Network Working Group M. Nottingham Internet-Draft August 22, 2024 Intended status: Standards Track Expires: February 23, 2025 HTTP Cache Groups draft-ietf-httpbis-cache-groups-latest Abstract This specification introduces a means of describing the relationships between stored responses in HTTP caches, "grouping" them by associating a stored response with one or more opaque strings. About This Document This note is to be removed before publishing as an RFC. Status information for this document may be found at . Discussion of this document takes place on the HTTP Working Group mailing list (), which is archived at . Working Group information can be found at . Source for this draft and an issue tracker can be found at . Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on February 23, 2025. Nottingham Expires February 23, 2025 [Page 1] Internet-Draft HTTP Cache Groups August 2024 Copyright Notice Copyright (c) 2024 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 (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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3 2. The Cache-Groups Response Header Field . . . . . . . . . . . 3 2.1. Identifying Grouped Responses . . . . . . . . . . . . . . 4 2.2. Cache Behaviour . . . . . . . . . . . . . . . . . . . . . 4 2.2.1. Invalidation . . . . . . . . . . . . . . . . . . . . 4 3. The Cache-Group-Invalidation Response Header Field . . . . . 4 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 4.1. HTTP Field Names . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 6.1. Normative References . . . . . . . . . . . . . . . . . . 6 6.2. Informative References . . . . . . . . . . . . . . . . . 6 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 1. Introduction HTTP caching [HTTP-CACHING] operates at the granularity of a single resource; the freshness of one stored response does not affect that of others. This granularity can make caching more efficient -- for example, when a page is composed of many assets that have different requirements for caching. However, there are also cases where the relationship between stored responses could be used to improve cache efficiency. For example, it is often necessary to invalidate a set of related resources. This might be because a state-changing request has side effects on other resources, or it might be purely for administrative convenience (e.g., "invalidate this part of the site"). Grouping Nottingham Expires February 23, 2025 [Page 2] Internet-Draft HTTP Cache Groups August 2024 responses together provides a dedicated way to express these relationships, instead of relying on things like URL structure. In addition to sharing invalidation events, the relationships indicated by grouping can also be used by caches to optimise their operation; for example, it could be used to inform the operation of cache eviction algorithms. Section 2 introduces a means of describing the relationships between a set of stored responses in HTTP caches by associating them with one or more opaque strings. It also describes how caches can use that information to apply invalidation events to members of a group. Section 3 introduces one new source of such events: a HTTP response header that allows a state-changing response to trigger a group invalidation. These mechanisms operate within a single cache, across the stored responses associated with a single origin server. They do not address this issues of synchronising state between multiple caches (e.g., in a hierarchy or mesh), nor do they facilitate association of stored responses from disparate origins. 1.1. Notational Conventions 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. This specification uses the following terminology from [STRUCTURED-FIELDS]: List, String, Parameter. 2. The Cache-Groups Response Header Field The Cache-Groups HTTP Response Header is a List of Strings [STRUCTURED-FIELDS]. Each member of the list is an opaque value that identifies a group that the response belongs to. HTTP/1.1 200 OK Content-Type: application/javascript Cache-Control: max-age=3600 Cache-Groups: "scripts" The ordering of members is not significant. Unrecognised Parameters MUST be ignored. Nottingham Expires February 23, 2025 [Page 3] Internet-Draft HTTP Cache Groups August 2024 Implementations MUST support at least 128 groups in a field value, with up to at least 128 characters in each member. Note that generic limitations on HTTP field lengths may constrain the size of this field value in practice. 2.1. Identifying Grouped Responses Two responses stored in the same cache are considered to have the same group when all of the following conditions are met: 1. They both contain a Cache-Groups response header field that contains the same String (in any position in the List), when compared character-by-character. 2. The both share the same URI origin (per Section 4.3.1 of [HTTP]). 2.2. Cache Behaviour 2.2.1. Invalidation A cache that invalidates a stored response MAY invalidate any stored responses that share groups (per Section 2.1) with that response. Cache extensions can explicitly strengthen the requirement above. For example, a targeted cache control header field [TARGETED] might specify that caches processing it are required to invalidate such responses. 3. The Cache-Group-Invalidation Response Header Field The Cache-Group-Invalidation response header field is a List of Strings [STRUCTURED-FIELDS]. Each member of the list is an opaque value that identifies a group that the response invalidates, per Section 2.2.1. For example, a POST request that has side effects on two cache groups could indicate that stored responses associated with either or both of those groups should be invalidated with: HTTP/1.1 200 OK Content-Type: text/html Cache-Group-Invalidation: "eurovision-results", "kylie-minogue" The Cache-Group-Invalidation header field MUST be ignored on responses to requests that have a safe method (e.g., GET; see Section 9.2.1 of [HTTP]). Nottingham Expires February 23, 2025 [Page 4] Internet-Draft HTTP Cache Groups August 2024 A cache that receives a Cache-Group-Invalidation header field on a response to an unsafe request MAY invalidate any stored responses that share groups (per Section 2.1) with any of the listed groups. Cache extensions can explicitly strengthen the requirement above. For example, a targeted cache control header field [TARGETED] might specify that caches processing it are required to respect the Cache- Group-Invalidation signal. The ordering of members is not significant. Unrecognised Parameters MUST be ignored. Implementations MUST support at least 128 groups in a field value, with up to at least 128 characters in each member. Note that generic limitations on HTTP field lengths may constrain the size of this field value in practice. 4. IANA Considerations IANA should perform the following tasks: 4.1. HTTP Field Names Enter the following into the Hypertext Transfer Protocol (HTTP) Field Name Registry: o Field Name: Cache-Groups o Status: permanent o Reference: RFC nnnn o Comments: o Field Name: Cache-Group-Invalidation o Status: permanent o Reference: RFC nnnn o Comments: 5. Security Considerations This mechanism allows resources that share an origin to invalidate each other. Because of this, origins that represent multiple parties (sometimes referred to as "shared hosting") might allow one party to Nottingham Expires February 23, 2025 [Page 5] Internet-Draft HTTP Cache Groups August 2024 group its resources with those of others, or to send signals which have side effects upon them. Shared hosts that wish to mitigate these risks can control access to the header fields defined in this specification. 6. References 6.1. Normative References [HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, . [HTTP-CACHING] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Caching", STD 98, RFC 9111, DOI 10.17487/RFC9111, June 2022, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [STRUCTURED-FIELDS] Nottingham, M. and P. Kamp, "Structured Field Values for HTTP", draft-ietf-httpbis-sfbis-06 (work in progress), April 2024. 6.2. Informative References [TARGETED] Ludin, S., Nottingham, M., and Y. Wu, "Targeted HTTP Cache Control", RFC 9213, DOI 10.17487/RFC9213, June 2022, . Appendix A. Acknowledgements Thanks to Stephen Ludin for his review and suggestions. Nottingham Expires February 23, 2025 [Page 6] Internet-Draft HTTP Cache Groups August 2024 Author's Address Mark Nottingham Prahran Australia Email: mnot@mnot.net URI: https://www.mnot.net/ Nottingham Expires February 23, 2025 [Page 7]