#?SuikaWiki/0.9 page-icon="HTTP"

[[#comment]]


* 仕様書から


** RFC 2068・2616 (HTTP/1.1) 14.9 Cache-Control

> The Cache-Control general-header field is used to specify directives
that MUST be obeyed by all caching mechanisms along the
request/response chain. The directives specify behavior intended to
prevent caches from adversely interfering with the request or
response. These directives typically override the default caching
algorithms. Cache directives are unidirectional in that the presence
of a directive in a request does not imply that the same directive [DEL[should]] [INS[is to]] be given in the response.

Cache-control 一般頭欄は全てのキャッシュ機構が要求・応答の鎖に沿って
従わ'''なければならない'''指示を指定するのに使います。
指示はキャッシュが要求や応答の不利な妨害となるのを防ぐのを
目的として振舞いを指定します。これらの指示は典型的には既定の
キャッシュ算法を上書きします。キャッシュ指示は一方向性のものであって、
要求に指示が使われていることは同じ指示が応答にも使われることを
暗示するものではありません。

> Note that HTTP/1.0 caches may not implement Cache-Control and [DEL[may]] [INS[might]]
only implement Pragma: no-cache (see section 14.32).

なお、 HTTP/1.0 キャッシュは Cache-Control を実装しておらず、
Pragma: no-cache だけを実装しているかもしれませぬ。

> Cache directives [DEL[must]] [INS[MUST]] be passed through by a proxy or gateway
application, regardless of their significance to that application,
since the directives [DEL[may]] [INS[might]] be applicable to all recipients along the
request/response chain. It is not possible to specify a cache-
directive for a specific cache.

キャッシュ指示は串や関門応用を通り抜けなければ'''なりません'''。
当該応用に対する意義の如何に関わらずです。なぜなら指示は
要求・応答の鎖にある全ての受信者に対して適用され得るからです。
特定キャッシュに対して cache-directive を指定することは不可能です。

>
- Cache-Control   = "Cache-Control" ":" 1#cache-directive
- cache-directive = cache-request-directive | cache-response-directive

>
[DEL[

[PRE[
          cache-request-directive =
                            "no-cache" [ "=" <"> 1#field-name <"> ]
                          | "no-store"
                          | "max-age" "=" delta-seconds
                          | "max-stale" [ "=" delta-seconds ]
                          | "min-fresh" "=" delta-seconds
                          | "only-if-cached"
                          | cache-extension
]PRE]
]DEL]
>
[INS[

[PRE[
    cache-request-directive =
           "no-cache"                          ; Section 14.9.1
         | "no-store"                          ; Section 14.9.2
         | "max-age" "=" delta-seconds         ; Section 14.9.3, 14.9.4
         | "max-stale" [ "=" delta-seconds ]   ; Section 14.9.3
         | "min-fresh" "=" delta-seconds       ; Section 14.9.3
         | "no-transform"                      ; Section 14.9.5
         | "only-if-cached"                    ; Section 14.9.4
         | cache-extension                     ; Section 14.9.6
]PRE]
]INS]
>
[DEL[

[PRE[
          cache-response-directive =
                            "public"
                          | "private" [ "=" <"> 1#field-name <"> ]
                          | "no-cache" [ "=" <"> 1#field-name <"> ]
                          | "no-store"
                          | "no-transform"
                          | "must-revalidate"
                          | "proxy-revalidate"
                          | "max-age" "=" delta-seconds
                          | cache-extension
]PRE]
]DEL]
>
[INS[

[PRE[
     cache-response-directive =
           "public"                               ; Section 14.9.1
         | "private" [ "=" <"> 1#field-name <"> ] ; Section 14.9.1
         | "no-cache" [ "=" <"> 1#field-name <"> ]; Section 14.9.1
         | "no-store"                             ; Section 14.9.2
         | "no-transform"                         ; Section 14.9.5
         | "must-revalidate"                      ; Section 14.9.4
         | "proxy-revalidate"                     ; Section 14.9.4
         | "max-age" "=" delta-seconds            ; Section 14.9.3
         | "s-maxage" "=" delta-seconds           ; Section 14.9.3
         | cache-extension                        ; Section 14.9.6
]PRE]
]INS]
>
- cache-extension = token [ "=" ( token | quoted-string ) ]

> When a directive appears without any 1#field-name parameter, the
directive applies to the entire request or response. When such a
directive appears with a 1#field-name parameter, it applies only to
the named field or fields, and not to the rest of the request or
response.  This mechanism supports extensibility; implementations of
future versions of the HTTP protocol [DEL[may]] [INS[might]] apply these directives to
header fields not defined in HTTP/1.1.

指示が 1#field-name パラメーターなしに現れる時、その指示は
要求又は応答全体に適用されます。その様な指示が 1#field-name
パラメーターつきで現れる時、これはその名前の欄(達)にのみ
適用され、要求や応答の残りには適用されません。この機構は拡張性をもちます。
将来の版の HTTP プロトコルの実装はこれらの指示を HTTP/1.1
で定義されていない頭欄に適用するかもしれません。

> The cache-control directives can be broken down into these general
categories:

cache-control 指示は次の一般分類に分けることが出来ます。

>
- [DEL[o]] [INS[-]]  Restrictions on what is cach[INS[e]]able; these may only be imposed by the origin server.
- [DEL[o]] [INS[-]] Restrictions on what may be stored by a cache; these may be imposed
by either the origin server or the user agent.
- [DEL[o]] [INS[-]] Modifications of the basic expiration mechanism; these may be
imposed by either the origin server or the user agent.
- [DEL[o]] [INS[-]] Controls over cache revalidation and reload; these may only be
imposed by a user agent.
- [DEL[o]] [INS[-]] Control over transformation of entities.
- [DEL[o]] [INS[-]] Extensions to the caching system.

- 何がキャッシュ可能かの制限。これは源サーバーのみ課せる。
- 何がキャッシュに蓄積可能かの制限。これは源サーバーと利用者代理者の
両者が課せる。
- 基本期限機構の編集。これは源サーバーと利用者代理者の
両者が課せる。
- キャッシュの再検証と再読み込みの制御。これは利用者代理者のみ課せる。
- 実体の転送に係る制御
- キャッシュ系統の拡張


*** 14.9.1 What is Cach[INS[e]]able [INS[何がキャッシュ可能か]]

> By default, a response is cach[INS[e]]able if the requirements of the request
method, request header fields, and the response status indicate that
it is cach[INS[e]]able. Section 13.4 summarizes these defaults for
cach[INS[e]]ability. The following Cache-Control response directives allow an
origin server to override the default cach[INS[e]]ability of a response:

既定では、応答はその要求 method, 要求頭欄及び応用状態がキャッシュ可能であると示していればキャッシュ可能です。
13.4節がキャッシュ可能性の既定をまとめています。
次の [CODE(HTTP)[Cache-Control]] 応答指示によって、
応答の既定のキャッシュ可能性を起源サーバーが上書きすることができます。

>
:public:
Indicates that the response [DEL[is cachable]] [INS[MAY be cached]] by any cache, even if it
would normally be non-cach[INS[e]]able or cach[INS[e]]able only within a non-shared
cache. (See also Authorization, section 14.8, for additional
details.)

その応答は、通常ならキャッシュ可能でないか又は非共有キャッシュ中でのみキャッシュ可能なキャッシュを含めてどんなキャッシュでもキャッシュして'''構いません'''。
(追加の詳細については14.8節の [CODE(HTTP)[[[Authorization]]]] をご覧下さい。)

>
:private:
Indicates that all or part of the response message is intended for a
single user and MUST NOT be cached by a shared cache. This allows an
origin server to state that the specified parts of the response are
intended for only one user and are not a valid response for requests
by other users. A private (non-shared) cache [DEL[may]] [INS[MAY]] cache the response.

応答メッセージの全部又は一部が単独の利用者向けのものであって、
共有キャッシュはキャッシュしては'''ならない'''ことを示します。
これによって起源サーバーは、応答の特定の部分が唯一の利用者向けのものであって他の利用者による要求に対する妥当な応答ではないと表明できます。
私的 (非共有) キャッシュは応答をキャッシュしても'''構いません'''。

> Note: This usage of the word private only controls where the
response may be cached, and cannot ensure the privacy of the
message content.

注意 : 単語 [CODE(HTTP)[private]] のこの用法は応答をキャッシュしても良いかを制御するだけであって、
メッセージ内容のプライバシーを保護することは出来ません。

>
:no-cache:[DEL[Indicates that all or part of the response message MUST NOT be cached anywhere.]] [INS[If the no-cache directive does not specify a field-name, then a cache MUST NOT use the response to satisfy a subsequent request without successful revalidation with the origin server.]]
This allows an origin server to prevent caching even by
caches that have been configured to return stale responses to client
requests.

;[DEL[応答メッセージの全部又は一部がどこにおいてもキャッシュ'''してはならない'''ことを示します。]] [INS[[CODE(HTTP)[no-cache]] 指令が [CODE(ABNF)[field-name]] を指定していなければ、キャッシュは起源サーバーの再検証で成功を得なければ後続の要求を満足するためにこの応答を使っては'''なりません'''。]]
この指令により、起源サーバーは、
クライアントの要求に対して腐った応答を返すように設定されているキャッシュでさえキャッシュすることを防ぐことができます。

[INS[

> If the no-cache directive does specify one or more field-names,
then a cache MAY use the response to satisfy a subsequent request,
subject to any other restrictions on caching. However, the
specified field-name(s) MUST NOT be sent in the response to a
subsequent request without successful revalidation with the origin
server. This allows an origin server to prevent the re-use of
certain header fields in a response, while still allowing caching
of the rest of the response.

[CODE(HTTP)[no-cache]] 指令が1つ以上の [CODE(ABNF)[field-name]]
を指定していれば、キャッシュは、キャッシュに関する他の制約の対象である
後続の要求を満足するのにこの応答を使っても'''構いません'''。
しかし、指定された [CODE(ABNF)[field-name]]
は、起源サーバーによる再検証で成功を得ない限り後続の要求に対する応答では送っては'''なりません'''。
これによって、起源サーバーは、応答中の特定の頭欄が再利用されることを防ぎつつも、
応答の残りをキャッシュすることを認めることが出来ます。
]INS]

> Note: Most HTTP/1.0 caches will not recognize or obey this directive.

注意 : ほとんどの HTTP/1.0 キャッシュはこの指令を認識も従いもしません。


*** 14.9.2 What May be Stored by Caches [INS[キャッシュは何を蓄積してもよいか]]

>
:[INS[no-store]]:
The purpose of the no-store directive is to prevent the inadvertent
release or retention of sensitive information (for example, on backup
tapes). The no-store directive applies to the entire message, and [DEL[may]] [INS[MAY]]
be sent either in a response or in a request. If sent in a request, a
cache MUST NOT store any part of either this request or any response
to it. If sent in a response, a cache MUST NOT store any part of
either this response or the request that elicited 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.

[CODE(HTTP)[no-store]] 指令の目的は、 (例えば、バックアップ・テープ上に)
繊細な情報が不注意で放出・記憶されるのを防ぐことです。
[CODE(HTTP)[no-store]] 指令はメッセージ全体に適用され、
応答中でも要求中でも送信して'''構いません'''。要求中で送る場合、
キャッシュはこの要求及びそれに対する応答のどの部分も蓄積しては'''なりません'''。
この指令は、非共有キャッシュにも共有キャッシュにも適用されます。
この文脈で、「蓄積'''してはならない'''」とは。
キャッシュが情報を非揮発性記憶装置に意図的に蓄積しては'''ならず'''、
その情報を送信した後に可及的速やかに揮発性の蓄積装置からも消去する最善の努力を行わなければ'''ならない'''ことを意味します。

> Even when this directive is associated with a response, users [DEL[may]] [INS[might]]
explicitly store such a response outside of the caching system (e.g.,
with a "Save As" dialog). History buffers [DEL[may]] [INS[MAY]] store such responses as
part of their normal operation.

この指令が応答に関連付けられている時であっても、利用者はそのような応答をキャッシュ・システムの外部で
(例えば「名前をつけて保存」対話によって)
陽に蓄積しても構いません。記憶バッファーはそのような応答を通常の操作で蓄積して構いません。

> The purpose of this directive is to meet the stated requirements of
certain users and service authors who are concerned about accidental
releases of information via unanticipated accesses to cache data
structures. While the use of this directive [DEL[may]] [INS[might]] improve privacy in
some cases, we caution that it is NOT in any way a reliable or
sufficient mechanism for ensuring privacy. In particular, malicious
or compromised caches [DEL[may]] [INS[might]] not recognize or obey this directive[DEL[;]][INS[,]] and
communications networks [DEL[may]] [INS[might]] be vulnerable to eavesdropping.

この指令に目的は、キャッシュのデータ構造に対する予想外の接続によって情報が偶発的に放出されることを心配する利用者やサービス著者の要求に応えることです。
この指令の使用は幾つかの場合に個人情報の扱いを改善するかもしれませんが、
プライバシーの確保には信頼可能で十分な仕組みでは'''ない'''ことを警告します。
特に、悪意のあるキャッシュや怪しげなキャッシュは、
この指令を認識したりしたがったりしないかもしれず、
通信網には盗聴の危険があるかもしれません。


*** 14.9.3 Modifications of the Basic Expiration Mechanism [INS[基本満期機構の修正]]

> The expiration time of an entity [DEL[may]] [INS[MAY]] be specified by the origin
server using the Expires header (see section 14.21). Alternatively,
it [DEL[may]] [INS[MAY]] be specified using the max-age directive in a response. [INS[When the max-age cache-control directive is present in a cached response, the response is stale if its current age is greater than the age value given (in seconds) at the time of a new request for that resource. The max-age directive on a response implies that the response is cacheable (i.e., "public") unless some other, more restrictive cache directive is also present.]]

> If a response includes both an Expires header and a max-age
directive, the max-age directive overrides the Expires header, even
if the Expires header is more restrictive. This rule allows an origin
server to provide, for a given response, a longer expiration time to
an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This [DEL[may]] [INS[might]] be
useful if certain HTTP/1.0 caches improperly calculate ages or
expiration times, perhaps due to desynchronized clocks.

[INS[

> Many HTTP/1.0 cache implementations will treat an Expires value that
is less than or equal to the response Date value as being equivalent
to the Cache-Control response directive "no-cache". If an HTTP/1.1
cache receives such a response, and the response does not include a
Cache-Control header field, it SHOULD consider the response to be
non-cacheable in order to retain compatibility with HTTP/1.0 servers.

> Note: An origin server might wish to use a relatively new HTTP
cache control feature, such as the "private" directive, on a
network including older caches that do not understand that
feature. The origin server will need to combine the new feature
with an Expires field whose value is less than or equal to the
Date value. This will prevent older caches from improperly
caching the response.

>
:s-maxage:
If a response includes an s-maxage directive, then for a shared
cache (but not for a private cache), 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 directive (see
section 14.9.4), i.e., that the shared cache must not use the
entry after it becomes stale to respond to a subsequent request
without first revalidating it with the origin server. The s-
maxage directive is always ignored by a private cache.
]INS]

> Note[DEL[:]] [INS[that]] most older caches, not compliant with this specification,
do not implement any [DEL[Cache-Control]] [INS[cache-control]] directives.  An origin server
wishing to use a [DEL[Cache-Control]] [INS[cache-control]] directive that restricts, but does not
prevent, caching by an HTTP/1.1-compliant cache [DEL[may]] [INS[MAY]] exploit the
requirement that the max-age directive overrides the Expires header,
and the fact that pre-HTTP/1.1-compliant caches do not observe the
max-age directive.

> Other directives allow a[DEL[n]] user agent to modify the basic expiration
mechanism. These directives [DEL[may]] [INS[MAY]] be specified on a request:

>
:max-age:
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.

>
:min-fresh:
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.

>
:max-stale:
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.

> If a cache returns a stale response, either because of a max-stale
directive on a request, or because the cache is configured to
override the expiration time of a response, the cache MUST attach a
Warning header to the stale response, using Warning [INS[1]]10 (Response is
stale).

[INS[

> A cache MAY be configured to return stale responses without
validation, but only if this does not conflict with any "MUST"-level
requirements concerning cache validation (e.g., a "must-revalidate"
cache-control directive).
> If both the new request and the cached entry include "max-age"
directives, then the lesser of the two values is used for determining
the freshness of the cached entry for that request.
]INS]


*** 14.9.4 Cache Revalidation and Reload Controls [INS[キャッシュ再検証及び再読み込み制御]]

> Sometimes a[DEL[n]] user agent [DEL[may]] [INS[might]] want or need to insist that a cache
revalidate its cache entry with the origin server (and not just with
the next cache along the path to the origin server), or to reload its
cache entry from the origin server. End-to-end revalidation [DEL[may]] [INS[might]] be
necessary if either the cache or the origin server has overestimated
the expiration time of the cached response. End-to-end reload may be
necessary if the cache entry has become corrupted for some reason.

> End-to-end revalidation may be requested either when the client does
not have its own local cached copy, in which case we call it
"unspecified end-to-end revalidation", or when the client does have a
local cached copy, in which case we call it "specific end-to-end
revalidation."

> The client can specify these three kinds of action using Cache-Control request directives:

>
:End-to-end reload:
The request includes a "no-cache" [DEL[Cache-Control]] [INS[cache-control]] directive or, for
compatibility with HTTP/1.0 clients, "Pragma: no-cache". [DEL[No field]] [INS[Field]]
names [DEL[may]] [INS[MUST NOT]] be included with the no-cache directive in a request. The
server MUST NOT use a cached copy when responding to such a
request.

>
:Specific end-to-end revalidation:
The request includes a "max-age=0" [DEL[Cache-Control]] [INS[cache-control] directive, which]]
forces each cache along the path to the origin server to revalidate
its own entry, if any, with the next cache or server. The initial
request includes a cache-validating conditional with the client's
current validator.

>
:Unspecified end-to-end revalidation:
The request includes "max-age=0" [DEL[Cache-Control]] [INS[cache-control] directive, which]]
forces each cache along the path to the origin server to revalidate
its own entry, if any, with the next cache or server. The initial
request does not include a cache-validating conditional; the first
cache along the path (if any) that holds a cache entry for this
resource includes a cache-validating conditional with its current
validator.

>
:[INS[max-age]]:
When an intermediate cache is forced, by means of a max-age=0
directive, to revalidate its own cache entry, and the client has
supplied its own validator in the request, the supplied validator [DEL[may]] [INS[might]]
differ from the validator currently stored with the cache entry. In
this case, the cache [DEL[may]] [INS[might]] use either validator in making its own
request without affecting semantic transparency.

However, the choice of validator may affect performance. The best
approach is for the intermediate cache to use its own validator when
making its request. If the server replies with 304 (Not Modified),
then the cache should return its now validated copy to the client
with a 200 (OK) response. If the server replies with a new entity and
cache validator, however, the intermediate cache [DEL[should]] [INS[can]] compare the
returned validator with the one provided in the client's request,
using the strong comparison function. If the client's validator is
equal to the origin server's, then the intermediate cache simply
returns 304 (Not Modified). Otherwise, it returns the new entity with
a 200 (OK) response.

If a request includes the no-cache directive, it [DEL[should not]] [INS[SHOULD NOT]] include
min-fresh, max-stale, or max-age.

>
:[INS[only-if-cached]]:
In some cases, such as times of extremely poor network connectivity,
a client may want a cache to return only those responses that it
currently has stored, and not to reload or revalidate with the origin
server. To do this, the client may include the only-if-cached
directive in a request. If it receives this directive, a cache SHOULD
either respond using a cached entry that is consistent with the other
constraints of the request, or respond with a 504 (Gateway Timeout)
status. However, 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.

>
:[INS[must-revalidate]]:
Because a cache [DEL[may]] [INS[MAY]] be configured to ignore a server's specified
expiration time, and because a client request [DEL[may]] [INS[MAY]] include a max-stale
directive (which has a similar effect), the protocol also includes a
mechanism for the origin server to require revalidation of a cache
entry on any subsequent use. When the must-revalidate directive is
present in a response received by a cache, that cache MUST NOT use
the entry after it becomes stale to respond to a subsequent request
without first revalidating it with the origin server. (I.e., the
cache [DEL[must]] [INS[MUST]] do an end-to-end revalidation every time, if, based solely
on the origin server's Expires or max-age value, the cached response is stale.)

> 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 [DEL[should]] [INS[SHOULD]] send the must-revalidate directive if and only if
failure to revalidate a request on the entity could result in
incorrect operation, such as a silently unexecuted financial
transaction.  Recipients MUST NOT take any automated action that
violates this directive, and MUST NOT automatically provide an
unvalidated copy of the entity if revalidation fails.

> Although this is not recommended, user agents operating under severe
connectivity constraints [DEL[may]] [INS[MAY]] violate this directive but, if so, MUST
explicitly warn the user that an unvalidated response has been
provided.  The warning MUST be provided on each unvalidated access,
and SHOULD require explicit user confirmation.

>
:[INS[proxy-revalidate]]:
The proxy-revalidate directive has the same meaning as the must-revalidate
directive, except that it does not apply to non-shared
user agent caches. It can be used on a response to an authenticated
request to permit the user's cache to store and later return the
response without needing to revalidate it (since it has already been
authenticated once by that user), while still requiring proxies that
service many users to revalidate each time (in order to make sure
that each user has been authenticated). Note that such authenticated
responses also need the public cache control directive in order to
allow them to be cached at all.


*** 14.9.5 No-Transform Directive [INS[非変換指令]]

>
:[INS[no-transform]]:
Implementers of intermediate caches (proxies) have found it useful to
convert the media type of certain entity bodies. A [INS[non-transparent]]
proxy might, for example, convert between image formats in order to save cache space
or to reduce the amount of traffic on a slow link. [DEL[HTTP has to date been silent on these transformations.]]

> Serious operational problems [DEL[have already occurred]] [INS[occur]], however, when
these transformations [DEL[have been]] [INS[are]] applied to entity bodies intended for
certain kinds of applications. For example, applications for medical
imaging, scientific data analysis and those using end-to-end
authentication, all depend on receiving an entity body that is bit
for bit identical to the original entity-body.

> Therefore, if a response includes the no-transform directive, an
intermediate cache or proxy MUST NOT change those headers that are
listed in section 13.5.2 as being subject to the no-transform
directive.  This implies that the cache or proxy [DEL[must not]] [INS[MUST NOT]]
change any aspect of the entity-body that is specified by these headers[DEL[.]][INS[,]] including the value of the entity-body itself.


*** 14.9.6 Cache Control Extensions [INS[キャッシュ制御拡張]]

> The Cache-Control header field can be extended through the use of one
or more cache-extension tokens, each with an optional assigned value.
Informational extensions (those which do not require a change in
cache behavior) [DEL[may]] [INS[MAY]] 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 which 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 [DEL[Cache-Control]] [INS[cache-control]] directives can be made without
requiring changes to the base protocol.

> This extension mechanism depends on a[INS[n]] 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 [DEL["]]community[DEL["]] which acts as a modifier to the [DEL["]]private[DEL["]] directive. We
define this new directive to mean that, in addition to any non-shared
cache, any cache which is shared only by members of the community
named within its value may cache the response. An origin server
wishing to allow the [DEL["]]UCI[DEL["]] community to use an otherwise private
response in their shared cache(s) [DEL[may]] [INS[could]] do so by including

>
- [SAMP(HTTP)[Cache-Control: private, community="UCI"]]

> A cache seeing this header field will act correctly even if the cache
does not understand the [DEL["]]community[DEL["]] cache-extension, since it will
also see and understand the [DEL["]]private[DEL["]] 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
cach[INS[e]]ability) such that the cache behavior will remain minimally
correct even if the cache does not understand the extension(s).


** RFC の部分の License

[[RFCのライセンス]]


* メモ

- [1] [[Google]] からの応答に [CODE(HTTP)[x-gzip-ok=""]] ってのがついてました。
- [2] ''Q234067 - HOWTO: Prevent Caching in Internet Explorer'' <http://web.archive.org/web/20010810040821/support.microsoft.com/support/kb/articles/Q234/0/67.ASP>: [[M$]] も [CODE(HTTP)[[[Pragma]]]] より [CODE(HTTP)[Cache-Control]] を推奨していました。ところで、この文書によると、 [CODE(HTTP)[Cache-Control: no-cache]] には対応しているけど [CODE(HTML)[<[CODE(HTMLe)[[[meta]]]] [CODE(HTMLa)[[[http-equiv]]]]="cache-control" [CODE(HTMLa)[[[content]]]]="no-cache">]] には対応していないとか。妥当な態度ですね。

[3]
[CODE(HTTP example code)@en[Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0]]
[SRC@en[[[XOOPS]]]]

([[名無しさん]] [sage] [WEAK[2005-08-18 10:50:21 +00:00]])