draft-ietf-httpbis-immutable-03.txt		draft-ietf-httpbis-immutable-latest.txt
	HTTP Working Group P. McManus		HTTP Working Group P. McManus
	Internet-Draft Mozilla		Internet-Draft Mozilla
	Intended status: Standards Track July 3, 2017		Intended status: Standards Track October 28, 2018
	Expires: January 4, 2018		Expires: May 1, 2026
	HTTP Immutable Responses		HTTP Immutable Responses
	draft-ietf-httpbis-immutable-03		draft-ietf-httpbis-immutable-03
	Abstract		Abstract
	The immutable HTTP response Cache-Control extension allows servers to		The immutable HTTP response Cache-Control extension allows servers to
	identify resources that will not be updated during their freshness		identify resources that will not be updated during their freshness
	lifetime. This assures that a client never needs to revalidate a		lifetime. This ensures that a client never needs to revalidate a
	cached fresh resource to be certain it has not been modified.		cached fresh resource to be certain it has not been modified.
	Note to Readers		Note to Readers
	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 http://httpwg.github.io/		Working Group information can be found at http://httpwg.github.io/
	[2]; source code and issues list for this draft can be found at		[2]; source code and issues list for this draft can be found at
	https://github.com/httpwg/http-extensions/labels/immutable [3].		https://github.com/httpwg/http-extensions/labels/immutable [3].
	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 http://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 January 4, 2018.		This Internet-Draft will expire on May 1, 2026.
	Copyright Notice		Copyright Notice
	Copyright (c) 2017 IETF Trust and the persons identified as the		Copyright (c) 2018 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
	(http://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
			1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
			1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
			2. The Immutable Cache-Control Extension . . . . . . . . . . . . 3
			2.1. About Intermediaries . . . . . . . . . . . . . . . . . . 4
			2.2. Example . . . . . . . . . . . . . . . . . . . . . . . . . 4
			3. Security Considerations . . . . . . . . . . . . . . . . . . . 4
			4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
			5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
			5.1. Normative References . . . . . . . . . . . . . . . . . . 5
			5.2. Informative References . . . . . . . . . . . . . . . . . 5
			5.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 5
			Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 6
			Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
	1. Introduction		1. Introduction
	HTTP's freshness lifetime mechanism [RFC7234] allows a client to		HTTP's freshness lifetime mechanism [RFC7234] allows a client to
	safely reuse a stored response to satisfy future requests for a		safely reuse a stored response to satisfy future requests for a
	specified period of time. However, it is still possible that the		specified period of time. However, it is still possible that the
	resource will be modified during that period.		resource will be modified during that period.
	For instance, a front page newspaper photo with a freshness lifetime		For instance, a front-page newspaper photo with a freshness lifetime
	of one hour would mean that no user would see a cached photo more		of one hour would mean that no user would see a cached photo more
	than one hour old. However, the photo could be updated at any time		than one hour old. However, the photo could be updated at any time,
	resulting in different users seeing different photos depending on the		resulting in different users seeing different photos depending on the
	contents of their caches for up to one hour. This is compliant with		contents of their caches for up to one hour. This is compliant with
	the caching mechanism defined in [RFC7234].		the caching mechanism defined in [RFC7234].
	Users that need to confirm there have been no updates to their cached		Users that need to confirm there have been no updates to their cached
	responses typically use the reload (or refresh) mechanism in their		responses typically use the reload (or refresh) mechanism in their
	user agents. This in turn generates a conditional request [RFC7232]		user agents. This in turn generates a conditional request [RFC7232],
	and either a new representation or, if unmodified, a 304 (Not		and either a new representation or, if unmodified, a 304 (Not
	Modified) response [RFC7232] is returned. A user agent that		Modified) response [RFC7232] is returned. A user agent that
	understands HTML and fetches its dependent sub-resources might issue		understands HTML and fetches its dependent sub-resources might issue
	hundreds of conditional requests to refresh all portions of a common		hundreds of conditional requests to refresh all portions of a common
	page [REQPERPAGE].		page [REQPERPAGE].
	However some content providers never create more than one variant of		However, some content providers never create more than one variant of
	a sub-resource, because they use "versioned" URLs. When these		a sub-resource, because they use "versioned" URLs. When these
	resources need an update they are simply published under a new URL,		resources need an update, they are simply published under a new URL,
	typically embedding an identifier unique to that version of the		typically embedding an identifier unique to that version of the
	resource in the path, and references to the sub-resource are updated		resource in the path, and references to the sub-resource are updated
	with the new path information.		with the new path information.
	For example, "https://www.example.com/101016/main.css" might be		For example, "https://www.example.com/101016/main.css" might be
	updated and republished as "https://www.example.com/102026/main.css",		updated and republished as "https://www.example.com/102026/main.css",
	with any links that reference it being changed at the same time.		with any links that reference it being changed at the same time.
	This design pattern allows a very large freshness lifetime to be used		This design pattern allows a very large freshness lifetime to be used
	for the sub-resource without guessing when it will be updated in the		for the sub-resource without guessing when it will be updated in the
	future.		future.
	Unfortunately, the user agent does not know when this versioned URL		Unfortunately, the user agent does not know when this versioned URL
	design pattern is used. As a result, user-driven refreshes still		design pattern is used. As a result, user-driven refreshes still
	translate into wasted conditional requests for each sub-resource as		translate into wasted conditional requests for each sub-resource as
	each will return 304 responses.		each will return 304 responses.
	The "immutable" HTTP response Cache-Control extension allows servers		The immutable HTTP response Cache-Control extension allows servers to
	to identify responses that will not be updated during their freshness		identify responses that will not be updated during their freshness
	lifetimes.		lifetimes.
	This effectively informs clients that any conditional request for		This effectively informs clients that any conditional request for
	that response can be safely skipped without worrying that it has been		that response can be safely skipped without worrying that it has been
	updated.		updated.
	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", "MAY", and "OPTIONAL" in this		"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	document are to be interpreted as described in [RFC2119].		"OPTIONAL" in this document are to be interpreted as described in BCP
			14 [RFC2119] [RFC8174] when, and only when, they appear in all
			capitals, as shown here.
	2. The immutable Cache-Control extension		2. The Immutable Cache-Control Extension
	When present in an HTTP response, the "immutable" Cache-Control		When present in an HTTP response, the immutable Cache-Control
	extension indicates that the origin server will not update the		extension indicates that the origin server will not update the
	representation of that resource during the freshness lifetime of the		representation of that resource during the freshness lifetime of the
	response.		response.
	Clients SHOULD NOT issue a conditional request during the response's		Clients SHOULD NOT issue a conditional request during the response's
	freshness lifetime (e.g. upon a reload) unless explicitly overridden		freshness lifetime (e.g., upon a reload) unless explicitly overridden
	by the user (e.g. a force reload).		by the user (e.g., a force reload).
	The immutable extension only applies during the freshness lifetime of		The immutable extension only applies during the freshness lifetime of
	the stored response. Stale responses SHOULD be revalidated as they		the stored response. Stale responses SHOULD be revalidated as they
	normally would be in the absence of immutable.		normally would be in the absence of the immutable extension.
	The immutable extension takes no arguments. If any arguments are		The immutable extension takes no arguments. If any arguments are
	present, they have no meaning, and MUST be ignored. Multiple		present, they have no meaning and MUST be ignored. Multiple
	instances of the immutable extension are equivalent to one instance.		instances of the immutable extension are equivalent to one instance.
	The presence of an immutable Cache-Control extension in a request has		The presence of an immutable Cache-Control extension in a request has
	no effect.		no effect.
	2.1. About Intermediaries		2.1. About Intermediaries
	An immutable response has the same semantic meaning when received by		An immutable response has the same semantic meaning when received by
	proxy clients as it does when received by User-Agent based clients.		proxy clients as it does when received by user-agent-based clients.
	Therefore proxies SHOULD skip conditionally revalidating fresh		Therefore, proxies SHOULD skip conditionally revalidating fresh
	responses containing the immutable extension unless there is a signal		responses containing the immutable extension unless there is a signal
	from the client that a validation is necessary (e.g. a no-cache		from the client that a validation is necessary (e.g., a no-cache
	Cache-Control request directive defined by Section 5.2.1.4 of		Cache-Control request directive defined in Section 5.2.1.4 of
	[RFC7234]).		[RFC7234]).
	A proxy that uses immutable to bypass a conditional revalidation can		A proxy that uses the immutable extension to bypass a conditional
	choose whether to reply with a 304 or 200 to its requesting client		revalidation can choose whether to reply with a 304 or 200 response
	based on the request headers the proxy received.		to its requesting client based on the request headers the proxy
			received.
	2.2. Example		2.2. Example
	Cache-Control: max-age=31536000, immutable		Cache-Control: max-age=31536000, immutable
	3. Security Considerations		3. Security Considerations
	The immutable mechanism acts as form of soft pinning and, as with all		The immutable mechanism acts as form of soft pinning and, as with all
	pinning mechanisms, creates a vector for amplification of cache		pinning mechanisms, creates a vector for amplification of cache
	corruption incidents. These incidents include cache poisoning		corruption incidents. These incidents include cache-poisoning
	attacks. Three mechanisms are suggested for mitigation of this risk:		attacks. Three mechanisms are suggested for mitigation of this risk:
	o Clients SHOULD ignore immutable from resources that are not part		o Clients SHOULD ignore the immutable extension from resources that
	of an authenticated context such as HTTPS. Authenticated		are not part of an authenticated context such as HTTPS.
	resources are less vulnerable to cache poisoning.		Authenticated resources are less vulnerable to cache poisoning.
	o User-Agents often provide two different refresh mechanisms: reload		o User agents often provide two different refresh mechanisms: reload
	and some form of force-reload. The latter is used to rectify		and some form of force-reload. The latter is used to rectify
	interrupted loads and other corruption. These reloads, typically		interrupted loads and other corruption. These reloads, typically
	indicated through no-cache request attributes, SHOULD ignore		indicated through no-cache request attributes, SHOULD ignore the
	immutable as well.		immutable extension as well.
	o Clients SHOULD ignore immutable for resources that do not provide		o Clients SHOULD ignore the immutable extension for resources that
	a strong indication that the stored response size is the correct		do not provide a strong indication that the stored response size
	response size such as responses delimited by connection close.		is the correct response size such as responses delimited by
			connection close.
	4. IANA Considerations		4. IANA Considerations
	Section 7.1 of [RFC7234] requires registration of the immutable		The immutable extension has been registered in the "Hypertext
	extension in the "Hypertext Transfer Protocol (HTTP) Cache Directive		Transfer Protocol (HTTP) Cache Directive Registry" per the guidelines
	Registry" with IETF Review.		described in Section 7.1 of [RFC7234].
	o Cache-Directive: immutable
	o Pointer to specification text: [this document]
	5. Acknowledgments		o Cache Directive: immutable
	Thank you to Ben Maurer for partnership in developing and testing		o Reference: RFC 8246
	this idea. Thank you to Amos Jeffries for help with proxy
	interactions and to Mark Nottingham for help with the documentation.
	6. References		5. References
	6.1. Normative References		5.1. Normative References
	[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>.
	[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer		[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
	Protocol (HTTP/1.1): Conditional Requests", RFC 7232,		Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
	DOI 10.17487/RFC7232, June 2014,		DOI 10.17487/RFC7232, June 2014,
	<https://www.rfc-editor.org/info/rfc7232>.		<https://www.rfc-editor.org/info/rfc7232>.
	[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,		[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
	Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",		Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
	RFC 7234, DOI 10.17487/RFC7234, June 2014,		RFC 7234, DOI 10.17487/RFC7234, June 2014,
	<https://www.rfc-editor.org/info/rfc7234>.		<https://www.rfc-editor.org/info/rfc7234>.
	6.2. Informative References		[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
			2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
			May 2017, <https://www.rfc-editor.org/info/rfc8174>.
			5.2. Informative References
	[REQPERPAGE]		[REQPERPAGE]
	"HTTP Archive", n.d.,		HTTP Archive, "Total Requests per Page",
	<http://httparchive.org/interesting.php#reqTotal>.		<http://httparchive.org/interesting.php#reqTotal>.
	6.3. 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] http://httpwg.github.io/		[2] http://httpwg.github.io/
	[3] https://github.com/httpwg/http-extensions/labels/immutable		[3] https://github.com/httpwg/http-extensions/labels/immutable
			Acknowledgments
			Thank you to Ben Maurer for partnership in developing and testing
			this idea. Thank you to Amos Jeffries for help with proxy
			interactions and to Mark Nottingham for help with the documentation.
	Author's Address		Author's Address
	Patrick McManus		Patrick McManus
	Mozilla		Mozilla
	Email: pmcmanus@mozilla.com		Email: mcmanus@ducksong.com
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