HTTP M. Nottingham
Internet-Draft Fastly
Intended status: Standards Track March 1, 2020
Expires: September 2, 2020
The Cache-Status HTTP Response Header Field
draft-ietf-httpbis-cache-header-03
Abstract
To aid debugging, HTTP caches often append headers to a response
detailing how they handled the request. This specification codifies
that practice and updates it for 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.
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This Internet-Draft will expire on September 2, 2020.
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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
3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Normative References . . . . . . . . . . . . . . . . . . 7
5.2. Informative References . . . . . . . . . . . . . . . . . 7
5.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
To aid debugging, HTTP caches often append headers to a response
detailing 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 = sh-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, to allow debugging of
the entire chain of caches handling the request.
Each list member identifies the cache that inserted that value, and
MUST have a type of either sh-string or sh-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 = sh-boolean
fwd = sh-token
fwd-status = sh-integer
ttl = sh-integer
stored = sh-boolean
collapsed = sh-boolean
key = sh-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 (possibly with modifications; e.g., if the request was
conditional, a 304 Not Modified could be generated from cache).
"hit" and "fwd" are exclusive; only one of them should appear on each
list member.
2.2. The fwd parameter
"fwd" indicates why the request went forward.
It can have one of the following values:
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 forward 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.
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
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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
4. 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.
However, if the Cache-Status header 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 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
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headers (as are present in the Web platform). When in doubt, the
Cache-Status header field can be omitted.
5. References
5.1. Normative References
[]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-13 (work in progress),
August 2019.
[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>.
[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>.
5.2. 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>.
5.3. 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
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Author's Address
Mark Nottingham
Fastly
Email: mnot@mnot.net
URI: https://www.mnot.net/
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