draft-ietf-httpbis-cache-header-00.txt   draft-ietf-httpbis-cache-header-01.txt 
HTTP Working Group M. Nottingham HTTP M. Nottingham
Internet-Draft Fastly Internet-Draft Fastly
Intended status: Standards Track January 27, 2019 Intended status: Standards Track November 2, 2019
Expires: July 31, 2019 Expires: May 5, 2020
The Cache HTTP Response Header The Cache-Status HTTP Response Header
draft-ietf-httpbis-cache-header-00 draft-ietf-httpbis-cache-header-01
Abstract Abstract
To aid debugging, HTTP caches often append headers to a response To aid debugging, HTTP caches often append headers to a response
detailing how they handled the request. This specification codifies detailing how they handled the request. This specification codifies
that practice and updates it for HTTP's current caching model. that practice and updates it for HTTP's current caching model.
Note to Readers Note to Readers
_RFC EDITOR: please remove this section before publication_ _RFC EDITOR: please remove this section before publication_
skipping to change at page 1, line 44 skipping to change at page 1, line 44
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This Internet-Draft will expire on July 31, 2019. This Internet-Draft will expire on May 5, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. The Cache HTTP Response Header . . . . . . . . . . . . . . . 3 2. The Cache-Status HTTP Response Header . . . . . . . . . . . . 3
3. Security Considerations . . . . . . . . . . . . . . . . . . . 6 2.1. The fwd parameter . . . . . . . . . . . . . . . . . . . . 4
4. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.2. The fwd-res parameter . . . . . . . . . . . . . . . . . . 4
4.1. Normative References . . . . . . . . . . . . . . . . . . 6 2.3. The fwd-stored parameter . . . . . . . . . . . . . . . . 4
4.2. Informative References . . . . . . . . . . . . . . . . . 7 2.4. The res-fresh parameter . . . . . . . . . . . . . . . . . 5
4.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.5. The cache-fresh parameter . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 2.6. The collapse-hit parameter . . . . . . . . . . . . . . . 5
2.7. The collapse-wait parameter . . . . . . . . . . . . . . . 5
2.8. 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction 1. Introduction
To aid debugging, HTTP caches often append headers to a response To aid debugging, HTTP caches often append headers to a response
detailing how they handled the request. detailing how they handled the request.
Unfortunately, the semantics of these headers are often unclear, and Unfortunately, the semantics of these headers are often unclear, and
both the semantics and syntax used vary greatly between both the semantics and syntax used vary greatly between
implementations. implementations.
This specification defines a single, new HTTP response header field, This specification defines a single, new HTTP response header field,
"Cache" for this purpose. "Cache-Status" 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
1.1. Notational Conventions 1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
This document uses ABNF as defined in [RFC5234], along with the "%s" This document uses ABNF as defined in [RFC5234], along with the "%s"
extension for case sensitivity defined in [RFC7405]. extension for case sensitivity defined in [RFC7405].
2. The Cache HTTP Response Header 2. The Cache-Status HTTP Response Header
The Cache HTTP response header indicates the handling of the request The Cache-Status HTTP response header indicates caches' handling of
corresponding to the response it occurs within by caches along the the request corresponding to the response it occurs within.
path.
Its value is a Parameterised List Its value is a List [I-D.ietf-httpbis-header-structure]:
[I-D.ietf-httpbis-header-structure]:
Cache = sh-param-list Cache-Status = sh-list
Each member of the parameterised list represents a cache that has Each member of the parameterised list represents a cache that has
handled the request. 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).
The first member of the list represents the cache closest to the Caches determine when it is appropriate to add the Cache-Status
origin server, and the last member of the list represents the cache header field to a response. Some might decide to add it to all
closest to the user agent (possibly including the user agent's cache responses, whereas others might only do so when specifically
itself, if it chooses to append a value). configured to, or when the request contains a header that activates a
debugging mode.
Caches determine when it is appropriate to add the Cache header field When adding a value to the Cache-Status header field, caches SHOULD
to a response. Some might decide to add it to all responses, whereas preserve the existing contents of the header, to allow debugging of
others might only do so when specifically configured to, or when the the entire chain of caches handling the request.
request contains a header that activates a debugging mode.
When adding a value to the Cache header field, caches SHOULD preserve The list members identify the cache that inserted the value, and MUST
the existing contents of the header, to allow debugging of the entire have a type of either sh-string or sh-token. Depending on the
chain of caches handling the request. 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.
Identifiers in the parameterised list members are expected to be Each member of the list can also have a number of parameters that
cache-actions: 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.
cache-action = %s"HIT_FRESH" fwd = sh-token
/ %s"HIT_STALE" fwd-res = sh-token
/ %s"HIT_REFRESH_MODIFIED" fwd-stored = sh-boolean
/ %s"HIT_REFRESH_NOT_MODIFIED" res-fresh = sh-integer
/ %s"HIT_REFRESH_STALE" cache-fresh = sh-integer
/ %s"MISS" collapse-hit = sh-boolean
/ %s"MISS_CLIENT" collapse-wait = sh-integer
/ %s"BYPASS" key = sh-string
/ %s"ERROR"
The semantics of cache-actions are: 2.1. The fwd parameter
o HIT_FRESH - The cache used a fresh stored response to satisfy the "fwd" indicates why the request went forward. If it is not present,
request without going forward the value defaults to "none".
o HIT_STALE - The cache used a stale stored response to satisfy the It can have one of the following values: * none - The request did not
request without going forward go forward; i.e., it was a hit, and was served from the cache. *
bypass - The cache was configured to not handle this request * 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 could not select a response based upon this
request's headers. * 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) *
res-stale - The cache was able to select a response for the request,
but it was stale * 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
o HIT_REFRESH_MODIFIED - The cache had a stale stored response, went 2.2. The fwd-res parameter
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, "fwd-res" indicates what the result of the forward request was. It
went forward to validate it, and used the stored response to is only valid when fwd is "res-stale" or "req-stale", and defaults to
satisfy the request "full" if not present when fwd is one of those values.
o HIT_REFRESH_STALE - The cache had a stale stored response, went It can have one of the following values: * full - indicates that the
forward to validate it, and encountered a problem, so the stored response was a complete response (any status code except 304 Not
response was used to satisfy the request Modified and 206 Partial Response) * partial - indicates that the
response was a 206 Partial Response * notmod - indicates that the
response was a 304 Not Modified
o MISS - The cache did not have a stored response, so the request 2.3. The fwd-stored parameter
was forwarded
o MISS_CLIENT - The client included request directives (e.g., "fwd-stored" indicates whether the cache stored the response; a true
Pragma, Cache-Control) that prevented the cache from returning a value indicates that it did. Only valid when fwd is not "none".
response, so the request was forwarded
o BYPASS - The cache was configured to forward the request without 2.4. The res-fresh parameter
attempting to use a stored response
o ERROR - The cache was unable to use a stored response or obtain "res-fresh" indicates the response's remaining freshness lifetime (as
one by going forward 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.
Caches SHOULD use the most specific cache-action to a given response, 2.5. The cache-fresh parameter
but are not required to use all cache-actions. Future updates to
this specification can add additional cache-actions.
Each member of the Cache header can also have any (or all, or none) "cache-fresh" indicates the response's remaining freshness lifetime
of the following parameters: 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.
node = sh-string If both cache-fresh and res-fresh appear as parameters on the same
fresh = sh-integer value, it implies that the cache freshness overrode the response
age = sh-integer freshness.
cacheable = sh-boolean
key = sh-string
latency = sh-integer
cl_nm = sh-boolean
Their semantics are: 2.6. The collapse-hit parameter
o "node" - a string identifying for the cache node. MAY be a "collapse-hit" indicates whether this request was collapsed together
hostname, IP address, or alias. 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.
o "fresh" - an integer indicating the cache's estimation of the 2.7. The collapse-wait parameter
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 "collapse-wait" indicates the amount of time that the cache held the
([RFC7234], Section 4.2.3) of this response in seconds. MUST be 0 request while waiting to see if it could be successfully collapsed,
or greater. as an integer number of milliseconds.
o "cacheable" - a boolean indicating whether the cache can store 2.8. The key parameter
this response, according to [RFC7234], Section 3 and any locally
applied configuration.
o "key" - a string representing the key that the cache has "key" conveys a representation of the cache key used for the
associated with this response. This might include the request response. Note that this may be implementation-specific.
URL, request headers, and other values.
o "latency" - an integer indicating the amount of time in 3. Examples
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 The most minimal cache hit:
had a 304 Not Modified status code.
While all of these parameters are OPTIONAL, caches are encouraged to Cache-Status: ExampleCache
use the 'node' parameter to identify themselves.
3. Security Considerations ... 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
4. Security Considerations
Information about a cache's content can be used to infer the activity Information about a cache's content can be used to infer the activity
of those using it. Generally, access to sensitive information in a of those using it. Generally, access to sensitive information in a
cache is limited to those who are authorised to access that cache is limited to those who are authorised to access that
information (using a variety of techniques), so this does not information (using a variety of techniques), so this does not
represent an attack vector in the general sense. represent an attack vector in the general sense.
However, if the Cache header is exposed to parties who are not However, if the Cache-Status header is exposed to parties who are not
authorised to obtain the response it occurs within, it could expose authorised to obtain the response it occurs within, it could expose
information about that data. information about that data.
For example, if an attacker were able to obtain the Cache header from For example, if an attacker were able to obtain the Cache-Status
a response containing sensitive information and access were limited header from a response containing sensitive information and access
to one person (or limited set of people), they could determine were limited to one person (or limited set of people), they could
whether that information had been accessed before. This is similar determine whether that information had been accessed before. This is
to the information exposed by various timing attacks, but is arguably similar to the information exposed by various timing attacks, but is
more reliable, since the cache is directly reporting its state. arguably more reliable, since the cache is directly reporting its
state.
Mitigations include use of encryption (e.g., TLS [RFC8446])) to Mitigations include use of encryption (e.g., TLS [RFC8446])) to
protect the response, and careful controls over access to response protect the response, and careful controls over access to response
headers (as are present in the Web platform). When in doubt, the headers (as are present in the Web platform). When in doubt, the
Cache header field can be omitted. Cache-Status header field can be omitted.
4. References 5. References
4.1. Normative 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.
[I-D.ietf-httpbis-header-structure] [I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP", Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-09 (work in progress), draft-ietf-httpbis-header-structure-13 (work in progress),
December 2018. August 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008, DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>. <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>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014, RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>. <https://www.rfc-editor.org/info/rfc7405>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
4.2. Informative References 5.2. Informative References
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>. <https://www.rfc-editor.org/info/rfc8446>.
4.3. URIs 5.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/ [1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] https://httpwg.org/ [2] https://httpwg.org/
[3] https://github.com/httpwg/http-extensions/labels/cache-header [3] https://github.com/httpwg/http-extensions/labels/cache-header
Author's Address Author's Address
Mark Nottingham Mark Nottingham
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