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HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Day Software
Obsoletes: 2616 (if approved) J. Gettys
Intended status: Standards Track Alcatel-Lucent
Expires: February 5, 2011 J. Mogul
HP
H. Frystyk
Microsoft
L. Masinter
Adobe Systems
P. Leach
Microsoft
T. Berners-Lee
W3C/MIT
Y. Lafon, Ed.
W3C
M. Nottingham, Ed.
J. Reschke, Ed.
greenbytes
August 4, 2010
HTTP/1.1, part 6: Caching
draft-ietf-httpbis-p6-cache-11
Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypermedia information
systems. 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). The current issues list is
at <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
documents (including fancy diffs) can be found at
<http://tools.ietf.org/wg/httpbis/>.
The changes in this draft are summarized in Appendix C.12.
Status of This Memo
Fielding, et al. Expires February 5, 2011 [Page 1]
Internet-Draft HTTP/1.1, Part 6 August 2010
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-
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 February 5, 2011.
Copyright Notice
Copyright (c) 2010 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 . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
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1.4. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 7
1.4.1. Core Rules . . . . . . . . . . . . . . . . . . . . . . 7
1.4.2. ABNF Rules defined in other Parts of the
Specification . . . . . . . . . . . . . . . . . . . . 7
2. Cache Operation . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Response Cacheability . . . . . . . . . . . . . . . . . . 7
2.1.1. Storing Partial and Incomplete Responses . . . . . . . 8
2.2. Constructing Responses from Caches . . . . . . . . . . . . 9
2.3. Freshness Model . . . . . . . . . . . . . . . . . . . . . 10
2.3.1. Calculating Freshness Lifetime . . . . . . . . . . . . 11
2.3.2. Calculating Age . . . . . . . . . . . . . . . . . . . 12
2.3.3. Serving Stale Responses . . . . . . . . . . . . . . . 13
2.4. Validation Model . . . . . . . . . . . . . . . . . . . . . 14
2.5. Request Methods that Invalidate . . . . . . . . . . . . . 14
2.6. Shared Caching of Authenticated Responses . . . . . . . . 15
2.7. Caching Negotiated Responses . . . . . . . . . . . . . . . 16
2.8. Combining Responses . . . . . . . . . . . . . . . . . . . 16
3. Header Field Definitions . . . . . . . . . . . . . . . . . . . 17
3.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 18
3.2.1. Request Cache-Control Directives . . . . . . . . . . . 18
3.2.2. Response Cache-Control Directives . . . . . . . . . . 20
3.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 22
3.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.6. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 26
4. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 28
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
5.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 28
5.2. Header Field Registration . . . . . . . . . . . . . . . . 29
6. Security Considerations . . . . . . . . . . . . . . . . . . . 29
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.1. Normative References . . . . . . . . . . . . . . . . . . . 30
8.2. Informative References . . . . . . . . . . . . . . . . . . 31
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 31
Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 31
Appendix C. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 33
C.1. Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 33
C.2. Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 33
C.3. Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 34
C.4. Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 34
C.5. Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 34
C.6. Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 34
C.7. Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 35
C.8. Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 35
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C.9. Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 35
C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 36
C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 36
C.12. Since draft-ietf-httpbis-p6-cache-10 . . . . . . . . . . . 37
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
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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.
Caching would be useless if it did not significantly improve
performance. The goal of caching in HTTP/1.1 is to reuse a prior
response message to satisfy a current request. In some cases, a
stored response can be reused without the need for a network request,
reducing latency and network round-trips; a "freshness" mechanism is
used for this purpose (see Section 2.3). Even when a new request is
required, it is often possible to reuse all or parts of the payload
of a prior response to satisfy the request, thereby reducing network
bandwidth usage; a "validation" mechanism is used for this purpose
(see Section 2.4).
1.2. Terminology
This specification uses a number of terms to refer to the roles
played by participants in, and objects of, HTTP caching.
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 cached 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.
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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 has an
equivalent copy of a representation.
shared cache
A cache that is accessible to more than one user. A non-shared
cache is dedicated to a single user.
1.3. Requirements
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].
An implementation is not compliant if it fails to satisfy one or more
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of the "MUST" or "REQUIRED" level requirements for the protocols it
implements. An implementation that satisfies all the "MUST" or
"REQUIRED" level and all the "SHOULD" level requirements for its
protocols is said to be "unconditionally compliant"; one that
satisfies all the "MUST" level requirements but not all the "SHOULD"
level requirements for its protocols is said to be "conditionally
compliant".
1.4. Syntax Notation
This specification uses the ABNF syntax defined in Section 1.2 of
[Part1] (which extends the syntax defined in [RFC5234] with a list
rule). 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),
HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
sequence of data), SP (space), VCHAR (any visible USASCII character),
and WSP (whitespace).
1.4.1. Core Rules
The core rules below are defined in Section 1.2.2 of [Part1]:
quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
token = <token, defined in [Part1], Section 1.2.2>
OWS = <OWS, defined in [Part1], Section 1.2.2>
1.4.2. ABNF Rules defined in other Parts of the Specification
The ABNF rules below are defined in other parts:
field-name = <field-name, defined in [Part1], Section 3.2>
HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
port = <port, defined in [Part1], Section 2.6>
pseudonym = <pseudonym, defined in [Part1], Section 9.9>
uri-host = <uri-host, defined in [Part1], Section 2.6>
2. Cache Operation
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
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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 headers, 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 (see Section 3.1 of [Part7]) does not
appear in the request, if the cache is shared, unless the response
explicitly allows it (see Section 2.6), and
o the response either:
* contains an Expires header (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
* 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).
In this context, a cache has "understood" a request method or a
response status code if it recognises it and implements any cache-
specific behaviour. In particular, 206 Partial Content responses
cannot be cached by an implementation that does not handle partial
content (see Section 2.1.1).
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.
2.1.1. Storing Partial and Incomplete Responses
A cache that receives an incomplete response (for example, with fewer
bytes of data than specified in a Content-Length header) can store
the response, but MUST treat it as a partial response [Part5].
Partial responses can be combined as described in Section 4 of
[Part5]; the result might be a full response or might still be
partial. A cache MUST NOT return a partial response to a client
without explicitly marking it as such using the 206 (Partial Content)
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status code.
A cache that does not support the Range and Content-Range headers
MUST NOT store incomplete or partial responses.
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 4.3 of [Part1]) 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 request-headers nominated by the stored response (if
any) match those presented (see Section 2.7), and
o the presented request and stored response are free from directives
that would prevent its use (see Section 3.2 and Section 3.4), 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).
When a stored response is used to satisfy a request without
validation, caches 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.
Requests with methods that are unsafe (Section 7.1.1 of [Part2]) MUST
be written through the cache to the origin server; i.e., a cache must
not 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 2.5.
Caches MUST use the most recent response (as determined by the Date
header) when more than one suitable response is stored. They can
also forward a request with "Cache-Control: max-age=0" or "Cache-
Control: no-cache" to disambiguate which response to use.
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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 (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
validate the cached response before reusing it for subsequent
requests.
Since origin servers do not always provide explicit expiration times,
HTTP caches MAY assign heuristic expiration times when explicit times
are not specified, employing algorithms that use other header values
(such as the Last-Modified time) to estimate a plausible expiration
time. The HTTP/1.1 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 might need to influence freshness calculation.
They can do this using several request cache directives, with the
effect of either increasing or loosening constraints on freshness.
See Section 3.2.1.
[[ISSUE-no-req-for-directives: there are not requirements directly
applying to cache-request-directives and freshness.]]
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 (Section 3.3) is present, use its
value minus the value of the Date response header, 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 8 of [Part2]: 200, 203, 206,
300, 301 and 410), a heuristic expiration time MAY be calculated.
Heuristics MUST NOT be used for response status codes that do not
explicitly allow it.
When a heuristic is used to calculate freshness lifetime, the cache
SHOULD attach a Warning header 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 (Section 6.6 of
[Part4]), the heuristic expiration value SHOULD be 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 URLs 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 response-header 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
(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, 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, in a form appropriate for arithmetic
operations. See Section 9.3 of [Part1] for the definition of the
Date header, and for requirements regarding responses without a
Date response header.
now
The term "now" means "the current value of the clock at the host
performing the calculation". Hosts that use HTTP, but especially
hosts running origin servers and caches, SHOULD use NTP
([RFC1305]) or some similar protocol to synchronize their 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; note this value MUST be
interpreted 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);
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;
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.
Caches 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).
Caches SHOULD NOT return stale responses unless they are disconnected
(i.e., it cannot contact the origin server or otherwise find a
forward path) or otherwise explicitly allowed (e.g., the max-stale
request directive; see Section 3.2.1).
Stale responses SHOULD have a Warning header with the 110 warn-code
(see Section 3.6). Likewise, the 112 warn-code SHOULD be sent on
stale responses if the cache is disconnected.
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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 SHOULD forward it to the requesting client without adding a
new Warning (but without removing any existing Warning headers). A
cache SHOULD NOT 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 2.7), 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, the cache SHOULD add an If-
Modified-Since header whose value is that of the Last-Modified header
from the selected (see Section 2.7) stored response, if available.
Additionally, the cache SHOULD add an If-None-Match header whose
value is that of the ETag header(s) from all responses stored for the
requested URI, if present. However, if any of the stored responses
contains only partial content, its entity-tag SHOULD NOT be included
in the If-None-Match header field unless the request is for a range
that would be fully satisfied by that stored response.
A 304 (Not Modified) response status code indicates that the stored
response can be updated and reused; see Section 2.8.
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 full response SHOULD be used to satisfy the
request and MAY replace the stored response.
If a cache receives a 5xx response while attempting to validate a
response, it MAY either forward this response to the requesting
client, or act as if the server failed to respond. In the latter
case, it MAY return a previously stored response (see Section 2.3.3).
2.5. Request Methods that Invalidate
Because unsafe methods (Section 7.1.1 of [Part2]) have the potential
for changing state on the origin server, intervening caches can use
them to keep their contents up-to-date.
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The following HTTP methods MUST cause a cache to invalidate the
effective Request URI (Section 4.3 of [Part1]) as well as the URI(s)
in the Location and Content-Location headers (if present):
o PUT
o DELETE
o POST
An invalidation based on a URI from a Location or Content-Location
header MUST NOT be performed if the host part of that URI differs
from the host part in the effective request URI (Section 4.3 of
[Part1]). This helps prevent denial of service attacks.
A cache that passes through requests for methods it does not
understand SHOULD invalidate the effective request URI (Section 4.3
of [Part1]).
Here, "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.
2.6. Shared Caching of Authenticated Responses
Shared caches MUST NOT use a cached response to a request with an
Authorization header (Section 3.1 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.
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2.7. 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 request-headers nominated
by the Vary header match in both the original request (i.e., that
associated with the stored response), and the presented request.
The selecting request-headers 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's
syntax
o combining multiple message-header fields with the same field name
(see Section 3.2 of [Part1])
o normalizing both header values in a way that is known to have
identical semantics, according to the header'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 request-headers is known
as the selected response.
If no selected response is available, the cache MAY forward the
presented request to the origin server in a conditional request; see
Section 2.4.
2.8. Combining Responses
When a cache receives a 304 (Not Modified) response or a 206 (Partial
Content) response (in this section, the "new" response"), it needs to
created an updated response by combining the stored response with the
new one, so that the updated response can be used to satisfy the
request, and potentially update the cached response.
If the new response contains an ETag, it identifies the stored
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response to use. [[TODO-mention-CL: might need language about
Content-Location here]][[TODO-select-for-combine: Shouldn't this be
the selected response?]]
If the new response's status code is 206 (partial content), both the
stored and new responses MUST have validators, and those validators
MUST match using the strong comparison function (see Section 4 of
[Part4]). Otherwise, the responses MUST NOT be combined.
The stored response headers are used as those of the updated
response, except that
o any stored Warning headers with warn-code 1xx (see Section 3.6)
MUST be deleted.
o any stored Warning headers with warn-code 2xx MUST be retained.
o any other headers provided in the new response MUST replace all
instances of the corresponding headers from the stored response.
The updated response headers MUST be used to replace those of the
stored response in cache (unless the stored response is removed from
cache). In the case of a 206 response, the combined representation
MAY be stored.
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" response-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 = "Age" ":" OWS Age-v
Age-v = delta-seconds
Age field-values are non-negative integers, representing time in
seconds.
delta-seconds = 1*DIGIT
If a cache receives a value larger than the largest positive integer
it can represent, or if any of its age calculations overflows, it
MUST transmit an Age header with a field-value of 2147483648 (2^31).
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Caches SHOULD use an arithmetic type of at least 31 bits of range.
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" general-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.
HTTP/1.1 caches 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).
Cache directives MUST be passed through by a proxy or gateway
application, 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-Control = "Cache-Control" ":" OWS Cache-Control-v
Cache-Control-v = 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
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no-cache
The no-cache request directive indicates that a stored response
MUST NOT be used 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 non-shared 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.
max-age
The max-age request directive indicates that the client is willing
to accept a response whose age is no greater than the specified
time in 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.
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no-transform
The no-transform request directive indicates that an intermediate
cache or proxy MUST NOT change the Content-Encoding, Content-Range
or Content-Type request headers, nor the request representation.
only-if-cached
The only-if-cached request directive indicates that the client
only wishes to return 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, such a request MAY be forwarded within that group of
caches.
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 the response MAY be
cached, even if it would normally be non-cacheable or cacheable
only within a non-shared cache. (See also Authorization, Section
3.1 of [Part7], for additional details.)
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 (non-shared) 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 headers. That is, the specified field-
names(s) MUST NOT be stored by a shared cache, whereas the
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remainder of the response message MAY be.
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.
If the no-cache response directive specifies one or more field-
names, this requirement is limited to the field-values associated
with the listed response headers. That is, the specified field-
name(s) MUST NOT be sent in the response to a subsequent request
without successful validation on 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 non-shared 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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must-revalidate
The must-revalidate response directive indicates that once it has
become stale, the response MUST NOT be used 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 an
HTTP/1.1 cache MUST obey the must-revalidate directive; in
particular, if the cache cannot reach the origin server for any
reason, it MUST generate a 504 (Gateway Timeout) response.
Servers SHOULD send the must-revalidate directive if and only if
failure to validate a request on the representation could result
in incorrect operation, such as a silently unexecuted financial
transaction.
proxy-revalidate
The proxy-revalidate response directive has the same meaning as
the must-revalidate response directive, except that it does not
apply to non-shared caches.
max-age
The max-age response directive indicates that 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. The s-maxage directive also implies the semantics of the
proxy-revalidate response directive.
no-transform
The no-transform response directive indicates that an intermediate
cache or proxy MUST NOT change the Content-Encoding, Content-Range
or Content-Type response headers, 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
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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.
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 non-shared
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.
Unrecognized cache directives MUST be ignored; 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.
Registrations MUST include the following fields:
o Cache Directive Name
o Pointer to specification text
Values to be added to this name space are subject to IETF review
([RFC5226], Section 4.1).
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The registry itself is maintained at
<http://www.iana.org/assignments/http-cache-directives>.
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 6.1 of [Part1]; it MUST be sent in rfc1123-date format.
Expires = "Expires" ":" OWS Expires-v
Expires-v = HTTP-date
For example
Expires: Thu, 01 Dec 1994 16:00:00 GMT
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.
HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in
the future.
HTTP/1.1 clients and caches MUST treat other invalid date formats,
especially including the value "0", as in the past (i.e., "already
expired").
3.4. Pragma
The "Pragma" general-header field is used to include implementation-
specific directives that might apply to any recipient along the
request/response chain. All pragma directives specify optional
behavior from the viewpoint of the protocol; however, some systems
MAY require that behavior be consistent with the directives.
Pragma = "Pragma" ":" OWS Pragma-v
Pragma-v = 1#pragma-directive
pragma-directive = "no-cache" / extension-pragma
extension-pragma = token [ "=" ( token / quoted-string ) ]
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When the no-cache directive is present in a request message, an
application SHOULD forward the request toward the origin server even
if it has a cached copy of what is being requested. This pragma
directive has the same semantics as the no-cache response directive
(see Section 3.2.2) and is defined here for backward compatibility
with HTTP/1.0. Clients SHOULD include both header fields when a no-
cache request is sent to a server not known to be HTTP/1.1 compliant.
HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
sent "Cache-Control: no-cache".
Note: Because the meaning of "Pragma: no-cache" as a response-
header field is not actually specified, it does not provide a
reliable replacement for "Cache-Control: no-cache" in a response.
This mechanism is deprecated; no new Pragma directives will be
defined in HTTP.
3.5. Vary
The "Vary" response-header field conveys the set of request-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 2.7.
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 2.7.
In uncacheable or stale responses, the Vary field value advises the
user agent about the criteria that were used to select the
representation.
Vary = "Vary" ":" OWS Vary-v
Vary-v = "*" / 1#field-name
The set of header fields named by the Vary field value is known as
the selecting request-headers.
Servers 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 request-headers (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. The "*" value MUST NOT be generated by a proxy server.
The field-names given are not limited to the set of standard request-
header fields defined by this specification. Field names are case-
insensitive.
3.6. Warning
The "Warning" general-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 headers 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 = "Warning" ":" OWS Warning-v
Warning-v = 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, 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 headers SHOULD order them with
this user agent behavior in mind. New Warning headers SHOULD be
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added after any existing Warning headers.
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 caches 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 caches 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 headers
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 in the message.
If an implementation 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
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.
110 Response is stale
SHOULD be included whenever the returned response is stale.
111 Revalidation failed
SHOULD be included if a cache returns a stale response because an
attempt to validate the response failed, due to an inability to
reach the server.
112 Disconnected operation
SHOULD be included if the cache is intentionally disconnected from
the rest of the network for a period of time.
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113 Heuristic expiration
SHOULD be included if the cache heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater
than 24 hours.
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.
214 Transformation applied
MUST be added by an intermediate 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.
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.
4. 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 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
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<http://www.iana.org/assignments/http-cache-directives> 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. Header Field Registration
The Message Header Field Registry located at <http://www.iana.org/
assignments/message-headers/message-header-index.html> 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
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a user believes that the information has been removed from the
network. Therefore, cache contents need to be protected as sensitive
information.
7. Acknowledgments
Much of the content and presentation of the caching design is due to
suggestions and comments from individuals including: Shel Kaphan,
Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
8. References
8.1. Normative References
[Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
and Message Parsing", draft-ietf-httpbis-p1-messaging-11
(work in progress), August 2010.
[Part2] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 2: Message
Semantics", draft-ietf-httpbis-p2-semantics-11 (work in
progress), August 2010.
[Part4] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional
Requests", draft-ietf-httpbis-p4-conditional-11 (work in
progress), August 2010.
[Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
Partial Responses", draft-ietf-httpbis-p5-range-11 (work
in progress), August 2010.
[Part7] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication",
draft-ietf-httpbis-p7-auth-11 (work in progress),
August 2010.
[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
Fielding, et al. Expires February 5, 2011 [Page 30]
Internet-Draft HTTP/1.1, Part 6 August 2010
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,
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 2.5)
Do not mention RFC 2047 encoding and multiple languages in Warning
headers anymore, as these aspects never were implemented.
(Section 3.6)
Appendix B. Collected ABNF
Age = "Age:" OWS Age-v
Age-v = delta-seconds
Cache-Control = "Cache-Control:" OWS Cache-Control-v
Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
cache-directive ] )
Expires = "Expires:" OWS Expires-v
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Expires-v = HTTP-date
HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
OWS = <OWS, defined in [Part1], Section 1.2.2>
Pragma = "Pragma:" OWS Pragma-v
Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
pragma-directive ] )
Vary = "Vary:" OWS Vary-v
Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
] ) )
Warning = "Warning:" OWS Warning-v
Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
] )
cache-directive = cache-request-directive / cache-response-directive
cache-extension = token [ "=" ( token / quoted-string ) ]
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 = <field-name, defined in [Part1], Section 3.2>
port = <port, defined in [Part1], Section 2.6>
pragma-directive = "no-cache" / extension-pragma
pseudonym = <pseudonym, defined in [Part1], Section 9.9>
quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
token = <token, defined in [Part1], Section 1.2.2>
uri-host = <uri-host, defined in [Part1], Section 2.6>
warn-agent = ( uri-host [ ":" port ] ) / pseudonym
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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)
C.1. Since RFC2616
Extracted relevant partitions from [RFC2616].
C.2. Since draft-ietf-httpbis-p6-cache-00
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer"
(<http://purl.org/NET/http-errata#trailer-hop>)
o <http://tools.ietf.org/wg/httpbis/trac/ticket/12>: "Invalidation
after Update or Delete"
(<http://purl.org/NET/http-errata#invalidupd>)
o <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
Informative references"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/48>: "Date reference
typo"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/49>: "Connection
header text"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative
references"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/66>: "ISO-8859-1
Reference"
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o <http://tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative up-
to-date references"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in
13.2.2"
Other changes:
o Use names of RFC4234 core rules DQUOTE and HTAB (work in progress
on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
C.3. Since draft-ietf-httpbis-p6-cache-01
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not
used"
Other changes:
o Get rid of duplicate BNF rule names ("host" -> "uri-host") (work
in progress on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
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 Registration
(<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
o Reference RFC 3984, and update header registrations for headers
defined in this document.
C.5. Since draft-ietf-httpbis-p6-cache-03
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/106>: "Vary header
classification"
C.6. Since draft-ietf-httpbis-p6-cache-04
Ongoing work on ABNF conversion
(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
o Use "/" instead of "|" for alternatives.
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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
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 <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
1xx Warn-Codes"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
13.5.1 and 13.5.2"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/138>: "The role of
Warning and Semantic Transparency in Caching"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods and
Caching"
In addition: Final work on ABNF conversion
(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
o Add appendix containing collected and expanded ABNF, reorganize
ABNF introduction.
C.8. Since draft-ietf-httpbis-p6-cache-06
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/161>: "base for
numeric protocol elements"
Affected issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/37>: Vary and non-
existant headers
C.9. Since draft-ietf-httpbis-p6-cache-07
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
1xx Warn-Codes"
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o <http://tools.ietf.org/wg/httpbis/trac/ticket/167>: "Content-
Location on 304 responses"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/169>: "private and
no-cache CC directives with headers"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/187>: "RFC2047 and
warn-text"
C.10. Since draft-ietf-httpbis-p6-cache-08
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/147>: "serving
negotiated responses from cache: header-specific canonicalization"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/197>: "Effect of CC
directives on history lists"
Affected issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/199>: Status codes
and caching
Partly resolved issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
13.5.1 and 13.5.2"
C.11. Since draft-ietf-httpbis-p6-cache-09
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/29>: "Age
calculation"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/168>: "Clarify
differences between / requirements for request and response CC
directives"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/174>: "Caching
authenticated responses"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/208>: "IANA registry
for cache-control directives"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/211>: "Heuristic
caching of URLs with query components"
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Partly resolved issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the
requested resource's URI"
C.12. Since draft-ietf-httpbis-p6-cache-10
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
entity / representation / variant terminology"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
removing the 'changes from 2068' sections"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
heuristic caching for new status codes"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
heuristic caching for new status codes"
o Clean up TODOs and prose in "Combining Responses."
Index
A
age 6
Age header 17
C
cache 5
Cache Directives
max-age 19, 22
max-stale 19
min-fresh 19
must-revalidate 22
no-cache 19, 21
no-store 19, 21
no-transform 20, 22
only-if-cached 20
private 20
proxy-revalidate 22
public 20
s-maxage 22
Cache-Control header 18
cacheable 5
E
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Expires header 24
explicit expiration time 5
F
first-hand 6
fresh 6
freshness lifetime 6
G
Grammar
Age 17
Age-v 17
Cache-Control 18
Cache-Control-v 18
cache-extension 18
cache-request-directive 18
cache-response-directive 20
delta-seconds 17
Expires 24
Expires-v 24
extension-pragma 24
Pragma 24
pragma-directive 24
Pragma-v 24
Vary 25
Vary-v 25
warn-agent 26
warn-code 26
warn-date 26
warn-text 26
Warning 26
Warning-v 26
warning-value 26
H
Headers
Age 17
Cache-Control 18
Expires 24
Pragma 24
Vary 25
Warning 26
heuristic expiration time 5
M
max-age
Cache Directive 19, 22
max-stale
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Cache Directive 19
min-fresh
Cache Directive 19
must-revalidate
Cache Directive 22
N
no-cache
Cache Directive 19, 21
no-store
Cache Directive 19, 21
no-transform
Cache Directive 20, 22
O
only-if-cached
Cache Directive 20
P
Pragma header 24
private
Cache Directive 20
proxy-revalidate
Cache Directive 22
public
Cache Directive 20
S
s-maxage
Cache Directive 22
stale 6
V
validator 6
Vary header 25
W
Warning header 26
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Authors' Addresses
Roy T. Fielding (editor)
Day Software
23 Corporate Plaza DR, Suite 280
Newport Beach, CA 92660
USA
Phone: +1-949-706-5300
Fax: +1-949-706-5305
EMail: fielding@gbiv.com
URI: http://roy.gbiv.com/
Jim Gettys
Alcatel-Lucent Bell Labs
21 Oak Knoll Road
Carlisle, MA 01741
USA
EMail: jg@freedesktop.org
URI: http://gettys.wordpress.com/
Jeffrey C. Mogul
Hewlett-Packard Company
HP Labs, Large Scale Systems Group
1501 Page Mill Road, MS 1177
Palo Alto, CA 94304
USA
EMail: JeffMogul@acm.org
Henrik Frystyk Nielsen
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052
USA
EMail: henrikn@microsoft.com
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Larry Masinter
Adobe Systems, Incorporated
345 Park Ave
San Jose, CA 95110
USA
EMail: LMM@acm.org
URI: http://larry.masinter.net/
Paul J. Leach
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052
EMail: paulle@microsoft.com
Tim Berners-Lee
World Wide Web Consortium
MIT Computer Science and Artificial Intelligence Laboratory
The Stata Center, Building 32
32 Vassar Street
Cambridge, MA 02139
USA
EMail: timbl@w3.org
URI: http://www.w3.org/People/Berners-Lee/
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)
EMail: mnot@mnot.net
URI: http://www.mnot.net/
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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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