draft-ietf-httpbis-cache-header-05.txt   draft-ietf-httpbis-cache-header-06.txt 
HTTP M. Nottingham HTTP M. Nottingham
Internet-Draft Fastly Internet-Draft Fastly
Intended status: Standards Track August 11, 2020 Intended status: Standards Track 13 January 2021
Expires: February 12, 2021 Expires: 17 July 2021
The Cache-Status HTTP Response Header Field The Cache-Status HTTP Response Header Field
draft-ietf-httpbis-cache-header-05 draft-ietf-httpbis-cache-header-06
Abstract Abstract
To aid debugging, HTTP caches often append header fields to a To aid debugging, HTTP caches often append header fields to a
response explaining how they handled the request. This specification response explaining how they handled the request. This specification
codifies that practice and updates it to align with HTTP's current codifies that practice and updates it to align with HTTP's current
caching model. caching model.
Note to Readers Note to Readers
_RFC EDITOR: please remove this section before publication_ _RFC EDITOR: please remove this section before publication_
Discussion of this draft takes place on the HTTP working group Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/ [1]. https://lists.w3.org/Archives/Public/ietf-http-wg/
(https://lists.w3.org/Archives/Public/ietf-http-wg/).
Working Group information can be found at https://httpwg.org/ [2]; Working Group information can be found at https://httpwg.org/
source code and issues list for this draft can be found at (https://httpwg.org/); source code and issues list for this draft can
https://github.com/httpwg/http-extensions/labels/cache-header [3]. be found at https://github.com/httpwg/http-extensions/labels/cache-
header (https://github.com/httpwg/http-extensions/labels/cache-
header).
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 12, 2021. This Internet-Draft will expire on 17 July 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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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-Status HTTP Response Header Field . . . . . . . . . 3 2. The Cache-Status HTTP Response Header Field . . . . . . . . . 3
2.1. The hit parameter . . . . . . . . . . . . . . . . . . . . 4 2.1. The hit parameter . . . . . . . . . . . . . . . . . . . . 4
2.2. The fwd parameter . . . . . . . . . . . . . . . . . . . . 4 2.2. The fwd parameter . . . . . . . . . . . . . . . . . . . . 4
2.3. The fwd-status parameter . . . . . . . . . . . . . . . . 5 2.3. The fwd-status parameter . . . . . . . . . . . . . . . . 5
2.4. The ttl parameter . . . . . . . . . . . . . . . . . . . . 5 2.4. The ttl parameter . . . . . . . . . . . . . . . . . . . . 5
2.5. The stored parameter . . . . . . . . . . . . . . . . . . 5 2.5. The stored parameter . . . . . . . . . . . . . . . . . . 5
2.6. The collapsed parameter . . . . . . . . . . . . . . . . . 5 2.6. The collapsed parameter . . . . . . . . . . . . . . . . . 5
2.7. The key parameter . . . . . . . . . . . . . . . . . . . . 5 2.7. The key parameter . . . . . . . . . . . . . . . . . . . . 5
2.8. The detail parameter . . . . . . . . . . . . . . . . . . 5 2.8. The detail parameter . . . . . . . . . . . . . . . . . . 6
3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Defining New Proxy-Status Parameters . . . . . . . . . . . . 7 4. Defining New Proxy-Status Parameters . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8 7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 9 7.2. Informative References . . . . . . . . . . . . . . . . . 9
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction 1. Introduction
To aid debugging, HTTP caches often append header fields to a To aid debugging, HTTP caches often append header fields to a
response explaining how they handled the request. Unfortunately, the response explaining how they handled the request. Unfortunately, the
semantics of these headers are often unclear, and both the semantics semantics of these headers are often unclear, and both the semantics
and syntax used vary greatly between implementations. and syntax used vary between implementations.
This specification defines a single, new HTTP response header field, This specification defines a new HTTP response header field, "Cache-
"Cache-Status" for this purpose. Status" for this purpose.
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-Status HTTP Response Header Field 2. The Cache-Status HTTP Response Header Field
The Cache-Status HTTP response header indicates caches' handling of The Cache-Status HTTP response header field indicates caches'
the request corresponding to the response it occurs within. handling of the request corresponding to the response it occurs
within.
Its value is a List [I-D.ietf-httpbis-header-structure]: Its value is a List [I-D.ietf-httpbis-header-structure], Section 3.1:
Cache-Status = sf-list Cache-Status = sf-list
Each member of the list represents a cache that has handled the Each member of the list represents a cache that has handled the
request. The first member of the list represents the cache closest request. The first member of the list represents the cache closest
to the origin server, and the last member of the list represents the 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 closest to the user (possibly including the user agent's cache
cache itself, if it chooses to append a value). itself, if it appends a value).
Caches determine when it is appropriate to add the Cache-Status Caches determine when it is appropriate to add the Cache-Status
header field to a response. Some might add it to all responses, header field to a response. Some might add it to all responses,
whereas others might only do so when specifically configured to, or whereas others might only do so when specifically configured to, or
when the request contains a header that activates a debugging mode. when the request contains a header field that activates a debugging
mode.
When adding a value to the Cache-Status header field, caches SHOULD When adding a value to the Cache-Status header field, caches SHOULD
preserve the existing contents of the header field, to allow preserve the existing field value, to allow debugging of the entire
debugging of the entire chain of caches handling the request. chain of caches handling the request.
Each list member identifies the cache that inserted that value, and Each list member identifies the cache that inserted it and MUST be a
MUST be a String or Token. Depending on the deployment, this might String or Token. Depending on the deployment, this might be a
be a product or service name (e.g., ExampleCache or "Example CDN"), a product or service name (e.g., ExampleCache or "Example CDN"), a
hostname ("cache-3.example.com"), and IP address, or a generated hostname ("cache-3.example.com"), an IP address, or a generated
string. string.
Each member of the list can also have parameters that describe that Each member of the list can have parameters that describe that
cache's handling of the request. While all of these parameters are cache's handling of the request. While these parameters are
OPTIONAL, caches are encouraged to provide as much information as OPTIONAL, caches are encouraged to provide as much information as
possible. possible.
This specification defines these parameters: This specification defines the following parameters:
hit = sf-boolean hit = sf-boolean
fwd = sf-token fwd = sf-token
fwd-status = sf-integer fwd-status = sf-integer
ttl = sf-integer ttl = sf-integer
stored = sf-boolean stored = sf-boolean
collapsed = sf-boolean collapsed = sf-boolean
key = sf-string key = sf-string
detail = sf-token / sf-string detail = sf-token / sf-string
2.1. The hit parameter 2.1. The hit parameter
"hit", when true, indicates that the request was satisfied by the "hit", when true, indicates that the request was satisfied by the
cache; i.e., it did not go forward, and the response was obtained cache; i.e., it was not forwarded, and the response was obtained from
from the cache. A response that originally was produced by the the cache. A response that was originally produced by the origin but
origin but was modified by the cache (for example, a 304 or 206 was modified by the cache (for example, a 304 or 206 status code) is
status code) is still considered a hit. still considered a hit, as long as it did not go forward (e.g., for
validation).
"hit" and "fwd" are exclusive; only one of them should appear on each "hit" and "fwd" are exclusive; only one of them should appear on each
list member. list member.
2.2. The fwd parameter 2.2. The fwd parameter
"fwd" indicates that the request went forward towards the origin, and "fwd" indicates that the request went forward towards the origin, and
why. why.
The following values are defined to explain why the request went The following parameter values are defined to explain why the request
forward: went forward, from most specific to least:
o uri-miss - The cache did not contain any responses that matched * bypass - The cache was configured to not handle this request
* method - The request method's semantics require the request to be
forwarded
* request - The cache was able to select a fresh response for the
request, but the request's semantics (e.g., Cache-Control request
directives) did not allow its use
* stale - The cache was able to select a response for the request,
but it was stale
* uri-miss - The cache did not contain any responses that matched
the request URI the request URI
o vary-miss - The cache contained a response that matched the * vary-miss - The cache contained a response that matched the
request URI, but could not select a response based upon this request URI, but could not select a response based upon this
request's headers and stored Vary headers. request's headers and stored Vary headers.
o miss - The cache did not contain any responses that could be used * miss - The cache did not contain any responses that could be used
to satisfy this request (to be used when an implementation cannot to satisfy this request (to be used when an implementation cannot
distinguish between uri-miss and vary-miss) distinguish between uri-miss and vary-miss)
o stale - The cache was able to select a response for the request, The most specific reason that the cache is aware of SHOULD be used.
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
2.3. The fwd-status parameter 2.3. The fwd-status parameter
"fwd-status" indicates what status code the next hop server returned "fwd-status" indicates what status code the next hop server returned
in response to the request. Only meaningful when "fwd" is present; in response to the request. Only meaningful when "fwd" is present;
if "fwd-status" is not present but "fwd" is, it defaults to the if "fwd-status" is not present but "fwd" is, it defaults to the
status code sent in the response. status code sent in the response.
This parameter is useful to distinguish cases when the next hop This parameter is useful to distinguish cases when the next hop
server sends a 304 Not Modified response to a conditional request, or server sends a 304 Not Modified response to a conditional request, or
a 206 Partial Response because of a range request. a 206 Partial Response because of a range request.
2.4. The ttl parameter 2.4. The ttl parameter
"ttl" indicates the response's remaining freshness lifetime as "ttl" indicates the response's remaining freshness lifetime as
calculated by the cache, as an integer number of seconds, measured calculated by the cache, as an integer number of seconds, measured
when the response is sent by the cache. This includes freshness when the response header section is sent by the cache. This includes
assigned by the cache; e.g., through heuristics, local configuration, freshness assigned by the cache; e.g., through heuristics, local
or other factors. May be negative, to indicate staleness. configuration, or other factors. May be negative, to indicate
staleness.
2.5. The stored parameter 2.5. The stored parameter
"stored" indicates whether the cache stored the response; a true "stored" indicates whether the cache stored the response; a true
value indicates that it did. Only meaningful when fwd is present. value indicates that it did. Only meaningful when fwd is present.
2.6. The collapsed parameter 2.6. The collapsed parameter
"collapsed" indicates whether this request was collapsed together "collapsed" indicates whether this request was collapsed together
with one or more other forward requests; if true, the response was with one or more other forward requests; if true, the response was
skipping to change at page 6, line 4 skipping to change at page 6, line 14
2.8. The detail parameter 2.8. The detail parameter
"detail" allows implementations to convey additional information not "detail" allows implementations to convey additional information not
captured in other parameters; for example, implementation-specific captured in other parameters; for example, implementation-specific
states, or other caching-related metrics. states, or other caching-related metrics.
For example: For example:
Cache-Status: ExampleCache; hit; detail=MEMORY Cache-Status: ExampleCache; hit; detail=MEMORY
The semantics of a detail parameter are always specific to the cache 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 that sent it; even if a member of details from another cache shares
the same name, it might not mean the same thing. the same name, it might not mean the same thing.
This parameter is intentionally limited. If an implementation needs This parameter is intentionally limited. If an implementation's
to convey additional information, they are encouraged to register developer or operator needs to convey additional information in an
extension parameters (see Section 4) or define another header field. interoperable fashion, they are encouraged to register extension
parameters (see Section 4) or define another header field.
3. Examples 3. Examples
The most minimal cache hit: The most minimal cache hit:
Cache-Status: ExampleCache; hit Cache-Status: ExampleCache; hit
... but a polite cache will give some more information, e.g.: ... but a polite cache will give some more information, e.g.:
Cache-Status: ExampleCache; hit; ttl=376 Cache-Status: ExampleCache; hit; ttl=376
skipping to change at page 7, line 17 skipping to change at page 7, line 22
New Cache-Status Parameters can be defined by registering them in the New Cache-Status Parameters can be defined by registering them in the
HTTP Cache-Status Parameters registry. HTTP Cache-Status Parameters registry.
Registration requests are reviewed and approved by a Designated Registration requests are reviewed and approved by a Designated
Expert, as per [RFC8126], Section 4.5. A specification document is Expert, as per [RFC8126], Section 4.5. A specification document is
appreciated, but not required. appreciated, but not required.
The Expert(s) should consider the following factors when evaluating The Expert(s) should consider the following factors when evaluating
requests: requests:
o Community feedback * Community feedback
o If the value is sufficiently well-defined * If the value is sufficiently well-defined
o Generic parameters are preferred over vendor-specific, * Generic parameters are preferred over vendor-specific,
application-specific or deployment-specific values. If a generic application-specific, or deployment-specific values. If a generic
value cannot be agreed upon in the community, the parameter's name value cannot be agreed upon in the community, the parameter's name
should be correspondingly specific (e.g., with a prefix that should be correspondingly specific (e.g., with a prefix that
identifies the vendor, application or deployment). identifies the vendor, application or deployment).
Registration requests should use the following template: Registration requests should use the following template:
o Name: [a name for the Cache-Status Parameter that matches key] * Name: [a name for the Cache-Status Parameter that matches key]
o Description: [a description of the parameter semantics and value] * Description: [a description of the parameter semantics and value]
o Reference: [to a specification defining this parameter] * Reference: [to a specification defining this parameter]
See the registry at https://iana.org/assignments/http-cache-status See the registry at https://iana.org/assignments/http-cache-status
[4] for details on where to send registration requests. (https://iana.org/assignments/http-cache-status) for details on where
to send registration requests.
5. IANA Considerations 5. IANA Considerations
Upon publication, please create the HTTP Cache-Status Parameters Upon publication, please create the HTTP Cache-Status Parameters
registry at https://iana.org/assignments/http-cache-status [5] and registry at https://iana.org/assignments/http-cache-status
populate it with the types defined in Section 2; see Section 4 for (https://iana.org/assignments/http-cache-status) and populate it with
its associated procedures. the types defined in Section 2; see Section 4 for its associated
procedures.
6. Security Considerations 6. Security Considerations
Attackers can use the information in Cache-Status to probe the Attackers can use the information in Cache-Status to probe the
behaviour of the cache (and other components), and infer the activity behaviour of the cache (and other components), and infer the activity
of those using the cache. The Cache-Status header field may not of those using the cache. The Cache-Status header field may not
create these risks on its own, but can assist attackers in exploiting create these risks on its own, but can assist attackers in exploiting
them. them.
For example, knowing if a cache has stored a response can help an For example, knowing if a cache has stored a response can help an
skipping to change at page 8, line 25 skipping to change at page 8, line 33
To avoid assisting such attacks, the Cache-Status header field can be To avoid assisting such attacks, the Cache-Status header field can be
omitted, only sent when the client is authorized to receive it, or omitted, only sent when the client is authorized to receive it, or
only send sensitive information (e.g., the key parameter) when the only send sensitive information (e.g., the key parameter) when the
client is authorized. client is authorized.
7. References 7. References
7.1. Normative 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 [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
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 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26, Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017, RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>. <https://www.rfc-editor.org/info/rfc8126>.
[I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", Work in Progress, Internet-Draft, draft-ietf-
httpbis-header-structure-19, 3 June 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-httpbis-
header-structure-19.txt>.
[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>.
[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>.
7.2. Informative References 7.2. Informative References
[ENTANGLE] [ENTANGLE] Kettle, J., "Web Cache Entanglement: Novel Pathways to
Kettle, J., "Web Cache Entanglement: Novel Pathways to
Poisoning", n.d., <https://i.blackhat.com/USA- Poisoning", n.d., <https://i.blackhat.com/USA-
20/Wednesday/us-20-Kettle-Web-Cache-Entanglement-Novel- 20/Wednesday/us-20-Kettle-Web-Cache-Entanglement-Novel-
Pathways-To-Poisoning-wp.pdf>. Pathways-To-Poisoning-wp.pdf>.
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-status
Author's Address Author's Address
Mark Nottingham Mark Nottingham
Fastly Fastly
made in Prahran VIC
Prahran, VIC
Australia Australia
Email: mnot@mnot.net Email: mnot@mnot.net
URI: https://www.mnot.net/ URI: https://www.mnot.net/
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