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RFC 9010

Routing for RPL (Routing Protocol for Low-Power and Lossy Networks) Leaves

Pages: ~36
Proposed Standard
Updates:  655067758505

Top   ToC   RFCv3-9010
P Thubert, Ed.
Cisco Systems, Inc.
M. Richardson
Sandelman Software Works
April 2021

Routing for RPL (Routing Protocol for Low-Power and Lossy Networks) Leaves


This specification provides a mechanism for a host that implements a routing-agnostic interface based on IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Neighbor Discovery to obtain reachability services across a network that leverages RFC 6550 for its routing operations. It updates RFCs 6550, 6775, and 8505.

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

Copyright Notice

Copyright (c) 2021 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 ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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Table of Contents

Top   ToC   RFCv3-9010

1.  Introduction

The design of Low-Power and Lossy Networks (LLNs) is generally focused on saving energy, which is the most constrained resource of all. Other design constraints, such as a limited memory capacity, duty cycling of the LLN devices, and low-power lossy transmissions, derive from that primary concern.
The IETF produced "[RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks]" [RFC 6550] to provide routing services for IPv6 [RFC 8200] within such constraints. RPL belongs to the class of distance-vector protocols -- which, compared to link-state protocols, limit the amount of topological knowledge that needs to be installed and maintained in each node -- and does not require convergence to avoid micro-loops.
To save signaling and routing state in constrained networks, RPL allows a path stretch (see [RFC 6687]), whereby 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, 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 data-plane inconsistency discovery, which avoids wasting energy in the proactive repair of unused paths.
For many of the nodes, though not all, the DODAG provides multiple forwarding solutions towards the root of the topology via so-called parents. RPL installs the routes proactively, but to adapt to fuzzy connectivity -- whereby the physical topology cannot be expected to reach a stable state -- it uses a lazy route maintenance operation that may only fix them reactively, upon actual traffic. The result is that RPL provides reachability for most of the LLN nodes, most of the time, but may not converge in the classical sense.
RPL can be deployed in conjunction with IPv6 Neighbor Discovery (ND) [RFC 4861] [RFC 4862] and IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) ND [RFC 6775] [RFC 8505] to maintain reachability within a Non-Broadcast Multi-Access (NBMA) multi-link subnet.
In that mode, IPv6 addresses are advertised individually as host routes. Some nodes may act as routers and participate in the forwarding operations, whereas others will only receive/originate packets, acting as hosts in the data plane. Per the terminology of [RFC 6550], an IPv6 host [RFC 8504] that is reachable over the RPL network is called a "leaf".
Section 2 of RFC 9008 defines the terms "RPL leaf", "RPL-Aware Leaf" (RAL), and "RPL-Unaware Leaf" (RUL). A RPL leaf is a host attached to one or more RPL routers; as such, it relies on the RPL router(s) to forward its traffic across the RPL domain but does not forward traffic from another node. As opposed to the RAL, the RUL does not participate in RPL and relies on its RPL router(s) to also inject the routes to its IPv6 addresses in the RPL domain.
A RUL may be unable to participate because it is very energy constrained or code-space constrained, or because it would be unsafe to let it inject routes in RPL. Using 6LoWPAN ND as opposed to RPL as the host-to-router interface limits the surface of the possible attacks by the RUL against the RPL domain. If all RULs and RPL-Aware Nodes (RANs) use 6LoWPAN ND for the neighbor discovery process, it is also possible to protect the address ownership of all nodes, including the RULs.
This document specifies how the router injects the host routes in the RPL domain on behalf of the RUL. Section 5 details how the RUL can leverage 6LoWPAN ND to obtain the routing services from the router. In that model, the RUL is also a 6LoWPAN Node (6LN) and the RPL-aware router is also a 6LoWPAN Router (6LR). Using the 6LoWPAN ND Address Registration mechanism, the RUL signals that the router must inject a host route for the Registered Address.
               |          Internet
            |     | <------------- 6LBR / RPL DODAG Root
            +-----+                     ^
               |                        |
         o    o   o  o                  | RPL
     o o   o  o   o  o     o    o       |
    o  o o  o o    o   o  o   o  o      |  +
    o   o      o     o   o   o    o     |
   o  o   o  o   o  o    o    o  o      | 6LoWPAN ND
      o  o  o  o        o   o           |
     o       o            o    o        v
   o      o     o <------------- 6LR / RPL Border Router
                                        | 6LoWPAN ND only
                u <------------- 6LN / RPL-Unaware Leaf
The RPL Non-Storing mode mechanism is used to extend the routing state with connectivity to the RULs even when the DODAG is operated in Storing mode. The unicast packet-forwarding operation by the 6LR serving a RUL is described in Section 4.1.1 of RFC 9008.
Examples of possible RULs include severely energy-constrained sensors such as window smash sensors (alarm system) and kinetically powered light switches. Other applications of this specification may include a smart grid network that controls appliances -- such as washing machines or the heating system -- in the home. Appliances may not participate in the RPL protocol operated in the smart grid network but can still interact with the smart grid for control and/or metering.
This specification can be deployed incrementally in a network that implements [RFC 9008]. Only the root and the 6LRs that connect the RULs need to be upgraded. The RPL routers on the path will only see unicast IPv6 traffic between the root and the 6LR.
This document is organized as follows:
  • Sections [3] and [4] present in a non-normative fashion the salient aspects of RPL and 6LoWPAN ND, respectively, that are leveraged in this specification to provide connectivity to a 6LN acting as a RUL across a RPL network.
  • Section 5 lists the requirements that a RUL needs to match in order to be served by a RPL router that complies with this specification.
  • Section 6 presents the changes made to [RFC 6550]; a new behavior is introduced whereby the 6LR advertises the 6LN's addresses in a RPL Destination Advertisement Object (DAO) message based on the ND registration by the 6LN, and the RPL DODAG root performs the Extended Duplicate Address Request / Extended Duplicate Address Confirmation (EDAR/EDAC) exchange with the 6LoWPAN Border Router (6LBR) on behalf of the 6LR; modifications are introduced to some RPL options and to the RPL Status to facilitate the integration of the protocols.
  • Section 7 presents the changes made to [RFC 9009]; the use of the Destination Cleanup Object (DCO) message is extended to the Non-Storing RPL Mode of Operation (MOP) to report asynchronous issues from the root to the 6LR.
  • Section 8 presents the changes made to [RFC 6775] and [RFC 8505]; the range of the Address Registration Option / Extended Address Registration Option (ARO/EARO) Status values is reduced to 64 values, and the remaining bits in the original status field are now reserved.
  • Sections [9] and [10] present the operation of this specification for unicast and multicast flows, respectively, and Section 11 presents associated security considerations.
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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 [RFC 2119] [RFC 8174] when, and only when, they appear in all capitals, as shown here.

2.2.  Glossary

This document uses the following abbreviations:
6LoWPAN Backbone Router
6LoWPAN Capability Indication Option
6LoWPAN Border Router
6LoWPAN Node (a low-power host or router)
6LoWPAN Routing Header
IPv6 over Low-Power Wireless Personal Area Network
6LoWPAN Router
Address-Protected Neighbor Discovery
Address Registration Option
Duplicate Address Confirmation
Duplicate Address Detection
Destination Advertisement Object (a RPL message)
Duplicate Address Request
Destination Cleanup Object (a RPL message)
DODAG Information Object (a RPL message)
Destination-Oriented Directed Acyclic Graph
Extended Address Registration Option
Extended Duplicate Address Confirmation
Extended Duplicate Address Request
Extended Unique Identifier
Low-Power and Lossy Network
Multicast Listener Discovery
RPL Mode of Operation
Neighbor Advertisement
Non-Broadcast Multi-Access
Neighbor Cache Entry
Neighbor Discovery
Neighbor Solicitation
Prefix Information Option
Router Advertisement
RPL-Aware Leaf
RPL-Aware Node (either a RPL router or a RPL-Aware Leaf)
Routing Header for IPv6 (type 3)
Registration Ownership Verifier
RPL Packet Information
Routing Protocol for Low-Power and Lossy Networks
RPL-Unaware Leaf
Source Address Validation Improvement
Stateless Address Autoconfiguration
Source Routing Header
Transaction ID (a sequence counter in the EARO)
Transit Information Option

2.3.  Related Documents

The terminology used in this document is consistent with, and incorporates the terms provided in, "[Terms Used in Routing for Low-Power and Lossy Networks]" [RFC 7102]. A glossary of classical 6LoWPAN abbreviations is given in Section 2.2. Other terms in use in LLNs are found in "[Terminology for Constrained-Node Networks]" [RFC 7228]. This specification uses the terms "6LN" and "6LR" to refer specifically to nodes that implement the 6LN and 6LR roles in 6LoWPAN ND and does not expect other functionality such as 6LoWPAN Header Compression [RFC 6282] from those nodes.
"RPL", "RPI", "RPL Instance" (indexed by a RPLInstanceID), "up", and "down" are defined in "[RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks]" [RFC 6550]. The RPI is the abstract information that RPL defines to be placed in data packets, e.g., as the RPL Option [RFC 6553] within the IPv6 Hop-By-Hop Header. By extension, the term "RPI" is often used to refer to the RPL Option itself. The DAO and DIO messages are also specified in [RFC 6550]. The DCO message is defined in [RFC 9009].
This document uses the terms "RUL", "RAN", and "RAL" consistently with [RFC 9008]. A RAN is either a RAL or a RPL router. As opposed to a RUL, a RAN manages the reachability of its addresses and prefixes by injecting them in RPL by itself.
In this document, readers will encounter terms and concepts that are discussed in the following documents:
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3.  RPL External Routes and Data-Plane Artifacts

RPL was initially designed to build stub networks whereby the only border router would be the RPL DODAG root (typically co-located with the 6LBR) and all the nodes in the stub would be RPL aware. But [RFC 6550] was also prepared to be extended for external routes ("targets" in RPL parlance), via the External ('E') flag in the Transit Information Option (TIO). External targets provide the ability to reach destinations that are outside the RPL domain and connected to the RPL domain via RPL border routers that are not the root. Section 4.1 of RFC 9008 provides a set of rules (summarized below) that must be followed for routing packets to and from an external destination. A RUL is a special case of an external target that is also a host directly connected to the RPL domain.
A 6LR that acts as a border router for external routes advertises them using Non-Storing mode DAO messages that are unicast directly to the root, even if the DODAG is operated in Storing mode. Non-Storing mode routes are not visible inside the RPL domain, and all packets are routed via the root. The RPL DODAG root tunnels the data packets directly to the 6LR that advertised the external route, which decapsulates and forwards the original (inner) packets.
The RPL Non-Storing MOP signaling and the associated IPv6-in-IPv6 encapsulated packets appear as normal traffic to the intermediate routers. Support of external routes only impacts the root and the 6LR. It can be operated with legacy intermediate routers and does not add to the amount of state that must be maintained in those routers. A RUL is an example of a destination that is reachable via an external route that happens to also be a host route.
The RPL data packets typically carry a Hop-by-Hop Header with a RPL Option [RFC 6553] that contains the RPI (the RPL Packet Information, as defined in Section 11.2 of RFC 6550). Unless the RUL already placed a RPL Option in the outer header chain, the packets from and to the RUL are encapsulated using an IPv6-in-IPv6 tunnel between the root and the 6LR that serves the RUL (see Sections 7 and 8 of [RFC 9008] for details). If the packet from the RUL has an RPI, the 6LR acting as a RPL border router rewrites the RPI to indicate the selected RPL Instance and set the flags, but it does not need to encapsulate the packet (see Section 9.2.2).
In Non-Storing mode, packets going down the DODAG carry a Source Routing Header (SRH). The IPv6-in-IPv6 encapsulation, the RPI, and the SRH are collectively called the "RPL artifacts" and can be compressed using the method defined in [RFC 8138]. Appendix A presents an example compressed format for a packet forwarded by the root to a RUL in a Storing mode DODAG.
The inner packet that is forwarded to the RUL may carry some RPL artifacts, e.g., an RPI if the original packet was generated with it, and an SRH in a Non-Storing mode DODAG. [RFC 9008] expects the RUL to support the basic IPv6 node requirements per [RFC 8504] and, in particular, the mandates in Sections 4.2 and 4.4 of [RFC 8200]. As such, the RUL is expected to ignore the RPL artifacts that may be left over -- either an SRH whose Segments Left is zero or a RPL Option in the Hop-by-Hop Header (which can be skipped when not recognized; see Section 5.3 for details).
A RUL is not expected to support the compression method defined in [RFC 8138]. For that reason, the border router (the 6LR here) uncompresses the packet before forwarding it over an external route to a RUL [RFC 9008].
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4.  6LoWPAN Neighbor Discovery

This section goes through the 6LoWPAN ND mechanisms that this specification leverages, as a non-normative reference to the reader. The full normative text is to be found in [RFC 6775], [RFC 8505], and [RFC 8928].

4.1.  Address Registration per RFC 6775

The classical IPv6 Neighbor Discovery (IPv6 ND) protocol [RFC 4861] [RFC 4862] was defined for serial links and transit media such as Ethernet. It is a reactive protocol that relies heavily on multicast operations for Address Discovery (aka address lookup) and Duplicate Address Detection (DAD).
"[Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)]" [RFC 6775] adapts IPv6 ND for operations over energy-constrained LLNs. The main functions of [RFC 6775] are to proactively establish the Neighbor Cache Entry (NCE) in the 6LR and to prevent address duplication. To that effect, [RFC 6775] introduces a unicast Address Registration mechanism that contributes to reducing the use of multicast messages compared to the classical IPv6 ND protocol.
[RFC 6775] also introduces the Address Registration Option (ARO), which is carried in the unicast Neighbor Solicitation (NS) and Neighbor Advertisement (NA) messages between the 6LoWPAN Node (6LN) and the 6LoWPAN router (6LR). It also defines the Duplicate Address Request (DAR) and Duplicate Address Confirmation (DAC) messages between the 6LR and the 6LBR). In an LLN, the 6LBR is the central repository of all the Registered Addresses in its domain and the source of truth for uniqueness and ownership.

4.2.  Extended Address Registration per RFC 8505

"[Registration Extensions for IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Neighbor Discovery]" [RFC 8505] updates RFC 6775 with a generic Address Registration mechanism that can be used to access services such as routing and ND proxy functions. To that effect, [RFC 8505] defines the Extended Address Registration Option (EARO), as shown in Figure 2:
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |     Type      |     Length    |    Status     |    Opaque     |
  |  Rsvd | I |R|T|     TID       |     Registration Lifetime     |
  |                                                               |
 ...          Registration Ownership Verifier (ROVR)             ...
  |                                                               |

4.2.1.  R Flag

[RFC 8505] introduces the R flag in the EARO. The Registering Node sets the R flag to indicate whether the 6LR should ensure reachability for the Registered Address. If the R flag is set to 0, then the Registering Node handles the reachability of the Registered Address by other means. In a RPL network, this means that either it is a RAN that injects the route by itself or it uses another RPL router for reachability services.
This document specifies how the R flag is used in the context of RPL. A RPL leaf that implements the 6LN functionality from [RFC 8505] requires reachability services for an IPv6 address if and only if it sets the R flag in the NS(EARO) used to register the address to a 6LR acting as a RPL border router. Upon receiving the NS(EARO), the RPL router generates a DAO message for the Registered Address if and only if the R flag is set to 1.
Section 9.2 specifies additional operations when the R flag is set to 1 in an EARO that is placed in either an NS message or an NA message.

4.2.2.  TID, "I" Field, and Opaque Field

When the T flag is set to 1, the EARO includes a sequence counter called the "Transaction ID" (TID), which is needed to fill the Path Sequence field in the RPL Transit Information Option (TIO). For this reason, support of [RFC 8505] by the RUL, as opposed to only [RFC 6775], is a prerequisite for this specification; this requirement is fully explained in Section 5.1. The EARO also transports an Opaque field and an associated "I" field that describes what the Opaque field transports and how to use it.
Section 9.2.1 specifies the use of the "I" field and the Opaque field by a RUL.

4.2.3.  Route Ownership Verifier

Section 5.3 of RFC 8505 introduces the Registration Ownership Verifier (ROVR) field, which has a variable length of 64 to 256 bits. The ROVR replaces the 64-bit Extended Unique Identifier (EUI-64) in the ARO [RFC 6775], which was used to uniquely identify an Address Registration with the link-layer address of the owner but provided no protection against spoofing.
"[Address-Protected Neighbor Discovery for Low-Power and Lossy Networks]" [RFC 8928] leverages the ROVR field as a cryptographic proof of ownership to prevent a rogue third party from registering an address that is already owned. The use of the ROVR field enables the 6LR to block traffic that is not sourced at an owned address.
This specification does not address how the protection offered by [RFC 8928] could be extended for use in RPL. On the other hand, it adds the ROVR to the DAO to build the proxied EDAR at the root (see Section 6.1), which means that nodes that are aware of the host route are also aware of the ROVR associated to the Target Address.

4.3.  EDAR/EDAC per RFC 8505

[RFC 8505] updates the DAR/DAC messages to EDAR/EDAC messages to carry the ROVR field. The EDAR/EDAC exchange takes place between the 6LR and the 6LBR. It is triggered by an NS(EARO) message from a 6LN to create, refresh, and delete the corresponding state in the 6LBR. The exchange is protected by the retry mechanism specified in Section 8.2.6 of RFC 6775, though in an LLN, a duration longer than the default value of the RetransTimer (RETRANS_TIMER) [RFC 4861] of 1 second may be necessary to cover the round-trip delay between the 6LR and the 6LBR.
RPL [RFC 6550] specifies a periodic DAO from the 6LN all the way to the root that maintains the routing state in the RPL network for the lifetime indicated by the source of the DAO. This means that for each address, there are two keep-alive messages that traverse the whole network: one to the root and one to the 6LBR.
This specification avoids the periodic EDAR/EDAC exchange across the LLN. The 6LR turns the periodic NS(EARO) from the RUL into a DAO message to the root on every refresh, but it only generates the EDAR upon the first registration, for the purpose of DAD, which must be verified before the address is injected in RPL. Upon the DAO message, the root proxies the EDAR exchange to refresh the state at the 6LBR on behalf of the 6LR, as illustrated in Figure 8 in Section 9.1.

4.3.1.  Capability Indication Option per RFC 7400

"[6LoWPAN-GHC: Generic Header Compression for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)]" [RFC 7400] defines the 6LoWPAN Capability Indication Option (6CIO), which enables a node to expose its capabilities in Router Advertisement (RA) messages.
[RFC 8505] defines a number of bits in the 6CIO; in particular:
The node is a 6LR.
The node is an IPv6 ND Registrar -- i.e., it supports registrations based on EARO.
The node is a Routing Registrar -- i.e., an IPv6 ND Registrar that also provides reachability services for the Registered Address.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |     Type      |   Length = 1  |     Reserved      |D|L|B|P|E|G|
   |                           Reserved                            |
A 6LR that provides reachability services for a RUL in a RPL network as specified in this document includes a 6CIO in its RA messages and set the L, P, and E flags to 1 as prescribed by [RFC 8505]; this is fully explained in Section 9.2.
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5.  Requirements for the RPL-Unaware Leaf

This document describes how RPL routing can be extended to reach a RUL. This section specifies the minimal RPL-independent functionality that the RUL needs to implement in order to obtain routing services for its addresses.

5.1.  Support of 6LoWPAN ND

To obtain routing services from a router that implements this specification, a RUL needs to implement [RFC 8505] and sets the "R" and "T" flags in the EARO to 1 as discussed in Sections [4.2.1] and [4.2.2], respectively. Section 9.2.1 specifies new behaviors for the RUL, e.g., when the R flag set to 1 in an NS(EARO) is not echoed in the NA(EARO), which indicates that the route injection failed.
The RUL is expected to request routing services from a router only if that router originates RA messages with a 6CIO that has the L, P, and E flags all set to 1 as discussed in Section 4.3.1, unless configured to do so. It is suggested that the RUL also implement [RFC 8928] to protect the ownership of its addresses.
A RUL that may attach to multiple 6LRs is expected to prefer those that provide routing services. The RUL needs to register with all the 6LRs from which it desires routing services.
Parallel Address Registrations to several 6LRs should be performed in a rapid sequence, using the same EARO for the same address. Gaps between the Address Registrations will invalidate some of the routes until the Address Registration finally shows on those routes.
[RFC 8505] introduces error Status values in the NA(EARO) that can be received synchronously upon an NS(EARO) or asynchronously. The RUL needs to support both cases and refrain from using the address when the Status value indicates a rejection (see Section 6.3).

5.2.  Support of IPv6 Encapsulation

Section 4.1.1 of RFC 9008 defines the rules for signaling an external destination (e.g., a RUL) and tunneling to its attachment router (designated as a 6LR). In order to terminate the IPv6-in-IPv6 tunnel, the RUL, as an IPv6 host, would have to be capable of decapsulating the tunneled packet and either drop the encapsulated packet if it is not the final destination or pass it to the upper layer for further processing. As indicated in Section 4.1 of RFC 9008, this is not mandated by [RFC 8504], and the IPv6-in-IPv6 tunnel from the root is terminated at the parent 6LR. It is thus not necessary for a RUL to support IPv6-in-IPv6 decapsulation.

5.3.  Support of the Hop-by-Hop Header

A RUL is expected to process an Option Type in a Hop-by-Hop Header as prescribed by Section 4.2 of RFC 8200. An RPI with an Option Type of 0x23 [RFC 9008] is thus skipped when not recognized.

5.4.  Support of the Routing Header

A RUL is expected to process an unknown Routing Header Type as prescribed by Section 4.4 of RFC 8200. This implies that the SRH, which has a Routing Type of 3 [RFC 6554], is ignored when Segments Left is zero. When Segments Left is non-zero, the RUL discards the packet and sends an ICMP Parameter Problem message with Code 0 to the packet's source address, pointing to the unrecognized Routing Type.
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6.  Enhancements to RFC 6550

This document specifies a new behavior whereby a 6LR injects DAO messages for unicast addresses (see Section 9) and multicast addresses (see Section 10) on behalf of leaves that are not aware of RPL. The RUL addresses are exposed as external targets [RFC 6550]. Conforming to [RFC 9008], IPv6-in-IPv6 encapsulation between the 6LR and the RPL DODAG root is used to carry the RPL artifacts and remove them when forwarding outside the RPL domain, e.g., to a RUL.
This document also synchronizes the liveness monitoring at the root and the 6LBR. The same lifetime value is used for both, and a single keep-alive message, the RPL DAO, traverses the RPL network. Another new behavior is introduced whereby the RPL DODAG root proxies the EDAR message to the 6LBR on behalf of the 6LR (see Section 8), for any leaf node that implements the 6LN functionality described in [RFC 8505].
Section 6.7.7 of RFC 6550 introduces the RPL Target option, which can be used in RPL control messages such as the DAO message to signal a destination prefix. This document adds capabilities for transporting the ROVR field (see Section 4.2.3) and the IPv6 address of the prefix advertiser when the Target is a shorter prefix. Their use is signaled by a new ROVR Size field being non-zero and a new "Advertiser address in Full (F)" flag set to 1, respectively; see Section 6.1.
This specification defines a new flag, "Root Proxies EDAR/EDAC (P)", in the RPL DODAG Configuration option; see Section 6.2.
Furthermore, this specification provides the ability to carry the EARO Status defined for 6LoWPAN ND in RPL DAO and DCO messages, embedded in a RPL Status; see Section 6.3.
Section 12 of RFC 6550 details RPL support for multicast flows when the RPL Instance is operated with a MOP setting of 3 ("Storing Mode of Operation with multicast support"). This specification extends the RPL DODAG root operation to proxy-relay the MLDv2 operation [RFC 3810] between the RUL and the 6LR; see Section 10.

6.1.  Updated RPL Target Option

This specification updates the RPL Target option to transport the ROVR that was also defined for 6LoWPAN ND messages. This enables the RPL DODAG root to generate the proxied EDAR message to the 6LBR.
The Target Prefix of the RPL Target option is left (high bit) justified and contains the advertised prefix; its size may be smaller than 128 when it indicates a prefix route. The Prefix Length field signals the number of bits that correspond to the advertised prefix; it is 128 for a host route or less in the case of a prefix route. This remains unchanged.
This specification defines the new 'F' flag. When it is set to 1, the size of the Target Prefix field MUST be 128 bits and it MUST contain an IPv6 address of the advertising node taken from the advertised prefix. In that case, the Target Prefix field carries two distinct pieces of information: a route that can be a host route or a prefix route, depending on the Prefix Length; and an IPv6 address that can be used to reach the advertising node and validate the route.
If the 'F' flag is set to 0, the Target Prefix field can be shorter than 128 bits, and it MUST be aligned to the next byte boundary after the end of the prefix. Any additional bits in the rightmost octet are filled with padding bits. Padding bits are reserved and set to 0 as specified in Section 6.7.7 of RFC 6550.
With this specification, the ROVR is the remainder of the RPL Target option. The size of the ROVR is indicated in a new ROVR Size field that is encoded to map one to one with the Code Suffix in the EDAR message (see Table 4 of [RFC 8505]). The ROVR Size field is taken from the Flags field, which is an update to the "RPL Target Option Flags" IANA registry.
The updated format is illustrated in Figure 4. It is backward compatible with the Target option defined in [RFC 6550]. It is recommended that the updated format be used as a replacement in new implementations in all MOPs in preparation for upcoming route ownership validation mechanisms based on the ROVR, unless the device or the network is so constrained that this is not feasible.
   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|Flg|ROVRsz | Prefix Length |
  |                                                               |
  |                Target Prefix (Variable Length)                |
  .                                                               .
  |                                                               |
 ...            Registration Ownership Verifier (ROVR)           ...
  |                                                               |
New fields:
1-bit flag. Set to 1 to indicate that the Target Prefix field contains the complete (128-bit) IPv6 address of the advertising node.
1-bit flag. Set to 1 to request that the root perform a proxy EDAR/EDAC exchange.
The 'X' flag can only be set to 1 if the DODAG is operating in Non-Storing mode and if the root sets the "Root Proxies EDAR/EDAC (P)" flag to 1 in the DODAG Configuration option; see Section 6.2.
The 'X' flag can be set for host routes to RULs and RANs; it can also be set for internal prefix routes if the 'F' flag is set, using the node's address in the Target Prefix field to form the EDAR, but it cannot be used otherwise.
Flg (Flags):
The 2 bits remaining unused in the Flags field are reserved for flags. The field MUST be initialized to 0 by the sender and MUST be ignored by the receiver.
ROVRsz (ROVR Size):
Indicates the size of the ROVR. It MUST be set to 1, 2, 3, or 4, indicating a ROVR size of 64, 128, 192, or 256 bits, respectively.
If a legacy Target option is used, then the value must remain 0, as specified in [RFC 6550].
In the case of a value above 4, the size of the ROVR is undetermined and this node cannot validate the ROVR; an implementation SHOULD propagate the whole Target option upwards as received to enable the verification by an ancestor that would support the upgraded ROVR.
Registration Ownership Verifier (ROVR):
This is the same field as in the EARO; see [RFC 8505].

6.2.  Additional Flag in the RPL DODAG Configuration Option

The DODAG Configuration option is defined in Section 6.7.6 of RFC 6550. Its purpose is extended to distribute configuration information affecting the construction and maintenance of the DODAG, as well as operational parameters for RPL on the DODAG, through the DODAG. This option was originally designed with four bit positions reserved for future use as 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 = 0x04 |Opt Length = 14| |P| | |A|       ...           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                     +
                                |4 bits |
This specification defines a new flag, "Root Proxies EDAR/EDAC (P)". The 'P' flag is encoded in bit position 1 of the reserved flags in the DODAG Configuration option (counting from bit 0 as the most significant bit), and it is set to 0 in legacy implementations as specified in Sections 20.14 and 6.7.6 of [RFC 6550], respectively.
The 'P' flag is set to 1 to indicate that the root performs the proxy operation, which implies that it supports this specification and the updated RPL Target option (see Section 6.1).
Section 4.1.3 of RFC 9008 updates [RFC 6550] to indicate that the definition of the flags applies to MOP values from zero (0) to six (6) only. For a MOP value of 7, the implementation MUST assume that the root performs the proxy operation.
The RPL DODAG Configuration option is typically placed in a DODAG Information Object (DIO) message. The DIO message propagates down the DODAG to form and then maintain its structure. The DODAG Configuration option is copied unmodified from parents to children. [RFC 6550] states that "Nodes other than the DODAG root MUST NOT modify this information when propagating the DODAG Configuration option." Therefore, a legacy parent propagates the 'P' flag as set by the root, and when the 'P' flag is set to 1, it is transparently flooded to all the nodes in the DODAG.

6.3.  Updated RPL Status

The RPL Status is defined in Section 6.5.1 of RFC 6550 for use in the DAO-ACK message. Values are assigned as follows:
Range Meaning
0 Success / Unqualified acceptance
1-127 Not an outright rejection
128-255 Rejection
Table 1: RPL Status per RFC 6550
The 6LoWPAN ND Status was defined for use in the EARO; see Section 4.1 of RFC 8505. This specification adds the ability to allow the carriage of 6LoWPAN ND Status values in RPL DAO and DCO messages, embedded in the RPL Status field.
To achieve this, the range of the ARO/EARO Status values is reduced to 0-63, which updates the IANA registry created for [RFC 6775]. This reduction ensures that the values fit within a RPL Status as shown in Figure 6. See Sections [12.2], [12.5], and [12.6] for the respective IANA declarations. These updates are reasonable because the associated registry relies on the Standards Action policy [RFC 8126] for registration and only values up to 10 are currently allocated.
    0 1 2 3 4 5 6 7
This specification updates the RPL Status with the following subfields:
1-bit flag. Set to 1 to indicate a rejection. When set to 0, a Status value of 0 indicates Success / Unqualified acceptance and other values indicate "Not an outright rejection" as per RFC 6550.
1-bit flag. Indicates the type of the RPL Status value.
Status Value:
6-bit unsigned integer.
If the 'A' flag is set to 1, this field transports a value defined for the 6LoWPAN ND EARO Status.
When the 'A' flag is set to 0, this field transports a Status value defined for RPL.
When building a DCO or a DAO-ACK message upon an IPv6 ND NA or an EDAC message, the RPL DODAG root MUST copy the 6LoWPAN ND status code unchanged in the RPL Status Value field and set the 'A' flag to 1. The RPL DODAG root MUST set the 'U' flag to 1 for all rejection and unknown status codes. The status codes in the 1-10 range [RFC 8505] are all considered rejections.
Reciprocally, upon a DCO or a DAO-ACK message from the RPL DODAG root with a RPL Status that has the 'A' flag set, the 6LR MUST copy the RPL Status value unchanged in the Status field of the EARO when generating an NA to the RUL.
Top   ToC   RFCv3-9010

7.  Enhancements to RFC 9009

[RFC 9009] defines the DCO message for RPL Storing mode only, with a link-local scope. All nodes in the RPL network are expected to support the specification, since the message is processed hop by hop along the path that is being cleaned up.
This specification extends the use of the DCO message to the Non-Storing MOP, whereby the DCO is sent end to end by the root directly to the RAN that injected the DAO message for the considered target. In that case, intermediate nodes do not need to support [RFC 9009]; they forward the DCO message as a plain IPv6 packet between the root and the RAN.
In the case of a RUL, the 6LR that serves the RUL acts as the RAN that receives the Non-Storing DCO. This specification leverages the Non-Storing DCO between the root and the 6LR that serves as the attachment router for a RUL. A 6LR and a root that support this specification MUST implement the Non-Storing DCO.
Top   ToC   RFCv3-9010

8.  Enhancements to RFCs 6775 and 8505

This document updates [RFC 6775] and [RFC 8505] to reduce the range of the ARO/EARO Status values to 64 values. The two most significant (leftmost) bits of the original ND Status field are now reserved; they MUST be set to 0 by the sender and ignored by the receiver.
This document also updates the behavior of a 6LR acting as a RPL router and of a 6LN acting as a RUL in the 6LoWPAN ND Address Registration as follows:
  • If the RPL DODAG root advertises the ability to proxy the EDAR/EDAC exchange to the 6LBR, the 6LR refrains from sending the keep-alive EDAR message. If it is separated from the 6LBR, the root regenerates the EDAR message to the 6LBR periodically, upon a DAO message that signals the liveliness of the address.
  • The use of the R flag is extended to the NA(EARO) to confirm whether the route was installed.
Top   ToC   RFCv3-9010

9.  Protocol Operations for Unicast Addresses

The description below assumes that the root sets the 'P' flag in the DODAG Configuration option and performs the EDAR proxy operation presented in Section 4.3.
If the 'P' flag is set to 0, the 6LR MUST generate the periodic EDAR messages and process the returned status as specified in [RFC 8505]. If the EDAC indicates success, the rest of the flow takes place as presented but without the proxied EDAR/EDAC exchange.
Section 9.1 provides an overview of the route injection in RPL, whereas Section 9.2 offers more details from the perspective of the different nodes involved in the flow.

9.1.  General Flow

This specification eliminates the need to exchange keep-alive EDAR and EDAC messages all the way from a 6LN to the 6LBR across a RPL mesh. Instead, the EDAR/EDAC exchange with the 6LBR is proxied by the RPL DODAG root upon the DAO message that refreshes the RPL routing state. The first EDAR upon a new Address Registration cannot be proxied, though, as it is generated for the purpose of DAD, which must be verified before the address is injected in RPL.
In a RPL network where the function is enabled, refreshing the state in the 6LBR is the responsibility of the root. Consequently, only addresses that are injected in RPL will be kept alive at the 6LBR by the RPL DODAG root. Since RULs are advertised using Non-Storing mode, the DAO message flow and the keep-alive EDAR/EDAC can be nested within the Address (re)Registration flow. Figure 7 illustrates that, for the first Address Registration, both the DAD and the keep-alive EDAREDAC exchanges happen in the same sequence.
   6LN/RUL            6LR   <6LR*>   Root               6LBR
      |<---Using ND--->|<--Using RPL->|<-----Using ND---->|
      |                |<-----------Using ND------------->|
      |                |              |                   |
      |   NS(EARO)     |              |                   |
      |--------------->|                                  |
      |                |            EDAR                  |
      |                |--------------------------------->|
      |                |                                  |
      |                |             EDAC                 |
      |                |<---------------------------------|
      |                |                                  |
      |                |   DAO(X=0)   |                   |
      |                |------------->|                   |
      |                |                                  |
      |                |    DAO-ACK   |                   |
      |                |<-------------|                   |
      |   NA(EARO)     |              |                   |
      |<---------------|              |                   |
      |                |              |                   |
This flow requires that the lifetimes and sequence counters in 6LoWPAN ND and RPL be aligned.
To achieve this, the Path Sequence and the Path Lifetime in the DAO message are taken from the Transaction ID and the Address Registration lifetime in the NS(EARO) message from the 6LN.
On the first Address Registration, illustrated in Figure 7 for RPL Non-Storing mode, the EDAR/EDAC exchange takes place as prescribed by [RFC 8505]. If the exchange fails, the 6LR returns an NA message with a non-zero status to the 6LN, the NCE is not created, and the address is not injected in RPL. Otherwise, the 6LR creates an NCE and injects the Registered Address in the RPL routing using a DAO/DAO-ACK exchange with the RPL DODAG root.
An Address Registration refresh is performed by the 6LN to keep the NCE in the 6LR alive before the lifetime expires. Upon the refresh of a registration, the 6LR reinjects the corresponding route in RPL before it expires, as illustrated in Figure 8.
   6LN/RUL   <-ND->   6LR   <-RPL->  Root   <-ND->      6LBR
      |                |              |                   |
      |   NS(EARO)     |              |                   |
      |--------------->|              |                   |
      |                |   DAO(X=1)   |                   |
      |                |------------->|                   |
      |                |              |       EDAR        |
      |                |              |------------------>|
      |                |              |       EDAC        |
      |                |              |<------------------|
      |                |    DAO-ACK   |                   |
      |                |<-------------|                   |
      |   NA(EARO)     |              |                   |
      |<---------------|              |                   |
This is what causes the RPL DODAG root to refresh the state in the 6LBR, using an EDAC message. In the case of an error in the proxied EDAR flow, the error is returned in the DAO-ACK using a RPL Status with the 'A' flag set to 1, which embeds a 6LoWPAN Status value as discussed in Section 6.3.
The 6LR may receive a requested DAO-ACK after it received an asynchronous Non-Storing DCO, but the non-zero status in the DCO supersedes a positive status in the DAO-ACK, regardless of the order in which they are received. Upon the DAO-ACK -- or the DCO, if one arrives first -- the 6LR responds to the RUL with an NA(EARO).
An issue may be detected later, e.g., the address moves to a different DODAG with the 6LBR attached to a different 6LoWPAN Backbone Router (6BBR); see Figure 5 in Section 3.3 of RFC 8929. The 6BBR may send a negative ND Status, e.g., in an asynchronous NA(EARO) to the 6LBR.
[RFC 8929] expects that the 6LBR is co-located with the RPL DODAG root, but if not, the 6LBR MUST forward the status code to the originator of the EDAR -- either the 6LR or the RPL DODAG root that proxies for it. The ND status code is mapped in a RPL Status value by the RPL DODAG root, and then back to an ND Status by the 6LR to the 6LN. Note that a legacy RAN that receives a Non-Storing DCO that it does not support will ignore it silently, as specified in Section 6 of RFC 6550. The result is that it will remain unaware that it is no longer reachable until its next RPL exchange happens. This situation will be cleared upon the next Non-Storing DAO exchange if the error is returned in a DAO-ACK.
Figure 9 illustrates this in the case where the 6LBR and the root are not co-located, and the root proxies the EDAR/EDAC flow.
6LN/RUL  <-ND->  6LR  <-RPL->  Root  <-ND->  6LBR  <-ND->  6BBR
   |              |             |              |             |
   |              |             |              |   NA(EARO)  |
   |              |             |              |<------------|
   |              |             |     EDAC     |             |
   |              |             |<-------------|             |
   |              |     DCO     |              |             |
   |              |<------------|              |             |
   |   NA(EARO)   |             |              |             |
   |<-------------|             |              |             |
   |              |             |              |             |
If the root does not proxy, then the EDAC with a non-zero status reaches the 6LR directly. In that case, the 6LR MUST clean up the route using a DAO with a Lifetime of 0, and it MUST propagate the status back to the RUL in an NA(EARO) with the R flag set to 0.
The RUL may terminate the registration at any time by using a Registration Lifetime of 0. This specification requires that the RPL Target option transport the ROVR. This way, the same flow as the heartbeat flow is sufficient to inform the 6LBR using the root as a proxy, as illustrated in Figure 8.
All or any combination of the 6LR, the root, and the 6LBR might be collapsed in a single node.

9.2.  Detailed Operation

The following sections specify the behavior of (1) the 6LN acting as a RUL, (2) the 6LR acting as a border router and serving the 6LN, (3) the RPL DODAG root, and (4) the 6LBR in the control flows that enable RPL routing back to the RUL, respectively.

9.2.1.  Perspective of the 6LN Acting as a RUL

This specification builds on the operation of a 6LoWPAN ND-compliant 6LN/RUL, which is expected to operate as follows:
  1. The 6LN selects a 6LR that provides reachability services for a RUL. This is signaled by a 6CIO in the RA messages with the L, P, and E flags set to 1 as prescribed by [RFC 8505].
  2. The 6LN obtains an IPv6 global address, via either (1) Stateless Address Autoconfiguration (SLAAC) [RFC 4862] based on a Prefix Information Option (PIO) [RFC 4861] found in an RA message or (2) some other means, such as DHCPv6 [RFC 8415].
  3. Once it has formed an address, the 6LN registers its address and refreshes its registration periodically, early enough within the lifetime of the previous Address Registration, as prescribed by [RFC 6775], to refresh the NCE before the lifetime indicated in the EARO expires. It sets the T flag to 1 as prescribed in [RFC 8505]. The TID is incremented each time and wraps in a lollipop fashion (see Section 5.2.1 of RFC 8505, which is fully compatible with Section 7.2 of RFC 6550).
  4. As stated in Section 5.2 of RFC 8505, the 6LN can register with more than one 6LR at the same time.In that case, all the fields in the EARO are set to the same valuefor all of the parallel Address Registrations, with the exceptionof the Registration Lifetime field and the R flag, which may be set todifferent values.The 6LN may cancel a subset of its registrations or may transfer a registration from one or more old 6LRs to one or more new 6LRs. To do so, the 6LN sends a series of NS(EARO) messages, all with the same TID, with a zero Registration Lifetime to the old 6LR(s) and with a non-zero Registration Lifetime to the new 6LR(s). In that process, the 6LN SHOULD send the NS(EARO) with a non-zero Registration Lifetime and ensure that at least one succeeds before it sends an NS(EARO) that terminates another registration. This avoids the churn related to transient route invalidation in the RPL network above the common parent of the involved 6LRs.
  5. Following Section 5.1 of RFC 8505, a 6LN acting as a RUL sets the R flag in the EARO of its registration(s) for which it requires routing services. If the R flag is not echoed in the NA, the RUL MUST assume that establishing the routing services via this 6LR failed, and it SHOULD attempt to use another 6LR. The RUL SHOULD ensure that one registration succeeds before setting the R flag to 0. In the case of a conflict with the preceding rule regarding the lifetime, the rule regarding the lifetime has precedence.
  6. The 6LN may use any of the 6LRs to which it registered as the default gateway. Using a 6LR to which the 6LN is not registered may result in packets dropped at the 6LR by a Source Address Validation Improvement (SAVI) function [RFC 7039] and thus is not recommended.
Even without support for RPL, the RUL may be configured with an opaque value to be provided to the routing protocol. If the RUL has knowledge of the RPL Instance into which the packet should be injected, then it SHOULD set the Opaque field in the EARO to the RPLInstanceID; otherwise, it MUST leave the Opaque field as 0.
Regardless of the setting of the Opaque field, the 6LN MUST set the "I" field to 0 to signal "topological information to be passed to a routing process", as specified in Section 5.1 of RFC 8505.
A RUL is not expected to produce RPL artifacts in the data packets, but it may do so. For instance, if the RUL has minimal awareness of the RPL Instance, then it can build an RPI. A RUL that places an RPI in a data packet SHOULD indicate the RPLInstanceID of the RPL Instance where the packet should be forwarded. It is up to the 6LR (e.g., by policy) to use the RPLInstanceID information provided by the RUL or rewrite it to the selected RPLInstanceID for forwarding inside the RPL domain. All the flags and the SenderRank field are set to 0 as specified by Section 11.2 of RFC 6550.

9.2.2.  Perspective of the 6LR Acting as a Border Router

A 6LR that provides reachability services for a RUL in a RPL network as specified in this document MUST include a 6CIO in its RA messages and set the L, P, and E flags to 1 as prescribed by [RFC 8505].
As prescribed by [RFC 8505], the 6LR generates an EDAR message upon reception of a valid NS(EARO) message for the registration of a new IPv6 address by a 6LN. If the initial EDAR/EDAC exchange succeeds, then the 6LR installs an NCE for the Registration Lifetime.
If the R flag is set to 1 in the NS(EARO), the 6LR SHOULD inject the host route in RPL, unless this is barred for other reasons, such as the saturation of the RPL parents. The 6LR MUST use RPL Non-Storing mode signaling and the updated Target option (see Section 6.1). To avoid a redundant EDAR/EDAC flow to the 6LBR, the 6LR SHOULD refrain from setting the 'X' flag. The 6LR MUST request a DAO-ACK by setting the 'K' flag in the DAO message. Successfully injecting the route to the RUL's address will be indicated via the 'U' flag set to 0 in the RPL Status of the DAO-ACK message.
For the registration refreshes, if the RPL DODAG root sets the 'P' flag in the DODAG Configuration option to 1, then the 6LR MUST refrain from sending the keep-alive EDAR; instead, it MUST set the 'X' flag to 1 in the Target option of the DAO messages, to request that the root proxy the keep-alive EDAR/EDAC exchange with the 6LBR (see Section 6); if the 'P' flag is set to 0, then the 6LR MUST set the 'X' flag to 0 and handle the EDAR/EDAC flow itself.
The Opaque field in the EARO provides a means to signal which RPL Instance is to be used for the DAO advertisements and the forwarding of packets sourced at the Registered Address when there is no RPI in the packet.
As described in [RFC 8505], if the "I" field is 0, then the Opaque field is expected to carry the RPLInstanceID suggested by the 6LN; otherwise, there is no suggested RPL Instance. If the 6LR participates in the suggested RPL Instance, then the 6LR MUST use that RPL Instance for the Registered Address.
If there is no suggested RPL Instance or if the 6LR does not participate in the suggested RPL Instance, it is expected that the packets coming from the 6LN "can unambiguously be associated to at least one RPL Instance" [RFC 6550] by the 6LR, e.g., using a policy that maps the 6-tuple to a RPL Instance.
The DAO message advertising the Registered Address MUST be constructed as follows:
  1. The Registered Address is signaled as the Target Prefix in the updated Target option in the DAO message; the Prefix Length is set to 128 but the 'F' flag is set to 0, since the advertiser is not the RUL. The ROVR field is copied unchanged from the EARO (see Section 6.1).
  2. The 6LR indicates one of its global or unique-local IPv6 unicast addresses as the Parent Address in the TIO associated with the Target option.
  3. The 6LR sets the External ('E') flag in the TIO to indicate that it is redistributing an external target into the RPL network.
  4. The Path Lifetime in the TIO is computed from the Registration Lifetime in the EARO. This operation converts seconds to the Lifetime Units used in the RPL operation. This creates the deployment constraint that the Lifetime Unit is reasonably compatible with the expression of the Registration Lifetime; e.g., a Lifetime Unit of 0x4000 maps the most significant byte of the Registration Lifetime to the Path Lifetime. In that operation, the Path Lifetime must be set to ensure that the path has a longer lifetime than the registration and also covers the round-trip time to the root. Note that if the Registration Lifetime is 0, then the Path Lifetime is also 0 and the DAO message becomes a No-Path DAO, which cleans up the routes down to the RUL's address; this also causes the root as a proxy to send an EDAR message to the 6LBR with a Lifetime of 0.
  5. The Path Sequence in the TIO is set to the TID value found in the EARO.
Upon receiving or timing out the DAO-ACK after an implementation-specific number of retries, the 6LR MUST send the corresponding NA(EARO) to the RUL. Upon receiving an asynchronous DCO message, it MUST send an asynchronous NA(EARO) to the RUL immediately but still be capable of processing the DAO-ACK if one is pending.
The 6LR MUST set the R flag to 1 in the NA(EARO) that it sends back to the 6LN if and only if the 'U' flag in the RPL Status is set to 0, indicating that the 6LR injected the Registered Address in the RPL routing successfully and that the EDAR proxy operation succeeded.
If the 'A' flag in the RPL Status is set to 1, the embedded Status value is passed back to the RUL in the EARO Status. If the 'U' flag is also set to 1, the registration failed for 6LoWPAN-ND-related reasons, and the NCE is removed.
An error injecting the route causes the 'U' flag to be set to 1. If the error is not related to ND, the 'A' flag is set to 0. In that case, the registration succeeds, but the RPL route is not installed. So, the NA(EARO) is returned with a status indicating success but the R flag set to 0, which means that the 6LN obtained a binding but no route.
If the 'A' flag is set to 0 in the RPL Status of the DAO-ACK, then the 6LoWPAN ND operation succeeded, and an EARO Status of 0 (Success) MUST be returned to the 6LN. The EARO Status of 0 MUST also be used if the 6LR did not attempt to inject the route but could create the binding after a successful EDAR/EDAC exchange or refresh it.
If the 'U' flag is set to 1 in the RPL Status of the DAO-ACK, then the route was not installed, and the R flag MUST be set to 0 in the NA(EARO). The R flag MUST be set to 0 if the 6LR did not attempt to inject the route.
In a network where Address-Protected Neighbor Discovery (AP-ND) is enabled, in the case of a DAO-ACK or a DCO transporting an EARO Status value of 5 (Validation Requested), the 6LR MUST challenge the 6LN for ownership of the address, as described in Section 6.1 of RFC 8928, before the registration is complete. This flow, illustrated in Figure 10, ensures that the address is validated before it is injected in the RPL routing.
6LN                                       6LR        Root        6LBR
 |                                         |           |           |
 |<--------------- RA ---------------------|           |           |
 |                                         |           |           |
 |------ NS(EARO) (ROVR=Crypto-ID) ------->|           |           |
 |                                         |           |           |
 |<-NA(EARO) (Status=Validation Requested)-|           |           |
 |                                         |           |           |
 |---- NS(EARO) and proof of ownership --->|           |           |
 |                                         |           |           |
 |                                <validate the proof> |           |
 |                                                     |           |
 |<------- NA(EARO) (Status=10) -----<if failed>       |           |
 |                                                     |           |
 |                                       <else>        |           |
 |                                         |           |           |
 |                                         |--------- EDAR ------->|
 |                                         |                       |
 |                                         |<-------- EDAC --------|
 |                                         |                       |
 |                                         |           |           |
 |                                         |-DAO(X=0)->|           |
 |                                         |           |           |
 |                                         |<- DAO-ACK-|           |
 |                                         |           |           |
 |<---------- NA(EARO) (Status=0) ---------|           |           |
 |                                         |           |           |
 |                                         |           |           |
 |------ NS(EARO) (ROVR=Crypto-ID) ------->|           |           |
 |                                         |-DAO(X=1)->|           |
 |                                         |           |-- EDAR -->|
 |                                         |           |           |
 |                                         |           |<-- EDAC --|
 |                                         |<- DAO-ACK-|           |
 |<---------- NA(EARO) (Status=0) ---------|           |           |
 |                                         |           |           |
If the challenge succeeded, then the operations continue as normal. In particular, a DAO message is generated upon the NS(EARO) that proves the ownership of the address. If the challenge failed, the 6LR rejects the registration as prescribed by AP-ND and may take actions to protect itself against Denial-Of-Service (DoS) attacks by a rogue 6LN; see Section 11.
The 6LR may, at any time, send a unicast asynchronous NA(EARO) with the R flag set to 0 to signal that it has stopped providing routing services, and/or with an EARO Status of 2 (Neighbor Cache Full) to signal that it removed the NCE. It may also send a final RA -- unicast or multicast -- with a router Lifetime field of 0, to signal that it will cease to serve as the router, as specified in Section 6.2.5 of RFC 4861. This may happen upon a DCO or a DAO-ACK message indicating that the path is already removed; otherwise, the 6LR MUST remove the host route to the 6LN using a DAO message with a Path Lifetime of 0.
A valid NS(EARO) message with the R flag set to 0 and a Registration Lifetime that is not zero signals that the 6LN wishes to maintain the binding but does not require (i.e., no longer requires) the routing services from the 6LR. Upon this message, if, due to a previous NS(EARO) with the R flag set to 1 the 6LR was injecting the host route to the Registered Address in RPL using DAO messages, then the 6LR MUST invalidate the host route in RPL using a DAO with a Path Lifetime of 0. It is up to the registering 6LN to maintain the corresponding route from then on, by either (1) keeping it active via a different 6LR or (2) acting as a RAN and managing its own reachability.
When forwarding a packet from the RUL into the RPL domain, if the packet does not have an RPI, the 6LR MUST encapsulate the packet to the root and add an RPI. If there is an RPI in the packet, the 6LR MUST rewrite the RPI, but it does not need to encapsulate.

9.2.3.  Perspective of the RPL DODAG Root

A RPL DODAG root MUST set the 'P' flag to 1 in the RPL DODAG Configuration option of the DIO messages that it generates (see Section 6) to signal that it proxies the EDAR/EDAC exchange and supports the updated RPL Target option.
Upon reception of a DAO message, for each updated RPL Target option (see Section 6.1) with the 'X' flag set to 1, the root MUST notify the 6LBR by using a proxied EDAR/EDAC exchange; if the RPL DODAG root and the 6LBR are integrated, an internal API can be used instead.
The EDAR message MUST be constructed as follows:
  1. The target IPv6 address from the RPL Target option is placed in the Registered Address field of the EDAR message;
  2. The Registration Lifetime is adapted from the Path Lifetime in the TIO by converting the Lifetime Units used in RPL into units of 60 seconds used in the 6LoWPAN ND messages;
  3. The TID value is set to the Path Sequence in the TIO and indicated with an ICMP code of 1 in the EDAR message;
  4. The ROVR in the RPL Target option is copied as is in the EDAR, and the ICMP Code Suffix is set to the appropriate value as shown in Table 4 of [RFC 8505], depending on the size of the ROVR field.
Upon receiving an EDAC message from the 6LBR, if a DAO is pending, then the root MUST send a DAO-ACK back to the 6LR. Otherwise, if the status in the EDAC message is not "Success", then it MUST send an asynchronous DCO to the 6LR.
In either case, the EDAC Status is embedded in the RPL Status with the 'A' flag set to 1.
The proxied EDAR/EDAC exchange MUST be protected with a timer whose appropriate duration and number of retries (1) are implementation dependent and (2) SHOULD be configurable, since the root and the 6LBR are typically nodes with a higher capacity and manageability than 6LRs. Upon timing out, the root MUST send an error back to the 6LR as above, using either a DAO-ACK or a DCO, as appropriate, with the 'A' and 'U' flags set to 1 in the RPL Status, and a RPL Status value of "6LBR Registry Saturated" [RFC 8505].

9.2.4.  Perspective of the 6LBR

The 6LBR is unaware that the RPL DODAG root is not the new attachment 6LR of the RUL, so it is not impacted by this specification.
Upon reception of an EDAR message, the 6LBR behaves as prescribed by [RFC 8505] and returns an EDAC message to the sender.
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10.  Protocol Operations for Multicast Addresses

Section 12 of RFC 6550 details RPL support for multicast flows. This support is activated by setting the MOP value to 3 ("Storing Mode of Operation with multicast support") in the DIO messages that form the DODAG. This section also applies if and only if the MOP of the RPL Instance is 3.
RPL support for multicast is not source specific and only operates as an extension to the Storing mode of operation for unicast packets. Note that it is the RPL model that the multicast packet is copied and transmitted as a Layer 2 unicast to each of the interested children. This remains true when forwarding between the 6LR and the listener 6LN.
"[Multicast Listener Discovery Version 2 (MLDv2) for IPv6]" [RFC 3810] provides an interface for a listener to register with multicast flows. In the MLD model, the router is a "querier", and the host is a multicast listener that registers with the querier to obtain copies of the particular flows it is interested in.
The equivalent of the first Address Registration happens as illustrated in Figure 11. The 6LN, as an MLD listener, sends an unsolicited Report to the 6LR. This enables it to start receiving the flow immediately and causes the 6LR to inject the multicast route in RPL.
   6LN/RUL                6LR             Root                   6LBR
      |                    |               |                       |
      | unsolicited Report |               |                       |
      |------------------->|               |                       |
      |                    | DAO           |                       |
      |                    |-------------->|                       |
      |                    |    DAO-ACK    |                       |
      |                    |<--------------|                       |
      |                    |               | <if not done already> |
      |                    |               |  unsolicited Report   |
      |                    |               |---------------------->|
      |                    |               |                       |
This specification does not change MLD but will operate more efficiently if the asynchronous messages for unsolicited Report and Done are sent by the 6LN as Layer 2 unicast to the 6LR, particularly on wireless.
The 6LR acts as a generic MLD querier and generates a DAO with the multicast address as the Target Prefix as described in Section 12 of RFC 6550. As for the unicast host routes, the Path Lifetime associated to the Target is mapped from the Query Interval and is set to be larger, to account for variable propagation delays to the root. The root proxies the MLD exchange as a listener with the 6LBR acting as the querier, so as to get packets from a source external to the RPL domain.
Upon a DAO with a Target option for a multicast address, the RPL DODAG root checks to see if it is already registered as a listener for that address, and if not, it performs its own unsolicited Report for the multicast address as described in Section 6.1 of RFC 3810. The Report is source independent, so there is no source address listed.
The equivalent of the registration refresh is pulled periodically by the 6LR acting as the querier. Upon the timing out of the Query Interval, the 6LR sends a Multicast Address Specific Query to each of its listeners, for each multicast address. The listeners respond with a Report. Based on the Reports, the 6LR maintains the aggregated list of all the multicast addresses for which there is a listener and advertises them using DAO messages as specified in Section 12 of RFC 6550. Optionally, the 6LR MAY send a General Query, where the Multicast Address field is set to 0. In that case, the multicast packet is passed as a Layer 2 unicast to each of the interested children.
Upon a Report, the 6LR generates a DAO with as many Target options as there are Multicast Address Records in the Report message, copying the Multicast Address field in the Target Prefix of the RPL Target option. The DAO message is a Storing mode DAO, passed to a selection of the 6LR's parents.
Asynchronously to this, a similar procedure happens between the root and a router, such as the 6LBR, that serves multicast flows on the link where the root is located. Again, the Query and Report messages are source independent. The root lists exactly once each multicast address for which it has at least one active multicast DAO state, copying the multicast address in the DAO state in the Multicast Address field of the Multicast Address Records in the Report message.
This is illustrated in Figure 12:
   6LN/RUL                6LR             Root                6LBR
      |                    |               |                    |
      |       Query        |               |                    |
      |<-------------------|               |                    |
      |       Report       |               |                    |
      |------------------->|               |                    |
      |                    | DAO           |                    |
      |                    |-------------->|                    |
      |                    |    DAO-ACK    |                    |
      |                    |<--------------|                    |
      |                    |               |       Query        |
      |                    |               |<-------------------|
      |                    |               |       Report       |
      |                    |               |------------------->|
      |                    |               |                    |
Note that all or any combination of the 6LR, the root, and the 6LBR might be collapsed in a single node, in which case the flow above happens internally, and possibly through internal API calls as opposed to messaging.
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11.  Security Considerations

It is worth noting that with [RFC 6550], every node in the LLN is RPL aware and can inject any RPL-based attack in the network. This specification improves this situation by isolating edge nodes that can only interact with the RPL routers using 6LoWPAN ND, meaning that they cannot perform RPL insider attacks.
The LLN nodes depend on the 6LBR and the RPL participants for their operation. A trust model must be put in place to ensure that the right devices are acting in these roles, so as to avoid such threats as black-holing (see Section 7 of RFC 7416), DoS attacks whereby a rogue 6LR creates a high churn in the RPL network by advertising and removing many forged addresses, or a bombing attack whereby an impersonated 6LBR would destroy state in the network by using a status code of 4 ("Removed") [RFC 8505].
This trust model could be, at a minimum, based on Layer 2 secure joining and link-layer security. This is a generic 6LoWPAN requirement; see Req-5.1 in Appendix B.5 of RFC 8505.
In a general manner, the Security Considerations sections of [RFC 6550], [RFC 7416], [RFC 6775], and [RFC 8505] apply to this specification as well.
In particular, link-layer security is needed to prevent DoS attacks whereby a rogue 6LN creates a high churn in the RPL network by constantly registering and deregistering addresses with the R flag set to 1 in the EARO.
[RFC 8928] updated 6LoWPAN ND with AP-ND. AP-ND protects the owner of an address against address theft and impersonation attacks in an LLN. Nodes supporting the extension compute a cryptographic identifier (Crypto-ID) and use it with one or more of their Registered Addresses. The Crypto-ID identifies the owner of the Registered Address and can be used to provide proof of ownership of the Registered Addresses. Once an address is registered with the Crypto-ID and proof of ownership is provided, only the owner of that address can modify the registration information, thereby enforcing SAVI. [RFC 8928] reduces even further the attack perimeter that is available to the edge nodes, and its use is suggested in this specification.
Additionally, the trust model could include role validation (e.g., using role-based authorization) to ensure that the node that claims to be a 6LBR or a RPL DODAG root is entitled to do so.
The Opaque field in the EARO enables the RUL to suggest a RPLInstanceID where its traffic is placed. It is also possible for an attacker RUL to include an RPI in the packet. This opens the door to attacks where a RPL Instance would be reserved for critical traffic, e.g., with a specific bandwidth reservation, that the additional traffic generated by a rogue may disrupt. The attack may be alleviated by traditional access control and traffic-shaping mechanisms where the 6LR controls the incoming traffic from the 6LN. More importantly, the 6LR is the node that injects the traffic in the RPL domain, so it has the final word on which RPL Instance is to be used for the traffic coming from the RUL, per its own policy. In particular, a policy can override the formal language that forces the use of the Opaque field or the rewriting of the RPI provided by the RUL, in a situation where the network administrator finds it relevant.
At the time of this writing, RPL does not have a route ownership validation model whereby it is possible to validate the origin of an address that is injected in a DAO. This specification makes a first step in that direction by allowing the root to challenge the RUL via the 6LR that serves it.
Section 6.1 indicates that when the length of the ROVR field is unknown, the RPL Target option must be passed on as received in RPL Storing mode. This creates a possible opening for using DAO messages as a covert channel. Note that DAO messages are rare, and overusing that channel could be detected. An implementation SHOULD notify the network management system when a RPL Target option is received with an unknown ROVR field size, to ensure that the network administrator is aware of the situation.
[RFC 9009] introduces the ability for a rogue common ancestor node to invalidate a route on behalf of the target node. In this case, the RPL Status in the DCO has the 'A' flag set to 0, and an NA(EARO) is returned to the 6LN with the R flag set to 0. This encourages the 6LN to try another 6LR. If a 6LR exists that does not use the rogue common ancestor, then the 6LN will eventually succeed gaining reachability over the RPL network in spite of the rogue node.
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12.  IANA Considerations

12.1.  Fixing the Address Registration Option Flags

Section 9.1 of RFC 8505 created a registry for the 8-bit Address Registration Option Flags field. IANA has renamed the first column of the table from "ARO Status" to "Bit Number".

12.2.  Resizing the ARO Status Values

Section 12 of RFC 6775 created the "Address Registration Option Status Values" registry with a range of 0-255.
This specification reduces that range to 0-63; see Section 6.3.
IANA has modified the "Address Registration Option Status Values" registry so that the upper bound of the unassigned values is 63. This document has been added as a reference. The registration procedure has not changed.

12.3.  New RPL DODAG Configuration Option Flag

IANA has assigned the following flag in the "DODAG Configuration Option Flags for MOP 0..6" registry [RFC 9008]:
Bit Number Capability Description Reference
1 Root Proxies EDAR/EDAC (P) RFC 9010
Table 2: New DODAG Configuration Option Flag
IANA has added this document as a reference for MOP 7 in the RPL "Mode of Operation" registry.

12.4.  RPL Target Option Flags Registry

This document modifies the "RPL Target Option Flags" registry initially created per Section 20.15 of RFC 6550. The registry now includes only 4 bits (Section 6.1) and lists this document as an additional reference. The registration procedure has not changed.
Section 6.1 also defines two new entries in the registry, as follows:
Bit Number Capability Description Reference
0 Advertiser address in Full (F) RFC 9010
1 Proxy EDAR Requested (X) RFC 9010
Table 3: RPL Target Option Flags Registry

12.5.  New Subregistry for RPL Non-Rejection Status Values

IANA has created a new subregistry for the RPL Non-Rejection Status values for use in the RPL DAO-ACK, DCO, and DCO-ACK messages with the 'A' flag set to 0 and the 'U' flag set to 1, under the "Routing Protocol for Low Power and Lossy Networks (RPL)" registry.
  • Possible values are 6-bit unsigned integers (0..63).
  • The registration procedure is IETF Review [RFC 8126].
  • The initial allocation is as indicated in Table 4:
Value Meaning Reference
0 Success / Unqualified acceptance RFC 6550 / RFC 9010
1..63 Unassigned
Table 4: Acceptance Values of the RPL Status

12.6.  New Subregistry for RPL Rejection Status Values

IANA has created a new subregistry for the RPL Rejection Status values for use in the RPL DAO-ACK and DCO messages with the 'A' flag set to 0 and the 'U' flag set to 1, under the "Routing Protocol for Low Power and Lossy Networks (RPL)" registry.
  • Possible values are 6-bit unsigned integers (0..63).
  • The registration procedure is IETF Review [RFC 8126].
  • The initial allocation is as indicated in Table 5:
Value Meaning Reference
0 Unqualified rejection RFC 9010
1 No routing entry RFC 9009
2..63 Unassigned
Table 5: Rejection Values of the RPL Status
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13.  References

13.1.  Normative References

S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
R. Vida, and L. Costa, "Multicast Listener Discovery Version 2 (MLDv2) for IPv6", RFC 3810, DOI 10.17487/RFC3810, June 2004,
T. Narten, E. Nordmark, W. Simpson, and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007,
T. Winter, P. Thubert, A. Brandt, J. Hui, R. Kelsey, P. Levis, K. Pister, R. Struik, JP. Vasseur, and R. Alexander, "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks", RFC 6550, DOI 10.17487/RFC6550, March 2012,
Z. Shelby, S. Chakrabarti, E. Nordmark, and C. Bormann, "Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)", RFC 6775, DOI 10.17487/RFC6775, November 2012,
JP. Vasseur, "Terms Used in Routing for Low-Power and Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January 2014,
C. Bormann, "6LoWPAN-GHC: Generic Header Compression for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400, November 2014,
M. Cotton, B. Leiba, and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017,
B. Leiba, "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017,
S. Deering, and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017,
T. Chown, J. Loughney, and T. Winters, "IPv6 Node Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504, January 2019,
P. Thubert, E. Nordmark, S. Chakrabarti, 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,
P. Thubert, B. Sarikaya, M. Sethi, and R. Struik, "Address-Protected Neighbor Discovery for Low-Power and Lossy Networks", RFC 8928, DOI 10.17487/RFC8928, November 2020,
M.I. Robles, M Richardson, 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,
R.A Jadhav, P Thubert, R.N Sahoo, and Z Cao, "Efficient Route Invalidation", RFC 9009, DOI 10.17487/RFC9009, April 2021,

13.2.  Informative References

S. Thomson, T. Narten, and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, DOI 10.17487/RFC4862, September 2007,
N. Kushalnagar, G. Montenegro, 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,
J. Hui, and P. Thubert, "Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, DOI 10.17487/RFC6282, September 2011,
J. Hui, and JP. Vasseur, "The Routing Protocol for Low-Power and Lossy Networks (RPL) Option for Carrying RPL Information in Data-Plane Datagrams", RFC 6553, DOI 10.17487/RFC6553, March 2012,
J. Hui, JP. Vasseur, D. Culler, and V. Manral, "An IPv6 Routing Header for Source Routes with the Routing Protocol for Low-Power and Lossy Networks (RPL)", RFC 6554, DOI 10.17487/RFC6554, March 2012,
E. Kim, D. Kaspar, C. Gomez, and C. Bormann, "Problem Statement and Requirements for IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Routing", RFC 6606, DOI 10.17487/RFC6606, May 2012,
J. Tripathi, J. de Oliveira, and JP. Vasseur, "Performance Evaluation of the Routing Protocol for Low-Power and Lossy Networks (RPL)", RFC 6687, DOI 10.17487/RFC6687, October 2012,
J. Wu, J. Bi, M. Bagnulo, F. Baker, and C. Vogt, "Source Address Validation Improvement (SAVI) Framework", RFC 7039, DOI 10.17487/RFC7039, October 2013,
C. Bormann, M. Ersue, and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, DOI 10.17487/RFC7228, May 2014,
T. Tsao, R. Alexander, M. Dohler, V. Daza, A. Lozano, and M. Richardson, "A Security Threat Analysis for the Routing Protocol for Low-Power and Lossy Networks (RPLs)", RFC 7416, DOI 10.17487/RFC7416, January 2015,
P. Thubert, and R. Cragie, "IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Paging Dispatch", RFC 8025, DOI 10.17487/RFC8025, November 2016,
P. Thubert, C. Bormann, L. Toutain, and R. Cragie, "IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138, April 2017,
T. Mrugalski, M. Siodelski, B. Volz, A. Yourtchenko, M. Richardson, S. Jiang, T. Lemon, and T. Winters, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 8415, DOI 10.17487/RFC8415, November 2018,
P. Thubert, C.E. Perkins, and E. Levy-Abegnoli, "IPv6 Backbone Router", RFC 8929, DOI 10.17487/RFC8929, November 2020,
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Appendix A.  Example Compression

Figure 13 illustrates the case in Storing mode where the packet is received from the Internet, then the root encapsulates the packet to insert the RPI and deliver it to the 6LR that is the parent and last hop to the final destination, which is not known to support [RFC 8138].
+-+ ... -+-+ ... +-+- ... -+-+ ... -+-+-+ ... +-+-+ ... -+ ... +-...
|11110001|SRH-6LoRH| RPI-  |IP-in-IP| NH=1      |11110CPP| UDP | UDP
|Page 1  |Type1 S=0| 6LoRH | 6LoRH  |LOWPAN_IPHC| UDP    | hdr |Payld
+-+ ... -+-+ ... +-+- ... -+-+ ... -+-+-+ ... +-+-+ ... -+ ... +-...
         <-4 bytes->                <-        RFC 6282        ->
                                    <-     No RPL artifact ...
The difference from the example presented in Figure 19 of [RFC 8138] is the addition of an SRH-6LoRH before the RPI-6LoRH to transport the compressed address of the 6LR as the destination address of the outer IPv6 header. In Figure 19 of [RFC 8138], the destination IP of the outer header was elided and was implicitly the same address as the destination of the inner header. Type 1 was arbitrarily chosen, and the size of 0 denotes a single address in the SRH.
In Figure 13, the source of the IPv6-in-IPv6 encapsulation is the root, so it is elided in the IPv6-in-IPv6 6LoRH. The destination is the parent 6LR of the destination of the encapsulated packet, so it cannot be elided. If the DODAG is operated in Storing mode, it is the single entry in the SRH-6LoRH and the SRH-6LoRH Size is encoded as 0. The SRH-6LoRH is the first 6LoRH in the chain. In this particular example, the 6LR address can be compressed to 2 bytes, so a Type of 1 is used. The result is that the total length of the SRH-6LoRH is 4 bytes.
In Non-Storing mode, the encapsulation from the root would be similar to that represented in Figure 13 with possibly more hops in the SRH-6LoRH and possibly multiple SRH-6LoRHs if the various addresses in the routing header are not compressed to the same format. Note that on the last hop to the parent 6LR, the RH3 is consumed and removed from the compressed form, so the use of Non-Storing mode vs. Storing mode is indistinguishable from the packet format.
The SRH-6LoRHs are followed by the RPI-6LoRH and then the IPv6-in-IPv6 6LoRH. When the IPv6-in-IPv6 6LoRH is removed, all the 6LoRH Headers that precede it are also removed. The Paging Dispatch [RFC 8025] may also be removed if there was no previous Page change to a Page other than 0 or 1, since the LOWPAN_IPHC is encoded in the same fashion in the default Page 0 and in Page 1. The resulting packet to the destination is the encapsulated packet compressed per [RFC 6282].
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The authors wish to thank Ines Robles, Georgios Papadopoulos, and especially Rahul Jadhav and Alvaro Retana for their reviews and contributions to this document. Also many thanks to Éric Vyncke, Erik Kline, Murray Kucherawy, Peter van der Stok, Carl Wallace, Barry Leiba, Julien Meuric, and especially Benjamin Kaduk and Elwyn Davies, for their reviews and useful comments during the IETF Last Call and the IESG review sessions.
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Authors' Addresses

Pascal Thubert

Cisco Systems, Inc.
45 Allee des Ormes - BP1200
MOUGINS - Sophia Antipolis   06254

Michael C. Richardson

Sandelman Software Works
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