ROLL                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                               R. Jadhav
Expires: December 21, 2018 June 24, 2019                                       Huawei Tech
                                                             M. Gillmore
                                                                   Itron
                                                            J. Pylakutty
                                                                   Cisco
                                                           June 19,
                                                       December 21, 2018

                  Root initiated routing state in RPL
                   draft-ietf-roll-dao-projection-04
                   draft-ietf-roll-dao-projection-05

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
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   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. June 24, 2019.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  References  Subset of a 6LoWPAN Glossary  . . . . . . . . . . . . . .   4
     2.3.  New Terms . . . . . . . . . .   4
     2.3.  Subset of a 6LoWPAN Glossary . . . . . . . . . . . . . .   4
     2.4.  New Terms .  References  . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Extending RFC 6550  . . . . . . . . . . . . . . . . . . . . .   5
   4.
     3.1.  RPL Instances . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  New RPL Control Message Options . . . . . . . . . . . . . . .   5
   5.   6
     3.3.  Projected DAO . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Non-storing
       3.3.1.  Non-Storing Mode Projected Route P-Route  . . . . . . . . . . . . . .   8
     5.2.
       3.3.2.  Storing-Mode Projected Route P-Route  . . . . . . . . . . . . . .   9
   6.  Applications . .  10
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   5.  IANA Considerations . . .  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 . . . . . . .  12
     5.1.  New RPL Control Codes . . . . . . . . . . . . . . . . .  15
   8.  Security Considerations .  12
     5.2.  Error in Projected Route ICMPv6 Code  . . . . . . . . . .  13
   6.  Acknowledgments . . . . . . . .  15
   9.  IANA Considerations . . . . . . . . . . . . . . .  13
   7.  References  . . . . . .  15
   10. Acknowledgments . . . . . . . . . . . . . . . . . . .  13
     7.1.  Normative References  . . . .  16
   11. References . . . . . . . . . . . . . .  14
     7.2.  Informative References  . . . . . . . . . . .  16
     11.1.  Normative References . . . . . .  15
   Appendix A.  Applications . . . . . . . . . . . . .  16
     11.2.  Informative References . . . . . . .  15
     A.1.  Loose Source Routing in Non-storing Mode  . . . . . . . .  15
     A.2.  Transversal Routes in storing and non-storing modes . . .  17
   Appendix A. B.  Examples . . . . . . . . . . . . . . . . . . . . . .  18
     A.1.  19
     B.1.  Using storing mode P-DAO in non-storing mode MOP  . . . .  18
     A.2.  19
     B.2.  Projecting a storing-mode transversal route . . . . . . .  19  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20  22

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
   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) (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

   P-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. P-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.

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

   RS:   Router Solicitation

2.4.

2.3.  New Terms

   Projected Route:

   P-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

2.4.  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].

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; options.  In Non-Storing Mode, the former indicates a
   node
   TIO option is used in the DAO message to be reached and indicate the latter specifies a immediate
   parent that can be used of a given path.  The TIO applies to reach the Target options that node.
   immedially preceed it.  Options may be factorized; one or more
   contiguous multiple TIOs apply may
   be present to indicate multiple routes to the one or more contiguous
   addressed indicated in the Target options Options that immediately precede
   the TIOs in the RPL message.

   This specification introduces 2 two 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 P-Route (in a
   fashion analogous to RPL Storing Mode) whereas the SRVIO installs a source-
   routing
   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

3.1.  RPL Control Message Options

   The format Instances

   It must be noted that RPL has a concept of RPOs is 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:

        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

   o  If the number of Via
         Addresses.

   Path Sequence:  8-bit unsigned integer.  When PCE needs to influence a RPL Target option is
         issued by the root of particular instance to add better
      routes in conformance with the DODAG (i.e. routing objectives in a DAO message), that
         root sets
      instance, it may do so.  When the Path Sequence and increments PCE modifies an existing
      instance then the Path Sequence
         each time it issues a RPL Target option with updated
         information.  The indicated sequence deprecates any state for added routes must not create a
         given Target loop in that was learned from
      instance.  This is achieved by always preferring 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 route obtained
      from the Configuration option) PCE over a route that
         the prefix is valid for learned via RPL.

   o  If the PCE installs a more specific (say, Traffic Engineered)
      route determination.  The period starts
         when between a new Path Sequence is seen.  A value of all one bits
         (0xFF) represents infinity.  A value particular pair of all zero bits (0x00)
         indicates nodes then it SHOULD use a loss
      Local Instance from the ingress node of reachability.  A DAO message that contains
         a Via Information option path.  A packet
      associated 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 instance will be elided if it is the same as that of
         (all of) the target(s).  In routed along that case, the Next-Hop Address
         could be expressed as the 8-bytes suffix only.

   An RPO MUST contain at least one Via Address, path 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 placed over a route by sending an extended DAO message called Global Instance again.  A packet that is
      placed on a
   Projected-DAO (P-DAO) to an arbitrary router Global Instance may be injected in the DODAG, indicating
   one or more sequence(s) of routers inside the DODAG via which Local Instance
      based on node policy and the
   target(s) indicated in Local Instance paramenters.

   In all cases, the Target Information Option(s) (TIO) can be
   reached.

   A P-DAO path is sent from a global address of the root to a global address
   of the recipient, and MUST be confirmed indicated by a DAO-ACK, which new Via Information option,
   and the flow is sent
   back similar 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 flow used to obtain loose source
   routing.

3.2.  New RPL Control Message Options

   The format 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 RPOs is determined using the 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 | 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 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .                     Via Address 1                             .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .                              ....                             .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       .                                                               .
       .                     Via Address n                             .
       .                                                               .
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 1: Via Information option format

   Option Type:  0x0A for VIO, 0x0B 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 (to be used as a
      source-routed path to confirmed by IANA)
   Option Length:  In bytes; variable, depending on the target.  The recipient number of the P-DAO Via
         Addresses.

   Path Sequence:  8-bit unsigned integer.  When a RPL Target option is
         issued by the ingress router root of the source-routed path.  Upon DODAG (i.e. in a Non-Storing
      Mode P-DAO, DAO message), that
         root sets the ingress router installs a source-routed state to
      the target Path Sequence and replies to increments the root directly with Path Sequence
         each time it issues a DAO-ACK
      message. RPL Target option with updated
         information.  The Storing Mode is discussed in section Section 5.2.  It uses indicated sequence deprecates any state for a
      VIO with one Via Address per consecutive hop,
         given Target that was learned from the ingress a previous sequence and adds
         to
      the egress any state that was learned for that sequence.

   Path Lifetime:  8-bit unsigned integer.  The length of time in
         Lifetime Units (obtained from the path, including Configuration option) that
         the list prefix is valid for route determination.  The period starts
         when a new Path Sequence is seen.  A value of all intermediate
      routers 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 the data path order.  The this document.

   Via Addresses indicate Address:  16 bytes.  IPv6 Address of the
      routers in which next hop towards the routing state to
         destination(s) indicated in the target have to be
      installed via option that immediately
         precede the next RPO.  Via Address Addresses are indicated in the VIO.  In normal
      operations, the P-DAO is propagated along the chain of Via Routers
      from the egress router order of
         the data path till from the ingress one, which
      confirms the installation to the root with egress nodes.

   An RPO MUST contain at least one Via Address, and a DAO-ACK message.
      Note that the root may Via Address MUST
   NOT be present more than once, otherwise the ingress and it may be the egress of
      the path, that it can also be neither but it cannot RPO MUST be both.

5.1.  Non-storing Mode ignored.

3.3.  Projected Route

   As illustrated in Figure 2, DAO

   This draft adds a P-DAO that carries an SRVIO enables capability to RPL whereby the root to install of a source-routed path towards DODAG
   projects a target route by sending an extended DAO message called a
   Projected-DAO (P-DAO) to an arbitrary router in any
   particular router; with this path information the router 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 add be
   reached.

   A P-DAO is sent from a
   source routed header reflecting global address of the P-route root to any packet for which a global address
   of the current destination either recipient, and MUST be confirmed by a DAO-ACK, which is sent
   back to a global address of the said target or root.

   A P-DAO message MUST contain at least one TIO and at least one RPO
   following it.  There can be reached
   via the target.

              ------+---------
                    |          Internet
                    |
                 +-----+
                 |     | Border Router
                 |     |  (RPL Root)
                 +-----+                   |  P  ^            |
                    |                      | at most one such sequence of TIOs and
   then RPOs.

   Like a classical DAO | ACK        | Loose
              o    o   o    o     router   V     |            | Source
          o o   o  o   o  o  o      o  o            | message, a 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, processed only if it is
   "new" per section 9.2.2.  "Generation of DAO Messages" of the router that installs a P-route
   does not have a connected route (a direct adjacency) to RPL
   specification [RFC6550]; this is determined using the next
   soure routed hop and fails to locate it Path Sequence
   information from the RPO as opposed to a neighbor or TIO.  Also, a neighbor Path Lifetime
   of a neighbor, then it MUST ensure 0 in an RPO indicates that it has another projected a route is to the next loose hop under the control be removed.

   There are two kinds of operation for the same route
   computation system, otherwise P-Routes, the P-DAO Storing Mode
   and the Non-Storing Mode.

   o  The Non-Storing Mode is rejected.

   When forwarding discussed in Section 3.3.1.  It uses an
      SRVIO that carries a packet list of Via Addresses to be used as a destination for which source-
      routed path to the router
   determines that routing happens via target.  The recipient of the target, P-DAO is the
      ingress router inserts of the source routing header in source-routed path.  Upon a Non-Storing Mode
      P-DAO, the packet ingress router installs a source-routed state to reach the target.  In the
   case of a loose source-routed path, there MUST be either a neighbor
   that is adjacent
      target and replies to the loose next hop, on which case the packet s
   forwarded to that neighbor, or root directly with 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 DAO-ACK message.

   o  The Storing Mode is dropped.

   In order to add discussed in Section 3.3.2.  It uses a source-routing header, the router encapsulates the
   packet VIO
      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 one Via Address in per consecutive hop, from the SRVIO, and ingress to the SRH
   would contain
      egress of the path, including the list of all intermediate routers
      in the remaining data path order.  The Via Addresses and then indicate the
   target.

   In practice, routers in
      which the router will normally use routing state to the "IPv6 over Low-Power
   Wireless Personal Area Network (6LoWPAN) Paging Dispatch" [RFC8025] target have to compress be installed via the RPL artifacts as indicated
      next Via Address in the "6LoWPAN Routing
   Header" [RFC8138] specification. VIO.  In that case, normal operations, the router indicates
   self as encapsulator in an IP-in-IP 6LoRH Header, and places P-DAO is
      propagated along the list chain of Via Addresses in Routers from the order egress router
      of the VIO and then the target in path till the
   SRH 6LoRH Header.

5.2.  Storing-Mode Projected Route

   As illustrated in Figure 3, ingress one, which confirms the Storing Mode projected iq used by installation
      to the root to install a routing state towards with a target in DAO-ACK message.  Note that the root may be the routers along
   a segment between an
      ingress and an it may be the egress router; this enables of the
   routers to forward along path, that segment any packet for which the next
   loose hop it can also be
      neither but it cannot be both.

   In case of a forwarding error along a P-Route, an ICMP error is sent
   to the said target, for instance root with a loose source routed new Code "Error in Projected Route" (See
   Section 5.2).  The root can then modify or remove the P-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 for which 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 loose hop that
   this node could not reach.  if a 6LoWPAN Routing Header (6LoRH)
   [RFC8138] is used to carry the target, or 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 3.3.1 and Section 3.3.2 respectively.

3.3.1.  Non-Storing Mode P-Route

   As illustrated in Figure 2, a packet for
   which 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 has can add a routing state
   source routed header reflecting the P-route to any packet for which
   the final 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            |  ^       | Projected P-DAO   . Route
         o  o o  o o    o   o   o  o  o             |  | DAO   | Route Source  . Path
         o   o    o  o     o  o    o  o  o          | ^        | Route   . From
        o  o   o  o   o         o   o o       v             | DAO    v Path    . Root
           o  o   LLN   o  o  o                                |             o    target       V         . To
          o       o               o    o              Loose Source Route Path                      | Desti-
        o          o             o     o                 From Root To Destination v                      | nation
                                      destination             V

                          LLN

                 Figure 3: 2: Projecting a Non-Storing Route

   A route

   In order to install the relevant routing state along the segment
   between an ingress and indicated by an egress routers, SRVIO may be loose, meaning that the root sends node
   that owns the next listed Via Address is not necessarily a unicast
   P-DAO message to neighbor.
   Without proper loop avoidance mechanisms, the egress router interaction of the loose
   source routing segment that must
   be installed.  The P-DAO message contains and other mechanisms may effectively cause loops.  In
   order to avoid those loops, if the ordered list of hops
   along router that installs a P-route
   does not have a connected route (a direct adjacency) to the segment next
   soure routed hop and fails to locate it as a direct sequence neighbor or a neighbor
   of Via Information options a neighbor, then it MUST ensure that are preceded by one or more RPL Target options it has another P-Route to which they
   relate.  Each Via Information option contains a Path Lifetime for
   which the state is to be maintained.

   The root sends
   next loose hop under the control of the same route computation
   system, otherwise the P-DAO directly is rejected.

   When forwarding a packet to a destination for which the egress node of the segment.
   In router
   determines that P-DAO, routing happens via the destination IP address matches target, the Via Address router inserts
   the source routing header in the last VIO.  This is how packet to reach the egress recognizes its role. target.  In a
   similar fashion, the ingress node recognizes its role as it matches
   Via Address in the first VIO.

   The egress node
   case 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 loose source-routed path, there MUST be
   already able to route either a neighbor
   that is adjacent to the target(s) loose next hop, on its own.  It may either be which case the target, or may have some existing information packet s
   forwarded to reach the
   target(s), such as a connected route that neighbor, 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 source-routed path 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 next
   hop; in the list of Via Information options, latter case, another encapsulation takes place and recursively the message
   process possibly recurses; otherwise the packet is propagated unchanged along dropped.

   In order to add a source-routing header, the sequence of routers
   indicated in router encapsulates the P-DAO, but in
   packet with an IP-in-IP header and a non-storing mode source routing
   header (SRH) [RFC6554].  In the reverse order, from egress to
   ingress.

   The address uncompressed form the source of the predecessor to
   packet would be used as destination of self, the
   propagated DAO message is found in destination would be the first Via Information option Address
   in the
   precedes SRVIO, and the one that SRH would contain the address list of the propagating node,
   which is used as source of remaining Via
   Addresses and then the packet.

   Upon receiving a propagated DAO, an intermediate router as well as target.

   In practice, the ingress router install a route towards will normally use the DAO target(s) via its
   successor "IPv6 over Low-Power
   Wireless Personal Area Network (6LoWPAN) Paging Dispatch" [RFC8025]
   to compress the RPL artifacts as indicated in the P-DAO; "6LoWPAN Routing
   Header" [RFC8138] specification.  In that case, the router locates the VIO that contains its
   address, and uses indicates
   self as next hop the address found encapsulator in an IP-in-IP 6LoRH Header, and places the list
   of Via Address
   field Addresses in the following VIO.  The router MAY install additional routes
   towards order of the addresses that are located in VIOs that are after VIO and then the
   next one, if any, but target in the
   SRH 6LoRH Header.

   In case of a conflict or a lack of resource, a
   route to forwarding error along a target installed by the root has precedence.

   The process recurses till Source Route path, the P-DAO is propagated node
   that fails to ingress router
   of the segment, which answers forward SHOULD send an ICMP error with a DAO-ACK code "Error in
   Source Routing Header" back to the root.

   Also, source of the path indicated 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 P-DAO may be loose,
   period of time and it SHOULD send an ICMP message with a Code "Error
   in which case the
   reachability Projected Route" to the next hop has root.  Failure to be asserted.  Each router follow these steps may
   result in packet loss and wasted resources along the source route
   path indicated in a P-DAO that is expected broken.

3.3.2.  Storing-Mode P-Route

   As illustrated in Figure 3, the Storing Mode projected iq used by the
   root to be able to reach its
   successor, either with a connected route (direct neighbor), or by
   routing, for instance following install a route installed previously by routing state towards a DAO
   or target in the routers along
   a P-DAO message.  If segment between an ingress and an egress router; this enables the
   routers to forward along that route is not connected then a recursive
   lookup may take place at segment any packet forwarding time to find for which the next
   loose 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 the said target, for instance a Via Information option is used to clean up loose source routed
   packet for which the state.  The P-DAO next loose hop 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 target, 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 packet for
   which the
   root, using router has a Destination Advertisement Object (DAO) that is unicast
   from the node directly routing state to the root, and the root typically builds a
   source routed path to a final destination down via the DODAG by recursively
   concatenating this information.
   target.

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

                       Figure 4: RPL non-storing mode of operation

   Based on 3: Projecting a route

   In order to install the parent-children relationships expressed in relevant routing state along the non-
   storing DAO messages,the root possesses topological information about segment
   between an ingress and an egress routers, the whole network, though this information is limited root sends a unicast
   P-DAO message to the
   structure egress router of the DODAG for which it is the destination.  A packet routing segment that is generated within must
   be installed.  The P-DAO message contains the domain will always reach ordered list of hops
   along the root, segment as a direct sequence of Via Information options
   that are preceded by one or more RPL Target options to which
   can then apply they
   relate.  Each Via Information option contains a source routing information Path Lifetime for
   which the state is to reach be maintained.

   The root sends the destination
   if P-DAO directly to the egress node of the segment.
   In that P-DAO, the destination is also IP address matches the Via Address in
   the DODAG.  Similarly, last VIO.  This is how the egress recognizes its role.  In a packet coming
   from
   similar fashion, the outside ingress node recognizes its role as it matches
   Via Address in the first VIO.

   The egress node of the domain for a destination 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 in a RPL domain reaches
   already able to route to the root. target(s) on its own.  It results that may either be
   the root, target, or then may have some associated centralized
   computation engine existing information to reach the
   target(s), such as a PCE, can determine the amount connected route or an already installed P-Route.
   If one of packets 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 a destination in all the RPL domain, and thus targets, then it forwards the amount of
   energy and bandwidth that is wasted for transmission, between itself
   and
   P-DAO with unchanged content to its loose predecessor in the destination, as well segment
   as indicated in the risk of fragmentation, any
   potential delays because list of a paths longer than necessary (shorter
   paths exist that would not traverse Via Information options, and recursively
   the root).

   As a network gets deep, message is propagated unchanged along the size sequence of routers
   indicated in the source routing header that P-DAO, but in the root must add reverse order, from egress 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
   ingress.

   The address of well-targeted routing state
   would allow to make the source routing operation loose as opposed predecessor to
   strict, and save packet size.  Limiting be used as destination of the packet size
   propagated DAO message is directly
   beneficial to found in the energy budget, but, mostly, it reduces Via Information option the chances
   of frame loss and/or packet fragmentation, which is highly
   detrimental to the LLN operation.  Because
   precedes the capability to store a
   routing state in every node is limited, one that contain the decision address of the propagating node,
   which route is installed where can only be optimized with a global knowledge used as source of the system, packet.

   Upon receiving a knowledge that the root or propagated DAO, 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 router as well as
   the source route headers in deep networks.  Once a P-DAO exchange
   has taken place for ingress router install a given target, if route towards the root operates DAO target(s) via its
   successor in non
   storing mode, then it may elide the sequence of routers P-DAO; the router locates the VIO that is
   installed contains its
   address, and uses as next hop the address found in the network from its source route headers to destination Via Address
   field in the following VIO.  The router MAY install additional routes
   towards the addresses that are reachable via located in VIOs that target, and are after the source route headers
   effectively become loose.

6.2.  Transversal Routes
   next one, if any, but in storing and non-storing modes

   RPL is optimized for Point-to-Multipoint (P2MP), root to leaves and
   Multipoint-to-Point (MP2P) leaves case of a conflict or a lack of resource, a
   route to root operations, whereby routes
   are always a target installed along by the RPL DODAG.  Transversal Peer root has precedence.

   The process recurses till the P-DAO is propagated 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 ingress router
   of the way up segment, which answers with a DAO-ACK to the root of the DODAG.  If root.

   Also, the destination is path indicated in a P-DAO may be loose, in which case the
      same DODAG,
   reachability to the root must encapsulate next hop has to be asserted.  Each router along
   the packet path indicated in a P-DAO is expected to place be able to reach its
   successor, either with a
      Routing Header connected route (direct neighbor), or by
   routing, for instance following a route installed previously by a DAO
   or a P-DAO message.  If that has the strict source route information down is not connected then a recursive
   lookup may take place at packet forwarding time to find the DODAG next hop
   to reach the destination.  This will be target(s).  If it does not and cannot reach the case even if next
   router in the
      destination is relatively close to P-DAO, the source and router MUST answer to the root is
      relatively far off.

   o  In storing mode, unless with a
   negative DAO-ACK indicating the destination successor that is a child unreachable
   (suggested status 11 to be confirmed by IANA).

   A Path Lifetime of the source,
      the packets will follow the default route 0 in a Via Information option is used to clean up
   the DODAG state.  The P-DAO is forwarded as well.
      If described above, but the destination DAO is in the same DODAG, they will eventually
      reach a common parent that has
   interpreted as a route to the destination; at
      worse, the common parent may also be No-Path DAO and results in cleaning up existing
   state as opposed to refreshing an existing one or installing a new
   one.

   In case of a forwarding error along a Storing Mode P-Route, the root.  From node
   that common
      parent, 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 will follow a path down
   loss and wasted resources along the DODAG P-Route that is
      optimized for the Objective Function broken.

4.  Security Considerations

   This draft uses messages that was 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 build the
      DODAG.

                      ------+--------- 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?

5.  IANA Considerations

5.1.  New RPL Control Codes

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

               +------+-------------------+---------------+
               |          Internet Code |
                    +-----+ Description       | Reference     | Border Router
               +------+-------------------+---------------+
               | 0x0A |  (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 Via               | This document |
               |      |                   |               |
               | 0x0B | Source-Routed Via | This document |
               +------+-------------------+---------------+

                             RPL Control Codes

   This document is engineered with a different objective.
   For that reason, earlier work at updating the IETF introduced registry created by RFC 6550 for the "Reactive
   Discovery
   RPL 3-bit Mode of Point-to-Point Routes in Low Power and Lossy Networks"
   [RFC6997], which specifies a distributed method for establishing
   optimized P2P routes. Operation (MOP) as follows:

   +-----------+----------------------------------------+--------------+
   | MOP value | Description                            | Reference    |
   +-----------+----------------------------------------+--------------+
   |     5     | Non-Storing mode of operation with     | This draft proposes an alternate based on a
   centralized route computation.

                 ------+---------         |          Internet
   |
                    +-----+           | P-Routes                               | Border Router document     |
   |  (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:                                        |              |
   |     6     | Storing mode of operation with         | This         |
   |           | P-Routes                               | document     |
   +-----------+----------------------------------------+--------------+

                           DIO Mode of operation

5.2.  Error in Projected Transversal Route ICMPv6 Code

   In some cases RPL will return an ICMPv6 error message when a message
   cannot be forwarded along a P-Route.  This specification enables to store source-routed or storing mode
   state ICMPv6 error message is
   "Error in intermediate routers, which enables to limit the stretch of 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 P2P route and maintain new code is allocated from
   the characteristics within ICMPv6 Code Fields Registry for ICMPv6 Message Type 1, for "Error
   in Projected Route", with a given SLA.  An
   example suggested code value of service using this mechanism oculd be a control loop that
   would 8, to be installed in a network that uses classical RPL
   confirmed by IANA.

6.  Acknowledgments

   The authors wish to acknowledge JP Vasseur and Patrick Wetterwald for
   asynchronous data collection.  In that case,
   their contributions to the P2P path may be
   installed in a different RPL Instance, with a different objective
   function. ideas developed here.

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  References
7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in certain
   proprietary implementations RFCs to sort out conflicts between multiple
   sources of routing information.  This draft conforms the instance
   model as follows:

   o  If 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 PCE needs to influence a particular instance to add better
      routes 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>.

   [RFC6553]  Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
              Power and Lossy Networks (RPL) Option for Carrying RPL
              Information in conformance Data-Plane Datagrams", RFC 6553,
              DOI 10.17487/RFC6553, March 2012,
              <https://www.rfc-editor.org/info/rfc6553>.

   [RFC6554]  Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with the routing objectives in that
      instance, it may do so.  When the PCE modifies an existing
      instance then the added routes must not create a loop 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 that
      instance.  This is achieved by always preferring a route obtained
      from the PCE RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over a route that is learned via RPL.

   o  If the PCE installs a more specific (say, Traffic Engineered)
      route between a particular pair TSCH mode
              of nodes then it SHOULD use IEEE 802.15.4", draft-ietf-6tisch-architecture-19 (work
              in progress), December 2018.

   [I-D.ietf-detnet-architecture]
              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", draft-ietf-
              detnet-architecture-10 (work in progress), December 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>.

   [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.  Applications

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

   A RPL implementation operating in a
      Local Instance from very constrained LLN typically
   uses the ingress node Non-Storing Mode of Operation as represented in Figure 4.
   In 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 mode, a Global Instance may be injected in the Local Instance
      based on RPL node policy and the Local Instance paramenters.

   In all cases, the path is indicated by indicates a new Via Information option,
   and parent-child relationship to the flow
   root, using a Destination Advertisement Object (DAO) that is similar to unicast
   from the flow used node directly 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, the root, and whatever security is deployed for the root typically builds a given
   network also applies
   source routed path to a destination down the flows in this draft.

9.  IANA Considerations

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

               +------+-------------------+---------------+ recursively
   concatenating this information.

              ------+---------
                    | Code          Internet
                    | Description
                 +-----+
                 | Reference     |
               +------+-------------------+---------------+ Border Router
                 | 0x0A     | Via  (RPL Root)
                 +-----+                      ^     | This document        |
                    |                         | DAO | ACK    |
              o    o   o    o                 | 0x0B     | Source-Routed Via        | This document Strict
          o o   o  o   o  o  o o   o          |
               +------+-------------------+---------------+

                             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 Source
         o  o o  o o    o   o   o  o  o       | Reference     |        |  value Route
         o   o    o  o     o  o    o  o  o    |     |        |
   +----------+------------------------------------------+-------------+
        o  o   o  o   o         o   o o       |    5     | Non-Storing mode of operation with       | This        |
   |          | Projected routes                         | document    |
   |          |                                          |             |
   |    6     | Storing     v        v
        o          o             o     o
                          LLN

                Figure 4: RPL non-storing mode of operation with Projected | This        |
   |          | routes                                   | document    |
   +----------+------------------------------------------+-------------+

                           DIO Mode of operation

10.  Acknowledgments

   The authors wish

   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 acknowledge JP Vasseur and Patrick Wetterwald the
   structure of the DODAG for
   their contributions 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 ideas developed here.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use destination
   if the destination is also in RFCs the DODAG.  Similarly, a packet coming
   from the outside of the domain for a destination that is expected 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.,
   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 R. Alexander, "RPL: IPv6 Routing Protocol thus the amount of
   energy and bandwidth that is wasted for
              Low-Power transmission, between itself
   and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.

   [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 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 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.,
   nodes that are many hops away.  In some use cases, a RPL network
   forms long lines 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 a limited amount 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 well-targeted routing state
   would allow to make 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., source routing operation loose as opposed to
   strict, 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., 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.

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 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.

                      ------+---------
                       |          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
              J. Martocci, 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 Low Power and Lossy Networks", RFC 6997,
              DOI 10.17487/RFC6997, August 2013,
              <https://www.rfc-editor.org/info/rfc6997>.

   [RFC7102]  Vasseur, JP., "Terms Used in Routing Networks"
   [RFC6997], which specifies a distributed method for Low-Power 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
              Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
              2014, <https://www.rfc-editor.org/info/rfc7102>. 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.

Appendix A. B.  Examples

A.1.

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 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.

   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.

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

   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)
                    +-----+
                     ^ Projected DAO-ACK 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 9: Projected DAO-ACK 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 10: Projected Transversal Route

Authors' Addresses

   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

   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

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

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