Using RPI Option Type, Routing Header for Source Routes, and IPv6-in-IPv6 Encapsulation in the RPL Data Plane
RFC 9008

Document Type RFC - Proposed Standard (April 2021; Errata)
Authors Ines Robles  , Michael Richardson  , Pascal Thubert 
Last updated 2021-04-27
Replaces draft-robles-roll-useofrplinfo
Stream Internet Engineering Task Force (IETF)
Formats plain text html xml pdf htmlized (tools) htmlized with errata bibtex
Reviews
Stream WG state Submitted to IESG for Publication (wg milestone: Mar 2020 - Initial Submission o... )
Document shepherd Peter Van der Stok
Shepherd write-up Show (last changed 2020-06-14)
IESG IESG state RFC 9008 (Proposed Standard)
Action Holders
(None)
Consensus Boilerplate Yes
Telechat date
Responsible AD Alvaro Retana
Send notices to Peter Van der Stok <consultancy@vanderstok.org>, aretana.ietf@gmail.com
IANA IANA review state Version Changed - Review Needed
IANA action state RFC-Ed-Ack


Internet Engineering Task Force (IETF)                       M.I. Robles
Request for Comments: 9008                                 UTN-FRM/Aalto
Updates: 6550, 6553, 8138                                  M. Richardson
Category: Standards Track                                            SSW
ISSN: 2070-1721                                               P. Thubert
                                                                   Cisco
                                                              April 2021

 Using RPI Option Type, Routing Header for Source Routes, and IPv6-in-
                IPv6 Encapsulation in the RPL Data Plane

Abstract

   This document looks at different data flows through Low-Power and
   Lossy Networks (LLN) where RPL (IPv6 Routing Protocol for Low-Power
   and Lossy Networks) is used to establish routing.  The document
   enumerates the cases where RPL Packet Information (RPI) Option Type
   (RFC 6553), RPL Source Route Header (RFC 6554), and IPv6-in-IPv6
   encapsulation are required in the data plane.  This analysis provides
   the basis upon which to design efficient compression of these
   headers.  This document updates RFC 6553 by adding a change to the
   RPI Option Type.  Additionally, this document updates RFC 6550 by
   defining a flag in the DODAG Information Object (DIO) Configuration
   option to indicate this change and updates RFC 8138 as well to
   consider the new Option Type when the RPL Option is decompressed.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9008.

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

Table of Contents

   1.  Introduction
     1.1.  Overview
   2.  Terminology and Requirements Language
   3.  RPL Overview
   4.  Updates to RFC 6550, RFC 6553, and RFC 8138
     4.1.  Updates to RFC 6550
       4.1.1.  Advertising External Routes with Non-Storing Mode
               Signaling
       4.1.2.  Configuration Options and Mode of Operation
       4.1.3.  Indicating the New RPI in the DODAG Configuration
               Option Flag
     4.2.  Updates to RFC 6553: Indicating the New RPI Option Type
     4.3.  Updates to RFC 8138: Indicating the Way to Decompress with
           the New RPI Option Type
   5.  Reference Topology
   6.  Use Cases
   7.  Storing Mode
     7.1.  Storing Mode: Interaction between Leaf and Root
       7.1.1.  SM: Example of Flow from RAL to Root
       7.1.2.  SM: Example of Flow from Root to RAL
       7.1.3.  SM: Example of Flow from Root to RUL
       7.1.4.  SM: Example of Flow from RUL to Root
     7.2.  SM: Interaction between Leaf and Internet
       7.2.1.  SM: Example of Flow from RAL to Internet
       7.2.2.  SM: Example of Flow from Internet to RAL
       7.2.3.  SM: Example of Flow from RUL to Internet
       7.2.4.  SM: Example of Flow from Internet to RUL
     7.3.  SM: Interaction between Leaf and Leaf
       7.3.1.  SM: Example of Flow from RAL to RAL
       7.3.2.  SM: Example of Flow from RAL to RUL
       7.3.3.  SM: Example of Flow from RUL to RAL
       7.3.4.  SM: Example of Flow from RUL to RUL
   8.  Non-Storing Mode
     8.1.  Non-Storing Mode: Interaction between Leaf and Root
       8.1.1.  Non-SM: Example of Flow from RAL to Root
       8.1.2.  Non-SM: Example of Flow from Root to RAL
       8.1.3.  Non-SM: Example of Flow from Root to RUL
       8.1.4.  Non-SM: Example of Flow from RUL to Root
     8.2.  Non-Storing Mode: Interaction between Leaf and Internet
       8.2.1.  Non-SM: Example of Flow from RAL to Internet
       8.2.2.  Non-SM: Example of Flow from Internet to RAL
       8.2.3.  Non-SM: Example of Flow from RUL to Internet
       8.2.4.  Non-SM: Example of Flow from Internet to RUL
     8.3.  Non-SM: Interaction between Leaves
       8.3.1.  Non-SM: Example of Flow from RAL to RAL
       8.3.2.  Non-SM: Example of Flow from RAL to RUL
       8.3.3.  Non-SM: Example of Flow from RUL to RAL
       8.3.4.  Non-SM: Example of Flow from RUL to RUL
   9.  Operational Considerations of Supporting RULs
   10. Operational Considerations of Introducing 0x23
   11. IANA Considerations
     11.1.  Option Type in RPL Option
     11.2.  Change to the "DODAG Configuration Option Flags"
            Subregistry
     11.3.  Change MOP Value 7 to Reserved
   12. Security Considerations
   13. References
     13.1.  Normative References
     13.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   RPL (IPv6 Routing Protocol for Low-Power and Lossy Networks)
   [RFC6550] is a routing protocol for constrained networks.  [RFC6553]
   defines the RPL Option carried within the IPv6 Hop-by-Hop Options
   header to carry the RPLInstanceID and quickly identify
   inconsistencies (loops) in the routing topology.  The RPL Option is
   commonly referred to as the RPL Packet Information (RPI), although
   the RPI is the routing information that is defined in [RFC6550] and
   transported in the RPL Option.  RFC 6554 [RFC6554] defines the "RPL
   Source Route Header" (RH3), an IPv6 extension header to deliver
   datagrams within a RPL routing domain, particularly in Non-Storing
   mode.

   These various items are referred to as RPL artifacts, and they are
   seen on all of the data plane traffic that occurs in RPL-routed
   networks; they do not, in general, appear on the RPL control plane at
   all, which is mostly hop-by-hop traffic (one exception being
   Destination Advertisement Object (DAO) messages in Non-Storing mode).

   It has become clear from attempts to do multi-vendor
   interoperability, and from a desire to compress as many of the above
   artifacts as possible, that not all implementers agree when artifacts
   are necessary, or when they can be safely omitted, or removed.

   The ROLL (Routing Over Low power and Lossy networks) Working Group
   analyzed how IPv6 rules [RFC2460] apply to the Storing and Non-
   Storing use of RPL.  The result was 24 data-plane use cases.  They
   are exhaustively outlined here in order to be completely unambiguous.
   During the processing of this document, new rules were published as
   [RFC8200], and this document was updated to reflect the normative
   changes in that document.

   This document updates [RFC6553], changing the value of the Option
   Type of the RPL Option to make routers compliant with [RFC8200]
   ignore this option when it is not recognized.

   A Routing Header Dispatch for IPv6 over Low-Power Wireless Personal
   Area Networks (6LoWPAN) (6LoRH) [RFC8138] defines a mechanism for
   compressing RPL Option information and Routing Header type 3 (RH3)
   [RFC6554], as well as an efficient IPv6-in-IPv6 technique.

   Most of the use cases described herein require the use of IPv6-in-
   IPv6 packet encapsulation.  When encapsulating and decapsulating
   packets, [RFC6040] MUST be applied to map the setting of the explicit
   congestion notification (ECN) field between inner and outer headers.
   Additionally, [TUNNELS] is recommended reading to explain the
   relationship of IP tunnels to existing protocol layers and the
   challenges in supporting IP tunneling.

   Unconstrained uses of RPL are not in scope of this document, and
   applicability statements for those uses may provide different advice,
   e.g., [ACP].

1.1.  Overview

   The rest of the document is organized as follows: Section 2 describes
   the terminology that is used.  Section 3 provides a RPL overview.
   Section 4 describes the updates to RFC 6553, RFC 6550, and RFC 8138.
   Section 5 provides the reference topology used for the use cases.
   Section 6 describes the use cases included.  Section 7 describes the
   Storing mode cases and Section 8 the Non-Storing mode cases.
   Section 9 describes the operational considerations of supporting RPL-
   unaware leaves.  Section 10 depicts operational considerations for
   the proposed change on RPI Option Type, Section 11 the IANA
   considerations, and then Section 12 describes the security aspects.

2.  Terminology and Requirements Language

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

   The following terminology defined in [RFC7102] applies to this
   document: LLN, RPL, RPL domain, and ROLL.

   Consumed:  A Routing Header is consumed when the Segments Left field
      is zero, which indicates that the destination in the IPv6 header
      is the final destination of the packet and that the hops in the
      Routing Header have been traversed.

   RPL Leaf:  An IPv6 host that is attached to a RPL router and obtains
      connectivity through a RPL Destination-Oriented Directed Acyclic
      Graph (DODAG).  As an IPv6 node, a RPL leaf is expected to ignore
      a consumed Routing Header, and as an IPv6 host, it is expected to
      ignore a Hop-by-Hop Options header.  Thus, a RPL leaf can
      correctly receive a packet with RPL artifacts.  On the other hand,
      a RPL leaf is not expected to generate RPL artifacts or to support
      IP-in-IP encapsulation.  For simplification, this document uses
      the standalone term leaf to mean a RPL leaf.

   RPL Packet Information (RPI):  The information defined abstractly in
      [RFC6550] to be placed in IP packets.  The term is commonly used,
      including in this document, to refer to the RPL Option [RFC6553]
      that transports that abstract information in an IPv6 Hop-by-Hop
      Options header.  [RFC8138] provides an alternate (more compressed)
      formatting for the same abstract information.

   RPL-Aware Node (RAN):  A device that implements RPL.  Please note
      that the device can be found inside the LLN or outside LLN.

   RPL-Aware Leaf (RAL):  A RPL-aware node that is also a RPL leaf.

   RPL-Unaware Node:  A device that does not implement RPL, thus the
      device is RPL unaware.  Please note that the device can be found
      inside the LLN.

   RPL-Unaware Leaf (RUL):  A RPL-unaware node that is also a RPL leaf.

   6LoWPAN Node (6LN):  [RFC6775] defines it as the following: "A
      6LoWPAN node is any host or router participating in a LoWPAN.
      This term is used when referring to situations in which either a
      host or router can play the role described."  In this document, a
      6LN acts as a leaf.

   6LoWPAN Router (6LR):  [RFC6775] defines it as the following: "An
      intermediate router in the LoWPAN that is able to send and receive
      Router Advertisements (RAs) and Router Solicitations (RSs) as well
      as forward and route IPv6 packets.  6LoWPAN routers are present
      only in route-over topologies."

   6LoWPAN Border Router (6LBR):  [RFC6775] defines it as the following:
      "A border router located at the junction of separate 6LoWPAN
      networks or between a 6LoWPAN network and another IP network.
      There may be one or more 6LBRs at the 6LoWPAN network boundary.  A
      6LBR is the responsible authority for IPv6 prefix propagation for
      the 6LoWPAN network it is serving.  An isolated LoWPAN also
      contains a 6LBR in the network, which provides the prefix(es) for
      the isolated network."

   Flag Day:  A flag day is caused when a network is reconfigured in a
      way that nodes running the older configuration cannot communicate
      with nodes running the new configuration.  An example of a flag
      day is when the ARPANET changed from IP version 3 to IP version 4
      on January 1, 1983 [RFC0801].  In the context of this document, a
      switch from RPI Option Type (0x63) to Option Type (0x23) presents
      as a disruptive changeover.  In order to reduce the amount of time
      for such a changeover, Section 4.1.3 provides a mechanism to allow
      nodes to be incrementally upgraded.

   Non-Storing Mode (Non-SM):  A RPL mode of operation in which the RPL-
      aware nodes send information to the root about their parents.
      Thus, the root knows the topology.  Because the root knows the
      topology, the intermediate 6LRs do not maintain routing state, and
      source routing is needed.

   Storing Mode (SM):  A RPL mode of operation in which RPL-aware nodes
      (6LRs) maintain routing state (of the children) so that source
      routing is not needed.

      |  Note: Due to lack of space in some tables, we refer to IPv6-in-
      |  IPv6 as IP6-IP6.

3.  RPL Overview

   RPL defines the RPL control message (control plane), which is an
   ICMPv6 message [RFC4443] with a Type of 155.  DIS (DODAG Information
   Solicitation), DIO (DODAG Information Object), and DAO (Destination
   Advertisement Object) messages are all RPL control messages but with
   different Code values.  A RPL stack is shown in Figure 1.

   +--------------+
   | Upper Layers |
   |              |
   +--------------+
   |   RPL        |
   |              |
   +--------------+
   |   ICMPv6     |
   |              |
   +--------------+
   |   IPv6       |
   |              |
   +--------------+
   |   6LoWPAN    |
   |              |
   +--------------+
   |   PHY-MAC    |
   |              |
   +--------------+

                            Figure 1: RPL Stack

   RPL supports two modes of Downward internal traffic: in Storing mode
   (SM), it is fully stateful; in Non-Storing mode (non-SM), it is fully
   source routed.  A RPL Instance is either fully Storing or fully Non-
   Storing, i.e., a RPL Instance with a combination of fully Storing and
   Non-Storing nodes is not supported with the current specifications at
   the time of writing this document.  External routes are advertised
   with non-SM messaging even in an SM network, see Section 4.1.1

4.  Updates to RFC 6550, RFC 6553, and RFC 8138

4.1.  Updates to RFC 6550

4.1.1.  Advertising External Routes with Non-Storing Mode Signaling

   Section 6.7.8 of [RFC6550] introduces the 'E' flag that is set to
   indicate that the 6LR that generates the DAO redistributes external
   targets into the RPL network.  An external target is a target that
   has been learned through an alternate protocol, for instance, a route
   to a prefix that is outside the RPL domain but reachable via a 6LR.
   Being outside of the RPL domain, a node that is reached via an
   external target cannot be guaranteed to ignore the RPL artifacts and
   cannot be expected to process the compression defined in [RFC8138]
   correctly.  This means that the RPL artifacts should be contained in
   an IP-in-IP encapsulation that is removed by the 6LR, and that any
   remaining compression should be expanded by the 6LR before it
   forwards a packet outside the RPL domain.

   This specification updates [RFC6550] to say that advertising external
   targets using Non-Storing mode DAO messaging even in a Storing mode
   network is RECOMMENDED.  This way, external routes are not advertised
   within the DODAG, and all packets to an external target reach the
   root like normal Non-Storing mode traffic.  The Non-Storing mode DAO
   informs the root of the address of the 6LR that injects the external
   route, and the root uses IP-in-IP encapsulation to that 6LR, which
   terminates the IP-in-IP tunnel and forwards the original packet
   outside the RPL domain free of RPL artifacts.

   In the other direction, for traffic coming from an external target
   into the LLN, the parent (6LR) that injects the traffic always
   encapsulates to the root.  This whole operation is transparent to
   intermediate routers that only see traffic between the 6LR and the
   root, and only the root and the 6LRs that inject external routes in
   the network need to be upgraded to add this function to the network.

   A RUL is a special case of external target when the target is
   actually a host, and it is known to support a consumed Routing Header
   and to ignore a Hop-by-Hop Options header as prescribed by [RFC8200].
   The target may have been learned through an external routing protocol
   or may have been registered to the 6LR using [RFC8505].

   In order to enable IP-in-IP all the way to a 6LN, it is beneficial
   that the 6LN supports decapsulating IP-in-IP, but that is not assumed
   by [RFC8504].  If the 6LN is a RUL, the root that encapsulates a
   packet SHOULD terminate the tunnel at a parent 6LR.  The root may
   encapsulate all the way to the RUL if it is aware that the RUL
   supports IP-in-IP decapsulation and the artifacts in the outer header
   chain.

   A node that is reachable over an external route is not expected to
   support [RFC8138].  Whether a decapsulation took place or not and
   even when the 6LR is delivering the packet to a RUL, the 6LR that
   injected an external route MUST undo the [RFC8138] compression on the
   packet before forwarding over that external route.

4.1.2.  Configuration Options and Mode of Operation

   Section 6.7.6 of [RFC6550] describes the DODAG Configuration option
   as containing a series of flags in the first octet of the payload.

   Anticipating future work to revise RPL relating to how the LLN and
   DODAG are configured, this document renames the IANA "DODAG
   Configuration Option Flags" subregistry so that it applies to Mode of
   Operation (MOP) values zero (0) through six (6) only, leaving the
   flags unassigned for MOP value seven (7).  The MOP is described in
   [RFC6550], Section 6.3.1.

   In addition, this document reserves MOP value 7 for future expansion.

   See Sections 11.2 and 11.3.

4.1.3.  Indicating the New RPI in the DODAG Configuration Option Flag

   In order to avoid a flag day caused by lack of interoperation between
   nodes of the new RPI Option Type (0x23) and old RPI Option Type
   (0x63), this section defines a flag in the DODAG Configuration
   option, to indicate when the new RPI Option Type can be safely used.
   This means that the flag is going to indicate the value of Option
   Type that the network will be using for the RPL Option.  Thus, when a
   node joins to a network, it will know which value to use.  With this,
   RPL-capable nodes know if it is safe to use 0x23 when creating a new
   RPL Option.  A node that forwards a packet with an RPI MUST NOT
   modify the Option Type of the RPL Option.

   This is done using a DODAG Configuration option flag that will signal
   "RPI 0x23 enable" and propagate through the network.  Section 6.3.1
   of [RFC6550] defines a 3-bit Mode of Operation (MOP) in the DIO Base
   Object.  The flag is defined only for MOP value between 0 to 6.

   For a MOP value of 7, a node MUST use the RPI 0x23 option.

   As stated in [RFC6550], the DODAG Configuration option is present in
   DIO messages.  The DODAG Configuration option distributes
   configuration information.  It is generally static, and it does not
   change within the DODAG.  This information is configured at the DODAG
   root and distributed throughout the DODAG with the DODAG
   Configuration option.  Nodes other than the DODAG root do not modify
   this information when propagating the DODAG Configuration option.

   Currently, the DODAG Configuration option in [RFC6550] states that
   the unused bits "MUST be initialized to zero by the sender and MUST
   be ignored by the receiver."  If the flag is received with a value
   zero, which is the default, then new nodes will remain compatible
   with RFC 6553 -- originating traffic with the old RPI Option Type
   value (0x63).  If the flag is received with a value of 1, then the
   value for the RPL Option MUST be set to 0x23.

   Bit number three of the Flags field in the DODAG Configuration option
   is to be used as shown in Table 1 (which is the same as Table 36 in
   Section 11 and is shown here for convenience):

             +============+=================+===============+
             | Bit number |   Description   |   Reference   |
             +============+=================+===============+
             |     3      | RPI 0x23 enable | This document |
             +------------+-----------------+---------------+

               Table 1: DODAG Configuration Option Flag to
                        Indicate the RPI Flag Day

   In the case of reboot, the node (6LN or 6LR) does not remember the
   RPI Option Type (i.e., whether or not the flag is set), so the node
   will not trigger DIO messages until a DIO message is received that
   indicates the RPI value to be used.  The node will use the value 0x23
   if the network supports this feature.

4.2.  Updates to RFC 6553: Indicating the New RPI Option Type

   This modification is required in order to be able to send, for
   example, IPv6 packets from a RPL-aware leaf to a RPL-unaware node
   through the Internet (see Section 7.2.1) without requiring IPv6-in-
   IPv6 encapsulation.

   Section 6 of [RFC6553] states, as shown in Table 2, that in the
   Option Type field of the RPL Option, the two high-order bits must be
   set to '01' and the third bit is equal to '1'.  The first two bits
   indicate that the IPv6 node must discard the packet if it doesn't
   recognize the Option Type, and the third bit indicates that the
   Option Data may change in route.  The remaining bits serve as the
   Option Type.

        +===========+===================+=============+===========+
        | Hex Value |    Binary Value   | Description | Reference |
        |           +=====+=====+=======+             |           |
        |           | act | chg |  rest |             |           |
        +===========+=====+=====+=======+=============+===========+
        |    0x63   |  01 |  1  | 00011 |  RPL Option | [RFC6553] |
        +-----------+-----+-----+-------+-------------+-----------+

                     Table 2: Option Type in RPL Option

   This document illustrates that it is not always possible to know for
   sure at the source whether a packet will travel only within the RPL
   domain or whether it will leave it.

   At the time [RFC6553] was published, leaking a Hop-by-Hop Options
   header in the outer IPv6 header chain could potentially impact core
   routers in the Internet.  So at that time, it was decided to
   encapsulate any packet with a RPL Option using IPv6-in-IPv6 in all
   cases where it was unclear whether the packet would remain within the
   RPL domain.  In the exception case where a packet would still leak,
   the Option Type would ensure that the first router in the Internet
   that does not recognize the option would drop the packet and protect
   the rest of the network.

   Even with [RFC8138], where the IPv6-in-IPv6 header is compressed,
   this approach yields extra bytes in a packet; this means consuming
   more energy and more bandwidth, incurring higher chances of loss, and
   possibly causing a fragmentation at the 6LoWPAN level.  This impacts
   the daily operation of constrained devices for a case that generally
   does not happen and would not heavily impact the core anyway.

   While the intention was and remains that the Hop-by-Hop Options
   header with a RPL Option should be confined within the RPL domain,
   this specification modifies this behavior in order to reduce the
   dependency on IPv6-in-IPv6 and protect the constrained devices.
   Section 4 of [RFC8200] clarifies the behavior of routers in the
   Internet as follows: "it is now expected that nodes along a packet's
   delivery path only examine and process the Hop-by-Hop Options header
   if explicitly configured to do so."

   When unclear about the travel of a packet, it becomes preferable for
   a source not to encapsulate, accepting the fact that the packet may
   leave the RPL domain on its way to its destination.  In that event,
   the packet should reach its destination and should not be discarded
   by the first node that does not recognize the RPL Option.  However,
   with the current value of the Option Type, if a node in the Internet
   is configured to process the Hop-by-Hop Options header, and if such a
   node encounters an Option Type with the first two bits set to 01 and
   the node conforms to [RFC8200], it will drop the packet.  Host
   systems should do the same, irrespective of the configuration.

   Thus, this document updates the Option Type of the RPL Option
   [RFC6553], naming it RPI Option Type for simplicity (Table 3): the
   two high order bits MUST be set to '00', and the third bit is equal
   to '1'.  The first two bits indicate that the IPv6 node MUST skip
   over this option and continue processing the header ([RFC8200],
   Section 4.2) if it doesn't recognize the Option Type, and the third
   bit continues to be set to indicate that the Option Data may change
   en route.  The rightmost five bits remain at 0x3(00011).  This
   ensures that a packet that leaves the RPL domain of an LLN (or that
   leaves the LLN entirely) will not be discarded when it contains the
   RPL Option.

   With the new Option Type, if an IPv6 (intermediate) node (RPL
   unaware) receives a packet with a RPL Option, it should ignore the
   Hop-by-Hop RPL Option (skip over this option and continue processing
   the header).  This is relevant, as it was mentioned previously, in
   the case that there is a flow from RAL to Internet (see
   Section 7.2.1).

   This is a significant update to [RFC6553].

      +===========+===================+=============+===============+
      | Hex Value |    Binary Value   | Description |   Reference   |
      |           +=====+=====+=======+             |               |
      |           | act | chg |  rest |             |               |
      +===========+=====+=====+=======+=============+===============+
      |    0x23   |  00 |  1  | 00011 |  RPL Option | This document |
      +-----------+-----+-----+-------+-------------+---------------+

                 Table 3: Revised Option Type in RPL Option

   Without the signaling described below, this change would otherwise
   create a lack of interoperation (flag day) for existing networks that
   are currently using 0x63 as the RPI Option Type value.  A move to
   0x23 will not be understood by those networks.  It is suggested that
   RPL implementations accept both 0x63 and 0x23 when processing the
   header.

   When forwarding packets, implementations SHOULD use the same value of
   RPI Type as was received.  This is required because the RPI Option
   Type does not change en route ([RFC8200], Section 4.2).  It allows
   the network to be incrementally upgraded and allows the DODAG root to
   know which parts of the network have been upgraded.

   When originating new packets, implementations should have an option
   to determine which value to originate with.  This option is
   controlled by the DODAG Configuration option (Section 4.1.3).

   The change of RPI Option Type from 0x63 to 0x23 makes all nodes that
   are compliant with Section 4.2 of [RFC8200] tolerant of the RPL
   artifacts.  There is no longer a need to remove the artifacts when
   sending traffic to the Internet.  This change clarifies when to use
   IPv6-in-IPv6 headers and how to address them: the Hop-by-Hop Options
   header containing the RPI MUST always be added when 6LRs originate
   packets (without IPv6-in-IPv6 headers), and IPv6-in-IPv6 headers MUST
   always be added when a 6LR finds that it needs to insert a Hop-by-Hop
   Options header containing the RPL Option.  The IPv6-in-IPv6 header is
   to be addressed to the RPL root when on the way up, and to the end
   host when on the way down.

   In the Non-Storing case, dealing with RPL-unaware leaf nodes is much
   easier as the 6LBR (DODAG root) has complete knowledge about the
   connectivity of all DODAG nodes, and all traffic flows through the
   root node.

   The 6LBR can recognize RPL-unaware leaf nodes because it will receive
   a DAO about that node from the 6LR immediately above that RPL-unaware
   node.

   The Non-Storing mode case does not require the Type change from 0x63
   to 0x23, as the root can always create the right packet.  The Type
   change does not adversely affect the Non-Storing case (see
   Section 4.1.3).

4.3.  Updates to RFC 8138: Indicating the Way to Decompress with the New
      RPI Option Type

   This modification is required in order to be able to decompress the
   RPL Option with the new Option Type of 0x23.

   The RPI-6LoRH header provides a compressed form for the RPL RPI; see
   [RFC8138], Section 6.  A node that is decompressing this header MUST
   decompress using the RPI Option Type that is currently active, that
   is, a choice between 0x23 (new) and 0x63 (old).  The node will know
   which to use based upon the presence of the flag in the DODAG
   Configuration option defined in Section 4.1.3.  For example, if the
   network is in 0x23 mode (by DIO option), then it should be
   decompressed to 0x23.

   Section 7 of [RFC8138] documents how to compress the IPv6-in-IPv6
   header.

   There are potential significant advantages to having a single code
   path that always processes IPv6-in-IPv6 headers with no conditional
   branches.

   In Storing mode, the scenarios where the flow goes from RAL to RUL
   and RUL to RUL include compression of the IPv6-in-IPv6 and RPI
   headers.  The IPv6-in-IPv6 header MUST be used in this case, and it
   SHOULD be compressed as specified in [RFC8138], Section 7.  Figure 2
   illustrates the case in Storing mode where the packet is received
   from the Internet, then the root encapsulates the packet to insert
   the RPI.  In that example, the leaf is not known to support RFC 8138,
   and the packet is encapsulated to the 6LR that is the parent and last
   hop to the final destination.

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

             Figure 2: RPI Inserted by the Root in Storing Mode

   In Figure 2, the source of the IPv6-in-IPv6 encapsulation is the
   root, so it is elided in the IP-in-IP 6LoRH.  The destination is the
   parent 6LR of the destination of the inner packet so it cannot be
   elided.  It is placed as the single entry in a Source Route Header
   6LoRH (SRH-6LoRH) as the first 6LoRH.  There is a single entry so the
   SRH-6LoRH Size is zero.  In that example, the Type is 1 so the 6LR
   address is compressed to two bytes.  This results in the total length
   of the SRH-6LoRH being four bytes.  The RPI-6LoRH and then the IP-in-
   IP 6LoRH follow.  When the IP-in-IP 6LoRH is removed, all the router
   headers that precede it are also removed.  The Paging Dispatch
   [RFC8025] may also be removed if there was no previous Page change to
   a Page other than 0 or 1, since the LOWPAN_IPHC is encoded in the
   same fashion in the default Page 0 and in Page 1.  The resulting
   packet to the destination is the inner packet compressed with
   [RFC6282].

5.  Reference Topology

   A RPL network in general is composed of a 6LBR, a Backbone Router
   (6BBR), a 6LR, and a 6LN as a leaf logically organized in a DODAG
   structure.

   Figure 3 shows the reference RPL topology for this document.  The
   nodes are labeled with letters so that they may be referenced in
   subsequent sections.  In the figure, 6LR represents a full router
   node.  The 6LN is a RPL-aware router or host (as a leaf).
   Additionally, for simplification purposes, it is supposed that the
   6LBR has direct access to Internet and is the root of the DODAG, thus
   the 6BBR is not present in the figure.

   The 6LN leaves marked as RAL (F, H, and I) are RPL nodes with no
   children hosts.

   The leaves marked as RUL (G and J) are devices that do not speak RPL
   at all (RPL unaware), but use Router Advertisements, 6LoWPAN
   Duplicate Address Request and Duplicate Address Confirmation (DAR/
   DAC), and 6LoWPAN Neighbor Discovery (ND) only to participate in the
   network [RFC8505].  In the document, these leaves (G and J) are also
   referred to as a RUL.

   The 6LBR (A) in the figure is the root of the Global DODAG.

                     +------------+
                     |  INTERNET  ----------+
                     |            |         |
                     +------------+         |
                                            |
                                            |
                                            |
                                          A |
                                      +-------+
                                      |6LBR   |
                          +-----------|(root) |-------+
                          |           +-------+       |
                          |                           |
                          |                           |
                          |                           |
                          |                           |
                          | B                         |C
                      +---|---+                   +---|---+
                      |  6LR  |                   |  6LR  |
            +---------|       |--+             +---       ---+
            |         +-------+  |             |  +-------+  |
            |                    |             |             |
            |                    |             |             |
            |                    |             |             |
            |                    |             |             |
            | D                  |  E          |             |
          +-|-----+          +---|---+         |             |
          |  6LR  |          |  6LR  |         |             |
          |       |    +------       |         |             |
          +---|---+    |     +---|---+         |             |
              |        |         |             |             |
              |        |         +--+          |             |
              |        |            |          |             |
              |        |            |          |             |
              |        |            |        I |          J  |
           F  |        | G          | H        |             |
        +-----+-+    +-|-----+  +---|--+   +---|---+     +---|---+
        |  RAL  |    | RUL   |  | RAL  |   |  RAL  |     | RUL   |
        |  6LN  |    |  6LN  |  | 6LN  |   |  6LN  |     |  6LN  |
        +-------+    +-------+  +------+   +-------+     +-------+

                     Figure 3: A Reference RPL Topology

6.  Use Cases

   In the data plane, a combination of RFC 6553, RFC 6554, and IPv6-in-
   IPv6 encapsulation are going to be analyzed for a number of
   representative traffic flows.

   The use cases describe the communication in the following cases:

   *  Between RPL-aware nodes with the root (6LBR)

   *  Between RPL-aware nodes with the Internet

   *  Between RUL nodes within the LLN (e.g., see Section 7.1.4)

   *  Inside of the LLN when the final destination address resides
      outside of the LLN (e.g., see Section 7.2.3)

   The use cases are as follows:

      Interaction between leaf and root:

         RAL to root

         root to RAL

         RUL to root

         root to RUL

      Interaction between leaf and Internet:

         RAL to Internet

         Internet to RAL

         RUL to Internet

         Internet to RUL

      Interaction between leaves:

         RAL to RAL

         RAL to RUL

         RUL to RAL

         RUL to RUL

   This document is consistent with the rule that a header cannot be
   inserted or removed on the fly inside an IPv6 packet that is being
   routed.  This is a fundamental precept of the IPv6 architecture as
   outlined in [RFC8200].

   As the Rank information in the RPI artifact is changed at each hop,
   it will typically be zero when it arrives at the DODAG root.  The
   DODAG root MUST force it to zero when passing the packet out to the
   Internet.  The Internet will therefore not see any SenderRank
   information.

   Despite being legal to leave the RPI artifact in place, an
   intermediate router that needs to add an extension header (e.g., RH3
   or RPL Option) MUST still encapsulate the packet in an (additional)
   outer IP header.  The new header is placed after this new outer IP
   header.

   A corollary is that an intermediate router can remove an RH3 or RPL
   Option only if it is placed in an encapsulating IPv6 header that is
   addressed _to_ this intermediate router.  When doing the above, the
   whole encapsulating header must be removed.  (A replacement may be
   added.)

   Both the RPL Option and the RH3 headers may be modified in very
   specific ways by routers on the path of the packet without the need
   to add and remove an encapsulating header.  Both headers were
   designed with this modification in mind, and both the RPL RH3 and the
   RPL Option are marked mutable but recoverable: so an IPsec
   Authentication Header (AH) can be applied across these headers, but
   it cannot secure the values that mutate.

   The RPI MUST be present in every single RPL data packet.

   Prior to [RFC8138], there was significant interest in creating an
   exception to this rule and removing the RPI for Downward flows in
   Non-Storing mode.  This exception covered a very small number of
   cases, and caused significant interoperability challenges while
   adding significant interest in the code and tests.  The ability to
   compress the RPI down to three bytes or less removes much of the
   pressure to optimize this any further.

   Throughout the following subsections, the examples are described in
   more detail in the first subsections, and more concisely in the later
   ones.

   The use cases are delineated based on the following IPV6 and RPL
   mandates:

      The RPI has to be in every packet that traverses the LLN.

      -  Because of the above requirement, packets from the Internet
         have to be encapsulated.

      -  A header cannot be inserted or removed on the fly inside an
         IPv6 packet that is being routed.

      -  Extension headers may not be added or removed except by the
         sender or the receiver.

      -  RPI and RH3 headers may be modified by routers on the path of
         the packet without the need to add and remove an encapsulating
         header.

      -  An RH3 or RPL Option can only be removed by an intermediate
         router if it is placed in an encapsulating IPv6 header, which
         is addressed to the intermediate router.

      -  The Non-Storing mode requires downstream encapsulation by the
         root for RH3.

   The use cases are delineated based on the following assumptions:

      This document assumes that the LLN is using the no-drop RPI Option
      Type (0x23).

      -  Each IPv6 node (including Internet routers) obeys [RFC8200], so
         that the 0x23 RPI Option Type can be safely inserted.

      -  All 6LRs obey [RFC8200].

      -  The RPI is ignored at the IPv6 destination (dst) node (RUL).

      -  In the use cases, we assume that the RAL supports IP-in-IP
         encapsulation.

      -  In the use cases, we don't assume that the RUL supports IP-in-
         IP encapsulation.

      -  For traffic leaving a RUL, if the RUL adds an opaque RPI, then
         the 6LR as a RPL Border Router SHOULD rewrite the RPI to
         indicate the selected Instance and set the flags.

      -  The description for RALs applies to RAN in general.

      -  Unconstrained uses of RPL are not in scope of this document.

      -  Compression is based on [RFC8138].

      -  The flow label [RFC6437] is not needed in RPL.

7.  Storing Mode

   In Storing mode (SM) (fully stateful), the sender can determine if
   the destination is inside the LLN by looking if the destination
   address is matched by the DIO's Prefix Information Option (PIO)
   option.

   Table 4 itemizes which headers are needed in each of the following
   scenarios.  It indicates whether an IPv6-in-IPv6 header must be added
   and to which destination it must be addressed:

   1.  the final destination (the RAL node that is the target (tgt)),

   2.  the "root", or

   3.  the 6LR parent of a RUL.

   In cases where no IPv6-in-IPv6 header is needed, the column states
   "No", and the destination is N/A (Not Applicable).  If the IPv6-in-
   IPv6 header is needed, the column shows "must".

   In all cases, the RPI is needed, since it identifies inconsistencies
   (loops) in the routing topology.  In general, the RH3 is not needed
   because it is not used in Storing mode.  However, there is one
   scenario (from the root to the RUL in SM) where the RH3 can be used
   to point at the RUL (Table 8).

   The leaf can be a router 6LR or a host, both indicated as 6LN.  The
   root refers to the 6LBR (see Figure 3).

   +=====================+==========+==============+==================+
   | Interaction between | Use Case | IPv6-in-IPv6 | IPv6-in-IPv6 dst |
   +=====================+==========+==============+==================+
   |     Leaf - Root     |  RAL to  |      No      |       N/A        |
   |                     |   root   |              |                  |
   |                     +----------+--------------+------------------+
   |                     | root to  |      No      |       N/A        |
   |                     |   RAL    |              |                  |
   |                     +----------+--------------+------------------+
   |                     | root to  |     must     |       6LR        |
   |                     |   RUL    |              |                  |
   |                     +----------+--------------+------------------+
   |                     |  RUL to  |     must     |       root       |
   |                     |   root   |              |                  |
   +=====================+----------+--------------+------------------+
   |   Leaf - Internet   |  RAL to  |     may      |       root       |
   |                     |   Int    |              |                  |
   |                     +----------+--------------+------------------+
   |                     |  Int to  |     must     |    RAL (tgt)     |
   |                     |   RAL    |              |                  |
   |                     +----------+--------------+------------------+
   |                     |  RUL to  |     must     |       root       |
   |                     |   Int    |              |                  |
   |                     +----------+--------------+------------------+
   |                     |  Int to  |     must     |       6LR        |
   |                     |   RUL    |              |                  |
   +=====================+----------+--------------+------------------+
   |     Leaf - Leaf     |  RAL to  |      No      |       N/A        |
   |                     |   RAL    |              |                  |
   |                     +----------+--------------+------------------+
   |                     |  RAL to  |    No(up)    |       N/A        |
   |                     |   RUL    +--------------+------------------+
   |                     |          |  must(down)  |       6LR        |
   |                     +----------+--------------+------------------+
   |                     |  RUL to  |   must(up)   |       root       |
   |                     |   RAL    +--------------+------------------+
   |                     |          |  must(down)  |       RAL        |
   |                     +----------+--------------+------------------+
   |                     |  RUL to  |   must(up)   |       root       |
   |                     |   RUL    +--------------+------------------+
   |                     |          |  must(down)  |       6LR        |
   +=====================+----------+--------------+------------------+

           Table 4: IPv6-in-IPv6 Encapsulation in Storing Mode

7.1.  Storing Mode: Interaction between Leaf and Root

   This section describes the communication flow in Storing mode (SM)
   between the following:

      RAL to root

      root to RAL

      RUL to root

      root to RUL

7.1.1.  SM: Example of Flow from RAL to Root

   In Storing mode, RPI [RFC6553] is used to send the RPLInstanceID and
   Rank information.

   In this case, the flow comprises:

   RAL (6LN) --> 6LR_i --> root (6LBR)

   For example, a communication flow could be: Node F (6LN) --> Node D
   (6LR_i) --> Node B (6LR_i) --> Node A root (6LBR)

   The RAL (Node F) inserts the RPI, and sends the packet to the 6LR
   (Node D), which decrements the Rank in the RPI and sends the packet
   up.  When the packet arrives at the 6LBR (Node A), the RPI is removed
   and the packet is processed.

   No IPv6-in-IPv6 header is required.

   The RPI can be removed by the 6LBR because the packet is addressed to
   the 6LBR.  The RAL must know that it is communicating with the 6LBR
   to make use of this scenario.  The RAL can know the address of the
   6LBR because it knows the address of the root via the DODAGID in the
   DIO messages.

   Table 5 summarizes which headers are needed for this use case.

            +===================+=========+=======+==========+
            |       Header      | RAL src | 6LR_i | 6LBR dst |
            +===================+=========+=======+==========+
            |   Added headers   |   RPI   |   --  |    --    |
            +===================+---------+-------+----------+
            |  Modified headers |    --   |  RPI  |    --    |
            +===================+---------+-------+----------+
            |  Removed headers  |    --   |   --  |   RPI    |
            +===================+---------+-------+----------+
            | Untouched headers |    --   |   --  |    --    |
            +===================+---------+-------+----------+

                Table 5: SM: Summary of the Use of Headers
                             from RAL to Root

7.1.2.  SM: Example of Flow from Root to RAL

   In this case, the flow comprises:

   root (6LBR) --> 6LR_i --> RAL (6LN)

   For example, a communication flow could be: Node A root (6LBR) -->
   Node B (6LR_i) --> Node D (6LR_i) --> Node F (6LN)

   In this case, the 6LBR inserts RPI and sends the packet down.  The
   6LR increments the Rank in the RPI (it examines the RPLInstanceID to
   identify the right forwarding table).  The packet is processed in the
   RAL, and the RPI is removed.

   No IPv6-in-IPv6 header is required.

   Table 6 summarizes which headers are needed for this use case.

            +===================+==========+=======+=========+
            |       Header      | 6LBR src | 6LR_i | RAL dst |
            +===================+==========+=======+=========+
            |   Added headers   |   RPI    |   --  |    --   |
            +===================+----------+-------+---------+
            |  Modified headers |    --    |  RPI  |    --   |
            +===================+----------+-------+---------+
            |  Removed headers  |    --    |   --  |   RPI   |
            +===================+----------+-------+---------+
            | Untouched headers |    --    |   --  |    --   |
            +===================+----------+-------+---------+

                Table 6: SM: Summary of the Use of Headers
                             from Root to RAL

7.1.3.  SM: Example of Flow from Root to RUL

   In this case, the flow comprises:

   root (6LBR) --> 6LR_i --> RUL (IPv6 dst node)

   For example, a communication flow could be: Node A (6LBR) --> Node B
   (6LR_i) --> Node E (6LR_n) --> Node G (RUL)

   6LR_i (Node B) represents the intermediate routers from the source
   (6LBR) to the destination (RUL), and 1 <= i <= n, where n is the
   total number of routers (6LR) that the packet goes through, from the
   6LBR (Node A) to the RUL (Node G).

   The 6LBR will encapsulate the packet in an IPv6-in-IPv6 header and
   prepend an RPI.  The IPv6-in-IPv6 header is addressed to the 6LR
   parent of the RUL (6LR_n).  The 6LR parent of the RUL removes the
   header and sends the packet to the RUL.

   Table 7 summarizes which headers are needed for this use case.

    +==================+===============+=========+=========+=========+
    |      Header      |    6LBR src   |  6LR_i  |  6LR_n  | RUL dst |
    +==================+===============+=========+=========+=========+
    |  Added headers   | IP6-IP6 (RPI) |    --   |    --   |    --   |
    +==================+---------------+---------+---------+---------+
    | Modified headers |       --      |   RPI   |    --   |    --   |
    +==================+---------------+---------+---------+---------+
    | Removed headers  |       --      |    --   | IP6-IP6 |    --   |
    |                  |               |         |  (RPI)  |         |
    +==================+---------------+---------+---------+---------+
    |    Untouched     |       --      | IP6-IP6 |    --   |    --   |
    |     headers      |               |         |         |         |
    +==================+---------------+---------+---------+---------+

       Table 7: SM: Summary of the Use of Headers from Root to RUL

   IP-in-IP encapsulation may be avoided for root-to-RUL communication.
   In SM, it can be replaced by a loose RH3 header that indicates the
   RUL.  In which case, the packet is routed to the 6LR as a normal SM
   operation, then the 6LR forwards to the RUL based on the RH3, and the
   RUL ignores both the consumed RH3 and the RPI, as in Non-Storing
   mode.

   Table 8 summarizes which headers are needed for this scenario.

   +===========+======+==============+===============+================+
   |   Header  | 6LBR |    6LR_i     |     6LR_n     |    RUL dst     |
   |           | src  | i=(1,..,n-1) |               |                |
   +===========+======+==============+===============+================+
   |   Added   | RPI, |      --      |       --      |       --       |
   |  headers  | RH3  |              |               |                |
   +===========+------+--------------+---------------+----------------+
   |  Modified |  --  |     RPI      |      RPI,     |       --       |
   |  headers  |      |              | RH3(consumed) |                |
   +===========+------+--------------+---------------+----------------+
   |  Removed  |  --  |      --      |       --      |       --       |
   |  headers  |      |              |               |                |
   +===========+------+--------------+---------------+----------------+
   | Untouched |  --  |     RH3      |       --      | RPI, RH3 (both |
   |  headers  |      |              |               |    ignored)    |
   +===========+------+--------------+---------------+----------------+

   Table 8: SM: Summary of the Use of Headers from Root to RUL without
                              Encapsulation

7.1.4.  SM: Example of Flow from RUL to Root

   In this case, the flow comprises:

   RUL (IPv6 src node) --> 6LR_1 --> 6LR_i --> root (6LBR)

   For example, a communication flow could be: Node G (RUL) --> Node E
   (6LR_1) --> Node B (6LR_i) --> Node A root (6LBR)

   6LR_i represents the intermediate routers from the source (RUL) to
   the destination (6LBR), and 1 <= i <= n, where n is the total number
   of routers (6LR) that the packet goes through, from the RUL to the
   6LBR.

   When the packet arrives from the RUL (Node G) to 6LR_1 (Node E), the
   6LR_1 will encapsulate the packet in an IPv6-in-IPv6 header with an
   RPI.  The IPv6-in-IPv6 header is addressed to the root (Node A).  The
   root removes the header and processes the packet.

   Table 9 summarizes which headers are needed for this use case where
   the IPv6-in-IPv6 header is addressed to the root (Node A).

    +==================+=========+===============+=========+==========+
    |      Header      | RUL src |     6LR_1     |  6LR_i  | 6LBR dst |
    +==================+=========+===============+=========+==========+
    |  Added headers   |    --   | IP6-IP6 (RPI) |    --   |    --    |
    +==================+---------+---------------+---------+----------+
    | Modified headers |    --   |       --      |   RPI   |    --    |
    +==================+---------+---------------+---------+----------+
    | Removed headers  |    --   |       --      |    --   | IP6-IP6  |
    |                  |         |               |         |  (RPI)   |
    +==================+---------+---------------+---------+----------+
    |    Untouched     |    --   |       --      | IP6-IP6 |    --    |
    |     headers      |         |               |         |          |
    +==================+---------+---------------+---------+----------+

        Table 9: SM: Summary of the Use of Headers from RUL to Root

7.2.  SM: Interaction between Leaf and Internet

   This section describes the communication flow in Storing mode (SM)
   between the following:

      RAL to Internet

      Internet to RAL

      RUL to Internet

      Internet to RUL

7.2.1.  SM: Example of Flow from RAL to Internet

   In this case, the flow comprises:

   RAL (6LN) --> 6LR_i --> root (6LBR) --> Internet

   For example, the communication flow could be: Node F (RAL) --> Node D
   (6LR_i) --> Node B (6LR_i) --> Node A root (6LBR) --> Internet

   6LR_i represents the intermediate routers from the source (RAL) to
   the root (6LBR), and 1 <= i <= n, where n is the total number of
   routers (6LR) that the packet goes through, from the RAL to the 6LBR.

   RPL information from RFC 6553 may go out to Internet as it will be
   ignored by nodes that have not been configured to be RPL aware.  No
   IPv6-in-IPv6 header is required.

   On the other hand, the RAL may insert the RPI encapsulated in an
   IPv6-in-IPv6 header to the root.  Thus, the root removes the RPI and
   sends the packet to the Internet.

      |  Note: In this use case, a leaf node is used, but this use case
      |  can also be applicable to any RPL-aware node type (e.g., 6LR).

   Table 10 summarizes which headers are needed for this use case when
   there is no encapsulation.  Note that the RPI is modified by 6LBR to
   set the SenderRank to zero in the case that it is not already zero.
   Table 11 summarizes which headers are needed when encapsulation to
   the root takes place.

      +===================+=========+=======+======+===============+
      |       Header      | RAL src | 6LR_i | 6LBR |  Internet dst |
      +===================+=========+=======+======+===============+
      |   Added headers   |   RPI   |   --  |  --  |       --      |
      +===================+---------+-------+------+---------------+
      |  Modified headers |    --   |  RPI  | RPI  |       --      |
      +===================+---------+-------+------+---------------+
      |  Removed headers  |    --   |   --  |  --  |       --      |
      +===================+---------+-------+------+---------------+
      | Untouched headers |    --   |   --  |  --  | RPI (Ignored) |
      +===================+---------+-------+------+---------------+

         Table 10: SM: Summary of the Use of Headers from RAL to
                      Internet with No Encapsulation

   +===============+===============+=========+=========+==============+
   |     Header    |    RAL src    |  6LR_i  |   6LBR  | Internet dst |
   +===============+===============+=========+=========+==============+
   | Added headers | IP6-IP6 (RPI) |    --   |    --   |      --      |
   +===============+---------------+---------+---------+--------------+
   |    Modified   |       --      |   RPI   |    --   |      --      |
   |    headers    |               |         |         |              |
   +===============+---------------+---------+---------+--------------+
   |    Removed    |       --      |    --   | IP6-IP6 |      --      |
   |    headers    |               |         |  (RPI)  |              |
   +===============+---------------+---------+---------+--------------+
   |   Untouched   |       --      | IP6-IP6 |    --   |      --      |
   |    headers    |               |         |         |              |
   +===============+---------------+---------+---------+--------------+

     Table 11: SM: Summary of the Use of Headers from RAL to Internet
                  with Encapsulation to the Root (6LBR)

7.2.2.  SM: Example of Flow from Internet to RAL

   In this case, the flow comprises:

   Internet --> root (6LBR) --> 6LR_i --> RAL (6LN)

   For example, a communication flow could be: Internet --> Node A root
   (6LBR) --> Node B (6LR_1) --> Node D (6LR_n) --> Node F (RAL)

   When the packet arrives from Internet to 6LBR, the RPI is added in a
   outer IPv6-in-IPv6 header (with the IPv6-in-IPv6 destination address
   set to the RAL) and sent to the 6LR, which modifies the Rank in the
   RPI.  When the packet arrives at the RAL, the packet is decapsulated,
   which removes the RPI before the packet is processed.

   Table 12 summarizes which headers are needed for this use case.

   +==================+==============+===============+=======+=========+
   |      Header      | Internet src |      6LBR     | 6LR_i | RAL dst |
   +==================+==============+===============+=======+=========+
   |  Added headers   |      --      | IP6-IP6 (RPI) |   --  |    --   |
   +==================+--------------+---------------+-------+---------+
   |     Modified     |      --      |       --      |  RPI  |    --   |
   |     headers      |              |               |       |         |
   +==================+--------------+---------------+-------+---------+
   |     Removed      |      --      |       --      |   --  | IP6-IP6 |
   |     headers      |              |               |       |  (RPI)  |
   +==================+--------------+---------------+-------+---------+
   |    Untouched     |      --      |       --      |   --  |    --   |
   |     headers      |              |               |       |         |
   +==================+--------------+---------------+-------+---------+

      Table 12: SM: Summary of the Use of Headers from Internet to RAL

7.2.3.  SM: Example of Flow from RUL to Internet

   In this case, the flow comprises:

   RUL (IPv6 src node) --> 6LR_1 --> 6LR_i --> root (6LBR) --> Internet

   For example, a communication flow could be: Node G (RUL) --> Node E
   (6LR_1) --> Node B (6lR_i) --> Node A root (6LBR) --> Internet

   The node 6LR_1 (i=1) will add an IPv6-in-IPv6 (RPI) header addressed
   to the root such that the root can remove the RPI before passing
   upwards.  In the intermediate 6LR, the Rank in the RPI is modified.

   The originating node will ideally leave the IPv6 flow label as zero
   so that the packet can be better compressed through the LLN.  The
   6LBR will set the flow label of the packet to a non-zero value when
   sending to the Internet.  For details, check [RFC6437].

   Table 13 summarizes which headers are needed for this use case.

   +===========+==========+=========+============+=========+==========+
   |   Header  | IPv6 src |  6LR_1  |   6LR_i    |   6LBR  | Internet |
   |           |  (RUL)   |         | i=(2,..,n) |         |   dst    |
   +===========+==========+=========+============+=========+==========+
   |   Added   |    --    | IP6-IP6 |     --     |    --   |    --    |
   |  headers  |          |  (RPI)  |            |         |          |
   +===========+----------+---------+------------+---------+----------+
   |  Modified |    --    |    --   |    RPI     |    --   |    --    |
   |  headers  |          |         |            |         |          |
   +===========+----------+---------+------------+---------+----------+
   |  Removed  |    --    |    --   |     --     | IP6-IP6 |    --    |
   |  headers  |          |         |            |  (RPI)  |          |
   +===========+----------+---------+------------+---------+----------+
   | Untouched |    --    |    --   |     --     |    --   |    --    |
   |  headers  |          |         |            |         |          |
   +===========+----------+---------+------------+---------+----------+

     Table 13: SM: Summary of the Use of Headers from RUL to Internet

7.2.4.  SM: Example of Flow from Internet to RUL

   In this case, the flow comprises:

   Internet --> root (6LBR) --> 6LR_i --> RUL (IPv6 dst node)

   For example, a communication flow could be: Internet --> Node A root
   (6LBR) --> Node B (6LR_i) --> Node E (6LR_n) --> Node G (RUL)

   The 6LBR will have to add an RPI within an IPv6-in-IPv6 header.  The
   IPv6-in-IPv6 encapsulating header is addressed to the 6LR parent of
   the RUL.

   Further details about this are mentioned in [RFC9010], which
   specifies RPL routing for a 6LN acting as a plain host and being
   unaware of RPL.

   The 6LBR may set the flow label on the inner IPv6-in-IPv6 header to
   zero in order to aid in compression [RFC8138] [RFC6437].

   Table 14 summarizes which headers are needed for this use case.

   +===========+==============+=========+==============+=========+=====+
   |   Header  |   Internet   |   6LBR  |    6LR_i     |  6LR_n  | RUL |
   |           |     src      |         | i=(1,..,n-1) |         | dst |
   +===========+==============+=========+==============+=========+=====+
   |   Added   |      --      | IP6-IP6 |      --      |    --   |  -- |
   |  headers  |              |  (RPI)  |              |         |     |
   +===========+--------------+---------+--------------+---------+-----+
   |  Modified |      --      |    --   |     RPI      |    --   |  -- |
   |  headers  |              |         |              |         |     |
   +===========+--------------+---------+--------------+---------+-----+
   |  Removed  |      --      |    --   |      --      | IP6-IP6 |  -- |
   |  headers  |              |         |              |  (RPI)  |     |
   +===========+--------------+---------+--------------+---------+-----+
   | Untouched |      --      |    --   |      --      |    --   |  -- |
   |  headers  |              |         |              |         |     |
   +===========+--------------+---------+--------------+---------+-----+

      Table 14: SM: Summary of the Use of Headers from Internet to RUL

7.3.  SM: Interaction between Leaf and Leaf

   This section describes the communication flow in Storing mode (SM)
   between the following:

      RAL to RAL

      RAL to RUL

      RUL to RAL

      RUL to RUL

7.3.1.  SM: Example of Flow from RAL to RAL

   In [RFC6550], RPL allows a simple, one-hop optimization for both
   Storing and Non-Storing networks.  A node may send a packet destined
   to a one-hop neighbor directly to that node.  See Section 9 of
   [RFC6550].

   When the nodes are not directly connected, then the flow comprises
   the following in the Storing mode:

   RAL src (6LN) --> 6LR_ia --> common parent (6LR_x) --> 6LR_id --> RAL
   dst (6LN)

   For example, a communication flow could be: Node F (RAL src) --> Node
   D (6LR_ia) --> Node B (6LR_x) --> Node E (6LR_id) --> Node H (RAL
   dst)

   6LR_ia (Node D) represents the intermediate routers from the source
   to the common parent 6LR_x (Node B), and 1 <= ia <= n, where n is the
   total number of routers (6LR) that the packet goes through, from the
   RAL (Node F) to the common parent 6LR_x (Node B).

   6LR_id (Node E) represents the intermediate routers from the common
   parent 6LR_x (Node B) to the destination RAL (Node H), and 1 <= id <=
   m, where m is the total number of routers (6LR) that the packet goes
   through, from the common parent (6LR_x) to the destination RAL (Node
   H).

   It is assumed that the two nodes are in the same RPL domain (that
   they share the same DODAG root).  At the common parent (Node B), the
   direction flag ('O' flag) of the RPI is changed (from decreasing
   ranks to increasing ranks).

   While the 6LR nodes will update the RPI, no node needs to add or
   remove the RPI, so no IPv6-in-IPv6 headers are necessary.

   Table 15 summarizes which headers are needed for this use case.

      +===========+=========+========+===============+========+=====+
      |   Header  | RAL src | 6LR_ia | 6LR_x (common | 6LR_id | RAL |
      |           |         |        |    parent)    |        | dst |
      +===========+=========+========+===============+========+=====+
      |   Added   |   RPI   |   --   |       --      |   --   |  -- |
      |  headers  |         |        |               |        |     |
      +===========+---------+--------+---------------+--------+-----+
      |  Modified |    --   |  RPI   |      RPI      |  RPI   |  -- |
      |  headers  |         |        |               |        |     |
      +===========+---------+--------+---------------+--------+-----+
      |  Removed  |    --   |   --   |       --      |   --   | RPI |
      |  headers  |         |        |               |        |     |
      +===========+---------+--------+---------------+--------+-----+
      | Untouched |    --   |   --   |       --      |   --   |  -- |
      |  headers  |         |        |               |        |     |
      +===========+---------+--------+---------------+--------+-----+

        Table 15: SM: Summary of the Use of Headers from RAL to RAL

7.3.2.  SM: Example of Flow from RAL to RUL

   In this case, the flow comprises:

   RAL src (6LN) --> 6LR_ia --> common parent (6LBR, the root) -->
   6LR_id --> RUL (IPv6 dst node)

   For example, a communication flow could be: Node F (RAL) --> Node D
   --> Node B --> Node A --> Node B --> Node E --> Node G (RUL)

   6LR_ia represents the intermediate routers from the source (RAL) to
   the common parent (the root), and 1 <= ia <= n, where n is the total
   number of routers (6LR) that the packet goes through, from the RAL to
   the root.

   6LR_id (Node E) represents the intermediate routers from the root
   (Node B) to the destination RUL (Node G).  In this case, 1 <= id <=
   m, where m is the total number of routers (6LR) that the packet goes
   through, from the root down to the destination RUL.

   In this case, the packet from the RAL goes to the 6LBR because the
   route to the RUL is not injected into the RPL SM.  Thus, the RAL
   inserts an RPI (RPI1) addressed to the root (6LBR).  The root does
   not remove the RPI1 (the root cannot remove an RPI if there is no
   encapsulation).  The root inserts an IPv6-in-IPv6 encapsulation with
   an RPI2 and sends it to the 6LR parent of the RUL, which removes the
   encapsulation and RPI2 before passing the packet to the RUL.

   Table 16 summarizes which headers are needed for this use case.

   +===========+=====+========+=========+========+=========+===========+
   |   Header  | RAL | 6LR_ia |   6LBR  | 6LR_id |  6LR_m  |  RUL dst  |
   |           | src |        |         |        |         |           |
   +===========+=====+========+=========+========+=========+===========+
   |   Added   | RPI1|   --   | IP6-IP6 |   --   |    --   |     --    |
   |  headers  |     |        |  (RPI2) |        |         |           |
   +===========+-----+--------+---------+--------+---------+-----------+
   |  Modified |  -- |  RPI1  |    --   |  RPI2  |    --   |     --    |
   |  headers  |     |        |         |        |         |           |
   +===========+-----+--------+---------+--------+---------+-----------+
   |  Removed  |  -- |   --   |    --   |   --   | IP6-IP6 |     --    |
   |  headers  |     |        |         |        |  (RPI2) |           |
   +===========+-----+--------+---------+--------+---------+-----------+
   | Untouched |  -- |   --   |   RPI1  |  RPI1  |   RPI1  |    RPI1   |
   |  headers  |     |        |         |        |         | (ignored) |
   +===========+-----+--------+---------+--------+---------+-----------+

        Table 16: SM: Summary of the Use of Headers from RAL to RUL

7.3.3.  SM: Example of Flow from RUL to RAL

   In this case, the flow comprises:

   RUL (IPv6 src node) --> 6LR_ia --> 6LBR --> 6LR_id --> RAL dst (6LN)

   For example, a communication flow could be: Node G (RUL) --> Node E
   --> Node B --> Node A --> Node B --> Node D --> Node F (RAL)

   6LR_ia (Node E) represents the intermediate routers from the source
   (RUL) (Node G) to the root (Node A).  In this case, 1 <= ia <= n,
   where n is the total number of routers (6LR) that the packet goes
   through, from the source to the root.

   6LR_id represents the intermediate routers from the root (Node A) to
   the destination RAL (Node F).  In this case, 1 <= id <= m, where m is
   the total number of routers (6LR) that the packet goes through, from
   the root to the destination RAL.

   The 6LR_1 (Node E) receives the packet from the RUL (Node G) and
   inserts the RPI (RPI1) encapsulated in an IPv6-in-IPv6 header to the
   root.  The root removes the outer header including the RPI (RPI1) and
   inserts a new RPI (RPI2) addressed to the destination RAL (Node F).

   Table 17 summarizes which headers are needed for this use case.

    +===========+=====+=========+========+=========+========+=========+
    |   Header  | RUL |  6LR_1  | 6LR_ia |   6LBR  | 6LR_id | RAL dst |
    |           | src |         |        |         |        |         |
    +===========+=====+=========+========+=========+========+=========+
    |   Added   |  -- | IP6-IP6 |   --   | IP6-IP6 |   --   |    --   |
    |  headers  |     |  (RPI1) |        |  (RPI2) |        |         |
    +===========+-----+---------+--------+---------+--------+---------+
    |  Modified |  -- |    --   |  RPI1  |    --   |  RPI2  |    --   |
    |  headers  |     |         |        |         |        |         |
    +===========+-----+---------+--------+---------+--------+---------+
    |  Removed  |  -- |    --   |   --   | IP6-IP6 |   --   | IP6-IP6 |
    |  headers  |     |         |        |  (RPI1) |        |  (RPI2) |
    +===========+-----+---------+--------+---------+--------+---------+
    | Untouched |  -- |    --   |   --   |    --   |   --   |    --   |
    |  headers  |     |         |        |         |        |         |
    +===========+-----+---------+--------+---------+--------+---------+

        Table 17: SM: Summary of the Use of Headers from RUL to RAL

7.3.4.  SM: Example of Flow from RUL to RUL

   In this case, the flow comprises:

   RUL (IPv6 src node) --> 6LR_1 --> 6LR_ia --> 6LBR --> 6LR_id --> RUL
   (IPv6 dst node)

   For example, a communication flow could be: Node G (RUL src) --> Node
   E --> Node B --> Node A (root) --> Node C --> Node J (RUL dst)

   Internal nodes 6LR_ia (e.g., Node E or Node B) is the intermediate
   router from the RUL source (Node G) to the root (6LBR) (Node A).  In
   this case, 1 <= ia <= n, where n is the total number of routers (6LR)
   that the packet goes through, from the RUL to the root. 6LR_1 applies
   when ia=1.

   6LR_id (Node C) represents the intermediate routers from the root
   (Node A) to the destination RUL (Node J).  In this case, 1 <= id <=
   m, where m is the total number of routers (6LR) that the packet goes
   through, from the root to the destination RUL.

   The 6LR_1 (Node E) receives the packet from the RUL (Node G) and adds
   the RPI (RPI1) in an IPv6-in-IPv6 encapsulation directed to the root.
   The root removes the outer header including the RPI (RPI1) and
   inserts a new RPI (RPI2) addressed to the 6LR parent of the RUL.

   Table 18 summarizes which headers are needed for this use case.

   +===========+===+=========+========+=========+========+=========+===+
   |   Header  |RUL|  6LR_1  | 6LR_ia |   6LBR  | 6LR_id |  6LR_n  |RUL|
   |           |src|         |        |         |        |         |dst|
   +===========+===+=========+========+=========+========+=========+===+
   |   Added   | --| IP6-IP6 |   --   | IP6-IP6 |   --   |    --   | --|
   |  headers  |   |  (RPI1) |        |  (RPI1) |        |         |   |
   +===========+---+---------+--------+---------+--------+---------+---+
   |  Modified | --|    --   |  RPI1  |    --   |  RPI2  |    --   | --|
   |  headers  |   |         |        |         |        |         |   |
   +===========+---+---------+--------+---------+--------+---------+---+
   |  Removed  | --|    --   |   --   | IP6-IP6 |   --   | IP6-IP6 | --|
   |  headers  |   |         |        |  (RPI1) |        |  (RPI2) |   |
   +===========+---+---------+--------+---------+--------+---------+---+
   | Untouched | --|    --   |   --   |    --   |   --   |    --   | --|
   |  headers  |   |         |        |         |        |         |   |
   +===========+---+---------+--------+---------+--------+---------+---+

        Table 18: SM: Summary of the Use of Headers from RUL to RUL

8.  Non-Storing Mode

   In Non-Storing mode (Non-SM) (fully source routed), the 6LBR (DODAG
   root) has complete knowledge about the connectivity of all DODAG
   nodes and all traffic flows through the root node.  Thus, there is no
   need for all nodes to know about the existence of RPL-unaware nodes.
   Only the 6LBR needs to act if compensation is necessary for RPL-
   unaware receivers.

   Table 19 summarizes which headers are needed in the following
   scenarios and indicates when the RPI, RH3, and IPv6-in-IPv6 header
   are to be inserted.  The last column depicts the target destination
   of the IPv6-in-IPv6 header: 6LN (indicated by "RAL"), 6LR (parent of
   a RUL), or the root.  In cases where no IPv6-in-IPv6 header is
   needed, the column indicates "No".  There is no expectation on RPL
   that RPI can be omitted because it is needed for routing, quality of
   service, and compression.  This specification expects that an RPI is
   always present.  The term "may(up)" means that the IPv6-in-IPv6
   header may be necessary in the Upward direction.  The term "must(up)"
   means that the IPv6-in-IPv6 header must be present in the Upward
   direction.  The term "must(down)" means that the IPv6-in-IPv6 header
   must be present in the Downward direction.

   The leaf can be a router 6LR or a host, both indicated as 6LN
   (Figure 3).  In Table 19, the (1) indicates a 6TiSCH case [RFC8180],
   where the RPI may still be needed for the RPLInstanceID to be
   available for priority/channel selection at each hop.

      +=============+========+=====+=====+==============+==========+
      | Interaction |  Use   | RPI | RH3 | IPv6-in-IPv6 | IP-in-IP |
      |   between   |  Case  |     |     |              |   dst    |
      +=============+========+=====+=====+==============+==========+
      | Leaf - Root | RAL to | Yes |  No |      No      |    No    |
      |             |  root  |     |     |              |          |
      |             +--------+-----+-----+--------------+----------+
      |             |  root  | Yes | Yes |      No      |    No    |
      |             | to RAL |     |     |              |          |
      |             +--------+-----+-----+--------------+----------+
      |             |  root  | Yes | Yes |      No      |   6LR    |
      |             | to RUL | (1) |     |              |          |
      |             +--------+-----+-----+--------------+----------+
      |             | RUL to | Yes |  No |     must     |   root   |
      |             |  root  |     |     |              |          |
      +=============+--------+-----+-----+--------------+----------+
      |    Leaf -   | RAL to | Yes |  No |   may(up)    |   root   |
      |   Internet  |  Int   |     |     |              |          |
      |             +--------+-----+-----+--------------+----------+
      |             | Int to | Yes | Yes |     must     |   RAL    |
      |             |  RAL   |     |     |              |          |
      |             +--------+-----+-----+--------------+----------+
      |             | RUL to | Yes |  No |     must     |   root   |
      |             |  Int   |     |     |              |          |
      |             +--------+-----+-----+--------------+----------+
      |             | Int to | Yes | Yes |     must     |   6LR    |
      |             |  RUL   |     |     |              |          |
      +=============+--------+-----+-----+--------------+----------+
      | Leaf - Leaf | RAL to | Yes | Yes |   may(up)    |   root   |
      |             |  RAL   |     |     +--------------+----------+
      |             |        |     |     |  must(down)  |   RAL    |
      |             +--------+-----+-----+--------------+----------+
      |             | RAL to | Yes | Yes |   may(up)    |   root   |
      |             |  RUL   |     |     +--------------+----------+
      |             |        |     |     |  must(down)  |   6LR    |
      |             +--------+-----+-----+--------------+----------+
      |             | RUL to | Yes | Yes |   must(up)   |   root   |
      |             |  RAL   |     |     +--------------+----------+
      |             |        |     |     |  must(down)  |   RAL    |
      |             +--------+-----+-----+--------------+----------+
      |             | RUL to | Yes | Yes |   must(up)   |   root   |
      |             |  RUL   |     |     +--------------+----------+
      |             |        |     |     |  must(down)  |   6LR    |
      +=============+--------+-----+-----+--------------+----------+

         Table 19: Headers Needed in Non-Storing Mode: RPI, RH3,
                        IPv6-in-IPv6 Encapsulation

8.1.  Non-Storing Mode: Interaction between Leaf and Root

   This section describes the communication flow in Non-Storing mode
   (Non-SM) between the following:

      RAL to root

      root to RAL

      RUL to root

      root to RUL

8.1.1.  Non-SM: Example of Flow from RAL to Root

   In Non-Storing mode, the leaf node uses default routing to send
   traffic to the root.  The RPI must be included since it contains the
   Rank information, which is used to avoid and/or detect loops.

   RAL (6LN) --> 6LR_i --> root(6LBR)

   For example, a communication flow could be: Node F --> Node D -->
   Node B --> Node A (root)

   6LR_i represents the intermediate routers from the source to the
   destination.  In this case, 1 <= i <= n, where n is the total number
   of routers (6LR) that the packet goes through, from the source (RAL)
   to the destination (6LBR).

   This situation is the same case as Storing mode.

   Table 20 summarizes which headers are needed for this use case.

            +===================+=========+=======+==========+
            |       Header      | RAL src | 6LR_i | 6LBR dst |
            +===================+=========+=======+==========+
            |   Added headers   |   RPI   |   --  |    --    |
            +===================+---------+-------+----------+
            |  Modified headers |    --   |  RPI  |    --    |
            +===================+---------+-------+----------+
            |  Removed headers  |    --   |   --  |   RPI    |
            +===================+---------+-------+----------+
            | Untouched headers |    --   |   --  |    --    |
            +===================+---------+-------+----------+

                 Table 20: Non-SM: Summary of the Use of
                         Headers from RAL to Root

8.1.2.  Non-SM: Example of Flow from Root to RAL

   In this case, the flow comprises:

   root (6LBR) --> 6LR_i --> RAL (6LN)

   For example, a communication flow could be: Node A (root) --> Node B
   --> Node D --> Node F

   6LR_i represents the intermediate routers from the source to the
   destination.  In this case, 1 <= i <= n, where n is the total number
   of routers (6LR) that the packet goes through, from the source (6LBR)
   to the destination (RAL).

   The 6LBR inserts an RH3 and an RPI.  No IPv6-in-IPv6 header is
   necessary as the traffic originates with a RPL-aware node, the 6LBR.
   The destination is known to be RPL aware because the root knows the
   whole topology in Non-Storing mode.

   Table 21 summarizes which headers are needed for this use case.

          +===================+==========+==========+==========+
          |       Header      | 6LBR src |  6LR_i   | RAL dst  |
          +===================+==========+==========+==========+
          |   Added headers   | RPI, RH3 |    --    |    --    |
          +===================+----------+----------+----------+
          |  Modified headers |    --    | RPI, RH3 |    --    |
          +===================+----------+----------+----------+
          |  Removed headers  |    --    |    --    | RPI, RH3 |
          +===================+----------+----------+----------+
          | Untouched headers |    --    |    --    |    --    |
          +===================+----------+----------+----------+

             Table 21: Non-SM: Summary of the Use of Headers
                             from Root to RAL

8.1.3.  Non-SM: Example of Flow from Root to RUL

   In this case, the flow comprises:

   root (6LBR) --> 6LR_i --> RUL (IPv6 dst node)

   For example, a communication flow could be: Node A (root) --> Node B
   --> Node E --> Node G (RUL)

   6LR_i represents the intermediate routers from the source to the
   destination.  In this case, 1 <= i <= n, where n is the total number
   of routers (6LR) that the packet goes through, from the source (6LBR)
   to the destination (RUL).

   In the 6LBR, the RH3 is added; it is then modified at each
   intermediate 6LR (6LR_1 and so on), and it is fully consumed in the
   last 6LR (6LR_n) but is left in place.  When the RPI is added, the
   RUL, which does not understand the RPI, will ignore it (per
   [RFC8200]); thus, encapsulation is not necessary.

   Table 22 summarizes which headers are needed for this use case.

   +===========+======+==============+===============+================+
   |   Header  | 6LBR |    6LR_i     |     6LR_n     |    RUL dst     |
   |           | src  | i=(1,..,n-1) |               |                |
   +===========+======+==============+===============+================+
   |   Added   | RPI, |      --      |       --      |       --       |
   |  headers  | RH3  |              |               |                |
   +===========+------+--------------+---------------+----------------+
   |  Modified |  --  |   RPI, RH3   |      RPI,     |       --       |
   |  headers  |      |              | RH3(consumed) |                |
   +===========+------+--------------+---------------+----------------+
   |  Removed  |  --  |      --      |       --      |       --       |
   |  headers  |      |              |               |                |
   +===========+------+--------------+---------------+----------------+
   | Untouched |  --  |      --      |       --      | RPI, RH3 (both |
   |  headers  |      |              |               |    ignored)    |
   +===========+------+--------------+---------------+----------------+

     Table 22: Non-SM: Summary of the Use of Headers from Root to RUL

8.1.4.  Non-SM: Example of Flow from RUL to Root

   In this case, the flow comprises:

   RUL (IPv6 src node) --> 6LR_1 --> 6LR_i --> root (6LBR) dst

   For example, a communication flow could be: Node G --> Node E -->
   Node B --> Node A (root)

   6LR_i represents the intermediate routers from the source to the
   destination.  In this case, 1 <= i <= n, where n is the total number
   of routers (6LR) that the packet goes through, from the source (RUL)
   to the destination (6LBR).  For example, 6LR_1 (i=1) is the router
   that receives the packets from the RUL.

   In this case, the RPI is added by the first 6LR (6LR_1) (Node E),
   encapsulated in an IPv6-in-IPv6 header, and modified in the
   subsequent 6LRs in the flow.  The RPI and the entire packet are
   consumed by the root.

   Table 23 summarizes which headers are needed for this use case.

     +===============+=========+==============+=======+==============+
     |     Header    | RUL src |    6LR_1     | 6LR_i |   6LBR dst   |
     +===============+=========+==============+=======+==============+
     | Added headers |    --   | IPv6-in-IPv6 |   --  |      --      |
     |               |         |    (RPI)     |       |              |
     +===============+---------+--------------+-------+--------------+
     |    Modified   |    --   |      --      |  RPI  |      --      |
     |    headers    |         |              |       |              |
     +===============+---------+--------------+-------+--------------+
     |    Removed    |    --   |      --      |   --  | IPv6-in-IPv6 |
     |    headers    |         |              |       |    (RPI)     |
     +===============+---------+--------------+-------+--------------+
     |   Untouched   |    --   |      --      |   --  |      --      |
     |    headers    |         |              |       |              |
     +===============+---------+--------------+-------+--------------+

      Table 23: Non-SM: Summary of the Use of Headers from RUL to Root

8.2.  Non-Storing Mode: Interaction between Leaf and Internet

   This section describes the communication flow in Non-Storing mode
   (Non-SM) between the following:

      RAL to Internet

      Internet to RAL

      RUL to Internet

      Internet to RUL

8.2.1.  Non-SM: Example of Flow from RAL to Internet

   In this case, the flow comprises:

   RAL (6LN) src --> 6LR_i --> root (6LBR) --> Internet dst

   For example, a communication flow could be: Node F (RAL) --> Node D
   --> Node B --> Node A --> Internet.  Having the RAL information about
   the RPL domain, the packet may be encapsulated to the root when the
   destination is not in the RPL domain of the RAL.

   6LR_i represents the intermediate routers from the source to the
   destination, and 1 <= i <= n, where n is the total number of routers
   (6LR) that the packet goes through, from the source (RAL) to the
   6LBR.

   In this case, the encapsulation from the RAL to the root is optional.
   The simplest case is when the RPI gets to the Internet (as the
   Table 24 shows it), knowing that the Internet is going to ignore it.

   The IPv6 flow label should be set to zero to aid in compression
   [RFC8138], and the 6LBR will set it to a non-zero value when sending
   towards the Internet [RFC6437].

   Table 24 summarizes which headers are needed for this use case when
   no encapsulation is used.  Table 25 summarizes which headers are
   needed for this use case when encapsulation to the root is used.

      +===================+=========+=======+======+===============+
      |       Header      | RAL src | 6LR_i | 6LBR |  Internet dst |
      +===================+=========+=======+======+===============+
      |   Added headers   |   RPI   |   --  |  --  |       --      |
      +===================+---------+-------+------+---------------+
      |  Modified headers |    --   |  RPI  | RPI  |       --      |
      +===================+---------+-------+------+---------------+
      |  Removed headers  |    --   |   --  |  --  |       --      |
      +===================+---------+-------+------+---------------+
      | Untouched headers |    --   |   --  |  --  | RPI (Ignored) |
      +===================+---------+-------+------+---------------+

         Table 24: Non-SM: Summary of the Use of Headers from RAL
                    to Internet with No Encapsulation

    +===========+===============+=======+==============+==============+
    |   Header  |    RAL src    | 6LR_i |     6LBR     | Internet dst |
    +===========+===============+=======+==============+==============+
    |   Added   | IP6v6-in-IPv6 |   --  |      --      |      --      |
    |  headers  |     (RPI)     |       |              |              |
    +===========+---------------+-------+--------------+--------------+
    |  Modified |       --      |  RPI  |      --      |      --      |
    |  headers  |               |       |              |              |
    +===========+---------------+-------+--------------+--------------+
    |  Removed  |       --      |   --  | IPv6-in-IPv6 |      --      |
    |  headers  |               |       |    (RPI)     |              |
    +===========+---------------+-------+--------------+--------------+
    | Untouched |       --      |   --  |      --      |      --      |
    |  headers  |               |       |              |              |
    +===========+---------------+-------+--------------+--------------+

        Table 25: Non-SM: Summary of the Use of Headers from RAL to
                  Internet with Encapsulation to the Root

8.2.2.  Non-SM: Example of Flow from Internet to RAL

   In this case, the flow comprises:

   Internet --> root (6LBR) --> 6LR_i --> RAL dst (6LN)

   For example, a communication flow could be: Internet --> Node A
   (root) --> Node B --> Node D --> Node F (RAL)

   6LR_i represents the intermediate routers from source to destination,
   and 1 <= i <= n, where n is the total number of routers (6LR) that
   the packet goes through, from the 6LBR to the destination (RAL).

   The 6LBR must add an RH3 header.  As the 6LBR will know the path and
   address of the target node, it can address the IPv6-in-IPv6 header to
   that node.  The 6LBR will zero the flow label upon entry in order to
   aid compression [RFC8138].

   Table 26 summarizes which headers are needed for this use case.

   +===========+==========+==============+==============+==============+
   |   Header  | Internet |     6LBR     |    6LR_i     |   RAL dst    |
   |           |   src    |              |              |              |
   +===========+==========+==============+==============+==============+
   |   Added   |    --    | IPv6-in-IPv6 |      --      |      --      |
   |  headers  |          |  (RH3, RPI)  |              |              |
   +===========+----------+--------------+--------------+--------------+
   |  Modified |    --    |      --      | IPv6-in-IPv6 |      --      |
   |  headers  |          |              |  (RH3, RPI)  |              |
   +===========+----------+--------------+--------------+--------------+
   |  Removed  |    --    |      --      |      --      | IPv6-in-IPv6 |
   |  headers  |          |              |              |  (RH3, RPI)  |
   +===========+----------+--------------+--------------+--------------+
   | Untouched |    --    |      --      |      --      |      --      |
   |  headers  |          |              |              |              |
   +===========+----------+--------------+--------------+--------------+

    Table 26: Non-SM: Summary of the Use of Headers from Internet to RAL

8.2.3.  Non-SM: Example of Flow from RUL to Internet

   In this case, the flow comprises:

   RUL (IPv6 src node) --> 6LR_1 --> 6LR_i --> root (6LBR) --> Internet
   dst

   For example, a communication flow could be: Node G --> Node E -->
   Node B --> Node A --> Internet

   6LR_i represents the intermediate routers from the source to the
   destination, and 1 <= i <= n, where n is the total number of routers
   (6LRs) that the packet goes through, from the source (RUL) to the
   6LBR, e.g., 6LR_1 (i=1).

   In this case, the flow label is recommended to be zero in the RUL.
   As the RUL parent adds RPL headers in the RUL packet, the first 6LR
   (6LR_1) will add an RPI inside a new IPv6-in-IPv6 header.  The IPv6-
   in-IPv6 header will be addressed to the root.  This case is identical
   to the Storing mode case (see Section 7.2.3).

   Table 27 summarizes which headers are needed for this use case.

    +===========+=========+=========+============+=========+==========+
    |   Header  | RUL src |  6LR_1  |   6LR_i    |   6LBR  | Internet |
    |           |         |         | i=(2,..,n) |         |   dst    |
    +===========+=========+=========+============+=========+==========+
    |   Added   |    --   | IP6-IP6 |     --     |    --   |    --    |
    |  headers  |         |  (RPI)  |            |         |          |
    +===========+---------+---------+------------+---------+----------+
    |  Modified |    --   |    --   |    RPI     |    --   |    --    |
    |  headers  |         |         |            |         |          |
    +===========+---------+---------+------------+---------+----------+
    |  Removed  |    --   |    --   |     --     | IP6-IP6 |    --    |
    |  headers  |         |         |            |  (RPI)  |          |
    +===========+---------+---------+------------+---------+----------+
    | Untouched |    --   |    --   |     --     |    --   |    --    |
    |  headers  |         |         |            |         |          |
    +===========+---------+---------+------------+---------+----------+

        Table 27: Non-SM: Summary of the Use of Headers from RUL to
                                  Internet

8.2.4.  Non-SM: Example of Flow from Internet to RUL

   In this case, the flow comprises:

   Internet src --> root (6LBR) --> 6LR_i --> RUL (IPv6 dst node)

   For example, a communication flow could be: Internet --> Node A
   (root) --> Node B --> Node E --> Node G

   6LR_i represents the intermediate routers from the source to the
   destination, and 1 <= i <= n, where n is the total number of routers
   (6LR) that the packet goes through, from the 6LBR to the RUL.

   The 6LBR must add an RH3 header inside an IPv6-in-IPv6 header.  The
   6LBR will know the path and will recognize that the final node is not
   a RPL-capable node as it will have received the connectivity DAO from
   the nearest 6LR.  The 6LBR can therefore make the IPv6-in-IPv6 header
   destination be the last 6LR.  The 6LBR will set to zero the flow
   label upon entry in order to aid compression [RFC8138].

   Table 28 summarizes which headers are needed for this use case.

   +===========+==========+============+============+============+=====+
   |   Header  | Internet |    6LBR    |   6LR_i    |   6LR_n    | RUL |
   |           |   src    |            |            |            | dst |
   +===========+==========+============+============+============+=====+
   |   Added   |    --    |  IP6-IP6   |     --     |     --     |  -- |
   |  headers  |          | (RH3, RPI) |            |            |     |
   +===========+----------+------------+------------+------------+-----+
   |  Modified |    --    |     --     |  IP6-IP6   |     --     |  -- |
   |  headers  |          |            | (RH3, RPI) |            |     |
   +===========+----------+------------+------------+------------+-----+
   |  Removed  |    --    |     --     |     --     |  IP6-IP6   |  -- |
   |  headers  |          |            |            |   (RH3,    |     |
   |           |          |            |            |    RPI)    |     |
   +===========+----------+------------+------------+------------+-----+
   | Untouched |    --    |     --     |     --     |     --     |  -- |
   |  headers  |          |            |            |            |     |
   +===========+----------+------------+------------+------------+-----+

    Table 28: Non-SM: Summary of the Use of Headers from Internet to RUL

8.3.  Non-SM: Interaction between Leaves

   This section describes the communication flow in Non-Storing mode
   (Non-SM) between the following:

      RAL to RAL

      RAL to RUL

      RUL to RAL

      RUL to RUL

8.3.1.  Non-SM: Example of Flow from RAL to RAL

   In this case, the flow comprises:

   RAL src --> 6LR_ia --> root (6LBR) --> 6LR_id --> RAL dst

   For example, a communication flow could be: Node F (RAL src) --> Node
   D --> Node B --> Node A (root) --> Node B --> Node E --> Node H (RAL
   dst)

   6LR_ia represents the intermediate routers from the source to the
   root, and 1 <= ia <= n, where n is the total number of routers (6LR)
   that the packet goes through, from the RAL to the root.

   6LR_id represents the intermediate routers from the root to the
   destination, and 1 <= id <= m, where m is the total number of the
   intermediate routers (6LR).

   This case involves only nodes in same RPL domain.  The originating
   node will add an RPI to the original packet and send the packet
   Upward.

   The originating node may put the RPI (RPI1) into an IPv6-in-IPv6
   header addressed to the root so that the 6LBR can remove that header.
   If it does not, then the RPI1 is forwarded down from the root in the
   inner header to no avail.

   The 6LBR will need to insert an RH3 header, which requires that it
   add an IPv6-in-IPv6 header.  It removes the RPI (RPI1), as it was
   contained in an IPv6-in-IPv6 header addressed to it.  Otherwise,
   there may be an RPI buried inside the inner IP header, which should
   be ignored.  The root inserts an RPI (RPI2) alongside the RH3.

   Networks that use the RPL point-to-point extension [RFC6997] are
   essentially Non-Storing DODAGs and fall into this scenario or the
   scenario given in Section 8.1.2, with the originating node acting as
   a 6LBR.

   Table 29 summarizes which headers are needed for this use case when
   encapsulation to the root takes place.

   Table 30 summarizes which headers are needed for this use case when
   there is no encapsulation to the root.  Note that in the Modified
   headers row, going up in each 6LR_ia only the RPI1 is changed.  Going
   down, in each 6LR_id the IPv6 header is swapped with the RH3 so both
   are changed alongside with the RPI2.

   +===========+=========+========+===============+=========+=========+
   |   Header  | RAL src | 6LR_ia |      6LBR     |  6LR_id | RAL dst |
   +===========+=========+========+===============+=========+=========+
   |   Added   | IP6-IP6 |   --   |  IP6-IP6 (RH3 |    --   |    --   |
   |  headers  |  (RPI1) |        | -> RAL, RPI2) |         |         |
   +===========+---------+--------+---------------+---------+---------+
   |  Modified |    --   |  RPI1  |       --      | IP6-IP6 |    --   |
   |  headers  |         |        |               |  (RH3,  |         |
   |           |         |        |               |  RPI2)  |         |
   +===========+---------+--------+---------------+---------+---------+
   |  Removed  |    --   |   --   |    IP6-IP6    |    --   | IP6-IP6 |
   |  headers  |         |        |     (RPI1)    |         |  (RH3,  |
   |           |         |        |               |         |  RPI2)  |
   +===========+---------+--------+---------------+---------+---------+
   | Untouched |    --   |   --   |       --      |    --   |    --   |
   |  headers  |         |        |               |         |         |
   +===========+---------+--------+---------------+---------+---------+

   Table 29: Non-SM: Summary of the Use of Headers from RAL to RAL with
                        Encapsulation to the Root

   +===========+======+========+=============+=============+===========+
   |   Header  | RAL  | 6LR_ia |     6LBR    |    6LR_id   |  RAL dst  |
   |           | src  |        |             |             |           |
   +===========+======+========+=============+=============+===========+
   |   Added   | RPI1 |   --   |   IP6-IP6   |      --     |     --    |
   |  headers  |      |        | (RH3, RPI2) |             |           |
   +===========+------+--------+-------------+-------------+-----------+
   |  Modified |  --  |  RPI1  |      --     |   IP6-IP6   |     --    |
   |  headers  |      |        |             |    (RH3,    |           |
   |           |      |        |             |    RPI2)    |           |
   +===========+------+--------+-------------+-------------+-----------+
   |  Removed  |  --  |   --   |      --     |      --     |  IP6-IP6  |
   |  headers  |      |        |             |             |   (RH3,   |
   |           |      |        |             |             |   RPI2)   |
   +===========+------+--------+-------------+-------------+-----------+
   | Untouched |  --  |   --   |     RPI1    |     RPI1    |    RPI1   |
   |  headers  |      |        |             |             | (Ignored) |
   +===========+------+--------+-------------+-------------+-----------+

      Table 30: Non-SM: Summary of the Use of Headers from RAL to RAL
                     without Encapsulation to the Root

8.3.2.  Non-SM: Example of Flow from RAL to RUL

   In this case, the flow comprises:

   RAL --> 6LR_ia --> root (6LBR) --> 6LR_id --> RUL (IPv6 dst node)

   For example, a communication flow could be: Node F (RAL) --> Node D
   --> Node B --> Node A (root) --> Node B --> Node E --> Node G (RUL)

   6LR_ia represents the intermediate routers from the source to the
   root, and 1 <= ia <= n, where n is the total number of intermediate
   routers (6LR).

   6LR_id represents the intermediate routers from the root to the
   destination, and 1 <= id <= m, where m is the total number of the
   intermediate routers (6LRs).

   As in the previous case, the RAL (6LN) may insert an RPI (RPI1)
   header, which must be in an IPv6-in-IPv6 header addressed to the root
   so that the 6LBR can remove this RPI.  The 6LBR will then insert an
   RH3 inside a new IPv6-in-IPv6 header addressed to the last 6LR_id
   (6LR_id = m) alongside the insertion of RPI2.

   If the originating node does not put the RPI (RPI1) into an IPv6-in-
   IPv6 header addressed to the root, then the RPI1 is forwarded down
   from the root in the inner header to no avail.

   Table 31 summarizes which headers are needed for this use case when
   encapsulation to the root takes place.  Table 32 summarizes which
   headers are needed for this use case when no encapsulation to the
   root takes place.

   +===========+=========+========+=========+=========+=========+=====+
   |   Header  | RAL src | 6LR_ia |   6LBR  |  6LR_id |  6LR_m  | RUL |
   |           |         |        |         |         |         | dst |
   +===========+=========+========+=========+=========+=========+=====+
   |   Added   | IP6-IP6 |   --   | IP6-IP6 |    --   |    --   |  -- |
   |  headers  |  (RPI1) |        |  (RH3,  |         |         |     |
   |           |         |        |  RPI2)  |         |         |     |
   +===========+---------+--------+---------+---------+---------+-----+
   |  Modified |    --   |  RPI1  |    --   | IP6-IP6 |    --   |  -- |
   |  headers  |         |        |         |  (RH3,  |         |     |
   |           |         |        |         |  RPI2)  |         |     |
   +===========+---------+--------+---------+---------+---------+-----+
   |  Removed  |    --   |   --   | IP6-IP6 |    --   | IP6-IP6 |  -- |
   |  headers  |         |        |  (RPI1) |         |  (RH3,  |     |
   |           |         |        |         |         |  RPI2)  |     |
   +===========+---------+--------+---------+---------+---------+-----+
   | Untouched |    --   |   --   |    --   |    --   |    --   |  -- |
   |  headers  |         |        |         |         |         |     |
   +===========+---------+--------+---------+---------+---------+-----+

   Table 31: Non-SM: Summary of the Use of Headers from RAL to RUL with
                        Encapsulation to the Root

   +===========+====+========+=========+=========+=========+===========+
   |   Header  |RAL | 6LR_ia |   6LBR  |  6LR_id |  6LR_n  |  RUL dst  |
   |           |src |        |         |         |         |           |
   +===========+====+========+=========+=========+=========+===========+
   |   Added   |RPI1|   --   | IP6-IP6 |    --   |    --   |     --    |
   |  headers  |    |        |  (RH3,  |         |         |           |
   |           |    |        |  RPI2)  |         |         |           |
   +===========+----+--------+---------+---------+---------+-----------+
   |  Modified | -- |  RPI1  |    --   | IP6-IP6 |    --   |     --    |
   |  headers  |    |        |         |  (RH3,  |         |           |
   |           |    |        |         |  RPI2)  |         |           |
   +===========+----+--------+---------+---------+---------+-----------+
   |  Removed  | -- |   --   |    --   |    --   | IP6-IP6 |     --    |
   |  headers  |    |        |         |         |  (RH3,  |           |
   |           |    |        |         |         |  RPI2)  |           |
   +===========+----+--------+---------+---------+---------+-----------+
   | Untouched | -- |   --   |   RPI1  |   RPI1  |   RPI1  |    RPI1   |
   |  headers  |    |        |         |         |         | (ignored) |
   +===========+----+--------+---------+---------+---------+-----------+

      Table 32: Non-SM: Summary of the Use of Headers from RAL to RUL
                     without Encapsulation to the Root

8.3.3.  Non-SM: Example of Flow from RUL to RAL

   In this case, the flow comprises:

   RUL (IPv6 src node) --> 6LR_1 --> 6LR_ia --> root (6LBR) --> 6LR_id
   --> RAL dst (6LN)

   For example, a communication flow could be: Node G (RUL) --> Node E
   --> Node B --> Node A (root) --> Node B --> Node E --> Node H (RAL)

   6LR_ia represents the intermediate routers from source to the root,
   and 1 <= ia <= n, where n is the total number of intermediate routers
   (6LR).

   6LR_id represents the intermediate routers from the root to the
   destination, and 1 <= id <= m, where m is the total number of the
   intermediate routers (6LR).

   In this scenario, the RPI (RPI1) is added by the first 6LR (6LR_1)
   inside an IPv6-in-IPv6 header addressed to the root.  The 6LBR will
   remove this RPI and add its own IPv6-in-IPv6 header containing an RH3
   header and an RPI (RPI2).

   Table 33 summarizes which headers are needed for this use case.

   +===========+=====+=========+========+=========+=========+=========+
   |   Header  | RUL |  6LR_1  | 6LR_ia |   6LBR  |  6LR_id | RAL dst |
   |           | src |         |        |         |         |         |
   +===========+=====+=========+========+=========+=========+=========+
   |   Added   |  -- | IP6-IP6 |   --   | IP6-IP6 |    --   |    --   |
   |  headers  |     |  (RPI1) |        |  (RH3,  |         |         |
   |           |     |         |        |  RPI2)  |         |         |
   +===========+-----+---------+--------+---------+---------+---------+
   |  Modified |  -- |    --   |  RPI1  |    --   | IP6-IP6 |    --   |
   |  headers  |     |         |        |         |  (RH3,  |         |
   |           |     |         |        |         |  RPI2)  |         |
   +===========+-----+---------+--------+---------+---------+---------+
   |  Removed  |  -- |    --   |   --   | IP6-IP6 |    --   | IP6-IP6 |
   |  headers  |     |         |        |  (RPI1) |         |  (RH3,  |
   |           |     |         |        |         |         |  RPI2)  |
   +===========+-----+---------+--------+---------+---------+---------+
   | Untouched |  -- |    --   |   --   |    --   |    --   |    --   |
   |  headers  |     |         |        |         |         |         |
   +===========+-----+---------+--------+---------+---------+---------+

     Table 33: Non-SM: Summary of the Use of Headers from RUL to RAL

8.3.4.  Non-SM: Example of Flow from RUL to RUL

   In this case, the flow comprises:

   RUL (IPv6 src node) --> 6LR_1 --> 6LR_ia --> root (6LBR) --> 6LR_id
   --> RUL (IPv6 dst node)

   For example, a communication flow could be: Node G --> Node E -->
   Node B --> Node A (root) --> Node C --> Node J

   6LR_ia represents the intermediate routers from the source to the
   root, and 1 <= ia <= n, where n is the total number of intermediate
   routers (6LR).

   6LR_id represents the intermediate routers from the root to the
   destination, and 1 <= id <= m, where m is the total number of the
   intermediate routers (6LR).

   This scenario is the combination of the previous two cases.

   Table 34 summarizes which headers are needed for this use case.

   +===========+===+=========+=======+=========+=========+=========+===+
   |   Header  |RUL|  6LR_1  | 6LR_ia|   6LBR  |  6LR_id |  6LR_m  |RUL|
   |           |src|         |       |         |         |         |dst|
   +===========+===+=========+=======+=========+=========+=========+===+
   |   Added   | --| IP6-IP6 |   --  | IP6-IP6 |    --   |    --   | --|
   |  headers  |   |  (RPI1) |       |  (RH3,  |         |         |   |
   |           |   |         |       |  RPI2)  |         |         |   |
   +===========+---+---------+-------+---------+---------+---------+---+
   |  Modified | --|    --   |  RPI1 |    --   | IP6-IP6 |    --   | --|
   |  headers  |   |         |       |         |  (RH3,  |         |   |
   |           |   |         |       |         |  RPI2)  |         |   |
   +===========+---+---------+-------+---------+---------+---------+---+
   |  Removed  | --|    --   |   --  | IP6-IP6 |    --   | IP6-IP6 | --|
   |  headers  |   |         |       |  (RPI1) |         |  (RH3,  |   |
   |           |   |         |       |         |         |  RPI2)  |   |
   +===========+---+---------+-------+---------+---------+---------+---+
   | Untouched | --|    --   |   --  |    --   |    --   |    --   | --|
   |  headers  |   |         |       |         |         |         |   |
   +===========+---+---------+-------+---------+---------+---------+---+

      Table 34: Non-SM: Summary of the Use of Headers from RUL to RUL

9.  Operational Considerations of Supporting RULs

   Roughly half of the situations described in this document involve
   leaf ("host") nodes that do not speak RPL.  These nodes fall into two
   further categories: ones that drop a packet that have RPI or RH3
   headers, and ones that continue to process a packet that has RPI and/
   or RH3 headers.

   [RFC8200] provides for new rules that suggest that nodes that have
   not been configured (explicitly) to examine Hop-by-Hop Options
   headers should ignore those headers and continue processing the
   packet.  Despite this, and despite the switch from 0x63 to 0x23,
   there may be nodes that predate RFC 8200 or are simply intolerant.
   Those nodes will drop packets that continue to have RPL artifacts in
   them.  In general, such nodes cannot be easily supported in RPL LLNs.

   There are some specific cases where it is possible to remove the RPL
   artifacts prior to forwarding the packet to the leaf host.  The
   critical thing is that the artifacts have been inserted by the RPL
   root inside an IPv6-in-IPv6 header, and that the header has been
   addressed to the 6LR immediately prior to the leaf node.  In that
   case, in the process of removing the IPv6-in-IPv6 header, the
   artifacts can also be removed.

   The above case occurs whenever traffic originates from the outside
   the LLN (the "Internet" cases above), and Non-Storing mode is used.
   In Non-Storing mode, the RPL root knows the exact topology (as it
   must create the RH3 header) and therefore knows which 6LR is prior to
   the leaf.  For example, in Figure 3, Node E is the 6LR prior to leaf
   Node G, or Node C is the 6LR prior to leaf Node J.

   Traffic originating from the RPL root (such as when the data
   collection system is co-located on the RPL root), does not require an
   IPv6-in-IPv6 header (in Storing or Non-Storing mode), as the packet
   is originating at the root, and the root can insert the RPI and RH3
   headers directly into the packet as it is formed.  Such a packet is
   slightly smaller, but can only be sent to nodes (whether RPL aware or
   not) that will tolerate the RPL artifacts.

   An operator that finds itself with a high amount of traffic from the
   RPL root to RPL-unaware leaves will have to do IPv6-in-IPv6
   encapsulation if the leaf is not tolerant of the RPL artifacts.  Such
   an operator could otherwise omit this unnecessary header if it was
   certain of the properties of the leaf.

   As the Storing mode cannot know the final path of the traffic,
   intolerant leaf nodes, which drop packets with RPL artifacts, cannot
   be supported.

10.  Operational Considerations of Introducing 0x23

   This section describes the operational considerations of introducing
   the new RPI Option Type of 0x23.

   During bootstrapping, the node receives the DIO with the information
   of RPI Option Type, indicating the new RPI in the DODAG Configuration
   option flag.  The DODAG root is in charge of configuring the current
   network with the new value, through DIO messages, and determining
   when all the nodes have been set with the new value.  The DODAG
   should change to a new DODAG version.  In case of rebooting, the node
   does not remember the RPI Option Type.  Thus, the DIO is sent with a
   flag indicating the new RPI Option Type.

   The DODAG Configuration option is contained in a RPL DIO message,
   which contains a unique Destination Advertisement Trigger Sequence
   Number (DTSN) counter.  The leaf nodes respond to this message with
   DAO messages containing the same DTSN.  This is a normal part of RPL
   routing; the RPL root therefore knows when the updated DODAG
   Configuration option has been seen by all nodes.

   Before the migration happens, all the RPL-aware nodes should support
   both values.  The migration procedure is triggered when the DIO is
   sent with the flag indicating the new RPI Option Type.  Namely, it
   remains at 0x63 until it is sure that the network is capable of 0x23,
   then it abruptly changes to 0x23.  The 0x23 RPI Option allows the
   sending of packets to non-RPL nodes.  The non-RPL nodes should ignore
   the option and continue processing the packets.

   As mentioned previously, indicating the new RPI in the DODAG
   Configuration option flag is a way to avoid the flag day (abrupt
   changeover) in a network using 0x63 as the RPI Option Type value.  It
   is suggested that RPL implementations accept both 0x63 and 0x23 RPI
   Option Type values when processing the header to enable
   interoperability.

11.  IANA Considerations

11.1.  Option Type in RPL Option

   This document updates the registration made in the "Destination
   Options and Hop-by-Hop Options" subregistry [RFC6553] from 0x63 to
   0x23 as shown in Table 35.

     +===========+===================+==============+===============+
     | Hex Value |    Binary Value   | Description  |   Reference   |
     |           +=====+=====+=======+              |               |
     |           | act | chg |  rest |              |               |
     +===========+=====+=====+=======+==============+===============+
     |    0x23   |  00 |  1  | 00011 |  RPL Option  | This document |
     +-----------+-----+-----+-------+--------------+---------------+
     |    0x63   |  01 |  1  | 00011 |  RPL Option  |   [RFC6553],  |
     |           |     |     |       | (DEPRECATED) | this document |
     +-----------+-----+-----+-------+--------------+---------------+

                   Table 35: Option Type in RPL Option

   The "DODAG Configuration Option Flags for MOP 0..6" subregistry is
   updated as follows (Table 36):

          +============+========================+===============+
          | Bit Number | Capability Description |   Reference   |
          +============+========================+===============+
          |     3      |    RPI 0x23 enable     | This document |
          +------------+------------------------+---------------+

                Table 36: DODAG Configuration Option Flag to
                         Indicate the RPI Flag Day

11.2.  Change to the "DODAG Configuration Option Flags" Subregistry

   IANA has changed the name of the "DODAG Configuration Option Flags"
   subregistry to "DODAG Configuration Option Flags for MOP 0..6".

   The subregistry references this document for this change.

11.3.  Change MOP Value 7 to Reserved

   IANA has changed the registration status of value 7 in the "Mode of
   Operation" subregistry from Unassigned to Reserved.  This change is
   in support of future work.

   This document is listed as a reference for this entry in the
   subregistry.

12.  Security Considerations

   The security considerations covered in [RFC6553] and [RFC6554] apply
   when the packets are in the RPL Domain.

   The IPv6-in-IPv6 mechanism described in this document is much more
   limited than the general mechanism described in [RFC2473].  The
   willingness of each node in the LLN to decapsulate packets and
   forward them could be exploited by nodes to disguise the origin of an
   attack.

   While a typical LLN may be a very poor origin for attack traffic (as
   the networks tend to be very slow, and the nodes often have very low
   duty cycles), given enough nodes, LLNs could still have a significant
   impact, particularly if the attack is targeting another LLN.
   Additionally, some uses of RPL involve large-backbone, ISP-scale
   equipment [ACP], which may be equipped with multiple 100 Gb/s
   interfaces.

   Blocking or careful filtering of IPv6-in-IPv6 traffic entering the
   LLN as described above will make sure that any attack that is mounted
   must originate from compromised nodes within the LLN.  The use of
   network ingress filtering [BCP38] on egress traffic at the RPL root
   will alert the operator to the existence of the attack as well as
   drop the attack traffic.  As the RPL network is typically numbered
   from a single prefix, which is itself assigned by RPL, network
   ingress filtering [BCP38] involves a single prefix comparison and
   should be trivial to automatically configure.

   There are some scenarios where IPv6-in-IPv6 traffic should be allowed
   to pass through the RPL root, such as the IPv6-in-IPv6 mediated
   communications between a new pledge and the Join Registrar/
   Coordinator (JRC) when using [BRSKI] and [ZEROTOUCH-JOIN].  This is
   the case for the RPL root to do careful filtering: it occurs only
   when the Join Coordinator is not co-located inside the RPL root.

   With the above precautions, an attack using IPv6-in-IPv6 tunnels can
   only be by a node within the LLN on another node within the LLN.
   Such an attack could, of course, be done directly.  An attack of this
   kind is meaningful only if the source addresses are either fake or if
   the point is to amplify return traffic.  Such an attack could also be
   done without the use of IPv6-in-IPv6 headers, by using forged source
   addresses instead.  If the attack requires bidirectional
   communication, then IPv6-in-IPv6 provides no advantages.

   Whenever IPv6-in-IPv6 headers are being proposed, there is a concern
   about creating security issues.  In the Security Considerations
   section of [RFC2473] (Section 9), it was suggested that tunnel entry
   and exit points can be secured by securing the IPv6 path between
   them.  This recommendation is not practical for RPL networks.
   [RFC5406] provides guidance on what on what additional details are
   needed in order to "Use IPsec".  While the use of Encapsulating
   Security Payload (ESP) would prevent source address forgeries, in
   order to use it with [RFC8138], compression would have to occur
   before encryption, as the [RFC8138] compression is lossy.  Once
   encrypted, there would be no further redundancy to compress.  These
   are minor issues.  The major issue is how to establish trust enough
   such that Internet Key Exchange Protocol Version 2 (IKEv2) could be
   used.  This would require a system of certificates to be present in
   every single node, including any Internet nodes that might need to
   communicate with the LLN.  Thus, using IPsec requires a global PKI in
   the general case.

   More significantly, the use of IPsec tunnels to protect the IPv6-in-
   IPv6 headers would, in the general case, scale with the square of the
   number of nodes.  This is a lot of resources for a constrained nodes
   on a constrained network.  In the end, the IPsec tunnels would be
   providing only BCP38-like origin authentication!  That is, IPsec
   provides a transitive guarantee to the tunnel exit point that the
   tunnel entry point did network ingress filtering [BCP38] on traffic
   going in.  Just doing origin filtering per BCP 38 at the entry and
   exit of the LLN provides a similar level of security without all the
   scaling and trust problems related to IPv6 tunnels as discussed in
   [RFC2473].  IPsec is not recommended.

   An LLN with hostile nodes within it would not be protected against
   impersonation within the LLN by entry/exit filtering.

   The RH3 header usage described here can be abused in equivalent ways.
   An external attacker may form a packet with an RH3 that is not fully
   consumed and encapsulate it to hide the RH3 from intermediate nodes
   and disguise the origin of traffic.  As such, the attacker's RH3
   header will not be seen by the network until it reaches the
   destination, which will decapsulate it.  As indicated in Section 4.2
   of [RFC6554], RPL routers are responsible for ensuring that an SRH is
   only used between RPL routers.  As such, if there is an RH3 that is
   not fully consumed in the encapsulated packet, the node that
   decapsulates it MUST ensure that the outer packet was originated in
   the RPL domain and drop the packet otherwise.

   Also, as indicated by Section 2 of [RFC6554], RPL Border Routers "do
   not allow datagrams carrying an SRH header to enter or exit a RPL
   routing domain."  This sentence must be understood as concerning non-
   fully-consumed packets.  A consumed (inert) RH3 header could be
   present in a packet that flows from one LLN, crosses the Internet,
   and enters another LLN.  Per the discussion in this document, such
   headers do not need to be removed.  However, there is no case
   described in this document where an RH3 is inserted in a Non-Storing
   network on traffic that is leaving the LLN, but this document should
   not preclude such a future innovation.

   In short, a packet that crosses the border of the RPL domain MAY
   carry an RH3, and if so, that RH3 MUST be fully consumed.

   The RPI, if permitted to enter the LLN, could be used by an attacker
   to change the priority of a packet by selecting a different
   RPLInstanceID, perhaps one with a higher energy cost, for instance.
   It could also be that not all nodes are reachable in an LLN using the
   default RPLInstanceID, but a change of RPLInstanceID would permit an
   attacker to bypass such filtering.  Like the RH3, an RPI is to be
   inserted by the RPL root on traffic entering the LLN by first
   inserting an IPv6-in-IPv6 header.  The attacker's RPI therefore will
   not be seen by the network.  Upon reaching the destination node, the
   RPI has no further meaning and is just skipped; the presence of a
   second RPI will have no meaning to the end node as the packet has
   already been identified as being at its final destination.

   For traffic leaving a RUL, if the RUL adds an uninitialized RPI
   (e.g., with a value of zero), then the 6LR as a RPL Border Router
   SHOULD rewrite the RPI to indicate the selected Instance and set the
   flags.  This is done in order to avoid the following scenarios: 1)
   The leaf is an external router that passes a packet that it did not
   generate and that carries an unrelated RPI, and 2) The leaf is an
   attacker or presents misconfiguration and tries to inject traffic in
   a protected Instance.  Also, this applies to the case where the leaf
   is aware of the RPL Instance and passes a correct RPI; the 6LR needs
   a configuration that allows that leaf to inject in that instance.

   The RH3 and RPIs could be abused by an attacker inside of the network
   to route packets in nonobvious ways, perhaps eluding observation.
   This usage appears consistent with a normal operation of [RFC6997]
   and cannot be restricted at all.  This is a feature, not a bug.

   [RFC7416] deals with many other threats to LLNs not directly related
   to the use of IPv6-in-IPv6 headers, and this document does not change
   that analysis.

   Nodes within the LLN can use the IPv6-in-IPv6 mechanism to mount an
   attack on another part of the LLN, while disguising the origin of the
   attack.  The mechanism can even be abused to make it appear that the
   attack is coming from outside the LLN, and unless countered, this
   could be used to mount a DDOS attack upon nodes elsewhere in the
   Internet.  See [DDOS-KREBS] for an example of such attacks already
   seen in the real world.

   If an attack comes from inside of LLN, it can be alleviated with SAVI
   (Source Address Validation Improvement) using [RFC8505] with
   [RFC8928].  The attacker will not be able to source traffic with an
   address that is not registered, and the registration process checks
   for topological correctness.  Notice that there is Layer 2
   authentication in most of the cases.  If an attack comes from outside
   LLN, IPv6-in-IPv6 can be used to hide inner routing headers, but by
   construction, the RH3 can typically only address nodes within the
   LLN.  That is, an RH3 with a CmprI less than 8 should be considered
   an attack (see Section 3 of [RFC6554]).

   Nodes outside of the LLN will need to pass IPv6-in-IPv6 traffic
   through the RPL root to perform this attack.  To counter, the RPL
   root SHOULD either restrict ingress of IPv6-in-IPv6 packets (the
   simpler solution), or it SHOULD walk the IP header extension chain
   until it can inspect the upper-layer payload as described in
   [RFC7045].  In particular, the RPL root SHOULD do network ingress
   filtering [BCP38] on the source addresses of all IP headers that it
   examines in both directions.

   Note: there are some situations where a prefix will spread across
   multiple LLNs via mechanisms such as the one described in [RFC8929].
   In this case, the network ingress filtering [BCP38] needs to take
   this into account, either by exchanging detailed routing information
   on each LLN or by moving the network ingress filtering [BCP38]
   further towards the Internet, so that the details of the multiple
   LLNs do not matter.

13.  References

13.1.  Normative References

   [BCP38]    Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

              <https://rfc-editor.org/info/bcp38>

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

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <https://www.rfc-editor.org/info/rfc6040>.

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

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

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

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

   [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", RFC 7045,
              DOI 10.17487/RFC7045, December 2013,
              <https://www.rfc-editor.org/info/rfc7045>.

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

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

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

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

13.2.  Informative References

   [ACP]      Eckert, T., Behringer, M. H., and S. Bjarnason, "An
              Autonomic Control Plane (ACP)", Work in Progress,
              Internet-Draft, draft-ietf-anima-autonomic-control-plane-
              30, 30 October 2020, <https://tools.ietf.org/html/draft-
              ietf-anima-autonomic-control-plane-30>.

   [BRSKI]    Pritikin, M., Richardson, M. C., Eckert, T., Behringer, M.
              H., and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", Work in Progress, Internet-
              Draft, draft-ietf-anima-bootstrapping-keyinfra-45, 11
              November 2020, <https://tools.ietf.org/html/draft-ietf-
              anima-bootstrapping-keyinfra-45>.

   [DDOS-KREBS]
              Goodin, D., "Record-breaking DDoS reportedly delivered by
              >145k hacked cameras", September 2016,
              <https://arstechnica.com/information-technology/2016/09/
              botnet-of-145k-cameras-reportedly-deliver-internets-
              biggest-ddos-ever/>.

   [RFC0801]  Postel, J., "NCP/TCP transition plan", RFC 801,
              DOI 10.17487/RFC0801, November 1981,
              <https://www.rfc-editor.org/info/rfc801>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <https://www.rfc-editor.org/info/rfc2460>.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
              December 1998, <https://www.rfc-editor.org/info/rfc2473>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC5406]  Bellovin, S., "Guidelines for Specifying the Use of IPsec
              Version 2", BCP 146, RFC 5406, DOI 10.17487/RFC5406,
              February 2009, <https://www.rfc-editor.org/info/rfc5406>.

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437,
              DOI 10.17487/RFC6437, November 2011,
              <https://www.rfc-editor.org/info/rfc6437>.

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

   [RFC6997]  Goyal, M., Ed., Baccelli, E., Philipp, M., Brandt, A., and
              J. Martocci, "Reactive Discovery of Point-to-Point Routes
              in Low-Power and Lossy Networks", RFC 6997,
              DOI 10.17487/RFC6997, August 2013,
              <https://www.rfc-editor.org/info/rfc6997>.

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

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

   [RFC8180]  Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
              IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
              Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
              May 2017, <https://www.rfc-editor.org/info/rfc8180>.

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

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

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

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

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

   [TUNNELS]  Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-intarea-tunnels-10, 12 September 2019,
              <https://tools.ietf.org/html/draft-ietf-intarea-tunnels-
              10>.

   [ZEROTOUCH-JOIN]
              Richardson, M., "6tisch Zero-Touch Secure Join protocol",
              Work in Progress, Internet-Draft, draft-ietf-6tisch-
              dtsecurity-zerotouch-join-04, 8 July 2019,
              <https://tools.ietf.org/html/draft-ietf-6tisch-dtsecurity-
              zerotouch-join-04>.

Acknowledgments

   This work is done thanks to the grant given by the StandICT.eu
   project.

   A special BIG thanks to C. M. Heard for the help with Section 4.
   Much of the editing in that section is based on his comments.

   Additionally, the authors would like to acknowledge the review,
   feedback, and comments of the following (in alphabetical order):
   Dominique Barthel, Robert Cragie, Ralph Droms, Simon Duquennoy, Cenk
   Guendogan, Rahul Jadhav, Benjamin Kaduk, Matthias Kovatsch, Gustavo
   Mercado, Subramanian Moonesamy, Marcela Orbiscay, Cristian Perez,
   Charlie Perkins, Alvaro Retana, Peter van der Stok, Xavier
   Vilajosana, Éric Vyncke, and Thomas Watteyne.

Authors' Addresses

   Maria Ines Robles
   Universidad Tecno. Nac.(UTN)-FRM, Argentina /Aalto University Finland
   Coronel Rodríguez 273
   M5500 Mendoza
   Provincia de Mendoza
   Argentina

   Email: mariainesrobles@gmail.com

   Michael C. Richardson
   Sandelman Software Works
   470 Dawson Avenue
   Ottawa ON K1Z 5V7
   Canada

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

   Pascal Thubert
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   06254 MOUGINS - Sophia Antipolis
   France

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