path: root/doc/rfc/rfc9685.txt

Internet Engineering Task Force (IETF)                   P. Thubert, Ed.
Request for Comments: 9685                                 November 2024
Updates: 4861, 6550, 6553, 8505, 9010                                   
Category: Standards Track                                               
ISSN: 2070-1721


Listener Subscription for IPv6 Neighbor Discovery Multicast and Anycast
                               Addresses

Abstract

   This document updates the 6LoWPAN extensions to IPv6 Neighbor
   Discovery (specified in RFCs 4861 and 8505) to enable a listener to
   subscribe to an IPv6 anycast or multicast address.  This document
   also updates the Routing Protocol for Low-Power and Lossy Networks
   (RPL) (specified in RFCs 6550 and 6553) to add a new Non-Storing
   multicast mode and new support for anycast addresses in Storing and
   Non-Storing modes.  This document extends RFC 9010 to enable a
   6LoWPAN Router (6LR) to inject the anycast and multicast addresses in
   RPL.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9685.

Copyright Notice

   Copyright (c) 2024 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
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   publication of this document.  Please review these documents
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   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
     2.1.  Requirements Language
     2.2.  Terminology from Relevant RFCs
     2.3.  Abbreviations
     2.4.  New Terms
   3.  Overview
   4.  Amending RFC 4861
   5.  Extending RFC 7400
   6.  Amending RFC 6550
     6.1.  Mandating the ROVR Field
     6.2.  Updating MOP 3
     6.3.  New Non-Storing Multicast MOP
     6.4.  RPL Anycast Operation
     6.5.  New Registered Address Type Indicator P-Field
     6.6.  New RPL Target Option P-Field
   7.  Extending RFC 8505
     7.1.  Placing the New P-Field in the EARO
     7.2.  Placing the New P-Field in the EDAR Message
     7.3.  Registration Extensions
   8.  Extending RFC 9010
   9.  Leveraging RFC 8928
   10. Consistent Uptime Option
   11. Operational Considerations
   12. Security Considerations
   13. Backward Compatibility
   14. IANA Considerations
     14.1.  New P-Field Values Registry
     14.2.  New EDAR Message Flags Registry
     14.3.  New Address Registration Option Flags
     14.4.  New RPL Target Option Flags
     14.5.  New RPL Mode of Operation
     14.6.  New 6LoWPAN Capability Bit
     14.7.  New Address Registration Option Status Values
     14.8.  New IPv6 Neighbor Discovery Option Format
   15. References
     15.1.  Normative References
     15.2.  Informative References
   Acknowledgments
   Author's Address

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 radio (when both transmitting
   or simply listening) is a major energy drain, and the LLN protocols
   must be adapted to allow the nodes to remain sleeping with the radio
   turned off at most times.

   "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks"
   [RFC6550] provides IPv6 [RFC8200] routing services within such
   constraints.  To save signaling and routing state in constrained
   networks, the RPL routing is only performed along a Destination-
   Oriented Directed Acyclic Graph (DODAG) that is optimized to reach a
   Root node, as opposed to along the shortest path between two peers,
   which may be a fuzzy concept anyway in a radio LLN.

   This stretches the routes between RPL nodes inside the DODAG for a
   vastly reduced amount of control traffic and routing state that would
   be required to operate an any-to-any shortest path protocol.
   Additionally, broken routes may be fixed lazily and on-demand based
   on data plane inconsistency discovery, which avoids wasting energy in
   the proactive repair of unused paths.

   RPL uses Destination Advertisement Object (DAO) messages to establish
   Downward routes.  DAO messages are an optional feature for
   applications that require point-to-multipoint (P2MP) or point-to-
   point (P2P) traffic.  RPL supports two modes of Downward traffic:
   Storing (fully stateful) or Non-Storing (fully source routed); see
   Section 9 of [RFC6550].  The mode is signaled in the Mode of
   Operation (MOP) field in the DODAG Information Object (DIO) messages
   and applies to the whole RPL Instance.

   Any given RPL Instance is either Storing or Non-Storing.  In both
   cases, P2P packets travel Up toward a DODAG root then Down to the
   final destination (unless the destination is on the Upward route).
   In the Non-Storing case, the packet will travel all the way to a
   DODAG root before traveling Down.  In the Storing case, the packet
   may be directed Down towards the destination by a common ancestor of
   the source and the destination prior to reaching a DODAG root.
   Section 12 of [RFC6550] details the Storing Mode of Operation with
   multicast support with source-independent multicast routing in RPL.

   The classical Neighbor Discovery (ND) protocol [RFC4861] [RFC4862]
   was defined for serial links and shared transit media such as
   Ethernet at a time when broadcast on those media types was cheap,
   while memory for neighbor cache was expensive.  It was thus designed
   as a reactive protocol that relies on caching and multicast
   operations for the Address Discovery (aka lookup) and Duplicate
   Address Detection (DAD) of IPv6 unicast addresses.  Those multicast
   operations typically impact every node on-link when at most one is
   really targeted.  This is a waste of energy and implies that all
   nodes are awake to hear the request, which is inconsistent with
   power-saving (sleeping) modes.

   The original specification for 6LoWPAN ND, "Neighbor Discovery
   Optimization for IPv6 over Low-Power Wireless Personal Area Networks
   (6LoWPANs)" [RFC6775], was introduced to avoid the excessive use of
   multicast messages and enable IPv6 ND for operations over energy-
   constrained nodes.  [RFC6775] changes the classical IPv6 ND model to
   proactively establish the Neighbor Cache Entry (NCE) associated to
   the unicast address of a 6LoWPAN Node (6LN) in one or more 6LoWPAN
   Routers (6LRs) that serve it.  To that effect, [RFC6775] defines a
   new Address Registration Option (ARO) that is placed in unicast
   Neighbor Solicitation (NS) and Neighbor Advertisement (NA) messages
   between the 6LN and the 6LR.

   "Registration Extensions for IPv6 over Low-Power Wireless Personal
   Area Network (6LoWPAN) Neighbor Discovery" [RFC8505] updates
   [RFC6775] so that a generic Address Registration mechanism can be
   used to access services such as routing and ND proxy and introduces
   the Extended Address Registration Option (EARO) for that purpose.
   This provides a routing-agnostic interface for a host to request that
   the router injects a unicast IPv6 address in the local routing
   protocol and provides return reachability for that address.

   "Routing for RPL (Routing Protocol for Low-Power and Lossy Networks)
   Leaves" [RFC9010] provides the router counterpart of the mechanism
   for a host that implements [RFC8505] to inject its unicast Unique
   Local Addresses (ULAs) and Global Unicast Addresses (GUAs) in RPL.
   Although RPL also provides multicast routing, 6LoWPAN ND supports
   only the registration of unicast addresses, and there is no
   equivalent of [RFC9010] to specify the 6LR behavior upon the
   subscription of one or more multicast addresses.

   "Multicast Listener Discovery Version 2 (MLDv2) for IPv6" [RFC3810]
   enables the router to learn which node listens to which multicast
   address, but as the classical IPv6 ND protocol, MLD relies on
   multicasting queries to all nodes, which is unfit for low-power
   operations.  As for IPv6 ND, it makes sense to let the 6LNs control
   when and how they maintain the state associated to their multicast
   addresses in the 6LR, e.g., during their own wake time.  In the case
   of a constrained node that already implements [RFC8505] for unicast
   reachability, it makes sense to extend that support to subscribe the
   multicast addresses they listen to.

   This specification Extends [RFC8505] and [RFC9010] by adding the
   capability for the 6LN to subscribe anycast and multicast addresses
   and for the 6LR to inject them in RPL when appropriate.  Note that
   due to the unreliable propagation of packets in the LLN, it cannot be
   guaranteed that any given packet is delivered once and only once.  If
   a breakage happens along the preferred parent tree that is normally
   used for multicast forwarding, the packet going Up may be rerouted to
   an alternate parent, leading to potential failures and duplications,
   whereas a packet going Down will not be delivered in the subtree.  It
   is up to the Upper Layer Protocols (ULPs) to cope with both
   situations.

2.  Terminology

2.1.  Requirements Language

   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.

   In addition, "Extends" and "Amends" are used as more specific terms
   for "Updates" per Section 3 of [UPDATES-TAG] as follows:

   Amends/Amended by:  This tag pair is used with an amending RFC that
          changes the amended RFC.  This could include bug fixes,
          behavior changes, etc.  This is intended to specify mandatory
          changes to the protocol.  The goal of this tag pair is to
          signal to anyone looking to implement the amended RFC that
          they MUST also implement the amending RFC.

   Extends/Extended by:  This tag pair is used with an extending RFC
          that defines an optional addition to the extended RFC.  This
          can be used by documents that use existing extension points or
          clarifications that do not change existing protocol behavior.
          This signals to implementers and protocol designers that there
          are changes to the extended RFC that they need to consider but
          not necessarily implement.

2.2.  Terminology from Relevant RFCs

   This document uses terms and concepts that are discussed in:

   *  "Neighbor Discovery for IP version 6 (IPv6)" [RFC4861],

   *  "IPv6 Stateless Address Autoconfiguration" [RFC4862],

   *  "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks"
      [RFC6550],

   *  "Neighbor Discovery Optimization for IPv6 over Low-Power Wireless
      Personal Area Networks (6LoWPANs)" [RFC6775],

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

   *  "Using RPI Option Type, Routing Header for Source Routes, and
      IPv6-in-IPv6 Encapsulation in the RPL Data Plane" [RFC9008].

2.3.  Abbreviations

   This document uses the following abbreviations:

   6CIO:  Capability Indication Option

   6LBR:  6LoWPAN Border Router

   6LN:   6LoWPAN Node

   6LR:   6LoWPAN Router

   ARO:   Address Registration Option

   DAC:   Duplicate Address Confirmation

   DAD:   Duplicate Address Detection

   DAO:   Destination Advertisement Object

   DAR:   Duplicate Address Request

   DIO:   DODAG Information Object

   DMB:   Direct MAC Broadcast

   DODAG:  Destination-Oriented Directed Acyclic Graph

   EARO:  Extended Address Registration Option

   EDAC:  Extended Duplicate Address Confirmation

   EDAR:  Extended Duplicate Address Request

   IR:    Ingress Replication

   LLN:   Low-Power and Lossy Network

   MLD:   Multicast Listener Discovery

   MOP:   Mode of Operation

   NA:    Neighbor Advertisement

   NCE:   Neighbor Cache Entry

   ND:    Neighbor Discovery

   NS:    Neighbor Solicitation

   RA:    Router Advertisement

   RAN:   RPL-Aware Node

   ROVR:  Registration Ownership Verifier (pronounced "rover")

   RPL:   Routing Protocol for Low-Power and Lossy Networks (pronounced
          "ripple")

   RS:    Router Solicitation

   RTO:   RPL Target Option

   RUL:   RPL-Unaware Leaf

   TID:   Transaction ID

   TIO:   Transit Information Option

2.4.  New Terms

   This document introduces the following terms:

   Origin:  The node that issued an anycast or multicast advertisement,
          in the form of either an NS(EARO) or a DAO(TIO, RTO) message.

   Merge/merging:  The action of receiving multiple anycast or multicast
          advertisements, either internally from self, in the form of an
          NS(EARO) message, or as a DAO(TIO, RTO) message, and
          generating a single DAO(TIO, RTO).  The RPL router maintains a
          state per origin for each advertised address and merges the
          advertisements for all subscriptions for the same address in a
          single advertisement.  A RPL router that merges multicast
          advertisements from different origins becomes the origin of
          the merged advertisement and uses its own values for the Path
          Sequence and Registration Ownership Verifier (ROVR) fields.

   Subscribe/subscription:  The special form of registration that
          leverages NS(EARO) to register (or subscribe to) a multicast
          or an anycast address.

3.  Overview

   This specification Extends [RFC8505] to provide a registration method
   (called "subscription" in this case) for anycast and multicast
   addresses.  The specification inherits the proof of ownership defined
   in [RFC8928] that already protects the address registration in
   [RFC8505] to also protect the new subscription mechanism.  [RFC8505]
   is agnostic to the routing protocol in which the address may be
   redistributed.

   As opposed to unicast addresses, there might be multiple
   registrations from multiple parties for the same address.  The router
   retains one registration per party for each multicast or anycast
   address but injects the route into the routing protocol only once for
   each address.  The injection happens asynchronously to the
   registration.  On the other hand, the validation exchange with the
   registrar (6LBR) is still needed if the router checks the right for
   the host to listen to the anycast or multicast address.

   Figure 1 depicts the registration of an anycast or a multicast
   address.  As shown, the 6LBR receives and accepts multiple EDAR
   messages for the same address, and the address being registered by
   multiple nodes is not treated as a duplication.

       6LoWPAN Node           6LR             6LBR
         (host1)           (router)        (registrar)
            |                  |               |
            |   DMB link       |               |
            |                  |               |
            |  ND-Classic RS   |               |
            |----------------->|               |
            |------------>     |               |
            |------------------------>         |
            |  ND-Classic RA   |               |
            |<-----------------|               |
            |                  |               |
            |  NS(EARO)        |               |
            |----------------->|               |
            |                  |     EDAR      |
            |                  |-------------->|
            |                  |               |
            |                  |     EDAC      |
            |                  |<--------------|
            |        NA(EARO)  |
            |<-----------------|<inject route> ->
            |                  |
                      ...
         (host2)           (router)           6LBR
            |  NS(EARO)        |               |
            |----------------->|               |
            |                  |               |
            |                  |     EDAR      |
            |                  |-------------->|
            |                  |               |
            |                  |     EDAC      |
            |                  |<--------------|
            |        NA(EARO)  |               |
            |<-----------------|               |
                      ...
         (host1)           (router)
            |  NS(EARO)        |               |
            |----------------->|               |
            |                  |               |
            |        NA(EARO)  |               |
            |<-----------------|               |
                      ...
            |                  |<maintain route> ->
                      ...

      Figure 1: Registration Flow for an Anycast or Multicast Address

   In classical networks, [RFC8505] may be used for an ND proxy
   operation as specified in [RFC8929] or redistributed in a full-
   fledged routing protocol such as what might be done in BGP for
   Ethernet VPN [MAC-SIGNALING] or in the Routing in Fat Trees (RIFT)
   protocol [RIFT].  The device mobility can be gracefully supported as
   long as the routers can exchange and make sense of the sequence
   counter in the TID field of the EARO.

   In the case of LLNs, RPL [RFC6550] is the routing protocol of choice
   and [RFC9010] specifies how the unicast address advertised with
   [RFC8505] is redistributed in RPL.  This specification also provides
   RPL extensions for anycast and multicast address operation and
   redistribution.  In the RPL case, and unless specified otherwise, the
   behavior is the same as it is for unicast for the 6LBR that acts as
   RPL Root, the intermediate routers Down the RPL graph, the 6LRs that
   act as access routers, and the 6LNs that are the RPL-unaware
   destinations.  In particular, forwarding a packet happens as
   specified in Section 11 of [RFC6550], including loop avoidance and
   detection, though in the multicast case, multiple copies might be
   generated.

   [RFC8505] is a prerequisite to this specification.  A node that
   implements this MUST also implement [RFC8505].  This specification
   modifies existing options and updates the associated behaviors to
   enable the registration for multicast addresses as an extension to
   [RFC8505].  As with the registration of a unicast address, the
   subscription to anycast and multicast addresses between a node and
   its router(s) is agnostic (meaning it is independent) from the
   routing protocol in which this information may be redistributed or
   aggregated by the router to other routers.  However, protocol
   extensions would be needed in the protocol when multicast services
   are not available.

   This specification also Extends [RFC6550] and [RFC9010] to add
   multicast ingress replication (IR) in Non-Storing mode and anycast
   support in both Storing and Non-Storing modes in the case of a route-
   over multilink subnet based on the RPL routing protocol.  A 6LR that
   implements the RPL extensions specified herein MUST also implement
   [RFC9010].

   Figure 2 illustrates the typical scenario of an LLN as a single IPv6
   subnet, with a 6LBR that acts as Root for RPL operations and
   maintains a registry of the active registrations as an abstract data
   structure called an "Address Registrar" for 6LoWPAN ND.

   The LLN may be a hub-and-spoke access link such as (Low-Power) Wi-Fi
   [IEEE-802.11] and (Low-Energy) Bluetooth [IEEE-802.15.1] or a Route-
   Over LLN such as the Wi-SUN [Wi-SUN] and IPv6 over the TSCH mode of
   IEEE 802.15.4 (6TiSCH) [RFC9030] meshes that leverage 6LoWPAN
   [RFC4919] [RFC6282] and RPL [RFC6550] over IEEE 802.15.4
   [IEEE-802.15.4].

                     |
         ----+-------+------------
             |     Wire side
          +------+
          | 6LBR |
          |(Root)|
          +------+
          o  o  o  Wireless side
      o   o o   o  o o
     o  o  o o   o  o  o
    o  o  o   LLN  o   +---+
      o  o   o  o   o  |6LR|
      o o  o o   o     +---+
       o   o   o o o  o    z
      o  o oo o  o        +---+
             o            |6LN|
                          +---+

                          Figure 2: Wireless Mesh

   A leaf acting as a 6LN registers its unicast addresses to a RPL
   router acting as a 6LR using a Layer 2 unicast NS message with an
   EARO as specified in [RFC8505].  The registration state is
   periodically renewed by the Registering Node before the lifetime
   indicated in the EARO expires.  As for unicast IPv6 addresses, the
   6LR uses an EDAR and then an EDAC exchange with the 6LBR to notify
   the 6LBR of the presence of the listeners.

   This specification updates the EARO with a new 2-bit field, the
   P-Field, as detailed in Section 7.1.  The existing R flag that
   requests reachability for the Registered Address gets new behavior.
   With this extension, the 6LNs can now subscribe to the anycast and
   multicast addresses they listen to, using a new P-Field in the EARO
   to signal that the registration is for a multicast address.  Multiple
   6LNs may subscribe the same multicast address to the same 6LR.  Note
   the use of the term "subscribe": this means that when using the EARO
   registration mechanism, a node registers the unicast addresses that
   it owns but subscribes to the multicast addresses that it listens to.

   With this specification, the 6LNs can also subscribe the anycast
   addresses they accept using a new P-Field in the EARO to signal that
   the registration is for an anycast address.  For multicast addresses,
   multiple 6LNs may subscribe the same anycast address to the same 6LR.

   If the R flag is set in the subscription of one or more 6LNs for the
   same address, the 6LR injects the anycast addresses and multicast
   addresses of a scope larger than the link-scope in RPL, based on the
   longest subscription lifetime across the active subscriptions for the
   address.

   In the RPL Storing Mode of Operation with multicast support
   (Section 12 of [RFC6550]), the DAO messages for the multicast address
   percolate along the RPL-preferred parent tree and mark a subtree that
   becomes the multicast tree for that multicast address, with 6LNs that
   subscribed to the address as the leaves.  As prescribed in Section 12
   of [RFC6550], the 6LR forwards a multicast packet as an individual
   unicast Medium Access Control (MAC) frame to each peer along the
   multicast tree, except to the node it received the packet from.

   In the new RPL Non-Storing Mode of Operation with ingress replication
   multicast support that is introduced here, the DAO messages announce
   the multicast addresses as Targets, and never as Transits.  The
   multicast distribution is an IR whereby the Root encapsulates the
   multicast packets to all the 6LRs that are transit for the multicast
   address, using the same source-routing header as for unicast targets
   attached to the respective 6LRs.

   LLN links are typically Direct MAC Broadcast (DMB) (see more in
   [IPv6-OVER-WIRELESS]) with no emulation to increase range (over
   multiple radio hops) or reliability.  In such links, broadcasting is
   unreliable and asynchronous transmissions force a listener to remain
   awake, so asynchronous broadcasting is generally inefficient.  Thus,
   the expectation is that whenever possible, the 6LRs deliver the
   multicast packets as individual unicast MAC frames to each of the
   6LNs that subscribed to the multicast address.  On the other hand, in
   a network where nodes do not sleep, asynchronous broadcasting may
   still help recovering faster when state is lost.

   With this specification, anycast addresses can be injected in RPL in
   both Storing and Non-Storing modes.  In Storing mode, the RPL router
   accepts DAO messages from multiple children for the same anycast
   address, but it only forwards a packet to one of the children.  In
   Non-Storing mode, the Root maintains the list of all the RPL nodes
   that announced the anycast address as Target, but it forwards a given
   packet to only one of them.

   Operationally speaking, deploying a new MOP means that one cannot
   update a live network.  The network administrator must create a new
   instance with MOP 5 and migrate nodes to that instance by allowing
   them to join it.

   For backward compatibility, this specification allows building a
   single DODAG signaled as MOP 1 that conveys anycast, unicast, and
   multicast packets using the same source-routing mechanism; see more
   in Section 11.

   It is also possible to leverage this specification between the 6LN
   and the 6LR for the registration of unicast, anycast, and multicast
   IPv6 addresses in networks that are not necessarily LLNs and/or where
   the routing protocol between the 6LR and its peer routers is not
   necessarily RPL.  In that case, the distribution of packets between
   the 6LR and the 6LNs may effectively rely on a broadcast or multicast
   support at the lower layer (e.g., using this specification as a
   replacement to MLD in an Ethernet-bridged domain and still using
   either a plain MAC-layer broadcast or snooping of this protocol to
   control the flooding).  It may also rely on overlay services to
   optimize the impact of Broadcast, Unknown, and Multicast (BUM)
   traffic over a fabric, e.g., registering with [MAC-SIGNALING] and
   forwarding with [RFC9574].

   For instance, it is possible to operate a RPL Instance in the new
   Non-Storing Mode of Operation with ingress replication multicast
   support (while possibly signaling a MOP of 1) and use "Multicast
   Protocol for Low-Power and Lossy Networks (MPL)" [RFC7731] for the
   multicast operation.  MPL floods the DODAG with the multicast
   messages independently of the RPL DODAG topologies.  Two variations
   are possible:

   *  In one possible variation, all the 6LNs set the R flag in the EARO
      for a multicast target, upon which the 6LRs send a unicast DAO
      message to the Root; the Root filters out the multicast messages
      for which there is no listener and only floods when a listener
      exists.

   *  In a simpler variation, the 6LNs do not set the R flag and the
      Root floods all the multicast packets over the whole DODAG.  Using
      a configuration mechanism, it is also possible to control the
      behavior of the 6LR to ignore the R flag.  It can be configured to
      either always or never send the DAO message and/or to control the
      Root and specify which groups it should flood or not flood.

   Note that if the configuration instructs the 6LR not to send the DAO
   message, then MPL can be used in conjunction with the RPL Storing
   mode as well.

4.  Amending RFC 4861

   Section 7.1 of [RFC4861] requires silently discarding NS and NA
   packets when the Target Address is a multicast address.  This
   specification Amends [RFC4861] by allowing the advertisement of
   multicast and anycast addresses in the Target Address field when the
   NS message is used for a registration, per Section 5.5 of [RFC8505].

5.  Extending RFC 7400

   This specification Extends "6LoWPAN-GHC: Generic Header Compression
   for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)"
   [RFC7400] by defining a new capability bit for use in the 6CIO.
   [RFC7400] was already extended by [RFC8505] for use in IPv6 ND
   messages.

   The new "Registration for Unicast, Multicast, and Anycast Addresses
   Supported" X flag indicates to the 6LN that the 6LR accepts unicast,
   multicast, and anycast address registrations as specified in this
   document and will ensure that packets for the Registered Address will
   be routed to the 6LNs that registered with the R flag set
   appropriately.

   Figure 3 illustrates the X flag in its position (8, counting 0 to 15
   in network order in the 16-bit array); see Section 14.6 for the IANA
   registration of capability bits.


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

                 Figure 3: New Capability Bits in the 6CIO

   New Option Field:

   X:  This is a 1-bit flag for "Registration for Unicast, Multicast,
      and Anycast Addresses Supported".

6.  Amending RFC 6550

   This specification Amends [RFC6550] to mandate the use of the ROVR
   field as the indication of the origin of a Target advertisement in
   RPL DAO messages, as specified as an option in Section 6.1 of
   [RFC9010].

   This specification Extends [RFC6550] with a new P-Field in the RPL
   Target Option (RTO).

   The specification also Amends the behaviors of the Modes of Operation
   MOP 1 and MOP 3 and Extends [RFC6550] with a new MOP 5.

6.1.  Mandating the ROVR Field

   For anycast and multicast advertisements (in NS or DAO messages),
   multiple origins may subscribe to the same address, in which case the
   multiple advertisements from the different or unknown origins are
   merged by the common parent; in that case, the common parent becomes
   the origin of the merged advertisements and uses its own ROVR value.
   On the other hand, a parent that propagates an advertisement from a
   single origin uses the original ROVR in the propagated RTO, as it
   does for unicast address advertisements, so the origin is recognized
   across multiple hops.

   [RFC6550] uses the Path Sequence in the Transit Information Option
   (TIO) to retain only the freshest unicast route and remove the stale
   ones, e.g., in the case of mobility.  [RFC9010] copies the
   Transaction ID (TID) from the EARO into the Path Sequence and the
   ROVR field into the associated RTO.  This way, it is possible to
   identify both the Registering Node and the order of registration in
   RPL for each individual advertisement, so the most recent path and
   lifetime values are used.

   This specification Extends [RFC6550] for anycast and multicast
   advertisements to require that the Path Sequence be used between, and
   only between, advertisements for the same Target and from the same
   origin (i.e., with the same ROVR value).  In that case, only the
   freshest advertisement is retained, but the freshness comparison
   cannot apply if the origin is not determined (i.e., the origin did
   not support this specification).

   [RFC6550] uses the Path Lifetime in the TIO to indicate the remaining
   time for which the advertisement is valid for unicast route
   determination, and a Path Lifetime value of 0 invalidates that route.
   [RFC9010] maps the Address Registration lifetime in the EARO and the
   Path Lifetime in the TIO so they are comparable when both forms of
   advertisements are received.

   The RPL router that merges multiple advertisements for the same
   anycast or multicast addresses MUST use and advertise the longest
   remaining lifetime across all the origins of the advertisements for
   that address.  When the lifetime expires, the router sends a no-path
   DAO message (i.e., the lifetime is 0) using the same value for the
   ROVR value as for the previous advertisements.  This value refers to
   either the single descendant that advertised the Target if there is
   only one or the router itself if there is more than one.

   Note that the Registration Lifetime, TID, and ROVR fields are also
   placed in the EDAR message so the state created by the EDAR is also
   comparable with that created upon an NS(EARO) or a DAO message.  For
   simplicity, the text below mentions only NS(EARO) but it also applies
   to EDAR.

6.2.  Updating MOP 3

   RPL supports multicast operations in the Storing Mode of Operation
   with multicast support (MOP 3), which provides source-independent
   multicast routing in RPL, as prescribed in Section 12 of [RFC6550].
   MOP 3 is a Storing Mode of Operation.  This operation builds a
   multicast tree within the RPL DODAG for each multicast address.  This
   specification provides additional details for the MOP 3.

   The expectation in MOP 3 is that the unicast traffic also follows the
   Storing Mode of Operation.  However, this is rarely the case in LLN
   deployments of RPL where the Non-Storing Mode of Operation (MOP 1) is
   the norm.  Though it is preferred to build separate RPL Instances,
   one in MOP 1 and one in MOP 3, this specification allows hybrid use
   of the Storing mode for multicast and the Non-Storing mode for
   unicast in the same RPL Instance, as is elaborated in more detail in
   Section 11.

   For anycast and multicast advertisements, including MOP 3, the ROVR
   field is placed in the RTO as specified in [RFC9010] for both MOP 3
   and MOP 5 as it is for unicast advertisements.

   Though it was implicit with [RFC6550], this specification clarifies
   that the freshness comparison based on the Path Sequence is not used
   when the origin cannot be determined, which occurs in the case of
   multiple subscriptions of a multicast or unicast address.  The
   comparison is to be used only between advertisements from the same
   origin, which is either an individual subscriber or a descendant that
   merged multiple advertisements.

   A RPL router maintains a remaining Path Lifetime for each DAO message
   that it receives for a multicast target and sends its own DAO message
   for that target with the longest remaining lifetime across its
   listening children.  If the router has only one descendant listening,
   it propagates the TID and ROVR as received.  Conversely, if the
   router merges multiple advertisements (possibly including one for
   itself as a listener), the router uses its own ROVR and TID values.
   This implies a possible transition of ROVR and TID values when the
   number of listening children changes from one to more or back to one,
   which should not be considered as an error or a change of ownership
   by the parents above.

6.3.  New Non-Storing Multicast MOP

   This specification adds a Non-Storing Mode of Operation with ingress
   replication multicast support RPL (as assigned by IANA; see
   Section 14.5) whereby the Non-Storing Mode DAO to the Root may
   advertise a multicast address in the RTO, whereas the TIO cannot.

   In that mode, the RPL Root performs an IR operation on the multicast
   packets.  This means that it transmits one copy of each multicast
   packet to each 6LR that is a transit for the multicast target, using
   the same source-routing header and encapsulation as it would for a
   unicast packet for a RPL-Unaware Leaf (RUL) attached to that 6LR.

   For the intermediate routers, the packet appears as any source-routed
   unicast packet.  The difference shows only at the 6LR, which
   terminates the source-routed path and forwards the multicast packet
   to all 6LNs that registered for the multicast address.

   For a packet that is generated by the Root, the Root builds a source-
   routing header as shown in Section 8.1.3 of [RFC9008], but for which
   the last and only the last address is multicast.  For a packet that
   is not generated by the Root, the Root encapsulates the multicast
   packet as per Section 8.2.4 of [RFC9008].  In that case, the outer
   header is purely unicast, and the encapsulated packet is purely
   multicast.

   For anycast and multicast advertisements in NA messages (at the 6LR)
   and DAO messages (at the Root), as discussed in Section 6.2, the
   freshness comparison based on the TID field is applied only between
   messages from the same origin, as determined by the same value in the
   ROVR field.

   The Root maintains a remaining Path Lifetime for each advertisement
   it receives, and a 6LR generates the DAO message for multicast
   addresses with the longest remaining lifetime across its registered
   6LNs, using its own ROVR and TID when multiple 6LNs have subscribed
   or when the 6LR is a subscriber.

   This specification allows enabling the operation in a MOP 1 brown
   field for this new mode as well; see more in Section 11.

6.4.  RPL Anycast Operation

   With multicast, the address has a recognizable format, and a
   multicast packet is to be delivered to all the active subscribers.
   In contrast, the format of an anycast address is not distinguishable
   from that of a unicast address.  A legacy node may issue a DAO
   message without setting the P-Field to 2; the unicast behavior may
   apply to anycast traffic within a portion of the network, but the
   packets will still be delivered.  That message will be
   undistinguishable from a unicast advertisement, and the anycast
   behavior in the data plane can only happen if all the nodes that
   advertise the same anycast address are synchronized with the same
   TID.  That way, the multiple paths can remain in the RPL DODAG.

   With the P-Field set to 2, this specification alleviates the issue of
   synchronizing the TIDs and ROVR fields.  As for multicast, the
   freshness comparison based on the TID (in the EARO) and the Path
   Sequence (in the TIO) is ignored unless the messages have the same
   origin; this is inferred by the same ROVR in the RTO and/or the EARO,
   and the latest value of the lifetime is retained for each origin.

   A RPL router that propagates an advertisement from a single origin
   uses the ROVR and Path Sequence from that origin, whereas a router
   that merges multiple subscriptions uses its own ROVR and Path
   Sequence and the longest lifetime over the different advertisements.
   A target is routed as anycast by a parent (or the Root) that received
   at least one DAO message for that target with the P-Field set to 2.

   As opposed to multicast, the anycast operation described herein
   applies to both addresses and prefixes, and the P-Field can be set to
   2 for both.  An external destination (such as an address or prefix)
   that may be injected as a RPL Target from multiple border routers
   should be injected as anycast in RPL to enable load balancing.  In
   contrast, a mobile target that is multihomed should be advertised as
   unicast over the multiple interfaces to favor the TID comparison
   instead of multipath load balancing.

   For either multicast or anycast, there can be multiple subscriptions
   from multiple origins, each using a different value of the ROVR field
   that identifies the individual subscription.  The 6LR maintains a
   subscription state per value of the ROVR for each multicast or
   anycast address, but it injects the route into RPL only once for each
   address.  In the case of a multicast address, this occurs only if its
   scope is larger than the link-scope (three or more).  Since the
   subscriptions are considered separate, the check on the TID that acts
   as the subscription sequence only applies to the subscription with
   the same ROVR.

   Like the 6LR, a RPL router in Storing mode propagates the merged
   advertisement to its parent(s) in DAO messages once and only once for
   each address, but it retains a routing table entry for each of the
   children that advertised the address.

   When forwarding multicast packets Down the DODAG, the RPL router
   copies all the children that advertised the address in their DAO
   messages.  In contrast, when forwarding anycast packets Down the
   DODAG, the RPL router MUST copy one and only one of the children that
   advertised the address in their DAO messages and forward it to one
   parent if there is no such child.

6.5.  New Registered Address Type Indicator P-Field

   The new Registered Address Type Indicator (RATInd) is created for use
   in the RTO, the EARO, and the header of EDAR messages.  The RATInd
   indicates whether an address is unicast, multicast, or anycast.  The
   new 2-bit P-Field is defined to transport the RATInd in different
   protocols.

   The P-Field can take the values shown in Table 2.

   The intent for the value of 3 is a prefix registration (see
   [REGISTRATION]), which is expected to be published after this
   specification.  At the time of this writing, RPL already advertises
   prefixes, and treats unicast addresses as prefixes with a length of
   128, so it does not need that new value.  On the other hand, 6LoWPAN
   ND does not, so the value of 3 (meaning prefix registration) will not
   be processed adequately.  As a result:

   *  When the value of 3 is received in an RTO (see Section 6.6), this
      value MUST be ignored by the receiver (meaning it is treated as a
      value of 0) but the message is processed normally (as per
      [RFC6550] and [RFC9010]).

   *  In the case of an EARO (see Section 7.1) or an EDAR (see
      Section 7.2), the message MUST be dropped, and the receiving node
      MAY either reply with a status of 12 "Invalid Registration" or
      remain silent.

6.6.  New RPL Target Option P-Field

   [RFC6550] recognizes a multicast address by its format (as specified
   in Section 2.7 of [RFC4291]) and applies the specified multicast
   operation if the address is recognized as multicast.  This
   specification updates [RFC6550] to add the 2-bit P-Field (see
   Section 6.5) to the RTO to indicate that the Target Address is to be
   processed as unicast, multicast, or anycast.

   *  An RTO that has the P-Field set to 0 is called a unicast RTO.

   *  An RTO that has the P-Field set to 1 is called a multicast RTO.

   *  An RTO that has the P-Field set to 2 is called an anycast RTO.

   The suggested position for the P-Field is 2 counting from 0 to 7 in
   network order as shown in Figure 4, based on Figure 4 of [RFC9010],
   which defines the flags in positions 0 and 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 = 0x05 | Option Length |F|X| P |ROVRsz | Prefix Length |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                Target Prefix (Variable Length)                |
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
    ...            Registration Ownership Verifier (ROVR)           ...
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 4: Format of the RPL Target Option (RTO)

   New and updated Option Field:

   P:  This is a 2-bit field; see Section 6.5.

7.  Extending RFC 8505

   This specification Extends [RFC8505] by adding the concept of a
   subscription for anycast and multicast addresses and creating a new
   field called the P-Field in the EARO and in the EDAR and EDAC
   messages to signal the type of registration.

7.1.  Placing the New P-Field in the EARO

   Section 4.1 of [RFC8505] defines the EARO as an extension to the ARO
   defined in [RFC6775].  This specification adds a new P-Field that is
   placed in the EARO flags and is set as follows:

   *  The P-Field is set to 1 to signal that the Registered Address is a
      multicast address.  When the P-Field is 1 and the R flag is set to
      1 as well, the 6LR that conforms to this specification joins the
      multicast stream (e.g., by injecting the address in the RPL
      multicast support that is extended in this specification for the
      Non-Storing mode).

   *  The P-Field is set to 2 to signal that the Registered Address is
      an anycast address.  When the P-Field is 2 and the R flag is 1,
      the 6LR that conforms to this specification injects the anycast
      address in the routing protocol(s) that it participates in (e.g.,
      in the RPL anycast support that is introduced in this
      specification for both the Storing and Non-Storing modes).

   Figure 5 illustrates the P-Field in its position (2, counting 0 to 7
   in network order in the 8-bit array); see Section 14.1 for the IANA
   registration of P-Field values.

      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     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Rsv| P | I |R|T|     TID       |     Registration Lifetime     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
    ...          Registration Ownership Verifier (ROVR)              ...
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 5: Extended Address Registration Option (EARO) Format

   New and updated Option Fields:

   Rsv:  This is a 2-bit field.  It is reserved and MUST be set to 0 and
      ignored by the receiver.

   P:  This is a 2-bit P-Field; see Section 6.5.

7.2.  Placing the New P-Field in the EDAR Message

   Section 4 of [RFC6775] provides the same format for DAR and DAC
   messages but the Status field is only used in DAC messages and has to
   be set to 0 in DAR messages.  [RFC8505] extends the DAC message as an
   EDAC but does not change the Status field in the EDAR.

   This specification repurposes the Status field in the EDAR as a Flags
   field.  It adds a new P-Field to the EDAR flags field to match the
   P-Field in the EARO and signal the new types of registration.  The
   EDAC message is not modified.

   Figure 6 illustrates the P-Field in its position (0, counting 0 to 7
   in network order in the 8-bit array); see Section 14.2 for the IANA
   registration of EDAR message flags.

      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      |CodePfx|CodeSfx|          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | P | Reserved  |     TID       |     Registration Lifetime     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
    ...            Registration Ownership Verifier (ROVR)           ...
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                       Registered Address                      +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 6: Extended Duplicate Address Request (EDAR) Message Format

   New and updated Option Fields:

   Reserved:  This is a 6-bit field.  It is reserved and MUST be set to
      0 and ignored by the receiver.

   P:  This is a 2-bit field; see Section 6.5.

7.3.  Registration Extensions

   [RFC8505] specifies the following behaviors:

   *  A router that expects to reboot may send a final RA message, upon
      which nodes should subscribe elsewhere or redo the subscription to
      the same router upon reboot.  In all other cases, a node reboot is
      silent.  When the node comes back to life, existing registration
      state might be lost if it was not persisted, e.g., in persistent
      memory.

   *  The registration method is specified only for unicast addresses.

   *  The 6LN must register all its ULAs and GUAs with an NS(EARO)
      message.

   *  The 6LN may set the R flag in the EARO to obtain return
      reachability services by the 6LR, e.g., through ND proxy
      operations or by injecting the route in a route-over subnet.

   *  the 6LR maintains a registration state per Registered Address,
      including an NCE with the Link-Layer Address (LLA) of the
      Registered Node (the 6LN here).

   This specification Amends the above behaviors and Extends them with
   the following behaviors:

   *  The concept of subscription is introduced for anycast and
      multicast addresses as an extension to the registration of a
      unicast address.  The respective operations are similar from the
      perspective of the 6LN, but they show important differences on the
      router side, which maintains a separate state for each origin and
      merges them in its own advertisements.

   *  New ARO Statuses are introduced to indicate a "Registration
      Refresh Request" and an "Invalid Registration" (see Table 8).

      The former status is used in asynchronous NA(EARO) messages to
      indicate to peer 6LNs that they are requested to reregister all
      addresses that were previously registered to the originating node.
      The NA message may be sent to a unicast or a multicast link-scope
      address and should be contained within the L2 range where nodes
      may have effectively registered or, respectively, subscribed to
      this router (e.g., a radio broadcast domain).  The latter is
      generic to any error in the EARO and is used, for example, to
      report that the P-Field is not consistent with the Registered
      Address in NS(EARO) and EDAR messages.

      A router that wishes to refresh its state (e.g., upon reboot or in
      any situation where it may have missed a registration or lost a
      registration state) SHOULD send an asynchronous NA(EARO) with this
      new status value.  Failure to do so will delay the recovery of the
      network until the next periodic registration by the attached 6LNs
      and packets may be lost until then.  That asynchronous multicast
      NA(EARO) MUST be sent to the all-nodes link-scope multicast
      address (ff02::1), and the Target MUST be set to the link-local
      address that was exposed previously by this node to accept
      registrations.

      The TID field in the multicast NA(EARO) message is the one
      associated to the Target and follows the same rules as the TID in
      the NS(EARO) message for the same Target; see Section 5.2 of
      [RFC8505], which points to Section 7.2 of [RFC6550] for the
      lollipop mechanism used in the TID operation.  It is incremented
      by the sender each time it sends a new series of NS and/or NA
      messages with the EARO about the Target.  The TID indicates a
      reboot when it is in the "straight" part of the lollipop, between
      the initial value and 255.  After that, the TID remains below 128
      as long as the device is alive.  An asynchronous multicast
      NA(EARO) with a TID below 128 MUST NOT be considered as indicating
      a reboot.

      The asynchronous multicast NA(EARO) indicating a "Registration
      Refresh Request" MAY be reissued a few times within a short
      period, to increase the chances that the message is received by
      all registered nodes despite the unreliable transmissions within
      the LLN; the TID MUST be incremented each time.  The receiver 6LN
      MUST consider that multiple NA(EARO) messages indicating a
      "Registration Refresh Request" from the same 6LR received within
      that short period with comparable and increasing TID values (i.e.,
      their absolute difference is less than the SEQUENCE_WINDOW; see
      Section 7.2 of [RFC6550]) are in fact indicative of the same
      request.  The 6LN MUST process one and only one of the series of
      messages.  If the TIDs are desynchronized (not comparable) or
      decreased, then the NA(EARO) is considered as a new request and it
      MUST be processed.

      The multicast NA(EARO) SHOULD be resent enough times for the TID
      to be issued with the value of 255 so the next NA(EARO) after the
      initial series is outside the lollipop and is not confused with a
      reboot.  By default, the TID initial setting after boot is 252,
      the SEQUENCE_WINDOW is 4, the duration of the short period is 10
      seconds, the interval between retries is 1 second, and the number
      of retries is 3.  To reach 255 at boot time, the sender MAY either
      issue at least 4 NA messages, skip a TID value, or start with a
      value that is more than 252.  The best values for the short
      period, the number of retries, and the TID initial setting depend
      on the environment and SHOULD be configurable.

   *  A new IPv6 ND Consistent Uptime Option (CUO) is introduced to be
      placed in IPv6 ND messages.  The CUO allows figuring out the state
      consistency between the sender and the receiver.  For instance, a
      node that rebooted needs to reset its uptime to 0.  A router that
      changed information like a prefix information option has to
      advertise an incremented state sequence.  To that effect, the CUO
      carries a Node State Sequence Information (NSSI) and a Consistent
      Uptime.  See Section 10 for the option details.

      A node that receives the CUO checks whether it is indicative of a
      desynchronization between peers.  A peer that discovers that a
      router has changed should reassess which addresses it formed based
      on the new PIOs from that router and resync the state that it
      installed in the router (e.g., the registration state for its
      addresses).  In the process, the peer may attempt to:

      -  form new addresses and register them,

      -  deprecate old addresses and deregister them using a Lifetime of
         0, and

      -  reform any potentially lost state (e.g., by registering again
         an existing address that it will keep using).

      A loss of state is inferred if the Consistent Uptime of the peer
      is less than the time since the state was installed, or if the
      NSSI is incremented for a Consistent Uptime.

   *  Registration for multicast and anycast addresses is now supported.
      The P-Field is added to the EARO to signal when the Registered
      Address is anycast or multicast.  The value of the P-Field is not
      consistent with the Registered Address if:

      -  the Registered Address is a multicast address (Section 2.4 of
         [RFC4291]) and the P-Field indicates a value that is not 1, or

      -  the Registered Address is not a multicast address and the
         P-Field indicates a value that is 1.

      If this occurs, then the message, NS(EARO) or EDAR, MUST be
      dropped, and the receiving node MAY either reply with a status of
      12 "Invalid Registration" or remain silent.

   *  The Status field in the EDAR message that was reserved and not
      used in [RFC8505] is repurposed to transport the flags to signal
      multicast and anycast.

   *  The 6LN MUST also subscribe all the IPv6 multicast addresses that
      it listens to, and it MUST set the P-Field to 1 in the EARO for
      those addresses.  The one exception is the all-nodes link-scope
      multicast address ff02::1 [RFC4291], which is implicitly
      registered by all nodes, meaning that all nodes are expected to
      accept messages sent to ff02::1 but are not expected to register
      it.

   *  The 6LN MAY set the R flag in the EARO to obtain the delivery of
      the multicast packets by the 6LR (e.g., by MLD proxy operations,
      or by injecting the address in a route-over subnet or in the
      Protocol Independent Multicast [RFC7761] protocol).

   *  The 6LN MUST also subscribe all the IPv6 anycast addresses that it
      supports, and it MUST set the P-Field in the EARO to 2 for those
      addresses.

   *  The 6LR and the 6LBR are extended to accept more than one
      subscription for the same address when it is anycast or multicast,
      since multiple 6LNs may subscribe to the same address of these
      types.  In both cases, the ROVR in the EARO uniquely identifies a
      registration within the namespace of the Registered Address.

   *  The 6LR MUST also consider that all the nodes that registered an
      address to it (as known by the Source Link-Layer Address Option
      (SLLAO)) also registered ff02::1 [RFC4291] to the all-nodes link-
      scope multicast address.

   *  The 6LR MUST maintain a subscription state per tuple (IPv6
      address, ROVR) for both anycast and multicast types of addresses.
      It SHOULD notify the 6LBR with an EDAR message, unless it
      determined that the 6LBR is legacy and does not support this
      specification.  In turn, the 6LBR MUST maintain a subscription
      state per tuple (IPv6 address, ROVR) for both anycast and
      multicast types of address.

8.  Extending RFC 9010

   [RFC9010] specifies the following behaviors:

   *  The 6LR has no specified procedure to inject multicast and anycast
      routes in RPL even though RPL supports multicast.

   *  Upon a registration with the R flag set to 1 in the EARO, the 6LR
      injects the address in the RPL unicast support.

   *  Upon receiving a packet directed to a unicast address for which it
      has an active registration, the 6LR delivers the packet as a
      unicast Layer 2 frame to the LLA of the node that registered the
      unicast address.

   This specification Extends [RFC9010] by adding the following
   behavior:

   *  Upon a subscription with the R flag and the P-Field both set to 1
      in the EARO, if the scope of the multicast address is above link-
      scope [RFC7346], then the 6LR injects the address in the RPL
      multicast support and sets the P-Field in the RTO to 1 as well.

   *  Upon a subscription with the R flag set to 1 and the P-Field set
      to 2 in the EARO, the 6LR injects the address in the new RPL
      anycast support and sets the P-Field to 2 in the RTO.

   *  Upon receiving a packet directed to a multicast address for which
      it has at least one subscription, the 6LR delivers a copy of the
      packet as a unicast Layer 2 frame to the LLA of each of the nodes
      that registered to that multicast address.

   *  Upon receiving a packet directed to an anycast address for which
      it has at least one subscription, the 6LR delivers a copy of the
      packet as a unicast Layer 2 frame to the LLA of exactly one of the
      nodes that registered to that multicast address.

9.  Leveraging RFC 8928

   "Address-Protected Neighbor Discovery for Low-Power and Lossy
   Networks" [RFC8928] was defined to protect the ownership of unicast
   IPv6 addresses that are registered with [RFC8505].

   With [RFC8928], it is possible for a node to autoconfigure a pair of
   public and private keys and use them to sign the registration of
   addresses that are either autoconfigured or obtained through other
   methods.

   The first hop router (the 6LR) may then validate a registration and
   perform source address validation on packets coming from the sender
   node (the 6LN).

   Anycast and multicast addresses are not owned by one node.  Multiple
   nodes may subscribe to the same address.  In that context, the method
   specified in [RFC8928] cannot be used with autoconfigured key pairs
   to protect a single ownership.

   For an anycast or a multicast address, it is still possible to
   leverage [RFC8928] to enforce the right to subscribe.  If [RFC8928]
   is used, a key pair MUST be associated with the address before it is
   deployed, and a ROVR MUST be generated from that key pair as
   specified in [RFC8928].  The address and the ROVR MUST then be
   installed in the 6LBR so it can recognize the address and compare the
   ROVR on the first subscription.

   The key pair MUST then be provisioned in each node that needs to
   subscribe to the anycast or multicast address, so the node can follow
   the steps in [RFC8928] to subscribe to the address.

10.  Consistent Uptime Option

   This specification introduces a new option that characterizes the
   uptime of the sender.  The option may be used by routers in RA
   messages and by any node in NS, NA, and RS messages.  It is used by
   the receiver to infer whether some state synchronization might be
   lost (e.g., due to reboot).

      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    | Exponent  |  Uptime Mantissa  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |S|U|   flags   |          NSSI         |     Peer NSSI         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 7: Consistent Uptime Option (CUO) Format

   Type:  Assigned by IANA; see Table 9.

   Length:  1

   Uptime Exponent:  A 6-bit unsigned integer and the Exponent to the
      base 2 of the uptime unit.

   Uptime Mantissa:  A 10-bit unsigned integer and the mantissa of the
      uptime value.

   S:  A 1-bit flag set to 1 to indicate that the sender is low-power
      and may sleep.

   U:  A 1-bit flag set to 1 to indicate that the Peer NSSI field is
      valid; it MUST be set to 0 when the message is not unicast and
      MUST be set to 1 when the message is unicast and the sender has an
      NSSI state for the intended receiver.

   flags:  6-bit flags that are reserved and that MUST be set to 0 by
      the sender and ignored by the receiver.

   NSSI:  A 12-bit unsigned integer that represents the Node State
      Sequence Information (NSSI).  It MUST be stored by the receiver if
      it has a dependency on information advertised or stored at the
      sender.

   Peer NSSI:  A 12-bit unsigned integer that echoes the last known NSSI
      from the peer.

   The Consistent Uptime indicates how long the sender has been
   continuously up and running (though possibly sleeping) without loss
   of state.  It is expressed by the Uptime Mantissa in units of 2 to
   the power of the Uptime Exponent in milliseconds.  The receiver
   derives the boot time of the sender as the current time minus the
   sender's Consistent Uptime.

   If the boot time of the sender is updated to a newer time, any state
   that the receiver installed in the sender before the reboot is
   probably lost.  The receiver MUST reassess all the state it installed
   in the server (e.g., any registration) and reinstall if it is still
   needed.

   The U flag not set in a unicast message indicates that the sender has
   lost all state from this node.  If the U flag is set, then the Peer
   NSSI field can be used to assess which changes the sender missed.
   For the other way around, any state that was installed in the
   receiver from information by the sender before it rebooted MUST be
   removed and may or may not be reinstalled later.

   The value of the uptime is reset to 0 at some point of the sender's
   reboot sequence, but it may not still be 0 when the first message is
   sent, so the receiver must not expect a value of 0 as the signal of a
   reboot.

               +----------+----------+------------+--------+
               | Mantissa | Exponent | Resolution | Uptime |
               +----------+----------+------------+--------+
               | 1        | 0        | 1 ms       | 1 ms   |
               +----------+----------+------------+--------+
               | 5        | 10       | 1 s        | 5 s    |
               +----------+----------+------------+--------+
               | 2        | 15       | 30 s       | 1 min  |
               +----------+----------+------------+--------+
               | 2        | 21       | 33 min     | 1 hour |
               +----------+----------+------------+--------+

                  Table 1: Consistent Uptime Rough Values

   The NSSI SHOULD be stored in persistent memory by the sender and
   incremented when it may have missed or lost state about a peer, or
   when it has updated some state in a fashion that will impact a peer
   (e.g., a host formed a new address or a router advertises a new
   prefix).  When persisting is not possible, then the NSSI is randomly
   generated.

   As long as the NSSI remains constant, the cross-dependent state (such
   as addresses in a host that depend on a prefix in a router) can
   remain stable, meaning less checks in the receiver.  Any change in
   the value of the NSSI is an indication that the sender updated some
   state and that the dependent state in the receiver should be
   reassessed (e.g., addresses that were formed based on an RA with a
   previous NSSI should be checked, or new addresses may be formed and
   registered).

11.  Operational Considerations

   With this specification, a RPL DODAG forms a realm, and multiple RPL
   DODAGs may be federated in a single RPL Instance administratively.
   This means that a multicast address that needs to span a RPL DODAG
   MUST use a scope of Realm-Local whereas a multicast address that
   needs to span a RPL Instance MUST use a scope of Admin-Local as
   discussed in Section 3 of [RFC7346], "IPv6 Multicast Address Scopes".

   "IPv6 Addressing of IPv4/IPv6 Translators" [RFC6052] enables
   embedding IPv4 addresses in IPv6 addresses.  The Root of a DODAG may
   leverage that technique to translate IPv4 traffic in IPv6 and route
   along the RPL domain.  When encapsulating a packet with an IPv4
   multicast Destination Address, it MUST use a multicast address with
   the appropriate scope, Realm-Local or Admin-Local.

   "Unicast-Prefix-based IPv6 Multicast Addresses" [RFC3306] enables
   forming 2^32 multicast addresses from a single /64 prefix.  If an
   IPv6 prefix is associated to an Instance or a RPL DODAG, this
   provides a namespace that can be used in any desired fashion.  For
   instance, it is possible for a standard defining organization to form
   its own registry and allocate 32-bit values from that namespace to
   network functions or device types.  When used within a RPL deployment
   that is associated with a /64 prefix, the IPv6 multicast addresses
   can be automatically derived from the prefix and the 32-bit value for
   either a Realm-Local or an Admin-Local multicast address as needed in
   the configuration.

   This specification introduces the new RPL MOP 5.  Operationally
   speaking, deploying a new RPL MOP means that one cannot update a live
   network.  The network administrator must create a new instance with
   MOP 5 and migrate nodes to that instance by allowing them to join it.

   In a "green field" deployment where all nodes support this
   specification, it is possible to deploy a single RPL Instance using a
   multicast MOP for unicast, multicast, and anycast addresses.

   In a "brown field" where legacy devices that do not support this
   specification coexist with upgraded devices, it is RECOMMENDED to
   deploy one RPL Instance in any MOP (typically MOP 1) for unicast that
   legacy nodes can join and a separate RPL Instance dedicated to
   multicast and anycast operations using a multicast MOP.

   To deploy a Storing mode multicast operation using MOP 3 in a RPL
   domain, it is required that the RPL routers that support MOP 3 have
   enough density to build a DODAG that covers all the potential
   listeners and includes the spanning multicast trees that are needed
   to distribute the multicast flows.  This might not be the case when
   extending the capabilities of an existing network.

   In the case of the new Non-Storing multicast MOP, arguably the new
   support is only needed at the 6LRs that will accept multicast
   listeners.  It is still required that each listener be able to reach
   at least one such 6LR, so the upgraded 6LRs must be deployed to cover
   all the 6LNs that need multicast services.

   Using separate RPL Instances for unicast traffic on the one hand and
   for anycast and multicast traffic on the other hand allows for the
   use of different objective functions; one favors the link quality Up
   for unicast collection and the other favors Downwards link quality
   for multicast distribution.

   However, this might be impractical in some use cases where the
   signaling and the state to be installed in the devices are very
   constrained, the upgraded devices are too sparse, or the devices do
   not support more multiple instances.

   When using a single RPL Instance, MOP 3 expects the Storing Mode of
   Operation for both unicast and multicast, which is an issue in
   constrained networks that typically use MOP 1 for unicast.  This
   specification allows a mixed mode that is signaled as MOP 1 in the
   DIO messages for backward compatibility, where limited multicast and/
   or anycast is available, under the following conditions:

   *  There MUST be enough density of the 6LRs that support the mixed
      mode to cover all the 6LNs that require multicast or anycast
      services.  In Storing mode, there MUST be enough density of the
      6LRs that support the mixed mode to also form a DODAG to the Root.

   *  The RPL routers that support the mixed mode are configured to
      operate in accordance with the desired operation in the network.

   *  The MOP signaled in the RPL DIO messages is MOP 1 to enable the
      legacy nodes to operate as leaves.

   *  The support of multicast and/or anycast in the RPL Instance SHOULD
      be signaled by the 6LRs to the 6LN using a 6CIO; see Section 5.

   *  Alternatively, the support of multicast in the RPL domain can be
      globally known by other means including configuration or external
      information such as support of a version of an industry standard
      that mandates it.  In that case, all the routers MUST support the
      mixed mode.

12.  Security Considerations

   This specification Extends [RFC8505] and [RFC9010] and leverages
   [RFC9008].  The security sections in these documents also apply to
   this document.  In particular, the link layer SHOULD be sufficiently
   protected to prevent rogue access.

   RPL [RFC6550] already supports routing on multicast addresses,
   whereby the endpoint that subscribes to the group by injecting the
   multicast address participates as a RPL-Aware Node (RAN) in the RPL.
   Using an extension of [RFC8505] as opposed to RPL to subscribe the
   address allows a RPL-Unaware Leaf (RUL) to subscribe as well.  As
   noted in [RFC9010], this provides a better security posture for the
   RPL network, since the nodes that do not really need to speak RPL, or
   are not trusted enough to inject RPL messages, can be forbidden from
   doing so, which bars a number of attack vectors from within RPL.
   Acting as an RUL, those nodes may still leverage the RPL network
   through the capabilities that are opened via ND operations.  With
   this specification, a node that needs multicast delivery can now
   obtain the service in a RPL domain while not being allowed to inject
   RPL messages.

   Compared to [RFC6550], this specification enables tracking the origin
   of the multicast subscription inside the RPL network.  This is a
   first step to enable a form of Route Ownership Validation (ROV) (see
   [RFC6811]) in RPL using the ROVR field in the EARO as proof of
   ownership.

   Section 9 leverages [RFC8928] to prevent a rogue node from
   registering a unicast address that it does not own.  The mechanism
   could be extended to anycast and multicast addresses if the values of
   the ROVR they use are known in advance, but how this is done is not
   in scope for this specification.  One way would be to authorize the
   ROVR of the valid users in advance.  A less preferred way would be to
   synchronize the ROVR and TID values across the valid subscribers as
   preshared key material.

   In the latter case, it could be possible to update the keys
   associated to an address in all the 6LNs, but the flow is not clearly
   documented and may not complete in due time for all nodes in LLN use
   cases.  It may be simpler to install an all-new address with new keys
   over a period of time, and switch the traffic to that address when
   the migration is complete.

13.  Backward Compatibility

   A legacy 6LN will not subscribe multicast addresses, and the service
   will be the same when the network is upgraded.  A legacy 6LR will not
   set the X flag in the 6CIO, and an upgraded 6LN will not subscribe
   multicast addresses.

   Upon receiving an EDAR message, a legacy 6LBR may not realize that
   the address being registered is anycast or multicast and will return
   that it is a duplicate in the EDAC status.  The 6LR MUST ignore a
   duplicate status in the EDAC for anycast and multicast addresses.

   As detailed in Section 11, it is possible to add multicast on an
   existing MOP 1 deployment.

   The combination of a multicast address and the P-Field set to 0 in an
   RTO in a MOP 3 RPL Instance is an indication to the receiver that
   supports this specification (the parent) that the sender (child) does
   not support this specification.  However, the RTO is accepted and
   processed as if the P-Field was set to 1 for backward compatibility.

   When the DODAG is operated in MOP 3, a legacy node will not set the
   P-Field and still expect multicast service as specified in Section 12
   of [RFC6550].  In MOP 3, an RTO that is received with a target that
   is multicast and the P-Field set to 0 MUST be considered as multicast
   and MUST be processed as if the P-Field is set to 1.

14.  IANA Considerations

   IANA has made changes under the "Internet Control Message Protocol
   version 6 (ICMPv6) Parameters" [IANA.ICMP] and "Routing Protocol for
   Low Power and Lossy Networks (RPL)" [IANA.RPL] registry groupings;
   see details in the subsections that follow.

14.1.  New P-Field Values Registry

   IANA has created a new "P-Field Values" registry under the "Internet
   Control Message Protocol version 6 (ICMPv6) Parameters" registry
   group to store the expression of the RATInd as a P-Field.

   The registration procedure is Standards Action [RFC8126].  The
   initial allocations are as indicated in Table 2:

       +-------+--------------------------------------+-----------+
       | Value | Registered Address Type Indicator    | Reference |
       +-------+--------------------------------------+-----------+
       | 0     | Registration for a Unicast Address   | RFC 9685  |
       +-------+--------------------------------------+-----------+
       | 1     | Registration for a Multicast Address | RFC 9685  |
       +-------+--------------------------------------+-----------+
       | 2     | Registration for an Anycast Address  | RFC 9685  |
       +-------+--------------------------------------+-----------+
       | 3     | Unassigned                           | RFC 9685  |
       +-------+--------------------------------------+-----------+

                     Table 2: P-Field Values Registry

14.2.  New EDAR Message Flags Registry

   IANA has created a new "EDAR Message Flags" registry under the
   "Internet Control Message Protocol version 6 (ICMPv6) Parameters"
   registry group.

   The registration procedure is IETF Review or IESG Approval [RFC8126].
   The initial allocations are as indicated in Table 3:

        +------------+------------------+------------------------+
        | Bit Number | Meaning          | Reference              |
        +------------+------------------+------------------------+
        | 0-1        | P-Field (2 bits) | RFC 9685, Section 14.1 |
        +------------+------------------+------------------------+
        | 2-7        | Unassigned       |                        |
        +------------+------------------+------------------------+

                   Table 3: EDAR Message Flags Registry

14.3.  New Address Registration Option Flags

   IANA has made an addition to the "Address Registration Option Flags"
   registry [IANA.ICMP.ARO.FLG] under the "Internet Control Message
   Protocol version 6 (ICMPv6) Parameters" registry group as indicated
   in Table 4:

        +------------+------------------+------------------------+
        | Bit Number | Description      | Reference              |
        +------------+------------------+------------------------+
        | 2-3        | P-Field (2 bits) | RFC 9685, Section 14.1 |
        +------------+------------------+------------------------+

              Table 4: New Address Registration Option Flags

14.4.  New RPL Target Option Flags

   IANA has made an addition to the "RPL Target Option Flags" registry
   [IANA.RPL.RTO.FLG] under the "Routing Protocol for Low Power and
   Lossy Networks (RPL)" registry group as indicated in Table 5:

     +------------+------------------------+------------------------+
     | Bit Number | Capability Description | Reference              |
     +------------+------------------------+------------------------+
     | 2-3        | P-Field (2 bits)       | RFC 9685, Section 14.1 |
     +------------+------------------------+------------------------+

                   Table 5: New RPL Target Option Flags

14.5.  New RPL Mode of Operation

   IANA has made an addition to the "Mode of Operation" registry
   [IANA.RPL.MOP] under the "Routing Protocol for Low Power and Lossy
   Networks (RPL)" registry group as indicated in Table 6:

       +-------+---------------------------------------+-----------+
       | Value | Description                           | Reference |
       +-------+---------------------------------------+-----------+
       | 5     | Non-Storing Mode of Operation with    | RFC 9685  |
       |       | ingress replication multicast support |           |
       +-------+---------------------------------------+-----------+

                     Table 6: New RPL Mode of Operation

14.6.  New 6LoWPAN Capability Bit

   IANA has made an addition to the "6LoWPAN Capability Bits" registry
   [IANA.ICMP.6CIO] under the "Internet Control Message Protocol version
   6 (ICMPv6) Parameters" registry group as indicated in Table 7:

     +-----+--------------------------------------------+-----------+
     | Bit | Description                                | Reference |
     +-----+--------------------------------------------+-----------+
     | 8   | X flag: Registration for Unicast,          | RFC 9685  |
     |     | Multicast, and Anycast Addresses Supported |           |
     +-----+--------------------------------------------+-----------+

                   Table 7: New 6LoWPAN Capability Bit

14.7.  New Address Registration Option Status Values

   IANA has made additions to the "Address Registration Option Status
   Values" registry [IANA.ICMP.ARO.STAT] under the "Internet Control
   Message Protocol version 6 (ICMPv6) Parameters" registry group as
   indicated in Table 8:

           +-------+------------------------------+-----------+
           | Value | Description                  | Reference |
           +-------+------------------------------+-----------+
           | 11    | Registration Refresh Request | RFC 9685  |
           +-------+------------------------------+-----------+
           | 12    | Invalid Registration         | RFC 9685  |
           +-------+------------------------------+-----------+

             Table 8: New Address Registration Option Status
                                  Values

14.8.  New IPv6 Neighbor Discovery Option Format

   IANA has made an addition to the "IPv6 Neighbor Discovery Option
   Formats" registry under the "Internet Control Message Protocol
   version 6 (ICMPv6) Parameters" registry group as indicated in
   Table 9:

              +------+--------------------------+-----------+
              | Type | Description              | Reference |
              +------+--------------------------+-----------+
              | 42   | Consistent Uptime Option | RFC 9685  |
              +------+--------------------------+-----------+

                Table 9: New IPv6 Neighbor Discovery Option
                                   Format

15.  References

15.1.  Normative References

   [IANA.ICMP]
              IANA, "Internet Control Message Protocol version 6
              (ICMPv6) Parameters",
              <https://www.iana.org/assignments/icmpv6-parameters>.

   [IANA.ICMP.6CIO]
              IANA, "6LoWPAN Capability Bits",
              <https://www.iana.org/assignments/icmpv6-parameters>.

   [IANA.ICMP.ARO.FLG]
              IANA, "Address Registration Option Flags",
              <https://www.iana.org/assignments/icmpv6-parameters>.

   [IANA.ICMP.ARO.STAT]
              IANA, "Address Registration Option Status Values",
              <https://www.iana.org/assignments/icmpv6-parameters>.

   [IANA.RPL] IANA, "Routing Protocol for Low Power and Lossy Networks
              (RPL)", <https://www.iana.org/assignments/rpl>.

   [IANA.RPL.MOP]
              IANA, "Mode of Operation",
              <https://www.iana.org/assignments/rpl>.

   [IANA.RPL.RTO.FLG]
              IANA, "RPL Target Option Flags",
              <https://www.iana.org/assignments/rpl>.

   [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>.

   [RFC3306]  Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
              Multicast Addresses", RFC 3306, DOI 10.17487/RFC3306,
              August 2002, <https://www.rfc-editor.org/info/rfc3306>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [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>.

   [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>.

   [RFC7346]  Droms, R., "IPv6 Multicast Address Scopes", RFC 7346,
              DOI 10.17487/RFC7346, August 2014,
              <https://www.rfc-editor.org/info/rfc7346>.

   [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>.

   [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>.

   [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>.

   [RFC8928]  Thubert, P., Ed., Sarikaya, B., Sethi, M., and R. Struik,
              "Address-Protected Neighbor Discovery for Low-Power and
              Lossy Networks", RFC 8928, DOI 10.17487/RFC8928, November
              2020, <https://www.rfc-editor.org/info/rfc8928>.

   [RFC9010]  Thubert, P., Ed. and M. Richardson, "Routing for RPL
              (Routing Protocol for Low-Power and Lossy Networks)
              Leaves", RFC 9010, DOI 10.17487/RFC9010, April 2021,
              <https://www.rfc-editor.org/info/rfc9010>.

   [RFC9030]  Thubert, P., Ed., "An Architecture for IPv6 over the Time-
              Slotted Channel Hopping Mode of IEEE 802.15.4 (6TiSCH)",
              RFC 9030, DOI 10.17487/RFC9030, May 2021,
              <https://www.rfc-editor.org/info/rfc9030>.

15.2.  Informative References

   [IEEE-802.11]
              IEEE, "IEEE Standard for Information Technology--
              Telecommunications and Information Exchange between
              Systems - Local and Metropolitan Area Networks--Specific
              Requirements - Part 11: Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) Specifications",
              DOI 10.1109/IEEESTD.2021.9363693, IEEE Std 802.11-2020,
              <https://ieeexplore.ieee.org/document/9363693>.

   [IEEE-802.15.1]
              IEEE, "IEEE Standard for Information technology - Local
              and metropolitan area networks - Specific requirements -
              Part 15.1a: Wireless Medium Access Control (MAC) and
              Physical Layer (PHY) specifications for Wireless Personal
              Area Networks (WPAN)", IEEE Std 802.15.1-2005,
              DOI 10.1109/IEEESTD.2005.96290,
              <https://ieeexplore.ieee.org/document/1490827>.

   [IEEE-802.15.4]
              IEEE, "IEEE Standard for Low-Rate Wireless Networks",
              DOI 10.1109/IEEESTD.2020.9144691, IEEE Std 802.15.4-2020,
              <https://ieeexplore.ieee.org/document/9144691>.

   [IPv6-OVER-WIRELESS]
              Thubert, P., Ed. and M. Richardson, "Architecture and
              Framework for IPv6 over Non-Broadcast Access", Work in
              Progress, Internet-Draft, draft-ietf-6man-ipv6-over-
              wireless-06, 23 May 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-6man-
              ipv6-over-wireless-06>.

   [MAC-SIGNALING]
              Thubert, P., Ed., Przygienda, T., and J. Tantsura, "Secure
              EVPN MAC Signaling", Work in Progress, Internet-Draft,
              draft-thubert-bess-secure-evpn-mac-signaling-04, 13
              September 2023, <https://datatracker.ietf.org/doc/html/
              draft-thubert-bess-secure-evpn-mac-signaling-04>.

   [REGISTRATION]
              Thubert, P., Ed., "IPv6 Neighbor Discovery Prefix
              Registration", Work in Progress, Internet-Draft, draft-
              ietf-6lo-prefix-registration-06, 9 November 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-6lo-
              prefix-registration-06>.

   [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>.

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              DOI 10.17487/RFC6052, October 2010,
              <https://www.rfc-editor.org/info/rfc6052>.

   [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>.

   [RFC6811]  Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
              Austein, "BGP Prefix Origin Validation", RFC 6811,
              DOI 10.17487/RFC6811, January 2013,
              <https://www.rfc-editor.org/info/rfc6811>.

   [RFC7731]  Hui, J. and R. Kelsey, "Multicast Protocol for Low-Power
              and Lossy Networks (MPL)", RFC 7731, DOI 10.17487/RFC7731,
              February 2016, <https://www.rfc-editor.org/info/rfc7731>.

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <https://www.rfc-editor.org/info/rfc7761>.

   [RFC8929]  Thubert, P., Ed., Perkins, C.E., and E. Levy-Abegnoli,
              "IPv6 Backbone Router", RFC 8929, DOI 10.17487/RFC8929,
              November 2020, <https://www.rfc-editor.org/info/rfc8929>.

   [RFC9008]  Robles, M.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", RFC 9008,
              DOI 10.17487/RFC9008, April 2021,
              <https://www.rfc-editor.org/info/rfc9008>.

   [RFC9574]  Rabadan, J., Ed., Sathappan, S., Lin, W., Katiyar, M., and
              A. Sajassi, "Optimized Ingress Replication Solution for
              Ethernet VPNs (EVPNs)", RFC 9574, DOI 10.17487/RFC9574,
              May 2024, <https://www.rfc-editor.org/info/rfc9574>.

   [RIFT]     Przygienda, A., Ed., Head, J., Ed., Sharma, A., Thubert,
              P., Rijsman, B., and D. Afanasiev, "RIFT: Routing in Fat
              Trees", Work in Progress, Internet-Draft, draft-ietf-rift-
              rift-24, 23 May 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-rift-
              rift-24>.

   [UPDATES-TAG]
              Kühlewind, M. and S. Krishnan, "Definition of new tags for
              relations between RFCs", Work in Progress, Internet-Draft,
              draft-kuehlewind-rswg-updates-tag-02, 8 July 2024,
              <https://datatracker.ietf.org/doc/html/draft-kuehlewind-
              rswg-updates-tag-02>.

   [Wi-SUN]   Robert, H., Liu, B., Zhang, M., and C. Perkins, "Wi-SUN
              FAN Overview", Work in Progress, Internet-Draft, draft-
              heile-lpwan-wisun-overview-00, 3 July 2017,
              <https://datatracker.ietf.org/doc/html/draft-heile-lpwan-
              wisun-overview-00>.

Acknowledgments

   This work is a production of an effective collaboration between the
   IETF 6lo WG and the Wi-Sun FAN WG.  Thanks to all in both WGs who
   contributed reviews and productive suggestions, in particular:
   Carsten Bormann, Paul Duffy, Klaus Hueske, Adnan Rashid, Rahul
   Jadhav, Gene Falendysz, Don Sturek, Dario Tedeschi, Saurabh Jain, and
   Chris Hett, with special thanks to Esko Dijk for his useful WGLC
   reviews and proposed changes.  Also many thanks to Éric Vyncke, Sandy
   Ginoza, Zaheduzzaman Sarker, Paul Wouters, Roman Danyliw, John
   Scudder, Dirk Von Hugo, Murray Kucherawy, Kyle Rose, Scott Kelly, and
   Dan Romascanu for their suggestions and comments during the IETF last
   call and IESG review cycle.

Author's Address

   Pascal Thubert (editor)
   06330 Roquefort-les-Pins
   France
   Email: pascal.thubert@gmail.com