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	<?rfc sortrefs="yes"?>	-ietf-roll-rnfd-07" number="9866" consensus="true" category="std" obsoletes="" u
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	<front>	<front>

	<title abbrev="RNFD">RNFD: Fast border router crash detection in RPL</title>	<title abbrev="RNFD">Root Node Failure Detector (RNFD): Fast Detection of
		Border Router Crashes in the Routing Protocol for Low-Power and Lossy
		Networks (RPL)</title>
		<seriesInfo name="RFC" value="9866"/>
	<author initials="K." surname="Iwanicki" fullname="Konrad Iwanicki">	<author initials="K." surname="Iwanicki" fullname="Konrad Iwanicki">
	<organization>University of Warsaw</organization>	<organization>University of Warsaw</organization>
	<address>	<address>
	<postal>	<postal>
	<street>Banacha 2</street>	<street>Banacha 2</street>
	<city>Warszawa</city>	<city>Warszawa</city>
	<code>02-097</code>	<code>02-097</code>
	<country>Poland</country>	<country>Poland</country>
	</postal>	</postal>
	<phone>+48 22 55 44 428</phone>	<phone>+48 22 55 44 428</phone>
	<email>iwanicki@mimuw.edu.pl</email>	<email>iwanicki@mimuw.edu.pl</email>
	</address>	</address>
	</author>	</author>

		<date year="2025" month="September"/>


	<date year="2025" month="March" day="08"/>	<area>RTG</area>
		<workgroup>roll</workgroup>
	<area>Internet</area>
	<workgroup>ROLL</workgroup>
	<keyword>Internet-Draft</keyword>


	<abstract>	<!-- [rfced Please insert any keywords (beyond those that appear in
		the title) for use on https://www.rfc-editor.org/search. -->


	<t>By and large, a correct operation of a network running RPL (the IPv6 routing	<keyword>example</keyword>
	protocol for low-power and lossy networks) requires border routers to be up.
	In many applications, it is beneficial for the nodes to detect a failure of a bo
	rder router as soon as possible to trigger fallback actions.
	This document specifies RNFD (the root node failure detector), an extension to R
	PL that expedites border router crash detection by having nodes collaboratively
	monitor the status of a given border router.
	The extension introduces an additional state at each node, a new type of RPL Con
	trol Message Options for synchronizing this state among different nodes, and the
	coordination algorithm itself.</t>


		<abstract>
		<t>By and large, correct operation of a network running the Routing
		Protocol for Low-Power and Lossy Networks (RPL) requires border routers to
		be
		up. In many applications, it is beneficial for the nodes to detect a
		failure of a border router as soon as possible to trigger fallback
		actions. This document specifies the Root Node Failure Detector (RNFD),
		an extension to RPL that expedites detection of border router crashes by
		having nodes collaboratively monitor the status of a given border
		router. The extension introduces an additional state at each node, a
		new type of RPL Control Message Option for synchronizing this state
		among different nodes, and the coordination algorithm itself.</t>
	</abstract>	</abstract>


	</front>	</front>


	<middle>	<middle>

		<section anchor="intro" numbered="true" toc="default">
		<name>Introduction</name>
		<t>RPL is an IPv6 routing protocol for Low-Power and Lossy Networks
		(LLNs) <xref target="RFC6550" format="default"/>. Such networks are
		usually constrained in device energy and channel capacity. They are
		formed largely of nodes that offer little processing power and memory,
		and links that are of variable qualities and support low data rates.
		Therefore, a significant challenge that a routing protocol for LLNs has
		to address is minimizing resource consumption without sacrificing
		reaction time to network changes.</t>


	<section anchor="intro" title="Introduction">	<t>One of the main design principles adopted in RPL to minimize node
		resource consumption is delegating much of the responsibility for
	<t>RPL is an IPv6 routing protocol for low-power and lossy networks (LLNs) <xref	routing to LLN Border Routers (LBRs). A network is organized into
	target="RFC6550"/>.	Destination-Oriented Directed Acyclic Graphs (DODAGs), each
	Such networks are usually constrained in device energy and channel capacity.	corresponding to an LBR and having all its paths terminate at the LBR.
	They are formed largely of nodes that offer little processing power and memory,	To this end, every node is dynamically assigned a Rank representing its
	and links that are of variable qualities and support low data rates.	distance to a given LBR, measured in some metric, with the LBR having
	Therefore, a significant challenge that a routing protocol for LLNs has to addre	the minimal Rank, which reflects its role as the DODAG root. The Ranks
	ss is minimizing resource consumption without sacrificing reaction time to netwo	allow each non-LBR node to select from among its neighbors (i.e., nodes
	rk changes.</t>	to which the node has links) those that are closer to the LBR than the
		node itself (i.e., the node's parents in the graph). The resulting DODAG
	<t>One of the main design principles adopted in RPL to minimize node resource co	paths, consisting of the node-parent links, are utilized for routing
	nsumption is delegating much of the responsibility for routing to LLN border rou	packets upward to the LBR and outside the LLN. They are also used by
	ters (LBRs).	nodes to periodically report their connectivity upward to the LBR, which
	A network is organized into destination-oriented directed acyclic graphs (DODAGs	allows for directing packets downward from the LBR to these
	), each corresponding to an LBR and having all its paths terminate at the LBR.	nodes (e.g., by means of source routing <xref target="RFC6554"
	To this end, every node is dynamically assigned a rank representing its distance	format="default"/>). All in all, not only do LBRs
	, measured in some metric, to a given LBR, with the LBR having the minimal rank,	participate in routing, but they also drive the process of DODAG construct
	which reflects its role as the DODAG root.	ion
	The ranks allow each non-LBR node to select from among its neighbors (i.e., node	and maintenance underlying the protocol.</t>
	s to which the node has links) those that are closer to the LBR than the node it	<t>To play this central role, LBRs are expected to be more capable than
	self: the node’s parents in the graph.	regular LLN nodes. They are assumed not to be constrained in computing
	The resulting DODAG paths, consisting of the node-parent links, are utilized for	power, memory, and energy, which often entails a more involved
	routing packets upward: to the LBR and outside the LLN.	hardware-software architecture and tethered power supply. However, this
	They are also used by nodes to periodically report their connectivity upward to	also makes them prone to failures, especially since
	the LBR, which allows in turn for directing packets downward, from the LBR to th	it is often difficult to ensure a backup power supply for every LBR in lar
	ese nodes, for instance, by means of source routing <xref target="RFC6554"/>.	ge deployments.</t>
	All in all, not only do LBRs participate in routing but also drive the process o	<section anchor="effects-of-lbr-crashes" numbered="true" toc="default">
	f DODAG construction and maintenance underlying the protocol.</t>	<name>Effects of LBR Crashes</name>
		<t>When an LBR crashes, or more generally, fails in a way that
	<t>To play this central role, LBRs are expected to be more capable than regular	prevents other nodes in its DODAG from communicating with it, the
	LLN nodes.	nodes also lose the ability to communicate with other Internet hosts.
	They are assumed not to be constrained in computing power, memory, and energy, w	In addition, a significant fraction of DODAG paths interconnecting the
	hich often entails a more involved hardware-software architecture and tethered p	nodes become invalid, as they pass through the dead LBR. The others
	ower supply.	also degenerate as a result of DODAG repair attempts, which are bound
	This, however, also makes them prone to failures, especially since in large depl	to fail. In effect, routing inside the DODAG also becomes largely
	oyments it is often difficult to ensure a backup power supply for every LBR.</t>	impossible. Consequently, it is desirable that an LBR crash be
		detected by the nodes fast, so that they can leave the broken DODAG
	<section anchor="effects-of-lbr-crashes" title="Effects of LBR Crashes">	and join another one or trigger additional application- or
		deployment-dependent fallback mechanisms, thereby minimizing the
	<t>When an LBR crashes or, more generally, fails in a way that prevents other no	negative impact of the disconnection.</t>
	des in its DODAG from communicating with it, the nodes also lose the ability to	<t>Since all DODAG paths lead to the corresponding LBR, detecting its
	communicate with other Internet hosts.	crash by a node entails dropping all parents and adopting an infinite
	In addition, a significant fraction of DODAG paths interconnecting the nodes bec	Rank, which reflects the node's inability to reach the dead LBR.
	ome invalid, as they pass through the dead LBR.	Depending on the deployment settings, the node can then remain in such
	The others also degenerate as a result of DODAG repair attempts, which are bound	a state, join a different DODAG, or even become the root of a
	to fail.	floating DODAG. In any case, however, achieving this state for all
	In effect, routing inside the DODAG also becomes largely impossible.	nodes is slow, can generate heavy traffic, and is difficult to
	Consequently, it is desirable that an LBR crash be detected by the nodes fast, s	implement correctly <xref target="Iwanicki16" format="default"/> <xref
	o that they can leave the broken DODAG and join another one or trigger additiona	target="Paszkowska19" format="default"/> <xref target="Ciolkosz19"
	l application- or deployment-dependent fallback mechanisms, thereby minimizing t	format="default"/>.</t>
	he negative impact of the disconnection.</t>

	<t>Since all DODAG paths lead to the corresponding LBR, detecting its crash by a
	node entails dropping all parents and adopting an infinite rank, which reflects
	the node’s inability to reach the dead LBR.
	Depending on the deployment settings, the node can then remain in such a state,
	join a different DODAG, or even become itself the root of a floating DODAG.
	In any case, however, achieving this state for all nodes is slow, can generate h
	eavy traffic, and is difficult to implement correctly <xref target="Iwanicki16"/
	> <xref target="Paszkowska19"/> <xref target="Ciolkosz19"/>.</t>

	<t>To start with, tearing down all DODAG paths requires each of the dead LBR’s n
	eighbors to detect that its link with the LBR is no longer up.
	Otherwise, any of the neighbors unaware of this fact can keep advertising a fini
	te rank and can thus be other nodes’ parent or ancestor in the DODAG: such nodes
	will incorrectly believe they have a valid path to the dead LBR.
	Detecting a crash of a link by a node normally happens when the node has observe
	d sufficiently many forwarding failures over the link.
	Therefore, considering the low-data-rate applications of LLNs, the period from t
	he crash to the moment of eliminating from the DODAG the last link to the dead L
	BR may be long.
	Subsequently learning by all nodes that none of their links can form any path le
	ading to the dead LBR also adds latency, partly due to parent changes that the n
	odes independently perform in attempts to repair their broken paths locally.
	Since a non-LBR node has only local knowledge of the network, potentially incons
	istent with that of other nodes, such parent changes often produce paths contain
	ing loops, which have to be broken before all nodes can conclude that no path to
	the dead LBR exists globally.
	Even with RPL’s dedicated loop detection mechanisms <xref target="RFC6553"/>, th
	is also requires traffic, and hence time.
	Finally, switching a parent or discovering a loop can also generate cascaded bur
	sts of control traffic, owing to the adaptive Trickle algorithm for exchanging D
	ODAG information <xref target="RFC6202"/>.
	Overall, the behavior of the network when handling an LBR crash is highly subopt
	imal, thereby not being in line with RPL’s goals of minimizing resource consumpt
	ion and reaction latencies.</t>

	</section>
	<section anchor="design-principles" title="Design Principles">

	<t>To address this issue, this document proposes an extension to RPL, dubbed Roo
	t Node Failure Detector (RNFD).
	To minimize the time and traffic required to handle an LBR crash, the RNFD algor
	ithm adopts the following design principles, derived directly from the previous
	observations:</t>

	<t><list style="numbers">
	<t>Explicitly coordinating LBR monitoring between nodes instead of relying onl
	y on the emergent behavior resulting from their independent operation.</t>
	<t>Avoiding probing all links to the dead LBR so as to reduce the tail latency
	when eliminating these links from the DODAG.</t>
	<t>Exploiting concurrency by prompting proactive checking for a possible LBR c
	rash when some nodes suspect such a failure may have taken place, which aims to
	further reduce the overall latency.</t>
	<t>Minimizing changes to RPL’s existing algorithms by operating in parallel an
	d largely independently (in the background), and introducing few additional assu
	mptions.</t>
	</list></t>

	<t>While these principles do improve RPL’s performance under a wide range of LBR
	crashes, their probabilistic nature precludes hard guarantees for all possible
	corner cases.
	In particular, in some scenarios, RNFD’s operation may result in false negatives
	, but these situations are peculiar and will eventually be handled by RPL’s own
	aforementioned mechanisms.
	Likewise, in some scenarios, notably involving highly unstable links, false posi
	tives may occur, but they can be alleviated as well.
	In any case, the principles also guarantee that RNFD can be deactivated at any t
	ime, if needed, in which case RPL’s operation is unaffected.</t>

	</section>
	<section anchor="other-solutions" title="Other Solutions">

	<t>Given the consequences of LBR failures, if possible, it is also worth conside
	ring other solutions to the problem.
	More specifically, power outages can be alleviated by provisioning redundant pow
	er sources or emergency batteries.
	Likewise, RPL’s so-called virtual DODAG roots can help tolerate some failures of
	individual LBRs.</t>

	<t>As mentioned previously, RNFD has been designed to be largely independent of
	those solutions, that is, rather than aiming to be their replacement, it can com
	plement them.
	In particular, the operation of RNFD with different variants of virtual DODAG ro
	ots is covered in <xref target="mgmnt_virt_dodag_roots"/>.</t>

	</section>
	</section>
	<section anchor="terms" title="Terminology">

	<t>The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”,
	“SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this d
	ocument are to be interpreted as described in BCP 14 <xref target="RFC2119"/> <x
	ref target="RFC8174"/> when, and only when, they appear in all capitals, as show
	n here.</t>

	<t>The Terminology used in this document is consistent with and incorporates tha
	t described in “Terms Used in Routing for Low-Power and Lossy Networks (LLNs)” <
	xref target="RFC7102"/>, “RPL: IPv6 Routing Protocol for Low-Power and Lossy Net
	works” <xref target="RFC6550"/>, and “The Routing Protocol for Low-Power and Los
	sy Networks (RPL) Option for Carrying RPL Information in Data-Plane Datagrams” <
	xref target="RFC6553"/>.
	Other terms in use in LLNs can be found in “Terminology for Constrained-Node Net
	works” <xref target="RFC7228"/>.</t>

	<t>In particular, the following acronyms appear in the document:</t>

	<t><list style="hanging">
	<t hangText='DIO'>
	DODAG Information Object (a RPL message)</t>
	<t hangText='DIS'>
	DODAG Information Solicitation (a RPL message)</t>
	<t hangText='DODAG'>
	Destination-Oriented Directed Acyclic Graph</t>
	<t hangText='LLN'>
	Low-power and Lossy Network</t>
	<t hangText='LBR'>
	LLN Border Router</t>
	</list></t>

	<t>In addition, the document introduces the following concepts:</t>

	<t><list style="hanging">
	<t hangText='Sentinel'>
	One of the two roles that a node can play in RNFD. For a given DODAG Version,
	a Sentinel node is a DODAG root’s neighbor that monitors the DODAG root’s status
	. There are normally multiple Sentinels for a DODAG root. However, being the DOD
	AG root’s neighbor need not imply being Sentinel.</t>
	<t hangText='Acceptor'>
	The other of the two roles that a node can play in RNFD. For a given DODAG Ver
	sion, an Acceptor node is a node that is not Sentinel.</t>
	<t hangText='Locally Observed DODAG Root’s State (LORS)'>
	A node’s local knowledge of the DODAG root’s status, specifying in particular
	whether the DODAG root is up.</t>
	<t hangText='Conflict-Free Replicated Counter (CFRC)'>
	Conceptually represents a dynamic set whose cardinality can be estimated. It d
	efines a partial order on its values and supports element addition and union. Th
	e union operation is order- and duplicate-insensitive, that is, idempotent, comm
	utative, and associative.</t>
	</list></t>

	</section>
	<section anchor="overview" title="Overview">

	<t>As mentioned previously, LBRs are DODAG roots in RPL, and hence a crash of an
	LBR is global in that it affects all nodes in the corresponding DODAG.
	Therefore, each node running RNFD for a given DODAG explicitly tracks the DODAG
	root’s current condition, which is referred to as Locally Observed DODAG Root’s
	State (LORS), and synchronizes its local knowledge with other nodes.</t>

	<t>Since monitoring the condition of the DODAG root is performed by tracking the
	status of its links (i.e., whether they are up or down), it can only be done by
	the root’s neighbors; other nodes must accept their observations.
	Consequently, depending on their roles, non-root nodes are divided in RNFD into
	two disjoint groups: Sentinels and Acceptors.
	A Sentinel node is a DODAG root’s neighbor that monitors its link with the root.
	The DODAG root thus normally has multiple Sentinels but being its neighbor need
	not imply being Sentinel.
	An Acceptor node is in turn a node that is not Sentinel.
	Acceptors thus mainly collect and propagate Sentinels’ observations.
	More information on Sentinel selection can be found in <xref target="mgmnt_roles
	_and_cfrc_lens"/>.</t>


	<section anchor="protocol-state-machine" title="Protocol State Machine">	<t>To start with, tearing down all DODAG paths requires each of the
		dead LBR's neighbors to detect that its link with the LBR is no longer
		up. Otherwise, any of the neighbors unaware of this fact can keep
		advertising a finite Rank and can thus be other nodes' parent or
		ancestor in the DODAG; such nodes will incorrectly believe they have a
		valid path to the dead LBR. Detecting a crash of a link by a node
		normally happens when the node has sufficiently observed many
		forwarding failures over the link. Therefore, considering the
		low-data-rate applications of LLNs, the period from the crash to the
		moment of eliminating the last link to the dead LBR from the DODAG may
		be long. Subsequently, learning by all nodes that none of their links
		can form any path leading to the dead LBR also adds latency, partly
		due to parent changes that the nodes independently perform in attempts
		to repair their broken paths locally. Since a non-LBR node has only
		local knowledge of the network, potentially inconsistent with that of
		other nodes, such parent changes often produce paths containing loops,
		which have to be broken before all nodes can conclude that no path to
		the dead LBR exists globally. Even with RPL's dedicated loop
		detection mechanisms <xref target="RFC6553" format="default"/>, this
		also requires traffic and hence time. Finally, switching a parent or
		discovering a loop can also generate cascaded bursts of control
		traffic, owing to the adaptive Trickle algorithm for exchanging DODAG
		information <xref target="RFC6206" format="default"/>. Overall, the
		behavior of the network when handling an LBR crash is highly
		suboptimal, thereby not being in line with RPL's goals of minimizing
		resource consumption and reaction latencies.</t>
		</section>
		<section anchor="design-principles" numbered="true" toc="default">
		<name>Design Principles</name>
		<t>To address this issue, this document proposes an extension to RPL,
		dubbed the "Root Node Failure Detector (RNFD)". To minimize the time
		and traffic required to handle an LBR crash, the RNFD algorithm adopts
		the following design principles, derived directly from the previous
		observations:</t>
		<ol spacing="normal" type="1"><li>
		<t>Explicitly coordinating LBR monitoring between nodes instead of
		relying only on the emergent behavior resulting from their
		independent operation.</t>
		</li>
		<li>
		<t>Avoiding probing all links to the dead LBR so as to reduce the
		tail latency when eliminating these links from the DODAG.</t>
		</li>
		<li>
		<t>Exploiting concurrency by prompting proactive checking for a
		possible LBR crash when some nodes suspect such a failure may have
		taken place, which aims to further reduce the overall latency.</t>
		</li>
		<li>
		<t>Minimizing changes to RPL's existing algorithms by operating in
		parallel and largely independently (in the background) and by
		introducing few additional assumptions.</t>
		</li>
		</ol>
		<t>While these principles do improve RPL's performance under a wide
		range of LBR crashes, their probabilistic nature precludes hard
		guarantees for all possible corner cases. In particular, in some
		scenarios, RNFD's operation may result in false negatives, but these
		situations are peculiar and will eventually be handled by RPL's own
		aforementioned mechanisms. Likewise, in some scenarios, notably
		involving highly unstable links, false positives may occur, but they
		can be alleviated as well. In any case, the principles also guarantee
		that RNFD can be deactivated at any time if needed, in which case
		RPL's operation is unaffected.</t>
		</section>
		<section anchor="other-solutions" numbered="true" toc="default">
		<name>Other Solutions</name>
		<t>Given the consequences of LBR failures, it is also
		worth considering other solutions to the problem. More specifically,
		power outages can be alleviated by provisioning redundant power
		sources or emergency batteries. Likewise, RPL's so-called virtual
		DODAG roots can help tolerate some failures of individual LBRs.</t>
		<t>As mentioned previously, RNFD has been designed to be largely
		independent of those solutions; that is, rather than aiming to be
		their replacement, RNFD can complement them. In particular, the
		operation of RNFD with different variants of virtual DODAG roots is
		covered in <xref target="mgmnt_virt_dodag_roots"
		format="default"/>.</t>
		</section>
		</section>
		<section anchor="terms" numbered="true" toc="default">
		<name>Terminology</name>
		<t>
		The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
		"<bcp14>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 document are to
		be
		interpreted as described in BCP 14 <xref target="RFC2119"/> <xref
		target="RFC8174"/> when, and only when, they appear in all capitals, as
		shown here.
		</t>
		<t>The terminology used in this document is consistent with and
		incorporates that described in "Terms Used in Routing for Low-Power
		and Lossy Networks" <xref target="RFC7102" format="default"/>, "RPL:
		IPv6 Routing Protocol for Low-Power and Lossy Networks" <xref
		target="RFC6550" format="default"/>, and "The Routing Protocol for
		Low-Power and Lossy Networks (RPL) Option for Carrying RPL Information
		in Data-Plane Datagrams" <xref target="RFC6553" format="default"/>.
		Other terms used in LLNs can be found in "Terminology for
		Constrained-Node Networks" <xref target="RFC7228"
		format="default"/>.</t>


	<t>The possible values of LORS and transitions between them are depicted in <xre	<t>In particular, the following acronyms appear in the document:</t>
	f target="fig_state_machine"/>.	<dl newline="false" spacing="normal">
	States “UP” and “GLOBALLY DOWN” can be attained by both Sentinels and Acceptors;	<dt>DIO:</dt>
	states “SUSPECTED DOWN” and “LOCALLY DOWN” — by Sentinels only.</t>	<dd>DODAG Information Object (a RPL message)</dd>
		<dt>DIS:</dt>
		<dd>DODAG Information Solicitation (a RPL message)</dd>
		<dt>DODAG:</dt>
		<dd>Destination-Oriented Directed Acyclic Graph</dd>
		<dt>LLN:</dt>
		<dd>Low-Power and Lossy Network</dd>
		<dt>LBR:</dt>
		<dd>LLN Border Router</dd>
		</dl>
		<t>In addition, the document introduces the following concepts:</t>
		<dl newline="false" spacing="normal">
		<dt>Sentinel:</dt>
		<dd>One of the two roles that a node can play in RNFD. For a given
		DODAG Version, a Sentinel node is a DODAG root's neighbor that
		monitors the DODAG root's status. There are normally multiple
		Sentinels for a DODAG root. However, being the DODAG root's neighbor
		need not imply being a Sentinel.</dd>
		<dt>Acceptor:</dt>
		<dd>The other of the two roles that a node can play in RNFD. For a
		given DODAG Version, an Acceptor node is a node that is not
		a Sentinel.</dd>
		<dt>Locally Observed DODAG Root's State (LORS):</dt>
		<dd>A node's local knowledge of the DODAG root's status, specifying in
		particular whether the DODAG root is up.</dd>
		<dt>Conflict-Free Replicated Counter (CFRC):</dt>
		<dd>Conceptually represents a dynamic set whose cardinality can be
		estimated. It defines a partial order on its values and supports
		element addition and union. The union operation is order- and
		duplicate-insensitive, that is, idempotent, commutative, and
		associative.</dd>
		</dl>
		</section>
		<section anchor="overview" numbered="true" toc="default">
		<name>Overview</name>
		<t>As mentioned previously, LBRs are DODAG roots in RPL; hence, a
		crash of an LBR is global in that it affects all nodes in the
		corresponding DODAG. Therefore, each node running RNFD for a given
		DODAG explicitly tracks the DODAG root's current condition, which is
		referred to as Locally Observed DODAG Root's State (LORS), and
		synchronizes its local knowledge with other nodes.</t>
		<t>Since monitoring the condition of the DODAG root is performed by
		tracking the status of its links (i.e., whether they are up or down), it
		can only be done by the root's neighbors; other nodes must accept their
		observations. Consequently, depending on their roles, non-root nodes
		are divided in RNFD into two disjoint groups: Sentinels and Acceptors.
		A Sentinel node is a DODAG root's neighbor that monitors its link with
		the root. Thus, the DODAG root normally has multiple Sentinels, but being
		its neighbor need not imply being a Sentinel. An Acceptor node is
		a node that is not a Sentinel. Acceptors thus mainly collect and
		propagate Sentinels' observations. More information on Sentinel
		selection can be found in <xref target="mgmnt_roles_and_cfrc_lens"
		format="default"/>.</t>


	<figure title="RNFD States and Transitions" anchor="fig_state_machine"><artwork>	<section anchor="protocol-state-machine" numbered="true" toc="default">
	<![CDATA[	<name>Protocol State Machine</name>
		<t>The possible values of LORS and transitions between them are
		depicted in <xref target="fig_state_machine" format="default"/>.
		States "UP" and "GLOBALLY DOWN" can be attained by both Sentinels and
		Acceptors; states "SUSPECTED DOWN" and "LOCALLY DOWN" can be attained
		by Sentinels only.</t>
		<figure anchor="fig_state_machine">
		<name>RNFD States and Transitions</name>
		<artwork name="" type="" align="left" alt=""><![CDATA[
	+---------------------------------------------------------+	+---------------------------------------------------------+
	\| \|---------------------------+ 3a \|	\| \|---------------------------+ 3a \|
	\| +-----------------+---------+ 3b \| \|	\| +-----------------+---------+ 3b \| \|
	\| \| 2b \| v v v	\| \| 2b \| v v v
	+-+----+-+ 1 +---------+-+ +-----------+ +-+------+-+	+-+----+-+ 1 +---------+-+ +-----------+ +-+------+-+
	\| UP +---->+ SUSPECTED +---->+ LOCALLY +---->+ GLOBALLY \|	\| UP +---->+ SUSPECTED +---->+ LOCALLY +---->+ GLOBALLY \|
	\| +<----+ DOWN \| 2a \| DOWN \| 3c \| DOWN \|	\| +<----+ DOWN \| 2a \| DOWN \| 3c \| DOWN \|
	+-+----+-+ 4a +-----------+ +-+---------+ +-+--------+	+-+----+-+ 4a +-----------+ +-+---------+ +-+--------+
	^ ^ \| \|	^ ^ \| \|
	\| \| 4b \| \|	\| \| 4b \| \|
	\| +---------------------------+ 5 \|	\| +---------------------------+ 5 \|

	+--------------------------------------------------+	+--------------------------------------------------+]]></artwork>
		</figure>
	]]></artwork></figure>	<t>To begin with, when any node joins a DODAG Version, the DODAG root
		must appear alive, so the node initializes RNFD with its LORS equal to
	<t>To begin with, when any node joins a DODAG Version, the DODAG root must appea	"UP". For a properly working DODAG root, the node remains in state
	r alive, so the node initializes RNFD with its LORS equal to “UP”.	"UP".</t>
	For a properly working DODAG root, the node remains in state “UP”.</t>	<t>However, when a node (acting as a Sentinel) starts suspecting that
		the root may have crashed, it changes its LORS to "SUSPECTED DOWN"
	<t>However, when a node — acting as Sentinel — starts suspecting that the root m	(transition 1 in <xref target="fig_state_machine" format="default"/>).
	ay have crashed, it changes its LORS to “SUSPECTED DOWN” (transition 1 in <xref	The transition from "UP" to "SUSPECTED DOWN" can happen based on the
	target="fig_state_machine"/>).	node's observations at either the data plane (for instance, link-layer
	The transition from “UP” to “SUSPECTED DOWN” can happen based on the node’s obse	triggers about missing hop-by-hop acknowledgments for packets
	rvations at either the data plane, for instance, link-layer triggers about missi	forwarded over the node's link to the root) or at the control plane (for
	ng hop-by-hop acknowledgments for packets forwarded over the node’s link to the	example, a significant growth in the number of Sentinels already
	root, or the control plane, for example, a significant growth in the number of S	suspecting the root to be dead). In state "SUSPECTED DOWN", the
	entinels already suspecting the root to be dead.	Sentinel node may verify its suspicion and/or inform other nodes about
	In state “SUSPECTED DOWN”, the Sentinel node may verify its suspicion and/or inf	the suspicion. When this has been done, it changes its LORS to
	orm other nodes about the suspicion.	"LOCALLY DOWN" (transition 2a). In some cases, the verification need
	When this has been done, it changes its LORS to “LOCALLY DOWN” (transition 2a).	not be performed, and as an optimization, a direct transition from
	In some cases, the verification need not be performed and, as an optimization, a	"UP" to "LOCALLY DOWN" (transition 2b) can be done instead.</t>
	direct transition from “UP” to “LOCALLY DOWN” (transition 2b) can be done inste	<t>If a sufficient number of Sentinels have their LORS equal to "LOCALLY
	ad.</t>	DOWN", all nodes (Sentinels and Acceptors) consent globally that the
		DODAG root must have crashed and set their LORS to "GLOBALLY DOWN",
	<t>If sufficiently many Sentinels have their LORS equal to “LOCALLY DOWN”, all n	irrespective of the previous value (transitions 3a, 3b, and 3c).
	odes — Sentinels and Acceptors — consent globally that the DODAG root must have	State "GLOBALLY DOWN" is terminal in that the only transition any node
	crashed and set their LORS to “GLOBALLY DOWN”, irrespective of the previous valu	can perform from this to another state (transition 5) takes place when
	e (transitions 3a, 3b, and 3c).	the node joins a new DODAG Version. When a node is in state "GLOBALLY
	State “GLOBALLY DOWN” is terminal in that the only transition any node can perfo	DOWN", RNFD forces RPL to maintain an infinite Rank and no parent,
	rm from this to another state (transition 5) takes place when the node joins a n	thereby preventing routing packets upward in the DODAG. In other
	ew DODAG version.	words, this state represents a situation in which all non-root nodes
	When a node is in state “GLOBALLY DOWN”, RNFD forces RPL to maintain an infinite	agree that the current DODAG Version is unusable; hence, to
	rank and no parent, thereby preventing routing packets upward in the DODAG.	recover, the root has to give a proof of being alive by initiating a
	In other words, this state represents a situation in which all non-root nodes ag	new DODAG Version.</t>
	ree that the current DODAG version is unusable, and hence, to recover, the root	<t>In contrast, if a node (either a Sentinel or Acceptor) is in state
	has to give a proof of being alive by initiating a new DODAG Version.</t>	"UP", RNFD does not influence RPL's packet forwarding; a node can
		route packets upward if it has a parent. The same is true for states
	<t>In contrast, if a node — either Sentinel or Acceptor — is in state “UP”, RNFD	"SUSPECTED DOWN" and "LOCALLY DOWN", attainable only by Sentinels.
	does not influence RPL’s packet forwarding: a node can route packets upward if	Finally, while in any of the two states, a Sentinel node may observe
	it has a parent.	some activity of the DODAG root and hence decide that its suspicion
	The same is true for states “SUSPECTED DOWN” and “LOCALLY DOWN”, attainable only	is a mistake. In such a case, it returns to state "UP" (transitions
	by Sentinels.	4a and 4b).</t>
	Finally, while in any of the two states, a Sentinel node may observe some activi	</section>
	ty of the DODAG root, and hence decide that its suspicion is a mistake.	<section anchor="counters-and-communication" numbered="true" toc="default"
	In such a case, it returns to state “UP” (transitions 4a and 4b).</t>	>
		<name>Counters and Communication</name>
	</section>	<t>To enable arriving at a global conclusion that the DODAG root has
	<section anchor="counters-and-communication" title="Counters and Communication">	crashed (i.e., transiting to state "GLOBALLY DOWN"), all nodes count
		locally and synchronize among each other the number of Sentinels
	<t>To enable arriving at a global conclusion that the DODAG root has crashed (i.	considering the root to be dead (i.e., those in state "LOCALLY DOWN").
	e., transiting to state “GLOBALLY DOWN”), all nodes count locally and synchroniz	This process employs structures referred to as Conflict-Free
	e among each other the number of Sentinels considering the root to be dead (i.e.	Replicated Counters (CFRCs). They are stored and modified
	, those in state “LOCALLY DOWN”).	independently by each node and are disseminated throughout the network
	This process employs structures referred to as conflict-free replicated counters	in options added to RPL link-local control messages: DODAG Information
	(CFRCs).	Objects (DIOs) and DODAG Information Solicitations (DISs). Upon
	They are stored and modified independently by each node and are disseminated thr	reception of such an option from its neighbor, a node merges the
	oughout the network in options added to RPL link-local control messages: DODAG I	received counter with its local one, thereby obtaining a new content
	nformation Objects (DIOs) and DODAG Information Solicitations (DISs).	for its local counter.</t>
	Upon reception of such an option from its neighbor, a node merges the received c	<t>The merging operation is idempotent, commutative, and associative.
	ounter with its local one, thereby obtaining a new content for its local counter	Moreover, all possible counter values are partially ordered. This
	.</t>	enables ensuring eventual consistency of the counters across all
		nodes, irrespective of the particular sequence of merges, shape of the
	<t>The merging operation is idempotent, commutative, and associative.	DODAG, or general network topology. In effect, as long as the network
	Moreover, all possible counter values are partially ordered.	is connected, all nodes will be able to arrive at the same conclusion
	This enables ensuring eventual consistency of the counters across all nodes, irr	regarding the DODAG root, in particular when no two Sentinels
	espective of the particular sequence of merges, shape of the DODAG, or general n	have a direct link with each other.</t>
	etwork topology.	<t>Each node in RNFD maintains two CFRCs for a DODAG:</t>
	In effect, as long as the network is connected, all nodes will be able to arrive	<dl>
	at the same conclusion regarding the DODAG root, in particular, even when no tw	<dt>PositiveCFRC:</dt><dd>Counts Sentinels that consider or have
	o Sentinels have a direct link with each other.</t>	previously considered the root node as alive in the current DODAG
		Version.</dd>
	<t>Each node in RNFD maintains two CFRCs for a DODAG:</t>	<dt>NegativeCFRC:</dt><dd>Counts Sentinels that consider or have
		previously considered the root node as dead in the current DODAG
	<t><list style="symbols">	Version.</dd>
	<t>PositiveCFRC, counting Sentinels that consider or have previously considere	</dl>
	d the root node as alive in the current DODAG Version,</t>	<t>The PositiveCFRC is always greater than or equal to the NegativeCFRC
	<t>NegativeCFRC, counting Sentinels that consider or have previously considere	in
	d the root node as dead in the current DODAG Version.</t>	terms of the partial order defined for the counters. The difference
	</list></t>	between the value of the PositiveCFRC and the value of the NegativeCFRC
		is
	<t>PositiveCFRC is always greater than or equal to the NegativeCFRC in terms of	thus nonnegative and estimates the number of Sentinels that still
	the partial order defined for the counters.	consider the DODAG root node as alive.</t>
	The difference between the value of PositiveCFRC and the value of NegativeCFRC i	</section>
	s thus nonnegative and estimates the number of Sentinels that still consider the	</section>
	DODAG root node as alive.</t>

	</section>
	</section>
	<section anchor="the-rnfd-option" title="The RNFD Option">

	<t>RNFD state synchronization between nodes takes place through the RNFD Option.
	It is a new type of RPL Control Message Options that is carried in link-local RP
	L control messages, notably DIOs and DISs.
	Its main task is allowing the receivers to merge their two CFRCs with the sender
	’s CFRCs.</t>

	<section anchor="general-cfrc-requirements" title="General CFRC Requirements">

	<t>CFRCs in RNFD MUST support the following operations:</t>

	<t><list style="hanging">
	<t hangText='value(c)'>
	Returns a nonnegative integer value corresponding to the number of nodes count
	ed by a given CFRC, c.</t>
	<t hangText='zero()'>
	Returns a CFRC that counts no nodes, that is, has its value equal to 0.</t>
	<t hangText='self()'>
	Returns a CFRC that counts only the node executing the operation.</t>
	<t hangText='infinity()'>
	Returns a CFRC that counts all possible nodes and represents a special value,
	infinity.</t>
	<t hangText='merge(c1, c2)'>
	Returns a CFRC that is a union of c1 and c2 (i.e., counts all nodes that are c
	ounted by either c1, c2, or both c1 and c2).</t>
	<t hangText='compare(c1, c2)'>
	Returns the result of comparing c1 to c2.</t>
	<t hangText='saturated(c)'>
	Returns TRUE if a given CFRC, c, is saturated (i.e., no more new nodes should
	be counted by it) or FALSE otherwise.</t>
	</list></t>

	<t>The partial ordering of CFRCs implies that the result of compare(c1, c2) can
	be either:</t>

	<t><list style="symbols">
	<t>smaller, if c1 is ordered before c2 (i.e., c2 counts all nodes that c1 coun
	ts and at least one node that c1 does not count);</t>
	<t>greater, if c1 is ordered after c2 (i.e., c1 counts all nodes that c2 count
	s and at least one node that c2 does not count);</t>
	<t>equal, if c1 and c2 are the same (i.e., they count the same nodes);</t>
	<t>incomparable, otherwise.</t>
	</list></t>

	<t>In particular, zero() is smaller than all other values and infinity() is grea
	ter than any other value.</t>

	<t>The properties of merging in turn can be formalized as follows for any c1, c2
	, and c3:</t>

	<t><list style="symbols">
	<t>idempotence: c1 = merge(c1, c1);</t>
	<t>commutativity: merge(c1, c2) = merge(c2, c1);</t>
	<t>associativity: merge(c1, merge(c2, c3)) = merge(merge(c1, c2), c3).</t>
	</list></t>

	<t>In particular, merge(c, zero()) always equals c while merge(c, infinity()) al
	ways equals infinity().</t>

	<t>There are many algorithmic structures that can provide the aforementioned pro
	perties of CFRC.
	Although in principle RNFD does not rely on any specific one, the option adopts
	so-called linear counting <xref target="Whang90"/>.</t>


	</section>	<section anchor="the-rnfd-option" numbered="true" toc="default">
	<section anchor="msg_format" title="Format of the Option">	<name>The RNFD Option</name>
		<t>RNFD state synchronization between nodes takes place through the RNFD
		Option. It is a new type of RPL Control Message Option that is carried
		in link-local RPL control messages, notably DIOs and DISs. Its main
		task is allowing the receivers to merge their two CFRCs with the
		sender's CFRCs.</t>
		<section anchor="general-cfrc-requirements" numbered="true" toc="default">
		<name>General CFRC Requirements</name>
		<t>CFRCs in RNFD <bcp14>MUST</bcp14> support the following operations:</
		t>
		<dl newline="true" spacing="normal">
		<dt>value(c)</dt>
		<dd>Returns a nonnegative integer value corresponding to the number
		of nodes counted by a given CFRC, c.</dd>
		<dt>zero()</dt>
		<dd>Returns a CFRC that counts no nodes, that is, has its value
		equal to 0.</dd>
		<dt>self()</dt>
		<dd>Returns a CFRC that counts only the node executing the operation.<
		/dd>
		<dt>infinity()</dt>
		<dd>Returns a CFRC that counts all possible nodes and represents a
		special value, infinity.</dd>
		<dt>merge(c1, c2)</dt>
		<dd>Returns a CFRC that is a union of c1 and c2 (i.e., counts all
		nodes that are counted by either c1, c2, or both c1 and c2).</dd>
		<dt>compare(c1, c2)</dt>
		<dd>Returns the result of comparing c1 to c2.</dd>
		<dt>saturated(c)</dt>
		<dd>Returns TRUE if a given CFRC, c, is saturated (i.e., no more new
		nodes should be counted by it); returns FALSE otherwise.</dd>
		</dl>
		<t>The partial ordering of CFRCs implies that the result of compare(c1,
		c2) can be either:</t>
		<ul spacing="normal">
		<li>
		<t>smaller, if c1 is ordered before c2 (i.e., c2 counts all nodes
		that c1 counts and at least one node that c1 does not count);</t>
		</li>
		<li>
		<t>greater, if c1 is ordered after c2 (i.e., c1 counts all nodes
		that c2 counts and at least one node that c2 does not count);</t>
		</li>
		<li>
		<t>equal, if c1 and c2 are the same (i.e., they count the same nodes
		); or</t>
		</li>
		<li>
		<t>incomparable, otherwise.</t>
		</li>
		</ul>
		<t>In particular, zero() is smaller than all other values, and infinity(
		) is greater than any other value.</t>
		<t>The properties of merging can be formalized as follows for
		any c1, c2, and c3:</t>
		<ul spacing="normal">
		<li>
		<t>idempotence: c1 = merge(c1, c1);</t>
		</li>
		<li>
		<t>commutativity: merge(c1, c2) = merge(c2, c1); and</t>
		</li>
		<li>
		<t>associativity: merge(c1, merge(c2, c3)) = merge(merge(c1, c2), c3
		).</t>
		</li>
		</ul>
		<t>In particular, merge(c, zero()) always equals c, while merge(c,
		infinity()) always equals infinity().</t>
		<t>There are many algorithmic structures that can provide the
		aforementioned properties of CFRC. Although in principle RNFD does
		not rely on any specific one, the option adopts so-called linear
		counting <xref target="Whang90" format="default"/>.</t>
		</section>


	<t>The format of the RNFD Option conforms to the generic format of RPL Control M	<section anchor="msg_format" numbered="true" toc="default">
	essage Options:</t>	<name>Format of the Option</name>


	<figure title="Format of the RNFD Option" anchor="fig_opt_format"><artwork><![CD	<t>The format of the RNFD Option conforms to the generic format of RPL C
	ATA[	ontrol Message Options (see <xref target="RFC6550" sectionFormat="of" section="6
		.7.1" format="default"/>):</t>
		<figure anchor="fig_opt_format">
		<name>Format of the RNFD Option</name>
		<artwork name="" type="" 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 = TBD1 \| Option Length \| \|	\| Type = 0x0E \| Option Length \| \|
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
	\| \|	\| \|
	+ +	+ +
	\| PosCFRC, NegCFRC (Variable Length*) \|	\| PosCFRC, NegCFRC (Variable Length*) \|
	. .	. .
	. .	. .

	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
		</figure>
	The '*' denotes that, if present, the fields have equal lengths.	<t>The "*" denotes that, if present, the fields have equal lengths.</t>
	]]></artwork></figure>

	<t><list style="hanging">
	<t hangText='Option Type'>
	TBD1</t>
	<t hangText='Option Length'>
	8-bit unsigned integer. Denotes the length of the option in octets excluding t
	he Option Type and Option Length fields. Its value MUST be even. A value of 0 de
	notes that RNFD is disabled in the current DODAG Version.</t>
	<t hangText='PosCFRC, NegCFRC'>
	Two variable-length, octet-aligned bit arrays carrying the sender’s PositiveCF
	RC and NegativeCFRC, respectively.</t>
	</list></t>

	<t>The length of the arrays constituting the PosCFRC and NegCFRC fields is the s
	ame and is derived from Option Length as follows.
	The value of Option Length is divided by 2 to obtain the number of octets each o
	f the two arrays occupies.
	The resulting number of octets is multiplied by 8 which yields an upper bound on
	the number of bits in each array.
	As the actual bit length of each of the arrays, the largest prime number less th
	an the upper bound is assumed.
	For example, if the value of Option Length is 16, then each array occupies 8 oct
	ets, and its actual bit length is 61, as this is the largest prime number less t
	han 64.</t>

	<t>Furthermore, for any bit equal to 1 in the NegCFRC, the bit with the same ind
	ex MUST be equal to 1 also in the PosCFRC.
	Any unused bits (i.e., the bits beyond the actual bit length of each of the arra
	ys) MUST be equal to 0.
	Finally, if PosCFRC has all its bits equal to 1, then NegCFRC MUST also have all
	its bits equal to 1.</t>

	<t>The CFRC operations are defined for such bit arrays of a given length as foll
	ows:</t>

	<t><list style="hanging">
	<t hangText='value(c)'>
	Returns the smallest integer value not less than -LT*ln(L0/LT), where ln() is
	the natural logarithm function, L0 is the number of bits equal to 0 in the array
	corresponding to <spanx style="verb">c</spanx> and LT is the bit length of the
	array.</t>
	<t hangText='zero()'>
	Returns an array with all bits equal to 0.</t>
	<t hangText='self()'>
	Returns an array with a single bit, selected uniformly at random, equal to 1.<
	/t>
	<t hangText='infinity()'>
	Returns an array with all bits equal to 1.</t>
	<t hangText='merge(c1, c2)'>
	Returns a bit array that constitutes a bitwise OR of c1 and c2, that is, a bit
	in the resulting array is equal to 0 only if the same bit is equal to 0 in both
	c1 and c2.</t>
	<t hangText='compare(c1, c2)'>
	Returns:</t>
	</list></t>

	<t><list style="symbols">
	<t>equal if each bit of c1 is equal to the corresponding bit of c2;</t>
	<t>less if c1 and c2 are not equal and, for each bit equal to 1 in c1, the cor
	responding bit in c2 is also equal to 1;</t>
	<t>greater if c1 and c2 are not equal and, for each bit equal to 1 in c2, the
	corresponding bit in c1 is also equal to 1;</t>
	<t>incomparable, otherwise.</t>
	</list></t>

	<t><list style="hanging">
	<t hangText='saturated(c)'>
	Returns TRUE, if more than RNFD_CFRC_SATURATION_THRESHOLD of the bits in c are
	equal to 1, or FALSE, otherwise.</t>
	</list></t>

	</section>
	</section>
	<section anchor="rnfd_behavior" title="RPL Router Behavior">

	<t>Although RNFD operates largely independently of RPL, it does need interact wi
	th RPL and the overall protocol stack.
	These interactions are described next and can be realized, for instance, by mean
	s of event triggers.</t>

	<section anchor="rnfd_behavior_roles" title="Joining a DODAG Version and Changin
	g the RNFD Role">

	<t>Whenever RPL running at a node joins a DODAG Version, RNFD — if active — MUST
	assume for the node the role of Acceptor.
	Accordingly, it MUST set its LORS to “UP” and its PositiveCFRC and NegativeCFRC
	to zero().</t>

	<t>The role may then change between Acceptor and Sentinel at any time.
	However, while a switch from Sentinel to Acceptor has no preconditions, for a sw
	itch from Acceptor to Sentinel to be possible, <spanx style="emph">all</spanx> o
	f the following conditions MUST hold:</t>

	<t><list style="numbers">
	<t>LORS is “UP”;</t>
	<t>saturated(PositiveCFRC) is FALSE;</t>
	<t>a neighbor entry for the DODAG root is present in RPL’s DODAG parent set;</
	t>
	<t>the neighbor is considered reachable via its link-local IPv6 address.</t>
	</list></t>

	<t>A role change also requires appropriate updates to LORS and CFRCs, so that th
	e node is properly accounted for.
	More specifically, when changing its role from Acceptor to Sentinel, the node MU
	ST add itself to its PositiveCFRC as follows.
	It MUST generate a new CFRC value, selfc = self(), and MUST replace its Positive
	CFRC, denoted oldpc, with newpc = merge(oldpc, selfc).
	In contrast, the effects of a switch from Sentinel to Acceptor vary depending on
	the node’s value of LORS before the switch:</t>

	<t><list style="symbols">
	<t>for “GLOBALLY DOWN”, the node MUST NOT modify its LORS, PositiveCFRC, and N
	egativeCFRC;</t>
	<t>for “LOCALLY DOWN”, the node MUST set its LORS to “UP” but MUST NOT modify
	its PositiveCFRC and NegativeCFRC;</t>
	<t>for “UP” and “SUSPECTED DOWN”, the node MUST set its LORS to “UP”, MUST NOT
	modify it PositiveCFRC, but MUST add itself to NegativeCFRC, that is, replace i
	ts NegativeCFRC, denoted oldnc, with newnc = merge(oldnc, selfc), where selfc is
	the counter generated with self() when the node last added itself to its Positi
	veCFRC.</t>
	</list></t>

	</section>
	<section anchor="rnfd_behavior_detection" title="Detecting and Verifying Problem
	s with the DODAG Root">

	<t>Only nodes that are Sentinels take active part in detecting crashes of the DO
	DAG Root; Acceptors just disseminate their observations, reflected in the CFRCs.
	</t>

	<t>The DODAG root monitoring SHOULD be based on both internal inputs, notably th
	e values of CFRCs and LORS, and external inputs, such as triggers from RPL and o
	ther protocols.
	External input monitoring SHOULD be performed preferably in a reactive fashion,
	also independently of RPL, and at both data plane and control plane.
	In particular, it is RECOMMENDED that RNFD be directly notified of events releva
	nt to the routing adjacency maintenance mechanisms on which RPL relies, such as
	Layer 2 triggers <xref target="RFC5184"/> or the Neighbor Unreachability Detecti
	on <xref target="RFC4861"/> mechanism.
	In addition, depending on the underlying protocol stack, there may be other pote
	ntial sources of such events, for instance, neighbor communication overhearing.
	In any case, only events concerning the DODAG root need be monitored.
	For example, RNFD can conclude that there may be problems with the DODAG root if
	it observes a lack of multiple consecutive L2 acknowledgments for packets trans
	mitted by the node via the link to the DODAG root.
	Internally, in turn, it is RECOMMENDED that RNFD take action whenever there is a
	change to its local CFRCs, so that a node can have a chance to participate in d
	etecting potential problems even when normally it would not exchange packets ove
	r the link with the DODAG root during some period.
	In particular, RNFD SHOULD conclude that there may be problems with the DODAG ro
	ot, when the fraction value(NegativeCFRC)/value(PositiveCFRC) has grown by at le
	ast RNFD_SUSPICION_GROWTH_THRESHOLD since the node last set its LORS to “UP”.</t
	>

	<t>Whenever having its LORS set to “UP” RNFD concludes — based on either externa
	l or internal inputs — that there may be problems with the link with the DODAG r
	oot, it MUST set its LORS to either “SUSPECTED DOWN” or, as an optimization, to
	“LOCALLY DOWN”.</t>

	<t>The “SUSPECTED DOWN” value of LORS is temporary: its aim is to give RNFD an a
	dditional opportunity to verify whether the link with the DODAG root is indeed d
	own.
	Depending on the outcome of such verification, RNFD MUST set its LORS to either
	“UP”, if the link has been confirmed not to be down, or “LOCALLY DOWN”, otherwis
	e.
	The verification can be performed, for example, by transmitting RPL DIS or ICMPv
	6 Echo Request messages to the DODAG root’s link-local IPv6 address and expectin
	g replies confirming that the root is up and reachable through the link.
	Care should be taken not to overload the DODAG root with traffic due to simultan
	eous probes, for instance, random backoffs can be employed to this end.
	It is RECOMMENDED that the “SUSPECTED DOWN” value of LORS is attained and verifi
	cation takes place if RNFD’s conclusion on the state of the DODAG root is based
	only on indirect observations, for example, the aforementioned growth of the CFR
	C values.
	In contrast, for direct observations, such as missing L2 acknowledgments, the ve
	rification MAY be skipped, with the node’s LORS effectively changing from “UP” d
	irectly to “LOCALLY DOWN”.</t>

	<t>For consistency with RPL, when detecting potential problems with the DODAG ro
	ot, RNFD also must make use of RPL’s independent knowledge.
	More specifically, a node MUST switch its LORS from “UP” or “SUSPECTED DOWN” dir
	ectly to “LOCALLY DOWN” if a neighbor entry for the DODAG root is removed from R
	PL’s DODAG parent set or the neighbor ceases to be considered reachable via its
	link-local IPv6 address.</t>

	<t>Finally, while having its LORS already equal to “LOCALLY DOWN”, a node may ma
	ke an observation confirming that its link with the DODAG root is actually up.
	In such a case, it SHOULD set its LORS back to “UP” but MUST NOT do this before
	the previous conditions 2–4 necessary for a node to change its role from Accepto
	r to Sentinel all hold (see <xref target="rnfd_behavior_roles"/>).</t>

	<t>To appropriately account for the node’s observations on the state of the DODA
	G root, the aforementioned LORS transitions are accompanied by changes to the no
	de’s local CFRCs as follows.
	Transitions between “UP” and “SUSPECTED DOWN” do not affect any of the two CFRCs
	.
	During a switch from “UP” or “SUSPECTED DOWN” to “LOCALLY DOWN”, in turn, the no
	de MUST add itself to its NegativeCFRC, as explained previously.
	By symmetry, if there is a transition from “LOCALLY DOWN” to “UP”, the node MUST
	add itself to its PositiveCFRC, again, as explained previously.</t>

	<t>Such changes to a node’s local CFRCs, if performed repeatedly due to incorrec
	t decisions regarding the status of the node’s link with the DODAG root, may lea
	d to those CFRCs becoming saturated.
	An implementation should thus try to minimize false-positive transitions from “U
	P” and “SUSPECTED DOWN” to “LOCALLY DOWN”.
	The exact approach depends on the specific solutions employed for assessing the
	state of a link.
	For instance, one can utilize additional mechanisms for increasing the confidenc
	e of individual decisions, such as during the aforementioned verification in the
	“SUSPECTED DOWN” state, or can limit the number of transitions per node, possib
	ly in an adaptive fashion.</t>

	</section>
	<section anchor="rnfd_behavior_consensus" title="Disseminating Observations and
	Reaching Agreement">

	<t>Nodes disseminate their observations by exchanging CFRCs in the RNFD Options
	embedded in link-local RPL control messages, notably DIOs and DISs.
	When processing such a received option, a node — acting as Sentinel or Acceptor
	— MUST update its PositiveCFRC and NegativeCFRC to respectively newpc = merge(ol
	dpc, recvpc) and newnc = merge(oldnc, recvnc), where oldpc and oldnc are the val
	ues of the node’s PositiveCFRC and NegativeCFRC before the update, while recvpc
	and recvnc are the received values of option fields PosCFRC and NegCFRC, respect
	ively.</t>

	<t>In effect, the node’s value of fraction value(NegativeCFRC)/value(PositiveCFR
	C) may change.
	If the fraction reaches at least RNFD_CONSENSUS_THRESHOLD (with value(PositiveCF
	RC) being greater than zero), then the node consents on the DODAG root being dow
	n.
	Accordingly, it MUST change its LORS to “GLOBALLY DOWN” and set its PositiveCFRC
	and NegativeCFRC both to infinity().</t>

	<t>The “GLOBALLY DOWN” value of LORS is terminal: the node MUST NOT change it an
	d MUST NOT modify its CFRCs until it joins a new DODAG Version.
	With this value of LORS, RNFD at the node MUST also prevent RPL from having any
	DODAG parent and advertising any Rank other than INFINITE_RANK.</t>

	<t>Since the RNFD Option is embedded, among others, in RPL DIO control messages,
	updates to a node’s CFRCs may affect the sending schedule of these messages, wh
	ich is driven by the DIO Trickle timer <xref target="RFC6206"/>.
	It is RECOMMENDED to use for RNFD a dedicated Trickle timer, different from RPL’
	s original DIO Trickle timer.
	In such a setting, whenever the dedicated timer fires and no DIO message contain
	ing the RNFD Option has been sent to the link-local all-RPL-nodes multicast IPv6
	address since the previous firing, the node sends a DIO message containing the
	RNFD Option to the address.
	The minimal and maximal interval sizes of the dedicated timer SHOULD NOT be smal
	ler than those of RPL’s original DIO Trickle timer.
	In contrast, in the absence of the dedicated Trickle timer for RNFD, an implemen
	tation SHOULD ensure that the RNFD Option is present in multicast DIO messages s
	ufficiently often to quickly propagate changes to the node’s CFRCs, and notably
	as soon as possible after a reset of the timer triggered by RNFD.
	In the remainder of this document, we will refer to the Trickle timer utilized b
	y RNFD — either the dedicated one or RPL’s original one, depending on the implem
	entation — simply as “Trickle timer”.
	In particular, a node MUST reset its Trickle timer when it changes its LORS to “
	GLOBALLY DOWN”, so that information about the detected crash of the DODAG root i
	s disseminated in the DODAG fast.
	Likewise, a node SHOULD reset its Trickle timer when any of its local CFRCs chan
	ges significantly.</t>

	</section>
	<section anchor="rnfd_behavior_root" title="DODAG Root’s Behavior">

	<t>The DODAG root node MUST assume the role of Acceptor in RNFD and MUST NOT eve
	r switch this role.
	It MUST also monitor its LORS and local CFRCs, so that it can react to various e
	vents.</t>

	<t>To start with, the DODAG root MUST generate a new DODAG Version, thereby rest
	arting the protocol, if it changes its LORS to “GLOBALLY DOWN”, which may happen
	when the root has restarted after a crash or the nodes have falsely detected it
	s crash.
	It MAY also generate a new DODAG Version if fraction value(NegativeCFRC)/value(P
	ositiveCFRC) approaches RNFD_CONSENSUS_THRESHOLD, so as to avoid potential inter
	ruptions to routing.</t>

	<t>Furthermore, the DODAG root SHOULD either generate a new DODAG Version or inc
	rease the bit length of its CFRCs if saturated(PositiveCFRC) becomes TRUE.
	This is a self-regulation mechanism that helps adjust the CFRCs to a potentially
	large number of Sentinels (see <xref target="mgmnt_roles_and_cfrc_lens"/>).</t>

	<t>In general, issuing a new DODAG Version effectively restarts RNFD.
	The DODAG root MAY thus perform this operation also in other situations.</t>

	</section>
	<section anchor="rnfd_behavior_deactivation" title="Activating and Deactivating
	the Protocol on Demand">

	<t>RNFD can be activated and deactivated on demand, once per DODAG Version.
	The particular policies for activating and deactivating the protocol are outside
	the scope of this document.
	However, the activation and deactivation MUST be done at the DODAG root node; ot
	her nodes MUST comply.</t>

	<t>More specifically, when a non-root node joins a DODAG Version, RNFD at the no
	de is initially inactive.
	The node MUST NOT activate the protocol unless it receives for this DODAG Versio
	n a valid RNFD Option containing some CFRCs, that is, having its Option Length f
	ield positive.
	In particular, if the option accompanies the message that causes the node to joi
	n the DODAG Version, the protocol MUST be active from the moment of the joining.
	RNFD then remains active at the node until it is explicitly deactivated or the n
	ode joins a new DODAG Version.
	An explicit deactivation MUST take place when the node receives an RNFD Option f
	or the DODAG Version with no CFRCs, that is, having its Option Length field equa
	l to zero.
	When explicitly deactivated, RNFD MUST NOT be reactivated unless the node joins
	a new DODAG Version.
	In particular, when the first RNFD Option received by the node has its Option Le
	ngth field equal to zero, the protocol MUST remain deactivated for the entire ti
	me the node belongs to the current DODAG Version.</t>

	<t>When RNFD at a node is initially inactive for a DODAG Version, the node MUST
	NOT attach any RNFD Option to the messages it sends (in particular, because it m
	ay not know the desired CFRC length — see <xref target="rnfd_behavior_cfrc_size"
	/>).
	When the protocol has been explicitly deactivated, the node MAY also decide not
	to attach the option to its outgoing messages.
	However, it is RECOMMENDED that it sends sufficiently many messages with the opt
	ion to the link-local all-RPL-nodes multicast IPv6 address to allow its neighbor
	s to learn that RNFD has been deactivated in the current DODAG version.
	In particular, it MAY reset its Trickle timer to this end but also MAY use some
	reactive mechanisms, for example, replying with a unicast DIO or DIS containing
	the RNFD Option with no CFRCs to a message from a neighbor that contains the opt
	ion with some CFRCs, as such a neighbor appears not to have learned about the de
	activation of RNFD.</t>

	</section>
	<section anchor="rnfd_behavior_cfrc_size" title="Processing CFRCs of Incompatibl
	e Lengths">

	<t>The merge() and compare() operations on CFRCs require both arguments to be co
	mpatible, that is, to have the same bit length.
	However, the processing rules for the RNFD Option (see <xref target="msg_format"
	/>) do not necessitate this.
	This fact is made use of not only in the mechanisms for activating and deactivat
	ing the protocol (see <xref target="rnfd_behavior_deactivation"/>), but also in
	mechanisms for dynamic adjustments of CFRCs, which aim to enable deployment-spec
	ific policies (see <xref target="mgmnt_roles_and_cfrc_lens"/>).
	A node thus must be prepared to receive the RNFD Option with fields PosCFRC and
	NegCFRC of a different bit length than the node’s own PositiveCFRC and NegativeC
	FRC.
	Assuming that it has RNFD active and that fields PosCFRC and NegCFRC in the opti
	on have a positive length, the node MUST react as follows.</t>

	<t>If the bit length of fields PosCFRC and NegCFRC is the same as that of the no
	de’s local PositiveCFRC and NegativeCFRC, then the node MUST perform the merges,
	as detailed previously (see <xref target="rnfd_behavior_consensus"/>).</t>

	<t>If the bit length of fields PosCFRC and NegCFRC is smaller than that of the n
	ode’s local PositiveCFRC and NegativeCFRC, then the node MUST ignore the option
	and MAY reset its Trickle timer.</t>

	<t>If the bit length of fields PosCFRC and NegCFRC is greater than that of the n
	ode’s local PositiveCFRC and NegativeCFRC, then the node MUST extend the bit len
	gth of its local CFRCs to be equal to that in the option and set the CFRCs as fo
	llows:</t>

	<t><list style="symbols">
	<t>If the node’s LORS is “GLOBALLY DOWN”, then both its local CFRCs MUST be se
	t to infinity().</t>
	<t>Otherwise, they both MUST be set to zero(), and the node MUST account for i
	tself in so initialized CFRCs. More specifically, if the node is Sentinel, then
	it MUST add itself to its PositiveCFRC, as detailed previously. In addition, if
	its LORS is “LOCALLY DOWN”, then it MUST also add itself to its NegativeCFRC, ag
	ain, as explained previously. Finally, the node MUST perform merges of its local
	CFRCs and the ones received in the option (see <xref target="rnfd_behavior_cons
	ensus"/>) and MAY reset its Trickle timer.</t>
	</list></t>

	<t>In contrast, if the node is unable to extend its local CFRCs, for example, be
	cause it lacks resources, then it MUST stop participating in RNFD, that is, unti
	l it joins a new DODAG Version, it MUST NOT send the RNFD Option and MUST ignore
	this option in received messages.</t>

	<t>A DODAG root node can be requested to increase the bit length of its CFRCs ex
	ternally, as part of the management policies (see <xref target="mgmnt_roles_and_
	cfrc_lens"/>).
	If it cannot fulfill such a request, then it is MUST NOT stop participating in R
	FND and SHOULD return an error to the requester instead.
	Otherwise, since it is always Acceptor, the above rules require it to extend bot
	h CFRCs to the requested length and to set them both to either zero() or infinit
	y(), depending on whether its LORS is, respectively, different from or equal to
	“GLOBALLY DOWN”.
	In the latter case, given the earlier rules governing the root’s behavior upon r
	eaching the “GLOBALLY DOWN” state (cf. <xref target="rnfd_behavior_root"/>), the
	root is also bound to eventually set its CFRCs to zero() and, in addition, gene
	rate a new DODAG Version and change its LORS back to “UP”.
	Therefore, these two steps can be optimized into one, meaning that effectively,
	irrespective of its LORS, when increasing the bit length of its CFRCs in respons
	e to an external request, the root also sets the CFRCs to zero().</t>

	</section>
	<section anchor="summary-of-rnfds-interactions-with-rpl" title="Summary of RNFD’
	s Interactions with RPL">

	<t>In summary, RNFD interacts with RPL in the following manner:</t>

	<t><list style="symbols">
	<t>While having its LORS equal to “GLOBALLY DOWN”, RNFD prevents RPL from rout
	ing packets and advertising upward routes in the corresponding DODAG (see <xref
	target="rnfd_behavior_consensus"/>).</t>
	<t>In some scenarios, RNFD triggers RPL to issue a new DODAG Version (see <xre
	f target="rnfd_behavior_root"/>).</t>
	<t>Depending on the implementation, RNFD may cause RPL’s DIO Trickle timer res
	ets (see <xref target="rnfd_behavior_consensus"/>, <xref target="rnfd_behavior_d
	eactivation"/>, and <xref target="rnfd_behavior_cfrc_size"/>).</t>
	<t>RNFD monitors events relevant to routing adjacency maintenance as well as t
	hose affecting RPL’s DODAG parent set (see <xref target="rnfd_behavior_roles"/>
	and <xref target="rnfd_behavior_detection"/>).</t>
	<t>Using RNFD entails embedding the RNFD Option into link-local RPL control me
	ssages (see <xref target="msg_format"/>).</t>
	</list></t>

	</section>
	<section anchor="rnfd_behavior_constants" title="Summary of RNFD’s Constants">

	<t>The following is a summary of RNFD’s constants:</t>

	<t><list style="hanging">
	<t hangText='RNFD_CONSENSUS_THRESHOLD'>
	A threshold concerning the value of fraction value(NegativeCFRC)/value(Positiv
	eCFRC). If the value at a Sentinel or Acceptor node reaches the threshold, then
	the node’s LORS is set to “GLOBALLY DOWN”, which implies that consensus has been
	reached on the DODAG root node being down (see <xref target="rnfd_behavior_cons
	ensus"/>). The default value of the threshold is 0.51, which indicates that a ma
	jority of Sentinels must consider the root to be down to reach the consensus. In
	general, the higher the value the longer the detection period but the lower the
	risk of false positives.</t>
	<t hangText='RNFD_SUSPICION_GROWTH_THRESHOLD'>
	A threshold concerning the value of fraction value(NegativeCFRC)/value(Positiv
	eCFRC). If the value at a Sentinel node grows at least by this threshold since t
	he time the node’s LORS was last set to “UP”, then the node’s LORS is set to “SU
	SPECTED DOWN” or “LOCALLY DOWN”, which implies that the node starts suspecting o
	r assumes a crash of the DODAG root (see <xref target="rnfd_behavior_detection"/
	>). The higher the value the longer the duration of detecting true crashes but t
	he lower the risk of increased traffic due to verifying false suspicions. The de
	fault value of the threshold is 0.12, which in sparse networks (up to 8 neighbor
	s per node) triggers a suspicion at a Sentinel node after just one other Sentine
	l starts considering the root as dead, while being gradually more conservative i
	n denser networks.</t>
	<t hangText='RNFD_CFRC_SATURATION_THRESHOLD'>
	A threshold concerning the percentage of bits set to 1 in a CFRC, c. If the pe
	rcentage for c is equal to or greater than this threshold, then saturated(c) ret
	urns TRUE, which hints the DODAG root to generate a new DODAG Version or increas
	e the bit length of the CFRCs (see <xref target="rnfd_behavior_root"/>). The def
	ault value of the threshold is 0.63. The higher the value is, the higher the pro
	bability of bit collisions, and hence the more erratic the results of function v
	alue(c) may be.</t>
	</list></t>


	<t>The means of configuring the constants at individual nodes are outside the sc	<!-- [rfced] Section 4.2: We note that "Type" appears in Figure 2, but that it
	ope of this document.</t>	is defined as "Option Type" in the definition list that follows. Are any
		updates needed for consistency?
		-->


	</section>	<dl newline="false" spacing="normal">
	</section>	<dt>Option Type:</dt>
	<section anchor="mgmnt" title="Manageability Considerations">	<dd>0x0E</dd>
		<dt>Option Length:</dt>
		<dd>8-bit unsigned integer. Denotes the length of the option in
		octets, excluding the Option Type and Option Length fields. Its value
		<bcp14>MUST</bcp14> be even. A value of 0 denotes that RNFD is
		disabled in the current DODAG Version.</dd>
		<dt>PosCFRC, NegCFRC:</dt>
		<dd>Two variable-length, octet-aligned bit arrays carrying the
		sender's PositiveCFRC and NegativeCFRC, respectively.</dd>
		</dl>
		<t>The length of the arrays constituting the PosCFRC and NegCFRC
		fields is the same and is derived from Option Length as follows. The
		value of Option Length is divided by 2 to obtain the number of octets
		each of the two arrays occupies. The resulting number of octets is
		multiplied by 8, which yields an upper bound on the number of bits in
		each array. As the actual bit length of each of the arrays, the
		largest prime number less than the upper bound is assumed. For
		example, if the value of Option Length is 16, then each array occupies
		8 octets, and its actual bit length is 61, as this is the largest
		prime number less than 64.</t>
		<t>Furthermore, for any bit equal to 1 in the NegCFRC, the bit with
		the same index <bcp14>MUST</bcp14> also be equal to 1 in the PosCFRC.
		Any unused bits (i.e., the bits beyond the actual bit length of each
		of the arrays) <bcp14>MUST</bcp14> be equal to 0. Finally, if PosCFRC
		has all its bits equal to 1, then NegCFRC <bcp14>MUST</bcp14> also
		have all its bits equal to 1.</t>
		<t>The CFRC operations are defined for such bit arrays of a given length
		as follows:</t>
		<dl newline="true" spacing="normal">
		<dt>value(c)</dt>
		<dd>Returns the smallest integer value not less than -LT*ln(L0/LT),
		where ln() is the natural logarithm function, L0 is the number of
		bits equal to 0 in the array corresponding to c, and LT is
		the bit length of the array.</dd>
		<dt>zero()</dt>
		<dd>Returns an array with all bits equal to 0.</dd>
		<dt>self()</dt>
		<dd>Returns an array with a single bit, selected uniformly at random,
		equal to 1.</dd>
		<dt>infinity()</dt>
		<dd>Returns an array with all bits equal to 1.</dd>
		<dt>merge(c1, c2)</dt>
		<dd>Returns a bit array that constitutes a bitwise OR of c1 and c2.
		That is, a bit in the resulting array is equal to 0 only if the same
		bit is equal to 0 in both c1 and c2.</dd>
		<dt>compare(c1, c2)</dt>
		<dd><t>Returns:</t>


	<t>RNFD is largely self-managed, with the exception of protocol activation and d	<ul spacing="normal">
	eactivation, as well as node role assignment and the related CFRC size adjustmen	<li>
	t, for which only the aforementioned mechanisms are defined, so as to enable ado	<t>equal, if each bit of c1 is equal to the corresponding bit of
	pting deployment-specific policies.	c2;</t>
	This section discusses the manageability issues.</t>	</li>
		<li>
		<t>less, if c1 and c2 are not equal, and for each bit equal to 1 in
		c1, the corresponding bit in c2 is also equal to 1;</t>
		</li>
		<li>
		<t>greater, if c1 and c2 are not equal, and for each bit equal to 1
		in c2, the corresponding bit in c1 is also equal to 1; or</t>
		</li>
		<li>
		<t>incomparable, otherwise.</t>
		</li>
		</ul>
		</dd>
		<dt>saturated(c)</dt>
		<dd>Returns TRUE if more than RNFD_CFRC_SATURATION_THRESHOLD of the
		bits in c are equal to 1; returns FALSE otherwise.</dd>
		</dl>
		</section>
		</section>


	<section anchor="mgmnt_roles_and_cfrc_lens" title="Role Assignment and CFRC Size	<section anchor="rnfd_behavior" numbered="true" toc="default">
	Adjustment">	<name>RPL Router Behavior</name>
		<t>Although RNFD operates largely independently of RPL, it does need to
		interact with RPL and the overall protocol stack. These interactions
		are described next and can be realized, for instance, by means of event
		triggers.</t>
		<section anchor="rnfd_behavior_roles" numbered="true" toc="default">
		<name>Joining a DODAG Version and Changing the RNFD Role</name>
		<t>Whenever RPL is running at a node and joins a DODAG Version, RNFD (if
		active) <bcp14>MUST</bcp14> assume the role of Acceptor for the node.
		Accordingly, it <bcp14>MUST</bcp14> set its LORS to "UP" and its
		PositiveCFRC and NegativeCFRC to zero().</t>


	<t>One approach to node role and CFRC size selection is to manually designate sp	<t>The role may then change between Acceptor and Sentinel at any time.
	ecific nodes as Sentinels in RNFD, assuming that they will have chances to satis	However, while a switch from Sentinel to Acceptor has no
	fy the necessary conditions for attaining this role (see <xref target="rnfd_beha	preconditions, in order for a switch from Acceptor to Sentinel to be pos
	vior_roles"/>), and fixing the CFRC bit length to accommodate these nodes.</t>	sible,
		<em>all</em> of the following conditions <bcp14>MUST</bcp14> hold:</t>
		<ol spacing="normal" type="1"><li>
		<t>LORS is "UP";</t>
		</li>
		<li>
		<t>saturated(PositiveCFRC) is FALSE;</t>
		</li>
		<li>
		<t>a neighbor entry for the DODAG root is present in RPL's DODAG par
		ent set; and</t>
		</li>
		<li>
		<t>the neighbor is considered reachable via its link-local IPv6 addr
		ess.</t>
		</li>
		</ol>
		<t>A role change also requires appropriate updates to LORS and CFRCs,
		so that the node is properly accounted for. More specifically, when
		changing its role from Acceptor to Sentinel, the node
		<bcp14>MUST</bcp14> add itself to its PositiveCFRC as follows. It
		<bcp14>MUST</bcp14> generate a new CFRC value, selfc = self(), and
		it <bcp14>MUST</bcp14> replace its PositiveCFRC, denoted oldpc, with
		newpc = merge(oldpc, selfc). In contrast, the effects of a switch
		from Sentinel to Acceptor vary depending on the node's value of LORS
		before the switch:</t>


	<t>Another approach is to automate the selection process: in principle, any node	<ul spacing="normal">
	satisfying the necessary conditions for becoming Sentinel (see <xref target="rn	<li>
	fd_behavior_roles"/>) can attain this role.	<t>For "GLOBALLY DOWN", the node <bcp14>MUST NOT</bcp14> modify
	However, in networks where the DODAG root node has many neighbors, this approach	its LORS, PositiveCFRC, and NegativeCFRC.</t>
	may lead to saturated(PositiveCFRC) quickly becoming TRUE, which — without addi	</li>
	tional measures — may degrade RNFD’s performance.	<li>
	This issue can be handled with a probabilistic solution: if PositiveCFRC becomes	<t>For "LOCALLY DOWN", the node <bcp14>MUST</bcp14> set its LORS
	saturated with little or no increase in NegativeCFRC, then a new DODAG Version	to "UP" but <bcp14>MUST NOT</bcp14> modify its PositiveCFRC and
	can be issued and a node satisfying the necessary conditions can become Sentinel	NegativeCFRC.</t>
	in this version only with probability 1/2.	</li>
	This process can be continued with the probability being halved in each new DODA	<li>
	G Version until PositiveCFRC is no longer quickly saturated.	<t>For "UP" and "SUSPECTED DOWN", the node <bcp14>MUST</bcp14> set
	Another solution is to increase, potentially multiple times the bit length of th	its LORS to "UP" and <bcp14>MUST NOT</bcp14> modify its PositiveCFRC
	e CFRCs by the DODAG root if PositiveCFRC becomes saturated with little or no gr	,
	owth in NegativeCFRC, which does not require issuing a new DODAG Version but len	but it <bcp14>MUST</bcp14> add itself to NegativeCFRC. That is, it <
	gthens the RNFD Option.	bcp14>MUST</bcp14>
	In this way, again, a sufficient bit length can be dynamically discovered or the	replace its NegativeCFRC, denoted oldnc, with newnc = merge(oldnc,
	root can conclude that a given bit length is excessive for (some) nodes and res	selfc), where selfc is the counter generated with self() when the
	ort to the previous solution.	node last added itself to its PositiveCFRC.</t>
	Increasing the bit length can be done, for instance, by doubling it, respecting	</li>
	the condition that it has to be a prime number (see <xref target="msg_format"/>)	</ul>
	.</t>	</section>
		<section anchor="rnfd_behavior_detection" numbered="true" toc="default">
		<name>Detecting and Verifying Problems with the DODAG Root</name>
		<t>Only nodes that are Sentinels take an active part in detecting crashe
		s
		of the DODAG root; Acceptors just disseminate their observations,
		reflected in the CFRCs.</t>


	<t>In either of the solutions, Sentinel nodes should preferably be stable themse	<t>The DODAG root monitoring <bcp14>SHOULD</bcp14> be based on both
	lves and have stable links to the DODAG root.	internal inputs, notably the values of CFRCs and LORS, and external
	Otherwise, they may often exhibit LORS transitions between “UP” and “LOCALLY DOW	inputs, such as triggers from RPL and other protocols. External input
	N” or switches between Acceptor and Sentinel roles, which gradually saturates CF	monitoring <bcp14>SHOULD</bcp14> be performed preferably in a reactive
	RCs.	fashion, also independently of RPL, and at both the data plane and contr
	Although as a mitigation the number of such transitions and switches per node MA	ol
	Y be limited, having Sentinels stable SHOULD be preferred.</t>	plane. In particular, it is <bcp14>RECOMMENDED</bcp14> that RNFD be
		directly notified of events relevant to the routing adjacency
		maintenance mechanisms on which RPL relies, such as Layer 2 (L2) trigger
		s
		<xref target="RFC5184" format="default"/> or the Neighbor
		Unreachability Detection <xref target="RFC4861" format="default"/>
		mechanism. In addition, depending on the underlying protocol stack,
		there may be other potential sources of such events, for instance,
		neighbor communication overhearing. In any case, only events
		concerning the DODAG root need to be monitored. For example, RNFD can
		conclude that there may be problems with the DODAG root if it observes
		a lack of multiple consecutive L2 acknowledgments for packets
		transmitted by the node via the link to the DODAG root. Internally, in
		turn,
		it is <bcp14>RECOMMENDED</bcp14> that RNFD take action
		whenever there is a change to its local CFRCs, so that a node can have
		a chance to participate in detecting potential problems even when
		normally it would not exchange packets over the link with the DODAG
		root during some period. In particular, RNFD <bcp14>SHOULD</bcp14>
		conclude that there may be problems with the DODAG root when the
		fraction value(NegativeCFRC)/value(PositiveCFRC) has grown by at least
		RNFD_SUSPICION_GROWTH_THRESHOLD since the node last set its LORS to
		"UP".</t>


	</section>	<t>Whenever, having its LORS set to "UP", RNFD concludes (based on either
	<section anchor="mgmnt_virt_dodag_roots" title="Virtual DODAG Roots">	external or internal inputs) that there may be problems with the
		link with the DODAG root, it <bcp14>MUST</bcp14> set its LORS either to "SUSPECT
		ED
		DOWN" or, as an optimization, to "LOCALLY DOWN".</t>
		<t>The "SUSPECTED DOWN" value of LORS is temporary: its aim is to give
		RNFD an additional opportunity to verify whether the link with the
		DODAG root is indeed down. Depending on the outcome of such
		verification, RNFD <bcp14>MUST</bcp14> set its LORS to either "UP", if
		the link has been confirmed not to be down, or "LOCALLY DOWN",
		otherwise. The verification can be performed, for example, by
		transmitting RPL DIS or ICMPv6 Echo Request messages to the DODAG
		root's link-local IPv6 address and expecting replies confirming that
		the root is up and reachable through the link. Care should be taken
		not to overload the DODAG root with traffic due to simultaneous
		probes, for instance, random backoffs can be employed to this end. It
		is <bcp14>RECOMMENDED</bcp14> that the "SUSPECTED DOWN" value of LORS
		be attained and verification take place if RNFD's conclusion on the
		state of the DODAG root is based only on indirect observations, for
		example, the aforementioned growth of the CFRC values. In contrast,
		for direct observations, such as missing L2 acknowledgments, the
		verification <bcp14>MAY</bcp14> be skipped, with the node's LORS
		effectively changing from "UP" directly to "LOCALLY DOWN".</t>
		<t>For consistency with RPL, when detecting potential problems with
		the DODAG root, RNFD also must make use of RPL's independent
		knowledge. More specifically, a node <bcp14>MUST</bcp14> switch its
		LORS from "UP" or "SUSPECTED DOWN" directly to "LOCALLY DOWN" if a
		neighbor entry for the DODAG root is removed from RPL's DODAG parent
		set or the neighbor ceases to be considered reachable via its
		link-local IPv6 address.</t>


	<t>RPL allows a DODAG to have a so-called virtual root, that is, a collection of	<t>Finally, while having its LORS already equal to "LOCALLY DOWN", a
	nodes coordinating to act as a single root of the DODAG.	node may make an observation confirming that its link with the DODAG
	The details of the coordination process are left open in the specification <xref	root is actually up. In such a case, it <bcp14>SHOULD</bcp14> set its
	target="RFC6550"/> but, from RNFD’s perspective, two possible realizations are	LORS back to "UP" but <bcp14>MUST NOT</bcp14> do this before
	worth consideration:</t>	conditions 2-4 in <xref
		target="rnfd_behavior_roles" format="default"/>, which are necessary for
		a node
		to change its role from Acceptor to Sentinel, all hold.</t>


	<t><list style="symbols">	<t>To appropriately account for the node's observations on the state
	<t>Just a single (primary) node of the nodes comprising the virtual root acts	of the DODAG root, the aforementioned LORS transitions are accompanied
	as the actual root of the DODAG. Only when this node fails, does another (backup	by changes to the node's local CFRCs as follows. Transitions between
	) node take over. As a result, at any time, at most one of the nodes comprising	"UP" and "SUSPECTED DOWN" do not affect either of the two CFRCs. In con
	the virtual root is the actual root.</t>	trast, during
	<t>More than one of the nodes comprising the virtual root act as actual roots	a switch from "UP" or "SUSPECTED DOWN" to "LOCALLY DOWN", the
	of the DODAG, all advertising the same Rank in the DODAG. When some of the nodes	node <bcp14>MUST</bcp14> add itself to its NegativeCFRC, as explained
	fail, the other nodes may or may not react in any specific way. In other words,	previously. By symmetry, if there is a transition from "LOCALLY DOWN"
	at any time, more than one node can be the actual root.</t>	to "UP", the node <bcp14>MUST</bcp14> add itself to its PositiveCFRC,
	</list></t>	as explained previously.</t>
		<t>Such changes to a node's local CFRCs, if performed repeatedly due
		to incorrect decisions regarding the status of the node's link with
		the DODAG root, may lead to those CFRCs becoming saturated. An
		implementation should thus try to minimize false-positive transitions
		from "UP" and "SUSPECTED DOWN" to "LOCALLY DOWN". The exact approach
		depends on the specific solutions employed for assessing the state of
		a link. For instance, one can utilize additional mechanisms for
		increasing the confidence of individual decisions, such as during the
		aforementioned verification in the "SUSPECTED DOWN" state, or can
		limit the number of transitions per node, possibly in an adaptive
		fashion.</t>
		</section>


	<t>In the first realization, RNFD’s operation is largely unaffected.	<section anchor="rnfd_behavior_consensus" numbered="true" toc="default">
	The necessary conditions for a node to become Sentinel (<xref target="rnfd_behav	<name>Disseminating Observations and Reaching Agreement</name>
	ior_roles"/>) guarantee that only the current primary root node is monitored by	<t>Nodes disseminate their observations by exchanging CFRCs in the
	the protocol.	RNFD Options embedded in link-local RPL control messages, notably DIOs
	This SHOULD be taken into account in the policies for node role assignment, CFRC	and DISs. When processing such a received option, a node (acting as a
	size selection, and, possibly, the setting of the two thresholds (<xref target=	Sentinel or Acceptor) <bcp14>MUST</bcp14> update its PositiveCFRC and
	"rnfd_behavior_constants"/>).	NegativeCFRC to newpc = merge(oldpc, recvpc) and newnc =
	Moreover, when a new primary has been elected, to avoid polluting CFRCs with obs	merge(oldnc, recvnc), respectively. Here, oldpc and oldnc are the values
	ervations on the previous primary, a new DODAG Version MUST be issued.</t>	of the
		node's PositiveCFRC and NegativeCFRC before the update, while recvpc
		and recvnc are the received values of option fields PosCFRC and
		NegCFRC, respectively.</t>
		<t>In effect, the node's value of the fraction
		value(NegativeCFRC)/value(PositiveCFRC) may change. If the fraction
		reaches at least RNFD_CONSENSUS_THRESHOLD (with value(PositiveCFRC)
		being greater than zero), then the node consents on the DODAG root
		being down. Accordingly, it <bcp14>MUST</bcp14> change its LORS to
		"GLOBALLY DOWN" and set its PositiveCFRC and NegativeCFRC both to
		infinity().</t>
		<t>The "GLOBALLY DOWN" value of LORS is terminal; the node <bcp14>MUST
		NOT</bcp14> change it and <bcp14>MUST NOT</bcp14> modify its CFRCs
		until it joins a new DODAG Version. With this value of LORS, RNFD at
		the node <bcp14>MUST</bcp14> also prevent RPL from having any DODAG
		parent and advertising any Rank other than INFINITE_RANK.</t>
		<t>Since the RNFD Option is embedded, among others, in RPL DIO control
		messages, updates to a node's CFRCs may affect the sending schedule of
		these messages, which is driven by the DIO Trickle timer <xref
		target="RFC6206" format="default"/>. It is <bcp14>RECOMMENDED</bcp14>
		to use a dedicated Trickle timer for RNFD that is different from RPL's
		original DIO Trickle timer. In such a setting, whenever the dedicated
		timer fires and no DIO message containing the RNFD Option has been
		sent to the link-local all-RPL-nodes multicast IPv6 address since the
		previous firing, the node sends a DIO message containing the RNFD
		Option to the address. The minimal and maximal interval sizes of the
		dedicated timer <bcp14>SHOULD NOT</bcp14> be smaller than those of
		RPL's original DIO Trickle timer. In contrast, in the absence of the
		dedicated Trickle timer for RNFD, an implementation
		<bcp14>SHOULD</bcp14> ensure that the RNFD Option is present in
		multicast DIO messages sufficiently often to quickly propagate changes
		to the node's CFRCs and, notably, as soon as possible after a reset of
		the timer triggered by RNFD. In the remainder of this document, we
		will refer to the Trickle timer utilized by RNFD (either the
		dedicated one or RPL's original one, depending on the implementation)
		simply as "Trickle timer". In particular, a node <bcp14>MUST</bcp14>
		reset its Trickle timer when it changes its LORS to "GLOBALLY DOWN",
		so that information about the detected crash of the DODAG root is
		disseminated in the DODAG fast. Likewise, a node
		<bcp14>SHOULD</bcp14> reset its Trickle timer when any of its local
		CFRCs change significantly.</t>
		</section>


	<t>In the second realization, the fact that the virtual root consists of multipl	<section anchor="rnfd_behavior_root" numbered="true" toc="default">
	e nodes is transparent to RNFD.	<name>DODAG Root's Behavior</name>
	Therefore, employing RNFD is such a setting can be beneficial only if the nodes	<t>The DODAG root node <bcp14>MUST</bcp14> assume the role of Acceptor
	comprising the virtual root may suffer from correlated crashes, for instance, du	in RNFD and <bcp14>MUST NOT</bcp14> ever switch this role. It
	e to global power outages.</t>	<bcp14>MUST</bcp14> also monitor its LORS and local CFRCs, so that it
		can react to various events.</t>
		<t>To start with, the DODAG root <bcp14>MUST</bcp14> generate a new
		DODAG Version, thereby restarting the protocol, if it changes its LORS
		to "GLOBALLY DOWN", which may happen when the root has restarted after
		a crash or the nodes have falsely detected its crash. It
		<bcp14>MAY</bcp14> also generate a new DODAG Version if the fraction
		value(NegativeCFRC)/value(PositiveCFRC) approaches
		RNFD_CONSENSUS_THRESHOLD, so as to avoid potential interruptions to
		routing.</t>
		<t>Furthermore, the DODAG root <bcp14>SHOULD</bcp14> either generate a
		new DODAG Version or increase the bit length of its CFRCs if
		saturated(PositiveCFRC) becomes TRUE. This is a self-regulation
		mechanism that helps adjust the CFRCs to a potentially large number of
		Sentinels (see <xref target="mgmnt_roles_and_cfrc_lens"
		format="default"/>).</t>
		<t>In general, issuing a new DODAG Version effectively restarts RNFD.
		Thus, the DODAG root <bcp14>MAY</bcp14> also perform this operation in
		other situations.</t>
		</section>


	</section>	<section anchor="rnfd_behavior_deactivation" numbered="true" toc="default"
	<section anchor="mgmnt_monitoring" title="Monitoring">	>
		<name>Activating and Deactivating the Protocol on Demand</name>
		<t>RNFD can be activated and deactivated on demand, once per DODAG
		Version. The particular policies for activating and deactivating the
		protocol are outside the scope of this document. However, the
		activation and deactivation <bcp14>MUST</bcp14> be done at the DODAG
		root node; other nodes <bcp14>MUST</bcp14> comply.</t>
		<t>More specifically, when a non-root node joins a DODAG Version, RNFD
		at the node is initially inactive. The node <bcp14>MUST NOT</bcp14>
		activate the protocol unless it receives for this DODAG Version a
		valid RNFD Option containing some CFRCs, that is, having its Option
		Length field positive. In particular, if the option accompanies the
		message that causes the node to join the DODAG Version, the protocol
		<bcp14>MUST</bcp14> be active from the moment of the joining. RNFD
		then remains active at the node until it is explicitly deactivated or
		the node joins a new DODAG Version. An explicit deactivation
		<bcp14>MUST</bcp14> take place when the node receives an RNFD Option
		for the DODAG Version with no CFRCs, that is, having its Option Length
		field equal to zero. When explicitly deactivated, RNFD <bcp14>MUST
		NOT</bcp14> be reactivated unless the node joins a new DODAG Version.
		In particular, when the first RNFD Option received by the node has its
		Option Length field equal to zero, the protocol <bcp14>MUST</bcp14>
		remain deactivated for the entire time the node belongs to the current
		DODAG Version.</t>


	<t>For monitoring the operation of RNFD, its implementation SHOULD provide the f	<t>When RNFD at a node is initially inactive for a DODAG Version, the
	ollowing information about a node:</t>	node <bcp14>MUST NOT</bcp14> attach any RNFD Option to the messages it
		sends (in particular, because it may not know the desired CFRC length;
		see <xref target="rnfd_behavior_cfrc_size" format="default"/>). When
		the protocol has been explicitly deactivated, the node
		<bcp14>MAY</bcp14> also decide not to attach the option to its
		outgoing messages. However, it is <bcp14>RECOMMENDED</bcp14> that it
		send a sufficient number of messages with the option to the link-local
		all-RPL-nodes multicast IPv6 address to allow its neighbors to learn
		that RNFD has been deactivated in the current DODAG Version. In
		particular, it <bcp14>MAY</bcp14> reset its Trickle timer to this end
		but <bcp14>MAY</bcp14> also use some reactive mechanisms. For example, i
		t <bcp14>MAY</bcp14>
		reply with a unicast DIO or DIS containing the RNFD Option with no
		CFRCs to a message from a neighbor that contains the option with some
		CFRCs, as such a neighbor appears not to have learned about the
		deactivation of RNFD.</t>
		</section>
		<section anchor="rnfd_behavior_cfrc_size" numbered="true" toc="default">
		<name>Processing CFRCs of Incompatible Lengths</name>
		<t>The merge() and compare() operations on CFRCs require both
		arguments to be compatible, that is, to have the same bit length.
		However, the processing rules for the RNFD Option (see <xref
		target="msg_format" format="default"/>) do not necessitate this. This
		fact is made use of not only in the mechanisms for activating and
		deactivating the protocol (see <xref
		target="rnfd_behavior_deactivation" format="default"/>), but also in
		mechanisms for dynamic adjustments of CFRCs, which aim to enable
		deployment-specific policies (see <xref
		target="mgmnt_roles_and_cfrc_lens" format="default"/>). A node thus
		must be prepared to receive the RNFD Option with fields PosCFRC and
		NegCFRC of a different bit length than the node's own PositiveCFRC and
		NegativeCFRC. Assuming that it has RNFD active and that fields
		PosCFRC and NegCFRC in the option have a positive length, the node
		<bcp14>MUST</bcp14> react as follows.</t>
		<t>If the bit length of fields PosCFRC and NegCFRC is the same as that
		of the node's local PositiveCFRC and NegativeCFRC, then the node
		<bcp14>MUST</bcp14> perform the merges, as detailed previously (see
		<xref target="rnfd_behavior_consensus" format="default"/>).</t>
		<t>If the bit length of fields PosCFRC and NegCFRC is smaller than
		that of the node's local PositiveCFRC and NegativeCFRC, then the node
		<bcp14>MUST</bcp14> ignore the option and <bcp14>MAY</bcp14> reset its
		Trickle timer.</t>
		<t>If the bit length of fields PosCFRC and NegCFRC is greater than
		that of the node's local PositiveCFRC and NegativeCFRC, then the node
		<bcp14>MUST</bcp14> extend the bit length of its local CFRCs to be
		equal to that in the option and set the CFRCs as follows:</t>
		<ul spacing="normal">
		<li>
		<t>If the node's LORS is "GLOBALLY DOWN", then both of its local
		CFRCs <bcp14>MUST</bcp14> be set to infinity().</t>
		</li>
		<li>
		<t>Otherwise, they both <bcp14>MUST</bcp14> be set to zero(), and
		the node <bcp14>MUST</bcp14> account for itself in so initialized
		CFRCs. More specifically, if the node is a Sentinel, then it
		<bcp14>MUST</bcp14> add itself to its PositiveCFRC, as detailed
		previously. In addition, if its LORS is "LOCALLY DOWN", then it
		<bcp14>MUST</bcp14> also add itself to its NegativeCFRC, as
		explained previously. Finally, the node <bcp14>MUST</bcp14>
		perform merges of its local CFRCs and the ones received in the
		option (see <xref target="rnfd_behavior_consensus"
		format="default"/>) and <bcp14>MAY</bcp14> reset its Trickle
		timer.</t>
		</li>
		</ul>
		<t>In contrast, if the node is unable to extend its local CFRCs, for
		example, because it lacks resources, then it <bcp14>MUST</bcp14> stop
		participating in RNFD. That is, until it joins a new DODAG Version, it
		<bcp14>MUST NOT</bcp14> send the RNFD Option and <bcp14>MUST</bcp14>
		ignore this option in received messages.</t>
		<t>A DODAG root node can be requested to increase the bit length of
		its CFRCs externally, as part of the management policies (see <xref
		target="mgmnt_roles_and_cfrc_lens" format="default"/>). If it cannot
		fulfill such a request, then it <bcp14>MUST NOT</bcp14> stop
		participating in RNFD and <bcp14>SHOULD</bcp14> return an error to the
		requester instead. Otherwise, since it is always an Acceptor, the above
		rules require it to extend both CFRCs to the requested length and to
		set them both to either zero() or infinity(), depending on whether its
		LORS is different from or equal to "GLOBALLY DOWN", respectively. In
		the latter case, given the earlier rules governing the root's behavior
		upon reaching the "GLOBALLY DOWN" state (cf. <xref
		target="rnfd_behavior_root" format="default"/>), the root is also
		bound to eventually set its CFRCs to zero() and, in addition, generate
		a new DODAG Version and change its LORS back to "UP". Therefore,
		these two steps can be optimized into one, meaning that effectively,
		irrespective of its LORS, when increasing the bit length of its CFRCs
		in response to an external request, the root also sets the CFRCs to
		zero().</t>
		</section>
		<section anchor="summary-of-rnfds-interactions-with-rpl" numbered="true" t
		oc="default">
		<name>Summary of RNFD's Interactions with RPL</name>
		<t>In summary, RNFD interacts with RPL in the following manner:</t>
		<ul spacing="normal">
		<li>
		<t>While having its LORS equal to "GLOBALLY DOWN", RNFD prevents
		RPL from routing packets and advertising upward routes in the
		corresponding DODAG (see <xref target="rnfd_behavior_consensus"
		format="default"/>).</t>
		</li>
		<li>
		<t>In some scenarios, RNFD triggers RPL to issue a new DODAG
		Version (see <xref target="rnfd_behavior_root"
		format="default"/>).</t>
		</li>
		<li>
		<t>Depending on the implementation, RNFD may cause RPL's DIO
		Trickle timer resets (see Sections <xref target="rnfd_behavior_conse
		nsus"
		format="counter"/>, <xref target="rnfd_behavior_deactivation"
		format="counter"/>, and <xref target="rnfd_behavior_cfrc_size"
		format="counter"/>).</t>
		</li>
		<li>
		<t>RNFD monitors events relevant to routing adjacency maintenance
		as well as those affecting RPL's DODAG parent set (see Sections <xre
		f
		target="rnfd_behavior_roles" format="counter"/> and <xref
		target="rnfd_behavior_detection" format="counter"/>).</t>
		</li>
		<li>
		<t>Using RNFD entails embedding the RNFD Option into link-local
		RPL control messages (see <xref target="msg_format"
		format="default"/>).</t>
		</li>
		</ul>
		</section>


	<t><list style="symbols">	<section anchor="rnfd_behavior_constants" numbered="true" toc="default">
	<t>whether the protocol is active,</t>	<name>Summary of RNFD's Constants</name>
	<t>whether LORS is “GLOBALLY DOWN”,</t>	<t>The following is a summary of RNFD's constants:</t>
	</list></t>


	<t>accompanied by the recommended monitoring parameters provided by RPL itself <	<dl newline="false" spacing="normal">
	xref target="RFC6550"/>, notably the DODAG Version number and the Rank.	<dt>RNFD_CONSENSUS_THRESHOLD:</dt>
	To offer even finer-grained visibility into RNFD’s state at the node, the implem	<dd>A threshold concerning the value of the fraction
	entation MAY in addition provide:</t>	value(NegativeCFRC)/value(PositiveCFRC). If the value at a Sentinel
		or Acceptor node reaches the threshold, then the node's LORS is set
		to "GLOBALLY DOWN", which implies that consensus has been reached on
		the DODAG root node being down (see <xref
		target="rnfd_behavior_consensus" format="default"/>). The default
		value of the threshold is 0.51, which indicates that a majority of
		Sentinels must consider the root to be down to reach the
		consensus. In general, when the value is higher, the detection period
		is longer, but the risk of false positives is lower.
		</dd>
		<dt>RNFD_SUSPICION_GROWTH_THRESHOLD:</dt>
		<dd>A threshold concerning the value of the fraction
		value(NegativeCFRC)/value(PositiveCFRC). If the value at a Sentinel
		node grows at least by this threshold since the time the node's LORS
		was last set to "UP", then the node's LORS is set to "SUSPECTED
		DOWN" or "LOCALLY DOWN", which implies that the node starts
		suspecting or assumes a crash of the DODAG root (see <xref
		target="rnfd_behavior_detection" format="default"/>). When the
		value is higher, the duration of detecting true crashes is longer,
		but the risk of increased traffic due to verifying false suspicions
		is lower. The default value of the threshold is 0.12, which in
		sparse networks (up to 8 neighbors per node) triggers a suspicion at
		a Sentinel node after just one other Sentinel starts considering the
		root as dead, while being gradually more conservative in denser
		networks.</dd>
		<dt>RNFD_CFRC_SATURATION_THRESHOLD:</dt>
		<dd>A threshold concerning the percentage of bits set to 1 in a
		CFRC, c. If the percentage for c is equal to or greater than this
		threshold, then saturated(c) returns TRUE, which hints the DODAG
		root to generate a new DODAG Version or increase the bit length of
		the CFRCs (see <xref target="rnfd_behavior_root"
		format="default"/>). The default value of the threshold is 0.63.
		When the value is higher, the probability of bit collisions is
		higher, and the results of function value(c) may thus be more
		erratic.</dd>
		</dl>
		<t>The means of configuring the constants at individual nodes are outsid
		e the scope of this document.</t>
		</section>
		</section>
		<section anchor="mgmnt" numbered="true" toc="default">
		<name>Manageability Considerations</name>
		<t>RNFD is largely self-managed, with the exception of protocol
		activation and deactivation, as well as node role assignment and the
		related CFRC size adjustment, for which only the aforementioned
		mechanisms are defined, so as to enable adopting deployment-specific
		policies. This section discusses the manageability issues.</t>
		<section anchor="mgmnt_roles_and_cfrc_lens" numbered="true" toc="default">
		<name>Role Assignment and CFRC Size Adjustment</name>


	<t><list style="symbols">	<t>One approach to node role and CFRC size selection is to manually
	<t>the assigned role (i.e., Sentinel or Acceptor),</t>	designate specific nodes as Sentinels in RNFD, assuming that they will
	<t>the exact value of LORS (i.e., “UP”, “SUSPECTED DOWN”, “LOCALLY DOWN”, or	have chances to satisfy the necessary conditions for attaining this
	“GLOBALLY DOWN”),</t>	role (see <xref target="rnfd_behavior_roles" format="default"/>), and
	<t>the two CFRCs (i.e., PositiveCFRC and NegativeCFRC),</t>	to fix the CFRC bit length to accommodate these nodes.</t>
	<t>the constants listed in <xref target="rnfd_behavior_constants"/>.</t>
	</list></t>


	</section>	<t>Another approach is to automate the selection process. In
	</section>	principle, any node satisfying the necessary conditions for becoming
	<section anchor="security" title="Security Considerations">	a Sentinel (see <xref target="rnfd_behavior_roles" format="default"/>)
		can attain this role. However, in networks where the DODAG root node
		has many neighbors, this approach may lead to saturated(PositiveCFRC)
		quickly becoming TRUE, which may
		degrade RNFD's performance without additional measures. This issue can
		be handled with a
		probabilistic solution: if PositiveCFRC becomes saturated with little
		or no increase in NegativeCFRC, then a new DODAG Version can be issued,
		and a node satisfying the necessary conditions can become a Sentinel in
		this version only with probability 1/2. This process can be continued
		with the probability being halved in each new DODAG Version until
		PositiveCFRC is no longer quickly saturated. Another solution is to
		increase, potentially multiple times, the bit length of the CFRCs by
		the DODAG root if PositiveCFRC becomes saturated with little or no
		growth in NegativeCFRC. This does not require issuing a new DODAG
		Version but lengthens the RNFD Option. In this way, a
		sufficient bit length can be dynamically discovered, or the root can
		conclude that a given bit length is excessive for (some) nodes and
		resort to the previous solution. Increasing the bit length can be
		done, for instance, by doubling it, respecting the condition that it
		has to be a prime number (see <xref target="msg_format"
		format="default"/>).</t>
		<t>In either of the solutions, Sentinel nodes should preferably be
		stable themselves and have stable links to the DODAG root. Otherwise,
		they may often exhibit LORS transitions between "UP" and "LOCALLY
		DOWN" or switches between Acceptor and Sentinel roles, which gradually
		saturates CFRCs. As a mitigation, the number of such
		transitions and switches per node <bcp14>MAY</bcp14> be limited; however
		,
		having Sentinels be stable <bcp14>SHOULD</bcp14> be preferred.</t>
		</section>
		<section anchor="mgmnt_virt_dodag_roots" numbered="true" toc="default">
		<name>Virtual DODAG Roots</name>


	<t>RNFD is an extension to RPL and is thus both vulnerable to and benefits from	<t>RPL allows a DODAG to have a so-called "virtual root", that is, a
	the security issues and solutions described in <xref target="RFC6550"/> and <xre	collection of nodes coordinating to act as a single root of the DODAG.
	f target="RFC7416"/>.	The details of the coordination process are left open in
	Its specification in this document does not introduce new traffic patterns or ne	<xref target="RFC6550" format="default"/>, but from
	w messages, for which specific mitigation techniques would be required beyond wh	RNFD's perspective, two possible realizations are worth
	at can already be adopted for RPL.</t>	consideration:</t>
		<ul spacing="normal">
		<li>
		<t>Just a single (primary) node of the nodes comprising the
		virtual root acts as the actual root of the DODAG. Only when this
		node fails does another (backup) node take over. As a result, at
		any time, at most one of the nodes comprising the virtual root is
		the actual root.</t>
		</li>
		<li>
		<t>More than one of the nodes comprising the virtual root act as
		actual roots of the DODAG, all advertising the same Rank in the
		DODAG. When some of the nodes fail, the other nodes may or may not
		react in any specific way. In other words, at any time, more than
		one node can be the actual root.</t>
		</li>
		</ul>


	<t>In particular, RNFD depends on information exchanged in the RNFD Option.	<t>In the first realization, RNFD's operation is largely unaffected.
	If the contents of this option were compromised, then failure misdetection may o	The necessary conditions for a node to become a Sentinel (<xref
	ccur.	target="rnfd_behavior_roles" format="default"/>) guarantee that only
	One possibility is that the DODAG root may be falsely detected as crashed, which	the current primary root node is monitored by the protocol. This
	would result in an inability of the nodes to route packets, at least until a ne	<bcp14>SHOULD</bcp14> be taken into account in the policies for node
	w DODAG Version is issued by the root.	role assignment, CFRC size selection, and, possibly, the setting of
	Another possibility is that a crash of the DODAG root may not be detected by RNF	the three thresholds (<xref target="rnfd_behavior_constants"
	D, in which case RPL would have to rely on its own mechanisms.	format="default"/>). Moreover, when a new primary has been elected,
	Moreover, compromising the contents of the RNFD Option may also lead to increase	a
	d DIO traffic due to Trickle timer resets.	new DODAG Version <bcp14>MUST</bcp14> be issued to avoid polluting CFRCs
	Consequently, RNFD deployments are RECOMMENDED to use RPL security mechanisms if	with observations on the previous primary.</t>
	there is a risk that control information might be modified or spoofed.</t>


	<t>In this context, RNFD’s two features are worth highlighting.	<t>In the second realization, the fact that the virtual root consists
	First, unless all neighbors of a DODAG root are compromised, a false positive ca	of multiple nodes is transparent to RNFD. Therefore, employing RNFD
	n always be detected by the root based on its local CFRCs.	in such a setting can be beneficial only if the nodes comprising the
	If the frequency of such false positives becomes problematic, RNFD can be disabl	virtual root may suffer from correlated crashes, for instance, due to
	ed altogether, for instance, until the problem has been diagnosed.	global power outages.</t>
	This procedure can be largely automated at LBRs.	</section>
	Second, some types of false negatives can also be detected this way.
	Those that pass undetected, in turn, are likely not to have major negative conse
	quences on RPL apart from the lack of improvement to its performance upon a DODA
	G root’s crash, at least if RPL’s other components are not attacked as well.</t>


	</section>	<section anchor="mgmnt_monitoring" numbered="true" toc="default">
	<section anchor="iana-considerations" title="IANA Considerations">	<name>Monitoring</name>
		<t>For monitoring the operation of RNFD, its implementation <bcp14>SHOUL
		D</bcp14> provide the following information about a node:</t>
		<ul spacing="normal">
		<li>
		<t>whether the protocol is active, and</t>
		</li>
		<li>
		<t>whether LORS is "GLOBALLY DOWN".</t>
		</li>
		</ul>


	<t>To represent the RNFD Option, IANA is requested to allocate the value TBD1 fr	<t>This information <bcp14>MUST</bcp14> be accompanied by the
	om the <eref target="https://www.iana.org/assignments/rpl/rpl.xhtml#control-mess	recommended monitoring parameters provided by RPL itself <xref
	age-options">“RPL Control Message Options” registry</eref> of the “Routing Proto	target="RFC6550" format="default"/>, notably the DODAG Version number
	col for Low Power and Lossy Networks (RPL)” registry group.</t>	and the Rank. To offer even finer-grained visibility into RNFD's
		state at the node, the implementation <bcp14>MAY</bcp14> also
		provide:</t>
		<ul spacing="normal">
		<li>
		<t>the assigned role (i.e., Sentinel or Acceptor),</t>
		</li>
		<li>
		<t>the exact value of LORS (i.e., "UP", "SUSPECTED DOWN", "LOCALLY
		DOWN", or "GLOBALLY DOWN"),</t>
		</li>
		<li>
		<t>the two CFRCs (i.e., PositiveCFRC and NegativeCFRC), and</t>
		</li>
		<li>
		<t>the constants listed in <xref target="rnfd_behavior_constants"
		format="default"/>.</t>
		</li>
		</ul>
		</section>
		</section>
		<section anchor="security" numbered="true" toc="default">
		<name>Security Considerations</name>
		<t>RNFD is an extension to RPL and thus is vulnerable to and
		benefits from the security issues and solutions described in <xref
		target="RFC6550" format="default"/> and <xref target="RFC7416"
		format="default"/>. Its specification in this document does not
		introduce new traffic patterns or new messages, for which specific
		mitigation techniques would be required beyond what can already be
		adopted for RPL.</t>
		<t>In particular, RNFD depends on information exchanged in the RNFD
		Option. If the contents of this option were compromised, then failure
		misdetection may occur. One possibility is that the DODAG root may be
		falsely detected as crashed, which would result in an inability of the
		nodes to route packets, at least until a new DODAG Version is issued by
		the root. Another possibility is that a crash of the DODAG root may not
		be detected by RNFD, in which case RPL would have to rely on its own
		mechanisms. Moreover, compromising the contents of the RNFD Option may
		also lead to increased DIO traffic due to Trickle timer resets.
		Consequently, RNFD deployments are <bcp14>RECOMMENDED</bcp14> to use RPL
		security mechanisms if there is a risk that control information might be
		modified or spoofed.</t>
		<t>In this context, two features of RNFD are worth highlighting. First,
		unless all neighbors of a DODAG root are compromised, a false positive
		can always be detected by the root based on its local CFRCs. If the
		frequency of such false positives becomes problematic, RNFD can be
		disabled altogether, for instance, until the problem has been diagnosed.
		This procedure can be largely automated at LBRs. Second, some types of
		false negatives can also be detected this way. Those that do pass
		undetected are likely not to have major negative consequences
		on RPL apart from the lack of improvement to its performance upon a
		DODAG root's crash, at least if RPL's other components are not attacked
		as well.</t>
		</section>


	</section>	<section anchor="iana-considerations" numbered="true" toc="default">
	<section anchor="acknowledgements" title="Acknowledgements">	<name>IANA Considerations</name>
		<t>IANA has allocated the following value
		in the "RPL
		Control Message Options" registry within the <eref
		target="https://www.iana.org/assignments/rpl">"Routing Protocol for
		Low Power and Lossy Networks (RPL)" registry group</eref>.</t>


	<t>The authors would like to acknowledge Piotr Ciolkosz and Agnieszka Paszkowska	<table anchor="options-iana">
	.	<name></name>
	Agnieszka contributed to deeper understanding and formally proving various aspec	<thead>
	ts of RPL’s behavior upon an LBR crash.	<tr>
	Piotr in turn developed a prototype implementation of RNFD dedicated for RPL to	<th>Value</th>
	verify earlier performance claims.</t>	<th>Meaning</th>
		<th>Reference</th>
		</tr>
		</thead>
		<tbody>
		<tr>
		<td>0x0E</td>
		<td>RNFD Option</td>
		<td>RFC 9866</td>
		</tr>
		</tbody>
		</table>


	</section>	</section>

	</middle>	</middle>


	<back>	<back>

		<references>
		<name>References</name>
		<references>
		<name>Normative References</name>
		<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2
		119.xml"/>
		<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6
		206.xml"/>
		<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6
		550.xml"/>
		<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6
		553.xml"/>
		<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6
		554.xml"/>
		<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8
		174.xml"/>
		</references>
		<references>
		<name>Informative References</name>
		<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4
		861.xml"/>
		<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5
		184.xml"/>
		<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7
		102.xml"/>
		<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7
		228.xml"/>
		<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7
		416.xml"/>


	<references title='Normative References'>	<reference anchor="Iwanicki16">
		<front>
	&RFC2119;	<title>RNFD: Routing-Layer Detection of DODAG (Root) Node Failures i
	&RFC6206;	n Low-Power Wireless Networks</title>
	&RFC6550;	<author initials="K." surname="Iwanicki" fullname="Konrad Iwanicki">
	&RFC6553;	<organization/>
	&RFC6554;	</author>
	&RFC8174;	<date year="2016"/>
		</front>
	</references>	<refcontent>2016 15th ACM/IEEE International Conference on Information
		Processing in Sensor Networks (IPSN), pp. 1-12</refcontent>
		<seriesInfo name="DOI" value="10.1109/IPSN.2016.7460720"/>
		</reference>


	<references title='Informative References'>	<reference anchor="Ciolkosz19">
		<front>
		<title>Integration of the RNFD Algorithm for Border Router Failure D
		etection with the RPL Standard for Routing IPv6 Packets</title>
		<author initials="P." surname="Ciolkosz" fullname="Piotr Ciolkosz">
		<organization/>
		</author>
		<date year="2019"/>
		</front>
		<refcontent>Master's Thesis, University of Warsaw</refcontent>
		</reference>


	&RFC4861;	<reference anchor="Paszkowska19">
	&RFC5184;	<front>
	&RFC6202;	<title>Failure Handling in RPL Implementations: An Experimental Qual
	&RFC7102;	itative Study</title>
	&RFC7228;	<author initials="A." surname="Paszkowska" fullname="Agnieszka Paszk
	&RFC7416;	owska">
	<reference anchor="Iwanicki16" >	<organization/>
	<front>	</author>
	<title>RNFD: Routing-layer detection of DODAG (root) node failures in low-po	<author initials="K." surname="Iwanicki" fullname="Konrad Iwanicki">
	wer wireless networks</title>	<organization/>
	<author initials="K." surname="Iwanicki" fullname="Konrad Iwanicki">	</author>
	<organization></organization>	<date year="2019"/>
	</author>	</front>
	<date year="2016"/>	<refcontent>Mission-Oriented Sensor Networks and Systems: Art and Scie
	</front>	nce, Springer International Publishing, pp. 49-95</refcontent>
	<seriesInfo name="In" value="IPSN 2016: Proceedings of the 15th ACM/IEEE Inter	<seriesInfo name="DOI" value="10.1007/978-3-319-91146-5_3"/>
	national Conference on Information Processing in Sensor Networks, IEEE, pp. 1--1	</reference>
	2"/>
	<seriesInfo name="DOI" value="10.1109/IPSN.2016.7460720"/>
	</reference>
	<reference anchor="Ciolkosz19" >
	<front>
	<title>Integration of the RNFD Algorithm for Border Router Failure Detection
	with the RPL Standard for Routing IPv6 Packets</title>
	<author initials="P." surname="Ciolkosz" fullname="Piotr Ciolkosz">
	<organization></organization>
	</author>
	<date year="2019"/>
	</front>
	<seriesInfo name="Master's Thesis," value="University of Warsaw"/>
	</reference>
	<reference anchor="Paszkowska19" >
	<front>
	<title>Failure Handling in RPL Implementations: An Experimental Qualitative
	Study</title>
	<author initials="A." surname="Paszkowska" fullname="Agnieszka Paszkowska">
	<organization></organization>
	</author>
	<author initials="K." surname="Iwanicki" fullname="Konrad Iwanicki">
	<organization></organization>
	</author>
	<date year="2019"/>
	</front>
	<seriesInfo name="In" value="Mission-Oriented Sensor Networks and Systems: Art
	and Science (Habib M. Ammari ed.), Springer International Publishing, pp. 49--
	95"/>
	<seriesInfo name="DOI" value="10.1007/978-3-319-91146-5_3"/>
	</reference>
	<reference anchor="Whang90" >
	<front>
	<title>A Linear-time Probabilistic Counting Algorithm for Database Applicati
	ons</title>
	<author initials="K.-Y." surname="Whang" fullname="Kyu-Young Whang">
	<organization></organization>
	</author>
	<author initials="B.T." surname="Vander-Zanden" fullname="Brad T. Vander-Zan
	den">
	<organization></organization>
	</author>
	<author initials="H.M." surname="Taylor" fullname="Howard M. Taylor">
	<organization></organization>
	</author>
	<date year="1990"/>
	</front>
	<seriesInfo name="In" value="ACM Transactions on Database Systems"/>
	<seriesInfo name="DOI" value="10.1145/78922.78925"/>
	</reference>


		<reference anchor="Whang90">
		<front>
		<title>A Linear-time Probabilistic Counting Algorithm for Database A
		pplications</title>
		<author initials="K.-Y." surname="Whang" fullname="Kyu-Young Whang">
		<organization/>
		</author>
		<author initials="B.T." surname="Vander-Zanden" fullname="Brad T. Va
		nder-Zanden">
		<organization/>
		</author>
		<author initials="H.M." surname="Taylor" fullname="Howard M. Taylor"
		>
		<organization/>
		</author>
		<date year="1990"/>
		</front>
		<refcontent>ACM Transactions on Database Systems (TODS), vol. 15, no.
		2, pp. 208-229</refcontent>
		<seriesInfo name="DOI" value="10.1145/78922.78925"/>
		</reference>
		</references>
	</references>	</references>

		<section anchor="acknowledgements" numbered="false" toc="default">
		<name>Acknowledgements</name>
		<t>The author would like to acknowledge <contact fullname="Piotr
		Ciolkosz"/> and <contact fullname="Agnieszka Paszkowska"/>. Agnieszka
		contributed to deeper understanding and formally proving various aspects
		of RPL's behavior upon an LBR crash. Piotr developed a
		prototype implementation of RNFD dedicated for RPL to verify earlier
		performance claims.</t>
		</section>
	</back>	</back>


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	</rfc>	</rfc>

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