Roll                                                             D. Popa
Internet-Draft                                               M. Gillmore
Intended status: Standards Track                              Itron, Inc
Expires: January 23, 2015                                     L. Toutain
                                                        Telecom Bretagne
                                                                  J. Hui
                                                                R. Ruben
                                                               K. Monden
                             Hitachi, Ltd., Yokohama Research Laboratory
                                                           July 22, 2014

Applicability Statement for the Routing Protocol for Low Power and Lossy
                     Networks (RPL) in AMI Networks


   This document discusses the applicability of RPL in Advanced Metering
   Infrastructure (AMI) networks.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on January 23, 2015.

Copyright Notice

   Copyright (c) 2014 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
   ( in effect on the date of

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   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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Required Reading  . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Out of scope requirements . . . . . . . . . . . . . . . .   4
   2.  Routing Protocol for LLNs (RPL) . . . . . . . . . . . . . . .   4
   3.  Description of AMI networks for electric meters . . . . . . .   5
     3.1.  Deployment Scenarios  . . . . . . . . . . . . . . . . . .   5
   4.  Smart Grid Traffic Description  . . . . . . . . . . . . . . .   7
     4.1.  Smart Grid Traffic Characteristics  . . . . . . . . . . .   7
     4.2.  Smart Grid Traffic QoS Requirements . . . . . . . . . . .   8
     4.3.  RPL applicability per Smart Grid Traffic Characteristics    9
   5.  Layer 2 applicability . . . . . . . . . . . . . . . . . . . .   9
     5.1.  IEEE Wireless Technology  . . . . . . . . . . . . . . . .   9
     5.2.  IEEE PowerLine Communication (PLC) technology . . . . . .  10
   6.  Using RPL to Meet Functional Requirements . . . . . . . . . .  10
   7.  RPL Profile . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  RPL Features  . . . . . . . . . . . . . . . . . . . . . .  11
       7.1.1.  RPL Instances . . . . . . . . . . . . . . . . . . . .  11
       7.1.2.  Storing vs. Non-Storing Mode  . . . . . . . . . . . .  11
       7.1.3.  DAO Policy  . . . . . . . . . . . . . . . . . . . . .  12
       7.1.4.  Path Metrics  . . . . . . . . . . . . . . . . . . . .  12
       7.1.5.  Objective Function  . . . . . . . . . . . . . . . . .  12
       7.1.6.  DODAG Repair  . . . . . . . . . . . . . . . . . . . .  12
       7.1.7.  Multicast . . . . . . . . . . . . . . . . . . . . . .  13
       7.1.8.  Security  . . . . . . . . . . . . . . . . . . . . . .  13
       7.1.9.  Peer-to-Peer communications . . . . . . . . . . . . .  13
     7.2.  Description of Layer-two features . . . . . . . . . . . .  13
       7.2.1.  IEEE 1901.2 PHY and MAC sub-layer features  . . . . .  13
       7.2.2.  IEEE 802.15.4 (g + e) PHY and MAC features  . . . . .  14
       7.2.3.  IEEE MAC sub-layer Security Features  . . . . . . . .  15
     7.3.  6LowPAN Options . . . . . . . . . . . . . . . . . . . . .  17
     7.4.  Recommended Configuration Defaults and Ranges . . . . . .  17
       7.4.1.  Trickle Parameters  . . . . . . . . . . . . . . . . .  17
       7.4.2.  Other Parameters  . . . . . . . . . . . . . . . . . .  18
   8.  Manageability Considerations  . . . . . . . . . . . . . . . .  19
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     9.1.  Security Considerations during initial deployment . . . .  20
     9.2.  Security Considerations during incremental deployment . .  20
   10. Other Related Protocols . . . . . . . . . . . . . . . . . . .  20

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   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     13.1.  Informative References . . . . . . . . . . . . . . . . .  20
     13.2.  Normative references . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   Advanced Metering Infrastructure (AMI) systems enable the
   measurement, configuration, and control of energy, gas and water
   consumption and distribution, through two-way scheduled, on
   exception, and on-demand communication.

   AMI networks are composed of millions of endpoints, including meters,
   distribution automation elements, and eventually home area network
   devices.  They are typically inter-connected using some combination
   of wireless and power-line communications, forming the so-called
   Neighbor Area Network (NAN) along with a backhaul network providing
   connectivity to "command-and-control" management software
   applications at the utility company back office.

1.1.  Requirements Language

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

1.2.  Required Reading

   [surveySG] gives an overview of Smart Grid architecture and related

   NAN can use wireless communication technology in which case is using,
   from the IEEE 802.15.4 standard family, the IEEE 802.15.4g PHY Layer
   amendment and IEEE 802.15.4e MAC sub-layer amendment, specifically
   adapted to smart grid networks.

   NAN can also use PLC (Power Line Communication) technology as an
   alternative to wireless communications.  Several standards for PLC
   technology have emerged, such as IEEE P1901.2.

   NAN can further use a mix of wireless and PLC technologies to
   increase the network coverage ratio, a critical requirement for AMI

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1.3.  Out of scope requirements

   The following are outside the scope of this document:

   o  Applicability statement for RPL in AMI networks composed of
      battery-powered devices (i.e., gas/water meters).

   o  Applicability statement for RPL in AMI networks composed of a mix
      of AC powered devices (i.e., electric meters) and battery-powered
      meters (i.e., gas/water meters).

   o  Applicability statement for RPL storing mode of operation in AMI

2.  Routing Protocol for LLNs (RPL)

   RPL provides routing functionality for mesh networks that can scale
   up to thousands of resource-constrained devices, interconnected by
   low power and lossy links, and communicating with the external
   network infrastructure through a common aggregation point(s) (e.g., a

   RPL builds a Directed Acyclic Graph (DAG) routing structure rooted at
   a LBR (LLN Border Router), ensures loop-free routing, and provides
   support for alternate routes, as well as, for a wide range of routing
   metrics and policies.

   RPL was designed to operate in energy-constrained environments and
   includes energy-saving mechanisms (e.g., Trickle timers) and energy-
   aware metrics.  RPL's ability to support multiple different metrics
   and constraints at the same time enables it to run efficiently in
   heterogeneous networks composed of nodes and links with vastly
   different characteristics [RFC6551].

   This document describes the applicability of RPL non-storing mode (as
   defined in [RFC6550]) to AMI deployments.  RPL was designed to meet
   the following application requirements:

   o  Routing Requirements for Urban Low-Power and Lossy Networks

   o  Industrial Routing Requirements in Low-Power and Lossy Networks

   o  Home Automation Routing Requirements in Low-Power and Lossy
      Networks [RFC5826].

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   o  Building Automation Routing Requirements in Low-Power and Lossy
      Networks [RFC5867].

   The Routing Requirements for Urban Low-Power and Lossy Networks are
   applicable to AMI networks as well.  The terminology used in this
   document is defined in [I-D.ietf-roll-terminology-13].

3.  Description of AMI networks for electric meters

   In many deployments, in addition to measuring energy consumption, the
   electric meter network plays a central role in the Smart Grid since
   the device enables the utility company to control and query the
   electric meters themselves and can serve as a backhaul for all other
   devices in the Smart Grid, e.g., water and gas meters, distribution
   automation and home area network devices.  Electric meters may also
   be used as sensors to monitor electric grid quality and to support
   applications such as Electric Vehicle charging.

   Electric meter networks can be composed of millions of smart meters
   (or nodes), each of which is resource-constrained in terms of
   processing power, storage capabilities, and communication bandwidth,
   due to a combination of factors including Federal Communications
   Commission (FCC) or other continents' regulations on spectrum use,
   American National Standards Institute (ANSI) standards or other
   continents' regulation on meter behavior and performance, on heat
   emissions within the meter, form factor and cost considerations.
   These constraints result in a compromise between range and
   throughput, with effective link throughput of tens to a few hundred
   kilobits per second per link, a potentially significant portion of
   which is taken up by protocol and encryption overhead when strong
   security measures are in place.

   Electric meters are often interconnected into multi-hop mesh
   networks, each of which is connected to a backhaul network leading to
   the utility company network through a network aggregation point,
   e.g., an LBR.

3.1.  Deployment Scenarios

   AMI networks are composed of millions of endpoints distributed across
   both urban and rural environments.  Such endpoints can include
   electric, gas, and water meters, distribution automation elements,
   and home area network devices.

   Devices in the network communicate directly with other devices in
   close proximity using a variety of low-power and/or lossy link
   technologies that are both wireless and wired (e.g., IEEE 802.15.4g,
   IEEE 802.15.4e, IEEE 1901.2, IEEE 802.11).  In addition to serving as

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   sources and destinations of packets, many network elements typically
   also forward packets and thus form a mesh topology.

   In a typical AMI deployment, groups of meters within physical
   proximity form routing domains, each in the order of a 1,000 to
   10,000 meters.  Thus, each electric meter mesh typically has several
   thousand wireless endpoints, with densities varying based on the area
   and the terrain.

                                          M   M   M   M  | M
             /-----------\            /---+---+---+---+--+-+- phase 1
    +----+   ( Internet  )    +----+ /   M    M    M    M
    |MDMS|---(           )----|LBR |/----+----+----+----+---- phase 2
    +----+   (   WAN     )    +----+\
              \----------/           \   M  M      M   M
                                      \--+--+----+-+---+----- phase 3
                                                  \   M   M

                      Figure 1: Typical NAN Topology

   A typical AMI network architecture (see figure Figure 1) is composed
   of a MDMS (Meter Data Management System) connected through IP network
   to a LBR, which can be located in the power substation or somewhere
   else in the field.  The power substation connects the households and
   buildings.  The physical topology of the electrical grid is a tree
   structure, either due to the 3 different power phases coming through
   the sub-station or just to the electrical network topology.  Meters
   (represented by a M in the previous figure) can also participate to a
   HAN (Home-Area Network).  The scope of this document is the
   communication between the LBR and the meters,i.e., the NAN segment.

   Node density can vary significantly.  For example, apartment
   buildings in urban centers may have hundreds of meters in close
   proximity, whereas rural areas may have sparse node distributions and
   include nodes that only have a small number of network neighbors.
   Each routing domain is connected to the larger IP infrastructure
   through one or more LBRs, which provide Wide Area Network (WAN)
   connectivity through various traditional network technologies, e.g.,
   Ethernet, cellular, private WiMAX-based WAN, optical fiber.  Paths in
   the mesh between a network node and the nearest LBR may be composed
   of several hops or even several tens of hops.  Powered from the main

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   line, electric meters have less energy constraints than battery
   powered devices, such as gas and water meters, and can afford the
   additional resources required to route packets.

   As a function of the the technology used to exchange information, the
   logical network topology will not necessarily match the eletric grid
   topology.  If meters exchange information through radio technologies
   such as in IEEE 802.15.4 family, the topology is a meshed network,
   where nodes belonging to the same DODAG can be connected to the grid
   through different substations.  If narrowband PLC technology is used,
   it will follow more or less the physical tree structure since
   diaphony may allow one phase to communicate with the other.  This is
   particularly true near the LBR.  Some mixt topology can also be
   observed, since some LBR may be strategically installed in the field
   to avoid all the communications going through a single LBR.
   Nethertheless, the short propagation range forces meters to relay the

4.  Smart Grid Traffic Description

4.1.  Smart Grid Traffic Characteristics

   In current AMI deployments, metering applications typically require
   all smart meters to communicate with a few head-end servers, deployed
   in the utility company data center.  Head-end servers generate data
   traffic to configure smart meter data reading or initiate queries,
   and use unicast and multicast to efficiently communicate with a
   single device (i.e., Point-to-Point (P2P) communications) or groups
   of devices respectively (i.e., Point-to-Multipoint (P2MP)
   communication).  The head-end server may send a single small packet
   at a time to the meters (e.g., a meter read request, a small
   configuration change, service switch command) or a series of large
   packets (e.g., a firmware download across one or even thousands of
   devices).  The frequency of large file transfers, e.g., firmware
   download of all metering devices, is typically much lower than the
   frequency of sending configuration messages or queries.  Each smart
   meter generates Smart Metering Data (SMD) traffic according to a
   schedule (e.g., periodic meter reads), in response to on-demand
   queries (e.g., on-demand meter reads), or in response to some local
   event (e.g., power outage, leak detection).  Such traffic is
   typically destined to a single head-end server.  The SMD traffic is
   thus highly asymmetric, where the majority of the traffic volume
   generated by the smart meters typically goes through the LBRs, and is
   directed from the smart meter devices to the head-end servers, in a
   MP2P fashion.  Current SMD traffic patterns are fairly uniform and
   well-understood.  The traffic generated by the head-end server and
   destined to metering devices is dominated by periodic meter reads,

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   while traffic generated by the metering devices is typically
   uniformly spread over some periodic read time-window.

   Smart metering applications typically do not have hard real-time
   constraints, but they are often subject to bounded latency and
   stringent reliability service level agreements.

   Distribution Automation (DA) applications typically involve a small
   number of devices that communicate with each other in a Point-to-
   Point (P2P) fashion, and may or may not be in close physical
   proximity.  DA applications typically have more stringent latency
   requirements than SMD applications.

   There are also a number of emerging applications such as electric
   vehicle charging.  These applications may require P2P communication
   and may eventually have more stringent latency requirements than SMD

4.2.  Smart Grid Traffic QoS Requirements

   As described previously, the two main traffic families in a NAN are:

   A) Meter-initiated traffic (Meter-to-head-end - M2HE)

   B) Head-end-initiated traffic (Head-end-to-meter - HE2M)

      B1)  request is sent in point-to-point to a specific meter

      B2)  request is sent in multicast to a subset of meters

      B3)  request is sent in multicast to all meters

   The M2HE are event-based, while the HE2M are mostly command-response.
   In most cases, M2HE traffic is more critical than HE2M one, but there
   can be exceptions.

   Regarding priority, traffic may also be decomposed into several
   classes :

   C1)  Highly Priority Critical traffic for Power System Outage,
      Pricing Events and Emergency Messages require a 98%+ packet
      delivery under 5 s.  Payload size < 100 bytes

   C2)  Critical Priority traffic Power Quality Events, Meter Service
      Connection and Disconnection require 98%+ packet delivery under
      10s.  Payload size < 150 bytes

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   C3)  Normal Priority traffic for System Events including Faults,
      Configuration and Security require 98%+ packet delivery under 30s.
      Payload size < 200 bytes

   C4)  Low Priority traffic for Recurrent Meter Reading require 98%+
      packet 2 hour delivery window 6 times per day.  Payload size < 400

   C5)  Background Priority traffic for firmware/software updates
      processed to 98%+ of devices with in 7 days.  Average firmware
      update is 1 MB.

4.3.  RPL applicability per Smart Grid Traffic Characteristics

   RPL non-storing mode of operation naturally support upstream and
   downstream forwarding of unicast traffic between the DODAG root and
   each DODAG node, and between DODAG nodes and DODAG root,

   Group communication model used in smart grid requires RPL non-storing
   mode of operation to support downstream forwarding of multicast
   traffic with a scope larger than link-local.  The DODAG root is the
   single device that injects multicast traffic, with a scope larger
   than link-local, into the DODAG.

   Altough not currently used in metering applications, support of peer-
   to-peer communications between DODAG nodes is identified as a key
   feature to be supported in smart grid networks.

5.  Layer 2 applicability

5.1.  IEEE Wireless Technology

   IEEE Std. 802.15.4g-2012 and IEEE 802.15.4e-2012 amendments to
   802.15.2-2011 standard have been specifically developed for smart
   grid networks.  They are the most common PHY and MAC layers used for
   wireless AMI network.  802.15.4g specifies multiple mode of operation
   (FSK, OQPSK and OFDM modulations) with speeds from 50kbps to 600kbps,
   and allows for transport of a full IPv6 packet (i.e., 1280 octets)
   without the need for upper layer segmentation and re-assembly.

   IEEE Std. 802.15.4e-2012 is an amendment to IEEE Std 802.15.4-2011
   that specifies additional media access control (MAC) behaviors and
   frame formats that allow IEEE 802.15.4 devices to support a wide
   range of industrial and commercial applications that were not
   adequately supported prior to the release of this amendment.  It is
   important to notice that 802.15.4e does not change the link-layer
   security scheme defined in 802.15.4-2011 (and 802.15.4-2006).

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5.2.  IEEE PowerLine Communication (PLC) technology

   The IEEE Std 1901.2-2013 standard specifies communications for low
   frequency (less than 500 kHz) narrowband power line devices via
   alternating current and direct current electric power lines.  IEEE
   Std 1901.2-2013 standard supports indoor and outdoor communications
   over low voltage line (line between transformer and meter, less than
   1000 V), through transformer low-voltage to medium-voltage (1000 V up
   to 72 kV) and through transformer medium-voltage to low-voltage power
   lines in both urban and in long distance (multi- kilometer) rural

   IEEE Std 1901.2 defines the PHY layer and the MAC sub-layer of the
   data link layer.  The MAC sub-layer endorses a sub-set of
   802.15.4-2006 and 802.15.4e-2012 MAC sub-layer features.

   IEEE Std 1901.2 PHY Layer bit rates are scalable up to 500 kbps
   depending on the application requirements and type of encoding used.

   IEEE Std 1901.2 MAC layer allows for transport of a full IPv6 packet
   (i.e., 1280 octets) without the need for upper layer segmentation and

   IEEE Std 1901.2 standard specifies link-layer security features that
   fully endorse 802.15.4-2006 MAC sub-layer security scheme.

6.  Using RPL to Meet Functional Requirements

   The functional requirements for most AMI deployments are similar to
   those listed in [RFC5548]:

   o  The routing protocol MUST be capable of supporting the
      organization of a large number of nodes into regions containing on
      the order of 10^2 to 10^4 nodes each.

   o  The routing protocol MUST provide mechanisms to support
      configuration of the routing protocol itself.

   o  The routing protocol SHOULD support and utilize the large number
      of highly directed flows to a few head-end servers to handle

   o  The routing protocol MUST dynamically compute and select effective
      routes composed of low-power and lossy links.  Local network
      dynamics SHOULD NOT impact the entire network.  The routing
      protocol MUST compute multiple paths when possible.

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   o  The routing protocol MUST support multicast and unicast
      addressing.  The routing protocol SHOULD support formation and
      identification of groups of field devices in the network.

   RPL supports the following features:

   o  Scalability: Large-scale networks characterized by highly directed
      traffic flows between each smart meter and the head-end servers in
      the utility network.  To this end, RPL builds a Directed Acyclic
      Graph (DAG) rooted at each LBR.

   o  Zero-touch configuration: This is done through in-band methods for
      configuring RPL variables using DIO messages, and DIO message

   o  The use of links with time-varying quality characteristics: This
      is accomplished by allowing the use of metrics that effectively
      capture the quality of a path (e.g., Expected Transmission Count
      (ETX)) and by limiting the impact of changing local conditions by
      discovering and maintaining multiple DAG parents, and by using
      local repair mechanisms when DAG links break.

7.  RPL Profile

7.1.  RPL Features

7.1.1.  RPL Instances

   RPL operation is defined for a single RPL instance.  However,
   multiple RPL instances can be supported in multi-service networks
   where different applications may require the use of different routing
   metrics and constraints, e.g., a network carrying both SDM and DA

7.1.2.  Storing vs. Non-Storing Mode

   In most scenarios, electric meters are powered by the grid they are
   monitoring and are not energy-constrained.  Instead, electric meters
   have hardware and communication capacity constraints that are
   primarily determined by cost, and secondarily by power consumption.
   As a result, different AMI deployments can vary significantly in
   terms of memory size, computation power and communication
   capabilities.  For this reason, the use of RPL storing or non-storing
   mode SHOULD be deployment specific.  When meters are memory
   constrained and cannot adequately store the route tables necessary to
   support hop-by-hop routing, RPL non-storing mode SHOULD be preferred.
   On the other hand, when nodes are capable of storing such routing
   tables, the use of storing mode may lead to reduced overhead and

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   route repair latency.  However, in high-density environments, storing
   routes can be challenging because some nodes may have to maintain
   routing information for a large number of descendents.  In this
   document we only focus on non-storing mode of operation.

7.1.3.  DAO Policy

   Two-way communication is a requirement in AMI systems.  As a result,
   nodes SHOULD send DAO messages to establish downward paths from the
   root to themselves.

7.1.4.  Path Metrics

   Smart metering deployments utilize link technologies that may exhibit
   significant packet loss and thus require routing metrics that take
   packet loss into account.  To characterize a path over such link
   technologies, AMI deployments can use the Expected Transmission Count
   (ETX) metric as defined in [RFC6551].

   Additional metrics may be defined in companion RFCs.

7.1.5.  Objective Function

   RPL relies on an Objective Function for selecting parents and
   computing path costs and rank.  This objective function is decoupled
   from the core RPL mechanisms and also from the metrics in use in the
   network.  Two objective functions for RPL have been defined at the
   time of this writing, OF0 [RFC6552] and MRHOF [RFC6719], both of
   which define the selection of a preferred parent and backup parents,
   and are suitable for AMI deployments.  Additional objective functions
   may be defined in companion RFCs.

7.1.6.  DODAG Repair

   To effectively handle time-varying link characteristics and
   availability, AMI deployments SHOULD utilize the local repair
   mechanisms in RPL.  Local repair is triggered by broken link
   detection.  The first local repair mechanism consists of a node
   detaching from a DODAG and then re-attaching to the same or to a
   different DODAG at a later time.  While detached, a node advertises
   an infinite rank value so that its children can select a different
   parent.  This process is known as poisoning and is described in
   Section&nbsp; of [RFC6550].  While RPL provides an option to form a
   local DODAG, doing so in AMI for electric meters is of little benefit
   since AMI applications typically communicate through a LBR.  After
   the detached node has made sufficient effort to send notification to
   its children that it is detached, the node can rejoin the same DODAG
   with a higher rank value.  The configured duration of the poisoning

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   mechanism needs to take into account the disconnection time
   applications running over the network can tolerate.  Note that when
   joining a different DODAG, the node need not perform poisoning.  The
   second local repair mechanism controls how much a node can increase
   its rank within a given DODAG Version (e.g., after detaching from the
   DODAG as a result of broken link or loop detection).  Setting the
   DAGMaxRankIncrease to a non-zero value enables this mechanism, and
   setting it to a value of less than infinity limits the cost of count-
   to-infinity scenarios when they occur, thus controlling the duration
   of disconnection applications may experience.

7.1.7.  Multicast

   Multicast support for RPL in non-storing mode are being developed in
   companion RFCs (see [draft-ietf-roll-trickle-mcast-06]).

7.1.8.  Security

   AMI deployments operate in areas that do not provide any physical
   security.  For this reason, the link layer, transport layer and
   application layer technologies utilized within AMI networks typically
   provide security mechanisms to ensure authentication,
   confidentiality, integrity, and freshness.  As a result, AMI
   deployments may not need to implement RPL's security mechanisms and
   could rely on link layer and higher layer security features.

7.1.9.  Peer-to-Peer communications

   Basic peer-to-peer capabilities are already defined in the RPL
   [RFC6550].  Additional mechanisms for peer-to-peer communications are
   proposed in companion RFCs (see [RFC6997]).

7.2.  Description of Layer-two features

7.2.1.  IEEE 1901.2 PHY and MAC sub-layer features

   The IEEE Std 1901.2 PHY layer is based on OFDM modulation and defines
   a time frequency interleaver over the entire PHY frame coupled with a
   Reed Solomon and Viterbi Forward Error Correction for maximum
   robustness.  Since the noise level in each OFDM sub-carrier can vary
   significantly IEEE 1901.2 specifies two complementary mechanisms
   allowing to fine-tune the robustness/performance tradeoff implicit in
   such systems.  More specifically, the first (coarse-grained)
   mechanism, defines the modulation from several possible choices
   (robust (super-ROBO, ROBO), BPSK, QPSK,...).  The second (fine-
   grained) maps the sub-carriers which are too noisy and deactivates

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   The existence of multiple modulations and dynamic frequency exclusion
   renders the problem of selecting a path between two nodes non-
   trivial, as the possible number of combinations increases
   significantly, e.g.  use a direct link with slow robust modulation,
   or use a relay meter with fast modulation and 12 disabled sub-
   carriers.  In addition, IEEE 1901.2 technology offers a mechanism
   (adaptive tone map) for periodic exchanges on the link quality
   between nodes to constantly react to channel fluctuations.  Every
   meter keeps a state of the quality of the link to each of its
   neighbors by either piggybacking the tone mapping on the data
   traffic, or by sending explicit tone map requests.

   IEEE 1901.2 MAC frame format shares most in common with the IEEE
   802.15.4-2006 MAC frame format [IEEE802.15.4], with a few exceptions
   described below.

   o  IEEE 1901.2 MAC frame is obtained by prepending a Segment Control
      Field to the IEEE 802.15.4-2006 MAC header.  One function of the
      Segment Control Field is to signal the use of the MAC sub-layer
      segmentation and reassembly.

   o  IEEE 1901.2 MAC frames uses only the 802.15.4 MAC addresses with a
      length of 16 and 64 bits.

   o  IEEE 1901.2 MAC sub-layer endorses the concept of Information
      Elements, as defined in IEEE 802.15.4e-2012 [IEEE802.15.4e].  The
      format and use of Information Elements are not relevant to RPL
      applicability statement.

   The IEEE 1901.2 PHY frame payload size varies as a function of the
   modulation used to transmit the frame and the strength of the Forward
   Error Correction scheme.

   The maximum IEEE 1901.2 PHY frame payload is 512 bytes.  The maximum
   IEEE 1901.2 MAC frame payload is 1280 bytes, which supports the IPv6
   minimum MTU requirement.

   When there is a mistmatch between the PHY frame payload size and the
   size of a MAC frame carrying an IPv6 packet, IEEE 1901.2 specifies a
   MAC sub-layer segmentation and re-assembly mechanism that provides a
   reliable one-hop transfer of that MAC frame segments.

7.2.2.  IEEE 802.15.4 (g + e) PHY and MAC features

   IEEE Std 802.15.4g defines multiple modes of operation, where each
   mode uses different modulation and has multiple data rates.
   Additionally, 802.15.4g PHY layer includes mechanisms to improve the
   robustness of the radio communications, such as data whitening and

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   Forward Error Correction coding.  The 802.15.4g PHY frame payload can
   carry up to 2048 octets.

   The IEEE Std 802.15.4g defines the following modulations: MR-FSK
   (Multi- Rate FSK), MR-OFDM (Multi-Rate OFDM) and MR-O-QPSK (Multi-
   Rate O-QPSK).  The (over-the-air) bit rates for these modulations
   range from 4.8 to 600kbps for MR-FSK, from 50 to 600kbps for MR-OFDM
   and from 6.25 to 500kbps for MR-O-QPSK.

   The MAC sub-layer running on top of a 4g radio link is based on IEEE
   802.15.4e.  The 802.15.4e MAC allows for a variety of modes for
   operation.  These include: Timetimeslotslotted channel hopping
   (TSCH), specifically designed for application domains such as process
   automation, Low latency deterministic networks (LLDN), for
   application domains such as factory automation, Deterministic and
   synchronous multi-channel extension (DSME), for general industrial
   and commercial application domains that includes Channel diversity to
   increase network robustness, and Asynchronous multi-channel
   adaptation (AMCA), for large infrastructure application domains.

   The MAC addressing scheme supports short (16-bits) addresses along
   with extended (64-bits) addresses.

   Information Elements, Enhanced Beacons and frame version 2, as
   defined in 802.15.4e, MUST be supported.

   Since the MAC frame payload size limitation is given by the 4g PHY
   frame payload size limitation (i.e.,2048 bytes) and MAC layer
   overhead (headers, trailers, Information Elements and security
   overhead), the MAC frame payload MUST able to carry a full IPv6
   packet of 1280 octets without upper layer fragmentation and re-

7.2.3.  IEEE MAC sub-layer Security Features

   Since IEEE 1901.2 standard is based on the 802.15.4-2006 MAC sub-
   layer and fully endorses the security scheme defined in
   802.15.4-2006, we only focus on description of IEEE 802.15.4 security

   The IEEE 802.15.4 specification was designed to support a variety of
   applications, many of which are security sensitive.  The IEEE
   802.15.4 provides four basic security services: message
   authentication, message integrity, message confidentiality, and
   freshness checks to avoid replay attacks.

   The 802.15.4 security layer is handled at the media access control
   layer, below 6LowPAN layer.  The application specifies its security

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   requirements by setting the appropriate control parameters into the
   radio/PLC stack.  The 802.15.4 defines four packet types: beacon
   frames, data frames, acknowledgments frame, and command frames for
   the media access control layer.  The 802.15.4 specification does not
   support security for acknowledgement frames; data frames, beacon
   frames and command frames can support integrity protection and
   confidentiality protection for the frames's data field.  An
   application has a choice of security suites that control the type of
   security protection that is provided for the transmitted MAC frame.
   The 802.15.4 specification defines eight different security suites,
   outlined bellow.  We can broadly classify the suites by the
   properties that they offer: no security, encryption only (AES-CTR),
   authentication only (AES-CBC-MAC), and encryption and authentication
   (AES-CCM).  Each category that supports authentication comes in three
   variants depending on the size of the MAC (Message Authentication
   Control) that it offers.  The MAC can be either 4, 8, or 16 bytes
   long.  Additionally, for each suite that offers encryption, the
   recipient can optionally enable replay protection.

   o  Null = No security.

   o  AES-CTR = Encryption only, CTR mode.

   o  AES-CBC-MAC-128 = No encryption, 128-bit MAC.

   o  AES-CBC-MAC-64 = No encryption, 64-bit MAC.

   o  AES-CCM-128 = Encryption and 128-bit MAC.

   o  AES-CCM-64 = Encryption and 64-bit MAC.

   o  AES-CCM-32 = Encryption and 32-bit MAC.

   To achieve authentication, any device can maintain an Access Control
   List (ACL) which is a list of trusted nodes from which the device
   wishes to receive data.  Data encryption is done by encryption of
   Message Authentication Control (MAC) frame payload using the key
   shared between two devices, or among a group of peers.  If the key is
   to be shared between two peers, it is stored with each entry in the
   ACL list; otherwise, the key is stored as the default key.  Thus, the
   device can make sure that its data can not be read by devices that do
   not possess the corresponding key.  However, device addresses are
   always transmitted unencrypted, which makes attacks that rely on
   device identity somewhat easier to launch.  Integrity service is
   applied by appending a Message Integrity Code (MIC) generated from
   blocks of encrypted message text.  This ensures that a frame can not
   be modified by a receiver device that does not share a key with the
   sender.  Finally, sequential freshness uses a frame counter and key

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   sequence counter to ensure the freshness of the incoming frame and
   guard against replay attacks.

   A cryptographic MAC is used to authenticate messages.  While longer
   MACs lead to improved resiliency of the code, they also make packet
   size larger and thus take up bandwidth in the network.  In
   constrained environments such as metering infrastructures, an optimum
   balance between security requirements and network throughput must be

7.3.  6LowPAN Options

   AMI implementations based on 1901.2 and 802.15.4(g+e) can utilize all
   of the IPv6 Header Compression schemes specified in [RFC6282]
   Section&nbsp;3 and all of the IPv6 Next Header compression schemes
   specified in [RFC6282] Section&nbsp;4, if reducing over the air/wire
   overhead is a requirement.  However, since the link-layer MTU in both
   wireless and PLC links supports the transmission of a full IPv6
   packet, the use of 6LowPAN fragmentation is NOT RECOMMENDED.

7.4.  Recommended Configuration Defaults and Ranges

7.4.1.  Trickle Parameters

   Trickle was designed to be density-aware and perform well in networks
   characterized by a wide range of node densities.  The combination of
   DIO packet suppression and adaptive timers for sending updates allows
   Trickle to perform well in both sparse and dense environments.  Node
   densities in AMI deployments can vary greatly, from nodes having only
   one or a handful of neighbors to nodes having several hundred
   neighbors.  In high density environments, relatively low values for
   Imin may cause a short period of congestion when an inconsistency is
   detected and DIO updates are sent by a large number of neighboring
   nodes nearly simultaneously.  While the Trickle timer will
   exponentially backoff, some time may elapse before the congestion
   subsides.  While some link layers employ contention mechanisms that
   attempt to avoid congestion, relying solely on the link layer to
   avoid congestion caused by a large number of DIO updates can result
   in increased communication latency for other control and data traffic
   in the network.  To mitigate this kind of short-term congestion, this
   document recommends a more conservative set of values for the Trickle
   parameters than those specified in [RFC6206].  In particular,
   DIOIntervalMin is set to a larger value to avoid periods of
   congestion in dense environments, and DIORedundancyConstant is
   parameterized accordingly as described below.  These values are
   appropriate for the timely distribution of DIO updates in both sparse
   and dense scenarios while avoiding the short-term congestion that
   might arise in dense scenarios.  Because the actual link capacity

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   depends on the particular link technology used within an AMI
   deployment, the Trickle parameters are specified in terms of the
   link's maximum capacity for transmitting link-local multicast
   messages.  If the link can transmit m link-local multicast packets
   per second on average, the expected time it takes to transmit a link-
   local multicast packet is 1/m seconds.

   DIOIntervalMin:  AMI deployments SHOULD set DIOIntervalMin such that
      the Trickle Imin is at least 50 times as long as it takes to
      transmit a link-local multicast packet.  This value is larger than
      that recommended in [RFC6206] to avoid congestion in dense urban
      deployments as described above.  In energy-constrained deployments
      (e.g., in water and gas battery-based routing infrastructure),
      DIOIntervalMin MAY be set to a value resulting in a Trickle Imin
      of several (e.g. 2) hours.

   DIOIntervalDoublings:  AMI deployments SHOULD set
      DIOIntervalDoublings such that the Trickle Imax is at least 2
      hours or more.  For very energy constrained deployments (e.g.,
      water and gas battery-based routing infrastructure),
      DIOIntervalDoublings MAY be set to a value resulting in a Trickle
      Imax of several (e.g., 2) days.

   DIORedundancyConstant:  AMI deployments SHOULD set
      DIORedundancyConstant to a value of at least 10.  This is due to
      the larger chosen value for DIOIntervalMin and the proportional
      relationship between Imin and k suggested in [RFC6206].  This
      increase is intended to compensate for the increased communication
      latency of DIO updates caused by the increase in the
      DIOIntervalMin value, though the proportional relationship between
      Imin and k suggested in [RFC6206] is not preserved.  Instead,
      DIORedundancyConstant is set to a lower value in order to reduce
      the number of packet transmissions in dense environments.

7.4.2.  Other Parameters

   o  AMI deployments SHOULD set MinHopRankIncrease to 256, resulting in
      8 bits of resolution (e.g., for the ETX metric).

   o  To enable local repair, AMI deployments SHOULD set MaxRankIncrease
      to a value that allows a device to move a small number of hops
      away from the root.  With a MinHopRankIncrease of 256, a
      MaxRankIncrease of 1024 would allow a device to move up to 4 hops

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8.  Manageability Considerations

   Network manageability is a critical aspect of smart grid network
   deployment and operation.  With millions of devices participating in
   the smart grid network, many requiring real-time reachability,
   automatic configuration, and lightweight network health monitoring
   and management are crucial for achieving network availability and
   efficient operation.  RPL enables automatic and consistent
   configuration of RPL routers through parameters specified by the
   DODAG root and disseminated through DIO packets.  The use of Trickle
   for scheduling DIO transmissions ensures lightweight yet timely
   propagation of important network and parameter updates and allows
   network operators to choose the trade-off point they are comfortable
   with respect to overhead vs.  reliability and timeliness of network
   updates.  The metrics in use in the network along with the Trickle
   Timer parameters used to control the frequency and redundancy of
   network updates can be dynamically varied by the root during the
   lifetime of the network.  To that end, all DIO messages SHOULD
   contain a Metric Container option for disseminating the metrics and
   metric values used for DODAG setup.  In addition, DIO messages SHOULD
   contain a DODAG Configuration option for disseminating the Trickle
   Timer parameters throughout the network.  The possibility of
   dynamically updating the metrics in use in the network as well as the
   frequency of network updates allows deployment characteristics (e.g.,
   network density) to be discovered during network bring-up and to be
   used to tailor network parameters once the network is operational
   rather than having to rely on precise pre- configuration.  This also
   allows the network parameters and the overall routing protocol
   behavior to evolve during the lifetime of the network.  RPL specifies
   a number of variables and events that can be tracked for purposes of
   network fault and performance monitoring of RPL routers.  Depending
   on the memory and processing capabilities of each smart grid device,
   various subsets of these can be employed in the field.

9.  Security Considerations

   Smart grid networks are subject to stringent security requirements as
   they are considered a critical infrastructure component.  At the same
   time, since they are composed of large numbers of resource-
   constrained devices inter-connected with limited-throughput links,
   many available security mechanisms are not practical for use in such
   networks.  As a result, the choice of security mechanisms is highly
   dependent on the device and network capabilities characterizing a
   particular deployment.  In contrast to other types of LLNs, in smart
   grid networks centralized administrative control and access to a
   permanent secure infrastructure is available.  As a result link-
   layer, transport-layer and/or application-layer security mechanisms
   are typically in place and using RPL's secure mode is not necessary.

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9.1.  Security Considerations during initial deployment

   During the manufacturing process, the meters are loaded with the
   appropriate security credentials (keys, certificates).  These
   security credentials are unique per device and only known by the
   device itself and the head-end security appliances.  The
   manufacturing security credentials are then used by the devices to
   authenticate with the system and negotiate operational security
   credentials, for both network and application layers.

9.2.  Security Considerations during incremental deployment

   If during the system operation a device fails or is compromised, it
   is replaced with a new device.  The new device does not take over the
   security identity of the replaced device.  The security credentials
   associated with the failed/compromised device are removed from the
   security appliances.

10.  Other Related Protocols

   This section is intentionally left blank.

11.  IANA Considerations

   This memo includes no request to IANA.

12.  Acknowledgements

   The authors would like to acknowledge the review, feedback, and
   comments of Jari Arkko, Dominique Barthel, Cedric Chauvenet, Yuichi
   Igarashi, Philip Levis, Jeorjeta Jetcheva, Nicolas Dejean, and JP

13.  References

13.1.  Informative References

              Gungor, V., Sahin, D., Kocak, T., Ergut, S., Buccella, C.,
              Cecati, C., and G. Hancke, "A Survey on Smart Grid
              Potential Applications and Communication Requirements",
              Feb 2013.

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              IEEE SA, "IEEE Standard for Information technology-- Local
              and metropolitan area networks-- Specific requirements--
              Part 15.4: Wireless Medium Access Control (MAC) and
              Physical Layer (PHY) Specifications for Low Rate Wireless
              Personal Area Networks (WPANs)", September 2006.

              IEEE SA, "IEEE Standard for Local and metropolitan area
              networks--Part 15.4: Low-Rate Wireless Personal Area
              Networks (LR-WPANs) Amendment 1: MAC sublayer", April

              IEEE SA, "IEEE Standard for Low-Frequency (less than 500
              kHz) Narrowband Power Line Communications for Smart Grid
              Applications", December 2013.

13.2.  Normative references

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

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

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, March 2011.

   [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.

   [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.

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

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

   [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.

   [RFC6552]  Thubert, P., "Objective Function Zero for the Routing
              Protocol for Low-Power and Lossy Networks (RPL)", RFC
              6552, March 2012.

   [RFC6719]  Gnawali, O. and P. Levis, "The Minimum Rank with
              Hysteresis Objective Function", RFC 6719, September 2012.

Authors' Addresses

   Daniel Popa
   Itron, Inc
   52, rue Camille Desmoulins
   Issy les Moulineaux  92130


   Matthew Gillmore
   Itron, Inc
   2111 N Molter Rd.
   Liberty Lake, WA  99019


   Laurent Toutain
   Telecom Bretagne
   2 rue de la Chataigneraie
   Cesson Sevigne  35510


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   Jonathan Hui
   170 West Tasman Drive
   San Jose, CA  95134


   Ruben Salazar
   30000 Mill Creek Ave # 100
   Alpharetta, GA  30022


   Kazuya Monden
   Hitachi, Ltd., Yokohama Research Laboratory
   292, Yoshida-cho, Totsuka-ku, Yokohama-shi
   Kanagawa-ken    244-0817


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