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
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  to this document.  Code Components extracted from this document must
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  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: [email protected]
  URI:   https://www.mnot.net/

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