Roll                                                           A. Brandt
Internet-Draft                                             Sigma Designs
Intended status: Informational                               E. Baccelli
Expires: June 4, 2015                                              INRIA
                                                               R. Cragie
                                                         P. van der Stok
                                                       December 01, 2014

   Applicability Statement: The use of the RPL protocol suite in Home
                    Automation and Building Control


   The purpose of this document is to provide guidance in the selection
   and use of protocols from the RPL protocol suite to implement the
   features required for control in building and home environments.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on June 4, 2015.

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Relationship to other documents . . . . . . . . . . . . .   4
     1.2.  Requirements language . . . . . . . . . . . . . . . . . .   4
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.4.  Required Reading  . . . . . . . . . . . . . . . . . . . .   5
     1.5.  Out of scope requirements . . . . . . . . . . . . . . . .   5
   2.  Deployment Scenario . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Network Topologies  . . . . . . . . . . . . . . . . . . .   6
     2.2.  Traffic Characteristics . . . . . . . . . . . . . . . . .   7
       2.2.1.  General . . . . . . . . . . . . . . . . . . . . . . .   8
       2.2.2.  Source-sink (SS) communication paradigm . . . . . . .   8
       2.2.3.  Publish-subscribe (PS, or pub/sub)) communication
               paradigm  . . . . . . . . . . . . . . . . . . . . . .   9
       2.2.4.  Peer-to-peer (P2P) communication paradigm . . . . . .   9
       2.2.5.  Peer-to-multipeer (P2MP) communication paradigm . . .   9
       2.2.6.  Additional considerations: Duocast and N-cast . . . .  10
       2.2.7.  RPL applicability per communication paradigm  . . . .  10
     2.3.  Layer-2 applicability . . . . . . . . . . . . . . . . . .  11
   3.  Using RPL to meet Functional Requirements . . . . . . . . . .  11
   4.  RPL Profile . . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.1.  RPL Features  . . . . . . . . . . . . . . . . . . . . . .  13
       4.1.1.  RPL Instances . . . . . . . . . . . . . . . . . . . .  13
       4.1.2.  Storing vs. Non-Storing Mode  . . . . . . . . . . . .  13
       4.1.3.  DAO Policy  . . . . . . . . . . . . . . . . . . . . .  13
       4.1.4.  Path Metrics  . . . . . . . . . . . . . . . . . . . .  14
       4.1.5.  Objective Function  . . . . . . . . . . . . . . . . .  14
       4.1.6.  DODAG Repair  . . . . . . . . . . . . . . . . . . . .  14
       4.1.7.  Multicast . . . . . . . . . . . . . . . . . . . . . .  14
       4.1.8.  Security  . . . . . . . . . . . . . . . . . . . . . .  15
       4.1.9.  P2P communications  . . . . . . . . . . . . . . . . .  15
       4.1.10. IPv6 address configuration  . . . . . . . . . . . . .  16
     4.2.  Layer 2 features  . . . . . . . . . . . . . . . . . . . .  16
       4.2.1.  Specifics about layer-2 . . . . . . . . . . . . . . .  16
       4.2.2.  Services provided at layer-2  . . . . . . . . . . . .  16
       4.2.3.  6LowPAN options assumed . . . . . . . . . . . . . . .  16
       4.2.4.  MLE and other things  . . . . . . . . . . . . . . . .  16
     4.3.  Recommended Configuration Defaults and Ranges . . . . . .  16
       4.3.1.  Trickle parameters  . . . . . . . . . . . . . . . . .  16
       4.3.2.  Other Parameters  . . . . . . . . . . . . . . . . . .  17
   5.  MPL Profile . . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.1.  Recommended configuration Defaults and Ranges . . . . . .  17

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       5.1.1.  Real-Time optimizations . . . . . . . . . . . . . . .  17
       5.1.2.  Trickle parameters  . . . . . . . . . . . . . . . . .  18
       5.1.3.  Other parameters  . . . . . . . . . . . . . . . . . .  19
   6.  Manageability Considerations  . . . . . . . . . . . . . . . .  19
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     7.1.  Security considerations during initial deployment . . . .  20
     7.2.  Security Considerations during incremental deployment . .  21
     7.3.  Security Considerations for P2P uses  . . . . . . . . . .  21
     7.4.  MPL routing . . . . . . . . . . . . . . . . . . . . . . .  21
     7.5.  RPL Security features . . . . . . . . . . . . . . . . . .  21
   8.  Other related protocols . . . . . . . . . . . . . . . . . . .  22
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  22
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     12.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Appendix A.  RPL shortcomings in home and building deployments  .  27
     A.1.  Risk of undesired long P2P routes . . . . . . . . . . . .  27
       A.1.1.  Traffic concentration at the root . . . . . . . . . .  28
       A.1.2.  Excessive battery consumption in source nodes . . . .  28
     A.2.  Risk of delayed route repair  . . . . . . . . . . . . . .  28
       A.2.1.  Broken service  . . . . . . . . . . . . . . . . . . .  28
   Appendix B.  Communication failures . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   The primary purpose of this document is to give guidance in the use
   of the 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.

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

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   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/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 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.

1.1.  Relationship to other documents

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

   In addition, an extension document has been produced specifically to
   provide a solution for reactive discovery of point-to-point routes in
   LLNs [RFC6997].  The present applicability document provides
   recommendations and requirements for the accompanying parameter value

   A common set of security threats are described in
   [I-D.ietf-roll-security-threats].  The applicability statements
   complement the security threats document by describing preferred
   security settings and solutions within the applicability statement
   conditions.  This applicability statement recommends more light
   weight security solutions and specify the conditions under which
   these solutions are appropriate.

1.2.  Requirements language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

1.3.  Terminology

   This document uses terminology from [RFC6997],
   [I-D.ietf-roll-trickle-mcast], [RFC7102], [IEEE802.15.4], and

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1.4.  Required Reading

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

1.5.  Out of scope requirements

   The considered network diameter is limited to a maximum diameter of
   10 hops and a typical diameter of 5 hops, which 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
   sub-networks.  Each sub-network in a building automation deployment
   typically contains tens to hundreds of nodes, and for critical
   operations may operate independently from the other sub-networks.

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

   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.

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   Monitoring of functional correctness is at least as important.
   Devices typically communicate their status regularly and send alarm
   messages notifying a malfunction of equipment or network.

   In building control, the infrastructure of the building management
   network can be shared with the security/access, the IP telephony, and
   the 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 for 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, which also lessens the impact on availability of the
   control network

2.1.  Network Topologies

   In general, the home automation network or building control network
   consists of wired and wireless sub-networks.  In large buildings
   especially, the wireless sub-networks can be connected to an IP
   backbone network where all infrastructure services are located, such
   as DNS, automation servers, etc.

   The wireless sub-network can be configured according to any of the
   following topologies:

   o  A stand-alone network of 10-100 nodes without 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 which
   wake up according to clock signals or external events.

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   In a building control network, for large installation with multiple
   border routers, sub-networks 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 sub-network: (i) a tree-shaped
   collection of routes spanning from a central building controller via
   the border router, on to destination nodes in the sub-network; and/or
   (ii) a flat, un-directed collection of intra-network routes between
   functionally related nodes in the sub-network.

   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

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 light-weight point-to-point control style; e.g.  Put-Ack
   or Get-Response.  There are however exceptions.  Bulk data transfer
   is used for firmware update 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

   Often, there is a direct physical relation 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 temporally 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 e.g.
   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.

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   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 1-hop and 2-hop radius would suffice to
   control the required lights.  The same argument holds for 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.

   An example of an office surface is shown in [office-light], and the
   current use of wireless lighting control products is shown in

2.2.1.  General

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

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 sub-network) which 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 be placed
   outside the building.

   With regards 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

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   respect to the location of the Fire detectors and the smoke dampers
   need to be put in place to meet the stringent delay requirements.

2.2.3.  Publish-subscribe (PS, or pub/sub)) communication paradigm

   This paradigm translates to a number of devices expressing their
   interest for 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 sub-network.  Peer-to-peer (P2P)
   traffic is a common traffic type in home automation networks.  Most
   building automation networks rely on P2P traffic, described in the
   next paragraph.  Other building automation networks rely on P2P
   control traffic between controls and a local controller box for
   advanced scene and 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 which 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

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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 the strict time constraints.
   The recommended multicast algorithm (e.g.
   [I-D.ietf-roll-trickle-mcast]) assures that messages are delivered to
   ALL intended destinations.

2.2.7.  RPL applicability per communication paradigm

   In the case of the SS paradigm applied to a wireless sub-network 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 sub-network 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 enabling P2P mode of
   operation [RFC6997] to reduce the code size and satisfy memory

   P2P-RPL [RFC6997] is required for all P2P and P2MP traffic taking
   place between nodes within a wireless sub-network (excluding the
   border router) to assure responsiveness.  Source and destination
   devices are typically physically close based on room layout.
   Consequently, most P2P and P2MP traffic is 1-hop or 2-hop traffic.
   Appendix A explains why P2P-RPL is preferable to RPL for this type of
   communication.  Appendix B explains why reliability measures such as
   multi-path routing are necessary even when 1-hop communication

   Additional advantages of P2P-RPL for home and building automation
   networks are, for example:

   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 msec time window.  When

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      connectivity needs to be restored, discovery is typically
      completed within seconds.  In most cases, an alternative (earlier
      discovered) route will work and route rediscovery is not

   o  Broadcast storms typically associated with route discovery for
      AODV 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 DIOs should be forwarded for this
      temporary DAG.  Something looking 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 is
   preferably deployed with source routing in non-storing mode as
   explained in Section 4.1.2.

   Multicast with MPL [I-D.ietf-roll-trickle-mcast] 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 adaption layers [RFC4944] and
   [I-D.ietf-6lo-lowpanz].  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 5 hops helps to
   achieve this even while using source routing.

   Dropped packets are often experienced in the targeted environments.
   ICMP, UDP and even TCP flows may benefit from link layer unicast
   acknowledgments and retransmissions.  Link layer unicast
   acknowledgments are compulsory 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], [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

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   path towards a desired destination that is neither the root of a DAG,
   nor a destination originating DAO signalling.  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] is necessary in home automation and building
   control networks, as point-to-point style traffic 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.

   When more reliability is required, P2P-RPL enables the establishment
   of multiple independent paths.  For 1-hop destinations this means
   that one 1-hop communication and a second 2-hop communication take
   place via a neighbouring node.  Such a pair of redundant
   communication paths can be achieved by using MPL where the source is
   a MPL forwarder, while a second MPL forwarder is 1 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 2nd
   forwarder.  The 2nd 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
   (typically15 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 is necessary 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
   is preferable to reduce memory consumption in repeaters with
   constrained memory when source routing is used.

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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 which 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

   In each of the above mentioned constrained deployments, a link layer
   node (e.g. coordinator or master) assumes the role as authoritative
   root node, transmitting singlecast RAs with a 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 is designed to be aware of sleeping nodes in order to
   support the distribution of updated global prefixes to such sleeping

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
   necessarily joins at least one RPL instance, as a new, temporary
   instance is created during each P2P-RPL route discovery operation.  A
   node can be designed to join multiple RPL instances.

4.1.2.  Storing vs. Non-Storing Mode

   Non-storing mode is necessary 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.  DAO messages are not acknowledged in networks composed
   of battery operated field devices in order to minimize the power

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   consumption overhead associated with path discovery.  The DAO
   messages build up a source route because the nodes are recommended to
   be in non-storing mode.

   If devices in LLNs participate in multiple RPL instances and DODAGs,
   it is highly recommended that both the RPLInstance ID and the DODAGID
   be included in the DAO.

4.1.4.  Path Metrics

   ETX is the recommended metric.  [RFC6551] provides other options.

   It is recommended that packets from asymmetric and/or unstable
   channels are deleted at layer 2.

4.1.5.  Objective Function

   OF0 is the recommended Objective Function.  Other Objective Functions
   should be used only 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 of
   [RFC6550].  Given that the majority of nodes in the building do not
   physically move around, creating new DODAGs should not happen

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;
   [I-D.ietf-roll-trickle-mcast] is the generally accepted protocol for
   home and building deployments.  This section relies heavily on the
   conclusions of [RT-MPL].

   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

   Guaranteeing timeliness is intimately related to the density of the
   MPL routers.  In ideal circumstances the message is propagated as a

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   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
   be repeated.  Repetition of the message can be inhibited by a small
   value of k.  To assure timeliness, the value of k should be chosen
   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
   channels, it is advisable to choose the network such that at least 2
   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 in
   the home is not possible to automatically configure the forwarder
   topology at the time of writing this document.

4.1.8.  Security

   In order to support low-cost devices and devices running on a
   battery, RPL uses either unsecured messages or secured messages.  If
   RPL is used with unsecured messages, link layer security is a minimum
   security requirement (see Section 7).  If RPL is used with secured
   messages, the following RPL security parameter values are

   o  T = '0': Do not use timestamp in the Counter Field.

   o  Algorithm = '0': Use CCM with AES-128

   o  KIM = '10': Use group key, Key Source present, Key Index present

   o  LVL = 0: Use MAC-32

4.1.9.  P2P communications

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

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4.1.10.  IPv6 address configuration

   Assigned IP addresses follow IETF standards to be routable and unique
   within the routing domain.

4.2.  Layer 2 features

   No particular requirements exist for layer 2 but for the ones 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.  6LowPAN options assumed

   Not applicable

4.2.4.  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.

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 = 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
   which affects the DIO, the change is ideally communicated to a node,
   n hops away, within n times Imin.  Often, dependent on the node
   density, packets are lost, or not sent,leading to larger delays.

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   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 during more than 10 minutes it can
   safely conclude 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

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
   to 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].

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   In addition, end to end delays and message losses are reduced, by
   adding a real-time layer between MPL and MAC to throw away too late
   messages and favour 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.

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

   o  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.  Imin

   The recommended value is Imin = 10 - 50 ms.

   When Imin is chosen 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.

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   The recommended value is Imax = 100 - 400 ms.

   The value of Imax is less important than the value of max_expiration.
   Given an Imin value of 10 ms, the 3rd MPL interval has a value of
   10*2*2 = 40 ms.  When Imin has a value of 40 ms, the 3rd interval has
   a value of 160 ms.  Given that more than 3 intervals are unnecessary,
   the Imax does not contribute much to the performance.

5.1.3.  Other parameters

   Other parameters are the k parameter and the max_expiration

   k > q (see condition above).  Under this condition and for small
   Imin, 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 which will probably not meet their deadline.

6.  Manageability Considerations

   At this moment it is not clear how homenets will be managed.
   Consequently it is not clear which tools will be used and which
   parameters must be exposed 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 MIBS, NETCONF modules, DHCP and
   others) with large differences between the ways an installation is

7.  Security Considerations

   This section refers to the security considerations of [RFC6997],
   [RFC6550], [I-D.ietf-roll-trickle-mcast], and the counter measures
   discussed in sections 6 and 7 of [I-D.ietf-roll-security-threats].

   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 which 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

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   securing RPL messages.  In both cases, the assumption is that the
   credential is a shared key.  Therefore, a mechanism is required which
   allows secure distribution of a shared key and configuration of
   network identity.  Both can rely on: (i) pre-installation using an
   out-of-band method, (ii) delivered securely when a device is
   introduced into the network or (iii) delivered securely by a trusted
   neighbouring device.  The shared key MUST be stored in a secure
   fashion which makes it difficult to be read by an unauthorized party.

   Securely delivering a key requires a delivery mechanism that has data
   origin authentication, confidentiality and integrity protection.  On
   reception of the delivered key, freshness of the delivered key needs
   to be ensured.  Securely storing a key requires a storage mechanism
   that has confidentiality and integrity protection and is only
   accessible by an authorized party.

   The network security domain is typically distinct from the
   application security domains within the network, of which there may
   be more than one.  For this reason, end-to-end security between
   applications is recommended by using DTLS [RFC6347] or TLS [RFC5246].

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 good practice to create a network of
   nodes which 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 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.  The Protocol for carrying Authentication for Network Access
   (PANA) Relay Element [RFC6345] in conjunction with PANA [RFC5191]
   provides a framework for network access and delivery of common link

   New approaches to initial security deployment are being developed in
   [I-D.kumar-dice-dtls-relay] and
   [I-D.richardson-6tisch--security-6top].  Other initiatives are likely
   to emerge in the context of minimal intervention configuration.  At
   this moment it is not clear what the final most widely deployed
   solution will be.

   For building control an installer will probably use an installation
   tool that establishes a secure communication path with the joining
   node.  In the home, nodes can be visually inspected by the home owner

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   and simple measures like pushing buttons simultaneously on joint and
   joining devices is probably sufficient.

   This recommendation is in line with the countermeasures described in
   section 6.1.1 of [I-D.ietf-roll-security-threats]

7.2.  Security Considerations during incremental deployment

   When nodes are lost, no additional security measures are needed, the
   network remains secure as before by not allowing the addition of new
   nodes.  New nodes can be added by using the same protocols used for
   initial deployment.  Some protocols may need a state change to a
   subset of the secured nodes, other protocols only need the presence
   of a "trusted" installation node [RFC6345], [RFC5191], or

7.3.  Security Considerations for P2P uses

   Refer to the security considerations of [RFC6997].  Many initiatives
   are under way to provide lighter weight security such as:
   [I-D.ietf-dice-profile] and [I-D.keoh-dice-multicast-security]

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 CoAP application as specified in
   [I-D.ietf-core-groupcomm].  An alternative is the creation of a MPL
   MIB and use of SNMPv3 [RFC3411] or equivalent techniques to specify
   the Multicast addresses in the MIB.  The application of security
   measures for the specification of the multicast addresses assures
   that the routing of MPL packets is secured.

7.5.  RPL Security features

   This section follows the structure of section 7, "RPL security
   features" of [I-D.ietf-roll-security-threats], where a thorough
   analysis of security threats and proposed counter measures relevant
   to RPL and MPL are done.

   In accordance with section 7.1 of [I-D.ietf-roll-security-threats],
   "Confidentiality features", a secured RPL protocol implements payload
   protection, as explained in Section 7 of this document.  The
   attributes key-length and life-time of the keys depend on operational
   conditions, maintenance and installation procedures.

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   Section 7.1 and Section 7.2 of this document recommend link-layer
   measures to assure integrity in accordance with section 7.2 of
   [I-D.ietf-roll-security-threats], "Integrity features".

   The provision of multiple paths recommended in section 7.3
   "Availability features" of [I-D.ietf-roll-security-threats] is also
   recommended from a reliability point of view.  Randomly choosing
   paths is a possibility.

   Key management discussed in section 7.4, "Key Management" of
   [I-D.ietf-roll-security-threats], is not standardized and discussions

   Section 7.5, "Considerations on Matching Application Domain Needs" of
   [I-D.ietf-roll-security-threats] applies as such.

8.  Other related protocols

   Application and transport protocols used in home and building
   automation domains are expected to mostly consist in 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

9.  IANA Considerations

   No considerations for IANA pertain to this document.

10.  Acknowledgements

   This document reflects discussions and remarks from several
   individuals including (in alphabetical order): Mukul Goyal, Sandeep
   Kumar, Jerry Martocci, Charles Perkins, Michael Richardson, and Zach

11.  Changelog

   Changes from version 0 to version 1.

   o  Adapted section structure to template.

   o  Standardized the reference syntax.

   o  Section 2.2, moved everything concerning algorithms to section
      2.2.7, and adapted text in 2.2.1-2.2.6.

   o  Added MPL parameter text to section 4.1.7 and section 4.3.1.

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   o  Replaced all TODO sections with text.

   o  Consistent use of border router, monitoring, home- and building

   o  Reformulated security aspects with references to other

   o  MPL and RPL parameter values introduced.

   Changes from version 1 to version 2.

   o  Clarified common characteristics of control in home and building.

   o  Clarified failure behaviour of point to point communication in

   o  Changed examples, more hvac and less lighting.

   o  Clarified network topologies.

   o  replaced reference to smart_object paper by reference to I-D.roll-

   o  Added a concise definition of secure delivery and secure storage

   o  text about securing network with PANA

   Changes from version 2 to version 3.

   o  Changed security section to follow the structure of security
      threats draft.

   o  Added text to DODAG repair sub-section

   Changes from version 3 to version 4.

   o  Renumbered sections and moved text to conform to applicability

   o  Extended MPL parameter value text

   o  Added references to building control products

   Changes from version 4 to version 5.

   o  Large editing effort to streamline text

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   o  Rearranged Normative and Informative references

   o  Replaced RFC2119 terminology by non-normative terminology

   o  Rearranged text of section 7, 7.1, and 7.2 to agree with the
      intention of section 7.2

   Changes from version 5 to version 6.

   o  Issues #162 - #166 addressed

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, September 2007.

   [RFC5191]  Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H., and A.
              Yegin, "Protocol for Carrying Authentication for Network
              Access (PANA)", RFC 5191, May 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5548]  Dohler, M., Watteyne, T., Winter, T., and D. Barthel,
              "Routing Requirements for Urban Low-Power and Lossy
              Networks", RFC 5548, May 2009.

   [RFC5673]  Pister, K., Thubert, P., Dwars, S., and T. Phinney,
              "Industrial Routing Requirements in Low-Power and Lossy
              Networks", RFC 5673, October 2009.

   [RFC5826]  Brandt, A., Buron, J., and G. Porcu, "Home Automation
              Routing Requirements in Low-Power and Lossy Networks", RFC
              5826, April 2010.

   [RFC5867]  Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
              "Building Automation Routing Requirements in Low-Power and
              Lossy Networks", RFC 5867, June 2010.

   [RFC6282]  Hui, J. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              September 2011.

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   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [RFC6550]  Winter, T., Thubert, P., 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, March 2012.

   [RFC6551]  Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D.
              Barthel, "Routing Metrics Used for Path Calculation in
              Low-Power and Lossy Networks", RFC 6551, March 2012.

   [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, March

   [RFC6997]  Goyal, M., Baccelli, E., Philipp, M., Brandt, A., and J.
              Martocci, "Reactive Discovery of Point-to-Point Routes in
              Low-Power and Lossy Networks", RFC 6997, August 2013.

   [RFC6998]  Goyal, M., 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,
              August 2013.

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, January 2014.

              Hui, J. and R. Kelsey, "Multicast Protocol for Low power
              and Lossy Networks (MPL)", draft-ietf-roll-trickle-
              mcast-11 (work in progress), November 2014.

              Tsao, T., Alexander, R., Dohler, M., Daza, V., Lozano, A.,
              and M. Richardson, "A Security Threat Analysis for Routing
              Protocol for Low-power and lossy networks (RPL)", draft-
              ietf-roll-security-threats-11 (work in progress), October

              "IEEE 802.15.4 - Standard for Local and metropolitan area
              networks -- Part 15.4: Low-Rate Wireless Personal Area
              Networks", <IEEE Standard 802.15.4>.

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   [G.9959]   "ITU-T G.9959 Short range narrow-band digital
              radiocommunication transceivers - PHY and MAC layer
              specifications", <ITU-T G.9959>.

12.2.  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,
              December 2002.

   [RFC6345]  Duffy, P., Chakrabarti, S., Cragie, R., Ohba, Y., and A.
              Yegin, "Protocol for Carrying Authentication for Network
              Access (PANA) Relay Element", RFC 6345, August 2011.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228, May 2014.

              Brandt, A. and J. Buron, "Transmission of IPv6 packets
              over ITU-T G.9959 Networks", draft-ietf-6lo-lowpanz-08
              (work in progress), October 2014.

              Tschofenig, H., "A Datagram Transport Layer Security
              (DTLS) 1.2 Profile for the Internet of Things", draft-
              ietf-dice-profile-05 (work in progress), October 2014.

              Keoh, S., Kumar, S., Garcia-Morchon, O., Dijk, E., and A.
              Rahman, "DTLS-based Multicast Security in Constrained
              Environments", draft-keoh-dice-multicast-security-08 (work
              in progress), July 2014.

              Kumar, S., Keoh, S., and O. Garcia-Morchon, "DTLS Relay
              for Constrained Environments", draft-kumar-dice-dtls-
              relay-02 (work in progress), October 2014.

              Rahman, A. and E. Dijk, "Group Communication for CoAP",
              draft-ietf-core-groupcomm-25 (work in progress), September

              Richardson, M., "6tisch secure join using 6top", draft-
              richardson-6tisch--security-6top-04 (work in progress),
              November 2014.

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

              Baccelli, E., Phillip, M., Brandt, A., Valev , H., and J.
              Buron , "Report on P2P-RPL Interoperability Testing",
              RR-7864 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

              Lighting, Philips., "OccuSwitch wireless", Brochure, http:
              /osw/MK_OSWNETBROC_5.pdf, May 2012.

              Clanton and Associates, ., "A Life Cycle Cost Evaluation
              of Multiple Lighting Control Strategies", Wireless
              Lighting Control,
              clanton_lighting_control_report_0411.pdf, February 2014.

   [RTN2011]  Holtman, K. and P. van der Stok, "Real-time routing for
              low-latency 802.15.4 control networks", International
              Workshop on Real-Time Networks; Euromicro Conference on
              Real-Time Systems, July 2011.

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

              Kastner, W., Neugschwandtner, G., Soucek, S., and H.
              Newman, "Communication Systems for Building Automation and
              Control", Proceedings of the IEEE Vol 93, No 6, June 2005.

Appendix A.  RPL shortcomings in home and building deployments

A.1.  Risk of undesired long P2P routes

   The DAG, being a tree structure is formed from a root.  If nodes
   residing in different branches have a need for communicating
   internally, DAG mechanisms provided in RPL [RFC6550] will propagate

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   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 just two router nodes but in unfortunate
   cases, RPL may send traffic three hops up and three hops down again.
   This leads to several undesired phenomena described in the following

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 RF systems any child node of the root is not just directing
   RF power downwards 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.  In 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 last DAO message was issued the
   destination module and the time one of the parents sends the next
   DIO, the destination cannot be reached.  There is no mechanism in RPL
   to initiate 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 250ms, 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

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   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.  While 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 on the connectivity between neighbouring nodes are
   discussed in [RTN2011] and [MEAS].

   The work is motivated by the measurements in literature which 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 an
   iso-strength line.  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 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, (3) a region without reception.  In the
   text below it is shown that installation of meshes with neighbours in
   the clear region is not sufficient.

   [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,

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

   Eight nodes were distributed over a surface of 30m2.  All nodes are
   at one hop distance from each other and are situated in the clear
   region of each other.  Each node sends messages to each of its
   neighbours, and repeats the message until it arrives.  The latency of
   the message was measured over periods of at least a week.  It is
   noticed that latencies longer than a second occurred without apparent
   reasons, but only during working days and never in the weekends.  Bad
   periods could last for minutes.  By sending messages via two paths:

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   (1) one hop path directly, and (2) two hop path via a randomly chosen
   neighbour, the probability of delays larger than 100 ms decreased

   The conclusion is that even for 1-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 2-hop path to increase the reliability
   of timely message transfer.

   [MEAS] confirms that temporary bad reception by close neighbours can
   occur within other types of areas.  Nodes were installed on the
   ceiling in a grid with a distance of 30-50 cm between nodes. 200
   nodes were distributed over an area of 10m x 5m.  It clearly
   transpired that with increasing distance the probability of reception
   decreases.  At the same time a few nodes furthest away from the
   sender had a high probability of message reception, while some close
   neighbours of the sender did not receive messages.  The patterns of
   clear reception nodes evolved over time.

   The conclusion is that even for direct neighbours reception can
   temporarily be bad during periods of several minutes.  For a reliable
   and timely communication it is imperative to have at least two
   communication paths available (e.g. two hop paths next to the 1-hop
   path for direct neighbours).

Authors' Addresses

   Anders Brandt
   Sigma Designs


   Emmanuel Baccelli


   Robert Cragie


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   Peter van der Stok


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