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Versions: (draft-thubert-roll-dao-projection) 00 01

ROLL                                                     P. Thubert, Ed.
Internet-Draft                                              J. Pylakutty
Intended status: Standards Track                                   Cisco
Expires: September 11, 2017                               March 10, 2017


                  Root initiated routing state in RPL
                   draft-ietf-roll-dao-projection-01

Abstract

   This document proposes a protocol extension to RPL that enables to
   install a limited amount of centrally-computed routes in a RPL graph,
   enabling loose source routing down a non-storing mode DODAG, or
   transversal routes inside the DODAG.  As opposed to the classical
   route injection in RPL that are injected by the end devices, this
   draft enables the root of the DODAG to projects the routes that are
   needed on the nodes where they should be installed.

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 http://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 September 11, 2017.

Copyright Notice

   Copyright (c) 2017 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
   (http://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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  New RPL Control Message Options . . . . . . . . . . . . . . .   3
     3.1.  Via Information Option  . . . . . . . . . . . . . . . . .   4
   4.  Projected DAO . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Non-storing Mode Projected DAO  . . . . . . . . . . . . .   6
     4.2.  Storing-Mode Projected DAO  . . . . . . . . . . . . . . .   8
   5.  Applications  . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Loose Source Routing in Non-storing Mode  . . . . . . . .  10
     5.2.  Transversal Routes in storing and non-storing modes . . .  11
   6.  RPL Instances . . . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     10.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  15
     A.1.  Using storing mode P-DAO in non-storing mode MOP  . . . .  16
     A.2.  Projecting a storing-mode transversal route . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The Routing Protocol for Low Power and Lossy Networks (LLN)(RPL)
   [RFC6550] 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 [I-D.ietf-6tisch-architecture] leverages RPL
   for its routing operation and considers the Deterministic Networking
   Architecture [I-D.ietf-detnet-architecture] 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.





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   Based on heuristics of usage, path length, and knowledge of device
   capacity and available resources such as battery levels and
   reservable buffers, a Path Computation Element ([PCE]) with a global
   visibility on the system could install additional P2P routes that are
   more optimized for the current needs as expressed by the objective
   function.

   This draft enables a RPL root, with optionally the assistance of a
   PCE, to install and maintain additional storing and non-storing mode
   routes within the RPL domain, along a selected set of nodes and for a
   selected duration, thus providing routes more suitable than those
   obtained with the distributed operation of RPL.  Those routes may be
   installed in either storing and non-storing modes RPL instances,
   resulting in potentially hybrid situations where the mode of the
   projected routes is different from that of the other routes in the
   instance.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The Terminology used in this document is consistent with and
   incorporates that described in `Terminology in Low power And Lossy
   Networks' [RFC7102] and [RFC6550].

3.  New RPL Control Message Options

   Section 6.7 of [RFC6550] specifies Control Message Options (CMO) to
   be placed in RPL messages such as the Destination Advertisement
   Object (DAO) message.  The RPL Target Option and the Transit
   Information Option (TIO) are such options; the former indicates a
   node to be reached and the latter specifies a parent that can be used
   to reach that node.  Options may be factorized; one or more
   contiguous TIOs apply to the one or more contiguous Target options
   that immediately precede the TIOs in the RPL message.

   This specification introduces a new Control Message Option, the Via
   Information option (VIO).  Like the TIO, the VIO MUST be preceded by
   one or more RPL Target options to which it applies.  Unlike the TIO,
   the VIO are not factorized: multiple contiguous Via options indicate
   an ordered sequence of routers to reach the target(s), presented in
   the order of the packet stream, source to destination, and in which a
   routing state must be installed.

   The Via Information option MUST contain at least one Via Address.




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3.1.  Via Information Option

   The Via Information option MAY be present in DAO messages, and 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Type = 0x0A | Option Length | Path Sequence | Path Lifetime |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .                     Via Address 1                             .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .                              ....                             .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .                     Via Address n                             .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                  Figure 1: Via Information option format

   Option Type:  0x0A (to be confirmed by IANA)

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

   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.





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   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 all one bits
         (0xFF) represents infinity.  A value of all zero bits (0x00)
         indicates a loss of reachability.  A DAO message that contains
         a Via Information option with a Path Lifetime of 0x00 for a
         Target is referred as a No-Path (for that Target) in this
         document.

   Via Address:  16 bytes.  IPv6 Address of the next hop towards the
         destination(s) indicated in the target option that immediately
         precede the VIO.  TBD: See how the /64 prefix can be elided if
         it is the same as that of (all of) the target(s).  In that
         case, the Next-Hop Address could be expressed as the 8-bytes
         suffix only, otherwise it is expressed as 16 bytes, at least in
         storing mode.

4.  Projected DAO

   This draft adds a capability to RPL whereby the root projects a route
   through an extended DAO message called a Projected-DAO (P-DAO) to an
   arbitrary router down the DODAG, indicating a next hop or a sequence
   of routers via which a certain destination indicated in the Target
   Information option may be reached.

   A P-DAO message MUST contain at least a Target Information option and
   at least one VIA Information option following it.

   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 VIO as opposed to a TIO.  Also, a Path Lifetime
   of 0 in a VIO indicates that a route is to be removed.

   There are two kinds of P-DAO, the storing mode and the non-storing
   mode ones.

      The non-storing mode P-DAO discussed in section Section 4.1 has a
      single VIO with one or more Via Addresses in it, the list of Via
      Addresses indicating the source-routed path to the target to be
      installed in the router that receives the message, which replies
      to the root directly with a DAO-ACK message.

      The storing mode P-DAO discussed in section Section 4.2 has at
      least two Via Information options with one Via Address each, for
      the ingress and the egress of the path, and more if there are
      intermediate routers.  The Via Addresses indicate the routers in



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      which the routing state to the target have to be installed via the
      next Via Address in the sequence of 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 path, that
      it can also be neither but it cannot be both.

   The root is expected to use these mechanisms optimally and with
   required parsimony to limit the state installed in the devices to fit
   within their resources, but how the root figures the amount of
   resources that is available in each device is out of scope for this
   document.

   In particular, the draft expects that the root has enough information
   about the capability for each node to store a number of routes, which
   can be discovered for instance using a Network Management System
   (NMS) and/or the RPL routing extensions specified in Routing for Path
   Calculation in LLNs [RFC6551].

   A route that is installed by a P-DAO is not necessarily installed
   along the DODAG, though how the root and the optional PCE obtain the
   additional topological information to compute other routes is out of
   scope for this document

4.1.  Non-storing Mode Projected DAO

   As illustrated in Figure 2, the non-storing mode P-DAO 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 path to any packet for which the
   current destination either is the said target or can be reached via
   the 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.
















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

                 Figure 2: Projecting a non-storing route

   A router that receives a non-storing P-DAO installs a source routed
   path towards each of the consecutive targets via a source route path
   indicated in the following VIO.

   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 order to do so, 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 VIO, 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 the 6LoWPAN Routing Header
   [I-D.ietf-roll-routing-dispatch] specification.  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.





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4.2.  Storing-Mode Projected DAO

   As illustrated in Figure 3, the storing mode P-DAO enables 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                 |     |        | Loose
          o o   o  o   o  o  o o   o          |  ^           | Source
         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             o     o

                          LLN

                       Figure 3: Projecting a route

   Based on available topological, usage and capabilities node
   information, the root or an associated PCE computes which segment
   should be optimized and which relevant state should be installed in
   which nodes.  The algorithm is out of scope but it is envisaged that
   the root could compute the ratio between the optimal path (existing
   path not traversing the root, and the current path), the application
   service level agreement (SLA) for specific flows that could benefit
   from shorter paths, the energy wasted in the network, local
   congestion on various links that would benefit from having flows
   routed along alternate paths.

   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




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   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,
   which 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
   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



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

5.  Applications

5.1.  Loose Source Routing in Non-storing Mode

   A RPL implementation operating in a very constrained LLN typically
   uses the non-storing mode of operation whereby 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.

              ------+---------
                    |          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 4: 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



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   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
   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.

5.2.  Transversal Routes in storing and non-storing modes

   RPL is optimized for Point-to-Multipoint (P2MP), root to leaves and
   Multipoint-to-Point (MP2P) leaves to root operations, whereby routes
   are always installed along the RPL DODAG.  Transversal Peer to Peer
   (P2P) routes in a RPL network will generally suffer from some stretch
   since routing between 2 peers always happens via a common parent, as
   illustrated in Figure 5:

   o  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



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      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.

   o  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.

                      ------+---------
                       |          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 5: 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.











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                 ------+---------
                       |          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 6: 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
   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.

6.  RPL Instances

   It must be noted that RPL has a concept of instance but does not have
   a concept of an administrative distance, which exists in certain
   proprietary implementations to sort out conflicts between multiple
   sources.  This draft conforms the instance model as follows:

   o  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.  When the PCE modifies an existing
      instance then the added routes must not create a loop in that
      instance.  This is achieved by always preferring a route obtained
      from the PCE over a route that is learned via RPL.

   o  If the PCE installs a more specific (Traffic Engineering) route
      between a particular pair of nodes then it should use a Local
      Instance from the ingress node of that path.  Only packets
      associated with that instance will be routed along that path.






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   In all cases, the path is indicated by a new Via Information option,
   and the flow is similar to the flow used to obtain loose source
   routing.

7.  Security Considerations

   This draft uses messages that are already present in [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.

8.  IANA Considerations

   This document updates the IANA registry for the Mode of Operation
   (MOP)

      4: Non-Storing with Projected routes [this]

   This document updates IANA registry for the RPL Control Message
   Options

      0x0A: Via descriptor [this]

9.  Acknowledgments

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

10.  References

10.1.  Normative References

   [I-D.ietf-roll-routing-dispatch]
              Thubert, P., Bormann, C., Toutain, L., and R. Cragie,
              "6LoWPAN Routing Header", draft-ietf-roll-routing-
              dispatch-05 (work in progress), October 2016.

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

   [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,
              <http://www.rfc-editor.org/info/rfc6550>.



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   [RFC6551]  Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
              and D. Barthel, "Routing Metrics Used for Path Calculation
              in Low-Power and Lossy Networks", RFC 6551,
              DOI 10.17487/RFC6551, March 2012,
              <http://www.rfc-editor.org/info/rfc6551>.

   [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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc8025>.

10.2.  Informative References

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-11 (work
              in progress), January 2017.

   [I-D.ietf-detnet-architecture]
              Finn, N. and P. Thubert, "Deterministic Networking
              Architecture", draft-ietf-detnet-architecture-00 (work in
              progress), September 2016.

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

   [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,
              <http://www.rfc-editor.org/info/rfc6997>.

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

Appendix A.  Examples








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A.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 7, 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 7: 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).

   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.




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   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.

A.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)
                    +-----+
                       | Projected 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 8: Projected 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.

   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.






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                 ------+---------
                       |          Internet
                       |
                    +-----+
                    |     | Border Router
                    |     |  (RPL Root)
                    +-----+
                     ^ Projected 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 9: Projected 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 10: Projected Transversal Route

Authors' Addresses











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   Pascal Thubert (editor)
   Cisco Systems
   Village d'Entreprises Green Side
   400, Avenue de Roumanille
   Batiment T3
   Biot - Sophia Antipolis  06410
   FRANCE

   Phone: +33 4 97 23 26 34
   Email: pthubert@cisco.com


   James Pylakutty
   Cisco Systems
   Cessna Business Park
   Kadubeesanahalli
   Marathalli ORR
   Bangalore, Karnataka  560087
   INDIA

   Phone: +91 80 4426 4140
   Email: mundenma@cisco.com





























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