< draft-ietf-httpbis-variants-03.txt		draft-ietf-httpbis-variants-04.txt >
	HTTP M. Nottingham		HTTP M. Nottingham
	Internet-Draft Fastly		Internet-Draft Fastly
	Updates: 7234 (if approved) July 1, 2018		Updates: 7234 (if approved) October 22, 2018
	Intended status: Standards Track		Intended status: Standards Track
	Expires: January 2, 2019		Expires: April 25, 2019
	HTTP Representation Variants		HTTP Representation Variants
	draft-ietf-httpbis-variants-03		draft-ietf-httpbis-variants-04
	Abstract		Abstract
	This specification introduces an alternative way to communicate a		This specification introduces an alternative way to communicate a
	secondary cache key for a HTTP resource, using the HTTP "Variants"		secondary cache key for a HTTP resource, using the HTTP "Variants"
	and "Variant-Key" response header fields. Its aim is to make HTTP		and "Variant-Key" response header fields. Its aim is to make HTTP
	proactive content negotiation more cache-friendly.		proactive content negotiation more cache-friendly.
	Note to Readers		Note to Readers
	skipping to change at page 1, line 49 ¶		skipping to change at page 1, line 49 ¶
	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 January 2, 2019.		This Internet-Draft will expire on April 25, 2019.
	Copyright Notice		Copyright Notice
	Copyright (c) 2018 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
	(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 40 ¶		skipping to change at page 2, line 40 ¶
	4.2. Check Vary . . . . . . . . . . . . . . . . . . . . . . . 10		4.2. Check Vary . . . . . . . . . . . . . . . . . . . . . . . 10
	4.3. Example of Cache Behaviour . . . . . . . . . . . . . . . 11		4.3. Example of Cache Behaviour . . . . . . . . . . . . . . . 11
	5. Origin Server Behaviour . . . . . . . . . . . . . . . . . . . 12		5. Origin Server Behaviour . . . . . . . . . . . . . . . . . . . 12
	5.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 12		5.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 12
	5.1.1. Single Variant . . . . . . . . . . . . . . . . . . . 12		5.1.1. Single Variant . . . . . . . . . . . . . . . . . . . 12
	5.1.2. Multiple Variants . . . . . . . . . . . . . . . . . . 13		5.1.2. Multiple Variants . . . . . . . . . . . . . . . . . . 13
	5.1.3. Partial Coverage . . . . . . . . . . . . . . . . . . 14		5.1.3. Partial Coverage . . . . . . . . . . . . . . . . . . 14
	6. Defining Content Negotiation Using Variants . . . . . . . . . 14		6. Defining Content Negotiation Using Variants . . . . . . . . . 14
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
	8. Security Considerations . . . . . . . . . . . . . . . . . . . 15		8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
	9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15		9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
	9.1. Normative References . . . . . . . . . . . . . . . . . . 16		9.1. Normative References . . . . . . . . . . . . . . . . . . 16
	9.2. Informative References . . . . . . . . . . . . . . . . . 16		9.2. Informative References . . . . . . . . . . . . . . . . . 16
	9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 17		9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 17
	Appendix A. Variants for Existing Content Negotiation Mechanisms 17		Appendix A. Variants for Existing Content Negotiation Mechanisms 17
	A.1. Accept . . . . . . . . . . . . . . . . . . . . . . . . . 17		A.1. Accept . . . . . . . . . . . . . . . . . . . . . . . . . 17
	A.2. Accept-Encoding . . . . . . . . . . . . . . . . . . . . . 18		A.2. Accept-Encoding . . . . . . . . . . . . . . . . . . . . . 18
	A.3. Accept-Language . . . . . . . . . . . . . . . . . . . . . 19		A.3. Accept-Language . . . . . . . . . . . . . . . . . . . . . 19
	Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 19		Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 19
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20		Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
	skipping to change at page 3, line 40 ¶		skipping to change at page 3, line 40 ¶
	GET /foo HTTP/1.1		GET /foo HTTP/1.1
	Host: www.example.com		Host: www.example.com
	Accept-Language: en;q=0.5, fr;q=1.0		Accept-Language: en;q=0.5, fr;q=1.0
	HTTP/1.1 200 OK		HTTP/1.1 200 OK
	Content-Type: text/html		Content-Type: text/html
	Content-Language: en		Content-Language: en
	Vary: Accept-Language		Vary: Accept-Language
	Transfer-Encoding: chunked		Transfer-Encoding: chunked
	[French content]		[English content]
	Provided that the cache has full knowledge of the semantics of		Provided that the cache has full knowledge of the semantics of
	Accept-Language and Content-Language, it will know that an English		Accept-Language and Content-Language, it will know that an English
	representation is available and might be able to infer that a French		representation is available and might be able to infer that a French
	representation is not available. But, it does not know (for example)		representation is not available. But, it does not know (for example)
	whether a Japanese representation is available without making another		whether a Japanese representation is available without making another
	request, incurring possibly unnecessary latency.		request, incurring possibly unnecessary latency.
	This specification introduces the HTTP Variants response header field		This specification introduces the HTTP Variants response header field
	(Section 2) to enumerate the available variant representations on the		(Section 2) to enumerate the available variant representations on the
	skipping to change at page 4, line 25 ¶		skipping to change at page 4, line 25 ¶
	Accept-Language: en;q=0.5, fr;q=1.0		Accept-Language: en;q=0.5, fr;q=1.0
	HTTP/1.1 200 OK		HTTP/1.1 200 OK
	Content-Type: text/html		Content-Type: text/html
	Content-Language: en		Content-Language: en
	Vary: Accept-Language		Vary: Accept-Language
	Variants: Accept-Language;de;en;jp		Variants: Accept-Language;de;en;jp
	Variant-Key: en		Variant-Key: en
	Transfer-Encoding: chunked		Transfer-Encoding: chunked
	[French content]		[English content]
	Proactive content negotiation mechanisms that wish to be used with		Proactive content negotiation mechanisms that wish to be used with
	Variants need to define how to do so explicitly; see Section 6. As a		Variants need to define how to do so explicitly; see Section 6. As a
	result, it is best suited for negotiation over request headers that		result, it is best suited for negotiation over request headers that
	are well-understood.		are well-understood.
	Variants also works best when content negotiation takes place over a		Variants also works best when content negotiation takes place over a
	constrained set of representations; since each variant needs to be		constrained set of representations; since each variant needs to be
	listed in the header field, it is ill-suited for open-ended sets of		listed in the header field, it is ill-suited for open-ended sets of
	representations.		representations.
	skipping to change at page 7, line 14 ¶		skipping to change at page 7, line 14 ¶
	3. The "Variant-Key" HTTP Header Field		3. The "Variant-Key" HTTP Header Field
	The Variant-Key HTTP response header field is used to indicate the		The Variant-Key HTTP response header field is used to indicate the
	values from the Variants header field that identify the		values from the Variants header field that identify the
	representation it occurs within.		representation it occurs within.
	Variant-Key = 1#available-values		Variant-Key = 1#available-values
	available-values = available-value *( ";" available-value )		available-values = available-value *( ";" available-value )
	Each member of the list contains the selected available-value(s), in		Each member of the list contains a set of selected available-value(s)
	the same order as the variants listed in the Variants header field.		that identify this representation, in the same order as the variants
			listed in the Variants header field.
	Therefore, Variant-Key MUST be the same length (in comma-separated		Therefore, each member of Variant-Key MUST be the same length (in
	members) as Variants, and each member MUST correspond in position to		semicolon-separated members) as Variants, and each member's
	its companion in Variants.		available-values MUST correspond in position to their companions in
			Variants.
	For example:		For example:
	Variants: Accept-Encoding;gzip;br, Accept-Language;en ;fr		Variants: Accept-Encoding;gzip;br, Accept-Language;en ;fr
	Variant-Key: gzip, fr		Variant-Key: gzip;fr
	This header pair indicates that the representation has a "gzip"		This header pair indicates that the representation has a "gzip"
	content-coding and "fr" content-language.		content-coding and "fr" content-language.
	A more complex example involves listing multiple available-values in		If the response can be used to satisfy more than one request), they
	a list member, to indicate that the response can be used to satisfy		can be listed in additional members. For example:
	requests with any of those values. For example:
	Variants: Accept-Encoding;gzip;br, Accept-Language;en ;fr		Variants: Accept-Encoding;gzip;br, Accept-Language;en ;fr
	Variant-Key: gzip;identity, fr		Variant-Key: gzip;fr, identity;fr
	indicates that this response can be used for requests whose Accept-		indicates that this response can be used for requests whose Accept-
	Encoding algorithm selects "gzip" or "identity", as long as the		Encoding algorithm selects "gzip" or "identity", as long as the
	Accept-Language algorithm selects "fr" - perhaps because there is no		Accept-Language algorithm selects "fr" - perhaps because there is no
	gzip-compressed French representation.		gzip-compressed French representation.
	This highlights an important aspect of Variant-Key; it is only used		When more than one Variant-Key value is in a response, the first one
	to indicate what request attributes are associated with the response		present MUST indicate the variant-key for the response it occurs
	containing it; this is different from headers like Content-Encoding,		within.
	which indicate attributes of the response itself.
	3.1. Generating a Variant-Key List		3.1. Generating a Variant-Key List
	This algorithm generates a list of normalised strings from Variant-		This algorithm generates a list of normalised strings from Variant-
	Key, suitable for comparison with values generated by Section 4.		Key, suitable for comparison with values generated by Section 4.
	Given stored-headers (a set of headers from a stored response), a		Given stored-headers (a set of headers from a stored response), a
	normalised list of variant-keys for that message can be generated by		normalised list of variant-keys for that message can be generated by
	following this algorithm:		following this algorithm:
	skipping to change at page 8, line 22 ¶		skipping to change at page 8, line 22 ¶
	field-values of stored-headers whose field-name is "Variant-Key"		field-values of stored-headers whose field-name is "Variant-Key"
	and joining them with a comma (",").		and joining them with a comma (",").
	3. Let value-list be the result of splitting variant-key-header on		3. Let value-list be the result of splitting variant-key-header on
	commas (",").		commas (",").
	4. For each value in value-list:		4. For each value in value-list:
	1. Remove all whitespace from value.		1. Remove all whitespace from value.
	2. Let items be the result of splitting value on ";".		2. Append value to variant-keys.
	3. append items to variant-keys.
	5. Return the result of running Compute Possible Keys (Section 4.1)		5. Return variant-keys.
	on variant-keys, an empty string and an empty list.
	4. Cache Behaviour		4. Cache Behaviour
	Caches that implement the Variants header field and the relevant		Caches that implement the Variants header field and the relevant
	semantics of the field-name it contains can use that knowledge to		semantics of the field-name it contains can use that knowledge to
	either select an appropriate stored representation, or forward the		either select an appropriate stored representation, or forward the
	request if no appropriate representation is stored.		request if no appropriate representation is stored.
	They do so by running this algorithm (or its functional equivalent)		They do so by running this algorithm (or its functional equivalent)
	upon receiving a request:		upon receiving a request:
	skipping to change at page 10, line 16 ¶		skipping to change at page 10, line 14 ¶
	2. For each value in values:		2. For each value in values:
	1. If key-stub is an empty string, let this-key be a copy of		1. If key-stub is an empty string, let this-key be a copy of
	value.		value.
	2. Otherwise:		2. Otherwise:
	1. Let this-key be a copy of key-stub.		1. Let this-key be a copy of key-stub.
	2. Append a comma (",") to this-key.		2. Append a semicolon (";") to this-key.
	3. Append value to this-key.		3. Append value to this-key.
	3. Let remaining-facets be a copy of all of the members of key-		3. Let remaining-facets be a copy of all of the members of key-
	facets except the first.		facets except the first.
	4. If remaining-facets is empty, append this-key to possible-		4. If remaining-facets is empty, append this-key to possible-
	keys.		keys.
	5. Otherwise, run Compute Possible Keys on remaining-facets,		5. Otherwise, run Compute Possible Keys on remaining-facets,
	skipping to change at page 11, line 19 ¶		skipping to change at page 11, line 17 ¶
	Note that implementation of the Vary header field varies in practice,		Note that implementation of the Vary header field varies in practice,
	and the algorithm above illustrates only one way to apply it. It is		and the algorithm above illustrates only one way to apply it. It is
	equally viable to forward the request if there is a request header		equally viable to forward the request if there is a request header
	listed in Vary but not Variants.		listed in Vary but not Variants.
	4.3. Example of Cache Behaviour		4.3. Example of Cache Behaviour
	For example, if the selected variants-header was:		For example, if the selected variants-header was:
	Variants: Accept-Language;en;fr,de, Accept-Encoding;gzip;br		Variants: Accept-Language;en;fr;de, Accept-Encoding;gzip;br
	and the request contained the headers:		and the request contained the headers:
	Accept-Language: fr;q=1.0, en;q=0.1		Accept-Language: fr;q=1.0, en;q=0.1
	Accept-Encoding: gzip		Accept-Encoding: gzip
	Then the sorted-variants would be:		Then the sorted-variants would be:
	[		[
	["fr", "en"] // prefers French, will accept English		["fr", "en"] // prefers French, will accept English
	skipping to change at page 13, line 42 ¶		skipping to change at page 13, line 42 ¶
	Host: www.example.net		Host: www.example.net
	Accept-Language: en;q=1.0, fr;q=0.5		Accept-Language: en;q=1.0, fr;q=0.5
	Accept-Encoding: gzip, br		Accept-Encoding: gzip, br
	HTTP/1.1 200 OK		HTTP/1.1 200 OK
	Content-Type: image/gif		Content-Type: image/gif
	Content-Language: en		Content-Language: en
	Content-Encoding: br		Content-Encoding: br
	Variants: Accept-Language;en;jp;de		Variants: Accept-Language;en;jp;de
	Variants: Accept-Encoding;br;gzip		Variants: Accept-Encoding;br;gzip
	Variant-Key: en, br		Variant-Key: en;br
	Vary: Accept-Language, Accept-Encoding		Vary: Accept-Language, Accept-Encoding
	Transfer-Encoding: chunked		Transfer-Encoding: chunked
	Here, the cache knows that there are two axes that the response		Here, the cache knows that there are two axes that the response
	varies upon; language and encoding. Thus, there are a total of nine		varies upon; language and encoding. Thus, there are a total of nine
	possible representations for the resource (including the identity		possible representations for the resource (including the identity
	encoding), and the cache needs to consider the selection algorithms		encoding), and the cache needs to consider the selection algorithms
	for both axes.		for both axes.
	Upon a subsequent request, if both selection algorithms return a		Upon a subsequent request, if both selection algorithms return a
	skipping to change at page 15, line 10 ¶		skipping to change at page 15, line 10 ¶
	order of preference, given the value of the request header		order of preference, given the value of the request header
	nominated above and an available-values list from the Variants		nominated above and an available-values list from the Variants
	header. If the result is an empty list, it implies that the cache		header. If the result is an empty list, it implies that the cache
	cannot satisfy the request.		cannot satisfy the request.
	Appendix A fulfils these requirements for some existing proactive		Appendix A fulfils these requirements for some existing proactive
	content negotiation mechanisms in HTTP.		content negotiation mechanisms in HTTP.
	7. IANA Considerations		7. IANA Considerations
	This specification registers two values in the Permanent Message		This specification registers the following entry in the Permanent
	Header Field Names registry established by [RFC3864]:		Message Header Field Names registry established by [RFC3864]:
	o Header field name: Variants		o Header field name: Variants
	o Applicable protocol: http		o Applicable protocol: http
	o Status: standard		o Status: standard
	o Author/Change Controller: IETF		o Author/Change Controller: IETF
	o Specification document(s): [this document]		o Specification document(s): [this document]
	o Related information:		o Related information:
			This specification registers the following entry in the Permanent
			Message Header Field Names registry established by [RFC3864]:
	o Header field name: Variant-Key		o Header field name: Variant-Key
	o Applicable protocol: http		o Applicable protocol: http
	o Status: standard		o Status: standard
	o Author/Change Controller: IETF		o Author/Change Controller: IETF
	o Specification document(s): [this document]		o Specification document(s): [this document]
	skipping to change at page 16, line 43 ¶		skipping to change at page 16, line 46 ¶
	<https://www.rfc-editor.org/info/rfc7234>.		<https://www.rfc-editor.org/info/rfc7234>.
	[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>.
	9.2. Informative References		9.2. Informative References
	[I-D.ietf-httpbis-client-hints]		[I-D.ietf-httpbis-client-hints]
	Grigorik, I., "HTTP Client Hints", draft-ietf-httpbis-		Grigorik, I., "HTTP Client Hints", draft-ietf-httpbis-
	client-hints-05 (work in progress), January 2018.		client-hints-06 (work in progress), July 2018.
	[RFC2295] Holtman, K. and A. Mutz, "Transparent Content Negotiation		[RFC2295] Holtman, K. and A. Mutz, "Transparent Content Negotiation
	in HTTP", RFC 2295, DOI 10.17487/RFC2295, March 1998,		in HTTP", RFC 2295, DOI 10.17487/RFC2295, March 1998,
	<https://www.rfc-editor.org/info/rfc2295>.		<https://www.rfc-editor.org/info/rfc2295>.
	[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration		[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
	Procedures for Message Header Fields", BCP 90, RFC 3864,		Procedures for Message Header Fields", BCP 90, RFC 3864,
	DOI 10.17487/RFC3864, September 2004,		DOI 10.17487/RFC3864, September 2004,
	<https://www.rfc-editor.org/info/rfc3864>.		<https://www.rfc-editor.org/info/rfc3864>.
	skipping to change at page 20, line 5 ¶		skipping to change at page 20, line 5 ¶
	Acknowledgements		Acknowledgements
	This protocol is conceptually similar to, but simpler than,		This protocol is conceptually similar to, but simpler than,
	Transparent Content Negotiation [RFC2295]. Thanks to its authors for		Transparent Content Negotiation [RFC2295]. Thanks to its authors for
	their inspiration.		their inspiration.
	It is also a generalisation of a Fastly VCL feature designed by		It is also a generalisation of a Fastly VCL feature designed by
	Rogier 'DocWilco' Mulhuijzen.		Rogier 'DocWilco' Mulhuijzen.
	Thanks to Hooman Beheshti, Ilya Grigorik and Jeffrey Yasskin for		Thanks to Hooman Beheshti, Ilya Grigorik, Leif Hedstrom, and Jeffrey
	their review and input.		Yasskin for their review and input.
	Author's Address		Author's Address
	Mark Nottingham		Mark Nottingham
	Fastly		Fastly
	Email: mnot@mnot.net		Email: mnot@mnot.net
	URI: https://www.mnot.net/		URI: https://www.mnot.net/
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