Diff: rfc9176.original.xml

	rfc9176.original.xml	rfc9176.xml

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	<front>	<front>

	<title>CoRE Resource Directory</title>	<title abbrev="CoRE Resource Directory">Constrained RESTful Environments (CoRE)
	<seriesInfo name="Internet-Draft" value="draft-ietf-core-resource-directory-	Resource
	28"/>	Directory</title>
		<seriesInfo name="RFC" value="9176"/>
	<author initials="C." surname="Amsüss" fullname="Christian Amsüss" role="edi tor">	<author initials="C." surname="Amsüss" fullname="Christian Amsüss" role="edi tor">
	<organization/>	<organization/>
	<address>	<address>

	<postal>
	<street>Hollandstr. 12/4</street>
	<code>1020</code>
	<country>Austria</country>
	</postal>
	<phone>+43-664-9790639</phone>
	<email>christian@amsuess.com</email>	<email>christian@amsuess.com</email>
	</address>	</address>
	</author>	</author>
	<author initials="Z." surname="Shelby" fullname="Zach Shelby">	<author initials="Z." surname="Shelby" fullname="Zach Shelby">

	<organization>ARM</organization>	<organization>Edge Impulse</organization>
	<address>	<address>
	<postal>	<postal>

	<street>150 Rose Orchard</street>	<street>3031 Tisch Way</street>
	<city>San Jose</city>	<city>San Jose</city>

	<code>95134</code>	<code>95128</code>
	<country>USA</country>	<country>United States of America</country>
	</postal>	</postal>

	<phone>+1-408-203-9434</phone>	<email>zach@edgeimpulse.com</email>
	<email>zach.shelby@arm.com</email>
	</address>	</address>
	</author>	</author>
	<author initials="M." surname="Koster" fullname="Michael Koster">	<author initials="M." surname="Koster" fullname="Michael Koster">

	<organization>SmartThings</organization>	<organization>PassiveLogic</organization>
	<address>	<address>
	<postal>	<postal>

	<street>665 Clyde Avenue</street>	<street>524 H Street</street>
	<city>Mountain View</city>	<city>Antioch</city>
	<code>94043</code>	<region>CA</region>
	<country>USA</country>	<code>94509</code>
		<country>United States of America</country>
	</postal>	</postal>
	<phone>+1-707-502-5136</phone>	<phone>+1-707-502-5136</phone>

	<email>Michael.Koster@smartthings.com</email>	<email>michaeljohnkoster@gmail.com</email>
	</address>	</address>
	</author>	</author>
	<author initials="C." surname="Bormann" fullname="Carsten Bormann">	<author initials="C." surname="Bormann" fullname="Carsten Bormann">

	<organization>Universitaet Bremen TZI</organization>	<organization>Universität Bremen TZI</organization>
	<address>	<address>
	<postal>	<postal>
	<street>Postfach 330440</street>	<street>Postfach 330440</street>
	<city>Bremen</city>	<city>Bremen</city>
	<code>D-28359</code>	<code>D-28359</code>
	<country>Germany</country>	<country>Germany</country>
	</postal>	</postal>
	<phone>+49-421-218-63921</phone>	<phone>+49-421-218-63921</phone>
	<email>cabo@tzi.org</email>	<email>cabo@tzi.org</email>
	</address>	</address>
	</author>	</author>
	<author initials="P." surname="van der Stok" fullname="Peter van der Stok">	<author initials="P." surname="van der Stok" fullname="Peter van der Stok">

	<organization abbrev="consultant">consultant</organization>	<organization>vanderstok consultancy</organization>
	<address>	<address>

	<phone>+31-492474673 (Netherlands), +33-966015248 (France)</phone>	<email>stokcons@bbhmail.nl</email>
	<email>consultancy@vanderstok.org</email>
	<uri>www.vanderstok.org</uri>
	</address>	</address>
	</author>	</author>

	<date year="2021" month="March" day="07"/>	<date year="2022" month="April"/>
	<area>Internet</area>	<area>Internet</area>
	<workgroup>CoRE</workgroup>	<workgroup>CoRE</workgroup>

	<keyword>CoRE, Web Linking, Resource Discovery, Resource Directory</keyword>	<keyword>CoRE</keyword>
		<keyword>Web Linking</keyword>
		<keyword>Resource Discovery</keyword>
		<keyword>Resource Directory</keyword>
	<abstract>	<abstract>

	<t>In many IoT applications, direct discovery of resources is not practica	<t>In many Internet of Things (IoT) applications, direct discovery of reso
	l	urces is not
	due to sleeping nodes, or networks where multicast traffic	practical due to sleeping nodes or networks where multicast traffic
	is inefficient. These problems can be solved by employing an entity called	is inefficient. These problems can be solved by employing an entity called
	a Resource Directory (RD), which contains information about resources held on	a Resource Directory (RD), which contains information about resources held on

	other servers, allowing lookups to be performed for those resources. The input t	other servers, allowing lookups to be performed for those resources. The input t
	o an RD is composed of links and the output is composed of links constructed fro	o an RD is composed of links, and the output is composed of links constructed fr
	m the information stored in the RD. This	om the information stored in the RD. This
	document specifies the web interfaces that an RD supports for web servers to dis	document specifies the web interfaces that an RD supports for web servers to dis
	cover the RD and to register, maintain, lookup	cover the RD and to register, maintain, look up,
	and remove information on resources. Furthermore, new target attributes useful	and remove information on resources. Furthermore, new target attributes useful
	in conjunction with an RD are defined.</t>	in conjunction with an RD are defined.</t>
	</abstract>	</abstract>

	<note>
	<name>Note to Readers</name>
	<t>Discussion of this document takes place on the
	CORE Working Group mailing list (core@ietf.org),
	which is archived at <eref target="https://mailarchive.ietf.org/arch/browse/co
	re/">https://mailarchive.ietf.org/arch/browse/core/</eref>.</t>
	<t>Source for this draft and an issue tracker can be found at
	<eref target="https://github.com/core-wg/resource-directory">https://github.co
	m/core-wg/resource-directory</eref>.</t>
	</note>
	</front>	</front>
	<middle>	<middle>
	<section anchor="introduction" numbered="true" toc="default">	<section anchor="introduction" numbered="true" toc="default">
	<name>Introduction</name>	<name>Introduction</name>

	<t>In the work on Constrained RESTful Environments (CoRE), a REST architec	<t>In the work on Constrained RESTful Environments (CoRE), a Representatio
	ture	nal State Transfer (REST) architecture
	suitable for constrained nodes (e.g. with limited RAM and ROM <xref target="RFC7	suitable for constrained nodes (e.g., with limited RAM and ROM <xref target="RFC
	228" format="default"/>)	7228" format="default"/>)
	and networks (e.g. 6LoWPAN <xref target="RFC4944" format="default"/>)	and networks (e.g., IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN)
		<xref target="RFC4944" format="default"/>)
	has been established and is used in	has been established and is used in

	Internet-of-Things (IoT) or	Internet of Things (IoT) or
	machine-to-machine (M2M) applications such as smart energy	machine-to-machine (M2M) applications, such as smart energy
	and building automation.</t>	and building automation.</t>
	<t>The discovery of resources offered by a constrained server is very impo rtant	<t>The discovery of resources offered by a constrained server is very impo rtant
	in machine-to-machine applications where there are no humans in the loop and	in machine-to-machine applications where there are no humans in the loop and
	static interfaces result in fragility. The discovery of resources provided by	static interfaces result in fragility. The discovery of resources provided by

	an HTTP Web Server is typically called Web Linking <xref target="RFC8288" format	an HTTP web server is typically called web linking <xref target="RFC8288" format
	="default"/>. The use of	="default"/>. The use of
	Web Linking for the description and discovery of resources hosted by	web linking for the description and discovery of resources hosted by
	constrained web servers is specified by the CoRE Link Format	constrained web servers is specified by the CoRE Link Format
	<xref target="RFC6690" format="default"/>. However, <xref target="RFC6690" forma t="default"/> only describes how to discover	<xref target="RFC6690" format="default"/>. However, <xref target="RFC6690" forma t="default"/> only describes how to discover
	resources from the web server that hosts them by querying	resources from the web server that hosts them by querying
	<tt>/.well-known/core</tt>. In many constrained scenarios, direct discovery of r esources is	<tt>/.well-known/core</tt>. In many constrained scenarios, direct discovery of r esources is

	not practical due to sleeping nodes, or networks where	not practical due to sleeping nodes or networks where
	multicast traffic is inefficient. These problems can be solved by employing	multicast traffic is inefficient. These problems can be solved by employing
	an entity called a Resource Directory (RD), which contains information about res ources held on	an entity called a Resource Directory (RD), which contains information about res ources held on
	other servers, allowing lookups to be performed for those resources.</t>	other servers, allowing lookups to be performed for those resources.</t>

	<t>This document specifies the web interfaces that an RD supports for web servers to discover the RD and to register, maintain, lookup	<t>This document specifies the web interfaces that an RD supports for web servers to discover the RD and to register, maintain, look up,
	and remove information on resources. Furthermore, new target attributes useful i n	and remove information on resources. Furthermore, new target attributes useful i n
	conjunction with an RD are defined. Although the examples in	conjunction with an RD are defined. Although the examples in

	this document show the use of these interfaces with CoAP <xref target="RFC7252" format="default"/>, they	this document show the use of these interfaces with the Constrained Application Protocol (CoAP) <xref target="RFC7252" format="default"/>, they
	can be applied in an equivalent manner to HTTP <xref target="RFC7230" format="de fault"/>.</t>	can be applied in an equivalent manner to HTTP <xref target="RFC7230" format="de fault"/>.</t>
	</section>	</section>
	<section anchor="terminology" numbered="true" toc="default">	<section anchor="terminology" numbered="true" toc="default">
	<name>Terminology</name>	<name>Terminology</name>

	<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",	<t>The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL"	>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>",
		"<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14
		>", "<bcp14>NOT RECOMMENDED</bcp14>", "<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL
		</bcp14>"
	in this	in this
	document are to be interpreted as described in BCP 14 <xref target="RFC2119" for mat="default"/> <xref target="RFC8174" format="default"/>	document are to be interpreted as described in BCP 14 <xref target="RFC2119" for mat="default"/> <xref target="RFC8174" format="default"/>
	when, and only when, they appear in all capitals, as shown here.</t>	when, and only when, they appear in all capitals, as shown here.</t>
	<t>The term "byte" is used in its now customary sense as a synonym for "oc tet".</t>	<t>The term "byte" is used in its now customary sense as a synonym for "oc tet".</t>
	<t>This specification requires readers to be familiar with all the terms a nd	<t>This specification requires readers to be familiar with all the terms a nd

	concepts that are discussed in <xref target="RFC3986" format="default"/>, <xref target="RFC8288" format="default"/> and <xref target="RFC6690" format="default"/ >. Readers should	concepts that are discussed in <xref target="RFC3986" format="default"/>, <xref target="RFC8288" format="default"/>, and <xref target="RFC6690" format="default" />. Readers should
	also be familiar with the terms and concepts discussed in <xref target="RFC7252" format="default"/>. To	also be familiar with the terms and concepts discussed in <xref target="RFC7252" format="default"/>. To
	describe the REST interfaces defined in this specification, the URI Template	describe the REST interfaces defined in this specification, the URI Template
	format is used <xref target="RFC6570" format="default"/>.</t>	format is used <xref target="RFC6570" format="default"/>.</t>
	<t>This specification makes use of the following additional terminology:</ t>	<t>This specification makes use of the following additional terminology:</ t>
	<dl newline="true">	<dl newline="true">

	<dt>	<dt>Resolve Against</dt>
	resolve against </dt>	<dd>The expression "a URI reference is <em>resolved against</em> a base
	<dd>	URI" is used
	<t>The expression "a URI-reference is <em>resolved against</em> a base	to describe the process of <xref target="RFC3986" section="5.2" sectionFo
	URI" is used	rmat="comma"/>.
	to describe the process of <xref target="RFC3986" format="default"/> Section 5.2	Noteworthy corner cases include the following: (1) if the URI refer
	. Noteworthy corner cases are	ence is a (full)
	that if the URI-reference is a (full) URI and resolved against any base URI, th	URI, resolving against any base URI gives the original
	at gives the original full URI, and	full URI and (2) resolving an empty URI reference gives the base
	that resolving an empty URI reference gives the base URI without any fragment id	URI without any fragment identifier.</dd>
	entifier.</t>	<dt>Resource Directory (RD)</dt>
	</dd>
	<dt>
	Resource Directory (RD) </dt>
	<dd>	<dd>

	<t>A web entity that stores information about web resources and implem	<t>A web entity that stores information about web resources and
	ents the	implements the REST interfaces defined in this specification for
	REST interfaces defined in this specification for discovery, for the creation, m	discovery, for the creation, maintenance, and removal of
	aintenance and removal of registrations, and for lookup	registrations, and for lookup of the registered resources.</t>
	of the registered resources.</t>
	</dd>	</dd>

	<dt>	<dt>Sector</dt>
	Sector </dt>
	<dd>	<dd>

	<t>In the context of an RD, a sector is a	<t>In the context of an RD, a sector is a logical grouping of endpoint
	logical grouping of endpoints.	s.</t>
	</t>
	<t>The abbreviation "d=" is used for the sector in query parameters fo r	<t>The abbreviation "d=" is used for the sector in query parameters fo r

	compatibility with deployed implementations.</t>	compatibility with deployed implementations.</t>
	</dd>	</dd>

	<dt>	<dt>Endpoint (EP)</dt>
	Endpoint </dt>
	<dd>	<dd>

	<t>Endpoint (EP) is a term used to describe a web server or client in	<t>Endpoint (EP) is a term used to describe a web server or client
	<xref target="RFC7252" format="default"/>.	in <xref target="RFC7252" format="default"/>. In the context of
	In the context of this specification an endpoint is used to describe a	this specification, an endpoint is used to describe a web server
	web server that registers resources to the RD. An endpoint	that registers resources to the RD. An endpoint is identified by its
	is identified by its endpoint name, which is included during registration,	endpoint name, which is included during registration, and has a
	and has a unique name within the associated sector of the registration.</t>	unique name within the associated sector of the registration.</t>
	</dd>	</dd>

	<dt>	<dt>Registration Base URI</dt>
	Registration Base URI </dt>
	<dd>	<dd>

	<t>The Base URI of a Registration is a URI that typically gives scheme	<t>The base URI of a registration is a URI that typically gives scheme
	and	and
	authority information about an Endpoint. The Registration Base URI is provided a	authority information about an endpoint. The registration base URI is p
	t registration time, and is used by the RD to	rovided at
	resolve relative references of the registration into URIs.</t>	registration time and is used by the RD to
		resolve relative references of the registration into URIs.</t>
	</dd>	</dd>

	<dt>	<dt>Target</dt>
	Target </dt>
	<dd>	<dd>

	<t>The target of a link is the destination address (URI) of the link.	<t>The target of a link is the destination address (URI) of the link.
	It is sometimes identified with "href=", or displayed as <tt><target></tt>	It is
	. Relative targets need resolving with respect to the Base URI (section 5.2 of <	sometimes identified with "href=" or displayed as <tt><target></t
	xref target="RFC3986" format="default"/>).	t>.
	</t>	Relative targets need resolving with respect to the base URI (<xref
	<t>This use of the term Target is consistent with <xref target="RFC828	target="RFC3986" sectionFormat="of" section="5.2"/>).</t>
	8" format="default"/>'s use of the term.</t>	<t>This use of the term "target" is consistent with the use in <xref t
		arget="RFC8288"
		format="default"/>.</t>
	</dd>	</dd>

	<dt>	<dt>Context</dt>
	Context </dt>
	<dd>	<dd>

	<t>The context of a link is the source address (URI) of the link,	<t>The context of a link is the source address (URI) of the link
	and describes which resource is linked to the target.	and describes which resource is linked to the target.
	A link's context is made explicit in serialized links as the "anchor=" attribute	A link's context is made explicit in serialized links as the "anchor="
	.	attribute.</t>
	</t>	<t>This use of the term "context" is consistent with the use in <xref
	<t>This use of the term Context is consistent with <xref target="RFC82	target="RFC8288"
	88" format="default"/>'s use of the term.</t>	format="default"/>.</t>
	</dd>	</dd>

	<dt>	<dt>Directory Resource</dt>
	Directory Resource </dt>
	<dd>	<dd>

	<t>A resource in the RD containing registration resources.</t>	<t>A directory resource is a resource in the RD containing registratio n resources.</t>
	</dd>	</dd>

	<dt>	<dt>Registration Resource</dt>
	Registration Resource </dt>
	<dd>	<dd>

	<t>A resource in the RD that contains information about an Endpoint an	<t>A registration resource is a resource in the RD that contains infor
	d its links.</t>	mation about an
		endpoint and its links.</t>
	</dd>	</dd>

	<dt>	<dt>Commissioning Tool (CT)</dt>
	Commissioning Tool </dt>
	<dd>	<dd>

	<t>Commissioning Tool (CT) is a device that assists during installatio	<t>A Commissioning Tool (CT) is a device that assists during installat
	n events	ion events
	by assigning values to parameters, naming endpoints and groups, or adapting	by assigning values to parameters, naming endpoints and groups, or adap
	the installation to the needs of the applications.</t>	ting
		the installation to the needs of the applications.</t>
	</dd>	</dd>

	<dt>	<dt>Registrant-EP</dt>
	Registrant-ep </dt>
	<dd>	<dd>

	<t>Registrant-ep is the endpoint that is registered into the RD. The r	<t>A registrant-EP is the endpoint that is registered into the RD. The
	egistrant-ep can register itself, or a CT registers the registrant-ep.</t>	registrant-EP can
		register itself, or a CT registers the registrant-EP.</t>
	</dd>	</dd>

	<dt>	<dt>Resource Directory Address Option (RDAO)</dt>
	RDAO </dt>
	<dd>	<dd>

	<t>Resource Directory Address Option.	<t>A Resource Directory Address Option (RDAO) is a new IPv6 Neighbor D
	A new IPv6 Neighbor Discovery option defined for announcing an RD's address.</t>	iscovery option defined
		for announcing an RD's address.</t>
	</dd>	</dd>
	</dl>	</dl>
	</section>	</section>
	<section anchor="arch" numbered="true" toc="default">	<section anchor="arch" numbered="true" toc="default">
	<name>Architecture and Use Cases</name>	<name>Architecture and Use Cases</name>
	<section anchor="principles" numbered="true" toc="default">	<section anchor="principles" numbered="true" toc="default">
	<name>Principles</name>	<name>Principles</name>
	<t>The RD is primarily a tool to make discovery operations more	<t>The RD is primarily a tool to make discovery operations more

	efficient than querying /.well-known/core on all connected devices, or across	efficient than querying <tt>/.well-known/core</tt> on all connected devices or a cross
	boundaries that would limit those operations.</t>	boundaries that would limit those operations.</t>
	<t>It provides information about resources hosted by other devices that could otherwise only be obtained by	<t>It provides information about resources hosted by other devices that could otherwise only be obtained by

	directly querying the /.well-known/core resource on these other devices, either	directly querying the <tt>/.well-known/core</tt> resource on these other devices
	by a unicast request or a multicast request.</t>	, either by a unicast request or a multicast request.</t>
	<t>Information SHOULD only be stored in the RD	<t>Information <bcp14>SHOULD</bcp14> only be stored in the RD
	if it can be obtained by querying the described device's	if it can be obtained by querying the described device's

	/.well-known/core resource directly.</t>	<tt>/.well-known/core</tt> resource directly.</t>
	<t>Data in the RD can only be provided by the	<t>Data in the RD can only be provided by the

	device which hosts those data or a dedicated Commissioning Tool (CT).	device that hosts the data or a dedicated Commissioning Tool (CT).
	These CTs act on behalf of endpoints too constrained, or generally	These CTs act on behalf of endpoints too constrained, or generally
	unable, to present that information themselves. No other client can modify data	unable, to present that information themselves. No other client can modify data
	in the RD. Changes to the information in the RD do not propagate automatically b ack to the web servers from where the information originated.</t>	in the RD. Changes to the information in the RD do not propagate automatically b ack to the web servers from where the information originated.</t>
	</section>	</section>
	<section anchor="architecture" numbered="true" toc="default">	<section anchor="architecture" numbered="true" toc="default">
	<name>Architecture</name>	<name>Architecture</name>
	<t>The RD architecture is illustrated in <xref target="fig-arch" format= "default"/>. An	<t>The RD architecture is illustrated in <xref target="fig-arch" format= "default"/>. An
	RD is used as a repository of registrations	RD is used as a repository of registrations
	describing resources hosted on other web servers, also called endpoints	describing resources hosted on other web servers, also called endpoints

	(EP).	(EPs).
	An endpoint is a web server associated with a scheme, IP address and port. A phy	An endpoint is a web server associated with a scheme, IP address, and port. A ph
	sical node may host one or more endpoints. The	ysical node may host one or more endpoints. The
	RD implements a set of REST interfaces for endpoints to register and maintain	RD implements a set of REST interfaces for endpoints to register and maintain

	RD registrations, and for endpoints to	RD registrations and for endpoints to
	lookup resources from the RD. An RD can be logically segmented by the use of Sec	look up resources from the RD. An RD can be logically segmented by the use of se
	tors.</t>	ctors.</t>
	<t>A mechanism to discover an RD using CoRE Link Format <xref target="RF C6690" format="default"/> is defined.</t>	<t>A mechanism to discover an RD using CoRE Link Format <xref target="RF C6690" format="default"/> is defined.</t>

	<t>Registrations	<t>Registrations in the RD are soft state and need to be periodically re
	in the RD are soft state and need to be periodically refreshed.</t>	freshed.</t>
	<t>An endpoint uses specific interfaces to register, update and remove a	<t>An endpoint uses specific interfaces to register, update, and remove
	registration. It is also possible for an RD to fetch Web Links	a registration. It is also possible for an RD to fetch web links
	from endpoints and add their contents to its registrations.</t>	from endpoints and add their contents to its registrations.</t>
	<t>At the first registration of an endpoint, a "registration resource" i s created,	<t>At the first registration of an endpoint, a "registration resource" i s created,
	the location of which is returned to the registering endpoint. The registering	the location of which is returned to the registering endpoint. The registering
	endpoint uses this registration resource to manage the contents of registrations .</t>	endpoint uses this registration resource to manage the contents of registrations .</t>

	<t>A lookup interface for discovering any of the Web Links stored in the RD is	<t>A lookup interface for discovering any of the web links stored in the RD is
	provided using the CoRE Link Format.</t>	provided using the CoRE Link Format.</t>
	<figure anchor="fig-arch">	<figure anchor="fig-arch">

	<name>The RD architecture.</name>	<name>The RD Architecture</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<artwork name="" type="ascii-art" align="left" alt=""><![CDATA[
	Registration Lookup	Registration Lookup
	Interface Interface	Interface Interface
	+----+ \| \|	+----+ \| \|
	\| EP \|---- \| \|	\| EP \|---- \| \|
	+----+ ---- \| \|	+----+ ---- \| \|
	--\|- +------+ \|	--\|- +------+ \|
	+----+ \| ----\| \| \| +--------+	+----+ \| ----\| \| \| +--------+
	\| EP \| ---------\|-----\| RD \|----\|-----\| Client \|	\| EP \| ---------\|-----\| RD \|----\|-----\| Client \|
	+----+ \| ----\| \| \| +--------+	+----+ \| ----\| \| \| +--------+
	--\|- +------+ \|	--\|- +------+ \|
	+----+ ---- \| \|	+----+ ---- \| \|
	\| CT \|---- \| \|	\| CT \|---- \| \|
	+----+	+----+

	]]></artwork>	]]></artwork>

		----+
	</figure>	</figure>

	<t>A Registrant-EP MAY keep concurrent registrations to more than one RD at the same time	<t>A registrant-EP <bcp14>MAY</bcp14> keep concurrent registrations to m ore than one RD at the same time
	if explicitly configured to do so,	if explicitly configured to do so,
	but that is not expected to be supported by typical EP implementations.	but that is not expected to be supported by typical EP implementations.
	Any such registrations are independent of each other.	Any such registrations are independent of each other.
	The usual expectation when multiple discovery mechanisms or addresses are config ured	The usual expectation when multiple discovery mechanisms or addresses are config ured
	is that they constitute a fall-back path for a single registration.</t>	is that they constitute a fall-back path for a single registration.</t>
	</section>	</section>
	<section anchor="ER-model" numbered="true" toc="default">	<section anchor="ER-model" numbered="true" toc="default">
	<name>RD Content Model</name>	<name>RD Content Model</name>

	<t>The Entity-Relationship (ER) models shown in <xref target="fig-ER-WKC " format="default"/> and <xref target="fig-ER-RD" format="default"/> model the c ontents of /.well-known/core and the RD respectively, with entity-relationship d iagrams <xref target="ER" format="default"/>. Entities (rectangles) are used for concepts that exist independently. Attributes (ovals) are used for concepts tha t exist only in connection with a related entity. Relations (diamonds) give a se mantic meaning to the relation between entities. Numbers specify the cardinality of the relations.</t>	<t>The Entity-Relationship (ER) models shown in Figures <xref target="fi g-ER-WKC" format="counter"/> and <xref target="fig-ER-RD" format="counter"/> mod el the contents of <tt>/.well-known/core</tt> and the RD respectively, with enti ty-relationship diagrams <xref target="ER" format="default"/>. Entities (rectang les) are used for concepts that exist independently. Attributes (ovals) are used for concepts that exist only in connection with a related entity. Relations (di amonds) give a semantic meaning to the relation between entities. Numbers specif y the cardinality of the relations.</t>
	<t>Some of the attribute values are URIs. Those values are always full U RIs and never relative references in the information model.	<t>Some of the attribute values are URIs. Those values are always full U RIs and never relative references in the information model.

	They can, however, be expressed as relative references in serializations, and of	However, they can be expressed as relative references in serializations, and the
	ten are.</t>	y often are.</t>
	<t>These models provide an abstract view of the information expressed in	<t>These models provide an abstract view of the information expressed in
	link-format documents and an RD. They cover the concepts, but not necessarily a	link-format documents and an RD. They cover the concepts but not necessarily al
	ll details of an RD's operation; they are meant to give an overview, and not be	l details of an RD's operation; they are meant to give an overview and not be a
	a template for implementations.</t>	template for implementations.</t>
	<figure anchor="fig-ER-WKC">	<figure anchor="fig-ER-WKC">

	<name>ER Model of the content of /.well-known/core</name>	<name>ER Model of the Content of <tt>/.well-known/core</tt></name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<artwork name="" type="ascii-art" align="left" alt=""><![CDATA[
	+----------------------+	+----------------------+
	\| /.well-known/core \|	\| /.well-known/core \|
	+----------------------+	+----------------------+
	\|	\|
	\| 1	\| 1
	////////\\\\\\\	////////\\\\\\\
	< contains >	< contains >
	\\\\\\\\///////	\\\\\\\\///////
	\|	\|
	\| 0+	\| 0+
	+--------------------+	+--------------------+
	\| link \|	\| link \|
	+--------------------+	+--------------------+
	\|	\|
	\| 1 oooooooo	\| 1 oooooooo
	+-----o target o	+-----o target o
	\| oooooooo	\| oooooooo
	oooooooooooo 0+ \|	oooooooooooo 0+ \|
	o target o--------+	o target o--------+
	o attribute o \| 0+ oooooo	o attribute o \| 0+ oooooo
	oooooooooooo +-----o rel o	oooooooooooo +-----o rel o
	\| oooooo	\| oooooo
	\|	\|
	\| 1 ooooooooo	\| 1 ooooooooo
	+-----o context o	+-----o context o
	ooooooooo	ooooooooo

	]]></artwork>	]]></artwork>
	</figure>	</figure>

	<t>The model shown in <xref target="fig-ER-WKC" format="default"/> model	<t><xref target="fig-ER-WKC" format="default"/> models the contents of
	s the contents of /.well-known/core which contains:</t>	<tt>/.well-known/core</tt>, which contains a set of links belonging to th
	<ul spacing="normal">	e hosting web server.</t>
	<li>a set of links belonging to the hosting web server</li>	<t>The web server is free to choose links it deems appropriate to be expo
	</ul>	sed in its <tt>/.well-known/core</tt>.
	<t>The web server is free to choose links it deems appropriate to be exp	Typically, the links describe resources that are served by the host, but the set
	osed in its <tt>/.well-known/core</tt>.	can also contain links to resources on other servers (see examples in <xref tar
	Typically, the links describe resources that are served by the host, but the set	get="RFC6690" sectionFormat="of" section="5"/>).
	can also contain links to resources on other servers (see examples in <xref tar
	get="RFC6690" format="default"/> page 14).
	The set does not necessarily contain links to all resources served by the host.< /t>	The set does not necessarily contain links to all resources served by the host.< /t>

	<t>A link has the following attributes (see <xref target="RFC8288" forma	<t>A link has the following attributes (see <xref target="RFC8288" secti
	t="default"/>):</t>	onFormat="of" section="5"/>):</t>
	<ul spacing="normal">	<ul>
	<li>	<li><t>Zero or more link relations:
	<t>Zero or more link relations: They describe relations between the	They describe relations between the link context and the link target.
	link context and the link target. </t>	</t>
	<t>	<t>In link-format serialization, they are expressed as space-separate
	In link-format serialization, they are expressed as space-separated values in th	d values in the
	e "rel" attribute, and default to "hosts".</t>	"rel" attribute and default to "hosts".</t>
	</li>	</li>
	<li>	<li><t>A link context URI: It defines the source of the relation, e.g.,
	<t>A link context URI: It defines the source of the relation, e.g. <	<em>who</em> "hosts" something.</t>
	em>who</em> "hosts" something. </t>	<t>In link-format serialization, it is expressed in the "anchor" attr
	<t>	ibute and defaults
	In link-format serialization, it is expressed in the "anchor" attribute and defa	to the Origin of the target (practically, the target with its path an
	ults to the Origin of the target (practically: the target with its path and late	d later components
	r components removed)</t>	removed).</t>
	</li>	</li>
	<li>	<li><t>A link target URI: It defines the destination of the relation (e
	<t>A link target URI: It defines the destination of the relation (e.	.g., <em>what</em> is hosted) and is
	g. <em>what</em> is hosted), and is the topic of all target attributes. </t>	the topic of all target attributes.</t>
	<t>	<t>In link-format serialization, it is expressed between angular brac
	In link-format serialization, it is expressed between angular brackets, and some	kets and
	times called the "href".</t>	sometimes called the "href".</t>
	</li>	</li>
	<li>Other target attributes (e.g. resource type (rt), interface (if),	<li>Other target attributes (e.g., resource type (rt), interface (if),
	or content format (ct)).	or content format (ct)): These provide additional information about the target
	These provide additional information about the target URI.</li>	URI.</li>
	</ul>	</ul>

	<figure anchor="fig-ER-RD">	<figure anchor="fig-ER-RD">
	<name>ER Model of the content of the RD</name>	<name>ER Model of the Content of the RD</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<artwork name="" type="ascii-art" align="left" alt=""><![CDATA[
	+--------------+	+--------------+
	+ RD +	+ RD +
	+--------------+	+--------------+
	\| 1	\| 1
	\|	\|
	\|	\|
	\|	\|
	\|	\|
	//////\\\\	//////\\\\
	< contains >	< contains >

	skipping to change at line 405 ¶	skipping to change at line 379 ¶
	oooooooo \| o target o-----+	oooooooo \| o target o-----+
	\| o attribute o \| 0+ oooooo	\| o attribute o \| 0+ oooooo
	ooooooooooo 0+ \| ooooooooooo +-----o rel o	ooooooooooo 0+ \| ooooooooooo +-----o rel o
	o endpoint o----+ \| oooooo	o endpoint o----+ \| oooooo
	o attribute o \|	o attribute o \|
	ooooooooooo \| 1 ooooooooo	ooooooooooo \| 1 ooooooooo
	+----o context o	+----o context o
	ooooooooo	ooooooooo
	]]></artwork>	]]></artwork>
	</figure>	</figure>

	<t>The model shown in <xref target="fig-ER-RD" format="default"/> models	<t><xref target="fig-ER-RD" format="default"/> models the contents of
	the contents of the RD which contains in addition to /.well-known/core:</t>	the RD, which contains, in addition to <tt>/.well-known/core</tt>, 0 to n
	<ul spacing="normal">	registrations of
	<li>0 to n Registrations of endpoints,</li>	endpoints.</t>
	</ul>	<t>A registration is associated with one endpoint. A registration define
	<t>A registration is associated with one endpoint. A registration define	s a set of links,
	s a set of links as defined for /.well-known/core. A Registration has six types	as defined for <tt>/.well-known/core</tt>. A registration has six types o
	of attributes:</t>	f attributes:</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>an endpoint name ("ep", a Unicode string) unique within a sector</ li>	<li>an endpoint name ("ep", a Unicode string) unique within a sector</ li>

	<li>a Registration Base URI ("base", a URI typically describing the sc	<li>a registration base URI ("base", a URI typically describing the sc
	heme://authority part)</li>	heme://authority part)</li>
	<li>a lifetime ("lt"),</li>	<li>a lifetime ("lt")</li>
	<li>a registration resource location inside the RD ("href"),</li>	<li>a registration resource location inside the RD ("href")</li>
	<li>optionally a sector ("d", a Unicode string)</li>	<li>optionally, a sector ("d", a Unicode string)</li>
	<li>optional additional endpoint attributes (from <xref target="iana-r egistry" format="default"/>)</li>	<li>optional additional endpoint attributes (from <xref target="iana-r egistry" format="default"/>)</li>
	</ul>	</ul>
	<t>The cardinality of "base" is currently 1;	<t>The cardinality of "base" is currently 1;

	future documents are invited to extend the RD specification to support multiple	future documents are invited to extend the RD specification to support multiple
	values (e.g. <xref target="I-D.silverajan-core-coap-protocol-negotiation" format	values (e.g., <xref target="I-D.silverajan-core-coap-protocol-negotiation" forma
	="default"/>).	t="default"/>).
	Its value is used as a Base URI when resolving URIs in the links contained in th	Its value is used as a base URI when resolving URIs in the links contained in th
	e endpoint.</t>	e endpoint.</t>
	<t>Links are modelled as they are in <xref target="fig-ER-WKC" format="d	<t>Links are modeled as they are in <xref target="fig-ER-WKC" format="de
	efault"/>.</t>	fault"/>.</t>
	</section>	</section>
	<section anchor="linklocal" numbered="true" toc="default">	<section anchor="linklocal" numbered="true" toc="default">

	<name>Link-local addresses and zone identifiers</name>	<name>Link-Local Addresses and Zone Identifiers</name>
	<t>Registration Base URIs can contain link-local IP addresses.	<t>Registration base URIs can contain link-local IP addresses.
	To be usable across hosts, those cannot be serialized to contain zone identifier	To be usable across hosts, those cannot be serialized to
	s (see <xref target="RFC6874" format="default"/> Section 1).</t>	contain zone identifiers (see <xref target="RFC6874"
		section="1" sectionFormat="comma"/>).</t>
	<t>Link-local addresses can only be used on a single link	<t>Link-local addresses can only be used on a single link

	(therefore RD servers cannot announce them when queried on a different link),	(therefore, RD servers cannot announce them when queried on a different link),
	and lookup clients using them need to keep track of which interface they got the m from.</t>	and lookup clients using them need to keep track of which interface they got the m from.</t>
	<t>Therefore, it is advisable in many scenarios	<t>Therefore, it is advisable in many scenarios

	to use addresses with larger scope if available.</t>	to use addresses with larger scopes, if available.</t>
	</section>	</section>
	<section anchor="cellular" numbered="true" toc="default">	<section anchor="cellular" numbered="true" toc="default">
	<name>Use Case: Cellular M2M</name>	<name>Use Case: Cellular M2M</name>
	<t>Over the last few years, mobile operators around the world	<t>Over the last few years, mobile operators around the world
	have focused on development of M2M solutions in order to	have focused on development of M2M solutions in order to
	expand the business to the new type of users: machines. The	expand the business to the new type of users: machines. The
	machines are connected directly to a mobile network using an appropriate	machines are connected directly to a mobile network using an appropriate

	embedded wireless interface (GSM/GPRS, WCDMA, LTE) or via a gateway providing	embedded wireless interface (GSM/General Packet Radio Service (GPRS), Wideband C
	short and wide range wireless interfaces.	ode Division Multiple Access (W-CDMA), LTE, etc.) or via a gateway providing
		short- and wide-range wireless interfaces.
	The ambition in such systems is to build them from reusable components.	The ambition in such systems is to build them from reusable components.

	These speed up development and deployment,	These speed up development and deployment
	and enable shared use of machines across different applications.	and enable shared use of machines across different applications.
	One crucial component of such systems	One crucial component of such systems
	is the discovery of resources (and thus the endpoints they are hosted on) capabl e of providing required	is the discovery of resources (and thus the endpoints they are hosted on) capabl e of providing required
	information at a given time or acting on instructions from the end users.</t>	information at a given time or acting on instructions from the end users.</t>
	<t>Imagine a scenario where endpoints installed on vehicles enable	<t>Imagine a scenario where endpoints installed on vehicles enable
	tracking of the position of these vehicles for fleet management purposes and all ow	tracking of the position of these vehicles for fleet management purposes and all ow

	monitoring of environment parameters. During the boot-up process	monitoring of environment parameters. During the boot-up process,
	endpoints register with an RD, which is hosted by the	endpoints register with an RD, which is hosted by the
	mobile operator or somewhere in the cloud. Periodically, these endpoints	mobile operator or somewhere in the cloud. Periodically, these endpoints
	update their registration and may modify resources they offer.</t>	update their registration and may modify resources they offer.</t>

	<t>When endpoints are not always connected, for example because they ent er	<t>When endpoints are not always connected, for example, because they en ter
	a sleep mode, a remote server is usually used to provide proxy access to	a sleep mode, a remote server is usually used to provide proxy access to

	the endpoints. Mobile apps or web applications for environment monitoring contac t the RD, look up the endpoints capable of providing information about the envir onment using an appropriate set of link parameters, obtain information on how to contact them (URLs of the proxy server), and then initiate interaction to obtai n information that is finally processed, displayed on the screen and usually sto red in a database. Similarly, fleet management systems provide	the endpoints. Mobile apps or web applications for environment monitoring contac t the RD, look up the endpoints capable of providing information about the envir onment using an appropriate set of link parameters, obtain information on how to contact them (URLs of the proxy server), and then initiate interaction to obtai n information that is finally processed, displayed on the screen, and usually st ored in a database. Similarly, fleet management systems provide
	the appropriate link parameters to the RD to look up for EPs deployed on	the appropriate link parameters to the RD to look up for EPs deployed on
	the vehicles the application is responsible for.</t>	the vehicles the application is responsible for.</t>
	</section>	</section>
	<section anchor="automation" numbered="true" toc="default">	<section anchor="automation" numbered="true" toc="default">
	<name>Use Case: Home and Building Automation</name>	<name>Use Case: Home and Building Automation</name>
	<t>Home and commercial building automation systems can benefit from the use	<t>Home and commercial building automation systems can benefit from the use
	of IoT web services. The discovery requirements of these applications are	of IoT web services. The discovery requirements of these applications are
	demanding. Home automation usually relies on run-time discovery to commission	demanding. Home automation usually relies on run-time discovery to commission

	the system, whereas in building automation a combination of professional	the system, whereas, in building automation, a combination of professional
	commissioning and run-time discovery is used. Both home and building automation	commissioning and run-time discovery is used. Both home and building automation

	involve peer-to-peer interactions between endpoints, and involve battery-powered	involve peer-to-peer interactions between endpoints and involve battery-powered
	sleeping devices.	sleeping devices.

	Both can use the common RD infrastructure to establish device interactions effic iently,	Both can use the common RD infrastructure to establish device interactions effic iently
	but can pick security policies suitable for their needs.</t>	but can pick security policies suitable for their needs.</t>

	<t>Two phases can be discerned for a network servicing the system: (1) i nstallation and (2) operation. During the operational phase, the network is conn ected to the Internet with a Border Router (e.g. a 6LoWPAN Border Router (6LBR), see <xref target="RFC6775" format="default"/>) and the nodes connected to the n etwork can use the Internet services that are provided by the Internet Provider or the network administrator. During the installation phase, the network is comp letely stand-alone, no Border Router is connected, and the network only supports the IP communication between the connected nodes. The installation phase is usu ally followed by the operational phase.	<t>Two phases can be discerned for a network servicing the system: (1) i nstallation and (2) operation. During the operational phase, the network is conn ected to the Internet with a border router (e.g., a 6LoWPAN Border Router (6LBR) <xref target="RFC6775" format="default"/>), and the nodes connected to the netw ork can use the Internet services that are provided by the IP or network adminis trator. During the installation phase, the network is completely stand-alone, no border router is connected, and the network only supports the IP communication between the connected nodes. The installation phase is usually followed by the o perational phase.
	As an RD's operations work without hard dependencies on names or addresses,	As an RD's operations work without hard dependencies on names or addresses,
	it can be used for discovery across both phases.</t>	it can be used for discovery across both phases.</t>
	</section>	</section>
	<section anchor="usecase-catalogues" numbered="true" toc="default">	<section anchor="usecase-catalogues" numbered="true" toc="default">
	<name>Use Case: Link Catalogues</name>	<name>Use Case: Link Catalogues</name>
	<t>Resources may be shared through data brokers that have no knowledge b eforehand	<t>Resources may be shared through data brokers that have no knowledge b eforehand
	of who is going to consume the data. An RD can be used to hold	of who is going to consume the data. An RD can be used to hold
	links about resources and services hosted anywhere to make them discoverable	links about resources and services hosted anywhere to make them discoverable
	by a general class of applications.</t>	by a general class of applications.</t>
	<t>For example, environmental and weather sensors that generate data for public	<t>For example, environmental and weather sensors that generate data for public

	consumption may provide data to an intermediary server, or broker. Sensor	consumption may provide data to an intermediary server or broker. Sensor
	data are published to the intermediary upon changes or at regular intervals.	data are published to the intermediary upon changes or at regular intervals.
	Descriptions of the sensors that resolve to links to sensor data may be publishe d	Descriptions of the sensors that resolve to links to sensor data may be publishe d
	to an RD. Applications wishing to consume the data can use	to an RD. Applications wishing to consume the data can use

	RD Lookup to discover and resolve links	RD lookup to discover and resolve links
	to the desired resources and endpoints. The RD service need	to the desired resources and endpoints. The RD service need
	not be coupled with the data intermediary service. Mapping of RDs	not be coupled with the data intermediary service. Mapping of RDs
	to data intermediaries may be many-to-many.</t>	to data intermediaries may be many-to-many.</t>

	<t>Metadata in web link formats like <xref target="RFC6690" format="defa ult"/> which may be internally stored as triples, or relation/attribute	<t>Metadata in web link formats, such as the one defined in <xref target ="RFC6690" format="default"/>, which may be internally stored as triples or rel ation/attribute
	pairs providing metadata about resource links, need to be supported by RDs. Exte rnal catalogues that are	pairs providing metadata about resource links, need to be supported by RDs. Exte rnal catalogues that are
	represented in other formats may be converted to common web linking formats for	represented in other formats may be converted to common web linking formats for
	storage and access by RDs. Since it is common practice for these	storage and access by RDs. Since it is common practice for these
	to be encoded in URNs <xref target="RFC8141" format="default"/>, simple and loss less structural transforms should	to be encoded in URNs <xref target="RFC8141" format="default"/>, simple and loss less structural transforms should
	generally be sufficient to store external metadata in RDs.</t>	generally be sufficient to store external metadata in RDs.</t>
	<t>The additional features of an RD allow sectors to be defined	<t>The additional features of an RD allow sectors to be defined
	to enable access to a particular set of resources from particular applications.	to enable access to a particular set of resources from particular applications.
	This provides isolation and protection of sensitive data when needed. Applicatio n groups with multicast addresses may be defined to support efficient data trans port.</t>	This provides isolation and protection of sensitive data when needed. Applicatio n groups with multicast addresses may be defined to support efficient data trans port.</t>
	</section>	</section>
	</section>	</section>
	<section anchor="rd-discovery-and-other-interface-independent-components" nu mbered="true" toc="default">	<section anchor="rd-discovery-and-other-interface-independent-components" nu mbered="true" toc="default">

	<name>RD discovery and other interface-independent components</name>	<name>RD Discovery and Other Interface-Independent Components</name>
	<t>This and the following sections define the required set of REST interfa ces between an RD,	<t>This and the following sections define the required set of REST interfa ces between an RD,

	endpoints and lookup clients. Although the examples throughout these sections as sume the use of	endpoints, and lookup clients. Although the examples throughout these sections a ssume the use of
	CoAP <xref target="RFC7252" format="default"/>, these REST interfaces can also b e realized using HTTP <xref target="RFC7230" format="default"/>.	CoAP <xref target="RFC7252" format="default"/>, these REST interfaces can also b e realized using HTTP <xref target="RFC7230" format="default"/>.
	The multicast discovery and simple registration operations are exceptions to tha t,	The multicast discovery and simple registration operations are exceptions to tha t,
	as they rely on mechanisms unavailable in HTTP.	as they rely on mechanisms unavailable in HTTP.
	In all definitions in these sections, both CoAP response codes (with dot notatio n) and HTTP response codes	In all definitions in these sections, both CoAP response codes (with dot notatio n) and HTTP response codes

	(without dot notation) are shown. An RD implementing this specification MUST sup port	(without dot notation) are shown. An RD implementing this specification <bcp14>M UST</bcp14> support
	the discovery, registration, update, lookup, and removal interfaces.</t>	the discovery, registration, update, lookup, and removal interfaces.</t>

	<t>All operations on the contents of the RD MUST be atomic and idempotent.	<t>All operations on the contents of the RD <bcp14>MUST</bcp14> be atomic
	</t>	and idempotent.</t>

	<t>For several operations, interface templates are given in list form;	<t>For several operations, interface templates are given in list form;

	those describe the operation participants, request codes, URIs, content formats and outcomes.	those describe the operation participants, request codes, URIs, content formats, and outcomes.
	Sections of those templates contain normative content about	Sections of those templates contain normative content about

	Interaction, Method, URI Template and URI Template Variables	Interaction, Method, URI Template, and URI Template Variables,
	as well as the details of the Success condition.	as well as the details of the Success condition.
	The additional sections	The additional sections

	on options like Content-Format and on Failure codes	for options (such as Content-Format) and for Failure codes
	give typical cases that an implementation of the RD should deal with.	give typical cases that an implementation of the RD should deal with.
	Those serve to illustrate the typical responses	Those serve to illustrate the typical responses

	to readers who are not yet familiar with all the details of CoAP based interface	to readers who are not yet familiar with all the details of CoAP-based interface
	s;	s;
	they do not limit what a server may respond under atypical circumstances.</t>	they do not limit how a server may respond under atypical circumstances.</t>
	<t>REST clients (registrant-EPs and CTs during registration and maintenanc	<t>REST clients (registrant-EPs and CTs during registration and maintenanc
	e, lookup clients, RD servers during simple registrations)	e, lookup clients, and RD servers during simple registrations)
	must be prepared to receive any unsuccessful code and act upon it	must be prepared to receive any unsuccessful code and act upon it

	according to its definition, options and/or payload to the best of their capabil ities,	according to its definition, options, and/or payload to the best of their capabi lities,
	falling back to failing the operation if recovery is not possible.	falling back to failing the operation if recovery is not possible.

	In particular, they SHOULD retry the request upon 5.03 (Service Unavailable; 503	In particular, they <bcp14>SHOULD</bcp14> retry the request upon 5.03 (Service U
	in HTTP)	navailable; 503 in HTTP)
	according to the Max-Age (Retry-After in HTTP) option,	according to the Max-Age (Retry-After in HTTP) option
	and SHOULD fall back to link-format when receiving 4.15 (Unsupported Content-For	and <bcp14>SHOULD</bcp14> fall back to link format when receiving 4.15 (Unsuppor
	mat; 415 in HTTP).</t>	ted Content-Format; 415 in HTTP).</t>
	<t>An RD MAY make the information submitted to it available to further	<t>An RD <bcp14>MAY</bcp14> make the information submitted to it available
	directories (subject to security policies on link confidentiality),	to further
		directories (subject to security policies on link confidentiality)
	if it can ensure that a loop does not form. The protocol used	if it can ensure that a loop does not form. The protocol used
	between directories to ensure loop-free operation is outside the scope of	between directories to ensure loop-free operation is outside the scope of
	this document.</t>	this document.</t>
	<section anchor="finding_an_rd" numbered="true" toc="default">	<section anchor="finding_an_rd" numbered="true" toc="default">
	<name>Finding a Resource Directory</name>	<name>Finding a Resource Directory</name>

	<t>A (re-)starting device may want to find one or more RDs	<t>A (re)starting device may want to find one or more RDs
	before it can discover their URIs. Dependent on the operational conditions, one or more of the techniques below apply.</t>	before it can discover their URIs. Dependent on the operational conditions, one or more of the techniques below apply.</t>

	<t>The device may be pre-configured to exercise specific mechanisms for	<t>The device may be preconfigured to exercise specific mechanisms for
	finding the RD:</t>	finding the RD:</t>
	<ol spacing="normal" type="1">	<ol spacing="normal" type="1">
	<li>It may be configured with a specific IP address for the RD. That IP	<li>It may be configured with a specific IP address for the RD. That IP
	address may also be an anycast address, allowing the network to	address may also be an anycast address, allowing the network to
	forward RD requests to an RD that is topologically close; each	forward RD requests to an RD that is topologically close; each
	target network environment in which some of these preconfigured	target network environment in which some of these preconfigured
	nodes are to be brought up is then configured with a route for this	nodes are to be brought up is then configured with a route for this
	anycast address that leads to an appropriate RD. (Instead of using	anycast address that leads to an appropriate RD. (Instead of using
	an anycast address, a multicast address can also be preconfigured.	an anycast address, a multicast address can also be preconfigured.
	The RD servers then need to configure one of their	The RD servers then need to configure one of their
	interfaces with this multicast address.)</li>	interfaces with this multicast address.)</li>
	<li>It may be configured with a DNS name for the RD and use DNS to ret urn	<li>It may be configured with a DNS name for the RD and use DNS to ret urn
	the IP address of the RD; it can find a DNS server to perform the lookup using t he usual mechanisms for finding DNS servers.</li>	the IP address of the RD; it can find a DNS server to perform the lookup using t he usual mechanisms for finding DNS servers.</li>

	<li>It may be configured to use a service discovery mechanism such as	<li>It may be configured to use a service discovery mechanism, such as
	DNS-SD, as outlined in <xref target="rd-using-dnssd" format="default"/>.</li>	DNS-based Service Discovery (DNS-SD), as outlined in <xref target="rd-using-dnss
		d" format="default"/>.</li>
	</ol>	</ol>
	<t>For cases where the device is not specifically configured with a way	<t>For cases where the device is not specifically configured with a way
	to find an RD, the network may want to provide a	to find an RD, the network may want to provide a
	suitable default.</t>	suitable default.</t>
	<ol spacing="normal" type="1">	<ol spacing="normal" type="1">

	<li>The IPv6 Neighbor Discovery option RDAO <xref target="rdao" format ="default"/> can do that.</li>	<li>The IPv6 Neighbor Discovery option RDAO (<xref target="rdao" forma t="default"/>) can do that.</li>
	<li>When DHCP is in use,	<li>When DHCP is in use,
	this could be provided via a DHCP option (no such option is defined	this could be provided via a DHCP option (no such option is defined
	at the time of writing).</li>	at the time of writing).</li>
	</ol>	</ol>
	<t>Finally, if neither the device nor the network offers any specific	<t>Finally, if neither the device nor the network offers any specific
	configuration, the device may want to employ heuristics to find a	configuration, the device may want to employ heuristics to find a
	suitable RD.</t>	suitable RD.</t>

	<t>The present specification does not fully define these heuristics, but	<t>The present specification does not fully define these heuristics but
	suggests a number of candidates:</t>	suggests a number of candidates:</t>
	<ol spacing="normal" type="1">	<ol spacing="normal" type="1">

	<li>In a 6LoWPAN, just assume the Border Router (6LBR) can act as an	<li>In a 6LoWPAN, just assume the 6LBR can act as an
	RD (using the ABRO option to find that <xref target="RFC6775" format="default"/>	RD (using the Authoritative Border Router Option (ABRO) to find that <xref targe
	).	t="RFC6775" format="default"/>).
	Confirmation can be obtained by sending a unicast to	Confirmation can be obtained by sending a unicast to
	<tt>coap://[6LBR]/.well-known/core?rt=core.rd*</tt>.</li>	<tt>coap://[6LBR]/.well-known/core?rt=core.rd*</tt>.</li>

	<li>In a network that supports multicast well, discovering the RD usin	<li>In a network that supports multicast well, discover the RD using
	g	a multicast query for <tt>/.well-known/core</tt>, as specified in CoRE Link
	a multicast query for /.well-known/core as specified in CoRE Link	Format <xref target="RFC6690" format="default"/>, and send a Multicast GET to
	Format <xref target="RFC6690" format="default"/>: Sending a Multicast GET to	<tt>coap://[ff0x::fe]/.well-known/core?rt=core.rd*</tt>. RDs within the
	<tt>coap://[MCD1]/.well-known/core?rt=core.rd*</tt>. RDs within the
	multicast scope will answer the query.</li>	multicast scope will answer the query.</li>
	</ol>	</ol>

	<t>When answering a multicast request directed at a link-local group,
		<t>When answering a multicast request directed at a link-local group,
	the RD may want to respond from a routable address;	the RD may want to respond from a routable address;
	this makes it easier for registrants to use one of their own routable addresse s for registration.	this makes it easier for registrants to use one of their own routable addresse s for registration.

	When <xref target="RFC6724" format="default"/> is used for source address select	When source addresses are selected using the mechanism described in <xref target
	ion,	="RFC6724" format="default"/>,
	this can be achieved by applying the changes of its Section 10.4,	this can be achieved by applying the changes of its Section <xref target="RFC672
	picking public addresses in its Section 5 Rule 7,	4" section="10.4" sectionFormat="bare"/>,
	and superseding rule 8 with preferring the source address's precedence.</t>	picking public addresses in Rule 7 of its Section <xref target="RFC6724" section
		="5" sectionFormat="bare"/>,
		and superseding Rule 8 with preferring the source address's precedence.</t>
	<t>As some of the RD addresses obtained by the methods listed here are	<t>As some of the RD addresses obtained by the methods listed here are

	just (more or less educated) guesses, endpoints MUST make use of any	just (more or less educated) guesses, endpoints <bcp14>MUST</bcp14> make use of any
	error messages to very strictly rate-limit requests to candidate IP	error messages to very strictly rate-limit requests to candidate IP
	addresses that don't work out. For example, an ICMP Destination	addresses that don't work out. For example, an ICMP Destination
	Unreachable message (and, in particular, the port unreachable code for	Unreachable message (and, in particular, the port unreachable code for
	this message) may indicate the lack of a CoAP server on the candidate	this message) may indicate the lack of a CoAP server on the candidate

	host, or a CoAP error response code such as 4.05 "Method Not Allowed"	host, or a CoAP error response code, such as 4.05 (Method Not Allowed),
	may indicate unwillingness of a CoAP server to act as a directory	may indicate unwillingness of a CoAP server to act as a directory
	server.</t>	server.</t>
	<t>The following RD discovery mechanisms are recommended:</t>	<t>The following RD discovery mechanisms are recommended:</t>
	<ul spacing="normal">	<ul spacing="normal">

	<li>In managed networks with border routers that need stand-alone oper	<li>In managed networks with border routers that need stand-alone oper
	ation, the RDAO option is recommended (e.g. operational phase described in <xref	ation, the RDAO is recommended (e.g., the operational phase described in <xref t
	target="automation" format="default"/>).</li>	arget="automation" format="default"/>).</li>
	<li>In managed networks without border router (no Internet services av	<li>In managed networks without border routers (no Internet services a
	ailable), the use of a preconfigured anycast address is recommended (e.g. instal	vailable), the use of a preconfigured anycast address is recommended (e.g., the
	lation phase described in <xref target="automation" format="default"/>).</li>	installation phase described in <xref target="automation" format="default"/>).</
	<li>In networks managed using DNS-SD, the use of DNS-SD for discovery	li>
	as described in <xref target="rd-using-dnssd" format="default"/> is recommended.	<li>In networks managed using DNS-SD, the use of DNS-SD for discovery,
	</li>	as described in <xref target="rd-using-dnssd" format="default"/>, is recommende
		d.</li>
	</ul>	</ul>

	<t>The use of multicast discovery in mesh networks is NOT RECOMMENDED.</ t>	<t>The use of multicast discovery in mesh networks is <bcp14>NOT RECOMME NDED</bcp14>.</t>
	<section anchor="rdao" numbered="true" toc="default">	<section anchor="rdao" numbered="true" toc="default">
	<name>Resource Directory Address Option (RDAO)</name>	<name>Resource Directory Address Option (RDAO)</name>
	<t>The Resource Directory Address Option (RDAO) carries	<t>The Resource Directory Address Option (RDAO) carries
	information about the address of the RD in RAs (Router Advertisements) of IPv6 N eighbor Discovery (ND),	information about the address of the RD in RAs (Router Advertisements) of IPv6 N eighbor Discovery (ND),

	similar to how RDNSS options <xref target="RFC8106" format="default"/> are sent. This information is	similar to how Recursive DNS Server (RDNSS) options <xref target="RFC8106" forma t="default"/> are sent. This information is
	needed when endpoints cannot discover the RD with a link-local	needed when endpoints cannot discover the RD with a link-local

	or realm-local scope multicast address, for instance because the	or realm-local scope multicast address, for instance, because the
	endpoint and the RD are separated by a Border Router	endpoint and the RD are separated by a
	(6LBR). In many circumstances the availability of DHCP cannot be guaranteed eith	6LBR. In many circumstances, the availability of DHCP cannot be guaranteed
	er	during commissioning of the network either. The presence and the use of the RD i
	during commissioning of the network. The presence and the use of the RD is	s
	essential during commissioning.</t>	essential during commissioning.</t>

	<t>It is possible to send multiple RDAO options in one message,	<t>It is possible to send multiple RDAOs in one message,
	indicating as many RD addresses.</t>	indicating as many RD addresses.</t>
	<t>The RDAO format is:</t>	<t>The RDAO format is:</t>
	<figure anchor="fig-rdao">	<figure anchor="fig-rdao">
	<name>Resource Directory Address Option</name>	<name>Resource Directory Address Option</name>

	<artwork name="" type="" align="left" alt=""><![CDATA[	<artwork name="" type="ascii-art" align="left" alt=""><![CDATA[
	0 1 2 3	0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	\| Type \| Length = 3 \| Reserved \|	\| Type \| Length = 3 \| Reserved \|
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	\| Valid Lifetime \|	\| Valid Lifetime \|
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	\| \|	\| \|
	+ +	+ +
	\| \|	\| \|
	+ RD Address +	+ RD Address +
	\| \|	\| \|
	+ +	+ +
	\| \|	\| \|
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

		]]></artwork>
		</figure>


	Fields:	<t>Fields:</t>


	Type: TBD38	<dl indent="18">
		<dt>Type:</dt>
		<dd>41</dd>


	Length: 8-bit unsigned integer. The length of	<dt>Length:</dt>
	the option in units of 8 bytes.	<dd> 8-bit unsigned integer. The length of
	Always 3.	the option in units of 8 bytes.
		Always 3.</dd>


	Reserved: This field is unused. It MUST be	<dt>Reserved:</dt>
	initialized to zero by the sender and	<dd>This field is unused. It <bcp14>MUST</bcp14> be
	MUST be ignored by the receiver.	initialized to zero by the sender and
		<bcp14>MUST</bcp14> be ignored by the receiver.</dd>


	Valid Lifetime: 32-bit unsigned integer. The length of	<dt>Valid Lifetime:</dt>
	time in seconds (relative to	<dd>32-bit unsigned integer. The length of
	the time the packet is received) that	time in seconds (relative to
	this RD address is valid.	the time the packet is received) that
	A value of all zero bits (0x0) indicates	this RD address is valid.
	that this RD address	A value of all zero bits (0x0) indicates
	is not valid anymore.	that this RD address
		is not valid anymore.</dd>


	RD Address: IPv6 address of the RD.	<dt>RD Address:</dt>
	]]></artwork>	<dd> IPv6 address of the RD.</dd>
	</figure>	</dl>
	</section>	</section>
	<section anchor="rd-using-dnssd" numbered="true" toc="default">	<section anchor="rd-using-dnssd" numbered="true" toc="default">

	<name>Using DNS-SD to discover a Resource Directory</name>	<name>Using DNS-SD to Discover a Resource Directory</name>
	<t>An RD can advertise its presence in DNS-SD <xref target="RFC6763" f ormat="default"/>	<t>An RD can advertise its presence in DNS-SD <xref target="RFC6763" f ormat="default"/>

	using the service name <tt>_core-rd._udp</tt> (for CoAP), <tt>_core-rd-dtls._udp	using the service names defined in this document: <tt>_core-rd._udp</tt
	</tt> (for CoAP over DTLS),	> (for CoAP), <tt>_core-rd-dtls._udp</tt>
	<tt>_core-rd._tcp</tt> (for CoAP over TCP) or <tt>_core-rd-tls._tcp</tt> (for Co	(for CoAP over DTLS), <tt>_core-rd._tcp</tt> (for CoAP over TCP), or
	AP over TLS)	<tt>_core-rd-tls._tcp</tt> (for CoAP over TLS).
	defined in this document.	(For the WebSocket transports of CoAP, no service is defined,
	(For the WebSocket transports of CoAP, no service is defined	as DNS-SD is typically unavailable in environments where CoAP over WebS
	as DNS-SD is typically unavailable in environments where CoAP over WebSockets is	ockets is
	used).</t>	used.)</t>
	<t>The selection of the service indicates the protocol used, and	<t>The selection of the service indicates the protocol used, and
	the SRV record points the client to a host name and port to use as a starting po	the SRV record points the client to a host name and port to use as a st
	int for the URI discovery steps of <xref target="discovery" format="default"/>.<	arting point for
	/t>	the "URI discovery" steps of <xref target="discovery" format="default"/
	<t>This section is a simplified concrete application of the more gener	>.</t>
	ic mechanism	<t>This section is a simplified, concrete application of the more gene
		ric mechanism
	specified in <xref target="I-D.ietf-core-rd-dns-sd" format="default"/>.</t>	specified in <xref target="I-D.ietf-core-rd-dns-sd" format="default"/>.</t>
	</section>	</section>
	</section>	</section>
	<section anchor="payload-content-formats" numbered="true" toc="default">	<section anchor="payload-content-formats" numbered="true" toc="default">
	<name>Payload Content Formats</name>	<name>Payload Content Formats</name>

	<t>RDs implementing this specification MUST support the	<t>RDs implementing this specification <bcp14>MUST</bcp14> support the
	application/link-format content format (ct=40).</t>	<tt>application/link-format</tt> content format (ct=40).</t>
	<t>RDs implementing this specification MAY support additional content fo	<t>RDs implementing this specification <bcp14>MAY</bcp14> support additi
	rmats.</t>	onal content formats.</t>
	<t>Any additional content format supported by an RD implementing this	<t>Any additional content format supported by an RD implementing this

	specification SHOULD be able to express all the information expressible in link-	specification <bcp14>SHOULD</bcp14> be able to express all the information expre
	format.	ssible in link format.
	It MAY be able to express information that is inexpressible in link-format,	It <bcp14>MAY</bcp14> be able to express information that is inexpressible in li
	but those expressions SHOULD be avoided where possible.</t>	nk format,
		but those expressions <bcp14>SHOULD</bcp14> be avoided where possible.</t>
	</section>	</section>
	<section anchor="discovery" numbered="true" toc="default">	<section anchor="discovery" numbered="true" toc="default">
	<name>URI Discovery</name>	<name>URI Discovery</name>
	<t>Before an endpoint can make use of an RD, it must first know the RD's address	<t>Before an endpoint can make use of an RD, it must first know the RD's address

	and port, and the URI path information for its REST APIs. This section defines	and port and the URI path information for its REST APIs. This section defines
	discovery of the RD and its URIs using the well-known interface of the	discovery of the RD and its URIs using the well-known interface of the

	CoRE Link Format <xref target="RFC6690" format="default"/> after having discover ed a host as described in <xref target="finding_an_rd" format="default"/>.</t>	CoRE Link Format <xref target="RFC6690" format="default"/> after having discover ed a host, as described in <xref target="finding_an_rd" format="default"/>.</t>
	<t>Discovery of the RD registration URI is performed by sending either a multicast or	<t>Discovery of the RD registration URI is performed by sending either a multicast or

	unicast GET request to <tt>/.well-known/core</tt> and including a Resource Type (rt)	unicast GET request to <tt>/.well-known/core</tt> and including a resource type (rt)
	parameter <xref target="RFC6690" format="default"/> with the value "core.rd" in the query string. Likewise, a	parameter <xref target="RFC6690" format="default"/> with the value "core.rd" in the query string. Likewise, a

	Resource Type parameter value of "core.rd-lookup*" is used to discover the	resource type parameter value of "core.rd-lookup*" is used to discover the
	URIs for RD Lookup operations, core.rd* is used to discover all URIs for RD oper	URIs for RD lookup operations, and "core.rd*" is used to discover all URIs for R
	ations.	D operations.
	Upon success, the response will contain a payload with	Upon success, the response will contain a payload with
	a link format entry for each RD function discovered, indicating the URI	a link format entry for each RD function discovered, indicating the URI

	of the RD function returned and the corresponding Resource Type. When performing	of the RD function returned and the corresponding resource type. When performing
	multicast discovery, the multicast IP address used will depend on the scope requ ired	multicast discovery, the multicast IP address used will depend on the scope requ ired
	and the multicast capabilities of the network (see <xref target="mc-registration " format="default"/>).</t>	and the multicast capabilities of the network (see <xref target="mc-registration " format="default"/>).</t>

	<t>An RD MAY provide hints about the content-formats it supports in the	<t>An RD <bcp14>MAY</bcp14> provide hints about the content formats it s
	links it exposes or registers, using the "ct" target attribute, as shown in the	upports in the links it exposes or registers, using the "ct" target attribute, a
	example below. Clients MAY use these hints to select alternate content-formats f	s shown in the example below. Clients <bcp14>MAY</bcp14> use these hints to sele
	or interaction with the RD.</t>	ct alternate content formats for interaction with the RD.</t>
	<t>HTTP does not support multicast and consequently only unicast discove	<t>HTTP does not support multicast, and, consequently, only unicast disc
	ry can be supported	overy can be
	at the using the HTTP <tt>/.well-known/core</tt> resource.</t>	supported using the HTTP <tt>/.well-known/core</tt> resource.</t>
	<t>RDs implementing this specification MUST support query filtering for	<t>RDs implementing this specification <bcp14>MUST</bcp14> support query
	the rt parameter as defined in <xref target="RFC6690" format="default"/>.</t>	filtering for
		the rt parameter, as defined in <xref target="RFC6690" format="default"/>.</t>
	<t>While the link targets in this discovery step are often expressed in path-absolute form,	<t>While the link targets in this discovery step are often expressed in path-absolute form,
	this is not a requirement.	this is not a requirement.

	Clients of the RD SHOULD therefore accept URIs of all schemes they support,	Clients of the RD <bcp14>SHOULD</bcp14> therefore accept URIs of all schemes the y support,
	both as URIs and relative references,	both as URIs and relative references,
	and not limit the set of discovered URIs to those hosted at the address used for URI discovery.</t>	and not limit the set of discovered URIs to those hosted at the address used for URI discovery.</t>
	<t>With security policies where the client requires the RD to be authori zed to act as an RD,	<t>With security policies where the client requires the RD to be authori zed to act as an RD,
	that authorization may be limited to resources on which the authorized RD advert ises the adequate resource types.	that authorization may be limited to resources on which the authorized RD advert ises the adequate resource types.

	Clients that have obtained links they can not rely on yet	Clients that have obtained links they cannot rely on yet
	can repeat the URI discovery step at the /.well-known/core resource of the indic	can repeat the "URI discovery" step at the <tt>/.well-known/core</tt> resource o
	ated host	f the indicated host
	to obtain the resource type information from an authorized source.</t>	to obtain the resource type information from an authorized source.</t>

	<t>The URI Discovery operation can yield multiple URIs of a given resour	<t>The URI discovery operation can yield multiple URIs of a given resour
	ce type.	ce type.
	The client of the RD can use any of the discovered addresses initially.</t>	The client of the RD can try out any of the discovered addresses.</t>
	<t>The discovery request interface is specified as follows	<t>The discovery request interface is specified as follows

	(this is exactly the Well-Known Interface of <xref target="RFC6690" format="defa	(this is exactly the well-known interface of <xref target="RFC6690" section="4"
	ult"/> Section 4,	sectionFormat="comma"/>,
	with the additional requirement that the server MUST support query filtering):</	with the additional requirement that the server <bcp14>MUST</bcp14> suppo
	t>	rt query filtering):</t>
	<dl>	<dl>
	<dt>	<dt>Interaction:</dt>
	Interaction: </dt>	<dd> EP, CT, or Client -> RD </dd>
	<dd>	<dt>Method:</dt>
	<t>EP, CT or Client -> RD</t>	<dd>GET</dd>
	</dd>	<dt>URI Template:</dt>
	<dt>	<dd><tt>/.well-known/core{?rt}</tt></dd>
	Method: </dt>	<dt>URI Template Variables:</dt>
	<dd>	<dd>
	<t>GET</t>	<t><br/></t>
	</dd>	<dl>
	<dt>	<dt>rt := </dt>
	URI Template: </dt>	<dd>Resource Type. <bcp14>SHOULD</bcp14> contain one of the values "core
	<dd>	.rd",
	<t>/.well-known/core{?rt}</t>	"core.rd-lookup*",
	</dd>	"core.rd-lookup-res", "core.rd-lookup-ep", or "core.rd*"</dd>
	<dt>	</dl>
	URI Template Variables: </dt>	</dd>
	<dd>	<dt>Accept:</dt>
	<dl>	<dd>absent, <tt>application/link-format</tt>, or any other media type represen
	<dt>	ting web links</dd>
	rt := </dt>	</dl>
	<dd>
	<t>Resource Type. SHOULD contain one of the values "core.rd", "c	<t>The following response is expected on this interface:</t>
	ore.rd-lookup*",
	"core.rd-lookup-res", "core.rd-lookup-ep", or "core.rd*"</t>	<dl>
	</dd>	<dt>Success:</dt>
	</dl>	<dd>2.05 (Content) or 200 (OK) with an
	</dd>	<tt>application/link-format</tt> or other web link payload containing one or m
	<dt>	ore matching entries for the RD resource.</dd>
	Accept: </dt>	</dl>
	<dd>
	<t>absent, application/link-format or any other media type represent	<t>The following example shows an endpoint discovering an RD using this interfac
	ing web links</t>	e,
	</dd>	thus learning that the directory resource location in this example is /rd and th
	</dl>	at the
	<t>The following response is expected on this interface:</t>	content format delivered by the server hosting the resource is <tt>application/l
	<dl>	ink-format</tt>
	<dt>	(ct=40). Note that it is up to the RD to choose its RD locations.</t>
	Success: </dt>
	<dd>	<figure anchor="example-discovery">
	<t>2.05 "Content" or 200 "OK" with an	<name>Example Discovery Exchange</name>
	application/link-format or other web link payload containing one or more matchin	<sourcecode type=""><![CDATA[
	g entries for the RD resource.</t>	Req: GET coap://[ff02::fe]/.well-known/core?rt=core.rd*
	</dd>
	</dl>
	<t>The following example shows an endpoint discovering an RD using this
	interface,
	thus learning that the directory resource location, in this example, is /rd, and
	that the
	content-format delivered by the server hosting the resource is application/link-
	format
	(ct=40). Note that it is up to the RD to choose its RD locations.</t>
	<figure anchor="example-discovery">
	<name>Example discovery exchange</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[
	Req: GET coap://[MCD1]/.well-known/core?rt=core.rd*

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	</rd>;rt=core.rd;ct=40,	</rd>;rt=core.rd;ct=40,
	</rd-lookup/ep>;rt=core.rd-lookup-ep;ct=40,	</rd-lookup/ep>;rt=core.rd-lookup-ep;ct=40,
	</rd-lookup/res>;rt=core.rd-lookup-res;ct=40	</rd-lookup/res>;rt=core.rd-lookup-res;ct=40

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>

	<t>The following example shows the way of indicating that a client may r	<t>The following example shows the way of indicating that a client may re
	equest	quest
	alternate content-formats. The Content-Format code attribute "ct" MAY include a	alternate content formats. The Content-Format code attribute "ct" <bcp14>
	space-separated sequence of Content-Format codes as specified in	MAY</bcp14>
	Section 7.2.1 of <xref target="RFC7252" format="default"/>, indicating that mult	include a space-separated sequence of Content-Format codes, as specified
	iple content-formats are available.	in <xref
	The example below shows the required Content-Format 40 (application/link-format)	target="RFC7252" section="7.2.1" sectionFormat="of"/>, indicating that mu
	indicated as well as a CBOR and JSON representation from <xref target="I-D.ietf-	ltiple
	core-links-json" format="default"/>	content formats are available. The example below shows the required Conte
	(which have no numeric values assigned yet, so they are shown as TBD64 and TBD50	nt-Format 40
	4 as in that draft).	(<tt>application/link-format</tt>) indicated, as well as Concise Binary O
	The RD resource locations /rd, and /rd-lookup are example values.	bject Representation (CBOR)
	The server in this example also indicates that it is capable of providing observ	and JSON representations in the style of <xref target="I-D.ietf-core-link
	ation on resource lookups.</t>	s-json" format="default"/>
		(for which the experimental values 65060 and 65050 are used in this examp
		le). The RD resource locations /rd
		and /rd-lookup are example values. The server in this example also indica
		tes that
		it is capable of providing observation on resource lookups.</t>
	<figure anchor="example-discovery-ct">	<figure anchor="example-discovery-ct">

	<name>Example discovery exchange indicating additional content-formats	<name>Example Discovery Exchange Indicating Additional Content-Formats
	</name>	</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET coap://[MCD1]/.well-known/core?rt=core.rd*	Req: GET coap://[ff02::fe]/.well-known/core?rt=core.rd*

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:

	</rd>;rt=core.rd;ct="40 65225",	</rd>;rt=core.rd;ct=40,
	</rd-lookup/res>;rt=core.rd-lookup-res;ct="40 TBD64 TBD504";obs,	</rd-lookup/res>;rt=core.rd-lookup-res;ct="40 65060 65050";obs,
	</rd-lookup/ep>;rt=core.rd-lookup-ep;ct="40 TBD64 TBD504"	</rd-lookup/ep>;rt=core.rd-lookup-ep;ct="40 65060 65050"
	]]></artwork>	]]></sourcecode>
	</figure>	</figure>

	<t>For maintenance, management and debugging,	<t>For maintenance, management, and debugging,
	it can be useful to identify the components that constitute the RD server.	it can be useful to identify the components that constitute the RD server.
	The identification can be used to find client-server incompatibilities,	The identification can be used to find client-server incompatibilities,

	supported features, required updates and other aspects.	supported features, required updates, and other aspects.
	The Well-Known interface described in Section 4 of <xref target="RFC6690" format	The well-known interface described in <xref target="RFC6690" section="4" section
	="default"/> can be used to find such data.</t>	Format="of"/> can be used to find such data.</t>
	<t>It	<t>It would typically be stored in an implementation information link
	would typically be stored in an implementation information link	(as described in <xref target="I-D.bormann-t2trg-rel-impl" format="default"/>).<
	(as described in <xref target="I-D.bormann-t2trg-rel-impl" format="default"/>):<	/t>
	/t>
	<figure anchor="example-impl-discovery">	<figure anchor="example-impl-discovery">

	<name>Example exchange of obtaining implementation information, using	<name>Example Exchange of Obtaining Implementation Information Using t
	the relation type currently proposed in the work-in-progress document</name>	he Relation Type Currently Proposed in <xref target="I-D.bormann-t2trg-rel-impl"
	<artwork name="" type="" align="left" alt=""><![CDATA[	format="default"/></name>
		<sourcecode type=""><![CDATA[
	Req: GET /.well-known/core?rel=impl-info	Req: GET /.well-known/core?rel=impl-info

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	<http://software.example.com/shiny-resource-directory/1.0beta1>;	<http://software.example.com/shiny-resource-directory/1.0beta1>;
	rel=impl-info	rel=impl-info

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>

	<t>Note that depending on the particular server's architecture,	<t>Note that, depending on the particular server's architecture,
	such a link could be anchored at the RD server's root	such a link could be anchored at the RD server's root

	(as in this example), or	(as in this example) or
	at individual RD components.	at individual RD components.
	The latter is to be expected when different applications	The latter is to be expected when different applications
	are run on the same server.</t>	are run on the same server.</t>
	</section>	</section>
	</section>	</section>
	<section anchor="registration" numbered="true" toc="default">	<section anchor="registration" numbered="true" toc="default">
	<name>Registration</name>	<name>Registration</name>

	<t>After discovering the location of an RD, a registrant-ep or CT MAY	<t>After discovering the location of an RD, a registrant-EP or CT <bcp14>M
	register the resources of the registrant-ep using the registration interface. Th	AY</bcp14>
	is interface	register the resources of the registrant-EP using the registration interface. Th
		is interface
	accepts a POST from an endpoint containing the list of resources to be added	accepts a POST from an endpoint containing the list of resources to be added
	to the directory as the message payload in the CoRE Link Format <xref target="RF C6690" format="default"/> or other representations of web links, along with quer y	to the directory as the message payload in the CoRE Link Format <xref target="RF C6690" format="default"/> or other representations of web links, along with quer y

	parameters indicating the name of the endpoint, and optionally the sector,	parameters indicating the name of the endpoint and, optionally, the sector,
	lifetime and base URI of the registration.	lifetime, and base URI of the registration.
	It is expected that other specifications will define further parameters (see	It is expected that other specifications will define further parameters (see

	<xref target="iana-registry" format="default"/>). The RD then creates a new regi stration resource in the RD and returns its location. The receiving endpoint MUS T use that	<xref target="iana-registry" format="default"/>). The RD then creates a new regi stration resource in the RD and returns its location. The receiving endpoint <bc p14>MUST</bcp14> use that
	location when refreshing registrations using this interface. Registration	location when refreshing registrations using this interface. Registration
	resources in the RD are kept active for the period indicated by the lifetime	resources in the RD are kept active for the period indicated by the lifetime
	parameter. The creating endpoint is responsible for refreshing the registration resource within this	parameter. The creating endpoint is responsible for refreshing the registration resource within this

	period using either the registration or update interface. The registration	period, using either the registration or update interface. The registration
	interface MUST be implemented to be idempotent, so that registering twice	interface <bcp14>MUST</bcp14> be implemented to be idempotent, so that registeri
		ng twice
	with the same endpoint parameters ep and d (sector) does not create multiple reg istration resources.</t>	with the same endpoint parameters ep and d (sector) does not create multiple reg istration resources.</t>
	<t>The following rules apply for a registration request targeting a given (ep, d) value pair:</t>	<t>The following rules apply for a registration request targeting a given (ep, d) value pair:</t>
	<ul spacing="normal">	<ul spacing="normal">

	<li>When the (ep, d) value pair of the registration-request is different from any existing registration,	<li>When the (ep, d) value pair of the registration request is different from any existing registration,
	a new registration is generated.</li>	a new registration is generated.</li>

	<li>When the (ep, d) value pair of the registration-request is equal to an existing registration,	<li>When the (ep, d) value pair of the registration request is equal to an existing registration,
	the content and parameters of the existing registration are replaced with the co ntent of the registration request.	the content and parameters of the existing registration are replaced with the co ntent of the registration request.

	Like the later changes to registration resources, security policies (<xref targe t="policies" format="default"/>) usually require such requests to come from the same device.</li>	As with changes to registration resources, security policies (<xref target="poli cies" format="default"/>) usually require such requests to come from the same de vice.</li>
	</ul>	</ul>
	<t>The posted link-format document can (and typically does) contain relati ve references	<t>The posted link-format document can (and typically does) contain relati ve references

	both in its link targets and in its anchors, or contain empty anchors.	both in its link targets and in its anchors; it can also contain empty anchors.
	The RD server needs to resolve these references in order to faithfully represent them in lookups.	The RD server needs to resolve these references in order to faithfully represent them in lookups.
	They are resolved against the base URI of the registration,	They are resolved against the base URI of the registration,

	which is provided either explicitly in the <tt>base</tt> parameter or constructe d implicitly from the requester's URI as constructed from its network address an d scheme.</t>	which is provided either explicitly in the <tt>base</tt> parameter or constructe d implicitly from the requester's URI, as constructed from its network address a nd scheme.</t>
	<t>For media types to which <xref target="limitedlinkformat" format="defau lt"/> applies	<t>For media types to which <xref target="limitedlinkformat" format="defau lt"/> applies

	(i.e. documents in application/link-format),	(i.e., documents in <tt>application/link-format</tt>),
	request bodies MUST be expressed in Limited Link Format.</t>	request bodies <bcp14>MUST</bcp14> be expressed in Limited Link Format.</t>
	<t>The registration request interface is specified as follows:</t>	<t>The registration request interface is specified as follows:</t>


	<dl>	<dl>

	<dt>	<dt>Interaction:</dt>
	Interaction: </dt>	<dd>EP or CT -> RD</dd>
	<dd>	<dt>Method:</dt>
	<t>EP or CT -> RD</t>	<dd>POST</dd>
	</dd>	<dt>URI Template:</dt>
	<dt>	<dd>{+rd}{?ep,d,lt,base,extra-attrs*}</dd>
	Method: </dt>	<dt>URI Template Variables:</dt>
	<dd>	<dd>
	<t>POST</t>	<t><br/></t>
	</dd>	<dl>
	<dt>	<dt>rd :=</dt>
	URI Template: </dt>	<dd>RD registration URI
	<dd>	(mandatory). This is the location of
	<t>{+rd}{?ep,d,lt,base,extra-attrs*}</t>	the RD, as obtained from discovery.</dd>
	</dd>	<dt>ep :=</dt>
	<dt>
	URI Template Variables: </dt>	<dd><t>Endpoint name (mostly mandatory). The endpoint name is an ide
	<dd>	ntifier
	<dl>	that <bcp14>MUST</bcp14> be unique within a sector.</t>
	<dt>
	rd := </dt>	<t>As the endpoint name is a Unicode string, it is
	<dd>	encoded in UTF-8 (and possibly percent encoded) during
	<t>RD registration URI	variable expansion (see <xref target="RFC6570" section="3.2.1"
	(mandatory). This is the location of	sectionFormat="comma"/>). The endpoint name <bcp14>MUST NOT</bcp14>
	the RD, as obtained from discovery.</t>	contain any
	</dd>	character in the inclusive ranges 0-31 or 127-159.</t>
	<dt>
	ep := </dt>	<t>The maximum length of this parameter is 63 bytes encoded in UTF-8
	<dd>	.</t>
	<t>Endpoint name (mostly mandatory). The endpoint name is an ident	<t>If the RD is configured to recognize the endpoint that is to be a
	ifier	uthorized to use
	that MUST be unique within a sector.	exactly
	</t>	one endpoint name, the RD assigns that name. In that case, giving the
	<t>As the endpoint name is a Unicode string,	endpoint name
	it is encoded in UTF-8 (and possibly pct-encoded) during variable expansion (see	becomes optional for the client; if the client gives any other endpoi
	<xref target="RFC6570" format="default"/> Section 3.2.1).	nt name, it is
	The endpoint name MUST NOT contain any character in the inclusive ranges 0-31 or	not authorized to perform the registration.</t>
	127-159.</t>
	<t>The maximum length of this parameter is 63 UTF-8 encoded bytes.
	</t>
	<t>If the RD is configured to recognize the endpoint to be authori
	zed to use exactly one endpoint name, the RD assigns that name.
	In that case, giving the endpoint name becomes optional for the client;
	if the client gives any other endpoint name, it is not authorized to perform the
	registration.</t>
	</dd>
	<dt>
	d := </dt>
	<dd>
	<t>Sector (optional). The sector to which this endpoint belongs.
	When this parameter is not present, the
	RD MAY associate the endpoint with a configured default sector
	(possibly based on the endpoint's authorization)
	or leave it empty.
	</t>
	<t>The sector is encoded like the ep parameter, and is limited to
	63 UTF-8 encoded bytes as well.</t>
	</dd>
	<dt>
	lt := </dt>
	<dd>
	<t>Lifetime (optional). Lifetime of the registration in seconds. R
	ange of 1-4294967295.
	If no lifetime is included in the initial registration, a default value of
	90000 (25 hours) SHOULD be assumed.</t>
	</dd>
	<dt>
	base := </dt>
	<dd>
	<t>Base URI (optional). This parameter sets the base URI of the re
	gistration, under which
	the relative links in the payload are to be interpreted. The specified URI typic
	ally does not have a path component of its own, and MUST be suitable as a base U
	RI to resolve any relative references given in the registration. The parameter i
	s therefore usually of the shape "scheme://authority" for
	HTTP and CoAP URIs.
	The URI SHOULD NOT have a query or fragment component
	as any non-empty relative part in a reference would remove those parts from the
	resulting URI.</t>
	</dd>
	<dt/>
	<dd>
	<t>In the absence of this parameter the scheme of the protocol, so
	urce address
	and source port of the registration request are assumed.
	The Base URI is consecutively constructed by concatenating the used protocol's s
	cheme
	with the characters "://", the requester's source address as an address
	literal and ":" followed by its port (if it was not the protocol's default
	one) in analogy to <xref target="RFC7252" format="default"/> Section 6.5.</t>
	</dd>
	<dt/>
	<dd>
	<t>This parameter is
	mandatory when the directory is filled by a third party such as an
	commissioning tool.</t>
	</dd>
	<dt/>
	<dd>
	<t>If the registrant-ep uses an ephemeral port to register with, i
	t MUST include the base
	parameter in the registration to provide a valid network path.</t>
	</dd>
	<dt/>
	<dd>
	<t>A registrant that cannot be reached by potential lookup clients
	at the address it registers from
	(e.g. because it is behind some form of Network Address Translation (NAT))
	MUST provide a reachable base address with its registration.</t>
	</dd>	</dd>

	<dt/>
	<dd>	<dt>d :=</dt>
	<t>If the Base URI contains a link-local IP literal, it MUST NOT c	<dd><t>Sector (optional). This is the sector to which this endpoint
	ontain a Zone Identifier,	belongs.
	and MUST be local to the link on which the registration request is received.</t>	When this parameter is not present, the
		RD <bcp14>MAY</bcp14> associate the endpoint with a configured defaul
		t sector
		(possibly based on the endpoint's authorization)
		or leave it empty.</t>
		<t>The sector is encoded like the ep parameter and is limited to 63 b
		ytes encoded in
		UTF-8 as well.</t>
	</dd>	</dd>

	<dt/>
	<dd>	<dt>lt := </dt>
		<dd>Lifetime (optional). This is the lifetime of the registration in
		seconds, with a
		range of 1-4294967295.
		If no lifetime is included in the initial registration, a default val
		ue of
		90000 (25 hours) <bcp14>SHOULD</bcp14> be assumed.</dd>
		<dt>base :=</dt>
		<dd>
		<t>Base URI (optional). This parameter sets the base URI of the reg
		istration, under
		which the relative links in the payload are to be interpreted. The
		specified URI
		typically does not have a path component of its own and <bcp14>MUST
		</bcp14> be
		suitable as a base URI to resolve any relative references given in
		the registration.
		The parameter is therefore usually of the shape "scheme://authority
		" for
		HTTP and CoAP URIs.
		The URI <bcp14>SHOULD NOT</bcp14> have a query or fragment componen
		t,
		as any non-empty relative part in a reference would remove those pa
		rts from the
		resulting URI.</t>
		<t>In the absence of this parameter, the scheme of the protocol, th
		e source address,
		and the source port of the registration request are assumed.
		The base URI is consecutively constructed by concatenating the used
		protocol's scheme
		with the characters "://", the requester's source address as an add
		ress literal,
		and ":" followed by its port (if it was not the protocol's default
		one). This is analogous to the process described in <xref target="R
		FC7252" section="6.5" sectionFormat="comma"/>.</t>
		<t>This parameter is
		mandatory when the directory is filled by a third party, such as a
		commissioning tool.</t>
		<t>If the registrant-EP uses an ephemeral port to register with, i
		t
		<bcp14>MUST</bcp14> include the base
		parameter in the registration to provide a valid network path.</t>
		<t>A registrant that cannot be reached by potential lookup clients
		at the address it
		registers from
		(e.g., because it is behind some form of Network Address Translatio
		n (NAT))
		<bcp14>MUST</bcp14> provide a reachable base address with its regis
		tration.</t>
		<t>If the base URI contains a link-local IP literal, it <bcp14>MUS
		T NOT</bcp14>
		contain a Zone Identifier
		and <bcp14>MUST</bcp14> be local to the link on which the registrat
		ion request is
		received.</t>
	<t>Endpoints that register with a base that contains a path compon ent	<t>Endpoints that register with a base that contains a path compon ent

	cannot efficiently express their registrations in Limited Link Format (<xref tar	cannot efficiently express their registrations in Limited Link Form
	get="limitedlinkformat" format="default"/>).	at (<xref
	Those applications should use different representations of links to which <xref	target="limitedlinkformat" format="default"/>).
	target="limitedlinkformat" format="default"/> is not applicable	Those applications should use different representations of links to
	(e.g. <xref target="I-D.hartke-t2trg-coral" format="default"/>).	which <xref
	<!-- or may just use unlimited link format if there is indication that the serve	target="limitedlinkformat" format="default"/> is not applicable
	r is not strict about it -->	(e.g., <xref target="I-D.ietf-core-coral" format="default"/>). </t>
	</t>	</dd>
	</dd>
	<dt>	<dt>extra-attrs :=</dt>
	extra-attrs := </dt>	<dd>Additional registration attributes (optional). The endpoint can
	<dd>	pass any
	<t>Additional registration attributes (optional). The endpoint can	parameter registered in <xref target="iana-registry" format="defaul
	pass any	t"/> to the
	parameter registered at <xref target="iana-registry" format="default"/> to the d	directory. If the RD is
	irectory. If the RD is	aware of the parameter's specified semantics, it processes the para
	aware of the parameter's specified semantics, it processes it accordingly.	meter accordingly.
	Otherwise, it MUST store the unknown key and its value(s) as an endpoint	Otherwise, it <bcp14>MUST</bcp14> store the unknown key and its val
	attribute for further lookup.</t>	ue(s) as an
	</dd>	endpoint attribute for further lookup.</dd>
	</dl>	</dl>
	</dd>	</dd>
	<dt>	<dt>Content-Format:</dt>
	Content-Format: </dt>	<dd><tt>application/link-format</tt> or any other indicated media type
	<dd>	representing web links</dd>
	<t>application/link-format or any other indicated media type represent	</dl>
	ing web links</t>	<t>The following response is expected on this interface:</t>
	</dd>	<dl>
	</dl>	<dt>Success:</dt>
	<t>The following response is expected on this interface:</t>	<dd>
	<dl>	<t>2.01 (Created) or 201 (Created). The Location-Path option or Locat
	<dt>	ion header
	Success: </dt>	field <bcp14>MUST</bcp14> be included in the response. This location
	<dd>	<bcp14>MUST</bcp14> be a stable identifier generated by the RD, as it
	<t>2.01 "Created" or 201 "Created". The Location-Path option or Locati	is used
	on header field	for all subsequent operations on this registration resource. The regi
	MUST be included in the response. This location MUST be a stable identifier	stration
	generated by the RD as it is used for all subsequent	resource location thus returned is for the purpose of updating the li
	operations on this registration resource. The registration resource location thu	fetime
	s returned is for the purpose of updating the lifetime	of the registration and for maintaining the content of the
	of the registration and for maintaining the content of the	registered links, including updating and deleting links.</t>
	registered links, including updating and deleting links.</t>	<t>A registration with an already-registered ep and d value pair
	</dd>	responds with the same success code and location as the original regi
	<dt/>	stration;
	<dd>	the set of links registered with the endpoint is replaced with the li
	<t>A registration with an already registered ep and d value pair	nks
	responds with the same success code and location as the original registration;	from the payload.</t>
	the set of links registered with the endpoint is replaced with the links	<t>The location <bcp14>MUST NOT</bcp14> have a query or fragment comp
	from the payload.</t>	onent,
	</dd>	as that could conflict with query parameters during the registration
	<dt/>	update
	<dd>	operation. Therefore, the Location-Query option <bcp14>MUST NOT</bcp1
	<t>The location MUST NOT have a query or fragment component,	4> be
	as that could conflict with query parameters during the Registration Update oper	present in a successful response.</t>
	ation.	</dd>
	Therefore, the Location-Query option MUST NOT be present in a successful respons	</dl>
	e.</t>
	</dd>	<t>If the registration fails, including request timeouts,
	</dl>	or if delays from Service Unavailable responses with Max-Age or Retry
	<t>If the registration fails, including request timeouts,	-After
	or if delays from Service Unavailable responses with Max-Age or Retry-After	accumulate to exceed the registrant's configured timeouts,
	accumulate to exceed the registrant's configured timeouts,	it <bcp14>SHOULD</bcp14> pick another registration URI from the "URI
	it SHOULD pick another registration URI from the "URI Discovery" step	discovery" step of <xref target="discovery"/>,
	and if there is only one or the list is exhausted,	and, if there is only one or the list is exhausted,
	pick other choices from the "Finding a Resource Directory" step.	pick other choices from the "finding a resource directory" step of <
	Care has to be taken to consider the freshness of results obtained earlier,	xref target="finding_an_rd"/>.
	e.g. of the result of a <tt>/.well-known/core</tt> response,	Care has to be taken to consider the freshness of results obtained ea
	the lifetime of an RDAO option and	rlier,
	of DNS responses.	e.g., the result of a <tt>/.well-known/core</tt> response,
	Any rate limits and persistent errors from the "Finding a Resource Directory" st	the lifetime of an RDAO, and DNS responses.
	ep	Any rate limits and persistent errors from the "finding a resource di
	must be considered for the whole registration time,	rectory" step
	not only for a single operation.</t>	must be considered for the whole registration time,
	<t>The following example shows a registrant-ep with the name "node1" regis	not only for a single operation.</t>
	tering	<t>The following example shows a registrant-EP with the name "node1"
	two resources to an RD using this interface. The location "/rd"	registering
	is an example RD location discovered in a request similar to <xref target="examp	two resources to an RD using this interface. The location "/rd"
	le-discovery" format="default"/>.</t>	is an example RD location discovered in a request similar to <xref
	<figure anchor="example-payload">	target="example-discovery" format="default"/>.</t>
	<name>Example registration payload</name>	<figure anchor="example-payload">
	<artwork name="" type="" align="left" alt=""><![CDATA[	<name>Example Registration Payload</name>
		<sourcecode type=""><![CDATA[
	Req: POST coap://rd.example.com/rd?ep=node1	Req: POST coap://rd.example.com/rd?ep=node1
	Content-Format: 40	Content-Format: 40
	Payload:	Payload:
	</sensors/temp>;rt=temperature-c;if=sensor,	</sensors/temp>;rt=temperature-c;if=sensor,
	<http://www.example.com/sensors/temp>;	<http://www.example.com/sensors/temp>;
	anchor="/sensors/temp";rel=describedby	anchor="/sensors/temp";rel=describedby

	Res: 2.01 Created	Res: 2.01 Created
	Location-Path: /rd/4521	Location-Path: /rd/4521

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>

	<t>An RD may optionally support HTTP. Here is an example of almost the sam	<t>An RD may optionally support HTTP. Here is an example of almost the sam
	e registration operation above, when done using HTTP.</t>	e registration operation above when done using HTTP.</t>

	<figure anchor="example-payload-http">	<figure anchor="example-payload-http">

	<name>Example registration payload as expressed using HTTP</name>	<name>Example Registration Payload as Expressed Using HTTP</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req:	Req:
	POST /rd?ep=node1&base=http://[2001:db8:1::1] HTTP/1.1	POST /rd?ep=node1&base=http://[2001:db8:1::1] HTTP/1.1
	Host: rd.example.com	Host: rd.example.com
	Content-Type: application/link-format	Content-Type: application/link-format

	</sensors/temp>;rt=temperature-c;if=sensor,	</sensors/temp>;rt=temperature-c;if=sensor,
	<http://www.example.com/sensors/temp>;	<http://www.example.com/sensors/temp>;
	anchor="/sensors/temp";rel=describedby	anchor="/sensors/temp";rel=describedby

	Res:	Res:
	HTTP/1.1 201 Created	HTTP/1.1 201 Created
	Location: /rd/4521	Location: /rd/4521

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	<section anchor="simple" numbered="true" toc="default">	<section anchor="simple" numbered="true" toc="default">
	<name>Simple Registration</name>	<name>Simple Registration</name>

	<t>Not all endpoints hosting resources are expected to know how to uploa d links to an RD as described in <xref target="registration" format="default"/>. Instead, simple endpoints can implement the Simple Registration approach descri bed in this section. An RD implementing this specification MUST implement Simple Registration. However, there may	<t>Not all endpoints hosting resources are expected to know how to uploa d links to an RD, as described in <xref target="registration" format="default"/> . Instead, simple endpoints can implement the simple registration approach descr ibed in this section. An RD implementing this specification <bcp14>MUST</bcp14> implement simple registration. However, there may
	be security reasons why this form of directory discovery would be disabled.</t>	be security reasons why this form of directory discovery would be disabled.</t>

	<t>This approach requires that the registrant-ep makes available the hos	<t>This approach requires that the registrant-EP makes available the hos
	ted resources	ted resources
	that it wants to be discovered, as links on its <tt>/.well-known/core</tt> inter	that it wants to be discovered as links on its <tt>/.well-known/core</tt> interf
	face as	ace, as
	specified in <xref target="RFC6690" format="default"/>.	specified in <xref target="RFC6690" format="default"/>.
	The links in that document are subject to the same limitations as the payload of a registration	The links in that document are subject to the same limitations as the payload of a registration
	(with respect to <xref target="limitedlinkformat" format="default"/>).</t>	(with respect to <xref target="limitedlinkformat" format="default"/>).</t>
	<ul spacing="normal">	<ul spacing="normal">

	<li>The registrant-ep finds one or more addresses of the directory ser ver as described in <xref target="finding_an_rd" format="default"/>.</li>	<li>The registrant-EP finds one or more addresses of the directory ser ver, as described in <xref target="finding_an_rd" format="default"/>.</li>
	<li>	<li>

	<t>The registrant-ep sends (and regularly refreshes with) a POST	<t>The registrant-EP sends (and regularly refreshes with) a POST
	request to the <tt>/.well-known/rd</tt> URI of the directory server of choice. T	request to the <tt>/.well-known/rd</tt> URI of the directory server o
	he body of the POST request is empty, and triggers the resource	f choice. The
	directory server to perform GET requests at the requesting registrant-ep's /.wel	body of the POST request is empty and triggers the resource
	l-known/core to obtain the link-format payload to register. </t>	directory server to perform GET requests (redone before lifetime expi
	<t>	ry) at the requesting registrant-EP's
	The registrant-ep includes the same registration parameters in the POST request	<tt>/.well-known/core</tt> to obtain the link-format payload to regis
	as it would with a regular registration per <xref target="registration" format="	ter. </t>
	default"/>. The registration base URI of the registration is taken from the regi	<t>The registrant-EP includes the same registration parameters in the
	strant-ep's network address (as is default with regular registrations). </t>	POST request as
	<t>	it would with a regular registration, per <xref target="registration"
	Example request from registrant-EP to RD (unanswered until the next step):</t>	format="default"/>. The registration base URI of the registration is
	</li>	taken from the
	</ul>	registrant-EP's network address (as is default with regular registrat
		ions). </t>
		<t>The following is an example request from the registrant-EP to the
		RD (unanswered
		until the next step):</t>

	<figure anchor="example-simple1">	<figure anchor="example-simple1">

	<name>First half example exchange of a simple registration</name>	<name>First-Half Example Exchange of a Simple Registration</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: POST /.well-known/rd?lt=6000&ep=node1	Req: POST /.well-known/rd?lt=6000&ep=node1
	(No payload)	(No payload)

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>

	<ul spacing="normal">	</li>
	<li>	<li>

	<t>The RD queries the registrant-ep's discovery resource to determin	<t>The RD queries the registrant-EP's discovery resource to determin
	e the success of the operation.	e the success of
	It SHOULD keep a cache of the discovery resource and not query it again as long	the operation. It <bcp14>SHOULD</bcp14> keep a cache of the discovery
	as it is fresh. </t>	resource and not
	<t>	query it again as long as it is fresh. </t>
	Example request from the RD to the registrant-EP:</t>	<t>The following is an example request from the RD to the registrant
	</li>	-EP:</t>
	</ul>
	<figure anchor="example-simple2">	<figure anchor="example-simple2">

	<name>Example exchange of the RD querying the simple endpoint</name>	<name>Example Exchange of the RD Querying the Simple Endpoint</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET /.well-known/core	Req: GET /.well-known/core
	Accept: 40	Accept: 40

	Res: 2.05 Content	Res: 2.05 Content
	Content-Format: 40	Content-Format: 40
	Payload:	Payload:
	</sen/temp>	</sen/temp>

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
		</li>
		</ul>

	<t>With this response, the RD would answer the previous step's request:< /t>	<t>With this response, the RD would answer the previous step's request:< /t>
	<figure anchor="example-simple3">	<figure anchor="example-simple3">

	<name>Second half example exchange of a simple registration</name>	<name>Second-Half Example Exchange of a Simple Registration</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Res: 2.04 Changed	Res: 2.04 Changed

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	<t>The sequence of fetching the registration content before sending a su ccessful response	<t>The sequence of fetching the registration content before sending a su ccessful response
	was chosen to make responses reliable,	was chosen to make responses reliable,
	and the point about caching was chosen to still allow very constrained registran ts.	and the point about caching was chosen to still allow very constrained registran ts.

	Registrants MUST be able to serve a GET request to <tt>/.well-known/core</tt> af	Registrants <bcp14>MUST</bcp14> be able to serve a GET request to <tt>/.well-kno
	ter having requested registration.	wn/core</tt> after having requested registration.
	Constrained devices MAY regard the initial request as temporarily failed when th	Constrained devices <bcp14>MAY</bcp14> regard the initial request as temporarily
	ey need RAM occupied by their own request to serve the RD's GET,	failed when they need RAM occupied by their own request to serve the RD's GET
	and retry later when the RD already has a cached representation of their discove ry resources.	and retry later when the RD already has a cached representation of their discove ry resources.

	Then, the RD can reply immediately and the registrant can receive the response.< /t>	Then, the RD can reply immediately, and the registrant can receive the response. </t>
	<t>The simple registration request interface is specified as follows:</t >	<t>The simple registration request interface is specified as follows:</t >

	<dl>	<dl>
	<dt>	<dt>Interaction:</dt>
	Interaction: </dt>	<dd>EP -> RD</dd>
	<dd>	<dt>Method: </dt>
	<t>EP -> RD</t>	<dd>POST</dd>
	</dd>	<dt>URI Template: </dt>
	<dt>	<dd><tt>/.well-known/rd{?ep,d,lt,extra-attrs*}</tt></dd>
	Method: </dt>
	<dd>
	<t>POST</t>
	</dd>
	<dt>
	URI Template: </dt>
	<dd>
	<t>/.well-known/rd{?ep,d,lt,extra-attrs*}</t>
	</dd>
	</dl>	</dl>

	<t>URI Template Variables are as they are for registration in <xref targ
	et="registration" format="default"/>.	<t>URI Template Variables are the same as for registration in <xref targ
		et="registration" format="default"/>.
	The base attribute is not accepted to keep the registration interface simple;	The base attribute is not accepted to keep the registration interface simple;
	that rules out registration over CoAP-over-TCP or HTTP that would need to specif y one.	that rules out registration over CoAP-over-TCP or HTTP that would need to specif y one.

	For some time during this document's development, the URI template <tt>/.well-kn	For some time during this document's development, the URI Template <tt>/.well-kn
	own/core{?ep,...}</tt> has been in use instead.</t>	own/core{?ep,...}</tt> was in use instead.</t>
	<t>The following response is expected on this interface:</t>	<t>The following response is expected on this interface:</t>
	<dl>
	<dt>	<dl>
	Success: </dt>	<dt>Success:</dt>
	<dd>	<dd>2.04 (Changed)</dd>
	<t>2.04 "Changed".</t>	</dl>
	</dd>	<t>For the second interaction triggered by the above, the
	</dl>	registrant-EP takes the role of server and the RD takes the role of
	<t>For the second interaction triggered by the above, the registrant-ep	client. (Note that this is exactly the well-known interface
	takes the role of server and the RD the role of client.	of <xref target="RFC6690" section="4" sectionFormat="comma"/>):
	(Note that this is exactly the Well-Known Interface of <xref target="RFC6690" fo
	rmat="default"/> Section 4):
	<!-- the above paragraph could just as well be any other text;
	what matters is that the tables above and below are clearly separated. -->
	</t>	</t>
	<dl>	<dl>

	<dt>	<dt>Interaction:</dt>
	Interaction: </dt>	<dd>RD -> EP</dd>
	<dd>	<dt>Method:</dt>
	<t>RD -> EP</t>	<dd>GET</dd>
	</dd>	<dt>URI Template:</dt>
	<dt>	<dd><tt>/.well-known/core</tt></dd>
	Method: </dt>
	<dd>
	<t>GET</t>
	</dd>
	<dt>
	URI Template: </dt>
	<dd>
	<t>/.well-known/core</t>
	</dd>
	</dl>	</dl>
	<t>The following response is expected on this interface:</t>	<t>The following response is expected on this interface:</t>
	<dl>	<dl>

	<dt>	<dt>Success:</dt>
	Success: </dt>	<dd>2.05 (Content)</dd>
	<dd>
	<t>2.05 "Content".</t>
	</dd>
	</dl>	</dl>

	<t>When the RD uses any authorization credentials to access the endpoint	<t>When the RD uses any authorization credentials to access the endpoint
	's discovery resource,	's discovery
	or when it is deployed in a location where third parties might reach it but not	resource or when it is deployed in a location where third parties might r
	the endpoint,	each it but not
	it SHOULD verify that the apparent registrant-ep intends to register with the gi	the endpoint, it <bcp14>SHOULD</bcp14> verify that the apparent registran
	ven registration parameters	t-EP intends to
	before revealing the obtained discovery information to lookup clients.	register with the given registration parameters
	An easy way to do that is to verify the simple registration request's sender add	before revealing the obtained discovery information to lookup clients.
	ress using the Echo option as described in <xref target="I-D.ietf-core-echo-requ	An easy way to do that is to verify the simple registration request's sen
	est-tag" format="default"/> Section 2.4.</t>	der address using
	<t>The RD MUST delete registrations created by simple registration after	the Echo option, as described in <xref target="RFC9175" section="2.4"
	the expiration of their lifetime. Additional operations on the registration res	sectionFormat="comma"/>.</t>
	ource cannot be executed because no registration location is returned.</t>	<t>The RD <bcp14>MUST</bcp14> delete registrations created by simple reg
		istration after the expiration of their lifetime. Additional operations on the r
		egistration resource cannot be executed because no registration location is retu
		rned.</t>
	</section>	</section>
	<section anchor="third-party-registration" numbered="true" toc="default">	<section anchor="third-party-registration" numbered="true" toc="default">

	<name>Third-party registration</name>	<name>Third-Party Registration</name>
	<t>For some applications, even Simple Registration may be too taxing	<t>For some applications, even simple registration may be too taxing
	for some very constrained devices, in particular if the security requirements	for some very constrained devices, in particular, if the security requirements
	become too onerous.</t>	become too onerous.</t>

	<t>In a controlled environment (e.g. building control), the RD	<t>In a controlled environment (e.g., building control), the RD
	can be filled by a third party device, called a Commissioning Tool (CT). The com	can be filled by a third-party device, called a Commissioning Tool (CT). The CT
	missioning	can fill the RD from a database or other means. For
	tool can fill the RD from a database or other means. For	that purpose scheme, the IP address and port of the URI of the registered device
	that purpose scheme, IP address and port of the URI of the registered device is	is
	the value of the "base" parameter of the registration described in <xref target ="registration" format="default"/>.</t>	the value of the "base" parameter of the registration described in <xref target ="registration" format="default"/>.</t>

	<t>It should be noted that the value of the "base" parameter applies to all the links of the registration and has consequences for the anchor value of t he individual links as exemplified in <xref target="weblink" format="default"/>. An eventual (currently non-existing) "base" attribute of the link is not affect ed by the value of "base" parameter in the registration.</t>	<t>It should be noted that the value of the "base" parameter applies to all the links of the registration and has consequences for the anchor value of t he individual links, as exemplified in <xref target="weblink" format="default"/> . A potential (currently nonexistent) "base" attribute of the link is not affec ted by the value of "base" parameter in the registration.</t>
	</section>	</section>
	<section anchor="operations-on-the-registration-resource" numbered="true" toc="default">	<section anchor="operations-on-the-registration-resource" numbered="true" toc="default">
	<name>Operations on the Registration Resource</name>	<name>Operations on the Registration Resource</name>

	<t>This section describes how the registering endpoint can maintain the registrations that it created. The registering endpoint can be the registrant-ep or the CT. The registrations are resources of the RD.</t>	<t>This section describes how the registering endpoint can maintain the registrations that it created. The registering endpoint can be the registrant-EP or the CT. The registrations are resources of the RD.</t>
	<t>An endpoint should not use this interface for registrations that it d id not create.	<t>An endpoint should not use this interface for registrations that it d id not create.

	This is usually enforced by security policies,	This is usually enforced by security policies,
	which in general require equivalent credentials for creation of and operations o	which, in general, require equivalent credentials for creation of and ope
	n a registration.</t>	rations on a
	<t>After the initial registration, the registering endpoint retains the	registration.</t>
	returned location of the registration resource for further operations, including	<t>After the initial registration, the registering endpoint retains the
	refreshing the registration in order to extend the lifetime and "keep-alive" th	returned location of the registration resource for further operations, including
	e registration. When the lifetime of the registration has expired, the RD SHOULD	refreshing the registration in order to extend the lifetime and "keep-alive" th
	NOT respond to discovery queries concerning this endpoint. The RD SHOULD contin	e registration. When the lifetime of the registration has expired, the RD <bcp14
	ue to provide access to the registration resource after a registration time-out	>SHOULD NOT</bcp14> respond to discovery queries concerning this endpoint. The R
	occurs in order to enable the registering endpoint to eventually refresh the reg	D <bcp14>SHOULD</bcp14> continue to provide access to the registration resource
	istration. The RD MAY eventually remove the registration resource for the purpos	after a registration timeout occurs in order to enable the registering endpoint
	e of garbage collection. If the registration resource is removed, the correspond	to eventually refresh the registration. The RD <bcp14>MAY</bcp14> eventually rem
	ing endpoint will need to be re-registered.</t>	ove the registration resource for the purpose of garbage collection. If the regi
	<t>The registration resource may also be used cancel the registration us	stration resource is removed, the corresponding endpoint will need to be reregis
	ing DELETE, and to perform further operations beyond the scope of this specifica	tered.</t>
	tion.</t>	<t>The registration resource may also be used to cancel the registration
		using DELETE and to perform further operations beyond the scope of this specifi
		cation.</t>
	<t>Operations on the registration resource are sensitive to reordering;	<t>Operations on the registration resource are sensitive to reordering;
	<xref target="freshness" format="default"/> describes how order is restored.</t>	<xref target="freshness" format="default"/> describes how order is restored.</t>
	<t>The operations on the registration resource are described below.</t>	<t>The operations on the registration resource are described below.</t>
	<section anchor="update" numbered="true" toc="default">	<section anchor="update" numbered="true" toc="default">
	<name>Registration Update</name>	<name>Registration Update</name>
	<t>The update interface is used by the registering endpoint to refresh or update its	<t>The update interface is used by the registering endpoint to refresh or update its
	registration with an RD. To use the interface, the registering endpoint sends a POST request to the registration resource returned by the initial registration o peration.</t>	registration with an RD. To use the interface, the registering endpoint sends a POST request to the registration resource returned by the initial registration o peration.</t>

	<t>An update MAY update registration parameters like lifetime, base UR I or others.	<t>An update <bcp14>MAY</bcp14> update registration parameters, such a s lifetime, base URI, or others.
	Parameters that are not being changed should not	Parameters that are not being changed should not
	be included in an update. Adding parameters that have not changed increases	be included in an update. Adding parameters that have not changed increases
	the size of the message but does not have any other implications.	the size of the message but does not have any other implications.

	Parameters are included as query parameters in an update operation as	Parameters are included as query parameters in an update operation, as
	in <xref target="registration" format="default"/>.</t>	in <xref target="registration" format="default"/>.</t>
	<t>A registration update resets the timeout of the registration to the (possibly	<t>A registration update resets the timeout of the registration to the (possibly

	updated) lifetime of the registration, independent of whether a <tt>lt</tt> para meter	updated) lifetime of the registration, independent of whether an <tt>lt</tt> par ameter
	was given.</t>	was given.</t>
	<t>If the base URI of the registration is changed in an update,	<t>If the base URI of the registration is changed in an update,
	relative references submitted in the original registration or later updates are resolved anew against the new base.</t>	relative references submitted in the original registration or later updates are resolved anew against the new base.</t>
	<t>The registration update operation only describes the use of POST wi th an empty payload.	<t>The registration update operation only describes the use of POST wi th an empty payload.
	Future standards might describe the semantics of using content formats and paylo ads	Future standards might describe the semantics of using content formats and paylo ads
	with the POST method to update the links of a registration (see <xref target="li nk-up" format="default"/>).</t>	with the POST method to update the links of a registration (see <xref target="li nk-up" format="default"/>).</t>
	<t>The update registration request interface is specified as follows:< /t>	<t>The update registration request interface is specified as follows:< /t>
	<dl>	<dl>

	<dt>	<dt>Interaction:</dt>
	Interaction: </dt>	<dd>EP or CT -> RD</dd>
	<dd>	<dt>Method:</dt>
	<t>EP or CT -> RD</t>	<dd>POST</dd>
	</dd>	<dt>URI Template:</dt>
	<dt>	<dd>{+location}{?lt,base,extra-attrs*}</dd>
	Method: </dt>	<dt>URI Template Variables:</dt>
	<dd>	<dd>
	<t>POST</t>	<t><br/></t>
	</dd>
	<dt>
	URI Template: </dt>
	<dd>
	<t>{+location}{?lt,base,extra-attrs*}</t>
	</dd>
	<dt>
	URI Template Variables: </dt>
	<dd>
	<dl>	<dl>

	<dt>	<dt>location :=</dt>
	location := </dt>	<dd>This is the location returned by the RD as a result of a suc
	<dd>	cessful
	<t>This is the Location returned by the RD as a result of a su	earlier registration.</dd>
	ccessful	<dt>lt :=</dt>
	earlier registration.</t>	<dd>Lifetime (optional). This is the lifetime of the registratio
	</dd>	n in seconds,
	<dt>	with a range of 1-4294967295. If no lifetime is included, the pre
	lt := </dt>	vious last
	<dd>	lifetime set on a previous update or the original registration
	<t>Lifetime (optional). Lifetime of the registration in second	(falling back to 90000) <bcp14>SHOULD</bcp14> be used.</dd>
	s. Range of 1-4294967295.	<dt>base :=</dt>
	If no lifetime is included, the previous last
	lifetime set on a previous update or the original registration
	(falling back to 90000) SHOULD be used.</t>
	</dd>
	<dt>
	base := </dt>
	<dd>	<dd>

	<t>Base URI (optional). This parameter updates the Base URI es	<t>Base URI (optional). This parameter updates the base URI es
	tablished in the	tablished in the
	original registration to a new value,	original registration to a new value and is subject to
	and is subject to	the same restrictions as in the registration.</t>
	the same restrictions as in the registration.
	</t>
	<t>If the parameter is set in an update, it is stored by the R D as the new	<t>If the parameter is set in an update, it is stored by the R D as the new

	Base URI under which to interpret the relative links present in the payload of t	base URI under which to interpret the relative links present in
	he original registration.</t>	the payload of
		the original registration.</t>
	<t>If the parameter is not set in the request but was set befo re, the previous	<t>If the parameter is not set in the request but was set befo re, the previous

	Base URI value is kept unmodified.</t>	base URI value is kept unmodified.</t>
	<t>If the parameter is not set in the request and was not set before either, the	<t>If the parameter is not set in the request and was not set before either, the

	source address and source port of the update request are stored as the	source address and source port of the update request are stored
	Base URI.</t>	as the
		base URI.</t>
	</dd>	</dd>

	<dt>	<dt>extra-attrs :=</dt>
	extra-attrs := </dt>
	<dd>	<dd>
	<t>Additional registration attributes (optional). As with the registration,	<t>Additional registration attributes (optional). As with the registration,

	the RD processes them if it knows their semantics. Otherwise, unknown	the RD processes them if it knows their semantics. Otherwise, u
	attributes are stored as endpoint attributes, overriding any previously	nknown
	stored endpoint attributes of the same key.</t>	attributes are stored as endpoint attributes, overriding any pr
	</dd>	eviously
	<dt/>	stored endpoint attributes of the same key.</t>
	<dd>	<t>Note that this default behavior does not allow removing an e
	<t>Note that this default behavior does not allow removing an	ndpoint attribute
	endpoint attribute in an update.	in an update. For attributes whose functionality depends on the
	For attributes whose functionality depends on the endpoints' ability to remove t	endpoints'
	hem in an update,	ability to remove them in an update,
	it can make sense to define a value whose presence is equivalent to the absence	it can make sense to define a value whose presence is equivalen
	of a value.	t to the absence
	As an alternative, an extension can define different updating rules for their at	of a value. As an alternative, an extension can define differen
	tributes.	t updating rules
	That necessitates either discovery of whether the RD is aware of that extension,	for their attributes. That necessitates either discovering whet
	or tolerating the default behavior.</t>	her the RD is
	</dd>	aware of that extension or tolerating the default behavior.</t>
		</dd>
	</dl>	</dl>

	</dd>	</dd>
	<dt>	<dt>Content-Format:</dt>
	Content-Format: </dt>	<dd>none (no payload)</dd>
	<dd>
	<t>none (no payload)</t>
	</dd>
	</dl>	</dl>
	<t>The following responses are expected on this interface:</t>	<t>The following responses are expected on this interface:</t>
	<dl>	<dl>

	<dt>	<dt>Success:</dt>
	Success: </dt>	<dd>2.04 (Changed) or 204 (No Content) if the update was successfull
	<dd>	y processed.</dd>
	<t>2.04 "Changed" or 204 "No Content" if the update was successful	<dt>Failure:</dt>
	ly processed.</t>	<dd>4.04 (Not Found) or 404 (Not Found). Registration does not exist
	</dd>	(e.g., may have
	<dt>	been removed).</dd>
	Failure: </dt>
	<dd>
	<t>4.04 "Not Found" or 404 "Not Found". Registration does not exis
	t (e.g. may have been removed).</t>
	</dd>
	</dl>	</dl>


	<t>If the registration update fails in any way, including "Not Found" and request timeouts,	<t>If the registration update fails in any way, including "Not Found" and request timeouts,
	or if the time indicated in a Service Unavailable Max-Age/Retry-After exceeds th e remaining lifetime,	or if the time indicated in a Service Unavailable Max-Age/Retry-After exceeds th e remaining lifetime,

	the registering endpoint SHOULD attempt registration again.</t>	the registering endpoint <bcp14>SHOULD</bcp14> attempt registration again.</t>
	<t>The following example shows how the registering endpoint resets the timeout on its registration resource at	<t>The following example shows how the registering endpoint resets the timeout on its registration resource at

	an RD using this interface with the example location value: /rd/4521.</t>	an RD using this interface with the example location value /rd/4521:</t>
	<figure anchor="example-update">	<figure anchor="example-update">

	<name>Example update of a registration</name>	<name>Example Update of a Registration</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: POST /rd/4521	Req: POST /rd/4521

	Res: 2.04 Changed	Res: 2.04 Changed

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	<t>The following example shows the registering endpoint updating its r egistration resource at	<t>The following example shows the registering endpoint updating its r egistration resource at

	an RD using this interface with the example location value: /rd/4521. The initia l registration by the registering endpoint set the following values:</t>	an RD using this interface with the example location value /rd/4521. The initial registration by the registering endpoint set the following values:</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>endpoint name (ep)=endpoint1</li>	<li>endpoint name (ep)=endpoint1</li>
	<li>lifetime (lt)=500</li>	<li>lifetime (lt)=500</li>

	<li>Base URI (base)=coap://local-proxy-old.example.com</li>	<li>base URI (base)=coap://local-proxy-old.example.com</li>
	<li>payload of <xref target="example-payload" format="default"/></li >	<li>payload of <xref target="example-payload" format="default"/></li >
	</ul>	</ul>
	<t>The initial state of the RD is reflected in the following request:< /t>	<t>The initial state of the RD is reflected in the following request:< /t>
	<figure anchor="example-update-base-lookup-pre">	<figure anchor="example-update-base-lookup-pre">

	<name>Example lookup before a change to the base address</name>	<name>Example Lookup Before a Change to the Base Address</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET /rd-lookup/res?ep=endpoint1	Req: GET /rd-lookup/res?ep=endpoint1

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	<coap://local-proxy-old.example.com/sensors/temp>;	<coap://local-proxy-old.example.com/sensors/temp>;
	rt=temperature-c;if=sensor,	rt=temperature-c;if=sensor,
	<http://www.example.com/sensors/temp>;	<http://www.example.com/sensors/temp>;
	anchor="coap://local-proxy-old.example.com/sensors/temp";	anchor="coap://local-proxy-old.example.com/sensors/temp";
	rel=describedby	rel=describedby

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>

	<t>The following example shows the registering endpoint changing the B ase URI to <tt>coaps://new.example.com:5684</tt>:</t>	<t>The following example shows the registering endpoint changing the b ase URI to <tt>coaps://new.example.com:5684</tt>:</t>
	<figure anchor="example-update-base">	<figure anchor="example-update-base">

	<name>Example registration update that changes the base address</nam	<name>Example Registration Update that Changes the Base Address</nam
	e>	e>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: POST /rd/4521?base=coaps://new.example.com	Req: POST /rd/4521?base=coaps://new.example.com

	Res: 2.04 Changed	Res: 2.04 Changed

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	<t>The consecutive query returns:</t>	<t>The consecutive query returns:</t>
	<figure anchor="example-update-base-lookup-post">	<figure anchor="example-update-base-lookup-post">

	<name>Example lookup after a change to the base address</name>	<name>Example Lookup After a Change to the Base Address</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET /rd-lookup/res?ep=endpoint1	Req: GET /rd-lookup/res?ep=endpoint1

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	<coaps://new.example.com/sensors/temp>;	<coaps://new.example.com/sensors/temp>;
	rt=temperature-c;if=sensor,	rt=temperature-c;if=sensor,
	<http://www.example.com/sensors/temp>;	<http://www.example.com/sensors/temp>;
	anchor="coaps://new.example.com/sensors/temp";	anchor="coaps://new.example.com/sensors/temp";
	rel=describedby	rel=describedby

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	</section>	</section>
	<section anchor="removal" numbered="true" toc="default">	<section anchor="removal" numbered="true" toc="default">
	<name>Registration Removal</name>	<name>Registration Removal</name>

	<t>Although RD registrations have soft state and will eventually timeo	<t>Although RD registrations have soft state and will eventually time
	ut after their	out after their
	lifetime, the registering endpoint SHOULD explicitly remove an entry from the RD	lifetime, the registering endpoint <bcp14>SHOULD</bcp14> explicitly remove an en
	if it	try from the RD if it
	knows it will no longer be available (for example on shut-down). This is	knows it will no longer be available (for example, on shutdown). This is
	accomplished using a removal interface on the RD by performing a DELETE on	accomplished using a removal interface on the RD by performing a DELETE on
	the endpoint resource.</t>	the endpoint resource.</t>
	<t>The removal request interface is specified as follows:</t>	<t>The removal request interface is specified as follows:</t>
	<dl>	<dl>

	<dt>	<dt>Interaction:</dt>
	Interaction: </dt>	<dd>EP or CT -> RD</dd>
	<dd>	<dt>Method:</dt>
	<t>EP or CT -> RD</t>	<dd>DELETE</dd>
	</dd>	<dt>URI Template:</dt>
	<dt>	<dd>{+location}</dd>
	Method: </dt>	<dt>URI Template Variables:</dt>
	<dd>
	<t>DELETE</t>
	</dd>
	<dt>
	URI Template: </dt>
	<dd>
	<t>{+location}</t>
	</dd>
	<dt>
	URI Template Variables: </dt>
	<dd>	<dd>

		<t><br/></t>
	<dl>	<dl>

	<dt>	<dt>location :=</dt>
	location := </dt>	<dd>This is the location returned by the RD as a result of a suc
	<dd>	cessful
	<t>This is the Location returned by the RD as a result of a su	earlier registration.</dd>
	ccessful
	earlier registration.</t>
	</dd>
	</dl>	</dl>
	</dd>	</dd>

	</dl>	</dl>
	<t>The following responses are expected on this interface:</t>
		<t> The following responses are expected on this interface:</t>
	<dl>	<dl>

	<dt>	<dt>Success:</dt>
	Success: </dt>	<dd>2.02 (Deleted) or 204 (No Content) upon successful deletion.</dd
	<dd>	>
	<t>2.02 "Deleted" or 204 "No Content" upon successful deletion</t>	<dt>Failure:</dt>
	</dd>	<dd>4.04 (Not Found) or 404 (Not Found). Registration does not exist
	<dt>	(e.g., may
	Failure: </dt>	already have been removed).</dd>
	<dd>
	<t>4.04 "Not Found" or 404 "Not Found". Registration does not exis
	t (e.g. may already have been removed).</t>
	</dd>
	</dl>	</dl>

	<t>The following examples shows successful removal of the endpoint fro
	m the RD with example location value /rd/4521.</t>	<t>The following example shows successful removal of the endpoint from
		the RD
		with example location value /rd/4521:</t>
	<figure anchor="example-removal">	<figure anchor="example-removal">

	<name>Example of a registration removal</name>	<name>Example of a Registration Removal</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: DELETE /rd/4521	Req: DELETE /rd/4521

	Res: 2.02 Deleted	Res: 2.02 Deleted

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	</section>	</section>
	<section anchor="link-up" numbered="true" toc="default">	<section anchor="link-up" numbered="true" toc="default">

	<name>Further operations</name>	<name>Further Operations</name>
	<t>Additional operations on the registration can be specified in futur	<t>Additional operations on the registration can be specified in futur
	e documents, for example:</t>	e documents, for
		example:</t>
	<ul spacing="normal">	<ul spacing="normal">

	<li>Send iPATCH (or PATCH) updates (<xref target="RFC8132" format="d	<li>Send iPATCH (or PATCH) updates <xref target="RFC8132" format="de
	efault"/>) to add, remove or change the links of a registration.</li>	fault"/> to add,
		remove, or change the links of a registration.</li>
	<li>Use GET to read the currently stored set of links in a registrat ion resource.</li>	<li>Use GET to read the currently stored set of links in a registrat ion resource.</li>
	</ul>	</ul>

	<t>Those operations are out of scope of this document, and will requir e media types suitable for modifying sets of links.</t>	<t>Those operations are out of scope of this document and will require media types suitable for modifying sets of links.</t>
	</section>	</section>
	<section anchor="freshness" numbered="true" toc="default">	<section anchor="freshness" numbered="true" toc="default">

	<name>Request freshness</name>	<name>Request Freshness</name>
	<t>Some security mechanisms usable with an RD allow out of order reque	<t>Some security mechanisms usable with an RD allow out-of-order reque
	st processing,	st processing
	or do not even mandate replay protection at all.	or do not even mandate replay protection at all.
	The RD needs to ensure that operations on the registration resource	The RD needs to ensure that operations on the registration resource
	are executed in an order that does not distort the client's intentions.</t>	are executed in an order that does not distort the client's intentions.</t>

	<t>This ordering of operations is expressed in terms of freshness as d efined in <xref target="I-D.ietf-core-echo-request-tag" format="default"/>.	<t>This ordering of operations is expressed in terms of freshness, as defined in <xref target="RFC9175" format="default"/>.
	Requests that alter a resource's state need to be fresh relative to the latest r equest	Requests that alter a resource's state need to be fresh relative to the latest r equest
	that altered that state in a conflicting way.</t>	that altered that state in a conflicting way.</t>

	<t>An RD SHOULD determine a request's freshness,	<t>An RD <bcp14>SHOULD</bcp14> determine a request's freshness
	and MUST use the Echo option if it requires request freshness and can not determ	and <bcp14>MUST</bcp14> use the Echo option if it requires request freshness and
	ine the it in any other way.	cannot determine it in any other way.
	An endpoint MUST support the use of the Echo option.	An endpoint <bcp14>MUST</bcp14> support the use of the Echo option.
	(One reason why an RD would not require freshness is when no relevant registrati	(One reason why an RD would not require freshness is when no relevant registrati
	on properties are covered by is security policies.)</t>	on properties are covered by its security policies.)</t>
	<section anchor="efficient-use-of-echo-by-an-rd" numbered="true" toc=" default">	<section anchor="efficient-use-of-echo-by-an-rd" numbered="true" toc=" default">

	<name>Efficient use of Echo by an RD</name>	<name>Efficient Use of Echo by an RD</name>
	<t>To keep latency and traffic added by the freshness requirements t o a minimum,	<t>To keep latency and traffic added by the freshness requirements t o a minimum,
	RDs should avoid naive (sufficient but inefficient) freshness criteria.</t>	RDs should avoid naive (sufficient but inefficient) freshness criteria.</t>
	<t>Some simple mechanisms the RD can employ are:</t>	<t>Some simple mechanisms the RD can employ are:</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>	<li>

	<t>State counter.	<t>State counter. The RD can keep a monotonous counter that incre
	The RD can keep a monotonous counter that increments whenever a registration cha	ments
	nges.	whenever a registration changes. For every registration resource,
	For every registration resource, it stores the post-increment value of that reso	it stores
	urce's last change.	the post-increment value of that resource's last change. Requests
	Requests altering them need to have at least that value encoded in their Echo op	altering
	tion,	them need to have at least that value encoded in their Echo optio
	and are otherwise rejected with a 4.01 Unauthorized and the current counter valu	n
	e as the Echo value.	and are otherwise rejected with a 4.01 (Unauthorized) and the cur
	If other applications on the same server use Echo as well,	rent
	that encoding may include a prefix indicating that it pertains to the RD's count	counter value as the Echo value. If other applications on the sam
	er. </t>	e server
	<t>	use Echo as well, that encoding may include a prefix indicating t
	The value associated with a resource needs to be kept across the removal of regi	hat it
	strations	pertains to the RD's counter.</t>
	if the same registration resource is to be reused. </t>	<t>The value associated with a resource needs to be kept across
	<t>	the removal
	The counter can be reset (and the values of removed resources forgotten)	of registrations if the same registration resource is to be reuse
	when all previous security associations are reset.	d.</t>
	<!-- It can also be reset more frequently in an epoch based scheme, but that d	<t>The counter can be reset (and the values of removed resources
	escription is too large to fit in the margin. --> </t>	forgotten)
	<t>	when all previous security associations are reset.</t>
	This is the "Persistent Counter" method of <xref target="I-D.ietf-core-echo-requ	<t>This is the "Persistent Counter" method of <xref target="RFC9
	est-tag" format="default"/> Appendix A.</t>	175"
		section="A" sectionFormat="comma"/>.</t>
	</li>	</li>
	<li>	<li>

	<t>Preemptive Echo values.	<t>Preemptive Echo values. The current state counter can be sent
	The current state counter can be sent in an Echo option not only when requests a	in an Echo
	re rejected with 4.01 Unauthorized,	option not only when requests are rejected with 4.01 (Unauthorize
	but also with successful responses.	d) but
	Thus, clients can be provided with Echo values sufficient for their next request	also with successful responses. Thus, clients can be provided wit
	on a regular basis. </t>	h Echo
	<t>	values sufficient for their next request on a regular basis. This
	While endpoints may discard received Echo values at leisure between requests,	is also described in <xref target="RFC9175" section="2.3" sectionFormat="of"/>
	they are encouraged to retain these values for the next request to avoid additio	</t>
	nal round trips.</t>	<t> While endpoints may discard received Echo values at leisure
		between
		requests, they are encouraged to retain these values for the next
		request
		to avoid additional round trips.</t>
	</li>	</li>

	<li>If the RD can ensure that only one security association has mo difying access to any registration at any given time,	<li>If the RD can ensure that only one security association has mo difying access to any registration at any given time
	and that security association provides order on the requests,	and that security association provides order on the requests,
	that order is sufficient to show request freshness.</li>	that order is sufficient to show request freshness.</li>
	</ul>	</ul>
	</section>	</section>
	<section anchor="examples-of-echo-usage" numbered="true" toc="default" >	<section anchor="examples-of-echo-usage" numbered="true" toc="default" >

	<name>Examples of Echo usage</name>	<name>Examples of Echo Usage</name>
	<t><xref target="example-freshness" format="default"/> shows the int eractions of an endpoint	<t><xref target="example-freshness" format="default"/> shows the int eractions of an endpoint
	that has forgotten the server's latest Echo value	that has forgotten the server's latest Echo value
	and temporarily reduces its registration lifetime:</t>	and temporarily reduces its registration lifetime:</t>
	<figure anchor="example-freshness">	<figure anchor="example-freshness">

	<name>Example update of a registration</name>	<name>Example Update of a Registration</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: POST /rd/4521?lt=7200	Req: POST /rd/4521?lt=7200

	Res: 4.01 Unauthorized	Res: 4.01 Unauthorized
	Echo: 0x0123	Echo: 0x0123


	(EP tries again immediately)	(EP tries again immediately.)

	Req: POST /rd/4521?lt=7200	Req: POST /rd/4521?lt=7200
	Echo: 0x0123	Echo: 0x0123

	Res: 2.04 Changed	Res: 2.04 Changed
	Echo: 0x0124	Echo: 0x0124


	(Later the EP regains its confidence in its long-term reachability)	(Later, the EP regains its confidence in its long-term reachability.)

	Req: POST /rd/4521?lt=90000	Req: POST /rd/4521?lt=90000
	Echo: 0x0124	Echo: 0x0124

	Res: 2.04 Changed	Res: 2.04 Changed
	Echo: 0x0247	Echo: 0x0247

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>

	<t>The other examples do not show Echo options for simplicity,
	and because they lack the context for any example values to have meaning.</t>	<t>The other examples do not show Echo options for two reasons: (1) f
		or simplicity
		and (2) because they lack the context for any example values to have meaning.</t
		>
	</section>	</section>
	</section>	</section>
	</section>	</section>
	</section>	</section>


	<section anchor="lookup" numbered="true" toc="default">	<section anchor="lookup" numbered="true" toc="default">
	<name>RD Lookup</name>	<name>RD Lookup</name>
	<t>To discover the resources registered with the RD,	<t>To discover the resources registered with the RD,
	a lookup interface must be provided. This lookup interface	a lookup interface must be provided. This lookup interface
	is defined as a default, and it is assumed that RDs may also support lookups	is defined as a default, and it is assumed that RDs may also support lookups

	to return resource descriptions in alternative formats (e.g. JSON or CBOR link f	to return resource descriptions in alternative formats (e.g., JSON or CBOR link
	ormat <xref target="I-D.ietf-core-links-json" format="default"/>)	format <xref target="I-D.ietf-core-links-json" format="default"/>)
	or using more advanced interfaces (e.g. supporting context or semantic	or use more advanced interfaces (e.g., supporting context- or semantic-based loo
	based lookup) on different resources that are discovered independently.</t>	kup) on different resources that are discovered independently.</t>
	<t>RD Lookup allows lookups for endpoints and resources	<t>RD lookup allows lookups for endpoints and resources
	using attributes defined in this document and for use with the CoRE	using attributes defined in this document and for use with the CoRE
	Link Format. The result of a lookup request is the list of links (if any)	Link Format. The result of a lookup request is the list of links (if any)

	corresponding to the type of lookup. Thus, an endpoint lookup MUST return a lis	corresponding to the type of lookup. Thus, an endpoint lookup <bcp14>MUST</bcp1
	t of endpoints and a resource lookup MUST return a list of links to resources.</	4> return a list of endpoints, and a resource lookup <bcp14>MUST</bcp14> return
	t>	a list of links to resources.</t>
	<t>The lookup type is selected by a URI endpoint, which is indicated by a	<t>The lookup type implemented by a lookup resource is indicated by a reso
	Resource Type as per <xref target="lookup-types" format="default"/> below:</t>	urce type, as per <xref target="lookup-types" format="default"/>:</t>
	<table anchor="lookup-types" align="center">	<table anchor="lookup-types" align="center">
	<name>Lookup Types</name>	<name>Lookup Types</name>
	<thead>	<thead>
	<tr>	<tr>
	<th align="left">Lookup Type</th>	<th align="left">Lookup Type</th>
	<th align="left">Resource Type</th>	<th align="left">Resource Type</th>
	<th align="left">Mandatory</th>	<th align="left">Mandatory</th>
	</tr>	</tr>
	</thead>	</thead>
	<tbody>	<tbody>

	skipping to change at line 1571 ¶	skipping to change at line 1462 ¶
	<td align="left">Mandatory</td>	<td align="left">Mandatory</td>
	</tr>	</tr>
	<tr>	<tr>
	<td align="left">Endpoint</td>	<td align="left">Endpoint</td>
	<td align="left">core.rd-lookup-ep</td>	<td align="left">core.rd-lookup-ep</td>
	<td align="left">Mandatory</td>	<td align="left">Mandatory</td>
	</tr>	</tr>
	</tbody>	</tbody>
	</table>	</table>
	<section anchor="resource-lookup" numbered="true" toc="default">	<section anchor="resource-lookup" numbered="true" toc="default">

	<name>Resource lookup</name>	<name>Resource Lookup</name>
	<t>Resource lookup results in links that are semantically equivalent to the links submitted to the RD by the registrant.	<t>Resource lookup results in links that are semantically equivalent to the links submitted to the RD by the registrant.
	The links and link parameters returned by the lookup are equal to the originally submitted ones,	The links and link parameters returned by the lookup are equal to the originally submitted ones,

	except that the target reference is fully resolved,	except that the target reference is fully resolved
	and that the anchor reference is fully resolved if it is present in the lookup r esult at all.</t>	and that the anchor reference is fully resolved if it is present in the lookup r esult at all.</t>
	<t>Links that did not have an anchor attribute in the registration are r eturned without an anchor attribute.	<t>Links that did not have an anchor attribute in the registration are r eturned without an anchor attribute.
	Links of which href or anchor was submitted as a (full) URI are returned with th e respective attribute unmodified.</t>	Links of which href or anchor was submitted as a (full) URI are returned with th e respective attribute unmodified.</t>
	<t>The above rules allow the client to interpret the response as links w ithout any further knowledge of the storage conventions of the RD.	<t>The above rules allow the client to interpret the response as links w ithout any further knowledge of the storage conventions of the RD.

	The RD MAY replace the registration base URIs with a configured intermediate pro xy, e.g. in the case of an HTTP lookup interface for CoAP endpoints.</t>	The RD <bcp14>MAY</bcp14> replace the registration base URIs with a configured i ntermediate proxy, e.g., in the case of an HTTP lookup interface for CoAP endpoi nts.</t>
	<t>If the base URI of a registration contains a link-local address,	<t>If the base URI of a registration contains a link-local address,

	the RD MUST NOT show its links unless the lookup was made from the	the RD <bcp14>MUST NOT</bcp14> show its links unless the lookup was made from th e
	link on which the registered endpoint can be reached.	link on which the registered endpoint can be reached.

	The RD MUST NOT include zone identifiers in the resolved URIs.</t>	The RD <bcp14>MUST NOT</bcp14> include zone identifiers in the resolved URIs.</t >
	</section>	</section>
	<section anchor="lookup-filtering" numbered="true" toc="default">	<section anchor="lookup-filtering" numbered="true" toc="default">

	<name>Lookup filtering</name>	<name>Lookup Filtering</name>
	<t>Using the Accept Option, the requester can control whether the return	<t>Using the Accept option, the requester can control whether the return
	ed list is returned in CoRE Link Format (<tt>application/link-format</tt>, defau	ed list is returned in CoRE Link Format (<tt>application/link-format</tt>, defau
	lt) or in alternate content-formats (e.g. from <xref target="I-D.ietf-core-links	lt) or in alternate content formats (e.g., from <xref target="I-D.ietf-core-link
	-json" format="default"/>).</t>	s-json" format="default"/>).</t>
	<t>Multiple search criteria MAY be included in a lookup. All included cr	<t>Multiple search criteria <bcp14>MAY</bcp14> be included in a lookup.
	iteria MUST match for a link to be returned. The RD MUST support matching with m	All included criteria <bcp14>MUST</bcp14> match for a link to be returned. The R
	ultiple search criteria.</t>	D <bcp14>MUST</bcp14> support matching with multiple search criteria.</t>
	<t>A link matches a search criterion if it has an attribute of the same
	name and the same value, allowing for a trailing "*" wildcard operator as in Sec	<t>A link matches a search criterion if it has an attribute of
	tion 4.1 of <xref target="RFC6690" format="default"/>.	the same name and the same value, allowing for a trailing "*"
	Attributes that are defined as <tt>relation-types</tt> (in the link-format ABNF)	wildcard operator, as in <xref target="RFC6690" section="4.1"
	match if the search value matches any of their values (see Section 4.1 of <xref	sectionFormat="of"/>. Attributes that are defined as
	target="RFC6690" format="default"/>; e.g. <tt>?if=tag:example.net,2020:sensor</	<tt>relation-types</tt> (in the link-format ABNF) match if the
	tt> matches <tt>;if="example.regname tag:example.net,2020:sensor";</tt>).	search value matches any of their values (see <xref target="RFC6690" sec
	A resource link also matches a search criterion if its endpoint would match the	tion="4.1" sectionFormat="of"/>;
	criterion, and vice versa, an endpoint link matches a search criterion if any of	for example, <tt>?if=tag:example.net,2020:sensor</tt> matches
	its resource links matches it.</t>	<tt>;if="example.regname tag:example.net,2020:sensor";</tt>.
	<t>Note that <tt>href</tt> is a valid search criterion and matches targe	A resource link also matches a search criterion if its
	t references. Like all search criteria, on a resource lookup it can match the ta	endpoint would match the criterion, and vice versa, an
	rget reference of the resource link itself, but also the registration resource o	endpoint link matches a search criterion if any of its
	f the endpoint that registered it.	resource links matches it.</t>
	Queries for resource link targets MUST be in URI form (i.e. not relative referen	<t>Note that <tt>href</tt> is a valid search criterion and matches targe
	ces) and are matched against a resolved link target. Queries for endpoints SHOUL	t references. Like all search criteria, on a resource lookup, it can match the t
	D be expressed in path-absolute form if possible and MUST be expressed in URI fo	arget reference of the resource link itself but also the registration resource o
	rm otherwise; the RD SHOULD recognize either.	f the endpoint that registered it.
	The <tt>anchor</tt> attribute is usable for resource lookups, and, if queried, M	Queries for resource link targets <bcp14>MUST</bcp14> be in URI form (i.e., not
	UST be in URI form as well.</t>	relative references) and are matched against a resolved link target. Queries for
	<t>Additional query parameters "page" and "count" are used to obtain loo	endpoints <bcp14>SHOULD</bcp14> be expressed in path-absolute form if possible
	kup results in specified increments using pagination, where count specifies how	and <bcp14>MUST</bcp14> be expressed in URI form otherwise; the RD <bcp14>SHOULD
	many links to return and page specifies which subset of links organized in seque	</bcp14> recognize either.
	ntial pages, each containing 'count' links, starting with link zero and page zer	The <tt>anchor</tt> attribute is usable for resource lookups and, if queried, <b
	o. Thus, specifying count of 10 and page of 0 will return the first 10 links in	cp14>MUST</bcp14> be in URI form as well.</t>
	the result set (links 0-9). Count = 10 and page = 1 will return the next 'page'	<t>Additional query parameters "page" and "count" are used to obtain loo
	containing links 10-19, and so on.	kup results in specified increments using pagination, where count specifies how
	Unlike block-wise transfer of a compelte result set,	many links to return and page specifies which subset of links organized in seque
		ntial pages, each containing 'count' links, starting with link zero and page zer
		o. Thus, specifying a count of 10 and page of 0 will return the first 10 links i
		n the result set (links 0-9). Specifying a count of 10 and page of 1 will return
		the next 'page' containing links 10-19, and so on.
		Unlike block-wise transfer of a complete result set,
	these parameters ensure that each chunk of results can be interpreted on its own .	these parameters ensure that each chunk of results can be interpreted on its own .

	This simplifies the processing,	This simplifies the processing
	but can result in duplicate or missed items when coinciding with changes from th e registration interface.</t>	but can result in duplicate or missed items when coinciding with changes from th e registration interface.</t>
	<t>Endpoints that are interested in a lookup result repeatedly or contin uously can use	<t>Endpoints that are interested in a lookup result repeatedly or contin uously can use

	mechanisms like ETag caching, resource observation (<xref target="RFC7641" forma t="default"/>),	mechanisms such as ETag caching, resource observation <xref target="RFC7641" for mat="default"/>,
	or any future mechanism that might allow more efficient observations of collecti ons.	or any future mechanism that might allow more efficient observations of collecti ons.

	These are advertised, detected and used according to their own specifications	These are advertised, detected, and used according to their own specifications
	and can be used with the lookup interface as with any other resource.</t>	and can be used with the lookup interface as with any other resource.</t>
	<t>When resource observation is used,	<t>When resource observation is used,

	every time the set of matching links changes, or the content of a matching link changes, the RD sends a notification with the matching link set.	every time the set of matching links changes or the content of a matching link c hanges, the RD sends a notification with the matching link set.
	The notification contains the successful current response to the given request,	The notification contains the successful current response to the given request,
	especially with respect to representing zero matching links	especially with respect to representing zero matching links
	(see "Success" item below).</t>	(see "Success" item below).</t>
	<t>The lookup interface is specified as follows:</t>	<t>The lookup interface is specified as follows:</t>
	<dl>	<dl>

	<dt>	<dt>Interaction:</dt>
	Interaction: </dt>	<dd>Client -> RD</dd>
	<dd>	<dt>Method:</dt>
	<t>Client -> RD</t>	<dd>GET</dd>
	</dd>	<dt>URI Template:</dt>
	<dt>	<dd>{+type-lookup-location}{?page,count,search*}</dd>
	Method: </dt>	<dt>URI Template Variables:</dt>
	<dd>	<dd>
	<t>GET</t>	<t><br/></t>
	</dd>
	<dt>
	URI Template: </dt>
	<dd>
	<t>{+type-lookup-location}{?page,count,search*}</t>
	</dd>
	<dt>
	URI Template Variables: </dt>
	<dd>
	<dl>	<dl>

	<dt>	<dt>type-lookup-location :=</dt>
	type-lookup-location := </dt>	<dd>RD lookup URI for a given lookup type (mandatory). The address
	<dd>	is
	<t>RD Lookup URI for a given lookup type (mandatory). The addres	discovered as described in <xref target="discovery" format="default
	s is	"/>.</dd>
	discovered as described in <xref target="discovery" format="default"/>.</t>	<dt>search := </dt>
	</dd>
	<dt>
	search := </dt>
	<dd>
	<t>Search criteria for limiting the number of results (optional)
	.</t>
	</dd>
	<dt/>
	<dd>
	<t>The search criteria are an associative array, expressed in a
	form-style query as per the URI template (see <xref target="RFC6570" format="def
	ault"/> Sections 2.4.2 and 3.2.8)</t>
	</dd>
	<dt>
	page := </dt>
	<dd>
	<t>Page (optional). Parameter cannot be used without the count
	parameter. Results are returned from result set in pages that contain
	'count' links starting from index (page * count). Page numbering starts
	with zero.</t>
	</dd>
	<dt>
	count := </dt>
	<dd>	<dd>

	<t>Count (optional). Number of results is limited to this parame	<t> Search criteria for limiting the number of results (optional)
	ter value. If	. The search
	the page parameter is also present, the response MUST only include 'count'	criteria are an associative array, expressed in a form-style quer
	links starting with the (page * count) link in the result set from the query. If	y,
	the count parameter is not present, then the response MUST return all matching	as per the URI Template (see <xref target="RFC6570" format="defaul
	links in the result set. Link numbering starts with zero.</t>	t"/>, Sections
		<xref target="RFC6570" section="2.4.2" sectionFormat="bare"/> and
		<xref
		target="RFC6570" section=" 3.2.8" sectionFormat="bare"/>).</t>
	</dd>	</dd>

		<dt>page :=</dt>
		<dd>Page (optional). This parameter cannot be used without the cou
		nt
		parameter. Results are returned from result set in pages that conta
		in
		'count' links starting from index (page * count). Page numbering st
		arts
		with zero.</dd>
		<dt>count := </dt>
		<dd>Count (optional). The number of results is limited to this par
		ameter value. If
		the page parameter is also present, the response <bcp14>MUST</bcp14
		> only include
		'count' links starting with the (page * count) link in the result s
		et from the
		query. If the count parameter is not present, then the response <bc
		p14>MUST</bcp14>
		return all matching links in the result set. Link numbering starts
		with zero. </dd>
	</dl>	</dl>

	</dd>	</dd>
	<dt>
	Accept: </dt>	<dt>Accept:</dt>
	<dd>	<dd>absent, <tt>application/link-format</tt>, or any other indicated m
	<t>absent, application/link-format or any other indicated media type	edia type representing web
	representing web links</t>	links</dd>
	</dd>
	</dl>	</dl>

	<t>The following responses codes are defined for this interface:</t>
		<t> The following responses codes are defined for this interface:</t>
	<dl>	<dl>

	<dt>	<dt>Success:</dt>
	Success: </dt>
	<dd>	<dd>

	<t>2.05 "Content" or 200 "OK" with an <tt>application/link-format</t	<t>2.05 (Content) or 200 (OK) with an <tt>application/link-format</t
	t> or other web link payload containing matching entries for the lookup.	t> or other
	</t>	web link payload containing matching entries for the lookup.</t>
	<t>The payload can contain zero links (which is an empty payload in	<t>The payload can contain zero links (which is an empty payload in
	<xref target="RFC6690" format="default"/> link format, but could also be <tt>[]<	the link format described in <xref
	/tt> in JSON based formats),	target="RFC6690" format="default"/> but could also be <tt>[]</tt>
	indicating that no entities matched the request.</t>	in JSON-based formats), indicating that no entities matched the reque
		st.</t>
	</dd>	</dd>
	</dl>	</dl>


	</section>	</section>
	<section anchor="resource-lookup-examples" numbered="true" toc="default">	<section anchor="resource-lookup-examples" numbered="true" toc="default">

	<name>Resource lookup examples</name>	<name>Resource Lookup Examples</name>
	<t>The examples in this section assume the existence of CoAP hosts with a default CoAP port 61616. HTTP hosts are possible and do not change the nature of the examples.</t>	<t>The examples in this section assume the existence of CoAP hosts with a default CoAP port 61616. HTTP hosts are possible and do not change the nature of the examples.</t>

	<t>The following example shows a client performing a resource lookup wit h the example resource look-up locations discovered in <xref target="example-dis covery" format="default"/>:</t>	<t>The following example shows a client performing a resource lookup wit h the example resource lookup locations discovered in <xref target="example-disc overy" format="default"/>:</t>
	<figure anchor="example-lookup-res">	<figure anchor="example-lookup-res">

	<name>Example a resource lookup</name>	<name>Example of a Resource Lookup</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET /rd-lookup/res?rt=tag:example.org,2020:temperature	Req: GET /rd-lookup/res?rt=tag:example.org,2020:temperature

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	<coap://[2001:db8:3::123]:61616/temp>;	<coap://[2001:db8:3::123]:61616/temp>;
	rt="tag:example.org,2020:temperature"	rt="tag:example.org,2020:temperature"

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	<t>A client that wants to be notified of new resources as they show up c an use	<t>A client that wants to be notified of new resources as they show up c an use

	observation:</t>	this observation:</t>
	<figure anchor="example-lookup-obs">	<figure anchor="example-lookup-obs">

	<name>Example an observing resource lookup</name>	<name>Example of an Observing Resource Lookup</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET /rd-lookup/res?rt=tag:example.org,2020:light	Req: GET /rd-lookup/res?rt=tag:example.org,2020:light
	Observe: 0	Observe: 0

	Res: 2.05 Content	Res: 2.05 Content
	Observe: 23	Observe: 23
	Payload: empty	Payload: empty

	(at a later point in time)	(at a later point in time)

	Res: 2.05 Content	Res: 2.05 Content
	Observe: 24	Observe: 24
	Payload:	Payload:
	<coap://[2001:db8:3::124]/west>;rt="tag:example.org,2020:light",	<coap://[2001:db8:3::124]/west>;rt="tag:example.org,2020:light",
	<coap://[2001:db8:3::124]/south>;rt="tag:example.org,2020:light",	<coap://[2001:db8:3::124]/south>;rt="tag:example.org,2020:light",
	<coap://[2001:db8:3::124]/east>;rt="tag:example.org,2020:light"	<coap://[2001:db8:3::124]/east>;rt="tag:example.org,2020:light"

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>

	<t>The following example shows a client performing a paginated resource lookup</t>	<t>The following example shows a client performing a paginated resource lookup:</t>
	<figure anchor="example-lookup-page">	<figure anchor="example-lookup-page">

	<name>Examples of paginated resource lookup</name>	<name>Example of Paginated Resource Lookup</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET /rd-lookup/res?page=0&count=5	Req: GET /rd-lookup/res?page=0&count=5

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	<coap://[2001:db8:3::123]:61616/res/0>;ct=60,	<coap://[2001:db8:3::123]:61616/res/0>;ct=60,
	<coap://[2001:db8:3::123]:61616/res/1>;ct=60,	<coap://[2001:db8:3::123]:61616/res/1>;ct=60,
	<coap://[2001:db8:3::123]:61616/res/2>;ct=60,	<coap://[2001:db8:3::123]:61616/res/2>;ct=60,
	<coap://[2001:db8:3::123]:61616/res/3>;ct=60,	<coap://[2001:db8:3::123]:61616/res/3>;ct=60,
	<coap://[2001:db8:3::123]:61616/res/4>;ct=60	<coap://[2001:db8:3::123]:61616/res/4>;ct=60

	Req: GET /rd-lookup/res?page=1&count=5	Req: GET /rd-lookup/res?page=1&count=5

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	<coap://[2001:db8:3::123]:61616/res/5>;ct=60,	<coap://[2001:db8:3::123]:61616/res/5>;ct=60,
	<coap://[2001:db8:3::123]:61616/res/6>;ct=60,	<coap://[2001:db8:3::123]:61616/res/6>;ct=60,
	<coap://[2001:db8:3::123]:61616/res/7>;ct=60,	<coap://[2001:db8:3::123]:61616/res/7>;ct=60,
	<coap://[2001:db8:3::123]:61616/res/8>;ct=60,	<coap://[2001:db8:3::123]:61616/res/8>;ct=60,
	<coap://[2001:db8:3::123]:61616/res/9>;ct=60	<coap://[2001:db8:3::123]:61616/res/9>;ct=60

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	<t>The following example shows a client performing a lookup of all resou rces	<t>The following example shows a client performing a lookup of all resou rces
	of all endpoints of a given endpoint type. It assumes that two endpoints (with e ndpoint	of all endpoints of a given endpoint type. It assumes that two endpoints (with e ndpoint
	names <tt>sensor1</tt> and <tt>sensor2</tt>) have previously registered with the ir respective	names <tt>sensor1</tt> and <tt>sensor2</tt>) have previously registered with the ir respective

	addresses <tt>coap://sensor1.example.com</tt> and <tt>coap://sensor2.example.com	addresses (<tt>coap://sensor1.example.com</tt> and <tt>coap://sensor2.example.co
	</tt>, and	m</tt>) and
	posted the very payload of the 6th response of section 5 of <xref target="RFC669	posted the very payload of the 6th response of <xref target="RFC6690" sectionFor
	0" format="default"/>.</t>	mat="of" section="5"/>.</t>
	<t>It demonstrates how absolute link targets stay unmodified, while rela tive ones	<t>It demonstrates how absolute link targets stay unmodified, while rela tive ones
	are resolved:</t>	are resolved:</t>
	<figure anchor="example-lookup-multiple">	<figure anchor="example-lookup-multiple">

	<name>Example of resource lookup from multiple endpoints</name>	<name>Example of a Resource Lookup from Multiple Endpoints</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET /rd-lookup/res?et=tag:example.com,2020:platform	Req: GET /rd-lookup/res?et=tag:example.com,2020:platform

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	<coap://sensor1.example.com/sensors>;ct=40;title="Sensor Index",	<coap://sensor1.example.com/sensors>;ct=40;title="Sensor Index",
	<coap://sensor1.example.com/sensors/temp>;rt=temperature-c;if=sensor,	<coap://sensor1.example.com/sensors/temp>;rt=temperature-c;if=sensor,
	<coap://sensor1.example.com/sensors/light>;rt=light-lux;if=sensor,	<coap://sensor1.example.com/sensors/light>;rt=light-lux;if=sensor,
	<http://www.example.com/sensors/t123>;rel=describedby;	<http://www.example.com/sensors/t123>;rel=describedby;
	anchor="coap://sensor1.example.com/sensors/temp",	anchor="coap://sensor1.example.com/sensors/temp",
	<coap://sensor1.example.com/t>;rel=alternate;	<coap://sensor1.example.com/t>;rel=alternate;
	anchor="coap://sensor1.example.com/sensors/temp",	anchor="coap://sensor1.example.com/sensors/temp",
	<coap://sensor2.example.com/sensors>;ct=40;title="Sensor Index",	<coap://sensor2.example.com/sensors>;ct=40;title="Sensor Index",
	<coap://sensor2.example.com/sensors/temp>;rt=temperature-c;if=sensor,	<coap://sensor2.example.com/sensors/temp>;rt=temperature-c;if=sensor,
	<coap://sensor2.example.com/sensors/light>;rt=light-lux;if=sensor,	<coap://sensor2.example.com/sensors/light>;rt=light-lux;if=sensor,
	<http://www.example.com/sensors/t123>;rel=describedby;	<http://www.example.com/sensors/t123>;rel=describedby;
	anchor="coap://sensor2.example.com/sensors/temp",	anchor="coap://sensor2.example.com/sensors/temp",
	<coap://sensor2.example.com/t>;rel=alternate;	<coap://sensor2.example.com/t>;rel=alternate;
	anchor="coap://sensor2.example.com/sensors/temp"	anchor="coap://sensor2.example.com/sensors/temp"

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	</section>	</section>
	<section anchor="ep-lookup" numbered="true" toc="default">	<section anchor="ep-lookup" numbered="true" toc="default">

	<name>Endpoint lookup</name>	<name>Endpoint Lookup</name>
	<t>The endpoint lookup returns links to and information about registrati on resources,	<t>The endpoint lookup returns links to and information about registrati on resources,

	which themselves can only be manipulated by the registering endpoint.</t>	which themselves can only be manipulated by the registering endpoint.</t>
	<t>Endpoint registration resources are annotated with their endpoint nam	<t>Endpoint registration resources are annotated with their endpoint nam
	es (ep), sectors (d, if present) and registration base URI (base; reports the re	es (ep), sectors
	gistrant-ep's address if no explicit base was given) as well as a constant resou	(d, if present), and registration base URI (base; reports the registrant-
	rce type (rt="core.rd-ep"); the lifetime (lt) is not reported.	EP's address if
	Additional endpoint attributes are added as target attributes to their endpoint	no explicit base was given), as well as a constant resource type (rt="cor
	link unless their specification says otherwise.</t>	e.rd-ep"); the
	<t>Links to endpoints SHOULD be presented in path-absolute form or, if r	lifetime (lt) is not reported.
	equired, as (full) URIs. (This ensures that the output conforms to Limited Link	Additional endpoint attributes are added as target attributes to their en
	Format as described in <xref target="limitedlinkformat" format="default"/>.)</t>	dpoint link
	<t>Base addresses that contain link-local addresses MUST NOT include zon	unless their specification says otherwise.</t>
	e identifiers,	<t>Links to endpoints <bcp14>SHOULD</bcp14> be presented in path-absolut
	and such registrations <!-- or "' base attributes" --> MUST NOT be	e form or, if
	shown unless the lookup was made from the same link from which the	required, as (full) URIs. (This ensures that the output conforms to Limit
	registration was made.</t>	ed Link Format,
	<t>While Endpoint Lookup does expose the registration resources,	as described in <xref target="limitedlinkformat" format="default"/>.)</t>
	the RD does not need to make them accessible to clients.	<t>Base addresses that contain link-local addresses <bcp14>MUST NOT</bcp
	Clients SHOULD NOT attempt to dereference or manipulate them.</t>	14> include zone
	<t>An RD can report registrations in lookup whose URI scheme and authori	identifiers, and such registrations <bcp14>MUST
	ty differ from the lookup resource's.	NOT</bcp14> be shown unless the lookup was made from the same link from w
	Lookup clients MUST be prepared to see arbitrary URIs as registration resources	hich the
	in the results	registration was made.</t>
	and treat them as opaque identifiers;
	the precise semantics of such links are left to future specifications.</t>	<t>While the endpoint lookup does expose the registration resources,
	<t>The following example shows a client performing an endpoint lookup li	the RD does not need to make them accessible to clients.
	mited to endpoints of endpoint type <tt>tag:example.com,2020:platform</tt>:</t>	Clients <bcp14>SHOULD NOT</bcp14> attempt to dereference or manipulate th
		em.</t>
		<t> An RD can report registrations in lookup whose URI scheme and
		authority differ from that of the lookup resource.
		Lookup clients <bcp14>MUST</bcp14> be prepared to see arbitrary URIs as r
		egistration
		resources in the results and treat them as opaque identifiers;
		the precise semantics of such links are left to future specifications.</t
		>
		<t>The following example shows a client performing an endpoint lookup tha
		t is limited to
		endpoints of endpoint type <tt>tag:example.com,2020:platform</tt>:</t>
	<figure anchor="example-lookup-ep">	<figure anchor="example-lookup-ep">

	<name>Examples of endpoint lookup</name>	<name>Example of Endpoint Lookup</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET /rd-lookup/ep?et=tag:example.com,2020:platform	Req: GET /rd-lookup/ep?et=tag:example.com,2020:platform

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	</rd/1234>;base="coap://[2001:db8:3::127]:61616";ep=node5;	</rd/1234>;base="coap://[2001:db8:3::127]:61616";ep=node5;
	et="tag:example.com,2020:platform";ct=40;rt=core.rd-ep,	et="tag:example.com,2020:platform";ct=40;rt=core.rd-ep,
	</rd/4521>;base="coap://[2001:db8:3::129]:61616";ep=node7;	</rd/4521>;base="coap://[2001:db8:3::129]:61616";ep=node7;
	et="tag:example.com,2020:platform";ct=40;d=floor-3;	et="tag:example.com,2020:platform";ct=40;d=floor-3;
	rt=core.rd-ep	rt=core.rd-ep

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	</section>	</section>
	</section>	</section>
	<section anchor="policies" numbered="true" toc="default">	<section anchor="policies" numbered="true" toc="default">

	<name>Security policies</name>	<name>Security Policies</name>
	<t>The security policies that are applicable to an RD strongly depend on t	<t>The security policies that are applicable to an RD strongly depend on t
	he application,	he application
	and are not set out normatively here.</t>	and are not set out normatively here.</t>
	<t>This section provides a list of aspects that applications should consid er when describing their use of the RD,	<t>This section provides a list of aspects that applications should consid er when describing their use of the RD,
	without claiming to cover all cases.	without claiming to cover all cases.

	It is using terminology of <xref target="I-D.ietf-ace-oauth-authz" format="defau	It uses terminology of <xref target="I-D.ietf-ace-oauth-authz" format="default"/
	lt"/>,	>,
	in which the RD acts as the Resource Server (RS), and both registrant-eps and lo	in which the RD acts as the Resource Server (RS), and both registrant-EPs and lo
	okup clients act as Clients (C) with support from an Authorization Server (AS),	okup clients act as Clients (C) with support from an Authorization Server (AS),
	without the intention of ruling out other (e.g. certificate / public-key infrast	without the intention of ruling out other schemes (e.g., those based on certific
	ructure (PKI) based) schemes.</t>	ates/Public Key Infrastructures (PKIs)).</t>
	<t>Any, all or none of the below can apply to an application.	<t>Any, all, or none of the below can apply to an application.
	Which are relevant depends on its protection objectives.</t>	Which are relevant depends on its protection objectives.</t>

	<t>Security policies are set by configuration of the RD, or by choice of t	<t>Security policies are set by configuration of the RD or by choice of th
	he implementation.	e implementation.
	Lookup clients (and, where relevant, endpoints) can only trust an RD to uphold t	Lookup clients (and, where relevant, endpoints) can only trust an RD to uphold t
	hem if it is authenticated,	hem if it is authenticated
	and authorized to serve as an RD according to the application's requirements.</t >	and authorized to serve as an RD according to the application's requirements.</t >
	<section anchor="secure-ep" numbered="true" toc="default">	<section anchor="secure-ep" numbered="true" toc="default">

	<name>Endpoint name</name>	<name>Endpoint Name</name>
	<t>Whenever an RD needs to provide trustworthy results to clients doing	<t>Whenever an RD needs to provide trustworthy results to clients doing
	endpoint lookup,	endpoint lookup
	or resource lookup with filtering on the endpoint name,	or resource lookup with filtering on the endpoint name,
	the RD must ensure that the registrant is authorized to use the given endpoint n ame.	the RD must ensure that the registrant is authorized to use the given endpoint n ame.
	This applies both to registration and later to operations on the registration re source.	This applies both to registration and later to operations on the registration re source.

	It is immaterial whether the client is the registrant-ep itself or a CT is doing	It is immaterial whether the client is the registrant-EP itself or a CT is doing
	the registration:	the registration.
	The RD cannot tell the difference, and CTs may use authorization credentials aut	The RD cannot tell the difference, and CTs may use authorization credentials aut
	horizing only operations on that particular endpoint name, or a wider range of e	horizing only operations on that particular endpoint name or a wider range of en
	ndpoint names.</t>	dpoint names.</t>
	<t>It is up to the concrete security policy to describe	<t>It is up to the concrete security policy to describe

	how endpoint name and sector are transported when certificates are used.	how the endpoint name and sector are transported when certificates are used.
	For example, it may describe how SubjectAltName dNSName entries are mapped to en dpoint and domain names.</t>	For example, it may describe how SubjectAltName dNSName entries are mapped to en dpoint and domain names.</t>
	<section anchor="arbitrary-ep" numbered="true" toc="default">	<section anchor="arbitrary-ep" numbered="true" toc="default">

	<name>Random endpoint names</name>	<name>Random Endpoint Names</name>
	<t>Conversely, in applications where the RD does not check the endpoin t name,	<t>Conversely, in applications where the RD does not check the endpoin t name,
	the authorized registering endpoint can generate a random number (or string) tha t identifies the endpoint.	the authorized registering endpoint can generate a random number (or string) tha t identifies the endpoint.
	The RD should then remember unique properties of the registrant,	The RD should then remember unique properties of the registrant,
	associate them with the registration for as long as its registration resource is active (which may be longer than the registration's lifetime),	associate them with the registration for as long as its registration resource is active (which may be longer than the registration's lifetime),
	and require the same properties for operations on the registration resource.</t>	and require the same properties for operations on the registration resource.</t>

	<t>Registrants that are prepared to pick a different identifier when t
	heir initial attempt	<t>Registrants that are prepared to pick a different identifier when their initi
		al attempt
	(or attempts, in the unlikely case of two subsequent collisions)	(or attempts, in the unlikely case of two subsequent collisions)

	at registration is unauthorized should pick an identifier at least twice as long	at registration is unauthorized should pick an identifier at least twice as long
	as the expected number of registrants;	as would be needed to enumerate the expected number of registrants;
	registrants without such a recovery options should pick significantly longer end	registrants without any such recovery options should pick significantly longer e
	point names (e.g. using UUID URNs <xref target="RFC4122" format="default"/>).</t	ndpoint names (e.g., using Universally Unique Identifier (UUID) URNs <xref targe
	>	t="RFC4122" format="default"/>).</t>
	</section>	</section>
	</section>	</section>

	<section anchor="entered-resources" numbered="true" toc="default">	<section anchor="entered-links" numbered="true" toc="default">
	<name>Entered resources</name>	<name>Entered Links</name>
	<t>When lookup clients expect that certain types of links can only origi nate from certain endpoints,	<t>When lookup clients expect that certain types of links can only origi nate from certain endpoints,
	then the RD needs to apply filtering to the links an endpoint may register.</t>	then the RD needs to apply filtering to the links an endpoint may register.</t>
	<t>For example, if clients use an RD to find a server that provides firm ware updates,	<t>For example, if clients use an RD to find a server that provides firm ware updates,
	then any registrant that wants to register (or update) links to firmware sources will need to provide suitable credentials to do so, independently of its endpoi nt name.</t>	then any registrant that wants to register (or update) links to firmware sources will need to provide suitable credentials to do so, independently of its endpoi nt name.</t>

	<t>Note that the impact of having undesirable links in the RD depends on	<t>Note that the impact of having undesirable links in the RD depends on
	the application:	the application.
	if the client requires the firmware server to present credentials as a firmware	If the client requires the firmware server to present credentials as a firmware
	server,	server,
	a fraudulent link's impact is limited to the client revealing its intention to o btain updates and slowing down the client until it finds a legitimate firmware s erver;	a fraudulent link's impact is limited to the client revealing its intention to o btain updates and slowing down the client until it finds a legitimate firmware s erver;
	if the client accepts any credentials from the server as long as they fit the pr ovided URI, the impact is larger.</t>	if the client accepts any credentials from the server as long as they fit the pr ovided URI, the impact is larger.</t>

	<t>An RD may also require that links are only registered if the registra
	nt is authorized to publish information about the anchor (or even target) of the	<t>An RD may also require that links are only registered if the registrant is au
	link.	thorized to publish information about the anchor (or even target) of the link.
	One way to do this is to demand that the registrant present the same credentials	One way to do this is to demand that the registrant present the same credentials
	as a client that they'd need to present if contacted as a server at the resourc	in its role as a registering client that it would need to present in its role a
	es' URI, which may include using the address and port that are part of the URI.	s a server when contacted at the resources' URI. These credentials may include u
		sing the address and port that are part of the URI.
	Such a restriction places severe practical limitations on the links that can be registered.</t>	Such a restriction places severe practical limitations on the links that can be registered.</t>
	<t>As above, the impact of undesirable links depends on the extent to wh ich the lookup client relies on the RD.	<t>As above, the impact of undesirable links depends on the extent to wh ich the lookup client relies on the RD.

	To avoid the limitations, RD applications should consider <!-- can we pull in RF C6919 to make this normative? --> prescribing that lookup clients only use the d iscovered information as hints,	To avoid the limitations, RD applications should consider prescribing that looku p clients only use the discovered information as hints
	and describe which pieces of information need to be verified because they impact the application's security.	and describe which pieces of information need to be verified because they impact the application's security.
	A straightforward way to verify such information is to request it again from an authorized server, typically the one that hosts the target resource.	A straightforward way to verify such information is to request it again from an authorized server, typically the one that hosts the target resource.

	That similar to what happens in <xref target="discovery" format="default"/> when the URI discovery step is repeated.</t>	That is similar to what happens in <xref target="discovery" format="default"/> w hen the "URI discovery" step is repeated.</t>
	</section>	</section>
	<section anchor="link-confidentiality" numbered="true" toc="default">	<section anchor="link-confidentiality" numbered="true" toc="default">

	<name>Link confidentiality</name>	<name>Link Confidentiality</name>
	<t>When registrants publish information in the RD that is not available	<t>When registrants publish information in the RD that is not available
	to any client that would query the registrant's /.well-known/core interface,	to any client that would query the registrant's <tt>/.well-known/core</tt> inter
	or when lookups to that interface are subject so stricter firewalling than looku	face,
	ps to the RD,	or when lookups to that interface are subject to stricter firewalling than looku
		ps to the RD,
	the RD may need to limit which lookup clients may access the information.</t>	the RD may need to limit which lookup clients may access the information.</t>
	<t>In this case, the endpoint (and not the lookup clients) needs to be c areful to check the RD's authorization.	<t>In this case, the endpoint (and not the lookup clients) needs to be c areful to check the RD's authorization.
	The RD needs to check any lookup client's authorization	The RD needs to check any lookup client's authorization
	before revealing information directly (in resource lookup)	before revealing information directly (in resource lookup)
	or indirectly (when using it to satisfy a resource lookup search criterion).</t>	or indirectly (when using it to satisfy a resource lookup search criterion).</t>
	</section>	</section>
	<section anchor="segmentation" numbered="true" toc="default">	<section anchor="segmentation" numbered="true" toc="default">
	<name>Segmentation</name>	<name>Segmentation</name>
	<t>Within a single RD, different security policies can apply.</t>	<t>Within a single RD, different security policies can apply.</t>
	<t>One example of this are multi-tenant deployments separated by the sec tor (d) parameter.	<t>One example of this are multi-tenant deployments separated by the sec tor (d) parameter.
	Some sectors might apply limitations on the endpoint names available,	Some sectors might apply limitations on the endpoint names available,
	while others use a random identifier approach to endpoint names and place limits on the entered links based on their attributes instead.</t>	while others use a random identifier approach to endpoint names and place limits on the entered links based on their attributes instead.</t>
	<t>Care must be taken in such setups to determine the applicable access control measures to each operation.	<t>Care must be taken in such setups to determine the applicable access control measures to each operation.
	One easy way to do that is to mandate the use of the sector parameter on all ope rations,	One easy way to do that is to mandate the use of the sector parameter on all ope rations,
	as no credentials are suitable for operations across sector borders anyway.</t>	as no credentials are suitable for operations across sector borders anyway.</t>
	</section>	</section>
	<section anchor="first-come-first-remembered-a-default-policy" numbered="t rue" toc="default">	<section anchor="first-come-first-remembered-a-default-policy" numbered="t rue" toc="default">

	<name>First-Come-First-Remembered: A default policy</name>	<name>"First Come First Remembered": A Default Policy</name>
	<t>The First-Come-First-Remembered policy is provided both as a referenc	<t>The "First Come First Remembered" policy is provided both as a refere
	e example for a security policy definition,	nce example for a
	and as a policy that implementations may choose to use as default policy in abse	security policy definition and as a policy that implementations may choos
	nce of other configuration.	e to use as
	It is designed to enable efficient discovery operations even in ad-hoc settings.	default policy in the absence of any other configuration. It is designed
	</t>	to enable efficient
	<t>Under this policy, the RD accepts registrations for any endpoint name	discovery operations even in ad hoc settings.</t>
	that is not assigned to an active registration resource,	<t>Under this policy, the RD accepts registrations for any endpoint name
		that is not assigned to an active registration resource
	and only accepts registration updates from the same endpoint.	and only accepts registration updates from the same endpoint.

	The policy is minimal in that towards lookup clients it does not make any of the	The policy is minimal in that it does not make any promises to lookup clients ab
	claims of <xref target="entered-resources" format="default"/> and <xref target=	out the claims of Sections <xref target="entered-links" format="counter"/> and <
	"link-confidentiality" format="default"/>,	xref target="link-confidentiality" format="counter"/>,
	and its claims on <xref target="secure-ep" format="default"/> are limited to the	and promises about the claims in <xref target="secure-ep" format="default"/> are
	lifetime of that endpoint's registration.	limited to the lifetime of that endpoint's registration.
	It does, however, guarantee towards any endpoint that for the duration of its re	It does however promise the endpoint that, for the duration of its registration,
	gistration, its links will be discoverable on the RD.</t>	its links will be discoverable on the RD.</t>
	<t>When a registration or operation is attempted, the RD MUST determine	<t>When a registration or operation is attempted, the RD <bcp14>MUST</bc
	the client's subject name or public key:</t>	p14> determine the client's subject name or public key:</t>
	<ul spacing="normal">	<ul spacing="normal">

	<li>If the client's credentials indicate any subject name that is cert	<li>If the client's credentials indicate any subject name that is cert
	ified by any authority which the RD recognizes (which may be the system's trust	ified by any authority that the RD recognizes (which may be the system's trust a
	anchor store), all such subject names are stored.	nchor store), all such subject names are stored.
	With CWT or JWT based credentials (as common with ACE), the Subject (sub) claim	With credentials based on CWT or JWT (as common with Authentication and Authoriz
	is stored as a single name, if it exists.	ation for Constrained Environments (ACE)), the Subject (sub) claim is stored as
		a single name, if it exists.
	With X.509 certificates, the Common Name (CN) and the complete list of SubjectAl tName entries are stored.	With X.509 certificates, the Common Name (CN) and the complete list of SubjectAl tName entries are stored.
	In both cases, the authority that certified the claim is stored along with the s ubject, as the latter may only be locally unique.</li>	In both cases, the authority that certified the claim is stored along with the s ubject, as the latter may only be locally unique.</li>
	<li>Otherwise, if the client proves possession of a private key, the m atching public key is stored.	<li>Otherwise, if the client proves possession of a private key, the m atching public key is stored.
	This applies both to raw public keys and to the public keys indicated in certifi cates that failed the above authority check.</li>	This applies both to raw public keys and to the public keys indicated in certifi cates that failed the above authority check.</li>
	<li>If neither is present, a reference to the security session itself is stored.	<li>If neither is present, a reference to the security session itself is stored.

	With (D)TLS, that is the connection itself, or the session resumption informatio n if available.	With (D)TLS, that is the connection itself or the session resumption information , if available.
	With OSCORE, that is the security context.</li>	With OSCORE, that is the security context.</li>
	</ul>	</ul>
	<t>As part of the registration operation, that information is stored alo ng with the registration resource.</t>	<t>As part of the registration operation, that information is stored alo ng with the registration resource.</t>

	<t>The RD MUST accept all registrations whose registration resource is n ot already active,	<t>The RD <bcp14>MUST</bcp14> accept all registrations whose registratio n resource is not already active,
	as long as they are made using a security layer supported by the RD.</t>	as long as they are made using a security layer supported by the RD.</t>
	<t>Any operation on a registration resource,	<t>Any operation on a registration resource,
	including registrations that lead to an existing registration resource,	including registrations that lead to an existing registration resource,

	MUST be rejected by the RD unless all the stored information is found in the new	<bcp14>MUST</bcp14> be rejected by the RD unless all the stored information is f
	request's credentials.</t>	ound in the new request's credentials.</t>
	<t>Note that even though subject names are compared in this policy,
		<t>Note that, even though subject names are compared in this policy,
	they are never directly compared to endpoint names,	they are never directly compared to endpoint names,

	and an endpoint can not expect to "own" any particular endpoint name outside of an active registration --	and an endpoint cannot expect to "own" any particular endpoint name outside of a n active registration --
	even if a certificate says so.	even if a certificate says so.
	It is an accepted shortcoming of this approach that the endpoint has no indicati on of whether the RD remembers it by its subject name or public key;	It is an accepted shortcoming of this approach that the endpoint has no indicati on of whether the RD remembers it by its subject name or public key;

	recognition by subject happens on a best-effort base (given the RD may not recog	recognition by subject happens on a best-effort basis (given the RD may not reco
	nize any authority).	gnize any authority).
	Clients MUST be prepared to pick a different endpoint name when rejected by the	Clients <bcp14>MUST</bcp14> be prepared to pick a different endpoint name when r
	RD initially or after a change in their credentials;	ejected by the RD initially or after a change in their credentials;
	picking an endpoint name as per <xref target="arbitrary-ep" format="default"/> i	picking an endpoint name, as per <xref target="arbitrary-ep" format="default"/>,
	s an easy option for that.</t>	is an easy option for that.</t>
	<t>For this policy to be usable without configuration, clients should no t set a sector name in their registrations.	<t>For this policy to be usable without configuration, clients should no t set a sector name in their registrations.
	An RD can set a default sector name for registrations accepted under this policy ,	An RD can set a default sector name for registrations accepted under this policy ,

	which is useful especially in a segmented setup where different policies apply t o different sectors.	which is especially useful in a segmented setup where different policies apply t o different sectors.
	The configuration of such a behavior, as well as any other configuration applica ble to such an RD	The configuration of such a behavior, as well as any other configuration applica ble to such an RD

	(i.e. the set of recognized authorities)	(i.e., the set of recognized authorities),
	is out of scope for this document.</t>	is out of scope for this document.</t>
	</section>	</section>
	</section>	</section>
	<section anchor="security-considerations" numbered="true" toc="default">	<section anchor="security-considerations" numbered="true" toc="default">
	<name>Security Considerations</name>	<name>Security Considerations</name>

	<t>The security considerations as described in Section 5 of <xref target="	<t>The security considerations as described in <xref target="RFC8288" sect
	RFC8288" format="default"/> and	ion="5" sectionFormat="of"/> and <xref target="RFC6690" section="6" sectionForma
	Section 6 of <xref target="RFC6690" format="default"/> apply. The <tt>/.well-kno	t="of"/> apply. The <tt>/.well-known/core</tt> resource may be
	wn/core</tt> resource may be	protected, e.g., using DTLS when hosted on a CoAP server, as described in
	protected e.g. using DTLS when hosted on a CoAP server as described in
	<xref target="RFC7252" format="default"/>.</t>	<xref target="RFC7252" format="default"/>.</t>

	<t>Access that is limited or affects sensitive data SHOULD be protected,	<t>Access that is limited or affects sensitive data <bcp14>SHOULD</bcp14>
	e.g. using (D)TLS or OSCORE (<xref target="RFC8613" format="default"/>;	be protected,
		e.g., using (D)TLS or OSCORE <xref target="RFC8613" format="default"/>;
	which aspects of the RD this affects depends on the security policies of the app lication (see <xref target="policies" format="default"/>).</t>	which aspects of the RD this affects depends on the security policies of the app lication (see <xref target="policies" format="default"/>).</t>
	<section anchor="seccons-discovery" numbered="true" toc="default">	<section anchor="seccons-discovery" numbered="true" toc="default">
	<name>Discovery</name>	<name>Discovery</name>
	<t>Most steps in discovery of the RD, and possibly its resources, are no t covered by CoAP's security mechanisms.	<t>Most steps in discovery of the RD, and possibly its resources, are no t covered by CoAP's security mechanisms.
	This will not endanger the security properties of the registrations and lookup i tself	This will not endanger the security properties of the registrations and lookup i tself

	(where the client requires authorization of the RD if it expects any security pr	(where the client requires authorization of the RD if it expects any security pr
	operties of the operation),	operties of the operation)
	but may leak the client's intention to third parties,	but may leak the client's intention to third parties
	and allow them to slow down the process.</t>	and allow them to slow down the process.</t>
	<t>To mitigate that, clients can retain the RD's address,	<t>To mitigate that, clients can retain the RD's address,

	use secure discovery options like configured addresses,	use secure discovery options (such as configured addresses),
	and send queries for RDs in a very general form (<tt>?rt=core.rd*</tt> rather th	and send queries for RDs in a very general form (e.g., <tt>?rt=core.rd*</tt> rat
	an <tt>?rt=core.rd-lookup-ep</tt>).</t>	her than <tt>?rt=core.rd-lookup-ep</tt>).</t>
	</section>	</section>
	<section anchor="endpoint_identification" numbered="true" toc="default">	<section anchor="endpoint_identification" numbered="true" toc="default">
	<name>Endpoint Identification and Authentication</name>	<name>Endpoint Identification and Authentication</name>

	<t>An Endpoint (name, sector) pair is unique within the set of endpoints	<t>An endpoint (name, sector) pair is unique within the set of endpoints
	registered by the RD. An Endpoint MUST NOT be identified by its protocol, port	registered by the RD. An endpoint <bcp14>MUST NOT</bcp14> be identified by its
	or IP	protocol, port, or IP
	address as these may change over the lifetime of an Endpoint.</t>	address, as these may change over the lifetime of an endpoint.</t>
	<t>Every operation performed by an Endpoint on an RD	<t>Every operation performed by an endpoint on an RD
	SHOULD be mutually authenticated using Pre-Shared Key, Raw Public Key or	<bcp14>SHOULD</bcp14> be mutually authenticated using a pre-shared key, a raw pu
	Certificate based security.</t>	blic key, or
	<t>Consider the following threat: two devices A and B are registered at	certificate-based security.</t>
	a single server. Both devices have unique, per-device credentials for use with D	<t>Consider the following threat: two devices, A and B, are registered a
	TLS to make sure that only parties with authorization to access A or B can do so	t a single server. Both devices have unique, per-device credentials for use with
	.</t>	DTLS to make sure that only parties with authorization to access A or B can do
		so.</t>
	<t>Now, imagine that a malicious device A wants to sabotage the device B . It uses its credentials during the DTLS exchange. Then, it specifies the	<t>Now, imagine that a malicious device A wants to sabotage the device B . It uses its credentials during the DTLS exchange. Then, it specifies the
	endpoint name of device B as the name of its own endpoint in device A. If the se rver does not check	endpoint name of device B as the name of its own endpoint in device A. If the se rver does not check
	whether the identifier provided in the DTLS handshake matches the	whether the identifier provided in the DTLS handshake matches the

	identifier used at the CoAP layer then it may be inclined to use the	identifier used at the CoAP layer, then it may be inclined to use the
	endpoint name for looking up what information to provision to the malicious devi ce.</t>	endpoint name for looking up what information to provision to the malicious devi ce.</t>

	<t>Endpoint authorization needs to be checked on registration and regist
	ration resource operations	<t>Endpoint authorization needs to be checked on registration and registration r
	independently of whether there are configured requirements on the credentials fo	esource operations
	r a given endpoint name (and sector; <xref target="secure-ep" format="default"/>	independently of whether there are configured requirements on the credentials fo
	)	r a given endpoint name and sector (<xref target="secure-ep" format="default"/>)
	or whether arbitrary names are accepted (<xref target="arbitrary-ep" format="def ault"/>).</t>	or whether arbitrary names are accepted (<xref target="arbitrary-ep" format="def ault"/>).</t>

	<t>Simple registration could be used to circumvent address-based access	<t>Simple registration could be used to circumvent address-based access
	control:	control.
	An attacker would send a simple registration request with the victim's address a	An attacker would send a simple registration request with the victim's address a
	s source address,	s the source address
	and later look up the victim's /.well-known/core content in the RD.	and later look up the victim's <tt>/.well-known/core</tt> content in the RD.
	Mitigation for this is recommended in <xref target="simple" format="default"/>.< /t>	Mitigation for this is recommended in <xref target="simple" format="default"/>.< /t>

	<t>The registration resource path is visible to any client that is allow ed endpoint lookup,	<t>The registration resource path is visible to any client that is allow ed endpoint lookup
	and can be extracted by resource lookup clients as well.	and can be extracted by resource lookup clients as well.
	The same goes for registration attributes that are shown as target attributes or lookup attributes.	The same goes for registration attributes that are shown as target attributes or lookup attributes.
	The RD needs to consider this in the choice of registration resource paths,	The RD needs to consider this in the choice of registration resource paths,

	and administrators or endpoint in their choice of attributes.</t>	as do administrators or endpoints in their choice of attributes.</t>
	</section>	</section>
	<section anchor="access-control" numbered="true" toc="default">	<section anchor="access-control" numbered="true" toc="default">
	<name>Access Control</name>	<name>Access Control</name>

	<t>Access control SHOULD be performed separately for the RD registration and Lookup	<t>Access control <bcp14>SHOULD</bcp14> be performed separately for the RD registration and lookup
	API paths, as different endpoints may be authorized to register	API paths, as different endpoints may be authorized to register

	with an RD from those authorized to lookup endpoints from the RD. Such access	with an RD from those authorized to look up endpoints from the RD. Such access
	control SHOULD be performed in as fine-grained a level as possible. For example	control <bcp14>SHOULD</bcp14> be performed in as fine-grained a level as possibl
	access control for lookups could be performed either at the sector, endpoint	e. For example,
		access control for lookups could be performed either at the sector, endpoint,
	or resource level.</t>	or resource level.</t>
	<t>The precise access controls necessary (and the consequences of failur e to enforce them)	<t>The precise access controls necessary (and the consequences of failur e to enforce them)
	depend on the protection objectives of the application and the security policies (<xref target="policies" format="default"/>) derived from them.</t>	depend on the protection objectives of the application and the security policies (<xref target="policies" format="default"/>) derived from them.</t>
	</section>	</section>
	<section anchor="denial-of-service-attacks" numbered="true" toc="default">	<section anchor="denial-of-service-attacks" numbered="true" toc="default">

	<name>Denial of Service Attacks</name>	<name>Denial-of-Service Attacks</name>
	<t>Services that run over UDP unprotected are vulnerable to unknowingly	<t>Services that run over UDP unprotected are vulnerable to unknowingly

	amplify and distribute a DoS attack as UDP does not require return	amplify and distribute a DoS attack, as UDP does not require a return
	routability check.	routability check.
	Since RD lookup responses can be significantly	Since RD lookup responses can be significantly
	larger than requests, RDs are prone to this.</t>	larger than requests, RDs are prone to this.</t>

	<t><xref target="RFC7252" format="default"/> describes this at length in its Section 11.3,	<t><xref target="RFC7252" format="default"/> describes this at length in its Section <xref target="RFC7252" section="11.3" sectionFormat="bare"/>,
	including some mitigation by using small block sizes in responses.	including some mitigation by using small block sizes in responses.

	The upcoming <xref target="I-D.ietf-core-echo-request-tag" format="default"/> up dates that	<xref target="RFC9175" format="default"/> updates that
	by describing a source address verification mechanism using the Echo option.</t>	by describing a source address verification mechanism using the Echo option.</t>

	<t>[ If this document is published together with or after I-D.ietf-core-
	echo-request-tag,
	the above paragraph is replaced with the following:</t>
	<t><xref target="RFC7252" format="default"/> describes this at length in
	its Section 11.3,
	and <xref target="I-D.ietf-core-echo-request-tag" format="default"/> (which upda
	tes the former)
	recommends using the Echo option to verify the request's source address.</t>
	<t>]</t>
	</section>	</section>
	<section anchor="skipping-freshness-checks" numbered="true" toc="default">	<section anchor="skipping-freshness-checks" numbered="true" toc="default">

	<name>Skipping freshness checks</name>	<name>Skipping Freshness Checks</name>
	<t>When RD based applications are built in which request freshness check	<t>When RD-based applications are built in which request freshness check
	s are not performed,	s are not performed,
	these concerns need to be balanced:</t>	these concerns need to be balanced:</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>	<li>
	<t>When alterations to registration attributes are reordered,	<t>When alterations to registration attributes are reordered,
	an attacker may create any combination of attributes ever set,	an attacker may create any combination of attributes ever set,
	with the attack difficulty determined by the security layer's replay properties. </t>	with the attack difficulty determined by the security layer's replay properties. </t>
	<t>	<t>
	For example, if <xref target="example-freshness" format="default"/> were conduct ed without freshness assurances,	For example, if <xref target="example-freshness" format="default"/> were conduct ed without freshness assurances,
	an attacker could later reset the lifetime back to 7200.	an attacker could later reset the lifetime back to 7200.

	<!-- That is, with DTLS without replay protection, or DTLS when causing a retran
	smission and swallowing one message,
	or with OSCORE when swallowing all of an update's retransmission but the ep late
	r tries again. -->
	Thus, the device is made unreachable to lookup clients.</t>	Thus, the device is made unreachable to lookup clients.</t>
	</li>	</li>
	<li>	<li>
	<t>When registration updates without query parameters	<t>When registration updates without query parameters

	(which just serve to restart the lifetime)	(which just serve to restart the lifetime) can be reordered,
	can be reordered,	an attacker can use intercepted messages to give the appearance of th
	an attacker can use intercepted messages to give the appearance of the device be	e device being
	ing alive to the RD. </t>	alive to the RD. </t>
	<t>	<t>This is unacceptable when the RD's security policy promises reacha
	This is unacceptable when when the RD's security policy promises reachability of	bility of
	endpoints	endpoints (e.g., when disappearing devices would trigger further inve
	(e.g. when disappearing devices would trigger further investigation),	stigation)
	but may be acceptable with other policies.</t>	but may be acceptable with other policies.</t>
	</li>	</li>
	</ul>	</ul>
	</section>	</section>
	</section>	</section>


	<section anchor="iana-considerations" numbered="true" toc="default">	<section anchor="iana-considerations" numbered="true" toc="default">
	<name>IANA Considerations</name>	<name>IANA Considerations</name>
	<section anchor="iana-rt" numbered="true" toc="default">	<section anchor="iana-rt" numbered="true" toc="default">
	<name>Resource Types</name>	<name>Resource Types</name>

	<t>IANA is asked to enter the following values into the Resource Type (r	<t>IANA has added the following values to the "Resource Type (rt=) Link
	t=) Link	Target Attribute Values"
	Target Attribute Values sub-registry of the Constrained Restful Environments	subregistry of the "Constrained RESTful Environments (CoRE) Parameters"
	(CoRE) Parameters registry defined in <xref target="RFC6690" format="default"/>:	registry defined in <xref target="RFC6690" format="default"/>:</t>
	</t>
	<table align="center">	<table align="center">
		<name>Additions to Resource Type (rt=) Link Target Attribute Values Subregistr
		y</name>
	<thead>	<thead>
	<tr>	<tr>
	<th align="left">Value</th>	<th align="left">Value</th>
	<th align="left">Description</th>	<th align="left">Description</th>
	<th align="left">Reference</th>	<th align="left">Reference</th>
	</tr>	</tr>
	</thead>	</thead>
	<tbody>	<tbody>
	<tr>	<tr>
	<td align="left">core.rd</td>	<td align="left">core.rd</td>
	<td align="left">Directory resource of an RD</td>	<td align="left">Directory resource of an RD</td>

	<td align="left">RFCTHIS <xref target="discovery" format="default" /></td>	<td align="left">RFC 9176, <xref target="discovery" format="defaul t"/></td>
	</tr>	</tr>
	<tr>	<tr>
	<td align="left">core.rd-lookup-res</td>	<td align="left">core.rd-lookup-res</td>
	<td align="left">Resource lookup of an RD</td>	<td align="left">Resource lookup of an RD</td>

	<td align="left">RFCTHIS <xref target="discovery" format="default" /></td>	<td align="left">RFC 9176, <xref target="discovery" format="defaul t"/></td>
	</tr>	</tr>
	<tr>	<tr>
	<td align="left">core.rd-lookup-ep</td>	<td align="left">core.rd-lookup-ep</td>
	<td align="left">Endpoint lookup of an RD</td>	<td align="left">Endpoint lookup of an RD</td>

	<td align="left">RFCTHIS <xref target="discovery" format="default" /></td>	<td align="left">RFC 9176, <xref target="discovery" format="defaul t"/></td>
	</tr>	</tr>
	<tr>	<tr>
	<td align="left">core.rd-ep</td>	<td align="left">core.rd-ep</td>
	<td align="left">Endpoint resource of an RD</td>	<td align="left">Endpoint resource of an RD</td>

	<td align="left">RFCTHIS <xref target="lookup" format="default"/>< /td>	<td align="left">RFC 9176, <xref target="lookup" format="default"/ ></td>
	</tr>	</tr>
	</tbody>	</tbody>
	</table>	</table>
	</section>	</section>
	<section anchor="ipv6-nd-resource-directory-address-option" numbered="true " toc="default">	<section anchor="ipv6-nd-resource-directory-address-option" numbered="true " toc="default">
	<name>IPv6 ND Resource Directory Address Option</name>	<name>IPv6 ND Resource Directory Address Option</name>

	<t>This document registers one new ND option type under the sub-registry	<t>IANA has registered one new ND option type in the "IPv6 Neighbor
	"IPv6 Neighbor Discovery Option Formats"	Discovery Option Formats" subregistry of the "Internet Control Message
	of the "Internet Control Message Protocol version 6 (ICMPv6) Parameters" registr	Protocol version 6 (ICMPv6) Parameters" registry:</t>
	y:</t>
	<ul spacing="normal">	<table>
	<li>Resource Directory Address Option (TBD38)</li>	<name>Addition to IPv6 Neighbor Discovery Option Formats Subregistry</name>
	</ul>	<thead>
	<t>[ The RFC editor is asked to replace TBD38	<tr>
	with the assigned number in the document;	<th>Type</th>
	the value 38 is suggested. ]</t>	<th>Description</th>
		<th>Reference</th>
		</tr>
		</thead>
		<tbody>
		<tr>
		<td>41</td>
		<td>Resource Directory Address Option</td>
		<td>RFC 9176</td>
		</tr>
		</tbody>
		</table>

	</section>	</section>


	<section anchor="iana-registry" numbered="true" toc="default">	<section anchor="iana-registry" numbered="true" toc="default">

	<name>RD Parameter Registry</name>	<name>RD Parameters Registry</name>
	<t>This specification defines a new sub-registry for registration and lo	<t>This specification defines a new subregistry for registration and
	okup	lookup parameters called "RD Parameters" within the "Constrained
	parameters called "RD Parameters" under "CoRE Parameters". Although this	RESTful Environments (CoRE) Parameters" registry. Although this
	specification defines a basic set of parameters, it is expected that other	specification defines a basic set of parameters, it is expected that
	standards that make use of this interface will define new ones.</t>	other standards that make use of this interface will define new
	<t>Each entry in the registry must include</t>	ones.</t>
		<t>Each entry in the registry must include:</t>
	<ul spacing="normal">	<ul spacing="normal">

	<li>the human readable name of the parameter,</li>	<li>the human-readable name of the parameter,</li>
	<li>the short name as used in query parameters or target attributes,</	<li>the short name, as used in query parameters or target attributes,<
	li>	/li>
	<li>indication of whether it can be passed as a query parameter at reg	<li>syntax and validity requirements (if any),</li>
	istration of endpoints, as a query parameter in lookups, or be expressed as a ta	<li>indication of whether it can be passed as a query parameter at reg
	rget attribute,</li>	istration of
	<li>syntax and validity requirements if any,</li>	endpoints, passed as a query parameter in lookups, or expressed as a ta
	<li>a description,</li>	rget
	<li>and a link to reference documentation.</li>	attribute,</li>
		<li>a description, and</li>
		<li>a link to reference documentation.</li>
	</ul>	</ul>

	<t>The query parameter MUST be both a valid URI query key <xref target="	<t>The query parameter <bcp14>MUST</bcp14> be both a valid URI query key
	RFC3986" format="default"/> and a token as used in <xref target="RFC8288" format	<xref target="RFC3986" format="default"/> and a token as used in <xref target="
	="default"/>.</t>	RFC8288" format="default"/>.</t>
	<t>The description must give details on whether the parameter can be upd	<t>The reference documentation must give details on whether the paramete
	ated, and how it is to be processed in lookups.</t>	r can be updated and how it is to be processed in lookups.</t>
	<t>The mechanisms around new RD parameters should be designed in such a	<t>The mechanisms around new RD parameters should be designed in such a
	way that they tolerate RD implementations that are unaware of the parameter and	way that they tolerate RD implementations that are unaware of the parameter and
	expose any parameter passed at registration or updates on in endpoint lookups. (	expose any parameter passed at registration or updates in endpoint lookups. (For
	For example, if a parameter used at registration were to be confidential, the re	example, if a parameter used at registration were to be confidential, the regis
	gistering endpoint should be instructed to only set that parameter if the RD adv	tering endpoint should be instructed to only set that parameter if the RD advert
	ertises support for keeping it confidential at the discovery step.)</t>	ises support for keeping it confidential at the discovery step.)</t>
	<t>Initial entries in this sub-registry are as follows:</t>	<t>Initial entries in this subregistry are as follows:</t>
	<table anchor="tab-registry" align="center">	<table anchor="tab-registry" align="center">

	<name>RD Parameters</name>	<name>New RD Parameters Registry</name>
	<thead>	<thead>
	<tr>	<tr>

	<th align="left">Full name</th>	<th align="left">Name</th>
	<th align="left">Short</th>	<th align="left">Short</th>
	<th align="left">Validity</th>	<th align="left">Validity</th>
	<th align="left">Use</th>	<th align="left">Use</th>
	<th align="left">Description</th>	<th align="left">Description</th>
	</tr>	</tr>
	</thead>	</thead>
	<tbody>	<tbody>
	<tr>	<tr>
	<td align="left">Endpoint Name</td>	<td align="left">Endpoint Name</td>
	<td align="left">ep</td>	<td align="left">ep</td>

	skipping to change at line 2143 ¶	skipping to change at line 2052 ¶
	<td align="left">d</td>	<td align="left">d</td>
	<td align="left">Unicode*</td>	<td align="left">Unicode*</td>
	<td align="left">RLA</td>	<td align="left">RLA</td>
	<td align="left">Sector to which this endpoint belongs</td>	<td align="left">Sector to which this endpoint belongs</td>
	</tr>	</tr>
	<tr>	<tr>
	<td align="left">Registration Base URI</td>	<td align="left">Registration Base URI</td>
	<td align="left">base</td>	<td align="left">base</td>
	<td align="left">URI</td>	<td align="left">URI</td>
	<td align="left">RLA</td>	<td align="left">RLA</td>

	<td align="left">The scheme, address and port and path at which th	<td align="left">The scheme, address, port, and path at which this
	is server is available</td>	server is
		available</td>
	</tr>	</tr>
	<tr>	<tr>
	<td align="left">Page</td>	<td align="left">Page</td>
	<td align="left">page</td>	<td align="left">page</td>
	<td align="left">Integer</td>	<td align="left">Integer</td>
	<td align="left">L</td>	<td align="left">L</td>
	<td align="left">Used for pagination</td>	<td align="left">Used for pagination</td>
	</tr>	</tr>
	<tr>	<tr>
	<td align="left">Count</td>	<td align="left">Count</td>
	<td align="left">count</td>	<td align="left">count</td>
	<td align="left">Integer</td>	<td align="left">Integer</td>
	<td align="left">L</td>	<td align="left">L</td>
	<td align="left">Used for pagination</td>	<td align="left">Used for pagination</td>
	</tr>	</tr>
	<tr>	<tr>
	<td align="left">Endpoint Type</td>	<td align="left">Endpoint Type</td>
	<td align="left">et</td>	<td align="left">et</td>
	<td align="left">	<td align="left">

	<xref target="et-description" format="default"/></td>	RFC 9176, <xref target="et-description" format="default"/></td>
	<td align="left">RLA</td>	<td align="left">RLA</td>

	<td align="left">Semantic type of the endpoint (see <xref target=" et-registry" format="default"/>)</td>	<td align="left">Semantic type of the endpoint (see RFC 9176, <xre f target="et-registry" format="default"/>)</td>
	</tr>	</tr>
	</tbody>	</tbody>
	</table>	</table>

	<t>(Short: Short name used in query parameters or target attributes. Val	<t>Where:</t>
	idity: Unicode* = 63 Bytes of UTF-8 encoded Unicode, with no control characters	<dl newline="false" spacing="normal">
	as per <xref target="registration" format="default"/>. Use: R = used at registra	<dt>Short:</dt>
	tion, L = used at lookup, A = expressed in target attribute.)</t>	<dd>Short name used in query parameters or target attributes</dd>

		<dt>Validity:</dt>
		<dd>
		<t><br/></t>
		<dl spacing="normal">
		<dt>Unicode* = </dt>
		<dd>up to 63 bytes of UTF-8-encoded Unicode, with no control charac
		ters as per
		<xref target="registration" format="default"/></dd>
		</dl>
		</dd>

		<dt>Use:</dt>
		<dd>
		<t><br/></t>
		<dl spacing="compact">
		<dt>R =</dt>
		<dd>used at registration</dd>
		<dt>L =</dt>
		<dd>used at lookup</dd>
		<dt>A =</dt>
		<dd>expressed in the target attribute</dd>

		</dl>
		</dd>
		</dl>

	<t>The descriptions for the options defined in this document are only su mmarized here.	<t>The descriptions for the options defined in this document are only su mmarized here.

	To which registrations they apply and when they are to be shown is described in the respective sections of this document.	To which registrations they apply and when they are to be shown are described in the respective sections of this document.
	All their reference documentation entries point to this document.</t>	All their reference documentation entries point to this document.</t>

	<t>The IANA policy for future additions to the sub-registry is "Expert R eview"	<t>The IANA policy for future additions to the subregistry is Expert Rev iew,
	as described in <xref target="RFC8126" format="default"/>. The evaluation should consider	as described in <xref target="RFC8126" format="default"/>. The evaluation should consider
	formal criteria,	formal criteria,

	duplication of functionality (Is the new entry redundant with an existing one?),	duplication of functionality (i.e., is the new entry redundant with an existing
	topical suitability (E.g. is the described property actually a property of the e	one?),
	ndpoint and not a property of a particular resource, in which case it should go	topical suitability (e.g., is the described property actually a property of the
	into the payload of the registration and need not be registered?),	endpoint and not a property of a particular resource, in which case it should go
	and the potential for conflict with commonly used target attributes (For example	into the payload of the registration and need not be registered?),
	, <tt>if</tt> could be used as a parameter for conditional registration if it we	and the potential for conflict with commonly used target attributes (e.g., <tt>i
	re not to be used in lookup or attributes, but would make a bad parameter for lo	f</tt> could be used as a parameter for conditional registration if it were not
	okup, because a resource lookup with an <tt>if</tt> query parameter could ambigu	to be used in lookup or attributes but would make a bad parameter for lookup bec
	ously filter by the registered endpoint property or the <xref target="RFC6690" f	ause a resource lookup with an <tt>if</tt> query parameter could ambiguously fil
	ormat="default"/> target attribute).</t>	ter by the registered endpoint property or the target attribute <xref target="RF
		C6690" format="default"/>).</t>
	<section anchor="et-description" numbered="true" toc="default">	<section anchor="et-description" numbered="true" toc="default">

	<name>Full description of the "Endpoint Type" RD Parameter</name>	<name>Full Description of the "Endpoint Type" RD Parameter</name>
	<t>An endpoint registering at an RD can describe itself with endpoint	<t>An endpoint registering at an RD can describe itself with
	types,	endpoint types, similar to how resources are described with resource
	similar to how resources are described with Resource Types in <xref target="RFC6	types in <xref target="RFC6690" format="default"/>. An endpoint
	690" format="default"/>.	type is expressed as a string, which can be either a URI or one of
	An endpoint type is expressed as a string, which can be either a URI or one of	the values defined in the "Endpoint Type (et=) RD Parameter Values"
	the values defined in the Endpoint Type sub-registry.	subregistry. Endpoint types can be passed in the <tt>et</tt> query
	Endpoint types can be passed in the <tt>et</tt> query parameter as part of extra	parameter as part of extra-attrs at the "registration" step of <xref t
	-attrs	arget="registration"/>, are shown
	at the Registration step,	on endpoint lookups using the <tt>et</tt> target attribute, and can
	are shown on endpoint lookups using the <tt>et</tt> target attribute,	be filtered for using <tt>et</tt> as a search criterion in resource
	and can be filtered for using <tt>et</tt> as a search criterion in resource and	and endpoint lookup. Multiple endpoint types are given as separate
	endpoint lookup.	query parameters or link attributes.</t>
	Multiple endpoint types are given as separate query parameters or link	<t>Note that the endpoint type differs from the resource type in that
	attributes.</t>	it uses multiple
	<t>Note that Endpoint Type differs from Resource Type in that it uses	attributes rather than space-separated values.
	multiple
	attributes rather than space separated values.
	As a result, RDs implementing this specification automatically support correct	As a result, RDs implementing this specification automatically support correct
	filtering in the lookup interfaces from the rules for unknown endpoint	filtering in the lookup interfaces from the rules for unknown endpoint
	attributes.</t>	attributes.</t>
	</section>	</section>
	</section>	</section>
	<section anchor="et-registry" numbered="true" toc="default">	<section anchor="et-registry" numbered="true" toc="default">

	<name>"Endpoint Type" (et=) RD Parameter values</name>	<name>Endpoint Type (et=) RD Parameter Values</name>
	<t>This specification establishes a new sub-registry under "CoRE Paramet	<t>This specification establishes a new subregistry called "Endpoint
	ers"	Type (et=) RD Parameter Values" within the "Constrained RESTful
	called '"Endpoint Type" (et=) RD Parameter values'.	Environments (CoRE) Parameters" registry.
	The registry properties (required policy, requirements, template) are identical	The registry properties (required policy, requirements, and template) are
	to those of the Resource Type parameters in <xref target="RFC6690" format="defau	identical to those of the "Resource Type (rt=) Link Target Attribute
	lt"/>, in short:</t>	Values" subregistry defined in <xref target="RFC6690" format="default"/>;
	<t>The review policy is IETF Review for values starting with "core", and	in short, the review policy is IETF Review for
	Specification Required for others.</t>	values starting with "core" and Specification Required for others.</t>
	<t>The requirements to be enforced are:</t>	<t>The requirements to be enforced are:</t>
	<ul spacing="normal">	<ul spacing="normal">

	<li>The values MUST be related to the purpose described in <xref targe	<li>The values <bcp14>MUST</bcp14> be related to the purpose described
	t="et-description" format="default"/>.</li>	in <xref target="et-description" format="default"/>.</li>
	<li>The registered values MUST conform to the ABNF reg-rel-type defini	<li>The registered values <bcp14>MUST</bcp14> conform to the ABNF reg-
	tion of	rel-type definition of
	<xref target="RFC6690" format="default"/> and MUST NOT be a URI.</li>	<xref target="RFC6690" format="default"/> and <bcp14>MUST NOT</bcp14> be a URI.<
		/li>
	<li>It is recommended to use the period "." character for segmentation .</li>	<li>It is recommended to use the period "." character for segmentation .</li>
	</ul>	</ul>

	<t>The registry initially contains one value:</t>	<t>The initial contents of the registry are as follows:</t>
	<ul spacing="normal">
	<li>"core.rd-group": An application group as described in <xref target	<table>
	="groups" format="default"/>.</li>	<name>New Endpoint Type (et=) RD Parameter Values Registry</name>
	</ul>	<thead>
		<tr>
		<th>Value</th>
		<th>Description</th>
		<th>Reference</th>
		</tr>
		</thead>
		<tbody>
		<tr>
		<td>core.rd-group</td>
		<td>An application group, as described in RFC 9176, <xref target="groups"
		format="default"/>.</td>
		<td>RFC 9176</td>
		</tr>
		</tbody>
		</table>

	</section>	</section>
	<section anchor="mc-registration" numbered="true" toc="default">	<section anchor="mc-registration" numbered="true" toc="default">
	<name>Multicast Address Registration</name>	<name>Multicast Address Registration</name>

	<t><!-- IANA has assigned -->	<t>
	IANA is asked to assign	IANA has assigned
	the following multicast addresses for use by CoAP nodes:</t>	the following multicast addresses for use by CoAP nodes:</t>
	<t>IPv4 -- "all CoRE Resource Directories" address MCD2 (suggestion: 22	<dl newline="false" spacing="normal">
	4.0.1.189), from the "IPv4	<dt>IPv4</dt>
	Multicast Address Space Registry". As the address is used for	<dd>-- "All CoRE Resource Directories" address 224.0.1.190, in the
	discovery that may span beyond a single network, it has come from	"Internetwork Control Block (224.0.1.0 - 224.0.1.255 (224.0.1/24))"
	the Internetwork Control Block (224.0.1.x) <xref target="RFC5771" format="	subregistry within the "IPv4 Multicast Address Space Registry". As
	default"/>.</t>	the address is used for discovery that may span beyond a single
	<t>IPv6 -- "all CoRE Resource Directories" address MCD1 (suggestions FF	network, it has come from the Internetwork Control Block (224.0.1.x)
	0X::FE), from the "IPv6 Multicast	<xref target="RFC5771" format="default"/>.</dd>
	Address Space Registry", in the "Variable Scope Multicast	<dt>IPv6</dt>
	Addresses" space (RFC 3307). Note that there is a distinct	<dd>-- "All CoRE Resource Directories" address ff0x::fe, in
	multicast address for each scope that interested CoAP nodes should	the "Variable Scope Multicast Addresses" subregistry within the "IPv6
	listen to; CoAP needs the Link-Local and Site-Local scopes only.</t>	Multicast Address Space Registry" <xref target="RFC3307"
	<t>[ The RFC editor is asked to replace MCD1 and MCD2	format="default"/>. Note that there is a distinct multicast address for
	with the assigned addresses throughout the document. ]</t>	each scope that interested CoAP nodes should
		listen to; CoAP needs the
		link-local and site-local scopes only.</dd>
		</dl>
	</section>	</section>


	<section anchor="well-known-uris" numbered="true" toc="default">	<section anchor="well-known-uris" numbered="true" toc="default">
	<name>Well-Known URIs</name>	<name>Well-Known URIs</name>

	<t>IANA is asked to permanently register	<t>IANA has registered the URI suffix "rd" in the "Well-Known URIs"
	<!-- IANA has permanently registered -->	registry as follows: </t>
	the URI suffix "rd"
	in the "Well-Known URIs" registry.	<table>
	The change controller is the IETF,	<name>Addition to Well-Known URIs Registry</name>
	this document is the reference.</t>	<thead>
		<tr>
		<th>URI Suffix</th>
		<th>Change Controller</th>
		<th>Reference</th>
		<th>Status</th>
		</tr>
		</thead>
		<tbody>
		<tr>
		<td>rd</td>
		<td>IETF</td>
		<td>RFC 9176</td>
		<td>permanent</td>
		</tr>
		</tbody>
		</table>

	</section>	</section>
	<section anchor="service-names-and-transport-protocol-port-number-registry " numbered="true" toc="default">	<section anchor="service-names-and-transport-protocol-port-number-registry " numbered="true" toc="default">

	<name>Service Names and Transport Protocol Port Number Registry</name>	<name>Service Name and Transport Protocol Port Number Registry</name>
	<t>IANA is asked to enter four new items into the Service Names and Tran	<t>IANA has added four new items to the "Service Name and Transport Prot
	sport Protocol Port Number Registry:</t>	ocol Port Number Registry" as follows:</t>
	<ul spacing="normal">
	<li>Service name: "core-rd", Protocol: "udp", Description: "Resource	<table>
	Directory accessed using CoAP"</li>	<name>Additions to Service Name and Transport Protocol Port Number Registry</n
	<li>Service name "core-rd-dtls", Protocol: "udp", Description: "Resour	ame>
	ce Directory accessed using CoAP over DTLS"</li>	<thead>
	<li>Service name: "core-rd", Protocol: "tcp", Description: "Resource	<tr>
	Directory accessed using CoAP over TCP"</li>	<th>Service Name</th>
	<li>Service name "core-rd-tls", Protocol: "tcp", Description: "Resourc	<th>Transport Protocol</th>
	e Directory accessed using CoAP over TLS"</li>	<th>Description</th>
	</ul>	<th>Reference</th>
	<t>All in common have this document as their reference.</t>	</tr>
		</thead>
		<tbody>
		<tr>
		<td>core-rd</td>
		<td>udp</td>
		<td>Resource Directory accessed using CoAP</td>
		<td>RFC 9176</td>
		</tr>
		<tr>
		<td>core-rd-dtls</td>
		<td>udp</td>
		<td>Resource Directory accessed using CoAP over DTLS</td>
		<td>RFC 9176</td>
		</tr>
		<tr>
		<td>core-rd</td>
		<td>tcp</td>
		<td>Resource Directory accessed using CoAP over TCP</td>
		<td>RFC 9176</td>
		</tr>
		<tr>
		<td>core-rd-tls</td>
		<td>tcp</td>
		<td>Resource Directory accessed using CoAP over TLS</td>
		<td>RFC 9176</td>
		</tr>
		</tbody>
		</table>

	</section>	</section>
	</section>	</section>
	<section anchor="examples" numbered="true" toc="default">	<section anchor="examples" numbered="true" toc="default">
	<name>Examples</name>	<name>Examples</name>

	<t>Two examples are presented: a Lighting Installation example in <xref ta rget="lt-ex" format="default"/> and a LwM2M example in <xref target="lwm2m-ex" f ormat="default"/>.</t>	<t>Two examples are presented: a lighting installation example in <xref ta rget="lt-ex" format="default"/> and a Lightweight M2M (LwM2M) example in <xref t arget="lwm2m-ex" format="default"/>.</t>
	<section anchor="lt-ex" numbered="true" toc="default">	<section anchor="lt-ex" numbered="true" toc="default">
	<name>Lighting Installation</name>	<name>Lighting Installation</name>

	<t>This example shows a simplified lighting installation which makes use	<t>This example shows a simplified lighting installation that makes use
	of	of
	the RD with a CoAP interface to facilitate the installation and start-up of	the RD with a CoAP interface to facilitate the installation and startup of
	the application code in the lights and sensors. In particular, the example	the application code in the lights and sensors. In particular, the example
	leads to the definition of a group and the enabling of the corresponding	leads to the definition of a group and the enabling of the corresponding

	multicast address as described in <xref target="groups" format="default"/>. No c	multicast address, as described in <xref target="groups" format="default"/>. No
	onclusions must be drawn on the realization of actual	conclusions must be drawn on the realization of actual
	installation or naming procedures, because the example only "emphasizes" some of	installation or naming procedures, because the example only emphasizes some of t
	the issues	he issues
	that may influence the use of the RD and does not pretend to be normative.</t>	that may influence the use of the RD and does not pretend to be normative.</t>
	<section anchor="lt-in-ch" numbered="true" toc="default">	<section anchor="lt-in-ch" numbered="true" toc="default">
	<name>Installation Characteristics</name>	<name>Installation Characteristics</name>
	<t>The example assumes that the installation is managed. That means th at a Commissioning	<t>The example assumes that the installation is managed. That means th at a Commissioning
	Tool (CT) is used to authorize the addition of nodes, name them, and name	Tool (CT) is used to authorize the addition of nodes, name them, and name
	their services. The CT can be connected to the installation in many ways:	their services. The CT can be connected to the installation in many ways:

	the CT can be part of the installation network, connected by WiFi to the	the CT can be part of the installation network, connected by Wi-Fi to the
	installation network, or connected via GPRS link, or other method.</t>	installation network, connected via GPRS link, or connected by another method.</
		t>
	<t>It is assumed that there are two naming authorities for the install ation:	<t>It is assumed that there are two naming authorities for the install ation:
	(1) the network manager that is responsible for the correct operation of	(1) the network manager that is responsible for the correct operation of

	the network and the connected interfaces, and (2) the lighting manager that	the network and the connected interfaces and (2) the lighting manager that
	is responsible for the correct functioning of networked lights and sensors.	is responsible for the correct functioning of networked lights and sensors.
	The result is the existence of two naming schemes coming from the two managing	The result is the existence of two naming schemes coming from the two managing
	entities.</t>	entities.</t>

	<t>The example installation consists of one presence sensor, and two l uminaries,	<t>The example installation consists of one presence sensor and two lu minaries,
	luminary1 and luminary2, each with their own wireless interface. Each luminary	luminary1 and luminary2, each with their own wireless interface. Each luminary

	contains three lamps: left, right and middle. Each luminary is accessible	contains three lamps: left, right, and middle. Each luminary is accessible
	through one endpoint. For each lamp a resource exists to modify the settings	through one endpoint. For each lamp, a resource exists to modify the settings
	of a lamp in a luminary. The purpose of the installation is that the presence	of a lamp in a luminary. The purpose of the installation is that the presence
	sensor notifies the presence of persons to a group of lamps. The group of	sensor notifies the presence of persons to a group of lamps. The group of

	lamps consists of: middle and left lamps of luminary1 and right lamp of luminary	lamps consists of the middle and left lamps of luminary1 and the right lamp of l
	2.</t>	uminary2.</t>
	<t>Before commissioning by the lighting manager, the network is instal	<t>Before commissioning by the lighting manager, the network is instal
	led and	led, and
	access to the interfaces is proven to work by the network manager.</t>	access to the interfaces is proven to work by the network manager.</t>
	<t>At the moment of installation, the network under installation is no t necessarily	<t>At the moment of installation, the network under installation is no t necessarily

	connected to the DNS infrastructure. Therefore, SLAAC IPv6 addresses are	connected to the DNS infrastructure. Therefore, Stateless Address Autoconfigurat
	assigned to CT, RD, luminaries and the sensor.	ion (SLAAC) IPv6 addresses are
		assigned to CT, RD, luminaries, and the sensor.
	The addresses shown in <xref target="interface-S" format="default"/> below stand in for these in the following examples.</t>	The addresses shown in <xref target="interface-S" format="default"/> below stand in for these in the following examples.</t>
	<table anchor="interface-S" align="center">	<table anchor="interface-S" align="center">

	<name>Addresses used in the examples</name>	<name>Addresses Used in the Examples</name>
	<thead>	<thead>
	<tr>	<tr>
	<th align="left">Name</th>	<th align="left">Name</th>
	<th align="left">IPv6 address</th>	<th align="left">IPv6 address</th>
	</tr>	</tr>
	</thead>	</thead>
	<tbody>	<tbody>
	<tr>	<tr>
	<td align="left">luminary1</td>	<td align="left">luminary1</td>
	<td align="left">2001:db8:4::1</td>	<td align="left">2001:db8:4::1</td>

	skipping to change at line 2322 ¶	skipping to change at line 2335 ¶
	<tr>	<tr>
	<td align="left">Presence sensor</td>	<td align="left">Presence sensor</td>
	<td align="left">2001:db8:4::3</td>	<td align="left">2001:db8:4::3</td>
	</tr>	</tr>
	<tr>	<tr>
	<td align="left">RD</td>	<td align="left">RD</td>
	<td align="left">2001:db8:4::ff</td>	<td align="left">2001:db8:4::ff</td>
	</tr>	</tr>
	</tbody>	</tbody>
	</table>	</table>

	<t>In <xref target="rd-en" format="default"/> the use of RD during ins tallation is	<t>In <xref target="rd-en" format="default"/>, the use of RD during in stallation is
	presented.</t>	presented.</t>
	</section>	</section>
	<section anchor="rd-en" numbered="true" toc="default">	<section anchor="rd-en" numbered="true" toc="default">

	<name>RD entries</name>	<name>RD Entries</name>
	<t>It is assumed that access to the DNS infrastructure is not always p ossible	<t>It is assumed that access to the DNS infrastructure is not always p ossible
	during installation. Therefore, the SLAAC addresses are used in this section.</t >	during installation. Therefore, the SLAAC addresses are used in this section.</t >
	<t>For discovery, the resource types (rt) of the devices are important . The	<t>For discovery, the resource types (rt) of the devices are important . The
	lamps in the luminaries have rt=tag:example.com,2020:light, and the presence sen sor has rt=tag:example.com,2020:p-sensor.	lamps in the luminaries have rt=tag:example.com,2020:light, and the presence sen sor has rt=tag:example.com,2020:p-sensor.

	The endpoints have names which are relevant to the light installation manager.	The endpoints have names that are relevant to the light installation manager.
	In this case luminary1, luminary2, and the presence sensor are located in	In this case, luminary1, luminary2, and the presence sensor are located in
	room 2-4-015, where luminary1 is located at the window and luminary2 and	room 2-4-015, where luminary1 is located at the window and luminary2 and
	the presence sensor are located at the door. The endpoint names reflect	the presence sensor are located at the door. The endpoint names reflect

	this physical location. The middle, left and right lamps are accessed via	this physical location. The middle, left, and right lamps are accessed via
	path /light/middle, /light/left, and /light/right respectively. The identifiers	path /light/middle, /light/left, and /light/right, respectively. The identifiers
	relevant to the RD are shown in <xref target="endpoint" format="default"/> below	relevant to the RD are shown in <xref target="endpoint" format="default"/>.</t>
	:</t>
	<table anchor="endpoint" align="center">	<table anchor="endpoint" align="center">

	<name>RD identifiers</name>	<name>RD Identifiers</name>
	<thead>	<thead>
	<tr>	<tr>
	<th align="left">Name</th>	<th align="left">Name</th>

	<th align="left">endpoint</th>	<th align="left">Endpoint</th>
	<th align="left">resource path</th>	<th align="left">Resource Path</th>
	<th align="left">resource type</th>	<th align="left">Resource Type</th>
	</tr>	</tr>
	</thead>	</thead>
	<tbody>	<tbody>
	<tr>	<tr>
	<td align="left">luminary1</td>	<td align="left">luminary1</td>
	<td align="left">lm_R2-4-015_wndw</td>	<td align="left">lm_R2-4-015_wndw</td>
	<td align="left">/light/left</td>	<td align="left">/light/left</td>
	<td align="left">tag:example.com,2020:light</td>	<td align="left">tag:example.com,2020:light</td>
	</tr>	</tr>
	<tr>	<tr>

	skipping to change at line 2393 ¶	skipping to change at line 2406 ¶
	<td align="left">tag:example.com,2020:light</td>	<td align="left">tag:example.com,2020:light</td>
	</tr>	</tr>
	<tr>	<tr>
	<td align="left">Presence sensor</td>	<td align="left">Presence sensor</td>
	<td align="left">ps_R2-4-015_door</td>	<td align="left">ps_R2-4-015_door</td>
	<td align="left">/ps</td>	<td align="left">/ps</td>
	<td align="left">tag:example.com,2020:p-sensor</td>	<td align="left">tag:example.com,2020:p-sensor</td>
	</tr>	</tr>
	</tbody>	</tbody>
	</table>	</table>

	<t>It is assumed that the CT has performed RD discovery and has receiv ed a response like the one in the <xref target="discovery" format="default"/> ex ample.</t>	<t>It is assumed that the CT has performed RD discovery and has receiv ed a response like the one in the example in <xref target="discovery" format="de fault"/>.</t>
	<t>The CT inserts the endpoints of the luminaries and the sensor in th e RD	<t>The CT inserts the endpoints of the luminaries and the sensor in th e RD
	using the registration base URI parameter (base) to specify the interface addres s:</t>	using the registration base URI parameter (base) to specify the interface addres s:</t>
	<figure anchor="example-lighting-1">	<figure anchor="example-lighting-1">

	<name>Example of registrations a CT enters into an RD</name>	<name>Example of Registrations a CT Enters into an RD</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: POST coap://[2001:db8:4::ff]/rd	Req: POST coap://[2001:db8:4::ff]/rd
	?ep=lm_R2-4-015_wndw&base=coap://[2001:db8:4::1]&d=R2-4-015	?ep=lm_R2-4-015_wndw&base=coap://[2001:db8:4::1]&d=R2-4-015
	Payload:	Payload:
	</light/left>;rt="tag:example.com,2020:light",	</light/left>;rt="tag:example.com,2020:light",
	</light/middle>;rt="tag:example.com,2020:light",	</light/middle>;rt="tag:example.com,2020:light",
	</light/right>;rt="tag:example.com,2020:light"	</light/right>;rt="tag:example.com,2020:light"

	Res: 2.01 Created	Res: 2.01 Created
	Location-Path: /rd/4521	Location-Path: /rd/4521


	skipping to change at line 2426 ¶	skipping to change at line 2439 ¶
	Res: 2.01 Created	Res: 2.01 Created
	Location-Path: /rd/4522	Location-Path: /rd/4522

	Req: POST coap://[2001:db8:4::ff]/rd	Req: POST coap://[2001:db8:4::ff]/rd
	?ep=ps_R2-4-015_door&base=coap://[2001:db8:4::3]&d=R2-4-015	?ep=ps_R2-4-015_door&base=coap://[2001:db8:4::3]&d=R2-4-015
	Payload:	Payload:
	</ps>;rt="tag:example.com,2020:p-sensor"	</ps>;rt="tag:example.com,2020:p-sensor"

	Res: 2.01 Created	Res: 2.01 Created
	Location-Path: /rd/4523	Location-Path: /rd/4523

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	<t>The sector name d=R2-4-015 has been added for an efficient lookup b ecause	<t>The sector name d=R2-4-015 has been added for an efficient lookup b ecause

	filtering on "ep" name is more awkward. The same sector name is communicated to	filtering on the "ep" name is more awkward. The same sector name is communicated to
	the two luminaries and the presence sensor by the CT.</t>	the two luminaries and the presence sensor by the CT.</t>
	<t>The group is specified in the RD. The base parameter is set to the site-local	<t>The group is specified in the RD. The base parameter is set to the site-local
	multicast address allocated to the group.	multicast address allocated to the group.
	In the POST in the example below, the resources supported by all group members a re published.</t>	In the POST in the example below, the resources supported by all group members a re published.</t>
	<figure anchor="example-lighting-2">	<figure anchor="example-lighting-2">

	<name>Example of a multicast group a CT enters into an RD</name>	<name>Example of a Multicast Group a CT Enters into an RD</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: POST coap://[2001:db8:4::ff]/rd	Req: POST coap://[2001:db8:4::ff]/rd
	?ep=grp_R2-4-015&et=core.rd-group&base=coap://[ff05::1]	?ep=grp_R2-4-015&et=core.rd-group&base=coap://[ff05::1]
	Payload:	Payload:
	</light/left>;rt="tag:example.com,2020:light",	</light/left>;rt="tag:example.com,2020:light",
	</light/middle>;rt="tag:example.com,2020:light",	</light/middle>;rt="tag:example.com,2020:light",
	</light/right>;rt="tag:example.com,2020:light"	</light/right>;rt="tag:example.com,2020:light"

	Res: 2.01 Created	Res: 2.01 Created
	Location-Path: /rd/501	Location-Path: /rd/501

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	<t>After the filling of the RD by the CT, the application in the lumin aries	<t>After the filling of the RD by the CT, the application in the lumin aries

	can learn to which groups they belong, and enable their interface for the	can learn to which groups they belong and enable their interface for the
	multicast address.</t>	multicast address.</t>
	<t>The luminary, knowing its sector and being configured to join any g roup	<t>The luminary, knowing its sector and being configured to join any g roup
	containing lights, searches for candidate groups and joins them:</t>	containing lights, searches for candidate groups and joins them:</t>
	<figure anchor="example-lighting-3">	<figure anchor="example-lighting-3">

	<name>Example of a lookup exchange to find suitable multicast addres	<name>Example of a Lookup Exchange to Find Suitable Multicast Addres
	ses</name>	ses</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep	Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep
	?d=R2-4-015&et=core.rd-group&rt=light	?d=R2-4-015&et=core.rd-group&rt=light

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	</rd/501>;ep=grp_R2-4-015;et=core.rd-group;	</rd/501>;ep=grp_R2-4-015;et=core.rd-group;
	base="coap://[ff05::1]";rt=core.rd-ep	base="coap://[ff05::1]";rt=core.rd-ep

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	<t>From the returned base parameter value, the luminary learns the mul ticast address	<t>From the returned base parameter value, the luminary learns the mul ticast address
	of the multicast group.</t>	of the multicast group.</t>
	<t>The presence sensor can learn the presence of groups that support r esources with rt=tag:example.com,2020:light in its own sector by sending the sam e request, as used by the luminary. The presence sensor learns the multicast add ress to use for sending messages to the luminaries.</t>	<t>The presence sensor can learn the presence of groups that support r esources with rt=tag:example.com,2020:light in its own sector by sending the sam e request, as used by the luminary. The presence sensor learns the multicast add ress to use for sending messages to the luminaries.</t>
	</section>	</section>
	</section>	</section>
	<section anchor="lwm2m-ex" numbered="true" toc="default">	<section anchor="lwm2m-ex" numbered="true" toc="default">
	<name>OMA Lightweight M2M (LwM2M)</name>	<name>OMA Lightweight M2M (LwM2M)</name>
	<t>OMA LwM2M is a profile for device services based on CoAP, providing i nterfaces and operations for device management and device service enablement.</t >	<t>OMA LwM2M is a profile for device services based on CoAP, providing i nterfaces and operations for device management and device service enablement.</t >

	<t>An LwM2M server is an instance of an LwM2M middleware service layer, containing an RD (<xref target="LwM2M" format="default"/> page 36f).</t>	<t>An LwM2M server is an instance of an LwM2M middleware service layer, containing an RD (<xref target="LwM2M" format="default"/>, starting at page 36). </t>
	<t>That RD only implements the registration interface, and no lookup is implemented.	<t>That RD only implements the registration interface, and no lookup is implemented.

	Instead, the LwM2M server provides access to the registered resources, in a simi	Instead, the LwM2M server provides access to the registered resources in a simil
	lar way to a reverse proxy.</t>	ar way to a reverse proxy.</t>
	<t>The location of the LwM2M Server and RD URI path is provided by the L	<t>The location of the LwM2M server and RD URI path is provided by the L
	wM2M Bootstrap process, so no dynamic discovery of the RD is used. LwM2M Servers	wM2M bootstrap process, so no dynamic discovery of the RD is used. LwM2M servers
	and endpoints are not required to implement the /.well-known/core resource.</t>	and endpoints are not required to implement the <tt>/.well-known/core</tt> reso
		urce.</t>
	</section>	</section>
	</section>	</section>

	<section anchor="acknowledgments" numbered="true" toc="default">
	<name>Acknowledgments</name>
	<t>Oscar Novo, Srdjan Krco, Szymon Sasin, Kerry Lynn, Esko Dijk, Anders
	Brandt, Matthieu Vial, Jim Schaad, Mohit Sethi, Hauke Petersen, Hannes Tschofeni
	g, Sampo Ukkola, Linyi
	Tian, Jan Newmarch, Matthias Kovatsch, Jaime Jimenez and Ted Lemon have provided
	helpful comments, discussions and ideas to improve and
	shape this document. Zach would also like to thank his colleagues from the
	EU FP7 SENSEI project, where many of the RD concepts were
	originally developed.</t>
	</section>
	<section anchor="changelog" numbered="true" toc="default">
	<name>Changelog</name>
	<t>changes from -27 to -28</t>
	<ul spacing="normal">
	<li>Security policies / link confidentiality: Point out the RD's obligat
	ions that follow from such a policy.</li>
	<li>Simple registration: clarify term "regular registration" by introduc
	ing it along with the reference to <xref target="registration" format="default"/
	></li>
	<li>Wording fix in first-come-first-remembered</li>
	<li>Wording fixes in RD definition</li>
	<li>Capitalization: Consistently using "registration resource"</li>
	</ul>
	<t>changes from -26 to -27</t>
	<ul spacing="normal">
	<li>In general, this addresses the points that were pointed out in
	https://mailarchive.ietf.org/arch/msg/core/xWLomwwhovkU-CPGNxnvs40BhaM/ as
	having "evolved from the review comments being discussed in the interim
	meetings", and the review comments from Esko Dijk that were largely entangled
	in these points.</li>
	<li>
	<t>Relaxation of the serialization rules for link-format </t>
	<t>
	The interpretation of RFC6690 used in <xref target="resolution-rules" format="de
	fault"/> was shown to be faulty.
	Along with a correction, the common implementations of link-format were surveyed
	again
	and it was found that the only one that employed the faulty interpretation can s
	till safely be upgraded.
	These were removed from the set considered for Limited Link Format,
	making the set of valid Limited Link Format documents larger. </t>
	<t>
	As a consequence, the prescribed serialization of RD output can be roughly halve
	d in bytes. </t>
	<t>
	There might be additional usage patterns that are possible with the new set of c
	onstraints,
	but there is insufficient implementation and deployment experience with them to
	warrant a change changes on that front at this point.
	The specification can later be extended compatibly to allow these cases and drop
	the requirement of Limited Link Format.</t>
	</li>
	<li>
	<t>Add Request freshness subsection </t>
	<t>
	It is now recommended (with security considerations on consequences of not doing
	it) to require ordering of RD operations. </t>
	<t>
	The Echo mechanism (previously suggested in various places but never exclusively
	) is the one prescribed way of getting this ordering,
	making the echo-request-tag reference normative.</t>
	</li>
	<li>
	<t>Improved expression about when an RD needs to verify simple registr
	ation. </t>
	<t>
	The simple wording missed the authorization part, and did not emphasize that thi
	s is a per-deployment property.</t>
	</li>
	<li>Point out the non-atomic properties of paginated access.</li>
	<li>Clarification around impl-info reference.</li>
	<li>Inconsistencies and extraneous quotings removed from examples.</li>
	</ul>
	<t>changes from -25 to -26</t>
	<ul spacing="normal">
	<li>
	<t>Security policies:
	</t>
	<ul spacing="normal">
	<li>The First-Come-First-Remembered policy is added as an example an
	d a potential default behavior.</li>
	<li>Clarify that the mapping between endpoint names and subject fiel
	ds is up to a policy that defines reliance on names, and give an example.</li>
	<li>Random EP names: Point that multiple collisions are possible but
	unlikely.</li>
	<li>
	<t>Add pointers to policies:
	</t>
	<ul spacing="normal">
	<li>RD replication: Point out that policies may limit that.</li>
	<li>Registration: Reword (ep, d) mapping to a previous registrat
	ion's resource that could have been read as another endpoint taking over an exis
	ting registration.</li>
	</ul>
	</li>
	<li>Clarify that the security policy is a property of the RD the any
	client may need to verify by checking the RD's authorization.</li>
	<li>Clarify how information from an untrusted RD can be verified</li
	>
	<li>Remove speculation about how in detail ACE scopes are obtained.<
	/li>
	</ul>
	</li>
	<li>
	<t>Security considerations:
	</t>
	<ul spacing="normal">
	<li>Generalize to all current options for security layers usable wit
	h CoAP (OSCORE was
	missing as the text predated RFC8613)</li>
	<li>Relax the previous SHOULD on secure access to SHOULD where prote
	ction is indicated by security policies
	(bringing the text in line with the -25 changes)</li>
	<li>Point out that failure to follow the security considerations has
	implications depending on the protection objective described with the security
	policies</li>
	<li>Shorten amplification mitigation</li>
	<li>Add note about information in Registration Resource path.</li>
	<li>Acknowledge that most host discovery operations are not secured;
	mention consequences and mitigation.</li>
	</ul>
	</li>
	<li>Abstract, introduction: removed "or disperse networks"</li>
	<li>
	<t>RD discovery:
	</t>
	<ul spacing="normal">
	<li>Drop the previously stated assumption that RDAO and any DHCP opt
	ions would only be used together with SLAAC and DHCP for address configuration,
	respectivly.</li>
	<li>Give concrete guidance for address selection based on RFC6724 wh
	en responding to multicasts</li>
	<li>
	<t>RDAO:
	</t>
	<ul spacing="normal">
	<li>Clarify that it is an option for RAs and not other ND messag
	es.</li>
	<li>Change Lifetime from 16-bit minutes to 32-bit seconds and sw
	ap it with Reserved (aligning it with RDNSS which it shares other properties as
	well).</li>
	</ul>
	</li>
	<li>Point out that clients may need to check RD authorization alread
	y in last discovery step</li>
	</ul>
	</li>
	<li>
	<t>Registration:
	</t>
	<ul spacing="normal">
	<li>Wording around "mostly mandatory" has been improved, conflicts c
	larified and sector default selection adjusted.</li>
	</ul>
	</li>
	<li>Simple registration: Rather than coopting POSTs to /.well-known/core
	, a new resource /.well-known/rd is registered.
	A historical note in the text documents the change.</li>
	<li>
	<t>Examples:
	</t>
	<ul spacing="normal">
	<li>Use example URIs rather than unclear reg names (unless it's RFC6
	690 examples, which were kept for continuity)</li>
	<li>The LwM2M example was reduced from an outdated explanation of th
	e complete LwM2M model to a summary of how RD is used in there, with a reference
	to the current specification.</li>
	<li>Luminary example: Explain example addresses</li>
	<li>Luminary example: Drop reference to coap-group mechanism that's
	becoming obsolete, and thus also to RFC7390</li>
	<li>Multicast addresses in the examples were changed from ff35:30:20
	01:db8::x to ff35:30:2001:db8:f1::8000:x; the 8000 is to follow RFC 3307, and th
	e f1 is for consistency with all the other example addresses where 2001:db8::/32
	is subnetted to 2001:db8:x::/48 by groups of internally consistent examples.</l
	i>
	</ul>
	</li>
	<li>
	<t>Use case text enhancements
	</t>
	<ul spacing="normal">
	<li>Home and building automation: Tie in with RD</li>
	<li>M2M: Move system design paragraph towards the topic of reusabili
	ty.</li>
	</ul>
	</li>
	<li>Various editorial fixes in response to Gen-ART and IESG reviews.</li
	>
	<li>Rename 'Full description of the "Endpoint Type" Registration Paramet
	er' section to '... RD Parameter'</li>
	<li>Error handling: Place a SHOULD around the likely cases, and make the
	previous "MUST to the best of their capabilities" a "must".</li>
	<li>impl-info: Add note about the type being WIP</li>
	<li>Interaction tables: list CTs as possible initiators where applicable
	</li>
	<li>Registration update: Relax requirement to not send parameters needle
	ssly</li>
	<li>Terminology: Clarify that the CTs' installation events can occur mul
	tiple times.</li>
	<li>Promote RFCs 7252, 7230 and 8288 to normative references</li>
	<li>Moved Christian Amsuess to first author</li>
	</ul>
	<t>changes from -24 to -25</t>
	<ul spacing="normal">
	<li>
	<t>Large rework of section 7 (Security policies) </t>
	<t>
	Rather than prescribing which data in the RD <em>is</em> authenticated (and how)
	,
	it now describes what applications built on an RD <em>can</em> choose to authent
	icate,
	show possibilities on how to do it and outline what it means for clients. </t>
	<t>
	This addresses Russ' Genart review points on details in the text in a rather bro
	ad fashion.
	That is because the discussion on the topic inside the WG showed that that text
	on security has been driven more review-by-review than by an architectural plan
	of the authors and WG.</t>
	</li>
	<li>Add concrete suggestions (twice as long as registrant number with re
	tries, or UUIDs without) for random endpoint names</li>
	<li>Point out that simple registration can have faked origins,
	RECOMMEND mitigation when applicable
	and suggest the Echo mechanism to implement it.</li>
	<li>Reference existing and upcoming specifications for DDOS mitigation i
	n CoAP.</li>
	<li>Explain the provenance of the example's multicast address.</li>
	<li>Make "SHOULD" of not manipulating foreign registrations a "should" a
	nd explain how it is enforced</li>
	<li>Clarify application of RFC6570 to search parameters</li>
	<li>Syntactic fixes in examples</li>
	<li>
	<t>IANA:
	</t>
	<ul spacing="normal">
	<li>Don't announce expected number of registrations (goes to write-u
	p)</li>
	<li>Include syntax as part of a field's validity in entry requiremen
	ts</li>
	</ul>
	</li>
	<li>
	<t>Editorial changes
	</t>
	<ul spacing="normal">
	<li>Align wording between abstract and introduction</li>
	<li>Abbreviation normalization: "ER model", "RD"</li>
	<li>RFC8174 boilerplate update</li>
	<li>Minor clarity fixes</li>
	<li>Markup and layouting</li>
	</ul>
	</li>
	</ul>
	<t>changes from -23 to -24</t>
	<ul spacing="normal">
	<li>Discovery using DNS-SD added again</li>
	<li>Minimum lifetime (lt) reduced from 60 to 1</li>
	<li>References added</li>
	<li>
	<t>IANA considerations
	</t>
	<ul spacing="normal">
	<li>added about .well-known/core resource</li>
	<li>added DNS-SD service names</li>
	<li>made RDAO option number a suggestion</li>
	<li>added "reference" field to endpoint type registry</li>
	</ul>
	</li>
	<li>Lookup: mention that anchor is a legitimate lookup attribute</li>
	<li>Terminology and example fixes</li>
	<li>Layout fixes, esp. the use of non-ASCII characters in figures</li>
	</ul>
	<t>changes from -22 to -23</t>
	<ul spacing="normal">
	<li>Explain that updates can not remove attributes</li>
	<li>Typo fixes</li>
	</ul>
	<t>changes from -21 to -22</t>
	<ul spacing="normal">
	<li>Request a dedicated IPv4 address from IANA (rather than sharing with
	All CoAP nodes)</li>
	<li>Fix erroneous examples</li>
	<li>
	<t>Editorial changes
	</t>
	<ul spacing="normal">
	<li>Add figure numbers to examples</li>
	<li>Update RD parameters table to reflect changes of earlier version
	s in the text</li>
	<li>Typos and minor wording</li>
	</ul>
	</li>
	</ul>
	<t>changes from -20 to -21</t>
	<t>(Processing comments during WGLC)</t>
	<ul spacing="normal">
	<li>Defer outdated description of using DNS-SD to find an RD to the defi
	ning document</li>
	<li>Describe operational conditions in automation example</li>
	<li>Recommend particular discovery mechanisms for some managed network s
	cenarios</li>
	</ul>
	<t>changes from -19 to -20</t>
	<t>(Processing comments from the WG chair review)</t>
	<ul spacing="normal">
	<li>Define the permissible characters in endpoint and sector names</li>
	<li>Express requirements on NAT situations in more abstract terms</li>
	<li>Shifted heading levels to have the interfaces on the same level</li>
	<li>Group instructions for error handling into general section</li>
	<li>Simple Registration: process reflowed into items list</li>
	<li>Updated introduction to reflect state of CoRE in general, reference
	RFC7228
	(defining "constrained") and use "IoT" term in addition to "M2M"</li>
	<li>Update acknowledgements</li>
	<li>
	<t>Assorted editorial changes
	</t>
	<ul spacing="normal">
	<li>Unify examples style</li>
	<li>Terminology: RDAO defined and not only expanded</li>
	<li>Add CT to <xref target="fig-arch" format="default"/></li>
	<li>Consistency in the use of the term "Content Format"</li>
	</ul>
	</li>
	</ul>
	<t>changes from -18 to -19</t>
	<ul spacing="normal">
	<li>link-local addresses: allow but prescribe split-horizon fashion when
	used,
	disallow zone identifiers</li>
	<li>Remove informative references to documents not mentioned any more</l
	i>
	</ul>
	<t>changes from -17 to -18</t>
	<ul spacing="normal">
	<li>Rather than re-specifying link format (Modernized Link Format), desc
	ribe a
	Limited Link Format that's the uncontested subset of Link Format</li>
	<li>Acknowledging the -17 version as part of the draft</li>
	<li>Move "Read endpoint links" operation to future specification like PA
	TCH</li>
	<li>Demote links-json to an informative reference, and removed them from
	exchange
	examples</li>
	<li>Add note on unusability of link-local IP addresses, and describe mit
	igation.</li>
	<li>Reshuffling of sections: Move additional operations and endpoint loo
	kup back
	from appendix, and groups into one</li>
	<li>Lookup interface tightened to not imply applicability for non link-f
	ormat
	lookups (as those can have vastly different views on link cardinality)</li>
	<li>Simple registration: Change sequence of GET and POST-response, ensur
	ing
	unsuccessful registrations are reported as such, and suggest how devices that
	would have required the inverse behavior can still cope with it.</li>
	<li>Abstract and introduction reworded to avoid the impression that reso
	urces are
	stored in full in the RD</li>
	<li>Simplify the rules governing when a registration resource can or mus
	t be
	changed.</li>
	<li>Drop a figure that has become useless due to the changes of and -13
	and -17</li>
	<li>Wording consistency fixes: Use "Registrations" and "target attribute
	s"</li>
	<li>Fix incorrect use of content negotiation in discovery interface desc
	ription
	(Content-Format -> Accept)</li>
	<li>State that the base attribute value is part of endpoint lookup even
	when
	implicit in the registration</li>
	<li>Update references from RFC5988 to its update RFC8288</li>
	<li>Remove appendix on protocol-negotiation (which had a note to be remo
	ved
	before publication)</li>
	</ul>
	<t>changes from -16 to -17</t>
	<t>(Note that -17 is published as a direct follow-up to -16, containing a
	single change to be discussed at IETF103)</t>
	<ul spacing="normal">
	<li>Removed groups that are enumerations of registrations and have dedic
	ated mechanism</li>
	<li>Add groups that are enumerations of shared resources and are a speci
	al case of endpoint registrations</li>
	</ul>
	<t>changes from -15 to -16</t>
	<ul spacing="normal">
	<li>Recommend a common set of resources for members of a group</li>
	<li>Clarified use of multicast group in lighting example</li>
	<li>Add note on concurrent registrations from one EP being possible but
	not expected</li>
	<li>Refresh web examples appendix to reflect current use of Modernized L
	ink Format</li>
	<li>Add examples of URIs where Modernized Link Format matters</li>
	<li>Editorial changes</li>
	</ul>
	<t>changes from -14 to -15</t>
	<ul spacing="normal">
	<li>Rewrite of section "Security policies"</li>
	<li>Clarify that the "base" parameter text applies both to relative refe
	rences
	both in anchor and href</li>
	<li>Renamed "Registree-EP" to Registrant-EP"</li>
	<li>Talk of "relative references" and "URIs" rather than "relative" and
	"absolute" URIs. (The concept of "absolute URIs" of <xref target="RFC3986" forma
	t="default"/> is not needed in RD).</li>
	<li>Fixed examples</li>
	<li>Editorial changes</li>
	</ul>
	<t>changes from -13 to -14</t>
	<ul spacing="normal">
	<li>Rename "registration context" to "registration base URI" (and "con"
	to
	"base") and "domain" to "sector" (where the abbreviation "d" stays for
	compatibility reasons)</li>
	<li>Introduced resource types core.rd-ep and core.rd-gp</li>
	<li>Registration management moved to appendix A, including endpoint and
	group lookup</li>
	<li>
	<t>Minor editorial changes
	</t>
	<ul spacing="normal">
	<li>PATCH/iPATCH is clearly deferred to another document</li>
	<li>Recommend against query / fragment identifier in con=</li>
	<li>Interface description lists are described as illustrative</li>
	<li>Rewording of Simple Registration</li>
	</ul>
	</li>
	<li>Simple registration carries no error information and succeeds immedi
	ately (previously, sequence was unspecified)</li>
	<li>Lookup: href are matched against resolved values (previously, this w
	as unspecified)</li>
	<li>Lookup: lt are not exposed any more</li>
	<li>con/base: Paths are allowed</li>
	<li>Registration resource locations can not have query or fragment parts
	</li>
	<li>Default life time extended to 25 hours</li>
	<li>clarified registration update rules</li>
	<li>lt-value semantics for lookup clarified.</li>
	<li>added template for simple registration</li>
	</ul>
	<t>changes from -12 to -13</t>
	<ul spacing="normal">
	<li>Added "all resource directory" nodes MC address</li>
	<li>Clarified observation behavior</li>
	<li>version identification</li>
	<li>example rt= and et= values</li>
	<li>domain from figure 2</li>
	<li>more explanatory text</li>
	<li>endpoints of a groups hosted by different RD</li>
	<li>
	<t>resolve RFC6690-vs-8288 resolution ambiguities:
	</t>
	<ul spacing="normal">
	<li>require registered links not to be relative when using anchor</l
	i>
	<li>return absolute URIs in resource lookup</li>
	</ul>
	</li>
	</ul>
	<t>changes from -11 to -12</t>
	<ul spacing="normal">
	<li>added Content Model section, including ER diagram</li>
	<li>removed domain lookup interface; domains are now plain attributes of
	groups and endpoints</li>
	<li>updated chapter "Finding a Resource Directory"; now distinguishes co
	nfiguration-provided, network-provided and heuristic sources</li>
	<li>improved text on: atomicity, idempotency, lookup with multiple param
	eters, endpoint removal, simple registration</li>
	<li>updated LWM2M description</li>
	<li>clarified where relative references are resolved, and how context an
	d anchor interact</li>
	<li>new appendix on the interaction with RFCs 6690, 5988 and 3986</li>
	<li>lookup interface: group and endpoint lookup return group and registr
	ation resources as link targets</li>
	<li>lookup interface: search parameters work the same across all entitie
	s</li>
	<li>removed all methods that modify links in an existing registration (P
	OST with payload, PATCH and iPATCH)</li>
	<li>removed plurality definition (was only needed for link modification)
	</li>
	<li>enhanced IANA registry text</li>
	<li>state that lookup resources can be observable</li>
	<li>More examples and improved text</li>
	</ul>
	<t>changes from -09 to -10</t>
	<ul spacing="normal">
	<li>removed "ins" and "exp" link-format extensions.</li>
	<li>removed all text concerning DNS-SD.</li>
	<li>removed inconsistency in RDAO text.</li>
	<li>suggestions taken over from various sources</li>
	<li>replaced "Function Set" with "REST API", "base URI", "base path"</li
	>
	<li>moved simple registration to registration section</li>
	</ul>
	<t>changes from -08 to -09</t>
	<ul spacing="normal">
	<li>clarified the "example use" of the base RD resource values /rd, /rd-
	lookup, and /rd-group.</li>
	<li>changed "ins" ABNF notation.</li>
	<li>various editorial improvements, including in examples</li>
	<li>clarifications for RDAO</li>
	</ul>
	<t>changes from -07 to -08</t>
	<ul spacing="normal">
	<li>removed link target value returned from domain and group lookup type
	s</li>
	<li>Maximum length of domain parameter 63 bytes for consistency with gro
	up</li>
	<li>removed option for simple POST of link data, don't require a .well-k
	nown/core resource to accept POST data and handle it in a special way; we alread
	y have /rd for that</li>
	<li>add IPv6 ND Option for discovery of an RD</li>
	<li>clarify group configuration section 6.1 that endpoints must be regis
	tered before including them in a group</li>
	<li>removed all superfluous client-server diagrams</li>
	<li>simplified lighting example</li>
	<li>introduced Commissioning Tool</li>
	<li>RD-Look-up text is extended.</li>
	</ul>
	<t>changes from -06 to -07</t>
	<ul spacing="normal">
	<li>added text in the discovery section to allow content format hints to
	be exposed in the discovery link attributes</li>
	<li>editorial updates to section 9</li>
	<li>update author information</li>
	<li>minor text corrections</li>
	</ul>
	<t>Changes from -05 to -06</t>
	<ul spacing="normal">
	<li>added note that the PATCH section is contingent on the progress of
	the PATCH method</li>
	</ul>
	<t>changes from -04 to -05</t>
	<ul spacing="normal">
	<li>added Update Endpoint Links using PATCH</li>
	<li>http access made explicit in interface specification</li>
	<li>Added http examples</li>
	</ul>
	<t>Changes from -03 to -04:</t>
	<ul spacing="normal">
	<li>Added http response codes</li>
	<li>Clarified endpoint name usage</li>
	<li>Add application/link-format+cbor content-format</li>
	</ul>
	<t>Changes from -02 to -03:</t>
	<ul spacing="normal">
	<li>Added an example for lighting and DNS integration</li>
	<li>Added an example for RD use in OMA LWM2M</li>
	<li>Added Read Links operation for link inspection by endpoints</li>
	<li>Expanded DNS-SD section</li>
	<li>Added draft authors Peter van der Stok and Michael Koster</li>
	</ul>
	<t>Changes from -01 to -02:</t>
	<ul spacing="normal">
	<li>Added a catalogue use case.</li>
	<li>Changed the registration update to a POST with optional link format
	payload. Removed the endpoint type update from the update.</li>
	<li>Additional examples section added for more complex use cases.</li>
	<li>New DNS-SD mapping section.</li>
	<li>Added text on endpoint identification and authentication.</li>
	<li>Error code 4.04 added to Registration Update and Delete requests.</l
	i>
	<li>Made 63 bytes a SHOULD rather than a MUST for endpoint name and reso
	urce type parameters.</li>
	</ul>
	<t>Changes from -00 to -01:</t>
	<ul spacing="normal">
	<li>Removed the ETag validation feature.</li>
	<li>Place holder for the DNS-SD mapping section.</li>
	<li>Explicitly disabled GET or POST on returned Location.</li>
	<li>New registry for RD parameters.</li>
	<li>Added support for the JSON Link Format.</li>
	<li>Added reference to the Groupcomm WG draft.</li>
	</ul>
	<t>Changes from -05 to WG Document -00:</t>
	<ul spacing="normal">
	<li>Updated the version and date.</li>
	</ul>
	<t>Changes from -04 to -05:</t>
	<ul spacing="normal">
	<li>Restricted Update to parameter updates.</li>
	<li>Added pagination support for the Lookup interface.</li>
	<li>Minor editing, bug fixes and reference updates.</li>
	<li>Added group support.</li>
	<li>Changed rt to et for the registration and update interface.</li>
	</ul>
	<t>Changes from -03 to -04:</t>
	<ul spacing="normal">
	<li>Added the ins= parameter back for the DNS-SD mapping.</li>
	<li>Integrated the Simple Directory Discovery from Carsten.</li>
	<li>Editorial improvements.</li>
	<li>Fixed the use of ETags.</li>
	<li>Fixed tickets 383 and 372</li>
	</ul>
	<t>Changes from -02 to -03:</t>
	<ul spacing="normal">
	<li>Changed the endpoint name back to a single registration parameter ep
	= and removed the h= and ins= parameters.</li>
	<li>Updated REST interface descriptions to use RFC6570 URI Template form
	at.</li>
	<li>Introduced an improved RD Lookup design as its own function set.</li
	>
	<li>Improved the security considerations section.</li>
	<li>Made the POST registration interface idempotent by requiring the ep=
	parameter to be present.</li>
	</ul>
	<t>Changes from -01 to -02:</t>
	<ul spacing="normal">
	<li>Added a terminology section.</li>
	<li>Changed the inclusion of an ETag in registration or update to a MAY.
	</li>
	<li>Added the concept of an RD Domain and a registration parameter for i
	t.</li>
	<li>Recommended the Location returned from a registration to be stable,
	allowing for endpoint and Domain information to be changed during updates.</li>
	<li>Changed the lookup interface to accept endpoint and Domain as query
	string parameters to control the scope of a lookup.</li>
	</ul>
	</section>
	</middle>	</middle>
	<back>	<back>


		<displayreference target="I-D.silverajan-core-coap-protocol-negotiation" to="COA
		P-PROT-NEG"/>
		<displayreference target="I-D.ietf-ace-oauth-authz" to="ACE-OAUTH-AUTHZ"/>
		<displayreference target="I-D.ietf-core-links-json" to="CORE-LINKS-JSON"/>
		<displayreference target="I-D.ietf-core-rd-dns-sd" to="CORE-RD-DNS-SD"/>
		<displayreference target="I-D.bormann-t2trg-rel-impl" to="T2TRG-REL-IMPL"/>
		<displayreference target="I-D.ietf-core-coral" to="CORE-CORAL"/>

	<references>	<references>
	<name>References</name>	<name>References</name>
	<references>	<references>
	<name>Normative References</name>	<name>Normative References</name>

	<reference anchor="RFC6690" target="https://www.rfc-editor.org/info/rfc6
	690">
	<front>
	<title>Constrained RESTful Environments (CoRE) Link Format</title>
	<seriesInfo name="DOI" value="10.17487/RFC6690"/>
	<seriesInfo name="RFC" value="6690"/>
	<author fullname="Z. Shelby" initials="Z." surname="Shelby">
	<organization/>
	</author>
	<date month="August" year="2012"/>
	<abstract>
	<t>This specification defines Web Linking using a link format for
	use by constrained web servers to describe hosted resources, their attributes, a
	nd other relationships between links. Based on the HTTP Link Header field defin
	ed in RFC 5988, the Constrained RESTful Environments (CoRE) Link Format is carri
	ed as a payload and is assigned an Internet media type. "RESTful" refers to the
	Representational State Transfer (REST) architecture. A well-known URI is defin
	ed as a default entry point for requesting the links hosted by a server. [STAND
	ARDS-TRACK]</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC2119" target="https://www.rfc-editor.org/info/rfc2
	119">
	<front>
	<title>Key words for use in RFCs to Indicate Requirement Levels</tit
	le>
	<seriesInfo name="DOI" value="10.17487/RFC2119"/>
	<seriesInfo name="RFC" value="2119"/>
	<seriesInfo name="BCP" value="14"/>
	<author fullname="S. Bradner" initials="S." surname="Bradner">
	<organization/>
	</author>
	<date month="March" year="1997"/>
	<abstract>
	<t>In many standards track documents several words are used to sig
	nify the requirements in the specification. These words are often capitalized.
	This document defines these words as they should be interpreted in IETF document
	s. This document specifies an Internet Best Current Practices for the Internet
	Community, and requests discussion and suggestions for improvements.</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC3986" target="https://www.rfc-editor.org/info/rfc3
	986">
	<front>
	<title>Uniform Resource Identifier (URI): Generic Syntax</title>
	<seriesInfo name="DOI" value="10.17487/RFC3986"/>
	<seriesInfo name="RFC" value="3986"/>
	<seriesInfo name="STD" value="66"/>
	<author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee
	">
	<organization/>
	</author>
	<author fullname="R. Fielding" initials="R." surname="Fielding">
	<organization/>
	</author>
	<author fullname="L. Masinter" initials="L." surname="Masinter">
	<organization/>
	</author>
	<date month="January" year="2005"/>
	<abstract>
	<t>A Uniform Resource Identifier (URI) is a compact sequence of ch
	aracters that identifies an abstract or physical resource. This specification d
	efines the generic URI syntax and a process for resolving URI references that mi
	ght be in relative form, along with guidelines and security considerations for t
	he use of URIs on the Internet. The URI syntax defines a grammar that is a supe
	rset of all valid URIs, allowing an implementation to parse the common component
	s of a URI reference without knowing the scheme-specific requirements of every p
	ossible identifier. This specification does not define a generative grammar for
	URIs; that task is performed by the individual specifications of each URI schem
	e. [STANDARDS-TRACK]</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC8126" target="https://www.rfc-editor.org/info/rfc8
	126">
	<front>
	<title>Guidelines for Writing an IANA Considerations Section in RFCs
	</title>
	<seriesInfo name="DOI" value="10.17487/RFC8126"/>
	<seriesInfo name="RFC" value="8126"/>
	<seriesInfo name="BCP" value="26"/>
	<author fullname="M. Cotton" initials="M." surname="Cotton">
	<organization/>
	</author>
	<author fullname="B. Leiba" initials="B." surname="Leiba">
	<organization/>
	</author>
	<author fullname="T. Narten" initials="T." surname="Narten">
	<organization/>
	</author>
	<date month="June" year="2017"/>
	<abstract>
	<t>Many protocols make use of points of extensibility that use con
	stants to identify various protocol parameters. To ensure that the values in th
	ese fields do not have conflicting uses and to promote interoperability, their a
	llocations are often coordinated by a central record keeper. For IETF protocols
	, that role is filled by the Internet Assigned Numbers Authority (IANA).</t>
	<t>To make assignments in a given registry prudently, guidance des
	cribing the conditions under which new values should be assigned, as well as whe
	n and how modifications to existing values can be made, is needed. This documen
	t defines a framework for the documentation of these guidelines by specification
	authors, in order to assure that the provided guidance for the IANA Considerati
	ons is clear and addresses the various issues that are likely in the operation o
	f a registry.</t>
	<t>This is the third edition of this document; it obsoletes RFC 52
	26.</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC6570" target="https://www.rfc-editor.org/info/rfc6
	570">
	<front>
	<title>URI Template</title>
	<seriesInfo name="DOI" value="10.17487/RFC6570"/>
	<seriesInfo name="RFC" value="6570"/>
	<author fullname="J. Gregorio" initials="J." surname="Gregorio">
	<organization/>
	</author>
	<author fullname="R. Fielding" initials="R." surname="Fielding">
	<organization/>
	</author>
	<author fullname="M. Hadley" initials="M." surname="Hadley">
	<organization/>
	</author>
	<author fullname="M. Nottingham" initials="M." surname="Nottingham">
	<organization/>
	</author>
	<author fullname="D. Orchard" initials="D." surname="Orchard">
	<organization/>
	</author>
	<date month="March" year="2012"/>
	<abstract>
	<t>A URI Template is a compact sequence of characters for describi
	ng a range of Uniform Resource Identifiers through variable expansion. This spec
	ification defines the URI Template syntax and the process for expanding a URI Te
	mplate into a URI reference, along with guidelines for the use of URI Templates
	on the Internet. [STANDARDS-TRACK]</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC6763" target="https://www.rfc-editor.org/info/rfc6
	763">
	<front>
	<title>DNS-Based Service Discovery</title>
	<seriesInfo name="DOI" value="10.17487/RFC6763"/>
	<seriesInfo name="RFC" value="6763"/>
	<author fullname="S. Cheshire" initials="S." surname="Cheshire">
	<organization/>
	</author>
	<author fullname="M. Krochmal" initials="M." surname="Krochmal">
	<organization/>
	</author>
	<date month="February" year="2013"/>
	<abstract>
	<t>This document specifies how DNS resource records are named and
	structured to facilitate service discovery. Given a type of service that a clie
	nt is looking for, and a domain in which the client is looking for that service,
	this mechanism allows clients to discover a list of named instances of that des
	ired service, using standard DNS queries. This mechanism is referred to as DNS-b
	ased Service Discovery, or DNS-SD.</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC7230" target="https://www.rfc-editor.org/info/rfc7
	230">
	<front>
	<title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Ro
	uting</title>
	<seriesInfo name="DOI" value="10.17487/RFC7230"/>
	<seriesInfo name="RFC" value="7230"/>
	<author fullname="R. Fielding" initials="R." role="editor" surname="
	Fielding">
	<organization/>
	</author>
	<author fullname="J. Reschke" initials="J." role="editor" surname="R
	eschke">
	<organization/>
	</author>
	<date month="June" year="2014"/>
	<abstract>
	<t>The Hypertext Transfer Protocol (HTTP) is a stateless applicati
	on-level protocol for distributed, collaborative, hypertext information systems.
	This document provides an overview of HTTP architecture and its associated ter
	minology, defines the "http" and "https" Uniform Resource Identifier (URI) schem
	es, defines the HTTP/1.1 message syntax and parsing requirements, and describes
	related security concerns for implementations.</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC7252" target="https://www.rfc-editor.org/info/rfc7
	252">
	<front>
	<title>The Constrained Application Protocol (CoAP)</title>
	<seriesInfo name="DOI" value="10.17487/RFC7252"/>
	<seriesInfo name="RFC" value="7252"/>
	<author fullname="Z. Shelby" initials="Z." surname="Shelby">
	<organization/>
	</author>
	<author fullname="K. Hartke" initials="K." surname="Hartke">
	<organization/>
	</author>
	<author fullname="C. Bormann" initials="C." surname="Bormann">
	<organization/>
	</author>
	<date month="June" year="2014"/>
	<abstract>
	<t>The Constrained Application Protocol (CoAP) is a specialized we
	b transfer protocol for use with constrained nodes and constrained (e.g., low-po
	wer, lossy) networks. The nodes often have 8-bit microcontrollers with small am
	ounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wir
	eless Personal Area Networks (6LoWPANs) often have high packet error rates and a
	typical throughput of 10s of kbit/s. The protocol is designed for machine- to-
	machine (M2M) applications such as smart energy and building automation.</t>
	<t>CoAP provides a request/response interaction model between appl
	ication endpoints, supports built-in discovery of services and resources, and in
	cludes key concepts of the Web such as URIs and Internet media types. CoAP is d
	esigned to easily interface with HTTP for integration with the Web while meeting
	specialized requirements such as multicast support, very low overhead, and simp
	licity for constrained environments.</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC8288" target="https://www.rfc-editor.org/info/rfc8
	288">
	<front>
	<title>Web Linking</title>
	<seriesInfo name="DOI" value="10.17487/RFC8288"/>
	<seriesInfo name="RFC" value="8288"/>
	<author fullname="M. Nottingham" initials="M." surname="Nottingham">
	<organization/>
	</author>
	<date month="October" year="2017"/>
	<abstract>
	<t>This specification defines a model for the relationships betwee
	n resources on the Web ("links") and the type of those relationships ("link rela
	tion types").</t>
	<t>It also defines the serialisation of such links in HTTP headers
	with the Link header field.</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="I-D.ietf-core-echo-request-tag" target="https://www.i
	etf.org/archive/id/draft-ietf-core-echo-request-tag-12.txt">
	<front>
	<title>CoAP: Echo, Request-Tag, and Token Processing</title>
	<seriesInfo name="Internet-Draft" value="draft-ietf-core-echo-reques
	t-tag-12"/>
	<author fullname="Christian Amsüss">
	</author>
	<author fullname="John Preuß Mattsson">
	<organization>Ericsson AB</organization>
	</author>
	<author fullname="Göran Selander">
	<organization>Ericsson AB</organization>
	</author>
	<date day="1" month="February" year="2021"/>
	<abstract>
	<t> This document specifies enhancements to the Constrained Appl
	ication
	Protocol (CoAP) that mitigate security issues in particular use
	cases. The Echo option enables a CoAP server to verify the freshness
	of a request or to force a client to demonstrate reachability at its
	claimed network address. The Request-Tag option allows the CoAP
	server to match block-wise message fragments belonging to the same
	request. This document updates RFC7252 with respect to the client
	Token processing requirements, forbidding non-secure reuse of Tokens
	to ensure binding of response to request when CoAP is used with a
	security protocol, and with respect to amplification mitigation,
	where the use of Echo is now recommended.


	</t>	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6690.
	</abstract>	xml"/>
	</front>	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.
	</reference>	xml"/>
	<reference anchor="RFC8174" target="https://www.rfc-editor.org/info/rfc8	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3986.
	174">	xml"/>
	<front>	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8126.
	<title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</ti	xml"/>
	tle>	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6570.
	<seriesInfo name="DOI" value="10.17487/RFC8174"/>	xml"/>
	<seriesInfo name="RFC" value="8174"/>	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6763.
	<seriesInfo name="BCP" value="14"/>	xml"/>
	<author fullname="B. Leiba" initials="B." surname="Leiba">	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7230.
	<organization/>	xml"/>
	</author>	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7252.
	<date month="May" year="2017"/>	xml"/>
	<abstract>	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8288.
	<t>RFC 2119 specifies common key words that may be used in protoco	xml"/>
	l specifications. This document aims to reduce the ambiguity by clarifying tha	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.9175.
	t only UPPERCASE usage of the key words have the defined special meanings.</t>	xml"/>
	</abstract>	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.
	</front>	xml"/>
	</reference>
	</references>	</references>
	<references>	<references>
	<name>Informative References</name>	<name>Informative References</name>

	<reference anchor="RFC6775" target="https://www.rfc-editor.org/info/rfc6
	775">	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6775.
	<front>	xml"/>
	<title>Neighbor Discovery Optimization for IPv6 over Low-Power Wirel	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6874.
	ess Personal Area Networks (6LoWPANs)</title>	xml"/>
	<seriesInfo name="DOI" value="10.17487/RFC6775"/>	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8132.
	<seriesInfo name="RFC" value="6775"/>	xml"/>
	<author fullname="Z. Shelby" initials="Z." role="editor" surname="Sh	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7641.
	elby">	xml"/>
	<organization/>
	</author>
	<author fullname="S. Chakrabarti" initials="S." surname="Chakrabarti
	">
	<organization/>
	</author>
	<author fullname="E. Nordmark" initials="E." surname="Nordmark">
	<organization/>
	</author>
	<author fullname="C. Bormann" initials="C." surname="Bormann">
	<organization/>
	</author>
	<date month="November" year="2012"/>
	<abstract>
	<t>The IETF work in IPv6 over Low-power Wireless Personal Area Net
	work (6LoWPAN) defines 6LoWPANs such as IEEE 802.15.4. This and other similar l
	ink technologies have limited or no usage of multicast signaling due to energy c
	onservation. In addition, the wireless network may not strictly follow the trad
	itional concept of IP subnets and IP links. IPv6 Neighbor Discovery was not des
	igned for non- transitive wireless links, as its reliance on the traditional IPv
	6 link concept and its heavy use of multicast make it inefficient and sometimes
	impractical in a low-power and lossy network. This document describes simple op
	timizations to IPv6 Neighbor Discovery, its addressing mechanisms, and duplicate
	address detection for Low- power Wireless Personal Area Networks and similar ne
	tworks. The document thus updates RFC 4944 to specify the use of the optimizati
	ons defined here. [STANDARDS-TRACK]</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC6874" target="https://www.rfc-editor.org/info/rfc6
	874">
	<front>
	<title>Representing IPv6 Zone Identifiers in Address Literals and Un
	iform Resource Identifiers</title>
	<seriesInfo name="DOI" value="10.17487/RFC6874"/>
	<seriesInfo name="RFC" value="6874"/>
	<author fullname="B. Carpenter" initials="B." surname="Carpenter">
	<organization/>
	</author>
	<author fullname="S. Cheshire" initials="S." surname="Cheshire">
	<organization/>
	</author>
	<author fullname="R. Hinden" initials="R." surname="Hinden">
	<organization/>
	</author>
	<date month="February" year="2013"/>
	<abstract>
	<t>This document describes how the zone identifier of an IPv6 scop
	ed address, defined as <zone_id> in the IPv6 Scoped Address Architecture (
	RFC 4007), can be represented in a literal IPv6 address and in a Uniform Resourc
	e Identifier that includes such a literal address. It updates the URI Generic S
	yntax specification (RFC 3986) accordingly.</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC8132" target="https://www.rfc-editor.org/info/rfc8
	132">
	<front>
	<title>PATCH and FETCH Methods for the Constrained Application Proto
	col (CoAP)</title>
	<seriesInfo name="DOI" value="10.17487/RFC8132"/>
	<seriesInfo name="RFC" value="8132"/>
	<author fullname="P. van der Stok" initials="P." surname="van der St
	ok">
	<organization/>
	</author>
	<author fullname="C. Bormann" initials="C." surname="Bormann">
	<organization/>
	</author>
	<author fullname="A. Sehgal" initials="A." surname="Sehgal">
	<organization/>
	</author>
	<date month="April" year="2017"/>
	<abstract>
	<t>The methods defined in RFC 7252 for the Constrained Application
	Protocol (CoAP) only allow access to a complete resource, not to parts of a res
	ource. In case of resources with larger or complex data, or in situations where
	resource continuity is required, replacing or requesting the whole resource is
	undesirable. Several applications using CoAP need to access parts of the resour
	ces.</t>
	<t>This specification defines the new CoAP methods, FETCH, PATCH,
	and iPATCH, which are used to access and update parts of a resource.</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC7641" target="https://www.rfc-editor.org/info/rfc7
	641">
	<front>
	<title>Observing Resources in the Constrained Application Protocol (
	CoAP)</title>
	<seriesInfo name="DOI" value="10.17487/RFC7641"/>
	<seriesInfo name="RFC" value="7641"/>
	<author fullname="K. Hartke" initials="K." surname="Hartke">
	<organization/>
	</author>
	<date month="September" year="2015"/>
	<abstract>
	<t>The Constrained Application Protocol (CoAP) is a RESTful applic
	ation protocol for constrained nodes and networks. The state of a resource on a
	CoAP server can change over time. This document specifies a simple protocol ex
	tension for CoAP that enables CoAP clients to "observe" resources, i.e., to retr
	ieve a representation of a resource and keep this representation updated by the
	server over a period of time. The protocol follows a best-effort approach for s
	ending new representations to clients and provides eventual consistency between
	the state observed by each client and the actual resource state at the server.</
	t>
	</abstract>
	</front>
	</reference>
	<reference anchor="ER">	<reference anchor="ER">
	<front>	<front>
	<title>The entity-relationship model--toward a unified view of data< /title>	<title>The entity-relationship model--toward a unified view of data< /title>

	<seriesInfo name="DOI" value="10.1145/320434.320440"/>	<author fullname="Peter Pin-Shan Chen" initials="P." surname="Chen">
	<seriesInfo name="ACM Transactions on Database Systems" value="Vol.
	1, pp. 9-36"/>
	<author fullname="Peter Pin-Shan Chen" initials="P." surname="Chen">
	<organization/>	<organization/>
	</author>	</author>

	<date month="March" year="1976"/>	<date month="March" year="1976"/>
	</front>	</front>
		<seriesInfo name="DOI" value="10.1145/320434.320440"/>
		<refcontent>ACM Transactions on Database Systems, Vol. 1, pp. 9-36</
		refcontent>
	</reference>	</reference>

	<reference anchor="I-D.silverajan-core-coap-protocol-negotiation" target
	="https://www.ietf.org/archive/id/draft-silverajan-core-coap-protocol-negotiatio
	n-09.txt">
	<front>
	<title>CoAP Protocol Negotiation</title>
	<seriesInfo name="Internet-Draft" value="draft-silverajan-core-coap-
	protocol-negotiation-09"/>
	<author fullname="Bilhanan Silverajan">
	<organization>TUT</organization>
	</author>
	<author fullname="Mert Ocak">
	<organization>Ericsson</organization>
	</author>
	<date day="2" month="July" year="2018"/>
	<abstract>
	<t> CoAP has been standardised as an application-level REST-base
	d
	protocol. When multiple transport protocols exist for exchanging
	CoAP resource representations, this document introduces a way forward
	for CoAP endpoints as well as intermediaries to agree upon alternate
	transport and protocol configurations as well as URIs for CoAP
	messaging. Several mechanisms are proposed: Extending the CoRE
	Resource Directory with new parameter types, introducing a new CoAP
	Option with which clients can interact directly with servers without
	needing the Resource Directory, and finally a new CoRE Link Attribute
	allowing exposing alternate locations on a per-resource basis.


	</t>	<xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.silvera
	</abstract>	jan-core-coap-protocol-negotiation.xml"/>
	</front>
	</reference>
	<reference anchor="I-D.ietf-ace-oauth-authz" target="https://www.ietf.or
	g/archive/id/draft-ietf-ace-oauth-authz-37.txt">
	<front>
	<title>Authentication and Authorization for Constrained Environments
	(ACE) using the OAuth 2.0 Framework (ACE-OAuth)</title>
	<seriesInfo name="Internet-Draft" value="draft-ietf-ace-oauth-authz-
	37"/>
	<author fullname="Ludwig Seitz">
	<organization>Combitech</organization>
	</author>
	<author fullname="Goeran Selander">
	<organization>Ericsson</organization>
	</author>
	<author fullname="Erik Wahlstroem">
	</author>
	<author fullname="Samuel Erdtman">
	<organization>Spotify AB</organization>
	</author>
	<author fullname="Hannes Tschofenig">
	<organization>Arm Ltd.</organization>
	</author>
	<date day="4" month="February" year="2021"/>
	<abstract>
	<t> This specification defines a framework for authentication an
	d
	authorization in Internet of Things (IoT) environments called ACE-
	OAuth. The framework is based on a set of building blocks including
	OAuth 2.0 and the Constrained Application Protocol (CoAP), thus
	transforming a well-known and widely used authorization solution into
	a form suitable for IoT devices. Existing specifications are used
	where possible, but extensions are added and profiles are defined to
	better serve the IoT use cases.


	</t>	<reference anchor='I-D.ietf-ace-oauth-authz'>
	</abstract>	<front>
	</front>	<title>Authentication and Authorization for Constrained Environments Using the O
	</reference>	Auth 2.0 Framework (ACE-OAuth)</title>
	<reference anchor="I-D.ietf-core-links-json" target="https://www.ietf.or	<author initials='L' surname='Seitz' fullname='Ludwig Seitz'>
	g/archive/id/draft-ietf-core-links-json-10.txt">	<organization />
	<front>	</author>
	<title>Representing Constrained RESTful Environments (CoRE) Link For	<author initials='G' surname='Selander' fullname='Göran Selander'>
	mat in JSON and CBOR</title>	<organization />
	<seriesInfo name="Internet-Draft" value="draft-ietf-core-links-json-	</author>
	10"/>	<author initials='E' surname='Wahlstroem' fullname='Erik Wahlstroem'>
	<author fullname="Kepeng LI">	<organization />
	<organization>Alibaba Group</organization>	</author>
	</author>	<author initials='S' surname='Erdtman' fullname='Samuel Erdtman'>
	<author fullname="Akbar Rahman">	<organization />
	<organization>InterDigital</organization>	</author>
	</author>	<author initials='H' surname='Tschofenig' fullname='Hannes Tschofenig'>
	<author fullname="Carsten Bormann">	<organization />
	<organization>Universitaet Bremen TZI</organization>	</author>
	</author>	<date month='November' day='8' year='2021'/>
	<date day="26" month="February" year="2018"/>	</front>
	<abstract>	<seriesInfo name='Internet-Draft' value='draft-ietf-ace-oauth-authz-46' />
	<t> JavaScript Object Notation, JSON (RFC 8259) is a text-based	<format type='TXT'
	data	target='http://www.ietf.org/internet-drafts/draft-ietf-ace-oauth-authz-4
	format which is popular for Web based data exchange. Concise Binary	6.txt' />
	Object Representation, CBOR (RFC7049) is a binary data format which	</reference>
	has been optimized for data exchange for the Internet of Things
	(IoT). For many IoT scenarios, CBOR formats will be preferred since
	it can help decrease transmission payload sizes as well as
	implementation code sizes compared to other data formats.


	Web Linking (RFC 8288) provides a way to represent links between Web	<reference anchor='I-D.ietf-core-links-json'>
	resources as well as the relations expressed by them and attributes	<front>
	of such a link. In constrained networks, a collection of Web links	<title>Representing Constrained RESTful Environments (CoRE) Link Format in JSON
	can be exchanged in the CoRE link format (RFC 6690). Outside of	and CBOR</title>
	constrained environments, it may be useful to represent these	<author initials='K' surname='Li' fullname='Kepeng Li'>
	collections of Web links in JSON, and similarly, inside constrained	<organization />
	environments, in CBOR. This specification defines a common format	</author>
	for this.	<author initials='A' surname='Rahman' fullname='Akbar Rahman'>
		<organization />
		</author>
		<author initials='C' surname='Bormann' fullname='Carsten Bormann' role="editor">
		<organization />
		</author>
		<date month='February' day='26' year='2018' />
		</front>
		<seriesInfo name='Internet-Draft' value='draft-ietf-core-links-json-10' />
		<format type='TXT'
		target='http://www.ietf.org/internet-drafts/draft-ietf-core-links-json-1
		0.txt' />
		</reference>


	</t>	<reference anchor='I-D.ietf-core-rd-dns-sd'>
	</abstract>	<front>
	</front>	<title>CoRE Resource Directory: DNS-SD mapping</title>
	</reference>	<author initials='P' surname='van der Stok' fullname='Peter van der Stok'>
	<reference anchor="I-D.ietf-core-rd-dns-sd" target="https://www.ietf.org	<organization />
	/archive/id/draft-ietf-core-rd-dns-sd-05.txt">	</author>
	<front>	<author initials='M' surname='Koster' fullname='Michael Koster'>
	<title>CoRE Resource Directory: DNS-SD mapping</title>	<organization />
	<seriesInfo name="Internet-Draft" value="draft-ietf-core-rd-dns-sd-0	</author>
	5"/>	<author initials='C' surname='Amsuess' fullname='Christian Amsuess'>
	<author fullname="Peter van der Stok">	<organization />
	<organization>Consultant</organization>	</author>
	</author>	<date month='July' day='7' year='2019' />
	<author fullname="Michael Koster">	</front>
	<organization>SmartThings</organization>	<seriesInfo name='Internet-Draft' value='draft-ietf-core-rd-dns-sd-05' />
	</author>	<format type='TXT'
	<author fullname="Christian Amsüss">	target='http://www.ietf.org/internet-drafts/draft-ietf-core-rd-dns-sd-05
	<organization>Energy Harvesting Solutions</organization>	.txt' />
	</author>	</reference>
	<date day="7" month="July" year="2019"/>
	<abstract>
	<t> Resource and service discovery are complementary. Resource
	discovery
	provides fine-grained detail about the content of a web server, while
	service discovery can provide a scalable method to locate servers in
	large networks. This document defines a method for mapping between
	CoRE Link Format attributes and DNS-Based Service Discovery records
	to facilitate the use of either method to locate RESTful service
	interfaces (APIs) in heterogeneous HTTP/CoAP environments.


	</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="LwM2M" target="https://openmobilealliance.org/RELEASE /LightweightM2M/V1_1-20180612-C/OMA-TS-LightweightM2M_Transport-V1_1-20180612-C. pdf">	<reference anchor="LwM2M" target="https://openmobilealliance.org/RELEASE /LightweightM2M/V1_1-20180612-C/OMA-TS-LightweightM2M_Transport-V1_1-20180612-C. pdf">
	<front>	<front>
	<title>Lightweight Machine to Machine Technical Specification: Trans port Bindings (Candidate Version 1.1)</title>	<title>Lightweight Machine to Machine Technical Specification: Trans port Bindings (Candidate Version 1.1)</title>
	<author>	<author>
	<organization>Open Mobile Alliance</organization>	<organization>Open Mobile Alliance</organization>
	</author>	</author>

	<date year="2018" month="June" day="12"/>	<date year="2018" month="June"/>
	</front>
	</reference>
	<reference anchor="RFC7228" target="https://www.rfc-editor.org/info/rfc7
	228">
	<front>
	<title>Terminology for Constrained-Node Networks</title>
	<seriesInfo name="DOI" value="10.17487/RFC7228"/>
	<seriesInfo name="RFC" value="7228"/>
	<author fullname="C. Bormann" initials="C." surname="Bormann">
	<organization/>
	</author>
	<author fullname="M. Ersue" initials="M." surname="Ersue">
	<organization/>
	</author>
	<author fullname="A. Keranen" initials="A." surname="Keranen">
	<organization/>
	</author>
	<date month="May" year="2014"/>
	<abstract>
	<t>The Internet Protocol Suite is increasingly used on small devic
	es with severe constraints on power, memory, and processing resources, creating
	constrained-node networks. This document provides a number of basic terms that
	have been useful in the standardization work for constrained-node networks.</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC4944" target="https://www.rfc-editor.org/info/rfc4
	944">
	<front>
	<title>Transmission of IPv6 Packets over IEEE 802.15.4 Networks</tit
	le>
	<seriesInfo name="DOI" value="10.17487/RFC4944"/>
	<seriesInfo name="RFC" value="4944"/>
	<author fullname="G. Montenegro" initials="G." surname="Montenegro">
	<organization/>
	</author>
	<author fullname="N. Kushalnagar" initials="N." surname="Kushalnagar
	">
	<organization/>
	</author>
	<author fullname="J. Hui" initials="J." surname="Hui">
	<organization/>
	</author>
	<author fullname="D. Culler" initials="D." surname="Culler">
	<organization/>
	</author>
	<date month="September" year="2007"/>
	<abstract>
	<t>This document describes the frame format for transmission of IP
	v6 packets and the method of forming IPv6 link-local addresses and statelessly a
	utoconfigured addresses on IEEE 802.15.4 networks. Additional specifications inc
	lude a simple header compression scheme using shared context and provisions for
	packet delivery in IEEE 802.15.4 meshes. [STANDARDS-TRACK]</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC8141" target="https://www.rfc-editor.org/info/rfc8
	141">
	<front>
	<title>Uniform Resource Names (URNs)</title>
	<seriesInfo name="DOI" value="10.17487/RFC8141"/>
	<seriesInfo name="RFC" value="8141"/>
	<author fullname="P. Saint-Andre" initials="P." surname="Saint-Andre
	">
	<organization/>
	</author>
	<author fullname="J. Klensin" initials="J." surname="Klensin">
	<organization/>
	</author>
	<date month="April" year="2017"/>
	<abstract>
	<t>A Uniform Resource Name (URN) is a Uniform Resource Identifier
	(URI) that is assigned under the "urn" URI scheme and a particular URN namespace
	, with the intent that the URN will be a persistent, location-independent resour
	ce identifier. With regard to URN syntax, this document defines the canonical s
	yntax for URNs (in a way that is consistent with URI syntax), specifies methods
	for determining URN-equivalence, and discusses URI conformance. With regard to
	URN namespaces, this document specifies a method for defining a URN namespace an
	d associating it with a namespace identifier, and it describes procedures for re
	gistering namespace identifiers with the Internet Assigned Numbers Authority (IA
	NA). This document obsoletes both RFCs 2141 and 3406.</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC6724" target="https://www.rfc-editor.org/info/rfc6
	724">
	<front>
	<title>Default Address Selection for Internet Protocol Version 6 (IP
	v6)</title>
	<seriesInfo name="DOI" value="10.17487/RFC6724"/>
	<seriesInfo name="RFC" value="6724"/>
	<author fullname="D. Thaler" initials="D." role="editor" surname="Th
	aler">
	<organization/>
	</author>
	<author fullname="R. Draves" initials="R." surname="Draves">
	<organization/>
	</author>
	<author fullname="A. Matsumoto" initials="A." surname="Matsumoto">
	<organization/>
	</author>
	<author fullname="T. Chown" initials="T." surname="Chown">
	<organization/>
	</author>
	<date month="September" year="2012"/>
	<abstract>
	<t>This document describes two algorithms, one for source address
	selection and one for destination address selection. The algorithms specify def
	ault behavior for all Internet Protocol version 6 (IPv6) implementations. They
	do not override choices made by applications or upper-layer protocols, nor do th
	ey preclude the development of more advanced mechanisms for address selection.
	The two algorithms share a common context, including an optional mechanism for a
	llowing administrators to provide policy that can override the default behavior.
	In dual-stack implementations, the destination address selection algorithm can
	consider both IPv4 and IPv6 addresses -- depending on the available source addr
	esses, the algorithm might prefer IPv6 addresses over IPv4 addresses, or vice ve
	rsa.</t>
	<t>Default address selection as defined in this specification appl
	ies to all IPv6 nodes, including both hosts and routers. This document obsolete
	s RFC 3484. [STANDARDS-TRACK]</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC8106" target="https://www.rfc-editor.org/info/rfc8
	106">
	<front>
	<title>IPv6 Router Advertisement Options for DNS Configuration</titl
	e>
	<seriesInfo name="DOI" value="10.17487/RFC8106"/>
	<seriesInfo name="RFC" value="8106"/>
	<author fullname="J. Jeong" initials="J." surname="Jeong">
	<organization/>
	</author>
	<author fullname="S. Park" initials="S." surname="Park">
	<organization/>
	</author>
	<author fullname="L. Beloeil" initials="L." surname="Beloeil">
	<organization/>
	</author>
	<author fullname="S. Madanapalli" initials="S." surname="Madanapalli
	">
	<organization/>
	</author>
	<date month="March" year="2017"/>
	<abstract>
	<t>This document specifies IPv6 Router Advertisement (RA) options
	(called "DNS RA options") to allow IPv6 routers to advertise a list of DNS Recur
	sive Server Addresses and a DNS Search List to IPv6 hosts.</t>
	<t>This document, which obsoletes RFC 6106, defines a higher defau
	lt value of the lifetime of the DNS RA options to reduce the likelihood of expir
	y of the options on links with a relatively high rate of packet loss.</t>
	</abstract>
	</front>	</front>
	</reference>	</reference>

	<reference anchor="I-D.bormann-t2trg-rel-impl" target="https://www.ietf.
	org/archive/id/draft-bormann-t2trg-rel-impl-02.txt">
	<front>
	<title>impl-info: A link relation type for disclosing implementation
	information</title>
	<seriesInfo name="Internet-Draft" value="draft-bormann-t2trg-rel-imp
	l-02"/>
	<author fullname="Carsten Bormann">
	<organization>Universität Bremen TZI</organization>
	</author>
	<date day="27" month="September" year="2020"/>
	<abstract>
	<t> For debugging, it is often helpful to have information about
	the
	implementation of a peer. The present specification defines a link
	relation type, "impl-info", that can be used to convey such
	information via self-description, such as in the "/.well-known/core"
	resource.


	</t>	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7228.
	</abstract>	xml"/>
	</front>	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4944.
	</reference>	xml"/>
	<reference anchor="I-D.hartke-t2trg-coral" target="https://www.ietf.org/	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8141.
	archive/id/draft-hartke-t2trg-coral-09.txt">	xml"/>
	<front>	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6724.
	<title>The Constrained RESTful Application Language (CoRAL)</title>	xml"/>
	<seriesInfo name="Internet-Draft" value="draft-hartke-t2trg-coral-09	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8106.
	"/>	xml"/>
	<author fullname="Klaus Hartke">
	<organization>Ericsson</organization>	<xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.bormann
	</author>	-t2trg-rel-impl.xml"/>
	<date day="8" month="July" year="2019"/>
	<abstract>	<xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.ietf-co
	<t> The Constrained RESTful Application Language (CoRAL) defines	re-coral.xml"/>
	a data
	model and interaction model as well as two specialized serialization	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4122.
	formats for the description of typed connections between resources on	xml"/>
	the Web ("links"), possible operations on such resources ("forms"),	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8613.
	as well as simple resource metadata.	xml"/>
		<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5771.
		xml"/>
		<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3849.
		xml"/>
		<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3306.
		xml"/>
		<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3307.
		xml"/>


	</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC4122" target="https://www.rfc-editor.org/info/rfc4
	122">
	<front>
	<title>A Universally Unique IDentifier (UUID) URN Namespace</title>
	<seriesInfo name="DOI" value="10.17487/RFC4122"/>
	<seriesInfo name="RFC" value="4122"/>
	<author fullname="P. Leach" initials="P." surname="Leach">
	<organization/>
	</author>
	<author fullname="M. Mealling" initials="M." surname="Mealling">
	<organization/>
	</author>
	<author fullname="R. Salz" initials="R." surname="Salz">
	<organization/>
	</author>
	<date month="July" year="2005"/>
	<abstract>
	<t>This specification defines a Uniform Resource Name namespace fo
	r UUIDs (Universally Unique IDentifier), also known as GUIDs (Globally Unique ID
	entifier). A UUID is 128 bits long, and can guarantee uniqueness across space a
	nd time. UUIDs were originally used in the Apollo Network Computing System and
	later in the Open Software Foundation\'s (OSF) Distributed Computing Environment
	(DCE), and then in Microsoft Windows platforms.</t>
	<t>This specification is derived from the DCE specification with t
	he kind permission of the OSF (now known as The Open Group). Information from e
	arlier versions of the DCE specification have been incorporated into this docume
	nt. [STANDARDS-TRACK]</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC8613" target="https://www.rfc-editor.org/info/rfc8
	613">
	<front>
	<title>Object Security for Constrained RESTful Environments (OSCORE)
	</title>
	<seriesInfo name="DOI" value="10.17487/RFC8613"/>
	<seriesInfo name="RFC" value="8613"/>
	<author fullname="G. Selander" initials="G." surname="Selander">
	<organization/>
	</author>
	<author fullname="J. Mattsson" initials="J." surname="Mattsson">
	<organization/>
	</author>
	<author fullname="F. Palombini" initials="F." surname="Palombini">
	<organization/>
	</author>
	<author fullname="L. Seitz" initials="L." surname="Seitz">
	<organization/>
	</author>
	<date month="July" year="2019"/>
	<abstract>
	<t>This document defines Object Security for Constrained RESTful E
	nvironments (OSCORE), a method for application-layer protection of the Constrain
	ed Application Protocol (CoAP), using CBOR Object Signing and Encryption (COSE).
	OSCORE provides end-to-end protection between endpoints communicating using Co
	AP or CoAP-mappable HTTP. OSCORE is designed for constrained nodes and networks
	supporting a range of proxy operations, including translation between different
	transport protocols.</t>
	<t>Although an optional functionality of CoAP, OSCORE alters CoAP
	options processing and IANA registration. Therefore, this document updates RFC
	7252.</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC5771" target="https://www.rfc-editor.org/info/rfc5
	771">
	<front>
	<title>IANA Guidelines for IPv4 Multicast Address Assignments</title
	>
	<seriesInfo name="DOI" value="10.17487/RFC5771"/>
	<seriesInfo name="RFC" value="5771"/>
	<seriesInfo name="BCP" value="51"/>
	<author fullname="M. Cotton" initials="M." surname="Cotton">
	<organization/>
	</author>
	<author fullname="L. Vegoda" initials="L." surname="Vegoda">
	<organization/>
	</author>
	<author fullname="D. Meyer" initials="D." surname="Meyer">
	<organization/>
	</author>
	<date month="March" year="2010"/>
	<abstract>
	<t>This document provides guidance for the Internet Assigned Numbe
	rs Authority (IANA) in assigning IPv4 multicast addresses. It obsoletes RFC 317
	1 and RFC 3138 and updates RFC 2780. This memo documents an Internet Best Curr
	ent Practice.</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC3849" target="https://www.rfc-editor.org/info/rfc3
	849">
	<front>
	<title>IPv6 Address Prefix Reserved for Documentation</title>
	<seriesInfo name="DOI" value="10.17487/RFC3849"/>
	<seriesInfo name="RFC" value="3849"/>
	<author fullname="G. Huston" initials="G." surname="Huston">
	<organization/>
	</author>
	<author fullname="A. Lord" initials="A." surname="Lord">
	<organization/>
	</author>
	<author fullname="P. Smith" initials="P." surname="Smith">
	<organization/>
	</author>
	<date month="July" year="2004"/>
	<abstract>
	<t>To reduce the likelihood of conflict and confusion when relatin
	g documented examples to deployed systems, an IPv6 unicast address prefix is res
	erved for use in examples in RFCs, books, documentation, and the like. Since si
	te-local and link-local unicast addresses have special meaning in IPv6, these ad
	dresses cannot be used in many example situations. The document describes the u
	se of the IPv6 address prefix 2001:DB8::/32 as a reserved prefix for use in docu
	mentation. This memo provides information for the Internet community.</t>
	</abstract>
	</front>
	</reference>
	<reference anchor="RFC3306" target="https://www.rfc-editor.org/info/rfc3
	306">
	<front>
	<title>Unicast-Prefix-based IPv6 Multicast Addresses</title>
	<seriesInfo name="DOI" value="10.17487/RFC3306"/>
	<seriesInfo name="RFC" value="3306"/>
	<author fullname="B. Haberman" initials="B." surname="Haberman">
	<organization/>
	</author>
	<author fullname="D. Thaler" initials="D." surname="Thaler">
	<organization/>
	</author>
	<date month="August" year="2002"/>
	</front>
	</reference>
	</references>	</references>
	</references>	</references>
	<section anchor="groups" numbered="true" toc="default">	<section anchor="groups" numbered="true" toc="default">
	<name>Groups Registration and Lookup</name>	<name>Groups Registration and Lookup</name>

	<t>The RD-Groups usage pattern allows announcing application groups inside an RD.</t>	<t>The RD-Group's usage pattern allows announcing application groups insid e an RD.</t>
	<t>Groups are represented by endpoint registrations.	<t>Groups are represented by endpoint registrations.

	Their base address is a multicast address,	Their base address is a multicast address,
	and they SHOULD be entered with the endpoint type <tt>core.rd-group</tt>.	and they <bcp14>SHOULD</bcp14> be entered with the endpoint type <tt>core.
	The endpoint name can also be referred to as a group name in this context.</t>	rd-group</tt>.
		The endpoint name can also be referred to as a group name in this context.
		</t>
	<t>The registration is inserted into the RD by a Commissioning Tool,	<t>The registration is inserted into the RD by a Commissioning Tool,

	which might also be known as a group manager here.	which might also be known as a group manager here.
	It performs third party registration and registration updates.</t>	It performs third-party registration and registration updates.</t>
	<t>The links it registers SHOULD be available on all members that join the	<t>The links it registers <bcp14>SHOULD</bcp14> be available on all member
	group.	s that join the group.
	Depending on the application, members that lack some resource	Depending on the application, members that lack some resources
	MAY be permissible if requests to them fail gracefully.</t>	<bcp14>MAY</bcp14> be permissible if requests to them fail gracefully.</t>
	<t>The following example shows a CT registering a group with the name "lig	<t>The following example shows a CT registering a group with the name "lig
	hts" which provides two resources.	hts", which provides two resources.
	The directory resource path /rd	The directory resource path /rd
	is an example RD location discovered in a request similar to <xref target="examp le-discovery" format="default"/>.	is an example RD location discovered in a request similar to <xref target="examp le-discovery" format="default"/>.

	The group address in the example is constructed from <xref target="RFC3849" form at="default"/>'s reserved 2001:db8:: prefix as a unicast-prefix based site-local address (see <xref target="RFC3306" format="default"/>.</t>	The group address in the example is constructed from the reserved 2001:db8:: pre fix in <xref target="RFC3849" format="default"/> as a unicast-prefix-based site- local address (see <xref target="RFC3306" format="default"/>).</t>
	<figure anchor="example-group-registration">	<figure anchor="example-group-registration">

	<name>Example registration of a group</name>	<name>Example Registration of a Group</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: POST coap://rd.example.com/rd?ep=lights&et=core.rd-group	Req: POST coap://rd.example.com/rd?ep=lights&et=core.rd-group
	&base=coap://[ff35:30:2001:db8:f1::8000:1]	&base=coap://[ff35:30:2001:db8:f1::8000:1]
	Content-Format: 40	Content-Format: 40
	Payload:	Payload:
	</light>;rt="tag:example.com,2020:light";	</light>;rt="tag:example.com,2020:light";
	if="tag:example.net,2020:actuator",	if="tag:example.net,2020:actuator",
	</color-temperature>;if="tag:example.net,2020:parameter";u=K	</color-temperature>;if="tag:example.net,2020:parameter";u=K

	Res: 2.01 Created	Res: 2.01 Created
	Location-Path: /rd/12	Location-Path: /rd/12

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	<t>In this example, the group manager can easily permit devices that have no	<t>In this example, the group manager can easily permit devices that have no

	writable color-temperature to join, as they would still respond to brightness	writable color-temperature to join, as they would still respond to brightness-ch
	changing commands. Had the group instead contained a single resource that sets	anging commands. Had the group instead contained a single resource that sets
	brightness and color temperature atomically, endpoints would need to support	brightness and color-temperature atomically, endpoints would need to support
	both properties.</t>	both properties.</t>
	<t>The resources of a group can be looked up like any other resource,	<t>The resources of a group can be looked up like any other resource,
	and the group registrations (along with any additional registration parameters)	and the group registrations (along with any additional registration parameters)
	can be looked up using the endpoint lookup interface.</t>	can be looked up using the endpoint lookup interface.</t>

	<t>The following example shows a client performing an endpoint lookup for all groups.</t>	<t>The following example shows a client performing an endpoint lookup for all groups:</t>
	<figure anchor="example-group-lookup">	<figure anchor="example-group-lookup">

	<name>Example lookup of groups</name>	<name>Example Lookup of Groups</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET /rd-lookup/ep?et=core.rd-group	Req: GET /rd-lookup/ep?et=core.rd-group

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	</rd/12>;ep=lights&et=core.rd-group;	</rd/12>;ep=lights&et=core.rd-group;
	base="coap://[ff35:30:2001:f1:db8::8000:1]";rt=core.rd-ep	base="coap://[ff35:30:2001:f1:db8::8000:1]";rt=core.rd-ep

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>

	<t>The following example shows a client performing a lookup of all resourc es of all endpoints (groups) with et=core.rd-group.</t>	<t>The following example shows a client performing a lookup of all resourc es of all endpoints (groups) with et=core.rd-group:</t>
	<figure anchor="example-group-lookup-res">	<figure anchor="example-group-lookup-res">

	<name>Example lookup of resources inside groups</name>	<name>Example Lookup of Resources Inside Groups</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET /rd-lookup/res?et=core.rd-group	Req: GET /rd-lookup/res?et=core.rd-group

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	<coap://[ff35:30:2001:db8:f1::8000:1]/light>;	<coap://[ff35:30:2001:db8:f1::8000:1]/light>;
	rt="tag:example.com,2020:light";	rt="tag:example.com,2020:light";
	if="tag:example.net,2020:actuator",	if="tag:example.net,2020:actuator",
	<coap://[ff35:30:2001:db8:f1::8000:1]/color-temperature>;	<coap://[ff35:30:2001:db8:f1::8000:1]/color-temperature>;
	if="tag:example.net,2020:parameter";u=K,	if="tag:example.net,2020:parameter";u=K,

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	</section>	</section>
	<section anchor="weblink" numbered="true" toc="default">	<section anchor="weblink" numbered="true" toc="default">

	<name>Web links and the Resource Directory</name>	<name>Web Links and the Resource Directory</name>
	<t>Understanding the semantics of a link-format document and its URI refer ences is	<t>Understanding the semantics of a link-format document and its URI refer ences is
	a journey through different documents (<xref target="RFC3986" format="default"/> defining URIs, <xref target="RFC6690" format="default"/>	a journey through different documents (<xref target="RFC3986" format="default"/> defining URIs, <xref target="RFC6690" format="default"/>

	defining link-format documents based on <xref target="RFC8288" format="default"/ > which defines Link header fields,	defining link-format documents based on <xref target="RFC8288" format="default"/ >, which defines Link header fields,
	and <xref target="RFC7252" format="default"/> providing the transport). This app endix summarizes	and <xref target="RFC7252" format="default"/> providing the transport). This app endix summarizes
	the mechanisms and semantics at play from an entry in <tt>/.well-known/core</tt> to a	the mechanisms and semantics at play from an entry in <tt>/.well-known/core</tt> to a
	resource lookup.</t>	resource lookup.</t>
	<t>This text is primarily aimed at people entering the field of Constraine d	<t>This text is primarily aimed at people entering the field of Constraine d
	Restful Environments from applications that previously did not use web	Restful Environments from applications that previously did not use web
	mechanisms.</t>	mechanisms.</t>
	<section anchor="a-simple-example" numbered="true" toc="default">	<section anchor="a-simple-example" numbered="true" toc="default">

	<name>A simple example</name>	<name>A Simple Example</name>
	<t>Let's start this example with a very simple host, <tt>2001:db8:f0::1< /tt>. A client	<t>Let's start this example with a very simple host, <tt>2001:db8:f0::1< /tt>. A client

	that follows classical CoAP Discovery (<xref target="RFC7252" format="default"/>	that follows classical CoAP discovery (<xref target="RFC7252" section="7" sectio
	Section 7), sends the	nFormat="comma"/>) sends the
	following multicast request to learn about neighbours supporting resources with	following multicast request to learn about neighbors supporting resources with
	resource-type "temperature".</t>	resource-type "temperature".</t>
	<t>The client sends a link-local multicast:</t>	<t>The client sends a link-local multicast:</t>
	<figure anchor="example-weblink-wkc">	<figure anchor="example-weblink-wkc">

	<name>Example of direct resource discovery</name>	<name>Example of Direct Resource Discovery</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET coap://[ff02::fd]:5683/.well-known/core?rt=temperature	Req: GET coap://[ff02::fd]:5683/.well-known/core?rt=temperature

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	</sensors/temp>;rt=temperature;ct=0	</sensors/temp>;rt=temperature;ct=0

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	<t>where the response is sent by the server, <tt>[2001:db8:f0::1]:5683</ tt>.</t>	<t>where the response is sent by the server, <tt>[2001:db8:f0::1]:5683</ tt>.</t>

	<t>While the client -- on the practical or implementation side -- can ju	<t>While a practical client side implementation might just go
	st go	ahead and create a new request to <tt>[2001:db8:f0::1]:5683</tt> with Uri-Path
	ahead and create a new request to <tt>[2001:db8:f0::1]:5683</tt> with Uri-Path:	<tt>sensors</tt> and <tt>temp</tt>, the full resolution steps for insertion into
	<tt>sensors</tt> and <tt>temp</tt>, the full resolution steps for insertion into	and retrieval from the RD without any shortcuts are as follows.</t>
	and retrieval from the RD without any shortcuts are:</t>
	<section anchor="resolveURI" numbered="true" toc="default">	<section anchor="resolveURI" numbered="true" toc="default">
	<name>Resolving the URIs</name>	<name>Resolving the URIs</name>

	<t>The client parses the single returned record. The link's target (so	<t>The client parses the single returned link. Its target (sometimes
	metimes	called "href") is <tt>/sensors/temp</tt>, which is a relative URI that needs res
	called "href") is "<tt>/sensors/temp</tt>", which is a relative URI that needs r	olving.
	esolving.
	The base	The base

	URI <coap://[ff02::fd]:5683/.well-known/core> is used to resolve the	URI <tt>coap://[ff02::fd]:5683/.well-known/core</tt> is used to resolve the
	reference /sensors/temp against.</t>	reference against <tt>/sensors/temp</tt>.</t>
	<t>The Base URI of the requested resource can be composed from the opt	<t>The base URI of the requested resource can be composed from the opt
	ions of the CoAP GET request by following the steps of	ions of the CoAP GET request by following the steps of
	<xref target="RFC7252" format="default"/> section 6.5 (with an addition at the e	<xref target="RFC7252" sectionFormat="comma" section="6.5"/> (with an addition a
	nd of 8.2) into	t the end of Section <xref target="RFC7252" sectionFormat="bare" section="8.2"/>
	"<tt>coap://[2001:db8:f0::1]/.well-known/core</tt>".</t>	) into
	<t>Because "<tt>/sensors/temp</tt>" starts with a single slash,	<tt>coap://[2001:db8:f0::1]/.well-known/core</tt>.</t>
	the record's target is resolved by replacing the path "<tt>/.well-known/core</tt	<t>Because <tt>/sensors/temp</tt> starts with a single slash,
	>"	the link's target is resolved by replacing the path <tt>/.well-known/core</tt>
	from the Base URI (section 5.2 <xref target="RFC3986" format="default"/>) with t	from the base URI (<xref target="RFC3986" sectionFormat="comma" section="5.2"/>)
	he relative target URI "<tt>/sensors/temp</tt>" into	with the relative target URI <tt>/sensors/temp</tt> into
	"<tt>coap://[2001:db8:f0::1]/sensors/temp</tt>".</t>	<tt>coap://[2001:db8:f0::1]/sensors/temp</tt>.</t>
	</section>	</section>
	<section anchor="interpreting-attributes-and-relations" numbered="true" toc="default">	<section anchor="interpreting-attributes-and-relations" numbered="true" toc="default">

	<name>Interpreting attributes and relations</name>	<name>Interpreting Attributes and Relations</name>
	<t>Some more information but the record's target can be obtained from	<t>Some more information about the link's target can be obtained from t
	the payload:	he payload:
	the resource type of the target is "temperature", and its content format is	the resource type of the target is "temperature", and its content format is
	text/plain (ct=0).</t>	text/plain (ct=0).</t>
	<t>A relation in a web link is a three-part statement that specifies a named relation between the so-called "context resource"	<t>A relation in a web link is a three-part statement that specifies a named relation between the so-called "context resource"
	and the target resource, like "<em>This page</em> has <em>its table	and the target resource, like "<em>This page</em> has <em>its table

	of contents</em> at <em>/toc.html</em>". In link format documents,	of contents</em> at <em>/toc.html</em>". In link-format documents,
	there is an implicit "host relation" specified with default parameter: rel="host	there is an implicit "host relation" specified with default parameter rel="hosts
	s".</t>	".</t>
	<t>In our example, the context resource of the link is implied to be "	<t>In our example, the context resource of the link is implied to be <
	coap:://[2001:db8:f0::1]"	tt>coap:://[2001:db8:f0::1]</tt>
	by the default value of the anchor (see <xref target="resolution-rules" format=" default"/>).	by the default value of the anchor (see <xref target="resolution-rules" format=" default"/>).
	A full English expression of the "host relation" is:</t>	A full English expression of the "host relation" is:</t>

	<t>'<tt>coap://[2001:db8:f0::1]</tt> is hosting the resource	<t indent="3"><tt>coap://[2001:db8:f0::1]</tt> is hosting the resource
	<tt>coap://[2001:db8:f0::1]/sensors/temp</tt>, which is of the resource type "te mperature" and	<tt>coap://[2001:db8:f0::1]/sensors/temp</tt>, which is of the resource type "te mperature" and

	can be accessed using the text/plain content format.'</t>	can be read in the text/plain content format.</t>
	</section>	</section>
	</section>	</section>
	<section anchor="a-slightly-more-complex-example" numbered="true" toc="def ault">	<section anchor="a-slightly-more-complex-example" numbered="true" toc="def ault">

	<name>A slightly more complex example</name>	<name>A Slightly More Complex Example</name>
	<t>Omitting the <tt>rt=temperature</tt> filter, the discovery query woul d	<t>Omitting the <tt>rt=temperature</tt> filter, the discovery query woul d

	have given some more records in the payload:</t>	have given some more links in the payload:</t>
	<figure anchor="example-weblink-wkc-extended">	<figure anchor="example-weblink-wkc-extended">

	<name>Extended example of direct resource discovery</name>	<name>Extended Example of Direct Resource Discovery</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET coap://[ff02::fd]:5683/.well-known/core	Req: GET coap://[ff02::fd]:5683/.well-known/core

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	</sensors/temp>;rt=temperature;ct=0,	</sensors/temp>;rt=temperature;ct=0,
	</sensors/light>;rt=light-lux;ct=0,	</sensors/light>;rt=light-lux;ct=0,
	</t>;anchor="/sensors/temp";rel=alternate,	</t>;anchor="/sensors/temp";rel=alternate,
	<http://www.example.com/sensors/t123>;anchor="/sensors/temp";	<http://www.example.com/sensors/t123>;anchor="/sensors/temp";
	rel=describedby	rel=describedby

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>

	<t>Parsing the third record, the client encounters the "anchor" paramete	<t>Parsing the third link, the client encounters the "anchor" parameter.
	r. It is	It is
	a URI relative to the Base URI of the request and is thus resolved to	a URI relative to the base URI of the request and is thus resolved to
	"<tt>coap://[2001:db8:f0::1]/sensors/temp</tt>".	<tt>coap://[2001:db8:f0::1]/sensors/temp</tt>.
	That is the context resource of the link, so the "rel" statement is not about	That is the context resource of the link, so the "rel" statement is not about

	the target and the Base URI any more, but about the target and the resolved	the target and the base URI any more but about the target and the resolved
	URI. Thus, the third record could be read as	URI. Thus, the third link could be read as:</t>
	"<tt>coap://[2001:db8:f0::1]/sensors/temp</tt> has an alternate representation a	<t indent="3"><tt>coap://[2001:db8:f0::1]/sensors/temp</tt> has an alternate rep
	t	resentation at
	<tt>coap://[2001:db8:f0::1]/t</tt>".</t>	<tt>coap://[2001:db8:f0::1]/t</tt>.</t>
	<t>Following the same resolution steps, the fourth record can be read as	<t>Following the same resolution steps, the fourth link can be read as <
	"<tt>coap://[2001:db8:f0::1]/sensors/temp</tt> is	tt>coap://[2001:db8:f0::1]/sensors/temp</tt> is
	described by <tt>http://www.example.com/sensors/t123</tt>".</t>	described by <tt>http://www.example.com/sensors/t123</tt>.</t>
	</section>	</section>
	<section anchor="enter-the-resource-directory" numbered="true" toc="defaul t">	<section anchor="enter-the-resource-directory" numbered="true" toc="defaul t">
	<name>Enter the Resource Directory</name>	<name>Enter the Resource Directory</name>
	<t>The RD tries to carry the semantics obtainable by classical	<t>The RD tries to carry the semantics obtainable by classical

	CoAP discovery over to the resource lookup interface as faithfully as possible.<	CoAP discovery over to the resource lookup interface as faithfully as pos
	/t>	sible.</t>
	<t>For the following queries, we will assume that the simple host has us	<t>For the following queries, we will assume that the simple host has use
	ed Simple	d simple
	Registration to register at the RD that was announced to it,	registration to register at the RD that was announced to it,
	sending this request from its UDP port <tt>[2001:db8:f0::1]:6553</tt>:</t>	sending this request from its UDP port <tt>[2001:db8:f0::1]:6553</tt>:</t
		>
	<figure anchor="example-weblink-simple">	<figure anchor="example-weblink-simple">

	<name>Example of a simple registration</name>	<name>Example of a Simple Registration</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: POST coap://[2001:db8:f01::ff]/.well-known/rd?ep=simple-host1	Req: POST coap://[2001:db8:f0::ff]/.well-known/rd?ep=simple-host1

	Res: 2.04 Changed	Res: 2.04 Changed

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>

	<t>The RD would have accepted the registration, and queried the	<t>The RD would have accepted the registration and queried the
	simple host's <tt>/.well-known/core</tt> by itself. As a result, the host is reg istered	simple host's <tt>/.well-known/core</tt> by itself. As a result, the host is reg istered
	as an endpoint in the RD with the name "simple-host1". The registration is	as an endpoint in the RD with the name "simple-host1". The registration is

	active for 90000 seconds, and the endpoint registration Base URI is	active for 90000 seconds, and the endpoint registration base URI is
	"<tt>coap://[2001:db8:f0::1]</tt>" following the resolution steps described in <	<tt>coap://[2001:db8:f0::1]</tt>, following the resolution steps described in <x
	xref target="resolveURI" format="default"/>. It should be remarked that the Base	ref target="resolveURI" format="default"/>. It should be remarked that the base
	URI constructed that way always yields a URI of the form: scheme://authority wi	URI constructed that way always yields a URI of the form scheme://authority with
	thout path suffix.</t>	out path suffix.</t>
	<t>If the client now queries the RD as it would previously have issued a multicast	<t>If the client now queries the RD as it would previously have issued a multicast
	request, it would go through the RD discovery steps by fetching	request, it would go through the RD discovery steps by fetching
	<tt>coap://[2001:db8:f0::ff]/.well-known/core?rt=core.rd-lookup-res</tt>, obtain	<tt>coap://[2001:db8:f0::ff]/.well-known/core?rt=core.rd-lookup-res</tt>, obtain
	<tt>coap://[2001:db8:f0::ff]/rd-lookup/res</tt> as the resource lookup endpoint, and	<tt>coap://[2001:db8:f0::ff]/rd-lookup/res</tt> as the resource lookup endpoint, and
	ask it for all temperature resources:</t>	ask it for all temperature resources:</t>
	<figure anchor="example-weblink-lookup-result">	<figure anchor="example-weblink-lookup-result">

	<name>Example exchange performing resource lookup</name>	<name>Example Exchange Performing Resource Lookup</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET coap://[2001:db8:f0::ff]/rd-lookup/res?rt=temperature	Req: GET coap://[2001:db8:f0::ff]/rd-lookup/res?rt=temperature

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	<coap://[2001:db8:f0::1]/sensors/temp>;rt=temperature;ct=0	<coap://[2001:db8:f0::1]/sensors/temp>;rt=temperature;ct=0

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	<t>This is not <em>literally</em> the same response that it would have r eceived from a	<t>This is not <em>literally</em> the same response that it would have r eceived from a
	multicast request, but it contains the equivalent statement:</t>	multicast request, but it contains the equivalent statement:</t>

	<t>'<tt>coap://[2001:db8:f0::1]</tt> is hosting the resource	<t indent="3"><tt>coap://[2001:db8:f0::1]</tt> is hosting the resource
	<tt>coap://[2001:db8:f0::1]/sensors/temp</tt>, which is of the resource type "te mperature" and	<tt>coap://[2001:db8:f0::1]/sensors/temp</tt>, which is of the resource type "te mperature" and

	can be accessed using the text/plain content format.'</t>	can be accessed using the text/plain content format.</t>
	<t>To complete the examples, the client could also query all resources h osted at	<t>To complete the examples, the client could also query all resources h osted at
	the endpoint with the known endpoint name "simple-host1":</t>	the endpoint with the known endpoint name "simple-host1":</t>
	<figure anchor="example-weblink-lookup-result-extended">	<figure anchor="example-weblink-lookup-result-extended">

	<name>Extended example exchange performing resource lookup</name>	<name>Extended Example Exchange Performing Resource Lookup</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	Req: GET coap://[2001:db8:f0::ff]/rd-lookup/res?ep=simple-host1	Req: GET coap://[2001:db8:f0::ff]/rd-lookup/res?ep=simple-host1

	Res: 2.05 Content	Res: 2.05 Content
	Payload:	Payload:
	<coap://[2001:db8:f0::1]/sensors/temp>;rt=temperature;ct=0,	<coap://[2001:db8:f0::1]/sensors/temp>;rt=temperature;ct=0,
	<coap://[2001:db8:f0::1]/sensors/light>;rt=light-lux;ct=0,	<coap://[2001:db8:f0::1]/sensors/light>;rt=light-lux;ct=0,
	<coap://[2001:db8:f0::1]/t>;	<coap://[2001:db8:f0::1]/t>;
	anchor="coap://[2001:db8:f0::1]/sensors/temp";rel=alternate,	anchor="coap://[2001:db8:f0::1]/sensors/temp";rel=alternate,
	<http://www.example.com/sensors/t123>;	<http://www.example.com/sensors/t123>;
	anchor="coap://[2001:db8:f0::1]/sensors/temp";rel=describedby	anchor="coap://[2001:db8:f0::1]/sensors/temp";rel=describedby

	]]></artwork>	]]></sourcecode>
	</figure>	</figure>
	<t>All the target and anchor references are already in absolute form the re, which	<t>All the target and anchor references are already in absolute form the re, which
	don't need to be resolved any further.</t>	don't need to be resolved any further.</t>

	<t>Had the simple host done an equivalent full registration with a base=	<t>Had the simple host done an equivalent full registration with a base=
	parameter (e.g.	parameter (e.g.,
	<tt>?ep=simple-host1&base=coap+tcp://simple-host1.example.com</tt>), that co	<tt>?ep=simple-host1&base=coap+tcp://sh1.example.com</tt>), that context wou
	ntext would	ld
	have been used to resolve the relative anchor values instead, giving</t>	have been used to resolve the relative anchor values instead, giving the followi
		ng and analogous links:</t>
	<figure anchor="example-weblink-lookup-result-base">	<figure anchor="example-weblink-lookup-result-base">

	<name>Example payload of a response to a resource lookup with a dedica	<name>Example Payload of a Response to a Resource Lookup with a Dedica
	ted base URI</name>	ted Base URI</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode type=""><![CDATA[
	<coap+tcp://simple-host1.example.com/sensors/temp>;rt=temperature;ct=0	<coap+tcp://sh1.example.com/sensors/temp>;rt=temperature;ct=0
	]]></artwork>	]]></sourcecode>
	</figure>	</figure>

	<t>and analogous records.</t>
	</section>	</section>
	<section anchor="resolution-rules" numbered="true" toc="default">	<section anchor="resolution-rules" numbered="true" toc="default">

	<name>A note on differences between link-format and Link header fields</ name>	<name>A Note on Differences between Link-Format and Link Header Fields</ name>
	<t>While link-format and Link header fields look very similar and are ba sed on the same	<t>While link-format and Link header fields look very similar and are ba sed on the same
	model of typed links, there are some differences between <xref target="RFC6690" format="default"/> and	model of typed links, there are some differences between <xref target="RFC6690" format="default"/> and
	<xref target="RFC8288" format="default"/>.	<xref target="RFC8288" format="default"/>.
	When implementing an RD or interacting with an RD,	When implementing an RD or interacting with an RD,

	care must be taken to follow the <xref target="RFC6690" format="default"/> behav	care must be taken to follow the behavior described in <xref target="RFC6690" fo
	ior	rmat="default"/>
	whenever application/link-format representations are used.</t>	whenever <tt>application/link-format</tt> representations are used.</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>	<li>
	<t>"Default value of anchor":	<t>"Default value of anchor":

	Both under <xref target="RFC6690" format="default"/> and <xref target="RFC8288" format="default"/>,	Under both <xref target="RFC6690" format="default"/> and <xref target="RFC8288" format="default"/>,
	relative references in the term inside the angle brackets (the target)	relative references in the term inside the angle brackets (the target)
	and the anchor attribute are resolved against the relevant base URI	and the anchor attribute are resolved against the relevant base URI

	(which usually is the URI used to retrieve the entity),	(which usually is the URI used to retrieve the entity)
	and independent of each other. </t>	and independent of each other. </t>
	<t>	<t>

	When, in an <xref target="RFC8288" format="default"/> Link header, the anchor at tribute is absent,	When, in a Link header <xref target="RFC8288" format="default"/>, the anchor att ribute is absent,
	the link's context is the URI of the selected representation	the link's context is the URI of the selected representation
	(and usually equal to the base URI). </t>	(and usually equal to the base URI). </t>
	<t>	<t>

	In <xref target="RFC6690" format="default"/> links, if the anchor attribute is a bsent,	In links per <xref target="RFC6690" format="default"/>, if the anchor attribute is absent,
	the default value is the Origin of	the default value is the Origin of

	(for all relevant cases: the URI reference <tt>/</tt> resolved against)	(for all relevant cases, the URI reference <tt>/</tt> resolved against)
	the link's target.</t>	the link's target.</t>
	</li>	</li>
	<li>	<li>
	<t>There is no percent encoding in link-format documents. </t>	<t>There is no percent encoding in link-format documents. </t>

	<t>	<t>A link-format document is a UTF-8-encoded string of Unicode chara
	A link-format document is a UTF-8 encoded string of Unicode characters and	cters and
	does not have percent encoding, while Link header fields are practically ASCII	does not have percent encoding, while Link header fields are practica
	strings that use percent encoding for non-ASCII characters, stating the	lly ASCII
	encoding explicitly when required. </t>	strings that use percent encoding for non-ASCII characters, stating t
	<t>	he
	For example, while a Link header field in a page about a Swedish city might read	encoding explicitly when required. </t>
	</t>	<t>For example, while a Link header field in a page about a Swedish c
	<artwork name="" type="" align="left" alt=""><![CDATA[	ity might
		read:</t>

		<sourcecode name="" type="http-message"><![CDATA[
	Link: </temperature/Malm%C3%B6>;rel=live-environment-data	Link: </temperature/Malm%C3%B6>;rel=live-environment-data

	]]></artwork>	]]></sourcecode>
	<t>	<t>

	a link-format document from the same source might describe the link as </t>	a link-format document from the same source might describe the link as: </t>
	<artwork name="" type="" align="left" alt=""><![CDATA[	<sourcecode name="" type=""><![CDATA[
	</temperature/Malmö>;rel=live-environment-data	</temperature/Malmö>;rel=live-environment-data

	]]></artwork>	]]></sourcecode>
	</li>	</li>
	</ul>	</ul>
	</section>	</section>
	</section>	</section>
	<section anchor="limitedlinkformat" numbered="true" toc="default">	<section anchor="limitedlinkformat" numbered="true" toc="default">
	<name>Limited Link Format</name>	<name>Limited Link Format</name>

	<t>The CoRE Link Format as described in <xref target="RFC6690" format="def ault"/>	<t>The CoRE Link Format, as described in <xref target="RFC6690" format="de fault"/>,
	has been interpreted differently by implementers,	has been interpreted differently by implementers,
	and a strict implementation	and a strict implementation
	rules out some use cases of an RD	rules out some use cases of an RD

	(e.g. base values with path components in combination with absent anchors).</t>	(e.g., base values with path components in combination with absent anchors).</t>
	<t>This appendix describes	<t>This appendix describes

	a subset of link format documents called Limited Link Format.	a subset of link format documents called the Limited Link Format.
	The one rule herein is not very limiting in practice --	The one rule herein is not very limiting in practice --

	all examples in RFC6690, and all deployments the authors are aware of already st	all examples in <xref target="RFC6690" format="default"/> and all deployments th
	ick to them --	e authors are aware of already stick to them --
	but ease the implementation of RD servers.</t>	but eases the implementation of RD servers.</t>
	<t>It is applicable to representations in the application/link-format medi	<t>It is applicable to representations in the <tt>application/link-format<
	a type,	/tt> media type
	and any other media types that inherit <xref target="RFC6690" format="default"/>	and any other media types that inherit <xref target="RFC6690" section="2.1" sect
	Section 2.1.</t>	ionFormat="comma"/>. </t>
	<t>A link format representation is in Limited Link format if,	<t>A link format representation is in the Limited Link Format if,
	for each link in it,	for each link in it,
	the following applies:</t>	the following applies:</t>
	<t>All URI references either follow the URI or the path-absolute ABNF rule of	<t>All URI references either follow the URI or the path-absolute ABNF rule of

	RFC3986 (i.e. target and anchor each either start with a scheme or with a	<xref target="RFC3986" format="default"/> (i.e., the target and anchor each ei ther start with a scheme or with a
	single slash).</t>	single slash).</t>

	<!-- LocalWords: lookups multicast lookup RESTful CoRE LoWPAN CoAP	</section>
	-->	<section anchor="acknowledgments" numbered="false" toc="default">
	<!-- LocalWords: microcontrollers URI DNS EP IP EPs discoverable	<name>Acknowledgments</name>
	-->	<t> <contact fullname="Oscar Novo"/>, <contact fullname="Srdjan Krco"/>, <c
	<!-- LocalWords: Metadata metadata lossless anycast ABRO RDNSS ICMP	ontact fullname="Szymon Sasin"/>, <contact fullname="Kerry Lynn"/>, <contact ful
	-->	lname="Esko Dijk"/>, <contact fullname="Anders
	<!-- LocalWords: DHCPv RD's DHCP RDs unicast JSON CBOR wildcard TLS	Brandt"/>, <contact fullname="Matthieu Vial"/>, <contact fullname="Jim Schaad"/>
	-->	, <contact fullname="Mohit Sethi"/>, <contact fullname="Hauke Petersen"/>, <cont
	<!-- LocalWords: subdomain substring prepending subtype DTLS UDP	act fullname="Hannes Tschofenig"/>, <contact fullname="Sampo Ukkola"/>, <contact
	-->	fullname="Linyi Tian"/>, <contact fullname="Jan Newmarch"/>, <contact fullname=
	<!-- LocalWords: routability NTP TCP WiFi GPRS FQDN SLAAC IPv SDOs	"Matthias Kovatsch"/>, <contact fullname="Jaime Jimenez"/>, and <contact fullnam
	-->	e="Ted Lemon"/> have provided helpful comments, discussions, and ideas to improv
	<!-- LocalWords: OMA LWM IETF OMNA API SMS MSISDN	e and shape this document. Zach would also like to thank his colleagues from the
	-->	EU FP7 SENSEI project, where many of the RD concepts were originally developed.
		</t>
	</section>	</section>
	</back>	</back>
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