< draft-ietf-httpbis-immutable-00.txt		draft-ietf-httpbis-immutable-01.txt >
	Network Working Group P. McManus		HTTP Working Group P. McManus
	Internet-Draft Mozilla		Internet-Draft Mozilla
	Intended status: Standards Track January 12, 2017		Intended status: Standards Track March 13, 2017
	Expires: July 16, 2017		Expires: September 14, 2017
	HTTP Immutable Responses		HTTP Immutable Responses
	draft-ietf-httpbis-immutable-00		draft-ietf-httpbis-immutable-01
	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 assures 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
			Discussion of this draft takes place on the HTTP working group
			mailing list (ietf-http-wg@w3.org), which is archived at
			https://lists.w3.org/Archives/Public/ietf-http-wg/ .
			Working Group information can be found at http://httpwg.github.io/ ;
			source code and issues list for this draft can be found at
			https://github.com/httpwg/http-extensions/labels/immutable .
	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.
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	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
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	This Internet-Draft will expire on July 16, 2017.		This Internet-Draft will expire on September 14, 2017.
	Copyright Notice		Copyright Notice
	Copyright (c) 2017 IETF Trust and the persons identified as the		Copyright (c) 2017 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
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	described in the Simplified BSD License.		described in the Simplified BSD License.
	1. Introduction		1. Introduction
	The HTTP freshness lifetime [RFC7234] caching attribute specifies		HTTP's freshness lifetime mechanism [RFC7234] allows a client to
	that a client may safely reuse a response to satisfy future requests		safely reuse a stored response to satisfy future requests for a
	over a specific period of time. It does not specify that the		specified period of time. However, it is still possible that the
	resource will be not 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 should see a photo more than one		of one hour would mean that no user would see a cached photo more
	hour old. However, the photo could be updated at any time resulting		than one hour old. However, the photo could be updated at any time
	in different users seeing different photos depending on the contents		resulting in different users seeing different photos depending on the
	of their caches for up to one hour. This is compliant with the		contents of their caches for up to one hour. This is compliant with
	caching mechanism defined in [RFC7234].		the caching mechanism defined in [RFC7234].
	Users that need to confirm there have been no updates to their		Users that need to confirm there have been no updates to their cached
	current cached resources typically invoke the reload (or refresh)		responses typically use the reload (or refresh) mechanism in their
	mechanism in the user agent. This in turn generates a conditional		user agents. This in turn generates a conditional request [RFC7232]
	request [RFC7232] and either a new representation or, if unmodified,		and either a new representation or, if unmodified, a 304 (Not
	a 304 response [RFC7231] is returned. A user agent that manages HTML		Modified) response [RFC7232] is returned. A user agent that
	and its dependent sub-resources may issue hundreds of conditional		understands HTML and fetches its dependent sub-resources might issue
	requests to refresh all portions of a common HTML page [REQPERPAGE].		hundreds of conditional requests to refresh all portions of a common
			page [REQPERPAGE].
	Through the use of the versioned URL design pattern some content		However some content providers never create more than one variant of
	providers never create more than one variant of a sub-resource. When		a sub-resource, because they use "versioned" URLs. When these
	these resources need an update they are simply published under a new		resources need an update they are simply published under a new URL,
	URL, typically embedding a variant identifier in the path, and		typically embedding an identifier unique to that version of the
	references to the sub-resource are updated with the new path		resource in the path, and references to the sub-resource are updated
	information.		with the new path information.
	For example, https://www.example.com/101016/main.css might be updated		For example, "https://www.example.com/101016/main.css" might be
	and republished as https://www.example.com/102026/main.css and the		updated and republished as "https://www.example.com/102026/main.css",
	html that references it is changed at the same time. This design		with any links that references it being changed at the same time.
	pattern allows a very large freshness lifetime to be applied to the		This design pattern allows a very large freshness lifetime to be used
	sub-resource without guessing when it will be updated in the future.		for the sub-resource without guessing when it will be updated in the
			future.
	Unfortunately, the user-agent is not aware of the versioned URL		Unfortunately, the user agent does not know when this versioned URL
	design pattern. User driven refresh events still translate into		design pattern is used. As a result, user-driven refreshes still
	wasted conditional requests for each sub-resource as each will return		translate into wasted conditional requests for each sub-resource as
	304 responses.		each will return 304 responses.
	The immutable HTTP response Cache-Control extension allows servers to		The "immutable" HTTP response Cache-Control extension allows servers
	identify resources that will not be updated during their freshness		to identify responses that will not be updated during their freshness
	lifetime. This effectively instructs the client that any conditional		lifetimes.
	request for a previously served variant of that resource may be
	safely skipped without worrying that it has been updated.		This effectively informs clients that any conditional request for
			that response can be safely skipped without worrying that it has been
			updated.
	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 MUST 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
			freshness lifetime (e.g. upon a reload) unless explicitly overridden
			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 response. Stale responses SHOULD be revalidated as they normally		the stored response. Stale responses SHOULD be revalidated as they
	would be in the absence of immutable.		normally would be in the absence of immutable.
	The immutable extension takes no arguments and if any arguments are		The immutable extension takes no arguments. If any arguments are
	present they have no meaning. Multiple instances of the immutable		present, they have no meaning, and MUST be ignored. Multiple
	extension are equivalent to one instance. The presence of an		instances of the immutable extension are equivalent to one instance.
	immutable Cache-Control extension in a request has no effect.		The presence of an immutable Cache-Control extension in a request has
			no effect.
	2.1. About Intermediaries		2.1. About Intermediaries
	An immutable response has the same semantic meaning for proxy clients		An immutable response has the same semantic meaning whe received by
	as it does for User-Agent based clients and they therefore MAY also		proxy clients as it does when received by User-Agent based clients.
	presume a conditional revalidation for a response marked immutable		Therefore proxies SHOULD skip conditionally revalidating fresh
	would return 304. A proxy client who uses immutable to anticipate a		responses containing the immutable extension unless there is a signal
	304 response may choose whether to reply with a 304 or 200 to its		from the client that a validation is necessary (e.g. a no-cache
	requesting client.		Cache-Control request directive).
			A proxy that uses immutable to bypass a conditional revalidation may
			choose whether to reply with a 304 or 200 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 for resources that are not part of		o Clients SHOULD ignore immutable from resources that are not part
	an authenticated context such as HTTPS. Authenticated resources		of an authenticated context such as HTTPS. Authenticated
	are less vulnerable to cache poisoning.		resources are less vulnerable to cache poisoning.
	o User-Agents often provide two different refresh mechanismss:		o User-Agents often provide two different refresh mechanisms: reload
	reload and some form of force-reload. The latter is used to		and some form of force-reload. The latter is used to rectify
	rectify interrupted loads and other corruption. These reloads,		interrupted loads and other corruption. These reloads, typically
	typically indicated through no-cache request attributes, should		indicated through no-cache request attributes, SHOULD ignore
	ignore immutable as well.		immutable as well.
	o Clients should ignore immutable for resources that do not provide		o Clients SHOULD ignore immutable for resources that do not provide
	a strong indication that the stored response size is the correct		a strong indication that the stored response size is the correct
	response size such as responses delimited by connection close.		response size such as responses delimited by connection close.
	4. IANA Considerations		4. IANA Considerations
	[RFC7234] sections 7.1 and 7.1.2 require registration of the		[RFC7234] sections 7.1 and 7.1.2 require registration of the
	immutable extension in the "Hypertext Transfer Protocol (HTTP) Cache		immutable extension in the "Hypertext Transfer Protocol (HTTP) Cache
	Directive Registry" with IETF Review.		Directive Registry" with IETF Review.
	o Cache-Directive: immutable		o Cache-Directive: immutable
	o Pointer to specification text: [this document]		o Pointer to specification text: [this document]
	5. Acknowledgments		5. Acknowledgments
	Thank you to Ben Maurer for partnership in developing and testing		Thank you to Ben Maurer for partnership in developing and testing
	this idea. Thank you to Amos Jeffries for help with proxy		this idea. Thank you to Amos Jeffries for help with proxy
	interactions.		interactions and to Mark Nottingham for help with the documentation.
	6. References		6. References
	6.1. Normative References		6.1. Normative References
	[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer		[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
	Protocol (HTTP/1.1): Semantics and Content", RFC 7231,		Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
	DOI 10.17487/RFC7231, June 2014,		DOI 10.17487/RFC7231, June 2014,
	<http://www.rfc-editor.org/info/rfc7231>.		<http://www.rfc-editor.org/info/rfc7231>.
	[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,
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