IPv6 Neighbor Discovery Multicast Address Listener Subscription

Status of This Memo

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

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

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This Internet-Draft will expire on 16 April 2023.¶

Copyright Notice

Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include 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

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 (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.¶

The "Routing Protocol for Low Power and Lossy Networks" [RFC6550] (RPL) 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 2 peers, whatever that would mean in each LLN.¶

This trades the quality of peer-to-peer (P2P) paths for a vastly reduced amount of control traffic and routing state that would be required to operate an any-to-any shortest path protocol. Additionally, broken routes may be fixed lazily and on-demand, based on dataplane inconsistency discovery, which avoids wasting energy in the proactive repair of unused paths.¶

Section 12 of [RFC6550] details the "Storing Mode of Operation with multicast support" with source-independent multicast routing in RPL.¶

The classical "IPv6 Neighbor Discovery (IPv6 ND) Protocol" [RFC4861] [RFC4862] was defined for serial links and shared transit media such as Ethernet at a time when broadcast was cheap on those media 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, which is a waste of energy, and imply that all nodes are awake to hear the request, which is inconsistent with power saving (sleeping) modes.¶

The original 6LoWPAN ND, "Neighbor Discovery Optimizations for 6LoWPAN networks" [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 the a 6LoWPAN Router(s) (6LR) that serves 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 6LoWPAN Neighbor Discovery" [RFC8505] updates [RFC6775] into a generic Address Registration mechanism that 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 provide return reachability for that address.¶

"Routing for RPL 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. But though 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 address.¶

The "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 to that support to subscribe the multicast addresses they listen to.¶

This specification extends [RFC8505] and [RFC9010] to add the capability for the 6LN to subscribe anycast and multicast addresses and for the 6LR to inject them in RPL when appropriate.¶

3. Overview

This specification inherits from [RFC6550], [RFC8505], and [RFC9010] to provide additional capabilities for anycast and multicast. Unless specified otherwise therein, the behavior of the 6LBR that acts as RPL Root, of the intermediate routers down the RPL graph, of the 6LR that act as access routers and of the 6LNs that are the RPL-unaware destinations, is the same as for unicast. In particular, forwarding a packet happens as specified in section 11 of [RFC6550], including loop avoidance and detection, though in the case of multicast multiple copies might be generated.¶

[RFC8505] is a pre-requisite to this specification. A node that implements this MUST also implement [RFC8505]. This specification does not introduce a new option; it modifies existing options and updates the associated behaviors to enable the Registration for Multicast Addresses as an extension to [RFC8505].¶

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

Figure 1 illustrates the classical situation of an LLN as a single IPv6 Subnet, with a 6LoWPAN Border Router (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 Std 802.11] and Bluetooth (Low Energy) [IEEE Std 802.15.1], or a Route-Over LLN such as the Wi-SUN and 6TiSCH meshes [Wi-SUN] that leverages 6LoWPAN [RFC4919][RFC6282] and RPL [RFC6550] over [IEEE Std 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|
                       +---+

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/EDAC exchange with the 6LBR to notify the 6LBR of the presence of the listeners.¶

This specification updates the EARO with a new two bit fields, 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 6LN may subscribe to the same multicast address to the same 6LR. Note the use of the term "subscribe": 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. As for multicast, multiple 6LN 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 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", 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 MAC frame to each peer along the multicast tree, excepting to the node it received the packet from.¶

In the new RPL "Non-Storing Mode of Operation with multicast support" that is introduced here, the DAO messages announce the multicast addresses as Targets though never as Transit. The multicast distribution is an ingress replication 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.¶

Broadcasting is typically unreliable in LLNs (no ack) and forces a listener to remain awake, so it generally discouraged. The expectation is thus that in either mode, the 6LRs deliver the multicast packets as individual unicast MAC frames to each of the 6LNs that subscribed to the multicast address.¶

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 from multiple children for the same anycast address, but 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 forwards a given packet to only one of them.¶

For backward compatibility, this specification allows to build a single DODAG signaled as MOP 1, that conveys anycast, unicast and multicast packets using the same source routing mechanism, 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 above 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 plain MAC-layer broadcast or snooping this protocol to control the flooding. It may also rely on overlay services to optimize the impact of Broadcast, Unknown and Multicast (BUM) over a fabric, e.g. registering with [I-D.thubert-bess-secure-evpn-mac-signaling] and forwarding with [I-D.ietf-bess-evpn-optimized-ir].¶

For instance, it is possible to operate a RPL Instance in the new "Non-Storing Mode of Operation with 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 there is.¶
• 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 configuration, it is also possible to control the behavior of the 6LR to ignore the R flag and 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, then MPL can really by used in conjunction with RPL Storing Mode as well.¶

4. Updating RFC 4861

Section 7.1 of [RFC4861] requires to silently discard NS and NA packets when the Target Address is a multicast address. This specification Amends [RFC4861] by allowing to advertise 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 xcast Address 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 2 illustrates the X flag in its suggested position (8, counting 0 to 15 in network order in the 16-bit array), to be confirmed by IANA.¶

    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                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

New Option Field:¶

X

1-bit flag: "Registration for Unicast, Multicast, and Anycast Addresses Supported"¶

6. Updating RFC 6550

[RFC6550] uses the Path Sequence in the Transit Information Option (TIO) to retain only the freshest unicast route and remove stale ones, e.g., in the case of mobility. [RFC9010] copies the TID from the EARO into the Path Sequence, and the ROVR field into the associated RPL Target Option (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 requires the use of the ROVR field as the indication of the origin of a Target advertisement in the RPL DAO messages, as specified in section 6.1 of [RFC9010]. 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 recognised across multiple hops.¶

This specification Extends [RFC6550] to require that, for anycast and multicast advertisements, the Path Sequence is 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 advertisement 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 (i.e. the lifetime is 0) using the same value for ROVR value as for the previous advertisements, that is either self or the single descendant that advertised the Target.¶

Note that the Registration Lifetime, TID and ROVR fields are also placed in the EDAR message so the state created by 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 applies also to EDAR.¶

   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)           ...
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7. Updating RFC 8505

   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                 ...
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  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                      +
 |                                                               |
 +                                                               +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

8. Updating RFC 9010

With [RFC9010]:¶

• The 6LR injects only unicast routes in RPL¶
• 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 the nodes that registered the unicast address.¶

This specification adds the following behavior:¶

• Upon a subscription with the R and the P field 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 M flag in the RTO.¶
• Upon a subscription with the R 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 a 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. Also, anycast and multicast addresses are not used to source traffic. In that context, the method specified in [RFC8928] cannot be used with autoconfigured keypairs 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 keypair MUST be associated with the address before it is deployed, and a ROVR MUST be generated from that keypair 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 keypair 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 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 NA, 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         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

To be assigned by IANA, see Table 10¶

Length

1¶

S

1-bit flag, set to 1 to indicate that the sender is low-power and may sleep.¶

U

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, reserved. MUST be set to 0 by the sender and ignored by the receiver.¶

NSSI

12-bits unsigned integer: The Node State Sequence Information, MUST be stored by the receiver if it has a dependency on information advertised or stored at the sender.¶

Peer NSSI

12-bits unsigned integer: Echoes the last known NSSI from the peer.¶

Uptime Exponent

6-bits unsigned integer: The 2-exponent of the uptime unit¶

Uptime Mantissa

10-bits unsigned integer: The mantissa of the uptime value¶

The Node Uptime indicates how long the sender has been continuously up and running without loss of state. It is expressed by the Uptime Mantissa in units of 2 at the power of the Uptime Exponent milliseconds. The receiver derives the boot time of the sender as the current Epoch the sender's Node Uptime. If the sender rebooted since last time state was installed, any state that was installed in the sender before it rebooted needs to be reassessed and reinstalled if it is still needed. 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 if the uptime is reset to 0 at some point of the sender's reboot sequence, but may not be still 0 when the first message is sent, so the receiver must not expect a value of 0 as the signal of a reboot.¶

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 has updated some state in a fashion that will that 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 loss.¶

Any change in the value of the NSSI from a node is an indication that the node updated some state and that the needful state should be reinstalled, e.g., addresses that where formed based on an RA with a previous NSSI should be reassessed, and the registration state updated in the peer.¶

11. Deployment considerations

With this specification, a RPL DODAG forms a realm, and multiple RPL DODAGs may 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 "IPv6 Multicast Address Scopes" [RFC7346].¶

"IPv6 Addressing of IPv4/IPv6 Translators" [RFC6052] enables to embed 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 an 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 to form 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. It is for instance 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.¶

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 co-exist with upgraded devices, it is RECOMMENDED to deploy one RPL Instance in any Mode of Operation (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 there is enough density of RPL routers that support MOP 3 to build a DODAG that covers all the potential listeners and include 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 can reach at least one such 6LR, so the upgraded 6LRs must be deployed to cover all the 6LN that need multicast services.¶

Using separate RPL Instances for in the one hand unicast traffic and in the other hand anycast and multicast traffic allows to use different objective function, one favoring the link quality up for unicast collection and one favoring downwards link quality for multicast distribution.¶

But 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 6LRs that support the mixed mode to cover the all the 6LNs that require multicast or anycast services. In Storing Mode, there MUST be enough density or 6LR that support the mixed mode to also form a DODAG to the Root.¶
• The RPL routers that support the mixed mode and are configured to operate in in accordance with the desired operation in the network.¶
• The MOP signaled in the RPL DODAG Information Object (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 such as 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 the security section of that document also applies to this document. In particular, the link layer SHOULD be sufficiently protected to prevent rogue access.¶

Section 9 leverages [RFC8928] to prevent an rogue node to register 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 is known in advance, but how this is done is not in scope for this specification. One way would be to authorize in a advance the ROVR of the valid users. A less preferred way could be to synchronize the ROVR and TID values across the valid subscribers as a 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 a 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 P field in the 6CIO and an upgraded 6LN will not subscribe multicast addresses.¶

Upon an EDAR message, a legacy 6LBR may not realize that the address being registered is anycast or multicast, and return that it is duplicate in the EDAC status. The 6LR MUST ignore a duplicate status in the EDAR 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 understood by the receiver that supports this specification (the parent) as an indication that the sender (child) does not support this specification, but 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 M bit set to 0 MUST be considered as multicast and MUST be processed as if the P field is set to 1.¶

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