< 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 ¶
	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 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.
	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
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	(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
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	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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