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Prahran Australia mnot@mnot.net https://www.mnot.net/

Web and Internet Transport HTTP HTTP, Caching, Invalidation 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 strings. About This Document 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 .

Introduction 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. introduces a means of describing the relationships between stored responses in HTTP caches, by associating those responses with one or more groups that reflect those relationships. It also describes how caches can use that information to apply invalidation events to members of a group. introduces one new source of such events: a HTTP response header field 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 (see ). They do not address the 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.

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 when, and only when, they appear in all capitals, as shown here. This specification uses the following terminology from : List, String, Parameter.

The Cache-Groups Response Header Field The Cache-Groups HTTP Response Header is a List of Strings (Sections and of ). Each member of the list is a value that identifies a group that the response belongs to. These strings are opaque -- while they might have some meaning to the server that creates them, the cache does not have any insight into their structure or content (beyond uniquely identifying a group). The ordering of members is not significant. Unrecognised Parameters are to be ignored. Implementations MUST support at least 32 groups in a field value, with up to at least 32 characters in each member. Note that generic limitations on HTTP field lengths may constrain the size of this field value in practice.

Identifying Grouped Responses Two responses stored in the same cache are considered to belong to 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 (case sensitive).
2. They both share the same URI origin (per ).

Cache Behaviour

Invalidation A cache that invalidates a stored response MAY invalidate any stored responses that share groups (per ) with that response. Cache extensions can explicitly strengthen the requirement above. For example, a targeted cache control header field might specify that caches processing it are required to invalidate such responses.

The Cache-Group-Invalidation Response Header Field The Cache-Group-Invalidation response header field is a List of Strings (Sections and of ). Each member of the list is a value that identifies a group that the response invalidates, per . For example, following a POST request that has side effects on two cache groups, the corresponding response could indicate that stored responses associated with either or both of those groups should be invalidated with: The Cache-Group-Invalidation header field MUST be ignored on responses to requests that have a safe method (e.g., GET; see ). 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 ) with any of the listed groups. Cache extensions can explicitly strengthen the requirement above. For example, a targeted cache control header field might specify that caches processing it are required to respect the Cache-Group-Invalidation signal. The ordering of members is not significant. Unrecognised Parameters are to be ignored. Implementations MUST support at least 32 groups in a field value, with up to at least 32 characters in each member. Note that generic limitations on HTTP field lengths may constrain the size of this field value in practice.

IANA Considerations IANA should perform the following tasks:

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:

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.

References Normative References The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230. The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages. This document obsoletes RFC 7234. This document describes a set of data types and associated algorithms that are intended to make it easier and safer to define and handle HTTP header and trailer fields, known as "Structured Fields", "Structured Headers", or "Structured Trailers". It is intended for use by specifications of new HTTP fields. This document obsoletes RFC 8941. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This specification defines a convention for HTTP response header fields that allow cache directives to be targeted at specific caches or classes of caches. It also defines one such header field, the CDN-Cache-Control response header field, which is targeted at content delivery network (CDN) caches.

Acknowledgements Thanks to Stephen Ludin for his review and suggestions.