Root initiated routing state in RPL
draft-ietf-roll-dao-projection-14

Document Type Active Internet-Draft (roll WG)
Authors Pascal Thubert  , Rahul Jadhav  , Matthew Gillmore 
Last updated 2020-10-02
Replaces draft-thubert-roll-dao-projection
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ROLL                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Updates: 6550 (if approved)                                  R.A. Jadhav
Intended status: Standards Track                             Huawei Tech
Expires: 5 April 2021                                        M. Gillmore
                                                                   Itron
                                                          2 October 2020

                  Root initiated routing state in RPL
                   draft-ietf-roll-dao-projection-14

Abstract

   This document updates RFC 6550 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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   This Internet-Draft will expire on 5 April 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
     2.2.  Glossary  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Other Terms . . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  References  . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Updating RFC 6550 . . . . . . . . . . . . . . . . . . . . . .   6
   4.  New RPL Control Messages and Options  . . . . . . . . . . . .   8
     4.1.  New P-DAO Request Control Message . . . . . . . . . . . .   8
     4.2.  New PDR-ACK Control Message . . . . . . . . . . . . . . .   9
     4.3.  Route Projection Options  . . . . . . . . . . . . . . . .  10
     4.4.  Sibling Information Option  . . . . . . . . . . . . . . .  12
   5.  Projected DAO . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Requesting a Track  . . . . . . . . . . . . . . . . . . .  15
     5.2.  Identifying a Track . . . . . . . . . . . . . . . . . . .  16
     5.3.  Forwarding Along a Track  . . . . . . . . . . . . . . . .  17
     5.4.  Non-Storing Mode Projected Route  . . . . . . . . . . . .  18
     5.5.  Storing Mode Projected Route  . . . . . . . . . . . . . .  19
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
     7.1.  New RPL Control Codes . . . . . . . . . . . . . . . . . .  21
     7.2.  New RPL Control Message Options . . . . . . . . . . . . .  22
     7.3.  SubRegistry for the Projected DAO Request Flags . . . . .  22
     7.4.  SubRegistry for the PDR-ACK Flags . . . . . . . . . . . .  23
     7.5.  Subregistry for the PDR-ACK Acceptance Status Values  . .  23
     7.6.  Subregistry for the PDR-ACK Rejection Status Values . . .  23
     7.7.  SubRegistry for the Route Projection Options Flags  . . .  24
     7.8.  SubRegistry for the Sibling Information Option Flags  . .  24
     7.9.  Error in Projected Route ICMPv6 Code  . . . . . . . . . .  25
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  25
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  25
   10. Informative References  . . . . . . . . . . . . . . . . . . .  26
   Appendix A.  Applications . . . . . . . . . . . . . . . . . . . .  27
     A.1.  Loose Source Routing  . . . . . . . . . . . . . . . . . .  27
     A.2.  Transversal Routes  . . . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

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1.  Introduction

   RPL, the "Routing Protocol for Low Power and Lossy Networks" [RPL]
   (LLNs), is a generic Distance Vector protocol that is well suited for
   application in a variety of low energy Internet of Things (IoT)
   networks.  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 Internet.  The Root is responsible to select
   the RPL Instance that is used to forward a packet coming from the
   Internet into the RPL domain and set the related RPL information in
   the packets. 6TiSCH uses RPL for its routing operations.

   The "6TiSCH Architecture" [6TiSCH-ARCHI] also leverages the
   "Deterministic Networking Architecture" [RFC8655] centralized model
   whereby the device resources and capabilities are exposed to an
   external controller which installs routing states into the network
   based on some objective functions that reside in that external
   entity.  With DetNet and 6TiSCH, the component of the controller that
   is responsible of computing routes is called a Path Computation
   Element ([PCE]).

   Based on heuristics of usage, path length, and knowledge of device
   capacity and available resources such as battery levels and
   reservable buffers, the PCE with a global visibility on the system
   can compute direct Peer to Peer (P2P) routes that are optimized for
   the needs expressed by an objective function.  This document
   specifies protocol extensions to RPL [RPL] that enable the Root of a
   main DODAG to install centrally-computed routes inside the DODAG on
   behalf of a PCE.

   This specification expects that the main RPL Instance is operated in
   RPL Non-Storing Mode of Operation (MOP) to sustain the exchanges with
   the Root.  In that Mode, the Root has enough information to build a
   basic DODAG topology based on parents and children, but lacks the
   knowledge of siblings.  This document adds the capability for nodes
   to advertise sibling information in order to improve the topological
   awareness of the Root.

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

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

   The concept of a Track was introduced in the 6TiSCH architecture, as
   a potentially complex path with redundant forwarding solutions along
   the way.  A node at the ingress of more than one Segment in a Track
   may use any combination of those Segments to forward a packet towards
   the Track Egress.

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

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

   The time scale at which the PCE (re)computes the Track can be long,
   using long-term statistical metrics to perform global optimizations
   at the scale of the whole network.  Conversely, the PSE makes
   forwarding decisions at the time scale of one or a small collection
   of packets, based on a knowledge that is limited in scope to the
   Track itself, so it can be refreshed at a fast pace.

   Projected Routes must be used with the parsimony to limit the amount
   of state that is installed in each device to fit within the device
   resources, and to maintain the amount of rerouted traffic within the
   capabilities of the transmission links.  The methods used to learn
   the node capabilities and the resources that are available in the
   devices and in the network are out of scope for this document.

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

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

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2.  Terminology

2.1.  Requirements Language

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

2.2.  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)
   LLN:  Low-Power and Lossy Network
   NMPR:  Non-Storing Mode Projected Route
   MOP:  RPL Mode of Operation
   P-DAO:  Projected DAO
   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
   RPO:  A Route Projection Option; it can be a VIO or an SRVIO.
   RTO:  RPL Target Option
   RUL:  RPL-Unaware Leaf
   SIO:  RPL Sibling Information Option
   SRVIO:  A Source-Routed Via Information Option, used in Non-Storing
      Mode P-DAO messages.
   SMPR:  Storing Mode Projected Route
   TIO:  RPL Transit Information Option
   VIO:  A Via Information Option, used in Storing Mode P-DAO messages.

2.3.  Other Terms

   Projected Route:  A Projected Route is a path segment that is
      computed remotely, and installed and maintained by a RPL Root.
   Projected DAO:  A DAO message used to install a Projected Route.
   Track:  A complex path with redundant Segments to a destination.
   TrackID:  A RPL Local InstanceID with the 'D' bit set.  The TrackID
      is associated with a IPv6 Address to the Track Egress Node.

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2.4.  References

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

3.  Updating RFC 6550

   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).  This specification extends the DAO
   message with the Projected DAO (P-DAO); a P-DAO message signals one
   or more Projected Route(s) using the new CMOs presented therein.

   A Projected Route can be an additional route of higher precedence
   within the main DODAG.  In that case, it is installed with a P-DAO
   using the parameters of the main DODAG, typically a global
   RPLInstanceID and the DODAGID field elided as shown in Section 6.4.1.
   of [RPL].

   A Projected Route can also be a Segment within a Track.  A stand-
   alone Segment can be used as a Serial Track.  Segments can also be
   combined to form a Complex Track.  The Root uses a local RPL Instance
   rooted at the Track Egress to signal the Track.  The local
   RPLInstanceID of the Track is called the TrackID, more in
   Section 5.2.  A P-DAO message for a Track signals the IPv6 Address of
   the Track Egress in the DODAGID field of the DAO Base Object, and the
   TrackID in the RPLInstanceID field, as shown in Figure 1.

      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|   Flags   |   Reserved    | DAOSequence   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +            IPv6 Address of the Track Egress                   +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Option(s)...
     +-+-+-+-+-+-+-+-+

                 Figure 1: Projected DAO Format for a Track

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

   New CMOs called the Route Projection Options (RPO) are introduced for
   use in P-DAO messages as a multihop alternative to the TIO.  One RPO
   is the Via Information Option (VIO); the VIO installs a state at each
   hop along a Storing Mode Projected Route (SMPR).  The other is the
   Source-Routed VIO (SRVIO); the SRVIO installs a source-routing state
   at the Segment ingress, which uses that state to encapsulate a packet
   with a Routing Header (RH) along a Non-Storing Mode Projected Route
   (NMPR).

   Like in a DAO message, the RTOs can be factorized in a P-DAO, but the
   RPOs cannot.  A P-DAO contains one or more RTOs that indicate the
   destinations that can be reached via the Track, and exactly one RPO
   that signals the sequence of nodes between the Track Ingress and the
   Track Egress, both included.  In Non-Storing Mode, the Root sends the
   P-DAO to the Track Ingress where the source-routing state is stored.
   In Storing Mode, the P-DAO is sent to the Track Egress and forwarded
   along the Segment in the reverse direction, installing a Storing Mode
   state at each hop.  In both cases the Track Ingress generates the P-
   DAO-ACK when the installation is successful.

   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 4.4.  The sibling selection process is out of scope.

   Two new RPL Control Messages are also introduced, to enable a RAN to
   request the establishment of a Track between self as the Track
   Ingress Node and a Track Egress.  The RAN 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 4.1 for more.  A positive PDR-ACK indicates that the Track
   was built and that the Roots commits to maintain the Track for the
   negotiated lifetime.  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.  The Root may use an asynchronous PDR-ACK with an negative
   status to indicate that the Track was terminated before its time.

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4.  New RPL Control Messages and Options

4.1.  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.
   Exactly one RTO MUST be present in a PDR.  The RTO signals the Track
   Egress, more in Section 5.1.

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

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

                     Figure 2: New P-DAO Request Format

   TrackID:  8-bit field indicating the RPLInstanceID associated with
      the Track.  It is set to zero upon the first request for a new
      Track and then to the TrackID once the Track was created, to
      either renew it of destroy it.

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

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

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

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

      A PDR with a fresher PDRSequence refreshes the lifetime, and a
      PDRLifetime of 0 indicates that the track should be destroyed.

   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.

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4.2.  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 3: New PDR-ACK Control Message Format

   TrackID:  The RPLInstanceID of the Track that was created.  The value
      of 0x00 is used to when no Track was created.

   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 4:

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

                       Figure 4: 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 4 and Table 5.

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

4.3.  Route Projection Options

   An RPO signals the ordered list of IPv6 Via Addresses that
   constitutes the hops of either a Serial Track or a Segment of a more
   Complex Track.  An RPO MUST contain at least one Via Address, and a
   Via Address MUST NOT be present more than once, otherwise the RPO
   MUST be ignored.  The format of the RPOs is as follows:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Type        | Option Length |     Flags     |   SegmentID   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Segm. Sequence | Seg. Lifetime |      SRH-6LoRH header         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .                     Via Address 1                             .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .                              ....                             .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .                     Via Address n                             .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 5: Route Projection Option format (uncompressed form)

   Option Type:  0x0B for VIO, 0x0C for SRVIO (to be confirmed by IANA)

   Option Length:  In bytes; variable, depending on the number of Via
      Addresses and the compression.

   SegmentID:  8-bit field that identifies a Segment within a Track or

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      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.

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

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

      The Segment information indicated in the RPO deprecates any state
      for the Segment indicated by the SegmentID within the indicated
      Track and sets up the new information.

      An RPO with a Segment Sequence that is not as fresh as the current
      one is ignored.

      An RPO for a given Track Egress with the same (TrackID, SegmentID,
      Segment Sequence) indicates a retry; it MUST NOT change the
      Segment and MUST be propagated or answered as the first copy.

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

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

      A P-DAO message that contains a Via Information option with a
      Segment Lifetime of zero for a Track Egress is referred as a No-
      Path (for that Track Egress) in this document.

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

   Via Address:  An IPv6 addresse along the Segment.

      In a VIO, the list is a strict path between direct neighbors,
      whereas for an SRVIO, the list may be loose, provided that each
      listed node has a path to the next listed node, e.g., via another
      Track.

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      In the case of a SMPR, or if [RFC8138] is not used in the data
      packets, then the Root MUST use only one SRH-6LoRH per RPO, and
      the compression is the same for all the addresses, as shown in
      Figure 5.

      In case of a NMPR, and if [RFC8138] is in use in the main DODAG,
      then the Root SHOULD optimize the size of the SRVIO; more than one
      SRH-6LoRH may be present, e.g., if the compression level changes
      inside the Segment and different SRH-6LoRH Types are required.

4.4.  Sibling Information Option

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

   The format of the SIO is as follows:

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

                Figure 6: Sibling Information Option Format

   Option Type:  0x0D (to be confirmed by IANA)

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

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   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 refernce is the Root of
      the main DODAG.

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

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

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

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

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

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

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

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

   Sibling Address:  2 to 16 bytes, the IPv6 Address of the sibling in a
      [RFC8138] compressed form as indicated by the Compression Type
      field.

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

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5.  Projected DAO

   This draft adds a capability to RPL whereby the Root of a DODAG
   projects a Track by sending one or more Projected-DAO (P-DAO)
   messages to selected routers in the DODAG.  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 Route
   Projection Option (RPO).  A P-DAO message MUST contain exactly one
   RPO, which is either a VIO or an SRVIO, and MUST follow one or more
   RTOs.  There can be at most one such sequence of RTO(s) and an RPO.

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

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

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

   *  The Non-Storing Mode is discussed in Section 5.4.  A Non-Storing
      Mode P-DAO carries an SRVIO with the loose list of Via Addresses
      that forms a source-routed Segment to the Track Egress.  The
      recipient of the P-DAO is the ingress router of the source-routed
      Segment.  The ingress router MUST install a source-routed state to
      the Track Egress and reply to the Root directly using a DAO-ACK
      message if requested to.

   *  The Storing Mode is discussed in Section 5.5.  A Storing Mode
      P-DAO carries a VIO with the strict list of Via Addresses from the
      ingress to the egress of the Segment in the data path order.  The
      routers listed in the Via Addresses, except the egress, MUST
      install a routing state to the Target(s) via the next Via Address
      in the VIO.  In normal operations, the P-DAO is propagated along
      the chain of Via Routers from the egress router of the path till
      the ingress one, which confirms the installation to the Root with
      a DAO-ACK message.  Note that the Root may be the ingress and it
      may be the egress of the Segment, that it can also be neither but
      it cannot be both.

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   In case of a forwarding error along a Projected Route, an ICMP error
   is sent to the Root with a new Code "Error in Projected Route" (See
   Section 7.9).  The Root can then modify or remove the Projected
   Route.  The "Error in Projected Route" message has the same format as
   the "Destination Unreachable Message", as specified in RFC 4443
   [RFC4443].

   The portion of the invoking packet that is sent back in the ICMP
   message SHOULD record at least up to the RH if one is present, and
   this hop of the RH SHOULD 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.

   The sender and exact operation depend on the Mode and is described in
   Section 5.4 and Section 5.5 respectively.

5.1.  Requesting a Track

   A Node is free to ask the Root for a new Track at any time.  This is
   done with a PDR message, that indicates in the Requested Lifetime
   field the duration for which the Track should be established.  Upon a
   PDR, the Root MAY install the necessary Segments, 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
   Track Egress signaled in the P-DAO.

   The Root is free to design the Track as it wishes, and to change the
   Segments overtime to serve the Track as needed, without notifying the
   resquesting Node.  The Segment Lifetime in the P-DAO messages does
   not need to be aligned to the Requested Lifetime in the PDR, or
   between P-DAO messages for different Segments.  The Root may use
   shorter lifetimes for the Segments and renew them faster than the
   Track is, or longer lifetimes in which case it will need to tear down
   the Segments if the Track is not renewed.

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

   If the Track fails and cannot be restored, the Root notifies the
   resquesting Node 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.

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5.2.  Identifying a Track

   RPL defines the concept of an Instance to signal an individual
   routing topology but does not have a concept of an administrative
   distance, which exists in certain proprietary implementations to sort
   out conflicts between multiple sources of routing information within
   one routing topology.

   This draft leverages the RPL Instance model as follows:

   *  The Root MAY use P-DAO messages to add better routes in the main
      (Global) Instance in conformance with the routing objectives in
      that Instance.  To achieve this, the Root MAY install an SMPR
      along a path down the main Non-Storing Mode DODAG.  This enables a
      loose source routing and reduces the size of the Routing Header,
      see Appendix A.1.

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

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

   *  The Root MAY also use P-DAO messages to install a specific (say,
      Traffic Engineered) path as a Serial or as a Complex Track, to a
      particular endpoint that is the Track Egress.  In that case, the
      Root MUST install a Local RPL Instance (see section 5 of [RPL]).

      In a that case, the TrackID MUST be unique for the Global Unique
      IPv6 Address (GUA) or Unique-Local Address (ULA) of the Track
      Egress that serves as DODAGID for the Track.  This way, a Track is
      uniquely identified by the tuple (Track Egress Address, TrackID)
      where the TrackID is always represented with the 'D' flag set.

      The Track Egress Address and the TrackID MUST be signaled in the
      P-DAO message as shown in Figure 1.

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5.3.  Forwarding Along a Track

   Sending a Packet within a RPL Local Instance requires the presence of
   an RPL Packet Information (RPI) (see [USEofRPLinfo]) in the outer
   IPv6 Header chain.  The RPI carries a local RPLInstanceID which, in
   association with the IPv6 final destination, indicates the RPL
   Instance that the packet follows.

   This draft leverages the RPL Forwarding model follows:

   *  The RPI carries a local RPLInstanceID called the TrackID, which,
      in association with the IPv6 final destination, indicates the
      Track along which the packet is forwarded.  The 'D' flag in the
      RPLInstanceID MUST be set to indicate that the final destination
      address in the IPv6 header owns the local RPLInstanceID, more in
      Section 5.3.

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

      In case of a NMPR, the outer chain of IPv6 Headers contains an
      IPv6 RH as well.  If it is not fully consumed, then the final
      destination is the last entry in the RH; else it is the
      Destination Address in the IPv6 Header.  When using the [RFC8138]
      compression, it is the last hop of the last SRH-6LoRH of the outer
      header in either case.

   *  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.  Else, i.e., if the Track Ingress is forwarding a
      packet into the Track, or if the the final destination is reached
      over the Track via the Track Egress but is located beyond it, then
      an IP-in-IP encapsulation is needed.

      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.  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.

      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.

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   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
   main instance, e.g., by setting the "T" flag [TURN-ON_RFC8138] in the
   RPL configuration option.

5.4.  Non-Storing Mode Projected Route

   As illustrated in Figure 7, a P-DAO that carries an SRVIO enables the
   Root to install a source-routed path towards a Track Egress in any
   particular router.

              ------+---------
                    |          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 Track        X
           o  o  o  o             o     Egress

          o       o               o    o
        o          o             o     o
                                      destination

                          LLN

                  Figure 7: Projecting a Non-Storing Route

   A route indicated by an SRVIO may be loose, meaning that the node
   that owns the next listed Via Address is not necessarily a neighbor.
   Without proper loop avoidance mechanisms, the interaction of loose
   source routing and other mechanisms may effectively cause loops.

   When forwarding a packet to a destination for which the router
   determines that routing happens via the Track Egress, the router
   inserts the source routing header in the packet with the destination
   set to the Track Egress.

   In order to signal the Segment, the router encapsulates the packet
   with an IP-in-IP header and a Routing Header as follows:

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   *  In the uncompressed form the source of the packet is this router,
      the destination is the first Via Address in the SRVIO, 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 SRVIO, 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].

   In the case of a loose source-routed path, there MUST be either a
   neighbor that is adjacent to 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 is Ingress; in the latter case, another
   encapsulation takes place and the process possibly recurses;
   otherwise the packet is dropped.

   In case of a 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 period of time and
   it SHOULD send an ICMP message with a Code "Error in Projected 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.

5.5.  Storing Mode Projected Route

   As illustrated in Figure 8, a P-DAO that carries a VIO enables the
   Root to install a stateful route towards a collection of Targets
   along a Segment between a Track Ingress and a Track Egress.

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           ------+---------
                 |          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                 From Root To Destination v

                        Figure 8: 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 VIO with the direct sequence of Via Addresses.  The VIO
   follows one or more RTOs indicating the Targets to which the Track
   leads.  The 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 VIO.  This is how the egress recognizes its role.  In
   a similar fashion, the ingress node recognizes its role as it matches
   first Via Address in the 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) on its own.  If one of the
   Targets is not known, the node MUST answer to the Root with a
   negative DAO-ACK listing the Target(s) that could not be located
   (suggested status 10 to be confirmed by IANA).

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

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   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
   VIO.  The router MAY install additional routes towards the VIA
   Addresses that are the VIO after the next one, if any, but in case of
   a conflict or a lack of resource, the route(s) to the Target(s) have
   precedence.

   If a router cannot reach its predecessor in the VIO, the router MUST
   answer to the Root with a negative DAO-ACK indicating the successor
   that is unreachable (suggested status 11 to be confirmed by IANA).

   The process continues till the P-DAO is propagated to ingress router
   of the Segment, which answers with a DAO-ACK to the Root.

   A Segment Lifetime of 0 in a Via Information option is used to clean
   up the state.  The P-DAO is forwarded as described above, but the DAO
   is interpreted as a No-Path DAO and results in cleaning up existing
   state as opposed to refreshing an existing one or installing a new
   one.

   In case of a forwarding error along an SMPR, the node that fails to
   forward SHOULD send an ICMP error with a code "Error in Projected
   Route" to the Root.  Failure to do so may result in packet loss and
   wasted resources along the Projected Route that is broken.

6.  Security Considerations

   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.

   TODO: should probably consider how P-DAO messages could be abused by
   a) rogue nodes b) via replay of messages c) if use of P-DAO messages
   could in fact deal with any threats?

7.  IANA Considerations

7.1.  New RPL Control Codes

   This document extends the IANA Subregistry created by RFC 6550 for
   RPL Control Codes as indicated in Table 1:

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

                      Table 1: New RPL Control Codes

7.2.  New RPL Control Message Options

   This document extends the IANA Subregistry created by RFC 6550 for
   RPL Control Message Options as indicated in Table 2:

     +=======+======================================+===============+
     | Value | Meaning                              | Reference     |
     +=======+======================================+===============+
     |  0x0B | Via Information option               | This document |
     +-------+--------------------------------------+---------------+
     |  0x0C | Source-Routed Via Information option | This document |
     +-------+--------------------------------------+---------------+
     |  0x0D | Sibling Information option           | This document |
     +-------+--------------------------------------+---------------+

                   Table 2: RPL Control Message Options

7.3.  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 3:

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

                         Table 3: Initial PDR Flags

7.4.  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

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

7.5.  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 4:

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

             Table 4: Acceptance values of the PDR-ACK Status

7.6.  Subregistry for the PDR-ACK Rejection Status Values

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

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

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

   *  Initial allocation is as indicated in Table 5:

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

              Table 5: Rejection values of the PDR-ACK Status

7.7.  SubRegistry for the Route Projection Options Flags

   IANA is requested to create a Subregistry for the 5-bit Route
   Projection Options (RPO) 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].  No bit is
   currently defined for the Route Projection Options (RPO) Flags.

7.8.  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 6:

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    +============+===================================+===============+
    | Bit number | Capability description            | Reference     |
    +============+===================================+===============+
    |     0      | Connectivity is bidirectional (B) | This document |
    +------------+-----------------------------------+---------------+

                        Table 6: Initial SIO Flags

7.9.  Error in Projected Route ICMPv6 Code

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

   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 Projected Route", with a suggested code value of 8, to be
   confirmed by IANA.

8.  Acknowledgments

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

9.  Normative References

   [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>.

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

   [RPL]      Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for

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              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.

   [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>.

10.  Informative References

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

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

   [6TiSCH-ARCHI]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", Work in Progress, Internet-Draft,
              draft-ietf-6tisch-architecture-29, 27 August 2020,
              <https://tools.ietf.org/html/draft-ietf-6tisch-
              architecture-29>.

   [RAW-ARCHI]
              Thubert, P., Papadopoulos, G., and R. Buddenberg,
              "Reliable and Available Wireless Architecture/Framework",
              Work in Progress, Internet-Draft, draft-pthubert-raw-
              architecture-04, 6 July 2020,
              <https://tools.ietf.org/html/draft-pthubert-raw-
              architecture-04>.

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   [TURN-ON_RFC8138]
              Thubert, P. and L. Zhao, "A RPL DODAG Configuration Option
              for the 6LoWPAN Routing Header", Work in Progress,
              Internet-Draft, draft-ietf-roll-turnon-rfc8138-17, 30
              September 2020, <https://tools.ietf.org/html/draft-ietf-
              roll-turnon-rfc8138-17>.

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

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

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

   [USEofRPLinfo]
              Robles, 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", Work in
              Progress, Internet-Draft, draft-ietf-roll-useofrplinfo-41,
              21 September 2020, <https://tools.ietf.org/html/draft-
              ietf-roll-useofrplinfo-41>.

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

Appendix A.  Applications

A.1.  Loose Source Routing

   A RPL implementation operating in a very constrained LLN typically
   uses the Non-Storing Mode of Operation as represented in Figure 9.
   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.

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              ------+---------
                    |          Internet
                    |
                 +-----+
                 |     | 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 9: 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 Root, or then some associated centralized
   computation engine such as a PCE, can determine the amount of packets
   that reach a destination in the RPL domain, and thus the amount of
   energy and bandwidth that is wasted for transmission, between itself
   and the destination, as well as the risk of fragmentation, any
   potential delays because of a paths longer than necessary (shorter
   paths exist that would not traverse the Root).

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

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

A.2.  Transversal Routes

   RPL is optimized for Point-to-Multipoint (P2MP) and Multipoint-to-
   Point (MP2P), whereby routes are always installed along the RPL DODAG
   respectively from and towards the DODAG Root.  Transversal Peer to
   Peer (P2P) routes in a RPL network will generally suffer from some
   elongated (stretched) path versus the best possible path, since
   routing between 2 nodes always happens via a common parent, as
   illustrated in Figure 10:

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

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

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                      ------+---------
                       |          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 10: Routing Stretch between S and D via common parent X

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

                 ------+---------
                       |          Internet
                       |
                    +-----+
                    |     | Border Router
                    |     |  (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 11: Projected Transversal Route

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

Authors' Addresses

   Pascal Thubert (editor)
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   06254 Mougins - Sophia Antipolis
   France

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

   Rahul Arvind Jadhav
   Huawei Tech
   Kundalahalli Village, Whitefield,
   Bangalore 560037
   Karnataka
   India

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

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

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

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