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+Network Working Group                                      M. Nottingham
+Internet-Draft                                          20 December 2024
+Intended status: Standards Track                                        
+Expires: 23 June 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
+   https://datatracker.ietf.org/doc/draft-ietf-httpbis-cache-groups/.
+
+   Discussion of this document takes place on the HTTP Working Group
+   mailing list (mailto:ietf-http-wg@w3.org), which is archived at
+   https://lists.w3.org/Archives/Public/ietf-http-wg/.  Working Group
+   information can be found at https://httpwg.org/.
+
+   Source for this draft and an issue tracker can be found at
+   https://github.com/httpwg/http-extensions/labels/cache-groups.
+
+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 23 June 2025.
+
+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 Revised BSD License text as
+   described in Section 4.e of the Trust Legal Provisions and are
+   provided without warranty as described in the Revised BSD License.
+
+Table of Contents
+
+   1.  Introduction
+     1.1.  Notational Conventions
+   2.  The Cache-Groups Response Header Field
+     2.1.  Identifying Grouped Responses
+     2.2.  Cache Behaviour
+       2.2.1.  Invalidation
+   3.  The Cache-Group-Invalidation Response Header Field
+   4.  IANA Considerations
+     4.1.  HTTP Field Names
+   5.  Security Considerations
+   6.  References
+     6.1.  Normative References
+     6.2.  Informative References
+   Appendix A.  Acknowledgements
+   Author's Address
+
+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
+   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.
+
+   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]).
+
+   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:
+
+   *  Field Name: Cache-Groups
+
+   *  Status: permanent
+
+   *  Reference: RFC nnnn
+
+   *  Comments:
+
+   *  Field Name: Cache-Group-Invalidation
+
+   *  Status: permanent
+
+   *  Reference: RFC nnnn
+
+   *  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
+   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,
+              <https://www.rfc-editor.org/rfc/rfc9110>.
+
+   [HTTP-CACHING]
+              Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
+              Ed., "HTTP Caching", STD 98, RFC 9111,
+              DOI 10.17487/RFC9111, June 2022,
+              <https://www.rfc-editor.org/rfc/rfc9111>.
+
+   [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/rfc/rfc2119>.
+
+   [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/rfc/rfc8174>.
+
+   [STRUCTURED-FIELDS]
+              Nottingham, M. and P. Kamp, "Structured Field Values for
+              HTTP", Work in Progress, Internet-Draft, draft-ietf-
+              httpbis-sfbis-06, 21 April 2024,
+              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
+              sfbis-06>.
+
+6.2.  Informative References
+
+   [TARGETED] Ludin, S., Nottingham, M., and Y. Wu, "Targeted HTTP Cache
+              Control", RFC 9213, DOI 10.17487/RFC9213, June 2022,
+              <https://www.rfc-editor.org/rfc/rfc9213>.
+
+Appendix A.  Acknowledgements
+
+   Thanks to Stephen Ludin for his review and suggestions.
+
+Author's Address
+
+   Mark Nottingham
+   Prahran
+   Australia
+   Email: mnot@mnot.net
+   URI:   https://www.mnot.net/