[要約] RFC 8246は、HTTP Immutable Responsesの仕様を定義しています。この仕様の目的は、キャッシュの効率を向上させ、リソースの不変性を確保することです。

Internet Engineering Task Force (IETF)                        P. McManus
Request for Comments: 8246                                       Mozilla
Category: Standards Track                                 September 2017
ISSN: 2070-1721

HTTP Immutable Responses




The immutable HTTP response Cache-Control extension allows servers to identify resources that will not be updated during their freshness lifetime. This ensures that a client never needs to revalidate a cached fresh resource to be certain it has not been modified.


Status of This Memo


This is an Internet Standards Track document.

これはInternet Standards Trackドキュメントです。

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

このドキュメントは、IETF(Internet Engineering Task Force)の製品です。これは、IETFコミュニティのコンセンサスを表しています。公開レビューを受け、インターネットエンジニアリングステアリンググループ(IESG)による公開が承認されました。インターネット標準の詳細については、RFC 7841のセクション2をご覧ください。

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8246.


Copyright Notice


Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

Copyright(c)2017 IETF Trustおよびドキュメントの作成者として識別された人物。全著作権所有。

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

この文書は、BCP 78およびIETF文書に関するIETFトラストの法的規定(https://trustee.ietf.org/license-info)の対象であり、この文書の発行日に有効です。これらのドキュメントは、このドキュメントに関するあなたの権利と制限について説明しているため、注意深く確認してください。このドキュメントから抽出されたコードコンポーネントには、Trust Legal Provisionsのセクション4.eに記載されているSimplified BSD Licenseのテキストが含まれている必要があり、Simplified BSD Licenseに記載されているように保証なしで提供されます。

Table of Contents


   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  The Immutable Cache-Control Extension . . . . . . . . . . . .   3
     2.1.  About Intermediaries  . . . . . . . . . . . . . . . . . .   4
     2.2.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6
1. Introduction
1. はじめに

HTTP's freshness lifetime mechanism [RFC7234] allows a client to safely reuse a stored response to satisfy future requests for a specified period of time. However, it is still possible that the resource will be modified during that period.


For instance, a front-page newspaper photo with a freshness lifetime of one hour would mean that no user would see a cached photo more than one hour old. However, the photo could be updated at any time, resulting in different users seeing different photos depending on the contents of their caches for up to one hour. This is compliant with the caching mechanism defined in [RFC7234].


Users that need to confirm there have been no updates to their cached responses typically use the reload (or refresh) mechanism in their user agents. This in turn generates a conditional request [RFC7232], and either a new representation or, if unmodified, a 304 (Not Modified) response [RFC7232] is returned. A user agent that understands HTML and fetches its dependent sub-resources might issue hundreds of conditional requests to refresh all portions of a common page [REQPERPAGE].

キャッシュされた応答が更新されていないことを確認する必要があるユーザーは、通常、ユーザーエージェントで再読み込み(または更新)メカニズムを使用します。これにより、条件付きリクエスト[RFC7232]が生成され、新しい表現か、変更されていない場合は304(Not Modified)レスポンス[RFC7232]が返されます。 HTMLを理解し、その従属サブリソースをフェッチするユーザーエージェントは、何百もの条件付きリクエストを発行して、共通ページのすべての部分をリフレッシュする場合があります[REQPERPAGE]。

However, some content providers never create more than one variant of a sub-resource, because they use "versioned" URLs. When these resources need an update, they are simply published under a new URL, typically embedding an identifier unique to that version of the resource in the path, and references to the sub-resource are updated with the new path information.


For example, "https://www.example.com/101016/main.css" might be updated and republished as "https://www.example.com/102026/main.css", with any links that reference it being changed at the same time.


This design pattern allows a very large freshness lifetime to be used for the sub-resource without guessing when it will be updated in the future.


Unfortunately, the user agent does not know when this versioned URL design pattern is used. As a result, user-driven refreshes still translate into wasted conditional requests for each sub-resource as each will return 304 responses.


The immutable HTTP response Cache-Control extension allows servers to identify responses that will not be updated during their freshness lifetimes.


This effectively informs clients that any conditional request for that response can be safely skipped without worrying that it has been updated.


1.1. Notational Conventions
1.1. 表記規則

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.


2. The Immutable Cache-Control Extension
2. 不変のキャッシュ制御拡張

When present in an HTTP response, the immutable Cache-Control extension indicates that the origin server will not update the representation of that resource during the freshness lifetime of the response.


Clients SHOULD NOT issue a conditional request during the response's freshness lifetime (e.g., upon a reload) unless explicitly overridden by the user (e.g., a force reload).

クライアントは、ユーザーによって明示的にオーバーライドされない限り(強制リロードなど)、応答のフレッシュネスの有効期間中(リロード時など)に条件付きリクエストを発行してはなりません(SHOULD NOT)。

The immutable extension only applies during the freshness lifetime of the stored response. Stale responses SHOULD be revalidated as they normally would be in the absence of the immutable extension.


The immutable extension takes no arguments. If any arguments are present, they have no meaning and MUST be ignored. Multiple instances of the immutable extension are equivalent to one instance. The presence of an immutable Cache-Control extension in a request has no effect.


2.1. About Intermediaries
2.1. 仲介業者について

An immutable response has the same semantic meaning when received by proxy clients as it does when received by user-agent-based clients. Therefore, proxies SHOULD skip conditionally revalidating fresh responses containing the immutable extension unless there is a signal from the client that a validation is necessary (e.g., a no-cache Cache-Control request directive defined in Section of [RFC7234]).


A proxy that uses the immutable extension to bypass a conditional revalidation can choose whether to reply with a 304 or 200 response to its requesting client based on the request headers the proxy received.


2.2. Example
2.2. 例

Cache-Control: max-age=31536000, immutable

キャッシュ制御:max-age = 31536000、不変

3. Security Considerations
3. セキュリティに関する考慮事項

The immutable mechanism acts as form of soft pinning and, as with all pinning mechanisms, creates a vector for amplification of cache corruption incidents. These incidents include cache-poisoning attacks. Three mechanisms are suggested for mitigation of this risk:


o Clients SHOULD ignore the immutable extension from resources that are not part of an authenticated context such as HTTPS. Authenticated resources are less vulnerable to cache poisoning.

o クライアントは、HTTPSなどの認証済みコンテキストの一部ではないリソースからの不変の拡張機能を無視する必要があります(SHOULD)。認証されたリソースは、キャッシュポイズニングに対して脆弱ではありません。

o User agents often provide two different refresh mechanisms: reload and some form of force-reload. The latter is used to rectify interrupted loads and other corruption. These reloads, typically indicated through no-cache request attributes, SHOULD ignore the immutable extension as well.

o ユーザーエージェントは、多くの場合、2つの異なる更新メカニズムを提供します。リロードと何らかの形式の強制リロードです。後者は、中断されたロードやその他の破損を修正するために使用されます。これらのリロードは、通常、キャッシュなしのリクエスト属性によって示され、不変の拡張も無視する必要があります(SHOULD)。

o Clients SHOULD ignore the immutable extension for resources that do not provide a strong indication that the stored response size is the correct response size such as responses delimited by connection close.

o クライアントは、保存された応答サイズが、接続のクローズによって区切られた応答などの正しい応答サイズであることを示す強力な指標を提供しないリソースの不変の拡張を無視する必要があります(SHOULD)。

4. IANA Considerations
4. IANAに関する考慮事項

The immutable extension has been registered in the "Hypertext Transfer Protocol (HTTP) Cache Directive Registry" per the guidelines described in Section 7.1 of [RFC7234].


o Cache Directive: immutable

o キャッシュディレクティブ:不変

o Reference: RFC 8246

o リファレンス:RFC 8246

5. References
5. 参考文献
5.1. Normative References
5.1. 引用文献

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.

[RFC2119] Bradner、S。、「要件レベルを示すためにRFCで使用するキーワード」、BCP 14、RFC 2119、DOI 10.17487 / RFC2119、1997年3月、<https://www.rfc-editor.org/info/ rfc2119>。

[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests", RFC 7232, DOI 10.17487/RFC7232, June 2014, <https://www.rfc-editor.org/info/rfc7232>.

[RFC7232]フィールディング、R。、エド。およびJ. Reschke編、「Hypertext Transfer Protocol(HTTP / 1.1):Conditional Requests」、RFC 7232、DOI 10.17487 / RFC7232、2014年6月、<https://www.rfc-editor.org/info/rfc7232> 。

[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234, DOI 10.17487/RFC7234, June 2014, <https://www.rfc-editor.org/info/rfc7234>.

[RFC7234] Fielding、R.、Ed。、Nottingham、M.、Ed。、and J. Reschke、Ed。、 "Hypertext Transfer Protocol(HTTP / 1.1):Caching"、RFC 7234、DOI 10.17487 / RFC7234、June 2014 、<https://www.rfc-editor.org/info/rfc7234>。

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8174] Leiba、B。、「RFC 2119キーワードの大文字と小文字のあいまいさ」、BCP 14、RFC 8174、DOI 10.17487 / RFC8174、2017年5月、<https://www.rfc-editor.org/info/ rfc8174>。

5.2. Informative References
5.2. 参考引用

[REQPERPAGE] HTTP Archive, "Total Requests per Page", <http://httparchive.org/interesting.php#reqTotal>.

[REQPERPAGE] HTTPアーカイブ、「ページごとの合計リクエスト数」、<http://httparchive.org/interesting.php#reqTotal>。



Thank you to Ben Maurer for partnership in developing and testing this idea. Thank you to Amos Jeffries for help with proxy interactions and to Mark Nottingham for help with the documentation.

このアイデアの開発とテストに協力してくれたBen Maurerに感謝します。プロキシの相互作用についてはAmos Jeffriesに、ドキュメントについてはMark Nottinghamに感謝します。

Author's Address


Patrick McManus Mozilla


   Email: mcmanus@ducksong.com