One document matched: draft-ietf-httpbis-p6-cache-09.txt
Differences from draft-ietf-httpbis-p6-cache-08.txt
HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Day Software
Obsoletes: 2616 (if approved) J. Gettys
Intended status: Standards Track One Laptop per Child
Expires: September 9, 2010 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
March 8, 2010
HTTP/1.1, part 6: Caching
draft-ietf-httpbis-p6-cache-09
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/11> 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.10.
Status of this Memo
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This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on September 9, 2010.
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
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the Trust Legal Provisions and are provided without warranty as
described in the 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
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. Caching Negotiated Responses . . . . . . . . . . . . . . . 15
2.7. 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 . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.6. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 25
4. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 28
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
5.1. Message Header Registration . . . . . . . . . . . . . . . 28
6. Security Considerations . . . . . . . . . . . . . . . . . . . 28
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.1. Normative References . . . . . . . . . . . . . . . . . . . 29
8.2. Informative References . . . . . . . . . . . . . . . . . . 30
Appendix A. Compatibility with Previous Versions . . . . . . . . 30
A.1. Changes from RFC 2068 . . . . . . . . . . . . . . . . . . 30
A.2. Changes from RFC 2616 . . . . . . . . . . . . . . . . . . 30
Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 31
Appendix C. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 32
C.1. Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 32
C.2. Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 32
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C.3. Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 33
C.4. Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 33
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 . . . . . . . . . . . 34
C.8. Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 35
C.9. Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 35
C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 35
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
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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 include 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 may 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 an entity should
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 is an
equivalent copy of an entity.
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:
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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 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 "public"
directive is present; see Section 3.2), 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
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[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)
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 Request-URI and that of the stored response match
([[TODO-Request-URI: Need to find a new term for this, as Part 1
doesn't define Request-URI anymore; the new term request-target
does not work for this. (see
<http://tools.ietf.org/wg/httpbis/trac/ticket/196>)]]), 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 2.6), 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).
[[TODO-method-cacheability: define method cacheability for GET, HEAD
and POST in p2-semantics.]]
When a stored response is used to satisfy a request, 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. [[DISCUSS-includes-validated: this currently includes
successfully validated responses.]]
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
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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.
[[TODO-header-properties: end-to-end and hop-by-hop headers, non-
modifiable headers removed; re-spec in p1]]
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
entity 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. This
means that the response is always stale, so that caches should
validate it before using it for subsequent requests. [[TODO-
response-stale: This wording may cause confusion, because the
response may still be served stale.]]
Since origin servers do not always provide explicit expiration times,
HTTP caches may also assign heuristic expiration times when they 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.
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Additionally, clients may 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.
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,
but a heuristic may be used; 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 of 200, 203, 206, 300, 301 or 410, a heuristic
expiration time can be calculated. Heuristics MUST NOT be used for
other response status codes.
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
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fraction might be 10%.
[[REVIEW-query-string-heuristics: took away HTTP/1.0 query string
heuristic uncacheability.]]
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 term "age_value" denotes the value of the Age header, in a form
appropriate for arithmetic operations.
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 (see Section 9.3 of [Part1]). The term "date_value"
denotes the value of the Date header, in a form appropriate for
arithmetic operations.
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.
A response's age can be calculated in two entirely independent ways:
1. now 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. age_value, if all of the caches along the response path implement
HTTP/1.1.
These are combined as
corrected_received_age = max(now - date_value, age_value)
When an Age value is received, it MUST be interpreted relative to the
time the request was initiated, not the time that the response was
received.
corrected_initial_age = corrected_received_age
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+ (now - request_time)
where "request_time" is the time (according to the local clock) when
the request that elicited this response was sent.
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.
In summary:
age_value - Age header field-value received with the response
date_value - Date header field-value received with the response
request_time - local time when the cache made the request
resulting in the stored response
response_time - local time when the cache received the response
now - current local time
apparent_age = max(0, response_time - date_value);
corrected_received_age = max(apparent_age, age_value);
response_delay = response_time - request_time;
corrected_initial_age = corrected_received_age + response_delay;
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.
If a cache receives a first-hand response (either an entire response,
or a 304 (Not Modified) response) that it would normally forward to
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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 2.6), 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 2.6) 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 2.7.
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 is used both to satisfy the
request and replace the stored response. [[TODO-req-missing: Should
there be a requirement here?]]
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.
The following HTTP methods MUST cause a cache to invalidate the
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Request-URI 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 Request-URI. This helps prevent denial of
service attacks.
[[TODO-def-host-part: "host part" needs to be specified better.]]
A cache that passes through requests for methods it does not
understand SHOULD invalidate the Request-URI.
Here, "invalidate" means that the cache will either remove all stored
responses related to the 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.
[[TODO-spec-success-invalidate: specify that only successful (2xx,
3xx?) responses invalidate.]]
2.6. 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
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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 normalisation that may 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.
2.7. 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.
If the new response contains an ETag, it identifies the stored
response to use. [[TODO-mention-CL: may need language about Content-
Location here]][[TODO-inm-mult-etags: cover case where INM with
multiple etags was sent]]
If the 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 from the stored response and the updated response.
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o any stored Warning headers with warn-code 2xx MUST be retained in
the stored response and the updated response.
o any headers provided in the new response MUST replace the
corresponding headers from the stored response.
If a header field-name in the new response matches more than one
header in the stored response, all such stored headers MUST be
replaced.
The updated response can [[TODO-is-req: requirement?]] be used to
replace the stored response in cache. In the case of a 206 response,
the combined entity-body MAY be stored.
[[ISSUE-how-head: discuss how to handle HEAD updates]]
3. Header Field Definitions
This section defines the syntax and semantics of HTTP/1.1 header
fields related to caching.
For entity-header fields, both sender and recipient refer to either
the client or the server, depending on who sends and who receives the
entity.
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).
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
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HTTP/1.0 caches may not implement the Age header field.
3.2. Cache-Control
The "Cache-Control" general-header field is used to specify
directives that MUST be obeyed by all 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.
Note that 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
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
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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 may 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 max-stale directive is also included, 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. [[TODO-staleness: of any staleness?
--mnot]]
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 intermediate
cache or proxy MUST NOT change the Content-Encoding, Content-Range
or Content-Type request headers, nor the request entity-body.
only-if-cached
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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. 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
remainder of the response message MAY be.
Note: This usage of the word private only controls where the
response may be stored, and 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.
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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 caching 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 may be vulnerable to eavesdropping.
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
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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 entity 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 entity-body.
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.
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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).
3.3. Expires
The "Expires" entity-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
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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 ) ]
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
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response can be used to satisfy a given request; see Section 2.6.
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 2.6.
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.
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;
it may only be generated by an origin 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 entity body 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.
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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
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 entity body or entity
headers that is not rectified by a validation (for example, a
lossy compression of the entity bodies) 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.
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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.
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 cache or proxy if it applies any
transformation changing the content-coding (as specified in the
Content-Encoding header) or media-type (as specified in the
Content-Type header) of the response, or the entity-body of the
response, unless this Warning code already appears in the
response.
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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 an entity 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. Message Header Registration
The Message Header Registry located at <http://www.iana.org/
assignments/message-headers/message-header-index.html> should 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
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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 should 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-09
(work in progress), March 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-09 (work in
progress), March 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-09 (work in
progress), March 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-09 (work
in progress), March 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-09 (work in progress),
March 2010.
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[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.
Appendix A. Compatibility with Previous Versions
A.1. Changes from RFC 2068
A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
was introduced to add this missing case. (Sections 2.1, 3.2).
Range request responses would become very verbose if all meta-data
were always returned; by allowing the server to only send needed
headers in a 206 response, this problem can be avoided.
(Section 2.7)
The Cache-Control: max-age directive was not properly defined for
responses. (Section 3.2.2)
Warnings could be cached incorrectly, or not updated appropriately.
(Section 2.3, 2.7, 3.2, and 3.6) Warning also needed to be a general
header, as PUT or other methods may have need for it in requests.
A.2. Changes from RFC 2616
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)
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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
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
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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
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>)
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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"
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.
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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.
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://trac.tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement
of 13.5.1 and 13.5.2"
o <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138>: "The role
of Warning and Semantic Transparency in Caching"
o <http://trac.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.
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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"
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"
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Index
A
age 6
Age header 17
C
cache 5
Cache Directives
max-age 19, 22
max-stale 19
min-fresh 19
must-revalidate 21
no-cache 18, 21
no-store 18, 21
no-transform 19, 22
only-if-cached 19
private 20
proxy-revalidate 22
public 20
s-maxage 22
Cache-Control header 18
cacheable 5
E
Expires header 23
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 23
Expires-v 23
extension-pragma 24
Pragma 24
pragma-directive 24
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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 23
Pragma 24
Vary 24
Warning 25
heuristic expiration time 5
M
max-age
Cache Directive 19, 22
max-stale
Cache Directive 19
min-fresh
Cache Directive 19
must-revalidate
Cache Directive 21
N
no-cache
Cache Directive 18, 21
no-store
Cache Directive 18, 21
no-transform
Cache Directive 19, 22
O
only-if-cached
Cache Directive 19
P
Pragma header 24
private
Cache Directive 20
proxy-revalidate
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Cache Directive 22
public
Cache Directive 20
S
s-maxage
Cache Directive 22
stale 6
V
validator 6
Vary header 24
W
Warning header 25
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
One Laptop per Child
21 Oak Knoll Road
Carlisle, MA 01741
USA
Email: jg@laptop.org
URI: http://www.laptop.org/
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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
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/
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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)
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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