HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Adobe
Obsoletes: 2616 (if approved) Y. Lafon, Ed.
Intended status: Standards Track W3C
Expires: September 13, 2012 M. Nottingham, Ed.
Rackspace
J. Reschke, Ed.
greenbytes
March 12, 2012
HTTP/1.1, part 6: Caching
draft-ietf-httpbis-p6-cache-19
Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypertext information
systems. HTTP has been in use by the World Wide Web global
information initiative since 1990. This document is Part 6 of the
seven-part specification that defines the protocol referred to as
"HTTP/1.1" and, taken together, obsoletes RFC 2616.
Part 6 defines requirements on HTTP caches and the associated header
fields that control cache behavior or indicate cacheable response
messages.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft should take place on the HTTPBIS working
group mailing list (ietf-http-wg@w3.org), which is archived at
.
The current issues list is at
and related
documents (including fancy diffs) can be found at
.
The changes in this draft are summarized in Appendix C.20.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 13, 2012.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Conformance and Error Handling . . . . . . . . . . . . . . 7
1.4. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 7
1.4.1. Core Rules . . . . . . . . . . . . . . . . . . . . . . 8
1.4.2. ABNF Rules defined in other Parts of the
Specification . . . . . . . . . . . . . . . . . . . . 8
1.5. Delta Seconds . . . . . . . . . . . . . . . . . . . . . . 8
2. Cache Operation . . . . . . . . . . . . . . . . . . . . . . . 8
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2.1. Response Cacheability . . . . . . . . . . . . . . . . . . 9
2.2. Constructing Responses from Caches . . . . . . . . . . . . 10
2.3. Freshness Model . . . . . . . . . . . . . . . . . . . . . 12
2.3.1. Calculating Freshness Lifetime . . . . . . . . . . . . 13
2.3.2. Calculating Age . . . . . . . . . . . . . . . . . . . 14
2.3.3. Serving Stale Responses . . . . . . . . . . . . . . . 16
2.4. Validation Model . . . . . . . . . . . . . . . . . . . . . 16
2.4.1. Freshening Responses with 304 Not Modified . . . . . . 17
2.5. Updating Caches with HEAD Responses . . . . . . . . . . . 18
2.6. Request Methods that Invalidate . . . . . . . . . . . . . 18
2.7. Shared Caching of Authenticated Responses . . . . . . . . 19
2.8. Caching Negotiated Responses . . . . . . . . . . . . . . . 19
2.9. Combining Partial Content . . . . . . . . . . . . . . . . 20
3. Header Field Definitions . . . . . . . . . . . . . . . . . . . 21
3.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 21
3.2.1. Request Cache-Control Directives . . . . . . . . . . . 22
3.2.2. Response Cache-Control Directives . . . . . . . . . . 24
3.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 26
3.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.6. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.6.1. 110 Response is Stale . . . . . . . . . . . . . . . . 31
3.6.2. 111 Revalidation Failed . . . . . . . . . . . . . . . 31
3.6.3. 112 Disconnected Operation . . . . . . . . . . . . . . 31
3.6.4. 113 Heuristic Expiration . . . . . . . . . . . . . . . 32
3.6.5. 199 Miscellaneous Warning . . . . . . . . . . . . . . 32
3.6.6. 214 Transformation Applied . . . . . . . . . . . . . . 32
3.6.7. 299 Miscellaneous Persistent Warning . . . . . . . . . 32
3.6.8. Warn Code Extensions . . . . . . . . . . . . . . . . . 32
4. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 32
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
5.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 33
5.2. Warn Code Registry . . . . . . . . . . . . . . . . . . . . 33
5.3. Header Field Registration . . . . . . . . . . . . . . . . 34
6. Security Considerations . . . . . . . . . . . . . . . . . . . 34
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
8.1. Normative References . . . . . . . . . . . . . . . . . . . 35
8.2. Informative References . . . . . . . . . . . . . . . . . . 35
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 36
Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 36
Appendix C. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 37
C.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 38
C.2. Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 38
C.3. Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 38
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C.4. Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 39
C.5. Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 39
C.6. Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 39
C.7. Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 39
C.8. Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 40
C.9. Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 40
C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 40
C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 41
C.12. Since draft-ietf-httpbis-p6-cache-10 . . . . . . . . . . . 41
C.13. Since draft-ietf-httpbis-p6-cache-11 . . . . . . . . . . . 42
C.14. Since draft-ietf-httpbis-p6-cache-12 . . . . . . . . . . . 42
C.15. Since draft-ietf-httpbis-p6-cache-13 . . . . . . . . . . . 42
C.16. Since draft-ietf-httpbis-p6-cache-14 . . . . . . . . . . . 42
C.17. Since draft-ietf-httpbis-p6-cache-15 . . . . . . . . . . . 43
C.18. Since draft-ietf-httpbis-p6-cache-16 . . . . . . . . . . . 43
C.19. Since draft-ietf-httpbis-p6-cache-17 . . . . . . . . . . . 43
C.20. Since draft-ietf-httpbis-p6-cache-18 . . . . . . . . . . . 43
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
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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.
1.1. Purpose
An HTTP cache is a local store of response messages and the subsystem
that controls its message storage, retrieval, and deletion. 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.
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 2.3, 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 cache response can
therefore reduce both latency and network transfers 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 2.4) or if the
origin is unavailable.
1.2. Terminology
This specification uses a number of terms to refer to the roles
played by participants in, and objects of, HTTP caching.
cache
A conformant implementation of a HTTP cache. Note that this
implies an HTTP/1.1 cache; this specification does not define
conformance for HTTP/1.0 caches.
shared cache
A cache that stores responses to be reused by more than one user;
usually (but not always) deployed as part of an intermediary.
private cache
A cache that is dedicated to a single user.
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cacheable
A response is cacheable if a cache is allowed to store a copy of
the response message for use in answering subsequent requests.
Even when a response is cacheable, there might be additional
constraints on whether a cache can use the stored copy to satisfy
a particular request.
explicit expiration time
The time at which the origin server intends that a representation
no longer be returned by a cache without further validation.
heuristic expiration time
An expiration time assigned by a cache when no explicit expiration
time is available.
age
The age of a response is the time since it was sent by, or
successfully validated with, the origin server.
first-hand
A response is first-hand if the freshness model is not in use;
i.e., its age is 0.
freshness lifetime
The length of time between the generation of a response and its
expiration time.
fresh
A response is fresh if its age has not yet exceeded its freshness
lifetime.
stale
A response is stale if its age has passed its freshness lifetime
(either explicit or heuristic).
validator
A protocol element (e.g., an entity-tag or a Last-Modified time)
that is used to find out whether a stored response is an
equivalent copy of a representation. See Section 2.1 of [Part4].
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strong validator
A validator that is defined by the origin server such that its
current value will change if the representation body changes;
i.e., an entity-tag that is not marked as weak (Section 2.3 of
[Part4]) or, if no entity-tag is provided, a Last-Modified value
that is strong in the sense defined by Section 2.2.2 of [Part4].
1.3. 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].
This document defines conformance criteria for several roles in HTTP
communication, including Senders, Recipients, Clients, Servers, User-
Agents, Origin Servers, Intermediaries, Proxies and Gateways. See
Section 2 of [Part1] for definitions of these terms.
An implementation is considered conformant if it complies with all of
the requirements associated with its role(s). Note that SHOULD-level
requirements are relevant here, unless one of the documented
exceptions is applicable.
This document also uses ABNF to define valid protocol elements
(Section 1.4). In addition to the prose requirements placed upon
them, Senders MUST NOT generate protocol elements that are invalid.
Unless noted otherwise, Recipients MAY take steps to recover a usable
protocol element from an invalid construct. However, HTTP does not
define specific error handling mechanisms, except in cases where it
has direct impact on security. This is because different uses of the
protocol require different error handling strategies; for example, a
Web browser may wish to transparently recover from a response where
the Location header field doesn't parse according to the ABNF,
whereby in a systems control protocol using HTTP, this type of error
recovery could lead to dangerous consequences.
1.4. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with the list rule extension defined in Section
1.2 of [Part1]. Appendix B shows the collected ABNF with the list
rule expanded.
The following core rules are included by reference, as defined in
[RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
(CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
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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).
1.4.1. Core Rules
The core rules below are defined in [Part1]:
OWS =
quoted-string =
token =
1.4.2. ABNF Rules defined in other Parts of the Specification
The ABNF rules below are defined in other parts:
field-name =
HTTP-date =
port =
pseudonym =
uri-host =
1.5. Delta Seconds
The delta-seconds rule specifies a non-negative integer, representing
time in seconds.
delta-seconds = 1*DIGIT
If an implementation receives a delta-seconds value larger than the
largest positive integer it can represent, or if any of its
subsequent calculations overflows, it MUST consider the value to be
2147483648 (2^31). Recipients parsing a delta-seconds value MUST use
an arithmetic type of at least 31 bits of range, and senders MUST NOT
send delta-seconds with a value greater than 2147483648.
2. Cache Operation
Proper cache operation preserves the semantics of HTTP transfers
([Part2]) while eliminating the transfer of information already held
in the cache. Although caching is an entirely OPTIONAL feature of
HTTP, we assume that reusing the cached response is desirable and
that such reuse is the default behavior when no requirement or
locally-desired 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.
Each cache entry consists of a cache key and one or more HTTP
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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 containing a
representation of the resource identified by the request target.
However, it is also possible to cache negative results (e.g., 404 not
found), incomplete results (e.g., 206 partial content), and responses
to safe methods other than GET if the method's definition allows such
caching and defines something suitable for use as a cache key.
The default 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, most implementations simply decline
other methods and use only the URI as the 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 2.8).
2.1. Response Cacheability
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 3.2) does not appear
in request or response header fields, and
o the "private" cache response directive (see Section 3.2.2) does
not appear in the response, if the cache is shared, and
o the "Authorization" header field (see Section 4.1 of [Part7]) does
not appear in the request, if the cache is shared, unless the
response explicitly allows it (see Section 2.7), and
o the response either:
* contains an Expires header field (see Section 3.3), or
* contains a max-age response cache directive (see
Section 3.2.2), or
* contains a s-maxage response cache directive and the cache is
shared, or
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* contains a Cache Control Extension (see Section 3.2.3) that
allows it to be cached, or
* has a status code that can be served with heuristic freshness
(see Section 2.3.1.1).
Note that any of the requirements listed above can be overridden by a
cache-control extension; see Section 3.2.3.
In this context, a cache has "understood" a request method or a
response status code if it recognizes it and implements any cache-
specific behavior.
Note that, in normal operation, most caches will not store a response
that has neither a cache validator nor an explicit expiration time,
as such responses are not usually useful to store. However, caches
are not prohibited from storing such responses.
A response message is considered complete when all of the octets
indicated by the message framing ([Part1]) are received prior to the
connection being closed. If the request is GET, the response status
is 200 (OK), and the entire response header block 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 ([Part5]) and combining the successful
response with the stored entry, as defined in Section 2.9. 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.
2.2. Constructing Responses from Caches
For a presented request, a cache MUST NOT return a stored response,
unless:
o The presented effective request URI (Section 5.5 of [Part1]) and
that of the stored response match, and
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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 2.8), and
o the presented request does not contain the no-cache pragma
(Section 3.4), nor the no-cache cache directive (Section 3.2.1),
unless the stored response is successfully validated
(Section 2.4), and
o the stored response does not contain the no-cache cache directive
(Section 3.2.2), unless it is successfully validated
(Section 2.4), and
o the stored response is either:
* fresh (see Section 2.3), or
* allowed to be served stale (see Section 2.3.3), or
* successfully validated (see Section 2.4).
Note that any of the requirements listed above can be overridden by a
cache-control extension; see Section 3.2.3.
When a stored response is used to satisfy a request without
validation, a cache MUST include a single Age header field
(Section 3.1) in the response with a value equal to the stored
response's current_age; see Section 2.3.2.
A cache MUST write through requests with methods that are unsafe
(Section 6.1.1 of [Part2]) to the origin server; i.e., a cache must
not 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 2.6.
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 a 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 on every use. A cache,
especially a shared cache, SHOULD use a mechanism, such as NTP
[RFC1305], to synchronize its clock with a reliable external
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standard.
2.3. Freshness Model
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 3.3) or the max-age response
cache directive (Section 3.2.2). 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 the cached response before reusing it for
subsequent requests (see Section 2.3.3).
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.
The calculation to determine if a response is fresh is:
response_is_fresh = (freshness_lifetime > current_age)
The freshness_lifetime is defined in Section 2.3.1; the current_age
is defined in Section 2.3.2.
Additionally, clients can influence freshness calculation -- either
constraining it relaxing it -- by using the max-age and min-fresh
request cache directives. See Section 3.2.1 for details.
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 4 for an explanation of the difference between
caches and history mechanisms.
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2.3.1. Calculating Freshness Lifetime
A cache can calculate the freshness lifetime (denoted as
freshness_lifetime) of a response by using the first match of:
o If the cache is shared and the s-maxage response cache directive
(Section 3.2.2) is present, use its value, or
o If the max-age response cache directive (Section 3.2.2) is
present, use its value, or
o If the Expires response header field (Section 3.3) 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 2.3.1.1.
Note that this calculation is not vulnerable to clock skew, since all
of the information comes from the origin server.
2.3.1.1. Calculating Heuristic Freshness
If no explicit expiration time is present in a stored response that
has a status code whose definition allows heuristic freshness to be
used (including the following in Section 7 of [Part2]: 200, 203, 206,
300, 301 and 410), a cache MAY calculate a heuristic expiration time.
A cache MUST NOT use heuristics to determine freshness for responses
with status codes that do not explicitly allow it.
When a heuristic is used to calculate freshness lifetime, a cache
SHOULD attach a Warning header field with a 113 warn-code to the
response if its current_age is more than 24 hours and such a warning
is not already present.
Also, if the response has a Last-Modified header field (Section 2.2
of [Part4]), caches are encouraged to use a heuristic expiration
value that is no more than some fraction of the interval since that
time. A typical setting of this fraction might be 10%.
Note: RFC 2616 ([RFC2616], Section 13.9) required that caches do
not calculate heuristic freshness for URIs with query components
(i.e., those containing '?'). In practice, this has not been
widely implemented. Therefore, servers are encouraged to send
explicit directives (e.g., Cache-Control: no-cache) if they wish
to preclude caching.
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2.3.2. Calculating Age
HTTP/1.1 uses the Age header field to convey the estimated age of the
response message when obtained from a cache. The Age field value is
the cache's estimate of the amount of time 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 3.1), in a form appropriate for arithmetic operation; or
0, if not available.
date_value
HTTP/1.1 requires origin servers to send a Date header field, if
possible, with every response, giving the time at which the
response was generated. The term "date_value" denotes the value
of the Date header field, in a form appropriate for arithmetic
operations. See Section 10.2 of [Part2] 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 cache SHOULD use NTP ([RFC1305])
or some similar protocol to synchronize its clocks to a globally
accurate time standard.
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:
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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 SHOULD be combined as
corrected_initial_age = max(apparent_age, corrected_age_value);
unless the cache is confident in the value of the Age header (e.g.,
because there are no HTTP/1.0 hops in the Via header), 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;
Additionally, to avoid common problems in date parsing:
o HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date
which appears to be more than 50 years in the future is in fact in
the past (this helps solve the "year 2000" problem).
o Although all date formats are specified to be case-sensitive,
recipients SHOULD match day, week and timezone names case-
insensitively.
o An HTTP/1.1 implementation MAY internally represent a parsed
Expires date as earlier than the proper value, but MUST NOT
internally represent a parsed Expires date as later than the
proper value.
o All expiration-related calculations MUST be done in GMT. The
local time zone MUST NOT influence the calculation or comparison
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of an age or expiration time.
o If an HTTP header field incorrectly carries a date value with a
time zone other than GMT, it MUST be converted into GMT using the
most conservative possible conversion.
2.3.3. 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 2.3.
A cache MUST NOT return 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 3.2.2).
A cache MUST NOT return 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 3.2.1).
A cache SHOULD append a Warning header field with the 110 warn-code
(see Section 3.6) to stale responses. Likewise, a cache SHOULD add
the 112 warn-code to stale responses if the cache is disconnected.
If a cache receives a first-hand response (either an entire response,
or a 304 (Not Modified) response) that it would normally forward to
the requesting client, and the received response is no longer fresh,
the cache can forward it to the requesting client without adding a
new Warning (but without removing any existing Warning header
fields). A cache shouldn't attempt to validate a response simply
because that response became stale in transit.
2.4. Validation Model
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 2.8), it can use the conditional
request mechanism [Part4] in the forwarded request to give the origin
server an opportunity to both select a valid stored response to be
used, and to update it. This process is known as "validating" or
"revalidating" the stored response.
When sending such a conditional request, a cache adds an If-Modified-
Since header field whose value is that of the Last-Modified header
field from the selected (see Section 2.8) stored response, if
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available.
Additionally, a cache can add an If-None-Match header field whose
value is that of the ETag header field(s) from all responses stored
for the requested URI, if present. However, if any of the stored
responses contains only partial content, the cache shouldn't include
its entity-tag in the If-None-Match header field unless the request
is for a range that would be fully satisfied by that stored 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 2.4.1.
o A full response (i.e., one with a response body) indicates that
none of the stored responses nominated in the conditional request
is suitable. Instead, the cache can use the full response to
satisfy the request and MAY replace the stored response(s).
o However, if a cache receives a 5xx 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, it can return a previously stored response (see
Section 2.3.3).
2.4.1. Freshening Responses with 304 Not Modified
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.
o If the new response contains a strong validator, then that strong
validator identifies the selected representation. All of the
stored responses with the same strong validator are selected. If
none of the stored responses contain the same strong validator,
then this new response corresponds to a new selected
representation and MUST NOT update the existing 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.
o If the new response does not include any form of validator, there
is only one stored response, and that stored response also lacks a
validator, then that stored response is selected.
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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 3.6);
o retain any Warning header fields in the stored response with warn-
code 2xx; and,
o use other header fields provided in the 304 response to replace
all instances of the corresponding header fields in the stored
response.
2.5. Updating Caches with HEAD Responses
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 is used to both invalidate and update
cached GET responses.
If one or more stored GET responses can be selected (as per
Section 2.8) for a HEAD request, and the Content-Length, ETag or
Last-Modified value of a HEAD response differs from that in a
selected GET response, the cache MUST consider that selected response
to be stale.
If the Content-Length, ETag and Last-Modified values of a HEAD
response (when present) are the same as that in a selected GET
response (as per Section 2.8), the cache SHOULD update the remaining
headers in the stored response using the following rules:
o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 3.6);
o retain any Warning header fields in the stored response with warn-
code 2xx; and,
o use other header fields provided in the response to replace all
instances of the corresponding header fields in the stored
response.
2.6. Request Methods that Invalidate
Because unsafe request methods (Section 6.1.1 of [Part2]) 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
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[Part1]) as well as the URI(s) in the Location and Content-Location
response header fields (if present) when a non-error response to a
request with an unsafe method is received.
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 [Part1]). This helps prevent denial of service attacks.
A cache MUST invalidate the effective request URI (Section 5.5 of
[Part1]) 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 or 3xx 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
returned in response to a subsequent request.
Note that this does not guarantee that all appropriate responses are
invalidated. For example, the request that caused the change at the
origin server might not have gone through the cache where a response
is stored.
2.7. Shared Caching of Authenticated Responses
A shared cache MUST NOT use a cached response to a request with an
Authorization header field (Section 4.1 of [Part7]) 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 3.2.2) have such an effect: must-revalidate,
public, 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 2.3.3) 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.
2.8. Caching Negotiated Responses
When a cache receives a request that can be satisfied by a stored
response that has a Vary header field (Section 3.5), 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
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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 [Part1])
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., re-ordering 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, and
subsequent requests to that resource can only be properly interpreted
by the origin server.
The stored response with matching selecting header fields is known as
the selected response.
If multiple selected responses are available, the most recent
response (as determined by the Date header field) is used; see
Section 2.2.
If no selected response is available, the cache can forward the
presented request to the origin server in a conditional request; see
Section 2.4.
2.9. 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 ([Part5]). 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 4.2 of [Part5].
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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 3.6);
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.
3. Header Field Definitions
This section defines the syntax and semantics of HTTP/1.1 header
fields related to caching.
3.1. 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 2.3.2.
Age = delta-seconds
Age field-values are non-negative integers, representing time in
seconds (see Section 1.5).
The presence of an Age header field in a response implies that a
response is not first-hand. However, the converse is not true, since
HTTP/1.0 caches might not implement the Age header field.
3.2. 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 3.2.3 for information about how
Cache-Control directives defined elsewhere are handled.
Note: HTTP/1.0 caches might not implement Cache-Control and might
only implement Pragma: no-cache (see Section 3.4).
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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.
Cache-Control = 1#cache-directive
cache-directive = cache-request-directive
/ cache-response-directive
cache-extension = token [ "=" ( token / quoted-string ) ]
3.2.1. Request Cache-Control Directives
cache-request-directive =
"no-cache"
/ "no-store"
/ "max-age" "=" delta-seconds
/ "max-stale" [ "=" delta-seconds ]
/ "min-fresh" "=" delta-seconds
/ "no-transform"
/ "only-if-cached"
/ cache-extension
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.
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.
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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.
max-age
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.
max-stale
The max-stale request directive indicates that the client is
willing to accept a response that has exceeded its expiration
time. If max-stale is assigned a value, then the client is
willing to accept a response that has exceeded its expiration time
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.
min-fresh
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.
no-transform
The no-transform request directive indicates that an intermediary
(whether or not it implements a cache) MUST NOT change the
Content-Encoding, Content-Range or Content-Type request header
fields, nor the request representation.
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.
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3.2.2. Response Cache-Control Directives
cache-response-directive =
"public"
/ "private" [ "=" DQUOTE 1#field-name DQUOTE ]
/ "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
/ "no-store"
/ "no-transform"
/ "must-revalidate"
/ "proxy-revalidate"
/ "max-age" "=" delta-seconds
/ "s-maxage" "=" delta-seconds
/ cache-extension
public
The public response directive indicates that a response whose
associated request contains an 'Authentication' header MAY be
stored (see Section 2.7).
private
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.
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.
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 implementations as if an unqualified
private directive was received; i.e., the special handling for the
qualified form is not widely implemented.
no-cache
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 return
stale responses.
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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.
Note: Most HTTP/1.0 caches will not recognize or obey this
directive. Also, no-cache response directives with field-names
are often handled by implementations as if an unqualified no-cache
directive was received; i.e., the special handling for the
qualified form is not widely implemented.
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.
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.
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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.
max-age
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.
s-maxage
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.
no-transform
The no-transform response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT change the
Content-Encoding, Content-Range or Content-Type response header
fields, nor the response representation.
3.2.3. 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.
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 standard directive are supplied, such that
applications that do not understand the new directive will default to
the behavior specified by the standard directive, and those that
understand the new directive will recognize it as modifying the
requirements associated with the standard directive. In this way,
extensions to the cache-control directives can be made without
requiring changes to the base protocol.
This extension mechanism depends on an HTTP cache obeying all of the
cache-control directives defined for its native HTTP-version, obeying
certain extensions, and ignoring all directives that it does not
understand.
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For example, consider a hypothetical new response directive called
"community" that acts as a modifier to the private directive. We
define this new directive to mean that, in addition to any private
cache, any cache that is shared only by members of the community
named within its value may 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 seeing this header field will act correctly even if the cache
does not understand the community cache-extension, since it will also
see and understand the private directive and thus default to the safe
behavior.
A cache MUST ignore unrecognized cache directives; it is assumed that
any cache directive likely to be unrecognized by an HTTP/1.1 cache
will be combined with standard directives (or the response's default
cacheability) such that the cache behavior will remain minimally
correct even if the cache does not understand the extension(s).
The HTTP Cache Directive Registry defines the name space for the
cache directives.
A registration MUST include the following fields:
o Cache Directive Name
o Pointer to specification text
Values to be added to this name space require IETF Review (see
[RFC5226], Section 4.1).
The registry itself is maintained at
.
3.3. Expires
The "Expires" header field gives the date/time after which the
response is considered stale. See Section 2.3 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 field-value is an absolute date and time as defined by HTTP-date
in Section 8 of [Part2]; a sender MUST use the rfc1123-date format.
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Expires = HTTP-date
For example
Expires: Thu, 01 Dec 1994 16:00:00 GMT
A cache MUST treat other invalid date formats, especially including
the value "0", as in the past (i.e., "already expired").
Note: If a response includes a Cache-Control field with the max-
age directive (see Section 3.2.2), that directive overrides the
Expires field. Likewise, the s-maxage directive overrides Expires
in shared caches.
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 most caches will evict a response far sooner
than that. Therefore, senders ought not produce them.
An origin server without a clock MUST NOT assign Expires values to a
response unless these values were associated with the resource by a
system or user with a reliable clock. It MAY assign an Expires value
that is known, at or before server configuration time, to be in the
past (this allows "pre-expiration" of responses without storing
separate Expires values for each resource).
3.4. 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 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 is not present in a request, the no-
cache request pragma-directive MUST have the same effect on caches as
if "Cache-Control: no-cache" were present (see Section 3.2.1).
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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.
3.5. Vary
The "Vary" header field conveys the set of header fields that were
used to select the representation.
Caches use this information, in part, to determine whether a stored
response can be used to satisfy a given request; see Section 2.8.
determines, while the response is fresh, whether a cache is permitted
to use the response to reply to a subsequent request without
validation; see Section 2.8.
In uncacheable or stale responses, the Vary field value advises the
user agent about the criteria that were used to select the
representation.
Vary = "*" / 1#field-name
The set of header fields named by the Vary field value is known as
the selecting header fields.
A server SHOULD include a Vary header field with any cacheable
response that is subject to server-driven negotiation. Doing so
allows a cache to properly interpret future requests on that resource
and informs the user agent about the presence of negotiation on that
resource. A server MAY include a Vary header field with a non-
cacheable response that is subject to server-driven negotiation,
since this might provide the user agent with useful information about
the dimensions over which the response varies at the time of the
response.
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A Vary field value of "*" signals that unspecified parameters not
limited to the header fields (e.g., the network address of the
client), play a role in the selection of the response representation;
therefore, a cache cannot determine whether this response is
appropriate. A proxy MUST NOT generate the "*" value.
The field-names given are not limited to the set of standard header
fields defined by this specification. Field names are case-
insensitive.
3.6. 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 message. 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
warn-text = quoted-string
warn-date = DQUOTE HTTP-date DQUOTE
Multiple warnings can be attached to a response (either by the origin
server or by a cache), including multiple warnings with the same code
number, only differing in warn-text.
When this occurs, the user agent SHOULD inform the user of as many of
them as possible, in the order that they appear in the response.
Systems that generate multiple Warning header fields are encouraged
to order them with this user agent behavior in mind. New Warning
header fields are added after any existing Warning headers fields.
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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 Warnings describe the freshness or validation status of the
response, and so 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 Warnings describe some aspect of the representation that is
not rectified by a validation (for example, a lossy compression of
the representation) and MUST NOT be deleted by a cache after
validation, unless a full response is returned, in which case they
MUST be.
If an implementation sends a message with one or more Warning header
fields to a receiver whose version is HTTP/1.0 or lower, then the
sender MUST include in each warning-value a warn-date that matches
the Date header field in the message.
If a system receives a message with a warning-value that includes a
warn-date, and that warn-date is different from the Date value in the
response, then that warning-value MUST be deleted from the message
before storing, forwarding, or using it. (preventing the consequences
of naive caching of Warning header fields.) If all of the warning-
values are deleted for this reason, the Warning header field MUST be
deleted 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.
3.6.1. 110 Response is Stale
A cache SHOULD include this whenever the returned response is stale.
3.6.2. 111 Revalidation Failed
A cache SHOULD include this when returning a stale response because
an attempt to validate the response failed, due to an inability to
reach the server.
3.6.3. 112 Disconnected Operation
A cache SHOULD include this if it is intentionally disconnected from
the rest of the network for a period of time.
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3.6.4. 113 Heuristic Expiration
A cache SHOULD include this if it heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater than
24 hours.
3.6.5. 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.
3.6.6. 214 Transformation Applied
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.
3.6.7. 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.
3.6.8. Warn Code Extensions
The HTTP Warn Code Registry defines the name space for warn codes.
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 name space require IETF Review (see
[RFC5226], Section 4.1).
The registry itself is maintained at
.
4. History Lists
User agents often have history mechanisms, such as "Back" buttons and
history lists, that can be used to redisplay a representation
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retrieved earlier in a session.
The freshness model (Section 2.3) does not necessarily apply to
history mechanisms. I.e., 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).
5. IANA Considerations
5.1. Cache Directive Registry
The registration procedure for HTTP Cache Directives is defined by
Section 3.2.3 of this document.
The HTTP Cache Directive Registry shall be created at
and be
populated with the registrations below:
+------------------------+------------------------------+
| Cache Directive | Reference |
+------------------------+------------------------------+
| max-age | Section 3.2.1, Section 3.2.2 |
| max-stale | Section 3.2.1 |
| min-fresh | Section 3.2.1 |
| must-revalidate | Section 3.2.2 |
| no-cache | Section 3.2.1, Section 3.2.2 |
| no-store | Section 3.2.1, Section 3.2.2 |
| no-transform | Section 3.2.1, Section 3.2.2 |
| only-if-cached | Section 3.2.1 |
| private | Section 3.2.2 |
| proxy-revalidate | Section 3.2.2 |
| public | Section 3.2.2 |
| s-maxage | Section 3.2.2 |
| stale-if-error | [RFC5861], Section 4 |
| stale-while-revalidate | [RFC5861], Section 3 |
+------------------------+------------------------------+
5.2. Warn Code Registry
The registration procedure for HTTP Warn Codes is defined by
Section 3.6.8 of this document.
The HTTP Warn Code Registry shall be created at
and be
populated with the registrations below:
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+-----------+----------------------------------+---------------+
| Warn Code | Short Description | Reference |
+-----------+----------------------------------+---------------+
| 110 | Response is Stale | Section 3.6.1 |
| 111 | Revalidation Failed | Section 3.6.2 |
| 112 | Disconnected Operation | Section 3.6.3 |
| 113 | Heuristic Expiration | Section 3.6.4 |
| 199 | Miscellaneous Warning | Section 3.6.5 |
| 214 | Transformation Applied | Section 3.6.6 |
| 299 | Miscellaneous Persistent Warning | Section 3.6.7 |
+-----------+----------------------------------+---------------+
5.3. Header Field Registration
The Message Header Field Registry located at shall be
updated with the permanent registrations below (see [RFC3864]):
+-------------------+----------+----------+-------------+
| Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+-------------+
| Age | http | standard | Section 3.1 |
| Cache-Control | http | standard | Section 3.2 |
| Expires | http | standard | Section 3.3 |
| Pragma | http | standard | Section 3.4 |
| Vary | http | standard | Section 3.5 |
| Warning | http | standard | Section 3.6 |
+-------------------+----------+----------+-------------+
The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force".
6. Security Considerations
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.
7. Acknowledgments
See Section 9 of [Part1].
8. References
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8.1. Normative References
[Part1] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"HTTP/1.1, part 1: URIs, Connections, and Message
Parsing", draft-ietf-httpbis-p1-messaging-19 (work in
progress), March 2012.
[Part2] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"HTTP/1.1, part 2: Message Semantics",
draft-ietf-httpbis-p2-semantics-19 (work in progress),
March 2012.
[Part4] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"HTTP/1.1, part 4: Conditional Requests",
draft-ietf-httpbis-p4-conditional-19 (work in progress),
March 2012.
[Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"HTTP/1.1, part 5: Range Requests and Partial Responses",
draft-ietf-httpbis-p5-range-19 (work in progress),
March 2012.
[Part7] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"HTTP/1.1, part 7: Authentication",
draft-ietf-httpbis-p7-auth-19 (work in progress),
March 2012.
[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.
8.2. Informative References
[RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation", RFC 1305, March 1992.
[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.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
Fielding, et al. Expires September 13, 2012 [Page 35]
Internet-Draft HTTP/1.1, Part 6 March 2012
May 2008.
[RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale
Content", RFC 5861, April 2010.
Appendix A. Changes from RFC 2616
Make the specified age calculation algorithm less conservative.
(Section 2.3.2)
Remove requirement to consider Content-Location in successful
responses in order to determine the appropriate response to use.
(Section 2.4)
Clarify denial of service attack avoidance requirement.
(Section 2.6)
Change ABNF productions for header fields to only define the field
value. (Section 3)
Do not mention RFC 2047 encoding and multiple languages in Warning
header fields anymore, as these aspects never were implemented.
(Section 3.6)
Appendix B. Collected ABNF
Age = delta-seconds
Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
cache-directive ] )
Expires = HTTP-date
HTTP-date =
OWS =
Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
pragma-directive ] )
Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
) )
Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
)
cache-directive = cache-request-directive / cache-response-directive
cache-extension = token [ "=" ( token / quoted-string ) ]
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cache-request-directive = "no-cache" / "no-store" / ( "max-age="
delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
"min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
cache-extension
cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
"no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
"must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
) / ( "s-maxage=" delta-seconds ) / cache-extension
delta-seconds = 1*DIGIT
extension-pragma = token [ "=" ( token / quoted-string ) ]
field-name =
port =
pragma-directive = "no-cache" / extension-pragma
pseudonym =
quoted-string =
token =
uri-host =
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
]
ABNF diagnostics:
; Age defined but not used
; Cache-Control defined but not used
; Expires defined but not used
; Pragma defined but not used
; Vary defined but not used
; Warning defined but not used
Appendix C. Change Log (to be removed by RFC Editor before publication)
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C.1. Since RFC 2616
Extracted relevant partitions from [RFC2616].
C.2. Since draft-ietf-httpbis-p6-cache-00
Closed issues:
o : "Trailer"
()
o : "Invalidation
after Update or Delete"
()
o : "Normative and
Informative references"
o : "Date reference
typo"
o : "Connection
header text"
o : "Informative
references"
o : "ISO-8859-1
Reference"
o : "Normative up-
to-date references"
o : "typo in
13.2.2"
Other changes:
o Use names of RFC4234 core rules DQUOTE and HTAB (work in progress
on )
C.3. Since draft-ietf-httpbis-p6-cache-01
Closed issues:
o : "rel_path not
used"
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Other changes:
o Get rid of duplicate BNF rule names ("host" -> "uri-host") (work
in progress on )
o Add explicit references to BNF syntax and rules imported from
other parts of the specification.
C.4. Since draft-ietf-httpbis-p6-cache-02
Ongoing work on IANA Message Header Field Registration
():
o Reference RFC 3984, and update header field registrations for
header fields defined in this document.
C.5. Since draft-ietf-httpbis-p6-cache-03
Closed issues:
o : "Vary header
classification"
C.6. Since draft-ietf-httpbis-p6-cache-04
Ongoing work on ABNF conversion
():
o Use "/" instead of "|" for alternatives.
o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
whitespace ("OWS") and required whitespace ("RWS").
o Rewrite ABNFs to spell out whitespace rules, factor out header
field value format definitions.
C.7. Since draft-ietf-httpbis-p6-cache-05
This is a total rewrite of this part of the specification.
Affected issues:
o : "Definition of
1xx Warn-Codes"
o : "Placement of
13.5.1 and 13.5.2"
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o : "The role of
Warning and Semantic Transparency in Caching"
o : "Methods and
Caching"
In addition: Final work on ABNF conversion
():
o Add appendix containing collected and expanded ABNF, reorganize
ABNF introduction.
C.8. Since draft-ietf-httpbis-p6-cache-06
Closed issues:
o : "base for
numeric protocol elements"
Affected issues:
o : "Vary and non-
existant headers"
C.9. Since draft-ietf-httpbis-p6-cache-07
Closed issues:
o : "Definition of
1xx Warn-Codes"
o : "Content-
Location on 304 responses"
o : "private and
no-cache CC directives with headers"
o : "RFC2047 and
warn-text"
C.10. Since draft-ietf-httpbis-p6-cache-08
Closed issues:
o : "serving
negotiated responses from cache: header-specific canonicalization"
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o : "Effect of CC
directives on history lists"
o : "Cache
Extensions can override no-store, etc."
Affected issues:
o : Status codes
and caching
Partly resolved issues:
o : "Placement of
13.5.1 and 13.5.2"
C.11. Since draft-ietf-httpbis-p6-cache-09
Closed issues:
o : "Age
calculation"
o : "Clarify
differences between / requirements for request and response CC
directives"
o : "Caching
authenticated responses"
o : "IANA registry
for cache-control directives"
o : "Heuristic
caching of URLs with query components"
Partly resolved issues:
o : "Term for the
requested resource's URI"
C.12. Since draft-ietf-httpbis-p6-cache-10
Closed issues:
o : "Clarify
entity / representation / variant terminology"
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o : "consider
removing the 'changes from 2068' sections"
o : "Allowing
heuristic caching for new status codes"
o Clean up TODOs and prose in "Combining Responses."
C.13. Since draft-ietf-httpbis-p6-cache-11
Closed issues:
o : "Text about
clock requirement for caches belongs in p6"
C.14. Since draft-ietf-httpbis-p6-cache-12
Closed issues:
o : "Header
Classification"
o : "Clarify
'public'"
C.15. Since draft-ietf-httpbis-p6-cache-13
Closed issues:
o : "untangle
ABNFs for header fields"
C.16. Since draft-ietf-httpbis-p6-cache-14
Closed issues:
o : "Mismatch Vary"
o : "Cache
Invalidation only happens upon successful responses"
o : "Recommend
minimum sizes for protocol elements"
o : "Proxies don't
'understand' methods"
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o : "Cache
Extensions can override no-store, etc."
o : "Pragma"
C.17. Since draft-ietf-httpbis-p6-cache-15
Closed issues:
o : "Motivate one-
year limit for Expires"
C.18. Since draft-ietf-httpbis-p6-cache-16
Closed issues:
o : "Document
HTTP's error-handling philosophy"
o : "Cache-Control
directive case sensitivity"
C.19. Since draft-ietf-httpbis-p6-cache-17
Closed issues:
o : "Interaction
of request and response Cache-Control"
o : "Refining age
for 1.1 proxy chains"
o : "warn-code
registry"
C.20. Since draft-ietf-httpbis-p6-cache-18
Closed issues:
o : "Combining
HEAD responses"
o : "Field names
in cache-control header arguments"
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Index
1
110 Response is Stale (warn code) 31
111 Revalidation Failed (warn code) 31
112 Disconnected Operation (warn code) 31
113 Heuristic Expiration (warn code) 32
199 Miscellaneous Warning (warn code) 32
2
214 Transformation Applied (warn code) 32
299 Miscellaneous Persistent Warning (warn code) 32
A
age 6
Age header field 21
C
cache 5
Cache Directives
max-age 23, 26
max-stale 23
min-fresh 23
must-revalidate 25
no-cache 22, 24
no-store 22, 25
no-transform 23, 26
only-if-cached 23
private 24
proxy-revalidate 26
public 24
s-maxage 26
cache entry 8
cache key 8
Cache-Control header field 21
cacheable 5
E
Expires header field 27
explicit expiration time 6
F
first-hand 6
fresh 6
freshness lifetime 6
G
Grammar
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Age 21
Cache-Control 22
cache-extension 22
cache-request-directive 22
cache-response-directive 24
delta-seconds 8
Expires 28
extension-pragma 28
Pragma 28
pragma-directive 28
Vary 29
warn-agent 30
warn-code 30
warn-date 30
warn-text 30
Warning 30
warning-value 30
H
Header Fields
Age 21
Cache-Control 21
Expires 27
Pragma 28
Vary 29
Warning 30
heuristic expiration time 6
M
max-age
Cache Directive 23, 26
max-stale
Cache Directive 23
min-fresh
Cache Directive 23
must-revalidate
Cache Directive 25
N
no-cache
Cache Directive 22, 24
no-store
Cache Directive 22, 25
no-transform
Cache Directive 23, 26
O
only-if-cached
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Cache Directive 23
P
Pragma header field 28
private
Cache Directive 24
private cache 5
proxy-revalidate
Cache Directive 26
public
Cache Directive 24
S
s-maxage
Cache Directive 26
shared cache 5
stale 6
strong validator 7
V
validator 6
strong 7
Vary header field 29
W
Warn Codes
110 Response is Stale 31
111 Revalidation Failed 31
112 Disconnected Operation 31
113 Heuristic Expiration 32
199 Miscellaneous Warning 32
214 Transformation Applied 32
299 Miscellaneous Persistent Warning 32
Warning header field 30
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/
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Yves Lafon (editor)
World Wide Web Consortium
W3C / ERCIM
2004, rte des Lucioles
Sophia-Antipolis, AM 06902
France
EMail: ylafon@w3.org
URI: http://www.raubacapeu.net/people/yves/
Mark Nottingham (editor)
Rackspace
EMail: mnot@mnot.net
URI: http://www.mnot.net/
Julian F. Reschke (editor)
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
Phone: +49 251 2807760
Fax: +49 251 2807761
EMail: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
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