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Versions: (draft-dt-roll-rpl) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 RFC 6550

Networking Working Group                                  T. Winter, Ed.
Internet-Draft
Intended status: Standards Track                         P. Thubert, Ed.
Expires: April 29, 2010                                    Cisco Systems
                                                        ROLL Design Team
                                                            IETF ROLL WG
                                                        October 26, 2009


      RPL: IPv6 Routing Protocol for Low power and Lossy Networks
                         draft-ietf-roll-rpl-04

Status of this Memo

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   This Internet-Draft will expire on April 29, 2010.

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Abstract

   Low power and Lossy Networks (LLNs) are a class of network in which
   both the routers and their interconnect are constrained: LLN routers
   typically operate with constraints on (any subset of) processing
   power, memory and energy (battery), and their interconnects are
   characterized by (any subset of) high loss rates, low data rates and
   instability.  LLNs are comprised of anything from a few dozen and up
   to thousands of LLN routers, and support point-to- point traffic
   (between devices inside the LLN), point-to-multipoint traffic (from a
   central control point to a subset of devices inside the LLN) and
   multipoint-to- point traffic (from devices inside the LLN towards a
   central control point).  This document specifies the IPv6 Routing
   Protocol for LLNs (RPL), which provides a mechanism whereby
   multipoint-to-point traffic from devices inside the LLN towards a
   central control point, as well as point-to-multipoint traffic from
   the central control point to the devices inside the LLN, is
   supported.  Support for point-to-point traffic is also available.

































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.1.  Design Principles  . . . . . . . . . . . . . . . . . . . .  6
     1.2.  Expectations of Link Layer Type  . . . . . . . . . . . . .  7
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Protocol Model . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  9
       3.1.1.  Topology Instance and Objectives . . . . . . . . . . .  9
       3.1.2.  Multipoint-to-Point Traffic Flows and DAGs . . . . . . 11
       3.1.3.  Point-to-Multipoint Traffic Flows  . . . . . . . . . . 11
       3.1.4.  Point-to-Point Traffic Flows . . . . . . . . . . . . . 12
     3.2.  Protocol Operation . . . . . . . . . . . . . . . . . . . . 12
       3.2.1.  DAG Construction . . . . . . . . . . . . . . . . . . . 12
       3.2.2.  Destination Advertisement  . . . . . . . . . . . . . . 15
     3.3.  Loop Avoidance and Stability . . . . . . . . . . . . . . . 17
       3.3.1.  Greediness and Rank-based Instabilities  . . . . . . . 17
       3.3.2.  DAG Loops  . . . . . . . . . . . . . . . . . . . . . . 18
       3.3.3.  DAO Loops  . . . . . . . . . . . . . . . . . . . . . . 18
       3.3.4.  Sibling Loops  . . . . . . . . . . . . . . . . . . . . 18
   4.  Routing Metrics and Constraints Used By RPL  . . . . . . . . . 18
   5.  RPL Protocol Specification . . . . . . . . . . . . . . . . . . 19
     5.1.  RPL Messages . . . . . . . . . . . . . . . . . . . . . . . 19
       5.1.1.  ICMPv6 RPL Control Message . . . . . . . . . . . . . . 19
       5.1.2.  DAG Information Solicitation (DIS) . . . . . . . . . . 20
       5.1.3.  DAG Information Object (DIO) . . . . . . . . . . . . . 20
       5.1.4.  Destination Advertisement Object (DAO) . . . . . . . . 27
     5.2.  Conceptual Data Structures . . . . . . . . . . . . . . . . 28
       5.2.1.  Candidate Neighbors Data Structure . . . . . . . . . . 28
       5.2.2.  Directed Acyclic Graphs (DAGs) Data Structure  . . . . 29
     5.3.  DAG Rank . . . . . . . . . . . . . . . . . . . . . . . . . 30
     5.4.  DAG Discovery and Maintenance  . . . . . . . . . . . . . . 31
       5.4.1.  DAG Discovery Rules  . . . . . . . . . . . . . . . . . 32
       5.4.2.  Reception and Processing of DIO messages . . . . . . . 36
       5.4.3.  DIO Transmission . . . . . . . . . . . . . . . . . . . 38
       5.4.4.  Trickle Timer for DIO Transmission . . . . . . . . . . 39
     5.5.  DAG Sequence Number Increment  . . . . . . . . . . . . . . 40
     5.6.  DAG Selection  . . . . . . . . . . . . . . . . . . . . . . 41
     5.7.  Administrative rank  . . . . . . . . . . . . . . . . . . . 41
     5.8.  Collision  . . . . . . . . . . . . . . . . . . . . . . . . 42
     5.9.  Guidelines for Objective Functions . . . . . . . . . . . . 42
       5.9.1.  Objective Function . . . . . . . . . . . . . . . . . . 42
       5.9.2.  Objective Function 0 (OF0) . . . . . . . . . . . . . . 44
     5.10. Establishing Routing State Outward Along the DAG . . . . . 46
       5.10.1. Destination Advertisement Operation  . . . . . . . . . 47
     5.11. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 54
       5.11.1. Host Basic Operation . . . . . . . . . . . . . . . . . 55
       5.11.2. Instance Forwarding  . . . . . . . . . . . . . . . . . 55



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       5.11.3. DAG Inconsistency Loop Detection . . . . . . . . . . . 56
       5.11.4. Sibling Loop Avoidance . . . . . . . . . . . . . . . . 56
       5.11.5. DAO Inconsistency Loop Detection and Recovery  . . . . 57
     5.12. Multicast Operation  . . . . . . . . . . . . . . . . . . . 57
     5.13. Maintenance of Routing Adjacency . . . . . . . . . . . . . 58
     5.14. Packet Forwarding  . . . . . . . . . . . . . . . . . . . . 59
   6.  RPL Constants and Variables  . . . . . . . . . . . . . . . . . 60
   7.  Manageability Considerations . . . . . . . . . . . . . . . . . 61
     7.1.  Control of Function and Policy . . . . . . . . . . . . . . 61
       7.1.1.  Initialization Mode  . . . . . . . . . . . . . . . . . 61
       7.1.2.  DIO Base option  . . . . . . . . . . . . . . . . . . . 61
       7.1.3.  Trickle Timers . . . . . . . . . . . . . . . . . . . . 62
       7.1.4.  DAG Sequence Number Increment  . . . . . . . . . . . . 63
       7.1.5.  Destination Advertisement Timers . . . . . . . . . . . 63
       7.1.6.  Policy Control . . . . . . . . . . . . . . . . . . . . 63
       7.1.7.  Data Structures  . . . . . . . . . . . . . . . . . . . 63
     7.2.  Information and Data Models  . . . . . . . . . . . . . . . 64
     7.3.  Liveness Detection and Monitoring  . . . . . . . . . . . . 64
       7.3.1.  Candidate Neighbor Data Structure  . . . . . . . . . . 64
       7.3.2.  Directed Acyclic Graph (DAG) Table . . . . . . . . . . 64
       7.3.3.  Routing Table  . . . . . . . . . . . . . . . . . . . . 65
       7.3.4.  Other RPL Monitoring Parameters  . . . . . . . . . . . 65
       7.3.5.  RPL Trickle Timers . . . . . . . . . . . . . . . . . . 66
     7.4.  Verifying Correct Operation  . . . . . . . . . . . . . . . 66
     7.5.  Requirements on Other Protocols and Functional
           Components . . . . . . . . . . . . . . . . . . . . . . . . 66
     7.6.  Impact on Network Operation  . . . . . . . . . . . . . . . 66
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 66
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 66
     9.1.  RPL Control Message  . . . . . . . . . . . . . . . . . . . 66
     9.2.  New Registry for RPL Control Codes . . . . . . . . . . . . 67
     9.3.  New Registry for the Control Field of the DIO Base
           Option . . . . . . . . . . . . . . . . . . . . . . . . . . 67
     9.4.  DAG Information Object (DIO) Suboption . . . . . . . . . . 68
     9.5.  Objective Code Point for the Default Objective
           Function OF0 . . . . . . . . . . . . . . . . . . . . . . . 68
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 68
   11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 69
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 70
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 70
     12.2. Informative References . . . . . . . . . . . . . . . . . . 71
   Appendix A.  Requirements  . . . . . . . . . . . . . . . . . . . . 72
     A.1.  Protocol Properties Overview . . . . . . . . . . . . . . . 72
       A.1.1.  IPv6 Architecture  . . . . . . . . . . . . . . . . . . 73
       A.1.2.  Typical LLN Traffic Patterns . . . . . . . . . . . . . 73
       A.1.3.  Constraint Based Routing . . . . . . . . . . . . . . . 73
     A.2.  Deferred Requirements  . . . . . . . . . . . . . . . . . . 74
   Appendix B.  Examples  . . . . . . . . . . . . . . . . . . . . . . 74



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     B.1.  Destination Advertisement  . . . . . . . . . . . . . . . . 76
     B.2.  Example: DAG Parent Selection  . . . . . . . . . . . . . . 77
     B.3.  Example: DAG Maintenance . . . . . . . . . . . . . . . . . 78
     B.4.  Example: Greedy Parent Selection and Instability . . . . . 79
   Appendix C.  Outstanding Issues  . . . . . . . . . . . . . . . . . 81
     C.1.  Additional Support for P2P Routing . . . . . . . . . . . . 81
     C.2.  Loop Detection . . . . . . . . . . . . . . . . . . . . . . 81
     C.3.  Destination Advertisement / DAO Fan-out  . . . . . . . . . 81
     C.4.  Source Routing . . . . . . . . . . . . . . . . . . . . . . 82
     C.5.  Address / Header Compression . . . . . . . . . . . . . . . 82
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 82








































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1.  Introduction

   Low power and Lossy Networks (LLNs) are made largely of constrained
   nodes (with limited processing power, memory, and sometimes energy
   when they are battery operated).  These routers are interconnected by
   lossy links, typically time supporting only low data rates, that are
   usually unstable with relatively low packet delivery rates.  Another
   characteristic of such networks is that the traffic patterns are not
   simply unicast, but in many cases point-to-multipoint or multipoint-
   to-point.  Furthermore such networks may potentially comprise up to
   thousands of nodes.  These characteristics offer unique challenges to
   a routing solution: the IETF ROLL Working Group has defined
   application-specific routing requirements for a Low power and Lossy
   Network (LLN) routing protocol, specified in
   [I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548].  This
   document specifies the IPv6 Routing Protocol for Low power and Lossy
   Networks (RPL).

1.1.  Design Principles

   RPL was designed with the objective to meet the requirements spelled
   out in [I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548].  Because
   those requirements are heterogeneous and sometimes incompatible in
   nature, the approach is first taken to design a protocol capable of
   supporting a core set of functionalities corresponding to the
   intersection of the requirements.  (Note: it is intended that as this
   design evolves optional features may be added to address some
   application specific requirements).  This is a key protocol design
   decision providing a granular approach in order to restrict the core
   of the protocol to a minimal set of functionalities, and to allow
   each implementation of the protocol to be optimized in terms of,
   e.g., minimizing required code space and use of limited computation
   resources.

   Multiple instances of the protocol can be operated at the same time
   in order to serve different and potentially antagonistic constraints.
   Instances run independently of one another with no required
   interaction.  A node might participate to multiple instances and
   route independently along the associated topologies.  This
   specification defines only the protocol operation for the node within
   one instance.  Consideration is given to default behavior that
   enables future extensions for the multiple instances and related
   policies.

   It must be noted that RPL is not restricted to the aforementioned
   applications and is expected to be used in other environments.  All



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   "MUST" application requirements that cannot be satisfied by RPL will
   be specifically listed in the Appendix A, accompanied by a
   justification.

   The core set of functionalities is to be capable of operating in the
   most severely constrained environments, with minimal requirements for
   memory, energy, processing, communication, and other consumption of
   limited resources from nodes.  Trade-offs inherent in the
   provisioning of protocol features will be exposed to the implementer
   in the form of configurable parameters, such that the implementer can
   further tweak and optimize the operation of RPL as appropriate to a
   specific application and implementation.  Finally, RPL is designed to
   consult implementation specific policies to determine, for example,
   the evaluation of routing metrics.

   A set of companion documents to this specification will provide
   further guidance in the form of applicability statements specifying a
   set of operating points appropriate to the Building Automation, Home
   Automation, Industrial, and Urban application scenarios.

1.2.  Expectations of Link Layer Type

   This specification does not rely on any particular features of a
   specific link layer technologies.  It is anticipated that an
   implementer should be able to operate RPL over a variety of different
   link layers, including but not limited to low power wireless or PLC
   (Power Line Communication) technologies.

   Implementers may find RFC 3819 [RFC3819] a useful reference when
   designing a link layer interface between RPL and a particular link
   layer technology.


2.  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 RFC
   2119 [RFC2119].

   This document requires readers to be familiar with the terminology
   described in `Terminology in Low power And Lossy Networks'
   [I-D.ietf-roll-terminology].








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   DAG:  Directed Acyclic Graph.  A directed graph having the property
         that all edges are oriented in such a way that no cycles exist.
         In the RPL context, all edges are contained in paths oriented
         toward and terminating at one or more root nodes (a DAG root,
         or sink- typically a Low power and Lossy Network Border Router
         (LBR)).  For the purpose of this document, the term DAG is
         often used to refer to a DAG Iteration as defined below.

   DAG Instance:  A DAG Instance is a set of possibly multiple
         Destination Oriented DAGs.  A network may have more than one
         DAG Instance, and a RPL router can participate to multiple DAG
         instances.  Each DAG Instance operates independently of other
         DAG Instances.  This document describes operation within a
         single DAG instance.

   InstanceID:  Unique identifier of a DAG Instance.

   Destination Oriented DAG:  A DAG rooted at a single destination,
         which is a node with no outgoing edges.  The tuple (InstanceID,
         DAGID) uniquely identifies a Destination Oriented DAG.  In the
         RPL context, a router can can belong to at most one Destination
         Oriented DAG per DAG Instance.

   DAGID:  The identifier of a DAG root.  The DAGID must be unique
         within the scope of a DAG Instance in the LLN.

   DAG Iteration:  The DAG that results from the iterative process that
         reshapes the Destination Oriented DAG upon a stimulation by the
         root.

   DAGSequenceNumber:  A sequential counter that is incremented by the
         root to form a new Iteration of a DAG.  A DAG Iteration is
         identified uniquely by the (InstanceID, DAGID,
         DAGSequenceNumber) tuple.

   DAG parent:  A parent of a node within a DAG is one of the immediate
         successors of the node on a path towards the DAG root.

   DAG sibling:  A sibling of a node within a DAG is defined in this
         specification to be any neighboring node which is located at
         the same rank within a DAG.  Note that siblings defined in this
         manner do not necessarily share a common parent.

   DAG root:  A DAG root is a node within the DAG that has no outgoing
         edges.  Because the graph is acyclic, by definition all DAGs
         must have at least one DAG root and all paths terminate at a
         DAG root.




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   Sub-DAG  The sub-DAG of a node is the set of other nodes in the DAG
         that might use a path towards the DAG root that contains the
         node.  Nodes in the sub-DAG of a node have a greater rank
         (although not all nodes of greater rank are in the sub-DAG).

   Grounded:  A DAG is grounded if it contains a DAG root offering
         connectivity to an external routed infrastructure such as the
         public Internet or a private core (non-LLN) IP network.

   Floating:  A DAG is floating if is not grounded.  A floating DAG is
         not expected to reach any additional external routed
         infrastructure such as the public Internet or a private core
         (non-LLN) IP network.

   Inward:  Inward refers to the direction from leaf nodes towards DAG
         roots, following the orientation of the edges within the DAG.

   Outward:  Outward refers to the direction from DAG roots towards leaf
         nodes, going against the orientation of the edges within the
         DAG.

   OCP:  Objective Code Point.  The Objective Code Point is used to
         indicate which Objective Function is in use in a DAG.  The
         Objective Code Point is further described in
         [I-D.ietf-roll-routing-metrics].

   OF:   Objective Function.  The Objective Function (OF) defines which
         routing metrics, optimization objectives, and related functions
         are in use in a DAG.  The Objective Function is further
         described in [I-D.ietf-roll-routing-metrics].

   Note that in this document, the terms `node' and `LLN router' are
   used interchangeably.


3.  Protocol Model

   The aim of this section is to describe RPL in the spirit of
   [RFC4101].  Protocol details can be found in further sections.

3.1.  Overview

3.1.1.  Topology Instance and Objectives

   A topology instance of RPL exists over the scope of an LLN in support
   of a particular application, or service, and is optimized according
   to a certain objective, as determined by an Objective Function (OF),
   and may be characterized by certain destination prefixes as well.  A



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   topology instance, or DAG Instance, may be administratively
   associated with an InstanceID.

   A single topology instance may comprise:

   o  a single Destination Oriented DAG with a single DAG root

      *  For example, a DAG optimized to minimize latency rooted at a
         single centralized lighting controller in a home automation
         application.

   o  multiple uncoordinated Destination Oriented DAGs with independent
      DAG roots (differing DAGIDs)

      *  For example, multiple data collection points in an urban data
         collection application that do not have an always-on backbone
         suitable to coordinate to form a single DAG, and further use
         the formation of multiple DAGs as a means to dynamically and
         autonomously partition the network.

   o  a single Destination Oriented DAG with multiple DAG roots
      coordinating over some backbone

      *  For example, multiple border routers operating with a reliable
         backbone, e.g. in support of a 6LowPAN application, that are
         capable to act as logically equivalent sinks to the same DAG.

   o  a combination of one of the above as suited to some application
      scenario

   The exact deployment scenario is determined as appropriate to the
   application and capabilities of the LLN nodes.  What is suitable for
   one deployment may not be possible or necessary for another.

   Traffic is bound to a specific DAG Instance by a marking in the flow
   label of the IPv6 header.  Traffic originating in support of a
   particular application may be tagged to follow an appropriate
   instance, for example to follow paths optimized for low latency or
   low energy.  The provisioning or automated discovery of a mapping
   between an InstanceID and a type or service of application traffic is
   beyond the scope of this specification.

   Conceptually a node running RPL may capable to maintain a membership
   in multiple DAG Instances in support of different application
   services and/or optimization objectives.  For example, one instance
   may optimize for minimizing latency and a separate orthogonal
   instance may optimize for minimizing energy.  This scenario does
   introduce some additional considerations, for example loop avoidance



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   and default routing behavior.  These considerations are beyond the
   scope of this specification and are intended to be elaborated on in a
   future revision of this or a companion specification.  As such, this
   specification will deal exclusively with the scenario where a node
   implements RPL in support of a single DAG Instance.

3.1.2.  Multipoint-to-Point Traffic Flows and DAGs

   Many of the dominant traffic flows in support of the LLN application
   scenarios are MP2P flows ([I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548]).  These
   flows are rooted at designated nodes that have some application
   significance, such as providing connectivity to an external routed
   infrastructure.  The term "external" is used to refer to the public
   Internet or a core private (non-LLN) IP network.

   LLN nodes running RPL will construct Directed Acyclic Graphs (DAGs)
   rooted at DAG roots, which may be naturally designated according to
   their application significance.  This structure provides the routing
   solution for the dominant MP2P traffic flows.  The DAG structure
   further provides each node potentially multiple successors for MP2P
   flows, which may be used for, e.g., local route repair or load
   balancing.

   Nodes running RPL are able to further restrict the scope of the
   routing problem by using the DAG as a reference topology.  By
   referencing a rank property that is related to the positions in the
   DAG, nodes are able to determine their positions in a DAG relative to
   each other.  This information is used by RPL in part to construct
   rules for movement relative to the DAG that endeavor to avoid loops.
   It is important to note that the rank property is derived from
   metrics, and not directly from the position in the DAG (Section 5.3).

3.1.3.  Point-to-Multipoint Traffic Flows

   As DAGs are organized, RPL will use a destination advertisement
   mechanism to build up routing tables in support of outward P2MP
   traffic flows.  This mechanism, using the DAG as a reference,
   distributes routing information across intermediate nodes (between
   the DAG leaves and the root), guided along the DAG, such that the
   routes toward destination prefixes in the outward direction may be
   set up.  As the DAG undergoes modification during DAG maintenance,
   the destination advertisement mechanism can be triggered to update
   the outward routing state.







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3.1.4.  Point-to-Point Traffic Flows

   A baseline support for P2P traffic in RPL is provided by the DAG, as
   P2P traffic may flow inward along the DAG until a common parent is
   reached that has stored an entry for the destination in its routing
   table and is capable of directing the traffic outward along the
   correct outward path.  RPL also provides support for the trivial case
   where a P2P destination may be a `one-hop' neighbor.  In the present
   document RPL does not specify nor preclude any additional mechanisms
   that may be capable to compute and install more optimal routes into
   LLN nodes in support of arbitrary P2P traffic according to some
   routing metric.

3.2.  Protocol Operation

3.2.1.  DAG Construction

3.2.1.1.  DAG Information Object (DIO)

   A DAG Information Object is defined and used by RPL in order to build
   and maintain a DAG.  This document defines an ICMPv6 Message Type RPL
   Control Message, which is capable to carry the DIO.  The DIO message
   conveys information about the DAG, including:

   o  A DAGID used to identify the DAG as sourced from the DAG root.
      The DAGID must be unique to a single DAG in the scope of the LLN.

   o  Objective Function identified by an Objective Code Point (OCP) as
      described below.

   o  Rank information used by nodes to determine their positions in the
      DAG relative to each other.

   o  Sequence number originated from the DAG root, used to aid in
      identification of dependent sub-DAGs and coordinate topology
      changes in a manner so as to avoid loops.

   o  Indications and configuration for the DAG, e.g. grounded or
      floating, administrative preference, ...

   o  A set of path metrics and constraints, as further described in
      [I-D.ietf-roll-routing-metrics].

   o  List of additional destination prefixes reachable inwards along
      the DAG.

   The DIO messages are issued whenever a change is detected to the DAG
   such that a node is able to determine that a region of the DAG has



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   become inconsistent.  As the DAG stabilizes the period at which RA
   messages occur is configured to taper off, reducing the steady-state
   overhead of DAG maintenance.  The periodic issue of DIO messages,
   along with the triggered DIO messages in response to inconsistency,
   is one feature that enables RPL to operate in the presence of
   unreliable links.

3.2.1.2.  Grounded and Floating DAGs

   Certain LLN nodes may offer connectivity to an external routed
   infrastructure in support of an application scenario.  These nodes
   are designated `grounded', and may serve as the DAG roots of a
   grounded DAG.  DAGs that do not have a grounded DAG root are floating
   DAGs.  In either case routes may be provisioned toward the DAG root,
   although in the floating case there is no expectation to reach an
   external infrastructure.  Some applications will include permanent
   floating DAGs.

3.2.1.3.  Administrative Preference

   An administrative preference may be associated with each DAG root,
   and thereby each DAG, in order that some DAGs in the LLN may be more
   preferred over other DAGs.  For example, a DAG root that is sinking
   traffic in support of a data collection application may be configured
   by the application to be very preferred.  A transient DAG, e.g. a DAG
   that is only existing until a permanent DAG is found, may be
   configured to be less preferred.  The administrative preference
   offers a way to engineer the formation of the DAG in support of the
   application.

3.2.1.4.  Objective Function (OF)

   The Objective Function (OF) conveys and controls the optimization
   objectives in use within the DAG.  The Objective Function is
   indicated by an Objective Code Point (OCP), and is further specified
   in [I-D.ietf-roll-routing-metrics].  Each instance of an allocated OF
   indicates:

   o  The set of metrics used within the DAG

   o  The method used for least cost path determination.

   o  The method used to compute DAG Rank

   o  The methods used to prepare metrics for propagation within a DIO
      message

   By using defined OCPs that are understood by all nodes in a



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   particular implementation, and by conveying them in the DIO message,
   RPL nodes may work to build optimized LLN using a variety of
   application and implementation specific metrics and goals.

   A default OF, OF0 (designated by OCP value of 0x0000), is specified
   with a well-defined default behavior.  OF0 may be used to define RPL
   behaviors in the case where a node encounters a DIO message
   containing a code point that it does not support, if allowed by
   policy.

3.2.1.5.  Distributed Algorithm Operation

   A high level overview of the distributed algorithm which constructs
   the DAG is as follows:

   o  Some nodes may be initially provisioned to act as DAG roots,
      either permanent or transient, with associated preferences.

   o  Nodes will maintain a data structure containing their candidate
      (viable) neighbors, as determined by the implementation.  This
      data structure will also track DAG information as learned from and
      associated with each neighbor.

   o  Nodes that are members of a DAG, including DAG roots, will
      multicast DIO messages as needed (when inconsistency is detected),
      to their link-local neighbors.  Nodes will also respond to DIS
      messages.

   o  Nodes that receive DIO messages may either discard the DIO based
      on several criteria, including rank-based loop avoidance rules, or
      process the DIO to maintain a position in an existing DAG or
      improve a position as according to an Objective Function (OF) and
      current path cost.

   o  Nodes manage a set of DAG Parents according to the rules specified
      by RPL.  This set is also augmented to include DAG siblings.

   o  DIO messages may be emitted more or less frequently as a function
      of DAG consistency.

   o  As less preferred DAGs encounter more preferred DAGs that offer
      equivalent or better optimization objectives for the same
      InstanceID, the nodes in the less preferred DAGs may leave to join
      the more preferred DAGs, finally leaving only the more preferred
      DAGs.  This is an illustration of the mechanism by which an
      application may engineer DAG construction.





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   o  The nodes provision routing table entries for the destinations
      specified by the DIO towards their DAG Parents.  Nodes may
      provision a DAG Parent as a default gateway.

3.2.2.  Destination Advertisement

   As RPL constructs DAGs, nodes may provision routes toward
   destinations advertised through DIO messages through their selected
   parents, and are thus able to send traffic inward along the DAG by
   forwarding to their selected parents.  However, this mechanism alone
   is not sufficient to support P2MP traffic flowing outward along the
   DAG from the DAG root toward nodes.  A destination advertisement
   mechanism is employed by RPL to build up routing state in support of
   these outward flows.  The destination advertisement mechanism may not
   be supported in all implementations, as appropriate to the
   application requirements.  A DAG root that supports using the
   destination advertisement mechanism to build up routing state will
   indicate such in the DIO message.  A DAG root that supports using the
   destination advertisement mechanism must be capable of allocating
   enough state to store the routing state received from the LLN.

3.2.2.1.  Destination Advertisement Object (DAO)

   A Destination Advertisement Object is defined and used by RPL in
   order to convey the destination information inward along the DAG
   toward the DAG root.  This document defines an ICMPv6 Message Type
   RPL Control Message, which is capable to carry the DAO.  The
   information conveyed in the DAO message includes the following:

   o  A lifetime and sequence counter to determine the freshness of the
      destination advertisement.

   o  Depth information used by nodes to determine how far away the
      destination (the source of the destination advertisement) is

   o  Prefix information to identify the destination, which may be a
      prefix, an individual host, or multicast listeners

   o  Reverse Route information to record the nodes visited (along the
      outward path) when the intermediate nodes along the path cannot
      support storing state for Hop-By-Hop routing.

3.2.2.2.  Destination Advertisement Operation

   As the DAG is constructed and maintained, nodes are capable to emit
   DAO messages to a subset of their DAG parents.





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3.2.2.2.1.  `One-Hop' Neighbors

   As a special case, a node may periodically emit a link-local
   multicast IPv6 DAO message advertising its locally available
   destination prefixes.  This mechanism allows for the one-hop
   neighbors of a node to learn explicitly of the prefixes on the node,
   and in some application specific scenarios this is desirable in
   support of provisioning a trivial `one-hop' route.  In this case,
   nodes that receive the multicast destination advertisement may use it
   to provision the one-hop route only, and not engage in any additional
   processing (so as not to engage the mechanisms used by a DAG parent).

3.2.2.2.2.  Operation in Support of Stateful Nodes

   When a (unicast) DAO message reaches a node capable of storing
   routing state, the node extracts information from the DAO message and
   updates its local database with a record of the DAO message and the
   neighbor that it was received from.  When the node later propagates
   DAO messages, it selects the best (least depth) information for each
   destination and conveys this information again in the form of DAO
   messages to a subset of its own DAG parents.  At this time the node
   may perform route aggregation if it is able, thus reducing the
   overall number of DAO messages.

3.2.2.2.3.  Operation in Support of Stateless Nodes

   When a (unicast) DAO message reaches a node incapable of storing
   additional state, the node must append the next-hop address (from
   which neighbor the DAO message was received) to a Reverse Route Stack
   carried within the DAO message.  The node then passes the DAO message
   on to one or more of its DAG parents without storing any additional
   state.

   When a node that is capable of storing routing state encounters a
   (unicast) DAO message with a Reverse Route Stack that has been
   populated, the node knows that the DAO message has traversed a region
   of nodes that did not record any routing state.  The node is able to
   detach and store the Reverse Route State and associate it with the
   destination described by the DAO message.  Subsequently the node may
   use this information to construct a source route in order to bridge
   the region of nodes that are unable to support Hop-By-Hop routing to
   reach the destination.

3.2.2.2.4.  Additional Considerations

   Further aggregations of DAO messages prefix reachability information
   by destinations are possible in order to support additional
   scalability.



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   A special case of an DAO message, termed a `no-DAO', may be used to
   tear down the routing state that has been established by the
   destination advertisement mechanism in case of, e.g., unreachability
   or other events that affect the outward routing state.

   A further example of the operation of the destination advertisement
   mechanism is available in Appendix B.1

3.3.  Loop Avoidance and Stability

   The goal of a guaranteed consistent and loop free global routing
   solution for an LLN may not be practically achieved given the real
   behavior and volatility of the underlying metrics.  The trade offs to
   achieve a stable approximation of global convergence may be too
   restrictive with respect to the need of the LLN to react quickly in
   response to the lossy environment.  Globally the LLN may be able to
   achieve a weak convergence, in particular as link changes are able to
   be handled locally and result in minimal changes to global topology.

   RPL does not aim to guarantee loop free path selection, or strong
   global convergence.  In order to reduce control overhead, in
   particular the expense of mechanisms such as count-to-infinity, RPL
   does try to avoid the creation of loops when undergoing topology
   changes.

   RPL includes rank-based mechanisms for detecting loops to ensure that
   packets make forward progress within the DAG and trigger DAG repair
   if necessary.

3.3.1.  Greediness and Rank-based Instabilities

   Once a node has joined a DAG, RPL disallows certain behaviors,
   including greediness, in order to prevent resulting instabilities in
   the DAG.

   If a node is allowed to be greedy and attempts to move deeper in the
   DAG, beyond its most preferred parent, in order to increase the size
   of the DAG parent set, then an instability can result.  This is
   illustrated in Figure 14.

   Suppose a node is willing to receive and process a DIO messages from
   a node in its own sub-DAG, and in general a node deeper than it.  In
   such cases a chance exists to create a feedback loop, wherein two or
   more nodes continue to try and move in the DAG in order to optimize
   against each other.  In some cases this will result in an
   instability.  It is for this reason that RPL mandates that a node
   never receive and process DIO messages from deeper nodes.  This rule
   creates an `event horizon', whereby a node cannot be influenced into



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   an instability by the action of nodes that may be in its own sub-DAG.

   A further example of the consequences of greedy operation, and
   instability related to processing DIO messages from nodes of greater
   rank, may be found in Appendix B.4

3.3.2.  DAG Loops

   A DAG loop may occur when a node detaches from the DAG and reattaches
   to a device in its prior sub-DAG.  This may happen in particular when
   DIO messages are missed.  Strict use of the DAG sequence number can
   eliminate this type of loop.

3.3.3.  DAO Loops

   A DAO loop may occur when the parent has a route installed upon
   receiving and processing a DAO message from a child, but the child
   has subsequently cleaned up the state.  This loop happens when a no-
   DAO was missed till a heartbeat cleans up all states.  RPL includes
   loop detection mechanisms that may mitigate the impact of DAO loops
   and trigger their repair.

   In the case where stateless DAO operation is used, i.e. source
   routing specifies the outwards routes, then DAO Loops should not
   occur on the stateless portions of the path.

3.3.4.  Sibling Loops

   Sibling loops could occur if a group of siblings kept choosing
   amongst themselves as successors such that a packet does not make
   forward progress.  This specification limits the number of times that
   sibling forwarding may be used at a given rank to prevent sibling
   loops.


4.  Routing Metrics and Constraints Used By RPL

   Routing metrics are used by routing protocols to compute the shortest
   paths.  Interior Gateway Protocols (IGPs) such as IS-IS ([RFC5120])
   and OSPF ([RFC4915]) use static link metrics.  Such link metrics can
   simply reflect the bandwidth or can also be computed according to a
   polynomial function of several metrics defining different link
   characteristics; in all cases they are static metrics.  Some routing
   protocols support more than one metric: in the vast majority of the
   cases, one metric is used per (sub)topology.  Less often, a second
   metric may be used as a tie-breaker in the presence of Equal Cost
   Multiple Paths (ECMP).  The optimization of multiple metrics is known
   as an NP complete problem and is sometimes supported by some



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   centralized path computation engine.

   In contrast, LLNs do require the support of both static and dynamic
   metrics.  Furthermore, both link and node metrics are required.  In
   the case of RPL, it is virtually impossible to define one metric, or
   even a composite, that will satisfy all use cases.

   In addition, RPL supports constrained-based routing where constraints
   may be applied to link and nodes.  If a link or a node does not
   satisfy a required constraint, it is `pruned' from the candidate list
   thus leading to a constrained shortest path.

   The set of supported link/node constraints and metrics is specified
   in [I-D.ietf-roll-routing-metrics].

   The role of the Objective Function is to advertise routing metrics
   and constraints in addition to the objectives used to compute the
   (constrained) shortest path.

   Example 1: Shortest path: path offering the shortest end-to-end delay

   Example 2: Constrained shortest path: the path that does traverse any
              battery-operated node and that optimizes the path
              reliability


5.  RPL Protocol Specification

5.1.  RPL Messages

5.1.1.  ICMPv6 RPL Control Message

   This document defines the RPL Control Message, a new ICMPv6 message.
   The RPL Control Message has the following general format, in
   accordance with [RFC4443]:


        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      |     Code      |          Checksum             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                         Message Body                          +
       |                                                               |

                       Figure 1: RPL Control Message




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   The RPL Control message is an ICMPv6 information message with a
   requested Type of 155.

   The Code will be used to identify RPL Control Messages as follows:

   o  0x01: DAG Information Solicitation (Section 5.1.2)

   o  0x02: DAG Information Object (Section 5.1.3)

   o  0x04: Destination Advertisement Object (Section 5.1.4)

5.1.2.  DAG Information Solicitation (DIS)

   The DAG Information Solicitation (DIS) message may be used to solicit
   a DAG Information Object from a RPL node.  Its use is analogous to
   that of a Router Solicitation; a node may use DIS to probe its
   neighborhood for nearby DAGs.  The DAG Information Solicitation
   carries no additional message body.

5.1.3.  DAG Information Object (DIO)

   The DAG Information Object carries a number of metrics and other
   information that allows a node to discover a DAG, select its DAG
   parents, and identify its siblings while employing loop avoidance
   strategies.

5.1.3.1.  DIO Base Option

   The DIO Base Option is a container option, which is always present,
   and might contain a number of suboptions.  The base option regroups
   the minimum information set that is mandatory in all cases.




















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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |G|D|A|0|0| Prf |   Sequence    |  InstanceID   |    DAGRank    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                            DAGID                              |
       +                                                               +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   sub-option(s)...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 2: DIO Base Option

   Control Field:  The DAG Control Field is currently allocated as
         follows:

         Grounded (G):  The Grounded (G) flag is set when the DAG root
               is offering connectivity to an external routed
               infrastructure such as the Internet.

         Destination Advertisement Trigger (D):  The Destination
               Advertisement Trigger (D) flag is set when the DAG root
               or another node in the successor chain decides to trigger
               the sending of destination advertisements in order to
               update routing state for the outward direction along the
               DAG, as further detailed in Section 5.10.  Note that the
               use and semantics of this flag are still under
               investigation.

         Destination Advertisement Supported (A):  The Destination
               Supported (A) bit is set when the DAG root is capable to
               support the collection of destination advertisement
               related routing state and enables the operation of the
               destination advertisement mechanism within the DAG.

         DAGPreference (Prf):  3-bit unsigned integer set by the DAG
               root to its preference and unchanged at propagation.
               DAGPreference ranges from 0x00 (least preferred) to 0x07
               (most preferred).  The default is 0 (least preferred).
               The DAG preference provides an administrative mechanism
               to engineer the self-organization of the LLN, for example
               indicating the most preferred LBR.  If a node has the
               option to join a more preferred DAG while still meeting



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               other optimization objectives, then the node will
               generally seek to join the more preferred DAG as
               determined by the OF.

         Unassigned bits of the Control Field are considered as
         reserved.  They MUST be set to zero on transmission and MUST be
         ignored on receipt.

   Sequence Number:  8-bit unsigned integer set by the DAG root,
         incremented according to a policy provisioned at the DAG root,
         and propagated with no change outwards along the DAG.  Each
         increment SHOULD have a value of 1 and may cause a wrap back to
         zero.

   InstanceID:  8-bit field indicating the topology instance associated
         with the DAG, as provisioned at the DAG root.

   DAGRank:  8-bit unsigned integer indicating the DAG rank of the node
         sending the DIO message.  The DAGRank of the DAG root is
         ROOT_RANK.  DAGRank is further described in Section 5.4.

   DAGID:  128-bit unsigned integer which uniquely identify a DAG.  This
         value is set by the DAG root.  The global IPv6 address of the
         DAG root can be used, however. the DAGID MUST be unique per DAG
         within the scope of the LLN.  In the case where a DAG root is
         rooting multiple DAGs the DAGID MUST be unique for each DAG
         rooted at a specific DAG root.

   The following values MUST NOT change during the propagation of DIO
   messages outwards along the DAG:
      Grounded (G)
      Destination Advertisement Supported (A)
      DAGPreference (Prf)
      Sequence
      InstanceID
      DAGID
   All other fields of the DIO message may be updated at each hop of the
   propagation.

5.1.3.1.1.  DIO Suboptions

   In addition to the minimum options presented in the base option,
   several suboptions are defined for the DIO message:

5.1.3.1.1.1.  Format






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        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
       |  Subopt. Type |         Subopt Length         | Subopt Data
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -

                  Figure 3: DIO Suboption Generic Format

   Suboption Type:  8-bit identifier of the type of suboption.  When
         processing a DIO message containing a suboption for which the
         Suboption Type value is not recognized by the receiver, the
         receiver MUST silently ignore the unrecognized option, continue
         to process the following suboption, correctly handling any
         remaining options in the message.

   Suboption Length:  16-bit unsigned integer, representing the length
         in octets of the suboption, not including the suboption Type
         and Length fields.

   Suboption Data:  A variable length field that contains data specific
         to the option.

   The following subsections specify the DIO message suboptions which
   are currently defined for use in the DAG Information Object.

   Implementations MUST silently ignore any DIO message suboptions
   options that they do not understand.

   DIO message suboptions may have alignment requirements.  Following
   the convention in IPv6, these options are aligned in a packet such
   that multi-octet values within the Option Data field of each option
   fall on natural boundaries (i.e., fields of width n octets are placed
   at an integer multiple of n octets from the start of the header, for
   n = 1, 2, 4, or 8).

5.1.3.1.1.2.  Pad1

   The Pad1 suboption does not have any alignment requirements.  Its
   format is as follows:


        0
        0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+
       |   Type = 0    |
       +-+-+-+-+-+-+-+-+

                              Figure 4: Pad 1



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   NOTE! the format of the Pad1 option is a special case - it has
   neither Option Length nor Option Data fields.

   The Pad1 option is used to insert one or two octets of padding in the
   DIO message to enable suboptions alignment.  If more than two octets
   of padding is required, the PadN option, described next, should be
   used rather than multiple Pad1 options.

5.1.3.1.1.3.  PadN

   The PadN option does not have any alignment requirements.  Its format
   is as follows:


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
       |   Type = 1    |         Subopt Length         | Subopt Data
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -

                              Figure 5: Pad N

   The PadN option is used to insert three or more octets of padding in
   the DIO message to enable suboptions alignment.  For N (N > 2) octets
   of padding, the Option Length field contains the value N-3, and the
   Option Data consists of N-3 zero-valued octets.  PadN Option data
   MUST be ignored by the receiver.

5.1.3.1.1.4.  DAG Metric Container

   The DAG Metric Container suboption may be aligned as necessary to
   support its contents.  Its format is as follows:


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
       |   Type = 2    |       Container Length        | DAG Metric Data
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -

                      Figure 6: DAG Metric Container

   The DAG Metric Container is used to report aggregated path metrics
   along the DAG.  The DAG Metric Container may contain a number of
   discrete node, link, and aggregate path metrics as chosen by the
   implementer.  The Container Length field contains the length in
   octets of the DAG Metric Data.  The order, content, and coding of the
   DAG Metric Container data is as specified in



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   [I-D.ietf-roll-routing-metrics].

   The processing and propagation of the DAG Metric Container is
   governed by implementation specific policy functions.

5.1.3.1.1.5.  Destination Prefix

   The Destination Prefix suboption does not have any alignment
   requirements.  Its format is as follows:


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Type = 3    |            Length             |Resvd|Prf|Resvd|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Prefix Lifetime                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Prefix Length |                                               |
       +-+-+-+-+-+-+-+-+                                               |
       |             Destination Prefix (Variable Length)              |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 7: DAG Destination Prefix

   The Destination Prefix suboption is used when the DAG root, or
   another node located inwards along the DAG on the path to the DAG
   root, needs to indicate that it offers connectivity to destination
   prefixes other than the default.  This may be useful in cases where
   more than one LBR is operating within the LLN and offering
   connectivity to different administrative domains, e.g. a home network
   and a utility network.  In such cases, upon observing the Destination
   Prefixes offered by a particular DAG, a node MAY decide to join
   multiple DAGs in support of a particular application.

   The Length is coded as the length of the suboption in octets,
   excluding the Type and Length fields.

   Prf is the Route Preference as in [RFC4191].  The reserved fields
   MUST be set to zero on transmission and MUST be ignored on receipt.

   The Prefix Lifetime is a 32-bit unsigned integer representing the
   length of time in seconds (relative to the time the packet is sent)
   that the Destination Prefix is valid for route determination.  A
   value of all one bits (0xFFFFFFFF) represents infinity.  A value of
   all zero bits (0x00000000) indicates a loss of reachability.



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   The Prefix Length is an 16-bit unsigned integer that indicates the
   number of leading bits in the destination prefix.

   The Destination Prefix contains Prefix Length significant bits of the
   destination prefix.  The remaining bits of the Destination Prefix, as
   required to complete the trailing octet, are set to 0.

   In the event that a DIO message may need to specify connectivity to
   more than one destination, the Destination Prefix suboption may be
   repeated.

5.1.3.1.1.6.  DAG Timer Configuration

   The DAG Timer Configuration suboption does not have any alignment
   requirements.  Its format is as follows:


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Type = 4    |            Length             | DIOIntDoubl.  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  DIOIntMin.   |
       +-+-+-+-+-+-+-+-+

                     Figure 8: DAG Timer Configuration

   The DAG Timer Configuration suboption is used to distribute
   configuration information for DAG Timer Operation through the DAG.
   The information communicated in this suboption is generally static
   and unchanging within the DAG, therefore it is not necessary to
   include in every DIO.  This suboption MAY be included periodically by
   the DAG Root, and SHOULD be included in response to a unicast
   request, e.g. a DAG Information Solicitation (DIS) message.

   The Length is coded as 2.

   DIOIntervalDoublings is an 8-bit unsigned integer.  Configured on the
   DAG root and used to configure the trickle timer governing when DIO
   message should be sent within the DAG.  DIOIntervalDoublings is the
   number of times that the DIOIntervalMin is allowed to be doubled
   during the trickle timer operation.

   DIOIntervalMin is an 8-bit unsigned integer.  Configured on the DAG
   root and used to configure the trickle timer governing when DIO
   message should be sent within the DAG.  The minimum configured
   interval for the DIO trickle timer in units of ms is
   2^DIOIntervalMin.  For example, a DIOIntervalMin value of 16ms is



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   expressed as 4.

5.1.4.  Destination Advertisement Object (DAO)

   The Destination Advertisement Object (DAO) is used to propagate
   destination information inwards along the DAG.  The RPL use of the
   DAO allows the nodes in the DAG to build up routing state for nodes
   contained in the sub-DAG in support of traffic flowing outward along
   the DAG.


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         DAO Sequence          |  InstanceID   |   DAO Rank    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          DAO Lifetime                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Route Tag                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Prefix Length |    RRCount    |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
       |                   Prefix (Variable Length)                    |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Reverse Route Stack (Variable Length)             |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 9: The Destination Advertisement Object (DAO)

   DAO Sequence:  Incremented by the node that owns the prefix for each
         new DAO message for that prefix.

   InstanceID:  8-bit field indicating the topology instance associated
         with the DAG, as learned from the DIO.

   DAO Rank:  Set by the node that owns the prefix and first issues the
         DAO message to its rank.

   DAO Lifetime:  32-bit unsigned integer.  The length of time in
         seconds (relative to the time the packet is sent) that the
         prefix is valid for route determination.  A value of all one
         bits (0xFFFFFFFF) represents infinity.  A value of all zero
         bits (0x00000000) indicates a loss of reachability.




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   Route Tag:  32-bit unsigned integer.  The Route Tag may be used to
         give a priority to prefixes that should be stored.  This may be
         useful in cases where intermediate nodes are capable of storing
         a limited amount of routing state.  The further specification
         of this field and its use is under investigation.

   Prefix Length:  Number of valid leading bits in the IPv6 Prefix.

   RRCount:  8-bit unsigned integer.  This counter is used to count the
         number of entries in the Reverse Route Stack.  A value of `0'
         indicates that no Reverse Route Stack is present.

   Prefix:  Variable-length field containing an IPv6 address or a prefix
         of an IPv6 address.  The Prefix Length field contains the
         number of valid leading bits in the prefix.  The bits in the
         prefix after the prefix length (if any) are reserved and MUST
         be set to zero on transmission and MUST be ignored on receipt.

   Reverse Route Stack:  Variable-length field containing a sequence of
         RRCount (possibly compressed) IPv6 addresses.  A node that adds
         on to the Reverse Route Stack will append to the list and
         increment the RRCount.

5.2.  Conceptual Data Structures

   The RPL implementation MUST maintain the following conceptual data
   structures in support of DAG discovery:

   o  A set of candidate neighbors

   o  For each DAG:

      *  A set of DAG parents and siblings

5.2.1.  Candidate Neighbors Data Structure

   The set of candidate neighbors is to be populated by neighbors that
   are discovered by the neighbor discovery mechanism and further
   qualified as statistically stable as per the mechanisms discussed in
   [I-D.ietf-roll-routing-metrics].  The candidate neighbors, and
   related metrics, should demonstrate stability/reliability beyond a
   certain threshold, and it is recommended that a local confidence
   value be maintained with respect to the neighbor in order to track
   this.  Implementations MAY choose to bound the maximum size of the
   candidate neighbor set, in which case a local confidence value will
   assist in ordering neighbors to determine which ones should remain in
   the candidate neighbor set and which should be evicted.




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   If Neighbor Unreachability Detection (NUD) determines that a
   candidate neighbor is no longer reachable, then it shall be removed
   from the candidate neighbor set.  In the case that the candidate
   neighbor has associated states in the DAG parent set or active DA
   entries, then the removal of the candidate neighbor shall be
   coordinated with tearing down these states.  All provisioned routes
   associated with the candidate neighbor should be removed.

5.2.2.  Directed Acyclic Graphs (DAGs) Data Structure

   At a given point of time, a DAG Iteration is uniquely identified by
   the tuple (DagID, InstanceID, DAGSequenceNumber) where a change in
   the sequence denotes the iteration of a given DAG over time.  When a
   single device is capable to root multiple DAGs in support of an
   application need for multiple optimization objectives it MUST produce
   a different and unique (DagID, InstanceID) pair for each of the
   multiple DAGs.

   For each DAG that a node is, or may become, a member of, the
   implementation MUST keep a DAG table with the following entries:

   o  InstanceID

   o  DAGID

   o  DAGSequenceNumber

   o  DAG Metric Container, including DAGObjectiveCodePoint

   o  A set of Destination Prefixes offered inwards along the DAG

   o  A set of DAG parents and siblings

   o  A timer to govern the sending of DIO messages for the DAG

   When a DAG is discovered for which no DAG data structure is
   instantiated, and the node wants to join, then the DAG data structure
   is instantiated.

   When the DAG parent set is depleted (i.e. the last DAG is removed),
   then the DAG data structure SHOULD be suppressed after the expiration
   of an implementation-specific local timer.  An implementation SHOULD
   delay before deallocating the DAG data structure in order to observe
   that the DAGSequenceNumber has incremented should any new DAG parents
   appear for the DAG.






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5.2.2.1.  DAG Parents/Siblings Structure

   When the DAG is self-rooted, the set of DAG parents/siblings is
   empty.

   In all other cases, for each node in the set, the implementation MUST
   keep a record of:

   o  a reference to the neighboring device which is the DAG parent or
      sibling

   o  a record of most recent information taken from the DAG Information
      Object last processed from the DAG parent

   DAG parents may be ordered, according to the OF.  When ordering DAG
   parents, in consultation with the OF, the most preferred DAG parent
   may be identified.  All current DAG parents must have a rank less
   than self.  All current DAG siblings must have a rank equal to self.

   When nodes are added to or removed from the DAG set the most
   preferred DAG parent may have changed.  The role of all the nodes in
   the list should be reevaluated.  In particular, any nodes having a
   rank greater than self after such a change must be evicted from the
   set.

   An implementation may choose to keep these records as an extension of
   the Default Router List (DRL).

5.3.  DAG Rank

   Based on the selection of DAG Parents, the metrics conveyed by the
   most preferred DAG parent, the nodes own metrics and configuration,
   and a related function defined by the OF, a node will be able to
   compute a value for its rank as a consequence of selecting a most
   preferred DAG parent.

   The rank value feeds back into the DAG parent selection according to
   a loop-avoidance strategy.  Once a DAG parent has been added, and a
   rank value for the node within the DAG has been computed, the nodes
   further options with regard to DAG parent selection and movement
   within the DAG are restricted in favor of loop avoidance.

   It is important to note that the DAG Rank is not itself a metric,
   although its value is derived from and influenced by the use of
   metrics to select DAG parents and take up a position in the DAG.  The
   only aim of the rank is to inform loop avoidance and detection.

   The computation of the DAG Rank MUST be done in such a way so as to



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   maintain the following properties for any nodes M and N that are
   neighbors in the LLN:

   DAGRank(M) is less than DAGRank(N):  In this case, M is probably
           located in a more preferred position than N in the DAG with
           respect to the metrics and optimizations defined by the
           objective code point.  In any fashion, Node M may safely be a
           DAG parent for Node N without risk of creating a loop.
           Further, for a node N, all parents in the DAG parent set must
           be of rank less than self's DAGRank(N).  In other words, the
           rank presented by a node N MUST be greater (deeper) than that
           presented by any of its parents.

   DAGRank(M) equals DAGRank(N):  In this case M and N are located
           positions of relatively the same optimality within the DAG.
           In some cases, Node M may be used as a successor by Node N,
           but with related chance of creating a loop that must be
           detected and broken by some other means.

   DAGRank(M) is greater than DAGRank(N):  In this case, then node M is
           located in a less preferred position than N in the DAG with
           respect to the metrics and optimizations defined by the
           objective code point.  Further, Node (M) may in fact be in
           Node (N)'s sub-DAG.  There is a higher risk to Node (N)
           selecting Node (M) as a DAG parent, as such a selection may
           create a loop.

   As an example, the DAG Rank could be computed in such a way so as to
   closely track ETX when the objective function is to minimize ETX, or
   latency when the objective function is to minimize latency, or in a
   more complicated way as appropriate to the objective code point being
   used within the DAG.

5.4.  DAG Discovery and Maintenance

   DAG discovery locates the nearest sink (aka root), as determined
   according to some metrics and constraints, and forms a Directed
   Acyclic Graph towards that sink, by identifying a set of DAG parents.
   During this process DAG discovery also identifies siblings, which may
   be used later to provide additional path diversity towards the DAG
   root.  DAG discovery enables nodes to implement different policies
   for selecting their DAG parents in the DAG by using implementation
   specific policy functions.  DAG discovery specifies a set of rules to
   be followed by all implementations in order to ensure interoperation.
   DAG discovery also standardizes the format that is used to advertise
   the most common information that is used in order to select DAG
   parents.




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   One of these information, the DAG rank, is used by DAG discovery to
   provide loop avoidance even if nodes implement different policies.
   The DAG Rank is computed as specified by the OF in use by the DAG,
   demonstrating the properties described in Section 5.3.  The rank
   should be computed in such a way so as to provide a comparable basis
   with other nodes which may not use the same metric at all.

   The DAG discovery procedures take into account a number of factors,
   including:

   o  RPL rules for loop avoidance based on DAGs and ranks

   o  The Objective Function

   o  The advertised metrics

   o  Local policy functions (e.g. a bounded number of candidate
      neighbors).

5.4.1.  DAG Discovery Rules

   In order to organize and maintain loopless structure, the DAG
   discovery implementation in the nodes MUST obey to the following
   rules and definitions:

5.4.1.1.  DAGs

   1.  DAG discovery instantiates LLN topologies that are each optimized
       for specific constraints and goals.  A topology assumes the shape
       of a DAG, and a DAG Instance is uniquely identified by its
       instanceID.

   2.  For reasons of scalability and operations of the protocol, a DAG
       Instance is partitioned into a set of DAGs rooted at a
       destination, aka Destination Oriented DAGs.  A destination is
       uniquely identified by a DAGID so a DAG rooted at a destination
       is uniquely identified by the pair (InstanceID, DAGID).

   3.  A Destination Oriented DAG is periodically reconstructed from the
       root, by incrementing a DAGSequenceNumber.  An Iteration of a
       Destination Oriented DAG is thus uniquely identified by the tuple
       (InstanceID, DAGID, DAGSequenceNumber).  Through this document,
       the graph formed by this iterative process is referred to as the
       DAG Iteration, or in short, the DAG.

   4.  The rank is defined within the scope of a DAG Iteration as an
       abstract coordinate to compare the relative position of nodes and
       ensure forward progress of the traffic.



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   5.  A node MUST belong at most to one DAG Iteration per InstanceID
       and MUST select all its parents and siblings within that same DAG
       Iteration.

5.4.1.2.  DAG Sequence Number

   1.  The DAGSequenceNumber is incremented by the root and flooded
       through DIOs.

   2.  The root floods a new DAGSequenceNumber periodically, at a rate
       that depends on the deployment.  This rate can be set to 0 if
       other methods such as loop detection are considered sufficient to
       solve the routing issues in that deployment.

   3.  The root MAY also flood a new DAGSequenceNumber on-demand.  The
       details of the mechanism to signal the root to do so are to be
       specified in a future revision of this document.

   4.  A parent that advertises the new DAGSequenceNumber can not
       possibly belong to the sub-DAG of a node that still advertises an
       older DAGSequenceNumber.  The node MAY thus attach to that parent
       regardless of the relative rank, and this situation is equivalent
       to jumping onto a different Destination Oriented DAG.

   5.  Thus, as a new DAGSequenceNumber spreads, a new DAG Iteration
       forms that supersedes the previous one.  During a
       DAGSequenceNumber transition, a node MAY decide to forward
       packets via 'future parents' that belong to the same Destination
       Oriented DAG (same InstanceID and DagID), but a more recent
       (incremented) DAGSequenceNumber.

5.4.1.3.  DAG Root

   1.  A node that does not have any DAG parent MAY become the root of
       its own floating DAG.  It's rank is ROOT_RANK.

   2.  A (non-LLN) router is considered connected to a grounded
       infrastructure at rank BASE_RANK.  A LLN node that is attached to
       such an infrastructure router is the DAG root of its own grounded
       DAG.  It's rank is ROOT_RANK.

   3.  In a deployment that uses a backbone link to federate a number of
       LLN roots, it is possible to run RPL over the backbone and use
       one router as a backbone root.  The backbone root exposes a rank
       of BASE_RANK over the backbone.  All the LLN roots that are
       parented to that backbone root, including the backbone root if it
       also serves as LLN root, expose a rank of ROOT_RANK over the LLN
       and act as multiple roots for a same DAG, coordinated by the



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       backbone root.

   4.  The DAG root exposes the DAG in the DIO message and LLN nodes
       propagate the DIO message outwards along the DAG.

5.4.1.4.  Moving Inside a DAG

   1.  A node moves when it changes its parent selection within the same
       DAG Iteration.  When a node moves (within its DAG) in a fashion
       that cause its rank to decrease, the node MUST abandon all
       parents and siblings with a rank larger than self, and MAY adopt
       as siblings nodes with the same rank.

   2.  A node MAY move at any time, with no delay, within its DAG when
       the move does not cause the node to increase its own DAG rank, as
       per the rank calculation indicated by the OF.

   3.  A node MUST NOT move outwards along a DAG that it is attached to,
       causing the DAG rank to increase.  If a node cannot stay within
       the DAG without a rank increase, then it MUST poison its routes
       as described in Section 5.4.1.6.

   4.  When DIO messages are received from other routers located at
       lesser rank in the same DAG, those routers are eligible for
       consideration as DAG parents.  DIO messages received from other
       routers located at the same rank in the same DAG may be
       considered as coming from siblings.  DIO messages that are
       received from other routers located at greater rank within the
       same DAG might cause greedy behaviors and loops; such a DIO is
       ignored unless:

       1.  The DIO comes from an existing parent or sibling; in which
           case that parent must be removed.

       2.  The DIO comes from a node that has better OF ratings than any
           parent known at this point; in that case, this potential
           parent MAY be remembered in order to jump at a better
           position when the next sequence is flooded.

5.4.1.5.  Jumping Onto Another DAG

   1.  A node jumps when it performs a new parent selection whereby its
       DAG Iteration changes within the same DAG Instance.  When a node
       jumps onto a new DAG Iteration, it MUST abandon all parents and
       siblings from its previous position.

   2.  A node MAY jump from its current DAG onto any other DAG that
       provides service for the same InstanceID if it is preferred by



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       the OF, for example for reasons such as connectivity, configured
       preference, free medium time, size, security, bandwidth, DAG
       rank, or whatever metrics the LLN uses.  This is allowed
       regardless of the rank that the node reaches in the new DAG.

   3.  A node that jumps should attempt to transmit all the packets
       received as part of the previous DAG along the previous DAG.  In
       other words, it should switch the parent set only after the
       outstanding packet queue of packets received prior to announcing
       the jump is exhausted.

   4.  Jumping back onto a previous DAG is equivalent to moving inside
       that DAG and obeys the same rules.  To satisfy this, a node
       detaching from a DAG SHOULD remember its DAG as identified by the
       tuple (InstanceID, DagID, DAGSequenceNumber) as well as its rank
       within that DAG for long as that DAG exists.

5.4.1.6.  Poisoning a Broken Path

   1.  A node SHOULD poison its inwards routes when it looses all of its
       current feasible parents, i.e. the set of DAG parents becomes
       depleted, and it can not jump onto an alternate DAG.

   2.  In order to poison its inwards routes, a node MAY stay at its
       position within its DAG (that is maintain its InstanceID, DagID,
       DAGSequenceNumber and Rank) but it SHOULD immediately advertise a
       rank of INFINITE_RANK in a DIO so as to force all its children to
       remove it from their parent list and try an alternate path.  The
       node SHOULD then wait for a new DAG Iteration (DAGSequenceNumber
       increment) before resuming its operation in the same Destination
       Oriented DAG.

   3.  Alternatively, a node MAY detach from its DAG.  A node that
       detaches becomes root of its own floating DAG and MUST
       immediately advertise its new situation in a DIO.

   4.  Either way, the route poisoning will recursively be flooded
       throughout the impacted sub-DAG as children lose their last
       parent in the original DAG.

   5.  The loss of a DIO message may interrupt the flooding.  This can
       be compensated by cheer repetition through the trickle algorithm.
       If that also fails, packet loops will be prevented by the
       detection mechanism described in Section 5.11.







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5.4.1.7.  Following a Parent

   1.  If a node that receives a DIO from one of its DAG parents
       indicating that the parent has left the DAG, it may either follow
       that parent or stay in its current DAG through an alternate DAG
       parent if that is possible.

   2.  If a DAG parent increases its rank such that the node rank would
       have to change, and if the node does not wish to follow (e.g. it
       has alternate options), then the DAG parent SHOULD be evicted
       from the DAG parent set.  If the DAG parent is the last in the
       DAG parent set, then the node SHOULD chose to follow it.

5.4.1.8.  DAG Inconsistency

   1.  When a node detects or causes a DAG inconsistency, as described
       in Section 5.4.4.2, then the node SHOULD send an unsolicited DIO
       message to its one-hop neighbors.  The DIO is updated to
       propagate the new DAG information.  Such an event MUST also cause
       the trickle timer governing the periodic sending of DIO messages
       to be reset.

5.4.2.  Reception and Processing of DIO messages

   When an DIO message is received from a source device named SRC, the
   receiving node must first determine whether or not the DIO message
   should be accepted for further processing, and subsequently present
   the DIO message for further processing if eligible.

   1.  If the DIO message is malformed, then the DIO message is not
       eligible for further processing and is silently discarded.  A RPL
       implementation MAY log the reception of a malformed DIO message.

   2.  If SRC is not a member of the candidate neighbor set, then the
       DIO is not eligible for further processing.  (Further evaluation/
       confidence of this neighbor is necessary)

   3.  If the DIO message advertises a DAG that the node is already a
       member of, then:

       *  If the rank of SRC as reported in the DIO message is lesser
          than that of the node within the DAG, then the DIO message
          MUST be considered for further processing.

       *  If the rank of SRC as reported in the DIO message is equal to
          that of the node within the DAG, then SRC is marked as a
          sibling and the DIO message is not eligible for further
          processing.



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       *  If the rank of SRC as reported in the DIO message is higher
          than that of the node within the DAG, and SRC is not a DAG
          parent, then the DIO message MUST NOT be considered for
          further processing

   4.  Even if not processed further, information from a DIO might be
       remembered for instance if SRC is preferable to the current
       parents per the OF selection process.

   5.  If SRC is a DAG parent for any other DAG that the node is
       attached to, then the DIO message MUST be considered for further
       processing (the DAG parent may have jumped).

   6.  If the DIO message advertises a DAG that offers a better (new or
       alternate) solution to an optimization objective desired by the
       node, then the DIO message MUST be considered for further
       processing.

5.4.2.1.  Overview of DIO Message Processing

      If the received DIO message is for a new/alternate DAG:

         If the node has sent an DIO message within the risk window as
         described in Section 5.8 then a collision has occurred; do not
         process the DIO message any further.

         If the SRC node is also a DAG parent for another DAG that the
         node is a member of, and if the new/alternate DAG is the same
         InstanceID as the other DAG, then the DAG parent is known to
         have jumped.

            Remove SRC as a DAG parent from the other DAG

            If the other DAG is now empty of candidate parents, then
            prepare to directly follow SRC into the new DAG by adding it
            as a DAG parent for the new DAG, else ignore the DIO message
            (do not follow the parent).

         Instantiate a data structure for the new/alternate DAG if
         necessary

         If the new/alternate DAG offers a better solution to the
         optimization objectives, then jump: copy the DIO information
         place the neighbor into the DAG parent set.

      If the DIO message is for a known/existing DAG:





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         Process the DIO message as per the rules in Section 5.4

   As DIO messages are received from candidate neighbors, the neighbors
   may be promoted to DAG parents by following the rules of DAG
   discovery as described in Section 5.4.  When a node places a neighbor
   into the DAG Parent set, the node becomes attached to the DAG through
   the new parent node.

   In the DAG discovery implementation, the most preferred parent should
   be used to restrict which other nodes may become DAG parents.  Some
   nodes in the DAG parent set may be of a rank less than or equal to
   the most preferred DAG parent.  (This case may occur, for example, if
   an energy constrained device is at a lesser rank but should be
   avoided as per an optimization objective, resulting in a more
   preferred parent at a greater rank).

5.4.3.  DIO Transmission

   Each node maintains a timer that governs when to multicast DIO
   messages.  This timer is implemented as a trickle timer operating
   over a variable interval.  Trickle timers are further detailed in
   Section 5.4.4.  The governing parameters for the timer should be
   configured consistently across the DAG, and are provided by the DAG
   root in the DIO message.  In addition to periodic DIO messages, each
   node may respond to a DIS message with a DIO message.

   o  When a node detects an inconsistency, it SHOULD reset the interval
      of the trickle timer to a minimum value, causing DIO messages to
      be emitted more frequently as part of a strategy to quickly
      correct the inconsistency.  Such inconsistencies may be, for
      example, an update to a key parameter (e.g. sequence number) in
      the DIO message or a loop detected when a node located inwards
      along the DAG forwards traffic outwards.  Inconsistencies are
      further detailed in Section 5.4.4.2.

   o  When a node enters a mode of consistent operation within a DAG,
      i.e.  DIO messages from its DAG parents are consistent and no
      other inconsistencies are detected, it may begin to open up the
      interval of the trickle timer towards a maximum value, causing DIO
      messages to be emitted less frequently, thus reducing network
      maintenance overhead and saving energy consumption.

   o  When a node is initialized, it MAY be configured to remain silent
      and not multicast any DIO messages until it has encountered and
      joined a DAG (perhaps initially probing for a nearby DAG with an
      DIS message).  Alternately, it may choose to root its own floating
      DAG and begin multicasting DIO messages using a default trickle
      configuration.  The second case may be advantageous if it is



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      desired for independent nodes to begin aggregating into scattered
      floating DAGs in the absence of a grounded node, for example in
      support of LLN installation and commissioning.

   Note that if multiple DAG roots are participating in the same DAG,
   i.e. offering DIO messages with the same DAGID, then they must
   coordinate with each other to ensure that their DIO messages are
   consistent when they emit DIO messages.  In particular the Sequence
   number must be identical from each DAG root, regardless of which of
   the multiple DAG roots issues the DIO message, and changes to the
   Sequence number should be issued at the same time.  The specific
   mechanism of this coordination, e.g. along a non-LLN network between
   DAG roots, is beyond the scope of this specification.

5.4.4.  Trickle Timer for DIO Transmission

   RPL treats the construction of a DAG as a consistency problem, and
   uses a trickle timer [Levis08] to control the rate of control
   broadcasts.

   For each DAG that a node is part of, the node must maintain a single
   trickle timer.  The required state contains the following conceptual
   items:

   I:    The current length of the communication interval

   T:    A timer with a duration set to a random value in the range
         [I/2, I]

   C:    Redundancy Counter

   I_min:  The smallest communication interval in milliseconds.  This
         value is learned from the DIO message as (2^DIOIntervalMin)ms.
         The default value is DEFAULT_DIO_INTERVAL_MIN.

   I_doublings:  The number of times I_min should be doubled before
         maintaining a constant rate, i.e.  I_max = I_min *
         2^I_doublings.  This value is learned from the DIO message as
         DIOIntervalDoublings.  The default value is
         DEFAULT_DIO_INTERVAL_DOUBLINGS.

5.4.4.1.  Resetting the Trickle Timer

   The trickle timer for a DAGID is reset by:

   1.  Setting I_min and I_doublings to the values learned from the DIO
       message.




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   2.  Setting C to zero.

   3.  Setting I to I_min.

   4.  Setting T to a random value as described above.

   5.  Restarting the trickle timer to expire after a duration T

   When node learns about a DAG through a DIO message and makes the
   decision to join it, it initializes the state of the trickle timer by
   resetting the trickle timer and listening.  Each time it hears a
   consistent DIO message for this DAG from a DAG parent, it MAY
   increment C.

   When the timer fires at time T, the node compares C to the redundancy
   constant, DEFAULT_DIO_REDUNDANCY_CONSTANT.  If C is less than that
   value, the node generates a new DIO message and multicasts it.  When
   the communication interval I expires, the node doubles the interval I
   so long as it has previously doubled it fewer than I_doubling times,
   resets C, and chooses a new T value.

5.4.4.2.  Determination of Inconsistency

   The trickle timer is reset whenever an inconsistency is detected
   within the DAG, for example:

   o  The node joins a new DAGID

   o  The node moves within a DAGID

   o  The node receives a modified DIO message from a DAG parent

   o  A DAG parent forwards a packet intended to move inwards,
      indicating an inconsistency and possible loop.

   o  A metric communicated in the DIO message is determined to be
      inconsistent, as according to a implementation specific path
      metric selection engine.

   o  The rank of a DAG parent has changed.

5.5.  DAG Sequence Number Increment

   The DAG root makes the sole determination of when to revise the
   DAGSequenceNumber by incrementing it upwards.  When the
   DAGSequenceNumber is increased an inconsistency results, causing DIO
   messages to be sent back outwards along the DAG to convey the change.
   The degree to which this mechanism is relied on may be determined by



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   the implementation- on one hand it may serve as a periodic heartbeat,
   refreshing the DAG states, and on the other hand it may result in a
   constant steady-state control cost overhead which is not desirable.

   Some implementations may provide an administrative interface, such as
   a command line, at the DAG root whereby the DAGSequenceNumber may be
   caused to increment in response to some policy outside of the scope
   of RPL.

   Other implementations may make use of a periodic timer to
   automatically increment the DAGSequenceNumber, resulting in a
   periodic DAG iteration at a rate appropriate to the application and
   implementation.  Other automated mechanisms to determine
   DAGSequenceNumber increments are also possible as appropriate to a
   deployment.

5.6.  DAG Selection

   The DAG selection is implementation and algorithm dependent.  Nodes
   SHOULD prefer to join DAGs for InstanceIDs advertising OCPs and
   destinations compatible with their implementation specific
   objectives.  In order to limit erratic movements, and all metrics
   being equal, nodes SHOULD keep their previous selection.  Also, nodes
   SHOULD provide a means to filter out a candidate parent whose
   availability is detected as fluctuating, at least when more stable
   choices are available.

   When connection to a fixed network is not possible or preferable for
   security or other reasons, scattered DAGs MAY aggregate as much as
   possible into larger DAGs in order to allow connectivity within the
   LLN.

   A node SHOULD verify that bidirectional connectivity and adequate
   link quality is available with a candidate neighbor before it
   considers that candidate as a DAG parent.

5.7.  Administrative rank

   When the DAG is formed under a common administration, or when a node
   performs a certain role within a community, it might be beneficial to
   associate a range of acceptable rank with that node.  For instance, a
   node that has limited battery should be a leaf unless there is no
   other choice, and may then augment the rank computation specified by
   the OF in order to expose an exaggerated rank.







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5.8.  Collision

   A race condition occurs if 2 nodes send DIO messages at the same time
   and then attempt to join each other.  This might happen, for example,
   between nodes which act as DAG root of their own DAGs.  In order to
   detect the situation, LLN Nodes time stamp the sending of DIO
   message.  Any DIO message received within a short link-layer-
   dependent period introduces a risk.  It is up to the implementation
   to define the duration of the risk window.

   There is risk of a collision when a node receives and processes a DIO
   within the risk window.  For example, it may occur that two nodes are
   associated with different DAGs and near-simultaneously send DIO
   messages, which are received and processed by both, and possibly
   result in both nodes simultaneously deciding to attach to each other.
   As a remedy, in the face of a potential collision, as determined by
   receiving a DIO within the risk window, the DIO message is not
   processed.  It is expected that subsequent DIOs would not cross.

5.9.  Guidelines for Objective Functions

5.9.1.  Objective Function

   An Objective Function (OF) allows for the selection of a DAG to join,
   and a number of peers in that DAG as parents.  The OF is used to
   compute an ordered list of parents.  The OF is also responsible to
   compute the rank of the device within the DAG.

   The Objective Function is specified in the DIO message within a DAG
   Metric Container using an Objective Code Point (OCP), as specified in
   [I-D.ietf-roll-routing-metrics], and indicates the method that must
   be used to compute the DAG (e.g. "minimize the path cost using the
   ETX metric and avoid `Blue' links").  The Objective Code Points are
   specified in [I-D.ietf-roll-routing-metrics].  This document
   specifies an Objective Function, OF0, in support of default
   operation.  In the case where the DIO does not include an OCP
   specification in the DAG Metric Container, OF0 MAY be presumed.

   Most Objective Functions are expected to follow the same abstract
   behavior:

   o  The parent selection is triggered each time an event indicates
      that a potential next hop information is updated.  This might
      happen upon the reception of a DIO message, a timer elapse, or a
      trigger indicating that the state of a candidate neighbor has
      changed.





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   o  An OF scans all the interfaces on the device.  Although there may
      typically be only one interface in most application scenarios,
      there might be multiple of them and an interface might be
      configured to be usable or not for RPL operation.  An interface
      can also be configured with a preference or dynamically learned to
      be better than another by some heuristics that might be link-layer
      dependent and are out of scope.  Finally an interface might or not
      match a required criterion for an Objective Function, for instance
      a degree of security.  As a result some interfaces might be
      completely excluded from the computation, while others might be
      more or less preferred.

   o  An OF scans all the candidate neighbors on the possible interfaces
      to check whether they can act as a router for a DAG.  There might
      be multiple of them and a candidate neighbor might need to pass
      some validation tests before it can be used.  In particular, some
      link layers require experience on the activity with a router to
      enable the router as a next hop.

   o  An OF computes self's rank by adding the step of rank to that
      candidate to the rank of that candidate.  The step of rank is
      computed by estimating the link as follows:

      *  The step of rank might vary from 1 to 16.

         +  1 indicates a unusually good link, for instance a link
            between powered devices in a mostly battery operated
            environment.

         +  4 indicates a `normal'/typical link, as qualified by the
            implementation.

         +  16 indicates a link that can hardly be used to forward any
            packet, for instance a radio link with quality indicator or
            expected transmission count that is close to the acceptable
            threshold.

      *  Candidate neighbors that would cause self's rank to increase
         are ignored

   o  Candidate neighbors that advertise an OF incompatible with the set
      of OF specified by the policy functions are ignored.

   o  As it scans all the candidate neighbors, the OF keeps the current
      best parent and compares its capabilities with the current
      candidate neighbor.  The OF defines a number of tests that are
      critical to reach the objective.  A test between the routers
      determines an order relation.



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      *  If the routers are roughly equal for that relation then the
         next test is attempted between the routers,

      *  Else the best of the 2 becomes the current best parent and the
         scan continues with the next candidate neighbor

      *  Some OFs may include a test to compare the ranks that would
         result if the node joined either router

   o  When the scan is complete, the preferred parent is elected and
      self's rank is computed as the preferred parent rank plus the step
      in rank with that parent.

   o  Other rounds of scans might be necessary to elect alternate
      parents and siblings.  In the next rounds:

      *  Candidate neighbors that are not in the same DAG are ignored

      *  Candidate neighbors that are of greater rank than self are
         ignored

      *  Candidate neighbors of an equal rank to self (siblings) are
         ignored

      *  Candidate neighbors of a lesser rank than self (non-siblings)
         are preferred

5.9.2.  Objective Function 0 (OF0)

   This document specifies a default objective function, called OF0,
   indicated by an OCP value of 0x0000.  OF0 is the default objective
   function of RPL, and can be used if allowed by the policy of the
   processing node when the OF indicated in the DIO message is unknown
   to the node.  If not allowed, then the DIO message is simply ignored
   and not processed by the node.  OF0 is notable in that it does not
   use physical metrics as described in [I-D.ietf-roll-routing-metrics],
   but is only based on abstract information from the DIO message such
   as rank and administrative preference.

   OF0 favors connectivity.  That is, the Objective Function is designed
   to find the nearest sink into a 'grounded' topology, and if there is
   none then join any network per order of administrative preference.
   The metric in use is the rank.

   OF0 selects a preferred parent and a backup next hop if one is
   available.  The backup next hop might be a parent or a sibling.  All
   the traffic is routed via the preferred parent.  When the link
   conditions do not let a packet through to the preferred parent, the



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   packet is passed to the backup next hop.

   The step of rank is 4 for each hop.

5.9.2.1.  Selection of the Preferred Parent

   As it scans all the candidate neighbors, OF0 keeps the parent that is
   the best for the following criteria (in order):

   1.   The interface must be usable and any administrative preference
        associated with the interface applies first.

   2.   A candidate that would cause the node to augment the rank in the
        current DAG is not considered.

   3.   A router that has been validated as usable, e.g. with a local
        confidence that has exceeded some pre-configured threshold, is
        better.

   4.   If none are grounded then a DAG with a more preferred
        administrative preference (DAGPreference) is better.

   5.   A router that offers connectivity to a grounded DAG is better.

   6.   A lesser resulting rank is better.

   7.   A DAG for which there is an alternate parent is better.  This
        check is optional.  It is performed by computing the backup next
        hop while assuming that this router won.

   8.   The DAG that was in use already is preferred.

   9.   The preferred parent that was in use already is better.

   10.  A router that has announced a DIO message more recently is
        preferred.

5.9.2.2.  Selection of the Backup Next Hop

   o  The interface must be usable and the administrative preference (if
      any) applies first.

   o  The preferred parent is ignored.

   o  Candidate neighbors that are not in the same DAG are ignored.

   o  Candidate neighbors with a higher rank are ignored.




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   o  Candidate neighbors of a better rank than self (non-siblings) are
      preferred.

   o  A router that has been validated as usable, e.g. with a local
      confidence that has exceeded some pre-configured threshold, is
      better.

   o  The router with a better router preference wins.

   o  The backup next hop that was in use already is better.

5.10.  Establishing Routing State Outward Along the DAG

   The destination advertisement mechanism supports the dissemination of
   routing state required to support traffic flows outward along the
   DAG, from the DAG root toward nodes.

   As a result of destination advertisement operation:

   o  DAG discovery establishes a DAG oriented toward a DAG root along
      which inward routes toward the DAG root are set up.

   o  Destination advertisement establishes outward routes along the
      DAG.  Such paths consist of:
      *  Hop-By-Hop routing state within islands of `stateful' nodes.
      *  Source Routing `bridges' across nodes that do not retain state.

   Destinations disseminated with the destination advertisement
   mechanism may be prefixes, individual hosts, or multicast listeners.
   The mechanism supports nodes of varying capabilities as follows:

   o  When nodes are capable of storing routing state, they may inspect
      destination advertisements and learn hop-by-hop routing state
      toward destinations by populating their routing tables with the
      routes learned from nodes in their sub-DAG.  In this process they
      may also learn necessary piecewise source routes to traverse
      regions of the LLN that do not maintain routing state.  They may
      perform route aggregation on known destinations before emitting
      Destination Advertisements.

   o  When nodes are incapable of storing routing state, they may
      forward destination advertisements, recording the reverse route as
      the go in order to support the construction of piecewise source
      routes.

   Nodes that are capable of storing routing state, and finally the DAG
   roots, are able to learn which destinations are contained in the sub-
   DAG below the node, and via which next-hop neighbors.  The



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   dissemination and installation of this routing state into nodes
   allows for Hop-By-Hop routing from the DAG root outwards along the
   DAG.  The mechanism is further enhance by supporting the construction
   of source routes across stateless `gaps' in the DAG, where nodes are
   incapable of storing additional routing state.  An adaptation of this
   mechanism allows for the implementation of loose-source routing.

   A special case, the reception of a destination advertisement
   addressed to a link-local multicast address, allows for a node to
   learn destinations directly available from its one-hop neighbors.

   A design choice behind advertising routes via destination
   advertisements is not to synchronize the parent and children
   databases along the DAG, but instead to update them regularly to
   recover from the loss of packets.  The rationale for that choice is
   time variations in connectivity across unreliable links.  If the
   topology can be expected to change frequently, synchronization might
   be an excessive goal in terms of exchanges and protocol complexity.
   The approach used here results in a simple protocol with no real
   peering.  The destination advertisement mechanism hence provides for
   periodic updates of the routing state, as cued by occasional RAs and
   other mechanisms, similarly to other protocols such as RIP [RFC2453].

5.10.1.  Destination Advertisement Operation

5.10.1.1.  Overview

   According to implementation specific policy, a subset or all of the
   feasible parents in the DAG may be selected to receive prefix
   information from the destination advertisement mechanism.  This
   subset of DAG parents shall be designated the set of DA parents.

   As DAO messages for particular destinations move inwards along the
   DAG, a sequence counter is used to guarantee their freshness.  The
   sequence counter is incremented by the source of the DAO message (the
   node that owns the prefix, or learned the prefix via some other
   means), each time it issues a DAO message for its prefix.  Nodes that
   receive the DAO message and, if scope allows, will be forwarding a
   DAO message for the unmodified destination inwards along the DAG,
   will leave the sequence number unchanged.  Intermediate nodes will
   check the sequence counter before processing a DAO message, and if
   the DAO is unchanged (the sequence counter has not changed), then the
   DAO message will be discarded without additional processing.
   Further, if the DAO message appears to be out of synch (the sequence
   counter is 2 or more behind the present value) then the DAO state is
   considered to be stale and may be purged, and the DAO message is
   discarded.  A depth is also added for tracking purposes; the depth is
   incremented at each hop as the DAO message is propagated up the DAG.



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   Nodes that are storing routing state may use the depth to determine
   which possible next-hops for the destination are more optimal.

   If destination advertisements are activated in the DIO message as
   indicated by the `D' bit, the node sends unicast destination
   advertisements to one of its DA parents, that is selected as most
   favored for incoming outwards traffic.  The node only accepts unicast
   destination advertisements from any nodes but those contained in the
   DA parent subset.

   Receiving a DIO message with the `D' destination advertisement bit
   set from a DAG parent stimulates the sending of a delayed destination
   advertisement back, with the collection of all known prefixes (that
   is the prefixes learned via destination advertisements for nodes
   lower in the DAG, and any connected prefixes).  If the Destination
   Advertisement Supported (A) bit is set in the DIO message for the
   DAG, then a destination advertisement is also sent to a DAG parent
   once it has been added to the DA parent set after a movement, or when
   the list of advertised prefixes has changed.

   A node that modifies its DAG Parent set may set the `D' bit in
   subsequent DIO propagation in order to trigger destination
   advertisements to be updated to its DAG Parents and other inward
   nodes on the DAG.  Additional recommendations and guidelines
   regarding the use of this mechanism are still under consideration and
   will be elaborated in a future revision of this specification.

   Destination advertisements may advertise positive (prefix is present)
   or negative (removed) DAO messages, termed as no-DAOs.  A no-DAO is
   stimulated by the disappearance of a prefix below.  This is
   discovered by timing out after a request (a DIO message) or by
   receiving a no-DAO.  A no-DAO is a conveyed as a DAO message with a
   DAO Lifetime of ZERO_LIFETIME.

   A node that is capable of recording the state information conveyed in
   a unicast DAO message will do so upon receiving and processing the
   DAO message, thus building up routing state concerning destinations
   below it in the DAG.  If a node capable of recording state
   information receives a DAO message containing a Reverse Route Stack,
   then the node knows that the DAO message has traversed one or more
   nodes that did not retain any routing state as it traversed the path
   from the DAO source to the node.  The node may then extract the
   Reverse Route Stack and retain the included state in order to specify
   Source Routing instructions along the return path towards the
   destination.  The node MUST set the RRCount back to zero and clear
   the Reverse Route Stack prior to passing the DAO message information
   on.




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   A node that is unable to record the state information conveyed in the
   DAO message will append the next-hop address to the Reverse Route
   Stack, increment the RRCount, and then pass the destination
   advertisement on without recording any additional state.  In this way
   the Reverse Route Stack will contain a vector of next hops that must
   be traversed along the reverse path that the DAO message has
   traveled.  The vector will be ordered such that the node closest to
   the destination will appear first in the list.  In such cases, if it
   is useful to the implementation to try and build up redundant paths,
   the node may choose to convey the destination advertisement to one or
   more DAG parents in order of preference as guided by an
   implementation specific policy.

   In some cases (called hybrid cases), some nodes along the path a
   destination advertisement follows inward along the DAG may store
   state and some may not.  The destination advertisement mechanism
   allows for the provisioning of routing state such that when a packet
   is traversing outwards along the DAG, some nodes may be able to
   directly forward to the next hop, and other nodes may be able to
   specify a piecewise source route in order to bridge spans of
   stateless nodes within the path on the way to the desired
   destination.

   In the case where no node is able to store any routing state as
   destination advertisements pass by, and the DAG root ends up with DAO
   messages that contain a completely specified route back to the
   originating node in the form of the inverted Reverse Route Stack.  A
   DAG root should not request (Destination Advertisement Trigger) nor
   indicate support (Destination Advertisement Supported) for
   destination advertisements if it is not able to store the Reverse
   Route Stack information in this case.

   The destination advertisement mechanism requires stateful nodes to
   maintain lists of known prefixes.  A prefix entry contains the
   following abstract information:

   o  A reference to the ND entry that was created for the advertising
      neighbor.

   o  The IPv6 address and interface for the advertising neighbor.

   o  The logical equivalent of the full destination advertisement
      information (including the prefixes, depth, and Reverse Route
      Stack, if any).

   o  A 'reported' Boolean to keep track whether this prefix was
      reported already, and to which of the DA parents.




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   o  A counter of retries to count how many DIO messages were sent on
      the interface to the advertising neighbor without reachability
      confirmation for the prefix.

   Note that nodes may receive multiple information from different
   neighbors for a specific destination, as different paths through the
   DAG may be propagating information inwards along the DAG for the same
   destination.  A node that is recording routing state will keep track
   of the information from each neighbor independently, and when it
   comes time to propagate the DAO message for a particular prefix to
   the DA parents, then the DAO information will be selected from among
   the advertising neighbors who offer the least depth to the
   destination.

   The destination advertisement mechanism stores the prefix entries in
   one of 3 abstract lists; the Connected, the Reachable and the
   Unreachable lists.

   The Connected list corresponds to the prefixes owned and managed by
   the local node.

   The Reachable list contains prefixes for which the node keeps
   receiving DAO messages, and for those prefixes which have not yet
   timed out.

   The Unreachable list keeps track of prefixes which are no longer
   valid and in the process of being deleted, in order to send DAO
   messages with zero lifetime (also called no-DAO) to the DA parents.

5.10.1.1.1.  Destination Advertisement Timers

   The destination advertisement mechanism requires 2 timers; the
   DelayDAO timer and the RemoveTimer.

   o  The DelayDAO timer is armed upon a stimulation to send a
      destination advertisement (such as a DIO message from a DA
      parent).  When the timer is armed, all entries in the Reachable
      list as well as all entries for Connected list are set to not be
      reported yet for that particular DA parent.

   o  The DelayDAO timer has a duration that is DEF_DAO_LATENCY divided
      by a multiple of the DAG rank of the node.  The intention is that
      nodes located deeper in the DAG should have a shorter DelayDAO
      timer, allowing DAO messages a chance to be reported from deeper
      in the DAG and potentially aggregated along sub-DAGs before
      propagating further inwards.





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   o  The RemoveTimer is used to clean up entries for which DAO messages
      are no longer being received from the sub-DAG.

      *  When a DIO message is sent that is requesting destination
         advertisements, a flag is set for all DAO entries in the
         routing table.

      *  If the flag has already been set for a DAO entry, the retry
         count is incremented.

      *  If a DAO message is received to confirm the entry, the entry is
         refreshed and the flag and count may be cleared.

      *  If at least one entry has reached a threshold value and the
         RemoveTimer is not running, the entry is considered to be
         probably gone and the RemoveTimer is started.

      *  When the RemoveTimer elapse, DAO messages with lifetime 0, i.e.
         no-DAOs, are sent to explicitly inform DA parents that the
         entries which have reached the threshold are no longer
         available, and the related routing states may be propagated and
         cleaned up.

   o  The RemoveTimer has a duration of min (MAX_DESTROY_INTERVAL,
      TBD(DIO Trickle Timer Interval)).

5.10.1.2.  Multicast Destination Advertisement Messages

   It is also possible for a node to multicast a DAO message to the
   link-local scope all-nodes multicast address FF02::1.  This message
   will be received by all node listening in range of the emitting node.
   The objective is to enable direct P2P communication, between
   destinations directly supported by neighboring nodes, without needing
   the RPL routing structure to relay the packets.

   A multicast DAO message MUST be used only to advertise information
   about self, i.e. prefixes in the Connected list or addresses owned by
   this node.  This would typically be a multicast group that this node
   is listening to or a global address owned by this node, though it can
   be used to advertise any prefix owned by this node as well.  A
   multicast DAO message is not used for routing and does not presume
   any DAG relationship between the emitter and the receiver; it MUST
   NOT be used to relay information learned (e.g. information in the
   Reachable list) from another node; information obtained from a
   multicast DAO MAY be installed in the routing table and MAY be
   propagated by a router in unicast DAOs.

   A node receiving a multicast DAO message addressed to FF02::1 MAY



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   install prefixes contained in the DAO message in the routing table
   for local use.  Such a node MUST NOT perform any other processing on
   the DAO message (i.e. such a node does not presume it is a DA
   parent).

5.10.1.3.  Unicast Destination Advertisement Messages from Child to
           Parent

   When sending a destination advertisement to a DA parent, a node
   includes the DAOs for prefix entries not already reported (since the
   last DA Trigger from an DIO message) in the Reachable and Connected
   lists, as well as no-DAOs for all the entries in the Unreachable
   list.  Depending on its policy and ability to retain routing state,
   the receiving node SHOULD keep a record of the reported DAO message.
   If the DAO message offers the best route to the prefix as determined
   by policy and other prefix records, the node SHOULD install a route
   to the prefix reported in the DAO message via the link local address
   of the reporting neighbor and it SHOULD further propagate the
   information in a DAO message.

   The DIO message from the DAG root is used to synchronize the whole
   DAG, including the periodic reporting of destination advertisements
   back up the DAG.  Its period is expected to vary, depending on the
   configuration of the trickle timer that governs the RAs.

   When a node receives a DIO message over an LLN interface from a DA
   parent, the DelayDAO is armed to force a full update.

   When the node broadcasts a DIO message on an LLN interface, for all
   entries on that interface:

   o  If the entry is CONFIRMED, it goes PENDING with the retry count
      set to 0.

   o  If the entry is PENDING, the retry count is incremented.  If it
      reaches a maximum threshold, the entry goes ELAPSED If at least
      one entry is ELAPSED at the end of the process: if the RemoveTimer
      is not running then it is armed with a jitter.

   Since the DelayDAO timer has a duration that decreases with the
   depth, it is expected to receive all DAO messages from all children
   before the timer elapses and the full update is sent to the DA
   parents.

   Once the RemoveTimer is elapsed, the prefix entry is scheduled to be
   removed and moved to the Unreachable list if there are any DA parents
   that need to be informed of the change in status for the prefix,
   otherwise the prefix entry is cleaned up right away.  The prefix



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   entry is removed from the Unreachable list when no more DA parents
   need to be informed.  This condition may be satisfied when a no-DAO
   is sent to all current DA parents indicating the loss of the prefix,
   and noting that in some cases parents may have been removed from the
   set of DA parents.

5.10.1.4.  Other Events

   Finally, the destination advertisement mechanism responds to a series
   of events, such as:

   o  Destination advertisement operation stopped: All entries in the
      abstract lists are freed.  All the routes learned from DAO
      messages are removed.

   o  Interface going down: for all entries in the Reachable list on
      that interface, the associated route is removed, and the entry is
      scheduled to be removed.

   o  Loss of routing adjacency: When the routing adjacency for a
      neighbor is lost, as per the procedures described in Section 5.13,
      and if the associated entries are in the Reachable list, the
      associated routes are removed, and the entries are scheduled to be
      destroyed.

   o  Changes to DA parent set: all entries in the Reachable list are
      set to not 'reported' and DelayDAO is armed.

5.10.1.5.  Aggregation of Prefixes by a Node

   There may be number of cases where a aggregation may be shared within
   a group of nodes.  In such a case, it is possible to use aggregation
   techniques with destination advertisements and improve scalability.

   Other cases might occur for which additional support is required:

   1.  The aggregating node is attached within the sub-DAG of the nodes
       it is aggregating for.

   2.  A node that is to be aggregated for is located somewhere else
       within the DAG, not in the sub-DAG of the aggregating node.

   3.  A node that is to be aggregated for is located somewhere else in
       the LLN.

   Consider a node M that is performing an aggregation, and a node N
   that is to be a member of the aggregation group.  A node Z situated
   above the node M in the DAG, but not above node N, will see the



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   advertisements for the aggregation owned by M but not that of the
   individual prefix for N. Such a node Z will route all the packets for
   node N towards node M, but node M will have no route to the node N
   and will fail to forward.

   Additional protocols may be applied beyond the scope of this
   specification to dynamically elect/provision an aggregating node and
   groups of nodes eligible to be aggregated in order to provide route
   summarization for a sub-DAG.

5.11.  Loop Detection

   RPL loop avoidance mechanisms are kept simple and designed to
   minimize churn and states.  Loops may form for a number of reasons,
   from control packet loss to sibling forwarding.  RPL includes a
   reactive loop detection technique that protects from meltdown and
   triggers repair of broken paths.

   RPL loop detection uses information that is placed into the packet in
   the flow label.  It assumes that the flow label may be overloaded for
   this purpose.  The flow label is constructed as follows:


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                               |O|S|R|D|  SenderRank   |  InstanceID   |
                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 10: RPL Flow Label

   Outwards 'O' bit:  1-bit flag indicating whether the packet is
         expected to progress inwards or outwards.  A router sets the
         'O' bit when the packet is expect to progress outwards (using
         DAO routes), and resets it when forwarding towards the root of
         the DAG.  A host MUST set the bit to 0.

   Sibling 'S' bit:  1-bit flag indicating whether the packet has been
         forwarded via a sibling at the present rank, and denotes a risk
         of a sibling loop.  A host sets the bit to 0.

   Rank-Error 'R' bit:  1-bit flag indicating whether a rank error was
         detected.  A rank error is detected when there is a mismatch in
         the relative ranks and the direction as indicated in the 'O'
         bit.  A host MUST set the bit to 0.






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   DAO-Error 'D' bit:  1-bit flag indicating whether a DAO error was
         detected.  An undetected DAO error would have resulted in an
         inward to outward transition that is not expected with this
         spec.  A host MUST set the bit to 0.

   SenderRank:  8-bit field indicating the rank of the sender.  A host
         MUST set the rank to INFINITE_RANK.  A router MUST place its
         own rank in the flow label when forwarding.

   InstanceID:  8-bit field indicating the DAG instance along which the
         packet is sent.

5.11.1.  Host Basic Operation

   It is expected that a host that does not participate to RPL in any
   fashion is configured to set the flow label to all zeroes in its
   outgoing packets.  The host MAY send a packet to any router
   regardless of the DAG and RPL operations at large.

   A host that participates to RPL SHOULD zero out all the flags, and it
   MUST set the sender rank to INFINITE_RANK.  If the host can map a
   flow to a given InstanceID then it MUST set the flow label
   accordingly.  Forwarding rules are the same for this host and a
   router, and are described in the next section.

5.11.2.  Instance Forwarding

   Instance IDs is used to avoid loops between DAGs from different
   origins.  DAGs that constructed for antagonistic constraints might
   contain paths that, if mixed together, would yield loops.  Those
   loops are avoided by forwarding a packet along the DAG that is
   associated to a given instance.

   The InstanceID is placed by the source in the flow label.  It is not
   meaningful if the packet has the flow label set to all zeroes.
   Otherwise it MUST match the DAG instance onto which the packet is
   placed by any node, be it a host or router.

   When a router receives a packet that is flagged with a given instance
   ID and the node can forward the packet along the DAG associated to
   that instance, then the router MUST do so and leave the instance ID
   flag unchanged.

   If any node can not forward a packet along the DAG associated to the
   instance ID in the flow label, then the node MAY either change the
   InstanceID to match a DAG that it is using for this packet or discard
   the packet.  That decision is based on a policy.




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   The default policy is as follows: if the node can forward along the
   DAG associated to the instance RPL_DEFAULT_INSTANCE then it should do
   so.  Otherwise it should drop the packet.

5.11.3.  DAG Inconsistency Loop Detection

   The DAG is inconsistent is the direction of a packet does not match
   the rank relationship.  A receiver detects an inconsistency if it
   receives a packet with either:

      the 'O' bit set (to outwards) from a node of a higher rank.

      the 'O' bit reset (for inwards) from a node of a lesser rank.

      the 'S' bit set (to sibling) from a node of a different rank.

   The propagation of a new sequence creates local inconsistencies.  In
   particular, it is possible for a router to forward a packet to a
   future parent (same instance, same DAGID, higher sequence) without a
   loop, regardless of the rank of that parent.  In that case, the
   sending router MUST present itself as a host on the future DAG and
   use a rank of INFINITE_RANK as it forwards the packets via a future
   parent to avoid a false positive.

   One inconsistency along the path is not considered as a critical
   error and the packet may continue.  But a second detection along the
   path of a same packet should not occur and the packet is dropped.

   This process is controlled by the Rank-Error bit in the Flow Label.
   When an inconsistency, is detected on a packet, if the Rank-Error bit
   was not set then the Rank-Error bit is set.  If it was set the packet
   is discarded and the trickle timer is reset.

5.11.4.  Sibling Loop Avoidance

   When a packet is forwarded along siblings, it cannot be checked for
   forward progress and may loop between siblings.  Experimental
   evidence has shown that one sibling hop can be very useful but is
   generally sufficient to avoid loops.  Based on that evidence, this
   specification enforces the simple rule that a packet may not make 2
   sibling hops in a row.

   When a host issues a packet or when a router forwards a packet to a
   non sibling, the Sibling bit in the packet must be reset.  When a
   router forwards to a sibling: if the Sibling bit was not set then the
   Sibling bit is set.  If the Sibling bit was set then the packet is
   discarded.  This does not denote a graph inconsistency but indicates
   that a new graph should probably be formed with a new sequence.



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5.11.5.  DAO Inconsistency Loop Detection and Recovery

   A DAO inconsistency happens when router that has an outwards DAO
   route via a child that is a remnant from an obsolete state that is
   not matched in the child.  With DAO inconsistency loop recovery, a
   packet can be used to recursively explore and cleanup the obsolete
   DAO states along a sub-DAG.

   In a general manner, a packet that goes outwards should never go
   inwards again.  So rather than routing inwards a packet with the
   Outwards bit set, the router MUST discard the packet.  If DAO
   inconsistency loop recovery is applied, then the router SHOULD send
   the packet to the parent that passed it with the DAO-Error bit set.

   Upon a packet with a DAO bit set, the parent MUST remove the routing
   states that caused forwarding to that child, clear DAO-Error bit and
   send the packet again.  The packet will make its way either to an
   alternate child or inwards to a parent.  If that parent still has an
   inconsistent DAO state via self, the process will recurse and that
   state will be cleaned up as well.

5.12.  Multicast Operation

   This section describes further the multicast routing operations over
   an IPv6 RPL network, and specifically how unicast DAOs can be used to
   relay group registrations inwards.  Wherever the following text
   mentions MLD, one can read MLDv2 or v3.

   As is traditional, a listener uses a protocol such as MLD with a
   router to register to a multicast group.

   Along the path between the router and the root of the DAG, MLD
   requests are mapped and transported as DAO messages within the RPL
   protocol; each hop coalesces the multiple requests for a same group
   as a single DAO message to the parent(s), in a fashion similar to
   proxy IGMP, but recursively between child router and parent up to the
   root.

   A router might select to pass a listener registration DAO message to
   its preferred parent only, in which case multicast packets coming
   back might be lost for all of its sub-DAG if the transmission fails
   over that link.  Alternatively the router might select to copy
   additional parents as it would do for DAO messages advertising
   unicast destinations, in which case there might be duplicates that
   the router will need to prune.

   As a result, multicast routing states are installed in each router on
   the way from the listeners to the root, enabling the root to copy a



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   multicast packet to all its children routers that had issued a DAO
   message including a DAO for that multicast group, as well as all the
   attached nodes that registered over MLD.

   For unicast traffic, it is expected that the grounded root of an RPL
   DAG terminates RPL and MAY redistribute the RPL routes over the
   external infrastructure using whatever routing protocol is used
   there.  For multicast traffic, the root MAY proxy MLD for all the
   nodes attached to the RPL routers (this would be needed if the
   multicast source is located in the external infrastructure).  For
   such a source, the packet will be replicated as it flows outwards
   along the DAG based on the multicast routing table entries installed
   from the DAO message.

   For a source inside the DAG, the packet is passed to the preferred
   parents, and if that fails then to the alternates in the DAG.  The
   packet is also copied to all the registered children, except for the
   one that passed the packet.  Finally, if there is a listener in the
   external infrastructure then the DAG root has to further propagate
   the packet into the external infrastructure.

   As a result, the DAG Root acts as an automatic proxy Rendezvous Point
   for the RPL network, and as source towards the Internet for all
   multicast flows started in the RPL LLN.  So regardless of whether the
   root is actually attached to the Internet, and regardless of whether
   the DAG is grounded or floating, the root can serve inner multicast
   streams at all times.

5.13.  Maintenance of Routing Adjacency

   The selection of successors, along the default paths inward along the
   DAG, or along the paths learned from destination advertisements
   outward along the DAG, leads to the formation of routing adjacencies
   that require maintenance.

   In IGPs such as OSPF [RFC4915] or IS-IS [RFC5120], the maintenance of
   a routing adjacency involves the use of Keepalive mechanisms (Hellos)
   or other protocols such as BFD ([I-D.ietf-bfd-base]) and MANET
   Neighborhood Discovery Protocol (NHDP [I-D.ietf-manet-nhdp]).
   Unfortunately, such an approach is not desirable in constrained
   environments such as LLN and would lead to excessive control traffic
   in light of the data traffic with a negative impact on both link
   loads and nodes resources.  Overhead to maintain the routing
   adjacency should be minimized.  Furthermore, it is not always
   possible to rely on the link or transport layer to provide
   information of the associated link state.  The network layer needs to
   fall back on its own mechanism.




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   Thus RPL makes use of a different approach consisting of probing the
   neighbor using a Neighbor Solicitation message (see [RFC4861]).  The
   reception of a Neighbor Advertisement (NA) message with the
   "Solicited Flag" set is used to verify the validity of the routing
   adjacency.  Such mechanism MAY be used prior to sending a data
   packet.  This allows for detecting whether or not the routing
   adjacency is still valid, and should it not be the case, select
   another feasible successor to forward the packet.

5.14.  Packet Forwarding

   When forwarding a packet to a destination, precedence is given to
   selection of a next-hop successor as follows:

   1.  In the scope of this specification, it is preferred to select a
       successor from a DAG that matches the InstanceID marked in the
       IPv6 header of the packet being forwarded.

   2.  If a local administrative preference favors a route that has been
       learned from a different routing protocol than RPL, then use that
       successor.

   3.  If there is an entry in the routing table matching the
       destination that has been learned from a multicast destination
       advertisement (e.g. the destination is a one-hop neighbor), then
       use that successor.

   4.  If there is an entry in the routing table matching the
       destination that has been learned from a unicast destination
       advertisement (e.g. the destination is located outwards along the
       sub-DAG), then use that successor.

   5.  If there is a DAG offering a route to a prefix matching the
       destination, then select one of those DAG parents as a successor.

   6.  If there is a DAG parent offering a default route then select
       that DAG parent as a successor.

   7.  If there is a DAG offering a route to a prefix matching the
       destination, but all DAG parents have been tried and are
       temporarily unavailable (as determined by the forwarding
       procedure), then select a DAG sibling as a successor.

   8.  Finally, if no DAG siblings are available, the packet is dropped.
       ICMP Destination Unreachable may be invoked.  An inconsistency is
       detected.

   TTL MUST be decremented when forwarding.  If the packet is being



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   forwarded via a sibling, then the TTL MAY be decremented more
   aggressively (by more than one) to limit the impact of possible
   loops.

   Note that the chosen successor MUST NOT be the neighbor that was the
   predecessor of the packet (split horizon), except in the case where
   it is intended for the packet to change from an inward to an outward
   flow, such as switching from DIO routes to DAO routes as the
   destination is neared.


6.  RPL Constants and Variables

   ZERO_LIFETIME  This is the special value of a lifetime that indicates
         immediate death and removal.  ZERO_LIFETIME has a value of 0.

   BASE_RANK  This is the rank for a virtual root that might be used to
         coordinate multiple roots.  BASE_RANK has a value of 0.

   ROOT_RANK  This is the rank for a DAG root.  ROOT_RANK has a value of
         1.

   INFINITE_RANK  This is the constant maximum for the rank.
         INFINITE_RANK has a value of 0xFF.

   RPL_DEFAULT_INSTANCE  This is the instance ID that is used by this
         protocol by a node without a policy to know any better.
         RPL_DEFAULT_INSTANCE has a value of 0.

   DEFAULT_DIO_INTERVAL_MIN  To be determined

   DEFAULT_DIO_INTERVAL_DOUBLINGS  To be determined

   DEF_DAO_LATENCY  To be determined

   MAX_DESTROY_INTERVAL  To be determined

   DIO Timer  One instance per DAG that a node is a member of.  Expiry
         triggers DIO message transmission.  Trickle timer with variable
         interval in [0, DIOIntervalMin..2^DIOIntervalDoublings].  See
         Section 5.4.4

   DAG Sequence Number Increment Timer  Up to one instance per DAG that
         the node is acting as DAG root of.  May not be supported in all
         implementations.  Expiry triggers revision of
         DAGSequenceNumber, causing a new series of updated DIO message
         to be sent.  Interval should be chosen appropriate to
         propagation time of DAG and as appropriate to application



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         requirements (e.g. response time vs. overhead).  See
         Section 5.5

   DelayDAO Timer  Up to one instance per DA parent (the subset of DAG
         parents chosen to receive destination advertisements) per DAG.
         Expiry triggers sending of DAO message to the DA parent.  The
         interval is to be proportional to DEF_DAO_LATENCY/(node rank),
         such that nodes of greater rank (further outward along the DAG)
         expire first, coordinating the sending of DAO messages to allow
         for a chance of aggregation.  See Section 5.10.1.1.1

   RemoveTimer  Up to one instance per DA entry per neighbor (i.e. those
         neighbors that have given DAO messages to this node as a DAG
         parent) Expiry triggers a change in state for the DA entry,
         setting up to do unreachable (No-DAO) advertisements or
         immediately deallocating the DA entry if there are no DA
         parents.  The interval is min(MAX_DESTROY_INTERVAL, TBD(DIO
         Trickle Timer Interval)).  See Section 5.10.1.1.1


7.  Manageability Considerations

   The aim of this section is to give consideration to the manageability
   of RPL, and how RPL will be operated in LLN beyond the use of a MIB
   module.  The scope of this section is to consider the following
   aspects of manageability: fault management, configuration, accounting
   and performance.

7.1.  Control of Function and Policy

7.1.1.  Initialization Mode

   When a node is first powered up, it may either choose to stay silent
   and not send any multicast DIO message until it has joined a DAG, or
   to immediately root a transient DAG and start sending multicast DIO
   messages.  A RPL implementation SHOULD allow configuring whether the
   node should stay silent or should start advertising DIO messages.

   Furthermore, the implementation SHOULD to allow configuring whether
   or not the node should start sending an DIS message as an initial
   probe for nearby DAGs, or should simply wait until it received RA
   messages from other nodes that are part of existing DAGs.

7.1.2.  DIO Base option

   RPL specifies a number of protocol parameters.

   A RPL implementation SHOULD allow configuring the following routing



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   protocol parameters, which are further described in Section 5.1.3.1:

   DAGPreference
   InstanceID
   DAGObjectiveCodePoint
   DAGID
   Destination Prefixes
   DIOIntervalDoublings
   DIOIntervalMin

   DAG Root behavior:  In some cases, a node may not want to permanently
         act as a DAG root if it cannot join a grounded DAG.  For
         example a battery-operated node may not want to act as a DAG
         root for a long period of time.  Thus a RPL implementation MAY
         support the ability to configure whether or not a node could
         act as a DAG root for a configured period of time.

   DAG Table Entry Suppression  A RPL implementation SHOULD provide the
         ability to configure a timer after the expiration of which the
         DAG table that contains all the records about a DAG is
         suppressed, to be invoked if the DAG parent set becomes empty.

7.1.3.  Trickle Timers

   A RPL implementation makes use of trickle timer to govern the sending
   of DIO message.  Such an algorithm is determined a by a set of
   configurable parameters that are then advertised by the DAG root
   along the DAG in DIO messages.

   For each DAG, a RPL implementation MUST allow for the monitoring of
   the following parameters, further described in Section 5.4.4:

   I

   T

   C

   I_min

   I_doublings:

   A RPL implementation SHOULD provide a command (for example via API,
   CLI, or SNMP MIB) whereby any procedure that detects an inconsistency
   may cause the trickle timer to reset.






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7.1.4.  DAG Sequence Number Increment

   A RPL implementation may allow by configuration at the DAG root to
   refresh the DAG states by updating the DAGSequenceNumber.  A RPL
   implementation SHOULD allow configuring whether or not periodic or
   event triggered mechanism are used by the DAG root to control
   DAGSequenceNumber change.

7.1.5.  Destination Advertisement Timers

   The following set of parameters of the DAO messages SHOULD be
   configurable:

   o  The DelayDAO timer

   o  The Remove timer

7.1.6.  Policy Control

   DAG discovery enables nodes to implement different policies for
   selecting their DAG parents.

   A RPL implementation SHOULD allow configuring the set of acceptable
   or preferred Objective Functions (OF) referenced by their Objective
   Codepoints (OCPs) for a node to join a DAG, and what action should be
   taken if none of a node's candidate neighbors advertise one of the
   configured allowable Objective Functions.

   A node in an LLN may learn routing information from different routing
   protocols including RPL.  It is in this case desirable to control via
   administrative preference which route should be favored.  An
   implementation SHOULD allow for specifying an administrative
   preference for the routing protocol from which the route was learned.

   A RPL implementation SHOULD allow for the configuration of the "Route
   Tag" field of the DAO messages according to a set of rules defined by
   policy.

7.1.7.  Data Structures

   Some RPL implementation may limit the size of the candidate neighbor
   list in order to bound the memory usage, in which case some otherwise
   viable candidate neighbors may not be considered and simply dropped
   from the candidate neighbor list.

   A RPL implementation MAY provide an indicator on the size of the
   candidate neighbor list.




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7.2.  Information and Data Models

   The information and data models necessary for the operation of RPL
   will be defined in a separate document specifying the RPL SNMP MIB.

7.3.  Liveness Detection and Monitoring

   The aim of this section is to describe the various RPL mechanisms
   specified to monitor the protocol.

   As specified in Section 5.2, an implementation must maintain a set of
   data structures in support of DAG discovery:

   o  The candidate neighbors data structure

   o  For each DAG:

      *  A set of DAG parents

7.3.1.  Candidate Neighbor Data Structure

   A node in the candidate neighbor list is a node discovered by the
   some means and qualified to potentially become of neighbor or a
   sibling (with high enough local confidence).  A RPL implementation
   SHOULD provide a way monitor the candidate neighbors list with some
   metric reflecting local confidence (the degree of stability of the
   neighbors) measured by some metrics.

   A RPL implementation MAY provide a counter reporting the number of
   times a candidate neighbor has been ignored, should the number of
   candidate neighbors exceeds the maximum authorized value.

7.3.2.  Directed Acyclic Graph (DAG) Table

   For each DAG, a RPL implementation MUST keep track of the following
   DAG table values:

   o  DAGID

   o  DAGObjectiveCodePoint

   o  A set of Destination Prefixes offered inwards along the DAG

   o  A set of DAG Parents

   o  timer to govern the sending of DIO messages for the DAG





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   o  DAGSequenceNumber

   The set of DAG parents structure is itself a table with the following
   entries:

   o  A reference to the neighboring device which is the DAG parent

   o  A record of most recent information taken from the DAG Information
      Object last processed from the DAG Parent

   o  A flag reporting if the Parent is a DA Parent as described in
      Section 5.10

7.3.3.  Routing Table

   For each route provisioned by RPL operation, a RPL implementation
   MUST keep track of the following:

   o  Destination Prefix

   o  Destination Prefix Length

   o  Lifetime Timer

   o  Next Hop

   o  Next Hop Interface

   o  Flag indicating that the route was provisioned from one of:

      *  Unicast DAO message

      *  DIO message

      *  Multicast DAO message

7.3.4.  Other RPL Monitoring Parameters

   A RPL implementation SHOULD provide a counter reporting the number of
   a times the node has detected an inconsistency with respect to a DAG
   parent, e.g. if the DAGID has changed.

   A RPL implementation MAY log the reception of a malformed DIO message
   along with the neighbor identification if avialable.







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7.3.5.  RPL Trickle Timers

   A RPL implementation operating on a DAG root MUST allow for the
   configuration of the following trickle parameters:

   o  The DIOIntervalMin expressed in ms

   o  The DIOIntervalDoublings

   A RPL implementation MAY provide a counter reporting the number of
   times an inconsistency (and thus the trickle timer has been reset).

7.4.  Verifying Correct Operation

   This section has to be completed in further revision of this document
   to list potential Operations and Management (OAM) tools that could be
   used for verifying the correct operation of RPL.

7.5.  Requirements on Other Protocols and Functional Components

   RPL does not have any impact on the operation of existing protocols.

7.6.  Impact on Network Operation

   To be completed.


8.  Security Considerations

   Security Considerations for RPL are to be developed in accordance
   with recommendations laid out in, for example,
   [I-D.tsao-roll-security-framework].


9.  IANA Considerations

9.1.  RPL Control Message

   The RPL Control Message is an ICMP information message type that is
   to be used carry DAG Information Objects, DAG Information
   Solicitations, and Destination Advertisement Objects in support of
   RPL operation.

   IANA has defined a ICMPv6 Type Number Registry.  The suggested type
   value for the RPL Control Message is 155, to be confirmed by IANA.






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9.2.  New Registry for RPL Control Codes

   IANA is requested to create a registry, RPL Control Codes, for the
   Code field of the ICMPv6 RPL Control Message.

   New codes may be allocated only by an IETF Consensus action.  Each
   code should be tracked with the following qualities:

   o  Code

   o  Description

   o  Defining RFC

   Three codes are currently defined:

        +------+----------------------------------+---------------+
        | Code | Description                      | Reference     |
        +------+----------------------------------+---------------+
        | 0x01 | DAG Information Solicitation     | This document |
        | 0x02 | DAG Information Object           | This document |
        | 0x04 | Destination Advertisement Object | This document |
        +------+----------------------------------+---------------+

                             RPL Control Codes

9.3.  New Registry for the Control Field of the DIO Base Option

   IANA is requested to create a registry for the Control field of the
   DIO Base Option.

   New bit numbers may be allocated only by an IETF Consensus action.
   Each bit should be tracked with the following qualities:

   o  Bit number (counting from bit 0 as the most significant bit)

   o  Capability description

   o  Defining RFC

   Four groups are currently defined:










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      +-------+-------------------------------------+---------------+
      |  Bit  | Description                         | Reference     |
      +-------+-------------------------------------+---------------+
      |   0   | Grounded DAG                        | This document |
      |   1   | Destination Advertisement Trigger   | This document |
      |   2   | Destination Advertisement Supported | This document |
      | 5,6,7 | DAG Preference                      | This document |
      +-------+-------------------------------------+---------------+

                           DIO Base Option Flags

9.4.  DAG Information Object (DIO) Suboption

   IANA is requested to create a registry for the DIO Base Option
   Suboptions

         +-------+------------------------------+---------------+
         | Value | Meaning                      | Reference     |
         +-------+------------------------------+---------------+
         |   0   | Pad1 - DIO Padding           | This document |
         |   1   | PadN - DIO suboption padding | This document |
         |   2   | DAG Metric Container         | This Document |
         |   3   | Destination Prefix           | This Document |
         |   4   | DAG Timer Configuration      | This Document |
         +-------+------------------------------+---------------+

            DAG Information Option (DIO) Base Option Suboptions

9.5.  Objective Code Point for the Default Objective Function OF0

   This specification specifies the Default Objective Function (called
   OF0) for which the OCP field of the OF object, as defined in
   [I-D.ietf-roll-routing-metrics], is equal to 0x0000

                    +-------+---------+---------------+
                    | Value | Meaning | Reference     |
                    +-------+---------+---------------+
                    |   0   | OF0     | This document |
                    +-------+---------+---------------+

                              OCP Allocation


10.  Acknowledgements

   The authors would like to acknowledge the review, feedback, and
   comments from Emmanuel Baccelli, Dominique Barthel, Yusuf Bashir,
   Mathilde Durvy, Manhar Goindi, Mukul Goyal, Anders Jagd, Quentin



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   Lampin, Jerry Martocci, Alexandru Petrescu, and Don Sturek.

   The authors would like to acknowledge the guidance and input provided
   by the ROLL Chairs, David Culler and JP Vasseur.

   The authors would like to acknowledge prior contributions of Robert
   Assimiti, Mischa Dohler, Julien Abeille, Ryuji Wakikawa, Teco Boot,
   Patrick Wetterwald, Bryan Mclaughlin, Carlos J. Bernardos, Thomas
   Watteyne, Zach Shelby, Caroline Bontoux, Marco Molteni, Billy Moon,
   and Arsalan Tavakoli, which have provided useful design
   considerations to RPL.


11.  Contributors

   RPL is the result of the contribution of the following members of the
   ROLL Design Team, including the editors, and additional contributors
   as listed below:

   JP Vasseur
   Cisco Systems, Inc
   11, Rue Camille Desmoulins
   Issy Les Moulineaux,   92782
   France

   Email: jpv@cisco.com


   Jonathan W. Hui
   Arch Rock Corporation
   501 2nd St. Ste. 410
   San Francisco, CA  94107
   USA

   Email: jhui@archrock.com


   Thomas Heide Clausen
   LIX, Ecole Polytechnique, France

   Phone: +33 6 6058 9349
   EMail: T.Clausen@computer.org
   URI:   http://www.ThomasClausen.org/


   Richard Kelsey
   Ember Corporation
   Boston, MA



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   USA

   Phone: +1 617 951 1225
   Email: kelsey@ember.com


   Philip Levis
   Stanford University
   358 Gates Hall, Stanford University
   Stanford, CA  94305-9030
   USA

   Email: pal@cs.stanford.edu


   Stephen Dawson-Haggerty
   UC Berkeley
   Soda Hall, UC Berkeley
   Berkeley, CA  94720
   USA

   Email: stevedh@cs.berkeley.edu


   Kris Pister
   Dust Networks
   30695 Huntwood Ave.
   Hayward,   94544
   USA

   Email: kpister@dustnetworks.com


   Anders Brandt
   Zensys, Inc.
   Emdrupvej 26
   Copenhagen, DK-2100
   Denmark

   Email: abr@zen-sys.com


12.  References

12.1.  Normative References

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



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12.2.  Informative References

   [I-D.ietf-bfd-base]
              Katz, D. and D. Ward, "Bidirectional Forwarding
              Detection", draft-ietf-bfd-base-09 (work in progress),
              February 2009.

   [I-D.ietf-manet-nhdp]
              Clausen, T., Dearlove, C., and J. Dean, "MANET
              Neighborhood Discovery Protocol (NHDP)",
              draft-ietf-manet-nhdp-10 (work in progress), July 2009.

   [I-D.ietf-roll-building-routing-reqs]
              Martocci, J., Riou, N., Mil, P., and W. Vermeylen,
              "Building Automation Routing Requirements in Low Power and
              Lossy Networks", draft-ietf-roll-building-routing-reqs-07
              (work in progress), September 2009.

   [I-D.ietf-roll-home-routing-reqs]
              Brandt, A., Buron, J., and G. Porcu, "Home Automation
              Routing Requirements in Low Power and Lossy Networks",
              draft-ietf-roll-home-routing-reqs-08 (work in progress),
              September 2009.

   [I-D.ietf-roll-routing-metrics]
              Vasseur, J. and D. Networks, "Routing Metrics used for
              Path Calculation in Low Power and Lossy Networks",
              draft-ietf-roll-routing-metrics-01 (work in progress),
              October 2009.

   [I-D.ietf-roll-terminology]
              Vasseur, J., "Terminology in Low power And Lossy
              Networks", draft-ietf-roll-terminology-02 (work in
              progress), October 2009.

   [I-D.tsao-roll-security-framework]
              Tsao, T., Alexander, R., Dohler, M., Daza, V., and A.
              Lozano, "A Security Framework for Routing over Low Power
              and Lossy Networks", draft-tsao-roll-security-framework-01
              (work in progress), September 2009.

   [Levis08]  Levis, P., Brewer, E., Culler, D., Gay, D., Madden, S.,
              Patel, N., Polastre, J., Shenker, S., Szewczyk, R., and A.
              Woo, "The Emergence of a Networking Primitive in Wireless
              Sensor Networks", Communications of the ACM, v.51 n.7,
              July 2008,
              <http://portal.acm.org/citation.cfm?id=1364804>.




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   [RFC2453]  Malkin, G., "RIP Version 2", STD 56, RFC 2453,
              November 1998.

   [RFC3819]  Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, July 2004.

   [RFC4101]  Rescorla, E. and IAB, "Writing Protocol Models", RFC 4101,
              June 2005.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, June 2007.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120, February 2008.

   [RFC5548]  Dohler, M., Watteyne, T., Winter, T., and D. Barthel,
              "Routing Requirements for Urban Low-Power and Lossy
              Networks", RFC 5548, May 2009.

   [RFC5673]  Pister, K., Thubert, P., Dwars, S., and T. Phinney,
              "Industrial Routing Requirements in Low-Power and Lossy
              Networks", RFC 5673, October 2009.


Appendix A.  Requirements

A.1.  Protocol Properties Overview

   RPL demonstrates the following properties, consistent with the
   requirements specified by the application-specific requirements
   documents.





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A.1.1.  IPv6 Architecture

   RPL is strictly compliant with layered IPv6 architecture.

   Further, RPL is designed with consideration to the practical support
   and implementation of IPv6 architecture on devices which may operate
   under severe resource constraints, including but not limited to
   memory, processing power, energy, and communication.  The RPL design
   does not presume high quality reliable links, and operates over lossy
   links (usually low bandwidth with low packet delivery success rate).

A.1.2.  Typical LLN Traffic Patterns

   Multipoint-to-Point (MP2P) and Point-to-multipoint (P2MP) traffic
   flows from nodes within the LLN from and to egress points are very
   common in LLNs.  Low power and lossy network Border Router (LBR)
   nodes may typically be at the root of such flows, although such flows
   are not exclusively rooted at LBRs as determined on an application-
   specific basis.  In particular, several applications such as building
   or home automation do require P2P (Point-to-Point) communication.

   As required by the aforementioned routing requirements documents, RPL
   supports the installation of multiple paths.  The use of multiple
   paths include sending duplicated traffic along diverse paths, as well
   as to support advanced features such as Class of Service (CoS) based
   routing, or simple load balancing among a set of paths (which could
   be useful for the LLN to spread traffic load and avoid fast energy
   depletion on some, e.g. battery powered, nodes).  Conceptually,
   multiple instances of RPL can be used to send traffic along different
   topology instances, the construction of which is governed by
   different Objective Functions (OF).  Details of RPL operation in
   support of multiple instances are beyond the scope of the present
   specification.

A.1.3.  Constraint Based Routing

   The RPL design supports constraint based routing, based on a set of
   routing metrics and constraints.  The routing metrics and constraints
   for links and nodes with capabilities supported by RPL are specified
   in a companion document to this specification,
   [I-D.ietf-roll-routing-metrics].  RPL signals the metrics,
   constraints, and related Objective Functions (OFs) in use in a
   particular implementation by means of an Objective Code Point (OCP).
   Both the routing metrics, constraints, and the OF help determine the
   construction of the Directed Acyclic Graphs (DAG) using a distributed
   path computation algorithm.





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A.2.  Deferred Requirements

   NOTE: RPL is still a work in progress.  At this time there remain
   several unsatisfied application requirements, but these are to be
   addressed as RPL is further specified.


Appendix B.  Examples

   Consider the example LLN physical topology in Figure 11.  In this
   example the links depicted are all usable L2 links.  Suppose that all
   links are equally usable, and that the implementation specific policy
   function is simply to minimize hops.  This LLN physical topology then
   yields the DAG depicted in Figure 12, where the links depicted are
   the edges toward DAG parents.  This topology includes one DAG, rooted
   by an LBR node (LBR) at rank 1.  The LBR node will issue DIO
   messages, as governed by a trickle timer.  Nodes (11), (12), (13),
   have selected (LBR) as their only parent, attached to the DAG at rank
   2, and periodically multicast DIOs.  Node (22) has selected (11) and
   (12) in its DAG parent set, and advertises itself at rank 3.  Node
   (22) thus has a set of DAG parents {(11), (12)} and siblings {((21),
   (23)}.


                                     (LBR)
                                     / | \
                                .---`  |  `----.
                               /       |        \
                            (11)------(12)------(13)
                             | \       | \       | \
                             |  `----. |  `----. |  `----.
                             |        \|        \|        \
                            (21)------(22)------(23)      (24)
                             |        /|        /|         |
                             |  .----` |  .----` |         |
                             | /       | /       |         |
                            (31)------(32)------(33)------(34)
                             |        /| \       | \       | \
                             |  .----` |  `----. |  `----. |  `----.
                             | /       |        \|        \|        \
                   .--------(41)      (42)      (43)------(44)------(45)
                  /         /         /| \       | \
            .----`    .----`    .----` |  `----. |  `----.
           /         /         /       |        \|        \
        (51)------(52)------(53)------(54)------(55)------(56)


   Note that the links depicted represent the usable L2 connectivity



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   available in the LLN.  For example, Node (31) can communicate
   directly with its neighbors, Nodes (21), (22), (32), and (41).  Node
   (31) cannot communicate directly with any other nodes, e.g. (33),
   (23), (42).  In this example these links offer bidirectional
   communication, and `bad' links are not depicted.

                      Figure 11: Example LLN Topology



                                     (LBR)
                                     / | \
                                .---`  |  `----.
                               /       |        \
                            (11)      (12)      (13)
                             | \       | \       | \
                             |  `----. |  `----. |  `----.
                             |        \|        \|        \
                            (21)      (22)      (23)      (24)
                             |        /|        /|         |
                             |  .----` |  .----` |         |
                             | /       | /       |         |
                            (31)      (32)      (33)      (34)
                             |        /| \       | \       | \
                             |  .----` |  `----. |  `----. |  `----.
                             | /       |        \|        \|        \
                   .--------(41)      (42)      (43)      (44)      (45)
                  /         /         /| \       | \
            .----`    .----`    .----` |  `----. |  `----.
           /         /         /       |        \|        \
        (51)      (52)      (53)      (54)      (55)      (56)


   Note that the links depicted represent directed links in the DAG
   overlaid on top of the physical topology depicted in Figure 11.  As
   such, the depicted edges represent the relationship between nodes and
   their DAG parents, wherein all depicted edges are directed and
   oriented `up' on the page toward the DAG root (LBR).  The DAG may
   provide default routes within the LLN, and serves as the foundation
   on which RPL builds further routing structure, e.g. through the
   destination advertisement mechanism.

                          Figure 12: Example DAG








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B.1.  Destination Advertisement

   Consider the example DAG depicted in Figure 12.  Suppose that Nodes
   (22) and (32) are unable to record routing state.  Suppose that Node
   (42) is able to perform prefix aggregation on behalf of Nodes (53),
   (54), and (55).

   o  Node (53) would send a DAO message to Node (42), indicating the
      availability of destination (53).

   o  Node (54) and Node (55) would similarly send DAO messages to Node
      (42) indicating their own destinations.

   o  Node (42) would collect and store the routing state for
      destinations (53), (54), and (55).

   o  In this example, Node (42) may then be capable of representing
      destinations (42), (53), (54), and (55) in the aggregation (42').

   o  Node (42) sends a DAO message advertising destination (42') to
      Node 32.

   o  Node (32) does not want to maintain any routing state, so it adds
      onto to the Reverse Route Stack in the DAO message and passes it
      on to Node (22) as (42'):[(42)].  It may send a separate DAO
      message to indicate destination (32).

   o  Node (22) does not want to maintain any routing state, so it adds
      on to the Reverse Route Stack in the DAO message and passes it on
      to Node (12) as (42'):[(42), (32)].  It also relays the DAO
      message containing destination (32) to Node 12 as (32):[(32)], and
      finally may send a DAO message for itself indicating destination
      (22).

   o  Node (12) is capable to maintain routing state again, and receives
      the DAO messages from Node (22).  Node (12) then learns:
      *  Destination (22) is available via Node (22)
      *  Destination (32) is available via Node (22) and the piecewise
         source route to (32)
      *  Destination (42') is available via Node (22) and the piecewise
         source route to (32), (42').

   o  Node (12) sends DAO messages to (LBR), allowing (LBR) to learn
      routes to the destinations (12), (22), (32), and (42'). (42),
      (53), (54), and (55) are available via the aggregation (42').  It
      is not necessary for Node (12) to propagate the piecewise source
      routes to (LBR).




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B.2.  Example: DAG Parent Selection

   For example, suppose that a node (N) is not attached to any DAG, and
   that it is in range of nodes (A), (B), (C), (D), and (E).  Let all
   nodes be configured to use an OCP which defines a policy such that
   ETX is to be minimized and paths with the attribute `Blue' should be
   avoided.  Let the rank computation indicated by the OCP simply
   reflect the ETX aggregated along the path.  Let the links between
   node (N) and its neighbors (A-E) all have an ETX of 1 (which is
   learned by node (N) through some implementation specific method).
   Let node (N) be configured to send RPL DIS messages to probe for
   nearby DAGs.

   o  Node (N) transmits a RPL DIS message.

   o  Node (B) responds.  Node (N) investigates the DIO message, and
      learns that Node (B) is a member of DAGID 1 at rank 4, and not
      `Blue'.  Node (N) takes note of this, but is not yet confident.

   o  Similarly, Node (N) hears from Node (A) at rank 9, Node (C) at
      rank 5, and Node (E) at rank 4.

   o  Node (D) responds.  Node (D) has a DIO message that indicates that
      it is a member of DAGID 1 at rank 2, but it carries the attribute
      `Blue'.  Node (N)'s policy function rejects Node (D), and no
      further consideration is given.

   o  This process continues until Node (N), based on implementation
      specific policy, builds up enough confidence to trigger a decision
      to join DAGID 1.  Let Node (N) determine its most preferred parent
      to be Node (E).

   o  Node (N) adds Node (E) (rank 4) to its set of DAG parents for
      DAGID 1.  Following the mechanisms specified by the OCP, and given
      that the ETX is 1 for the link between (N) and (E), Node (N) is
      now at rank 5 in DAGID 1.

   o  Node (N) adds Node (B) (rank 4) to its set of DAG parents for
      DAGID 1.

   o  Node (N) is a sibling of Node (C), both are at rank 5.

   o  Node (N) may now forward traffic intended for the default
      destination inward along DAGID 1 via nodes (B) and (E).  In some
      cases, e.g. if nodes (B) and (E) are tried and fail, node (N) may
      also choose to forward traffic to its sibling node (C), without
      making inward progress but with the intention that node (C) or a
      following successor can make inward progress.  Should Node (C) not



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      have a viable parent, it should never send the packet back to Node
      (N) (to avoid a 2-node loop).

B.3.  Example: DAG Maintenance


          :                      :                      :
          :                      :                      :
         (A)                    (A)                    (A)
          |\                     |                      |
          | `-----.              |                      |
          |        \             |                      |
         (B)       (C)          (B)       (C)          (B)
                    |                      |             \
                    |                      |              `-----.
                    |                      |                     \
                   (D)                    (D)                    (C)
                                                                  |
                                                                  |
                                                                  |
                                                                 (D)

              -1-                    -2-                    -3-


                        Figure 13: DAG Maintenance

   Consider the example depicted in Figure 13-1.  In this example, Node
   (A) is attached to a DAG at some rank d.  Node (A) is a DAG parent of
   Nodes (B) and (C).  Node (C) is a DAG parent of Node (D).  There is
   also an undirected sibling link between Nodes (B) and (C).

   In this example, Node (C) may safely forward to Node (A) without
   creating a loop.  Node (C) may not safely forward to Node (D),
   contained within it's own sub-DAG, without creating a loop.  Node (C)
   may forward to Node (B) in some cases, e.g. the link (C)->(A) is
   temporarily unavailable, but with some chance of creating a loop
   (e.g. if multiple nodes in a set of siblings start forwarding
   `sideways' in a cycle) and requiring the intervention of additional
   mechanisms to detect and break the loop.

   Consider the case where Node (C) hears a DIO message from a Node (Z)
   at a lesser rank and superior position in the DAG than node (A).
   Node (C) may safely undergo the process to evict node (A) from its
   DAG parent set and attach directly to Node (Z) without creating a
   loop, because its rank will decrease.

   Now consider the case where the link (C)->(A) becomes nonviable, and



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   node (C) must move to a deeper rank within the DAG:

   o  Node (C) must first detach from the DAG by removing Node (A) from
      its DAG parent set, leaving an empty DAG parent set.  Node (C) may
      become the root of its own floating, less preferred, DAG.

   o  Node (D), hearing a modified DIO message from Node (C), follows
      Node (C) into the floating DAG.  This is depicted in Figure 13-2.
      In general, any node with no other options in the sub-DAG of Node
      (C) will follow Node (C) into the floating DAG, maintaining the
      structure of the sub-DAG.

   o  Node (C) hears a DIO message with an incremented DAGSequenceNumber
      from Node (B) and determines it is able to rejoin the grounded DAG
      by reattaching at a deeper rank to Node (B).  Node (C) adds Node
      (B) to its DAG parent set.  Node (C) has now safely moved deeper
      within the grounded DAG without creating any loops.

   o  Node (D), and any other sub-DAG of Node (C), will hear the
      modified DIO message sourced from Node (C) and follow Node (C) in
      a coordinated manner to reattach to the grounded DAG.  The final
      DAG is depicted in Figure 13-3

B.4.  Example: Greedy Parent Selection and Instability


         (A)                    (A)                    (A)
          |\                     |\                     |\
          | `-----.              | `-----.              | `-----.
          |        \             |        \             |        \
         (B)       (C)          (B)        \            |        (C)
                                  \        |            |        /
                                   `-----. |            | .-----`
                                          \|            |/
                                          (C)          (B)

              -1-                    -2-                    -3-


                  Figure 14: Greedy DAG Parent Selection

   Consider the example depicted in Figure 14.  A DAG is depicted in 3
   different configurations.  A usable link between (B) and (C) exists
   in all 3 configurations.  In Figure 14-1, Node (A) is a DAG parent
   for Nodes (B) and (C), and (B)--(C) is a sibling link.  In
   Figure 14-2, Node (A) is a DAG parent for Nodes (B) and (C), and Node
   (B) is also a DAG parent for Node (C).  In Figure 14-3, Node (A) is a
   DAG parent for Nodes (B) and (C), and Node (C) is also a DAG parent



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   for Node (B).

   If a RPL node is too greedy, in that it attempts to optimize for an
   additional number of parents beyond its preferred parent, then an
   instability can result.  Consider the DAG illustrated in Figure 14-1.
   In this example, Nodes (B) and (C) may most prefer Node (A) as a DAG
   parent, but are operating under the greedy condition that will try to
   optimize for 2 parents.

   When the preferred parent selection causes a node to have only one
   parent and no siblings, the node may decide to insert itself at a
   slightly higher rank in order to have at least one sibling and thus
   an alternate forwarding solution.  This does not deprive other nodes
   of a forwarding solution and this is considered acceptable
   greediness.

   o  Let Figure 14-1 be the initial condition.

   o  Suppose Node (C) first is able to leave the DAG and rejoin at a
      lower rank, taking both Nodes (A) and (B) as DAG parents as
      depicted in Figure 14-2.  Now Node (C) is deeper than both Nodes
      (A) and (B), and Node (C) is satisfied to have 2 DAG parents.

   o  Suppose Node (B), in its greediness, is willing to receive and
      process a DIO message from Node (C) (against the rules of RPL),
      and then Node (B) leaves the DAG and rejoins at a lower rank,
      taking both Nodes (A) and (C) as DAG parents.  Now Node (B) is
      deeper than both Nodes (A) and (C) and is satisfied with 2 DAG
      parents.

   o  Then Node (C), because it is also greedy, will leave and rejoin
      deeper, to again get 2 parents and have a lower rank then both of
      them.

   o  Next Node (B) will again leave and rejoin deeper, to again get 2
      parents

   o  And again Node (C) leaves and rejoins deeper...

   o  The process will repeat, and the DAG will oscillate between
      Figure 14-2 and Figure 14-3 until the nodes count to infinity and
      restart the cycle again.

   o  This cycle can be averted through mechanisms in RPL:

      *  Nodes (B) and (C) stay at a rank sufficient to attach to their
         most preferred parent (A) and don't go for any deeper (worse)
         alternate parents (Nodes are not greedy)



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      *  Nodes (B) and (C) do not process DIO messages from nodes deeper
         than themselves (because such nodes are possibly in their own
         sub-DAGs)


Appendix C.  Outstanding Issues

   This section enumerates some outstanding issues that are to be
   addressed in future revisions of the RPL specification.

C.1.  Additional Support for P2P Routing

   In some situations the baseline mechanism to support arbitrary P2P
   traffic, by flowing inward along the DAG until a common parent is
   reached and then flowing outward, may not be suitable for all
   application scenarios.  A related scenario may occur when the outward
   paths setup along the DAG by the destination advertisement mechanism
   are not be the most desirable outward paths for the specific
   application scenario (in part because the DAG links may not be
   symmetric).  It may be desired to support within RPL the discovery
   and installation of more direct routes `across' the DAG.  Such
   mechanisms need to be investigated.

C.2.  Loop Detection

   It is under investigation to complement the loop avoidance strategies
   provided by RPL with a loop detection mechanism that may be employed
   when traffic is forwarded.

C.3.  Destination Advertisement / DAO Fan-out

   When DAO messages are relayed to more than one DAG parent, in some
   cases a situation may be created where a large number of DAO messages
   conveying information about the same destination flow inward along
   the DAG.  It is desirable to bound/limit the multiplication/fan-out
   of DAO messages in this manner.  Some aspects of the Destination
   Advertisement mechanism remain under investigation, such as behavior
   in the face of links that may not be symmetric.

   In general, the utility of providing redundancy along outwards routes
   by sending DAO messages to more than one parent is under
   investigation.

   The use of suitable triggers, such as the `D' bit, to trigger DA
   operation within an affected sub-DAG, is under investigation.
   Further, the ability to limit scope of the affected depth within the
   sub-DAG is under investigation (e.g. if a stateful node can proxy for
   all nodes `behind' it, then there may be no need to propagate the



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   triggered `D' bit further).

C.4.  Source Routing

   In support of nodes that maintain minimal routing state, and to make
   use of the collection of piecewise source routes from the destination
   advertisement mechanism, there needs to be some investigation of a
   mechanism to specify, attach, and follow source routes for packets
   traversing the LLN.

C.5.  Address / Header Compression

   In order to minimize overhead within the LLN it is desirable to
   perform some sort of address and/or header compression, perhaps via
   labels, addresses aggregation, or some other means.  This is still
   under investigation.


Authors' Addresses

   Tim Winter (editor)

   Email: wintert@acm.org


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

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com


   ROLL Design Team
   IETF ROLL WG

   Email: rpl-authors@external.cisco.com










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