Diff: rfc9875.original

	rfc9875.original	rfc9875.txt


	HTTP M. Nottingham	Internet Engineering Task Force (IETF) M. Nottingham
	Internet-Draft 16 May 2025	Request for Comments: 9875 September 2025
	Intended status: Standards Track	Category: Standards Track
	Expires: 17 November 2025	ISSN: 2070-1721

	HTTP Cache Groups	HTTP Cache Groups

	draft-ietf-httpbis-cache-groups-07

	Abstract	Abstract

	This specification introduces a means of describing the relationships	This specification introduces a means of describing the relationships
	between stored responses in HTTP caches, "grouping" them by	between stored responses in HTTP caches, "grouping" them by
	associating a stored response with one or more strings.	associating a stored response with one or more 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	Status of This Memo


	This Internet-Draft is submitted in full conformance with the	This is an Internet Standards Track document.
	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	This document is a product of the Internet Engineering Task Force
	and may be updated, replaced, or obsoleted by other documents at any	(IETF). It represents the consensus of the IETF community. It has
	time. It is inappropriate to use Internet-Drafts as reference	received public review and has been approved for publication by the
	material or to cite them other than as "work in progress."	Internet Engineering Steering Group (IESG). Further information on
		Internet Standards is available in Section 2 of RFC 7841.


	This Internet-Draft will expire on 17 November 2025.	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/rfc9875.

	Copyright Notice	Copyright Notice

	Copyright (c) 2025 IETF Trust and the persons identified as the	Copyright (c) 2025 IETF Trust and the persons identified as the
	document authors. All rights reserved.	document authors. All rights reserved.

	This document is subject to BCP 78 and the IETF Trust's Legal	This document is subject to BCP 78 and the IETF Trust's Legal

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

	Table of Contents	Table of Contents


	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2	1. Introduction
	1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3	1.1. Notational Conventions
	2. The Cache-Groups Response Header Field . . . . . . . . . . . 3	2. The Cache-Groups Response Header Field
	2.1. Identifying Grouped Responses . . . . . . . . . . . . . . 4	2.1. Identifying Grouped Responses
	2.2. Cache Behaviour . . . . . . . . . . . . . . . . . . . . . 4	2.2. Cache Behaviour
	2.2.1. Invalidation . . . . . . . . . . . . . . . . . . . . 4	2.2.1. Invalidation
	3. The Cache-Group-Invalidation Response Header Field . . . . . 4	3. The Cache-Group-Invalidation Response Header Field
	4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5	4. IANA Considerations
	4.1. HTTP Field Names . . . . . . . . . . . . . . . . . . . . 5	5. Security Considerations
	5. Security Considerations . . . . . . . . . . . . . . . . . . . 6	6. References
	6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6	6.1. Normative References
	6.1. Normative References . . . . . . . . . . . . . . . . . . 6	6.2. Informative References
	6.2. Informative References . . . . . . . . . . . . . . . . . 6	Acknowledgements
	Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 7	Author's Address
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7

	1. Introduction	1. Introduction

	HTTP caching [HTTP-CACHING] operates at the granularity of a single	HTTP caching [HTTP-CACHING] operates at the granularity of a single
	resource; the freshness of one stored response does not affect that	resource; the freshness of one stored response does not affect that
	of others. This granularity can make caching more efficient -- for	of others. This granularity can make caching more efficient -- for
	example, when a page is composed of many assets that have different	example, when a page is composed of many assets that have different
	requirements for caching.	requirements for caching.

	However, there are also cases where the relationship between stored	However, there are also cases where the relationship between stored

	skipping to change at page 3, line 23 ¶	skipping to change at line 90 ¶
	indicated by grouping can also be used by caches to optimise their	indicated by grouping can also be used by caches to optimise their
	operation; for example, it could be used to inform the operation of	operation; for example, it could be used to inform the operation of
	cache eviction algorithms.	cache eviction algorithms.

	Section 2 introduces a means of describing the relationships between	Section 2 introduces a means of describing the relationships between
	stored responses in HTTP caches, by associating those responses with	stored responses in HTTP caches, by associating those responses with
	one or more groups that reflect those relationships. It also	one or more groups that reflect those relationships. It also
	describes how caches can use that information to apply invalidation	describes how caches can use that information to apply invalidation
	events to members of a group.	events to members of a group.


	Section 3 introduces one new source of such events: a HTTP response	Section 3 introduces one new source of such events: an HTTP response
	header field that allows a state-changing response to trigger a group	header field that allows a state-changing response to trigger a group
	invalidation.	invalidation.

	These mechanisms operate within a single cache, across the stored	These mechanisms operate within a single cache, across the stored
	responses associated with a single origin server (see Section 2.1).	responses associated with a single origin server (see Section 2.1).
	They do not address the issues of synchronising state between	They do not address the issues of synchronising state between
	multiple caches (e.g., in a hierarchy or mesh), nor do they	multiple caches (e.g., in a hierarchy or mesh), nor do they
	facilitate association of stored responses from disparate origins.	facilitate association of stored responses from disparate origins.

	1.1. Notational Conventions	1.1. Notational Conventions

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

	This specification uses the following terminology from	This specification uses the following terminology from

	[STRUCTURED-FIELDS]: List, String, Parameter.	[STRUCTURED-FIELDS]: List, String, and Parameter.

	2. The Cache-Groups Response Header Field	2. The Cache-Groups Response Header Field

	The Cache-Groups HTTP Response Header is a List of Strings (Sections	The Cache-Groups HTTP Response Header is a List of Strings (Sections
	3.1 and 3.3.1 of [STRUCTURED-FIELDS]). Each member of the list is a	3.1 and 3.3.1 of [STRUCTURED-FIELDS]). Each member of the list is a
	value that identifies a group that the response belongs to. These	value that identifies a group that the response belongs to. These
	strings are opaque -- while they might have some meaning to the	strings are opaque -- while they might have some meaning to the
	server that creates them, the cache does not have any insight into	server that creates them, the cache does not have any insight into
	their structure or content (beyond uniquely identifying a group).	their structure or content (beyond uniquely identifying a group).


	skipping to change at page 4, line 37 ¶	skipping to change at line 152 ¶
	2. They both share the same URI origin (per Section 4.3.1 of	2. They both share the same URI origin (per Section 4.3.1 of
	[HTTP]).	[HTTP]).

	2.2. Cache Behaviour	2.2. Cache Behaviour

	2.2.1. Invalidation	2.2.1. Invalidation

	A cache that invalidates a stored response MAY invalidate any stored	A cache that invalidates a stored response MAY invalidate any stored
	responses that share groups (per Section 2.1) with that response.	responses that share groups (per Section 2.1) with that response.
	Note that further grouped invalidations are not triggered by a	Note that further grouped invalidations are not triggered by a

	grouped invalidation; i.e., this mechanism does not "cascade."	grouped invalidation; i.e., this mechanism does not "cascade".

	Cache extensions can explicitly strengthen the requirement above.	Cache extensions can explicitly strengthen the requirement above.
	For example, a targeted cache control header field [TARGETED] might	For example, a targeted cache control header field [TARGETED] might
	specify that caches processing it are required to invalidate such	specify that caches processing it are required to invalidate such
	responses.	responses.

	3. The Cache-Group-Invalidation Response Header Field	3. The Cache-Group-Invalidation Response Header Field

	The Cache-Group-Invalidation response header field is a List of	The Cache-Group-Invalidation response header field is a List of
	Strings (Sections 3.1 and 3.3.1 of [STRUCTURED-FIELDS]). Each member	Strings (Sections 3.1 and 3.3.1 of [STRUCTURED-FIELDS]). Each member

	skipping to change at page 5, line 37 ¶	skipping to change at line 198 ¶
	The ordering of members is not significant. Unrecognised Parameters	The ordering of members is not significant. Unrecognised Parameters
	are to be ignored.	are to be ignored.

	Implementations MUST support at least 32 groups in a field value,	Implementations MUST support at least 32 groups in a field value,
	with up to at least 32 characters in each member. Note that generic	with up to at least 32 characters in each member. Note that generic
	limitations on HTTP field lengths may constrain the size of this	limitations on HTTP field lengths may constrain the size of this
	field value in practice.	field value in practice.

	4. IANA Considerations	4. IANA Considerations


	IANA should perform the following tasks:	IANA has added the following entries to the "Hypertext Transfer
		Protocol (HTTP) Field Name Registry":
	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	Field Name: Cache-Groups
		Status: permanent
		Reference: RFC 9875


	* Comments:	Field Name: Cache-Group-Invalidation
		Status: permanent
		Reference: RFC 9875

	5. Security Considerations	5. Security Considerations

	This mechanism allows resources that share an origin to invalidate	This mechanism allows resources that share an origin to invalidate
	each other. Because of this, origins that represent multiple parties	each other. Because of this, origins that represent multiple parties
	(sometimes referred to as "shared hosting") might allow one party to	(sometimes referred to as "shared hosting") might allow one party to

	group its resources with those of others, or to send signals which	group its resources with those of others or to send signals that have
	have side effects upon them.	side effects upon them.

	Shared hosts that wish to mitigate these risks can control access to	Shared hosts that wish to mitigate these risks can control access to
	the header fields defined in this specification.	the header fields defined in this specification.

	6. References	6. References

	6.1. Normative References	6.1. Normative References

	[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,	[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
	Ed., "HTTP Semantics", STD 97, RFC 9110,	Ed., "HTTP Semantics", STD 97, RFC 9110,
	DOI 10.17487/RFC9110, June 2022,	DOI 10.17487/RFC9110, June 2022,

	<https://www.rfc-editor.org/rfc/rfc9110>.	<https://www.rfc-editor.org/info/rfc9110>.

	[HTTP-CACHING]	[HTTP-CACHING]
	Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,	Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
	Ed., "HTTP Caching", STD 98, RFC 9111,	Ed., "HTTP Caching", STD 98, RFC 9111,
	DOI 10.17487/RFC9111, June 2022,	DOI 10.17487/RFC9111, June 2022,

	<https://www.rfc-editor.org/rfc/rfc9111>.	<https://www.rfc-editor.org/info/rfc9111>.

	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,	Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,	DOI 10.17487/RFC2119, March 1997,

	<https://www.rfc-editor.org/rfc/rfc2119>.	<https://www.rfc-editor.org/info/rfc2119>.

	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,

	May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.	May 2017, <https://www.rfc-editor.org/info/rfc8174>.

	[STRUCTURED-FIELDS]	[STRUCTURED-FIELDS]
	Nottingham, M. and P. Kamp, "Structured Field Values for	Nottingham, M. and P. Kamp, "Structured Field Values for
	HTTP", RFC 9651, DOI 10.17487/RFC9651, September 2024,	HTTP", RFC 9651, DOI 10.17487/RFC9651, September 2024,

	<https://www.rfc-editor.org/rfc/rfc9651>.	<https://www.rfc-editor.org/info/rfc9651>.

	6.2. Informative References	6.2. Informative References

	[TARGETED] Ludin, S., Nottingham, M., and Y. Wu, "Targeted HTTP Cache	[TARGETED] Ludin, S., Nottingham, M., and Y. Wu, "Targeted HTTP Cache
	Control", RFC 9213, DOI 10.17487/RFC9213, June 2022,	Control", RFC 9213, DOI 10.17487/RFC9213, June 2022,

	<https://www.rfc-editor.org/rfc/rfc9213>.	<https://www.rfc-editor.org/info/rfc9213>.


	Appendix A. Acknowledgements	Acknowledgements

	Thanks to Stephen Ludin for his review and suggestions.	Thanks to Stephen Ludin for his review and suggestions.

	Author's Address	Author's Address

	Mark Nottingham	Mark Nottingham
	Prahran	Prahran
	Australia	Australia
	Email: mnot@mnot.net	Email: mnot@mnot.net
	URI: https://www.mnot.net/	URI: https://www.mnot.net/

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