ROLL                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Updates: 6554 (if approved)                                  R.A. Jadhav
Intended status: Standards Track                             Huawei Tech                             R.A. Jadhav
Expires: 28 January 25 March 2022                                       Huawei Tech
                                                             M. Gillmore
                                                                   Itron
                                                            27 July
                                                       21 September 2021

                  Root initiated routing state in RPL
                   draft-ietf-roll-dao-projection-19
                   draft-ietf-roll-dao-projection-20

Abstract

   This document extends RFC 6550 and 6550, RFC 6553 6553,and RFC 8138 to enable a RPL
   Root to install and maintain Projected Routes within its DODAG, along
   a selected set of nodes that may or may not include self, for a
   chosen duration.  This potentially enables routes that are more
   optimized or resilient than those obtained with the classical
   distributed operation of RPL, either in terms of the size of a
   Routing Header or in terms of path length, which impacts both the
   latency and the packet delivery ratio.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5   4
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5   4
     2.2.  Glossary  .  References  . . . . . . . . . . . . . . . . . . . . . . .   5   4
     2.3.  Other Terms  Glossary  . . . . . . . . . . . . . . . . . . . . . . .   6
     2.4.  References .   4
     2.4.  Domain Terms  . . . . . . . . . . . . . . . . . . . . . .   6   5
   3.  Extending RFC 6550  Context and Goal  . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Projected DAO . .  RPL Applicability . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Sibling Information Option  RPL Routing Modes . . . . . . . . . . . . . . . . . . . .   8
     3.3.  P-DAO Request  On Tracks . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.4.  Extending the RPI  Serial Track Signaling  . . . . . . . . . . . . . . . . .  10
       3.4.1.  Using Storing Mode Segments . . . . .   9 . . . . . . . .  11
       3.4.2.  Using Non-Storing Mode joining Tracks . . . . . . . .  17
     3.5.  Complex Tracks  . . . . . . . . . . . . . . . . . . . . .  24
     3.6.  Scope and Expectations  . . . . . . . . . . . . . . . . .  26
   4.  Extending existing RFCs . . . . . . . . . . . . . . . . . . .  28
     4.1.  Extending RFC 6553 6550  . . . . . . . . . . . . . . . . . . .  28
       4.1.1.  Projected DAO . .   9
   5. . . . . . . . . . . . . . . . . . .  28
       4.1.2.  Via Information Option  . . . . . . . . . . . . . . .  30
       4.1.3.  Sibling Information Option  . . . . . . . . . . . . .  30
       4.1.4.  P-DAO Request . . . . . . . . . . . . . . . . . . . .  30
       4.1.5.  Extending the RPI . . . . . . . . . . . . . . . . . .  30
     4.2.  Extending RFC 8138 6553  . . . . . . . . . . . . . . . . . . .  31
     4.3.  Extending RFC 8138  . . .  10
   6. . . . . . . . . . . . . . . . .  32
   5.  New RPL Control Messages and Options  . . . . . . . . . . . .  11
     6.1.  33
     5.1.  New P-DAO Request Control Message . . . . . . . . . . . .  11
     6.2.  33
     5.2.  New PDR-ACK Control Message . . . . . . . . . . . . . . .  12
     6.3.  34
     5.3.  Via Information Options . . . . . . . . . . . . . . . . .  13
     6.4.  36
     5.4.  Sibling Information Option  . . . . . . . . . . . . . . .  16
   7.  Projected DAO . . . . . . . .  39
   6.  Root Initiated Routing State  . . . . . . . . . . . . . . . .  18
     7.1.  41
     6.1.  Requesting a Track  . . . . . . . . . . . . . . . . . . .  19
     7.2.  41
     6.2.  Identifying a Track . . . . . . . . . . . . . . . . . . .  20
     7.3.  42
     6.3.  Installing a Track  . . . . . . . . . . . . . . . . . . .  21
       7.3.1.  Storing-Mode P-Route  . . .  43
       6.3.1.  Signaling a Projected Route . . . . . . . . . . . . .  22
       7.3.2.  Non-Storing-Mode  44
       6.3.2.  Installing a Track Segment with a Storing Mode
               P-Route . . . . . . . . . . . . . .  24
     7.4.  Forwarding Along a Track  . . . . . . . . . . . . . . . .  26
   8.  Profiles .  45
       6.3.3.  Installing a Track Leg with a Non-Storing Mode
               P-Route . . . . . . . . . . . . . . . . . . . . . . .  47
     6.4.  Tearing Down a P-Route  . .  27
   9.  Example Track Signaling . . . . . . . . . . . . . . .  49
     6.5.  Maintaining a Track . . . .  28
     9.1.  Using Storing-Mode Segments . . . . . . . . . . . . . . .  29
       9.1.1.  Stitched Segments  49
       6.5.1.  Maintaining a Track Segment . . . . . . . . . . . . .  50
       6.5.2.  Maintaining a Track Leg . . . . .  29
       9.1.2.  External routes . . . . . . . . . .  50
     6.6.  Encapsulating and Forwarding Along a Track  . . . . . . .  51
     6.7.  Compression of the RPL Artifacts  . .  31
       9.1.3.  Segment Routing . . . . . . . . . .  53
   7.  Lesser Constrained Variations . . . . . . . . .  32
     9.2.  Using Non-Storing-Mode joining Tracks . . . . . . .  55
     7.1.  Storing Mode Main DODAG . . .  34
       9.2.1.  Stitched Tracks . . . . . . . . . . . . . .  55
     7.2.  A Track as a Full DODAG . . . . .  34
       9.2.2.  External routes . . . . . . . . . . . .  57
   8.  Profiles  . . . . . . .  36
       9.2.3.  Segment Routing . . . . . . . . . . . . . . . . . . .  38
   10.  58
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  41
   11.  60
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  41
     11.1.  60
     10.1.  New Elective 6LoWPAN Routing Header Type . . . . . . . .  41
     11.2.  61
     10.2.  New Critical 6LoWPAN Routing Header Type . . . . . . . .  42
     11.3.  61
     10.3.  New Subregistry For The RPL Option Flags . . . . . . . .  42
     11.4.  61
     10.4.  New RPL Control Codes  . . . . . . . . . . . . . . . . .  43
     11.5.  62
     10.5.  New RPL Control Message Options  . . . . . . . . . . . .  43
     11.6.  62
     10.6.  SubRegistry for the Projected DAO Request Flags  . . . .  43
     11.7.  63
     10.7.  SubRegistry for the PDR-ACK Flags  . . . . . . . . . . .  44
     11.8.  63
     10.8.  Subregistry for the PDR-ACK Acceptance Status Values . .  44
     11.9.  63
     10.9.  Subregistry for the PDR-ACK Rejection Status Values  . .  44
     11.10.  64
     10.10. SubRegistry for the Via Information Options Flags  . . .  45
     11.11.  64
     10.11. SubRegistry for the Sibling Information Option Flags . .  45
     11.12.  65
     10.12. New Destination destination Advertisement Object Flag  . . . . . . .  46
     11.13. Error in Projected Route  65
     10.13. New ICMPv6 Error Code  . . . . . . . . . .  46
   12. . . . . . . .  65
     10.14. New RPL Rejection Status values  . . . . . . . . . . . .  66
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  46
   13.  66
   12. Normative References  . . . . . . . . . . . . . . . . . . . .  46
   14.  66
   13. Informative References  . . . . . . . . . . . . . . . . . . .  47  68
   Appendix A.  Applications . . . . . . . . . . . . . . . . . . . .  49  70
     A.1.  Loose Source Routing  . . . . . . . . . . . . . . . . . .  49  70
     A.2.  Transversal  East-West Routes  . . . . . . . . . . . . . . . . . . .  51 .  72
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  52  73

1.  Introduction

   RPL, the "Routing Protocol for Low Power and Lossy Networks" [RPL]
   (LLNs), is a generic an anisotropic Distance Vector protocol that is well well-
   suited for application in a variety of low energy Internet of Things
   (IoT)
   networks. networks where stretched P2P paths are acceptable vs. the
   signaling and state overhead involved in maintaining shortest paths
   across.

   RPL forms Destination destination Oriented Directed Acyclic Graphs (DODAGs) in
   which the Root often acts as the Border Router router to connect the RPL
   domain to the Internet.  The Root is responsible to select
   the RPL Instance IP backbone and routes along that is used to forward a packet coming from the
   Internet into graph up, towards the RPL domain
   Root, and set down towards the related RPL information nodes.

   With this specification, a Path Computation Element [PCE] in an
   external controller interacts with the packets. 6TiSCH uses RPL for its routing operations. Root to compute centrally
   shorter Peer to Peer (P2P) paths within a pre-existing RPL Main
   DODAG.  The "6TiSCH Architecture" [6TiSCH-ARCHI] also leverages the
   "Deterministic Networking Architecture" [RFC8655] centralized model
   whereby the device resources and capabilities are exposed topological information that is passed to an
   external controller which installs routing states into the network
   based on some objective functions PCE is
   derived from the DODAG that reside is already available at the Root in RPL
   Non-Storing Mode.  This specification introduces protocol extensions
   that external
   entity.  With DetNet and 6TiSCH, the component of enrich the controller topological information that is responsible of computing routes is called a Path Computation
   Element ([PCE]). available at the Root
   and passed to the PCE.

   Based on heuristics of usage, path length, and knowledge of device
   capacity and available resources
   such as battery levels and reservable buffers, buffers in the nodes, the PCE
   with a global visibility on the system can compute direct Peer to Peer (P2P) optimize the computed
   routes that are optimized for the needs expressed by an objective function.  This document
   specifies protocol extensions to RPL [RPL] that enable application needs, including the Root of a
   main DODAG capability to install centrally-computed routes inside the DODAG on
   behalf of a PCE. provide
   path redundancy.  This specification expects that the main RPL Instance is operated in
   RPL Non-Storing Mode of Operation (MOP) to sustain the exchanges with
   the Root.  In that Mode, the Root has enough information to build a
   basic DODAG topology based on parents and children, but lacks the
   knowledge of siblings.  This document adds the capability for nodes
   to advertise sibling information in order to improve the topological
   awareness of the Root.

   As opposed to the classical RPL operations where routes are injected
   by the Target nodes, the also introduces protocol
   extensions that enable the Root of a
   DODAG to project the routes that are needed onto the nodes where they
   should be installed.  This specification uses the term Projected
   Route to refer to those routes.  Projected Routes can be used to
   reduce the size of the source routing headers with loose source
   routing operations down translates the main computed paths into
   RPL DODAG.  Projected Routes can
   also be used to build transversal routes for route optimization and
   Traffic Engineering purposes, between nodes of the DODAG.

   A Projected Route may be installed in either Storing and Non-Storing
   Mode, potentially resulting in hybrid situations where the Mode of
   the Projected Route is different from that of the main RPL Instance.
   A Projected Route may be a stand-alone end-to-end path or a Segment
   in a more complex forwarding graph called a Track.

   The concept of a Track was introduced in the 6TiSCH architecture, install them as
   a potentially complex path with redundant forwarding solutions along
   the way.  With this specification, a Track is a DODAG formed by a RPL
   local Instance that is rooted at the Track Ingress.  If there is a
   single Track Egress, then the Track is reversible to form another
   DODAG by reversing the direction of each edge.  A node at the ingress
   of more than one Segment in a Track may use one or more of these
   Segments to forward a packet inside the Track.

   The "Reliable and Available Wireless (RAW) Architecture/Framework"
   [RAW-ARCHI] defines the Path Selection Engine (PSE) that adapts the
   use of the path redundancy within a Track to defeat the diverse
   causes of packet loss.

   The PSE is a dataplane extension of the PCE; it controls the
   forwarding operation of the packets within a Track, using Packet ARQ,
   Replication, Elimination, and Overhearing (PAREO) functions over the
   Track segments, to provide a dynamic balance between the reliability
   and availability requirements of the flows and the need to conserve
   energy and spectrum.

   The time scale at which the PCE (re)computes the Track can be long,
   using long-term statistical metrics to perform global optimizations
   at the scale of the whole network.  Conversely, the PSE makes
   forwarding decisions at the time scale of one or a small collection
   of packets, based on a knowledge that is limited in scope to the
   Track itself, so it can be refreshed at a fast pace. Projected Routes must be used with the parsimony to limit the amount
   of state that is installed in each device to fit within the device
   resources, and to maintain the amount of rerouted traffic within the
   capabilities of the transmission links.  The methods used to learn
   the node capabilities and the resources that are available in the
   devices and in (aka P-Routes) inside the network are out
   DODAG on behalf of scope for this document.

   This specification uses the RPL Root as a proxy to the PCE.  The PCE
   may be collocated with the Root, or may reside in an external
   Controller.

   In that case, the PCE exchanges control messages with the Root over a
   Southbound API that is out of scope for this specification.  The
   algorithm to compute the paths and the protocol used by an external
   PCE to obtain the topology of the network from the Root are also out
   of scope.

2.  Terminology

2.1.  Requirements Language

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

2.2.  References

   In this document, readers will encounter terms and concepts that are
   discussed in the "Routing Protocol for Low Power and Lossy Networks"
   [RPL], the "6TiSCH Architecture" [6TiSCH-ARCHI], the "Deterministic
   Networking Architecture" [RFC8655], the "Reliable and Available
   Wireless (RAW) Architecture/Framework" [RAW-ARCHI], and "Terminology
   in Low power And Lossy Networks" [RFC7102].

2.3.  Glossary

   This document often uses the following acronyms:

   CMO:  Control Message Option
   DAO:  Destination  destination Advertisement Object
   DAG:  Directed Acyclic Graph
   DODAG:  Destination-Oriented  destination-Oriented Directed Acyclic Graph; A DAG with only
      one vertex (i.e., node) that has no outgoing edge (i.e., link)
   GUA:  IPv6 Global Unicast Address
   LLN:  Low-Power and Lossy Network
   MOP:  RPL Mode of Operation
   P-DAO:  Projected DAO
   P-Route:  Projected Route
   PDR:  P-DAO Request
   RAN:  RPL-Aware Node (either a RPL Router router or a RPL-Aware Leaf)
   RAL:  RPL-Aware Leaf
   RH:  Routing Header
   RPI:  RPL Packet Information
   RTO:  RPL Target Option
   RUL:  RPL-Unaware Leaf
   SIO:  RPL Sibling Information Option
   SR-VIO:
   ULA:  IPv6 Unique Local Address
   NSM-VIO:  A Source-Routed Via Information Option, used in Non-Storing- Non-Storing
      Mode P-DAO messages.
   TIO:  RPL Transit Information Option
   SF-VIO:
   SM-VIO:  A strict Via Information Option, used in Storing-Mode Storing Mode P-DAO
      messages.
   VIO:  A Via Information Option; it can be a SF-VIO SM-VIO or an SR-VIO.

2.3.  Other NSM-VIO.

2.4.  Domain Terms

   Projected Route:  A RPL Projected Route P-Route is a RPL route that is computed
      remotely by a PCE, and installed and maintained by a RPL Root on
      behalf of the PCE.
   Segment:  A strict sequence of node along which a route is installed.
      With this specification, a Segment  It is installed by the Root as a state that signals that
      destinations (aka Targets) are reachable along a sequence of the
      main DODAG using Storing-Mode P-DAO messages.
      nodes.
   Projected DAO:  A DAO message used to install a Projected Route.
   Track:  A DODAG P-Route.
   Path:  Quoting section 1.1.3 of [INT-ARCHI]: "At a given moment, all
      the IP datagrams from a particular source host to a particular
      destination host will typically traverse the same sequence of
      gateways.  We use the term "path" for this sequence.  Note that provides a complex
      path from or is uni-directional; it is not unusual to have different paths
      in the two directions between a given host pair.".
      Section 2 of [I-D.irtf-panrg-path-properties] points to a Root longer,
      more modern definition of path, which begins as follows: " A
      sequence of adjacent path elements over which a packet can be
      transmitted, starting and ending with a node.  A path is
      unidirectional.  Paths are time-dependent, i.e., the sequence of
      path elements over which packets are sent from one node to another
      may change.  A path is defined between two nodes.  "
      It follows that the general acceptance of a path is a linear
      sequence of nodes, as opposed to a multi-dimensional graph.  In
      the destination context of this document, a path is observed by following one
      copy of a packet that is injected in a Track and possibly
      replicated within.
   Track:  A networking graph that can be followed to transport packets
      with equivalent treatment; as opposed to the DODAG.  The Root definition of a path
      above, a Track Track is not necessarily linear.  It may contain
      multiple paths that may fork and rejoin, and may enable the RAW
      Packet ARQ, Replication, Elimination, and Overhearing (PAREO)
      operations.
      This specification builds Tracks that are DODAGs oriented towards
      a Track Ingress, and the forward direction for packets is down the DODAG, East-
      West from the Track Ingress to one of the possibly multiple Track
      Egress Nodes. Nodes, which is also down the DODAG.
      The DODAG Track may be strictly connected, meaning that the vertices are
      adjacent, or loosely connected, meaning that the vertices are
      connected using Segments that are associated to the same Track.
      With this specification, a Track is installed by the Root of the
      main DODAG using Non-Storing-Mode P-DAO messages.
   TrackID:  A RPL Local InstanceID with that identifies a Track using the D bit set to 0.
      namespace owned ny the Track Ingress.  The TrackID is associated
      with the IPv6 Address of the Track Ingress that is used to signal the DODAG Root, as
      DODAGID, and together they form a unique identification of the
      Track (see the definition of DODAGID in section 2 of [RPL].

2.4.  References

   In this document, readers will encounter terms and concepts
   Serial Track:  A Track that are
   discussed in the "Routing Protocol for Low Power and Lossy Networks"
   [RPL] and "Terminology in Low power And Lossy Networks" [RFC7102].

3.  Extending RFC 6550
3.1.  Projected DAO

   Section 6 of [RPL] introduces the RPL Control Message Options (CMO),
   including has only one path.
   Stand-Alone:  A single P-DAO that fully defines a Track, e.g., a
      Serial Track installed with a single Storing Mode Via Information
      option (SM-VIO).
   subTrack:  A Track within a Track.  As the RPL Target Option (RTO) and Transit Non-Storing Mode Via
      Information Option
   (TIO), which option (NSM-VIO) can be placed in RPL messages such as the Destination
   Advertisement Object (DAO).  This specification extends the DAO
   message with the Projected DAO (P-DAO); only signal a P-DAO message signals loose sequence of
      nodes, it takes a
   Projected Route number of them to one or more Targets using signal a complex Track.  Each
      NSM-VIO for the new CMOs presented
   therein.  This specification enables to combine one or more Projected
   Routes into same TrackId but a DODAG called different Segment ID signals a Track,
      different subTracks that is traversed to reach the
   Targets.

   The Track is assimilated with Ingress adds to the DODAG formed for topology.
   Track Leg:  An end-to-end East-West serial path that can be a Local RPL
   Instance.  The local RPLInstanceID of the Track
      by itself or a subTrack of a complex Track.  With this
      specification, a Leg is called is installed by the
   TrackID, more in Section 7.2.  A Root of the main DODAG
      using Non-Storing Mode P-DAO message for messages, and it is expressed as a
      loose sequence of nodes that are joined by Track signals
   the TrackID in the RPLInstanceID field.  The Segments.
   Track Ingress Segment:  A serial path formed by a strict sequence of nodes,
      along which a P-Route is
   signaled in installed.  With this specification, a
      Segment is typically installed by the DODAGID field Root of the Projected DAO Base Object; that
   field is elided in main DODAG using
      Storing Mode P-DAO messages.  A Segment used as the case topological
      edge of a Track.  Since this specification builds only DODAGs, all
      Segments are oriented from Ingress (East) to Egress (West), as
      opposed to the main general RAW model, which allows North/South
      Segments that can be bidirectional.

3.  Context and Goal
3.1.  RPL Instance.  The Track
   Ingress Applicability

   RPL is optimized for situations where the power is scarce, the Root of
   bandwidth constrained and the Track, as shown in Figure 1. transmissions unreliable.  This specification defines matches
   the new "Projected DAO" (P) flag.  The 'P'
   flag use case of an IoT LLN where RPL is encoded in bit position 2 (to be confirmed by IANA) typically used today, but
   also situations of high relative mobility between the
   Flags field nodes in the DAO Base Object.  The Root MUST set it to 1 in
   network (aka swarming), e.g., within a
   Projected DAO message.  Otherwise it MUST be set to 0.  It is variable set to
   0 in legacy implementations as specified respectively in Sections
   20.11 and 6.4 of [RPL]. .

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    TrackID    |K|D|P|  Flags  |   Reserved    | DAOSequence   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +               IPv6 Address vehicles with
   a similar global motion, or a toon of drones.

   To reach this goal, RPL is primarily designed to minimize the Track Ingress               +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Option(s)...
     +-+-+-+-+-+-+-+-+

                    Figure 1: Projected DAO Base Object

   New fields:

   TrackID:  In control
   plane activity, that is the case relative amount of a P-DAO, routing protocol
   exchanges vs. data traffic, and the RPLInstanceID field is called
      TrackID.  This amount of state that is a naming convenience but
   maintained in each node.  RPL does not change the
      semantics need converge, and format provides
   connectivity to most nodes most of the RPLInstanceID that is used as TrackID.

   P:  1-bit flag (position to time.

   RPL may form multiple topologies called instances.  Instances can be confirmed by IANA).
   created to enforce various optimizations through objective functions,
   or to reach out through different Root Nodes.  The 'P' flag is set concept of
   objective function allows to 1 by adapt the Root activity of the routing
   protocol to signal a Projected DAO, the use case, e.g., type, speed, and it quality of the LLN
   links.

   RPL instances operate as ships in the night, unbeknownst of one
   another.  The RPL Root is set responsible to 0 otherwise.

   In RPL Non-Storing Mode, select the TIO and RTO are combined in a DAO
   message RPL Instance that
   is used to inform forward a packet coming from the DODAG Root of all Backbone into the edges RPL
   domain and set the related RPL information in the DODAG, which
   are formed by packets. 6TiSCH
   leverages RPL for its distributed routing operations.

   To reduce the directed parent-child relationships.  Options may
   be factorized; multiple RTOs may be present to signal routing exchanges, RPL leverages an anisotropic
   Distance Vector approach, which does not need a collection global knowledge of
   children that can be reached via
   the parent(s) indicated in topology, and only optimizes the
   TIO(s) that follows routes to and from the RTOs. RPL Root,
   allowing P2P paths to be stretched.  Although RPL installs its routes
   proactively, it only maintains them lazily, in reaction to actual
   traffic, or as a slow background activity.

   This specification generalizes is simple and efficient in situations where the
   case of a parent that can be used traffic is
   mostly directed from or to reach a child with that central node, such as the control
   traffic between routers and a controller of a
   whole Track through which Software Defined
   Networking (SDN) infrastructure or an Autonomic Control Plane (ACP).

   But stretch in P2P routing is counter-productive to both children reliability
   and siblings latency as it introduces additional delay and chances of loss.
   As a result, [RPL] is not a good fit for the Track
   Egress are reachable.

   New CMOs called use cases listed in the Via Information Options (VIO) are introduced for
   RAW use cases document [USE-CASES], which demand high availability
   and reliability, and as a consequence require both short and diverse
   paths.

3.2.  RPL Routing Modes

   RPL first forms a default route in P-DAO messages each node towards the a Root, and
   those routes together coalesce as a multihop alternative Directed Acyclic Graph upwards.
   RPL then constructs routes to so-called Targets in the TIO.  One VIO
   is the Stateful VIO (SF-VIO); reverse
   direction, down the SF-VIO installs Storing-Mode
   Projected Route along same DODAG.  So do so, a strict segment. RPL Instance can be
   operated either in RPL Storing or Non-Storing Mode of Operation (MOP)
   The other is the Source-
   Routed VIO (SR-VIO); default route towards the SR-VIO installs Root is maintained aggressively and may
   change while a Non-Storing-Mode Projected
   Route at packet progresses without causing loops, so the Track Ingress, which uses that state to encapsulate a packet with
   will still reach the Root.

   In Non-Storing Mode, each node advertises itself as a Routing Header (RH) Target directly
   to the Track Egress.

   Like in a DAO message, Root, indicating the RTOs can parents that may be factorized in a P-DAO, but used to reach self.
   Recursively, the
   Via Information Options cannot.  A P-DAO contains one or more RTOs Root builds and maintains an image of the whole
   DODAG in memory, and leverages that indicate abstraction to compute source
   route paths for the packets to their destinations that can be reached via down the Track, and
   exactly one VIO that signals DODAG.
   When a sequence node changes its point(s) of nodes.  In Non-Storing
   Mode, attachment to the DODAG, it takes
   single unicast packet to the Root sends along the P-DAO default route to update
   it, and the Track Ingress where the source-
   routing state connectivity is stored.  In Storing Mode, the P-DAO restored immediately; this mode is sent to
   preferable for use cases where internet connectivity is dominant, or
   when, like here, the
   Track Egress and forwarded along Root controls the Segment network activity in the reverse
   direction, installing a nodes.

   In Storing Mode state to Mode, the Track Egress at routing information percolates upwards, and each hop.  In both cases
   node maintains the Track Ingress is routes to the owner subDAG of its descendants down the Track,
   and it generates
   DODAG.  The maintenance is lazy, either reactive upon traffic or as a
   slow background process.  Packets flow via the P-DAO-ACK when common parent and the installation
   routing stretch is successful.

3.2.  Sibling Information Option

   This specification adds another CMO called reduced vs. Non-Storing, for a better P2P
   connectivity, while the Sibling Information
   Option (SIO) that internet connectivity is used restored more
   slowly, time for the DV operation to operate hop-by-hop.

   Either way, the RPL routes are injected by the Target nodes, in a
   distributed fashion.  To complement RPL Aware Node (RAN) to advertise a
   selection of its candidate neighbors as siblings and eliminate routing
   stretch, this specification introduces an hybrid mode that combines
   Storing and Non-Storing operations to build and project routes onto
   the Root, more nodes where they should be installed.  This specification uses
   the term P-Route to refer to those routes.

   A P-Route may be installed in
   Section 6.4.  The sibling selection process is out either Storing and Non-Storing Mode,
   potentially resulting in hybrid situations where the Mode of scope.  The
   expectation the P-
   Route is that a node reports a Sibling Address in a SIO based
   on an active address registration [RFC8505] different from that sibling for
   that address with the 'R' flag not set in of the EARO.  The node may
   assess RPL Main DODAG.  P-Routes can be
   used as stand-alone segments to reduce the liveliness size of the sibling at any time by performing source routing
   headers with loose source routing operations down the main RPL DODAG.
   P-Routes can also be combined with other P-Routes to form a
   registration for one of its own addresses, either more
   complex forwarding graph called a link local or Track.

3.3.  On Tracks

   A Track is typically a
   global one, depending on whether the node expects the sibling collection of parallel loose source routed
   sequences of nodes from Ingress to
   perform a matching advertisement Egress, forming so-called Track
   Legs, that are joined with strict Segments of other nodes.  The Legs
   are expressed in its own SIO.

3.3.  P-DAO Request

   Two new RPL Control Messages are also introduced, Non-Storing Modes and require an encapsulation
   to enable add a RAN to
   request Source Route Header, whereas the establishment of a Segments are expressed in
   Storing Mode.

   A Serial Track comprises provides only one path between self as the Track Ingress Node and
   Egress.  It comprises at most one Leg. A Stand-Alone Segment defines
   implicitly a Serial Track Egress.  The RAN makes from its request by
   sending a new P-DAO Request (PDR) Message to the Root.  The Root
   confirms with a new PDR-ACK message back Ingress to the requester RAN, see
   Section 6.1 for more. Egress.

   A positive PDR-ACK indicates that the complex Track
   was built and forms a graph that the Roots commits provides a collection of potential
   paths to maintain the Track provide redundancy for the
   negotiated lifetime.  In the case of packets, either as a complex Track, each Segment is
   maintained independently and asynchronously by the Root, with its own
   lifetime collection
   of Legs that may be shorter, the same, parallel or longer than that cross at certain points, or as a more
   generic DODAG.

   The concept of a Track was introduced in the
   Track.  The Root may use an asynchronous PDR-ACK with an negative
   status to indicate "6TiSCH Architecture"
   [6TiSCH-ARCHI], as a collection of potential paths that leverage
   redundant forwarding solutions along the way.  With this
   specification, a Track was terminated before its time.

3.4.  Extending the RPI

   Sending forms DODAG that is directed towards a Packet within Track
   Ingress.  If there is a RPL Local Instance requires single Track Egress, then the presence Track is
   reversible to form another DODAG by reversing the direction of each
   edge.  A node at the abstract RPL Packet Information (RPI) described in section 11.2. Ingress of [RPL] more than one Segment in the outer IPv6 Header chain (see [USEofRPLinfo]).  The
   RPI carries a local RPLInstanceID which, in association with either
   the source Track may
   use one or more of these Segments to forward a packet inside the destination address in the IPv6 Header, indicates
   Track.

   Section 5.1. of [RPL] describes the RPL Instance that the packet follows.

   This specification extends [RPL] and its encoding.
   There can be up to create 128 global RPL Instances, for which there can be
   one or more DODAGs, and there can be 64 local RPL Instances, with a new flag that signals
   namespace that a packet is forwarded along indexed by a projected route.

   Projected-Route 'P':  1-bit flag.  It is set to 1 if this packet DODAGID, where the DODAGID is
      sent over a projected route and set to 0 otherwise.

4.  Extending RFC 6553

   "The RPL Option for Carrying RPL Information in Data-Plane Datagrams"
   [RFC6553]describes the RPL Option for use among RPL routers to
   include Unique
   Local Address (ULA) or a Global Unicast Address (GUA) of the abstract RPL Packet Information (RPI) described in
   section 11.2. Root of [RPL] in data packets.

   The
   the DODAG.

   A Track is normally associated with a Local RPL Option Instance which
   RPLInstanceID is commonly referred to used as the RPI though the RPI is
   really the abstract information that is transported TrackID, more in Section 6.2.  A Track
   Leg may also be used as a subTrack that extends the RPL
   Option.  [USEofRPLinfo] updated the Option Type from 0x63 to 0x23.

   This specification modifies main DODAG.
   In that case, the RPL Option TrackID is set to encode the 'P' flag as
   follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |  Option Type  |  Opt Data Len |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |O|R|F|P|0|0|0|0| global RPLInstanceID |          SenderRank           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         (sub-TLVs)                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 2: Extended RPL Option Format

   Option Type:  0x23 or 0x63, see [USEofRPLinfo]

   Opt Data Len:  See [RFC6553]

   'O', 'R' and 'F' flags:  See [RFC6553].  Those flags MUST be set of the
   main DODAG, which suffices to 0
      by identify the sender and ignored by routing topology.  As
   opposed to local RPL instances, the receiver if Track Ingress that encapsulates
   the 'P' flag packets over a subtrack is set.

   Projected-Route 'P':  1-bit flag as defined in Section 3.4.

   RPLInstanceID:  See [RFC6553].  Indicates not Root, and that the TrackId if source address
   of the 'P' flag encapsulated packet is set.

   SenderRank:  See [RFC6553].  This field MUST be set not used to 0 by determine the
      sender and ignored by Track.

   A P-DAO message for a Track signals the receiver if TrackID in the 'P'flag is set.

5.  Extending RFC 8138

   Section 6.3 of [RFC8138] presents RPLInstanceID
   field.  In the formats case of a local RPL Instance, the 6LoWPAN Routing
   Header address of type 5 (RPI-6LoRH) that compresses the RPI for normal RPL
   operation.  The format of Track
   Ingress to be used as source to encapsulated packets along the RPI-6LoRH Track
   is not suited for Projected
   routes since signaled in the O,R,F flags are not used and DODAGID field of the Rank Projected DAO Base Object;
   that field is unknown and
   ignored. elided in the case of the RPL Main DODAG, see Figure 3.

3.4.  Serial Track Signaling

   This specification introduces a new 6LoRH, the P-RPI-6LoRH, with a
   type concept of 7.  The P-RPI-6LoRH header is usually a P-Route along either a Critical 6LoWPAN
   Routing Header, but it can be elective as well if an SRH-6LoRH
   Track Leg or a Segment.  A P-Route is
   present installed and controls the routing decision.

   The P-RPI-6LoRH is designed to compress the RPI along RPL Projected
   Routes.  It sformat is as follows:

                0                   1                   2
                0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               |1|0|E| Length  | 6LoRH Type 7  | RPLInstanceID |
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 3: P-RPI-6LoRH Format

   Elective 'E':  See [RFC8138].  The 'E' flag is set to 1 to indicate maintained using
   an Elective 6LoRH, meaning that it can be ignored when forwarding.

6.  New extended RPL Control Messages and Options

6.1.  New P-DAO Request Control Message

   The P-DAO Request (PDR) DAO message is sent by a Node in the main DODAG
   to the Root.  It is called a request to establish or refresh Projected DAO (P-DAO), and a
   Track where
   this node is Track Ingress.  The source IPv6 address composed of the PDR
   signals the Track Ingress combination of one or more P-Routes.  This
   specification introduces the requested Track, and the TrackID is
   indicated Via Information Option (VIO) to signal a
   sequence of hops in a Leg or a Segment in the message itself. P-DAO messages, either
   in Storing Mode (SM-VIO) or NON-Storing Mode (NSM-VIO).  One and only P-DAO
   messages contains a single VIO, associated to one or more RPL Target Option
   MUST be present in
   Options that signal the message.  The RTO signals destination IPv6 addresses that can reached
   along the Track.

   Before diving deeper into Track Egress,
   more in Section 7.1.

   The RPL Control Code Legs and Segments signaling and
   operation, this section provides examples of what how route
   projection works through variations of a simple example.  In this
   simple example we show host routes though RPL Targets can be
   prefixes.  Say we want to build a Serial Track from node A to E in
   Figure 1, so A can route packets to E's neighbors F and G along A, B,
   C, D and E as opposed to via the Root:

                                                 /===> F
                   A ===> B ===> C ===> D===> E <
                                                 \===> G

                         Figure 1: Reference Track

   Conventionally we use ==> to represent a strict hop and --> for a
   loose hop.  We use -to- like in C==>D==>E-to-F to represent coma-
   separated Targets, e.g., F is a Target for Segment C==>D==>E.  In
   this example, A is Track Ingress, E is Track Egress.  C is a
   stitching point.  F and G are "external" Targets for the Track, and
   become reachable from A via the Track A(ingress) to E (Egress and
   implicit Target in Non-Storing Mode) leading to F and G (explicit
   Targets).

   In a general manner the desired outcome is as follows:

   *  Targets are E, F, and G

   *  P-DAO 1 signals C==>D==>E
   *  P-DAO 2 signals A==>B==>C

   *  P-DAO 3 signals F and G via the A-->E Track

   P-DAO 3 may be ommitted if P-DAO 1 and 2 signal F and G as Targets.

   Loose sequences of hops must be expressed in Non-Storing Mode, so
   P-DAO 3 contains a NSM-VIO.  With this specification, the DODAGID to
   be used by the Ingress as source address is signaled if needed in the
   DAO base object, the via list starts at the first loose hop and
   matches the source route header, and the Egress of a Non-Storing Mode
   P-DAO is an implicit Target that is not listed in the RTO.

3.4.1.  Using Storing Mode Segments

   A==>B==>C and C==>D==>E are segments of a same Track.  Note that the
   Storing Mode signaling imposes strict continuity in a segment, since
   the P-DAO is passed hop by hop, as a classical DAO is, along the
   reverse datapath that it signals.  One benefit of strict routing is
   that loops are avoided along the Track.

3.4.1.1.  Stitched Segments

   In this formulation:

   *  P-DAO 1 signals C==>D==>E-to-F,G

   *  P-DAO 2 signals A==>B==>C-to-F,G

   Storing Mode P-DAO 1 is sent to E and when it is succesfully
   acknowledged, Storing Mode P-DAO 2 is sent to C, as follows:

           +====================+==============+==============+
           |       Field        | P-DAO 1 to E | P-DAO 2 to C |
           +====================+==============+==============+
           |        Mode        | Storing      | Storing      |
           +--------------------+--------------+--------------+
           |   Track Ingress    | A            | A            |
           +--------------------+--------------+--------------+
           | (DODAGID, TrackID) | (A, 129)     | (A, 129)     |
           +--------------------+--------------+--------------+
           |     SegmentID      | 1            | 2            |
           +--------------------+--------------+--------------+
           |        VIO         | C, D, E      | A, B, C      |
           +--------------------+--------------+--------------+
           |      Targets       | F, G         | F, G         |
           +--------------------+--------------+--------------+

                         Table 1: P-DAO Messages

   As a result the RIBs are set as follows:

         +======+=============+=========+=============+==========+
         | Node | destination | Origin  | Next Hop(s) | TrackID  |
         +======+=============+=========+=============+==========+
         |  E   | F, G        | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  D   | E           | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | F, G        | P-DAO 1 | E           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  C   | D           | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | F, G        | P-DAO 1 | D           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  B   | C           | P-DAO 2 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | F, G        | P-DAO 2 | C           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  A   | B           | P-DAO 2 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | F, G        | P-DAO 2 | B           | (A, 129) |
         +------+-------------+---------+-------------+----------+

                            Table 2: RIB setting

   Packets originated by A to F or G do not require an encapsulation as
   the RPI can be placed in the native header chain.  For packets that
   it routes, A must encapsulate to add the RPI that signals the
   trackID; the outer headers of the packets that are forwarded along
   the Track have the following settings:

    +========+===================+===================+================+
    | Header | IPv6 Source Addr. | IPv6 Dest.  Addr. | TrackID in RPI |
    +========+===================+===================+================+
    | Outer  |         A         |       F or G      |    (A, 129)    |
    +--------+-------------------+-------------------+----------------+
    | Inner  |       X != A      |       F or G      |      N/A       |
    +--------+-------------------+-------------------+----------------+

                      Table 3: Packet Header Settings

   As an example, say that A has a packet for F.  Using the RIB above:

   *  From P-DAO 2: A forwards to B and B forwards to C.

   *  From P-DAO 1: C forwards to D and D forwards to E.

   *  From Neighbor Cache Entry: C delivers the packet to F.

3.4.1.2.  External routes

   In this example, we consider F and G as destinations that are
   external to the Track as a DODAG, as discussed in section 4.1.1. of
   [RFC9008].  We then apply the directives for encapsulating in that
   case, more in Section 6.6.

   In this formulation, we set up the Track Leg explicitly, which
   creates less routing state in intermediate hops at the expense of
   larger packets to accommodate source routing:

   *  P-DAO 1 signals C==>D==>E-to-E

   *  P-DAO 2 signals A==>B==>C-to-E

   *  P-DAO 3 signals F and G via the A-->E-to-F,G Track

   Storing Mode P-DAO 1 and 2, and Non-Storing Mode P-DAO 3, are sent to
   E, C and A, respectively, as follows:

    +====================+==============+==============+==============+
    |                    | P-DAO 1 to E | P-DAO 2 to C | P-DAO 3 to A |
    +====================+==============+==============+==============+
    |        Mode        | Storing      | Storing      | Non-Storing  |
    +--------------------+--------------+--------------+--------------+
    |   Track Ingress    | A            | A            | A            |
    +--------------------+--------------+--------------+--------------+
    | (DODAGID, TrackID) | (A, 129)     | (A, 129)     | (A, 129)     |
    +--------------------+--------------+--------------+--------------+
    |     SegmentID      | 1            | 2            | 3            |
    +--------------------+--------------+--------------+--------------+
    |        VIO         | C, D, E      | A, B, C      | E            |
    +--------------------+--------------+--------------+--------------+
    |      Targets       | E            | E            | F, G         |
    +--------------------+--------------+--------------+--------------+

                          Table 4: P-DAO Messages

   Note in the above that E is not an implicit Target in Storing mode,
   so it must be added in the RTO.

   As a result the RIBs are set as follows:

         +======+=============+=========+=============+==========+
         | Node | destination | Origin  | Next Hop(s) | TrackID  |
         +======+=============+=========+=============+==========+
         |  E   | F, G        | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  D   | E           | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  C   | D           | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | E           | P-DAO 1 | D           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  B   | C           | P-DAO 2 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | E           | P-DAO 2 | C           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  A   | B           | P-DAO 2 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | E           | P-DAO 2 | B           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | F, G        | P-DAO 3 | E           | (A, 129) |
         +------+-------------+---------+-------------+----------+

                            Table 5: RIB setting

   Packets from A to E do not require an encapsulation.  The outer
   headers of the packets that are forwarded along the Track have the
   following settings:

   +========+===================+====================+================+
   | Header | IPv6 Source Addr. | IPv6 Dest.  Addr.  | TrackID in RPI |
   +========+===================+====================+================+
   | Outer  |         A         |         E          |    (A, 129)    |
   +--------+-------------------+--------------------+----------------+
   | Inner  |         X         | E (X != A), F or G |      N/A       |
   +--------+-------------------+--------------------+----------------+

                     Table 6: Packet Header Settings

   As an example, say that A has a packet for F.  Using the RIB above:

   *  From P-DAO 3: A encapsulates the packet the Track signaled by
      P-DAO 3, with the outer header above.  Now the packet destination
      is E.

   *  From P-DAO 2: A forwards to B and B forwards to C.

   *  From P-DAO 1: C forwards to D and D forwards to E; E decapsulates
      the packet.

   *  From Neighbor Cache Entry: C delivers packets to F or G.

3.4.1.3.  Segment Routing

   In this formulation leverages Track Legs to combine Segments and form
   a Graph.  The packets are source routed from a Segment to the next to
   adapt the path.  As such, this can be seen as a form of Segment
   Routing [RFC8402]:

   *  P-DAO 1 signals C==>D==>E-to-E

   *  P-DAO 2 signals A==>B-to-B,C

   *  P-DAO 3 signals F and G via the A-->C-->E-to-F,G Track

   Storing Mode P-DAO 1 and 2, and Non-Storing Mode P-DAO 3, are sent to
   E, B and A, respectively, as follows:

    +====================+==============+==============+==============+
    |                    | P-DAO 1 to E | P-DAO 2 to B | P-DAO 3 to A |
    +====================+==============+==============+==============+
    |        Mode        | Storing      | Storing      | Non-Storing  |
    +--------------------+--------------+--------------+--------------+
    |   Track Ingress    | A            | A            | A            |
    +--------------------+--------------+--------------+--------------+
    | (DODAGID, TrackID) | (A, 129)     | (A, 129)     | (A, 129)     |
    +--------------------+--------------+--------------+--------------+
    |     SegmentID      | 1            | 2            | 3            |
    +--------------------+--------------+--------------+--------------+
    |        VIO         | C, D, E      | A, B         | C, E         |
    +--------------------+--------------+--------------+--------------+
    |      Targets       | E            | C            | F, G         |
    +--------------------+--------------+--------------+--------------+

                          Table 7: P-DAO Messages

   Note in the above that the Segment can terminate at the loose hop as
   used in the example of P-DAO 1 or at the previous hop as done with
   P-DAO 2.  Both methods are possible on any Segment joined by a loose
   Track Leg. P-DAO 1 generates more signaling since E is the Segment
   Egress when D could be, but has the benefit that it validates that
   the connectivity between D and E still exists.

   As a result the RIBs are set as follows:

         +======+=============+=========+=============+==========+
         | Node | destination | Origin  | Next Hop(s) | TrackID  |
         +======+=============+=========+=============+==========+
         |  E   | F, G        | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  D   | E           | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  C   | D           | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | E           | P-DAO 1 | D           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  B   | C           | P-DAO 2 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  A   | B           | P-DAO 2 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | C           | P-DAO 2 | B           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | E, F, G     | P-DAO 3 | C, E        | (A, 129) |
         +------+-------------+---------+-------------+----------+

                            Table 8: RIB setting

   Packets originated at A to E do not require an encapsulation, but
   carry a SRH via C.  The outer headers of the packets that are
   forwarded along the Track have the following settings:

   +========+===================+====================+================+
   | Header | IPv6 Source Addr. | IPv6 Dest.  Addr.  | TrackID in RPI |
   +========+===================+====================+================+
   | Outer  |         A         |  C till C then E   |    (A, 129)    |
   +--------+-------------------+--------------------+----------------+
   | Inner  |         X         | E (X != A), F or G |      N/A       |
   +--------+-------------------+--------------------+----------------+

                     Table 9: Packet Header Settings

   As an example, say that A has a packet for F.  Using the RIB above:

   *  From P-DAO 3: A encapsulates the packet the Track signaled by
      P-DAO 3, with the outer header above.  Now the destination in the
      IPv6 Header is C, and a SRH signals the final destination is E.

   *  From P-DAO 2: A forwards to B and B forwards to C.

   *  From P-DAO 3: C processes the SRH and sets the destination in the
      IPv6 Header to E.

   *  From P-DAO 1: C forwards to D and D forwards to E; E decapsulates
      the packet.

   *  From the Neighbor Cache Entry: C delivers packets to F or G.

3.4.2.  Using Non-Storing Mode joining Tracks

   In this formulation:

   *  P-DAO 1 signals C==>D==>E-to-F,G

   *  P-DAO 2 signals A==>B==>C-to-C,F,G

   A==>B==>C and C==>D==>E are Tracks expressed as Non-Storing P-DAOs.

3.4.2.1.  Stitched Tracks

   Non-Storing Mode P-DAO 1 and 2 are sent to C and A respectively, as
   follows:

           +====================+==============+==============+
           |                    | P-DAO 1 to C | P-DAO 2 to A |
           +====================+==============+==============+
           |        Mode        | Non-Storing  | Non-Storing  |
           +--------------------+--------------+--------------+
           |   Track Ingress    | C            | A            |
           +--------------------+--------------+--------------+
           | (DODAGID, TrackID) | (C, 131)     | (A, 131)     |
           +--------------------+--------------+--------------+
           |     SegmentID      | 1            | 1            |
           +--------------------+--------------+--------------+
           |        VIO         | D, E         | B, C         |
           +--------------------+--------------+--------------+
           |      Targets       | F, G         | E, F, G      |
           +--------------------+--------------+--------------+

                         Table 10: P-DAO Messages

   As a result the RIBs are set as follows:

         +======+=============+=========+=============+==========+
         | Node | destination | Origin  | Next Hop(s) | TrackID  |
         +======+=============+=========+=============+==========+
         |  E   | F, G        | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  D   | E           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  C   | D           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  "   | E, F, G     | P-DAO 1 | D, E        | (C, 131) |
         +------+-------------+---------+-------------+----------+
         |  B   | C           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  A   | B           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  "   | C, E, F, G  | P-DAO 2 | B, C        | (A, 131) |
         +------+-------------+---------+-------------+----------+

                           Table 11: RIB setting

   Packets originated at A to E, F and G do not require an
   encapsulation, though it is preferred that A encapsulates and C
   decapsulates.  Either way, they carry a SRH via B and C, and C needs
   to encapsulate to E, F, or G to add an SRH via D and E.  The
   encapsulating headers of packets that are forwarded along the Track
   between C and E have the following settings:

    +========+===================+===================+================+
    | Header | IPv6 Source Addr. | IPv6 Dest.  Addr. | TrackID in RPI |
    +========+===================+===================+================+
    | Outer  |         C         |  D till D then E  |    (C, 131)    |
    +--------+-------------------+-------------------+----------------+
    | Inner  |         X         |     E, F, or G    |      N/A       |
    +--------+-------------------+-------------------+----------------+

              Table 12: Packet Header Settings between C and E

   As an example, say that A has a packet for F.  Using the RIB above:

   *  From P-DAO 2: A encapsulates the packet with destination of F in
      the Track signaled by P-DAO 2.  The outer header has source A,
      destination B, an SRH that indicates C as the next loose hop, and
      a RPI indicating a TrackId of 131 from A's namespace, which is
      distinct from TrackId of 131 from C's.

   *  From the SRH: Packets forwarded by B have source A, destination C,
      a consumed SRH, and a RPI indicating a TrackId of 131 from A's
      namespace.  C decapsulates.

   *  From P-DAO 1: C encapsulates the packet with destination of F in
      the Track signaled by P-DAO 1.  The outer header has source C,
      destination D, an SRH that indicates E as the next loose hop, and
      a RPI indicating a TrackId of 131 from C's namespace.  E
      decapsulates.

3.4.2.2.  External routes

   In this formulation:

   *  P-DAO 1 signals C==>D==>E-to-E

   *  P-DAO 2 signals A==>B==>C-to-C,E

   *  P-DAO 3 signals F and G via the A-->E-to-F,G Track

   Non-Storing Mode P-DAO 1 is sent to C and Non-Storing Mode P-DAO 2
   and 3 are sent A, as follows:

    +====================+==============+==============+==============+
    |                    | P-DAO 1 to C | P-DAO 2 to A | P-DAO 3 to A |
    +====================+==============+==============+==============+
    |        Mode        | Non-Storing  | Non-Storing  | Non-Storing  |
    +--------------------+--------------+--------------+--------------+
    |   Track Ingress    | C            | A            | A            |
    +--------------------+--------------+--------------+--------------+
    | (DODAGID, TrackID) | (C, 131)     | (A, 129)     | (A, 141)     |
    +--------------------+--------------+--------------+--------------+
    |     SegmentID      | 1            | 1            | 1            |
    +--------------------+--------------+--------------+--------------+
    |        VIO         | D, E         | B, C         | E            |
    +--------------------+--------------+--------------+--------------+
    |      Targets       | E            | E            | F, G         |
    +--------------------+--------------+--------------+--------------+

                          Table 13: P-DAO Messages

   As a result the RIBs are set as follows:

         +======+=============+=========+=============+==========+
         | Node | destination | Origin  | Next Hop(s) | TrackID  |
         +======+=============+=========+=============+==========+
         |  E   | F, G        | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  D   | E           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  C   | D           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  "   | E           | P-DAO 1 | D, E        | (C, 131) |
         +------+-------------+---------+-------------+----------+
         |  B   | C           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  A   | B           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  "   | C, E        | P-DAO 2 | B, C        | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | F, G        | P-DAO 3 | E           | (A, 141) |
         +------+-------------+---------+-------------+----------+

                           Table 14: RIB setting

   The encapsulating headers of packets that are forwarded along the
   Track between C and E have the following settings:

    +========+===================+===================+================+
    | Header | IPv6 Source Addr. | IPv6 Dest.  Addr. | TrackID in RPI |
    +========+===================+===================+================+
    | Outer  |         C         |  D till D then E  |    (C, 131)    |
    +--------+-------------------+-------------------+----------------+
    | Middle |         A         |         E         |    (A, 141)    |
    +--------+-------------------+-------------------+----------------+
    | Inner  |         X         |     E, F or G     |      N/A       |
    +--------+-------------------+-------------------+----------------+

                      Table 15: Packet Header Settings

   As an example, say that A has a packet for F.  Using the RIB above:

   *  From P-DAO 3: A encapsulates the packet with destination of F in
      the Track signaled by P-DAO 3.  The outer header has source A,
      destination E, and a RPI indicating a TrackId of 141 from A's
      namespace.  This recurses with:

   *  From P-DAO 2: A encapsulates the packet with destination of E in
      the Track signaled by P-DAO 2.  The outer header has source A,
      destination B, an SRH that indicates C as the next loose hop, and
      a RPI indicating a TrackId of 129 from A's namespace.

   *  From the SRH: Packets forwarded by B have source A, destination C
      , a consumed SRH, and a RPI indicating a TrackId of 129 from A's
      namespace.  C decapsulates.

   *  From P-DAO 1: C encapsulates the packet with destination of E in
      the Track signaled by P-DAO 1.  The outer header has source C,
      destination D, an SRH that indicates E as the next loose hop, and
      a RPI indicating a TrackId of 131 from C's namespace.  E
      decapsulates.

3.4.2.3.  Segment Routing

   In this formulation:

   *  P-DAO 1 signals C==>D==>E-to-E

   *  P-DAO 2 signals A==>B-to-C

   *  P-DAO 3 signals F and G via the A-->C-->E-to-F,G Track

   Non-Storing Mode P-DAO 1 is sent to C and Non-Storing Mode P-DAO 2
   and 3 are sent A, as follows:

    +====================+==============+==============+==============+
    |                    | P-DAO 1 to C | P-DAO 2 to A | P-DAO 3 to A |
    +====================+==============+==============+==============+
    |        Mode        | Non-Storing  | Non-Storing  | Non-Storing  |
    +--------------------+--------------+--------------+--------------+
    |   Track Ingress    | C            | A            | A            |
    +--------------------+--------------+--------------+--------------+
    | (DODAGID, TrackID) | (C, 131)     | (A, 129)     | (A, 141)     |
    +--------------------+--------------+--------------+--------------+
    |     SegmentID      | 1            | 1            | 1            |
    +--------------------+--------------+--------------+--------------+
    |        VIO         | D, E         | B            | C, E         |
    +--------------------+--------------+--------------+--------------+
    |      Targets       |              | C            | F, G         |
    +--------------------+--------------+--------------+--------------+

                          Table 16: P-DAO Messages

   As a result the RIBs are set as follows:

         +======+=============+=========+=============+==========+
         | Node | destination | Origin  | Next Hop(s) | TrackID  |
         +======+=============+=========+=============+==========+
         |  E   | F, G        | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  D   | E           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  C   | D           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  "   | E           | P-DAO 1 | D, E        | (C, 131) |
         +------+-------------+---------+-------------+----------+
         |  B   | C           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  A   | B           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  "   | C           | P-DAO 2 | B, C        | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | E, F, G     | P-DAO 3 | C, E        | (A, 141) |
         +------+-------------+---------+-------------+----------+

                           Table 17: RIB setting

   The encapsulating headers of packets that are forwarded along the
   Track between A and B have the following settings:

    +========+===================+===================+================+
    | Header | IPv6 Source Addr. | IPv6 Dest.  Addr. | TrackID in RPI |
    +========+===================+===================+================+
    | Outer  |         A         |  B till D then E  |    (A, 129)    |
    +--------+-------------------+-------------------+----------------+
    | Middle |         A         |         C         |    (A, 141)    |
    +--------+-------------------+-------------------+----------------+
    | Inner  |         X         |     E, F or G     |      N/A       |
    +--------+-------------------+-------------------+----------------+

                      Table 18: Packet Header Settings

   The encapsulating headers of packets that are forwarded along the
   Track between B and C have the following settings:

    +========+===================+===================+================+
    | Header | IPv6 Source Addr. | IPv6 Dest.  Addr. | TrackID in RPI |
    +========+===================+===================+================+
    | Outer  |         A         |         C         |    (A, 141)    |
    +--------+-------------------+-------------------+----------------+
    | Inner  |         X         |     E, F or G     |      N/A       |
    +--------+-------------------+-------------------+----------------+

                      Table 19: Packet Header Settings

   The encapsulating headers of packets that are forwarded along the
   Track between C and E have the following settings:

    +========+===================+===================+================+
    | Header | IPv6 Source Addr. | IPv6 Dest.  Addr. | TrackID in RPI |
    +========+===================+===================+================+
    | Outer  |         C         |  D till D then E  |    (C, 131)    |
    +--------+-------------------+-------------------+----------------+
    | Middle |         A         |         E         |    (A, 141)    |
    +--------+-------------------+-------------------+----------------+
    | Inner  |         X         |     E, F or G     |      N/A       |
    +--------+-------------------+-------------------+----------------+

                      Table 20: Packet Header Settings

   As an example, say that A has a packet for F.  Using the RIB above:

   *  From P-DAO 3: A encapsulates the packet with destination of F in
      the Track signaled by P-DAO 3.  The outer header has source A,
      destination C, an SRH that indicates E as the next loose hop, and
      a RPI indicating a TrackId of 141 from A's namespace.  This
      recurses with:

   *  From P-DAO 2: A encapsulates the packet with destination of C in
      the Track signaled by P-DAO 2.  The outer header has source A,
      destination B, and a RPI indicating a TrackId of 129 from A's
      namespace.  B decapsulates forwards to C based on a sibling
      connected route.

   *  From the SRH: C consumes the SRH and makes the destination E.

   *  From P-DAO 1: C encapsulates the packet with destination of E in
      the Track signaled by P-DAO 1.  The outer header has source C,
      destination D, an SRH that indicates E as the next loose hop, and
      a RPI indicating a TrackId of 131 from C's namespace.  E
      decapsulates.

3.5.  Complex Tracks

   To increase the reliability of the P2P transmission, this
   specification enables to build a collection of Legs between the same
   Ingress and Egress Nodes and combine them with the same TrackID, as
   shown in Figure 2.  Legs may cross at loose hops edges or remain
   parallel.

   The Segments that join the loose hops of a Leg are installed with the
   same TrackID as the Leg. But each individual Leg and Segment has its
   own P-RouteID which allows to manage it separately.  When Legs cross
   within respsective Segment, the next loose hop (the current
   destination of the packet) indicates which Leg is being followed and
   a Segment that can reach that next loose hop is selected.

           CPF               CPF          CPF                 CPF

                          Southbound API

      _-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-
    _-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-

                         +----------+
                         | RPL Root |
                         +----------+
                           (      )
                 (                                  )
           (              DODAG                              )
             (                                           )
     (                                                         )
                                                                     )
     <-    Leg 1            B,                            E ->
     <--- Segment 1 A,B ---> <------- Segment 2 C,D,E ------->

                FWD  --z  Relay --z   FWD  --z   FWD          Target 1
            z-- Node  z--  Node  z--  Node  z--  Node --z     /
         --z    (A)        (B) \      (C)        (D)  z--    /
   Track                        \                       Track
   Ingress                    Segment 5                 Egress - Tgt 2
     (I)                           \                     (E)
         --z                        \                 z--    \
          z-- FWD   --z  FWD  --z  Relay --z  FWD  --z        \
              Node   z-- Node  z-- Node   z-- Node            Target 3
              (F)        (G)       (H)        (J)

     <------ Segment 3 F,G,H ------> <---- Segment 4 J,E ---->
     <-      Leg 2                  H,                    E ->

     <--- Segment 1 A,B ---> <- S5-> <---- Segment 4 J,E ---->
     <-      Leg 3          B,      H,                    E ->
                                                                     )
      (
                 (                                        )

                       Figure 2: Segments and Tracks

   Note that while this specification enables to build both Segments
   inside a Leg (aka East-West), such as Segment 2 above which is within
   Leg 1, and Inter-Leg Segments (aka North-South), such as Segment 2
   above which joins Leg 1 and Leg 2, it does not signal to the Ingress
   which Inter-Leg Segments are available, so the use of North-South
   Segments and associated PAREO functions is curently limited.  The
   only possibility available at this time is to define overlapping Legs
   as illustrated in Figure 2, with Leg 3 that is congruent with Leg 1
   till node B and congruent with Leg 2 from node H on, abstracting
   Segment 5 as an East-West Segment.

   DetNet Forwarding Nodes only understand the simple 1-to-1 forwarding
   sublayer transport operation along a segment whereas the more
   sophisticated Relay nodes can also provide service sublayer functions
   such as Replication and Elimination.  One possible mapping between
   DetNet and this specification is to signal the Relay Nodes as the
   hops of a Leg and the forwarding Nodes as the hops in a Segment that
   join the Relay nodes as illustrated in Figure 2.

3.6.  Scope and Expectations

   This specification expects that the RPL Main DODAG is operated in RPL
   Non-Storing Mode to sustain the exchanges with the Root.  Based on
   its comprehensive knowledge of the parent-child relationship, the
   Root can form an abstracted view of the whole DODAG topology.  This
   document adds the capability for nodes to advertise additional
   sibling information to complement the topological awareness of the
   Root to be passed on to the PCE, and enable the PCE to build more /
   better paths that traverse those siblings.

   P-Routes require resources such as routing table space in the routers
   and bandwidth on the links; the amount of state that is installed in
   each node must be computed to fit within the node's memory, and the
   amount of rerouted traffic must fit within the capabilities of the
   transmission links.  The methods used to learn the node capabilities
   and the resources that are available in the devices and in the
   network are out of scope for this document.  The method to capture
   and report the LLN link capacity and reliability statistics are also
   out of scope.  They may be fetched from the nodes through network
   management functions or other forms of telemetry such as OAM.

   The "6TiSCH Architecture" [6TiSCH-ARCHI] leverages a centralized
   model that is similar to that of "Deterministic Networking
   Architecture" [RFC8655], whereby the device resources and
   capabilities are exposed to an external controller which installs
   routing states into the network based on its own objective functions
   that reside in that external entity.  With DetNet and 6TiSCH, the
   component of the controller that is responsible of computing routes
   is a PCE.  The PCE computes its routes based on its own objective
   functions such as described in [RFC4655], and typically controls the
   routes using the PCE Protocol (PCEP) by [RFC5551].  While this
   specification expects a PCE and while PCEP might effectively be used
   between the Root and the PCE, the control protocol between the PCE
   and the Root is out of scope.

   This specification expects a single PCE with a full view of the
   network.  Distributing the PCE function for a large network is out of
   scope.  This specification uses the RPL Root as a proxy to the PCE.
   The PCE may be collocated with the Root, or may reside in an external
   Controller.  In that case, the protocol between the Root and the PCE
   is out of scope and abstracted by / mapped to RPL inside the DODAG;
   one possibility is for the Root to transmit the RPL DAOs with the
   SIOs that detail the parent/child and sibling information.

   The algorithm to compute the paths and the protocol used by the PCE
   and the metrics and link statistics involved in the computation are
   also out of scope.  The effectiveness of the route computation by the
   PCE depends on the quality of the metrics that are reported from the
   RPL network.  Which metrics are used and how they are reported is out
   of scope, but the expectation is that they are mostly of long-term,
   statistical nature, and provide visibility on link throughput,
   latency, stability and availability over relatively long periods.

   The "Reliable and Available Wireless (RAW) Architecture/Framework"
   [RAW-ARCHI] extends the definition of Track, as being composed of
   East-West directional segments and North-South bidirectional
   segments, to enable additional path diversity, using Packet ARQ,
   Replication, Elimination, and Overhearing (PAREO) functions over the
   available paths, to provide a dynamic balance between the reliability
   and availability requirements of the flows and the need to conserve
   energy and spectrum.. This specification prepares for RAW by setting
   up the Tracks, but only forms DODAGs, which are composed of
   aggregated end-to-end loose source routed Legs, joined by strict
   routed Segments, all oriented East-West.

   The RAW Architecture defines a dataplane extension of the PCE called
   the Path Selection Engine (PSE), that adapts the use of the path
   redundancy within a Track to defeat the diverse causes of packet
   loss.  The PSE controls the forwarding operation of the packets
   within a Track This specification can use but does not impose a PSE
   and does not provide the policies that wouldselect which packets are
   routed through which path within a Track, IOW, how the PSE may use
   the path redundancy within the Track.  By default, the use of the
   available redundancy is limited to simple load balancing, and all the
   segments are East-West unidirectional only.

   A Track may be set up to reduce the load around the Root, or to
   enable urgent traffic to flow more directly.  This specification does
   not provide the policies that would decide which flows are routed
   through which Track.  In a Non-Storing Mode RPL Instance, the Main
   DODAG provides a default route via the Root, and the Tracks provide
   more specific routes to the Track Targets.

4.  Extending existing RFCs

4.1.  Extending RFC 6550

   This specification extends RPL [RPL] to enable the Root to install
   East-West routes inside a Main DODAG that is operated as non-Storing
   Mode.  A Projected DAO (P-DAO) message (see Section 4.1.1) contains a
   new Via Information Option that installs a strict or a loose sequence
   of hops to form respectively a Track Segment or a Track Leg.  A new
   P-DAO Request (PDR) message (see Section 5.1) enables a Track Ingress
   to request the Track from the Root for which it is the Root and it
   owns the address that serves as TrackID, as well as the associated
   namespace from which it selects the TrackID.  In the context of this
   specification, the installed route appears as a more specific route
   to the Track Targets, and the Track Ingress routes the packets
   towards the Targets via the Track using the longest match as usual.

   To ensure that the PDR and P-DAO messages can flow at most times, it
   is RECOMMENDED that the nodes involved in a Track mantain multiple
   parents in the Main DODAG, advertise them all to the Root, and use
   them in turn to retry similar packets.  It is also RECOMMENDED that
   the Root uses diverse source route paths to retry similar messages ot
   the nodes in the Track.

4.1.1.  Projected DAO

   Section 6 of [RPL] introduces the RPL Control Message Options (CMO),
   including the RPL Target Option (RTO) and Transit Information Option
   (TIO), which can be placed in RPL messages such as the destination
   Advertisement Object (DAO).  A DAO message signals routing
   information to one or more Targets indicated in RTOs, providing one
   hop information at a time in the TIO.  A Projected DAO (P-DAO) is a
   special DAO message generated by the Root to install a P-Route formed
   of multiple hops in its DODAG.  This provides a RPL-based method to
   install the Tracks as expected by the 6TiSCH Architecture
   [6TiSCH-ARCHI] as a collection of multiple P-Routes.

   The P-DAO is signaled with a new "Projected DAO" (P) flag, see
   Figure 3.  The 'P' flag is encoded in bit position 2 (to be confirmed
   by IANA) of the Flags field in the DAO Base Object.  The Root MUST
   set it to 1 in a Projected DAO message.  Otherwise it MUST be set to
   0.  It is set to 0 in Legacy implementations as specified
   respectively in Sections 20.11 and 6.4 of [RPL]

   The P-DAO is control plane signaling and should not be stuck behind
   high traffic levels.  The expectation is that the P-DAO message is
   sent as high QoS level, above that of data traffic, typically with
   the Network Control precedence.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    TrackID    |K|D|P|  Flags  |   Reserved    | DAOSequence   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                   DODAGID field set to the                    |
     +               IPv6 Address of the Track Ingress               +
     |              used to source encapsulated packets              |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Option(s)...
     +-+-+-+-+-+-+-+-+

                    Figure 3: Projected DAO Base Object

   New fields:

   TrackID:  The local or global RPLInstanceID of the DODAG that serves
      as Track, more in Section 6.2

   P:  1-bit flag (position to be confirmed by IANA).

      The 'P' flag is set to 1 by the Root to signal a Projected DAO,
      and it is set to 0 otherwise.

   In RPL Non-Storing Mode, the TIO and RTO are combined in a DAO
   message to inform the DODAG Root of all the edges in the DODAG, which
   are formed by the directed parent-child relationships.  Options may
   be factorized; multiple RTOs may be present to signal a collection of
   children that can be reached via the parent(s) indicated in the
   TIO(s) that follows the RTOs.  This specification generalizes the
   case of a parent that can be used to reach a child with that of a
   whole Track through which children and siblings of the Track Egress
   are reachable.

4.1.2.  Via Information Option

   New CMOs called the Via Information Options (VIO) are introduced for
   use in P-DAO messages as a multihop alternative to the TIO, more in
   Section 5.3.  One VIO is the stateful Storing Mode VIO (SM-VIO); an
   SM-VIO installs a strict hop-by-hop P-Route called a Track Segment.
   The other is the Non-Storing Mode VIO (NSM-VIO); the NSM-VIO installs
   a loose source-routed P-Route called a Track Leg at the Track
   Ingress, which uses that state to encapsulate a packet IPv6_in_IPv6
   with a new Routing Header (RH) to the Track Egress, more in
   Section 6.6.

   A P-DAO contains one or more RTOs to indicate the Target
   (destinations) that can be reached via the P-Route, followed by
   exactly one VIO that signals the sequence of nodes to be followed,
   more in Section 6.  There are two modes of operation for the
   P-Routes, the Storing Mode and the Non-Storing Mode, see
   Section 6.3.2 and Section 6.3.3 respectively for more.

4.1.3.  Sibling Information Option

   This specification adds another CMO called the Sibling Information
   Option (SIO) that is used by a RPL Aware Node (RAN) to advertise a
   selection of its candidate neighbors as siblings to the Root, more in
   Section 5.4.  The SIO is placed in DAO messages that are sent
   directly to the Root of the main DODAG.

4.1.4.  P-DAO Request

   Two new RPL Control Messages are also introduced, to enable a RPL-
   Aware Node to request the establishment of a Track between self as
   the Track Ingress Node and a Track Egress.  The node makes its
   request by sending a new P-DAO Request (PDR) Message to the Root.
   The Root confirms with a new PDR-ACK message back to the requester
   RAN, see Section 5.1 for more.

4.1.5.  Extending the RPI

   Sending a Packet within a RPL Local Instance requires the presence of
   the abstract RPL Packet Information (RPI) described in section 11.2.
   of [RPL] in the outer IPv6 Header chain (see [RFC9008]).  The RPI
   carries a local RPLInstanceID which, in association with either the
   source or the destination address in the IPv6 Header, indicates the
   RPL Instance that the packet follows.

   This specification extends [RPL] to create a new flag that signals
   that a packet is forwarded along a P-Route.

   Projected-Route 'P':  1-bit flag.  It is set to 1 in the RPI that is
      added in the encapsulation when a packet is sent over a Track.  It
      is set to 0 when a packet is forwarded along the main Track,
      including when the packet follows a Segment that joins loose hops
      of the Main DODAG.  The flag is not mutable en-route.

   The encoding of the 'P' flag in native format is shown in Section 4.2
   while the compressed format is indicated in Section 4.3.

4.2.  Extending RFC 6553

   "The RPL Option for Carrying RPL Information in Data-Plane Datagrams"
   [RFC6553]describes the RPL Option for use among RPL routers to
   include the abstract RPL Packet Information (RPI) described in
   section 11.2. of [RPL] in data packets.

   The RPL Option is commonly referred to as the RPI though the RPI is
   really the abstract information that is transported in the RPL
   Option.  [RFC9008] updated the Option Type from 0x63 to 0x23.

   This specification modifies the RPL Option to encode the 'P' flag as
   follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |  Option Type  |  Opt Data Len |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |O|R|F|P|0|0|0|0| RPLInstanceID |          SenderRank           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         (sub-TLVs)                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4: Extended RPL Option Format

   Option Type:  0x23 or 0x63, see [RFC9008]

   Opt Data Len:  See [RFC6553]

   'O', 'R' and 'F' flags:  See [RFC6553].  Those flags MUST be set to 0
      by the sender and ignored by the receiver if the 'P' flag is set.

   Projected-Route 'P':  1-bit flag as defined in Section 4.1.5.

   RPLInstanceID:  See [RFC6553].  Indicates the TrackId if the 'P' flag
      is set, as discussed in Section 4.1.1.

   SenderRank:  See [RFC6553].  This field MUST be set to 0 by the
      sender and ignored by the receiver if the 'P'flag is set.

4.3.  Extending RFC 8138

   The 6LoWPAN Routing Header [RFC8138] specification introduces a new
   IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN)
   [RFC6282] dispatch type for use in 6LoWPAN route-over topologies,
   which initially covers the needs of RPL data packet compression.

   Section 4 of [RFC8138] presents the generic formats of the 6LoWPAN
   Routing Header (6LoRH) with two forms, one Elective that can be
   ignored and skipped when the router does not understand it, and one
   Critical which causes the packet to be dropped when the router cannot
   process it.  The 'E' Flag in the 6LoRH indicates its form.  In order
   to skip the Elective 6LoRHs, their format imposes a fixed expression
   of the size, whereas the size of a Critical 6LoRH may be signaled in
   variable forms to enable additional optimizations.

   When the [RFC8138] compression is used, the Root of the Main DODAG
   that sets up the Track also constructs the compressed routing header
   (SRH-6LoRH) on behalf of the Track Ingress, which saves the
   complexities of optimizing the SRH-6LoRH encoding in constrained
   code.  The SRH-6LoRH is signaled in the NSM-VIO, in a fashion that it
   is ready to be placed as is in the packet encapsulation by the Track
   Ingress.

   Section 6.3 of [RFC8138] presents the formats of the 6LoWPAN Routing
   Header of type 5 (RPI-6LoRH) that compresses the RPI for normal RPL
   operation.  The format of the RPI-6LoRH is not suited for P-Routes
   since the O,R,F flags are not used and the Rank is unknown and
   ignored.

   This specification introduces a new 6LoRH, the P-RPI-6LoRH that can
   be used in either Elective or Critical 6LoRH form, see Table 21 and
   Table 22 respectively.  The new 6LoRH MUST be used as a Critical
   6LoRH, unless an SRH-6LoRH is present and controls the routing
   decision, in which case it MAY be used in Elective form.

   The P-RPI-6LoRH is designed to compress the RPI along RPL P-Routes.
   Its format is as follows:

                0                   1                   2
                0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               |1|0|E| Length  |  6LoRH Type   | RPLInstanceID |
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        Figure 5: P-RPI-6LoRH Format

   Type:  IANA is requested to define the same value of the type for
      both Elective and Critical forms.  A type of 8 is suggested.

   Elective 'E':  See [RFC8138].  The 'E' flag is set to 1 to indicate
      an Elective 6LoRH, meaning that it can be ignored when forwarding.

   RPLInstanceID :  In the context of this specification, the
      RPLInstanceID field signals the TrackID, see Section 3.3 and
      Section 6.2 .

   Section 6.7 details how a a Track Ingress leverages the P-RPI-6LoRH
   Header as part of the encapsulation of a packet to place it into a
   Track.

5.  New RPL Control Messages and Options

5.1.  New P-DAO Request Control Message

   The P-DAO Request (PDR) message is sent by a Node in the Main DODAG
   to the Root.  It is a request to establish or refresh a Track where
   this node is Track Ingress, and signals whether an acknowledgment
   called PDR-ACK is requested or not.  A positive PDR-ACK indicates
   that the Track was built and that the Roots commits to maintain the
   Track for the negotiated lifetime.

   The Root may use an asynchronous PDR-ACK with an negative status to
   indicate that the Track was terminated before its time.  A status of
   "Transient Failure" (see Section 10.9) is an indication that the PDR
   may be retried after a reasonable time that depends on the
   deployment.  Other negative status values indicate a permanent error;
   the tentative must be abandoned until a corrective action is taken at
   the application layer or through network management.

   The source IPv6 address of the PDR signals the Track Ingress to-be of
   the requested Track, and the TrackID is indicated in the message
   itself.  One and only one RPL Target Option MUST be present in the
   message.  The RTO signals the Track Egress, more in Section 6.1.

   The RPL Control Code for the PDR is 0x09, to be confirmed by IANA.
   The format of PDR Base Object is as follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    TrackID    |K|R|   Flags   |  ReqLifetime  | PDRSequence   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Option(s)...
     +-+-+-+-+-+-+-+-+

                     Figure 4: 6: New P-DAO Request Format

   TrackID:  8-bit field.  In the context of this specification, the
      TrackID field indicating signals the RPLInstanceID associated with of the Track. DODAG formed by the
      Track, see Section 3.3 and Section 6.2.  To allocate a new Track,
      the Ingress Node must provide a value that is not in use at this
      time.

   K:  The 'K' flag is set to indicate that the recipient is expected to
      send a PDR-ACK back.

   R:  The 'R' flag is set to request a Complex Track for redundancy.

   Flags:  Reserved.  The Flags field MUST initialized to zero by the
      sender and MUST be ignored by the receiver

   ReqLifetime:  8-bit unsigned integer.  The requested lifetime for the
      Track expressed in Lifetime Units (obtained from the DODAG
      Configuration option).

      A PDR with a fresher PDRSequence refreshes the lifetime, and a
      PDRLifetime of 0 indicates that the track Track should be destroyed. destroyed,
      e.g., when the application that requested the Track terminates.

   PDRSequence:  8-bit wrapping sequence number, obeying the operation
      in section 7.2 of [RPL].  The PDRSequence is used to correlate a
      PDR-ACK message with the PDR message that triggered it.  It is
      incremented at each PDR message and echoed in the PDR-ACK by the
      Root.

6.2.

5.2.  New PDR-ACK Control Message

   The new PDR-ACK is sent as a response to a PDR message with the 'K'
   flag set.  The RPL Control Code for the PDR-ACK is 0x0A, to be
   confirmed by IANA.  Its format is as follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    TrackID    |     Flags     | Track Lifetime|  PDRSequence  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | PDR-ACK Status|                Reserved                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Option(s)...
     +-+-+-+-+-+-+-+

                Figure 5: 7: New PDR-ACK Control Message Format

   TrackID:  The RPLInstanceID  Set to the TrackID indicated in the TrackID field of the Track
      PDR messages that was created.  The value
      of 0x00 is used to when no Track was created. this replies to.

   Flags:  Reserved.  The Flags field MUST initialized to zero by the
      sender and MUST be ignored by the receiver

   Track Lifetime:  Indicates that remaining Lifetime for the Track,
      expressed in Lifetime Units; the value of zero (0x00) indicates
      that the Track was destroyed or not created.

   PDRSequence:  8-bit wrapping sequence number.  It is incremented at
      each PDR message and echoed in the PDR-ACK.

   PDR-ACK Status:  8-bit field indicating the completion.  The PDR-ACK
      Status is substructured as indicated in Figure 6: 8:

                                 0 1 2 3 4 5 6 7
                                +-+-+-+-+-+-+-+-+
                                |E|R|  Value    |
                                +-+-+-+-+-+-+-+-+

                       Figure 6: 8: PDR-ACK status Format

      E:  1-bit flag.  Set to indicate a rejection.  When not set, the
         value of 0 indicates Success/Unqualified acceptance Acceptance and other
         values indicate "not an outright rejection".
      R:  1-bit flag.  Reserved, MUST be set to 0 by the sender and
         ignored by the receiver.
      Status Value:  6-bit unsigned integer.  Values depending on the
         setting of the 'E' flag, see Table 27 and Table 28.

   Reserved:  The Reserved field MUST initialized to zero by the sender
      and MUST be ignored by the receiver

6.3.

5.3.  Via Information Options

   A VIO signals the ordered list of IPv6 Via Addresses that constitutes
   the hops of either a Serial Track or Leg (using Non-Storing Mode) a Segment (using
   storing mode) of a more Complex Track.  A Storing Mode P-DAO contains one Storing
   Mode VIO MUST contain at least (SM-VIO) whereas a Non-Storing Mode P-DAO contains one Via Address, Non-
   Storing Mode VIO (NSM-VIO)

   The duration of the validity of a VIO is indicated in a Segment
   Lifetime field.  A P-DAO message that contains a VIO with a Segment
   Lifetime of zero is referred as a No-Path P-DAO.

   The VIO contains one or more SRH-6LoRH header(s), each formed of a
   SRH-6LoRH head and a collection of compressed Via
   Address Addresses, except
   in the case of a Non-Storing Mode No-Path P-DAO where the SRH-6LoRH
   header is not present.

   In the case of a SM-VIO, or if [RFC8138] is not used in the data
   packets, then the Root MUST NOT be present more than once, otherwise use only one SRH-6LoRH per Via
   Information Option, and the compression is the same forall the
   addresses, as shown in Figure 9, for simplicity.

   In case of an NSM-VIO and if [RFC8138] is in use in the Main DODAG,
   the Root SHOULD optimize the size of the NSM-VIO if using different
   SRH-6LoRH Types make the VIO MUST globally shorter; this means that more
   than one SRH-6LoRH may be
   ignored. present.

   The format of the Via Information Options is as follows:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Type        | Option Length |     Flags     |   SegmentID   P-RouteID   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Segm. Sequence | Seg. Lifetime |        SRH-6LoRH header head         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .           Via Address 1 (compressed by RFC 8138)              .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .                              ....                             .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .           Via Address n (compressed by RFC 8138)              .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .              Additional SRH-6LoRH Header(s)                   .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                              ....                             .

                  Figure 7: 9: VIO format (uncompressed form)

   Option Type:  0x0B  0x0E for SF-VIO, 0x0C SM-VIO, 0x0F for SR-VIO NSM-VIO (to be confirmed by
      IANA)
      IANA), see =Table 25

   Option Length:  In bytes;  8-bit unsigned integer, representing the length in
      octets of the option, not including the Option Type and Length
      fields, see section 6.7.1. of [RPL]; the Option Length is
      variable, depending on the number of Via Addresses and the compression.

   SegmentID:
      compression applied.

   P-RouteID:  8-bit field that identifies a Segment within component of a Track or the main
      Main DODAG as indicated by the TrackID field.  The value of 0 is
      used to signal a Serial Track, i.e., made of a single segment. segment/Leg.
      In an SM-VIO, the P-RouteID indicates an actual Segment.  In an an
      NSM-VIO, it indicates a Leg, that is a serial subTrack that is
      added to the overall topology of the Track.

   Segment Sequence:  8-bit unsigned integer.  The Segment Sequence
      obeys the operation in section 7.2 of [RPL] and the lollipop
      starts at 255.

      When the Root of the DODAG needs to refresh or update a Segment in
      a Track, it increments the Segment Sequence individually for that
      Segment.

      The Segment information indicated in the VIO deprecates any state
      for the Segment indicated by the SegmentID P-RouteID within the indicated
      Track and sets up the new information.

      A VIO with a Segment Sequence that is not as fresh as the current
      one is ignored.

      A VIO for a given DODAGID with the same (TrackID, SegmentID, P-RouteID,
      Segment Sequence) indicates a retry; it MUST NOT change the
      Segment and MUST be propagated or answered as the first copy.

   Segment Lifetime:  8-bit unsigned integer.  The length of time in
      Lifetime Units (obtained from the Configuration option) that the
      Segment is usable.

      The period starts when a new Segment Sequence is seen.  The value
      of 255 (0xFF) represents infinity.  The value of zero (0x00)
      indicates a loss of reachability.

      A P-DAO message that contains a VIO with a Segment Lifetime of
      zero is referred as a No-Path P-DAO in this document.

   SRH-6LoRH header: head:  The first 2 bytes of the (first) SRH-6LoRH as shown
      in Figure 6 of [RFC8138].  A  As an example, a 6LoRH Type of 4 means
      that the VIA Addresses are provided in full with no compression.

   Via Address:  An IPv6 address ULA or GUA of a node along the Segment.

      In a SF-VIO,  The
      VIO contains one or more IPv6 Via Addresses listed in the list is a strict path between direct neighbors, datapath
      order from the Segment Ingress to Egress, both included. Egress.  The router list is expressed in a
      compressed form as signaled by the preceding SRH-6LoRH header.

      In a Storing Mode P-DAO that processes updates or removes a section of an SF-VIO MAY create additional routing state
      towards
      already existing Segment, the nodes after self via list in the node immediately after self SM-VIO may represent
      only the section of the Segment that is being updated; at the
      extreme, the SM-VIO updates only one node, in which case it
      contains only one IPv6 address.  In all other cases, the SF-VIO, but list in
      the VIO MUST be complete.

      In the case of memory shortage an SM-VIO, the routes to list indicates a sequential (strict)
      path through direct neighbors, the
      Targets have precedence since they are complete list starts at Ingress
      and ends at Egress, and the ones that nodes listed in the router
      commits to store. VIO, including the
      Egress, MAY be considered as implicit Targets.

      In the case of an SR-VIO, NSM-VIO, the complete list includes the egress but not can be loose and
      excludes the ingress
      node.  It starts Ingress node, starting at the first loose hop and ends
      ending at a Track Egress.  In
      that case, Egress; the Track Egress MUST be considered as
      an implicit Target, so it MUST NOT be listed it signaled in a RPL Target
      Option.  The
      list in an SR-VIO may be loose, provided that each listed node has
      a path to the next listed node, e.g., via a segment or another
      Track.

      In the case of a SF-VIO, or if [RFC8138] is not used in the data
      packets, then the Root MUST use only one SRH-6LoRH per Via
      Information Option, and the compression is the same for all the
      addresses, as shown in Figure 7.

      In case of an SR-VIO, and if [RFC8138] is in use in the main
      DODAG, then the Root SHOULD optimize the size of the SR-VIO; more
      than one SRH-6LoRH may be present, e.g., if the compression level
      changes inside the Segment and different SRH-6LoRH Types are
      required.  The content of the SR-VIO starting at the first SRH-
      6LoRH header is thus verbatim the one that the Track Ingress
      places in the packet encapsulation to reach the Track Ingress.

6.4.

5.4.  Sibling Information Option

   The Sibling Information Option (SIO) provides indication on siblings
   that could be used by the Root to form Projected Routes. P-Routes.  One or more SIO(s)
   may be placed in the DAO messages that are sent to the Root in
   Non-Storing Non-
   Storing Mode.

   To advertise a neighbor node, the router MUST have an active Address
   Registration from that sibling using [RFC8505], for an address (ULA
   or GUA) that serves as identifier for the node.  If this router also
   registers an address to that sibling, and the link has similar
   properties in both directions, only the router with the lowest
   Interface ID in its registered address needs report the SIO, and the
   Root will assume symmetry.

   The format of the SIO is as follows:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Type        | Option Length |Comp.|B|D|Flags| |D| Flags |Comp.|    Opaque     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Step of Rank       |          Reserved             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .       Sibling DODAGID (if the D flag not set)               .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .                     Sibling Address                           .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 8: 10: Sibling Information Option Format

   Option Type:  0x0D  0x10 for SIO (to be confirmed by IANA) IANA), see =Table 25

   Option Length:  In bytes, the size of the option.

   Compression Type:  3-bit  8-bit unsigned integer.  This is integer, representing the SRH-6LoRH
      Type as defined in figure 7 length in section 5.1
      octets of [RFC8138] that
      corresponds to the compression used for option, not including the Sibling Address Option Type and
      its DODAGID if resent.  The Compression reference is the Root Length
      fields, see section 6.7.1. of
      the main DODAG. [RPL].

   Reserved for Flags:  MUST be set to zero by the sender and MUST be
      ignored by the receiver.

   B:  1-bit flag that is set to indicate that the connectivity to the
      sibling is bidirectional and roughly symmetrical.  In that case,
      only one of the siblings may report the SIO for the hop.  If 'B'
      is not set then the SIO only indicates connectivity from the
      sibling to this node, and does not provide information on the hop
      from this node to the sibling.

   D:  1-bit flag that is set to indicate that sibling belongs to the
      same DODAG.  When not set, the Sibling DODAGID is indicated.

   Flags:  Reserved.  The Flags field MUST initialized to zero by the
      sender and MUST be ignored by the receiver

   Opaque:  MAY be used to carry information that the node and the Root
      understand, e.g., a particular representation of the Link
      properties such as a proprietary Link Quality Information for
      packets received from the sibling.  An industrial Alliance that
      uses RPL for a particular use / environment MAY redefine the use
      of this field to fit its needs.

   Compression Type:  3-bit unsigned integer.  This is the SRH-6LoRH
      Type as defined in figure 7 in section 5.1 of [RFC8138] that
      corresponds to the compression used for the Sibling Address and
      its DODAGID if resent.  The Compression reference is the Root of
      the Main DODAG.

   Step of Rank:  16-bit unsigned integer.  This is the Step of Rank
      [RPL] as computed by the Objective Function between this node and
      the sibling.

   Reserved:  The Reserved field MUST initialized to zero by the sender
      and MUST be ignored by the receiver

   Sibling DODAGID:  2 to 16 bytes, the DODAGID of the sibling in a
      [RFC8138] compressed form as indicated by the Compression Type
      field.  This field is present if and only if the D flag is not
      set.

   Sibling Address:  2 to 16 bytes, an IPv6 Address of the sibling, with
      a scope that MUST be make it reachable from the Root, e.g., it
      cannot be a Link Local Address.  The IPv6 address is encoded in
      the [RFC8138] compressed form indicated by the Compression Type
      field.

   An SIO MAY be immediately followed by a DAG Metric Container.  In
   that case the DAG Metric Container provides additional metrics for
   the hop from the Sibling to this node.

7.  Projected DAO

   This draft adds a capability to RPL whereby the

6.  Root of a main DODAG
   installs Initiated Routing State

6.1.  Requesting a Track as a collection of Projected Routes, using a
   Projected-DAO (P-DAO) message to maintain each individual route.  The
   P-DAO signals a collection of Targets in

   This specification introduces the RPL Target Option(s)
   (RTO).  Those Targets can be reached via a sequence of routers
   indicated in a VIO.  A P-DAO message MUST contain exactly one VIO,
   which is either a SF-VIO or an SR-VIO, and MUST follow one or more
   RTOs.  There can be at most one such sequence of RTO(s) and an Via
   Information Option.  A track is identified by a tuple DODAGID,
   TrackID and each route within a Track is indexed PDR message, used by a SegmentID.

   A P-DAO MUST be sent from the address of the Root that serves as
   DODAGID for the main DODAG.  It MUST be sent an LLN node to a GUA or a ULA of
   either the ingress or the egress of the Segment, more below.  If the
   'K' Flag is present in the P-DAO, and unless the P-DAO does not reach
   it,
   request the ingress formation of the Segment is the node that acknowledges the
   message, using a DAO-ACK that MUST be sent back to the address that
   serves as DODAGID new Track for the main DODAG.

   Like a classical DAO message, a P-DAO causes a change of state only
   if it is "new" per section 9.2.2.  "Generation of DAO Messages" of
   the RPL specification [RPL]; which this node is determined using the Segment
   Sequence information from the VIO as opposed to the Path Sequence
   from a TIO.  Also, a Segment Lifetime of 0 in a VIO indicates Ingress.
   Note that the projected route associated to the Segment is to be removed.

   There are two kinds of operation namespace for the Projected Routes, the
   Storing Mode and the Non-Storing Mode.

   *  The Non-Storing Mode is discussed in Section 7.3.2.  A Non-Storing
      Mode P-DAO carries an SR-VIO with the loose list of Via Addresses
      that forms a source-routed Segment to the Track Egress.  The
      recipient of the P-DAO TrackID is owned by the Track Ingress; it MUST install a
      source-routed state to the Track Egress Ingress node,
   and reply to the Root
      directly using a DAO-ACK message if requested to.

   *  The Storing Mode is discussed in Section 7.3.1.  A Storing Mode
      P-DAO carries a SF-VIO with the strict list of Via Addresses from
      the ingress to the egress of the Segment in the data path order.
      The routers listed in the Via Addresses, except the egress, MUST
      install a routing state to the Target(s) via the next Via Address in the SF-VIO.  In normal operations, the P-DAO is propagated
      along the chain of Via Routers from the egress router of the path
      till the ingress one, which confirms the installation to the Root
      with a DAO-ACK message.

   In case absence of a forwarding error along a Projected Route, an ICMP error
   is sent to the Root with a new Code "Error in Projected Route" (See
   Section 11.13).  The Root can then modify or remove the Projected
   Route.  The "Error in Projected Route" message has the same format as
   the "Destination Unreachable Message", as specified in RFC 4443
   [RFC4443].

   The portion of the invoking packet that is sent back in the ICMP
   message SHOULD record at least up to the RH if one is present, and
   this hop of the RH SHOULD PDR, there must be consumed by this node so that the
   destination in the IPv6 header is some procedure for the next hop that this node could
   not reach.  if a 6LoWPAN Routing Header (6LoRH) [RFC8138] is used
   Root to
   carry the IPv6 routing information assign TrackIDs in the outer header then that
   whole 6LoRH information SHOULD be present in the ICMP message.

   The sender and exact operation depend on the Mode and is described namespace while avoiding collisions,
   more in Section 7.3.2 and Section 7.3.1 respectively.

7.1.  Requesting a Track

   A Node is free to ask the Root for a new Track for which it will be
   Ingress at any time.  This is done with a 6.2.

   The PDR message, that indicates signals the desired TrackID and the duration for which the
   Track should be established.  Upon a PDR, the Root MAY install the necessary
   Segments,
   Track as requested, in which case it answers with a PDR-ACK
   indicating the granted Track Lifetime.  All the Segments MUST be of a
   same mode, either Storing or Non-Storing.  All the Segments MUST be
   created with the same TrackID and the same DODAGID signaled in the
   P-DAO.

   The Root is free to design designs the Track as it wishes, sees best, and to change updates / changes the
   Segments overtime to serve the Track as needed, without notifying needed.  There is no
   notification to the
   resquesting Node. requesting node when those changes happen.  The
   Segment Lifetime in the P-DAO messages does not need to be aligned to
   the Requested Lifetime in the PDR, or between P-DAO messages for
   different Segments.  The Root may use shorter lifetimes for the
   Segments and renew them faster than the Track is, or longer lifetimes
   in which case it will need to tear down the Segments if the Track is
   not renewed.

   When the Track Lifetime that was returned in the PDR-ACK is close to
   elapse,
   elapse - vs. the resquesting Node needs trip time from the node to the Root, the requesting
   node SHOULD resend a PDR using the TrackID in the PDR-ACK to extend
   the lifetime of the Track, else the Track will time out and the Root
   will tear down the whole structure.

   If the Track fails and cannot be restored, the Root notifies the
   resquesting Node
   requesting node asynchronously with a PDR-ACK with a Track Lifetime
   of 0, indicating that the Track has failed, and a PDR-ACK Status
   indicating the reason of the fault.

7.2.

6.2.  Identifying a Track

   RPL defines the concept of an Instance to signal an individual
   routing topology but does not have a concept of an administrative
   distance, which exists in certain proprietary implementations to sort
   out conflicts between topology, and multiple sources topologies can coexist in the same
   network.  The RPLInstanceID is tagged in the RPI of routing information within
   one routing topology. every packet to
   signal which topology the packet actually follows.

   This draft leverages the RPL Instance model as follows:

   *  The Root MAY use P-DAO messages to add better routes in the main
      (Global) RPL Instance in conformance with the routing objectives
      in that Instance.

      To achieve this, the Root MAY install an Storing-
      Mode P-Route a Segment along a path down
      the main Non-Storing Mode DODAG.  This enables a loose source
      routing and reduces the size of the Routing Header, see
      Appendix A.1.  The Root MAY also install a Track Leg across the
      Main DODAG to complement the routing topology.

      When adding an Storing-Mode a P-Route to the main RPL Instance, Main DODAG, the Root MUST set the
      RPLInstanceID field of the P-DAO message Base Object (see section 6.4.1.
      of [RPL]) to the RPLInstanceID of the main Main DODAG, and MUST NOT use
      the DODAGID field.  A Projected Route P-Route provides a longer match to the
      Target Address than the default route via the Root, so it is
      preferred.

      Once the Projected Route is installed, the intermediate nodes
      listed in the SF-VIO after first one (i.e.  The ingress) can be
      elided from the RH in packets sent along the Segment signaled in
      the P-DAO.  The resulting loose source routing header indicates
      (one of) the Target(s) as the next entry after the ingress.

   *  The Root MAY also use P-DAO messages to install a specific Track as an
      independent routing topology (say, Traffic Engineered) path as a Serial or as a Complex Track, to a achieve
      particular endpoint that is the Track Egress.  In that case, routing characteristics from an Ingress to an Egress
      Endpoints.  To achieve this, the Root MUST install set up a Local local RPL
      Instance (see section 5 of [RPL]), and the Local RPLInstanceID is called
      serves as TrackID.

      In that case, the  The TrackID MUST be unique for the Global Unique IPv6 Address (GUA) ULA or Unique-Local Address (ULA)
      GUA of the Track Ingress that serves as DODAGID for the Track.

      This way, a Track is uniquely identified by the tuple (DODAGID,
      TrackID) where the TrackID is always represented with the D flag
      set to 0 (see also section 5.1. of [RPL]), indicating when used in
      an RPI that the source address of the IPv6 packet signals the
      DODAGID.

      The P-DAO Base Object MUST indicate the tuple (DODAGID, TrackID)
      that identifies the Track as shown in Figure 3, and the P-RouteID
      that identifies the P-Route MUST be signaled in the VIO as shown
      in Figure 9.

      The Track Ingress is the root of the DODAG ID formed by the local
      RPL Instance.  It owns the namespace of its TrackIDs, so it can
      pick any unused value to request a new Track with a PDR.  The  In a
      particular deployment where PDR are not used, the namespace can be
      delegated to the main Root, which can assign the TrackIDs for the
      Tracks it creates without collision.

      With this specification, the Root is aware of all the active
      Tracks, so it can also pick any unused value to form Tracks
      without a PDR.  To avoid a collision of the Root and the Track
      Ingress picking the same value at the same time, it is RECOMMENDED
      that the Track Ingress starts allocating the ID value of the Local
      RPLInstanceID (see section 5.1. of [RPL]) used as
      TrackIDs with the value 0 incrementing, while the Root starts with
      63 decrementing.

      This way, a Track is uniquely identified [RPL]) used as TrackIDs with
      the value 0 incrementing, while the Root starts with 63
      decrementing.

6.3.  Installing a Track

   A Serial Track can be installed by a single P-Route that signals the
   sequence of consecutive nodes, either in Storing Mode as a single-
   Segment Track, or in Non-Storing Mode as a single-Leg Track.  A
   single-Leg Track can be installed as a loose Non-Storing Mode
   P-Route, in which case the next loose entry must recursively be
   reached over a Serial Track.

   A Complex Track can be installed as a collection of P-Routes with the
   same DODAGID and Track ID.  The Ingress of a Non-Storing Mode P-Route
   is the owner and Root of the DODAGID.  The Ingress of a Storing Mode
   P-Route must be either the owner of the DODAGID, or a hop of a Leg of
   the same Track.  In the latter case, the Targets of the P-Route must
   include the next hop of the Leg if there is one, to ensure forwarding
   continuity.  In the case of a Complex Track, each Segment is
   maintained independently and asynchronously by the Root, with its own
   lifetime that may be shorter, the same, or longer than that of the
   Track.

   A route along a Track for which the TrackID is not the RPLInstanceID
   of the Main DODAG MUST be installed with a higher precedence than the
   routes along the Main DODAG, meaning that:

   *  Longest match MUST be the prime comparison for routing.

   *  In case of equal length match, the route along the Track MUST be
      preferred vs. the one along the Main DODAG.

   *  There SHOULD NOT be 2 different Tracks leading to the same Target
      from same Ingress node, unless there's a policy for selecting
      which packets use which Track; such policy is out of scope.

   *  A packet that was routed along a Track MUST NOT be routed along
      the main DODAG again; if the destination is not reachable as a
      neighbor by the node where the packet exits the Track then the
      packet MUST be dropped.

6.3.1.  Signaling a Projected Route

   This draft adds a capability whereby the Root of a main RPL DODAG
   installs a Track as a collection of P-Routes, using a Projected-DAO
   (P-DAO) message for each individual Track Leg or Segment.  The P-DAO
   signals a collection of Targets in the RPL Target Option(s) (RTO).
   Those Targets can be reached via a sequence of routers indicated in a
   VIO.

   Like a classical DAO message, a P-DAO causes a change of state only
   if it is "new" per section 9.2.2.  "Generation of DAO Messages" of
   the RPL specification [RPL]; this is determined using the Segment
   Sequence information from the VIO as opposed to the Path Sequence
   from a TIO.  Also, a Segment Lifetime of 0 in a VIO indicates that
   the P-Route associated to the Segment is to be removed.  There are
   two Modes of operation for the P-Routes, the Storing and the Non-
   Storing Modes.

   A P-DAO message MUST be sent from the address of the Root that serves
   as DODAGID for the Main DODAG.  It MUST contain either exactly one
   sequence of one or more RTOs followed one VIO, or any number of
   sequences of one or more RTOs followed by one or more TIOs.  The
   former is the tuple (DODAGID,
      TrackID) normal expression for this specification, where as the TrackID is always represented with the D flag
      set
   latter corresponds to 0.

      The Track Egress Address and the TrackID MUST be signaled in the
      P-DAO message as shown variation for lesser constrained
   environments described in Figure 1.

7.3.  Installing a Track Section 7.2.

   A Storing-Mode P-DAO contains an SF-VIO that signals the strict
   sequence of consecutive nodes to form a segment between creates or updates a segment
   ingress and Track Leg MUST be sent to a segment egress (both included).  It installs GUA or
   a route ULA of
   a higher precedence along the segment towards Ingress of the Targets indicated
   in Leg; it must contain the Target Options.  The segment is included full list of
   hops in a DODAG indicated
   by the Leg unless the Leg is being removed.  A P-DAO Base Object, that may be the one formed by the main RPL
   Instance, or
   creates a new Track associated with Segment MUST be sent to a local RPL Instance.  A Track
   Egress is signaled as GUA or a Target in ULA of the P-DAO,
   Segment Egress and as MUST signal the last entry is
   an SF-VIO full list of hops in Segment; a last segment towards that Egress.

   A Non-Storing-Mode
   P-DAO signals that updates (including deletes) a strict or loose sequence section of nodes
   between a Segment MUST be
   sent to the Track Ingress (excluded) first node after the modified Segment and a Track Egress (included).
   It installs a source-routed path signal the full
   list of a higher precedence within hops in the
   Track indicated by section starting at the P-DAO Base Object, towards node that immediately
   precedes the Targets
   indicated modified section.

   In Non-Storing Mode, as discussed in Section 6.3.3, the Target Options.  The source-routed path requires a
   Source-Routing header which implies an encapsulation to add Root sends
   the SRH P-DAO to an existing packet.

   The next entry in the sequence must be either a neighbor of Track Ingress where the
   previous entry, or reachable as a Target via another Projected Route,
   either Storing or Non-Storing.  If it source-routing state is reachable over a Storing
   Mode Projected Route, the next entry
   applied, whereas in Storing Mode, the loose sequence P-DAO is sent to the
   Target of a previous segment and the ingress of a next segment; the
   segments are associated with last node
   on the same Track, which avoids installed path and forwarded in the need of
   an encapsulation.  Conversely, if it is reachable over reverse direction,
   installing a Non-Storing Storing Mode Projected Route, the next loose source routed hop of state at each hop, as discussed in
   Section 6.3.2.  In both cases the inner Track Ingress is a Target of a previous Track and the ingress owner of a next the
   Track, which requires a de- and a re-encapsulation.

   A Serial Track is installed by a single Projected Routes that signals it generates the sequence of consecutive nodes, either in Storing or Non-Storing
   Mode. P-DAO-ACK when the installation is
   successful.

   If can be a loose Non-Storing Mode Projected Route, the 'K' Flag is present in which
   case the next loose entry P-DAO, the P-DAO must recursively be reached over a Serial
   Track.

   A Complex Track can be installed as
   acknowledged using a collection of Projected Routes
   with DAO-ACK that is sent back to the same DODAGID and Track ID.  The Ingress address of a Non-Storing
   Mode Projected Route must be the owner of
   Root from which the DODAGID.  The Ingress
   of a Storing Mode Projected Route must be either P-DAO was received.  In most cases, the owner first
   node of the
   DODAGID, Leg, Segment, or the egress updated section of the Segment is the
   node that sends the acknowledgment.  The exception to the rule is
   when an intermediate node in a Segment fails to forward a preceding Storing
   Mode Projected Route P-DAO to the previous node in the same Track. SM-VIO.

   In a No-Path Non-Storing Mode P-DAO, the latter case, SRH-6LoRH MUST NOT be
   present in the Targets of NSM-VIO; the Projected
   Route must be Targets of state in the preceding Projected Route to ensure Ingress is erased
   regardless.  In all other cases, a VIO MUST contain at least one Via
   Address, and a Via Address MUST NOT be present more than once, which
   would create a loop.

   A node that
   they are visible from processes a VIO MAY verify whether one of these
   conditions happen, and when so, it MUST ignore the track Ingress.

7.3.1.  Storing-Mode P-Route

   Profile 1 extends P-DAO and reject
   it with a RPL operation Rejection Status of "Error in a Non-Storing Mode network with
   Storing-Mode Projected Routes VIO" in the DAO-ACK, see
   Section 10.14.

   Other errors than those discussed explicitely that install segments along prevent the main
   DODAG and enable to loose source routing between
   installing the Root and route are acknowledged with a RPL Rejection Status of
   "Unqualified Rejection" in the
   targets.

7.3.1.1. DAO-ACK.

6.3.2.  Installing a Storing-Mode Track Segment with a Storing Mode P-Route

   As illustrated in Figure 9, 11, a Storing Mode P-DAO that carries a SF-VIO enables the
   Root to install installs a stateful route
   along the Segment signaled by the SM-VIO towards a collection of the Targets
   along a
   indicated in the Target Options.  The Segment between is to be included in a Track Ingress and
   DODAG indicated by the P-DAO Base Object, that may be the one formed
   by the RPL Main DODAG, or a Track Egress using associated with a
   projected DAO Message. local RPL
   Instance.

           ------+---------
                 |          Internet
                 |
              +-----+
              |     | Border Router router
              |     |  (RPL Root)
              +-----+                      |     ^                   |
                 |                         | DAO | ACK               |
           o    o   o    o                 |     |                   |
       o o   o  o   o  o  o o   o          |  ^       | Projected    .
      o  o o  o o    o   o   o  o  o       |  | DAO   | Route        .
      o   o    o  o     o  o    o  o  o    | ^        |              .
     o  o   o  o   o         o   o o       v | DAO    v              .
     o          o   LLN   o   o     o                                |
         o o   o        o     o              Loose Source Route Path |
      o       o      o    o                 From Root To Destination                                          v

                       Figure 9: 11: Projecting a route

   In order to install the relevant routing state along the Segment ,
   the Root sends a unicast P-DAO message to the Track Egress router of
   the routing Segment that is being installed.  The P-DAO message
   contains a SF-VIO SM-VIO with the direct strict sequence of Via Addresses.  The SF- SM-
   VIO follows one or more RTOs indicating the Targets to which the
   Track leads.  The SF-VIO SM-VIO contains a Segment Lifetime for which the
   state is to be maintained.

   The Root sends the P-DAO directly to the egress Egress node of the Segment.
   In that P-DAO, the destination IP address matches the last Via
   Address in the SF-VIO. SM-VIO.  This is how the egress Egress recognizes its role.
   In a similar fashion, the ingress Segment Ingress node recognizes its role as
   it matches first Via Address in the SF-VIO. SM-VIO.

   The Egress node of the Segment is the only node in the path that does
   not install a route in response to the P-DAO; it is expected to be
   already able to route to the Target(s) based on its own. existing tables.
   If one of the Targets is not known, the node MUST answer to the Root
   with a
   negative DAO-ACK listing the unreachable Target(s) that could not be located
   (suggested in an RTO and a
   rejection status 10 to be confirmed by IANA). of "Unreachable Target".

   If the egress Egress node can reach all the Targets, then it forwards the
   P-DAO with unchanged content to its loose predecessor in the Segment as
   indicated in the list of Via Information options, and recursively the
   message is propagated unchanged along the sequence of routers
   indicated in the P-DAO, but in the reverse order, from egress Egress to
   ingress.
   Ingress.

   The address of the predecessor to be used as destination of the
   propagated DAO message is found in the Via Address the precedes the
   one that contain the address of the propagating node, which is used
   as source of the message.

   Upon receiving a propagated DAO, all except the Egress Router router MUST
   install a route towards the DAO Target(s) via their successor in the
   SF-VIO.
   SM-VIO.  A router that cannot store the routes to all the Targets in
   a P-DAO MUST reject the P-DAO by sending a DAO-ACK to the Root with a
   Rejection Status of "Out of Resources" as opposed to forwarding the
   DAO to its predecessor in the list.  The router MAY install
   additional routes towards the VIA Addresses that are the SF-VIO SM-VIO after the next one,
   self, if any, but in case of a conflict or a lack of resource, the
   route(s) to the Target(s)
   have precedence. are the ones that must be installed in
   priority.

   If a router cannot reach its predecessor in the SF-VIO, SM-VIO, the router
   MUST answer send the DAO-ACK to the Root with a negative DAO-ACK indicating the
   successor that is unreachable (suggested status 11 to be confirmed by
   IANA). Rejection Status of
   "Predecessor Unreachable".

   The process continues till the P-DAO is propagated to ingress Ingress router
   of the Segment, which answers with a DAO-ACK to the Root.  The Root
   always expects a DAO-ACK, either from the Track Ingress with a
   positive status or from any node along the segment with a negative
   status.  If the DAO-ACK is not received, the Root may retry the DAO
   with the same TID, or tear down the route.

7.3.1.2.  Maintaining and Tearing Down

6.3.3.  Installing a Track Leg with a Storing-Mode Non-Storing Mode P-Route

   A Segment Lifetime of 0

   As illustrated in Figure 12, a VIO is used to clean up the state.  The Non-Storing Mode P-DAO is forwarded as described above, but the DAO is interpreted as
   a No-Path DAO and results in cleaning up existing state as opposed to
   refreshing an existing one or installing installs a new one.

   Note that the continuity of
   source-routed path within the segment may be broken; this happens
   if Track indicated by the bidirectional connectivity between contiguous hops of P-DAO Base
   Object, towards the
   segment is lost.  In that case Targets indicated in the Root needs Target Options.  The
   source-routed path requires a Source-Routing header which implies an
   IP-in-IP encapsulation to send add the projected
   DAO SRH to one or more intermediate node(s) as opposed an existing packet.  It is
   sent to the egress
   node, indicating Track Ingress which creates a portion of segment that is being replaced or
   cleaned up.  At tunnel associated with the extreme,
   Track, and connected routes over the P-DAO updates only one node, tunnel to the Targets in
   which case it contains only one VIO.

   In case of a forwarding error along an Storing-Mode P-Route, the node
   that fails to forward SHOULD send an ICMP error with
   RTO.  The tunnel encapsulation MUST incorporate a code "Error in
   Projected Route" to routing header via
   the Root.  Failure to do so may result list addresses listed in packet
   loss and wasted resources along the Projected Route that is broken.

7.3.2.  Non-Storing-Mode P-Route

   As illustrated VIO in Figure 10, a P-DAO that carries an SR-VIO enables the Root to install a source-routed path from a same order.  The content
   of the NSM-VIO starting at the first SRH-6LoRH header MUST be used
   verbatim by the Track Ingress towards when it encapsulates a Target along packet to
   forward it over the main DODAG. Track.

              ------+---------
                    |          Internet
                    |
                 +-----+
                 |     | Border Router router
                 |     |  (RPL Root)
                 +-----+                    |  P  ^ ACK
                    |        Track          | DAO |
              o    o   o  o  Ingress X      V     |   X
          o o   o  o   o  o     o   X   o             X Source
         o  o o  o o    o   o  o    X  o  o           X Routed
         o   o    °  o     o   o   o X     o          X Segment
        o  o   o  o   o  o    o  o     X Track Egress       X
           o  o  o  o             o     Egress    |
                                     Target
          o       o     LLN          o    o
        o          o             o     o
                                      destination

                          LLN

                 Figure 10: 12: Projecting a Non-Storing Route

   The next entry in the source-routed path must be either a neighbor of
   the previous entry, or reachable as a Target via another P-Route,
   either Storing or Non-Storing, which implies that the nested P-Route
   has to be installed before the loose sequence is, and that P-Routes
   must be installed from the last to the first along the datapath.  For
   instance, a Segment of a Track must be installed before the Leg(s) of
   the same Track that use it, and stitched Segments must be installed
   in order from the last that reaches to the Targets to the first.

   If the next entry in the loose sequence is reachable over a Storing
   Mode P-Route, it MUST be the Target of a Segment and the Ingress of a
   next segment, both already setup; the segments are associated with
   the same Track, which avoids the need of an additional encapsulation.
   For instance, in Section 3.4.1.3, Segments A==>B-to-C and
   C==>D==>E-to-F must be installed with Storing Mode P-DAO messages 1
   and 2 before the Track A-->C-->E-to-F that joins them can be
   installed with Non-Storing Mode P-DAO 3.

   Conversely, if it is reachable over a Non-Storing Mode P-Route, the
   next loose source-routed hop of the inner Track is a Target of a
   previously installed Track and the Ingress of a next Track, which
   requires a de- and a re-encapsulation when switching the outer Tracks
   that join the loose hops.  This is examplified in Section 3.4.2.3
   where Non-Storing Mode P-DAO 1 and 2 install strict Tracks that Non-
   Storing Mode P-DAO 3 joins as a super Track.  In such a case, packets
   are subject to double IP-in-IP encapsulation.

6.4.  Tearing Down a P-Route

   A P-DAO with a lifetime of 0 is interpreted as a No-Path DAO and
   results in cleaning up existing state as opposed to refreshing an
   existing one or installing a new one.  To tear down a Track, the Root
   must tear down all the Track Segments and Legs that compose it one by
   one.

   Since the state about a Leg of a Track is located only the Ingress
   Node, the Root cleans up the Leg by sending an NSM-VIO to the Ingress
   indicating the TrackID and the P-RouteID of the Leg being removed, a
   Segment Lifetime of 0 and a newer Segment Sequence.  The SRH-6LoRH
   with the Via Addresses in the NSM-VIO are not needed and MUST be
   omitted.  Upon that NSM-VIO, the Ingress node removes all state for
   that Track if any, and replies positively anyway.

   The Root cleans up a section of a Segment by sending an SM-VIO to the
   last node of the Segment, with the TrackID and the P-RouteID of the
   Segment being updated, a Segment Lifetime of zero (0) and a newer
   Segment Sequence.  The Via Addresses in the SM-VIO indicates the
   section of the Segment being modified, from the first to the last
   node that is impacted.  This can be the whole Segment if it is
   totally removed, or a sequence of one or more nodes that have been
   bypassed by a Segment update.

   The No-Path P-DAO is forwarded normally along the reverse list, even
   if the intermediate node does not find a Segment state to clean up.
   This results in cleaning up the existing Segment state if any, as
   opposed to refreshing an existing one or installing a new one.

6.5.  Maintaining a Track

   Repathing a Track Segment or Leg may cause jitter and packet
   misordering.  For critical flows that require timely and/or in-order
   delivery, it might be necessary to deploy the PAREO functions
   [RAW-ARCHI] over a highly redundant Track..  This specification
   allows to use more than one Leg for a Track, and 1+N packet
   redundancy.

   This section provides the steps to ensure that no packet is lost due
   to the operation itself.  This is ensured by installing the new
   section from its last node to the first, so when an intermediate node
   installs a route along the new section, all the downstream nodes in
   the section have already installed their own.  The disabled section
   is removed when the packets in-flight are forwarded along the new
   section as well.

6.5.1.  Maintaining a Track Segment

   To modify a section of a Segment between a first node and a second,
   downstream node (which can be the Ingress and Egress), while
   conserving those nodes in the Segment, the Root sends an SM-VIO to
   the second node indicating the sequence of nodes in the new section
   of the Segment.  The SM-VIO indicates the TrackID and the P-RouteID
   of the Segment being updated, and a newer Segment Sequence.  The
   P-DAO is propagated from the second to the first node and on the way,
   it updates the state on the nodes that are common to the old and the
   new section of the Segment and creates a state in the new nodes.

   When the state is updated in an intermediate node, that node might
   still receive packets that were in flight from the Ingress to self
   over the old section of the Segment.  Since the remainder of the
   Segment is already updated, the packets are forwarded along the new
   version of the Segment from that node on.

   After a reasonable time to enable the deprecated sections to empty,
   the root tears down the remaining section(s) of the old segments are
   teared down as described in Section 6.4.

6.5.2.  Maintaining a Track Leg

   This specification allows to add Legs to a Track by sending a Non-
   Storing Mode P-DAO to the Ingress associated to the same TrackID, and
   a new Segment ID.  If the Leg is loose, then the Segments that join
   the hops must be created first.  It makes sense to add a new Leg
   before removing one that is misbehaving, and switch to the new Leg
   before removing the old.

   It is also possible to update a Track Leg by sending a Non-Storing
   Mode P-DAO to the Ingress with the same Segment ID, an incremented
   Segment Sequence, and the new complete listy of hops in the NSM-VIO.
   Updating a live Leg means changing one or more of the intermediate
   loose hops, and involves laying out new Segments from and to the new
   loose hops before the NSM-VIO for the new Leg is issued.

   Packets that are in flight over the old version of the Track Leg
   still follow the old source route path over the old Segments.  After
   a reasonable time to enable the deprecated Segments to empty, the
   root tears down those Segments as described in Section 6.4.

6.6.  Encapsulating and Forwarding Along a Track

   When forwarding a packet to a destination for which the a router
   determines that routing happens via a Track Target, for which it is Ingress,
   the router
   inserts must encapsulated the packet using IP-in-IP to add the
   Source Routing Header in the packet with the final destination at set to the Track
   Egress.

   In order  Though fragmentation is possible in a 6LoWPAN LLN, e.g.,
   using [6LoWPAN], [RFC8930], and/or [RFC8931], it is RECOMMENDED to signal
   allow an MTU that is larger than 1280 in the Segment, main DODAG and allows
   for the router encapsulates additional headers while exposing only 1280 to the 6LoWPAN
   Nodes as indicated by section 4 of [6LoWPAN].

   All properties of a Track operations are inherited form the main RPL
   Instance that is used to install the Track.  For instance, the use of
   compression per [RFC8138] is determined by whether it is used in the
   RPL Main DODAG, e.g., by setting the "T" flag [TURN-ON_RFC8138] in
   the RPL configuration option.

   The Track Ingress that places a packet in a Track encapsulates it
   with an IP-in-IP header and header, a Routing Header, and an IPv6 Hop-by-Hop
   Option Header that contains the RPL Packet Information (RPI) as
   follows:

   *  In the uncompressed form the source of the packet is the address
      that this router, router uses as DODAGID for the Track, the destination is
      the first Via Address in the SR-VIO, NSM-VIO, and the RH is a Source
      Routing Header (SRH) [RFC6554] that contains the list of the
      remaining Via Addresses terminating by the Track Egress.

   *  The preferred alternate in a network where 6LoWPAN Header
      Compression [RFC6282] is used is to leverage "IPv6 over Low-Power
      Wireless Personal Area Network (6LoWPAN) Paging Dispatch"
      [RFC8025] to compress the RPL artifacts as indicated in [RFC8138].

      In that case, the source routed header is the exact copy of the
      (chain of) SRH-6LoRH found in the SR-VIO, NSM-VIO, also terminating by the
      Track Egress.  The RPI-6LoRH is appended next, followed by an IP-
      in-IP 6LoRH Header that indicates the Ingress Router router in the
      Encapsulator Address field, see as a similar case Figure 20 of
      [TURN-ON_RFC8138].

   To signal the Track in the packet, this specification leverages the
   RPL Forwarding model follows:

   *  In the data packets, the Track DODAGID and the TrackID MUST be
      respectively signaled as the IPv6 Source Address and the
      RPLInstanceID field of the RPI that MUST be placed in the outer
      chain of IPv6 Headers.

      The RPI carries a local RPLInstanceID called the TrackID, which,
      in association with the DODAGID, indicates the Track along which
      the packet is forwarded.

      The D flag in the RPLInstanceID MUST be set to 0 to indicate that
      the source address in the IPv6 header is set ot the DODAGID, more
      in Section 6.2.

   *  This draft conforms to the principles of [RFC9008] with regards to
      packet forwarding and encapsulation along a Track, as follows:

      -  With this draft, the Track is a RPL DODAG.  From the
         perspective of that DODAG, the Track Ingress is the Root, the
         Track Egress is a RPL-Aware 6LR, and neighbors of the Track
         Egress that can be reached via the Track, but are external to
         it, are external destinations and treated as RPL-Unaware Leaves
         (RULs).  The encapsulation rules in [RFC9008] apply.

      -  If the Track Ingress is the originator of the packet and the
         Track Egress is the destination of the packet, there is no need
         for an encapsulation.

      -  So the Track Ingress must encapsulate the traffic that it did
         not originate, and add an RPI.

      A packet that is being routed over the RPL Instance associated to
      a first Non-Storing Mode Track MAY be placed (encapsulated) in a
      second Track to cover one loose hop of the first Track as
      discussed in more details Section 3.4.2.3.  On the other hand, a
      Storing Mode Track must be strict and a packet that it placed in a
      Storing Mode Track MUST follow that Track till the Track Egress.

   The forwarding of a packet along a track will fail if the Track
   continuity is broken,e.g.:

   *  In the case of a strict path along a Segment, if the next strict
      hop is not reachable, the packet is dropped.

   *  In the case of a loose source-routed path, when the loose next hop
      is not a neighbor, there MUST must be either a
   segment for Segment of the same Track to the
      that loose next hop, on which hop.  When that is the case the packet is
      forwarded to the next hop along that segment, or a common neighbor
      with the loose next hop, on which case the packet is forwarded to
      that neighbor, or another Track to the loose next hop for which
      this node or a neighbor is Ingress.  In Ingress; in the latter last case, another
      encapsulation takes place and the process possibly recurses;
      otherwise the packet is dropped.

   *  When a Track Egress extracts a packet from a Track (decapsulates
      the packet), the destination of the inner packet must be either
      this node or a direct neighbor, or a Target of another Segment of
      the same Track for which this node is Ingress, otherwise the
      packet MUST be dropped.

   In case of a failure forwarding a packet along a Segment, e.g., the
   next hop is unreachable, the node that discovers the fault MUST send
   an ICMPv6 Error message [RFC4443] to the Root, with a new Code "Error
   in P-Route" (See Section 10.13).  The Root can then repair by
   updating the broken Segment and/or Tracks, and in the case of a
   broken Segment, remove the leftover sections of the segment using SM-
   VIOs with a lifetime of 0 indicating the section ot one or more nodes
   being removed (See Section 6.5).

   In case of a permanent forwarding error along a Source Route path,
   the node that fails to forward SHOULD send an ICMP error with a code
   "Error in Source Routing Header" back to the source of the packet, as
   described in section 11.2.2.3. of [RPL].  Upon this message, the
   encapsulating node SHOULD stop using the source route path for a
   reasonable period of time which duration depends on the deployment,
   and it SHOULD send an ICMP message with a Code "Error in Projected Route" P-Route" to
   the Root.  Failure to follow these steps may result in packet loss
   and wasted resources along the source route path that is broken.

7.4.  Forwarding Along

   Either way, the ICMP message MUST be throttled in case of consecutive
   occurrences.  It MUST be sourced at the ULA or a GUA that is used in
   this Track

   This draft leverages for the RPL Forwarding model follows:

   *  In source node, so the data packets, Root can establish where the Track DODAGID
   error happened.

   The portion of the invoking packet that is sent back in the ICMP
   message SHOULD record at least up to the RH if one is present, and
   this hop of the TrackID MUST RH SHOULD be
      respectively signaled as consumed by this node so that the
   destination in the IPv6 Source Address and header is the
      RPLInstanceID field of next hop that this node could
   not reach.  if a 6LoWPAN Routing Header (6LoRH) [RFC8138] is used to
   carry the RPI IPv6 routing information in the outer header then that MUST
   whole 6LoRH information SHOULD be placed present in the outer
      chain ICMP message.

6.7.  Compression of IPv6 Headers.

      The RPI carries a local RPLInstanceID called the TrackID, which, RPL Artifacts

   When using [RFC8138] in association with the DODAGID, indicates Main DODAG operated in Non-Storing Mode
   in a 6LoWPAN LLN, a typical packet that circulates in the Track Main DODAG
   is formatted as shown in Figure 13, representing the case where :

   +-+ ... -+- ... -+- ... -+-+- ... +-+-+-+ ... +-+-+ ... -+ ... +-...
   |11110001|  SRH- | RPI-  | IP-in-IP | NH=1      |11110CPP| UDP | UDP
   | Page 1 | 6LoRH | 6LoRH |  6LoRH   |LOWPAN_IPHC| UDP    | hdr |Payld
   +-+ ... -+- ... -+- ... -+-+- ... +-+-+-+ ... +-+-+ ... -+ ... +-...
                                        <=        RFC 6282      =>
             <================ Inner packet ==================== = =

           Figure 13: A Packet as Forwarded along which the Main DODAG

   Since there is no page switch between the encapsulated packet and the
   encapsulation, the first octet of the compressed packet that acts as
   page selector is forwarded.

      The D flag actually removed at encapsulation, so the inner
   packet used in the RPLInstanceID MUST be set to 0 descriptions below start with the SRH-6LoRH, and
   is verbatim the packet represented in Figure 13 from the second octet
   on.

   When encapsulating that inner packet to indicate place it in the Track, the
   first header that the Ingress appends at the head of the inner packet
   is an IP-in-IP 6LoRH Header; in that header, the encapsulator
   address, which maps to the IPv6 source address in the uncompressed
   form, contains a GUA or ULA IPv6 header address of the Ingress node that
   serves as DODAG ID for the Track, expressed in the compressed form
   and using the DODAGID of the Main DODAG as compression reference.  If
   the address is set ot compressed to 2 bytes, the DODAGID, more resulting value for the
   Length field shown in Section 7.4.

   *  This draft conforms Figure 14 is 3, meaning that the principles SRH-6LoRH as a
   whole is 5-octets long.

      0                   1                   2
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-     ...     -+
     |1|0|1| Length  | 6LoRH Type 6  |  Hop Limit    | Track DODAGID |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-     ...     -+

                    Figure 14: The IP-in-IP 6LoRH Header

   At the head of the resulting sequence of [USEofRPLinfo] bytes, the track Ingress
   then adds the RPI that carries the TrackID as RPLinstanceID as a P-
   RPI-6LoRH Header, as illustrated in Figure 5, using the TrackID as
   RPLInstanceID.  Combined with regards the IP-in-IP 6LoRH Header, this allows
   to identify the Track without ambiguity.

   The SRH-6LoRH is then added at the head of the resulting sequence of
   bytes as a verbatim copy of the content of the SR-VIO that signaled
   the selected Track Leg.

        0                   1
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5  ..         ..        ..
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-    -+-    -+ ... +-    -+
       |1|0|0|  Size   |6LoRH Type 0..4| Hop1 | Hop2 |     | HopN |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-    -+-    -+ ... +-    -+
                                                 Where N = Size + 1

                      Figure 15: The SRH 6LoRH Header

   The format of the resulting encapsulated packet forwarding and encapsulation in [RFC8138]
   compressed form is illustrated in Figure 16:

   +-+ ... -+-+-+- ... -+-+-+- ... -+-+-+-+-+- ... +-+-+-+-+-+-+- ...
   | Page 1 |  SRH-6LoRH  | P-RPI-6LoRH | IP-in-IP 6LoRH | Inner Packet
   +-+ ... -+-+-+- ... -+-+-+- ... -+-+-+-+-+- ... +-+-+-+-+-+-+- ...

    Signals :  Loose Hops :    TrackID  :  Track DODAGID :

               Figure 16: A Packet as Forwarded along a Track.

      -  In Track

7.  Lesser Constrained Variations

7.1.  Storing Mode Main DODAG

   This specification expects that case, the Track Main DODAG is operated in Non-
   Storing Mode.  The reasons for that limitation are mostly related to
   LLN operations, power and spectrum conservation:

   *  In Non-Storing Mode The Root already possesses the DODAG, DODAG topology,
      so the Track Ingress additional topological information is reduced to the
      siblings.

   *  The downwards routes are updated with unicast messages to the
      Root, which ensures that the Root can reach back to the LLN nodes
      after a repair faster than in the case of Storing Mode.  Also the
      Root can control the use of the path diversity in the DODAG to
      reach to the LLN nodes.  For both reasons, Non-Storing Mode
      provides better capabilities for the Root to maintain the
      P-Routes.

   *  When the Main DODAG is operated in Non-Storing Mode, P-Routes
      enable loose Source Routing, which is only an advantage in that
      mode.  Storing Mode does not use Source Routing Headers, and does
      not derive the Track Egress same benefits from this capability.

   On the other hand, since RPL is a RAL, and neighbors Layer-3 routing protocol, its
   applicability extends beyond LLNs to a generic IP network.  RPL
   requires fewer resources than alternative IGPs like OSPF, ISIS,
   EIGRP, BABEL or RIP at the expense of a route stretch vs. the Track
         Egress
   shortest path routes to a destination that those protocols compute.
   P-Routes add the capability to install shortest and/or constrained
   routes to special destinations such as discussed in section A.9.4. of
   the ANIMA ACP [RFC8994].

   In a powered and wired network, when enough memory to store the
   needed routes is available, the RPL Storing Mode proposes a better
   trade-off than the Non-Storing, as it reduces the route stretch and
   lowers the load on the Root.  In that case, the control path between
   the Root and the LLN nodes is highly available compared to LLNs, and
   the nodes can be reached via to maintain the P-Routes at most times.

   This section specifies the additions that are needed to support
   Projected Routes when the Main DODAG is operated in Storing Mode.  As
   long as the RPI can be processed adequately by the dataplane, the
   changes to this specification are limited to the DAO message.  The
   Track structure, routes and forwarding operations remain the same.

   In Storing Mode, the Root misses the Child to Parent relationship
   that forms the Main DODAG, as well as the sibling information.  To
   provide that knowledge the nodes in the network MUST send additional
   DAO messages that are RULs. unicast to the Root as Non-Storing DAO messages
   are.

   In the DAO message, the originating router advertises a set of
   neighbor nodes using Sibling Information Options (SIO)s, regardless
   of the relative position in the DODAG of the advertised node vs. this
   router.

   The
         encapsulation rules DAO message MUST be formed as follows:

   *  The originating router is identified by the source address of the
      DAO.  That address MUST be the one that this router registers to
      neighbor routers so the Root can correlate the DAOs from those
      routers when they advertise this router as their neighbor.  The
      DAO contains one or more sequences of one Transit Information
      Option and one or more Sibling Information Options.  There is no
      RPL Target Option so the Root is not confused into adding a
      Storing Mode route to the Target.

   *  The TIO is formed as in [USEofRPLinfo] apply.

      -  If Storing Mode, and the Track Ingress Parent Address is
      not present.  The Path Sequence and Path Lifetime fields are
      aligned with the originator values used in the Address Registration of the packet
      node(s) advertised in the SIO, as explained in Section 9.1. of
      [RFC9010].  Having similar values in all nodes allows to factorise
      the TIO for multiple SIOs as done with [RPL].

   *  The TIO is followed by one or more SIOs that provide an address
      (ULA or GUA) of the advertised neighbor node.

   But the RPL routing information headers may not be supported on all
   type of routed network infrastructures, especially not in high-speed
   routers.  When the RPI is not be supported in the dataplane, there
   cannot be local RPL Instances and RPL can only operate as a single
   topology (the Main DODAG).  The RPL Instance is that of the Main
   DODAG and the Ingress node that encapsulates is not the Root.  The
   routes along the Tracks are alternate routes to those available along
   the Main DODAG.  They MAY conflict with routes to children and MUST
   take precedence in the routing table.  The Targets MUST be adjacent
   to the Track Egress to avoid loops that may form if the packet is
   reinjected in the destination Main DODAG.

7.2.  A Track as a Full DODAG

   This specification builds parallel or crossing Track Legs as opposed
   to a more complex DODAG with interconnections at any place desirable.
   The reason for that limitation is related to constrained node
   operations, and capability to store large amount of topological
   information and compute complex paths:

   *  With this specification, the packet, there is node in the LLN has no topological
      awareness, and does not need
         for an to maintain dynamic information about
      the link quality and availability.

   *  The Root has a complete topological information and statistical
      metrics that allow it or its PCE to perform a global optimization
      of all Tracks in its DODAG.  Based on that information, the Root
      computes the Track Leg and predigest the source route paths.

   *  The node merely selects one of the proposed paths and applies the
      associated pre-computed routing header in the encapsulation.

      -  So  This
      alleviates both the complexity of computing a path and the
      compressed form of the routing header.

   The "Reliable and Available Wireless (RAW) Architecture/Framework"
   [RAW-ARCHI] actually expects the PSE at the Track Ingress must encapsulate to react to
   changes in the traffic that it did
         not originate, forwarding conditions along the Track, and add an RPI in reroute
   packets to maintain the required degree of reliability.  To achieve
   this, the PSE need the full richness of a DODAG to form any fashion.

      A packet path that is being routed over
   could make meet the SLAs.

   This section specifies the RPL Instance associated additions that are needed to
      a first Non-Storing Mode turn the
   Track MAY be placed (encapsulated) in into a
      second Track full DODAG and enable the main Root to cover one loose hop of provide the first Track.  On
   necessary topological information to the
      other hand, a Storing Mode Track must be strict and a packet Ingress.  The
   expectation is that
      it placed the metrics that the PSE uses are of an order
   other than that of the PCE, because of the difference of time scale
   between routing and forwarding, mor e in a Storing Mode Track MUST follow [RAW-ARCHI].  It follows
   that Track till the
      Track Egress.

      When a Track Egress extracts a packet PSE will learn the metrics it needs from a Track (decapsulates an alternate
   source, e.g., OAM frames.

   To pass the packet), topological information to the Destination of Ingress, the inner packet MUST be either
      this node or a direct neighbor, or Root uses a Target of another Segment
   P-DAO messages that contains sequences of Target and Transit
   Information options that collectively represent the Track, expressed
   in the same Track for which this node fashion as in classical Non-Storing Mode.  The difference
   as that the Root is ingress, otherwise the
      packet MUST be dropped.

   All properties of a Track operations are inherited form source as opposed to the main RPL
   Instance destination, and can
   report information on many Targets, possibly the full Track, with one
   P-DAO.

   Note that is used to install the Track.  For instance, Path Sequence and Lifetime in the use of
   compression per [RFC8138] is determined TIO are selected by whether it is used
   the Root, and that the Target/Transit information tupples in the
   main instance, e.g.,
   P-DAO are not those received by setting the "T" flag [TURN-ON_RFC8138] Root in the
   RPL configuration option. DAO messages about
   the said Targets.  The Track may follow sibling routes and does not
   need to be congruent with the Main DODAG.

8.  Profiles

   This document provides a set of tools that may or may not be needed
   by an implementation depending on the type of application it serves.
   This sections described profiles that can be implemented separately
   and can be used to discriminate what an implementation can and cannot
   do.  This section describes profiles that enable to implement only a
   portion of this specification to meet a particular use case.

   Profiles 0 to 2 operate in the Main RPL Instance and do not require
   the support of local RPL Instances or the indication of the RPL
   Instance in the data plane.  Profile 3 and above leverage Local RPL
   Instances to build arbitrary Tracks rooted at the Track Ingress and
   using its namespace for TrackID.

   Profiles 0 and 1 are REQUIRED by all implementations that may be used
   in LLNs; this enables to use Storing Mode to reduce the size of the
   Source Route Header in the most common LLN deployments.  Profile 2 is
   RECOMMENDED in high speed / wired environment to enable traffic
   Engineering and network automation.  All the other profile /
   environment combinations are OPTIONAL.

   Profile 0  Profile 0 is the legacy Legacy support of [RPL] Non-Storing Mode. Mode,
      with default routing Northwards (up) and strict source routing
      Southwards (down the main DOAG).  It provides the minimal common
      functionality that must be implemented as a prerequisite to all
      the Track-supporting profiles.  The other Profiles extend Profile
      0 with selected capabilities that this specification introduces on
      top.

   Profile 1 (Storing-Mode (Storing Mode P-Route Segments along the main Main DODAG)  Profi
      le 1 does not create new paths; compared to Profile 0, it combines
      Storing and Non-
      Storing Non-Storing Modes to balance the size of the routing header Routing
      Header in the packet and the amount of state in the intermediate
      routers in a Non-Storing Mode RPL DODAG.

   Profile 2 (Non-Storing-Mode (Non-Storing Mode P-Route Segments along the main Main DODAG)  P
      rofile 2 extends Profile 0 with Strict Source-Routing Non-Storing- Non-Storing
      Mode Projected Routes P-Routes along the main DODAG. Main DODAG, which is the same as Profile 1
      but using NSM VIOs as opposed to SM VIOs.  Profile 2 provides the
      same capability to compress the SRH in packets down the main Main DODAG
      as Profile 1, but it require an encapsulation, in order to insert
      an additional SRH between the loose source routing hops.

   Profile 3  Profile 3 and above build  In that
      case, the Tracks MUST be installed as subTracks of the Main DODAG,
      the main RPL Instance MUST be used as TrackID, and the Ingress
      node that do encapsulates is not necessarily
      follow the main DODAG. Root as it does not own the
      DODAGID.

   Profile 3  In order to form the best path possible, those Profiles
      require the support of Sibling Information Option to inform the
      Root of additional possible hops.  Profile 3 extends Profile 1
      with additional Storing-Mode Projected Routes Storing Mode P-Routes that install segments that
      do not follow the main Main DODAG.  If the Segment Ingress (in the SF-VIO) SM-
      VIO) is the same as the IPv6 Address of the Track Ingress (in the
      projected DAO base Object), the P-DAO creates an implicit Track
      between the Segment Ingress and the Segment Egress.

   Profile 4  Profile 4 extends Profile 2 with Strict Source-Routing
      Non-Storing-Mode Projected Routes
      Non-Storing Mode P-Routes to form East-West Tracks that are inside
      the main
      DODAG. Main DODAG but do not necessarily follow it.  A Track is
      formed as one or more strict source source routed paths between
      the Root that is the Track Ingress, and the Track Egress that is
      the last node node.

   Profile 5  Profile 5 Combines Profile 4 with Profile 1 and enables to
      loose source routing between the Ingress and the Egress of the
      Track.  As in Profile 1, Storing-Mode Projected Routes Storing Mode P-Routes connect the dots in
      the loose source route.

   Profile 6  Profile 6 Combines Profile 4 with Profile 2 and also
      enables to loose source routing between the Ingress and the Egress
      of the Track.

9.  Example Track Signaling

   The remainder of the section provides an example of how a Track can
   be signaled

                                                  ===> F
                   A ===> B ===> C ===> D===> E <
                                                  ===> G

                         Figure 11: Reference Track

   A is Track ingress, E is track Egress.  C is stitching point.  F and
   G are E's neighbors, "external" to the Track, and reachable from A
   over the Track A->E.

   In a general manner we want:

   *  P-DAO 1 signals C==>B==>E

   *  P-DAO 2 signals A==>B==>C
   *  P-DAO 3 signals F and G via the A==>E Track

   P-DAO 3 being loose, it can only be non-storing.  Note that since the
   Root is always the ingress of a Track, and all SR-VIOs are now Track,
   the Root being signaled in the DAO base object can now be elided in
   the VIA list in SR-VIO.  This enables the construction by the main
   root of the RFC 8138 optimized SRH-6LoRH in the SR-VIO, to be placed
   as is

   Profile 7  Profile 7 implements profile 5 in the packet by the Root.

9.1.  Using Storing-Mode Segments

   A==>B==>C and C==>D==>E are segments of a same Track.  Note Main DODAG that the
   storing mode signaling imposes strict continuity in a segment.  One
   benefit of strict routing is that loops are avoided along the Track.

9.1.1.  Stitched Segments

   Storing-Mode P-DAO 1 and 2 are sent to E and C, as follows:

              +===============+==============+==============+
              |     Field     | P-DAO 1 to E | P-DAO 2 to C |
              +===============+==============+==============+
              |      Mode     | Storing      |
      operated in Storing      |
              +---------------+--------------+--------------+
              | Track Ingress | A            | A            |
              +---------------+--------------+--------------+
              |    TrackID    | (A, 129)     | (A, 129)     |
              +---------------+--------------+--------------+
              |      VIO      | C, D, E      | A, B, C      |
              +---------------+--------------+--------------+
              |    Targets    | E, F, G      | E, F, G      |
              +---------------+--------------+--------------+

                          Table 1: P-DAO Messages

   As a result the RIBs are set Mode as follows:

         +======+=============+=========+=============+==========+
         | Node | Destination | Origin  | Next Hop(s) | TrackID  |
         +======+=============+=========+=============+==========+
         |  E   | F, G        | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  D   | E           | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | F, G        | P-DAO 1 | E           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  C   | D           | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | E, F, G     | P-DAO 1 | D           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  B   | C           | P-DAO 2 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | E, F, G     | P-DAO 2 | C           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  A   | B           | P-DAO 2 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  A   | E, F, G     | P-DAO 2 | B           | (A, 129) |
         +------+-------------+---------+-------------+----------+

                            Table 2: RIB setting

   E recognizes that it is the Track Egress because it is both a Target
   and a Segment Endpoint.

   Packets originated by A to E, F, or G, do not require an
   encapsulation.  In any fashion, the outer headers of the packets that
   are forwarded along the Track have the following settings:

    +========+===================+===================+================+
    | Header | IPv6 Source Addr. | IPv6 Dest.  Addr. | TrackID presented in RPI |
    +========+===================+===================+================+
    | Outer  |         A         |     E, F or G     |    (A, 129)    |
    +--------+-------------------+-------------------+----------------+
    | Inner  |       X != A      |     E, F or G     |      N/A       |
    +--------+-------------------+-------------------+----------------+

                      Table 3: Packet header settings Section 7.1.  As an example, say that A has a packet for F.  Using the RIB above:

   *  From P-DAO 2: A forwards to B and B forwards to C.

   *  From P-DAO 1: C forwards to D and D forwards to E.

   *  From Neighbor Cache Entry: C delivers the packet to F.

9.1.2.  External routes

   Storing-Mode P-DAO in
      Profile 1 and 2, and Non-Storing-Mode P-DAO 3, are sent to
   E, C and A, respectively, as follows:

      +===============+==============+==============+==============+
      |               | P-DAO 1 to E | P-DAO 2 to C | P-DAO 3 to A |
      +===============+==============+==============+==============+
      |      Mode     | Storing      | Storing      | Non-Storing  |
      +---------------+--------------+--------------+--------------+
      | Track Ingress | A            | A            | A            |
      +---------------+--------------+--------------+--------------+
      |    TrackID    | (A, 129)     | (A, 129)     | (A, 129)     |
      +---------------+--------------+--------------+--------------+
      |      VIO      | C, D, E      | A, B, C      | E            |
      +---------------+--------------+--------------+--------------+
      |    Targets    | E            | E            | F, G         |
      +---------------+--------------+--------------+--------------+

                         Table 4: P-DAO Messages

   As a result the RIBs are set as follows:

         +======+=============+=========+=============+==========+
         | Node | Destination | Origin  | Next Hop(s) | TrackID  |
         +======+=============+=========+=============+==========+
         |  E   | F, G        | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  D   | E           | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  C   | D           | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | E           | P-DAO 1 | D           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  B   | C           | P-DAO 2 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | E           | P-DAO 2 | C           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  A   | B           | P-DAO 2 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  A   | E           | P-DAO 2 | B           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  A   | F, G        | P-DAO 3 | E           | (A, 129) |
         +------+-------------+---------+-------------+----------+

                            Table 5: RIB setting

   Packets from A to E do not require an encapsulation.  In any fashion, is the outer headers RPLInstanceID of the packets that are forwarded along the Track
   have the following settings:

   +========+===================+====================+================+
   | Header | IPv6 Source Addr. | IPv6 Dest.  Addr.  | TrackID in RPI |
   +========+===================+====================+================+
   | Outer  |         A         |         E          |    (A, 129)    |
   +--------+-------------------+--------------------+----------------+
   | Inner  |         X         | E (X != A), F or G |      N/A       |
   +--------+-------------------+--------------------+----------------+

                     Table 6: Packet header settings

   As an example, say that A has Main
      DODAG.  Longest match rules decide whether a packet for F.  Using the RIB above:

   *  From P-DAO 3: A encapsulates the packet the Track signaled by
      P-DAO 3, with the outer header above.  Now the packet destination is E.

   *  From P-DAO 2: A forwards to B and B forwards to C.

   *  From P-DAO 1: C forwards to D and D forwards to E; E decapsulates
      the packet.

   *  From Neighbor Cache Entry: C delivers packets to F or G.

9.1.3.  Segment Routing

   Storing-Mode P-DAO 1 and 2, and Non-Storing-Mode P-DAO 3, are sent to
   E, B and A, respectively, as follows:

      +===============+==============+==============+==============+
      |               | P-DAO 1 to E | P-DAO 2 to B | P-DAO 3 to A |
      +===============+==============+==============+==============+
      |      Mode     | Storing      | Storing      | Non-Storing  |
      +---------------+--------------+--------------+--------------+
      | Track Ingress | A            | A            | A            |
      +---------------+--------------+--------------+--------------+
      |    TrackID    | (A, 129)     | (A, 129)     | (A, 129)     |
      +---------------+--------------+--------------+--------------+
      |      VIO      | C, D, E      | A, B         | C, E         |
      +---------------+--------------+--------------+--------------+
      |    Targets    | E            | B, C         | F, G         |
      +---------------+--------------+--------------+--------------+

                         Table 7: P-DAO Messages

   As a result the RIBs are set as follows:

         +======+=============+=========+=============+==========+
         | Node | Destination | Origin  | Next Hop(s) | TrackID  |
         +======+=============+=========+=============+==========+
         |  E   | F, G        | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  D   | E           | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  C   | D           | P-DAO 1 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | E           | P-DAO 1 | D           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  B   | C           | P-DAO 2 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  A   | B           | P-DAO 2 | Neighbor    | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | C           | P-DAO 2 | B           | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | E, F, G     | P-DAO 3 | C, E        | (A, 129) |
         +------+-------------+---------+-------------+----------+

                            Table 8: RIB setting

   Packets from A to E do not require an encapsulation, but carry a SRH
   via C.  In any fashion, the outer headers of the packets that are
   forwarded along
      the Track have the following settings:

   +========+===================+====================+================+
   | Header | IPv6 Source Addr. | IPv6 Dest.  Addr.  | TrackID in RPI |
   +========+===================+====================+================+
   | Outer  |         A         |  C till C then E   |    (A, 129)    |
   +--------+-------------------+--------------------+----------------+
   | Inner  |         X         | E (X != A), F Main DODAG or G |      N/A       |
   +--------+-------------------+--------------------+----------------+

                     Table 9: Packet header settings

   As an example, say that A has a packet for F.  Using the RIB above:

   *  From P-DAO 3: A encapsulates the packet the Track signaled by
      P-DAO 3, with the outer header above.  Now the destination rerouted in the
      IPv6 Header is C, and a SRH signals the final destination track.

   Profile 8  Profile 8 is E.

   *  From P-DAO 2: A forwards to B and B forwards to C.

   *  From P-DAO 3: C processes the SRH and sets the destination offered in the
      IPv6 Header to E.

   *  From P-DAO 1: C forwards to D and D forwards to E; E decapsulates
      the packet.

   *  From the Neighbor Cache Entry: C delivers packets to F or G.

9.2.  Using Non-Storing-Mode joining Tracks

   A==>B==>C and C==>D==>E are Tracks expressed as non-storing P-DAOs.

9.2.1.  Stitched Tracks

   Non-Storing Mode P-DAO 1 and 2 are sent to C and A respectively, as
   follows:

              +===============+==============+==============+
              |               | P-DAO 1 to C | P-DAO 2 to A |
              +===============+==============+==============+
              |      Mode     | Non-Storing  | Non-Storing  |
              +---------------+--------------+--------------+
              | Track Ingress | C            | A            |
              +---------------+--------------+--------------+
              |    TrackID    | (C, 131)     | (A, 129)     |
              +---------------+--------------+--------------+
              |      VIO      | D, E         | B, C         |
              +---------------+--------------+--------------+
              |    Targets    | F, G         | E, F, G      |
              +---------------+--------------+--------------+

                          Table 10: P-DAO Messages

   As a result the RIBs are set as follows:

         +======+=============+=========+=============+==========+
         | Node | Destination | Origin  | Next Hop(s) | TrackID  |
         +======+=============+=========+=============+==========+
         |  E   | F, G        | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  D   | E           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  C   | D           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  "   | E, F, G     | P-DAO 1 | D, E        | (C, 131) |
         +------+-------------+---------+-------------+----------+
         |  B   | C           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  A   | B           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  "   | C, E, F, G  | P-DAO 2 | B, C        | (A, 129) |
         +------+-------------+---------+-------------+----------+

                           Table 11: RIB setting

   Packets from A to E, F and G do not require an encapsulation, though
   it is preferred that A encapsulates and C decapsulates.  Either way,
   they carry a SRH via B and C, and C needs to encapsulate to E, F, or
   G to add an SRH via D and E.  The encapsulating headers preparation of packets
   that are forwarded along the Track between C RAW work, and E have the following
   settings:

    +========+===================+===================+================+
    | Header | IPv6 Source Addr. | IPv6 Dest.  Addr. | TrackID in RPI |
    +========+===================+===================+================+
    | Outer  |         C         |  D till D then E  |    (C, 131)    |
    +--------+-------------------+-------------------+----------------+
    | Inner  |         X         |     E, F, or G    |      N/A       |
    +--------+-------------------+-------------------+----------------+

                      Table 12: Packet header settings

   As an example, say that A has a packet
      for F.  Using the RIB above:

   *  From P-DAO 2: A encapsulates the packet with destination of F in
      the Track signaled by P-DAO 2.  The outer header has source A,
      destination B, use cases where an SRH that indicates C as the next loose hop, and
      a RPI indicating a TrackId of 129 from A's namespace.

   *  From the SRH: Packets forwarded by B have source A, destination C
      , a consumed SRH, and a RPI indicating a TrackId of 129 from A's
      namespace.  C decapsulates.

   *  From P-DAO 1: C encapsulates the packet with destination of F arbitrary node in the Track signaled by P-DAO 1.  The outer header has source C,
      destination D, an SRH that indicates E as the next loose hop, and
      a RPI indicating a TrackId of 131 from C's namespace.  E
      decapsulates.

9.2.2.  External routes

   Non-Storing Mode P-DAO 1 is sent to C and Non-Storing Mode P-DAO 2
   and 3 are sent A, as follows:

      +===============+==============+==============+==============+
      |               | P-DAO 1 to C | P-DAO 2 to A | P-DAO 3 to A |
      +===============+==============+==============+==============+
      |      Mode     | Non-Storing  | Non-Storing  | Non-Storing  |
      +---------------+--------------+--------------+--------------+
      | Track Ingress | C            | A            | A            |
      +---------------+--------------+--------------+--------------+
      |    TrackID    | (C, 131)     | (A, 129)     | (A, 141)     |
      +---------------+--------------+--------------+--------------+
      |      VIO      | D, E         | B, C         | E            |
      +---------------+--------------+--------------+--------------+
      |    Targets    | E            | E            | F, G         |
      +---------------+--------------+--------------+--------------+

                         Table 13: P-DAO Messages

   As a result network can afford
      the RIBs are set same code complexity as follows:

         +======+=============+=========+=============+==========+
         | Node | Destination | Origin  | Next Hop(s) | TrackID  |
         +======+=============+=========+=============+==========+
         |  E   | F, G        | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  D   | E           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  C   | D           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  "   | E           | P-DAO 1 | D, E        | (C, 131) |
         +------+-------------+---------+-------------+----------+
         |  B   | C           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  A   | B           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  "   | C, E        | P-DAO 2 | B, C        | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | F, G        | P-DAO 3 | E           | (A, 141) |
         +------+-------------+---------+-------------+----------+

                           Table 14: RIB setting

   The encapsulating headers of packets that are forwarded along the
   Track between C and E have the following settings:

    +========+===================+===================+================+
    | Header | IPv6 Source Addr. | IPv6 Dest.  Addr. | TrackID in RPI |
    +========+===================+===================+================+
    | Outer  |         C         |  D till D then E  |    (C, 131)    |
    +--------+-------------------+-------------------+----------------+
    | Middle |         A         |         E         |    (A, 141)    |
    +--------+-------------------+-------------------+----------------+
    | Inner  |         X         |     E, F or G     |      N/A       |
    +--------+-------------------+-------------------+----------------+

                      Table 15: Packet header settings

   As an example, say that A has a packet for F.  Using the RIB above:

   *  From P-DAO 3: A encapsulates the packet with destination of F RPL Root in
      the Track signaled by P-DAO 3.  The outer header has source A,
      destination E, and a RPI indicating traditional
      deployment.  It offers a TrackId of 141 from A's
      namespace.  This recurses with:

   *  From P-DAO 2: A encapsulates the packet with destination of E in full DODAG visibility to the Track signaled by P-DAO 2.  The outer header has source A,
      destination B, an SRH that indicates C
      Ingress as the next loose hop, and
      a RPI indicating a TrackId of 129 from A's namespace.

   *  From the SRH: Packets forwarded by B have source A, destination C
      , a consumed SRH, and a RPI indicating a TrackId of 129 from A's
      namespace.  C decapsulates.

   *  From P-DAO 1: C encapsulates the packet with destination of E specified in Section 7.2 in
      the Track signaled by P-DAO 1.  The outer header has source C,
      destination D, an SRH that indicates E as the next loose hop, and
      a RPI indicating a TrackId of 131 from C's namespace.  E
      decapsulates.

9.2.3.  Segment Routing

   Non-Storing Mode P-DAO 1 is sent to C and Non-Storing Mode P-DAO 2
   and 3 are sent A, as follows:

      +===============+==============+==============+==============+
      |               | P-DAO 1 to C | P-DAO 2 to A | P-DAO 3 to A |
      +===============+==============+==============+==============+
      |      Mode     | Non-Storing  | Non-Storing  | Non-Storing  |
      +---------------+--------------+--------------+--------------+
      | Track Ingress | C            | A            | A            |
      +---------------+--------------+--------------+--------------+
      |    TrackID    | (C, 131)     | (A, 129)     | (A, 141)     |
      +---------------+--------------+--------------+--------------+
      |      VIO      | D, E         | B            | C, E         |
      +---------------+--------------+--------------+--------------+
      |    Targets    |              | C            | F, G         |
      +---------------+--------------+--------------+--------------+

                         Table 16: P-DAO Messages

   As a result the RIBs are set as follows:

         +======+=============+=========+=============+==========+
         | Node | Destination | Origin  | Next Hop(s) | TrackID  |
         +======+=============+=========+=============+==========+
         |  E   | F, G        | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  D   | E           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  C   | D           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  "   | E           | P-DAO 1 | D, E        | (C, 131) |
         +------+-------------+---------+-------------+----------+
         |  B   | C           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  A   | B           | ND      | Neighbor    | Any      |
         +------+-------------+---------+-------------+----------+
         |  "   | C           | P-DAO 2 | B, C        | (A, 129) |
         +------+-------------+---------+-------------+----------+
         |  "   | E, F, G     | P-DAO 3 | C, E        | (A, 141) |
         +------+-------------+---------+-------------+----------+

                           Table 17: RIB setting

   The encapsulating headers of packets that are forwarded along the
   Track between A and B have the following settings:

    +========+===================+===================+================+
    | Header | IPv6 Source Addr. | IPv6 Dest.  Addr. | TrackID in RPI |
    +========+===================+===================+================+
    | Outer  |         A         |  B till D then E  |    (A, 129)    |
    +--------+-------------------+-------------------+----------------+
    | Middle |         A         |         C         |    (A, 141)    |
    +--------+-------------------+-------------------+----------------+
    | Inner  |         X         |     E, F or G     |      N/A       |
    +--------+-------------------+-------------------+----------------+

                      Table 18: Packet header settings

   The encapsulating headers of packets that are forwarded along the
   Track between B and C have the following settings:

    +========+===================+===================+================+
    | Header | IPv6 Source Addr. | IPv6 Dest.  Addr. | TrackID in RPI |
    +========+===================+===================+================+
    | Outer  |         A         |         C         |    (A, 141)    |
    +--------+-------------------+-------------------+----------------+
    | Inner  |         X         |     E, F or G     |      N/A       |
    +--------+-------------------+-------------------+----------------+

                      Table 19: Packet header settings

   The encapsulating headers of packets that are forwarded along the
   Track between C Main
      DODAG.

   Profile 9  Profile 9 combines profiles 7 and E have the following settings:

    +========+===================+===================+================+
    | Header | IPv6 Source Addr. | IPv6 Dest.  Addr. | TrackID in RPI |
    +========+===================+===================+================+
    | Outer  |         C         |  D till D then E  |    (C, 131)    |
    +--------+-------------------+-------------------+----------------+
    | Middle |         A         |         E         |    (A, 141)    |
    +--------+-------------------+-------------------+----------------+
    | Inner  |         X         |     E, F or G     |      N/A       |
    +--------+-------------------+-------------------+----------------+

                      Table 20: Packet header settings

   As an example, say that A has a packet for F.  Using the RIB above:

   *  From P-DAO 3: A encapsulates the packet with destination of F in 8, operating the Track signaled by P-DAO 3.  The outer header has source A,
      destination C, an SRH that indicates E
      as the next loose hop, and
      a RPI indicating a TrackId of 141 from A's namespace.  This
      recurses with:

   *  From P-DAO 2: A encapsulates the packet with destination of C in
      the Track signaled by P-DAO 2.  The outer header has source A,
      destination B, and a RPI indicating a TrackId of 129 from A's
      namespace.  B decapsulates forwards to C based on full DODAG within a sibling
      connected route.

   *  From the SRH: C consumes the SRH and makes the destination E.

   *  From P-DAO 1: C encapsulates the packet with destination of E in Storing Mode Main DODAG, using only the Track signaled by P-DAO 1.  The outer header has source C,
      destination D, an SRH that indicates E
      Main DODAG RPLInstanceID as the next loose hop, and
      a RPI indicating a TrackId of 131 from C's namespace.  E
      decapsulates.

10. TrackID.

9.  Security Considerations

   It is worth noting that with [RPL], every node in the LLN is RPL-
   aware and can inject any RPL-based attack in the network.  This draft
   uses messages that are already present in RPL [RPL] with optional
   secured versions.  The same secured versions may be used with this
   draft, and whatever security is deployed for a given network also
   applies to the flows in this draft.

   The LLN nodes depend on the 6LBR and the RPL participants for their
   operation.  A trust model must be put in place to ensure that the
   right devices are acting in these roles, so as to avoid threats such
   as black-holing, (see [RFC7416] section 7).  This trust model could
   be at a minimum based on a Layer-2 Secure joining and the Link-Layer
   security.  This is a generic 6LoWPAN requirement, see Req5.1 in
   Appendix B.5 of [RFC8505].

   In a general manner, the Security Considerations in [RPL], and
   [RFC7416] apply to this specification as well.  The Link-Layer
   security is needed in particular to prevent Denial-Of-Service attacks
   whereby a rogue router creates a high churn in the RPL network by
   constantly injected forged P-DAO messages and using up all the
   available storage in the attacked routers.

   Additionally, the trust model could include a role validation (e.g.,
   using a role-based authorization) to ensure that the node that claims
   to be a RPL Root is entitled to do so.  That trust should propagate
   from egress Egress to ingress Ingress in the case of a Storing Mode P-DAO.

11.

   This specification suggests some validation of the VIO to prevent
   basic loops by avoiding that a node appears twice.  But that is only
   a minimal protection.  Arguably, an attacker tha can inject P-DAOs
   can reroute any traffic and deplete critical resources such as
   spectrum and battery in the LLN rapidly.

10.  IANA Considerations

11.1.
10.1.  New Elective 6LoWPAN Routing Header Type

   This document updates the IANA registry titled "Elective 6LoWPAN
   Routing Header Type" that was created for [RFC8138] and assigns the
   following value:

                  +=======+=============+===============+

              +===============+=============+===============+
              |     Value     | Description | Reference     |
                  +=======+=============+===============+
              +===============+=============+===============+
              |   7 8 (Suggested) | P-RPI-6LoRH | This document |
                  +-------+-------------+---------------+
              +---------------+-------------+---------------+

                   Table 21: New Elective 6LoWPAN Routing
                                Header Type

11.2.

10.2.  New Critical 6LoWPAN Routing Header Type

   This document updates the IANA registry titled "Critical 6LoWPAN
   Routing Header Type" that was created for [RFC8138] and assigns the
   following value:

                  +=======+=============+===============+

              +===============+=============+===============+
              |     Value     | Description | Reference     |
                  +=======+=============+===============+
              +===============+=============+===============+
              |   7 8 (Suggested) | P-RPI-6LoRH | This document |
                  +-------+-------------+---------------+
              +---------------+-------------+---------------+

                   Table 22: New Critical 6LoWPAN Routing
                                Header Type

11.3.

10.3.  New Subregistry For The RPL Option Flags

   IANA is required to create a subregistry for the 8-bit RPL Option
   Flags field, as detailed in Figure 2, 4, under the "Routing Protocol for
   Low Power and Lossy Networks (RPL)" registry.  The bits are indexed
   from 0 (leftmost) to 7.  Each bit is tracked Tracked with the following
   qualities:

   *  Bit number (counting from bit 0 as the most significant bit)

   *  Indication When Set

   *  Reference

   Registration procedure is "Standards Action" [RFC8126].  The initial
   allocation is as indicated in Table 26:

           +============+======================+===============+

         +===============+======================+===============+
         |   Bit number  | Indication When Set  | Reference     |
           +============+======================+===============+
         +===============+======================+===============+
         |       0       | Down 'O'             | [RFC6553]     |
           +------------+----------------------+---------------+
         +---------------+----------------------+---------------+
         |       1       | Rank-Error (R)       | [RFC6553]     |
           +------------+----------------------+---------------+
         +---------------+----------------------+---------------+
         |       2       | Forwarding-Error (F) | [RFC6553]     |
           +------------+----------------------+---------------+
         +---------------+----------------------+---------------+
         | 3 (Suggested) | Projected-Route (P)  | This document |
           +------------+----------------------+---------------+
         +---------------+----------------------+---------------+

                       Table 23: Initial PDR Flags

11.4.

10.4.  New RPL Control Codes

   This document extends the IANA Subregistry created by RFC 6550 for
   RPL Control Codes as indicated in Table 24:

          +======+=============================+===============+

    +==================+=============================+===============+
    |       Code       | Description                 | Reference     |
          +======+=============================+===============+
    +==================+=============================+===============+
    | 0x09 (Suggested) | Projected DAO Request (PDR) | This document |
          +------+-----------------------------+---------------+
    +------------------+-----------------------------+---------------+
    | 0x0A (Suggested) | PDR-ACK                     | This document |
          +------+-----------------------------+---------------+
    +------------------+-----------------------------+---------------+

                     Table 24: New RPL Control Codes

11.5.

10.5.  New RPL Control Message Options

   This document extends the IANA Subregistry created by RFC 6550 for
   RPL Control Message Options as indicated in Table 25:

          +=======+============================+===============+

    +==================+=============================+===============+
    |      Value       | Meaning                     | Reference     |
          +=======+============================+===============+
    +==================+=============================+===============+
    |  0x0B 0x0E (Suggested) | Stateful VIO (SF-VIO) (SM-VIO)       | This document |
          +-------+----------------------------+---------------+
    +------------------+-----------------------------+---------------+
    |  0x0C 0x0F (Suggested) | Source-Routed VIO (SR-VIO) (NSM-VIO) | This document |
          +-------+----------------------------+---------------+
    +------------------+-----------------------------+---------------+
    |  0x0D 0x10 (Suggested) | Sibling Information option  | This document |
          +-------+----------------------------+---------------+
    +------------------+-----------------------------+---------------+

                  Table 25: RPL Control Message Options

11.6.

10.6.  SubRegistry for the Projected DAO Request Flags

   IANA is required to create a registry for the 8-bit Projected DAO
   Request (PDR) Flags field.  Each bit is tracked Tracked with the following
   qualities:

   *  Bit number (counting from bit 0 as the most significant bit)

   *  Capability description

   *  Reference

   Registration procedure is "Standards Action" [RFC8126].  The initial
   allocation is as indicated in Table 26:

          +============+========================+===============+
          | Bit number | Capability description | Reference     |
          +============+========================+===============+
          |     0      | PDR-ACK request (K)    | This document |
          +------------+------------------------+---------------+
          |     1      | Requested path should  | This document |
          |            | be redundant (R)       |               |
          +------------+------------------------+---------------+

                        Table 26: Initial PDR Flags

11.7.

10.7.  SubRegistry for the PDR-ACK Flags

   IANA is required to create an subregistry for the 8-bit PDR-ACK Flags
   field.  Each bit is tracked Tracked with the following qualities:

   *  Bit number (counting from bit 0 as the most significant bit)

   *  Capability description

   *  Reference

   Registration procedure is "Standards Action" [RFC8126].  No bit is
   currently defined for the PDR-ACK Flags.

11.8.

10.8.  Subregistry for the PDR-ACK Acceptance Status Values

   IANA is requested to create a Subregistry for the PDR-ACK Acceptance
   Status values.

   *  Possible values are 6-bit unsigned integers (0..63).

   *  Registration procedure is "Standards Action" [RFC8126].

   *  Initial allocation is as indicated in Table 27:

            +-------+------------------------+---------------+
            | Value | Meaning                | Reference     |
            +-------+------------------------+---------------+
            | 0     | Unqualified acceptance Acceptance | This document |
            +-------+------------------------+---------------+

            Table 27: Acceptance values of the PDR-ACK Status

11.9.

10.9.  Subregistry for the PDR-ACK Rejection Status Values

   IANA is requested to create a Subregistry for the PDR-ACK Rejection
   Status values.

   *  Possible values are 6-bit unsigned integers (0..63).

   *  Registration procedure is "Standards Action" [RFC8126].

   *  Initial allocation is as indicated in Table 28:

             +-------+-----------------------+---------------+
             | Value | Meaning               | Reference     |
             +-------+-----------------------+---------------+
             | 0     | Unqualified rejection Rejection | This document |
             +-------+-----------------------+---------------+
             | 1     | Transient Failure     | This document |
             +-------+-----------------------+---------------+

              Table 28: Rejection values of the PDR-ACK Status

11.10.

10.10.  SubRegistry for the Via Information Options Flags

   IANA is requested to create a Subregistry for the 5-bit Via
   Information Options (Via Information Option) Flags field.  Each bit
   is tracked Tracked with the following qualities:

   *  Bit number (counting from bit 0 as the most significant bit)

   *  Capability description

   *  Reference

   Registration procedure is "Standards Action" [RFC8126].  No bit is
   currently defined for the Via Information Options (Via Information
   Option) Flags.

11.11.

10.11.  SubRegistry for the Sibling Information Option Flags

   IANA is required to create a registry for the 5-bit Sibling
   Information Option (SIO) Flags field.  Each bit is tracked Tracked with the
   following qualities:

   *  Bit number (counting from bit 0 as the most significant bit)

   *  Capability description

   *  Reference

   Registration procedure is "Standards Action" [RFC8126].  The initial
   allocation is as indicated in Table 29:

    +============+===================================+===============+

          +===============+========================+===========+
          |   Bit number  | Capability description | Reference |
    +============+===================================+===============+
          +===============+========================+===========+
          | 0 (Suggested) | Connectivity is bidirectional (B) "D" flag: Sibling in   | This      |
          |               | same DODAG as Self     | document  |
    +------------+-----------------------------------+---------------+
          +---------------+------------------------+-----------+

                       Table 29: Initial SIO Flags

11.12.

10.12.  New Destination destination Advertisement Object Flag

   This document modifies the "Destination "destination Advertisement Object (DAO)
   Flags" registry initially created in Section 20.11 of [RPL] .

   Section 3.1 4.1.1 also defines one new entry in the Registry as follows:

          +---------------+------------------------+-----------+
          | Bit Number    | Capability Description | Reference |
          +---------------+------------------------+-----------+
          | 2 (suggested) (Suggested) | Projected DAO (P)      | THIS RFC  |
          +---------------+------------------------+-----------+

              Table 30: New Destination destination Advertisement Object
                                (DAO) Flag

11.13.  Error in Projected Route

10.13.  New ICMPv6 Error Code

   In some cases RPL will return an ICMPv6 error message when a message
   cannot be forwarded along a Projected Route.  This ICMPv6 error
   message is "Error in Projected Route". P-Route.

   IANA has defined an ICMPv6 "Code" Fields Registry for ICMPv6 Message
   Types.  ICMPv6 Message Type 1 describes "Destination "destination Unreachable"
   codes.  This specification requires that a new code is allocated from
   the ICMPv6 Code Fields Registry for ICMPv6 Message Type 1, for "Error
   in Projected Route", P-Route", with a suggested code value of 8, to be confirmed by
   IANA.

12.

10.14.  New RPL Rejection Status values

   This specification updates the Subregistry for the "RPL Rejection
   Status" values under the RPL registry, as follows:

          +---------------+-------------------------+-----------+
          | Value         | Meaning                 | Reference |
          +---------------+-------------------------+-----------+
          | 2 (Suggested) | Out of Resources        | THIS RFC  |
          +---------------+-------------------------+-----------+
          | 3 (Suggested) | Error in VIO            | THIS RFC  |
          +---------------+-------------------------+-----------+
          | 4 (Suggested) | Predecessor Unreachable | THIS RFC  |
          +---------------+-------------------------+-----------+
          | 5 (Suggested) | Unreachable Target      | THIS RFC  |
          +---------------+-------------------------+-----------+
          | 6..63         | Unassigned              |           |
          +---------------+-------------------------+-----------+

                Table 31: Rejection values of the RPL Status

11.  Acknowledgments

   The authors wish to acknowledge JP Vasseur, Remy Liubing, James
   Pylakutty
   Pylakutty, and Patrick Wetterwald for their contributions to the
   ideas developed here.

13.  Many thanks to Dominique Barthel and SVR Anand
   for their global contribution to 6TiSCH, RAW and this document, as
   well as text suggestions that were incorporated, and to Michael
   Richardson for his useful recommendations based on his global view of
   the system.  Also special thanks Toerless Eckert for his deep review,
   with many excellent suggestions that improved the readability and
   well as the content of the specification.

12.  Normative References

   [INT-ARCHI]
              Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [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>.

   [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>.

   [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
              Computation Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

   [RFC5551]  Gellens, R., Ed., "Lemonade Notifications Architecture",
              RFC 5551, DOI 10.17487/RFC5551, August 2009,
              <https://www.rfc-editor.org/info/rfc5551>.

   [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>.

   [RPL]      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>.

   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

14.

13.  Informative References

   [6LoWPAN]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <https://www.rfc-editor.org/info/rfc4944>.

   [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>.

   [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>.

   [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>.

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

   [RAW-ARCHI]
              Thubert, P., Papadopoulos, G. Z., and L. Berger, "Reliable
              and Available Wireless Architecture/Framework", Work in
              Progress, Internet-Draft, draft-ietf-raw-architecture-00,
              12 draft-ietf-raw-architecture-01,
              28 July 2021, <https://datatracker.ietf.org/doc/html/
              draft-ietf-raw-architecture-00>.
              draft-ietf-raw-architecture-01>.

   [USE-CASES]
              Papadopoulos, G. Z., Thubert, P., Theoleyre, F., and C. J.
              Bernardos, "RAW use cases", Work in Progress, Internet-
              Draft, draft-ietf-raw-use-cases-02, 12 July 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-raw-use-
              cases-02>.

   [TURN-ON_RFC8138]
              Thubert, P., Ed. and L. Zhao, "A Routing Protocol for Low-
              Power and Lossy Networks (RPL) Destination-Oriented
              Directed Acyclic Graph (DODAG) Configuration Option for
              the 6LoWPAN Routing Header", RFC 9035,
              DOI 10.17487/RFC9035, April 2021,
              <https://www.rfc-editor.org/info/rfc9035>.

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,
              <https://www.rfc-editor.org/info/rfc8655>.

   [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>.

   [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>.

   [USEofRPLinfo]

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,
              <https://www.rfc-editor.org/info/rfc8655>.

   [RFC8930]  Watteyne, T., Ed., Thubert, P., Ed., and C. Bormann, "On
              Forwarding 6LoWPAN Fragments over a Multi-Hop IPv6
              Network", RFC 8930, DOI 10.17487/RFC8930, November 2020,
              <https://www.rfc-editor.org/info/rfc8930>.

   [RFC8931]  Thubert, P., Ed., "IPv6 over Low-Power Wireless Personal
              Area Network (6LoWPAN) Selective Fragment Recovery",
              RFC 8931, DOI 10.17487/RFC8931, November 2020,
              <https://www.rfc-editor.org/info/rfc8931>.

   [RFC8994]  Eckert, T., Ed., Behringer, M., Ed., and S. Bjarnason, "An
              Autonomic Control Plane (ACP)", RFC 8994,
              DOI 10.17487/RFC8994, May 2021,
              <https://www.rfc-editor.org/info/rfc8994>.

   [RFC9008]  Robles, M.I., Richardson, M., and P. Thubert, "Using RPI
              Option Type, Routing Header for Source Routes, and IPv6-
              in-IPv6 Encapsulation in the RPL Data Plane", RFC 9008,
              DOI 10.17487/RFC9008, April 2021,
              <https://www.rfc-editor.org/info/rfc9008>.

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

   [I-D.irtf-panrg-path-properties]
              Enghardt, T. and C. Krähenbühl, "A Vocabulary of Path
              Properties", Work in Progress, Internet-Draft, draft-irtf-
              panrg-path-properties-03, 9 July 2021,
              <https://datatracker.ietf.org/doc/html/draft-irtf-panrg-
              path-properties-03>.

   [PCE]      IETF, "Path Computation Element",
              <https://datatracker.ietf.org/doc/charter-ietf-pce/>.
              <https://dataTracker.ietf.org/doc/charter-ietf-pce/>.

Appendix A.  Applications

A.1.  Loose Source Routing

   A RPL implementation operating in a very constrained LLN typically
   uses the Non-Storing Mode of Operation as represented in Figure 12. 17.
   In that mode, a RPL node indicates a parent-child relationship to the
   Root, using a Destination destination Advertisement Object (DAO) that is unicast
   from the node directly to the Root, and the Root typically builds a
   source routed path to a destination down the DODAG by recursively
   concatenating this information.

              ------+---------
                    |          Internet
                    |
                 +-----+
                 |     | Border Router router
                 |     |  (RPL Root)
                 +-----+                      ^     |        |
                    |                         | DAO | ACK    |
              o    o   o    o                 |     |        | Strict
          o o   o  o   o  o  o o   o          |     |        | Source
         o  o o  o o    o   o   o  o  o       |     |        | Route
         o   o    o  o     o  o    o  o  o    |     |        |
        o  o   o  o   o         o   o o       |     v        v
        o          o             o     o
                          LLN

                Figure 12: 17: RPL Non-Storing Mode of operation

   Based on the parent-children relationships expressed in the non-
   storing Non-
   Storing DAO messages,the Root possesses topological information about
   the whole network, though this information is limited to the
   structure of the DODAG for which it is the destination.  A packet
   that is generated within the domain will always reach the Root, which
   can then apply a source routing information to reach the destination
   if the destination is also in the DODAG.  Similarly, a packet coming
   from the outside of the domain for a destination that is expected to
   be in a RPL domain reaches the Root.

   It results that the Root, or then some associated centralized
   computation engine such as a PCE, can determine the amount of packets
   that reach a destination in the RPL domain, and thus the amount of
   energy and bandwidth that is wasted for transmission, between itself
   and the destination, as well as the risk of fragmentation, any
   potential delays because of a paths longer than necessary (shorter
   paths exist that would not traverse the Root).

   As a network gets deep, the size of the source routing header that
   the Root must add to all the downward packets becomes an issue for
   nodes that are many hops away.  In some use cases, a RPL network
   forms long lines and a limited amount of well-Targeted well-targeted routing state
   would allow to make the source routing operation loose as opposed to
   strict, and save packet size.  Limiting the packet size is directly
   beneficial to the energy budget, but, mostly, it reduces the chances
   of frame loss and/or packet fragmentation, which is highly
   detrimental to the LLN operation.  Because the capability to store a
   routing state in every node is limited, the decision of which route
   is installed where can only be optimized with a global knowledge of
   the system, a knowledge that the Root or an associated PCE may
   possess by means that are outside of the scope of this specification.

   This specification enables to store a Storing Mode state in
   intermediate routers, which enables to limit the excursion of the
   source route headers in deep networks.  Once a P-DAO exchange has
   taken place for a given Target, if the Root operates in non Storing
   Mode, then it may elide the sequence of routers that is installed in
   the network from its source route headers to destination that are
   reachable via that Target, and the source route headers effectively
   become loose.

A.2.  Transversal  East-West Routes

   RPL is optimized for Point-to-Multipoint (P2MP) and Multipoint-to-
   Point (MP2P), whereby routes are always installed North-South (aka
   up/down) along the RPL DODAG respectively from and towards the DODAG
   Root.  Transversal  Peer to Peer (P2P) East-West routes in a RPL network will
   generally suffer from some elongated (stretched) path versus the best possible a direct
   (optimized) path, since routing between 2 two nodes always happens via
   a common parent, as illustrated in Figure 13: 18:

                      ------+---------
                       |          Internet
                       |
                    +-----+
                    |     | Border router
                    |     |  (RPL Root)
                    +-----+
                       X
                 ^    v   o    o
             ^ o   o  v   o  o  o o   o
            ^  o o  o v    o   o   o  o  o
            ^   o    o  v     o  o    o  o  o
           S  o   o  o   D         o   o o
           o          o             o     o
                             LLN

       Figure 18: Routing Stretch between S and D via common parent X
                          along North-South Paths

   *  In Storing Mode, unless the destination is a child of the source,
      the packets will follow the default route up the DODAG as well.
      If the destination is in the same DODAG, they will eventually
      reach a common parent that has a route to the destination; at
      worse, the common parent may also be the Root.  From that common
      parent, the packet will follow a path down the DODAG that is
      optimized for the Objective Function that was used to build the
      DODAG.

   *  in Non-Storing Mode, all packets routed within the DODAG flow all
      the way up to the Root of the DODAG.  If the destination is in the
      same DODAG, the Root must encapsulate the packet to place an RH
      that has the strict source route information down the DODAG to the
      destination.  This will be the case even if the destination is
      relatively close to the source and the Root is relatively far off.

                      ------+---------
                       |          Internet
                       |
                    +-----+
                    |     | Border Router
                    |     |  (RPL Root)
                    +-----+
                       X
                 ^    v   o    o
             ^ o   o  v   o  o  o o   o
            ^  o o  o v    o   o   o  o  o
            ^   o    o  v     o  o    o  o  o
           S  o   o  o   D         o   o o
           o          o             o     o
                             LLN

       Figure 13: Routing Stretch between S and D via common parent X

   It results that it is often beneficial to enable transversal East-West P2P
   routes, either if the RPL route presents a stretch from shortest
   path, or if the new route is engineered with a different objective,
   and that it is even more critical in Non-Storing Mode than it is in
   Storing Mode, because the routing stretch is wider.  For that reason,
   earlier work at the IETF introduced the "Reactive Discovery of
   Point-to-Point Routes in Low Power and Lossy Networks" [RFC6997],
   which specifies a distributed method for establishing optimized P2P
   routes.  This draft proposes an alternate based on a centralized
   route computation.

                 ------+---------
                       |          Internet
                       |
                    +-----+
                    |     | Border Router router
                    |     |  (RPL Root)
                    +-----+
                       |
                 o    o   o    o
             o o   o  o   o  o  o o   o
            o  o o  o o    o   o   o  o  o
            o   o    o  o     o  o    o  o  o
           S>>A>>>B>>C>>>D         o   o o
           o          o             o     o
                             LLN

           Figure 14: Projected Transversal 19: More direct East-West Route between S and D

   This specification enables to store source-routed or Storing Mode
   state in intermediate routers, which enables to limit the stretch of
   a P2P route and maintain the characteristics within a given SLA.  An
   example of service using this mechanism oculd be a control loop that
   would be installed in a network that uses classical RPL for
   asynchronous data collection.  In that case, the P2P path may be
   installed in a different RPL Instance, with a different objective
   function.

Authors' Addresses

   Pascal Thubert (editor)
   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
   Rahul Arvind Jadhav
   Huawei Tech
   Kundalahalli Village, Whitefield,
   Bangalore 560037
   Karnataka
   India

   Phone: +91-080-49160700
   Email: rahul.ietf@gmail.com

   Matthew Gillmore
   Itron, Inc
   Building D
   2111 N Molter Road
   Liberty Lake,  99019
   United States

   Phone: +1.800.635.5461
   Email: matthew.gillmore@itron.com