Internet Engineering Task Force (IETF) J. Hou

Internet Engineering Task Force (IETF) J. Hou
Request for Comments: 9354 B. Liu
Category: Standards Track Huawei Technologies
ISSN: 2070-1721 Y-G. Hong
Daejeon University
X. Tang
SGEPRI
C. Perkins
Lupin Lodge
January 2023

Transmission of IPv6 Packets over Power Line Communication (PLC)
Networks

Abstract

Power Line Communication (PLC), namely using electric power lines for
indoor and outdoor communications, has been widely applied to support
Advanced Metering Infrastructure (AMI), especially smart meters for
electricity. The existing electricity infrastructure facilitates the
expansion of PLC deployments due to its potential advantages in terms
of cost and convenience. Moreover, a wide variety of accessible
devices raises the potential demand of IPv6 for future applications.
This document describes how IPv6 packets are transported over
constrained PLC networks, such as those described in ITU-T G.9903,
IEEE 1901.1, and IEEE 1901.2.

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
https://www.rfc-editor.org/info/rfc9354.

Copyright Notice

Copyright (c) 2023 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
(https://trustee.ietf.org/license-info) in effect on the date of
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include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
2. Requirements Notation and Terminology
3. Overview of PLC
3.1. Protocol Stack
3.2. Addressing Modes
3.3. Maximum Transmission Unit
3.4. Routing Protocol
4. IPv6 over PLC
4.1. Stateless Address Autoconfiguration
4.2. IPv6 Link-Local Address
4.3. Unicast Address Mapping
4.3.1. Unicast Address Mapping for IEEE 1901.1
4.3.2. Unicast Address Mapping for IEEE 1901.2 and ITU-T
G.9903
4.4. Neighbor Discovery
4.5. Header Compression
4.6. Fragmentation and Reassembly
5. Internet Connectivity Scenarios and Topologies
6. Operations and Manageability Considerations
7. IANA Considerations
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Authors' Addresses

1. Introduction

The idea of using power lines for both electricity supply and
communication can be traced back to the beginning of the last
century. Using the existing power grid to transmit messages, Power
Line Communication (PLC) is a good candidate for supporting various
service scenarios such as in houses and offices, in trains and
vehicles, in smart grids, and in Advanced Metering Infrastructure
(AMI) [SCENA]. The data-acquisition devices in these scenarios share
common features such as fixed position, large quantity of nodes, low
data rate, and low power consumption.

Although PLC technology has evolved over several decades, it has not
been fully adapted for IPv6-based constrained networks. The
resource-constrained scenarios related to the Internet of Things
(IoT) lie in the low voltage PLC networks with most applications in
the area of AMI, vehicle-to-grid communications, in-home energy
management, and smart street lighting. IPv6 is important for PLC
networks, due to its large address space and efficient address
autoconfiguration.

This document provides a brief overview of PLC technologies. Some of
them have LLN (Low-Power and Lossy Network) characteristics, i.e.,
limited power consumption, memory, and processing resources. This
document specifies the transmission of IPv6 packets over those
constrained PLC networks. The general approach is to adapt elements
of the 6LoWPAN (IPv6 over Low-Power Wireless Personal Area Network)
and 6lo (IPv6 over Networks of Resource-constrained Nodes)
specifications, such as those described in [RFC4944], [RFC6282],
[RFC6775], and [RFC8505], to constrained PLC networks.

2. Requirements Notation and Terminology

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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

This document uses the following acronyms and terminologies:

6BBR: 6LoWPAN Backbone Router

6LBR: 6LoWPAN Border Router

6lo: IPv6 over Networks of Resource-constrained Nodes

6LoWPAN: IPv6 over Low-Power Wireless Personal Area Network

6LR: 6LoWPAN Router

AMI: Advanced Metering Infrastructure

BBPLC: Broadband Power Line Communication

Coordinator: A device capable of relaying messages

DAD: Duplicate Address Detection

EUI: Extended Unique Identifier

IID: Interface Identifier

LLN: Low-Power and Lossy Network

MTU: Maximum Transmission Unit

NBPLC: Narrowband Power Line Communication

PAN: Personal Area Network

PANC: PAN Coordinator, a coordinator that also acts as the primary
controller of a PAN

PLC: Power Line Communication

PLC device: An entity that follows the PLC standards and implements
the protocol stack described in this document

RA: Router Advertisement

RPL: Routing Protocol for Low-Power and Lossy Networks

Below is a mapping table of the terminology between [IEEE_1901.2],
[IEEE_1901.1], [ITU-T_G.9903], and this document.

+=================+=============+===============+===============+
| IEEE 1901.2 | IEEE 1901.1 | ITU-T G.9903 | This document |
+=================+=============+===============+===============+
| PAN Coordinator | Central | PAN | PAN |
| | Coordinator | Coordinator | Coordinator |
+-----------------+-------------+---------------+---------------+
| Coordinator | Proxy | Full-Function | Coordinator |
| | Coordinator | Device | |
+-----------------+-------------+---------------+---------------+
| Device | Station | PAN Device | PLC Device |
+-----------------+-------------+---------------+---------------+

Table 1: Terminology Mapping between PLC Standards

3. Overview of PLC

PLC technology enables convenient two-way communications for home
users and utility companies to monitor and control electrically
connected devices such as electricity meters and street lights. PLC
can also be used in smart home scenarios, such as the control of
indoor lights and switches. Due to the large range of communication
frequencies, PLC is generally classified into two categories:
Narrowband PLC (NBPLC) for automation of sensors (which have a low
frequency band and low power cost) and Broadband PLC (BBPLC) for home
and industry networking applications.

Various standards have been addressed on the Media Access Control
(MAC) and Physical (PHY) layers. For example, standards for BBPLC
(1.8-250 MHz) include IEEE 1901 and ITU-T G.hn, and standards for
NBPLC (3-500 kHz) include ITU-T G.9902 (G.hnem), ITU-T G.9903
(G3-PLC) [ITU-T_G.9903], ITU-T G.9904 (PRIME), IEEE 1901.2 (a
combination of G3-PLC and PRIME PLC) [IEEE_1901.2], and IEEE 1901.2a
(an amendment to IEEE 1901.2) [IEEE_1901.2a].

IEEE 1901.1 [IEEE_1901.1], a PLC standard that is aimed at the medium
frequency band of less than 12 MHz, was published by the IEEE
standard for Smart Grid Powerline Communication Working Group (SGPLC
WG). IEEE 1901.1 balances the needs for bandwidth versus
communication range and is thus a promising option for 6lo
applications.

This specification is focused on IEEE 1901.1, IEEE 1901.2, and ITU-T
G.9903.

3.1. Protocol Stack

The protocol stack for IPv6 over PLC is illustrated in Figure 1. The
PLC MAC and PLC PHY layers correspond to the layers described in IEEE
1901.1, IEEE 1901.2, or ITU-T G.9903. The 6lo adaptation layer for
PLC is illustrated in Section 4. For multihop tree and mesh
topologies, a routing protocol is likely to be necessary. The routes
can be built in mesh-under mode at Layer 2 or in route-over mode at
Layer 3, as explained in Sections 3.4 and 4.4.

+----------------------------------------+
| Application Layer |
+----------------------------------------+
| Transport Layer |
+----------------------------------------+
| |
| IPv6 Layer |
| |
+----------------------------------------+
| Adaptation Layer for IPv6 over PLC |
+----------------------------------------+
| PLC MAC Layer |
+----------------------------------------+
| PLC PHY Layer |
+----------------------------------------+

Figure 1: PLC Protocol Stack

3.2. Addressing Modes

Each PLC device has a globally unique long address of 48 bits
[IEEE_1901.1] or 64 bits [IEEE_1901.2] [ITU-T_G.9903] and a short
address of 12 bits [IEEE_1901.1] or 16 bits [IEEE_1901.2]
[ITU-T_G.9903]. The long address is set by the manufacturer
according to the IEEE EUI-48 MAC address or the IEEE EUI-64 address.
Each PLC device joins the network by using the long address and
communicates with other devices by using the short address after
joining the network. Short addresses can be assigned during the
onboarding process, by the PANC or the JRC (join registrar/
coordinator) in CoJP (Constrained Join Protocol) [RFC9031].

3.3. Maximum Transmission Unit

The Maximum Transmission Unit (MTU) of the MAC layer determines
whether fragmentation and reassembly are needed at the adaptation
layer of IPv6 over PLC. IPv6 requires an MTU of 1280 octets or
greater; thus, for a MAC layer with an MTU lower than this limit,
fragmentation and reassembly at the adaptation layer are required.

The IEEE 1901.1 MAC supports upper-layer packets up to 2031 octets.
The IEEE 1901.2 MAC layer supports an MTU of 1576 octets (the
original value of 1280 bytes was updated in 2015 [IEEE_1901.2a]).
Though these two technologies can support IPv6 originally without
fragmentation and reassembly, it is possible to configure a smaller
MTU in a high-noise communication environment. Thus, the 6lo
functions, including header compression, fragmentation, and
reassembly, are still applicable and useful.

The MTU for ITU-T G.9903 is 400 octets, which is insufficient for
supporting IPv6's MTU. For this reason, fragmentation and reassembly
are required for G.9903-based networks to carry IPv6.

3.4. Routing Protocol

Routing protocols suitable for use in PLC networks include:

* RPL (Routing Protocol for Low-Power and Lossy Networks) [RFC6550]
is a Layer 3 routing protocol. AODV-RPL [AODV-RPL] updates RPL to
include reactive, point-to-point, and asymmetric routing. IEEE
1901.2 specifies Information Elements (IEs) with MAC layer
metrics, which can be provided to a Layer 3 routing protocol for
parent selection.

* IEEE 1901.1 supports the mesh-under routing scheme. Each PLC node
maintains a routing table, in which each route entry comprises the
short addresses of the destination and the related next hop. The
route entries are built during the network establishment via a
pair of association request/confirmation messages. The route
entries can be changed via a pair of proxy change request/
confirmation messages. These association and proxy change
messages must be approved by the central coordinator (PANC in this
document).

* LOADng (Lightweight On-demand Ad hoc Distance vector routing
protocol, Next Generation) is a reactive protocol operating at
Layer 2 or Layer 3. Currently, LOADng is supported in ITU-T
G.9903 [ITU-T_G.9903], and the IEEE 1901.2 standard refers to
ITU-T G.9903 for LOAD-based networks.

4. IPv6 over PLC

A PLC node distinguishes between an IPv6 PDU and a non-IPv6 PDU based
on the equivalent of an Ethertype in a Layer 2 PLC PDU. [RFC7973]
defines an Ethertype of "A0ED" for LoWPAN encapsulation, and this
information can be carried in the IE field in the MAC header of
[IEEE_1901.2] or [ITU-T_G.9903]. And regarding [IEEE_1901.1], the IP
packet type has been defined with the corresponding MAC Service Data
Unit (MSDU) type value 49. And the 4-bit Internet Protocol version
number in the IP header helps to distinguish between an IPv4 PDU and
an IPv6 PDU.

6LoWPAN and 6lo standards, as described in [RFC4944], [RFC6282],
[RFC6775], and [RFC8505], provide useful functionality, including
link-local IPv6 addresses, stateless address autoconfiguration,
neighbor discovery, header compression, fragmentation, and
reassembly. However, due to the different characteristics of the PLC
media, the 6LoWPAN adaptation layer, as it is, cannot perfectly
fulfill the requirements of PLC environments. These considerations
suggest the need for a dedicated adaptation layer for PLC, which is
detailed in the following subsections.

4.1. Stateless Address Autoconfiguration

To obtain an IPv6 Interface Identifier (IID), a PLC device performs
stateless address autoconfiguration [RFC4944]. The autoconfiguration
can be based on either a long or short link-layer address.

The IID can be based on the device's 48-bit MAC address or its EUI-64
identifier [EUI-64]. A 48-bit MAC address MUST first be extended to
a 64-bit IID by inserting 0xFFFE at the fourth and fifth octets as
specified in [RFC2464]. The IPv6 IID is derived from the 64-bit IID
by inverting the U/L (Universal/Local) bit [RFC4291].

For IEEE 1901.2 and ITU-T G.9903, a 48-bit "pseudo-address" is formed
by the 16-bit PAN ID, 16 zero bits, and the 16-bit short address as
follows:

16_bit_PAN:0000:16_bit_short_address

Then, the 64-bit IID MUST be derived by inserting the 16-bit 0xFFFE
into as follows:

16_bit_PAN:00FF:FE00:16_bit_short_address

For the 12-bit short addresses used by IEEE 1901.1, the 48-bit
pseudo-address is formed by a 24-bit NID (Network Identifier,
YYYYYY), 12 zero bits, and a 12-bit TEI (Terminal Equipment
Identifier, XXX) as follows:

YYYY:YY00:0XXX

The 64-bit IID MUST be derived by inserting the 16-bit 0xFFFE into
this 48-bit pseudo-address as follows:

YYYY:YYFF:FE00:0XXX

As investigated in [RFC7136], aside from the method discussed in
[RFC4291], other IID-generation methods defined by the IETF do not
imply any additional semantics for the Universal/Local (U/L) bit (bit
6) and the Individual/Group bit (bit 7). Therefore, these two bits
are not reliable indicators. Thus, when using an IID derived by a
short address, the operators of the PLC network can choose whether or
not to comply with the original meaning of these two bits. If they
choose to comply with the original meaning, these two bits MUST both
be set to zero, since the IID derived from the short address is not
global. In order to avoid any ambiguity in the derived IID, these
two bits MUST NOT be a valid part of the PAN ID (for IEEE 1901.2 and
ITU-T G.9903) or NID (for IEEE 1901.1). For example, the PAN ID or
NID must always be chosen so that the two bits are zeros or the high
six bits in PAN ID or NID are left shifted by two bits. If they
choose not to comply with the original meaning, the operator must be
aware that these two bits are not reliable indicators, and the IID
cannot be transformed back into a short link-layer address via a
reverse operation of the mechanism presented above. However, the
short address can still be obtained via the Unicast Address Mapping
mechanism described in Section 4.3.

For privacy reasons, the IID derived from the MAC address (with
padding and bit clamping) SHOULD only be used for link-local address
configuration. A PLC host SHOULD use the IID derived from the short
link-layer address to configure IPv6 addresses used for communication
with the public network; otherwise, the host's MAC address is
exposed. As per [RFC8065], when short addresses are used on PLC
links, a shared secret key or version number from the Authoritative
Border Router Option [RFC6775] can be used to improve the entropy of
the hash input. Thus, the generated IID can be spread out to the
full range of the IID address space while stateless address
compression is still allowed. By default, the hash algorithm SHOULD
be SHA256, using the version number, the PAN ID or NID, and the short
address as the input arguments, and the 256-bit hash output is
truncated into the IID by taking the high 64 bits.

4.2. IPv6 Link-Local Address

The IPv6 link-local address [RFC4291] for a PLC interface is formed
by appending the IID, as defined above, to the prefix FE80::/64 (see
Figure 2).

10 bits 54 bits 64 bits
+----------+-----------------------+----------------------------+
|1111111010| (zeros) | Interface Identifier |
+----------+-----------------------+----------------------------+

Figure 2: IPv6 Link-Local Address for a PLC Interface

4.3. Unicast Address Mapping

The address-resolution procedure for mapping IPv6 unicast addresses
into PLC link-layer addresses follows the general description in
Section 7.2 of [RFC4861]. [RFC6775] improves this procedure by
eliminating usage of multicast NS (Neighbor Solicitation). The
resolution is realized by the NCEs (neighbor cache entries) created
during the address registration at the routers. [RFC8505] further
improves the registration procedure by enabling multiple LLNs to form
an IPv6 subnet and by inserting a link-local address registration to
better serve proxy registration of new devices.

4.3.1. Unicast Address Mapping for IEEE 1901.1

The Source Link-Layer Address and Target Link-Layer Address options
for IEEE_1901.1 used in the NS and Neighbor Advertisement (NA) have
the following form.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length=1 | NID :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
:NID (continued)| Padding (all zeros) | TEI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: Unicast Address Mapping for IEEE 1901.1

Option fields:

Type: 1 for Source Link-Layer Address and 2 for Target Link-Layer
Address.

Length: The length of this option (including Type and Length fields)
in units of 8 octets. The value of this field is 1 for the 12-bit
IEEE 1901.1 PLC short addresses.

NID: 24-bit Network Identifier

Padding: 12 zero bits

TEI: 12-bit Terminal Equipment Identifier

4.3.2. Unicast Address Mapping for IEEE 1901.2 and ITU-T G.9903

The Source Link-Layer Address and Target Link-Layer Address options
for IEEE_1901.2 and ITU-T G.9903 used in the NS and NA have the
following form.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length=1 | PAN ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding (all zeros) | Short Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: Unicast Address Mapping for IEEE 1901.2

Option fields:

Type: 1 for Source Link-Layer Address and 2 for Target Link-Layer
Address.

Length: The length of this option (including Type and Length fields)
in units of 8 octets. The value of this field is 1 for the 16-bit
IEEE 1901.2 PLC short addresses.

PAN ID: 16-bit PAN IDentifier

Padding: 16 zero bits

Short Address: 16-bit short address

4.4. Neighbor Discovery

Neighbor discovery procedures for 6LoWPAN networks are described in
[RFC6775] and [RFC8505]. These optimizations support the
registration of sleeping hosts. Although PLC devices are
electrically powered, sleeping mode SHOULD still be used for power
saving.

For IPv6 prefix dissemination, Router Solicitations (RSs) and Router
Advertisements (RAs) MAY be used as per [RFC6775]. If the PLC
network uses route-over mode, the IPv6 prefix MAY be disseminated by
the Layer 3 routing protocol, such as RPL, which may include the
prefix in the DIO (DODAG Information Object) message. As per
[RFC9010], it is possible to have PLC devices configured as RPL-
unaware leaves, which do not participate in RPL at all, along with
RPL-aware PLC devices. In this case, the prefix dissemination SHOULD
use the RS/RA messages.

For dissemination of context information, RAs MUST be used as per
[RFC6775]. The 6LoWPAN context option (6CO) MUST be included in the
RA to disseminate the Context IDs used for prefix and/or address
compression.

For address registration in route-over mode, a PLC device MUST
register its addresses by sending a unicast link-local NS to the 6LR.
If the registered address is link local, the 6LR SHOULD NOT further
register it to the registrar (6LBR or 6BBR). Otherwise, the address
MUST be registered via an ARO (Address Registration Option) or EARO
(Extended Address Registration Option) included in the DAR (Duplicate
Address Request) [RFC6775] or EDAR (Extended Duplicate Address
Request) [RFC8505] messages. For PLC devices compliant with
[RFC8505], the 'R' flag in the EARO MUST be set when sending NSs in
order to extract the status information in the replied NAs from the
6LR. If DHCPv6 is used to assign addresses or the IPv6 address is
derived from the unique long or short link-layer address, Duplicate
Address Detection (DAD) SHOULD NOT be utilized. Otherwise, DAD MUST
be performed at the 6LBR (as per [RFC6775]) or proxied by the routing
registrar (as per [RFC8505]). The registration status is fed back
via the DAC (Duplicate Address Confirmation) or EDAC (Extended
Duplicate Address Confirmation) message from the 6LBR and the NA from
the 6LR. Section 6 of [RFC8505] shows how devices that only behave
as specified in [RFC6775] can work with devices that have been
updated per [RFC8505].

For address registration in mesh-under mode, since all the PLC
devices are link-local neighbors to the 6LBR, DAR/DAC or EDAR/EDAC
messages are not required. A PLC device MUST register its addresses
by sending a unicast NS message with an ARO or EARO. The
registration status is fed back via the NA message from the 6LBR.

4.5. Header Compression

IPv6 header compression in PLC is based on [RFC6282] (which updates
[RFC4944]). [RFC6282] specifies the compression format for IPv6
datagrams on top of IEEE 802.15.4; therefore, this format is used for
compression of IPv6 datagrams within PLC MAC frames. For situations
when the PLC MAC MTU cannot support the 1280-octet IPv6 packet, the
headers MUST be compressed according to the encoding formats
specified in [RFC6282], including the Dispatch Header, the
LOWPAN_IPHC, and the compression residue carried inline.

For IEEE 1901.2 and ITU-T G.9903, the IP header compression follows
the instruction in [RFC6282]. However, additional adaptation MUST be
considered for IEEE 1901.1 since it has a short address of 12 bits
instead of 16 bits. The only modification is the semantics of the
"Source Address Mode" and the "Destination Address Mode" when set as
"10" in Section 3.1 of [RFC6282], which is illustrated as follows.

SAM: Source Address Mode:

If SAC=0: Stateless compression

10: 16 bits. The first 112 bits of the address are elided. The
value of the first 64 bits is the link-local prefix padded with
zeros. The following 64 bits are 0000:00ff:fe00:0XXX, where
0XXX are the 16 bits carried inline, in which the first 4 bits
are zero.

If SAC=1: Stateful context-based compression

10: 16 bits. The address is derived using context information and
the 16 bits carried inline. Bits covered by context
information are always used. Any IID bits not covered by
context information are taken directly from their corresponding
bits in the mapping between the 16-bit short address and the
IID as provided by 0000:00ff:fe00:0XXX, where 0XXX are the 16
bits carried inline, in which the first 4 bits are zero. Any
remaining bits are zero.

DAM: Destination Address Mode:

If M=0 and DAC=0: Stateless compression

10: 16 bits. The first 112 bits of the address are elided. The
value of the first 64 bits is the link-local prefix padded with
zeros. The following 64 bits are 0000:00ff:fe00:0XXX, where
0XXX are the 16 bits carried inline, in which the first 4 bits
are zero.

If M=0 and DAC=1: Stateful context-based compression

10: 16 bits. The address is derived using context information and
the 16 bits carried inline. Bits covered by context
information are always used. Any IID bits not covered by
context information are taken directly from their corresponding
bits in the mapping between the 16-bit short address and the
IID as provided by 0000:00ff:fe00:0XXX, where 0XXX are the 16
bits carried inline, in which the first 4 bits are zero. Any
remaining bits are zero.

4.6. Fragmentation and Reassembly

The constrained PLC MAC layer provides the functions of fragmentation
and reassembly. However, fragmentation and reassembly are still
required at the adaptation layer if the MAC layer cannot support the
minimum MTU demanded by IPv6, which is 1280 octets.

In IEEE 1901.1 and IEEE 1901.2, the MAC layer supports payloads as
big as 2031 octets and 1576 octets, respectively. However, when the
channel condition is noisy, smaller packets have a higher
transmission success rate, and the operator can choose to configure
smaller MTU at the MAC layer. If the configured MTU is smaller than
1280 octets, the fragmentation and reassembly defined in [RFC4944]
MUST be used.

In ITU-T G.9903, the maximum MAC payload size is fixed to 400 octets,
so to cope with the required MTU of 1280 octets by IPv6,
fragmentation and reassembly at the 6lo adaptation layer MUST be
provided as specified in [RFC4944].

[RFC4944] uses a 16-bit datagram tag to identify the fragments of the
same IP packet. [RFC4963] specifies that at high data rates, the
16-bit IP identification field is not large enough to prevent
frequent incorrectly assembled IP fragments. For constrained PLC,
the data rate is much lower than the situation mentioned in
[RFC4963]; thus, the 16-bit tag is sufficient to assemble the
fragments correctly.

5. Internet Connectivity Scenarios and Topologies

The PLC network model can be simplified to two kinds of network
devices: PAN Coordinator (PANC) and PLC device. The PANC is the
primary coordinator of the PLC subnet and can be seen as a primary
node; PLC devices are typically PLC meters and sensors. The address
registration and DAD features can also be deployed on the PANC, for
example, the 6LBR [RFC6775] or the routing registrar [RFC8505]. IPv6
over PLC networks are built as tree, mesh, or star topologies
according to the use cases. Generally, each PLC network has one
PANC. In some cases, the PLC network can have alternate coordinators
to replace the PANC when the PANC leaves the network for some reason.
Note that the PLC topologies in this section are based on logical
connectivity, not physical links. The term "PLC subnet" refers to a
multilink subnet, in which the PLC devices share the same address
prefix.

The star topology is common in current PLC scenarios. In single-hop
star topologies, communication at the link layer only takes place
between a PLC device and a PANC. The PANC typically collects data
(e.g., a meter reading) from the PLC devices and then concentrates
and uploads the data through Ethernet or cellular networks (see
Figure 5). The collected data is transmitted by the smart meters
through PLC, aggregated by a concentrator, and sent to the utility
and then to a Meter Data Management System for data storage,
analysis, and billing. This topology has been widely applied in the
deployment of smart meters, especially in apartment buildings.

PLC Device PLC Device
\ / +---------
\ / /
\ / +
\ / |
PLC Device ------ PANC ===========+ Internet
/ \ |
/ \ +
/ \ \
/ \ +---------
PLC Device PLC Device

<---------------------->
PLC subnet (IPv6 over PLC)

Figure 5: PLC Star Network Connected to the Internet

A tree topology is useful when the distance between a device A and
the PANC is beyond the PLC-allowed limit and there is another device
B in between able to communicate with both sides. Device B in this
case acts as both a PLC device and a Coordinator. For this scenario,
the link-layer communications take place between device A and device
B, and between device B and PANC. An example of a PLC tree network
is depicted in Figure 6. This topology can be applied in smart
street lighting, where the lights adjust the brightness to reduce
energy consumption while sensors are deployed on the street lights to
provide information such as light intensity, temperature, and
humidity. The data-transmission distance in the street lighting
scenario is normally above several kilometers; thus, a PLC tree
network is required. A more sophisticated AMI network may also be
constructed into the tree topology that is depicted in [RFC8036]. A
tree topology is suitable for AMI scenarios that require large
coverage but low density, e.g., the deployment of smart meters in
rural areas. RPL is suitable for maintenance of a tree topology in
which there is no need for communication directly between PAN
devices.

PLC Device
\ +---------
PLC Device A \ /
\ \ +
\ \ |
PLC Device B -- PANC ===========+ Internet
/ / |
/ / +
PLC Device---PLC Device / \
/ +---------
PLC Device---PLC Device

<------------------------->
PLC subnet (IPv6 over PLC)

Figure 6: PLC Tree Network Connected to the Internet

Mesh networking in PLC has many potential applications and has been
studied for several years. By connecting all nodes with their
neighbors in communication range (see Figure 7), a mesh topology
dramatically enhances the communication efficiency and thus expands
the size of PLC networks. A simple use case is the smart home
scenario where the ON/OFF state of air conditioning is controlled by
the state of home lights (ON/OFF) and doors (OPEN/CLOSE). AODV-RPL
[AODV-RPL] enables direct communication between PLC devices, without
being obliged to transmit frames through the PANC, which is a
requirement often cited for the AMI infrastructure.

PLC Device---PLC Device
/ \ / \ +---------
/ \ / \ /
/ \ / \ +
/ \ / \ |
PLC Device--PLC Device---PANC ===========+ Internet
\ / \ / |
\ / \ / +
\ / \ / \
\ / \ / +---------
PLC Device---PLC Device

<------------------------------->
PLC subnet (IPv6 over PLC)

Figure 7: PLC Mesh Network Connected to the Internet

6. Operations and Manageability Considerations

Constrained PLC networks are not managed in the same way as
enterprise networks or carrier networks. Constrained PLC networks,
like the other IoT networks, are designed to be self-organized and
self-managed. The software or firmware is flashed into the devices
before deployment by the vendor or operator. And during the
deployment process, the devices are bootstrapped, and no extra
configuration is needed to get the devices connected to each other.
Once a device becomes offline, it goes back to the bootstrapping
stage and tries to rejoin the network. The onboarding status of the
devices and the topology of the PLC network can be visualized via the
PANC. The recently formed IOTOPS WG in the IETF aims to identify the
requirements in IoT network management, and operational practices
will be published. Developers and operators of PLC networks should
be able to learn operational experiences from this WG.

7. IANA Considerations

This document has no IANA actions.

8. Security Considerations

Due to the high accessibility of power grids, PLC might be
susceptible to eavesdropping within its communication coverage, e.g.,
one apartment tenant may have the chance to monitor the other smart
meters in the same apartment building. Link-layer security
mechanisms, such as payload encryption and device authentication, are
designed in the PLC technologies mentioned in this document.
Additionally, an on-path malicious PLC device could eavesdrop or
modify packets sent through it if appropriate confidentiality and
integrity mechanisms are not implemented.

Malicious PLC devices could paralyze the whole network via DoS
attacks, e.g., keep joining and leaving the network frequently or
sending multicast routing messages containing fake metrics. A device
may also inadvertently join a wrong or even malicious network,
exposing its data to malicious users. When communicating with a
device outside the PLC network, the traffic has to go through the
PANC. Thus, the PANC must be a trusted entity. Moreover, the PLC
network must prevent malicious devices from joining the network.
Thus, mutual authentication of a PLC network and a new device is
important, and it can be conducted during the onboarding process of
the new device. Methods include protocols such as the TLS/DTLS
Profile [RFC7925] (exchanging pre-installed certificates over DTLS),
the Constrained Join Protocol (CoJP) [RFC9031] (which uses pre-shared
keys), and Zero-Touch Secure Join [ZEROTOUCH] (an IoT version of the
Bootstrapping Remote Secure Key Infrastructure (BRSKI), which uses an
Initial Device Identifier (IDevID) and a Manufacturer Authorized
Signing Authority (MASA) service to facilitate authentication). It
is also possible to use Extensible Authentication Protocol (EAP)
methods such as the one defined in [RFC9140] via transports like
Protocol for Carrying Authentication for Network Access (PANA)
[RFC5191]. No specific mechanism is specified by this document, as
an appropriate mechanism will depend upon deployment circumstances.
In some cases, the PLC devices can be deployed in uncontrolled
places, where the devices may be accessed physically and be
compromised via key extraction. The compromised device may be still
able to join in the network since its credentials are still valid.
When group-shared symmetric keys are used in the network, the
consequence is even more severe, i.e., the whole network or a large
part of the network is at risk. Thus, in scenarios where physical
attacks are considered to be relatively highly possible, per-device
credentials SHOULD be used. Moreover, additional end-to-end security
services are complementary to the network-side security mechanisms,
e.g., if a device is compromised and has joined in the network, and
then it claims itself as the PANC and tries to make the rest of the
devices join its network. In this situation, the real PANC can send
an alarm to the operator to acknowledge the risk. Other behavior-
analysis mechanisms can be deployed to recognize the malicious PLC
devices by inspecting the packets and the data.

IP addresses may be used to track devices on the Internet; such
devices can often in turn be linked to individuals and their
activities. Depending on the application and the actual use pattern,
this may be undesirable. To impede tracking, globally unique and
non-changing characteristics of IP addresses should be avoided, e.g.,
by frequently changing the global prefix and avoiding unique link-
layer derived IIDs in addresses. [RFC8065] discusses the privacy
threats when an IID is generated without sufficient entropy,
including correlation of activities over time, location tracking,
device-specific vulnerability exploitation, and address scanning.
And an effective way to deal with these threats is to have enough
entropy in the IID compared to the link lifetime. Consider a PLC
network with 1024 devices and a link lifetime is 8 years, according
to the formula in [RFC8065], an entropy of 40 bits is sufficient.
Padding the short address (12 or 16 bits) to generate the IID of a
routable IPv6 address in the public network may be vulnerable to deal
with address scans. Thus, as suggested in Section 4.1, a hash
function can be used to generate a 64-bit IID. When the version
number of the PLC network is changed, the IPv6 addresses can be
changed as well. Other schemes such as limited lease period in
DHCPv6 [RFC8415], Cryptographically Generated Addresses (CGAs)
[RFC3972], Temporary Address Extensions [RFC8981], Hash-Based
Addresses (HBAs) [RFC5535], or semantically opaque addresses
[RFC7217] SHOULD be used to enhance the IID privacy.

9. References

9.1. Normative References

[IEEE_1901.1]
IEEE, "IEEE Standard for Medium Frequency (less than 12
MHz) Power Line Communications for Smart Grid
Applications", DOI 10.1109/IEEESTD.2018.8360785, IEEE
Std 1901.1, May 2018,
<https://ieeexplore.ieee.org/document/8360785>.

[IEEE_1901.2]
IEEE, "IEEE Standard for Low-Frequency (less than 500 kHz)
Narrowband Power Line Communications for Smart Grid
Applications", DOI 10.1109/IEEESTD.2013.6679210, IEEE
Std 1901.2, December 2013,
<https://ieeexplore.ieee.org/document/6679210>.

[ITU-T_G.9903]
ITU-T, "Narrowband orthogonal frequency division
multiplexing power line communication transceivers for
G3-PLC networks", ITU-T Recommendation G.9903, August
2017, <https://www.itu.int/rec/T-REC-G.9903>.

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

[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,
<https://www.rfc-editor.org/info/rfc2464>.

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.

[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.

[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>.

[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,
<https://www.rfc-editor.org/info/rfc6282>.

[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,
<https://www.rfc-editor.org/info/rfc6550>.

[RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
Bormann, "Neighbor Discovery Optimization for IPv6 over
Low-Power Wireless Personal Area Networks (6LoWPANs)",
RFC 6775, DOI 10.17487/RFC6775, November 2012,
<https://www.rfc-editor.org/info/rfc6775>.

[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
February 2014, <https://www.rfc-editor.org/info/rfc7136>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8505] Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
Perkins, "Registration Extensions for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Neighbor
Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
<https://www.rfc-editor.org/info/rfc8505>.

9.2. Informative References

[AODV-RPL] Perkins, C. E., Anand, S.V.R., Anamalamudi, S., and B.
Liu, "Supporting Asymmetric Links in Low Power Networks:
AODV-RPL", Work in Progress, Internet-Draft, draft-ietf-
roll-aodv-rpl-15, 30 September 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-roll-
aodv-rpl-15>.

[EUI-64] IEEE Standards Association, "Guidelines for Use of
Extended Unique Identifier (EUI), Organizationally Unique
Identifier (OUI), and Company ID (CID)", August 2017,
<https://standards.ieee.org/wp-
content/uploads/import/documents/tutorials/eui.pdf>.

[IEEE_1901.2a]
IEEE, "IEEE Standard for Low-Frequency (less than 500 kHz)
Narrowband Power Line Communications for Smart Grid
Applications - Amendment 1",
DOI 10.1109/IEEESTD.2013.6679210, IEEE Std 1901.2a,
October 2015,
<https://ieeexplore.ieee.org/document/7286946>.

[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, DOI 10.17487/RFC3972, March 2005,
<https://www.rfc-editor.org/info/rfc3972>.

[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963,
DOI 10.17487/RFC4963, July 2007,
<https://www.rfc-editor.org/info/rfc4963>.

[RFC5191] Forsberg, D., Ohba, Y., Ed., Patil, B., Tschofenig, H.,
and A. Yegin, "Protocol for Carrying Authentication for
Network Access (PANA)", RFC 5191, DOI 10.17487/RFC5191,
May 2008, <https://www.rfc-editor.org/info/rfc5191>.

[RFC5535] Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535,
DOI 10.17487/RFC5535, June 2009,
<https://www.rfc-editor.org/info/rfc5535>.

[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<https://www.rfc-editor.org/info/rfc7217>.

[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016,
<https://www.rfc-editor.org/info/rfc7925>.

[RFC7973] Droms, R. and P. Duffy, "Assignment of an Ethertype for
IPv6 with Low-Power Wireless Personal Area Network
(LoWPAN) Encapsulation", RFC 7973, DOI 10.17487/RFC7973,
November 2016, <https://www.rfc-editor.org/info/rfc7973>.

[RFC8036] Cam-Winget, N., Ed., Hui, J., and D. Popa, "Applicability
Statement for the Routing Protocol for Low-Power and Lossy
Networks (RPL) in Advanced Metering Infrastructure (AMI)
Networks", RFC 8036, DOI 10.17487/RFC8036, January 2017,
<https://www.rfc-editor.org/info/rfc8036>.

[RFC8065] Thaler, D., "Privacy Considerations for IPv6 Adaptation-
Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
February 2017, <https://www.rfc-editor.org/info/rfc8065>.

[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.

[RFC8981] Gont, F., Krishnan, S., Narten, T., and R. Draves,
"Temporary Address Extensions for Stateless Address
Autoconfiguration in IPv6", RFC 8981,
DOI 10.17487/RFC8981, February 2021,
<https://www.rfc-editor.org/info/rfc8981>.

[RFC9010] Thubert, P., Ed. and M. Richardson, "Routing for RPL
(Routing Protocol for Low-Power and Lossy Networks)
Leaves", RFC 9010, DOI 10.17487/RFC9010, April 2021,
<https://www.rfc-editor.org/info/rfc9010>.

[RFC9031] Vučinić, M., Ed., Simon, J., Pister, K., and M.
Richardson, "Constrained Join Protocol (CoJP) for 6TiSCH",
RFC 9031, DOI 10.17487/RFC9031, May 2021,
<https://www.rfc-editor.org/info/rfc9031>.

[RFC9140] Aura, T., Sethi, M., and A. Peltonen, "Nimble Out-of-Band
Authentication for EAP (EAP-NOOB)", RFC 9140,
DOI 10.17487/RFC9140, December 2021,
<https://www.rfc-editor.org/info/rfc9140>.

[SCENA] Cano, C., Pittolo, A., Malone, D., Lampe, L., Tonello, A.,
and A. Dabak, "State of the Art in Power Line
Communications: From the Applications to the Medium", IEEE
Journal on Selected Areas in Communications, Volume 34,
Issue 7, DOI 10.1109/JSAC.2016.2566018, July 2016,
<https://ieeexplore.ieee.org/document/7467440>.

[ZEROTOUCH]
Richardson, M., "6tisch Zero-Touch Secure Join protocol",
Work in Progress, Internet-Draft, draft-ietf-6tisch-
dtsecurity-zerotouch-join-04, 8 July 2019,
<https://datatracker.ietf.org/doc/html/draft-ietf-6tisch-
dtsecurity-zerotouch-join-04>.

Acknowledgements

We gratefully acknowledge suggestions from the members of the IETF
6lo Working Group. Great thanks to Samita Chakrabarti and Gabriel
Montenegro for their feedback and support in connecting the IEEE and
ITU-T sides. The authors thank Scott Mansfield, Ralph Droms, and Pat
Kinney for their guidance in the liaison process. The authors wish
to thank Stefano Galli, Thierry Lys, Yizhou Li, Yuefeng Wu, and
Michael Richardson for their valuable comments and contributions.
The authors wish to thank Carles Gomez for shepherding this document.
The authors also thank Paolo Volpato for delivering the presentation
at IETF 113. Sincere acknowledgements to the valuable comments from
the following reviewers: Dave Thaler, Dan Romascanu, Murray
Kucherawy, Benjamin Kaduk, Alvaro Retana, Éric Vyncke, Meral
Shirazipour, Roman Danyliw, and Lars Eggert.

Authors' Addresses

Jianqiang Hou
Huawei Technologies
101 Software Avenue,
Nanjing
210012
China
Email: [email protected]

Bing Liu
Huawei Technologies
Haidian District
No. 156 Beiqing Rd.
Beijing
100095
China
Email: [email protected]

Yong-Geun Hong
Daejeon University
Dong-gu
62 Daehak-ro
Daejeon
34520
Republic of Korea
Email: [email protected]

Xiaojun Tang
State Grid Electric Power Research Institute
19 Chengxin Avenue
Nanjing
211106
China
Email: [email protected]

Charles E. Perkins
Lupin Lodge
Email: [email protected]