< draft-ietf-httpbis-cache-header-03.txt		draft-ietf-httpbis-cache-header-04.txt >
	HTTP M. Nottingham		HTTP M. Nottingham
	Internet-Draft Fastly		Internet-Draft Fastly
	Intended status: Standards Track March 1, 2020		Intended status: Standards Track August 5, 2020
	Expires: September 2, 2020		Expires: February 6, 2021
	The Cache-Status HTTP Response Header Field		The Cache-Status HTTP Response Header Field
	draft-ietf-httpbis-cache-header-03		draft-ietf-httpbis-cache-header-04
	Abstract		Abstract
	To aid debugging, HTTP caches often append headers to a response		To aid debugging, HTTP caches often append header fields to a
	detailing how they handled the request. This specification codifies		response explaining how they handled the request. This specification
	that practice and updates it for HTTP's current caching model.		codifies that practice and updates it to align with 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_
	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/ [1].
	Working Group information can be found at https://httpwg.org/ [2];		Working Group information can be found at https://httpwg.org/ [2];
	skipping to change at page 1, line 44 ¶		skipping to change at page 1, line 45 ¶
	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 September 2, 2020.		This Internet-Draft will expire on February 6, 2021.
	Copyright Notice		Copyright Notice
	Copyright (c) 2020 IETF Trust and the persons identified as the		Copyright (c) 2020 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(https://trustee.ietf.org/license-info) in effect on the date of		(https://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 2, line 32 ¶		skipping to change at page 2, line 32 ¶
	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
	3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 5		2.8. The detail parameter . . . . . . . . . . . . . . . . . . 5
	4. Security Considerations . . . . . . . . . . . . . . . . . . . 6		3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
	5. References . . . . . . . . . . . . . . . . . . . . . . . . . 7		4. Defining New Proxy-Status Parameters . . . . . . . . . . . . 7
	5.1. Normative References . . . . . . . . . . . . . . . . . . 7		5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
	5.2. Informative References . . . . . . . . . . . . . . . . . 7		6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
	5.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 7		7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8		7.1. Normative References . . . . . . . . . . . . . . . . . . 8
			7.2. Informative References . . . . . . . . . . . . . . . . . 9
			7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9
			Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
	1. Introduction		1. Introduction
	To aid debugging, HTTP caches often append headers to a response		To aid debugging, HTTP caches often append header fields to a
	detailing how they handled the request.		response explaining how they handled the request. Unfortunately, the
			semantics of these headers are often unclear, and both the semantics
	Unfortunately, the semantics of these headers are often unclear, and		and syntax used vary greatly between implementations.
	both the semantics and syntax used vary greatly between
	implementations.
	This specification defines a single, new HTTP response header field,		This specification defines a single, new HTTP response header field,
	"Cache-Status" for this purpose.		"Cache-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
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	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 indicates caches' handling of
	the request corresponding to the response it occurs within.		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]:
	Cache-Status = sh-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 client (possibly including the user agent's
	cache itself, if it chooses to append a value).		cache itself, if it chooses to append 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 decide to add it to all		header field to a response. Some might decide to add it to all
	responses, whereas others might only do so when specifically		responses, whereas others might only do so when specifically
	configured to, or when the request contains a header that activates a		configured to, or when the request contains a header that activates a
	debugging mode.		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, to allow debugging of		preserve the existing contents of the header field, to allow
	the entire chain of caches handling the request.		debugging of the entire 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 that value, and
	MUST have a type of either sh-string or sh-token. Depending on the		MUST be a String or Token. Depending on the deployment, this might
	deployment, this might be a product or service name (e.g.,		be a product or service name (e.g., ExampleCache or "Example CDN"), a
	ExampleCache or "Example CDN"), a hostname ("cache-3.example.com"),		hostname ("cache-3.example.com"), and IP address, or a generated
	and 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 also have parameters that describe that
	cache's handling of the request. While all of these parameters are		cache's handling of the request. While all of 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 these parameters:
	hit = sh-boolean		hit = sf-boolean
	fwd = sh-token		fwd = sf-token
	fwd-status = sh-integer		fwd-status = sf-integer
	ttl = sh-integer		ttl = sf-integer
	stored = sh-boolean		stored = sf-boolean
	collapsed = sh-boolean		collapsed = sf-boolean
	key = sh-string		key = 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 did not go forward and the response was obtained from
	from the cache (possibly with modifications; e.g., if the request was		the cache. A response that originally was produced by the origin but
	conditional, a 304 Not Modified could be generated from cache).		was modified by the cache (for example, a 304 or 206 status code) is
			still considered a hit.
	"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 why the request went forward.		"fwd" indicates that the request went forward towards the origin, and
			why.
	It can have one of the following values:		The following values are defined to explain why the request went
			forward:
	o uri-miss - The cache did not contain any responses that matched		o 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		o 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		o 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
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	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 is sent by the cache. This includes freshness
	assigned by the cache; e.g., through heuristics, local configuration,		assigned by the cache; e.g., through heuristics, local configuration,
	or other factors. May be negative, to indicate staleness.		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 forward response; a		"stored" indicates whether the cache stored the response; a true
	true value indicates that it did. Only meaningful when fwd is		value indicates that it did. Only meaningful when fwd is present.
	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
	successfully reused; if not, a new request had to be made. If not		successfully reused; if not, a new request had to be made. If not
	present, the request was not collapsed with others. Only meaningful		present, the request was not collapsed with others. Only meaningful
	when fwd is present.		when fwd is present.
	2.7. The key parameter		2.7. The key parameter
	"key" conveys a representation of the cache key used for the		"key" conveys a representation of the cache key used for the
	response. Note that this may be implementation-specific.		response. Note that this may be implementation-specific.
			2.8. The detail parameter
			"detail" allows implementations to convey additional information not
			captured in other parameters; for example, implementation-specific
			states, or other caching-related metrics.
			For example:
			Cache-Status: ExampleCache; hit; detail=MEMORY
			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
			the same name, it might not mean the same thing.
			This parameter is intentionally limited. If an implementation needs
			to convey additional information, 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
	A stale hit just has negative freshness:		A stale hit just has negative freshness:
	Cache-Status: ExampleCache; hit; ttl=-412		Cache-Status: ExampleCache; hit; ttl=-412
	Whereas a complete miss is:		Whereas a complete miss is:
	Cache-Status: ExampleCache; fwd=uri-miss		Cache-Status: ExampleCache; fwd=uri-miss
	A miss that successfully validated on the back-end server:		A miss that successfully validated on the back-end server:
	skipping to change at page 6, line 30 ¶		skipping to change at page 7, line 5 ¶
	A miss that the cache attempted to collapse, but couldn't:		A miss that the cache attempted to collapse, but couldn't:
	Cache-Status: ExampleCache; fwd=uri-miss; collapsed=?0		Cache-Status: ExampleCache; fwd=uri-miss; collapsed=?0
	Going through two layers of caching, both of which were hits, and the		Going through two layers of caching, both of which were hits, and the
	second collapsed with other requests:		second collapsed with other requests:
	Cache-Status: OriginCache; hit; ttl=1100; collapsed,		Cache-Status: OriginCache; hit; ttl=1100; collapsed,
	"CDN Company Here"; hit; ttl=545		"CDN Company Here"; hit; ttl=545
	4. Security Considerations		4. Defining New Proxy-Status Parameters
			New Cache-Status Parameters can be defined by registering them in the
			HTTP Cache-Status Parameters registry.
			Registration requests are reviewed and approved by a Designated
			Expert, as per [RFC8126], Section 4.5. A specification document is
			appreciated, but not required.
			The Expert(s) should consider the following factors when evaluating
			requests:
			o Community feedback
			o If the value is sufficiently well-defined
			o Generic parameters are preferred over vendor-specific,
			application-specific or deployment-specific values. If a generic
			value cannot be agreed upon in the community, the parameter's name
			should be correspondingly specific (e.g., with a prefix that
			identifies the vendor, application or deployment).
			Registration requests should use the following template:
			o Name: [a name for the Cache-Status Parameter that matches key]
			o Description: [a description of the parameter semantics and value]
			o Reference: [to a specification defining this parameter]
			See the registry at https://iana.org/assignments/http-cache-status
			[4] for details on where to send registration requests.
			5. IANA Considerations
			Upon publication, please create the HTTP Cache-Status Parameters
			registry at https://iana.org/assignments/http-cache-statuses [5] and
			populate it with the types defined in Section 2; see Section 4 for
			its associated procedures.
			6. 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-Status header is exposed to parties who are not		However, if the Cache-Status header field is exposed to parties who
	authorised to obtain the response it occurs within, it could expose		are not authorised to obtain the response it occurs within, it could
	information about that data.		expose information about that data.
	For example, if an attacker were able to obtain the Cache-Status		For example, if an attacker were able to obtain the Cache-Status
	header from a response containing sensitive information and access		header field from a response containing sensitive information and
	were limited to one person (or limited set of people), they could		access were limited to one person (or limited set of people), they
	determine whether that information had been accessed before. This is		could determine whether that information had been accessed before.
	similar to the information exposed by various timing attacks, but is		This is similar to the information exposed by various timing attacks,
	arguably more reliable, since the cache is directly reporting its		but is arguably more reliable, since the cache is directly reporting
	state.		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		header fields (as are present in the Web platform). When in doubt,
	Cache-Status header field can be omitted.		the Cache-Status header field can be omitted.
	5. References		7. References
	5.1. Normative References		7.1. Normative References
	[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 Field Values for
	draft-ietf-httpbis-header-structure-13 (work in progress),		HTTP", draft-ietf-httpbis-header-structure-19 (work in
	August 2019.		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		[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>.
	[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>.
			[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
			Writing an IANA Considerations Section in RFCs", BCP 26,
			RFC 8126, DOI 10.17487/RFC8126, June 2017,
			<https://www.rfc-editor.org/info/rfc8126>.
	[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>.
	5.2. Informative References		7.2. 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>.
	5.3. URIs		7.3. 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
			[4] https://iana.org/assignments/http-cache-status
			[5] https://iana.org/assignments/http-cache-statuses
	Author's Address		Author's Address
	Mark Nottingham		Mark Nottingham
	Fastly		Fastly
			made in
			Prahran, VIC
			Australia
	Email: mnot@mnot.net		Email: mnot@mnot.net
	URI: https://www.mnot.net/		URI: https://www.mnot.net/
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