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Versions: 00 01 02 03 04 05 draft-ietf-6lo-plc

6Lo Working Group                                                 J. Hou
Internet-Draft                                       Huawei Technologies
Intended Status: Standards Track                               Y-G. Hong
Expires: September 11, 2017                                         ETRI
                                                                 X. Tang
                                                                  SGEPRI
                                                          March 10, 2017


             Transmission of IPv6 Packets over PLC Networks
                          draft-hou-6lo-plc-00

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 the smart
   meters for electricity.  With the inherent advantage of existing
   electricity infrastructure, PLC is expanding deployments all over the
   world, indicating the potential demand of IPv6 for future
   applications.  As part of this technology, Narrowband PLC (NBPLC) is
   focused on the low-bandwidth and low-power scenarios, including
   current standards such as IEEE 1901.2 and ITU-T G.9903.  This
   document describes how IPv6 packets are transported over constrained
   PLC networks.

Status of this Memo

   This Internet-Draft is submitted to IETF 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 http://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 September 11, 2017.

Copyright Notice

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




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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Requirements Notation and Terminology  . . . . . . . . . . . .  3
   3.  Overview of PLC  . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Protocol Stack . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Addressing Modes . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  Maximum Transmission Unit  . . . . . . . . . . . . . . . .  6
   4.  Specification of IPv6 over Narrowband PLC  . . . . . . . . . .  6
     4.1.  IEEE 1901.2  . . . . . . . . . . . . . . . . . . . . . . .  6
       4.1.1.  Stateless Address Autoconfiguration  . . . . . . . . .  6
       4.1.2.  IPv6 Link Local Address  . . . . . . . . . . . . . . .  7
       4.1.3.  Unicast and Multicast Address Mapping  . . . . . . . .  7
       4.1.4.  Header Compression . . . . . . . . . . . . . . . . . .  8
       4.1.5.  Fragmentation and Reassembly . . . . . . . . . . . . .  9
     4.2.  ITU-T G.9903 . . . . . . . . . . . . . . . . . . . . . . .  9
       4.2.1.  Stateless Address Autoconfiguration  . . . . . . . . .  9
       4.2.2.  IPv6 Link Local Address  . . . . . . . . . . . . . . .  9
       4.2.3.  Unicast and Multicast Address Mapping  . . . . . . . . 10
       4.2.4.  Header Compression . . . . . . . . . . . . . . . . . . 11
       4.2.5.  Fragmentation and Reassembly . . . . . . . . . . . . . 11
       4.2.6.  Extension at 6lo Adaptation Layer  . . . . . . . . . . 12
   5.  Internet Connectivity Scenarios and Topologies . . . . . . . . 12
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   7.  Security Consideration . . . . . . . . . . . . . . . . . . . . 14
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16

1.  Introduction

   The idea of using power line for both electricity supply and
   communication can be traced back to the beginning of the last
   century.  With the obvious advantage of existing power grid, PLC is a
   good candidate for supporting various service scenarios such as in
   houses and offices, in trains and vehicles, in smart grid and



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   advanced metering infrastructure (AMI).  Such applications cover the
   smart meters for electricity, gas and water that share the common
   features like fixed position, large quantity, low data rate, and long
   life time.

   Although PLC technology has an evolution history of several decades,
   the adaptation of PLC for IP based constrained networks is not fully
   developed.  The 6lo related scenarios lie in the low voltage PLC
   networks with most applications in the area of Advanced Metering
   Infrastructure, Vehicle-to-Grid communications, in-home energy
   management and smart street lighting.  It is of great importance to
   deploy IPv6 for PLC devices for its large address space and quick
   addressing.  In addition, due to various existing PLC standards, a
   comparison among them is needed to facilitate the selection of the
   most applicable PLC standard in certain using scenarios.

   The following sections provide a brief overview of PLC, then describe
   transmission of IPv6 packets over PLC networks.  The general approach
   is to adapt elements of the 6LoWPAN specifications [RFC4944],
   [RFC6282], and [RFC6775] to constrained PLC networks.  Similar 6LoPLC
   adaptation layer was previously proposed in [draft-popa-6lo-6loplc],
   however, with the same purpose, this document provides more updated,
   structured and instructive information for the deployment of IPv6
   over 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
   [RFC2119].

   Below are the terms used in this document:

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

   AMI: Advanced Metering Infrastructure

   BBPLC: Broadband Power Line Communication

   BR: Border Router

   HDPLC: High Definition Power Line Communication

   IID: Interface Identifier

   LAN: Local Area Network




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   LOADng: Lightweight On-demand Ad-hoc Distance-vector Routing Protocol
   Next Generation

   MSDU: MAC Service Data Unit

   MTU: Maximum Transmission Unit

   NBPLC: Narrowband Power Line Communication

   OFDM: Orthogonal Frequency Division Multiplexing

   PLC: Power Line Communication

   PSDU: PHY Service Data Unit

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

   WAN: Wide Area Network

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 its
   large range of communication frequencies, PLC is generally classified
   into two categories: Narrowband PLC (NBPLC) for automation of
   sensors, 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. IEEE 1901 and ITU-
   T G.hn for BBPLC (1.8-250 MHz), IEEE 1901.2, ITU-T G.9902 (G.hnem),
   ITU-T G.9903 (G3-PLC) and ITU-T G.9904 (PRIME) for NBPLC (3-500 kHz)
   and the recent proposal for the IEEE 1901.1 standard aiming at the
   frequency band of 2-12 MHz.

   Narrowband PLC is a very important branch of PLC technology due to
   its low frequency band and low power cost.  So far the recent PLC
   standards, ITU-T G.9903 (G3-PLC) and IEEE 1901.2, are dominating as
   two of the most robust schemes available.  Different networking
   methods exist in different NBPLC standards.  The formation of a ITU-T
   G.9903 network is based on a MAC Layer routing protocol called LOADng
   (Lightweight On-demand Ad-hoc Distance-vector Routing Protocol Next
   Generation).  Different from ITU-T G.9903, IEEE 1901.2 enables a
   variable structure of the MAC layer which can alternatively apply
   layer-2 or layer-3 mesh networking.  IEEE 1901.2 enables a
   coexistence mode with ITU-T G.9903 using layer-2 LOADng protocol, and
   on the other hand it allows the adaptation of layer-3 RPL protocol
   (IPv6 Routing Protocol for Low-Power and Lossy Networks).




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   The IEEE 1901.1 WG is currently working on a new PLC standard, IEEE
   1901.1, which focuses on the frequency band of 2-12 MHz [IEEE
   1901.1].  This promising medium-frequency PLC standard, known as PLC-
   IOT, is suitable for 6lo applications thus mentioned in this
   document.  Details on this standard is to be determined.

3.1.  Protocol Stack

   The protocol stack for IPv6 over PLC is illustrated in Figure 1 that
   contains the following elements from bottom to top: PLC PHY Layer,
   PLC MAC Layer, Adaptation layer for IPv6 over PLC, IPv6 Layer,
   TCP/UDP Layer and Application Layer.  The PLC MAC/PHY layer
   corresponds to a certain PLC standard such as IEEE 1901.2 or ITU-T
   G.9903.  For the Broadband PLC cases, the adaptation layer for IPv6
   over PLC MAY not be used unless in some certain specifications.  The
   deployment of the 6lo adaptation layer are specified in section 4
   according to different standards.  Routing protocol like RPL on
   Network layer is optional according to the specified PLC standard,
   for example IEEE 1901.2 MAY use RPL protocol while ITU-T G.9903 MUST
   NOT.

                 +----------------------------------------+
                 |           Application Layer            |
                 +----------------------------------------+
                 |                TCP/UDP                 |
                 +----------------------------------------+
                 |                                        |
                 |                  IPv6                  |
                 |                                        |
                 |----------------------------------------|
                 |   Adaptation layer for IPv6 over PLC   |
                 +----------------------------------------+
                 |             PLC MAC Layer              |
                 |   (IEEE 1901.2 MAC/ITU-T G.9903 MAC)   |
                 +----------------------------------------+
                 |             PLC PHY Layer              |
                 |   (IEEE 1901.2 PHY/ITU-T G.9903 PHY)   |
                 +----------------------------------------+

                        Figure 1: PLC Protocol Stack

3.2.  Addressing Modes

   Two addressing modes are enabled in PLC including the IEEE 64-bit
   extended address and the 16-bit short address which is unique within
   the PAN [IEEE 1901.2, ITU-T G.9903].  Physical addressing uses a
   globally unique 64-bit address to represent each node on the
   powerline.  This is useful when initializing a system because the



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   nodes do not have unique logical addresses on power up.  Logical
   addressing uses 16-bit short address to represent each node on the
   powerline with a much lower latency and higher throughput.  Note that
   in ITU-T G.9930, though two addressing modes are enabled, only 16-bit
   addressing is supported in mesh routing.

3.3.  Maximum Transmission Unit

   Maximum Transmission Unit (MTU) of MAC layer is an important
   parameter that determines the applicability of fragmentation and
   reassembly at the adaptation layer of IPv6 over PLC.  IPv6 requires
   that every link in the Internet have 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.

   As a wired communication technology, the MTU of PLC MAC layer is
   normally higher than wireless technology based on IEEE 802.15.4.  The
   IEEE 1901.2 MAC layer supports the MTU of 1576 octets (the original
   value 1280 byte was updated in 2015 [IEEE 1901.2a]).  The MTU for
   ITU-T G.9903 is 400 octets, insufficient for supporting complete IPv6
   packets.  For this concern, fragmentation/reassembly in [RFC 4944]
   MUST be enabled for the G.9903-based scenarios (details can be found
   in section 4.2.5).

4.  Specification of IPv6 over Narrowband PLC

   Due to the narrow bandwidth and low data rate in NBPLC, a 6lo
   adaptation layer is needed to support the transmission of IPv6
   packets.  6LoWPAN standards [RFC 4944], [RFC 6775], and [RFC 6282]
   provides useful functionality including link-local IPv6 addresses,
   stateless address auto-configuration, neighbor discovery and header
   compression.  These standards are referred in the specifications of
   the 6lo adaptation layer which is illustrated in the following
   subsections.

4.1.  IEEE 1901.2

4.1.1.  Stateless Address Autoconfiguration

   An IEEE 1901.2 device performs stateless address autoconfiguration
   according to [RFC 4944] so as to obtain an IPv6 Interface Identifier
   (IID).  In the 16-bit short addressing mode, the 64-bit IID SHALL be
   derived by insert 16-bit "FFEE" into a "pseudo 48-bit address" which
   is formed by the 16-bit PAN ID, 16-bit zero and the 16-bit short
   address as follows:

       16_bit_PAN:00FF:FE00:16_bit_short_address




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   Considering that this derived IID is not globally unique, the
   "Universal/Local" (U/L) bit (7th bit) SHALL be set to zero.

   For the EUI-64 addressing mode, as per [RFC 2464], the Interface
   Identifier is then formed from by complementing the U/L bit,
   generally setting to 1, since an interface's built-in address is
   expected to be globally unique.

4.1.2.  IPv6 Link Local Address

   The IPv6 link-local address [RFC4291] for an IEEE 1901.2 interface is
   formed by appending the Interface Identifier, 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 in IEEE 1901.2

4.1.3.  Unicast and Multicast Address Mapping

   The address resolution procedure for mapping IPv6 unicast addresses
   into IEEE 1901.2 link-layer addresses follows the general description
   in section 7.2 of [RFC4861], unless otherwise specified.

   The Source/Target Link-layer Address option has the following forms
   when the link layer is IEEE 1901.2 and the addresses are EUI-64 or
   16-bit short addresses, respectively.





















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    0                   1              0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length=2   |  |     Type      |    Length=1   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |  |     16-bit short Address      |
   +-         IEEE 1901.2         -+  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            EUI-64             |  |           Padding             |
   +-                             -+  +-                             -+
   |                               |  |         (all zeros)           |
   +-           Address           -+  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   +-           Padding           -+
   |                               |
   +-         (all zeros)         -+
   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 3: Unicast Address Mapping in IEEE 1901.2

   Option fields:

   Type: 1 for Source Link-layer address and 2 for Target Link-layer
   address.

   Length: This is the length of this option (including the type and
   length fields) in units of 8 octets.  The value of this field is 2 if
   using EUI-64 addresses, or 1 if using 16-bit short addresses.

   IEEE 1901.2 Address: The 64-bit IEEE 1901.2 address, or the 16-bit
   short address.  This is the address the interface currently responds
   to.  This address may be different from the built-in address used to
   derive the Interface Identifier, because of privacy or security
   (e.g., of neighbor discovery) considerations.

   Multicast address mapping is not supported in IEEE 1901.2.  A link-
   local multicast only reaches neighbors within direct physical
   connectivity.  IEEE 1901.2 excludes the functionality of multicast
   either in [RFC 4944] or in coexistence modes with G3-PLC and PRIME.
   However, IEEE 1901.2 supports the required MTU by IPv6, eliminating
   the need of fragmentation and reassembly at the 6lo adaptation layer,
   so the multicast functionality in this case is applicable and is
   RECOMMENDED in this document.

4.1.4.  Header Compression




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   The IEEE 1901.2 PHY layer supports a maximum PSDU (PHY Service Data
   Unit) of 512 octets while the allowed PHY payload is smaller and can
   change dynamically based on channel conditions.  Due to the limited
   PHY payload, header compression at 6lo adaptation layer is of great
   importance and MUST be applied.  The compression of IPv6 datagrams
   within IEEE 1901.2 frames refers to [RFC 6282], which updates [RFC
   4944].  Header compression as defined in RFC6282 which specifies the
   fragmentation methods for IPv6 datagrams on top of IEEE 802.15.4, is
   REQUIRED in this document as the basis for IPv6 header compression in
   IEEE 1901.2.  All headers MUST be compressed according to RFC6282
   encoding formats.

4.1.5.  Fragmentation and Reassembly

   To cope with the mismatch between the size of the PHY frame payload
   and the size of the MAC Service Data Unit (MSDU), IEEE 1901.2 MAC
   layer provides the functionality of segmentation and reassembly.  A
   Segment Control Field is defined in the MAC frame header regardless
   of whether segmentation is required.  This process segments a MAC
   layer datagram into multiple fragments and provides a reliable one-
   hop transfer of the resulting fragments.  However, for the 6lo
   adaptation layer, since IEEE 1901.2 naturally supports a MAC payload
   of 1280 octets, the minimum MTU of IPv6, there is no need for
   fragmentation and reassembly for the IPv6 packet transmission.  This
   document specifies that, in the IPv6 packet transmission over IEEE
   1901.2, fragmentation and reassembly in [RFC 4944] MUST NOT be used.

4.2.  ITU-T G.9903

4.2.1.  Stateless Address Autoconfiguration

   The stateless address auto-configuration in ITU-T G.9903 also refers
   to [RFC 4944] with the following selections: The 64-bit interface
   identifier shall be derived from a "pseudo 48-bit address" formed
   with the PAN identifier and the short address as follows:
   0xYYYY:00FF:FE00:XXXX where 0xYYYY is the PAN identifier and XXXX is
   the short address.  Additional care shall be taken when choosing a
   PAN identifier so as not to interfere with I/G and U/L bits of the
   interface identifier.  If the PAN identifiers are chosen randomly,
   then the U/L and I/G bits (7th and 8th bits) shall be set to zero
   [ITU-T G.9903].

4.2.2.  IPv6 Link Local Address

   In ITU-T G.9903, the formation of IPv6 link-local address follows the
   same process as IEEE 1901.2 (see section 4.1.2) by appending the
   Interface Identifier (IID) to the prefix FE80::/64.




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4.2.3.  Unicast and Multicast Address Mapping

   The address resolution procedure for mapping IPv6 unicast addresses
   into ITU-T G.9903 link-layer addresses follows the general
   description in section 7.2 of [RFC4861], unless otherwise specified.
   Source/Target link-layer address option field SHOULD contain the EUI-
   64 address or the combined address with PAN ID and 16-bit short
   address of the source or target device as below.  Note that the
   format of the Target Link-layer address in ITU-T G.9903 (see Figure
   4) is specified according to the Annex E of [ITU-T G.9903].

    0                   1              0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length=2   |  |     Type      |    Length=1   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |  |            PAN ID             |
   +-        ITU-T G.9903         -+  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            EUI-64             |  |     16-bit short Address      |
   +-                             -+  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Address            |  |         (all zeros)           |
   +-                             -+  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   +-           Padding           -+
   |                               |
   +-         (all zeros)         -+
   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 4: Unicast Address Mapping in ITU-T G.9903

   Option fields:

   Type: 1 for Source Link-layer address and 2 for Target Link-layer
   address.

   Length: This is the length of this option (including the type and
   length fields) in units of 8 octets.  The value of this field is 2 if
   using EUI-64 addresses, or 1 if using 16-bit short addresses.

   ITU-T G.9903 Address: The 64-bit IEEE 1901.2 address, or the 16-bit
   short address.  This is the address the interface currently responds
   to.  This address may be different from the built-in address used to
   derive the Interface Identifier, because of privacy or security
   (e.g., of neighbor discovery) considerations.




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   The address resolution procedure for mapping IPv6 multicast addresses
   into ITU-T G.9903 link-layer addresses follows the general
   description in [RFC 4944] and MUST only be used in a mesh-enabled
   network.  An IPv6 packet with a multicast destination address (DST),
   consisting of the sixteen octets DST[1] through DST[16], is
   transmitted to the following 802.15.4 16-bit multicast address (see
   Figure 5):

                     0                   1
                     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    |1 0 0|DST[15]* |    DST[16]    |
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 5: Multicast Address Mapping

   Here, DST[15]* refers to the last 5 bits in octet DST[15], that is,
   bits 3-7 within DST[15].  The initial 3-bit pattern of "100" follows
   the 16-bit address format for multicast addresses (see Section 12 of
   [RFC 4944]).

4.2.4.  Header Compression

   Header compression as defined in [RFC6282], which specifies the
   compression format for IPv6 datagrams on top of IEEE 802.15.4, is
   REQUIRED in this document as the basis for IPv6 header compression in
   ITU-T G.9903.  All headers MUST be compressed according to [RFC6282]
   encoding formats.

4.2.5.  Fragmentation and Reassembly

   Similar to IEEE 1901.2, Segment Control Field is also defined in the
   ITU-T G.9903 MAC frame header, and the functionality of fragmentation
   and reassembly is also enabled at the G.9903 MAC layer.  However, the
   maximum MAC payload size is fixed to 400 octets at the present ITU-T
   G.9903 recommendation, thus to cope with the required MTU of 1280
   octets by IPv6, fragmentation and reassembly at 6lo adaptation layer
   MUST be provided referring to [RFC 4944].

   To avoid the duplicate fragmentation at both 6lo adaptation layer and
   ITU-T G.9903 MAC layer, an optional way is to limit the MAC payload
   size so that the MSDU can fit the PHY payload without MAC layer
   fragmentation.  However, the number of data bytes of the PHY payload
   can change dynamically based on channel conditions (see section 9.3
   in [ITU-T G.9903]), so the best solution is incrementing the allowed
   MAC payload size above 1280 octets so as to avoid the use of
   fragmentation and reassembly at 6lo adaptation layer.




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4.2.6.  Extension at 6lo Adaptation Layer

   Apart from the 6Lo headers specified in [RFC 4944], an additional
   command frame header is defined for the mesh routing procedure which
   appears in the following order: Mesh addressing header, Broadcast
   header, Fragmentation header, Command frame header [ITU-T G.9903].

   Figure 6 shows an example of the command frame: The ESC header type
   (01000000b) indicates an additional dispatch byte follows (see [RFC
   4944] and [RFC 6282]).  Then this 1-octet dispatch field is used as
   the Command frame header and filled with the Command ID.  This header
   shall be in the last position if more than one header is present in
   the frame.  The Command ID can be classified into 4 types:

    - LOADng message (0x01)

    - LoWPAN bootstrapping protocol message (0x02)

    - Reserved by ITU-T (0x03-0x0F)

    - CMSR protocol messages (0X10-0X1F)

             0                   1                   2
        Bits 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
            +---------------+---------------+-----------------
            |ESC header type|               |
            |               |   Command ID  | Command Payload
            |  01 000000b   |               |
            +---------------+---------------+-----------------

         Figure 6: Command Frame Header Format of ITU-T G.9903

   For the Mesh addressing type and header, it is worthy to note that
   the value of the HopsLeft field SHALL not exceed adpMaxHops.  When
   the originator and final destination devices are neighbors (i.e., the
   next hop address equals the final destination address and the next
   hop address is present in the originator's neighbor table), the mesh
   header shall be omitted in the frame.

5.  Internet Connectivity Scenarios and Topologies

   The network model can be simplified to two kinds of network devices:
   Border Router (BR) and Node.  BR is the coordinator of the PLC subnet
   and can be seen as a master node while Nodes are typically PLC meters
   and sensors.  The IPv6 over PLC networks SHOULD be built as tree,
   mesh or star according to the specified using scenarios.  Every
   network requires at least one BR to communicate with each nodes.




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   One common topology in the current PLC scenarios is star.  In this
   case, the communication at the link layer only takes place between a
   node and a BR.  The BR collects data (e.g. smart meter reading) from
   different nodes, and then concentrates and uploads the data through
   Ethernet or LPWAN (see Figure 7).  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.

                       Node      Node
                         \        /           +---------
                          \      /           /
                           \    /           +
                            \  /            |
                Node ------  BR =========== |  Internet
                            /  \            |
                           /    \           +
                          /      \           \
                         /        \           +---------
                       Node      Node

                <---------------------->
                      PLC subnet

          Figure 7: PLC Star Network connected to the Internet

   Tree topology is used when the distance between a node A and BR is
   beyond the PLC allowed limit while there is another node B in between
   able to communicate with both sides.  Node B in this case acts both
   as a 6lo Node and a Proxy Coordinator (PCO).  For this scenario, the
   link layer communications take place between node A and node B, and
   between node B and BR.  An example of PLC tree network is depicted in
   Figure 8.  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 give information
   such as wind speed, 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 as depicted in [RFC 8036].











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                            Node
                              \                   +---------
                    Node       \                 /
                      \         \               +
                       \         \              |
              Node --- Node ----- BR ========== |  Internet
                       /         /              |
                      /         /               +
           Node --- Node       /                 \
                              /                   +---------
                   Node --- Node

           <------------------------->
                    PLC subnet

         Figure 8: PLC Tree Network connected to the Internet

   Mesh networking in PLC is still under development but of great
   potential applications.  By connecting all nodes with their neighbors
   in communication range (see Figure 9), mesh topology dramatically
   enhances the communication efficiency and thus expands the size of a
   PLC network.  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).

                    Node ----- Node
                    / \        / \                   +---------
                   /   \      /   \                 /
                  /     \    /     \               +
                 /       \  /       \              |
      Node --- Node ---- Node ----- BR =========== |  Internet
                 \       /  \       /              |
                  \     /    \     /               +
                   \   /      \   /                 \
                    \ /        \ /                   +---------
                    Node ----- Node

      <--------------------------------->
                   PLC subnet

         Figure 9: PLC Mesh Network connected to the Internet

6.  IANA Considerations

   There are no IANA considerations related to this document.

7.  Security Consideration




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   This document has no security consideration beyond those in [RFC
   4944] and [RFC 6282].

8.  References

8.1.  Normative References

   [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>.

   [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>.

   [RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
             Networks", RFC 2464, December 1998, <http://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, <http://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, September 2007, <http://www.rfc-
             editor.org/info/rfc4944>.

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

8.2.  Informative References

   [draft-popa-6lo-6loplc] Popa, D. and J.H. Hui, "6LoPLC: Transmission
             of IPv6 Packets over IEEE 1901.2 Narrowband Powerline
             Communication Networks", draft-popa-6lo-6loplc-ipv6-over-



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             ieee19012-networks-00, March 2014,
             <https://tools.ietf.org/html/draft-popa-6lo-6loplc-ipv6-
             over-ieee19012-networks-00>.

   [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, work in progress,
             <http://sites.ieee.org/sagroups-1901-1>.

   [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>.

   [ITU-T G.9960] International Telecommunication Union, "Unified high-
             speed wireline-based home networking transceivers - System
             architecture and physical layer specification", ITU-T
             G.9960, December 2011, <https://www.itu.int/rec/T-REC-
             G.9960>.

   [ITU-T G.9961] International Telecommunication Union, "Unified high-
             speed wireline-based home networking transceivers - Data
             link layer specification", ITU-T G.9961, June 2010,
             <https://www.itu.int/rec/T-REC-G.9961>.

   [RFC 8036] Cam-Winget, N., 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, January 2017, <http://www.rfc-
             editor.org/info/rfc8036>.

9.  Acknowledgments

   Authors wish to thank Yizhou Li and Yuefeng Wu for their valuable
   comments and contributions.

Authors' Addresses

   Jianqiang Hou
   Huawei Technologies
   101 Software Avenue,
   Nanjing 210012
   China

   Phone: +86 15852944235
   Email: houjianqiang@huawei.com



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

   Phone: +82 42 860 6557
   Email: yghong@etri.re.kr


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

   Phone: +86-25-81098508
   Email: itc@sgepri.sgcc.com.cn

































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