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Routing for RPL Leaves
draft-ietf-roll-unaware-leaves-15

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9010.
Authors Pascal Thubert , Michael Richardson
Last updated 2020-04-28 (Latest revision 2020-04-15)
Replaces draft-thubert-roll-unaware-leaves
RFC stream Internet Engineering Task Force (IETF)
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Reviews
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Associated WG milestone
Mar 2020
Initial submission of routing for RPL Leaves draft to the IESG
Document shepherd Rahul Jadhav
Shepherd write-up Show Last changed 2020-04-21
IESG IESG state Became RFC 9010 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Alvaro Retana
Send notices to JADHAV Rahul <rahul.ietf@gmail.com>
draft-ietf-roll-unaware-leaves-15
ROLL                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Updates: 6550, 8505 (if approved)                          M. Richardson
Intended status: Standards Track                               Sandelman
Expires: 17 October 2020                                   15 April 2020

                         Routing for RPL Leaves
                   draft-ietf-roll-unaware-leaves-15

Abstract

   This specification extends RFC6550 and RFC8505 to provide routing
   services to Hosts called RPL Unaware Leaves that implement 6LoWPAN ND
   but do not participate to RPL.  This specification also enables the
   RPL Root to proxy the 6LoWPAN keep-alive flows in its DODAG.

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 17 October 2020.

Copyright Notice

   Copyright (c) 2020 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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  References  . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Glossary  . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  6LoWPAN Neighbor Discovery  . . . . . . . . . . . . . . . . .   7
     3.1.  RFC 6775 Address Registration . . . . . . . . . . . . . .   7
     3.2.  RFC 8505 Extended Address Registration  . . . . . . . . .   7
       3.2.1.  R Flag  . . . . . . . . . . . . . . . . . . . . . . .   8
       3.2.2.  TID, I Field and Opaque Fields  . . . . . . . . . . .   8
       3.2.3.  ROVR  . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  RFC 8505 Extended DAR/DAC . . . . . . . . . . . . . . . .   9
       3.3.1.  RFC 7400 Capability Indication Option . . . . . . . .   9
   4.  Updating RFC 6550 . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Updating RFC 8505 . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Requirements on the RPL-Unware Leaf . . . . . . . . . . . . .  11
     6.1.  Support of 6LoWPAN ND . . . . . . . . . . . . . . . . . .  11
     6.2.  External Routes and RPL Artifacts . . . . . . . . . . . .  12
       6.2.1.  Support of IPv6 Encapsulation . . . . . . . . . . . .  13
       6.2.2.  Support of the HbH Header . . . . . . . . . . . . . .  13
       6.2.3.  Support of the Routing Header . . . . . . . . . . . .  13
   7.  Updated RPL Status  . . . . . . . . . . . . . . . . . . . . .  13
   8.  Updated RPL Target option . . . . . . . . . . . . . . . . . .  14
   9.  Protocol Operations for Unicast Addresses . . . . . . . . . .  15
     9.1.  General Flow  . . . . . . . . . . . . . . . . . . . . . .  15
       9.1.1.  In RPL Non-Storing-Mode . . . . . . . . . . . . . . .  16
       9.1.2.  In RPL Storing-Mode . . . . . . . . . . . . . . . . .  19
     9.2.  Detailed Operation  . . . . . . . . . . . . . . . . . . .  21
       9.2.1.  By the RUL Acting as 6LN  . . . . . . . . . . . . . .  21
       9.2.2.  By the RPL Border Router Acting as 6LR  . . . . . . .  22
       9.2.3.  By the RPL Root . . . . . . . . . . . . . . . . . . .  24
       9.2.4.  By the 6LBR . . . . . . . . . . . . . . . . . . . . .  25
   10. Protocol Operations for Multicast Addresses . . . . . . . . .  26
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  28
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
     12.1.  Resizing the ARO Status values . . . . . . . . . . . . .  29
     12.2.  New DODAG Configuration Option Flag  . . . . . . . . . .  29
     12.3.  RPL Target Option Flags  . . . . . . . . . . . . . . . .  29
     12.4.  New Subregistry for the RPL Non-Rejection Status
            values . . . . . . . . . . . . . . . . . . . . . . . . .  29
     12.5.  New Subregistry for the RPL Rejection Status values  . .  30
   13. acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  30
   14. Normative References  . . . . . . . . . . . . . . . . . . . .  30
   15. Informative References  . . . . . . . . . . . . . . . . . . .  32
   Appendix A.  Example Compression  . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

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1.  Introduction

   The design of Low Power and Lossy Networks (LLNs) is generally
   focused on saving energy, which is the most constrained resource of
   all.  Other design constraints, such as a limited memory capacity,
   duty cycling of the LLN devices and low-power lossy transmissions,
   derive from that primary concern.

   The IETF produced the "Routing Protocol for Low Power and Lossy
   Networks" [RFC6550] (RPL) to provide IPv6 [RFC8200] routing services
   within such constraints.  RPL belongs to the class of Distance-Vector
   protocols, which, compared to link-state protocols, limit the amount
   of topological knowledge that needs to be installed and maintained in
   each node.

   To save signaling and routing state in constrained networks, RPL
   allows a routing stretch (see [RFC6687]), whereby routing is only
   performed along an acyclic graph optimized to reach a Root node, as
   opposed to straight along a shortest path between 2 peers, whatever
   that would mean in a given LLN.  This trades the quality of peer-to-
   peer (P2P) paths for a vastly reduced amount of control traffic and
   routing state that would be required to operate a any-to-any shortest
   path protocol.  Finally, broken routes may be fixed lazily and on-
   demand, based on dataplane inconsistency discovery, which avoids
   wasting energy in the proactive repair of unused paths.

   To provide alternate paths in lossy networks, RPL forms Direction-
   Oriented Directed Acyclic Graphs (DODAGs) using DODAG Information
   Solicitation (DIS) and DODAG Information Object (DIO) messages.  For
   many of the nodes, though not all, a DODAG provides multiple
   forwarding solutions towards the Root of the topology via so-called
   parents.  RPL is designed to adapt to fuzzy connectivity, whereby the
   physical topology cannot be expected to reach a stable state, with a
   lazy control that creates the routes proactively, but may only fix
   them reactively, upon actual traffic.  The result is that RPL
   provides reachability for most of the LLN nodes, most of the time,
   but may not converge in the classical sense.

   [RFC6550] provides unicast and multicast routing services to RPL-
   Aware nodes (RANs), either as a collection tree or with routing back.
   In the latter case, an RAN injects routes to itself using Destination
   Advertisement Object (DAO) messages sent either to parent-nodes, in
   the RPL Storing Mode, or to the Root indicating their parent, in the
   Non-Storing Mode.  This process effectively forms a DODAG back to the
   device that is a subset of the DODAG to the Root with all links
   reversed.

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   RPL can be deployed as an extension to IPv6 Neighbor Discovery (ND)
   [RFC4861][RFC4862] and 6LoWPAN ND [RFC6775][RFC8505] to maintain
   reachability within a Non-Broadcast Multi-Access (NBMA) subnet.  In
   that mode, some nodes may act as Routers and participate to the
   forwarding operations whereas others will only terminate packets,
   acting as Hosts in the data-plane.  In [RFC6550] terms, a Host that
   is reachable over the RPL network is called a Leaf.

   "When to use RFC 6553, 6554 and IPv6-in-IPv6" [USEofRPLinfo]
   introduces the term RPL-Aware-Leaf (RAL) for a Leaf that injects
   routes in RPL to manage the reachability of its own IPv6 addresses.
   In contrast, the term RPL-Unaware Leaf (RUL) designates a Leaf that
   does not participate to RPL at all.  A RUL is an IPv6 Host [RFC8504]
   that needs a RPL-Aware Router to obtain routing services over the RPL
   network.

   This specification leverages the Address Registration mechanism
   defined in 6LoWPAN ND to enable a RUL as a 6LoWPAN Node (6LN) to
   interface with a RPL-Aware Router as a 6LoWPAN Router (6LR) to
   request that the 6LR injects the relevant routing information for the
   Registered Address in the RPL domain on its behalf.  A RUL may be
   unable to participate because it is very energy-constrained, or
   because it is unsafe to let it inject routes in RPL, in which case
   using 6LowPAN ND as the interface for the RUL limits the surface of
   the possible attacks and optionally protects the address ownership.

   The Non-Storing Mode mechanisms are used to extend the routing state
   with connectivity to RULs even when the DODAG is operated in Storing-
   Mode DODAGs.  The unicast packet forwarding operation by the 6LR
   serving a 6LN that is a RPL Leaf is described in [USEofRPLinfo].

   Examples of routing-agnostic 6LNs include lightly-powered sensors
   such as window smash sensor (alarm system), and kinetically powered
   light switches.  Other applications of this specification may include
   a smart grid network that controls appliances - such as washing
   machines or the heating system - in the home.  Appliances may not
   participate to the RPL protocol operated in the Smartgrid network but
   can still interact with the Smartgrid for control and/or metering.

2.  Terminology

2.1.  BCP 14

   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.

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2.2.  References

   The Terminology used in this document is consistent with and
   incorporates that described in "Terms Used in Routing for Low-Power
   and Lossy Networks (LLNs)" [RFC7102].  A glossary of classical
   6LoWPAN acronyms is given in Section 2.3.  Other terms in use in LLNs
   are found in "Terminology for Constrained-Node Networks" [RFC7228].

   "RPL", the "RPL Packet Information" (RPI), "RPL Instance" (indexed by
   a RPLInstanceID) are defined in "RPL: IPv6 Routing Protocol for
   Low-Power and Lossy Networks" [RFC6550].  The RPI is the abstract
   information that RPL defines to be placed in data packets, e.g., as
   the RPL Option [RFC6553] within the IPv6 Hop-By-Hop Header.  By
   extension the term "RPI" is often used to refer to the RPL Option
   itself.  The DODAG Information Solicitation (DIS), Destination
   Advertisement Object (DAO) and DODAG Information Object (DIO)
   messages are also specified in [RFC6550].  The Destination Cleanup
   Object (DCO) message is defined in [EFFICIENT-NPDAO].

   This document uses the terms RPL-Unaware Leaf (RUL) and RPL Aware
   Leaf (RAL) consistently with [USEofRPLinfo].  The term RPL-Aware Node
   (RAN) is introduced to refer to a node that is either an RAL or a RPL
   Router.  As opposed to a RUL, an RAN manages the reachability of its
   addresses and prefixes by injecting them in RPL by itself.

   In this document, readers will encounter terms and concepts that are
   discussed in the following documents:

   Classical IPv6 ND:  "Neighbor Discovery for IP version 6" [RFC4861]
      and "IPv6 Stateless Address Autoconfiguration" [RFC4862],

   6LoWPAN:  "Problem Statement and Requirements for IPv6 over Low-Power
      Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606] and
      "IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs):
      Overview, Assumptions, Problem Statement, and Goals" [RFC4919],
      and

   6LoWPAN ND:  Neighbor Discovery Optimization for Low-Power and Lossy
      Networks [RFC6775], "Registration Extensions for 6LoWPAN Neighbor
      Discovery" [RFC8505], and "Address Protected Neighbor Discovery
      for Low-power and Lossy Networks" [AP-ND] .

2.3.  Glossary

   This document often uses the following acronyms:

   AR:  Address Resolution (aka Address Lookup)

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   6CIO:  6LoWPAN Capability Indication Option

   6LN:  6LoWPAN Node (a Low Power Host or Router)

   6LR:  6LoWPAN Router

   (E)ARO:  (Extended) Address Registration Option

   (E)DAR:  (Extended) Duplicate Address Request

   (E)DAC:  (Extended) Duplicate Address Confirmation

   DAD:  Duplicate Address Detection

   DAO:  Destination Advertisement Object (a RPL message)

   DCO:  Destination Cleanup Object (a RPL message)

   DIS:  DODAG Information Solicitation (a RPL message)

   DIO:  DODAG Information Object (a RPL message)

   DODAG:  Destination-Oriented Directed Acyclic Graph

   LLN:  Low-Power and Lossy Network

   NA:  Neighbor Advertisement

   NCE:  Neighbor Cache Entry

   ND:  Neighbor Discovery

   NS:  Neighbor Solicitation

   RA:  Router Advertisement

   ROVR:  Registration Ownership Verifier

   RPI:  RPL Packet Information

   RAL:  RPL-Aware Leaf

   RAN:  RPL-Aware Node (either a RPL Router or a RPL-Aware Leaf)

   RUL:  RPL-Unaware Leaf

   TID:  Transaction ID (a sequence counter in the EARO)

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3.  6LoWPAN Neighbor Discovery

3.1.  RFC 6775 Address Registration

   The classical "IPv6 Neighbor Discovery (IPv6 ND) Protocol" [RFC4861]
   [RFC4862] was defined for transit media such a Ethernet.  It is a
   reactive protocol that relies heavily on multicast operations for
   address discovery (aka lookup) and duplicate address detection (DAD).

   "Neighbor Discovery Optimizations for 6LoWPAN networks" [RFC6775]
   adapts IPv6 ND for operations over energy-constrained LLNs.  The main
   functions of [RFC6775] are to proactively establish the Neighbor
   Cache Entry (NCE) in the 6LR and to prevent address duplication.  To
   that effect, [RFC6775] introduces a new unicast Address Registration
   mechanism that contributes to reducing the use of multicast messages
   compared to the classical IPv6 ND protocol.

   [RFC6775] defines a new Address Registration Option (ARO) that is
   carried in the unicast Neighbor Solicitation (NS) and Neighbor
   Advertisement (NA) messages between the 6LoWPAN Node (6LN) and the
   6LoWPAN Router (6LR).  It also defines the Duplicate Address Request
   (DAR) and Duplicate Address Confirmation (DAC) messages between the
   6LR and the 6LoWPAN Border Router (6LBR).  In an LLN, the 6LBR is the
   central repository of all the Registered Addresses in its domain and
   the source of truth for uniqueness and ownership.

3.2.  RFC 8505 Extended Address Registration

   "Registration Extensions for 6LoWPAN Neighbor Discovery" [RFC8505]
   updates the behavior of RFC 6775 to enable a generic Address
   Registration to services such as routing and ND proxy, and defines
   the Extended Address Registration Option (EARO) as shown in Figure 1:

      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    |    Status     |    Opaque     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Rsvd | I |R|T|     TID       |     Registration Lifetime     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
    ...             Registration Ownership Verifier                 ...
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 1: EARO Option Format

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3.2.1.  R Flag

   [RFC8505] introduces the "R" flag in the EARO.  The Registering Node
   sets the "R" flag to indicate whether the 6LR should ensure
   reachability for the Registered Address.  If the "R" flag is not set,
   then the Registering Node handles the reachability of the Registered
   Address by other means, which means in a RPL network that it is an
   RAN or that it uses another RPL Router for reachability services.

   This document specifies how the "R" flag is used in the context of
   RPL.  A 6LN is a RUL that requires reachability services for an IPv6
   address if and only if it sets the "R" flag in the NS(EARO) used to
   register the address to a RPL border router acting as 6LR.  Upon
   receiving the NS(EARO), the RPL router generates a DAO message for
   the Registered Address if and only if the "R" flag is set.

3.2.2.  TID, I Field and Opaque Fields

   The EARO also includes a sequence counter called Transaction ID
   (TID), which maps to the Path Sequence Field found in Transit Options
   in RPL DAO messages.  This is the reason why the support of [RFC8505]
   by the RUL as opposed to only [RFC6775] is a prerequisite for this
   specification (more in Section 6.1).  The EARO also transports an
   Opaque field and an "I" field that describes what the Opaque field
   transports and how to use it.  Section 9.2.1 specifies the use of the
   "I" field and of the Opaque field by a RUL.

3.2.3.  ROVR

   Section 5.3. of [RFC8505] introduces the Registration Ownership
   Verifier (ROVR) field of variable length from 64 to 256 bits.  The
   ROVR is a replacement of the EUI-64 in the ARO [RFC6775] that was
   used to identify uniquely an Address Registration with the Link-Layer
   address of the owner, but provided no protection against spoofing.

   "Address Protected Neighbor Discovery for Low-power and Lossy
   Networks" [AP-ND] leverages the ROVR field as a cryptographic proof
   of ownership to prevent a rogue third party from misusing the
   address.  [AP-ND] adds a challenge/response exchange to the [RFC8505]
   Address Registration and enables Source Address Validation by a 6LR
   that will drop packets with a spoofed address.

   This specification does not address how the protection by [AP-ND]
   could be extended to RPL.  On the other hand, it adds the ROVR to the
   DAO to build the proxied EDAR at the Root (see Section 8), which
   means that nodes that are aware of the Host route to the 6LN are made
   aware of the associated ROVR as well.

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3.3.  RFC 8505 Extended DAR/DAC

   [RFC8505] updates the periodic DAR/DAC exchange that takes place
   between the 6LR and the 6LBR using Extended DAR/DAC messages which
   can carry a ROVR field of variable size.  The exchange is triggered
   by an NS(EARO) message and is intended to create, refresh and delete
   the corresponding state in the 6LBR for a lifetime that is indicated
   by the 6LN.  It is protected by the ARQ mechanism specified in 8.2.6
   of [RFC6775], though in an LLN, a duration longer than the
   RETRANS_TIMER [RFC4861] of 1 second may be necessary to cover the
   Turn Around Trip delay from the 6LR to the 6LBR.

   RPL [RFC6550] specifies a periodic DAO from the 6LN all the way to
   the Root that maintains the routing state in the RPL network for the
   lifetime indicated by the source of the DAO.  This means that for
   each address, there are two keep-alive messages that traverse the
   whole network, one to the Root and one to the 6LBR.

   This specification removes the extraneous keep-alive across the LLN.
   The 6LR turns the periodic Address Registration from the RUL into a
   DAO message to the Root on every refresh, but it only generates the
   EDAR upon the first registration, for the purpose of DAD.  Upon a
   refresher DAO, the Root proxies the EDAR exchange to refresh the
   state at the 6LBR on behalf of the 6LR, as illustrated in Figure 7.

3.3.1.  RFC 7400 Capability Indication Option

   "6LoWPAN-GHC: Generic Header Compression for IPv6 over Low-Power
   Wireless Personal Area Networks (6LoWPANs)" [RFC7400] defines the
   6LoWPAN Capability Indication Option (6CIO) that enables a node to
   expose its capabilities in Router Advertisement (RA) messages.
   [RFC8505] defines a number of bits in the 6CIO, in particular:

   L:  Node is a 6LR.
   E:  Node is an IPv6 ND Registrar -- i.e., it supports registrations
      based on EARO.
   P:  Node is a Routing Registrar, -- i.e., an IPv6 ND Registrar that
      also provides reachability services for the Registered Address.

       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  |     Reserved      |D|L|B|P|E|G|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Reserved                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Figure 2: 6CIO flags

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   A 6LR that can provide reachability services for a RUL in a RPL
   network as specified in this document SHOULD include a 6CIO in its RA
   messages and set the L, P and E flags as prescribed by [RFC8505], see
   Section 6.1 for the behavior of the RUL.

4.  Updating RFC 6550

   This document specifies a new behavior whereby a 6LR injects DAO
   messages for unicast addresses (see Section 9) and multicast
   addresses (see Section 10) on behalf of leaves that are not aware of
   RPL.  The addresses are exposed as external targets [RFC6550].  Per
   [USEofRPLinfo], an IP-in-IP encapsulation that terminates at the RPL
   Root is used to remove RPL artifacts and compression techniques that
   may not be processed correctly outside of the RPL domain.

   This document also synchronizes the liveness monitoring at the Root
   and the 6LBR.  A same value of lifetime is used for both, and a
   single keep-alive message, the RPL DAO, traverses the RPL network.  A
   new behavior is introduced whereby the RPL Root proxies the EDAR
   message to the 6LBR on behalf of the 6LR (more in Section 5), for any
   6LN, RUL or RAN.

   RPL defines a configuration option that is registered to IANA in
   section 20.14. of [RFC6550].  This specification defines a new flag
   "Root Proxies EDAR/EDAC" (P) that is encoded in one of the reserved
   control bits in the option.  The new flag is set to indicate that the
   Root performs the proxy operation and that all nodes in the RPL
   network must refrain from renewing the 6LBR state directly.  The bit
   position of the "P" flag is indicated in Section 12.2.

   Section 6.3.1.  of [RFC6550] defines a 3-bit Mode of Operation (MOP)
   in the DIO Base Object.  The new "P" flag is defined only for MOP
   value between 0 to 6.  For a MOP value of 7 or above, the flag MAY
   indicate something different and MUST NOT be interpreted as "Root
   Proxies EDAR/EDAC" unless the specification of the MOP indicates to
   do so.

   The RPL Status defined in section 6.5.1. of [RFC6550] for use in the
   DAO-Ack message is extended to be used in the DCO messages
   [EFFICIENT-NPDAO] as well.  Furthermore, this specification enables
   to use a RPL Status to transport the IPv6 ND Status defined for use
   in the EARO, more in Section 7.

   Section 6.7. of [RFC6550] introduces the RPL Control message Options
   such as the RPL Target Option that can be included in a RPL Control
   message such as the DAO.  Section 8 updates the RPL Target Option to
   optionally transport the ROVR used in the IPv6 Registration (see
   Section 3.2.3) so the RPL Root can generate a full EDAR message.

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5.  Updating RFC 8505

   This document updates [RFC8505] to introduce the anonymous EDAR and
   NS(EARO) messages.  The anonymous messages are used for backward
   compatibility.  The anonymous messages are recognizable by a zero
   ROVR field and can only be used as a refresher for a pre-existing
   state associated to the Registered Address.  More specifically, an
   anonymous message can only increase the lifetime and/or increment the
   TID of an existing state at the 6LBR.

   Upon the renewal of a 6LoWPAN ND Address Registration, this
   specification changes the behavior of a RPL Router acting as 6LR for
   the registration.  If the Root indicates the capability to proxy the
   EDAR/EDAC exchange to the 6LBR then the 6LR refrains from sending an
   EDAR message; if the Root is separated from the 6LBR, the Root
   regenerates the EDAR message to the 6LBR upon a DAO message that
   signals the liveliness of the Address.  The regenerated message is
   anonymous iff the DAO is a legacy message that does not carry a ROVR
   as specified in Section 8.

6.  Requirements on the RPL-Unware Leaf

   This document provides RPL routing for a RUL, that is a 6LN acting as
   an IPv6 Host and not aware of RPL.  Still, a minimal RPL-independent
   functionality is required from the RUL to obtain routing services.

6.1.  Support of 6LoWPAN ND

   In order to obtain routing services from a 6LR, a RUL MUST implement
   [RFC8505] and set the "R" flag in the EARO.  The RUL SHOULD support
   [AP-ND] and use it to protect the ownership of its addresses.  The
   RUL MUST NOT request routing services from a 6LR that does not
   originate RA messages with a CIO that has the L, P, and E flags all
   set as discussed in Section 3.3.1.

   A RUL that has multiple potential routers MUST prefer those that
   provide routing services.  The RUL MUST register to all the 6LRs from
   which it desires routing services.  If there are no available
   routers, the connection of the RUL fails.  The Address Registrations
   SHOULD be performed in an RApid sequence, using the exact same EARO
   for a same Address.  Gaps between the Address Registrations will
   invalidate some of the routes till the Address Registration finally
   shows on those routes as well.

   [RFC8505] introduces error Status values in the NA(EARO) which can be
   received synchronously upon an NS(EARO) or asynchronously.  The RUL
   MUST support both cases and MUST refrain from using the address when
   the Status value indicates a rejection.

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6.2.  External Routes and RPL Artifacts

   Section 4.1. of [USEofRPLinfo] provides a set of rules that MUST be
   followed for the routing operations to a RUL.

   A 6LR that is upgraded to act as a border router for external routes
   advertises them using Non-Storing Mode DAO messages that are unicast
   directly to the Root, even if the DODAG is operated in Storing Mode.
   Non-Storing Mode routes are not visible inside the RPL domain and all
   packets are routed via the Root.  An upgraded Root tunnels the
   packets directly to the 6LR that advertised the external route which
   decapsulates and forwards the original (inner) packet.

   The RPL Non-Storing Mode signaling and the associated IP-in-IP
   encapsulated packets are normal traffic for the intermediate Routers.
   The support of external routes only impacts the Root and the 6LR.  It
   can be operated with legacy intermediate routers and does not add to
   the amount of state that must be maintained in those routers.  A RUL
   is an example of a destination that is reachable via an external
   route which happens to be a Host route.

   The RPL data packets always carry a Hop-by-Hop Header to transport a
   RPL Packet Information (RPI) [RFC6550].  So unless the RUL originates
   its packets with an RPI, the 6LR needs to tunnel them to the Root to
   add the RPI.  As a rule of a thumb and except for the very special
   case above, the packets from and to a RUL are always encapsulated
   using an IP-in-IP tunnel between the Root and the 6LR that serves the
   RUL (see sections 7.1.4, 7.2.3, 7.2.4, 7.3.3, 7.3.4, 8.1.3, 8.1.4,
   8.2.3, 8.2.4, 8.3.3 and 8.3.4 of [USEofRPLinfo] for details).

   In Non-Storing Mode, packets going down carry a Source Routing Header
   (SRH).  The IP-in-IP encapsulation, the RPI and the SRH are
   collectively called the "RPL artifacts" and can be compressed using
   [RFC8138].  Figure 14 presents an example compressed format for a
   packet forwarded by the Root to a RUL in a Storing Mode DODAG.

   The inner packet that is forwarded to the RUL may carry some RPL
   artifacts, e.g., an RPI if the original packet was generated with it
   and possibly an SRH in a Non-Storing Mode DODAG.  [USEofRPLinfo]
   expects the RUL to support the basic "IPv6 Node Requirements"
   [RFC8504].  In particular the RUL is expected to ignore the RPL
   artifacts that are either consumed or not applicable to a Host.

   A RUL is not expected to support the compression method defined in
   [RFC8138].  Unless configured otherwise, the border router MUST
   uncompress the outgoing packet before forwarding over an external
   route, even if it is not the destination of the incoming packet, and
   even when delivering to a RUL.

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6.2.1.  Support of IPv6 Encapsulation

   Section 2.1 of [USEofRPLinfo] sets the rules for forwarding IP-in-IP
   either to the final 6LN or to a parent 6LR.  In order to enable IP-
   in-IP to the 6LN in Non-Storing Mode, the 6LN must be able to
   decapsulate the tunneled packet and either drop the inner packet if
   it is not the final destination, or pass it to the upper layer for
   further processing.  Unless it is aware that the RUL can handle IP-
   in-IP properly, the Root that encapsulates a packet to a RUL
   terminates the IP-in-IP tunnel at the parent 6LR . For that reason,
   it is beneficial but not necessary for a RUL to support IP-in-IP.

6.2.2.  Support of the HbH Header

   A RUL is expected to process an unknown Option Type in a Hop-by-Hop
   Header as prescribed by section 4.2 of [RFC8200].  This means in
   particular that an RPI with an Option Type of 0x23 [USEofRPLinfo] is
   ignored when not understood.

6.2.3.  Support of the Routing Header

   A RUL is expected to process an unknown Routing Header Type as
   prescribed by section 4.4 of [RFC8200].  This means in particular
   that Routing Header with a Routing Type of 3 [RFC6554] is ignored
   when the Segments Left is zero, and the packet is dropped otherwise.

7.  Updated RPL Status

   The RPL Status is defined in section 6.5.1. of [RFC6550] for use in
   the DAO-Ack message and values are assigned as follows:

               +---------+--------------------------------+
               | Range   | Meaning                        |
               +=========+================================+
               | 0       | Success/Unqualified acceptance |
               +---------+--------------------------------+
               | 1-127   | Not an outright rejection      |
               +---------+--------------------------------+
               | 128-255 | Rejection                      |
               +---------+--------------------------------+

                     Table 1: RPL Status per RFC 6550

   This specification extends the scope of the RPL Status to be used in
   RPL DCO messages.  Furthermore, this specification enables to carry
   the IPv6 ND Status values defined for use in the EARO and initially
   listed in table 1 of [RFC8505] in a RPL Status.

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   Section 12.1 reduces the range of EARO Status values to 0-63 ensure
   that they fit within a RPL Status as shown in Figure 3.

                               0
                               0 1 2 3 4 5 6 7
                              +-+-+-+-+-+-+-+-+
                              |E|A|  Value    |
                              +-+-+-+-+-+-+-+-+

                        Figure 3: RPL Status Format

   RPL Status subfields:

   E:  1-bit flag.  Set to indicate a rejection.  When not set, a value
      of 0 indicates Success/Unqualified acceptance and other values
      indicate "not an outright rejection" as per RFC 6550.

   A:  1-bit flag.  Indicates the type of the Status value.

   Status Value:  6-bit unsigned integer.  If the 'A' flag is set this
      field transports a Status value defined for IPv6 ND EARO.  When
      the 'A' flag is not set, the Status value is defined in a RPL
      extension.

   When building a DCO or a DAO-ACK message upon an IPv6 ND NA or a DAC
   message, the RPL Root MUST copy the ARO Status unchanged in a RPL
   Status with the 'A' bit set.  The RPL Root MUST set the 'E' flag for
   all values in range 1-10 which are all considered rejections.

   Conversely, the 6LR MUST copy the value of the RPL Status unchanged
   in the EARO of an NA message that is built upon a RPL Status with the
   'A' bit set in a DCO or a DAO-ACK message.

8.  Updated RPL Target option

   This specification updates the RPL Target option to transport the
   ROVR.  This enables the RPL Root to generate a full EDAR message as
   opposed to an anonymous EDAR that has restricted properties.

   The Target Prefix field MUST be aligned to the next 4-byte boundary
   after the size indicated by the Prefix Length.  If necessary the
   transported prefix MUST be padded with zeros.

   With this specification the ROVR is the remainder of the RPL Target
   Option.  The size of the ROVR is indicated in a new ROVR Size field
   that is encoded to map one-to-one with the Code Suffix in the EDAR
   message (see table 4 of [RFC8505]).

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   The modified format is illustrated in Figure 4.  It is backward
   compatible with the Target Option in [RFC6550] and SHOULD be used as
   a replacement.

      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 = 0x05 | Option Length |ROVRsz | Flags | Prefix Length |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                Target Prefix (Variable Length)                |
      .                Aligned to 4-byte boundary                     .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
     ...            Registration Ownership Verifier (ROVR)           ...
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 4: Updated Target Option

   New fields:

   ROVRsz:  Indicates the Size of the ROVR.  It MAY be 1, 2, 3, or 4,
      denoting a ROVR size of 64, 128, 192, or 256 bits, respectively.

   Registration Ownership Verifier (ROVR):  This is the same field as in
      the EARO, see [RFC8505]

9.  Protocol Operations for Unicast Addresses

   The description below assumes that the Root sets the "P" flag in the
   DODAG Configuration Option and performs the EDAR proxy operation.

9.1.  General Flow

   This specification eliminates the need to exchange keep-alive
   Extended Duplicate Address messages, EDAR and EDAC, all the way from
   a 6LN to the 6LBR across a RPL mesh.  Instead, the EDAR/EDAC exchange
   with the 6LBR is proxied by the RPL Root upon a DAO message that
   refreshes the RPL routing state.  Any combination of the logical
   functions of 6LR, Root and 6LBR might be collapsed in a single node.

   To achieve this, the lifetimes and sequence counters in 6LoWPAN ND
   and RPL are aligned.  In other words, the Path Sequence and the Path
   Lifetime in the DAO message are taken from the Transaction ID and the
   Address Registration lifetime in the NS(EARO) message from the 6LN.

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   In a RPL network where the function is enabled, refreshing the state
   in the 6LBR is the responsibility of the Root.  Consequently, only
   addresses that are injected in RPL will be kept alive by the RPL
   Root.  In a same fashion, if an additional routing protocol is
   deployed on a same network, that additional routing protocol may need
   to handle the keep alive procedure for the addresses that it serves.

   On the first Address Registration, illustrated in Figure 5 for RPL
   Non-Storing Mode, the Extended Duplicate Address exchange takes place
   as prescribed by [RFC8505].  If the exchange fails, the 6LR returns
   an NA message with a negative status to the 6LN, the NCE is not
   created and the address is not injected in RPL.  If it is successful,
   the 6LR creates an NCE and injects the Registered Address in RPL
   using DAO/DAO-ACK exchanges all the way to the RPL DODAG Root.

   The 6LN signals the termination of a registration with a 6LR using an
   NS(EARO) with a Registration Lifetime set to 0.  Upon this, the 6LR
   MUST perform an EDAR/EDAC exchange to clean up the state at the 6LBR,
   as illustrated in Figure 8, unless it uses the ROVR in the RPL Target
   Option and, in Storing Mode, it is propagated to the Root.

9.1.1.  In RPL Non-Storing-Mode

   In Non-Storing Mode, the DAO message flow can be nested within the
   Address Registration flow as illustrated in Figure 5.

      6LN/RUL            6LR            Root               6LBR
         |                |              |                   |
         |   NS(EARO)     |              |                   |
         |--------------->|                                  |
         |                |          Extended DAR            |
         |                |--------------------------------->|
         |                |                                  |
         |                |          Extended DAC            |
         |                |<---------------------------------|
         |                |      DAO     |                   |
         |                |------------->|                   |
         |                |              | (anonymous) EDAR  |
         |                |              |------------------>|
         |                |              |       EDAC        |
         |                |              |<------------------|
         |                |    DAO-ACK   |                   |
         |                |<-------------|                   |
         |   NA(EARO)     |              |                   |
         |<---------------|              |                   |
         |                |              |                   |

           Figure 5: First Registration Flow in Non-Storing Mode

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   An issue may be detected later, e.g., the address moves within the
   LLN or to a different Root on a backbone [6BBR].  In that case the
   value of the status that indicates the issue can be passed from
   6LoWPAN ND to RPL and back as illustrated in Figure 6.

      6LN/RUL              6LR           Root              6LBR
         |                  |             |                  |
         |                  |             | NA(EARO, Status) |
         |                  |             |<-----------------|
         |                  | DCO(Status) |                  |
         |                  |<------------|                  |
         | NA(EARO, Status) |             |                  |
         |<-----------------|             |                  |
         |                  |             |                  |

                        Figure 6: Asynchronous Issue

   An Address re-Registration is performed by the 6LN to maintain the
   NCE in the 6LR alive before lifetime expires.  Upon an Address re-
   Registration, as illustrated in Figure 7, the 6LR redistributes the
   Registered Address NS(EARO) in RPL.

      6LN/RUL            6LR            Root               6LBR
         |                |              |                   |
         |   NS(EARO)     |              |                   |
         |--------------->|                                  |
         |                |      DAO     |                   |
         |                |------------->|                   |
         |                |              | (anonymous) EDAR  |
         |                |              |------------------>|
         |                |              |       EDAC        |
         |                |              |<------------------|
         |                |    DAO-ACK   |                   |
         |                |<-------------|                   |
         |   NA(EARO)     |              |                   |
         |<---------------|              |                   |

            Figure 7: Next Registration Flow in Non-Storing Mode

   This causes the RPL DODAG Root to refresh the state in the 6LBR with
   an EDAC message or an anonymous EDAC if the ROVR is not indicated in
   the Target Option.  In both cases, the EDAC message sent in response
   by the 6LBR contains the actual value of the ROVR field for that
   Address Registration.  In case of an error on the proxied EDAR flow,
   the error MUST be returned in the DAO-ACK - if one was requested -
   using a RPL Status with the 'A' flag set that imbeds a 6LoWPAN Status
   value as discussed in Section 7.

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   If the Root could not return the negative Status in the DAO-ACK then
   it sends an asynchronous Destination Cleanup Object (DCO) message
   [EFFICIENT-NPDAO] to the 6LR by placing the negative Status in the
   RPL Status with the 'A' flag set.  Note that if both are used in a
   short interval of time, the DAO-ACK and DCO messages are not
   guaranteed to arrive in the same order at the 6LR.

   The 6LR may receive a requested DAO-ACK even after it received a DCO,
   but the negative Status in the DCO supercedes a positive Status in
   the DAO-ACK regardless of the order in which they are received.  Upon
   the DAO-ACK - or the DCO if it arrives first - the 6LR responds to
   the RUL with an NA(EARO).  If the RPL Status has the 'A' flag set,
   then the ND Status is extracted and passed in the EARO; else, if the
   'E' flag is set, indicating a rejection, then the status 4 "Removed"
   is used; else, the ND Status of 0 indicating "Success" is used.

   The RUL may terminate the registration at anytime by using a
   Registration Lifetime of 0.  This specification expects that the RPL
   Target option transports a ROVR.  If that is the case, the normal
   heartbeat flow is sufficient to inform the 6LBR using the Root as
   proxy as illustrated in Figure 7.  If the 6LR could not add the ROVR
   to the DAO message, then it MUST inform the 6LBR separately using as
   illustrated in Figure 8.

      6LN/RUL            6LR            Root               6LBR
         |                |              |                   |
         |NS(EARO,Lifet=0)|              |                   |
         |--------------->|                                  |
         |                |          Extended DAR            |
         |                |--------------------------------->|
         |                |                                  |
         |                |          Extended DAC            |
         |                |<---------------------------------|
         |                |DAO (Lifeti=0)|                   |
         |                |------------->|                   |
         |                |              |  anonymous EDAR   |
         |                |              |------------------>|
         |                |              |       EDAC        |
         |                |              |<------------------|
         |                |    DAO-ACK   |                   |
         |                |<-------------|                   |
         |   NA(EARO)     |              |                   |
         |<---------------|              |                   |
         |                |              |                   |
         |           <Remove NCE>        |                   |
         |                |              |                   |

       Figure 8: Last Registration Flow in Non-Storing Mode, No ROVR

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9.1.2.  In RPL Storing-Mode

   In RPL Storing Mode, the DAO-ACK is optional.  When it is used, it is
   generated by the RPL parent, which does not need to wait for the
   grand-parent to send the acknowledgment.  A successful DAO-ACK is not
   a guarantee that the DAO has yet reached the Root or that the EDAR
   was successfully proxied by the Root.

   The 6LR uses the EDAR/EDAC exchange as in Non-Storing Mode, for the
   initial registration, and also possibly at the termination in the
   case the 6LR could not add the ROVR to the RPL Target option of the
   DAO message.

   6LN/RUL         6LR             6LR            Root              6LBR
    |               |               |               |                  |
    |   NS(EARO)    |               |               |                  |
    |-------------->|               |               |                  |
    |               |                 Extended DAR                     |
    |               |------------------------------------------------->|
    |               |                                                  |
    |               |                 Extended DAC                     |
    |               |<-------------------------------------------------|
    |   NA(EARO)    |               |               |                  |
    |<--------------|               |               |                  |
    |               |      DAO      |               |                  |
    |               |-------------->|               |                  |
    |               |    DAO-ACK    |               |                  |
    |               |<--------------|               |                  |
    |               |               |      DAO      |                  |
    |               |               |-------------->|                  |
    |               |               |    DAO-ACK    |                  |
    |               |               |<--------------|                  |
    |               |               |               | (anonymous) EDAR |
    |               |               |               |----------------->|
    |               |               |               |   EDAC(ROVR)     |
    |               |               |               |<-----------------|
    |               |               |               |                  |

             Figure 9: First Registration Flow in Storing Mode

   The Storing Mode of RPL does not provide and end-to-end confirmation
   that a DAO reached the root.  When the 6LR has just joined, and later
   if DAO messages are lost before reaching the Root, the 6LR might not
   be reachable back from the Root.  Performing an EDAR/EDAC exchange on
   behalf of a RUL provides that confirmation.  On the other hand, if
   the 6LR retries an EDAR and never gets and EDAC back, it SHOULD
   resend a DAO to become reachable again, before it tries another
   sequence of EDAR.

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   6LN/RUL         6LR             6LR            Root              6LBR
    |               |               |               |                  |
    |   NS(EARO)    |               |               |                  |
    |-------------->|               |               |                  |
    |   NA(EARO)    |               |               |                  |
    |<--------------|               |               |                  |
    |               |      DAO      |               |                  |
    |               |-------------->|               |                  |
    |               |    DAO-ACK    |               |                  |
    |               |<--------------|               |                  |
    |               |               |      DAO      |                  |
    |               |               |-------------->|                  |
    |               |               |    DAO-ACK    |                  |
    |               |               |<--------------|                  |
    |               |               |               | (anonymous) EDAR |
    |               |               |               |----------------->|
    |               |               |               |   EDAC(ROVR)     |
    |               |               |               |<-----------------|
    |               |               |               |                  |

             Figure 10: Next Registration Flow in Storing Mode

   If the keep-alive fails, or an asynchronous issue is reported, the
   path can be cleaned up asynchronously using a DCO message
   [EFFICIENT-NPDAO] as illustrated in Figure 11 and described in
   further details in Section 9.2.3.

   6LN/RUL            6LR           6LR          Root              6LBR
    |                  |             |             |                  |
    |                  |             |             | NA(EARO, Status) |
    |                  |             |             |<-----------------|
    |                  |             |             |                  |
    |                  |             | DCO(Status) |                  |
    |                  |             |<------------|                  |
    |                  |             |             |                  |
    |                  | DCO(Status) |             |                  |
    |                  |<------------|             |                  |
    |                  |             |             |                  |
    | NA(EARO, Status) |             |             |                  |
    |<-----------------|             |             |                  |
    |                  |             |             |                  |

                      Figure 11: Issue in Storing Mode

   In the case illustrated here, the issue is actually detected in the
   ND protocol and reported in the State of a NA(EARO) message.  That
   statis is transported in the DCO message as a RPL Status with the 'A'
   and typically the 'E' flags set.

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9.2.  Detailed Operation

9.2.1.  By the RUL Acting as 6LN

   This specification does not alter the operation of a 6LoWPAN ND-
   compliant 6LN, and a RUL is expected to operate as follows:

   1.  The 6LN obtains an IPv6 global address, either using Stateless
       Address Autoconfiguration (SLAAC) [RFC4862] based on a Prefix
       Information Option (PIO) [RFC4861] found in an RA message, or
       some other means such as DHCPv6 [RFC3315].

   2.  Once it has formed an address, the 6LN (re)registers its address
       periodically, within the Lifetime of the previous Address
       Registration, as prescribed by [RFC6775] and [RFC8505], to
       refresh the NCE before the lifetime indicated in the EARO
       expires.  The TID is incremented each time and wraps in a
       lollipop fashion (see section 5.2.1 of [RFC8505] which is fully
       compatible with section 7.2 of [RFC6550]).

   3.  As stated in section 5.2 of [RFC8505], the 6LN can register to
       more than one 6LR at the same time.  In that case, it MUST use
       the same value of TID for all of the parallel Address
       Registrations.  The 6LN should send the registration(s) with a
       non-zero Registration Lifetime and ensure that one succeeds
       before it terminates other registrations to maintain the state in
       the network and at the 6LBR and minimize the churn.

   4.  Following section 5.1 of [RFC8505], a 6LN acting as a RUL sets
       the "R" flag in the EARO of at least one registration, whereas
       acting as an RAN it never does.  If the "R" flag is not echoed in
       the NA, the RUL SHOULD attempt to use another 6LR.  The 6LN
       should send the registration(s) with the "R" flag set and ensure
       that one succeeds before it sends the registrations with the flag
       reset.  In case of a conflict with the preceeding rule on
       lifetime, the rule on lifetime has precedence.

   5.  The 6LN may use any of the 6LRs to which it registered as default
       gateway.  Using a 6LR to which the 6LN is not registered may
       result in packets dropped at the 6LR by a Source Address
       Validation function (SAVI) so it is not recommended.

   Even without support for RPL, a RUL may be aware of opaque values to
   be provided to the routing protocol.  If the RUL has a knowledge of
   the RPL Instance the packet should be injected into, then it SHOULD
   set the Opaque field in the EARO to the RPLInstanceID, else it MUST
   leave the Opaque field to zero.

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   Regardless of the setting of the Opaque field, the 6LN MUST set the
   "I" field to zero to signal "topological information to be passed to
   a routing process" as specified in section 5.1 of [RFC8505].

   A RUL is not expected to produce RPL artifacts in the data packets,
   but it MAY do so.  For instance, if the RUL has a minimal awareness
   of the RPL Instance then it can build an RPI.  A RUL that places an
   RPI in a data packet MUST indicate the RPLInstanceID of the RPL
   Instance where the packet should be forwarded.  All the flags and the
   Rank field are set to zero as specified by section 11.2 of [RFC6550].

9.2.2.  By the RPL Border Router Acting as 6LR

   Also as prescribed by [RFC8505], the 6LR generates an EDAR message
   upon reception of a valid NS(EARO) message for the registration of a
   new IPv6 Address by a 6LN.  If the initial EDAR/EDAC exchange
   succeeds, then the 6LR installs an NCE for the Registration Lifetime.
   For the refreshes of the registration, if the RPL Root has indicated
   that it proxies the keep-alive EDAR/EDAC exchange with the 6LBR (see
   Section 4), the 6LR MUST refrain from sending the keep-alive EDAR
   itself.

   If the "R" flag is set in the NS(EARO), the 6LR SHOULD attempt to
   inject the host route in RPL, unless this is barred for other
   reasons, like a saturation of the network or if its RPL parent.  The
   6LR MUST set "R" flag in the NA(EARO) back if and only if it
   successfully injected the Registered Address in RPL.

   The 6LR may at any time send a unicast asynchronous NA(EARO) with the
   "R" flag reset to signal that it stops providing routing services,
   and/or with the EARO Status 2 "Neighbor Cache full" to signal that it
   removes the NCE.  It may also send a final RA, unicast or multicast,
   with a Router Lifetime field of zero, to signal that it stops serving
   as router, as specified in section 6.2.5 of [RFC4861].

   The Opaque field in the EARO hints the 6LR on the RPL Instance that
   SHOULD be used for the DAO advertisements, and for the forwarding of
   packets sourced at the registered address when there is no RPI in the
   packet, in which case the 6LR MUST encapsulate the packet to the Root
   adding an RPI in the outer header.  If the Opaque field is zero, the
   6LR is free to use the default RPL Instance (zero) for the registered
   address or to select an Instance of its choice.

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   if the "I" field is not zero, then the 6LR MUST consider that the
   Opaque field is zero.  If the Opaque field is not zero, then it is
   expected to carry a RPLInstanceID for the RPL Instance suggested by
   the 6LN.  If the 6LR does not participate to the associated Instance,
   then the 6LR MUST consider that the Opaque field is zero; else, that
   is if the 6LR participates to the suggested Instance, then the 6LR
   SHOULD use that Instance for the registered address.

   The DAO message advertising the Registered Address MUST be
   constructed as follows:

   1.  The Registered Address is signaled as Target Prefix in the RPL
       Target Option in the DAO message; the Prefix Length is set to 128

   2.  RPL Non-Storing Mode is to be used.  The 6LR indicates one of its
       global or unique-local IPv6 unicast addresses as the Parent
       Address in the associated RPL Transit Information Option (TIO)

   3.  the External 'E' flag in the TIO is set to indicate that the 6LR
       redistributes an external target into the RPL network

   4.  the Path Lifetime in the TIO is computed from the Lifetime in the
       EARO Option.  This adapts it to the Lifetime Units used in the
       RPL operation; note that if the lifetime is 0, then the 6LR
       generates a No-Path DAO message that cleans up the routes down to
       the Address of the 6LN; this also causes the Root as a proxy to
       send an EDAR message to the 6LBR with a Lifetime of 0.

   5.  the Path Sequence in the TIO is set to the TID value found in the
       EARO option.

   The NCE is removed if the 6LR tries to inject the route is RPL and
   fails for reasons related to ND, which is recognized by both the 'E'
   and the 'A' flags set in the RPL Status of the DAO-ACK or the DCO, as
   detailed below.

   Otherwise, success injecting the route is assumed if a DAO-ACK was
   not requested or if it is received with a RPL Status that is not a
   rejection (i.e., the 'E' flag not set).

   In case of success, if the 'A' flag is set in the RPL Status of the
   DAO-ACK, then the 6LR MUST use the Status Value in the RPL Status for
   the Status in the NA(EARO), else a Status of 0 (Success) is returned.

   The status of 0 MUST also be used if the 6LR could not even try to
   inject the route - note that the "R" flag is reset in that case.

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   In a network where Address Protected Neighbor Discovery (AP-ND) is
   enabled, in case of a DAO-ACK or a DCO indicating transporting an
   EARO Status Value of 5 (Validation Requested), the 6LR MUST challenge
   the 6LN for ownership of the address, as described in section 6.1 of
   [AP-ND], before the Registration is complete.  This ensures that the
   address validated before it is injected in RPL.

   If the challenge succeeds then the operations continue as normal.  In
   particular a DAO message is generated upon the NS(EARO) that proves
   the ownership of the address.  If the challenge failed, the 6LR
   rejects the registration as prescribed by AP-ND and may take actions
   to protect itself against DoS attacks by a rogue 6LN, see Section 11.

   The other rejection codes indicate that the 6LR failed to inject the
   address into the RPL network.  If an EARO Status is transported, the
   6LR MUST send a NA(EARO) to the RUL with that Status value, and the
   "R" flag not set.  Similarly, upon receiving a DCO message indicating
   that the address of a RUL should be removed from the routing table,
   the 6LR issues an asynchronous NA(EARO) to the RUL with the embedded
   ND Status value if there was one, and the "R" flag not set.

   If a 6LR receives a valid NS(EARO) message with the "R" flag reset
   and a Registration Lifetime that is not 0, and the 6LR was
   redistributing the Registered Address due to previous NS(EARO)
   messages with the flag set, then it MUST stop injecting the address.
   It is up to the Registering 6LN to maintain the corresponding route
   from then on, either keeping it active via a different 6LR or by
   acting as an RAN and managing its own reachability.

9.2.3.  By the RPL Root

   A RPL Root SHOULD set the "P" flag in the RPL configuration option of
   the DIO messages that it generates (see Section 4) to signal that it
   proxies the keep-alive EDAR/EDAC echange.  The remainder of this
   section assumes that it does.

   Upon reception of a DAO message, for each RPL Target option that
   creates or updates an existing RPL state, the Root notifies the 6LBR.
   This can be done using an internal API if they are co-located, or
   using a proxied EDAR/EDAC exchange if they are separated.

   If the RPL Target option transports a ROVR, then the Root MUST use it
   to build a full EDAR message; else, an anonymous EDAR is used with
   the ROVR field set to zero.

   The EDAR message MUST be constructed as follows:

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   1.  The Target IPv6 address from the RPL Target Option is placed in
       the Registered Address field of the EDAR message;

   2.  the Registration Lifetime is adapted from the Path Lifetime in
       the TIO by converting the Lifetime Units used in RPL into units
       of 60 seconds used in the 6LoWPAN ND messages;

   3.  the TID value is set to the Path Sequence in the TIO and
       indicated with an ICMP code of 1 in the EDAR message;

   4.  If the ROVR is present in the RPL Target option, it is copied as
       is in the EDAR and the ICMP Code Suffix is set to the appropriate
       value as shown in Table 4 of [RFC8505] depending on the size of
       the ROVR field; else, the ROVR field in the EDAR is set to zero
       indicating an anonymous EDAR.

   Upon a Status value in an EDAC message that is not "Success", the
   Root SHOULD destroy the formed paths using either a DAO-ACK (in Non-
   Storing Mode) or a DCO downwards as specified in [EFFICIENT-NPDAO].
   Failure to destroy the former path would result in Stale routing
   state and local black holes if the address belongs to another party
   elsewhere in the network.  The RPL Status value that maps the 6LoWPAN
   ND Status value MUST be embedded in the RPL Status in the DCO.

9.2.4.  By the 6LBR

   Upon reception of an EDAR message with the ROVR field set to a non-
   zero value, the 6LBR acts as prescribed by [RFC8505].  If the ROVR is
   set to 0, indicating an anonymous EDAR, the 6LBR MUST act as below:

   1.  The 6LBR checks whether an entry exists for the address.  If the
       entry does not exist, the 6LBR MUST NOT create the entry, and it
       MUST answer with a Status "Removed" in the EDAC message.  If the
       entry exists, the 6LBR computes whether the TID in the EDAR
       message is fresher than the one in the entry as prescribed in
       section 4.2.1. of [RFC8505], and continues as follows:

   2.  If the anonymous EDAR message is fresher, the 6LBR updates the
       TID in the entry, restarts the heartbeat timer for the entry, and
       answers with a Status "Success" in the EDAC message.  If the
       value of the Registration Lifetime is smaller than the value in
       the entry, then the latter value MUST be used for the heartbeat;
       this means in particular that the Registration Lifetime of 0 is
       ignored.  Conversely, if the duration of the Lifetime is extended
       by the Registration Lifetime in the EDAR message, it is used for
       the hearbeat and to the value in the entry is updated.

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   3.  If the TID in the entry is the same or fresher, the 6LBR does not
       update the entry, and answers with a Status "Success" and "Moved"
       in the EDAC message, respectively.

   The EDAC that is constructed is the same as if the anonymous EDAR was
   a full EDAR, but for the ROVR that is set to zero.

10.  Protocol Operations for Multicast Addresses

   Section 12 of [RFC6550] details the RPL support for multicast flows.
   This support is not source-specific and only operates as an extension
   to the Storing Mode of Operation for unicast packets.  Note that it
   is the RPL model that the multicast packet is passed as a Layer-2
   unicast to each of the interested children.  This remains true when
   forwarding between the 6LR and the listener 6LN.

   "Multicast Listener Discovery (MLD) for IPv6" [RFC2710] and its
   updated version "Multicast Listener Discovery Version 2 (MLDv2) for
   IPv6" [RFC3810] provide an interface for a listener to register to
   multicast flows.  MLDv2 is backwards compatible with MLD, and adds in
   particular the capability to filter the sources via black lists and
   white lists.  In the MLD model, the Router is a "querier" and the
   Host is a multicast listener that registers to the querier to obtain
   copies of the particular flows it is interested in.

   On the first Address Registration, as illustrated in Figure 12, the
   6LN, as an MLD listener, sends an unsolicited Report to the 6LR in
   order to start receiving the flow immediately.

      6LN/RUL                6LR             Root                6LBR
         |                    |               |                    |
         | unsolicited Report |               |                    |
         |------------------->|               |                    |
         |     <L2 ack>       | DAO           |                    |
         |                    |-------------->|                    |
         |                    |    DAO-ACK    |                    |
         |                    |<--------------| <if not listening> |
         |                    |               | unsolicited Report |
         |                    |               |------------------->|
         |                    |               |                    |

                Figure 12: First Multicast Registration Flow

   Since multicast Layer-2 messages are avoided, it is important that
   the asynchronous messages for unsolicited Report and Done are sent
   reliably, for instance using a Layer-2 acknowledgment, or attempted
   multiple times.

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   The 6LR acts as a generic MLD querier and generates a DAO for the
   multicast target.  The lifetime of the DAO is set to be in the order
   of the Query Interval, yet larger to account for variable propagation
   delays.

   The Root proxies the MLD exchange as a listener with the 6LBR acting
   as the querier, so as to get packets from a source external to the
   RPL domain.  Upon a DAO with a multicast target, the RPL Root checks
   if it is already registered as a listener for that address, and if
   not, it performs its own unsolicited Report for the multicast target.

   An Address re-Registration is pulled periodically by 6LR acting as
   querier.  Note that the message may be sent unicast to all the known
   individual listeners.  Upon a time out of the Query Interval, the 6LR
   sends a Query to each of its listeners, and gets a Report back that
   is mapped into a DAO, as illustrated in Figure 13:

      6LN/RUL                6LR             Root                6LBR
         |                    |               |                    |
         |       Query        |               |                    |
         |<-------------------|               |                    |
         |       Report       |               |                    |
         |------------------->|               |                    |
         |                    | DAO           |                    |
         |                    |-------------->|                    |
         |                    |    DAO-ACK    |                    |
         |                    |<--------------|                    |
         |                    |               |                    |
         |                    |               |       Query        |
         |                    |               |<-------------------|
         |                    |               |       Report       |
         |                    |               |------------------->|
         |                    |               |                    |
         |                    |               |                    |

                     Figure 13: Next Registration Flow

   Note that any of the functions 6LR, Root and 6LBR might be collapsed
   in a single node, in which case the flow above happens internally,
   and possibly through internal API calls as opposed to messaging.

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11.  Security Considerations

   First of all, it is worth noting that with [RFC6550], every node in
   the LLN is RPL-aware and can inject any RPL-based attack in the
   network.  This specification isolates edge nodes that can only
   interact with the RPL routers using 6LoWPAN ND, meaning that they
   cannot perform RPL insider attacks. 6LoWPAN ND can optionally provide
   SAVI features, which reduces even more the attack perimeter that is
   available to the edge nodes.

   The LLN nodes depend on the 6LBR and the RPL participants for their
   operation.  A trust model must be put in place to ensure that the
   right devices are acting in these roles, so as to avoid threats such
   as black-holing, (see [RFC7416] section 7) or bombing attack whereby
   an impersonated 6LBR would destroy state in the network by using the
   "Removed" Status code.

   This trust model could be at a minimum based on a Layer-2 Secure
   joining and the Link-Layer security.  This is a generic 6LoWPAN
   requirement, see Req5.1 in Appendix of [RFC8505].

   Additionally, the trust model could include a role validation to
   ensure that the node that claims to be a 6LBR or a RPL Root is
   entitled to do so.

   The anonymous EDAR message does not carry a valid Registration Unique
   ID [RFC8505] in the form of a ROVR and may be played by any node on
   the network without the need to know the ROVR.  The 6LBR MUST NOT
   create an entry based on a anonymous EDAR and it MUST NOT decrease
   the value of the lifetime.  All it can do is refresh the lifetime and
   the TID of an existing entry.  So the message cannot be used to
   create a binding state in the 6LBR but it can be used to maintain one
   active longer than expected.

   Note that a full EDAR message with a lifetime of 0 will destroy that
   state and the anonymous message will not recreate it.  Note also that
   a rogue that has access to the network can attack the 6LBR with other
   (forged) addresses and ROVR, and that this is a much easier DoS
   attack than trying to keep existing state alive longer.

   At the time of this writing RPL does not have a zerotrust model
   whereby it is possible to validate the origin of an address that is
   injected in a DAO.  This specification makes a first step in that
   direction by allowing the Root to challenge the RUL by the 6LR that
   serves it.

12.  IANA Considerations

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12.1.  Resizing the ARO Status values

   IANA is requested to modify the Address Registration Option Status
   Values Registry as follows: The unassigned values range is reduced
   from 11-255 to 11-63.

12.2.  New DODAG Configuration Option Flag

   This specification updates the Registry for the "DODAG Configuration
   Option Flags" that was created for [RFC6550] as follows:

          +------------+----------------------------+-----------+
          | Bit Number | Capability Description     | Reference |
          +============+============================+===========+
          | 1          | Root Proxies EDAR/EDAC (P) | THIS RFC  |
          +------------+----------------------------+-----------+

                Table 2: New DODAG Configuration Option Flag

12.3.  RPL Target Option Flags

   Section 20.15 of [RFC6550] creates a registry for the 8-bit RPL
   Target Option Flags field.  This specification reduces the field to 4
   bits.  The IANA is requested to reduce the size of the registry
   accordingly.

12.4.  New Subregistry for the RPL Non-Rejection Status values

   This specification creates a new Subregistry for the RPL Non-
   Rejection Status values for use in RPL DAO-ACK and DCO messages with
   the 'A' flag reset, under the ICMPv6 parameters registry.

   *  Possible values are 6-bit unsigned integers (0..63).

   *  Registration procedure is "Standards Action" [RFC8126].

   *  Initial allocation is as indicated in Table 3:

              +-------+------------------------+-----------+
              | Value | Meaning                | Reference |
              +=======+========================+===========+
              | 0     | Unqualified acceptance | RFC 6550  |
              +-------+------------------------+-----------+

               Table 3: Acceptance values of the RPL Status

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12.5.  New Subregistry for the RPL Rejection Status values

   This specification creates a new Subregistry for the RPL Rejection
   Status values for use in RPL DAO-ACK and RCO messages with the 'A'
   flag reset, under the ICMPv6 parameters registry.

   *  Possible values are 6-bit unsigned integers (0..63).

   *  Registration procedure is "Standards Action" [RFC8126].

   *  Initial allocation is as indicated in Table 4:

             +-------+-----------------------+---------------+
             | Value | Meaning               | Reference     |
             +=======+=======================+===============+
             | 0     | Unqualified rejection | This document |
             +-------+-----------------------+---------------+

                Table 4: Rejection values of the RPL Status

13.  acknowledgments

   The authors wish to thank Georgios Papadopoulos and Rahul Jadhav for
   their early reviews of and contributions to this document

14.  Normative References

   [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>.

   [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710,
              DOI 10.17487/RFC2710, October 1999,
              <https://www.rfc-editor.org/info/rfc2710>.

   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,
              <https://www.rfc-editor.org/info/rfc3810>.

   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals",
              RFC 4919, DOI 10.17487/RFC4919, August 2007,
              <https://www.rfc-editor.org/info/rfc4919>.

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   [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>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [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>.

   [RFC6553]  Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
              Power and Lossy Networks (RPL) Option for Carrying RPL
              Information in Data-Plane Datagrams", RFC 6553,
              DOI 10.17487/RFC6553, March 2012,
              <https://www.rfc-editor.org/info/rfc6553>.

   [RFC6554]  Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with the Routing Protocol
              for Low-Power and Lossy Networks (RPL)", RFC 6554,
              DOI 10.17487/RFC6554, March 2012,
              <https://www.rfc-editor.org/info/rfc6554>.

   [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>.

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

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7400]  Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
              IPv6 over Low-Power Wireless Personal Area Networks
              (6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400, November
              2014, <https://www.rfc-editor.org/info/rfc7400>.

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   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8138]  Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
              "IPv6 over Low-Power Wireless Personal Area Network
              (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
              April 2017, <https://www.rfc-editor.org/info/rfc8138>.

   [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>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [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>.

   [AP-ND]    Thubert, P., Sarikaya, B., Sethi, M., and R. Struik,
              "Address Protected Neighbor Discovery for Low-power and
              Lossy Networks", Work in Progress, Internet-Draft, draft-
              ietf-6lo-ap-nd-20, 9 March 2020,
              <https://tools.ietf.org/html/draft-ietf-6lo-ap-nd-20>.

   [USEofRPLinfo]
              Robles, I., Richardson, M., and P. Thubert, "Using RPI
              Option Type, Routing Header for Source Routes and IPv6-in-
              IPv6 encapsulation in the RPL Data Plane", Work in
              Progress, Internet-Draft, draft-ietf-roll-useofrplinfo-38,
              23 March 2020, <https://tools.ietf.org/html/draft-ietf-
              roll-useofrplinfo-38>.

   [EFFICIENT-NPDAO]
              Jadhav, R., Thubert, P., Sahoo, R., and Z. Cao, "Efficient
              Route Invalidation", Work in Progress, Internet-Draft,
              draft-ietf-roll-efficient-npdao-17, 30 October 2019,
              <https://tools.ietf.org/html/draft-ietf-roll-efficient-
              npdao-17>.

15.  Informative References

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   [RFC6606]  Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
              Statement and Requirements for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Routing",
              RFC 6606, DOI 10.17487/RFC6606, May 2012,
              <https://www.rfc-editor.org/info/rfc6606>.

   [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>.

   [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>.

   [RFC6687]  Tripathi, J., Ed., de Oliveira, J., Ed., and JP. Vasseur,
              Ed., "Performance Evaluation of the Routing Protocol for
              Low-Power and Lossy Networks (RPL)", RFC 6687,
              DOI 10.17487/RFC6687, October 2012,
              <https://www.rfc-editor.org/info/rfc6687>.

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

   [RFC8025]  Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Paging Dispatch",
              RFC 8025, DOI 10.17487/RFC8025, November 2016,
              <https://www.rfc-editor.org/info/rfc8025>.

   [RFC8504]  Chown, T., Loughney, J., and T. Winters, "IPv6 Node
              Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
              January 2019, <https://www.rfc-editor.org/info/rfc8504>.

   [6BBR]     Thubert, P., Perkins, C., and E. Levy-Abegnoli, "IPv6
              Backbone Router", Work in Progress, Internet-Draft, draft-
              ietf-6lo-backbone-router-20, 23 March 2020,
              <https://tools.ietf.org/html/draft-ietf-6lo-backbone-
              router-20>.

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Appendix A.  Example Compression

   Figure 14 illustrates the case in Storing Mode where the packet is
   received from the Internet, then the Root encapsulates the packet to
   insert the RPI and deliver to the 6LR that is the parent and last hop
   to the final destination, which is not known to support [RFC8138].

   +-+ ... -+-+ ... +-+- ... -+-+ ... -+-+-+ ... +-+-+ ... -+ ... +-...
   |11110001|SRH-6LoRH| RPI-  |IP-in-IP| NH=1      |11110CPP| UDP | UDP
   |Page 1  |Type1 S=0| 6LoRH | 6LoRH  |LOWPAN_IPHC| UDP    | hdr |Payld
   +-+ ... -+-+ ... +-+- ... -+-+ ... -+-+-+ ... +-+-+ ... -+ ... +-...
            <-4 bytes->                <-        RFC 6282        ->
                                       <-     No RPL artifact ...

           Figure 14: Encapsulation to Parent 6LR in Storing Mode

   The difference with the example presented in Figure 19 of [RFC8138]
   is the addition of a SRH-6LoRH before the RPI-6LoRH to transport the
   compressed address of the 6LR as the destination address of the outer
   IPv6 header.  In the original example the destination IP of the outer
   header was elided and was implicitly the same address as the
   destination of the inner header.  Type 1 was arbitrarily chosen, and
   the size of 0 denotes a single address in the SRH.

   In Figure 14, the source of the IP-in-IP encapsulation is the Root,
   so it is elided in the IP-in-IP 6LoRH.  The destination is the parent
   6LR of the destination of the inner packet so it cannot be elided.
   In Storing Mode, it is placed as the single entry in an SRH-6LoRH as
   the first 6LoRH.  Since there is a single entry so the SRH-6LoRH Size
   is 0.  In this particular example, the 6LR address can be compressed
   to 2 bytes so a Type of 1 is used.  It results that the total length
   of the SRH-6LoRH is 4 bytes.

   In Non-Storing Mode, the encapsulation from the Root would be similar
   to that represented in Figure 14 with possibly more hops in the SRH-
   6LoRH and possibly multiple SRH-6LoRHs if the various addresses in
   the routing header are not compressed to the same format.  Note that
   on the last hop to the parent 6LR, the RH3 is consumed and removed
   from the compressed form, so the use of Non-Storing Mode vs.  Storing
   Mode is indistinguishable from the packet format.

   The SRH-6LoRHs are followed by RPI-6LoRH and then the IP-in-IP 6LoRH.
   When the IP-in-IP 6LoRH is removed, all the 6LoRH Headers that
   precede it are also removed.  The Paging Dispatch [RFC8025] may also
   be removed if there was no previous Page change to a Page other than
   0 or 1, since the LOWPAN_IPHC is encoded in the same fashion in the
   default Page 0 and in Page 1.  The resulting packet to the
   destination is the inner packet compressed with [RFC6282].

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Authors' Addresses

   Pascal Thubert (editor)
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   06254 Mougins - Sophia Antipolis
   France

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com

   Michael C. Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca
   URI:   http://www.sandelman.ca/

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