The Cache-Status HTTP Response Header Field
draft-ietf-httpbis-cache-header-04
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9211.
|
|
|---|---|---|---|
| Author | Mark Nottingham | ||
| Last updated | 2020-08-05 | ||
| Replaces | draft-nottingham-cache-header | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
ARTART Telechat review
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by Martin Dürst
Ready w/issues
GENART Last Call review
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by Paul Kyzivat
Ready w/issues
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
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| IESG | IESG state | Became RFC 9211 (Proposed Standard) | |
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draft-ietf-httpbis-cache-header-04
HTTP M. Nottingham
Internet-Draft Fastly
Intended status: Standards Track August 5, 2020
Expires: February 6, 2021
The Cache-Status HTTP Response Header Field
draft-ietf-httpbis-cache-header-04
Abstract
To aid debugging, HTTP caches often append header fields to a
response explaining how they handled the request. This specification
codifies that practice and updates it to align with HTTP's current
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 (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].
Working Group information can be found at https://httpwg.org/ [2];
source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/cache-header [3].
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 https://datatracker.ietf.org/drafts/current/.
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 February 6, 2021.
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Copyright Notice
Copyright (c) 2020 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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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 . . . . . . . . . . . . . . . . . . . . 5
2.5. The stored parameter . . . . . . . . . . . . . . . . . . 5
2.6. The collapsed parameter . . . . . . . . . . . . . . . . . 5
2.7. The key parameter . . . . . . . . . . . . . . . . . . . . 5
2.8. The detail parameter . . . . . . . . . . . . . . . . . . 5
3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Defining New Proxy-Status Parameters . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 9
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
To aid debugging, 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 greatly between implementations.
This specification defines a single, new HTTP response header field,
"Cache-Status" for this purpose.
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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 ABNF as defined in [RFC5234], along with the "%s"
extension for case sensitivity defined in [RFC7405].
2. The Cache-Status HTTP Response Header Field
The Cache-Status HTTP response header indicates caches' handling of
the request corresponding to the response it occurs within.
Its value is a List [I-D.ietf-httpbis-header-structure]:
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 client (possibly including the user agent's
cache itself, if it chooses to append a value).
Caches determine when it is appropriate to add the Cache-Status
header field to a response. Some might decide to add it to all
responses, whereas others might only do so when specifically
configured to, or when the request contains a header that activates a
debugging mode.
When adding a value to the Cache-Status header field, caches SHOULD
preserve the existing contents of the header field, to allow
debugging of the entire chain of caches handling the request.
Each list member identifies the cache that inserted that value, and
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"), and IP address, or a generated
string.
Each member of the list can also have parameters that describe that
cache's handling of the request. While all of these parameters are
OPTIONAL, caches are encouraged to provide as much information as
possible.
This specification defines these parameters:
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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 did not go forward and the response was obtained from
the cache. A response that originally was produced by the origin but
was modified by the cache (for example, a 304 or 206 status code) is
still 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
why.
The following values are defined to explain why the request went
forward:
o uri-miss - The cache did not contain any responses that matched
the request URI
o 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.
o 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)
o stale - The cache was able to select a response for the request,
but it was stale
o request - The cache was able to select a fresh response for the
request, but client request headers (e.g., Cache-Control request
directives) did not allow its use
o bypass - The cache was configured to not handle this request
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2.3. The fwd-status parameter
"fwd-status" indicates what status code the next hop server returned
in response to the 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 due to a range request.
2.4. The ttl parameter
"ttl" indicates the response's remaining freshness lifetime as
calculated by the cache, as an integer number of seconds, measured
when the response is sent by the cache. This includes freshness
assigned by the cache; e.g., through heuristics, local configuration,
or other factors. May be negative, to indicate staleness.
2.5. The stored parameter
"stored" indicates whether the cache stored the response; 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; 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 used for the
response. Note that this may be implementation-specific.
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
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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 needs
to convey additional information, they are encouraged to register
extension parameters (see Section 4) or define another header field.
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 layers of caching, both of which were hits, and the
second collapsed with other requests:
Cache-Status: OriginCache; hit; ttl=1100; collapsed,
"CDN Company Here"; hit; ttl=545
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4. Defining New Proxy-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
requests:
o Community feedback
o If the value is sufficiently well-defined
o 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:
o Name: [a name for the Cache-Status Parameter that matches key]
o Description: [a description of the parameter semantics and value]
o Reference: [to a specification defining this parameter]
See the registry at https://iana.org/assignments/http-cache-status
[4] for details on where to send registration requests.
5. IANA Considerations
Upon publication, please create the HTTP Cache-Status Parameters
registry at https://iana.org/assignments/http-cache-statuses [5] and
populate it with the types defined in Section 2; see Section 4 for
its associated procedures.
6. Security Considerations
Information about a cache's content can be used to infer the activity
of those using it. Generally, access to sensitive information in a
cache is limited to those who are authorised to access that
information (using a variety of techniques), so this does not
represent an attack vector in the general sense.
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However, if the Cache-Status header field is exposed to parties who
are not authorised to obtain the response it occurs within, it could
expose information about that data.
For example, if an attacker were able to obtain the Cache-Status
header field from a response containing sensitive information and
access were limited to one person (or limited set of people), they
could determine whether that information had been accessed before.
This is similar to the information exposed by various timing attacks,
but is arguably more reliable, since the cache is directly reporting
its state.
Mitigations include use of encryption (e.g., TLS [RFC8446])) to
protect the response, and careful controls over access to response
header fields (as are present in the Web platform). When in doubt,
the Cache-Status header field can be omitted.
7. References
7.1. Normative References
[I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", draft-ietf-httpbis-header-structure-19 (work in
progress), June 2020.
[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>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
[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>.
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7.2. Informative References
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
7.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] https://httpwg.org/
[3] https://github.com/httpwg/http-extensions/labels/cache-header
[4] https://iana.org/assignments/http-cache-status
[5] https://iana.org/assignments/http-cache-statuses
Author's Address
Mark Nottingham
Fastly
made in
Prahran, VIC
Australia
Email: mnot@mnot.net
URI: https://www.mnot.net/
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