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.
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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/