The Cache-Status HTTP Response Header Field

To aid debugging, HTTP caches often append header fields to a response explaining how they handled the request. This specification codifies that practice and updates it to align with HTTP's current caching model.¶

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 October 22, 2021.¶

Copyright (c) 2021 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.¶

To aid debugging, HTTP caches often append header fields to a response explaining how they handled the request. Unfortunately, the semantics of these headers are often unclear, and both the semantics and syntax used vary between implementations.¶

This specification defines a new HTTP response header field, "Cache-Status" for this purpose.¶

The Cache-Status HTTP response header field indicates caches' handling of the request corresponding to the response it occurs within.¶

Each member of the list represents a cache that has handled the request. The first member of the list represents the cache closest to the origin server, and the last member of the list represents the cache closest to the user (possibly including the user agent's cache itself, if it appends a value).¶

The Cache-Status header field is only applicable to responses that are generated by an origin server. An intermediary SHOULD NOT append a Cache-Status member to responses that it generates, even if that intermediary contains a cache, except when the generated response is based upon a stored response (e.g., a 304 Not Modified or 206 Partial Content).¶

Caches determine when it is appropriate to add the Cache-Status header field to a response. Some might add it to all responses, whereas others might only do so when specifically configured to, or when the request contains a header field that activates a debugging mode.¶

When adding a value to the Cache-Status header field, caches SHOULD preserve the existing field value, to allow debugging of the entire chain of caches handling the request.¶

Each list member identifies the cache that inserted it and MUST be a String or Token. Depending on the deployment, this might be a product or service name (e.g., ExampleCache or "Example CDN"), a hostname ("cache-3.example.com"), an IP address, or a generated string.¶

Each member of the list can have parameters that describe that cache's handling of the request. While these parameters are OPTIONAL, caches are encouraged to provide as much information as possible.¶

New Cache-Status Parameters can be defined by registering them in the HTTP Cache-Status Parameters registry.¶

Registration requests are reviewed and approved by a Designated Expert, as per [RFC8126], Section 4.5. A specification document is appreciated, but not required.¶

• Community feedback
• If the value is sufficiently well-defined
• Generic parameters are preferred over vendor-specific, application-specific, or deployment-specific values. If a generic value cannot be agreed upon in the community, the parameter's name should be correspondingly specific (e.g., with a prefix that identifies the vendor, application or deployment).

• Name: [a name for the Cache-Status Parameter that matches key]
• Description: [a description of the parameter semantics and value]
• Reference: [to a specification defining this parameter]

See the registry at https://iana.org/assignments/http-cache-status for details on where to send registration requests.¶

Upon publication, please create the HTTP Cache-Status Parameters registry at https://iana.org/assignments/http-cache-status and populate it with the types defined in Section 2; see Section 4 for its associated procedures.¶

Attackers can use the information in Cache-Status to probe the behaviour of the cache (and other components), and infer the activity of those using the cache. The Cache-Status header field may not create these risks on its own, but can assist attackers in exploiting them.¶

For example, knowing if a cache has stored a response can help an attacker execute a timing attack on sensitive data. Exposing the cache key can help an attacker understand modifications to the cache key, which may assist cache poisoning attacks. See [ENTANGLE] for details.¶

The underlying risks can be mitigated with a variety of techniques (e.g., use of encryption and authentication; avoiding the inclusion of attacker-controlled data in the cache key), depending on their exact nature.¶

To avoid assisting such attacks, the Cache-Status header field can be omitted, only sent when the client is authorized to receive it, or only send sensitive information (e.g., the key parameter) when the client is authorized.¶