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Internet Engineering Task Force (IETF) M. Nottingham Request for Comments: 9211 Fastly Category: Standards Track June 2022 ISSN: 2070-1721

            The Cache-Status HTTP Response Header Field

Abstract

 To aid debugging, HTTP caches often append header fields to a
 response, explaining how they handled the request in an ad hoc
 manner.  This specification defines a standard mechanism to do so
 that is aligned with HTTP's caching model.

Status of This Memo

 This is an Internet Standards Track document.

 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.

 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/rfc9211.

Copyright Notice

 Copyright (c) 2022 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-Status HTTP Response Header Field
   2.1.  The hit Parameter
   2.2.  The fwd Parameter
   2.3.  The fwd-status Parameter
   2.4.  The ttl Parameter
   2.5.  The stored Parameter
   2.6.  The collapsed Parameter
   2.7.  The key Parameter
   2.8.  The detail Parameter
 3.  Examples
 4.  Defining New Cache-Status Parameters
 5.  IANA Considerations
 6.  Security Considerations
 7.  References
   7.1.  Normative References
   7.2.  Informative References
 Author's Address

1. Introduction

 To aid debugging (both by humans and automated tools), HTTP caches
 often append header fields to a response explaining how they handled
 the request.  Unfortunately, the semantics of these header fields 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 with standardized syntax and semantics.

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 document uses the following terminology from Section 3 of
 [STRUCTURED-FIELDS] to specify syntax and parsing: List, String,
 Token, Integer, and Boolean.

 This document also uses terminology from [HTTP] and [HTTP-CACHING].

2. The Cache-Status HTTP Response Header Field

 The Cache-Status HTTP response header field indicates how caches have
 handled that response and its corresponding request.  The syntax of
 this header field conforms to [STRUCTURED-FIELDS].

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

 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.  See Section 6 for related security considerations.

 An intermediary SHOULD NOT append a Cache-Status member to responses
 that it generates locally, even if that intermediary contains a
 cache, unless the generated response is based upon a stored response
 (e.g., 304 (Not Modified) and 206 (Partial Content) are both based
 upon a stored response).  For example, a proxy generating a 400
 response due to a malformed request will not add a Cache-Status
 value, because that response was generated by the proxy, not the
 origin server.

 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 this
 identifier 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.

 This specification defines the following parameters.

2.1. The hit Parameter

 The value of "hit" is a Boolean that, when true, indicates that the
 request was satisfied by the cache; that is, it was not forwarded,
 and the response was obtained from the cache.

 A response that was originally produced by the origin but was
 modified by the cache (for example, a 304 or 206 status code) is
 still considered a hit, as long as it did not go forward (e.g., for
 validation).

 A response that was in cache but not able to be used without going
 forward (e.g., because it was stale or partial) is not considered a
 hit.  Note that a stale response that is used without going forward
 (e.g., because the origin server is not available) can be considered
 a hit.

 "hit" and "fwd" are exclusive; only one of them should appear on each
 list member.

2.2. The fwd Parameter

 "fwd", when present, indicates that the request went forward towards
 the origin; its value is a Token that indicates why.

 The following parameter values are defined to explain why the request
 went forward, from most specific to least:

 bypass:  The cache was configured to not handle this request.

 method:  The request method's semantics require the request to be
    forwarded.

 uri-miss:  The cache did not contain any responses that matched the
    request URI.

 vary-miss:  The cache contained a response that matched the request
    URI, but it could not select a response based upon this request's
    header fields and stored Vary header fields.

 miss:  The cache did not contain any responses that could be used to
    satisfy this request (to be used when an implementation cannot
    distinguish between uri-miss and vary-miss).

 request:  The cache was able to select a fresh response for the
    request, but the request's semantics (e.g., Cache-Control request
    directives) did not allow its use.

 stale:  The cache was able to select a response for the request, but
    it was stale.

 partial:  The cache was able to select a partial response for the
    request, but it did not contain all of the requested ranges (or
    the request was for the complete response).

 The most specific reason known to the cache SHOULD be used, to the
 extent that it is possible to implement.  See also [HTTP-CACHING],
 Section 4.

2.3. The fwd-status Parameter

 The value of "fwd-status" is an Integer that indicates which status
 code (see [HTTP], Section 15) the next-hop server returned in
 response to the forwarded request.  The fwd-status parameter is only
 meaningful when fwd is present.  If fwd-status is not present but the
 fwd parameter is, it defaults to the status code sent in the
 response.

 This parameter is useful to distinguish cases when the next-hop
 server sends a 304 (Not Modified) response to a conditional request
 or a 206 (Partial Content) response because of a range request.

2.4. The ttl Parameter

 The value of "ttl" is an Integer that indicates the response's
 remaining freshness lifetime (see [HTTP-CACHING], Section 4.2.1) as
 calculated by the cache, as an integer number of seconds, measured as
 closely as possible to when the response header section is sent by
 the cache.  This includes freshness assigned by the cache through,
 for example, heuristics (see [HTTP-CACHING], Section 4.2.2), local
 configuration, or other factors.  It may be negative, to indicate
 staleness.

2.5. The stored Parameter

 The value of "stored" is a Boolean that indicates whether the cache
 stored the response (see [HTTP-CACHING], Section 3); a true value
 indicates that it did.  The stored parameter is only meaningful when
 fwd is present.

2.6. The collapsed Parameter

 The value of "collapsed" is a Boolean that indicates whether this
 request was collapsed together with one or more other forward
 requests (see [HTTP-CACHING], Section 4).  If true, the response was
 successfully reused; if not, a new request had to be made.  If not
 present, the request was not collapsed with others.  The collapsed
 parameter is only meaningful when fwd is present.

2.7. The key Parameter

 The value of "key" is a String that conveys a representation of the
 cache key (see [HTTP-CACHING], Section 2) used for the response.
 Note that this may be implementation specific.

2.8. The detail Parameter

 The value of "detail" is either a String or a Token that allows
 implementations to convey additional information not captured in
 other parameters, such as implementation-specific states or other
 caching-related metrics.

 For example:

 Cache-Status: ExampleCache; hit; detail=MEMORY

 The semantics of a detail parameter are always specific to the cache
 that sent it; even if a details parameter from another cache shares
 the same value, it might not mean the same thing.

 This parameter is intentionally limited.  If an implementation's
 developer or operator needs to convey additional information in an
 interoperable fashion, they are encouraged to register extension
 parameters (see Section 4) or define another header field.

3. Examples

 The following is an example of a minimal cache hit:

 Cache-Status: ExampleCache; hit

 However, a polite cache will give some more information, e.g.:

 Cache-Status: ExampleCache; hit; ttl=376

 A stale hit just has negative freshness, as in this example:

 Cache-Status: ExampleCache; hit; ttl=-412

 Whereas this is an example of a complete miss:

 Cache-Status: ExampleCache; fwd=uri-miss

 This is an example of a miss that successfully validated on the
 backend server:

 Cache-Status: ExampleCache; fwd=stale; fwd-status=304

 This is an example of a miss that was collapsed with another request:

 Cache-Status: ExampleCache; fwd=uri-miss; collapsed

 This is an example of a miss that the cache attempted to collapse,
 but couldn't:

 Cache-Status: ExampleCache; fwd=uri-miss; collapsed=?0

 The following is an example of going through two separate layers of
 caching, where the cache closest to the origin responded to an
 earlier request with a stored response, and a second cache stored
 that response and later reused it to satisfy the current request:

 Cache-Status: OriginCache; hit; ttl=1100,
               "CDN Company Here"; hit; ttl=545

 The following is an example of going through a three-layer caching
 system, where the closest to the origin is a reverse proxy (where the
 response was served from cache); the next is a forward proxy
 interposed by the network (where the request was forwarded because
 there wasn't any response cached with its URI, the request was
 collapsed with others, and the resulting response was stored); and
 the closest to the user is a browser cache (where there wasn't any
 response cached with the request's URI):

 Cache-Status: ReverseProxyCache; hit
 Cache-Status: ForwardProxyCache; fwd=uri-miss; collapsed; stored
 Cache-Status: BrowserCache; fwd=uri-miss

4. Defining New Cache-Status Parameters

 New Cache-Status parameters can be defined by registering them in the
 "HTTP Cache-Status" registry.

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

 The expert(s) should consider the following factors when evaluating
 requests:

• 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).

 Registration requests should use the following template:

 Name:  [a name for the Cache-Status parameter's key; see
    Section 3.1.2 of [STRUCTURED-FIELDS] for syntactic requirements]

 Type:  [the Structured Type of the parameter's value; see
    Section 3.1.2 of [STRUCTURED-FIELDS]]

 Description:  [a description of the parameter's semantics]

 Reference:  [to a specification defining this parameter, if
    available]

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

5. IANA Considerations

 IANA has created the "HTTP Cache-Status" registry at
 <https://www.iana.org/assignments/http-cache-status> and populated it
 with the types defined in Section 2; see Section 4 for its associated
 procedures.

 IANA has added the following entry in the "Hypertext Transfer
 Protocol (HTTP) Field Name Registry" defined in [HTTP], Section 18.4:

 Field name:  Cache-Status
 Status:  permanent
 Reference:  RFC 9211

6. Security Considerations

 Attackers can use the information in Cache-Status to probe the
 behavior 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 it 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.

 Additionally, 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., using encryption and authentication and avoiding the inclusion
 of attacker-controlled data in the cache key), depending on their
 exact nature.  Note that merely obfuscating the key does not mitigate
 this risk.

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

7. References

7.1. 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/info/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/info/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/info/rfc2119>.

 [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
            Writing an IANA Considerations Section in RFCs", BCP 26,
            RFC 8126, DOI 10.17487/RFC8126, June 2017,
            <https://www.rfc-editor.org/info/rfc8126>.

 [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/info/rfc8174>.

 [STRUCTURED-FIELDS]
            Nottingham, M. and P-H. Kamp, "Structured Field Values for
            HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
            <https://www.rfc-editor.org/info/rfc8941>.

7.2. Informative References

 [ENTANGLE] Kettle, J., "Web Cache Entanglement: Novel Pathways to
            Poisoning", September 2020,
            <https://portswigger.net/research/web-cache-entanglement>.

Author's Address

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