RFC 9211

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


   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

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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

   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",
   "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

   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

   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

   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

   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

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

   *  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

   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

   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,

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

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

   [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,

   [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>.

              Nottingham, M. and P-H. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,

7.2.  Informative References

   [ENTANGLE] Kettle, J., "Web Cache Entanglement: Novel Pathways to
              Poisoning", September 2020,

Author's Address

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