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RFC 6997 - Reactive Discovery of Point-to-Point Routes in Low-Po


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Internet Engineering Task Force (IETF)                     M. Goyal, Ed.
Request for Comments: 6997                  Univ. of Wisconsin Milwaukee
Category: Experimental                                       E. Baccelli
ISSN: 2070-1721                                               M. Philipp
                                                                   INRIA
                                                               A. Brandt
                                                           Sigma Designs
                                                             J. Martocci
                                                        Johnson Controls
                                                             August 2013

              Reactive Discovery of Point-to-Point Routes
                    in Low-Power and Lossy Networks

Abstract

   This document specifies a point-to-point route discovery mechanism,
   complementary to the Routing Protocol for Low-power and Lossy
   Networks (RPL) core functionality.  This mechanism allows an IPv6
   router to discover "on demand" routes to one or more IPv6 routers in
   a Low-power and Lossy Network (LLN) such that the discovered routes
   meet specified metrics constraints.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6997.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1. Introduction ....................................................4
   2. The Use Cases ...................................................4
   3. Terminology .....................................................5
   4. Applicability ...................................................6
   5. Functional Overview .............................................7
   6. P2P Route Discovery Mode of Operation ..........................10
      6.1. Setting a P2P Mode DIO ....................................10
   7. P2P Route Discovery Option (P2P-RDO) ...........................15
   8. The P2P Discovery Reply Object (P2P-DRO) .......................18
      8.1. Secure P2P-DRO ............................................20
      8.2. Setting a P2P-RDO Carried in a P2P Discovery Reply
           Object ....................................................21
   9. P2P-RPL Route Discovery by Creating a Temporary DAG ............21
      9.1. Joining a Temporary DAG ...................................21
      9.2. Trickle Operation for P2P Mode DIOs .......................22
      9.3. Processing a P2P Mode DIO .................................24
      9.4. Additional Processing of a P2P Mode DIO at an
           Intermediate Router .......................................26
      9.5. Additional Processing of a P2P Mode DIO at the Target .....27
      9.6. Processing a P2P-DRO at an Intermediate Router ............28
      9.7. Processing a P2P-DRO at the Origin ........................30
   10. The P2P Discovery Reply Object Acknowledgement (P2P-DRO-ACK) ..31
   11. Secure P2P-RPL Operation ......................................32
   12. Packet Forwarding along a Route Discovered Using P2P-RPL ......33
   13. Interoperability with Core RPL ................................34
   14. Security Considerations .......................................34
   15. IANA Considerations ...........................................36
      15.1. Additions to Mode of Operation ...........................36
      15.2. Additions to RPL Control Message Options .................36
      15.3. Additions to RPL Control Codes ...........................36
   16. Known Issues and Future Work ..................................37
   17. Acknowledgements ..............................................37
   18. References ....................................................38
      18.1. Normative References .....................................38
      18.2. Informative References ...................................38

1.  Introduction

   Targeting Low-power and Lossy Networks (LLNs), the IPv6 Routing
   Protocol for LLNs (RPL) [RFC6550] provides paths along a Directed
   Acyclic Graph (DAG) rooted at a single router in the network.
   Establishment and maintenance of a DAG are performed by routers in
   the LLN using Destination-Oriented DAG (DODAG) Information Object
   (DIO) messages.  When two arbitrary routers (neither of which is the
   DAG's root) need to communicate, the data packets are restricted to
   travel only along the links in the DAG.  Such point-to-point (P2P)
   routing functionality may not be sufficient for several home
   automation [RFC5826] and building automation [RFC5867] applications,
   due to the following reasons:

   o  The need to pre-establish routes: Each potential destination in
      the network must declare itself as such ahead of the time a source
      needs to reach it.

   o  The need to route only along the links in the DAG: A DAG is built
      to optimize the routing cost to reach the root.  Restricting P2P
      routes to use only the in-DAG links may result in significantly
      suboptimal routes and severe traffic congestion near the DAG root.

   This document describes an extension to core RPL (i.e., the RPL
   functionality described in [RFC6550]) that enables an IPv6 router in
   the LLN to discover routes to one or more IPv6 routers in the LLN "on
   demand".  The discovered routes may not be the best available but are
   guaranteed to meet the specified routing metric constraints.  Thus,
   such routes are considered "good enough" from the application's
   perspective.  This reactive P2P route discovery mechanism is
   henceforth referred to as P2P-RPL.

   A mechanism to measure the end-to-end cost of an existing route is
   specified in [RFC6998].  As discussed in Section 4, measuring the
   end-to-end cost of an existing route may help in deciding whether to
   initiate the discovery of a better route using P2P-RPL and the metric
   constraints to be used for this purpose.

2.  The Use Cases

   One use case, common in home [RFC5826] and commercial building
   [RFC5867] environments, involves a device (say, a remote control)
   that suddenly needs to communicate with another device (say, a lamp)
   to which it does not already have a route (and whose network address
   it knows a priori).  In this case, the remote control must be able to
   discover a route to the lamp "on demand".

   Another use case, common in a commercial building environment,
   involves a large LLN deployment where P2P communication along a
   particular DAG among hundreds (or thousands) of routers creates
   severe traffic congestion near that DAG's root.  In this case, it is
   desirable to discover direct routes between various source-
   destination pairs that do not pass through the DAG's root.

   Other use cases involve scenarios where energy or latency constraints
   are not satisfied by the P2P routes along an existing DAG because
   they involve traversing many more routers than necessary to reach the
   destination.

3.  Terminology

   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
   [RFC2119].

   Additionally, this document uses terminology from [RFC6550] and
   [RFC6554].  Further terminology may be found in [ROLL-TERMS].  This
   document introduces the following terms:

   Origin:  The IPv6 router initiating the P2P-RPL route discovery.

   Target:  The IPv6 router at the other end point of the P2P route(s)
      to be discovered.  A P2P-RPL route discovery can discover routes
      to multiple Targets at the same time.

   Intermediate Router:  An IPv6 router that is neither the Origin nor a
      Target.

   Forward direction:  The direction from the Origin to the Target.

   Reverse direction:  The direction from the Target to the Origin.

   Forward Route:  A route in the Forward direction.

   Reverse Route:  A route in the Reverse direction.

   Bidirectional Route:  A route that can be used in both Forward and
      Reverse directions.

   Ingress-only Interface:  A network interface that can only receive
      packets.

   Egress-only Interface:  A network interface that can only send
      packets.

   Source Route:  A complete and ordered list of routers that can be
      used by a packet to travel from a source to a destination node.

   Hop-by-hop Route:  The route characterized by each router on the
      route using its routing table to determine the next hop on the
      route.

   RPL Security Configuration:  The values for the Counter is Time,
      Security Algorithm, Key Identifier Mode, and Security Level
      fields, as defined in Section 6.1 of [RFC6550], inside the
      Security section of a secure RPL control message.

4.  Applicability

   A route discovery using P2P-RPL may be performed by an Origin when no
   route exists between itself and the Target(s) or when the existing
   routes do not satisfy the application requirements.  P2P-RPL is
   designed to discover Hop-by-hop or Source Routes to one or more
   Targets such that the discovered routes meet the specified
   constraints.  In some application contexts, the constraints that the
   discovered routes must satisfy are intrinsically known or can be
   specified by the application.  For example, an Origin that expects
   its Targets to be less than 5 hops away may use "hop-count < 5" as
   the constraint.  In other application contexts, the Origin may need
   to measure the cost of the existing route to a Target to determine
   the constraints.  For example, an Origin that measures the total
   expected transmission count (ETX) along its current route to a Target
   to be 20 may use "ETX < x*20", where x is a fraction that the Origin
   chooses, as the constraint.  A mechanism to measure the cost of an
   existing route between two IPv6 routers is specified in [RFC6998].
   If there is no existing route between the Origin and the Target(s) or
   the cost measurement for the existing routes fails, the Origin will
   have to guess the constraints to be used in the initial route
   discovery.  Once the initial route discovery succeeds or fails, the
   Origin will have a better estimate for the constraints to be used in
   the subsequent route discovery.

   P2P-RPL may result in discovery of better P2P routes than those
   available along a global DAG designed to optimize routing cost to the
   DAG's root.  The improvement in route quality depends on a number of
   factors, including the network topology, the "distance" between the
   Origin and the Target (in terms of the routing metrics in use), and
   the prevalent conditions in the network.  In general, a P2P-RPL route
   may be better than the one along a global DAG if the Origin and the
   Target are nearby.  Similarly, a P2P-RPL route may not be much better
   than the one along a global DAG if the Origin and the Target are far
   apart.  Note that even when P2P-RPL routes are not much better than
   those along a global DAG, P2P-RPL routes may still be able to avoid

   congestion that might occur near the root if the routing takes place
   only along a global DAG.  In general, the cost associated with a
   P2P-RPL route discovery (in terms of the control messages -- mostly
   DIOs -- generated) increases with the distance between the Origin and
   the Target.  However, it is possible to limit the cost of route
   discovery by carefully setting the routing constraints, the Trickle
   parameters (which govern DIO generation), and the time duration for
   which a router maintains its membership in the temporary DAG created
   for the route discovery.  A network designer may take into
   consideration both the benefits (potentially better routes; no need
   to maintain routes proactively; avoid congestion near the global
   DAG's root) and costs when using P2P-RPL.  The latency associated
   with a P2P-RPL route discovery again depends on the distance between
   the Origin and the Target and on the Trickle parameters.

   Like core RPL [RFC6550], P2P-RPL operation requires that links have
   bidirectional reachability.  For this reason, the routers
   participating in a P2P-RPL route discovery must ensure that

   o  Links that do not have bidirectional reachability do not become
      part of the route being discovered; and

   o  IPv6 addresses belonging to Ingress-only (or Egress-only)
      Interfaces do not become part of the route being discovered.

5.  Functional Overview

   This section contains a high-level description of P2P-RPL.

   A P2P-RPL route discovery takes place by forming a DAG rooted at the
   Origin.  As is the case with core RPL, P2P-RPL uses IPv6 link-local
   multicast DIO messages to establish a DAG.  However, unlike core RPL,
   this DAG is temporary in nature.  The routes are discovered and
   installed while the DAG is alive.  Once the specified duration of
   their membership in the DAG is over, the routers leave the DAG, and
   hence the DAG ceases to exist.  However, the installed routes are
   retained for their specified lifetime (which is different than the
   specified duration of a router's membership in the DAG) even though
   the DAG that caused their installation no longer exists.  In P2P-RPL,
   the sole purpose of DAG creation is to discover routes to the
   Target(s), and DIOs serve as the route discovery messages.  Each
   router joining the DAG determines a rank for itself in the DAG and
   ignores the subsequent DIOs received from lower-ranked (higher in
   numerical value) neighbors.  Thus, the route discovery messages
   propagate away from the Origin rather than return to it.  As in core
   RPL, DIO generation at a router is controlled by a Trickle timer
   [RFC6206], which allows a router to avoid generating unnecessary
   messages while providing protection against packet loss.  P2P-RPL

   also uses the routing metrics [RFC6551], Objective Functions, and
   packet-forwarding framework [RFC6554] [RFC6553] developed for
   core RPL.

   An Origin may use P2P-RPL to discover routes to one or more Targets
   identified by one or more unicast/multicast addresses.  P2P-RPL
   allows for the discovery of one Hop-by-hop Route or up to four Source
   Routes per Target.  The discovered routes are guaranteed to meet the
   specified routing metric constraints but may not be the best
   available.  P2P-RPL may fail to discover any route if the specified
   routing constraints are overly strict.

   The Origin initiates a P2P-RPL route discovery by forming a temporary
   DAG rooted at itself.  The DIOs used to create the temporary DAG are
   identified by a new Mode of Operation (P2P Route Discovery mode,
   defined in Section 6).  The DIOs listing the P2P Route Discovery mode
   as the Mode of Operation are henceforth referred to as the P2P mode
   DIOs.  A P2P mode DIO always carries exactly one P2P Route Discovery
   Option (P2P-RDO, defined in Section 7) in which the Origin specifies
   the following information:

   o  The IPv6 address of a Target.  This could be a unicast address or
      a multicast address.  Any additional Targets may be specified by
      including one or more RPL Target options [RFC6550] inside the DIO.

   o  The nature of the route(s) to be discovered: Hop-by-hop or Source
      Routes.  This specification allows for the discovery of one
      Hop-by-hop Route or up to four Source Routes per Target.

   o  The desired number of routes (if Source Routes are being
      discovered).

   o  Whether the Target(s) should send P2P Discovery Reply Object
      (P2P-DRO) messages (defined in Section 8) back to the Origin on
      receiving a DIO message.  A P2P-DRO message carries a discovered
      Source Route back to the Origin or establishes a Hop-by-hop Route
      between the Origin and the Target.

   A P2P-RDO also includes the best route from the Origin that the
   router, generating the P2P mode DIO, has seen so far.

   A P2P mode DIO MAY also carry:

   o  One or more Metric Container options to specify:

      *  The relevant routing metrics.

      *  The constraints that the discovered route must satisfy.  These
         constraints also limit how far the DIO messages may travel.

   o  One or more RPL Target options to specify additional unicast or
      multicast Targets.

   As the routers join the temporary DAG, they keep track of the best
   route(s) (so far from the Origin) they have seen and advertise these
   routes, along with the corresponding routing metrics, in their P2P
   mode DIOs.  A router, including the Target(s), discards a received
   P2P mode DIO if the aggregated routing metrics on the route
   advertised by the DIO do not satisfy the listed constraints.  These
   constraints can be used to limit the propagation of P2P mode DIO
   messages.  A router may also discard a received P2P mode DIO if it
   does not wish to be a part of the discovered route due to limited
   resources or due to policy reasons.

   When a Target receives a P2P mode DIO, it contains inside the P2P-RDO
   a complete Source Route from the Origin to this Target.  Since the
   links in the discovered route have bidirectional reachability
   (Section 7), the Target may use the discovered route to reach the
   Origin.  Thus, a router that provides a particular service in the LLN
   (e.g., an outside temperature server) could initiate a P2P-RPL route
   discovery listing all its potential clients as Targets, thereby
   allowing the clients to discover a Source Route back to the server.
   In this case, the Origin (the server) might want to disable the
   generation of P2P-DRO messages by the Targets (the clients).  If the
   Origin has requested that P2P-DRO messages be sent back, the Target
   may select the discovered route in the received DIO for further
   processing, as described next.  This document does not specify a
   particular method for the Target to use to select a route for further
   processing.  Example methods include selecting any route that meets
   the constraints or selecting the best route(s) discovered over a
   certain time period.

   If one or more Source Routes are being discovered, the Target sends
   the selected Source Route(s) to the Origin via P2P-DRO messages, with
   one P2P-DRO message carrying one discovered route.  On receiving a
   P2P-DRO message, the Origin stores the discovered route in its
   memory.  This specification allows the Origin to discover up to four
   Source Routes per Target, thereby allowing the Origin to have
   sufficient ready-to-use alternatives should one or more of these

   routes fail.  If a Hop-by-hop Route is being discovered, the Target
   sends a P2P-DRO message containing the selected route to the Origin.
   The P2P-DRO message travels back to the Origin along the selected
   route, establishing state for the Forward Route in the routers on
   the path.

   The Target may request that the Origin acknowledge the receipt of a
   P2P-DRO message by sending back a P2P-DRO Acknowledgement
   (P2P-DRO-ACK) message (defined in Section 10).  The Origin unicasts a
   P2P-DRO-ACK message to the Target.  If the Target does not receive
   the requested P2P-DRO-ACK within a certain time interval of sending a
   P2P-DRO, it resends the P2P-DRO message (up to a certain number of
   times) carrying the same route as before.

   The use of Trickle timers to delay the propagation of DIO messages
   may cause some nodes to generate these messages even when the desired
   routes have already been discovered.  In order to preempt the
   generation of such unnecessary messages, the Target may set a "Stop"
   flag in the P2P-DRO message to let the nodes in the LLN know about
   the completion of the route discovery process.  The routers receiving
   such a P2P-DRO should not generate any more DIOs for this temporary
   DAG, nor should they process any received DIOs for this temporary DAG
   in the future.  However, such routers must still process the P2P-DROs
   received for this temporary DAG.

6.  P2P Route Discovery Mode of Operation

   This section specifies a new RPL Mode of Operation (MOP), P2P Route
   Discovery mode (or P2P mode, for short), with value 4.  A DIO message
   listing P2P mode as the MOP is identified as performing a P2P-RPL
   route discovery by creating a temporary DAG.  A P2P mode DIO MUST
   carry exactly one P2P Route Discovery Option (P2P-RDO, specified in
   Section 7).

6.1.  Setting a P2P Mode DIO

   The Base object in a P2P mode DIO message MUST be set in the
   following manner:

   o  RPLInstanceID: RPLInstanceID MUST be a local value as described in
      Section 5.1 of [RFC6550].  The Origin chooses the RPLInstanceID to
      be used for a particular route discovery in accordance with the
      following rules:

      *  The Origin SHOULD NOT reuse a RPLInstanceID for a route
         discovery if some routers might still maintain membership in
         the DAG that the Origin had initiated for the previous route
         discovery using this RPLInstanceID.  As described in Section 7,

         a router's membership in a DAG created for a P2P-RPL route
         discovery lasts for the time duration (say, 't' seconds)
         indicated by the L field inside the P2P-RDO.  In general, there
         is no upper bound on the time duration by when all the routers
         have left the DAG created for a P2P-RPL route discovery.  In
         the specific case where the discovered route must be at most
         'n' hops in length, all the routers must have left the DAG
         "(n+1)*t" seconds after its initiation by the Origin.  In
         practice, all the routers should have joined the DAG within 't'
         seconds of its initiation (since the route discovery must
         complete while the Origin still belongs to the DAG), and hence
         all the routers should have left the DAG within "2*t" seconds
         of its initiation.  Hence, it is usually sufficient that the
         Origin wait for twice the duration indicated by the L field
         inside the P2P-RDO used for the previous route discovery before
         reusing the RPLInstanceID for a new route discovery.
         Individual P2P-RPL deployments are encouraged to share their
         experience with various RPLInstanceID reuse policies to help
         guide the development of a Standards Track version of the
         protocol.

      *  When initiating a new route discovery to a particular Target,
         the Origin MUST NOT reuse the RPLInstanceID used in a previous
         route discovery to this Target if the state created during the
         previous route discovery might still exist in some routers.
         Note that it is possible that the previous route discovery did
         not succeed yet some routers still ended up creating state.
         The Default Lifetime and Lifetime Unit parameters in the DODAG
         Configuration Option specify the lifetime of the state that the
         routers, including the Origin and the Target, maintain for a
         Hop-by-hop or Source Route discovered using P2P-RPL.  Suppose
         this lifetime is 'X' seconds.  As discussed above, any state
         created during the previous route discovery was likely created
         within "2*t" seconds of its initiation.  Hence, it is
         sufficient that the Origin lets a time duration equal to
         "X+2*t" seconds pass since the initiation of the previous route
         discovery before initiating a new route discovery to the same
         Target using the same RPLInstanceID.

   o  Version Number: This field MUST be set to zero.  The temporary DAG
      used for P2P-RPL route discovery does not exist long enough to
      have new versions.

   o  Grounded (G) Flag: This flag MUST be set to one.  Unlike a global
      RPL instance, the concept of a floating DAG, used to provide
      connectivity within a sub-DAG detached from a grounded DAG, does
      not apply to a local RPL instance.  Hence, an Origin MUST always
      set the G flag to one when initiating a P2P-RPL route discovery.

      Further, item 3 of Section 8.2.2.2 in [RFC6550] does not apply,
      and a node MUST NOT initiate a new DAG if it does not have any
      parent left in a P2P-RPL DAG.

   o  Mode of Operation (MOP): This field MUST be set to four,
      corresponding to P2P Route Discovery mode.

   o  Destination Advertisement Trigger Sequence Number (DTSN): This
      field MUST be set to zero on transmission and ignored on
      reception.

   o  DODAGPreference (Prf): This field MUST be set to zero (least
      preferred).

   o  DODAGID: This field MUST be set to an IPv6 address of the Origin.

   o  The other fields in the DIO Base object can be set in the desired
      fashion as per the rules described in [RFC6550].

   A received P2P mode DIO MUST be discarded if it does not follow the
   above-listed rules regarding the RPLInstanceID, Version Number,
   G flag, MOP, and Prf fields inside the Base object.

   The DODAG Configuration Option inside a P2P mode DIO MUST be set in
   the following manner:

   o  The Origin MUST set the MaxRankIncrease parameter to zero to
      disable local repair of the temporary DAG.  A received P2P mode
      DIO MUST be discarded if the MaxRankIncrease parameter inside the
      DODAG Configuration Option is not zero.

   o  The Origin SHOULD set the Trickle parameters
      (DIOIntervalDoublings, DIOIntervalMin, DIORedundancyConstant) as
      recommended in Section 9.2.

   o  The Origin sets the Default Lifetime and Lifetime Unit parameters
      to indicate the lifetime of the state that the routers, including
      the Origin and the Target(s), maintain for a Hop-by-hop or Source
      Route discovered using P2P-RPL.

   o  The Origin sets the other fields in the DODAG Configuration
      Option, including the Objective Code Point (OCP) identifying the
      Objective Function, in the desired fashion as per the rules
      described in [RFC6550].

   o  As discussed in Section 14, P2P-RPL does not distinguish between
      the "preinstalled" and "authenticated" security modes described in
      [RFC6550].  Consequently, the Origin MUST set the Authentication
      Enabled (A) flag to zero.  A received P2P mode DIO MUST be
      discarded if the A flag inside the DODAG Configuration Option is
      not zero.

   o  An Intermediate Router (or a Target) MUST set various fields in
      the DODAG Configuration Option in the outgoing P2P mode DIOs to
      the values they had in the incoming P2P mode DIOs for this DAG.

   A default DODAG Configuration Option takes effect if a P2P mode DIO
   does not carry an explicit one.  The default DODAG Configuration
   Option has the following parameter values:

   o  Authentication Enabled: 0

   o  DIOIntervalMin: 6, which translates to 64 ms as the value for the
      Imin parameter in a Trickle operation.  This value is roughly one
      order of magnitude larger than the typical transmission delay on
      IEEE 802.15.4 links and corresponds to the recommendation in
      Section 9.2 for well-connected topologies.

   o  DIORedundancyConstant: 1.  See the discussion in Section 9.2.

   o  MaxRankIncrease: 0 (to disable local repair of the temporary DAG).

   o  Default Lifetime: 0xFF, to correspond to infinity.

   o  Lifetime Unit: 0xFFFF, to correspond to infinity.

   o  Objective Code Point: 0, i.e., OF0 [RFC6552] is the default
      Objective Function (OF).

   o  The remaining parameters have default values as specified in
      [RFC6550].

   Individual P2P-RPL deployments are encouraged to share their
   experience with these default values to help guide the development of
   a Standards Track version of the protocol.

   The routing metrics and constraints [RFC6551] used in P2P-RPL route
   discovery are included in one or more Metric Container options
   [RFC6550] inside the P2P mode DIO.  Note that a DIO need not include
   a Metric Container if OF0 is the Objective Function in effect.  In
   that case, a P2P mode DIO may still specify an upper limit on the
   maximum rank, that a router may have in the temporary DAG, inside
   the P2P-RDO.

   A P2P mode DIO:

   o  MUST carry one (and only one) P2P-RDO.  The P2P-RDO allows for the
      specification of one unicast or multicast address for the Target.
      A received P2P mode DIO MUST be discarded if it does not contain
      exactly one P2P-RDO.

   o  MAY carry one or more RPL Target options to specify additional
      unicast/multicast addresses for the Target.  If a unicast address
      is specified, it MUST be a global address or a unique-local
      address.

   o  MAY carry one or more Metric Container options to specify routing
      metrics and constraints.

   o  MAY carry one or more Route Information Options [RFC6550].  In the
      context of P2P-RPL, a Route Information Option advertises to the
      Target(s) the Origin's connectivity to the prefix specified in the
      option.

   o  MAY carry one DODAG Configuration Option.  If a P2P mode DIO does
      not carry an explicit DODAG Configuration Option, the default
      DODAG Configuration Option defined in this section is considered
      to be in effect.

   A RPL option other than those listed above MUST be ignored when found
   inside a received P2P mode DIO and MUST NOT be included in the P2P
   mode DIOs that the receiving router generates.

   In accordance with core RPL, a P2P mode DIO MUST propagate via link-
   local multicast.  The IPv6 source address in a P2P mode DIO MUST be a
   link-local address, and the IPv6 destination address MUST be the
   link-local multicast address all-RPL-nodes [RFC6550].  A P2P mode DIO
   MUST be transmitted on all interfaces the router has in this RPL
   routing domain [RFC6554].

7.  P2P Route Discovery Option (P2P-RDO)

   This section defines a new RPL control message option: the P2P Route
   Discovery Option (P2P-RDO).

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Type = 0x0a | Option Length |R|H| N | Compr | L |MaxRank/NH |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                         TargetAddr                            |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                       Address[1..n]                           |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 1: Format of the P2P Route Discovery Option (P2P-RDO)

   The format of a P2P Route Discovery Option (P2P-RDO) is illustrated
   in Figure 1.  A P2P mode DIO and a P2P-DRO message (defined in
   Section 8) MUST carry exactly one P2P-RDO.  A P2P-RDO consists of the
   following fields:

   o  Option Type: 0x0a.

   o  Option Length: This field is an 8-bit unsigned integer
      representing the length in octets of the option, not including the
      Option Type and Option Length fields.

   o  Reply (R): The Origin sets this flag to one to allow the Target(s)
      to send P2P-DRO messages back to the Origin.  If this flag is set
      to zero, a Target MUST NOT generate any P2P-DRO messages.

   o  Hop-by-hop (H): This flag is valid only if the R flag is set to
      one.  The Origin sets this flag to one if it desires Hop-by-hop
      Routes.  The Origin sets this flag to zero if it desires Source
      Routes.  This specification allows for the establishment of one
      Hop-by-hop Route or up to four Source Routes per Target.  The
      Hop-by-hop Route is established in the Forward direction, i.e.,
      from the Origin to the Target.  This specification does not allow
      for the establishment of Hop-by-hop Routes in the Reverse
      direction.

   o  Number of Routes (N): This field is valid only if the R flag is
      set to one and the H flag is set to zero, i.e., the Targets are
      allowed to generate P2P-DRO messages carrying discovered Source
      Routes back to the Origin.  In this case, the value in the N field
      plus one indicates the number of Source Routes that each Target
      should convey to the Origin.  When Hop-by-hop Routes are being
      discovered, the N field MUST be set to zero on transmission and
      ignored on reception.

   o  Compr: This field is a 4-bit unsigned integer indicating the
      number of prefix octets that are elided from the Target field and
      the Address vector.  For example, the Compr value will be zero if
      full IPv6 addresses are carried in the Target field and the
      Address vector.

   o  Lifetime (L): This is a 2-bit field that indicates the exact
      duration that a router joining the temporary DAG, including the
      Origin and the Target(s), MUST maintain its membership in the DAG.
      A router MUST leave the temporary DAG once the time elapsed since
      it joined reaches the value indicated by this field.  The mapping
      between the value in this field and the duration of the router's
      membership in the temporary DAG is as follows:

      *  0x00: 1 second

      *  0x01: 4 seconds

      *  0x02: 16 seconds

      *  0x03: 64 seconds

      The Origin sets this field based on its expectation regarding the
      time required for the route discovery to complete, which includes
      the time required for the DIOs to reach the Target(s) and the
      P2P-DROs to travel back to the Origin.  The time required for the
      DIOs to reach the Target(s) would in turn depend on the Trickle
      parameters (Imin and the redundancy constant) as well as the
      expected distance (in terms of hops and/or ETX) to the Target(s).
      While deciding on the value in this field, the Origin should also
      take into account the fact that all routers joining the temporary
      DAG would need to stay in the DAG for this much time.

   o  MaxRank/NH:

      *  When a P2P-RDO is included in a P2P mode DIO, this field
         indicates the upper limit on the integer portion of the rank
         (calculated using the DAGRank() macro defined in [RFC6550])
         that a router may have in the temporary DAG being created.  An
         Intermediate Router MUST NOT join a temporary DAG being created
         by a P2P mode DIO if the integer portion of its rank would be
         equal to or higher (in numerical value) than the MaxRank limit.
         A Target can join the temporary DAG at a rank whose integer
         portion is equal to the MaxRank.  A router MUST discard a
         received P2P mode DIO if the integer part of the advertised
         rank equals or exceeds the MaxRank limit.  A value of 0 in this
         field indicates that the MaxRank is infinity.

      *  When a P2P-RDO is included in a P2P-DRO message, this field
         indicates the index of the next-hop (NH) address inside the
         Address vector.

   o  TargetAddr: This is an IPv6 address of the Target after eliding
      Compr number of prefix octets.  When the P2P-RDO is included in a
      P2P mode DIO, this field may contain a unicast address or a
      multicast address.  If a unicast address is specified, it MUST be
      a global address or a unique-local address.  Any additional Target
      addresses can be specified by including one or more RPL Target
      options [RFC6550] in the DIO.  When the P2P-RDO is included in a
      P2P-DRO, this field MUST contain a unicast global or unique-local
      IPv6 address of the Target generating the P2P-DRO.

   o  Address[1..n]: This is a vector of IPv6 addresses representing a
      complete route so far in the Forward direction:

      *  Each element in the Address vector has size (16 - Compr) octets
         and MUST contain a valid global or unique-local IPv6 address
         with the first Compr octets elided.

      *  The total number of elements inside the Address vector is given
         by n = (Option Length - 2 - (16 - Compr))/(16 - Compr).

      *  The IPv6 address that a router adds to the vector MUST belong
         to the interface on which the router received the DIO
         containing this P2P-RDO.  Further, this interface MUST NOT be
         an Ingress-only Interface.  This allows the route accumulated
         in the Address vector to be a Bidirectional Route that can be
         used by a Target to send a P2P-DRO message to the Origin.

      *  The Address vector MUST carry the accumulated route in the
         Forward direction, i.e., the first element in the Address
         vector must contain the IPv6 address of the router next to the
         Origin, and so on.

      *  The Origin and Target addresses MUST NOT be included in the
         Address vector.

      *  A router adding its address to the vector MUST ensure that none
         of its addresses already exist in the vector.  A Target
         specifying a complete route in the Address vector MUST ensure
         that the vector does not contain any address more than once.

      *  The Address vector MUST NOT contain any multicast addresses.

8.  The P2P Discovery Reply Object (P2P-DRO)

   This section defines two new RPL control message types: the P2P
   Discovery Reply Object (P2P-DRO), with code 0x04; and the Secure
   P2P-DRO, with code 0x84.  A P2P-DRO serves one of the following
   functions:

   o  carries a discovered Source Route from a Target to the Origin;

   o  establishes a Hop-by-hop Route as it travels from a Target to the
      Origin.

   A P2P-DRO message can also serve the function of letting the routers
   in the LLN know that a P2P-RPL route discovery is complete and no
   more DIO messages need to be generated for the corresponding
   temporary DAG.  A P2P-DRO message MUST carry one (and only one)
   P2P-RDO whose TargetAddr field MUST contain a unicast IPv6 address of
   the Target that generates the P2P-DRO.  A P2P-DRO message MUST travel
   from the Target to the Origin via link-local multicast along the
   route specified inside the Address vector in the P2P-RDO, as included
   in the P2P-DRO.  The IPv6 source address in a P2P-DRO message MUST be
   a link-local address, and the IPv6 destination address MUST be the
   link-local multicast address all-RPL-nodes [RFC6550].  A P2P-DRO
   message MUST be transmitted on all interfaces the router has in this
   RPL routing domain [RFC6554].

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | RPLInstanceID |    Version    |S|A|Seq|     Reserved          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                         DODAGID                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Option(s)...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...

     Figure 2: Format of the Base P2P Discovery Reply Object (P2P-DRO)

   The format of the base P2P Discovery Reply Object (P2P-DRO) is shown
   in Figure 2.  A base P2P-DRO consists of the following fields:

   o  RPLInstanceID: This field provides the RPLInstanceID of the
      temporary DAG used for route discovery.

   o  Version: This field provides the Version of the temporary DAG used
      for route discovery.  Since a temporary DAG always has value zero
      for the Version, this field MUST always be set to zero.

   o  Stop (S): This flag, when set to one by a Target, indicates that
      the P2P-RPL route discovery is over.  All the routers receiving
      such a P2P-DRO, including those not listed in the route carried
      inside a P2P-RDO,

      *  SHOULD NOT process any more DIOs received for this
         temporary DAG;

      *  SHOULD NOT generate any more DIOs for this temporary DAG;

      *  SHOULD cancel any pending DIO transmissions for this
         temporary DAG.

      Note that the Stop flag serves to stop further DIO
      generation/processing for a P2P-RPL route discovery but does not
      affect the processing of P2P-DRO messages at either the Origin or
      the Intermediate Routers.  In other words, a router (the Origin or
      an Intermediate Router) MUST continue to process the P2P-DRO
      messages even if an earlier P2P-DRO message (with the same
      RPLInstanceID and DODAGID fields) had the Stop flag set to one.
      When set to zero, this flag does not imply anything and MUST be
      ignored on reception.

   o  Ack Required (A): This flag, when set to one by the Target,
      indicates that the Origin MUST unicast a P2P-DRO-ACK message
      (defined in Section 10) to the Target when it receives the
      P2P-DRO.

   o  Sequence Number (Seq): This 2-bit field indicates the sequence
      number for the P2P-DRO.  This field is relevant when the A flag is
      set to one, i.e., the Target requests an acknowledgement from the
      Origin for a received P2P-DRO.  The Origin includes the
      RPLInstanceID, the DODAGID, and the Sequence Number of the
      received P2P-DRO inside the P2P-DRO-ACK message it sends back to
      the Target.

   o  Reserved: These bits are reserved for future use.  These bits MUST
      be set to zero on transmission and MUST be ignored on reception.

   o  DODAGID: This field provides the DODAGID of the temporary DAG used
      for route discovery.  The DODAGID also identifies the Origin.  The
      RPLInstanceID, the Version, and the DODAGID together uniquely
      identify the temporary DAG used for route discovery and can be
      copied from the DIO message advertising the temporary DAG.

   o  Options: The P2P-DRO message:

      *  MUST carry one (and only one) P2P-RDO that MUST specify a
         complete route between the Target and the Origin.  A received
         P2P-DRO message MUST be discarded if it does not contain
         exactly one P2P-RDO.

      *  MAY carry one or more Metric Container options that contain the
         aggregated routing metrics values for the route specified in
         the P2P-RDO.

      A RPL option other than those listed above MUST be ignored when
      found inside a received P2P-DRO message.

8.1.  Secure P2P-DRO

   A Secure P2P-DRO message follows the format shown in Figure 7 of
   [RFC6550], where the base format is the base P2P-DRO shown in
   Figure 2.

8.2.  Setting a P2P-RDO Carried in a P2P Discovery Reply Object

   A P2P Discovery Reply Object MUST carry one (and only one) P2P-RDO,
   which MUST be set as defined in Section 7.  Specifically, the
   following fields MUST be set as follows:

   o  Reply (R): This flag MUST be set to zero on transmission and
      ignored on reception.

   o  Hop-by-Hop (H): The H flag in the P2P-RDO included in a P2P-DRO
      message MUST have the same value as the H flag in the P2P-RDO
      inside the corresponding DIO message.

   o  Number of Routes (N): This field MUST be set to zero on
      transmission and ignored on reception.

   o  Lifetime (L): This field MUST be set to zero on transmission and
      ignored on reception.

   o  MaxRank/NH: This field indicates the index of the next-hop address
      in the Address vector.  When a Target generates a P2P-DRO message,
      the NH field is set to n = (Option Length - 2 - (16 - Compr))/
      (16 - Compr).

   o  TargetAddr: This field MUST contain a unicast global or unique-
      local IPv6 address of the Target generating the P2P-DRO.

   o  Address[1..n]: The Address vector MUST contain a complete route
      between the Origin and the Target such that the first element in
      the vector contains the IPv6 address of the router next to the
      Origin and the last element contains the IPv6 address of the
      router next to the Target.

9.  P2P-RPL Route Discovery by Creating a Temporary DAG

   This section details the P2P-RPL route discovery operation.

9.1.  Joining a Temporary DAG

   All the routers participating in a P2P-RPL route discovery, including
   the Origin and the Target(s), MUST join the temporary DAG being
   created for that purpose.  When a router joins a temporary DAG
   advertised by a P2P mode DIO, it MUST maintain its membership in the
   temporary DAG for the duration indicated by the L field inside the
   P2P-RDO.  The only purpose of a temporary DAG's existence is to
   facilitate the P2P-RPL route discovery process.  The temporary DAG
   MUST NOT be used to route data packets.  In other words, joining a
   temporary DAG does not allow a router to provision routing table

   entries listing the router's parents in the temporary DAG as the next
   hops (i.e., the last bullet point in Section 3.2.8 of [RFC6550] is
   not applicable when the DAG is a temporary DAG created for the
   purpose of a P2P-RPL route discovery).

   Given the nature of a temporary DAG created for a P2P-RPL route
   discovery, this document disallows the solicitation of P2P mode DIOs
   using DODAG Information Solicitation (DIS) messages as described in
   [RFC6550].  A router participating in a P2P-RPL route discovery MUST
   NOT reset its Trickle timer, which controls the transmission of P2P
   mode DIOs in response to a multicast DIS.  Also, the router MUST NOT
   send a P2P mode DIO in response to a unicast DIS.  In other words,
   the rules in Section 8.3 of [RFC6550] regarding a router's response
   to a multicast/unicast DIS are not applicable for P2P mode DIOs.

   A router MUST detach from the temporary DAG created for a P2P-RPL
   route discovery once the duration of its membership in the DAG has
   reached the value indicated by the L field inside the P2P-RDO.  After
   receiving a P2P-DRO with the Stop flag set to one, a router SHOULD
   NOT send or process any more DIOs for this temporary DAG and SHOULD
   also cancel any pending DIO transmissions.

9.2.  Trickle Operation for P2P Mode DIOs

   A RPL router uses a Trickle timer [RFC6206] to control DIO
   transmissions.  The Trickle control of DIO transmissions provides
   quick resolution of any "inconsistency" while avoiding redundant DIO
   transmissions.  The Trickle algorithm also imparts protection against
   loss of DIOs due to inherent lack of reliability in LLNs.  When
   controlling the transmissions of a P2P mode DIO, a Trickle timer
   SHOULD follow the following rules:

   o  The receipt of a P2P mode DIO that allows the router to advertise
      a better route (in terms of the routing metrics and the OF in use)
      than before is considered "inconsistent" and hence resets the
      Trickle timer.  Note that the first receipt of a P2P mode DIO
      advertising a particular temporary DAG is always considered an
      "inconsistent" event.

   o  The receipt of a P2P mode DIO from a parent in the temporary DAG
      is considered neither "consistent" nor "inconsistent" if it does
      not allow the router to advertise a better route than before.
      Thus, the receipt of such DIOs has no impact on the Trickle
      operation.  Note that this document does not impose any
      requirements on how a router might choose its parents in the
      temporary DAG.

   o  The receipt of a P2P mode DIO is considered "consistent" if the
      source of the DIO is not a parent in the temporary DAG and either
      of the following conditions is true:

      *  The DIO advertises a better route than the router but does not
         allow the router to advertise a better route itself; or

      *  The DIO advertises a route as good as the route (to be)
         advertised by the router.

      Note that the Trickle algorithm's DIO suppression rules are in
      effect at all times.  Hence, a P2P-RPL router may suppress a DIO
      transmission even if it has not made any DIO transmissions yet.

   o  The receipt of a P2P mode DIO that advertises a worse route than
      what the router advertises (or would advertise when it gets a
      chance to generate its DIO) is considered neither "consistent" nor
      "inconsistent", i.e., the receipt of such a DIO has no impact on
      the Trickle operation.

   o  The Imin parameter SHOULD be set taking into account the
      connectivity within the network.  For highly connected networks, a
      small Imin value (on the order of the typical transmission delay
      for a DIO) may lead to congestion in the network as a large number
      of routers reset their Trickle timers in response to the first
      receipt of a DIO from the Origin.  These routers would generate
      their DIOs within the Imin interval and cause additional routers
      to reset their Trickle timers and generate more DIOs.  Thus, for
      highly connected networks, the Imin parameter SHOULD be set to a
      value at least one order of magnitude larger than the typical
      transmission delay for a DIO.  For sparsely connected networks,
      the Imin parameter can be set to a value that is a small multiple
      of the typical transmission delay for a DIO.  Note that the Imin
      value has a direct impact on the time required for a P2P-RPL route
      discovery to complete.  In general, the time required for a
      P2P-RPL route discovery would increase approximately linearly with
      the value of the Imin parameter.  Since the route discovery must
      complete while the Origin still belongs to the temporary DAG
      created for that purpose, the Origin should set the time duration
      for which a router maintains its membership in the temporary DAG
      (indicated by the L field inside the P2P-RDO) to a large enough
      value, taking into account the Imin value as well as the expected
      distance (in terms of hops and/or ETX) to the Target(s).

   o  The Imax parameter SHOULD be set to a large value (several orders
      of magnitude higher than the Imin value) and is unlikely to be
      critical for P2P-RPL operation.  This is because the first receipt
      of a P2P mode DIO for a particular temporary DAG is considered an
      inconsistent event and would lead to the resetting of the Trickle
      timer duration to the Imin value.  Given the temporary nature of
      the DAGs used in P2P-RPL, the Trickle timer may not get a chance
      to increase much.

   o  The recommended value of redundancy constant "k" is 1.  With this
      value of "k", a DIO transmission will be suppressed if the router
      receives even a single "consistent" DIO during a timer interval.
      This setting for the redundancy constant is designed to reduce the
      number of messages generated during a route discovery process and
      is suitable for environments with low or moderate packet loss
      rates.  However, this setting may result in an increase in the
      time required for the route discovery process to complete.  A
      higher value for the redundancy constant may be more suitable in

      *  environments with high packet loss rates; or

      *  deployments where the time required for the route discovery
         process to complete needs to be as small as possible; or

      *  deployments where specific destinations are reachable only
         through specific Intermediate Routers (and hence these
         Intermediate Routers should not suppress their DIOs).

      A particular deployment should take into account the above-
      mentioned factors when deciding on the value of the redundancy
      constant.

   Individual P2P-RPL deployments are encouraged to share their
   experience with these rules to help guide the development of a
   Standards Track version of the protocol.  Applicability Statements
   that specify the use of P2P-RPL MUST provide guidance for setting
   Trickle parameters, particularly Imin and the redundancy constant.

9.3.  Processing a P2P Mode DIO

   The rules for DIO processing and transmission as described in
   Section 8 of RPL [RFC6550] apply to P2P mode DIOs as well, except as
   modified in this document.  In particular, in accordance with
   Section 8.2.3 of RPL [RFC6550], a received P2P mode DIO MUST be
   discarded if it is malformed, according to the rules specified in
   this document and in [RFC6550].

   The following rules for processing a received P2P mode DIO apply to
   both Intermediate Routers and the Target.

   A router SHOULD discard a received P2P mode DIO with no further
   processing if it does not have bidirectional reachability with the
   neighbor that generated the received DIO.  Note that bidirectional
   reachability does not mean that the link must have the same values
   for a routing metric in both directions.  A router SHOULD calculate
   the values of the link-level routing metrics included in the received
   DIO, taking into account the metric's value in both Forward and
   Reverse directions.  Bidirectional reachability along a discovered
   route allows the Target to use this route to reach the Origin.  In
   particular, the P2P-DRO messages travel from the Target to the Origin
   along a discovered route.

   A router MUST discard a received P2P mode DIO with no further
   processing:

   o  if the DIO advertises INFINITE_RANK as defined in Section 17
      of [RFC6550]

   o  if the integer part of the rank advertised in the DIO equals or
      exceeds the MaxRank limit listed in the P2P Route Discovery Option

   o  if the routing metric values do not satisfy one or more of the
      mandatory route constraints listed in the DIO or if the router
      cannot evaluate the mandatory route constraints, e.g., if the
      router does not support the metrics used in the constraints

   o  if the router previously received a P2P-DRO message with the same
      RPLInstanceID and DODAGID as the received DIO and with the Stop
      flag set to one

   The router MUST check the Target addresses listed in the P2P-RDO and
   any RPL Target options included in the received DIO.  If one of its
   IPv6 addresses is listed as a Target address or if it belongs to the
   multicast group specified as one of the Target addresses, the router
   considers itself a Target and processes the received DIO as specified
   in Section 9.5.  Otherwise, the router considers itself an
   Intermediate Router and processes the received DIO as specified in
   Section 9.4.

9.4.  Additional Processing of a P2P Mode DIO at an Intermediate Router

   An Intermediate Router MUST discard a received P2P mode DIO with no
   further processing

   o  if the DIO is received on an Ingress-only Interface; or

   o  if the receiving interface does not have a global or unique-local
      IPv6 address configured with the address prefix implied by the
      Compr field in the P2P-RDO inside the received DIO; or

   o  if the router cannot uniquely identify the address prefix implied
      by the Compr field in the P2P-RDO (this might happen if the
      receiving interface has multiple global/unique-local IPv6
      addresses, each configured with a different address prefix); or

   o  if adding its IPv6 address to the route in the Address vector
      inside the P2P-RDO would result in the route containing multiple
      addresses belonging to this router.

   On receiving a P2P mode DIO, an Intermediate Router MUST do the
   following.  The router MUST determine whether this DIO advertises a
   better route than the router itself and whether the receipt of the
   DIO would allow the router to advertise a better route than before.
   Accordingly, the router SHOULD consider this DIO as
   consistent/inconsistent from the Trickle perspective, as described in
   Section 9.2.  Note that the route comparison in a P2P-RPL route
   discovery is performed using the parent selection rules of the OF in
   use as specified in Section 14 of RPL [RFC6550].  If the received DIO
   would allow the router to advertise a better route, the router MUST
   add a unicast IPv6 address of the receiving interface (after eliding
   Compr prefix octets) to the route in the Address vector inside the
   P2P-RDO and remember this route for inclusion in its future DIOs.

   When an Intermediate Router adds an IPv6 address to a route, it MUST
   ensure that

   o  the IPv6 address is a unicast global or unique-local IPv6 address
      assigned to the interface on which the DIO containing the route
      was received;

   o  the IPv6 address was configured with the address prefix implied by
      the Compr field in the P2P-RDO inside the received DIO.

   To improve the diversity of the routes being discovered, an
   Intermediate Router SHOULD keep track of multiple routes (as long as
   all these routes are the best seen so far), one of which SHOULD be
   selected in a uniform random manner for inclusion in the P2P-RDO

   inside the router's next DIO.  Note that the route accumulation in a
   P2P mode DIO MUST take place even if the Origin does not want any
   P2P-DRO messages to be generated (i.e., the R flag inside the P2P-RDO
   is set to zero).  This is because the Target may still be able to use
   the accumulated route as a Source Route to reach the Origin.

9.5.  Additional Processing of a P2P Mode DIO at the Target

   The Target MAY remember the discovered route contained in the P2P-RDO
   in the received DIO for use as a Source Route to reach the Origin.
   The lifetime of this Source Route is specified by the Default
   Lifetime and Lifetime Unit parameters inside the DODAG Configuration
   Option currently in effect.  This lifetime can be extended (or
   shortened) appropriately, following a hint from an upper-layer
   protocol.

   If the Reply flag inside the P2P-RDO in the received DIO is set to
   one, the Target MUST select one or more discovered routes and send
   one or more P2P-DRO messages, carrying one discovered route each,
   back to the Origin.  If the H flag inside the P2P-RDO is set to one,
   the Target needs to select one route and send a P2P-DRO message along
   this route back to the Origin.  As this P2P-DRO message travels back
   to the Origin, the routers on the path establish a hop-by-hop routing
   state, thereby establishing a Hop-by-hop Route in the Forward
   direction.  If the H flag is set to zero, the number of Source Routes
   to be selected (and the number of P2P-DRO messages to be sent back)
   is given by one plus the value of the N field in the P2P-RDO.  The
   Target may select the discovered route inside the received DIO as one
   or more of the routes that would be carried inside a P2P-DRO message
   back to the Origin.  This document does not prescribe a particular
   method for the Target to select the routes.  Example methods include
   selecting each route that meets the specified routing constraints
   until the desired number of routes has been selected, or selecting
   the best routes discovered over a certain time period.  If multiple
   routes are to be selected, the Target SHOULD avoid selecting routes
   that have large segments in common.

   If the Target selects the route contained in the P2P-RDO in the
   received DIO, it sends a P2P-DRO message back to the Origin
   (identified by the DODAGID field in the DIO).  The P2P-DRO message
   MUST include a P2P-RDO that contains the selected route inside the
   Address vector.  Various fields inside the P2P-RDO MUST be set as
   specified in Section 8.2.  The Target MAY set the A flag inside the
   P2P-DRO message to one if it desires the Origin to send back a
   P2P-DRO-ACK message on receiving the P2P-DRO.  In this case, the
   Target waits for the duration of P2P_DRO_ACK_WAIT_TIME for the
   P2P-DRO-ACK message to arrive.  Failure to receive the P2P-DRO-ACK
   message within this time duration causes the Target to retransmit the

   P2P-DRO message.  The Target MAY retransmit the P2P-DRO message in
   this fashion up to MAX_P2P_DRO_RETRANSMISSIONS times.  Both
   P2P_DRO_ACK_WAIT_TIME and MAX_P2P_DRO_RETRANSMISSIONS are
   configurable parameters to be chosen based on the characteristics of
   individual deployments.  Note that all P2P-DRO transmissions and
   retransmissions MUST take place while the Target is still a part of
   the temporary DAG created for the route discovery.  A Target MUST NOT
   transmit a P2P-DRO if it no longer belongs to this DAG.

   The Target MAY set the Stop flag inside the P2P-DRO message to one if

   o  this router is the only Target specified in the corresponding DIO,
      i.e., the corresponding DIO specified a unicast address of the
      router as the TargetAddr inside the P2P-RDO with no additional
      Targets specified via RPL Target options; and

   o  the Target has already selected the desired number of routes.

   The Target MAY include a Metric Container option in the P2P-DRO
   message.  This Metric Container contains the end-to-end routing
   metric values for the route specified in the P2P-RDO.  The Target
   MUST transmit the P2P-DRO message via a link-local multicast.

   A Target MUST NOT forward a P2P mode DIO any further if no other
   Targets are to be discovered, i.e., if a unicast IPv6 address (of
   this Target) is specified as the TargetAddr inside the P2P-RDO and no
   additional Targets are specified via RPL Target options inside the
   DIOs for this route discovery.  Otherwise, the Target MUST generate
   DIOs for this route discovery as an Intermediate Router would.

9.6.  Processing a P2P-DRO at an Intermediate Router

   If the DODAGID field in the received P2P-DRO does not list a router's
   own IPv6 address, the router considers itself an Intermediate Router
   and MUST process the received message in the following manner:

   o  The router MUST discard the received P2P-DRO with no further
      processing if it does not belong to the temporary DAG identified
      by the RPLInstanceID and the DODAGID fields in the P2P-DRO.

   o  If the Stop flag inside the received P2P-DRO is set to one, the
      router SHOULD NOT send or receive any more DIOs for this temporary
      DAG and SHOULD cancel any pending DIO transmissions.

   o  The router MUST ignore any Metric Container options contained in
      the P2P-DRO message.

   o  If an Address[NH] element inside the P2P-RDO lists the router's
      own unicast IPv6 address, the router is a part of the route
      carried in the P2P-RDO.  In this case, the router MUST do the
      following:

      *  To prevent loops, the router MUST discard the P2P-DRO message
         with no further processing if the Address vector in the P2P-RDO
         includes multiple IPv6 addresses assigned to the router's
         interfaces.

      *  If the H flag inside the P2P-RDO is set to one, the router MUST
         store the state for the Forward Hop-by-hop Route carried inside
         the P2P-RDO.  This state consists of:

         +  the RPLInstanceID and the DODAGID fields of the P2P-DRO

         +  the route's destination, the Target (identified by the
            TargetAddr field inside the P2P-RDO)

         +  the IPv6 address of the next hop, Address[NH+1] (unless the
            NH value equals the number of elements in the Address
            vector, in which case the Target itself is the next hop)

         This Hop-by-hop routing state MUST expire at the end of the
         lifetime specified by the Default Lifetime and Lifetime Unit
         parameters inside the DODAG Configuration Option used in P2P
         mode DIOs for this route discovery.

      *  If the router already maintains a Hop-by-hop state listing the
         Target as the destination and carrying the same RPLInstanceID
         and DODAGID fields as the received P2P-DRO, and the next-hop
         information in the state does not match the next hop indicated
         in the received P2P-DRO, the router MUST discard the P2P-DRO
         message with no further processing.  Note that this situation
         would occur in the following two cases:

         +  When the route listed in the Address vector inside the
            P2P-RDO contains a previously undetected loop.  In this
            case, this rule causes the P2P-DRO messages to be discarded.

         +  When a Hop-by-hop Route between the Origin and the Target,
            previously established using the same RPLInstanceID and
            DODAGID as the route currently being established, still
            exists and at least partially overlaps the route currently
            being established.

      *  The router MUST decrement the NH field inside the P2P-RDO and
         send the P2P-DRO message further via link-local multicast.

9.7.  Processing a P2P-DRO at the Origin

   When a router receives a P2P-DRO message that lists its IPv6 address
   in the DODAGID field, the router recognizes itself as the Origin for
   the corresponding P2P-RPL route discovery, notes the Target that
   originated this message (from the TargetAddr field inside the
   P2P-RDO), and processes the message in the following manner:

   o  The Origin MUST discard the received P2P-DRO with no further
      processing if it no longer belongs to the temporary DAG identified
      by the RPLInstanceID and the DODAGID fields in the P2P-DRO.

   o  If the Stop flag inside the received P2P-DRO is set to one, the
      Origin SHOULD NOT generate any more DIOs for this temporary DAG
      and SHOULD cancel any pending DIO transmissions.

   o  If the P2P-RDO inside the P2P-DRO has the H flag set to zero, the
      Address vector inside the P2P-RDO contains a Source Route to this
      Target.  The Origin MUST set the lifetime of this Source Route to
      the value specified by the Default Lifetime and Lifetime Unit
      parameters inside the DODAG Configuration Option in the P2P mode
      DIOs used for this route discovery.  This lifetime could be
      extended (or shortened) appropriately, following a hint from an
      upper-layer protocol.

   o  If the P2P-RDO inside the P2P-DRO has the H flag set to one, the
      P2P-DRO message is establishing a Hop-by-hop Route to this Target,
      and the Origin MUST store in its memory the state for this
      Hop-by-hop Route in the manner described in Section 9.6.  This
      Hop-by-hop routing state MUST expire at the end of the lifetime
      specified by the Default Lifetime and Lifetime Unit parameters
      inside the DODAG Configuration Option used in P2P mode DIOs for
      this route discovery.  A Standards Track version of P2P-RPL may
      consider specifying a signaling mechanism that will allow the
      Origin to extend (or shorten) the lifetime of a P2P-RPL Hop-by-hop
      Route, following a suitable hint from an upper-layer protocol.

   o  If the received P2P-DRO message contains one or more Metric
      Container options, the Origin MAY store the values of the routing
      metrics associated with the discovered route in its memory.  This
      information may be useful in formulating the constraints for any
      future P2P-RPL route discovery to this Target.

   o  If the A flag is set to one in the received P2P-DRO message, the
      Origin MUST generate a P2P-DRO-ACK message as described in
      Section 10 and unicast the message to the Target.  The Origin MAY
      use the route just discovered to send the P2P-DRO-ACK message to
      the Target.  Section 12 describes how a packet may be forwarded
      along a Source/Hop-by-hop Route discovered using P2P-RPL.

10.  The P2P Discovery Reply Object Acknowledgement (P2P-DRO-ACK)

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | RPLInstanceID |    Version    |Seq|        Reserved           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                         DODAGID                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 3: Format of the Base P2P Discovery Reply Object
                       Acknowledgement (P2P-DRO-ACK)

   A P2P-DRO message may fail to reach the Origin due to a number of
   reasons.  Unlike the DIO messages, which benefit from Trickle-
   controlled retransmissions, the P2P-DRO messages are prone to loss
   due to unreliable packet transmission in LLNs.  Since a P2P-DRO
   message travels via link-local multicast, it cannot use link-level
   acknowledgements to improve the reliability of its transmission.
   Also, an Intermediate Router may drop the P2P-DRO message (e.g.,
   because of its inability to store the state for the Hop-by-hop Route
   that the P2P-DRO is establishing).  To protect against the potential
   failure of a P2P-DRO message to reach the Origin, the Target MAY
   request that the Origin send back a P2P-DRO Acknowledgement
   (P2P-DRO-ACK) message on receiving a P2P-DRO message.  Failure to
   receive such an acknowledgement within the P2P_DRO_ACK_WAIT_TIME
   interval of sending the P2P-DRO message forces the Target to resend
   the message (as described in Section 9.5).

   This section defines two new RPL control message types: the P2P-DRO
   Acknowledgement (P2P-DRO-ACK), with code 0x05; and the Secure
   P2P-DRO-ACK, with code 0x85.  A P2P-DRO-ACK message MUST travel as a
   unicast message from the Origin to the Target.  The IPv6 source and
   destination addresses used in a P2P-DRO-ACK message MUST be global or
   unique-local.  The format of a base P2P-DRO-ACK message is shown in
   Figure 3.  Various fields in a P2P-DRO-ACK message MUST have the same
   values as the corresponding fields in the P2P-DRO message.  The field
   marked as "Reserved" MUST be set to zero on transmission and MUST be

   ignored on reception.  A Secure P2P-DRO-ACK message follows the
   format shown in Figure 7 of [RFC6550], where the base format is the
   same as the base P2P-DRO-ACK shown in Figure 3.

11.  Secure P2P-RPL Operation

   Each RPL control message type, including those defined in this
   document, has a secure version.  A secure RPL control message is
   identified by the value 1 in the most significant bit of the Code
   field.  Each secure RPL control message contains a Security section
   (see Figures 7 and 8 of [RFC6550]) whose contents are described in
   Section 6.1 of [RFC6550].  Sections 6.1, 10, and 19 of [RFC6550]
   describe core RPL's security apparatus.  These sections are
   applicable to P2P-RPL's secure operation as well, except as
   constrained in this section.

   Core RPL allows a router to decide locally on a per-packet basis
   whether to use security and, if yes, what Security Configuration (see
   definition in Section 3) to use (the only exception being the
   requirement to send a Secure DIO in response to a Secure DIS; see
   Section 10.2 of [RFC6550]).  In contrast, this document requires that
   routers participating in a P2P-RPL route discovery follow the
   Origin's lead regarding security.  The Origin decides whether to use
   security, and the particular Security Configuration to be used for
   this purpose.  All the routers participating in this route discovery
   MUST generate only secure control messages if the Origin so decides
   and MUST use for this purpose the Security Configuration that the
   Origin chose.  The Origin MUST NOT set the "Key Identifier Mode"
   field inside the chosen Security Configuration to value 1, since this
   setting indicates the use of a per-pair key, which is not suitable
   for securing messages that travel by (link-local) multicast (e.g.,
   DIOs) or that travel over multiple hops (e.g., P2P-DROs).  The Origin
   MUST use the chosen Security Configuration to secure all the control
   messages (DIOs and P2P-DRO-ACKs) it generates.

   A router MUST NOT join the temporary DAG being created for a P2P-RPL
   route discovery if:

   o  it receives both secure and unsecure DIOs or Secure DIOs with
      different Security Configurations pertaining to this route
      discovery (i.e., referring to the same RPLInstanceID and DODAGID
      combination) prior to joining; or

   o  it cannot use the Security Configuration found in the Secure DIOs
      pertaining to this route discovery.

   When a router (an Intermediate Router or a Target) joins a temporary
   DAG being created using Secure DIOs, it MUST remember the common
   Security Configuration used in the received Secure DIOs and MUST use
   this configuration to secure all the control messages (DIOs and
   P2P-DROs) it generates.

   If an Intermediate Router (or a Target) encounters a control message
   (a DIO or a P2P-DRO or a P2P-DRO-ACK) pertaining to this route
   discovery that is either not secure or does not follow the Security
   Configuration the router remembers for this route discovery, the
   router MUST enter the "lock down" mode for the remainder of its stay
   in this temporary DAG.  An Intermediate Router (or a Target) in the
   "lock down" mode MUST NOT generate or process any control messages
   (irrespective of the Security Configuration used) pertaining to this
   route discovery.  If the Origin receives a control message (a
   P2P-DRO) that does not follow the Security Configuration the Origin
   has chosen for this route discovery, it MUST discard the received
   message with no further processing.

12.  Packet Forwarding along a Route Discovered Using P2P-RPL

   An Origin uses the Source Routing Header (SRH) [RFC6554] to send a
   packet along a Source Route discovered using P2P-RPL.

   Travel along a Hop-by-hop Route, established using P2P-RPL, requires
   specifying the RPLInstanceID and the DODAGID (of the temporary DAG
   used for the route discovery) to identify the route.  This is because
   a P2P-RPL route discovery does not use globally unique RPLInstanceID
   values, and hence both the RPLInstanceID (a local value assigned by
   the Origin) and the DODAGID (an IPv6 address of the Origin) are
   required to uniquely identify a P2P-RPL Hop-by-hop Route to a
   particular destination.

   An Origin includes a RPL option [RFC6553] inside the IPv6 Hop-by-Hop
   Options header of a packet to send it along a Hop-by-hop Route
   established using P2P-RPL.  For this purpose, the Origin MUST set the
   DODAGID of the temporary DAG used for the route discovery as the
   source IPv6 address of the packet.  Further, the Origin MUST specify
   inside the RPL option the RPLInstanceID of the temporary DAG used for
   the route discovery and set the O flag inside the RPL option to one.
   On receiving this packet, an Intermediate Router checks the O flag
   and correctly infers the source IPv6 address of the packet as the
   DODAGID of the Hop-by-hop Route.  The router then uses the DODAGID,
   the RPLInstanceID, and the destination address to identify the
   routing state to be used to forward the packet further.

13.  Interoperability with Core RPL

   This section describes how RPL routers that implement P2P-RPL
   interact with RPL routers that do not.  In general, P2P-RPL operation
   does not affect core RPL operation, and vice versa.  However, core
   RPL does allow a router to join a DAG as a leaf node even if it does
   not understand the Mode of Operation (MOP) used in the DAG.  Thus, a
   RPL router that does not implement P2P-RPL may conceivably join a
   temporary DAG being created for a P2P-RPL route discovery as a leaf
   node and maintain its membership even though the DAG no longer
   exists.  This may impose a drain on the router's memory.  However,
   such RPL-only leaf nodes do not interfere with P2P-RPL route
   discovery, since a leaf node may only generate a DIO advertising an
   INFINITE_RANK and all routers implementing P2P-RPL are required to
   discard such DIOs.  Note that core RPL does not require that a router
   join a DAG whose MOP it does not understand.  Moreover, RPL routers
   in a particular deployment may have strict restrictions on the DAGs
   they may join, thereby mitigating the problem.

   The P2P-RPL mechanism described in this document works best when all
   the RPL routers in the LLN implement P2P-RPL.  In general, the
   ability to discover routes, as well as the quality of discovered
   routes, would deteriorate with the fraction of RPL routers that
   implement P2P-RPL.

14.  Security Considerations

   In general, the security considerations for the operation of P2P-RPL
   are similar to those for the operation of RPL (as described in
   Section 19 of the RPL specification [RFC6550]).  Sections 6.1 and 10
   of [RFC6550] describe RPL's security framework, which provides data
   confidentiality, authentication, replay protection, and delay
   protection services.  This security framework can also be used in
   P2P-RPL after taking into account the constraints specified in
   Section 11.  P2P-RPL requires that all routers participating in a
   secure route discovery use the Security Configuration chosen by the
   Origin.  The intention is to avoid compromising the overall security
   of a route discovery due to some routers using a weaker Security
   Configuration.  With the "lock down" mechanism as described in
   Section 11 in effect, it is unlikely that an Origin would accept a
   route discovered under a Security Configuration other than the one it
   intended.  Any attempt to use a different Security Configuration
   (than the one the Origin intended) is likely to result, in the worst
   case, in the failure of the route discovery process.  In the best-
   case scenario, any such attempt by a rogue router would result in its
   neighbors entering the "lock down" mode and acting as firewalls to
   allow the route discovery to proceed in the remaining network.

   The RPL specification [RFC6550] describes three modes of security:
   unsecured, preinstalled, and authenticated.  In the unsecured mode,
   secure control messages are not used, and the only available security
   is the security provided by the link-layer protocols.  In the
   preinstalled mode, all the nodes use a preinstalled group key to join
   a secure DAG as the "routers" or "hosts", where the term "router"
   means a node that is capable of forwarding packets received from its
   parents or children in the DAG, and the term "host" refers to nodes
   that cannot function as "routers".  In the authenticated mode, the
   nodes can join a secure DAG as "hosts" using the preinstalled key but
   then need to authenticate themselves to a key server to obtain the
   key that will allow them to work as "routers".  The temporary DAG
   created for a P2P-RPL discovery cannot be used for routing packets.
   Hence, it is not meaningful to say that a node joins this DAG as a
   "router" or a "host" in the sense defined above.  Hence, in P2P-RPL,
   there is no distinction between the preinstalled and authenticated
   modes.  A router can join a temporary DAG created for a secure
   P2P-RPL route discovery only if it can support the Security
   Configuration in use, which also specifies the key in use.  It does
   not matter whether the key is preinstalled or dynamically acquired.
   The router must have the key in use before it can join the DAG being
   created for a secure P2P-RPL route discovery.

   If a rogue router can support the Security Configuration in use (in
   particular, if it knows the key in use), it can join the secure
   P2P-RPL route discovery and cause various types of damage.  Such a
   rogue router could advertise false information in its DIOs in order
   to include itself in the discovered route(s).  It could generate
   bogus P2P-DRO messages carrying bad routes or maliciously modify
   genuine P2P-DRO messages it receives.  A rogue router acting as the
   Origin could launch denial-of-service attacks against the LLN
   deployment by initiating fake P2P-RPL route discoveries; in this type
   of scenario, RPL's authenticated mode of operation, where a node can
   obtain the key to use for a P2P-RPL route discovery only after proper
   authentication, would be useful.

   Since a P2P-DRO message travels along a Source Route specified inside
   the message, some of the security concerns that led to the
   deprecation of Type 0 routing headers [RFC5095] may apply.  To avoid
   the possibility of a P2P-DRO message traveling in a routing loop,
   this document requires that each Intermediate Router confirm that the
   Source Route listed inside the message does not contain any routing
   loop involving itself before the router could forward the message
   further.  As specified in Section 9.6, this check involves the router
   making sure that its IPv6 addresses do not appear multiple times
   inside the Source Route with one or more other IPv6 addresses in
   between.

15.  IANA Considerations

15.1.  Additions to Mode of Operation

   This document defines a new Mode of Operation, entitled "P2P Route
   Discovery Mode of Operation" (see Section 6), assigned a value of 4
   from the "Mode of Operation" space [RFC6550].

     +-------+---------------------------------------+---------------+
     | Value |              Description              |   Reference   |
     +-------+---------------------------------------+---------------+
     |   4   | P2P Route Discovery Mode of Operation | This document |
     +-------+---------------------------------------+---------------+

                             Mode of Operation

15.2.  Additions to RPL Control Message Options

   This document defines a new RPL option: "P2P Route Discovery" (see
   Section 7), assigned a value of 0x0a from the "RPL Control Message
   Options" space [RFC6550].

               +-------+---------------------+---------------+
               | Value |       Meaning       |   Reference   |
               +-------+---------------------+---------------+
               |  0x0a | P2P Route Discovery | This document |
               +-------+---------------------+---------------+

                        RPL Control Message Options

15.3.  Additions to RPL Control Codes

   This document defines the following new RPL messages:

   o  "P2P Discovery Reply Object" (see Section 8), assigned a value of
      0x04 from the "RPL Control Codes" space [RFC6550].

   o  "Secure P2P Discovery Reply Object" (see Section 8.1), assigned a
      value of 0x84 from the "RPL Control Codes" space [RFC6550].

   o  "P2P Discovery Reply Object Acknowledgement" (see Section 10),
      assigned a value of 0x05 from the "RPL Control Codes"
      space [RFC6550].

   o  "Secure P2P Discovery Reply Object Acknowledgement" (see
      Section 10), assigned a value of 0x85 from the "RPL Control Codes"
      space [RFC6550].

   +--------+----------------------------------------+-----------------+
   |  Code  |              Description               |    Reference    |
   +--------+----------------------------------------+-----------------+
   |  0x04  |       P2P Discovery Reply Object       |  This document  |
   |  0x84  |   Secure P2P Discovery Reply Object    |  This document  |
   |  0x05  |       P2P Discovery Reply Object       |  This document  |
   |        |            Acknowledgement             |                 |
   |  0x85  |   Secure P2P Discovery Reply Object    |  This document  |
   |        |            Acknowledgement             |                 |
   +--------+----------------------------------------+-----------------+

                             RPL Control Codes

16.  Known Issues and Future Work

   This document is presented as an Experimental specification to
   facilitate P2P-RPL's deployment in LLN scenarios where reactive P2P
   route discovery is considered useful or necessary.  It is anticipated
   that, once sufficient operational experience has been gained, this
   specification will be revised to progress it on to the Standards
   Track.  Experience reports regarding P2P-RPL implementation and
   deployment are encouraged, particularly with respect to:

   o  Secure P2P-RPL operation (Section 11);

   o  Rules governing Trickle operation (Section 9.2);

   o  Values in the default DODAG Configuration Option (Section 6.1);

   o  The RPLInstanceID reuse policy (Section 6.1);

   o  Utility and implementation complexity of allowing multiple Target
      addresses in a P2P-RPL route discovery.

17.  Acknowledgements

   The authors gratefully acknowledge the contributions of the following
   individuals (in alphabetical order) in the development of this
   document: Dominique Barthel, Jakob Buron, Cedric Chauvenet, Thomas
   Clausen, Robert Cragie, Ralph Droms, Adrian Farrel, Stephen Farrell,
   Brian Haberman, Ted Humpal, Richard Kelsey, Phil Levis, Charles
   Perkins, Joseph Reddy, Michael Richardson, Zach Shelby, Martin
   Stiemerling, Pascal Thubert, Hristo Valev, and JP Vasseur.

18.  References

18.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, March 2011.

   [RFC6550]  Winter, T., Thubert, P., 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, March 2012.

   [RFC6551]  Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D.
              Barthel, "Routing Metrics Used for Path Calculation in
              Low-Power and Lossy Networks", RFC 6551, March 2012.

   [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,
              March 2012.

18.2.  Informative References

   [RFC5095]  Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
              of Type 0 Routing Headers in IPv6", RFC 5095,
              December 2007.

   [RFC5826]  Brandt, A., Buron, J., and G. Porcu, "Home Automation
              Routing Requirements in Low-Power and Lossy Networks",
              RFC 5826, April 2010.

   [RFC5867]  Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
              "Building Automation Routing Requirements in Low-Power and
              Lossy Networks", RFC 5867, June 2010.

   [RFC6552]  Thubert, P., "Objective Function Zero for the Routing
              Protocol for Low-Power and Lossy Networks (RPL)",
              RFC 6552, March 2012.

   [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,
              March 2012.

   [RFC6998]  Goyal, M., Ed., Baccelli, E., Brandt, A., and J. Martocci,
              "A Mechanism to Measure the Routing Metrics along a Point-
              to-Point Route in a Low-Power and Lossy Network",
              RFC 6998, August 2013.

   [ROLL-TERMS]
              Vasseur, JP., "Terminology in Low power And Lossy
              Networks", Work in Progress, March 2013.

Authors' Addresses

   Mukul Goyal (editor)
   University of Wisconsin Milwaukee
   3200 N. Cramer St.
   Milwaukee, WI  53201
   USA

   Phone: +1-414-229-5001
   EMail: mukul@uwm.edu

   Emmanuel Baccelli
   INRIA

   Phone: +33-169-335-511
   EMail: Emmanuel.Baccelli@inria.fr
   URI:   http://www.emmanuelbaccelli.org/

   Matthias Philipp
   INRIA

   Phone: +33-169-335-511
   EMail: matthias-philipp@gmx.de

   Anders Brandt
   Sigma Designs
   Emdrupvej 26A, 1.
   Copenhagen, Dk-2100
   Denmark

   Phone: +45-29609501
   EMail: abr@sdesigns.dk

   Jerald Martocci
   Johnson Controls
   507 E. Michigan Street
   Milwaukee, WI  53202
   USA

   Phone: +1-414-524-4010
   EMail: jerald.p.martocci@jci.com

 

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