ROLL                                                     P. Thubert, Ed.
Internet-Draft                                                     Cisco
Updates: 6550, 8505 (if approved)                           July 4, 2019                          M. Richardson
Intended status: Standards Track                               Sandelman
Expires: January 5, March 2, 2020                                   August 30, 2019

                         Routing for RPL Leaves
                   draft-ietf-roll-unaware-leaves-02
                   draft-ietf-roll-unaware-leaves-03

Abstract

   This specification leverages 6LoWPAN ND extends RFC6550 and RFC8505 to provide a unicast and
   multicast routing service services in a RPL domain to 6LNs that are plain
   hosts and do not participate to RPL.

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   This Internet-Draft will expire on January 5, March 2, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  References  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Glossary  . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  6LoWPAN Neighbor Discovery  . . . . . . . . . . . . . . . . .   6
   4.  Updating RFC 6550 . . . . . . . . . . . . . . . . . . . . . .   7   8
   5.  Updating RFC 8505 . . . . . . . . . . . . . . . . . . . . . .   7   9
   6.  Dependencies on the  6LN Requirements to be a RPL-Unware Leaf  . . . . . . . . . .   9
     6.1.  Support of 6LoWPAN ND . . . . . . . . .   8 . . . . . . . . .   9
     6.2.  External Routes and RPL Artifacts . . . . . . . . . . . .  10
       6.2.1.  Support of the HbH Header . . . . . . . . . . . . . .  10
       6.2.2.  Support of the Routing Header . . . . . . . . . . . .  10
       6.2.3.  Support of IPv6 Encapsulation . . . . . . . . . . . .  11
   7.  Updated RPL Target option . . . . . . . . . . . . . . . . . .  11
   8.  Protocol Operations for Unicast Addresses . . . . . . . . . .   9
     7.1.  12
     8.1.  General Flow  . . . . . . . . . . . . . . . . . . . . . .   9
     7.2.  6LN Operation  12
       8.1.1.  In RPL Non-Storing-Mode . . . . . . . . . . . . . . .  12
       8.1.2.  In RPL Storing-Mode . . . . . . . . .  11
     7.3.  6LR . . . . . . . .  14
     8.2.  Operation . . . . . . . . . . . . . . . . . . . . . .  12
     7.4.  RPL Root Operation . .  15
       8.2.1.  By the 6LN  . . . . . . . . . . . . . . . . .  13
     7.5.  6LBR Operation . . . .  15
       8.2.2.  By the 6LR  . . . . . . . . . . . . . . . . .  14
   8.  Protocol Operations for Multicast Addresses . . . .  16
       8.2.3.  By the RPL Root . . . . .  15
   9.  Implementation Status . . . . . . . . . . . . . .  18
       8.2.4.  By the 6LBR . . . . . .  17
   10. Security Considerations . . . . . . . . . . . . . . .  19
   9.  Protocol Operations for Multicast Addresses . . . .  17
   11. IANA Considerations . . . . .  20
   10. Implementation Status . . . . . . . . . . . . . . . .  17
   12. Acknowledgments . . . .  22
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   13. References  22
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
     12.1.  RPL Target Option Flags  . . . .  17
     13.1.  Normative References . . . . . . . . . . . .  22
     12.2.  New Subsubregistry for the Status values of the RPL DAO-
            ACK Message  . . . . . .  17
     13.2.  Informative References . . . . . . . . . . . . . . . .  22
   13. Acknowledgments .  19
   Appendix A.  Example Compression . . . . . . . . . . . . . . . .  20
   Author's Address . . . . . .  23
   14. References  . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   The design of Low Power and Lossy Networks (LLNs) is generally
   focused on saving energy, which . . . . . . .  23
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     14.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Appendix A.  Example Compression  . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

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 is a 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 a
   DODAG 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
   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 any-to-
   any shortest path protocol.  Finally, broken routes may be fixed
   lazily and on-
   demand, 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.  It results  The result is that RPL provides
   reachability for most of the LLN nodes, most of the time, but does 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 self itself using Destination Advertisement Object
   (DAO) messages sent to either their parents 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.

   When a routing protocol such as RPL is used to maintain reachability
   within a Non-Broadcast Multi-Access (NBMA) subnet, some nodes may act
   as routers and participate to the routing operations whereas others
   may be plain hosts.  In RPL [RFC6550] terms, a plain host that does not
   participate to is reachable
   over the routing protocol RPL network is called a Leaf.  It must be
   noted

   "When to use RFC 6553, 6554 and IPv6-in-IPv6"
   [I-D.ietf-roll-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 6LN could RPL-Unaware Leaf (RUL)
   designates a leaf does not participate to RPL and inject DAO routes to
   self, but refrain from advertising DIO and get children. at all.  In that case,
   the 6LN is still a plain host but not a Leaf. that needs an interface to its RPL router to
   obtain routing services over the LLN.  This specification enables a
   RPL-Unaware Leaf (RUL) to announce itself as a host and demand request that the 6LR
   6LRs that accepts accept the registration also inject 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
   Leaf 6LN is described in "When to use RFC 6553, 6554 and IPv6-in-IPv6" [I-D.ietf-roll-useofrplinfo].  This document adds the capability by a
   6LR to advertise the Global, Unique-Local and Multicast IPv6
   address(es) of the 6LN in the RPL protocol.

   Examples of routing-agnostic 6LN may include lightly-powered sensors
   such as window smash sensor (alarm system), or the kinetically
   powered light switch.  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.  Applicances
   may not participate to the RPL protocol operated in the smart grid
   network but can still receive control packet from the smart grid.

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.

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].

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

   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
   "octet".

   "RPL", the "RPL Packet Information" (RPI) and (RPI), "RPL Instance", DIO, DAO
   and DIS messages are Instance" (indexed by
   a RPLInstanceID)are defined in the "RPL: IPv6 Routing Protocol for
   Low-Power Low-
   Power and Lossy Networks" [RFC6550] specification. .  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
   [I-D.ietf-roll-efficient-npdao].

   This document introduces uses the term terms RPL-Unaware Leaf (RUL) to refer to
   a node that uses a RPL router (without necessarily knowing it) as 6LR and depends on that router to obtain reachability for its addresses
   inside the RPL domain.  On the contrary, the term RPL-Aware Aware
   Leaf (RAL) consistently with [I-D.ietf-roll-useofrplinfo].  The term
   RPL-Aware Node (RAN) is used introduced to refer to a host node that is either
   a RAL or a router that participates to RPL
   and advertises router.  As opposed to a RUL, a RAN manages the
   reachability of its addresses of 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

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

   o  "IPv6 Stateless Address Autoconfiguration" [RFC4862],

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

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

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

   o  "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)

   6BBR: 6LoWPAN Backbone Router (proxy ND)

   6LBR: 6LoWPAN Border Router (an Address Registrar that is
         authoritative on DAD)

   6LN:  6LoWPAN Node (a Low Power host or router)

   6LR:  6LoWPAN Router

   6CIO: Capability Indication Option

   (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

   DCO:  Destination Cleanup Object
   DIS:  DODAG Information Solicitation

   DIO:  DODAG Information Object

   DODAG:  Destination-Oriented Directed Acyclic Graph

   LLN:  Low-Power and Lossy Network

   NA:   Neighbor Advertisement

   NCE:  Neighbor Cache Entry

   ND:   Neighbor Discovery

   NDP:  Neighbor Discovery Protocol

   NS:   Neighbor Solicitation

   RA:   Router Advertisement

   ROVR: Registration Ownership Verifier (pronounced rover)

   RPI:  RPL Packet Information (an Option in the Hop-By_Hop header) Header)

   RAL:  RPL-Aware Leaf

   RS:   Router Solicitation

   RPL:  IPv6 Routing Protocol for LLNs (pronounced ripple)

   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

   The IPv6 [RFC8200]Neighbor "IPv6 Neighbor Discovery (IPv6 ND) Protocol Protocol" (NDP) suite
   [RFC4861] [RFC4862] was defined for fast 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 address
   registration mechanism that contributes to reduce 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 domain.

   "Registration Extensions for 6LoWPAN Neighbor Discovery" [RFC8505]
   updates the behavior of RFC 6775 to enable a generic registration to
   routing services and defines an Extended ARO (EARO).  The format of
   the EARO is 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

   The 'R'

   [RFC8505] specifies the use of the R flag that is set if in the EARO by the
   Registering Node.  With [RFC8505], the Registering Node expects that sets the R
   flag to indicate whether the 6LR
   ensures should ensure reachability for the
   Registered Address, e.g., by means of routing or proxying ND.

   The EARO also includes a sequence counter called Transaction ID
   (TID), which maps
   Adapted to the Path Sequence Field found in Transit Options
   in RPL DAO messages.  It is this specification, this means that a prerequisite 6LN operates as a
   RUL for this specification.

   Finally, the EARO transports an Opaque field and an 'I' field that
   describes what IPv6 address iff it sets the Opaque field transports and how to use it.  This
   specification requires that R flag in the I field is left to 0 and NS(EARO) used
   to use register the
   Opaque field to carry address.  If the RPL InstanceID if one R flag is known, else to
   leave not set, then the Opaque field
   Registering Node is expected to zero.

4.  Updating RFC 6550

   This be a RAN that handles the
   reachability of the Registered Address by itself.  Conversely, this
   document specifies a new behavior whereby of a RPL router acting as 6LR injects DAO
   messages for unicast addresses registered through the updated 6LoWPAN
   ND [RFC8505] on behalf of 6LN nodes
   registration 6LR that are not RPL-aware.

   Upon the renewal of a 6LoWPAN ND registration, this specification
   changes depends on the behavior setting of the 6LR as follows.  If the 'R' R flag is set, in the 6LR injects
   NS(EARO).  The RPL router generates a DAO targeting message for the Registered Address, and refrains
   from sending a DAR message.  the DAR/DAC exchange that refreshes
   Address upon an NS(EARO) iff the
   state R flag in the 6LBR happens instead between the RPL Root and the 6LBR.
   In that flow, EARO is set.

   The EARO also includes a sequence counter called Transaction ID
   (TID), which maps to the Path Sequence Field found in Transit Options
   in RPL Root acts as a proxy on behalf of DAO messages.  This is the 6LR upon reason why the reception support of [RFC8505]
   by the DAO propagation initiated at the 6LR.

5.  Updating RFC 8505

   The behavior defined in RUL as opposed to only [RFC6775] is a prerequisite for this
   specification whereby the 6LR (more in Section 6.1).  The EARO also transports an
   Opaque field and an "I" field that
   processes describes what the registration advertises Opaque field
   transports and how to use it.

   Section 8.2.1 specifies the Registered Address in DAO
   messages use of the R flag, of the "I" field and bypasses
   of the DAR/DAC process Opaque field by a RUL.

   "Address Protected Neighbor Discovery for Low-power and Lossy
   Networks" [I-D.ietf-6lo-ap-nd] protects the renewal ownership of an address
   and enables a
   registration, challenge that is only triggered leveraged by an NS(EARO) this specification.  It
   also enables Source Address Validation by a 6LR that has the 'R' flag
   set.  If will drop the 'R' flag
   packets that are sourced at an address that is not set, then the Registering Node is
   expected to be a RAN router that handles the reachability of the
   Registered Address by itself. registered.

4.  Updating RFC 6550

   This document also specifies a keep-alive EDAR message new behavior whereby a 6LR injects DAO
   messages for unicast addresses (see Section 8) and multicast
   addresses (see Section 9) 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
   Root
   artifacts and compression techniques that may use not be processed
   correctly outside of the RPL domain.  This specification updates RPL
   [RFC6550] to maintain an existing state mandate that External Routes are advertised using Non-
   Storing Mode signaling even in the 6LBR upon receiving
   DAO messages.  The keep-alive EDAR message may only act as a
   refresher and can only update Storing-Mode network in order to
   inform the Lifetime and root of the TID address of the state
   in 6LR that terminates the 6LBR.

   This document similarly IP-in-
   IP tunnel.

   [RFC8505] specifies a keep-alive NS(EARO) message periodic EDAR/EDAC exchange that takes place
   between the RPL Root may use to maintain an existing state in 6LR and the 6LBR.  It is triggered by a 6BBR upon
   receiving DAO messages.  The keep-alive NS(EARO) message may only act
   as a refresher
   and can only update the Lifetime is intended to create and then refresh the TID of the corresponding state in
   the 6BBR.

   As prescribed by [RFC8505], 6LBR for a lifetime that is indicated by the 6LN.  Conversely,
   RPL router SHOULD NOT set [RFC6550] specifies a periodic DAO that maintains the 'R' flag.

6.  Dependencies on routing
   state in the 6LN

   This document provides RPL routing network for a 6LN acting as a plain host
   and not aware of RPL.  Still, a minimal RPL-independent functionality lifetime that is expected from indicated by the 6LN in order
   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 operate properly as a RLU; in
   particular:

   o the 6LN MUST implement [RFC8505] Root and set one to the 'R' flag in 6LBR.

   This document synchronizes the EARO
      option.  The 'R' flag is used to determine whether liveness monitoring at the Registering
      Node Root and
   the 6LBR.  A same value of lifetime is used for both, and a RUL, not aware of single
   keep alive message, the RPL operation in DAO, traverses the network, and
      thus does not participate to it. RPL network.  A 6LN new
   behavior is considered introduced whereby the RPL Root proxies the EDAR message
   to be a RUL
      if and only if it sets the 'R' flag in 6LBR on behalf of the EARO.

   o 6LR (more in Section 5).  [RFC6550] is
   updated with new RPL data packets are often encapsulated using IP Status values for use in IP DAO-ACK and DCO that
   map the 6LoWNAN ND values defined 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.

   o  An arbitrary 6LN is expected to support IPv6-in-IPv6 encapsulation
      when it Table 1 of [RFC8505].  The
   Resulting set is shown in Table 1.  The Status code are listed in the destination
   same order and DAO-ACK Status code of 128 maps to 6LoWPAN ND Status
   Code of 1.

   Section 5.3. of [RFC8505] introduces the outer header.  If the 6LN is Registration Ownership
   Verifier (ROVR) of a
      host, it is expected variable length from 64 to drop 256 bits.  A ROVR is
   created by the inner packet if it is not Registering Node and associated to the
      destination registration of the inner header.

   o  An arbitrary 6LN
   an IPv6 Address.  It is expected used to process an unknown detect a duplication (DAD) and may
   also enable the Registering Node to prove its ownership of the
   Registered Address [I-D.ietf-6lo-ap-nd].  Section 6.7. of [RFC6550]
   introduces the RPL Control Message Options such as the RPL Target
   Option Type that can be included in a Hop-by-Hop Header RPL Control Message such as prescribed by section 4.2 of [RFC8200]. the DAO.
   This means in particular that an RPI with an document updates the RPL Target Option Type of 0x23
      [I-D.ietf-roll-useofrplinfo] is ignored when not understood.

   o  An arbitrary 6LN is expected to process an unknown Routing Header
      Type as prescribed by section 4.4 of [RFC8200].  This means optionally transport a
   ROVR, more in
      particular Section 7.  This enables the RPL Root to generate a
   full EDAR Message as opposed to a keep-alive EDAR that Routing Header with has restricted
   properties.

5.  Updating RFC 8505

   This document updates [RFC8505] to introduce a Routing Type of 3 [RFC6553]
      is ignored keep-alive EDAR
   message and a keep-alive NS(EARO) message.  The keep-alive messages
   are used for backward compatibility, when the Segments Left is zero, DAO does not transport
   a ROVR as specified in Section 7.  The keep-alive messages have a
   zero ROVR field and dropped otherwise.

   o  When IP-in-IP is can only be used and to refresh a pre-existing state
   associated to the outer headers terminate at Registered Address.  More specifically, a keep-
   alive message can only increase the 6LR
      that generated lifetime and/or increment the DAO, then TID
   of the 6LR decapsulates existing state in a 6LBR.

   Upon the packet to renewal of a 6LoWPAN ND registration, this specification
   changes the 6LN ( see Appendix A behavior of a RPL router acting as 6LR for the format in Storing Mode).  In that
      case
   registration as follows: if the 6LN gets a packet that is free of RPL artifacts.  IP-in-
      IP Root indicates the capability to
   proxy the 6LR MUST be used if EDAR/EDAC exchange to the 6LN cannot handle or ignore 6LBR then the
      RPL artifacts or 6LR refrains from
   sending an EDAR message.  If the way they are compressed [RFC8138].  It SHOULD
      be used it there Root is a particular bandwidth or power constraint at separated from the 6LN that justifies saving 6LBR, the encapsulation at
   Root regenerates the last hop.

   o  In order EDAR message to save the IP-in-IP encapsulation and to support Storing
      Mode of operation, it is preferred 6LBR upon a DAO message that
   signals the liveliness of the Address.

6.  6LN can ignore an RPI
      and consume Requirements to be a RPL-Unware Leaf

   This document provides RPL routing header in both the native for a RUL, that is a 6LN acting as
   a plain host and compressed
      forms.  In order to enable IP-in-IP to not aware of RPL.  Still, a minimal RPL-independent
   functionality is expected from the 6LN in Non-Storing Mode,
      it is also order to obtain routing
   services from the 6LR.

6.1.  Support of interest that 6LoWPAN ND

   A RUL MUST implement [RFC8505] and set the 6LN supports decapsulating IP-in-
      IP R flag in both forms.  But since the preferred behaviour when using
      IP-in-IP EARO option.
   A 6LN is that considered to be a RUL if and only if it sets the outter headers terminate at R flag in
   the 6LR,
      supporting this capability EARO.

   A RUL SHOULD implement [RFC8505] and set the R flag in the EARO
   option.  A 6LN is secondary.

7.  Protocol Operations for Unicast Addresses

7.1.  General Flow

   This specification enables considered to save be a RUL if and only if it sets the R
   flag in the EARO.

   [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 suggested by [RFC8505] depending on the Status value.

   A RUL SHOULD supports [I-D.ietf-6lo-ap-nd] to protect the ownership
   of its addresses.

6.2.  External Routes and RPL Artifacts

   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 8 for the 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.

   IP-in-IP to the 6LR MUST be used if the final destination cannot
   handle or ignore the RPL artifacts or the way they are compressed
   [RFC8138].  An External route indicates by default a node or a prefix
   that is not known to handle or ignore the RPL artifacts.  The
   RECOMMENDED behaviour when using IP-in-IP to an External route is
   that the outer headers terminate at the 6LR that injected the
   External route.  Non-Storing Mode signaling MUST be used to inject
   External routes to the Root in order to advertise the 6LR that is
   associated to a RUL.

   In order to save the IP-in-IP encapsulation and to support Storing
   Mode of operation, it is preferred that the 6LN can ignore an RPI and
   consume a routing header in both the native and [RFC8138]-compressed
   forms.  In order to enable IP-in-IP to a 6LN in Non-Storing Mode, it
   is also of interest that the 6LN supports decapsulating IP-in-IP in
   both forms.

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
   [I-D.ietf-roll-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

   A RUL may support IPv6-in-IPv6 decapsulation when it is the
   destination of the outer header but that is not assumed by [RFC8504].
   If the 6LN is a RUL, it may be able to drop the inner packet if it is
   not the destination of the inner header.  By default the IP-in-IP
   tunnel should terminate at the parent 6LR so supporting this
   capability in a RUL is secondary.

7.  Updated RPL Target option

   This specification updates the RPL Target option to transport the
   ROVR as illustrated in Figure 2.  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.

      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 2: Updated Target Option

   New fields:

   RVRsz:          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]

8.  Protocol Operations for Unicast Addresses

8.1.  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
   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.  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.

   From the perspective of the 6LN, the registration flow happens
   transparently; it is not delayed by the proxy RPL operation, so the
   device does not need to change the amount of time it waits based upon
   whether RPL proxy operation happens or not.

   On the first registration, illustrated in Figure 3, from the
   perspective of the 6LR in Non-Storing Mode, the Extended Duplicate
   Address message takes place as prescribed by [RFC8505].  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.

   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.

8.1.1.  In RPL Non-Storing-Mode

   In Non-Storing Mode, the flows can be nested as illustrated in
   Figure 3 and it is possible to carry information such as an updated
   lifetime from the 6LBR all the way 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 3: First Registration Flow in Non-Storing Mode

   A re-registration is performed by the exchange of Extended Duplicate
   Address messages, EDAR and EDAC, from a 6LN all the way to maintain the 6LBR
   across NCE in the
   6LR alive before lifetime expires.  Upon a re-registration, as
   illustrated in Figure 4, the 6LR redistributes the Registered Address
   NS(EARO) in RPL.

   This causes the RPL mesh, for DODAG Root to refresh the sole purpose of refreshing an existing state in the 6LBR.  Instead, 6LBR with
   a keep-alive EDAC message.  The keep-alive EDAC lacks the EDAR/EDAC exchange
   Registration Ownership Verifier (ROVR) information, since it is proxied by the not
   present in RPL Root upon a DAO messages, but the EDAC message sent in response by
   the 6LBR contains the actual value of the ROVR field for that refreshes
   registration.  This enables the RPL routing state.  To
   achieve this, Root to perform the lifetimes proxy-
   registration for the Registered Address and sequence counters in 6LoWPAN ND attract traffic captured
   over the backbone by the 6BBR and
   RPL are aligned. route it back to the device.

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

           Figure 4: Next Registration Flow in Non-Storing Mode

   In other words, case of an error on the Path Sequence and keep-alive EDAR flow, the Path
   Lifetime error SHOULD be
   returned in the DAO message are derived from DAO-ACK - if one was requested - using the Transaction ID mapping of
   RPL Status and 6LoWPAN Status values discussed in Section 4.

   If the registration lifetime Root could not return the negative Status in the NS(EARO) DAO-ACK then
   it sends an asynchronous Destination Cleanup Object (DCO) message from the 6LN.

   From the perspective of the 6LN,
   [I-D.ietf-roll-efficient-npdao] to the registration flow happens
   transparently; it is not delayed by 6LR indicating the proxy RPL operation, so issue with
   the
   device does not need to wait more whether RPL proxy operation happens
   or not.  The flows below mapped Status value.  Note that if both are RPL Non-Storing Mode examples.  In
   Storing Mode, used in a short
   interval of time, the DAO ACK may not be present, DAO-ACK and the DAO DCO messages
   cascade from child to parent all the way are not guaranteed to
   arrive in the DODAG Root.

   On same order at the first registration, illustrated in Figure 2, from 6LR.  So the
   perspective 6LR must still expect a
   DAO-ACK even if it received a DCO while it was waiting for an
   acknowledgement for a short period of time, but the 6LR negative status
   in Non-Storing Mode, the Extended Duplicate
   Address message takes place as prescribed by [RFC8505].  When
   successful, the flow creates DCO supercedes a Neighbor Cache Entry (NCE) positive status in the 6LR,
   and the 6LR injects DAO-ACK regardless of
   the Registered Address order in RPL using DAO/DAO-ACK
   exchanges all which they are received.

   Upon the way DAO-ACK - or the DCO if it arrives first - the 6LR responds
   to the RPL DODAG Root.  The protocol does not
   carry RUL with a specific information NA(EARO) and the 6LoWPAN ND Status value that is
   mapped from the Extended Duplicate Address
   messages were already exchanged, so RPL status in the Root proxies them anyway.
   Note that RPL message.  An asynchronous DCO
   is also mapped in an asynchronous NA(EARO) to the RUL with a mapped
   Status value.

8.1.2.  In RPL Storing-Mode

   In Storing Mode Mode, the DAO ACK DAO-ACK is optional.  When it is used, it is
   generated from by the parent
   that RPL parent, which does not necessary need to wait for the grand parent
   grand-parent to acknowledge, so send the acknowledgement.  A successful DAO-ACK is no
   not a guarantee that the DAO has yet reached the Root or that the
   keep-alive EDAR has succeeded.  On

   If the other hand, keep alive fails, the flows can be nested in Non-Storing Mode, and it path is possible to carry information such cleaned up asynchronously using
   a DCO message [I-D.ietf-roll-efficient-npdao] as an updated lifetime from the
   6LBR all the way to the 6LN. illustrated in
   Figure 5 and described in further details in Section 8.2.3.

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

                     Figure 2: First Registration Flow

   A re-registration is performed by the 6LN to maintain the NCE in the
   6LR alive before lifetime expires.  Upon a re-registration, as
   illustrated in Figure 3, the 6LR redistributes the Registered Address
   NS(EARO) in RPL.  This causes the RPL DODAG Root to refresh the state
   in the 6LBR with a keep-alive EDAC message.  The keep-alive EDAC
   lacks the Registration Ownership Verifier (ROVR) information, since
   it is not present in RPL DAO messages, but the EDAC message sent in
   response by the 6LBR contains the actual value of the ROVR field for
   that registration.  This enables the RPL Root to perform the proxy-
   registration for the Registered Address and attract traffic captured
   over the backbone by the 6BBR and route it back to the device.

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

             Figure 3: 5: Next Registration Flow

   Note that any of the functions 6LR, Root and 6LBR might be collapsed
   in a single node, in which case Storing Mode

8.2.  Operation

8.2.1.  By the flow above happens internally,
   and possibly through internal API calls as opposed to messaging.

7.2. 6LN Operation

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

   o  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].

   o  Once it has formed an address, the 6LN (re)registers its address
      periodically, within the Lifetime of the previous registration, as
      prescribed by [RFC8505].

   o  Upon each consecutive registration, the 6LN MUST increase the TID
      field.

   o  If the 6LN is aware of the RPL Instance the packet should be
      injected into, then it SHOULD set the Opaque field to the
      InstanceID, else it MUST leave the Opaque field to zero.  In any
      fashion the 6LN MUST set the 'I' field to zero. [RFC6775].

   o  A 6LN acting as a RUL MUST set sets the 'R' R flag in the EARO whereas a 6LN
      acting as a RAN SHOULD NOT does not set the 'R' flag. R flag as prescribed by [RFC8505]
      section 5.1.

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

   o  The 6LN MAY can register to more than one 6LR at the same time.  In
      that case, a same value of TID is used for each registration.

   o  The 6LN MAY may use any of the 6LRs to which it register to forward
      its packets.

   o  Using a 6LR to which the 6LN is not expected to registered may
      result in packets dropped by a Source Address Validation function.

   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 so 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.  In any fashion the 6LN MUST set the
   "I" field to zero to indicate that topological information to be
   passed to a routing process as specified in [RFC8505] section 5.1.

   A RUL is not expected to produce RPL artifacts in the data packets.

7.3. 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
   [RFC6550].

8.2.2.  By the 6LR Operation

   Also as prescribed by [RFC8505], the 6LR generates a DAR message upon
   reception of a valid NS(EARO) message for the registration of a new
   IPv6 Address by a 6LN.  If the Duplicate Address exchange succeeds,
   then the 6LR installs a Neighbor Cache Entry (NCE).  If the 'R' 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' R flag in
   the ARO EARO of the response NA message. 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. fashion
   (see section 5.2.1 of [RFC8505] which is fully compatible with
   section 7.2 of [RFC6550]).  As long as the 'R' R flag is set and this
   router can still manage the reachability of Registered Address, the
   6LR keeps setting the 'R' R flag in the EARO of the response NA message,
   but the exchange of Extended Duplicate Address messages is skipped.

   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 RPL Packet
   Information (RPI) in the packet, in which case the 6LR SHOULD add one
   to the packet.  if the 'I' "I" field is not zero, then the 6LR MUST
   consider that the Opaque field is left to zero.  If the Opaque field is not
   set to zero, then it should carry a RPL InstanceID 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 left to zero. empty.  If the Opaque field left to zero, is empty, the 6LR is free to use the
   default 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.

   Upon a successful NS/NA(EARO) exchange: if the 'R' 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.

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

   o  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 128 128;

   o  the External 'E' flag in the Transit Information Option (TIO)
      associated to the Target Option is set to indicate that the 6LR
      redistributes an external target into the RPL network.  This  When the
      Root has to use an IP-in-IP [I-D.ietf-roll-useofrplinfo], then
      this flag indicates the IP-in-IP should be addressed to this node;

   o  the Path Lifetime in the TIO is
      how 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;

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

   o  Additionally, in Non-Storing Mode the 6LR indicates one of its
      global IPv6 unicast addresses as the Parent Address in the TIO.

   If a DAO-ACK is not requested, or has a Status that is less than 128,
   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 with a status of 132 (Validation
   Requested) the 6LR challenges the Root knows 6LN for ownership of the address,
   as described in Non-Storing Mode to use IP-in-IP and
      terminate section 6.1 of [RFC8505].  If the outters headers at challenge succeeds
   then the 6LR that operations continue as normal.  In particular a DAO message
   is generated upon the DAO.

   o NS(EARO) that proves the Path Lifetime in ownership of the TIO is computed
   address.  If the challenge failed the 6LR MUST refrain from injecting
   the Lifetime address in the
      EARO Option RPL and may take actions to adapt it protect itself against DoS
   attacks by a rogue 6LN, see Section 11

   Other status values above 128 indicate that the 6LR failed to inject
   the Lifetime Units used in address into the RPL
      operation.  Note network.  In that if case the lifetime is 0, then the 6LR generates MUST send
   a No-Path DAO message that cleans up the routes down NA(EARO) to the
      Address of the 6LN.

   o RUL with the Path Sequence in mapped Status value.  If for any other
   reason the TIO is set 6LR fails to inject the TID value found in address into the
      EARO option.

   o  Additionally, in Non-Storing Mode RPL network, the
   6LR indicates one of its
      global IPv6 unicast addresses as the Parent Address in SHOULD send a NA(EARO) to the TIO. RUL with a status of 2 (Out of
   Storage) which indicates a possibility to retry later.

   If a 6LR receives a valid NS(EARO) message with the 'R' 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.

7.4.

   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 mapped Status value.

8.2.3.  By the RPL Root Operation

   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 route. RPL Root.  Upon reception of a DAO
   message
   message, for each RPL Target option that creates or updates an
   existing RPL state:

   o  the Root notifies the 6LBR using an internal API if they are
      collocated, co-
      located, or performs a keep-alive DAR/DAC an EDAR/EDAC exchange on behalf of the registering node 6LR if
      they are separated.

   o  In an extended topology with  If the Target option transports a Backbone Link, ROVR, then
      the Root notifies
      the 6LBR by proxying MUST use it to build a full EDAR message as the 6LR
      would.  Else, a keep-alive NS(EARO) on behalf of the 6LN
      that owns the address indicated in the Target Option.

   The keep-alive EDAR and is used with the NS(EARO) messages ROVR field set to
      zero.

   An EDAR message MUST be constructed as follows:

   o  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

   o  the ROVR field in the keep-alive EDAR is set to 64-bits of all
      ones to indicate that it is not provided and this is a keep-alive
      EDAR.  The actual value of the ROVR for that registration is
      returned by the 6LBR in an EDAC, and used in the proxy NS(EARO). respectively;

   o  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. messages;

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

   o  the TID value is set to the Path Sequence in the TIO.  The 'T'
      flag TIO and indicated
      with an ICMP code of 1 are in the EDAR message;

   o  when present in the RPL Target option, the ROVR field is used as
      is in the NS(EARO) EDAR and the DAR
      message, respectively.

   Upon 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 status keep-alive EDAR.  The
      actual value of the ROVR for that registration is expected from
      the 6LBR in the response EDAC.

   Upon a DAC Status value in an EDAC message that is not "Success", the
   Root MAY SHOULD destroy the formed paths using either a No-Path DAO DAO-ACK (in Non-
   Storing Mode) or a DCO downwards as specified in
   [I-D.ietf-roll-efficient-npdao].

   In Non-Storing Mode, the outer IPv6 header that is used by the Root  Failure to transport destroy the source routing information former path
   would result in data packets down Stale routing state and local black holes if the
   DODAG has
   address belongs to another party elsewhere in the 6LR network.  The RPL
   Status value that serves the 6LN as final destination.  This
   way, when the final 6LR decapsulates maps the outer header, it also
   removes all 6LowpAN ND status value MUST be placed in
   the RPL artifacts from DCO.

8.2.4.  By the packet.

7.5. 6LBR Operation

   Upon reception of a DAR an EDAR message with the Owner Unique ID ROVR field is set to all ones, 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 DAC 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 DAC 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.

8.

   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
   registration.

9.  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 registration, as illustrated in Figure 4, 6, 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 times.

   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 echange as listener with the 6BBR 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    DAO-ACK    |                    |
         |                    |<--------------|                    |
         |                    |               | <if not listening> |
         |                    |               | unsolicited Report |
         |                    |               |------------------->|
         |                    |               |                    |
         |                    |               |                    |

                Figure 4: 6: First Multicast Registration Flow

   A re-registration is pulled by 6LR acting as querier.  Note that the
   message may 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 5, 7,

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

                     Figure 5: 7: 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.

9.

10.  Implementation Status

10.

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.

11.

   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.  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.

12.2.  New Subsubregistry for the Status values of the RPL DAO-ACK
       Message

   This specification creates a new subsubregistry for the Status values
   of the RPL DAO-ACK Message, under the ICMPv6 parameters registry.

   o  Possible values are 8-bit unsigned integers (0..255).

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

   o  Initial allocation is as indicated in Table 1:

    +---------+--------------------------------------+----------------+
    |  Value  |               Meaning                | Defining Spec  |
    +---------+--------------------------------------+----------------+
    |    0    |        Unqualified acceptance        |    RFC6550     |
    |         |                                      |                |
    |  1-127  |      Reserved for Warning Codes      |    RFC6550     |
    |         |                                      |                |
    |   128   |          Duplicate Address           |    This RFC    |
    |         |                                      |                |
    |   129   |            Out of Storage            |    This RFC    |
    |         |                                      |                |
    |   130   |                Moved                 |    This RFC    |
    |         |                                      |                |
    |   131   |               Removed                |    This RFC    |
    |         |                                      |                |
    |   132   |         Validation Requested         |    This RFC    |
    |         |                                      |                |
    |   133   |       Duplicate Source Address       |    This RFC    |
    |         |                                      |                |
    |   134   |        Invalid Source Address        |    This RFC    |
    |         |                                      |                |
    |   135   |   Address topologically incorrect    |    This specification has no requirement on IANA.

12. RFC    |
    |         |                                      |                |
    |   136   |       6LBR Registry saturated        |    This RFC    |
    |         |                                      |                |
    |   137   |          Validation Failed           |    This RFC    |
    |         |                                      |                |
    | 138-192 | Reserved for 6LoWPAN ND code mapping |    This RFC    |
    |         |                                      |                |
    | 193-255 |  Reserved for other Rejection Codes  |    RFC6550     |
    +---------+--------------------------------------+----------------+

             Table 1: Status values of the RPL DAO-ACK Message

13.  Acknowledgments

   The author wishes authors wish to thank Michael Richardson and Georgios Papadopoulos for their early
   reviews of and contributions to this document

13.

14.  References

13.1.

14.1.  Normative References

   [I-D.ietf-6lo-ap-nd]
              Thubert, P., Sarikaya, B., Sethi, M., and R. Struik,
              "Address Protected Neighbor Discovery for Low-power and
              Lossy Networks", draft-ietf-6lo-ap-nd-12 (work in
              progress), April 2019.

   [I-D.ietf-roll-efficient-npdao]
              Jadhav, R., Thubert, P., Sahoo, R., and Z. Cao, "Efficient
              Route Invalidation", draft-ietf-roll-efficient-npdao-15
              (work in progress), July 2019.

   [I-D.ietf-roll-useofrplinfo]
              Robles, I., Richardson, M., and P. Thubert, "Using RPL
              Option Type, Routing Header for Source Routes and IPv6-in-
              IPv6 encapsulation in the RPL Data Plane", draft-ietf-
              roll-useofrplinfo-31 (work in progress), August 2019.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

13.2.

14.2.  Informative References

   [I-D.ietf-roll-efficient-npdao]
              Jadhav, R., Thubert, P., Sahoo, R., and Z. Cao, "Efficient
              Route Invalidation", draft-ietf-roll-efficient-npdao-14
              (work in progress), July 2019.

   [I-D.ietf-roll-useofrplinfo]
              Robles, I., Richardson, M., and P. Thubert, "Using RPL
              Option Type, Routing Header for Source Routes and IPv6-in-
              IPv6 encapsulation in the RPL Data Plane", draft-ietf-
              roll-useofrplinfo-30 (work in progress), June 2019.

   [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>.

   [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>.

   [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>.

Appendix A.  Example Compression

   Figure 6 8 illustrates the case in Storing mode where the packet is
   received from the Internet, then the root 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 transpotr transport the
   destination address of the outer IPv6 header.

   +-+ ... -+-+ ... +-+- ... -+-+- -+-+ ... +-+-+-+ -+-+-+ ... +-+-+ ... -+ ... +-...
   |11110001|SRH-6LoRH| RPI-  | IP-in-IP |  |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 6: 8: Encapsulation to Parent 6LR in Storing Mode

   In Figure 6, 8, 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 Root would be similar
   to that represented in Figure 6 8 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
   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].

Author's Address

Authors' Addresses
   Pascal Thubert (editor)
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   MOUGINS - Sophia Antipolis  06254
   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/