[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-thubert-roll-dao-projection) 00 01 02 03 04 05 06 07 08 09

ROLL                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Updates: 6550 (if approved)                                  R.A. Jadhav
Intended status: Standards Track                             Huawei Tech
Expires: 7 May 2020                                          M. Gillmore
                                                                   Itron
                                                         4 November 2019


                  Root initiated routing state in RPL
                   draft-ietf-roll-dao-projection-08

Abstract

   This document enables 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 source-route header or in terms of path
   length, which impacts both the latency and the packet delivery ratio.
   Projected Routes may be installed in either Storing and Non-Storing
   Modes Instances of the classical RPL operation, resulting in
   potentially hybrid situations where the mode of some Projected Routes
   is different from that of the other routes in the RPL Instance.

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 7 May 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




Thubert, et al.            Expires 7 May 2020                   [Page 1]


Internet-Draft               DAO Projection                November 2019


   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Subset of a 6LoWPAN Glossary  . . . . . . . . . . . . . .   4
     2.3.  Other Terms . . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  References  . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Extending RFC 6550  . . . . . . . . . . . . . . . . . . . . .   6
   4.  Identifying a Path  . . . . . . . . . . . . . . . . . . . . .   7
   5.  New RPL Control Messages and Options  . . . . . . . . . . . .   7
     5.1.  New P-DAO Request Control Message . . . . . . . . . . . .   7
     5.2.  New PDR-ACK Control Message . . . . . . . . . . . . . . .   8
     5.3.  Route Projection Options  . . . . . . . . . . . . . . . .  10
     5.4.  Sibling Information Option  . . . . . . . . . . . . . . .  11
   6.  Projected DAO . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Non-Storing Mode Projected Route  . . . . . . . . . . . .  14
     6.2.  Storing-Mode Projected Route  . . . . . . . . . . . . . .  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  New RPL Control Codes . . . . . . . . . . . . . . . . . .  18
     8.2.  New RPL Control Message Options . . . . . . . . . . . . .  18
     8.3.  New SubRegistry for the Projected DAO Request (PDR)
           Flags . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.4.  New SubRegistry for the PDR-ACK Flags . . . . . . . . . .  19
     8.5.  New Subregistry for the PDR-ACK Acceptance Status
           values  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     8.6.  New Subregistry for the PDR-ACK Rejection Status
           values  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     8.7.  New SubRegistry for the Route Projection Options (RPO)
           Flags . . . . . . . . . . . . . . . . . . . . . . . . . .  20
     8.8.  New SubRegistry for the Sibling Information Option
           (SIO) Flags . . . . . . . . . . . . . . . . . . . . . . .  21
     8.9.  Error in Projected Route ICMPv6 Code  . . . . . . . . . .  21
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  21
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  22
   11. Informative References  . . . . . . . . . . . . . . . . . . .  22
   Appendix A.  Applications . . . . . . . . . . . . . . . . . . . .  23
     A.1.  Loose Source Routing in Non-storing Mode  . . . . . . . .  23



Thubert, et al.            Expires 7 May 2020                   [Page 2]


Internet-Draft               DAO Projection                November 2019


     A.2.  Transversal Routes in storing and non-storing
           modes . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . .  27
     B.1.  Using storing mode P-DAO in non-storing mode MOP  . . . .  27
     B.2.  Projecting a storing-mode transversal route . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   RPL, the "Routing Protocol for Low Power and Lossy Networks"
   [RFC6550] (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.

   The 6TiSCH architecture [6TiSCH-ARCHI] leverages RPL for its routing
   operations and considers the Deterministic Networking Architecture
   [RFC8655] as one possible 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, a PCE with a global visibility on the system can
   compute P2P routes that are more optimized for the current needs as
   expressed by the objective function.

   This draft proposes a protocol extension to RPL that enables the Root
   to install a limited amount of centrally-computed routes in a RPL
   graph, on behalf of a PCE that may be collocated or separated from
   the Root.  Those extensions enable loose source routing down in RPL
   Non-Storing Mode and transversal routes inside the DODAG regardless
   of the RPL Mode of Operation (MOP).

   As opposed to the classical RPL operations where routes are injected
   by the Target nodes, the protocol extension enables 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.  A Projected Route may be a stand-
   alone path to a Target or a segment in a complex Track [6TiSCH-ARCHI]
   that provides redundant forwarding solutions to a destination to



Thubert, et al.            Expires 7 May 2020                   [Page 3]


Internet-Draft               DAO Projection                November 2019


   improve reliability and availability of the wireless transmissions
   [RAW-PS].

   Projected Routes must be used with the parsimony to limit the amount
   of state that is installed in each device to fit within its
   resources, and to limit the amount of rerouted traffic to fit within
   the capabilities of the transmission links.  The method 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.

   In RPL Non-Storing Mode, the Root has enough information to build a
   basic DODAG topology.  This document adds the capability for nodes to
   advertise sibling information in order to improve the topological
   awareness of the Root.  This specification uses the RPL Root as a
   proxy to the PCE.  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 out of scope for this document.

2.  Terminology

2.1.  BCP 14

   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.  Subset of a 6LoWPAN Glossary

   This document often uses the following acronyms:

   6BBR:  6LoWPAN Backbone Router

   6LBR:  6LoWPAN Border Router

   6LN:  6LoWPAN Node

   6LR:  6LoWPAN Router

   DAD:  Duplicate Address Detection

   DODAG:  Destination-Oriented Directed Acyclic Graph

   LLN:  Low-Power and Lossy Network

   NA:  Neighbor Advertisement




Thubert, et al.            Expires 7 May 2020                   [Page 4]


Internet-Draft               DAO Projection                November 2019


   NCE:  Neighbor Cache Entry

   ND:  Neighbor Discovery

   NDP:  Neighbor Discovery Protocol

   NS:  Neighbor Solicitation

   RPL:  IPv6 Routing Protocol for LLNs [RFC6550]

   CMO:  Control Message Option

   DAO:  Destination Advertisement Object

   VIO:  A Via Information Option, used in Storing Mode P-DAO messages.

   SRVIO:  A Source-Routed Via Information Option, used in Non-Storing
      Mode P-DAO messages.

   RPO:  A Route Projection Option; it can be a VIO or an SRVIO.

   P-DAO:  A Projected DAO is a DAO message sent by the RPL Root to
      install a Projected Route.

   RTO:  RPL Target Option

   RAN:  RPL-Aware Node

   RA:  Router Advertisement

   RS:  Router Solicitation


2.3.  Other Terms


   Projected Route:  A Projected Route is a serial path that is computed
      and installed remotely by a RPL Root.

   Track:  The term Track is used in this document to refer to a complex
      path, e.g., a DODAG, that incorporates redundant Projected Routes
      towards a destination for increased reliability, high availability
      and load balancing.

2.4.  References

   In this document, readers will encounter terms and concepts that are
   discussed in the following documents:



Thubert, et al.            Expires 7 May 2020                   [Page 5]


Internet-Draft               DAO Projection                November 2019


   *  "Routing Protocol for Low Power and Lossy Networks" [RFC6550], and

   *  "Terminology in Low power And Lossy Networks" [RFC7102].


3.  Extending RFC 6550

   This specification introduces two new RPL Control Messages to enable
   a RPL Aware Node (RAN) to request the establisment of a path from
   self to a Target.  A RAN may request the installation of a path 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 with a
   completion status once it is done installing the path.  See
   Section 5.1 for more.

   Section 6.7 of [RFC6550] specifies RPL Control Message Options (CMO)
   to be placed in RPL messages such as the Destination Advertisement
   Object (DAO) message.  The RPL Target Option (RTO) and the Transit
   Information Option (TIO) are such options.  In Non-Storing Mode, the
   TIO option is used in the DAO message to indicate a parent within a
   DODAG.  The TIO applies to the RTOs that immedially preceed it in the
   message.  Options may be factorized; multiple TIOs may be present to
   indicate multiple routes to the one or more contiguous addresses
   indicated in the RTOs that immediately precede the TIOs in the RPL
   message.

   This specification introduces two new CMOs referred to as Route
   Projection Options (RPO) to install Projected Routes.  One RPO is the
   Via Information Option (VIO) and the other is the Source-Routed VIO
   (SRVIO).  The VIO installs a route on each hop along a Projected
   Route (in a fashion analogous to RPL Storing Mode) whereas the SRVIO
   installs a source-routing state at the ingress node, which uses that
   state to encapsulate a packet with an IPv6 Routing Header in a
   fashion similar to RPL Non-Storing Mode.  Like the TIO, the RPOs MUST
   be preceded by exactly one RTO to which they apply, and they can be
   factorized: multiple contiguous RPOs indicate alternate paths to the
   Target, more in Section 5.3.

   This specification also introduces a new CMO to enable a RAN to
   advertise (some of) its siblings to the Root, using a new Sibling
   Information Option (SIO) as specified in Section 5.4.










Thubert, et al.            Expires 7 May 2020                   [Page 6]


Internet-Draft               DAO Projection                November 2019


4.  Identifying a Path

   It must be noted that RPL has a concept of Instance to represent
   different routing topologies 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 conforms
   the Instance model as follows:

   *  If the PCE needs to influence a particular Instance to add better
      routes in conformance with the routing objectives in that
      Instance, it may do so as long as it does not create a loop.  A
      Projected Route is always preferred over a route that is learned
      via RPL.

   *  A PCE that installs a more specific (say, Traffic Engineered) and
      possibly complex path (aka a Track) towards a particular Target
      MUST use a Local RPL Instance (see section 5 of [RFC6550])
      associated to that Target to identify the path.  We refer to that
      Local RPLInstanceID as TrackID.  A projected path is uniquely
      identified within the RPL domain by the tuple (Target address,
      TrackID).  When packet is placed on a Track, a RPL Packet
      Information (RPI) is added with the TrackID as RPLInstanceID.  The
      RPLInstanceID has the 'D' flag set, indicating that the
      destination address in the IPv6 header is the Target that is used
      to identify the Track.

   *  A packet that is routed over a projected path MUST NOT be placed
      over a different RPL Instance again.  A packet that is placed on a
      Global Instance MAY be injected in a Local Instance based on a
      network policy and the Local Instance configuration.

   A Projected Route is a serial path that may the whole path or a
   segment in a complex Track, in which case multiple Projected Routes
   are installed with the stuple (Target address, TrackID), and a node
   that is present on more than one segment in a Track may be able to
   use either of the Projected Routes to forward towards the Target.
   The selection of the best route in a Track at forwarding time is out
   of scope for this document.  [RAW-PS] elaborates on that particular
   problem.

5.  New RPL Control Messages and Options

5.1.  New P-DAO Request Control Message

   The PDR is sent to the Root to request a new Path.  Exactly one
   Target Options MUST be present.




Thubert, et al.            Expires 7 May 2020                   [Page 7]


Internet-Draft               DAO Projection                November 2019


   The format of P-DAO Request (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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          | RPLInstanceID |K|R|   Flags   |  PDRLifetime  | PDRSequence   |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |   Option(s)...
          +-+-+-+-+-+-+-+-+


                     Figure 1: New P-DAO Request Format

   TrackID:  8-bit field indicating the topology Instance 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 indicate that the Requested path should be
      redundant.

   PDRLifetime:  8-bit unsigned integer.  The requested lifetime for the
      Track expressed in Lifetime Units (obtained from the 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.  The PDRSequence obeys
      the operation in section 7.2 of [RFC6550].  It is incremented at
      each PDR message and echoed in the PDR-ACK by the Root.  The
      PDRSequence is used to correlate a PDR-ACK message with the PDR
      message that triggeted it.

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.  Its format is as follows:











Thubert, et al.            Expires 7 May 2020                   [Page 8]


Internet-Draft               DAO Projection                November 2019


         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    | PDR-ACK Status|     Flags     | Track Lifetime|
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |  PDRSequence  |                Reserved                       |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |  Option(s)...
        +-+-+-+-+-+-+-+

                Figure 2: New PDR-ACK Control Message Format

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

   PDR-ACK Status:  Indicates the completion.  Substructured as
      indicated in Figure 3.

   Track Lifetime:  Indicates that remaining Lifetime for the Track, 0
      if 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.

   The PDR-ACK Status is further substructured as follows:

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

                      Figure 3: PDR-ACK status Format

   The PDR-ACK Status subfields are:

   E:  1-bit flag.  Set to indicate a rejection.  When not set, a 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 depedning on the
      setting of the 'E' flag as indicated respectively in Table 4 and
      Table 5.





Thubert, et al.            Expires 7 May 2020                   [Page 9]


Internet-Draft               DAO Projection                November 2019


5.3.  Route Projection Options

   The RPOs indicate a series of IPv6 addresses that can be compressed
   using the method defined in the "6LoWPAN Routing Header" [RFC8138]
   specification using the address of the Root found in the DODAGID
   field of DIO messages as Compression Reference.

   An RPO indicates a Projected Route that can be a serial Track in full
   or a segment of a more complex Track.  The Track is identified by a
   RPLInstanceID that is either Global or local to the Target of the
   Track.

   The format of 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 |Comp.|  Flags  |    TrackID    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Path Lifetime | Path Sequence |     Reserved                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .                     Via Address 1                             .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .                              ....                             .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .                     Via Address n                             .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                  Figure 4: Via Information option format

   Option Type:  0x0A for VIO, 0x0B for SRVIO (to be confirmed by IANA)





Thubert, et al.            Expires 7 May 2020                  [Page 10]


Internet-Draft               DAO Projection                November 2019


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

   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 all the Via Addresses.

   TrackID:  8-bit field indicating the topology Instance associated
      with the Track.

   Path Lifetime:  8-bit unsigned integer.  The length of time in
      Lifetime Units (obtained from the Configuration option) that the
      prefix is valid for route determination.  The period starts when a
      new Path Sequence is seen.  A value of 255 (0xFF) represents
      infinity.  A value of zero (0x00) indicates a loss of
      reachability.  A DAO message that contains a Via Information
      option with a Path Lifetime of zero for a Target is referred as a
      No-Path (for that Target) in this document.

   Path Sequence:  8-bit unsigned integer.  When a RPL Target option is
      issued by the Root of the DODAG (i.e. in a DAO message), that Root
      sets the Path Sequence and increments the Path Sequence each time
      it issues a RPL Target option with updated information.  The
      indicated sequence deprecates any state for a given Target that
      was learned from a previous sequence and adds to any state that
      was learned for that sequence.

   Via Address:  2 to 16 bytes, a compressed IPv6 Address.  A Via
      Address indicates the next hop within the path towards the
      destination(s) that is indicated in the Target option that
      immediately precede the RPO in the DAO message.  Via Addresses are
      indicated in the order of the path from the ingress to the egress
      nodes.  All Via addresses are expressed in the same size as
      indicated by the Compression Type.

   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.

5.4.  Sibling Information Option

   The Sibling Information Option (SIO) provides indication on siblings
   that could be used by the Root to form Projected Routes.  The format
   of SIOs is as follows:








Thubert, et al.            Expires 7 May 2020                  [Page 11]


Internet-Draft               DAO Projection                November 2019


        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| Flags |    Opaque     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Step of Rank       |       Reserved                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .                     Sibling Address                           .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                Figure 5: Sibling Information Option Format

   Option Type:  0x0C (to be confirmed by IANA)

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

   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.

   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.

   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 industraial 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
      [RFC6550] as computed by the Objective Function between this node
      and the sibling.

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



Thubert, et al.            Expires 7 May 2020                  [Page 12]


Internet-Draft               DAO Projection                November 2019


   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.

6.  Projected DAO

   This draft adds a capability to RPL whereby the Root of a DODAG
   projects a route by sending an extended DAO message called a
   Projected-DAO (P-DAO) to an arbitrary router in the DODAG, indicating
   one or more sequence(s) of routers inside the DODAG via which the
   Target(s) indicated in the RPL Target Option(s) (RTO) can be reached.

   A P-DAO is sent from a global address of the Root to a global address
   of the recipient, and MUST be confirmed by a DAO-ACK, which is sent
   back to a global address of the Root.

   A P-DAO message MUST contain at least one RTO and at least one RPO
   following it.  There can be at most one such sequence of RTOs and
   then RPOs.

   Like a classical DAO message, a P-DAO is processed only if it is
   "new" per section 9.2.2.  "Generation of DAO Messages" of the RPL
   specification [RFC6550]; this is determined using the Path Sequence
   information from the RPO as opposed to a TIO.  Also, a Path Lifetime
   of 0 in an RPO indicates that a route 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 6.1.  It uses an
      SRVIO that carries a list of Via Addresses to be used as a source-
      routed path to the Target.  The recipient of the P-DAO is the
      ingress router of the source-routed path.  Upon a Non-Storing Mode
      P-DAO, the ingress router installs a source-routed state to the
      Target and replies to the Root directly with a DAO-ACK message.

   *  The Storing Mode is discussed in Section 6.2.  It uses a VIO with
      one Via Address per consecutive hop, from the ingress to the
      egress of the path, including the list of all intermediate routers
      in the data path order.  The Via Addresses indicate the routers in
      which the routing state to the Target have to be installed 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



Thubert, et al.            Expires 7 May 2020                  [Page 13]


Internet-Draft               DAO Projection                November 2019


      to the Root with a DAO-ACK message.  Note that the Root may be the
      ingress and it may be the egress of the path, that it can also be
      neither but it cannot be both.


   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 8.9).  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 routing header if
   one is present, and the routing header 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 outter
   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 6.1 and Section 6.2 respectively.

6.1.  Non-Storing Mode Projected Route

   As illustrated in Figure 6, a P-DAO that carries an SRVIO enables the
   Root to install a source-routed path towards a Target in any
   particular router; with this path information the router can add a
   source routed header reflecting the Projected Route to any packet for
   which the current destination either is the said Target or can be
   reached via the Target.

              ------+---------
                    |          Internet
                    |
                 +-----+
                 |     | Border Router
                 |     |  (RPL Root)
                 +-----+                   |  P  ^            |
                    |                      | DAO | ACK        | Loose
              o    o   o    o     router   V     |            | Source
          o o   o  o   o  o  o      o  o            | P-DAO   . Route
         o  o o  o o    o   o   o  o  o             | Source  . Path
         o   o    o  o     o  o    o  o  o          | Route   . From
        o  o   o  o   o         o   o o             | Path    . Root
           o  o  o  o             o    Target       V         . To
          o       o               o    o                      | Desti-
        o          o             o     o                      | nation
                                      destination             V

                          LLN



Thubert, et al.            Expires 7 May 2020                  [Page 14]


Internet-Draft               DAO Projection                November 2019


                  Figure 6: 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.  In
   order to avoid those loops, if the router that installs a Projected
   Route does not have a connected route (a direct adjacency) to the
   next soure routed hop and fails to locate it as a neighbor or a
   neighbor of a neighbor, then it MUST ensure that it has another
   Projected Route to the next loose hop under the control of the same
   route computation system, otherwise the P-DAO is rejected.

   When forwarding a packet to a destination for which the router
   determines that routing happens via the Target, the router inserts
   the source routing header in the packet to reach the Target.  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 s
   forwarded to that neighbor, or a source-routed path to the loose next
   hop; in the latter case, another encapsulation takes place and the
   process possibly recurses; otherwise the packet is dropped.

   In order to add a source-routing header, the router encapsulates the
   packet with an IP-in-IP header and a non-storing mode source routing
   header (SRH) [RFC6554].  In the uncompressed form the source of the
   packet would be self, the destination would be the first Via Address
   in the SRVIO, and the SRH would contain the list of the remaining Via
   Addresses and then the Target.

   In practice, the router will normally use the "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 router indicates self as encapsulator in an IP-in-IP 6LoRH
   Header, and places the list of Via Addresses in the order of the VIO
   and then the Target in the SRH 6LoRH Header.

   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 [RFC6550].  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.






Thubert, et al.            Expires 7 May 2020                  [Page 15]


Internet-Draft               DAO Projection                November 2019


6.2.  Storing-Mode Projected Route

   As illustrated in Figure 7, the Storing Mode route projection is used
   by the Root to install a routing state towards a Target in the
   routers along a segment between an ingress and an egress router; this
   enables the routers to forward along that segment any packet for
   which the next loose hop is the said Target, for Instance a loose
   source routed packet for which the next loose hop is the Target, or a
   packet for which the router has a routing state to the final
   destination via the Target.

              ------+---------
                    |          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 7: Projecting a route

   In order to install the relevant routing state along the segment
   between an ingress and an egress routers, the Root sends a unicast
   P-DAO message to the egress router of the routing segment that must
   be installed.  The P-DAO message contains the ordered list of hops
   along the segment as a direct sequence of Via Information options
   that are preceded by one or more RPL Target options to which they
   relate.  Each Via Information option contains a Path 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 Via Address in
   the last VIO.  This is how the egress recognizes its role.  In a
   similar fashion, the ingress node recognizes its role as it matches
   Via Address in the first 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



Thubert, et al.            Expires 7 May 2020                  [Page 16]


Internet-Draft               DAO Projection                November 2019


   already able to route to the Target(s) on its own.  It may either be
   the Target, or may have some existing information to reach the
   Target(s), such as a connected route or an already installed
   Projected Route.  If one of the Targets cannot be located, 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.

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

   Upon receiving a propagated DAO, an intermediate router as well as
   the ingress router install a route towards the DAO Target(s) via its
   successor in the P-DAO; the router locates the VIO that contains its
   address, and uses as next hop the address found in the Via Address
   field in the following VIO.  The router MAY install additional routes
   towards the addresses that are located in VIOs that are after the
   next one, if any, but in case of a conflict or a lack of resource, a
   route to a Target installed by the Root has precedence.

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

   Also, the path indicated in a P-DAO may be loose, in which case the
   reachability to the next hop has to be asserted.  Each router along
   the path indicated in a P-DAO is expected to be able to reach its
   successor, either with a connected route (direct neighbor), or by
   routing, for Instance following a route installed previously by a DAO
   or a P-DAO message.  If that route is not connected then a recursive
   lookup may take place at packet forwarding time to find the next hop
   to reach the Target(s).  If it does not and cannot reach the next
   router in the P-DAO, 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).

   A Path 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




Thubert, et al.            Expires 7 May 2020                  [Page 17]


Internet-Draft               DAO Projection                November 2019


   state as opposed to refreshing an existing one or installing a new
   one.

   In case of a forwarding error along a Storing Mode Projected Route,
   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.

7.  Security Considerations

   This draft uses messages that are already present in RPL [RFC6550]
   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?

8.  IANA Considerations

8.1.  New RPL Control Codes

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

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

                      Table 1: New RPL Control Codes

8.2.  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:










Thubert, et al.            Expires 7 May 2020                  [Page 18]


Internet-Draft               DAO Projection                November 2019


     +-------+--------------------------------------+---------------+
     | 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

8.3.  New SubRegistry for the Projected DAO Request (PDR) 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:

          +------------+------------------------+---------------+
          | 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

8.4.  New 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 7 May 2020                  [Page 19]


Internet-Draft               DAO Projection                November 2019


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

8.5.  New Subregistry for the PDR-ACK Acceptance Status values

   IANA is requested to create a new 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

8.6.  New Subregistry for the PDR-ACK Rejection Status values

   IANA is requested to create a new 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 5:

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

              Table 5: Rejection values of the PDR-ACK Status

8.7.  New SubRegistry for the Route Projection Options (RPO) Flags

   IANA is requested to create a new 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)



Thubert, et al.            Expires 7 May 2020                  [Page 20]


Internet-Draft               DAO Projection                November 2019


   *  Capability description

   *  Reference

   Registration procedure is "Standards Action" [RFC8126].  No bit is
   currently defined for the Route Projection Options (RPO) Flags.

8.8.  New SubRegistry for the Sibling Information Option (SIO) 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:

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

                        Table 6: Initial SIO Flags

8.9.  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.

9.  Acknowledgments

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



Thubert, et al.            Expires 7 May 2020                  [Page 21]


Internet-Draft               DAO Projection                November 2019


10.  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>.

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

   [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>.

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

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

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

   [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>.

11.  Informative References





Thubert, et al.            Expires 7 May 2020                  [Page 22]


Internet-Draft               DAO Projection                November 2019


   [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-28, 29 October 2019,
              <https://tools.ietf.org/html/draft-ietf-6tisch-
              architecture-28>.

   [RAW-PS]   Thubert, P. and G. Papadopoulos, "Reliable and Available
              Wireless Problem Statement", Work in Progress, Internet-
              Draft, draft-pthubert-raw-problem-statement-04, 23 October
              2019, <https://tools.ietf.org/html/draft-pthubert-raw-
              problem-statement-04>.

   [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>.

   [PCE]      IETF, "Path Computation Element", November 2019,
              <https://datatracker.ietf.org/doc/charter-ietf-pce/>.

Appendix A.  Applications

A.1.  Loose Source Routing in Non-storing Mode

   A RPL implementation operating in a very constrained LLN typically
   uses the Non-Storing Mode of Operation as represented in Figure 8.
   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 7 May 2020                  [Page 23]


Internet-Draft               DAO Projection                November 2019


              ------+---------
                    |          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 8: 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).

   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




Thubert, et al.            Expires 7 May 2020                  [Page 24]


Internet-Draft               DAO Projection                November 2019


   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 source-routed or 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 in storing and non-storing modes

   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 9:

   *  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 a
      Routing Header 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.













Thubert, et al.            Expires 7 May 2020                  [Page 25]


Internet-Draft               DAO Projection                November 2019


                      ------+---------
                       |          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 9: 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.
   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 10: Projected Transversal Route

   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



Thubert, et al.            Expires 7 May 2020                  [Page 26]


Internet-Draft               DAO Projection                November 2019


   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.

Appendix B.  Examples

B.1.  Using storing mode P-DAO in non-storing mode MOP

   In non-storing mode, the DAG Root maintains the knowledge of the
   whole DODAG topology, so when both the source and the destination of
   a packet are in the DODAG, the Root can determine the common parent
   that would have been used in storing mode, and thus the list of nodes
   in the path between the common parent and the destination.  For
   Instance in the diagram shown in Figure 11, if the source is node 41
   and the destination is node 52, then the common parent is node 22.

              ------+---------
                    |          Internet
                    |
                 +-----+
                 |     | Border Router
                 |     |  (RPL Root)
                 +-----+
                  | \  \____
                 /   \       \
               o 11   o 12     o  13
              /       |       /  \
            o 22      o 23   o 24  o 25
           /  \       | \      \
         o 31   o 32  o   o     o 35
        /      /      |    \    |    \
       o 41   o 42    o     o   o 45   o 46
       |      |       |     |    \     |
       o 51   o 52    o 53  o     o 55 o 56

                          LLN

          Figure 11: Example DODAG forming a logical tree topology

   With this draft, the Root can install a storing mode routing states
   along a segment that is either from itself to the destination, or
   from one or more common parents for a particular source/destination
   pair towards that destination (in this particular example, this would
   be the segment made of nodes 22, 32, 42).





Thubert, et al.            Expires 7 May 2020                  [Page 27]


Internet-Draft               DAO Projection                November 2019


   In the example below, say that there is a lot of traffic to nodes 55
   and 56 and the Root decides to reduce the size of routing headers to
   those destinations.  The Root can first send a DAO to node 45
   indicating Target 55 and a Via segment (35, 45), as well as another
   DAO to node 46 indicating Target 56 and a Via segment (35, 46).  This
   will save one entry in the routing header on both sides.  The Root
   may then send a DAO to node 35 indicating Targets 55 and 56 a Via
   segment (13, 24, 35) to fully optimize that path.

   Alternatively, the Root may send a DAO to node 45 indicating Target
   55 and a Via segment (13, 24, 35, 45) and then a DAO to node 46
   indicating Target 56 and a Via segment (13, 24, 35, 46), indicating
   the same DAO Sequence.

B.2.  Projecting a storing-mode transversal route

   In this example, say that a PCE determines that a path must be
   installed between node S and node D via routers A, B and C, in order
   to serve the needs of a particular application.

   The Root sends a P-DAO with a Target option indicating the
   destination D and a sequence Via Information option, one for S, which
   is the ingress router of the segment, one for A and then for B, which
   are an intermediate routers, and one for C, which is the egress
   router.

                 ------+---------
                       |          Internet
                       |
                    +-----+
                    |     | Border Router
                    |     |  (RPL Root)
                    +-----+
                       | P-DAO message to C
                 o    |   o    o
             o o   o |    o  o  o o   o
            o  o o  | o    o   o   o  o  o
            o   o   V  o     o  o    o  o  o
           S  A  B  C   D         o   o o
           o          o             o     o
                             LLN

                         Figure 12: P-DAO from Root

   Upon reception of the P-DAO, C validates that it can reach D, e.g.
   using IPv6 Neighbor Discovery, and if so, propagates the P-DAO
   unchanged to B.




Thubert, et al.            Expires 7 May 2020                  [Page 28]


Internet-Draft               DAO Projection                November 2019


   B checks that it can reach C and of so, installs a route towards D
   via C.  Then it propagates the P-DAO to A.

   The process recurses till the P-DAO reaches S, the ingress of the
   segment, which installs a route to D via A and sends a DAO-ACK to the
   Root.

                 ------+---------
                       |          Internet
                       |
                    +-----+
                    |     | Border Router
                    |     |  (RPL Root)
                    +-----+
                     ^ P-DAO-ACK from S
                 /    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 13: P-DAO-ACK to Root

   As a result, a transversal route is installed that does not need to
   follow the DODAG structure.

                 ------+---------
                       |          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 14: Projected Transversal Route

Authors' Addresses




Thubert, et al.            Expires 7 May 2020                  [Page 29]


Internet-Draft               DAO Projection                November 2019


   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 7 May 2020                  [Page 30]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/