see
Section 3.2 of [RFC7230])
o normalizing both header field values in a way that is known to
have identical semantics, according to the header field's
specification (e.g., reordering field values when order is not
significant; case-normalization, where values are defined to be
case-insensitive)
If (after any normalization that might take place) a header field is
absent from a request, it can only match another request if it is
also absent there.
A Vary header field-value of "*" always fails to match.
The stored response with matching selecting header fields is known as
the selected response.
If multiple selected responses are available (potentially including
responses without a Vary header field), the cache will need to choose
one to use. When a selecting header field has a known mechanism for
doing so (e.g., qvalues on Accept and similar request header fields),
that mechanism MAY be used to select preferred responses; of the
remainder, the most recent response (as determined by the Date header
field) is used, as per Section 4.
If no selected response is available, the cache cannot satisfy the
presented request. Typically, it is forwarded to the origin server
in a (possibly conditional; see Section 4.3) request.
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" in the cache, it can be used to satisfy
subsequent requests without contacting the origin server, thereby
improving efficiency.
The primary mechanism for determining freshness is for an origin
server to provide an explicit expiration time in the future, using
either the Expires header field (Section 5.3) or the max-age response
directive (Section 5.2.2.8). 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 cache directives in a
request to constrain or relax freshness calculations for the
corresponding response (Section 5.2.1).
When calculating freshness, to avoid common problems in date parsing:
o Although all date formats are specified to be case-sensitive, a
cache recipient SHOULD match day, week, and time-zone names
case-insensitively.
o 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.
o A cache recipient MUST NOT allow local time zones to influence the
calculation or comparison of an age or expiration time.
o A cache recipient SHOULD consider a date with a zone abbreviation
other than GMT or UTC to be invalid for calculating expiration.
Note that freshness applies only to cache operation; it cannot be
used to force a user agent to refresh its display or reload a
resource. See 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 using the first match of the
following:
o If the cache is shared and the s-maxage response directive
(Section 5.2.2.9) is present, use its value, or
o If the max-age response directive (Section 5.2.2.8) is present,
use its value, or
o If the Expires response header field (Section 5.3) is present, use
its value minus the value of the Date response header field, or
o 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 not vulnerable to clock skew, since all
of the information comes from the origin server.
When there is more than one value present for a given directive
(e.g., two Expires header fields, multiple Cache-Control: max-age
directives), the directive's value is considered invalid. Caches are
encouraged to consider responses that have invalid freshness
information to be stale.
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 header field
values (such as the Last-Modified time) to estimate a plausible
expiration time. This specification does not provide specific
algorithms, but does impose worst-case constraints on their results.
A cache MUST NOT use heuristics to determine freshness when an
explicit expiration time is present in the stored response. Because
of the requirements in Section 3, this means that, effectively,
heuristics can only be used on responses without explicit freshness
whose status codes are defined as cacheable by default (see Section
6.1 of [RFC7231]), and those responses without explicit freshness
that have been marked as explicitly cacheable (e.g., with a "public"
response directive).
If the response has a Last-Modified header field (Section 2.2 of
[RFC7232]), caches are encouraged to use a heuristic expiration value
that is no more than some fraction of the interval since that time.
A typical setting of this fraction might be 10%.
When a heuristic is used to calculate freshness lifetime, a cache
SHOULD generate a Warning header field with a 113 warn-code (see
Section 5.5.4) in the response if its current_age is more than 24
hours and such a warning is not already present.
Note: Section 13.9 of [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 preclude 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 response was
generated or validated by the origin server. In essence, the Age
value is the sum of the time that the response has been resident in
each of the caches along the path from the origin server, plus the
amount of time it has been in transit along network paths.
The following data is used for the age calculation:
age_value
The term "age_value" denotes the value of the Age header field
(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
7.1.1.2 of [RFC7231] for the definition of the Date header field,
and for requirements regarding responses without it.
now
The term "now" means "the current value of the clock at the host
performing the calculation". A host ought to use NTP ([RFC5905])
or some similar protocol to synchronize its clocks to Coordinated
Universal Time.
request_time
The current value of the clock at the host at the time the request
resulting in the stored response was made.
response_time
The current value of the clock at the host at the time the
response was received.
A response's age can be calculated in two entirely independent ways:
1. the "apparent_age": response_time minus date_value, if the local
clock is reasonably well synchronized to the origin server's
clock. If the result is negative, the result is replaced by
zero.
2. the "corrected_age_value", if all of the caches along the
response path implement HTTP/1.1. A cache MUST interpret this
value relative to the time the request was initiated, not the
time that the response was received.
apparent_age = max(0, response_time - date_value);
response_delay = response_time - request_time;
corrected_age_value = age_value + response_delay;
These are combined as
corrected_initial_age = max(apparent_age, corrected_age_value);
unless the cache is confident in the value of the Age header field
(e.g., because there are no HTTP/1.0 hops in the Via header field),
in which case the corrected_age_value MAY be used as the
corrected_initial_age.
The current_age of a stored response can then be calculated by adding
the amount of time (in seconds) since the stored response was last
validated by the origin server to the corrected_initial_age.
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-store" or "no-cache"
cache directive, a "must-revalidate" cache-response-directive, or an
applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
see Section 5.2.2).
A cache MUST NOT send stale responses unless it is disconnected
(i.e., it cannot contact the origin server or otherwise find a
forward path) or doing so is explicitly allowed (e.g., by the
max-stale request directive; see Section 5.2.1).
A cache SHOULD generate a Warning header field with the 110 warn-code
(see Section 5.5.1) in stale responses. Likewise, a cache SHOULD
generate a 112 warn-code (see Section 5.5.3) in stale responses if
the cache is disconnected.
A cache SHOULD NOT generate a new Warning header field when
forwarding a response that does not have an Age header field, even if
the response is already stale. A cache need not validate a response
that merely became stale in transit.
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 selected; see Section 4.1), it can use the conditional
request mechanism [RFC7232] in the forwarded request to give the next
inbound server an opportunity to select a valid stored response to
use, updating the stored metadata in the process, or to replace the
stored response(s) with a new response. This process is known as
"validating" or "revalidating" the stored response.
4.3.1. Sending a Validation Request
When sending a conditional request for cache validation, a cache
sends one or more precondition header fields containing validator
metadata from its stored response(s), which is then compared by
recipients to determine whether a stored response is equivalent to a
current representation of the resource.
One such validator is the timestamp given in a Last-Modified header
field (Section 2.2 of [RFC7232]), which can be used in an
If-Modified-Since header field for response validation, or in an
If-Unmodified-Since or If-Range header field for representation
selection (i.e., the client is referring specifically to a previously
obtained representation with that timestamp).
Another validator is the entity-tag given in an ETag header field
(Section 2.3 of [RFC7232]). 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).
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 one of
its stored 200 (OK) or 206 (Partial Content) responses, the cache
SHOULD evaluate any applicable conditional header field preconditions
received in that request with respect to the corresponding validators
contained within the selected response. A cache MUST NOT evaluate
conditional header fields that are only applicable to an origin
server, found in a request with semantics that cannot be satisfied
with a cached response, or applied to a target resource for which it
has no stored responses; such preconditions are likely intended for
some other (inbound) server.
The proper evaluation of conditional requests by a cache depends on
the received precondition header fields and their precedence, as
defined in Section 6 of [RFC7232]. The If-Match and
If-Unmodified-Since conditional header fields are not applicable to a
cache.
A request containing an If-None-Match header field (Section 3.2 of
[RFC7232]) indicates that the client wants to validate one or more of
its own stored responses in comparison to whichever stored response
is selected by the cache. If the field-value is "*", or if the
field-value is a list of entity-tags and at least one of them matches
the entity-tag of the selected stored response, a cache recipient
SHOULD generate a 304 (Not Modified) response (using the metadata of
the selected stored response) instead of sending that stored
response.
When a cache decides to revalidate its own stored responses for a
request that contains an If-None-Match list of entity-tags, the cache
MAY combine the received list with a list of entity-tags from its own
stored set of responses (fresh or stale) and send the union of the
two lists as a replacement If-None-Match header field value in the
forwarded request. If a stored response contains only partial
content, the cache MUST NOT include its entity-tag in the union
unless the request is for a range that would be fully satisfied by
that partial stored response. If the response to the forwarded
request is 304 (Not Modified) and has an ETag header field value with
an entity-tag that is not in the client's list, the cache MUST
generate a 200 (OK) response for the client by reusing its
corresponding stored response, as updated by the 304 response
metadata (Section 4.3.4).
If an If-None-Match header field is not present, a request containing
an If-Modified-Since header field (Section 3.3 of [RFC7232])
indicates that the client wants to validate one or more of its own
stored responses by modification date. A cache recipient SHOULD
generate a 304 (Not Modified) response (using the metadata of the
selected stored response) if one of the following cases is true: 1)
the selected stored response has a Last-Modified field-value that is
earlier than or equal to the conditional timestamp; 2) no
Last-Modified field is present in the selected stored response, but
it has a Date field-value that is earlier than or equal to the
conditional timestamp; or, 3) neither Last-Modified nor Date is
present in the selected stored response, but the cache recorded it as
having been received at a time earlier than or equal to the
conditional timestamp.
A cache that implements partial responses to range requests, as
defined in [RFC7233], also needs to evaluate a received If-Range
header field (Section 3.2 of [RFC7233]) with respect to its selected
stored response.
4.3.3. Handling a Validation Response
Cache handling of a response to a conditional request is dependent
upon its status code:
o A 304 (Not Modified) response status code indicates that the
stored response can be updated and reused; see Section 4.3.4.
o A full response (i.e., one with a payload body) indicates that
none of the stored responses nominated in the conditional request
is suitable. Instead, the cache MUST use the full response to
satisfy the request and MAY replace the stored response(s).
o However, if a cache receives a 5xx (Server Error) response while
attempting to validate a response, it can either forward this
response to the requesting client, or act as if the server failed
to respond. In the latter case, the cache MAY send a previously
stored response (see Section 4.2.4).
4.3.4. Freshening Stored Responses upon Validation
When a cache receives a 304 (Not Modified) response and already has
one or more stored 200 (OK) responses for the same cache key, the
cache needs to identify which of the stored responses are updated by
this new response and then update the stored response(s) with the new
information provided in the 304 response.
The stored response to update is identified by using the first match
(if any) of the following:
o If the new response contains a strong validator (see Section 2.1
of [RFC7232]), then that strong validator identifies the selected
representation for update. All of the stored responses with the
same strong validator are selected. If none of the stored
responses contain the same strong validator, then the cache MUST
NOT use the new response to update any stored responses.
o If the new response contains a weak validator and that validator
corresponds to one of the cache's stored responses, then the most
recent of those matching stored responses is selected for update.
o If the new response does not include any form of validator (such
as in the case where a client generates an If-Modified-Since
request from a source other than the Last-Modified response header
field), and there is only one stored response, and that stored
response also lacks a validator, then that stored response is
selected for update.
If a stored response is selected for update, the cache MUST:
o delete any Warning header fields in the stored response with
warn-code 1xx (see Section 5.5);
o retain any Warning header fields in the stored response with
warn-code 2xx; and,
o use other header fields provided in the 304 (Not Modified)
response to replace all instances of the corresponding header
fields in the stored response.
4.3.5. Freshening Responses via HEAD
A response to the HEAD method is identical to what an equivalent
request made with a GET would have been, except it lacks a body.
This property of HEAD responses can be used to invalidate or update a
cached GET response if the more efficient conditional GET request
mechanism is not available (due to no validators being present in the
stored response) or if transmission of the representation body is not
desired even if it has changed.
When a cache makes an inbound HEAD request for a given request target
and receives a 200 (OK) response, the cache SHOULD update or
invalidate each of its stored GET responses that could have been
selected for that request (see Section 4.1).
For each of the stored responses that could have been selected, if
the stored response and HEAD response have matching values for any
received validator fields (ETag and Last-Modified) and, if the HEAD
response has a Content-Length header field, the value of
Content-Length matches that of the stored response, the cache SHOULD
update the stored response as described below; otherwise, the cache
SHOULD consider the stored response to be stale.
If a cache updates a stored response with the metadata provided in a
HEAD response, the cache MUST:
o delete any Warning header fields in the stored response with
warn-code 1xx (see Section 5.5);
o retain any Warning header fields in the stored response with
warn-code 2xx; and,
o use other header fields provided in the HEAD response to replace
all instances of the corresponding header fields in the stored
response and append new header fields to the stored response's
header section unless otherwise restricted by the Cache-Control
header field.
4.4. Invalidation
Because unsafe request methods (Section 4.2.1 of [RFC7231]) such as
PUT, POST or DELETE have the potential for changing state on the
origin server, intervening caches can use them to keep their contents
up to date.
A cache MUST invalidate the effective Request URI (Section 5.5 of
[RFC7230]) as well as the URI(s) in the Location and Content-Location
response header fields (if present) when a non-error status code is
received in response to an unsafe request method.
However, a cache MUST NOT invalidate a URI from a Location or
Content-Location response header field if the host part of that URI
differs from the host part in the effective request URI (Section 5.5
of [RFC7230]). This helps prevent denial-of-service attacks.
A cache MUST invalidate the effective request URI (Section 5.5 of
[RFC7230]) when it receives a non-error response to a request with a
method whose safety is unknown.
Here, a "non-error response" is one with a 2xx (Successful) or 3xx
(Redirection) status code. "Invalidate" means that the cache will
either remove all stored responses related to the effective request
URI or will mark these as "invalid" and in need of a mandatory
validation before they can be sent in response to a subsequent
request.
Note that this does not guarantee that all appropriate responses are
invalidated. For example, a state-changing request might invalidate
responses in the caches it travels through, but relevant responses
still might be stored in other caches that it has not.
5. Header Field Definitions
This section defines the syntax and semantics of HTTP/1.1 header
fields related to caching.
5.1. Age
The "Age" header field conveys the sender's estimate of the amount of
time since the response was generated or successfully validated at
the origin server. Age values are calculated as specified in
Section 4.2.3.
Age = delta-seconds
The Age field-value is a non-negative integer, representing time in
seconds (see Section 1.2.1).
The presence of an Age header field implies that the response was not
generated or validated by the origin server for this request.
However, lack of an Age header field does not imply the origin was
contacted, since the response might have been received from an
HTTP/1.0 cache that does not implement Age.
5.2. Cache-Control
The "Cache-Control" header field is used to specify directives for
caches along the request/response chain. Such cache directives are
unidirectional in that the presence of a directive in a request does
not imply that the same directive is to be given in the response.
A cache MUST obey the requirements of the Cache-Control directives
defined in this section. See Section 5.2.3 for information about how
Cache-Control directives defined elsewhere are handled.
Note: Some HTTP/1.0 caches might not implement Cache-Control.
A proxy, whether or not it implements a cache, MUST pass cache
directives through in forwarded messages, regardless of their
significance to that application, since the directives might be
applicable to all recipients along the request/response chain. It is
not possible to target a directive to a specific cache.
Cache directives are identified by a token, to be compared
case-insensitively, and have an optional argument, that can use both
token and quoted-string syntax. For the directives defined below
that define arguments, recipients ought to accept both forms, even if
one is documented to be preferred. For any directive not defined by
this specification, a recipient MUST accept both forms.
Cache-Control = 1#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 Cache-Control Directives
5.2.1.1. max-age
Argument syntax:
delta-seconds (see Section 1.2.1)
The "max-age" request directive indicates that the client is
unwilling to accept a response whose age is greater than the
specified number of seconds. Unless the max-stale request directive
is also present, the client is not willing to accept a stale
response.
This directive uses the token form of the argument syntax: e.g.,
'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the
quoted-string form.
5.2.1.2. max-stale
Argument syntax:
delta-seconds (see Section 1.2.1)
The "max-stale" request directive indicates that the client is
willing to accept a response that has exceeded its freshness
lifetime. If max-stale is assigned a value, then the client is
willing to accept a response that has exceeded its freshness lifetime
by no more than the specified number of seconds. If no value is
assigned to max-stale, then the client is willing to accept a stale
response of any age.
This directive uses the token form of the argument syntax: e.g.,
'max-stale=10' not 'max-stale="10"'. A sender SHOULD NOT generate
the quoted-string form.
5.2.1.3. min-fresh
Argument syntax:
delta-seconds (see Section 1.2.1)
The "min-fresh" request directive indicates that the client is
willing to accept a response whose freshness lifetime is no less than
its current age plus the specified time in seconds. That is, the
client wants a response that will still be fresh for at least the
specified number of seconds.
This directive uses the token form of the argument syntax: e.g.,
'min-fresh=20' not 'min-fresh="20"'. A sender SHOULD NOT generate
the quoted-string form.
5.2.1.4. no-cache
The "no-cache" request directive indicates that a cache MUST NOT use
a stored response to satisfy the request without successful
validation on the origin server.
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 an intermediary
(whether or not it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of [RFC7230].
5.2.1.7. only-if-cached
The "only-if-cached" request directive indicates that the client only
wishes to obtain a stored response. If it receives this directive, a
cache SHOULD either respond using a stored response that is
consistent with the other constraints of the request, or respond with
a 504 (Gateway Timeout) status code. If a group of caches is being
operated as a unified system with good internal connectivity, a
member cache MAY forward such a request within that group of caches.
5.2.2. Response Cache-Control Directives
5.2.2.1. must-revalidate
The "must-revalidate" response directive indicates that once it has
become stale, a cache MUST NOT use the response to satisfy subsequent
requests without successful validation on the origin server.
The must-revalidate directive is necessary to support reliable
operation for certain protocol features. In all circumstances a
cache MUST obey the must-revalidate directive; in particular, if a
cache cannot reach the origin server for any reason, it MUST generate
a 504 (Gateway Timeout) response.
The must-revalidate directive ought to be used by servers if and only
if failure to validate a request on the representation could result
in incorrect operation, such as a silently unexecuted financial
transaction.
5.2.2.2. no-cache
Argument syntax:
#field-name
The "no-cache" response directive indicates that the response MUST
NOT be used to satisfy a subsequent request without successful
validation on the origin server. This allows an origin server to
prevent a cache from using it to satisfy a request without contacting
it, even by caches that have been configured to send stale responses.
If the no-cache response directive specifies one or more field-names,
then a cache MAY use the response to satisfy a subsequent request,
subject to any other restrictions on caching. However, any header
fields in the response that have the field-name(s) listed MUST NOT be
sent in the response to a subsequent request without successful
revalidation with the origin server. This allows an origin server to
prevent the re-use of certain header fields in a response, while
still allowing caching of the rest of the response.
The field-names given are not limited to the set of header fields
defined by this specification. Field names are case-insensitive.
This directive uses the quoted-string form of the argument syntax. A
sender SHOULD NOT generate the token form (even if quoting appears
not to be needed for single-entry lists).
Note: Although it has been back-ported to many implementations, some
HTTP/1.0 caches will not recognize or obey this directive. Also,
no-cache response directives with field-names are often handled by
caches as if an unqualified no-cache directive was received; i.e.,
the special handling for the qualified form is not widely
implemented.
5.2.2.3. no-store
The "no-store" response directive indicates that a cache MUST NOT
store any part of either the immediate request or response. This
directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a
best-effort attempt to remove the information from volatile storage
as promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might
be vulnerable to eavesdropping.
5.2.2.4. no-transform
The "no-transform" response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of [RFC7230].
5.2.2.5. public
The "public" response directive indicates that any cache MAY store
the response, even if the response would normally be non-cacheable or
cacheable only within a private cache. (See Section 3.2 for
additional details related to the use of public in response to a
request containing Authorization, and Section 3 for details of how
public affects responses that would normally not be stored, due to
their status codes not being defined as cacheable by default; see
Section 4.2.2.)
5.2.2.6. private
Argument syntax:
#field-name
The "private" response directive indicates that the response message
is intended for a single user and MUST NOT be stored by a shared
cache. A private cache MAY store the response and reuse it for later
requests, even if the response would normally be non-cacheable.
If the private response directive specifies one or more field-names,
this requirement is limited to the field-values associated with the
listed response header fields. That is, a shared cache MUST NOT
store the specified field-names(s), whereas it MAY store the
remainder of the response message.
The field-names given are not limited to the set of header fields
defined by this specification. Field names are case-insensitive.
This directive uses the quoted-string form of the argument syntax. A
sender SHOULD NOT generate the token form (even if quoting appears
not to be needed for single-entry lists).
Note: This usage of the word "private" only controls where the
response can be stored; it cannot ensure the privacy of the message
content. Also, private response directives with field-names are
often handled by caches as if an unqualified private directive was
received; i.e., the special handling for the qualified form is not
widely implemented.
5.2.2.7. proxy-revalidate
The "proxy-revalidate" response directive has the same meaning as the
must-revalidate response directive, except that it does not apply to
private caches.
5.2.2.8. max-age
Argument syntax:
delta-seconds (see Section 1.2.1)
The "max-age" response directive indicates that the response is to be
considered stale after its age is greater than the specified number
of seconds.
This directive uses the token form of the argument syntax: e.g.,
'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the
quoted-string form.
5.2.2.9. s-maxage
Argument syntax:
delta-seconds (see Section 1.2.1)
The "s-maxage" response directive indicates that, in shared caches,
the maximum age specified by this directive overrides the maximum age
specified by either the max-age directive or the Expires header
field. The s-maxage directive also implies the semantics of the
proxy-revalidate response directive.
This directive uses the token form of the argument syntax: e.g.,
's-maxage=10' not 's-maxage="10"'. A sender SHOULD NOT generate the
quoted-string form.
5.2.3. Cache Control Extensions
The Cache-Control header field can be extended through the use of one
or more cache-extension tokens, each with an optional value. A cache
MUST ignore unrecognized cache directives.
Informational extensions (those that do not require a change in cache
behavior) can be added without changing the semantics of other
directives.
Behavioral extensions are designed to work by acting as modifiers to
the existing base of cache directives. Both the new directive and
the old directive are supplied, such that applications that do not
understand the new directive will default to the behavior specified
by the old directive, and those that understand the new directive
will recognize it as modifying the requirements associated with the
old directive. In this way, extensions to the existing cache-control
directives can be made without breaking deployed caches.
For example, consider a hypothetical new response directive called
"community" that acts as a modifier to the private directive: in
addition to private caches, any cache that is shared only by members
of the named community is allowed to cache the response. An origin
server wishing to allow the UCI community to use an otherwise private
response in their shared cache(s) could do so by including
Cache-Control: private, community="UCI"
A cache that recognizes such a community cache-extension could
broaden its behavior in accordance with that extension. A cache that
does not recognize the community cache-extension would ignore it and
adhere to the private directive.
5.3. Expires
The "Expires" 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 field does not imply that the original
resource will change or cease to exist at, before, or after that
time.
The Expires value is an HTTP-date timestamp, as defined in Section
7.1.1.1 of [RFC7231].
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 field with the max-age
directive (Section 5.2.2.8), a recipient MUST ignore the Expires
field. Likewise, if a response includes the s-maxage directive
(Section 5.2.2.9), a shared cache recipient MUST ignore the Expires
field. In both these cases, the value in Expires is only intended
for recipients that have not yet implemented the Cache-Control field.
An origin server without a clock MUST NOT generate an Expires field
unless its value represents a fixed time in the past (always expired)
or its value has been associated with the resource by a system or
user with a reliable clock.
Historically, HTTP required the Expires field-value to be no more
than a year in the future. While longer freshness lifetimes are no
longer prohibited, extremely large values have been demonstrated to
cause problems (e.g., clock overflows due to use of 32-bit integers
for time values), and many caches will evict a response far sooner
than that.
5.4. Pragma
The "Pragma" header field allows backwards compatibility with
HTTP/1.0 caches, so that clients can specify a "no-cache" request
that they will understand (as Cache-Control was not defined until
HTTP/1.1). When the Cache-Control header field is also present and
understood in a request, Pragma is ignored.
In HTTP/1.0, Pragma was defined as an extensible field for
implementation-specified directives for recipients. This
specification deprecates such extensions to improve interoperability.
Pragma = 1#pragma-directive
pragma-directive = "no-cache" / extension-pragma
extension-pragma = token [ "=" ( token / quoted-string ) ]
When the Cache-Control header field is not present in a request,
caches MUST consider the no-cache request pragma-directive as having
the same effect as if "Cache-Control: no-cache" were present (see
Section 5.2.1).
When sending a no-cache request, a client ought to include both the
pragma and cache-control directives, unless Cache-Control: no-cache
is purposefully omitted to target other Cache-Control response
directives at HTTP/1.1 caches. For example:
GET / HTTP/1.1
Host: www.example.com
Cache-Control: max-age=30
Pragma: no-cache
will constrain HTTP/1.1 caches to serve a response no older than 30
seconds, while precluding implementations that do not understand
Cache-Control from serving a cached response.
Note: Because the meaning of "Pragma: no-cache" in responses is
not specified, it does not provide a reliable replacement for
"Cache-Control: no-cache" in them.
5.5. Warning
The "Warning" header field is used to carry additional information
about the status or transformation of a message that might not be
reflected in the status code. This information is typically used to
warn about possible incorrectness introduced by caching operations or
transformations applied to the payload of the message.
Warnings can be used for other purposes, both cache-related and
otherwise. The use of a warning, rather than an error status code,
distinguishes these responses from true failures.
Warning header fields can in general be applied to any message,
however some warn-codes are specific to caches and can only be
applied to response messages.
Warning = 1#warning-value
warning-value = warn-code SP warn-agent SP warn-text
[ SP warn-date ]
warn-code = 3DIGIT
warn-agent = ( uri-host [ ":" port ] ) / pseudonym
; the name or pseudonym of the server adding
; the Warning header field, for use in debugging
; a single "-" is recommended when agent unknown
warn-text = quoted-string
warn-date = DQUOTE HTTP-date DQUOTE
Multiple warnings can be generated in a response (either by the
origin server or by a cache), including multiple warnings with the
same warn-code number that only differ in warn-text.
A user agent that receives one or more Warning header fields SHOULD
inform the user of as many of them as possible, in the order that
they appear in the response. Senders that generate multiple Warning
header fields are encouraged to order them with this user agent
behavior in mind. A sender that generates new Warning header fields
MUST append them after any existing Warning header fields.
Warnings are assigned three digit warn-codes. The first digit
indicates whether the Warning is required to be deleted from a stored
response after validation:
o 1xx warn-codes describe the freshness or validation status of the
response, and so they MUST be deleted by a cache after validation.
They can only be generated by a cache when validating a cached
entry, and MUST NOT be generated in any other situation.
o 2xx warn-codes describe some aspect of the representation that is
not rectified by a validation (for example, a lossy compression of
the representation) and they MUST NOT be deleted by a cache after
validation, unless a full response is sent, in which case they
MUST be.
If a sender generates one or more 1xx warn-codes in a message to be
sent to a recipient known to implement only HTTP/1.0, the sender MUST
include in each corresponding warning-value a warn-date that matches
the Date header field in the message. For example:
HTTP/1.1 200 OK
Date: Sat, 25 Aug 2012 23:34:45 GMT
Warning: 112 - "network down" "Sat, 25 Aug 2012 23:34:45 GMT"
Warnings have accompanying warn-text that describes the error, e.g.,
for logging. It is advisory only, and its content does not affect
interpretation of the warn-code.
If a recipient that uses, evaluates, or displays Warning header
fields receives a warn-date that is different from the Date value in
the same message, the recipient MUST exclude the warning-value
containing that warn-date before storing, forwarding, or using the
message. This allows recipients to exclude warning-values that were
improperly retained after a cache validation. If all of the
warning-values are excluded, the recipient MUST exclude the Warning
header field as well.
The following warn-codes are defined by this specification, each with
a recommended warn-text in English, and a description of its meaning.
The procedure for defining additional warn codes is described in
Section 7.2.1.
5.5.1. Warning: 110 - "Response is Stale"
A cache SHOULD generate this whenever the sent response is stale.
5.5.2. Warning: 111 - "Revalidation Failed"
A cache SHOULD generate this when sending a stale response because an
attempt to validate the response failed, due to an inability to reach
the server.
5.5.3. Warning: 112 - "Disconnected Operation"
A cache SHOULD generate this if it is intentionally disconnected from
the rest of the network for a period of time.
5.5.4. Warning: 113 - "Heuristic Expiration"
A cache SHOULD generate this if it heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater than
24 hours.
5.5.5. Warning: 199 - "Miscellaneous Warning"
The warning text can include arbitrary information to be presented to
a human user or logged. A system receiving this warning MUST NOT
take any automated action, besides presenting the warning to the
user.
5.5.6. Warning: 214 - "Transformation Applied"
This Warning code MUST be added by a proxy if it applies any
transformation to the representation, such as changing the
content-coding, media-type, or modifying the representation data,
unless this Warning code already appears in the response.
5.5.7. Warning: 299 - "Miscellaneous Persistent Warning"
The warning text can include arbitrary information to be presented to
a human user or logged. A system receiving this warning MUST NOT
take any automated action.
6. History Lists
User agents often have history mechanisms, such as "Back" buttons and
history lists, that can be used to redisplay a representation
retrieved earlier in a session.
The freshness model (Section 4.2) does not necessarily apply to
history mechanisms. That is, a history mechanism can display a
previous representation even if it has expired.
This does not prohibit the history mechanism from telling the user
that a view might be stale or from honoring cache directives (e.g.,
Cache-Control: no-store).
7. IANA Considerations
7.1. 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
<http://www.iana.org/assignments/http-cache-directives>.
7.1.1. Procedure
A registration MUST include the following fields:
o Cache Directive Name
o Pointer to specification text
Values to be added to this namespace require IETF Review (see
[RFC5226], Section 4.1).
7.1.2. Considerations for New Cache Control Directives
New extension directives ought to consider defining:
o What it means for a directive to be specified multiple times,
o When the directive does not take an argument, what it means when
an argument is present,
o When the directive requires an argument, what it means when it is
missing,
o Whether the directive is specific to requests, responses, or able
to be used in either.
See also Section 5.2.3.
7.1.3. Registrations
The registry has been populated with the registrations below:
+------------------------+----------------------------------+
| Cache Directive | Reference |
+------------------------+----------------------------------+
| max-age | Section 5.2.1.1, Section 5.2.2.8 |
| max-stale | Section 5.2.1.2 |
| min-fresh | Section 5.2.1.3 |
| must-revalidate | Section 5.2.2.1 |
| no-cache | Section 5.2.1.4, Section 5.2.2.2 |
| no-store | Section 5.2.1.5, Section 5.2.2.3 |
| no-transform | Section 5.2.1.6, Section 5.2.2.4 |
| only-if-cached | Section 5.2.1.7 |
| private | Section 5.2.2.6 |
| proxy-revalidate | Section 5.2.2.7 |
| public | Section 5.2.2.5 |
| s-maxage | Section 5.2.2.9 |
| stale-if-error | [RFC5861], Section 4 |
| stale-while-revalidate | [RFC5861], Section 3 |
+------------------------+----------------------------------+
7.2. Warn Code Registry
The "Hypertext Transfer Protocol (HTTP) Warn Codes" registry defines
the namespace for warn codes. It has been created and is now
maintained at <http://www.iana.org/assignments/http-warn-codes>.
7.2.1. Procedure
A registration MUST include the following fields:
o Warn Code (3 digits)
o Short Description
o Pointer to specification text
Values to be added to this namespace require IETF Review (see
[RFC5226], Section 4.1).
7.2.2. Registrations
The registry has been populated with the registrations below:
+-----------+----------------------------------+---------------+
| Warn Code | Short Description | Reference |
+-----------+----------------------------------+---------------+
| 110 | Response is Stale | Section 5.5.1 |
| 111 | Revalidation Failed | Section 5.5.2 |
| 112 | Disconnected Operation | Section 5.5.3 |
| 113 | Heuristic Expiration | Section 5.5.4 |
| 199 | Miscellaneous Warning | Section 5.5.5 |
| 214 | Transformation Applied | Section 5.5.6 |
| 299 | Miscellaneous Persistent Warning | Section 5.5.7 |
+-----------+----------------------------------+---------------+
7.3. Header Field Registration
HTTP header fields are registered within the "Message Headers"
registry maintained at
<http://www.iana.org/assignments/message-headers/>.
This document defines the following HTTP header fields, so the
"Permanent Message Header Field Names" registry has been updated
accordingly (see [BCP90]).
+-------------------+----------+----------+-------------+
| Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+-------------+
| Age | http | standard | Section 5.1 |
| Cache-Control | http | standard | Section 5.2 |
| Expires | http | standard | Section 5.3 |
| Pragma | http | standard | Section 5.4 |
| Warning | http | standard | Section 5.5 |
+-------------------+----------+----------+-------------+
The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force".
8. 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 messaging
[RFC7230] and semantics [RFC7231].
Caches expose additional potential vulnerabilities, since the
contents of the cache represent an attractive target for malicious
exploitation. Because cache contents persist after an HTTP request
is complete, an attack on the cache can reveal information long after
a user believes that the information has been removed from the
network. Therefore, cache contents need to be protected as sensitive
information.
In particular, various attacks might be amplified by being stored in
a shared cache; such "cache poisoning" attacks use the cache to
distribute a malicious payload to many clients, and are especially
effective when an attacker can use implementation flaws, elevated
privileges, or other techniques to insert such a response into a
cache. One common attack vector for cache poisoning is to exploit
differences in message parsing on proxies and in user agents; see
Section 3.3.3 of [RFC7230] for the relevant requirements.
Likewise, implementation flaws (as well as misunderstanding of cache
operation) might lead to caching of sensitive information (e.g.,
authentication credentials) that is thought to be private, exposing
it to unauthorized parties.
Furthermore, the very use of a cache can bring about privacy
concerns. For example, if two users share a cache, and the first one
browses to a site, the second may be able to detect that the other
has been to that site, because the resources from it load more
quickly, thanks to the cache.
Note that the Set-Cookie response header field [RFC6265] does not
inhibit caching; a cacheable response with a Set-Cookie header field
can be (and often is) used to satisfy subsequent requests to caches.
Servers who wish to control caching of these responses are encouraged
to emit appropriate Cache-Control response header fields.
9. Acknowledgments
See Section 10 of [RFC7230].
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, June 2014.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
June 2014.
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
June 2014.
[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
RFC 7233, June 2014.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235, June 2014.
10.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[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, June 1999.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale
Content", RFC 5861, April 2010.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011.
Appendix A. Changes from RFC 2616
The specification has been substantially rewritten for clarity.
The conditions under which an authenticated response can be cached
have been clarified. (Section 3.2)
New status codes can now define that caches are allowed to use
heuristic freshness with them. Caches are now allowed to calculate
heuristic freshness for URIs with query components. (Section 4.2.2)
The algorithm for calculating age is now less conservative. Caches
are now required to handle dates with time zones as if they're
invalid, because it's not possible to accurately guess.
(Section 4.2.3)
The Content-Location response header field is no longer used to
determine the appropriate response to use when validating.
(Section 4.3)
The algorithm for selecting a cached negotiated response to use has
been clarified in several ways. In particular, it now explicitly
allows header-specific canonicalization when processing selecting
header fields. (Section 4.1)
Requirements regarding denial-of-service attack avoidance when
performing invalidation have been clarified. (Section 4.4)
Cache invalidation only occurs when a successful response is
received. (Section 4.4)
Cache directives are explicitly defined to be case-insensitive.
Handling of multiple instances of cache directives when only one is
expected is now defined. (Section 5.2)
The "no-store" request directive doesn't apply to responses; i.e., a
cache can satisfy a request with no-store on it and does not
invalidate it. (Section 5.2.1.5)
The qualified forms of the private and no-cache cache directives are
noted to not be widely implemented; for example, "private=foo" is
interpreted by many caches as simply "private". Additionally, the
meaning of the qualified form of no-cache has been clarified.
(Section 5.2.2)
The "no-cache" response directive's meaning has been clarified.
(Section 5.2.2.2)
The one-year limit on Expires header field values has been removed;
instead, the reasoning for using a sensible value is given.
(Section 5.3)
The Pragma header field is now only defined for backwards
compatibility; future pragmas are deprecated. (Section 5.4)
Some requirements regarding production and processing of the Warning
header fields have been relaxed, as it is not widely implemented.
Furthermore, the Warning header field no longer uses RFC 2047
encoding, nor does it allow multiple languages, as these aspects were
not implemented. (Section 5.5)
This specification introduces the Cache Directive and Warn Code
Registries, and defines considerations for new cache directives.
(Section 7.1 and Section 7.2)
Appendix B. Imported ABNF
The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
character).
The rules below are defined in [RFC7230]:
OWS = <OWS, see [RFC7230], Section 3.2.3>
field-name = <field-name, see [RFC7230], Section 3.2>
quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>
token = <token, see [RFC7230], Section 3.2.6>
port = <port, see [RFC7230], Section 2.7>
pseudonym = <pseudonym, see [RFC7230], Section 5.7.1>
uri-host = <uri-host, see [RFC7230], Section 2.7>
The rules below are defined in other parts:
HTTP-date = <HTTP-date, see [RFC7231], Section 7.1.1.1>
Appendix C. Collected ABNF
In the collected ABNF below, list rules are expanded as per Section
1.2 of [RFC7230].
Age = delta-seconds
Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
cache-directive ] )
Expires = HTTP-date
HTTP-date = <HTTP-date, see [RFC7231], Section 7.1.1.1>
OWS = <OWS, see [RFC7230], Section 3.2.3>
Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
pragma-directive ] )
Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
)
cache-directive = token [ "=" ( token / quoted-string ) ]
delta-seconds = 1*DIGIT
extension-pragma = token [ "=" ( token / quoted-string ) ]
field-name = <field-name, see [RFC7230], Section 3.2>
port = <port, see [RFC7230], Section 2.7>
pragma-directive = "no-cache" / extension-pragma
pseudonym = <pseudonym, see [RFC7230], Section 5.7.1>
quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>
token = <token, see [RFC7230], Section 3.2.6>
uri-host = <uri-host, see [RFC7230], Section 2.7>
warn-agent = ( uri-host [ ":" port ] ) / pseudonym
warn-code = 3DIGIT
warn-date = DQUOTE HTTP-date DQUOTE
warn-text = quoted-string
warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
]
Index
1
110 (warn-code) 31
111 (warn-code) 31
112 (warn-code) 31
113 (warn-code) 31
199 (warn-code) 32
2
214 (warn-code) 32
299 (warn-code) 32
A
age 11
Age header field 21
C
cache 4
cache entry 5
cache key 5-6
Cache-Control header field 21
D
Disconnected Operation (warn-text) 31
E
Expires header field 28
explicit expiration time 11
F
fresh 11
freshness lifetime 11
G
Grammar
Age 21
Cache-Control 22
cache-directive 22
delta-seconds 5
Expires 28
extension-pragma 29
Pragma 29
pragma-directive 29
warn-agent 29
warn-code 29
warn-date 29
warn-text 29
Warning 29
warning-value 29
H
Heuristic Expiration (warn-text) 31
heuristic expiration time 11
M
max-age (cache directive) 22, 26
max-stale (cache directive) 22
min-fresh (cache directive) 22
Miscellaneous Persistent Warning (warn-text) 32
Miscellaneous Warning (warn-text) 32
must-revalidate (cache directive) 24
N
no-cache (cache directive) 23, 25
no-store (cache directive) 23, 24
no-transform (cache directive) 23, 25
O
only-if-cached (cache directive) 23
P
Pragma header field 29
private (cache directive) 25
private cache 4
proxy-revalidate (cache directive) 26
public (cache directive) 25
R
Response is Stale (warn-text) 30
Revalidation Failed (warn-text) 31
S
s-maxage (cache directive) 27
shared cache 4
stale 11
strong validator 18
T
Transformation Applied (warn-text) 32
V
validator 16
W
Warning header field 29
Authors' Addresses
Roy T. Fielding (editor)
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
EMail: fielding@gbiv.com
URI: http://roy.gbiv.com/
Mark Nottingham (editor)
Akamai
EMail: mnot@mnot.net
URI: http://www.mnot.net/
Julian F. Reschke (editor)
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
EMail: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
ISO Plain Language
On this page
PROBLEMThe web repeats the same requests over and over, and fetching each one again from the origin server is slow and wasteful.
MECHANISMA cache keeps a copy of a response and serves it again while it is fresh. When the copy has expired, the cache asks the origin server whether it is still valid, using a validator such as a Last-Modified date or an ETag, and refreshes its copy from the answer.
WHAT IT DEFINESThe RFC defines when a response may be stored at all, how its age is calculated, and a set of Cache-Control directives for both ends to say what a cache may do. It has been obsoleted by RFC 9111.
§1What an HTTP Cache Is
HTTP is typically used for distributed information systems, where performance can be improved by the use of response caches. This document defines the parts of HTTP/1.1 that concern 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. It stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests. Any client or server may employ a cache, though a server that is acting as a tunnel cannot use one. A shared cache stores responses to be reused by more than one user and is usually deployed as part of an intermediary. A private cache is dedicated to a single user and is often deployed as part of a user agent.
The goal of caching in HTTP/1.1 is to improve performance significantly by reusing a prior response to satisfy a current request. A stored response is considered fresh if it can be reused without validation, which means checking with the origin server to see whether the cached response remains valid for this request. A fresh response reduces both latency and network overhead each time it is reused. A cached response that is not fresh might still be reusable, either because it can be freshened by validation or because the origin is unavailable.
Proper cache operation preserves the semantics of HTTP transfers while eliminating the transfer of information the cache already holds. Caching is an entirely optional feature, but reusing a cached response is desirable and is the default when nothing prevents it, so the requirements are aimed at preventing a cache from storing a response that cannot be reused or from reusing a stored one inappropriately. Each cache entry consists of a cache key and one or more responses for prior requests that used the same key. The most common form is a 200 response to a GET request, but permanent redirects, negative results such as 404, incomplete results such as 206, and responses to other methods may also be cached. The primary cache key is the request method and the target URI, though many caches only cache GET responses and use just the URI. When a request target is subject to content negotiation, its cache entry may hold several stored responses, each distinguished by a secondary key holding the values of the original request's selecting header fields.
A cache is a box of saved answers. When the same question is asked again, the answer comes out of the box instead of from the far end of the network. The rules exist so that a cache never hands back something that has gone out of date without first finding out whether it still applies.
§3When a Response May Be Stored
A cache must not store a response unless several conditions hold. The request method must be one the cache understands and that is defined as cacheable, and the response status code must be one the cache understands. The no-store directive must not appear in the request or the response. The private response directive must not appear in the response if the cache is shared. The Authorization header field must not appear in the request if the cache is shared, unless the response explicitly allows such responses to be stored. And the response must either carry an Expires header field, carry a max-age directive, carry an s-maxage directive when the cache is shared, carry a cache control extension that allows it to be cached, have a status code that is cacheable by default, or carry a public directive.
Any of those requirements can be overridden by a cache-control extension. In normal operation some caches will not store a response that has neither a cache validator nor an explicit expiration time, since such responses are not usually useful to store, but the specification does not forbid it.
An incomplete response is one where the connection closed before all the octets the framing indicated were received. If the method is GET, the status is 200 and the whole header section has arrived, a cache may store the incomplete body provided the entry is recorded as incomplete; a 206 response may be stored the same way. A cache must not store incomplete or partial-content responses if it does not support the Range and Content-Range header fields or does not understand the range units used in them. A cache must not use an incomplete response to answer a request unless it has been completed or the request asks for a range wholly inside it, and it must not send a partial response to a client without marking it with the 206 status code.
Authenticated requests get extra protection. A shared cache must not use a cached response to a request carrying an Authorization header field to satisfy any later request, unless a directive in the response allows it. The directives that have that effect are must-revalidate, public and s-maxage. Responses carrying must-revalidate or s-maxage also cannot be served stale by shared caches, so a response with either max-age=0, must-revalidate or s-maxage=0 cannot satisfy a later request without being revalidated at the origin.
There is a list of reasons a cache must refuse to keep an answer: the server asked it not to, the answer is personal, the request was authenticated, or there is no way to tell when the answer goes out of date. A half-received answer may be kept, but only if the cache knows how to finish it.
§4Serving and Validating a Stored Response
A cache must not reuse a stored response unless the presented effective request URI matches the stored one, the stored response's request method allows it to be used for the presented request, the selecting header fields the stored response nominates match, the presented request contains neither the no-cache pragma nor the no-cache directive unless the stored response is successfully validated, the stored response does not itself carry the no-cache directive unless it is validated, and the stored response is fresh, allowed to be served stale, or successfully validated.
When a stored response is used without validation, the cache must generate an Age header field, replacing any present, holding the stored response's current age. A cache must write through requests with unsafe methods, which means it may not generate a reply before it has forwarded the request and received the matching response; such requests might also invalidate responses the cache already holds. When more than one suitable response is stored, the cache must use the most recent one, as determined by the Date header field, and it can forward the request with max-age=0 or no-cache to disambiguate. A cache with no working clock must not use stored responses without revalidating them on every use.
The selecting header fields, and so the secondary key, come from the Vary header field. When a stored response has a Vary, the cache must not use it unless all the fields Vary nominates match in both the original request and the presented one. Two requests' selecting fields match only if the first can be transformed into the second by adding or removing whitespace where the syntax allows it, by combining multiple header fields with the same name, or by normalizing both values in a way known to have identical semantics, such as reordering where order does not matter or case-normalizing where values are case-insensitive. A field absent from a request after normalization can only match another request where it is also absent. A Vary value of * always fails to match. If several responses are selected, a known mechanism such as qvalues on Accept may pick a preferred one, and of the remainder the most recent is used. If no response is selected, the request goes to the origin server.
When a cache holds responses for a requested URI but cannot serve any of them, because they are stale or because none can be selected, it can use the conditional request mechanism in the forwarded request. That gives the next inbound server the chance to confirm that a stored response is still equivalent to the current representation, updating the stored metadata in the process, or to replace it with a new response. The validators used are the timestamp in a Last-Modified header field, which can go in If-Modified-Since for validation or in If-Unmodified-Since or If-Range for selecting a representation, and the entity-tag in an ETag header field, which can go in If-None-Match for validation or in If-Match or If-Range for selecting a representation.
A cache that receives a conditional request must evaluate the applicable preconditions against the validators in the response it has selected, but must not evaluate preconditions that only apply to an origin server, that belong to a request whose semantics cannot be met from a cache, or that apply to a resource the cache holds nothing for; those are probably meant for another server. The If-Match and If-Unmodified-Since fields are not applicable to a cache at all. When the answer comes back, a 304 means the stored response can be updated and reused, a full response means none of the nominated stored responses was suitable, so the cache must use the full response and may replace what it had, and a 5xx leaves the cache free either to forward the error or to treat it as a failure to respond, in which case it may send a stored response instead.
When the stored copy has gone stale, the cache sends the request on with a note asking whether the page it already has is still the current one. The server answers with a short not modified message, and the cache refreshes its copy from the metadata that came back, or with the whole new answer if the page really did change.
ClientCacheOrigin server
- GET /resourceThe client asks for the resource.
- GET /resource, conditionalThe stored copy is stale, so the cache forwards the request with a validator such as If-None-Match or If-Modified-Since.
- 304 Not ModifiedThe origin says the copy the cache holds is still current.
- 200 OK with AgeThe cache serves the stored copy and adds an Age header saying how old it is.
§4.2Freshness, Age and Stale Responses
A response is fresh when its age has not yet passed its freshness lifetime, and stale when it has. The freshness lifetime is the length of time between the response being generated by the origin server and its expiration time. An explicit expiration time is the moment the origin server intends the response to stop being usable without further validation; a heuristic expiration time is one a cache assigns when the server gave no explicit time. The age of a response is the time since it was generated by, or successfully validated with, the origin server.
The main way freshness is set is for the origin server to give an explicit expiration time in the future, with either the Expires header field or the max-age directive. A server that wants every request validated can set the expiration time in the past, and a compliant cache will then validate a stale response before reusing it. The test is simply whether the freshness lifetime is greater than the current age. Clients can send the max-age or min-fresh directives to constrain or relax the calculation.
The freshness lifetime is calculated by taking the first of these that applies: the s-maxage value, if the cache is shared and s-maxage is present; the max-age value, if max-age is present; the Expires value minus the Date value, if Expires is present; and otherwise nothing explicit, in which case a heuristic may apply. Because all the information comes from the origin server, this calculation is not vulnerable to clock skew. If more than one value is present for a directive, the directive's value is considered invalid, and caches are encouraged to treat a response with invalid freshness information as stale.
A cache may assign a heuristic expiration time when the server gave none, using other header values such as the Last-Modified time to estimate a plausible one. The specification gives no algorithm but imposes worst-case limits: heuristics must not be used when an explicit expiration time is present, and when the response has a Last-Modified field, the heuristic value should be no more than some fraction of the interval since that time, a typical setting being 10 per cent. A cache that uses a heuristic should add a Warning header field with the 113 warn-code when the response is more than 24 hours old.
The Age header field carries the sender's estimate of the time since the response was generated or validated at the origin server. In essence the value is the sum of the time the response has spent in each cache along the path plus the time in transit. It is computed from the Age field already present, the Date field, the time the request was made and the time the response arrived: the apparent age is the response time minus the date value, floored at zero; the response delay is the response time minus the request time; the corrected age value is the incoming Age value plus that delay; and the corrected initial age is the larger of the apparent age and the corrected age value, unless the cache trusts the Age field, in which case it may use the corrected age value alone. The current age adds the time the response has been resident in the cache since it arrived.
A cache must not generate a stale response if an explicit directive forbids it, such as no-store, no-cache, must-revalidate, or an applicable s-maxage or proxy-revalidate. It must not send stale responses at all unless it is disconnected from the origin or the request explicitly allows it, for example with the max-stale directive. A cache should add the 110 warn-code to stale responses, and the 112 warn-code when it is disconnected.
Freshness is a simple comparison: how old is this copy, against how long is it good for. The server normally says how long, the cache works out how old, and if the numbers say the copy is too old the cache has to check with the server before handing it out.
§5.2The Directives That Control Caching
The Cache-Control header field carries directives for caches along the request and response chain. Such directives are unidirectional: a directive in a request does not imply the same directive in the response. A proxy, whether or not it implements a cache, must pass cache directives through in forwarded messages, and it is not possible to target a directive at a particular cache.
The request-side directives are max-age, which says the client will not accept a response older than the given number of seconds, and, unless max-stale is also present, will not accept a stale response at all; max-stale, which says the client will accept a response that has exceeded its freshness lifetime, by no more than the given number of seconds if a value is given or of any age if not; min-fresh, which says the client wants a response that will still be fresh for at least the given number of seconds; no-cache, which says the cache must not use a stored response without validating it at the origin; no-store, which says the cache must not store any part of the request or of the response, and which is explicitly not a reliable privacy mechanism; no-transform, which says an intermediary must not transform the payload; and only-if-cached, which says the client wants only a stored response, so the cache should answer from its store if it can and otherwise return a 504 Gateway Timeout.
The response-side directives are must-revalidate, which says a stale response cannot be reused without successful validation, and which a cache must obey in all circumstances, generating a 504 if it cannot reach the origin server at all; no-cache, which says the response cannot be used to satisfy a later request without validation, and which, when it lists field names, means those header fields must not be sent again without revalidation while the rest of the response may be; no-store; no-transform; public, which lets any cache store the response even where it would normally not be allowed to; private, which says the response is for a single user and must not be stored by a shared cache, or, when it lists field names, protects only those fields; proxy-revalidate, which means the same as must-revalidate but does not apply to private caches; max-age, which says the response is stale once its age exceeds the given seconds; and s-maxage, which in shared caches overrides both max-age and Expires and implies proxy-revalidate.
The Expires header field gives the date and time after which the response is considered stale. Its presence says nothing about whether the resource will change or stop existing. A cache recipient must read an invalid date, including the value 0, as a time in the past. If a response carries max-age, the Expires field must be ignored, and if it carries s-maxage, a shared cache must ignore it as well. The Pragma header field exists for backwards compatibility with HTTP/1.0 caches: when Cache-Control is present and understood, Pragma is ignored, and when it is absent, a no-cache pragma has the same effect as a no-cache directive. A cache may add extension directives, and must ignore any it does not recognise.
The Warning header field carries extra information about the status or transformation of a message that the status code does not reflect, typically to warn about possible incorrectness introduced by caching. Using a warning rather than an error code distinguishes these responses from true failures. Warn codes have three digits, and the first digit says whether the warning must be deleted from a stored response after validation: 1xx codes describe freshness or validation status and must be deleted, and can only be generated by a cache while validating; 2xx codes describe an aspect of the representation that validation does not fix and must not be deleted unless a full response is sent.
Both ends can state rules with Cache-Control. The client can say how old an answer it will accept; the server can say how long the answer may be kept, whether it may be kept by a shared cache, and whether it must be checked even when it looks usable. The older Pragma and Expires headers exist for the same job and for compatibility with HTTP/1.0.
- no-storeThe cache must not store any part of the request or the response. It is not a reliable privacy mechanism.
- privateThe response is for a single user and must not be stored by a shared cache. With field names, only those fields are protected.
- publicAny cache may store the response, even one that would normally not be allowed to.
- must-revalidateOnce stale, the response cannot be reused without checking with the origin server. A cache that cannot reach the origin must answer 504.
- max-ageThe response is stale once its age is greater than this many seconds.
- s-maxageIn shared caches this overrides max-age and Expires, and it implies proxy-revalidate.
- 110Response is Stale — A cache should generate this whenever the response it sends is stale.
- 111Revalidation Failed — A cache should generate this when sending a stale response because it could not reach the server to validate it.
- 112Disconnected Operation — A cache should generate this if it is intentionally disconnected from the rest of the network.
- 113Heuristic Expiration — A cache should generate this if it guessed a freshness lifetime over 24 hours and the response is older than that.
- 214Transformation Applied — A proxy must add this if it transforms the representation.
- 299Miscellaneous Persistent Warning — Arbitrary information for a person or a log. A receiving system must take no automated action.
§8Security Considerations
Caches expose additional potential vulnerabilities, because their contents are an attractive target and they persist after an HTTP request is complete. An attack on a cache can reveal information long after a user believes the information has left the network, so cache contents need to be protected as sensitive information.
Attacks may be amplified by being stored in a shared cache. Cache poisoning attacks use the cache to distribute a malicious payload to many clients, and are especially effective when an attacker can use implementation flaws, elevated privileges or other techniques to insert such a response into a cache. One common vector is to exploit differences in message parsing between proxies and user agents. Implementation flaws, or a misunderstanding of how caches operate, may also lead to sensitive information such as authentication credentials being cached although it was thought to be private.
The use of a cache brings privacy concerns of its own. If two users share a cache and the first browses to a site, the second may be able to detect that the first has been there, because the resources from it load more quickly. The Set-Cookie response header field does not inhibit caching: a cacheable response that carries a Set-Cookie field can be, and often is, used to satisfy later requests, so servers that want to control this are encouraged to send appropriate Cache-Control response header fields.
A shared cache is a place where answers from one user can be handed to another, so it needs care. It also makes a good target: one poisoned answer can reach many people at once, and it keeps a copy of things long after the user thinks they have gone.
§7References
[RFC7230]RFC 7230Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, June 2014.June 2014[RFC7231]RFC 7231Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.June 2014[RFC7232]RFC 7232Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests", RFC 7232, June 2014.June 2014[RFC7233]RFC 7233Wild, M. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Range Requests", RFC 7233, June 2014.June 2014[RFC7235]RFC 7235Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Header Fields", RFC 7235, June 2014.June 2014[RFC5234]RFC 5234Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.January 2008