| Rfc | 8036 |
|---|
| Title | Applicability Statement for the Routing Protocol for Low-Power and
Lossy Networks (RPL) in Advanced Metering Infrastructure (AMI)
Networks |
|---|
| Author | N. Cam-Winget, Ed., J. Hui, D. Popa |
|---|
| Date | January 2017 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
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.
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
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.
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
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.
|
+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
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.
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)
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
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.
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.
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]).
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.
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
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
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
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.
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
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.
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.
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>.
[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>.
[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>.
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