HTTP M. Nottingham
Internet-Draft Fastly
Intended status: Standards Track November 4, 2019
Expires: May 7, 2020
The Cache-Status HTTP Response Header
draft-ietf-httpbis-cache-header-02
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 May 7, 2020.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. The Cache-Status HTTP Response Header . . . . . . . . . . . . 3
2.1. The fwd parameter . . . . . . . . . . . . . . . . . . . . 4
2.2. The fwd-res parameter . . . . . . . . . . . . . . . . . . 4
2.3. The fwd-stored parameter . . . . . . . . . . . . . . . . 5
2.4. The res-fresh parameter . . . . . . . . . . . . . . . . . 5
2.5. The cache-fresh parameter . . . . . . . . . . . . . . . . 5
2.6. The collapse-hit parameter . . . . . . . . . . . . . . . 5
2.7. The collapse-wait parameter . . . . . . . . . . . . . . . 5
2.8. The key parameter . . . . . . . . . . . . . . . . . . . . 6
3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Normative References . . . . . . . . . . . . . . . . . . 7
5.2. Informative References . . . . . . . . . . . . . . . . . 8
5.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 8
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
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 parameterised 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 agent (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.
The list members identify the cache that inserted the 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 a number of 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.
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fwd = sh-token
fwd-res = sh-token
fwd-stored = sh-boolean
res-fresh = sh-integer
cache-fresh = sh-integer
collapse-hit = sh-boolean
collapse-wait = sh-integer
key = sh-string
2.1. The fwd parameter
"fwd" indicates why the request went forward. If it is not present,
the value defaults to "none".
It can have one of the following values:
o none - The request did not go forward; i.e., it was a hit, and was
served from the cache.
o bypass - The cache was configured to not handle this request
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.
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 res-stale - The cache was able to select a response for the
request, but it was stale
o req-stale - 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
2.2. The fwd-res parameter
"fwd-res" indicates what the result of the forward request was. It
is only valid when fwd is "res-stale" or "req-stale", and defaults to
"full" if not present when fwd is one of those values.
It can have one of the following values:
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o full - indicates that the response was a complete response (any
status code except 304 Not Modified and 206 Partial Response)
o partial - indicates that the response was a 206 Partial Response
o notmod - indicates that the response was a 304 Not Modified
2.3. The fwd-stored parameter
"fwd-stored" indicates whether the cache stored the response; a true
value indicates that it did. Only valid when fwd is not "none".
2.4. The res-fresh parameter
"res-fresh" indicates the response's remaining freshness lifetime (as
per [I-D.ietf-httpbis-cache], Section 4.2.1), as an integer number of
seconds. This does not include freshness assigned by the cache (see
"cache-fresh"). May be negative, to indicate staleness.
2.5. The cache-fresh parameter
"cache-fresh" indicates the response's remaining freshness lifetime
as calculated by the cache, as an integer number of seconds. This
includes freshness assigned by the cache; e.g., through heuristics,
local configuration, or other factors. May be negative, to indicate
staleness.
If both cache-fresh and res-fresh appear as parameters on the same
value, it implies that the cache freshness overrode the response
freshness.
2.6. The collapse-hit parameter
"collapse-hit" 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.
2.7. The collapse-wait parameter
"collapse-wait" indicates the amount of time that the cache held the
request while waiting to see if it could be successfully collapsed,
as an integer number of milliseconds.
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2.8. 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
... but a polite cache will give some more information, e.g.:
Cache-Status: ExampleCache; res-fresh=376
A "negative" hit (i.e., the cache imposed its own freshness
lifetime):
Cache-Status: ExampleCache; cache-fresh=415
A stale hit just has negative freshness:
Cache-Status: ExampleCache; res-fresh=-412
Whereas a complete miss is:
Cache-Status: ExampleCache; fwd=uri-miss
A miss that validated on the back-end server:
Cache-Status: ExampleCache; fwd=res-stale; fwd-res=notmod
A miss that was collapsed with another request:
Cache-Status: ExampleCache; fwd=uri-miss; collapse-hit=?1
A miss that the cache attempted to collapse, but couldn't:
Cache-Status: ExampleCache; fwd=uri-miss;
collapse-hit=?0; collapse-wait=240
Going through two layers of caching, both of which were hits, and the
second collapsed with other requests:
Cache-Status: "CDN Company Here"; res-fresh=545,
OriginCache; cache-fresh=1100; collapse-hit=?1
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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
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
[I-D.ietf-httpbis-cache]
Fielding, R., Nottingham, M., and J. Reschke, "HTTP
Caching", draft-ietf-httpbis-cache-05 (work in progress),
July 2019.
[]
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>.
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[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
Author's Address
Mark Nottingham
Fastly
Email: mnot@mnot.net
URI: https://www.mnot.net/
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