< draft-ietf-httpbis-cache-header-02.txt		draft-ietf-httpbis-cache-header-03.txt >
	HTTP M. Nottingham		HTTP M. Nottingham
	Internet-Draft Fastly		Internet-Draft Fastly
	Intended status: Standards Track November 4, 2019		Intended status: Standards Track March 1, 2020
	Expires: May 7, 2020		Expires: September 2, 2020
	The Cache-Status HTTP Response Header		The Cache-Status HTTP Response Header Field
	draft-ietf-httpbis-cache-header-02		draft-ietf-httpbis-cache-header-03
	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
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	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 May 7, 2020.		This Internet-Draft will expire on September 2, 2020.
	Copyright Notice		Copyright Notice
	Copyright (c) 2019 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
	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-Status HTTP Response Header . . . . . . . . . . . . 3		2. The Cache-Status HTTP Response Header Field . . . . . . . . . 3
	2.1. The fwd parameter . . . . . . . . . . . . . . . . . . . . 4		2.1. The hit parameter . . . . . . . . . . . . . . . . . . . . 4
	2.2. The fwd-res parameter . . . . . . . . . . . . . . . . . . 4		2.2. The fwd parameter . . . . . . . . . . . . . . . . . . . . 4
	2.3. The fwd-stored parameter . . . . . . . . . . . . . . . . 5		2.3. The fwd-status parameter . . . . . . . . . . . . . . . . 5
	2.4. The res-fresh parameter . . . . . . . . . . . . . . . . . 5		2.4. The ttl parameter . . . . . . . . . . . . . . . . . . . . 5
	2.5. The cache-fresh parameter . . . . . . . . . . . . . . . . 5		2.5. The stored parameter . . . . . . . . . . . . . . . . . . 5
	2.6. The collapse-hit parameter . . . . . . . . . . . . . . . 5		2.6. The collapsed parameter . . . . . . . . . . . . . . . . . 5
	2.7. The collapse-wait parameter . . . . . . . . . . . . . . . 5		2.7. The key parameter . . . . . . . . . . . . . . . . . . . . 5
	2.8. The key parameter . . . . . . . . . . . . . . . . . . . . 6		3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 5
	3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6		4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
	4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
	5. References . . . . . . . . . . . . . . . . . . . . . . . . . 7		5. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
	5.1. Normative References . . . . . . . . . . . . . . . . . . 7		5.1. Normative References . . . . . . . . . . . . . . . . . . 7
	5.2. Informative References . . . . . . . . . . . . . . . . . 8		5.2. Informative References . . . . . . . . . . . . . . . . . 7
	5.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 8		5.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 7
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 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.
	skipping to change at page 3, line 16 ¶		skipping to change at page 3, line 16 ¶
	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		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 = sh-list
	Each member of the parameterised list represents a cache that has		Each member of the list represents a cache that has handled the
	handled the request. The first member of the list represents the		request. The first member of the list represents the cache closest
	cache closest to the origin server, and the last member of the list		to the origin server, and the last member of the list represents the
	represents the cache closest to the user agent (possibly including		cache closest to the client (possibly including the user agent's
	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, to allow debugging of
	the entire chain of caches handling the request.		the entire chain of caches handling the request.
	The list members identify the cache that inserted the value, and MUST		Each list member identifies the cache that inserted that value, and
	have a type of either sh-string or sh-token. Depending on the		MUST have a type of either sh-string or sh-token. Depending on the
	deployment, this might be a product or service name (e.g.,		deployment, this might be a product or service name (e.g.,
	ExampleCache or "Example CDN"), a hostname ("cache-3.example.com"),		ExampleCache or "Example CDN"), a hostname ("cache-3.example.com"),
	and IP address, or a generated string.		and IP address, or a generated string.
	Each member of the list can also have a number of parameters that		Each member of the list can also have parameters that describe that
	describe that cache's handling of the request. While all of these		cache's handling of the request. While all of these parameters are
	parameters are OPTIONAL, caches are encouraged to provide as much		OPTIONAL, caches are encouraged to provide as much information as
	information as possible.		possible.
	fwd = sh-token		This specification defines these parameters:
	fwd-res = sh-token
	fwd-stored = sh-boolean
	res-fresh = sh-integer
	cache-fresh = sh-integer
	collapse-hit = sh-boolean
	collapse-wait = sh-integer
	key = sh-string
	2.1. The fwd parameter		hit = sh-boolean
			fwd = sh-token
			fwd-status = sh-integer
			ttl = sh-integer
			stored = sh-boolean
			collapsed = sh-boolean
			key = sh-string
	"fwd" indicates why the request went forward. If it is not present,		2.1. The hit parameter
	the value defaults to "none".
	It can have one of the following values:		"hit", when true, indicates that the request was satisfied by the
			cache; i.e., it did not go forward, and the response was obtained
			from the cache (possibly with modifications; e.g., if the request was
			conditional, a 304 Not Modified could be generated from cache).
	o none - The request did not go forward; i.e., it was a hit, and was		"hit" and "fwd" are exclusive; only one of them should appear on each
	served from the cache.		list member.
	o bypass - The cache was configured to not handle this request		2.2. The fwd parameter
			"fwd" indicates why the request went forward.
			It can have one of the following values:
	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.		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
	distinguish between uri-miss and vary-miss)		distinguish between uri-miss and vary-miss)
	o res-stale - The cache was able to select a response for the		o stale - The cache was able to select a response for the request,
	request, but it was stale		but it was stale
	o req-stale - The cache was able to select a fresh response for the		o request - The cache was able to select a fresh response for the
	request, but client request headers (e.g., Cache-Control request		request, but client request headers (e.g., Cache-Control request
	directives) did not allow its use		directives) did not allow its use
	2.2. The fwd-res parameter		o bypass - The cache was configured to not handle this request
	"fwd-res" indicates what the result of the forward request was. It
	is only valid when fwd is "res-stale" or "req-stale", and defaults to
	"full" if not present when fwd is one of those values.
	It can have one of the following values:
	o full - indicates that the response was a complete response (any
	status code except 304 Not Modified and 206 Partial Response)
	o partial - indicates that the response was a 206 Partial Response
	o notmod - indicates that the response was a 304 Not Modified
	2.3. The fwd-stored parameter		2.3. The fwd-status parameter
	"fwd-stored" indicates whether the cache stored the response; a true		"fwd-status" indicates what status code the next hop server returned
	value indicates that it did. Only valid when fwd is not "none".		in response to the request. Only meaningful when "fwd" is present;
			if "fwd-status" is not present but "fwd" is, it defaults to the
			status code sent in the response.
	2.4. The res-fresh parameter		This parameter is useful to distinguish cases when the next hop
			server sends a 304 Not Modified response to a conditional request, or
			a 206 Partial Response due to a range request.
	"res-fresh" indicates the response's remaining freshness lifetime (as		2.4. The ttl parameter
	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.
	2.5. The cache-fresh parameter		"ttl" indicates the response's remaining freshness lifetime as
			calculated by the cache, as an integer number of seconds, measured
			when the response is sent by the cache. This includes freshness
			assigned by the cache; e.g., through heuristics, local configuration,
			or other factors. May be negative, to indicate staleness.
	"cache-fresh" indicates the response's remaining freshness lifetime		2.5. The stored parameter
	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.
	If both cache-fresh and res-fresh appear as parameters on the same		"stored" indicates whether the cache stored the forward response; a
	value, it implies that the cache freshness overrode the response		true value indicates that it did. Only meaningful when fwd is
	freshness.		present.
	2.6. The collapse-hit parameter		2.6. The collapsed parameter
	"collapse-hit" 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.		present, the request was not collapsed with others. Only meaningful
			when fwd is present.
	2.7. The collapse-wait parameter
	"collapse-wait" indicates the amount of time that the cache held the
	request while waiting to see if it could be successfully collapsed,
	as an integer number of milliseconds.
	2.8. 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.
	3. Examples		3. Examples
	The most minimal cache hit:		The most minimal cache hit:
	Cache-Status: ExampleCache		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; res-fresh=376		Cache-Status: ExampleCache; hit; ttl=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:		A stale hit just has negative freshness:
	Cache-Status: ExampleCache; res-fresh=-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 validated on the back-end server:		A miss that successfully validated on the back-end server:
	Cache-Status: ExampleCache; fwd=res-stale; fwd-res=notmod		Cache-Status: ExampleCache; fwd=stale; fwd-status=304
	A miss that was collapsed with another request:		A miss that was collapsed with another request:
	Cache-Status: ExampleCache; fwd=uri-miss; collapse-hit=?1		Cache-Status: ExampleCache; fwd=uri-miss; collapsed
	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;		Cache-Status: ExampleCache; fwd=uri-miss; collapsed=?0
	collapse-hit=?0; collapse-wait=240
	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: "CDN Company Here"; res-fresh=545,		Cache-Status: OriginCache; hit; ttl=1100; collapsed,
	OriginCache; cache-fresh=1100; collapse-hit=?1		"CDN Company Here"; hit; ttl=545
	4. Security Considerations		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-Status header is exposed to parties who are not		However, if the Cache-Status header is exposed to parties who are not
	skipping to change at page 7, line 34 ¶		skipping to change at page 7, line 11 ¶
	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-Status header field can be omitted.		Cache-Status header field can be omitted.
	5. References		5. References
	5.1. 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-13 (work in progress),		draft-ietf-httpbis-header-structure-13 (work in progress),
	August 2019.		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>.
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