IPv6 Neighbor Discovery Multicast Address Registration

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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) to provide 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 a given LLN.¶

This trades the quality of peer-to-peer (P2P) paths for a vastly reduced amount of control traffic and routing state that would be required to operate 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 Address Discovery (aka Lookup) andsime 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 Glocal 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 registration 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 register the multicast addresses they listen to.¶

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

3. Overview

[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 with [RFC8505].¶

This specification also extends [RFC6550] and [RFC9010] in the case of a route-over multilink subnet based on the RPL routing protocol. 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 mesh [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 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.¶

With this specification, the 6LNs can now register for the multicast addresses they listen to, using a new M flag in the EARO to signal a registration for a multicast address. Multiple 6LN can register for the same multicast address to the same 6LR. Note the use of the term "for", a node registers the unicast addresses that it owns, but registers for multicast addresses that it listens to.¶

If the R flag is set in the registration of one or more 6LNs for the same multicast address, the 6LR injects the multicast address in the RPL multicast support, based on the longest registration lifetime across those 6LNs.¶

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 registered for it as the leaves.¶

As prescribed in section 12 of [RFC6550], the 6LR forward the multicast packets as individual unicast MAC frames to each child that advertised the multicast address in its DAO message . In most LLNs a broadcast is unreliable (no ack) and forces a listener to remain awake, so it is expected that the 6LR also delivers the multicast packet as individual unicast MAC frames to each of the 6LNs that registered for the multicast address.¶

In the new RPL "Non-Storing Mode of Operation with multicast support" that is introduced here, the DAO messages register multicast addresses as Targets, though never as Transit. The multicast distribution is hub-and-spoke from the Root to all the 6LRs that are transit for the multicast address, using the same source-routing header as for unicast targets attahced to the 6LR, but for the ultimate entry that is the multicast address.¶

With this specification, the 6LNs can also register for the anycast addresses they listen to, using a new A flag in the EARO to signal a registration for an anycast address. Multiple 6LN can register for the same anycast address to the same 6LR, but the RPL routing ensures that only one of the 6LN gets the particular packet.¶

4. Extending RFC 7400

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

The new "Registration for Multicast Address Supported" (M) flag indicates to the 6LN that the 6LR accepts multicast address registrations as specified in this document and will ensure that packets for the multicast Registered Address will be routed to the 6LNs that registered with the R flag set.¶

Figure 2 illustrates the M 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   |M|A|D|L|B|P|E|G|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Reserved                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

New Option Field:¶

M

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

5. Updating RFC 6550

   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|A|M|ROVRsz | Prefix Length |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  |                Target Prefix (Variable Length)                |
  .                                                               .
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
 ...            Registration Ownership Verifier (ROVR)           ...
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6. 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|A|M| I |R|T|     TID       |     Registration Lifetime     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
 ...             Registration Ownership Verifier                 ...
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7. 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 behaviour:¶

• Upon a registration with the R and the M flags set to 1 in the EARO, the 6LR injects the address in the RPL multicast support.¶
• Upon receiving a packet directed to a multicast address for which it has at least one registration, 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.¶

8. 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 use a scope of Realm-Local whereas a multicast address that needs to span a RPL Instance MUST use 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 leveerage 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 form a multicast address and use the appropriate scope, Realm-Local or Admin-Local.¶

"Unicast-Prefix-based IPv6 Multicast Addresse" [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" deployement 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.¶

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 so as to cover all the 6LN that need multicast services.¶

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 operations and operating for the multicast MOP.¶

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 favouring the link quality up for unicast collection and one farouring 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 teh 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 4.¶
• 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.¶

9. 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.¶

10. Backward Compatibility

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

As detailed in Section 8, it is possible to add multicast on an existing MOP 1 deployment,¶

The combination of a multicast address and the M flag 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 M flag was set for backward compatibility.¶

When the DODAG is operated in MOP 3, a legacy node may fail will not set the M flag 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 M flag is set.¶

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