Transmission of IPv6 Packets over PLC Networks
draft-ietf-6lo-plc-06

Document Type Active Internet-Draft (6lo WG)
Authors Jianqiang Hou  , Bing Liu  , Yong-Geun Hong  , Xiaojun Tang  , Charles Perkins 
Last updated 2021-04-20
Replaces draft-hou-6lo-plc
Stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication
Document shepherd Carles Gomez
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Send notices to Carles Gomez <carlesgo@entel.upc.edu>
IANA IANA review state Version Changed - Review Needed
6Lo Working Group                                                 J. Hou
Internet-Draft                                                    B. Liu
Intended status: Standards Track                     Huawei Technologies
Expires: October 22, 2021                                      Y-G. Hong
                                                                    ETRI
                                                                 X. Tang
                                                                  SGEPRI
                                                              C. Perkins
                                                             Lupin Lodge
                                                          April 20, 2021

             Transmission of IPv6 Packets over PLC Networks
                         draft-ietf-6lo-plc-06

Abstract

   Power Line Communication (PLC), namely using the electric-power lines
   for indoor and outdoor communications, has been widely applied to
   support Advanced Metering Infrastructure (AMI), especially smart
   meters for electricity.  The inherent advantage of existing
   electricity infrastructure facilitates the expansion of PLC
   deployments, and 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 ITU-T G.9903, IEEE 1901.1 and IEEE 1901.2.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 22, 2021.

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Copyright Notice

   Copyright (c) 2021 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
   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
   2.  Requirements Notation and Terminology . . . . . . . . . . . .   3
   3.  Overview of PLC . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Protocol Stack  . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Addressing Modes  . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Maximum Transmission Unit . . . . . . . . . . . . . . . .   6
     3.4.  Routing Protocol  . . . . . . . . . . . . . . . . . . . .   7
   4.  IPv6 over PLC . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Stateless Address Autoconfiguration . . . . . . . . . . .   8
     4.2.  IPv6 Link Local Address . . . . . . . . . . . . . . . . .   9
     4.3.  Unicast Address Mapping . . . . . . . . . . . . . . . . .   9
       4.3.1.  Unicast Address Mapping for IEEE 1901.1 . . . . . . .   9
       4.3.2.  Unicast Address Mapping for IEEE 1901.2 and ITU-T
               G.9903  . . . . . . . . . . . . . . . . . . . . . . .  10
     4.4.  Neighbor Discovery  . . . . . . . . . . . . . . . . . . .  11
     4.5.  Header Compression  . . . . . . . . . . . . . . . . . . .  12
     4.6.  Fragmentation and Reassembly  . . . . . . . . . . . . . .  12
   5.  Internet Connectivity Scenarios and Topologies  . . . . . . .  13
   6.  Operations and Manageability Considerations . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   8.  Security Consideration  . . . . . . . . . . . . . . . . . . .  16
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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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.  With the advantage of existing power grid, 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 grid and advanced metering infrastructure (AMI)
   [SCENA].  The data acquisition devices in these scenarios share
   common features such as fixed position, large quantity, 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 IoT related scenarios lie in the low voltage PLC
   networks with most applications in the area of Advanced Metering
   Infrastructure (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 efficent address auto-
   configuration.

   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 [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
   BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document uses the following acronyms and terminologies:

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

   6lo:  IPv6 over Networks of Resource-constrained Nodes

   AMI:  Advanced Metering Infrastructure

   BBPLC:  Broadband Power Line Communication

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   CID:  Context ID

   Coordinator:  A device capable of relaying messages.

   DAD:  Duplicate Address Detection

   EV:   Electric Vehicle

   IID:  IPv6 Interface Identifier

   IPHC: IP Header Compression

   LAN:  Local Area Network

   LLN:  Low power and Lossy Network

   MSDU: MAC Service Data Unit

   MTU:  Maximum Transmission Unit

   NBPLC:  Narrowband Power Line Communication

   OFDM: Orthogonal Frequency Division Multiplexing

   PANC: PAN Coordinator, a coordinator which 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 draft.

   PSDU: PHY Service Data Unit

   RPL:  IPv6 Routing Protocol for Low-Power and Lossy Networks

   RA:   Router Advertisement

   WAN:  Wide Area Network

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      The terminology used in this draft is aligned with IEEE 1901.2

   +---------------+----------------+------------------+---------------+
   |  IEEE 1901.2  |  IEEE 1901.1   |   ITU-T G.9903   | This document |
   +---------------+----------------+------------------+---------------+
   |      PAN      |    Central     | PAN Coordinator  |      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 electric plugged
   devices such as electricity meters and street lights.  Due to the
   large range of communication frequencies, PLC is generally classified
   into two categories: Narrowband PLC (NBPLC) for automation of sensors
   (which have low frequency band and low power cost), and Broadband PLC
   (BBPLC) for home and industry networking applications.

   Various standards have been addressed on the MAC and PHY layers for
   this communication technology, e.g., BBPLC (1.8-250 MHz) including
   IEEE 1901 and ITU-T G.hn, and NBPLC (3-500 kHz) including ITU-T
   G.9902 (G.hnem), ITU-T G.9903 (G3-PLC) [ITU-T_G.9903], ITU-T G.9904
   (PRIME), IEEE 1901.2 [IEEE_1901.2] (combination of G3-PLC and PRIME
   PLC) and IEEE 1901.2a [IEEE_1901.2a] (an amendment to IEEE 1901.2).

   A new PLC standard IEEE 1901.1 [IEEE_1901.1], which aims at the
   medium frequency band of less than 12 MHz, has been 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/PHY layer corresponds to IEEE 1901.1, IEEE 1901.2 or ITU-T
   G.9903.  The 6lo adaptation layer for PLC is illustrated in

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   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
   Section 3.4.

                    +----------------------------------------+
                    |           Application Layer            |
                    +----------------------------------------+
                    |                TCP/UDP                 |
                    +----------------------------------------+
                    |                                        |
                    |                  IPv6                  |
                    |                                        |
                    +----------------------------------------+
                    |   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-bit
   ([IEEE_1901.1]) or 64-bit ([IEEE_1901.2], [ITU-T_G.9903]) and a short
   address of 12-bit ([IEEE_1901.1]) or 16-bit ([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)
   [I-D.ietf-6tisch-minimal-security].

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 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 the MTU of 1576 octets (the
   original value of 1280 bytes was updated in 2015 [IEEE_1901.2a]).

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   Though these two technologies can support IPv6 natively without
   fragmentation and reassembly, it is possible to configure a smaller
   MTU in 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, insufficient for supporting
   IPv6's MTU.  For this reason, fragmentation and reassembly is
   required for G.9903-based networks to adapt IPv6.

3.4.  Routing Protocol

   Routing protocols suitable for use in PLC networks include:

   o  RPL (Routing Protocol for Low-Power and Lossy Networks) [RFC6550]
      is a layer 3 routing protocol.  AODV-RPL [I-D.ietf-roll-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 L3 routing protocol
      for parent selection.

   o  IEEE 1901.1 supports L2 routing.  Each PLC node maintains a L2
      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).

   o  LOADng 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

   6LoWPAN and 6lo standards [RFC4944], [RFC6282], [RFC6775], and
   [RFC8505] provides useful functionality including link-local IPv6
   addresses, stateless address auto-configuration, 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.

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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 Interface ID by inserting 0xFFFE at the fourth and fifth
   octets as specified in [RFC2464].  The IPv6 IID is derived from the
   64-bit Interface ID by inverting the U/L 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.
   Then, the 64-bit Interface ID MUST be derived by inserting 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 24-bit NID (Network IDentifier, YYYYYY),
   12 zero bits and a 12-bit TEI (Terminal Equipment Identifier, XXX).
   The 64-bit Interface ID MUST be derived by inserting 16-bit 0xFFFE
   into this 48-bit pseudo-address as follows:

       YYYY:YYFF:FE00:0XXX

   Since the derived Interface ID is not global, the "Universal/Local"
   (U/L) bit (7th bit) and the Individual/Group bit (8th bit) MUST both
   be set to zero.  In order to avoid any ambiguity in the derived
   Interface ID, these two bits MUST NOT be used to generate the PANID
   (for IEEE 1901.2 and ITU-T G.9903) or NID (for IEEE 1901.1).  In
   other words, the PANID or NID MUST always be chosen so that these
   bits are zeros.

   For privacy reasons, the IID derived from the MAC address SHOULD only
   be used for link-local address configuration.  A PLC host SHOULD use
   the IID derived from the link-layer short address to configure the
   IPv6 address 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.

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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.  The resolution is realized by the
   NCEs (neighbor cache entry) 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/Target Link-layer Address options for IEEE_1901.1 used in
   the Neighbor Solicitation and Neighbor Advertisement 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.

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   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

   In order to avoid the possibility of duplicated IPv6 addresses, the
   value of the NID MUST be chosen so that the 7th and 8th bits of the
   first byte of the NID are both zero.

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

   The Source/Target Link-layer Address options for IEEE_1901.2 and
   ITU-T G.9903 used in the Neighbor Solicitation and Neighbor
   Advertisement 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

   In order to avoid the possibility of duplicated IPv6 addresses, the
   value of the PAN ID MUST be chosen so that the 7th and 8th bits of
   the first byte of the PAN ID are both zero.

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4.4.  Neighbor Discovery

   Neighbor discovery procedures for 6LoWPAN networks are described in
   Neighbor Discovery Optimization for 6LoWPANs [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 address prefix dissemination, Router Solicitations (RS) and
   Router Advertisements (RA) MAY be used as per [RFC6775].  If the PLC
   network uses route-over, the IPv6 prefix MAY be disseminated by the
   layer 3 routing protocol, such as RPL, which may includes the prefix
   in the DIO message.  As per [I-D.ietf-roll-unaware-leaves], it is
   possible to have PLC devices configured as RPL-unaware-leaves, which
   do not participate to RPL at all, along with RPL-aware PLC devices.
   In this case, the prefix dissemination SHOULD use the RS/RA messages.

   For context information dissemination, Router Advertisements (RA)
   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 unicast link-local Neighbor
   Solicitation to the 6LR.  If the registered address is link-local,
   the 6LR SHOULD NOT further register it to the registrar (6LBR, 6BBR).
   Otherwise, the address MUST be registered via an ARO or EARO included
   in the DAR ([RFC6775]) or EDAR ([RFC8505]) messages.  For RFC8505
   compliant PLC devices, the 'R' flag in the EARO MUST be set when
   sending Neighbor Solicitaitons in order to extract the status
   information in the replied Neighbor Advertisements from the 6LR.  If
   DHCPv6 is used to assign addresses or the IPv6 address is derived
   from unique long or short link layer address, Duplicate Address
   Detection (DAD) MUST NOT be utilized.  Otherwise, the DAD MUST be
   performed at the 6LBR (as per [RFC6775]) or proxied by the routing
   registrar (as per [RFC8505]).  The registration status is feedbacked
   via the DAC or EDAC message from the 6LBR and the Neighbor
   Advertisement (NA) from the 6LR.

   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 feedbacked via the NA message from the 6LBR.

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4.5.  Header Compression

   The compression of IPv6 datagrams within PLC MAC frames refers to
   [RFC6282], which updates [RFC4944].  Header compression as defined in
   [RFC6282] which specifies the compression format for IPv6 datagrams
   on top of IEEE 802.15.4, is the basis for IPv6 header compression in
   PLC.  For situations when PLC MAC MTU cannot support the 1280-octet
   IPv6 packet, headers MUST be compressed according to [RFC6282]
   encoding formats.

   For IEEE 1901.2 and 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" when set as "10" in the section 3.1 of
   [RFC6282], which is illustrated as following.

   SAM: Source Address Mode:

   If SAC=0: Stateless compression

   10:   12 bits.  The first 116 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
         XXX are the 12 bits carried in-line.

   If SAC=1: stateful context-based compression

   10:   12 bits.  The address is derived using context information and
         the 12 bits carried in-line.  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 12-bit to IID mapping given by 0000:00ff:fe00:0XXX,
         where XXX are the 12 bits carried inline.  Any remaining bits
         are zero.

4.6.  Fragmentation and Reassembly

   Constrained PLC MAC layer provides the function of fragmentation and
   reassembly, however, fragmentation and reassembly is 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, it is possible to configure smaller MTU
   at the MAC layer.  If the configured MTU is smaller than 1280

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   octects, 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 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 constranied PLC,
   the data rate is much lower than the situation mentioned in RFC4963,
   thus the 16-bit tag is sufficient to assemble the fragements
   correctly.

5.  Internet Connectivity Scenarios and Topologies

   The PLC network model can be simplified to two kinds of network
   devices: PAN Coordinator (PANC) and PAN Device.  The PANC is the
   primary coordinator of the PLC subnet and can be seen as a master
   node; PAN Devices are typically PLC meters and sensors.  The PANC
   also serves as the Routing Registrar for proxy registration and DAD
   procedures, making use of the updated registration procedures in
   [RFC8505].  IPv6 over PLC networks are built as tree, mesh or star
   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 PAN Device and a PANC.  The PANC typically collects data
   (e.g., a meter reading) from the PAN devices, and then concentrates
   and uploads the data through Ethernet or LPWAN (see Figure 5).  The
   collected data is transmitted by the smart meters through PLC,
   aggregated by a concentrator, 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.

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                   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
   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 both as a PAN 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 PLC tree network is
   depicted in Figure 6.  This topology can be applied in the 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, humidity.
   Data transmission distance in the street lighting scenario is
   normally above several kilometers thus the PLC tree network is
   required.  A more sophisticated AMI network may also be constructed
   into the tree topology which 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.

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                          PLC Device
                               \                   +---------
                  PLC Device    \                 /
                       \         \               +
                        \         \              |
                   PLC Device -- 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 is of great potential applications and has
   been studied for several years.  By connecting all nodes with their
   neighbors in communication range (see Figure 7), 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
   enables direct PAN device to PAN device communication, without being
   obliged to transmit frames through the PANC, which is a requirement
   often cited for 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

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6.  Operations and Manageability Considerations

   The constrained PLC networks are not managed in the same way as the
   enterprise network or carrier network.  The constrained PLC networks
   as the other IoT networks, are designed to be self-organized and
   self-managed.  The software or firmware is flushed 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 device connected to each other.
   Once a device becomes offline, it goes back to the bootstrapping
   stage and tries to rejoin the network.  The onboard status of the
   devices and the topology of the PLC network can be visualized via the
   gateway.  The recently-formed iotops WG in IETF is aming to design
   more features for the management of IOT networks.

7.  IANA Considerations

   There are no IANA considerations related to this document.

8.  Security Consideration

   Due to the high accessibility of power grid, 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.  Thus link layer security
   mechanisms are designed in the PLC technologies mentioned in this
   document.

   Malicious PLC devices could paralyze the whole network via DOS
   attacks, e.g., keep joining and leaving the network frequently, or
   multicast routing messages containing fake metrics.  A device may
   also join a wrong or even malicious network, exposing its data to
   illegal users.  Mutual authentication of network and new device can
   be conducted during the onboarding process of the new device.
   Methods include protocols such as [RFC7925] (exchanging pre-installed
   certificates over DTLS) , [I-D.ietf-6tisch-minimal-security] (which
   uses pre-shared keys), and
   [I-D.ietf-6tisch-dtsecurity-zerotouch-join] (which uses IDevID and
   MASA service).  It is also possible to use EAP methods such as
   [I-D.ietf-emu-eap-noob] via transports like PANA [RFC5191].  No
   specific mechanism is specified by this document as an appropriate
   mechanism will depend upon deployment circumstances.

   IP addresses may be used to track devices on the Internet; such
   devices can 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

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   frequently changing the global prefix and avoiding unique link-layer
   derived IIDs in addresses.  [RFC8065] discusses the privacy threats
   when interface identifiers (IID) are generated without sufficient
   entropy, including correlation of activities over time, location
   tracking, device-specific vulnerability exploitation, and address
   scanning.  Schemes such as limited lease period in DHCPv6 [RFC3315],
   Cryptographically Generated Addresses (CGAs) [RFC3972], privacy
   extensions [RFC4941], Hash-Based Addresses (HBAs) [RFC5535], or
   semantically opaque addresses [RFC7217] SHOULD be considered to
   enhance the IID privacy.

9.  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.  Authors thank Scott Mansfield, Ralph Droms, Pat Kinney
   for their guidance in the liaison process.  Authors wish to thank
   Stefano Galli, Thierry Lys, Yizhou Li, Yuefeng Wu and Michael
   Richardson for their valuable comments and contributions.

10.  References

10.1.  Normative References

   [IEEE_1901.1]
              IEEE-SA Standards Board, "Standard for Medium Frequency
              (less than 15 MHz) Power Line Communications for Smart
              Grid Applications", IEEE 1901.1, May 2018,
              <https://ieeexplore.ieee.org/document/8360785>.

   [IEEE_1901.2]
              IEEE-SA Standards Board, "IEEE Standard for Low-Frequency
              (less than 500 kHz) Narrowband Power Line Communications
              for Smart Grid Applications", IEEE 1901.2, October 2013,
              <https://standards.ieee.org/findstds/
              standard/1901.2-2013.html>.

   [ITU-T_G.9903]
              International Telecommunication Union, "Narrowband
              orthogonal frequency division multiplexing power line
              communication transceivers for G3-PLC networks",
              ITU-T G.9903, February 2014,
              <https://www.itu.int/rec/T-REC-G.9903>.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <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>.

   [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>.

   [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>.

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10.2.  Informative References

   [EUI-64]   IEEE-SA Standards Board, "Guidelines for 64-bit Global
              Identifier (EUI-64) Registration Authority", IEEE EUI-64,
              March 1997, <https://standards.ieee.org/content/dam/ieee-
              standards/standards/web/documents/tutorials/eui.pdf>.

   [I-D.ietf-6tisch-dtsecurity-zerotouch-join]
              Richardson, M., "6tisch Zero-Touch Secure Join protocol",
              draft-ietf-6tisch-dtsecurity-zerotouch-join-04 (work in
              progress), July 2019.

   [I-D.ietf-6tisch-minimal-security]
              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Constrained Join Protocol (CoJP) for 6TiSCH", draft-ietf-
              6tisch-minimal-security-15 (work in progress), December
              2019.

   [I-D.ietf-emu-eap-noob]
              Aura, T., Sethi, M., and A. Peltonen, "Nimble out-of-band
              authentication for EAP (EAP-NOOB)", draft-ietf-emu-eap-
              noob-03 (work in progress), December 2020.

   [I-D.ietf-roll-aodv-rpl]
              Anamalamudi, S., Zhang, M., Perkins, C., Anand, S., and B.
              Liu, "AODV based RPL Extensions for Supporting Asymmetric
              P2P Links in Low-Power and Lossy Networks", draft-ietf-
              roll-aodv-rpl-08 (work in progress), May 2020.

   [I-D.ietf-roll-unaware-leaves]
              Thubert, P. and M. Richardson, "Routing for RPL Leaves",
              draft-ietf-roll-unaware-leaves-30 (work in progress),
              January 2021.

   [IEEE_1901.2a]
              IEEE-SA Standards Board, "IEEE Standard for Low-Frequency
              (less than 500 kHz) Narrowband Power Line Communications
              for Smart Grid Applications - Amendment 1", IEEE 1901.2a,
              September 2015, <https://standards.ieee.org/findstds/
              standard/1901.2a-2015.html>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <https://www.rfc-editor.org/info/rfc3315>.

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

   [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>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <https://www.rfc-editor.org/info/rfc4941>.

   [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>.

   [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>.

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   [SCENA]    Cano, C., Pittolo, A., Malone, D., and L. Lampe, "State of
              the Art in Power Line Communications: From the
              Applications to the Medium", July 2016,
              <https://ieeexplore.ieee.org/document/7467440>.

Authors' Addresses

   Jianqiang Hou
   Huawei Technologies
   101 Software Avenue,
   Nanjing 210012
   China

   Email: houjianqiang@huawei.com

   Bing Liu
   Huawei Technologies
   No. 156 Beiqing Rd. Haidian District,
   Beijing 100095
   China

   Email: remy.liubing@huawei.com

   Yong-Geun Hong
   Electronics and Telecommunications Research Institute
   161 Gajeong-Dong Yuseung-Gu
   Daejeon 305-700
   Korea

   Email: yghong@etri.re.kr

   Xiaojun Tang
   State Grid Electric Power Research Institute
   19 Chengxin Avenue
   Nanjing 211106
   China

   Email: itc@sgepri.sgcc.com.cn

   Charles E. Perkins
   Lupin Lodge

   Email: charliep@computer.org

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