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RFC 7733

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

Part 1 of 2, p. 1 to 22
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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.

Page 2 
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

Top      ToC       Page 17 
   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.

Top      ToC       Page 18 
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.

Top      ToC       Page 21 
   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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