





Internet Engineering Task Force (IETF)                N. Cam-Winget, Ed.
Request for Comments: 8036                                 Cisco Systems
Category: Standards Track                                         J. Hui
ISSN: 2070-1721                                                     Nest
                                                                 D. Popa
                                                              Itron, Inc
                                                            January 2017


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

Abstract

   This document discusses the applicability of the Routing Protocol for
   Low-Power and Lossy Networks (RPL) in Advanced Metering
   Infrastructure (AMI) networks.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8036.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   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 . . . . . . .   4
     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 Power Line Communication (PLC) Technology  . . . . .   9
   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.  DAO Policy  . . . . . . . . . . . . . . . . . . . . .  11
       7.1.3.  Path Metrics  . . . . . . . . . . . . . . . . . . . .  11
       7.1.4.  Objective Function  . . . . . . . . . . . . . . . . .  12
       7.1.5.  DODAG Repair  . . . . . . . . . . . . . . . . . . . .  12
       7.1.6.  Multicast . . . . . . . . . . . . . . . . . . . . . .  12
       7.1.7.  Security  . . . . . . . . . . . . . . . . . . . . . .  13
     7.2.  Description of Layer-2 Features . . . . . . . . . . . . .  13
       7.2.1.  IEEE 1901.2 PHY and MAC Sub-layer Features  . . . . .  13
       7.2.2.  IEEE 802.15.4 (Amendments G and 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  . . . . . . . . . . . . . . . .  18
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     9.1.  Security Considerations during Initial Deployment . . . .  20
     9.2.  Security Considerations during Incremental Deployment . .  20
     9.3.  Security Considerations Based on RPL's Threat Analysis  .  20
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  21
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     11.2.  Informative references . . . . . . . . . . . . . . . . .  22
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24





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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
   (HAN) devices.  They are typically interconnected using some
   combination of wireless and power line communications, thus 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

1.2.  Required Reading

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

   A NAN can use wireless communication technology, which is based on
   the IEEE 802.15.4 standard family: more specifically, the Physical
   Layer (PHY) amendment [IEEE.802.15.4g] and the Media Access Control
   (MAC) sub-layer amendment [IEEE.802.15.4e], which are adapted to
   smart grid networks.

   NAN can also use Power Line Communication (PLC) technology as an
   alternative to wireless communications.  Several standards for PLC
   technology have emerged, such as [IEEE.1901.2].

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











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1.3.  Out-of-Scope Requirements

   The following are outside the scope of this document:

   o  Applicability statement for RPL [RFC6550] 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 devices powered by alternating current (i.e., electric meters)
      and battery-powered meters (i.e., gas/water meters).

   o  Applicability statement for RPL storing mode of operation in AMI
      networks.

2.  Routing Protocol for LLNs (RPL)

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

   RPL builds a Directed Acyclic Graph (DAG) routing structure rooted at
   an LBR, 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.  The Routing Requirements
   for Urban Low-Power and Lossy Networks [RFC5548] are applicable to
   AMI networks as well.  The terminology used in this document is
   defined in [RFC7102].

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 HAN devices.  Electric meters may also be used as




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   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 regulations on spectrum
   use; on meter behavior and performance; and 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 HAN 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, and [IEEE.802.11]).  In addition to
   serving as 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.












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                                                         |
                                                         +M
                                                         |
                                          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
                                                    +--+---+--
                                  <----------------------------->
                                           RPL

                      Figure 1: Typical NAN Topology

   A typical AMI network architecture (see Figure 1) is composed of a
   Meter Data Management System (MDMS) connected through an IP network
   to an 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 three different power phases coming
   through the substation or just to the electrical network topology.
   Meters (represented by a M in the previous figure) can also
   participate in a HAN.  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
   may 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 WAN based on Worldwide Interoperability
   for Microwave Access (WiMAX), and 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 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 technology used to exchange information, the
   logical network topology will not necessarily match the electric grid
   topology.  If meters exchange information through radio technologies
   such as in the IEEE 802.15.4 family, then the topology is a meshed




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   network where nodes belonging to the same Destination-Oriented DAG
   (DODAG) can be connected to the grid through different substations.
   If narrowband PLC technology is used, it will more or less follow the
   physical tree structure since crosstalk may allow one phase to
   communicate with the other.  This is particularly true near the LBR.
   Some mixed topology can also be observed since some LBRs may be
   strategically installed in the field to avoid all the communications
   going through a single LBR.  Nevertheless, the short propagation
   range forces meters to relay the information.

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 that are
   deployed in the utility company data center.  Head-end servers
   generate data traffic to configure smart 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, or a 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 or 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
   Mesh Peer-to-Peer (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 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 service level agreements about reliability.






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   Distribution Automation (DA) applications typically involve a small
   number of devices that communicate with each other in a 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
   applications.

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 P2P 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 divided into several classes:

   C1)  High-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 for Power Quality Events and Meter
        Service Connection and Disconnection requires 98%+ packet
        delivery under 10s (payload size < 150 bytes)

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

   C4)  Low Priority traffic for Recurrent Meter Reading requires 98%+
        packet 2-hour delivery window 6 times per day (payload size <
        400 bytes)





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   C5)  Background Priority traffic for firmware/software updates
        processed to 98%+ of devices within 7 days (average firmware
        update is 1 MB)

4.3.  RPL Applicability per Smart Grid Traffic Characteristics

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

   The group communication model used in smart grid requires the 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.

5.  Layer-2 Applicability

5.1.  IEEE Wireless Technology

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

   IEEE Std 802.15.4e is an amendment to IEEE Std 802.15.4 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 IEEE 802.15.4e does not change the link-layer security
   scheme defined in the last two updates to IEEE Std 802.15.4 (e.g.,
   2006 and 2011 amendments).

5.2.  IEEE Power Line Communication (PLC) Technology

   IEEE Std 1901.2 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 supports indoor
   and outdoor communications over a low voltage line (the line between
   transformer and meter, which is less than 1000 V) through a
   transformer of low-voltage to medium-voltage (1000 V up to 72 kV) and
   through a transformer of medium-voltage to low-voltage power lines in




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   both urban and in long distance (multi-kilometer) rural
   communications.

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

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

   The 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 reassembly.

   IEEE Std 1901.2 specifies the necessary link-layer security features
   that fully endorse the IEEE 802.15.4 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].  This section informally highlights some
   of the similarities:

   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
      scalability.

   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.

   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.









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   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 (DODAG Information Object)
      messages and DIO message options [RFC6550].

   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 SMD and DA
   traffic.

7.1.2.  DAO Policy

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

7.1.3.  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 ETX metric as defined in
   [RFC6551].

   Additional metrics may be defined in companion RFCs.






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7.1.4.  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, Objective Function 0 (OF0) [RFC6552] and
   Minimum Rank with Hysteresis Objective Function (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.

7.1.5.  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 reattaching 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 8.2.2.5 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 an LBR.  After
   the detached node has made sufficient effort to send a 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 mechanism needs to take into account the disconnection time
   that 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.

7.1.6.  Multicast

   Multicast support for RPL in non-storing mode are being developed in
   companion RFCs (see [RFC7731]).







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7.1.7.  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; they
   MUST include, at a minimum, link-layer security such as that defined
   by IEEE 1901.2 and IEEE 802.15.4.

7.2.  Description of Layer-2 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 subcarrier can vary
   significantly, IEEE 1901.2 specifies two complementary mechanisms
   that allow fine-tuning of 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, and so on).  The
   second (fine-grained) mechanism maps the subcarriers that are too
   noisy and deactivates them.

   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 subcarriers.  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.

   The IEEE 1901.2 MAC frame format shares most in common with the IEEE
   802.15.4 MAC frame format [IEEE.802.15.4].  A few exceptions are
   described below.

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





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

   o  The IEEE 1901.2 MAC sub-layer endorses the concept of Information
      Elements, as defined in [IEEE.802.15.4e].  The format and use of
      Information Elements are not relevant to the 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 IEEE 1901.2 PHY MTU size is variable and dependent on the PHY
   settings in use (e.g., bandwidth, modulation, tones, etc).  As quoted
   from the IEEE 1901.2 specification:

      For CENELEC A/B, if MSDU size is more than 247 octets for robust
      OFDM (ROBO) and Super-ROBO modulations or more than 239 octets for
      all other modulations, the MAC layer shall divide the MSDU into
      multiple segments as described in 5.3.7.  For FCC and ARIB, if the
      MSDU size meets one of the following conditions: a) For ROBO and
      Super-ROBO modulations, the MSDU size is more than 247 octets but
      less than 494 octets, b) For all other modulations, the MSDU size
      is more than 239 octets but less than 478 octets.

7.2.2.  IEEE 802.15.4 (Amendments G and 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, the 802.15.4g PHY layer includes mechanisms to improve
   the robustness of the radio communications, such as data whitening
   and Forward Error Correction coding.  The 802.15.4g PHY frame payload
   can carry up to 2048 octets.

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

   o  Timetimeslotslotted Channel Hopping (TSCH): specifically designed
      for application domains such as process automation





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   o  Low-Latency Deterministic Networks (LLDN): for application domains
      such as factory automation.

   o  Deterministic and Synchronous Multi-channel Extension (DSME): for
      general industrial and commercial application domains that
      includes channel diversity to increase network robustness.

   o  Asynchronous Multi-channel Adaptation (AMCA): for large
      infrastructure application domains.

   The MAC addressing scheme supports short (16-bit) addresses along
   with extended (64-bit) addresses.  These addresses are assigned in
   different ways and are specified by specific standards organizations.
   Information Elements, Enhanced Beacons, and frame version 2, as
   defined in IEEE 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
   reassembly.

7.2.3.  IEEE MAC Sub-layer Security Features

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

   The IEEE 802.15.4 specification was designed to support a variety of
   applications, many of which are security sensitive.  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 the 6LowPAN (IPv6 over Low-Power Wireless Personal Area
   Network) layer.  The application specifies its security requirements
   by setting the appropriate control parameters into the radio/PLC
   stack.  IEEE 802.15.4 defines four packet types: beacon frames, data
   frames, acknowledgment frames, 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' 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.  Each security suite
   offers a different set of security properties and guarantees, and



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   ultimately offers different MAC frame formats.  The 802.15.4
   specification defines eight different security suites, outlined
   below.  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 Message Authentication Code 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

   Note that AES-CCM-32 is the most commonly used cipher in these
   deployments today.

   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 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 cannot 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 cannot
   be modified by a receiver device that does not share a key with the
   sender.  Finally, sequential freshness uses a frame counter and key
   sequence counter to ensure the freshness of the incoming frame and
   guard against replay attacks.

   A cryptographic Message Authentication Code (or keyed MIC) is used to
   authenticate messages.  While longer MICs lead to improved resiliency



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   of the code, they also make the 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 found.

7.3.  6LowPAN Options

   AMI implementations based on IEEE 1901.2 and 802.15.4 (amendments g
   and e) can utilize all of the IPv6 Header Compression schemes
   specified in Section 3 of [RFC6282] and all of the IPv6 Next Header
   compression schemes specified in Section 4 of [RFC6282], if reducing
   over the air/wire overhead is a requirement.

7.4.  Recommended Configuration Defaults and Ranges

7.4.1.  Trickle Parameters

   Trickle [RFC6206] 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 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.




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   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.

   DIOIntervalDoublings:  AMI deployments SHOULD set
      DIOIntervalDoublings such that the Trickle Imax is at least 2
      hours or more.

   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
      away.

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 with which 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



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   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, they are composed of large numbers of resource-constrained
   devices interconnected with limited-throughput links.  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 both
   centralized administrative control and access to a permanent secure
   infrastructure are available.  As a result, smart grid networks are
   deployed with security mechanisms such as link-layer, transport-
   layer, and/or application-layer security mechanisms; while it is best
   practice to secure all layers, using RPL's secure mode may not be
   necessary.  Failure to protect any of these layers can result in
   various attacks; a lack of strong authentication of devices in the
   infrastructure can lead to uncontrolled and unauthorized access.
   Similarly, failure to protect the communication layers can enable
   passive (in wireless mediums) attacks as well as man-in-the-middle
   and active attacks.

   As this document describes the applicability of RPL non-storing mode,
   the security considerations as defined in [RFC6550] also apply to
   this document and to AMI deployments.








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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 and certificates).  The
   configured security credentials during manufacturing are used by the
   devices to authenticate with the system and to further 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 known to be
   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.

9.3.  Security Considerations Based on RPL's Threat Analysis

   [RFC7416] defines a set of security considerations for RPL security.
   This document defines how it leverages the device's link-layer and
   application-layer security mechanisms to address the threats as
   defined in Section 6 of [RFC7416].

   Like any secure network infrastructure, an AMI deployment's ability
   to address node impersonation and active man-in-the-middle attacks
   rely on a mutual authentication and authorization process.  To enable
   strong mutual authentication, all nodes, from smart meters to nodes
   in the infrastructure, must have a credential.  The credential may be
   bootstrapped at the time the node is manufactured but must be
   appropriately managed and classified through the authorization
   process.  The management and authorization process ensures that the
   nodes are properly authenticated and behaving or 'acting' in their
   assigned roles.

   Similarly, to ensure that data has not been modified, confidentiality
   and integrity at the suitable layers (e.g., the link layer, the
   application layer, or both) should be used.

   To provide the security mechanisms to address these threats, an AMI
   deployment MUST include the use of the security schemes as defined by
   IEEE 1901.2 (and IEEE 802.15.4) with IEEE 802.15.4 defining the
   security mechanisms to afford mutual authentication, access control
   (e.g., authorization), and transport confidentiality and integrity.







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10.  Privacy Considerations

   Privacy of information flowing through smart grid networks are
   subject to consideration.  An evolving set of recommendations and
   requirements are being defined by different groups and consortiums;
   for example, the U.S. Department of Energy issued a document [DOEVCC]
   defining a process and set of recommendations to address privacy
   issues.  As this document describes the applicability of RPL, the
   privacy considerations as defined in [PRIVACY] and [EUPR] apply to
   this document and to AMI deployments.

11.  References

11.1.  Normative References

   [IEEE.1901.2]
              IEEE, "IEEE Standard for Low-Frequency (less than 500 kHz)
              Narrowband Power Line Communications for Smart Grid
              Applications", IEEE 1901.2-2013,
              DOI 10.1109/ieeestd.2013.6679210, December 2013,
              <http://ieeexplore.ieee.org/servlet/
              opac?punumber=6679208>.

   [IEEE.802.15.4]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks--Part 15.4: Low-Rate Wireless Personal Area
              Networks (LR-WPANs)", IEEE 802.15.4-2011,
              DOI 10.1109/ieeestd.2011.6012487, September 2011,
              <http://ieeexplore.ieee.org/servlet/
              opac?punumber=6012485>.

   [IEEE.802.15.4e]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks--Part 15.4: Low-Rate Wireless Personal Area
              Networks (LR-WPANs) Amendment 1: MAC sublayer", IEEE
              802.15.4e-2012, DOI 10.1109/ieeestd.2012.6185525, April
              2012, <http://ieeexplore.ieee.org/servlet/
              opac?punumber=6185523>.

   [IEEE.802.15.4g]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks--Part 15.4: Low-Rate Wireless Personal Area
              Networks (LR-WPANs) Amendment 3: Physical Layer (PHY)
              Specifications for Low-Data-Rate, Wireless, Smart Metering
              Utility Networks", IEEE 802.15.4g-2012,
              DOI 10.1109/ieeestd.2012.6190698, April 2012,
              <http://ieeexplore.ieee.org/servlet/
              opac?punumber=6190696>.



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

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

   [surveySG] 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",
              IEEE Transactions on Industrial Informatics Volume 9,
              Issue 1, pp. 28-42, DOI 10.1109/TII.2012.2218253, February
              2013.

11.2.  Informative references

   [DOEVCC]   "Voluntary Code of Conduct (VCC) Final Concepts and
              Principles", January 2015,
              <http://energy.gov/sites/prod/files/2015/01/f19/VCC%20Conc
              epts%20and%20Principles%202015_01_08%20FINAL.pdf>.

   [EUPR]     "Information for investors and data controllers", June
              2016, <https://ec.europa.eu/energy/node/1748>.

   [IEEE.802.11]
              IEEE, "IEEE Standard for Information technology--
              Telecommunications and information exchange between
              systems Local and metropolitan area networks--Specific
              requirements Part 11: Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) Specifications",
              IEEE 802.11-2012, DOI 10.1109/ieeestd.2012.6178212, March
              2012, <https://standards.ieee.org/getieee802/
              download/802.11-2012.pdf>.

   [PRIVACY]  Thaler, D., "Privacy Considerations for IPv6 Adaptation
              Layer Mechanisms", Work in Progress, draft-ietf-6lo-
              privacy-considerations-04, October 2016.

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




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

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

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

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

   [RFC6719]  Gnawali, O. and P. Levis, "The Minimum Rank with
              Hysteresis Objective Function", RFC 6719,
              DOI 10.17487/RFC6719, September 2012,
              <http://www.rfc-editor.org/info/rfc6719>.

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

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

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













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Acknowledgements

   Matthew Gillmore, Laurent Toutain, Ruben Salazar, and Kazuya Monden
   were contributors and noted as authors in earlier versions of this
   document.  The authors would also 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 Vasseur.

Authors' Addresses

   Nancy Cam-Winget (editor)
   Cisco Systems
   3550 Cisco Way
   San Jose, CA  95134
   United States of America

   Email: ncamwing@cisco.com


   Jonathan Hui
   Nest
   3400 Hillview Ave
   Palo Alto, CA  94304
   United States of America

   Email: jonhui@nestlabs.com


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

   Email: daniel.popa@itron.com















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