< draft-nottingham-link-hint-00.txt		draft-nottingham-link-hint-01.txt >
	Network Working Group M. Nottingham		Network Working Group M. Nottingham
	Internet-Draft June 10, 2013		Internet-Draft March 27, 2018
	Intended status: Informational		Intended status: Informational
	Expires: December 12, 2013		Expires: September 28, 2018
	HTTP Link Hints		HTTP Link Hints
	draft-nottingham-link-hint-00		draft-nottingham-link-hint-01
	Abstract		Abstract
	This memo specifies "HTTP Link Hints", a mechanism for annotating Web		This memo specifies "HTTP Link Hints", a mechanism for annotating Web
	links to HTTP(S) resources with information that otherwise might be		links to HTTP(S) resources with information that otherwise might be
	discovered by interacting with them.		discovered by interacting with them.
	Note to Readers		Note to Readers
	This draft should be discussed on the apps-discuss mailing list; see		_RFC EDITOR: please remove this section before publication_
	[apps-discuss].
	Status of this Memo		The issues list for this draft can be found at
			https://github.com/mnot/I-D/labels/link-hint [1].
			The most recent (often, unpublished) draft is at
			https://mnot.github.io/I-D/link-hint/ [2].
			Recent changes are listed at https://github.com/mnot/I-D/commits/gh-
			pages/link-hint [3].
			See also the draft's current status in the IETF datatracker, at
			https://datatracker.ietf.org/doc/draft-nottingham-link-hint/ [4].
			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 December 12, 2013.		This Internet-Draft will expire on September 28, 2018.
	Copyright Notice		Copyright Notice
	Copyright (c) 2013 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		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 3		1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
	2. HTTP Link Hints . . . . . . . . . . . . . . . . . . . . . . . 4		2. HTTP Link Hints . . . . . . . . . . . . . . . . . . . . . . . 4
	3. Pre-Defined HTTP Link Hints . . . . . . . . . . . . . . . . . 4		3. Pre-Defined HTTP Link Hints . . . . . . . . . . . . . . . . . 4
	3.1. allow . . . . . . . . . . . . . . . . . . . . . . . . . . 4		3.1. allow . . . . . . . . . . . . . . . . . . . . . . . . . . 5
	3.2. formats . . . . . . . . . . . . . . . . . . . . . . . . . 5		3.2. formats . . . . . . . . . . . . . . . . . . . . . . . . . 5
	3.3. links . . . . . . . . . . . . . . . . . . . . . . . . . . 5		3.3. links . . . . . . . . . . . . . . . . . . . . . . . . . . 5
	3.4. accept-post . . . . . . . . . . . . . . . . . . . . . . . 6		3.4. accept-post . . . . . . . . . . . . . . . . . . . . . . . 6
	3.5. accept-patch . . . . . . . . . . . . . . . . . . . . . . . 6		3.5. accept-patch . . . . . . . . . . . . . . . . . . . . . . 7
	3.6. accept-ranges . . . . . . . . . . . . . . . . . . . . . . 7		3.6. accept-ranges . . . . . . . . . . . . . . . . . . . . . . 7
	3.7. accept-prefer . . . . . . . . . . . . . . . . . . . . . . 7		3.7. accept-prefer . . . . . . . . . . . . . . . . . . . . . . 7
	3.8. precondition-req . . . . . . . . . . . . . . . . . . . . . 7		3.8. precondition-req . . . . . . . . . . . . . . . . . . . . 8
	3.9. auth-schemes . . . . . . . . . . . . . . . . . . . . . . . 7		3.9. auth-schemes . . . . . . . . . . . . . . . . . . . . . . 8
	3.10. status . . . . . . . . . . . . . . . . . . . . . . . . . . 8		3.10. status . . . . . . . . . . . . . . . . . . . . . . . . . 9
	4. Security Considerations . . . . . . . . . . . . . . . . . . . 8		4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
	5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8		5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
	5.1. HTTP Link Hint Registry . . . . . . . . . . . . . . . . . 8		5.1. HTTP Link Hint Registry . . . . . . . . . . . . . . . . . 9
	6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9		6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
	6.1. Normative References . . . . . . . . . . . . . . . . . . . 9		6.1. Normative References . . . . . . . . . . . . . . . . . . 10
	6.2. Informative References . . . . . . . . . . . . . . . . . . 10		6.2. Informative References . . . . . . . . . . . . . . . . . 11
	Appendix A. Representing Link Hints in Link Headers . . . . . . . 11		6.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 11
	Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 12		Appendix A. Representing Link Hints in Link Headers . . . . . . 12
	Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . . 12		Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 13
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12		Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
	1. Introduction		1. Introduction
	Clients can discover a variety of information about a HTTP		HTTP [RFC7230] clients can discover a variety of information about a
	[I-D.ietf-httpbis-p1-messaging] resource by interacting with it. For		resource by interacting with it. For example, the methods supported
	example, the methods supported can be learned through the Allow		can be learned through the Allow response header field, and the need
	response header field, whereas the need for authentication is		for authentication is conveyed with a 401 Authentication Required
	conveyed with a 401 Authentication Required status code.		status code.
	In some situations, it can be beneficial to know this information		In some situations, it can be beneficial to know this information
	before interacting with the resource; not only can it save time		before interacting with the resource; not only can it save time
	(through reduced round trips), but it can also affect the choices		(through reduced round trips), but it can also affect the choices
	given to the code or user driving the interaction.		given to the code or user driving the interaction.
	For example, a user interface that presents the data from an HTTP-		For example, a user interface that presents the data from an HTTP-
	based API might need to know which resources the user has write		based API might need to know which resources the user has write
	access to, so that it can present the appropriate interface.		access to, so that it can present the appropriate interface.
	This specification defines a vocabulary of "HTTP link hints" that		This specification defines a vocabulary of "HTTP link hints" that
	allow such metadata about HTTP resources to be attached to Web links		allow such metadata about HTTP resources to be attached to Web links
	[RFC5988], thereby making it available before the link is followed.		[RFC8288], thereby making it available before the link is followed.
	It also establishes a registry for future hints.		It also establishes a registry for future hints.
	It does not recommend a single serialisation format for link hints;		It does not recommend a single way to express link hints; rather, it
	rather, it is expected that this will be done by individual link		is expected that this will be done by individual link serialisations
	serialisations that use hints (whether they be in a representation		(see [RFC8288], Section 3.4.1). However, Appendix A does recommend
	body, message headers or elsewhere). However, Appendix A does		how to include link hints in the existing Link HTTP header field.
	recommend how to include link hints in the existing Link HTTP header
	field.
	Hints are just that - they are not a "contract", and are to only be		Hints are just that - they are not a "contract", and are to only be
	taken as advisory. The runtime behaviour of the resource always		taken as advisory. The runtime behaviour of the resource always
	overrides hinted information.		overrides hinted information.
	For example, a client might receive a hint that the PUT method is		For example, a client might receive a hint that the PUT method is
	allowed on all "widget" resources. This means that generally, the		allowed on all "widget" resources. This means that generally, the
	client can PUT to them, but a specific resource might reject a PUT		client can PUT to them, but a specific resource might reject a PUT
	based upon access control or other considerations.		based upon access control or other considerations.
	More fine-grained information might be gathered by interacting with		More fine-grained information might also be gathered by interacting
	the resource (e.g., via a GET), or by another resource "containing"		with the resource (e.g., via a GET), or by another resource
	it (such as a "widgets" collection) or describing it (e.g., one		"containing" it (such as a "widgets" collection) or describing it
	linked to it with a "describedby" link relation).		(e.g., one linked to it with a "describedby" link relation).
	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. HTTP Link Hints		2. HTTP Link Hints
	A HTTP link hint is a (key, value) tuple that describes the target		A HTTP link hint is a (key, value) tuple that describes the target
	resource of a Web link [RFC5988], or the link itself. The value's		resource of a Web link [RFC8288], or the link itself. The value's
	canonical form is a JSON [RFC4627] data structure, whose form is		canonical form is a JSON [RFC8259] data structure specific to that
	defined by the hint's definition.		hint.
	Typically, they are serialised in links as target attributes.		Typically, link hints are serialised in links as target attributes
			([RFC8288], Section 3.4.1).
	In JSON-based formats, this can be achieved by simply serialising		In JSON-based formats, this can be achieved by simply serialising
	link hints as an object; for example:		link hints as an object; for example:
	{		{
	"_links": {		"_links": {
	"self": {		"self": {
	"href": "/orders/523",		"href": "/orders/523",
	"hints": {		"hints": {
	"allow": ["GET", "POST"],		"allow": ["GET", "POST"],
	"accept-post": {		"accept-post": {
	"application/example+json":		"application/example+json":
	{}		{}
	}		}
	}		}
	}		}
	}		}
	}		}
	In other link formats, this requires a mapping from the canonical		In other link formats, this requires a mapping from the canonical
	JSON data model. One such mapping for the Link HTTP header is		JSON data model. One such mapping is described in Appendix A for the
	described in Appendix A.		Link HTTP header field.
	The information in a link hint SHOULD NOT be considered valid for		The information in a link hint SHOULD NOT be considered valid for
	longer than the freshness lifetime ([I-D.ietf-httpbis-p6-cache]) of		longer than the freshness lifetime ([RFC7234], Section 4.2) of the
	the representation that the link occurred within, and in some cases,		representation that the link occurred within, and in some cases, it
	it might be valid for a considerably shorter period.		might be valid for a considerably shorter period.
	Likewise, the information in a link hint is specific to the link it		Likewise, the information in a link hint is specific to the link it
	is attached to. This means that if a representation is specific to a		is attached to. This means that if a representation is specific to a
	particular user, the hints on links in that representation are also		particular user, the hints on links in that representation are also
	specific to that user.		specific to that user.
	3. Pre-Defined HTTP Link Hints		3. Pre-Defined HTTP Link Hints
	3.1. allow		3.1. allow
	o Hint Name: allow		o Hint Name: allow
	o Description: Hints the HTTP methods that can be used to interact		o Description: Hints the HTTP methods that can be used to interact
	with the target resource; equivalent to the Allow HTTP response		with the target resource; equivalent to the Allow HTTP response
	header.		header.
	o Content Model: array (of strings)		o Content Model: array (of strings)
	o Specification: [this document]		o Specification: [this document]
	Content MUST be an array of strings, containing HTTP methods.		Content MUST be an array of strings, containing HTTP methods.
	3.2. formats		3.2. formats
	o Hint Name: formats		o Hint Name: formats
	o Description: Hints the representation type(s) that the target		o Description: Hints the representation type(s) that the target
	resource can produce and consume, using the GET and PUT (if		resource can produce and consume, using the GET and PUT (if
	allowed) methods respectively.		allowed) methods respectively.
	o Content Model: object		o Content Model: object
	o Specification: [this document]		o Specification: [this document]
	Content MUST be an object, whose keys are media types, and values are		Content MUST be an object, whose keys are media types, and values are
	objects.		objects.
	The object MAY have a "links" member, whose value is an object		The object MAY have a "links" member, whose value is an object
	representing links (in the sense of [RFC5988]) whose context is any		representing links (in the sense of [RFC8288]) whose context is any
	document that uses that format. Generally, this will be schema or		document that uses that format. Generally, this will be schema or
	profile ([RFC6906]) information. The "links" member has the same		profile ([RFC6906]) information. The "links" member has the same
	format as the "links" hint.		format as the "links" hint.
	Furthermore, the object MAY have a "deprecated" member, whose value		Furthermore, the object MAY have a "deprecated" member, whose value
	is either true or false, indicating whether support for the format		is either true or false, indicating whether support for the format
	might be removed in the near future.		might be removed in the near future.
	All other members of the object are under control of the		All other members of the object are under control of the
	corresponding media type's definition.		corresponding media type's definition.
	skipping to change at page 5, line 41 ¶		skipping to change at page 5, line 49 ¶
	Furthermore, the object MAY have a "deprecated" member, whose value		Furthermore, the object MAY have a "deprecated" member, whose value
	is either true or false, indicating whether support for the format		is either true or false, indicating whether support for the format
	might be removed in the near future.		might be removed in the near future.
	All other members of the object are under control of the		All other members of the object are under control of the
	corresponding media type's definition.		corresponding media type's definition.
	3.3. links		3.3. links
	o Hint Name: links		o Hint Name: links
	o Description: Hints at links whose context is the target resource.		o Description: Hints at links whose context is the target resource.
	o Content Model: object		o Content Model: object
	o Specification: [this document]		o Specification: [this document]
	The "links" hint contains links (in the sense of [RFC5988]) whose		The "links" hint contains links (in the sense of [RFC8288]) whose
	context is the hinted target resource, which are stable for the		context is the hinted target resource, which are stable for the
	lifetime of the hint.		lifetime of the hint.
	Content MUST be an object, whose member names are link relations		Content MUST be an object, whose member names are link relations
	([RFC5988]) and values are objects that MUST have an "href" member		([RFC8288]) and values are objects that MUST have an "href" member
	whose value is a URI-reference ([RFC3986], using the original link as		whose value is a URI-reference ([RFC3986], using the original link as
	the base for resolution) for the link hint's target resource, and MAY		the base for resolution) for the link hint's target resource, and MAY
	itself contain link hints, serialised as the value for a "hints"		itself contain link hints, serialised as the value for a "hints"
	member.		member.
	For example:		For example:
	"links": {		"links": {
	"edit-form": {		"edit-form": {
	"href": "./edit",		"href": "./edit",
	skipping to change at page 7, line 8 ¶		skipping to change at page 7, line 27 ¶
	Content MUST be an array of strings, containing media types. Note		Content MUST be an array of strings, containing media types. Note
	that there is no opportunity to communicate format-specific hints for		that there is no opportunity to communicate format-specific hints for
	PATCH formats.		PATCH formats.
	When this hint is present, "PATCH" SHOULD be listed in the "allow"		When this hint is present, "PATCH" SHOULD be listed in the "allow"
	hint.		hint.
	3.6. accept-ranges		3.6. accept-ranges
	o Hint Name: accept-ranges		o Hint Name: accept-ranges
	o Description: Hints the range-specifier(s) available for the target		o Description: Hints the range-specifier(s) available for the target
	resource; equivalent to the Accept-Ranges HTTP response header		resource; equivalent to the Accept-Ranges HTTP response header
	[I-D.ietf-httpbis-p5-range].		[RFC7233].
	o Content Model: array (of strings)		o Content Model: array (of strings)
	o Specification: [this document]		o Specification: [this document]
	Content MUST be an array of strings, containing HTTP range-		Content MUST be an array of strings, containing HTTP range-
	specifiers.		specifiers.
	3.7. accept-prefer		3.7. accept-prefer
	o Hint Name: accept-prefer		o Hint Name: accept-prefer
	o Description: Hints the preference(s) [I-D.snell-http-prefer] that
	the target resource understands (and might act upon) in requests.		o Description: Hints the preference(s) [RFC7240] that the target
			resource understands (and might act upon) in requests.
	o Content Model: array (of strings)		o Content Model: array (of strings)
	o Specification: [this document]		o Specification: [this document]
	Content MUST be an array of strings, contain preferences. Note that,		Content MUST be an array of strings, contain preferences. Note that,
	by its nature, a preference can be ignored by the server.		by its nature, a preference can be ignored by the server.
	3.8. precondition-req		3.8. precondition-req
	o Hint Name: precondition-req		o Hint Name: precondition-req
	o Description: Hints that the target resource requires state-		o Description: Hints that the target resource requires state-
	changing requests (e.g., PUT, PATCH) to include a precondition, as		changing requests (e.g., PUT, PATCH) to include a precondition, as
	per [I-D.ietf-httpbis-p4-conditional], to avoid conflicts due to		per [RFC7232], to avoid conflicts due to concurrent updates.
	concurrent updates.
	o Content Model: array (of strings)		o Content Model: array (of strings)
	o Specification: [this document]		o Specification: [this document]
	Content MUST be an array of strings, with possible values "etag" and		Content MUST be an array of strings, with possible values "etag" and
	"last-modified" indicating type of precondition expected.		"last-modified" indicating type of precondition expected.
	See also the 428 Precondition Required status code ([RFC6585]).		See also the 428 Precondition Required status code ([RFC6585]).
	3.9. auth-schemes		3.9. auth-schemes
	o Hint Name: auth-schemes		o Hint Name: auth-schemes
	skipping to change at page 7, line 46 ¶		skipping to change at page 8, line 25 ¶
	o Specification: [this document]		o Specification: [this document]
	Content MUST be an array of strings, with possible values "etag" and		Content MUST be an array of strings, with possible values "etag" and
	"last-modified" indicating type of precondition expected.		"last-modified" indicating type of precondition expected.
	See also the 428 Precondition Required status code ([RFC6585]).		See also the 428 Precondition Required status code ([RFC6585]).
	3.9. auth-schemes		3.9. auth-schemes
	o Hint Name: auth-schemes		o Hint Name: auth-schemes
	o Description: Hints that the target resource requires		o Description: Hints that the target resource requires
	authentication using the HTTP Authentication Framework		authentication using the HTTP Authentication Framework [RFC7235].
	[I-D.ietf-httpbis-p7-auth].
	o Content Model: array (of objects)		o Content Model: array (of objects)
	o Specification: [this document]		o Specification: [this document]
	Content MUST be an array of objects, each with a "scheme" member		Content MUST be an array of objects, each with a "scheme" member
	containing a string that corresponds to a HTTP authentication scheme,		containing a string that corresponds to a HTTP authentication scheme,
	and optionally a "realms" member containing an array of zero to many		and optionally a "realms" member containing an array of zero to many
	strings that identify protection spaces that the resource is a member		strings that identify protection spaces that the resource is a member
	of.		of.
	For example:		For example:
	skipping to change at page 9, line 11 ¶		skipping to change at page 9, line 46 ¶
	Link hints are generic; that is, they are potentially applicable to		Link hints are generic; that is, they are potentially applicable to
	any HTTP resource, not specific to one application of HTTP, nor to		any HTTP resource, not specific to one application of HTTP, nor to
	one particular format. Generally, they ought to be information that		one particular format. Generally, they ought to be information that
	would otherwise be discoverable by interacting with the resource.		would otherwise be discoverable by interacting with the resource.
	Hint names MUST be composed of the lowercase letters (a-z), digits		Hint names MUST be composed of the lowercase letters (a-z), digits
	(0-9), underscores ("_") and hyphens ("-"), and MUST begin with a		(0-9), underscores ("_") and hyphens ("-"), and MUST begin with a
	lowercase letter.		lowercase letter.
	Hint content MUST be described in terms of JSON values ([RFC4627],		Hint content MUST be described in terms of JSON values ([RFC8259],
	Section 2.1).		Section 3).
	Hint semantics SHOULD be described in terms of the framework defined		Hint semantics SHOULD be described in terms of the framework defined
	in [RFC5988].		in [RFC8288].
	New hints are registered using the Expert Review process described in		New hints are registered using the Expert Review process described in
	[RFC5226] to enforce the criteria above. Requests for registration		[RFC8126] to enforce the criteria above. Requests for registration
	of new resource hints are to use the following template:		of new resource hints are to use the following template:
	o Hint Name: [hint name]		o Hint Name: [hint name]
	o Description: [a short description of the hint's semantics]		o Description: [a short description of the hint's semantics]
	o Content Model: [valid JSON value types; see RFC627 Section 2.1]		o Content Model: [valid JSON value types; see RFC627 Section 2.1]
	o Specification: [reference to specification document]		o Specification: [reference to specification document]
	Initial registrations are enumerated in Section 3. The "rel", "rev",		Initial registrations are enumerated in Section 3. The "rel", "rev",
	"hreflang", "media", "title", and "type" hint names are reserved, so		"hreflang", "media", "title", and "type" hint names are reserved, so
	as to avoid potential clashes with link serialisations.		as to avoid potential clashes with link serialisations.
	6. References		6. References
	6.1. Normative References		6.1. Normative References
	[I-D.ietf-httpbis-p1-messaging]
	Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
	(HTTP/1.1): Message Syntax and Routing",
	draft-ietf-httpbis-p1-messaging-22 (work in progress),
	February 2013.
	[I-D.ietf-httpbis-p6-cache]
	Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
	Transfer Protocol (HTTP/1.1): Caching",
	draft-ietf-httpbis-p6-cache-22 (work in progress),
	February 2013.
	[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, March 1997.		Requirement Levels", BCP 14, RFC 2119,
			DOI 10.17487/RFC2119, March 1997,
			<https://www.rfc-editor.org/info/rfc2119>.
	[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform		[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
	Resource Identifier (URI): Generic Syntax", STD 66,		Resource Identifier (URI): Generic Syntax", STD 66,
	RFC 3986, January 2005.		RFC 3986, DOI 10.17487/RFC3986, January 2005,
			<https://www.rfc-editor.org/info/rfc3986>.
	[RFC4627] Crockford, D., "The application/json Media Type for		[RFC5789] Dusseault, L. and J. Snell, "PATCH Method for HTTP",
	JavaScript Object Notation (JSON)", RFC 4627, July 2006.		RFC 5789, DOI 10.17487/RFC5789, March 2010,
			<https://www.rfc-editor.org/info/rfc5789>.
	[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an		[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status
	IANA Considerations Section in RFCs", BCP 26, RFC 5226,		Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
	May 2008.		<https://www.rfc-editor.org/info/rfc6585>.
	[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.		[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
			Protocol (HTTP/1.1): Message Syntax and Routing",
			RFC 7230, DOI 10.17487/RFC7230, June 2014,
			<https://www.rfc-editor.org/info/rfc7230>.
	6.2. Informative References		[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
			Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
			DOI 10.17487/RFC7232, June 2014,
			<https://www.rfc-editor.org/info/rfc7232>.
	[I-D.ietf-httpbis-p4-conditional]		[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
	Fielding, R. and J. Reschke, "Hypertext Transfer Protocol		"Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
	(HTTP/1.1): Conditional Requests",		RFC 7233, DOI 10.17487/RFC7233, June 2014,
	draft-ietf-httpbis-p4-conditional-22 (work in progress),		<https://www.rfc-editor.org/info/rfc7233>.
	February 2013.
	[I-D.ietf-httpbis-p5-range]		[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
	Fielding, R., Lafon, Y., and J. Reschke, "Hypertext		Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
	Transfer Protocol (HTTP/1.1): Range Requests",		RFC 7234, DOI 10.17487/RFC7234, June 2014,
	draft-ietf-httpbis-p5-range-22 (work in progress),		<https://www.rfc-editor.org/info/rfc7234>.
	February 2013.
	[I-D.ietf-httpbis-p7-auth]		[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
	Fielding, R. and J. Reschke, "Hypertext Transfer Protocol		Protocol (HTTP/1.1): Authentication", RFC 7235,
	(HTTP/1.1): Authentication", draft-ietf-httpbis-p7-auth-22		DOI 10.17487/RFC7235, June 2014,
	(work in progress), February 2013.		<https://www.rfc-editor.org/info/rfc7235>.
	[I-D.snell-http-prefer]		[RFC7240] Snell, J., "Prefer Header for HTTP", RFC 7240,
	Snell, J., "Prefer Header for HTTP",		DOI 10.17487/RFC7240, June 2014,
	draft-snell-http-prefer-18 (work in progress),		<https://www.rfc-editor.org/info/rfc7240>.
	January 2013.
	[RFC5789] Dusseault, L. and J. Snell, "PATCH Method for HTTP",		[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	RFC 5789, March 2010.		2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
			May 2017, <https://www.rfc-editor.org/info/rfc8174>.
	[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status		[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
	Codes", RFC 6585, April 2012.		Interchange Format", STD 90, RFC 8259,
			DOI 10.17487/RFC8259, December 2017,
			<https://www.rfc-editor.org/info/rfc8259>.
			[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
			DOI 10.17487/RFC8288, October 2017,
			<https://www.rfc-editor.org/info/rfc8288>.
			6.2. Informative References
	[RFC6906] Wilde, E., "The 'profile' Link Relation Type", RFC 6906,		[RFC6906] Wilde, E., "The 'profile' Link Relation Type", RFC 6906,
	March 2013.		DOI 10.17487/RFC6906, March 2013,
			<https://www.rfc-editor.org/info/rfc6906>.
	[apps-discuss]		[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
	IETF, "IETF Apps-Discuss Mailing List", n.d.,		Writing an IANA Considerations Section in RFCs", BCP 26,
	<https://www.ietf.org/mailman/listinfo/apps-discuss>.		RFC 8126, DOI 10.17487/RFC8126, June 2017,
			<https://www.rfc-editor.org/info/rfc8126>.
			6.3. URIs
			[1] https://github.com/mnot/I-D/labels/link-hint
			[2] https://mnot.github.io/I-D/link-hint/
			[3] https://github.com/mnot/I-D/commits/gh-pages/link-hint
			[4] https://datatracker.ietf.org/doc/draft-nottingham-link-hint/
	Appendix A. Representing Link Hints in Link Headers		Appendix A. Representing Link Hints in Link Headers
	A link hint can be represented in a Link header ([RFC5988], Section		A link hint can be represented in a Link header ([RFC8288],
	5) as a link-extension.		Section 3) as a link-extension.
	When doing so, the JSON of the hint's content SHOULD be normalised to		When doing so, the JSON of the hint's content SHOULD be normalised to
	reduce extraneous spaces (%x20), and MUST NOT contain horizontal tabs		reduce extraneous spaces (%x20), and MUST NOT contain horizontal tabs
	(%x09), line feeds (%x0A) or carriage returns (%x0D). When they are		(%x09), line feeds (%x0A) or carriage returns (%x0D). When they are
	part of a string value, these characters MUST be escaped as described		part of a string value, these characters MUST be escaped as described
	in [RFC4627] Section 2.5; otherwise, they MUST be discarded.		in [RFC8259] Section 7; otherwise, they MUST be discarded.
	Furthermore, if the content is an array or an object, the surrounding		Furthermore, if the content is an array or an object, the surrounding
	delimiters MUST be removed before serialisation. In other words, the		delimiters MUST be removed before serialisation. In other words, the
	outermost object or array is represented without the braces ("{}") or		outermost object or array is represented without the braces ("{}") or
	brackets ("[]") respectively, but this does not apply to inner		brackets ("[]") respectively, but this does not apply to inner
	objects or arrays.		objects or arrays.
	For example, the two JSON values below are those of the fictitious		For example, the two JSON values below are those of the fictitious
	"example" and "exmaple1" hints, respectively:		"example" and "example1" hints, respectively:
	"The Example Value"		"The Example Value"
	1.2		1.2
	In a Link header, they would be serialised as:		In a Link header, they would be serialised as:
	Link: </>; rel="sample"; example="The Example Value";		Link: </>; rel="sample"; example="The Example Value";
	example1=1.2		example1=1.2
	A more complex, single value for "example":		A more complex, single value for "example":
	skipping to change at page 12, line 11 ¶		skipping to change at page 13, line 11 ¶
	Link: </>; rel="sample"; example="\"foo\", -1.23, true,		Link: </>; rel="sample"; example="\"foo\", -1.23, true,
	[\"charlie\", \"bennet\"], {"cat": \"thor\"}, false"		[\"charlie\", \"bennet\"], {"cat": \"thor\"}, false"
	Appendix B. Acknowledgements		Appendix B. Acknowledgements
	Thanks to Jan Algermissen, Mike Amundsen, Bill Burke, Graham Klyne,		Thanks to Jan Algermissen, Mike Amundsen, Bill Burke, Graham Klyne,
	Leif Hedstrom, Jeni Tennison, Erik Wilde and Jorge Williams for their		Leif Hedstrom, Jeni Tennison, Erik Wilde and Jorge Williams for their
	suggestions and feedback.		suggestions and feedback.
	Appendix C. Open Issues
	The following is a list of placeholders for open issues.
	o Resource Hints
	* indicate a POST to 201 Created pattern
	* indicate an "action" POST
	* outbound links
	* forms?
	o Representation Hints
	* format profiles
	* schema?
	* deprecation
	Author's Address		Author's Address
	Mark Nottingham		Mark Nottingham
	Email: mnot@mnot.net		Email: mnot@mnot.net
	URI: http://www.mnot.net/		URI: http://www.mnot.net/
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