wdiff rfc9866.original rfc9866.txt

ROLL

Internet Engineering Task Force (IETF) K. Iwanicki
Internet-Draft
Request for Comments: 9866 University of Warsaw
Intended status:
Category: Standards Track 8 March 2025
Expires: 9 September 2025

RNFD:
ISSN: 2070-1721

Root Node Failure Detector (RNFD): Fast border router crash detection Detection of Border Router
Crashes in RPL
draft-ietf-roll-rnfd-07 the Routing Protocol for Low-Power and Lossy Networks (RPL)

Abstract

By and large, a correct operation of a network running RPL (the IPv6
routing protocol the Routing
Protocol for low-power Low-Power and lossy networks) 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 RNFD (the root
node failure detector), the Root Node
Failure Detector (RNFD), an extension to RPL that expedites detection
of border router crash detection 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
Options
Option for synchronizing this state among different nodes, and the
coordination algorithm itself.

Status of This Memo

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Internet Standards is available in Section 2 of six months RFC 7841.

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This Internet-Draft will expire on 9 September 2025.
https://www.rfc-editor.org/info/rfc9866.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Effects of LBR Crashes . . . . . . . . . . . . . . . . . 3
1.2. Design Principles . . . . . . . . . . . . . . . . . . . . 4
1.3. Other Solutions . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Protocol State Machine . . . . . . . . . . . . . . . . . 7
3.2. Counters and Communication . . . . . . . . . . . . . . . 8
4. The RNFD Option . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. General CFRC Requirements . . . . . . . . . . . . . . . . 9
4.2. Format of the Option . . . . . . . . . . . . . . . . . . 10
5. RPL Router Behavior . . . . . . . . . . . . . . . . . . . . . 12
5.1. Joining a DODAG Version and Changing the RNFD Role . . . 12
5.2. Detecting and Verifying Problems with the DODAG Root . . 13
5.3. Disseminating Observations and Reaching Agreement . . . . 15
5.4. DODAG Root’s Root's Behavior . . . . . . . . . . . . . . . . . . 16
5.5. Activating and Deactivating the Protocol on Demand . . . 17
5.6. Processing CFRCs of Incompatible Lengths . . . . . . . . 18
5.7. Summary of RNFD’s RNFD's Interactions with RPL . . . . . . . . . 19
5.8. Summary of RNFD’s RNFD's Constants . . . . . . . . . . . . . . . 20
6. Manageability Considerations . . . . . . . . . . . . . . . . 21
6.1. Role Assignment and CFRC Size Adjustment . . . . . . . . 21
6.2. Virtual DODAG Roots . . . . . . . . . . . . . . . . . . . 22
6.3. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 22
7. Security Considerations . . . . . . . . . . . . . . . . . . . 23
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.1.
9.1. Normative References . . . . . . . . . . . . . . . . . . 24
10.2.
9.2. Informative References . . . . . . . . . . . . . . . . . 25
Acknowledgements
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 26

1. Introduction

RPL is an IPv6 routing protocol for low-power Low-Power and lossy networks Lossy Networks
(LLNs) [RFC6550]. 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.

One of the main design principles adopted in RPL to minimize node
resource consumption is delegating much of the responsibility for
routing to LLN border routers Border Routers (LBRs). A network is organized into
destination-oriented directed acyclic graphs
Destination-Oriented Directed Acyclic Graphs (DODAGs), each
corresponding to an LBR and having all its paths terminate at the
LBR. To this end, every node is dynamically assigned a rank
representing its distance, measured in some metric, distance to a given LBR, measured in some metric,
with the LBR having the minimal rank, which reflects its role as the
DODAG root. The ranks allow each non-LBR node to select from among
its neighbors (i.e., nodes to which the node has links) those that
are closer to the LBR than the node itself: itself (i.e., the node’s node's parents
in the
graph. graph). The resulting DODAG paths, consisting of the node-parent node-
parent links, are utilized for routing packets upward: upward to the LBR and
outside the LLN. They are also used by nodes to periodically report
their connectivity upward to the LBR, which allows in turn for directing
packets downward, downward from the LBR to these nodes, for nodes (for instance, by means
of source routing [RFC6554]. [RFC6554]). All in all, not only do LBRs
participate in routing routing, but they also drive the process of DODAG
construction and maintenance underlying the protocol.

To play this central role, LBRs are expected to be more capable than
regular LLN nodes. They are assumed not to be constrained in
computing power, memory, and energy, which often entails a more
involved hardware-software architecture and tethered power supply.
This, however,
However, this also makes them prone to failures, especially since in
large deployments it
is often difficult to ensure a backup power supply for every LBR. LBR in
large deployments.

1.1. Effects of LBR Crashes

When an LBR crashes or, crashes, or more generally, fails in a way that prevents
other nodes in its DODAG from communicating with it, the nodes also
lose the ability to communicate with other Internet hosts. In
addition, a significant fraction of DODAG paths interconnecting the
nodes become invalid, as they pass through the dead LBR. The others
also degenerate as a result of DODAG repair attempts, which are bound
to fail. In effect, routing inside the DODAG also becomes largely
impossible. Consequently, it is desirable that an LBR crash be
detected by the nodes fast, so that they can leave the broken DODAG
and join another one or trigger additional application- or
deployment-dependent fallback mechanisms, thereby minimizing the
negative impact of the disconnection.

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 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 heavy traffic, and is difficult to
implement correctly [Iwanicki16] [Paszkowska19] [Ciolkosz19].

To start with, tearing down all DODAG paths requires each of the dead
LBR’s
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’ nodes' parent or
ancestor in the DODAG: 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 observed 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 from the DODAG the last link to the dead LBR from the DODAG
may be long. Subsequently 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
RPL's dedicated loop detection mechanisms [RFC6553], this also
requires traffic, 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 [RFC6202]. Overall, the behavior of the network when
handling an LBR crash is highly suboptimal, thereby not being in line
with RPL’s RPL's goals of minimizing resource consumption and reaction
latencies.

1.2. Design Principles

To address this issue, this document proposes an extension to RPL,
dubbed Root the "Root Node Failure Detector (RNFD). (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:

1. Explicitly coordinating LBR monitoring between nodes instead of
relying only on the emergent behavior resulting from their
independent operation.

2. Avoiding probing all links to the dead LBR so as to reduce the
tail latency when eliminating these links from the DODAG.

3. 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.

4. Minimizing changes to RPL’s RPL's existing algorithms by operating in
parallel and largely independently (in the background), background) and by
introducing few additional assumptions.

While these principles do improve RPL’s 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 RNFD's operation may result in false negatives, but these
situations are peculiar and will eventually be handled by RPL’s 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, time if needed, in
which case RPL’s RPL's operation is unaffected.

1.3. Other Solutions

Given the consequences of LBR failures, if possible, 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 RPL's so-called virtual DODAG roots
can help tolerate some failures of individual LBRs.

As mentioned previously, RNFD has been designed to be largely
independent of those solutions, solutions; that is, rather than aiming to be
their replacement, it RNFD can complement them. In particular, the
operation of RNFD with different variants of virtual DODAG roots is
covered in Section 6.2.

2. Terminology

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”,
“SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
“OPTIONAL”
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

The Terminology terminology used in this document is consistent with and
incorporates that described in “Terms "Terms Used in Routing for Low-Power
and Lossy Networks (LLNs)” Networks" [RFC7102], “RPL: "RPL: IPv6 Routing Protocol for
Low-Power Low-
Power and Lossy Networks” Networks" [RFC6550], and “The "The Routing Protocol for
Low-Power and Lossy Networks (RPL) Option for Carrying RPL
Information in Data-Plane Datagrams” Datagrams" [RFC6553]. Other terms in use used in
LLNs can be found in “Terminology "Terminology for Constrained-Node Networks” Networks"
[RFC7228].

In particular, the following acronyms appear in the document:

DIO

DIO: DODAG Information Object (a RPL message)

DIS

DIS: DODAG Information Solicitation (a RPL message)

DODAG

DODAG: Destination-Oriented Directed Acyclic Graph

LLN Low-power

LLN: Low-Power and Lossy Network

LBR

LBR: LLN Border Router

In addition, the document introduces the following concepts:

Sentinel

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 root's neighbor
that monitors the DODAG root’s root's status. There are normally
multiple Sentinels for a DODAG root. However, being the DODAG
root’s
root's neighbor need not imply being a Sentinel.

Acceptor

Acceptor: 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.

Locally Observed DODAG Root’s Root's State (LORS) (LORS): A node’s node's local knowledge
of the DODAG root’s root's status, specifying in particular whether the
DODAG root is up.

Conflict-Free Replicated Counter (CFRC) (CFRC): 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.

3. Overview

As mentioned previously, LBRs are DODAG roots in RPL, and hence 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 root's current condition, which is
referred to as Locally Observed DODAG Root’s Root's State (LORS), and
synchronizes its local knowledge with other nodes.

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 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 root's neighbor that monitors
its link with the root. The Thus, the DODAG root thus normally has multiple
Sentinels
Sentinels, but being its neighbor need not imply being a Sentinel.
An Acceptor node is in turn a node that is not a Sentinel. Acceptors thus
mainly collect and propagate Sentinels’ Sentinels' observations. More
information on Sentinel selection can be found in Section 6.1.

3.1. Protocol State Machine

The possible values of LORS and transitions between them are depicted
in Figure 1. States “UP” "UP" and “GLOBALLY DOWN” "GLOBALLY DOWN" can be attained by both
Sentinels and Acceptors; states “SUSPECTED DOWN” "SUSPECTED DOWN" and “LOCALLY DOWN” — "LOCALLY DOWN"
can be attained by Sentinels only.

+---------------------------------------------------------+
| |---------------------------+ 3a |
| +-----------------+---------+ 3b | |
| | 2b | v v v
+-+----+-+ 1 +---------+-+ +-----------+ +-+------+-+
| UP +---->+ SUSPECTED +---->+ LOCALLY +---->+ GLOBALLY |
| +<----+ DOWN | 2a | DOWN | 3c | DOWN |
+-+----+-+ 4a +-----------+ +-+---------+ +-+--------+
^ ^ | |
| | 4b | |
| +---------------------------+ 5 |
+--------------------------------------------------+

Figure 1: RNFD States and Transitions

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 “UP”. "UP". For a properly working DODAG root, the node remains in
state
“UP”. "UP".

However, when a node — acting (acting as Sentinel — a Sentinel) starts suspecting that
the root may have crashed, it changes its LORS to “SUSPECTED DOWN” "SUSPECTED DOWN"
(transition 1 in Figure 1). The transition from “UP” "UP" to “SUSPECTED
DOWN” "SUSPECTED
DOWN" can happen based on the node’s node's observations at either the data
plane, for
plane (for instance, link-layer triggers about missing hop-by-hop
acknowledgments for packets forwarded over the node’s node's link to the
root,
root) or at the control plane, for plane (for example, a significant growth in
the number of Sentinels already suspecting the root to be dead. dead). In
state
“SUSPECTED DOWN”, "SUSPECTED DOWN", the Sentinel node may verify its suspicion
and/or inform other nodes about the suspicion. When this has been
done, it changes its LORS to “LOCALLY DOWN” "LOCALLY DOWN" (transition 2a). In some
cases, the verification need not be performed and, performed, and as an
optimization, a direct transition from “UP” "UP" to “LOCALLY DOWN” "LOCALLY DOWN"
(transition 2b) can be done instead.

If sufficiently many a sufficient number of Sentinels have their LORS equal to “LOCALLY
DOWN”, "LOCALLY
DOWN", all nodes — Sentinels (Sentinels and Acceptors — Acceptors) consent globally that the
DODAG root must have crashed and set their LORS to “GLOBALLY
DOWN”, "GLOBALLY DOWN",
irrespective of the previous value (transitions 3a, 3b, and 3c).
State “GLOBALLY DOWN” "GLOBALLY DOWN" is terminal in that the only transition any
node can perform from this to another state (transition 5) takes
place when the node joins a new DODAG version. When a node is in
state “GLOBALLY DOWN”, "GLOBALLY DOWN", RNFD forces RPL to maintain an infinite rank
and no parent, thereby preventing routing packets upward in the
DODAG. In other words, this state represents a situation in which
all non-root nodes agree that the current DODAG version is unusable,
and unusable;
hence, to recover, the root has to give a proof of being alive by
initiating a new DODAG Version.

In contrast, if a node — either (either a Sentinel or Acceptor — Acceptor) is in state
“UP”,
"UP", RNFD does not influence RPL’s RPL's packet forwarding: forwarding; a node can
route packets upward if it has a parent. The same is true for states
“SUSPECTED DOWN”
"SUSPECTED DOWN" and “LOCALLY DOWN”, "LOCALLY DOWN", attainable only by Sentinels.
Finally, while in any of the two states, a Sentinel node may observe
some activity of the DODAG root, root and hence decide that its suspicion
is a mistake. In such a case, it returns to state “UP” "UP" (transitions
4a and 4b).

3.2. Counters and Communication

To enable arriving at a global conclusion that the DODAG root has
crashed (i.e., transiting to state “GLOBALLY DOWN”), "GLOBALLY DOWN"), all nodes count
locally and synchronize among each other the number of Sentinels
considering the root to be dead (i.e., those in state “LOCALLY
DOWN”). "LOCALLY
DOWN"). This process employs structures referred to as conflict-free
replicated counters Conflict-Free
Replicated Counters (CFRCs). They are stored and modified
independently by each node and are disseminated throughout the
network in options added to RPL link-local control messages: DODAG
Information Objects (DIOs) and DODAG Information Solicitations
(DISs). Upon reception of such an option from its neighbor, a node
merges the received counter with its local one, thereby obtaining a
new content for its local counter.

The merging operation is idempotent, commutative, and associative.
Moreover, all possible counter values are partially ordered. This
enables ensuring eventual consistency of the counters across all
nodes, irrespective of the particular sequence of merges, shape of
the DODAG, or general network topology. In effect, as long as the
network is connected, all nodes will be able to arrive at the same
conclusion regarding the DODAG root, in particular, even particular when no two
Sentinels have a direct link with each other.

Each node in RNFD maintains two CFRCs for a DODAG:

* PositiveCFRC, counting

PositiveCFRC: Counts Sentinels that consider or have previously
considered the root node as alive in the current DODAG Version,

* NegativeCFRC, counting Version.

NegativeCFRC: Counts Sentinels that consider or have previously
considered the root node as dead in the current DODAG Version.

The PositiveCFRC is always greater than or equal to the NegativeCFRC
in terms of the partial order defined for the counters. The
difference between the value of the PositiveCFRC and the value of the
NegativeCFRC is thus nonnegative and estimates the number of
Sentinels that still consider the DODAG root node as alive.

4. The RNFD Option

RNFD state synchronization between nodes takes place through the RNFD
Option. It is a new type of RPL Control Message Options 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 sender's CFRCs.

4.1. General CFRC Requirements

CFRCs in RNFD MUST support the following operations:

value(c)
Returns a nonnegative integer value corresponding to the number of
nodes counted by a given CFRC, c.

zero()
Returns a CFRC that counts no nodes, that is, has its value equal
to 0.

self()
Returns a CFRC that counts only the node executing the operation.

infinity()
Returns a CFRC that counts all possible nodes and represents a
special value, infinity.

merge(c1, c2)
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).

compare(c1, c2)
Returns the result of comparing c1 to c2.

saturated(c)
Returns TRUE if a given CFRC, c, is saturated (i.e., no more new
nodes should be counted by it) or it); returns FALSE otherwise.

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

* 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);

* 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);

* equal, if c1 and c2 are the same (i.e., they count the same
nodes); or

* incomparable, otherwise.

In particular, zero() is smaller than all other values values, and
infinity() is greater than any other value.

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

* idempotence: c1 = merge(c1, c1);

* commutativity: merge(c1, c2) = merge(c2, c1); and

* associativity: merge(c1, merge(c2, c3)) = merge(merge(c1, c2),
c3).

In particular, merge(c, zero()) always equals c c, while merge(c,
infinity()) always equals infinity().

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 [Whang90].

4.2. Format of the Option

The format of the RNFD Option conforms to the generic format of RPL
Control Message Options:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = TBD1 0x0E | Option Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+ +
| PosCFRC, NegCFRC (Variable Length*) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Format of the RNFD Option

The '*' "*" denotes that, if present, the fields have equal lengths.

Figure 2: Format of the RNFD Option

Option Type TBD1 Type: 0x0E

Option Length Length: 8-bit unsigned integer. Denotes the length of the
option in octets octets, excluding the Option Type and Option Length
fields. Its value MUST be even. A value of 0 denotes that RNFD
is disabled in the current DODAG Version.

PosCFRC, NegCFRC NegCFRC: Two variable-length, octet-aligned bit arrays
carrying the sender’s sender's PositiveCFRC and NegativeCFRC, respectively.

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 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.

Furthermore, for any bit equal to 1 in the NegCFRC, the bit with the
same index MUST also be equal to 1 also in the PosCFRC. Any unused bits
(i.e., the bits beyond the actual bit length of each of the arrays)
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.

The CFRC operations are defined for such bit arrays of a given length
as follows:

value(c)
Returns the smallest integer value not less than -LT*ln(L0/
LT), -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 c, and LT is the bit
length of the array.

zero()
Returns an array with all bits equal to 0.

self()
Returns an array with a single bit, selected uniformly at random,
equal to 1.

infinity()
Returns an array with all bits equal to 1.

merge(c1, c2)
Returns a bit array that constitutes a bitwise OR of c1 and c2, that 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.

compare(c1, c2)
Returns:

* equal equal, if each bit of c1 is equal to the corresponding bit of
c2;

* less less, if c1 and c2 are not equal and, equal, and for each bit equal to 1
in c1, the corresponding bit in c2 is also equal to 1;

* greater greater, if c1 and c2 are not equal and, equal, and for each bit equal to
1 in c2, the corresponding bit in c1 is also equal to 1; or

* incomparable, otherwise.

saturated(c)
Returns TRUE, TRUE if more than RNFD_CFRC_SATURATION_THRESHOLD of the
bits in c are equal to 1, or
FALSE, 1; returns FALSE otherwise.

5. RPL Router Behavior

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.

5.1. Joining a DODAG Version and Changing the RNFD Role

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

The role may then change between Acceptor and Sentinel at any time.
However, while a switch from Sentinel to Acceptor has no
preconditions, in order for a switch from Acceptor to Sentinel to be
possible, _all_ of the following conditions MUST hold:

1. LORS is “UP”; "UP";

2. saturated(PositiveCFRC) is FALSE;

3. a neighbor entry for the DODAG root is present in RPL’s RPL's DODAG
parent set; and

4. the neighbor is considered reachable via its link-local IPv6
address.

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 MUST add itself
to its PositiveCFRC as follows. It MUST generate a new CFRC value,
selfc = self(), and it MUST replace its PositiveCFRC, denoted oldpc, 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 node's value
of LORS before the switch:

* for “GLOBALLY DOWN”, For "GLOBALLY DOWN", the node MUST NOT modify its LORS,
PositiveCFRC, and NegativeCFRC; NegativeCFRC.

* for “LOCALLY DOWN”, For "LOCALLY DOWN", the node MUST set its LORS to “UP” "UP" but MUST
NOT modify its PositiveCFRC and NegativeCFRC; NegativeCFRC.

* for “UP” For "UP" and “SUSPECTED DOWN”, "SUSPECTED DOWN", the node MUST set its LORS to “UP”, "UP"
and MUST NOT modify it its PositiveCFRC, but it MUST add itself to
NegativeCFRC, that
NegativeCFRC. That is, it MUST replace its NegativeCFRC, denoted oldnc, NegativeCFRC (denoted
oldnc) with newnc = merge(oldnc, selfc), where selfc is the
counter generated with self() when the node last added itself to
its PositiveCFRC.

5.2. Detecting and Verifying Problems with the DODAG Root

Only nodes that are Sentinels take an active part in detecting
crashes of the DODAG Root; root; Acceptors just disseminate their
observations, reflected in the CFRCs.

The DODAG root monitoring SHOULD be based on both internal inputs,
notably the values of CFRCs and LORS, and external inputs, such as
triggers from RPL and other protocols. External input monitoring
SHOULD be performed preferably in a reactive fashion, also
independently of RPL, and at both the data plane and control plane.
In particular, it is RECOMMENDED that RNFD be directly notified of
events relevant to the routing adjacency maintenance mechanisms on
which RPL relies, such as Layer 2 (L2) triggers [RFC5184] or the
Neighbor Unreachability Detection [RFC4861] 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
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
detecting potential problems even when normally it would not exchange
packets over the link with the DODAG root during some period. In
particular, RNFD SHOULD conclude that there may be problems with the
DODAG root, 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”. "UP".

Whenever having its LORS is set to “UP” "UP" and RNFD concludes — based (based on either
external or internal inputs — inputs) that there may be problems with the link
with the DODAG root, it MUST set its LORS to either “SUSPECTED
DOWN” to "SUSPECTED DOWN"
or, as an optimization, to “LOCALLY DOWN”. "LOCALLY DOWN".

The “SUSPECTED DOWN” "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 MUST set its LORS to either “UP”, "UP", if the link has
been confirmed not to be down, or “LOCALLY DOWN”, "LOCALLY DOWN", otherwise. The
verification can be performed, for example, by transmitting RPL DIS
or ICMPv6 Echo Request messages to the DODAG root’s 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 RECOMMENDED that
the “SUSPECTED DOWN” "SUSPECTED DOWN" value of LORS is be attained and verification takes take
place if RNFD’s 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 MAY be skipped, with the
node’s
node's LORS effectively changing from “UP” "UP" directly to “LOCALLY
DOWN”. "LOCALLY
DOWN".

For consistency with RPL, when detecting potential problems with the
DODAG root, RNFD also must make use of RPL’s RPL's independent knowledge.
More specifically, a node MUST switch its LORS from “UP” "UP" or
“SUSPECTED DOWN”
"SUSPECTED DOWN" directly to “LOCALLY DOWN” "LOCALLY DOWN" if a neighbor entry for
the DODAG root is removed from RPL’s RPL's DODAG parent set or the neighbor
ceases to be considered reachable via its link-local IPv6 address.

Finally, while having its LORS already equal to “LOCALLY DOWN”, "LOCALLY DOWN", a
node may make 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”
"UP" but MUST NOT do this before the previous conditions 2–4 2-4
necessary for a node to change its role from Acceptor to Sentinel all
hold (see Section 5.1).

To appropriately account for the node’s node's observations on the state of
the DODAG root, the aforementioned LORS transitions are accompanied
by changes to the node’s node's local CFRCs as follows. Transitions between
“UP”
"UP" and “SUSPECTED DOWN” "SUSPECTED DOWN" do not affect any either of the two CFRCs.
During a switch from “UP” "UP" or “SUSPECTED DOWN” "SUSPECTED DOWN" to “LOCALLY DOWN”, in turn, "LOCALLY DOWN", the
node MUST add itself to its NegativeCFRC, as explained previously.
By symmetry, if there is a transition from “LOCALLY
DOWN” "LOCALLY DOWN" to “UP”, "UP",
the node MUST add itself to its PositiveCFRC, again, as explained
previously.

Such changes to a node’s node's local CFRCs, if performed repeatedly due to
incorrect decisions regarding the status of the node’s 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” "UP" and “SUSPECTED DOWN” "SUSPECTED DOWN" to “LOCALLY DOWN”. "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” "SUSPECTED DOWN" state, or can
limit the number of transitions per node, possibly in an adaptive
fashion.

5.3. Disseminating Observations and Reaching Agreement

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 (acting as a
Sentinel or Acceptor — Acceptor) MUST update its PositiveCFRC and NegativeCFRC
to respectively newpc = merge(oldpc, recvpc) and newnc = merge(oldnc, recvnc), where
respectively. Here, oldpc and oldnc are the values of the node’s node's
PositiveCFRC and NegativeCFRC before the update, while recvpc and
recvnc are the received values of option fields PosCFRC and NegCFRC,
respectively.

In effect, the node’s 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 MUST change its LORS to “GLOBALLY DOWN” "GLOBALLY DOWN"
and set its PositiveCFRC and NegativeCFRC both to infinity().

The “GLOBALLY DOWN” "GLOBALLY DOWN" value of LORS is terminal: terminal; the node MUST NOT
change it and 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.

Since the RNFD Option is embedded, among others, in RPL DIO control
messages, updates to a node’s node's CFRCs may affect the sending schedule
of these messages, which is driven by the DIO Trickle timer
[RFC6206]. It is RECOMMENDED to use for RNFD a dedicated Trickle
timer, timer for
RNFD that is different from RPL’s 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 SHOULD NOT
be smaller than those of RPL’s RPL's original DIO Trickle timer. In
contrast, in the absence of the dedicated Trickle timer for RNFD, an
implementation SHOULD ensure that the RNFD Option is present in
multicast DIO messages sufficiently often to quickly propagate
changes to the node’s CFRCs, and notably 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 (either
the dedicated one or RPL’s RPL's original one, depending on the
implementation —
implementation) simply as “Trickle timer”. "Trickle timer". In particular, a node
MUST reset its Trickle timer when it changes its LORS to “GLOBALLY
DOWN”, "GLOBALLY
DOWN", so that information about the detected crash of the DODAG root
is disseminated in the DODAG fast. Likewise, a node SHOULD reset its
Trickle timer when any of its local CFRCs changes change significantly.

5.4. DODAG Root’s Root's Behavior

The DODAG root node MUST assume the role of Acceptor in RNFD and MUST
NOT ever switch this role. It MUST also monitor its LORS and local
CFRCs, so that it can react to various events.

To start with, the DODAG root MUST generate a new DODAG Version,
thereby restarting the protocol, if it changes its LORS to “GLOBALLY
DOWN”, "GLOBALLY
DOWN", which may happen when the root has restarted after a crash or
the nodes have falsely detected its crash. It MAY 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.

Furthermore, the DODAG root SHOULD 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 Section 6.1).

In general, issuing a new DODAG Version effectively restarts RNFD.
The
Thus, the DODAG root MAY thus also perform this operation also in other
situations.

5.5. Activating and Deactivating the Protocol on Demand

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;
other nodes MUST comply.

More specifically, when a non-root node joins a DODAG Version, RNFD
at the node is initially inactive. The node MUST NOT 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 MUST 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 MUST 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 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 Length field equal to zero, the
protocol MUST remain deactivated for the entire time the node belongs
to the current DODAG Version.

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 may not know the desired CFRC length — length; see
Section 5.6). 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 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 MAY
reset its Trickle timer to this end but also MAY also use some reactive
mechanisms, for
mechanisms. For example, replying it MAY 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.

5.6. Processing CFRCs of Incompatible Lengths

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 Section 4.2) do not
necessitate this. This fact is made use of not only in the
mechanisms for activating and deactivating the protocol (see
Section 5.5), but also in mechanisms for dynamic adjustments of
CFRCs, which aim to enable deployment-specific policies (see
Section 6.1). 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 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 MUST react as follows.

If the bit length of fields PosCFRC and NegCFRC is the same as that
of the node’s node's local PositiveCFRC and NegativeCFRC, then the node MUST
perform the merges, as detailed previously (see Section 5.3).

If the bit length of fields PosCFRC and NegCFRC is smaller than that
of the node’s node's local PositiveCFRC and NegativeCFRC, then the node MUST
ignore the option and MAY reset its Trickle timer.

If the bit length of fields PosCFRC and NegCFRC is greater than that
of the node’s node's local PositiveCFRC and NegativeCFRC, then the node MUST
extend the bit length of its local CFRCs to be equal to that in the
option and set the CFRCs as follows:

* If the node’s node's LORS is “GLOBALLY DOWN”, "GLOBALLY DOWN", then both of its local
CFRCs MUST be set to infinity().

* Otherwise, they both MUST be set to zero(), and the node MUST
account for itself in so initialized CFRCs. More specifically, if
the node is a Sentinel, then it MUST add itself to its
PositiveCFRC, as detailed previously. In addition, if its LORS is “LOCALLY
DOWN”,
"LOCALLY DOWN", then it MUST also add itself to its NegativeCFRC,
again, as explained previously. Finally, the node MUST perform
merges of its local CFRCs and the ones received in the option (see
Section 5.3) and MAY reset its Trickle timer.

In contrast, if the node is unable to extend its local CFRCs, for
example, because it lacks resources, then it MUST stop participating
in RNFD, that RNFD. That is, until it joins a new DODAG Version, it MUST NOT
send the RNFD Option and MUST ignore this option in received
messages.

A DODAG root node can be requested to increase the bit length of its
CFRCs externally, as part of the management policies (see
Section 6.1). If it cannot fulfill such a request, then it is MUST NOT
stop participating in RFND RNFD and SHOULD 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, respectively, is different from or equal to “GLOBALLY
DOWN”. "GLOBALLY DOWN",
respectively. In the latter case, given the earlier rules governing
the
root’s root's behavior upon reaching the “GLOBALLY DOWN” "GLOBALLY DOWN" state (cf.
Section 5.4), 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”. "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().

5.7. Summary of RNFD’s RNFD's Interactions with RPL

In summary, RNFD interacts with RPL in the following manner:

* While having its LORS equal to “GLOBALLY DOWN”, "GLOBALLY DOWN", RNFD prevents RPL
from routing packets and advertising upward routes in the
corresponding DODAG (see Section 5.3).

* In some scenarios, RNFD triggers RPL to issue a new DODAG Version
(see Section 5.4).

* Depending on the implementation, RNFD may cause RPL’s RPL's DIO Trickle
timer resets (see Section Sections 5.3, Section 5.5, and Section 5.6).

* RNFD monitors events relevant to routing adjacency maintenance as
well as those affecting RPL’s RPL's DODAG parent set (see Section Sections 5.1
and Section 5.2).

* Using RNFD entails embedding the RNFD Option into link-local RPL
control messages (see Section 4.2).

5.8. Summary of RNFD’s RNFD's Constants

The following is a summary of RNFD’s RNFD's constants:

RNFD_CONSENSUS_THRESHOLD

RNFD_CONSENSUS_THRESHOLD: A threshold concerning the value of the
fraction value(NegativeCFRC)/value(PositiveCFRC). If the value at
a Sentinel or Acceptor node reaches the threshold, then the node’s node's
LORS is set to “GLOBALLY DOWN”, "GLOBALLY DOWN", which implies that consensus has
been reached on the DODAG root node being down (see Section 5.3).
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, the higher the value value, the longer the
detection period but the lower the risk of false positives.

RNFD_SUSPICION_GROWTH_THRESHOLD

RNFD_SUSPICION_GROWTH_THRESHOLD: 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 node's LORS was last set to “UP”, "UP", then the node’s node's
LORS is set to “SUSPECTED DOWN” "SUSPECTED DOWN" or “LOCALLY DOWN”, "LOCALLY DOWN", which implies
that the node starts suspecting or assumes a crash of the DODAG
root (see Section 5.2). The higher the value value, the longer the
duration of detecting true crashes but the lower the risk of
increased traffic due to verifying false suspicions. 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.

RNFD_CFRC_SATURATION_THRESHOLD

RNFD_CFRC_SATURATION_THRESHOLD: 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 Section 5.4).
The default value of the threshold is 0.63. The higher the value is, value,
the higher the probability of bit collisions, collisions and hence the more
erratic the results of function value(c) may be.

The means of configuring the constants at individual nodes are
outside the scope of this document.

6. Manageability Considerations

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.

6.1. Role Assignment and CFRC Size Adjustment

One approach to node role and CFRC size selection is to manually
designate specific nodes as Sentinels in RNFD, assuming that they
will have chances to satisfy the necessary conditions for attaining
this role (see Section 5.1), and fixing the CFRC bit length to
accommodate these nodes.

Another approach is to automate the selection process: in process. In principle,
any node satisfying the necessary conditions for becoming a Sentinel
(see Section 5.1) 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 — without
additional measures — may degrade RNFD’s performance.
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 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 times, the bit length of the CFRCs by the DODAG
root if PositiveCFRC becomes saturated with little or no growth in NegativeCFRC, which
NegativeCFRC. This does not require issuing a new DODAG Version but
lengthens the RNFD Option. In this way, again, a sufficient bit
length can be dynamically discovered 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 Section 4.2).

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” "UP" and
“LOCALLY DOWN”
"LOCALLY DOWN" or switches between Acceptor and Sentinel roles, which
gradually saturates CFRCs. Although as As a mitigation mitigation, the number of such
transitions and switches per node MAY be limited, limited; however, having
Sentinels be stable SHOULD be preferred.

6.2. Virtual DODAG Roots

RPL allows a DODAG to have a so-called virtual root, "virtual root", that is, a
collection of nodes coordinating to act as a single root of the
DODAG. The details of the coordination process are left open in the
specification [RFC6550] but,
[RFC6550], but from RNFD’s RNFD's perspective, two possible realizations are
worth consideration:

* 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,
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.

* 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.

In the first realization, RNFD’s RNFD's operation is largely unaffected.
The necessary conditions for a node to become a Sentinel
(Section 5.1) guarantee that only the current primary root node is
monitored by the protocol. This SHOULD be taken into account in the
policies for node role assignment, CFRC size selection, and,
possibly, the setting of the two thresholds (Section 5.8). Moreover,
when a new primary has been elected, a new DODAG Version MUST be
issued to avoid polluting CFRCs with observations on the previous primary, a new DODAG Version MUST be issued.
primary.

In the second realization, the fact that the virtual root consists of
multiple nodes is transparent to RNFD. Therefore, employing RNFD is in
such a setting can be beneficial only if the nodes comprising the
virtual root may suffer from correlated crashes, for instance, due to
global power outages.

6.3. Monitoring

For monitoring the operation of RNFD, its implementation SHOULD
provide the following information about a node:

* whether the protocol is active, and

* whether LORS is “GLOBALLY DOWN”, "GLOBALLY DOWN".

This information is accompanied by the recommended monitoring
parameters provided by RPL itself [RFC6550], notably the DODAG
Version number and the Rank. To offer even finer-grained visibility
into RNFD’s RNFD's state at the node, the implementation MAY in addition also provide:

* the assigned role (i.e., Sentinel or Acceptor),

* the exact value of LORS (i.e., “UP”, “SUSPECTED DOWN”, “LOCALLY
DOWN”, "UP", "SUSPECTED DOWN", "LOCALLY
DOWN", or “GLOBALLY DOWN”), "GLOBALLY DOWN"),

* the two CFRCs (i.e., PositiveCFRC and NegativeCFRC), and

* the constants listed in Section 5.8.

7. Security Considerations

RNFD is an extension to RPL and is thus both is vulnerable to and benefits
from the security issues and solutions described in [RFC6550] and
[RFC7416]. 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.

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
RECOMMENDED to use RPL security mechanisms if there is a risk that
control information might be modified or spoofed.

In this context, RNFD’s 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 pass undetected, in turn, 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 root's crash, at least if RPL’s RPL's other
components are not attacked as well.

8. IANA Considerations

To represent the RNFD Option,

IANA is requested to allocate has allocated the following value
TBD1 from in the “RPL "RPL Control Message Options”
Options" registry
(https://www.iana.org/assignments/rpl/rpl.xhtml#control-message-
options) of within the “Routing "Routing Protocol for Low Power and
Lossy Networks
(RPL)” (RPL)" registry group.

9. Acknowledgements

The authors would like to acknowledge Piotr Ciolkosz and Agnieszka
Paszkowska. Agnieszka contributed to deeper understanding and
formally proving various aspects of RPL’s behavior upon an LBR crash.
Piotr in turn developed a prototype implementation of group
(https://www.iana.org/assignments/rpl).

+=======+=============+===========+
| Value | Meaning | Reference |
+=======+=============+===========+
| 0x0E | RNFD dedicated
for RPL to verify earlier performance claims.

10. Option | RFC 9866 |
+-------+-------------+-----------+

Table 1

9. References

10.1.

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
"The Trickle Algorithm", RFC 6206, DOI 10.17487/RFC6206,
March 2011, <https://www.rfc-editor.org/info/rfc6206>.

[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.

[RFC6553] Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
Power and Lossy Networks (RPL) Option for Carrying RPL
Information in Data-Plane Datagrams", RFC 6553,
DOI 10.17487/RFC6553, March 2012,
<https://www.rfc-editor.org/info/rfc6553>.

[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554,
DOI 10.17487/RFC6554, March 2012,
<https://www.rfc-editor.org/info/rfc6554>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.2.

9.2. Informative References

[Ciolkosz19]
Ciolkosz, P., "Integration of the RNFD Algorithm for
Border Router Failure Detection with the RPL Standard for
Routing IPv6 Packets", Master's Thesis, University of
Warsaw, 2019.

[Iwanicki16]
Iwanicki, K., "RNFD: Routing-layer detection Routing-Layer Detection of DODAG
(root) node failures
(Root) Node Failures in low-power wireless networks",
In IPSN 2016: Proceedings of the Low-Power Wireless Networks", 2016
15th ACM/IEEE International Conference on Information
Processing in Sensor Networks, IEEE, Networks (IPSN), pp. 1--12, 1-12,
DOI 10.1109/IPSN.2016.7460720, 2016,
<https://doi.org/10.1109/IPSN.2016.7460720>.

[Paszkowska19]
Paszkowska, A. and K. Iwanicki, "Failure Handling in RPL
Implementations: An Experimental Qualitative Study", In
Mission-Oriented Sensor Networks and Systems: Art and
Science (Habib M. Ammari ed.),
Science, Springer International Publishing, pp. 49--95, 49-95,
DOI 10.1007/978-3-319-91146-5_3, 2019,
<https://doi.org/10.1007/978-3-319-91146-5_3>.

[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.

[RFC5184] Teraoka, F., Gogo, K., Mitsuya, K., Shibui, R., and K.
Mitani, "Unified Layer 2 (L2) Abstractions for Layer 3
(L3)-Driven Fast Handover", RFC 5184,
DOI 10.17487/RFC5184, May 2008,
<https://www.rfc-editor.org/info/rfc5184>.

[RFC6202] Loreto, S., Saint-Andre, P., Salsano, S., and G. Wilkins,
"Known Issues and Best Practices for the Use of Long
Polling and Streaming in Bidirectional HTTP", RFC 6202,
DOI 10.17487/RFC6202, April 2011,
<https://www.rfc-editor.org/info/rfc6202>.

[RFC7102] Vasseur, JP., "Terms Used in Routing for Low-Power and
Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
2014, <https://www.rfc-editor.org/info/rfc7102>.

[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.

[RFC7416] Tsao, T., Alexander, R., Dohler, M., Daza, V., Lozano, A.,
and M. Richardson, Ed., "A Security Threat Analysis for
the Routing Protocol for Low-Power and Lossy Networks
(RPLs)", RFC 7416, DOI 10.17487/RFC7416, January 2015,
<https://www.rfc-editor.org/info/rfc7416>.

[Whang90] Whang, K.-Y., Vander-Zanden, B.T., and H.M. Taylor, "A
Linear-time Probabilistic Counting Algorithm for Database
Applications", In ACM Transactions on Database Systems, Systems
(TODS), vol. 15, no. 2, pp. 208-229,
DOI 10.1145/78922.78925, 1990,
<https://doi.org/10.1145/78922.78925>.

Acknowledgements

The author would like to acknowledge Piotr Ciolkosz and 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.

Author's Address

Konrad Iwanicki
University of Warsaw
Banacha 2
02-097 Warszawa
Poland
Phone: +48 22 55 44 428
Email: iwanicki@mimuw.edu.pl