Root initiated routing state in RPL
draft-ietf-roll-dao-projection-06

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Last updated 2019-05-24
Replaces draft-thubert-roll-dao-projection
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ROLL                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Updates: 6550, 6553, 8138 (if approved)                        R. Jadhav
Intended status: Standards Track                             Huawei Tech
Expires: November 25, 2019                                   M. Gillmore
                                                                   Itron
                                                            J. Pylakutty
                                                                   Cisco
                                                            May 24, 2019

                  Root initiated routing state in RPL
                   draft-ietf-roll-dao-projection-06

Abstract

   This document extends RFC 6550, RFC 6553 and RFC 8138 and enable 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.  In constrast with classical
   routes 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 November 25, 2019.

Copyright Notice

   Copyright (c) 2019 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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  New Terms . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  References  . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Extending RFC 6550  . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  RPL Instances . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  New RPL Control Message Options . . . . . . . . . . . . .   5
     3.3.  RPI for Projected Routes  . . . . . . . . . . . . . . . .   7
     3.4.  Projected DAO . . . . . . . . . . . . . . . . . . . . . .   7
       3.4.1.  Non-Storing Mode P-Route  . . . . . . . . . . . . . .   9
       3.4.2.  Storing-Mode P-Route  . . . . . . . . . . . . . . . .  10
   4.  Extending RFC 8138  . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Elective RPI 6LoRH  . . . . . . . . . . . . . . . . . . .  13
   5.  Extending RFC 6553  . . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Uncompressed RPL Option . . . . . . . . . . . . . . . . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  New Elective 6LoWPAN Routing Header Type  . . . . . . . .  14
     7.2.  New RPL Control Codes . . . . . . . . . . . . . . . . . .  15
     7.3.  Error in Projected Route ICMPv6 Code  . . . . . . . . . .  15
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Appendix A.  Applications . . . . . . . . . . . . . . . . . . . .  18
     A.1.  Loose Source Routing in Non-storing Mode  . . . . . . . .  18
     A.2.  Transversal Routes in storing and non-storing modes . . .  19
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . .  21
     B.1.  Using storing mode P-DAO in non-storing mode MOP  . . . .  21
     B.2.  Projecting a storing-mode transversal route . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   The "Routing Protocol for Low Power and Lossy Networks" [RFC6550]
   (LLN)(RPL) is a generic Distance Vector protocol that is well suited
   low energy Internet of Things (IoT) networks.  RPL forms Destination

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   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) 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.

   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 P-Routes.

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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.  New Terms

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

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

   This specification introduces 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 P-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).

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3.1.  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.

3.2.  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 255 (0xFF)
         represents infinity.  A value of zero (0x00) indicates a loss
         of reachability.  A DAO message that contains a Via Information

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         option with a Path Lifetime of zero 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.

   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.

3.3.  RPI for Projected Routes

   RPL [RFC6550], Section 11.2, specifies the RPL Packet Information
   (RPI) as a set of fields that are placed by RPL routers in IP packets
   to identify the RPL Instance, detect anomalies and trigger corrective
   actions.

   In particular, the SenderRank, which is the scalar metric computed by
   a specialized Objective Function such as described in [RFC6552],
   indicates the Rank of the sender and is modified at each hop.  The
   SenderRank field is used to validate that the packet progresses in
   the expected direction, either upwards or downwards, along the DODAG.

   RPL defines the "RPL Option for Carrying RPL Information in Data-
   Plane Datagrams" [RFC6553] to transport the RPI, which is carried in
   an IPv6 Hop-by-Hop Options Header [RFC8200], typically consuming
   eight bytes per packet.

   This specification updates [RFC6550] as follows.  When using
   projected routes, the Rank is useless and SHOULD be set to 0 in the
   non-compressed form, and can be elided in the compressed form (see
   Section 4.1).  In a same fashion, the O, R, and F flags that are
   defined in Section 11.2 of [RFC6550] are not used for packets that
   follow a projected route and they MUST be reset.  A new flag is
   added, the P flag that indicates that the packet is injected along a
   projected route.

3.4.  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.

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   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 P-Routes, the Storing Mode
   and the Non-Storing Mode.

   o  The Non-Storing Mode is discussed in Section 3.4.1.  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.

   o  The Storing Mode is discussed in Section 3.4.2.  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.

   In case of a forwarding error along a P-Route, an ICMP error is sent
   to the root with a new Code "Error in Projected Route" (See
   Section 7.3).  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 that is sent back in
   the ICMP message SHOULD record at least up to the routing header if
   one is present, and the routing header SHOULD be consumed by this
   node so that the destination in the IPv6 header is the next hop that
   this node could not reach.  if a 6LoWPAN Routing Header (6LoRH)
   [RFC8138] is used to carry the IPv6 routing information in the outter
   header then that whole 6LoRH information SHOULD be present in the

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   ICMP message.  The sender and exact operation depend on the Mode and
   is described in Section 3.4.1 and Section 3.4.2 respectively.

3.4.1.  Non-Storing Mode P-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
   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 P-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

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   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.

   +-+ ... -+-+ ... +-+- ... -+-+-     ...      -+-+-+- ... -+-+ ...
   |11110001|SRH-6LoRH| ERPI- | IP-in-IP  Encap  | NH=1      |11110CPP|
   |Page 1  |Type1 S=2| 6LoRH |  6LoRH   sulator |LOWPAN_IPHC| UDP    |
   +-+ ... -+-+ ... +-+- ... -+-+-     ...      -+-+-+- ... -+-+ ...
            <-RFC8138-><-This-><----RFC 8138----><-----RFC 6282------->
                         RFC      5 to 19 bytes     No RPL artifact

               Figure 3: Example Compressed Packet with SRH.

   In case of a forwarding error along a Source Route path, the node
   that fails to forward SHOULD send an ICMP error with a code "Error in
   Source Routing Header" back to the source of the packet, as described
   in section 11.2.2.3. of [RFC6550].  Upon this message, the
   encapsulating node SHOULD stop using the source route path for a
   period of time and it SHOULD send an ICMP message with a Code "Error
   in Projected Route" to the root.  Failure to follow these steps may
   result in packet loss and wasted resources along the source route
   path that is broken.

3.4.2.  Storing-Mode P-Route

   As illustrated in Figure 4, 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

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   which the router has a routing state to the final destination via the
   target.

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

                       Figure 4: 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 P-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).

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

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

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

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

   Also, the path indicated in a P-DAO may be loose, in which case the
   reachability to the next hop has to be asserted.  Each router along
   the path indicated in a P-DAO is expected to be able to reach its
   successor, either with a connected route (direct neighbor), or by
   routing, for instance following a route installed previously by a DAO
   or a P-DAO message.  If that route is not connected then a recursive
   lookup may take place at packet forwarding time to find the next hop
   to reach the target(s).  If it does not and cannot reach the next
   router in the P-DAO, the router MUST answer to the root with a
   negative DAO-ACK indicating the successor that is unreachable
   (suggested status 11 to be confirmed by IANA).

   A Path Lifetime of 0 in a Via Information option is used to clean up
   the state.  The P-DAO is forwarded as described above, but the DAO is
   interpreted as a No-Path DAO and results in cleaning up existing
   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 node
   that fails to forward SHOULD send an ICMP error with a code "Error in
   Projected Route" to the root.  Failure to do so may result in packet
   loss and wasted resources along the P-Route that is broken.

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4.  Extending RFC 8138

4.1.  Elective RPI 6LoRH

   [RFC8138] defines a Critical 6LoRH to compress the RPL RPI found in
   normal packets inside a RPL domain, the RPI-6LoRH.

   this specification introduces the ERPI-6LoRH header that MUST be used
   to compress the RPI in packets that follow a projected route.  As
   discussed in Section 3.3, the Rank and the O, R, anf F flags are
   always set to 0 and can be elided.  The new P flag is always set and
   can also be elided.  It results that in general only the RPL
   InstanceID is necessary in the compressed form.

   This specification adds an optimization whereby the local
   RPLInstanceID 0 for the source of the packet (the encapsulator when
   using IP in IP) can be elided.  This is the case where the
   RPLInstanceID is encoded as binary b10000000, decimal 128, in the
   non-compressed form.

   The ERPI-6LoRH header is Elective since it does not contain
   information that is critical to the routing and it can be ignored
   when not understood.  The resulting format is illustrated in Figure 5
   below:

     0                   1                   2
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |1|0|1| Length  | 6LoRH Type 5  | RPLInstanceID |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 5: A ERPI-6LoRH carrying a RPLInstanceID

   The ERPI-6LoRH header is identifies by a 6LoRH Type of 5 (to be
   confirmed by IANA), which is the same value as the RPI-6LoRH but in
   the Elective namespace.If the RPLInstanceID is a local RPLInstanceID
   0 for the source of the packet then it MUST be elided and the length
   MUST be set to 0.  Else the length MUST be set to 1 to indicate that
   the ERPI-6LoRH carries a RPLinstanceID.

5.  Extending RFC 6553

5.1.  Uncompressed RPL Option

   [RFC6553] defines a format for the RPI that is suitable for
   transporting in the IPv6 Hop-by-Hop Header [RFC8200].  This

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   specification introduces a new flag in the RPI that must be encoded
   in any format includeing uncompressed.

   The updated format for the RPL Option is presented in Figure 6.

        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
                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                       |  Option Type  |  Opt Data Len |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |O|R|F|P|0|0|0|0| RPLInstanceID |          SenderRank           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         (sub-TLVs)                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 6: RPL Option

   New fields:

   P:              1-bit flag; indicates that the packet is routed along
                   a projected route.

6.  Security Considerations

   This draft uses messages that are already present in RPL [RFC6550]
   with optional secured versions.  The same secured versions may be
   used with this draft, and whatever security is deployed for a given
   network also applies to the flows in this draft.

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

7.  IANA Considerations

7.1.  New Elective 6LoWPAN Routing Header Type

   This specification assigns a new value (to be confirmed by IANA) in
   the Elective 6LoWPAN Routing Header Type Registry created for RFC
   8138 as below:

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             +---------------+-------------+----------------+
             |     Value     |   Meaning   | Defining Spec  |
             +---------------+-------------+----------------+
             | 5 (suggested) |  ERPI-6LoRH | This document  |
             +---------------+-------------+----------------+

             Table 1: New Elective 6LoWPAN Routing Header Type

7.2.  New RPL Control Codes

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

               +------+-------------------+---------------+
               | 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 value | Description                            | Reference    |
   +-----------+----------------------------------------+--------------+
   |     5     | Non-Storing mode of operation with     | This         |
   |           | P-Routes                               | document     |
   |           |                                        |              |
   |     6     | Storing mode of operation with         | This         |
   |           | P-Routes                               | document     |
   +-----------+----------------------------------------+--------------+

                           DIO Mode of operation

7.3.  Error in Projected Route ICMPv6 Code

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

   IANA has defined an ICMPv6 "Code" Fields Registry for ICMPv6 Message
   Types.  ICMPv6 Message Type 1 describes "Destination Unreachable"
   codes.  This specification requires that a new code is allocated from
   the ICMPv6 Code Fields Registry for ICMPv6 Message Type 1, for "Error

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

8.  Acknowledgments

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

9.  References

9.1.  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>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

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

   [RFC6552]  Thubert, P., Ed., "Objective Function Zero for the Routing
              Protocol for Low-Power and Lossy Networks (RPL)",
              RFC 6552, DOI 10.17487/RFC6552, March 2012,
              <https://www.rfc-editor.org/info/rfc6552>.

   [RFC6553]  Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
              Power and Lossy Networks (RPL) Option for Carrying RPL
              Information in 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 Protocol
              for Low-Power and Lossy Networks (RPL)", RFC 6554,
              DOI 10.17487/RFC6554, March 2012,
              <https://www.rfc-editor.org/info/rfc6554>.

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   [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>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

9.2.  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-20 (work
              in progress), March 2019.

   [I-D.ietf-detnet-architecture]
              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", draft-ietf-
              detnet-architecture-13 (work in progress), May 2019.

   [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>.

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Appendix A.  Applications

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

   A RPL implementation operating in a very constrained LLN typically
   uses the Non-Storing Mode of Operation as represented in Figure 7.
   In that mode, a RPL node indicates a parent-child relationship to the
   root, using a Destination Advertisement Object (DAO) that is unicast
   from the node directly to the root, and the root typically builds a
   source routed path to a destination down the DODAG by recursively
   concatenating this information.

              ------+---------
                    |          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 7: RPL non-storing mode of operation

   Based on the parent-children relationships expressed in the non-
   storing DAO messages,the root possesses topological information about
   the whole network, though this information is limited to the
   structure of the DODAG for which it is the destination.  A packet
   that is generated within the domain will always reach the root, which
   can then apply a source routing information to reach the destination
   if the destination is also in the DODAG.  Similarly, a packet coming
   from the outside of the domain for a destination that is expected to
   be in a RPL domain reaches the root.

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

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

   This specification enables to store 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 8:

   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

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      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 8: 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 9: Projected Transversal Route

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

Appendix B.  Examples

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

   In non-storing mode, the DAG root maintains the knowledge of the
   whole DODAG topology, so when both the source and the destination of
   a packet are in the DODAG, the root can determine the common parent
   that would have been used in storing mode, and thus the list of nodes
   in the path between the common parent and the destination.  For
   instance in the diagram shown in Figure 10, 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 10: 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

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   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.

B.2.  Projecting a storing-mode transversal route

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

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

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

                        Figure 11: P-DAO from root

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   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)
                    +-----+
                     ^ 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 12: P-DAO-ACK to root

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

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                 ------+---------
                       |          Internet
                       |
                    +-----+
                    |     | Border Router
                    |     |  (RPL Root)
                    +-----+
                       |
                 o    o   o    o
             o o   o  o   o  o  o o   o
            o  o o  o o    o   o   o  o  o
            o   o    o  o     o  o    o  o  o
           S>>A>>>B>>C>>>D         o   o o
           o          o             o     o
                             LLN

                  Figure 13: 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

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   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

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