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The Cache-Status HTTP Response Header Field

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9211.
Author Mark Nottingham
Last updated 2022-06-08 (Latest revision 2021-08-17)
Replaces draft-nottingham-cache-header
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Associated WG milestone
Submit Cache-Status Header
Document shepherd Tommy Pauly
Shepherd write-up Show Last changed 2021-04-20
IESG IESG state Became RFC 9211 (Proposed Standard)
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Consensus boilerplate Yes
Telechat date (None)
Responsible AD Francesca Palombini
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IANA IANA review state IANA OK - Actions Needed
IANA action state RFC-Ed-Ack
HTTP                                                       M. Nottingham
Internet-Draft                                                    Fastly
Intended status: Standards Track                          17 August 2021
Expires: 18 February 2022

              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.

Note to Readers

   _RFC EDITOR: please remove this section before publication_

   Discussion of this draft takes place on the HTTP working group
   mailing list (, which is archived at

   Working Group information can be found at
   (; source code and issues list for this draft can
   be found at
   header (

Status of This Memo

   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

   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 18 February 2022.

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Copyright Notice

   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 (
   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  The Cache-Status HTTP Response Header Field . . . . . . . . .   3
     2.1.  The hit parameter . . . . . . . . . . . . . . . . . . . .   4
     2.2.  The fwd parameter . . . . . . . . . . . . . . . . . . . .   4
     2.3.  The fwd-status parameter  . . . . . . . . . . . . . . . .   5
     2.4.  The ttl parameter . . . . . . . . . . . . . . . . . . . .   6
     2.5.  The stored parameter  . . . . . . . . . . . . . . . . . .   6
     2.6.  The collapsed parameter . . . . . . . . . . . . . . . . .   6
     2.7.  The key parameter . . . . . . . . . . . . . . . . . . . .   6
     2.8.  The detail parameter  . . . . . . . . . . . . . . . . . .   6
   3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Defining New Cache-Status Parameters  . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

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 headers 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.

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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 ABNF as defined in [RFC5234], with rules prefixed
   with "sf-" and the "key" rule as defined in [STRUCTURED-FIELDS].  It
   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 ([STRUCTURED-FIELDS], Section 3.1):

   Cache-Status   = sf-list

   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).

   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.

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   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 (""), 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:

   hit          = sf-boolean
   fwd          = sf-token
   fwd-status   = sf-integer
   ttl          = sf-integer
   stored       = sf-boolean
   collapsed    = sf-boolean
   key          = sf-string
   detail       = sf-token / sf-string

2.1.  The hit parameter

   "hit", when true, indicates that the request was satisfied by the
   cache; i.e., 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" indicates that the request went forward towards the origin, and

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   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 could not select a response based upon this
      request's headers and stored Vary headers.

   *  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 that the cache is aware of SHOULD be used,
   to the extent that it is possible to implement.  See also
   [HTTP-CACHING], Section 4.

2.3.  The fwd-status parameter

   "fwd-status" indicates what status code (see [HTTP], Section 15) the
   next hop server returned in response to the forwarded request.  Only
   meaningful when "fwd" is present; if "fwd-status" is not present but
   "fwd" 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 Response because of a range request.

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2.4.  The ttl parameter

   "ttl" 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; e.g., through heuristics (see
   [HTTP-CACHING], Section 4.2.2), local configuration, or other
   factors.  May be negative, to indicate staleness.

2.5.  The stored parameter

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

2.6.  The collapsed parameter

   "collapsed" 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.  Only meaningful when fwd is present.

2.7.  The key parameter

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

2.8.  The detail parameter

   "detail" allows implementations to convey additional information not
   captured in other parameters; for example, 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 member of details from another cache shares
   the same name, 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.

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3.  Examples

   The most minimal cache hit:

   Cache-Status: ExampleCache; hit

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

   Cache-Status: ExampleCache; hit; ttl=376

   A stale hit just has negative freshness:

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

   Whereas a complete miss is:

   Cache-Status: ExampleCache; fwd=uri-miss

   A miss that successfully validated on the back-end server:

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

   A miss that was collapsed with another request:

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

   A miss that the cache attempted to collapse, but couldn't:

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

   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

   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):

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   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 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.

   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 that matches the
      'key' ABNF rule]

   *  Description: [a description of the parameter semantics and value]

   *  Reference: [to a specification defining this parameter, if

   See the registry at
   ( for details on where
   to send registration requests.

5.  IANA Considerations

   Upon publication, please create the HTTP Cache-Status Parameters
   registry at
   ( and populate it with
   the types defined in Section 2; see Section 4 for its associated

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   Also, please create the following entry in the Hypertext Transfer
   Protocol (HTTP) Field Name Registry defined in [HTTP], Section 18.4:

   *  Field name: Cache-Status

   *  Status: permanent

   *  Specification document: [this document]

   *  Comments:

6.  Security Considerations

   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

   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., use of encryption and authentication; 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
   only send sensitive information (e.g., the key parameter) when the
   client is authorized.

7.  References

7.1.  Normative References

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

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   [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,

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

   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-17, 25 July 2021,

              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Caching", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-cache-17, 25 July 2021,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

7.2.  Informative References

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

Author's Address

   Mark Nottingham
   Prahran VIC


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