Internet DRAFT - draft-nottingham-cache-header
draft-nottingham-cache-header
Network Working Group M. Nottingham
Internet-Draft Fastly
Intended status: Standards Track September 7, 2018
Expires: March 11, 2019
The Cache HTTP Response Header
draft-nottingham-cache-header-00
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_
The issues list for this draft can be found at
https://github.com/mnot/I-D/labels/cache-header [1].
The most recent (often, unpublished) draft is at
https://mnot.github.io/I-D/cache-header/ [2].
Recent changes are listed at https://github.com/mnot/I-D/commits/gh-
pages/cache-header [3].
See also the draft's current status in the IETF datatracker, at
https://datatracker.ietf.org/doc/draft-nottingham-cache-header/ [4].
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 11, 2019.
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Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. The Cache HTTP Response Header . . . . . . . . . . . . . . . 3
3. Security Considerations . . . . . . . . . . . . . . . . . . . 6
4. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Normative References . . . . . . . . . . . . . . . . . . 6
4.2. Informative References . . . . . . . . . . . . . . . . . 7
4.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
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" for this purpose.
For example:
Cache: HIT_FRESH; node="reverse-proxy.example.com:80";
key="https://example.com/foo|Accept-Encoding:gzip",
HIT_STALE; node="FooCDN parent"; fresh=-45; age=200; latency=3,
MISS; node="FooCDN edge"; fresh=-45; age=200; latency=98
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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 HTTP Response Header
The Cache HTTP response header indicates the handling of the request
corresponding to the response it occurs within by caches along the
path.
Its value is a Parameterised List
[I-D.ietf-httpbis-header-structure]:
Cache = sh-param-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 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 header field, caches SHOULD preserve
the existing contents of the header, to allow debugging of the entire
chain of caches handling the request.
Identifiers in the parameterised list members are expected to be
cache-actions:
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cache-action = %s"HIT_FRESH"
/ %s"HIT_STALE"
/ %s"HIT_REFRESH_MODIFIED"
/ %s"HIT_REFRESH_NOT_MODIFIED"
/ %s"HIT_REFRESH_STALE"
/ %s"MISS"
/ %s"MISS_CLIENT"
/ %s"BYPASS"
/ %s"ERROR"
The semantics of cache-actions are:
o HIT_FRESH - The cache used a fresh stored response to satisfy the
request without going forward
o HIT_STALE - The cache used a stale stored response to satisfy the
request without going forward
o HIT_REFRESH_MODIFIED - The cache had a stale stored response, went
forward to validate it, and used the new response to satisfy the
request
o HIT_REFRESH_NOT_MODIFIED - The cache had a stale stored response,
went forward to validate it, and used the stored response to
satisfy the request
o HIT_REFRESH_STALE - The cache had a stale stored response, went
forward to validate it, and encountered a problem, so the stored
response was used to satisfy the request
o MISS - The cache did not have a stored response, so the request
was forwarded
o MISS_CLIENT - The client included request directives (e.g.,
Pragma, Cache-Control) that prevented the cache from returning a
response, so the request was forwarded
o BYPASS - The cache was configured to forward the request without
attempting to use a stored response
o ERROR - The cache was unable to use a stored response or obtain
one by going forward
Caches SHOULD use the most specific cache-action to a given response,
but are not required to use all cache-actions. Future updates to
this specification can add additional cache-actions.
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Each member of the Cache header can also have any (or all, or none)
of the following parameters:
node = sh-string
fresh = sh-integer
age = sh-integer
cacheable = sh-boolean
key = sh-string
latency = sh-integer
cl_nm = sh-boolean
Their semantics are:
o "node" - a string identifying for the cache node. MAY be a
hostname, IP address, or alias.
o "fresh" - an integer indicating the cache's estimation of the
freshness lifetime ([RFC7234], Section 4.2.1) of this response in
seconds, including any locally applied configuration. MAY be
negative.
o "age" - an integer indicating the cache's estimation of the age
([RFC7234], Section 4.2.3) of this response in seconds. MUST be 0
or greater.
o "cacheable" - a boolean indicating whether the cache can store
this response, according to [RFC7234], Section 3 and any locally
applied configuration.
o "key" - a string representing the key that the cache has
associated with this response. This might include the request
URL, request headers, and other values.
o "latency" - an integer indicating the amount of time in
milliseconds between the receipt of a complete set of request
headers and sending the complete set of response headers of this
response, from the viewpoint of the cache. Note that this may not
include buffering time in transport protocols and similar delays.
o "cl_nm" - a boolean indicating whether the response to the client
had a 304 Not Modified status code.
While all of these parameters are OPTIONAL, caches are encouraged to
use the 'node' parameter to identify themselves.
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3. 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 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 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 header field can be omitted.
4. References
4.1. Normative References
[I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-07 (work in progress),
July 2018.
[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>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
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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>.
4.2. 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>.
4.3. URIs
[1] https://github.com/mnot/I-D/labels/cache-header
[2] https://mnot.github.io/I-D/cache-header/
[3] https://github.com/mnot/I-D/commits/gh-pages/cache-header
[4] https://datatracker.ietf.org/doc/draft-nottingham-cache-header/
Author's Address
Mark Nottingham
Fastly
Email: mnot@mnot.net
URI: https://www.mnot.net/
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