draft-reschke-http-jfv-05.txt		draft-reschke-http-jfv-06.txt
	Network Working Group J. Reschke		Network Working Group J. Reschke
	Internet-Draft greenbytes		Internet-Draft greenbytes
	Intended status: Standards Track December 16, 2016		Intended status: Standards Track June 25, 2017
	Expires: June 19, 2017		Expires: December 27, 2017
	A JSON Encoding for HTTP Header Field Values		A JSON Encoding for HTTP Header Field Values
	draft-reschke-http-jfv-05		draft-reschke-http-jfv-06
	Abstract		Abstract
	This document establishes a convention for use of JSON-encoded field		This document establishes a convention for use of JSON-encoded field
	values in HTTP header fields.		values in HTTP header fields.
	Editorial Note (To be removed by RFC Editor before publication)		Editorial Note (To be removed by RFC Editor before publication)
	Distribution of this document is unlimited. Although this is not a		Distribution of this document is unlimited. Although this is not a
	work item of the HTTPbis Working Group, comments should be sent to		work item of the HTTPbis Working Group, comments should be sent to
	the Hypertext Transfer Protocol (HTTP) mailing list at		the Hypertext Transfer Protocol (HTTP) mailing list at ietf-http-
	ietf-http-wg@w3.org [1], which may be joined by sending a message		wg@w3.org [1], which may be joined by sending a message with subject
	with subject "subscribe" to ietf-http-wg-request@w3.org [2].		"subscribe" to ietf-http-wg-request@w3.org [2].
	Discussions of the HTTPbis Working Group are archived at		Discussions of the HTTPbis Working Group are archived at
	<http://lists.w3.org/Archives/Public/ietf-http-wg/>.		<http://lists.w3.org/Archives/Public/ietf-http-wg/>.
	XML versions and latest edits for this document are available from		XML versions and latest edits for this document are available from
	<http://greenbytes.de/tech/webdav/#draft-reschke-http-jfv>.		<http://greenbytes.de/tech/webdav/#draft-reschke-http-jfv>.
	The changes in this draft are summarized in Appendix C.5.		The changes in this draft are summarized in Appendix E.9.
	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 http://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 June 19, 2017.		This Internet-Draft will expire on December 27, 2017.
	Copyright Notice		Copyright Notice
	Copyright (c) 2016 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.
	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
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	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 . . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	2. Data Model and Format . . . . . . . . . . . . . . . . . . . . 3		2. Data Model and Format . . . . . . . . . . . . . . . . . . . . 4
	3. Sender Requirements . . . . . . . . . . . . . . . . . . . . . 4		3. Sender Requirements . . . . . . . . . . . . . . . . . . . . . 5
	4. Recipient Requirements . . . . . . . . . . . . . . . . . . . . 5		4. Recipient Requirements . . . . . . . . . . . . . . . . . . . 5
	5. Using this Format in Header Field Definitions . . . . . . . . 5		5. Using this Format in Header Field Definitions . . . . . . . . 5
	6. Deployment Considerations . . . . . . . . . . . . . . . . . . 5		6. Deployment Considerations . . . . . . . . . . . . . . . . . . 6
	7. Interoperability Considerations . . . . . . . . . . . . . . . 5		7. Interoperability Considerations . . . . . . . . . . . . . . . 6
	7.1. Encoding and Characters . . . . . . . . . . . . . . . . . 5		7.1. Encoding and Characters . . . . . . . . . . . . . . . . . 6
	7.2. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 6		7.2. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 6
	7.3. Object Constraints . . . . . . . . . . . . . . . . . . . . 6		7.3. Object Constraints . . . . . . . . . . . . . . . . . . . 7
	8. Internationalization Considerations . . . . . . . . . . . . . 6		8. Internationalization Considerations . . . . . . . . . . . . . 7
	9. Security Considerations . . . . . . . . . . . . . . . . . . . 6		9. Security Considerations . . . . . . . . . . . . . . . . . . . 7
	10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7		10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
	10.1. Normative References . . . . . . . . . . . . . . . . . . . 7		10.1. Normative References . . . . . . . . . . . . . . . . . . 7
	10.2. Informative References . . . . . . . . . . . . . . . . . . 7		10.2. Informative References . . . . . . . . . . . . . . . . . 8
	Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 8		Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 10
	A.1. Content-Length . . . . . . . . . . . . . . . . . . . . . . 8		A.1. Content-Length . . . . . . . . . . . . . . . . . . . . . 10
	A.2. Content-Disposition . . . . . . . . . . . . . . . . . . . 9		A.2. Content-Disposition . . . . . . . . . . . . . . . . . . . 10
	A.3. WWW-Authenticate . . . . . . . . . . . . . . . . . . . . . 10		A.3. WWW-Authenticate . . . . . . . . . . . . . . . . . . . . 11
	A.4. Accept-Encoding . . . . . . . . . . . . . . . . . . . . . 11		A.4. Accept-Encoding . . . . . . . . . . . . . . . . . . . . . 12
	Appendix B. Discussion . . . . . . . . . . . . . . . . . . . . . 12		Appendix B. Use of JSON Field Value Encoding in the Wild . . . . 13
	Appendix C. Change Log (to be removed by RFC Editor before		B.1. W3C Reporting API Specification . . . . . . . . . . . . . 14
	publication) . . . . . . . . . . . . . . . . . . . . 12		B.2. W3C Clear Site Data Specification . . . . . . . . . . . . 14
	C.1. Since draft-reschke-http-jfv-00 . . . . . . . . . . . . . 12		B.3. W3C Feature Policy Specification . . . . . . . . . . . . 14
	C.2. Since draft-reschke-http-jfv-01 . . . . . . . . . . . . . 12		Appendix C. Relation to HTTP 'Key' Header Field . . . . . . . . 14
	C.3. Since draft-reschke-http-jfv-02 . . . . . . . . . . . . . 13		Appendix D. Discussion . . . . . . . . . . . . . . . . . . . . . 14
	C.4. Since draft-reschke-http-jfv-03 . . . . . . . . . . . . . 13		Appendix E. Change Log (to be removed by RFC Editor before
	C.5. Since draft-reschke-http-jfv-04 . . . . . . . . . . . . . 13		publication) . . . . . . . . . . . . . . . . . . . . 14
	C.5.1. Since draft-ietf-httpbis-jfv-00 . . . . . . . . . . . 13		E.1. Since draft-reschke-http-jfv-00 . . . . . . . . . . . . . 15
	C.5.2. Since draft-ietf-httpbis-jfv-01 . . . . . . . . . . . 13		E.2. Since draft-reschke-http-jfv-01 . . . . . . . . . . . . . 15
	C.5.3. Since draft-ietf-httpbis-jfv-02 . . . . . . . . . . . 13		E.3. Since draft-reschke-http-jfv-02 . . . . . . . . . . . . . 15
	Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . . 13		E.4. Since draft-reschke-http-jfv-03 . . . . . . . . . . . . . 15
			E.5. Since draft-reschke-http-jfv-04 . . . . . . . . . . . . . 15
			E.6. Since draft-ietf-httpbis-jfv-00 . . . . . . . . . . . . . 15
			E.7. Since draft-ietf-httpbis-jfv-01 . . . . . . . . . . . . . 15
			E.8. Since draft-ietf-httpbis-jfv-02 . . . . . . . . . . . . . 15
			E.9. Since draft-reschke-http-jfv-05 . . . . . . . . . . . . . 16
			Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 16
			Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16
	1. Introduction		1. Introduction
	Defining syntax for new HTTP header fields ([RFC7230], Section 3.2)		Defining syntax for new HTTP header fields ([RFC7230], Section 3.2)
	is non-trivial. Among the commonly encountered problems are:		is non-trivial. Among the commonly encountered problems are:
	o There is no common syntax for complex field values. Several well-		o There is no common syntax for complex field values. Several well-
	known header fields do use a similarly looking syntax, but it is		known header fields do use a similarly looking syntax, but it is
	hard to write generic parsing code that will both correctly handle		hard to write generic parsing code that will both correctly handle
	valid field values but also reject invalid ones.		valid field values but also reject invalid ones.
	skipping to change at page 3, line 43 ¶		skipping to change at page 4, line 5 ¶
	format that can be used in definitions of new header fields, where		format that can be used in definitions of new header fields, where
	the goals were:		the goals were:
	o to be compatible with header field recombination when fields occur		o to be compatible with header field recombination when fields occur
	multiple times in a single message (Section 3.2.2 of [RFC7230]),		multiple times in a single message (Section 3.2.2 of [RFC7230]),
	and		and
	o not to use any problematic characters in the field value (non-		o not to use any problematic characters in the field value (non-
	ASCII characters and certain whitespace characters).		ASCII characters and certain whitespace characters).
			Note: [HSTRUCT], a work item of the IETF HTTP Working Group, is a
			different attempt to address this set of problems -- it tries to
			identify and formalize common field structures in existing header
			fields; the syntax defined over there would usually lead to a more
			compact notation.
	2. Data Model and Format		2. Data Model and Format
	In HTTP, header fields with the same field name can occur multiple		In HTTP, header fields with the same field name can occur multiple
	times within a single message (Section 3.2.2 of [RFC7230]). When		times within a single message (Section 3.2.2 of [RFC7230]). When
	this happens, recipients are allowed to combine the field values		this happens, recipients are allowed to combine the field values
	using commas as delimiter. This rule matches nicely JSON's array		using commas as delimiter. This rule matches nicely JSON's array
	format (Section 5 of [RFC7159]). Thus, the basic data model used		format (Section 5 of [RFC7159]). Thus, the basic data model used
	here is the JSON array.		here is the JSON array.
	Header field definitions that need only a single value can restrict		Header field definitions that need only a single value can restrict
	skipping to change at page 5, line 31 ¶		skipping to change at page 5, line 47 ¶
	("]") octet, then		("]") octet, then
	3. run the resulting octet sequence through a JSON parser.		3. run the resulting octet sequence through a JSON parser.
	The result of the parsing operation is either an error (in which case		The result of the parsing operation is either an error (in which case
	the header field values needs to be considered invalid), or a JSON		the header field values needs to be considered invalid), or a JSON
	array.		array.
	5. Using this Format in Header Field Definitions		5. Using this Format in Header Field Definitions
	[[anchor5: Explain what a definition of a new header field needs to		Specifications defining new HTTP header fields need to take the
	do precisely to use this format, mention must-ignore extensibility]]		considerations listed in Section 8.3.1 of [RFC7231] into account.
			Many of these will already be accounted for by using the format
			defined in this specification.
			Readers of HTTP-related specifications frequently expect an ABNF
			definition of the field value syntax. This is not really needed
			here, as the actual syntax is JSON text, as defined in Section 2 of
			[RFC7159].
			A very simple way to use this JSON encoding thus is just to cite this
			specification -- specifically the "json-field-value" ABNF production
			defined in Section 2 -- and otherwise not to talk about the details
			of the field syntax at all.
			An alternative approach is just to repeat the ABNF-related parts from
			Section 2.
			This frees the specification from defining the concrete on-the-wire
			syntax. What's left is defining the field value in terms of a JSON
			array. An important aspect is the question of extensibility, e.g.
			how recipients ought to treat unknown field names. In general, a
			"must ignore" approach will allow protocols to evolve without
			versioning or even using entire new field names.
	6. Deployment Considerations		6. Deployment Considerations
	This JSON-based syntax will only apply to newly introduced header		This JSON-based syntax will only apply to newly introduced header
	fields, thus backwards compatibility is not a problem. That being		fields, thus backwards compatibility is not a problem. That being
	said, it is conceivable that there is existing code that might trip		said, it is conceivable that there is existing code that might trip
	over double quotes not being used for HTTP's quoted-string syntax		over double quotes not being used for HTTP's quoted-string syntax
	(Section 3.2.6 of [RFC7230]).		(Section 3.2.6 of [RFC7230]).
	7. Interoperability Considerations		7. Interoperability Considerations
	skipping to change at page 6, line 27 ¶		skipping to change at page 7, line 22 ¶
	with [ECMA-262], Section 24.3.1.1).		with [ECMA-262], Section 24.3.1.1).
	Furthermore, ordering of object members is not significant and can		Furthermore, ordering of object members is not significant and can
	not be relied upon.		not be relied upon.
	8. Internationalization Considerations		8. Internationalization Considerations
	In HTTP/1.1, header field values are represented by octet sequences,		In HTTP/1.1, header field values are represented by octet sequences,
	usually used to transmit ASCII characters, with restrictions on the		usually used to transmit ASCII characters, with restrictions on the
	use of certain control characters, and no associated default		use of certain control characters, and no associated default
	character encoding, nor a way to describe it ([RFC7230], Section		character encoding, nor a way to describe it ([RFC7230],
	3.2). HTTP/2 does not change this.		Section 3.2). HTTP/2 does not change this.
	This specification maps all characters which can cause problems to		This specification maps all characters which can cause problems to
	JSON escape sequences, thereby solving the HTTP header field		JSON escape sequences, thereby solving the HTTP header field
	internationalization problem.		internationalization problem.
	Future specifications of HTTP might change to allow non-ASCII		Future specifications of HTTP might change to allow non-ASCII
	characters natively. In that case, header fields using the syntax		characters natively. In that case, header fields using the syntax
	defined by this specification would have a simple migration path (by		defined by this specification would have a simple migration path (by
	just stopping to require escaping of non-ASCII characters).		just stopping to require escaping of non-ASCII characters).
	skipping to change at page 7, line 4 ¶		skipping to change at page 7, line 46 ¶
	Using JSON-shaped field values is believed to not introduce any new		Using JSON-shaped field values is believed to not introduce any new
	threads beyond those described in Section 12 of [RFC7159], namely the		threads beyond those described in Section 12 of [RFC7159], namely the
	risk of recipients using the wrong tools to parse them.		risk of recipients using the wrong tools to parse them.
	Other than that, any syntax that makes extensions easy can be used to		Other than that, any syntax that makes extensions easy can be used to
	smuggle information through field values; however, this concern is		smuggle information through field values; however, this concern is
	shared with other widely used formats, such as those using parameters		shared with other widely used formats, such as those using parameters
	in the form of name/value pairs.		in the form of name/value pairs.
	10. References		10. References
	10.1. Normative References		10.1. Normative References
	[RFC0020] Cerf, V., "ASCII format for network interchange",		[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
	STD 80, RFC 20, DOI 10.17487/RFC0020, October 1969,		RFC 20, DOI 10.17487/RFC0020, October 1969,
	<http://www.rfc-editor.org/info/rfc20>.		<http://www.rfc-editor.org/info/rfc20>.
	[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for		[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
	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,
	<http://www.rfc-editor.org/info/rfc5234>.		<http://www.rfc-editor.org/info/rfc5234>.
	[RFC7159] Bray, T., "The JavaScript Object Notation (JSON)		[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data
	Data Interchange Format", RFC 7159, DOI 10.17487/		Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
	RFC7159, March 2014,		2014, <http://www.rfc-editor.org/info/rfc7159>.
	<http://www.rfc-editor.org/info/rfc7159>.
	[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext		[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
	Transfer Protocol (HTTP/1.1): Message Syntax and		Protocol (HTTP/1.1): Message Syntax and Routing",
	Routing", RFC 7230, DOI 10.17487/RFC7230,		RFC 7230, DOI 10.17487/RFC7230, June 2014,
	June 2014,		<http://www.rfc-editor.org/info/rfc7230>.
	<http://www.rfc-editor.org/info/rfc7230>.
	[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext		[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
	Transfer Protocol (HTTP/1.1): Semantics and		Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
	Content", RFC 7231, DOI 10.17487/RFC7231,		DOI 10.17487/RFC7231, June 2014,
	June 2014,		<http://www.rfc-editor.org/info/rfc7231>.
	<http://www.rfc-editor.org/info/rfc7231>.
	[RFC7493] Bray, T., Ed., "The I-JSON Message Format",		[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
	RFC 7493, DOI 10.17487/RFC7493, March 2015,		DOI 10.17487/RFC7493, March 2015,
	<http://www.rfc-editor.org/info/rfc7493>.		<http://www.rfc-editor.org/info/rfc7493>.
	10.2. Informative References		10.2. Informative References
	[ECMA-262] Ecma International, "ECMA-262 6th Edition, The		[CLEARSITE]
	ECMAScript 2015 Language Specification",		West, M., "Clear Site Data", W3C Working Draft WD-clear-
	Standard ECMA-262, June 2015,		site-data-20160720, July 2016,
	<http://www.ecma-international.org/ecma-262/6.0/>.		<http://www.w3.org/TR/2016/WD-clear-site-data-20160720/>.
	[ISO-8859-1] International Organization for Standardization,		Latest version available at <http://www.w3.org/TR/clear-
	"Information technology -- 8-bit single-byte coded		site-data/>.
	graphic character sets -- Part 1: Latin alphabet
	No. 1", ISO/IEC 8859-1:1998, 1998.
	[KEY] Fielding, R. and M. Nottingham, "The Key HTTP		[ECMA-262]
	Response Header Field", draft-ietf-httpbis-key-01		Ecma International, "ECMA-262 6th Edition, The ECMAScript
	(work in progress), March 2016.		2015 Language Specification", Standard ECMA-262, June
			2015, <http://www.ecma-international.org/ecma-262/6.0/>.
	[RFC5987] Reschke, J., "Character Set and Language Encoding		[FEATUREPOL]
	for Hypertext Transfer Protocol (HTTP) Header Field		Clelland, I., "Clear Site Data", W3C Draft Community Group
	Parameters", RFC 5987, DOI 10.17487/RFC5987,		Report , June 2017, <https://wicg.github.io/feature-
	August 2010,		policy/>.
	<http://www.rfc-editor.org/info/rfc5987>.
	[RFC6266] Reschke, J., "Use of the Content-Disposition Header		[HSTRUCT] Kamp, P-H., "HTTP Header Common Structure", draft-ietf-
	Field in the Hypertext Transfer Protocol (HTTP)",		httpbis-header-structure-01 (work in progress), April
	RFC 6266, DOI 10.17487/RFC6266, June 2011,		2017.
	<http://www.rfc-editor.org/info/rfc6266>.
	[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in		[ISO-8859-1]
	Internationalization in the IETF", BCP 166,		International Organization for Standardization,
	RFC 6365, DOI 10.17487/RFC6365, September 2011,		"Information technology -- 8-bit single-byte coded graphic
	<http://www.rfc-editor.org/info/rfc6365>.		character sets -- Part 1: Latin alphabet No. 1", ISO/
			IEC 8859-1:1998, 1998.
	[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext		[KEY] Fielding, R. and M. Nottingham, "The Key HTTP Response
	Transfer Protocol (HTTP/1.1): Authentication",		Header Field", draft-ietf-httpbis-key-01 (work in
	RFC 7235, DOI 10.17487/RFC7235, June 2014,		progress), March 2016.
	<http://www.rfc-editor.org/info/rfc7235>.
	[XMLHttpRequest] WhatWG, "XMLHttpRequest",		[REPORTING]
	<https://xhr.spec.whatwg.org/>.		Grigorik, I. and M. West, "Reporting API 1", W3C Group
			Note NOTE-reporting-1-20160607, June 2016,
			<http://www.w3.org/TR/2016/NOTE-reporting-1-20160607/>.
	URIs		Latest version available at <http://www.w3.org/TR/
			reporting-1/>.
	[1] <mailto:ietf-http-wg@w3.org>		[RFC5987] Reschke, J., "Character Set and Language Encoding for
			Hypertext Transfer Protocol (HTTP) Header Field
			Parameters", RFC 5987, DOI 10.17487/RFC5987, August 2010,
			<http://www.rfc-editor.org/info/rfc5987>.
	[2] <mailto:ietf-http-wg-request@w3.org?subject=subscribe>		[RFC6266] Reschke, J., "Use of the Content-Disposition Header Field
			in the Hypertext Transfer Protocol (HTTP)", RFC 6266,
			DOI 10.17487/RFC6266, June 2011,
			<http://www.rfc-editor.org/info/rfc6266>.
			[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
			Internationalization in the IETF", BCP 166, RFC 6365,
			DOI 10.17487/RFC6365, September 2011,
			<http://www.rfc-editor.org/info/rfc6365>.
			[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
			Protocol (HTTP/1.1): Authentication", RFC 7235,
			DOI 10.17487/RFC7235, June 2014,
			<http://www.rfc-editor.org/info/rfc7235>.
			[XMLHttpRequest]
			WhatWG, "XMLHttpRequest", <https://xhr.spec.whatwg.org/>.
	Appendix A. Examples		Appendix A. Examples
	This section shows how some of the existing HTTP header fields would		This section shows how some of the existing HTTP header fields would
	look like if they would use the format defined by this specification.		look like if they would use the format defined by this specification.
	A.1. Content-Length		A.1. Content-Length
	"Content-Length" is defined in Section 3.3.2 of [RFC7230], with the		"Content-Length" is defined in Section 3.3.2 of [RFC7230], with the
	field value's ABNF being:		field value's ABNF being:
	Content-Length = 1*DIGIT		Content-Length = 1*DIGIT
	So the field value is similar to a JSON number ([RFC7159], Section		So the field value is similar to a JSON number ([RFC7159],
	6).		Section 6).
	Content-Length is restricted to a single field instance, as it		Content-Length is restricted to a single field instance, as it
	doesn't use the list production (as per Section 3.2.2 of [RFC7230]).		doesn't use the list production (as per Section 3.2.2 of [RFC7230]).
	However, in practice multiple instances do occur, and the definition		However, in practice multiple instances do occur, and the definition
	of the header field does indeed discuss how to handle these cases.		of the header field does indeed discuss how to handle these cases.
	If Content-Length was defined using the JSON format discussed here,		If Content-Length was defined using the JSON format discussed here,
	the ABNF would be something like:		the ABNF would be something like:
	Content-Length = #number		Content-Length = #number
	skipping to change at page 11, line 26 ¶		skipping to change at page 12, line 43 ¶
	codings = content-coding / "identity" / "*"		codings = content-coding / "identity" / "*"
	weight = OWS ";" OWS "q=" qvalue		weight = OWS ";" OWS "q=" qvalue
	qvalue = ( "0" [ "." 0*3DIGIT ] )		qvalue = ( "0" [ "." 0*3DIGIT ] )
	/ ( "1" [ "." 0*3("0") ] )		/ ( "1" [ "." 0*3("0") ] )
	An example for a complex header field value given in the definition		An example for a complex header field value given in the definition
	of the header field is:		of the header field is:
	gzip;q=1.0, identity; q=0.5, *;q=0		gzip;q=1.0, identity; q=0.5, *;q=0
	Due to the defaulting rules for the quality value ([RFC7231], Section		Due to the defaulting rules for the quality value ([RFC7231],
	5.3.1), this could also be written as:		Section 5.3.1), this could also be written as:
	gzip, identity; q=0.5, *; q=0		gzip, identity; q=0.5, *; q=0
	A JSON representation could be:		A JSON representation could be:
	[		[
	{		{
	"gzip" : {		"gzip" : {
	}		}
	},		},
	skipping to change at page 12, line 23 ¶		skipping to change at page 13, line 44 ¶
	would be mapped to a member whose name is given by the string		would be mapped to a member whose name is given by the string
	literal, and the value is an empty object.		literal, and the value is an empty object.
	For what it's worth, one of the most common cases for 'Accept-		For what it's worth, one of the most common cases for 'Accept-
	Encoding' would become:		Encoding' would become:
	"gzip", "deflate"		"gzip", "deflate"
	which would be only a small overhead over the original format.		which would be only a small overhead over the original format.
	Appendix B. Discussion		Appendix B. Use of JSON Field Value Encoding in the Wild
			Since work started on this document, various specifications have
			adopted this format. At least one of these moved away after the HTTP
			Working Group decided to focus on [HSTRUCT] (see thread starting at
			<https://lists.w3.org/Archives/Public/ietf-http-
			wg/2016OctDec/0505.html>).
			The sections below summarize the current usage of this format.
			B.1. W3C Reporting API Specification
			Defined in W3C Note "Reporting API 1" (Section 3.1 of [REPORTING]).
			Still in use in latest editor copy as of June 2017.
			B.2. W3C Clear Site Data Specification
			Defined in W3C Working Draft "Clear Site Data" (Section 2.1 of
			[CLEARSITE]). Latest Editor's Draft replaces the use of JSON with a
			custom syntax (see <https://lists.w3.org/Archives/Public/ietf-http-
			wg/2017AprJun/0214.html> for feedback).
			B.3. W3C Feature Policy Specification
			Defined in W3C Draft Community Group Report "Feature Policy"
			(Section 6.1 of [FEATUREPOL]). Previously relied on this document,
			now replicates the syntax into a custom ABNF defined in a separate
			section (Section 5.1 of [FEATUREPOL]).
			Appendix C. Relation to HTTP 'Key' Header Field
			[KEY] aims to improve the cacheability of responses that vary based
			on certain request header fields, addressing lack of granularity in
			the existing "Vary" response header field ([RFC7231], Section 7.1.4).
			If the JSON-based format described by this document gains popularity,
			it might be useful to add a JSON-aware "Key Parameter" (see
			Section 2.3 of [KEY]).
			Appendix D. Discussion
	This approach uses a default of "JSON array", using implicit array		This approach uses a default of "JSON array", using implicit array
	markers. An alternative would be a default of "JSON object". This		markers. An alternative would be a default of "JSON object". This
	would simplify the syntax for non-list-typed header fields, but all		would simplify the syntax for non-list-typed header fields, but all
	the benefits of having the same data model for both types of header		the benefits of having the same data model for both types of header
	fields would be gone. A hybrid approach might make sense, as long as		fields would be gone. A hybrid approach might make sense, as long as
	it doesn't require any heuristics on the recipient's side.		it doesn't require any heuristics on the recipient's side.
	Note: a concrete proposal was made by Kazuho Oku in <https://		Note: a concrete proposal was made by Kazuho Oku in
	lists.w3.org/Archives/Public/ietf-http-wg/2016JanMar/0155.html>.		<https://lists.w3.org/Archives/Public/ietf-http-
			wg/2016JanMar/0155.html>.
	[[anchor15: Use of generic libs vs compactness of field values..]]
	[[anchor16: Mention potential "Key" header field extension ([KEY]).]]
	Appendix C. Change Log (to be removed by RFC Editor before publication)		[[CREF1: Use of generic libs vs compactness of field values..]]
	C.1. Since draft-reschke-http-jfv-00		Appendix E. Change Log (to be removed by RFC Editor before publication)
			E.1. Since draft-reschke-http-jfv-00
	Editorial fixes + working on the TODOs.		Editorial fixes + working on the TODOs.
	C.2. Since draft-reschke-http-jfv-01		E.2. Since draft-reschke-http-jfv-01
	Mention slightly increased risk of smuggling information in header		Mention slightly increased risk of smuggling information in header
	field values.		field values.
	C.3. Since draft-reschke-http-jfv-02		E.3. Since draft-reschke-http-jfv-02
	Mention Kazuho Oku's proposal for abbreviated forms.		Mention Kazuho Oku's proposal for abbreviated forms.
	Added a bit of text about the motivation for a concrete JSON subset		Added a bit of text about the motivation for a concrete JSON subset
	(ack Cory Benfield).		(ack Cory Benfield).
	Expand I18N section.		Expand I18N section.
	C.4. Since draft-reschke-http-jfv-03		E.4. Since draft-reschke-http-jfv-03
	Mention relation to KEY header field.		Mention relation to KEY header field.
	C.5. Since draft-reschke-http-jfv-04		E.5. Since draft-reschke-http-jfv-04
	Between June and December 2016, this was a work item of the HTTP		Between June and December 2016, this was a work item of the HTTP
	working group (see		working group (see <https://datatracker.ietf.org/doc/draft-ietf-
	<https://datatracker.ietf.org/doc/draft-ietf-httpbis-jfv/>). Work		httpbis-jfv/>). Work (if any) continues now on
	(if any) continues now on
	<https://datatracker.ietf.org/doc/draft-reschke-http-jfv/>.		<https://datatracker.ietf.org/doc/draft-reschke-http-jfv/>.
	Changes made while this was a work item of the HTTP Working Group:		Changes made while this was a work item of the HTTP Working Group:
	C.5.1. Since draft-ietf-httpbis-jfv-00		E.6. Since draft-ietf-httpbis-jfv-00
	Added example for "Accept-Encoding" (inspired by Kazuho's feedback),		Added example for "Accept-Encoding" (inspired by Kazuho's feedback),
	showing a potential way to optimize the format when default values		showing a potential way to optimize the format when default values
	apply.		apply.
	C.5.2. Since draft-ietf-httpbis-jfv-01		E.7. Since draft-ietf-httpbis-jfv-01
	Add interop discussion, building on I-JSON and ECMA-262 (see		Add interop discussion, building on I-JSON and ECMA-262 (see
	<https://github.com/httpwg/http-extensions/issues/225>).		<https://github.com/httpwg/http-extensions/issues/225>).
	C.5.3. Since draft-ietf-httpbis-jfv-02		E.8. Since draft-ietf-httpbis-jfv-02
	Move non-essential parts into appendix.		Move non-essential parts into appendix.
	Updated XHR reference.		Updated XHR reference.
	Appendix D. Acknowledgements		E.9. Since draft-reschke-http-jfv-05
			Add meat to "Using this Format in Header Field Definitions".
			Add a few lines on the relation to "Key".
			Summarize current use of the format.
			Acknowledgements
	Thanks go to the Hypertext Transfer Protocol Working Group		Thanks go to the Hypertext Transfer Protocol Working Group
	participants.		participants.
	Author's Address		Author's Address
	Julian F. Reschke		Julian F. Reschke
	greenbytes GmbH		greenbytes GmbH
	Hafenweg 16		Hafenweg 16
	Muenster, NW 48155		Muenster, NW 48155
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