Internet Engineering Task Force (IETF) A. Brandt

Internet Engineering Task Force (IETF) A. Brandt
Request for Comments: 7733 Sigma Designs
Category: Standards Track E. Baccelli
ISSN: 2070-1721 INRIA
R. Cragie
ARM Ltd.
P. van der Stok
Consultant
February 2016

Applicability Statement: The Use of the Routing Protocol
for Low-Power and Lossy Networks (RPL) Protocol Suite
in Home Automation and Building Control

Abstract

The purpose of this document is to provide guidance in the selection
and use of protocols from the Routing Protocol for Low-Power and
Lossy Networks (RPL) protocol suite to implement the features
required for control in building and home environments.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7733.

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Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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Table of Contents

1. Introduction ....................................................4
1.1. Relationship to Other Documents ............................5
1.2. Terminology ................................................6
1.3. Required Reading ...........................................6
1.4. Requirements That Are Out of Scope .........................6
2. Deployment Scenario .............................................6
2.1. Network Topologies .........................................7
2.2. Traffic Characteristics ....................................8
2.2.1. General .............................................9
2.2.2. Source-Sink (SS) Communication Paradigm ............10
2.2.3. Publish-Subscribe (PS, or Pub/Sub)
Communication Paradigm .............................10
2.2.4. Peer-to-Peer (P2P) Communication Paradigm ..........10
2.2.5. Peer-to-Multipeer (P2MP) Communication Paradigm ....11
2.2.6. Additional Considerations: Duocast and N-Cast ......11
2.2.7. RPL Applicability per Communication Paradigm .......11
2.3. Layer 2 Applicability .....................................13
3. Using RPL to Meet Functional Requirements ......................13
4. RPL Profile ....................................................14
4.1. RPL Features ..............................................14
4.1.1. RPL Instances ......................................15
4.1.2. Storing vs. Non-Storing Mode .......................15
4.1.3. DAO Policy .........................................15
4.1.4. Path Metrics .......................................15
4.1.5. Objective Function .................................16
4.1.6. DODAG Repair .......................................16
4.1.7. Multicast ..........................................16
4.1.8. Security ...........................................17
4.1.9. P2P Communications .................................21
4.1.10. IPv6 Address Configuration ........................21
4.2. Layer 2 Features ..........................................21
4.2.1. Specifics about Layer 2 ............................21
4.2.2. Services Provided at Layer 2 .......................21
4.2.3. IPv6 over Low-Power Wireless Personal Area
Network (6LoWPAN) Options Assumed ..................21
4.2.4. Mesh Link Establishment (MLE) and Other Things .....21
4.3. Recommended Configuration Defaults and Ranges .............21
4.3.1. Trickle Parameters .................................22
4.3.2. Other Parameters ...................................22
5. MPL Profile ....................................................23
5.1. Recommended Configuration Defaults and Ranges .............23
5.1.1. Real-Time Optimizations ............................23
5.1.2. Trickle Parameters .................................23
5.1.3. Other Parameters ...................................24
6. Manageability Considerations ...................................25

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7. Security Considerations ........................................25
7.1. Security Considerations during Initial Deployment .........26
7.2. Security Considerations during Incremental Deployment .....27
7.3. Security Considerations for P2P Implementations ...........27
7.4. MPL Routing ...............................................27
7.5. RPL Security Features .....................................27
8. Other Related Protocols ........................................28
9. References .....................................................28
9.1. Normative References ......................................28
9.2. Informative References ....................................32
Appendix A. RPL Shortcomings in Home and Building Deployments .....35
A.1. Risk of Undesirable Long P2P Routes ........................35
A.1.1. Traffic Concentration at the Root ......................35
A.1.2. Excessive Battery Consumption in Source Nodes ..........35
A.2. Risk of Delayed Route Repair ...............................35
A.2.1. Broken Service .........................................36
Appendix B. Communication Failures ................................36
Acknowledgements ..................................................38
Authors' Addresses ................................................38

1. Introduction

The primary purpose of this document is to give guidance in the use
of the Routing Protocol for Low-Power and Lossy Networks (RPL)
protocol suite in two application domains:

o Home automation

o Building automation

The guidance is based on the features required by the requirements
documents "Home Automation Routing Requirements in Low-Power and
Lossy Networks" [RFC5826] and "Building Automation Routing
Requirements in Low-Power and Lossy Networks" [RFC5867],
respectively. The Advanced Metering Infrastructure is also
considered where appropriate. The applicability domains distinguish
themselves in the way they are operated, their performance
requirements, and the most likely network structures. An abstract
set of distinct communication paradigms is then used to frame the
applicability domains.

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Home automation and building automation application domains share a
substantial number of properties:

o In both domains, the network can be disconnected from the ISP and
must still continue to provide control to the occupants of the
home or building. Routing needs to be possible independent of the
existence of a border router.

o Both domains are subject to unreliable links but require instant
and very reliable reactions. This has an impact on routing
because of timeliness and multipath routing.

The differences between the two application domains mostly appear in
commissioning, maintenance, and the user interface, which do not
typically affect routing. Therefore, the focus of this applicability
document is on reliability, timeliness, and local routing.

It should be noted that adherence to the guidance in this document
does not necessarily guarantee fully interoperable solutions in home
automation networks and building control networks and that additional
rigorous and managed programs will be needed to ensure
interoperability.

1.1. Relationship to Other Documents

The Routing Over Low power and Lossy networks (ROLL) working group
has specified a set of routing protocols for Low-Power and Lossy
Networks (LLNs) [RFC6550]. This applicability text describes a
subset of those protocols and the conditions under which the subset
is appropriate, and it provides recommendations and requirements for
the accompanying parameter value ranges.

In addition, [RFC6997] was written specifically as an extension to
core RPL [RFC6550] and provides a solution for reactive discovery of
point-to-point routes in LLNs. The present applicability document
provides recommendations and requirements for the accompanying
parameter value ranges.

[RFC7416] describes a common set of security threats. The
applicability statements provided in Section 4.1.8.2.2 of this
document complement [RFC7416] by describing preferred security
settings and solutions within the applicability statement conditions.
This applicability statement recommends lighter-weight security
solutions appropriate for home and building environments and
indicates why these solutions are appropriate.

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1.2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

Additionally, this document uses terminology from [RFC6997],
[RFC7731], [RFC7102], [IEEE802.15.4], and [RFC6550].

1.3. Required Reading

Applicable requirements are described in [RFC5826] and [RFC5867]. A
survey of the application field is described in [BC-Survey].

1.4. Requirements That Are Out of Scope

The considered network diameter is limited to a maximum diameter of
10 hops and a typical diameter of five hops; this captures the most
common cases in home automation and building control networks.

This document does not consider the applicability of RPL-related
specifications for urban and industrial applications [RFC5548]
[RFC5673], which may exhibit significantly larger network diameters.

2. Deployment Scenario

The use of communications networks in buildings is essential to
satisfy energy-saving regulations. Environmental conditions of
buildings can be adapted to suit the comfort of the individuals
present inside. Consequently, when no one is present, energy
consumption can be reduced. Cost is the main driving factor behind
deployment of wireless networking in buildings, especially in the
case of retrofitting, where wireless connectivity saves costs
incurred due to cabling and building modifications.

A typical home automation network is comprised of less than
100 nodes. Large building deployments may span 10,000 nodes, but to
ensure uninterrupted service of light and air conditioning systems in
individual zones of the building, nodes are typically organized in
subnetworks. Each subnetwork in a building automation deployment
typically contains tens to hundreds of nodes and, for critical
operations, may operate independently from the other subnetworks.

The main purpose of the home or building automation network is to
provide control over light and heating/cooling resources. User
intervention via wall controllers is combined with movement, light
and temperature sensors to enable automatic adjustment of window
blinds, reduction of room temperature, etc. In general, the sensors

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and actuators in a home or building typically have fixed physical
locations and will remain in the same home or building automation
network.

People expect an immediate and reliable response to their presence or
actions. For example, a light not switching on after entry into a
room may lead to confusion and a profound dissatisfaction with the
lighting product.

Monitoring of functional correctness is at least as important as
timely responses. Devices typically communicate their status
regularly and send alarm messages to notify users or implementers
that a malfunction of controlled equipment or a controlled network
has occurred.

In building control, the infrastructure of the building management
network can be shared with security/access, Internet Protocol (IP)
telephony, and fire/alarm networks. This approach has a positive
impact on the operation and cost of the network; however, care should
be taken to ensure that the availability of the building management
network does not become compromised beyond the ability of critical
functions to perform adequately.

In homes, the entertainment network for audio/video streaming and
gaming has different requirements, where the most important
requirement is the need for high bandwidth not typically needed for
home or building control. It is therefore expected that the
entertainment network in the home will mostly be separate from the
control network, as this will also lessen the impact on the
availability of the control network.

2.1. Network Topologies

In general, the home automation network or building control network
consists of wired and wireless subnetworks. In large buildings in
particular, the wireless subnetworks can be connected to an IP
backbone network where all infrastructure services (e.g., Domain Name
System (DNS), automation servers) are located.

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The wireless subnetwork can be configured according to any of the
following topologies:

o A stand-alone network of 10-100 nodes without a border router.
This typically occurs in the home with a stand-alone control
network, in low-cost buildings, and during installation of
high-end control systems in buildings.

o A connected network with one border router. This configuration
will happen in homes where home appliances are controlled from
outside the home, possibly via a smart phone, and in many building
control scenarios.

o A connected network with multiple border routers. This will
typically happen in installations of large buildings.

Many of the nodes are battery powered and may be sleeping nodes that
wake up according to clock signals or external events.

In a building control network, for a large installation with multiple
border routers, subnetworks often overlap both geographically and
from a wireless coverage perspective. Due to two purposes of the
network -- (i) direct control and (ii) monitoring -- there may exist
two types of routing topologies in a given subnetwork:
(i) a tree-shaped collection of routes spanning from a central
building controller via the border router, on to destination nodes in
the subnetwork, and (ii) a flat, undirected collection of
intra-network routes between functionally related nodes in the
subnetwork.

The majority of nodes in home and building automation networks are
typically Class 0 devices [RFC7228], such as individual wall
switches. Only a few nodes (such as multi-purpose remote controls)
are more expensive Class 1 devices, which can afford more memory
capacity.

2.2. Traffic Characteristics

Traffic may enter the network originating from a central controller,
or it may originate from an intra-network node. The majority of
traffic is of a lightweight point-to-point control style, e.g.,
Put-Ack or Get-Response. There are, however, exceptions. Bulk data
transfer is used for firmware updates and logging, where firmware
updates enter the network and logs leave the network. Group
communication is used for service discovery or to control groups of
nodes, such as light fixtures.

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Often, there is a direct physical relationship between a controlling
sensor and the controlled equipment. For example, the temperature
sensor and room controller are located in the same room, sharing the
same climate conditions. Consequently, the bulk of senders and
receivers are separated by a distance that allows one-hop direct path
communication. A graph of the communication will show several fully
connected subsets of nodes. However, due to interference, multipath
fading, reflection, and other transmission mechanisms, the one-hop
direct path may be temporarily disconnected. For reliability
purposes, it is therefore essential that alternative n-hop
communication routes exist for quick error recovery. (See Appendix B
for motivation.)

Looking over time periods of a day, the networks are very lightly
loaded. However, bursts of traffic can be generated by, for example,
incessant pushing of the button of a remote control, the occurrence
of a defect, and other unforeseen events. Under those conditions,
the timeliness must nevertheless be maintained. Therefore, measures
are necessary to remove any unnecessary traffic. Short routes are
preferred. Long multi-hop routes via the border router should be
avoided whenever possible.

Group communication is essential for lighting control. For example,
once the presence of a person is detected in a given room, lighting
control applies to that room only, and no other lights should be
dimmed or switched on/off. In many cases, this means that a
multicast message with a one-hop and two-hop radius would suffice to
control the required lights. The same argument holds for Heating,
Ventilating, and Air Conditioning (HVAC) and other climate-control
devices. To reduce network load, it is advisable that messages to
the lights in a room are not distributed any further in the mesh than
necessary, based on intended receivers.

[Office-Light] provides an example of an office space, and
[OccuSwitch] describes the current use of wireless lighting control
products.

2.2.1. General

Although air conditioning and other environmental-control
applications may accept response delays of tens of seconds or longer,
alarm and light control applications may be regarded as soft
real-time systems. A slight delay is acceptable, but the perceived
quality of service degrades significantly if response times exceed
250 ms. If the light does not turn on at short notice, a user may
activate the controls again, thus causing a sequence of commands such
as Light{on,off,on,off,...} or Volume{up,up,up,up,up,...}. In

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addition, the repetitive sending of commands creates an unnecessary
loading of the network, which in turn increases the poor
responsiveness of the network.

2.2.2. Source-Sink (SS) Communication Paradigm

This paradigm translates to many sources sending messages to the same
sink, sometimes reachable via the border router. As such,
Source-Sink (SS) traffic can be present in home and building
networks. The traffic may be generated by environmental sensors
(often present in a wireless subnetwork) that push periodic readings
to a central server. The readings may be used for pure logging or,
more often, processed to adjust light, heating, and ventilation.
Alarm sensors may also generate SS-style traffic. The central server
in a home automation network will be connected mostly to a wired
network segment of the home network, although it is likely that cloud
services will also be used. The central server in a building
automation network may be connected to a backbone or placed outside
the building.

With regard to message latency, most SS transmissions can tolerate
worst-case delays measured in tens of seconds. Fire detectors,
however, represent an exception; for example, special provisions with
respect to the location of the fire detectors and smoke dampers need
to be put in place to meet stringent delay requirements that are
measured in seconds.

2.2.3. Publish-Subscribe (PS, or Pub/Sub) Communication Paradigm

This paradigm translates to a number of devices expressing their
interest in a service provided by a server device. For example, a
server device can be a sensor delivering temperature readings on the
basis of delivery criteria, like changes in acquisition value or age
of the latest acquisition. In building automation networks, this
paradigm may be closely related to the SS paradigm, given that
servers, which are connected to the backbone or outside the building,
can subscribe to data collectors that are present at strategic places
in the building automation network. The use of PS will probably
differ significantly from installation to installation.

2.2.4. Peer-to-Peer (P2P) Communication Paradigm

This paradigm translates to a device transferring data to another
device often connected to the same subnetwork. Peer-to-Peer (P2P)
traffic is a common traffic type in home automation networks. Most
building automation networks rely on P2P traffic as described in the
next paragraph. Other building automation networks rely on P2P
control traffic between controls and a local controller box for

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advanced group control. A local controller box can be further
connected to service control boxes, thus generating more SS or PS
traffic.

P2P traffic is typically generated by remote controls and wall
controllers that push Control Messages directly to light or heat
sources. P2P traffic has a stringent requirement for low latency,
since P2P traffic often carries application messages that are invoked
by humans. As mentioned in Section 2.2.1, application messages
should be delivered within a few hundred milliseconds, even when
connections fail momentarily.

2.2.5. Peer-to-Multipeer (P2MP) Communication Paradigm

This paradigm translates to a device sending a message as many times
as there are destination devices. Peer-to-Multipeer (P2MP) traffic
is common in home and building automation networks. Often, a
thermostat in a living room responds to temperature changes by
sending temperature acquisitions to several fans and valves
consecutively. This paradigm is also closely related to the PS
paradigm in the case where a single server device has multiple
subscribers.

2.2.6. Additional Considerations: Duocast and N-Cast

This paradigm translates to a device sending a message to many
destinations in one network transfer invocation. Multicast is well
suited for lighting where a presence sensor sends a presence message
to a set of lighting devices. Multicast increases the probability
that the message is delivered within strict time constraints. The
recommended multicast algorithm (e.g., [RFC7731]) provides a
mechanism for delivering messages to all intended destinations.

2.2.7. RPL Applicability per Communication Paradigm

In the case of the SS paradigm applied to a wireless subnetwork to a
server reachable via a border router, the use of RPL [RFC6550] in
non-storing mode is appropriate. Given the low resources of the
devices, source routing will be used from the border router to the
destination in the wireless subnetwork for messages generated outside
the mesh network. No specific timing constraints are associated with
the SS-type messages, so network repair does not violate the
operational constraints. When no SS traffic takes place, it is good
practice to load only RPL code that enables the P2P mode of operation
[RFC6997] to reduce the code size and satisfy memory requirements.

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To assure responsiveness, P2P-RPL [RFC6997] is required for all P2P
and P2MP traffic taking place between nodes within a wireless
subnetwork (excluding the border router). Source and destination
devices are typically physically close, based on room layout.
Consequently, most P2P and P2MP traffic is one-hop or two-hop
traffic. Appendix A identifies shortcomings of using RPL for this
type of communication; these shortcomings are counteracted through
the use of P2P-RPL. Appendix B explains why reliability measures
such as multipath routing are necessary even when one-hop
communication dominates.

Examples of additional advantages of P2P-RPL for home and building
automation networks are as follows:

o Individual wall switches are typically inexpensive Class 0 devices
[RFC7228] with extremely low memory capacities. Multi-purpose
remote controls for use in a home environment typically have more
memory, but such devices are asleep when there is no user
activity. P2P-RPL reactive discovery allows a node to wake up and
find new routes within a few seconds, while memory-constrained
nodes only have to keep routes to relevant targets.

o The reactive discovery features of P2P-RPL ensure that commands
are normally delivered within the 250 ms time window. When
connectivity needs to be restored, discovery is typically
completed within seconds. In most cases, an alternative route (a
route that was discovered earlier) will work and route rediscovery
is not necessary.

o Broadcast storms typically associated with route discovery for the
Ad hoc On-Demand Distance Vector (AODV) [RFC3561] are less
disruptive for P2P-RPL. P2P-RPL has a "Stop" bit, which is set by
the target of a route discovery to notify all other nodes that no
more Destination-Oriented Directed Acyclic Graph (DODAG)
Information Object (DIO) messages should be forwarded for this
temporary DAG. Something that looks like a broadcast storm may
happen when no target is responding; however, in this case, the
Trickle suppression mechanism kicks in, limiting the number of DIO
forwards in dense networks.

Due to the limited memory of the majority of devices, P2P-RPL SHOULD
be deployed with source routing in non-storing mode, as explained in
Section 4.1.2.

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Multicast with the Multicast Protocol for Low-Power and Lossy
Networks (MPL) [RFC7731] is preferably deployed for N-cast over the
wireless network. Configuration constraints that are necessary to
meet reliability and timeliness with MPL are discussed in
Section 4.1.7.

2.3. Layer 2 Applicability

This document applies to [IEEE802.15.4] and [G.9959], which are
adapted to IPv6 by the adaptation layers [RFC4944] and [RFC7428].
Other Layer 2 technologies, accompanied by an "IP-over-Foo"
specification, are also relevant, provided there is no frame size
issue and there are link-layer acknowledgements.

The above-mentioned adaptation layers leverage on the compression
capabilities of [RFC6554] and [RFC6282]. Header compression allows
small IP packets to fit into a single Layer 2 frame, even when source
routing is used. A network diameter limited to five hops helps to
achieve this, even while using source routing.

Dropped packets are often experienced in the targeted environments.
Internet Control Message Protocol (ICMP), User Datagram Protocol
(UDP), and even Transmission Control Protocol (TCP) flows may benefit
from link-layer unicast acknowledgements and retransmissions.
Link-layer unicast acknowledgements SHOULD be enabled when
[IEEE802.15.4] or [G.9959] is used with RPL and P2P-RPL.

3. Using RPL to Meet Functional Requirements

Several features required by [RFC5826] and [RFC5867] challenge the
P2P paths provided by RPL. Appendix A reviews these challenges. In
some cases, a node may need to spontaneously initiate the discovery
of a path towards a desired destination that is neither the root of a
DAG nor a destination originating Destination Advertisement Object
(DAO) signaling. Furthermore, P2P paths provided by RPL are not
satisfactory in all cases because they involve too many intermediate
nodes before reaching the destination.

P2P-RPL [RFC6997] SHOULD be used in home automation and building
control networks, as traffic of a point-to-point style is substantial
and route repair needs to be completed within seconds. P2P-RPL
provides a reactive mechanism for quick, efficient, and root-
independent route discovery/repair. The use of P2P-RPL furthermore
allows data traffic to avoid having to go through a central region
around the root of the tree and drastically reduces path length
[SOFT11] [INTEROP12]. These characteristics are desirable in home
and building automation networks because they substantially decrease
unnecessary network congestion around the root of the tree.

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When more reliability is required, P2P-RPL enables the establishment
of multiple independent paths. For one-hop destinations, this means
that one one-hop communication and a second two-hop communication
take place via a neighboring node. Such a pair of redundant
communication paths can be achieved by using MPL, where the source is
an MPL Forwarder while a second MPL Forwarder is one hop away from
both the source and the destination node. When the source multicasts
the message, it may be received by both the destination and the
second MPL Forwarder. The second MPL Forwarder forwards the message
to the destination, thus providing two routes from sender to
destination.

To provide more reliability with multiple paths, P2P-RPL can maintain
two independent P2P source routes per destination, at the source.
Good practice is to use the paths alternately to assess their
existence. When one P2P path has failed (possibly only temporarily),
as described in Appendix B, the alternative P2P path can be used
without discarding the failed path. The failed P2P path, unless
proven to work again, can be safely discarded after a timeout
(typically 15 minutes). A new route discovery is done when the
number of P2P paths is exhausted due to persistent link failures.

4. RPL Profile

P2P-RPL SHOULD be used in home automation and building control
networks. Its reactive discovery allows for low application response
times, even when on-the-fly route repair is needed. Non-storing mode
SHOULD be used to reduce memory consumption in repeaters with
constrained memory when source routing is used.

4.1. RPL Features

An important constraint on the application of RPL is the presence of
sleeping nodes.

For example, in a stand-alone network, the master node (or
coordinator) providing the logical Layer 2 identifier and unique node
identifiers to connected nodes may be a remote control that returns
to sleep once new nodes have been added. Due to the absence of the
border router, there may be no global routable prefixes at all.
Likewise, there may be no authoritative always-on root node, since
there is no border router to host this function.

In a network with a border router and many sleeping nodes, there may
be battery-powered sensors and wall controllers configured to contact
other nodes in response to events and then return to sleep. Such
nodes may never detect the announcement of new prefixes via
multicast.

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In each of the above-mentioned constrained deployments, a link-layer
node (e.g., coordinator or master) SHOULD assume the role of an
authoritative root node, transmitting unicast Router Advertisement
(RA) messages with a Unique Local Address (ULA) prefix information
option to nodes during the joining process to prepare the nodes for a
later operational phase, where a border router is added.

A border router SHOULD be designed to be aware of sleeping nodes in
order to support the distribution of updated global prefixes to such
sleeping nodes.

4.1.1. RPL Instances

When operating P2P-RPL on a stand-alone basis, there is no
authoritative root node maintaining a permanent RPL DODAG. A node
MUST be able to join at least one RPL Instance, as a new, temporary
instance is created during each P2P-RPL route discovery operation. A
node MAY be designed to join multiple RPL Instances.

4.1.2. Storing vs. Non-Storing Mode

Non-storing mode MUST be used to cope with the extremely constrained
memory of a majority of nodes in the network (such as individual
light switches).

4.1.3. DAO Policy

Nodes send DAO messages to establish downward paths from the root to
themselves. In order to minimize the power consumption overhead
associated with path discovery, DAO messages are not acknowledged in
networks composed of battery-operated field devices. The DAO
messages build up a source route because the nodes MUST be in
non-storing mode.

If devices in LLNs participate in multiple RPL Instances and DODAGs,
both the RPLInstance ID and the DODAGID SHOULD be included in
the DAO.

4.1.4. Path Metrics

Expected Transmission Count (ETX) is the RECOMMENDED metric.
[RFC6551] provides other options.

Packets from asymmetric and/or unstable links SHOULD be deleted at
Layer 2.

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4.1.5. Objective Function

Objective Function Zero (OF0) [RFC6552] MUST be the Objective
Function. Other Objective Functions MAY be used when dictated by
circumstances.

4.1.6. DODAG Repair

Since P2P-RPL only creates DODAGs on a temporary basis during route
repair or route discovery, there is no need to repair DODAGs.

For SS traffic, local repair is sufficient. The accompanying process
is known as "poisoning" and is described in Section 8.2.2.5 of
[RFC6550]. Given that the majority of nodes in the building do not
physically move around, creating new DODAGs should not happen
frequently.

4.1.7. Multicast

Commercial lighting deployments may have a need for multicast to
distribute commands to a group of lights in a timely fashion.
Several mechanisms exist for achieving such functionality; [RFC7731]
is the RECOMMENDED protocol for home and building deployments. This
section relies heavily on the conclusions of [RT-MPL].

At reception of a packet, the MPL Forwarder starts a series of
consecutive Trickle timer intervals, where the first interval has a
minimum size of Imin. Each consecutive interval is twice as long as
the former, with a maximum value of Imax. There is a maximum number
of intervals given by max_expiration. For each interval of length I,
a time t is randomly chosen in the period [I/2, I]. For a given
packet, p, MPL counts the number of times it receives p during the
period [0, t] in a counter c. At time t, MPL rebroadcasts p when
c < k, where k is a predefined constant with a value k > 0.

The density of forwarders and the frequency of message generation are
important aspects to obtain timeliness during control operations.
A high frequency of message generation can be expected when a
remote-control button is incessantly pressed or when alarm situations
arise.

Guaranteeing timeliness is intimately related to the density of the
MPL routers. In ideal circumstances, the message is propagated as a
single wave through the network, such that the maximum delay is
related to the number of hops times the smallest repetition interval
of MPL. Each forwarder that receives the message passes the message
on to the next hop by repeating the message. When several copies of
a message reach the forwarder, it is specified that the copy need not

Brandt, et al. Standards Track [Page 16]

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be repeated. Repetition of the message can be inhibited by a small
value of k. To assure timeliness, the chosen value of k should be
high enough to make sure that messages are repeated at the first
arrival of the message in the forwarder. However, a network that is
too dense leads to a saturation of the medium that can only be
prevented by selecting a low value of k. Consequently, timeliness is
assured by choosing a relatively high value of k but assuring at the
same time a low enough density of forwarders to reduce the risk of
medium saturation. Depending on the reliability of the network
links, it is advisable to configure the density of the network such
that at least two forwarders per hop repeat messages to the same set
of destinations.

There are no rules about selecting forwarders for MPL. In buildings
with central management tools, the forwarders can be selected, but at
the time of this writing it is not possible to automatically
configure the forwarder topology in the home.

4.1.8. Security

RPL MAY use unsecured RPL messages to reduce message size. If there
is a single node that uses unsecured RPL messages, link-layer
security MUST be used on all nodes. Therefore, all RPL messages MUST
be secured using:

o RPL message security, or

o Link-layer security, or

o Both RPL message security and link-layer security

A symmetric key is used to secure a RPL message using either RPL
message security or link-layer security. The symmetric key MUST be
distributed or established in a secure fashion. There may be more
than one symmetric key in use by any node at any one time. The same
symmetric key MUST NOT be used for both RPL message security and
link-layer security between two peer nodes.

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4.1.8.1. Symmetric Key Distribution

The scope of symmetric key distribution MUST be no greater than the
network itself, i.e., a group key. This document describes what
needs to be implemented to meet this requirement. The scope of
symmetric key distribution MAY be smaller than the network -- for
example:

o A pairwise symmetric key between two peers.

o A group key shared between a subset of nodes in the network.

4.1.8.2. Symmetric Key Distribution Mechanism

The authentication mechanism as described in Section 6.9 of
[ZigBeeIP] SHALL be used to securely distribute a network-wide
symmetric key.

The purpose of the authentication procedure is to provide mutual
authentication resulting in:

o Preventing untrusted nodes without appropriate credentials from
joining the trusted network.

o Preventing trusted nodes with appropriate credentials from joining
an untrusted network.

There is an Authentication Server, which is responsible for
authenticating the nodes on the network. If the authentication is
successful, the Authentication Server sends the network security
material to the joining node through the Protocol for Carrying
Authentication for Network Access (PANA) [RFC5191] [RFC6345]. The
joining node becomes a full participating node in the network and is
able to apply Layer 2 security to RPL messages using the distributed
network key.

The joining node does not initially have access to the network
security material. Therefore, it is not able to apply Layer 2
security to the packets exchanged during the authentication process.
The enforcement point rules at the edge of the network ensure that
the packets involved in PANA authentication are processed even though
they are unsecured at the Medium Access Control (MAC) layer. The
rules also ensure that any other incoming traffic that is not secured
at the MAC layer is discarded and is not forwarded.

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4.1.8.2.1. Authentication Stack

Authentication can be viewed as a protocol stack as a layer
encapsulates the layers above it.

o Transport Layer Security (TLS) [RFC5246] MUST be used at the
highest layer of the authentication stack and carries the
authentication exchange. There is one cipher suite based on a
Pre-Shared Key (PSK) [RFC6655] and one cipher suite based on
Elliptic Curve Cryptography (ECC) [RFC7251].

o Extensible Authentication Protocol-TLS (EAP-TLS) [RFC5216] MUST be
used at the next layer to carry the TLS records for the
authentication protocol.

o EAP [RFC3748] MUST be used to provide the mechanisms for mutual
authentication. EAP requires a way to transport EAP packets
between the joining node and the node on which the Authentication
Server resides. These nodes are not necessarily in radio range of
each other, so it is necessary to have multi-hop support in the
EAP transport method. PANA [RFC5191] [RFC6345], which operates
over UDP, MUST be used for this purpose. [RFC3748] specifies the
derivation of a session key using the EAP key hierarchy; only the
EAP Master Session Key shall be derived, as [RFC5191] specifies
that it is used to set up keys for PANA authentication and
encryption.

o PANA [RFC5191] and a PANA relay [RFC6345] MUST be used at the next
layer:

* The joining node MUST act as the PANA Client (PaC).

* The parent edge router node MUST act as a PANA Relay Element
(PRE) according to [RFC6345], unless it is also the
Authentication Server. All routers at the edge of the network
MUST be capable of functioning in the PRE role.

* The Authentication Server node MUST act as the PANA
Authentication Agent (PAA). The Authentication Server MUST be
able to handle packets relayed according to [RFC6345].

This network authentication process uses link-local IPv6 addresses
for transport between the new node and its parent. If the parent is
not the Authentication Server, it MUST then relay packets from the
joining node to the Authentication Server and vice versa, using the
PANA relay mechanism [RFC6345]. The joining node MUST use a
link-local address based on its EUI-64 as the source address for
initial PANA authentication message exchanges.

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4.1.8.2.2. Applicability Statements

The following applicability statements describe the relationship
between the various specifications.

4.1.8.2.2.1. Applicability Statement for PSK TLS

[RFC6655] contains Authenticated Encryption with Associated Data
(AEAD) TLS cipher suites that are very similar to [RFC5487], whose
AEAD part is detailed in [RFC5116]. [RFC5487] references both
[RFC5288] and the original PSK cipher suite document [RFC4279], which
references RFC 2246, which was eventually replaced by [RFC5246],
which defines the TLS 1.2 messages.

4.1.8.2.2.2. Applicability Statement for ECC TLS

[RFC7251] contains AEAD TLS cipher suites that are very similar to
[RFC5289], whose AEAD part is detailed in [RFC5116]. [RFC5289]
references the original ECC cipher suite document [RFC4492], which
references RFC 2246, which was eventually replaced by [RFC5246],
which defines the TLS 1.2 messages.

4.1.8.2.2.3. Applicability Statement for EAP-TLS and PANA

[RFC5216] specifies how [RFC3748] is used to package [RFC5246] TLS
records into EAP packets. [RFC5191] provides transportation for the
EAP packets and the network-wide key carried in an encrypted
Attribute-Value Pair (AVP) as specified in [RFC6786]. The proposed
Pseudorandom Function (PRF) and authentication (AUTH) hashes based on
SHA-256 are represented as specified in [RFC7296] and detailed in
[RFC4868].

4.1.8.2.3. Security Using RPL Message Security

If RPL is used with secured messages [RFC6550], the following RPL
security parameter values SHOULD be used:

o Counter is Time (T) flag = 0: Do not use the timestamp in the
Counter field. Counters based on timestamps are typically more
applicable to industrial networks, where strict timing
synchronization between nodes is often implemented. Home and
building networks typically do not implement such strict timing
synchronization; therefore, a monotonically increasing counter is
more appropriate.

o Algorithm = 0: Use Counter with the Cipher Block Chaining Message
Authentication Code (CBC-MAC Mode) (CCM) with AES-128. This is
the only assigned mode at present.

Brandt, et al. Standards Track [Page 20]

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o Key Identifier Mode (KIM) = 10: Use a group key, Key Source
present, and Key Index present. Given the relatively confined
perimeter of a home or building network, a group key is usually
sufficient to protect RPL messages sent between nodes. The use of
the Key Source field allows multiple group keys to be used within
the network.

o Security Level (LVL) = 0: Use MAC-32. This is recommended, as
integrity protection for RPL messages is the basic requirement.
Encryption is unlikely to be necessary, given the relatively
non-confidential nature of RPL message payloads.

4.1.9. P2P Communications

[RFC6997] MUST be used to accommodate P2P traffic, which is typically
substantial in home and building automation networks.

4.1.10. IPv6 Address Configuration

Assigned IP addresses MUST be routable and unique within the routing
domain [RFC5889].

4.2. Layer 2 Features

No particular requirements exist for Layer 2, except for those cited
in the "IP-over-Foo" RFCs (see Section 2.3).

4.2.1. Specifics about Layer 2

Not applicable

4.2.2. Services Provided at Layer 2

Not applicable

4.2.3. IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN)
Options Assumed

Not applicable

4.2.4. Mesh Link Establishment (MLE) and Other Things

Not applicable

4.3. Recommended Configuration Defaults and Ranges

The following sections describe the recommended parameter values for
P2P-RPL and Trickle.

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4.3.1. Trickle Parameters

Trickle is used to distribute network parameter values to all nodes
without stringent time restrictions. The recommended Trickle
parameter values are:

o DIOIntervalMin 4, which translates to 16 ms

o DIOIntervalDoublings 14

o DIORedundancyConstant 1

When a node sends a changed DIO, this is an inconsistency and forces
the receiving node to respond within Imin. So, when something
happens that affects the DIO, the change is ideally communicated to a
node that is n hops away, within n times Imin. Often, depending on
the node density, packets are lost or are not sent, leading to larger
delays.

In general, we can expect DIO changes to propagate within 1 to
3 seconds within the envisaged networks.

When nothing happens, the DIO sending interval increases to
4.37 minutes, thus drastically reducing the network load. When a
node does not receive DIO messages for more than 10 minutes, it can
safely conclude that the connection with other nodes has been lost.

4.3.2. Other Parameters

This section discusses the P2P-RPL parameters.

P2P-RPL [RFC6997] provides the features requested by [RFC5826] and
[RFC5867]. P2P-RPL uses a subset of the frame formats and features
defined for RPL [RFC6550] but may be combined with RPL frame flows in
advanced deployments.

The recommended parameter values for P2P-RPL are:

o MinHopRankIncrease 1

o MaxRankIncrease 0

o MaxRank 6

o Objective Function: OF0

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5. MPL Profile

MPL is used to distribute values to groups of devices. Using MPL,
based on the Trickle algorithm, timeliness should also be guaranteed.
A deadline of 200 ms needs to be met when human action is followed by
an immediately observable action such as switching on lights. The
deadline needs to be met in a building where the number of hops from
seed to destination varies between 1 and 10.

5.1. Recommended Configuration Defaults and Ranges

5.1.1. Real-Time Optimizations

When the network is heavily loaded, MAC delays contribute
significantly to the end-to-end delays when MPL intervals between 10
and 100 ms are used to meet the 200 ms deadline. It is possible to
set the number of buffers in the MAC to 1 and set the number of
back-off repetitions to 1. The number of MPL repetitions compensates
for the reduced probability of transmission per MAC invocation
[RT-MPL].

In addition, end-to-end delays and message losses are reduced by
adding a real-time layer between MPL and MAC to throw away the
earliest messages (exploiting the MPL message numbering) and favor
the most recent ones.

5.1.2. Trickle Parameters

This section proposes values for the Trickle parameters used by MPL
for the distribution of packets that need to meet a 200 ms deadline.
The probability of meeting the deadline is increased by (1) choosing
a small Imin value, (2) reducing the number of MPL intervals, thus
reducing the load, and (3) reducing the number of MPL Forwarders to
also reduce the load.

The consequence of this approach is that the value of k can be larger
than 1 because network load reduction is already guaranteed by the
network configuration.

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Under the condition that the density of MPL repeaters can be limited,
it is possible to choose low MPL repeat intervals (Imin) connected to
k values such that k > 1. The minimum value of k is related to:

o The value of Imin. The length of Imin determines the number of
packets that can be received within the listening period of Imin.

o The number of repeaters receiving the broadcast message from the
same forwarder or seed. These repeaters repeat within the same
Imin interval, thus increasing the c counter.

Within the first MPL interval, a limited number, q, of messages can
be transmitted. Assuming a 3 ms transmission interval, q is given by
q = Imin / 3. Assuming that at most q message copies can reach a
given forwarder within the first repeat interval of length Imin, the
related MPL parameter values are suggested in the following sections.

5.1.2.1. Imin

The recommended value is Imin = 10 to 50 ms.

When the chosen Imin value is much smaller, the interference between
the copies leads to significant losses, given that q is much smaller
than the number of repeated packets. With much larger intervals, the
probability that the deadline will be met decreases with increasing
hop count.

5.1.2.2. Imax

The recommended value is Imax = 100 to 400 ms.

The value of Imax is less important than the value of max_expiration.
Given an Imin value of 10 ms, the third MPL interval has a value of
10 * 2 * 2 = 40 ms. When Imin has a value of 40 ms, the third
interval has a value of 160 ms. Given that more than three intervals
are unnecessary, Imax does not contribute much to performance.

5.1.3. Other Parameters

Other parameters are the k parameter and the max_expiration
parameter.

k > q (see condition above). Under this condition, and for a small
Imin value, a value of k = 2 or k = 3 is usually sufficient to
minimize the losses of packets in the first repeat interval.

max_expiration = 2 - 4. Higher values lead to more network load
while generating copies that will probably not meet their deadline.

Brandt, et al. Standards Track [Page 24]

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6. Manageability Considerations

At this time, it is not clear how homenets will be managed.
Consequently, it is not clear which tools will be used and which
parameters must be visible for management.

In building control, management is mandatory. It is expected that
installations will be managed using the set of currently available
tools (including IETF tools like Management Information Base (MIB)
modules, Network Configuration Protocol (NETCONF) modules, Dynamic
Host Configuration Protocol (DHCP), and others), with large
differences between the ways an installation is managed.

7. Security Considerations

This section refers to the security considerations of [RFC6997],
[RFC6550], and [RFC7731], as well as some attacks and countermeasures
as discussed in Sections 6 and 7, respectively, of [RFC7416].

Communications network security is based on providing integrity
protection and encryption to messages. This can be applied at
various layers in the network protocol stack, based on using various
credentials and a network identity.

The credentials that are relevant in the case of RPL are (i) the
credential used at the link layer in the case where link-layer
security is applied (see Section 7.1) or (ii) the credential used for
securing RPL messages. In both cases, the assumption is that the
credential is a shared key. Therefore, there MUST be a mechanism in
place that allows secure distribution of a shared key and
configuration of a network identity. Both MAY be done using
(i) pre-installation using an out-of-band method, (ii) secure
delivery when a device is introduced into the network, or
(iii) secure delivery by a trusted neighboring device, as described
in Section 4.1.8.1. The shared key MUST be stored in a secure
fashion that will make it difficult to be read by an unauthorized
party.

This document mandates that a Layer 2 mechanism be used during
initial and incremental deployment. Please see the following
sections.

Brandt, et al. Standards Track [Page 25]

RFC 7733 RPL in Home and Building February 2016

7.1. Security Considerations during Initial Deployment

Wireless mesh networks are typically secured at the link layer in
order to prevent unauthorized parties from accessing the information
exchanged over the links. It is a basic practice to create a network
of nodes that share the same keys for link-layer security and exclude
nodes sending unsecured messages. With per-message data origin
authentication, it is possible to prevent unauthorized nodes from
joining the mesh.

At initial deployment, the network is secured by consecutively
securing nodes at the link layer, thus building a network of secured
nodes. Section 4.1.8.2 describes a mechanism for building a network
of secured nodes.

This document does not specify a multicast security solution.
Networks deployed with this specification will depend upon Layer 2
security to prevent outsiders from sending multicast traffic. It is
recognized that this does not protect this control traffic from
impersonation by already-trusted devices. This is an area for a
future specification.

For building control, an installer will use an installation tool that
establishes a secure communication path with the joining node. It is
recognized that the recommendations for initial deployment as
discussed in this section do not cover all building requirements,
such as selecting -- independent of network topology -- the node to
be secured.

It is expected that a set of protocol combinations will evolve within
currently existing alliances of building control manufacturers. Each
set satisfies the installation requirements of installers, operators,
and manufacturers of building control networks in a given
installation context, e.g., lighting deployment in offices, HVAC
installation, incremental addition of equipment in homes, and others.

In the home, nodes can be visually inspected by the home owner.
Also, a simple procedure, e.g., pushing buttons simultaneously on an
already-secured device and an unsecured joining device, is usually
sufficient to ensure that the unsecured joining device is
authenticated securely, configured securely, and paired
appropriately.

This recommendation is in line with the countermeasures described in
Section 7.1 of [RFC7416].

Brandt, et al. Standards Track [Page 26]

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7.2. Security Considerations during Incremental Deployment

Once a network is operational, new nodes need to be added, or nodes
fail and need to be replaced. When a new node needs to be added to
the network, the new node is added to the network via an assisting
node in the manner described in Section 7.1.

On detection of a compromised node, all trusted nodes need to have
their symmetric keys that are known to be shared with the compromised
node rekeyed, and the trusted network is built up as described in
Section 7.1.

7.3. Security Considerations for P2P Implementations

Refer to the security considerations of [RFC6997].

7.4. MPL Routing

The routing of MPL is determined by the enabling of the interfaces
for specified multicast addresses. The specification of these
addresses can be done via a Constrained Application Protocol (CoAP)
application as specified in [RFC7390]. An alternative is the
creation of an MPL MIB and the use of the Simple Network Management
Protocol (SNMPv3) [RFC3411] or equivalent techniques to specify the
multicast addresses in the MIB. For secure dissemination of MPL
packets, Layer 2 security SHOULD be used, and the configuration of
multicast addresses as described in this section MUST be secure.

7.5. RPL Security Features

This section refers to the structure of Section 8 ("RPL Security
Features") of [RFC7416]. [RFC7416] provides a thorough analysis of
security threats and proposed countermeasures relevant to RPL
and MPL.

In accordance with Section 8.1 ("Confidentiality Features") of
[RFC7416], RPL message security implements payload protection, as
explained in Section 7 of this document. The attributes for key
length and lifetime of the keys depend on operational conditions,
maintenance, and installation procedures.

Sections 7.1 and 7.2 of this document recommend link-layer security
to assure integrity in accordance with Section 8.2 ("Integrity
Features") of [RFC7416].

The provision of multiple paths recommended in Section 8.3
("Availability Features") of [RFC7416] is also recommended from a
reliability point of view. Randomly choosing paths MAY be supported.

Brandt, et al. Standards Track [Page 27]

RFC 7733 RPL in Home and Building February 2016

A mechanism for key management, as discussed in Section 8.4 ("Key
Management") of [RFC7416], is provided in Section 4.1.8.2 of this
document.

8. Other Related Protocols

Application and transport protocols used in home and building
automation domains are expected to mostly consist of CoAP over UDP,
or equivalents. Typically, UDP is used for IP transport to keep down
the application response time and bandwidth overhead. CoAP is used
at the application layer to reduce memory footprint and bandwidth
requirements.

9. References

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,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
<http://www.rfc-editor.org/info/rfc3748>.

[RFC4279] Eronen, P., Ed., and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, DOI 10.17487/RFC4279, December 2005,
<http://www.rfc-editor.org/info/rfc4279>.

[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492,
DOI 10.17487/RFC4492, May 2006,
<http://www.rfc-editor.org/info/rfc4492>.

[RFC4868] Kelly, S. and S. Frankel, "Using HMAC-SHA-256,
HMAC-SHA-384, and HMAC-SHA-512 with IPsec", RFC 4868,
DOI 10.17487/RFC4868, May 2007,
<http://www.rfc-editor.org/info/rfc4868>.

[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<http://www.rfc-editor.org/info/rfc4944>.

Brandt, et al. Standards Track [Page 28]

RFC 7733 RPL in Home and Building February 2016

[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<http://www.rfc-editor.org/info/rfc5116>.

[RFC5191] Forsberg, D., Ohba, Y., Ed., Patil, B., Tschofenig, H.,
and A. Yegin, "Protocol for Carrying Authentication for
Network Access (PANA)", RFC 5191, DOI 10.17487/RFC5191,
May 2008, <http://www.rfc-editor.org/info/rfc5191>.

[RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216,
March 2008, <http://www.rfc-editor.org/info/rfc5216>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.

[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
DOI 10.17487/RFC5288, August 2008,
<http://www.rfc-editor.org/info/rfc5288>.

[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with
SHA-256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
DOI 10.17487/RFC5289, August 2008,
<http://www.rfc-editor.org/info/rfc5289>.

[RFC5487] Badra, M., "Pre-Shared Key Cipher Suites for TLS with
SHA-256/384 and AES Galois Counter Mode", RFC 5487,
DOI 10.17487/RFC5487, March 2009,
<http://www.rfc-editor.org/info/rfc5487>.

[RFC5548] Dohler, M., Ed., Watteyne, T., Ed., Winter, T., Ed., and
D. Barthel, Ed., "Routing Requirements for Urban Low-Power
and Lossy Networks", RFC 5548, DOI 10.17487/RFC5548,
May 2009, <http://www.rfc-editor.org/info/rfc5548>.

[RFC5673] Pister, K., Ed., Thubert, P., Ed., Dwars, S., and T.
Phinney, "Industrial Routing Requirements in Low-Power and
Lossy Networks", RFC 5673, DOI 10.17487/RFC5673,
October 2009, <http://www.rfc-editor.org/info/rfc5673>.

[RFC5826] Brandt, A., Buron, J., and G. Porcu, "Home Automation
Routing Requirements in Low-Power and Lossy Networks",
RFC 5826, DOI 10.17487/RFC5826, April 2010,
<http://www.rfc-editor.org/info/rfc5826>.

Brandt, et al. Standards Track [Page 29]

RFC 7733 RPL in Home and Building February 2016

[RFC5867] Martocci, J., Ed., De Mil, P., Riou, N., and W. Vermeylen,
"Building Automation Routing Requirements in Low-Power and
Lossy Networks", RFC 5867, DOI 10.17487/RFC5867,
June 2010, <http://www.rfc-editor.org/info/rfc5867>.

[RFC6282] Hui, J., Ed., and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<http://www.rfc-editor.org/info/rfc6282>.

[RFC6345] Duffy, P., Chakrabarti, S., Cragie, R., Ohba, Y., Ed., and
A. Yegin, "Protocol for Carrying Authentication for
Network Access (PANA) Relay Element", RFC 6345,
DOI 10.17487/RFC6345, August 2011,
<http://www.rfc-editor.org/info/rfc6345>.

[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,
<http://www.rfc-editor.org/info/rfc6550>.

[RFC6551] Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
and D. Barthel, "Routing Metrics Used for Path Calculation
in Low-Power and Lossy Networks", RFC 6551,
DOI 10.17487/RFC6551, March 2012,
<http://www.rfc-editor.org/info/rfc6551>.

[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,
<http://www.rfc-editor.org/info/rfc6554>.

[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655,
DOI 10.17487/RFC6655, July 2012,
<http://www.rfc-editor.org/info/rfc6655>.

[RFC6786] Yegin, A. and R. Cragie, "Encrypting the Protocol for
Carrying Authentication for Network Access (PANA)
Attribute-Value Pairs", RFC 6786, DOI 10.17487/RFC6786,
November 2012, <http://www.rfc-editor.org/info/rfc6786>.

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RFC 7733 RPL in Home and Building February 2016

[RFC6997] Goyal, M., Ed., Baccelli, E., Philipp, M., Brandt, A., and
J. Martocci, "Reactive Discovery of Point-to-Point Routes
in Low-Power and Lossy Networks", RFC 6997,
DOI 10.17487/RFC6997, August 2013,
<http://www.rfc-editor.org/info/rfc6997>.

[RFC6998] Goyal, M., Ed., Baccelli, E., Brandt, A., and J. Martocci,
"A Mechanism to Measure the Routing Metrics along a
Point-to-Point Route in a Low-Power and Lossy Network",
RFC 6998, DOI 10.17487/RFC6998, August 2013,
<http://www.rfc-editor.org/info/rfc6998>.

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

[RFC7251] McGrew, D., Bailey, D., Campagna, M., and R. Dugal,
"AES-CCM Elliptic Curve Cryptography (ECC) Cipher Suites
for TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014,
<http://www.rfc-editor.org/info/rfc7251>.

[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296,
October 2014, <http://www.rfc-editor.org/info/rfc7296>.

[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,
<http://www.rfc-editor.org/info/rfc7416>.

[RFC7731] Hui, J. and R. Kelsey, "Multicast Protocol for Low-Power
and Lossy Networks (MPL)", RFC 7731, DOI 10.17487/RFC7731,
February 2016, <http://www.rfc-editor.org/info/rfc7731>.

Brandt, et al. Standards Track [Page 31]

RFC 7733 RPL in Home and Building February 2016

[IEEE802.15.4]
IEEE, "IEEE Standard for Local and metropolitan area
networks--Part 15.4: Low-Rate Wireless Personal Area
Networks (LR-WPANs)", IEEE 802.15.4,
DOI 10.1109/ieeestd.2011.6012487,
<http://ieeexplore.ieee.org/servlet/
opac?punumber=6012485>.

[G.9959] International Telecommunication Union, "Short range
narrow-band digital radiocommunication transceivers - PHY,
MAC, SAR and LLC layer specifications", ITU-T
Recommendation G.9959, January 2015,
<http://www.itu.int/rec/T-REC-G.9959>.

9.2. Informative References

[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
DOI 10.17487/RFC3411, December 2002,
<http://www.rfc-editor.org/info/rfc3411>.

[RFC3561] Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc
On-Demand Distance Vector (AODV) Routing", RFC 3561,
DOI 10.17487/RFC3561, July 2003,
<http://www.rfc-editor.org/info/rfc3561>.

[RFC5889] Baccelli, E., Ed., and M. Townsley, Ed., "IP Addressing
Model in Ad Hoc Networks", RFC 5889, DOI 10.17487/RFC5889,
September 2010, <http://www.rfc-editor.org/info/rfc5889>.

[RFC6552] Thubert, P., Ed., "Objective Function Zero for the Routing
Protocol for Low-Power and Lossy Networks (RPL)",
RFC 6552, DOI 10.17487/RFC6552, March 2012,
<http://www.rfc-editor.org/info/rfc6552>.

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

[RFC7390] Rahman, A., Ed., and E. Dijk, Ed., "Group Communication
for the Constrained Application Protocol (CoAP)",
RFC 7390, DOI 10.17487/RFC7390, October 2014,
<http://www.rfc-editor.org/info/rfc7390>.

Brandt, et al. Standards Track [Page 32]

RFC 7733 RPL in Home and Building February 2016

[RFC7428] Brandt, A. and J. Buron, "Transmission of IPv6 Packets
over ITU-T G.9959 Networks", RFC 7428,
DOI 10.17487/RFC7428, February 2015,
<http://www.rfc-editor.org/info/rfc7428>.

[SOFT11] Baccelli, E., Philipp, M., and M. Goyal, "The P2P-RPL
Routing Protocol for IPv6 Sensor Networks: Testbed
Experiments", Proceedings of the 19th Annual Conference on
Software Telecommunications and Computer Networks, Split,
Croatia, September 2011.

[INTEROP12]
Philipp, M., Baccelli, E., Brandt, A., Valev, H., and J.
Buron, "Report on P2P-RPL Interoperability Testing", INRIA
Research Report RR-7864, January 2012.

[RT-MPL] van der Stok, P., "Real-Time multicast for wireless mesh
networks using MPL", White paper, April 2014,
<http://www.vanderstok.org/papers/Real-time-MPL.pdf>.

[OccuSwitch]
Philips lighting Electronics, "OccuSwitch Wireless
(brochure)", May 2012,
<http://www.philipslightingcontrols.com/assets/
cms/uploads/files/osw/MK_OSWNETBROC_5.pdf>.

[Office-Light]
Clanton and Associates, Inc., "Wireless Lighting Control -
A Life Cycle Cost Evaluation of Multiple Lighting Control
Strategies", February 2014, <http://www.daintree.net/
wp-content/uploads/2014/02/
clanton_lighting_control_report_0411.pdf>.

[RTN2011] Holtman, K. and P. van der Stok, "Real-time routing for
low-latency 802.15.4 control networks", 23rd Euromicro
Conference on Real-Time Systems, Porto, Portugal,
July 2011.

[MEAS] Holtman, K., "Connectivity loss in large scale
IEEE 802.15.4 network", Private Communication,
November 2013.

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RFC 7733 RPL in Home and Building February 2016

[BC-Survey]
Kastner, W., Neugschwandtner, G., Soucek, S., and H.
Newmann, "Communication Systems for Building Automation
and Control", Proceedings of the IEEE, Vol. 93, No. 6,
DOI 10.1109/JPROC.2005.849726, June 2005.

[ZigBeeIP]
ZigBee Alliance, "ZigBee IP specification", ZigBee
document 095023r34, March 2014, <http://www.zigbee.org/>.

Brandt, et al. Standards Track [Page 34]

RFC 7733 RPL in Home and Building February 2016

Appendix A. RPL Shortcomings in Home and Building Deployments

A.1. Risk of Undesirable Long P2P Routes

The DAG, being a tree structure, is formed from a root. If nodes
residing in different branches need to communicate internally, DAG
mechanisms provided in RPL [RFC6550] will propagate traffic towards
the root, potentially all the way to the root, and down along another
branch [RFC6998]. In a typical example, two nodes could reach each
other via only two router nodes, but in some unfortunate cases, RPL
may send traffic three hops up and three hops down again. This leads
to several undesirable phenomena, as described in the following
sections.

A.1.1. Traffic Concentration at the Root

If many P2P data flows have to move up towards the root to get down
again in another branch, there is an increased risk of congestion the
nearer to the root of the DAG the data flows. Due to the broadcast
nature of radio frequency (RF) systems, any child node of the root is
not only directing RF power downwards in its sub-tree but just as
much upwards towards the root, potentially jamming other MP2P traffic
leaving the tree or preventing the root of the DAG from sending P2MP
traffic into the DAG because the listen-before-talk link-layer
protection kicks in.

A.1.2. Excessive Battery Consumption in Source Nodes

Battery-powered nodes originating P2P traffic depend on the route
length. Long routes cause source nodes to stay awake for longer
periods before returning to sleep. Thus, a longer route translates
proportionally (more or less) into higher battery consumption.

A.2. Risk of Delayed Route Repair

The RPL DAG mechanism uses DIO and DAO messages to monitor the health
of the DAG. On rare occasions, changed radio conditions may render
routes unusable just after a destination node has returned a DAO
indicating that the destination is reachable. Given enough time, the
next Trickle timer-controlled DIO/DAO update will eventually repair
the broken routes; however, this may not occur in a timely manner
appropriate to the application. In an apparently stable DAG,
Trickle timer dynamics may reduce the update rate to a few times
every hour. If a user issues an actuator command, e.g., light on in
the time interval between the time that the last DAO message was
issued the destination module and the time that one of the parents
sends the next DIO, the destination cannot be reached. There is no

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RFC 7733 RPL in Home and Building February 2016

mechanism in RPL to initiate the restoration of connectivity in a
reactive fashion. The consequence is a broken service in home and
building applications.

A.2.1. Broken Service

Experience from the telecom industry shows that if the voice delay
exceeds 250 ms, users start getting confused, frustrated, and/or
annoyed. In the same way, if the light does not turn on within the
same period of time, a home control user will activate the controls
again, causing a sequence of commands such as
Light{on,off,off,on,off,...} or Volume{up,up,up,up,up,...}. Whether
the outcome is nothing or some unintended response, this is
unacceptable. A controlling system must be able to restore
connectivity to recover from the error situation. Waiting for an
unknown period of time is not an option. Although this issue was
identified during the P2P analysis, it applies just as well to
application scenarios where an IP application outside the LLN
controls actuators, lights, etc.

Appendix B. Communication Failures

Measurements of connectivity between neighboring nodes are discussed
in [RTN2011] and [MEAS].

The work is motivated by the measurements in literature that affirm
that the range of an antenna is not circle symmetric but that the
signal strength of a given level follows an intricate pattern around
the antenna, and there may be holes within the area delineated by a
polar plot. It is reported that communication is not symmetric:
reception of messages from node A by node B does not imply reception
of messages from node B by node A. The quality of the signal
fluctuates over time, and also the height of the antenna within a
room can have consequences for the range. As a function of the
distance from the source, three regions are generally recognized:
(1) a clear region with excellent signal quality, (2) a region with
fluctuating signal quality, and (3) a region without reception.
Installation of meshes with neighbors in the clear region is not
sufficient, as described below.

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RFC 7733 RPL in Home and Building February 2016

[RTN2011] extends existing work by:

o Observations over periods of at least a week,

o Testing links that are in the clear region,

o Observation in an office building during working hours, and

o Concentrating on one-hop and two-hop routes.

Eight nodes were distributed over a surface of 30 square meters. All
nodes are at a one-hop distance from each other, and all are situated
in each other's clear region. Each node sends messages to each of
its neighbors and repeats the message until it arrives. The latency
of the message was measured over periods of at least a week. It was
noticed that latencies longer than a second occurred without any
apparent reason, but only during working days and never during the
weekends. Bad periods could last for minutes. By sending messages
via two paths -- (1) a one-hop path directly and (2) a two-hop path
via a randomly chosen neighbor -- the probability of delays larger
than 100 ms decreased significantly.

The conclusion is that even for one-hop communication between
not-too-distant "line of sight" nodes, there are periods of low
reception in which communication deadlines of 200 ms are exceeded.
It pays to send a second message over a two-hop path to increase the
reliability of timely message transfer.

[MEAS] confirms that temporary bad reception by close neighbors can
occur within other types of areas. Nodes were installed on the
ceiling in a grid with a distance of 30-50 cm between them.
Two hundred nodes were distributed over an area of 10 m x 5 m. It
clearly transpired that with increasing distance the probability of
reception decreased. At the same time, a few nodes furthest away
from the sender had a high probability of message reception, while
some close neighbors of the sender did not receive messages. The
patterns of nodes experiencing good reception evolved over time.

The conclusion here is that even for direct neighbors reception can
temporarily be bad for periods of several minutes. For reliable and
timely communication, it is imperative to have at least two
communication paths available (e.g., two-hop paths next to the
one-hop path for direct neighbors).

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RFC 7733 RPL in Home and Building February 2016

Acknowledgements

This document reflects discussions and remarks from several
individuals, including (in alphabetical order) Stephen Farrell, Mukul
Goyal, Sandeep Kumar, Jerry Martocci, Catherine Meadows, Yoshihiro
Ohba, Charles Perkins, Yvonne-Anne Pignolet, Michael Richardson, Ines
Robles, Zach Shelby, and Meral Sherazipour.

Authors' Addresses

Anders Brandt
Sigma Designs

Email: [email protected]

Emmanuel Baccelli
INRIA

Email: [email protected]

Robert Cragie
ARM Ltd.
110 Fulbourn Road
Cambridge CB1 9NJ
United Kingdom

Email: [email protected]

Peter van der Stok
Consultant

Email: [email protected]

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