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ROLL                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Updates: draft-ietf-roll-efficient-npdao, 6550,            M. Richardson
         8505 (if approved)                                    Sandelman
Intended status: Standards Track                            12 June 2020
Expires: 14 December 2020


                         Routing for RPL Leaves
                   draft-ietf-roll-unaware-leaves-18

Abstract

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

Status of This Memo

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

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

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

   This Internet-Draft will expire on 14 December 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  References  . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Glossary  . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  6LoWPAN Neighbor Discovery  . . . . . . . . . . . . . . . . .   7
     3.1.  RFC 6775 Address Registration . . . . . . . . . . . . . .   7
     3.2.  RFC 8505 Extended Address Registration  . . . . . . . . .   7
       3.2.1.  R Flag  . . . . . . . . . . . . . . . . . . . . . . .   8
       3.2.2.  TID, I Field and Opaque Fields  . . . . . . . . . . .   8
       3.2.3.  ROVR  . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  RFC 8505 Extended DAR/DAC . . . . . . . . . . . . . . . .   9
       3.3.1.  RFC 7400 Capability Indication Option . . . . . . . .   9
   4.  Updating RFC 6550 . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Updating draft-ietf-roll-efficient-npdao  . . . . . . . . . .  11
   6.  Updating RFC 8505 . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Requirements on the RPL-Unware Leaf . . . . . . . . . . . . .  11
     7.1.  Support of 6LoWPAN ND . . . . . . . . . . . . . . . . . .  11
     7.2.  External Routes and RPL Artifacts . . . . . . . . . . . .  12
       7.2.1.  Support of IPv6 Encapsulation . . . . . . . . . . . .  13
       7.2.2.  Support of the HbH Header . . . . . . . . . . . . . .  13
       7.2.3.  Support of the Routing Header . . . . . . . . . . . .  13
   8.  Updated RPL Status  . . . . . . . . . . . . . . . . . . . . .  13
   9.  Updated RPL Target Option . . . . . . . . . . . . . . . . . .  14
   10. Protocol Operations for Unicast Addresses . . . . . . . . . .  15
     10.1.  General Flow . . . . . . . . . . . . . . . . . . . . . .  16
     10.2.  Detailed Operation . . . . . . . . . . . . . . . . . . .  18
       10.2.1.  Perspective of the RUL Acting as 6LN . . . . . . . .  18
       10.2.2.  Perspective of the Border Router Acting as 6LR . . .  19
       10.2.3.  Perspective of the RPL Root  . . . . . . . . . . . .  22
       10.2.4.  Perspective of the 6LBR  . . . . . . . . . . . . . .  22
   11. Protocol Operations for Multicast Addresses . . . . . . . . .  23
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  24
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
     13.1.  Fixing the Address Registration Option Flags . . . . . .  25
     13.2.  Resizing the ARO Status values . . . . . . . . . . . . .  25
   14. New DODAG Configuration Option Flag . . . . . . . . . . . . .  26
   15. New RPL Target Option Flag  . . . . . . . . . . . . . . . . .  26
   16. New Subregistry for the RPL Non-Rejection Status values . . .  26
   17. New Subregistry for the RPL Rejection Status values . . . . .  27
   18. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  27
   19. Normative References  . . . . . . . . . . . . . . . . . . . .  27
   20. Informative References  . . . . . . . . . . . . . . . . . . .  29
   Appendix A.  Example Compression  . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32




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1.  Introduction

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

   The IETF produced the "Routing Protocol for Low Power and Lossy
   Networks" [RFC6550] (RPL) to provide IPv6 [RFC8200] routing services
   within such constraints.  RPL belongs to the class of Distance-Vector
   protocols, which, compared to link-state protocols, limit the amount
   of topological knowledge that needs to be installed and maintained in
   each node, and does not require convergence to avoid micro-loops.

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

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

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





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

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

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

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

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

2.  Terminology

2.1.  BCP 14

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.



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2.2.  References

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

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

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

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

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

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

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

2.3.  Glossary

   This document often uses the following acronyms:

   AR:  Address Resolution (aka Address Lookup)




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   6CIO:  6LoWPAN Capability Indication Option

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

   6LR:  6LoWPAN Router

   (E)ARO:  (Extended) Address Registration Option

   (E)DAR:  (Extended) Duplicate Address Request

   (E)DAC:  (Extended) Duplicate Address Confirmation

   DAD:  Duplicate Address Detection

   DAO:  Destination Advertisement Object (a RPL message)

   DCO:  Destination Cleanup Object (a RPL message)

   DIS:  DODAG Information solicitation (a RPL message)

   DIO:  DODAG Information Object (a RPL message)

   DODAG:  Destination-Oriented Directed Acyclic Graph

   LLN:  Low-Power and Lossy Network

   NA:  Neighbor Advertisement

   NCE:  Neighbor Cache Entry

   ND:  Neighbor Discovery

   NS:  Neighbor solicitation

   RA:  Router Advertisement

   ROVR:  Registration Ownership Verifier

   RPI:  RPL Packet Information

   RAL:  RPL-Aware Leaf

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

   RUL:  RPL-Unaware Leaf

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




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3.  6LoWPAN Neighbor Discovery

3.1.  RFC 6775 Address Registration

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

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

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

3.2.  RFC 8505 Extended Address Registration

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


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Length    |    Status     |    Opaque     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Rsvd | I |R|T|     TID       |     Registration Lifetime     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
    ...             Registration Ownership Verifier                 ...
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 1: EARO Option Format



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3.2.1.  R Flag

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

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

3.2.2.  TID, I Field and Opaque Fields

   When the "T" flag is set, the EARO includes a sequence counter called
   Transaction ID (TID), which maps to the Path Sequence Field found in
   the RPL Transit Option.  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 7.1).  The EARO also
   transports an Opaque field and an associated "I" field that describes
   what the Opaque field transports and how to use it.  Section 10.2.1
   specifies the use of the "I" field and of the Opaque field by a RUL.

3.2.3.  ROVR

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

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

   This specification does not address how the protection by [AP-ND]
   could be extended to RPL.  On the other hand, it adds the ROVR to the
   DAO to build the proxied EDAR at the Root (see Section 9), which
   means that nodes that are aware of the Host route to the 6LN are made
   aware of the associated ROVR as well.



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3.3.  RFC 8505 Extended DAR/DAC

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

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

   This specification avoids the periodic EDAR/EDAC exchange across the
   LLN.  The 6LR turns the periodic NS(EARO) from the RUL into a DAO
   message to the Root on every refresh, but it only generates the EDAR
   upon the first registration, for the purpose of DAD, which must be
   verified before the address is injected in RPL.  Upon the DAO
   message, the Root proxies the EDAR exchange to refresh the state at
   the 6LBR on behalf of the 6LR, as illustrated in Figure 7.

3.3.1.  RFC 7400 Capability Indication Option

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |   Length = 1  |     Reserved      |D|L|B|P|E|G|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Reserved                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Figure 2: 6CIO flags



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   A 6LR that can provide reachability services for a RUL in a RPL
   network as specified in this document SHOULD include a 6CIO in its RA
   messages and set the L, P and E flags as prescribed by [RFC8505], see
   Section 7.1 for the corresponding behavior of the RUL.

4.  Updating RFC 6550

   This document specifies a new behavior whereby a 6LR injects DAO
   messages for unicast addresses (see Section 10) and multicast
   addresses (see Section 11) on behalf of leaves that are not aware of
   RPL.  The RUL addresses are exposed as external targets [RFC6550].
   Conforming [USEofRPLinfo], an IP-in-IP encapsulation between the 6LR
   and the RPL Root is used to carry the RPL artifacts and remove them
   when forwarding outside the RPL domain, e.g., to a RUL.

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

   Section 6.7.7 of [RFC6550] introduces RPL Target Option, which can be
   used in RPL Control messages such as the DAO message to signal a
   destination prefix.  Section 9 adds the capabilities to transport the
   ROVR field (see Section 3.2.3) and the IPv6 Address of the prefix
   advertiser when the Target is a shorter prefix, signaled by a new "F"
   flag.  The position of the "F" flag is indicated in Section 15.

   Section 6.7.6 of [RFC6550] defines the DODAG Configuration option
   with reserved flags.  This specification defines the new "Root
   Proxies EDAR/EDAC" (P) flag and encodes it in one of these reserved
   flags.  The "P" flag is set to indicate that the Root performs the
   proxy operation, which implies that it supports the Updated RPL
   Target Option (see Section 9).  The position of the "P" flag is
   indicated in Section 14.

   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 be
   redefined 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 placed in DCO messages
   [EFFICIENT-NPDAO] as well.  Furthermore, this specification enables
   to carry the EARO Status defined for 6LoWPAN ND in RPL DAO and DCO
   messages, embedded in a RPL Status, more in Section 8.



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5.  Updating draft-ietf-roll-efficient-npdao

   [EFFICIENT-NPDAO] defines the DCO for RPL Storing Mode only, with a
   link-local scope.  This specification extends its use to the Non-
   Storing MOP, whereby the DCO is sent unicast by the Root directly to
   the RAN that injected the DAO message for the considered target.

   This specification leverages the DCO between the Root and the 6LR
   that serves as attachment router for a RUL.

6.  Updating RFC 8505

   This document updates [RFC8505] to change the behavior of a RPL
   Router acting as 6LR in the 6LoWPAN ND Address Registration of a RUL
   acting as 6LN.  If the RPL Root advertise the capability to proxy the
   EDAR/EDAC exchange to the 6LBR, the 6LR refrains from sending the
   keep-alive EDAR message.  Instead, if it is separated from the 6LBR,
   the Root regenerates the EDAR message to the 6LBR periodically, upon
   a DAO message that signals the liveliness of the Address.

7.  Requirements on the RPL-Unware Leaf

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

7.1.  Support of 6LoWPAN ND

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

   A RUL that may attach to multiple 6LRs MUST prefer those that provide
   routing services.  The RUL MUST register to all the 6LRs from which
   it desires routing services.

   Parallel Address Registrations to several 6LRs SHOULD be performed in
   an rapid sequence, using the exact same EARO for the same Address.
   Gaps between the Address Registrations will invalidate some of the
   routes till the Address Registration finally shows on those routes.

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



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7.2.  External Routes and RPL Artifacts

   Section 4.1 of [USEofRPLinfo] provides a set of rules detailed below
   that MUST be followed for routing packets from and to a RUL.

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

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

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

   In Non-Storing Mode, packets going down carry a Source Routing Header
   (SRH).  The IP-in-IP encapsulation, the RPI and the SRH are
   collectively called the "RPL artifacts" and can be compressed using
   [RFC8138].  Figure 10 presents an example compressed format for a
   packet forwarded by the Root to a RUL in a Storing Mode DODAG.

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

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



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7.2.1.  Support of IPv6 Encapsulation

   Section 2.1 of [USEofRPLinfo] defines the rules for tunneling either
   to the final destination (6LN) or to its attachment router (6LR).  To
   terminate the IP-in-IP tunnel, the 6LN, as an IPv6 Host, 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 by other means that the RUL
   can handle IP-in-IP properly, which is not mandated by [RFC8504], the
   Root terminates the IP-in-IP tunnel at the parent 6LR.  It is thus
   not necessary for a RUL to support IP-in-IP decapsulation.

7.2.2.  Support of the HbH Header

   A RUL is expected to process an Option Type in a Hop-by-Hop Header as
   prescribed by section 4.2 of [RFC8200].  This means that the RPI with
   an Option Type of 0x23 [USEofRPLinfo] must be skipped when not
   recognized.

7.2.3.  Support of the Routing Header

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

8.  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

   The 6LoWPAN ND Status was defined for use in the EARO and the
   currently defined values are listed in table 1 of [RFC8505].  This
   specification enables to carry the 6LoWPAN ND Status values in RPL
   DAO and DCO messages, embedded in the RPL Status field.




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   To achieve this, Section 13.2 reduces the range of the EARO Status
   values to 0-63 to ensure that they fit within a RPL Status as shown
   in Figure 3.

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

                        Figure 3: RPL Status Format

   The following RPL Status subfields are defined:

   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 for RPL.

   When building a DCO or a DAO-ACK message upon an IPv6 ND NA or a EDAC
   message, the RPL Root MUST copy the 6LoWPAN ND Status unchanged in
   the RPL Status and set the 'A' bit.  The RPL Root MUST set the 'E'
   flag for Values in range 1-10 which are all considered rejections.

   Reciprocally, upon a DCO or a DAO-ACK message from the RPL Root with
   a RPL Status that has the 'A' bit set, the 6LR MUST copy the RPL
   Status Value unchanged in the Status field of the EARO when
   generating an NA to the RUL.

9.  Updated RPL Target Option

   This specification updates the RPL Target Option to transport the
   ROVR that was also defined for 6LoWPAN ND messages.  This enables the
   RPL Root to generate the proxied EDAR message to the 6LBR.

   The new "F" flag is set to indicate that the Target Prefix field
   contains the address of the advertising node in full, in which case
   the length of the Target Prefix field is 16 bytes regardless of the
   value of the Prefix Length field.

   If the "F" flag is reset, the Target Prefix field MUST be aligned to
   the next byte boundary after the size (expressed in bits) indicated
   by the Prefix Length field.  Padding bits are reserved and set to 0
   as prescribed by section 6.7.7 of [RFC6550].



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

   The modified format is illustrated in Figure 4.  It is backward
   compatible with the Target Option in [RFC6550] and SHOULD be used as
   a replacement in new implementations even for Storing Mode operations
   in preparation for upcoming security mechanisms based in the ROVR.

      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 |F|Flags| Prefix Length |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                Target Prefix (Variable Length)                |
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
     ...            Registration Ownership Verifier (ROVR)           ...
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 4: Updated Target Option

   New fields:

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

   F:  1-bit flag.  Set to indicate that Target Prefix field contains an
      Address of prefix advertiser in full.

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

10.  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.

   If the "P" flag is reset, the 6LR MUST generate the periodic EDAR
   messages and process the returned status as specified in [RFC8505].
   If the EDAC indicates success, the rest of the flow takes place as
   presented but without the proxied EDAR/EDAC exchange.





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10.1.  General Flow

   This specification eliminates the need to exchange keep-alive
   Extended Duplicate Address messages, EDAR and EDAC, all the way from
   a 6LN to the 6LBR across a RPL mesh.  Instead, the EDAR/EDAC exchange
   with the 6LBR is proxied by the RPL Root upon the DAO message that
   refreshes the RPL routing state.  The first EDAR upon a new
   Registration cannot be proxied, though, as it serves for the purpose
   of DAD, which must be verified before the address is injected in RPL.

   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 at the 6LBR by
   the RPL Root.

   Since RULs are advertised using Non-Storing Mode, the DAO message
   flow and the keep alive EDAR/EDAC can be nested within the Address
   (re)Registration flow.  Figure 5 illustrates that for the first
   Registration, both the DAD and the keep-alive EDAR/EDAC exchanges
   happen in the same sequence.

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

                   Figure 5: First RUL Registration Flow

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



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

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

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

                        Figure 6: Asynchronous Issue

   An Address re-Registration is performed by the 6LN to maintain the
   NCE in the 6LR alive before lifetime expires.  Upon the refresh of an
   Address re-Registration, as illustrated in Figure 7, the 6LR injects
   the Registered Address in RPL.

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

                    Figure 7: Next RUL Registration Flow





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   This is what causes the RPL Root to refresh the state in the 6LBR,
   using an EDAC message.  In case of an error in the proxied EDAR flow,
   the error is returned in the DAO-ACK using a RPL Status with the 'A'
   flag set that imbeds a 6LoWPAN Status Value as discussed in
   Section 8.

   The 6LR may receive a requested DAO-ACK after it received an
   asynchronous DCO, but the negative Status in the DCO supersedes a
   positive Status in the DAO-ACK regardless of the order in which they
   are received.  Upon the DAO-ACK - or the DCO if one arrives first -
   the 6LR responds to the RUL with an NA(EARO).

   The RUL MAY terminate the registration at any time by using a
   Registration Lifetime of 0.  This specification requires that the RPL
   Target Option transports the ROVR.  This way, the same flow as the
   heartbeat flow is sufficient to inform the 6LBR using the Root as
   proxy as illustrated in Figure 7.

   Any combination of the logical functions of 6LR, Root and 6LBR might
   be collapsed in a single node.

10.2.  Detailed Operation

10.2.1.  Perspective of the RUL Acting as 6LN

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

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

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

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



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

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

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

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

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

10.2.2.  Perspective of the Border Router Acting as 6LR

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

   If the "R" flag is set in the NS(EARO), the 6LR MUST inject the Host
   route in RPL, unless this is barred for other reasons, such as the
   saturation of the RPL parents.  The 6LR MUST use a RPL Non-Storing
   Mode signaling and the updated Target Option (see Section 9).  The
   6LR MUST request a DAO-ACK by setting the 'K' flag in the DAO
   message.  Success injecting the route to the RUL is indicated by the
   'E' flag set to 0 in the RPL status of the DAO-ACK message.



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

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

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

   1.  The Registered Address is signaled as Target Prefix in the
       updated Target Option in the DAO message; the Prefix Length is
       set to 128.  The ROVR field is copied unchanged from the EARO
       (see Section 9).

   2.  The 6LR indicates one of its global or unique-local IPv6 unicast
       addresses as the Parent Address in the RPL Transit Information
       Option (TIO) associated with the Target Option

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

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

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

   Upon receiving the DAO-ACK or an asynchronous DCO message, the 6LR
   MUST send the NA(EARO) to the RUL.

   The 6LR MUST set "R" flag in the NA(EARO) back if and only if the 'E'
   flag is reset, indicating that the 6LR injected the Registered
   Address in the RPL routing successfully and that the EDAR proxy
   operation succeeded.



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   If the 'A' flag in the RPL Status is set, the embedded Status Value
   is passed back to the RUL in the EARO Status.  If the 'E' flags is
   also set, the registration failed for 6LoWPAN ND related reasons, and
   the NCE is removed.

   If the 'A' flag is not set in the RPL Status of the DAO-ACK, then the
   6LoWPAN ND operation succeeded and an EARO Status of 0 (Success) MUST
   be returned to the RUL, even if the 'E' flag is set in the RPL
   Status.  The EARO Status of 0 MUST also be used if the 6LR could not
   even try to inject the route.

   This means that, in case of an error injecting the route that is not
   related to ND, the registration succeeds but the RPL route is not
   installed, which is signaled by the "R" flag reset.  It is up to the
   6LN to keep the binding with the 6LR or destroy it.

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

   If the challenge succeeds then the operations continue as normal.  In
   particular a DAO message is generated upon the NS(EARO) that proves
   the ownership of the address.  If the challenge failed, the 6LR
   rejects the registration as prescribed by AP-ND and may take actions
   to protect itself against DoS attacks by a rogue 6LN, see Section 12.

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

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








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10.2.3.  Perspective of the RPL Root

   A RPL Root SHOULD set the "P" flag in the RPL DODAG Configuration
   Option of the DIO messages that it generates (see Section 4) to
   signal that it proxies the EDAR/EDAC exchange and supports the
   Updated RPL Target option.  The remainder of this section assumes
   that it does.

   Upon reception of a DAO message, for each updated RPL Target Option
   (see Section 9) that creates or updates an existing RPL state, the
   Root MUST notify the 6LBR.  This can be done using an internal API if
   they are integrated, or using a proxied EDAR/EDAC exchange if they
   are separate entities.

   The EDAR message MUST be constructed as follows:

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

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

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

   4.  The ROVR in the RPL Target Option is copied as is in the EDAR and
       the ICMP Code Suffix is set to the appropriate value as shown in
       Table 4 of [RFC8505] depending on the size of the ROVR field.

   Upon receiving an EDAC message from the 6LBR, if a DAO is pending,
   then the Root MUST send a DAO-ACK back to the 6LR.  Else, if the
   Status in the EDAC message is not "Success", then it MUST send an
   asynchronous DCO to the 6LR.

   In either case, the EDAC Status is embedded in the RPL Status with
   the 'A' flag set.

10.2.4.  Perspective of the 6LBR

   The 6LBR is unaware that the RPL Root is not the new attachment 6LR
   of the RUL, so it is not impacted by this specification.

   Upon reception of an EDAR message, the 6LBR acts as prescribed by
   [RFC8505] and returns an EDAC message to the sender.






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11.  Protocol Operations for Multicast Addresses

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

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

   On the first Address Registration, as illustrated in Figure 8, the
   6LN, as an MLD listener, sends an unsolicited Report to the 6LR in
   order to start receiving the flow immediately.

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

                Figure 8: First Multicast Registration Flow

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

   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.




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   The Root proxies the MLD exchange as a listener with the 6LBR acting
   as the querier, so as to get packets from a source external to the
   RPL domain.

   Upon a DAO with a multicast target, the RPL Root checks if it is
   already registered as a listener for that address, and if not, it
   performs its own unsolicited Report for the multicast target.

   An Address re-Registration is pulled periodically by 6LR acting as
   querier.  Note that the message may be sent unicast to all the known
   individual listeners.

   Upon the timing 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 9:

      6LN/RUL                6LR             Root                6LBR
         |                    |               |                    |
         |       Query        |               |                    |
         |<-------------------|               |                    |
         |       Report       |               |                    |
         |------------------->|               |                    |
         |     <L2 ack>       |               |                    |
         |                    | DAO           |                    |
         |                    |-------------->|                    |
         |                    |    DAO-ACK    |                    |
         |                    |<--------------|                    |
         |                    |               |                    |
         |                    |               |       Query        |
         |                    |               |<-------------------|
         |                    |               |       Report       |
         |                    |               |------------------->|
         |                    |               |                    |
         |                    |               |                    |

                      Figure 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.

12.  Security Considerations

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



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   6LoWPAN ND can optionally provide SAVI features with [AP-ND], which
   reduces even more the attack perimeter that is available to the edge
   nodes.

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

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

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

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

13.  IANA Considerations

13.1.  Fixing the Address Registration Option Flags

   Section 9.1 of [RFC8505] creates a Registry for the 8-bit Address
   Registration Option Flags field.

   IANA is requested to rename the first column of the table from "ARO
   Status" to "Bit number".

13.2.  Resizing the ARO Status values

   Section 12 of [RFC6775] creates the Address Registration Option
   Status Values Registry with a range 0-255.

   This specification reduces that range to 0-63.

   IANA is requested to reduce the upper bound of the unassigned values
   in the Address Registration Option Status Values Registry from -255
   to -63.






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14.  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

15.  New RPL Target Option Flag

   Section 20.15 of [RFC6550] creates a Registry for the 8-bit "RPL
   Target Option Flags" field.  IANA is requested to reduce the size of
   the field in the Registry to 4 bits.  This specification also defines
   a new entry in the Registry as follows:

        +------------+--------------------------------+-----------+
        | Bit Number | Capability Description         | Reference |
        +============+================================+===========+
        | 1          | Advertiser Address in Full (F) | THIS RFC  |
        +------------+--------------------------------+-----------+

                    Table 3: New RPL Target Option Flag

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

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

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

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

   *  Initial allocation is as indicated in Table 4:

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

               Table 4: Acceptance values of the RPL Status




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17.  New Subregistry for the RPL Rejection Status values

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

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

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

   *  Initial allocation is as indicated in Table 5:

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

                Table 5: Rejection values of the RPL Status

18.  Acknowledgments

   The authors wish to thank Ines Robles, Georgios Papadopoulos and
   especially Rahul Jadhav for their reviews and contributions to this
   document.

19.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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

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

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



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   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

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

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.

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

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

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

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

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

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



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

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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

   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
              <https://www.rfc-editor.org/info/rfc8505>.

   [AP-ND]    Thubert, P., Sarikaya, B., Sethi, M., and R. Struik,
              "Address Protected Neighbor Discovery for Low-power and
              Lossy Networks", Work in Progress, Internet-Draft, draft-
              ietf-6lo-ap-nd-23, 30 April 2020,
              <https://tools.ietf.org/html/draft-ietf-6lo-ap-nd-23>.

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

   [EFFICIENT-NPDAO]
              Jadhav, R., Thubert, P., Sahoo, R., and Z. Cao, "Efficient
              Route Invalidation", Work in Progress, Internet-Draft,
              draft-ietf-roll-efficient-npdao-18, 15 April 2020,
              <https://tools.ietf.org/html/draft-ietf-roll-efficient-
              npdao-18>.

20.  Informative References




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

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

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,
              <https://www.rfc-editor.org/info/rfc6282>.

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

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

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

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

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









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Appendix A.  Example Compression

   Figure 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].

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

           Figure 10: Encapsulation to Parent 6LR in Storing Mode

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

   In Figure 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.
   If the DODAG is operated in Storing Mode, it is the single entry in
   the SRH-6LoRH and the SRH-6LoRH Size is encoded as 0.  The SRH-6LoRH
   is the first 6LoRH in the chain.  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 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.

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



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Authors' Addresses

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

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


   Michael C. Richardson
   Sandelman Software Works

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

































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