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Versions: (draft-thubert-roll-dao-projection) 00 01 02 03 04

ROLL                                                     P. Thubert, Ed.
Internet-Draft                                                     Cisco
Intended status: Standards Track                               R. Jadhav
Expires: December 21, 2018                                   Huawei Tech
                                                            J. Pylakutty
                                                                   Cisco
                                                           June 19, 2018


                  Root initiated routing state in RPL
                   draft-ietf-roll-dao-projection-04

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 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 December 21, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  References  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Subset of a 6LoWPAN Glossary  . . . . . . . . . . . . . .   4
     2.4.  New Terms . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Extending RFC 6550  . . . . . . . . . . . . . . . . . . . . .   5
   4.  New RPL Control Message Options . . . . . . . . . . . . . . .   5
   5.  Projected DAO . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Non-storing Mode Projected Route  . . . . . . . . . . . .   8
     5.2.  Storing-Mode Projected Route  . . . . . . . . . . . . . .   9
   6.  Applications  . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Loose Source Routing in Non-storing Mode  . . . . . . . .  11
     6.2.  Transversal Routes in storing and non-storing modes . . .  13
   7.  RPL Instances . . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     11.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  18
     A.1.  Using storing mode P-DAO in non-storing mode MOP  . . . .  18
     A.2.  Projecting a storing-mode transversal route . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   The "Routing Protocol for Low Power and Lossy Networks" [RFC6550]
   (LLN)(RPL) 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



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   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.

   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 to install and maintain projected
   routes (P-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 than those
   obtained with the 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.  P-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 P-routes is different from that of the other
   routes in the RPL Instance.

   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 algorithm used to
   compute the paths and the protocol used to learn the topology of the
   network and the resources that are available in devices and in the
   network are out of scope for this document.  Possibly with the
   assistance of a Path Computation Element ([PCE]) that could have a
   better visibility on the larger system, the root computes which
   segment could be optimized and uses this draft to install the
   corresponding projected routes.

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.





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2.2.  References

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

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

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

2.3.  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

   6CIO: Capability Indication Option

   EARO: (Extended) Address Registration Option -- (E)ARO

   EDAR: (Extended) Duplicate Address Request -- (E)DAR

   EDAC: (Extended) Duplicate Address Confirmation -- (E)DAC

   DAD:  Duplicate Address Detection

   DODAG:  Destination-Oriented Directed Acyclic Graph

   LLN:  Low-Power and Lossy Network

   NA:   Neighbor Advertisement

   NCE:  Neighbor Cache Entry

   ND:   Neighbor Discovery

   NDP:  Neighbor Discovery Protocol

   NS:   Neighbor Solicitation

   RPL:  IPv6 Routing Protocol for LLNs (pronounced ripple) [RFC6550]

   RA:   Router Advertisement



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   RS:   Router Solicitation

2.4.  New Terms

   Projected Route:  A route that is installed remotely by a RPL root.

3.  Extending RFC 6550

   Section 6.7 of RPL [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 2 new Control Message Options referred
   to as Route Projection Options (RPO).  One RPO is the 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 it to insert a routing
   header in a fashion similar to Non-Storing Mode.

   Like the TIO, the RPOs MUST be preceded by one or more RPL Target
   Options to which they apply, and they can be factorized: multiple
   contiguous RPOs indicate alternate paths to the target(s).

4.  New RPL Control Message Options

   The format of RPOs is as follows:



















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        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 | Path Sequence | Path Lifetime |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .                     Via Address 1                             .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .                              ....                             .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .                     Via Address n                             .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                  Figure 1: Via Information option format

   Option Type:  0x0A for VIO, 0x0B for SRVIO (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.

   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



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         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 RPO.  Via Addresses are indicated in the order of
         the data path from the ingress to the egress nodes.  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.

   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.  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 Target Information Option(s) (TIO) 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 TIO and at least one RPO
   following it.  There can be at most one such sequence of TIOs 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 Section 5.1.  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.



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      The Storing Mode is discussed in section Section 5.2.  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 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.

5.1.  Non-storing Mode Projected Route

   As illustrated in Figure 2, 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 P-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

                 Figure 2: 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 P-route



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   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 the "6LoWPAN Routing
   Header" [RFC8138] 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.

5.2.  Storing-Mode Projected Route

   As illustrated in Figure 3, the Storing Mode projected iq 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.









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


                       Figure 3: 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
   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



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

6.  Applications

6.1.  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 4.
   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



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   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
   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



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   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.

6.2.  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
      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.












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

       Figure 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.

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



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   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.

7.  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 of routing information.  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 (say, Traffic Engineered)
      route between a particular pair of nodes then it SHOULD use a
      Local Instance from the ingress node of that path.  A packet
      associated with that instance will be routed along that path and
      MUST NOT be placed over a Global Instance again.  A packet that is
      placed on a Global Instance may be injected in the Local Instance
      based on node policy and the Local Instance paramenters.

   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.

8.  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.

9.  IANA Considerations

   This document extends the IANA registry created by RFC 6550 for RPL
   Control Codes as follows:







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               +------+-------------------+---------------+
               | Code | Description       | Reference     |
               +------+-------------------+---------------+
               | 0x0A | Via               | This document |
               |      |                   |               |
               | 0x0B | Source-Routed Via | This document |
               +------+-------------------+---------------+

                             RPL Control Codes

   This document is updating the registry created by RFC 6550 for the
   RPL 3-bit Mode of Operation (MOP) as follows:

   +----------+------------------------------------------+-------------+
   |   MOP    | Description                              | Reference   |
   |  value   |                                          |             |
   +----------+------------------------------------------+-------------+
   |    5     | Non-Storing mode of operation with       | This        |
   |          | Projected routes                         | document    |
   |          |                                          |             |
   |    6     | Storing mode of operation with Projected | This        |
   |          | routes                                   | document    |
   +----------+------------------------------------------+-------------+

                           DIO Mode of operation

10.  Acknowledgments

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

11.  References

11.1.  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>.

   [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>.





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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,
              <https://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,
              <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>.

11.2.  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-14 (work
              in progress), April 2018.

   [I-D.ietf-detnet-architecture]
              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", draft-ietf-
              detnet-architecture-05 (work in progress), May 2018.

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





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   [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>.

Appendix A.  Examples

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



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   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.

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.




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   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)
                    +-----+
                     ^ 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


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

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