ROLL                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                             R.A. Jadhav
Expires: 29 May 2022                                         Huawei Tech
                                                             M. Gillmore
                                                                   Itron
                                                        25 November 2021


                  Root initiated routing state in RPL
                   draft-ietf-roll-dao-projection-22

Abstract

   This document extends RFC 6550, RFC 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

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 29 May 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.



Thubert, et al.            Expires 29 May 2022                  [Page 1]


Internet-Draft               DAO Projection                November 2021


   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     2.2.  References  . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Glossary  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Domain Terms  . . . . . . . . . . . . . . . . . . . . . .   5
       2.4.1.  Projected Route . . . . . . . . . . . . . . . . . . .   6
       2.4.2.  Projected DAO . . . . . . . . . . . . . . . . . . . .   6
       2.4.3.  Path  . . . . . . . . . . . . . . . . . . . . . . . .   6
       2.4.4.  Routing Stretch . . . . . . . . . . . . . . . . . . .   6
       2.4.5.  Track . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Context and Goal  . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  RPL Applicability . . . . . . . . . . . . . . . . . . . .   9
     3.2.  RPL Routing Modes . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Requirements  . . . . . . . . . . . . . . . . . . . . . .  11
       3.3.1.  Loose Source Routing  . . . . . . . . . . . . . . . .  11
       3.3.2.  East-West Routes  . . . . . . . . . . . . . . . . . .  13
     3.4.  On Tracks . . . . . . . . . . . . . . . . . . . . . . . .  14
       3.4.1.  Building Tracks With RPL  . . . . . . . . . . . . . .  15
       3.4.2.  Tracks and RPL Instances  . . . . . . . . . . . . . .  15
     3.5.  Serial Track Signaling  . . . . . . . . . . . . . . . . .  16
       3.5.1.  Using Storing Mode Segments . . . . . . . . . . . . .  17
       3.5.2.  Using Non-Storing Mode joining Tracks . . . . . . . .  24
     3.6.  Complex Tracks  . . . . . . . . . . . . . . . . . . . . .  31
     3.7.  Scope and Expectations  . . . . . . . . . . . . . . . . .  33
       3.7.1.  External Dependencies . . . . . . . . . . . . . . . .  33
       3.7.2.  Positioning vs. Related IETF Standards  . . . . . . .  33
   4.  Extending existing RFCs . . . . . . . . . . . . . . . . . . .  35
     4.1.  Extending RFC 6550  . . . . . . . . . . . . . . . . . . .  35
       4.1.1.  Projected DAO . . . . . . . . . . . . . . . . . . . .  36
       4.1.2.  Via Information Option  . . . . . . . . . . . . . . .  37
       4.1.3.  Sibling Information Option  . . . . . . . . . . . . .  37
       4.1.4.  P-DAO Request . . . . . . . . . . . . . . . . . . . .  38
       4.1.5.  Extending the RPI . . . . . . . . . . . . . . . . . .  38
       4.1.6.  Additional Flag in the RPL DODAG Configuration
               Option  . . . . . . . . . . . . . . . . . . . . . . .  38
     4.2.  Extending RFC 6553  . . . . . . . . . . . . . . . . . . .  39
     4.3.  Extending RFC 8138  . . . . . . . . . . . . . . . . . . .  40
   5.  New RPL Control Messages and Options  . . . . . . . . . . . .  41
     5.1.  New P-DAO Request Control Message . . . . . . . . . . . .  42



Thubert, et al.            Expires 29 May 2022                  [Page 2]


Internet-Draft               DAO Projection                November 2021


     5.2.  New PDR-ACK Control Message . . . . . . . . . . . . . . .  43
     5.3.  Via Information Options . . . . . . . . . . . . . . . . .  44
     5.4.  Sibling Information Option  . . . . . . . . . . . . . . .  47
   6.  Root Initiated Routing State  . . . . . . . . . . . . . . . .  49
     6.1.  RPL Network Setup . . . . . . . . . . . . . . . . . . . .  49
     6.2.  Requesting a Track  . . . . . . . . . . . . . . . . . . .  49
     6.3.  Identifying a Track . . . . . . . . . . . . . . . . . . .  50
     6.4.  Installing a Track  . . . . . . . . . . . . . . . . . . .  51
       6.4.1.  Signaling a Projected Route . . . . . . . . . . . . .  52
       6.4.2.  Installing a Track Segment with a Storing Mode
               P-Route . . . . . . . . . . . . . . . . . . . . . . .  53
       6.4.3.  Installing a Track Leg with a Non-Storing Mode
               P-Route . . . . . . . . . . . . . . . . . . . . . . .  55
     6.5.  Tearing Down a P-Route  . . . . . . . . . . . . . . . . .  57
     6.6.  Maintaining a Track . . . . . . . . . . . . . . . . . . .  57
       6.6.1.  Maintaining a Track Segment . . . . . . . . . . . . .  58
       6.6.2.  Maintaining a Track Leg . . . . . . . . . . . . . . .  58
     6.7.  Encapsulating and Forwarding Along a Track  . . . . . . .  59
     6.8.  Compression of the RPL Artifacts  . . . . . . . . . . . .  61
   7.  Lesser Constrained Variations . . . . . . . . . . . . . . . .  63
     7.1.  Storing Mode Main DODAG . . . . . . . . . . . . . . . . .  63
     7.2.  A Track as a Full DODAG . . . . . . . . . . . . . . . . .  65
   8.  Profiles  . . . . . . . . . . . . . . . . . . . . . . . . . .  66
   9.  Backwards Compatibility . . . . . . . . . . . . . . . . . . .  68
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  69
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  69
     11.1.  RPL DODAG Configuration Option Flag  . . . . . . . . . .  69
     11.2.  Elective 6LoWPAN Routing Header Type . . . . . . . . . .  70
     11.3.  Critical 6LoWPAN Routing Header Type . . . . . . . . . .  70
     11.4.  Subregistry For The RPL Option Flags . . . . . . . . . .  70
     11.5.  RPL Control Codes  . . . . . . . . . . . . . . . . . . .  71
     11.6.  RPL Control Message Options  . . . . . . . . . . . . . .  71
     11.7.  SubRegistry for the Projected DAO Request Flags  . . . .  72
     11.8.  SubRegistry for the PDR-ACK Flags  . . . . . . . . . . .  72
     11.9.  Subregistry for the PDR-ACK Acceptance Status Values . .  73
     11.10. Subregistry for the PDR-ACK Rejection Status Values  . .  73
     11.11. SubRegistry for the Via Information Options Flags  . . .  73
     11.12. SubRegistry for the Sibling Information Option Flags . .  74
     11.13. destination Advertisement Object Flag  . . . . . . . . .  74
     11.14. New ICMPv6 Error Code  . . . . . . . . . . . . . . . . .  75
     11.15. RPL Rejection Status values  . . . . . . . . . . . . . .  75
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  76
   13. Normative References  . . . . . . . . . . . . . . . . . . . .  76
   14. Informative References  . . . . . . . . . . . . . . . . . . .  77
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  80






Thubert, et al.            Expires 29 May 2022                  [Page 3]


Internet-Draft               DAO Projection                November 2021


1.  Introduction

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

   RPL forms destination Oriented Directed Acyclic Graphs (DODAGs) in
   which the Root often acts as the Border router to connect the RPL
   domain to the IP backbone and routes along that graph up, towards the
   Root, and down towards the nodes.

   With this specification, an abstract routing function called a Path
   Computation Element [PCE] (e.g., located in an central controller or
   collocated with the Root) interacts with the RPL Root to compute Peer
   to Peer (P2P) paths within a pre-existing RPL Main DODAG.  The
   topological information that is passed to the PCE is derived from the
   DODAG that is already available at the Root in RPL Non-Storing Mode.
   This specification introduces protocol extensions that enrich the
   topological information that is available at the Root and passed to
   the PCE.

   Based on usage, path length, and knowledge of available resources
   such as battery levels and reservable buffers in the nodes, the PCE
   with a global visibility on the system can optimize the computed
   routes for the application needs, including the capability to provide
   path redundancy.  This specification also introduces protocol
   extensions that enable the Root to translates the computed paths into
   RPL and install them as Projected Routes (aka P-Routes) inside the
   DODAG on behalf of a PCE.

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.









Thubert, et al.            Expires 29 May 2022                  [Page 4]


Internet-Draft               DAO Projection                November 2021


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 Advertisement Object
   DAG:  Directed Acyclic Graph
   DODAG:  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 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
   ULA:  IPv6 Unique Local Address
   NSM-VIO:  A Source-Routed Via Information Option, used in Non-Storing
      Mode P-DAO messages.
   SLO:  Service Level Objective
   TIO:  RPL Transit Information Option
   SM-VIO:  A strict Via Information Option, used in Storing Mode P-DAO
      messages.
   VIO:  A Via Information Option; it can be a SM-VIO or an NSM-VIO.

2.4.  Domain Terms

   This specification uses the following terminology:








Thubert, et al.            Expires 29 May 2022                  [Page 5]


Internet-Draft               DAO Projection                November 2021


2.4.1.  Projected Route

   A RPL 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.  It is
   installed as a state that signals that destinations (aka Targets) are
   reachable along a sequence of nodes.

2.4.2.  Projected DAO

   A DAO message used to install a P-Route.

2.4.3.  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 a path 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 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 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.

2.4.4.  Routing Stretch

   RPL is anisotropic, meaning that it is directional, or more exactly
   polar.  RPL does not behave the same way "down" with multicast DIO
   messages that form the DODAG and "up" with unicast DAO messages that
   follow the DODAG.  This is in contrast with traditional IGPs that
   operate the same in all directions and are thus called isotropic.

   The term Routing Stretch denotes the length of a path, as compared
   with a shortest path, which can be a abstract concepts in RPL when
   the metrics are statistical and dynamic, and the concept of short
   varies with the Objective Function.



Thubert, et al.            Expires 29 May 2022                  [Page 6]


Internet-Draft               DAO Projection                November 2021


   The RPL DODAG optimizes the P2MP (from Root) and MP2P (to Root)
   paths, but the P2P (node to node) traffic has to follow the same
   DODAG.  Following the DODAG, the RPL datapath passes via a common
   parent in Storing Mode and via the Root in Non-Storing Mode.  This
   typically involves more hops and more latency than the minimum
   possible for a direct P2P path that an isotropic protocol would
   compute.  We refer to this elongated path as stretched.

2.4.5.  Track

   A networking graph that can be followed to transport packets with
   equivalent treatment; as opposed to the definition of a path above, a
   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.

             A ==> B ==> C -=- F ==> G ==> H     T1       I: Ingress
           /              \   /              \ /          E: Egress
         I                  O                 E -=- T2    T1, T2, T3:
           \              /   \              / \            External
             P ==> Q ==> R -=- T ==> U ==> V     T3         Targets

            I ==> A ==> B ==> C : a segment to targets F and O

               I --> F --> E : a leg to targets T1, T2, T3

              I, A, B, C, F, G, H, E : a path to T1, T2, T3

                    Figure 1: A Track and its Components

   This specification builds Tracks that are DODAGs oriented towards a
   Track Ingress, and the forward direction for packets (aka East-West)
   is from the Track Ingress to one of the possibly multiple Track
   Egress Nodes, which is also down the DODAG.

   The 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.

2.4.5.1.  TrackID

   A RPL Local InstanceID that identifies a Track using the namespace
   owned by the Track Ingress.  The TrackID is associated with the IPv6
   Address of the Track Ingress that is used as DODAGID, and together
   they form a unique identification of the Track (see the definition of
   DODAGID in section 2 of [RPL].





Thubert, et al.            Expires 29 May 2022                  [Page 7]


Internet-Draft               DAO Projection                November 2021


2.4.5.2.  namespace

   The term namespace is used to refer to the scope of the TrackID.  The
   TrackID is locally significant within its namespace.  The namespace
   is identified by the DODAGID for the Track.  The tuple (DODAGID,
   TrackID) is globally unique.

2.4.5.3.  Serial Track

   A Track that has only one path.

2.4.5.4.  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).

2.4.5.5.  Stitching

   This specification using the term stitching to indicate that a track
   is piped to another one, meaning that traffic out of the first is
   injected in the other.

2.4.5.6.  subTrack

   A Track within a Track.  As the Non-Storing Mode Via Information
   option (NSM-VIO) can only signal a loose sequence of nodes, it takes
   a number of them to signal a complex Track.  Each NSM-VIO for the
   same TrackId but a different Segment ID signals a different subTracks
   that the Track Ingress adds to the topology.

2.4.5.7.  Leg

   An end-to-end East-West serial path that can be a Track by itself or
   a subTrack of a complex Track.  With this specification, a Leg is is
   installed by the Root of the main DODAG using Non-Storing Mode P-DAO
   messages, and it is expressed as a loose sequence of nodes that are
   joined by Track Segments.

2.4.5.8.  Segment

   A serial path formed by a strict sequence of nodes, along which a
   P-Route is installed.  With this specification, a Segment is
   typically installed by the Root of the main DODAG using Storing Mode
   P-DAO messages.  A Segment used as the 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
   general RAW model, which allows North/South Segments that can be
   bidirectional.



Thubert, et al.            Expires 29 May 2022                  [Page 8]


Internet-Draft               DAO Projection                November 2021


2.4.5.8.1.  Section of a Segment

   A continuous subset of a segment that may be replaced while the
   segment remains. for instance, in segment A=>B=>C=>D=>E=>F, say that
   the link C to D might be misbehaving.  The section B=>C=>D=>E in the
   segment may be replaced by B=>C'=>D'=>E to route around the problem.
   The segment becomes A=>B=>C'=>D'=>E=>F.

2.4.5.8.2.  Segment Routing and SRH

   The terms Segment Routing and SRH refer to using source-routing to
   hop over segments.  In a Non-Storing mode RPL domain, the SRH is
   typically a RPL Source Route Header (the IPv6 RH of type 3) as
   defined in [RFC6554].

   If the network is a 6LoWPAN Network, the expectation is that the SRH
   is compressed and encoded as a 6LoWPAN Routing Header (6LoRH), as
   specified in section 5 of [RFC8138].

   On the other hand, if the RPL Network is less constrained and
   operated in Storing Mode, as discussed in Section 7.1, the Segment
   Routing operation and the SRH could be as specified in [RFC8754].
   This specification applies equally to both forms of source routing
   and SRH.

3.  Context and Goal

3.1.  RPL Applicability

   RPL is optimized for situations where the power is scarce, the
   bandwidth constrained and the transmissions unreliable.  This matches
   the use case of an IoT LLN where RPL is typically used today, but
   also situations of high relative mobility between the nodes in the
   network (aka swarming), e.g., within a variable set of vehicles with
   a similar global motion, or a toon of drones.

   To reach this goal, RPL is primarily designed to minimize the control
   plane activity, that is the relative amount of routing protocol
   exchanges vs. data traffic, and the amount of state that is
   maintained in each node.  RPL does not need converge, and provides
   connectivity to most nodes most of the time.

   RPL may form multiple topologies called instances.  Instances can be
   created to enforce various optimizations through objective functions,
   or to reach out through different Root Nodes.  The concept of
   objective function allows to adapt the activity of the routing
   protocol to the use case, e.g., type, speed, and quality of the LLN
   links.



Thubert, et al.            Expires 29 May 2022                  [Page 9]


Internet-Draft               DAO Projection                November 2021


   RPL instances operate as ships passing in the night, unbeknownst of
   one another.  The RPL Root is responsible to select the RPL Instance
   that is used to forward a packet coming from the Backbone into the
   RPL domain and set the related RPL information in the packets. 6TiSCH
   leverages RPL for its distributed routing operations.

   To reduce the routing exchanges, RPL leverages an anisotropic
   Distance Vector approach, which does not need a global knowledge of
   the topology, and only optimizes the routes to and from the 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 is simple and efficient in situations where the traffic is
   mostly directed from or to a central node, such as the control
   traffic between routers and a controller of a Software Defined
   Networking (SDN) infrastructure or an Autonomic Control Plane (ACP).

   But stretch in P2P routing is counter-productive to both reliability
   and latency as it introduces additional delay and chances of loss.
   As a result, [RPL] is not a good fit for the use cases listed in the
   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 each node towards the a Root, and
   those routes together coalesce as a Directed Acyclic Graph upwards.
   RPL then constructs routes to destinations signaled as Targets in the
   reverse direction, down the same DODAG.  So do so, a RPL Instance can
   be operated either in RPL Storing or Non-Storing Mode of Operation
   (MOP).  The default route towards the Root is maintained aggressively
   and may change while a packet progresses without causing loops, so
   the packet will still reach the Root.

   In Non-Storing Mode, each node advertises itself as a Target directly
   to the Root, indicating the parents that may be used to reach self.
   Recursively, the Root builds and maintains an image of the whole
   DODAG in memory, and leverages that abstraction to compute source
   route paths for the packets to their destinations down the DODAG.
   When a node changes its point(s) of attachment to the DODAG, it takes
   single unicast packet to the Root along the default route to update
   it, and the connectivity is restored immediately; this mode is
   preferable for use cases where internet connectivity is dominant, or
   when, like here, the Root controls the network activity in the nodes.





Thubert, et al.            Expires 29 May 2022                 [Page 10]


Internet-Draft               DAO Projection                November 2021


   In Storing Mode, the routing information percolates upwards, and each
   node maintains the routes to the subDAG of its descendants down the
   DODAG.  The maintenance is lazy, either reactive upon traffic or as a
   slow background process.  Packets flow via the common parent and the
   routing stretch is reduced vs. Non-Storing, for a better P2P
   connectivity.  On the other hand, a new route takes a longer time to
   propagate to the Root, time for the Distance-Vector protocol to
   operate hop-by-hop, and the Internet connectivity is restored more
   slowly upon movement.

   Either way, the RPL routes are injected by the Target nodes, in a
   distributed fashion.  To complement RPL and eliminate routing
   stretch, this specification introduces an hybrid mode that combines
   Storing and Non-Storing operations to build and project routes onto
   the nodes where they should be installed.  This specification uses
   the term Projected Route (P-Route) to refer to those routes.

   A P-Route may be installed in either Storing and Non-Storing Mode,
   potentially resulting in hybrid situations where the Mode of the P-
   Route is different from that of the RPL Main DODAG.  P-Routes can be
   used as stand-alone segments to reduce the size of the 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 more
   complex forwarding graph called a Track.

3.3.  Requirements

3.3.1.  Loose Source Routing

   A RPL implementation operating in a very constrained LLN typically
   uses the Non-Storing Mode of Operation as represented in Figure 2.
   In that mode, a RPL node indicates a parent-child relationship to the
   Root, using a 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.















Thubert, et al.            Expires 29 May 2022                 [Page 11]


Internet-Draft               DAO Projection                November 2021


                 +-----+
                 |     | Border 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 2: RPL Non-Storing Mode of operation

   Based on the parent-children relationships expressed in the 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 wireless
   bandwidth near the Root is the gating factor for all transmissions
   towards or within the domain, and that the Root is a single point of
   failure for all connectivity to nodes within its domain.

   The RPL Root must add a source routing header to all downward
   packets.  As a network grows, the size of the source routing header
   augments with the depth of the nodes.  In some use cases, a RPL
   network forms long lines along physical structures such as streets
   for lighting.  Limiting the packet size is directly beneficial to the
   energy budget, but, mostly, it reduces the chances of frame loss and
   packet fragmentation, which are highly detrimental to the LLN
   operation.  A limited amount of well-targeted routing state would
   allow the source routing operation to be loose as opposed to strict,
   and save packet size.  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.

   Being on path for all packets in Non-Storing mode, the Root may
   determine the number of P2P packets in its RPL domain per source and
   destination, the latency incurred, and the amount of energy and
   bandwidth that is consumed to reach the self and then down, including
   a possible fragmentation when encapsulating larger packets.  Enabling



Thubert, et al.            Expires 29 May 2022                 [Page 12]


Internet-Draft               DAO Projection                November 2021


   a shorter path that would not traverse the Root for select P2P
   source/destinations may improve the latency, lower the consumption of
   constrained resources, free bandwidth at the bottleneck near the
   Root, improve the delivery ratio and reduce the latency for those P2P
   flows with a global benefit for all flows of reducing the load at the
   Root.

   This requirement is to store a routing state associated with the Main
   DODAG in selected RPL routers, to limit the excursion of the source
   route headers in deep networks.  The Root may elide the sequence of
   routers that is installed in the network from its source route
   header, which becomes loose while it is strict in [RPL].

3.3.2.  East-West Routes

   [RPL] optimizes Point-to-Multipoint (P2MP) routes from the Root,
   Multipoint-to-Point (MP2P) routes to the DODAG Root, and Internet
   access when the Root also serves as Border Router.  All routes are
   installed North-South (aka up/down) along the RPL DODAG.  Peer to
   Peer (P2P) East-West routes in a RPL network will generally suffer
   from some elongated (stretched) path versus a direct (optimized)
   path, since routing between two nodes always happens via a common
   parent, as illustrated in Figure 3:

                 ------+---------
                       |          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 3: Routing Stretch between S and D via common parent X
                          along North-South Paths

   As described in [RFC9008], the amount of stretch depends on the Mode
   of Operation:

   *  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



Thubert, et al.            Expires 29 May 2022                 [Page 13]


Internet-Draft               DAO Projection                November 2021


      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.

   *  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.

   It results that it is often beneficial to enable 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.

                    +-----+
                    |     | Border 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 4: More direct East-West Route between S and D

   The requirement is to install additional routes in the RPL routers,
   to reduce the stretch of some P2P routes and maintain the
   characteristics within a given SLO, e.g., in terms of latency and/or
   reliability.

3.4.  On Tracks






Thubert, et al.            Expires 29 May 2022                 [Page 14]


Internet-Draft               DAO Projection                November 2021


3.4.1.  Building Tracks With RPL

   The concept of a Track was introduced in the "6TiSCH Architecture"
   [6TiSCH-ARCHI], as a collection of potential paths that leverage
   redundant forwarding solutions along the way.  This can be a DODAG or
   a more complex structure that is only partially acyclic (e.g., per
   packet).

   With this specification, a Track is shaped as a DODAG, and following
   the directed edges leads to a Track Ingress.  Storing Mode P-DAO
   messages follow the direction of the edges to set up routes for
   traffic that flows the other way, towards the Track Egress(es).  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.

   A RPL Track is a collection of (one or more) parallel loose source
   routed sequences of nodes ordered from Ingress to Egress, each
   forming a Track Leg.  The nodes that are directly connected,
   reachable via existing Tracks as illustrated in Section 3.5.2.3 or
   joined with strict Segments of other nodes as shown in
   Section 3.5.1.3.  The Legs are expressed in RPL Non-Storing Mode and
   require an encapsulation to add a Source Route Header, whereas the
   Segments are expressed in RPL Storing Mode.

   A Serial Track comprises provides only one path between Ingress and
   Egress.  It comprises at most one Leg. A Stand-Alone Segment
   implicitly defines a Serial Track from its Ingress to Egress.

   A complex Track forms a graph that provides a collection of potential
   paths to provide redundancy for the packets, either as a collection
   of Legs that may be parallel or cross at certain points, or as a more
   generic DODAG.

3.4.2.  Tracks and RPL Instances

   Section 5.1. of [RPL] describes the RPL Instance and its encoding.
   There can be up to 128 global RPL Instances, for which there can be
   one or more DODAGs, and there can be 64 local RPL Instances, with a
   namespace that is indexed by a DODAGID, where the DODAGID is a Unique
   Local Address (ULA) or a Global Unicast Address (GUA) of the Root of
   the DODAG.  Bit 0 (most significant) is set to 1 to signal a Local
   RPLInstanceID, as shown in Figure 5.  By extension, this
   specification expresses the value of the RPLInstanceID as a single
   integer between 128 and 191, representing both the Local
   RPLInstanceID in 0..63 and Bit 0 set.




Thubert, et al.            Expires 29 May 2022                 [Page 15]


Internet-Draft               DAO Projection                November 2021


                  0 1 2 3 4 5 6 7
                 +-+-+-+-+-+-+-+-+
                 |1|D|   ID      |  Local RPLInstanceID in 0..63
                 +-+-+-+-+-+-+-+-+

                   Figure 5: Local RPLInstanceID Encoding

   A Track is normally associated with a Local RPL Instance which
   RPLInstanceID is used as the TrackID, more in Section 6.3.  A Track
   Leg may also be used as a subTrack that extends the RPL main DODAG.
   In that case, the TrackID is set to the global RPLInstanceID of the
   main DODAG, which suffices to identify the routing topology.  As
   opposed to local RPL instances, the Track Ingress that encapsulates
   the packets over a subtrack is not Root, and that the source address
   of the encapsulated packet is not used to determine the Track.

3.5.  Serial Track Signaling

   This specification enables to set up a P-Route along either a Track
   Leg or a Segment.  A P-Route is installed and maintained by the Root
   of the main DODAG using an extended RPL DAO message called a
   Projected DAO (P-DAO), and a Track is composed of the combination of
   one or more P-Routes.

   A P-DAO message for a Track signals the TrackID in the RPLInstanceID
   field.  In the case of a local RPL Instance, the address of the Track
   Ingress is used as source to encapsulate packets along the Track.
   The Track is signaled in the DODAGID field of the Projected DAO Base
   Object, see Figure 8.

   This specification introduces the Via Information Option (VIO) to
   signal a sequence of hops in a Leg or a Segment in the P-DAO
   messages, either in Storing Mode (SM-VIO) or Non-Storing Mode (NSM-
   VIO).  One P-DAO messages contains a single VIO, associated to one or
   more RPL Target Options that signal the destination IPv6 addresses
   that can reached along the Track, more in Section 5.3.

   Before diving deeper into Track Legs and Segments signaling and
   operation, this section provides examples of what how route
   projection works through variations of a simple example.  This simple
   example illustrates the case of host routes, though RPL Targets can
   be prefixes.

   Say we want to build a Serial Track from node A to E in Figure 6, 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:





Thubert, et al.            Expires 29 May 2022                 [Page 16]


Internet-Draft               DAO Projection                November 2021


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


                         Figure 6: Reference Track

   Conventionally we use ==> to represent a strict hop and --> for a
   loose hop.  We use "-to-", such as 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).

   Figure 5 depicts the format of the RPLInstanceID encoding for a local
   RPLInstanceID .

   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.

3.5.1.  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.






Thubert, et al.            Expires 29 May 2022                 [Page 17]


Internet-Draft               DAO Projection                November 2021


3.5.1.1.  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:





















Thubert, et al.            Expires 29 May 2022                 [Page 18]


Internet-Draft               DAO Projection                November 2021


         +======+=============+=========+=============+==========+
         | 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: E delivers the packet to F.




Thubert, et al.            Expires 29 May 2022                 [Page 19]


Internet-Draft               DAO Projection                November 2021


3.5.1.2.  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 6.7.

   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:









Thubert, et al.            Expires 29 May 2022                 [Page 20]


Internet-Draft               DAO Projection                November 2021


         +======+=============+=========+=============+==========+
         | 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.



Thubert, et al.            Expires 29 May 2022                 [Page 21]


Internet-Draft               DAO Projection                November 2021


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

3.5.1.3.  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:







Thubert, et al.            Expires 29 May 2022                 [Page 22]


Internet-Draft               DAO Projection                November 2021


         +======+=============+=========+=============+==========+
         | 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.





Thubert, et al.            Expires 29 May 2022                 [Page 23]


Internet-Draft               DAO Projection                November 2021


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

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

3.5.2.  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.

3.5.2.1.  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:












Thubert, et al.            Expires 29 May 2022                 [Page 24]


Internet-Draft               DAO Projection                November 2021


         +======+=============+=========+=============+==========+
         | 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.



Thubert, et al.            Expires 29 May 2022                 [Page 25]


Internet-Draft               DAO Projection                November 2021


   *  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.

3.5.2.2.  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:













Thubert, et al.            Expires 29 May 2022                 [Page 26]


Internet-Draft               DAO Projection                November 2021


         +======+=============+=========+=============+==========+
         | 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.



Thubert, et al.            Expires 29 May 2022                 [Page 27]


Internet-Draft               DAO Projection                November 2021


   *  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.

3.5.2.3.  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:









Thubert, et al.            Expires 29 May 2022                 [Page 28]


Internet-Draft               DAO Projection                November 2021


         +======+=============+=========+=============+==========+
         | 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:












Thubert, et al.            Expires 29 May 2022                 [Page 29]


Internet-Draft               DAO Projection                November 2021


    +========+===================+===================+================+
    | 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.





Thubert, et al.            Expires 29 May 2022                 [Page 30]


Internet-Draft               DAO Projection                November 2021


3.6.  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 7.  Legs may cross at the edges of loose hops 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 it to be managed separately.  When Legs
   cross within respective 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.





































Thubert, et al.            Expires 29 May 2022                 [Page 31]


Internet-Draft               DAO Projection                November 2021


           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 7: 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



Thubert, et al.            Expires 29 May 2022                 [Page 32]


Internet-Draft               DAO Projection                November 2021


   as illustrated in Figure 7, 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.

3.7.  Scope and Expectations

3.7.1.  External Dependencies

   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.

3.7.2.  Positioning vs. Related IETF Standards

3.7.2.1.  Extending 6TiSCH

   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.

3.7.2.2.  Mapping to DetNet

   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.






Thubert, et al.            Expires 29 May 2022                 [Page 33]


Internet-Draft               DAO Projection                November 2021


   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 7.

3.7.2.3.  Leveraging PCE

   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 [RFC5440].  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 also 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.

3.7.2.4.  Providing for RAW

   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.





Thubert, et al.            Expires 29 May 2022                 [Page 34]


Internet-Draft               DAO Projection                November 2021


   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.  The resulting Track is also a
   DODAG for which the Track Ingress is the Root, the owner the address
   that serves as DODAGID and authoritative for the associated namespace
   from which the TrackID is selected.  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.









Thubert, et al.            Expires 29 May 2022                 [Page 35]


Internet-Draft               DAO Projection                November 2021


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 8.  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 8: Projected DAO Base Object

   New fields:

   TrackID:  The local or global RPLInstanceID of the DODAG that serves
      as Track, more in Section 6.3




Thubert, et al.            Expires 29 May 2022                 [Page 36]


Internet-Draft               DAO Projection                November 2021


   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.  The DAO
   message signals to the Root that a given parent can be used to reach
   a given child.  The P-DAO message generalizes the DAO to signal to
   the Track Ingress that a Track for which it is Root can be used to
   reach children and siblings of the Track Egress.  In both cases,
   options may be factorized and multiple RTOs may be present to signal
   a collection of children that can be reached through the parent or
   the Track, respectively.

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 6.7.

   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 6.4.2 and Section 6.4.3 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.









Thubert, et al.            Expires 29 May 2022                 [Page 37]


Internet-Draft               DAO Projection                November 2021


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.

4.1.6.  Additional Flag in the RPL DODAG Configuration Option

   The DODAG Configuration Option is defined in Section 6.7.6 of [RPL].
   Its purpose is extended to distribute configuration information
   affecting the construction and maintenance of the DODAG, as well as
   operational parameters for RPL on the DODAG, through the DODAG.  This
   Option was originally designed with 4 bit positions reserved for
   future use as Flags.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 0x04 |Opt Length = 14|D| | | |A|       ...           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                     +
                                     |4 bits |

            Figure 9: DODAG Configuration Option (Partial View)




Thubert, et al.            Expires 29 May 2022                 [Page 38]


Internet-Draft               DAO Projection                November 2021


   This specification defines a new flag "Projected Routes Support" (D).
   The 'D' flag is encoded in bit position 0 of the reserved Flags in
   the DODAG Configuration Option (this is the most significant bit)(to
   be confirmed by IANA but there's little choice).  It is set to 0 in
   legacy implementations as specified respectively in Sections 20.14
   and 6.7.6 of [RPL].

   The 'D' flag is set to 1 to indicate that this specification is
   enabled in the network and that the Root will install the requested
   Tracks when feasible upon a PDR message.

   Section 4.1.2. of [RFC9008] updates [RPL] to indicate that the
   definition of the Flags applies to Mode of Operation values from zero
   (0) to six (6) only.  For a MOP value of 7, the implementation MUST
   consider that the Root accepts PDR messages and will install
   Projected Routes.

   The RPL DODAG Configuration option is typically placed in a DODAG
   Information Object (DIO) message.  The DIO message propagates down
   the DODAG to form and then maintain its structure.  The DODAG
   Configuration option is copied unmodified from parents to children.
   xref target='RFC6550'/> states that:

   |  Nodes other than the DODAG root MUST NOT modify this information
   |  when propagating the DODAG Configuration option.

   Therefore, a legacy parent propagates the 'D' flag as set by the
   root, and when the 'D' flag is set to 1, it is transparently flooded
   to all the nodes in the DODAG.

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:








Thubert, et al.            Expires 29 May 2022                 [Page 39]


Internet-Draft               DAO Projection                November 2021


      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 10: 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.









Thubert, et al.            Expires 29 May 2022                 [Page 40]


Internet-Draft               DAO Projection                November 2021


   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 22 and
   Table 23 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 11: 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 3.4 and
      Section 6.3 .

   Section 6.8 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




Thubert, et al.            Expires 29 May 2022                 [Page 41]


Internet-Draft               DAO Projection                November 2021


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 11.10) 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 6.2.

   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 12: New P-DAO Request Format

   TrackID:  8-bit field.  In the context of this specification, the
      TrackID field signals the RPLInstanceID of the DODAG formed by the
      Track, see Section 3.4 and Section 6.3.  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



Thubert, et al.            Expires 29 May 2022                 [Page 42]


Internet-Draft               DAO Projection                November 2021


   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 should be 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.

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 13: New PDR-ACK Control Message Format

   TrackID:  Set to the TrackID indicated in the TrackID field of the
      PDR messages that 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 14:




Thubert, et al.            Expires 29 May 2022                 [Page 43]


Internet-Draft               DAO Projection                November 2021


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

                       Figure 14: PDR-ACK status Format

      E:  1-bit flag.  Set to indicate a rejection.  When not set, the
         value of 0 indicates Success/Unqualified 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 28 and Table 29.

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

5.3.  Via Information Options

   A VIO signals the ordered list of IPv6 Via Addresses that constitutes
   the hops of either a Leg (using Non-Storing Mode) a Segment (using
   storing mode) of a Track.  A Storing Mode P-DAO contains one Storing
   Mode VIO (SM-VIO) whereas a Non-Storing Mode P-DAO contains one 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 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 use only one SRH-6LoRH per Via
   Information Option, and the compression is the same forall the
   addresses, as shown in Figure 15, 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 globally shorter; this means that more
   than one SRH-6LoRH may be present.

   The format of the Via Information Options is as follows:





Thubert, et al.            Expires 29 May 2022                 [Page 44]


Internet-Draft               DAO Projection                November 2021


        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     |   P-RouteID   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Segm. Sequence | Seg. Lifetime |        SRH-6LoRH head         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .           Via Address 1 (compressed by RFC 8138)              .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .                              ....                             .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .           Via Address n (compressed by RFC 8138)              .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .              Additional SRH-6LoRH Header(s)                   .
       |                                                               |
       .                              ....                             .


                 Figure 15: VIO format (uncompressed form)

   Option Type:  0x0E for SM-VIO, 0x0F for NSM-VIO (to be confirmed by
      IANA), see =Table 26

   Option Length:  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 applied.

   P-RouteID:  8-bit field that identifies a component of a Track or the
      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/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.





Thubert, et al.            Expires 29 May 2022                 [Page 45]


Internet-Draft               DAO Projection                November 2021


      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 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, 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.

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

   Via Address:  An IPv6 ULA or GUA of a node along the Segment.  The
      VIO contains one or more IPv6 Via Addresses listed in the datapath
      order from Ingress to Egress.  The list is expressed in a
      compressed form as signaled by the preceding SRH-6LoRH header.

      In a Storing Mode P-DAO that updates or removes a section of an
      already existing Segment, the list in the 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 list in
      the VIO MUST be complete.

      In the case of an SM-VIO, the list indicates a sequential (strict)
      path through direct neighbors, the complete list starts at Ingress
      and ends at Egress, and the nodes listed in the VIO, including the
      Egress, MAY be considered as implicit Targets.








Thubert, et al.            Expires 29 May 2022                 [Page 46]


Internet-Draft               DAO Projection                November 2021


      In the case of an NSM-VIO, the complete list can be loose and
      excludes the Ingress node, starting at the first loose hop and
      ending at a Track Egress; the Track Egress MUST be considered as
      an implicit Target, so it MUST NOT be signaled in a RPL Target
      Option.

5.4.  Sibling Information Option

   The Sibling Information Option (SIO) provides indication on siblings
   that could be used by the Root to form P-Routes.  One or more SIO(s)
   may be placed in the DAO messages that are sent to the Root in 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 |S| Flags |Comp.|    Opaque     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Step in Rank       |          Reserved             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .       Sibling DODAGID (if the D flag not set)               .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .                     Sibling Address                           .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 16: Sibling Information Option Format



Thubert, et al.            Expires 29 May 2022                 [Page 47]


Internet-Draft               DAO Projection                November 2021


   Option Type:  0x10 for SIO (to be confirmed by IANA), see =Table 26

   Option Length:  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].

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

   S:  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 in Rank:  16-bit unsigned integer.  This is the Step in Rank
      [RPL] as computed by the Objective Function between this node and
      the sibling, that reflects the abstract Rank increment that would
      be computed by the OF if the sibling was the preferred parent.

   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.





Thubert, et al.            Expires 29 May 2022                 [Page 48]


Internet-Draft               DAO Projection                November 2021


   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.

6.  Root Initiated Routing State

6.1.  RPL Network Setup

   To avoid the need of Path MTU Discovery, 6LoWPAN links are normally
   defined with a MTU of 1280 (see section 4 of [6LoWPAN]).  Injecting
   packets in a Track typically involves an IP-in-IP encapsulation and
   additional IPv6 Extension Headers.  This may cause a fragmentation if
   the resulting packets exceeds the MTU that is defined for the RPL
   domain.

   Though fragmentation is possible in a 6LoWPAN LLN, e.g., using
   [6LoWPAN], [RFC8930], and/or [RFC8931], it is RECOMMENDED to allow an
   MTU that is larger than 1280 in the main DODAG and allows for the
   additional headers while exposing only 1280 to the 6LoWPAN Nodes.

6.2.  Requesting a Track

   This specification introduces the PDR message, used by an LLN node to
   request the formation of a new Track for which this node is Ingress.
   Note that the namespace for the TrackID is owned by the Ingress node,
   and in the absence of a PDR, there must be some procedure for the
   Root to assign TrackIDs in that namespace while avoiding collisions,
   more in Section 6.3.

   The PDR signals the desired TrackID and the duration for which the
   Track should be established.  Upon a PDR, the Root MAY install the
   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 designs the Track as it sees best, and updates / changes the
   Segments overtime to serve the Track as needed.  There is no
   notification to the 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.





Thubert, et al.            Expires 29 May 2022                 [Page 49]


Internet-Draft               DAO Projection                November 2021


   When the Track Lifetime that was returned in the PDR-ACK is close to
   elapse - vs. the 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
   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.

6.3.  Identifying a Track

   RPL defines the concept of an Instance to signal an individual
   routing topology, and multiple topologies can coexist in the same
   network.  The RPLInstanceID is tagged in the RPI of 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 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
      Section 3.3.1.  The Root MAY also install a Track Leg across the
      Main DODAG to complement the routing topology.

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

   *  The Root MAY also use P-DAO messages to install a Track as an
      independent routing topology (say, Traffic Engineered) to achieve
      particular routing characteristics from an Ingress to an Egress
      Endpoints.  To achieve this, the Root MUST set up a local RPL
      Instance (see section 5 of [RPL]), and the Local RPLInstanceID
      serves as TrackID.  The TrackID MUST be unique for the IPv6 ULA or
      GUA of the Track Ingress that serves as DODAGID for the Track.







Thubert, et al.            Expires 29 May 2022                 [Page 50]


Internet-Draft               DAO Projection                November 2021


      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 8, and the P-RouteID
      that identifies the P-Route MUST be signaled in the VIO as shown
      in Figure 15.

      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.  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.

6.4.  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.





Thubert, et al.            Expires 29 May 2022                 [Page 51]


Internet-Draft               DAO Projection                November 2021


   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.

6.4.1.  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 normal expression for this specification, where as the
   latter corresponds to the variation for lesser constrained
   environments described in Section 7.2.

   A P-DAO that creates or updates a Track Leg MUST be sent to a GUA or
   a ULA of the Ingress of the Leg; it must contain the full list of
   hops in the Leg unless the Leg is being removed.  A P-DAO that
   creates a new Track Segment MUST be sent to a GUA or a ULA of the



Thubert, et al.            Expires 29 May 2022                 [Page 52]


Internet-Draft               DAO Projection                November 2021


   Segment Egress and MUST signal the full list of hops in Segment; a
   P-DAO that updates (including deletes) a section of a Segment MUST be
   sent to the first node after the modified Segment and signal the full
   list of hops in the section starting at the node that immediately
   precedes the modified section.

   In Non-Storing Mode, as discussed in Section 6.4.3, the Root sends
   the P-DAO to the Track Ingress where the source-routing state is
   applied, whereas in Storing Mode, the P-DAO is sent to the last node
   on the installed path and forwarded in the reverse direction,
   installing a Storing Mode state at each hop, as discussed in
   Section 6.4.2.  In both cases the Track Ingress is the owner of the
   Track, and it generates the P-DAO-ACK when the installation is
   successful.

   If the 'K' Flag is present in the P-DAO, the P-DAO must be
   acknowledged using a DAO-ACK that is sent back to the address of the
   Root from which the P-DAO was received.  In most cases, the first
   node of the Leg, Segment, or 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 Storing
   Mode P-DAO to the previous node in the SM-VIO.

   In a No-Path Non-Storing Mode P-DAO, the SRH-6LoRH MUST NOT be
   present in the NSM-VIO; the state in the 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 processes a VIO MAY verify whether one of these
   conditions happen, and when so, it MUST ignore the P-DAO and reject
   it with a RPL Rejection Status of "Error in VIO" in the DAO-ACK, see
   Section 11.15.

   Other errors than those discussed explicitely that prevent the
   installing the route are acknowledged with a RPL Rejection Status of
   "Unqualified Rejection" in the DAO-ACK.

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

   As illustrated in Figure 17, a Storing Mode P-DAO installs a route
   along the Segment signaled by the SM-VIO towards the Targets
   indicated in the Target Options.  The Segment is to be included in a
   DODAG indicated by the P-DAO Base Object, that may be the one formed
   by the RPL Main DODAG, or a Track associated with a local RPL
   Instance.





Thubert, et al.            Expires 29 May 2022                 [Page 53]


Internet-Draft               DAO Projection                November 2021


           ------+---------
                 |          Internet
                 |
              +-----+
              |     | Border 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                                          v

                       Figure 17: 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 SM-VIO with the strict sequence of Via Addresses.  The SM-
   VIO follows one or more RTOs indicating the Targets to which the
   Track leads.  The SM-VIO contains a Segment Lifetime for which the
   state is to be maintained.

   The Root sends the P-DAO directly to the Egress node of the Segment.
   In that P-DAO, the destination IP address matches the last Via
   Address in the SM-VIO.  This is how the Egress recognizes its role.
   In a similar fashion, the Segment Ingress node recognizes its role as
   it matches first Via Address in the 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 existing tables.
   If one of the Targets is not known, the node MUST answer to the Root
   with a DAO-ACK listing the unreachable Target(s) in an RTO and a
   rejection status of "Unreachable Target".

   If the Egress node can reach all the Targets, then it forwards the
   P-DAO with unchanged content to its 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 to
   Ingress.





Thubert, et al.            Expires 29 May 2022                 [Page 54]


Internet-Draft               DAO Projection                November 2021


   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 MUST
   install a route towards the DAO Target(s) via their successor in the
   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 SM-VIO after
   self, if any, but in case of a conflict or a lack of resource, the
   route(s) to the Target(s) are the ones that must be installed in
   priority.

   If a router cannot reach its predecessor in the SM-VIO, the router
   MUST send the DAO-ACK to the Root with a Rejection Status of
   "Predecessor Unreachable".

   The process continues till the P-DAO is propagated to 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.

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

   As illustrated in Figure 18, a Non-Storing Mode P-DAO installs a
   source-routed path within the Track indicated by the P-DAO Base
   Object, towards the Targets indicated in the Target Options.  The
   source-routed path requires a Source-Routing header which implies an
   IP-in-IP encapsulation to add the SRH to an existing packet.  It is
   sent to the Track Ingress which creates a tunnel associated with the
   Track, and connected routes over the tunnel to the Targets in the
   RTO.  The tunnel encapsulation MUST incorporate a routing header via
   the list addresses listed in the VIO in the same order.  The content
   of the NSM-VIO starting at the first SRH-6LoRH header MUST be used
   verbatim by the Track Ingress when it encapsulates a packet to
   forward it over the Track.










Thubert, et al.            Expires 29 May 2022                 [Page 55]


Internet-Draft               DAO Projection                November 2021


              ------+---------
                    |          Internet
                    |
                 +-----+
                 |     | Border 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 Egress       X
           o  o  o  o             o    |
                                     Target
          o       o     LLN          o    o
        o          o             o     o

                 Figure 18: 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 3.5.1.3, 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 3.5.2.3
   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.





Thubert, et al.            Expires 29 May 2022                 [Page 56]


Internet-Draft               DAO Projection                November 2021


6.5.  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 on 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; it SHOULD not
   be placed in the message and MUST be ignored by the receiver.  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.

6.6.  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.





Thubert, et al.            Expires 29 May 2022                 [Page 57]


Internet-Draft               DAO Projection                November 2021


6.6.1.  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 6.5.

6.6.2.  Maintaining a Track Leg

   This specification allows the Root 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 becoming excessively lossy,
   and switch to the new Leg before removing the old.  Dropping a Track
   before the new one is installed would reroute the traffic via the
   root; this may augment the latency beyond acceptable thresholds, and
   load the network near the root.  This may also cause loops in the
   case of stitched Tracks; the packets that cannot be injected in the
   second Track may be routed back at reinjected at the Ingress of the
   first.

   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 list 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 6.5.



Thubert, et al.            Expires 29 May 2022                 [Page 58]


Internet-Draft               DAO Projection                November 2021


6.7.  Encapsulating and Forwarding Along a Track

   When injecting a packet in a Track, the Ingress router must
   encapsulate the packet using IP-in-IP to add the Source Routing
   Header with the final destination set to the Track Egress.

   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, 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 uses as DODAGID for the Track, the destination is
      the first Via Address in the 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 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 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.





Thubert, et al.            Expires 29 May 2022                 [Page 59]


Internet-Draft               DAO Projection                November 2021


      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 6.3.

   *  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 3.5.2.3.  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 be a Segment of the same Track to
      that loose next 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 the last case, another
      encapsulation takes place and the process possibly recurses;
      otherwise the packet is dropped.








Thubert, et al.            Expires 29 May 2022                 [Page 60]


Internet-Draft               DAO Projection                November 2021


   *  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 11.14).  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 6.6).

   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 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.

   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 for the source node, so the Root can establish where the
   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 RH SHOULD be consumed by this node so that the
   destination in the IPv6 header is the next hop that this node could
   not reach.  if a 6LoWPAN Routing Header (6LoRH) [RFC8138] is used to
   carry the IPv6 routing information in the outer header then that
   whole 6LoRH information SHOULD be present in the ICMP message.

6.8.  Compression of the RPL Artifacts

   When using [RFC8138] in the Main DODAG operated in Non-Storing Mode
   in a 6LoWPAN LLN, a typical packet that circulates in the Main DODAG
   is formatted as shown in Figure 19, representing the case where :








Thubert, et al.            Expires 29 May 2022                 [Page 61]


Internet-Draft               DAO Projection                November 2021


   +-+ ... -+- ... -+- ... -+-+- ... +-+-+-+ ... +-+-+ ... -+ ... +-...
   |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 19: A Packet as Forwarded along 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 actually removed at encapsulation, so the inner
   packet used in the descriptions below start with the SRH-6LoRH, and
   is verbatim the packet represented in Figure 19 from the second octet
   on.

   When encapsulating that inner packet to 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 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 compressed to 2 bytes, the resulting value for the
   Length field shown in Figure 20 is 3, meaning that the 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 20: The IP-in-IP 6LoRH Header

   At the head of the resulting sequence of bytes, the track Ingress
   then adds the RPI that carries the TrackID as RPLinstanceID as a P-
   RPI-6LoRH Header, as illustrated in Figure 11, using the TrackID as
   RPLInstanceID.  Combined with 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.







Thubert, et al.            Expires 29 May 2022                 [Page 62]


Internet-Draft               DAO Projection                November 2021


        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 21: The SRH 6LoRH Header

   The format of the resulting encapsulated packet in [RFC8138]
   compressed form is illustrated in Figure 22:

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

    Signals :  Loose Hops :    TrackID  :  Track DODAGID :

               Figure 22: A Packet as Forwarded along a Track

7.  Lesser Constrained Variations

7.1.  Storing Mode Main DODAG

   This specification expects that the 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 topology,
      so the 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 same benefits from this capability.

   On the other hand, since RPL is a Layer-3 routing protocol, its
   applicability extends beyond LLNs to a generic IP network.  RPL
   requires fewer resources than alternative IGPs like OSPF, ISIS,



Thubert, et al.            Expires 29 May 2022                 [Page 63]


Internet-Draft               DAO Projection                November 2021


   EIGRP, BABEL or RIP at the expense of a route stretch vs. the
   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 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.
   Since there is no capability negotiation, the expectation is that all
   the nodes in the network support this specification in the same
   fashion, or are configured the same way through management.

   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 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 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.








Thubert, et al.            Expires 29 May 2022                 [Page 64]


Internet-Draft               DAO Projection                November 2021


   *  The TIO is formed as in Storing Mode, and the Parent Address is
      not present.  The Path Sequence and Path Lifetime fields are
      aligned with the values used in the Address Registration of the
      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 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 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 node in the LLN has no topological
      awareness, and does not need 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.  This
      alleviates both the complexity of computing a path and the
      compressed form of the routing header.








Thubert, et al.            Expires 29 May 2022                 [Page 65]


Internet-Draft               DAO Projection                November 2021


   The "Reliable and Available Wireless (RAW) Architecture/Framework"
   [RAW-ARCHI] actually expects the PSE at the Track Ingress to react to
   changes in the forwarding conditions along the Track, and reroute
   packets to maintain the required degree of reliability.  To achieve
   this, the PSE need the full richness of a DODAG to form any path that
   could make meet the Service Level Objective (SLO).

   This section specifies the additions that are needed to turn the
   Track into a full DODAG and enable the main Root to provide the
   necessary topological information to the Track Ingress.  The
   expectation is that 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 [RAW-ARCHI].  It follows
   that the PSE will learn the metrics it needs from an alternate
   source, e.g., OAM frames.

   To pass the topological information to the Ingress, the Root uses a
   P-DAO messages that contains sequences of Target and Transit
   Information options that collectively represent the Track, expressed
   in the same fashion as in classical Non-Storing Mode.  The difference
   as that the Root is the source as opposed to the destination, and can
   report information on many Targets, possibly the full Track, with one
   P-DAO.

   Note that the Path Sequence and Lifetime in the TIO are selected by
   the Root, and that the Target/Transit information tupples in the
   P-DAO are not those received by the Root in the 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



Thubert, et al.            Expires 29 May 2022                 [Page 66]


Internet-Draft               DAO Projection                November 2021


   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 support of [RPL] Non-Storing 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 P-Route Segments along the Main DODAG)  Profi
      le 1 does not create new paths; compared to Profile 0, it combines
      Storing and Non-Storing Modes to balance the size of the 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 P-Route Segments along the Main DODAG)  P
      rofile 2 extends Profile 0 with Strict Source-Routing Non-Storing
      Mode P-Routes along the 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 DODAG
      as Profile 1, but it require an encapsulation, in order to insert
      an additional SRH between the loose source routing hops.  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 encapsulates is not the 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 P-Routes that install segments that
      do not follow the Main DODAG.  If the Segment Ingress (in the 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 P-Routes to form East-West Tracks that are inside
      the 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.

   Profile 5  Profile 5 Combines Profile 4 with Profile 1 and enables to




Thubert, et al.            Expires 29 May 2022                 [Page 67]


Internet-Draft               DAO Projection                November 2021


      loose source routing between the Ingress and the Egress of the
      Track.  As in Profile 1, 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.

   Profile 7  Profile 7 implements profile 5 in a Main DODAG that is
      operated in Storing Mode as presented in Section 7.1.  As in
      Profile 1 and 2, the TrackID is the RPLInstanceID of the Main
      DODAG.  Longest match rules decide whether a packet is sent along
      the Main DODAG or rerouted in a track.

   Profile 8  Profile 8 is offered in preparation of the RAW work, and
      for use cases where an arbitrary node in the network can afford
      the same code complexity as the RPL Root in a traditional
      deployment.  It offers a full DODAG visibility to the Track
      Ingress as specified in Section 7.2 in a Non-Storing Mode Main
      DODAG.

   Profile 9  Profile 9 combines profiles 7 and 8, operating the Track
      as a full DODAG within a Storing Mode Main DODAG, using only the
      Main DODAG RPLInstanceID as TrackID.

9.  Backwards Compatibility

   This specification can operate in a mixed network where some nodes
   support it and some do not.  There are restructions, though.  All
   nodes that need to process a P-DAO must support this specification.
   As discussed in Section 3.7.1, how the root knows whether the nodes
   capabilities and whether it support this specification is out of
   scope.

   This specification defines the 'D' flag in the RPL DODAG
   Configuration Option (see Section 4.1.6) to signal that the RPL nodes
   can request the creation of Tracks.  The requester may not know
   whether the Track can effectively be constructed, and whether enough
   nodes along the preferred paths support this specification.
   Therefore it makes sense to only set the 'D' flags in DIO when the
   conditions of success are in place, in particular when all the nodes
   that could be on path of tracks are upgraded.









Thubert, et al.            Expires 29 May 2022                 [Page 68]


Internet-Draft               DAO Projection                November 2021


10.  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.

   With this specification, only the Root may generate P-DAO messages.
   PDR messages may only be sent to the Root.  This specification
   expects that the communication with the Root is authenticated but
   does enforce which method is used.

   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 to Ingress in the case of a Storing Mode P-DAO.

   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.

11.  IANA Considerations

11.1.  RPL DODAG Configuration Option Flag

   IANA is requested to assign a flag from the "DODAG Configuration
   Option Flags for MOP 0..6" [RFC9010] registry as follows:





Thubert, et al.            Expires 29 May 2022                 [Page 69]


Internet-Draft               DAO Projection                November 2021


       +---------------+------------------------------+-----------+
       | Bit Number    | Capability Description       | Reference |
       +---------------+------------------------------+-----------+
       | 0 (suggested) | Projected Routes Support (D) | THIS RFC  |
       +---------------+------------------------------+-----------+

              Table 21: New DODAG Configuration Option Flag

   IANA is requested to add [THIS RFC] as a reference for MOP 7 in the
   RPL Mode of Operation registry.

11.2.  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     |
              +===============+=============+===============+
              | 8 (Suggested) | P-RPI-6LoRH | This document |
              +---------------+-------------+---------------+

                   Table 22: New Elective 6LoWPAN Routing
                                Header Type

11.3.  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     |
              +===============+=============+===============+
              | 8 (Suggested) | P-RPI-6LoRH | This document |
              +---------------+-------------+---------------+

                   Table 23: New Critical 6LoWPAN Routing
                                Header Type

11.4.  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 10, 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 with the
   following qualities:



Thubert, et al.            Expires 29 May 2022                 [Page 70]


Internet-Draft               DAO Projection                November 2021


   *  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 27:

         +===============+======================+===============+
         |   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 24: Initial PDR Flags

11.5.  RPL Control Codes

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

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

                     Table 25: New RPL Control Codes

11.6.  RPL Control Message Options

   This document extends the IANA Subregistry created by RFC 6550 for
   RPL Control Message Options as indicated in Table 26:









Thubert, et al.            Expires 29 May 2022                 [Page 71]


Internet-Draft               DAO Projection                November 2021


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

                  Table 26: RPL Control Message Options

11.7.  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 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 27:

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

                        Table 27: Initial PDR Flags

11.8.  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 with the following qualities:

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

   *  Capability description

   *  Reference



Thubert, et al.            Expires 29 May 2022                 [Page 72]


Internet-Draft               DAO Projection                November 2021


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

11.9.  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 28:

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

            Table 28: Acceptance values of the PDR-ACK Status

11.10.  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 29:

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

              Table 29: Rejection values of the PDR-ACK Status

11.11.  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 with the following qualities:



Thubert, et al.            Expires 29 May 2022                 [Page 73]


Internet-Draft               DAO Projection                November 2021


   *  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.

11.12.  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 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 30:

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

                       Table 30: Initial SIO Flags

11.13.  destination Advertisement Object Flag

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

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











Thubert, et al.            Expires 29 May 2022                 [Page 74]


Internet-Draft               DAO Projection                November 2021


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

              Table 31: New destination Advertisement Object
                                (DAO) Flag

11.14.  New ICMPv6 Error Code

   In some cases RPL will return an ICMPv6 error message when a message
   cannot be forwarded along a P-Route.

   IANA has defined an ICMPv6 "Code" Fields Registry for ICMPv6 Message
   Types.  ICMPv6 Message Type 1 describes "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 P-Route", with a suggested code value of 8, to be confirmed by
   IANA.

11.15.  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 32: Rejection values of the RPL Status










Thubert, et al.            Expires 29 May 2022                 [Page 75]


Internet-Draft               DAO Projection                November 2021


12.  Acknowledgments

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

13.  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>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [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>.






Thubert, et al.            Expires 29 May 2022                 [Page 76]


Internet-Draft               DAO Projection                November 2021


   [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>.

   [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>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

14.  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>.



Thubert, et al.            Expires 29 May 2022                 [Page 77]


Internet-Draft               DAO Projection                November 2021


   [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-01,
              28 July 2021, <https://datatracker.ietf.org/doc/html/
              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-03, 20 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-raw-use-
              cases-03>.

   [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>.

   [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>.







Thubert, et al.            Expires 29 May 2022                 [Page 78]


Internet-Draft               DAO Projection                November 2021


   [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>.

   [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. Kraehenbuehl, "A Vocabulary of Path
              Properties", Work in Progress, Internet-Draft, draft-irtf-
              panrg-path-properties-04, 25 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-irtf-panrg-
              path-properties-04>.



Thubert, et al.            Expires 29 May 2022                 [Page 79]


Internet-Draft               DAO Projection                November 2021


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

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















Thubert, et al.            Expires 29 May 2022                 [Page 80]