ROLL                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Updates: 6550, 8505 (if approved)                          M. Richardson
Intended status: Standards Track                               Sandelman
Expires: 18 June 30 July 2020                                    27 January 2020                                   16 December 2019

                         Routing for RPL Leaves


   This specification extends RFC6550 and RFC8505 to provide unicast and
   multicast routing services in a RPL domain to 6LNs that are plain
   Hosts and do not participate to RPL, and enables the RPL Root to
   proxy the EDAR/EDAC flow on behalf of the RULs and RANs 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

   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 18 June 30 July 2020.

Copyright Notice

   Copyright (c) 2019 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 (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  References  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Glossary  . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  6LoWPAN Neighbor Discovery  . . . . . . . . . . . . . . . . .   7
     3.1.  RFC 6775  . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  RFC 8505 Extended ARO . . . . . . . . . . . . . . . . . .   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 . . . . . . . . . . . . . . . . . . . . . .   9  10
   5.  Updating RFC 8505 . . . . . . . . . . . . . . . . . . . . . .  10  11
   6.  Requirements on the RPL-Unware Leaf . . . . . . . . . . . . .  10  11
     6.1.  Support of 6LoWPAN ND . . . . . . . . . . . . . . . . . .  11
     6.2.  External Routes and RPL Artifacts . . . . . . . . . . . .  11  12
       6.2.1.  Support of the HbH Header . . . . . . . . . . . . . .  12  13
       6.2.2.  Support of the Routing Header . . . . . . . . . . . .  12  13
       6.2.3.  Support of IPv6 Encapsulation . . . . . . . . . . . .  12  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 . . . . . . . . . . . . . . .  15  16
       9.1.2.  In RPL Storing-Mode . . . . . . . . . . . . . . . . .  18  19
     9.2.  Detailed Operation  . . . . . . . . . . . . . . . . . . .  18  19
       9.2.1.  By the 6LN  . . . . . . . . . . . . . . . . . . . . .  19  20
       9.2.2.  By the 6LR  . . . . . . . . . . . . . . . . . . . . .  20  21
       9.2.3.  By the RPL Root . . . . . . . . . . . . . . . . . . .  22  23
       9.2.4.  By the 6LBR . . . . . . . . . . . . . . . . . . . . .  23  24
   10. Protocol Operations for Multicast Addresses . . . . . . . . .  23  24
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  25  26
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26  27
     12.1.  New DODAG Configuration Option Flag  . . . . . . . . . .  26  27
     12.2.  RPL Target Option Flags  . . . . . . . . . . . . . . . .  26  27
     12.3.  New Subregistry for the RPL Non-Rejection Status
            values . . . . . . . . . . . . . . . . . . . . . . . . .  26  27
     12.4.  New Subregistry for the RPL Rejection Status values  . .  26  27
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  27  28
   14. Normative References  . . . . . . . . . . . . . . . . . . . .  27  28
   15. Informative References  . . . . . . . . . . . . . . . . . . .  29  30
   Appendix A.  Example Compression  . . . . . . . . . . . . . . . .  30  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31  32

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
   protocol, which, compared to link-state protocols, limits the amount
   of topological knowledge that needs to be installed and maintained in
   each node.

   In order to operate 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.

   In order to cope with lossy transmissions, RPL forms Direction-
   Oriented Directed Acyclic Graphs (DODAGs) using DODAG Information
   Solicitation (DIS) and DODAG Information Object (DIO) messages.  For
   most 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 routes proactively but only fixes them when
   they are used by actual traffic.

   The result is that RPL provides reachability for most of the LLN
   nodes, most of the time, but may not really converge in the classical
   sense.  RPL provides unicast and multicast routing services back to
   RPL-Aware nodes (RANs).

   A RAN will inject routes to itself using Destination Advertisement
   Object (DAO) messages sent to either parent-nodes in Storing Mode or
   to the Root indicating their parent in 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.

   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, a RPL-Unaware Leaf (RUL) designates a Leaf does not
   participate to RPL at all.  A RUL is a plain Host that needs an
   interface to its RPL Router to obtain routing services over the LLN.

   This specification enables a RUL that is a 6LoWPAN Node (6LN) to
   announce itself as a Host to its 6LoWPAN Router (6LR) in the 6LoWPAN
   ND Address Address Registration, and to request that the 6LR injects
   the relevant routing information for the Registered Address in the
   RPL domain on its behalf.  The unicast packet forwarding operation by
   the 6LR serving a 6LN that is a RPL Leaf is described in

   Examples of routing-agnostic 6LN may include lightly-powered sensors
   such as window smash sensor (alarm system), and kinetically powered
   light switches.  Other application 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 receive control packet from the Smartgrid.

2.  Terminology

2.1.  BCP 14

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

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.

   The term "byte" is used in its now customary sense as a synonym for

   "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 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

   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 a RAL or a RPL
   Router.  As opposed to a RUL, a RAN manages the reachability of its
   addresses and prefixes by injecting them in RPL by itself.

   Other terms in use in LLNs are found in Terminology for
   Constrained-Node Networks [RFC7228].

   Readers are expected to be familiar with all the terms and concepts
   that are discussed in

   *  "Neighbor Discovery for IP version 6" [RFC4861],

   *  "IPv6 Stateless Address Autoconfiguration" [RFC4862],

   *  "Problem Statement and Requirements for IPv6 over Low-Power
      Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606],

   *  "IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs):
      Overview, Assumptions, Problem Statement, and Goals" [RFC4919],

   *  "Neighbor Discovery Optimization for Low-power and Lossy Networks"
      [RFC6775], and

   *  "Registration Extensions for IPv6 over Low-Power Wireless Personal
      Area Network (6LoWPAN) Neighbor Discovery" [RFC8505].

2.3.  Glossary

   This document often uses the following acronyms:

   AR:  Address Resolution (aka Address Lookup)


   6CIO:  6LoWPAN Border Router 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 (the abstract information RPL places in
      data packets as the RPL Option within the IPv6 Hop-By-Hop Header,
      and by extension the RPL Option itself)

   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)

3.  6LoWPAN Neighbor Discovery

3.1.  RFC 6775

   The "IPv6 Neighbor Discovery (IPv6 ND) Protocol" suite [RFC4861]
   [RFC4862] was defined for transit media such a Ethernet, and relies
   heavily on multicast operations for address discovery and duplicate
   address detection (DAD).  "Neighbor Discovery Optimizations for
   6LoWPAN networks" [RFC6775] (6LoWPAN ND) adapts IPv6 ND for
   operations over energy-constrained LLNs.  In particular, 6LoWPAN ND
   introduces a unicast Host Registration mechanism that contributes to
   reducing the use of multicast messages that are present in the
   classical IPv6 ND protocol.

   6LoWPAN ND 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).  6LoWPAN ND 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

   The main functions of [RFC6775] are to proactively establish the
   Neighbor Cache Entry in the 6LR and to avoid address duplication.
   There is no concept of registering the address for an external

3.2.  RFC 8505 Extended ARO

   "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

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, e.g., by means of routing or
   proxying ND.  If the "R" flag is not set, then the Registering Node
   is expected to be a RAN that handles the reachability of the
   Registered Address by itself.

   This document specifies how the "R" flag is used in the context of
   RPL.  A 6LN operates as a RUL for an IPv6 address iff it sets the "R"
   flag in the EARO used to register the address.  The RPL Router
   generates a DAO message for the Registered Address upon an NS(EARO)
   iff the "R" flag in the EARO is set.  Conversely, this document
   specifies the behavior of a RPL Router acting as 6LR that depends on
   the setting of the "R" flag in the EARO.

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, which means that nodes
   that are aware of the Host route to the 6LN are now aware of the
   associated ROVR as well.

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.  The
   Extended Duplicate Address messages can carry a ROVR field of
   variable size.  The periodic EDAR/EDAC exchange is triggered by a
   NS(EARO) message and is intended to create and then refresh the
   corresponding state in the 6LBR for a lifetime that is indicated by
   the 6LN.  Conversely, 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 a lifetime that is indicated by the source of the
   DAO.  This means that there are two periodic messages that traverse
   the whole network to indicate that an address is still reachable, one
   to the Root and one to the 6LBR.

   This specification saves the support of RPL in a 6LN called a RUL and
   avoids an extraneous periodic flow across the LLN.  The RUL only
   needs to perform a [RFC8505] Address Registration to the 6LR.  The
   6LR turns it into a DAO message to the Root on behalf of the RUL.
   Upon the new DAO, the Root proxies the EDAR exchange to the 6LBR on
   behalf of the 6LR.  This is illustrated in Figure 5. 6.

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 Addres

       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

   A RUL can get reachability services from a 6LR if and only if the L,
   P and E flags are set in a 6CIO originated by the 6LR.  A 6LR that
   can provide reachability services for a RUL node in a RPL network as
   specified in this document sets the L, P and E flags.

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 Targets are exposed as External addresses.  An IP-in-IP
   encapsulation that terminates at the border 6LR 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 heartbeat 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 network
   must refrain from renewing the 6LBR state directly.  The bit position
   of the "P" flag is indicated in Section 12.1.

   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.

5.  Updating RFC 8505

   This document updates [RFC8505] to introduce a keep-alive EDAR
   message and a keep-alive NS(EARO) message.  The keep-alive messages
   are used for backward compatibility, when the DAO does not transport
   a ROVR as specified in Section 8.  The keep-alive messages have a
   zero ROVR field and can only be used to refresh a pre-existing state
   associated to the Registered Address.  More specifically, a keep-
   alive message can only increase the lifetime and/or increment the TID
   of the existing state in a 6LBR.

   Upon the renewal of a 6LoWPAN ND Address Registration, this
   specification changes the behavior of a RPL Router acting as 6LR for
   the Address Registration as follows.  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.

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 in order to obtain routing
   services from the 6LR.

6.1.  Support of 6LoWPAN ND

   In order to obtain routing services from a RPL 6LR, a RUL MUST implement
   [RFC8505] and set the "R" flag in the EARO option.  The RUL MAY
   request routing services from a 6LR if and only if the L, R and E
   flags are all set in the 6CIO [RFC7400] originated by the 6LR.

   The RUL MUST register to all the 6LRs from which it expects to get requests routing
   services.  The Address Registrations SHOULD be performed in a 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 refrain from using the Registered Address
   as specified by [RFC8505] depending on the Status value.

   A RUL SHOULD support [AP-ND] to protect the ownership of its

6.2.  External Routes and RPL Artifacts

   Section 4.1. of [USEofRPLinfo] provides a set of rules that MUST be
   followed when forwarding packets over an external route:

   RPL data packets are often encapsulated using IP-in-IP and in Non-
   Storing Mode, packets going down will carry an SRH as well.  RPL data
   packets also typically carry a Hop-by-Hop Header to transport a RPL
   Packet Information (RPI) [RFC6550].  These additional headers are
   called RPL artifacts.

   When IP-in-IP is used and the outer headers terminate at a 6LR down
   the path (see Figure 9 10 for the compressed format in Storing Mode),
   then the 6LR decapsulates the IP-in-IP and the packet that is
   forwarded to the external destination is free of RPL artifacts - but
   possibly an RPI if packet was generated by a RAN in the same RPL
   domain as the destination RUL.

   Non-Storing Mode DAO messages are used to signal external routes to
   the Root, even if the DODAG is operated in Storing Mode.  This
   enables to advertise the 6LR that injects the route for use as tunnel
   endpoint in the data path.

   For all external routes, the Root should use an IP-in-IP tunnel to
   that 6LR, with the RPL artifacts in the outer header to be stripped
   by the 6LR.  The IP-in-IP encapsulation may be avoided in Storing
   Mode if the path to the external destination beyond the 6LR is known
   to handle or ignore the RPL artifacts properly [RFC8200].

   A RUL is an example of a destination that is reachable via an
   external (Host) route for which IP-in-IP tunneling may be avoided as
   it ignores the RPI and the consumed SRH artifacts.  The use of non-
   Storing Mode signaling in Storing Mode and the associated IP-in-IP
   encapsulation are transparent to intermediate Routers that only see
   packets back and forth between the Root and the 6LR and do not need a
   special support for external routes.

   The RUL may not support IP-in-IP tunneling [RFC8504], so if IP-in-IP
   is used, and unless the Root as a better knowledge, the tunnel should
   terminate at the 6LR that injected the external route to the RUL.

   Additionally, the RUL is not expected to support the compression
   method defined in [RFC8138].  The 6LR that injected the route MUST
   uncompress the packet before forwarding over an external route, even
   when delivering to a RUL, even when it is not the destination in the
   outer header of the incoming packet, unless configured to do

6.2.1.  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.

6.2.2.  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 [RFC6553] is ignored
   when the Segments Left is zero, and dropped otherwise.

6.2.3.  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.

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.  Only EARO Status
   values in the range 0-63 can be transported.

   The resulting RPL Status is as follows:

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

                        Figure 2: 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

   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.  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

8.  Updated RPL Target option

   This specification updates the RPL Target option to transport the
   ROVR as illustrated in Figure 3. 4.  This enables the RPL Root to
   generate a full EDAR Message as opposed to a keep-alive EDAR that has
   restricted properties.

   The Target Prefix MUST be aligned to the next 4-byte boundary after
   the size indicated by the Prefix Length. if necessary it is padded
   with zeros.  The size of the ROVR is indicated in a new ROVR Type
   field that is encoded to map the CodePfx in the EDAR message (see
   section 4.2 of [RFC8505]).

   With this specification the ROVR is the remainder of the RPL Target
   Option.  The format 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 3: 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 enables to save the exchange of Extended Duplicate
   Address messages, EDAR and EDAC, from a 6LN all the way to the 6LBR
   across a RPL mesh, for the sole purpose of refreshing an existing
   state in the 6LBR.  Instead, the EDAR/EDAC exchange is proxied by the
   RPL Root upon a DAO message that refreshes the RPL routing state.

   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.

   In that flow, the RPL Root acts as a proxy to refresh the state in
   the 6LBR.  The proxy operation applies to both RUL and RAN.  This
   means that 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 4 5 and
   Figure 6 7 for RPL Non-Storing and Storing Mode respectively, the
   Extended Duplicate Address exchange takes place as prescribed by
   [RFC8505].  Any of the functions 6LR, Root and 6LBR might be
   collapsed in a single node.

   When successful, the flow creates a Neighbor Cache Entry (NCE) in the
   6LR, and the 6LR injects the Registered Address in RPL using DAO/DAO-
   ACK exchanges all the way to the RPL DODAG Root.  The protocol does
   not carry a specific information that the Extended Duplicate Address
   messages were already exchanged, so the Root proxies them anyway.

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 4 5 and it is
   possible to carry information such as an updated lifetime from the
   6LBR all the way back to the 6LN.

        6LN              6LR            Root               6LBR
         |                |              |                   |
         |   NS(EARO)     |              |                   |
         |--------------->|                                  |
         |                |          Extended DAR            |
         |                |--------------------------------->|
         |                |                                  |
         |                |          Extended DAC            |
         |                |<---------------------------------|
         |                |      DAO     |                   |
         |                |------------->|                   |
         |                |              | (keep-alive) EDAR |
         |                |              |------------------>|
         |                |              |       EDAC        |
         |                |              |<------------------|
         |                |    DAO-ACK   |                   |
         |                |<-------------|                   |
         |   NA(EARO)     |              |                   |
         |<---------------|              |                   |
         |                |              |                   |
             (in case if an Error not reported in DAO-ACK)
         |                |              |                   |
         |                |      DCO     |                   |
         |                |<-------------|                   |
         |   NA(EARO)     |              |                   |
         |<---------------|              |                   |
         |                |              |                   |

           Figure 4: 5: First Registration Flow in Non-Storing Mode

   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 5, 6, the 6LR redistributes the
   Registered Address NS(EARO) in RPL.

        6LN              6LR            Root               6LBR
         |                |              |                   |
         |   NS(EARO)     |              |                   |
         |--------------->|                                  |
         |                |      DAO     |                   |
         |                |------------->|                   |
         |                |              | (keep-alive) EDAR |
         |                |              |------------------>|
         |                |              |       EDAC        |
         |                |              |<------------------|
         |                |    DAO-ACK   |                   |
         |                |<-------------|                   |
         |   NA(EARO)     |              |                   |
         |<---------------|              |                   |

            Figure 5: 6: 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 a keep-alive EDAC if the ROVR is not indicated in
   the Target Option.  In any case, 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 SHOULD 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.

   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 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 still 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 a 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

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 acknowledgement.  A successful DAO-ACK is
   not a guarantee that the DAO has yet reached the Root or that the
   keep-alive EDAR has succeeded.

   6LN             6LR             6LR            Root              6LBR
    |               |               |               |                 |
    |   NS(EARO)    |               |               |                 |
    |-------------->|               |               |                 |
    |   NA(EARO)    |               |               |                 |
    |<--------------|               |               |                 |
    |               |               |               |                 |
    |               |      DAO      |               |                 |
    |               |-------------->|               |                 |
    |               |    DAO-ACK    |               |                 |
    |               |<--------------|               |                 |
    |               |               |               |                 |
    |               |               |      DAO      |                 |
    |               |               |-------------->|                 |
    |               |               |    DAO-ACK    |                 |
    |               |               |<--------------|                 |
    |               |               |               |                 |
    |               |               |               | keep-alive EDAR |
    |               |               |               |---------------->|
    |               |               |               |   EDAC(ROVR)    |
    |               |               |               |<----------------|
    |               |               |               |                 |
                             (in case if an Error)
    |               |               |               |                 |
    |               |              DCO              |                 |
    |               |<------------------------------|                 |
    |   NA(EARO)    |               |               |                 |
    |<--------------|               |               |                 |
    |               |               |               |                 |

              Figure 6: 7: Next Registration Flow in Storing Mode

   If the keep alive fails, the path is cleaned up asynchronously using
   a DCO message [EFFICIENT-NPDAO] as illustrated in Figure 6 7 and
   described in further details in Section 9.2.3.

9.2.  Detailed Operation
9.2.1.  By the 6LN

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

   *  The 6LN obtains an IPv6 global address, for instance using
      autoconfiguration [RFC4862] based on a Prefix Information Option
      (PIO) [RFC4861] found in a Router Advertisement message or by some
      other means such as DHCPv6 [RFC3315].

   *  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].

   *  A 6LN acting as a RUL sets the "R" flag in the EARO whereas a 6LN
      acting as a RAN does not set the "R" flag as prescribed by
      [RFC8505] section 5.1.

   *  Upon each consecutive Address Registration, the 6LN increases the
      TID field in the EARO, as prescribed by [RFC8505] section 5.2.

   *  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 may use any of the 6LRs to which it register to forward
      its packets.  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).

   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.

   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 and can build an RPI.  A RUL that places an RPI in a
   data packet MUST indicate the RPLInstanceID that corresponds to the
   RPL Instance the packet should be injected into.  All the flags and
   the Rank field are set to zero as specified by section 11.2 of

9.2.2.  By the 6LR

   Also as prescribed by [RFC8505], the 6LR generates a DAR message upon
   reception of a valid NS(EARO) message for the Address Registration of
   a new IPv6 Address by a 6LN.  If the Duplicate Address exchange
   succeeds, then the 6LR installs an NCE.  If the "R" flag was set in
   the EARO of the NS message, and this 6LR can manage the reachability
   of Registered Address, then the 6LR sets the "R" flag in the EARO of
   the NA message that is sent in response.

   From then on, the 6LN periodically sends a new NS(EARO) to refresh
   the NCE state before the lifetime indicated in the EARO expires, with
   TID that is incremented each time till it wraps in a lollipop fashion
   (see section 5.2.1 of [RFC8505] which is fully compatible with
   section 7.2 of [RFC6550]).  As long as the R flag is set and this
   Router can still manage the reachability of Registered Address, the
   6LR keeps setting the "R" flag in the EARO of the response NA
   message, but the exchange of Extended Duplicate Address messages is

   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 enacapsulate the packet to the
   Root adding an RPI in the outer header.  if the "I" field is not
   zero, then the 6LR MUST consider that the Opaque field is zero.  If
   the Opaque field is not set to zero, then it should carry a
   RPLInstanceID for the 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.  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; 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:

   *  The Registered Address is placed in a RPL Target Option in the DAO
      message as the Target Prefix, and the Prefix Length is set to 128;

   *  RPL Non-Storing Mode is used, and the 6LR indicates one of its
      global IPv6 unicast addresses as the Parent Address in the RPL
      Transit Information Option (TIO) associated to the Target Option.

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

   *  the Path Lifetime in the TIO is computed from the Lifetime in the
      EARO Option to adapt 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;

   *  the Path Sequence in the TIO is set to the TID value found in the
      EARO option;

   Upon a successful NS/NA(EARO) exchange: if the "R" flag was set in
   the EARO of the NS message, then the 6LR SHOULD inject the Registered
   Address in RPL by sending a DAO message on behalf of the 6LN; else
   the 6LR MUST NOT inject the Registered Address into RPL.

   If a DAO-ACK is not requested, or has a Status that is not a
   rejection, indicating the DAO was accepted respectively by a parent
   in Storing Mode or by the Root in non-Storing Mode, the 6LR replies
   with a NA(EARO) to the RUL with a Status of 0 (Success).

   In case of a DAO-ACK or a DCO indicating a rejection and transporting
   an EARO Status Value of 5 (Validation Requested) the 6LR challenges
   the 6LN for ownership of the address, as described in section 6.1 of
   [RFC8505].  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 MUST refrain from injecting the address in RPL 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.  If for
   any other reason the 6LR fails to inject the address into the RPL
   network, the 6LR SHOULD send a NA(EARO) to the RUL with a Status of 2
   (Out of Storage) which indicates a possibility to retry later.
   Similarly, upon 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 a 6LR receives a valid NS(EARO) message with the "R" flag reset
   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 Node to maintain the
   corresponding route from then on, either keeping it active by sending
   further DAO messages, or destroying it using a No-Path DAO.

9.2.3.  By the RPL Root

   In RPL Storing Mode of Operation (MOP), the DAO message is propagated
   from child to parent all the way to the Root along the DODAG,
   populating routing state as it goes.  In Non-Storing Mode, The DAO
   message is sent directly to the RPL Root.  Upon reception of a DAO
   message, for each RPL Target option that creates or updates an
   existing RPL state:

   *  the Root notifies the 6LBR using an internal API if they are co-
      located, or performs an EDAR/EDAC exchange on behalf of the 6LR if
      they are separated.  If the Target option transports a ROVR, then
      the Root MUST use it to build a full EDAR message as the 6LR
      would.  Else, a keep-alive EDAR is used with the ROVR field set to

   An EDAR message MUST be constructed as follows:

   *  The Target IPv6 address from in the RPL Target Option is placed in
      the Registered Address field of the EDAR message and in the Target
      field of the NS message, respectively;

   *  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;

   *  the RPL Root indicates its own MAC Address as Source Link Layer
      Address (SLLA) in the NS(EARO);

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

   *  when present in the RPL Target option, the ROVR field is used 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 length of
      the ROVR field.  If it is not present the ROVR field in the EDAR
      is set to zero indicating that this is a keep-alive 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 placed in the DCO.

9.2.4.  By the 6LBR

   Upon reception of an EDAR message with the ROVR field is set to zero
   indicating a keep-alive EDAR, the 6LBR checks whether an entry exists
   for the and computes whether the TID in the DAR message is fresher
   than that in the entry as prescribed in section 4.2.1. of [RFC8505].

   If the entry does not exist, the 6LBR does not create the entry, and
   answers with a Status "Removed" in the EDAC message.

   If the entry exists but is not fresher, the 6LBR does not update the
   entry, and answers with a Status "Success" in the EDAC message.

   If the entry exists and the TID in the DAR message is fresher, the
   6LBR updates the TID in the entry, and if the lifetime of the entry
   is extended by the Registration Lifetime in the DAR message, it also
   updates the lifetime of the entry.  In that case, the 6LBR replies
   with a Status "Success" in the DAC message.

   The EDAC that is constructed is the same as if the keep-alive EDAR
   was a full EDAR, and includes the ROVR that is associated to the
   Address Registration.

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 if 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 7, 8, the
   6LN, as an MLD listener, sends an unsolicited Report to the 6LR in
   order to start receiving the flow immediately.  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 an Layer-2 acknoledgement, or attempted multiple

   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

   The Root proxies the MLD echange as 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.

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

                Figure 7: 8: First Multicast Registration Flow

   An Address re-Registration is pulled 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 8: 9:

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

                      Figure 8: 9: 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.

11.  Security Considerations

   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 access control, or could provide role validation as
   well.  This is a generic 6LoWPAN requirement, see Req5.1 in
   Appendix of [RFC8505].

   The keep-alive EDAR message does not carry a valid Registration
   Unique ID [RFC8505] and it cannot be used to create a binding state
   in the 6LBR.  The 6LBR MUST NOT create an entry based on a keep-alive
   EDAR that does not match an existing entry.  All it can do is refresh
   the lifetime and the TID of an existing entry.

   At the time of this writing RPL does not have a zerotrust model
   whereby the 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

12.1.  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

12.2.  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

12.3.  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 RCO Messages,
   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

12.4.  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, 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 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,

   [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710,
              DOI 10.17487/RFC2710, October 1999,

   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,

   [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,

   [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,

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

   [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,

   [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,

   [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,

   [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, <>.

   [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,

   [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, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

   [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,

   [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-12, 10 April 2019,
              ietf-6lo-ap-nd-13, 6 January 2020,

              Robles, I., Richardson, M., and P. Thubert, "Using RPL 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-32,
              4 November 2019, draft-ietf-roll-useofrplinfo-34,
              20 January 2020, <

              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,

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
              2014, <>.

15.  Informative References

   [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,

   [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, <>.

   [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,

   [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,

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,

   [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,

   [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,

   [RFC8504]  Chown, T., Loughney, J., and T. Winters, "IPv6 Node
              Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
              January 2019, <>.

Appendix A.  Example Compression

   Figure 9 10 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].
   The difference with the format presented in Figure 19 of [RFC8138] is
   the addition of a SRH-6LoRH before the RPI-6LoRH to transport the
   destination address of the outer IPv6 header.

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

           Figure 9: 10: Encapsulation to Parent 6LR in Storing Mode

   In Figure 9, 10, 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 9 10 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.

   Follows the RPI-6LoRH and then the IP-in-IP 6LoRH.  When the IP-in-IP
   6LoRH is removed, all the Router headers that precede it are also

   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].

Authors' Addresses

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

   Phone: +33 497 23 26 34

   Michael C. Richardson
   Sandelman Software Works